宇生核素深度剖面暴露测年的原理与应用

宇生核素暴露测年技术是目前第四纪地貌年代测定最常用的方法之一,由于难以定量化继承性核素以及侵蚀对样品暴露年代的影响(通常假设继承性和侵蚀的影响为0),其测年结果难以反映地貌的真实暴露年代。通过测定地貌剖面不同深度样品的宇生核素浓度,建立“宇生核素浓度—深度”剖面曲线,可同时定量化研究地貌体的暴露年龄、继承性宇生核素浓度以及侵蚀速率,有效地弥补了宇生核素暴露测年中继承性核素以及侵蚀速率不确定性的缺陷。基于蒙特卡洛方法的宇生核素深度剖面暴露测年模型(简称蒙特卡洛深度剖面模型)是最常用的计算模式之一,然而,国内关于该方法的原理及其应用研究相对较少,大大限制了该方法的广泛使用。笔者等详细阐述了蒙特卡洛深度剖面模型的原理及其在冰川地貌、河流阶地、冲洪积扇等地貌测年中的应用。此外,介绍了深度剖面模型的复现、非稳态模型、线性反演模型等其他几种计算模式并进一步探讨了蒙特卡洛深度剖面模型暴露测年技术的野外采样方法,以使其能够广泛应用于第四纪地貌学和年代学研究中。     

青藏高原地区宇生核素暴露年代数据存在问题探讨

青藏高原是第四纪古冰川研究的理想区域也是宇宙成因核素(terrestrial cosmogenic nuclides,TCN)暴露测年技术应用的天然实验场所。然而,现有的TCN测年数据与先前学者基于其他测年手段的研究结果不一致,显得相对年轻。为了探索其原因,本文尝试对青藏高原1594个TCN测年数据进行统计分析。研究结果表明:1测年样品中97%的样品是漂砾样品,测年数据中约有93%的年代数据小于130 ka;2 280组(n≥3)漂砾样品年代数据中大约76%的漂砾组数据变异系数大于10%,而基岩和羊背石样品组测年数据变异系数较低、相对集中;3冰碛垄表面漂砾样品的不等时暴露与后期侵蚀可能是造成TCN年代数据结果偏年轻的主要原因。本研究可为青藏高原地区冰川地貌TCN暴露年代研究提供重要启示。     

原地宇生核素埋藏测年法: 最新进展及其在中国早期人类遗址年代研究中的应用前景

本文介绍 26Al/ 10Be 埋藏测年的基本原理和最新进展,并分析了其数据误差的主要来源: 仪器测量引起的计数统计误差和相关物理常数引起的系统误差。该法最重要的假设前提为"简单恒态侵蚀模式",同一层位多个石英质砾石 26Al- 10Be 等时线和 21 Ne-26Al- 10Be 三核素埋藏测年法有可能揭示样品是否符合这一假设前提。取自周口店第1 地点第8~9 层的3 件石英质石制品和1 个石英砂样的 26Al- 10Be 等时线过坐标原点,且其斜率给出的年代与单个样品年代的加权平均值在误差范围内吻合,为这些样品没有系统显著偏老倾向的有力证据。宇生核素埋藏测年法理化基础坚实,独立于其他测年技术,应用范围正值我国年代研究的"盲段",样品为广泛分布的石英矿物,应可为我国早更新世人类遗址年代系列的建立作出重要贡献。     

宇生核素26Al/10Be埋藏测年法铝化学分析程序的改进

原地宇生核素26Al/10Be埋藏测年法是最近十多年涌现的测年新技术之一,广泛应用于地貌演化、古人类等研究领域,Al回收率及其纯度很大程度上决定着年代结果的精度.美国Purdue大学现用Al分离纯化程序有一些步骤尚待完善.本文通过条件实验,对该程序提出如下修改建议:①用38 ～ 75 μm阴离子交换树脂取代原用的75 ～ 150 μm树脂,以减少淋洗液(0.05 mol/L草酸-0.5 mol/L盐酸)体积并实现Al与主要干扰元素Fe、Ti的定量分离;②用阳离子交换法从草酸-盐酸中提取Al,以规避较为冗长的化学法破坏草酸.模拟样品的分析结果表明,经改进的两步骤可实现Al近于定量的回收,全流程Al回收率达91％-+5％,纯度为98％.     

