The Ca and Mg isotope record of the Cryogenian Trezona carbon isotope excursion
The Ca and Mg isotope record of the Cryogenian Trezona carbon isotope excursion
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DOI：
10.1016/j.epsl.2021.117002
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发表时间：
2021-05-28
影响因子：
5.3
通讯作者：
Higgins, John A.
中科院分区：
文献类型：
--
作者：
Ahm, Anne-Sofie C.;Bjerrum, Christian J.;Higgins, John A.
在五个不同的大陆上，在晚寒武纪马里诺冰期（>635 Ma）之前沉积的台地碳酸盐中记录到了碳同位素漂移，代表了16- 18%的碳同位素值变化。基于空间和时间的可重复性，该漂移先前被解释为在下降到雪球地球之前跟踪全球海洋中溶解的无机碳的碳同位素组成。然而，在现代有限的浅海和淡水环境中，碳同位素值也有类似的大范围，这在很大程度上独立于开阔洋化学，而是反映了当地的过程。在这项研究中，我们结合联合收割机的钙，镁，锶同位素地球化学与碳酸盐成岩作用的数值模型，解开的程度，其中的Treatment偏移反映了全球海水化学与当地浅水平台环境的变化。我们的分析表明，最极端的碳同位素值（类似于千分之-10对千分之+10）保存在前平台文石，是新形态的方解石在沉积物缓冲条件下，并记录了主要的碳同位素组成的平台顶部表面沃茨。与此相反，在早期流体缓冲成岩作用期间发生改变的碳酸盐岩中记录了Treatment偏移的下降和恢复，并且通常被石化。我们还发现，最低点的Treatrium偏移与硅质沉积物的部分增加，而从偏移的恢复与碳酸盐的相对增加。这种关系表明，极端负同位素的平台文石发生在一致的时期，增加输入的silicicorbital沉积物，水深的变化，并可能营养平台环境。虽然在新元古代台地碳酸盐岩中产生极负碳同位素值的过程仍然是个谜，我们推测这些偏移反映了在强烈的初级生产力时期，与CO2侵入台地沃茨相关的动力学同位素效应。(C)2021年，任作者。由爱思唯尔公司出版
The Trezona carbon isotope excursion is recorded on five different continents in platform carbonates deposited prior to the end-Cryogenian Marinoan glaciation (>635 Ma) and represents a change in carbon isotope values of 16-18%. Based on the spatial and temporal reproducibility, the excursion previously has been interpreted as tracking the carbon isotopic composition of dissolved inorganic carbon in the global ocean before the descent into a snowball Earth. However, in modern restricted shallow marine and freshwater settings, carbon isotope values have a similarly large range, which is mostly independent from open ocean chemistry and instead reflects local processes. In this study, we combine calcium, magnesium, and strontium isotope geochemistry with a numerical model of carbonate diagenesis to disentangle the degree to which the Trezona excursion reflects changes in global seawater chemistry versus local shallow-water platform environments. Our analysis demonstrates that the most extreme carbon isotope values (similar to-10 parts per thousand versus +10 parts per thousand) are preserved in former platform aragonite that was neomorphosed to calcite during sediment-buffered conditions and record the primary carbon isotope composition of platform-top surface waters. In contrast, the downturn and recovery of the Trezona excursion are recorded in carbonates that were altered during early fluid-buffered diagenesis and commonly are dolomitized. We also find that the nadir of the Trezona excursion is associated with a fractional increase in siliciclastic sediments, whereas the recovery from the excursion correlates with a relative increase in carbonate. This relationship suggests that the extreme negative isotopic shift in platform aragonite occurred in concert with periods of increased input of siliciclastic sediments, changes in water depth, and possibly nutrients to platform environments. Although the process for generating extremely negative carbon isotope values in Neoproterozoic platform carbonates remains enigmatic, we speculate that these excursions reflect kinetic isotope effects associated with CO2 invasion in platform waters during periods of intense primary productivity. (C) 2021 The Author(s). Published by Elsevier B.V.