Last Glacial to Holocene Changes in South Atlantic Deep Water Circulation

Abstract

A set of 55 benthic foraminiferal stable carbon and oxygen isotope time series, including 28 new records, is presented from the South Atlantic Ocean between 6°N and 47°S. We compiled these records with published data of the eastern North Atlantic to reconstruct the Atlantic deepwater circulation for the Last Glacial Maximum (19–23 ka) and the Late Holocene (0–4 ka) times. To better understand the spatial distribution of deep and bottom water masses, we assigned these records to three North-South sections representing the western South Atlantic, the central Atlantic east of the Mid-Atlantic Ridge, and the eastern marginal Atlantic. Corrections of up to +0.4‰ are suggested for several benthic δ13C values of cores located in high-productivity areas, to adjust for phytodetritus-induced depletion of especially glacial values. As a result of this new compilation, no shift of NADW to intermediate depth during the last glacial maximum is evident in the eastern and western marginal Atlantic. Instead, the core of an 13C-enriched water mass spreading southward to at least 30°S between 1200 and 1900 m points to a source of this water mass close to the Isthmus of Gibraltar, indicated by δ13C-values of up to 1.8‰. Therefore, we interpret this layer as an extended tongue of the Mediterranean Outflow Water. Below, a layer of glacial NADW is shown to flow southward at about the same depth interval or even deeper than it does today, although slightly depleted in 13C and less extended in water column. The admixing of NADW into the circumantarctic deepwater belt occurred a few degrees farther north than today, marked by a steep gradient in glacial δ13C between 30° and 40° S. From these gradients we derive a local formation of Southern Ocean deep water in the zone of extended winter sea-ice coverage south of the polar front. The spreading of this newly formed water mass, however, is restricted to the Atlantic basins south of Walvis Ridge and Rio-Grande Rise, where only a small amount of nutrient-enriched deep water passes across these barriers into the northern basins. Converted into nutrient concentrations, the new carbon isotope data set gives only a slight increase in the nutrient inventory of the deep Atlantic, in good agreement with previously published Cd/Ca data.