Continental freshwater carbonate coated grains: oncoids in Quaternary deposits of the Serra da Bodoquena region, Central-West Brazil

Abstract

Oncoid nucleation and growth have been attributed to microorganism activity, commonly cyanobacteria, resulting in the trapping of organic, bioclastic or lithoclastic nuclei by concentric laminations. However, the factors controlling these processes are not well understood, especially regarding freshwater precipitation systems. Freshwater oncoids from alluvial terraces underlying paludal tufas in Quaternary carbonate deposits in Central-West Brazil (Bodoquena downstream plains) were submitted to petrological studies by optical microscope, x-ray diffraction and fluorescence, scanning electron microscopy and C&O stable isotopes analyses. Biogenic structures where well-preserved cyanobacteria EPS sheath structures suggest in vivo precipitation by CO2 concentrating mechanisms, whereas smooth rhomb calcite crystals likely indicate post mortem precipitation. The four identified morphologic features of spherical and subspherical oncoid types suggest growing in a shallow water body, with constant laminar flow and the absence of predators. The chemical and isotopic composition of the oncoids may represent nucleation and growth under relatively milder climatic conditions than today. This oncoid nucleation system may bring valuable information about climatic changes in Central-West Brazil (Tropical South Hemisphere) at the end of the Pleistocene and the beginning of the Holocene.

KEYWORDS:
freshwater oncoids; continental carbonates; cyanobacteria calcification; microbial; EPS sheath structures