Determination of organic degradation rates in 100 My old sediments: Application to Cretaceous black shale intervals from Demerara Rise, ODP Leg 207
Résumé
Thick organic-rich, Albian to Santonian, black shale sequences were recovered from Ocean Drilling Program Sites 1261 and 1257 on the Demerara Rise, offshore Surinam, at depths of 564 and 174 m below seafloor (bsf) respectively. Total organic carbon (TOC) concentration varies between 3.9 and 17.3 wt%. Rock-Eval pyrolysis parameters and elemental analysis indicate that the kerogen samples are of Type II, thermally immature and with well preserved organic matter. The hydrogen index (HI) values fluctuate between 608 and 765 mg hydrocarbon (HC)/g TOC and correlate well with C/N and the Sorg/C atomic ratios, indicating varying early diagenesis conditions between samples. Higher C/N and Sorg/C values for kerogen from Site 1261 suggest more reducing depositional conditions than at Site 1257. Long term organic diagenesis is marked by a linear decrease in the oxygen index (OI) at Site 1261, from 34 to 22 mg CO2/g TOC. Ester and carboxyl functional groups are the main target of the degradation as inferred from the decrease in CO and OCC Fourier transform infrared spectroscopy (FTIR) bands as well as in the CHOH and the OCH3/CHxN bands in 13C nuclear magnetic resonance (NMR) spectra. Degradation rate at Site 1261, determined combining quantitative NMR spectra and bulk elemental characterization, is between 1.7 and 12.1 µmol/m3/yr per 1% TOC, at the lower end of reported rates in a similar context. Kerogen samples from Site 1257 do not show any decrease in oxygen content with depth, indicating significantly lower degradation rate. We propose that the long term exposure to moderate temperature (30 °C) at Site 1261 activated the organic matter, thereby sustaining microbial diagenesis.