δ13C values of carbonate nodules across the Permian–Triassic boundary in the Karoo Supergroup (South Africa) reflect a stinking sulfurous swamp, not atmospheric CO 2

Abstract

Paleosol macromorphology, petrography and stable isotope geochemistry of carbonate nodules from Permo–Triassic (P–T) boundary strata of the central Karoo basin, South Africa are presented in order to evaluate their utility as geochemical archives of Permian–Triassic paleoatmospheric chemistry and their potential to provide a terrestrial carbon isotope chemostratigraphy for correlation with contemporaneous global marine δ 13C records. Paleosol morphologies across P–T boundary strata of the central Karoo basin indicate the region was a poorly drained, seasonally to continuously flooded bottomland. Radiaxial calcite, microspar and micrite microtextures are observed in the calcite nodules associated with P–T paleosol profiles. Micritic calcite is typical of soil carbonate that forms in well-drained soils characterized by open-system gaseous diffusion between the soil and global atmosphere, whereas more coarsely crystalline textures and radiaxial calcite are more typical of crystallization in phreatic environments with limited gaseous diffusion between the soil and global atmosphere. In this regard, only those nodules composed of micritic calcite can be considered as a potentially reliable source of proxy information for paleoatmospheric δ 13C values across the P–T boundary. The δ 13C values of all carbonate nodules from two overlapping measured sections near Carlton Heights, South Africa range from − 24.4‰ to − 1.8‰, whereas micritic calcite nodule δ 13C values range from − 7.0‰ to − 21.3‰. Comparison of calcite δ 13C values with penecontemporaneous organic matter δ 13C values indicate that calcite δ 13C values < − 10.6‰ cannot provide a record of atmospheric δ 13C values. Rather, these more negative carbonate nodule δ 13C values likely reflect calcite crystallization under poorly drained, swampy conditions characterized by a semi-closed chemical system that evolved independently of Earth's troposphere. Therefore, previously reported negative carbon isotope shift(s) measured from carbonate nodules in the Karoo basin P–T strata do not provide a record of atmospheric δ 13C, and probably do not provide a viable carbon isotope stratigraphy that can be realistically correlated with global P–T marine carbon isotope stratigraphies.