1

Lead isotopes in the Eastern Equatorial Pacific record

Quaternary migration of the South Westerlies

Sylvain Pichat1,2*

, Wafa Abouchami2,3

, Stephen J. G. Galer2

1 Laboratoire de Géologie de Lyon (LGL-TPE), Ecole Normale Supérieure de Lyon, CNRS

UMR 5276, 69007 Lyon, France

2 Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany

3 Westfälische Wilhelms Universität, Institut für Mineralogie, 48149 Münster, Germany

* Corresponding author.

Tel.: +33 (0)472 728 792; Fax: +33 (0)472 728 677; E-mail address: spichat@ens-lyon.fr

(S. Pichat)

Highlights:

Dust sources to the Eastern Equatorial Pacific evaluated using a 160-kyr Pb isotope

record

The two main sources are from South America, in the Central and Southern Andes

EEP Pb isotope and Antarctica temperature records are strongly correlated

Dust contributions modulated by glacial-interglacial latitudinal shift of the South

Westerlies

2

Abstract

The influence of atmospheric dust on climate and biogeochemical cycles in the oceans

is well understood but poorly quantified. Glacial atmospheric dust loads were generally

greater than those during the Holocene, as shown, for example, by the covariation of dust

fluxes in the Equatorial Pacific and Antarctic ice cores. Nevertheless, it remains unclear

whether these increases in dust flux were associated with changes in sources of dust, which

would in turn suggest variations in wind patterns, climate or paleo-environment. Such

questions can be answered using radiogenic isotope tracers of dust provenance.

Here, we present a 160-kyr high-precision lead isotope time-series of dust input to the

Eastern Equatorial Pacific (EEP) from core ODP Leg 138, Site 849 (0°11.59'N,

110°31.18'W). The Pb isotope record, combined with Nd isotope data, rules out contributions

from Northern Hemisphere dust sources, north of the Intertropical Convergence Zone, such

as Asia or North Africa/Sahara; similarly, eolian sources in Australia, Central America, the

Northern Andes and Patagonia appear insignificant based upon the radiogenic isotope data.

Fluctuations in Pb isotope ratios throughout the last 160 kyr show, instead, that South

America remained the prevailing source of dusts to the EEP. There are two distinct South

American Pb isotope end-members, constrained to be located in the south Central Volcanic

Zone (CVZ, 22°S-27.5°S) and the South Volcanic Zone (SVZ, 33°S-43°S), with the former

most likely originating in the Atacama Desert. Dust availability in the SVZ appears to be

related to the weathering of volcanic deposits and the development of ash-derived Andosols,

and influenced by local factors that might include vegetation cover. Variations in the dust

fluxes from the two sources are in phase with both the dust flux and temperature records from

Antarctican ice cores. We show that the forcing of dust provenance over time in the EEP

overall is influenced by high-southerly-latitude climate conditions, leading to changes in the

latitudinal position and strength of the South Westerlies as well as the coastal winds that blow

northward along the Chilean margin. The net result is a modulation of dust emission from the

Atacama Desert and the SVZ via a northward migration of the South Westerlies during cold

periods and southward retreat during glacial terminations.

1. Introduction

Dust plays an important role in the whole Earth system − as recently reviewed by

Maher et al. (2010) − as it directly controls the radiative balance of the Earth, which in turn

affects the overall hydrological cycle (Claquin et al., 2003). Dust input also significantly

modifies oceanic biogeochemical cycles, in particular by delivering key biolimiting

micronutrients, such as iron, for the operation of the “biological carbon pump” (Sigman and

Boyle, 2000). A better understanding and knowledge of dust sources is thus crucial for

studies attempting to reconstruct Quaternary climate variations and for modeling climate

changes in the future (Bauer and Ganopolski, 2010).

Records from ice cores, marine and terrestrial sediment, along with modeling studies,

indicate that glacial periods – and in particular the Last Glacial Maximum (LGM) − were

“dustier” overall compared to the Holocene (e.g. Kohfeld and Harrison, 2001). Similarly,

records of dust flux intensity in the Eastern Equatorial Pacific (EEP) show a greater

“dustiness” during glacial times (Winckler et al., 2008). Many mechanisms have been

invoked to explain the increased glacial dust flux and the possible related changes in dust

sources for a given area. These include global factors such as wind strength (Lunt and

Valdes, 2002a; Mahowald et al., 2006; Werner et al., 2002), or gustiness (Lunt and Valdes,

2002b; McGee et al., 2010), as well as local conditions such as vegetation cover or soil

3

moisture (Lunt and Valdes, 2002a; Mahowald et al., 2006), erosion of fine sediment by

glaciers (Mahowald et al., 2006; Sugden et al. 2009), exposure of land due to lower sea-level

(Mahowald et al., 2011), aridity (Prospero et al., 2002), precipitation changes (Werner et al.,

2002), and snow/ice cover (Lunt and Valdes, 2002b). Identifying the factors controlling dust

production at a specific location is achievable with knowledge of dust emission sources and

their variability through time, and this is the primary aim of this study in the EEP.

While the main dust sources to the Northern Hemisphere are relatively well known

(Engelstaedter et al., 2006; Ginoux et al., 2012; Nakai et al., 1993; Prospero et al., 2002), the

limited amount of data available from the EEP render identifying dust sources difficult. For

this reason, modeling (Luo et al., 2003; Mahowald et al., 2011; Tanaka and Chiba, 2006),

mineralogical (Hyeong et al., 2005), geochemical (Hyeong et al., 2005; Nakai et al., 1993),

and radiogenic isotope (Grousset and Biscaye, 2005; Jones et al., 2000; Nakai et al., 1993;

Pettke et al., 2002; Stancin et al., 2006; Xie and Marcantonio, 2012) studies have proposed a

plethora of potential source areas: South and Central America, East Asia, North

Africa/Sahara, and Australia. The uncertainties in the identification of dust source areas for

the EEP is due to the methods used in dust-models for defining the “sources”, which may

overlook minor dust emitting regions, the limited datasets available for the EEP itself, and the

difficulty in unequivocally pinpointing a given source based on geochemical data alone.

Constraining sources of eolian dust, and assessing their evolution in space and time, is

most easily done using radiogenic isotope tracers. Lead isotopes in deep-sea sediments have

been used to infer the source of dust blown into the oceans (e.g. Jones et al., 2000; Grousset

and Biscaye, 2005; Stancin et al., 2006, 2008). Further, glacial-interglacial variations in Pb

isotope variations have been documented in bulk sediments from the tropical Atlantic Ocean,

and related to latitudinal shifts in the Intertropical Convergence Zone (ITCZ) (Abouchami

and Zabel, 2003), demonstrating the potential of Pb isotopes for reconstructing paleo-dust

provenance and wind trajectories.

In this study, we present a 160-kyr Pb isotope record from core ODP Leg 138 Site 849

- hereafter referred to as ODP849 - located in the EEP, along with data from the nearby site-

survey core VNTR01-8PC. The Pb isotope data are used to determine the provenance of dust

contributing to the EEP, how source area contributions varied over the last two glacial-

interglacial cycles, and how this can be related to Southern Hemisphere climate. Recent

radiogenic isotope data on ODP cores in the EEP, including ODP849, have been interpreted

as reflecting a predominant contribution of an Australian source of dust, south of the ITCZ,

over the last 25 ka (Xie and Marcantonio, 2012). Our study extends the Pb isotope record of

ODP849 back to 160 kyrs and provides additional new constraints on the sources of dust to

the EEP.

We show that Pb isotopic compositions in core ODP849 display sharp changes

coincident with glacial-interglacial transitions and that the variations over the last 160 kyr,

overall, are in phase with the Antarctica temperature record. We argue that South America is

the dominant source of dust to the EEP, and that the sharp changes throughout the record

reflect climate forcing on the two main sources in the Andes: the South Volcanic Zone (SVZ)

and the south Central Volcanic Zone (South CVZ). Dust input from these two areas appears

to be modulated by latitudinal migration and intensity of the South Westerlies, driven

ultimately by changes in southern hemisphere high-latitude temperatures.

4

2. Materials and methods

2.1. Sampling

Samples from two sediment cores located in the EEP were analyzed in this study:

core ODP849 (0°11.59'N, 110°31.18'W, 3851 m water depth) and core VNTR01-8PC

(0.037°N, 110.48°W, 3791 m water depth), the site-survey piston core for core ODP849. Due

to their remote location, the terrigenous fraction of the sediment is largely dominated by

eolian dust (Murray et al., 1995; Winckler et al., 2008). The ODP849 age-depth model (Mix

et al., 1995) is based on the comparison between the benthic foraminifera Cibicides

wuellerstorfi δ18

O record and the SPECMAP stack (Imbrie et al., 1984). For VNTR01-8PC,

we used the age-depth model of Pisias and Mix (1997). Core ODP849 was selected because 230

Th and 232

Th data used to reconstruct the true vertical sedimentary fluxes are available on

the samples studied (Pichat et al., 2004) (see Appendix A, Section A.1), and dust flux

variations have been documented (Winckler et al., 2008). A sample of ash from Chimborazo

volcano (1°28’S, 78°49’W), collected in an ice core drilled at one of the summits (Cumbre

Ventimilla) in 2000, was also analyzed to characterize the Pb isotope signature of potential

ash inputs nearby.

2.2. Methods

2.2.1. Origin of the Pb isotope signal in the bulk sediment assessed by leaching experiments

Leaching experiments were conducted on several samples from core ODP849 in order

to determine the isotopic compositions and partitioning of Pb between the various fractions of

the sediment. While our study is mostly focused on Pb isotopes, we also assessed the

partitioning of Nd in core ODP849 and analyzed Nd and Sr isotopes on detrital fractions

and/or ferromanganese oxide leachates of some samples (see Appendix A, Section A.1 for a

more detailed description).

2.2.2. Pb isotope measurements

Lead isotope data were obtained on 150-500 mg of bulk sediment dissolved using a

mixture of concentrated HNO3 and HF. Lead was separated from other elements by two-step

anion exchange chromatography on 100 µl-sized columns (Biorad AG1-X8, 100-200 mesh)

using HNO3-HBr mixtures (Lugmair and Galer, 1992). The isotope measurements were done

by either thermal ionization mass spectrometry (TIMS) or Multiple-Collector Inductively-

Coupled Plasma Mass-Spectrometer (MC-ICP-MS) (see Table 1 for details). During this

study, Pb procedural blanks, measured routinely alongside each set of 5 to 8 samples, were

lower than 40 pg for the TIMS measurements (8 procedural blanks) and 30-100 pg for the

MC-ICP-MS measurements (3 procedural blanks). The blanks are negligible compared to the

amount of Pb extracted, namely 75-600 ng for TIMS analyses and 300-700 ng for MC-ICP-

MS analyses. Lead concentrations in the bulk sediment were measured by quadrupole ICP-

MS on a separate aliquot (see Table 1 for details).

sample method age (ka) depth 206Pb/

204Pb ± 2σ 207

Pb/204

Pb ± 2σ 208Pb/

204Pb ± 2σ Pb (ppm) ± 2σ N

ODP849 (rmcd)

