A hydrothermal origin for isotopically anomalous cap dolostone cements from south China

No way out for snowball Earth Earth's emergence from perhaps its most severe ice age, the Marinoan 'snowball' glaciation around 635 million years ago, is thought to be linked with a massive release of trapped methane — largely based on evidence of a characteristic isotopic signature in calcite deposits in rock layers formed at the time. Bristow et al . propose that, instead, these calcites were formed by hydrothermal fluids at least 1.6 million years after the deposition of the surrounding strata, and that their signature derives from the thermogenic oxidation of hydrothermal methane from elsewhere. This withdraws a key line of evidence underpinning our understanding of how temperate conditions resumed following this extreme glaciation. The release of methane into the atmosphere through destabilization of clathrates is a positive feedback mechanism capable of amplifying global warming trends that may have operated several times in the geological past 1 , 2 , 3 . Such methane release is a hypothesized cause or amplifier for one of the most drastic global warming events in Earth history, the end of the Marinoan ‘snowball Earth’ ice age, ∼635 Myr ago 4 , 5 , 6 , 7 . A key piece of evidence supporting this hypothesis is the occurrence of exceptionally depleted carbon isotope signatures (δ 13 C PDB down to −48‰; ref. 8 ) in post-glacial cap dolostones (that is, dolostone overlying glacial deposits) from south China; these signatures have been interpreted as products of methane oxidation at the time of deposition 5 , 6 , 8 . Here we show, on the basis of carbonate clumped isotope thermometry, 87 Sr/ 86 Sr isotope ratios, trace element content and clay mineral evidence, that carbonates bearing the 13 C-depleted signatures crystallized more than 1.6 Myr after deposition of the cap dolostone. Our results indicate that highly 13 C-depleted carbonate cements grew from hydrothermal fluids and suggest that their carbon isotope signatures are a consequence of thermogenic methane oxidation at depth. This finding not only negates carbon isotope evidence for methane release during Marinoan deglaciation in south China, but also eliminates the only known occurrence of a Precambrian sedimentary carbonate with highly 13 C-depleted signatures related to methane oxidation in a seep environment. We propose that the capacity to form highly 13 C-depleted seep carbonates, through biogenic anaeorobic oxidation of methane using sulphate, was limited in the Precambrian period by low su