Sedimentary and geochemical indicators of productivity and oxygen contents in modern and ancient basins: The Holocene Black Sea as the “type” anoxic basin

Abstract

Data on the accumulation rates of organic carbon, total phosphorus and reduced sulfur over the past 7000 yr. from modern abyssal Black Sea sediments are examined as geochemical monitors of variations in biologic productivity and the development of deep-water masses. Variations in organic-carbon accumulation rates at both intermediate- and deep-water sites are believed to be controlled by the combined effects of enhanced preservation of organic matter under anoxic bottom conditions and the regulation of primary productivity in surface waters by nutrient availability. Estimates of variations in water-column and pore-water organic carbon consumption by sulfate-reducing bacteria can be made using average carbon and sulfur accumulation rates for various depositional episodes and are examined in the light of changes in the thickness of the H2S zone in the water column, the amount of available sulfate within this zone, and the reactiveness of particulate organic matter. Sulfur and phosphorus accumulation rates are used to estimate the maximum amount of phosphorus regenerated from organic matter during bacterial sulfate reduction processes. Total phosphorus accumulation rates are found to markedly increase over the part 1000 yr. and are not proportionally paralled by organic carbon accumulation rates. These accelerated phosphorus accumulation rates appear to stem from organic matter degradation, adsorption on surfaces of CaCO3, and perhaps diffusion into sediments from bottom waters and authigenic precipitation of phosphate minerals.Accumulation rates of organic carbon and phosphorus in Black Sea sediments are low in comparison to those typical of modern highly productive upwelling zones (Peru, South West Africa/Namibia). Accumulation rates of reduced sulfur and the ratios of reduced sulfur to organic carbon burial may be higher in anoxic basins than in oxic ones. Knowledge of accumulation rates, therefore, rather than concentrations alone, may help to differentiate black shales which resulted from enhanced organic matter preservation under anoxic conditions from those resulting from high biologic productivity with or without accompanying oxygen deficiencies.