EAGER: Microscale d34S Analyses in Pyrites to Distinguish Environmental and Biological Drivers of Isotopic Variability

EAGER：对黄铁矿进行微尺度 d34S 分析，以区分同位素变异的环境和生物驱动因素

• 批准号: 2048986
• 负责人: David Fike
• 金额: $ 9.84万
• 依托单位: Washington University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-03-15 至 2023-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2048986&HistoricalAwards=false
• 关键词: EAGER; Microscale; d34S; Analyses; Pyrites

Much of our understanding of changes in Earth’s environments and the co-evolution of its biosphere is based on geochemical signatures within the rock record. This project specifically seeks to extract deeper insights into the environmental information preserved in one of the most commonly used geochemical proxies: the sulfur isotope composition of pyrite, a mineral commonly formed in marine sediments. It has become apparent that the traditional analysis of the bulk isotopic composition of pyrite in sediments is inadequate because it averages environmental information contained within individual pyrite grains. However, by analyzing a representative suite of individual pyrite grains in sediments, investigators propose that they can distinguish both the direct imprint of biological activity and the degree of subsequent overprinting acquired after sediment deposition. This grain-specific approach can transform the ability to distinguish between competing biological and environmental processes that together give rise to the more commonly measured bulk geobiological signatures. This work will include mentoring of an undergraduate as part of the Students and Teachers as Research Scientists (STARS) Program, which offers incoming high-school seniors an opportunity to work within a laboratory research setting. In addition to gaining experience using cutting-edge techniques, the students are taught to express the results and significance of their research orally and in a research paper.Reconstructions of past environmental conditions and biological activity are often based on stable isotope proxies whose interpretations are inherently non-unique. In this project, researchers will develop and refine a new approach centered upon integrating traditional bulk (cm-scale) measurements of pyrite with micron-scale in-situ analyses in a petrographic context using secondary ion mass spectrometry (SIMS). They hypothesize that acquiring the individual sulfur isotopic values from a representative population of pyrite grains within each sample will enable a more rigorous reconstruction of depositional environments and, for the first time, distinguish between biological controls (i.e., isotopic fractionation during microbial sulfur cycling) and environmental controls (e.g., impact of sedimentation rate, organic carbon loading, etc.) that regulate the diffusive exchange between porewaters and the overlying water column. They seek to characterize and minimize any offset between the bulk value and the SIMS average, which could arise from several sources related to grain size, insufficient sampling density of individual pyrites, intragrain isotopic variability, or analytical artifacts in SIMS. They will demonstrate the technical feasibility of this approach and address the theoretical soundness of the hypothesis by analyzing samples from two contrasting systems: methane-seep sediments from Santa Monica Basin and mid-Pleistocene sediments from the Crotone Basin, Italy. In the former, the depositional environment has been stable and the porewater redox gradient is known, providing a control to investigate potentially variable biological fractionation and whether textural differences are diagnostic of different precipitation mechanisms. Samples in the latter location represent a wide range of depositional environments, exhibit large variations in the bulk slfur isotope values (-44 to +24‰), and include a wide range in petrographic textures and pyrite grain size and shape. Specific outcomes that will make this microanalytical approach more robust include: developing a mathematical algorithm to predict the sampling size (n) needed to accurately represent a sample distribution given the bulk sulfur isotope value and number of textures observed in SEM, refining their protocols to allow for analysing smaller grains while retaining 1‰ precision, developing an iterative ion imaging-sputtering method to incrementally analyze the 3D structure of grains and isolate any oxidized rims that may contribute to artifacts. The successful development and testing of this approach will provide a framework that allows for the unique identification of biological fractionation during microbial sulfur cycling as well as the degree to which the associated sediments have been impacted by closed-system processes during pyrite formation. This transformative framework could be widely applied to a host of geobiological problems in modern environments and the sedimentary record.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.

我们对地球环境变化和生物圈共同进化的理解，大部分是基于岩石记录中的地球化学特征。该项目旨在更深入地了解保存在最常用的地球化学代用指标中的环境信息：黄铁矿的硫同位素组成，黄铁矿是一种常见于海洋沉积物中的矿物。很明显，沉积物中黄铁矿的整体同位素组成的传统分析是不够的，因为它平均的环境信息包含在单个黄铁矿颗粒。然而，通过分析沉积物中一组具有代表性的黄铁矿颗粒，研究人员提出，他们可以区分生物活动的直接印记和沉积物沉积后获得的后续叠印程度。 这种特定于谷物的方法可以改变区分竞争性生物和环境过程的能力，这些过程共同产生更常见的测量批量地球生物学特征。 这项工作将包括指导本科生作为学生和教师作为研究科学家（STARS）计划的一部分，该计划为即将到来的高中毕业生提供在实验室研究环境中工作的机会。除了获得使用尖端技术的经验外，学生还被教导口头和研究论文表达他们的研究结果和意义。过去环境条件和生物活动的重建通常基于稳定同位素代理，其解释本质上是非唯一的。在这个项目中，研究人员将开发和完善一种新的方法，该方法集中在使用二次离子质谱法（西姆斯）在岩相学背景下将黄铁矿的传统批量（厘米级）测量与微米级原位分析相结合。他们假设，从每个样品中的黄铁矿颗粒的代表性群体中获得单个硫同位素值将能够更严格地重建沉积环境，并首次区分生物控制（即，微生物硫循环过程中的同位素分馏）和环境控制（例如，沉积速率、有机碳负荷等的影响）调节孔隙水和上覆水柱之间的扩散交换。他们寻求表征和最大限度地减少散装值和西姆斯平均值之间的任何偏移，这可能是由于与粒度相关的几个来源，个别黄铁矿的采样密度不足，粒内同位素变异性或西姆斯中的分析工件。他们将证明这种方法的技术可行性，并通过分析两个对比系统的样本来解决该假设的理论合理性：圣莫尼卡盆地的甲烷渗漏沉积物和意大利克罗托内盆地的中更新世沉积物。在前者中，沉积环境一直是稳定的，孔隙水氧化还原梯度是已知的，提供了一个控制调查潜在的可变生物分馏和纹理差异是否是不同的沉淀机制的诊断。在后一个位置的样品代表了广泛的沉积环境，表现出大量的硫同位素值（-44至+24‰）的变化，并包括广泛的岩石结构和黄铁矿粒度和形状。将使这种微观分析方法更加强大的具体成果包括：开发一种数学算法，以预测准确代表样品分布所需的采样量（n），给出了大量硫同位素值和SEM中观察到的纹理数量，改进了他们的方案，以允许分析较小的颗粒，同时保持1‰的精度，开发迭代离子成像-溅射方法，以增量分析晶粒的3D结构，并隔离可能导致伪影的任何氧化边缘。这种方法的成功开发和测试将提供一个框架，允许在微生物硫循环过程中的生物分馏的独特识别，以及在何种程度上相关的沉积物已受到封闭系统的过程中黄铁矿形成的影响。这一变革性的框架可以广泛应用于现代环境和沉积记录中的许多地球生物学问题。该奖项反映了NSF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。