洞穴石笋初始234U/238U值变化的古气候记录意义

通过对我国西南地区3个洞穴9根大型石笋272个初始234U/238U数据的分析研究,发现石笋初始234U/238U值长尺度变化与海洋沉积SPECMAP曲线δ18O记录有一定的正相关关系,与北纬25°夏季太阳辐射能量变化曲线呈一定的负相关关系,石笋初始234U/238U值的变化在冰期时波动强烈,而在间冰期波动相对平缓,在间冰期和冰期(间冰阶和冰阶)转化阶段该值呈跳跃状态变化。末次冰期及全新世阶段石笋初始234U/238U变化记录了该时段内的BA暖期和YD突变冷事件;全新世8200 a BP、7200 a BP、5200 a BP、4200 a BP、2800 a BP、1400 a BP 发生的几次较强冷事件在石笋初始234U/238U值都有相应记录,并且和长尺度的变化规律一致,冷事件发生时石笋初始234U/238U值偏重,暖期偏轻。封闭系统形成的洞穴石笋初始234U/238U变化类似于同地区洞穴石笋的δ18O对气候变化的记录特征,在我国西南地区两者与夏季风变化的强弱呈现一种负相关的关系,洞穴石笋初始234U/238U值可以作为一个有用的古气候替代指标来研究古降水的变化。     

湖北三宝洞石笋生长速率及其古气候意义

石笋生长速率通常被作为反映古气候或古环境变化的有效代用指标。本文以湖北三宝洞22万年以来17支石笋为材料,实测了190个230Th年龄,其生长曲线显示MIS 1和MIS 5 阶段平均生长速率较大,超过70μm/a; MIS 2,MIS 3,MIS 4和MIS 6阶段生长速率较慢,均低于25μm/a,甚至停止生长。间冰期平均生长速率比冰期增加了1~2倍以上,且具有"爆发式"快速生长的特点。以5000年为步长的同一洞穴17支石笋生长频率支持平均生长速率的研究结果,说明在轨道尺度上较大的石笋生长速率能够有效地指示暖湿的气候条件。研究结果表明,间冰期高海平面和强太阳辐射增强了研究区季风环流,提高了地表植被覆盖率,有利于洞穴岩溶水过饱和,从而导致间冰期多支石笋连续高速生长。     

我国西北地区盐湖中的铀234U/238U比值示踪研究

内蒙古二连盆地、鄂尔多斯盆地典型含铀碳酸型盐湖水、岩两相234U/238U综合分析表明,碳酸型盐湖铀来源于大气降水和潜水对盐湖盆地周围中生代到现代富铀沉积物的溶滤、浸出,具有快速、近源物质来源特点。盐湖卤水和对应沉积物234U/238U值一般为0.8～1.2,湖卤水和潜卤水（晶间卤水）-岩两相中的铀处于沉积平衡状态。早白垩世～上新世含膏盐地层对比研究证实了富铀岩层234U/238U值随铀含量增大而减小,并趋近于1。室内盐湖水蒸发模拟实验发现残余卤水、沉积物234U/238U具有随蒸发程度增大逐渐减小的变化特征。盐湖现代沉积物物相研究发现铀主要以碳酸铀酰和吸附形式赋存在富含有机物和碎屑成分的含盐粘土沉积中,铀在盐类晶体中含量极少,仅存在于封闭水和结晶水中。卤水和沉积物ARu值是盐湖铀源及铀含量水平的指示标志之一。     

