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Quaternary Science Reviews 29 (2010) 2579e2590

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Quaternary Science Reviews

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Glacial/interglacial changes in nutrient supply and stratification in the westernsubarctic North Pacific since the penultimate glacial maximum

Brigitte G. Brunelle a,*, Daniel M. Sigman a, Samuel L. Jaccard b, Lloyd D. Keigwin c, Birgit Plessen d,Georg Schettler d, Mea S. Cook e, Gerald H. Haug b,f

aDepartment of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton, NJ 08544, USAbGeological Institute, Eidgenössische Technische Hochschule, Sonneggstrasse 5, Zürich CH-8092, SwitzerlandcDepartment of Marine Geology and Geophysics, Woods Hole Oceanographic Institution, McLean 207A, Woods Hole, MA 02543, USAdGeoforschungszentrum Potsdam, Telegrafenberg, C 327, Potsdam D-14473, GermanyeDepartment of Geosciences, Williams College, 947 Main Street, Williamstown, MA 01267, USAf Leibniz Center for Earth Surface and Climate Studies, Institute for Geosciences Potsdam University, 14476 Potsdam, Germany

a r t i c l e i n f o

Article history:Received 14 July 2009Received in revised form3 March 2010Accepted 12 March 2010

* Corresponding author. Tel.: þ1 609 240 8865; faxE-mail addresses: bgbrunelle@gmail.com (B.G. Bru

(D.M. Sigman), samuel.jaccard@erdw.ethz.ch (S.L. JaccaKeigwin), Birgit.Plessen@gfz-potsdam.de (B. Plepotsdam.de (G. Schettler), Mea.S.Cook@williams.eduerdw.ethz.ch (G.H. Haug).

0277-3791/$ e see front matter Published by Elseviedoi:10.1016/j.quascirev.2010.03.010

a b s t r a c t

In piston cores from the open subarctic Pacific and the Okhotsk Sea, diatom-bound d15N (d15Ndb),biogenic opal, calcium carbonate, and barium were measured from coretop to the previous glacialmaximum (MIS 6). Glacial intervals are generally characterized by high d15Ndb (w8&) and lowproductivity, whereas interglacial intervals have a lower d15Ndb (5.7e6.3&) and indicate high biogenicproductivity. These data extend the regional swath of evidence for nearly complete surface nutrientutilization during glacial maxima, consistent with stronger upper water column stratification throughoutthe subarctic region during colder intervals. An early deglacial decline in d15Ndb of 2& at w17.5 ka,previously observed in the Bering Sea, is found here in the open subarctic Pacific record and arguably alsoin the Okhotsk, and a case can be made that a similar decrease in d15Ndb occurred in both regions at theprevious deglaciation as well. The early deglacial d15Ndb decrease, best explained by a decrease in surfacenutrient utilization, appears synchronous with southern hemisphere-associated deglacial changes andwith the Heinrich 1 event in the North Atlantic. This d15Ndb decrease may signal the initial deglacialweakening in subarctic North Pacific stratification and/or a deglacial increase in shallow subsurfacenitrate concentration. If the former, it would be the North Pacific analogue to the increase in verticalexchange inferred for the Southern Ocean at the time of Heinrich Event 1. In either case, the lack of anyclear change in paleoproductivity proxies during this interval would seem to require an early deglacialdecrease in the iron-to-nitrate ratio of subsurface nutrient supply or the predominance of light limitationof phytoplankton growth during the deglaciation prior to Bølling-Allerød warming.

Published by Elsevier Ltd.

1. Introduction

Multiple studies have suggested that biological productivity wasgreatly reduced during the last ice age in the major nutrient-richpolar ocean regions, namely, the Antarctic sector of the SouthernOcean and the subarctic North Pacific, including the Bering andOkhotsk Seas [Mortlock et al., 1991; Kumar et al., 1995; Francois

: þ1 609 258 1274.nelle), sigman@princeton.edurd), lkeigwin@whoi.edu (L.D.ssen), Georg.Schettler@gfz-(M.S. Cook), gerald.haug@

r Ltd.

et al., 1997; Narita et al., 2002; Kienast et al., 2004; Jaccard et al.,2005; Okazaki et al., 2005a,b; Galbraith et al., 2007]. However,paleoproductivity data alone cannot distinguish between the twopotential causes of the observed reduction in biogenic accumula-tion: 1) a decrease in the utilization of the surface nutrient pool bybiota, as might result from more extensive summertime sea icecover or the formation of a deeper mixed layer [Elderfield andRickaby, 2000; Anderson et al., 2002], or 2) a decrease in the rateat which nutrients are supplied to the surface, as might result froma more strongly stratified upper water column [Francois et al.,1997]. Reconstructing the fraction of surface nutrients utilized inthe polar oceans over the last glacial cycle can discern betweenthese two opposing interpretations of the paleoproductivity data.Ocean ventilation by regions of incomplete nutrient consumption,

B.G. Brunelle et al. / Quaternary Science Reviews 29 (2010) 2579e25902580

such as the Antarctic and subarctic North Pacific, tends to ventdeeply sequestered CO2 to the atmosphere. Stronger regionalstratification of the upper water column in these regions wouldreduce their potential to form deep water. Thus, the secondscenario e the “stratification hypothesis” e would, in regions ofcurrent deep water formation such as the Antarctic, implya reduction in CO2 release because of both their reduced ventilationof the interior and the increase in the efficiency of their local bio-logical pump.

Sedimentary nitrogen isotopes provide a link to the nutrientstatus of the surface ocean, recording the extent to which biotaconsume the surface nitrate pool. Phytoplankton preferentiallyconsume 14N-nitrate [Pennock et al., 1996; Waser et al., 1998],leaving the surface nitrate pool enriched in 15N [Wu et al., 1997;Sigman et al., 1999b]. As a result, as phytoplankton progressivelyutilize a given nitrate pool, the d15N of their organic matterincreases [Altabet and Francois, 1994; Altabet and Francois, 2001;Lourey et al., 2003] (d15N¼ {[(15N/14Nsample)/(15N/14Nreference)]� 1}*1000, where the reference is atmospheric N2). This organic matteris then exported to the seafloor and its isotopic signal becomesincorporated into the sediment record [Altabet and Francois, 1994;Farrell et al., 1995; Francois et al., 1997]. Assuming a constantisotope effect for nitrate assimilation and no change in sourcenitrate d15N, downcore reconstruction of sinking flux d15N shouldhold information on changes in the extent of nitrate utilization inthe surface ocean (see Brunelle et al., 2007 for a discussion of theassumptions; the (N) isotope effect, 3, of nitrate assimilation isdefined here as (14k/15k� 1)*1000&, where 14k and 15k are the ratecoefficients of the utilization reactions for the 14N- and 15N-bearingforms of nitrate, respectively).

