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Improved time control on Cretaceous coastal deposits: new results from Sr isotope measurements using laser ablation
Authors:Stefan Burla  Felix Oberli  Ulrich Heimhofer  Uwe Wiechert  Helmut Weissert
Institution:Geological Institute, ETH Zürich, Universitätsstrasse 6, CH-8092 Zürich, Switzerland;;Institute of Isotope Geochemistry and Mineral Resources, ETH Zürich, Clausiusstr. 25, CH-8092 Zürich, Switzerland;;Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Universitätsstr. 150, D-44801 Bochum, Germany;;Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, D-12249 Berlin, Germany;;Geological Institute, ETH Zürich, Universitätsstrasse 16, CH-8092 Zürich, Switzerland
Abstract:Sr-isotope measurements of unaltered oyster shell calcite using laser-ablation multi-collector ICPMS techniques represent a powerful tool capable of establishing a chronostratigraphic framework for fossil near-shore deposits. This approach is tested on two shoal-water successions from the Portuguese Algarve and Lusitanian basins covering Late Barremian to Albian strata. 87Sr/86Sr values derived from low-Mg calcite of oyster shells fluctuate between 0.70717 ± 0.00001 and 0.70746 ± 0.00002; absolute values as well as stratigraphic trends match well with the global open-marine seawater signal. The new Sr-isotope results in combination with existing bio- and chemostratigraphic records allow for identification of an Early Aptian biocalcification crisis at both studied localities and show that this event has severely affected coastal environments along the evolving Atlantic. Siliciclastic intervals in the Algarve section are interpreted as near-shore equivalents of enhanced detrital shedding into shelf and deep-sea environments as a consequence of increased chemical weathering rates during humid climate episodes.
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