Undergraduate Research Opportunities Program (UROP), Earth & Environmental Sciences, University of Minnesota DuluthRecords of sea level are important for scientists studying climate and geology in the past. Sea level is correlated with how much water is stored in glaciers and ice sheets, which indicates what the climate was like at that time. Past climate records are the key to understanding the future as Earth's climate changes. One method of documenting past sea level is with radiometric uranium-thorium (U-Th) dating of fossil corals. Corals make a good proxy for sea level because they need to grow near the surface of the water, where light is most abundant. Coral concentrate 238U from seawater in their skeleton (Gallup, 2015). Over time, 238U decays to 234U and then to 230Th at a predictable rate. When they first form, corals do not contain any 230Th, because it is insoluble. Thus, the concentration of 230Th in fossil coral skeletons allows us to determine their ages (Gallup, 2015). The elevation that the sample was taken from is then presumably the sea level at the time that coral was alive. In some cases, the coral is at a higher elevation due to tectonic uplift, but this can be corrected for. It's important for the sea level record to be accurate, since it serves as a reference for other research. Thus, it's crucial that U-Th dating methods be as accurate as possible. However, coral fossils are susceptible to alteration after their deposition, which can cause remobilization of 238U and 230Th (Gallup, 2015). The result is a shift in the apparent age of the sample. The quality of the sample can be inferred by looking at the ratio of 234U to its parent, 238U. The ratio at the time of deposition, the initial 234U value, can be calculated from the ratio at the time of analysis. How far it is from the accepted value for seawater at the time of deposition is an indicator of sample quality. Corals with a larger surface area are more susceptible to alteration, since they have more area for chemical and physical weathering. Five different types of coral, most notably Porites, Favids, and Leptids, were studied. Out of these, Porites has highest surface area (El-Sorogy, et. al, 2013), and is therefore more susceptible to alteration over time
