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

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。

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 MFK1 A 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 perspectives Based 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.