ODP004-rep1 (1) 18.9148 11 15.6573 11 38.5358 30

ODP004-rep2 (1) 18.9154 12 15.6578 11 38.5368 31

ODP004 average 3.1 0.04 18.9151 15.6575 38.5363 1.92 0.63 2

ODP012-A (1) 18.9908 13 15.6701 14 38.5358 37

ODP012-B (1) 18.9918 9 15.6706 11 38.5371 35

ODP012 average 4.3 0.12 18.9913 15.6703 38.5364 6.58 1.40 2

ODP032 (1) 8.2 0.32 18.7582 11 15.6204 12 38.5035 36 1.46 0.08 2

ODP042-A (1) 18.5921 26 15.5874 32 38.4602 106

ODP042-B (1) 18.5912 13 15.5873 15 38.4597 47

ODP042 average 10.5 0.42 18.5916 15.5874 38.4599 0.94 0.15 3

ODP052-A-rep1 (1) 18.5988 13 15.5906 15 38.4771 49

ODP052-A-rep2 (1) 18.6007 8 15.5929 10 38.4848 30

ODP052-B-rep1 (1) 18.5983 34 15.5914 37 38.4840 114

ODP052-B-rep2 (1) 18.5983 23 15.5914 20 38.4839 52

ODP052 average 12.5 0.52 18.5990 15.5916 38.4825 1.07 0.16 1

ODP062 (1) 14.5 0.62 18.6197 8 15.6002 8 38.5305 26 1.28 0.41 2

ODP072-A (2) 18.6429 13 15.6080 17 38.5590 55

ODP072-B (3) 18.6389 32 15.6043 28 38.5482 76

ODP072 average 17.1 0.74 18.6411 15.6063 38.5543 1.12 0.19 1

ODP082-A-rep1 (1) 18.6638 14 15.6147 16 38.5926 49

ODP082-A-rep2 (1) 18.6634 14 15.6136 16 38.5915 49

ODP082 average 18.9 0.82 18.6636 15.6142 38.5920 1.18 0.11 3

ODP102-A (3) 18.7138 34 15.6216 27 38.5756 68

ODP102-B (3) 18.7126 33 15.6239 28 38.5770 64

ODP102-C (3) 18.7114 25 15.6220 25 38.5791 58

ODP102 average 23.6 1.02 18.7126 15.6225 38.5772 1.28 0.34 3

ODP122-A (1) 18.6418 32 15.6057 26 38.5603 69

ODP122-B (1) 18.6443 48 15.6067 52 38.5621 162

ODP122 average 28.6 1.22 18.6431 15.6062 38.5612 0.76 0.04 3

ODP131 (3) 31.4 1.31 18.6284 30 15.6010 26 38.5358 60 0.86 0.08 2

ODP142-rep1 (3) 18.6820 23 15.6108 26 38.5502 56

ODP142-rep2 (3) 18.6783 30 15.6148 25 38.5512 59

ODP142 average 34.4 1.42 18.6802 15.6128 38.5507 1.12 0.31 3

ODP152a (1) 37.9 1.53 18.6293 13 15.6009 13 38.5417 38 0.71 0.16 1

ODP162-rep1 (1) 18.6353 9 15.6013 10 38.5379 32

ODP162-rep2 (1) 18.6342 9 15.5999 10 38.5332 31

ODP162-rep3 (1) 18.6338 10 15.5994 12 38.5317 38

ODP162 average 41.1 1.62 18.6345 15.6002 38.5342 0.92 0.16 2

ODP170 (3) 44.1 1.70 18.6381 73 15.6022 62 38.5410 150 0.55 0.08 1

ODP182 (1) 48.6 1.82 18.6258 8 15.6000 9 38.5268 28 0.84 0.04 2

ODP192-rep1 (3) 18.6320 22 15.6016 25 38.5384 63

ODP192-rep2 (3) 18.6257 72 15.6042 70 38.5424 155

ODP192 average 52.2 1.92 18.6288 15.6029 38.5404 1.86 0.36 1

ODP202 (1) 55.8 2.02 18.6591 7 15.6053 7 38.5384 22 1.63 0.39 3

ODP211 (1) 59.2 2.11 18.6208 9 15.5963 10 38.5118 30 0.63 0.11 1

ODP222-A (1) 18.6418 26 15.6026 32 38.5467 106

ODP222-B (3) 18.6414 38 15.6033 35 38.5512 79

ODP222 average 63.2 2.22 18.6416 15.6030 38.5490 1.10 0.20 2

ODP232 (1) 66.4 2.32 18.6496 13 15.6011 14 38.5520 44 1.64 0.21 1

Table 1. Pb isotopic composition and concentration for the two cores (ODP849 and VNTR01-8PC) and ashes from

Chimborazo analyzed in this study.

sample method age (ka) depth 206Pb/

204Pb ± 2σ 207

Pb/204

Pb ± 2σ 208Pb/

204Pb ± 2σ Pb (ppm) ± 2σ N

ODP849 (continued) (rmcd)

ODP242-rep1 (1) 18.6327 45 15.5985 56 38.5283 182

ODP242-rep2 (1) 18.6322 10 15.5979 10 38.5263 31

ODP242 average 69.4 2.41 18.6324 15.5982 38.5273 1.51 0.18 2

ODP252 (1) 72.6 2.52 18.6174 13 15.5932 13 38.5000 39 1.39 0.39 2

ODP272-rep1 (1) 18.6570 18 15.6060 22 38.5677 70

ODP272-rep2 (1) 18.6583 9 15.6076 9 38.5731 27

ODP272 average 78.8 2.72 18.6577 15.6068 38.5704 1.37 0.16 1

ODP292 (1) 85.3 2.92 18.6615 12 15.6052 12 38.5638 34 1.58 0.32 2

ODP331-rep1 (1) 18.6423 18 15.6037 17 38.5514 44

ODP331-rep2 (1) 18.6418 18 15.6026 17 38.5504 46

ODP331 average 101.3 3.31 18.6420 15.6031 38.5509 1.23 0.21 1

ODP342-rep1 (3) 18.6365 33 15.6010 29 38.5467 69

ODP342-rep2 (3) 18.6380 40 15.6027 35 38.5530 92

ODP342 average 106.5 3.42 18.6373 15.6019 38.5499 1.32 0.31 3

ODP362-rep1 (1) 18.6382 18 15.6019 20 38.5552 61

ODP362-rep2 (1) 18.6386 13 15.6023 12 38.5567 32

ODP362 average 114.7 3.61 18.6384 15.6021 38.5559 1.52 0.16 1

ODP382 (1) 122.0 3.82 18.6012 8 15.5863 8 38.4753 24 1.17 0.14 1

ODP403 (1) 127.9 4.03 18.6121 7 15.5883 7 38.4846 20 1.22 0.19 1

ODP431 (1) 134.5 4.31 18.6468 8 15.6015 8 38.5539 23 1.10 0.16 1

ODP472-rep1 (1) 18.6693 16 15.6103 14 38.5991 37

ODP472-rep2 (2) 18.6699 11 15.6125 14 38.5956 46

ODP472 average 144.2 4.71 18.6696 15.6114 38.5973 1.34 0.23 1

ODP513-rep1 (3) 18.6484 52 15.6055 47 38.5602 134

ODP513-rep2 (3) 18.6495 30 15.6065 25 38.5637 64

ODP513 average 155.8 5.13 18.6489 15.6060 38.5619 1.09 0.21 1

VNTR01-8PC (cm)

VNTR-8 9 (1) 4.2 9-10 18.6335 11 15.6013 10 38.5307 29 0.99 0.18 1

VNTR-8 13 -rep1 (1) 18.6122 9 15.5937 9 38.5084 26

VNTR-8 13 -rep2 (1) 18.6109 12 15.5926 12 38.5057 31

VNTR-8 13 average 6.0 13-14 18.6116 15.5931 38.5070 1.03 0.20 1

VNTR-8 31 (1) 14.0 31-32 18.5923 10 15.5849 66 38.4591 28 1.17 0.15 1

Chimborazo ashes

Chimbarazo -rep1 (2) 19.0172 14 15.6674 18 38.8678 59

Chimbarazo -rep2 (2) 19.0174 14 15.6673 18 38.8669 58

Chimbarazo average n/a n/a 19.0173 15.6674 38.8673 7.32 0.39 1

Depth: rmcd: revised composite depth for ODP849 in meters (Hagelberg et al., 1995).

Pb isotopes measurements: -A, -B, -C indicate replicate dissolutions. -rep#: replicate measurement number. -

average: average value used for the sample. Method: (1) measurements made by TIMS (Finnigan MAT 261) using the

triple spike technique (Galer, 1999), (2) measurements made by TIMS (ThermoFisher Triton) using the triple spike

technique, (3) measurements made by MC-ICP-MS (Nu HR, Nu instruments) using Tl-doping (White et al., 2000). All Pb

isotopic data were normalized to NIST SRM 981 values of Galer and Abouchami (1998). The average standard values

for measurements made by Finnigan MAT 261 on NIST SRM 981 were:206

Pb/204

Pb = 16.9413 ± 19,207

Pb/204

Pb =

15.4978 ± 21 and208

Pb/204

Pb = 36.7285 ± 56 (n = 75). For measurements made by ThermoFisher Triton:206

Pb/204

Pb =

16.9423 ± 24,207

Pb/204

Pb = 15.5001 ± 22 and208

Pb/204

Pb = 36.7281 ± 54 (n = 13). These values are consistent within

error with the ratios reported by Galer and Abouchami (1998). For MC-ICP-MS measurements, the 2σ standard error on

NIST SRM 981 measured every two samples is 160, 175 and 200 ppm for 206

Pb/204

Pb, 207

Pb/204

Pb and 208

Pb/204

Pb,

Pb concentrations were measured by quadrupole ICP-MS (7700 series, Agilent Technologies). Uncertainty was

assessed with repeated measurements of standard reference marine sediment USGS MAG-1. All the MAG-1 Pb

concentration measurements were within error of the certified reference value: 24 ± 3 ppm. The last column gives the

number (N) of replicate measurement made for concentration.

5

3. Results

3.1. Anthropogenic Pb contamination of the Holocene section in core ODP849

The Pb isotopic compositions and concentrations of the bulk sediment and the

leachates are reported in Table 1 and Appendix A (Tables A.1, A.2), respectively. The three

youngest Holocene samples (8.2 ka to present) of core ODP849 have elevated Pb

concentrations and highly radiogenic 206

Pb/204

Pb and 207

Pb/204

Pb ratios, unlike those of the

remaining samples. These features also contrast with those of the samples of similar age from

core VNTR01-8PC, indicating contamination of the ODP849 Holocene samples by lead-rich

material whose anthropogenic origin is discussed in Appendix A, Section A.2. Since we are

solely interested in the natural Pb isotope signal, only samples older than 8.2 ka from core

ODP849 (i.e. below 0.32 rmcd; Table 1), along with the Holocene samples from core

VNTR01-8PC, will be considered further below.

3.2. Origin of Pb in Eastern Equatorial Pacific core ODP849

The leaching experiments show that the residues have Pb isotopic compositions

similar to those of the bulk sediment (Fig. 1). In addition, the variability among the different

fractions from a single sample is much lower than the overall variability observed over the

whole 160-kyr bulk sediment record. This observation is consistent with the distribution of

Pb amongst the different phases (see Appendix A, Section A.3 for a detailed discussion).

Only a small proportion of Pb, between 0.5 and 5.4% of the total Pb, is carried by the

carbonate fraction (AcA leachate). The ferromanganese fraction (HH leachate) accounts for

8.0 to 22.6 % of the total Pb, while most of the Pb (70.1 - 91.6 %) is associated with the

terrigenous fraction of the sediment (residue) (Appendix A, Table A.1).

Since the Pb isotopic compositions of the terrigenous fraction are virtually

undistinguishable from those of the corresponding bulk sediment (Fig. 1), the bulk

sedimentary record can be considered to monitor closely the composition of the eolian inputs

to the EEP.

