Long-term soil erosion derived from in-situ Be-10 and inventories of meteoric Be-10 in deeply weathered soils in southern Brazil

摘要

Meteoric Be-10 is commonly used as a geochemical tracer of soil erosion and regolith residence time over long time scales (> 103 yr). The acidic conditions of tropical soils are less favorable for meteoric Be-10 retention, and make a translation of meteoric Be-10 inventories into surface erosion rates complex. In this empirical study, we evaluate the potential loss of meteoric Be-10 in the weathering zone. Along a toposequence in southern Brazil, three regolith profiles were sampled to explore the mobility of Be in the soil system. The sequential chemical extractions of four reactive fractions indicate that the amorphous oxy-hydroxide and crystalline oxide fractions are the main carriers of meteoric Be-10. Our data from the Brazilian toposequence reveal significantly higher inventories of meteoric Be-10 in the slope concavities compared to the upslope positions. This demonstrates that chemical leaching and deep percolation of meteoric Be-10 within the soil profile leads to vertical and lateral redistribution of meteoric Be-10 across the hillslope. Our data show that more than 50% of the delivered meteoric Be-10 is leached from the upslope regolith profiles. After correcting the inventories for incomplete retention of meteoric Be-10, surface erosion rates are derived for the slope convexities. Upslope erosion rates are below 5 mm/kyr and agree with the in-situ Be-10-derived denudation rates. Our results suggest that soil and sediment fluxes can accurately be derived from meteoric Be-10 inventories in well-developed soils, when the meteoric Be-10 mobility is constrained using differential mass balances.