During Ocean Anoxic Event 2 (OAE2, ~94 Ma), organic carbon burial in marine sediments increased dramatically, yet the amplification mechanism remains incompletely quantified. Raven et al. (2018) proposed that changes in the locus of organic matter sulfurization — rapid, polysulfide-driven sulfur incorporation into fresh organic matter in sinking particles and at the sediment-water interface versus gradual, sulfide-driven sulfurization in deeper sediments, modulated by vertical movement of the chemocline — can drive OAE2-scale organic carbon burial without large productivity swings. This problem is the unresolved quantitative core of that framework: the effective sulfur-to-carbon ratio (S:C) achieved by rapid sulfurization under natural particle and sediment conditions is not tightly constrained (the original mass-balance model assumed 4–5% for rapid sulfurization, whereas 48-hour experiments on natural sinking particles reach only ~2%); the sulfurization–preservation framework has been coupled to mechanistic Earth-system models only in a simplified, water-column-only form with prescribed rate constants; the proposed molecular-product signatures diagnostic of reaction locus lack targeted experimental validation (including the hypothesized ammonia dependence of disulfide formation); and the relative contributions of sulfurization versus mineral protection to organic carbon preservation across depositional settings remain unquantified. Later work has generalized the mechanism to four OAE2 sections, modern anoxic basins, and the Proterozoic, but these quantitative and mechanistic-coupling questions remain open.

Background

Ocean Anoxic Event 2 (OAE2), spanning the Cenomanian–Turonian boundary (~94 million years ago), was one of the largest carbon-cycle perturbations of the Phanerozoic: widespread ocean deoxygenation, a sharp positive carbon-isotope excursion, and greatly enhanced burial of organic carbon (OC) in marine black shales. Explanations have long been divided between supply-side controls (elevated primary productivity from volcanogenic nutrients) and preservation-side controls. Raven et al. (2018) provided a preservation-side mechanism with observational and modeling support. At the Pont d'Issole section (France), interbedded organic-rich shales and organic-lean limestones differ by ~78% in OC content (carbonate-free basis). Using co-registered total organic carbon (TOC), organic sulfur-to-carbon ratios (S:C, the molar ratio of organic sulfur to organic carbon), the sulfur-isotope composition of organic sulfur (δ34S of organic matter), and synchrotron S K-edge X-ray absorption near-edge spectroscopy (XANES, which resolves the oxidation-state speciation of sedimentary sulfur into reduced forms such as alkyl sulfides and disulfides versus oxidized forms such as sulfonates and sulfate esters), they showed the two lithologies carry distinct sulfurization signatures: TOC-rich shales record rapid, open-system sulfurization near the sediment-water interface, while TOC-lean limestones record slower, closed-system sulfurization deeper in the sediments. Organic matter (OM) sulfurization is the abiotic reaction of reduced sulfur species (polysulfides and bisulfide, HS−HS^-) with functionalized organic molecules (carbonyls, conjugated double bonds), which makes OM sulfur-rich and far more resistant to microbial degradation; it operates in two regimes — rapid (days-scale) reactions between polysulfides and fresh OM near redox interfaces, previously observed in sinking particles of the Cariaco Basin (Raven et al. 2016), and gradual (thousand-year-scale) reactions between sulfide and degraded OM in deeper sediments. A three-box mass-balance model of the water column, sediment–water interface, and underlying sediments, parameterized with Cariaco Basin fluxes and a standard multi-G reactivity scheme (organic matter divided into classes of different degradation reactivity), reproduced the shale–limestone TOC contrast by varying only the vertical position of the chemocline (the depth horizon separating oxic from sulfidic waters): removing sulfurization from the water column and surface sediments lowered modeled preservation by 55–77%. However, the assigned molar S:C ratios — 4–5% for rapid and 1–3% for gradual sulfurization — were only loosely constrained, the chemocline position was imposed rather than mechanistically predicted, and the paper itself listed the quantitative and mechanistic loose ends that define the present problem. Hülse, Arndt and Ridgwell (2019) subsequently implemented water-column OM sulfurization in the cGENIE Earth-system model (a 3-D ocean circulation model coupled to marine carbon, oxygen, phosphorus, and sulfur biogeochemistry) for OAE2 conditions, showing that rapid sulfurization (rate constant ksulf≥105 M−1yr−1k_{\mathrm{sulf}}\geq 10^{5}\,\mathrm{M^{-1}yr^{-1}}) can enhance OC burial by up to roughly threefold and reduce euxinic (sulfidic water column) volume by up to ~80%, but described this coupling as only a first step, with prescribed rate constants, water-column-only sulfurization, and no sediment-locus or sea-level resolution.

Problem Statement

Quantitatively establish how the locus, kinetics, and preservation stoichiometry of organic matter sulfurization control organic carbon burial in OAE2-type environments with fluctuating redox conditions. This comprises four linked components that jointly serve this single objective: (1) constrain the effective S:C ratio (the molar ratio of organic sulfur to organic carbon in sedimentary organic matter) achieved by rapid sulfurization of natural sinking particles and surface sediments under natural conditions, replacing the loosely assigned 4–5% (rapid) and 1–3% (gradual) values of the original mass-balance model, given that 48-hour experiments on natural particles so far reach only ~2%; (2) couple the sulfurization–preservation framework to mechanistic oceanographic models in which chemocline position and its dynamics emerge from productivity, circulation, nutrient and oxygen budgets, and sea-level change — extending the first, water-column-only Earth-system implementation (Hülse et al. 2019) to a locus-resolved, S:C-sensitive representation; (3) test the proposed diagnostic links between sulfur-speciation products and reaction locus — polysulfide-derived alkyl sulfides versus HS−HS^--derived products, and disulfide formation with its hypothesized dependence on ammonia availability — through targeted experiments varying oxidant availability, ammonia, and polysulfide chemistry, building on the corroboration of the locus framework at four OAE2 sections provided by Raven et al. (2019); and (4) disentangle the relative contributions of sulfurization and non-sulfur preservation mechanisms (mineral adsorption and protection, clay and iron association) to organic carbon preservation in different depositional settings. Each component is answerable by a defensible, source-grounded result for its own question; resolving the objective does not require completing every multi-year program, and component (3) requires the design of a discriminating test rather than new laboratory work.

References

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