Upon incorporation into the sediment, particularly in open oceanregimes, the isotopic compositionoforganicnitrogen is susceptible toalteration by diagenetic processes, so that, depending on the ocean-ographic and sedimentary environment, surface sediment d15N maybe elevated by up to 5& relative to the sinking particulate nitrogenpool [Altabet and Francois, 1994; Altabet, 1996; Altabet et al., 1999;Sachs et al., 1999]. This raises the concern that changes in bulk sedi-ment d15N interpreted as changes in surface ocean nitrate utilizationmay in fact be due to changes in the extent of diagenetic alterationover time. Inorganic nitrogen bound as ammonium in clay mineralsand altered terrestrial organic matter, both potentially resilient todiagenesis, can also contaminate the oceanic signal, even in deep seasediments (e.g. Schubert andCalvert, 2001and references therein). Toavoid these concerns, recent work has sought to isolate andmeasurethe isotopic composition of organic nitrogen bound within micro-fossils [Shemesh et al., 1993; Sigman et al., 1999a; Crosta andShemesh, 2002; Crosta et al., 2002; Robinson et al., 2004; Robinsonet al., 2005; Brunelle et al., 2007], which is thought to be protectedfromdecompositionbybacteria [King,1977;Swift andWheeler,1992;Kröger et al., 2000; Ingalls et al., 2003; Poulsen et al., 2003; Ingallset al., 2004]. The use of microfossil-bound isotopes does introducenew uncertainties, the nature of which ongoing research aims toelucidate [Robinson et al., 2004; Robinson et al., 2005; Brunelle et al.,2007]. We do not yet fully understand the controls on the d15N rela-tionships (1) between the integrated sinkingfluxanddiatombiomassand (2) between diatom biomass and frustule-bound N. However,given that microfossil-bound N removes known and potentiallysevere sourcesof artifact fromNisotopestudies, it is avaluable tool forinvestigating the history of sinking particulate d15N.

Previous N isotope studies employing bulk sediments, diatom-bound nitrogen, or both have suggested that nitrate utilizationtended to increase during the last glacial maximum (LGM) in theAntarctic sector of the Southern Ocean, the Bering Sea, and theopen subarctic North Pacific [Shemesh et al., 1993; Francois et al.,1997; Sigman et al., 1999a,b; Crosta and Shemesh, 2002; Robinson

et al., 2004; Brunelle et al., 2007; Galbraith et al., 2008; Robinsonand Sigman, 2008]. These studies have suggested that if produc-tivity in these regions was lower during the LGM, but utilizationwas the same or greater, then the supply of nutrients to thesurface must have been reduced (as follows from the relationshipthat utilization equals nitrate uptake divided by gross nitratesupply). This result suggests that the observed reduction inproductivity during the LGM was due to a decrease in the supplyof nutrients, consistent with a more strongly stratified watercolumn.

Modern nutrient utilization in the high nutrient, low chlorophyll(HNLC) regions of the subarctic North Pacific is thought to belimited, at least in part, by an insufficient supply of iron to thesystem [Martin et al., 1990; Tsuda et al., 2003], which is partiallysupplied to the subarctic surface via aerial deposition [Fung et al.,2000] and inputs from the shallow continental margin [Lam andBishop, 2008]. Therefore, if aerial iron deposition to the polarocean surface was at least as high during the LGM as it is today (andit was very likely greater, e.g. Rohling et al., 2003), then a reductionin the delivery of nitrate and iron to the surface from below,upwelled in a constant ratio, would have led to a net increase in theiron-to-nitrate ratio in the surface ocean. An increase in this ratioshould have led to the utilization of a greater fraction of theavailable nitrate pool, which is consistent with the N isotope resultsto date. Indeed, the diatom-bound N isotope data from the BeringSea suggest that surface nitrate utilization was nearly completeduring the LGM [Brunelle et al., 2007].

The transition from a highly stratified, low surface nutrient, lowproductivity regime in the LGM to a more productive, high surfacenutrient regime in the Holocene appears to have been composed oftwo distinct transitional states. The first, so far only recognized inthe Bering Sea [Brunelle et al., 2007], occurs upon the initial declinein benthic foraminiferal d18O, at approximately 17.5 ka, and roughlycoincident with Southern Hemisphere warming. This period ischaracterized by low biogenic accumulation (not clearly distin-guishable from that of the LGM) but a diatom-bound and bulksediment d15N that is lower than in the LGM, possibly indicatingreduced nutrient utilization at this time. The second major degla-cial interval, which has been recognized across the subarctic Pacific,occurs at the Bølling-Allerød (BA) warming event at w15 ka; it ischaracterized by dramatic peaks in biogenic accumulation [Keigwinet al., 1992; Ternois et al., 2001; Sato et al., 2002; Crusius et al.,2004; Seki et al., 2004; Brunelle et al., 2007; Galbraith et al.,2007] and in bulk sediment and diatom-bound d15N [Nakatsukaet al., 1995; Brunelle et al., 2007; Galbraith et al., 2008].Numerous studies have investigated the BA productivity peak. Incontrast, the early deglacial interval between 17.5 and 15 ka has notyet been broadly identified across the subarctic Pacific, and its placein the transition from glacial to interglacial state is much less clear.That the strong negative correlation between productivity anddiatom-bound d15N that characterizes much of the last glacial-interglacial cycle breaks down during this interval is intriguing initself. In addition, the d15N decrease appears to be synchronouswith Southern Hemisphere warming and the Heinrich 1 coldinterval of the North Atlantic, and it precedes a remarkable intervalof high productivity in the region.

This study seeks to evaluate the degree to which the diatom-bound N isotope results for the Bering Sea during the last glacial/interglacial cycle apply across the broader western subarctic NorthPacific and back to the previous ice age (stage 6). An additional aimof this study is to better characterize the deglacial state across thesubarctic Pacific, in particular, the “anomalous” 17 ka event in theBering Sea record characterized by low d15Ndb and low biogenic fluxand its relationship to the broadly recognized BA productivity peakthat follows it.

