Collaborative Research: Towards a Better Understanding of Tl Isotope Cycling under Different Redox Conditions

作为重建地球古代海洋中分子氧(O2)历史的工具，铊(Tl)元素的势头正在增强。重建这段历史很重要，因为地球古代海洋中氧气的可用性在地球上生命的起源和进化中发挥了关键的支配作用。此外，现代海洋的持续脱氧将影响地球上的每一个人，如果我们了解地球过去发生的类似事件，我们就可以更好地预测和准备这种脱氧。热释光用于跟踪海洋O2的变化源于这样一个事实，即其同位素因与锰(锰)氧化物矿物的相互作用而强烈分馏，而锰(锰)氧化物矿物目前只有在氧气存在的地方才形成和埋藏在海洋沉积物中。事实上，许多研究表明，保存在古代海岩中的Tl同位素比率可以提供有关海洋中过去氧气可获得性的重要信息。然而，我们对现代Tl同位素循环的了解还很不完整，这主要是因为在今天的环境中发现的Tl丰度极低，这使得收集足够的材料进行准确的Tl同位素分析变得困难。PI在科德角微咸水池塘(Siders Pond)的初步工作表明，在Tl丰度很低的环境中生成水、颗粒和沉积物的高质量Tl同位素数据是可行的。此外，这项工作的早期结果提供了有关连接Tl同位素与锰氧化物以及锰氧化物与O2-连接的重要新信息，这些连接可能也存在于古代海洋环境中。PI计划升级他们在Siders Pond的工作，并将其扩展到明尼苏达州的两个淡水湖(戴明湖和斯蒂尔湖)。这项工作的结果将极大地提高我们对现代Tl同位素循环的理解，进而使我们能够更有信心地使用Tl同位素重建地球过去的海洋氧合作用。这项研究将由一名博士后研究员领导，并将进一步促进几名本科生暑期实习生的教育。与这项研究相关的K-12推广工作将向大波士顿地区的学生和教师介绍生物地球化学和地球科学。在过去的五年里，古代海洋沉积岩中TL同位素比率的测量速度迅速加快，因为有理由认为它们可以跟踪过去海洋氧合作用的变化。不幸的是，指导和磨练解释所必需的现代Tl同位素研究没有跟上古代应用的步伐。PI从氧化还原分层的微咸水池塘(Siders Pond，Cood角)获得的初步数据表明，即使在非常低的Tl浓度下，也可以在自然环境中产生高质量的Tl同位素数据。此外，这些初步数据表明，Tl同位素与当地的锰(Mn)氧化物循环之间存在着强烈的、动态的联系。这些结果突出了氧化锰矿物--而不是O2--在驱动Tl同位素分馏效应方面的直接作用。PIS提议扩大他们对Siders Pond的TL同位素调查，并将目标对准明尼苏达州另外两个具有不同地球化学特征的淡水环境(戴明湖和斯蒂尔湖)。在初步数据的指导下，PI预测，在这些额外环境中的Tl同位素循环也将与当地的Mn循环最直接地耦合，相对较少或没有受到其他Tl相互作用的影响。指导这项研究的目的是(1)更好地了解和量化不同氧化还原条件下Tl及其同位素在水和颗粒之间的分配；(2)更好地了解和量化不同氧化还原条件下Tl及其同位素在沉积物中的保留情况。这些目标将通过实地工作、痕量金属和同位素比率测量以及基于同步加速器的技术相结合来实现。对现代TL同位素循环的更好理解只会有助于加强对古代沉积TL同位素比率的解释--以及它们与过去海洋氧合的联系。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。

The element thallium (Tl) is gaining momentum as a tool for reconstructing the history of molecular oxygen (O2) in Earth’s ancient oceans. Reconstructing this history is important because the availability of O2 in Earth’s ancient oceans played a key governing role in the origin and evolution of life on our planet. Furthermore, ongoing deoxygenation of modern oceans will affect every human on Earth, and we can better predict and prepare for this deoxygenation if we understand comparable events in Earth’s past. The utility of Tl to track changes in marine O2 stems from the fact that its isotopes are strongly fractionated by interactions with manganese (Mn) oxide minerals, which are formed and buried in marine sediments today only where O2 is present. Indeed, many studies show that Tl isotope ratios preserved in ancient marine rocks can provide important information about past O2 availability in the ocean. Yet, our understanding of the modern Tl isotope cycle is far from complete, due largely to the extremely low abundances of Tl found in environments today, which make it difficult to collect enough material for accurate Tl isotopic analysis. The PIs’ preliminary work in a brackish pond on Cape Cod (Siders Pond) show that it is feasible to generate quality Tl isotope data for water, particles, and sediments in an environment with very low Tl abundances. Furthermore, early results from this work provide important new information about the links that connect Tl isotopes to Mn oxides, and Mn oxides to O2 – links that were probably also present in ancient marine environments. It is the PIs’ plan to upscale their work in Siders Pond and extend it to two freshwater lakes in Minnesota (Deming and Steel lakes). The results of this work will vastly improve our understanding of the modern Tl isotope cycle, in-turn allowing for more confident reconstructions of Earth’s past ocean oxygenation using Tl isotopes. This research will be led by a postdoctoral investigator and will further the education of several undergraduate summer interns. K-12 outreach efforts associated with this research will introduce students and teachers in the greater Boston area to biogeochemistry and Earth science. Measurements of Tl isotope ratios in ancient marine sedimentary rocks have rapidly accelerated over the past half-decade because there is reason to think they can track changes in past ocean oxygenation. Unfortunately, the modern Tl isotope investigations necessary to guide and hone interpretations have not kept pace with the ancient applications. The PIs’ preliminary data from a redox-stratified and brackish pond (Siders Pond, Cape Cod) show that it is feasible, even under very low Tl concentrations, to generate quality Tl isotope data for waters, particles, and sediments in a natural setting. Moreover, these preliminary data identify a strong and temporally dynamic link connecting Tl isotopes to local manganese (Mn) oxide cycling. These results highlight the direct role that Mn oxide minerals – and not O2 – play in driving Tl isotope fractionation effects. The PIs are proposing to expand their Tl isotope investigation of Siders Pond, and also to target two additional geochemically distinct freshwater settings in Minnesota (Deming and Steel lakes). Guided by the preliminary data, the PIs predict that Tl isotope cycling in these additional settings will also be most directly coupled to local Mn cycling, with comparatively little to no effects being driven by other Tl interactions. Objectives guiding this research are (1) to better understand and quantify how Tl and its isotopes are partitioned between waters and particles under different redox conditions, and (2) to better understand and quantify how Tl and its isotopes are retained in sediments under different redox conditions. These objectives will be addressed via a combination of fieldwork, trace metal and isotope ratio measurements, and synchrotron-based techniques. An improved understanding of modern Tl isotope cycling will only serve to strengthen interpretations of ancient sedimentary Tl isotope ratios – and their connections to past ocean oxygenation.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.