Improved quantification of the global emissions of deep carbon through volcanism is one key objective of the Deep Carbon Observatory’s DECADE (Deep Carbon Degassing) initiative supported by the Reservoirs and Fluxes Community. One quantification approach measures deep carbon degassing in very active but as yet undocumented remote volcanic regions, the Vanuatu island arc in the southwest Pacific, for example. In a recent issue of the Journal of Volcanology and Geothermal Research , an international team led by DECADE board of directors members Patrick Allard (IPGP, France), Alessandro Aiuppa (Palermo University, Italy) and Hiroshi Shinohara (JSG, Japan) demonstrated that the Ambrym basaltic volcano in central Vanuatu arc ranks among the top-three known persistent emitters of volcanic gas at the global scale.
The Ambrym volcano consists of a huge (500 km3) submarine shield-pile rising 1,800 m above the ocean floor, topped by a 12km wide and young (two thousand years) Hawaiian-type caldera. Two major cones in this caldera, Benbow and Marum, host recurrent basaltic lava lakes, degassing from which sustains voluminous gas plumes (pictured above left). Researchers recently detected a very high SO2 emission rate from Ambrym (Bani et al., 2009, 2012). Despite challenging accessibility (pictured right), the international team determined for the first time the overall degassing budget of major gases, trace elements, and short-lived radioactive daughters of 222radon from Ambrym, combining both ground-based and airborne field measurements and lab analyses with various techniques. Their results reveal prodigious emission rates of magma-derived volatiles from this basaltic arc volcano, ranging from 5 to 10% of current global volcanic flux estimates of H2O, CO2, HCl, Cu, Cd, Au, Cs, Tl, 210Po and 210Pb, between 10% and 17% for SO2, HF, HBr, Hg, and over 30% for Ag, Se and Sn (shown below). CO2 flux, in particular, averages approximately 10 Gg per day. Such emission rates characterize Ambrym as one of the three most powerful gas emitters amongst persistently degassing subaerial volcanoes on Earth, together with Nyiragongo (East-African rift) and Etna (Sicily).
Simultaneous melt inclusion studies show that Ambrym basalt is not anomalously enriched in volatiles. Instead, voluminous gas released from the volcano results from a very high basalt supply/degassing rate (25 m3 s-1), from a 0.5 km3 large and approximately 4km deep magma reservoir that may be entirely renewed in about 240 days. Hence, current global estimates of volcanic carbon and other volatile emissions will now have to account for the previously unknown volatile contribution of Ambrym. New investigations of analogous volcanoes in other poorly documented regions of the southwest Pacific might reveal larger volcanic carbon emission rates than currently expected.
This study was supported by the French National Agency for Research (ANR contracts 06-CATT-002, ‘Arc Vanuatu’, and 06-CATT-012-01, ‘VOLGASPEC’), Palermo University (European Research Council, GA305377, Pi: A.A.), INGV-Pisa and the Geological Survey of Japan. This work was realized in collaboration with our Vanuatu scientific partner GEOHAZARD (Port Vila).
Images: Upper left: Two volcanoes in one: volcanic plumes from Benbow (right) and Marum (left), active cones of Ambrym (source: NASA, 2013/08/09). Middle right: Eastern inner slopes of Ambrym’s Benbow cone (1160 m asl, ~300 m deep) and location of its active lava lake. (Patrick Allard). Lower left: Ambrym volatile fluxes as percentages of global fluxes from subaerial volcanism (GVF; Patrick Allard).