3.3. Temporal Pb isotopic variations in Eastern Equatorial Pacific core ODP849

Lead isotopic compositions of the bulk ODP849 sediment are reported in Table 1 and

plotted in Figs. 2A-C. Lead isotope variations are observed over the 160-kyr record, with

radiogenic values generally occurring during glacials, such as the end of Marine Isotope

Stage (MIS) 6, MIS 4, and LGM, while sharp decreases happen at glacial terminations (Fig.

2A-C). During MIS 6, Pb isotope ratios are radiogenic and decrease sharply during

Termination II, reaching minimum values during the climate optimum of MIS 5.5. The MIS

5.5/5.4 transition is marked by a return to high radiogenic Pb isotope ratios. From MIS 5.4 to

the middle of MIS 5.1, the values remain relatively stable, although the temporal resolution

of our record over this section is too low to distinguish stadial/interstadial variations. There is

a sharp decrease before the MIS 5/4 transition followed by an increase until the middle of

MIS 4. Then, values decrease to a minimum at the MIS 4/3 transition. During MIS 3, all three

isotope ratios exhibit a general increase, with the most radiogenic values recorded around 20

ka close to the MIS 3/2 transition. During the last deglaciation, Pb isotope ratios decrease

sharply with minimum values recorded at around 10 ka; thereafter, a steady increase with

time is observed.

15.62

15.61

15.60

15.59

15.58

160140120100806040200

18.72

18.68

18.64

18.60

38.60

38.56

38.52

38.48

5.0

4.5

4.0

3.5

3.0

206 P

b/20

4 Pb

age (ka)

208 P

b/20

4 Pb

207 P

b/20

4 Pb

δ18O

(‰)

A

D

B

C

ODP849

MIS2 MIS4 MIS5 MIS6MIS3Holocene

VNTR01-8PCODP849

38.60

38.56

38.52

38.48

38.44

160140120100806040200

-14-12-10-8-6-4-20246

1200

800

400

00.350.300.250.200.150.10

Tem

pera

ture

EP

ICA

Dom

e C

(°C

)

208 P

b/20

4 Pb

age (ka)

ODP849 Pbisotopic composition

EPICA Dome C temperature

Dus

t EP

ICA

Dom

e C

(ng/

g)

232 T

h flu

x (µ

g/cm

2 /ka)

VNTR01-8PC Pb isotopic composition

MIS2 MIS4 MIS5 MIS6MIS3Holocene

B

C

D

EPICA Dome C dust

ODP849 232Th flux

A

cold

warm

18.57

18.61

18.65

18.69

18.73

0 10 20 30 40 50 60

206 P

b/20

4 Pb

age (ka)

38.45

38.49

38.53

38.57

38.61

0 10 20 30 40 50 60

ODP849 bulk averageAcA

HH

residue

AcAHH1HH2residue

208 P

b/20

4 Pb

ODP849 bulk

A

B

procedure 1: procedure 2:Fig. 1. Leaching experiments. (A) 206Pb/204Pb vs. age and (B) 208Pb/204Pb vs. age for ODP849 samples: bulk and leaching experiments. AcA: fraction dissolved in acetic acid, HH: fraction dissolved in hydroxylamine hydrochlo-ride, residue: residue after leachings (see section 2.2.1. for details). Error bars (2σ) shown only when bigger than signs.

Fig. 2. Pb isotopes records and benthic δ18O record for ODP849. Temporal variations in (A) 206Pb/204Pb, (B) 207Pb/204Pb, and (C) 208Pb/204Pb, for the two sediment cores analyzed in this study. The three youngest ODP849 samples are not plotted here and have been discarded for the reasons outlined in Section 3.1. (D) Benthic δ18O record (C. wuellerstorfi) for core ODP849 (Mix et al., 1995). Green and yellow areas correspond to cold marine isotope stages and stadials of MIS 5, respectively.

Fig. 3. Temporal variations in Pb isotopes, dust and temperature. (A) 208Pb/204Pb in ODP849 (blue) and VNTR01-8PC (green). (B) Antarctica temperature record from EPICA Dome C ice core (Jouzel et al., 2007). Temperature is expressed as a deviation from the average value taken over the last 1000 years. (C) Dust concentration record in the EPICA Dome C ice core (Lambert et al., 2008). (D) 232Th flux normalized to 230Th. Solid diamonds: data from Pichat et al. (2004), except for the three points older than 100 ka which are unpublished data. Open diamonds: data from Winckler et al. (2008). Green and yellow areas correspond to cold marine isotope stages and stadials of MIS 5, respectively.

6

Lead isotopic compositions from the EEP in core ODP849 co-vary remarkably with

the benthic δ18

O signal, which primarily reflects global ice volume (Fig. 2D). The high-

amplitude Pb isotope variations coincide systematically with transitions from glacial to

interglacial conditions and vice versa. Interpreting why lead isotopic variations from the EEP

are in phase with global climate proxy records will form the major focus of the remainder of

this contribution.

To first order, the 208

Pb/204

Pb record of core ODP849 co-varies with the dust content

of EPICA Dome C ice core in Antarctica (Fig. 3A,C). In particular, three main peaks of dust

– at the end of MIS 6, during MIS 4 and at the MIS 3/2 transition – are associated with more

radiogenic Pb isotopic compositions in ODP849. Similarly, the global increase in dust

content in Antarctica ice during MIS 3 is mirrored by an increasing trend of 208

Pb/204

Pb ratios

in core ODP849.

The correspondence between the ODP849 Pb isotope record and the Antarctica

temperature and dust flux records (Fig. 3A-C) is particularly striking. Further, variations in

both 208

Pb/204

Pb ratios and dust concentration in Antarctica are closely followed by the 232

Th

flux normalized to 230

Th, a proxy for dust inputs (Winckler et al., 2008) (Fig. 3D). The fact

that proxies of dust provenance (Pb isotopes) and dust flux (232

Th) to the EEP change in

parallel implies an intimate association between availability and delivery mechanism of dust

in the Southern Hemisphere.

4. Discussion

4.1. Assessing the number of dust sources to the Eastern Equatorial Pacific

Because potential source areas of dust have distinct Pb isotope signatures, these can

be used to pinpoint the provenance of terrigenous inputs to the EEP. Lead isotopes have the

useful property of distinguishing mixing of two or more source end-members in Pb isotope

spaces. Because of the greater variability in 206

Pb/204

Pb and 208

Pb/204

Pb overall in core

ODP849 compared to the 207

Pb/204

Pb ratio, we will mainly focus the discussion on the 208

Pb/204

Pb vs. 206

Pb/204

Pb relationship.

In Pb isotope space, the dataset forms two linear correlations which intersect at the

unradiogenic end (Fig. 4). One array is defined by the majority (n = 27) of ODP849 and

VNTR01-8PC samples, the second by three “outliers”: ODP102 (23.6 ka), 142 (34.4 ka) and

202 (55.8 ka). Further, Principal Component Analysis (PCA) of the dataset indicates that

98.7% of the variance can be accounted for by just two principal components (Appendix A,

Fig. A.4), thus requiring three end-members to explain virtually all the Pb isotope variability

observed in ODP849. Hence, the two Pb isotope arrays are best interpreted in terms of

mixing lines, whereby there are two different radiogenic sources of dust and a common

unradiogenic source. Nevertheless, only two sources of dust predominate at this location over

the past 160 kyr since most of the data lie along a single regression line (main array in Fig.

4). The secondary Pb array (3 samples only) is however significant, being confirmed by

duplicates/triplicates measurements and validated by the PCA.

4.2. Constraining dust sources to the Eastern Equatorial Pacific

Having identified that three distinct sources of dust contribute to the EEP, we now

attempt to determine their origin by comparing the ODP849 Pb isotope dataset with literature

data from potential source areas. Database construction is outlined in Appendix A, Section

38.3

38.4

38.5

38.6

38.7

38.8

38.9

39.0

18.50 18.55 18.60 18.65 18.70 18.75 18.80 18.85 18.90

SVZ

south CVZ

23.6 ka34.4 ka

55.8 ka

ODP849 main arrayODP849 secondary arrayVNTR01-8PCminerotrophic peat soils

208 P

b/20

4 Pb

206Pb/204Pb

-4.2

-4.0

-3.8

-3.6

-3.4

-3.2

-3.0

-2.8

-2.6

εNd

140120100806040200age (ka)

this study

Xie and Marcantonio (2012)

SVZ: - 0.3 ± 2.4

south CVZ: - 5.8 ± 3.6

T1 T2

Fig. 4. 208Pb/204Pb vs. 206Pb/204Pb plot of the two cores measured in this study. Two arrays can be distinguished: a main array (dark blue dotted line, underlined by a gray field) composed by 27 samples of ODP849 and the 3 samples of VNTR01-8PC, and a secondary array (clear blue dotted line) defined by 3 samples from ODP849: ODP102, ODP142, ODP202 correspon-ding to ages of 23.6, 34.4 and 55.8 ka, respectively. The average values of the Pb isotopic composition of volcanic rocks from the South Volcanic Zone (SVZ) and the south Central Volcanic Zone (South CVZ) are indicated by a red square and a dark blue circle, respectively. The mixing line (black) between the SVZ and the South CVZ dust sources is also shown. Ticks along the line represent 10% increments. Along the main array, during cold periods, the Pb isotopic compositions shift towards more radiogenic values (blue arrow) in the direction of the average value of the South CVZ while during warm periods, the shift occurs in the opposite direction towards the unradiogenic composition of the SVZ (red arrow). The Pre-an-thropogenic Holocene data (purple diamonds) for two minerotrophic peat soils located in the Chilean Lake District (38°S-42°S) are from De Vleeschouwer et al. (2008) (see Section 4.5 for details). Note that the SVZ end-member and the CLD peat data also match in a 207Pb/204Pb vs. 206Pb/204Pb plot (not shown).

Fig. 6. Temporal variations of the Nd isotopic compositions in the terrigenous fraction (residue) of ODP849 sediments (see Section 2.2.1. for details). Blue circles: this study; red squares: Xie and Marcantonio (2012). The red and blue arrows indicate the shift in the εNd record due to a change in the relative contribution of the two predominant dust sources to the EEP: towards the SVZ and the South CVZ, respectively. The εNd range for each source is given next to each arrow (see Section 3.4. for details). Green areas correspond to cold marine isotope stages. T1 and T2 indicate Terminations 1 and 2, respectively.

7

A.4. Based on the Pb isotope systematics (Fig. 5), South America appears to be the main dust

source for the EEP. It is also clear from Fig. 5 that Central America, Asia, Australia and

Africa can be excluded as primary sources of dust to the EEP.

Despite the large range in Pb isotopic compositions of various Asian end-members

(Appendix A, Section A.4), they mostly lie above the ODP849 main array (Fig. 5) and thus

do not appear to be suitable candidates for dust sources to the EEP. The lack of an Asian dust

signal in the EEP is corroborated by radiogenic isotope data from surface sediments (Stancin

et al., 2006) which suggest that Asian dust influence is largely confined to the western and

central parts of the Pacific, north of the ITCZ.

A predominance of Australian-derived dust to the EEP over the last 25 kyr was

suggested by Xie and Marcantonio (2012) based on Nd isotope data in several ODP cores at

~110°W. However, such contributions are quite inconsistent with the high-precision Pb

isotopic compositions reported here for ODP849 which are unlike those of any presently-

known Australian sources of dust (Fig. 5).

