Brachiopod δ³⁴S_(CAS) microanalyses indicate a dynamic, climate-influenced Permo-Carboniferous sulfur cycle

Abstract

Early isotopic studies of sulfate in carbonate minerals (carbonate associated sulfate; CAS) suggested that carbonates can provide a reliable, well-dated archive of the marine sulfur cycle through time. However, subsequent research has shown that diagenetic alteration can impose highly heterogeneous CAS sulfur isotopic compositions (δ³⁴S_(CAS)) among different carbonate phases within sediments. Such alteration necessitates targeted sampling of well-preserved, primary carbonate phases. Here, we present a new record of Carboniferous and Early Permian brachiopod δ³⁴S_(CAS) generated from over 130 measurements of microsampled brachiopod shells. Our record refines existing brachiopod δ³⁴S_(CAS) records and confirms a large, ∼6.5‰ δ³⁴S_(CAS) decrease in the Early Carboniferous. Importantly, the record also features a novel 3–5‰ increase in δ³⁴S_(CAS) near the Serpukhovian-Bashkirian boundary (323.4 Ma) that coincides with carbonate δ¹³C and δ¹⁸O increases. Variability in δ³⁴S_(CAS) is minor both within (≤0.3‰) and among (≤2‰) individual co-depositional brachiopod specimens. A taxon-specific δ³⁴S_(CAS) offset is present one species (Composita subtilita) that also exhibits a δ¹³C offset, supporting the existence of biological "vital effects" on δ³⁴S_(CAS). Geologic evidence and mathematical modeling of the Permo-Carboniferous carbon and sulfur cycles suggest that changes in the burial ratio of organic carbon to pyrite sulfur (RC:S) are insufficient to explain the observed mid-Carboniferous δ³⁴S_(CAS) record. We find that changes in the ³⁴S depletion of pyrite relative to seawater sulfate (ε³⁴) or in the δ³⁴S of the input to the ocean (δ³⁴S_(in)) are also needed. Large additions of O₂ from organic carbon burial during the Permo-Carboniferous cannot be entirely compensated for with sulfur cycle changes; lower than modern late Visean pO₂ and/or additional O₂ sinks are needed to keep pO₂ at plausible levels. Based on the geologic context surrounding our record's mid-Carboniferous δ³⁴S_(CAS) increase, we advocate for simultaneous changes in pyrite burial, ε³⁴, and δ³⁴S_(in), driven by sea level or tectonically induced changes in environments of sulfur burial, as a viable mechanism to produce rapid seawater δ³⁴S changes.

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