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Last Glacial to Holocene changes in South Atlantic deep water circulation

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Citation:
Bickert, T. and Mackensen, A. (2004): Last Glacial to Holocene changes in South Atlantic deep water circulation , In: Wefer, G., Mulitza, S., Rathmeyer, V., (eds.), The South Atlantic in the Late Quaternary - Reconstruction of Material Budget and Current Systems, pp. 671-693 .
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Abstract:

A set of 55 benthic foraminiferal stable carbon and oxygen isotope time series, including 28new records, is presented from the South Atlantic Ocean between 6°N and 47°S. We compiled theserecords with published data of the eastern North Atlantic to reconstruct the Atlantic deepwatercirculation for the Last Glacial Maximum (19-23 ka) and the Late Holocene (0-4 ka) times. To betterunderstand the spatial distribution of deep and bottom water masses, we assigned these records tothree North-South sections representing the western South Atlantic, the central Atlantic east of theMid-Atlantic Ridge, and the eastern marginal Atlantic. Corrections of up to +0.4 per mil are suggested forseveral benthic d13C values of cores located in high-productivity areas, to adjust for phytodetritusinduceddepletion of especially glacial values. As a result of this new compilation, no shift of NADWto intermediate depth during the last glacial maximum is evident in the eastern and western marginalAtlantic. Instead, the core of an 13C-enriched water mass spreading southward to at least 30°Sbetween 1200 and 1900 m points to a source of this water mass close to the Isthmus of Gibraltar,indicated by d13C-values of up to 1.8 per mil. Therefore, we interpret this layer as an extended tongue ofthe Mediterranean Outflow Water. Below, a layer of glacial NADW is shown to flow southward atabout the same depth interval or even deeper than it does today, although slightly depleted in 13Cand less extended in water column. The admixing of NADW into the circumantarctic deepwater beltoccurred a few degrees farther north than today, marked by a steep gradient in glacial d13C between 30° and 40°S. From these gradients we derive a local formation of Southern Ocean deep water in thezone of extended winter sea-ice coverage south of the polar front. The spreading of this newlyformed 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 barriersinto the northern basins. Converted into nutrient concentrations, the new carbon isotope data setgives only a slight increase in the nutrient inventory of the deep Atlantic, in good agreement withpreviously published Cd/Ca data.

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