Carbonates in natural and geotechnical settings - Chemical sediments as environmental archives

The diverse geoscientific topics of this publication are centered around the carbonate system and in particular on anhydrous and hydrous calcium carbonates and the distinct polymorphs of CaCO3. A broad range of physicochemical processes from sub-microscopic to regional scale opens the discussion on fundamental and applied research aspects connecting modern geochemistry with geology. The deposition of carbonate minerals in various natural and (geo)technical settings represents a chemical-sedimentary archive capturing site-specific natural and human-made environmental conditions. Different environmental parameters – changing temporally and spatially – can be reconstructed from the evolving mineral deposits. An in-depth process understanding of fluid-solid (e.g. water-rock) interaction is a key to mani-fold applications. This involves an advanced understanding of carbonate precipitation (growth) dynamics, inorganic or microbially-mediated crystallization mechanisms and the resulting material characteristics and their environmental dependencies. State-of-the-art and mostly high spatial resolution laboratory analytical geochemical and imaging techniques are utilized. This includes computer-controlled micromill and laser ablation based solid material sampling strategies in combination with mass spectrometric, electron and X-ray interactive analyses of elemental concentrations and isotopic ratios and fractionation, as well as of variable crystal growth and fabrics. Radiometric uranium-thorium age and rate determination and multiply-substituted isotopologue (clumped isotopes) measurements yielded valuable information. Field-based periodical or on-site and online environmental monitoring mainly of fluid phases applying automated data loggers constitutes an integral part in some ofthe studies presented.

Applying geochemical approaches and tools to geotechnical settings is focused on unwanted mineral deposits – dominantly carbonate scaling – impairing geothermalenergy production from deep aquifers or artificial water channels of different settings. This includes reduced (thermal) water and energy transfers from deep wells, in pipelines and heat exchangers, as well as in tunnel and surface drainages. The concept of ‘Scaling Forensics’ represents a multi-proxy high-resolution analyticalapproach of the carbonate scale materials and was developed and applied in order to evaluate and adapt unfavourable site-specific production conditions. Scale depositing geogenic and operational environmental processes determining the scaling progress and scale material characteristics (e.g. growth rate, consistency) are investigated. Specific environmental conditions such as production cessations/restarts, the problem of scale-fragment formation or the distinct role of solid/fluid phase nterfaces (e.g. corrosion layers, growth surfaces) and related processes (e.g. crystal nucleation, CO2/H2O outgassing, particle mobilization) are highlighted. The formation mechanisms of geologically young veins filling fractures of the Austrian ‘Erzberg’ iron ore deposit in Styria and their possible relation to gravitational or tectonic ass movements, as well as to variable regional climate conditions in a sensitive Alpine setting are discussed. Karst areas, caves and speleothems were investigated n the context of stalagmite growth dynamics and the reconstruction of past changes in air temperature and meteoric precipitation. Using selected and environmentally well-constrained natural and technical Ca-carbonate materials established as well as newly developed isotope tracers (e.g. clumped isotope thermometer and fluid rovenance tracer) can be evaluated and calibrated.