NE Taiwan Research Articles

Neotectonics of the volcanic Kuei-Shan Tao island, and geodynamic implications (NE Taiwan - SW Okinawa Trough)

What are the geodynamic processes during the transition from the onshore NE Taiwan collision to the offshore southwest Okinawa back-arc basin opening associated to the Ryu-Kyu subduction? What is the local neotectonic scheme of this transition (e.g.: structural sketch map of the outcropping volcanic edifice highlighting major faults and their associated earthquakes)? These are some of the fundamental questions adressed to the Kuei-Shan Tao volcanic island (KST hereafter) which is the unique emerged volcanic outcrops situated within this geodynamic transition area.Several incompatible KST geological mappings had been published without any faults, nor dykes, nor feeders (Ichikawa, 1934; Hsu, 1963; and Chiu et al., 2010) that needed to be updated and completed on the structural point of view. In order to do so, we acquired a new high resolution UAS-drone topography (Digital Surface Model) through photogrammetric processing, with a ground resolution <10 cm. We analyse and interprete it in detail using morphostructural photo-interpretation methods. Field works on KST is restricted due to technical and administrative reasons, so we compare our morphostructural map to the shoreline outcrops observed from a boat survey. Then, we have updated the Kuei-Shan Tao geological mapping (lava flows and pyroclastic falls), and the structural scheme as well as the major erosionnal landslide processes. Taking into account the re-interpretation of surroundings offshore bathymetry, old and new seismic profiles, the different drillings done in the Kuei-Chia northern flank, previous geophysical works, the existing massive andesite datings), as well as the inferred stress regimes deduced from the earthquake's focal mechanisms, we propose a KST neotectonic map. We propose also a new scenario for the recent KST volcanic evolution. Kuei-Shan Tao geology and geodynamics may have a so great importance for the 0.5 millions citizens of the so close flat lying Ilan Plain in terms of natural hazards (eruptions, tsunamis, earthquakes…).

Evidence for human-environment interactions as the driver for the abandonment of long-term Neolithic occupation at the Wansan site (NE Taiwan) around the 7th century BCE

The purpose of this study is to investigate how the Neolithic Wansan people in the Yilan Plain in NE Taiwan interacted with their environment from ca. 2070 to 540 BCE. As an island prone to natural disaster, Taiwanese scholars tend to attribute the abandonment of prehistoric settlements to the influence of climate change and overlook the contribution of human activity to environmental change. This research investigates the temporal-spatial variation of the intra-site scale archaeological material excavated at the Neolithic Wansan site, which is located on a small isolated hill, to explore the complexity of how humans interacted with their surroundings on a daily basis. It applies Bayesian modelling to a set of radiocarbon dates to construct a refined chronology for the prehistoric human habitation. Based on the analysis of stratigraphy, radiocarbon dating, and the spatial layout of archaeological features and objects, I demonstrate how the Neolithic Wansan people established the settlement by modifying the sloping terrain into several terraces, carving the bedrock to store goods and bury their deceased, and utilizing rocks from the hill to make tools and assemble coffins, hearths, and other alignments needed for daily life. However, these activities potentially worsened the already fragile sloped environment of this small hill. Together with the rainy climate, the long-term exploitation of this fragile and sloping terrain probably forced the Wansan people to invest time and energy in sustaining their environment. Growing requirements for maintaining the hill environment may have eventually motivated the Neolithic inhabitants to leave their settlement by 540 BCE. Although well-dated paleoenvironmental records from suitable archives covering the Neolithic period for the study region are still scarce, there is initial evidence that continuously increasing winter monsoon precipitation may have further enhanced human-driven erosional processes on the hill. The research thus demonstrates that together with paleoclimate reconstructions, study of archaeological sites can provide us with a more nuanced and direct understanding of the relationship between humans and their environment.

