Multi-Colony Calibrations of Coral Ba/Ca With a Contemporaneous In Situ Seawater Barium Record
Friday, 19 December 2014
The coral skeleton barium to calcium ratio (Ba/Cacoral), a proxy for seawater barium concentration (BaSW), has been interpreted as an upwelling tracer based on the “nutrient like” depth profile of BaSW. However, terrestrial runoff and differences between the vertical distribution of BaSW and that of the major nutrients in the upper water column can compromise the reliability of Ba/Cacoral as an upwelled nutrient proxy in some coastal locations. For example, a 6-month time series record of BaSW, SST, SSS, and surface nutrients in the Gulf of Panamá shows that maximum seasonal inputs from terrestrial runoff occur during the upwelling minimum. In contrast to nutrients, BaSW variations resulting from upwelling are overwhelmed by the runoff influence at this location. High resolution Ba/Cacoral records from multiple colonies of Porites lobata, Pavona gigantea, and Pavona clavus corals growing during this period record the nearly two-fold runoff-induced change in BaSW in Gulf of Panamá surface waters as an ~20-70% increase in skeletal Ba/Ca, providing an opportunity to calibrate the Ba proxy with a contemporaneous BaSW record. Compiling the Ba/Cacoral records from three co-located colonies of each species into taxa-specific composite regressions reveal statistically significant correlations with the BaSW time-series (r2=0.7; p <0.001). The application of the Pavona clavus calibration to a previously published Galapágos Ba/Cacoral record and offsets in regression variables between taxa demonstrate the necessity of using taxa-specific calibrations for quantitative BaSW reconstructions. Similar studies using longer high-resolution multi-colony records could further our understanding of intercolony-reproducibility and reduce the uncertainty in the regressions. These results support the application of Ba/Cacoral to reconstruct past changes in BaSW and underscore the value of combining multiple skeletal proxies to provide a complete picture of surface water biogeochemistry.