EVALUATING ORGANIC MATTER SULFURIZATION AS A MECHANISM OF ENHANCED BURIAL OF REDUCED CARBON AND SULFUR ACROSS THE TOARCIAN OCEANIC ANOXIC EVENT

Mudrocks with high organic carbon contents have been the focus of intensive study because they are key economic resources as well as frequently recording intervals of extreme environmental change in Earth history. In particular, the Mesozoic is known to contain multiple instances of widespread deposition of organic-rich mudrocks that are associated with the expansion of oceanic anoxia called Oceanic Anoxic Events (OAEs). A fundamental debate exists as to what mechanism exerts the dominant control on the formation of organic-rich mudrocks: enhanced organic matter production or preservation. An important consideration is that organic matter preservation is not governed solely by oceanic anoxia and may be controlled by other processes that alter organic matter inhibiting degradation. Organic matter sulfurization (OMS) is one such process that directly enhances preservation potential by making organic matter less bioavailable. OMS operates most rapidly within euxinic water columns (e.g., anoxic with free hydrogen sulfide). Thus, OMS may help us to better understand the cycling and burial of reduced carbon and sulfur across episodes of expanded oceanic anoxia.

Here we investigate OMS as a potential pathway of enhanced preservation of organic matter during an OAE. Specifically, we investigated this process across the Toarcian Oceanic Anoxic Event (T-OAE, ~183 Ma) of the Early Jurassic. We will present δ³⁴S_org and organic matter S:C ratio data from the event as recorded within three basins of the European epicontinental seaway: the Cleveland Basin (UK), the Paris Basin (Luxembourg), and the Southern German Basin (Switzerland). We find that the ratio of S:C in organic matter increases systematically with TOC across the three sections, maxing out at 3% S:C; and pyrite sulfur dominates as the reduced sulfur flux across all sections. When compared to other Mesozoic OAEs we see similar overall positive correlations between increased S:C ratio of organic matter with TOC, but much less sulfurization across the Lower Jurassic as compared to Cretaceous OAE II (8% S:C of organic matter and organic sulfur dominates over pyrite burial). Our results for the Mesozoic show that OMS is not a consistent feature across all OAEs and further work is needed to constrain the importance of OMS on reduced sulfur and carbon burial in deep time.