1999 GSA Annual Meeting -- Denver, Colorado
1999 GSA Annual Meeting -- Denver, Colorado
Author(s): MEYERS, Stephen R., SAGEMAN, Bradley B., HOLLANDER, David J., Department of Geological Sciences, Northwestern University, Evanston, IL 60208, meyers@earth.nwu.edu
Keywords: bioassemblage, biomarker, KWIS, Milankovitch, OAE
The Cenomanian-Turonian (C-T) hemipelagic deposits of the Cretaceous Western Interior Seaway (KWIS) are characterized by lithologic, faunal, and geochemical expressions of Milankovitch cyclicity. Siliciclastic dilution, moderated by high latitude obliquity-forcing of the hydrologic cycle, has been proposed as a contributing mechanism for generation of the stratigraphic cycles. Alternatively, changes in low latitude surface water conditions forced by precession may have moderated productivity of calcareous phytoplankton (i.e., coccolithophores), or influenced ecological relationships within the pelagic community (i.e., modulated the relative abundance of calcareous taxa via competition with diatoms). In order to test for changes in gross primary productivity during these cycles, and for changes in the bioassemblages and/or dominant primary producers, molecular paleontological techniques (lipid extractions) were applied to core samples of the Greenhorn Formation, central Western Interior. Since this interval also includes Oceanic Anoxic Event II (OAE II), core samples within, prior to, and following OAE II were molecularly analyzed to determine the influence of this global event on productivity intensity and preservation/bacterial reworking in the KWIS. Comparison of biomarker records with published skeletal data for diversity/abundance trends in coccolithophores, dinoflagellates, and planktonic foraminifera were used to ground truth the molecular paleontological approach. Molecular results prior to, within, and following the OAE II show a similar distribution of isoprenoids, hopanes, and steranes, suggesting an overall similarity in the bioassemblage. However, differences in the relative abundance of the biomarkers found in the samples indicate changes in the taxonomic dominance of photoautotrophs/heterotrophs, and the relative importance of bacterial reworking. The changes in taxonomic dominance across OAE II will be discussed in the context of understanding the linkages between Milankovitch cycles, climate, ocean chemistry, KWIS circulation, and associated biologic production.
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