我国西北地区盐湖中234U/238U比值与铀关系研究——以内蒙古地区碳酸盐型盐湖为例

内蒙古二连盆地、鄂尔多斯盆地典型含铀碳酸型盐湖水、岩两相2344U/23898U综合分析表明,该类盐湖铀来源于降水和潜水对盐湖盆地周围中生代以来富铀沉积物的溶滤、浸出,具有快速、近源物质来源特点.盐湖卤水和对应沉积物234 U/238U比值一般为0.8～1.2,盐湖卤水和潜卤水(晶间卤水)岩两相中的铀处于沉积平衡状态.含铀盐湖水、沉积物的234 U/238U比值随其铀含量增大而减小,并趋近于1,富铀盐湖水及沉积物234U/238U比值介于0.9～1.5之间.室内盐湖水蒸发模拟实验发现,残余卤水、沉积物234U/238U、铀总量具有随蒸发程度增大逐渐减小的变化特征.铀主要以碳酸铀酰络合物和吸附形式赋存在富含有机物和碎屑成分的含盐粘土沉积中.卤水和沉积物234 U/238U比值是盐湖铀源及铀含量水平的有效指示标志之一.     

TIMS测定铀矿石样品中^234U/^238U、^230Th/^232Th、^228Ra/^226Ra的方法研究

研究了TIMS测定铀矿石样品中234U/238U、230Th/232Th、228Ra/226Ra的方法。建立了铀矿石密闭混酸一次溶样的方法和采用阴离子、阳离子和Sr特效树脂逐级离子交换分离纯化U、Th和Ra的流程,满足了TIMS测量要求。测定结果表明：100～1000 ng的天然铀中234U/238U,其测量精密度从静态多接收的2.34%提高到动态多接收的0.47%;对230Th与232Th丰度接近、质量为1μg左右的钍,采用三带点样技术和法拉第多接收技术测定230Th/232Th,其内精度平均值为0.0048%,外精度为0.028%;采用单带加钽发射剂,ETP跳峰测定50～100 fg镭-228稀释剂中的228Ra/226Ra,其内精度小于0.10%,外精度小于0.20%。比较TIMS和HR-ICP-MS、α能谱法测定234U/238U、230Th/232Th、228Ra/226Ra结果,三者结果相吻合。TIMS测量法样品用量少、快速、准确、精密度高,是U、Th、Ra同位素比值测定方法的又一补充。     

洞穴碳酸盐230Th-234U-238U测年初始钍校正的等时线研究

初始钍的校正是不纯沉积碳酸盐230Th 234U 238U测年的一个重要方面，洞穴石笋初始Th校正通常采用230Th/232Th的原子比值为（4.4±2.2）×10-6。多年来4种不同的等时线方法应用到不纯碳酸盐测年初始Th的校正中，其中全样品的等时线方法是目前公认的较为完善的一种方法。通过测定云南同一石笋2个不同层位的9个样品的U、Th同位素组成,并进行等时线分析，结果表明2层样品初始钍230Th/232Th原子比值分别为（3.5±2.8）×10-6和（10.6±2.2）×10-6，这说明即使在同一地区，由于其混入Th来源的复杂性，230Th/232Th初始比值在同一样品不同层位仍可能有较大的差异，因此不能够完全采用单一层位等时线获得的初始值对整个样品进行校正。考虑到在实验测量过程中，232Th含量的过高对于230Th的准确测定也有很大的影响，应尽量选择纯净石笋样品进行测年研究，在样品的选择和前处理过程中就减少初始钍的影响，这对获得高精度的测年结果有着重要的意义。     

湖北清江榨洞石笋双波长反射光谱特征及其古气候意义

本文提出一种表征洞穴石笋纹层颜色的新方法———双波长分光光度法,用于研究湖北清江榨洞2号石笋的反射光谱特征并探讨其古气候的替代意义。双波长分光光度法能消除因样品表面性质等差异而引起的测量误差,提高分析测试的精密度和准确性,有利于真实反映石笋沉积纹层的颜色特征并揭示其内在机制。研究结果表明,石笋纹层450nm/700nm的反射光谱特征能较好地反映洞穴碳酸盐中痕量铁。由于铁是气候控制下地球表层各种作用的产物,可灵敏指示气候条件,因而石笋纹层的反射光谱是一个潜在的高分辨率古气候替代指标。     

