Department of Geoscience

We discuss a radiocarbon study of sediment samples collected from Nanfu terrace in western Taiwan. From these, we extracted humic acids (HA) and humin from the very fine and coarse grain-size fractions using a standard acid- alkali-acid pretreatment. The humin extracts were combusted at 400 and 1100oC by stepped-combustion, to yield a low-temperature (LT) carbon component and a high-temperature (HT) carbon component. We compare the ages of the LT and HT humin fractions to the HA fractions, in samples collected at 2 depths within the Nanfu terrace. As in previous stepped-combustion studies on sediments, we find that the HA ages are the youngest on average, and overlap the LT ages, and that the carbon contained in the HT fraction is always distinctly older than the LT and HA ages. To better understand the relationship between 14C age and combustion temperature, we conducted an incremental stepped-combustion experiment with one of the samples (1E) using 50oC steps that ranged from 300 to 1100oC. The 14C results of the stepped-combustion products show a clear division between 2 isotopically identifiable carbon constituents, from carbon released below 400oC and carbon released above 550oC. By comparing the d13C and 14C results, we find evidence for a third carbon isotopic component in the humin that is released when combusted at ~500oC.

Radiocarbon has been widely used in determining the age of various geological materials and events. The validity of ages, particularly for sediments, has often been questioned due to the incorporation of carbon-bearing components with different abundances of 13 C and 14 C. This study examines carbon contained in clay minerals (kaolinite, illite and illite-smectite) extracted by temperature-stepped combustion experiments , ranging from 350 C to 1100 C. Distinct carbon components were identified in the samples using both their 13 C and 14 C contents. Two carbon end-members were observed for all of the clay minerals: a low temperature component (<400 C), relatively depleted in 13 C and enriched in 14 C; and a high temperature component (>550 C), relatively enriched in 13 C and depleted in 14 C. The high temperature carbon component is relatively old and not released until the clay is completely oxidized. This observation suggests that this carbon was incorporated into the clay mineral when it first formed by weathering. We applied the same procedure to sediments collected from terraces along the Kueichung River, in southwestern Taiwan, sieved to <64 mm. We obtained results similar to the pure clay minerals, with distinct low temperature (relatively young) and high temperature (relatively old) end members. One of the sediments also contained a third carbon component, possibly detrital carbon. The radiocarbon results on these various carbon fractions provide a range of dates enabling us to place constraints on the timing of terrace development in Taiwan.

In this article, we present a study of seawater 129I/127I time-series data from several coastal sites in Taiwan, including Yehliu Geopark (north–northeast coast), Kaohsiung (southwest coast), and Zhuwei Fishing Village (northwest coast). The objective of this study was to document how 129I/127I responds to known seasonal variations in the surface ocean currents that carry 129I to each of these sites. The responses were shown to be quite distinct. The Zhuwei site, across Mainland China, had elevated spring and summer 129I/127I values, with abrupt peaks that reflected transient cross-strait currents that carry seawater with elevated 129I to the northwest coast of Taiwan. The Yehliu site, which receives all of its seawater from the Kuroshio Current, had relatively low and uniform 129I/127I values year-round. The Kaohsiung site showed a summertime minimum that likely stems from upwelling upstream induced by oceanic eddies associated with the Kuroshio intrusion. To our knowledge, these a...

