LATE HOLOCENE HYDROCHEMICAL CHANGES RECORDED IN NHECOLÂNDIA POND STRATIGRAPHY, BRAZILIAN PANTANAL WETLANDS

Nhecolândia is one of the most famous yet enigmatic landscapes in the heterogenous Pantanal wetlands of western Brazil. The region is renowned for its biodiversity and highly valued for its ecosystem services. Situated on the southern fringe of the Taquari River megafan, Nhecolândia is marked by about 10,000 ponds that are readily observable on satellite images, and consequently numerous of hypotheses, absent of ground-truth, have been posited regarding their origins. A number of studies have demonstrated that the compositions of pond waters vary spatially, with freshwater, oligosaline and saline ponds juxtaposed in close proximity to each other. The timing and mechanisms of pond formation remain unknown, however. Here, we use radiocarbon dating, stratigraphy, and multi-indicator analyses (sponge spicules, organic carbon, carbonate content, and particle size) of sediment cores from six broadly distributed saline ponds to assess the chronology of pond development and the environmental history of the Nhecolândia. The shallow stratigraphy of each pond is characterized by a tan, massive-to-thinly bedded basal sandy unit overlain by a dark green massive muddy unit. The sand-to-mud contact is commonly sharp, and radiocarbon constrains the timing of this transition to ~3.4 – 1.4 cal ka. Spicules of C. heterosclera, coupled with preservation of organic carbon and fine detrital particle sizes, confirm that muds just above the contact reflect sedimentation in permanent freshwater ponds. By contrast, the data suggest that highly ephemeral ponds were present during deposition of the sandy units. Several other paleo-records from the Pantanal region document recovery from acute drought in the mid-late Holocene, which may explain the pronounced change in depositional environment. A second important paleolimnological transition is captured within the muddy units with the disappearance of the freshwater sponge community, which is tentatively dated to ~1.7-0.8 cal ka. We interpret this change to be the result of an evolution in pond hydrochemistry, perhaps linked to the onset of high alkalinity and electrical conductivity, which are not ecologically conducive for C. heterosclera. Ongoing research is aimed at expanding biological and geochemical proxy evidence for Late Quaternary paleoenvironments in Nhecolândia.