QUANTITATIVE STRONTIUM ISOTOPE MODELS FOR WEATHERING, PEDOGENESIS ANDBIOGEOCHEMICAL CYCLING

Isotopes of strontium (Sr) are a useful tracer for weathering, atmosph eric fluxes, cation biocycling, and pedogenesis. We present basic mode ls for application of strontium isotopes to the soil-vegetation-atmosp here system. The mathematical formulations fall into the general categ ories of: (1) steady-state models, in which isotopic ratios remain con stant over the time scale of interest; and (2) time-dependent models, in which isotope ratios change through time. In the steady-state model s, fluxes of Sr and other elements to the system are constant. Steady- state models can be used to infer short-term weathering rates from riv er and stream isotope compositions, to determine fluxes to a single- o r multiple-layer soil exchange/solution system, and to quantify nutrie nt fluxes to vegetation. Time-dependent models involve a change in iso topic ratio from some initial value to a new value over the time perio d of interest; in some cases, the change may represent a shift from on e 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-depend ent models include identifying the dominant cation sources to an evolv ing 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 pare nt material isotopic signature) and New Mexico (with an atmospheric is otopic 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 prof ile. (C) 1998 Elsevier Science B.V.