研究论文 正式出版 版本 2 Vol 11 (1) : 66-71 2020
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洛川红黏土-黄土记录的风向变化及磁组构与磁化率对气候变化的敏感性对比
Changes of the wind direction recorded by red clay and loess in Luochuan and comparation of the sensitivity of magnetic fabric and magnetic susceptibility to climate change
: 2019 - 02 - 16
: 2019 - 05 - 10
: 2019 - 05 - 15
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摘要&关键词
摘要:磁组构分析是研究季风风向的一种常用手段。对洛川黄土剖面黄土-红黏土界线附近地层进行磁组构研究表明洛川剖面新近纪—第四纪过渡时期,风向整体呈南北向,且从红黏土沉积阶段到黄土沉积阶段,风向呈现顺时针方向的微小偏移,这可能与新近纪到第四纪的气候变化背景有关。另外,磁化率各向异性度P、磁线理L、磁面理F等参数曲线与磁化率曲线的对比显示磁化率各向异性P、L、F参数的变化稍超前于磁化率曲线的变化,这与蓝田段家坡剖面的磁组构研究结果类似。导致以上结果的原因尚未确定,但可能与磁组构信息被记录于沉积物的速度快于沉积物磁化率值的改变有关。
关键词:黄土;红黏土;磁组构;风向;磁化率各向异性
Abstract & Keywords
Abstract: Background, aim, and scope The stratum proximal to the loess/red-clay boundary represents an ideal carrier for researches on climate change from the Neogene Period to the Quaternary Period. As a part of the climate change, the information contained in the deposition that documents winding direction changes can be obtained through a magnetic fabric analysis. We have obtained preliminary experimental results in a magnetic fabric study at the Duanjiapo section. However, the credibility of these results is inadequate due to the limitations of a single sampling location, which prompts us to perform a similar study at the Luochuan section. Materials and methods In this paper, the Luochuan section is selected as the sampling location, which is situated in the Luochuan Loess National Geopark. Block-shaped samples were taken from the stratum proximal to the loess/red-clay boundary (9 m in total). The AMS of the samples were measured using a Kappa Bridge MFK1A multi-frequency magnetic susceptibility instrument in the paleomagnetic laboratory, Institute of Earth Environment of CAS. Each sample was rotated and measured along three orthogonal planes, and the AMS ellipsoid was then calculated by the least-squares method (Zhu et al, 2004; Zhang et al, 2010). The anisotropy parameters that we obtained was calculated by software, following the method of Jelinek (1981). To compare with ancient wind directions in history, we also obtained modern observational meteorological records in Luochuan region. Results Most of the samples exhibit a characteristic of 1.002≤P≤1.032, typical of aeolian sediments (Liu et al, 1988, 1990). We found that compared with curves of variables pertaining to the anisotropy of magnetic susceptibility (AMS), the magnetic susceptibility curve exhibits a “offset” of about 10—20 cm (hysteresis of the magnetic susceptibility curve). From the red clay deposition stage to the loess deposition stage, the wind directions generally exhibited a clockwise change; the distribution of daily maximum wind directions between 1973 and 2016 at the Luochuan section was basically dominated by north-south winds. In particular, north winds occurred most frequently, which is roughly consistent with the distribution of wind directions during the red clay stage, but slightly different from that of the loess stage. Discussion We have attempted to analyze the cause of the offset between the AMS variable curves and the magnetic susceptibility curve. The phenomenon in which dust and magnetic mineral particles were arranged along the direction of the transport dynamics occurred simultaneously with the deposition effect; along with the effects of subsequent precipitation and sedimentary coverage, these matters could be rapidly concreted to document and save information of ancient winds. However, changes in pedogenesis intensity - the major factor influencing magnetic susceptibility - took place in a relatively slow process requiring much longer time. Further, the intermediate- and long-distance transport of dust may also be relevant (the intermediate- and short-distance refers to a distance entailing a small difference of magnetic susceptibility values of soil surfaces between the source regions of dust and the region of dust deposition). Conclusions The AMS variable curves obtained at the Luochuan section are generally correlated to the changes in the susceptibility curve, yet presenting a certain earliness compared with the latter. This is consistent with the research result of the Duanjiapo section. Results of both sections indicate that the magnetic fabrics of deposition are more sensitive to climate changes than magnetic susceptibility. In terms of the Luochuan section, the distribution of winds directions obtained during the red clay deposition stage more closely resembles that of the modern ones, as compared with that obtained during the loess deposition stage. Additionally, the transitional zone between the loess and red-clay has a bottom boundary of an alluvial layer more than ~10 cm thick, which may be due to a submerge event. Recommendations and perspectivesBased on the results of this research, susceptibility curve has a weaker sensitivity than magnetic fabrics in reflecting climate changes. Therefore, we believe that the magnetic fabric can be a potential better indicator to find the initial inflection point of climate change.
