Beryllium cycling through deciduous trees and implications for meteoric 10Be systematics

Abstract

Application of meteoric 10Be inventories and 10Be/9Be ratios to determine surface ages, erosion rates, and denudation rates requires understanding the conditions under which meteoric 10Be functions as an open vs. closed system near the Earth's surface. Here, we investigate biologically mediated beryllium isotope mobility in an oak-hickory forest in eastern North America. We report meteoric 10Be and 9Be concentrations in soils, roots, and leaves of four deciduous tree species. The data show that beryllium is readily taken-up from the soil by roots, cycled through leaves, and redeposited on the forest floor with autumn leaf abscission. Hickory (genus Carya) dominates beryllium cycling with a spatially averaged annual 10Be flux to the soil surface through litterfall that is four times higher than the atmospheric 10Be deposition rate. Biocycling may thus modulate the shape of meteoric 10Be concentration and 10Be/9Be ratio depth profiles in soils. We present a biocycling model that describes beryllium uptake by roots, redeposition on the surface through litterfall, and retention according to an exponential adsorption rule. This model reproduces the observed form of meteoric 10Be and 9Be depth profiles and the beryllium isotope ratio in foliage at the study site. The model also indicates that both declining and bulge-type meteoric 10Be depth profiles may result from biocycling, even where beryllium loss by leaching is significant, implying that declining meteoric 10Be depth profiles may not be indicative of quantitative 10Be retention. We recommend that biocycling be considered in models of beryllium isotope systematics where hickory or other beryllium accumulating taxa are present and where litter production rates are high.