The Benthic Boundary Layer: Radioactivity and oceanographic data from the GEOSTAR-2 observatory

Full text: GEOSTAR-2 (GEophysical and Oceanographic STation for Abyssal Research) is the first European deep-sea observatory for geophysical and environmental monitoring at seabed becoming operative in 2000. It was deployed in September 2000 from the Italian R/V Urania, in the southern Tyrrhenian Sea, between the Sicilian coast and the island of Ustica, at a depth of about 1900 m. After 206 days, in April 2001, the observatory was recovered. More than 4100 hours of data were recorded continuously. This mission represented the longest lasting experiment using a complex module, with an intelligent unit, deployed at great depth. The sensors used for this mission (two magnetometers, a gravity meter, a hydrophone, a Doppler currentmeter, a single-point currentmeter, an automatic water sampler for laboratory geochemical analysis, a CTD and a transmissometer) were continuously controlled and managed by a data acquisition and control system able to transmit the data via surface buoy and radio or satellite link to on-shore operators. The seawater samples have been analysed for the following parameters: - gas concentration: CO2, CH4, He, Ne; - helium isotopic composition: R/Ra (3He/4He sample / 3He/4He atm ). CO2 and CH4 were analyzed by gas chromatography with micro-TCD. Helium analysis was carried out with a Perkin Elmer 8500 gas-chromatograph (Flame Ionization Detector and Hot Wire Detector; 5ppmv as detection limit; analytical errors of ± 5%). The 3He/4He isotopic ratio, was determined by a static mass spectrometer (VG5400TFT, VG Isotopes; typical uncertainties are about ±1% for 3He/4He ratios in the range of atmospheric values; below ∫ 0.1% for high-3He samples and below ±3% for low-3He (radiogenic) samples). The environmental radioactivity analyses of seawater samples, obtained by the automatic water sampler, has been performed in the framework of ERMES research project (Environmental Radioactivity Monitoring for Earth Sciences) to trace environmental changes in the Benthic Boundary Layer (BBL) seawater. The analysis of radionuclides was carried out by means of gamma spectrometry with coaxial high purity Germanium (HPGe) detectors having volumes ranging from 200 to 500 cm3 and a total background rate in the energy range [(60/2700) keV] varying from (221±2) to (980±10) counts/days depending on the detector. Each seawater sample has been measured for about ten days using a polystyrene box of 70 mm diameter and 30 mm height. Oceanographic data evidenced ocean-lithosphere interactions in the 1900 m deep BBL, distinguishing two water masses with different origin and, possibly, benthic residence time. Environmental radioactivity analyses shown a BBL characterised by a colder Western Water (WW), which is episodically displaced by the cascading of the warmer Eastern Overflow Water (EOW). (author)