Transformation, Fluxes and Impacts of Dissolved Metals from Shallow Water Hydrothermal Vents on Nearby Ecosystem Offshore of Kueishantao (NE Taiwan)

Hydrothermal vents are one of the important sources of major or trace elements in the ocean. The elemental fluxes, however, may be dynamic due to coastal processes and hydrothermal plumes, especially in shallow-water hydrothermal vents. We collected water samples by using the trace-metal clean technique inside and outside two shallow-water hydrothermal vents (white vent: low temperature, high pH; and yellow vent: high temperature, low pH) off Kueishantao Islet, Taiwan, China via SCUBA divers. We analyzed these samples for their hydro-chemical parameters and dissolved elements (Fe, Mn, Mg, V, Cu, and Mo) thereafter. Our results show that dissolved metals’ concentrations were significantly different between the two vents, with higher Mn and Fe in the White Vent than in the Yellow Vent, likely due to the decreased affinity of the dissolved metals for particles in the white vent. We estimated the plume fluxes of dissolved metals from the hydrothermal mouth by multiplying in situ hydrothermal discharge flowrates with metals’ concentrations inside the vents, which were: 1.09~7.02 × 104 kg Mg, 0.10~1.23 kg Fe, 0.08~28 kg Mn, 33.4~306 g V, 2.89~77.7 g Cu, and 54.3~664 g Mo, annually. The results further indicate that such plumes probably have impacted nearby seawater due to coastal currents and particle desorption during transport. Furthermore, the concentrations of biogenic elements could be further modified in seawater, and potentially impact nearby ecosystems on a larger scale. Our study provides information with which to further understand metal redeployment in submarine shallow nearby ecosystems.

Boron Isotopic Analysis of Representative Atmospheric Aerosols Derived From Long-Range Transported/Local Emission on an Islet Offshore NE Taiwan

To identify pollutant origins and their potential sources from either long-range transported (LRT) or local emissions, the Boron (B) concentrations and the B isotopic compositions (δ11B) in the water-soluble fraction of aerosols were determined. These aerosols were collected from an islet offshore in northeastern Taiwan, Peng Chia Yu (PCY), a non-residential islet, from January 1998 to March 2000. This islet may be influenced by various pollutants or chemical transport during monsoon periods, but suffers minimal human perturbation locally. The B in the specimens falls to 0.3–1.63 ng m−3 during the SW monsoon seasons when compared to 0.46–2.56 ng m−3 in the NE monsoon. However, the δ11B results show no clear variations in both monsoon seasons (10.7–24.3‰), regardless of differences in air mass origin. A two end-member mixing scenario is proposed to explain our observations using the obtained δ11B and 1/[B] results. The ocean endmember is characterized by high 1/[B] and high δ11B; while other endmember is from continental endmember or anthropogenic contributions that are characterized by of 1/[B] and lowδ11B (δ11B &amp;lt; 10‰). Based on these chemical and isotopic results, we found aerosol emissions in northern Taiwan are characterized as low B with nearly constant δ11B, when compared with long-range transported continental endmembers derived from nearby regions. This study provides preliminary B and δ11B levels in aerosols derived from LRT/local emissions and discusses potential monsoonal effects on aerosols offshore of NE Taiwan.

Potential geothermal structure inferred from the electrical resistivity and seismic reflection models in the western Ilan Plain, NE Taiwan

The Ilan Plain is identified as the western extension of the Okinawa Trough in the northeastern Taiwan subduction system and is considered one of the most active geothermal areas in Taiwan. An east-west extending magma-like body caused by the westward of the Okinawa Trough had been revealed from seismic and geomagnetic studies. Two geological models have been suggested as an extensional strike-slip fault and normal fault models for the deep geothermal field in NE Taiwan. The difference of the main fault in the two geology models is the active normal strike-slip fault or inactive thrust fault and its dip directions. We investigated the electrical resistivity structure beneath the Ilan Plain using audio-magnetotelluric data to delineate the location of its geothermal reservoirs. The 3D electrical resistivity model is highly correlated with the normal fault model. The electrical resistivity model shows that the high conductivity anomaly distributes in the depths of 500-1,100 m south of the drilled geothermal observation wells. We correlate the anomalous high conductor with the reservoir and it could be controlled by the Chuoshui Fault and the Chukeng Fault. The meteoric water may propagate down through the high-permeability fault system that creates the geothermal system.

