Enhancement of accumulation and anoxic degradation of organic matter controlled by cyclic productivity: a model

The aim of the study is to determine the relative influences of the genetic sources, the export efficiency, and the anaerobic degradation of organic carbon as factors controlling short-term cyclic organic-rich sedimentation in areas of high marine productivity. A cyclic organic sequence of the Kimmeridge Clay Formation of Yorkshire (U.K) was studied as an example of ancient sedimentation. This formation outcropping in the Cleveland basin near Marton is known as a lateral equivalent of the major oil source rock unit of the North Sea. On the basis of petrographic and geochemical results, a new model is proposed to explain the short-term cyclic sedimentation of organic carbon under continuously anoxic bottom conditions. The model assumes that the cyclicity is primarily controlled by variations of mineral-free phytoplanktonic productivity. Such variations are thought to induce changes in carbon recycling in the photic zone so that the export efficiency of the metabolizable organic matter is modified. A high sulphate reduction intensity relative to the organic matter flux is observed for the highest productivity periods, which also reflects an enhancement of export efficiency. As the redox conditions did not change significantly, such variations in sulphate reduction intensity must be attributed not only to quantitative variations in the export productivity, but also to its qualitative variations, with more metabolizable organic matter reaching the anoxic domain during high productivity periods. Biosedimentary processes depending on productivity, such as aggregation, are thought to play an important role in reducing the transit time of metabolizable organic matter to the sediment. Finally, the model implies that the amplitude of export productivity variations probably were larger than that of TOC variations recorded in sediment which are mainly due to biologically refractory organic matter from plankton.