中国盐湖沉积年代学研究进展

盐湖沉积记录了区域的气候和水文变化,是重要的古气候研究对象。年代学是盐湖古气候研究最重要的一项内容,是后续几乎所有工作的基础。盐湖沉积最常用的定年方法有₁₄C定年、铀系定年、光释光(OSL)定年、古地磁定年。受各种定年方法自身的局限性以及盐湖沉积特有的沉积特征,存在不同方法测出的年龄差异较大的现象。准确测定盐湖沉积的年代还较为困难,一定程度制约了盐湖古气候研究的发展。由于盐湖沉积有机质含量低,易受现代碳的污染,其₁₄C年代老于30 cal ka BP时,测出的年龄可能已饱和,需要谨慎对待。未来需要加强铀系定年和光释光定年等方法在盐湖沉积中的基础研究,并开发新的更好的测年方法,提高盐湖沉积测年的准确度,为深入开展盐湖古气候变化及成盐成矿规律研究提供坚实基础。     

柴达木盆地西台吉乃尔盐湖沉积的年代学研究

柴达木盆地中部西台吉乃尔盐湖成因及其年代框架还存在争议。通过应用光释光测年法（OSL）和放射性碳测年法（AMS¹⁴C）测定两支分别来自该盐湖中部及东部边缘区域的岩芯，获得4个OSL和5个AMS¹⁴C年代数据。结果表明：（1） 对于老于30 ka的样品放射性碳测年法存在明显的年代低估，而对于年轻样品则存在约4.0 ka的碳库效应，相对而言光释光测年方法具有一定的合理性。（2） 两支岩芯顶部粉砂质粘土层形成于约0.3 ka，中部含粉砂质粘土盐层形成于大约4.0 ka，底部粉砂质粘土层则至少形成于70 ka 之前。     

吉兰泰盐湖的形成及指示的环境意义

选择位于季风边缘区的吉兰泰盐湖为研究对象,利用OSL和14C测年手段重建湖面波动历史,探讨盐湖形成年代与环境变化过程。通过大范围的野外调查发现,在该湖泊南部地区发育高于现代干盐滩9 m的湖滨砂沉积,上覆风成砂沉积,其中湖滨沉积物的OSL年代为6.7 ka BP前后,表明当时该湖泊处于浅水状态,此后湖水经历了快速的下降过程。对于盐湖中心剖面点盐层之下粘土层的14C测年显示,在5 500 aBP以来,集聚大量的石膏、芒硝和石盐等蒸发盐类,该湖泊进入成盐期。通过区域对比发现,吉兰泰湖面快速下降阶段与季风边缘区中全新世干旱事件发生时间相对应。研究认为,吉兰泰盐类矿物的开始沉积,反映了中全新世阿拉善高原区域环境的突然变化,可能指示着5.5 Cal ka BP前后干旱事件的发生。     

柴达木盆地昆特依盐湖磁化率特征及古气候意义

昆特依盐湖是柴达木盆地盐湖演化和古气候研究的重要对象。前人对昆特依盐湖钻孔的年代学和地球化学组成特征开展过一些研究,但由于采样间距大,限制了对成盐演化过程的深入理解。而且,目前关于昆特依盐湖磁化率特征的研究未见报道。本文对昆特依盐湖ZK1402钻孔(35 m深)的₁₄C年代和磁化率进行了初步研究。结果表明：(1)钻孔由含盐碎屑层和盐层的互层所组成。钻孔的低频磁化率和高频磁化率的变化极为相似。盐层的低频磁化率值低,含盐碎屑层的低频磁化率值高。低频磁化率清晰地记录到古盐湖经历了四次显著的成盐期。(2)低频磁化率和频率磁化率(百分比形式)呈负相关关系。(3)钻孔24.8 m深处的加速器质谱(AMS)₁₄C校正年代为31780 ± 442 cal a BP,33.0 m深处的年代(19259 ± 207 cal a BP)出现倒转,表明钻孔下部的₁₄C年龄可能受到现代碳污染而接近饱和。     

