HIGH-RESOLUTION SULFUR ISOTOPE RECORDS OF EARLY AND MIDDLE PALEOZOIC SEAWATER AS RECORDED IN LIMESTONES AND DOLOSTONES OF MIDCONTINENT NORTH AMERICA

Carbonate-associated sulfate (CAS) has proven value as a paleoceanographic proxy for the sulfur isotope composition of sulfate in seawater. CAS is sulfate substituted into the crystal lattice of calcite or dolomite in place of the carbonate group, with concentrations typically in the ppm range. Past studies of modern sediments and comparisons between ancient CAS data and coeval evaporite deposits confirm the validity of the approach. Our ongoing work is extending this calibration to the modern lime muds of south Florida. While CAS concentrations may be compromised by diagenesis, primary isotopic values appear to be buffered from appreciable alteration. On geologic time scales, trends for the S isotope composition of the ocean track the net burial versus weathering of pyrite; this balance is a primary modulator of the oxygen content of the atmosphere. Previous models for the ancient atmosphere have been forced to rely on deposits of gypsum, anhydrite and barite, which are inherently patchy in their temporal and spatial distributions. By contrast, CAS provides unprecedented opportunity to generate continuous, high-resolution S isotope data using cratonal carbonate sequences.

Several workers have used CAS to produce high-resolution isotopic records of seawater sulfate in the Precambrian. These data reveal rapid shifts in S isotope values over short time periods, which are a product of low and variable concentrations of sulfate in the Precambrian ocean and thus of oxygen in the atmosphere. Surprisingly, no investigator has attempted a similarly high-resolution study of CAS isotopic variability in the Phanerozoic. Absent such studies, we are left with significant gaps in our knowledge of S cycling and corresponding evolution of earth-surface oxygenation. To address deficiencies in the Paleozoic sulfur record, we will present CAS data from selected limestones and dolostones of Midcontinent North America: the John Wash Limestone (Cambrian, Nevada), the Gasconade (Ordovician, Missouri), Upper Keyser, New Creek, and Corriganville (Silurian-Devonian, West Virginia), and Joana Limestone (Mississippian, Nevada). These data, generated at meter-scale intervals and spanning the early-mid Paleozoic, will ultimately refine numerical modeling of a critical time in biospheric evolution.