Spatial Variation in Thermal Stress Experienced by Barnacles on Rocky Shores: The Interplay Between Geographic Variation, Tidal Cycles and Microhabitat Temperatures

Thermal stress is an important driver of species’ distribution in the intertidal zone and, with the forecasted increasing frequency of extreme high temperatures associated with climate change, is likely to play an even greater role in the future. To better understand the scales at which thermal stress impacts organisms, we used biomimetic temperature loggers (robobarnacles) to measure latitudinal variation in estimated barnacle body temperatures (Tetraclita spp.) and evaluated the influences of large, geographic, and smaller scale, microhabitat variation on temperatures experienced. Robobarnacles were deployed at nine sites along the West Pacific and South China Sea coast (five sites in Taiwan, three in Hong Kong and one in Thailand, spanning 13-25 N) from May to September 2013. Estimated body temperatures did not follow a latitudinal gradient; instead, they revealed a mosaic of hot (e.g., NE Taiwan and Thailand) and cooler sites (e.g., two sites in Hong Kong). The hot sites were characterized by frequent occurrences of ‘heat stress’ events (estimated body temperatures  40C for  2 hours which would result in  50% Tetraclita entering coma). There was a correlation between hourly air temperatures and robo-temperatures, suggesting that air temperature together with solar radiation and thermal radiation re-emited by the rocky substrate drove the observed spatial robo-temperature variation. Air temperature is, therefore, an important contributor in affecting the body temperature of sessile intertidal species in the tropical and subtropical W Pacific and South China Sea and can be a good predictor for body temperature and thermal stress of intertidal barnacles.

Biogeochemistry and dynamics of particulate organic matter in a shallow-water hydrothermal field (Kueishantao Islet, NE Taiwan)

The particulate organic matter (POM) of shallow-water hydrothermal fields has been studied in the context of food web reconstruction, but the processes governing its biogeochemistry and dynamics are poorly explored. Here, we investigate the POM in the Kueishantao hydrothermal field using chemical and hydrodynamic approaches. The depletion of total suspended matter, lower C/N ratios, and higher carbon isotopic values of particulate organic carbon (δ13CPOC) in the vertical plumes relative to values derived from Si-based models were attributed to the hydraulic sorting of the vented particles, which tend to have more N and 13C in the fine fraction. The particulate organic carbon (POC), unlike the total suspended matter, was enriched in the vertical plumes and explained by physicochemical processes rather than biological addition. The POC-enriched plume-top water was found to be a better endmember than the vent fluids to explain particle mixing in the lateral plume. Physical mixing played a steering role in shaping the particle chemistry of the lateral plumes, but markedly 13C-enriched POC was still observable in several near-vent, low-to-intermediate-Si plume waters, implying locally enhanced primary production of at least 0.1–0.4 mg C/m3/h. The presence of eddies, confirmed by flow field measurements, should have contributed to the detection of biogeochemical anomalies via extending the retention time of plume water to 1–2 h. The dominating mixing process resulted in decoupling between the δ13CPOC signatures and carbonate chemistry in this shallow-water hydrothermal plume.

An analysis of the subsurface fault systems with audio-magnetotelluric surveys in the western Ilan Plain of NE Taiwan

An analysis of the subsurface fault systems with audio-magnetotelluric surveys in the western Ilan Plain of NE Taiwan

High‐Resolution 3‐D P Wave Velocity Structures Under NE Taiwan and Their Tectonic Implications

AbstractFirst P wave arrival‐time data from local earthquakes recorded by a dense geophone array deployed on the Ilan Plain and by existing permanent stations were combined to invert for high‐resolution P wave velocity structures under northeast Taiwan. With relatively high resolution, we were able to examine the structures in more detail and to investigate their significance and tectonic implications. We introduce two distinct groups of proposals for mechanisms of subduction polarity flipping in Taiwan, referred to as the “tear model” and “breakoff model.” While the predicted Philippine Sea Plate boundaries differ between the two models, those of the breakoff model and the related subducting indenter model are geodynamically similar. The surface junction and the west edge of the imaged high Vp anomalous Philippine Sea Plate comply with those predicted by the subducting indenter model and thus favor the breakoff model over the tear model. While the observed high Vp anomalous region in the mantle wedge can be explained as eclogitization of previously subducted crust, eclogitization of the overriding continental crustal roots cannot be ruled out. Those beneath the Taipei Basin and the Tatun Volcano Group exhibit a pattern potentially connected to the low Vp anomalies in the mantle wedge, suggesting the involvement of the Philippine Sea slab, either by asthenospheric upwelling due to extensional collapse or by fluid migration due to slab dehydration. Those beneath the Ilan Plain exhibit a low Vp pattern extending to deeper origins in the eastern offshore region, suggesting a connection with the opening of the Okinawa Trough.

Evaluations of an ocean bottom electro-magnetometer and preliminary results offshore NE Taiwan

Abstract. The first stage of field experiments involving the design and construction of a low-power consumption ocean bottom electro-magnetometer (OBEM) has been completed, which can be deployed for more than 180 d on the seafloor with a time drift of less than 0.95 ppm. To improve the performance of the OBEM, we rigorously evaluated each of its units, e.g., the data loggers, acoustic parts, internal wirings, and magnetic and electric sensors, to eliminate unwanted events such as unrecovered or incomplete data. The first offshore deployment of the OBEM together with ocean bottom seismographs (OBSs) was performed in NE Taiwan, where the water depth is approximately 1400 m. The total intensity of the magnetic field (TMF) measured by the OBEM varied in the range of 44 100–44 150 nT, which corresponded to the proton magnetometer measurements. The daily variations in the magnetic field were recorded using the two horizontal components of the OBEM magnetic sensor. We found that the inclinations and magnetic data of the OBEM varied with two observed earthquakes when compared to the OBS data. The potential fields of the OBEM were slightly, but not obviously, affected by the earthquakes.

Volatile Chalcophile Elements in Native Sulfur from a Submarine Hydrothermal System at Kueishantao, Offshore NE Taiwan

We analyzed sulfur isotopes, trace elements and chalcophile elements (Se, Te, As, Sb, and Hg) in the native sulfur matrix from the Kueishantao hydrothermal system and conducted a systematic micro-analytical investigation. The sulfur matrix lacked all measured metals (e.g., Fe, Cu) and rare earth elements (REEs) while being significantly enriched in Te, As, Se (750–1500 ppm), Sb (around 100 ppm) and some Hg. The δ34S data (0.2–2.4‰) suggest a magmatic source leached from igneous rocks and a small contribution of seawater sulfates to the sulfur in hydrothermal deposits. Correlations between Te, As, Sb, and S (r2 = 0.30–0.61) indicate that these elements behave coherently in magmatic-hydrothermal processes. The enrichment factors and content ratios of these elements demonstrate their abundance in the sulfur matrix and minor fractionation after being partitioned into the metallic melt and forming a separate vapor phase to transport. Our study focuses on the native sulfur matrix in a shallow-water volcanic hydrothermal system, to which relatively little attention has previously been paid. This will expand our understanding of hydrothermal precipitates. The study of volatile chalcophile elements in the matrix will provide significant information about their sources, distributions and other geochemical behaviors in magmatic-hydrothermal processes and help to understand the Kueishantao hydrothermal circulation better.

Fates of vent CO2 and its impact on carbonate chemistry in the shallow-water hydrothermal field offshore Kueishantao Islet, NE Taiwan

Increasing public awareness of anthropogenic CO2 emissions and consequent global change has stimulated the development of pragmatic approaches for the study of shallow-water CO2 vents and seeps as natural laboratories of CO2 perturbations. How CO2 propagates from the emission sites into surrounding environments (ocean and atmosphere), and its effects on seawater carbonate chemistry, have never been studied from a mechanistic perspective. Here, we combine experimental and modeling approaches to investigate the carbonate chemistry of a shallow-water hydrothermal field offshore Kueishantao Islet, NE Taiwan. A simple Si-based mixing model is used to trace hydrothermal fluid mixing with seawater along convection pathways. The estimated vent fluid component in the near-vent region is generally <1%. We further employed a modified bubble-plume model to examine gas bubble-aqueous phase interaction. We explain the dissolved inorganic carbon characteristics of the vertical plume as a synergistic interaction between CO2 gas dissolution and fluid entrainment. The bubble-plume model provides a conservative estimate of the flushing time (tens of minutes) for water in the near-vent region. The acidic, dissolved inorganic carbon-rich water in the lateral buoyant plume readily releases CO2, but mixing with seawater rapidly quenches its degassing potential, so that hydrothermal carbon is retained in the ocean. Ebullition, governed by initial bubble size distribution, is the key mechanism for vent CO2 to exit the seawater carbonate system.

Crustal stress field in Ilan Plain, NE Taiwan and the westernmost Okinawa trough-deduced from seismic stress inversion

Crustal stress field in Ilan Plain, NE Taiwan and the westernmost Okinawa trough-deduced from seismic stress inversion

Pseudo-3D seismic imaging of Geolin Mounds hydrothermal field in the Southern Okinawa Trough offshore NE Taiwan

Pseudo-3D seismic imaging of Geolin Mounds hydrothermal field in the Southern Okinawa Trough offshore NE Taiwan

Benthic foraminiferal fauna and sediment provenance of Eocene syn-rift sequences in Taiwan: implication for onset of Asian epi-continental marginal seas off China coast

Early Cenozoic rift basins developed commonly on the Mesozoic basement along the SE Asia Continent. However, Eocene–Oligocene sequences were only exposed widely onshore Taiwan. We study benthic foraminifera and determine U–Pb geochronology of detrital zircon grains in the Early Cenozoic sequence onshore Taiwan for understanding if there was any significant change in faunal content and sediment provenance between the Eocene syn-rift sequences and the overlying Oligocene post-rift strata across the breakup unconformity. The Eocene syn-rift sequences are mostly of thick, cross-bedded sandstones of fluvial-deltaic depositions without marine fauna. Intensive study on the Eocene stratigraphy in more than ten sections all over the Taiwan mountain belt in the last three decades, shallow marine benthic foraminifera of Gaudryina hayasakai-Hanzawaia asterizana-Planularia okinoshimaensis-Heterolepa praecinta assemblage were found only in very limited exposures in the Western Foothills, central Taiwan. This study shows that the Eocene benthic foraminiferal assemblage is similar to the Shihtsaoan fauna in the overlying Oligocene post-rift marine strata. This indicates no significant faunal break across the breakup unconformity. As a consequence, Gaudryina hayasakai Zone of the Shihtsaoan age can extend downward to the Eocene and the Shihtsaoan Stage would be equivalent to the Hsuehshanian Stage established previously from northern Taiwan. The Taiwan Eocene marine sequences could be deposited in a normal-fault bounded narrow seaway or gulf connecting southeastward to the open Huatung Basin. U–Pb geochronology of detrital zircons in the Taiwan Eocene syn-rift sandstones shows a prominent age peak centered in Yenshanian (100–155 Ma) similar to the Paleocene–Oligocene fluvial-deltaic deposits in the East China Sea and is also comparable to the U–Pb zircon age spectrum of modern river sands of the Fujian Province, SE China. Petrographic study also suggests a prevailing sediment provenance of the Cathaysia granites and volcanics. These Cathaysia detritals were transported to the Taiwan Eocene grabens through some SE-flowing rivers. Structural reconstructions in previous study indicated that the mountain belt with the Eocene sequences in NE Taiwan had been shortened by imbricated thrusting westward for 160–200 km wide during the arc-continent collision in the last 6.5 Ma. Integrating paleontology and sedimentology evidence reveal that the Eocene coast was located along the modern shelf break of the East China Sea ~ 350–400 km east off the modern Chinese coast line. This implies that in the Eocene time (~ 40 Ma) there was no wide Asian epi-continental marginal seas off Chinese coast until the onset sedimentation of the Oligocene post-rift strata during opening of the South China Sea in the Early Oligocene time (~ 33 Ma).

Magmatic pulses of the Tatun Volcano Group, northern Taiwan, revisited: Constraints from zircon U-Pb ages and Hf isotopes

The initiation and duration of magmatism in the Tatun Volcano Group (TVG), likely resulting from extensional collapse of the northern Taiwan mountain belt, remains an important issue of debates in the Taiwan orogeny. Here we report a detailed study of U-Pb ages determined by SIMS and LA-ICPMS, together with Hf isotopic data, of zircon separates from a total of 22 extrusive rocks from the TVG lava flow and additional two andesites from Keelungyu (KLY), a volcanic islet off NE Taiwan. Magmatic zircons from the TVG are characterized by uniform and high Hf isotopic ratios, with εHf(T) mostly from +20 to +10. Their overall 206Pb/238U ages suggest that the TVG magmatism was initiated ∼3 Ma but more active ≤0.8 Ma with a flare up ≤0.35 Ma. Magmatic zircons from the two KLY samples reveal identical 206Pb/238U ages (0.41 Ma), coupled with Hf isotopes [εHf(T): +25 to +9] comparable to those of the TVG. Thus, KLY is proposed as part of the TVG. Consequently, we argue that the main body of the TVG formed ≤0.8 Ma by post-collisional volcanism, but the mechanism resulting in the magmatic episodes ≥1.0 Ma remains obscure due to the scarcity of rock exposures.