Our data, instead, strongly support models of dust emission and transport which show

that little, if any, Australian or Asian dust reaches the EEP (Li et al., 2008; Mahowald et al.,

2011; Tanaka and Chiba, 2006). These models predict that dust reaching the EEP on the

equator at around 110°W originates mainly from South America and possibly also from

North African sources. Although North African sources are globally the most significant in

terms of dust flux (Ginoux et al., 2012; Prospero et al., 2002), the ODP849 arrays lie below

the Pb isotopic compositions of potential North Africa/Saharan sources, effectively ruling out

dust supply from these sources (Fig. 5).

These inferences are substantiated by the Nd isotopic compositions of the detrital

fractions from ODP849 (Appendix A, Table A.3) which are far more radiogenic (εNd = -3.9 to

-2.9) than those of both Chinese loess (εNd = -10.3 ± 1.4, n = 14; Pettke et al., 2000) and

Saharan sources (εNd = -10 to -15; Grousset et al., 1998; Grousset and Biscaye, 2005;

Skonieczny et al., 2011; 2013; Abouchami et al., 2013). The contribution of unradiogenic Nd

from either Asian or African sources, mixed-in during long-range transport, can only be very

minor to account for the narrow range in εNd reported here and by Xie and Marcantonio

(2012) in the EEP (Fig. 6). Indeed, Kumar et al. (2014) have shown that the characteristic

unradiogenic Nd isotope signature of African dust is carried as far as the Caribbean without

significant alteration of the signal.

In summary, on the basis of both Pb and Nd isotopes, we conclude that dust from

North African and/or Asian sources constitutes only a negligible fraction of the dust loads

over the EEP.

4.3. Particulate Pb inputs from the Pacific Equatorial Undercurrent

The Pacific Equatorial Undercurrent (EUC; a.k.a. Cromwell Current) is a fast-moving

shallow subsurface current running west-to-east from Papua New Guinea across the entire

Pacific Ocean (Lukas and Firing, 1984). The EUC transports particulate and dissolved

material from the Papua New Guinea region, at least up to 140°W in the Central Equatorial

Pacific (Coale et al., 1996; Lacan and Jeandel, 2001; Ziegler et al., 2008). ODP849 is located

much farther to the east (30° in longitude) and Xie and Marcantonio (2012) rejected an EUC

contributions at ~110°W during the last 25 kyr on the basis of Nd isotope data. The Pb

isotope systematics of core ODP849 supports this view for the whole 160 kyr record.

Basically, the Pb isotopic compositions of Papua New Guinea terranes and the submarine

Bismarck Arc, New Britain Arc and Manus Basin (Park et al., 2010, and references therein)

are quite inconsistent with the end-members determined here for ODP849 sediments (Fig. 4).

8

For a given 206

Pb/204

Pb, the 208

Pb/204

Pb and 207

Pb/204

Pb ratios are far too low (not shown).

This implies that most of the Pb from the Papua New Guinea area was removed from the

EUC before reaching the EEP at ~110°W. Potential contribution of Pb from this non-eolian

source of particulates can thus be excluded.

4.4. Location and relative contribution of South American dust sources

Recent models of dust production and dispersal predict that South American sources

contribute a large proportion of the total eolian inputs to the EEP (Li et al., 2008; Mahowald

et al., 2011; Tanaka and Chiba, 2006). The Pb isotopic data for ODP849 presented here fully

agree with these models for a South American origin. Below, we will try to pinpoint the

South American source areas more precisely.

We have subdivided the western part of South America into six Andean sub-domains

(Figs. 5, 7) based upon Pb isotope variations with latitude which, in turn, are related to crustal

thickness (Faure, 2001 and references therein). These are, from north to south: the North

Volcanic Zone (NVZ: 5°N-2°S), the Central Volcanic Zone (CVZ, north: 14.5°S-19.5°S,

transitional: 20.5°S-21.5°S, and south: 22°S-27.5°S), the Southern Volcanic Zone (SVZ:

33°S-43°S) and Patagonia (45°S-55°S).

The Pb isotope systematics (Fig. 5) show that northern Andes (NVZ, north and

transitional CVZ) and Patagonia are unlikely to contribute much dust to the EEP and can be

effectively ruled out as potential sources. Indeed, both models and data suggest that

Patagonian dust is instead directed to the southeast, towards the Southern Ocean and

Antarctica (Basile et al., 1997; Gaiero et al., 2007; Li et al., 2008; Sugden et al., 2009).

The main binary mixing Pb array (Figs. 4, 5) constrains the composition and location

of the predominant source components in South America to be an unradiogenic Pb end-

member and a radiogenic one, whose compositions match well those of the SVZ (33°-43°S)

and the South CVZ (22°-27.5°S), respectively. These two components also have suitable Nd

isotope compositions. Specifically, the average εNd in the South CVZ is -5.8 ± 3.6 (n = 21)

(Mamani et al., 2008), while dust, soils and river sediments from the SVZ and Patagonia have

εNd of -0.3 ± 2.4 (n = 19; Gaiero et al., 2007). Mixtures of dust from such sources would have

εNd lying between ca. -6 and 0 that could readily account for the range measured in core

ODP849 (εNd = -3.9 to -2.9; Fig. 6).

Thus, the combined Pb and Nd isotope records of core ODP849 point conclusively at

the SVZ and the South CVZ regions in South America as the predominant sources of dust to

the EEP, south of the ITCZ (Figs. 4, 5).

4.5. Comparison with modern South American dust sources

There is a paucity of data on modern South American dust sources, notably because

dust emissions are much lower than from Asia or the Sahara (Ginoux et al., 2012).

Information on South American dust emissions mostly relies on satellite observations which

indicate that the most important natural sources are confined to the western part of the

continent (e.g. Ginoux et al., 2012). The largest is situated in the Atacama Desert (Chile,

~20°-30°S), with lesser contributions from the Nazca (~15°S) and Sechura (~5°S) deserts of

Peru (Fig. 7). A further, more diffuse, dust source is located in the Argentinian Andes at

around 32°-38°S (Fig. 7). Satellite observations, however, are often impaired by the

cloudiness over the Andes especially at high latitudes, and by the rapid dispersion and

dilution of dust plumes (Gassó et al., 2010). One example is that Patagonia does not appear as

208 P

b/20

4 Pb

206Pb/204Pb

transitional CVZ

Potential dust sources:

north CVZ

NVZ

Central America

SVZ

south CVZ

Other source data:

North Africa/Sahara

Australia

Asia loess

America end-members

Data from this study:

Patagonia

ODP849 main array

ODP849 secondary array

VNTR01-8PC

Chimborazo ash

SVZ

south CVZ Asia end-members

2D-histograms fields (America):

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

18 18.2 18.4 18.6 18.8 19 19.2 19.4

38.2

38.4

38.6

38.8

39

39.2

39.4

CentralAmerica

northCVZ

North Africa / Sahara

transitionalCVZ

NVZ

Australia

CA

SA

NA

Asia (P)

Asia (S)Asia loess

Bodélé (Chad)

SVZ

southCVZ

Average values:

Fig. 5. 208Pb/204Pb vs. 206Pb/204Pb plot of the two cores - ODP849 (blue circles) and VNTR01-8PC (green circles) - measured in this study together with potential sources of dust to the EEP (see Appendix A, Section A.4 for the construction of each database): 1) South America divided in six subdomains: North Volcanic Zone (NVZ), northern, transitional and southern Central Volcanic Zone (CVZ), South Volcanic Zone (SVZ), and Patagonia (see Section 4.4 for the definition of each subdomains), 2) Central America, 3) North Africa, 4) Australia, and 5) Asia. The main array for ODP849 and VNTR01-8PC data is shown in dark blue. The average values of the Pb isotopic composition of volcanic rocks from the SVZ and the South CVZ are indicated by a red square and a clear blue circle, respectively. The mixing line (black) between the SVZ and the South CVZ dust sources is also shown. Ticks along the line represent 10% increments. We used 2D histogram contours when enough data were available: NVZ (purple), north CVZ (brown), transitional CVZ (yellow), South CVZ (black), SVZ (red), and Central America (green). Australia and North Africa / Sahara are represented by a yellow and a purple field, respectively. Patagonian data are represented by black open squares. Asian loess (Biscaye et al., 1997; Jones et al., 2000) are represented by dark green squares: SA, CA, and NA are, respectively, the South, Central and North America end-members defined by Stancin et al. (2006). The two Asian end-members defined by (P) Pettke et al. (2002) and (S) Stancin et al. (2006) are repre-sented by light green squares. Note that the Chimborazo volcano ash sample (yellow square) analyzed in this study plots in the field defined by the NVZ, indicating that volcanic material and mineral aerosols have similar Pb isotopic compositions.

9

an emission region in satellite observations at all (Fig. 12 in Ginoux et al., 2012) even though

modeling and field studies indicate it is a major contributor of dust to Antarctica (Basile et al,

1997; Gaiero et al., 2007; Li et al., 2008; Sugden et al., 2009). Short-term storm events that

play an important role in dust emissions might also be difficult to capture by satellites in

cloud-covered areas, notably in the southern Andes (Gaiero et al., 2013).

In terms of the geology, the Atacama Desert source (20°-30°S) lies in the transitional

and South CVZ (Fig. 7). This source corresponds remarkably well to the radiogenic mixing

end-member, South CVZ, identified independently based on the Pb isotope systematics alone

in core ODP849 (Figs. 4, 5). By contrast, the Nazca and Sechura deserts are situated too far

north to account for the unradiogenic SVZ end-member. The Argentinian source, located on

the eastern flank of the Andes, can be ruled out given the present day wind patterns over this

area (Appendix A, Fig. A.5) since dust transport westwards over the topographic barrier of

the Andes would be highly improbable.

Under modern conditions, the SVZ (33-43°S) (Fig. 7), like Patagonia, is not picked up

by satellites as a major dust emitter. However, Santiago, the capital of Chile (33.5°S), is built

in a large (~100 km by 40 km) sedimentary basin filled with volcanic sediment derived from

the erosion of the Andes and is currently under semi-arid conditions (Morata et al., 2008).

This basin could potentially provide dust with the required SVZ isotopic compositions to the

EEP. South of Santiago precipitation increases rapidly southwards to reach values higher than

1000 mm/yr south of 34°S (Veit, 1996). This intense precipitation causes strong chemical

erosion, notably due to spring snow melt, that deposits fine-grained sediment on the slopes of

the Andes and in the coastal area (Pepin et al., 2010). In addition, around half of the western

slopes of the Andes and the coastal plains between 36-42°S is covered by 3 to 6 m-thick

Andosols (Wright, 1965). These Andosols have a grain-size mode of ~20 μm and are thought

to be formed from both weathering and direct accumulation of ash from the stratovolcanoes

located nearby (e.g. Bertrand and Fagel, 2008; Wright, 1965). Thus, the Andosols would

have geochemical characteristics similar to those of the SVZ. At present, the plains in the

Chilean Lake District (CLD) (38°S-42°S) and Archipelago de Chiloe (42°S-43°S) are mostly

vegetation-covered below ca. 1000-1250m in altitude (Denton et al., 1999; Moreno et al.,

1999), hence dust emitted south of the precipitation divide (34°S) must come from higher

altitudes such as from the slopes of the Andean cordilleras.

Fortunately, a definitive association of the EEP unradiogenic Pb end-member with

dust sources on the western flanks of the Andes in the SVZ is possible. Holocene mineral

dust records from two minerotrophic peat soils situated in the CLD have shown that the

dominant source of dust to the peats is the local Andosols (De Vleeschouwer et al., 2008).

Figure 4 shows that the pre-industrial Pb isotopic compositions of dust in the two peat cores

are a perfect match to those of the SVZ end-member found in ODP849. This implies that a

fine dust fraction of SVZ composition exists, that has been the dominant emission source on

the western Andean flanks locally throughout the Holocene. The “SVZ” end-member

material in the EEP is almost certainly derived from this source.