The chemical composition of the Gaoping River in Taiwan reflects the weathering of both silicate and carbonate rocks found in its metasedimentary catchment. Major dissolved ion chemistry and radiocarbon signatures of dissolved inorganic carbon (DIC) reveal the importance of pyrite-derived sulphuric acid weathering on silicates and carbonates. Two-thirds of the dissolved load of the Gaoping River derives from sulphuric acid-mediated weathering of rocks within its catchment. This is reflected in the lowest reported signatures DI 14 C for a small mountainous river (43 to 71 percent modern carbon), with rock-derived carbonate constituting a 14 C-free DIC source. Using an inverse modelling approach integrating riverine major dissolved ion chemistry and DI 14 C, we provide quantitative constraints of mineral weathering pathways and calculate atmospheric CO 2 fluxes resulting from the erosion of the Taiwan orogeny over geological timescales. The results reveal that weathering on Taiwan releases 0.31 ± 0.12 MtC/yr, which is offset by burial of terrestrial biospheric organic carbon in offshore sediments. The latter tips the balance with respect to the total CO 2 budget of Taiwan such that the overall system acts as a net sink, with 0.24 ± 0.13 MtC/yr of atmospheric CO 2 consumed over geological timescales.

With uranium rich skeletons and density bands similar to their surface coral counterparts, deep-sea scleractinia are a promising archive of past climate. To improve the utility of fossil samples as monitors of deep ocean variability, we have measured 210 Pb and 226 Ra activities in a variety of modern specimens to constrain the range of growth rates. Mechanical and chemical cleaning of each sample are required to isolate the radionuclides trapped in the coral skeleton from surface contaminants. However, in many cases mechanically cleaned samples show the same overall growth rate as parallel transects of samples subjected to the full chemical and mechanical cleaning but with much higher overall activities. Three samples of Desmophyllum cristagalli show a range of vertical extension rates from 0.5 mm/yr to 2 mm/yr, consistent with previous estimates. A single Enallopsammia rostrata from the North Atlantic is over 100 years old. Its average radial growth rate is 0.07 mm/yr, and the clear banding in this direction is not consistent with annual periodicity. A minimum vertical extension rate of 5 mm/yr is estimated from the 210 Pb data. Both of these species are found in the fossil record and, with the growth rates determined here, can record about 100 years of climate change. The growth rates will allow the reconstruction of climate at subdecadal resolution in D. cristagalli and even higher resolution in E. rostrata.

In this study we use micro-sampling techniques to explore diagenetic processes in carbonates. These processes are important as they can affect the accuracy of U-series chronometry. Fission track maps of deep-sea scleractinian corals show a 3-fold difference between the minimum and maximum [U] in modern corals, which is reduced to a factor of 2 in fossil corals. We use micromilling and MC-ICP-MS to make detailed analyses of the [U] and δ 234 U initial distributions in corals from 218 ka to modern. Within each fossil coral we observe a large range of δ 234 U initial values, with high δ 234 U initial values typically associated with low [U]. A simple model shows that this observation is best explained by preferential movement of alpha-decay produced 234 U atoms (alpharecoil diffusion). Open-system addition of 234 U may occur when alpha-recoil diffusion is coupled with a high [U] surface layer, such as organic material. This process can result in large, whole-coral δ 234 U initial elevations with little effect on the final age. The diagenetic pathways that we model are relevant both to shallow-water and deep-sea scleractinian corals since both exhibit primary [U] heterogeneity and may be subject to U-addition. Lab ID Database ID 238 U (ppm) 230 Th (ppt) 232 Th (ppb) δ 234 U initial, raw (‰) (232 Th/ 238 U) act Age raw (yrs) Age corr (yrs) δ 234 U initial, corr (‰)

The Okinawa Trough is a young, intracontinental backarc basin that has formed behind the Ryukyu arc±trench system since late Miocene time. In the Southernmost Part of the Okinawa Trough (SPOT), a cluster of active submarine volcanoes delineates a volcanic belt, which is located only *100 km above the Wadati± Benioff zone. We report herein new major and trace element data for the SPOT volcanic rocks. These rocks show a compositional range from medium-K andesite to rhyolite. Their geochemical characteristics are similar to those of pre-backarc rifting volcanic rocks from the central Ryukyu arc, and different from those of backarc basin lavas from the Middle Okinawa Trough and the post-backarc rifting Ryukyu arc volcanics. Therefore, despite being topographically contiguous with the rest of the Trough, the SPOT that developed in the Quaternary is not a simple backarc basin but instead an embryonic rift zone in which early arc volcanism occurs as a result of the Ryukyu subduction.

Radiocarbon (14 C) is a useful tracer for surface ocean circulation and mixing, which reflects air-sea CO 2 exchange. We present radiocarbon marine reservoir ages (R) and corrections (ΔR) in Holocene inferred from 18 paired 14 C and 230 Th ages on fossil corals from Lanyu Island offshore eastern Taiwan. The results show large fluctuations in the ΔR value, with averages of-330 and-5 14 C yr for 6000-5100 yr BP and the past 150 years, respectively. The extremely young R in the mid-Holocene indicate a well-equilibrated North Equatorial Current (NEC), likely stemmed from enhanced air-sea interactions and strengthened Pacific Walker circulation. This suggests a larger E-W gradient across the Equatorial Pacific and hence a La Niña-like condition, consistent with both model simulations and other paleo-proxy records. Combining the ΔR records in the northern South China Sea, the results imply an increasing influence of the NEC water on the subtropical western Pacific since the mid-Holocene.

In the southernmost Ryukyu subduction zone, slab‐guiding behavior from intermediate‐depth earthquakes is well documented with a low‐frequency (&lt;2 Hz) first P arrival followed by sustained high‐frequency (3–10 Hz) wave trains. Such waves developed by propagating along a long path within the slab are expected to have high sensitivity to anisotropy within the slab. We determine shear wave splitting parameters from 178 intraplate events that are deeper than 100 km. The possible slab‐anisotropy‐associated polarization pattern shows the fast direction at N65°E and delay time of 0.13–0.45 s. This is stronger than the previously documented crust effect (&lt;0.1 s), similar to the mantle wedge effect (0.28 s in average), but weaker than the upper mantle effect (1.3 s in average) in Taiwan. The fast axis reflects the fossil spreading direction of Philippine Sea plate with minor clockwise rotation due to the collision to Eurasian plate.

The February 2018 M w 6.4 earthquake in eastern Taiwan caused extensive damage in Hualien City. Although damaging earthquakes are common in this region, there are relatively few permanent seismic stations deployed. Two days after the mainshock, we deployed 70 temporary seismic stations around Hualien City for 12 days (8-19 February), and station spacing was 1-5 km. During this time, 2192 aftershocks were located from which 580 focal mechanisms were determined. The aftershock sequence extended about 25 km southwestward from the epicenter of the mainshock into the Longitudinal Valley and to depths between 5 and 15 km. Earthquake hypocenters indicate that the aftershocks took place along a near-vertical to steeply west-dipping plane in the north that becomes more diffuse in the south. Focal mechanisms are predominantly extensional, different than the left-lateral strike slip with thrust-component faulting of the mainshock. Very few events occurred in the uppermost crust at depths of less than 5 km, and their focal mechanisms are left-lateral strike-slip faulting. From the mainshock to the aftershocks, the stress changed rapidly from a north-northwest-oriented compressional axis (P axis) to the same direction for the extensional axis (T axis). Electronic Supplement: Tables of aftershocks and focal mechanisms obtained in this study and figures of M w determination via manual spectral analysis and result of automatic spectral analysis.

We perform a systematic detection of deep tectonic tremor beneath the southern Central Range in Taiwan using a dense, small‐aperture seismic array. Deployed in February 2011, the array has been recording tectonic tremor nearly continuously for 134 days, including tremor triggered by the 2011 Mw 9.0 Tohoku‐Oki earthquake. We use broadband frequency–wavenumber beamforming and moving‐window grid‐search methods to compute array parameters for all continuous data and identify tremor as coherent nonimpulsive seismic signals with deep incidence angles. The obtained array parameters closely match with those of relocated local earthquakes and triggered tremor bursts located by the waveform envelope correlation and clustering (WECC) method, indicating the robustness of our array technique. During the 134‐day study period, we detect tremor for 44 days, with a total duration of 1481 min, three to six times as long as the detection of tremor by the WECC method. We find a relative quiescence in ambient tremor activity for about 20 days following the 2011 Tohoku‐Oki earthquake, suggesting that dynamic stresses from the distant mainshock triggered most tremor close to failure, resulting in a temporary lack of tremor activity. In some cases, we observe rapid tremor migration with a speed at the order of 40–50  km/hr that is similar to the speed of fast tremor migration along dip on narrow streaks in Japan and Cascadia. Our results suggest that dense array techniques are capable of capturing detailed spatiotemporal evolutions of tremor behaviors in southern Taiwan.

