Paleoclimatic and paleoenvironmental significance of late Pleistocene pedogenic carbonates ('Caliche Cordao') from the coastal Pampa of southern Brazil

The pedogenic carbonates found in Pleistocene units of the coastal Brazilian Pampa, informally designated as 'Caliche Corda similar to o', include rhizocretions, soft masses and isolated or coalescent nodules, either isolated within the sediment or forming discrete horizons, the most developed horizons (Ca-01 and Ca-02) stretching for hundreds of meters along the banks of Chuy Creek. Petrographic analyses show micritic (Alpha-type) matrix in most samples, with few signs of recrystallization as circumgranular spar rims or cracked sand grains. Stable carbon (delta 13C) and oxygen (delta 18O) isotope values in samples from the lower horizon Ca-01 exhibit small variation and indicate C3dominated plant assemblages and carbonate precipitation from relatively 18O-enriched water, whereas the delta 13C and delta 18O in samples from the upper horizon (Ca-02) exhibit wider variation, higher proportion of C4 plants and precipitation from 18O-depleted water in some samples. The delta 18O variations can be explained by variable degrees of evaporative enrichment, or to relative contributions of 18O-depleted (tropical) or 18O-enriched (subtropical) rainfall, based on the similarity with estimates from fossils of mastodons. Nodules from other sites are similar, but larger isotopic variations indicate carbonate precipitation at distinct times and environmental conditions, and a few exhibit partial recrystallization as larger spar calcite crystals. Luminescence ages of -74.7 and -71.3 ka of quartz grains in two nodules from Ca-01 indicate precipitation during MIS 4, whereas ages of -52 ka from Ca-02, and >= 30 ka in nodules from the shores of Mirim Lagoon indicate carbonate formation between MIS 3 and MIS 2. The probable source of carbonate was calcium released from eolian-sourced silty dust produced by Andean glaciers and transported from loess deposits in Argentina during cold and dry intervals of the last glacial, when arid/semiarid environments expanded from Patagonia across the Pampas up to southern Brazil. The variations of delta 13C in calcretes formed between MIS 4 and 2 point to changes in plant assemblages that could have contributed for the local extinction of the mammalian megafauna.