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The first shallow hydrate sample off southwest Taiwan has been collected at core site MD18-3542 in cruise MD214. Attractive features are present at ~5.5 m core depth where an unconformity lies, and sediment properties are distinctively different above and below the layer. In the work, we perform rock magnetic analyses to study the core magnetic behavior, including magnetic susceptibility (MS), hysteresis parameters, and X-ray diffraction spectra. Combining the core features and rock magnetic results with the shallow hydrate formation off southwest Taiwan, we consider that variation of the core magnetic property could relate to the pyritization of magnetic minerals caused by the dissociated gas, and summarize the gas distribution at the shallow strata: Abundant methane with hydrogen sulfide trapped below the unconformity could form an anoxic setting and activate the pyritization. Dissociated gas may have also leaked to its above layer, featuring the MS anomaly at ~3.5 m core depth. Magnetite appears in addition to iron sulfides at the depth between ~6.5 and ~10.0 m, indicating a transition layer and implying the insufficiency of the dissociated gas to complete the pyritization. Iron sulfides become dominant again at the core bottom, revealing that the dissociated gas mainly migrates from deep strata.

Changes in Earth’s eccentricity, obliquity, and precession can cause quasi-cyclic variations in Earth’s climate that may be preserved in sedimentary archives. Recent research has shown that shallow-marine paleoclimate archives in mid- to low-latitude regions have the potential to preserve changes in hydroclimate driven by precession, given sufficient space for sediment accumulation and a high sedimentation rate. Our integrated stratigraphic study of the Kueichulin Formation in Taiwan’s Western Foreland Basin (WFB) uses magnetostratigraphy, biostratigraphy and astrochronology to constrain time, with the aim to assess how the evolution of a rapidly uplifting mountain range affected the preservation of climate cycles in the shallow-marine record.Using time-series analysis of two sets of gamma-ray borehole data from the late Miocene to Pliocene Kueichulin Formation (WFB), we found that despite increasing monsoon intensities between 8 and 3 Ma, the preservation of precession-driven East Asian Summer Monsoon variability was low during the early stages of Taiwan orogenesis (before 5.4 Ma). Prior to 5.4 Ma, the Taiwan Strait had not yet formed. Consequently, the southeastern margin of Eurasia was open to the Pacific Ocean, and so the depositional environments in the WFB were susceptible to reworking by large waves. This led to the preservation of low-frequency eccentricity and obliquity, but not higher-frequency precession.Despite increasing basin subsidence from 5.4 to 4.9 Ma, the preservation of orbital oscillations is low. This is attributed to either low sedimentation rates at deeper water depths, which could obscure variations in sediment input or result in cycles below the resolution of the gamma-ray logging tool, or cycles not being detectible in the gamma-ray proxy record due to a lack of contrasting lithology. After 4.9 Ma and up to 3.2 Ma, the Taiwan orogen became the dominant sediment source for the WFB, and rapid growth of the orogen shielded the WFB from high-energy waves generated in the Pacific Ocean. The increased sediment influx and the formation of a semi-sheltered strait, combined with increased space for sediment accumulation in the WFB, resulted in enhanced preservation of precession-driven East Asian Summer Monsoon variability.

SUMMARY Prior work on rotational remanent magnetization (RRM) and rotational anhysteretic remanent magnetization (ARMROT) has demonstrated promise for magnetic mineral identification in earth materials. One challenge has been to calibrate the measurements to magnetic mineral types and microstructural controls, since previous studies have used differing spin rates, alternating field (AF) intensities and decay times, which hinders a comparison of data sets. Using a RAPID magnetometer we show that the range of usable practical rotation rates is 0.25–3 Hz [rps] which allows a wide range of RRM and ARMROT characteristics to be utilized (at 100 mT AF field, 100 μT bias field). Sets of magnetic mineral extracts from sediments, and well characterized rock samples that contain the key magnetic minerals magnetite, pyrrhotite and greigite are used for a calibration of the RRM-ARMROT behaviour. Detrital pyrrhotite and pyrrhotite-bearing phyllites have largely small positive effective field (Bg) values (up to 6 μT), with differences in Bg and ARMROT ratios at 0.5 and 2.5 Hz [rps] allowing grain size discrimination. The positive Bg values, and changes in RRM and ARMROT with rotation rates allow distinction of pyrrhotite from magnetite and diagenetic greigite. Diagenetic greigite has Bg values of –83 to –109 μT (at 0.5 Hz [rps]) and unusual RRM variation at low rotation rates caused by anisotropy affects. In contrast to previous work, based on crushed and sized natural magnetite at high spin rates, Bg for single domain magnetite from intact bacterial magnetofossils from Upper Cretaceous Chalk has some of the lowest Bg (0–1 μT) and displays a steep decline in ARMROT with increasing rotation rates. A simple tool for particle size characterization of magnetite may be the ratio of ARMROT at spin rates 2.5 and 0.5 Hz [rps]. Stability of RRM is better studied using RRM acquisition with increasing AF field intensity, since static demagnetization imparts a nuisance gyroremanence along the field axis. Mineral microstructure, dislocations and particle interactions are likely additional effects on RRM behaviour that need more investigation.

