Pyrolysis-combustion C dating of soil organic matter

Radiocarbon (¹⁴C) dating of total soil organic matter (SOM) often yields results inconsistent with the stratigraphic sequence. The onerous chemical extractions for SOM fractions do not always produce satisfactory ¹⁴C dates. In an effort to develop an alternative method, the pyrolysis-combustion technique was investigated to partition SOM into pyrolysis volatile (Py-V) and pyrolysis residue (Py-R) fractions. The Py-V fractions obtained from a thick glacigenic loess succession in Illinois yielded ¹⁴C dates much younger but more reasonable than the counterpart Py-R fractions for the soil residence time. Carbon isotopic composition (δ¹³C) was heavier in the Py-V fractions, suggesting a greater abundance of carbohydrate- and protein-related constituents, and δ¹³C was lighter in the Py-R fractions, suggesting more lignin- and lipid-related constituents. The combination of ¹⁴C dates and δ¹³C values indicates that the Py-V fractions are less biodegradation resistant and the Py-R fractions are more biodegradation resistant. The pyrolysis-combustion method provides a less cumbersome approach for ¹⁴C dating of SOM fractions. With further study, this method may become a useful tool for analyzing unlithified terrestrial sediments when macrofossils are absent.     

Evidence for Holocene environmental changes in the northern Fertile Crescent provided by pedogenic carbonate coatings

Holocene environmental changes in the northern Fertile Crescent remain poorly understood because of the scarcity of local proxy records in the region. In this study we investigated pedogenic (soil-formed) carbonate coatings on stones at the Pre-Pottery Neolithic site Göbekli Tepe as an indicator of local early-mid Holocene environmental changes. The ¹⁴C ages and stable isotopic composition of carbon and oxygen in thin (0.2–0.3 mm thick) pedogenic carbonate lamina indicate two main periods of coating formation: the early-Holocene (ca. 10000–6000 cal yr BP) and the mid-Holocene (ca. 6000–4000 cal yr BP). During the first period, there was an inverse relationship between δ¹³C and δ¹⁸O curves: a decrease in δ¹³C values coincide with an increase in δ¹⁸O values. For this period a trend towards higher temperatures is suggested. In the mid-Holocene, the mean rate of coating growth was 2–3 times higher than in the early Holocene. Both δ¹³C and δ¹⁸O reached their maximum values during this time and the direction of changes of the δ¹³C and δ¹⁸O curves became similar. The combination of data suggests that this period was the most humid in the Holocene and on average warmer than the early Holocene. At ca. 4000 cal yr BP secondary accumulation of carbonate ceased, presumably reflecting a shift to a more arid climate.     

Cross dating (Th/U-C) of calcite covering prehistoric paintings in Borneo

We present the first application of cross-dating (Th/U measured by thermo-ionization mass spectrometry (TIMS) and ¹⁴C measured by accelerator mass spectrometry (AMS)) of calcite covering prehistoric paintings. Th/U age estimates of cave drapery range from 9800 to 27,300 yr B.P. while conventional ¹⁴C age is estimated between 9900 and 7610 yr B.P. depending on the dead carbon correction. The age discrepancy is attributed to a disturbance of Th/U and/or ¹⁴C geochemical systems, showing the limits of the geochronological approach applied to this kind of material. For the Th/U system, the poor consistency of U data (U content, ²³⁴U/²³⁸U activity ratios) and apparent ages argue for open system conditions. For ¹⁴C system, variation of the dead carbon fraction (dcf) and a possible mixing of successive generations of calcite could account for age discrepancy. Nevertheless, one sample shows concordant ages for the two methods. Compatible ages through corrections for open system conditions are assumed for other samples. Then, the cross-dating suggests 9900 yr as the minimum age of the piece of drapery; the underlying painting must be older. This study of rock art demonstrates the presence of a Pleistocene population before 9900 yr in the southeast of Borneo, whereas previously the only population in evidence in this area was of Austronesian type from ∼5000 to 6000 yrs ago.     

