NSF GEO-NERC: Constraining the oxic marine sink of novel metal isotope proxies to underpin paleoceanographic reconstructions

地球上生命的进化与海洋中的条件密切相关，包括氧气水平。了解海洋何时开始含氧，以及氧气水平如何随时间变化，对于了解我们星球的历史非常重要。海洋沉积物中的化学线索可以用来解释过去的情况。目前正在研究的用于指示过去氧气水平的工具包括某些微量元素的同位素含量的变化，包括铊、钼、铀和锌。在这些工具能够可靠地应用于过去的沉积物之前，我们必须了解这些元素在当今海洋中是如何循环的，以及它们是如何被纳入沉积物的。虽然迄今为止的大多数工作都在研究这些元素和同位素是如何被纳入贫氧沉积物中的，但这项研究将在遍布世界海洋的富氧沉积物中测量这些同位素。这项研究将提供海洋同位素循环的基本界限，以更好地了解现代海洋化学，并最终了解生命和海洋地球化学如何在地球历史上共同进化。这项研究将有助于三名早期职业科学家的专业发展和培训，并促进国际合作。这是一个由国家科学基金会地球科学理事会（NSF/GEO）和联合王国国家环境研究理事会（NERC）通过NSF/GEO-NERC牵头机构协议共同资助的项目。该协议允许美国/英国提交一份联合提案，并由研究者拥有最大预算比例的机构进行同行评审。在成功地共同确定一个奖项后，每个机构为预算的一部分和与本国有关的调查人员提供资金，该项目旨在了解海洋中铊（Tl）、钼（Mo）、铀（U）和锌（Zn）同位素的现代循环，并通过限制含氧深海沉积物汇来改善对古海洋学代用指标的解释。这一目标将通过分析在每个海洋盆地收集的73个海洋沉积物岩心顶部样品的新组装的一套含氢组分的Tl，Mo，U和Zn同位素组成来实现。沉积物的自生成分将使用旨在仅溶解自生矿物的部分溶解技术来分离。这些方法的鲁棒性将使用散装沉积物矿物组合（通过XRD获得）以及散装和部分溶解样品的主要和微量元素组成进行彻底测试。生成的数据集将使海洋氧输出通量的大小（230 Th约束）和稳定同位素分馏的Tl，Mo，U和Zn的评价。一项试点研究表明，这些元素的同位素分馏在沉积物中的铁氧化物和锰氧化物之间是不同的，这在铁锰结壳和结核中也是独特的。通过将试点研究扩展到全球范围，我们将能够评估如何在海水中的铊，钼，铀和锌同位素先前公布的记录可能需要重新解释，由于不同的同位素分馏模式在氧化海洋sink.This奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

The evolution of life on Earth is closely tied to conditions, including oxygen levels, in the oceans. Understanding when the oceans first became oxygenated, and how oxygen levels have varied through time, is important to understanding the history of our planet. Chemical clues in marine sediments can be useful in deciphering past conditions. Tools that are currently being investigated as indicators of past oxygen levels include changes in the isotope content of certain trace elements, including thallium, molybdenum, uranium, and zinc. Before these tools can reliably be applied to past sediments, it is important that we understand how these elements are cycled in the present-day ocean, and how they are incorporated into sediments. While most work to date has looked at how these elements and isotopes are incorporated into oxygen-poor sediments, this study would measure these isotopes in a collection of oxygen-rich sediments that spans the world’s oceans. This study would provide essential bounds on marine isotope cycles to better understand modern ocean chemistry and, ultimately, how life and ocean geochemistry co-evolved over Earth history. This study would contribute to the professional development and training of three early-career scientists, and foster international collaboration. This is a project that is jointly funded by the National Science Foundation’s Directorate of Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the United Kingdom (UK) via the NSF/GEO-NERC Lead Agency Agreement. This Agreement allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award, each Agency funds the proportion of the budget and the investigators associated with its own country.This project seeks to understand the modern day cycling of thallium (Tl), molybdenum (Mo), uranium (U) and zinc (Zn) isotopes in the ocean and improve interpretations of paleoceanographic proxies by constraining the oxic deep-sea sediment sink. This goal will be achieved by analyzing the Tl, Mo, U and Zn isotope compositions of the hydrogenous component in a newly assembled set of 73 marine sediment core top samples collected across every ocean basin. The authigenic component of the sediments will be isolated using partial dissolution techniques aimed at only dissolving authigenic minerals. The robustness of these methods will be thoroughly tested using bulk sediment mineral assemblages (obtained by XRD) as well as major and trace element compositions of bulk and partially dissolved samples. The generated dataset will enable an evaluation of the marine oxic output flux magnitude (constrained with 230Th) and stable isotope fractionation for Tl, Mo, U and Zn. A pilot study suggests that the isotope fractionation of these elements is different between Fe-oxides and Mn-oxides within the sediment, which are also unique from ferromanganese crusts and nodules. By expanding the pilot study to a global scale, we will be able to evaluate how previously published records of Tl, Mo, U and Zn isotopes in seawater may need to be reinterpreted due to diverse isotope fractionation patterns in the oxic marine sink.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.