高分辨电感耦合等离子体质谱法测定铀矿石样品中234U/238U、230Th/232Th和228Ra/226Ra同位素比值 

采用氢氟酸-硝酸-盐酸混合酸密闭消解含铀矿石样品,用阴离子交换树脂、阳离子交换树脂和锶特效树脂逐级分离富集铀、钍和镭。使用高分辨电感耦合等离子体质谱(HR-ICPMS)测定分离纯化液中234U/238U2、30Th/232Th和228Ra/226Ra同位素。比值的测量精密度取决于比值的大小和对应核素浓度的大小。对质量浓度为10 ng/mL天然铀测量液,234U/238U的测量精密度优于1.2%;对230Th质量浓度为0.6ng/mL且230Th和232Th质量浓度接近的测量液,230Th/232Th的测量精密度为1.2%;对228Ra质量浓度为0.48 pg/mL且228Ra和226Ra质量浓度接近的测量液,228Ra/226Ra的测量精密度为4.0%。     

古气候变化的石笋同位素记录研究——以桂林盘龙洞为例

本文以桂林盘龙洞的研究工作为例，选择大型石笋剖面，在沉积学分析学研究的基础上，采用目前最先进的测年技术———ＡＭＳ１４Ｃ法，配合Ｕ系及稳定同位素地球化学综合手段，系统提取气候信息，建立了桂林地区３．６万ａ来气候变化的基本模式。该区末次冰期３．２～３．６万ａ时，年平均气温最低８～９℃。全新世在总的暖和、潮湿气候背景下，可划分出由暖到冷的３个气候旋回，每个旋回持续约３０００ａ左右，暖期年平均气温高于现今３～４℃，冷期低于现今３℃左右，可鉴别的次一级冷期可与我国５０００ａ物候记录资料进行对比。     

辽宁暖和洞石笋δ13C记录的古气候环境意义初探

基于辽宁暖和洞石笋NH33铀系测年结果,利用900条年纹层及302个碳、氧同位素和灰度数据,建立全新世大暖期内近1000年的石笋δ13C与灰度序列(8638±60 - 7748±53 aBP)。碳同位素与灰度序列细节特征变化趋势上的一致性,证实了δ13C所指示的古气候意义。在百年尺度上,石笋δ13C主要响应于温度变化,由地表生物量的变化驱动。石笋δ13C序列可进一步划分为3个气候段:(1)8655 – 8366 aBP,气候偏热;(2)8366 – 8048 aBP,气候转冷;(3)8048 – 7756 aBP,气候相对温暖。这与同时段孢粉记录具有很好的一致性。其中,8366-8048aBP的冷期与北大西洋地区“8.2 kaBP冷事件”相吻合。石笋δ13C记录中也包含一定的非气候因素,主要由渗水速率变化引起围岩CaCO3溶解量的变化造成。将石笋δ13C和灰度数据进行回归分析,可有效地剔除这一水动力因素的影响。      

A study of234U/238U ratios in groundwaters of Taiyuan area,Shanxi Province

Uranium contents and²³⁴U/²³⁸U ratios have been determined on 29 water samples from the Taiyuan area, Shanxi Province. The results show that the same artesian aquifer has similar uranium contents and²³⁴U/²³⁸U activity ratios, and the deeper aquifers have higher A. R. values but lower uranium contents. The A. R. values increase slightly towards groundwater flow in the artesian aquifers dominated by oxidizing ground waters. The Lancun Spring and the famous Jinci Spring belong to two different karst groundwater systems, i.e., the east and west karst groundwater systems. The recharge area of the Lancun Spring should cover the wide limestone outcrops of middle Ordovician in the northeast. The Ordovician fissure-karst ground water to the Jinci Spring is extensively mixed with fissure water in Carboniferous-Jurassic formations and seepage water from the Fenhe River.     

