Chang-Sik Cheong

Luminescence dating of well-sorted marine terrace sediments on the southeastern coast of Korea

Abstract Along the southeastern coastline of the Korean peninsula, well-developed marine terraces are found at various elevations. The ages of these terraces, and the time of deposition of the terrace sediments are important to our understanding of the geological history of this area during the Quaternary period, and represent a unique record of the regional tectonic activity. Previous efforts to establish a chronology using optically stimulated luminescence (OSL) methods have produced controversial results, particularly because of stratigraphic inconsistency and poor reproducibility. In this paper, the application of OSL dating based on the single-aliquot regenerative-dose (SAR) protocol for quartz is investigated. The dependence of equivalent dose on the preheat and cut-heat temperatures (thermal treatment of the regeneration and test-doses, respectively) are examined. Linearly modulated luminescence signals from chemically cleaned quartz samples are used to identify the presence of a thermally unstable component with a large optical cross-section (component A′), which in part affects the ability to correct for sensitivity changes during measurements, and thus the reliability of the equivalent dose estimates. In some samples, a higher heat treatment after the test-dose is shown to improve our ability to measure a dose given in the laboratory before any heat treatment (dose recovery test). This higher temperature treatment effectively removes component A′, and hence improves sensitivity correction. Furthermore, the samples were broadly divided into poorly sorted and well-sorted, based on field evidence. The poorly sorted samples contain friable, weathered gravel clasts, which is a likely post-depositional source of quartz grains. In general, these grains will not have been zeroed prior to deposition, and so the poorly sorted samples are rejected from further age studies. Results obtained from the well-sorted samples are reproducible at each sampling location, and give ages grouping broadly into 50–70 and 110–120xa0ka, but laterally discontinuous on a scale of tens of km. Our OSL results for the younger group are supported by radiocarbon ages from overlying terrestrial deposits. It is concluded that these results point to considerable tectonic activity in the southeast of Korea during the Late Pleistocene.

The resolution of stratigraphic inconsistency in the luminescence ages of marine terrace sediments from Korea

Several sets of marine terraces are exposed along the southeastern coast of the Korean peninsula. The formation ages of these terraces have attracted considerable attention because they provide essential information on local crustal stability. Over the last few years considerable effort has been put into the determination of these ages using optically stimulated luminescence (OSL) dating of the marine sediments from which the terraces were built. For several sites, the results of dose recovery tests and other OSL characteristics, including the dependence of equivalent dose on heat treatment prior to stimulation, have been described in detail elsewhere (Quat. Sci. Rev. 22 (2003) 407). However, it has been found that the samples from one site exhibit various undesirable OSL characteristics, which result in stratigraphically inconsistent OSL ages. In this paper, we investigate these characteristics, and use luminescence component separation to resolve this inconsistency. The resolved OSL ages obtained as such are then used for the discussion on the local crustal stability of the southeastern coast of Korea during the Late Pleistocene.

Determination of slip rate by optical dating of fluvial deposits from the Wangsan fault, SE Korea

Abstract The time-integrated slip rate in fault zones can be determined if the deformed deposits are reliably dated. Here, we report optically stimulated luminescence (OSL) ages of Late Pleistocene fluvial deposits cut by the Wangsan fault, southeastern Korea, which displaces a hanging wall block of about 28xa0m. Five sandy samples of the deformed Quaternary deposits were dated by quartz OSL using the single aliquot regenerative-dose (SAR) protocol. Three samples taken from the footwall block show stratigraphically consistent OSL ages of 54±7, 76±5 and 90±6xa0ka, from top to bottom. Two samples collected from the same layer in the hanging wall block show reproducible OSL ages of 81±5 and 82±5xa0ka, which are also in good agreement with the stratigraphic relationships. Our OSL ages yield an average sedimentation rate of the Quaternary deposits as around 0.04xa0mmxa0a −1 , and a minimum value of time-integrated slip rate as 0.52xa0mmxa0a −1 . This minimum slip rate is considerably higher than those reported earlier for Quaternary faults in southeastern Korea. The youngest OSL age (54±7xa0ka) constrains the maximum value of the recurrence interval of the fault movement.

Determination of sedimentation rate of a recently deposited tidal flat, western coast of Korea, using IRSL dating.

Abstract Luminescence dating using infrared stimulation of K-feldspar grains was undertaken to determine the sedimentation rate of a recently deposited tidal flat on the western coast of Korea. Five samples were collected from a core of the tidal flat. The very low luminescence intensities in the surface sample indicate that the material was well bleached at deposition. Recuperation before and after preheating was investigated on natural and laboratory-bleached surface samples and considered in age calculations. The five IRSL age estimates show a reasonable stratigraphic correlation and allow the calculation of the sedimentation rate of 6.1±0.4xa0cmxa0yr −1 . We conclude that infrared stimulated luminescence dating can contribute significantly to the determination of sedimentation rates of tidal flats.

230Th/234U dating of Holocene mollusk shells from Jeju Island, Korea, by multiple collectors inductively coupled plasma mass spectrometry

The230Th/234U ages of fossil mollusk shells collected from the Sinyangri and Hamori Formations, the youngest stratigraphic units on Jeju Island, Korea, were estimated using multiple collectors inductively coupled plasma mass spectrometry. Seven aragonite shells yielded230Th/234U ages ranging from 3434±40 yr to 4980±33 yr (2σm), in concordance with radiocarbon ages for samples containing little232Th. Our data suggest that the230Th/234U method can be a potentially useful tool for dating Holocene mollusks, provided that there is no evidence for severe recrystallization, cementation, and considerable amount of common thorium. An early uptake of uranium immediately after burial and subsequent maintenance of closed system for uranium and thorium can be assumed for our mollusk samples. This study, together with previous radiocarbon and optical dating results, demonstrates that the Hamori Formation where human footprints were recently discovered deposited during the middle Holocene.

Simultaneous multiple collector-ICP-MS measurement of Nd isotopic composition and Sm/Nd ratio in geological reference materials by interference corrections and external calibration using matrix-matched standards

This study introduces an improved multiple collector-ICP-MS protocol to simultaneously determine the Nd isotopic composition and Sm/Nd ratio of geological samples without isotope dilution. Our pretreatment procedure simply consists of sample digestion and single-step group separation of Sm and Nd. The Nd isotopic composition was conventionally corrected for Sm isobaric interference by using the exponential mass fractionation law and selected Sm isotopic compositions, and the Sm/Nd ratio was externally corrected by the standard-sample bracketing method. This protocol yielded the corrected 143Nd/144Nd ratios of the USGS GRMs (BHVO-2, BCR-2, AGV-2, and RGM-1) mostly in good agreement with previously recommended values. By contrast, their Sm/Nd ratios were consistently higher (up to 3.4%) than the recommended values when the measured ratios were corrected simply using data of an aqueous standard solution. Further calibration using a matrix-matched GRM as a secondary standard significantly reduced the discrepancies with previous ratios to within a 0.3% difference. Therefore, we recommend that the matrix of calibration standard should be matched with that of the samples as closely as possible in the inter-elemental ratio measurement of geological samples.

Mineral ages and zircon Hf isotopic composition of the Andong ultramafic complex: implications for the evolution of Mesozoic subduction system and subcontinental lithospheric mantle beneath SE Korea

The Phanerozoic subduction system of the Korean peninsula is considered to have been activated by at least Middle Permian time. The geochemically arc-like Andong ultramafic complex (AUC) occurring along the border between the Precambrian Yeongnam massif and the Cretaceous Gyeongsang back-arc basin provides a rare opportunity for direct study of the pre-Cretaceous mantle wedge lying above the subduction zone. The tightly constrained SHRIMP U–Pb age of zircons extracted from orthopyroxenite specimens (222.1±1.0 Ma) is indistinguishable from the Ar/Ar age of coexisting phlogopite (220±6 Ma). These ages represent the timing of suprasubduction zone magmatism likely in response to the sinking of cold and dense oceanic lithosphere and the resultant extensional strain regime in a nascent arc environment. The nearly coeval occurrence of a syenite-gabbro-monzonite suite in the SW Yeongnam massif also suggests an extensional tectonic setting along the continental margin side during Late Triassic time. The relatively enriched ɛ Hf range of dated zircons (+6.2 to −0.6 at 222 Ma) is in contrast to previously reported primitive Sr–Nd–Hf isotopic features of Cenozoic mantle xenoliths from Korea and eastern China. This enrichment is not ascribed to contamination by the hypothetical Palaeozoic crust beneath SE Korea, but is instead attributable to metasomatism of the lithospheric mantle during the earlier subduction of the palaeo-Pacific plate. Most AUC zircons show a restricted core-to-rim spread of ɛ Hf values, but some grains testify to the operation of open-system processes during magmatic differentiation.

Reevaluation of Th and U concentrations in marine sediment reference materials using isotope dilution MC-ICP-MS: towards the analytical improvements in dose rate estimation for luminescence dating

The thorium and uranium concentrations of four marine sediment reference materials (CRMs NIST 1646a, NRCC HISS-1, MESS-3 and PACS-2) were determined using isotope dilution-multiple collector-inductively coupled plasmas-mass spectrometry (ID-MC-ICP-MS), aiming at helping the assessment of reliability in dose rate estimation for luminescence dating. Quintuplicate analyses of the CRMs revealed their intrinsic heterogeneity in the concentration of both elements (up to 23% and 8% for thorium and uranium, respectively, in 2σ standard error level) for individual sub-samples of about 100 mg. The thorium and uranium concentrations of all CRMs determined in this study were significantly lower (up to 45%) than the provided or reported informative values, requiring reevaluation of their reference concentrations. The use of informative thorium and uranium concentrations in these CRMs as standard values for elemental analysis of unknown samples will result in overestimated dose rates, and thus age underestimation when applied to luminescence dating techniques.

Strontium isotope geochemistry of archaeological human tooth enamel excavated from Jeonju, southwestern Korea

Strontium isotopes of archaeological skeletal tissues have provided useful information to trace past human migration. This study presents strontium isotope data of human tooth enamel excavated from a group of medieval Joseon tombs in Jeonju, southwestern Korea. The enamel samples are divided into two isotopic populations yielding average 87Sr/86Sr of 0.71077 ± 0.00031 (group I) and 0.71233 ± 0.00032 (group II). Considering the gender distribution, 87Sr/86Sr ratios of group I may better represent the local value of biologically available strontium. Contrasting 87Sr/86Sr patterns of acetic acid extracts and residues of enamel fragments from a male of group I and a female of group II suggest their different residential mobility during childhood. The 87Sr/86Sr range of group I enamel is considerably lower than that reported from basement granites in the Jeonju area. Strontium in this group may have been conveyed from rainfall, and soluble soil fractions composed mainly of low-Rb/Sr minerals such as plagioclase.