Relationships between growth-bands and paleoenvironmental proxies Sr/Ca and Mg/Ca in hypercalcified sponge: A micro-laser induced breakdown spectroscopy approach

Long classified as coral hydrozoans, Chaetetids are now considered as a group of very shallow marine hyper calcified sponges. As corals do, they grow by adding calcium carbonate under their pellicular living body in such a rhythmic and regular way that we suspect that sclerochronology is possible, based on the hypothetical annual growth bands of their skeleton. Regarding to its accurate lateral resolution of 5 μm by laser shot, micro-LIBS study was chosen to check its potential for such application. The LIBS apparatus is composed of a microscope coupled with a 266 nm Nd-YAG laser, delivering a 4 mJ-power per shot, and an ICCD camera. The acquisition of the spectra is made via the SE 200 spectrograph, on the large 190 nm to 1100 nm wavelength range. The entire longitudinal thin section of the specimen was analysed from the bottom to the top of the Ptychochaetetes section in a multi-elementary cartography for Ca–Mg–Sr elements. Sodium and Barium were also detected but trace amounts. The Mg/Ca concentrations are mainly between 400 and 600 mmol/mol considering an average value for each profile. This study shows that during the Ptychochaetetes growth, an obvious time-dependent heterogeneity in the chemical Mg/Ca and Sr/Ca composition can be observed. These variations demonstrate the possible use of this method for sclerochronological studies.