R
Quaternary Research 60 (2003) 142–148 www.elsevier.com/locate/yqres
a,
a
b
c a
Department of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL 60115, USA b
Center for Climate Research, 1225 W. Dayton St., University of Wisconsin at Madison, Madison, WI 53706-1695, USA c
Department of Geology, Grand Valley State University, Allendale, MI 49401, USA
Received 27 August 2002
Abstract
Records from South America show that modern ENSO (El Nino-Southern Oscillation) did not exist 7000 cal yr B.P. and has developed progressively since then. There has been little information available on oceanic conditions in the eastern equatorial Pacific (EEP) to constrain explanations for ENSO onset. We report quantitative observations on thermocline and mixed-layer conditions in the EEP during ENSO start up. We found important changes in both the thermocline and the mixed layer, indicating increased upwelling of cooler waters since 7000 cal yr B.P. This resulted from change in the source and/or properties of waters supplying the Equatorial Undercurrent, which feeds upwelling along the equator and the Peru margin. Modeling shows that ENSO is sensitive to subsurface conditions in the eastern equatorial Pacific and that the changes in the thermocline we observed were driven by extratropical processes, giving these a role in conditioning the development of ENSO. This is in contrast to models that call for control of equatorial Pacific oceanography by tropical processes only. These infer stronger upwelling and cooler surface waters for the EEP during the mid-Holocene, which is not supported by our results.
© 2003 University of Washington. Published by Elsevier Inc. All rights reserved.
Introduction
The existence of ENSO variability in the equatorial Pacific depends on ocean and atmospheric processes both at the eastern and western ends of that ocean. These processes modulate the trade winds. In the west this occurs through the development of atmospheric convection; in the east it occurs in response to upwelling and conditions in the ther-
mocline (Philander, 1990). Recent paleoclimate records in
Peru (Sandweiss et al., 1996), Ecuador (Rodbell et al.,
1999), the Galapagos Islands (Riedinger, et al., 2002), and
the western Pacific (Tudhope et al., 2001) indicate that
ENSO variability did not become active until the late Holocene (after 6000 yrs ago). The background climate state and the physical mechanisms for this change in ENSO have
remained poorly constrained and controversial (Sandweiss et al., 1996; DeVries et al., 1999; Clement et al., 1999; Liu et al., 2000). although data available for the western Pacific
* Corresponding author.
E-mail address: paul@geol.niu.edu (P. Loubere).
indicate little Holocene variability in surface ocean conditions
(Tudhope et al., 2001) the Holocene state of the open ocean
eastern equatorial Pacific and the equatorial thermocline are unknown. Here, we present the first quantitative evidence for eastern equatorial Pacific thermocline and mixed-layer conditions since the early Holocene. Our results show that oceanographic changes in the eastern tropical thermocline and surface layer parallel the development of ENSO.
We reconstruct thermocline mixed-layer temperatures and nutrient contents, along with surface ocean biological productivity, for the eastern equatorial Pacific using benthic and planktonic foraminiferal stable isotopes and planktonic
and benthic assemblage data for three deep-sea cores (Fig.
1; we also refer to data from a fourth core, V19-30). Sea-
surface temperature (SST) is important in controlling trade winds. In the Pacific a critical factor is the SST gradient from east to west across the ocean. During the Holocene,
SST in the western Pacific does not appear to have changed
much (Tudhope et al., 2001), so the east to west gradient
would depend on any changes that occured in the EEP. The
SST in the eastern equatorial Pacific will be sensitive to
0033-5894/03/$ – see front matter © 2003 University of Washington. Published by Elsevier Inc. All rights reserved.
doi:10.1016/S0033-5894(03)00092-9

P. Loubere et al. / Quaternary Research 60 (2003) 142–148 143
Fig. 1. Core locations in the eastern equatorial Pacific. Solid contours show average annual productivity (gC/m
2
/yr); dashed contours show mean annual sea surface temperature.
thermocline temperatures and the rate of upwelling. Oxygen isotope values from shallow dwelling planktonic foraminifera along with planktonic assemblage data monitor SST.
