Gypsum degradation in Italy with respect to climatic, textural and erosional conditions

Abstract

Many experimental studies have been carried out on the meteorological degradation of carbonate outcrops, but to date only a few, without statistical relevance, on the same phenomenon on gypsum. To measure the lowering of gypsum surfaces and to understand the controlling factors, a high number of measurements that can be compared to other results are needed. The Micro Erosion Meter (MEM) method was used because the tablet method, used in the past few years, has been found not suitable for gypsum. Furthermore, use of the MEM limits the systematic error to less than 0.05 mm. This paper presents statistical results for gypsum dissolution using the MEM. In the last 9 years in Italy, over 4,000 degradation data have been collected, covering 15 gypsum lithotypes differing in petrography, texture and chronostratigraphy. The measurements were made at 26 different test locations and more than 220 measurement points, covering the whole of Italy. The test locations were chosen on the basis of their different climatic and morphologic conditions: the rainfalls ranging from 1,300 mm/yr. to less than 250 mm/yr. The mean degradation for the whole area, after excluding values linked to climatic or limiting lithologic or climatic situations, is 0.91 +/- 0.36 mm/1,000 mm rain, which corresponds to an average degradation rate for the Mediterranean area of 0.70 +/- 0.17 mm/yr. The observed gypsum degradation ranged between 0.21 and 2.66 mm/1,000 mm rain. The wide range in the degradation values experienced in this research directly reflects the dramatically different environmental conditions in each of the experimental locations. It was concluded that the fundamental parameter in gypsum degradation is gypsum dissolution. This parameter reaches 85% of the theoretical solubility of gypsum in the Mediterranean basin. The second most important parameter for gypsum degradation is mechanical erosion, which in turn depends upon the lithological and textural composition of the sample and upon local climate. The water flow conditions may also be important, because gypsum solubility becomes significant only when a turbulent flow is present. Finally, the relatively high value of annual degradation is the first experimental evidence that the karst cycle in gypsum is fast, no outcrop of this rock surviving more than a few hundred thousand years if exposed to meteorological agents.