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Identification of methane origins remains a challenging work as current diagnostic signals are often not sufficient to resolve individual formation and post‐formation processes. To address such a knowledge gap in a tectonically active and fragmented terrain, samples from mud volcanoes, gas seeps, and springs distributed along structural features onshore and offshore of Taiwan were analyzed for their isotopic compositions of methane, nitrogen, helium, dissolved inorganic carbon, CO2, and water. Our analyses yielded Δ13CH3D and Δ12CH2D2 values ranging between +1.9‰ and +7.8‰ and between +3.0‰ and +19.9‰, respectively. A portion of the samples were characterized by values that represent the thermodynamic equilibrium at temperatures of 99°–260°C. These temperature estimates, together with the bulk isotopic compositions and local geothermal gradients (25°–30°C/km), suggest that methane was formed by thermal maturation of organic matter at depths of 2–9 km below the land surface and channeled upward along faults. Other samples were found to deviate from equilibrium by varying degrees. Considering the geological background, helium isotopic ratios, and nitrogen isotopologue compositions, and methanogens detected at some sites, these gases are either abiotic in origin or a mixture of microbial and thermogenic sources. Regardless of whether the equilibrium of methane isotopologues was reached, few sites hosted by sedimentary formations were characterized by mantle‐like helium signatures, indicating decoupled origins and potential degassing of helium from the relic igneous source. Overall, these results suggest the extraction of methane and other gases from multiple depths from strata fragmented by fault displacement in an active orogenic belt. Plain Language Summary: Mud volcanoes and gas seeps are distributed primarily in active tectonic regimes and represent the surface expression of subsurface fracture networks. They provide rapid access to probing subsurface characteristics for the generations of methane, other gases, and water. Identifying the exact mechanisms of methane formation remains a challenging work due to the lack of diagnostic signals with sufficient resolving power. This study employed a wide range of isotopic tools to constrain the origins of gases and water from mud volcanoes and seeps in Taiwan, where dual subductions and associated arc‐continent collision between the Philippine Sea and Eurasian plates have enabled the plumbing of gases and water in accordance with structure development and stratum deformation. Our results reveal a spectrum of geochemical characteristics that point to diverse formation mechanisms and conditions. In particular, deeply sourced thermogenic methane (from depths of 2–9 km) appears to constitute a major gas component at most sites, and is mixed by various degrees with abiotic methane pertinent to igneous bodies emplaced during subduction/collision, or microbial methane generated at shallow depths. Overall, the results address the temporal and spatial variations in gas origins prone to terrain fragmentation and dynamic structural control associated with mountain building. Key Points: Mud volcanoes and gas seeps provide rapid access to probing subsurface fluid characteristics in tectonically active regionsThermogenic methane constitutes a major component of hydrocarbons and forms at temperatures of 99°–260°CA wide spectrum of geochemical characteristics indicates diverse formation mechanisms and conditions that constrain gas and water origins [ABSTRACT FROM AUTHOR]

Mud volcano (MV) is one of the most important channels for deep seated volatile materials to migrate to Earth's surface in the sedimentary basins at subduction zones. It plays a meaningful role of elemental budget in hydrosphere. Waters of MV fluid emitted from 46 mud pools in MV Lei-Gong-Huo (LGH) in eastern Taiwan were sampled from October 2015 to July 2016 monthly. Previous research indicates that the chemical characteristic of fluid expelled by MV LGH is dominated by two-end-member mixing of water--rock interaction with sedimentary and igneous rocks (Chao et al., Front Earth Sci 9:750436, 2022). Trace elements of 165 mud pool samples, including Al, Co, Ni, Cu, Zn, As, Br, Rb, Mo, Sb, I, Cs, Pb and U, were measured. The results show that waters of MV LGH reveal similar chemical characteristics to low-temperature ridge-flank hydrothermal spring and marine pore water in anoxic conditions coupled with the addition of clay dehydration. Principle component analysis (PCA) with previous published variables (Chao et al., Front Earth Sci 9:750436, 2022) indicates the important factors are the source of water--rock interaction, the surface ion exchange, the redox condition of mud pool, and the evaporation. In summary, the concentration of trace elements in the waters expelled by MV LGH is in the range of nM to pM. Their behaviors are dominated by secondary factors during and after migration such as surface ion exchange in response to changes in pressure and temperature, redox state of the mud pool, and evaporation. Trace elements in MV water are not suitable for fluid source or provenance studies. [ABSTRACT FROM AUTHOR]

Highlights • 13CDH 3 and 12CD 2 H 2 were measured independently on a Panorama mass spectrometer. • 13CDH 3 and 12CD 2 H 2 geothermometers give CH 4 formation temperature of 150 °C. • Thermogenic CH 4 is not in thermal equilibrium with hosting liquid mud. Abstract Resolved measurements of the relative abundances of 13CDH 3 and 12CD 2 H 2 from a Taiwan mud volcano confirm that thermogenic CH 4 with a formation temperature of 150 °C is bubbling from a pool of liquid mud. Analysis for both doubly-substituted molecules provides confirmation that isotopic exchange equilibrium was achieved between methane isotopologues. Methane extracted from the headspace of core samples taken from sediments deposited proximal to the bubbling pool of mud shows small departures from isotopic equilibrium. [ABSTRACT FROM AUTHOR]

In the present study, some mud volcanoes in Pingtung and Kaohsiung areas were selected. The selected mud volcanoes are located above the mud diaper system in southern Taiwan. Three different sampling techniques were used to collect samples in and around the said mud volcanoes. The results show that river water samples from different places fall on the meteoric line. Mud samples are found to be heavier in oxygen isotope while hydrogen isotope remains the same. Soil gas samples were also collected at the depth of 1 m and dissolved gases from the mud volcanic site (or nearby) were collected for gas chromatography analyses. Soil gas samples and dissolved gas from mud pool and groundwater were analyzed for radon concentration using RAD7 (solid-state nuclear track detector) bi-weekly. Long-term investigation will be needed to understand the relationship of mud volcanoes the eruption cycle with gas composition variations. It will also help to understand their relationship with tectonic activities in the region. [ABSTRACT FROM AUTHOR]

