北京大学AMS 14 C 国际比对样品测量*

介绍了北京大学加速器质谱(AMS)实验室参加国际原子能机构 (IAEA) 组织的第五届¹⁴ C国际比对样品测量的过程和结果,包括样品制备、小型¹⁴ C测量加速器质谱计装置和比对测量结果的统计分析等。北京大学的测量结果与IAEA加权统计平均值的偏差在1σ之内, 这一结果较客观地体现了我国¹⁴ C AMS年代测定在国际同类实验室中所处的地位。     

泥炭样品的AMS 14 C年龄测定: 全样、植物残体和孢粉浓缩物*

沉积物¹⁴ C的年龄测定一直是第四纪年代学研究的热点。文章对中国干旱、半干旱地区的内蒙古库伦泥炭剖面的泥炭全样、植物残体和孢粉浓缩物进行了AMS ¹⁴ C测年比较研究。从泥炭沉积物中提取用于AMS ¹⁴ C测年的孢粉浓缩物的关键步骤包括:过筛(125μm,63μm和10μm),重液浮选(比重1.9)及在180倍的体视显微镜下进行手工挑选。测年结果表明:孢粉浓缩物的测年值比同一层位的泥炭沉积物全样或植物残体的测年值老225~340年; 植物残体与孢粉浓缩物的测年结果较为接近。但是,距地表24~27cm处孢粉浓缩物给出了610~780A.D.的年龄值,远老于其下样点的年龄,而泥炭全样为现代样品,二者相差1255年,这可能与该样品孢粉纯度较低、掺杂有一些老碳物质燃烧后形成的微粒杂质有关。因此,提高孢粉纯度对孢粉浓缩物¹⁴ C测年至关重要。     

柴达木盆地察尔汗贝壳堤剖面年代学研究*

利用常规¹⁴ C、加速器(AMS)和²³⁰ Th等测年方法,对位于青藏高原东北部柴达木盆地察尔汗古湖贝壳堤剖面化石贝壳、湖泊沉积(包括现代沉积)样品有机质(包括碱性残留和胡敏酸组分)、碳酸盐(CaCO₃)和剖面顶部石盐(NaCl)晶体在不同实验室进行了系统的实验研究,探讨了不同样品测年结果的可靠性和精确性,并通过与已有研究结果的对比,对柴达木盆地贝壳堤剖面记录的察尔汗古湖高湖面演化历史进行了探讨,得出察尔汗古湖高湖面形成于约39.7~17.5 ¹⁴ C kaB.P. ; 研究证明柴达木盆地沉积物中有机质含量很低,且主要来源于菌藻类等低等藻类和微生物,受老碳和溶解物质影响强烈,因此必须经过多种方法综合测年才能比较可靠地确定所测样品的形成年代; 测年结果的不确定性不仅来源于样品自身元素(同位素)的特性和沉积过程中再沉积作用的影响,也可能源自于放射性¹⁴ C产生过程的不稳定性; 对比发现贝壳化石老于同层位有机质¹⁴ C年龄15~18ka,这可能与柴达木盆地所处的特殊地理位置和巨大的古湖泊面积及水体有关; 同时,不平衡铀系测年在确定开放体系矿物晶体年代应用中的可靠性尚须进一步深入分析,其机理也还需要探讨和实验研究。     

宇宙射线通量和气候变迁与地球磁场强度变化的关系

通过分析近30000年以来地球磁场强度和大气中¹⁴C 含量变化,讨论了¹⁴C年龄偏离实际值的原因,提出以地球磁场强度随时间的变化率校正¹⁴C 年龄T_c 偏离实际年龄T_t 的公式,即T_t=T_c-b(T_c-T_?|_F（1）ΔF/ΔT。用这一公式校正了¹⁴C 测年技术给出的新仙女木事件发生的时限,并获得了与树轮年龄和U-Th 年龄一致的结果。本文还对12000年以来地球磁场强度变化与气候变迁的相关性做了探讨。     

