EAR-Climate: Estimating Seawater Boron Isotope Ratios from Halite Evaporites

这项研究将通过解决海水中硼的组成如何随时间变化而变化，改善对3600万年前海洋pH值和大气二氧化碳浓度的估计。我们需要知道海水中硼的组成，以便利用化石壳中的少量硼来确定它们生长的水的pH值以及与水平衡的二氧化碳。该项目将分析岩盐沉积物，其中硼存在于盐晶体中捕获的少量蒸发海水中，以及矿物质中。通过分析这些盐沉积物中的硼，并确定古代海水中硼的组成，这项研究将使化石壳中的硼转化为更准确的大气二氧化碳估计，这将提高对气候和二氧化碳之间联系的理解。该项目将支持两名早期职业女性科学家和一名未被充分代表的少数族裔科学家，研究人员将与芝加哥南部的一个高中教育推广项目分享她们在这些主题上的科学专业知识。该项目将研究卤石蒸发岩地层中硼的行为，以确定含硼的相（例如流体包裹体或散布在卤石中的微量矿物质），并限制古代海水中硼的稳定同位素组成。通过测量几个含流体包裹体的岩盐亚样品的主要元素组成，建立了大量溶解岩盐样品中含有硼的各种矿物和流体组分之间的混合关系，从而可以计算流体包裹体的端元组成和古代海水的硼同位素组成。实验沉淀的岩盐、天然盐芯以及最终来自西班牙的约3600万年前的海洋岩盐样本将在本项目中进行研究。电感耦合等离子体质谱法将在芝加哥大学的地球化学设施中建立硼的稳定同位素分析。本研究除了对海洋pH和碳酸盐体系提供独立约束外，还将确定硼在石膏中的分布系数，解决硼地球化学循环的长期演化问题。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。

This research will improve estimates of ocean pH and atmospheric carbon dioxide concentrations ~36 million years ago by resolving how the composition of boron in seawater has changed over time. The composition of boron in seawater needs to be known in order use the small amounts of boron in fossil shells to determine the pH of the water they grew in and the carbon dioxide that would be in equilibrium with that water. This project will analyze rock salt deposits where boron is present in small amounts of evaporated seawater that are captured within the salt crystals, as well as within minerals. By analyzing the boron in these salt deposits and determining the composition of boron in ancient seawater, this research will allow the boron in fossil shells to be translated into more accurate estimates for atmospheric carbon dioxide, which will improve understanding about the connection between climate and carbon dioxide. The project will support two early-career women scientists and one under-represented minority scientist, and the researchers will share their scientific expertise on these topics with a high-school educational outreach program on the South Side of Chicago.The project will investigate the behavior of boron in halite evaporite formations to determine the phases that host boron (e.g. fluid inclusions or trace minerals disseminated in the halite) and constrain the stable isotopic composition of boron in ancient seawater. By measuring major element compositions across several halite subsamples bearing fluid inclusions, mixing relationships between the various mineral and fluid components that contribute boron to bulk dissolved halite samples will be established, which will allow for calculation of the endmember composition of the fluid inclusions and the boron isotopic composition of ancient seawater. Experimentally precipitated halites, natural salt cores, and ultimately a ~36 million year old marine halite sample from Spain will be studied in this project. Stable isotope analysis of boron by inductively coupled plasma mass spectrometry will be established in the geochemical facilities at the University of Chicago. In addition to providing an independent constraint on ocean pH and the carbonate system, this research will also determine the distribution coefficient of boron in gypsum and resolve the long-term evolution of the boron geochemical cycle.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.