An authigenic response to Ediacaran surface oxidation: remarkable micron-scale isotopic heterogeneity revealed by SIMS
Cui, H.; Kitajima, K.; Orland, I.J.; Baele, J.-M.; Xiao, S.; Kaufman, A.J.; Denny, A.; Spicuzza, M.J.; Fournelle, J.H.; Valley, J.W. (2022). An authigenic response to Ediacaran surface oxidation: remarkable micron-scale isotopic heterogeneity revealed by SIMS. Precambr. Res. 377: 106676. https://dx.doi.org/10.1016/j.precamres.2022.106676
In: Precambrian research. ELSEVIER SCIENCE BV: Amsterdam; London; New York; Oxford; Tokyo. ISSN 0301-9268; e-ISSN 1872-7433, more
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| Author keywords |
Shuram excursion; Secondary ion mass spectrometry; Microbial sulfate reduction; Anaerobic oxidation of methane; Carbon isotopes; Authigenic carbonate; Deep-time carbon cycle |
| Authors | | Top |
- Cui, H., more
- Kitajima, K.
- Orland, I.J.
- Baele, J.-M., more
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- Xiao, S.
- Kaufman, A.J.
- Denny, A.
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- Spicuzza, M.J.
- Fournelle, J.H.
- Valley, J.W.
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| Abstract |
The Ediacaran Shuram excursion (SE) records a global decrease in carbonate carbon isotope (δ13Ccarb) values from +6‰ down to ca. –10‰, representing the largest δ13Ccarb negative anomaly in Earth history. While the SE is widely recorded in the upper Doushantuo Formation of South China, it shows highly variable δ13Ccarb profiles among correlative sections. This inconsistent expression of the SE challenges the conventional view of a homogeneous marine dissolved inorganic carbon (DIC) reservoir. A potential process that could explain δ13Ccarb variability is local mineralization of isotopically distinct authigenic carbonates near the sediment–water interface during early diagenesis. However, a direct test of such authigenic carbonates is still limited. Here, following a recent study on the SE in an intra-shelf environment, we revisited an outer-shelf section, identified and analyzed μm-scale, syn-depositional authigenic calcite cements via integrated cathodoluminescence (CL), micro-X-ray fluorescence (μXRF), scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), and secondary ion mass spectrometry (SIMS). Our new SIMS results reveal remarkable micron-scale heterogeneity of δ13Ccarb in authigenic calcite cements, including extremely negative values down to –37.5‰ (VPDB). We interpret these calcite cements as methane-derived authigenic calcite (MDAC) resulting from microbial sulfate reduction (MSR) and anaerobic oxidation of methane (AOM) during early diagenesis. Based on the new results, we propose that the heterogeneous SE in South China — manifest on micrometer, centimeter, and basinal scales — was modulated by methane oxidation under variable local redox and water depth conditions. The SE, therefore, was coupled with different degrees of methane oxidation in individual basins, and globally triggered by enhanced seawater sulfate during an atmospheric oxygenation event. In light of this study, the potential role of redox variability in methane oxidation during the SE may have been underestimated. Our study demonstrates that integrated SIMS-SEM analysis can distinguish different generations of isotopically distinct carbonates otherwise undetected by conventional analysis, and is thus an effective approach to assess the origin and diagenetic history of δ13Ccarb anomalies in the sedimentary record. |
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