Lichenometric dating using Rhizocarpon subgenus Rhizocarpon in the Patagonian Andes, Argentina

This study represents the first attempt to develop and apply lichenometric dating curves of Rhizocarpon subgenus Rhizocarpon for dating glacier fluctuations in the Patagonian Andes. Six glaciers were studied along the Patagonian Andes. Surfaces of known ages (historical evidences and tree-ring analyses) were used as control sites to develop indirect lichenometric dating curves. Dating curves developed for the studied glaciers show the same general logarithmic form, indicating that growth rate of subgenus Rhizocarpon decreases over time. The strong west–east precipitation gradient across the Andean Cordillera introduces statistically significant differences in the growth curves, with faster growth rates in the moist west sites than the drier eastern sites. Latitudinal difference among the studied glaciers does not appear to be a major factor regulating lichen growth rates. Therefore, we developed two lichenometric curves for dating glacier fluctuations in wetter and drier sites in the Patagonian Andes during the past 450 yrs. Application of the developed curves to moraine dating allowed us to complement glacial chronologies previously obtained by tree-ring analyses. A first chronosequence for moraine formation in the Torrecillas Glacier (42°S) is presented. Our findings confirm the utility of lichenometry to date deglaciated surfaces in the Patagonian Andes.     

常规放射性14C测年金属系统装置研发和优化

放射性同位素14C测年技术广泛应用于第四纪地质学、考古学、海洋学和古气候等学科.常规14C测年,是采用β衰变低本底液体闪烁计数仪记录一定时间间隔内一定量样品中14C原子衰变数目的方法,由于仪器操作简单、方便,在不受样品量限制的情况下,测试的精度能够满足测年的要求.14C测年液闪法样品制样前处理系统主要在真空玻璃系统中完成,但真空玻璃系统容易出现破碎、断裂,每个实验室需要配备专门的玻璃焊耗师傅维护和维修真空玻璃系统,制约着14C测年技术的发展.文章探讨了常规14C制样系统的升级改造,建立了一套金属系统、不用玻璃焊量,无油污染、安装方便、拆卸灵活的常规14C测年样品制备系统,该套系统解决了常规14C发展中遇到的难点问题,使得常规14C测年技术得到更好的推广和应用.     

Precise radiocarbon dating of Late-Glacial cooling in mid-latitude South America

Variability of atmospheric ¹⁴C content often complicates radiocarbon-based chronologies; however, specific features such as periods of constant ¹⁴C age or steep changes in radiocarbon ages can be useful stratigraphic markers. The Younger Dryas event in the Northern Hemisphere is one of those periods, showing conspicuous ¹⁴C wiggles. Although the origin of those variations is not fully understood, we can make practical use of them and determine: (i) whether the Younger Dryas was global in extent; if so, (ii) were the initial cooling and the final warming synchronous worldwide; and (iii) what are the implications of these similarities/differences? Here we report high-resolution AMS ¹⁴C chronologies from the mid-latitudes of South America that pinpoint a cool episode between 11,400 and 10,200¹⁴C yr B.P. The onset of the final cool episode of the Late Glacial in the southern mid-latitudes, i.e., the Huelmo/Mascardi Cold Reversal, preceded the onset of the Younger Dryas cold event by ∼550 calendar years. Both events ended during a radiocarbon-age plateau at ∼10,200¹⁴C yr B.P. Thus, the Huelmo/Mascardi Cold Reversal encompasses the Younger Dryas, as well as a couple of short-term cool/warm oscillations that immediately preceded its onset in the North Atlantic region.     

Geochronology of Ailuropoda–Stegodon fauna and Gigantopithecus in Guangxi Province, southern China

Pleistocene faunas from south China are difficult to subdivide based on the long temporal ranges of many taxa and a reduced number of genera in comparison to faunas from temperate north China. In south China, the Ailuropoda–Stegodon fauna is a very general one and includes a relatively stable suite of genera that apparently persisted for long periods of time. These attributes have made constraining its time range difficult. Application of electron spin resonance (ESR) dating of tooth enamel constrains the ages well where uranium uptake was minor. Where uranium uptake into teeth was significant, an approach combining ESR and ²³⁰Th/²³⁴U isotopic analysis also yields excellent ages. Previous estimates of early, middle and late Pleistocene time ranges previously determined by biostratigraphic seriation for the Ailuropoda–Stegodon fauna are confirmed in all cases but are made more precise with our approach, including specific time ranges for certain archaic taxa. Absolute dating also yields an extended time range for Gigantopithecus blacki of 1200 to 310 ka.     

Precise 40Ar/39Ar dating of volcanic tuffs within the upper Messinian sequences in the Melilla carbonate complex (NE Morocco): implications for the Messinian Salinity Crisis

The Melilla carbonate complex (NE Morocco) is the only area of the Paleo-Mediterranean Sea where volcanic activity was present throughout most of the Messinian. ⁴⁰Ar/³⁹Ar dating of volcanic tuffs interbedded within the upper Messinian sedimentary deposits, known as the Terminal Carbonate Complex (TCC), yields accurate ages of paleoenvironmental and sea-level changes related to the Messinian Salinity Crisis. The new chronologic data (1) provide an average of 5.95–5.99 Ma for the base of the TCC, thus being synchronous with the onset of the Messinian Salinity Crisis, (2) demonstrate for the first time that the basal unconformity of the TCC does not represent a hiatus of long duration, (3) define a precise time line at 5.87±0.02 Ma (2σ) corresponding to sedimentary rocks exhibiting a lateral transition between continental and marine deposits typical of the TCC and (4) yield evidence that emersion of the Melilla platform during deposition of the TCC is partly related to tectono-magmatic activity. An erosional surface, capping the TCC deposits in the Melilla basin, is related to the major Messinian Mediterranean drawdown. The duration of the hiatus, associated with this surface, is estimated to be at most 450 kyr, but is probably shorter.     

准噶尔盆地周边硅质岩激光40Ar/39Ar法定年

由于较低的钾元素含量以及过剩氩的存在,长期以来对硅质岩的40Ar/39Ar定年一直存在较大难度。近年来,由于仪器水平的不断提高,新实验技术和方法的应用,特别是激光全熔40Ar/39Ar定年技术的应用,40Ar/39Ar定年方法具有了足够高的测试精度和稳定的低本底水平,可以满足测试极低钾元素含量的硅质岩样品的要求。利用多组矿物颗粒测试数据计算等时线年龄的方法可以很好地去除过剩氩对硅质岩年龄的影响。本文利用激光全熔40Ar/39Ar定年方法对新疆准噶尔盆地边缘的两个硅质岩样品进行了定年研究。采自白碱滩地区的08BJT-3样品的年龄测试结果为294±14Ma,该年龄结果与硅质岩样品所处的晚石炭世地层沉积年代基本一致。采自卡拉麦里地区的KML-2样品的年龄测试结果为266±14Ma,该年龄结果与强烈变形改造硅质岩样品的卡拉麦里构造变形带活动年代十分一致,表明激光全熔40Ar/39Ar定年方法可以准确地对硅质岩进行定年。     

Laser probe Ar/Ar dating: History and development from a technical perspective

The ⁴⁰Ar/³⁹Ar method using a laser probe opened the door to microscale measurements and diffusion profiles frozen in samples. In the first decade since the initial application of a laser for ⁴⁰Ar/³⁹Ar dating in 1973, practical applications have been few. This is due not only to the fact that the laser and vacuum technologies were immature but that mass spectrometry was also in its infancy. In those days, the sensitivity of a mass spectrometer was generally insufficient to measure the small amount of argon degassed from a geological sample by a laser. These problems have subsequently been solved by new technologies. To understand their current status, a brief history of their development is outlined. This outline focuses on the required detection limit in micro scale measurement, practical approaches for accurate measurement are explained through examples in our laboratory specifically relating to the technical aspects of ⁴⁰Ar/³⁹Ar dating.     

