Quantifying glacial moraine age, denudation, and soil mixing with cosmogenic nuclide depth profiles

Abstract

Glacial boulders and soils on moraines are often dated to quantify the timing of glaciations and/or rates of chemical weathering in moraine chronosequences. A common assumption is that moraine crest erosion and soil mixing are unimportant. However, several studies suggest moraine denudation may be substantial. We evaluate the magnitude of moraine denudation and soil mixing in the Pinedale (∼21 ka) and Bull Lake (∼140 ka) moraines at Fremont Lake (Wyoming, United States) using cosmogenic nuclide depth profiles and a numerical model of moraine erosion and nuclide production and decay. Depth profiles indicate mixing of the surface layer from 40 to 60 cm depth. Age constraints from depth profile dating of cosmogenic nuclides result in ages of 17–24 ka for the Pinedale moraine and 70–127.5 ka for the Bull Lake moraine. These ages are comparable to ages based on the Lal and Chen approach which uses a sample from the mixed layer and the unmixed layer. However, the Lal and Chen approach is difficult to apply in real environments as the mixing depth needs to be determined independently. We find our best fit model based ages for the Bull Lake moraine are younger than independent age constraints from boulder exposure dating due to incomplete mixing. Exposure age constraints from boulders are more straightforward for moraines than ages based on depth profile dating. Finally, we find that moraine crests do significantly erode and are mixed, suggesting that previous weathering and dust accumulation rate studies on moraines provide minimum estimates for these processes.