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The Indo-Pacific Warm Pool (IPWP) plays a crucial role in influencing climate dynamics both in the tropics and globally. Yet, there is an ongoing controversy concerning the evolution of surface temperatures in the IPWP since the Pliocene, which is fueled by contradictory proxy evidence. Temperature reconstructions using TEX86 indicate a gradual cooling by ∼2°C from the Pliocene to today while Mg/Ca-based studies using planktonic foraminifera do not report any long-term trends. A bias in Mg/Ca records due to seawater chemistry changes has been suggested as an explanation for this proxy mismatch. Here, we present data from two independent foraminifera-based temperature proxies, Mg/Ca and clumped isotopes (Δ47), measured on the same samples from IODP Site U1488 in the IPWP. We reconstructed mixed layer and subsurface temperatures and find very good agreement among Mg/Ca and Δ47 when applying a minor correction for changing Mg/Ca ratios of seawater. Diagenetic effects could influence Δ47 but the evaluation of foraminifera preservation at Site U1488 suggests that this effect is unlikely to have masked a long-term trend in the data. While remaining uncertainties prevent us from fully ruling out particular hypotheses, our study adds evidence that mixed layer temperatures likely did not cool substantially, while subsurface temperatures cooled more strongly since the Pliocene. The substantial Pleistocene cooling previously observed in TEX86 data is consistent with this finding when interpreting it as a combined surface and subsurface signal. publishedVersion

Sea surface temperature (SST) of the eastern equatorial Pacific is a key component of tropical oceanic and atmospheric circulation with global teleconnections. Forcing factors such as local and high-latitude insolation changes, ice sheet size and albedo feedbacks, and greenhouse gas radiation have been proposed as controls of long-term eastern tropical Pacific SST, though the precise role each mechanism plays is not fully known on glacial-interglacial or longer timescales. Here proposed mechanisms are evaluated by comparing orbital-scale records of eastern Pacific SST with forcing variability over the past 1.5 Ma. The primary SST records are a compilation of new and existing data from Ocean Drilling Program Site 1239 at the northeastern margin of the modern eastern Pacific cold tongue and Site 846 SST within the cold tongue. Using time series analysis, we test previously proposed mechanisms for control of long-term tropical SST change and SST gradients in the eastern Pacific. We find that within statistical uncertainties, in the precession band eastern Pacific SST is consistent with direct forcing by equatorial radiation changes in the tropical cold season (summer-fall) rather than inversely correlated as previously suggested. In the obliquity band high-latitude solar forcing leads or is in phase with eastern equatorial Pacific SST, while in the eccentricity band atmospheric greenhouse gas concentrations are closely associated with cold tongue SST. Pleistocene eastern Pacific SST gradients indicate that the gradient on the northern margin of the cold tongue strengthened through the mid-Pleistocene transition, a result compatible with the cold tongue becoming more focused at ~900–650 ka.

The relationship between climate, biological productivity, and nutrient flux is of considerable interest in the subarctic Pacific, which represents an important high-nitrate, low-chlorophyll region. While previous studies suggest that changes in iron supply and/or physical ocean stratification could hypothetically explain orbital-scale fluctuations in subarctic Pacific nutrient utilization and productivity, previous records of nutrient utilization are too short to evaluate these relationships over many glacial-interglacial cycles. We present new, high-resolution records of sedimentary δ15N, which offer the first opportunity to evaluate systematic, orbital-scale variations in subarctic Pacific nitrate utilization from 1.2 Ma. Nitrate utilization was enhanced during all glacials, varied with orbital-scale periodicity since the mid-Pleistocene transition, was strongly correlated with enhanced aeolian dust and low atmospheric CO2, but was not correlated with productivity. These results suggest that glacial stratification, rather than iron fertilization, systematically exerted an important regional control on nutrient utilization and air-sea carbon flux.

