Development of proxy seawater records from gorgonian coral skeletons
Information about the history of Earth’s climate system is largely derived from the chemical composition of biogenic skeletons preserved in the geologic record. The isotope and elemental composition of scleractinian corals have proven to be useful tools for palaeoceanographic reconstruction, providing information about the surface and deep oceans on time scales of decades to millennia. The potential of gorgonian corals (Cnidaria: Octocorallia) as archives of past oceanic conditions is less well explored. Gorgonians are widely distributed throughout the world’s oceans from the shallows to great depths, and with banded skeletons formed of organic gorgonin and calcite, the possibility exists to measure previously unobtainable proxy records. This Ph.D. focuses on the development of new methods for extracting proxy information from gorgonian coral skeletons and investigating the potential of gorgonians to record the conditions of the seawater in which they grew. Bermudian shallow water gorgonian colonies (Pseudoplexaura porosa, Plexaurella dichotoma and Plexaurella nutans) were sampled and their axial skeleton structure examined. The periodicity of skeletal banding was investigated using a series of staining and isotopic marking experiments, as well as the measurement of a radiocarbon time series. Mg/Ca and Sr/Ca ratios were measured in skeletal calcite using SIMS ion microprobe. Mg/Ca ratios show an overall, positive correlation with annual sea surface temperatures (SSTs) that is strongest in the autumn months (October–December). Interannual variability in Sr/Ca does not follow SST and may be influenced primarily by growth rate. A technique was developed to measure Tritium (3H) in skeletal gorgonin. Tritium, produced by nuclear weapons testing in the 1950s and 1960s, is a valuable diagnostic tool for ocean circulation models because it was produced in a spike-like fashion and has a strong north-south hemispheric contrast. A significant limitation to the utility of tritium observations in ocean circulation models is our imperfect knowledge of the time history of surface water concentrations. The technique developed here proves that gorgonian corals have the potential to provide a detailed time history of surface water tritium concentrations at a variety of locations. This thesis has successfully combined a wide range of techniques, some newly developed, to illustrate the potential of gorgonian corals as recorders of seawater conditions in the recent past. Because of their wide geographic and depth distribution, as well as their organic and inorganic skeletal components, gorgonian corals, although largely over looked thus far, should prove important archives of past oceanic conditions in the future.