Exploring the potential of moonmilk speleothems as archives of palaeo-environmental information

Moonmilk is an intriguing cave deposit that has a soft, malleable texture composed of two types of fibrous calcite crystals: needle fibre calcite and calcite nanofibres. These crystals create a 3D-mesh with extremely high porosity, giving moonmilk a water content of up to 95wt%. The exact mode of formation of moonmilk is still elusive, though microbial activity has often been invoked as a means of precipitating these fibrous calcite crystals. Our previous research has shown that moonmilk is rich in microbial biomass and nutrients. However, water dripping from moonmilk has very low levels of adenosine triphosphate (ATP), indicating limited microbial activity. Furthermore, we have also found the moonmilk dripwater to be at or near calcite saturation (SIC ≈ 0), hampering a strictly geochemical formation mechanism and indicating that another, yet unexplained, process may be at play in the formation of fibrous calcite. Active and fossil moonmilk can be found in caves displaying different morphologies: accretionary coatings on cave walls, stalactites of characteristic flat base, gour-pools or subaqueous fluffy deposits. Classical speleothems, such as stalagmites, are well-known as excellent archives of palaeoclimate proxy data. Formation of speleothems typically requires water percolating through a well-developed soil and carbonate host-rock to subsequently precipitate calcite in a cave environment. In similarity with classical speleothems, moonmilk has the potential to contain palaeo-environmental information. However, moonmilk forms under particular environmental conditions – often cold, alpine settings – where traditional speleothem formation is not possible. Therefore, palaeo-environmental proxy data from moonmilk could improve the spatial-temporal cover of palaeoclimate data, in particularly, covering mountainous regions that are highly sensitive to environmental changes. However, the potential of moonmilk as a possible palaeo-environmental archive has yet to be utilised since knowledge of its mode of formation, the incorporation of proxy signals, and how these are retained over time are yet to be fully explored. Petrographic, geochemical and microbiologic analyses will be carried out on moonmilk samples from two caves; Snežna jama na Raduhi (1556 m a.s.l., ~70 km N of Ljubljana) and Košelevka Cave (634 m a.s.l., 25 km WSW of Ljubljana). A variety of moonmilk types will be sampled, at various depths within the deposits, for radiocarbon dating, and stable isotopes (δ13C and δ18O) and trace element analyses. Furthermore, we will extract the moonmilk dripwater and interstitial water and analyse stable water isotopes (δD and δ18O). Our novel adapted cryogenic Scanning Electron Microscopy method will enable us to visualise the original texture of moonmilk, avoiding desiccation artefacts, to characterise moonmilk textures, mineral–microbe relationships and to detect formation mechanisms and subsequent diagenesis. Detailed cave air and water monitoring will be carried out in both caves to characterise the environmental conditions under which moonmilk is forming. The modern cave conditions can then be compared with the proxy data obtained from the currently forming moonmilk to assess how the environmental signals are encoded into the speleothem. Subsequently, the modern geochemical data will be contrasted with that of the older moonmilk deposits, which together with petrographic studies of possible diagenesis, will enable a full exploration into the potential of moonmilk as a novel palaeoclimate proxy archive. This would be a major advance in the field of speleothem palaeo-climatology by unlocking the huge potential of a new archive material.