M w 6.4 Meinong earthquake occurred on 6 February 2016 in southern Taiwan, resulting in more than one hundred casualties and several collapsed buildings. The aftershocks occurred mostly at mid-to-lower crustal depths (10-30 km), related to a blind fault system. However, several centimeters of cosesimic surface uplift within the Liushuang, Erhchungli, and GutingKeng Formations, composed mainly of mudstone, was recorded from the InSAR results. The uplifted pattern is similar to that of GPS and leveling data from 2000-2010, which indicates the deformation is accomplished by creeping due to the shallow mudstone structure related to mud diapir. Previous studies have shown limited information about the shallow structure in this region due to few deployed seismic stations. We deployed 36 temporary seismic stations (~5 km spacing) in this study around one month after the main shock to obtain a 3-D shear wave shallow crustal velocity structure using ambient noise tomography. The reliable periods of group and phase velocities from Rayleigh waves were 0.6-5 s, corresponding to around 0-5 km at depths. As a result, the low S-wave pattern speeds at 0-4 km depths correspond to the uplift region from both InSAR data for the coseismic period and GPS and leveling data for the interseismic period. The results from this study are compatible with the reflected seismic profile. The results show that with dense seismic array deployment we can obtain high subsurface image resolution to link the relationship between the surface observations to the subsurface structures.

and EAS for their support and encouragement. I would like to take this opportunity to thank many people in several institutes in Taiwan and Japan for helping with the field deployment and providing valuable data used in this study: for the earthquake swarm catalog. I also wish to thank the Ministry of Education of Taiwan for proving the government scholarship to support my study at Georgia Tech. Last and not least, I would like to acknowledge the support provided by my dear family and my former mentors: Dr.

The Solomon Islands is one of the most seismically active areas in the southern Pacific with high earthquake hazard potential. The regional seismic network, equipped with six broadband seismic stations, was constructed as late as October 2018. On January 27 and 29, 2020, two moderate earthquakes, Mw 6.3 and 6.0, respectively, occurred in the southeastern Solomon Islands. The entire foreshock-main-shock-aftershock sequence was recorded by this seismic network for exploring the seismogenic structures. Based on the spacial distribution of the foreshock-aftershock sequence, the interaction of the subduction and transform zones between the Pacific and the Australia plates could lead to the near-vertical dip-slip tear slab. Confirmed with PREM and the new 1D velocity model for testing the robustness of the earthquake locations, a seismic gap at depths from 25 to 35 km is observed as the “jelly sandwich” rheology of the continental crust of the Australia plate.

The Solomon Islands is one of the most seismically active areas in the southern Pacific with high earthquake hazard potential. The regional seismic network, equipped with six broadband seismic stations, was constructed as late as October 2018. On January 27 and 29, 2020, two moderate earthquakes, Mw 6.3 and 6.0, respectively, occurred in the southeastern Solomon Islands. The entire foreshock-main-shock-aftershock sequence was recorded by this seismic network for exploring the seismogenic structures. Based on the spacial distribution of the foreshock-aftershock sequence, the interaction of the subduction and transform zones between the Pacific and the Australia plates could lead to the near-vertical dip-slip tear slab. Confirmed with PREM and the new 1D velocity model for testing the robustness of the earthquake locations, a seismic gap at depths from 25 to 35 km is observed as the "jelly sandwich" rheology of the continental crust of the Australia plate.

