Insights into sulfate sources and water availability in the Atacama Desert through triple oxygen, strontium, and sulfur isotopes
Authors: Klipsch Swea , Herwartz Daniel , Voigt Claudia , Münker Carsten , Chong Guillermo , Böttcher Michael Ernst , Staubwasser Michael 
Published: March 2022

&lt;p&gt;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 &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt;, &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt;, &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr, and &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;SO4 &lt;/sub&gt;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&amp;#176;S and 25&amp;#176;S. Additionally, lacustrine gypsum and sulfate extracted from groundwater feeding the Salar de Llamar&amp;#225; and sodium sulfates from the Salar del Huasco were analyzed.&lt;/p&gt;&lt;p&gt;Sulfur from the ocean comprise high &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;SO4&lt;/sub&gt; values compared with low &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;SO4&lt;/sub&gt; volcanic sulfate allowing to estimate the marine sulfur contribution to the total sulfate sample. &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;SO4&lt;/sub&gt; decreases with distance from the coast principally confirming previously published results [1]. Because Sr substitutes for Ca in Ca-Sulfates, &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr follows similar systematics, at least for samples taken within the coastal fog zone (&lt;1200 m). &amp;#160;However, &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;SO4&lt;/sub&gt; and &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr of samples taken above 1200 m are decoupled indicating sulfate dissolution and re-precipitation or deposition of supra-regional Ca-rich aerosols with high &lt;sup&gt;87&lt;/sup&gt;Sr/&lt;sup&gt;86&lt;/sup&gt;Sr values.&lt;/p&gt;&lt;p&gt;Positive &amp;#8710;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt; values observed in all analyzed samples (0.1&amp;#8240; to 1.1&amp;#8240;) suggest a significant contribution from secondary atmospheric sulfate (SAS) to Atacama Desert soils. Distinct mass-independent &lt;sup&gt;17&lt;/sup&gt;O 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 O&lt;sub&gt;3&lt;/sub&gt; or H&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;2&lt;/sub&gt;. Within our dataset we can distinguish between a SAS&lt;sub&gt;(DMS)&lt;/sub&gt; endmember, comprising high &amp;#8710;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt; and &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;SO4&lt;/sub&gt; and a SAS&lt;sub&gt;Atacama&lt;/sub&gt; endmember comprising moderate &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt; and low &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;SO4&lt;/sub&gt;. Highest &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt; values, interpreted to represent a pure SAS&lt;sub&gt;Atacama&lt;/sub&gt; 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 &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt; values results from 1) dilution of the positive &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt; from&lt;sub&gt;&lt;/sub&gt;SAS by marine and/or terrestrial sulfate with &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt; &amp;#8776; 0&amp;#8240;, and 2) resetting of &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt; due to biological sulfate reduction and reoxidation. Lowest &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt; values are observed in sulfates from salars and soils from alluvial fans.&lt;/p&gt;&lt;p&gt;In general, &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4 &lt;/sub&gt;and&lt;sub&gt;&lt;/sub&gt;&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O&lt;sub&gt;SO4 &lt;/sub&gt;of our data show an inverse relationship reflecting not only the source contributions but also biological sulfate cycling. Thus, large &amp;#916;&lt;sup&gt;17&lt;/sup&gt;O&lt;sub&gt;SO4&lt;/sub&gt; anomalies (&amp;#8776;1&amp;#8240;) that suggest a dominant contribution from SAS&lt;sub&gt;Atacama&lt;/sub&gt;, also indicate the relative absence of biologically processed sulfate and thus, low water availability.&lt;/p&gt;&lt;p&gt;[1] Rech &lt;em&gt;et al.&lt;/em&gt; (2003), Geochim. Cosmochim. Acta &lt;strong&gt;67&lt;/strong&gt;, 575-586&lt;/p&gt;