Collision between the Philippine Sea Plate and the Eurasian Plate in the late Miocene-early Pliocene resulted in the uplift of Taiwan, and lithospheric flexure to the west formed the adjacent Western Foreland Basin (WFB). Petrographic studies of late Miocene to Recent sediment and sedimentary strata in the WFB indicate that Taiwan was the main sediment source to the WFB in the early- to mid-Pleistocene, and prior to this, sediments were assumed to be derived primarily from the Eurasian continent. However, uplift of Taiwan began significantly earlier, and sediment derived from the island should reflect the onset and acceleration of uplift and subsequent erosion.To resolve the timing of changes in sediment sources in the WFB, we present clay mineralogy, carbon and nitrogen geochemistry, and magnetic susceptibility data from the late Miocene to late Pliocene Kueichulin Fm that outcrops along the Da’an River in western Taiwan. This formation is composed of the lower Kuantaoshan Member, the middle Shihliufen Shale, and the upper Yutengping Sandstone. Clay mineralogy shows an upward increase in illite and illite crystallinity, and a decrease in chlorite and kaolinite starting at the base of the Shihliufen Shale, and this suggests that rapid erosion of Taiwan became a major sediment source to the WFB between the late Miocene and early Pliocene (Shihliufen Shale). δ13Corg­­­ and C/N ratios preserve the dominance of Taiwan-derived sediment in the early Pliocene where there is a marked change from dominantly marine- to dominantly terrestrially sourced carbon at the transition from the Shihliufen Shale to the overlying Yutengping Sandstone. Finally, a rapid decrease (>50%) in magnetic susceptibility across the Shihliufen/Yutengping boundary indicates a significant dilution of magnetic minerals deposited in the WFB by the high flux of non-magnetic minerals delivered from the Taiwan orogenic belt. Together, these datasets record a major shift in sediment source to the WFB during the late Miocene to early Pliocene, and that Taiwan became the dominant source of sediment supply to the WFB by the early Pliocene, approximately two million years earlier than previously thought.

The stratigraphic records of shallow-marine environments are not commonly regarded as excellent climate archives because of their presumed temporal incompleteness. However, a recent study of lower Pleistocene strata in the Western Foreland Basin, Taiwan, reveals high-resolution records of past climate oscillations preserved within shallow-marine strata. Deriving such narratives is made possible because of the high accommodation and sedimentation rates in the basin, which enhanced the completeness of the stratigraphic record.Here, we astrochronologically tune the Chinshui Shale and the lower part of the Cholan Formation of the Western Foreland Basin from approximately 3.5 to 2 Ma. These strata are calibrated to global deep-sea stable oxygen isotope (δ18O) records with established time scales detailing global climate change during the studied time period. The Chinshui Shale is mudstone-dominated and was deposited mostly in offshore settings, while the Cholan Formation comprises mainly heterolithic strata deposited in shallower settings (i.e., offshore transition, nearshore) of the paleo-Taiwan Strait. The data used herein are two borehole gamma-ray profiles through the Chinshui Shale and the Cholan Formation that have a proximal-distal relation to Taiwan. High gamma-ray values reflect clay-rich intervals and correlate to lower values of δ18O in the global reference records. Low gamma-ray values point to sand-rich packages and correlate with higher values of δ18O.Preliminary results show that the alternating clay-rich to sand-rich deposits during the late Pliocene to early Pleistocene are orbitally paced. The results allow us to i) tune the upper Pliocene–lower Pleistocene Chinshui Shale and lower part of the Cholan Formation, ii) refine the magneto-biostratigraphic framework established for this time interval in the Western Foreland Basin of Taiwan, and iii) lay the groundwork for connecting climatic changes in Taiwan during this time period to the wider frame of global climate change.

The Plio-Pleistocene Lichi Mélange in the Coastal Range of eastern Taiwan offers an excellent opportunity to study processes of mélange development at the continent-ocean interface of an active arc-continent collision. This paper presents new results of detailed geologic mapping, lithofacies analysis, magneto-biostratigraphy, paleocurrent, and paleoslope analyses in the southern Coastal Range to investigate the origins and significance of this mélange. The results show that the Lichi Mélange consists of mass-transport deposits including well-stratified block-in-matrix beds (olistostromes), extra-formational blocks (olistoliths), and broken formation with abundant soft-sediment deformation features that transition laterally into distal mega-slump beds and pebbly mudstones (subaqueous debrites). Abundant observations of depositional contacts and interbedding of mélange with contemporary (ca. 4–1 Ma) flysch units of the Fanshuliao and Paliwan formations confirm their sedimentary origin. Compacted sedimentological shear fabrics in olistostromal facies are broadly parallel to internal stratification and bedding, and are readily distinguishable from cross-cutting brittle fault zones related to post ~1 Ma west-vergent thrust faults. Paleoslope and paleocurrent analyses record down-slope gravity-driven transport toward the east and southeast.The data provide evidence for a polygenetic origin of the Lichi Mélange, in which sedimentary mass-wasting deposits are overprinted by younger tectonic shear zones. Slide blocks, conglomerate clasts, and detrital sand were all derived from an eroding source in the east-vergent eastern retrowedge of the Taiwan collisional orogen. The source area included tectonically accreted fragments of the two converging plates that represent shallow-crustal equivalents of the Miocene Yuli Belt and Eastern Slates exposed in the modern Central Range. Reconstructed stratigraphic panels record eastward progradation of olistostromal facies over distal basinal flysch deposits, which we infer resulted from eastward (oceanward) migration of a steep submarine slope at the leading edge of the retrowedge orogenic front. Thus, the Coastal Range basin evolved as a migrating retro-foredeep basin that formed on top of older, pre-collisional volcanic arc and forearc crust. These results demonstrate a unique type of sedimentary basin that is formed and then rapidly inverted at a convergent continent-ocean interface during the transition from intra-oceanic subduction to arc-continent collision. This revised history of the Lichi Mélange provides a new perspective on the dynamics of rapid crustal mixing and tectonic recycling at the convergent suture of an active arc-continent collision system.

