δ13C depth profiles from paleosols across the Permian-Triassic boundary: Evidence for methane release

Abstract

Stable carbon isotopic analyses of organic carbon (δ 13 C) in individual paleosol profiles from Permian–Triassic sequences of Antarctica reveal systematic isotopic variations with profile depth. These variations are in many cases analogous to those in modern soils, which are functions of redox conditions, soil development, and degree and type of microbial decay. In modern soils, these isotopic depth functions develop independently from vegetation changes (C 3 versus C 4 vegetation) and can be diagnostic of soil orders. This study shows that soil-intrinsic functions can be preserved in the δ 13 C values of paleosols as old as 260 Ma and constitute valuable data for paleoecological interpretations. A large carbon isotopic offset of as much as 10‰ in whole paleosol profiles across the Permian-Triassic boundary indicates significant changes in the soil biogeochemistry and the soil-atmosphere system. Early Triassic paleosols are distinctive in their extremely low δ 13 C values (to −42‰) and often show an anomalous δ 13 C depth distribution compared to both Permian paleosols and modern soils. Highly depleted δ 13 C values, as the ones in Early Triassic paleosols, are suggested to be associated with microbial methane oxidation (methanotrophy). This hypothesis implies increased methane concentrations in the Early Triassic soil-atmosphere system. Increased atmospheric methane was probably partly responsible for the global carbon isotopic shift documented in marine and terrestrial sediments across the Permian–Triassic boundary.