Modeling the impacts of diagenesis on carbonate paleoredox proxies
Modeling the impacts of diagenesis on carbonate paleoredox proxies
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DOI：
10.1016/j.gca.2022.09.021
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发表时间：
2022-09
影响因子：
5
通讯作者：
K. Lau;D. Hardisty
中科院分区：
文献类型：
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作者：
K. Lau;D. Hardisty
碳酸盐沉积记录的地球化学对于了解碳和氧化还原敏感元素的古海水演化是非常宝贵的。然而，碳酸盐地球化学古氧化还原代理的应用可能会受到限制，我们的能力，以识别记录的影响，同沉积和沉积后成岩作用与孔隙流体的地球化学成分，这是不同的上覆海水，从可变的氧化还原条件，流体来源，沉积物-与海水缓冲条件。我们扩展了一个数值框架，建立了钙同位素的应用，用于识别沉积物与流体缓冲的变化在文石方解石重结晶广泛应用的碳酸盐古氧化还原代理：碘比（I/Ca），铈异常（Ce/Ce*），和碳酸盐相关的铬，铀，硫同位素。我们模拟端元还原和氧化流体成岩的情况下，而不是将孔隙流体的地球化学演化模型内。结果表明，早期，相对未演化的海水（“海水缓冲”）的流体缓冲成岩作用代表了保存所有代理的理想方案，包括C同位素。相反，上覆水体高度演化的氧化还原条件下孔隙流体中的流体缓冲蚀变可能会改变原生碳酸盐地球化学。模型校准记录从巴哈马斜和Unda核心表明，每个代理是唯一敏感的成岩蚀变的给定风格，与Ce/Ce* 最强大的和I/Ca比最敏感的早期成岩蚀变。我们建议多种策略，包括与C，O和Ca同位素数据的关系，可以利用它来确定给定代理的原始海水地球化学的保存。最后，我们将成岩模型预测与二叠纪/三叠纪边界的已发表记录进行比较，以展示如何将地层地球化学模式与成岩模型结果的解释结合起来，以支持长期海水信号的解释。为了提高我们对成岩作用对碳酸盐岩古氧化还原代理的影响的理解，需要额外的实验约束，基于现场的观察和多代理数据集。
The geochemistry of the carbonate sedimentary record is invaluable for understanding the ancient seawater evolution of carbon and redox-sensitive elements. However, the application of carbonate geochemical paleoredox proxies can be limited by our ability to recognize records impacted by syn- and post-depositional diagenesis with pore fluids that have geochemical compositions which are distinct from the overlying seawater—resulting from variable redox conditions, fluid sources, and sediment- vs seawater-buffered conditions. We extend a numerical framework, established for the application of Ca isotopes, for recognizing sediment- vs fluid-buffered alterations during aragonite-to-calcite recrystallization for widely applied carbonate paleoredox proxies: iodine ratios (I/Ca), cerium anomaly (Ce/Ce*), and carbonate-associated Cr, U, and S isotopes. We model endmember reducing and oxidizing fluid diagenetic scenarios, as opposed to incorporating biogeochemical evolution of pore fluids within the model. The results reveal that early, fluid-buffered diagenesis of relatively unevolved seawater (“seawater-buffered”) represents the ideal scenario for preservation for all proxies, including C isotopes. Conversely, fluid-buffered alteration in pore fluids with redox conditions highly evolved from the overlying water column is likely to alter the primary carbonate geochemistry. Model calibration against records from the Bahamas Clino and Unda cores suggests each proxy is uniquely sensitive to a given style of diagenetic alteration, with Ce/Ce* the most robust and I/Ca ratios most sensitive to early diagenetic alteration. We recommend multiple strategies, including relationships with C, O, and Ca isotope data, that can be leveraged to identify the preservation of primary seawater geochemistry for a given proxy. Last, we compare the diagenetic model predictions with published records from the Permian/Triassic boundary to demonstrate how incorporating stratigraphic geochemical patterns alongside interpretation of diagenetic model results can support interpretation of secular seawater signals. To improve our understanding of the effects of diagenesis on carbonate paleoredox proxies, additional experimental constraints, field-based observations, and multi-proxy datasets are needed.