Insights into sulfate sources and water availability in the Atacama Desert through triple oxygen, strontium, and sulfur isotopes

Calcium sulfates are the dominating salts but the respective sulfate sources are debated. In order to quantify the relative contribution and spatial distribution of sulfate sources and to identify biological sulfate recycling processes, we analyzed δ¹⁸O_SO4, Δ¹⁷O_SO4, ⁸⁷Sr/⁸⁶Sr, and δ³⁴S_SO4 of sulfate from Atacama Desert soils (Chile). Surface samples were taken along four W-E transects from the Pacific coast to the Pre-Andean Cordillera between 19.5°S and 25°S. Additionally, lacustrine gypsum and sulfate extracted from groundwater feeding the Salar de Llamará and sodium sulfates from the Salar del Huasco were analyzed.

Sulfur from the ocean comprise high δ³⁴S_SO4 values compared with low δ³⁴S_SO4 volcanic sulfate allowing to estimate the marine sulfur contribution to the total sulfate sample. δ³⁴S_SO4 decreases with distance from the coast principally confirming previously published results [1]. Because Sr substitutes for Ca in Ca-Sulfates, ⁸⁷Sr/⁸⁶Sr follows similar systematics, at least for samples taken within the coastal fog zone (<1200 m).  However, δ³⁴S_SO4 and ⁸⁷Sr/⁸⁶Sr of samples taken above 1200 m are decoupled indicating sulfate dissolution and re-precipitation or deposition of supra-regional Ca-rich aerosols with high ⁸⁷Sr/⁸⁶Sr values.

Positive ∆¹⁷O_SO4 values observed in all analyzed samples (0.1‰ to 1.1‰) suggest a significant contribution from secondary atmospheric sulfate (SAS) to Atacama Desert soils. Distinct mass-independent ¹⁷O anomalies of SAS originate from atmospheric oxidation of reduced sulfur species from volcanic or anthropogenic emissions, or biogenic sulfur gases such as dimethyl sulfide (DMS) by O₃ or H₂O₂. Within our dataset we can distinguish between a SAS_(DMS) endmember, comprising high ∆¹⁷O_SO4 and δ³⁴S_SO4 and a SAS_Atacama endmember comprising moderate Δ¹⁷O_SO4 and low δ³⁴S_SO4. Highest Δ¹⁷O_SO4 values, interpreted to represent a pure SAS_Atacama endmember, are observed in samples from the Coastal Cordillera of the southernmost transect which is generally higher than the present maximum level of fog advection (1200 m). Lowering of Δ¹⁷O_SO4 values results from 1) dilution of the positive Δ¹⁷O_SO4 fromSAS by marine and/or terrestrial sulfate with Δ¹⁷O_SO4 ≈ 0‰, and 2) resetting of Δ¹⁷O_SO4 due to biological sulfate reduction and reoxidation. Lowest Δ¹⁷O_SO4 values are observed in sulfates from salars and soils from alluvial fans.

In general, Δ¹⁷O_SO4 andδ¹⁸O_SO4 of our data show an inverse relationship reflecting not only the source contributions but also biological sulfate cycling. Thus, large Δ¹⁷O_SO4 anomalies (≈1‰) that suggest a dominant contribution from SAS_Atacama, also indicate the relative absence of biologically processed sulfate and thus, low water availability.

[1] Rech et al. (2003), Geochim. Cosmochim. Acta 67, 575-586