Carbon isotopic evidence for organic matter oxidation in soils of the Old Red Sandstone (Silurian to Devonian, South Wales, UK)

Petrographic and calcrete carbon isotope data from seasonally waterlogged Upper Silurian (Přídolí) to Lower Devonian (Pragian) palaeo-Vertisols of the Old Red Sandstone, South Wales, UK, are presented. The δ¹³C values mostly range from -9 to -12‰ (VPDB), suggesting that the soils were inhabited by abundant vegetation that when oxidized (perhaps with microbial assistance) resulted in CO₂-rich soils. Such soils would favour calcrete precipitation through equilibration of soil zone CO₂ with the relatively lower atmospheric pCO₂. However, reliably estimating palaeoatmospheric pCO₂ using these carbon isotope data is a challenge.