Collaborative Research: On how the Bengal-Nicobar fan deposition influenced carbonate cementation in the incoming sediment to the Sumatra subduction zone (IODP Exp 362)

碳酸盐胶结物的沉淀，如方解石，改变了沉积物的物理力学性质。根据沉积物的性质及其相对于地震活动带（如俯冲带）的位置，碳酸盐胶结可能会降低孔隙度和渗透率，并增加沉积剖面的刚性。它还可以影响断层上的滑动和地震破裂过程。俯冲带中随着埋藏和温度升高的沉积物蚀变过程（即成岩作用）中碳酸盐的沉淀也为碳提供了一个重要的汇，碳可以从剖面中更深的蚀变岩石中释放出来，也可以通过脱挥发和其他水-岩相互作用过程释放出来。因此，俯冲带沉积物中的碳酸盐沉淀对碳循环具有重要影响。本文研究了苏门答腊岛俯冲带海底扇沉积中碳酸盐胶结物在硅质碎屑岩中的形成。将研究与硅屑沉积物蚀变及其碳酸盐胶结有关的过程，以及与微生物介导的碳循环有关的过程。研究活动包括使用最先进的成像技术对沉积物和水泥颗粒的矿物和结构关系进行岩石学调查。样品的地球化学，包括碳、氧、锶的稳定同位素分析和沉积物及其组成材料的微量金属成分，将进行尝试和识别导致碳酸盐胶结物形成的不同过程的特征。该研究还将包括描述粒间孔隙空间和晶界关系的工作，特别是关于孔隙度、渗透率和矿物溶解特征的变化。项目目标包括：(1)确定碳酸盐胶结物在俯冲带硅质碎屑沉积包体孔隙度降低和岩石性质演化中的作用；(2)评价碳酸盐水泥沉淀对碳循环的影响；(3)确定俯冲带沉积物形成碳酸盐胶结的热化学条件。这项工作的更广泛影响包括学生培训和将研究纳入课程。这些研究人员将与本国机构相关的组织合作，吸引在科学领域代表性不足的群体的成员，并让他们参与研究。调查人员将与学生和教育工作者一起开发用于中学教育的材料。该项目支持两名性别在科学领域代表性不足的研究人员，其中一名是少数族裔学生的榜样。尽管我们对碳循环驱动的碳酸盐形成有了深入的了解，无论是生物成因碳酸盐的再结晶还是甲烷衍生自生碳酸盐的沉淀，另一个可能显著影响沉积物胶结物形成的机制是，在成岩作用期间硅酸盐矿物蚀变产生碳酸氢盐和有利于碳酸盐矿物沉淀的阳离子。成岩碳酸盐沉淀有效地固碳。由于沉积物中的碳酸盐可能在成岩作用过程中通过脱挥发和/或碳酸钙的流体溶解从俯冲岩石中释放出来，因此了解俯冲带输入沉积物序列中的碳酸盐-碳库存对于限制地球表面碳储层的输入非常重要。成岩碳酸盐胶结物的形成改变了储层的物理力学性质，改变了储层的渗透性和力学强度。海相体系的胶结过程和硅酸盐风化作用对自生碳酸盐形成的作用尚不清楚。本文研究了碳-硅耦合体系在俯冲相关硅质沉积岩心生成碳酸盐胶结物和沉积含碳矿物中的相对作用。之所以选择来自苏门答腊岛俯冲带尼科巴扇的沉积物样本，是因为它们表现出明显的碳酸盐胶结作用，这些样本是最近在国际海洋发现计划（IODP）对印度洋的钻探探险中收集的。研究的重点是验证Nicobar扇碳酸盐胶结物是由于非碳酸盐硅质碎屑蚀变产生碱性而在埋藏和成岩过程中形成的假说。研究任务包括：(1)比较传统碳循环途径与硅酸盐-碳酸盐耦合体系形成碳酸盐的相对作用；(2)识别沉积物中导致碳酸盐形成的化学反应；(3)低估了海底硅酸盐蚀变促进碳酸盐沉淀的条件。沉积物样品将进行岩石学检查，并通过场发射扫描电子显微镜进行x射线元素测绘和阴极发光成像。样品也将进行地球化学分析。除微量元素组成分析外，还将进行高分辨率碳、氧和锶同位素和团块稳定同位素分析。数据将用于确定地下海洋硅酸盐在碳酸盐胶结地层中的作用，并确定与之相关的沉积物结构/孔隙度变化。研究结果将有助于我们理解在经历甲烷生成的以硅塑料为主的系统中流体-岩石反应、元素循环和孔隙度降低之间的相互依赖性，并将提高我们对全球碳收支、沉积物流变学和俯冲带断裂的理解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The precipitation of carbonate cements, like calcite, changes the physical and mechanical properties of sediments. Depending on the nature of the sediment and its location relative to zones of seismic activity, like subduction zones, carbonate cementation may decrease porosity and permeability and increase sedimentary section rigidity. It can also impact the slip on faults and earthquake rupture processes. Precipitation of carbonate during the alteration of sediments in subduction zones with burial and increasing temperature (i.e., diagenesis) can also provide an important sink for carbon, which may be released from rocks undergoing alteration deeper in the section or by devolatilization and other water-rock interaction processes. Thus, carbonate precipitation in subduction zone sediments can have a significant impact on the carbon cycle. This research examines the formation of carbonate cements in siliciclastic rocks in a major submarine fan deposit within the Sumatra subduction zone. Processes related to the alteration of siliciclastic sediments and their cementation by carbonate will be studied as will processes related to microbially-mediated carbon cycling. Research activities include petrographic investigation of mineral and textural relationships of sediment and cement grains using state-of-the-art imaging technologies. The geochemistry of samples, including stable isotope analyses of carbon, oxygen, and strontium and trace metal compositions of the sediments and their constituent materials will be carried out to try and identify signatures of different processes leading to the formation of carbonate cements. The study will also include work to characterize intergranular pore spaces and grain boundary relations, especially with regard to changes in porosity, permeability, and mineral dissolution features. Project goals include (1) determining the role of carbonate cements in porosity reduction and rock property evolution in subduction zone siliciclastic sediment packages; (2) evaluating the impact of carbonate cement precipitation on the carbon cycle; and (3) determining the thermal and chemical conditions that lead to carbonate cement formation in subduction zone sediments. Broader impacts of the work include student training and incorporation of research into courses. The investigators will work with organizations associated with their home institutions to attract members of groups underrepresented in the sciences and engage them in the research. The investigators will work with students and educators to develop materials to be used in secondary education settings. The project supports two investigators whose genders are underrepresented in the sciences, one of whom serves as a role model for minority students.Whereas we have a solid understanding of carbonate formation driven solely by carbon cycling, either recrystallization of biogenic carbonate or precipitation of methane-derived authigenic carbonate, another mechanism that may significantly influence the formation of sediment cements is where silicate mineral alteration during diagenesis produces bicarbonate and cations that favor precipitation of carbonate minerals. Diagenetic carbonate precipitation efficiently sequesters carbon. Because carbonate in sediments may be released during diagenesis from subducted rocks by devolatization and/or fluid-induced dissolution of calcium carbonate, knowledge of carbonate-carbon inventories in input sediment sequences to subduction zones is important to constrain the inputs to Earth's surface carbon reservoirs. Furthermore, the formation of diagenetic carbonate cements changes the physical and mechanical properties of the hosting sediment, altering its permeability and mechanical strength. The cementation process and the role of silicate weathering on authigenic carbonate formation in marine systems is not well understood. This work documents the relative roles of the coupled carbon-silica system in generating carbonate cements and depositing carbon-bearing minerals in cores of subduction-related siliciclastic sediments. Sediment samples from the Nicobar Fan from the Sumatra subduction zone were selected because they exhibit significant carbonate cementation, having been recently collected on an International Ocean Discovery Program (IODP) drilling expedition to the Indian Ocean. The research focuses on testing the hypothesis that Nicobar Fan carbonate cements formed at depth during burial and diagenesis due to the generation of alkalinity from the alteration of non-carbonate siliciclastic material. Research tasks include (1) comparing the relative roles of carbonate formation through traditional carbon cycling pathways with those via the proposed coupled silicate-carbonate system; (2) identifying chemical reactions in the sediments that lead to carbonate formation; and (3) understating conditions that promote carbonate precipitation from subsurface marine silicate alteration. Sediment samples will be examined petrographically and subjected to X-ray elemental mapping and cathodoluminescence imaging by field-emission scanning electron microscopy. Samples will also be analyzed geochemically. High-resolution carbon, oxygen and strontium isotopes and clumped stable isotope analyses will be carried out in addition to analyses of trace element composition. Data will be used to determine the role of subsurface marine silicates in carbonate cement formation and identify sediment fabric/porosity changes associated with it. Results will contribute to our understanding of the interdependence among fluid-rock reactions, element cycling, and porosity reduction in siliciclastic-dominated systems that experience methanogenesis and will improve our understanding of global carbon budgets, sediment rheology, and faulting in subduction zones.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.