Stability of Upper Ordovician to middle Silurian marine sulfur isotopes recorded in brachiopod carbonate-associated sulfate

During key intervals of evolution and mass extinction in the Late Ordovician and Silurian periods, glaciations and orogenies have been hypothesized to affect the ventilation and composition of Earth's oceans. Major carbon isotope excursions coincident with climatic and biologic changes may have been caused by the development of euxinic conditions or changes in the composition of rocks undergoing weathering. If widespread or long-lasting, marine euxinia could produce spatially variable sulfur isotopic compositions of marine sulfate and weathering changes could produce temporally variable compositions. However, the quality of all proxy archives for ancient seawater sulfate is variable, requiring careful consideration of primary and diagenetic sources of isotopic variability. To examine marine sulfur isotopic variability, we measured the sulfur isotopic composition of carbonate-associated sulfate incorporated into well-preserved brachiopods from the Cincinnati Arch, USA, Anticosti Island, Canada, and Gotland, Sweden.

These brachiopods are among the best-preserved carbonate specimens of this age and were deposited in three different tectonic settings on two different paleocontinents. Brachiopod preservation was assessed by petrographic metrics and new and previously published trace metal and carbonate clumped isotope analyses. We used a recently developed aqueous sulfate analytical technique, which dramatically decreases sample demands relative to traditional methods, to make the first replicate measurements of individual brachiopods.

Ordovician and Silurian brachiopods have a spatially and temporally invariant sulfur isotopic composition of ca. 25‰ VCDT. We observed up to a 4‰ sulfur isotope range among replicates within single brachiopods that clumped isotope data indicates predates shallow burial. This variability may be primary and related to vital effects during biomineralization or infaunal modes of life, or be earliest marine diagenetic. Nonetheless, it can account for all the sulfur isotope change over 30 Myr, suggesting that seawater sulfate was globally well-mixed and temporally invariant. Therefore, changes in ocean ventilation or the composition of weathering rock were likely not severe or long-lasting enough to affect the global sulfur budget.