Hydrothermal activity on the Reykjanes Ridge: the Steinahóll vent-field at 63°06′N

We have completed a systematic survey for hydrothermal activity along the Reykjanes Ridge from 57°45′N to 63°09′N. Shipboard determinations of total dissolvable Mn (TDMn) and dissolved Si, CH4 and H2 have revealed evidence for only one site of hydrothermal activity along this section of ridge crest, the Steinahóll vent-field at 63°06′N [1]. Our measurements confirm this site to be a stable hydrothermal convection cell of at least two years' duration. The site is situated in just 250–350 m of seawater and, unlike deeper-sited vent-fields (e.g. TAG, 3650 m, 26°N MAR), is notable for the formation of bubble-rich plumes which have been imaged using a high-frequency (38 kHz) echo-sounder. High dissolved gas concentrations of up to 18 nmol/l CH4 and 30 nmol/l H2 coincide with these bubble-rich plumes. The Steinahóll plume is also characterised by high total dissolvable Mn (TDMn) anomalies (up to 60 nmol/l) and dissolved Si anomalies (∼ 2 μmol/l) yielding molar TDMn/CH4 and TDMn/Si ratios of ∼ 3.5 and 0.03, respectively, directly comparable to the TAG hydrothermal plume. The absence of any indication of hydrothermal activity along the remainder of the Reykjanes Ridge is intriguing. Between 11°N and 40°N on the Mid-Atlantic Ridge, previous studies have revealed evidence for hydrothermally active sites approximately every 150 km. For the 300 km of ridge crest studied in detail here (750 km total) only one site has been found. Thus, despite its proximity to the Iceland hot-spot, the incidence of hydrothermal activity along this section of ridge crest appears to be only 20–50% of that measured elsewhere along the slow-spreading Mid-Atlantic Ridge.