Mitigation of Extreme Ocean Anoxic Event Conditions by Organic Matter Sulfurization

Past occurrences of widespread and severe anoxia in the ocean have frequently been associated with abundant geological evidence for free hydrogen sulfide (H ₂ S) in the water column, so-called euxinic conditions. Free H ₂ S may react with, and modify, the chemical structure of organic matter settling through the water column and in marine sediments, with hypothesized implications for carbon sequestration. Here, taking the example of Ocean Anoxic Event 2, we explore the potential impact of organic matter sulfurization on marine carbon and oxygen cycling by means of Earth system modeling. Our model experiments demonstrate that rapid sulfurization (k _sulf ≥ = 10 ⁵  M ⁻¹  year ⁻¹ ) of organic matter in the water column can drive a more than 30% enhancement of organic carbon preservation and burial in marine sediments and hence help accelerate climate cooling and Ocean Anoxic Event 2 recovery. As a consequence of organic matter sulfurization, we also find that H ₂ S can be rapidly scavenged and the euxinic ocean volume reduced by up to 80%—helping reoxygenate the ocean as well as reducing toxic H ₂ S emissions to the atmosphere, with potential implications for the kill mechanism at the end-Permian. Finally, we find that the addition of organic matter sulfurization induces a series of additional feedbacks, including further atmospheric CO ₂ drawdown and ocean reoxygenation by the creation of a previously unrecognized net source of alkalinity to the ocean as H ₂ S is scavenged and buried.