冰碛垄表面碎屑物质宇生核素暴露测年可行性研究

原地生宇宙成因核素(宇生核素)~(10)Be和~(26)Al暴露测年技术已广泛应用于各种地貌年代测定,尤其是在冰川地貌年代测定中备受青睐。多数学者通常选择冰碛垄表面散布的冰川漂砾作为测年对象。然而,冰川漂砾在暴露测年研究中存在暴露后期翻转、不等时暴露等问题,不仅影响冰川地貌测年的精度,而且难以获得较老的冰川地貌年代。冰碛垄表面碎屑物质,一直附着于冰碛垄"表面",可以规避冰川漂砾测年存在的不等时暴露问题,而且可以减少测试样本量、降低测试费用。因此,本文尝试探讨利用冰碛垄表面碎屑物质进行暴露测年研究的可行性,可望为较老冰川地貌暴露测年研究提供新的思路。     

青藏高原第四纪冰川的宇宙核素暴露年龄首次测定

青藏高原在第四纪时期究竟发育了几次冰期及时代研究深受国内外地球科学家关注,然而,老冰碛物的年代测定问题一直未突破。近年来发展成熟的就地成因宇宙核素(10Be,26Al)及宇宙气体(21Ne)的表面暴露年龄测定方法为冰碛物测年提供了条件。我们对青藏高原东部及腹地的第四纪冰川漂砾进行了核素年龄测试研究,并初步讨论了高原第四纪冰川发育及古环境变迁。      

青藏高原东南部稻城古冰帽南缘第四纪冰川活动的宇生核素年代研究

通常认为青藏高原东南部第四纪曾发育过一个冰帽——稻城冰帽,然而关于这个冰帽发育的年代和变化的过程,还缺乏足够的年代数据的支撑。本文选自稻城第四纪古冰帽南缘雄古附近的冰碛垄、冰碛台地、羊背石和冰川漂砾进行了宇生核素 10Be和 26Al暴露年代测定。结果显示,稻城古冰帽南缘至少发生过5次冰川前进事件,每次冰川前进到达最大规模的时间分别为 19.0±0.7~19.8±0.7kaB.P.,25.8±0.6~27.4±0.7kaB.P.,52.5±0.9~59.6±1.1kaB.P.,162.2±2.7~287.4±9.9kaB.P.和大于 480.2±2.2kaB.P. 。根据年龄数据以及空间位置关系的分析,稻城古冰帽很可能形成于约800~600kaB.P.之间,此后冰川的规模呈现出随着时间不断缩小的趋势,可能与该区气候的变干有关。MIS 3的冰川规模比MIS 2大,可能与MIS 3时期气候比MIS 2相对湿润有关。     

蒙山国家地质公园拦马墙砾石堆积体暴露测年研究

山东蒙山国家地质公园佛塔谷堆积了大量的巨砾,其形成原因存在着很大的争论。探讨蒙山佛塔谷砾石堆积体的形成年代有助于了解蒙山环境演化和气候变化。关于佛塔谷砾石的微地貌已有相关作者探讨,本文在前人研究的基础上从拦马墙砾石堆积体的宇生核素~(10)Be暴露年代以及全新世以来的气候环境和人类文化等方面进行探讨。研究结果表明:①拦马墙砾石宇生核素~(10)Be暴露年代结果为5.0～6.0 ka BP左右,不支持先前发表的"8.2 ka BP拦马冰期"的结论;②全新世以来山东蒙山区域处于一个暖湿的环境下,尽管有几次降温事件,但是不足以提供形成冰川的条件。     

原地生成宇宙成因核素暴露和埋藏年龄计算方法的统一

原地生成宇宙成因核素测年技术计算暴露和埋藏年龄以往分属于不同的计算方法体系,采用的计算等式不同。实际上,无论是暴露过程还是埋藏过程,都涉及了核素的新生和已生成核素的减少,其浓度计算方法原理上有相通之处。考察被研究样品历史上某时点的状态,侵蚀使样品距地表深度不断减小,埋藏使样品距地表深度不断增加,刚好是一种相反的过程,而原地生成宇宙成因核素的生成速率与距地表深度呈指数函数形式相关。本文按照原地生成宇宙成因核素浓度与暴露时间、侵蚀速率间关系的计算原理,推导出在匀速埋藏过程中,计算被测试样品的浓度随时间的变化时,以样品目前生成速率为标准,可将埋藏速率(β)视为负侵蚀速率(-ε),从而将暴露年龄和埋藏年龄的计算等式统一起来。非匀速埋藏情况,通过求导计算,仍可视埋藏为负侵蚀。在埋藏速率和侵蚀速率用同一参数指代,埋藏速率可视为负侵蚀速率的基础上,推导出不同埋藏(侵蚀)模式下样品的核素浓度计算等式。理论上根据不同埋藏(侵蚀)模式,可计算复杂地表演化过程样品的埋藏(暴露)年龄。

     

