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Alkenones and multiproxy strategies in
paleoceanographic studies
A. C. MixCollege of Oceanic and Atmospheric Sciences, Oregon State University, Oceanography Administrative Building 104,
Corvallis, Oregon 97331-5503 ([email protected])
E. BardCentre Europe'en de Recherche et d'Enseignement en Ge'osciences de l'Environnement, CNRS-Universite d'Aix-
Marseille III, Aix-en-Provence, France ([email protected])
G. EglintonEarth Sciences Department, University of Bristol, Bristol, England, United Kingdom ([email protected])
L. D. KeigwinWoods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 ([email protected])
A. C. RaveloInstitute of Ocean Sciences, University of California, Santa Cruz, California 95060 ([email protected])
Y. RosenthalInstitute of Marine and Coastal Studies, Rutgers University, New Brunswick, New Jersey 08903
[1] Abstract: We assess multiproxy strategies involving alkenone-based and other paleoceanographic
indicators in an effort to define uncertainties and to advance understanding of past oceanic and climatic
changes. Looking to the future, we recommend a parallel strategy of applying proxies to the geologic
record while continuously seeking refinements in methods and understanding of mechanisms that
control each proxy. A multiproxy strategy increases confidence in results and provides oceanographic
context for more effective interpretation of processes. A combination of proxies allows derivation of
additional properties, such as upper ocean paleosalinity or paleo-pCO2, for a more complete view of the
oceanographic and climate mechanisms involved in changing Earth's environment.
Keywords: Alkenone; isotope; foraminifera; paleotemperature; carbon dioxide; paleosalinity.
Index terms: Paleoceanography; paleoclimatology; geochemistry; organic marine chemistry.
Received February 22, 2000; Revised October 22, 2000; Accepted October 23, 2000;
Published November 22, 2000.
Mix, A. C., E. Bard, G. Eglinton, L. D. Keigwin, A. C. Ravelo, and Y. Rosenthal, 2000. Alkenones and multiproxy
strategies in paleoceanographic studies, Geochem. Geophys. Geosyst., vol. 1, Paper number 2000GC000056 [11,165
words, 4 figures, 1 table]. Published November 22, 2000.
Theme: Alkenones Guest Editor: John Hayes
G3G3GeochemistryGeophysics
Geosystems
Published by AGU and the Geochemical Society
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES
GeochemistryGeophysics
Geosystems
Review
Volume 1
November 22, 2000
Paper number 2000GC000056
ISSN: 1525-2027
Copyright 2000 by the American Geophysical Union
1. Introduction
[2] Measures of concentration, relative abun-
dance, and other properties of alkenones, spe-
cific long-chain organic molecules produced in
the ocean by haptophyte algae (such as certain
species of Coccolithophoridae), offer great
potential as proxy indices of climate change
in marine sediments. Paleoceanographers rely
on proxy data, which in the geologic record
substitute for the kind of instrumental measures
made in today's world. No proxies are perfect.
All have analytical uncertainties, and all have
biases associated with real-world recording
processes. Although an ideal proxy would be
dependable and predictable based on primary
control by laws of thermodynamics, this situa-
tion is rarely realized because most paleocea-
nographic proxy data are at least partly
controlled by poorly understood biological
processes.
[3] Biological processes in the water or sedi-
ment column create some biases associated
with the ecological or physiological responses
of the organisms with which a proxy is asso-
ciated. Such factors underscore the value of
multiple measurements. Because secondary
influences on proxies such as the biological
or postdepositional effects are likely to differ,
multiproxy studies can help to identify the
primary signals of interest. Here we review
the status of such multiproxy studies in the
context of alkenone-based measurements. Our
assessment of future research strategies stems
from discussion at the National Science Foun-
dation sponsored workshop Alkenone-Based
Paleoceanographic Indicators, hosted by
Woods Hole Oceanographic Institution and
the U.S. National Academy of Sciences in
October 1999.
[4] We begin with the conviction that all of the
proxies discussed here are useful. Where they
agree, they confirm and increase confidence in
results. Where they disagree, they lead us to
assess the reasons for their differences, yielding
new insights into underlying processes. Con-
sistent with our focus on alkenone-based
proxies, we ask the question, `̀ How can other
proxies, combined with alkenone indices, help
to constrain the interpretation of paleoceano-
graphic and paleoclimatic processes?'' We seek
to identify key research opportunities that
exploit multiple proxies to gain confidence in
results with complementary data or in combi-
nation to derive important properties that can-
not be reconstructed with a single proxy
measure.
[5] Following early studies that identified the
promise of alkenones as paleoceanographic
proxies [e.g., Volkman et al., 1980a, 1980b;
Marlowe et al., 1984; Brassell et al., 1986a,
1986b; Prahl and Wakeham, 1987] measure-
ments made in modern (core top) sediments
have supported the concept of using the Uk037
index, (e.g., [C37:2]/([C37:2]+[C37:3]), as a rea-
sonable measure of mean annual sea surface
temperature (SST) [Herbert et al., 1998; MuÈller
et al., 1998]. No significant diagenetic effects
on the Uk037 index have been detected [Prahl et
al., 1989; Madureira et al., 1995; Teece et al.,
1998]. Patterns of temperature change based on
the index appear to behave in systematic ways
through the last few hundred thousand years
[Schneider et al., 1996, 1999; Bard et al.,
1997]. The alkenones thus provide powerful
tools for estimating past changes in ocean
temperatures on a global scale, except for
regions in which alkenone productivity and
contents in sediments are very low, such as
the centers of subtropical gyres, and areas in
which temperatures may exceed the limits of
temperature calibration, such as the polar
oceans or tropical warm pools.
[6] As with all geologic proxies, care must be
taken to evaluate potential biases and errors on
a case-by-case basis. Second-order problems
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
regarding the use of alkenone-based measure-
ments as indices of sea surface properties
include the following:
1. Observations of peak alkenone production
at subsurface depths suggest that alkenone
indices may not record sea surface condi-
tions [Prahl et al., 1993; Okouchi et al.,
1999].
2. Observations of seasonal blooms in alke-
none production suggest that alkenone
indices may not record annual-average
conditions [Prahl et al., 1993].
