CHEMOSTRATIGRAPHY OF AN ORDOVICIAN-SILURIAN SHALLOW WATER CARBONATE PLATFORM: IMPLICATIONS FOR THE INTERPRETATION OF δ13C EXCURSIONS UNDER GLACIOEUSTATIC EXPOSURE SURFACES

The use of carbon isotope stratigraphy for stratigraphic correlation depends on synchronous changes in the carbon isotope ratios (δ¹³C_carb) preserved in carbonate-dominated strata. Such changes are commonly interpreted to reflect primary secular variation in ocean chemistry. This correlation technique has been widely applied, especially in Precambrian strata in which biostratigraphic correlation is often unreliable or impossible. Negative carbon isotope excursions associated with Neoproterozoic global glaciation have been reinterpreted recently as artifacts of meteoric diagenesis. Such processes are well understood and documented on carbonate platforms developed during Pliocene-Pleistocene glacial cycles; their impact in the more distant past is less certain. Here we examine δ¹³C_carb stratigraphy from multiple sections of a shallow water carbonate platform in Nevada deposited during the Ordovician-Silurian (O/S) icehouse. We do not find evidence for isotopic alteration (i.e., ¹³C depletion) akin to that observed in Pliocene-Pleistocene platforms, even in strata that contain multiple subaerial exposure surfaces associated with glacioeustatic sea level fall, including a major unconformity at the O/S boundary and a 50 m deep system of karst pipes under a Lower Silurian exposure surface. The rocks have depleted concentrations of Sr (10-120 ppm) and carbonate-associated sulfate (<150 ppm), and the oxygen isotope ratios do not covary with δ¹³C_carb. Despite field, petrographic, and geochemical evidence for interaction with meteoric fluids none of the exposure surfaces are associated with a significant negative δ¹³C_carb anomaly, and δ¹³C_carb values vary throughout the entire succession between -2‰ and +3‰. Chemostratigraphic correlation of the Nevada sections with strata in the Cincinnati Arch and Estonia are consistent with biostratigraphic constraints. These results suggest that Pliocene-Pleistocene diagenetic processes may not be perfectly analogous for interpreting the δ¹³C_carb records of carbonate platforms deposited prior to the establishment of an extensive terrestrial biosphere in the Silurian and Devonian.