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The Eastern Weddell Gyre- an overlooked high productivity region

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Geibert, W. , Usbeck, R. , Webb, A. , Hanfland, C. , Schwarz, J. N. and Ansorge, I. (2004): The Eastern Weddell Gyre- an overlooked high productivity region , Talk at the Open Science Conference of the XXVIII SCAR Meeting, 25-31 July 2004, Bremen, Germany (Terra Nostra Abstract Volume of XXVIII SCAR Meeting). .
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The Eastern Weddell Gyre an overlooked high productivity regionThe Weddell Gyre (WG) is generally known as a region of low bioproductivity. This view has hitherto been supported by relatively low chlorophyll concentrations as detected by satellites, high nutrient concentrations at the sea surface, and very low sedimentation rates in the underlying sediments. However, recent results from inverse modeling suggest a completely different situation for the Eastern part of the Weddell Gyre, from about 10° E to 35° E. The high productivity proposed by the model in the Eastern Weddell Gyre (EWG) is mainly a consequence of a substantial nutrient depletion at the surface, and a corresponding oxygen consumption in deeper layers, as known from the very few data available for that region. Satellite data do not show elevated chlorophyll concentrations in the EWG compared to neighbouring regions.In order to contribute new data from this poorly know region, and to help understanding the contrasting satellite and model data, Polarstern expedition ANT XX/2 into the EWG was carried out in the season 2002/03. A synoptical investigation (Synpart project) of productivity-related ocean properties was performed by the Alfred Wegener Institute together with scientists from the University Cape Town during the cruise. The measurements include fluorimetric detection of chlorophyll in the water column, determination of nutrient and oxygen concentrations, 234Th as a tracer for particle concentration and export, tracers for shallow and deep water circulation (228Ra, 227Ac). In addition, samples from the sediment surface were taken to allow for the determination of particle fluxes into the sediments by means of 230Th. This dataset gives insights into particle flux from the very surface of the ocean to the deep-sea floor. Additionally, data of pCO2 and a comprehensive oceanographic dataset are available from the expedition.The results clearly confirm the strong nutrient depletion in the surface waters of the EWG, in our case observed at 23°E, from about 62°30Ž S to 57°30Ž S. High concentrations of particulate 234Th are found there, in some cases in combination with a depletion of 234Th, which means export of particles from the sea surface. The fluorimetric determination of chlorophyll-a revealed that deep chlorophyll maxima were responsible for the contrasting satellite and model data. In some cases, high nutrient depletion together with deep chlorophyll was found where hardly any chlorophyll was visible at the sea surface by satellite. Compared to other parts of the Weddell Gyre, high opal concentrations in surface sediments of the EWG suggest that at least a fraction part of this productivity reaches the sea-floor (see presentation by Seeberg-Elverfeldt, this meeting). As the feature of deep chlorophyll maxima was not only found in the Eastern Weddell Gyre, but observed throughout the Weddell Gyre, there are implications for the interpretation of satellite images from the entire region. The productivity of the Weddell Gyre, especially of its Eastern part, must be considered to be currently substantially underestimated.

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