A submarine mud volcano (MV) known as TY1 is one of the largest conical structures found offshore southwestern Taiwan. Active gas bubble emissions at the flat crest of the mud volcano disperse sediment particles into the water column. Changes in the mud volcanism of TY1 were revealed by radiograph and grain size analysis of the sediment cores. 210 Pb geochronology was applied to the near-seafloor sediments to reveal the detailed mud volcanism from the deposition rate. In a 58-cm long sediment core taken from the northern margin of the crest, known as TY1-N.170, the size and frequency of the mud clasts decrease upward. The clasts are smaller than 8 mm above a sediment depth of 34 cm below the sea floor (cmbsf). Sediments between 0 and 19 cmbsf are massive, and particles larger than 2 mm are absent. An enrichment of coarse, silt-sized particles in the massive sediment unit and the restricted distribution of the unit suggest that the massive unit was generated by re-deposition of sediment particles that were dispersed into the water column by gas bubble emission. These characteristics suggest that during the last mud volcanism of TY1, there was a decrease in mud eruption energy, and gas bubble emission became the main activity. In core TY1-S.440, taken from the southern slope, a massive sediment unit enriched with coarse, silt-sized particles, is intercalated between mud breccia structures. This suggests repeated mud breccia flows caused by TY1. The excess 210 Pb activity present in the massive sediment unit of TY1-N.170 suggests activation of gas bubble emission. In the massive unit, a decrease in excess 210 Pb activity appears upward toward the seafloor. The highest value is 3.1 dpm/g at 19.8 cmbsf (26.4 g/cm 2 in cumulative mass); values lower than 1.0 dpm/g are distributed 2.8–6.8 cmbsf (3.6–8.8 g/cm 2 ). The upwardly decreasing trend is opposite that of the reference core from which a reasonable areal sedimentation rate was obtained using a constant-flux constant-sedimentation model. The anomalous depth profile at the massive unit indicates an increasing rate of sediment deposition, which originated from the activation of gas bubble emission. [ABSTRACT FROM AUTHOR]

The Shin-Yan-Ny-Hu (SYNH) MVs are typical subaerial mud volcanoes in southwestern Taiwan. The lipid biomarkers and their stable carbon isotopes were firstly examined to delineate the mud breccia sources, depositional environments, thermal maturity, probable source strata and the migration framework of volcanic fluid. The sediment organic matter within active SYNH MVs principally stemmed from the petroleum derivatives, bacteria and terrigenous C 3 plants suggested by the following indicators: the CPI of n -alkanes (∼1); the distribution and δ13C values of saturate, fatty acids and the plot of Pr/ n -C 17 vs. Ph/ n -C 18 ratios. The Pr/Ph ratios, the plot of Pr/ n -C 17 vs. Ph/ n -C 18 and the plot of Pr/Ph vs. O/C 30 Hopane ratios implied that the sediment organic matter was deposited in a marine deltaic sedimentary environment under an oxic depositional environment. The C 30 βα/C 30 αβ, T max , 22 S/(22 S + 22 R) ratios of homohopane (C 31) and bishomohopane (C 32), MPI 1 , MPI 2 , F 1 , F 2 , T max vs. HI and the Pr/ n -C 17 vs. Ph/ n -C 18 ratios revealed that the Type III organic material was early thermal maturity and the burial depth of mud breccia matrix was estimated around 4–5 km bsf. • The organic matter of SYNH MVs was derived from petroleum derivatives, bacteria and terrestrial higher plants. • The mud breccia matrix was deposited in a marine deltaic environment under an oxidized sedimentary environment. • The source organic matter was early thermal maturity, and the burial depth of mud breccia matrix was about 4–5 km bsf [ABSTRACT FROM AUTHOR]

Mud volcanoes are the most dynamic and unstable sedimentary structures in the areas of tectonic compression like the subduction zones. In this study, we comprehensively analyzed the distribution of minerals as well as diversity, abundance and metabolic potential of the microbial communities of major mud volcanic groups across Taiwan namely Chu-kou Fault (CKF), Gu-ting-keng Anticline (GTKA), Chi-shan Fault (CSF), and Longitudinal Valley Fault (LVF). The mud volcano fluids recorded relatively higher Na and Cl contents than the other elements, particularly in the CKF and GTKA groups. The highest microbial diversity and richness were observed in the CSF group, followed by the GTKA group, whereas the lowest microbial diversity was observed in the CKF and LVF groups. Proteobacteria were common in all the sampling sites, except WST-7 and WST-H (Wu-Shan-Ting) of the CSF group, which were abundant in Chloroflexi. The halophilic genus Alterococcus was abundant in the Na-and Cl-rich CL-A sites of the CKF group. Sulfurovum was dominant in the CLHS (Chung-Lun hot spring) site of the CKF group and was positively correlated with sulfur/thiosulfate respiration, which might have resulted in a higher expression of these pathways in the respective group. Aerobic methane-oxidizing microbial communities, such as Methylobacter , Methylomicrobium , Methylomonas , and Methylosoma , constituted a dominant part of the LVF and CSF groups, except for the YNH-A and YNH–B (Yang-Nyu-Hu) sites. The WST-7 and JS sites were abundant in both methane-producing and methane-oxidizing microbial communities. The LGH-F1 (Lei-Gong-Huo) site was dominated by both methanotrophic and methylotrophic genera, such as Methylomicrobium and Methylophaga , respectively. Methylotrophy, methanotrophs, and hydrocarbon-degrading pathways were more abundant in the LVF and CSF groups but not in the remaining groups. The results of this study extend our knowledge of the diversity, abundance, and metabolic functions of prokaryotes in major terrestrial mud volcanoes in Taiwan. [ABSTRACT FROM AUTHOR]

