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丹江中游龙泉村黄土—古土壤剖面风化成壤特征研究
【作者】 王海燕；
【导师】 庞奖励；
【作者基本信息】 陕西师范大学 ， 自然地理学， 2017， 硕士
【摘要】 黄土-古土壤沉积序列作为记录古气候演变的信息载体得到越来越多的关注。丹江中游流域位于秦岭南侧,属于北亚热带向暖温带过渡的季风气候区,对气候环境变化较为敏感。丹江流域的盆地地带分布面积不等的厚层风成黄土,是该区古气候环境演变信息的良好载体,目前对这些黄土的研究十分有限,许多问题亟待解决,例如这些黄土的地层序列、物理化学性质、风化成壤特征,等等。在野外考察的基础上,选择龙泉村(LQC)黄土剖面为研究对象,对其地层序列、理化性质(色度、磁化率、烧失量、粒度)、元素组成、风化成壤规律等进行了研究。获得以下几方面的认识:(1)龙泉村剖面地层序列自上而下依次为:表土层(MS)→古土壤(S0)→过渡性黄土(Lt)→马兰黄土(L1)→古土壤(S1)→黄土(L2)。表土 MS(0～40 cm),浊棕色(7.5YR5/3),粘土粉砂质地,团粒结构,多植物根系;古土壤S0(40～220 cm),浊棕色(7.5YR5/4),粘土质地,典型棱块状结构,致密紧实,棕色粘土胶膜普遍发育;过渡性黄土Lt(220～320 cm),浊黄橙色(10YR6/4),粉砂质地,块状结构;马兰黄土L1(320～664 cm),浊黄橙色(10YR6/3),均匀块状结构,较疏松;古土壤S1(664～1000cm),亮棕色(7.5YR5/6),粘土质地,典型的棱块状结构,致密紧实,坚硬,无胶膜,颜色较S0偏红;黄土 L2(1000～?),黄棕色(10YR5/6),粉砂质地,均匀的块状结构,较疏松。(2)龙泉村剖面中黄土与古土壤的理化指标特征差异明显。具体表现为:①黄土 L1/L2 的 L*和 b*(52.54/51.47,17.44/17.06)高于古土壤 Si/S0(49.18/48.39,17.04/16.15),a*与 L*、b*变化趋势相反,古土壤 So/S1(9.07/9.49)高于黄土 L2/L1(8.92/8.81)。②黄土的磁化率在42.4～181.5×10-8m3·kg-1之间,平均值为138.5× 10-8m3·kg-1,其中 L1的平均值为 153.0X 10-8m3·kg-1,L2 的平均值为 127.0X 10-8 m3·kg-1。古土壤的磁化率介于38.3～333.8×10-8m3·kg-1之间,平均值为183.3×10-8 m3·kg-1,其中 S0 的平均值为 184.2×10-8m3·kg-1,S1的平均值为 237.7×10-8m3·kg-1。③黄土烧失量的平均含量低于古土壤。烧失量在不同地层的大小排序为:S1(45.97 g·kg-1)>S0(42.69 g·kg-1)>L2(40.54 g·kg-1)>L1(37.25 g·kg-1)。④粒度组成主要以粉砂(2～63μm)含量(80.93%)为主,粘粒(<2 μm)次之(10.30%),黄土相对于古土壤粒级偏粗,黄土 L1、L2的粘粒(<2 μm)含量分别为7.96%、8.07%,古土壤S0、S1的粘粒(<2μm)含量分别为12.84%、14.68%,各层中值粒径(Md)和平均粒径(Mz)大小排序均为:L1>L2>S0>S1。(3)在古土壤Si和S0中A1的丰度分别为154.9 g·kg-1和154.0 g·kg-1,而马兰黄土 L1和黄土 L2中分别为150.0 g·kg-1和147.8 g·kg-1,古土壤明显高于黄土。Fe的含量分布趋势特征与A1 一致;Si在古土壤S1和So中丰度分别为649.6 g·kg-1和651.6g·kg-1,而马兰黄土L1和黄土L2中分别为 656.1 g·kg-1和 663.6 g·kg-1;K在整个剖面中含量相差较小;马兰黄土 L1中Ca、Na元素平均含量均为10.7 g·kg-1,Mg的含量为20.0 g·kg-1,三者达到剖面的最大值。Ba在古土壤Si含量最高(688.0 mg·kg-1);Ni和Cu的含量分布趋势一致,均在古土壤S1中高于黄土(L1/L2);Sr元素在古土壤So中平均含量最低(83.3 mg·kg-1),马兰黄土L1中平均含量最高(108.5 mg·kg-1)。(4)不同元素在古土壤中呈现出不同的活性,Al、Fe、Si、Mg呈现不同程度的富集,而Na、Ca则发生迁移,常量迁移能力顺序为:Na>Ca>K>Mg>Si>Al>Fe;As、Rb、Ni、Zu、Ba、Cu元素发生不同程度的富集,而Sr则发生强烈的迁移,微量迁移能力强弱排序为:Sr>Cu>Ba>Zn>Ni>Rb>As。(5)丹江黄土处于中等化学风化程度阶段,Na、Ca淋失严重,古土壤的风化程度强于黄土。具体表现为:化学蚀变指数CIA(72.59)指示龙泉村黄土剖面整体处于中等风化程度阶段;A-CN-K三角图显示该剖面经历了以斜长石为主的风化过程;淋溶系数(ba)、退碱系数(a)均呈L1>L2>S1>S0,Na2O/K20呈L1(0.39)>L2(0.33)>Si=S0(0.32),Si02/Al203 呈 L2(4.49)>L1(4.38)>S0(4.24)>S1(4.20),四个指标均呈黄土(L1/L2)高于古土壤(So/S1),而残积系数(Ki)变化与上述指标相反,五者共同表明古土壤风化要强于黄土。(6)黄土古土壤的风化成壤特征变化记录了古气候演变规律。末次间冰期(125000-75000 aB.P.),气候经历了回暖→变冷→回暖的过程,其中在870～930 cm深度处,水热组合最佳,成壤作用最强;末次冰期(75000-11500 aB.P.),气候整体上处于干冷状态,降水较少,气温较低,风化成壤作用较弱;全新世早期(11500-8500 aB.P.),气候可能经历了一个气候异常时期;全新世中期(8500-3100 aB.P.),气候温暖湿润,降水较多,风化成壤作用较强;全新世晚期(3100 aB.P.-至今),气候受人为活动的影响较大。(7)对比丹江黄土与黄土高原黄土的化学元素及风化程度,发现二者Al、Si均较高,除Ca外的化学元素变异系数均小于17%,丹江黄土 Al、Fe、K、Si含量高于黄土高原,Ca、Mg、Na元素含量低于黄土高原,反映丹江黄土中元素淋失更多,经历的风化淋溶作用更强。丹江马兰黄土 L1的磁化率(153.0×10-8m3·kg-1)高于洛川黄土(82.7×10-8m3·kg-1),古土壤 S1和 S0 的磁化率(237.7×10-8m3·kg-1、184.2×10-8m3·kg-1)高于洛川古土壤(159.0×10-8m3·kg-1);马兰黄土 L1 的 CIA(70.00)、残积系数(Ki:2.20)均高于洛川黄土(63.77、1.64),古土壤S1和S0 的 CIA、Ki(72.85、2.53,75.01、2.76)均高于洛川古土壤(67.36、1.94);马兰黄土 L1的Na/K(0.39)、淋溶系数(0.43)、退碱系数(0.23)均低于洛川黄土(0.57、0.64、0.33),古土壤 S0(0.32、0.33、0.16)和古土壤 S1(0.32、0.37、0.18)均低于洛川古土壤(0.44、0.54、0.25)。上述指标均显示两地区古土壤风化要强于黄土,指示两地区黄土经历了相似的沉积发育过程,但丹江黄土经历的风化程度更强,气候更暖湿。
