Deep water variability on the Southern Agulhas Plateau over the past 1.5 Ma

Jens.Gruetzner [ at ]


An important element of the global ocean thermohaline circulation is the oceanic connection between the Indian and South Atlantic Oceans off South Africa. Variable amounts of warm, salt-enriched South Indian Ocean waters enter the South Atlantic, the so-called ‘warm water return route’, and provide a source for heat and salt to the Atlantic thermocline that ultimately preconditions the Atlantic meridional overturning circulation for convection in the north, the formation of North Atlantic Deep Water (NADW). This eastward surface return flow is compensated at depth by a westward setting deep flow into the southern Indian Ocean that consists of NADW exiting the South Atlantic and Southern Source Waters (SSW), influenced by the Antarctic Circumpolar Current (ACC). Here we present a high-resolution multi-proxy record of deep water variability from sediment core MD02-2588 (2907 m water depth) and IODP Site U1475 (2669 m water depth) both recovered from the southern flank of the Agulhas Plateau in the southernmost South Atlantic. The location is close to the interface between NADW and SSW in the Southern Ocean enabling the reconstruction of the timing and amplitude of changes in southward advection of NADW and Southern Ocean circulation. We concentrate on identifying the phasing between changes in ice volume, the location of surface ocean fronts, deep ventilation and near-bottom flow speeds over the past 1.5 Ma – across the Middle Pleistocene transition. Our benthic carbon isotope record from MD02-2588/Site U1475 strongly suggest that there was a continued mid-depth northern source water influence over the southern Agulhas Plateau during glacial periods of the past 1.5 Ma. Nonetheless, significantly increased near bottom flow speeds, ~5–10 cm s−1 (3–7 μm coarser), during glacial periods indicates that there must be additional controls on physical ventilation. We suggest that vigor of near bottom currents on the Southern Agulhas Plateau is likely influenced by the orbital scale meridional expansion and contraction of the ACC and its associated surface fronts.

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Conference (Poster)
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AGU Fall Meeting, 10 Dec 2018 - 14 Dec 2018, Washington, D.C., U.S.A..
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Hall, I. R. , Starr, A. , Barker, S. , Hemming, S. R. , LeVay, L. J. , Jiménez-Espejo, F. , Gruetzner, J. , Lathika, N. and Expedition 361 Scientists, T. (2018): Deep water variability on the Southern Agulhas Plateau over the past 1.5 Ma , AGU Fall Meeting, Washington, D.C., U.S.A., 10 December 2018 - 14 December 2018 .

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