Tantalizing evidence for the glacial North Atlantic bottom water

Abstract

The global Meridional Overturning Circulation (MOC) is one of the key components of the global climate system. Over timescales of 102 to 103 y, MOC connects the cold, carbon-rich waters in the deep ocean with warmer, carbon-poorer surface waters through localized deep water formation and more widespread upwelling of deep water. By redistributing heat near the surface and affecting carbon storage at depth and therefore atmospheric pCO2, MOC exerts a strong control on the global climate. A long-standing puzzle in paleoclimatology is the configuration of MOC in the Atlantic basin during the cold climate conditions of the last glacial period. The Atlantic is of special interest, because the North Atlantic Deep Water (NADW), a key driver of the global MOC, forms there today. In PNAS, Keigwin and Swift (1) present intriguing new carbon isotope data from a very deep site in the western North Atlantic that indicate the presence of hitherto unidentified bottom water sourced in the glacial North Atlantic. In the modern ocean, two water masses dominate the deep Atlantic Ocean: NADW and the Antarctic Bottom Water (AABW) (Fig. 1 A ). NADW is composed of two varieties. The upper NADW is formed in the Labrador Sea by wintertime open ocean convection that may reach as deep as 2,000 m. The denser, lower NADW originates in the Nordic Seas through open ocean convection and mixing with Arctic Ocean water. It overflows southward over the Greenland−Iceland−Scotland Ridge. Both varieties flow southward as the deep western boundary current of the Atlantic basin in the depth range of 1,500 m to 4,000 m. In contrast, AABW originates on the continental shelves of Antarctica, where brine rejection during sea ice formation in coastal polynyas increases the seawater density. AABW flows northward in the Atlantic basin beneath NADW. Fig. 1. Schematic diagram of AMOC. … [↵][1]1Email: katsumi{at}umn.edu. [1]: #xref-corresp-1-1