Deep water mass geometry in the glacial Atlantic Ocean: A review of constraints from the paleonutrient proxy Cd/Ca

Abstract

Paleonutrient proxies currently provide the strongest constraints on the past spatial distribution of deep water masses. We review the state of knowledge derived from the trace metal proxy Cd/Ca for the Atlantic Ocean during the Last Glacial Maximum (LGM). We compile published benthic foraminiferal Cd/Ca data, supplemented with new data, to reconstruct meridional Cd sections through the Holocene and LGM Atlantic. Holocene Cd/Ca reflects the modern water masses North Atlantic Deep Water (NADW), Antarctic Bottom Water (AABW), and Antarctic Intermediate Water (AAIW) reasonably well, except for anomalously high values in the intermediate to mid‐depth far North Atlantic. LGM Cd/Ca clearly shows that NADW was replaced by the shallower Glacial North Atlantic Intermediate Water (GNAIW), with significant northward expansion of AABW. The boundary level between GNAIW and AABW was very abrupt, occurring between ∼2200 and 2500 m modern water depth. Combined Cd/Ca and Cibicidoidesδ13C data allow us to also calculate the air‐sea signature of δ13C (δ13Cas). The LGM Atlantic can be explained entirely by mixing between high‐δ13Cas GNAIW and low‐δ13Cas AABW. Negative δ13Cas values in glacial AABW were likely caused by poor ventilation during formation, probably associated with extensive sea ice coverage. Overall, our knowledge base would benefit from improved data coverage in the South Atlantic and Southern Ocean, and a better understanding of ancillary effects on the Cd/Ca proxy itself.