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.
Detailed information on stable isotope measurement methods, descriptions of the samples, and carbon and oxygen isotope data are available at www.geolsoc.org.uk/SUP18754.