Where In The Coral Is The Magnesium (and other trace elements) ?

Corals record information on reef health, historical ocean temperatures, flood events, and global climate change. In the scientific literature, time-series geochemical analyses of coral data (stable isotopes, trace elements, rare earth elements) have been based on sampling intervals greater than 1 millimeter. Within the last decade, new technologies have emerged that allow collection of time-series geochemical data on a micrometer scale. The laser ablation ICP/MS is an instrument that analyzes solid samples on a micrometer scale and simultaneously measures numerous trace elements. Using laser ablation ICP/MS in conjunction with the Scanning Electron Microscope (SEM) we have examined the sites and potential problems of geochemical sampling within the coral skeletal structure. Spot sampling (50 micrometer sampling size (spot size) with 85 micrometer spacing between samples) was conducted on parallel, longitudinal endothecal, exothecal and corallite wall structures. The locations were chosen based on SEM mosaic images of Montastrea faveolata. The analysis revealed values and intervals of data for trace elements including Mg, Al, Ba, Cu, and U that varied downcore between the three separate coral structures. Depth (age) intervals that showed an abundance of Mg (and other elements) within the exothecal area may be absent or substantially lower in either or both the endothecal and wall areas within the same intervals and vice versa. In addition, some areas that exhibit particularly high Mg peaks can be correlated with the location of hexagonal crystals identified with the SEM. These crystals are forming adjacent to "normal" acicular aragonite crystals, but have only been found within the exothecal areas of the coral. These results suggest that biomineralization of aragonite and the existence and location of trace elements within the skeleton are controlled separately by: 1) the coral polyp that lives in the endothecal portion of the skeleton, 2) the colonial tissue within the exothecal portion of the skeleton, and 3) the wall that separates these two areas. The micro-architecture of these three areas also differs and may explain the variations of trace element amounts found at different depths. It is well known that corals allow insight to the historical record of our oceans and global climate, however, this study shows that care must be taken to during sampling to ensure consistent data reporting