Carbon pump dynamics and limited organic carbon burial during OAE1a

Oceanic Anoxic Events (OAEs) are conspicuous intervals in the geologic record that are associated with the deposition of organic carbon (OC)-rich marine sediment, linked to extreme biogeochemical perturbations, and characterized by widespread ocean deoxygenation. Mechanistic links between the marine biological carbon pump (BCP), redox conditions, and organic carbon burial during OAEs, however, remain poorly constrained. In this work we reconstructed the BCP in the western Tethys Ocean across OAE1a (similar to 120 Mya) using sediment geochemistry and OC mass accumulation rates (OCAcc). We find that OCAcc were between 0.006 and 3.3 gC m(-2) yr(-1), with a mean value of 0.79 +/- 0.78 SD gC m(-2) yr(-1)-these rates are low and comparable to oligotrophic regions in the modern oceans. This challenges longstanding assumptions that oceanic anoxic events are intervals of strongly elevated organic carbon burial. Numerical modelling of the BCP, furthermore, reveals that such low OC fluxes are only possible with either or both low to moderate OC export fluxes from ocean surface waters, with rates similar to oligotrophic (nutrient-poor, < 30 gC m(-2) yr(-1)) and mesotrophic (moderate-nutrients,similar to 50-100 gC m(-2) yr(-1)) regions in the modern ocean, and stronger than modern vertical OC attenuation. The low OC fluxes thus reflect a relatively weak BCP. Low to moderate productivity is further supported by palaeoecological and geochemical evidence and was likely maintained through nutrient limitation that developed in response to the burial and sequestration of phosphorus in association with iron minerals under ferruginous (anoxic iron-rich) ocean conditions. Without persistently high productivity, ocean deoxygenation during OAE1a was more likely driven by other physicochemical and biological factors including ocean warming, changes in marine primary producer community composition, and fundamental shifts in the efficiency of the BCP with associated effects and feedbacks.

Automated summary

Oceanic Anoxic Events (OAEs) are prominent periods in the geological record characterized by the deposition of organic carbon-rich marine sediments and widespread ocean deoxygenation. However, the mechanistic links between the marine biological carbon pump (BCP), redox conditions, and organic carbon burial during OAEs are still poorly understood. This study reconstructs the BCP during OAE1a using sediment geochemistry and OC mass accumulation rates (OCAcc) and finds that the rates are low and comparable to oligotrophic regions in the modern oceans. Numerical modeling further suggests that the weak BCP and low to moderate productivity during OAE1a were likely due to nutrient limitation caused by the burial and sequestration of phosphorus in association with iron minerals under ferruginous ocean conditions. Ocean deoxygenation during OAE1a was likely driven by factors such as ocean warming, changes in the marine primary producer community composition, and shifts in the efficiency of the BCP.