3. The finding of different relationships of
Uk037 to temperature in different species or
strains of haptophyte algae in laboratory
cultures leads to uncertainties in the
choice of temperature calibrations applied
to the geologic record in some areas
[Conte et al., 1998].
4. Kinetic effects of cellular growth, observed
to affect both the calibration of the Uk037
index relative to temperature and d13C
values within the alkenone fraction, suggest
that these measures may be subject to
ecological biases [Bidigare et al., 1997;
Epstein et al., 1998].
5. Owing to erosion and redeposition at the
seafloor, fine-grained particles (likely car-
riers of alkenones in the geologic record)
may in some areas such as sediment drifts
be contaminated with material that is older
or from another geographic area [Laine
and Hollister, 1981; Keigwin and Jones,
1989].
6. Bioturbation in the sediment column,
which appears to be strongest for fine
particles that carry alkenones [Wheatcroft,
1992], may disrupt the stratigraphic
record of alkenone indices relative to
other proxy data carried on larger sedi-
mentary particles.
[7] Alkenone-based proxies are not the only
ones subject to the sorts of challenges noted
above. Essentially all paleoceanographic proxy
data are affected by these or analogous pro-
blems or biases. Paleoceanographers often live
with these problems, limiting their choice of
study materials and interpretations to the parts
of the record that are robust. They constantly
strive to reduce or mitigate such problems by
improving analytical methods or by developing
complementary data sets to confirm or reject
inferences based on a single proxy. Over the
years this mode of operation has yielded dra-
matic improvements in proxies and deep
insights into the operation of a constantly
changing Earth system.
2. Opportunities for Multiproxy
Studies
[8] In the following sections we outline oppor-
tunities for multiproxy studies that combine
alkenone-based indices with other proxies in a
way that provides unique information. This can
be viewed in two ways. First, application of
two or more proxies thought to record similar
aspects of the environment (SST, for instance)
enhances confidence in a result. Analysis of
differences in results offers the potential of
richer, more complete insights into each proxy
by helping to identify specific conditions asso-
ciated with preferred habitats. Second, analysis
of a combination of proxies that examine
different aspects of the environment provides
an oceanographic context, which can help to
reveal biases or overprints on the primary
signal. Other strategies for combining proxies
allow derivation of new indices, such as paleo-
pCO2 or paleosalinity. The discussion begins
by addressing opportunities to mitigate or capi-
talize on some of the challenges discussed
above. This is not a comprehensive review.
Our intent is to provide key examples to
illustrate a general strategy of using multiple
proxies to gain richer understanding of the
environment.
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
3. Water Column Structure
[9] Tests or calibrations of paleoceanographic
proxies often compare measures from modern
(core top) sediments to historical atlas data or
from sediment traps to coeval water column
data. The strength of core top tests is the broad
geographic range of available test sites. How-
ever, high-quality cores that preserve the sedi-
ment-water interface and have sufficiently high
accumulation rates to verify (based on 14C
dating) the presence of modern sediments
remain limited. Sediment trap materials are
guaranteed to represent the modern sediment
rain within a given year but offer less geo-
graphic coverage, sometimes lack suitable
water column data, and are unlikely to repre-
sent long-term average conditions at a site.
Nevertheless, both strategies are useful.
[10] When observed at large spatial scales,
alkenone measurements in modern (core top)
sediments support the concept of using the Uk037
index as a reasonable measure of SST [Herbert
et al., 1998; MuÈller et al., 1998]. In some
regions, however, detailed analysis of core tops
suggests that geologic records of Uk037 may be
biased due to seasonal and/or subsurface alke-
none production [e.g., Okouchi et al., 1999].
Water column and sediment trap samples pro-
vide evidence for production of alkenones in
association with subsurface chlorophyll max-
ima, which are often observed within the pyc-
nocline or thermocline [Prahl et al., 1993]. For
example, off Oregon, Uk037 growth temperatures
were 58±88C lower than SST values during a
summer-dominated bloom in September 1998
(Figure 1). Confirming this pattern, tempera-
tures recorded by Uk037 in core top sediments
here are similar to sea surface temperatures in
the winter [Prahl et al., 1995; Doose et al.,
1997]. Little biological production occurs here
in winter, but these findings are consistent with
a well-known regional pattern of phytoplankton
growth 40±80 m below the sea surface in
summer, associated with a deep chlorophyll
maximum [Prahl et al., 1993]. In the northeast
Pacific this biological feature is often found
near the top of the so-called Shallow Salinity
Minimum water, which forms near the North
Pacific Subpolar Front in winter [Kenyon,
1978; Talley, 1985].
[11] Among planktonic organisms that carry
paleoceanographic information, subsurface
growth is not unique to alkenones. In a transect
similar to that studied by Prahl et al. [1993],
some species of foraminifera were found living
in roughly the same depths and seasons as the
alkenone producers [Ortiz and Mix, 1992; Ortiz
et al., 1995]. For example, living (protoplasm
full) shells of the planktonic foraminifer Neo-
globoquadrina dutertrei, captured in plankton
tows off Oregon in September 1989, were most
abundant in subsurface habitats offshore (Fig-
ure 1). Subsurface maxima such as these reflect
this species need for both food (most abundant
in a subsurface maximum) and light (for algal
symbionts, most abundant at the sea surface).
The broad similarity of depths occupied by the
phytoplankton and zooplankton offers hope
that multiproxy comparison of tracers in these
different organisms is possible.
[12] Information from other proxies can place
Uk037 temperature estimates into the context of
vertical and seasonal gradients of temperature
within the euphotic zone and may open oppor-
tunities to estimate important water column
properties such as thermocline temperature.
For example, planktonic foraminifera and radi-
olarians, the zooplankton groups most com-
monly preserved in the geologic record,
include many different species with distinct
depth and seasonal preferences [Deuser et al.,
1981; Fairbanks et al., 1982; Sautter and
Thunell, 1989; Ortiz and Mix, 1992; Welling
and Pisias, 1998]. Distribution of species in the
water column and their seasonal succession
clearly depend on water column structure and
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
food supply in addition to SST [Mix, 1989a;
Ravelo and Fairbanks, 1992; deVernal et al.,
1993; Ortiz et al., 1995, 1996; Kohfeld et al.,
1996; Watkins et al., 1996, 1998; Pisias et al.,
1997; Ravelo and Andreasen, 1999].
