【作者】 钟艳霞；

【导师】 陈发虎；

【作者基本信息】 兰州大学 ， 自然地理学， 2008， 博士

【摘要】 末次冰期是距现代最近的一次冰期,末次冰期气候环境变化的深入研究,无疑会对未来气候环境变化研究提供重要的参考。末次冰期间冰段因其气候环境的特殊性而备受研究者的关注。我国青藏高原的古里雅冰芯及西部地区诸多湖泊均记录到了深海氧同位素三阶段（MIS3）特殊的气候环境变化。作为重要地质载体的黄土沉积,在古气候重建过程中发挥了巨大的作用,研究成果丰富,但是对于黄土高原西部地区MIS3古气候环境变化的研究相对缺乏。既有的黄土沉积中总有机碳同位素研究在MIS3时期东西部表现出较大的差异,特别是关于末次冰期黄土沉积中总有机碳同位素变化的控制因素存在诸多争议。本研究利用黄土高原西部地区末次冰期以来黄土沉积中总有机碳同位素及多指标的综合研究来恢复和重建MIS3气候环境变化历史,在多剖面末次冰期以来黄土总有机碳同位素对比分析的基础上,探讨黄土总有机碳同位素变化的时空差异,追踪黄土高原西部地区末次冰期总有机碳同位素变化的主控因素;为了更好地恢复末次冰期以来黄土高原植被类型的变化,在开展黄土高原西部地区现代植被和表土有机类脂化合物——正构烷烃分布模式研究的基础上,根据黄土高原西部地区MIS3及全新世黄土沉积中正构烷烃的分布模式,结合黄土总有机碳同位素分析结果,初步恢复和重建了黄土高原西部地区MIS3及全新世植被类型,并获得如下认识:1、末次冰期以来黄土高原黄土总有机碳同位素变化的时空差异性分析发现:在冰期间冰期尺度上黄土高原东西部地区黄土总有机碳同位素变化一致,表现出冰期总有机碳同位素偏轻,间冰期偏重,在空间上从东往西黄土沉积中总有机碳同位素逐渐偏轻;末次冰期总有机碳同位素变化,黄土高原东西部存在较大差异,东部地区在末次冰期内部仍然遵循冰期间冰期的变化趋势,即间冰段相对偏重,早晚冰阶相对偏轻;但是本研究区三个剖面的研究结果显示,黄土高原西部末次冰期间冰段黄土总有机碳同位素相对早晚冰阶偏轻,与东部地区相反。2、末次冰期以来黄土总有机碳同位素变化主控因素的确定:冰期间冰期尺度上黄土高原西部地区黄土总有机碳同位素变化与东部地区相一致,主要由C₃、C₄植物相对丰度变化引起;末次冰期,黄土高原东部地区黄土总有机碳同位素变化的控制因素与冰期间冰期尺度上相一致,但黄土高原西部地区黄土总有机碳同位素变化并非为C₃、C₄植物相对丰度变化所致。将黄土高原西部地区三个剖面末次冰期总有机碳同位素结果与末次冰期温度、降水、大气CO₂浓度变化进行对比分析,发现末次冰期的低温导致黄土高原西部地区同期几乎无C₄植物发育,植被类型为单一的C₃植物。因此,末次冰期黄土高原西部地区总有机碳同位素的变化主要为单一C₃植被对降水量变化的响应,而间冰段黄土总有机碳同位素变化所记录的为单一C₃植物受降水增加的影响而偏轻,并非东部地区所表现的C₃、C₄植物丰度变化导致的,其控制因素与东部地区不同。温度变化虽然对C₃植物有机碳同位素产生影响,但并非主要控制因素。3、黄土高原西部地区剖面（曹岘、大地湾、塬堡）总有机碳同位素、粒度、磁化率等多指标分析结果显示MIS3黄土高原西部地区环境变化同样存在三个亚阶段,但是在MIS3内部,早期降水相对晚期更为增加,变化趋势与深海氧同位素指示的全球普遍变化趋势相一致,与西部湖泊记录的晚期相对较高降水不同。古里雅冰芯记录到MIS3晚期高温和青藏高原及中国西部诸多湖泊记录的高降水,在黄土高原西部地区剖面也有所记录,但是强度不如早期。4、黄土高原西部地区现代植物及表土有机类脂化合物——正构烷烃分布模式研究表明:10种木本植物正构烷烃高碳数分布都以C₂₇或C₂₉为主峰,3种草本植物正构烷烃的高碳数分布均以C₃₁或C₃₃为主峰,表现出与木本植物完全不同的分布特征,与前人研究相一致;对于现代表土的研究发现,表土中正构烷烃的分布与上覆植被有直接性的关系,对于木本植被除灌木草本含量较高的混生林,正构烷烃出现以C₂₇和C₃₁为主峰的分布外,其余都是以C₂₇或C₂₉为主峰,说明混生林中由于草本植物量的高输入导致了表土正构烷烃的分布差异,草地表土表现出同样的特征,例如对于周围落叶植被含量丰富的草地,其接受到的落叶植物输入大于草本输入,使其主峰出现差异。对面积较大的纯草地表土,其正构烷烃的分布即表现为草本植物输入的特征,以C₃₁为主峰。对于争议较大的松科植物正构烷烃的分布,本研究区三种常见松科植物都显示以C₂₉或C₂₇为主峰,仍然表现出木本植物的特征。5、黄土高原西部地区三个剖面MIS3有机类脂化合物——正构烷烃的分布模式显示,除曹岘剖面两个样品以C₂₉为主峰,C₃₁含量次之外,其余样品都表现出以C₃₁为主峰,而C₂₇含量最低(C₂₇＜C₂₉＜C₃₁),这与该地区以纯草本为植被类型的现代表土样品的分布特征相一致,而与现代木本表土样品的分布特征不同,也与当地草本植物正构烷烃的分布相一致,与木本植物正构烷烃的分布不同,初步说明MIS3黄土高原西部地区发育以草本为主的植被类型,部分区域有木本荒漠植物分布。结合剖面总有机碳同位素研究结果,初步判断MIS3黄土高原西部地区发育的植被类型以C₃草本为主。6、秦安地区两个不同成因的全新世剖面分析结果显示,在整个剖面全新世时期分子标志化合物中高碳数正构烷烃的分布特征相同,都表现为以C₃₁为主峰,而C₂₇含量最低(C₂₇＜C₂₉＜C₃₁),这与该地区以纯草本为植被类型的现代表土样品的分布特征相同,而现代木本表土样品的分布特征不同,也与当地草本植物正构烷烃的分布相一致,与木本植物正构烷烃的分布不同。表明全新世时期该地区发育的植被类型是以草原植被为主。更多还原