The Plio-Pleistocene orogeny resulted in the exhumation of a regional metamorphic belt, comprising low- to high-grade metapelites in the Central Range, Taiwan. In this study, we analyze the distributions of B isotopes along a prograde metamorphic profile (argillite to greenschist) in the western Central Range to constrain the effects of metamorphic fluids generated during dehydration on the redistribution of the mobile elements. From west to east, peak metamorphic temperatures increase from ~200 °C to 320 °C. B concentrations and δ11B values generally decrease to the east with ~50% loss of B, and the δ11B values roughly decrease from −5‰ to −15‰. The X-ray diffraction (XRD) analysis results of these mineral compositions, except for calcite, show no correlation with B contents and δ11B. And the mixing monitoring of mineralogical compositions cannot account for the observed B contents and δ11B coherent and W-E distribution trend. As the B isotopes may be fractionated between rock and fluid during dehydration, a Rayleigh distillation equation was applied into a model under similar conditions. The results indicate that the measured δ11B values are consistent with the modeled equilibrium of fluid-rock B isotope fractionation within the constrained temperature ranges, thereby supporting the hypothesis that dehydration processes act as a major control on the distribution of B isotopes in the low-grade metapelites. This model also estimates a δ11B value of 5.8‰ for the integrated expelled fluids when getting totally 50% loss of B. Any B sequestering by tourmalines in meta-sediment is an important factor to retain B to a greater depth in subducting slabs. However, our results suggest this factor is not significant for the low-grade metapelites. This study constrains the effect of dehydration on B flux and B isotope distributions during low-grade metamorphism and how the heterogenetic mineralogical compositions may affect its variations, and may contribute to a better understanding of B mass cycling in subduction environments.

Influenced by rapid tectonic activities and frequent typhoons, Taiwan is unique in having extremely high rates of uplift, precipitation, denudation, and sedimentation. These dynamic surface processes not only influence landscape evolution and river behavior, but severely affect human societies both at present and in the past. Particularly, mass-movement and flooding events caused by earthquakes and/or typhoons may introduce a huge amount of sediments into river systems, and result in natural hazards due to consequent river-aggradation. The Iron Age Kiwulan Site is a recently discovered archaeological site situated on the margin of the Lanyang Plain in northeastern Taiwan. In its deposits, a cultural hiatus centered on 1150–1350 CE was recognized suggesting that the settlement was abandoned for two hundred years before being recolonized. To find the cause of this cultural break, the river-aggradation history since 800 CE has been reconstructed by a source-to-sink approach using multiple lines of evidence from the Lanyang Source-to-Sink System, northeastern Taiwan. Two particular river-aggradation events that occurred during 875–925 CE and 1400–1500 CE were traced from their source (upland river terrace), through floodplain lakes to their ultimate sink (the Okinawa Trough and Hoping Basin of the Pacific Ocean). However, as these extreme river-aggradation events occurred either significantly earlier or later than the cultural hiatus, dramatic climate and earthquake events are not likely to be the direct causes for the cultural hiatus. Instead, river migration might have been the reason for the abandonment and return of the Kiwulan settlements.

The Pleistocene was a phase of global cooling of the Earth through which glacial-interglacial cycles occurred, and the growth and decay of the ice-sheets resulted in quasi-cyclic sea-level fluctuations driven by orbital forcing. Despite that summer insolation is mostly controlled by precession, the records of the glacial cycles showcase a significant periodicity of ~41 kyrs during the Early Pleistocene forced by Earth’s obliquity (tilt) that varies the latitudinal distribution of insolation especially in high latitudes. The dominance of obliquity over precession in marine archives is commonly attributed to the in-phase effect of obliquity-related insolation versus the opposite-phased influence of precession, which may cancel out the summer insolation signal received by the southern and northern hemispheres.Here, we present a clastic shallow marine record from the Cholan Formation (Early Pleistocene; Taiwan). Facies analysis indicates that quasi-cyclic deposition occurred in shoreface to offshore environments in the paleo-Taiwan Strait. The magnetobiostratigraphic framework indicates that the studied section occurs in the lower part of the Matuyama subchron (1.925 - 2.595 Ma) close to the lower limit of the Olduvai (1.925 Ma) normal polarity subchron. Comparison of the stratigraphy to a d18O isotope record of benthic foraminifera and orbital curves of precession and obliquity at the time of sediment accumulation reveals a good correlation between depositional cycles and the Northern Hemisphere summer insolation, demonstrating precession dominated sea-level fluctuations during the Early Pleistocene. These results underpin recent findings suggesting that d18O isotope records of benthic foraminifera have a more significant precession signal than previously described. This study also demonstrates that shallow-marine stratigraphic successions in high-accommodation and high-sedimentation basins can be outstanding climate archives, possibly even preserving sediment flux responding to half-precession cycles.

The southwestern Ryukyu Trench represents the ultimate sink of sediments shed from Taiwan into the Philippine Sea, which are mainly transported to the trench by turbidity currents via submarine canyons. Here, we present trench turbidites intercalated with hemipelagites in a gravity pilot core and a piston core acquired on the Ryukyu Trench floor at 6147 m water depth. We performed X-ray fluorescence core scans (ITRAX profiles), magnetic measurements, and organic geochemistry analyses to discriminate turbidites from hemipelagites. We identified 36 turbidites (0.9–4.2 cm thick) based on visual core descriptions and Ca/Fe ratios in the ITRAX profiles. Three of these turbidites show magnetic signatures indicating the presence of pyrrhotite and peaks in the magnetic susceptibility profile, suggesting that Taiwan-sourced sediments are transported to the Ryukyu Trench by long-runout turbidity currents. Pyrrhotite is also present in hemipelagites of the upper part of the retrieved cores, indicating a dominant sediment source in Taiwan over the last several thousand years. Ca/Fe and Zr/Rb ratios in the ITRAX profiles mark distal turbidites (about 1–3 cm thick), and Zr/Rb peaks mainly reflect grain size changes. Detailed analyses of a representative turbidite show good correlation between Ca/Fe and Zr/Rb peaks with upward-coarsening and upward-fining trends that delimit the turbidite. Sedimentary organic matter in hemipelagites is characterized by higher total organic carbon and total nitrogen contents and higher δ13C values than that in turbidites. Our multi-proxy approach employing high-resolution XRF core scans to differentiate turbidites from hemipelagites contributes to establishing a comprehensive view of modern trench sedimentation from Taiwan to the southwestern Ryukyu Trench.

