Chemostratigraphy of an Ordovician-Silurian carbonate platform: δ13C records below glacioeustatic exposure surfaces

The use of carbon isotope stratigraphy to construct time lines for stratigraphic correlation requires synchronous changes in carbon isotope ratios (delta C-13) to be preserved in carbonate-dominated strata. Such changes are commonly interpreted to reflect primary secular variation in ocean chemistry. However, negative delta C-13 anomalies developed in Pliocene-Pleistocene carbonate platforms following glacioeustatic sea-level fall due to remineralization of terrestrial biomass during meteoric diagenesis. These anomalies are similar in structure and magnitude to some Neoproterozoic delta C-13 records, opening the possibility that the Neoproterozoic d(13)C anomalies have a meteoric origin derived from a large terrestrial biosphere. Here we test the hypothesis that a large terrestrial biosphere existed prior to the Silurian-Devonian land-plant radiation by examining delta C-13 records of subaerial exposure surfaces formed in a shallow-water carbonate platform during the Ordovician-Silurian icehouse. The exposure surfaces include an unconformity at the Ordovician-Silurian boundary with terra rossa and dissolution-collapse breccia, and a lower Silurian quartz sand layer feeding a 50-m-deep system of karst pipes. There is no evidence for delta C-13 depletion beneath either exposure surface. Strontium concentrations in the rocks are low (10-120 ppm) and covary with delta O-18; oxygen isotope ratios, however, do not positively correlate with delta C-13. Our results suggest that there was no significant terrestrial biosphere during Ordovician-Silurian time, and by extension, that Neoproterozoic negative carbon isotope anomalies cannot be explained by meteoric diagenesis.