Sulfur cycle in the Valles Caldera volcanic complex, New Mexico - Letter 2: Aqueous sulfate budget and implications for hydrological transport on early Mars

Abstract

Abstract There is a large disagreement about what sulfur sources participated in formation and transport of aqueous sulfate (SO 4 2 − ) on early Mars. Previous geochemical models mainly focused on volcanic degassing and atmospheric deposition via acid rainfall. Generally, it is unclear whether chemical weathering of sulfur mineralization present in volcanic bedrock might have been an important source of aqueous SO 4 2 − on Mars. Therefore, we selected the watershed of Valles Caldera to better constrain the quantities (fluxes) of SO 4 2 − from chemical weathering compared to direct oxidation of hydrothermal sulfur-rich (H2S) gases and atmospheric deposition. Our results show that in the presence of various oxidants (e.g., oxygen, metals) the SO 4 2 − contributions from these sources significantly vary between dry and wet seasons. In the dry seasons of Nov 2012 and May 2013, the aqueous SO 4 2 − flux from bedrock weathering was the highest (5992 to 8142 kg/month; ∼81% of total flux) with minor SO 4 2 − contributions from atmospheric deposition (1110 to 1508 kg/month; ∼15%) and hydrothermal H2S gas emission (314 to 404 kg/month; ∼4%). Conversely, in the wet season (Mar 2014) the bedrock weathering showed SO 4 2 − fluxes of 4070 kg/month (∼34%) and gas emission of 6088 kg/month (51%) compared to smaller inputs from atmospheric deposition (1796 kg/month; ∼15%). The volume of water interacting with a particular sulfur reservoir appears to define the amount (flux) of SO 4 2 − transported in the aqueous system of active volcanoes. While H2S emission more effectively increased SO 4 2 − fluxes in surface water under wet conditions, the SO 4 2 − fluxes from chemical weathering only slightly varied between investigated wet and dry seasons. We demonstrate that in addition to hydrothermal sulfur emissions the chemical weathering should be included in geochemical source-to-sink models of sulfur cycles on Mars, particularly when assuming prevailing dry surface conditions in the past.