Calcium biomineralizations associated with bioclastic deposits in coastal pedostratigraphic sequences of the southeastern Pampean Plain, Argentina

Abstract

The biomineralization process is genetically controlled, and it is the result of the metabolic activity of different organisms. Microorganisms, plants and animals produce calcium biomineralizations, calcium oxalates and carbonates being the most representative. The Quaternary pedosedimentary sequences of the southeastern coast of Buenos Aires province evolved from bioclastic and loess sediments, reworked by water and/or wind action. Calcium biomineralizations play an important role in the development of soils and contribute to differentiate sedimentary levels affected by pedogenesis. This work aims to characterize calcium biomineralizations in bioclastic, loessic and fluvio-eolian pedosedimentary sequences, typical of coastal environments of the southeastern Buenos Aires province. Modal soil profiles were defined in pedosedimentary sequences of the fluvio-eolian and coastal plains, in which disturbed and undisturbed samples were analyzed. Samples were analyzed at different scales of resolution: mesoscopic, microscopic and submicroscopic, using optical microscopy and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX). Organic matter content, pH , particle size distribution , mineralogy and calcium content were also quantified. Three geopedological units were differentiated in a sequence developed on loess mantle deposits: dunes and interdunes, coastal lagoon/paleo-coastal lagoon and shell ridges. In soils with incipient development, calcium biomineralizations are scarce, mainly associated with isolated bioclasts. In soils with more pedological development, bioclasts are affected by bioerosion through microorganism action (fungi and algae ), and subsequently calcium re-precipitated as secondary oxalates and carbonates (biomineralizations ). These biomineralizations also weakly add or bind skeletal components, incorporating themselves into the matrix of soils and sediments. The type and diversity of calcium biomineralizations increase directly in relation with time and pedogenetic evolution; so, these biomineralizations have been determinant in the origin, evolution and resistance to natural and anthropic degradation of the late Quaternary pedosedimentary sequences of southeastern Buenos Aires province, Argentina.