PAIRED SULFUR AND CARBON ISOTOPE STRATIGRAPHY FOR THE LATE LUDFORDIAN (SILURIAN) OF GOTLAND, SWEDEN: IMPLICATIONS FOR ENVIRONMENTAL, OCEANOGRAPHIC, AND BIOTIC CHANGE

The Late Silurian was a time of widespread environmental, oceanographic, and biotic change associated with one of the largest perturbations of the global carbon cycle in the Phanerozoic. The Lau extinction event, although of lesser magnitude than the “big five” Phanerozoic mass extinctions, has strikingly similar sedimentary architecture and faunal changes. Causal mechanism(s) linking this Late Silurian extinction event to a large positive carbon isotope excursion (CIE), paleoceanographic change, and climate remain poorly understood. One potential mechanism that could link the carbon cycle perturbation to biotic, sedimentologic, environmental fluctuations is large-scale expansion of oceanic anoxic and sulfidic conditions in the late Ludfordian. It is thought that the redox state of the oceans are directly linked to the overall burial versus oxidation of organic matter and pyrite.

Here we present carbonate-associated sulfate (δ³⁴S_CAS) paired with carbonate δ¹³C_carb and organic matter δ¹³C_org from an expanded carbonate-dominated late Ludfordian sequence from the Baltic Basin on the Island of Gotland, Sweden. The globally recognized Lau CIE is recorded within this δ¹³C_carb data (≥ +7‰), and for the first time coeval positive excursions in δ¹³C_org (~5‰ shift) and δ³⁴S_CAS (≥ 20–35‰ shift) are documented. These preliminary results support our working hypothesis that this late Ludlow geochemical event is linked to large amounts of organic matter and pyrite being buried likely under reduced oxygen ocean settings, rather than being a weathering driven event from widespread exposure of carbonate platforms during sea level lowstand. Peak δ³⁴S_CAS values are coincident with declining δ¹³C values in the post-event strata of the Hamra/Sundre formations and this apparent offset likely reflect different residence times of carbon versus sulfur and their responses to declining organic carbon and pyrite burial. These isotopic data provide the first records of sulfur cycle dynamics through this Late Silurian extinction event. Regardless of the ultimate cause(s), geochemical modeling and further isotope stratigraphy studies are needed, specifically in areas where sufficient pre– and post–δ¹³C excursion strata exist, to completely document the sulfur and carbon cycle dynamics related to this biogeochemical event.