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 mu m 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 out 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 run to I 100 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 (luring 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. (c)-2007 Elsevier B.V. All rights reserved.