HYDROTHERMAL ACTIVITY ON THE REYKJANES RIDGE - THE STEINAHOLL VENT-FIELD AT 63-DEGREES-06'N

We have completed a systematic survey for hydrothermal activity along the Reykjanes Ridge from 57-degrees-45'N to 63-degrees-09'N. Shipboard determinations of total dissolvable Mn (TDMn) and dissolved Si, CH4 a nd H-2 have revealed evidence for only one site of hydrothermal activi ty along this section of ridge crest, the Steinaholl vent-field at 63- degrees-06'N [1]. Our measurements confirm this site to be a stable hy drothermal convection cell of at least two years' duration. The site i s situated in just 250-350 m of seawater and, unlike deeper-sited vent -fields (e.g. TAG, 3650 m, 26-degrees-N MAR), is notable for the forma tion of bubble-rich plumes which have been imaged using a high-frequen cy (38 kHz) echo-sounder. High dissolved gas concentrations of up to 1 8 nmol/I CH4 and 30 nmol/l H-2 coincide with these bubble-rich plumes. The Steinaholl plume is also characterised by high total dissolvable Mn (TDMn) anomalies (up to 60 nmol/l) and dissolved Si anomalies (appr oximately mumol/l) yielding molar TDMn/CH4 and TDMn/Si ratios of appro ximately 3.5 and 0.03, respectively, directly comparable to the TAG hy drothermal plume. The absence of any indication of hydrothermal activi ty along the remainder of the Reykjanes Ridge is intriguing. Between 1 1-degrees-N and 40-degrees-N on the Mid-Atlantic Ridge, previous studi es have revealed evidence for hydrothermally active sites approximatel y 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 proximit y to the Iceland hot-spot, the incidence of hydrothermal activity alon g this section of ridge crest appears to be only 20-50% of that measur ed elsewhere along the slow-spreading Mid-Atlantic Ridge.