Hydrogeochemical characteristics of hot springs exposed from fault zones in western Guangdong and their ^(14)C age correction

Hot springs are natural exposed points of the hydrothermal system. The hydrogeochemistry of hot springs can be used to interpret the formation of the hydrothermal system; and the ^(14)C dating can be used to evaluate the renewability of the hydrothermal system. The hot springs exposed from fault zones in western Guangdong are classified as granite fissure water and clastic rock fissure water, which are sampled and tested. The results of water chemistry analysis show that hot spring water is mainly HCO_3-Na type in the beginning, while the mixing of seawater leads to the increase of Cl^-. Hydrogen and oxygen isotopes indicate that these hot springs mainly come from atmospheric precipitation, and water-rock interactions produce oxygen isotope exchange reactions, where a significant "oxygen drift" phenomenon can be observed. The relationship between δ^(13)C and HCO_3^- indicates that there is a deep source of CO_2 "dead carbon" in hot spring water. This systematic error is not considered in the existing ^(14)C dating correction models. The ^(14)C age of the deep source "dead carbon" correction proposed in this paper is close to the ^(14)C age of the reverse chemical simulation correction, the Gonfiantinie model, and the Mook model. The deep source "dead carbon" correction method can improve the systematic error. Therefore, the ^(14)C age corrected by the deep source "dead carbon" may be more representative in terms of the actual age of geothermal water.

基于反向地球化学模拟技术的深层地下水^(14)C年龄校正——以挠力河以北典型水流路径为例

地下水年龄的确定是水循环研究的重要内容,也是估计地下水水循环速度和定量评价地下水可更新能力的重要依据。以挠力河流域以北地区为例,利用反向水文地球化学模拟技术,识别出影响研究区内深层地下水地球化学演化的主要作用,并进行深层地下水^(14)C年龄的校正。研究认为,控制挠力河流域南部湿地区深层地下水化学演化的主要反应为方解石、黑云母、二氧化碳的溶解反应、高岭土的沉淀反应以及Ca-Na阳离子的交换反应;北部农业区主要发生了黑云母、二氧化碳的溶解及高岭土、方解石的沉淀和Ca-Na阳离子的交换反应。地下水^(14)C年龄校正结果表明,北部农业区深层地下水^(14)C年龄(9176a)明显老于南部湿地区(6097a),年龄校正结果与Vogel统计模型^(14)C年龄校正结果较接近。南部湿地区地下水循环速率为11.16 m/a,北部农业区地下水循环速率为2.19 m/a。研究结果对研究区地下水资源评价和制定农业区和湿地区地下水合理开发利用模式具有重要借鉴意义。

加速器质谱^(14)C断代

着重介绍加速器质谱计（ＡＭＳ）１４Ｃ断代方法，北京大学近三年来ＡＭＳ１４Ｃ断代工作。

Tri-Carb 2250 CA液体闪烁谱仪的性能与初步应用

本文介绍了 Tri-Carb 2250 CA 液体闪烁谱仪的性能及其运行一年的经验。内容主要包括以样品谱指数(SIS)和外标准转换谱指数(tSIE)作为猝灭指示参数进行的猝灭校正,效率示踪 DPM(ET-DPM),低水平氚测量的性能检验与评价,以及仪器使用中需注意的问题。

Radiocarbon Dating of Vertisols in China

Vertisols,which are mainly developed on fiuvial and lacustrine deposits and basalt,are extensively distributed in China. 66 samples of them for radiocarbon dating were collected across 5 provinces and 1 autonomous region, ranging from warm-temperate to subtropical and tropical zones in China.The soil organic matter was dated via surfaCe horizon, black soil horizon and dark-colour horizon of the vertisols,whereas carbonate through calcareous concretions and dispersed carbonate in soil profile using radiocarbon method. The present article elucidates the dates of genetic horizons,and of surfaCe and buried vertisols as well. 14C dating indicates that the surface vertisols were formed during the end of the late Pleistocene,ca.15600 years ago.Correlation between depth of sampling and measured 14C age shows that age characteristics of the vertisols of China are close to those of West Germany,Italy,Turnisia,Australia and Argeentia. However, two buried vertisols developed on fluvial and lacustrine deposits were formed during different geological periods.The buried soils in warm-temperate zone were formed in the mid Holocene, while those in subtropics were buried at 12930 years B.P.,and formed in the end of the late Pleistocene.

