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Based on seismic reflection profiles and multi-beam bathymetric data, a large submarine landslide named Fangliao Slide is mapped for the first time off SW Taiwan. The Fangliao Slide occurred on the continental slope to the west of the Fangliao Canyon at water depths between 420 and 900 m. The seafloor of the Fangliao Slide has a gentle slope angle (~1 - 2°). The landslide covers an area of ~15 km length and ~10 km width and a volume of ~26 km3. The headwall of the landslide has ~30 m vertical offset at the southern flank of mud diapir MD7-1, and the sidewalls are bounded by fault A in the west and faults C and D in the east. The sliding area is composed of five bathymetric terraces, indicating that the slope failures have occurred several times. The Fangliao Slide can be divided into an upper domain and a lower domain, separated at the water depth of ~600 m where the gas hydrate off SW Taiwan becomes dissociate. The initial slope failure of the Fangliao Slide was probably linked to mud diapirism of MD7-1 and the slope failure in the lower domain was probably augmented by the gas hydrate dissociation. The seafloor morphology in the lower domain is therefore more corrugated than in the upper domain.

In this study, we used pore water dissolved inorganic carbon (DIC), SO 4 2− , Ca 2+ and Mg 2+ gradients at the sulfate-methane transition zone (SMTZ) to estimate biogeochemical fluxes for cored sediments collected offshore SW Taiwan. Net DIC flux changes (Δ DIC-Prod ) were applied to determine the proportion of sulfate consumption by organic matter oxidation (heterotrophic sulfate reduction) and anaerobic oxidation of methane (AOM), and to determine reliable CH 4 fluxes at the SMTZ. Our results show that SO 4 2− profiles are mainly controlled by AOM rather than heterotrophic sulfate reduction. Refinement of CH 4 flux estimates enhance our understanding of methane abundance from deep carbon reservoirs to the SMTZ. Concentrations of chloride (Cl − ), bromide (Br − ) and iodide (I − ) dissolved in pore water were used to identify potential sources that control fluid compositions and the behavior of dissolved ions. Constant Cl − concentrations throughout ∼30 m sediment suggest no influence of gas hydrates for the compositions within the core. Bromide (Br − ) and Iodine (I − ) concentrations increase with sediment depth. The I − /Br − ratio appears to reflect organic matter degradation. SO 4 2− concentrations decrease with sediment depth at a constant rate, and sediment depth profiles of Br − and I − concentrations suggests diffusion as the main transport mechanism. Therefore diffusive flux calculations are reasonable. Coring sites with high CH 4 fluxes are more common in the accretionary wedge, amongst thrust faults and fractures, than in the passive continental margin offshore southwestern Taiwan. AOM reactions are a major sink for CH 4 passing upward through the SMTZ and prevent high methane fluxes in the water column and to the atmosphere.

Submarine canyons are major conduits for delivery of sediments originating from the continental margin into the deep sea. Here we analyze the effects of changing sea levels and climate (e.g. sediment supply) on the frequency of turbidites over the last 26 kyr. Our study is based on high resolution age controls for turbidites in core MD178-10-3291 from a water depth of 2070 m on the flank of the Gaoping Canyon offshore in SW Taiwan. Unlike most other submarine canyons worldwide, the head of the Gaoping Canyon has remained connected to the river mouth during the recent flooding of the continental shelf. Our results reveal that turbidity currents are less frequent during periods of sea-level lowstand. In contrast, turbidity currents have been more frequent since ~ 12 kyr BP, during the final stage of rising sea level and sea-level highstands. This may be the result of increased terrestrial sediment delivery due to enhanced rainfall intensity. Moreover, comparing to other source-to-sink systems, the turbidite occurrence in the flank of Gaoping Canyon through the last glacial cycle might have been more strongly influenced by climatic changes due to short sediment storage/response time.

In order to identify the mud diapirs and mud volcanoes off SW Taiwan, we have examined ∼1500 km long MCS profiles and related marine geophysical data. Our results show ten quasi-linear mud diapirs, oriented NNE–SSW to N–S directions. Thirteen mud volcanoes are identified from the multibeam bathymetric data. These mud volcanoes generally occur on tops of the diapiric structures. Moreover, the active mud flow tracks out of mud volcanoes MV1, MV3 and MV6 are observed through the high backscatter intensity stripes on the sidescan sonar images. The heights of the cone-shaped mud volcanoes range from 65 m to 345 m, and the diameters at base from 680 m to 4100 m. These mud volcanoes have abrupt slopes between 5.3° and 13.6°, implying the mudflow is active and highly viscous. In contrast, the flat crests of mud volcanoes are due to relative lower-viscosity flows. The larger cone-shaped mud volcanoes located at deeper water depths could be related to a longer eruption history. The formation of mud diapirs and volcanoes in the study area are ascribed to the overpressure in sedimentary layers, compressional tectonic forces and gas-bearing fluids. Especially, the gas-bearing fluid plays an important role in enhancing the intrusion after the diapirism as a large amount of gas expulsions is observed. The morphology of the upper Kaoping Slope is mainly controlled by mud diapiric intrusions.