In order to identify the mud diapirs and mud volcanoes off SW Taiwan, we have examined ~1500km 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 65m to 345m, and the diameters at base from 680m to 4100m. 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. [ABSTRACT FROM AUTHOR]

Highlights: [•] We combine the land and marine magnetic data to regional magnetic anomaly maps. [•] We present the first Curie point depth and thermal gradient map of whole Taiwan. [•] Special shallow Curie point depth at southern Taiwan by hot springs and mud volcanoes. [•] Curie point depth is high relationship with the heat flow data measurements. [•] Shallow Curie point depth and high heat flow are located at continent-arc boundaries. [Copyright &y& Elsevier]

Abstract: In this study, the geochemical characteristics of mud volcanoes were investigated to seek their possible linkage with arsenic mobilization in the region. Among the mud volcanoes of southern Taiwan, the largest and well-preserved active mud volcanoes are situated in Wushanting (WST) and Hsiaokunshui (HKS). These two selected active mud volcanoes are exhaling mainly methane (CH4 >95%) along with other gases like O2, CO2, CO, SO2, H2S, NO, and N2. The content of CH4 in the HKS active mud volcano is very close to that of the WST mud volcano. High Na+ and Cl− concentrations in mud liquids and mudstone deposits indicate their origin in a marine depositional source. However, a newly erupted active mud volcano in WST reflects the presence of a high level of SO4 2−. Significant variations (p ≤0.05) in trace and transition element (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Sr, Mo, Cd and Ba) concentrations in the liquids and sediments of mud volcanoes are noticed at different eruption periods, suggesting different elemental compositions and release mechanisms in different locations. The weathering of micas as well as clay minerals seems to be responsible for the enrichment in trace metals in mud volcano sediments and fluids. Several trace elements (V, Cr, Mn, Fe, Co, Ni, and Zn) are strongly bound to the sediments, but concentrations of Cu, Se, and Sr are significantly higher in the fluids. The presence of higher As contents in the sediment of HKS was observed in contrast to its fluid. As a reducing agent, humic substances seem to play an important role in binding with trace elements and particularly As in the sediments. Fourier Transform Infrared (FTIR) spectra of humic substances from muds showed the presence of possible functional groups of primary and secondary amines, ureas, urethanes (amide) and silica. [Copyright &y& Elsevier]

Abstract: The continental margin off SW Taiwan is in an incipient stage of orogeny and contains numerous active mud diapirs and mud volcanoes. Gas emissions out of the seabed off SW Taiwan are revealed by acoustic images from 38kHz echo sounders or sub-bottom profilers. However, the mechanism for gas emissions is still poorly understood. In this study, we show that the gas emissions out of the seabed and the associated tremors off SW Taiwan are strongly correlated with the ocean tides, especially with the diurnal and semidiurnal constituents. Particularly, for each tidal day the strong gas emissions (expulsions) and long-duration tremors occur mainly during both the rising periods to the higher high tides and the falling periods to the lower low tides. Both the gas emissions and tremor activities are generally quiescent near the tidal datum. On average, the daily intensity of gas emissions and the magnitude of tremors are positively proportional to the daily tidal range; a larger slope site implies a greater concentration of gas in the shallow sediments. Most of the individual tremors and long-duration tremors observed off SW Taiwan occur as results of the strong gas expulsions out of the gas-bearing seabed. The high-frequency components of the induced tremors quickly decay and are followed by horizontal low-frequency (~7Hz) harmonic oscillations of the seabed. Long-duration tremors may enhance seafloor instability and increase the potential for submarine landslides. [Copyright &y& Elsevier]

Fluid and mud samples were collected from Wushanting (WST), Hsiaokunshui (HKS), Yenshuikeng (YSK), Kuantzeling (KZL), and Kunshuiping (KSP) mud volcanoes of southwestern Taiwan. Concentrations of major ions and trace elements in mud volcanic fluids were analyzed to find the possible linkage to elevated arsenic (As) concentrations in the Chianan plain groundwater. The elevated Na, K, and Cl concentrations indicated possible marine origin of the fluids. The trace element concentrations in the mud volcanic fluids were generally low, but the As concentrations were up to 0.12 mg/L. High contents of As, Fe, and Mn were observed in the mud samples. Saturation index calculations indicated that both carbonate and oxide minerals acted as potential sinks for As in the mud volcanic fluids. Arsenic in the dewatering fluids and muds may be transported by the subsurface flow and surface streams as suspended solids and eventually deposited in the Chianan plain aquifers. Under reducing conditions, As may be released from the host minerals (such as Fe- and Mn-oxides/hydroxides), thereby causing widespread groundwater As pollution. [ABSTRACT FROM AUTHOR]

Summary Microbial communities responsible for methane cycling in mud volcanoes onshore are poorly characterized. This study analysed bubbling fluids and cored sediments retrieved from a mud volcano in eastern Taiwan. The pore water profiles revealed that methane concentrations generally increased with depth and changed dramatically at different depth intervals at different sites. The methane concentrations were inversely correlated with Fe2+/Mn2+ concentrations and δ13C values of methane, marking iron/manganese-methane transition zones in the sediment cores. Archaeal communities were dominated by ANME-2a members and methylotrophic methanogens, whereas bacterial communities consisted primarily of Proteobacteria, Firmicutes and Bacteroidetes. The 16S rRNA gene copy numbers of ANME-2a and Desulfuromonas/ Pelobacter populations varied by two to three orders of magnitude along the profile and exhibited a pattern comparable with those of Fe2+ and δ13C values of methane. These lines of evidence suggest a coupling between anaerobic methanotrophy and metal reduction in the metal-methane transition zones under sulfate-deficient conditions, a metabolic scheme contrasting with that observed in marine cold seeps. Anaerobic methanotrophs proliferate by removing methane produced from in situ methanogenesis and originating from the deep source. Methane finally emitted into the atmosphere is quantitatively and isotopically altered by various microbial processes compartmentalized at different depth intervals. [ABSTRACT FROM AUTHOR]

Field investigations following the 2003 Mw = 6.8 Chengkung earthquake in eastern Taiwan revealed some interesting observations of surface geological processes closely related to the co-seismic deformation. We discovered that the Tama Fault, which is about 15 km east of the causative Chihshang Fault, underwent shortening of about 15.5 mm locally in 2001 - 2006, particularly during the 2003 earthquake. This shows that ESE-WNW compression affects the upper crust of the Coastal Range and produces significant shortening in addition to that of the major Chihshang Fault. On the hanging wall of the Chihshang Fault, we also found vigorous activities of the two major mud volcanoes during the main shock, lasting several days. To the north, the Luoshan Mud Volcano, a large mud basin, erupted noisily with water and gases during the earthquake. To the south, in the Leikunghuo Mud Volcano, two sets of fractures, one aligned with the N16°E right-lateral fault and the other with the N80°E left-lateral fault, occurred during the earthquake. This conjugate system revealed a strike-slip stress regime with NE-SW compression and NW-SE extension. We interpret it to be the result of local stress permutation rather than regional tectonic stress. We conclude that deformation did occur inside of the Coastal Range, especially during the co-seismic event. Therefore, a better understanding of the internal deformation of the Coastal Range is an important target for future studies, particularly across three mapped faults: the Yungfeng, Tuluanshan and Tama faults. We also want to draw attention to the stress analysis in the mud volcanoes area, where the local stress perturbation plays an important role. [ABSTRACT FROM AUTHOR]

Fluid and mud samples collected from Hsiaokunshui (HKS), Wushanting (WST), Yenshuikeng (YSK), Kunshuiping (KSP), Liyushan (LYS), and Sinyangnyuhu (SYNH) mud volcanoes of southwestern Taiwan were characterized for major ions, humic substances (HS) and trace elements concentrations. The relationship between the release of arsenic (As) and activities of sulfate-reducing bacteria has been assessed to understand relevant geochemical processes in the mud volcanoes. Arsenic (0.02–0.06 mg/L) and humic substances (4.13 × 10−4 to 1.64 × 10−3 mM) in the fluids of mud volcanoes showed a positive correlation (r = 0.99, p < 0.05) except in Liyushan mud volcano. Arsenic and iron in mud sediments formed two separate groups i) high As, but low Fe in HKS, WST, and SYNH; and ii) low As, but high Fe in the YSK, KSP, and LYS mud volcanoes. The EhS.H.E. values of the mud volcano liquids were characterized by mild to strongly reducing conditions. The HKS, SYNH, and WST mud volcanoes (near the Chishan Fault) belongs to strong reducing environment (−33 to −116 mV), whereas the LYS, YSK, and KSP mud volcanoes located near the coastal plain are under mild reducing environment (−11 to 172 mV). At low Eh values mud volcanoes, saturation index (SI) values of poorly crystalline phases such as amorphous ferric hydroxide indicate understaturation, whereas saturation is reached in relatively high EhS.H.E. values mud volcanoes. Arsenic contents in sediments are low, presumably due to its release to fluids (As/Fe ratio in YSK, KSP, and LYS sediment: 4.86 × 10−4−6.20 × 10−4). At low EhS.H.E. values (mild to strong reducing environment), arsenic may co-precipitate with sulfides as a consequence of sulfate reduction (As/Fe ratios in WST, HKS, and SYNH sediments: 0.42–0.69). [ABSTRACT FROM AUTHOR]

Abstract: Mud volcanoes are important conduits for deep fluids to migrate upward, providing an important window to study fluid/sediment interactions at depth in accretionary prisms. Fluids emitting from mud volcanoes in southern and eastern Taiwan were analyzed for dissolved chemical composition and B isotopes to delineate the sources and chemical alteration occurring during fluid migration. Concentrations of dissolved major elements (Cl, Na, K, Ca, Mg, and SO4) in most of the mud volcano fluids were lower than seawater concentrations, except for the high Ca samples in eastern Taiwan, which were probably affected by fluids/igneous rock interactions. The high concentrations of B, Ba, and Li in the mud volcano fluids are consistent with a scenario that involves intense sediment alteration and clay dehydration at depth. The reaction temperatures estimated using chemical thermometers fall within the range of 70–170°C, with some variation by location. In nearby mud volcano sites, the concentration of dissolved major elements varied little, but the B stable isotope compositions varied by as much as 20‰, supporting the argument that dissolved B concentrations were over-printed with effects of sediment adsorption. This explains the occurrence of B enriched and heavy δ11B fluids at moderate depths in subduction zones. It is probable that the fluids in mud volcanoes in Taiwan originated from 2 to 5km depth and are influenced by processes involving tectonic shortening, sediment alteration, and clay dehydration, and subsequent retrograde adsorption during fluid migration. [Copyright &y& Elsevier]

Because of the increasing public awareness of the greenhouse effect, geological emissions of methane (GEM) have gained more attention. The Intergovernmental Panel on Climate Change included GEM in the natural sources of methane category in the Fourth Assessment Report in 2007. The methane flux from mud volcanoes (MVs) is the second leading source within GEM. However, given that gas flux from MVs has been measured directly only in Sicily and Azerbaijan, the global methane estimation is still a conservative assumption based on limited data. The behavior of MV activities is seldom reported. This study attempts to estimate annual methane emissions through MVs by using a video recorder to record MV activities along the Chishan Fault and determining the bubble size and flux rate of each MV. Observation results, after a 1 year observation period, indicate that the annual emissions of the 19 MVs along the Chishan Fault are 0.1–0.2% of the global methane flux from MVs, highlighting that the methane emission from these MVs should not be neglected. In addition, the continuous monitoring of a particular MV disclosed periodic characteristics of MV activities. These results manifest the spatial and temporal variations of MV activities along the Chishan Fault. The spatial variations are related to the extensional kinematics of the Chishan fault, while the temporal variation relates to the earth-tide-related MV activities. The estimated annual methane emissions from the study are far more than those of previous studies and are still considered conservative. Results of this study may provide valuable information for researchers attempting to estimate the quantity of GEM. [ABSTRACT FROM AUTHOR]

Abstract: A three-year geophysical investigation was carried out in the Wu-Shan-Ding mud volcano nature reserve in southwest Taiwan. This study utilized the very low-frequency electromagnetic (VLF-EM) method with a recent data processing technique, ensemble empirical mode decomposition (EEMD), to investigate the subsurface structure of the mud volcanic site through the single profile modeling and the surface 2D contour analysis. With the help of this new data processing technique, the signal of VLF-EM data was enhanced to an extent that a regularized inversion with finite-element forward modeling scheme can be applied to map the subsurface resistivity image of a single profile with satisfactory accuracy. This study successfully predicted the evolving of a gryphon, and proposed a model showing that the mechanism of fissure eruptions is more appropriate than a point source model for the shallow structure of the Wu-Shan-Ding mud volcano. The results also indicate that the Chi-shan fault is still active and that it traverses the study area with a nearly vertical up-thrown angle. [Copyright &y& Elsevier]

Abstract: Both land and offshore mud volcanoes have been investigated thoroughly in southwestern Taiwan in terms of morphological expression, spatial distribution, composition, and evasion flux of extruded material. The interactive relationship between mud volcanoes and their associated structures, however, has rarely been examined. This study proposes a release bend model that explains the spatial distribution and relative activity of mud volcanoes along the Chishan Fault, southwestern Taiwan. The surface movement of the Chishan Fault and mud volcano activity were monitored by GPS stations and gas flux measurements, respectively. The resistivity sub-structure of strata across the fault was investigated by the magnetotelluric (MT) method and resistivity image profiling (RIP). Analytical results support the contention that extensional activity at the release bend provides openings to fissures associated with the Chishan Fault, thereby increasing the potential for a mud volcano. The proposed model may facilitate future exploration of mud volcanoes. [Copyright &y& Elsevier]

Abstract: Mud volcanoes are important pathways for CH4 emission from deep buried sediments; however, the importance of gas fluxes have hitherto been neglected in atmospheric source budget considerations. In this study, gas fluxes have been monitored to examine the stability of their chemical compositions and fluxes spatially, and stable C isotopic ratios of CH4 were determined, for several mud volcanoes on land in Taiwan. The major gas components are CH4 (>90%), “air” (i.e. N2 +O2 +Ar, 1–5%) and CO2 (1–5%) and these associated gas fluxes varied slightly at different mud volcanoes in southwestern Taiwan. The Hsiao-kun-shui (HKS) mud volcano emits the highest CH4 concentration (CH4 >97%). On the other hand, the Chung-lun mud volcano (CL) shows CO2 up to 85%, and much lower CH4 content (<37%). High CH4 content (>90%) with low CO2 (<0.2%) are detected in the mud volcano gases collected in eastern Taiwan. It is suggestive that these gases are mostly of thermogenic origin based on C1 (methane)/C2 (ethane)+C3 (propane) and δ 13CCH4 results, with the exception of mud volcanoes situated along the Gu-ting-keng (GTK) anticline axis showing unique biogenic characteristics. Only small CH4 concentration variations, <2%, were detected in four on-site short term field-monitoring experiments, at Yue-shi-jie A, B, Kun-shui-ping and Lo-shan A. Preliminary estimation of CH4 emission fluxes for mud volcanoes on land in Taiwan fall in a range between 980 and 2010 tons annually. If soil diffusion were taken into account, the total amount of mud volcano CH4 could contribute up to 10% of total natural CH4 emissions in Taiwan. [Copyright &y& Elsevier]

Abstract: The supply of natural gas from mud volcanoes is commonly attributed to the migration of gas-bearing fluids from a deeper source area. However, the relative importance of these gas sources is not well known. Onshore mud volcanoes in Taiwan are mainly located in the Foothills of southwestern Taiwan and in the Coastal Range of eastern Taiwan, generally along structurally weak zones. Geochemical analyses of gases and sediments from 17 mud volcanoes were undertaken in this study. Using molecular and stable isotopic compositions, hydrocarbon gases were characterized by type and source maturity. Source rock assessments and nanofossil analyses were carried out to evaluate the potential, and to infer the origin of sediments. A particular effort was directed towards determining the genetic relationships between mud volcanic gas and associated sediments. Gases from Taiwan’s mud volcanoes are predominantly CH4-enriched, with the exception of a few mud volcanoes where CO2 predominates. The origins of hydrocarbon gases in these systems range from microbial to thermally over-mature sources; while, excess CO2 may be derived from an inorganic origin as indicated by measured δ13 values. The integration of geochemistry and local geology has yielded models with which to evaluate possible mechanisms for the formation of mud volcanoes. [Copyright &y& Elsevier]

Abstract: Mud volcanoes distributed in the southern part of Taiwan are believed to be sourced from an accretionary prism located along the collision boundary between the Philippine Sea plate and the Asian Continental plate. Fluid and mud samples have been collected from Wushanting and Hsiaokunshui mud volcanoes during October 2004, March and June 2005, and analyzed for major ions, trace elements, and stable isotopes. The results show that the mud volcano fluids are enriched in Na+ and Cl− but are depleted in Ca2+ and Mg2+. The chemical composition and oxidation state of the fluids fluctuate seasonally, with ionic contents being notably higher during the dry pre-monsoon period. The enriched ionic (e.g., Na+, , K+, Mg2+ and Cl−) compositions of the exchangeable fractions indicate that the muds were sourced from a marine depositional environment. δD and δ18O values indicate that the mud volcano fluids may have been modified by chemical exchange with 18O-rich crustal rocks and possibly originated from mixing of deep brines with circulating meteoric water. The spatial and temporal distribution of ions in the muds suggests a varied mixing regime within the mud volcanoes. Incubation study of the muds shows the leaching of soluble salts under aerobic conditions, which is generally consistent with the fluid geochemical characteristics. Moreover, elevated trace element (e.g., As, Zn, Cu and Mn) concentrations are also observed in the fluids and muds. Geochemical correlations between As, Zn and Cu in the muds suggest prevailing SO4-reducing conditions at depth. The relatively low Eh values (range: −50 to −30mV) of the fluids near the surface would favor bacterial Fe reduction and mobilization of As. The dewatering of mud volcanoes may represent a significant source of groundwater As in the nearby Chianan plain. [Copyright &y& Elsevier]

Many mud volcanoes are distributed along the tectonic sutures in southern Taiwan and can be divided into five zones based on their relative positions in different tectonic domains. Most active mud volcanoes are exhaling methane-dominated gases. Nevertheless, some gases show unusual carbon dioxide-dominated and/or nitrogen-excess compositions. This implies that there are multiple sources for the gas compositions of mud volcanoes in Taiwan.For better understanding the total amount of exhalation gases and its flux, the gas flow and compositions were continuously measured in the interval of two minutes at Chung-lun (CL) bubbling mud pool for a few months. The major compositions of gases exhaling from this site were 75~90% of CO2 and 5~12% of CH4. The amount of gases exhaling from the mud pool can be estimated to be about 1.4 ton/year for CH4 and 28 ton/year for CO2, respectively. The preliminary results of exhaling gas flux from the major vents of representative active mud volcanoes, yielded an estimated total CH4 output of the mud volcanoes in Taiwan of ca. 29 ton/year during quiescent period. [ABSTRACT FROM AUTHOR]

Taiwan is located at the collision boundary between the Philippine Sea Plate and the Asian Continental Plate and is one of the most active orogenic belts in the world. Fluids sampled from 9 sub-aerial mud volcanoes distributed along two major geological structures in southwestern Taiwan, the Chishan fault and the Gutingkeng anticline, were analyzed to evaluate possible sources of water and the degree of fluid-sediment interaction at depth in an accretionary prism. Overall, the Taiwanese mud volcano fluids are characterized by high Cl contents, up to 347 mM, suggesting a marine origin from actively de-watering sedimentary pore waters along major structures on land. The fluids obtained from the Gutingkeng anticline, as well as from the Coastal Plain area, show high Cl, Na, K, Ca, Mg and NH4, but low SO4 and B concentrations. In contrast, the Chishan fault fluids are much less saline (1/4 seawater value), but show much heavier O isotope compositions (δ18O=5.1–6.5 ‰). A simplified scenario of mixing between sedimentary pore fluids and waters affected by clay dehydration released at depth can explain several crucial observations including heavy O isotopes, radiogenic Sr contents (87Sr/86Sr=0.71136–0.71283), and relatively low salinities in the Chishan fluids. Gases isolated from the mud volcanoes are predominantly CH4 and CO2, where the CH4–C isotopic compositions show a thermogenic component of δ13C=−38 ‰. These results demonstrate that active mud volcano de-watering in Taiwan is a direct product of intense sediment accretion and plate collision in the region. [Copyright &y& Elsevier]

The microbial communities inhabiting mud volcanoes have received more attention due to their noteworthy impact on the global methane cycle. However, the impact of temporal effects of volcanic eruptions on the microbial community's diversity and functions remain poorly characterized. This study aimed to underpin the temporal variations in the bacterial community's diversity and PICRUSt-predicted functional profile changes of mud volcanic sites located in southern Taiwan using 16S rRNA gene sequencing. The physicochemical analysis showed that the samples were slightly alkaline and had elevated levels of Na+, Cl−, and SO42−. Comparatively, the major and trace element contents were distinctly higher, and tended to be increased in the long-period samples. Alpha diversity metrics revealed that the bacterial diversity and abundance were lesser in the initial period, but increased over time. Instead, day 96 and 418 samples showed reduced bacterial abundance, which may have been due to the dry spell that occurred before each sampling. The initial-period samples were significantly abundant in haloalkaliphilic marine-inhabiting, hydrocarbon-degrading bacterial genera such as Marinobacter, Halomonas, Marinobacterium, and Oceanimonas. Sulfur-reducing bacteria such as Desulfurispirillum and Desulfofarcimen were found dominant in the mid-period samples, whereas the methanogenic archaeon Methanosarcina was abundant in the long-period samples. Unfortunately, heavy precipitation encountered during the mid and long periods may have polluted the volcanic site with animal pathogens such as Desulfofarcimen and Erysipelothrix. The functional prediction results showed that lipid biosynthesis and ubiquinol pathways were significantly abundant in the initial days, and the super pathway of glucose and xylose degradation was rich in the long-period samples. The findings of this study highlighted that the temporal effects of a mud volcanic eruption highly influenced the bacterial diversity, abundance, and functional profiles in our study site. [ABSTRACT FROM AUTHOR]

[Display omitted] • Coseismic surface pop-up of the 2016 Meinong earthquake is due to crust volumetric strain change. • The coseismic strain change is characterized by shallow dilatation and underlying contraction. • Mud diapiring under pop-up is likely triggered by 'squeezing" contract strain at base of reservoir. We investigated the 2016 Meinong earthquake (Mw 6.4) in southwestern Taiwan, which caused surface pop-up in an area of 10x15 km2 with maximum uplift of 12 cm, where lies an array of mud volcanoes and possible underlying mud diapir. We calculated 3D strain tensor in a 3D mesh with 5x5x2 km grids in the epicentral area induced by the Coulomb stress change due to coseismic fault slip. We obtained substantial contraction strain (10−5–10−6) that occurred in a lobe showing "squeezing" at the depth of 5–14 km below the surface pop-up area. Dilatation strain (10−5–10−6) occurred at shallow level (0–3 km) with a radial pattern around the surface pop-up area. Combining with local geology, which is composed of Mio-Pliocene ~5-km-thick mudstone in a fold-thrust belt, we interpret that the 2016 Meinong coseismic surface pop-up was closely related to mud diapirs/volcanoes, which were likely reactivated by sudden increase of fluid pore-pressure in the basal reservoir (at 5–6 km depth) and dilatation in the shallow level. We also explored the potential effects of the Coulomb stress transfer on nearby receiver faults – including three arrays of mud diapir, the regional decollement, a suspected backthrust and one thrust close to the pop-up area. Our results show that the Coulomb stress transfer a) favors NNE-trending mud diapirs in the coseismic pop-up area, with a combination of clamping stress changes at 5–6 km depth and unclamping stress changes at 0–4 km depth, and b) it does not favor triggered thrust slip on the regional thrusts. [ABSTRACT FROM AUTHOR]

• Aerobic oxidation of alkanes was observed in hydrothermal fluids during storage. • The residual CH 4 shows the highest ever reported δ13C of up to +243‰. • The ε C for C 1 , C 2 , and C 3 oxidation were −37.1‰, −14.8‰ and −4.7‰, respectively. • The ε H for methane was −281 ± 187‰, while the Λ value was 8.4 ± 4.6. • Aerobic oxidation could produce carbon isotope reversal of the residual alkanes. Aerobic oxidation of short-chain alkanes was observed in gas samples from the Lutao intertidal hydrothermal vents in Taiwan, during storage at 20 °C for up to 29 months without adding bacterial strains and replenishing substrates. The carbon isotope fractionation factors (ε C) of methane (C 1), ethane (C 2), and propane (C 3), were calculated using the Rayleigh fractionation equation to be −37.1 ± 7.5‰, −14.8 ± 4.8‰, and −4.7 ± 5.2‰, respectively. The hydrogen isotope fractionation factor (ε H) of methane was determined to be −281 ± 187‰. DNA sequencing of the 16S rRNA gene in the vent fluids suggests that aerobic oxidation is dominated by methanotrophs of the genera Methylomicrobium and Methylophaga, which use the ribulose monophosphate pathway (RuMP). The degrees of isotope fractionation (ε C and ε H values) herein are larger than previously reported values, possibly due to the limited O 2 supply and low abundance of aerobic methane-oxidizing bacteria in the experiments. Since the fractionation factor of methane is higher than those of ethane and propane, the aerobic oxidation of thermogenic or microbial alkanes could produce a carbon isotope reversal, which is frequently noted as a trait of abiotic hydrocarbons. This work demonstrates that in addition to anaerobic microbial oxidation, aerobic oxidation with a low cell density can also produce significant isotope fractionation of alkanes in geological closed/semi-closed environments and open flow reaction systems that are characterized by moderate temperatures and a limited supply of substrates and O 2 ; these environments include cold seeps, mud volcanoes, and low-temperature hydrothermal plumes/aquifers/reservoirs. [ABSTRACT FROM AUTHOR]

Earthquakes can influence the activity of mud volcanoes and other fluid expulsion systems, which may erupt or change their activity shortly after the seismic event. This study investigates the influence of static and dynamic stresses, produced by some important earthquakes in Taiwan, on fluid expulsion systems that have coseismically responded to such events. Peak dynamic stresses have been estimated at the considered seepage features using available peak ground velocities, while computation of static stress changes has been performed considering the orientation of the feeder systems that are structurally controlling mud volcano development. Feeder dike orientations have been determined on the basis of the geometric characteristics of mud volcanoes (e.g., volcano elongation, vent alignments), and their spatial distribution shows some correlation with regional structures and the active tectonic stress field. The responses of Taiwanese fluid expulsion systems to earthquakes are correlated with the amplitude of dynamic stresses generated by seismic waves, while static stresses have or have not contributed to the triggering depending on the case. This observation suggests that static stresses are unlikely to be a necessary causative mechanism, as seepage systems do respond to earthquakes even when changes in static stress are unfavourable. • Earthquake-related stresses that have influenced the activity of some Taiwan mud volcanoes have been investigated. • Peak ground velocities have been used to evaluate peak dynamic stresses at the considered seepage features. • Static stress changes have been evaluated using the orientation of subsurface feeder dike systems of mud volcanoes. • Responses of seepage features are correlated with the amplitude of dynamic stresses produced by the passing seismic waves. • Static stresses have or have not contributed to the triggering, and thus they are unlikely to be a prerequisite mechanism. [ABSTRACT FROM AUTHOR]

