Uncovering the spatial heterogeneity of Ediacaran carbon cycling
Uncovering the spatial heterogeneity of Ediacaran carbon cycling
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DOI：
10.1111/gbi.12222
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发表时间：
2017-03
期刊：
影响因子：
3.7
通讯作者：
C. Li;D. Hardisty;G. Luo;J. Huang;T. Algeo;Cheng Meng;W. Shi;Z. An;J. Tong;S. Xie;
中科院分区：
文献类型：
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作者：
C. Li;D. Hardisty;G. Luo;J. Huang;T. Algeo;Cheng Meng;W. Shi;Z. An;J. Tong;S. Xie;
埃迪卡拉纪碳循环（6.35 - 5.41亿年前）的记录包括舒兰偏移（SE），这是地球历史上最大的负碳酸盐碳同位素偏移（低至-12 ‰）。由于难以解释海水溶解无机碳δ 13 C的全球极端下降，这种偏移的性质仍然是谜。本文通过对华南扬子地台埃迪卡拉纪陡山沱组沿着近端-远端剖面的碳酸盐和有机碳同位素（δ 13 Ccarb和δ 13 Corg）记录的研究，验证了SE的空间变化。与预期相反，我们的研究结果表明，这种偏移的幅度和形态及其与共存的δ 13 Corg的关系在整个平台上是高度异质的。综合的地球化学、矿物学、岩相学和地层学证据表明，SE是一个主要的海洋特征。数据汇编表明，SE还伴随着全球范围内碳酸盐相关硫酸盐（CAS）δ 34 S的平行负移以及87 Sr/86 Sr比值和沿海CAS浓度的增加，表明SE期间大陆风化和沿海海洋硫酸盐浓度升高。根据这些观测结果，我们提出了一个非均质氧化模型来解释陡山沱的SE和共存的δ 13 Corg记录的高度空间异质性，并可能与其他地区的SE相关。在这个模型中，我们推断通过SE持续的海洋氧化还原分层，但氧化剂的可用性增加（例如，O2和硫酸盐）仅限于边缘近地表海洋环境。有限的时空范围内的氧化提供了一种机制，以驱动非均质氧化的地下还原碳主要在大陆架地区。无论驱动SE的机制如何，未来的模型必须考虑本研究中提出的δ 13 C空间异质性的证据。
Records of the Ediacaran carbon cycle (635–541 million years ago) include the Shuram excursion (SE), the largest negative carbonate carbon isotope excursion in Earth history (down to −12‰). The nature of this excursion remains enigmatic given the difficulties of interpreting a perceived extreme global decrease in the δ13C of seawater dissolved inorganic carbon. Here, we present carbonate and organic carbon isotope (δ13Ccarb and δ13Corg) records from the Ediacaran Doushantuo Formation along a proximal‐to‐distal transect across the Yangtze Platform of South China as a test of the spatial variation of the SE. Contrary to expectations, our results show that the magnitude and morphology of this excursion and its relationship with coexisting δ13Corg are highly heterogeneous across the platform. Integrated geochemical, mineralogical, petrographic, and stratigraphic evidence indicates that the SE is a primary marine signature. Data compilations demonstrate that the SE was also accompanied globally by parallel negative shifts of δ34S of carbonate‐associated sulfate (CAS) and increased 87Sr/86Sr ratio and coastal CAS concentration, suggesting elevated continental weathering and coastal marine sulfate concentration during the SE. In light of these observations, we propose a heterogeneous oxidation model to explain the high spatial heterogeneity of the SE and coexisting δ13Corg records of the Doushantuo, with likely relevance to the SE in other regions. In this model, we infer continued marine redox stratification through the SE but with increased availability of oxidants (e.g., O2 and sulfate) limited to marginal near‐surface marine environments. Oxidation of limited spatiotemporal extent provides a mechanism to drive heterogeneous oxidation of subsurface reduced carbon mostly in shelf areas. Regardless of the mechanism driving the SE, future models must consider the evidence for spatial heterogeneity in δ13C presented in this study.