Age of the Tananwan Formation in Northern Taiwan: A Reexamination of the Magnetostratigraphy and Calcareous Nannofossil Biostratigraphy

Over the past two decades, the succession strata of the Tananwan Formation exposed horizontally on the Linkou Tableland in northwestern Taiwan has been either dated to a short time interval of 0.9 - 0.7 Ma or assigned to a vague long period within the Matuyama Epoch covering probably both the Olduvai and Jaramillo normal events. In this study, a reexamination of magnetostratigraphy and calcareous nannofossil biostratigraphy of this formation was carried out. The results show that the strata with stable remanent magnetizations have reversed magnetic polarities only, and the layers with marine sedimentary facies consistently contain specimens of large Gephyrocapsa. These updated results suggest that the Tananwan Formation was deposited within the interval of 1.46 - 1.24 Ma, corresponding to a short period of reversed magnetic polarity of the Matuyama Epoch

The Low-Temperature Besnus Magnetic Transition: Signals Due to Monoclinic and Hexagonal Pyrrhotite

The low‐temperature magnetic properties of the many pyrrhotite varieties have not been studied extensively. Monoclinic pyrrhotite (Fe7S8) goes through the Besnus transition at ~30–34 K, which is used widely to diagnose its presence in bulk samples. Other pyrrhotite polytypes are assumed to be antiferromagnetic, although it has been suggested occasionally that some may also have remanence‐carrying capabilities. Here we compare the magnetic properties of monoclinic (4M) and hexagonal (3T) pyrrhotite at low temperatures. The 4M pyrrhotite records a Besnus transition consistently. Despite not recording a Besnus transition, 3T pyrrhotite has a magnetic remanence at room temperature and has distinctive room‐ and low‐temperature magnetic properties that cannot be explained by known or unidentified impurities (with abundances 700 mT) and stable coercivities below 50 K. The importance of this mineral in fossil or active gas hydrate and methane venting environments makes it important to develop a more detailed understanding of its occurrences and magnetic properties.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

Rotational remanent magnetization as a magnetic mineral diagnostic tool at low rotation rates

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.</jats:p

Rotational remanent magnetisation as a magnetic mineral diagnostic tool at low rotation rates

Summary Prior work on rotational remanent magnetisation (RRM) and rotational anhysteretic remanent magnetisation (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 datasets. Using a RAPID magnetometer we show that the range of usable practical rotation rates is 0.25 to 3 Hz [rps] which allows a wide range of RRM and ARMROT characteristics to be utilised (at 100 mT AF field, 100μT bias field). Sets of magnetic mineral extracts from sediments, and well characterised 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, 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 characterisation 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 demagnetisation 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

Insolation-paced sea level and sediment flux during the early Pleistocene in Southeast Asia

Global marine archives from the early Pleistocene indicate that glacial-interglacial cycles, and their corresponding sea-level cycles, have predominantly a periodicity of ~ 41 kyrs driven by Earth’s obliquity. Here, we present a clastic shallow-marine record from the early Pleistocene in Southeast Asia (Cholan Formation, Taiwan). The studied strata comprise stacked cyclic successions deposited in offshore to nearshore environments in the paleo-Taiwan Strait. The stratigraphy was compared to both a δ18O isotope record of benthic foraminifera and orbital parameters driving insolation at the time of deposition. Analyses indicate a strong correlation between depositional cycles and Northern Hemisphere summer insolation, which is precession-dominated with an obliquity component. Our results represent geological evidence of precession-dominated sea-level fluctuations during the early Pleistocene, independent of a global ice-volume proxy. Preservation of this signal is possible due to the high-accommodation creation and high-sedimentation rate in the basin enhancing the completeness of the stratigraphic record

Extremely rapid up-and-down motions of island arc crust during arc-continent collision

10 pagesMountain building and the rock cycle often involve large vertical crustal motions, but their rates and timescales in unmetamorphosed rocks remain poorly understood. We utilize high-resolution magneto-biostratigraphy and backstripping analysis of marine deposits in an active arc-continent suture zone of eastern Taiwan to document short cycles of vertical crustal oscillations. A basal unconformity formed on Miocene volcanic arc crust in an uplifting forebulge starting ~6 Ma, followed by rapid foredeep subsidence at 2.3–3.2 mm yr−1 (~3.4–0.5 Ma) in response to oceanward-migrating flexural wave. Since ~0.8–0.5 Ma, arc crust has undergone extremely rapid (~9.0–14.4 mm yr−1) uplift to form the modern Coastal Range during transpressional strain. The northern sector may have recently entered another phase of subsidence related to a subduction polarity reversal. These transient vertical crustal motions are under-detected by thermochronologic methods, but are likely characteristic of continental growth by arc accretion over geologic timescales

Detection of noninteracting single domain particles using first-order reversal curve diagrams

We present a highly sensitive and accurate method for quantitativedetection and characterization of noninteracting or weakly interactinguniaxial single domain particles (UNISD) in rocks and sediments. Themethod is based on high-resolution measurements of first-order reversalcurves (FORCs). UNISD particles have a unique FORC signature that can beused to isolate their contribution among other magnetic components. Thissignature has a narrow ridge along the H(c) axis of the FORC diagram,called the central ridge, which is proportional to the switching fielddistribution of the particles. Therefore, the central ridge is directlycomparable with other magnetic measurements, such as remanentmagnetization curves, with the advantage of being fully selective to SDparticles, rather than other magnetic components. This selectivity isunmatched by other magnetic unmixing methods, and offers usefulapplications ranging from characterization of SD particles forpaleointensity studies to detecting magnetofossils and ultrafineauthigenically precipitated minerals in sediments

Complicated Magnetic Mineral Assemblages in Marine Sediments Offshore of Southwestern Taiwan: Possible Influence of Methane Flux on the Early Diagenetic Process

In this paper, we present the results of down core variations in magnetic susceptibility and magnetic mineral composition of fifteen piston cores retrieved from the active continental margin offshore of southwestern Taiwan, where a wide distribution of bottom simulating reflectors (BSRs) related to gas hydrate layers has been detected. X-ray analysis on magnetic extracts from the cores indicates that detrital magnetite and authigenic greigite, in various proportions, are the dominant magnetic minerals in sediments. Non-magnetic, authigenic pyrite is generally associated with greigite, but it may co-exist with magnetite or may solely occur in sediments. Consequently, the sediment sequences of the fifteen piston cores have complicated magnetic mineral assemblages that result in various magnetic susceptibility profiles. For core segments containing detrital magnetite as the only magnetic mineral, values of magnetic susceptibility are moderate with small variations (8 - 15 Ă 10-6 SI), which can be regarded as the susceptibility backgrounds for the initially deposited sediments. However, high magnetic susceptibilities relative to the backgrounds were found in core segments where magnetic mineral is enriched in greigite. Magnetic susceptibilities lower than the backgrounds were found in segments where neither magnetite nor greigite were detected. The complicated occurrence of magnetic minerals reveals that their host sediments at different levels have suffered various degrees of early diagenetic process ranging from oxic to anoxic conditions. Due to very low sedimentary organic matter content in the study area, detrital magnetite persisted in sediments that were subjected to initially oxic (or sub-oxic) conditions. However, the sedimentary organic matter is apparently insufficient for providing reducing environments for the formation or enrichment of authigenic iron sulfides. Instead, we propose that gaseous methane derived from gas hydrates in deeper sedimentary layers should be the most likely source of extra organic matter. The different flux intensities of this gas either through slow diffusion or rapid venting resulted in various anoxic conditions, which caused the dissolution or survival of magnetite and the formation or enrichment of greigite and pyrite