ASSESSING CATION DEPOSITION AND REMOBILIZATION IN ARID SOILS OF THE ATACAMA DESERT, CHILE

The persistence of hyper-arid conditions in the Atacama Desert, Chile over the past ~2 Ma has resulted in the accumulation of unique, salt-rich soils.  Surface studies have clearly shown that the chemical and isotopic composition of these soils varies geographically with distance from their sources.  In order to evaluate cation deposition and mobility in highly arid environments, strontium isotope ratios were measured at two sites in arid and hyper-arid climatic zones.  For each site, we examined isotopic variations of Sr associated with both sulfate and carbonate with depth in the profile.  Samples were first leached thoroughly with ultrapure water to completely remove sulfate salts (primarily gypsum), followed by a leaching with dilute acetic acid to extract carbonate salts.  

In the arid Copiapo profile (~20 mm mean annual precipitation), a trend of decreasing ⁸⁷Sr/⁸⁶Sr in the carbonate fraction with depth is clearly evident, with values ranging from ~0.7075 at the surface to 0.7066 at 200 cm depth.  In contrast, the sulfate leach shows a smaller range of values and no consistent trend with depth.  In the hyper-arid Yungay profile (<2 mm rain/yr) about 500 km to the north, the Sr isotope composition of the sulfate fraction decreases monotonically from 0.7075 near the surface to 0.7071 at 135 cm.  There is a hint of a similar trend in two carbonate fraction samples from this profile.  In both profiles, the shifts in Sr isotope ratio fall within the range of values expected for atmospheric deposition with an ultimate Andean source (e.g., salts from groundwater-fed salars).  The systematic shifts observed with depth in both soils could result from changes in the sources of salts to the profile, or from slight increases in the amount of seawater-derived Sr (in the form of sea salt particles, either dry deposited or dissolved in precipitation or coastal fog) over time.  Lower values at Copiapo may reflect a contribution from weathering of local granites (total ⁸⁷Sr/⁸⁶Sr ~ 0.7035), which has not occurred at the hyperarid site.  The lack of a clear trend in the Copiapo sulfate fraction indicates post-depositional redistribution of gypsum during infrequent precipitation events.

 

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