Hydrothermal methane plumes in the Mariana back-arc spreading centre

Abstract

Submarine hydrothermal vents were first unequivocally detected by measurement of ³He/⁴He ratio anomalies coupled with deviations from ambient T–S relationships on the Galapagos spreading centre^1,2. Since then the existence of hydrothermal vents along the entire length of the East Pacific Rise has been demonstrated by shipboard observations of ³He, methane, and manganese in the effluent plumes^3–5. We have extended our studies of vent fields at mid-ocean-ridge spreading centres to search for effluent plumes in back-arc basins, which are also thought to be active regions of seafloor spreading where new oceanic crust is formed⁶, and thus where rifting and Assuring along the axis of extension should provide similar opportunities for the downward penetration of sea water and the development of active hydrothermal systems. We have detected large plumes of methane-enriched water in the Mariana Trough back-arc basin⁷ and also in the summit crater of Loihi Seamount⁷ at the present site of the Hawaiian hotspot, thus establishing the widespread existence of ‘off-ridge’ hydro-thermal vents in the ocean. However, although the Loihi vents produce plumes enriched in both ³He and methane⁷, we find that, surprisingly, the Mariana vents are enriched in methane without a corresponding enrichment in ³He. This is the first time such a ‘decoupling’ of these two effluent plume tracers has been observed.