The natural reserve of the Wushangding mud volcano in the Yanchao of Kaohsiung is the first natural reserve for conserving the active mud volcano landscape in Taiwan. In recent years, domestic sightseeing tourism and eco-tourism has increased the pressure on the nature reserve. For a better nature reserve management, it is important to understand the short-term changes of mud volcano landscape and the factors that may cause the changes. Total station and LiDAR techniques were both used for mud volcano landscape monitoring in this reserve before, but time-consuming and high costs make short-term repeat monitoring less possible. With the rapid development of unmanned aerial vehicle (UAV) technology, it is possible to carry out a quick and low budget landscape monitoring with high resolution results for mud volcano landscape.Due to the small area of the Wushangding Mud Volcano natural reserve (less than 5 ha), we used UAV to take photographs from a flying height of 30 meters with >80% image overlap. The latitude, longitude, and elevation of the ground control points are measured by the total station with the coordinate reference system of TWD97. Then the images are processed by the Agisoft Photoscan software to create digital elevation model (DEM) for terrain difference comparison. We will carry out one survey approximately every 2 to 4 weeks for one year, and collect the monthly number of the visitors to the reserve and the meteorological data from the meteorological stations nearby to discuss the main factors that may attribute to the changes of mud volcano topography. [ABSTRACT FROM AUTHOR]

Mud volcanoes are one of the most important conduits for deep seated materials to migrate upward in sedimentary basins, convergent margin, and subduction zones. Understanding its temporal and spatial characteristics and variations provides us the important information on fluid sources and the environments in the depth. Mud volcano Lei-Gong-Huo (LGH) is a unique mud volcano which is located on the Mélange Formation lying on the andesitic volcanic arc. Fluids emitted from 47 mud volcanoes in LGH mud volcano area were sampled and measured their major, trace elements with 87Sr/86Sr ratios monthly form October 2015 to July 2016. Major elements of the fluids are Cl, Na and Ca, which are distributed between 291 and 376 mM, 131 and 289 mM, and 48.9 and 313 mM, respectively. High content of B, Ba, Mn, and Sr with relative low concentration of S and alkalinity are also detected. Comparing with seawater, LGH fluids have lower Na/Cl, K/Cl, and Mg/Cl but higher Ca/Cl ratios, indicating water-rock interaction of igneous rock and paleo-seawater at source region. This interpretation has further supported by Sr isotopes, which show low value of 87Sr/86Sr ratio down to 0.70710. The result of spatial distribution showing strong negative correlation between Na and Ca concentration and Ca and 87Sr/86Sr ratios indicates two end-member mixing is the major chemical characteristic. The fluid interacts with igneous rock carrying high Ca, low Na and low 87Sr/86Sr ratio while those interact with sedimentary rock carrying low Ca, high Na and high 87Sr/86Sr ratio. The source from igneous region dominates eastern part of the mud volcanoes in LGH while sedimentary source dominates western part. Most mud volcanoes show mixing behavior between two sources. Results of time series chemical composition show no significant variation, indicating no source contribution variation during the sampling period. In summary, fluids emitted from mud volcanoes in LGH are originated from two sources, which are water-rock interaction of igneous rock with paleo-seawater from the east and sedimentary rock from the west at depth, resulting from complex geologic background of mélange formation. The contributions from two sources are stable within the period of a year. [ABSTRACT FROM AUTHOR]

The Kenting Plateau has documented unusual topographic low relief surfaces that straddle the topographic crest of the northern Manila accretionary prism. It has a dimension of 35 km x 30 km and the water depth ranges from 60 m to 600 m. In the northern part, the plateau is comprised of an even surface having an average water depth of 500 m while in the SE part, there is an elevated flat area "Kuroshio Knoll", having a water depth of 60-70 m and a dimension of 3 km x 7 km. In order to document the submarine erosion, we have combined bathymetric data, Acoustic Doppler Current Profiler (ADCP) data and reflection seismic data with samples (gravity cores and gravels) collected from the plateau and its nearby perched basins. The southern part of the plateau comprises of hard rocks and relatively few mud diapirs as compared to the northern part which is abundant in mud diapirs along with a few mud volcanoes. One branch of the Kuroshio Current passes through this plateau and enters into the South China Sea. The thickness of the Kuroshio current is around 600 m. The velocity of the Kuroshio Current observed from the ADCP data on the Plateau surface reaches up to 0.5 m/s and up to 1 m/s on top of the Kuroshio Knoll, indicating erosion on the plateau surface, in particular on the Kuroshio Knoll. The P-wave velocity of the gravels collected from the plateau ranges from 3.2 - 4.5 km/s, implying the burial depth of parent rocks of the gravels in the range of 2.3-4 km below the seafloor. The parent rocks have been uplifted due to erosion of the plateau, as a result of isostatic rebound. The sand and silt contents of the cores collected in basins around the plateau show a decreasing trend in grain size away from the plateau, suggesting that submarine erosion is effective, thereby accumulating coarser sediments on the plateau and removing fine particles from the plateau, depositing in distal regions. Along with the erosion-induced isostatic rebound, the seismic lines in the plateau show the doubly-vergent thrusts on both sides of the plateau, which contribute towards the growth of the prism. [ABSTRACT FROM AUTHOR]

Mud volcanoes provide an accessible channel through which deep subsurface environments can be observed. The manner in which deeply sourced materials shape biogeochemical processes and microbial communities in such geological features remains largely unknown. This study characterized redox transitions, biogeochemical fluxes and microbial communities for samples collected from a methane-rich mud volcano in southwestern Taiwan. Our results indicated that oxygen penetration was confined within the upper 4 mm of fluids/muds and counteracted by the oxidation of pyrite, dissolved sulfide, methane and organic matter at various degrees. Beneath the oxic zone, anaerobic sulfur oxidation, sulfate reduction, anaerobic methanotrophy and methanogenesis were compartmentalized into different depths in the pool periphery, forming a metabolic network that efficiently cycles methane and sulfur. Community members affiliated with various Proteobacteria capable of aerobic oxidation of sulfur, methane and methyl compounds were more abundant in the anoxic zone with diminished sulfate and high methane. These findings suggest either the requirement of alternative electron acceptors or a persistent population that once flourished in the oxic zone. Overall, this study demonstrates the distribution pattern for a suite of oxidative and reductive metabolic reactions along a steep redox gradient imposed by deep fluids in a mud volcano ecosystem. [ABSTRACT FROM AUTHOR]