Applications of Cosmogenic He-3 and Ne-21 Dating to Glacial Moraines in Antarctica and California

Abstract:

The depositional age of moraines can be determined through cosmogenic nuclide exposure dating. These ages are useful in establishing a glacial history of an area and ascribing age constraints to transport processes. Be-10 is the most common nuclide used for exposure dating today, but this method is both expensive and time consuming because it requires analysis by accelerator mass spectrometry (AMS). He-3 and Ne-21 can be analyzed using noble gas mass spectrometry, which is more cost efficient than AMS and requires less chemical preparation. We collected samples from areas in Moraine Canyon, Antarctica (86.10° S, 157.75° W), which is a dry valley in the Transantarctic Mountains. Dolerite boulders along a transect of recessional moraines were sampled in the typical fashion of using a large piece of the boulder for analysis. Pyroxene minerals have been separated from these samples following the method of Bromley et al. (2014) using hydrofluoric acid. Exposure ages will be calculated from the He-3 concentrations in them. In the Mono Lake area of California, moraines were sampled from Bloody Canyon and McGee Creek sites. Instead of collecting a sample from an individual boulder, we collected approximately 25 granitic pebbles (1-3 cm) from 4-6 sites along the crest of the moraines following the method of Briner (2009). Each suite of pebbles was crushed together, and quartz minerals were separated from the agglomeration of pebbles. Cosmogenic Ne-21 will be measured from these samples to determine their exposure age. From these two field sites, we will use He-3 and Ne-21 to better understand the timing and extent of glaciation in these areas.