We report the radiocarbon age, chemistry, and mineralogy of sinter
deposits from Castle and Giant Geyser in the Upper Geyser Basin of
Yellowstone National Park. At each geyser, we sampled following the
stratigraphy of the older terrace and younger cone. We analyzed 15
samples with x-ray diffraction, x-ray tomography, x-ray fluorescence,
scanning electron microscopy, thin section microscopy, 3D imaging, and
loss on ignition. Castle Geyser's terrace and Giant Geyser's cone are
composed of more increasingly more mature sinter. The concentrations of
Na, K, Cs, and Ga decrease in the sinter with decreasing H2O and
increasing SiO2, therefore water and trace element concentrations
correlate with stratigraphic position, and sinters exhibit progressive
dehydration with increasing age. However, radiocarbon dates resulted in
stratigraphically out-of-sequence ages. This suggests both physical
mixing and the influence of magmatically dead carbon. By isolating and
dating individual pieces of externally sourced organic material, we
report ages as upper bounds---increasing the known age of Castle Geyser
cone to 2,000 yr BP. Additionally, we demonstrate that Castle Geyser's
shield is mineralogically distinct from and at least 1,000 years older
than it's cone.