Export production is an important component of the carbon cycle, modulating the climate system by transferring CO2 from the atmosphere to the deep ocean via the biological pump. Here we use barite accumulation rates to reconstruct export production in the eastern equatorial Pacific over the past 4.3 Ma. We find that export production fluctuated considerably on multiple time scales. Export production was on average higher (51 g C m−2 yr−1) during the Pliocene than the Pleistocene (40 g C m−2 yr−1), decreasing between 3 and 1 Ma (from more than 60 to 20 g C m−2 yr−1) followed by an increase over the last million years. These trends likely reflect basin-scale changes in nutrient inventory and ocean circulation. Our record reveals decoupling between export production and temperatures on these long (million years) time scale. On orbital time scales, export production was generally higher during cold periods (glacial maxima) between 4.3 and 1.1 Ma. This could be due to stronger wind stress and higher upwelling rates during glacial periods. A shift in the timing of maximum export production to deglaciations is seen in the last ~1.1 million years. Results from this study suggest that, in the eastern equatorial Pacific, mechanisms that affect nutrient supply and/or ecosystem structure and in turn carbon export on orbital time scales differ from those operating on longer time scales and that processes linking export production and climate-modulated oceanic conditions changed about 1.1 million years ago. These observations should be accounted for in climate models to ensure better predictions of future climate change.

The Bering Strait provides a shallow connection that allows freshwater to flow from the North Pacific into the North Atlantic, but this passage was closed during past glacials when sea level was at least 50 m lower than at present. Climate models investigating Bering Strait closure predict that this mechanism increases the salinity in the North Atlantic and reduces the salinity in the North Pacific, inducing a Pacific-Atlantic seesaw in meridional overturning circulation and poleward heat flux. However, the Pacific circulation response to Bering Strait closure, and thus the seesaw theory, has not been tested by long paleoceanographic records. We present long records of foraminiferal δ18O and δ13C from Integrated Ocean Drilling Program Site U1342 in the Bering Sea, which provide the first evidence of enhanced North Pacific Intermediate Water when the Bering Strait was closed during each of the extreme glacials of the last 1.2 Myr. These results suggest that orbital-scale variations in North Pacific Intermediate Water are coherent and in phase with variations in upper North Atlantic Deep Water, but are unrelated to changes in lower North Atlantic Deep Water. Together, these results provide evidence for systematic, orbital-scale variability in North Pacific Ocean circulation and may challenge the idea of an orbital-scale Pacific-Atlantic seesaw.

We generated new, long, high-resolution, climate proxy records from Deep Sea Drilling Project (DSDP) Sites 607 and 609 in the subpolar North Atlantic over the interval 225–970 ka, which have pronounced variability at periods f > 1/7 kyr) variance in our North Atlantic climate proxy records is shown, using bispectral and cross-bispectral methods, to be explainable as harmonics and/or combination tones of orbital-scale climatic variability of the North Atlantic region itself. Thus the timing and amplitude of high-frequency climate change in the North Atlantic region appears to be a nonlinear function of variations in high-latitude climate at Milankovitch frequencies.

Foraminifer count and multispecies isotopic data from Ocean Drilling Program (ODP) Sites 806 (0°N, 165°E) and 847 (0°N, 95°W) are presented for the last 6.4 m.y. Faunal evidence shows a decline of taxa of Miocene origin between 4.5 and 4.0 Ma at both sites. Prior to this decline, Globoturborotalita species and Neogloboquadrina acostaensis dominated Sites 806 and 847 assemblages, respectively. Globoturborotalitids were not replaced by other thermocline dwellers after 4.2 Ma, which suggests a deepening of the thermocline and thickening of the warm pool at Site 806. At Site 847 in the eastern “cold tongue,” N. acostaensis was gradually replaced by N. dutertrei. After 4.2 Ma, there was also a large increase (from 0.7 to 1.3‰) in the difference between the δ18O values of Globigerinoides sacculifer, a surface dweller, and Neogloboquadrina dutertrei, a seasonal thermocline dweller at Site 847; these data, in conjunction with faunal evidence, suggest that the temperature gradient in the upper photic zone increased significantly, largely because of subsurface cooling. The events in planktonic foraminifer evolution and ecology suggest that between 4.5 and 4.0 Ma a significant step was made toward development of the modern east-west gradient in surface hydrography. The timing of these events suggests that they were related to the closing of the Central American Seaway and subsequent changes in meridional temperature gradients and/or changes in air-sea interactions that modified tropical winds.

