Geochemical signatures in the coral Montastraea: Modern and mid-Holocene perspectives

In the first phase of this project, four decades of monthly resolved geochemical variations from two massive heads of Montastraea were used to explore the reproducibility of the geochemical signal in these two corals from Looe Key, Florida. The coral d18O and d13C records of the two corals have statistically indistinguishable mean values, which is not the case for the coral Sr/Ca records implying that nonenvironmental factors are influencing coral Sr/Ca. Calibration equations relating coral geochemistry variations to environmental variations at Looe Key are different from previously published equations for Montastraea. These calibration differences are not related to growth-related kinetic effects, but may reflect variations in seawater chemistry in the coastal waters of the Florida Keys. Additional studies are needed to identify the causes of the observed geochemical variability. In the second phase of this study, fourteen decades of monthly resolved geochemical variations in another Montastraea coral from Looe Key, Florida were compared to records of sea-surface temperature (SST). Coral Sr/Ca and d18O variations have a weak relationship with variations in SST and skeletal extension rates; however, many events in the Sr/Ca and d18O records are coincident with anomalies in SST, growth, or precipitation. Strong coupling exists between Sr/Ca and d18O in both anomaly and mean annual perspectives, which reflects the combined influence of SST and growth related processes on the geochemical signal. Separating these impacts proved to be problematic due to modest agreements with each forcing variable. In the final phase of this study, geochemical records from three, mid-Holocene(~5 ka) fossil Montastraea corals from the Dry Tortugas, Florida were compared with geochemical records from modern Montastraea corals from the same region to investigate temporal changes in climate. Stable isotopic records show significant changes through time, which can be interpreted in terms of environmental variation; however, large inter-coral variability between modern specimens of Montastraea precludes meaningful assessment of Sr/Ca. The pattern and mean d18O values in the fossil corals reflects changes in both temperature and salinity are reminiscent of centennial-scale variability present in other records from this region