High resolution coral Cd measurements using LA-ICP-MS and ID-ICP-MS: Calibration and interpretation

Cadmium (Cd) concentrations in coral skeleton track oceanic upwelling, which brings Cd-rich deep water to the surface. In this study we report sub-monthly measurements of [Cd]coral measured via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in three species of coral (Porites lobata, Pavona gigantea, and Pavona clavus) from an upwelling site in the Gulf of Panama. Although single LA tracks in all species yielded variable results, the average of multiple tracks mirrored changes in in situ seawater Cd ([Cd]sw), with an offset between peaks. In addition, [Cd]coral determined by LA-ICP-MS and isotope dilution (ID)-ICP-MS was compared in P. clavus and found to be highly correlated, despite consistently lower concentrations measured using the ID method. Two 5.5-month long high-resolution P. clavus [Cd]coral records revealed that the maximum [Cd]coral values occurred after the sea surface temperature minimum, at the same time as the seawater phosphate concentration and coral P/Ca maximum, and approximately one month before peak [Cd]sw. Based on these findings we hypothesize that corals are actively taking up Cd and incorporating it into their skeleton, and that [Cd]sw only increases once Cd is no longer being rapidly removed from the water column via biological processes. After accounting for a 1-month offset between the coral and seawater [Cd] maxima, both LA- and ID-derived [Cd]coral values were significantly correlated to [Cd]sw, providing a calibration for this [Cd]sw proxy in P. clavus. A three year high-resolution P. clavus LA-[Cd]coral record demonstrated the potential of [Cd]coral to generate a high-resolution [Cd]sw record. However, due to the natural variability in high-resolution LA-ICPMS [Cd]coral values within and among coral colonies, three laser tracks per colony and at least three colonies of P. clavus need to be analyzed to produce a reliable coral [Cd]coral record.