Sponge spicules as indicators of paleoenvironmental changes in island deposits – Upper Paraná River, Brazil
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Sediments of fluvial islands are important records for paleoenvironmental and paleoclimatic reconstruction, especially those related to the late Pleistocene and Holocene. The Upper Paraná River (UPR) has numerous paleoenvironmental studies using different approaches – e.g.: sedimentary facies analysis, geomorphological evolution, and evolution of sandbars. This is the first study to use freshwater sponge spicules preserved in fluvial sediments to characterize the environments and processes involved in the island formation. Sponge spicules analysis can be used as a complement of sedimentary facies. We studied two islands that, despite their geographical proximity and similarity, had significant differences in their paleoenvironmental evolution. We identified nineteen species of sponges of lotic and lentic habitats. The sediments from Bandeirantes Island have highly fragmented sponge spicules and species of lotic habitats from the late Pleistocene (14,620 cal yr BP). In addition, from 7380 to 1220 cal yr BP, the occurrence of spongiofacies of Tubella variabilis and Radiospongilla amazonensis indicates a lentic environment in the island. These spongiofacies suggests that Bandeirantes Island had an island-lake environment in the mid-Holocene. After 1220 cal yr BP, occurred the lake colmatation by overbank river flow. However, the sediments from Grande Island only contain lotic sponge spicule fragments. Furthermore, the fragmentation of sponge spicules is associated with transport and remobilization during the Late Pleistocene (12,430 cal yr BP). Finally, there was intermittent deposition and phases of removal of the material due to fluvial erosion after 2570 cal yr BP with rare and dispersed occurrences of freshwater sponges associated with sedimentary aggradation by overbank flow.