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Assessing the Primary Nature of the Neoproterozoic d13C Record Using Clumped Isotope Thermometry

Abstract

Neoproterozoic large negative carbon isotope excursions remain an enigmatic feature of the long-term carbon isotopic record yet their temporal distribution predating the appearance and diversification of complex animals in the fossil record demands an improved understanding of their origin. There is considerable uncertainty on whether these extreme excursions reflect primary perturbations in marine DIC, or whether they are post-depositional features. A variety of diagenetic processes have been proposed to generate the observed record including mechanisms relating to sea level drawdown, burial diagenesis in the presence of hydrocarbons and precipitation of authigenic carbonates. We examine perplexing aspects of the largest negative excursion on record—the Ediacaran Shuram Excursion—including: (1) co-variation in d13C and d18O carbonate, (2) elevated trace metal signatures, and (3) its global distribution. We combine bulk and micro-scale analysis of isotopic composition using carbonate clumped isotope thermometry and SIMS, and trace metal enrichments using ICP-OES, XANES and electron microprobe measurements. We find there is little evidence for large-scale isotopic re-ordering associated with open system diagenesis. Instead the bulk of the d18Omin trend can be explained by a two-step change caused by 1) a temperature increase and fluid composition change from enriched evaporative fluids to open marine fluids and 2) a mineralogical shift from dolomite to calcite. The excursion is correlated with an increase in fine-grained, poorly-weathered detrital sediment that accounts for the majority of the increase in bulk trace metal enrichment. Additionally, SIMS analysis indicates there is no difference in the d13C of authigenic phases versus primary carbonate phases suggesting the d13C of the fluid was not modified during burial. The results of this study suggest these rocks preserve their primary isotopic character and the extreme depletion reflects a secular change in marine DIC, which supports previous hypotheses that this excursion records a critical event that may have had a role in the appearance of Ediacaran fauna. Together, techniques such as clumped isotope thermometry and micro-scale chemical imaging provide an approach that holds promise for a range of questions relating to carbonates through time.