Active methane release in the early Cambrian seafloor? Clues from Ba isotope

Abstract

The Cambrian Explosion is characterized by a global greenhouse climate, but the mechanism behind this warming event remains unclear, impeding our understanding of their essential link. To address this, an integrated study of δ138Ba, δ34S, Scanning Electron Micrograph, and Energy Dispersive Spectrum was carried out on the lower Cambrian shales from a range of water depths in the Nanhua Basin of South China. These shales are characterized by variably elevated Ba concentrations (>10,000 ppm), primarily hosted by barite and Ba-feldspar, with the latter found only in a deep-water setting. The Ba-isotopic compositions of the study shales (δ138Ba, −0.15‰ to +0.60‰), along with the large size and euhedral shape of barite crystals and associated soft-sediment deformation, indicate that barite and Ba-feldspar were formed in porewaters close to the sulfate-methane-transition zone during early diagenesis under sulfate-rich and sulfate-poor conditions respectively. We propose that low sulfate in the early Cambrian ocean, coupled with the high organic matter content of the sediment, might result in large methane emissions from the sediment to the atmosphere and in turn, an early Cambrian greenhouse climate for the Cambrian Explosion.