巴丹吉林沙漠湖泊年轻沉积物14C测年初步研究

沙漠湖泊沉积物是研究气候环境变化的珍贵材料,湖泊岩芯年代学研究是重建古气候和古环境的基础和前提,然而其年代的测定存在诸多困难,尤其是年轻湖泊沉积物测年一直是当前地质年代学研究的难题。~(14)C测年作为一种传统的测年方法已被广泛应用于各类湖泊年轻沉积物的年代测定。巴丹吉林沙漠湖泊年轻沉积物能否利用~(14)C方法进行测年,其可靠性如何,需进行研究和验证。选择巴丹吉林沙漠腹地的两支岩芯(BR-1和YD-1),对其进行了~(14)C测年,并与已有的~(210)P_(buns)及其推算年代进行了对比。结果表明:1)沙漠湖泊中的植物残体来源存在较大的随机性,可能是原有的更老沉积层中的植物残体的再沉积,测年结果偏老约1000年。2)湖泊岩芯中有机沉积物样品的年代结果基本符合沉积规律,但受硬水效应的影响,其年代整体偏老400~900年。3)与~(210)P_(buns)年代对比,两种不同测年物质所测得的年代均存在不同程度的偏老现象,表明岩芯所在湖泊~(14)C年龄受多种因素影响,总体存在很大的不确定性。在测年材料极为稀缺的沙漠丘间湖区,利用岩芯中的植物残体、沉积物有机质进行~(14)C测年,可能受老沉积层中测年材料再沉积、湖水碳库效应等因素的影响,~(14)C年代存在诸多不确定性。因此,在对巴丹吉林沙漠丘间湖泊年轻沉积物测年研究过程中应谨慎运用。     

柴达木盆地昆特依盐湖磁化率特征及古气候意义

昆特依盐湖是柴达木盆地盐湖演化和古气候研究的重要对象。前人对昆特依盐湖钻孔的年代学和地球化学组成特征开展过一些研究,但由于采样间距大,限制了对成盐演化过程的深入理解。目前关于昆特依盐湖磁化率特征的研究尚未见报道。对昆特依盐湖ZK1402钻孔(35 m深)的14C年代和磁化率进行了初步研究。结果表明:1)钻孔由含盐碎屑层和盐层的互层所组成。盐层的低频磁化率值低,含盐碎屑层的低频磁化率值高。低频磁化率清晰地记录到古盐湖经历了4次显著的成盐期。钻孔的低频磁化率和高频磁化率的变化极为相似。2)低频磁化率和频率磁化率(百分比形式)呈负相关关系。3)钻孔9.6 m深处的加速器质谱(AMS)14C校正年代为23 333±255 cal a BP,24.8 m深处的年代为31 780±442 cal a BP,33.0 m深处的年代(19 259±207 cal a BP)出现倒转。钻孔24.8 m和33.0 m深处的14C年龄可能被低估。     

湖泊碳库效应及校正方法

湖泊作为过去环境变化的信息载体,可以很好地提供高分辨率的连续记录,因此常被用来进行定年研究。湖泊沉积物放射性碳定年由于缺乏陆源植物残体、碳屑等可靠的定年材料,多选用其他材料（如全样有机质、水生残体及各类壳体）,但是这些材料通常受到“碳库效应”（The reservoir effect）的影响,而使得年代结果失真,导致难以建立高精度的年代框架,成为这一领域研究放射性碳年代应用的瓶颈。因此,如何对湖相沉积物¹⁴C年代结果进行有效的碳库效应校正,对高精度年代序列的建立具有重要意义。本文综览了近几十年来针对湖泊碳库效应进行研究所取得的一系列研究成果,详细总结了影响湖泊碳库效应的各种因素,讨论了碳库效应对湖泊沉积物定年结果的影响;分析了目前对湖泊碳库的各类校正方法,包括使用同一测年手段、不同测年材料建立点上碳库大小以及采用不同测年手段对碳库的校正等。本文为湖泊沉积物高精度年代序列的建立提供了一定的参考。     

