Active methane release from the early Cambrian seafloor? Clues from Ba isotopes

The Cambrian Explosion coincided with 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 Microscopy, and Energy Dispersive Spectroscopy 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, may have resulted in large methane emissions from the sediment to the atmosphere, generating an early Cambrian greenhouse climate at the time of the Cambrian Explosion. •Ba in lower Cambrian deposits present mainly in both barite and Ba-feldspar.•Barite and Ba-feldspar formed respectively in sulfate-rich and -poor SMTZ porewaters.•Sulfur isotopes suggest a sulfate-depleted condition in early Cambrian deep ocean.•Limited seawater sulfate resulted in a shallow SMTZ in the marine sediment column.•Methane release from sediment contributed to early Cambrian greenhouse climate.