AMS14C法在地学中应用的发展近况

本文介绍了国外利用AMS¹⁴C法研究极冰、海水中溶解的碳、大气中含碳,深海沉积含碳物等的最新进展和成果。这些成果表明,由于AMS¹⁴C法测定需要的样品量仅是常规测定用量的千分之一,因而为在地学领域中的应用开辟了广阔的前景。我国的AMS¹⁴C方法研究已经起步,新方法的建成和使用必将推动有关学科的进一步发展。     

10 Be在大洋边缘海洋学中的应用及模型

主要讨论宇宙射线成因核素¹⁰Be（T _1/2＝ 1．5Ma）在大洋边缘海洋学尤其是中国近海海洋研究中的应用。在过去的近20年中，在中国开展的¹⁰Be研究在黄土堆积年龄及地层对比方面获得了诸多成果，但在海洋方面的应用研究距国际水平仍有一定的差距，尚需进一步加强。综述了海洋环境中¹⁰Be作为一个地球化学示踪剂的研究现状，着重介绍¹⁰Be在中国东部海域的收支平衡模式以及讨论¹⁰Be在太平洋西部边缘海及岛弧地区的应用前景。     

14C测年技术新进展

¹⁴C测年技术的进展主要表现为3个方面：1)¹⁴C常规测定技术向高精度发展比较成熟，目前已可以普及；2)加速器质谱技术的建立和普及，使测定要求的样品碳量减少到mg级，甚至μg级，虽然目前的精度尚未超过常规方法，但由于所需样品量极少，测定时间短而工效高，大大拓宽了应用范围；3)高精度¹⁴C－树轮年龄校正曲线的建立，不但可将样品的¹⁴C年龄转换到日历年龄，而且对有时序的系列样品的¹⁴C年龄数据通过曲线拟合方法转换到日历年龄时年龄误差大为缩小。     

湖泊沉积物的14 C和光释光测年* ——以固城湖为例

富含有机质的湖泊沉积物被认为是¹⁴ C测年建立古环境记录年代标尺的理想材料,光释光测年方法近年开始应用于水成沉积物的定年。应用¹⁴ C和光释光两种方法对江苏固城湖湖心钻孔岩芯进行了年龄测定,结果表明全样有机质的¹⁴ C年龄与石英的光释光年龄存在系统差异,后者较前者年轻约2000年。系统光释光测年研究排除了光释光年龄低估的可能性,所以,二者的差异可能是湖泊沉积物碳库效应的反映。     

博斯腾湖湖泊沉积物光释光年代测量*

使用光释光年代学的单片再生法测量了博斯腾湖沉积剖面中碳酸盐泥及粉砂质泥底部的浅湖相灰色粉细砂和风成沙的年龄,对剖面上部碳酸盐层中陆生植物残体进行了AMS ¹⁴ C测年。通过不同测片的等效剂量(D_e)值的分布状况评价了样品的晒褪程度,选择不随灵敏度校正后的自然释光信号变化的相对集中的等效剂量(D_e)值计算了样品的埋藏年龄。通过这些年龄结果的对比,发现石英矿物的OSL年龄和AMS ¹⁴ C年龄在地层上是一致的,表明尽管在浅湖相细砂中存在不完全晒褪,但根据相对较小而集中的D_e值计算得到的年龄结果是可靠的。这些年龄结果和地层资料揭示末次冰消期以来至早全新世,博斯腾湖处于无水干盆地向深水湖泊转化的浅水湖泊状态,现代深水博斯腾湖大约形成于距今8ka前后。     

第四纪骨化石样品的多方法对比测年

本文报道作者用¹⁴C、铀系和ESR等多种测年技术对比测定第四纪骨化石样品年龄结果,对比分析骨化石中各含碳组分¹⁴C年龄的异同。在此基础上讨论诸测年方法的可靠性和测年精度,分析哪种含碳组分最能代表骨质样品的真实年龄。对晚于40 000aB.P.的骨质样品,作者倾向于样品中氨基酸的¹⁴C测年,而对更老的样品,铀系法应优先被选用。本文还对北京周口店山顶洞遗址骨化石样品中不同含碳组分的¹⁴C测年结果做了讨论。     