罗北洼地CK-2孔沉积旋回研究

该论文为硕士学位论文 ,于 1999年 5月在中国科学院青海盐湖研究所完成。罗北洼地晚更新世以来的岩芯中 K 、Mg2 和 Na 相对含量能比较真实地反映该区的古气候演变过程。随着旋回地层学研究的深入发展 ,比较准确的绝对年龄要求迫在眉睫。当前 ,盐湖年龄的测定过程中 ,用 14 C绝对年龄法遇到的最大困难是难以从岩芯中获得足够多的合格碳样 ;另外 ,14 C绝对法的测年上限为 5万年 ,不能测定年龄超过 5万年的样品 ,因而使得盐湖研究者必须寻求更好的年龄测定手段。该论文通过对罗布泊盐湖北部罗北洼地钻孔 CK- 2岩芯中各层位沉积物岩性分析…     

洞穴次生碳酸盐稳定同位素古气候重建研究进展

近年来,洞穴次生碳酸盐沉积物因其有着湖泊沉积物、泥炭等无法比拟的测年优势,日渐成为陆地古气候重建的良好研究材料.通过铀系定年,可以获得过去60万年来精确的日历年时间序列,突破了其他陆地沉积物14C定年(5万年)的瓶颈.氧碳同位素作为洞穴石笋的古气候待用指标具有全球对比性,由于其高精度独立的时间标尺和无需校正同位素信号日渐成为其他气候记录对比的基石.在同位素平衡分馏条件下,石笋δ18O反映了当地气候的变化,在季风区一般反映降雨的变化,在欧洲则更多反映温度的变化.通过石笋1δ8O与北大西洋冰漂碎屑事件以及甲烷的对比关系,可以将海洋钻孔和两极冰芯的记录调谐到独立时间标尺的石笋记录上,进而可以探讨全球联系及其驱动机制.影响石笋1δ3C变化的因素比较复杂,一般在轨道尺度上指示了植被类型C3/C4比率的变化,而在更短时间尺度内主要受到土壤CO2活力的影响,进而可以反映当地降水和温度的变化.尽管洞穴石笋研究工作取得了较大进展,但仍然有诸多机理问题尚未解决,在介绍洞穴沉积物稳定同位素古气候重建研究现状的基础上,对今后的研究方向进行一定的探讨.     

X射线衍射在盐湖研究中的应用

对X射线衍射仪的工作原理、物相定性、定量分析方法进行了扼要介绍,同时详细论述了X射线衍射仪在盐湖地质(盐类矿物、盐湖粘土矿物、盐湖沉积矿物与古气候古环境关系)、盐湖化工(镁水泥、锂离子电池和晶须材料)和水盐体系相图研究中的应用。     

Paleoenvironmental implications of new OSL dates on the formation of the “Shell Bar” in the Qaidam Basin,northeastern Qinghai-Tibetan Plateau

A geological feature in the Qaidam Basin known as the “Shell Bar” contains millions of freshwater clam shells buried in situ. Since the 1980s, this feature in the now hyper-arid basin has been interpreted to be lake deposits that provide evidence for a warmer and more humid climate than present during late marine isotope stage 3 (MIS 3). Global climate during late MIS 3 and the last glacial maximum, however, was cold and dry, with much lower sea levels. We re-investigated the feature geomorphologically and sedimentologically, and employed optically stimulated luminescence (OSL) dating to verify the chronology of the sediments. We interpret the Shell Bar to be a remnant of a river channel formed by a stream that ran across an exposed lake bed during a regressive lake phase. Deflation of the surrounding older, fine-grained lacustrine deposits has left the fluvial channel sediments topographically inverted, indicating the erosive nature of the landscape. Luminescence ages place the formation of the Shell Bar in MIS 5 (~113–99 ka), much older than previous radiocarbon ages of <40 ka BP, but place the paleoclimatic inferences more in accord with other regional and global climate proxy records. We present a brief review of the age differences derived from ¹⁴C and OSL dating of some critical sections that were thought to represent a warmer and more humid climate than present during late MIS 3. We attribute the differences to underestimation of ¹⁴C ages. We suggest that ¹⁴C ages older than ~25 ka BP may require re-investigation, especially dates on samples from arid regions.     

