HR: 0800h
AN: PP51B-0602    [Abstracts]
TI: A 3 Permil Change in Carbon Isotope Fractionation Between Carbonates and Organic Matter: the Oxfordian Turning Point in Climate and Oceanography
AU: * Louis-Schmid, B
EM: beat.louis@erdw.ethz.ch
AF: Geological Institute ETH Zurich, ETH Zentrum, Zurich, 8092 Switzerland
AU: Rais, P
EM: pauline.rais@erdw.ethz.ch
AF: Geological Institute ETH Zurich, ETH Zentrum, Zurich, 8092 Switzerland
AU: Bernasconi, S M
EM: stefano.bernasconi@erdw.ethz.ch
AF: Geological Institute ETH Zurich, ETH Zentrum, Zurich, 8092 Switzerland
AU: Weissert, H
EM: helmut.weissert@erdw.ethz.ch
AF: Geological Institute ETH Zurich, ETH Zentrum, Zurich, 8092 Switzerland
AB: The Oxfordian (Late Jurassic) sedimentary record is characterized by a positive excursion in the δ¹³C of carbonates with an amplitude similar to that of excursions accompanying the Cretaceous oceanic anoxic events (e.g. Valanginian). It has been recognized in different paleoenvironments and is assumed to reflect global changes in the carbon cycle. Shales rich in organic matter are a common feature of the Oxfordian, whereas the following parts of the Late Jurassic are known as "carbonate times". This suggests that the carbon isotope excursion is due to changes in the burial-ratio of organic matter vs. carbonates. However, this excursion is also time-equivalent to major changes in other geochemical tracers like the strontium isotope ratio, indicating changes in hydrothermal activity and/or continental weathering. In addition, the distribution of hardgrounds and sedimentary gaps indicate significant changes in ocean-current patterns in the Early-Middle Oxfordian. These processes are likely to have co-influenced the evolution of C-isotope ratios in the Oxfordian. In order to evaluate the evolution of carbon cycle and climate in the Late Jurassic, we established a carbon isotope curve in a hemipelagic section in SE-France. The studied sediments were deposited in the Alpine Tethys seaway close to the opening Atlantic ocean. High and continuous sedimentation rates allow to construct high-resolution carbonate and organic carbon isotope records. The isotope excursion achieved its peak values within the mid-Oxfordian Transversarium ammonite zone. This is when in many parts of the Western Tethys carbonate accumulation rates strongly increased. At the time of the steepest rise in δ¹³C of the carbonates, the δ¹³C of organic matter undergoes a strong shift to more negative values. The resulting change of more than 3‰ in the C-isotope fractionation between carbonates and organic matter could indicate a rise in pCO₂ in surface waters and in the atmosphere of the order of 50%. If this hypothesis will be confirmed by C-isotope measurements on individual organic compounds, other processes such as increased volcanic activity need to be included into the history of the Oxfordian. Thus, the mid-Oxfordian positive excursion in C-isotopes marks one of the major turning points in Mesozoic oceanography and climate.
DE: 0428 Carbon cycling (4806)
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 4912 Biogeochemical cycles, processes, and modeling (0412, 0414, 0793, 1615, 4805)
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005