Terrestrial carbonates: mineral products of geobiological processes in the critical zone

The proposed project deals with a large group of terrestrial carbonate deposits, which form in soils and caves through a variety of physical, chemical, biological and geological processes that operate at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. The studied interface corresponds to the unsaturated (vadose) belowground part of the Critical Zone – a system defined from the outer extent of vegetation down to the lower limits of groundwater.
The project will focus on fibrous microcrystalline calcite (FMC), one of the most common and widespread forms of secondary calcium carbonate in vadose terrestrial environments (caves, soils, surficial sediments.). FMC is characterized by nonequilibrium crystal shapes composed of low-Mg calcite. The origin of FMC crystals has generally been attributed either to purely physicochemical processes or to microbial biomineralisation. The current biogenic hypotheses are largely based on common co-occurrence of fibrous calcite and microbial features, and the similarity in dimensions and morphology of calcite fibres and fungal hyphae and filamentous bacteria. Morphological arguments for biogenicity have been supported by geochemical and petrographic data, considered as biosignatures preserved in certain forms of FMC.
The purpose of the proposed project is to examine precipitation mechanisms of FMC using in-situ and laboratory investigation of calcite moonmilk speleothems, actively precipitating Slovenian karst caves, and analogous deposits from a fundamentally different setting – Mediterranean calcareous soils and calcretes. The research is aimed at testing a hypothesis that FMC may form through non-classical crystal growth pathways. The main objectives are: 1) to perform a comparative micromorphological study of fibrous microcrystalline calcite from a maximum diversity of terrestrial settings; 2) to extract an characterise extracellular polymeric substances (EPS) present in FMC sediment interstitial waters; 3) to analyse stable isotopes of carbon and oxygen of FMC samples from different microenvironments; 4) to perform microbiological characterisation of cave and soil FMC deposits using environmental metagenomics; and 5) to test crystallisation processes of FMC in vitro in the presence of previously identified polymers.
The anticipated study sites in Slovenian caves are targeted because they include all different microenvironments of FMC precipitation. All the caves is close to the laboratory and easily accessible throughout the year and will be used as a main field site for regular sampling of water and mineral precipitates, as well as for continuous microclimate determination. Comparative material will be provided from Mediterranean regions, known for active accumulations of FMC carbonates in extensive root systems.
The research will contribute to the understanding of fundamental principles of calcium carbonate deposition in near-surface terrestrial settings, an inherent subject of sedimentology. The addressed problems are on the interface of several scientific fields from geomicrobiology, cave & karst studies, soil science, and crystallography: expected results will thus bear significance in all these disciplines.