Carbon dioxide concentration in temperate climate caves and parent soils over an altitudinal gradient and its influence on speleothem growth and fabrics

Abstract

Carbon dioxide (CO 2 ) concentrations in caves and parent soils in the Italian Alps have been studied along a 2100 m altitudinal range -corresponding to a mean annual temperature (MAT) range of 12°C -in order to investigate the relationship between MAT, soil pCO 2 and cave air pCO 2 , and to test the influence of soil pCO 2 on speleothem growth and fabric to ultimately gain insight into their palaeoclimatic significance in temperate climate settings.Our findings indicate that soil CO 2 is linearly correlated to MAT and its mean annual concentration is described by the equation: soil CO 2 (ppmv) = 1112 + 460 MAT. Soil pCO 2 can also be exponentially correlated to actual evapotranspiration. The pCO 2 in the aquifer is linearly correlated to MAT at the infiltration site and is more influenced by summer soil pCO 2 . Cave air CO 2 in the innermost part of the caves exhibits a similar seasonal pattern, and commonly reaches concentrations of about 15% to 35%, with respect to the corresponding soil values, and is exponentially correlated to the MAT at the infiltration site.The combination of these parameters (soil pCO 2 , dripwater pCO 2 and cave air pCO 2 ) results in speleothem growth and controls their fabrics which are typical of four MAT/elevation belts broadly corresponding to the present-day vegetation zones. In the lower montane zone [100-800 m above sea level (a.s.l.)] speleothems mostly consist of columnar fabric, in the upper montane zone (800-1600 m a.s.l.) both columnar and dendritic fabrics are common, the Subalpine zone (1600-2200 m a.s.l.) is characterized mostly by moonmilk deposits, whereas in the Alpine zone (above 2200 m a.s.l.) no speleothems are forming today. Therefore, fabric changes in fossil speleothem can potentially be used to reconstruct MAT changes in temperate climate karst areas.

A penultimate glacial climate record from southern Hungary

“…There is, however, a strong firstorder climate control on the d 13 C value whereby an increase in humidity and temperature enhances soil bioproductivity and the partial pressure of soil carbon dioxide (e.g. Genty et al, 2001;Borsato et al, 2015). This results in lower d 13 C values in the dissolved inorganic carbon of the drip water and eventually in the speleothem (Fairchild and Baker, 2012).…”

Hydroclimate of the Last Glacial Maximum and deglaciation in southern Australia's arid margin interpreted from speleothem records (23–15 ka)

“…This suggests that there was less input of colloidal organic matter from the soil zone. Furthermore, clean, compact columnar calcite in well-monitored caves of temperate alpine settings, has been documented to form at low supersaturation (Frisia et al, 2000;Borsato et al, 2015Borsato et al, , 2016 and temperatures ranging from 13.2 to 9.2 • C (Borsato et al, 2015). By contrast, open columnar formation is enhanced when temperature drops (in the range 9.2 to 4.2 • C) if supersaturation remains low, as impurities are more effective in blocking growth sites.…”

Late quaternary speleogenesis and landscape evolution in the northern Apennine evaporite areas

“…Enhanced rainfall and humidity is the most suitable condition for the development of pervasive vegetation in the piedmont hillslopes. From a geochemical point of view, this means that the overabundance of biogenic CO 2 released by the pedogenic layers at the surface has a key impact in the production of carbonate speleothems once the waters percolate into the gypsum caves (Borsato et al, 2015).…”