On biospeleothems from a Venezuelan tepui cave: U-Th dating, growth rates, and morphology

Abstract

Seven silica biospeleothems from Cueva Charles Brewer, Chimantá Plateau, Venezuela have been successfully U-Th dated despite very low U and high detrital Th concentrations. Growth rates are low, between ~100 to ~800 µm/ka, and are greater closer to water level. Dates in unaltered material are in good stratigraphic order, but secondary silicification may compromise the U-Th system, yielding unreliable sequences of ages. Detritally-enriched layers correlate with global climate cycles of the Late Quaternary, in particular the cooler, drier phases of MIS 5d, 5a, and 4. SEM studies indicate that the peloidal material is made up of silica nano-particles assembled to form hollow tubules ~1 µm in diameter. Secondary silicification inside and outside the tubules fills most of the pores. Barite crystals are deposited close to the silicified core material. Chemical analyses suggest that the white peloidal material is deposited by stream-generated foam, while the silicified material and dark core material are closer to rock composition.