Combining meteoric and in situ 10Be for depositional flux estimations along a climate gradient

Along an extreme climate gradient from arid to humid in the Chilean Coastal Cordillera, we investigated soil profiles using the meteoric cosmogenic radionuclide ¹⁰Be and its ratio to stable ⁹Be. The isotope ratio ¹⁰Be(meteoric)/⁹Be combines an atmospheric flux tracer to Earth’s surface with stable ⁹Be that is released by weathering. In contrast to the well-established in situ ¹⁰Be method, the meteoric variety of ¹⁰Be is not limited to quartz-bearing lithologies and requires less sample material. However, to calculate denudation rates from ¹⁰Be(meteoric)/⁹Be knowing the depositional flux to Earth’s surface, F¹⁰Be_met, is an essential prerequisite.

We applied the in situ ¹⁰Be technique for denudation rate estimations on soil profiles (D_insitu) and compared the results with ¹⁰Be(meteoric)/⁹Be-derived denudation rates (D_met). Our results show that D_met in conjunction with F¹⁰Be_met derived from a general circulation (GCM) model (ECHAM5-HAM; Heikkilä, 2007) are consistent with D_insitu at the mediterranean (¹⁰Be_insitu: 126 ± 32 t km^-2 yr^-1, ¹⁰Be(meteoric)/⁹Be: 170 ± 40 t km^-2 yr^-1) and humid (¹⁰Be_insitu: 38 ± 10 t km^-2 yr^-1, ¹⁰Be(meteoric)/⁹Be: 31 ± 8 t km^-2 yr^-1) study sites, both with mean annual precipitation (MAP) > 400 mm (mediterranean: 440 mm yr^-1, humid: 1080 mm yr^-1). However, at the arid (MAP: 10 mm yr^-1) and semi-arid (MAP: 90 mm yr^-1) study sites, when using a GCM-based F¹⁰Be_met, D_met exceed D_insitu by a factor of up to 170. The cause for this offset may lie in an overestimation of F¹⁰Be_met at low precipitation. Indeed, when using D_insitu as benchmark and solving for F¹⁰Be_met instead, a much lower flux results.

These results suggest a lower precipitation limit (~400 mm yr^-1) for the use of GCM-derived F¹⁰Be_met especially in dry regions. In such arid and semi-arid regions with high spatial climate variability the depositional flux derived from coarse-resolution GCM-models overestimates the input of meteoric ¹⁰Be to Earth’s surface either by wet or dry deposition. In settings with sufficient precipitation above this limit the GCM-derived flux is suited to determine denudation rates using ¹⁰Be(meteoric)/⁹Be.

Heikkilä, U., 2007, Modeling of the atmospheric transport of the cosmogenic radionuclides ¹⁰Be and ⁷Be using the ECHAM5-HAM general circulation model: ETH Zurich, doi:10.3929/ETHZ-A-005560259.