[13] Statistical methods for estimating water
column properties from an array of species
assemblages build on the pioneering work of
Imbrie and Kipp [1971]. The strength of such
whole-fauna methods lies in their ability to
predict large-scale patterns of ocean properties,
using statistical relationships to `̀ see through''
local biases such as the depth habitat of indi-
vidual species. In theory, statistical transfer
functions that estimate mean annual SST do
1216
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16 161412
10
8
100200300400500600
0
50
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150
200
Distance from coast (km)
September 1989: Temperature, 42oN
Dep
th (m
)
+
+ +
G
M N32.6 32.8
33.4
32.832.432.6
33.032.8
33.4
32.632.8
100200300400500600
0
50
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September 1989: Salinity, 42oN
Distance from coast (km)
+
+ +
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M N
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0 0 0 1 2 3 45 10 15 202 4 6 830241260 5
200
Dep
th (m
)
Shells m-3 Shells m-3 Shells m-3 Shells m-3
572 km 220 km289 km 121 km
Figure 1. (top) Water column temperature and salinity profiles in a conductivity-temperature-depth transectalong 428N off southern Oregon in September 1989 [Ortiz et al., 1995]. Labeled points note the apparentdepths recorded by Uk0
37 temperatures [Prahl et al., 1993] in sediment trap samples collected in September,1988. G is the `̀ gyre'' site (9.38C), M is the `̀ midway'' site (11.38C), and N is the `̀ nearshore'' site (10.48C).Uk0
37 temperatures suggest subsurface formation of alkenones, consistent with the location of deep chlorophyllmaxima along a density surface of 25.0±25.2 st. Solid triangles denote locations of depth-stratified planktontow samples. (bottom) Abundance of living (protoplasm full) shells of the planktonic foraminiferNeogloboquadrina dutertrei, captured in plankton tows along 428N (distances offshore noted) in September1989 [Ortiz et al., 1995], records subsurface habitats offshore that are broadly similar to those recorded bythe alkenones.
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
not require the fauna or flora to live at the sea
surface, in all seasons, or even to be controlled
mechanistically by temperature. The method
does require that whatever properties actually
control species distributions are linearly related
to SST. If this relationship varies through time,
then estimates may be biased. In spite of nearly
30 years of continuous study, new statistical
methods continue to be developed [Hutson,
1979; Prell, 1985; Pflaumann et al., 1996;
Ortiz and Mix, 1997; Malmgren and Nordlund,
1997; Waelbroeck et al., 1998; Mix et al.,
1999]. Analysis of transfer functions based on
multiple fossil groups provides a useful check
on the results from any individual proxy [Mol-
fino et al., 1982; Pisias and Mix, 1997].
[14] Identification of fossil species assemblages
and geochemical measurements on several spe-
cies of foraminifera are commonly used to
provide information regarding water mass
properties (such as temperature) at the sea sur-
face, seasonal ranges in temperature, biological
productivity, and pycnocline depth or structure
[Imbrie and Kipp, 1971; Mix, 1989a, 1989b;
Abrantes, 1992; Watkins et al., 1996, 1998;
Andreasen and Ravelo, 1997; Mulitza et al.,
1997; Pisias and Mix, 1997; Cayre et al.,
1999a; Wolff et al., 1999]. Theory suggests that
prediction of multiple environmental properties
is possible from rich, multispecies data sets
[Imbrie and Kipp, 1971] but does not offer
definitive guidance about which environmental
properties are most important. Debate con-
tinues regarding the potential for biases in
estimates and the difficulty in uniquely assign-
ing variations in the fauna to any particular
environmental property, in part because many
environmental properties covary in the modern
ocean [Watkins and Mix, 1998]. Nevertheless,
most paleoceanographers accept that species
assemblages can yield useful quantitative or
semiquantitative indices of upper ocean tem-
perature, as well as estimates of either biologi-
cal productivity (as reflected in food supply) or
pycnocline structure (noting that these two
variables tend to be correlated in the modern
ocean). In addition, stable isotope and trace
metal/calcium ratio measurements made on
different species of planktonic foraminifera
(such as those known to tolerate oligotrophic
conditions and others limited to food-rich con-
ditions associated with nutrient-replete subsur-
face waters) can be used to detect changes in
the position of nutricline relative to the thermo-
cline and water column stratification [Fair-
banks et al., 1982; Ravelo and Fairbanks,
1992; Farrell et al., 1995].
[15] Faunal or isotopic estimates related to
pycnocline depth or structure would add value
to alkenone-based temperature estimates by
placing the Uk037 index into an appropriate
oceanographic context [Chapman et al.,
1996]. Consider the effects of thermocline
depth on the Uk037 record of temperature (Figure
2). Here we use water column data for annual-
average distributions of temperature, nitrate,
and chlorophyll (World Ocean Atlas -98,
[Ocean Climate Laboratory, 1999]) to model
such effects. We assume, for purposes of this
sensitivity test, that alkenones are produced in
the water column proportional to observed
chlorophyll concentrations. We calculate the
temperature expected to be recorded by alke-
nones as the chlorophyll-weighted average
temperature in the water column. This exercise
is intended as a sensitivity test. Although chlor-
ophyll is a reasonably good qualitative measure
of phytoplankton concentration and production
at low latitudes, subsurface chlorophyll max-
ima are not always associated with highest total
production [Cullen, 1982], and it is unclear
how chlorophyll data relate to total alkenone
production in many settings.
