Element Cycling in the Critical Zone as Viewed by New Isotope Tools

Abstract

Isotope Geochemists are dedicating substantial effort at developing isotope ratio-based tools that are now beginning to provide detailed insights into the processes that cycle elements between the compartments of the weathering zone. The first group makes use of the metal and metalloid elements of which the stable isotope ratios shift mainly when a) secondary minerals are formed; b) elements are cycled through higher plants. Distinct stable isotope ratios emerge for a given cycled element as a function of the isotope fractionation factor of the process involved and of the elements’ relative fluxes through these compartments (e.g. soil water, secondary minerals, plants, remaining primary minerals). A second tool provides absolute fluxes from the weathering zone: The ratio of the meteoric cosmogenic nuclide 10Be (T1/2 = 1.39 My) to the stable isotope 9Be combines a tracer of roughly known flux to the Earth's surface (over time scales characteristic of weathering) with one that depends on its release rate from rock by weathering. The resulting 10Be/9Be isotope ratio discloses both the Earth surfaces’ denudation rate and its weathering intensity. For both tools, a set of steady state mass balance equations describe these systems from the scale of a soil column over the hillslope scale to an entire river basin.