B.G. Brunelle et al. / Quaternary Science Reviews 29 (2010) 2579e2590 2581

2. Materials

Sediment core PC13 was raised from the Northern EmperorSeamounts, located in the open western subarctic Pacific, duringLeg 6 of the R/V ThomasWashington Roundabout expedition in 1988(Fig. 1; 49.7181�N, 168.3019�E, 2393 m). Sediment core GGC27 wasraised from the Akademia Nauk Rise in the Okhotsk Sea duringcruise 25 of the R/VAkademik Aleksandr Nesmeyanov in 1993 (Fig. 1;49.6011�N, 150.1797�E, 995 m). Both cores are archived at WoodsHole Oceanographic Institution.

Sediment chronology of the cores derives from foraminiferal d18Ostratigraphies of the benthic genus Uvigerina [Keigwin, 1998] as wellas fromseveral radiocarbondates. The PC13 data serieswas extendedtow700 cmdowncore, and supplementedwithpilot core analyses ofthe Holocene, using the same methods as earlier [Keigwin, 1998]. Intotal, PC13 has five 14C dates: at 16 cm (8.38 ka cal y) and 52.5 cm(14.20 ka cal y) measured on Globigerina bulloides; at 61 cm (15.18 kacal y) measured on amixed sample of Neogloboquadrina pachydermaandG.bulloides; andat80.5 cm(18.49 kacaly)and100.5 cm(19.84 kacal y) measured on N. pachyderma. GGC27 has three 14C dates: at42 cm (11.15 ka cal y) and 48 cm (13.82 ka cal y), both measured onG. bulloides, and at70 cm (19.15 ka cal y),measuredonN. pachyderma.Radiocarbon dates were converted into calendar years usingCalib5.0.1 (http://radiocarbon.pa.qub.ac.uk/calib, Stuiver et al. (1998)using DR¼ 400 y); see supplementary data for uncorrected 14Cmeasurements. The assignment of the stage 6/5 transition in PC13 istentative due to occasional reversals in d18O during these intervals.However, the rise in d15Ndb and CaCO3 concentration associatedwithTermination II at 555 cm suggests that this point is analogous to thecommencement of the BA during Termination I. The timescaledeveloped for the LGM-Termination I portion of PC13 is based on thetwo radiocarbon dates described above and correlations with opaland CaCO3 accumulation in nearbywell-dated cores RAMA 44PC andODP 882, respectively [Crusius et al., 2004; Galbraith et al., 2008;Jaccard et al., 2009]. The timescales developed for both GGC27and PC13 are uncertain after w11 ka; coretop ages are estimatedby comparison with nearby cores (4 ka for PC13 and 1 ka for GGC27[Keigwin, 1998]). When these ages are applied, accumulation

Fig. 1. Map of summer surface nitrate (JulyeSeptember; [Garcia et al., 2006]) showinglocation of cores sites: GGC27 in the Okhotsk Sea and PC13 in the open westernsubarctic Pacific (this study; see Materials section), and JPC17 in the central Bering Sea[Brunelle et al., 2007]. The map was plotted using GEOMAR’s online map creationprogram.

rates approximate those inferred for the last interglacial (MIS 5e; datanot shown).

3. Methods

The isotopic composition of diatom-bound organic N wasmeasured at Princeton University using the method described byRobinson et al. (2004), with modifications described in Robinsonet al. (2005) and Brunelle et al. (2007). The method detailed inRobinson et al. (2004) involves 1) physical separation of the diatomfraction from the bulk sediment [Sigman et al., 1999a], 2) removalof labile organic matter coating the diatom frustules by oxidationwith hydrogen peroxide, 3) conversion of organic nitrogen to NO3

�

by persulfate oxidation [Knapp and Sigman, 2003], 4) measure-ment of NO3

� concentration by chemiluminescence [Braman andHendrix, 1989], and 5) conversion of NO3

� to N2O by the denitri-fier method [Sigman et al., 2001], withmeasurement of the isotopiccomposition of the N2O by gas chromotography-isotope ratio massspectrometry using a modified Thermo GasBench II and Delta Plus[Casciotti et al., 2002]. Robinson et al. (2005) added a reductivecleaning step between steps 1 and 2 above (using dithionite-citricacid), as a precaution to prevent interference of metal oxides withthe subsequent oxidative cleaning. Brunelle et al. (2007) introducedoxidation by heated perchloric acid between steps 2 and 3 above toachieve a complete oxidation of refractory organicmatter from opalderived from low-opal sediments, in which changes in the opalgeochemistry are apparent (Haojia Ren, unpublished results).

Method replicates of d15Ndb in this study were generally betterthan 0.4& 1SD. However, the record from the Okhotsk Sea is muchnoisier than its open subarctic counterpart, and both recordsappear noisier (and replication worse) than in the Bering Searecord. This issue is described below in the context of the results.

The isotopic composition of bulk sedimentary Nwas analyzed atGFZ Potsdam using an elemental analyzer (NC2500 Carlo Erba)coupled with a ConFlo III interface on a stable isotope ratio massspectrometer (Thermo DELTAþXL). Replicate analyses indicateda standard deviation of 0.2& for d15N. Total carbon (TC) and totalorganic carbon (TOC) concentrations were also measured byelemental analyzer. CaCO3 concentration was determined indi-rectly by differencing measurements of TC and TOC to obtain totalinorganic carbon content (TIC). CaCO3 concentration was thendetermined by multiplying TIC by 8.33.

Trace elements were analyzed by ICP-AES (IRIS, ThermoElemental) at the GFZ. Digestion of 0.25 g sample aliquots, asapplied in the GFZ-laboratory, includes 1) carbonate dissolutionand wet-oxidation of organics (HNO3, 3 h, 130 �C), 2) oxidation ofrefractory organic compounds (HClO4, 5 h, 160�), 3) silicate disso-lution (HF, 2 days, 70 �C), 4) treatment with HClO4 (2 h, 210 �C), and5) addition of HCl and final dissolution to a volume of 50 ml. Acidconcentrations and the typical major element matrix of the finaldigestion solutions were considered for the preparation of themulti-element standard solutions. The mean relative standarddeviation for the measurement of Ba and Al is below 1%. Analyticaluncertainty can be slightly above 1% due to variable matrixcomposition. Biogenic barium (Babio) was calculated by assuminga constant detrital Ba/Al ratio of 0.0075 (Babio¼

PBae0.0075Al)

[Dymond et al., 1992].Th-normalized fluxes were calculated from measurements of

Th-excess activity according to Francois et al. (2004). 230Th wasmeasured at the University of British Columbia by isotopic dilutionusing 229Th spike on a single collector, sector field ICP-MS,following the procedure described by Choi et al. (2001). Briefly, thesediment samples were spiked and equilibrated with 229Th prior tototal digestion in HNO3, HF and HClO4. An aliquot was analyzeddirectly for 232Th and 238U using 229Th and 236U spikes, respectively.

