Oceanographic dynamics and the end of the last interglacial in the subpolar North Atlantic

The last interglacial interval was terminated by the inception of a long, progressive glaciation that is attributed to astronomically influenced changes in the seasonal distribution of sunlight over the earth. However, the feedbacks, internal dynamics, and global teleconnections associated with declining northern summer insolation remain incompletely understood. Here we show that a crucial early step in glacial inception involves the weakening of the subpolar gyre (SPG) circulation of the North Atlantic Ocean. Detailed new records of microfossil foraminifera abundance and stable isotope ratios in deep sea sediments from Ocean Drilling Program site 984 south of Iceland reveal repeated, progressive cold water-mass expansions into subpolar latitudes during the last peak interglacial interval, marine isotope substage 5e. These movements are expressed as a sequence of progressively extensive southward advances and subsequent retreats of a hydrographic boundary that may have been analogous to the modern Arctic front, and associated with rapid changes in the strength of the SPG. This persistent millennial-scale oceanographic oscillation accompanied a long-term cooling trend at a time of slowly declining northern summer insolation, providing an early link in the propagation of those insolation changes globally, and resulting in a rapid transition from extensive regional warmth to the dramatic instability of the subsequent ∼100 ka

Decreased influence of Antarctic intermediate water in the tropical Atlantic during North Atlantic cold events

© The Author(s), 2014. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Earth and Planetary Science Letters 389 (2014): 200-208, doi:10.1016/j.epsl.2013.12.037.Antarctic Intermediate Water (AAIW) is a key player in the global ocean circulation, contributing to the upper limb of the Atlantic Meridional Overturning Circulation (AMOC), and influencing interhemispheric heat exchange and the distribution of salinity, nutrients and carbon. However, the deglacial history of AAIW flow into the North Atlantic is controversial. Here we present a multicore-top neodymium isotope calibration, which confirms the ability of unclean foraminifera to faithfully record bottom water neodymium isotopic composition (εNdεNd) values in their authigenic coatings. We then present the first foraminifera-based reconstruction of εNdεNd from three sediment cores retrieved from within modern AAIW, in the western tropical North Atlantic. Our records reveal similar glacial and interglacial contributions of AAIW, and a pronounced decrease in the AAIW fraction during North Atlantic deglacial cold episodes, Heinrich Stadial 1 (HS1) and Younger Dryas (YD). Our results suggest two separate phases of reduced fraction of AAIW in the tropical Atlantic during HS1, with a greater reduction during early HS1. If a reduction in AAIW fraction also reflects reduced AMOC strength, this finding may explain why, in many regions, there are two phases of hydrologic change within HS1, and why atmospheric CO2 rose more rapidly during early than late HS1. Our result suggesting less flow of AAIW into the Atlantic during North Atlantic cold events contrasts with evidence from the Pacific, where intermediate-depth εNdεNd records may indicate increased flow of AAIW into the Pacific during the these same events. Antiphased εNdεNd behavior between intermediate depths of the North Atlantic and Pacific implies that the flow of AAIW into Atlantic and Pacific seesawed during the last deglaciation.This work was supported by US NSF grants and a Lawrence J. Pratt and Melinda M. Hall Endowed Fund for Interdisciplinary Research Award to D.W.O. and W.B.C. and by a Taiwan NSC Postdoctoral Fellowship (NSC98-2917-I-564-132) to K.F.H

What do benthic δ13C and δ18O data tell us about Atlantic circulation during Heinrich Stadial 1?