Oxygen isotope values from deeper dwelling taxa reflect thermocline temperatures. Carbon isotope values from the planktonic foraminifera along with benthic foraminiferal assemblages reflect the nutrient content of the waters and planktonic community biological productivity, allowing us to follow changes in upwelling.
Methods
The foraminiferal species we selected for analysis were chosen for their ecologic characteristics. Globigerinoides
ruber has been shown to calcify in the mixed layer and live within the upwelling systems of the equatorial Pacific
(Watkins et al., 1996, 1998). Neogloboquadrina dutertrei
isotope values represent a consistent position within the
thermocline of the eastern equatorial Pacific (Fairbanks et al., 1982; Curry et al., 1983; Ravelo and Fairbanks, 1992;
Loubere, 2001). Uvigerina species are a common benthic
taxon group in the EEP which have been used previously for detailed studies of abyssal isotopic values. This group does not show differential isotopic fractionation by size fraction or species within the study area.
We used data from four cores recovered in the EEP (Fig.
1; Y71-3-11MG3, 0.28° S, 83.275° W, 2656 m; V19-28,
2.47° S, 84.65° W, 2670 m; VNTR01-13MG, 3.09° S,

144 P. Loubere et al. / Quaternary Research 60 (2003) 142–148
Fig. 2. Paleoenvironmental data for the eastern equatorial Pacific. (a) Benthic reference oxygen (solid line) and carbon (dashed) isotopes. All plots except
(e) are for core Y71-3-11. For (d): Species %, N. pachyderma (dextral) is a cool water form and N. dutertrei is a warmer water taxon. Open triangles are the N. dutertrei/N. pachyderma (R) ratio for VNTR01-13— higher ratio
⫽ higher SST. (e) Records for cores V19-28 and VNTR01-13GC. V19-28 productivity shown as solid boxes; VNTR01-13 productivity as open boxes. For age dating, three radiocarbon measurements were made on core
Y71-3-11MG3 and two were made on V19-28 (see Methods).
90.825° W, 3304 m; V19-30, 3.38° S, 83.35° W, 3091 m).
Core Y71-3-11MG3 is a gravity multicore. We sampled it at
5cm intervals and picked between 6 and 12 G. ruber, 6 to 10
N. dutertrei and 8 to 10 Uvigerina specimens for isotopic analysis. The benthic isotopic data reflect global ocean shifts through time, and the planktonic data reflect both the global signal and changes local to the EEP. To get the local signal only, we present our values as the difference between coeval planktonic and benthic results, a procedure that cancels isotopic shifts due to global, rather than EEP, events.
All planktonic samples were at least duplicated, and for important intervals were replicated up to six times. The G.
ruber were picked from the 400 to 600-
␮ m, and the N.
dutertrei from the
⬎
500-
␮ m, size fractions. These size intervals have been found to yield consistent results for both oxygen and carbon isotopes in the EEP. Isotopic analyses were conducted at the Department of Geology at the University of California, Davis. The analytical error was
⫹
/
⫺
0.03‰ for oxygen and
⫹
/
⫺
0.02‰ for carbon. The mean difference for replicates of G. ruber was 0.25‰ for oxygen and 0.15‰ for carbon (42 pairs). Those values for
N. duterteri were 0.24‰ and 0.11‰, respectively (26 pairs).
Planktonic foraminiferal species percentages were deter-
mined for cores Y71-3-11MG3 and VNTR01-13GC (Fig.
1). An average of 550 individuals were counted from a
representative split of the
⬎
149-
␮ m fraction of each sample. The counts for N. dutertrei and N. pachyderma are for these taxa sensu stricto, intermediate forms were put into an
“intergrade” category.
Paleoproductivity estimates were made on cores V19-28
and VNTR01-13GC (Fig. 1) based on a benthic foraminif-
eral transfer function, using methods described in Loubere and Fariduddin (1999).