[16] In our sensitivity tests, at sites in which the
thermocline is always deeper than the euphotic
zone (Figure 2a±2c) the chlorophyll-weighted
temperatures approximate the contrast in SST
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
150
0
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6oC
1oC
5oC
5oC
7oC
15 2520
15 2520
15 2520
Temperature( oC)
Temperature( oC)
Temperature( oC)
0 2 4 6 8 10
0 10 20 30 40
0 10 20 30 40
Nitrate Chlorophyll
Nitrate
Nitrate
Chlorophyll
Chlorophyll0.0 0.1 0.2 0.3 0.4
0.0 0.2 0.60.4
0.0 0.1 0.2 0.3
Dep
th (
m)
Dep
th (
m)
Dep
th (
m)
a b c
d e f
g h i
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
between sites (or by inference, through time at
a site). Absolute temperature estimates are
slightly colder than surface values because such
sites generally have a deep chlorophyll max-
imum associated with a deep nutricline. In such
oceanographic settings the Uk037 index would
complement species-based estimates of paleo-
temperatures and agreement would yield con-
fidence in the result.
[17] At sites with a shallow thermocline (Figure
2d±2f), the chlorophyll-weighted temperatures
significantly underestimate both absolute SSTs
and the SST difference between the sites (or by
inference, through time at a site). The Uk037
index could, however, yield insight into tem-
peratures within the thermocline. This context
would present an exciting opportunity to under-
stand the properties of water masses that venti-
late the pycnocline and act as source waters in
upwelling systems, for example, in comparison
to studies of paleoproductivity.
[18] In a test case comparing sites with dif-
ferent thermocline depths and similar SSTs
(Figure 2g±2i) the chlorophyll-weighted tem-
peratures misrepresent the lack of SST con-
trast between sites. If a site changed through
time between these two conditions, alkenones
would likely provide insight into the sense of
pycnocline change (apparent cooling would
imply pycnocline shoaling). This application
could be used to confirm estimates based on
other proxies such as the comparison of
shallow-dwelling and deep-dwelling species
of foraminifera.
[19] In each of the hypothetical cases noted
above, the use of alkenone measurements in
concert with other proxies would add value and
convert a potential problem of interpretation
into an opportunity for rich insight into ocea-
nographic processes responsible for changes in
the observations.
4. Ecosystem Structure
[20] Laboratory cultures show that different
species or strains of alkenone-producing algae
yield significantly different relationships
between Uk037 and temperature [Conte et al.,
1998]. Although evidence for some of the
culture-based temperature relationships is rare
in sediment core tops, it remains possible that
ecosystem changes in the past would favor one
strain or another. Such changes, if undetected,
could lead to biases in the geologic record of
temperature change.
Figure 2. Water column profiles of annual-average temperature T, nitrate N, and chlorophyll C (opensymbols [from Ocean Climate Laboratory, 1999]); are used to explore hypothetical influences of thermoclinedepth on temperatures recorded by alkenone proxies. This exercise is intended as a sensitivity test. Atlaschlorophyll data are limited to 100-m depth, and we assume exponential decrease at greater depths. Here weassume that alkenones are produced in the water column proportional to observed chlorophyll distributions,and calculate the chlorophyll-weighted average temperature in the water column (solid symbols). (a±c) T, N,and C at two sites with a deep pycnocline, with warm SST (squares, 108S, 1358W), and cool SST (diamonds,288N, 1378W). Here chlorophyll-weighted temperatures (solid symbols, shown at chlorophyll-weighteddepths) approximately record the SST contrast of 78C between sites, but slightly underestimate absolute SSTdue to subsurface production. (d±f) T, N, and C at two sites with a shallow thermocline, one with warm SST(squares, 98N, 918W), and one with cool SST (diamonds, 38S, 828W). Here chlorophyll-weightedtemperatures (solid symbols) underestimate the SST contrast of 58C between sites as well as the absolute SSTvalues due to subsurface production. (g±i) T, N, and C at two sites with similar SST but contrast inthermocline depth, one deep (squares, 108S, 1358W), and one shallow (diamonds, 98N, 918W). Herechlorophyll-weighted temperatures (solid symbols) would predict unrealistic SST contrast of 58C betweensites, but if the primary control by pycnocline depth was recognized based on other proxies, the recordedtemperatures would provide a useful confirmation of changes in the pycnocline.
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
[21] Attempts to assess changes in the domi-
nance of alkenone producers based on the
preserved record of coccolith species abun-
dances [MuÈller et al., 1997; Weaver et al.,
1999] so far suggest no significant biases in
Uk037 temperature estimates related to species
dominance changes. Such tests have only been
made in a few locations, however, and should
be extended. Species abundance data from
other fossil groups such as foraminifera, radi-
olarians, diatoms, and others may also add
value as a record of the structure of ecosystems
or water column processes that influence the
dominant strain of algae that produce alke-
nones. As a preliminary step, we see value in
investigating relationships of residuals in Uk037
temperature estimates (i.e., the difference
between inferred temperatures and atlas values)
with species distributions (based on raw spe-
cies data in all fossil groups in addition to
derived properties such as transfer-function
temperature estimates) in the same core top
sediment samples.
[22] This strategy for assessing ecosystem com-
position can also be applied to multimolecular
proxies, i.e., those based on the abundances of
a range of organic compounds analyzed in the
same samples. Alkenone distributions (e.g.,
unsaturation in n-C37, C38, and C39) and those
of other biosynthetically related compounds
(e.g., alkenonates, alkenes) probably vary
among the present-day species (and strains)
that synthesize them. Several hundred biomar-
kers are now known to occur virtually ubiqui-
tously in marine sediments [Brassell, 1993].
Some are rather specific in their origin, as are
the alkenones (e.g., dinosterol comes from
dinoflagellates), while others are of rather gen-
eral biological origin (e.g., cholesterol is pre-
sent in a broad range of zooplankton and other
animal groups). Some biomarkers are clearly of
terrigenous origin (e.g., long straight chain
lipids such as n-C29 and n-C31 alkanes and n-
C28 and n-C30 alkanols, from land plants).
Relative or absolute abundances and fluxes of
biomarkers such as these can provide valuable
indications of the inputs of the various groups
of organisms through time (molecular stratigra-
phy) or space (molecular mapping). However,
relationships between biomarker production
and sediment input remain poorly calibrated
for many organic compounds.