Dissolved Uranium and 234U/238U in the Yamuna and the Chambal Rivers, India

Dissolved uranium concentration and ²³⁴U/²³⁸U activity ratio have been measured in two distinctly different Indian drainage systems: the Yamuna headwaters in the Himalaya and the Chambal river system in the plains to study the weathering and mobility of uranium in these watersheds. The dissolved uranium in the Chambal river system ranges from 0.2 to 1.74 μg L⁻¹ during September (tail end of monsoon), whereas in the Yamuna river system, its concentration varies from 0.1 to 3.18 μg L⁻¹ during October (post-monsoon) and from 0.09 to 3.61 μg L⁻¹ in June (summer). In the Yamuna main stream, uranium is highest at its source and decreases steadily along its course, from 3.18 μg L⁻¹ at Hanuman Chatti to 0.67 μg L⁻¹ at Batamandi, at the base of the Himalaya. This decrease results mainly from mixing of the Yamuna mainstream with its tributaries, which are lower in uranium. The high concentration of uranium at Hanuman Chatti is derived from weathering of the Higher Himalayan Crystalline series (HHC) and associated accessary minerals, which may include uranium-mineralised zones. The ²³⁴U/²³⁸U activity ratios in the samples from the Chambal watershed are in the range of 1.15±0.05 to 1.67±0.04; whereas in the Yamuna the ratios vary from 0.95±0.03 to 1.56±0.07, during post-monsoon and from 0.98±0.01 to 1.30±0.03, during summer. The relatively high ²³⁴U/²³⁸U activity ratios in the Yamuna system are in its tributaries from the lower reaches viz., the Amlawa, Aglar, Bata, Tons and the Giri. It is estimated that ~9×10³ and ~12 × 10³ kg of dissolved uranium are transported annually from the Yamuna at Batamandi and the Chambal at Udi, respectively. This corresponds to uranium weathering rates of 0.9 and 0.09 kg U km⁻² y⁻¹ in the basins of the Yamuna and the Chambal headwaters. This study confirms that uranium weathering rate in the Himalaya is far in excess (by about an order of magnitude) of the global average value of ~0.08 kg U km⁻² y⁻¹.     

Spatial and temporal variability of 234U/238U activity ratios in the Shu River,Central Asia

This study presents the temporal and spatial variability of ²³⁴U/²³⁸U activity ratios in the Shu River and provides interpretation to explain the downstream changes of uranium and the ²³⁴U/²³⁸U activity ratios in the study area. The positive linear correlation (R ² = 0.98, p < 0.001) between uranium concentration and specific electrical conductance is consistent with rock weathering and leaching as the major contributor of dissolved uranium in the studied area of the river. The ²³⁴U/²³⁸U activity ratio ranged between ~1.6 in the upper reaches of the river to ~1.15 furthest downstream. Activity ratios at specific sampling points do not show significant seasonal variability.     

Use of 234U and 238U isotopes to evaluate contamination of near-surface groundwater with uranium-mill effluent: a case study in south-central Colorado, U.S.A.

The ²³⁴U/²³⁸U alpha activity ratio (AR) was determined in 47 samples of variably uraniferous groundwater from the vicinity of a uranium mill near Cañon City, Colorado. The results illustrate that uranium isotopes can be used to determine the distribution of uranium contamination in groundwater and to indicate processes such as mixing and chemical precipitation that affect uranium concentrations. Highly to moderately contaminated groundwater samples collected from the mill site and land immediately downgradient from the mill site contain more than 100?μg/l of dissolved uranium and typically have AR values in the narrow range of 1.0–1.06. Other samples from the shallow alluvial aquifer farther downgradient from the mill contain 10–100?μg/l uranium and plot along a broad trend of increasing AR (1.06–1.46) with decreasing uranium concentration. The results are consistent with mixing of liquid mill waste (AR≈1.0) with alluvial groundwater of small, but variable, uranium concentrations and AR of 1.3–1.5. In the alluvial aquifer, the spatial distribution of wells with AR values less than 1.3 is consistent with previous estimates of the probable distribution of contamination, based on water chemistry and hydrology. Wells more distant from the area of probable contamination have AR values that are consistently greater than 1.3 and are indicative of little or no contamination. The methodology of this study can be extended usefully to similar sites of uranium mining, milling, or processing provided that local geohydrologic settings promote uranium mobility and that introduced uranium contamination is isotopically distinct from that of local groundwater.     