We now evaluate possible modern sources for the secondary Pb isotope array (Fig. 4).

Here, the SVZ source is clearly suitable as the unradiogenic Pb end-member, in common with

the main array. The radiogenic end-member likely originates in either the transitional CVZ or

the NVZ (Fig. 5). The former might imply dust supplied from the northern part of the

Atacama Desert, whereas a NVZ origin would possibly coincide with the Sechura Desert

(~5°S) source. Either way, the reason for a shift in the radiogenic Pb dust source at specific

times (23.6, 34.4 and 55.8 ka) over the 160-kyr record remains puzzling. Clearly, the South

CVZ appears to have been occasionally replaced by a more northerly dust source, though this

inference remains a matter for speculation pending additional work. In Sections 4.6 and 4.7,

120˚W 100˚W 80˚W 60˚W

60˚S

40˚S

20˚S

EQODP849

Chimborazo

NVZ

north CVZ

transitionalCVZsouthCVZ

SVZ

SWW

CW

STW Nazca

Sechura

Atacama

SVZ dust sources

ArgentinianAndes

0.4

0.3

0.2

0.1

0.0160140120100806040200

total

232 T

h flu

x (m

g/cm

2 /ka)

age (ka)

south CVZ

SVZ

MIS2 MIS4 MIS5 MIS6MIS3Holocene

Fig. 7. Map of the studied area with a sketch illustrating the mechanism of glacial-interglacial changes in dust sources to the EEP in relation to the wind systems. ODP849 position is indicated by a black circle and the red triangle indicates the position of the Chimborazo. The main wind systems are indicated: the wide arrows show the position and the direction of the Southeast Trade Winds (STW), the thin black arrows show the southerly Chilean coastal winds (CW): the area of maximum wind intensity is marked by a red area for warm periods and a blue area for cold periods, and the wide gray arrows indicate the position of the South Westerlies Winds (SWW) core. The dashed red (blue) line indicates the northward extension of the South Westerly wind belt during southern hemisphere warm (cold) periods. The subdivision made for South America (see Section 4.4) are indicated: NVZ (North Volcanic Zone), north, transitional and south CVZ (Central Volcanic Zone) and SVZ (South Volcanic Zone). The dotted fields indicate the modern sources of dust (see Section 4.5): the Sechura, Nazca, and Atacama deserts, the Argentinian Andes, and the SVZ dust sources (see Sections 4.5. and 4.7 for details).

Fig. 8. Fluxes of dust from the South CVZ (line with blue circles) and SVZ (line with red squares) in a simple binary mixing model. The thin lines around the South CVZ and SVZ fluxes represent a ±10% uncertainty in the contribution of each source in the binary mixing model (Fig. 4). Green areas correspond to cold marine isotope stages.

10

we will focus on the two primary sources of dust in our ODP849 record: the SVZ and the

South CVZ.

4.6. Mechanisms of dust production and delivery to the Eastern Equatorial Pacific

Overall, the picture drawn based purely upon Pb isotopes in ODP849 (Figs. 4, 5)

implies that the distribution of modern, natural South American dust sources has been a long-

term active feature throughout the 160-kyr record in core ODP849. But, why does emission

from two particular sources in South America dominate over others? Further, what are the

transport and delivery mechanisms to the EEP? These two questions can be addressed

considering the modern configuration, and are important to resolve in order to interpret the

glacial-interglacial Pb isotopic variations in ODP849 in the context of local paleo-

environment or paleoclimate .

The South CVZ is situated, with the Atacama Desert, in coastal Chile, which is the

driest place on Earth. At present, this region is influenced by the Southeast Trade Winds (Fig.

7) which blow from around 30°S to the equator and are responsible for deflation of dust from

the South CVZ and its transport to the EEP (Fig. 7). The hyper-aridity of the Atacama Desert,

which began ~15 to 25 Ma ago (Dunai et al., 2005; Evenstar et al., 2009), is the most likely

reason why it is such an important and persistent source of dust in the ODP849 record.

The SVZ is currently located under the northern part of the Southern Westerly wind

belt (Fig. 7 and Appendix A, Fig. A.5). Southerly coastal winds can develop along the coast

of Chile (Garreaud and Falvey, 2009), north of the core of the southern westerly wind belt

(49°-62°S), as illustrated by the northward component of the winds along the western side of

South America between 43°S and the equator (Appendix A, Fig. A.5). In austral

spring/summer, coastal winds are frequent, extending southward to ~43°S (Appendix A, Fig.

A.5); the intensity maximum lies between 38°S and 28°S (Garreaud and Falvey, 2009),

corresponding to an area located over the SVZ. During the autumn/winter, due to the

northward shift of the South Westerly wind belt, coastal winds are weaker and less frequent

and shift northward by about 4 to 5° in latitude with an intensity maximum that extends

southward to ~34°S, i.e. an area covering the north part of the SVZ.

These northward flowing coastal winds presumably deflate and transport fine-grained

Andosol material from the slopes of the Andean cordilleras to the north, where this dust is

entrained into the Southeast Trade Winds and delivered to the EEP.

An alternative idea is that fine-grained SVZ ash might be incorporated directly into

the coastal winds during eruptions, and from there routed to the EEP. Such a scenario would

also explain why dust emission from the SVZ region is not seen by satellite imagery as a

continuous source. Furthermore, the six most active Chilean stratovolcanos over the last half

millennium are all located in the SVZ. Nevertheless, Huybers and Langmuir (2009) have

shown that the eruption frequency in the SVZ increased dramatically in passing from the last

glacial to post-glacial and modern times. Thus, if transport of SVZ material to the EEP is

mediated by ash fall, one would expect a far greater dust flux at the EEP today compared to

that at the last glacial. This, however, is exactly the opposite of what is seen in the EEP dust

flux record (Fig. 3D).

Since the dust flux to the EEP anti-correlates with eruption frequency in the SVZ, the

prime mechanism for delivery of fine-grained material is unlikely to be ash falls. Rather,

deflation of fine-grained Andosols material in the SVZ region appears the most likely starting

point, consistent with the continuous supply of local dusts to peats in the area over the

Holocene (De Vleeschouwer et al., 2008).

11

4.7. Potential causes of the Quaternary modulation of South American dust sources

The fact that two main sources of South American dust contributed to the EEP

throughout the entire last 160 kyr implies a relatively stable regional wind pattern over this

period. Yet fluctuations in the relative contribution of the two main dust sources did occur

(Fig. 3A). Simple binary mixing calculations indicate that the SVZ was the most important

source of dust to the EEP, overall, contributing 60 to 95% of the total Pb (Figs. 4, 5). By

comparison, the relative contributions from the South CVZ were smaller throughout the

record, but proportionally increased during colder periods, reaching up to about 40%.

Using the dust flux record at site ODP849 (Fig. 3D) in combination with the mixing

proportions inferred above enables us to reconstruct the temporal variability in the dust fluxes

from the two sources supplying the EEP, as shown in Fig. 8. Clearly, the SVZ dust flux

always dominated throughout the record and peaked sharply at the two glacial terminations.

There is also a marked, but smoother, increase at the MIS 4-MIS 3 transition. By comparison,

dust flux from the South CVZ was enhanced during glacial MIS 2, MIS 6 and, to a lesser

extent MIS 4. Importantly, this implies that the mixing proportions as well as the fluxes from

the two dust sources did vary according to the prevailing climatic conditions.

In terms of paleoclimate, a change in dust production or source may arise from several

factors. The most obvious of these are the prevailing wind direction and intensity (McGee et

al., 2010; Werner et al., 2002), as well as the presence of a suitable size-fraction for deflation

and transport (Prospero et al., 2002). Other local factors may also control dust availability

such as changes in vegetation (Lunt and Valdes, 2002a; Mahowald et al., 2006) or ice/snow

cover (Lunt and Valdes, 2002b), precipitation rates (Werner et al., 2002), and production of

glacigenic material (Bullard, 2013; Sugden et al. 2009). Clearly, the availability of deflatable

material from both source areas must have been influential in determining the variability in

Pb isotopic compositions and dust fluxes observed in the EEP over the last 160 kyr.

The persisting hyperaridity of the Atacama source (South CVZ) over the last 15-25

Ma (Dunai et al., 2005; Evenstar et al., 2009) implies that local factors had little impact on

dust production. In addition, Ammann et al. (2001) have shown that the regions between

24°S and 27°S in the South CVZ were ice-free and remained dry during the LGM, thus

providing an unabated source of dust to the EEP. Hence, the most likely explanation for

increased glacial dust flux (Fig. 8) from this area is stronger or gustier winds.

During glacial periods in the SVZ, precipitation doubled and temperature decreased

by 6-8°C compared to modern values leading to glacier development over the Andean

cordilleras (Denton et al., 1999; Moreno et al., 1999). The frontal lobes of these glaciers

brought large amounts of glacigenic material from the Andes to the CLD and Archipelago de

Chiloe (Andersen et al., 1999; Denton et al., 1999). Vegetation cover in active outwash plains

is often scarce and dust emission rates under cold and wet conditions in modern environments

like Iceland can be as high as in desert areas (Bullard, 2013). By analogy, similar conditions

in the SVZ during glacial periods would shift the dust source area from its modern-like

position on the western slopes of the Andean cordilleras to the active outwash plains in the

CLD and Archipelago de Chiloe without significantly altering its Pb isotopic signature.

The striking overall similarities between the ODP849 Pb isotope record and the dust

and temperature records in Antarctica (Fig. 3A-C) is unlikely to be purely coincidental;

rather, it suggests a link between changes in dust provenance as recorded by Pb isotopes and

variations in the climate of the southern part of the Southern Hemisphere.

Modeling and observations have shown that the South Westerlies can migrate up to

several degrees in latitude towards the equator during cold periods, affecting both ocean and

atmosphere dynamics (e.g. Bard and Rickaby, 2009; Lamy et al., 2001, 2010; Moreno et al.,

1999; Stuut and Lamy 2004; Toggweiler et al., 2006; Valero-Garces et al., 2005). In

12

particular, alkenone sea-surface temperature records under the Agulhas Current (Bard and

Rickaby, 2009) and pollen records in the CLD (Moreno et al., 1999) indicate a glacial

northward shift of the South Westerlies by 7 to 9° of latitude. The latitudinal migration of the

South Westerlies is strongly correlated with southern hemisphere temperature (e.g.

Toggweiler et al., 2006). The correlation observed between the Antarctica temperature record

and the Pb isotope record in core ODP849 (Fig. 3A-B) is thus entirely consistent with a

modulation of dust sources and transport by the South Westerlies.

By analogy with modern seasonal changes in wind patterns (see Section 4.6. and

Appendix A, Fig. A.5), we advance the working hypothesis that during cold periods, general

shifting of the South Westerlies toward the equator pushes the zone of maximum coastal

wind intensity northwards (blue area in Fig. 7) over an area covering the north of the SVZ

and the South CVZ. Consequently, the contribution of the South CVZ to the dust loads

delivered to the EEP would be proportionally increased, while the SVZ dust flux would be

maintained or slightly increased if dust supplied from the outwash plains augmented (Fig. 8).

During warmings, notably Terminations I and II, the South Westerlies belt moves

southward (red area in Fig. 7), as does the area of maximum intensity for coastal winds.