The radiocarbon age of organic carbon in marine surface sediments

Long-term carbon cycling and climate change are strongly dependent on organic carbon (OC) burial in marine sediments. Radiocarbon (¹⁴C) has been widely used to constrain the sources, sinks, and processing of sedimentary OC. To elucidate the dominant controls on the radiocarbon content of total organic carbon (¹⁴C_TOC) accumulating in surface sediments we construct a box model that predicts ¹⁴C_TOC in the sediment mixed layer (measured as fraction modern, Fm). Our model defines three distinct OC pools (“degradable,” “semi-labile,” and “refractory”) and assumes that ¹⁴C_TOC flux to sediments is exclusively derived from surface ocean primary productivity, and hence follows a “generic” surface ocean dissolved inorganic carbon (DIC) bomb curve. Model predictions are compared to a set of 75 surface sediment samples, which span a wide geographic range and reflect diverse water column and depositional conditions, and for which sedimentation rate and mixed-layer depth are well characterized. Our model overestimates the Fm value for a majority (65%) of these sites, especially at shallow water depths and for sites characterized by depleted δ¹³C_TOC values. The model is most sensitive to sedimentation rate and mixed-layer depth. Therefore, slight changes to these parameters can lead to a match between modeled and measured Fm values at many sites. Yet, in some cases, we find that measured Fm values cannot be simulated without large and unrealistic changes to sedimentation rate and mixed-layer depth. These results point to sources of pre-aged OC to surface sediments and implicate soil-derived terrestrial OC, reworked marine OC, and/or anthropogenic carbon as important components of the organic matter present in surface sediments. This approach provides a valuable framework within which to explore controls on sedimentary organic matter composition and carbon burial over a range of spatial and temporal scales.     

Large-volume ultrafiltration for the study of radiocarbon signatures and size vs. age relationships in marine dissolved organic matter

In recent decades, tangential-flow ultrafiltration (UF) technology has become a primary tool for isolating large amounts of “ultrafiltered” marine dissolved organic carbon (UDOC; 0.1 μm to ∼1 nm) for the detailed characterization of DOC chemical composition and radiocarbon (Δ¹⁴C) signatures. However, while total DOC Δ¹⁴C values are generally thought to be quite similar in the world ocean, previous studies have reported widely different Δ¹⁴C values for UDOC, even from very similar ocean regions, raising questions about the relative “reactivity” of high molecular weight (HMW) DOC. Specifically, to what degree do variations in DOM molecular weight (MW) vs. composition alter its relative persistence, and therefore HMW DOC Δ¹⁴C values?In this study we evaluate the effects of varying proportions of HMW vs. low molecular weight (LMW) DOC on UDOC Δ¹⁴C values. Using concentration factor (CF) as a proxy for MW distributions, we modeled the retention of both OC and Δ¹⁴C in several very large CF experiments (CF >3000), from three depths (20, 670, and 915 m) in the North Pacific Subtropical Gyre (NPSG). The resulting DOC and Δ¹⁴C UF permeation coefficients generally increase with depth, consistent with mass balance trends, indicating very significant permeation of LMW, ¹⁴C-depleted DOC at depth, and higher recoveries of Δ¹⁴C-enriched, HMW DOC in the surface. In addition, changes in CF during sample concentration and ionic strength during sample diafiltration had very large and predictable impacts on UDOC Δ¹⁴C values.Together these results suggest that previously reported disparities in UDOC Δ¹⁴C values are reconciled by linked trends of Δ¹⁴C content vs. MW. At low CFs, UDOC samples have similar Δ¹⁴C values to total DOC. In contrast, UDOC samples collected at extremely high CFs (and after diafiltration) have more positive Δ¹⁴C values. We demonstrate that the observed relationships between UDOC Δ¹⁴C and CF derived from our data can directly explain offsets in all previously published UDOC Δ¹⁴C values for the NPSG. While CF is not traditionally considered in UF studies, our results indicate it can substantially influence the interpretation of UDOC ¹⁴C “age”, and thus reactivity, in the marine environment. In addition, our results indicate that CF can in fact be used as a proxy for average MW. We suggest that a variable-CF-UF approach, coupled with molecular-level Δ¹⁴C analyses, presents a new tool for studying relationships between molecular size, age, and “labile” DOC distributions in the ocean.     