湖北宜昌香溪河流域环境同位素特征及其水循环意义

为研究鄂西南岩溶山区的水循环过程,以湖北宜昌香溪河流域为研究对象,通过现场调查并结合环境同位素,对香溪河流域地表水和地下水进行了取样,通过测定其氢氧同位素组成,分析了同位素变化特征以及流域地下水和地表水的转换关系及其水循环特征。流域水中δD、δ18 O值组成分析表明:流域内各种水体主要分布在当地大气降水线的附近,构成斜率明显小于雨水线的蒸发线,3个子流域δD、δ18 O值的富集程度为:南阳河流域<古夫河流域<高岚河流域。南阳河流域上游受神农架山区地方性大气降水控制。响水洞和响龙洞(暗河出口)水中氘过量参数(d)值分别反映出不同的地下径流途径与滞留时间、水岩反应强度。子流域同位素沿程变化的特征反映出:在上游段,水来源不同以及地表水和地下水转换频繁是δD、δ18 O值变化的主要影响因素;在中下游段,流域内地下水流入河流,河水流量逐渐增大,不同的水源混合均匀,经过一定的蒸发作用,δD、δ18 O值的变幅趋于稳定。     

鲕粒粒度特征及其指示意义

沉积物的粒度资料是确定沉积环境的重要依据,一直以来多应用在碎屑岩方面,较少用于碳酸盐岩的分析。以北京西山寒武系张夏组鲕粒石灰岩为研究对象,在野外实测、室内薄片粒度统计的基础上,通过分析鲕粒粒度参数、粒度频率曲线及概率累积曲线特征,明确了鲕粒粒度在分析鲕粒灰岩沉积环境上的重要作用,并建立了下苇甸鲕粒粒度特征与鲕粒滩在空间上的耦合关系。研究结果表明:鲕粒粒径大小、分选系数、峰度及概率累积曲线组分截点可以很好地反映鲕粒沉积水动力环境;一般情况下,水动力环境较稳定的间歇高能滩沉积的灰泥鲕粒石灰岩鲕粒粒径较小,分选好,峰度尖锐,频率曲线主要为单峰态,鲕粒以悬浮搬运为主;水动力较为动荡的高能滩沉积的亮晶鲕粒石灰岩鲕粒粒径较大,分选较差,峰度多为平坦、中等,频率曲线主要为多峰态,概率累积曲线多为一跳一悬两段式;鲕粒的粒径与鲕粒圈层类型密切相关,粒径较大的鲕粒多以同心鲕、同心放射鲕为主,粒径较小的鲕粒多为放射鲕。研究结果为分析颗粒石灰岩的沉积环境提供了理论和依据。     

234U/238U evidence for local recharge and patterns of ground-water flow in the vicinity of Yucca Mountain,Nevada, USA