A combination of radiocarbon and oxygen isotope records provided us with time control. Dates on picked specimens of N. dutertrei were obtained from five samples
(see Fig. 2; NOSAMS Accession number 30531, Y71-3-
11MG3, 20 cm
⫽
6970 unadjusted
14
C yr B.P.; 30532,
Y71-3-11MG3, 20 cm
⫽
6990
11MG3, 45 cm
⫽
9880
⫽
7040
14
14
14
C yr B.P.; 30533, Y71-3-
C yr B.P.; 30534, V19-28, 28 cm
C yr B.P.; 30535, V19-28, 71 cm, 11,150
14
C yr
B.P.). These had an average error of 44 yrs, replication of one sample gave a difference of 20 yrs. They were corrected
by 600 yrs (Toggweiler et al., 1991) and converted to
“calendar” year equivalents (Bard et al., 1990). For
VNTR01-13GC, sample ages are estimated based on oxygen isotope stratigraphy (unpublished data) yielding a Holocene sedimentation rate quite similar to OPD846B (located at the same spot) which has a very good oxygen
isotope stratigraphy (Mix et al., 1995).
Results
Results appear in Figure 2. These show two patterns, one
associated with the thermocline and one with mixed-layer conditions. The carbon isotope results for the planktonic species in Y71-3-11 correlate with the oxygen isotope record of the thermocline species, N. dutertrei, and with the paleoproductivity record seen in cores V19-28 and

P. Loubere et al. / Quaternary Research 60 (2003) 142–148 145
Fig. 3. Regional patterns of ocean and continental change in the eastern tropical Pacific area compared to the seasonal and annual insolation anomaly at the equator (note that the seasonal insolation anomaly in the subtropics is qualitatively similar to that at the equator), and the winter insolation anomaly at 45°S.
The carbon isotope record reflects EEP thermocline change; the oxygen isotope record reflects mixed-layer temperature variation (pattern is also seen in
planktonic assemblage data). Ice core isotope values reflect air temperature in the high Andes (Thompson et al., 1995); more negative
⫽ cooler. The intervals
marked for the Galapagos (Colinvaux, 1972)
⫽
higher lake level (a tracer for warm ocean conditions); and for Haiti (Hodell et al., 1991)
⫽ higher lake level with a higher precipitation/evaporation ratio.
VNTR01-13GC. These show a younger Dryas age event with apparently lower thermocline temperature (N. dutertrei oxygen isotopes), higher thermocline nutrient content, and higher biological productivity (V19-28). This is followed by the reversal of these conditions through the mid-Holocene until about 7000 cal yr B.P. Note that the mid-Holocenepositive carbon isotope shift is also seen in the record of
core V19-30 (Fig. 1; Shackleton and Pisias, 1985). The
warming represented by the N. dutertrei oxygen isotope data for the mid-Holocene will be a minimum value since this species deepens its habitat in lower productivity settings
(such as those inferred for the mid-Holocene interval, Fig.
2; the cooling effect due to habitat shift (Loubere, 2001) will
be about 2°C; the habitat depth shift behavior of N. dutertrei reduces the magnitude of temperature changes recorded by this species’ oxygen isotope values). The younger Dryas to mean mid-Holocene oxygen isotope shift is 0.44‰ (with a max of 0.59‰). This could represent a warming of at least
2° (about 0.21‰ per °C). After about 7000 cal yr B.P., the data indicate cooling in the thermocline, increasing nutrient concentrations, and increasing productivity. The oxygen isotope shift between the mid-Holocene and late Holocene means is 0.29‰ (t test two-tail sigma
⫽
0.004 (20 d f )), with a maximum shift of 0.75‰, giving a mid- to late-Holocene cooling between 1 and 4°C.
The mixed-layer record shows a related but different pattern. The oxygen isotope record of the mixed layer species, G. ruber, and the assemblage percentages of temperature-sensitive species in Y71-3-11 show cooler conditions during the early Holocene followed by an apparent warm interval from 10,000 to about 7000 cal yr B.P. Then, cooling from the mid- to the late Holocene is indicated. The shift in oxygen isotope means between the mid- and late Holocene is 0.24‰ (t test two-tail sigma
⫽
0.010 (19 d f )) with a maximum shift of about 0.55‰. This would represent a temperature change of 1 to 2°C. The cooling is also indi-
cated by the planktonic foraminiferal assemblages (Fig. 2,
cores Y71-3-11 and VNTR01-13) which show the highest proportion of warm water species during the mid-Holocene.
