Using the Smithsonian Volcano database, it is seen that these CO2-hotspots occur above seafloor features which are suspected to issue CO2, CH4 and occasionally large amounts of heat (especially for FH and EH). Here, it can be seen that the TH occurs over a deep-water accretionary subduction wedge. This is a collision zone, where huge amounts of oceanic sediments pile up before they sink into and are swallowed up beneath the island masses to the north (Fig. 2). In such settings, it is well-known that continuous seepage of methane occurs out of the seafloor. Therefore, it is here speculated that the underwater and aerial oxidation of this excess methane gas provides the regional CO2-anomaly detected by OCO2.
The seafloor beneath the FH is also highly tectonized (Fig. 3), but in a completely different fashion to that of the TH. At Fiji, there are both colliding plates and rifting zones. The whole region is highly contorted and there are lots of seepage, both hot vents and cold, methane-dominated vents. Transmittal of methane and CO2 to the atmosphere is likely also here.