雷州半岛第四纪火山岩激光40Ar/39Ar等时线定年研究

雷州半岛是我国新生代火山岩最重要的分布地区之一,火山活动主要集中在中晚更新世。前人对雷州火山岩的年代学研究以K-Ar法为主。研究表明,雷州火山岩测年结果大致分布在0.38～3.04Ma范围内。根据地层和火山岩层的叠置关系,雷州第四纪火山岩由于覆盖在被确定是1.87Ma和0.76Ma沉积的地层之上,故火山岩年龄应小于该地层年龄。K-Ar法定年结果与雷州地区地层叠置关系存在矛盾。本文通过对雷州半岛第四纪火山岩进行野外考察及采样,利用激光40Ar/39Ar年代学方法进行了精细定年。结果表明,雷州火山岩的喷发主要集中18万年前后。定年结果还表明,对于年轻样品,基于尼尔值计算的K-Ar年龄及40Ar/39Ar表观年龄偏老,等时线年龄相对较为可靠。对同一样品的斑晶、基质作斑晶-基质等时线计算,只有在斑晶基质满足同源条件时才有意义。本文首次提出,通过对比未照射样品的初始36Ar/38Ar值的均一性,以检验样品是否同源,确认斑晶-基质等时线年龄的可信度。据此,等时线的处理方法可以推广应用于特定区域内全部同源同时样品。     

内蒙古大青山逆冲推覆体系中生代逆冲构造活动的40Ar-39Ar定年

内蒙古大青山呼和浩特段北侧发育3条走向北东东、指向北西的逆冲断层,并与被其分割的3个逆冲席体及一个原地系构成大青山逆冲推覆体系。逆冲断层上盘底部发育较深层次的糜棱岩,下盘顶部多发育低温的千糜岩。本研究在构造地质调查基础上,结合宏-微观岩石矿物学分析,采用⁴⁰Ar-³⁹Ar定年对该逆冲体系的活动时间进行约束。逆冲断层带内3个千糜岩绢云母⁴⁰Ar-³⁹Ar年龄范围为120~119Ma,另一样品给出了120Ma的概率统计峰值年龄。千糜岩为低温同变形变质产物,细粒绢云母为同变形新生矿物,其⁴⁰Ar-³⁹Ar年龄可代表变形年龄。侵位在断层内弱变形的花岗闪长岩为同构造晚期侵位,角闪石⁴⁰Ar-³⁹Ar年龄限定其冷却时间下限为121Ma,概率统计峰值年龄为119Ma。逆冲断层上盘底部发育较高温的糜棱岩,而低温千糜岩的形成时间应属于变形后期。因此,120Ma至119Ma期间,大青山逆冲推覆体系的逆冲作用应已是处于变形晚期。     

西藏夏垅铅锌银矿床绢云母40Ar/39Ar年龄及其地质意义

西藏冈底斯成矿带位于班公湖-怒江缝合带与雅鲁藏布江缝合带之间,从北向南依次可划分为3个成矿亚带:勒青拉-洞中松多铅锌银多金属成矿亚带,驱龙-甲马-邦铺斑岩铜钼成矿亚带,克鲁-冲木达斑岩-矽卡岩铜钼金成矿亚带。驱龙-甲马-邦铺成矿亚带内矿床的类型以斑岩型为主,部分伴生有矽卡岩型及热液脉型矿床。夏垅矿床位于驱龙-甲马-邦铺斑岩铜钼成矿亚带的西段,产于黑云母二长花岗岩基中,属于隐爆角砾岩型铅锌银多金属矿床。对夏垅矿床内与石英、方铅矿、闪锌矿密切共生的绢云母进行了⁴⁰Ar-³⁹Ar定年,确定其坪年龄及反等时线年龄分别为(23.56±0.22) Ma和(23.9±1.6) Ma。该矿床作为隐爆角砾岩型铅锌银多金属矿床的发现以及冈底斯火山岩自西向东逐渐变年轻的时空迁移规律说明在冈底斯成矿带的西侧存在着岩浆活动。夏垅矿床成矿年龄的精确测定把驱龙-甲马-邦铺斑岩铜钼成矿亚带向西延伸了120 km,增大了该成矿亚带的找矿潜力,为在该成矿亚带内寻找此类铅锌银多金属矿床提供了理论及实际依据。     

High-resolution210Pb dating of Lake Constance sediments: Stable lead in Lake Constance

