Four new hydrothermal sites on the Southwest Indian Ridge and the Central Indian Ridge located by Chinese DY115-19-Leg 3 cruise

After discovering the first black smoker at the ultraslow spreading Southwest Indian Ridge (SWIR), Chinese R/V Dayang Yihao continued its expedition with the third leg across the Indian Ocean. The leg started in Durban, South Africa on 15 March 2007 and finished in Fremantle, Australia on 24 April 2007. This 39-day leg was funded and sponsored by China Ocean Mineral Resources R & D Association (COMRA) and led by Dr. Xiqiu Han, Second Institute of Oceanography, SOA. The objective of this leg was mainly to look for new active hydrothermal sites along the ultraslow spreading segment of the SWIR and the moderately-fast spreading segment of the Central Indian Ridge (CIR).

During this leg, geophysical mapping was performed using a Kongsberg EM120 multibeam system, Geometric G-880G magnenometer, and GSS-2 and S-133 gravitimeters. To get higher resolution maps for the investigation area, the beam angle was reduced to 60°. TV-guided grab and gravity corer were used for geological sampling. A deep-sea video camera system was used for ocean floor observation. A 24-bottle CTD was deployed for water sampling. In-situ hydrothermal plume detection was conducted using MAPRs (Miniature Autonomous Plume Recorder), Mets (Methane Sensor), and CTD sensors attached to the cable of deep-towed instruments. USBL-navigation (Ultra Short Baseline) was used whenever possible to obtain the most accurate location of the under-water instrument. Assisted by GIS, the under-water instruments and the sampling locations were monitored real-time, which greatly improved the quality of data and samples obtained.

In total four new hydrothermal sites were found, three at the SWIR, and one located at the CIR. Each site showed intense signals of turbidity, methane and temperature anomalies in the water column, and hydrothermally influenced sediments and rocks, anemones and white crabs were also occasionally observed at these sites. In addition, two ultramafic complexes were located and sampled off-axis of the ridge at SWIR; interestingly, one is located 40km away from the central rift valley.

The ultramafic rocks, basalts, sulfides and sediments collected will serve to study the hydrothermalism along the SWIR and CIR. The water column anomalies will serve to understand the distribution of plumes and constrain the hydrothermal sites discovered. However, more detailed and higher resolution survey work will be needed in future cruises to locate active venting sites.