Applying stable cosmogenic 21Ne to understand surface processes in deep geological time (107-108 yr)

This work sets out to test the applicability of stable cosmogenic 21Ne for quantifying the rates of surface processes over time scales of 107-108 yr and the potential limitations and pitfalls associated with such time spans. First, we examine several processes in addition to in-situ production during exposure that affect the final measured concentration of 21Ne. We calculate the magnitude of 21Ne produced by interaction with secondary cosmic ray particles after burial (muogenic Ne) and by non-cosmogenic sources (nucleogenic Ne). We also evaluate the fraction of 21Ne lost through diffusion out of the quartz crystal as a function of time and temperature (depth). We then apply our calculations to 21Ne concentrations measured in sediments that were deposited along the northern passive margin of Gondwana during the late Precambrian, Cambrian, and Lower Cretaceous. In light of the measured concentrations in the sediment samples, we discuss the limitations imposed by our calculations and show that 21Ne concentrations measured in Lower Cretaceous samples can be interpreted in terms of surface exposure times or average erosion rates during the time of erosion and transport. In contrast, concentrations measured in Cambrian and Precambrian samples are limited in their use as surface process indicators although they still yield valuable geological information. We conclude that this novel application of in situ stable cosmogenic nuclides holds the potential as a tool for quantifying surface processes and understanding landscape evolution during the deep geological past and provides insight into macro-scale processes that have shaped Earth over the past hundreds of millions of years.