Geochemical data (CH4, SO42−, I−, Cl−, particulate organic carbon (POC), δ13C-CH4, and δ13C-CO2) are presented from the upper 30 m of marine sediment on a tectonic submarine accretionary wedge offshore southwest Taiwan. The sampling stations covered three ridges (Tai-Nan, Yung-An, and Good Weather), each characterized by bottom simulating reflectors, acoustic turbidity, and different types of faulting and anticlines. Sulfate and iodide concentrations varied little from seawater-like values in the upper 1–3 m of sediment at all stations; a feature that is consistent with irrigation of seawater by gas bubbles rising through the soft surface sediments. Below this depth, sulfate was rapidly consumed within 5–10 m by anaerobic oxidation of methane (AOM) at the sulfate-methane transition. Carbon isotopic data imply a mainly biogenic methane source. A numerical transport-reaction model was used to identify the supply pathways of methane and estimate depth-integrated turnover rates at the three ridges. Methane gas ascending from deep layers, facilitated by thrusts and faults, was by far the dominant term in the methane budget at all sites. Differences in the proximity of the sampling sites to the faults and anticlines mainly accounted for the variability in gas fluxes and depth-integrated AOM rates. By comparison, methane produced in situ by POC degradation within the modeled sediment column was unimportant. This study demonstrates that the geochemical trends in the continental margins offshore SW Taiwan are closely related to the different geological settings.

Source at https://doi.org/10.1002/2017GC006798 To systematically quantify the production, consumption, and migration of methane, 210 sediment cores were collected from offshore southwestern Taiwan and analyzed for their gas and aqueous geochemistry. These data, combined with published results, were used to calculate the diffusive methane fluxes across different geochemical transitions and to develop scenarios of mass balance and constrain deep microbial and thermogenic methane production rates within the accretionary prism. The results showed that methane diffusive fluxes ranged from 2.71 × 10−3 to 2.78 × 10−1 and from –1.88 × 10−1 to 3.97 mmol m−2 d−1 at the sulfate‐methane‐transition‐zone (SMTZ) and sediment‐seawater interfaces, respectively. High methane fluxes tend to be associated with structural features, suggesting a strong structural control on the methane transport. A significant portion of ascending methane (>50%) is consumed by anaerobic oxidation of methane at the SMTZ at most sites, indicating effective biological filtration. Gas compositions and isotopes revealed a transition from the predominance of microbial methane in the passive margin to thermogenic methane at the upper slope of the active margin and onshore mud volcanoes. Methane production and consumption at shallow depths were nearly offset with a small fraction of residual methane discharged into seawater. The flux imbalance arose primarily due to the larger production of methane through deep microbial and thermogenic processes at a magnitude of 1512–43,096 Tg Myr−1 and could be likely accounted for by the sequestration of methane into hydrate forms, and clay absorption.

Widely distributed bottom simulating reflectors (BSRs) imply the potential existence of gas hydrates offshore southwestern Taiwan. To compare the distribution of methane concentrations along passive and active margins in the region, bottom waters and cored sediments were collected during four cruises from 2005 to 2006. The results reveal that sites with high methane concentrations are predominantly distributed in the active margin and site GS5 is the only site that contains very high methane concentrations in the passive margin of studied area. Anomalously high methane fluxes still can be obtained from the calculation of diffusive methane flux, although there might be some gas leakage during or after sampling procedures. The profiles of methane and sulfate concentration reveal very shallow depths of the sulfate–methane interface (SMI) at some sites. There is evidence that sulfate reduction is mainly driven by the process of anaerobic methane oxidation. Thus, sulfate fluxes can be used as a proxy for methane fluxes through the use of diffusion equations; and the results show that the fluxes are very high in offshore southwestern Taiwan. The depths of the SMI are different at sites GH6 and C; however, both methane profiles reveal parallel methane gradients below the SMI. This might be because of methane migration to surface sediments from the same reservoir with the same diffusion rates. Although BSRs are widely distributed both in the active margin and in the passive margin, most sites with high methane concentrations have been found in the active margin. Therefore, the specific tectonic settings in offshore SW Taiwan might strongly control the stability of gas hydrates, and thus affect the methane concentrations and fluxes of the sediments and sea waters. Furthermore, the carbon isotopic composition of methane shows that a biogenic gas source is dominant at shallower depth; however, some thermogenic gases might be introduced through the fracture/fault zones from deeper source in the active region of studied area.