[23] Opportunities exist to collect major sets of
biomarker data as part of the alkenone analyses
with comparatively little additional effort. The
requirements are that the analytical steps,
which can be largely automated, be restricted
to extraction, simple fractionation, derivatiza-
tion, identification, and quantification by gas
chromatography mass spectrometry (GCMS) or
other methods. A conservative estimate of the
resolving power of this approach is that 50±100
biomarkers could be quantified in semiauto-
matic fashion, with little additional cost relative
to the measurement of just a few compounds
(Table 1).
[24] Multimolecular procedures of this type are
routinely employed in large numbers of ana-
lyses for pollutants, as in Environmental Pro-
tection Agency (EPA) studies. Extensive data
sets on molecular abundance may be manipu-
lated statistically in conjunction with other
types of proxy data in attempts to relate eco-
system components (species and strains of alga
or other organisms) with covarying biomarker
distributions. Such chemometric strategies
[e.g., Brassell et al., 1986a] mirror those used
for multivariate statistical analyses and transfer
function calculations based on other fossil
groups [e.g., Imbrie and Webb, 1981; Imbrie
et al., 1989]. Challenges will arise as these
strategies are developed, in part because some
organic compounds are much more vulnerable
to diagenetic alteration than are others. Never-
theless, examination of a variety of compounds
with different diagenetic properties may yield
insights into these secondary processes, in a
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
way that will benefit primary environmental
reconstructions.
[25] Looking farther into the future, we see
great promise in rapidly improving methods
of genetic sequence analysis, which may even-
tually be able to detect the genetic makeup of
the organisms that produce alkenones or other
organic compounds. Suitable methods that
would be applicable to sedimentary materials
do not exist at present, but development may be
possible. If such a strategy could identify
unambiguously the relative abundance of the
various strains of haptophyte algae responsible
for making the biomarker molecules preserved
in deep-sea sediments, it would be relatively
straightforward to choose the proper tempera-
ture calibration scheme for a paleotemperature
estimate.
5. Paleosalinity
[26] For many years, paleoceanographers have
sought viable proxies of regional salinity varia-
tions in the ocean. The combination of salinity
and temperature estimates would enable calcu-
lation of water densities, important in the high
latitudes for discerning sources of intermediate-
and deep-water ventilation. In the low latitudes,
salinity estimates also help to constrain the
balance between precipitation and evaporation
and to infer freshwater transports between
ocean basins. Near coastlines, paleosalinity
estimates would help record the history of river
fluxes associated with climate changes on land.
The importance of having a proxy for paleosa-
linity cannot be overstated. In models, even
small changes in the flux of fresh water to the
ocean in the high latitudes, or the transport of
water vapor between ocean basins in the low
latitudes, have dramatic effects on global ther-
mohaline circulation and global climate [Rahm-
storf, 1995]. Even a semiquantitative salinity
proxy would be valuable.
[27] Presently, there is no direct geologic proxy
for paleosalinity. Estimates are derived from
multiple proxies, for example, by combining
measurements of d18O in the calcite shells of
planktonic organisms such as foraminifera with
independent estimates of paleotemperature
[Imbrie et al., 1973; Broecker, 1989; Duplessy
et al., 1993; Rostek et al., 1993; Wang et al.,
1995; Cayre et al., 1999b] after correction for
the imprint of changing ice volume on global
oceanic d18O budgets [Labeyrie et al., 1987;
Mix, 1987; Shackleton, 1987]. For example,
comparison of Uk037 temperature estimates with
foraminiferal d18O data led Rostek et al. [1993]
to propose large changes in Indian Ocean
salinity related to monsoon strength (Figure
3). In practice, rigorous estimation of paleosa-
linity has proven quite difficult, as propagation
of errors from each part of the proxy adds
uncertainties to the result [Schmidt, 1999a,
1999b]. Comparison of multiple independent
proxies for salinity would provide important
crosschecks on results.
[28] Two primary challenges have so far limited
the application of isotopic salinity estimates:
uncertainties in the constancy of the relation-
ship between salinity and the oxygen isotopic
composition of water, and the precision of
Table 1. Classes of Biomarkers and Numbers of Compounds as Candidates of Multimolecular Proxies
Biomarker Class Compounds
C37 ± 39 alkenones, related alkenoates, and alkenes �15Sterols (e.g., dinosterol, cholesterol) �20Long chain n-alkanes, alkanols, and alkanoics (methyl esters), diols, and ketols �40Phytol, diplopterol, and other triterpenoids (e.g., amyrins, hopenes) �10
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
temperature estimates applicable to foraminif-
eral shells [Rohling and Bigg, 1998]. Progress
is being made on the first challenge in model-
ing studies that include both water and isotopic
transports in the atmosphere and ocean [Juillet-
Leclerc et al., 1997; Schmidt, 1998; Jouzel et
al., 2000; Delaygue et al., 2000]. Multiproxy
estimates of temperature may improve esti-
mates of paleosalinity by narrowing the range
of uncertainty for temperature corrections. Uk037
indices will play a role in this effort. Other new
methods will also help. For example, estimates
of calcification temperature based on proxies
such as Mg/Ca within the same shells as the
isotope measurements now appear promising
[NuÈrnberg et al., 1996; Hastings et al., 1998;
Lea et al., 1999, 2000; Mashiotta et al., 1999;
Rosenthal et al., 2000a; Elderfield and Gans-
sen, 2000].
[29] Similar strategies to derive paleosalinity
estimates from temperature estimates and oxy-
gen isotope data may come from the combina-
tion of the Uk037 index and d18O in the coccolith
fraction in deep-sea sediments. Isotopic dise-
quilibrium effects in coccoliths from oceanic
sediments remain poorly known, however,
because the small size of coccoliths makes it
nearly impossible to isolate species [Dudley
and Goodney, 1979]. New technologies for
analyses of isotopes and trace metals of small
particles by laser ablation may overcome such
difficulties in the future.
[30] Preliminary data on the distribution of the
alkenone n-C37:4 suggest an intriguing correla-
tion to sea surface salinity [Rosell-MeleÂ, 1998].