The Early Pliocene is the most recent interval in which equilibrium oceanic conditions can be studied in the context of global warmth relative to today. To characterize thermohaline circulation during this warm period (4.4-3.1 Ma) we combined new benthic foraminiferal isotopic data from the southeast Atlantic with published data, and reconstructed Pacific and Atlantic Ocean nutrient distributions. The data indicate enhanced ventilation of the deep Atlantic and intermediate Pacific during the Early Pliocene. Enhanced ventilation implies that delivery of surface water to high latitudes in western boundary currents of North Pacific and Atlantic subtropical gyres was probably also enhanced. Future modeling of this warm period needs to reconcile reduced meridional surface temperature gradients with enhanced subtropical gyre and thermohaline circulation.

High-resolution (∼3–4 kyr) planktonic stable isotope stratigraphies from Site 925 drilled on Ceara Rise (Ocean Drilling Program Leg 154) are used to investigate the role of the western equatorial Atlantic sea surface hydrography in early Pliocene (3.2–4.7 Ma) climate change. Oxygen isotope results from Globigerinoides sacculifer, a mixed layer dweller, suggest that equatorial sea surface temperatures were cooler than today by ∼2°–3°C, consistent with relatively strong northward advection of heat away from the equator. Oxygen isotope results from Neogloboquadrina dutertrei suggest that over the long term this thermocline dweller tracks global ice volume fluctuations. Stable isotope gradients between the two planktonic species throughout the entire interval imply a stable warm pool in the western equatorial Atlantic. We observe a rapid (3.4 kyr) decrease in G. sacculifer and the N. dutertrei δ18O values at 4.36 Ma that may reflect a freshening of the sea surface, a direct response to a southward displacement of the Intertropical Convergence Zone, and perhaps, an indirect response to restricted flow through the Central American Seaway.

We evaluated the response of eastern equatorial Pacific upper water column hydrography to the Panama seaway closure and the initiation of large-scale northern hemisphere glaciation, The δ18O gradient between Globorotalia tumida and Globigerinoides sacculifer indicates a general shoaling of the thermocline between 4.2 and 3.0 Ma which may be related to a recorded decrease in southeast trade wind strength. The δ13C gradients suggest that surface water nutrient concentrations have increased during the last 5 Myr and that upwelling intermediate waters began to change sources at ∼3.2 Ma. Changes in the trans-Pacific east-west temperature and nutrient gradients at 4.0 and 1.5 Ma are coincident with the seaway closure and a major phase of northern hemisphere glaciation. Changes in the sensitivity of surface circulation to Milankovitch forcing, δ18O records linearly related to orbital variations only after 3.2 Ma, are associated with the seaway closure. Pleistocene initiation of 100 and 41 kyr cycles in the thermocline depth proxy may suggest that the hydrographic response to Milankovitch forcing is enhanced by air-sea interactions which maintain the relatively steep Pleistocene trans-Pacific gradients. The δ13C records are dominated by the 41 kyr period and are usually coherent with high-latitude climate, suggesting that associated changes in the global carbon reservoir dominate the tropical δ13C signals.