Glaciers in the SVZ retreat, leaving behind large outwash plains which are potentially large

dust emitting areas. The increased dust availability combined with stronger winds enhances

the dust flux from the SVZ during glacial terminations. A concomitant reduction of the dust

flux from the South CVZ (Fig. 8) due to decreased wind intensity over this area leads to a

shift towards less radiogenic Pb isotopic compositions as recorded in ODP849 (red arrow in

Fig. 4). Such a mechanism is supported by the Nd isotope record which points to more

radiogenic values during Terminations I and II, consistent with a migration of the dust

emitting source from the South CVZ to the SVZ (blue and red arrow in Fig. 6, respectively).

After deglaciation, we hypothesize that SVZ dust production area shifts progressively from

the outwash plains that are progressively covered by vegetation to the poorly-vegetated

Andosol-covered areas at higher altitudes, which might explain the decrease in SVZ flux

(Fig. 8).

4.8. Comparison with South American paleoclimate records

Our dust record for ODP849 spans 160 kyr and covers several glacial-interglacial

cycles. On-shore paleoenvironmental and climatic records for southern South America

include pollen archives, geomorphology/pedology, geochemistry as well as paleolake level

stands for different latitudes (e.g. Kilian and Lamy, 2012; Lamy et al., 2010; Moreno et al.,

1999; Valero-Garces et al., 2005; Veit, 1996). Generally these records have much higher

resolution and more limited timespan, i.e. over the LGM and/or the Holocene making

comparison with the 160-kyr Pb record in ODP849 problematic.

The inference that the ODP849 dust record is largely driven by global climatic forcing

happening at southerly latitudes is, however, supported by longer-term off-shore sedimentary

proxy and SST records along the Chilean continental slope (e.g. Lamy et al., 2004; Stuut and

Lamy, 2004). In particular, in core GeoB 3375 located at ~28°S, Stuut and Lamy (2004)

found that a proxy for aridity correlates strongly with the 23-kyr precessional cycle. Such

features are not found in the ODP849 Pb isotope record (Fig. 3A), suggesting that the aridity

in the Atacama (South CVZ) region does not exert a strong control. Rather, the ODP849 Pb

isotope record shares strong similarities with the SST record from core ODP1233 further

south at ~41°S which matches global glacial-interglacial Antarctic climate changes and has

been attributed to the latitudinal shift of the South Westerlies (Lamy et al., 2004).

13

Altogether, this provides strong indications that high-latitude climate variations in the

southern hemisphere and concomitant changes in the strength and location of the South

Westerlies in Southern Chile are key to understanding dust inputs to the EEP during the

Quaternary.

5. Conclusions

Our 160-kyr lead isotope record in marine core ODP849 documents that the

provenance of eolian dust deposited in the Eastern Equatorial Pacific (EEP) changed in a

systematic way over time. Two distinct sources of dust ultimately derived from volcanic

material in western South America dominated over this time period. More precisely, these

sources lie in the Chilean Andes in the South Central Volcanic Zone (South CVZ, 22°S-

27.5°S, Atacama Desert) and the Southern Volcanic Zone (SVZ, 33°S-43°S), the latter being

the most important one.

Over the last two glacial-interglacial cycles, the relative contributions from the SVZ

and South CVZ sources appear to be modulated by the regional climate in southern South

America, and vary in-phase with temperature records derived from Antarctic ice cores. The

variability in dust flux from these two sources over the last 160 kyr appears to be primarily

driven by the latitudinal shift of the South Westerlies, which also modulate local factors that

influence dust production. During cold periods, the northward displacement of the South

Westerlies leads to increased contributions from the South CVZ (Atacama Desert) to the

EEP, while during climate warmings, notably during Terminations I and II, the southward

retreat of the South Westerlies favors the more southerly source in the SVZ.

The relationship identified here between temperature-driven latitudinal displacement

of the South Westerlies and sources of dust to the EEP over time deserves further attention as

similar high-precision southern hemisphere records become available. Our results

demonstrate that radiogenic isotope data can provide important constraints on variability in

source provenance through time as well as simply identifying source emission areas and their

fluxes. Such information is paramount for understanding dust feedbacks on climate change

and global biogeochemical cycles.

Acknowledgements

Part of this project was supported by a CNRS – INSU ECLIPSE II grant to S. Pichat.

W. Abouchami was funded partly by the Leibniz Prize awarded by the DFG to K. Mezger. A.

Hofmann is thanked for providing financial support for S. Pichat during his stay at MPIC and

comments on an early draft of the manuscript. We thank P. Telouk for maintaining the ICP-

MS lab in Lyon in state-of-the-art condition, E. Albalat for maintaining the clean lab in Lyon

in perfect condition, H. Feldmann and P. Jaeckel for their help with the TIMS and in the

clean labs in Mainz. The pink student team, S. Darfeuil and E. Poupart, is thanked for help

with the leaching experiments. This research used samples provided by the Ocean Drilling

Program (ODP849) and the Oregon State University Marine Geology Repository (VNTR01-

8PC). We are grateful to H. Bonnaveira for providing the samples of Chimborazo ash. We

thank the Editor G.M. Henderson, M. Paul and two anonymous reviewers for their helpful

comments.

14

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White, W.M., Albarede, F., Telouk, P., 2000. High-precision analysis of Pb isotope ratios by

multi-collector ICP-MS. Chem. Geol. 167, 257-270.

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glacial-interglacial dust fluxes in the equatorial Pacific and Antarctica. Science 320, 93-

96.

Wright, C.A., 1965. The volcanic ash soils of Chile, in: FAO (Ed.). Report No. 2017, Rome,

p. 201.

Xie, R.C., Marcantonio, F., 2012. Deglacial dust provenance changes in the Eastern

Equatorial Pacific and implications for ITCZ movement. Earth Planet. Sci. Lett. 317–

318, 386-395.

Ziegler, C.L., Murray, R.W., Plank, T., Hemming, S.R., 2008. Sources of Fe to the equatorial

Pacific Ocean from the Holocene to Miocene. Earth Planet. Sci. Lett. 270, 258-270.

1

Appendix A. Supplementary information

Lead isotopes in the Eastern Equatorial Pacific record Quaternary

migration of the South Westerlies

Sylvain Pichat1,2*

, Wafa Abouchami2,3

, Stephen J. G. Galer2

1 Laboratoire de Géologie de Lyon (LGL-TPE), Ecole Normale Supérieure de Lyon, CNRS UMR 5276, 69007

Lyon, France 2 Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany

3 Westfälische Wilhelms Universität, Institut für Mineralogie, 48149 Münster, Germany

* Corresponding author, e-mail address: spichat@ens-lyon.fr

Supplementary sections:

p 2. Section A.1. Methods

p 3. Table A1. Pb concentrations from the leaching experiments

p 4. Table A2. Pb isotopic compositions from the leaching experiments

p 5. Table A3. Sr and Nd isotopic compositions

p 6. Section A.2. Pb contamination of the upper Holocene section in core ODP849

p 7. Table A4. Pb concentrations in leaching experiments on ODP849 ‘young’ samples

p 8. Fig. A1. PCA on ODP849 and VNTR01-8PC

P 9. Fig. A2. 206

Pb/207

Pb vs. 208

Pb/207

Pb for ODP849

p 10. Section A.3. Origin of the Pb, Sr and Nd in ODP849

p 11. Fig. A3. Sr isotopic compositions in ODP849

p 12. Section A.4. Pb isotopes database construction

p 14. Fig. A4. PCA on ODP849 and VNTR01-8PC uncontaminated samples

P 15. Fig. A5. Wind pattern over South American

P16. References for Appendix A

2

Section A.1. Methods

Leaching experiments

A sequential leaching procedure, modified from that of Rutberg et al. (2005), was used to

isolate the carbonates, ferromanganese oxi-hydroxides, and detrital fractions of the sediment.

Around 300 mg of sediment powder was decarbonated by addition of 6 ml AcA (1 M acetic

acid buffered at pH=5 with sodium acetate). Samples were left for 20 min to let CO2 evolve with

frequent manual shaking and then mechanically agitation for 4h (1000 rpm). After centrifugation, the

supernatant (AcA fraction) was collected. The procedure was repeated twice on the residual fraction,

with 3 ml, then 2 ml AcA and 2h agitation each time. No CO2 evolution was observed upon addition of

AcA. The AcA fractions were combined. Residue was rinsed with 18.2 MΩ water.

Ferromanganese coatings and micro-nodules potentially contain high amounts of Pb (100 to

1300 ppm), which is mainly of seawater origin (Abouchami et al., 1997; Chow and Patterson, 1959).

Two leaching procedures, consisting of a single HH step (procedure 1) and two successive HH steps

(procedure 2), were tested to evaluate the effectiveness of the method in removing the ferromanganese

fractions from the sediment. For “procedure 1”, 6 ml HH (0.1M hydroxylamine hydrochloride

buffered at pH=4) was added to remove the ferromanganese fraction by mechanical agitation (4h). For

“procedure 2”, we used two HH steps (HH1 and HH2 in Appendix A, Tables A.1 and A.2), thus

doubling the amount of HH. The AcA and HH leachates and the residue were subsequently processed

through chemical separation and analyzed for Pb, Sr and Nd concentrations and isotopic compositions

(Appendix A. Tables A.1 to A.3).

The Pb blanks for the various steps are: 550 pg, 330 pg, 200 pg and 75 pg for the AcA, HH1,

HH2 and residue fractions, respectively. The blank is only significant for the AcA fraction were it

represent 3 to 5.5 % of the total Pb analyzed.

Reconstruction of “true” vertical flux by 230

Th normalization

230Th normalization can be used as a reference for estimating “true” vertical sedimentary

fluxes (e.g. Pichat et al., 2004), i.e. a flux that is not affected by sediment redistribution after

deposition due to focusing or winnowing. Because 230

Th is highly particle-reactive and is efficiently

scavenged from the water column by settling particles, the flux of scavenged 230

Th to the seafloor

approximates its known production rate in the water column and provides a good estimate of the true

vertical sedimentary flux.

H2O leachings

Pb concentration, ppb

sample H2O residue total bulk H2O % residue % bulk/total

ODP012 75 5325 5400 6575 1.4 98.6 1.22

ODP032 34 1201 1235 1462 2.7 97.3 1.18

ODP082 26 1084 1111 1177 2.4 97.6 1.06

ODP152a 11 673 684 707 1.6 98.4 1.03

ODP182 15 865 881 841 1.7 98.3 0.95

AcA, HH (procedure 1)

Pb concentration, ppb

sample AcA HH residue total bulk AcA % HH % residue % bulk/total

ODP042 4.7 95 843 943 944 0.5 10.1 89.4 1.00

ODP062 25.8 202 900 1127 1277 2.3 17.9 79.8 1.13

ODP082 5.4 93 1067 1165 1177 0.5 8.0 91.6 1.01

ODP182 32.4 101 477 611 841 5.3 16.5 78.2 1.38 3

AcA, HH (procedure 2)

sample AcA H2O HH 1 HH 2 H2O residue total bulk AcA % HH 1 % HH 2 % H2O total % residue % bulk/total

Pb concentration, ppb % of Pb in each fraction

ODP102 60.5 2.8 116 91 11 862 1143 1283 5.3 10.2 7.9 1.2 75.4 1.12

ODP122 47.0 6.8 128 68 10 609 869 764 5.4 14.7 7.8 1.9 70.1 0.88

ODP222 33.8 1.5 115 55 8 1079 1293 1099 2.6 8.9 4.3 0.7 83.5 0.85

Nd concentration, ppm % of Nd in each fraction

ODP102 4.11 0.01 0.47 0.30 0.02 1.83 6.75 6.04 60.9 7.0 4.5 0.6 27.1 1.12

ODP122 3.11 0.02 0.83 0.23 0.02 1.11 5.32 4.88 58.5 15.6 4.2 0.8 20.9 1.09

ODP222 7.05 0.05 2.23 0.78 0.04 3.45 13.59 15.70 51.9 16.4 5.7 0.6 25.4 0.87

Table A.1. Pb concentrations in the various fractions obtained from the leaching experiments.