Keywords:  loess; red clay; magnetic fabric; wind direction; anisotropy of magnetic susceptibility
岩石或沉积物中磁性矿物定向分布排列的特点称为磁组构(magnetic fabric),在宏观上表现为岩石或沉积物,即磁化率各向异性(of magnetic fabric,简称AMS)。磁化率各向异性,可以通过磁化率各向异性椭球体的最大轴Kmax 、中间轴Kint 和最小轴Kmin 的大小及其各自的方向来描述。沉积物的磁组构通常可以反映沉积物沉积时的搬运动力信息黄孝刚和孙继敏Zhang 2010Ge 2014Gong 2015。曾对蓝田段家坡剖面黄土红黏土界线附近层位进行磁组构研究,得到了一些初步的结果(1)自新近纪到第四纪蓝田地区主要风向由南北向分量占优势变为东西分量占优势(2)所研究的蓝田段家坡剖面相关地层磁化率曲线与磁化率各向异性各参数曲线存在约3060 cm的错位现象。同时,尝试探讨了导致这些现象的可能原因,但是受限于采样剖面的单一性,研究结果的可靠性尚待验证,因此在经典的洛川黄土剖面开展了与蓝田段家坡剖面类似的研究工作,以期获得更多的认识。


图1   洛川剖面位置
Fig.1 Location of Luochuan section, China
1   采样及实验测量
选择洛川剖面作为研究对象,采样剖面位于洛川黄土地质公园内,集中采集了黄土红黏土界线附近地层(共9 m)的块状样品。在野外将地层自上而下(自下而上)划分如下(按相对深度):
0—130 cm:为一厚层黄土的底部,采样地点由于背阴相对潮湿,土层外观呈暗黄色,缺乏黏性。
130 — 240 cm:暗红棕色古土壤,具棱柱状构造、黏粒胶膜。
240 — 310 cm:黄土,钙结核零星分布。
320 — 480 cm:暗红色棱柱状古土壤,钙结核零星分布,可见铁锰胶膜。
480 — 620 cm:黄土,颜色较浅,整层缺乏黏性。其中480 — 490 cm 层位质地易碎,分布有弱发育的结核,550 — 600 cm 的层位钙化明显,土质较硬且多分布大块结核。
620 — 710 cm:黄土- 红黏土过渡带,层位土质为细粉砂质、缺乏黏性,更接近黄土,随沉积年代由老到新,土层颜色呈现逐渐由红色转变为黄色的特征,即从颜色上看又具有红黏土的特征。
710 — 720 cm: 胶泥,深红棕色,疑为冲积层, 潮湿部位质地均匀、表层干后裂缝密集分布。
720 — 1120 cm:红黏土,整体颜色深红,各层位形态、质地差异相对较小,部分层位发育有结核。
样品采回后, 采用卡帕桥MFK1A 多频磁化率仪测量其磁化率各向异性,测量精度为2×10−8 SI,外场强度为200 A ∙ m−1,测试频率为976 Hz,每个样品都沿3 个正交面进行了旋转测量,磁化率各向异性椭球体由最小二乘法获得(Zhu et al,2004;Zhang et al,2010),体积磁化率κm 以及磁化率各向异性几个重要参数(形状因子T、各向异性度P、磁面理F、磁线理L)可由程序通过Jelinek 方法自动算出(Jelinek,1981)。
2实验结果与讨论
洛川剖面黄土- 红黏土序列磁化率曲线及磁化率各向异性各参数测试值曲线如图2。
由图2 可以看出:绝大部分样品F<1.02,P<1.032,呈现出典型的风成沉积物特征(Liu et al,1988,1990)。P F 二者线性相关较高,相关系数为R2 = 0.85(图3b),表明磁化率各向异性的主导因素是磁面理。L 值、F 值和P 值在新近系红黏土阶段波动,整体没有表现出大范围的升高或降低,而在黄土阶段,L 值、F 值和P 值均表现出与地层划分有一定的相关性,整体来看,黄土阶段的L 值、F 值和P 值较古土壤阶段的L值、F 值和P 值有升高的趋势,越是典型的黄土,F 值和P 值升高越明显;这些变化特征与沉积发生时的粉尘搬运动力有关,间接反映了气候的变化。


图2   洛川剖面体积磁化率曲线(a)、形状因子T 曲线(b)、各向异性度P 曲线(c)、磁面理F 曲线(d)、磁线理L 曲线(e)
Fig.2 The magnetic measurement results of loess specimens from Luochuan section are shown as a function of relative depth: volumicmagnetic susceptibility (a), AMS shape parameter curve (b), AMS degree curve (c), magnetic foliation curve (d), magnetic lineation curve (e)
橙色阴影部分代表古土壤或红黏土等气候相对暖湿的阶段,黄色- 橙色阴影渐变部分为黄色粉砂土- 红色粉砂土过渡层,紫色阴影部分表示冲积层。Warm stages are denoted as orange bars, a silty soil transition layer between L34 and red clay is denoted as yellow bars. The waterlainloess layers are denoted as purple bars.


图3   磁化率各向异性度P 与磁性线理L、面理F 相关关系图
Fig.3 P-L (corrected AMS degree versus AMS Lineation parameter) and P-F (corrected AMS degree versus AMS Foliation parameter) diagram
结合黄土地层划分和磁化率各向异性各参数曲线的形态,将磁化率曲线及磁化率各向异性各参数曲线划分为若干阶段(如图2 阴影部分与非阴影部分的交替),发现磁化率曲线与磁化率各向异性曲线存在约10 — 20 cm 的“错位”。对于该现象,有以下初步的解释,即:粉尘及磁性矿物颗粒顺着搬运营力方向排列的现象,在沉积作用发生时就在同步进行,伴随着后期的降水、沉积覆盖等作用可以较快固结并得以记录和保存古风向信息。而影响磁化率高低的主要因素——成壤强度的改变,则是一个相对缓慢的过程,需要更久时间才能完成。除此之外,也可能与粉尘的中短距离搬运有关(中短距离是指粉尘源区与粉尘沉积区两地表土磁化率值相差不大的距离)。
另外,在所采样品序列710 — 720 cm(黄土红黏土界线)附近有一P、F、L 值均异常高的层位(图2c、d、e),说明磁基质颗粒定向排列程度较高,应该是对应一次古沉积动力较强的事件。经对比原始采样记录,该层位厚约10 cm,上下界线明显,土质亦区别于其上下地层,为深红棕色胶泥,野外采样时曾初步判断为冲积层,现根据实测数据及Liu et al(1988)关于水流搬运再沉积黄土的磁化率各向异性特征研究,基本可以确定该层位为水流搬运再沉积产物。也就是说,在新近纪晚期(经与Sun et al(2006)建立的灵台-赵家川黄土剖面年代序列对比,该层位年龄约为2.8 Ma BP)洛川地区可能发生过一次区域性淹没事件(河湖相沉积),这次事件所处地层为L34 与红黏土过渡带底部,与之前在蓝田段家坡剖面研究发现的河湖相沉积事件(位于L33)处于不同的地层,因此并非同一事件,另外洛川剖面中该冲积层的厚度明显比位于蓝田段家坡剖面L33 内的冲积层厚度大,且更为平整、均一。
黄土沉积物的磁化率各向异性最大轴的偏角方向与风向有着很好的相关性,为了了解黄土和红黏土沉积阶段记录的季风风向,进一步绘制了黄土阶段和红黏土阶段的磁化率各向异性椭球体Kmax、Kmin 赤平投影图及Kmax 偏角玫瑰图(图4),其中Kmax 偏角玫瑰图的绘制过程参考Zhu et al(2004)和Zhang et al(2010)的方法剔除掉了那些可信度不足95% 以及可能受到较强烈扰动(Kmin倾角<60°)的点。


图4   洛川黄土样品最大轴Kmax(蓝色方块)和最小轴Kmin(粉色圆点)的赤平投影图(a), 洛川红黏土样品最大轴Kmax(蓝色方块)和最小轴Kmin(粉色圆点)的赤平投影图(b), 洛川黄土样品Kmax 偏角玫瑰图(c),洛川红黏土样品Kmax 偏角玫瑰图(d)
Fig.4 Stereographic projection of Kmax (blue squares), Kmin (pink dots) of loess samples in Luochuan (a), stereographic projection ofKmax (blue squares), Kmin (pink dots) of red clay samples in Luochuan (b), wind rose map traced by direction of Kmax of loess samplesin Luochuan (c), wind rose map traced by direction of Kmax of red clay samples in Luochuan (d)
由图4可以看出自红黏土沉积阶段到黄土沉积阶段,风向整体呈现出一个顺时针方向的变化,这与蓝田段家坡剖面同期呈现逆时针的风向变化不同;但两剖面均出现了盛行风向的变化,这可能是新近纪到第四纪的全球气候变化在局部地区的一种表现。
从新近纪末相对暖湿的气候变为第四纪冷暖交替的气候,这种变化是全球性的,其过程必然伴随着全球热量分布原有平衡的打破及再平衡的建立,而热量传输路径及强度也会在全球热量再平衡中发生变化。热量传输的主要载体是风和洋流,在陆地上则主要是风。气候是大气物理特征的长期平均状态,主要以冷、暖、干、湿等特征来衡量,当然风向作为一个气象要素,其长期特征也属于气候范畴。关于气候和风向的变化,两者不分先后,风、洋流其实就是冷、暖和干、湿两对重要气候特征之间的衔接环节,全球不同地区的冷暖变化直接导致风和洋流的改变,进而影响到具体某地区的干湿状况。在新近纪到第四纪全球气候变化的大背景下,洛川和蓝田地区风向发生改变可能就是全球热量传输路径诸多改变中的一部分。