Active normal faults and submarine landslides in the Keelung Shelf off NE Taiwan

Active normal faults and submarine landslides in the Keelung Shelf off NE Taiwan

Monitoring interseismic activity on the Ilan Plain (NE Taiwan) using Small Baseline PS-InSAR, GPS and leveling measurements: partitioning from arc-continent collision and backarc extension

Monitoring interseismic activity on the Ilan Plain (NE Taiwan) using Small Baseline PS-InSAR, GPS and leveling measurements: partitioning from arc-continent collision and backarc extension

Magmatic-like fluid source of the Chingshui geothermal field, NE Taiwan evidenced by carbonate clumped-isotope paleothermometry

The Chingshui geothermal field, a moderate-temperature and water-dominated hydrothermal system, was the site of the first geothermal power plant in Taiwan. Many geological, geophysical and geochemical studies using more than 21 drilled wells have been performed since the 1960s. However, there are still controversies regarding the heat and fluid sources due to the tectonically complicated geological setting. To clarify the heat and fluid sources, we analyzed clumped isotopes with carbon and oxygen isotopic compositions of calcite scaling in geothermal wells and veins on outcrops and calculated the δ18O values of the source fluids. Two populations of δ18O values were calculated: −5.8±0.8‰ VSMOW from scaling in the well and −1.0±1.6‰ to 10.0±1.3‰ VSMOW from outcropping calcite veins, indicative of meteoric and magmatic fluid sources, respectively. Meanwhile, two hydrothermal reservoirs at different depths have been identified by magnetotelluric (MT) imaging with micro-seismicity underneath this area. As a result, we propose a two-reservoir model: the shallow reservoir provides fluids from meteoric water for the scaling sampled from wells, whereas the deep reservoir provides magmatic fluids from deep marble decarbonization recorded in outcropping calcite veins.

Disentangling Natural and Anthropogenic Signals in Lacustrine Records: An Example from the Ilan Plain, NE Taiwan

The impact of human activities has been increasing to a degree where humans now outcompete many natural processes. When interpreting environmental and climatic changes recorded in natural archives on historical time scales, it is therefore important to be able to disentangle the relative contribution of natural and anthropogenic processes. Lake Meihua on the Ilan Plain in northeastern Taiwan offers a particularly suitable opportunity to test how human activities known from historical records can be recorded in lacustrine sediment. For this purpose, three cores from Lake Meihua have been studied by a multiproxy approach, providing the first decadal-resolution lacustrine records covering the past 150 years in Taiwan. Profiles of excess 210Pb, 137Cs and 239,240Pu from two short cores (MHL-09-01 and MHL-11-02) allowed a precise chronology to be established. The presence of a yellow, earthy layer with lower levels of organic material coincide with the record of land development associated with the construction of the San-Chin-Gong Temple during AD 1970-1982. Furthermore, in the lower part of the cores, the upwards increasing trend of inc/coh, TOC, TOC/TN, and grain size, coupled with the palynological data (increase of Alnus, Mallotus, Trema and herbs) from the nearby core MHL-5A with radiocarbon chronology, suggest that the area surrounding the lake has been significantly affected by agricultural activities since the arrival of Chinese settlers around ~AD 1874. In sum, this study demonstrates that this suite of lacustrine sediments in northeastern Taiwan has recorded human activities in agreement with historical documents, and that different human activities will leave distinct sedimentological, geochemical, and palynological signatures in the sedimentary archives. Therefore, multiproxy reconstructions are important to capture the complex nature of human-environmental interactions. A better understanding of the weathering and erosion response to human activities can also provide useful information for sustainable land-use management.