^(14)C measurement of forest soils in Dinghushan Biosphere Reserve

Organic carbon in forest soils of Qingyunsi and Wukesong profiles can be divided into fast and slow components. Δ14C values of these profiles decrease with increasing of depth. The Δ14C values in 30—40 cm depth interval of Wukesong profile are decreasing sharply until a very low value,showing that a strong geological environment change occurred about 1 560 years ago. The 14C apparent ages of Wukesong profile show that the coniferous and broad-leaf mixed forests around Wukesong profile have been developing since 425 a BP, which is consistent with historical documents. The penetrating depths of "bomb 14C" in Qingyunsi and Wukesong profiles are 10 and 20 cm, respectively.

Luminescence Chronology and Radiocarbon Reservoir Age Determination of Lacustrine Sediments from the Heihai Lake,NE Qinghai-Tibetan Plateau and Its Paleoclimate Implications

The accurately determining the lake ^14C reservoir age has a crucial significance for climatic reconstruction. In this study, the optically stimulated luminescence （OSL） dating method is employed to date samples from highstand lacustrine sediments, palaeoshoreline, fluvial terrace, and the alluvial fan of the Heihai Lake catchment. Accelerator mass spectrometry （AMS） 14C dating was also used to date fossil plants from highstand lacustrine sediments. Based on the calculations of linear regression with OSL against radiocarbon ages for same layers of two sections, the quantitative ^14C reservoir ages were estimated to lie between 3 353 and 3 464 yr during the 1.8 to 2.4 ka, which showed temporal variation. The sources of old carbon are the dissolution of carbonate bedrocks distributed along the Kunlun Mountain. The OSL ages of the different members of the hydatogen sedimentary system at Heihai Lake catchment indicate that a stronger hydrologic condition occurred from 3.0±0.2 to 1.8±0.2 ka, with a maximum lake level of 9 m higher than present. This humid stage was widely recorded in different sediments on the QTP and Chinese Loess Plateau （CLP）, indicating its broad synchronicity across the Asian Summer Monsoon region. The enhanced East Asian Summer Monsoon （EASM） and the Indian Summer Monsoon （ISM） resulted in the increase of moisture availability for the Heihai Lake area during this stage.

Effect of dead carbon on the^(14)C dating of the speleothem

Based on the comparison of dating results among high-precision TIMS U-series and AMS 14C as well as the published 14C dating results and their band counting ages (i.e. calendar ages), this paper discusses the effect of dead carbon on the speleothem 14C dating. The result shows that the fraction of incorporated dead carbon during the forma- tion of speleothem varies. The change in the fraction of dead carbon would result in big deviation in the 14C age of the speleothem. It is indispensable to take the dead carbon into consideration when dating the speleothem using the 14C method or studying the atmospheric 14C concentration dur- ing the past with the speleothem.

Estimating Change in Sedimentary Organic Carbon Content During Mangrove Restoration in Southern China Using Carbon Isotopic Measurements

Based on total carbon(C) and C isotopes in sediment cores,sedimentary organic carbon(SOC) was quantified in three types of mangrove sites(barren flat sites without mangroves,mangrove plantations,and natural mangrove forests),which were considered to represent a continuum from least restored to most restored sites in southern China.SOC densities in the barren sites,plantations,and natural forests were 90,170 and 288 Mg ha 1,respectively.We inferred that mangrove restoration increased SOC accumulation in coastal areas.At 0-70 cm depth,SOC δ 13 C values in both mangrove sites ranged from 27.37‰ to 23.07‰ and exhibited gradual enrichment with depth.In contrast,the values in the barren flat sites remained around 22.19‰ and fluctuated slightly with depth.At 0-60 cm,the 14 C ages of the SOC in the barren flat site,the natural mangrove site,and the artificial mangrove site ranged from 1 397 to 2 608,255 to 2 453,and 391 to 2 512 years BP,respectively.In both types of mangrove sites but not in the barren flat sites,the enrichment of δ 13 C with depth was related to increases in SOC decay and SOC age with depth.According to analysis of 14 C age,much of the mangrove-derived C was transported and stored at 0-60 cm depth under anaerobic conditions in both mangrove sites.The sediments of mangrove forests in southern China sequester large quantities of SOC during mangrove restoration.

CLIMO-ENVIRONMENT CHANGES IN NORTH XINJIANG SINCE LATE PLEISTOCENE

According to the information on fossil species and sedimentary features of the Late Pleistocene sediments in north Xinjiang, we may successfully learn the history of climatic changes during the past 120,000 yr which provides reasonable scientific evidence of vital importance for predicting the trend of future climate and evolution of environment as well as their influence on production arrangements.