【Abstract】 As the important information carrier which recorded the past environmental changes,the loess-palaeosol sequences has been used in the study of the evolution of palaeoclimate in the global change.The mid-reaches of Danjiang River Valley is located in the south of Qinling Mountain,which is a transition area between the North Temperate Zone and northern subtropical region,belong to sensitive climate change zone.Although covered a thick layer of eolian loess in the Danjiang River Basin,it is limited about the research for the loess deposition.For example,what are the loess stratigraphic sequences in this region?How does the weathering and pedogenic intensity under the north subtroptical monsoon climates?And so on.On the basis of field investigation,we choose the Longquan Village profile as the object of the study.Based on above the field and laboratory analysis,we study the stratigraphic sequences,physical and chemical properties including chromaticity,magnetic susceptibility,loess-on-ignition and grain size,element content as well as pedogenic process and their climate implications.Here,we present the results:(1)The stratigraphic sequence of the loess are mordem soil(MS)→ palaeosol layer(S0)→ transitional loess layer(Lt)→ Malan loess(L1)→palaeosol layer(S1)→loess layer(L2)from top to bottom.Detailed stratigraphy characteristics are presented as follows:Modern soil(MS,0-40 cm),dull brown(7.5YR5/3),clay silt,granular structure and plant roots develop;Palaeosol layer(So,40～220 cm),dull brown(7.5YR5/4),clay silt,columnar-blocky structure,relative firm,abundant dull brown clay film develop widely;Transitional loess layer(Lt,220-320 cm),dull yellow orange(10YR6/4),silt,block-massive structure;Malan loess layer(L1,320-664 cm),dull yellow orange(10YR6/3),silt,block-massive structure,loose;Palaeosol layer(S1,664～1000 cm),dull brown(7.5YR5/6),clay silt,columnar-blocky structure,firm and more red than So;Loess layer(L2,1000～?),yellow orange(10YR5/6),silt,block-massive structure and looser.(2)Physical and chemical properties in the loess-palaeosol sequences were different obviously at the LQC site:①The values of L*and b*in the L1/L2(52.54/51.47,17.44/17.06)are higher than that are in S1/S0(49.18/48.39,17.04/16.15).a*mean value is lower in the loess layer than the palaeosol layer,which is contrary with L*and b*.The values of a*in the L2 and L1 are 8.92,8.81 and 9.07,9.49 in the S0 and S1.②The range of magnetic susceptibility is 42.4～181.5 ×10-8 m3 ·kg-1 and the average value is 138.5 ×10-8m3·kg-1 in loess layer.Magnetic susceptibility have more lower values in the loess units than palaeosol units.The value is 153.0 × 10-8 m3 kg-1 in Li and 127.0 ×10-8 m3 kg-1 in L2.However,The range is 38.3-331.8 X 10-8m3 kg-1 in palaeosol layer and the average value is 183.3 ×10-8m3·kg-1,Which is 184.2 × 10-8m3·kg-1 in S0 and 237.7×10-8 m3·kg-1 in Si.