--> Abstract: A New Model for the Production of Synchronous Changes in the Carbon Isotopic Composition of Platform Derived Sediments, by Peter K. Swart and Gregor P. Eberli; #90078 (2008)

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A New Model for the Production of Synchronous Changes in the Carbon Isotopic Composition of Platform Derived Sediments

Peter K. Swart and Gregor P. Eberli
Marine Geology and Geophysics, University of Miami, Miami, FL

It has been recognized that synchronous global changes in the δ13C of carbonate sediments deposited on the margins of carbonate ramps and platforms can take place without invoking variations in the burial of organic carbon. For example, the bulk δ13C of sediments collected from the Bahamas, Maldives, Great Barrier Reef, and Great Australian Bight, all show a decrease in the δ13C over the past 5 to 10 Ma. In comparison, the δ13C of the global ocean, as measurement in pelagic foraminifera, shows an increase over the same period. Although the original platform material at these sites was largely aragonite, and the this eventually changes to low-Mg calcite, variations in the δ13C do not arise as a result of diagenesis. This is because such changes from aragonite to calcite occur under closed system conditions and as such there is insufficient organic carbon or oxidants to change the δ13C significantly. Instead the variations in δ13C arise as result of the admixture of varying amounts of shallow water carbonates, which possess relatively positive carbon isotopic compositions (+ 2 to +5 ‰), and pelagic carbonates which have δ13C values close to zero. It is proposed that synchronous variations in δ13C at the geographically separate locations result from a combination of two processes, (i) the progradation and backstepping of the platform margins in response to global sea-level changes, and (ii) the change from relatively negative calcite dominated shallow water carbonates to aragonitic sediments with more positive δ13C values. Superimposed on these changes are variations in the burial of organic carbon. This new model proposed to account for variations in the δ13C of platform derived carbonate sediments has important implications for the use of the δ13C values as stratigraphic markers and for modeling changes in the organic carbon cycle throughout geological time.

 

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas