Guest essay by Martin Hovland, Geophysiscist and Professor Emeritus, Center for Geobiology, University of Bergen, Norway
The newly released satellite OCO2-data indicates that there is CO2 input in tectonically active oceanic areas. This becomes evident by pairing seafloor topography and tectonic data with the recently published OCO2-results. Thus, in the released OCO2 dataset, showing the average atmospheric concentration of CO2 over a period of about 6 weeks late in 2014, there are three curious, relatively week, but distinct CO2-hotspots over oceanic regions:
1) The Timor CO2-hotspot
2) the Fiji CO2-hotspot, and
3) the Emperor CO2-hotspot, see Fig. 1.
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.
Because of the highly tectonized seafloor also underlying the Emperor CO2-hotspot, it is speculated that there is excess CO2 given off by the ocean also in this area. The effect of excess heat and gases seeping out of the seafloor hotspots, was illustrated already in 1988, in Fig. 10.4, by Hovland and Judd (in the book: “Seabed Pockmarks and Seepages: Impact on Geology, Biology and the Marine Environment”). A modified version of this conceptual idea is provided in Fig. 5.