We evaluate the relationship between ten surface ocean (0–300 m) hydrographic parameters and the spatial distribution of factor-analyzed core top planktonic foraminiferal abundances in the tropical Pacific Ocean (24°N–24°S) for core tops

Abundances of 12 species of planktonic foraminifera collected in two plankton tows from the east tropical Atlantic are compared to the chlorophyll content and the temperature of the sea water from which they were collected. As expected from previous work in the tropics, all dominant tropical species occur in greatest abundance within the photic zone. Many species occur in greatest abundances in the seasonal thermocline in association with the maximum chlorophyll concentration, while a few algal symbiont-bearing species occur in greatest abundance in the mixed layer. The δ18O measurements of planktonic foraminifera shells from core top sediment samples confirm the vertical stratification within the photic zone that is suggested by the relationship between hydrography and abundances found in the plankton tows and found in the statistical study by Ravelo et al. [1990]. Comparison between the measured δ18O values of planktonic foraminifera with the predicted δ18O profiles of the overlying water column at three core locations indicate that species abundances in the sediment record the seasonally integrated conditions of the photic zone and suggests that the abundance of a species in the sediment depends on whether the preferred ecological conditions of that species may be found within the photic zone of the overlying water column sometime during the year. Species which calcify below the photic zone have only trace relative abundances. Finally, it appears that the total range of δ18O values of the dominant species approximates the predicted annual δ18O of calcite range in the upper 80 m of the water column.

Geochemical sea surface temperature (SST) proxies such as the magnesium to calcium ratio (Mg/Ca) in foraminifera and the alkenone unsaturation index (UK′37) are becoming widely used in pre-Pleistocene climate records. This study quantitatively compares previously published Mg/Ca and UK′37 data from Ocean Drilling Program (ODP) site 847 in the eastern equatorial Pacific to assess the utility of these proxies to reconstruct tropical SST over the last 5 Ma. Foraminiferal Mg/Ca–SST calibrations that include a dissolution correction are most appropriate at this location because they provide SST estimates for the youngest sample that are close to modern mean annual SST. The long-term trends in the two records are remarkably similar and confirm a ∼3.5°C cooling trend from the early Pliocene warm period to the late Pleistocene noted in previous work. Absolute temperature estimates are similar for both proxies when errors in the dissolution correction used to estimate SST from Mg/Ca are taken into account. Comparing the two SST records at ODP site 847 to other records in the region shows that the eastern equatorial Pacific was 2–4°C warmer during the early Pliocene compared to today.

In the tropical Atlantic, planktonic foraminfera species are vertically distributed with highest abundances occurring in the photic zone (approximately 0–100 m). The tropical Atlantic thermocline dips from east to west and varies seasonally due to changes in the southeast and northeast trade winds. In the east, the thermocline is in the photic zone, and in the west, the well-mixed surface layer extends below the photic zone most of the year. As expected from species vertical distributions in plankton tows, the species assemblages on the seafloor are correlated to the hydrographic conditions of the overlying surface ocean layer. A new technique to reconstruct past tropical Atlantic (20°N to 20°S) photic zone hydrography and surface wind field uses faunal assemblage data from deep-sea cores. Planktonic foraminifera abundances in core tops correlate with observations of modern photic zone hydrography defined here as seasonal temperature variation and mixed layer depth. The hydrography is mathematically described using empirical orthogonal function (EOF) analysis of annual temperature range as a function of depth. Factor analysis of 29 species of planktonic foraminifera from 118 core tops produces three factors. The factors correlate to mixed layer depth and the two EOF modes. The ocean model of the Atlantic ocean produces similar map patterns of the EOF modes. Therefore the model can be used to simulate hydrographic changes to compare with faunal predicted past hydrographic changes. Since the ocean model is wind driven, this approach provides a way of evaluating the validity of estimates of past wind stress changes and the contribution of these changes to the faunal changes in the past. A double wind stress run indicates that the central and eastern equatorial and southeast regions of the study area are most sensitive to wind stress increases. Factor analysis of the foraminifera abundances from the last glacial maximum (LGM) shows that species associations change downcore and demonstrates how the methods developed in this study can be applied. Comparison of the double wind stress experiment and the LGM faunal changes indicates some areas of significant agreement suggesting that faunal changes may reflect thermocline structure response to the LGM wind field. Discrepancies may reflect the fact that uniform changes in the north and south trade wind strengths did not occur at the LGM.

