Factors controlling the burial of organic carbon in laminated and bioturbated sediments off NW Mexico: Implications for hydrocarbon preservation

Factors controlling the burial of organic carbon (OC) in Late Quaternary se diments on the NW Mexican continental margin are assessed using a suite of box and piston cores strategically located an the shelf-slope rise with res pect to the intense oxygen minimum in this region. An OC concentration maxi mum occurs on the mid-slope, below the core of an intense water-column O-2 minimum, due to current winnowing on the outer shelf, the preferential accu mulation of organic matter in fine-grained deposits, and the offshore decre ase in the settling flux of organic detritus. The organic matter at all wat er depths is overwhelmingly marine. Hydrogen indices (HT) are higher on the slope (>300 mgHC/g TOC) than on the shelf (<300 mg HC/g TOC), where curren t winnowing has promoted organic matter degradation, but there is no differ ence in HI in slope sediments accumulating under well oxygenated and O-2-de ficient conditions. The degree of winnowing appears to be the primary facto r affecting the preservational quality of organic matter deposited on this margin. Rates of accumulation of OC and opal are all higher in the interglacial int ervals when compared with the glacial deposits over the last 140,000 yr. Ho wever, matrix-corrected HI values in the mid- and lower-slope cores are inv ariant and are similar to values in the laminated intervals from the oxygen -minimum site. Thus, cyclic changes in organic carbon accumulation on this margin have been controlled by production variations rather than differenti al preservation. HI values in Late Quaternary sediments from several contin ental margins, including NW Mexico, and euxinic basins correspond to type L I kerogen, irrespective of bottom water O-2 concentrations. Therefore, the preservation of oil-prone kerogen in productive margin settings does not ap pear to be restricted to sediments deposited under conditions of low bottom water O-2 concentrations as envisioned in models of petroleum source-rock deposition. Copyright (C) 1999 Elsevier Science Ltd.