Controls on the age of vascular plant biomarkers in Black Sea sediments

Transfer of organic carbon (OC) from the terrestrial to the oceanic carbon pool is largely driven by riverine and aeolian transport. Before transport, however, terrigenous organic matter can be retained in intermediate terrestrial reservoirs such as soils. Using compound-specific radiocarbon analysis of terrigenous biomarkers their average terrestrial residence time can be evaluated.Here we show compound-specific radiocarbon (¹⁴C) ages of terrigenous biomarkers and bulk ¹⁴C ages accompanied by geochemical proxy data from core top samples collected along transects in front of several river mouths in the Black Sea. ¹⁴C ages of long chain n-alkanes, long chain n-fatty acids and total organic carbon (TOC) are highest in front of the river mouths, correlating well with BIT (branched and isoprenoid tetraether) indices, which indicates contribution of pre-aged, soil-derived terrigenous organic matter. The radiocarbon ages decrease further offshore towards locations where organic matter is dominated by marine production and aeolian input potentially contributes terrigenous organic matter. Average terrestrial residence times of vascular plant biomarkers deduced from n-C₂₉₊₃₁ alkanes and n-C₂₈₊₃₀ fatty acids ages from stations directly in front of the river mouths range from 900 ± 70 years to 4400 ± 170 years. These average residence times correlate with size and topography in climatically similar catchments, whereas the climatic regime appears to control continental carbon turnover times in morphologically similar drainage areas of the Black Sea catchment. Along-transect data imply petrogenic contribution of n-C₂₉₊₃₁ alkanes and input via different terrigenous biomarker transport modes, i.e., riverine and aeolian, resulting in aged biomarkers at offshore core locations. Because n-C₂₉₊₃₁ alkanes show contributions from petrogenic sources, n-C₂₈₊₃₀ fatty acids likely provide better estimates of average terrestrial residence times of vascular plant biomarkers. Moreover, sedimentary n-C₂₈ and n-C₃₀ fatty acids appear clearly much less influenced by autochthonous sources than n-C₂₄ and n-C₂₆ fatty acids as indicated by increasing radiocarbon ages with increasing chain-length and are, thus, more representative as vascular plant biomarkers.     

Radiocarbon reservoir age of high latitude North Atlantic surface water during the last deglacial

The radiocarbon reservoir age of high latitude North Atlantic Ocean surface water is essential for linking the continental and marine climate records, and is expected to vary according to changes in North Atlantic deep water (NADW) production. Measurements from this region also provide important input and/or tests of oceanic radiocarbon using 3-D global ocean circulation models. Here, we present a surface water radiocarbon reservoir age record of the high latitude western North Atlantic for the deglacial period via the use of fossil cold-water corals growing in waters that are rapidly exchanged with nearby surface waters. The reservoir age of high latitude North Atlantic surface waters was computed from the radiocarbon age difference between our radiocarbon calibration record (http://radiocarbon.LDEO.columbia.edu) and our marine radiocarbon data. ²³⁰Th/²³⁴U/²³⁸U dates provide the absolute coral ages. Our high latitude North Atlantic Ocean reservoir age data combined with recalculated reservoir ages based on published coexisting terrestrial and marine material and Vedde ash radiocarbon dates from central and eastern North Atlantic show modern values (380±140 year, n=14) during the Bolling and Allerod warm period and a 200 year increase in reservoir age (590±130 year, n=10) during the entire Younger Dryas (YD) cold episode. The reservoir age then decreased to 270±20 year (n=2) at the Preboreal/YD transition, although the dates are too sparse for us to be confident in this estimate. We are not able to resolve the timing of the transition to increased reservoir ages from the mid-Allerod to the YD due to the relatively small change and correspondingly large uncertainty in the estimates. The atmospheric Δ¹⁴C record derived from our atmospheric radiocarbon record displays a 40 per mil increase from 12,900 to 12,650 cal years BP, coincident with the shift to high reservoir ages in the early YD cold event. Intrusion of ¹⁴C depleted Antarctic Intermediate Water (AAIW) to the high latitude North Atlantic and reduction of NADW formation are possible causes for the coincident shift to high reservoir ages in the North Atlantic surface ocean and increased atmospheric Δ¹⁴C during the beginning of the YD event.     

