Speaker: Hadar Cohen Sadon, UC Santa Cruz
Title: The Role of the Reactive Iron-to-Sulfate Ratio in Organic Matter Preservation and the δ³⁴S Record of Sedimentary Rocks
Time: Wednesday, Oct 22 at 12:00pm PST
Location: EMS B210
Abstract: Microbial sulfate reduction (MSR) occurs in oxygen-depleted environments and is one of the most effective processes shaping global sulfur and carbon cycles. During early diagenesis, H2S produced by MSR reacts with organic matter (OM) via abiotic sulfurization, and/or with Fe to form pyrite. Abiotic sulfurization enhances the long-term preservation of OM. While sulfurization intensity, indicated by the organic S/C ratio, has been linked to OM preservation, the pathways through which sulfurization occurs play a crucial role. Intermolecular sulfurization leads to the formation of sulfur cross-linkages (e.g., sulfide bonds), which stabilize OM by forming larger macromolecules, whereas intramolecular sulfurization results in the formation of cyclic organic sulfur compounds (e.g., thiophenes), which contribute less to OM stabilization.
In this study, a set of organic-rich sedimentary rocks from marine and lacustrine environments, spanning geological periods from the Devonian-Mississippian (~416-359 Ma) to the Late Eocene (56-33 Ma) was examined. A comprehensive quantitative and isotopic analysis of sulfur in OM and pyrite, together with carbon and iron analyses, was used to investigate the competition between pyritization and sulfurization and their role in OM preservation. Organic-S structure was evaluated using a thermal proxy, Tmax-S [ºC], that indicates sulfur bond distribution within the OM.
Results show that the ratio of reactive iron to reduced sulfur fractions in the rock (FeR/Sorg+py [mol/mol]) governs the quantitative and isotopic partitioning of S between OM and pyrite. Furthermore, both the FeR/Sorg+py ratio and the isotopic gap between OM and pyrite (Δ34Sorg-py) correlate with Tmax-S (R2= 0.7 and 0.8, respectively), suggesting that the reactive iron-to-sulfate ratio controled sulfurization pathways. Combined with a kinetic model simulating δ34S evolution under varying environmental conditions, and an extensive d34S data compilation from the literature, these observations suggest that fluctuations in δ34Spy and δ34Sorg values throughout Earth’s history were primarily driven by variations in the reactive iron-to-sulfate ratio in the depositional environment. Furthermore, we propose that the reactive iron-to-sulfate ratio plays a critical role in controlling the contribution of sulfurization to organic matter preservation.
This seminar series is supported by the Casey Moore Fund.