加速器质谱计在第四纪年代学研究中的应用

介绍了加速器质谱计测试长寿命宇宙成因核素＾１０Ｂｅ，＾３６Ｃｌ，＾３２Ｓｉ，＾１４Ｃ，＾４１Ｃａ，＾１２９Ｉ的基本原理，根据国内外资料以及自己的研究成果，着重讨论了ＡＭＳ在第四纪地质年代学，古环境示踪，气候变化等研究领域的应用和前景。     

Neogene magnetic polarity stratigraphy of some fluviatile Siwalik sections, Nepal

The magnetostratigraphy of five new sections through the Neogene Siwalik Group of Nepal is presented. Rock magnetic experiments and detailed thermal demagnetization experiments prove that haematite is the carrier of a primary DRM or a PDRM. After stepwise thermal demagnetization of specimens from all sections, directions of characteristic remanent magnetization were obtained. The results show a positive reversal test and exhibit inclination errors of about 20. Due to gaps in exposures and extremely variable demagnetization behaviour of the haematite-bearing sediments, the resulting polarity sequences are poorly defined for some parts of the sections. However, double-sampled parts of some sections yield similar results. A correlation with the Surai Khola section (Appel, Rösler & Corvinus 1991) is proposed, based on rock magnetic parameters. AMS results from three of the sections prove the existence of primary sedimentary magnetic fabrics in these sections with systematic orientations of minimum and maximum susceptibility axes. Correlation with a standard polarity timescale indicates that the ages of all new sections lie within the age limits of the Surai Khola section.     

河北平原第四系地下水^36Cl年龄研究

本文报道了河北平原第四系地下水~(36)Cl研究的样品采集、制备,加速器质谱计(AMS)测定及地下水~(36)Cl年龄的计算原理和结果。     

Non-scaled analogue modelling of AMS development during viscous flow: A simulation on diapir-like structures

Development of magnetic fabric within a diapirically ascending columnar body was investigated using non-scaled analogue model made of plaster of Paris containing small amount of fine-grained homogeneously mixed magnetite. The apparatus for the modelling consists of a manual squeezer with calibrated spring and a Perspex container. Set of weak coloured layers at the bottom of the container was forced to intrude overlying fine-grained sand through a hole in a board attached to the squeezer. The development of AMS fabric is correlated with complex flow pattern indicated by coloured and originally horizontal plaster layers. Strongly constrictional and vertical fabric in the base and in the lower domain of the diapir resulting from convergent and upwards flows is overprinted by subhorizontal oblate fabrics due to vertical flattening and initial divergent flow in the apical parts. The measured AMS fabrics are compared with natural examples of magmatic stocks and dykes.     

Intra-shoreface erosion in response to rapid sea-level fall: depositional record of a tectonically uplifted strand plain, Pacific coast of Japan

This study documents lowering of the surf zone (i.e. the upper shoreface) leading to intra-shoreface erosion, following two rapid relative sea-level falls along a tectonically uplifted coast during the Holocene, and the characteristics of the resultant prograding shoreface deposits. These findings are based on high-resolution analysis and radiocarbon dating of three new drill cores obtained from the Kujukuri strand plain, Pacific coast of eastern Japan, combined with previously published borehole data and information on modern shoreline profile adjustments. A shallowing-upward sandy succession composed of lower and upper shoreface facies, foreshore and backshore facies was recognized in the drill cores. Two rapid falls in relative sea-level at 2·3 to 2·6 and 1·8 to 2·0 ka are recorded by downstepping of the base of the foreshore facies, and farther seawards by the lowering of an erosional boundary between the upper and lower shoreface facies. Superimposed bed profiles of an adjacent modern beach define an envelope, the base of which reflects shore-normal migration of longshore bars and troughs. The base of the envelope represents an erosional surface that divides the surface mobile layer above from preserved deposits beneath. The surface is concave upwards and steeper than the mean beach profile, and exhibits a flat platform approximately at the lower limit of the upper shoreface equating to the storm surf zone. The seaward transition of this surface, rather than the mean equilibrium profile, controls the metre-scale to decimetre-scale internal structure of the Kujukuri shoreface deposits. Depositional models for sea-level fall based on an exponential equilibrium profile do not adequately account for the presence and migration of longshore bars and troughs.     

Nine unusually large tsunami deposits from the past 4000 years at Kiritappu marsh along the southern Kuril Trench

Large earthquakes along the Kuril subduction zone in northern Japan are known to have caused damaging tsunami, although there is a little information on historical earthquakes and tsunami in this area because no documents exist before the 19th century that might refer to tsunami events. To determine the likely timing and size of future events we need information on their recurrence intervals and to do this for the prehistoric past we have investigated sediments located in the Kiritappu marsh in eastern Hokaido that we interpret as laid down by tsunami. Using reliable multiple lines of evidence from sedimentological, geomorphological, micropaleontological, and chronological results, we identify 13 tsunami sands. Two of these lie within a peat bed above a historical tephra, Ta-a (AD 1739); the upper one probably corresponds to the AD 1843 Tempo Tokachi-oki earthquake (M 8.2) tsunami, and the lower to either the AD 1952 Tokachi-oki earthquake (M 8.2) tsunami or the AD 1960 Chilean earthquake (M 9.5) tsunami. Underlying are 11 prehistoric tsunami sand beds (nine large sand beds and two smaller sand beds) deposited during the past 4000 years. Because of the wide spatial distribution of the large sand beds, and inundation distances inland of between 1200 to 3000 m, we suggest that they record unusually large tsunamis along the Kuril subduction zone. According to our analyses, these tsunami sands were derived from the coastal area and, although they do not show clear graded bedding, they commonly have gradational upper boundaries and erosional bases and include internal sedimentary structures such as plane beds, dunes, and current ripples, reflecting bedload transportation. Based on our results we calculate the recurrence interval of unusually large earthquakes (probably M 8.6) along the Kuril subduction zone as about 365–553 years and estimate the youngest large event to have occurred in the 17th century.     

