Jean Lynch-Stieglitz

The delivery of nutrients from intermediate waters that form in the Southern Ocean is thought to be a key control on tropical ocean surface productivity. In this paper, we present geochemical evidence that an increase in low‐latitude productivity during the Last Interglacial (LIG) was driven by an increase in the preformed nutrient content of Subantarctic Mode Water (SAMW). We generated records of benthic foraminiferal δ ¹³ C, δ ¹⁸ O, Cd/Ca and Mg/Li which are used to reconstruct seawater cadmium, dissolved oxygen, and temperature from a core site in the Florida Straits. The Florida Straits is a location of mixing between SAMW and Northern Component Water, the ratio of which is dependent on the strength of the Atlantic Meridional Overturning Circulation. We find that Late LIG seawater cadmium—which in today's ocean is correlated to phosphate—was substantially higher than the Late Holocene (LH) average at this location, while apparent oxygen utilization was similar during these two periods. Thus, we invoke higher preformed phosphate in the Florida Straits during the Late LIG relative to the LH. Increased SAMW preformed phosphate could be the result of reduced Antarctic Zone winter mixed layer residence time and greater Southern Ocean surface nutrient supply during the Late LIG compared to the LH, as supported by published reconstructions of Southern Ocean biogeochemistry and dynamics. We therefore hypothesize that higher SAMW preformed phosphate would cause an increase in the transport of nutrients into the low latitudes, thereby increasing productivity there.

The tropical Pacific climate has an outsized impact on global climate, yet future projections are poorly constrained. Data‐model comparisons from the mid‐Pliocene warm period (3.3 million years ago) can help investigate warm climate dynamics and evaluate model behavior. Here we compare proxy records to PlioMIP2 models and a model with modified cloud albedo. Relative to modern, the mid‐Pliocene warm period records show subsurface warming across the tropical Pacific, strong eastern Pacific surface warming and weak western Pacific surface warming. Using clustering analyses to group model behavior relative to the proxy data, we find the model cluster with the best fit with the proxy data has enhanced warming in mid‐latitude thermocline source water regions which connect to the equator through the ventilated thermocline. Our study shows tropical ocean heat content during the mid‐Pliocene warm period was higher than today and has broad implications for the ocean's ability to absorb anthropogenic heat.

The carbonate ”clumped” isotope thermometer (Δ47) in foraminifera is increasingly being used to reconstruct ocean temperature. Here we address several less understood aspects of the proxy using a large dataset comprising new and reprocessed data. The Δ47-temperature relationship in foraminifera (n = 260) is described by Δ47 = 0.0374 {plus minus} 0.0013 106/T2 + 0.1744 {plus minus}0.0154, and in inorganic calcite (n = 118) by Δ47 = 0.0392 {plus minus} 0.0014 106/T2 + 0.1547 {plus minus}0.0165. Dataset-related differences explain only 11% of the variance; non-thermal effects explain up to 13% of the variance. We address the paucity of benthic data, establishing with more certainty that temperature sensitivity is indistinguishable from planktics and inorganic calcite. The large benthic dataset resolves a previously uncharacterized correlation with [CO32-] that is small (ΔΔ47/ΔCO32- slope = 0.00019 {plus minus}0.00004, n = 66; R2 = 0.315, p <0.01). We report a multivariate regression to account for both temperature and Δ[CO32] for all benthics (epifaunal and infaunal), with Δ47 = 0.152 {plus minus} 0.049 + 0.03865 {plus minus}0.00376 x 106/T2 + 0.000150 {plus minus}0.0000601Δ[CO32-]. We apply these regressions to published Cenozoic benthic Δ47 data, with the multivariate benthic equation yielding temperature and δ18Osw values more consistent with independent proxies, models, and the broader understanding of ocean and cryosphere dynamics under different conditions, including across the Eocene-Oligocene Transition and the Early Eocene Climatic Optimum. In total, this work enables the application of clumped isotopes to foraminifera with a more realistic understanding of uncertainties.

Abundant proxy records suggest a profound reorganization of the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM, ~21,000 y ago), with the North Atlantic Deep Water (NADW) shoaling significantly relative to the present-day (PD) and forming Glacial North Atlantic Intermediate Water (GNAIW). However, almost all previous observational and modeling studies have focused on the zonal mean two-dimensional AMOC feature, while recent progress in the understanding of modern AMOC reveals a more complicated three-dimensional structure, with NADW penetrating from the subpolar North Atlantic to lower latitude through different pathways. Here, combining ²³¹ Pa/ ²³⁰ Th reconstructions and model simulations, we uncover a significant change in the three-dimensional structure of the glacial AMOC. Specifically, the mid-latitude eastern pathway (EP), located east of the Mid-Atlantic Ridge and transporting about half of the PD NADW from the subpolar gyre to the subtropical gyre, experienced substantial intensification during the LGM. A greater portion of the GNAIW was transported in the eastern basin during the LGM compared to NADW at the PD, resulting in opposite ²³¹ Pa/ ²³⁰ Th changes between eastern and western basins during the LGM. Furthermore, in contrast to the wind-steering mechanism of EP at PD, the intensified LGM EP was caused primarily by the rim current forced by the basin-scale open-ocean convection over the subpolar North Atlantic. Our results underscore the importance of accounting for three-dimensional oceanographic changes to achieve more accurate reconstructions of past AMOC.