[1] Given the importance of upwelling processes to coastal productivity and regional climate, it is critical to study the role of upwelling regions within the context of global climate change. We generated sea surface temperature (SST) records for the last 5 million years in three important upwelling regions: the eastern equatorial Pacific, the California margin, and the Peru margin. Prior to ∼3.0 Ma, SSTs at all sites were significantly warmer than today (by 3–9°C), indicating that cold upwelling regions that characterize the modern Pacific Ocean did not exist in the early Pliocene warm period (4.6 to 3.1 Ma), Earth's most recent period of sustained global warmth. Alkenone, phosphorus, and organic carbon mass accumulation rate records indicate that changes in productivity and SST were decoupled and that upwelling of nutrient enriched water occurred even when SSTs were warm during the early Pliocene. Thus the long-term trends in SST are likely explained by changes in the temperature of upwelled water rather than in the strength of upwelling-favorable winds alone. The fact that gradual cooling of upwelling regions began before the onset of significant Northern Hemisphere glaciation provides further evidence that the growth of ice sheets and their influence on atmospheric winds alone can not explain the cooling of upwelling regions. Our results suggest that the long-term average SSTs of upwelling regions are influenced by global changes in the depth and/or temperature of the ventilated thermocline.

[1] We present the first continuous records from 0 to 5 Ma (in 0.333 m.y. integrated time steps) of paired boron/calcium (B/Ca) ratios and boron isotopes (δ11B) in the planktonic foraminifera Globogerinoides sacculifer (without sacc) from a site in the western equatorial Pacific Ocean (Ocean Drilling Program Site 806). These measurements, the first made in conjunction with calcification temperature (magnesium/calcium ratios) and average shell mass measurements, indicate that pH is not the sole environmental variable controlling B in planktonic foraminiferal calcite. Our data are consistent with calcification temperature exerting a primary control on B concentration and isotopic composition in planktonic foraminifera. If so, calcification temperature must be taken into account if pH for past oceans and atmospheric pCO2 are to be estimated from B isotope measurements in foraminiferal calcite. Doing so will substantially increase the uncertainty of pH estimates. Although this work was designed as a temporal study, its results define new aspects of calibrating the δ11B paleo-pH tracer.

[1] We developed and applied an analytical system for the simultaneous determination of magnesium/calcium, strontium/calcium, and manganese/calcium ratios in biogenic carbonates using a Perkin-Elmer 4300 Optima DV inductively coupled plasma-optical emission spectrophotometer in radial mode. Intensity ratio responses to calibration standard elemental ratios were linear over suitable working ranges. We evaluated instrumental response using solution replicates, and we analyzed planktonic foraminifera in multiple runs. Ca concentrations were kept in a relatively narrow range, typically ∼0.6–2.4 mM Ca, although we found no resolvable Ca matrix effect over a much wider range. Each analysis required a minimum sample volume of 300 μl and took 3.5 min instrument time. An analytical run of 30 samples plus standards, blanks, and control materials typically took ∼5–6 hours. Reproducibility (interrun precision, 1s %RSD) for solutions was 1.2%, 1.5%, and 1.2% for Mg/Ca, Sr/Ca, and Mn/Ca ratios, far smaller than reproducibility for foraminiferal replicates over multiple analytical runs. There was no independent reference material for assessing accuracy. Detection limits were equivalent for an 0.5 mM Ca solution to 0.058 mmol/mol for Mg/Ca, 0.015 mmol/mol for Sr/Ca, and ∼0.005–0.020 mmol/mol for Mn/Ca. Precautions included limiting length of time the peristaltic tubing for the autosampler was used, avoiding Mg contamination from plastic ware, and screening out results from samples with very low Ca concentrations (

[1] 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.