Dating silica sinter (geyserite): A cautionary tale

We describe a new effort to date hydrothermal silica sinter deposits (geyserite) from the Upper Geyser Basin of Yellowstone National Park using 14C of co-deposited organic matter, U-series and cosmogenic 10Be methods. A majority of the samples were collected from stratigraphic sections, mainly at Riverside, Giant, and Castle Geysers. Ages obtained from 41 14C analyses range from modern to 12.1 cal ka BP. Nearly all the 14C ages show inconsistencies with their stratigraphic positions, and several replicate 14C analyses from the same sample result in significantly different ages. The δ13C values of the organic material in the sinter range from -26.6‰ to -12.7‰. The more enriched values are attributed to microbial fixation of dissolved inorganic carbon (DIC), which has heavier δ13C values and is 14C-depleted relative to atmospheric CO2, leading to apparent older ages. U-series analyses on 4 samples yielded ages between 2.2 and 7.4 ka. Large 230Th/U age uncertainties in the sinter, due to low uranium concentrations along with elevated 232Th and associated initial 230Th, make these ages imprecise for use on Holocene deposits. A single cosmogenic 10Be exposure age of 596±18 ka is considerably older than the age of underlying rhyolite and is thus unreliable. This apparent old age results from contamination by meteoric 10Be trapped in the opal that overprints the very small amount of cosmogenic 10Be. By presenting the problems we encountered and discussing their probable cause, this paper highlights the difficulty in obtaining reliable, high-precision geochronological data necessary to use sinter deposits as paleoenvironmental and paleo-hydrothermal archives.