Moonmilk is a well-known carbonate speleothem mainly consisting of calcite and other CaCO3 polymorphs with varying crystal morphology that forms on the walls of natural and anthropogenic underground environments, especially those found in carbonate bedrock. The genesis of moonmilk is poorly understood, both abiotic and biogenic processes may contribute to its formation, but a large number of studies demonstrated that the metabolic activity of microbial communities plays a major role in the minerogenesis of such deposits. However, the physicochemical properties of the host rocks significantly affect the moonmilk formation regulating the environmental conditions under which the bacterial communities grow as well as influencing the mineral composition of speleothems. Close to the town of Tarquinia (Central Italy), the area where is located the Etruscan necropolis of Monterozzi hosts numerous hypogean environments where moonmilk speleothems diffusively proliferate. Due to the heterogeneous lithology of bedrock (Macco Formation), consisting of calcarenite and hybrid sandstone, this restricted area offers a favourable chance to investigate the role of bedrock in the genesis of the moonmilk.
The main goals of this project are: 1) to provide a stratigraphic reconstruction of the Macco Formation and a detailed characterization of the main lithofacies in terms of mineral, chemical and C, O, Sr and Ca isotopic composition and physical properties also to support the archaeological studies and the restoration works on the Monterozzi necropolis; 2) to analyse the morphological, mineralogic and chemical features of the moonmilk concretions in relation to the variable physicochemical characters of the geologic substrate in order to also achieve a better understanding of how all these parameters can influence the microbial ecology.
The origin of the moonmilk is still debated, its formation has been ascribed either to physicochemical processes or biotic processes or a combination of both. Over the last two decades, this topic is being widely studied especially with regards to the biomineralization processes that may be involved in the formation of moonmilk and the characterization of the microbial communities associated to such speleothems. However, although the influence of the bedrock on the mineral composition, chemistry and morphology of the moonmilk deposits and on the microbial activity is generally accepted, in-dept knowledge of the physicochemical parameters that regulate such interrelationships is needed. In this respect, the project offers the opportunity to elucidate the role of bedrock in the genesis of the moonmilk inspecting, in a restricted area, the carbonate concretions grown on lithologically different bedrocks and providing a detailed characterization of the physical and chemical properties of the bedrocks. An array of investigation methods will be used for this purpose. Field survey and sampling, facies analysis, biostratigraphic analysis eventually implemented by isotope stratigraphy along with photogrammetric 3D modelling will be employed to reconstruct the stratigraphic architecture of the Macco Formation that is not yet fully defined. The petrophysical features of the bedrock lithologies will be analysed through polarized light microscopy, porosity determination with helium porosimeter, X-ray computed tomography, X-ray diffraction, scanning electron microscopy in order to highlight how bedrock microstructural features (e.g. grain-size, porosity and permeability) control water percolation, gas exchanges and diffusion of bacterial colonies. Mineral chemistry by EMPA and ICP-AES and ICP-MS elemental and C, O, Sr and Ca isotopic composition of the bedrock and the moonmilk spleothems will give an insight into the processes regulating the energy supply for microbial growth.
The strength of this project also lies in the different expertise of the components and the collaborators involved in the project that may provide an integrated stratigraphic, paleontological, petrographic, mineralogical and geochemical approach. Moreover, the research team will operate in synergy with another research group of Sapienza University that is working on the same area to characterize the microbial ecosystem associated to moonmilk deposits.