Environmental conditions controlling the chemical weathering of the Madara Horseman monument, NE Bulgaria

The conditions that led to the deterioration of the Madara Horseman sculpture, a relief carved in a sandy limestone cliff in north-east Bulgaria, were studied, with most emphasis on the chemical weathering at the stone-atmosphere interface. Total deposition sampled close to the monument contains high concentrations of NH(4)(+) and K(+), which shows the influence of the dense vegetation in the area, and Ca(2+), which reflects the cliff's weathering products. Ca(2+) is also one of the most important constituents (next to aluminosilicates) of the atmospheric particulate collected at the monument. S-rich particles are the most abundant particles in the size range < 0.5 mu m. High amounts of biogenic particles (K-P-S-rich) were found, especially for particles < 1.0 mu m. The chemical, mineralogical and petrographic characteristics of the stratum that holds the monument were studied down to a depth of 20 cm. Two distinctive processes are active at the stone-atmosphere interface. Firstly, a weathering crust (ca. 2 mm thickness), consisting mainly of calcite and gypsum and enriched with Cl(-), K(+) and NO(3)(-), is formed in the rain-sheltered areas of the monument. Secondly, karstic dissolution in the interior of the rock substrate leads to the formation of a hard carbonate crust at the surface of the sculpture. This natural surface hardening leads to a state of 'auto-conservation' of the monument. The atmospheric composition seems to have only a limited influence on the deterioration of the Madara Horseman. Far more important are the micro-climatic conditions and the action of lichens. (C) 2001 Editions scientifiques et medicales Elsevier SAS