The experimental results indicate that the plagioclase and pyroxenes in the treated samples show micropits and microfractures. Small crystals readily suffered from alteration, as compared with large ones, and the susceptibility of the minerals to the acid solution is decreased in the order of plagioclase, augite, hypersthene and opaque minerals. Plagioclase, which is the most reactive mineral in the experiments, shows an increase of SiO2, but a decrease of Al2O3, CaO and Na2O as the experiments proceeded. The chemical change of the treated andesite, it indicates that the relative amounts of SiO2 ,T iO2, MgO,FeO, MnO and K2O increase, while those of Al2O3, CaO, Na2O and P2O5decrease with an increase of the experimental duration. The relative mobility of chemical elements listed in decreasing order is P, Ca, Al, Na, Si, K, Mg, Fe, Mn and Ti in terms of the Kv alue. It is no wonder that Ca, Al and Na are more mobile than others because these el- ements are readily released into solution from the more reactive plagioclase, while Mg, Fe etc. still stay in the less reactive pyroxenes and opaque minerals.

Thirteen mud volcanoes are identified from the multibeam bathymetric data in the gas hydrate potential area of the upper Kaoping Slope, off southwest Taiwan. The heights of the mud volcanoes range from 65 m to 345 m and the size of their bases range from 680 m to 4,100 m in diameter. The slopes of the mud volcanoes are very steep (from 5.3° to 13.6°), suggesting that the mud volcanoes are fed by high-viscosity flows. ROV observations reveal active eruptions of the mud volcanoes MV1, MV5 and MV12, and two modes of eruption (explosive and effusive eruptions) were identified. The MV1 and MV5 are characterized by explosive eruptions and the eruptive cycles are about 3~5 and 5~8 minutes, respectively. The MV12 shows an effusive eruption, characterized by the continuous outpouring of mud together with gas plume. The gas plumes were found on tops of mud volcanoes MV1, MV4, MV5, MV10 and MV12, as indicated by the high-frequency sonar (EK500, EK60 sonars and multibeam echo sounder) images. The mud flows on the flank of six mud volcanoes (MV1, MV3, MV5, MV6, MV9 and MV10), illustrated by the high backscatter intensity bands from the sidescan sonar images. As evidenced by the results of investigation, the mud volcanoes in the upper Kaoping Slope off SW Taiwan are very active.

Bottom simulating reflector (BSR) is a key indicator for the presence of gas hydrate beneath the sea floor. Widely distributed BSRs have been observed in the area offshore of southwestern Taiwan where the active accretionary complex meets with the passive China continental margin. In order to better understand the distribution and characters of the gas hydrate in the region, closely spaced (1.86-km line spacing) multichannel seismic reflection surveys have been conducted in recent years under the support of the Central Geological Survey, ROC. Over 10000 km of multichannel seismic reflection profiles have been collected that cover an area of about 10000 km^2 offshore of southwestern Taiwan. BSRs can be identified along 50% of the seismic profiles that we collected. A newly compiled BSR distribution map suggests that gas hydrates are distributed both in the passive margin of the China continental slope as well as in the submarine Taiwan accretionary wedge, from water depths of 500 to over 3000 m. Gas hydrates are most concentrated underneath anticlinal ridges in the accretionary wedge, and underneath the slope ridges of the passive continental margin that were formed due to sedimentary processes. Active fluid activities are evident from various features observed on seismic reflection and chirp sonar profiles, such as mud volcanoes, gas plumes and gas charged shallow sedimentary layers. Fluid migration model has been established from a set of pseudo 3D seismic reflection data. The predicted locations of high fluid flux correlate well with those interpreted from geochemical analyses that show very high methane concentrations and very shallow sulfate-methane interfaces (SMI). This demonstrates the importance of structural control over gas hydrate emplacement. From the observed gas hydrate distribution and characters, the area offshore of southwestern Taiwan provides an ideal place to study and compare the formation and migration of gas hydrates under different tectonic settings.