SEVERAL ORDERS OF MAGNITUDE: GEOCHEMICAL VARIABILITY OF YELLOWSTONE HYDROTHERMAL ECOSYSTEMS

The surface expressions of the Yellowstone hydrothermal system include thousands of warm and hot springs that support extraordinarily diverse microbial communities. Underlying this biological diversity is geochemical diversity induced by the variable influence of many processes including infiltration of rain and melted snow, mineral dissolution and precipitation, gas injection, phase separation, fluid mixing, evaporation, and microbial metabolism, all of which can happen to various extents spatially and temporally. The consequences include hot spring pH values from < 2 to > 9, and specific conductances from hundreds to tens of thousands µS cm^-1. Accompanying these variations are total elemental concentrations ranging over several orders of magnitude throughout the periodic table. Using concentrations of chloride and sulfate as guides to fluid sources, the deeply sourced hydrothermal fluids of Yellowstone are relatively enriched in Li, Na, Rb, Cs, F, Br, Ga, As, Mo, W, Sb, and relatively depleted in Mg, Sr, Ba, Al, Pb, U, transition metals, and rare-earth elements, all of which are relatively enriched in acidic fluids at steam-influenced sites, together with ammonium. Solutes that defy easy categorization by fluid sources include Be, Ca, K, B, P, nitrate, nitrite, sulfide, O₂, H₂, CH₄ and CO. Taken together, these variations combine to yield large differences in elemental ratios. As an example, ratios of As to P vary from 1000:1 to 1:1000. Energy supplies that support chemotrophic microbial communities also vary by several orders of magnitude depending on where the water in a hot spring has been. Chemotrophs everywhere can get a bold metabolic energy return by oxidizing sulfide or ammonia with O₂. In contrast, oxidizing Fe⁺² with oxygen, while a rich energy source at low pH, is vastly diminished in ecosystems dominated by deeply sourced hydrothermal fluids, perhaps owing to the enhanced abiotic oxidation rate as pH increases. Meanwhile in strongly acidic fluids, Fe⁺² oxidation with sulfate to form iron oxide and sulfide minerals is incapable of supplying energy despite the elevated concentrations of dissolved iron and sulfate. Ultimately, geologic processes have geochemical consequences that enable microbial responses.