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

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. 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. 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. 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 sulphate concentrations in sea water. As a consequence, although clathrate destabilization may or may not have had a role in the exit from the 'snowball' state, it would not have left extreme carbon isotope signals in cap dolostones.

Mineralogical constraints on the paleoenvironments of the Ediacaran Doushantuo Formation.

Assemblages of clay minerals are routinely used as proxies for paleoclimatic change and paleoenvironmental conditions in Phanerozoic rocks. However, this tool is rarely applied in older sedimentary units. In this paper, the clay mineralogy of the Doushantuo Formation in South China is documented, providing constraints on depositional conditions of the Ediacaran Yangtze platform that host the earliest animal fossils in the geological record. In multiple sections from the Yangtze Gorges area, trioctahedral smectite (saponite) and its diagenetic products (mixed-layer chlorite/smectite, corrensite, and chlorite) are the dominant clays through the lower 80 m of the formation and constitute up to 30 wt% of the bulk rock. Saponite is interpreted as an in situ early diagenetic phase that formed in alkaline conditions (pH > or = 9). The absence of saponite in stratigraphically equivalent basin sections, 200-400 km to the south, indicates that alkaline conditions were localized in a nonmarine basin near the Yangtze Gorges region. This interpretation is consistent with crustal abundances of redox-sensitive trace elements in saponitic mudstones deposited under anoxic conditions, as well as a 10 per thousand difference in the carbon isotope record between Yangtze Gorges and basin sections. Our findings suggest that nonmarine environments may have been hospitable for the fauna preserved in the Yangtze Gorges, which includes the oldest examples of animal embryo fossils and acanthomorphic acritarchs.