The sedimentation rate in the northeastern South China Sea (SCS) is high and it therefore offers an opportunity for a high-resolution paleoceanographic study. This study is based on high-resolution AMS 14C dating on forams and oxygen isotope data of two planktonic foraminifera species (Globigerinoides ruber and Neogloboquadrina dutertrei) from the sediment core, MD18-3568, collected from the northeastern SCS, to reconstruct upper-ocean stratification since 35 ka.The marine sediment core MD18-3568 is located on the accretionary wedge off SW Taiwan at a water depth of 1,315 m, the whole core is dominated by hemipelagic sediments and is of 20.7 m in length. Samples for AMS 14C dating were selected at roughly 2 ka interval with a total of 16 samples. The ages show a continuously younging-upward trend with bottom of this core around 35,000 years BP. Samples for high-resolution oxygen isotope measurements were selected at a nominal 500-year age interval. The difference in δ18O between G. ruber (mixed layer dwelling species) and N. dutertrei (thermocline dwelling species) is used to reconstruct the upper ocean stratification with large difference indicating significant ocean stratification and vice versa. The results show moderate upper ocean stratification during 35-24 ka, and it became less stratified during the Last Glacial Maximum (LGM, 23-19 ka). During the deglacial stage, the stratification gradually became stronger until the early Holocene (12-9 ka), and it has kept strong upper-ocean stratification since 9 ka. Literature has documented less rainfall intensity during the LGM and heavy rainfall during the Holocene in southern Taiwan. We interpret the upper-ocean stratification in the NE South China Sea near Taiwan is linked to the amount of freshwater inputs from Taiwan. Less Taiwan freshwater input during the LGM led to a weak stratified upper ocean and a large amount of freshwater input from Taiwan led to a strong upper-ocean stratification during the Holocene.

Smythite (Fe9S11) is an occasionally reported magnetic iron sulphide mineral that occurs in varied geological settings and co-occurs commonly with other magnetic iron sulphide minerals. Determining the magnetic properties of smythite is important to understand its geological distribution and paleomagnetic and environmental magnetic significance. We have identified sedimentary smythite from three locations in Taiwan (one terrestrial and two marine), which suggest that smythite forms in methanic diagenetic environments into which sulfide has been reintroduced. We report the magnetic properties of our purest smythite sample and compare them with those of other magnetic iron sulfide minerals. The magnetization of smythite is controlled by multi-axial anisotropy, with magnetic easy axes that lie within the crystallographic basal plane. Smythite has stable magnetic properties with no low-temperature magnetic transition. The magnetic properties of smythite at elevated temperatures are dominated by thermal alteration, which precludes Curie temperature determination. Hysteresis and coercivity properties of stable single domain smythite are similar to those of greigite at, and below, room temperature. In contrast to greigite, and similar to pyrrhotite polytypes, smythite crystals occur as hexagonal plates. This morphological contrast facilitates discrimination of smythite from greigite in electron microscope observations, but it does not assist discrimination from pyrrhotite. Similar magnetic and morphological properties between smythite and other magnetic iron sulfides means that diagnostic mineralogical analyses (e.g., X-ray diffraction) are needed to identify these minerals. Further work is needed to obtain pure samples to develop a comprehensive domain state dependent magnetic property framework for smythite.

Marginal areas off southwestern Taiwan have been widely considered a high potential reservoir of gas hydrates based on several geophysical, geological, and geochemical investigations since the past decades. First gas hydrate sample has been collected on 21 June 2018 during the cruise MD214 at the core site MD18-3542 on the South Yung-An East Ridge. In the study, we focus on magnetic properties of this MD core. The most attractive feature in the magnetic susceptibility is an abrupt drop recorded at about 4 meters core depth. To clarify and identify the dominant magnetic mineral in the core, hysteresis loop parameters were first measured and then presented on the Day Plot, and further the X-ray diffraction analysis was applied to the selected core samples. Based on the magnetic results, the clear drop in the magnetic susceptibility is related to the change of dominant magnetic minerals in core sediments. Before about 4 meters core depth, the dominant magnetic mineral remains detrital magnetite. Below the depth, however, core sediments should have been infected by methane released by gas hydrate dissociation. Authigenic greigite and pyrite have become dominant, and therefore low magnetic susceptibility appears below 4 meters core depth.

Kung-Suan Ho and Wei-Teh Jiang kindly supplied monoclinic pyrrhotite crystals from museum and ore mineral collections, respectively. C. S. H. acknowledges funding from the Central Geological Survey and Ministry of Science and Technology, R.O.C., through grants 104-5226904000-02-01 and MOST 105-2116-M-001-018, respectively. A. P. R. acknowledges funding from the Australian Research Council through grant DP160100805.

The Taiwan orogen is the result of the collision of the Luzon Volcanic Arc pushed northwestward by the Philippine Sea Plate against the Chinese Continental Margin since the Late Miocene and is still uplifting nowadays. The fold-and-thrust belt in northern Taiwan shows an arcuate shape from SW to NE: the strike of the main tectonic structures changes from N 0 ° E to N 70 ° E. Such curvature has not yet been properly interpreted so far. In this study we used paleomagnetic analyses to decipher the tectonic evolution along the fold-and-thrust belt in northern Taiwan, which reveals complex magnetization patterns both in ante-folding and post-folding formations. We also interpret that the fold-and-thrust belt in northern Taiwan has experienced two main rotational events: a 30 - 60 ± 14 ° clockwise rotation followed by a 30 ± 7 ° counterclockwise rotation during the ages of 4.6–1.5 Ma.

