Chemical fractionation in benthic fauna from Saya de Malha Bank (Mascarene Plateau, western Indian Ocean): Vital and habitat effects

Abstract

The temperature, salinity, nutrient availability and dissolved oxygen characterizing ocean water-masses and circulation patterns are essential environmental controlling parameters of marine benthic assemblages. This work presents for the first time the geochemical composition of different taxonomic groups of benthic assemblages from the Saya de Malha Bank, Mascarene Plateau (Indian Ocean). The studied stations, located in water masses at different water depths, are perfect spots for studying biogeochemical proxies of diverse taxonomic groups to elucidate the impact of the vital factor as opposed to environmental or oceanographic factors. Forty-two specimens were studied, including scleractinian corals, bryozoans, brachiopods, bivalves, gastropods, scaphopods, and echinoids. Specimens were taphonomically analyzed in order to discard those with a poor preservation that might affect the geochemical assays. Geochemical analyses of the specimens involved high resolution inductively coupled plasma-mass spectrometry for main and trace elements. According to the results, (1) the content of trace elements in the shells is primarily controlled by their mineralogy: i.e. aragonitic, low-Mg calcite, and high-Mg calcite. (2) Vital effects related to metabolism play a major role in the co-precipitation and adsorption of trace elements in the shells, especially for Ca, Mg, Sr, Ba, and Zn. (3) The environmental parameters (habitat effect) such as oxygenation, primary productivity, salinity, but mainly temperature, at different water depths were found to be related to the growth rate of the specimens and the incorporation of trace elements. In the shallower station, the Li/Mg ratio is lower and the content in U is higher among the selected taxa, which is related to an increased growth rate in warmer waters.