Mercury Stable Isotopes as a Proxy of Photic Zone Euxinia

该项目的目标是开发一种新的方法，以确定地球历史上富含硫化氢的沃茨到达阳光容易穿透的海洋浅层（称为“透光区“）的时期。“这是海洋中生物最活跃的部分。特别是，透光层是地球上每年约25%的光合作用发生的地方。当富含硫化氢的沃茨进入透光层时，它们会破坏动物的生命，还可能阻碍光合作用，对海洋生态系统造成大规模破坏。地球过去的许多大规模喷发可能与透光层中的硫化氢有关，但海洋化学和生态系统如何联系的细节却知之甚少，因为在地质记录中很少有好的方法来检测浅水硫化物。因此，这项研究对于了解地球历史上的生命历程具有根本的重要性。然而，这项研究也与预测我们的未来有关，因为硫化氢在浅沃茨的出现现在更有可能是由于人类活动。其中包括过度使用化肥和全球变暖。因此，这项研究可能有助于阐明随着人类对海洋化学的影响加速，可能会发生的各种生态系统变化。作为工作的一部分，该团队还将开发一种新的数字教育资源，向学生传授海洋化学随时间的变化。具体而言，研究人员将探索使用汞（Hg）稳定同位素作为地球古代海洋中透光带euxinia（PZE）的代表。当缺氧、富含硫化物的沃茨扩展到透光层时，就会发生PZE。虽然这种情况在今天很少见，但PZE被认为在一些古老的海洋中广泛存在，对海洋和陆地生态系统可能产生深远的影响。然而，在过去，发展令人信服的证据PZE一直很困难。在这一建议中，研究人员假设，汞稳定同位素组成的海洋沉积岩可能作为一个新的代理PZE。这种新的代理的基础是，汞进行独特的质量独立的同位素分馏（MIF）在水溶液中的汞（II）的光还原时，由还原硫配体络合。具体地，结合到含氧配体（例如，羧基）导致阳性MIF（在残余Hg（II）中富集奇数质量数同位素199 Hg和201 Hg）。相反，结合到还原的硫配体（例如，巯基）产生阴性MIF。与此基本原理相一致的是，对中元古代页岩中汞同位素组成的初步研究表明，在氧化条件下沉积的沉积物中的MIF为零到轻微的正值，而在硫化物条件下沉积的沉积物中的MIF为明显的负值，从而支持了这一假设。研究人员将通过以下方式进一步验证这一新的替代指标：1）研究现代浅水富硫化物环境中的汞地球化学循环和同位素分馏，这是PZE的现代模拟; 2）测量有PZE证据的古代沉积岩中的汞同位素组成。这项研究将得到对硫化物水中汞光化学和相应同位素分馏的第一次实验研究的补充，这些研究将通过合作进行。研究人员还将在同一套岩石样品中检查新的铼（Re）同位素氧化还原代理，这些岩石样品将被分析汞同位素，这可能有利于解释两种同位素系统。Re同位素是一种非常新颖的古氧化还原代理，可以潜在地区分亚氧和缺氧/缺氧条件。因此，汞和铼同位素的耦合研究应该提供一个更完整的图片的氧化还原结构的古代海洋，提供更好的约束，这两个proxities.This奖项反映了NSF的法定使命，并已被认为是值得的支持，通过评估使用基金会的智力价值和更广泛的影响审查标准。

The goal of this project is to develop a new approach to identify periods in Earth history when hydrogen sulfide-rich waters reached the shallow depths of the oceans which sunlight easily penetrates, known as the "photic zone." This is the part of the ocean in which biology is most vigorous. In particular, the photic zone is where about 25% of photosynthesis happens on Earth each year. When hydrogen sulfide-rich waters enter the photic zone, they devastate animal life and may also hinder photosynthesis, causing massive disruption to ocean ecosystems. Many mass extinctions in Earth's past may have been associated with hydrogen sulfide in the photic zone, but the details of how ocean chemistry and ecosystems are linked are poorly understood because there are few good ways to detect shallow water sulfide in the geologic record. This research is therefore of fundamental importance to understanding the course of life through Earth's history. However, the research is also relevant to predicting our future because the occurrence of hydrogen sulfide in shallow waters is now more likely due to human activities. These include excess use of fertilizers and global warming. This research therefore may help to shed light on the sorts of ecosystem changes that might be expected as the human impact on ocean chemistry accelerates. As part of the work this team will also develop a novel digital education resource to teach students about changes in ocean chemistry through time.Specifically, investigators will explore the use of mercury (Hg) stable isotopes as a proxy for photic zone euxinia (PZE) in Earth's ancient oceans. PZE occurs when anoxic, sulfide-rich waters expand into the photic zone. Although this condition is rare today, PZE is thought to have been widespread in some ancient oceans with potentially profound effects on marine and terrestrial ecosystems. However, developing convincing evidence of PZE in the past has been difficult. In this proposal, investigators hypothesize that Hg stable isotope compositions in marine sedimentary rocks may serve as a new proxy of PZE. The basis of this novel proxy is that Hg undergoes unique mass independent isotope fractionation (MIF) during aqueous photoreduction of Hg(II) when complexed by reduced sulfur ligands. Specifically, photoreduction of Hg(II) bound to oxygen-containing ligands (e.g., carboxyl) leads to positive MIF (enrichment of odd-mass-number isotopes, 199Hg and 201Hg in the residual Hg(II)). In contrast, photoreduction of Hg(II) bound to reduced sulfur ligands (e.g., thiol) produces negative MIF. Consistent with this rationale, a pilot study on Hg isotope compositions in Mesoproterozoic shales showed zero to slightly positive MIF in sediments deposited under oxic conditions and distinct negative MIF under sulfidic conditions, thus supporting the hypothesis. Researchers will further validate this emerging new proxy by: 1) investigating Hg biogeochemical cycles and isotope fractionation in a modern shallow-water sulfide-rich environment, which serves as a modern analog of PZE; and 2) measuring Hg isotope compositions in ancient sedimentary rocks that have documented evidence of PZE. This research will be complemented by the first experimental studies on Hg photochemistry in sulfidic water and the corresponding isotope fractionation, which will be carried out through a collaboration. The researchers will also examine the new rhenium (Re) isotope redox proxy in the same suites of rock samples that will be analyzed for Hg isotopes, which might benefit the interpretation of both isotope systems. Re isotopes are a very novel paleoredox proxy that can potentially distinguish between suboxic and anoxic/euxinic conditions. Thus, the coupled study of Hg and Re isotopes should provide a more complete picture of the redox structure of the ancient ocean, providing better constraints on both proxies.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.