Current denudation rates in dolostone karst from central Spain: Implications for the formation of unroofed caves

Abstract

Rock tablets of known weight were buried in the soil of a karst region in Central Spain to evaluate the carbonate weathering during a period of a year. The experiment was conducted at two different soil depths: 5–10 and 50–55cm from the surface. The parental rock used in the experiment is composed of dolomite and magnesite with variable proportion of accessory minerals and minor elements. Soil mineral and chemical composition as well as its texture was also characterized. Meteorological conditions at the site together with temperature and CO2 in both soil levels were monitored. Sets of tablets were retrieved after 6 and 12months of the start of the experiment to account for seasonal weathering. Different lithologies do not exhibit significant differences in weathering, although a large inter-sample variability is attributed to variable size and distribution of the porosity. Results show an enhanced weathering during the wet and cold season that accounts for 78±1% of the total annual weathering. Rock tablets examined under scanning electron microscopy prior and after exposure to natural environment show that most of the material lost occurred along cracks, edges or large pores. Although dissolution is a common process, most of the weathering is due to crystal detachment. Rock tablets at the depth of 5–10cm were weathered 68±1% more than those set at 50–55cm from the surface. Higher soil moisture and concentration of CO2 were found deeper in the soil, which likely enhanced the dissolution of carbonate. However, physical weathering dominated weight loss of rock tablets at both soil depths; especially at the 5–10cm level where soil thermal and moisture cycles were more frequent and greater. Denudation rate calculated from the 12months set provides values of 2.48±1.07μm/yr and 1.75±0.66μm/yr at the depths of 5–10 and 50–55cm, respectively. Since the conditions at the average contact between soil and bedrock are similar to those at the 50–55cm depth, we consider that this is a more reliable denudation rate for the studied location during the studied period. The calculated weathering rate suggests that denudation has a limited contribution to the thinning of bedrock over caves at this site. Therefore, we consider that the formation of unroofed caves in this region most likely results from the thinning of bedrock cover over caves due to collapse of blocks from their ceilings.