Minor and trace elements in sclerosponge Ceratoporella nicholsoni: Biogenic aragonite near the inorganic endmember?

The trace and minor element composition have been investigated in the accretionary skeletons of two specimens of the Caribbean sclerosponge species, Ceratoporella nicholsoni; one from Tongue of the Ocean, Bahamas (TOTO), and the other from Pear Tree Bottom, Jamaica (PTB). Both sclerosponges were analyzed at high resolution using laser ablation ICP-MS (LA-ICP-MS) and verified using wet chemical methods (ICP-OES). The shallow PTB sclerosponge (25 m) was analyzed for a short, temperature-calibrated record of trace and minor elements while the TOTO sclerosponge (146 m) was analyzed for a century long record from subsurface waters. The resulting Ba/Ca, Mg/Ca, Pb/Ca, and U/Ca ratios were compared with Sr/Ca ratios, which have been shown to reliably record seasonal-scale temperature change in sclerosponges, close to equilibrium precipitation of inorganic aragonite. Sclerosponge aragonite incorporates minor and trace elements according to general mineralogic constraints with less "vital" effects than other calcium carbonate secreting organisms. This is perhaps most obvious in the behavior of Mg/Ca ratios which show an inverse correlation to temperature unlike any other calcite or aragonite organism, despite having Mg concentrations (∼ 1 mmol/mol) within published estimates of equilibrium aragonite. Ratios of Mg/Ca and Ba/Ca show a strong positive correlation despite large differences in ionic radii and U/Ca values (∼ 3.15 μmol/mol) are 2.5x higher in sclerosponge aragonite than in aragonite from corals, supporting earlier work. Ratios of U/Ca show an insignificant correlation with temperature-dependent Sr/Ca ratios (r² = 0.05), supporting existing theories that temperature signals recorded by U/Ca ratios can be convolved with pH, [CO₃^{- 2}], and phosphate signals. Differences between sclerosponge aragonite and other calcium carbonate arising from siliceous spicule contamination are unlikely because of the small mole percent of silicon in the aragonite. Small differences between the two sclerosponges were noticed in Pb/Ca and Sr/Ca because of the average ages of the sampled material and average temperatures of the ambient environment, respectively. Anomalies between LA-ICP-MS and ICP-OES were noticed in measurements of Ba/Ca and Pb/Ca due to standardization differences for LA-ICP-MS (Ba/Ca) and measurements close to the instrument detection limits of Pb (ICP-OES).