5th UF Water Institute Symposium Abstract

Submitter's Name Katherine Tison
Session Name Poster Session - Coastal Waters
Poster Number 16
Author(s) Katherine Tison,  University of Florida (Presenting Author)
  Andrea Dutton,  University of Florida
  Improving accuracy of past sea level chronologies: Testing assumptions about seawater uranium isotope composition
  To understand how coastal water resources will be affected by sea-level rise, it is imperative to constrain the potential rates of sea-level rise in a warmer world. Data of sea-level rise that occurred during past warm periods can be used to constrain potential rates of rise. One common technique used to reconstruct sea level is to determine the age and elevation of fossil corals that lived near the sea surface to develop a history of past sea level position. One challenge to using fossil corals as a proxy for past sea-level position is that the skeletal aragonite that is dated using the U-series radiometric dating technique can undergo geochemical alteration. Modern coral skeletons have the same uranium isotope composition (234U) as ambient seawater during growth. Because geochemical alteration tends to impart anomalously high 234U values, if the 234U at the time of growth is significantly different from modern seawater, then the data is thrown out based on the assumption that the coralline aragonite has been altered. Here we test the assumption that modern corals have the same 234U value as modern, open ocean seawater samples that display remarkably homogeneous 234U values. Although the mean value of modern coral data is close to that of open-ocean seawater, the variability is greater than that expected from analytical uncertainty alone, suggesting that there may be an additional source of variability. Riverine 234U can be much higher than that of open-ocean seawater and in coastal zones this may cause elevated 234U values of both the water and the corals growing there. To test the null hypothesis that seawater in coral reef environments is equal to that of the open ocean, we will measure the 234U of seawater samples collected from modern reef settings around the globe.