Variability of organic carbon isotope and C/N in the Hemudu Area,Hangzhou Bay and its environmental implications in the Holocene

With the purpose of tracing the variations of the organic matter sources of sediments, a sample column (25.96 m in size) from the Hemudu Area of Hangzhou Bay was put through AMS¹⁴C dating and biogeochemical analysis. TOC and TN presented similar variation trends, suggesting the same and stable sources of organic matter; the bad correlation between the grain size and TOC content indicated that the organic matter occurrence was neither controlled by the grain size nor the surface absorption of the fine particles, but it may has something to do with the complicated sedimentary hydrodynamic force, the input of organic matter and microbial action. Judging on the basis of C/N ratio and δ¹³C_TOC, the organic carbon in sediments was a mixture of terrigenous and marine organic carbon, testifying to the land-sea interaction characteristic of the study area. The indexes experienced abrupt changes at about 6.5 ka BP, when the lighter terrigenous organic carbon made an increased proportion to the heavier marine organic carbon. The phenomenon reflected the enclosure of the lagoon and the reduced exchange interactions with the seawater of open seas.     

Transport of terrestrial organic carbon to the oceans by rivers: re-estimating flux- and burial rates

 This study re-estimates one important component in the global carbon cycle: the modern global fluviatile organic carbon discharge- and burial rates. According to these results, approximately 430×10¹² g of terrestrial organic carbon are transported to the ocean in modern times. This amount is higher than the latest estimates but takes into account new data from Oceania not previously considered in global flux studies. However, only the minor amount of 10% or approximately 43×10¹² gC year^–1 is most likely buried in marine sediments. This amount is similar to the burial of marine organic carbon in the coastal ocean (55×10¹² gC year^–1). Adding both estimates gives approximately 100×10¹² gC year^–1, which is the value calculated by Berner (1982) for "terrestrial" deltaic-shelf sediments. However, the results in this study suggest that on a global scale the organic carbon content in coastal ocean sediments is not solely of terrestrial origin but a mixture of nearly equal amounts of marine and terrestrial organic carbon. The major part of the terrestrial organic carbon that enters the ocean by rivers (approximately 400×10¹² gC year^–1) seems to be either (a) remineralised in the ocean, whereas the mechanism by which the terrestrial organic carbon is oxidised in the ocean are unknown; or (b) is dispersed throughout the oceans and accumulates in pelagic sediments. Received: 9 November 1998 / Accepted: 25 May 1999     

Radiocarbon ages of pedogenic carbonate nodules from Coimbatore region,Tamil Nadu

In this paper we discuss the limitation of radiocarbon dates on the pedogenic calcic nodules formed in situ within the vertisols in the upland region of Coimbatore, Tamil Nadu. The radiocarbon ages were obtained using low-level scintillation counters and the dates range from ∼24 to 31 ¹⁴C kyrs BP. The ages correlate with the marine isotope stage of Late MIS3. However, since the calcic nodules are pedogenised and formed in an open system, the ¹⁴C ages should only be considered as estimates and not absolute ages because of the possibility of open-system behaviour with respect to carbon. Thus, we express caution in the interpretation of these and other radiocarbon ages obtained on pedogenic carbonate nodules. Multiple sub-mm size subsamples could provide more reliable age estimates.     