继承性宇生核素对暴露测年结果影响的定量分析方法探讨

宇生核素暴露测年过程中,通常假设在样品最后一次暴露前,样品中的宇生核素（继承性宇生核素）浓度为0。然而,大量的测年数据研究发现,样品的暴露年代结果会受到继承性核素的影响从而高估地貌的真实年代。因此,如何降低继承性核素对暴露年代结果的影响或者定量分析继承性核素的影响程度不仅可为地貌演化提供准确的年代数据,而且对宇生核素暴露测年技术的研究具有重要意义。因此,本文以宇生核素暴露测年技术在冰川地貌中应用为例,通过分析继承性核素的研究概况,并结合宇生核素暴露测年原理,探讨继承性核素对测年结果影响的定量分析方法。研究结果表明： ① 通过样品中 n ( 26 Al)/ n ( 10 Be) 值\[即同一样品中宇生核素Al 26 与Be 10 浓度（单位为atom/g）的比值\]以及同一地貌位置多个的样品年代数据分布情况可初步判断测年结果是否受到继承性核素的影响;② 通过现代冰川冰碛物中宇生核素的浓度可以定量分析继承性核素对暴露测年结果的影响; ③ 通过计算冰碛垄顶部和一定深度（＞2~3 m）样品的宇生核素浓度差,可以减少继承性核素的影响。本研究内容对冰川地貌宇生核素暴露测年具有重要意义。     

Multiple cosmogenic nuclides document complex Pleistocene exposure history of glacial drifts in Terra Nova Bay (northern Victoria Land, Antarctica)

Geomorphological and glacial geological surveys and multiple cosmogenic nuclide analyses (¹⁰Be, ²⁶Al, and ²¹Ne) allowed us to reconstruct the chronology of variations prior to the last glacial maximum of the East Antarctic Ice Sheet (EAIS) and valley glaciers in the Terra Nova Bay region. Glacially scoured coastal piedmonts with round-topped mountains occur below the highest local erosional trimline. They represent relict landscape features eroded by extensive ice overriding the whole coastal area before at least 6 Ma (pre-dating the build-up of the Mt. Melbourne volcanic field). Since then, summit surfaces were continuously exposed and well preserved under polar condition with negligible erosion rates on the order of 17 cm/Ma. Complex older drifts rest on deglaciated areas above the younger late-Pleistocene glacial drift and below the previously overridden summits. The combination of stable and radionuclide isotopes documents complex exposure histories with substantial periods of burial combined with minimal erosion. The areas below rounded summits were repeatedly exposed and buried by ice from local and outlet glaciers. The exposure ages of the older drift(s) indicate multiple Pleistocene glacial cycles, which did not significantly modify the pre-existing landscape.     

宇宙成因核素26Al/10Be埋藏测年法在宁夏沙坡头黄河砾石阶地年代研究中的应用

本文利用宇宙成因核素26Al/10Be埋藏测年法对宁夏沙坡头地区黄河左岸的T5、T6、T8和T9阶地开展了年代学研究。采用简单快速埋藏模式获得4级阶地的26Al/10Be埋藏年龄分别约为0.25Ma、0.36Ma、0.59Ma和1.06Ma,如果考虑后期剥蚀作用的影响（E≈5m/Ma）,则4级阶地的年龄分别约为0.42Ma、0.39Ma、1.02Ma和1.59Ma。本研究未发现砾石大小与10Be、26Al核素浓度及26Al/10Be比值有明显相关性。     

Stone run (block stream) formation in the Falkland Islands over several cold stages,deduced from cosmogenic isotope (10Be and 26Al) surface exposure dating

Cosmogenic isotope (¹⁰Be and ²⁶Al) surface exposure dating has been applied to valley‐axis and hillslope stone runs (relict periglacial block streams) and their source outcrops in the Falkland Islands, South Atlantic. The data indicate that stone runs are considerably older landforms than previously envisaged and afford no evidence that they are a product of the Last Glacial Maximum; the samples range in apparent ¹⁰Be age from 42k to 731k yr BP, but some of these are minima. The results indicate that valley‐axis stone runs may be up to 700–800k yr old, have simple exposure histories and are composite landforms that developed over several cold stages. Analyses of some hillslope and outcrop samples also demonstrate simple exposure histories with ¹⁰Be ages from 42k to 658k yr BP. In contrast, isotopic ratios from other hillslope and outcrop samples reveal they have had a complex exposure history involving periods of burial or shielding; the samples range in ¹⁰Be age from 59k to 569k yr BP and these are regarded as minimum age estimates. Larger stone runs may be older than smaller runs and there is a possibility that stone runs older than 800k yr exist in other parts of the Falklands. The assertion that glaciation in the Falklands was restricted to the highest uplands is supported by the data, and the potential for age determination of other boulder‐strewn and bedrock landforms, using cosmogenic isotope analysis, in order to extend the geochronology of Quaternary events and processes is noted. Copyright © 2008 John Wiley & Sons, Ltd.     

