Relation between sedimentation rate and burial of organic phosphorus and organic carbon in marine sediments

Abstract

Organic carbon and organic phosphorus concentrations in marine sediment cores with known sedimentation rates were determined and collected from the literature. With the exception of organic-rich sediments from upwelling areas, organic phosphorus concentrations in marine sediments do not correlate linearly with organic carbon concentrations. The C/P ratios of organic matter buried in marine sediments vary systematically with sedimentation rate, over the range studied (0.0001 to 2.5 cm/y). Low C/P ratios (< 200 atomic) occur in sediments with sedimentation rates both less than ∼0.002 cm/y and greater than ∼ 1 cm/y. Higher C/P ratios (up to 600) are found at intermediate sedimentation rates. A simple steady state model is developed for the early diagenesis of organic carbon and organic phosphorus in marine sediments. The processes included are sediment burial, pore water diffusion, and bacterial decomposition of organic matter by oxic respiration and sulfate reduction. Preferential regeneration of phosphorus relative to carbon is assumed to take place during oxic respiration. The modeling shows that both organic matter preservation and the degree of elemental fractionation between phosphorus and carbon depend strongly on the overall sedimentation rate. Very high sedimentation rates (coastal-deltaic environment) result in a good preservation of deposited organic matter with an initial C/P ratio close to the Redfield value. At very slow sedimentation rates (pelagic environment), the oxidation of the sedimentary organic matter is nearly complete and produces a low C/P residual organic material. This residual organic matter may represent either refractory detrital organic compounds enriched in phosphorus or in situ synthesized low C/P bacterial remains. The high C/P ratios at intermediate sedimentation rates (slope and shelf environment) reflect the preferential regeneration of phosphorus during the incomplete decomposition of the metabolizable organic matter.