Anaerobic microbial respiration as a link between 2 carbonate platform drowning and Ocean Anoxic Events

The deposition of carbonate rocks is closely tied to Earth’s climate and ocean chemistry. Healthy carbonate platforms produce sediment at a rate that usually keeps up with accommodation changes due to tectonic subsidence and sea level rise. In contrast, platform ‘drowning’ during Ocean Anoxic Events (OAEs) has long been considered a physical expression of biogeochemical changes that reduce shallow-water sedimentation rates. Identifying the exact mechanism(s) that contribute to platform drowning are critical for understanding the nature and duration of environmental disruptions during these events.

Here we present a new model for long-term platform drowning based on changing oceanic gradients in alkalinity and carbonate saturation states. Well-oxygenated oceans are characterized by steep gradients in saturation state with high rates of carbonate ‘over-production’ in the surface ocean and dissolution in the deep ocean. Under reducing conditions, anaerobic microbial metabolisms act to reduce these gradients so that there is less overproduction in the surface ocean which may manifest locally as slower accumulation rates in tropical shallow-water settings. Simple box models show that this is a quasi-steady state process that lasts as long for as long an anoxic condition persist, effectively coupling the timescales of carbonate sedimentation and redox changes. We posit that redox-based changes in ocean gradients act alongside other kill mechanisms to produce the diversity of platform drowning patterns observed in the rock record both in Meseozoic OAEs and for older hyperthermal events.

Smith B. P., Kerans C. & Fischer W. W., in review. Anaerobic microbial respiration as a link between 2 carbonate platform drowning and Ocean Anoxic Events. Article.

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