AcA: fraction dissolved in acetic acid, HH: fraction dissolved in hydroxylamine hydrochloride, res: residue after leachings. Measurement

method is described in Table 1. The Pb concentrations of the bulk from Table 1 are reported for comparison. The percentage of Pb

contained in each fraction is also given. The ratio of the concentration of Pb in the bulk to the concentration of the sum of the various

fractions is reported in the last column.

sample method age (ka) 206Pb/

204Pb ± 2σ 207

Pb/204

Pb ± 2σ 208Pb/

204Pb ± 2σ

H2O leachings

ODP012 H2O (1) 4.33 18.9812 60 15.6664 51 38.5388 131

ODP012 res -rep1 (1) 4.33 18.9924 12 15.6669 14 38.5286 43

ODP012 res -rep2 (1) 4.33 18.9923 11 15.6668 12 38.5284 39

ODP012 res average 18.9924 15.6668 38.5285

ODP032 res -rep1 (1) 8.22 18.7391 21 15.6182 24 38.4947 79

ODP032 res -rep2 (1) 8.22 18.7393 13 15.6184 13 38.4953 42

ODP032 res average 18.7392 15.6183 38.4950

ODP082 res (1) 18.93 18.6634 11 15.6131 12 38.5910 39

ODP152a res (1) 37.93 18.6295 16 15.6010 17 38.5417 42

ODP182 res (1) 48.56 18.6259 13 15.6001 13 38.5269 36

AcA, HH (procedure 1)

ODP082 AcA (1) 18.93 18.6693 221 15.6118 186 38.5709 467

ODP042 HH (1) 10.51 18.5862 13 15.5895 15 38.4634 46

ODP062 HH (2) 14.54 18.6115 35 15.6000 43 38.5135 140

ODP082 HH (1) 18.93 18.6588 18 15.6155 19 38.5862 58

ODP182 HH (1) 48.56 18.6164 17 15.5995 18 38.5127 57

ODP042 res -rep1 (1) 10.51 18.5949 14 15.5910 16 38.4735 50

ODP042 res -rep2 (3) 10.51 18.5929 6 15.5893 5 38.4668 15

ODP042 res average 18.5939 15.5902 38.4702

ODP062 res (2) 14.54 18.6221 41 15.6052 51 38.5330 169

ODP082 res -rep1 (1) 18.93 18.6640 29 15.6162 35 38.5943 113

ODP082 res -rep2 (3) 18.93 18.6611 14 15.6119 11 38.5838 28

ODP082 res average 18.6626 15.6141 38.5891

AcA, HH (procedure 2)

ODP102 AcA (3) 23.57 18.7227 43 15.6355 37 38.5374 90

ODP122 AcA (3) 28.65 18.6173 37 15.6054 32 38.5236 78

ODP222 AcA (3) 63.18 18.6054 104 15.6136 91 38.5291 262

ODP102 HH1 (3) 23.57 18.7057 17 15.6212 15 38.5688 36

ODP122 HH1 (3) 28.65 18.6381 24 15.6118 19 38.5618 45

ODP222 HH1 -rep1 (3) 63.18 18.6358 11 15.6067 10 38.5499 24

ODP222 HH1 -rep2 (3) 63.18 18.6365 29 15.6081 24 38.5507 59

ODP222 HH1 average 18.6362 15.6074 38.5503

ODP102 HH2 (3) 23.57 18.6912 20 15.6192 19 38.5568 45

ODP122 HH2 (3) 28.65 18.6131 33 15.6042 31 38.5226 76

ODP222 HH2 (3) 63.18 18.6232 24 15.6032 21 38.5262 54

ODP102 res -rep1 (3) 23.57 18.7146 16 15.6226 15 38.5853 32

ODP102 res -rep2 (3) 23.57 18.7063 18 15.6199 16 38.5817 39

ODP102 res average 18.7104 15.6213 38.5835

ODP122 res -rep1 (3) 28.65 18.6500 16 15.6115 13 38.5732 33

ODP122 res -rep2 (3) 28.65 18.6442 23 15.6083 18 38.5650 43

ODP122 res average 18.6471 15.6099 38.5691

ODP222 res -rep1 (3) 63.18 18.6394 32 15.6028 22 38.5493 65

ODP222 res -rep2 (3) 63.18 18.6423 11 15.6037 8 38.5504 24

ODP222 res average 18.6409 15.6033 38.5498

Table A.2. Pb isotopic composition in the various fractions obtained from the leaching

experiments.

AcA: fraction dissolved in acetic acid, HH: fraction dissolved in hydroxylamine hydrochloride, res:

residue after leachings. Measurement methods and symbols as in Table 1.

4

sample age, ka 87Sr/

86Sr ± 2σ 143

Nd/144

Nd ± 2σ eNd ± 2σ

ferromanganese fraction (HH-leachate)

ODP042 HH -rep1 10.51 0.709194 14

ODP042 HH -rep2 10.51 0.709095 15

ODP042 HH average 10.51 0.709145

ODP062 HH 14.54 0.709180 12

ODP082 HH -rep1 18.93 0.709176 8

ODP082 HH -rep2 18.93 0.709084 10

ODP082 HH average 18.93 0.709130

ODP182 HH 48.56 0.709118 13

terrigenous fraction (residue)

ODP042 res 10.51 0.512475 13 -3.18 0.25

ODP052 res 12.52 0.709055 6 0.512442 8 -3.82 0.15

ODP082 res A 18.93 0.709081 6 0.512451 7 -3.64 0.14

ODP082 res B 18.93 0.709011 6 0.512444 8 -3.79 0.15

ODP082 res average 18.93 0.709046 0.512448 -3.71

ODP102 res 23.57 0.709102 8

ODP122 res 28.65 0.709079 8 0.512447 10 -3.72 0.20

ODP170 res 44.12 0.708900 7 0.512490 6 -2.89 0.12

ODP182 res 48.56 0.709073 7 0.512459 5 -3.49 0.10

ODP222 res 63.18

ODP382 res 121.97 0.709054 7 0.512466 10 -3.36 0.19

ODP472 res 144.21 0.709022 7 0.512441 7 -3.85 0.14

Table A.3. 87

Sr/88

Sr in the hydroxylamine hydrochloride (HH)-leached fraction and 87

Sr/88

Sr, 143

Nd/144

Nd, and εNd in the residue (res) of sediment samples of core

ODP849 (see section 2.2.1. for details).

Sr isotopic ratios were analyzed by TIMS at the Max Planck Institute for Chemistry (MPI-C)

(Finnigan MAT 261 for HH and ThermoFisher Triton for residues) and normalized to SRM98787

Sr/88

Sr = 0.710250. The average SRM987 standard value measured during the session of

measurement was87

Sr/86

Sr=0.710217 ± 41 (n = 9) on the MAT 261 and 0.710279 ± 3 (n = 5)

on the Triton. Nd isotopes were measured by TIMS (ThermoFisher Triton) at the MPI-C as

NdO+

ions using a method developed in-house which consists of loading the sample on a

tungsten filament with a TaF5 activator. Multiple measurements of the La Jolla Nd standard

during the period of analysis yielded143

Nd/144

Nd = 0.511844 ± 14 (n = 11). A, B indicate

replicate leachings and dissolutions. -rep#: replicate measurement number.

5

6

Section A.2. Pb contamination of the upper Holocene section in core

ODP849

We discuss here the peculiar results obtained from the Holocene section of core ODP 849

(Table 1). The three youngest Holocene samples (8.5 ka to present) of core ODP849 have elevated Pb

concentrations (average: 3.32 μg/g) relative to those of the remaining samples (average: 1.18 μg/g, n =

30). In particular, the highest value of 6.58 μg/g is measured for sample ODP012 at 4.3 ka. In contrast,

the average Pb concentration measured in core VNTR01-8PC in the Holocene (1.06 μg/g) is

comparable to that of the majority of the ODP849 core samples. The high Pb content of the youngest

three ODP849 samples is also associated with highly radiogenic 206

Pb/204

Pb (18.758 - 18.992) and

207Pb/

204Pb (15.620 - 15.671) ratios compared to those found deeper in the core (Table 1). These

isotopic compositions are also more radiogenic than those recorded from the same time interval in the

site-survey piston core VNTR01-8PC, while 208

Pb/204

Pb ratios are similar.

Both Pb isotope ratios and Pb concentrations in the Holocene section of core ODP849 point to

the presence of a “spurious” signal. Principal Component Analysis (PCA) of the ODP849 Pb isotopic

compositions (Appendix A, Fig. A.1) shows that the first two principal components account for 70.0

and 29.7% of the variance. In the plane defined by these two components, the ODP849 data form three

distinct alignments. The first consists of 27 samples ranging in age between 10.5 and 155.8 ka, the

second is represented by just three samples − ODP102 (23.6 ka), 142 (34.4 ka) and 202 (55.8 ka) −

and the third is defined by the three youngest Holocene samples (ODP004, 012 and 032). This third

array is parallel to the axis of the first principal component, indicating that the variance of the three

youngest samples clearly has a different origin than that of the remaining samples (n = 30).

The Pb signal in the upper part of core ODP849 may have been acquired either via 1) input

from a “natural” lead-rich source during the Holocene, or 2) contamination by anthropogenic Pb

during core handling and/or storage. The first hypothesis can be definitively ruled out, because core

VNTR01-8PC – which is the core ODP849 site-survey piston core nearby – does not show similar

elevated Pb concentrations nor radiogenic 206

Pb/204

Pb and 207

Pb/204

Pb ratios. Consequently,

anthropogenic Pb contamination appears to be the most plausible explanation for the signature seen in

the upper Holocene section of core ODP849. The anthropogenic fraction of Pb is generally regarded as

relatively labile compared to the natural mineral component (Desboeuf et al., 2005). Thus, to confirm

the hypothesis of anthropogenic contamination, we performed a series of sequential extraction on these

three samples (Appendix A, Table A.4). The Pb concentrations in the AcA-leachate for ODP004,

ODP012, and ODP032 are, respectively, 27, 187, and 12 times higher than those of other samples in

the core. We calculated a “true” bulk concentration making the hypothesis that all the Pb extracted in

7

the AcA leachate is of anthropogenic origin. We found that the “true” bulk concentrations (Appendix

A, Table A.4) are within the range of those measured in the rest of the core. All these results point to

an anthropogenic contamination of the three youngest samples of core ODP849 analyzed in this study.

In addition, full replicate isotope analyses of sample ODP012 (dissolution, chromatographic separation

and mass spectrometry) give reproducible results (Table 1), suggesting that the contamination

occurred prior to chemical processing and analysis of the sample, and thus, is an integral part of the

bulk material.

In order to evaluate the provenance of the Pb “contaminant”, the data are compared to

potential anthropogenic sources of Pb in the conventional 206

Pb/207

Pb - 208

Pb/207

Pb space in Appendix

A, Fig. A.2. All samples analyzed – ODP849 samples older than 10 ka and the VNTR01-8PC samples

– plot along the same array and outside the field of present-day USA lead or aerosols collected in

North, Central and South America. By contrast, the three youngest samples from core ODP849

(ODP004, 012, 032) lie between the fields of USA alkyl lead from the 70’s (Chow et al., 1975) and

USA lead from the early 90’s (Simonetti et al., 2000), and are close to Pb isotopic compositions of late

1990’s aerosols from the USA (Bollhöffer and Rosman, 2001) or Central America (Bollhöffer and

Rosman, 2000) (Appendix A, Fig. A.2). These observations strongly suggest that the Pb in the three

youngest samples is predominantly anthropogenic in origin.

