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

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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.