③Loess units have lower average content of loess-on-ignition relative to palaeosol units.The values are respecitively:S1(45.97 g·kg-1)>S0(42.69 g.kg-1)>L2(40.54 g·kg-1)>Li(37.25 g.kg-1).④This loess sequence is mainly composed of silt(2-63 μm)with an average proportion of 80.93%and the clay(<2 μm)accounts for 10.30%in total composition.The average particle size(Mz)and median diameter(Md)all indicates the grain size was increased by the sequence of L1>L2>S0>S1.The sequence’s grain-size composition varies from bottom to top stratigraphic unit,and the palaeosol is smaller in grain size than that of their loess stratum.These properties reflect that the weathering intensity in palaeosol is stronger than in loess.(3)The characteristics of element distribution in Longquan Village profile:the average content of Al in the constant element is higher in Sl(154.9 g·kg-1),S0(154.0 g·kg-1)than Li(150.0 g·kg-1)and L2(147.8 g·kg-1).The average content of Fe is consistent with that of A1 in the profile.The average content of Si element in S1(649.6 g kg-1)and S1(651.6 g kg-1)is lower than the Li(656.1 g kg-1)and L2(663.6 g kg-1).The average content of K has a small difference in whole profile.The average content of Ca and Na in Li is 10.7 g kg-1,reaching the maximum value of profile.The content of Ba(688.0 mg·kg-1)in trace elements is the highest in S1 unit.Ni and Cu elements have same distribution characteristics in the profile,and are higher in Si than L1/L2.The average content of Sr element(83.3 mg·kg-1)is the lowest in S0 unit,while is the highest in L1(108.5 mg kg-1).(4)The migration variability of major elements and trace elements which are normalized by K in the palaeosol units presents different trend at the LQC site.Detail characteristics are as follows:the elements of Al,Fe,Si and Mg in constant elements show different extent of enrichment characteristics,while Na,Ca elements are migrated.the contents of As,Rb,Ni,Zu,Ba and Cu in the trace elements were enriched in different degrees,while Sr elements are strongly migrated.The migration trend of major elements is:Na>Ca>K>Mg>Si>Al>Fe;In addition,The migration trend of trace elements is:Sr>Cu>Ba>Zn>Ni>Rb>As.(5)The chemical weathering of the loess in the Danjiang River was in the stage of intermediate weathering,with Ca,Na loss seriously and potassium feldspar changed a little.Chemical weathering parameters CIA(72.59)indicated the chemical weathering of the loess in LQC was in the stage of primary transition to medium.And the A-CN-K diagram shows that the major elements Ca,Na rapidly leach out,while K relative stability.The orders of the two chemical weathering parameters ba and a were in L1>L2>Si>S0.The orders of Na2O/K2O was Li(0.39)>L2(0.33)>S1=S0(0.32)and Si02/Al2O3 was L2(4.49)>Li(4.38)>S0(4.24)>Si(4.20),which all indicated the weathering of the palaeosol(S0/S1)were stronger than the loess(L1/L2).