The onset of the mid-Holocene SST warm interval clearly lags warming and other changes in the thermocline. However, the interval correlates well with other regional indicators of climate in the equatorial Americas, as shown in
Figure 3.
Mid-Holocene SST warming in the EEP is, at first glance, not supported by measurements of planktonic fora-

146 P. Loubere et al. / Quaternary Research 60 (2003) 142–148 miniferal Mg/Ca ratios in a core next to the Galapagos
Islands (Koutavas et al., 2002) where a cooling of about
0.5°C is inferred. In applying their geochemical method the authors dismiss contradictory evidence on the Holocene to
Last Glacial Maximum timescale (e.g., Pisias and Mix,
1997; Mix et al., 1999) as well as Holocene studies from the coast of South America (e.g., Sandweiss et al., 1996; Andrus et al., 2002). Our evidence is based on six independent
lines of evidence for the mid-Holocene and is compatible with the coastal data. The controversy might be resolved by considering the ecology of the species (G. sacculifer) analyzed for Mg/Ca and the oceanographic changes we propose
for the Holocene. Watkins et al. (1996, 1998) show that G.
sacculifer inhabits the subtropical gyre and is adapted to a low nutrient, deep thermocline environment. The species is scarce along the equator where it composes generally less than 2% of surface sediment assemblages we have examined. It seems possible that during the late Holocene G.
sacculifer contributed to the equatorial sediments predominantly during strong El Nino events when hydrographic conditions mimicked its natural habitat. Hence, the species
Mg/Ca ratios would reflect the warm extreme of equatorial conditions. A modern analogy to this is found in the sedi-
ment trap record of the Panama Basin (Thunell et al., 1983)
where the flux of G. sacculifer is lowest during the spring upwelling season and highest during the summer-fall period with a thick well mixed layer. During the mid-Holocene we infer that equatorial conditions became more subtropical gyre-like and G. sacculifer would have been a more consistent inhabitant of the water column (our data show higher abundances of subtropical species in mid- compared to late-Holocene sediments), so its geochemistry would tend to reflect average rather than extreme conditions. This could lead to a small decrease in apparent SST, especially if abetted by changes in the species’ water column positioning
(G. sacculifer calcifies over a considerable depth range, into the thermocline) and reproductive behavior as conditions
shift (Nurnberg et al., 1996, show that gametogenesis influ-
ences Mg/Ca ratios in G. sacculifer).
Oceanographic conditions and the onset of ENSO
Our results indicate that during the mid-Holocene interval, when ENSO was diminished, the EEP thermocline was warmer, nutrient contents of thermocline and surface waters were lower, biological productivity was lower, and mixedlayer temperatures were somewhat warmer than at present.
Through the late Holocene surface and thermocline waters have cooled, nutrient contents have risen, and biological productivity has increased. These results are supported by
coastal observations along South America (Sandweiss et al.,
1996, 2001; Andrus et al., 2002). ENSO development over the last 7000 yrs (Sandweiss et al., 1996, 2001; Rodbell et al., 1999) appears to have followed mixed-layer and ther-
mocline cooling, and associated changes in thermocline
properties. This agrees with theory (Liu et al., 2000), modeling (Liu et al., 2000; Timmermann et al., 1999), and
observations (Trenbreth and Hoar, 1996), indicating that
ENSO is sensitive to thermocline conditions. Our proxy record of the equatorial thermocline in the EEP provides the first observational evidence that the evolution of ENSO since the early Holocene could be influenced by changes in the mean thermocline state. The warmer equatorial thermocline produces a deeper mixed layer, which reduces the efficiency of equatorial upwelling cooling. This weakens the coupling between the tropical ocean and the atmosphere
and in turn reduces the amplitude of ENSO (Liu et al.,
2000). Therefore, we propose that the dramatic reduction of
ENSO before the late Holocene was caused by the combined effect of the change in mean equatorial thermocline along with the enhanced seasonal forcing due to the North-
ern Hemisphere monsoon (Liu et al., 2000) and local insolation forcing (Clement et al., 1999) that prevailed during
early to mid-Holocene time.
