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

Abstract

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.

Evolution of Neoproterozoic Wonoka–Shuram Anomaly-aged carbonates: Evidence from clumped isotope paleothermometry

“…Similar arguments as those presented in another publication (Bristow et al, 2011) are relevant to this study and indicate that end-member mixing probably did not overprint an equilibrium clumped isotope signature in some Neoproterozoic successions. Mixing can be discounted as it would require end-member solutions or carbonate minerals with extremely exotic bulk isotope compositions (much greater than ±20‰) and there is no evidence for such exotic carbonate minerals or former fluids in these Neoproterozoic successions.…”

“…Clumped isotope-derived temperatures can be combined with ␦ 18 O carb to uniquely determine ␦ 18 O w through application of an appropriate carbonate-water oxygen isotope fractionation factor (Kim and O'Neil, 1997;Vasconcelos et al, 2005). The tool has been used for a range of applications including the study of primary and diagenetic processes in terrestrial and marine samples of a wide range of ages (Came et al, 2007;Affek et al, 2008;Dennis and Schrag, 2010;Eagle et al, 2010Eagle et al, , 2011Eagle et al, , 2013Passey et al, 2010;Tripati et al, 2010Tripati et al, , 2014Bristow et al, 2011;Ferry et al, 2011;Finnegan et al, 2011;Huntington et al, 2011;Keating-Bitonti et al, 2011;Loyd et al, 2012aLoyd et al, , 2013aLoyd et al, , 2014Passey and Henkes, 2012;Swanson et al, 2012;Dale et al, 2014).…”

“…These 47 values were converted to temperature using the same procedure as a previous study (Bristow et al, 2011) by applying an acid digestion fractionation factor to account for digestion at 90 • C and using theoretical calibrations . Published empirical calibrations extend from 1 to 65 • C (Ghosh et al, 2006;Dennis et al, 2011;Zaarur et al, 2013;Tang et al, 2014), and application to these samples would require a large extrapolation well beyond this temperature range.…”

Extensive, uplift-related and non-fault-controlled spar precipitation in the Permian Capitan Formation

“…Vital effects can be discounted because the analyzed carbonates are abiotic in origin. CO 2 degassing and end-member mixing are unlikely to be influencing the Δ 47 data based on the arguments of Bristow et al (2012), as carbonates experiencing CO 2 degassing should exhibit elevated δ 13 C carb (Fantidis and Ehhalt, 1970;Hendy, 1971;Affek et al, 2008). If anything, all carbonates examined here exhibit low δ 13 C carb compositions.…”

“…In addition, it is difficult to imagine significant CO 2 degassing particularly because these spars formed at depth, isolated from the atmosphere (or cave systems). Mixing can also be discounted as it would require end-member solutions or carbonate minerals with extremely exotic bulk isotope compositions [much greater/less than ±20‰ (Bristow et al, 2012)] and there is no evidence for such exotic carbonate minerals or former fluids in the Capitan or other units in close spatial association.…”