Spectral luminescence and geochemistry of coral aragonite: Effects of whole-core treatment

Abstract

Luminescent and geochemical properties of coral skeletons are increasingly used for time-series analysis to resolve past and ongoing changes in environmental and climatic conditions. Corals also contain non-skeletal matter which not only quenches luminescence but is also reported to compromise stable isotope and trace element composition. In order to understand the origin and magnitude of these potential sources of error we tested whether three commonly used cleaning treatments (NaOCl, H2O2 and HNO3) improved measurement accuracy of 1) luminescence intensities and spectral ratios, 2) stable isotope composition (δ18O, δ13C), and 3) trace element ratios (Sr/Ca, Mg/Ca, U/Ca, Y/Ca and Ba/Ca). Whole core treatment with concentrated reagent grade NaOCl at pH=9 significantly increased luminescence intensities, reduced the scatter in spectral ratios, revealed clear seasonality in the obscured tissue layer and removed (in)organic patchy contaminants throughout coral cores. Acidic agents (H2O2 and HNO3) caused severe dissolution of the aragonite skeleton and strongly affected the luminescence signature. Meanwhile, NaOCl did not affect absolute values and seasonal amplitudes in δ18O, δ13C and some trace element ratios. Treatment with reagent grade NaOCl revealed seasonal amplitudes for Sr/Ca and Mg/Ca in the tissue layer, and strongly improved Ba/Ca results. We show that a non-skeletal barium phase can be present in coral segments and suggest that the associated Ba/Ca-spikes may have resulted from anoxia, sulfate reduction, dissolution and re-precipitation of extra crystalline barite (BaSO4) in a redox front. Based on our findings we conclude that the use of an appropriate standardized cleaning protocol prior to analysis is recommended to reduce potential sources of errors and allow for accurate cross-comparison of coral records analyzed at different laboratories.