Quantitative strontium isotope models for weathering, pedogenesis and biogeochemical cycling

Abstract

Isotopes of strontium (Sr) are a useful tracer for weathering, atmospheric fluxes, cation biocycling, and pedogenesis. We present basic models for application of strontium isotopes to the soil–vegetation–atmosphere system. The mathematical formulations fall into the general categories of: (1) steady-state models, in which isotopic ratios remain constant over the time scale of interest; and (2) time-dependent models, in which isotope ratios change through time. In the steady-state models, fluxes of Sr and other elements to the system are constant. Steady-state models can be used to infer short-term weathering rates from river and stream isotope compositions, to determine fluxes to a single- or multiple-layer soil exchange/solution system, and to quantify nutrient fluxes to vegetation. Time-dependent models involve a change in isotopic ratio from some initial value to a new value over the time period of interest; in some cases, the change may represent a shift from one steady-state situation to a new one after a shift in one or more of the fluxes feeding the system. Examples of applications of time-dependent models include identifying the dominant cation sources to an evolving soil exchange/solution system, and calculating weathering rates by measuring the isotopic compositions of primary soil minerals. We use time-dependent models to explain differences in the isotopic ratios of labile and carbonate Sr from arid sites in Hawaii (with a basalt parent material isotopic signature) and New Mexico (with an atmospheric isotopic signature). These models suggest that the difference is due to a combination of low atmospheric strontium fluxes and high weathering rates in the Hawaiian profile compared to the New Mexico calcrete profile.