COSMOGENIC 36CL IN MAGNETITE: A NEW TOOL FOR EROSION RATES

Cosmogenic nuclides are widely used to assess catchment-averaged erosion rates, although the method has been largely limited to ¹⁰Be and ²⁶Al in quartz. This is because (a) quartz is resistant to weathering, and (b) erosion rate calculations require a uniform exponential decrease in production rates with depth. Erosion rates have remained difficult to measure in mafic landscapes due to a lack of quartz and also due to rapid weathering of minerals such as olivine.

Chlorine-36 may be measured in a variety of mafic phases, but its application to erosion rates has been hampered by its complex set of production pathways, which can include low-energy neutron capture on chlorine that renders the depth-dependence non-uniform, non-exponential and sensitive to water content.

Here we attempt to circumvent these problems by isolating ³⁶Cl in magnetite, a mineral that is resistant to chemical weathering and low in chloride, so that production rates are expected to decrease exponentially with depth.

The first obstacle is to accurately determine production rates in this mineral.

We are measuring ³⁶Cl in magnetite and ¹⁰Be in co-existing quartz from late-Pleistocene age, granitic glacial erratics from the eastern Sierra Nevada, California and in banded iron formations from the Quadrilá­tero Ferrífero in Minas Gerais, Brazil. Assuming continuous exposure for the erratics and steady state erosion for the BIF, we will present an estimate of the production rate of ³⁶Cl on iron. We are also using ³⁶Cl in magnetite to determine erosion rates in several watersheds, including Fort Sage, California where erosion rates are already known from the mineral quartz.