ORGANIC GEOCHEMISTRY OF THE EARLY APTIAN OAE1A FROM SHATSKY RISE (ODP LEG 198)

Series of mid-Cretaceous organic-rich claystones and porcellanites were recovered during ODP Leg 198 on Shatsky Rise in the west-central Pacific. The abundance, nature, and origin of the organic matter in select intervals were examined by shipboard procedures employing elemental analysis, Rock-Eval pyrolysis, and gas chromatography-mass spectrometry (GC-MS). Remarkable amounts of organic carbon (up to 35% Corg) occur within the early Aptian Oceanic Anoxic Event (OAE 1a) at Sites 1207 and 1213, which are among the highest values ever recorded for pelagic Cretaceous sequences. They attest to the extraordinary nature of the depositional conditions that led to enhanced sequestration of type I kerogen rich in algal and bacterial organic matter. The biomarker composition of the intervals provides clear testimony of the exceptional preservation of organic matter, both in its abundance and unaltered character. The hydrocarbon and ketone biomarkers show stratigraphic consistency in the compositions of compounds attributed to eukaryotes (notably sterenes and sterones), but variability in prokaryotic (especially hopanoid) distributions and abundances. The constituents included unprecedented series of 2-methyl-17b(H),21b(H) hopanes and hopanones diagnostic of cyanobacterial contributions. Alkenones, components only biosynthesized by haptophyte algae, occur in one of the lower Aptian samples, which extends the geological record of the occurrence of these paleotemperature proxies by 15 My. These preliminary results provide evidence of the biological origins of the organic matter in selected intervals of the early Aptian and help describe their environment of deposition, including the dysoxic conditions. They suggest changes in prokaryote populations related to the conditions that led to increased sequestration of organic matter and help describe the phytoplankton communities extant during OAE 1a.

The authors of this paper include the ODP Leg 198 Shipboard Scientific Party: T.J. Bralower, I. Premoli-Silva, M.J. Malone, M.A.Arthur, K. Averyt, P. Bown, J.E.T. Channell, L.J. Clarke, A.L. Dutton, J.W. Eleson, T.D. Frank, S. Gylesjö, H. Hancock, H. Kano, R.M. Leckie, K.M. Marsaglia, J. McGuire, K.T. Moe, M.R. Petrizzo, S. Robinson, U. Röhl, W.W. Sager, K. Takeda, D. Thomas, T. Williams, and J.C. Zachos.