Читать книгу Monument Future - Siegfried Siegesmund - Страница 331

Gypsum formation in Taya Caves is triggered by the dissolution of the calcareous bioclasts and pyrite crystals in the rock, releasing Ca and S in solution. This is validated by the occurrence of gypsum solely on the fossiliferous rock type, which is also rich in pyrite – diagenetic or secondary, often included inside the shells. Pyrite has been thought to be the precursor also of the sulfate-rich efflorescences in Yoshimi Hundred Caves (Oyama & Chigira 1999), whereas about the gypsum origin 225on Oya stone we need further investigations. The ubiquity of gypsum can be explained in terms of the relevant microenvironmental conditions. The deepest levels of the studied underground sites, the most environmentally isolated, may have a nearly constant RH of about 100 %. This value, under the influence of external airflows, gets significantly lower and more variable closer to the cave entrances, the microclimatic monitoring reveals (Fig. 4). Gypsum has an extremely high deliquescence relative humidity (DRH) – higher than 99 % (Charola et al. 2007) – so that is stable in many environments, even very humid, provided that the substrate is not wet.

Figure 4: Microclimatic monitoring during summer and early fall (at different distances from the entrances of the underground sites and outdoors).

With broader fluctuations and lower values of RH, the crystallization of a number of other phases, typically with lower DRH, can occur. Emblematic are the findings on Oya stone, which disclose the exclusive presence of gypsum in the deep, extremely humid quarry levels, whereas the main component of the efflorescences in the middle levels or semi-underground quarries is mirabilite (DRH = 97 % at 15 °C), then replaced by thenardite (DRH = 86 %) in the dryer external environment (Steiger & Asmussen 2008). The efflorescence formation may vary temporally, other than spatially, with the climatic season: like for the sulfates in Yoshimi Hundred Caves, almost all crystallizing during the dry winter season. The conditions of lower and variable RH are associated with more frequent cycles of crystallization/dissolution, hydration/dehydration, hygroscopic adsorption/desorption, and more damaging mechanical stresses generated on the stone. The rate of stone decay related to the simple interaction with liquid water, instead, upsurges in summer and early fall, when most precipitations are concentrated and typhoons may occur.

The results suggest that the chemistry of salt weathering is often controlled by the contribution of the rock-forming minerals. Another source that is worth mentioning is related to the efflorescences in Taya Caves that, given their composition and the presence of a cultivated land above the cave vault, is possibly represented by the agricultural chemicals used therein, which migrated downwards in solution.