Fine-scale phosphorus distribution in coral skeletons: Combining X-ray mapping by electronprobe microanalysis and LA-ICP-MS

Increased terrestrial phosphorus runoff is a major environmental problem that has been linked to deteriorating reef health. Unfortunately, long-term records of phosphorus are limited. Whilst phosphorus captured in coral skeletons could provide us with an archive of phosphorus variability, the mode of incorporation is poorly understood. In order to document phosphorus levels, we used laser ablation inductively coupled plasma mass spectrometry, followed by X-ray mapping of phosphorus in the skeleton at micron scale (~3-5 microns) using Electronprobe microanalysis. We recorded high phosphorus (≤8,700 ppm) in the living tissue zone associated with phosphorus-rich residues lining the internal pore network surfaces. The skeleton in the tissue zone had low, uniform levels of phosphorus, similar to older sections of the core (<50 ppm). Below the organic tissue layer, P was incorporated homogenously in the skeleton for extended periods (e. g., 5 mm growth bands). However, sections of the core (~1 cm down-core) displayed fine-scale elevated phosphorus concentrations associated with the presence of phosphorus-rich, often elongate (10-100 μm long), heterogeneities within the skeleton, the origin of these phosphorus-rich heterogeneities and their mode of incorporation requires further attention. In conclusion, these results support the continued development of this promising potential nutrient proxy.