DO AUTHIGENIC CARBONATE CEMENTS EXPLAIN THE NEOPROTEROZOIC (AND PERMIAN) CARBON ISOTOPE RECORDS?

In 1996, Richard Bambach and his colleagues speculated that large magnitude negative carbon isotope shifts and anomalous carbonate precipitation in both the Neoproterozoic and the Late Permian intervals were associated with the overturn of anoxic deep oceans and the introduction of CO₂ into surface environments. While this model provided a plausible kill mechanism for Late Permian marine and terrestrial organisms, there was no apparent connection of the isotope anomaly with continental glaciation as there was for the Neoproterozoic. Subsequent models based on sulfur isotope, iron speciation, and biomarker data indicate that the spread of oceanic H₂S into the photic zone resulted in Late Permian mass mortality, and new data from the terminal Ediacaran Period support the view that the earliest animals may have dealt with similar environmental constraints. The most profound negative carbon cycle anomaly in the Neoproterozoic was the Shuram event. Analogous to the Late Permian event, the Shuram was preceded by a global sea level fall and accompanied by coupled depletions in ¹³C and ³⁴S and a dramatic increase in ⁸⁷Sr/⁸⁶Sr in seawater proxies. Based on these observations, it is plausible that in both intervals widespread eutrophication of shallow seawater was driven by the enhanced delivery of nutrients and sulfate through continental weathering, resulting in the accumulation of phosphorite. In the phosphatic middle Ediacaran Doushantuo Formation of South China, we have uncovered carbon and strontium isotope evidence for the presence of methane-derived authigenic calcite cements closely associated with the Shuram anomaly. Carbon isotope compositions of these pervasive cements are as low as -35‰; their strontium isotope values are consistent with well-preserved bedded limestone lower in the sedimentary succession -- arguing against a hydrothermal origin for the cements. If correct, this may represent the oldest methane cold seep environment on Earth. We speculate that for both the Shuram and the Late Permian events anaerobic oxidation of methane by sulfate reducing bacteria resulted in the production of ¹³C depleted authigenic cements, and caused the spread of shallow water euxinic conditions and the demise of animals unable to cope with the toxin.