Raman geothermometry of carbonaceous material in the basal Ediacaran Doushantuo cap dolostone: The thermal history of extremely negative δ13C signatures in the aftermath of the terminal Cryogenian snowball Earth glaciation

Abstract

Extremely negative δ13C values (as low as −48‰) of blocky calcite cements filling cavities, sheet-cracks, and fractures in the basal Ediacaran cap dolostone of the Doushantuo Formation in South China have been considered as evidence for methane release possibly associated with gas hydrate destabilization in the aftermath of the terminal Cryogenian snowball Earth glaciation. Alternatively, these extremely negative δ13C values may have resulted from oxidation of thermogenic methane derived from localized hydrothermal activity long after the deposition of the Doushantuo cap dolostone. The latter interpretation was based on carbonate clumped isotope thermometry suggesting that the precipitation of extremely 13C-depleted calcite occurred at temperatures up to 476°C (mean=378°C, n=4) whereas carbonate fabrics with moderately negative δ13C values (generally>−10‰) were formed at much lower temperatures (mean=112°C, n=4). To assess these competing hypotheses, we systematically investigated the petrography, carbonate carbon and oxygen isotopes, and carbonaceous material (CM) Raman spectroscopy of the Doushantuo cap carbonate. Raman geothermometry suggests that CM associated with different microfabrics and δ13C signatures—including sedimentary fabrics with moderately negative δ13C values (e.g., peloids, stromatolite laminae, microbial mat fragments), diagenetic fabrics with moderately negative δ13C values (e.g., some void-filling or crack-filling cements), and diagenetic fabrics with extremely negative δ13C values (e.g., void-filling or crack-filling blocky and bladed calcites)—experienced temperatures generally <300°C (largely 200–300°C). Contrary to temperature estimates based on carbonate clumped isotopes, CM Raman geothermometry reveals that different microfabrics show largely overlapping paleotemperatures and that there is no apparent relationship between δ13C signatures and paleotemperatures. Whereas the origins of these extremely negative δ13C values remain unresolved, the Raman geothermometric data challenge the previous interpretation that they were derived from thermogenic methane related to localized hydrothermal activity at temperatures up to almost 500°C that only affected the thermal history of the calcite with extremely negative δ13C values. Instead, the new data suggest that the analyzed microfabrics experienced the same peak temperatures <300°C, probably related to the burial history of the host rocks or pervasive hydrothermal activity that affected all microfabrics in the Doushantuo cap dolostone.