A Predictive Model for Locating Early Holocene Archaeological Sites Based on Raised Shell‐Bearing Strata in Southeast Alaska,USA

A predictive model for locating early Holocene archaeological sites in southern Southeast Alaska was developed based on shell‐bearing raised marine deposits. Fieldwork included coring of select‐raised marine strata, measuring their elevations, and radiocarbon dating the associated shell samples within the cores. A subset of the data was used to produce a relative sea‐level curve spanning the Holocene. The relative sea‐level curve suggests that sites favorable for habitation between 9200 and 7000 ¹⁴C yr B.P. should be found 16–22 ± 1 m above present zero tide. The sea‐level curve and new high‐resolution digital elevation models allowed reconstruction of past shorelines at various elevations. Surveys to test the model found and recorded over 70 archaeological sites from present sea level up to 32 m above present zero tide. Eleven new sites were within the targeted elevation range and radiocarbon dated to 9280–6890 ¹⁴C yr B.P. Initial investigations indicate these older sites are rich in microblade and pebble tool technology. The new early Holocene sites indicate more extensive early maritime settlement of Alaska than implied by previous studies and contribute to our understanding of the early movement of people into North America.     

Distribution,sediment magnetism and geochemistry of the Saksunarvatn (10 180 ± 60 cal. yr BP) tephra in marine,lake, and terrestrial sediments,northwest Iceland

In 1997, seismic surveys in the troughs off northwest and north Iceland indicated the presence of a major, regional sub‐bottom reflector that can be traced over large areas of the shelf. Cores taken in 1997, and later in 1999 on the IMAGES V cruise, penetrated through the reflector. In core MD99‐2269 in Húnaflóaáll, this reflector is shown to be represented by a basaltic tephra with a geochemical signature and radiocarbon age correlative with the North Atlantic‐wide Saksunarvatn tephra. We trace this tephra throughout northwest Iceland in a series of marine and lake cores, as well as in terrestrial sediments; it forms a layer 1 to 25 cm thick of fine‐ to medium‐grained basaltic volcanic shards. The base of the tephra unit is always sharp but visual inspection and other measurements (carbonate and total organic carbon weight %) indicate a more diffuse upper boundary associated with bioturbation and with sediment reworking. Off northwest Iceland the Saksunarvatn tephra has distinct sediment magnetic properties. This is evident as a dramatic reduction in magnetic susceptibility, an increase in the frequency dependant magnetic susceptibility and ‘hard’ magnetisation in a −0.1T IRM backfield. Geochemical analyses from 11 sites indicate a tholeiitic basalt composition, similar to the geochemistry of a tephra found in the Greenland ice‐core that dates to 10 180 ± 60 cal. yr BP, and which was correlated with the 9000 ¹⁴C yr BP Saksunarvatn tephra. We present accelerator mass spectrometry ¹⁴C dates from the marine sites, which indicate that the ocean reservoir correction is close to ca. 400 yr at 9000 ¹⁴C yr BP off northwest Iceland. Copyright © 2002 John Wiley & Sons, Ltd.     

Comparative characterization of humic substances from the open ocean, estuarine water and fresh water

Humic substances were isolated from ocean, estuarine water and fresh water using a two column array of XAD-8 and XAD-4 resins in series. The extracted fulvic acids and XAD-4 fraction from different origins were characterized using UV–vis., molecular fluorescence, Fourier transform infrared (FTIR) spectroscopy and cross polarization magic angle spinning (CPMAS)-¹³C nuclear magnetic resonance (NMR) spectroscopy. The isolation procedure allowed us to obtain the necessary amount of sample for characterization, even in the case of open ocean water, which has a very low amount of dissolved organic carbon (DOC). Humic substances from the open ocean showed the lowest chromophore and fluorophore contents and showed relatively greater fluorescence at lower wavelengths than those from fresh water. FTIR and ¹³C NMR spectra highlighted the idea that humic substances from a marine environment have a more branched aliphatic structure and less aromatic structure than those highly influenced by terrestrial sources. The spectra also suggest that the open ocean humic substances have a higher content of olefinic carbons than aromatic- or alkyl-substituted carbons.