ALBERT

Description: A sequence of accelerator mass spectrometry (AMS) 14C dates performed on benthic and planktonic foraminifera from a northeast Atlantic deep-sea core (MD99-2334K; 37°48'N, 10°10'W; 3146 m) permit the reconstruction of deep water “14C ventilation ages” across the last deglaciation. The records from MD99-2334K have been placed on the GISP2 timescale via the synchrony of temperature changes recorded in the Greenland ice cores and in North Atlantic planktonic delta18O(cc) (calcite delta18O). On the basis of a range of estimates for past source water Delta14C, this permits the estimation of 14C projection ventilation ages for comparison with benthic-planktonic 14C age differences. Although the accurate estimation of past ventilation ages is precluded by unknown deep water Delta14C source signatures, and by uncertainty regarding the extent of deep water mixing, it is clear that deep water ventilation in the northeast Atlantic was significantly reduced during the last glaciation, increased abruptly coincident with the Bølling-Allerød warming, and rapidly became reduced again during the Younger Dryas cold reversal. The character of these changes is consistent with a varying dominance of North Atlantic Deep Water (NADW) versus Antarctic Bottom Water (AABW). Parallel benthic delta13C, deep water temperature (T(dw)), and deep water delta18O (delta18O(dw)) estimates support this inference. The fact that deglacial changes in the deep water radiocarbon content of the northeast Atlantic run parallel to opposite changes in atmospheric radiocarbon content, and in parallel with Greenland temperature fluctuations, unequivocally implicates changes in ocean circulation in deglacial climate evolution and illustrates the capacity for the deep ocean to respond and contribute to abrupt climate change.

Description: Paired measurements of Mg/Ca and delta18O(cc) (calcite delta18O) in benthic foraminifera from a deep-sea core recovered on the Iberian Margin (MD99-2334K; 37°48'N, 10°10'W; 3,146 m) have been performed in parallel with planktonic delta18O(cc) analyses and counts of ice-rafted debris (IRD). The synchrony of temperature changes recorded in the Greenland ice cores and in North Atlantic planktonic delta18O(cc) allows the proxy records from MD99-2334K to be placed confidently on the GISP2 time-scale. This correlation is further corroborated by AMS 14C-dates. Benthic Mg/Ca measurements in MD99-2334K permit the reconstruction of past deep-water temperature (T(dw)) changes since ~34 cal. ka BP (calendar kiloyears before present). Using these T(dw) estimates and parallel benthic delta18O(cc) measurements, a record of deep-water delta18O (delta18O(dw)) has been calculated. Results indicate greatly reduced T(dw) in the deep Northeast Atlantic during the last glaciation until ~15 cal. ka BP, when T(dw) warmed abruptly to near-modern values in parallel with the onset of the Bølling-Allerød interstadial. Subsequently, Tdw reverted to cold glacial values between ~13.4 and ~11.4 cal. ka BP, in parallel with the Younger Dryas cold reversal and the H0 ice-rafting event. Similar millennial-scale T(dw) changes also occurred during the last glaciation. Indeed, throughout the last ~34 cal. ka, millennial delta18O(dw) and T(dw) changes have remained well coupled and are linked with IRD pulses coincident with Heinrich events 3, 2, 1, and the Younger Dryas, when transitions to lower T(dw) and delta18O(dw) conditions occurred. In general, millennial T(dw) and delta18O(dw) variations recorded in MD99-2334K describe an alternation between colder, low-delta18O(dw) and warmer, high delta18O(dw) conditions, which suggests the changing local dominance of northern-sourced North Atlantic Deep Water (NADW) versus southern-sourced Antarctic Bottom Water (AABW). The observed similarity of the T(dw) and GISP2 delta18O(ice) records would therefore suggest a common component of variability resulting from the coupling of NADW formation and Greenland climate. A link between Greenland stadials and the incursion of cold, low-delta18O(dw) AABW in the deep Northeast Atlantic is thus implied, which contributes to the relationship between Greenland climate and the millennial benthic delta18O(cc) signal since ~34 cal. ka BP.