Secondary ion mass spectrometer geochemical analysis of secondary infilled areas in Stylaster erubescens

Paleoceanographers utilize geochemical proxies, such as oxygen isotopic ratios (delta18O), that are recorded in coral colonies to reconstruct past ocean conditions. Massive hermatypic corals precipitate skeletal aragonite with a delta18O composition that is in disequilibrium with seawater; therefore, compromising their use as a direct proxy for the delta 18O of seawater (delta18Oseawater). Previous studies on the family Stylasteridae, an ahermantypic coral, demonstrate that these corals precipitate aragonite in isotopic equilibrium with seawater. Stylasterid corals contain infilled pore spaces of secondary aragonite within the skeletal microstructure, presumably precipitated later in time. The delta 18O values of these infilled areas have not been studied in detail to determine if they complicate paleoceanographic reconstructions. Stylaster erubescens coral colonies were collected from the Charleston Bump (31.4°N, --78.8°W), in the Blake Plateau. Infilled and primary skeletal material was analyzed for delta18O values using secondary ion mass spectrometry (SIMS) on the micrometer scale. SIMS analysis determined that nine of the eleven infilled areas have delta18O values within analytical precision (+/-0.4‰, 2sigma) of the adjacent primary skeleton. The two infilled areas with differences in primary and secondary delta18O values outside 2sigma analytical precision have delta18O values that are both higher and lower than the surrounding primary skeleton suggesting environmental changes between the time of initial precipitation and later infilling are the source of the variation. The primary skeleton is composed of 1.9% infilled area by area; therefore, when using conventional millimeter-scale sampling infilled areas would not produce a detectable offset in the geochemical record, in this coral species in this location. These results suggest S. erubescens can be used as an environmental proxy data source in those regions and times where environmental change is minimal and coral contain few infilled pore spaces.