The cause of this relationship remains un-
known, but one option is a biological effect
based on differences in the saturation index of
n-C37 alkenones of species that survive in
conditions of low salinities relative to species
prevalent at higher salinities. At present, the
cause of this relationship is little more than
speculation, and other methods to identify the
28
27
26
25
-3
-2
-1
0
-3
-2
-1
39
37
35
330 50 100 175125 1507525
AGE (kyr BP)
SS
T(U
k’ 3
7o C
)δ18
OG
.ru
be
r
(o /oo
PD
B)
δ18O
(o /oo
PD
B)
Sal
inity
(o /oo
PS
U)
a
b
c
d
Figure 3. The combination of Uk037 temperature
estimates and foraminiferal d18O data estimatechanges in paleosalinity in Indian Ocean coreMD900963, 5804^0N, 73853^0E [Rostek et al.,1993]. (a) Uk0
37 SST estimates, based on thecalibration of Prahl and Wakeham [1987]. (b) Thed18O values of the surface-dwelling foraminiferaGlobigerinoides ruber. (c) Smoothed d18O of G.ruber (solid line), compared to an estimate of globaloceanic d18O history associated with changing icevolume (dashed line, following Labeyrie et al.[1987]). (d) Estimates of surface-water paleosalinityat the site of MD900963 based on temperature andice-volume corrections to the d18O of G. ruber, anda modern salinity of 35 PSU. Upper and lower linesreflect the range of slopes in the relationship of d18Oand salinity observed in surface waters of themodern Indian Ocean [Rostek et al., 1993].
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
organisms creating the alkenone molecules
(genetic sequencing, or additional molecular
information) may be needed. Nevertheless, we
see value in exploring relationships of this and
other types of organic molecules to salinity and
to a rich array of environmental properties in
general. If the relationships to salinity involve
the presence of different species or strains of
alkenone producers, resolving these effects
may yield further insights into interpretations
of temperature from unsaturation indices.
6. Paleo-pCO2
[31] The carbon isotopic composition of alke-
nones or other organic compounds combined
with those of foraminifera offer the potential to
reconstruct the distribution of aqueous CO2
concentrations in the upper ocean (Figure 4).
In concert with paleotemperature measure-
ments these data can be used to estimate atmo-
spheric pCO2 values that would be in
equilibrium with surface waters. This is com-
monly referred to as paleo-pCO2 [Jasper and
Hayes, 1990; Jasper et al., 1994]. The calcula-
tion of paleo-pCO2 from the carbon isotopic
composition of alkenones depends, among
others, on an accurate estimation of the tem-
perature at the precise season and depth of the
alkenone production. This is best done with
Figure 4. Multiproxy measurements of paleo-pCO2 in core W8402A-14GC from the equatorialPacific Ocean. (a) Equatorial Pacific paleo-pCO2
estimates based on d13C and temperature data (opensquares from Jasper et al. [1994]) and similarvalues corrected for growth rate effects estimatedfrom Sr/Ca (solid circles from Stoll and Schrag[2000], error bars reflect the estimated range ofgrowth rates). Solid line without symbols is atmo-spheric pCO2 history as recorded in the Vostok icecore [Petit et al., 1999]. (b) �pCO2, the differencebetween equatorial Pacific paleo-pCO2 estimatesand atmospheric values (open squares withoutgrowth rate corrections, solid circles with growthrate corrections and error bars as in Figure 4a).Positive values of �pCO2 indicate that theequatorial Pacific was a consistent source of CO2
to the atmosphere over the past �250 ka. (c) Uk037
temperature estimates in core W8402A-14GC [Lyleet al., 1992]. (d) Sea surface PO4 concentrations,estimated from Sr/Ca in the coccolith fraction ofcore W8402A-14GC, used for growth rate correc-tions on alkenone d13C [Stoll and Schrag, 2000].
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
alkenone proxy Uk037. Even if Uk0
37 records a
subsurface or seasonally biased temperature
estimate, it is the most appropriate temperature
to use in estimating pCO2 from alkenone iso-
topic data because it is carried in the same
molecule and thus is likely to record corre-
sponding environmental conditions.
[32] Metabolic processes within cells and
within populations may affect alkenone indices.
For example, some studies suggest potential
impacts of cell growth rate on alkenones [Bidi-
gare et al., 1997], which complicate estimation
of dissolved CO2 concentrations based on d13C
[Popp et al., 1999], and perhaps also tempera-
ture estimates based on the Uk037 index [Epstein
et al., 1998]. Application of additional proxies
offer the potential to constrain growth rates in
the geologic record and thus to correct for such
effects. Empirical evidence from the water
column suggests that population growth rates
are related to nutrient contents of water masses
[Bidigare et al., 1999]. On the basis of these
relationships, molecular indices of growth rate
are under development.
[33] Several geologic proxies offer the potential
to constrain the environmental or ecosystem
controls on growth rates of the biota. Proxies
exist for upper ocean nutrient concentrations
based on trace metal/calcium ratios in plank-
tonic foraminifera [Boyle, 1981; Keigwin and
Boyle, 1989], although effects of algal sym-
bionts [Mashiotta et al., 1997] and temperature
[Rickaby and Elderfield, 1999] may also influ-
ence such estimates. Nutrient utilization esti-
mates have been based on d15N of sedimentary
organic matter [Altabet and Francois, 1994].
Primary productivity estimates are based on
transfer function approaches using planktonic
foraminiferal, calcareous nannofossil, diatom,
or dynocyst species data [Mix, 1989a; Abrantes,
1992; Cayre et al., 1999a; Beaufort et al.,
1999], benthic foraminiferal species percen-
tages [Loubere and Fariduddin, 1999] or
abundance [Herguera and Berger, 1991], or
mass accumulation rates of biogenic materials
or chemical tracers [Dymond et al., 1992; Lyle
et al., 1992].
[34] Preliminary culture experiments suggest
that Sr/Ca in calcite liths may reflect the growth
rate of the coccolithophorid Emiliania huxleyi
[Rosenthal et al., 2000b], perhaps owing to a
kinetic effect on calcification rate. Because E.
huxleyi is a known alkenone producer, Sr/Ca in
this species may, in turn, reflect the growth rate
of the alkenone-producing flora. A recent study
of surface sediments across a latitudinal trans-
ect in the equatorial Pacific suggests that Sr/Ca
of the bulk coccolith fraction, cleaned to the
extent possible of contaminants associated with
Fe and Mn oxyhydroxides, covaries with mod-
ern spatial patterns of primary productivity
[Stoll and Schrag, 2000].
