On the potential use of magnesium and strontium concentrations as ecological indicators in the calcite skeleton of the red coral (Corallium rubrum)

The variability of magnesium, strontium and calcium concentrations in the t wo skeleton types (sclerites and axis) of the red coral (Corallium rubrum) was assessed by using X-ray fluorescence spectroscopy and microprobe analys is as a prerequisite for their use as ecological indicators. Axis cross sec tions showed light and dark circular growth bands corresponding to fast and slow growth, respectively. Using microprobe analyses the Mg and Sr concent rations ranged from 2.8 to 3.0% and from 0.21 to 0.29%, respectively, in th e sclerites and from 2.4 to 3.0% and from 0.1 to 0.28%, respectively, in th e axis. Mg/Ca and Sr/Ca ratios varied within sclerites (average, 12% for bo th ratios) and within the axis of single colonies (average, 20% for Mg/Ca a nd 48% for Sr/Ca). Sr/Ca ratios in the axis were lower in fast-growing bran ch tips than in older, more basal parts of the colonies, whereas the Mg/Ca ratio did not differ significantly between colony regions. Mg/Ca and Sr/Ca ratios in the axis decreased significantly with depth, and we estimated an increase of the Mg/Ca ratio of 0.004-0.006 per degree Celsius. In all depth layers, Sr/Ca ratios showed a significant direct relationship with skeleto n density in axis cross sections, whereas there was no significant relation ship for Mg/Ca. Overall, our data indicate that temperature promotes the in corporation of Mg in C. rubrum as in other calcite skeletons, whereas Sr co ncentrations are inversely related to growth rate. This preliminary study s uggests that Mg and Sr concentrations in the axis of the red coral have a s trong potential as ecological indicator for temperature and growth rate.