The use of carbon isotope stratigraphy to construct time lines for stratigraphic correlation requires synchronous changes in carbon isotope ratios (δ13C) to be preserved in carbonate-dominated strata. Such changes are commonly interpreted to reflect primary secular variation in ocean chemistry. However, negative δ13C 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 δ13C records, opening the possibility that the Neoproterozoic δ13C 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 δ13C 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 δ13C depletion beneath either exposure surface. Strontium concentrations in the rocks are low (10–120 ppm) and covary with δ18O; oxygen isotope ratios, however, do not positively correlate with δ13C. 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.