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

我们对地球环境的变化及其生物圈的共同进化的了解，很大程度上是基于岩石记录中的地球化学特征。该项目专门寻求对保存在最常用的地球化学指标之一中的环境信息进行更深入的了解：黄铁矿的硫同位素组成，这是一种通常在海洋沉积物中形成的矿物。很明显，对沉积物中黄铁矿的整体同位素组成的传统分析是不充分的，因为它平均了单个黄铁矿颗粒中所含的环境信息。然而，通过分析沉积物中具有代表性的一套单独的黄铁矿颗粒，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随后叠印的程度。这种特定于谷物的方法可以改变区分相互竞争的生物和环境过程的能力，这些过程共同产生了更普遍测量的整体地球生物学特征。这项工作将包括作为学生和教师作为研究科学家(STAR)计划的一部分来指导本科生，该计划为即将入学的高三学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，学生还被教导以口头和研究论文的形式表达他们研究的结果和意义。重建过去的环境条件和生物活性通常基于稳定的同位素替代品，其解释本身并不独特。在这个项目中，研究人员将开发和改进一种新的方法，其核心是将传统的黄铁矿体积(厘米级)测量与使用二次离子质谱仪(SIMS)的岩石学背景下的微米级原位分析相结合。他们假设，从每个样品中具有代表性的黄铁矿颗粒群中获取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(即微生物硫循环期间的同位素分馏)和环境控制(例如沉积速率、有机碳负荷等的影响)。它们调节着孔隙水和上覆水柱之间的扩散交换。他们试图表征并尽量减少块状值与SIMS平均值之间的任何偏差，这种偏差可能是由几个来源引起的，这些来源与颗粒大小、单个黄铁矿的采样密度不足、颗粒内同位素变异性或SIMS中的分析伪影有关。他们将论证这种方法的技术可行性，并通过分析来自两个不同系统的样本来说明这一假说的理论合理性：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。前者沉积环境稳定，孔隙水氧化还原梯度已知，为研究潜在的可变生物分馏和结构差异是否能诊断不同的沉积机制提供了控制。后一位置的样品代表了广泛的沉积环境，总体硫同位素变化很大(-44至+24‰)，包括广泛的岩石结构和黄铁矿颗粒大小和形状。将使这一微观分析方法更加稳健的具体成果包括：开发出一种数学算法来预测准确表示样品分布所需的采样大小(N)，给定整体硫同位素值和在扫描电子显微镜中观察到的纹理数量，改进其协议以允许分析更小的颗粒，同时保持1‰的精度，开发出迭代离子成像-溅射方法来逐步分析颗粒的三维结构，并分离出任何可能造成伪影的氧化轮缘。这一方法的成功开发和测试将提供一个框架，以便能够唯一地识别微生物硫循环过程中的生物分馏作用，以及在黄铁矿形成过程中相关沉积物受到闭合系统过程影响的程度。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一系列地质生物学问题。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.