TRACE ELEMENT DIFFERENCES AMONG SKELETAL STRUCTURES IN MONTASTRAEA FAVEOLATA

Corals provide invaluable geochemical data on historical variations in sea surface temperatures (SST), carbon and oxygen levels, and extreme local events. Laser ablation inductively coupled mass spectrometry (LA ICP-MS) is a relatively new, high-resolution geochemical technique that analyzes solid samples on a micrometer scale and simultaneously measures numerous trace elements. This method has been successfully used on the coral Porites to discern annual/seasonal historical SST and local pollution events. Using Porites, the laser generates an “average” of all morphological features during analysis due to the coral’s small and compact skeletal structure. In contrast, Montastraea faveolata is characterized by a large and "open" skeletal framework and easily allows the high-resolution laser to focus on individual structures within the coral skeleton.

Vertical transects were sampled along three parallel skeletal structures in M. faveolata: endothecal (location of polyp growth), exothecal (location of colonial growth) and the corallite wall that separates the two areas. The analyses revealed that abundances of trace metals including Mg, Al and Pb varied inconsistently among the three adjacent structures. If the laser is not precisely aligned with the corallite wall during the analysis and drifts into either the endothecal or exothecal areas, significant changes in trace element amounts can be recorded and may be misinterpreted as a sign of environmental change. Magnesium displays one of the most prominent differences between structures, due primarily to the existence of high Mg crystals (possible brucite). Research is continuing into the origin of the high amounts of Mg in the skeleton, which may be the result of Mg being released into the system during the degradation of chlorophyll to phaeophytin.

Laser ablation used in conjunction with large framework corals can be invaluable for assessing the geochemistry of specific skeletal features at high resolution. However, depending upon laser beam size and sample location on M. faveolata, differences in trace element constituents between adjacent structures can complicate interpretations of historical trends if the operator is not careful during analysis.