Quantifying the importance of authigenic carbonate in the ancient carbon cycle

在地球历史研究中，可以说没有任何元素比碳更受关注，因为它具有行星恒温器和大气氧气来源的双重作用。由于直接量化质量通量的挑战，特别是在深时间，沉积相的碳同位素测量（13 C）已成为研究全球碳循环的替代品。然而，最近的工作已经挑战了用于解释13 C记录的传统范例，表明自生碳酸盐，从厌氧有机质降解过程中释放的碳中沉淀出来，是碳的定量重要的第三个汇-特别是在pupper-low O2浓度的时期，例如在前寒武纪和中生代的短暂缺氧期间。如果这个模型是正确的，那么我们对碳循环和全球氧化还原预算的理解需要在根本层面上得到纠正。然而，测试这个模型是具有挑战性的，因为自生碳酸盐可以散布在大量的埋藏硅质沉积物。因此，与其将地球化学代用指标视为地质质量通量经验量化的替代品，这些方法必然是互补的。例如，北美上地壳的4D模型Macrostrat（macrostrat.org）的分析显示，泥盆纪末陆架碳酸盐生产崩溃，被硅质碎屑取代。这种情况预计将产生理想的边界条件的自生碳酸盐汇重新出现在否则氧化世界。*该提案概述了一个独特的和创新的方法组合-实地工作，同位素地球化学，计算能力-以评估深时间自生杠杆的强度。实地工作，既寻找这些泥盆系末碳酸盐和约束海水化学成分是一个一阶模型的测试。在加拿大前山脉，泥盆纪末和石炭纪早期的地层厚达数公里。工作将包括同时记录地层剖面和对台地碳酸盐岩及其盆地等效物进行地球化学取样。收集的样本将在维多利亚大学进行13 C和微量元素丰度分析。开发的地球化学数据将被用来模拟自生碳酸盐的重要性，结合这些测量的沉积物成分与沉积质量通量估计来自Macrostrat。这些工作将利用新产生的数据和使用机器阅读和数据挖掘技术从文献中提取的数据（geodeepdive.org）。不可避免的是，这种计算方法将揭示数据差距并提出新的假设，从而为进一步的实地和实验室工作提供新的方向和动力。

In Earth history research, arguably no element has received more attention than carbon, owing to its dual role as a planetary thermostat and source of atmospheric oxygen. Due to the challenges of direct quantification of mass fluxes, especially in deep time, carbon isotopic measurements (13C) of sedimentary phases have emerged as proxies to study the global carbon cycle. Recent work, however, has challenged the traditional paradigm used to interpret 13C records by suggesting that authigenic carbonate, precipitated from carbon released during anaerobic organic matter degradation, is a quantitatively important third sink for carbon - especially during times of putatively lower O2 concentrations, such as during the Precambrian and transient anoxia in the Phanerozoic.******If this model is correct, then our understanding of the carbon cycle and global redox budgets needs to be redressed at a fundamental level. Testing this model is challenging, however, because authigenic carbonate can be disseminated over a large volume of buried siliciclastic sediment. Thus, rather than viewing geochemical proxies as replacements for empirical quantification of geological mass fluxes, these approaches are necessarily complementary. For example, analyses from Macrostrat, a 4D model of the North American upper crust (macrostrat.org), reveal an end-Devonian crash in shelf carbonate production, with replacement by siliciclastics. This scenario is predicted to produce ideal boundary conditions for the reemergence of an authigenic carbonate sink in an otherwise oxic world.******This proposal outlines a unique and innovative combination of approaches - field work, isotope geochemistry, computational abilities - to assess the strength of the authigenic lever in deep time. Field work that both searches for these end-Devonian carbonates and constrains the composition of seawater chemistry is a first-order test of the model. In the Canadian Front Ranges, end Devonian and early Carboniferous strata are kilometers thick. Work will involve concurrent logging of stratigraphic sections and geochemical sampling of both platformal carbonates and their basinal equivalents. Collected samples will be analyzed for 13C and trace element abundances at the University of Victoria. The geochemical data developed will be used to model the importance of authigenic carbonate by combining these measurements of sediment composition with the sedimentary mass flux estimates derived from Macrostrat. These efforts will utilize both the newly produced data and data extracted from the literature using machine reading and data mining techniques (geodeepdive.org). Inevitably, this computational approach will reveal data gaps and pose new hypotheses, thereby providing new directions and motivation for further field and laboratory work.