Hydrothermal amorphous silica, barite and orpiment from the crater area of seamount (SM-13) off Nicobar island, Andaman sea: Indications for the development of a new hydrothermal field

Abstract

The Andaman sea characterized by trench-arc-backarc elements (Andaman-Nicobar island-arc, the Andaman back-arc spreading center, the seamount complex, and the back-arc basin) is a complex active basin and is known for hydrothermal activity in some of the seamounts (subaerial and submarine) of the volcanic arc. In the present study, 13 submarine seamounts (SM1- SM13) in the southern Andaman volcanic arc, extending from 6°47′N: 94°43′E to 7° 56′ N: 94° 2.47′E, were surveyed onboard RV Sindhu Sadhana (November 2021) to find the evidences for the hydrothermal activity. Out of the 13 seamounts, at one crater seamount (SM-13) we have observed evidence for hydrothermal activity characterized by plumes rich in dissolved gases (mainly CO2) in the water column, live chemosymbiotic organisms (Bathymodiolus species), and neoforming minerals such as amorphous silica, barite and orpiment. We observed two plumes rich in dissolved gases (mainly CO2), called GP1 and GP2, in the water column at two different water depths i.e. 400 and 380 mbsl respectively. Geochemical analysis of the water samples shows enhanced TCO2 concentrations within the plumes rich in dissolved gases (mainly CO2) observed in sub-bottom profiler data, however, the dissolved methane concentrations do not show any enrichment when compared with ambient seawater concentrations. Apart from the enhancement in the TCO2 concentrations, pH and total alkalinity data also show fluctuations within the same depth zone which may be attributed to the hydrothermal plume exhausting from the crater seamount. The occurrence of live chemosymbiotic organism Bathymodiolus species indicate an active and continuous supply of H2S. The precipitation of neoforming hydrothermal minerals such as amorphous silica, tabular barite, and orpiment on the surface of the rock samples further supports the hydrothermal activity. Previous studies carried out in 2007 have reported hydrothermal ferromanganese oxides precipitated from low-temperature hydrothermal fluids (Surya prakash et al., 2012) and inferred SM-13 as a dormant submarine volcano (Kamesh Raju et al., 2012). However, after 14 years, we observe volatile emissions to the water column, hydrothermal mineral precipitation and live chemosymbiotic organisms from the same seamount. These observations of the present study infer development of a new hydrothermal field in the least explored Andaman Sea. This study adds up a new active site to the global hydrothermal inventory which may allow better estimation of global hydrothermal flux and its role in oceanic elemental cycle.