B.G. Brunelle et al. / Quaternary Science Reviews 29 (2010) 2579e25902582

The remaining solution was used for 230Th separation by anion-exchange resins (AG1-X8) and subsequently analyzed by ICP-MS inlow resolution mode. Initial excess activities (230Thxs,0) wereobtained after corrections for: 1) the detrital 230Th inferred fromthe 232Th content of the sediment and using a 238U/232Th activityratio for the lithogenic fraction of 0.7 [Hendersen and Anderson,2003]; 2) the decay since the time of sediment deposition esti-mated from the age model; and 3) the diagenetic addition of 230Thderived from authigenic U.

Biogenic opal concentration was determined at Princeton andUBC by alkaline extraction of silica [Mortlock and Froelich, 1989].Dissolved Si concentrations in the extract were determined bymolybdate-blue spectrophotometry. Replicate measurementsindicate a reproducibility of �3%.

4. Results

4.1. Nitrogen isotopes

4.1.1. Open subarctic Pacificd15Ndb in PC13 is highest during cold intervals (between 7 and

8& during stages 2 and 6) and lowest during full interglacialconditions (between 4 and 6& during early stage 5 and the Holo-cene), with d15Ndb generally increasing as glaciation progresses(Fig. 2). However, a significant mid-glacial event is marked by lowd15Ndb (w4.5&) at w350 cm, followed by a rapid rise to w8& atw300 cm, perhaps corresponding to stage 3.

Deglaciations are characterized by initially low then subse-quently high d15Ndb. Termination I begins with an interval of lowd15Ndb (w6&), commencing at w17.5 ka and coincident with theinitial drop in d18O, and concluding at the transition into the BA atw15 ka (Figs. 2 and 4). Associated with the BA, d15Ndb increasessharply (tow8&) and then declines into the Holocene. TerminationII is also characterized by an initial decrease in d15Ndb, followed bya rise in d15Ndb and in the concentrations of CaCO3 and Babio (Fig. 4).

In general, bulk sediment d15N follows d15Ndb; however, theamplitude of the d15Ndb record is greater (4e8.5& compared to4e7&), and some features are much more distinct in the d15Ndbprofile (most importantly, the d15N difference between the LGMand the Holocene).

4.1.2. Okhotsk SeaSimilar to the open subarctic record, GGC27 in the Okhotsk

generally exhibits high d15Ndb during cold intervals (w7-8& duringstages 2 and 6) and lower d15Ndb in warmer intervals (w6& duringthe Holocene and early stage 5), again with d15Ndb increasing asglaciation progresses (Fig. 3). The overall amplitude of the record isnoticeably smaller than in the open subarctic Pacific, which mayresult from greater surface nutrient utilization at this site today (seediscussion below).

The record from the Okhotsk Sea appears noisier than that fromthe subarctic Pacific, and both are in turn noisier than our recordfrom the Bering Sea (Fig. 4) [Brunelle et al., 2007]. To some degree,this is consistent with the current nutrient utilization and thus thepotential for an increase during the ice age: modern nutrientconsumption is 80% or higher in the Sea of Okhotsk, indicating thepotential for only a small ice age increase (Fig. 1, Table 1). However,the apparent noisiness of the records is also negatively correlatedwith their overall opal content, with w5%, 10%, and 20% in ice agesections from the Okhotsk, open subarctic Pacific, and Bering Sea,respectively. When the opal fraction is low, as occurs in glacialstages of both records reported here (and in the Okhotsk Sea inparticular), samples may have been more easily contaminated bynon-diatomaceous opal-like phases, and extreme diagenetic effectson the characteristics of diatom opal can occur when it is present as

only a trace fraction of the sediment [Dixit et al., 2001]. Given theapparently inherent variability, we are not confident in small-scalefeatures within the lowest-opal intervals. However, the occurrenceof high diatom d15N during glacial maxima is coherent for all threerecords, during both the LGM and stage 6.

4.2. Productivity proxies

Both the concentration and Th-normalized accumulation ratesof biogenic opal, biogenic barium, and calcium carbonate are usedto infer changes in export production. As the concentration data aregenerally consistent on a qualitative level with changes in Th-normalized accumulation, the concentration data are used to inferchanges in productivity when accumulation data are not available,as is the case for most samples predating stage 2.

4.2.1. Open subarctic PacificIn general, productivity tends to be high during warm intervals

(stages 5, 3, and 1) and low during cold intervals (stages 6 and 2,and possibly stage 4) in PC13 (Fig. 2). However, the productivityproxies do exhibit some variation. For instance, CaCO3 and Babioappear to increase before opal during early stage 5. Also, while allproductivity proxies peak during the BA, only Babio and CaCO3

remain relatively high during the Holocene, with opal accumula-tion decreasing to near-LGM rates.

4.2.2. Okhotsk SeaThe opal, Babio, and CaCO3 records all suggest high productivity

during early stage 5 and the Holocene. However, only the Babiorecord indicates moderately high productivity throughout stage 5and during stage 3 (Fig. 3). One interpretation of these data is thatonly soft-bodied organisms flourished at these times; another(perhaps more plausible) interpretation is that the relativelymodest fluxes of CaCO3 and opal at these times were poorlypreserved in the sediment. All export flux proxies are extremelylow during cold stages 6, 4, and 2.

During Termination I, the rise in Babio appears to lead theincrease in opal and perhaps also that in CaCO3, as observedpreviously (Fig. 4) [Sato et al., 2002]. This Babio increase occurs atw14 ka, about 1 kyr after the pervasive subarctic productivity peak.The robustness of this apparent diachrony should be tested insubsequent work. As noted by Sato et al. (2002), unlike in openwestern subarctic North Pacific records (e.g., PC13), the export fluxproxies remain high through the Holocene.

5. Discussion

5.1. Enhanced nutrient utilization during colder times

In general, a strong anti-correlation exists between d15Ndb andbiogenic flux in both the Okhotsk and open subarctic Pacificrecords. Previous studies from the Bering Sea [Brunelle et al., 2007]and the open subarctic Pacific [Galbraith et al., 2008] have likewiseindicated a correlation between high sedimentary d15N and lowproductivity during colder climates. These studies have suggestedthat this relationship is consistent with enhanced upper watercolumn stratification in these regions during glaciation.

