Holocene limnological changes in saline and freshwater lakes, Lower Nhecolândia, Pantanal, Brazil

The lower Nhecolândia region, in the south of the Pantanal, contains thousands of shallow freshwater and saline-alkaline lakes isolated by sandy ridges. To understand the paleoenvironment, sediment cores from B02SR (freshwater) and 07SR (saline-alkaline) lakes were analyzed, employing a combination of ¹⁴C dating, microfossils, geochemical, elemental, and isotopic analyses. The 07SR core recovered Late Pleistocene sediments (~ 23,440 cal yrs BP), and the B02SR core Middle Holocene sediments (~ 6080 cal yrs BP). The base of the cores consists of bedded sands with no organic matter, sponge spicules, or diatoms. Phytoliths suggest the presence of cerrado vegetation with seasonal floods, suggestive of a periodically inundated distal floodplain. We interpret that the two lakes sustain perennial alkaline geochemical conditions between ~ 3080 and ~ 1330 cal yrs BP. The Lake B02SR transitioned to slightly acidic waters with low electrical conductivity from ~ 1330 cal yrs BP to the present, probably associated with a connection to ephemeral shallow or perennial channels. Lake 07SR maintained consistent water chemistry throughout the record, suggesting that an isolated drainage pattern remained unchanged creating persistent alkaline conditions. Our results suggest that lake chemical changes were spatially variable in lower Nhecolândia in the Holocene, which has implications for ecosystem services.