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ABSTRACT: Geochemical Evidence for Global Changes in Climate, pCO2, and Organic Carbon Burial at Cenomanian/Turonian Boundary

Walter E. Dean, Michael A. Arthur, Lisa M. Pratt

One of the most significant events in the Cretaceous record of organic carbon (OC) burial is the short (< 1 m.y.) but intense period of global organic productivity and OC burial that accompanied a major global rise in sea level at the Cenomanian/Turonian (C/T) boundary (about 91 Ma). The C/T event was nearly synchronous throughout the Atlantic and Tethys basins as well as in higher latitude epicontinental seas. The increased rate of OC burial resulted in massive transfer of carbon from the oceans and atmosphere to marine sediments that contain as much as 50% OC in some areas. This massive removal of isotopically light (13C-depleted) OC is recorded as a progressive increase in ^dgr13C of carbonate carbon by an average of about 2^pmil and as an incr ase in ^dgr13C of marine organic matter from values of about -27 to -28^pmil (typical of pre-C/T organic matter) to values as high as -21^pmil at the C/T boundary. We calculate that the increased rate of OC burial at the C/T boundary was sufficient to strip the atmosphere of CO2 within several hundred thousand years. These calculations ignore feedback from oceanic and crustal carbon reservoirs but do suggest that there may have been a significant reduction in pCO2 at the C/T boundary. A reduction in pCO2 has two major implications. First, lower pCO2 may have caused global cooling, as suggested by the oxygen isotopic composition of inoceramids from northwest Europe. Second, decreased availability of dissolved pCO2 may have ffected the isotopic fractionation of OC by phytoplankton, as suggested by experimental studies. The C/T event illustrates how changes in tectonism and sea level can induce significant, rapid paleoenvironmental changes and biotic turnover that affect the partitioning of carbon between sediment and oceanic reservoirs.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990