Meteoric cosmogenic Beryllium-10 adsorbed to river sediment and soil: Applications for Earth-surface dynamics

摘要

Rainfall scavenges meteoric cosmogenic Be-10 from the atmosphere. Be-10 falls to the Earth's surface, where it binds tightly to sediment particles in non-acidic soils over the life-span of those soils. As such, meteoric Be-10 has the potential to be an excellent geochemical tracer of erosion and stability of surfaces in a diverse range of natural settings. Meteoric Be-10 has great potential as a recorder of first-order erosion rates and soil residence times. Even though this tracer was first developed in the late 1980s and showed great promise as a geomorphic tool, it was sidelined in the past two decades with the rise of the "sister nuclide", in situ Be-10, which is produced at a known rate inside quartz minerals. Since these early days, substantial progress has been made in several areas that now shed new light on the applicability of the meteoric variety of this cosmogenic nuclide. Here, we revisit the potential of this tracer and we summarize the progress: (1) the atmospheric production and fallout is now described by numeric models, and agrees with present-day measurements and paleo-archives such as from rain and ice cores; (2) short-term fluctuations in solar modulation of cosmic rays or in the delivery of Be-10 are averaged out over the time scale soils accumulate; (3) in many cases, the delivery of Be-10 is not dependent on the amount of precipitation: (4) we explore where Be-10 is retained in soils and sediment; (5) we suggest a law to account for the strong grain-size dependence that controls adsorption and the measured nuclide concentrations: and (6) we present a set of algebraic expressions that allows calculation of both soil or sediment ages and erosion rates from the inventory of meteoric Be-10 distributed through a vertical soil column. The mathematical description is greatly simplified if the accumulation of Be-10 is at a steady state with its export through erosion. In this case, a surface sample allows for the calculation of an erosion rate. Explored further, this approach allows calculation of catchment-wide erosion rates from river sediment, similar to the approach using Be-10 produced in situ. In contrast to the in situ Be-10 approach, however, these analyses can be performed on any sample of fine-grained material, even where no quartz minerals are present. Therefore, this technique may serve as a tool to date sediment where no other chronometer is available, to track particle sources and to measure Earth-surface process rates in soil, suspended river sediment, and fine-grained sedimentary deposits.