TECTONIC SETTING DRIVES GEOCHEMICAL COMPOSITION IN TERRESTRIAL HOT SPRINGS

Hot springs are the surface expression of underground hydrothermal systems. Their geochemical composition is a direct consequence of water-rock reaction as hydrothermal fluid circulates in the crust. Fluid composition may be further altered by near surface groundwater mixing, underground phase separation, and surface input. It follows that different tectonic settings lead to variability in element abundance and speciation, which in turn influences element availability and the habitability of springs. To investigate the range of attainable compositions, we analyzed more than two hundred hot springs for approximately one hundred geochemical parameters from three tectonic settings; continental hot spot rhyolitic systems in Yellowstone National Park (YNP), divergent basaltic systems in Iceland, and subduction zone andesitic systems in Ecuador. The three rock types represented are broadly differentiated by silica and cation content. Hot spring major and trace elements can vary by more than five orders of magnitude at a given pH range, even within one tectonic setting. The YNP springs contain elevated silica, Zn, and Al and the greatest geochemical variability relative to the other settings. Icelandic springs are typically dilute compared to the other settings as indicated by low conductivity and total major element abundances. These springs are below saturation levels for many of the bedrock minerals, suggesting short residence times and incomplete reaction progress for water rock interactions. Despite the comparatively dilute composition, some solutes show the most variability and are occasionally highly abundant, most notably Mn, Fe, Zn, and H₂. The Ecuadorian springs have unexpected compositions based on host rock composition. Nearly all contain abundant inorganic carbon, leading to bicarbonate buffered systems centered around pH 6.3 ± 0.5. Additionally, elevated Na, K, and Cl relative to host rock content possibly resulted from seawater entrainment during subduction. Speciation in this setting leads to elevated As, Fe, and Mn concentrations that could help to fuel microbial metabolism in these springs. Taken as a whole, these data constrain hot spring compositions globally and provide a framework to better understand factors that influenced the diversification of microbial life in each setting.