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（？生物学？）长期（？地质学？）碳循环微生物席是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找到的最早的生命证据（长达35亿年）。 席层中微生物的代谢可以改变岩石圈和生物圈界面处矿物溶解和沉淀之间的平衡。形成岩石的微生物席，特别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）因素相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），如本提案中研究的模型系统，提供了独特的机会，研究由微生物沉淀的碳酸盐的早期地球化学变化的表面附近的垫。这一过程被称为早期成岩作用，它对岩石记录的形成非常重要。微生物在靠近垫表面的地方最活跃，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，但可以大大改变有机碳和碳酸盐矿物的性质。我们建议研究产生生物碳酸盐矿物的地球化学和微生物反应的耦合。我们选择研究一种独特的微生物垫系统，该系统约有一米厚，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-地球化学反应相结合的结果是层状碳酸盐岩，它与化石记录中典型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岩早期成岩作用的贡献。这将揭示最初的碳酸盐沉淀的哪一部分仍然保存在岩石记录从最初的沉淀。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。使用最先进的技术，研究改变沉积物深度的过程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，该项目提供了微生物碳酸盐如何储存无机碳（即，二氧化碳）变成一个长-？停留时间？碳酸盐储层（所谓的碳封存）。它还通过揭示地质微生物机制打开了一扇关于过去的窗户，这些机制可以在早期成岩作用期间以及进一步在岩石记录中保存生命的痕迹（微生物特征）。这些方面是了解现代和过去碳循环，早期生命发展以及其他行星上可能存在生命的关键特征。

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.