Collaborative Research: Calibration, Validation, and Application of Barium Isotopes in Marine Barite as a Tracer of the Marine Carbon Cycle

海洋的生物泵指的是一系列化学、物理和生物过程，这些过程将碳和营养物质从阳光照射的上层海洋重新分配到深海和海底。在全球范围内，这种泵在调节空气-海洋二氧化碳平衡方面发挥着重要作用，从而影响地球的气候。然而，重建海洋生物泵的强度和效率的波动是无法直接完成的，因为有太多的变量在起作用，海洋是如此之大。因此，使用地球化学指标，称为代理，对复杂过程具有特定的可测量响应。大多数海洋生物泵的地球化学指标只对当地条件敏感。本研究涉及一种新型地球化学示踪剂（矿物重晶石中钡的稳定同位素）的勘探、校准和验证，该示踪剂被假设对区域尺度的生物地球化学过程敏感。为了检验这种示踪剂，重晶石将从赤道太平洋的海洋沉积物岩心中分离出来。重晶石的出现被广泛解释为反映了与海洋生物泵有关的局部生物地球化学过程。因此，建议的代理具有同时揭示局部和区域尺度过程信息的好处，尽管代理信号的可靠解释首先需要在现代环境中进行校准和验证。本研究的目的是在赤道太平洋的一组岩心中对这一代理进行校准和验证。预计钡同位素方法将弥合生物地球化学的尺度鸿沟，并能够更准确地重建整个地球历史上的生物碳泵。这项工作的更广泛影响包括学生培训，通过开发新的生物地球化学代理来建立科学基础设施，参与科学中代表性不足的群体的学生的研究，支持早期职业科学家，以及支持科学中性别代表性不足的研究人员。本研究的重点是研究海洋重晶石钡同位素分析在区域空间尺度上重建海洋碳循环的效用。该项目涉及双管齐下的方法。研究的第一阶段需要通过对赤道太平洋现代岩心顶部沉积物的广泛调查来校准代理本身。将分析位于同一位置的沉积物和孔隙水的样本。重晶石将通过酸消解和其他分离技术从岩心中分离出来，并使用扫描电子显微镜及其集成化学分析仪检测纯度。孔隙水将通过标准湿化学技术进行分析。在确保重晶石样品的纯度后，将测量每个岩心顶部的重晶石的钡稳定同位素。研究的第二阶段通过对岩心内同位矿物分离和孔隙水地球化学分析来评估成岩蚀变过程中代理物的保真度。这一阶段的工作是测试埋在海底后是否发生了钡同位素变化，以及发生了多少变化。第一阶段和第二阶段的结果将用于确定海洋重晶石的钡同位素组成是否忠实地记录了海洋生物地球化学。如果是这样，该项目将成功校准和验证一种重建区域尺度海洋碳循环的新方法，这将在一系列时间尺度上具有实用性。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The ocean's biological pump refers to a set of chemical, physical, and biological processes that redistribute carbon and nutrients from the sunlit upper ocean to the deep ocean and seafloor. Globally, this pump has an important role in modulating the air-sea balance of carbon dioxide and can thus impact Earth's climate. However, reconstructing fluctuations in the strength and efficiency of the oceans biological pump cannot be done directly because there are so many variables in play and the ocean is so large. Thus, geochemical indicators, called proxies, that have a specific measurable response to a complex process are used. Most geochemical proxies for the ocean biological pump are sensitive only to local conditions. This research involves the exploration, calibration, and validation of a novel geochemical tracer (the stable isotopes of barium in the mineral barite) that is hypothesized to be sensitive to regional-scale biogeochemical processes. To examine this tracer, barite will be separated from marine sediment cores from the Equatorial Pacific. Barite's occurrence is widely interpreted as reflecting local biogeochemical processes related to the ocean's biological pump. Thus, the proposed proxy has the benefit of simultaneously revealing information on both local- and regional-scale processes, though reliable interpretation of proxy signals first requires calibration and validation in modern settings. This research sets out to do the calibration and validation of this proxy in a set of cores from the Equatorial Pacific Ocean. It is anticipated that the barium isotope approach will bridge the biogeochemical scaling divide and enable more accurate reconstructions of the biological carbon pump throughout Earth's history. Broader impacts of the work include student training, building infrastructure for science by developing a new biogeochemical proxy, engagement in the research of students from groups underrepresented in the sciences, support an early career scientist, and support of a researcher whose gender is underrepresented in the sciences. This research focuses on examining the utility of barium-isotopic analyses of marine barite to reconstruct the ocean's carbon cycle over regional spatial scales. The project involves a two pronged approach. Phase one of the research entails calibration of the proxy itself by means of an extensive survey of modern core-top sediments from the Equatorial Pacific. Samples of co-located sediments and pore waters will be analyzed. Barite will be separated from the cores via acid digestion and other separation techniques and examined for purity using a scanning electron microscope and its integrated chemical analyzer. Pore waters will be analyzed by standard wet-chemical techniques. After ensuring purity of the barite samples, the barium stable isotopes of the barite will be measured for each of the core tops. Phase two of the research assesses the fidelity of the proxy during diagenetic alteration down core via the analysis of co-located mineral separates and pore water geochemistry. This stage of the work tests whether and how much barium-isotopic modification occurs after burial on the seafloor. Results from phase one and two will be used to establish whether the barium-isotopic composition of marine barite faithfully records ocean biogeochemistry. If so, this project will have successfully calibrated and validated a novel means to reconstruct regional-scale marine carbon cycling, which will have utility over a range of timescales.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.