[35] Under an assumption that Sr/Ca variations
in the bulk coccolith fraction track growth rates
of the alkenone-producing haptophytes, Stoll
and Schrag [2000] use downcore data to cor-
rect for growth rate changes on the carbon
isotopic paleo-pCO2 estimates of Jasper et al.
[1994], which include estimates of temperature
based on the Uk037 index [Lyle et al., 1992].
These new calculations confirm the earlier
inference that the equatorial Pacific was always
a regional source of CO2 to the atmosphere;
i.e., local paleo-pCO2 values were consistently
higher than atmospheric pCO2 values in the
Vostok ice core [Petit et al., 1999]. The new
calculations suggest even greater CO2 outgas-
sing from the region and a long-term drift from
much higher pCO2 values in the past (espe-
cially >150 kyr BP) to lower values with the
last 100 ka. Significant uncertainties in the Sr/
Ca growth rate corrections include possible
changes in the Sr content of seawater through
time, which remains controversial [Stoll and
Schrag, 1998; Martin et al., 2000]. Clearly,
more work is needed to test the efficacy of
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
these multiproxy approaches in constraining
regional sources and sinks of CO2 in the
paleoceanographic record. Nevertheless, we
are encouraged that qualitative or semiquanti-
tative corrections for growth rates on alkenone
d13C data and paleo-pCO2 estimates may be
possible.
[36] Relatively little data exist on the chemical
or isotopic composition of coccolith species in
the geologic record. This reflects the difficulty
of isolating the calcite plates formed by indi-
vidual species of coccoliths, because of their
small size. New technologies for isolating such
particles or for analyzing them individually
within a mixed sample may be possible, based
on advances in small sample techniques such as
microprobe or laser ablation inductively coup-
led plasma±mass spectrometer (ICP±MS).
Recently developed analytical methods also
offer the opportunity to measure multiple ele-
mental ratios rapidly (e.g., Cd/Ca, Ba/Ca, Mg/
Ca, and others), thereby making the multiproxy
approach much easier to apply [Rosenthal et al.,
1999]. With these new technologies we expect
further progress on development of growth
rate corrections and other isotopic proxies
related to paleo-pCO2 estimates. In spite of
large uncertainties in several aspects of these
measurements and their interpretation, estab-
lishing regional patterns of paleo-pCO2 to
assess changes in sources and sinks of atmo-
spheric CO2 in the ocean would be of such
enormous benefit that these studies are worth
pursuing, recognizing that estimates will im-
prove as knowledge of controls on the proxies
is refined.
7. Seafloor Processes
[37] Alkenones, as hydrophobic compounds
originally part of the cells of coccolithophores,
should have an affinity for fine-grained sedi-
ments through adsorption on clays and silts, or
absorption in organic particles. Although rea-
sonable, this conjecture is unproven, and one of
our recommendations is for studies to confirm
which phases in the sediment actually carry the
alkenone signal.
[38] A present focus of paleoceanographers is
on rapidly accumulating sequences in sediment
drifts and continental margin sediments as a
means of obtaining high-resolution paleoceano-
graphic records [Sachs and Lehman, 1999]. We
worry, however, that artifacts may be intro-
duced into alkenone-based (and other) environ-
mental histories in such settings because fine-
grained sediments in drift deposits may be
transported downslope or across an ocean basin
by turbidity currents or may be advected by
bottom currents in deep recirculating gyres.
[39] For example, more than 30 years ago, it
was recognized that fine-grained sediments rich
in hematite all along the U.S. Atlantic conti-
nental margin were introduced by the St. Lawr-
ence river system and redistributed by deep-sea
contour currents [Heezen et al., 1966]. These
reddish sediments (which also include pre-
Pleistocene coccoliths) extend as far as Ber-
muda Rise, where excess fines accumulate in
drifts [Laine and Hollister, 1981; McCave et
al., 1982; Suman and Bacon, 1989]. Rapid
changes in carbonate contents of Bermuda Rise
sediments reflect changing input of fine-
grained terrigenous sediments from eastern
Canada [Keigwin and Jones, 1989]. Thus
changes in the fine sediments at Bermuda Rise
reflect variability of high-latitude climate and
deep-water circulation, rather than local condi-
tions in the subtropical gyre. If alkenones were
carried along with these fine-grained sedi-
ments, an intermittent cold artifact associated
with waters from the continental slope could
occur that might mimic high-latitude climate
changes [Sachs and Lehman, 1999]. Likewise,
the presence of shallow-water microfossils in
deep-water sequences from continental margins
provides clear evidence of downslope transport
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
that could carry an anomalous signal. Measures
of reworked tracers appropriate to each site
(such as shallow-water fossils in continental
margin settings or mineralogical tracers of fine-
grained sediment sources) will help to identify
problems associated with sediment transport.
[40] Bioturbation induced by benthic organisms
moves clay- and silt-sized particles more read-
ily than sand-sized particles [Wheatcroft,
1992]. Thus there is a potential for bias in
alkenone records relative to records based on
sand-sized foraminifera. For example, in ultra-
high resolution studies the alkenone record of
brief climate change events may be smoothed
or even shifted in depth compared to estimates
based on foraminiferal species or stable isotope
records. This would not be a large problem in
laminated sediments that have not been biotur-
bated or for low-resolution studies in which the
depth spacing between samples is >10 cm.
Indeed, downcore scanning based on reflec-
tance spectroscopy suggests that significant
information can be preserved at the scale of a
few centimeters in spite of bioturbation effects.
Barring effects of sediment transport noted
above, meaningful records of climatic variabil-
ity on the scale of a few centuries can be
produced where sedimentation rate exceeds
10 cm kaÿ1 [Chapman and Shackleton, 1998].
[41] Deconvolving the effects of bioturbation
on sedimentary signals is not straightforward.
In part this reflects a lack of quantitative indices
of bioturbation intensity through time, except
in isolated cases such as where thin ash layers
are present [Ruddiman and Glover, 1972].
