The Weddell Gyre history from marine sedimentary records

gerhard.kuhn [ at ]


The Weddell Gyre (WG) history is closely related to the formation of the Antarctic Circumpolar Current (ACC), which may have developed after the opening of the Tasman Gateway and Drake Passage between Antarctica and the adjoining continents in the upper Eocene (Huber et al., 2004; Francis et al., 2008). We have only very limited information from a couple of deep-sea drill sites about proto Weddell Gyre conditions, but the available data and models indicate that its establishment was associated with oceanic cooling (Mackensen & Ehrmann, 1992; Cristini et al., 2012). Studies on sediment cores from the Atlantic sector of the Southern Ocean and from the Weddell Sea provide information on the history and glacial/interglacial variability of the ACC and the WG since the early Oligocene. How does ACC flow speed interact with WG dynamics? Is the WG circulation independent from the ACC or is it related to seasonal and/or continuous sea ice coverage? Is its circulation related to brine formation in polynyas and subsequent supercooling of these brines below floating ice shelves, i.e., to the formation of Weddell Sea Bottom Water (WSBW) and Antarctic Bottom Water (AABW)? Was there a brine formation without floating ice shelves during glacials? Is the configuration of deep outflow of WSBW relevant for WG dynamics? Very little information has been provided to these questions up to now. The initiation of circumpolar circulation in the upper Eocene changed the oceanography and the sedimentary record drastically. In addition to an increase in the supply of ice-berg rafted debris (IBRD) to the Southern Ocean, the supply of the clay minerals chlorite and illite, which are mainly formed by physical weathering, increased on the expense of smectite, which is produced under humid and warm climatic conditions (Ehrmann & Mackensen, 1992). Decreasing atmospheric pCO2, changes in Southern Ocean deep water ventilation, and primary productivity have been recorded in several marine sediment proxies. After the middle Miocene Climatic Optimum, the Antarctic ice sheet build-up intensified and clear glacial/interglacial cycles during the Pliocene have been described from Gunnerus Ridge in the southeastern part of the WG (Hillenbrand and Ehrmann, 2003). After ~3.6 Ma, coinciding with the onset of widespread glaciation on the Northern Hemisphere, the composition of sediments deposited in the Weddell Sea changed significantly. High biogenic opal contents in the older sediments point to a favorably productive environment with sea ice coverage possibly only during winter (Hillenbrand and Ehrmann, 2005). Interglacials and even extreme warm interglacials, during which the West Antarctic Ice Sheet (WAIS) may have collapsed (Naish et al., 2009), have dominated the deposition under the WG during this time. The Antarctic glaciers may have been warm based and may have flowed faster, while little or no ice shelves surrounded the Weddell Sea. This should have reduced the formation of brines and thus WSBW/AABW production. The late Pliocene and lower Pleistocene sediments recorded cold conditions with low primary productivity, but with partially (mainly on seamounts) high carbonate contents from planktic foraminifera. Brine formation took place as it does today, mainly during interglacials. The East and West Antarctic ice sheets covered most of the shelf areas during glacial periods. Therefore, the brine and bottom water production was reduced as it is indicated by a decrease of grain-size in glacial-time sediments recovered from the AABW outflow area in the deep western Weddell Sea (Brehme, 1992; Fütterer et al., 1988; Grünig, 13 1991; Pudsey 1992). Alkalinity of Weddell Sea waters increased from interglacial to glacial periods, deepening the carbonate compensation depth during glacials (Rickaby et al., 2010). Oxygen and carbon isotopes from planktonic and calcareous benthic foraminifera, geochemical and clay mineral composition and other environmental proxies in sediment records from the continental margin in the Weddell Sea vary with the Pleistocene glacial-interglacial cycles and provide good archives for past changes in climate and ice sheet dynamics (Grobe & Mackensen, 1992; Anderson & Andrews, 1999; Diekmann et al., 2003; Smith et al., 2010; Weber et al., 2011; Hillenbrand et al., 2012; Stolldorf et al., 2012). We will present and discuss environmental conditions and sedimentological processes in the Weddell Sea for climatic conditions similar to present, warmer than today, and colder than today.

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Conference (Talk)
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Event Details
International Weddell Gyre Workshop at the Hanse Wissenschaftskolleg (Hanse Institute for Advanced Study), 17 Sep 2012 - 19 Sep 2012, Delmenhorst, Germany.
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Kuhn, G. and Hillenbrand, C. D. (2012): The Weddell Gyre history from marine sedimentary records , International Weddell Gyre Workshop at the Hanse Wissenschaftskolleg (Hanse Institute for Advanced Study), Delmenhorst, Germany, 17 September 2012 - 19 September 2012 .

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