Abstract
AbstractPast occurrences of widespread and severe anoxia in the ocean have frequently been associated with abundant geological evidence for free hydrogen sulfide (H2S) in the water column, so‐called euxinic conditions. Free H2S 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 (ksulf≥ = 105 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 H2S can be rapidly scavenged and the euxinic ocean volume reduced by up to 80%—helping reoxygenate the ocean as well as reducing toxic H2S 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 CO2 drawdown and ocean reoxygenation by the creation of a previously unrecognized net source of alkalinity to the ocean as H2S is scavenged and buried.
Highlights
Oceanic Anoxic Events (OAEs) are severe perturbations of global biogeochemical cycling associated with widespread deposition of laminated organic matter-rich sediments under anoxic/euxinic conditions—so-called black shales (Schlanger & Jenkyns, 1976)
We are able to forward proxy model key sedimentological features rather than having to make the assumption that black shale occurrence in the geological record can be equated with modeled occurrence of ocean floor anoxia, as was previously the state-of-the-art in paleo Earth system modeling (Monteiro et al, 2012)
Prior to the onset of OAE2, total organic carbon (TOC) contents were already elevated in the proto-North Atlantic Ocean with contents that can reach up to 10 wt% locally (e.g., Kuypers et al, 2002; Sinninghe Damsté & Köster, 1998)
Summary
Oceanic Anoxic Events (OAEs) are severe perturbations of global biogeochemical cycling associated with widespread deposition of laminated organic matter-rich sediments under anoxic/euxinic conditions—so-called black shales (Schlanger & Jenkyns, 1976). Envisaged as encompassing a series of three major and five minor events that occurred during the Jurassic and Cretaceous periods (Jenkyns, 2010), evidence for the transient occurrence of extensive ocean anoxia/euxinia has been found associated with the Late Ordovician, Late Devonian, end-Permian, and Early Jurassic extinction events (Meyer & Kump, 2008). Thought to be important to the occurrence of euxinic conditions are a warm climate and a nutrient-trapping geography, such as in restricted basins that were predisposed to euxinia (Meyer & Kump, 2008). Plausible OAE triggering mechanisms are known, even if not necessarily explored yet in any great mechanistic and dynamical detail. This leaves the primary outstanding question: What is (are) the dominant mechanism(s) by which the Earth system recovers from such extreme conditions?
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