Pisias [1983] argued based on time series
analysis of geologic records that bioturbation
intensity increases with sedimentation rate,
implying a result that even sediments accumu-
lating slowly would preserve a useful geologic
record. Some progress in developing tracers of
bioturbation intensity has come with studies
that suggest a link to total food supply (i.e.,
biological productivity of overlying waters)
[Trauth et al., 1997], which also may leave
an imprint on the benthic species assemblages
[Loubere, 1989].
[42] Bioturbation models typically treat only the
part of mixing that can be simulated as a
diffusive process [Peng et al., 1979], which
occurs typically within a few centimeters of the
sediment-water interface. In addition, burrows
are commonly preserved below the well-mixed
surface layer. The presence of burrows and
other bioturbation structures may be detected
by careful visual description or digital imaging
of the sediments [Ortiz and Rack, 1999] or by
systematic X-ray analysis of sediment cores. In
many cases, such data can help investigators to
avoid major artifacts associated with burrows at
the time of sampling.
[43] Bioturbation is potentially damaging when
materials carrying a chemical tracer are moved
from intervals of high abundance into barren or
nearly barren zones. This is called the `̀ biotur-
bation-abundance couple'' [Broecker et al.,
1984]. Its effects have been explored in the
context of radiocarbon dating [Bard et al.,
1987; Broecker et al., 1999] and stable isotope
records [Mix and Ruddiman, 1985]. Such stu-
dies require precise data on abundance (relative
to total sediment) of the material carrying a
paleoceanographic tracer. Even if the effects of
bioturbation cannot be removed completely
from measured records using mathematical
models, potential problems can be identified
if reliable abundance data are available. Thus
we recommend that that absolute abundances
of alkenones (and other proxy carriers such as
microfossil shells) be reported for studies in
which bioturbation may be a problem.
8. Summary and Recommendations
[44] Great opportunities exist for application of
multiple proxies to the record of past changes
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
in the oceans and global climate. Multiproxy
strategies can help one to `̀ see'' through the
biological processes that mediate most paleo-
ceanographic records. In many cases, the exact
biological mechanisms or chemical pathways
controlling paleoceanographic proxies are un-
certain. We suggest a strategy of application of
proxies to the geologic record in parallel with
continuous efforts to refine understanding of
mechanisms and calibration of methods. Our
experience is that this two-pronged approach
leads to rapid advancement of knowledge.
[45] In the simplest case, application of multiple
proxies to estimate a single environmental
property, such as SST, helps gain confidence
in the result by reducing errors. Differences
between estimates based on multiple proxies
are equally important, as they lead to deeper
understanding based on the oceanographic con-
text of each measurement. Thus we recommend
analysis of residuals between estimates based
on multiple proxies or between proxy estimates
and actual (atlas or measured) environmental
values as a path toward greater insight into the
true nature of each proxy. In an example of
oceanographic context we note that faunal or
isotopic estimates of pycnocline depth or struc-
ture would add value to alkenone-based tem-
perature estimates by placing the Uk037 index into
an appropriate oceanographic context. Informa-
tion on overall ecosystem structure based on
faunal and floral species assemblages, or iden-
tification of alkenone producers via a biomar-
ker or genetic strategy, would constrain the
choice of temperature calibrations and thus
improve paleotemperature estimates.
[46] A more complicated multiproxy strategy
involves the use of dissimilar proxies to derive
oceanic properties that cannot be assessed with
any single measurement. Examples in which
progress is likely are estimating paleosalinity
and paleo-pCO2. Significant uncertainties are
involved in both properties. Nevertheless, both
are so important that continued study is war-
ranted. For paleosalinity estimates, multiproxy
temperature estimates (including faunal trans-
fer functions, Uk037, and trace metal indices)
coupled to oxygen isotope data in foraminifera
are needed. We also recommend detailed study
of the coccolith fraction, using emerging tech-
nologies for small-sample chemical and isoto-
pic analysis. On the basis of preliminary
relationships between alkenone n-C37:4 and
salinity we believe exploration of full chro-
matograms for multimolecular geochemical
evidence of salinity change and other envir-
onmental effects is warranted. Estimates of
paleo-pCO2 call for a geologic proxy for cell
or population growth rate, and we suggest
further explorations of proxies for nutrients,
nutrient utilization, biological productivity,
and calcification rate.
[47] As a first step toward identifying biases in
the sedimentary record associated with sedi-
ment transport and bioturbation, we recom-
mend research to identify the particles that
carry alkenones, as well as a practice of report-
ing absolute abundances of alkenones (and
other particles carrying environmental signals
for other proxies).
[48] Confirmation that multiproxy strategies
actually work in the geologic record requires
coordinated study of core top sediments. This is
easier to recommend than to accomplish, as
high-quality core top samples are relatively rare
commodities. We thus recommend that field
programs involving coring make special efforts
to archive and share high-quality samples of the
sediment-water interface. The best available
tool for this purpose is the multicore, which is
fortunately available in many institutions.
Where possible, high-quality core tops should
be dated using 14C and other stratigraphic
methods to verify the presence of essentially
modern sediments and if possible to establish
sedimentation rates.
GeochemistryGeophysicsGeosystems G3G3 mix et al.: alkenones and multiproxy strategies 2000GC000056
[49] Finally, we note that the level of scrutiny
the alkenone community is applying to their
proxies, as exemplified by the Woods Hole
workshop of 1999, serves as a model for other
communities. One of our recommendations is
for workers on other data types to join together
in a similar fashion to intercalibrate, refine, and
revise their proxies as needed for a richer and
more complete view of ocean and climate
history.
Acknowledgments
[50] We thank T. I. Eglinton, M. H. Conte, G. Eglinton,
and J. M. Hayes, organizers of the NSF sponsored
workshop Alkenone-Based Paleoceanographic Indica-
tors. We also thank the workshop participants for
spirited discussions and the U.S. National Academy of
Sciences and Woods Hole Oceanographic Institution for
facilitating this effort. Reviews by W. Curry, M.
Feldberg, L. Labeyrie, F. Prahl, and R. Thunell improved
the manuscript.
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