Mercury Stable Isotopes as a Proxy of Photic Zone Euxinia

汞稳定同位素作为光区 Euxinia 的代表

• 批准号: 1760203
• 负责人: Ariel Anbar
• 金额: $ 42.04万
• 依托单位: Arizona State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-06-01 至 2024-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1760203&HistoricalAwards=false
• 关键词: Mercury; Stable; Isotopes; Proxy; Photic

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.

该项目的目的是开发一种新的方法，以识别地球历史上富含硫化氢的水到达海洋浅水深处的时期，阳光很容易穿透，称为“光学区”。这是生物学最有力的海洋部分。特别是，每年地球上大约有25％的光合作用发生。当富含硫化氢的水进入光学区域时，它们会破坏动物的生命并可能阻碍光合作用，从而大大破坏海洋生态系统。地球过去的许多大规模灭绝可能与光区中的硫化氢有关，但是对海洋化学和生态系统的联系的细节知之甚少，因为几乎没有什么好方法可以检测地质记录中的浅水硫化物。因此，这项研究对于通过地球历史理解生活的过程至关重要。但是，这项研究也与预测我们的未来有关，因为现在硫化氢在浅水中的发生更有可能是由于人类活动所致。这些包括过量使用肥料和全球变暖。因此，这项研究可能有助于阐明随着人类对海洋化学影响加速的影响，可能会预期的生态系统变化。作为工作的一部分，该团队还将开发一种新颖的数字教育资源，以随着时间的推移向学生传授海洋化学的变化。研究人员将探索汞（HG）稳定的同位素用作地球上的Euxinia（PZE）的代理。当缺氧，富含硫化物的水扩展到光学区域时，就会发生PZE。尽管今天这种情况很少见，但人们认为PZE在一些古老的海洋中广泛存在，对海洋和陆地生态系统具有深远的影响。但是，过去很难发展令人信服的PZE证据。在该提案中，研究人员假设海洋沉积岩中的HG稳定同位素组成可能是PZE的新代理。这种新的代理的基础是，当通过还原的硫配体复合时，HG在Hg（II）的水性光递录期间经历独特的质量独立同位素分馏（MIF）。具体而言，与含氧配体（例如，羧基）结合的Hg（II）的光元导致阳性MIF（在残留Hg（II）中，奇数质量数量同位素的富集，199hg和201Hg）。相反，与硫磺配体降低（例如硫醇）结合的Hg（II）的光摄取会产生负MIF。与这种基本原理一致，中菜植物学页岩中对HG同位素组成的试点研究显示，在硫磺条件下沉积的沉积物中的沉积物中的MIF零至略有正阳性，因此在硫酸条件下具有明显的负MIF，从而支持了假设。研究人员将通过以下方式进一步验证这一新出现的新代理人：1）在现代浅水硫化物富含硫化物的环境中研究HG生物地球化学周期和同位素分馏，该环境是PZE的现代类似物； 2）在古老的沉积岩中测量已经记录了PZE证据的HG同位素组成。这项研究将由硫化水中HG光化学和相应的同位素分馏的首次实验研究补充，该研究将通过协作进行。研究人员还将检查新的rhenium（RE）同位素氧化还原代理，以相同的岩石样品套件进行分析，这些岩石样品将对HG同位素进行分析，这可能会受益于两种同位素系统的解释。 RE同位体是一种非常新颖的古质子代理，可以潜在地区分亚氧化和缺氧/活生素条件。因此，对HG和RE同位素的耦合研究应提供对古代海洋的氧化还原结构的更完整的了解，从而为这两个代理提供了更好的限制。该奖项反映了NSF的法定任务，并被认为是值得通过基金会的知识分子优点和更广泛影响的审查标准来通过评估来支持的。

项目成果

Mercury isotope evidence for marine photic zone euxinia across the end-Permian mass extinction

• DOI: 10.1038/s43247-023-00821-6
• 发表时间: 2023-05
• 期刊: Communications Earth & Environment
• 作者: Ruoyu Sun;Yi Liu;J. Sonke;Feifei Zhang;Yaqiu Zhao;Yonggen Zhang;Jiubin Chen;Cong‐Qiang Liu

Mercury isotope evidence for recurrent photic-zone euxinia triggered by enhanced terrestrial nutrient inputs during the Late Devonian mass extinction

• DOI: 10.1016/j.epsl.2023.118175
• 发表时间: 2023-07
• 期刊: Earth and Planetary Science Letters
• 影响因子: 5.3
• 作者: Wang Zheng;G. Gilleaudeau;T. Algeo;Yaqiu Zhao;Yi Song;Yuanming Zhang;S. Sahoo;A. Anbar;S. Carmichael;S. Xie;Cong-Qiang Liu;Jiubin Chen