(6)The characteristics of weathering and pedogenesis can reflect the evolution of palaeoclimate in the loess-palaeosol sequences.The last interglacial period(125000-75000 aB.P.),the climate envolution expericened warming → cooling → warming again progress.At the depth of 870～930 cm,hydro-thermal characteristics are the best,and the strongest pedogenesis process.The last glacial period(75000-11500 aB.P.),the climate is in a dry and cold state as a whole,and weak weathering and pedogenesis process.The early-Holocene period(11500-8500 aB.P.)may have experienced a climate anomaly period.The mid-Holocene period(8500-3100 aB.P.),warm and humid climate,relatively strong weathering and pedogenesis process.The late-Holocene period(since 3100 aB.P.),local climate is more affected by human activities.(7)Compared the composition of chemical elements and the degree of the weathering of Danjiang loess and the Chinese Loess Plateau,it had obtained some similarities and differences.The average contents of Al and Si are high in this two rigions.The CV(coefficient of variation)of chemical elements is extrmenly low(<17%)except Ca,which suggests that the composition of either major elements or trace elements is very consistent within the whole profile.The average content of Al,Fe,K in Danjiang loess are higher than that in the Chinese Loess Plateau,while the average content of Ca,Mg,Na are lower.It is reflected that unstable compositions of the loess in the Danjiang River had been leached more,where it experienced the stronger leaching.The magnetic susceptibility(153.0 × 10-8 m3 · kg-1)of the L1 in the Danjiang River is higher the Luochuan,which is 82.7×10-8 m3·kg-1 of loess layer.The average values of Si(237.7 × 10-8 m3 ·kg-1)and So(184.2 × 10-8 m3 ·kg-1)are both higer than the palaeosol layer in Luochuan(159.0×10-8 m3·kg-1).The chemcial index of alteration(CIA:70.00),Ki(2.20)of the L1 in the Danjiang River are higher than that in the loess of Luochuan,Where are 63.77,1.64 in loess.The value of CIA and Ki in S1(72.85,2.53)and S0(75.01,2.76)are higher than that in the palaeosol of Luochuan,where are 67.36,1.94 in palaeosol.However,Na/K(0.39),ba(0.43),a(0.23)are lower than that in the loess of Luochuan,Where are 0.57,0.64,0.33 in loess.At the same time,The value of Na/K,ba and a in So(0.32,0.33,0.16)and S1(0.32,0.37,0.18)are higher than that in the palaeosol of Luochuan,where are 0.44,0.54,0.25 in palaeosol.It shows that the weathering of the palaeosol were more stronger than the loess.It also reflected that the loess experienced similar sedimentary process in Danjiang River and the Chinese Loess Plateau.However,the weathering of the loess in Danjiang River was stronger with its higher weathering indexs,which testified the ancient climate was more warm and wet in Danjiang River.