Past sedimentary organic matter accumulation and degradation controlled by productivity

Abstract

CHANGES in bottom-water oxygen concentration are commonly invoked to account for past changes in sedimentary organic carbon accumulation, as organic matter is more readily preserved in anoxic conditions^1–5. On the other hand, upper-water productivity has been shown to be the main controlling factor for accumulation of organic matter in modern sediments^6,7. It is therefore important to understand whether productivity significantly influenced accumulation patterns in the past, for example during the formation of petroleum source rocks. Here we present a study of a 150-Myr-old sequence containing short-term cycles in organic carbon content from the Kimmeridge Clay formation in Northern England. The lack of bioturbation and the fine-scale laminations throughout this section demonstrate that the sequence was deposited under continuously anoxic bottom-water conditions, so that oxic excursions cannot account for the cyclic accumulation pattern. We show that in this cycle, increased planktonic productivity appears to have caused both increased sedimentary accumulation of refractory organic material, and increased anaerobic degradation of metobolizable organic material. Thus, upper-water productivity should be considered as an important factor in controlling past as well as present accumulation patterns.