New investigation of Longqi hydrothermal field by submersible Jiaolong
Chunhui Tao, Jin Liang
Key Laboratory of Submarine Geosciences, the Second Institute of Oceanography, SOA
No.36, Baochubei Rd., Hangzhou, 310012, China
Xiang Xiao
Shanghai Jiao Tong University, China
Hongjun Yu
National Deep Sea Center, China
Xiangyang Li
China Ocean Mineral Resources R & D Association, China
Chinese Cruise 35th Science Party, Leg II
Since the first discovery of Longqi Field（49°39′E, 37°47′S）in 2007 by Chinese COMRA 19th Cruise, the first active hydrothermal vent field at the ultraslow spreading Southwest Indian Ridge has attracted widely interest. Over the course of Leg II of Chinese COMRA Cruise 35th, via 6 dives (89, 90 and 93 to 96), we surveyed Longqi Field by submersible Jiaolong for the first time.
A large number of vents with different temperature were newly found in pre-defined 'M zone' (Tao et al., 2012), and the venting area of 'S zone' was expanded in this cruise. The distribution extents of hydrothermal activity and sulfide in both 'M' and 'S' zones were approximately 200m×200m, respectively.
Hydrothermal fluids from different vents vary in physical and geochemical characteristics. The maximum temperature up to 379.3℃ was obtained at vent site DFF6 (49.6484°E, 37.7801°S) of 'M zone', with salinity 40‰, pH 3.42 (measured at 26.5℃ on board) and DO 14.3mg/L. The fluid with low temperature yields salinity 32‰, pH 6.25 (measured at 21.9℃ on board). The fluid from vent site DFF3(49.6494°E，37.7828°S), measured 352℃, yields the highest salinity up to 45‰, pH 3.21 (measured at 21.9℃ on board) and DO 12.9mg/L. Results of continuous temperature observation on DFF6 high-temperature vent and DFF11 low-temperature diffuse flow area, show that there are semidiurnal tide cycles. Temperature variation range of diffuse flow area was greater than that of high-temperature vent. Paroxysmal of venting was also observed in diffuse flow area.
More than 20 species of vent-associated fauna, including Bathymodiolus mussel, gastropods, stalked barnacles, Alvinocaridid shrimps, Kiwa, polychaeta, sea anemones, holothurians and fishes, were observed and mostly sampled at 7 vent sites (high temperature vents DFF1, DFF3, DFF6 and DFF12; low temperature areas DFF2, DFF5 and DFF11). Low-temperature vent sites are dominated by black (scaly-foot) and brown gastropods, with patches of stalked barnacles and mussels on less active areas. Alvinocaridid shrimps are visually most common in high-temperature vent sites, with polynoid polychaetes and sea anemones occasionally observed.
According to our survey, It is indicated that the scale of Longqi vent field is one of the largest hydrothermal vent fields on the mid-ocean ridges. The large amount, long periodical, extensive eruption vents discovered in this field, herald great opportunities for geologists, geochemists and biologists.
Acknowledgements
We thank the captain and crew of the Leg II and III of Chinese Cruise 35th on R/V XiangYangHong 9, and the submersible Jiaolong group, who contributed to the success of these Legs. Thanks also go to Dr. Kang Ding for joining this Cruise. This research was funded by the China Ocean Mineral R&D Association.
Keywords: hydrothermal field; high and low temperature vents; the Southwest Indian Ridge
