Theoretical and Practical Development of Aragonite Li/Mg as a Deep Sea Paleotemperature Proxy

古海洋学旨在了解海洋在过去气候变化中的作用。主要目标之一是改进对未来气候变化的预测。重建的一个重要因素是深海的温度。今天，海洋温度与深海环流和海洋储存温室气体二氧化碳远离大气层有关。然而，现有的工具或深海温度的“代理人”具有相对较大的不确定性，阻碍了对过去气候的理解。这项研究将进一步开发一种有前途的新替代物，其基础是被称为底栖有孔虫的海底生物的微观外壳中锂和镁元素的含量。该研究将包括实验室实验、海底样本校准以及对最后一个冰河时代的测试应用。将为研究生和本科生参与研究提供支持。过去海洋纬向翻转环流的变化反映在温度和盐度上（即，深海的结构（密度），对碳储存以及区域和全球气候都有重要影响。生物成因碳酸钙的元素比值被广泛用作古温度指标，但其精度和准确性往往受到与生物成矿作用有关的次生影响的限制。该项目将开发和应用一种新的工具，用于重建深海过去的温度。以前的工作表明，生物成因文石中锂和镁（Li/Mg）之间的比例几乎完全由温度（r2 0.9）解释，阻碍大多数元素代理的所谓“生命效应”的影响最小。更广泛的应用，这一工具将受益于更强的理论基础，改善现场校准，并应用于一个重要的古海洋学问题。为此，将测试四个假设：（1）在给定温度下，生物成因文石Li/Ca对Mg/Ca等温线的斜率（即，Li/Mg值）仅取决于温度依赖的非生物分配系数KLi/Ca和KMg/Ca。(2)非生物文石锂和镁的分配系数是速率依赖性的，但生物Li/Mg是受动力学影响最小。(3)在寒冷的沃茨中，世界性的底栖有孔虫Hoeglundina elegans Li/Mg对温度的响应比珊瑚的指数拟合稍陡。(4)在末次盛冰期（LGM，19-23 ka）期间，接近冰点（-2至-1 °C）的南极底层水占据了大西洋2-2.5 km深度以下，并被温暖（0至1 °C）的北大西洋冰川中层水覆盖。假设（1）和（2）的检验将通过实验室降水实验进行，假设（3）将通过岩心顶部校准进行，假设（4）将通过大西洋的LGM深度剖面进行。该奖项反映了NSF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 field of paleoceanography seeks to understand the ocean’s role in past climate changes. One of the primary goals is to improve projections of future climate change. An important factor to reconstruct is the temperature of the deep ocean. Today ocean temperature is related to deep ocean circulation and the ocean’s storage of the greenhouse gas carbon dioxide away from the atmosphere. However, existing tools or ‘proxies’ for deep ocean temperature have relatively large uncertainties that hinder the understanding of past climates. This research will further develop a promising new proxy, based on the amounts of the elements lithium and magnesium in the microscopic shells of seafloor organisms called benthic foraminifera. The study will include laboratory experiments, calibration with seafloor samples, and a test application to the last ice age. Support will be provided for graduate and undergraduate students to participate in the research. Past changes in the ocean’s meridional overturning circulation are reflected in the temperature and salinity (i.e., density) structure of the deep ocean, with important consequences for carbon storage and for regional and global climates. Elemental ratios in biogenic calcium carbonates are widely applied as paleotemperature proxies, but precision and accuracy are often limited by secondary influences related to biomineralization. This project will develop and apply a nascent tool for reconstructing the past temperature of the deep ocean. Previous work has shown that the ratio between lithium and magnesium (Li/Mg) in biogenic aragonite is almost entirely explained by temperature (r2 0.9), with minimal influence from the so-called ‘vital effects’ that hamper most elemental proxies. Broader application of this tool will benefit from stronger theoretical grounding, improved field calibration, and application to an important paleoceanographic problem. To those ends, four hypotheses will be tested: (1) At a given temperature, the slope of the biogenic aragonite Li/Ca vs. Mg/Ca isotherm (i.e., the Li/Mg value) depends only on the temperature-dependent abiotic partition coefficients KLi/Ca and KMg/Ca. (2) Abiotic aragonite Li and Mg partition coefficients are rate-dependent, but biogenic Li/Mg is minimally affected by kinetics. (3) In cold waters, the response of the cosmopolitan benthic foraminifer Hoeglundina elegans Li/Mg to temperature is slightly steeper than the exponential fit that characterizes corals. (4) During the Last Glacial Maximum (LGM, 19-23 ka), near-freezing (-2 to -1 °C) Antarctic Bottom Water occupied the Atlantic below ~2-2.5 km depth, and was overlain by warmer (0 to 1 °C) Glacial North Atlantic Intermediate Water. Testing of Hypotheses (1) and (2) will be through laboratory precipitation experiments, Hypothesis (3) through core top calibration, and Hypothesis (4) through LGM depth transects in the Atlantic.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.