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We have collected two shallow gas hydrate samples at two sites having different geological settings off southwest Taiwan during the cruise MD214 in 2018. The first core site, MD18-3542, is on the South Yuan-An East Ridge at ~ 1200 m water deep, where a structural unconformity covered by fine-silt sediments appears at ~ 5.5 m below the seafloor. The second core site, MD18-3543, is close to the Good-Weather Ridge at ~ 1100 m water deep, where a gas-related pockmark structure and authigenic carbonates are present at shallow strata with fine-silt sediments near the seafloor. Sediment properties of core MD18-3542 are distinctively different above and below the layer corresponding to the unconformity. Both cores show obvious gaps or voids in the lower core halves. The core features could be linked to the dissociated methane upward migrating from deep strata. Core site settings with upwelling methane would favor the formation of shallow gas hydrates. At site MD18-3542, the shallow hydrate could be formed due to high concentration methane kept beneath the unconformity covered by fine-silt sediments. At site MD18-3543, the shallow hydrate could be formed due to an extremely high flux of upwelling methane trapped either beneath the authigenic carbonates or fine-silt sediments.

The southern Okinawa Trough has been widely considered an important area with hydrothermal activity. Several active hydrothermal fields have been reported, especially around the Yonaguni Knoll IV. In this study, we collected marine sediment cores around the Yonaguni Knoll IV by using the R/V Ocean Research 1. Core sites with and without gas disturbance were selected based on the single-beam bathymetry (Chirp) by using the onboard echo sounder system. For the sites away from gas disturbance, which are generally considered showing the background situation in the southern Okinawa Trough, variations of the magnetic susceptibility in these cores are relatively stable with values about between 20 × 10-5 and 60 × 10-5 SI. As for the sites with clear gas-features detected by the Chirp sonar, the signature with dramatically changing magnetic susceptibility is observed in the cores. In general, gasfeatures are considered linking to the hydrothermal activity in the southern Okinawa Trough. Magnetic signature with such abruptly changing magnetic susceptibility could also be considered being caused by authigenic iron sulfides associated with the hydrothermal alternation. Therefore, the rock magnetic anomaly could be suggested as an indicator revealing the hydrothermal activity in the southern Okinawa Trough.

The Okinawa Trough (OT) is widely recognized as an area with extensive volcanism and hydrothermal activity. However, finding new hydrothermal vent sites in this region remains a technical challenge due to their small sizes. A deep-towed magnetic survey can acquire short-wavelength signals, which can provide useful information on the shallow subsurface structures of hydrothermal systems. In 2017, a deep-towed magnetic survey was conducted in selected area of the southern OT. After data processing and removing the regional magnetic field, crustal magnetization map was obtained by assuming a magnetized layer with a constant thickness of 500 m whose top surface is the seafloor. In addition, onboard echo sounder signals (EK-60) suggest the presence of gas plumes emanating from the seafloor within the water column. Some of these emanating gas plumes are concentrated in several small areas that correlate well with magnetization lows in the study region. The deep-towed magnetic survey results and other geochemical/geophysical observation results suggest that the Geolin Mounds (GLM) site and site Penglai Fault Zone (PFZ) are the highly potential hydrothermal areas in the southern OT.

The westernmost Okinawa Trough back-arc basin is located to the north of the Ryukyu islands and is situated above the northward dipping Ryukyu subducted slab. In the northern continental margin of the Okinawa Trough, the continental slope between the Keelung Valley and the Mein-Hua Submarine Canyon shows a steep angle and future slope failures are expected. The question is how slope failures will proceed? A sudden deep-seated slump or landslide would probably cause local tsunami and hit northern coast of Taiwan. To understand the probable submarine landslides, we conducted multi-channel seismic reflection, sub-bottom profilers, and multi-beam bathymetry surveys off NE Taiwan. Two general trends of shallow crustal faults are observed. The NE-SW trending faults generally follow the main structural trend of the Taiwan mountain belt. These faults are products of inversion tectonics of reverse faults from the former collisional thrust faults to post-collisional normal faults. Another trend of roughly E-W faults is consistent with the current N-S extension of the southern Okinawa Trough. The fault offsets in the eastern portion of the study area are more pronounced. No obvious basal surface of sliding is found beneath the continental margin. We conclude that the movement of the submarine landslides in the Keelung Shelf off northeastern Taiwan could be in a spread type. The submarine landslides mainly occur in the continental slope area and it is more obvious in the east than in the west of the Keelung Shelf.

Methane emissions out of the seabed could seriously affect Earthâs climate and are usually associated with the dissociation of gas hydrates stored in marine sediments on the continental margins. Spatially, gas emissions out of the seafloor are not evenly distributed in continental margins. Gas emissions out of the seabed generally occur through submarine mud volcanoes and gas seeps. To understand the seabed gas emissions off SW Taiwan, we investigate the distributions of active submarine mud volcanoes, gas seeps, and gas plumes off SW Taiwan. We examine all of the available sub-bottom profiler and EK echo sounder data. We identified 19 submarine mud volcanoes, 220 gas seeps, and 295 gas plumes. The gas emissions are generally distributed at the crests of mud diapiric ridges. Most of the active mud volcanoes and gas seeps cluster at the KASMVG (Kaoping submarine mud volcanoes group) area. We speculate that the intensive mud volcanism and gas seepage at the KASMVG area are ascribed to submarine channel erosion along the continental slope base. The erosion causes a deep V-shaped channel and a steep BSR (Bottom-Simulating Reflector) slope curve across the continental margin. The upward migration rate of free gas beneath the BSR is thus increased and intensifies mud volcanism and gas seepage at the KASMVG area. The gas seeps can reduce the slope stability and generate small-scale slides. The development of mud volcanoes in an area could effectively disturb the seabed morphology so that large-scale submarine landslides cannot easily happen.

The Taiwan mountain belt has been created due to the collision between the Philippine Sea Plate and the Eurasian Plate. Northernmost Taiwan and its offshore area are now under post-collisional collapse. The post-collisional magmatism is distributed around northern Taiwan. Here we first report a submarine volcano, named Keelung Submarine Volcano, existing in the near-shore area of northern Taiwan. The high 3He/4He ratios in the collected seawater samples suggest that the magma of the Keelung Submarine Volcano is derived from a mantle source. Geometrically, both the Keelung Submarine Volcano and the Tatun Volcano Group are situated above the western border of the subducted Philippine Sea Plate and may have a same magma source. Both volcanic areas belong to the northern Taiwan volcanic zone, instead of the Ryukyu volcanic front. The Keelung Submarine Volcano has been rotated clockwise ∼48° after its formation, which implies that the Keelung Submarine Volcano has formed before the Luzon arc collided against northern Taiwan. Consequently, the post-collisional model to explain the formation of the northern Taiwan volcanic zone is questionable. As indicated by numerous shallow earthquakes and persistent emissions of the volcanic gases out of the seafloor around the volcanic cone, the Keelung Submarine Volcano is as active as the Tatun Volcano Group. For the sake of volcanic hazard assessment, it is essential to monitor the activity of the Keelung Submarine Volcano.

Bottom simulating reflectors (BSRs), high methane flux, shallow sulfide/methane interfaces, fluids venting from the seafloor, authigenic carbonates within sediments, methane reefs, and self-biomes are common seafloor features in the area off southwest Taiwan. The geophysical and geochemical signatures of these features suggest a high potential for gas hydrate (GH) reservoirs in the region. The BSRs are typically interpreted as the boundary between free gases and solid hydrate, whereas the upper reaches of the hydrate stability zone and the distribution of gas hydrate in shallow sediments are not well understood. This study shows the first results of a marine controlled-source electromagnetic survey, conducted in the offshore area of SW Taiwan in 2010. The survey aimed to provide electrical resistivity information of the shallow sediments. Three target areas were surveyed: (1) an area to the southeast of the Xiaoliuchiu Island (gas seep G96), (2) an area in the west of the Yung-An Ridge (YAR) and (3) an area in the northwest of the Good Weather Ridge (GWR). In total, fourteen survey lines with a total length of 72 km were completed. Our preliminary results show that relatively high resistivity anomalies occur within pockmarks and at gas seepage sites. The apparent resistivity is estimated to be about 1 Ohm-m higher than background in G96 and YAR sites, while an anomaly up to 2 Ohm-m is found in the GWR. At gas seep site G96, the high resistivity anomaly may be due to the existence of authigenic carbonates; whereas, the high resistivity anomaly in the NW of the GWR site may also be due to the existence of gas hydrate in the shallow seabed. Based on the resistivity anomaly, the gas hydrate saturation is about 16% in the shallow sediments below the pockmark area in the northwest side of the GWR site.