Discerning primary versus diagenetic signals in carbonate carbon and oxygen isotope records: An example from the Permian–Triassic boundary of Iran
Discerning primary versus diagenetic signals in carbonate carbon and oxygen isotope records: An example from the Permian–Triassic boundary of Iran
复制标题
DOI：
10.1016/j.chemgeo.2015.12.013
复制
发表时间：
2016-03
期刊：
影响因子：
3.9
通讯作者：
Martin Schobben;C. Ullmann;L. Leda;D. Korn;U. Struck;W. Reimold;A. Ghaderi;T. Algeo;C. Korte-C.-Kort
中科院分区：
文献类型：
--
作者：
Martin Schobben;C. Ullmann;L. Leda;D. Korn;U. Struck;W. Reimold;A. Ghaderi;T. Algeo;C. Korte-C.-Kort
二叠纪-三叠纪界线（PTB）的沉积序列以显著的负碳同位素偏移为特征。在世界各地许多地点的化石（如腕足动物）和块状碳酸盐中都发现了这种偏移，通常被认为与全球碳循环扰动有关。氧同位素也显示出跨PTB的负偏移，但由于δ18O更倾向于成岩叠印（特别是在块状碳酸盐中），这些数据通常不用于古环境分析。本文分析了Kuh-e-Ali Bashi和Zal （NW伊朗）PTB序列的体岩、腕足动物δ13C和δ18O，以及牙形石δ18O，以评价主要海相同位素信号的成岩叠印。结果表明，利用配对的C-O同位素和Mn - Sr浓度不足以识别块状物质中的成岩蚀变，因为δ13C -δ18O共变可能是由于环境因素而不是成岩作用，Sr/Ca和Mn/Ca比值可以作为块状岩石岩性的函数而变化。δ13C剖面对比表明，整体碳酸盐均发生了不同程度的改变，但总体上的体岩趋势与腕足动物数据相似，系统偏移为- 1.2（±0.4）‰。这表明块状碳酸盐岩的一级δ13C趋势总体上是稳健的，但小规模碳同位素波动的意义是不确定的，特别是当这种波动与岩性变化有关时。PTB层段在研究剖面中以低碳酸盐“边界粘土”为标志，可能特别容易发生成岩蚀变，例如通过晚期白云化作用。岩石体与保存较好的化石（腕足类和牙形刺类）的氧同位素对比表明，前者的氧同位素偏移值为- 2.1（±0.4）‰。成岩模拟表明，这些偏移主要是埋藏温度~ 50 ~ 80℃、水岩比< 10时早期成岩作用的产物。在早期成岩作用中沉积的自生碳酸盐代表了全球范围内同位素轻碳的潜在主要汇，迄今为止受到的关注相对较少。
Sedimentary successions across the Permian–Triassic boundary (PTB) are marked by a prominent negative carbon isotope excursion. This excursion, found in both fossil (e.g., brachiopod) and bulk carbonate at many sites around the world, is generally considered to be related to a global carbon cycle perturbation. Oxygen isotopes also show a negative excursion across the PTB, but because δ18O is more prone to diagenetic overprint (especially in bulk carbonate), these data are often not used in palaeoenvironmental analyses. In the present study, bulk-rock and brachiopod δ13C and δ18O, as well as conodont δ18O, were analyzed in PTB successions at Kuh-e-Ali Bashi and Zal (NW Iran) in order to evaluate diagenetic overprints on primary marine isotopic signals. The results show that the use of paired C–O isotopes and Mn–Sr concentrations is not sufficient to identify diagenetic alteration in bulk materials, because δ13C–δ18O covariation can be due to environmental factors rather than diagenesis, and Sr/Ca and Mn/Ca ratios can vary as a function of bulk-rock lithology. Comparison of δ13C profiles shows that all bulk carbonate is altered to some degree, although the general bulk-rock trend mimics that of the brachiopod data with a systematic offset of − 1.2(± 0.4)‰. This suggests that the first-order δ13C trend in bulk carbonate is generally robust but that the significance of small-scale carbon isotope fluctuations is uncertain, especially when such fluctuations are linked to lithologic variation. The PTB interval, which is marked by a low-carbonate ‘Boundary Clay’ in the study sections, may be especially prone to diagenetic alteration, e.g., via late-stage dolomitization. Comparison of oxygen-isotope profiles for bulk rock and well-preserved fossils (both brachiopods and conodonts) shows that the former are offset by − 2.1(± 0.4)‰. Diagenetic modeling suggests that these offsets were the product mainly of early diagenesis at burial temperatures of ~ 50–80 °C and water/rock ratios of < 10. Authigenic carbonates precipitated during early diagenesis represent a potentially major sink for isotopically light carbon at a global scale that has received relatively little attention to date.