RUI: Can water in coral skeletons serve as an archive of environmental conditions?

Saint Olaf College, Northfield MN

Investigators

Abstract

This proposal seeks to answer the question: where does water exist in coral skeletons and what can this water tell us about global environmental change? The composition of fossil coral skeletons provides records of past environmental change which are used to understand drivers of past climate variability and contextualize modern climate change. The most common sources of such records target the geochemical composition of the mineral that corals produce. For example, measurements of Sr/Ca in coral skeletons allow us to reconstruct past temperature because Sr substitutes for Ca in the coral skeleton’s mineral aragonite lattice in a temperature-dependent manner. However, the mineral aragonite is not the only phase that exists in coral skeletons. For decades, it has been known that coral aragonite contains ~1% by weight of water. While this water has been investigated for its potential to trap samples of ancient seawater, the water has never been visualized in situ in the coral skeleton. The gap in our understanding of how and where coral skeletons archive water limits the ability to leverage the presence of water in corals as a paleoclimate archive. This research will create 2D and 3D maps of water in lab-grown coral skeletons using nanoscale imaging techniques. Proposed experiments will also fingerprint the origin of water in coral skeletons and test the ability of coral skeletons to retain water after the skeleton is formed. Ultimately, this work could open the door to using water in coral skeletons as an archive of past seawater conditions, helping to develop better models/forecasts and conservation efforts for coral reefs. The work will also support an early career scientist, transitioning to mid-career, at a Primarily Undergraduate Institution. The project will provide opportunities for at least ten St. Olaf College undergraduate students from a range of STEM disciplines to engage in sustained research with mentorship designed to support underrepresented groups in STEM. Additionally, the PI will co-create a science communication conference for academic and non-academic participants with St. Olaf’s public-facing Institute for Freedom and Community. Water in coral skeletons has been investigated for its paleoproxy potential, implicated as a facilitator of diagenetic transformations, and linked to the presence of ‘vital effects’ (i.e. geochemical patterns in coral aragonite that differ from those expected for inorganic aragonite). At present, however, the scientific community lacks knowledge about where this water exists and its origin. Specifically, water has never yet been visualized in coral skeletons and its exact origin and form is not confidently known. This project seeks to improve the understanding of water’s distribution and origin in coral skeletons by creating 2D and 3D maps of the distribution of water in cultured cold-water corals (Balanophyllia elegans) grown in deuterated water using Nanoscale Infrared Spectroscopy (AFM-IR) and Atom Probe Tomography (APT), respectively. Given concerns about contamination with ambient water, the analysis of corals cultured in deuterated water will ensure that these analyses identify water incorporated into the coral skeleton during growth, and not water introduced by contamination during sample preparation. Additionally, this work will test the ability of corals to retain intra-skeletal water incorporated at the time of calcification using deuterium isotope immersion experiments with natural corals that will subsequently be analyzed using thermogravimetric analysis coupled with fourier transform infrared spectroscopy. These analyses will help determine whether the isotopic composition of water in the sample (whether it is free or bound to organics) has been overprinted during the immersion process. Finally, the proposed work will test if coral water records coral metabolism, seawater properties, or a combination of both. Specifically, this work will produce cultured corals grown in solutions bubbled with Ar gas having different isotopic compositions in order to ground-truth whether coral skeletons record dissolved noble gasses in seawater. It will also produce cultured samples fed with deuterium-labeled food in order to explore the impact of coral metabolism on the isotopic composition of water measured in the coral skeletons. If the proposed work shows that water in corals can be used as an archive of seawater isotopic composition, coral metabolism, and/or noble gasses, it could lead to big-picture impacts such as improving our understanding of hydrological variability through new records of seawater δ18O or the response of coral metabolism to thermal stress and ocean acidification. 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.