Organic carbon burial during OAE2 driven by changes in the locus of organic matter sulfurization

Abstract

Ocean Anoxic Event 2 (OAE2) was a period of dramatic disruption to the global carbon cycle when massive amounts of organic matter (OM) were buried in marine sediments via complex and controversial mechanisms. Here we investigate the role of OM sulfurization, which makes OM less available for microbial respiration, in driving variable OM preservation in OAE2 sedimentary strata from Pont d’Issole (France). We find correlations between the concentration, S:C ratio, S-isotope composition, and sulfur speciation of OM suggesting that sulfurization facilitated changes in carbon burial at this site as the chemocline moved in and out of the sediments during deposition. These patterns are reproduced by a simple model, suggesting that small changes in primary productivity could drive large changes in local OM burial in environments poised near a critical redox threshold. This amplifying mechanism may be central to understanding the magnitude of global carbon cycle response to environmental perturbations.

Mitigation of Extreme Ocean Anoxic Event Conditions by Organic Matter Sulfurization

“…Our Earth system model‐based investigation supports the hypothesis that rapid OM sulfurization (i.e., k sulf ≥ 10 5 M −1 year −1 ) is important to the formation of Cretaceous black shales in productive and/or semirestricted, euxinic environments (Arndt et al, ; Kolonic et al, ; Raven et al, ). Assuming a plausible range of oceanic H 2 S concentrations between 10 and 100 μM, a threshold sulfurization rate constant of 10 5 M −1 year −1 for the onset of globally important impacts translates to a first‐order rate constant of 1.0–10.0 year −1 .…”

“…Intense sulfurization of black shale OM has been previously described for sediments from Tarfaya Basin (e.g., Kolonic et al, , ), Demerara Rise (e.g., Böttcher et al, ; Hetzel et al, ), Pont d'Issole (Raven et al, ), or the Jurassic Kimmeridge Clay Formation (e.g., Sinninghe Damsté et al, ; van Kaam‐Peters et al, , ; van Dongen et al, ). At all these sites, intense OM sulfurization is linked to enhanced OM burial.…”

“…As a consequence, rapid water column OM sulfurization could be an initially very efficient mechanism for drawing down CO 2 and H 2 S and thus inducing the Earth system's recovery from OAEs. However, although sulfurization of OM is considered by some authors a globally significant process for preserving organic compounds (Arndt et al, ; Raven et al, ; Sinninghe Damsté et al, ), the dynamic interplay between primary productivity, ocean redox conditions, OM sulfurization, and its preservation in the sediments has yet to be examined.…”

Chemical and Isotopic Evidence for Organic Matter Sulfurization in Redox Gradients Around Mangrove Roots

“…Alkyl sulfides and disulfides are the major products of sulfurization in Little Ambergris Cay sediments, similar to observations from laboratory sulfurization experiments (Amrani and Aizenshtat, 2004), modern sites (Kohnen et al, 1991;Eglinton et al, 1994) and ancient rocks (Raven et al, 2018(Raven et al, , 2019. At the same time, however, more oxidized forms are also significant contributors to OS Res in these sediments.…”

“…In the latter case, this means that OM Res contains 167 C atoms for every added S, far more than can be causally linked to preservation due to a single sulfurization event. The amount of C that can be effectively preserved for each sulfur addition has been estimated at roughly 8-30 C atoms (Raven et al, 2018). Accordingly, in a sample with an S:C ratio of 0.6 mol%, OM sulfurization (as a mechanism for enhancing hydrolysis resistance) accounts for essentially all of the S in OM Res but only 5-18% of the C. The remainder of C in OM Res represents OM that is naturally hydrolysis resistant, 13 C-enriched, and S-poor.…”