Gas Chemistry of Submarine Hydrothermal Venting at Maug Caldera, Mariana Arc

Abstract:

Maug volcano consists of 3 islands that define the perimeter of a submerged caldera that was formed by an explosive eruption. The caldera reaches a depth of ~225 meters, and has a prominent central cone or pinnacle that ascends within 20 meters of the sea surface. Our exploration of Maug began in 2003, when a single hydrocast in the caldera detected a strong suspended particle and helium plume reaching a maximum of δ³He = 250% at ~180 meters depth, clearly indicating hydrothermal activity within the caldera. In 2004 we returned armed with the ROPOS ROV, and two ROPOS dives discovered and sampled low temperature (~4 °C) diffuse venting associated with bacterial mats on the NE flank of the central pinnacle at 145 m depth. Samples collected with titanium gas tight bottles were badly diluted with ambient seawater but allowed an estimate of end-member ³He/⁴He of 7.3 Ra. Four vertical casts lowered into the caldera in 2004 all had a strong ³He signal (δ³He = 190%) at 150-190 meters depth.

A recent expedition in 2014 focused on the shallow (~10 m) gas venting along the caldera interior. Scuba divers were able to collect samples of the gas bubbles using evacuated SS bottles fitted with plastic funnels. The gas samples had a consistent ~170 ppm He, 8 ppmNe, 60% CO₂, 40%N₂, and 0.8% Ar, and an end-member ³He/⁴He ratio of 6.9 Ra. This ³He/⁴He ratio falls within the range for typical arc volcanoes. The rather high atmospheric component (N₂, Ar, Ne) in these samples is not contamination but appears to be derived from subsurface exchange between the ascending CO₂ bubbles and air saturated seawater. A single vertical cast in 2014 had a maximum δ³He = 55% at 140 m depth, much lower than in 2003 and 2004. This decrease is possibly due to recent flushing of the caldera by a storm event, or may reflect a decrease in the deep hydrothermal activity. This area of shallow CO₂ venting in Maug caldera is of particular interest as a natural laboratory for studying the effects of ocean acidification on corals.