VALIDATING EROSION RATES FROM 36CL IN MAGNETITE

Cosmogenic nuclides provide a powerful means with which to quantify rates of erosion and landscape evolution on Quaternary timescales. The preferred nuclide/target mineral pair for erosion rates is ¹⁰Be/quartz. This is because (a) quartz is common in many lithologies and resistant to weathering, (b) the production rate of ¹⁰Be in quartz is well-constrained, and (c) there is a well-established procedure for chemically isolating the in-situ produced ¹⁰Be signal. However, quartz is not always present, limiting the scope of this method.

Magnetite is also an attractive target mineral candidate. It has production pathways for several cosmogenic nuclides including ³⁶Cl and, like quartz, is both resistant to weathering and geologically widespread. Furthermore, magnetite is strongly ferrimagnetic, which allows large, high purity mineral separates to be easily obtained using magnetic separation techniques.

Here we evaluate the suitability of ³⁶Cl in magnetite for catchment-averaged erosion rates by comparing ³⁶Cl in magnetite to ¹⁰Be in quartz at six small, headwater catchments in the Sierra Nevada region. We find that the two mineral/nuclide pairs give the same erosion rate within 10% and 2σ analytical uncertainty at five of the six catchments and differ by approximately 30% in the remaining catchment. We ascribe this difference to heterogeneity in the distribution of quartz, magnetite, and/or erosion rates within the catchment. The generally good accordance between erosion rates determined from the two minerals implies that magnetite may be effectively used in lieu of quartz. We suggest that magnetite may be a suitable mineral for extending the scope of the cosmogenic erosion rate method to andesites, basalts, and other intermediate to mafic rocks with a paucity of quartz.