EVIDENCE FROM CATSKILL PEDOGENIC CARBONATES FOR A RAPID LATE DEVONIAN DECREASE IN ATMOSPHERIC CARBON DIOXIDE CONCENTRATIONS

ABSTRACT: Carbon isotopic data from calcitic pedogenic nodules in Catskill redbeds in south-central New York suggest that atmospheric PCO2 in the Middle to Late Devonian was 7 to 16 times that of today, but was decreasing faster than previous data and models have indicated. These data come nodules, nodules with any depositional carbonate, and bedded carbonates were excluded. The mean 813C of the Givetian nodules is-6.61%0 vs. PDB, whereas the mean 813C of the slightly younger Frasnian nodules is-7.930/00 vs. PDB. Application ofCerling's (1992) paleobarometer to these data indicates that atmospheric CO2 concentrations decreased from a range of2900 to 5200 ppmV in the Givetian to 2300 to 3200 ppm V in the Frasnian. These results suggest that atmospheric CO2 dropped rapidly in the Late Devonian, perhaps causing global cooling that led to South American glaciation and that contributed to Late Devonian extinction. The results also suggest that PCO2 changes alone cannot account for the observed temporal variation in the mineralogy of abiotic marine carbonates