Abyssal Atlantic circulation during the Last Glacial Maximum: Constraining the ratio between transport and vertical mixing

The ocean’s role in regulating atmospheric carbon dioxide on glacial‐interglacial timescales remains an unresolved issue in paleoclimatology. Reduced mixing between deep water masses may have aided oceanic storage of atmospheric CO_2 during the Last Glacial Maximum (LGM), but data supporting this idea have remained elusive. The δ^(13)C of benthic foraminifera indicate the Atlantic Ocean was more chemically stratified during the LGM, but the nonconservative nature of δ^(13)C complicates interpretation of the LGM signal. Here we use benthic foraminiferal δ^(18)O as a conservative tracer to constrain the ratio of meridional transport to vertical diffusivity in the deep Atlantic. Our calculations suggest that the ratio was at least twice as large at the LGM. We speculate that the primary cause was reduced mixing between northern and southern component waters, associated with movement of this water mass boundary away from the zone of intense mixing near the seafloor. The shallower water mass boundary yields an order of magnitude increase in the volume of southern component water, suggesting its residence time may have increased substantially. Our analysis supports the idea that an expanded volume of Antarctic Bottom Water and limited vertical mixing enhanced the abyssal ocean’s ability to trap carbon during glacial times

Carbon isotope offsets between benthic foraminifer species of the genus Cibicides (Cibicidoides) in the glacial sub-Antarctic Atlantic

©2016. American Geophysical Union. All Rights Reserved. Epibenthic foraminifer δ 13 C measurements are valuable for reconstructing past bottom water dissolved inorganic carbon δ 13 C (δ 13 C DIC ), which are used to infer global ocean circulation patterns. Epibenthic δ 13 C, however, may also reflect the influence of 13 C-depleted phytodetritus, microhabitat changes, and/or variations in carbonate ion concentrations. Here we compare the δ 13 C of two benthic foraminifer species, Cibicides kullenbergi and Cibicides wuellerstorfi, and their morphotypes, in three sub-Antarctic Atlantic sediment cores over several glacial-interglacial transitions. These species are commonly assumed to be epibenthic, living above or directly below the sediment-water interface. While this might be consistent with the small δ 13 C offset that we observe between these species during late Pleistocene interglacial periods (Δδ 13 C = −0.19 ± 0.31‰, N = 63), it is more difficult to reconcile with the significant δ 13 C offset that is found between these species during glacial periods (Δδ 13 C = −0.76 ± 0.44‰, N = 44). We test possible scenarios by analyzing Uvigerina spp. δ 13 C and benthic foraminifer abundances: (1) C. kullenbergi δ 13 C is biased to light values either due to microhabitat shifts or phytodetritus effects and (2) C. wuellerstorfi δ 13 C is biased to heavy values, relative to long-term average conditions, for instance by recording the sporadic occurrence of less depleted deepwater δ 13 C DIC . Neither of these scenarios can be ruled out unequivocally. However, our findings emphasize that supposedly epibenthic foraminifer δ 13 C in the sub-Antarctic Atlantic may reflect several factors rather than being solely a function of bottom water δ 13 C DIC . This could have a direct bearing on the interpretation of extremely light South Atlantic δ 13 C values at the Last Glacial Maximum

The Atlantic Ocean at the last glacial maximum: 1. Objective mapping of the GLAMAP sea-surface conditions

Recent efforts of the German paleoceanographic community have resulted in a unique data set of reconstructed sea-surface temperature for the Atlantic Ocean during the Last Glacial Maximum, plus estimates for the extents of glacial sea ice. Unlike prior attempts, the contributing research groups based their data on a common definition of the Last Glacial Maximum chronozone and used the same modern reference data for calibrating the different transfer techniques. Furthermore, the number of processed sediment cores was vastly increased. Thus the new data is a significant advance not only with respect to quality, but also to quantity. We integrate these new data and provide monthly data sets of global sea-surface temperature and ice cover, objectively interpolated onto a regular 1°x1° grid, suitable for forcing or validating numerical ocean and atmosphere models. This set is compared to an existing subjective interpolation of the same base data, in part by employing an ocean circulation model. For the latter purpose, we reconstruct sea surface salinity from the new temperature data and the available oxygen isotope measurements

Sea surface temperatures of the western Arabian Sea during the last deglaciation.

In this study we present a sea surface temperature (SST) record from the western Arabian Sea for the last\ud 20,000 years. We produced centennial-scale d18O and Mg/Ca SST time series of core NIOP929 with focus on\ud the glacial-interglacial transition. The western Arabian Sea is influenced by the seasonal NE and SW monsoon\ud wind systems. Lowest SSTs occur during the SW monsoon season because of upwelling of cold water, and\ud highest SSTs can be found in the low-productivity intermonsoon season. The Mg/Ca-based temperature record\ud reflects the integrated SST of the SW and NE monsoon seasons. The results show a glacial-interglacial SST\ud difference of 2C, which is corroborated by findings from other Arabian Sea cores. At 19 ka B.P. a yet\ud undescribed warm event of several hundred years duration is found, which is also reflected in the d18O record. A\ud second centennial-scale high SST/low d18O event is observed at 17 ka B.P. This event forms the onset of the\ud stepwise yet persistent trend toward Holocene temperatures. Highest Mg/Ca-derived SSTs in the NIOP929\ud record occurred between 13 and 10 ka B.P. Interglacial SST is 24C, indicating influence of upwelling. The\ud onset of Arabian Sea warming occurs when the North Atlantic is experiencing minimum temperatures. The rapid\ud temperature variations at 19, 17, and 13 ka B.P. are difficult to explain with monsoon changes alone and are\ud most likely also linked to regional hydrographic changes, such as trade wind induced variations in warm water\ud advection