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

The Plio-Pleistocene Lichi Melange in the Coastal Range of eastern Taiwan offers an excellent opportunity to study processes of melange development at the continent-ocean interface of an active arc-continent collision. This paper presents new results of detailed geologic mapping, lithofacies analysis, magneto-biostratigraphy, paleocurrent, and paleoslope analyses in the southern Coastal Range to investigate the origins and significance of this melange. The results show that the Lichi Melange consists of mass-transport deposits including well-stratified block-in-matrix beds (olistostromes), extra-formational blocks (olistoliths), and broken formation with abundant soft-sediment deformation features that transition laterally into distal mega-slump beds and pebbly mudstones (subaqueous debrites). Abundant observations of depositional contacts and interbedding of melange with contemporary (ca. 4–1 Ma) flysch units of the Fanshuliao and Paliwan formations confirm their sedimentary origin. Compacted sedimentological shear fabrics in olistostromal facies are broadly parallel to internal stratification and bedding, and are readily distinguishable from cross-cutting brittle fault zones related to post ~1 Ma west-vergent thrust faults. Paleoslope and paleocurrent analyses record down-slope gravity-driven transport toward the east and southeast. The data provide evidence for a polygenetic origin of the Lichi Melange, in which sedimentary mass-wasting deposits are overprinted by younger tectonic shear zones. Slide blocks, conglomerate clasts, and detrital sand were all derived from an eroding source in the east-vergent eastern retrowedge of the Taiwan collisional orogen. The source area included tectonically accreted fragments of the two converging plates that represent shallow-crustal equivalents of the Miocene Yuli Belt and Eastern Slates exposed in the modern Central Range. Reconstructed stratigraphic panels record eastward progradation of olistostromal facies over distal basinal flysch deposits, which we infer resulted from eastward (oceanward) migration of a steep submarine slope at the leading edge of the retrowedge orogenic front. Thus, the Coastal Range basin evolved as a migrating retro-foredeep basin that formed on top of older, pre-collisional volcanic arc and forearc crust. These results demonstrate a unique type of sedimentary basin that is formed and then rapidly inverted at a convergent continent-ocean interface during the transition from intra-oceanic subduction to arc-continent collision. This revised history of the Lichi Melange provides a new perspective on the dynamics of rapid crustal mixing and tectonic recycling at the convergent suture of an active arc-continent collision system.

International audience; We conducted a paleomagnetic study on the Early Permian volcanic and sedimentary rocks, and the Neoproterozoic mafic dikes in the Yili Block, NW China. Magnetite and hematite were proven to be the principal magnetic remanence carriers. Demagnetizations revealed stable characteristic remanence magnetizations with a sole reversed magnetic polarity. The magnetic remanence of only the Early Permian strata turned out to be primary based on positive fold tests; meanwhile, the magnetic remanence age of the mafic dikes is ambiguous. Accordingly, the first Early Permian paleomagnetic pole for the Yili Block is calculated at λ = 81.5°N, φ = 256.5°E, N = 11, and A95 = 10.9°. Comparisons of this new pole with published ones from the Yili, Tarim, and South Junggar blocks provide new quantitative constraints on late Paleozoic kinematic evolution of the SW Central Asian Orogenic Belt: (1) Between the Yili and Tarim blocks, significant relative movement took place along major strike‐slip faults during the Late Carboniferous to Early Permian (580 ± 290 km) and the Early to Late Permian (585 ± 340 km), and the displacement rate increased from the Late Carboniferous to Early Permian (~19.3 ± 9.7 mm/yr) to the Early to Late Permian (~29.3 ± 17.0 mm/yr); (2) a significant relative rotation of 28.3° ± 18.3° in the Late Permian, and a lateral displacement of 630 ± 295 km after the Late Permian occurred between the Yili and South Junggar blocks. The significant strike‐slip movements played an important role in the formation of the Central Asian Orogenic Belt and should be considered with great attention in tectonic and paleogeographic reconstructions.

International audience

We utilized reflection seismic and bathymetric data to infer the canyon-infilling, fold uplift, and gas hydrate occurrences beneath the frontal fold at the toe of the accretionary wedge, offshore SW Taiwan. The lateral migrating paleo-Penghu canyons has cut across the frontal fold with six distinct canyon/channel incisions marked by channel infills. The longitudinal bathymetric profile along the modern canyon course shows a knickpoint of ~300 m relief at this frontal fold, indicating that the rate of fold uplift is greater than that of canyon incision. The age for the initial thrusting of this fontal fold is around 240 kyr ago, as estimated by using the maximum thickness of growth strata of this fold divided by the sedimentation rate obtained from a nearby giant piston core. Bottom simulating reflector (BSR) on seismic sections indicates the base of gas hydrate stability zone. Beneath the frontal fold, there is a widespread occurrence of BSRs, suggesting the highly probable existence of substantial quantities of gas hydrates. A seismic flat spot and a few push-down reflectors below BSR are found lying beneath the anticlinal axis with bathymetric four-way dip closure. The flat spot, cutting across a series of dipping reflections beneath BSR, may indicate the contact between free gas and its underlying formation water. The push-down reflectors beneath BSRs are interpreted to result from abundant free gas hosted beneath the gas hydrate stability zone. The multiple paleo-canyon infills seen along and beneath the frontal fold and above BSRs may provide thick porous sands to host gas hydrates in the frontal fold.

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

Fluxes of the meteoric cosmogenic radionuclide 10 Be vary with changes in the incoming cosmic rays modulated by geomagnetic field intensity variations. The variability in the 10 Be flux can be used to synchronize ice cores, as well as marine sediments, by comparison with the relative paleointensity variations of the geomagnetic field. However, lock-in of the paleomagnetic signal at some depth below the sediment–water interface in marine sediments through acquisition of a post-depositional remanent magnetization (PDRM) adds uncertainty to synchronization. Despite the long history of such studies, the magnitude of the PDRM lock-in depth remains controversial. In this article, we present clear evidence for a downward offset of the paleointensity minimum relative to the 10 Be flux anomaly at the Matuyama–Brunhes (M–B) geomagnetic polarity boundary, which we interpret to result from a ∼ 15 cm PDRM lock in depth. This lock-in depth indicates that up to several tens of thousands years of age offset probably occurs when a paleomagnetic record is used for dating marine sediments, and the age of the M–B boundary should be revised to ca . 10 kyr younger, which is consistent with a younger ice core derived age of 770 ± 6 ka (2 σ ). This cosmogenic age tuning strategy will contribute to refining paleomagnetic-based age models for marine sediments and identifying of lead-lag relationships for global abrupt environmental changes.

Kaohsiung City and its neighborhood in the southwestern coastal plain of Taiwan have suffered serious air pollution since the region became the largest center for heavy-industry on the island. In order to unravel the air pollution history of the region, four 210Pb- and 137Cs-dated sediment box cores recovered in 2006 from offshore of this area were chosen for magnetic and petrographic analyses. The data were used to distinguish changes in concentration, composition and grain size of magnetic particles in the sediments due to inputs of anthropogenic magnetic spherules. Sedimentation rates have been reasonably constant for the last one hundred years, except at the core tops which were affected by a turbidite layer induced by a typhoon in 2005. Down-core profiles of mass-specific magnetic susceptibility (χ) and saturation isothermal remanent magnetization (SIRM) are similar among the cores, and reflect similar trends to magnetic spherule counts. This reveals that χ and SIRM of modern marine sediments can be used as air pollution indicators for nearby industrialized upwind areas. The studied record indicates that industrialization of the area was gradual during 1950–1980 and boomed afterward, resulting in a high production of airborne magnetic spherules, which is consistent with evidence for poor air quality at that time. Optical and scanning electron microscopic (SEM) surveys of magnetic extracts indicate that the magnetic spherules have grain sizes ranging from a few micrometers up to 50 μm and consist mainly of iron oxides with variable Si, Al, and Ca contents. X-ray diffraction analysis on magnetic extracts from different depths in the cores further indicates that magnetite and pyrrhotite, which are derived from terrigenous detritus, form the magnetic constituents of the sediments before the area was industrialized. In contrast, during the industrial boom, anthropogenic magnetite and hematite spherules became the dominant magnetic particles in the sediments. Down-core profiles of hard isothermal remanent magnetization (HIRM) below the turbidite layer also reveal similar trends to the corresponding magnetic spherule counts, which indicate that the concentration of hematite in the sediments is also closely related to the extent of air pollution. In addition, relatively low values of χARM/χ, which are indicative of coarse magnetic grains, started to occur when large magnetite spherules became significant during the industrialized period. The air pollution history elucidated from our sediment core data not only reflects the development of Kaohsiung from a small village to a highly industrialized metropolitan area in the 20th century, but it is also consistent with the most recent air pollution trends revealed by real time air quality measurements of PM10. Our results demonstrate the usefulness of magnetic parameters for delineating the air pollution history of coastal marine sediments down-wind of nearby industrialized regions.

Monoclinic pyrrhotite (Fe7S8) is widely claimed to carry magnetizations acquired during early diagenesis in anoxic sedimentary environments. In contrast, geochemical literature indicates that pyrrhotite formation is extremely slow below 180 °C, which makes it a highly unlikely carrier of early diagenetic remanences in sediments. This view is confirmed by the occurrence of late diagenetic Fe7S8 in ancient sediments and the general lack of Fe7S8 in modern sediments. Horng et al. [C.S. Horng, M. Torii, K.S. Shea, S.J. Kao, Inconsistent magnetic polarities between greigite- and pyrrhotite/magnetite-bearing marine sediments from the Tsailiao-chi section, southwestern Taiwan, Earth Planet. Sci. Lett. 164 (1998) 467–481.] documented the presence of Fe7S8 that carries a magnetic signal indistinguishable from that of detrital magnetite in Pleistocene marine sediments from Taiwan. We tested the possibility that the Fe7S8 could have a detrital origin by conducting a source-to-sink study and found Fe7S8 in metamorphic rocks of the Taiwan Central Range and in material eroded from these rocks in the transportation pathway and in the depositional sink. This confirms that the Fe7S8 has a detrital origin. Rapid transportation from source to sink (e.g., by typhoon-associated flood events) probably assists preservation of the Fe7S8, which might otherwise oxidize during transportation. The widespread occurrence of exhumed metamorphic rocks in orogenic belts around the world makes them a likely source of Fe7S8 in marginal sedimentary basins. Detrital Fe7S8 should therefore be more routinely considered to be responsible for paleomagnetic records when it is present in sediments with partial metamorphic provenance.

The importance of the magnetic iron sulfide minerals, greigite (Fe3S4) and pyrrhotite (Fe7S8), is often underappreciated in geochemical studies because they are metastable with respect to pyrite (FeS2). Based on magnetic properties and X-ray diffraction analysis, previous studies have reported widespread occurrences of these magnetic minerals along with magnetite (Fe3O4) in two thick Plio-Pleistocene marine sedimentary sequences from southwestern Taiwan. Different stratigraphic zones were classified according to the dominant magnetic mineral assemblages (greigite-, pyrrhotite-, and magnetite-dominated zones). Greigite and pyrrhotite are intimately associated with fine-grained sediments, whereas magnetite is more abundant in coarse-grained sediments. We measured total organic carbon (TOC), total sulfur (TS), total iron (FeT), 1N HCl extractable iron (FeA), and bulk sediment grain size for different stratigraphic zones in order to understand the factors governing the formation and preservation of the two magnetic iron sulfide minerals. The studied sediments have low TS/FeA weight ratios (0.03-0.2), far below that of pyrite (1.15), which indicates that an excess of reactive iron was available for pyritization. Observed low TS (0.05-0.27%) is attributed to the low organic carbon contents (TOC=0.25-0.55%), which resulted from dilution by rapid terrigenous sedimentation. The fine-grained sediments also have the highest FeT and FeA values. We suggest that under conditions of low organic carbon provision, the high iron activity in the fine-grained sediments may have removed reduced sulfur so effectively that pyritization was arrested or retarded, which, in turn, favored preservation of the intermediate magnetic iron sulfides. The relative abundances of reactive iron and labile organic carbon appear to have controlled the transformation pathway of amorphous FeS into greigite or into pyrrhotite. Compared to pyrrhotite-dominated sediments, greigite-dominated sediments are finer-grained and have higher FeA but lower TS. We suggest that diagenetic environments with higher supply of reactive iron, lower supply of labile organic matter, and, consequently, lower sulfide concentration result in relatively high Eh conditions, which favor formation of greigite relative to pyrrhotite

