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The Agulhas Drift, southwest Indian Ocean: a unique feature for ocean current reconstruction.

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Schlüter, P. and Uenzelmann-Neben, G. (2006): The Agulhas Drift, southwest Indian Ocean: a unique feature for ocean current reconstruction. , Inkaba ye Africa Workshop, 1 June - 3 June 2006, Potsdam, Germany. .
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Abstract:

The Earth`s climate is controlled by many different factors with large scale ocean currents playing a significant role. Especially thermohaline circulation with water masses like the Antarctic Bottom Water (AABW) or the North Atlantic Deep Water (NADW), are a motor for maintaining the exchange of warm and cold water masses. Knowing the former current paths improves the knowledge on former climate conditions. One way to reconstruct these palaeocurrents is a seismostratigraphic analysis of sedimentary structures because they comprise an archive of transport and deposition by ocean currents. South of South Africa in the Transkei Basin the giant elongated Agulhas Drift is located, which, including underlying sediments, is a unique feature for ocean current reconstruction over the past 30 Myrs. Here, the northern hemisphere originating NADW and the southern hemisphere originating AABW meet and built up this drift body. To map the exact location and dimensions of the Agulhas Drift we gathered about 2800 km of high resolution reflection seismic data in 2005. The analysis of the data showed the effect of large scale current depositions. In total, there are at least 4 major sedimentary phases to observe, which seem to be mainly triggered by the influence of NADW and AABW. The first stage represents ongoing deep sea sedimentation in Middle Cretaceous times. Later stages are separated by discordances, which represent the onset of AABW and NADW, among others, triggered by the opening of the Drake Passage gateway (~35 Ma ago) and the closure of the Panama Isthmus (~3 Ma ago). During our investigations we found another quite large drift body located underneath the Agulhas Drift. Corresponding to its shape and position, it must have been deposited by currents flowing in a more north-southerly direction, perpendicular to recent flow paths of NADW and AABW, which move in W-E direction. Most probably we assume, this drift has been built up by a then N-S flowing branch of the AABW, roughly 15 17 Ma ago. The overlying Agulhas Drift consists of an asymmetrical body, thickening to the north and the west. Its northward migrating crest is interpreted to represent still ongoing current variations of NADW and AABW.

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