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，我们的目标是调查叠层石作为档案的适用性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。为了达到这些目标，我们要调查的微量元素和钼和铀同位素组成的现代和古代（二叠纪，新元古代和新-古太古代）叠层石。许多碳酸盐岩直接记录了其水源的Mo和U同位素组成。然而，基质层似乎由微生物群落组成，除了蓝细菌之外，还包括能够还原金属的各种代谢，例如硫酸盐和铁还原剂，以及潜在的微生物。几个现代叠层石显示氧化还原敏感的金属在更深的缺氧层，如钼显着富集。因此，与其他碳酸盐岩相比，叠层石可能具有Mo和U还原的特征同位素信号。最近的实验研究表明，U还原过程中（UIV和UVI之间）的特征性强U同位素分馏可能仅限于生物U还原，这可能是现代近地表缺氧环境中的主导机制，如深黑海和Cariaco盆地。因此，这种局限性的海洋盆地的沉积物往往表现出钼和铀同位素特征的相关性。与本提案中计划的近期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相关的Mo和U同位素特征，或者R2）它们是否更愿意将其水源的同位素特征存档？在R1）情况下，层积物可能是重建金属还原代谢的工具，而在R2）情况下，它们可能是海水（和大气）氧化还原演化的记录器。与早元古代和太古代叠层石研究有关的关键问题是：A1）在这些叠层石中保存了它们沉积过程中产生的同位素特征; A2）它们能告诉我们关于海洋的氧化还原演化和/或金属还原生物的首次出现的什么信息？古代样品的选择，以代表各种早期地球的关键间隔，但也调查成岩蚀变的影响（包括。钼和铀同位素特征上的硅化作用。此外，叠层石的签名将与来自同一时期的其他碳酸盐和富含有机质的页岩中记录的签名进行比较，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.