If one accepts the premise that iron supply has limited algalgrowth in the subarctic Pacific over recent glacial/interglacialcycles, then increased nutrient utilization upon stratification alsorequires that oceanographic changes reduced iron supply less thanit did the nitrate supply. Such a change in fact seems likely. Bothaerial and margin iron inputs are important in the modernsubarctic Pacific, and dust input was unlikely to have decreasedduring the ice ages (indeed, it likely increased [Rohling et al., 2003;

Fig. 2. Down-core record in open subarctic Pacific core PC13 of foraminiferal d18O (vs. PDB, measured on Uvigerina), d15Ndb (vs. air; averages in closed circles and method replicatesin open circles), bulk sediment d15N (vs. air, diamonds), the sedimentary concentrations of biogenic opal, biogenic barium, and calcium carbonate (lines), and Th-normalized massaccumulation rates of the same (squares; in g cm�2 kyr�1). Shaded bars mark major climate intervals, as inferred from the d18O stratigraphy and five radiocarbon dates (four datesare indicated by arrows; in calendar ka): peak glacial, including stages 2 and 6, terminations 1 and 2 (T1 and T2; each termination is divided into an early deglacial and a peakdeglacial interval, separated by a dotted line), and peak interglacial, including stages 1 and 5e; in the case of the 5/6 transition, sedimentary concentrations were also used to definetemporal intervals.

B.G. Brunelle et al. / Quaternary Science Reviews 29 (2010) 2579e2590 2583

Shigemitsu et al., 2007]). Thus, the iron-to-macronutrient ratio inthe surface ocean should have increased, allowing for morecomplete utilization of the reduced nutrient supply, as supportedby the N isotope data. Alternatively, if improved light conditionsalone are adequate to drive more complete extraction of nitrate inthe subarctic Pacific, then the shoaling of the mixed layer uponstratification (e.g., Haug et al., 2005) would have been adequate initself to explain the observations.

The anti-correlation of diatom-bound d15N and biogenic fluxfirst observed in the Bering Sea [Brunelle et al., 2007] is observedhere at additional core sites over a broad region of the westernsubarctic Pacific, effectively strengthening the evidence for strat-ification of the entire oceanographic domain poleward of theKuroshio extension. Evidence from bulk sediment N isotopesalready exists for increased nutrient utilization in the open

subarctic Pacific [Galbraith et al., 2008], but it included subtractionof a background change in subsurface nitrate d15N based onmeasurements from the eastern subarctic Pacific. Even withoutsuch a correction, the diatom-bound N isotope data reported hereindicate higher nutrient utilization during the last (and previous)ice age.

That the amplitude of the d15Ndb change from the LGM to thelate Holocene is smaller in the Okhotsk record than in the opensubarctic record can be at least partially explained by the factthat nutrient utilization in the Okhotsk is more complete today.Whereas surface nitrate is drawn down to w15 mM in latesummer in both the open subarctic Pacific and the Bering Sea,surface nitrate is 2e5 mM at our Okhotsk Sea core site [Garciaet al., 2006]. Greater utilization in the modern Okhotsk is alsosupported by coretop d15Ndb, which is 1.3& heavier than

Fig. 3. Down-core record in Okhotsk Sea core GGC27 of foraminiferal d18O (vs. PDB, measured on Uvigerina), d15Ndb (vs. air; averages in closed circles and method replicates in opencircles), d15Nbulk (vs. air, diamonds), sedimentary concentrations of biogenic opal, biogenic barium, and calcium carbonate content (lines), and Th-normalized mass accumulationrates of the same (squares; in g cm�2 kyr�1). Shaded bars mark major climate intervals, as inferred from the d18O stratigraphy and three radiocarbon dates (indicated by arrows; incalendar ka): peak glacial, including stages 2 and 6, terminations 1 and 2 (T1 and T2; each termination is divided into an early deglacial and a peak deglacial interval, separated bya dotted line), and peak interglacial, including stages 1 and 5e.

B.G. Brunelle et al. / Quaternary Science Reviews 29 (2010) 2579e25902584

observed in the Bering Sea (6.4& vs 5.1&, cores GGC27 and JPC17respectively; see Table 1).

The N-isotope data suggest that surface nitrate utilization wasnearly complete across the subarctic Pacific during the LGM (Table1). Assuming that the change in d15Ndb from the LGM to the Holo-cene is equivalent to the change in sinking N d15N over this interval,utilization during the LGMwas>90% in the Bering and Okhotsk and>80% in the open subarctic (Table 1; see footnote regarding the opensubarctic site). These estimates are susceptible to uncertaintiesinherent in the application of the d15Ndb proxy, as well as assump-tions regarding ice age conditions (e.g., the d15N of the nitratesupply). Nevertheless, the consistency of the isotope and paleo-productivity results across this vast region supports the view thatstronger stratification and nearly complete nitrate utilization werepervasive features of the subarctic Pacific during the last ice age.

This conclusion is remarkable when one considers the oceano-graphic differences that exist among the marginal seas and the

open ocean in this region, both today and in the past. The chemicaland physical conditions prevailing in the modern marginal seasgenerally favor higher levels of productivity than occur in the opensubarctic Pacific. Yet, during the LGM, all productivity proxies weresignificantly reduced, implying that the damping mechanismoccurred on a regional scale and worked to remove distinctions[Narita et al., 2002; Sato et al., 2002; Kienast et al., 2004; Seki et al.,2004; Jaccard et al., 2005; Okazaki et al., 2005a,b; Brunelle et al.,2007; Galbraith et al., 2007]. Modern regional variations inwintertime nitrate supplymay have beenmuted during the LGM byregionally stronger perennial stratification (e.g., a strongerpermanent halocline). Moreover, today, the degree of nutrientutilization varies across the subarctic Pacific, possibly due to vari-able iron inputs from the atmosphere and margin. That utilizationis nearly complete during glaciation in all of these areas may be dueto an increased Fe-to-macronutrient ratio of the total nutrient inputto surface waters, the result of reduced vertical supply of nutrients

Fig. 4. Down-core records of d15Ndb, concentrations of biogenic opal, biogenic barium, and calcium carbonate (lines), and Th-normalized mass accumulation rates of the same(squares; in g cm�2 kyr�1) versus age for Termination 1 (aec) and versus depth for Termination 2 (def) in Okhotsk Sea core GGC27, open subarctic Pacific core PC13, and Bering Seacore JPC17, respectively. In aec, dark bars correspond to the early deglacial interval (17.5e14.9 ka) and light bars roughly correspond to the Bølling-Allerød warm interval(14.9e12.9 ka). In d-f, bars correspond to the early deglacial, peak deglacial, and peak interglacial intervals.

