Abstract

In March/April 2004, submersible dives with the remotely-operated vehicle ROPOS discovered an unusual CO₂-rich hydrothermal system near the summit of NW Eifuku, a submarine volcano located at 21.49° N, 144.04° E in the northern Mariana Arc. Although several sites of hydrothermal discharge were located on NW Eifuku, the most intense venting was found at 1600-m depth at the Champagne site, slightly west of the volcano summit. The Champagne site was found to be discharging two distinct fluids into the ocean: a) several small white chimneys were emitting milky 103° C gas-rich hydrothermal fluid with at least millimolar levels of H₂S and b) cold (< 4° C) droplets coated with a milky skin were rising slowly from the sediment. These droplets were later determined to consist mainly of liquid CO₂, with H₂S as a probable secondary component. The droplets were sticky, and did not tend to coalesce into larger droplets, even though they adhered to the ROV like clumps of grapes. The film coating the droplets was assumed to be CO₂ hydrate (or clathrate) which is known to form whenever liquid CO₂ contacts water under these P,T conditions. Samples of the 103° C hydrothermal fluids were collected in special gas-tight titanium sampling bottles that were able to withstand the high internal pressures created by the dissolved gases. The Champagne hydrothermal fluids contained a surprising 2.3 moles/kg of CO₂, an order of magnitude higher than any CO₂ values previously reported for submarine hydrothermal fluids. The overall gas composition was 87% CO₂, < 0.1% CH₄, < 2 ppm H₂, 0.012 mM/kg ⁴He, with the remaining 13% (322 mM/kg) assumed to be sulfur gases (H₂S, SO₂, etc.). (Additional analyses planned will confirm the speciation of this sulfur gas component). The helium had R/R_A = 7.3, typical of subduction zone systems (R = ³He/⁴He and R_A = R_air). Isotopic analysis of the CO₂ yielded δでるた ¹³C = -1.75 ‰ , much heavier than the -6.0 ‰ typical for carbon in MOR vent fluids. The C/³He ratio was ~2.2 x 10¹⁰, an order of magnitude higher than the average value of 2 x 10⁹ found in MOR vent fluids. The δでるた ¹³C and C/³He values suggest a substantial contribution to the carbon from subducted carbonates rather than mantle carbon. The Champagne site is only the second locality where liquid CO₂ has been observed venting into the deep sea (the other reported location is in the Okinawa Trough, see Sakai et al., 1990). Because of the presence of liquid CO₂ in proximity to hydrothermal organisms, the Champagne site may prove to be a valuable natural laboratory for studying the effects of high CO₂ concentrations on marine ecosystems.