Last-glacial-maximum North Atlantic deep water: on, off or somewhere in-between?

Abstract

Various papers have been published during the past decade concerning Last Glacial Maximum (LGM) North Atlantic Deep Water (NADW) flow. Using somewhat different methods, they have produced somewhat contradictory results. This review considers both apparent and real conflicts concerning the data and their interpretation, and attempts to resolve them. Despite the earlier (contradictory) interpretations, currently there is a widespread belief that nutrient concentrations in deep cores from the North Atlantic increased during glacial times and that concentrations in the upper-deep and intermediate waters decreased at least slightly. It is also clear that further north in the basin (particularly at upper-deep and intermediate depths), nutrient concentrations were as low or perhaps even lower than those seen today. Data from the Caribbean Sea, ventilated by intermediate waters through an approximately 1800 m sill, indicate that lower nutrient levels were also found at intermediate depths in the North and Tropical Atlantic; this data is supported by continental margin data. The recontoured 8 13 C data of Duplessy et al. ( Paleoceanography 3, 343—360 (1988)) remain a valid expression of the broadscale LGM Atlantic nutrient distribution. Data from the South Atlantic has been the most contradictory to date, but recent 8 13 c evidence from a low-productivity South Atlantic site supports Cd data indicating a relative stability in the nutrient chemistry of waters that are presently influenced by low-nutrient NADW. Sedimentary 231 Pa/ 230 Th data appear to require the continued export of Atlantic-generated 231 Pa from the Atlantic into the Southern Ocean. Finally, radiocarbon evidence from paired planktonic/benthic foraminifera indicates that the ventilation time of the North Atlantic remained low and that the ventilation time of the entire ocean did not change much beyond the uncertainty of the 14 C data. Taken together, this evidence suggests that the NADW became ‘Glacial North Atlantic Deep/Intermediate Water’ (GNAIDW) during glacial times, with perhaps a greater flux through intermediate waters than currently combined with a lesser flux through deeper waters. Although one cannot say much with confidence about the total GNAIDW flux, the data are consistent with a persistent but perhaps somewhat diminished role for NADW in the global thermohaline circulation during glacial times. A review of recent evidence concerning the response of the deep North Atlantic during the Younger Dryas concludes that there is no inconsistency between the new evidence and the occurence of a Younger Dryas NADW event in the deep western North Atlantic.