B.G. Brunelle et al. / Quaternary Science Reviews 29 (2010) 2579e2590 2585

coupled with continued (if not enhanced) dust and margin ironsupply [Robinson et al., 2005].

Furthermore, during glaciation, the effects of sea ice formationandmelting would have beenmore keenly felt by themarginal seasthan by the open ocean. In particular, the more vigorous sea iceproduction inferred for the Bering and the Okhotsk during the LGM[Sancetta, 1983; Sancetta et al., 1985; Shiga and Koizumi, 2000;Gorbarenko et al., 2003; Katsuki et al., 2003; Katsuki andTakahashi, 2005; Sakamoto et al., 2005] might have been expec-ted to induce 1) an increase in productivity due to enhanced winter

mixing and nutrient supply associatedwith the destratifying effectsof sea ice formation, 2) an increase in productivity due to stabili-zation of the mixed layer and the supply of iron from melting iceduring the summer, or 3) a decrease in productivity due tosummertime light limitation from ice cover. Given the varyingdistance of the studied sites from sea ice (GGC27 in the Okhotskbeing most proximal and PC13 the least), a strong role for theselocal sea ice effects should have led to substantially differentpaleoproductivity and d15Ndb records. Our three records are farfrom identical; however, the lack of dramatic differences among

Table 1Surface nitrate utilization during the LGM.

OkhotskGGC27

Open SubarcticPC13

BeringJPC17

Summer surface nitratea 5 mM 15 mM 15 mMModern surface utilizationb 80% 40% 40%Coretop bulk d15N, observed 6.8& 6.2& 5.1&Coretop d15Ndb, observed 6.4& 5.7& 5.1&Modern d15N of integrated sinking

nitrogen, calculatedc4.5& 2.7& 2.7&

Coretop d15Ndb e calculated d15N ofintegrated sinking nitrogend

1.9& 3.0& 2.4&

LGM d15Ndb, observed 7.4& 7.8& 8.3&Observed LGM d15Ndb e observed

coretop d15Ndb

1.0& 2.1& 3.2&

LGM d15N of integrated sinkingnitrogen, calculatede

5.5& 4.8& 5.9&

LGM surface utilizationf 93% 84% 96%

a Estimated from surface nitrate concentrations overlying each site in latesummer [Garcia et al., 2006].

b Surface nitrate utilization is calculated as 1�NO3�summer/NO3

�winter, where

NO3�winter equals 25 mM [Brunelle et al., in preparation].

c The modern d15N of integrated sinking N is calculated using the integratedproduct formula for Rayleigh fractionation (d15Nintegrated product¼ d15Nsource

NO3�þ 3*f*ln(f)/(1� f) [Mariotti et al., 1981], where d15Nsource NO3- is 6.5& and theisotope effect (3) is 5& [Brunelle et al., in preparation], and f is the fraction of nitrateremaining in the surface, i.e. NO3

�summer/NO3

�winter).

d The offset between the coretop d15Ndb and the calculated d15N of integrated bulkN is likely due to fractionation internal to the diatom [Brunelle et al., 2007].

e LGM surface utilization is calculated in the following manner: the difference ofthe coretop d15Ndb and the LGM d15Ndb is added to the calculated modern d15N ofsinking N to obtain the LGM d15N of sinking N; this value is then substituted into theintegrated product equation of Rayleigh fractionation to solve for the fraction of theinitial nitrate remaining after consumption.

f The LGM surface utilization calculated for the open subarctic Pacific may beunderestimated. Coretop d15Ndb in PC13 is likely only mid-Holocene in age, and thed15N of modern sinking diatom-bound nitrogen may be lower (as is the case for lateHolocene sediments in the Bering), which would yield a higher degree of utilizationin the LGM.

B.G. Brunelle et al. / Quaternary Science Reviews 29 (2010) 2579e25902586

them, beyond those expected from their modern surface nitrateconcentrations, again implies a regional scale change, inconsistentwith local sea ice forcing.

5.2. Deglaciation

We and others have previously noted and discussed a remark-able deglacial productivity maximum that occurs throughout thesubarctic North Pacific during the Bølling-Allerød, which is alsomarked by a maximum in d15N [Keigwin et al., 1992; Ternois et al.,2001; Sato et al., 2002; Crusius et al., 2004; Seki et al., 2004; Cooket al., 2005; Brunelle et al., 2007; Galbraith et al., 2007; Jaccardet al., 2009]. Previous work has suggested that the d15N increasemay be generated by 1) enhanced surface nutrient utilizationassociated with the observed productivity increase, and/or 2) anincrease in the d15N of nitrate feeding surface waters due toregional subsurface denitrification during this time (suggested byeastern tropical North Pacific records [Ganeshram et al., 1995] andsediment lamination on the Bering Shelf [Cook et al., 2005])[Brunelle et al., 2007]. While the explanation for the deglacial d15Nincrease is equivocal, and while the evolution from these condi-tions into the Holocene remains unclear, upper ocean conditionsduring the BA event were different from those of the ice age, asproductivity was notably higher in the former.

Sandwiched between the LGM and the BA is another notable butless studied early deglacial interval. Starting at w17.5 ka, d15Ndbdecreases to near-Holocene values of w5.5&, from d15Ndb> 8&during the LGM (Fig. 4). Despite the significant decline in d15Ndbduring this interval, the biogenic flux data suggest that productivityremained fairly constant and low. As with the BA-correlative event,

this is a clear departure from the broader glacial/interglacial anti-correlation between d15Ndb (nutrient utilization) and productivity.This deglacial d15Ndb decrease was previously observed in theBering Sea record, although it was sampled by fewermeasurements(Fig. 4). A similar drop is also apparent in records of bulk sedimentd15N [Sigman et al., 1993] and calculated organic nitrogen d15N[Shigemitsu et al., 2008] in the open western subarctic Pacific. Inthe Okhotsk, d15Ndb similarly decreases in the early deglaciation,although its subsequent rise in the BA is late (while bulk d15N showsmore consistent timing; Fig. 3); this pattern is also reproduced ina nearby record of bulk sediment d15N [Sigman et al., 1993]. The lagin the Okhotsk d15Ndb increase aside, all of our records seem torecord the early deglacial event.

