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Using deep-sea corals to test the role of the deep Southern Ocean in ocean circulation and the regulation of atmospheric carbon dioxide

Final Report Summary - DEEPOCEANGLACIALCO2 (Using deep-sea corals to test the role of the deep Southern Ocean in ocean circulation and the regulation of atmospheric carbon dioxide)

Modern pre-anthropogenic atmospheric carbon dioxide concentrations were in the range of 280 ppm, whereas those during the last glacial maximum about 21 thousand years ago were about a third lower at ∼180 ppm. Hence, even before mankind started to burn excessive amounts of fossil fuels, our planet underwent natural variability in atmospheric carbon dioxide concentrations. The only effective natural mechanism to add or remove this large amount of carbon to the atmosphere on the time scales involved (thousands of years) is by means of carbon dioxide degassing from the deep oceans. If more carbon was indeed stored in the deep oceans during the last glacial period then evidence for this carbon reservoir should exist in deep marine fossil records.

Several lines of evidence suggest this deep ocean reservoir existed, but results as yet remain controversial and patchy. In this context, the deep Southern Ocean potentially plays an important role but so far suitable paleoceanographic archives to test this hypothesis are largely missing. Today the far Southern Ocean represents a source of carbon dioxide to the atmosphere because of upwelling of deep water rich in respired carbon around Antarctica. During the last glacial period sea ice extended much further north and the Antarctic circumpolar current-driving westerly winds were shifted towards the lower latitudes further north, providing the boundary conditions in which such an isolated deep ocean reservoir rich in respired carbon could develop.

The aim of our project 'DeepOceanGlacialCO2' was to use a collection of rare deep sea corals from the Amundsen sector of the Southern Ocean to test this hypothesis. Since no published work existed on the usage of this particular species of deep-sea corals for absolute age dating, the first part of our project consisted of testing whether reliable absolute coral ages could be obtained employing the so-called U-series disequilibria dating technique.

Our findings were not ideal, but nevertheless highly relevant for scientific groups working with deep-sea corals since our calibration approach will provide a thorough testing routine for uranium-series dating of fossil deep-sea organisms and its associated limitations. Since the corals can still be reasonably well dated using the coral radiocarbon content, we can detect in which environment these marine organisms calcified.

Whether glacial or deglacial fossil corals grew in waters containing significantly larger amounts of dissolved carbon dioxide compared with their modern counterparts, is now being investigated using the coral boron isotopic composition. This approach is based on the seawater pH-dependent incorporation of borate ion into the coral skeleton. Finally, as an add-on the coral Nd isotopic composition can be reliably measured as well providing additional constraints about water mass mixing proportions of Atlantic versus Pacific-derived waters in the Antarctic circumpolar current.

Through continued funding of the PI Prof. Derek Vance this project is continuing. The fellowship recipient Dr Marcus Gutjahr participated in scientific cruise ANT-XXVI/3 onboard the RV Polarstern to the Southern Ocean in spring 2010 in order to sample more coral material. Collaborations with scientists at the AWI in Bremerhaven, Germany (Dr Gerhard Kuhn), the BAS in Cambridge, UK (Dr Claus-Dieter Hillenbrand), the Zoological Society of London, UK (Dr Alex Rogers) and the NOC in Southampton, UK (Dr Gavin Foster) are ongoing.

Results to date are encouraging. While the deep water dissolved carbon dioxide concentrations appear to have remained relatively constant and similar to present-day over the present interglacial (i.e. the past 10 000 years), late deglacial corals appear to have grown in waters that recently degassed carbon dioxide to the atmosphere. We are optimistic that the new coral material sampled on the RV Polarstern cruise ANT-XXVI/3 in spring 2010 will provide more samples of adequate age.