An authigenic response to Ediacaran surface oxidation: Remarkable micron-scale isotopic heterogeneity revealed by SIMS
An authigenic response to Ediacaran surface oxidation: Remarkable micron-scale isotopic heterogeneity revealed by SIMS
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DOI：
10.1016/j.precamres.2022.106676
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发表时间：
2022-05-31
影响因子：
3.8
通讯作者：
Valley, John W.
中科院分区：
文献类型：
--
作者：
Cui, Huan;Kitajima, Kouki;Valley, John W.
Ediacaran Shuram偏移（SE）记录了全球碳酸盐碳同位素（δ 13 Ccarb）值从+6%o下降到约60%。-10%o，代表地球历史上最大的δ 13 Ccarb负异常。华南陡山沱组上部广泛记录了SE，但在相关剖面中，13 Ccarb三角洲剖面变化较大。这种不一致的表达的SE挑战了传统的观点，均匀的海洋溶解无机碳（DIC）水库。一个潜在的过程，可以解释三角洲13 Ccarb的变化是当地矿化的同位素不同的自生碳酸盐沉积物-水界面附近的早期成岩作用。然而，这种自生碳酸盐的直接测试仍然有限。在这里，继最近的一项研究在大陆架内环境中的SE，我们重新访问了外大陆架部分，确定和分析微米级，同沉积自生方解石水泥通过集成阴极发光（CL），微X射线荧光（亩XRF），扫描电子显微镜（SEM），能量色散谱（EDS），和二次离子质谱（西姆斯）。我们新的西姆斯结果揭示了自生方解石胶结物中δ 13 Ccarb的显著的微米级不均匀性，包括低至-37.5%o（VPDB）的极负值。我们解释这些方解石胶结物甲烷衍生自生方解石（MDAC）的微生物硫酸盐还原（MSR）和甲烷厌氧氧化（AOM）在早期成岩作用。基于新的研究结果，我们提出，在中国南方的非均质SE-表现在微米，厘米和盆地尺度-调制的甲烷氧化在可变的局部氧化还原和水深条件下。因此，SE在个别盆地中与不同程度的甲烷氧化相耦合，并且在大气氧化事件期间由增强的海水硫酸盐在全球范围内触发。根据这项研究，在SE过程中甲烷氧化的氧化还原变化的潜在作用可能被低估了。我们的研究表明，综合SIMS-SEM分析可以区分不同世代的同位素不同的碳酸盐，否则无法检测到的常规分析，因此是一种有效的方法来评估三角洲13 Ccarb异常的沉积记录的起源和成岩历史。
The Ediacaran Shuram excursion (SE) records a global decrease in carbonate carbon isotope (delta 13Ccarb) values from +6%o down to ca. -10%o, representing the largest delta 13Ccarb negative anomaly in Earth history. While the SE is widely recorded in the upper Doushantuo Formation of South China, it shows highly variable delta 13Ccarb 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 delta 13Ccarb 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 mu m-scale, syn-depositional authigenic calcite cements via integrated cathodoluminescence (CL), micro-X-ray fluorescence (mu XRF), scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), and secondary ion mass spectrometry (SIMS). Our new SIMS results reveal remarkable micron-scale heterogeneity of delta 13Ccarb in authigenic calcite cements, including extremely negative values down to -37.5%o (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 delta 13Ccarb anomalies in the sedimentary record.