<p><strong>Abstract.</strong> 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 (<i>δ</i>¹⁸O_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 <i>δ</i>¹⁸O signature of sponge spicules from a single freshwater species (<i>Metania spinata</i>) grown on natural and artificial supports over nine months in a small Brazilian pond (Lagoa Verde, northwestern Minas Gerais). The <i>δ</i>¹⁸O_silica values were obtained using the infrared (IR) laser-heating fluorination technique following a controlled isotopic exchange (CIE). The <i>δ</i>¹⁸O values (<i>δ</i>¹⁸O_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 <i>δ</i>¹⁸O_water values over the months of growth were weighted using a sponge growth coefficient previously established for <i>Metania spinata</i>. The <i>δ</i>¹⁸O_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&amp;permil; °C^&amp;ndash;1 (<i>R</i>² = 0.63), when <i>δ</i>¹⁸O_water values and water temperature at the time of sample collection were considered. Such a positive temperature coefficient clearly indicates that the freshwater sponge <i>Metania spinata</i> 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 <i>δ</i>¹⁸O 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 <i>δ</i>¹⁸O_silica values from the spongillites of northwestern Minas Gerais as a direct proxy for past <i>δ</i>¹⁸O_water and/or temperature changes.</p>

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Adverse natural conditions will, generally, induce gemmulation in freshwater sponges. Because of this environmental dependence, gemmoscleres are given exceptional value in taxonomic, ecological and paleoenvironmental studies. Other spicules categories such as microscleres and beta megascleres have received little attention with regard to their occurrence and function during the sponge biological cycle. Metania spinata, a South American species common to bog waters in the Cerrado biome, produces alpha and beta megascleres, microscleres and gemmoscleres. To detect the environmental factors triggering the production of all these kinds of spicules, the species annual seasonal cycle was studied. Artificial substrates were devised, supplied with gemmules and placed in Lagoa Verde pond which contained a natural population of M. spinata. Field monitoring was conducted for eight months in order to observe the growth of sponges and spicules formation. Samples of water were taken monthly for physical and chemical parameters determination. The appearance of the alpha megascleres was sequentially followed by that of microscleres, gemmoscleres and beta megascleres. The first ones built the new sponge skeleton, the last three were involved in keeping inner moisture in the sponge body or its gemmules. The water level, temperature and the silicon (Si) concentration in the pond were the most important factors related to this sequential production of spicules, confirming environmental reconstructions based on the presence or absence of alpha megascleres and gemmoscleres in past sediments.

ResumoImportantes processos erosivos após o Cretáceo foram responsáveis pela evolução da paisagem na região de João Pinheiro, onde ocorrem os depósitos de espongilito. O avanço desses processos, aliados às rochas carbonáticas do substrato, permitiu o desenvolvimento de feições cársticas negativas, onde foram instaladas as lagoas formadoras dos depósitos de espongilito. Com base no tratamento de imagens de satélite aliado a trabalhos de campo, quatro morfodomínios geomorfológicos foram identificados na área: i) morfodomínio 1, representado por platôs associados aos arenitos do Grupo Areado, apresenta as maiores altitudes da área; ii) morfodomínio 2, que constitui uma área dissecada relacionada aos pelitos do Grupo Areado; iii) morfodomínio 3, representado por superfícies de erosão associadas às rochas do Grupo Bambui e Pré-Bambuí, sendo sobrepostas por sedimentos terciários/quaternários, onde se encontram as lagoas; iv) morfodomínio 4, constitui vales em calha que contêm as principais drenagens da região (rios da Prata e Paracatu) segundo um padrão meandrante, com feição geomorfológica fluvial de rios underfit. Esses vales cortam a superfície de aplainamento (morfodomínio 3) que contêm sedimentos pleistocênicos, caracterizando o morfodomínio mais recente. Palavras-chave: