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Ice or fire? Constraining the origin of isotopically anomalous cap carbonate cements by integrated μXRF, SEM, and SIMS


Huan Cui
AMGC, Vrije Universiteit Brussel


Hypothesis: It was hypothesized that the Marinoan glaciation (i.e., Snowball Earth) was terminated by the massive release of methane through clathrate destabilization at ~635 Ma. A key piece of evidence supporting this hypothesis is the finding of methane-derived (δ13Ccarb values of microdrilled carbonates as low as –48‰) authigenic carbonate cements (MDACCs) from the Marinoan cap carbonates in the basal Doushantuo Formation, South China. However, a more recent study based on clumped isotope analysis proposes that these MDACCs are hydrothermal (T as high as 476 °C) in origin.

Results and Discussion: To further constrain the controversial origins of these critical MDACCs, we conducted a detailed investigation via integrated μXRF, SEM and SIMS. We found a 60‰ range of δ13Ccarb values in the samples with positive δ13Ccarb values as high as +6.3‰ exclusively in coarse dolomite crystals and negative δ13Ccarb values as low as –53.8‰ in MDACCs. Both the positive δ13Ccarb values and the lowest δ13Ccarb values are revealed in this study for the first time. The combined μXRF and SEM analyses show that the low-δ13C MDACCs are Mn-rich and replace—therefore postdate—the isotopically heavy and Mn-poor dolomite crystals. The dolomites are typically euhedral and anomalously large (up to 200 μm), with dolomite cores partly or almost completely replaced by low-δ13C calcite. Thus, the high-δ13C dolomites formed during very late burial diagenesis and the MDACCs formed later replacing the pre-existing dolomite.

Conclusion: We conclude that the MDACCs are post-depositional in origin and formed in the Marinoan cap carbonates during very late diagenesis. In light of this study, we argue that the MDACCs cannot be used to infer the occurrence of methane clathrate destabilization for the termination of the Marinoan glaciation. The role of methane hydrates as a main driver in terminating the Marinoan glaciation should be re-assessed.