Genesis and microstratigraphy of calcite coralloids analysed by high resolution imaging and petrography V. Vanghi ⁎, S. Frisia, A. Borsato School of Environmental and Life Sciences, The University of Newcastle, Callaghan 2308, NSW, Australia abstract article info Article history: Received 26 March 2017 Received in revised form 1 August 2017 Accepted 2 August 2017 Available online 05 August 2017 Editor: Dr. B. Jones The genesis of calcite coralloid speleothems from Lamalunga cave (Southern Italy) is here investigated from a purely petrographic perspective, which constitutes the basis for any subsequent chemical investigation. Lamalunga cave coralloids formed on bones and debris on the floor of the cave. They consist of elongated colum- nar crystals whose elongation progressively increases from the flanks to the tips of the coralloid, forming a suc- cession of lens-shaped layers, which may be separated by micrite or impurity-rich layers. Organic molecules are preferentially concentrated toward the centre of convex lenses as highlighted by epifluorescence. Their occur- rence on cave floor, lens-shaped morphology and concentration of impurities toward the apex of the convex lenses supports the hypothesis that their water supply was hydroaerosol, generated by the fragmentation of cave drips. Evaporation and degassing preferentially occurred on tips, enhancing the digitated morphology and trapping the organic molecules and impurities, carried by the hydroaerosol, between the growing crystals which became more elongated. Micrite layers, that cap some coralloid lenses, likely identify periods when decreasing in hydroaerosol resulted in stronger evaporation and higher supersaturation with respect to calcite of the parent film of fluid. This interpretation of coralloid formation implies that these speleothems can be used to extract hydroclimate information. © 2017 Elsevier B.V. All rights reserved. Keywords: Coralloid speleothems Fabrics Calcite Hydroaerosol 1. Introduction In Lamalunga cave (Southern Italy), which is best known for the discovery of a complete fossilized Neanderthal skeleton, coralloids are the most common speleothem type, and due to their association with bones, they had been used to date the fossil (Lari et al., 2015). Coralloid speleothems, also known as cave popcorns (Hill and Forti, 1997), consist of knobs with digitate form, usually growing in clusters. Their internal structure is characterized by layers of calcium carbonate, which gradu- ally thicken from the depressions toward the protrusions. Coralloids speleothems are one of the most common speleothem type found in caves after stalactites, stalagmites and flowstones (Hill and Forti, 1997). Usually, coralloids are calcite (Gradzinski and Unrug, 1960; Webb, 1994; Hill and Forti, 1997; Niggemann et al., 1997; Dublyansky and Dublyansky, 1998; Cuevas-González et al., 2010; Leél-Őssy et al., 2011; Merino et al., 2014; Richter et al., 2015; Ammari et al., 2016) but they can also be aragonite (Thrailkill, 1968), in most of the cases al- ternating with calcite (Bar-Matthews et al., 1991; Cañaveras et al., 2001; Ortega et al., 2005; Caddeo et al., 2015; Martín Pérez et al., 2015). Speleothem-bearing caves can also develop in non carbonate bedrocks yielding silicate speleothems and among them, coralloids have also been reported. Siliceous coralloids have been found in sandstone (Wray, 1997; Lundberg et al., 2010), granite (Willems et al., 2002) and also lava tube (Miller et al., 2015, 2016). Nucleation and growth of coralloids are controlled by “substrate selection” Self and Hill (2003), a process described as when “the mineral individual (or mineral aggregate) growing from a convex substrate protrusion during competitive growth will continue its growth at the expense of its neighbours growing from flat or concave surfaces”. One of the main differences with other conventional speleothems is that coral- loids ontogenesis is not linked to dripping water. Different mechanisms have been invoked to explain coralloid formation, which include: 1) spray from splashing drops producing hydroaerosols, which in turn supplies the fluid that feeds the nascent coralloid (Gadoros and Cser, 1986; Dublyansky and Dublyansky, 1998), 2) high evaporative condi- tions and capillary film of water, which moves from the base toward the prominence of the speleothem (Gradzinski and Unrug, 1960; Self and Hill, 2003; Caddeo et al., 2015) and 3) strong CO 2 degassing (Thrailkill, 1976). Evaporation and CO 2 degassing of the parent water is greater at the speleothem protuberance, resulting in greater car- bonate precipitation at the apex of convex surfaces (Thrailkill, 1976; Caddeo et al., 2015), due to the high surface to volume ratio. The apex can then be larger than the coralloid base resulting in a grape- like form (Hill and Forti, 1997). Ventilation is believed to favour the growth of coralloids showing a preferred orientation facing upwind direction, by enhancing degassing and, consequently, the saturation state of the film of fluid relative to a phase of calcium carbonate (Onac Sedimentary Geology 359 (2017) 16–28 ⁎ Corresponding author. E-mail address: valentina.vanghi@uon.edu.au (V. Vanghi). http://dx.doi.org/10.1016/j.sedgeo.2017.08.001 0037-0738/© 2017 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo