2019, Vol.40
2 中国科学院地球环境研究所, 黄土与第四纪地质国家重点实验室, 陕西 西安 710061;
3 遵义师范学院, 贵州 遵义 563002;
4 中国科学院第四纪科学与全球变化卓越创新中心, 陕西 西安 710061)
沉积物粒度是沉积学研究中的一个重要物理指标，可以指示物源区性质、物质搬运过程和沉积环境等信息，因测试简单快捷且测试结果不受生物化学作用影响，是重建古环境的重要代用指标之一，目前已广泛应用于黄土[1~7]、海洋[8~10]和湖泊[11~31]等地质载体研究中。在湖泊研究中，沉积物粒度一般指示区域降水[13~16, 20, 24, 26, 28~29, 31]以及湖泊水位高低变化[11~12, 18~19, 23, 25, 30]等环境信息。但在一些特殊湖泊中并非如此，例如更尕海[21]粒度＞63 μm组分指示尘暴活动，卡拉库里湖[22]沉积物粒度指示冰川进退。另一方面，湖泊沉积物粒度在不同时间尺度和分辨率上的环境指示意义并不相同[15]。通常认为，在长尺度、低分辨率(百年、千年尺度)序列中，粒度偏粗指示湖泊水位较低，说明气候干旱；粒度偏细指示湖泊水位较高，反映气候湿润[11~12, 19, 23, 25, 30]。但在长尺度研究中，沉积物粒度较粗，有时也可指示湖泊水位较高和区域降水的增多。例如，Peng等[16]重建了岱海过去10000年以来东亚季风强弱变化引起的降水变化，结果表明沉积物平均粒径较小，反映东亚季风较弱，区域降水较少；反之，指示东亚季风增强和降水增多。这种降水变化也得到该钻孔孢粉记录的印证。Liu等[18]认为乌伦古湖沉积物粒度粗颗粒物质增多指示湖区降水增多、径流量增加，从而导致湖泊水位升高；反之，指示湖泊水位降低。这也得到孢粉研究结果的印证。而在短尺度、高分辨率(年际、十年尺度)气候序列中，现有的解释是：粒度偏粗指示降水较多、气候湿润，粒度偏细则指示降水较少、气候干旱[13~15]。但因湖泊面积大小、地形、流域植被覆盖及其他影响因素的不同，粒度粗细指示的气候意义也有所差异。本文以六盘山朝那湫为例，通过粒度与其他指标之间对比印证，揭示湖泊沉积物粒度在短时间尺度上的气候环境指示意义。
1 方法 1.1 研究区概况与采样朝那湫(又称六盘山天池)处在黄土高原西部六盘山南端西麓(图 1a)，距庄浪县城东北约30 km，是一个小型高山堰塞淡水湖泊，因其位于高山，位置较偏远，受人类活动影响较弱，是研究古气候变化的良好点位。朝那湫湖面海拔2430 m，湖面面积约0.02 km2，流域面积约0.2 km2，最大水深约9 m；湖泊季节性排水，湖水主要由降水补给(是葫芦河源头之一)。湖泊流域年均降水量为677 mm，年平均气温为3.4 ℃[32]。朝那湫东、北、南三面环山，湖盆形似“卧蚕”状(图 1b)，目前流域植被覆盖度较高，主要以灌木和草甸为主。湖区地表出露基岩为红色砂岩，地表径流携带的陆源碎屑物质是朝那湫沉积物的主要来源[33]。
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图 1 研究区概况 (a)朝那湫位置示意图；(b)朝那湫湖泊形状、地势及采样点位置示意图 Fig. 1 Overview of study site. (a)Location of Lake Chaonaqiu; (b)The shape, topography surrounding Lake Chaonaqiu(from Google Earth)and the locations of sampling sites |
2012年9月，我们用奥地利重力钻在湖心(35°15′53.08″N，106°18′35.99″E)选择两个采样点采集沉积物岩芯4根(每个采样点采集2根岩芯作为平行岩芯，采样点位置见图 1b所示)，岩芯分别命名为CNQ12-1 (73 cm)、CNQ12-2 (130 cm)、CNQ12-3 (70 cm)和CNQ12-4 (138 cm)，岩芯CNQ12-1和CNQ12-2在采样点1，CNQ12-3和CNQ12-4在采样点2，每个采样点的两根岩芯间距在0.5~1.0 m之间，所采岩芯沉积物-水界面清晰，未受扰动。通过野外观察和室内分析，4根岩芯岩性变化趋势基本一致，岩性主要为粉砂质沉积物。岩芯CNQ12-1 (73 cm)颜色变化表现为黑色与褐色相间，上部7 cm为黑色，7~16 cm为褐色，16~27 cm为黑色，向下为褐色与黑色互层(图 2c)，我们现场将岩芯CNQ12-1进行1 cm等间距分样，获得样品73个，密封低温冷藏。其余岩芯运回实验室备用，在搬运及后期实验室储存过程中发生了一定程度的压实作用；另外，在实验室将岩芯CNQ12- 4按1 cm间距分样，由于在分样过程中损失样品较多，未能建立准确的质量深度和年代模型。本文对岩芯CNQ12-4进行137Cs活度和粒度指标测试，以检验岩芯CNQ12-1的137Cs年代学(137Cs活度峰形)和粒度序列的可靠性。
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图 2 朝那湫岩芯岩性和年代模型 (a)岩芯CNQ12-1的137Cs和210Pbex活度变化.；(b)岩芯CNQ12-4的137Cs和210Pbex活度变化；(c)岩芯CNQ12-1的137Cs年代模型 Fig. 2 Lithology characteristics and age model of Lake Chaonaqiu core. (a)137Cs and 210Pbex activities of core CNQ12-1; (b)137Cs and 210Pbex activities of core CNQ12-4; (c)137Cs age model of core CNQ12-1 |
使用高纯锗γ能谱仪(ORTEC，GWL-250-15)测定岩芯CNQ12-1和CNQ12-4样品137Cs与210Pbex(过剩210Pb)活度，测试误差 < 10 %。岩芯CNQ12-1和CNQ12-4均开展粒度测试，仪器为Mastersizer-2000型激光粒度仪，测量粒径范围为0.02~2000 μm，样品前处理程序及方法参照文献[34]进行。岩芯CNQ12-1适量样品用1 mol/L高氯酸(HClO4)去除碳酸盐后用去离子水反复洗至中性，利用元素分析仪(vario EL Ⅲ)测试样品TOC(总有机碳)和TN(总氮)含量，实验误差 < 0.2 %。TOC与TN含量两者比值乘以1.167(氮原子和碳原子的原子量比值)得到样品的C/N比值。总有机碳(δ13Corg)测定仪器为连续流同位素比率质谱(Flash EA 1112 Series-Finnigan Delta Plus XP)，实验误差 < 0.1 ‰。测定岩芯CNQ12-1奇数编号样品元素组成，测试仪器为荷兰生产的X射线荧光光谱仪(PANalytical)，实验误差 < 0.01 %。本文TOC、TN以及δ13Corg指标测试在中国科学院地球化学研究所完成，其余指标测试均在中国科学院地球环境研究所完成。
2 结果 2.1 年代两岩芯137Cs和210Pbex活度变化趋势一致(图 2a和2b)，表明测试结果可靠。两岩芯137Cs活度峰高且呈单峰(图 2a和2b)，其分布形态与经典的全球大气137Cs沉降模式[35~36]相似，表明该湖137Cs主要来自大气沉降，且后期改造对137Cs峰形影响较弱。因此，朝那湫137Cs活度峰可用作理想的时标峰。但是，两岩芯137Cs活度峰值出现位置(深度)略有不同(图 2a和2b)，这可能与两岩芯的采样点沉积速率差异以及岩芯CNQ12-4在后期储存过程中受到一定程度的压实作用有关。
另一个现象是两岩芯210Pbex活度存在较大次级波动；两岩芯210Pbex活度均呈典型的对数分布形态，但二者绝对值相差较大(图 2a和2b)。这一方面反应了不同采样点接收到的陆源输入组分差异；另一方面也暗示该湖泊生物作用对210Pbex可能有较强的洗脱效应，这种洗脱效应可能与在云南程海[37]观察到的洗脱效应相似。另外，两岩芯210Pbex活度对数分布形态还间接表明两岩芯表层沉积物为真实的表层沉积物(即表层沉积物未在人为干扰情况下缺失或未在采样过程中缺失)。鉴于朝那湫沉积物210Pbex受到后期生物洗脱效应可能较明显，本文未使用210Pbex计算年代。
岩芯CNQ12-1的137Cs活度峰值出现在几何深度17.5 cm处，我们将其视为1964年时标(图 2a)，结合该岩芯质量累积深度(质量深度)求算出沉积物平均质量堆积速率为0.0852 g/(cm2·a)，据此计算出岩芯CNQ12-1记录的是1743~2012 A.D.、时长为268年的沉积序列(图 2c)。
2.2 指标岩芯CNQ12-1平均粒径变化在10.06~18.30 μm之间，平均值约为13.19 μm(图 3a)。沉积物粒径组分以粉砂(4~63 μm)为主，其中细粉砂(4~16 μm)含量约在40.20 % ~58.00 %之间，平均含量约为46.60 %；粗粉砂(16~63 μm)含量在19.20 % ~37.00 %之间，平均含量约为28.0 %；粘土(< 4 μm)含量在17.10 % ~30.90 %之间，平均含量约为24.16 %；砂级(＞63 μm)含量在约0~4.80 %之间，平均值约为1.26 %。岩芯CNQ12-4平均粒径变化在9.36~15.55 μm之间，平均值约为12.17 μm(图 3a)。岩芯CNQ12-1沉积物粒度频率分布呈显著的单峰曲线(图 3b)，表明沉积过程单一。这是因为朝那湫无河流进入，湖泊沉积物主要来自流域地表径流携带的陆源碎屑物质。岩芯CNQ12-4与CNQ12-1粒度地层变化趋势的相似性(图 3a)，有力地证明岩芯CNQ12-1粒度曲线的可靠性。
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图 3 朝那湫岩芯粒度序列(a)与分布特征(b) (a)橙色曲线表示岩芯CNQ12-1，蓝色曲线表示岩芯CNQ12-4；(b)岩芯CNQ12-1粒度频率分布 Fig. 3 Grain size sequences (a) and grain size frequency distribution (b) of Lake Chaonaqiu cores. The orange and blue curves in (a) indicate core CNQ12-1 and core CNQ12-4, respectively, and (b) indicate grain size distribution pattern in core CNQ12- |
岩芯CNQ12-1的δ13Corg在-32.48 ‰ ~-26.95 ‰之间，平均值为-30.49 ‰；TOC含量在2.77 % ~12.23 %之间，平均值约为6.29 %；TN含量在0.20 % ~0.91 %之间，平均值约为0.50 %；C/N比值在9.51~16.49之间，平均值为12.68(图 4给出的是去趋势后的C/N比值曲线)；SiO2含量在64.22 % ~78.89 %之间，平均值约为68.27 %。岩芯CNQ12-1各指标以及与其邻近区域记录指标[38~42]总体上均表现出较好的对应关系(图 4)。沉积物δ13Corg和SiO2与粒度变化趋势相同，沉积物粒度越粗(细)，对应的δ13Corg偏正(负)、SiO2越高(低)。TOC、TN和C/N比值(去除地层趋势)变化趋势相同，但均与粒度变化趋势相反，粒度越粗(细)，TOC、TN和C/N比值均越低(高)。
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图 4 朝那湫岩芯CNQ12-1沉积物粒度指标与其他指标及邻近区域其他记录对比 迭山树轮[38]、泰和山树轮SPEI[39](SPEI为标准化降水蒸发指数，数值越大表示湿润程度越高)和榆中干湿指数[40~42]平滑曲线均为11年滑动平均；浅绿色阴影表示干旱气候时段 Fig. 4 Comparison of the grain size with other indices of core CNQ12-1 in Lake Chaonaqiu and other records in neighboring regions. The smoothed curve of Dieshan Mt. precipitation[38], Taihe Mt. SPEI[39](Standardised Precipitation Evapotranspiration Index, higher value indicates a wetter climate condition)and the Yuzhong drought/flood index[40~42]are all 11 a running means. The light green bars indicate periods of drier climate |
湖泊沉积物粒径的粗细直接反映沉积水动力条件，而湖泊水动力条件不仅受湖泊面积、形状等因素影响，还受气候和流域植被等因素控制[43~44]。受湖泊水动力条件控制，沉积物粒径自湖岸到湖心逐渐变细。一般认为，粗粒沉积物指示湖泊面积退缩、水位较低的干旱气候，细粒沉积物指示湖泊面积扩张、水位较高的湿润气候[11~12, 19, 23, 25, 30]。这是因为在气候干旱期，湖泊面积收缩，采样点距离湖岸较近，水动力增强，携带的粗颗粒增多，沉积物里粒度偏粗；在气候湿润期，湖泊面积扩张，采样点距离湖岸较远，水动力减弱，细颗粒物质更易于沉积，沉积物粒度偏细。朝那湫是一个小型高山湖泊，湖水主要靠降水补给，湖岸西端地势较低有一出水口(见图 1b)，雨季湖水外溢，几乎不存在湖泊水位大幅度波动的可能性，因此水位变化对粒度分布的影响较小，湖泊流域降雨量变化可能是控制粒度粗细的重要因素。朝那湫处在黄土高原西部地区，虽然流域降雨量反映为半湿润气候环境，但黄土高原所处的气候带没有变化，因此朝那湫流域降水具备干旱-半干旱区气候特点：降水年内分配不均具有集中性(常以强降雨或暴雨形式出现)，无论年降水量增多或减少，但是降水特点(降水集中)没有发生变化。另有研究表明，黄土高原地区在干旱气候条件下降水的集中性则会更加突出[45]。另外，黄土高原地区植被覆盖与降水量呈正相关关系[46~48]。例如，信忠保等[46]研究了黄土高原地区1981~2006年植被覆盖与降水量之间的关系，结果表明：多雨期植被覆盖上升，而在降水减少期，植被覆盖则偏低。对朝那湫而言，气候偏湿期，降水量较多，流域较高的植被覆盖不但固定了地表土壤，而且削弱了地表径流搬运能力，较多细颗粒物质随地表径流入湖沉积，因此沉积物粒径偏细；气候偏干期，降水量减少，流域植被覆盖度偏低，地表裸露，地表土壤较为松散，一次集中性降雨过程造就搬运能力较强的地表径流，较多粗颗粒物质入湖沉积，导致沉积物粒径偏粗。
朝那湫沉积物有机代用指标和化学元素进一步印证了粒度的环境指示意义。湖泊沉积物中的TOC含量是描述沉积物中有机质丰度的基本指标[49]，湖泊沉积物有机质分为内生有机质(来源于水生植物[50])和陆源有机质(来源于流域植被[50])，C/N比值高(低)指示陆源(内生)有机质贡献高[51]。由于湖泊内生有机质通常在光合作用过程中部分使用溶解无机碳合成光合初产物，因此其δ13Corg相对于陆源有机质δ13Corg而言通常较高，沉积物总有机质δ13Corg偏负，指示陆源有机质贡献增大；沉积物总有机质δ13Corg偏正，指示湖泊内生有机质贡献增大。因此，沉积物TOC、C/N比值、δ13Corg联合可以很好地指示沉积物有机质的相对贡献[52~55]。朝那湫湖泊沉积物主要来源于陆源碎屑物质，降水增多，流域植被发育好，沉积物中陆源有机质增多，TOC含量和C/N比值升高，δ13Corg偏负；降水减少，流域植被覆盖较低，进入湖泊的陆源有机质减少，沉积物TOC含量和C/N比值降低，δ13Corg偏正。因此，朝那湫沉积物有机代用指标能够反映湖区降水量变化。湖泊沉积物Si元素在多项研究中被认为是来自外源碎屑物质，一般与外源物质的输入有关，其含量与降水量、流域土壤侵蚀等有密切联系[56~59]，且Si与粗粉砂和砂级颗粒有关[58]。岩芯CNQ12-1粒度、有机代用指标和Si元素反映的气候变化序列与迭山树轮[38]、泰和山树轮[39]以及榆中干湿指数序列[40~42]有良好的对应关系(图 4)。例如，1770 A.D.前后、1835 A.D.前后、1875 A.D.前后和1970 A.D.前后粒度偏粗、δ13Corg偏正、TOC和C/N比均偏低，表明该时期气候偏干，同期树轮记录与榆中干湿指数均证实了这4次干旱事件(图 4)。在气候偏湿期，陆源有机质贡献增大，沉积物TOC、TN含量和C/N比值升高、δ13Corg偏负、Si元素含量降低，对应时期沉积物粒径偏细；在气候偏干期，陆源有机质贡献减小，沉积物TOC、TN含量、C/N比值均降低、δ13Corg偏正、Si元素含量升高，对应时期沉积物粒径偏粗。
4 结论湖泊沉积物粒度不仅在不同时间尺度和分辨率上的环境指示意义并不相同，受湖泊面积大小、地形、流域植被覆盖及其他因素影响，粒度粗细指示的气候意义也有所差异，因此在不同的湖泊研究中应充分考虑影响沉积物粒度的各种因素。基于六盘山朝那湫湖泊岩芯CNQ12-1 (73 cm)的137Cs测年，建立过去近268年来高分辨率年代序列。该岩芯粒度指标与其他环境代用指标(TOC、C/N比值、Si元素等)的对比结果显示，沉积物粒度能够较好地反映流域降水变化历史，并记录了4次典型的干旱气候事件(1770 A.D.前后、1835 A.D.前后、1875 A.D.前后和1970 A.D.前后)，这与邻近地区其他高分辨率气候记录有良好的对应关系。本文揭示了湖泊沉积物粒度在短时间尺度研究中的特殊指示意义：粒度偏粗指示降水较少、气候干旱，粒度偏细指示降水较多、气候湿润。朝那湫湖泊沉积物粒度在短时间尺度上的气候指示意义与现有的解释不同，这主要是由流域降水特点和植被覆盖程度决定的。
致谢: 感谢审稿专家提出的建设性修改意见，向编辑部各位老师的工作表示敬意!
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2 State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, Shaanxi;
3 Zunyi Normal College, Zunyi, 563002, Guizhou;
4 Center for Excellence in Quaternary Science and Global Change, Chinese Academy of Sciences, Xi'an 710061, Shaanxi)
Abstract
In short time scales and high resolution (annual or decadal scales) studies, the variations of grain size in lacustrine sediments are often used to indicate the changes of paleoclimate, namely, coarse-grained sediments reflect elevated precipitation and thus wetter climatic conditions; while fine-grained sediments indicate lower precipitation and thus drier climatic conditions. However, the paleoclimatic implications of grain size in lacustrine sediment reveal a complex spatial pattern in different lakes due to site-specific conditions, such as lake/catchment ratio, vegetation cover, and other factors. Thus, it should limit our understanding of long-term paleoclimate variability. Here, the paleoclimatic implications in short time scales of grain-sizes of Lake Chaonaqiu sediments were discussed, based on the characteristics, the geographical location and the significances of multiple proxy indices from lake sediments of Lake Chaonaqiu.Lake Chaonaqiu, a small open alpine lake in the Liupan Mt. (the lake area is ca. 0.02 km2and the catchment area is ca. 0.2 km2, altitude in 2430 m a. s. l.), is located on the western Loess Plateau (LP), China. A 73-cm long sediment core was obtained in the centre of the lake (35°15'53.08"N, 106°18'35.99"E) using a 60-mm UWTTEC gravity corer, in September 2012. The lithology is mainly consistent of lacustrine black and brown silt. The chronology was well established by 137Cs activities for the past 268 years. A total of 73 samples were taken at 1 cm intervals for grain-size, TOC, TN, C/N ratio and δ13Corg analysis, and 37 samples were collected at 2 cm intervals for XRF analysis.The sediments was mainly composed of silt (4~63 μm), fine silt (4~16 μm) and coarse silt (16~63 μm) accounted for 40.20%~58.00% and 19.20%~37.00% respectively, the clay (< 4 μm) accounted for 17.10%~30.90%, and the content of sand component (>63 μm) was the lowest, ranging from 0~4.80%. Based on the well correlation of grain-size and other proxy indices of Lake Chaonaqiu sediments, we suggested that the variations of grain-size of Lake Chaonaqiu sediments could effectively indicate paleo-precipitation history in the study area. Fine-grained size indicates humid climate conditions, because the densely vegetation cover in the catchment during wetter period not only stabilizes the surface soil, but also weakens the load capacity of surface runoff, which result in fine-grained size entering into the lake. However, coarse-grained size indicates arid climate conditions, because the sparsely vegetation cover during drier periods exposes more soil surface area, and produces a less cohesive soil, even relatively low-intensity rainfall could create a stronger surface runoff load capacity, resulting in coarse-grained size entering into the lake. Therefore, four drought climate events at Lake Chaonaqiu area were identified (e.g., ca. 1770 A.D., 1835 A.D., 1875 A.D., and 1970 A.D.), which correlate well with the records of tree rings and historical documents in neighboring regions.