PARALLEL, HIGH-RESOLUTION CARBON AND SULFUR ISOTOPE RECORDS AND THEIR USE IN RESOLVING THE MECHANISMS BEHIND REGIONAL AND GLOBAL ISOTOPE EXCURSIONS

Analyses of carbonate rocks permit the generation of parallel, high-resolution carbon, sulfur and strontium isotope records for seawater spanning geologic history. An emerging proxy, carbonate-associated sulfate (CAS), has been shown to faithfully track the sulfur isotope composition of seawater in both modern and ancient environments. Because of the links between the carbon and sulfur cycles, , high resolution, CAS-based S isotope records have the potential to shed essential mechanistic light on the causes and the global versus regional implications of carbon isotope excursions observed in the geological record.

Results from the Late Cambrian SPICE event preserved at Shingle Pass, Nevada, show parallel positive C (4 per mil) and S (25 per mil) isotope excursions, likely recording a large-scale marine organic carbon burial event with sympathetic pyrite burial. Carbon and sulfur records from other events throughout the Paleozoic show a variety of different and evolving relationships. A Silurian-Devonian boundary section at Strait Creek, West Virginia, reveals a positive C excursion of 6 per mil but with an enigmatic S isotope relationship. Specifically, a possible negative S excursion (15 per mil) seems to be in phase with the large, positive C shift. By contrast, a Kinderhookian-Osagean (Early Mississippian) section in the Confusion Range, Utah, records invariant S isotope ratios across a C isotope excursion of 6 per mil. Mid-to-late Paleozoic records may be dominated by terrestrial organic C burial, which would lack parallel pyrite burial. Alternatively, the coupling of mid-to-late Paleozoic, high-resolution C-S isotope records may have weakened with decreasing sensitivity of the seawater S isotope reservoir to flux terms, the result of marine sulfate concentration increasing through the Paleozoic. Additional, ongoing high-resolution CAS work includes Pennsylvanian (Missourian) cyclothems and the Ordovician-Silurian boundary as part of our effort to deconvolve the drivers behind local versus global isotope effects under different and evolving oceanic conditions.