Abstract

Dissolved concentrations of CH₄, H₂ and Mn have been measured in a hydrothermal plume above the shallow water (c. 250m) Steinahóll vent site at 63°N on the Reykjanes Ridge. Samples were taken in October 1990 and June 1993. In 1990 dissolved CH₄ levels of up to 108 nmol/l and dissolved H₂ levels of up to 42 nmol/l were recorded. By 1993 these had declined to a maximum of 18nmol/l for CH₄ and 30nmol/l for H₂. Re-occupation of the site in 1993 included a higher density of sampling, suggesting that the fall in measured levels of these dissolved gases was not a sampling artefact. The high dissolved CH₄ and H₂ concentrations reported for 1990 were measured immediately after an episode of earthquake activity which may have resulted in the injection of magmatic gases into the local hydrothermal system. By 1993 the dissolved CH₄ levels had returned to values more typical of submarine hydrothermal systems elsewhere on the Mid-Atlantic Ridge. Dissolved H₂ levels remained high at Steinahóll, relative to other mid-ocean ridge sites, and it is believed that this is due to a combination of inorganic and biogenic processes. The gas chemistry of the Steinahóll hydrothermal site bears some similarities to that of the geothermal waters on the nearby Reykjanes Peninsula, Iceland, whereas the high levels of dissolved Mn in the plume are more typical of a submarine hydrothermal system. It appears, therefore, that the Steinahóll hydrothermal site may represent a transition between submarine and subaerial geothermal systems in its gas chemistry.