On 18 September 2022, the M W 6.9 Chihshang earthquake struck the south half of the Longitudinal Valley, Taiwan, and caused severe damage. A precise and rapid report for the distribution of aftershock sequence after a devastating earthquake provides key information for deciphering the seismogenic structure in the source region. The utilization of deep-learning methodologies for earthquake event detection offers a significant acceleration in data analysis. In this study, we use SeisBlue, a deep-learning platform/package, to extract the whole earthquake sequence from September to October 2022, including the M W 6.5 Guanshan foreshock, the M W 6.9 mainshock, over 14,000 aftershocks, and 866 foal mechanisms from two sets of broadband networks. After applying hypoDD for earthquakes, the distribution of aftershock sequence clearly depicts not only the Central Range Fault and the Longitudinal Valley Fault but also several local, shallow tectonic structures that have not been observed along the southern Longitudinal Valley.

The first 30 min of the aftershock sequence following the 2022 September 17, M L 6.4, Guanshan, Taiwan earthquake is investigated by the back-projection (BP) method. Based on the Automatic Gain Control (AGC) to process individual earthquake record and sort waveforms by epicentral distance, 80 events are identified near the Guanshan earthquake epicenter to verify its temporal variation. However, most of these events cannot clearly image by the BP method to determine its spatial locations for its small amplitude and short time separation. In total, 29 aftershocks are imaged by this study to identify its spatial locations during this time period. The relocated events are compared with the Central Weather Bureau (CWB) rapid report catalogue. Only 5 events have been reported by the CWB rapid report catalogue. These unreported hidden aftershocks are identified to distribute surrounding the epicenter and extended to the east. This extracted information of hidden aftershocks is crucial for assessing the potential for future large aftershocks and estimating the associated seismic hazards in a given region, and provide extra message to evaluate the source physics in general. 1. Propose and verify earthquake location using the back-projection method. 2. Discover 29 hidden events with 30 min after the Guanshan earthquake. 3. Provide extra information to study following large aftershocks and to estimate its associated seismic hazards.

Sedimentary nitrogen isotope (d 15 N sed) in Santa Barbara Basin (SBB) has been mostly interpreted as the record of the eastern tropical North Pacific (ETNP) intermediate water denitrification process. Nevertheless, debate remains regarding sources and control mechanisms of d 15 N sed signal in SBB. Multiproxy analyses including d 15 N sed , total organic carbon (TOC), total nitrogen (TN), C/N ratio, and marine biomarkers were performed on a 46-cm sediment core (SBB-190629) collected from SBB in 2019. The core was dated with varve counting and 210 Pb dating method, showing a depositional history of 1938-2019 CE with a sedimentation rate of 0.564 cm/year. The findings show that the d 15 N sed record (at~0.25-year resolution) ranges from 6.24‰ to 7.43‰, which was affected by both local and remote forcing. The long-term variations of the SBB d 15 N sed signature show a general decreasing trend from 1940 to the late 1980s, low values during 1980~2000, and an increase afterward, which is thought to reflect changes in ETNP denitrification induced by the strength of tropical trade winds. Our results also reveal a series of abrupt annual to multiannual changes, superimposed on the long-term variation mentioned above. The SBB local d 15 N signal (Dd 15 N SBB) is accessed by using the deviation from the mean d 15 N (Dd 15 N) of SBB-190629 to subtract the Dd 15 N of the ETNP. The Dd 15 N SBB record compares well with redox-sensitive proxies (Re/Mo ratio and C 29 stanol/stenol ratio) from the SBB bottom water and with the OC marine content calculated based on the C/N ratio mixing model, indicating that the Dd 15 N SBB is mainly controlled by bottom water denitrification, which was induced by the change of upwelling intensity and marine productivity. Since Frontiers in Marine Science frontiersin.org 01