Measurement of Ambient,Interstitial, and Residual Aerosol Particles on a Mountaintop Site in Central Sweden using an Aerosol Mass Spectrometer and a CVI

The Aerodyne aerosol mass spectrometer (Q-AMS) was coupled with a counterflow virtual impactor (CVI) for the first time to measure cloud droplet residuals of warm tropospheric clouds on Mt. Åreskutan in central Sweden in July 2003. Operating the CVI in different operational modes generated mass concentration and species-resolved mass distribution data for non-refractory species of the ambient, interstitial, and residual aerosol. The ambient aerosol measurements revealed that the aerosol at the site was mainly influenced by long-range transport and regional photochemical generation of nitrate and organic aerosol components. Four different major air masses were identified for the time interval of the experiment. While two air masses that approached the site from northeastern Europe via Finland showed very similar aerosol composition, the other two air masses from polar regions and the British Islands had a significantly different composition. During cloud events the larger aerosol particles were found to be activated into cloud droplets. On a mass basis the activation cut-off diameter was approximately 150 nm for nitrate and organics dominated particles and 200 nm for sulfate dominated particles. Generally nitrate and organics were found to be activated into cloud droplets with higher efficiency than sulfate. While a significant fraction of the nitrate in ambient particles was organic nitrates or nitrogen-containing organic species, the nitrate found in the cloud droplet residuals was mainly ammonium nitrate. After passage of clouds the ambient aerosol size distribution had shifted to smaller particle sizes due to the predominantly activation of larger aerosol particles without a significant change in the relative composition of the ambient aerosol.     

河北平原三河S9孔岩心特征及第四纪地层划分研究

第四纪地层年代学格架的建立对区域地层对比及古地理环境演化研究具有重要意义。通过分析河北大厂凹陷北部第四系标准孔S9钻孔岩心沉积物颜色、结构构造、岩性组合特征,应用测井曲线、古地磁、AMS ¹⁴C和OSL测年方法,对S9钻孔剖面进行详细地层划分、沉积相分析和地层年代学格架建立。将S9孔揭示的松散沉积物分为4套岩性组合:第1套岩性组合代表洪积扇沉积;第2套岩性组合代表辫状河沉积;第3套岩性组合代表洪积扇沉积;第4套岩性组合代表泛滥平原沉积和河床沉积。通过对古地磁实验结果的分析得出, 0~145.3 m为古地磁极性带的布容正极性时,145.3~212.6 m为古地磁极性带的松山反极性时,212.6~251.8 m为古地磁极性带的高斯正极性时。以磁性地层为主导,以岩性地层特征、¹⁴C和OSL测年数据为辅,对S9孔第四纪岩性地层进行初步划分,其第四纪地层的下更新统、中更新统和上更新统的底界对应深度分别为210.9 m、145.4 m和24.05 m。     

Dating basal peat: The geochronology of peat initiation revisited

Attributing the start of peat growth to an absolute timescale requires dating the bottom of peat deposits overlying mineral sediment, often called the basal peat. Peat initiation is reflected in the stratigraphy as a gradual transition from mineral sediment to increasingly organic material, up to where it is called peat. So far, varying criteria have been used to define basal peat, resulting in divergent approaches to date peat initiation. The lack of a universally applicable and quantitative definition, combined with multiple concerns that have been raised previously regarding the radiocarbon dating of peat, may result in apparent ages that are either too old or too young for the timing of peat initiation. Here, we aim to formulate updated recommendations for dating peat initiation. We provide a conceptual framework that supports the use of the organic matter (OM) gradient for a quantitative and reproducible definition of the mineral-to-peat transition (i.e., the stratigraphical range reflecting the timespan of the peat initiation process) and the layer defined as basal peat (i.e., the stratigraphical layer that is defined as the bottom of a peat deposit). Selection of dating samples is often challenging due to poor preservation of plant macrofossils in basal peat, and the representativity of humic and humin dates for the age of basal peat is uncertain. We therefore analyse the mineral-to-peat transition based on three highly detailed sequences of radiocarbon dates, including dates of plant macrofossils and the humic and humin fractions obtained from bulk samples. Our case study peatland in the Netherlands currently harbours a bog vegetation, but biostratigraphical analyses show that during peat initiation the vegetation was mesotrophic. Results show that plant macrofossils provide the most accurate age in the mineral-to-peat transition and are therefore recommendable to use for ¹⁴C dating basal peat. If these are unattainable, the humic fraction provides the best alternative and is interpreted as a terminus-ante-quem for peat initiation. The potential large age difference between dates of plant macrofossils and humic or humin dates (up to ∼1700 years between macrofossil and humic ages, and with even larger differences for humins) suggests that studies reusing existing bulk dates of basal peat should take great care in data interpretation. The potentially long timespan of the peat initiation process (with medians of ∼1000, ∼1300 and ∼1500 years within our case study peatland) demonstrates that choices regarding sampling size and resolution need to be well substantiated. We summarise our findings as a set of recommendations for dating basal peats, and advocate the widespread use of OM determination to obtain a low-cost, quantitative and reproducible definition of basal peat that eases intercomparison of studies.