Abstract
Cave environment allows long-term processes of rock weathering and chemical deposition that cannot occur on Earth surface directly exposed to meteoric and external biochemical agents. Apart from the common carbonate speleothems, chemical precipitation from infiltration water can also produce phosphate-rich formations usually occurring as parietal dark crusts or spheroidal nodules. Despite the potential purposes of these kind of deposits as paleoenvironmental proxies, they have been still poorly investigated by cave scientists. Monte Corchia cave (NW Tuscany, Central Italy) is one of the most studied caves in the world, particularly for paleoclimate reconstructions from calcite speleothems. Several samples of parietal formations were collected in relict phreatic and epiphreatic passages at different altitudinal levels that reflect different evolutionary stages of this large cave system. Samples were analyzed by diffractometry and SEM-EDS possibly revealing the occurrence of hydroxyapatite or fluorapatite mixed with Fe/Mn incrustations and allogenic clastic particles. Crusts often cover the entire section of relict phreatic or epiphreatic passages and can be related to precipitation during waterfilled phases. Phosphate nodules are almost entirely composed by hydroxyapatite or fluoroapatite and could be the result of long-term chemical (or bio-chemical) precipitation in air-filled environments.
Highlights
Caves are highly conservative environments where the effects of surface processes reveal themselves in depositional records that can be either clastic or chemical
Performed on a limited number of samples, our research indicates that the crusts and calcium phosphate nodules (i) are exclusively found in relict conduits of phreatic or epiphreatic origin, (ii) usually cover flow dissolution forms, (iii) are often associated with silicoclastic deposits of partially allogenic nature
Present seepage waters are free of dissolved phosphates (Montigiani et al, 1998; Mantelli et al, 2015b). All these elements suggest that: (i) P in the Corchia system has an ancient and allochthonous origin, (ii) it was transported into the karst system by waters coming from the surface, and (iii) that calcium phosphate was formed by in situ chemical precipitation in waterfilled environments with stagnant or very slowly flowing water
Summary
Caves are highly conservative environments where the effects of surface processes (concerning climate, hydrology, morphology, biology, etc.) reveal themselves in depositional records that can be either clastic (alluvial deposits) or chemical (speleothems). Other types of cave deposits are still poorly studied, potentially interesting as paleo-environmental proxies. Among these are gypsum crystals (Gázquez et al, 2013, 2020) and other uncommon speleothems such as Fe and Mn oxide deposits (e.g., Rossi et al, 2010; Frierdich et al, 2011). Phosphates are among the most frequent minerals in caves (Hill and Forti, 1997) They are the product of chemical reactions between guano, bones or other animal residues and cave bedrock, calcite concretions or sediments (e.g., Audra et al, 2019, and references therein). Phosphates are often found in caves where there are no relevant deposits of guano, usually occurring in the form of thin patinas or crusts that cover the rock or cave deposits, or as small spheroidal nodules grown on the walls (Onac and Forti, 2011; Audra et al, 2019)
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