Thew17.5 ka interval is of particular interest for reasons beyondthe breakdown of the d15Ndb/productivity anti-correlation. It alsocoincides with the H1 event in the North Atlantic, early warming inAntarctica, a sharp drop in atmospheric radiocarbon as well asoceanic radiocarbon changes, and the initial rise in atmosphericCO2 [Monnin et al., 2001; McManus et al., 2004; Hughen et al.,2006; Jouzel et al., 2007; Marchitto et al., 2007]. Some authorsargue for North Pacific environmental changes at this time,including surface ocean warming and a relative increase in surfaceocean radiocarbon content [Sarnthein et al., 2006, 2007; Gebhardtet al., 2008]. However, data from other records put nearly all of theapparent surface and deep North Pacific changes at the BA eventthat follows [Galbraith et al., 2007]. Below, we review the possibleorigins of the d15Ndb decrease and attempt to relate it to otherobservations to gain insight into its meaning.

To begin, it should be considered whether this decline in d15Ndbduring the early deglaciation might be caused solely by a decreasein the d15N of nitrate being fed to the subarctic Pacific surface. Giventhemagnitude of the d15Ndb decrease in both the Bering Sea and theopen subarctic Pacific (w2&), it is difficult to identify a mechanismthat could lower source nitrate d15N to this extent. Moreover,sedimentary d15N records from the eastern tropical Pacific and theCalifornia margin show no clear sign of a decrease at this time;instead, d15N increases over this interval (Fig. 5). Thus, the d15Ndbdecrease is best explained as a decrease in nitrate utilization.

As discussed above, the iron-to-nitrate supply ratio is thought tolimit the extent of nitrate utilization in the subarctic surface [Martinet al., 1990; Tsuda et al., 2003]. A decline in iron input during thisinterval could, therefore, lower nitrate utilization. However,a decrease in iron input, from the atmosphere or themargins, seemsunlikely at this time [Rohling et al., 2003; Shigemitsu et al., 2007;Jaccard et al., 2009]. Assuming a role for iron limitationthroughout the deglacial transition, the lack of a clear productivitychange associatedwith the d15Ndb decrease suggests that the nitratesupply from the subsurface increased without any large change iniron supply. This might occur if 1) the nitrate concentration ofsubsurfacewaters increasedwhile the iron content of the subsurfaceheld constant, and/or 2) vertical exchange increased (i.e. stratifica-tion weakened or wind-driven upwelling increased) while the ironcontent of the subsurface water dropped. In either case, the iron-to-nitrate ratio of waters supplied to the surfacewould have decreasedfrom the LGM state to account for the decline in nitrate utilization.

With regard to the first possibility, the Southern Oceanmay havestarted to supply high-[NO3

�], low-iron intermediate-depthwater tothe Pacific at this time. Incomplete nutrient utilization in theSubantarctic Zone of the Southern Ocean today leads to thesubduction of nutrient-rich waters that flow equatorward and fuelproductivity in the low latitude equatorial upwelling regions, aswellas the subarctic Pacific to some extent [Sarmiento et al., 2004].During the LGM, d15Ndb and paleoproductivity data from theSubantarctic sector of the SouthernOcean suggest that these surfacenutrients were more completely consumed; this would have

Fig. 5. Profile of a) Byrd ice d18O from Antarctica [Blunier and Brook, 2001]; b) d15Ndb in open subarctic Pacific core PC13, Okhotsk core GGC27, and Bering Sea core JPC17; and c) bulksediment d15N in Mexican margin core NH22P [Ganeshram et al., 1995] and California margin core 1017E [Hendy et al., 2004].

B.G. Brunelle et al. / Quaternary Science Reviews 29 (2010) 2579e2590 2587

lowered the nutrient content of the intermediate waters being sentnorthward across the Pacific [Robinson et al., 2005]. After w18 ka,the fraction of nutrients utilized in the Subantarctic surface declinedsteeply, possibly due to reduced aerial iron inputs at this time [Wolffet al., 2006; Winckler et al., 2008; Martinez-Garcia et al., 2009],which may have raised the nitrate concentration of the subductingintermediate waters feeding the low latitude thermocline withoutincreasing its iron concentration, yielding a lower iron-to-nitrateratio in new intermediate water. Moreover, the formation ofsouthern sourced intermediate and mode water appears to haveincreased sharplyatw17.5 ka [PahnkeandZahn, 2005;Muratli et al.,2010]. These changes may explain various eastern tropical Pacificchanges at w17.5 ka [De Pol-Holz et al., 2006; Kienast et al., 2006;Marchitto et al., 2007; Robinson et al., 2007].

Alternatively, low iron-to-nitrate water may have been impor-ted from deep water. The continuous scavenging of iron from thewater column tends to lower the iron-to-nitrate ratio of old deepwater [Archer and Johnson, 2000; Parekh et al., 2004]. The suddenimport of such water would have tended to lower nitrate utilizationin surface waters, with uncertain effects on productivity. Oneargument against this alternative is the current lack of evidence fora change in the chemistry of deep subarctic North Pacific wateruntil the BA [Galbraith et al., 2007]. However, given the largevolume of North Pacific deepwater, it still seems possible that there

was a modest increase in its exchange with overlying waters at thetime period of the d15Ndb minimum.

Of course, iron may not always be the dominant control on thedegree of nutrient utilization in the subarctic North Pacific.A breakdown inNorth Pacific stratificationduring the early deglacialinterval, before significant warming in the region [Keigwin et al.,1992], may have resulted in mean annual mixed layer depths fargreater than those observed today. If so, light limitation of phyto-plankton growth may have trumped the role of iron, preventing anincrease inproductivityevenas the supplyof bothnitrate and iron tothe euphotic zone increased at 17.5 ka. In this framework, theincreased production at the Bølling-Allerød could be explained bythe alleviation of light limitation due to the warming-inducedshoaling of the summertime mixed layer at that time. A majorweakness in this explanation is the uniqueness that it attributes tosubarctic biogeochemical conditions during the H1-correlativeinterval, as iron limitation appears to dominate today [Tsuda et al.,2003]. Still, we find it more plausible than a deglacial decrease inthe shallow subsurface iron-to-nutrient ratio.

