Tracking the Provenance of Soil Nutrients in Arid to Hyperarid Soils of the Atacama Desert, Chile, Using Strontium Isotopes

The Atacama Desert, Chile, provides an ideal site to investigate the flux and redistribution of soil nutrients in an endmember environment severely depleted in water and largely devoid of life.  Due to the low rate of precipitation and weathering, the primary source of nutrients is eolian deposition.  We present down-profile strontium isotope data from three sites ranging from hyperarid to arid (~2-20 mm annual rainfall).  Strontium can be used as a proxy for the provenance of calcium and other cation nutrients.  In the Atacama Desert, likely sources of Sr are Andean volcanics (⁸⁷Sr/⁸⁶Sr = 0.705-0.708), regional salars possibly derived from weathering of these rocks, and marine aerosols from occasional fog events (0.7092). In the wetter end of the climate spectrum, Sr from the weathering of Mesozoic granitic rocks also contributes to soluble Ca and Sr.

Samples from each of the horizons were leached with water (for soluble salts, primarily gypsum) and acetic acid (for carbonate), and selected residues were completely dissolved using HF.  All profiles show a general trend of decreasing ⁸⁷Sr/⁸⁶Sr with depth for both water and acetic acid leachates.  The profile from the Altamira site, intermediate both geographically and in annual rainfall, yields ⁸⁷Sr/⁸⁶Sr  ratios of 0.7054-0.7066, significantly lower than those of the other profiles (0.7068-0.7075).  Limited insoluble residue data from this site show that the silicate portion falls in the same isotopic range as the leachates; we hypothesize that the silicate material and salts are derived from a related source area.  The systematic shifts in ⁸⁷Sr/⁸⁶Sr exhibited by all profiles indicate changes in salt provenance, possibly correlating with climate, over time.  The true shift in input values of ⁸⁷Sr/⁸⁶Sr may be masked by mixing with incremental downward transport through the profile, as demonstrated for calcium by Ewing et al. (2008, GCA 72, 1096).