Indian Ocean hydrothermal systems: Seafloor hydrothermal activities, physical and chemical characteristics of hydrothermal fluids, and vent-associated biological communities

Abstract

In the nearly 40 years since the discovery of the deep-sea hydrothermal vent site at the alápagos spreading center, more than 300 sites of high-temperature hydrothermal venting have been discovered and investigated around the world. Surprisingly, however, most of these sites are located in the Pacific and Atlantic Oceans, whereas only five hydrothermal vent sites have been discovered in the Indian Ocean. During the TAIGA project, we conducted four research cruises to investigate four of the five Indian Ocean hydrothermal vent sites (two of which were newly discovered during one of the TAIGA cruises) located along the Central Indian Ridge (CIR). The results of geological and geochemical analyses demonstrate wide variation in fluid chemistry, reflecting the diverse geological background of the CIR hydrothermal vent fields. Although the CIR is an intermediate-spreading ridge, the geological and geochemical features of the Kairei hydrothermal field appear to be similar to ultramafic rock-hosted hydrothermal fields found along the slow-spreading Mid-Atlantic Ridge (MAR). By contrast, the Dodo hydrothermal field shares similarities with the hydrothermal vent sites found along the fast-spreading East Pacific Rise. The Solitaire and Edmond hydrothermal fields are characterized by high pH and Cl levels, respectively, although the geological background underlying the unusual chemistry of their hydrothermal fluids is still uncertain. Additionally, extensive microbiological analyses of the Kairei hydrothermal vent site revealed that its microbial communities are affected by the chemical characteristics of the hydrothermal vent fluids. Macrofaunal analyses also revealed new faunal data for the Indian Ocean hydrothermal vents, including novel genera and families that are potentially indigenous to the Indian Ocean hydrothermal systems. In particular, the discovery and characterization of a new morphotype of “scaly-foot” gastropod raises the question of the mechanism and physiological role of iron sulfide mineralization. The results of our investigations extended knowledge of the Indian Ocean hydrothermal systems including geochemical variations of hydrothermal fluids, their relationships to the geological background, the biodiversity and biogeography of the hydrothermal vent-associated microbial and faunal communities. This, in turn, will provide important insight into the relationships among geological backgrounds, hydrothermal processes, and biological activities, not only at the Indian Ocean hydrothermal vents but also in global midocean ridge hydrothermal systems.