Assessing the geochemical composition of the cold-water coral Enallopsammia rostrata as a proxy archive for the paleo-oceanographic evolution in the western Indian Ocean

Cold-water corals (CWC) have been identified as a geochemical proxy archive for paleo-oceanographic reconstructions of intermediate water masses, due to their long lifespan (several hundred years) and their globally widespread occurrence. To date, the descriptions of CWC appearances in the Indian Ocean are only sparse and therefore require comprehensive geochemical investigations of these taxa within this region. In this study, we compare the spatial and temporal variations in the chemical composition to the structural and growth-related features of CWCs to assess their application as proxy archives to reconstruct recent changes in the intermediate water masses in the West Indian Ocean.

During research cruise SO306 with RV Sonne two living CWC colonies of Enallopsammia rostrata (Pourtalès, 1878) were collected in the northern part of the Mozambique Channel around the island of Mayotte. The corals originate from intermediate water depths (600 to 900 m) within the transition zone of South Indian Central Water (SICW) and the underlying Red Sea Water (RSW).

The chemical composition of different branches from each colony was analysed using EPMA mapping (Mg, Ca, S, Sr) and LA-ICP-MS line scans (Li, Mg, Ca, Sr, U). U/Th dating enable the determination of ages and growth rates of the individual colonial parts, allowing the sclerochronological alignment of the geochemical data. The obtained long-term (> 100 years) and spatially high-resolution data reveal pronounced cyclic variability in E/Ca ratios (e.g. Sr/Ca). However, using existing proxy calibrations along these data do not meet recorded environmental observations, emphasising the necessity for species-specific adjustments.