Journal of Conference Abstracts

Near Seafloor Distribution of Hydrothermal Tracers (Sulfide, Methane, Manganese) in Lucky Strike and Menez Gwen Segments

Joël Radford Knoery Dept. DRO/GM, IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

radford@ifremer.fr

J.-L. Charlou Dept. DRO/GM, IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

M. Aballéa Dept. DRO/GM, IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

J. P. Donval Dept. DRO/GM, IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

Y. Fouquet Dept. DRO/GM, IFREMER, Centre de Brest, BP 70, 29280 Plouzané Cédex, France

Introduction

One of the goals of the DIVA-1 cruise was to examine the near seafloor distribution of hydrothermal tracers near Lucky Strike vent field. By analyzing the samples on board ship, maps of hydrothermal tracer anomalies can be prepared in near real-time. The availability of such maps after each dive may enhance the effectiveness of submersible hydrothermal exploration, and thus be useful in locating unknown vent fields. This was a second goal of DIVA-1.

Methods

To meet the above objectives, a miniature rosette (pylon and 16 1.2-l bottles) was secured in front of Nautile's sampling basket. Bottle closure time, depth and water column parameters were recorded using a CTD interfaced to this water sampling system. As soon as possible after each dive, subsamples were drawn and analyzed on board ship (methane, sulfide) or stored for shore-based analysis (total dissolvable manganese (TDM)). Methane samples (250 ml) were analyzed using He-stripping/gas chromatography. Water samples for TDM determinations (60 ml) were acidified using quartz-distilled HCl and analyzed ca. 12 months later using flow injection analysis and colorimetric detection. Sulfide samples (10 ml) were analyzed using an original method based on flow-injection analysis (fluorimetric detection) with a precision better than 5% and a 0.1 nmol/l detection limit.

Results

Part of one dive at Lucky Strike (37°17'N) was devoted to sample collection for a near seafloor (<5 m off the bottom) systematic survey of the vent field area. The Lucky Strike vent field is characterized by the emplacement of focused vents on the outside perimeter of the circular lava lake. Data from the survey showed that near seafloor concentrations of methane and TDM are higher near known vents than above the lava lake. A combination of processes can explain these data; emplacement of diffuse vents preferentially with focused vents, and lateral dispersion of the buoyant plume. Part of another dive examined buoyant plume geochemistry above Tour Eiffel. The buoyant plume data indicate that TDM appears to behave conservatively, while most of the vented sulfide is removed.

Near seafloor water samples were collected in the axial graben of the Menez Gwen ("white hill") segment (37°50'N) and enabled the discovery of the Menez Gwen vent field during the second of five exploration dives devoted to this segment. Discrete water samples and continuous temperature recordings show important tracer concentration anomalies and localized increases in water temperature (ca. 0.5°C) in the axial graben. This suggests that additional venting is occurring on the western wall of the axial graben, in the area called Menez Torret ("split mountain").

Conclusions

The combination of instrumental and analytical tools used during DIVA-1 was helpful in the description of plume dispersion processes on the scale of a vent field. Furthermore, this approach was shown to be very effective when used as a tool for hydrothermal exploration at the 1-10 km scale. DIVA-1 cruise results showed that sulfide can be used as a water column tracer of hydrothermal processes.