5.3. The Bølling-Allerød

The sharp increase in productivity upon the BAwarming occursthroughout the subarctic Pacific, as recorded by opal, CaCO3, and

B.G. Brunelle et al. / Quaternary Science Reviews 29 (2010) 2579e25902588

biogenic barium accumulation in the open subarctic Pacific [thisstudy; Keigwin et al., 1992; Crusius et al., 2004; Galbraith et al.,2007; Jaccard et al., 2009], by CaCO3 and to some extent opalaccumulation in the Bering Sea [Brunelle et al., 2007], and bybiogenic Ba accumulation in the Okhotsk Sea [Sato et al., 2002]. Atall sites studied so far, it is associated with an increase in bulksediment and diatom-bound d15N.

As discussed above, it is uncertain as to whether the N isotopeincrease at the onset of the BA event was driven dominantly bya greater denitrification signal in the subsurface nitrate or anincrease in nutrient utilization in the subarctic Pacific surface. InOkhotsk record GGC27, d15Ndb appears to increase after the onset ofthe BA (i.e. some time after 13.8 ka). For unknown reasons, anincrease in bulk sediment d15N and biogenic barium accumulationoccurs slightly earlier (at w13.8 ka) but still appears to occur laterelative to the BA event as it is represented in the open subarcticNorth Pacific. If confirmed in other records, this delay in theOkhotsk would imply that the d15N increase is at least partiallydriven by local nitrate utilization, with the utilization increaseoccurring later in the Okhotsk. However, Okhotsk Sea exportproduction increases into the d15N maximum but then appears toremain high after it. This makes a pure nitrate utilization expla-nation for the d15N maximum seem unlikely and causes us tocontinue to favor denitrification as the best explanation for the BAd15N maximum [Brunelle et al., 2007].

Several potential causes for the exceptional productivity recor-ded during the BA have been proposed, including increasednutrient inputs frommelting ice and enhanced upper water columnstability due to freshwater inputs [Keigwin et al., 1992]. Stableisotope and radiocarbon data suggest a significant increase in deepocean ventilation in the North Pacific at the onset of the BA[Keigwin, 1998; Galbraith et al., 2007]. The compiled data ofGalbraith et al. (2007) suggest that, during the BA, regeneratednutrients were shifted from the abyss into the mid-depth ocean,a change that could explain or contribute to the increases insubarctic Pacific productivity and North Pacific denitrification.However, this unidirectional change does not clearly explain thesubsequent Holocene declines in productivity and d15Ndb at sitessuch as PC13. Opal productivity apparently increased four-foldduring the BA in the open western subarctic Pacific relative to theHolocene [this study; Crusius et al., 2004; Galbraith et al., 2007;Jaccard et al., 2009]. In the modern western subarctic Pacific,approximately 60e90% of surface silicic acid is consumed by theend of the growing season [Harrison et al., 2004]. Even with thepossibility of more complete nutrient utilization during the BA,such a high level of opal productivity would have required anadditional input of silicate to the system relative to today.

6. Conclusions

This study presents two new records of d15Ndb from the subarcticPacific, along with a suite of paleoproductivity proxies, that qualita-tivelymatch previousfindings in a core of similar resolution from theBering Sea. The new d15Ndb data, inparticular those from theOkhotskSea, show a smaller and less clear glacial/interglacial d15Ndb differ-ence. In theOkhotsk, this canbepartlyattributed to thehighdegreeofsurface nitrate consumption today in that region, but it also probablyindicates methodological difficulties in sediments with less than 10%opal (w5% in the glacial sections of the Okhotsk). Regardless, glacialelevation of d15Ndb still prevails in all records, extending the regionalswath of evidence for a strong inverse correlation between surfacenutrient utilization and biological productivity over the last twoglacial/interglacial cycles in the subarctic Pacific. That extremely lowproductivity and apparently nearly complete surface nutrient utili-zation prevailed at all examined sites during glacial maxima suggest

that upper water column stratification dominated nutrient cyclingduring these times.

The early deglacial decline in d15Ndb, which is observed atmultiple sites and at both terminations 1 and 2, may signal theinitial deglacial weakening in subarctic North Pacific stratificationand/or an increase in shallow subsurface nitrate concentration. Thelack of any clear change in paleoproductivity proxies during thisinterval would seem to require a simultaneous decrease in the iron-to-nitrate ratio of subsurface waters or a critical role for lightlimitation of phytoplankton growth during these intervals.

There is much interest in the inverse behavior between NorthAtlantic and Southern Ocean overturning as a route to explainingthe pattern of warming and the timing of carbon dioxide rise at theend of the last ice age. It has been argued that the Heinrich 1 eventshut down deep North Atlantic overturning and increased over-turning in the Southern Ocean [Marchitto et al., 2007; Sigman et al.,2007; Anderson et al., 2009]. Numerical model experiments indi-cate that both the Southern Ocean and the North Pacific tend tobehave inversely to the North Atlantic, with both increasing theiroverturning when North Atlantic overturning declines or whenconditions favor a more globally distributed pattern of overturning[de Boer et al., 2007, 2008].

In this context, the low-d15N interval may be the North Pacificanalogue to the increase in vertical exchange inferred for theSouthern Ocean at the time of Heinrich 1. In the Southern Ocean,this H1-associated interval is apparent as an increase in opalproductivity, whereas the low-d15N interval described here for theNorth Pacific is associated with low productivity. However, the H1event brought circum-northern hemisphere cooling, more severethan even LGM conditions.With increased vertical exchange duringthe winter and a severely cold climate, failure to develop a shal-lower summertime mixed layer may well have prevented anincrease in productivity at that time. In this context, the followingBølling-Allerød warm interval may have brought the climateamelioration needed for productivity to increase.

If the 17.5 ka event involved increased vertical exchange, then itargues for changes in North Pacific stratification that occurred withthe same timing as the Southern Ocean. If instead the event recordsan increase in subsurface nitrate concentration and in its nitrate-to-iron ratio, then it argues that early deglacial changes altered thenutrient distributions of the North Pacific but not the physicalexchanges in the region. This would shift the onset of subarcticNorth Pacific destratification to the BA event warming or thereafter,calling for a physical mechanism that fits this later deglacial timing.

Acknowledgments

Support for this workwas provided by NSF grants OCE-0136449,OCE-9981479, and ANT-0453680, the Deutsche For-schungsgemeinschaft (DFG), and by a NDSEG fellowship to B.G.B.Work conducted aboard the USCG Healy (Healy 0202) was fundedby grant OPP-9912122. D. M. S. also gratefully acknowledgessupport from the Alexander von Humboldt Foundation, throughthe Friedrich Wilhelm Bessel Award.

Appendix. Supplementary data

Supplementary information associated with this article can befound in the online version, at doi:10.1016/j.quascirev.2010.03.010.

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