Surface exposure dating of the Great Aletsch Glacier Egesen moraine system,western Swiss Alps,using the cosmogenic nuclide 10Be

Egesen moraines throughout the Alps mark a glacial advance that has been correlated with the Younger Dryas cold period. Using the surface exposure dating method, in particular the measurement of the cosmogenic nuclide ¹⁰Be in rock surfaces, we attained four ages for boulders on a prominent Egesen moraine of Great Aletsch Glacier, in the western Swiss Alps. The ¹⁰Be dates range from 10 460±1100 to 9040±1020 yr ago. Three ¹⁰Be dates between 9630±810 and 9040±1020 yr ago are based upon samples from the surfaces of granite boulders. Two ¹⁰Be dates, 10 460±1100 and 9910±970 yr ago, are based upon a sample from a quartz vein at the surface of a schist boulder. In consideration of the numerous factors that can influence apparently young ¹⁰Be dates and the scatter within the data, we interpret the weighted mean of four boulder ages, 9640±430 yr (including the weighted mean of two ¹⁰Be dates of the quartz vein), as a minimum age of deposition of the moraine. All ¹⁰Be dates from the Great Aletsch Glacier Egesen moraine are consistent with radiocarbon dates of nearby bog‐bottom organic sediments, which provide minimum ages of deglaciation from the moraine. The ¹⁰Be dates from boulders on the Great Aletsch Glacier Egesen moraine also are similar to ¹⁰Be dates from Egesen moraines of Vadret Lagrev Glacier on Julier Pass, in the eastern Swiss Alps. Both the morphology of the Great Aletsch Glacier Egesen moraine and the comparison with ¹⁰Be dates from the inner Vadret Lagrev Egesen moraine support the hypothesis that the climatic cooling that occurred during the Younger Dryas cold episode influenced the glacial advance that deposited the Great Aletsch Glacier Egesen moraine. Because of the large size and slow response time of Great Aletsch Glacier, we suggest that the Great Aletsch Glacier Egesen moraine was formed during the last glacial advance of the multiphased Egesen cold period, the Kromer stage, during the Preboreal chron. Copyright © 2004 John Wiley & Sons, Ltd.     

Investigating the last deglaciation of the Scandinavian Ice Sheet in southwest Sweden with 10Be exposure dating

In this study we have obtained 17 cosmogenic exposure ages from three well‐developed moraine systems – Halland Coastal Moraines (HCM), Göteborg Moraine (GM) and Levene Moraine (LM) – which were formed during the last deglaciation in southwest Sweden by the Scandinavian Ice Sheet (SIS). The inferred ages of the inner HCM, GM and LM are 16.7 ± 1.6, 16.1 ± 1.4 and 13.6 ± 1.4 ka, respectively, which is slightly older than previous estimates of the deglaciation based on the minimum limiting radiocarbon ages and pollen stratigraphy. During this short interval from 16.7 ± 1.6 to 13.6 ± 1.4 ka a large part (100–125 km) of the marine‐based sector of the SIS in southwest Sweden was deglaciated, giving an average ice margin retreat between 20 to 50 m a^?1. The inception of the deglaciation pre‐dated the Bølling/Allerød warming, the rapid sea level rise at 14.6 cal. ka BP and the first inflow of warm Atlantic waters into Skagerrak. We suggest that ice retreat in southwest Sweden is mainly a dynamical response governed by the disintegration of the Norwegian Channel Ice Stream and not primarily driven by climatic changes. Copyright © 2011 John Wiley & Sons, Ltd.     

Importance of sampling across an assemblage of glacial landforms for interpreting cosmogenic ages of deglaciation

Deglaciation chronologies for some sectors of former ice sheets are relatively poorly constrained because of the paucity of features or materials traditionally used to constrain the timing of deglaciation. In areas without good deglaciation varve chronologies and/or without widespread occurrence of material that indicates the start of earliest organic radiocarbon accumulations suitable for radiocarbon dating, typically only general patterns and chronologies of deglaciation have been deduced. However, mid-latitude ice sheets that had warm-based conditions close to their margins often produced distinctive deglaciation landform assemblages, including eskers, deltas, meltwater channels and aligned lineation systems. Because these features were formed or significantly altered during the last glaciation, boulder or bedrock samples from them have the potential to yield reliable deglaciation ages using terrestrial cosmogenic nuclides (TCN) for exposure age dating. Here we present the results of a methodological study designed to examine the consistency of TCN-based deglaciation ages from a range of deglaciation landforms at a site in northern Norway. The strong coherence between exposure ages across several landforms indicates great potential for using TCN techniques on features such as eskers, deltas and meltwater channels to enhance the temporal resolution of ice-sheet deglaciation chronologies over a range of spatial scales.