Comparison of radiocarbon and OSL dating methods for a Late Quaternary sediment core from Lake Ulaan,Mongolia

Both radiocarbon and optically stimulated luminescence (OSL) dating methods were applied to test their suitability for establishing a chronology of arid-zone lacustrine sediments using a 5.88-m-long core drilled from Lake Ulaan, southern Mongolia. Although the radiocarbon and OSL ages agree in some samples, the radiocarbon ages are older than the corresponding OSL ages at the 550-cm depth horizon (late Pleistocene) and in the 100–300-cm interval (early to late Holocene). In the early to late Holocene, radiocarbon ages are consistently older than OSL ages by 4,100–5,800 years, and in the late Pleistocene by 2,700–3,000 years. Grain-size analysis of early to late Holocene sediments and one late Pleistocene sediment sample (550-cm depth) indicates that eolian processes were the dominant sediment-transport mechanism. Also, two late Pleistocene sediments samples (from 400- to 500-cm depths) are interpreted to have been deposited by both eolian and glaciofluvial processes. Accordingly, the radiocarbon ages that were older than the corresponding OSL ages during the Holocene seem to have been a consequence of the influx of ¹⁴C-deficient carbon delivered from adjacent soils and Paleozoic carbonate rocks by the westerly winds, a process that is also active today. In addition to the input of old reworked carbon by eolian processes, the late Pleistocene sediments were also influenced by old carbon delivered by deglacial meltwater. The results of this study suggest that when eolian sediment transport is suspected, especially in lakes of arid environments, the OSL dating method is superior to the radiocarbon dating method, as it eliminates a common ‘old-carbon’ error problem.     

New progress and problems of Quaternary moraine dating in the Tibetan Plateau

Since the 20th century, numerous Quaternary moraine dating methods have emerged, including lichenometric, moraine 14C, quartz sand thermoluminescence （TL）, electron spin resonance （ESR）, optically stimulated luminescence （OSL） and 10Be, 26A1, 36C1, 3H, 21Ne nuclide dating methods. These dating methods are widely applied to determine moraine ages and have provided a large dataset. Unfortunately each method has its defects. In this paper, we will review these various dating methods and provide some comments.     

Luminescence-dating zeroing tests in Lake Hoare, Taylor Valley, Antarctica

In two of the perennially ice-covered lakes in the McMurdo Dry Valleys, lakes Hoare and Bonney in the Taylor Valley, bottom water has ¹⁴C ages of 2.7 ka and 10 ka (respectively), rendering ¹⁴C ages of bottom sediments highly problematic. Consequently, we tested the effectiveness of thermoluminescence (TL) zeroing in polymineral fine silt material from several depositional environments around and on the lake (stream suspensions, ice-surface sand dune, and silty sand from near the top of the more-than-3m-thick ice). We also conducted TL and infrared-stimulated-luminescence (IRSL) dating tests on material from three box cores recovered from the bottom of Lake Hoare, in a transect away from the abutting Canada Glacier. We observed effective zeroing of light-sensitive TL in suspended silt from one input stream and less effective zeroing from another stream. We observed effective zeroing of light-sensitive TL also in silt from a glacier-proximal eolian ice-surface dune and from sand from within the upper 5 cm of ice. In contrast, in box-core 1, the bottom sediment yielded minimum TL apparent ages of 1500-2600 yrs, with no discernable stratigraphic depth trend. IRSL dating applied to the same box-core samples produced significantly lower age estimates, ranging from ~600 ± 200 yrs to 1440 ± 270 yrs top-to-bottom, an improvement over the depth-constant ~2200 yrs TL ages. In two other cores closer to the Canada Glacier, IRSL ages from ~600 ± 200 yrs (top) to ~ 2900± 300 yrs (at depth) were measured. Not only are the IRSL ages a significant improvement over the TL results, but the near-core-top IRSL ages are also a dramatic improvement over the ¹⁴C results (~2.7 ka). IRSL dating has a demonstrated potential to supplant ¹⁴C dating for such antarctic lacustrine deposits.