Approximately synchronous with the onset of Heinrich Stadial 1 (HS1), δ13C decreased throughout most of the upper (~1000–2500 m) Atlantic, and at some deeper North Atlantic sites. This early deglacial δ13C decrease has been alternatively attributed to a reduced fraction of high-δ13C North Atlantic Deep Water (NADW) or to a decrease in the NADW δ13C source value. Here we present new benthic δ18O and δ13C records from three relatively shallow (~1450–1650 m) subpolar Northeast Atlantic cores. With published data from other cores, these data form a depth transect (~1200–3900 m) in the subpolar Northeast Atlantic. We compare Last Glacial Maximum (LGM) and HS1 data from this transect with data from a depth transect of cores from the Brazil Margin. The largest LGM-to-HS1 decreases in both benthic δ13C and δ18O occurred in upper waters containing the highest NADW fraction during the LGM. We show that the δ13C decrease can be explained entirely by a lower NADW δ13C source value, entirely by a decrease in the proportion of NADW relative to Southern Ocean Water, or by a combination of these mechanisms. However, building on insights from model simulations, we hypothesize that reduced ventilation due to a weakened but still active Atlantic Meridional Overturning Circulation also contributed to the low δ13C values in the upper North Atlantic. We suggest that the benthic δ18O gradients above ~2300 m at both core transects indicate the depth to which heat and North Atlantic deglacial freshwater had mixed into the subsurface ocean by early HS1

Surface climate signals transmitted rapidly to deep North Atlantic throughout last millennium

Instrumental observations of subsurface ocean warming imply that ocean heat uptake has slowed 20th-century surface warming. We present high-resolution records from subpolar North Atlantic sediments that are consistent with instrumental observations of surface and deep warming/freshening and in addition reconstruct the surface-deep relation of the last 1200 years. Sites from ~1300 meters and deeper suggest an ~0.5 degrees celsius cooling across the Medieval Climate Anomaly to Little Ice Age transition that began ~1350 ± 50 common era (CE), whereas surface records suggest asynchronous cooling onset spanning ~600 years. These data suggest that ocean circulation integrates surface variability that is transmitted rapidly to depth by the Atlantic Meridional Ocean Circulation, implying that the ocean moderated Earth’s surface temperature throughout the last millennium as it does today

Strengthening of the Northeast Monsoon over the Flores Sea, Indonesia, at the time of Heinrich event 1

Paleoclimate evidence from South America and Asia has been interpreted to indicate that tropical rainfall migrated southward during the Northern Hemisphere cooling associated with Heinrich stadial 1 (HS1), an event of massive iceberg discharge to the North Atlantic ca. 18–15 ka. Although arid conditions associated with such a shift are well documented in southern Asia, as far south as Borneo, debate still exists regarding the precipitation response in southern Indonesia and Australia during HS1. This study utilizes concentrations of the long-lived nuclide 232Th as a proxy for detrital riverine input and 230Th normalization to estimate the history of preserved fluxes reaching the seafloor in the Flores Sea, located between southern Sulawesi and the Lesser Sunda Islands, Indonesia. Because the only source of 232Th to the ocean is continental minerals, this proxy is a robust indicator of continental weathering. The 230Th normalized burial fluxes of lithogenic and biogenic matter demonstrate that both detrital and biogenic fluxes in the Flores Sea were higher during HS1 than any other period in the past 22 k.y. High detrital fluxes indicate enhanced precipitation runoff from surrounding landmasses during a period of maximum southward shift of the Intertropical Convergence Zone. This study further constrains the northern limit of enhanced rainfall associated with a southward shift of Australian monsoon-related rainfall at the time of HS1 and highlights the value of 232Th as a proxy of continental input to deep-sea sediment records

Climate stability during the Pliocene warm period

We present a high-resolution climate record from a sediment core spanning an 80-kyr interval of time during the mid-Pliocene epoch, when warmer conditions and lower global ice volume prevailed worldwide. Oxygen and carbon isotope analyses were made on benthic and planktonic foraminifera from ODP Site 981 in the North Atlantic. The amplitude and approximate recurrence interval of suborbital variations in these records are comparable to those of Holocene and marine isotope stage 11 (MIS 11) records from the North Atlantic. We conclude that the mid-Pliocene warm interval was a time of relative climatic stability. These results suggest that warmer climatic conditions alone may not necessarily enhance variability in the climate system, a finding that may facilitate predictions of 21st century climatic response to anthropogenic warming

Seawater cadmium in the Florida Straits over the Holocene and implications for Upper AMOC variability

Author Posting. © American Geophysical Union, 2022. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography and Paleoclimatology 37, (2022): e2021PA004379, https://doi.org/10.1029/2021pa004379.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.This work was funded by the NSF Graduate Research Fellowship DGE-1148903 (SV) and NSF grant OCE-1459563 and OCE-1851900 (JLS)

