Preprints
https://doi.org/10.5194/bgd-10-12887-2013
https://doi.org/10.5194/bgd-10-12887-2013
 
07 Aug 2013
07 Aug 2013
Status: this preprint was under review for the journal BG but the revision was not accepted.

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

M. C. Matteuzzo1,2, A. Alexandre2, A. F. D. C. Varajão1, C. Volkmer-Ribeiro3, A. C. S. Almeida4, C. A. C. Varajão1, C. Vallet-Coulomb2, C. Sonzogni2, and H. Miche2 M. C. Matteuzzo et al.
  • 1Federal University of Ouro Preto, Department of Geology, Ouro Preto-MG, CEP 35400-000, Brazil
  • 2Aix-Marseille Université, CNRS, IRD, CEREGE UMR7330, 13545 Aix en Provence, France
  • 3Museum of Natural Sciences of the Zoobotanic Fundation of Rio Grande do Sul, Porto Alegre-RS, CEP 90690-000, Brazil
  • 4Federal Institute of Minas Gerais, Ouro Preto-MG, CEP 35400-000, 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 (δ18Osilica) 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 δ18Osilica values were obtained using the infrared (IR) laser-heating fluorination technique following a controlled isotopic exchange (CIE). The δ18O values (δ18Owater) 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 δ18Owater values over the months of growth were weighted using a sponge growth coefficient previously established for Metania spinata. The δ18Osilica values of sponges grown simultaneously and on similar substrates were scattered. No relationships were observed between the Δ18Osilica-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 δ18Owater 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 δ18Osilica values from the spongillites of northwestern Minas Gerais as a direct proxy for past δ18Owater and/or temperature changes.

M. C. Matteuzzo et al.

 
Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement

M. C. Matteuzzo et al.

M. C. Matteuzzo et al.

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