Two sediment cores (BO90/13b and BO90/17b) from Lake Constance were investigated by-spectrometry for²¹⁰Pb,¹³⁴Cs,¹³⁷Cs,²⁴¹Am,²³⁴Th, and other members of the²³⁸U decay chain. The sediments were dated using the constant-flux model for²¹⁰Pb, and accumulation rates were determined. These range from 0.04 to 0.65g/cm ²/yr (BO90/13b) and 0.04 to 0.8g/cm ²/yr (BO90/17b), respectively. The mean accumulation rate amounts to 0.16g/cm ²/yr for both cores. The cores had already been dated by lamination counting and reconstruction of high-water events at the Limnological Institute at Constance, so that a very precise time scale was available. Both ages derived are in agreement within statistical error up to 1900, which means dating with the constant-flux model for²¹⁰Pb was confirmed up to that age. The position of the maxima of bomb cesium and americium confirm the stratigraphic and²¹⁰Pb datings. With²⁴¹Am a further radioactive isotope is available, which can, due to the half-life of²⁴¹Pu (t _1/2=14.4yr) be detected now by-spectrometry and can serve as an additional time indicator, the maximum being dated at 1963. By applying the various time scales, the depth profiles of stable lead and zinc of core BO90/13b were dated. Both heavy metals show a very significant maximum located beneath the layer of the maxima of bomb cesium and americium, showing that these maxima are older than those of the bomb isotopes. It is remarkable in this context that the maximum of zinc concentration occurs a little later than that of stable lead. Similar concentration profiles are observable in core BO90/17b and other, older sediment cores (CS6-CS10) on a transect across the lake. In contrast to a former assumption, the depth profile of stable lead in Lake Constance sediments does not reflect the anthropogenic gasoline lead emissions into the atmosphere for Germany, their maximum being dated at 1971.     

Revision of the conchostracan genera Cratostracus and Porostracus from Cretaceous deposits in north-east China

The diagnoses of the conchostracan genera Cratostracus and Porostracus are revised following a re-examination of their type species under a scanning electron microscope. In Cratostracus each growth line has a serrated lower margin. Radial lirae branch into smaller ridges that merge with each other to give the upper part of each growth band in the ventral region of the carapace a slightly undulating surface. Cross-bars are present between these lirae. In Porostracus the radial lirae on the growth bands near the umbo are long and relatively widely spaced. There is a fine reticulum between them in the umbonal part of the carapace and cross-bars on the ventral surface. The ornament of Porostracus indicates that it can be assigned to the family Halysestheriidae rather than to the Afrograptidae, in which it has been placed previously. Morphological analysis of most of the species of the two genera and closely related Orthestheria (Migransia) under the scanning electron microscope is required before their stratigraphic potential can be fully realized.     

A history of the ‘Zambian copper flower', Becium centraliafricanum (B. homblei)

This contribution is a short history of the Zambian copper flower, Becium homblei (recently renamed B. centraliafricanum that has attracted attention as a plant associated with Cu-rich soils. It has a peculiar discontinuous distribution in Central Africa which has been explained in terms of biotype depletion. A field investigation into the ecology of the species shows that it is able to tolerate soil Cu concentrations of up to 15,000 μg/g (ppm), and soil nickel concentrations of nearly 5000 μg/g. B. homblei is also found on areas where soil metals are in trace quantities, and where soil bases, particularly Ca, are low. In spite of its tolerance to a wide range of edaphic conditions, the distribution of the species is very restricted in Zimbabwe, and this is almost certainly due to severe interspecific competition with a closely related species, B. obovatum, which is common on soils not unusually enriched in heavy metals. Pioneering work on geobotanical prospecting by use of Becium homblei was carried out by the late G. Woodward and others in the 1950s and 1960s. This species was used successfully for geobotanical prospecting for Cu. This present report is a brief history of these pioneering studies     

Origins and antiquity of the island fox (Urocyon littoralis) on California's Channel Islands

The island fox (Urocyon littoralis) is one of few reportedly endemic terrestrial mammals on California's Channel Islands. Questions remain about how and when foxes first colonized the islands, with researchers speculating on a natural, human-assisted, or combined dispersal during the late Pleistocene and/or Holocene. A natural dispersal of foxes to the northern Channel Islands has been supported by reports of a few fox bones from late Pleistocene paleontological localities. Direct AMS ¹⁴C dating of these “fossil” fox bones produced dates ranging from ∼ 6400 to 200 cal yr BP, however, postdating human colonization of the islands by several millennia. Although one of these specimens is the earliest securely dated fox from the islands, these new data support the hypothesis that Native Americans introduced foxes to all the Channel Islands in the early to middle Holocene. However, a natural dispersal for the original island colonization cannot be ruled out until further paleontological, archaeological, and genetic studies (especially aDNA [ancient DNA]) are conducted.     

