Biomineralization in Deep-sea Corals: Empirical Tracer Calibration Coupled to a Mechanistic Model of Skeletal Growth

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Our direct record of how the climate has changed is limited to the relatively recent period of time where calibrated instruments (thermometers, hygrometers, wind vanes, etc.) have been deployed. This instrumental record covers the last 100-150 years. If we want to exploit the large changes in past climate to better understand how the system works, we need archives capable of recording 'proxies' of the variables we care about. Far and away the most successful archive of past climate from the oceans comes from the shells of calcium carbonate secreting organisms. Shells from open ocean dwellers like foraminifera and coccoliths make up a large percentage of the seafloor sediments. Archives also include the annually banded skeletons of corals and mollusks. However, all of these incorporate proxies for past climate (Mg/Ca as a thermometer for instance) through the filter of biomineralization. These organisms modify the chemistry of their skeletons in only partially known ways, and these 'vital effects' hinder our ability to decode how climate has changed in the past. This project aims to better understand this biomineralization process through a combination of well-posed observations and a newly successful model of the calcification process. This project will focus on deep-sea corals because these animals live in an environment today that does not change very much. This makes them a good 'lab rat' since we can study a modern organism in the wild and still have controlled experiments. The broader impacts of this project include support for a graduate student to work directly on the research and outreach through public lectures and videos to reach a wide range of audiences.The deep-sea coral D. dianthus has a regular banding pattern and correlated geochemical tracer values that follow the banding. These patterns, and the ubiquity and usefulness of the archive, make this species an ideal target for further tracer development. Growing in near constant conditions, variations in stable isotopes and Me/Ca ratios within this coral's skeleton can only be due to 'vital effects'. This project will develop better thermometers and carbonate system proxies in D. dianthus using a two-pronged approach. The existing modern sample collection contains 41 individuals from a wide range of temperature, carbonate and phosphate concentrations. Bulk measurements of Li, B, Mg, Ca, Sr, Ba, and U using a multi-element ICP-MS method will give the empirical relationship between skeleton and seawater. Microanalysis of individual skeletons using SIMS and NanoSIMS will elucidate the internal structure of the geochemistry and add del11B. A new model of the biomineralization process in the coral's Extracellular Calcifying Fluid shows that it can correctly predict the linear relationship between del18O and del13C seen in D. dianthus and a wide array of marine calcifiers. This project will expand this model to include Me/Ca ratios and to test it against the SIMS and bulk analysis observations. The goal of the project is to create better tracers through improved characterization of the biomineralization process. While the model is already showing its worth, the plan to collect an empirical modern sample calibration dataset ensures that it can also improve modern calibrations.