Reconstructing the spatial patterns in thermocline depth is critical for understanding ocean‐atmosphere interactions. Previous foraminiferal proxies of thermocline depth focus on gradients between planktonic foraminifera living in the surface and subsurface ocean. However, both thermocline depth changes and stratification changes will impact this measure. In this study, we outline a method for reconstructing the tropical upper ocean vertical water column profile, enabling the separate assessment of thermocline depth and stratification changes. This method uses oxygen isotope data from surface and sub‐surface calcifying planktonic foraminifera (Globigerinoides ruber albus, Globorotalia tumida, Neogloboquadrina dutertrei, and Pulleniatina obliquiloculata) as well as data from benthic foraminifera from a core site below the thermocline. Using newly generated and compiled oxygen isotope data from Holocene‐aged marine sediments, we construct vertical profiles at 20 core sites in the Tropical Pacific Ocean. Quantitative estimates of thermocline depth along with error ranges from Monte Carlo simulations are extracted from the reconstructed profiles. There is a strong correlation between reconstructed Holocene and climatological thermocline depth, but the East‐West contrast in the depth of the thermocline is underestimated by 30%. Incorporating benthic information in thermocline estimates results in a dramatic improvement in the reconstruction of spatial gradients in thermocline depth compared to a simpler proxy, the difference in oxygen isotope ratio between a deeper calcifying planktonic species and the surface species, G. ruber.

The high rate of biological productivity in the North Atlantic is stimulated by the advective supply of nutrients into the region via the Gulf Stream (nutrient stream). It has been proposed that the projected future decline in the Atlantic Meridional Overturning Circulation (AMOC) will cause a reduction in nutrient supply and resulting productivity. In this work, we examine how the nutrient stream changed over the Younger Dryas climate reversal that marked the transition out of the last ice age. Gulf Stream nutrient content decreased, and oxygen content increased at the Florida Straits during this time of weakened AMOC. The decreased nutrient stream was accompanied by a reduction in biological productivity at higher latitudes in the North Atlantic, which supports the link postulated in theoretical and modeling studies.

We compare radiocarbon (¹⁴C) ages of coexisting planktonic foraminifera species from sediment cores VM12-107 and KNR166-2-26JPC from the Equatorial Atlantic Ocean for three time periods (Holocene, Heinrich Stadial 1, last glacial maximum). We find a maximum inter-species difference of 1200¹⁴C yr. On average, the¹⁴C ages deviate by ∼300 yr betweenGlobigerinoides ruberand other species. In most cases, this exceeds the analytical uncertainty range of the measurements and thus renders the choice of species for generating age models as important as sample weight. While modern stratified water-column profiles imply an increase in¹⁴C ages with water depth, we observe an expected parallel increase of¹⁴C ages and δ¹⁸O only at VM12-107. The mismatch between¹⁴C ages and δ¹⁸O at KNR166-2-26JPC likely results from the effects of bioturbation and the hydrographic setting. The largest difference in¹⁴C ages between mixed-layer versus thermocline-calcifying planktonic foraminifera are observed during Heinrich Stadial 1 despite a decrease in upper-ocean stratification at that time. This difference is likely the result of inconsistent increases in¹⁴C reservoir ages during times of reduced overturning circulation masking the potential of¹⁴C ages of coexisting planktonic foraminifera to reflect the density stratification of the water column.

Upwelling deep waters in the Southern Ocean release biologically sequestered carbon into the atmosphere, contributing to the relatively high atmospheric CO₂ levels during interglacial climate periods. Paleoceanographic evidence suggests this “CO₂ leak” was lessened during the last glacial maximum (LGM), potentially due to increased stratification, weaker and equatorward‐shifted winds, and/or enhanced biological carbon export. The collective influences of these mechanisms on the ocean's biological pump efficiency and amount of atmospheric CO₂ can be quantified by determining preformed phosphate of deep waters. We quantify preformed PO₄ (P_pre,AOU) and preformed () of LGM bottom waters using a compilation of published paleo‐temperature, nutrient and oxygen estimates from benthic foraminifera. Our results show that preformed phosphate of the Pacific and Indian deep oceans was reduced by about −0.53 ± 0.13 μM and suggest that much (64 ± 28 ppmv) of the Glacial‐Interglacial CO₂ drawdown resulted from changes in the ocean's biological pump efficiency. Once carbonate compensation is accounted for, this can explain the entire CO₂ drawdown (87 ± 40 ppmv). Preformed shows similar results. The reconstructed LGM P_pre,AOU and oxygen are qualitatively consistent with the changes produced by a suite of numerical sensitivity experiments that roughly simulate three proposed mechanisms for an increase in LGM biological pump efficiency: an increase in biological activity, a decrease in wind‐driven upwelling, and an increase in stratification in the Southern Ocean.

Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the global redistribution of heat and precipitation during both abrupt and longer‐term climate shifts. Over the next century, AMOC is projected to weaken due to greenhouse gas warming, though projecting its future behavior is dependent on a better understanding of how AMOC changes are forced. Seeking to resolve an apparent contradiction of AMOC trends from paleorecords of the more recent past, we reconstruct seawater cadmium, a nutrient‐like tracer, in the Florida Straits over the last ∼8,000 years, with emphasis on the last millennium. The gradual reduction in seawater Cd over the last 8,000 years could be due to a reduction in AMOC, consistent with cooling Northern Hemisphere temperatures and a southward shift of the Intertropical Convergence Zone. However, it is difficult to reconcile this finding with evidence for an increase in geostrophic flow through the Florida Straits over the same time period. We combine data from intermediate water depth sediment cores to extend this record into the Common Era at sufficient resolution to address the broad scale changes of this time period. There is a small decline in the Cd concentration in the Late Little Ice Age relative to the Medieval Climate Anomaly, but this change was much smaller than the changes observed over the Holocene and on the deglaciation. This suggests that any trend in the strength of AMOC over the last millennium must have been very subtle.