By Edmund Gable

Faculty Mentor: Dr. Pamela Grothe

Abstract

The response of warming and freshening oceanic conditions in the CTP from rising greenhouse gas concentrations is unknown because instrumental SST and SSS measurements only date back to 1950 C.E. With such a short modern record, it is important to develop proxies for SST and SSS that can provide accurate reconstructions of climate and extend this short observational period deeper into the past, to before the Industrial Era. My objectives are: 1) to assess the reliability of untested coral-species as a climate proxy using geochemical records of oxygen isotopes (δ18O) and strontium calcium ratios (Sr/Ca) and 2) to develop a more reliable SST geochemical proxy using the novel Strontium-uranium (Sr-U) method. Long-lived reef-building Scleractinia boulder corals have calcium carbonate skeletons that grow outwards at rates up to 15 mm/year. The chemical composition of the coral skeleton changes in relation to the environmental conditions of the seawater, such as temperature, salinity, light availability, and nutrients. Therefore, high-resolution geochemical data provide valuable information on the environmental conditions at the time the coral was living. A commonly used proxy for SST is the trace element ratios of strontium and calcium (Sr/Ca). Sr/Ca is inversely related to SST and insensitive to conditions such as salinity, light, and nutrients, making it a viable proxy for temperature (Schrag et al., 1999). Currently, in the CTP, these geochemical proxies are only extensively calibrated to the Scleractinia genus Porites. While Porites corals are abundant in the CTP, it is challenging to find enough samples of a single genus in time and space to reproduce a longer continuous climate record. Thus, I analyzed the capability of a novel species of boulder coral, Hydnophora microconos, to track climate using paired δ18O and Sr/Ca proxies. The results of my research revealed that while δ18O and Sr/Ca proxies were capable of accurately tracking SST over time, each individual coral sample required a different linear calibration between the proxy and SST. This is due to vital effects, which mandate that every coral colony will incorporate ocean chemistry slightly differently in response to environmental parameters due to differences in growth rates and genomes. I am currently developing methods of SST tracking using the Strontium-Uranium thermometer, which isolates the SST component from differences in carbonate chemistry due to vital effects by pairing Sr/Ca with U/Ca measurements. This will standardize the calibration of Sr/Ca measurements to SST, massively expanding the reliable coral archive to deepen our understanding of climatic processes. In addition, I will test the ability of UMW’s new ICP-MS instrument to measure precise levels of trace metals, expanding the methodology of the Sr-U thermometer to make it more accessible to Universities without high power instruments.