为了更好地与历史时期的古风向进行对比,获取了洛川地区现代气象观测记录(图5),发现洛川剖面1973 — 2016 年的日最大风向分布基本以南北向风为主,尤其是北风的出现频率最高,与红黏土阶段的风向分布大致接近,而与黄土阶段的风向分布稍有不同,这表明现代季风格局可能与新近纪末期的季风格局更为接近。


图5   洛川地区1973—2016 年的日最大风速的风向观测记录 分布情况(数据来自中国气象数据网,http://data.cma.cn/data/ cdcdetail/dataCode/SURF_CLI_CHN_MUL_DAY_V3.0.html/)
Fig.5 Distribution of direction of wind with daily maximumwind speed from 1973 to 2016 (data was from ChinaMeteorological Data Service Center, http://data.cma.cn/data/cdcdetail/dataCode/SURF_CLI_CHN_MUL_DAY_V3.0.html/)
3   结论
(1)洛川剖面所获得的磁化率各向异性各参数曲线整体与磁化率曲线变化相关,但细节上较磁化率曲线呈现出一定的超前,这与蓝田段家坡剖面的研究结果相同(Xie et al,2016),两个剖面的研究结果均表明沉积物磁组构对气候变化的敏感性要高于磁化率。
(2)对于本研究中的洛川剖面相关地层,自红黏土沉积阶段到黄土沉积阶段,风向整体呈现出一个顺时针方向的变化。与黄土阶段所获得的风向相比,红黏土阶段所获得的风向分布更接近于现代风向。
东亚地区是较典型的季风气候区,气候的变化与季风演化密切相关,气候变化既包括温度、降水的改变,又包括风力风向等要素的改变。根据本文的研究结果,磁化率曲线在反映气候变化上的敏感性不及磁组构,磁组构主要反映沉积颗粒的排布状况,可能受到各种沉积要素的影响,对搬运动力条件(如风力、风向)变化敏感,沉积颗粒一旦被新的沉积物有效覆盖将不会再受近地面搬运动力的影响。磁化率则主要反映成壤强度(土壤发育强度),而水、热条件变化对成壤强度的改变具有时间累积效应,是一个相对缓慢的过程而非一蹴而就。这可能是沉积物磁组构对气候变化的敏感性要高于磁化率的主要原因。
周卫健院士对本研究提供了支持,样品磁 化率各向异性的测量得到了中科院地球环境研究 所强小科研究员级高级工程师的指导和徐肖龙、 李欣桐实验员的帮助,中科院地球环境研究所鲜 锋副研究员、程鹏高级工程师对本文提出了修改 建议,在此一并致谢。
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稿件与作者信息
谢兴俊
XIE Xingjun
谢兴俊, E-mail: xiexj@ieecas.cn
常秋芳
CHANG Qiufang
孔祥辉
KONG Xianghui
中央高校基本科研业务费专项资金(2017NT04);北京师范大学学科交叉建设项目资金(2017NJCB03);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开放基金课题(SKLLQG1702);中国科学院“西部之光”人才培养引进计划(XAB2016B03)
Fundamental Research Funds for the Central Universities (2017NT04); Interdiscipline Research Funds of Beijing Normal University (2017NJCB03); State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, CAS (SKLLQG1702); CAS “Light of West China” Program (XAB2016B03)
出版历史
出版时间: 2019年5月15日 (版本2
参考文献列表中查看
地球环境学报
Journal of Earth Environment