HR: 1340h
AN: B33B-0264    [Abstracts]
TI: Biomarker changes across the Toarcian (early Jurassic) ocean anoxic event
AU: * Cohen, A M
EM: amcohen@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Grocke, D R
EM: grocke@mcmaster.ca
AF: McMaster University, General Science Building, Hamilton, ON L8S 4K1 Canada
AU: Grosjean, E
EM: egrosjea@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Summons, R E
EM: rsummons@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Rothman, D H
EM: dan@segovia.mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AB: The Early Toarcian oceanic anoxic event (in the Jurassic, about 183 million years ago) is marked by the global distribution of black shales rich in organic carbon. It is recorded in the geochemical record with high concentrations of organic carbon and isotopic excursions in carbonate carbon, organic carbon and nitrogen. Although there are many hypotheses regarding this anoxic event, its causes and consequences are still not well understood. Here we investigate the evolution of molecular fossils, or biomarkers, across the Toarcian ocean anoxic event in order to elucidate the dynamics of interactions within the carbon cycle during this time. The biomarkers of thirteen samples which span the bulk organic carbon isotopic excursion are studied in detail. To infer the interaction between the primary and secondary reservoirs of oceanic organic carbon, we have analysed time series of isoprenoid (pristane and phytane) and n-alkane (n-C$_{17}$ and n-C$_{18}$) isotopic compositions. In addition to the isotopic analyses, we trace the evolution of distributions of all hydrocarbon biomarkers. Samples are from the high-resolution and well-studied Hawsker Bottoms section. Preliminary results show that the isotopes of pristane and phytane and the n-alkanes do not trace the negative excursion of bulk $\delta^{13}$C$_{org}$. In addition, there is a change in isotopic ordering between the n-alkanes and the isoprenoids.
DE: 4802 Anoxic environments
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
DE: 1615 Biogeochemical processes (4805)
DE: 0400 Biogeosciences
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting