The potential of bamboo corals to record environmental conditions in their calcitic skeletons
The ocean’s physicochemical variability is not well understood despite its importance for the world’s climate because of rare instrumental time series. This issue can be resolved through natural archives that recorded past environmental conditions such as corals. Bamboo corals, thriving in water depths down to 4000 m, offer the potential to record environmental parameters over a wide range of oceanic conditions. This dissertation provides a detailed look into the chemical composition of bamboo coral skeletons to distinguish between environmental and biomineralisation controls over the skeletal composition. The ability of bamboo corals to record seawater temperature and nutrient availability was investigated by measuring the distribution of Mg and Ba, respectively, in the internode of an Atlantic specimen. The mean Mg/Ca composition of a bamboo coral can be used to reconstruct the ambient seawater temperature while mean Ba/Ca was found to record [Ba]SW. Conversely, the micron-scale skeletal Mg and Ba composition is strongly impacted by coral physiology. The use of skeletal Na and S concentrations as environmental proxies in marine calcifiers is debated and was therefore evaluated in two Atlantic bamboo corals. The microscale distribution of Na and S can show a concentric structure and zig zag patterns as found for Mg. The heterogeneity indicates a main control of the coral physiology over the skeletal composition. The B elemental as well as its isotopic composition in calcium carbonates have been used to study the ambient pH of the corals’ calcifying fluid (CF). The B/C and B isotopic spatial distribution in an Atlantic and a Pacific bamboo coral were measured in the bulk internode material and the purified calcite fraction. Our results suggest that besides CF pH, the skeletal boron isotopic composition of bamboo corals might be influenced by isotopically light borate influx to the CF, organic matter induced isotope fractionation or kinetic isotope effects.