Large isotopic variability at the micron-scale in ‘Shuram’ excursion carbonates from South Australia
Large isotopic variability at the micron-scale in ‘Shuram’ excursion carbonates from South Australia
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DOI：
10.1016/j.epsl.2020.116211
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发表时间：
2020-05
影响因子：
5.3
通讯作者：
J. Husson;B. Linzmeier;K. Kitajima;A. Ishida;A. Maloof;B. Schoene;S. Peters;J. Valley
中科院分区：
文献类型：
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作者：
J. Husson;B. Linzmeier;K. Kitajima;A. Ishida;A. Maloof;B. Schoene;S. Peters;J. Valley
埃迪卡拉纪（635 ~ 5.41亿年前）海相沉积物δ 13c负偏移较大，其中碳酸盐δ 13c值达到- 12‰（VPDB）。被称为“舒拉姆”短途旅行，许多工作者将这一δ 13c记录解释为对全球碳循环的前所未有的扰动，导致人们猜测与动物生命广泛同期上升的因果关系。其他人将δ 13c信号解释为成岩作用的产物，从而将其与理解后生动物进化的相关性降到最低。在这里，我们展示了扫描电镜成像和二次离子质谱（SIMS）测量的原位δ 13c和δ 18o值，以评估南澳大利亚Wonoka组的这些相互竞争的假设。偏移最小值的结果表明，圆形方解石沉积颗粒的δ 13c值在−12.8 ~−10.6‰之间，δ 18o值在−17.8 ~−15.5‰之间（VPDB）。自面状白云岩似乎在开放的沉积孔隙空间中畅通无阻地生长。这些早期白云岩为早自生白云岩，其δ 13c值可达+ 5‰，要求地层流体δ 13c组成与盆地水或大块沉积物有很大不同。总之，这些结果为晚期成岩叠印在Wonoka产生“Shuram”偏移的假设提供了很少的证据。相反，它们表明在地表环境中存在一个大的碳同位素梯度，浅水能够沉淀具有非常低的δ 13 C（低至- 12‰）的碳酸盐，而较深的陆架和/或海洋孔隙水产生具有正碳同位素值（高达+ 5‰）的碳酸盐。由于在广泛分布的埃迪卡拉盆地中发现了类似程度的负同位素偏移，因此这种梯度很可能是该时期陆架环境的特征，并且一个尚不清楚的全球过程导致了浅水碳酸盐岩的“Shuram”偏移。
Abstract Ediacaran-aged (635–541 million years ago) marine sediments contain a large negative carbon isotope (δ 13 C) excursion, in which carbonate δ 13 C values reach− 12‰(VPDB). Known as the ‘Shuram’excursion, many workers have interpreted this δ 13 C record as an unprecedented perturbation to the global carbon cycle, leading to speculation about a causal connection to the broadly contemporaneous rise of animal life. Others have interpreted the δ 13 C signal as a product of diagenesis, thereby minimizing its relevance for understanding the evolution of metazoans. Here, we present SEM imaging and in-situ δ 13 C and δ 18 O values measured by secondary ion mass spectrometry (SIMS) to assess these competing hypotheses in the Wonoka Formation of South Australia. Our results from the minimum of the excursion show that rounded sedimentary grains of calcite have δ 13 C values between− 12.8 to− 10.6‰ and δ 18 O values between− 17.8 to− 15.5‰(VPDB). Euhedral dolomite that appears to have grown unimpeded in open sedimentary pore spaces also is present. These early-stage dolomites are interpreted as early authigenic in origin and have δ 13 C values that reach+ 5‰, requiring a formation fluid with a substantially different δ 13 C composition from basin waters or bulk sediment. Together, these results provide little evidence for the hypothesis that a late diagenetic overprint has generated the ‘Shuram’excursion in the Wonoka. Instead, they suggest the presence of a large carbon isotopic gradient in the surface environment, with shallow waters capable of precipitating carbonates with very low δ 13 C (down to− 12‰) and deeper shelf and/or marine pore waters generating carbonates with positive carbon isotope values (up to+ 5‰). Because negative isotope excursions of similar magnitude are found in widely dispersed Ediacaran basins, it is likely that this gradient was characteristic of shelf environments of this period and that a still-unknown global process led to the ‘Shuram’excursion in shallow water carbonates.