Incorporation of Dissolved Heavy Metals Into the Skeleton of Porites Corals Based on Multi-Element Culturing Experiments

Abstract

Anthropogenic activities increase the level of dissolved heavy metals in some tropical near-shore environments threatening reef ecosystems. The skeleton of stony corals like Porites species potentially provides a high-resolution geochemical archive for past heavy metal concentrations, with potentially century long records revealing baseline values before large-scale human disturbance. However, few data exist for heavy metal partitioning into coral skeleton aragonite. To address this, culturing experiments exposing Porites lobata and Porites lichen to a mixture of dissolved Cr, Mn, Ni, Cu, Zn, Ag, Cd, Sn, Hg, and Pb over a wide concentration range have been performed. Water samples were taken frequently to monitor changes in the heavy metal concentration. Laser ablation ICP-MS measurements of the coral aragonite revealed metal concentrations that were positively correlated with Cr, Mn, Ni, Zn, Ag, Cd, and Pb concentrations in seawater. The D_TE values for most metals appear dependent on the seawater metal content, approximating a power law, and therefore stabilize at higher seawater metal/Ca ratios. The partitioning of Pb into the coral skeleton is a notable exception, with D_Pb being stable around 2 to 1 across a large range of “natural” to highly polluted seawater Pb concentrations. This and the general agreement with partition coefficients estimated by previous work suggests that the reconstruction of the heavy metal concentration in seawater for ecosystem monitoring is possible. However, the high variability within and between coral colonies requires further study and suggests that multiple records from multiple coral colonies should be combined to obtain robust reconstructions.