Abstract
Gas emissions from the seabed are favored by tectonically active settings and their distribution is often linked to the nearby faults. Here we use the multi-beam echo-sounder (MBES) and the multi-channel seismic (MCS) data and a sediment core to show multiple gas emissions near the fault complex out of the shelf of the Mid-Okinawa Trough. The features indicating the gas emissions include 1) a set of the conical positive reliefs at the seabed, 2) the bundle-shaped clusters of the high-backscattering intensities in the water column, and 3) the sub-circular medium-to high-backscattering patches at the level of the seabed. These features together show that the free gases can escape from the marine sediments then rise in the water column at present, while some other gases trapped in the sub-seafloor sediment might contribute to the precipitation of the authigenic carbonates in the past. The spatial relationship between the gas emissions and the faults suggests that the faulting driven by the back-arc extension should provide the permeable migration pathways for the gas emissions to operate, and thus determines where most of them could potentially occur. The area surrounding the restraining bend concentrates part of the gas emissions rather than along the fault lines, due to the lateral compression and the structural complexity. This is demonstrated by the results of the numerical model of finite element method (FEM), which shows two gas emissions are within the compressed zone of the modeled restraining step-over. This study provides new evidence of the role of the tectonic stresses in determining the sites of degassing of marine sediments.
Highlights
Gas emission in the marine environment has attracted significan research attention over the past few decades
The gas plumes are compelling evidence of the present-day emission of the gas bubbles from the seabed. Their presence indicates that the free gas bubbles can rise through the water column by hundreds of meters before breaking up
It has to be stated that the frequency of 12 kHz used in this study for data acquisition is not high enough to reveal all of the emissions of gas bubbles, for those consisting of dispersed gas bubbles
Summary
Gas emission in the marine environment has attracted significan research attention over the past few decades. As a member of the potent greenhouse gases, methane can sometimes reach the shallow seawater by forming ascending streams of gas bubbles and possibly enter the atmosphere (McGinnis et al, 2006; Westbrook et al, 2009). This fate of methane has been questioned by the study in the Arctic Ocean, in which enhanced methane concentration was detected close to the gas emissions near the seabed but not in the seawater near the sea level (Myhre et al, 2016).
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