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Well‐dated benthic foraminifer oxygen isotopic records (δ18O) from different water depths and locations within the Atlantic Ocean exhibit distinct patterns and significant differences in timing over the last deglaciation. This has two implications: on the one hand, it confirms that benthic δ18O cannot be used as a global correlation tool with millennial‐scale precision, but on the other hand, the combination of benthic isotopic records with independent dating provides a wealth of information on past circulation changes. Comparing new South Atlantic benthic isotopic data with published benthic isotopic records, we show that (1) circulation changes first affected benthic δ18O in the 1000–2200 m range, with marked decreases in benthic δ18O taking place at ∼17.5 cal. kyr B.P. (ka) due to the southward propagation of brine waters generated in the Nordic Seas during Heinrich Stadial 1 (HS1) cold period; (2) the arrival of δ18O‐depleted deglacial meltwater took place later at deeper North Atlantic sites; (3) hydrographic changes recorded in North Atlantic cores below 3000 m during HS1 do not correspond to simple alternations between northern‐ and southern‐sourced water but likely reflect instead the incursion of brine‐generated deep water of northern as well as southern origin; and (4) South Atlantic waters at ∼44°S and ∼3800 m depth remained isolated from better‐ventilated northern‐sourced water masses until after the resumption of North Atlantic Deep Water (NADW) formation at the onset of the Bølling‐Allerod, which led to the propagation of NADW into the South Atlantic.
Rapid changes in ocean circulation and climate have been observed in marine-sediment and ice cores over the last glacial period and deglaciation, highlighting the non-linear character of the climate system and underlining the possibility of rapid climate shifts in response to anthropogenic greenhouse gas forcing. To date, these rapid changes in climate and ocean circulation are still not fully explained. One obstacle hindering progress in our understanding of the interactions between past ocean circulation and climate changes is the difficulty of accurately dating marine cores. Here, we present a set of 92 marine sediment cores from the Atlantic Ocean for which we have established age-depth models that are consistent with the Greenland GICC05 ice core chronology, and computed the associated dating uncertainties, using a new deposition modeling technique. This is the first set of consistently dated marine sediment cores enabling paleoclimate scientists to evaluate leads/lags between circulation and climate changes over vast regions of the Atlantic Ocean. Moreover, this data set is of direct use in paleoclimate modeling studies.
Epibenthic foraminifer δ13C measurements are valuable for reconstructing past bottom water dissolved inorganic carbon δ13C (δ13CDIC), which are used to infer global ocean circulation patterns. Epibenthic δ13C, however, may also reflect the influence of 13C‐depleted phytodetritus, microhabitat changes, and/or variations in carbonate ion concentrations. Here we compare the δ13C 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 δ13C offset that we observe between these species during late Pleistocene interglacial periods (Δδ13C = −0.19 ± 0.31‰, N = 63), it is more difficult to reconcile with the significant δ13C offset that is found between these species during glacial periods (Δδ13C = −0.76 ± 0.44‰, N = 44). We test possible scenarios by analyzing Uvigerina spp. δ13C and benthic foraminifer abundances: (1) C. kullenbergi δ13C is biased to light values either due to microhabitat shifts or phytodetritus effects and (2) C. wuellerstorfi δ13C is biased to heavy values, relative to long‐term average conditions, for instance by recording the sporadic occurrence of less depleted deepwater δ13CDIC. Neither of these scenarios can be ruled out unequivocally. However, our findings emphasize that supposedly epibenthic foraminifer δ13C in the sub‐Antarctic Atlantic may reflect several factors rather than being solely a function of bottom water δ13CDIC. This could have a direct bearing on the interpretation of extremely light South Atlantic δ13C values at the Last Glacial Maximum.
Two millennial-scale northern hemisphere stadials are recorded during the last deglaciation, Heinrich Stadial 1 (HS1) and the Younger Dryas (YD) (Björck et al., 1998). These are separated by a relatively warm period in the northern hemisphere, the Bølling-Allerød (B/A), which is synchronous with a cooling in the southern hemisphere, the Antarctic Cold Reversal (ACR)
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