Insights into sulfate sources and water availability in the Atacama Desert through triple oxygen, strontium, and sulfur isotopes
Résumé
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 δ18OSO4, Δ17OSO4, 87Sr/86Sr, and δ34SSO4 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 δ34SSO4 values compared with low δ34SSO4 volcanic sulfate allowing to estimate the marine sulfur contribution to the total sulfate sample. δ34SSO4 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, δ34SSO4 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 Δ17OSO4 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 Δ17OSO4 and δ34SSO4 and a SASAtacama endmember comprising moderate Δ17OSO4 and low δ34SSO4. Highest Δ17OSO4 values, interpreted to represent a pure SASAtacama 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 Δ17OSO4 values results from 1) dilution of the positive Δ17OSO4 fromSAS by marine and/or terrestrial sulfate with Δ17OSO4 ≈ 0‰, and 2) resetting of Δ17OSO4 due to biological sulfate reduction and reoxidation. Lowest Δ17OSO4 values are observed in sulfates from salars and soils from alluvial fans.In general, Δ17OSO4 andδ18OSO4 of our data show an inverse relationship reflecting not only the source contributions but also biological sulfate cycling. Thus, large Δ17OSO4 anomalies (≈1‰) that suggest a dominant contribution from SASAtacama, 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