内蒙古呼和浩特变质核杂岩韧性拆离带40Ar-39Ar定年及其构造含义

晚中生代,内蒙古大青山依次经历晚侏罗世盘羊山逆冲推覆、早白垩世呼和浩特变质核杂岩伸展、早白垩世大青山逆冲推覆断层及早白垩世以来高角度正断层复杂构造演化。其中,呼和浩特变质核杂岩韧性剪切带的冷却时间和抬升机制的制约尚不明确。本文在野外考察和显微构造分析基础上,采用逐步加热40Ar-39Ar定年法对韧性剪切带内不同单矿物的冷却年龄进行了测定。角闪石、白云母、黑云母和钾长石单矿物40Ar-39Ar冷却年龄处于120~116Ma之间。结合已有年龄数据及单矿物封闭温度,构建了韧性剪切带的冷却曲线。结果表明,韧性剪切带在122~115Ma期间存在一个明显的快速冷却过程。这一阶段快速冷却是与变质核杂岩拆离断层相关核部杂岩拆离折返作为大青山逆冲推覆断层上盘抬升的结果。     

喜马拉雅东构造结高压麻粒岩PT轨迹、锆石U-Pb定年及其地质意义

喜马拉雅东构造结出露了一套基性高压麻粒岩,其峰期矿物组合为石榴石+单斜辉石+石英+金红石+斜长石,利用相平衡计算其峰期温压条件为904℃、1.37GPa,利用锆石U-Pb定年方法确定其变质年龄为20.7±2.3Ma。角闪斜方辉石麻粒岩为其第一阶段退变产物,其变质矿物组合为斜方辉石+角闪石+斜长石+石英+钛铁矿+磁铁矿,温压条件为压力小于0.6GPa,温度为720~760℃。角闪岩相退变矿物组合为角闪石+斜长石+石英+钛铁矿+磁铁矿,温度小于745℃,压力小于0.6GPa。在角闪斜方辉石麻粒岩中变质锆石获得的定年结果为9.38±0.22M,根据锆石中角闪石+斜长石+石英的矿物包体特征,确定该年龄代表角闪岩相退变质年龄。据此,确定了喜马拉雅东构造结基性高压麻粒岩的PTt轨迹为顺时针2阶段折返过程,即第一阶段发生在20Ma左右的由高压麻粒岩相到角闪岩相退变阶段,第二阶段发生在9Ma左右的从角闪岩相深度折返到地表的阶段,计算得到其折返速率分别为2.4mm/y和2.3mm/y,这2个阶段的折返与目前通常认为的青藏高原2个主要抬升阶段是基本一致的。     

年轻火山岩的定年:进展与问题

年轻火山岩定年对人类的起源与进化、地质年表建立、古环境演化、火山灾害、岩浆演化及地球动力学等研究非常重要。由于年轻样品放射性成因子体积累较少、大气氩含量高,微量的过剩氩极难识别,使得年轻火山岩及其沉积地层高精度定年一直是地质年代学中最具挑战性的课题之一。过去几年,年轻火山岩定年在技术上的进展主要体现在定年精度和准确度不断提升,尤其是利用新型多接收稀有气体质谱对单颗粒样品进行逐步升温~(40)Ar/~(39)Ar定年,是方法学上最重要的进展之一。本文总结了年轻火山岩~(40)Ar/~(39)Ar定年的现状、影响因素和相关定年技术及应用,以引起大家的关注和思考,共同提高我国年轻火山岩定年的技术水平。利用新型多接收质谱和创新的实验方法,通过对全球重要火山喷发事件和年轻国际标准样的研究,广泛开展实验室间的对比是提高国内~(40)Ar/~(39)Ar年代学水平并在年轻火山岩定年中取得突破的重要途径,也是当前~(40)Ar/~(39)Ar年代学研究的重要方向。