Mg/Ca and Sr/Ca ratios of Porites coral skeleton: Evaluation of the effect of skeletal growth rate

Abstract

We examined whether the Mg/Ca and Sr/Ca ratios of Porites coral skeletons are influenced by skeletal growth rate. A colony of Porites australiensis was collected from the Ryukyu Islands, Japan, and analyzed for the elemental ratios along two synchronous growth axes with different extension/calcification rates. The results showed little difference in the Mg/Ca and Sr/Ca profiles between the two growth axes. The Mg/Ca and Sr/Ca profiles have synchronous annual cycles with a broad peak in summer and a narrow trough in winter. This shape suggests that skeletal extension is faster in summer than in winter. A detailed description of the skeletal growth is given in combination with (1) X-ray image of skeletal density banding, (2) skeletal density measurements, and (3) the depth of skeleton occupied by organic tissue layer (i.e., polyp layer). We concluded that both the Mg/Ca and Sr/Ca ratios are independent of skeletal growth rate at least in the approximate range of 11–20 mm/year (skeletal extension rate) when bulk density of the skeleton is 1.1–1.2 mg/mm3, or 13–23 mg mm–2 year–1 (calcification rate). It is also concluded that the Mg/Ca ratio may be susceptible to biological/metabolic effect.