Formation and Early Diagenesis of Carbonate Deposits: The Role of Microbes and Extracellular Organic Matter (EOM)

微生物生态系统提供了短期（生物）和长期（地质）碳循环之间的关键联系。微生物垫就是在沉积物表面发现的这种生态系统的一个例子。这些垫子被认为类似于我们在岩石记录中发现的最早的生命证据（高达35亿年前）。垫层中微生物的代谢可以改变岩石圈和生物圈交界面矿物溶解和降水的平衡。形成岩石的微生物席，尤其是产生碳酸盐的微生物席，正好位于这个界面上。因此，它们是研究有机碳通过内在（微生物过程）和外在（环境）因素结合转化为碳酸盐矿物的理想模型系统。形成岩石的生态系统（称为微生物岩），如本文所研究的模式系统，提供了独特的机会来研究近地垫表面微生物沉淀的碳酸盐的早期生物地球化学变化。这一过程被称为早期成岩作用，对岩石记录的形成非常重要。微生物在靠近垫子表面的地方最活跃，但在深处，它们有充裕的时间，它们的新陈代谢虽然非常缓慢，但可以极大地改变有机碳和碳酸盐矿物的性质。我们建议研究地球化学和微生物反应的耦合作用，以产生生物成因的碳酸盐矿物。我们选择研究一个独特的微生物垫系统，它大约一米厚，由数百层碳酸盐矿物组成，由有机碳层隔开。我们还将研究这些矿物随深度的地质变化，以及有机碳化学性质的变化。微生物-地球化学反应的综合结果是一层状碳酸盐岩，与化石记录中通常发现的细层状泥晶叠层石非常相似。我们的目标是确定微生物和物理化学过程对微生物碳酸盐早期成岩作用的贡献。这将揭示出最初的碳酸盐沉淀物的哪一部分被保存在最初沉淀的岩石记录中。换句话说，我们将能够直接确定微生物及其产物在矿物成岩作用中的作用。利用最先进的技术，对深层沉积物变化过程的研究是理解这些微生物碳酸盐向化石记录缓慢过渡的关键一步。此外，该项目还提供了微生物碳酸盐如何将无机碳（即二氧化碳）储存成长-？停留时间?碳酸盐储层（所谓的固碳）。它还通过揭示地质微生物机制，打开了一扇了解过去的窗口，可以在早期成岩作用中保存生命的痕迹（微生物特征），并进一步在岩石记录中保存。这些方面是理解现代和过去的碳循环、早期生命的发展以及其他星球上可能存在的生命的关键特征。

Microbial ecosystems provide a critical link between the short-term (?biological?) and the long-term (?geological?) carbon cycle. Microbial mats are an example of such ecosystems found at the surface of sediments. These mats are believed to resemble the earliest evidence of life that we can find in the rock record (up to 3.5 billion years old). The metabolism of microbes present in the mats can change the balance between mineral dissolution and precipitation at the interface between the lithosphere and the biosphere. Rock-forming microbial mats, especially the ones producing carbonates, are precisely situated at this interface. Therefore, they are ideal model systems to study the transformation of organic carbon into carbonate minerals through a combination of intrinsic (microbial processes) and extrinsic (the environment) factors. The rock-forming ecosystems (called microbialites), like the model system studied in this proposal, provide the unique opportunity to study the early biogeochemical changes of the carbonates precipitated by the microbes near the surface of the mat. This process, called early diagenesis, is very important in forming of the rock record. The microbes are most active near the surface of the mat, but at depth they have the luxury of time and their metabolism, albeit very slow, can greatly change the properties of both the organic carbon and the carbonate minerals. We propose to investigate the coupling of geochemical and microbiological reactions that produce biogenic carbonate minerals. We chose to investigate a unique microbial mat system that is approximately one meter thick and comprised of hundred of layers of carbonate minerals separated by layers of organic carbon. We also will study the geologic changes of these minerals with depth, and coupled to this, the changes in chemical properties of the organic carbon. The result of the combined microbial-geochemical reactions is a layered carbonate rock that is very similar to the finely-laminated micritic stromatolites, which are typically found in the fossil record. Our goal is to determine the contribution of microbial and physicochemical processes to early diagenesis of microbial carbonates. This will shed light on which fraction of the initial carbonate precipitate remains preserved in the rock record from the initial precipitation. In other words, we will be able to determine the role of microbes and their products in the diagenesis of minerals directly. Using state-of-the-art techniques, study of the processes altering the sediments at depth represents a critical step in the understanding of the slow transition of these microbial carbonates toward the fossil record. Furthermore, this project provides insight of how microbial carbonates can store inorganic carbon (i.e., CO2) into a long-?residence time? carbonate reservoir (so-called carbon sequestration). It also opens a window on the past by revealing geomicrobial mechanisms that could preserve traces of life (microbial signatures) during early diagenesis and further in the rock record. These aspects are key features for understanding modern and past carbon cycles, development of early life as well as possible life on other planets.