Subannual coral Ba/Ca variability in Siderastreasiderea from the northwest Gulf of Mexico.
Supervisors: Kristine L. DeLong (Louisiana State University), Thierry Correge (Univ. Bordeaux)
Geochemistry of a coral skeleton varies with marine environmental changes with time, thus creating an archive to infer past environmental changes. Coral Ba/Ca is a proxy for seawater barium content (Ba) that is complicated by multiple factors including upwelling, terrigenous input, and sea surface productivity. This study examines subannual variations of Ba/Ca and Sr/Ca for two time intervals (1929–1942 and 1976–2005) in a Siderastreasidereacoral from West Flower Garden Bank (27.8⁰N, 93.8⁰W) located 185 km away from the Louisiana and Texas coast, far from terrigenous runoff and coastal upwelling. The regular seasonal coral Ba/Ca cycles are quasi-in-phase with sea surface temperature and thus coral Sr/Ca (a temperature proxy) but with a different seasonal pattern, and the coral Ba/Ca lacks the large anomalies commonly seen in corals located in upwelling areas or near rivers. Chlorophyll-a concentration, an indicator of sea surface productivity, which varies seasonally in the Gulf of Mexico (GoM), negatively correlates with coral Ba/Ca (r = –0.56, p < 0.05) representing 31.4% of the coral Ba/Ca seasonal cycle. Ba behaves like a nutrient; therefore, an increase in primary production in the winter months reduces Ba and decrease coral Ba/Ca values. There is a significant (p < 0.01) increase in mean coral Ba/Ca of 1.76 ±0.40 (2σ) μmol/mol between the two time intervals examined. After the first offshore oil discovery in the GoM in 1947, large-scale operations escalated in the late 1970s. Offshore oil exploration uses barite (BaSO4) in drilling muds that are released into the seafloor as waste and thus could be the source for increased coral Ba/Ca and Ba in the GoM. Annual coral Ba/Ca from 1976–2005 corresponds to changes in annual barite consumption in the United States, where the GoM is the largest oil field. There is a relatively small but regular increase in the winter to early spring in coral Ba/Ca, especially from 1929–1942, that reflects wind-driven upwelling when the winds are stronger and is dampened or missing after the 1980s oil boom in the GoM. Additionally, production waters from oil platforms also add barium and heavy metals into the marine environment; therefore, coral Ba/Ca may potentially be a tracer for marine environmental impacts caused by the offshore oil industry as well as ocean biological productivity.