Data Constraints on Glacial Atlantic Water Mass Geometry and Properties

Abstract

The chemical composition of benthic foraminifera from marine sediment cores provides information on how glacial subsurface water properties differed from modern, but separating the influence of changes in the origin and end‐member properties of subsurface water from changes in flows and mixing is challenging. Spatial gaps in coverage of glacial data add to the uncertainty. Here we present new data from cores collected from the Demerara Rise in the western tropical North Atlantic, including cores from the modern tropical phosphate maximum at Antarctic Intermediate Water (AAIW) depths. The results suggest lower phosphate concentration and higher carbonate saturation state within the phosphate maximum than modern despite similar carbon isotope values, consistent with less accumulation of respired nutrients and carbon, and reduced air‐sea gas exchange in source waters to the region. An inversion of new and published glacial data confirms these inferences and further suggests that lower preformed nutrients in AAIW, and partial replacement of this still relatively high‐nutrient AAIW with nutrient‐depleted, carbonate‐rich waters sourced from the region of the modern‐day northern subtropics, also contributed to the observed changes. The results suggest that glacial preformed and remineralized phosphate were lower throughout the upper Atlantic, but deep phosphate concentration was higher. The inversion, which relies on the fidelity of the paleoceanographic data, suggests that the partial replacement of North Atlantic sourced deep water by Southern Ocean Water was largely responsible for the apparent deep North Atlantic phosphate increase, rather than greater remineralization.

Constraining the Variability of the Atlantic Meridional Overturning Circulation During the Holocene

“…Painstaking efforts have been devoted to reconstruct changes in AMOC dynamics in the recent geological past in order to better predict its future evolution. These investigations provide ample evidence for large-scale glacial and deglacial AMOC reorganizations as inferred from independent proxy approaches (Howe, Piotrowski, Noble, et al, 2016;Lippold et al, 2016;Lynch-Stieglitz, 2017;Marchitto & Broecker, 2006;McManus et al, 1999;Oppo et al, 2018;Praetorius et al, 2008;Roberts et al, 2010). While there is a general sense that AMOC variability was subdued during the Holocene compared with the last glacial termination (Keigwin & Boyle, 2000;Oppo et al, 2003), proxy evidence is often contradictory.…”

Atlantic Circulation and Ice Sheet Influences on Upper South Atlantic Temperatures During the Last Deglaciation

“…Today, these sites are located just below the low salinity core of Antarctic Intermediate Waters (AAIW) and within the mixing gradient with high salinity NADW (supporting information Figure S1). Concentrations of PO 4 and Cd are elevated in AAIW relative to NADW (Middag et al, ; Figure ), in part because Circumpolar Deep Water (CDW) contributes nutrients to AAIW (Sarmiento et al, ) and in part because remineralization in the AAIW formation region increases the PO 4 values and reduces δ 13 C values (Oppo et al, ). Middepth sediment core PC‐CAM61 (22.52°S, 39.90°W) was collected from 1,890‐m water depth, within the modern extent of NADW and near its upper boundary with AAIW.…”

Constraints on the Northwestern Atlantic Deep Water Circulation From 231Pa/230Th During the Last 30,000 Years

“…Stable carbon isotope data (δ 13 C) from a depth transect along the BBOR show lighter δ 13 C values with increasing water depth interpreted as propagating SSW during the LGM (Evans & Hall, ; Keigwin, ). However, in the light of recent findings of past water mass distributions in the western Atlantic basin from a δ 13 C and Nd isotope perspective, an extensive advance of SSW into the North Atlantic may need to be seen more critically (Gebbie, ; Howe et al, ; Oppo et al, ; Spooner et al, ).…”

“…Spooner et al () therefore suggested that the boundary between GNAIW and SSW at 30°S was situated somewhere between 2,600‐ and ~4,000‐m water depth possibly in deeper than in shallow depths. This boundary is rather expected to descend toward 30°N instead of being stable on such a long distance due to the increasing dominance of NSW with shorter distance to the NADW formation areas (Gebbie, ; Howe et al, ; Oppo et al, ). Accordingly, a strong inflow of SSW in the deep northwestern Atlantic basin during the LGM appears unlikely.…”