QUANTIFYING ORGANIC CARBON BURIAL DURING THE TOARCIAN OCEANIC ANOXIC EVENT

There were numerous global climatic perturbations to the ocean-atmosphere system during the Mesozoic. One of the best studied of these events is the Toarcian OAE (~183 Ma) that resulted from the large influx of carbon dioxide from volcanic activity. This had cascading effects on the environment, including a warmer, wetter climate and large-scale marine deoxygenation. These environmental feedbacks resulted in the burial of significant amounts of organic carbon (OC) in sedimentary deposits and a punctuated 3-to-4‰ positive carbon isotope excursion recorded in the sedimentary record. Constraining the amount of OC buried during this event is critical to constraining global carbon and oxygen budgets and associated climatic feedbacks. Here, we seek to quantify the overall amount of OC buried during this event using two different methods: Numerical geochemical modeling of the carbon cycle and compiling global mass accumulation rates (MAR) of OC derived from the preserved stratigraphic record.

Numerical modeling of the carbon cycle, using the carbon isotope record, provides quantitative estimates for global OC burial. However, these non-unique solutions yield a large range of estimates of OC buried: 300 to 800 Tg. These can be compared to those derived from the known stratigraphic and geographic distribution of Toarcian deposits. Here, we present a global database of more than 100 Toarcian stratigraphic successions with TOC data and calculated mass accumulation rates (MAR) of OC from each locality. OC burial rates from these locations were extrapolated to larger geographic areas with similar depositional environments; this ultimately provides an estimate for total preserved OC. While the OC MAR documented at these sites can account for significant portion of the OC buried during the OAE, there are still large discrepancies between mapping estimates and those derived from geochemical modeling. These differences likely are due to the preserved sedimentary record predominantly consisting of marginal marine and epicontinental deposits and the record from some geographic regions remaining unknown. These discrepancies imply there may be significant OC burial associated with this event that either is present in understudied regions, was later destroyed by plate tectonics, or lost during hydrocarbon expulsion.