Stability of North Atlantic water masses in face of pronounced climate variability during the Pleistocene

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 19 (2004): PA2008, doi:10.1029/2003PA000921.Geochemical profiles from the North Atlantic Ocean suggest that the vertical δ13C structure of the water column at intermediate depths did not change significantly between glacial and interglacial time over much of the Pleistocene, despite large changes in ice volume and iceberg delivery from nearby landmasses. The most anomalous δ13C profiles are from the extreme interglaciations of the late Pleistocene. This compilation of data suggests that, unlike today (an extreme interglaciation), the two primary sources of northern deep water, Norwegian-Greenland Sea and Labrador Sea/subpolar North Atlantic, had different characteristic δ13C values over most of the Pleistocene. We speculate that the current open sea ice conditions in the Norwegian-Greenland Sea are a relatively rare occurrence and that the high-δ13C deep water that forms in this region today is geologically unusual. If northern source deep waters can have highly variable δ13C, then this likelihood must be considered when inferring past circulation changes from benthic δ13C records.National Science Foundation grants OCE-0118005 and OCE-0118001, which supported MER and DWO

Simulating the 128-ka Antarctic climate response to Northern Hemisphere ice sheet melting using the isotope-enabled HadCM3

Warmer than present Antarctic and Southern Ocean temperatures during the last interglacial, approximately 128,000 years ago, have been attributed to changes in north‐south ocean heat transport, causing opposing hemispheric temperature anomalies. We investigate the magnitude of Antarctic warming and Antarctic ice core isotopic enrichment in response to Northern Hemisphere meltwater input during the early last interglacial. A 1,600‐year HadCM3 simulation driven by 0.25 Sv of meltwater input reproduces 50–60% of the peak Southern Ocean summer sea surface temperature anomaly, sea ice retreat, and ice core isotope enrichment. We also find a robust increase in the proportion of cold season precipitation during the last interglacial, leading to lower isotopic values at the Antarctic ice core sites. These results suggest that a HadCM3 simulation including 0.25 Sv for 3,000–4,000 years would reconcile the last interglacial observations, providing a potential solution for the last interglacial missing heat problem

Southwest Atlantic water mass evolution during the last deglaciation

The rise in atmospheric CO2 during Heinrich Stadial 1 (HS1; 14.5–17.5 kyr B.P.) may have been driven by the release of carbon from the abyssal ocean. Model simulations suggest that wind‐driven upwelling in the Southern Ocean can liberate 13C‐depleted carbon from the abyss, causing atmospheric CO2 to increase and the δ13C of CO2 to decrease. One prediction of the Southern Ocean hypothesis is that water mass tracers in the deep South Atlantic should register a circulation response early in the deglaciation. Here we test this idea using a depth transect of 12 cores from the Brazil Margin. We show that records below 2300 m remained 13C‐depleted until 15 kyr B.P. or later, indicating that the abyssal South Atlantic was an unlikely source of light carbon to the atmosphere during HS1. Benthic δ18O results are consistent with abyssal South Atlantic isolation until 15 kyr B.P., in contrast to shallower sites. The depth dependent timing of the δ18O signal suggests that correcting δ18O for ice volume is problematic on glacial terminations. New data from 2700 to 3000 m show that the deep SW Atlantic was isotopically distinct from the abyss during HS1. As a result, we find that mid‐depth δ13C minima were most likely driven by an abrupt drop in δ13C of northern component water. Low δ13C at the Brazil Margin also coincided with an ~80‰ decrease in Δ14C. Our results are consistent with a weakening of the Atlantic meridional overturning circulation and point toward a northern hemisphere trigger for the initial rise in atmospheric CO2 during HS1.Key PointsDeep SW Atlantic was unlikely source of light carbon to atmosphere during HS1Mid‐depth isotopic anomalies due to change in northern component waterNorthern component water had robust influence in South Atlantic during HS1Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/111970/1/palo20190.pd

The 8200 year B.P. event in the slope water system, western subpolar North Atlantic

Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 20 (2005): PA2003, doi:10.1029/2004PA001074.Stable isotope, trace metal, alkenone paleothermometry, and radiocarbon methods have been applied to sediment cores in the western subpolar North Atlantic between Hudson Strait and Cape Hatteras to reveal the history of climate in that region over the past ∼11 kyr. We focus on cores from the Laurentian Fan, which is known to have rapid and continuous accumulation of hemipelagic sediment. Although results among our various proxy data are not always in agreement, the weight of the evidence (alkenone sea surface temperature (SST), δ18O and abundance of Globigerinoides ruber) indicates a continual cooling of surface waters over Laurentian Fan, from about 18°C in the early Holocene to about 8°C today. Alternatively, Mg/Ca data on planktonic foraminifera indicate no systematic change in Holocene SST. The inferred long-term decrease in SST was probably driven by decreasing seasonality of Northern Hemisphere insolation. Two series of proxy data show the gradual cooling was interrupted by a two-step cold pulse that began 8500 years ago, and lasted about 700 years. Although this event is associated with the final deglaciation of Hudson Bay, there is no δ18O minimum anywhere in the Labrador Sea, yet there is some evidence for it as far south as Cape Hatteras. Finally, although the 8200 year B.P. event has been implicated in decreasing North Atlantic ventilation, and hence widespread temperature depression on land and at sea, we find inconsistent evidence for a change at that time in deep ocean nutrient content at ∼4 km water depth.Funding for JPS was from the NOAA Climate and Global Change Program (NA 16GP2679), NSF-Earth System History (0116940), the Jeptha H. and Emily V. Wade Award for Research, and a Henry L. and Grace Doherty Professorship. LDK and YR were funded by NSF grant OCE-0117149