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早期海洋和大气氧化还原演化和II)能够还原金属的微生物出现档案的适用性。为了达到这些目的，我们希望研究近代和古代（二叠纪、新元古代和新-古太古代）叠层石的微量元素、Mo和u同位素组成。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，叠层石似乎由微生物群落组成，除了蓝藻外，还包括各种能够减少金属的代谢，如硫酸盐和铁还原剂，以及潜在的反硝化生物。一些现代叠层石在较深的缺氧层中显示出明显的氧化还原敏感金属富集，如Mo。因此，与其他碳酸盐相比，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，在uv和UVI之间的U还原过程中，特征强烈的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（如深黑海和卡里亚科盆地）的主要机制。因此，这些受限制的大洋盆地的沉积物经常表现出Mo和U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：1)它们是否记录了与金属还原相关的Mo和U同位素特征，或2)它们是否存档了其水源的同位素特征？在R1情况下，叠层石可能代表重建金属还原代谢外观的工具，而在R2情况下，它们可能代表海水（和大气）氧化还原演化的记录仪。与早期元古代和太古宙叠层石研究相关的关键问题是：A1)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)它们能告诉我们关于海洋氧化还原演化和/或金属还原生物首次出现的什么信息？选择古代样品是为了代表各种早期地球关键层，但也为了研究成岩蚀变（包括白云化和硅化）对Mo和U同位素特征的影响。此外，叠层岩的特征将与同期其他碳酸盐岩和富有机质页岩的特征进行比较，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.