Assessing the relationship between the δ¹⁸O signatures of siliceous sponge spicules and water in a~tropical lacustrine environment (Minas Gerais, Brazil)

Abstract. Siliceous sponge spicules constitute an important siliceous component of lacustrine sediments, together with widespread diatom frustules. In contrast to diatom frustules, siliceous spicules are formed in sponges in an enzymatic way. Previous attempts to use their oxygen isotopic signature (δ¹⁸O_silica) as a paleoenvironmental proxy have led to contradictory conclusions. These attempts demonstrated the need to further assess whether sponges form their silica in oxygen isotopic equilibrium with water. For this reason, we measured the δ¹⁸O signature of sponge spicules from a single freshwater species (Metania spinata) grown on natural and artificial supports over nine months in a small Brazilian pond (Lagoa Verde, northwestern Minas Gerais). The δ¹⁸O_silica values were obtained using the infrared (IR) laser-heating fluorination technique following a controlled isotopic exchange (CIE). The δ¹⁸O values (δ¹⁸O_water) and temperature of the pond water were periodically measured and reconstructed over the course of the sponge growth. Assuming that silica may form continuously in the spicules, temperature and δ¹⁸O_water values over the months of growth were weighted using a sponge growth coefficient previously established for Metania spinata. The δ¹⁸O_silica values of sponges grown simultaneously and on similar substrates were scattered. No relationships were observed between the Δ¹⁸O_silica-water and water temperature when the reconstructed values were considered. Conversely, a positive correlation was obtained, with a coefficient of 0.3‰ °C^–1 (R² = 0.63), when δ¹⁸O_water values and water temperature at the time of sample collection were considered. Such a positive temperature coefficient clearly indicates that the freshwater sponge Metania spinata does not form its siliceous spicules in oxygen isotopic equilibrium with the pond water. Instead, one or several biologically controlled kinetic fractionation mechanisms may be in play during the various steps of silica formation. Our results suggest that the latest precipitation gives its δ¹⁸O imprint to the entire spicules assemblage. The amplitude of the apparent fractionations increases with temperature, but other controlling parameters, such as dissolved Si concentration and nutrient availability, co-varying with temperature may intervene. These results prevent the use of δ¹⁸O_silica values from the spongillites of northwestern Minas Gerais as a direct proxy for past δ¹⁸O_water and/or temperature changes.