Shifting modes of iron sulfidization at the onset of OAE-2 drive regional shifts in pyrite delta S-34 records

Total reduced inorganic S isotope ratios (delta S-34(CRS)) shift toward more negative values across much of the southern North Atlantic just before the onset of the Cenomanian-Turonian Ocean Anoxic Event (OAE-2). At the same time, there is no parallel isotopic change in the significantly larger pool of kerogen (organic) S, which indicates that the distribution and S-isotope composition of sulfide in the environment likely did not drive the change in delta S-34(CRS). Here, we investigate possible explanations for the negative shift in delta S-34(CRS) values and their divergence from organic S by isolating iron sulfides for morphological identification and grain-specific isotopic analysis using secondary ion mass spectrometry (SIMS). In pre- and syn-OAE-2 sedimentary rocks from Demerara Rise, we find four distinct morphologies of iron sulfides: pyrite framboids (1-20 mu m diameter), irregular pyrite aggregates (1-38 mu m diameter), large cemented pyrite aggregates (-60 mu m diameter), and irregular and cemented aggregates of the pyrite polymorph marcasite (1-45 mu m diameter). These different textural groups have distinct S-isotopic compositions that are largely consistent through the onset of OAE-2. As such, the secular change in bulk delta S-34(CRS) values likely reflects the changing proportions of these phases stratigraphically across OAE-2. All textural groups feature resolvable intra-grain delta S-34 variability, suggesting that the environments in which they formed were characterized by dynamic sulfide delta S-34 values and/or by partial closed-system distillation. We use grain-specific delta S-34 distributions to rule out shoaling of the chemocline within the sediments as a mechanism for the observed decrease in delta S-34(CRS). Instead, we propose that changes in the reactivity of the iron species delivered to Demerara Rise over the similar to 200 kyr leading up to the onset of OAE-2 impacted the relative contributions of pyrite with S-isotope signatures reflecting the water column, shallow sediments, and deeper sediments to the bulk sedimentary delta S-34(CRS) value. Specifically, the change in iron reactivity at the onset of OAE-2 favored the production of S-34-depleted large, cemented aggregates and framboids at the expense of more S-34-enriched irregular aggregates. Our results underscore that bulk delta S-34(CRS) measurements integrate multiple reduced phases that form via distinct reaction mechanisms and potentially in different parts of the depositional environment. Grain-specific SIMS analyses dramatically enrich our ability to interpret pyrite isotopic patterns in the geologic record.