We test a recent suggestion that orbital eccentricity has influenced the geomagnetic field by performing a range of spectral analyses, including estimation of statistical confidence limits, on a 2.2-million-year paleomagnetic inclination record from the western Caroline Basin (WCB). Our analyses indicate that the claimed ∼100-kyr signal, while present over discrete portions of the WCB paleomagnetic record, is not statistically significant for the entire record and is not modulated by the 404-kyr eccentricity component as would be expected if the signal was influenced by orbital eccentricity. This signal also has highly variable phase compared to orbital eccentricity variations and the paleomagnetic record shows no statistically significant coherency with the orbital signal. Our results from a nearby coeval western Philippine Sea record support the present analysis. Together, these data suggest that the hypothesis of orbital energization of the geomagnetic field has yet to be convincingly demonstrated.

We have analyzed carbon and oxygen isotopes of two groups of planktonic foraminiferal shells from sediments near the Pliocene/Pleistocene (P/P) and the Miocene/Pliocene (M/P) boundaries, in the Lower Gutingkeng Formation in southwestern Taiwan. The P/P shells display heavier isotopic compositions ( δ 18 O =−1.5‰, δ 13 C =1.5‰ ) over a narrow range, suggesting a pristine environmental signature. On the other hand, the M/P shells are much lighter ( δ 18 O =−3 to −7‰, δ 13 C =1 to −1.5‰ ), and more scattered, associated with secondary carbonate diagenesis. Our detailed SEM examination confirms this proposition. The P/P shells display a clean micro-texture and are free from carbonate recrystallization. However, there are abundant secondary calcite crystals in the M/P shells. Therefore, the extreme shift of δ18O and δ13C in foraminiferal shells provides a guideline for evaluation of diagenetic artifacts of deeply buried sediments that have been uplifted on land.

In several recent published studies, paleomagnetic results from greigite-bearing sediments reveal characteristic remanences that are anti-parallel to those carried by coexisting detrital magnetic minerals and polarities that are opposite to those expected for the age of the rock unit. These observations have important implications for the reliability of paleomagnetic data from greigite-bearing sediments. We have investigated the origin of such contradictory magnetic polarities by studying the formation mechanisms of greigite in mudstones from the Lower Gutingkeng Formation, southwestern Taiwan. Scanning electron microscope observations indicate that the Gutingkeng greigite has three modes of occurrence, including nodular, framboidal and matrix greigite. Microtextural observations, including transection of bedding by iron-sulfide nodules with no deviation of sediment textures, the presence of partially dissolved edges around detrital and early diagenetic phases, and neoformation of greigite and Fe-rich clays around detrital phyllosilicates, indicate that all three types of greigite have a diagenetic origin that post-dates early diagenetic pyrite. In addition, paleomagnetic data yield contradictory polarities even for greigite-bearing sister samples from the same stratigraphic horizon. The data are collectively interpreted to indicate that neoformation of the Gutingkeng greigite occurred after partial dissolution of syngenetic or early diagenetic pyrite. The timing of greigite formation can apparently vary enough to give contradictory polarities for different greigite components even within a single stratigraphic horizon. Direct petrographic observation of authigenic magnetic iron-sulfide phases, as carried out in this study, can provide important constraints on formation mechanisms and timing of remanence acquisition for these minerals and suggests that care should be taken when interpreting magnetostratigraphic data from greigite-bearing sediments.

Lithofacies analysis, magnetostratigraphy, and seismic profiles of Pliocene-Pleistocene foreland basin deposits of Taiwan provide a framework to evaluate the stratigraphic development of a collisional marine foreland basin. We have recognized several scales of stratigraphic packages and unconformities in deposits of the Taiwan foreland basin. Small-scale (20 to 150 m thick) stratigraphic sequences contain upward-shallowing, marine lithofacies successions that are bracketed by thin coquina sandstones. We interpret the small-scale stratigraphic packages as “parasequences” in the traditional sequence stratigraphy model, the thin coquina sandstones representing marine-flooding intervals. The average duration of individual small-scale packages was in the range of 37.5 k.y., on the basis of our magnetostratigraphy. These sequences are interpreted as the product of eustatic sea- level change possibly related to the orbital time series of obliquity. Intermediate-scale stratigraphic sequences are 150 to 1000 m thick and are bounded by unconformities that are well exposed in outcrop and can be clearly identified in seismic sections. The unconformity surfaces have several hundred meters of relief and represent periods of major fluvial valley incision in the foreland basin. One of the unconformities is locally an angular one that we interpret as representing a growth structure that formed during structural uplift of the proximal margin of the foreland basin at ca. 1.25 Ma. Across this angular unconformity, there were marked increases in rates of sediment accumulation and tectonic subsidence in the foreland basin. Other major unconformities that bound intermediate-scale stratigraphic sequences are high-relief disconformities. These unconformities may be the product of eustatic changes, because there has been little change in rates of sediment accumulation and tectonic subsidence across these unconformities. The duration of individual, intermediate-scale packages ranges from ∼100 000 to 700 000 yr, on the basis of magnetostratigraphy and biostratigraphy. We interpret the intermediate-scale sequences as “sequences” in the traditional sequence stratigraphy model. Our analysis of the Pliocene-Pleistocene deposits of the Taiwan foreland basin has several implications for understanding the stratigraphic evolution of this collisional marine foreland basin. (1) Deposition in the Taiwan foreland basin appears to have been punctuated by at least five episodes of erosion and major fluvial valley incision. Large volumes of sediment were eroded from the proximal margin of the foreland basin and transported to more distal parts of the foreland basin or to depocenters outside the foreland basin system during all stages of basin development. (2) The presence of high-relief unconformities and growth structures in the Pliocene-Pleistocene foreland basin deposits suggests a well-developed wedge-top depozone in the foreland basin system. (3) The Pliocene- Pleistocene strata of the foreland basin of Taiwan record ∼2.3 m.y. of deposition, on the basis of our magnetostratigraphy. Sediment accumulation rate was on the order of ∼950 m/m.y. during the earlier stages of basin development. During the later stages of basin development, sediment accumulation rate increased to ∼1900 m/m.y. Sediment accumulation rates in the collisional marine foreland basin of Taiwan are much higher than previously published rates from more extensively studied retroarc foreland basins and collisional nonmarine foreland basins.

