Comparison of Atlantic and Pacific paleochemical records for the last 215,000 years: changes in deep ocean circulation and chemical inventories

Abstract

Detailed Cd/Ca and δ 13C data have been obtained for benthic foraminifera from western North Atlantic and Equatorial Pacific sediment cores. In the modern ocean, both tracers are closely linked to nutrient distributions. The sedimentary records for both tracers indicate that bottom waters overlying the Atlantic site have been nutrient-depleted relative to those at the Pacific site over the last 215,000 years. From this evidence it is reasonable to infer that there has been a continuous net flux of nutrient-depleted water from the western North Atlantic into the Pacific. This exchange has undergone significant fluctuations, with the export of nutrient-depleted Atlantic water diminishing by about a factor of two relative to the inflow from Southern Ocean sources. Over the last 215,000 years, carbon isotope fluctuations in both regions are dominated by variable storage of isotopically light carbon on continents with a lesser contribution from these deep ocean circulation changes. The cadmium signal in the North Atlantic is dominated by changes in deep ocean circulation patterns; cadmium shows less variability in the Pacific which may reflect changes in the global average cadmium content or minor changes in deep Pacific circulation patterns. Using these records to estimate global averages, it appears that glacial ocean water had 22% more Cd and 0.46‰ less 13C than the modern ocean. These numbers are estimates which may be revised as more data become available, although they are not likely to be as much as 20% in error for Cd or 0.2‰ for 13C. Relative North Atlantic Deep Water (NADW) formation rates are modulated with a significant 41 kyr periodicity linked to obliquity-induced variations in high latitude insolation; NADW lags 8 ± 2 kyr behind insolation, however.