10Be/9Be and 26Al/10Be support a late Miocene burial age for basal Gray Fossil Site sediments

Abstract

We provide 2 independent radioisotopic age estimates for cored basal sediments of the Gray Fossil Site using cosmogenic nuclides. The first estimate uses meteoric 10 Be/ 9 Be from the bottom of the GFS-1 core, as well as from modern local grasses, to constrain the deposition of basal GFS sinkhole complex sediments to 6.60 ± 0.85 Ma. We corroborated this age estimate using in-situ 10 Be and 26 Al in quartz sands from the GFS-1 core. This estimate provided a looser constraint than the 10 Be met / 9 Be approach, yielding a minimum burial age for the basal sediments of 4.43 ± 0.34 Ma. These independent geochronometers provide evidence that the deepest GFS sediments are at least early Pliocene in age, and likely date to the late Miocene.