Uranium concentrations and ²³⁴U/²³⁸U ratios in saturated-zone and perched ground water were used to investigate hydrologic flow and downgradient dilution and dispersion in the vicinity of Yucca Mountain, a potential high-level radioactive waste disposal site. The U data were obtained by thermal ionization mass spectrometry on more than 280 samples from the Death Valley regional flow system. Large variations in both U concentrations (commonly 0.6–10 μg l⁻¹) and ²³⁴U/²³⁸U activity ratios (commonly 1.5–6) are present on both local and regional scales; however, ground water with ²³⁴U/²³⁸U activity ratios from 7 up to 8.06 is restricted largely to samples from Yucca Mountain. Data from ground water in the Tertiary volcanic and Quaternary alluvial aquifers at and adjacent to Yucca Mountain plot in 3 distinct fields of reciprocal U concentration versus ²³⁴U/²³⁸U activity ratio correlated to different geographic areas. Ground water to the west of Yucca Mountain has large U concentrations and moderate ²³⁴U/²³⁸U whereas ground water to the east in the Fortymile flow system has similar ²³⁴U/²³⁸U, but distinctly smaller U concentrations. Ground water beneath the central part of Yucca Mountain has intermediate U concentrations but distinctive ²³⁴U/²³⁸U activity ratios of about 7–8. Perched water from the lower part of the unsaturated zone at Yucca Mountain has similarly large values of ²³⁴U/²³⁸U. These U data imply that the Tertiary volcanic aquifer beneath the central part of Yucca Mountain is isolated from north-south regional flow. The similarity of ²³⁴U/²³⁸U in both saturated- and unsaturated-zone ground water at Yucca Mountain further indicates that saturated-zone ground water beneath Yucca Mountain is dominated by local recharge rather than regional flow. The distinctive ²³⁴U/²³⁸U signatures also provide a natural tracer of downgradient flow. Elevated ²³⁴U/²³⁸U in ground water from two water-supply wells east of Yucca Mountain are interpreted as the result of induced flow from 40 a of ground-water withdrawal. Elevated ²³⁴U/²³⁸U in a borehole south of Yucca Mountain is interpreted as evidence that natural downgradient flow is more likely to follow southerly paths in the structurally anisotropic Tertiary volcanic aquifer where it becomes diluted by regional flow in the Fortymile system.     

Scavenging and particle residence times determined from 234Th/238U disequilibria in the coastal waters of Mecklenburg Bay

Activities of the naturally occurring radionuclide ²³⁴Th were determined in water samples of Mecklenburg Bay (SW Baltic Sea) using a new Th-specific diatomite adsorption technique followed by liquid scintillation spectrometry. Activities of “dissolved” (operationally defined as Th in the centrifugate) and particulate ²³⁴Th varied in the range of 1.4–6.9 and 0.9–9.3 mBq l⁻¹, respectively. A significant correlation between K_d and SPM concentration was found. From this particle-concentration effect, the “colloidal pumping” model predicts that 98% of the “dissolved” Th is associated with colloids rather than being truly dissolved. Relative to calculated activities of the parent nuclide ²³⁸U in the Bay, the ²³⁴Th data yielded mean ²³⁴Th scavenging residence times in the range of 1.2–9.7 days. Particulate ²³⁴Th activities are inversely correlated to SPM concentrations. Particle residence times ranged from a few days in winter up to 20 days in spring characterized by less intense bottom currents. The hydrodynamic regime is the master variable controlling scavenging of Th and other similarly particle-reactive elements in Mecklenburg Bay.     

Analysis of the atmospheric 14C record spanning the past 50,000 years derived from high-precision 230Th/234U/238U, 231Pa/235U and 14C dates on fossil corals

Changes in the geomagnetic field intensity, solar variability, and the internal changes of the carbon cycle are believed to be the three controlling factors of past atmospheric radiocarbon (¹⁴C) concentrations (denoted as Δ¹⁴C). Of these three, it is believed that the field intensity is the dominant factor. We analyze an atmospheric Δ¹⁴C record spanning the past 50,000 years based on previously-published ²³⁰Th/²³⁴U/²³⁸U and ¹⁴C dates of fossil corals from Kiritimati, Barbados, Araki and Santo Islands, and identify the role of the Laschamp geomagnetic field excursion on the long term trend of the Δ¹⁴C record. There is a general consistency between the coral Δ¹⁴C record and the Δ¹⁴C output from carbon cycle models based on the global ¹⁴C production estimates. High-precision, high-accuracy ²³⁰Th/²³⁴U/²³⁸U dates and redundant ²³¹Pa/²³⁵U dates anchor the timing of this Δ¹⁴C record. We propose that a significant fraction of the long-term Δ¹⁴C trend may be due to inaccuracies in the generally accepted ¹⁴C decay constant. The uncertainty in estimating the shape of ¹⁴C beta spectrum below 20 keV leads to one of the greatest errors in decay constant estimates. Once the ¹⁴C half-life is validated via redundant techniques, Δ¹⁴C records will provide a better opportunity to examine the roles of carbon cycle and ¹⁴C production influences.