Stromatolites as archives for metal mobilization and early life metabolisms? Uranium and Mo isotope studies of modern and Archean stromatolites and carbonates.

在拟议的项目中，我们的目标是研究叠层石作为档案的适宜性，用于i)早期海洋和大气的氧化还原演化和ii)能够还原金属的微生物的出现。为了达到这些目的，我们想要研究现代和古代(二叠纪、新元古代和新太古代)叠层石的微量元素和钼、铀同位素组成。许多碳酸盐岩直接记录其水源的钼和铀同位素组成。然而，叠层石似乎由微生物群落组成，除了蓝藻外，还包括各种能够还原金属的代谢产物，如硫酸盐和铁还原物质，以及潜在的进行反硝化作用的微生物。几种现代叠层石在更深的缺氧层，如钼，显示出对氧化还原敏感的金属的显著富集性。因此，与其他碳酸盐不同，叠层石可能具有钼和铀还原的特征同位素信号。最近的实验研究表明，铀还原过程中(UIV和UVI之间)特征强烈的铀同位素分馏可能仅限于生物铀还原，这可能是现代近地表缺氧环境(如深海黑海和加勒比盆地)的主要机制。因此，这些受限制的大洋盆地的沉积物经常显示出钼和铀同位素特征的相关性。与本提案中计划的最近叠层石调查相关的关键问题是：r1)它们是否记录了与金属还原有关的Mo和U同位素特征；或r2)它们是否更愿意将其水源的同位素特征存档？在情况r1)中，叠层石可能是重建金属还原代谢现象的工具，而在情况r2)中，它们可能是海水(和大气)氧化还原演化的记录器。与早元古代和太古宙叠层石研究有关的关键问题是：A1)在它们沉积过程中产生的同位素特征是否保存在这些叠层石中；A2)它们能告诉我们什么关于海洋的氧化还原演化和/或金属还原生物体的首次出现？选择古代样品是为了代表各种早期地球关键时期，也是为了调查成岩蚀变的影响(包括。白云岩化和硅化)在钼和铀同位素特征上。此外，叠层岩的特征将与同一时期其他碳酸盐和富含有机的页岩中记录的特征进行比较，以优化古氧化还原演化的重建。

In the proposed project, we aim to investigate the suitability of stromatoilites as archive for I) the redox evolution of early oceans and atmosphere and II) for the appearance of microorganism that were capable to reduce metals. In order to reach these aims, we want to investigate the trace element- and Mo- and U-isotope compositions of recent and ancient (Permian, Neoproterozoic and Neo- to Paleoarchean) stromatolites. Many carbonates directly record the Mo and U isotope composition of their water source. However, stromatolites appear to consist of microbial communities, including beside cyanobacteria also a variety of metabolisms that are capable to reduce metals, such as sulfate and iron reducers and potentially also denitrification-performing organisms. Several modern stromatolites show significant enrichment of redox-sensitive metals in the deeper anoxic layers, such as Mo. Thus, in contrast to other carbonates, stromatolites may bear characteristic isotopic signals of Mo and U reduction. Recent experimental studies imply that the characteristically strong U isotope fractionation during U reduction (between UIV and UVI) may be limited to biotic U reduction, which is likely the dominant mechanism in modern near-surface anoxic environments, such as the deep Black Sea and Cariaco Basin. As a result, sediments of such restricted ocean basins frequently show a correlation of Mo and U isotope signatures. The key-questions, related to the investigation of recent stromatolites planned in this proposal are R1) do they record Mo and U isotopic signatures associated with metal reduction or R2) do they rather archive the isotopic signatures of their water source? In case R1) stromatolites may represent a tool to reconstruct the appearance of metal-reducing metabolisms, while in case R2) they may represent recorders for the redox evolution of seawater (and the atmosphere). The key-questions, related to the investigation of the early Proterozoic and Archean stromatolites are: A1) are the isotopic signatures generated during their deposition preserved in these stromatolites and A2) what can they tell us about the redox-evolution of the oceans and/ or the first appearance of metal reducing organisms? Ancient samples are selected in order to represent a variety of early Earth key intervals, but also to investigate effects of diagenetic alteration (incl. dolomitisation and silification) on the Mo and U isotope signatures. Furthermore, the signatures of the stromatilites will be compared with signatures recorded in other carbonates and organic-rich shales from the same period, in order to optimize reconstructions of the paleo-redox evolution.