Table A.4. Pb concentrations in the three youngest samples of core ODP849

analyzed in this study.

Pb concentration, ppm

sample AcA bulk calculated bulk(1)

ODP004 0.796 1.92 1.126 ODP012 5.616 6.58 0.959 ODP032 0.363 1.46 1.098

(1)

"true" bulk sediment concentration calculated by subtracting the labile Pb fraction

extracted in the AcA leachate to the measured bulk sediment concentration without leaching

(see text for details)

Using the relationship in 20x

Pb/204

Pb (x = 6, 7 or 8) space vs. 1/Pb (not shown), the Pb isotopic

composition of the anthropogenic end-member for core ODP849 is constrained to lie in the range

206Pb/

204Pb = 19.07-19.10,

207Pb/

204Pb = 15.688-15.692, and

208Pb/

204Pb = 38.53-38.56. Such

compositions are consistent with those from aerosols from the central/eastern part of the USA

(Argonne, New York, South Holland, Tampa) and central Mexico (Santa Ana) (Appendix A, Fig.

A.2), confirming once more that the Holocene section of core ODP849 has suffered anthropogenic

contamination.

ODP102

ODP202ODP142

component 1 (70%)

com

pone

nt 2

(29.

7%)

component 2 (29.7%)

com

pone

nt 3

(0.3

%)

com

pone

nt 3

(0.3

%)

ODP032 ODP004

ODP012

1

2

0

-1

-2-2 0 2 4

-2 0 2 4

1

-1

0

1

-1

0

1 20-1-2

A

B C

207Pb/204Pb

208Pb/204Pb

206Pb/204Pb

207Pb/204Pb

208Pb/204Pb

206Pb/204Pb

207Pb/204Pb

208Pb/204Pb

206Pb/204Pb

Fig. A.1. (A-C) Plots of the Principal Component Analysis of ODP849 and VNTR01-8PC in Pb isotope space, shown as blue circles. The projections of the unitary vectors on each isotopic axis are shown as red circles. The relative contribution of each principal component to the total variance is indicated in percent.

8

component 1 (70%)

1.14

1.16

1.18

1.20

1.22

1.24

2.42 2.43 2.44 2.45 2.46 2.47 2.48

alkyllead USA 70’s

SA

lead USA 90’s

ODP849 (ODP004 to 032)ODP849

calculated end-membersChimborazo ashesVNTR01-8PC

208Pb/207Pb

206 P

b/20

7 Pb

CanadaUSACentral America

This study: Aerosols:

South America

NY

SH

AR

TPTP

ODP849 (Holocene, LGM)Xie and Marcantonio (2012):

Fig. A.2. Anthropogenic Pb contamination in the upper section of ODP849. 206Pb/207Pb vs. 208Pb/207Pb comparing our data with present-day aerosols from North, Central and South America including samples collected in Argonne (AR), New York (NY), Tampa (TP), Santa Anna (SA), and South Holland (SH) (Bollhöffer and Rosman, 2000; 2001). The fields of USA alkyllead of the 70’s (Chow et al., 1975) and USA Pb emission of the 90’s (Simonetti et al., 2000) are also repre-sented.

9

10

Section A.3. Origin of the Pb, Sr and Nd in ODP849

The Pb isotopic composition could not be measured on all AcA leachates due to the low

amount of Pb recovered from this fraction. Significant differences between the Pb isotopic

compositions of the carbonate and the bulk sediment were observed though with no clear systematics

(Fig. 1). The absence of a systematic and reproducible difference can be easily attributed to variable

blank contributions which are non-negligible given the low amount of Pb in the AcA leachates

(Appendix A, Section A.1). Alternatively, variable partial dissolution of other phases, detrital or

ferromanganese oxides, could possibly be responsible for this effect. The HH leachates

(ferromanganese fraction) show on average slightly less radiogenic Pb isotopic compositions than

those of the bulk sediments (Fig. 1). The Sr isotope ratios of the HH leachates were measured to

validate the seawater origin of the Fe-Mn fraction and show comparable values to that of present-day

seawater (87

Sr/86

Sr = 0.709175) (Appendix A, Table A.3 and Fig. A.3). Thus, we are confident that the

HH leaching has efficiently removed the ferromanganese fraction from the sediment.

About 21 to 27% of the total Nd is carried by the terrigenous fraction of the sediment

(Appendix A, Table A.1). Nd is mainly carried by carbonates (52 to 61%) while the ferromanganese

fraction represents 12 to 22%. Both the Nd and Sr isotopic compositions measured in the terrigenous

fraction (residue) of the ODP849 sediment are in agreement with those reported by Xie and

Marcantonio (2012) on the < 63 μm-sieved residual fraction of ODP849 (Fig. 1 and Appendix A, Fig.

A.3). Our results show that despite not being sieved, the Nd and Sr isotopic compositions of the

residual fraction of our samples are representative of those of the terrigenous inputs to the EEP.

Barite incorporates seawater Sr. Thus, in areas of high productivity such as the EEP where

sediment barite concentration is higher, the Sr isotopic signal is displaced towards seawater values

(Xie and Marcantonio, 2012). Indeed, the values that we measured in the terrigenous fraction of the

sediment (residue) are lower than those measured in the ferromanganese fraction (HH) (Appendix A,

Fig. A.3). However, the interpretation of the Sr signal in terms of the origin of the terrigenous fraction

is difficult because productivity has varied over glacial/interglacial time-scale in the EEP (Pichat et al.,

2004), thus there is a variable influence of barite in the terrigenous fraction.

140120100806040200

0.70920

0.70915

0.70910

0.70905

0.70900

0.70895

0.70890

0.70885

86S

r/87S

r

age (ka)

seawater

residueferromanganese

Xie and Marcantonio (2012):

This study:

residue with DTPA residue without DTPA

Fig. A.3. Temporal variations of the Sr isotopic compositions in the terrig-enous (residue) and ferromanganese (HH) fractions of ODP849 sediments (see Appendix A, Sections A.1 and A.3 for details). Data in blue are this study. Data in red and purple are from Xie and Marcantonio (2012). DTPA is used to leach barite from the sediment. The dashed horizontal line repre-sents the present-day seawater. Green areas correspond to cold marine isotope stages.

11

12

Section A.4. Pb isotopes database construction

To decipher between the various potential sources areas of dust to the Eastern Equatorial

Pacific, we used Pb isotopic composition from the literature. As described in the Introduction, these

areas include South and Central America, East Asia, North Africa and Australia (see Section 1 of the

main text for details). Data compilation used for our study is given at http://www.pangaea.de.

South America: data compilation from the GEOROC database (http://georoc.mpch-

mainz.gwdg.de/georoc/). We filtered the GEOROC output using the following procedure:

1) Incomplete Pb isotope datasets, i.e. no 208

Pb/204

Pb or no 206

Pb/204

Pb, were excluded.

2) Duplicate or triplicate values from different papers of the same author or group of authors

have been removed. This filtering step reduces considerably the total number of data available for the

region of interest.

3) Leachates/residue couples were excluded because they cannot be compared to other data

and because there is no means to calculate the bulk value.

4) Pb data from ores, minerals, xenoliths and inclusions were also excluded because they

represent a small fraction of the erodible material and/or were comings from underground mines.

Overall the value of the Pb isotopic composition of these types of material is not related to the

geographic origin because the geological process that formed them is not linked to the thickness of the

continental crust.

5) Basement rocks were also excluded because there is little spatial variation in the Pb isotopic

composition contrary to volcanic rocks. Thus, they do not provide a good means to distinguish

between the potential dust sources. However, including the basement data does not change notably the

average values for each sub-domain defined in our study but increase the data scattering, i.e. the

standard deviation.

We calculated the average value and the 2D histogram for each of the six subdomains of South

America (see Section 4.4. for details).

Central America: Central American volcanic arc compilation from GEOROC (incomplete

datasets and duplicate values were removed) and Galapagos Islands data from White et al. (1993). For

the latter, the Pb isotopic compositions were renormalized to the value of Galer and Abouchami

(1998).

North Africa / Sahara: we used the Saharan dust end-member defined by Abouchami and

Zabel (2003), data from North African granitoids (Juteau et al., 1986), and sediments from the Bodélé

Depression in Chad (Abouchami et al., 2013).

13

Australia: fluvial clays (< 2 μm) from the Murray Darling Basin (De Deckker et al., 2010a),

fine fraction extracted from loess and soils from southeastern Australia (Vallelonga et al., 2010), and

volcanic rocks (Ewart et al., 1977, Ewart, 1982) and basalts (Zhang et al., 2001) from Queensland.

Contaminated samples (De Deckker et al., 2010b), incomplete datasets and duplicate values were

removed.

Asia: loess data from Biscaye et al. (1997) and Jones et al. (2000). Asian end-member

estimated values are based on measurements made on deep-sea sediments (Pettke et al., 2002, Stancin

et al., 2006). Thus, the end-members determined from these studies are not “pure” end-members but

contains already a small portion of other dust sources. These latter values have been renormalized to

the value of Galer and Abouchami (1998).

A full reference list for all the data used in the database is given at http://www.pangaea.de.

-4 -2 0 2 4-1

-0.5

0

0.5

1

1.5co

mpo

nent

2 (4

.7%

)

-1 0 1-1

-0.5

0

0.5

1

-4 -2 0 2 4-1

-0.5

0

0.5

1

component 1 (94.0%)

com

pone

nt 3

(1.3

%)

ODP102

ODP202

ODP142

207Pb/204Pb

component 1 (94.0%)

component 2 (4.7%)

com

pone

nt 3

(1.3

%)

A

B C

207Pb/204Pb

207Pb/204Pb

208Pb/204Pb

208Pb/204Pb208Pb/204Pb

206Pb/204Pb

206Pb/204Pb 206Pb/204Pb

Fig. A.4. (A-C) Plots of the Principal Component Analysis of the uncontaminated samples of ODP849 and VNTR01-8PC, shown as blue circles. The projections of the unitary vectors on each isotopic axis are shown as red circles. The relative contribution of each principal component to the total variance is indicated in percent. The proportion of the total variance accounted for by the third component is almost negligible (B, C). Two principal components accounts for 98.7 % of the variance (A), thus three geochemical end-members explain the Pb isotopic composition of ODP849. (A) In the plane of the first two components, two separate alignments can be identified. One is defined by 27 samples of ODP849 plus the three samples of VNTR01-8PC, the other by the three outliers of ODP849: ODP102, 142 and 202. This subdivision is consistent with the correlations seen in the 208Pb/204Pb vs. 206Pb/204Pb plots, each of which can be explained by binary mixing (see section 4.1 and Fig. 4).

14

austral summer (DJFM)

Zona

l com

pone

nt (U

-win

d)

austral winter (JJAS) averaged 1948-2013Mean V-wind m/s

Mean U-wind m/s

Mean V-wind m/s Mean V-wind m/s

Mean U-wind m/s Mean U-wind m/s

Fig. A.5. Wind pattern over South America. Zonal component (U-wind, upper panel) and meridional component (V-wind, lower panel) of winds over South America for austral summer, austral winter, and averaged from 1948 to 2013 based on data from the NCEP/NCAR reanalysis project (Kalnay et al., 1996).

Mer

idio

nal c

ompo

nent

(V-w

ind)

15

16

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