INFLUENCE OF WATER COLUMN ANOXIA ON THE BURIAL AND PRESERVATION OF CARBON AND PHOSPHORUS IN MARINE SHALES

Organic P and organic C concentrations were measured in several well-characterized Phanerozoic marine shale sequences with the primary focus being the Camp Run Member of the Devonian-Mississippian New Albany Shale. Sequences were selected with close spatial association of bioturbated and laminated sediments which reflect deposition from oxic and anoxic waters, respectively. Average organic C/P mole ratios calculated from the data are 150 for bioturbated shales and 3900 for the laminated shales of the New Albany. Differences in the extent and mechanisms of early-diagenesis related to the oxygenation of waters at the sediment-water interface can account for the C/P ratios of buried organic matter. Low C/P ratios of bioturbated sediments are attributed to (1) the increased ability of bacteria to store P in well-oxygenated environments which leads to the production of low C/P ratio bacterial biomass and (2) the extensive oxidation of sedimentary organic matter resulting in the formation of residual organic phases with low C/P ratios. High organic C/P ratios of laminated sediments are explained by a combination of (1) the limited ability of bacteria to store P under anoxic conditions, (2) extensive P regeneration from sedimentary organic matter, (3) enhanced C preservation relative to the bioturbated shales. It is also shown that relative to organic C, laminated shales are not as effective a P sink as are bioturbated shales. This provides a mechanism in anoxic environments for burying large quantities of organic matter without simultaneously sequestering the P needed to sustain further productivity.