S-ISOTOPE ANALYSIS OF THE MID-ORDOVICIAN SAN JUAN AND TABLE HEAD FORMATIONS, ARGENTINA AND NEWFOUNDLAND: EVIDENCE FOR HIGH-RESOLUTION REDOX CYCLING IN THE EARLY PALEOZOIC

Biospheric evolution is recorded, in part, by the isotopic record of carbon and sulfur, wherein enhanced burial of organic carbon results in increased oxygenation, oxidative weathering of crustal materials, and delivery of sulfate to the ocean. In turn, the extent of sulfate reduction depends on both oceanic redox state and the availability of an organic carbon source. To explore C-S linkages in the Early Paleozoic, we have constructed high-resolution δ¹³C and δ³⁴S curves for the Middle Ordovician (Arenig-Llanvirn) San Juan Formation (SJF), Argentine Precordillera and Table Head Formation (THF), Western Newfoundland. Whereas the SJF represents lithologically variable, shallow subtidal carbonate deposition, the THF records lithologically monotonous, deeper water deposition. δ¹³C curves for both formations show restricted δ¹³C variation, in agreement with the composite curve of Saltzman (2005), suggesting deposition under possibly nutrient-limited marine conditions. Stratigraphic variation in δ¹³C also confirms that the THF is, as a whole, younger than the SJF.

δ³⁴S curves were constructed using carbonate-associated trace sulfate (CAS), which should represent the composition of marine sulfate at the time of deposition. δ³⁴S curves for both formations show rapid (20-30m) isotopic shifts of ~6‰ that are superimposed over a lower-order signal. In the SJF, lower-order variation is recorded as a 10‰ positive shift occurring over ~200m followed by a return to original values. Similar high-order variation in δ³⁴S exhibited in both the SJF and THF suggests: 1) that recorded CAS isotopic signals are independent of depositional environment, 2) that high-order isotopic variation may be a global phenomenon, and 3) that the mechanism responsible for this high-order isotopic variation acted on oceanographic, rather than geologic, time scales. We suggest that high-order δ³⁴S variation reflects transient changes in the extent of oceanic bottom-water anoxia, resulting in variable redox cycling (BSR and sulfide oxidation) in deep-ocean environments. To further explore if δ³⁴S variation resulted from changes in redox cycling, sulfate-δ¹⁸O curves will be constructed and the time-dependence of observed isotopic variation will be determined via high-resolution U-Pb dating of numerous bentonites sampled in the SJF.