Deglacial variability in the surface return flow of the Atlantic meridional overturning circulation

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 23 (2008): PA1217, doi:10.1029/2007PA001450.Benthic foraminiferal Cd/Ca from a Florida Current sediment core documents the history of the northward penetration of southern source waters within the surface return flow of the Atlantic meridional overturning circulation (AMOC). Cd seawater estimates (CdW) indicate that intermediate-depth southern source waters crossed the equator and contributed to the Florida Current during the Bølling-Allerød warm period of the last deglaciation, consistent with evidence of only a modest AMOC reduction compared to today. The CdW estimates also provide the first paleoceanographic evidence of a reduction in the influence of intermediate-depth southern source waters within the Florida Current during the Younger Dryas, a deglacial cold event characterized by a weak North Atlantic AMOC. Our results reveal a close correspondence between the northward penetration of intermediate-depth southern source waters and the influence of North Atlantic Deep Water, suggesting a possible link between intermediate-depth southern source waters and the strength of the Atlantic AMOC.This work was funded by the NSF and the WHOI Ocean and Climate Change Institute

Tropical Atlantic climate response to low-latitude and extratropical sea-surface temperature : a Little Ice Age perspective

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 36 (2009): L11703, doi:10.1029/2009GL038677.Proxy reconstructions and model simulations suggest that steeper interhemispheric sea surface temperature (SST) gradients lead to southerly Intertropical Convergence Zone (ITCZ) migrations during periods of North Atlantic cooling, the most recent of which was the Little Ice Age (LIA; ∼100–450 yBP). Evidence suggesting low-latitude Atlantic cooling during the LIA was relatively small (<1°C) raises the possibility that the ITCZ may have responded to a hemispheric SST gradient originating in the extratropics. We use an atmospheric general circulation model (AGCM) to investigate the relative influence of low-latitude and extratropical SSTs on the meridional position of the ITCZ. Our results suggest that the ITCZ responds primarily to local, low-latitude SST anomalies and that small cool anomalies (<0.5°C) can reproduce the LIA precipitation pattern suggested by paleoclimate proxies. Conversely, even large extratropical cooling does not significantly impact low-latitude hydrology in the absence of ocean-atmosphere interaction.This work was supported by NSF grants OCE 0623364 and ATM 033746 as well as the student research fund of MIT’s Department of Earth, Atmospheric and Planetary Science

Comparison of equatorial Pacific sea surface temperature variability and trends with Sr/Ca records from multiple corals

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 31 (2016): 252–265, doi:10.1002/2015PA002897.Coral Sr/Ca is widely used to reconstruct past ocean temperatures. However, some studies report different Sr/Ca-temperature relationships for conspecifics on the same reef, with profound implications for interpretation of reconstructed temperatures. We assess whether these differences are attributable to small-scale oceanographic variability or “vital effects” associated with coral calcification and quantify the effect of intercolony differences on temperature estimates and uncertainties. Sr/Ca records from four massive Porites colonies growing on the east and west sides of Jarvis Island, central equatorial Pacific, were compared with in situ logger temperatures spanning 2002–2012. In general, Sr/Ca captured the occurrence of interannual sea surface temperature events but their amplitude was not consistently recorded by any of the corals. No long-term trend was identified in the instrumental data, yet Sr/Ca of one coral implied a statistically significant cooling trend while that of its neighbor implied a warming trend. Slopes of Sr/Ca-temperature regressions from the four different colonies were within error, but offsets in mean Sr/Ca rendered the regressions statistically distinct. Assuming that these relationships represent the full range of Sr/Ca-temperature calibrations in Jarvis Porites, we assessed how well Sr/Ca of a nonliving coral with an unknown Sr/Ca-temperature relationship can constrain past temperatures. Our results indicate that standard error of prediction methods underestimate the actual error as we could not reliably reconstruct the amplitude or frequency of El Niño–Southern Oscillation events as large as ± 2°C. Our results underscore the importance of characterizing the full range of temperature-Sr/Ca relationships at each study site to estimate true error.This study was supported by an NSF Graduate Research Fellowship to A.A. and by NSF-OCE-0926986 and NSF-OCE-1031971.2016-08-0