The Pantanal is the world's largest tropical wetland and a biodiversity hotspot, yet its response to Quaternary environmental change is unclear. To address this problem, sediment cores from shallow lakes connected to the Upper Paraguay River (PR) were analyzed and radiocarbon dated to track changes in sedimentary environments. Stratal relations, detrital particle size, multiple biogeochemical indicators, and sponge spicules suggest fluctuating lake-level lowstand conditions between ~ 11,000 and 5300 cal yr BP, punctuated by sporadic and in some cases erosive flood flows. A hiatus has been recorded from ~ 5300 to 2600 cal yr BP, spurred by confinement of the PR within its channel during an episode of profound regional drought. Sustained PR flooding caused a transgression after ~ 2600 cal yr BP, with lake-level highstand conditions appearing during the Little Ice Age. Holocene PR flood pulse dynamics are best explained by variability in effective precipitation, likely driven by insolation and tropical sea-surface temperature gradients. Our results provide novel support for hypotheses on: (1) stratigraphic discontinuity of floodplain sedimentary archives; (2) late Holocene methane flux from Southern Hemisphere wetlands; and (3) pre-colonial indigenous ceramics traditions in western Brazil.
Sediments obtained by vibrocoring from Samambaia Lake (22º36'S / 53º23'W) in Taquarussu, Mato Grosso do Sul State, Brazil, were dated using thermoluminescence. Grain size and organic content analysis were performed, and the freshwater sponge spicules present in the sediments were identified. The age of sediments ranged between TL 4,350 ± 210 and TL 32,740 ± 163 years BP. Spicules of the sponges Dosilia pydanieli, Metania spinata, Radiospongila amazonensis, Corvospongilla sp. and Oncosclera sp. were found. Analyses suggest an alternation of lotic and lentic phases, in agreement with earlier lines of evidence regarding the existence of paleodrainage in the region during the Late Pleistocene to the Mid-Holocene, as well as previously described paleoclimatic conditions. This work reinforces the reliability of the use of freshwater sponge spicules as proxy data in the proposition of paleoenvironmental reconstructions. The term spongofacies is introduced for the sedimentary sequences in which spicules of freshwater sponge species predominate and indicate specific paleoenvironmental conditions.
The objective of this research is to examine the history of lentic ecosystem salinity in the southern Pantanal wetlands (Brazil). The timing and controls on hydrochemical changes were inferred using sponge spicule and diatom paleoecology on a Holocene-aged sediment core from Nhecolândia, a lake district situated on a fossil lobe of the Taquari megafan. The oldest portion of the core contains Heterorotula fistula spicules, indicative of an ephemeral freshwater lake that existed until * 4.6 cal ka BP. Benthic diatoms of the genus Gomphonema and Eunotia appeared * 3.2 cal ka BP, indicating a shallow and dystrophic environment. A transition to a more permanent lake that hosted freshwater sponges (e.g., Corvoheteromeyenia spp.), and diatom assemblages (e.g., Cyclotella meneghiniana, Aulacoseira pantanalensis) endured until * 1.3 cal year BP; after this time, most sponges and planktic diatoms disappear from the sedimentary record. High abundances of Anomoeoneis sphaerophora and Craticula guaykuruorum in the latest Holocene reflect a transition to a hyperalkaline, saline lake environment. The results suggest that Nhecolândia's saline lakes may evolve from freshwater precursors due to local (biochemical) and regional (geo-climatic) controls on water availability, which has implications for patterns of biodiversity and ecosystems services in Pantanal.
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