Source: Stewart et al., 2001

Strontium vs. Calcium

• Both are alkaline earth elements with +2 valence charge

• Strontium – Atomic number = 38– Ionic Radius = 1.18 angstroms

• Calcium– Atomic number = 20– Ionic Radius = 1.00 angstroms

Source: Kennedy et al., 2002

Source: Capo et al., 1998

Source: Capo et al., 1998

Strontium Isotope System

• Indicator of both age and geochemical origin– Older rocks with same initial Rb/Sr will have

higher 87Sr/86Sr than younger ones.– Rocks of a given age composed of different

minerals will show differentiation in strontium ratios

– The combination of these factors allows strontium to be used a tracer of cation source

Source: Capo et al., 1998

Strontium as a Tracer of Soil Cation Source: Weathering vs. Atmospheric Deposition

• Mass Balance – Two Member Mixing

• Must have sufficient difference between the two end members

• Must assume steady-state weathering or use more complex models to simulate changes in end members

Source: Bullen et al., 1997

Source: White et al., 1999

Source: Blum et al., 2002

Source: Kennedy et al., 2002

Figure 2: Location of proposed field sites in the San Juan Mountains in relation to the western US deserts.

Field Sites

87/86Sr 143/144Nd εNd

Surface rocks 0.707397 (0.000013) 0.512216 -8.23

Dust Events

April 1, 2005 0.720471 (0.000013) 0.512089 -10.71

April 4, 2005 0.716402 (0.000012) 0.512084 -10.81

April 8, 2005 0.714878 (0.000009) 0.512104 -10.42

May 9, 2005 0.713525 (0.000007) 0.512103 -10.44

Source: Chadwick et al., 1999

Source: Stewart et al., 2001