Siliceous-sulphate rock coatings at Zhenzhu Spring, Tengchong, China: the integrated product of acid-fog deposition, spring water capillary action, and dissolution

Siliceous-sulphate rock coatings were observed at Zhenzhu Spring, an acid sulphate hot spring in the Tengchong volcanic field, China. These rock coatings are mainly formed of gypsum and amorphous silica. Some alum-(K), voltaite, α-quartz and muscovite were also found. Four different laminae are developed in the rock coatings: gypsum layer, tight siliceous layer, tabular siliceous layer and siliceous debris layer. The gypsum layer is located at the top of the rock coatings, while other siliceous layers appear below the gypsum layer. Geochemical modelling of the fluids was performed to identify the mechanisms responsible for the formation of gypsum and amorphous silica. The results indicated that the occurrence of gypsum is related to the acid-fog deposition and amorphous silica mainly originates from spring water. Fog deposition provided the rock coatings with abundant SO42− and Ca, and the subsequent complete evaporation of the condensed fluids produced gypsum. Seasonal climate change (especially variation in rainfall) determines the fluctuations of capillary action and dissolution. Rainfall events in the wet season led to periods of non-precipitating gypsum and promoted the capillary rise of the spring water. Slightly diluted capillary water (a small amount of rainwater) covered the rock coatings, formed a tight siliceous layer on the rock-coating surface and/or filled the pores among the gypsum crystals forming many tabular siliceous aggregates. Heavy rainfall (high dilution), however, resulted in non-precipitating amorphous silica and accelerated the gypsum dissolution, leaving tabular pores around tabular siliceous aggregates and forming a tabular siliceous layer.

Strategies for 14C Dating the Oxtotitlán Cave Paintings, Guerrero, Mexico

ABSTRACTOxtotitlán Cave paintings have been considered among the earliest in Mesoamerica on stylistic grounds, but confirmation of this hypothesis through absolute dating has not been attempted until now. We describe the application of advanced radiocarbon strategies developed for situations such as caves with high carbon backgrounds. Using a low-temperature plasma oxidation system, we dated both the ancient paint and the biogenic rock coatings that cover the paint layers at Oxtotitlán. Our research has significantly expanded the time frame for the production of polychrome rock paintings encompassing the Early Formative and Late Formative/Early Classic periods, statistically spanning a long era from before ca. 1500 cal B.C. to cal A.D. 600.