ORGANIC MATTER DIAGENESIS IN SURFACE SEDIMENTS: FACTORS CONTROLLING ORGANIC MATTER PASSAGE FROM THE BIOSPHERE TO THE GEOSPHERE: A TRIBUTE TO MARY JO BAEDECKER

Aquatic sediments serve as an intense reactor through which organic matter derived from the overlying water column begins a passage ultimately leading to the formation of sedimentary rocks. Microbial processes are efficient in processing approximately 99% of this organic rain and modifying the organic matter ultimately deposited to surface sediments. The consequences of this efficient reactor are profound. The organic matter that is eventually preserved is the source of fossil fuels and provides insight into the Earth’s history. The balance between loss by remineralization, preservation by burial, and weathering of uplifted kerogen-containing sedimentary rocks inextricably links the global carbon, oxygen and sulfur cycles. It is these global implications that drive the need to understand the biogeochemical processes that determine the character and quantity of organic matter that is either degraded or preserved. In this talk, we will present quantitative information for the degradation of lipid biomarker compounds first derived from Cape Lookout Bight, NC. These results show that lipid biomarker compounds encompass a range in reactivity and that this spectrum of reactivity must be considered in applications of organic biomarkers. We will then extend these results to more recent studies conducted in the York River, VA investigating organic matter degradation in sediments influenced by biological and physical mixing. These results demonstrate that the intense sediment disturbance promotes degradation of more recalcitrant organic matter, presumably due to prolonged exposure to oxygen and other electron acceptors (e.g., NO3, Mn and Fe oxides) as well as removal of metabolites and/or co-metabolism with newly introduced fresh organic matter. In contrast, the degradation of more labile constituents is equally facilitated by biological and physical disturbance. Building on Dr. Mary Jo Baedecker’s seminal work on organic compound diagenesis that provided a foundation for studies utilizing compound level approaches, the lipid biomarker approach offers key insights into the processes driving the remineralization and preservation of sedimentary organic matter.