The oxidation of petrogenic organic carbon: a source of CO2 during transient warming events?

The terrestrial carbon cycle has long been discussed under a framework that focuses on inorganic carbon (i.e. the balance between solid Earth degassing and silicate weathering). Therefore, the role of organic carbon has remained poorly constrained in both the present and past. A recent study highlighted the importance of rock-derived “petrogenic” organic carbon (OC_petro), suggesting that the amount of CO₂ released during the exhumation and mobilisation of OC_petro may be comparable to that from volcanism. To determine the response of OC_petro to future climate change, warming events in the geologic record can be investigated. For example, there are biomarker-based evidence for up to an order-of-magnitude increase in the burial of OC_petro in shallow-marine sediments dated to the Paleocene-Eocene thermal maximum (PETM; ∼56 Ma). However, estimates of the proportion of OC_petro lost via oxidation are unavailable due to the lack of suitable techniques.

Raman spectroscopy assesses differences in the crystallinity of OC_petro, allowing the distinction between graphitised and disordered carbon. Modern river systems have shown a shift towards a dominance of graphite downstream, as disordered carbon are more susceptible to oxidation. Here, we explore whether Raman spectroscopy can be used to reconstruct OC_petro oxidation in the past. During the PETM, there is an increase of graphite in the mid-Atlantic Coastal Plain, indicating enhanced OC_petro oxidation. This is consistent with signs of intensified physical erosion and enhanced OC_petro delivery. On the other hand, the distribution of graphitised carbon vs. disordered carbon (and biomarkers) do not change in the Arctic Ocean, implying spatial variability. This study demonstrates, for the first time, the utility of Raman spectroscopy as a novel tool to evaluate OC_petro oxidation in a geological context. Applying this approach to quantify oxidation rates require further ground truthing in settings with different degrees of weathering.