The Strytan Hydrothermal Field (SHF) is located in Eyjafjord, northern Iceland, exhibits off-axis, alkaline (pH ~ 10), hot (up to 78°C), submarine hydrothermal venting, building a 55 meter high saponite tower. In the context of origin of life and early Earth analogs, the SHF has received a considerable amount of attention. We’ve investigated the geochemistry of hydrothermal fluids from three sites within Eyjafjördur: Big Strytan, Arnarnestrytan, and Hrisey, with the intention of understanding better the subsurface geochemical processes taking place at the site which lead to the observed fluid compositions. Overall, the geochemistry at each site was similar with a few exceptions for the Hrisey site. The pH values were all alkaline, ranging from 9.16 to 10.22, making this one of the few, known submarine and alkaline hydrothermal springs. Temperatures ranged from 66 to 78 °C, whereas salinity at each site was low, ranging from 0.5 to 14.5 % seawater.

Two main processes control the composition of hydrothermal fluids: 1) mixing with seawater, and 2) water-rock interactions. Sodium, Mg, K, Ca (except for Hrisey), Cl, Br, B, and Sr are enriched and linearly correlate with one another on a mixing line with seawater values as an end point. Subtracting the seawater component, however, indicates significantly elevated concentrations of many major and trace elements. End-member ‘reservoir’ compositions are 2.4 mM Na, 3 to 27 uM K, 40 to 120 uM Ca (except for H1, which was much higher in Ca), 10 to 25 uM B, and overall high concentrations of trace elements. These enrichment/depletion patterns are hypothesized to occur as a result of water-rock interactions in a closed system: recharge of meteoric water occurs in the mountains to the south of Eyjafjördur, followed by low temperature alteration of plagioclase, pyroxene and olivine in basalt, and precipitation of calcite which removes most Ca2+ and bicarbonate species. This explains the observed high pH, variable Ca concentrations, and low DIC.

Methane (CH4), H2, and CO concentrations were all elevated relative to normal seawater. Furthermore, a range of d13C-CH4 were measured, suggesting not only potentially methanogenesis, but also the possibility that some of the CH4 has an abiotic origin (values heavier than -20‰). Weathering of pyroxene contained within the basalt may be producing H2, which subsequently reacts with CO or CO2 to form CH4. The abiotic production of H2 and CH4 in the SHF is intriguing, as it broadens the range of potential life, and origin of life, environments significantly. Thus far, coastal environments with shallow-sea hydrothermal venting have not been considered.