Carbon isotopes in continental weathering environments and variations in ancient atmospheric CO 2 pressure

Abstract

Abundance and carbon isotope data from an Fe(CO 3)OH component in apparent solid solution in oolitic goethites have been used to infer ancient atmospheric CO 2 pressures. A test of the validity of these estimates might be comparisons of the carbon isotope compositions of Fe(CO 3)OH in oolitic goethites with time-equivalent pedogenic calcites. Temporal trends of the oolitic goethite and pedogenic calcite δ 13C values are generally similar, but time-equivalent samples from each of these two groups are not common in the existing data. To facilitate discussion of the concept, comparisons were made of available goethite and calcite samples even though ages of the compared samples in each pair were not identical. In four out of the five comparisons, Fe(CO 3)OH abundance and δ 13C data were combined with pedogenic calcite δ 13C data to calculate physically reasonable soil CO 2 concentrations for the ancient calcitic soids. This suggests that the compared oolitic goethite and pedogenic calcite systems were responding to the same global scale phenomenon (i.e., atmospheric CO 2). Atmospheric P CO 2 as determined from the goethites in these four “well-behaved” cases ranged from values indistinguishable from modern (within analytical uncertainty) to values up to approximately 16 time modern (modern atmospheric P CO 2 was taken to be 10 −3.5 atm). One interpretation of the fifth, “anomalous”, comparison is that atmospheric CO 2 levels increased from about 3 times modern to about 18 times modern from the Triassic into the Early Jurassic. This inferred value for the P CO 2 of the Early Jurassic atmosphere is not uniquely constrained by the existing data and needs to be substantiated. However, even considerably lower Early Jurassic atmospheric P CO 2 values of 6 to 9 times modern (i.e., 1/3 to 1/2 of the estimated value of 18 times modern) would still indicate significant differences between the global carbon cycles then and now. These results highlight the need for more research on the behavior of the atmosphere during and after the Triassic-Jurassic transition.