Paired organic matter and pyrite δ34S records reveal mechanisms of carbon, sulfur, and iron cycle disruption during Ocean Anoxic Event 2

Abstract

The sulfur (S) isotope composition of pyrite in the sedimentary record has played an important part in our understanding of the evolution of biogeochemical cycles throughout Earth history. However, the kinetics of pyritization are complex and depend strongly on the reactivity and mineralogy of available iron. As a second major sink for sulfide in anoxic sediments, organic matter (OM) provides essential context for reconstructing the distribution and isotopic composition of environmental sulfide. To first order, roughly parallel pyrite and OM d 34 S profiles reflect changes in sulfide, while independent patterns require alternative explanations, including changes in iron availability or OM characteristics. We apply this framework to Ocean Anoxic Event 2 (OAE-2, ~94 Mya), a period of enhanced burial of reduced C and S (in OM and pyrite) that has been associated with an expansion of reducing marine conditions. We present paired S-isotope records for pyrite and OM along with profiles of OM S:C ratio and S redox speciation from four wellcharacterized lithologic sections with a range of depositional environments (Pont d'Issole, Cismon, Tarfaya Basin, and Demerara Rise) to reconstruct both local redox structure and global mechanisms impacting the C, S and Fe cycles around OAE-2.

Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging: Extracting local paleo‐environmental information from modern and ancient sediments

“…Sediments from the Cismon section are thought to represent nearly the opposite end‐member case to Santa Barbara Basin, with very low bulk CRS δ 34 S values (avg. –42.7‰) . These values have been interpreted to represent pyrite formation in a system that was open to the diffusive supply of sulfate, so a narrow intra‐sample range of δ 34 S values is expected.…”

“…Finally, sediments from the Demerara Rise are thought to represent an intermediate case between the Cismon section and the Santa Barbara Basin, with a corresponding bulk CRS δ 34 S value of −24.4‰ for our sample. This black shale sample (422.8 m depth, ODP Leg 207, Hole 1258a) was deposited under a euxinic water column, so sulfate consumption by MSR probably outpaced the diffusive replenishment of sulfate . Therefore, a larger intra‐sample range of δ 34 S values relative to the Cismon section sample is expected.…”

“…As with the Cismon section sample, optical microscope images of the mounted extract from the Demerara Rise (Figure S10, supporting information) display the presence of both euhedral marcasite (in this case mostly in irregular aggregates) and framboidal pyrite, as supported by laser Raman microprobe spot analyses and SEM (Figure S8, supporting information). Pyrite and marcasite grains measured by SIMS had average δ 34 S values of −24.9 ± 11.2‰ (1σ; n = 45) and −26.1 ± 6.1‰ (1σ; n = 19; Figure ), compared with a bulk δ 34 S composition of −24.4 ± 0.2‰ . In contrast to those from the Cismon section sample, pyrites from the Demerara sample sometimes featured larger, more easily‐resolved intra‐grain variation in the δ 34 S value (average 1σ = ±6.0‰, compared with an average intra‐grain standard error of ±1.6‰, 1σ; Figure ; Figures S11A–S11D, supporting information).…”

“…Another way to generate more acidic pore waters than are typical for marine sediments is oxic organic matter respiration . This process could have been exacerbated by the high organic loading at both sites . Future work using SIMS will provide further constraints on the relative abundance of marcasite and the genetic relationship(s) between pyrite and marcasite in black shales.…”

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

“…Sulfurization is frequently a major contributor to OM preservation in OM-rich, anoxic, open marine sediments, in both modern and ancient systems (Francois, 1987;Eglinton et al, 1994;Wakeham et al, 1995;Raven et al, 2019). Recent work has found that sulfurization occurs more rapidly, and in a broader range of environments, than previously realized, including hydrothermal systems (Gomez-Saez et al, 2016), sinking marine particles (Raven et al, 2016(Raven et al, , 2019, and surface sediments exposed to variable redox conditions (Jessen et al, 2017). However, relatively little is known about how this process impacts the preservation of carbon in coastal ecosystems.…”