Magnetic minerals in six samples of oceanic basalts of the transition zone and upper sheeted dikes from Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) Hole 504B, Leg 83, were studied by methods of rock magnetism and transmission electron microscopy (TEM). TEM observations showed that the magnetic mineral in these basalts is end-member magnetite (TMO) of extremely fine-grain size (30–100 nm) primarily in the range of pseudosingle-domain magnetite, consistent with the rock magnetic properties including hysteresis parameters, Curie temperature, and low-temperature measurements (Verwey transition). Magnetite formed by two different processes: (1) oxidation-“exsolution,” true exsolution, and hydrothermal alteration, and (2) oxidation-exsolution, a second stage of oxidation-exsolution, and hydrothermal alteration. The primary titanomagnetite (TM60-70) that crystallized from the melt thus evolved to end-member magnetite coexisting with titanite (sphene), kassite, ulvospinel (TM ∼ 87), and ilmenite on a submicroscopic scale. On the basis of the formation mechanisms of the magnetic carrier, the primary titanomagnetite (TM ∼ 60) with Curie temperature of ∼180°C did not acquire thermoremanent magnetization (TRM) in these basalts. Instead, the Ti-bearing magnetite (TM ∼ 10–20) that formed as oxidized or exsolved lamellae acquired its first thermal chemical remanent magnetization (CRM) at ∼ 500–400°C during subsolidus cooling. Upon the onset of hydrothermal alteration the recrystallized end-member magnetite acquired a second CRM. The natural remanent magnetization of the basalts from the transition zone and upper sheeted dikes is therefore characteristic of CRMs that were acquired when titanomagnetite altered, in part, to magnetite during subsolidus cooling and hydrothermal alteration close to the ridge axis.

To establish a magnetobiostratigraphy for a 620-m-thick middle to late Pleistocene mudstone sequence in the Lower Gutingkeng Formation of the Tsailiao-chi (TLC) section in southwestern Taiwan, we conducted paleomagnetic and mineral magnetic measurements, together with sediment granulometry and calcareous nannofossil identification. Paleomagnetic samples from 65 sites revealed two types of thermal demagnetization (25–400°C) behavior: (1) single-component stable characteristic remanence in magnetite- and pyrrhotite-dominated samples (Type S), and (2) abrupt changes in polarity when samples with significant greigite concentrations were heated above 320–340°C (Type C). The characteristic polarities derived from Type S samples and from magnetite-dominated Type C samples (obtained above 340°C) are consistent with those determined from nannofossil biostratigraphy. This implies that the NRM carried by magnetite and pyrrhotite is reliable. The essentially antiparallel remanence components in Type C samples below 340°C are attributed to greigite. The almost antiparallel direction could have resulted from delayed formation of greigite, but in this case, the different direction of this component must have resulted from variable remanence lock-in times. Alternatively, the opposite polarities may result from self-reversal, which warrants further investigation. Pyrrhotite and greigite may have both formed authigenically, but there is no clear explanation for the observed differences in direction.

By using bulk samples, rock magnetic measurements were performed to discriminate between pyrrhotite- and greigite-bearing shallow marine sediments that are now uplifted above sea level in southwestern Taiwan. Thermal demagnetization of a composite isothermal remanent magnetization (IRM) was found to be effective in differentiating between the two types of sediments. To check the thermal instability and estimate the true unblocking temperature (TB) spectra of sediments containing these minerals, saturation IRMs (SIRMs) were imparted at each temperature step during demagnetization. While pyrrhotite-bearing samples showed unambiguous TB temperature spectra, greigite-bearing samples underwent considerable alteration which is responsible for most of the decrease in magnetization during thermal demagnetization. Such thermal instability of greigite is a practical and important clue for its identification. Zero-field warming of IRM from 5 to 300 K sensitively indicates the presence of pyrrhotite and trace magnetite in bulk samples without any magnetic separation.

[1] The use of pyrrhotite as an index mineral to delineate metamorphic isograds has long been proposed. However, details of the occurrence of pyrrhotite in different metamorphic facies and its implications have rarely been explored. Here, by simple characterization of magnetic properties and mineral identification, we demonstrate that pyrrhotite is widely distributed in low-grade metamorphic terranes of Taiwan's Central Range and in sediments derived therefrom. By coupling the distribution of pyrrhotite in rocks with that in dated sediment strata, we have studied the denudation history of Taiwan's orogen from a source-to-sink perspective. We suggest that pyrrhotite is a potential tracer for studying surface processes in orogens with high denudation rates similar to that in Taiwan.