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Advanced Taiwan Ocean Prediction System

Tseng, Tsu-Lun - ALL News | 2018-05-16 | Count:108

Title:  Mid-depth Atlantic Water Circulation across the last deglaciation


The Atlantic Meridional Overturning Circulation (AMOC), the system of warm-to-cold water transformation that results in North Atlantic Deep Water (NADW) production, redistributes heat, nutrients, salt and carbon in the global ocean, and therefore has a major influence on Earth’s climate. Antarctic Intermediate Water (AAIW) is an essential limb of the AMOC, and supplies North Atlantic waters that are exported at depth. However, existing reconstructions have yielded conflicting results on the history of mid-depth water circulation across the most recent glacial termination. Here we present foraminiferal neodymium isotope data (εNd), a quasi-conservative water mass tracer, from intermediate-depth cores collected from the western tropical North Atlantic (Demerara Rise, 671-1100 m) and South Atlantic (Brazil Margin, 1050-1500m) covering the past 25 kyrs. The foraminiferal εNd records from the Demerara Rise suggest a pronounced decrease in the AAIW fraction during the North Atlantic deglacial iceberg discharge events, Heinrich Stadial 1 (HS1, 17.5-14.7 kyr BP) and Younger Dryas (YD, 12.8-11.7 kyr BP), the two intervals with possible “shut downs” in production of NADW. The foraminiferal records from the Brazil Margin reveal no change in seawater Nd isotopes during the two cold events. We therefore conclude that there is no evidence for greater incursion of AAIW into the North Atlantic during either YD or HS1. On the other hand, based on our new deglacial εNd records in the mid-depth (~ 2000 m) South Atlantic, the finding that there was little change in water mass provenance in the mid-depth South Atlantic between the Last Glacial Maximum (23-19 kyr BP) and HS1, despite decreased overturning, suggests that the rate of production of mid-depth southern-sourced water mass decreased in concert with decreased production of northern-sourced intermediate water at the onset of HS1. Consequently, we propose that even drastic changes in the strength of AMOC need not cause a significant change in South Atlantic mid-depth water mass proportions.

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