Timing and mechanisms of oceanographic change
The timing of changes in the EEP thermocline and SST appears to have been controlled by multiple factors through the Holocene. Events we record for the thermocline correlate with insolation changes at mid to high latitudes in the
southern hemisphere, as shown in Figure 3. The connection
between the EEP thermocline and higher latitudes lies with the source of waters feeding the equatorial undercurrent
(EUC). The EUC flows from west to east along the equator and supplies subsurface water for upwelling. The undercurrent is divided into two parts: shallow subsurface waters with a source principally in the southern subtropical gyre
(Liu et al., 1994; Huang and Liu, 1999), and deeper sub-
surface waters with a source in the southwest subantarctic
Pacific (Toggweiler et al., 1991). It is the latter which
supplies the bulk of new nutrients driving eastern equatorial
Pacific biological productivity. Figure 3 shows that the
record of winter insolation in the southern hemisphere correlates with our thermocline records. Winter insolation controls the characteristics and circulation of subducted ther-
mocline waters. Our results support modeling work (Liu et al., 2000), indicating that southern hemisphere boreal winter
warming during the mid-Holocene resulted in warmer thermocline waters entering the tropics. Further, these waters had lower nutrient contents than at present and supported lower biological productivity. This latter observation is sup-
ported by data from the coast of Peru (Andrus et al., 2002).
Since there is no evidence that subantarctic nutrient con-
centrations were lower at this time (Hodell et al., 2000), it
appears that the subtropical source (lower nutrients) became relatively more important during the mid-Holocene. As this source is warmer than the subantarctic source, a change in relative importance of the sources would enhance an insolation-driven thermocline warming. Cooling in the thermo-

cline since 7000 cal yr B.P. follows a decreasing higher southern latitude winter temperature anomaly and coincident with this, nutrient concentrations have risen in subsurface waters indicating increased importance of the subantarctic source.
Finally, temperatures in the mixed layer have not simply followed those in the thermocline. Instead, the mixed layer species oxygen isotope record and planktonic species assemblage data indicate a warmer interval which lags mid-
Holocene events in the thermocline, but which correlates to
indicators of warmth in the tropical Americas (Fig. 3).
Coastal South American records also show SST warmth at
this time (Sandweiss et al., 1996; Andrus et al., 2002).
Mid-Holocene SST warming appears to have been delayed until the negative austral summer insolation anomaly dimin-
ished (Fig. 3). Exploratory results from a coupled ocean
atmosphere GCM (FOAM by Liu, 2002, http://ccr.aos.
wisc.edu/-behling) suggest that in the central equatorial
Pacific, the mid-Holocene SST was colder than today due to strengthened tradewinds and reduced mean annual insolation. Nevertheless, in the eastern equatorial Pacific the mid-
Holocene SST in the model was warmer than present, especially in austral summer. In the boreal summer, the warmer SST appears produced by increased local equatorial
insolation (Fig. 3). In the austral summer, the warmer SST
seems associated with reduced coastal upwelling and a weaker subtropical high which is produced by reduced
southern hemisphere insolation during this season (Fig. 3).
Further studies, however, are needed to understand SST changes across the tropical Pacific during the Holocene.
Acknowledgments
We gratefully acknowledge the National Science Foundation support for the core repositories we used; and the help of core curators at Oregon State University School of
Oceanography (June Padman, Bobbi Conard) and Lamont
Doherty Earth Observatory (Rusty Lotti-Bond). Dr. H.
Spero kindly made his isotope laboratory available to us.
This work was partly supported by NSF Grant OCE
9617713 and by the Graduate School at Northern Illinois
University. This is CCR Contribution Number 822.
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