Questioning the role of methane in the wake of a snowball Earth: Insights from isotopically anomalous cap dolostone cements with a complex diagenetic history

Abstract

Methane has been widely regarded as an important source of greenhouse gas that modulated Earth’s paleoclimate. Notably, it has been proposed that a massive release of methane via clathrate destabilization may have played a critical role during and/or in the immediate aftermath of the Marinoan deglaciation. One key piece of supporting evidence for this hypothesis comes from the isotopically anomalous (δ13Ccarb < –30 ‰) methane-derived authigenic calcite (MDAC) cements within post-Marinoan cap dolostones in South China. However, the origin of these MDAC cements remains hotly contested, including a clumped isotope study that reinterpreted them as resulting from late hydrothermal fluids. To further evaluate these critical yet controversial cements, a detailed investigation by secondary ion mass spectrometry (SIMS) was conducted. Here we report the largest range of δ13Ccarb values (–53.1 ‰ to +6.3 ‰) yet measured in Precambrian carbonates. Our study shows that the MDAC cements are post-depositional, void-filling, and rich in Mn. Importantly, integrated petrographic and SIMS results consistently show that MDAC cements post-date disrupted dolomite laminae that bear surprisingly positive δ13Ccarbvalues up to +6.3 ‰. This is the first report that authigenic carbonates with positive δ13Ccarb signals are found within post-Marinoan cap dolostone. Although the precise age of these authigenic carbonates remains ambiguous, integrated chemostratigraphic and SEM-SIMS results suggest that the dolomite laminae formed after the deposition of the uppermost cap dolostone interval, when the seawater δ13C had already evolved to positive values; and the MDAC formed even later. The dolomite laminae and MDAC cements, therefore, represent distinct post-depositional, exogenous, diagenetic carbon signals that are irrelevant to the deglaciation. Our new results challenge the hypothesis that methane played a central role at the end of or in the immediate aftermath of the Marinoan glaciation. Rather, the infiltration of methane-bearing fluids within cap dolostones could have occurred much later.