【Abstract】 As the last glaciation is the most recent Ice Age,it is very important to understand the last glacial climat change to predict future climat change.Due to the special characteristics of the climatic and environmental change,the research on the Marine Isotope Stage 3（MIS3） attracts numerous researchers’ attention.In China,the distinct climate change in MIS3 is demonstrated by records from Guliya ice core in Qinghai-Tibetan Plateau and a number of lake sediments in west China.Even loess deposition plays an important role in reconstructing the past climate change of the whole globe,the research on palaeoclimatic and palaeoenvironmental change during MIS3 in the western Loess Plateau is still relatively undeveloped.The totalδ¹³Corgs results of loess from east and west Loess Plateau are shown very different during MIS3,since there is still a hot debate about the controling factors of the totalδ¹³Corgs variation in loess deposition through the last glacial period.In this study,we integrated the totalδ¹³Corgs and other multiple proxy records from loess deposits to reconstruct the last glacial climatic and environmental change during MIS3 in the western Loess Plateau.The spatial and temporal variation of the totalδ¹³Corgs of loess is established,together with a exploration of the corresponding controlling factors,under a close comparison of multiple profile records since the last glaciation. Thus we get a good understanding of the evolutionary history of vegetation types in the western Loess Plateau since the last glaciation.1,The spatial-temporal variation of the totalδ¹³Corgs in Loess Plateau since the last glaciation displays the following features:（1） the totalδ¹³Corgs is consistent in east and west Loess Plateau on the glacial-interglacial scale;（2） In glacial period the totalδ¹³Corgs become lighter,but heavier in interglacial period;（3） During the last glaciation the totalδ¹³Corgs changes in Loess Plateau are different from east to west. The totalδ¹³Corgs changes of the last glaciation in the eastern Loess Plateau have the same characters as on the glacial-interglacial scale,which is relatively heavier in MIS3 but lighter in MIS2 and MIS4.However,the results from the three sections in the western Loess Plateau show that the totalδ¹³Corgs in loess is relatively lighter in MIS3 but heavier in MIS2 and MIS4.2,The dominant controlling factors of the totalδ¹³Corgs changes in loess since the last glaciation are explored as follows:（1） From glaciation to interglaciation the changes of totalδ¹³Corgs is consistent between west and east Loess plateau due to relative abundance changes of C₃ and C₄ plants.（2） In the last glaciation,the controlling factors of the totalδ¹³Corgs from the eastern Loess Plateau show the same pattern with the glacial-interglacial scale,which are relative abundance changes of C₃ and C₄ plants.But the controlling factors of totalδ¹³Corgs changes in the western Loess Plateau are something else.By comparing the totalδ¹³Corgs from the three sections in the western Loess Plateau with the records of temperature,precipitation and the concerntration of atmospheric CO₂ during the last glaciation,we found that, as the temperature of last glaciation was too low for C₄ plants to grow,the vegetation was represented by pure C₃ plants.Then the totalδ¹³Corgs change is caused by the pure C₃ vegetation in response to precipitation variation.Therefore,the totalδ¹³Corgs change in C₃ plants gradually becomes lighter under the increasing precipitation.So we conclude that during the last galciation the controling factors of the totalδ¹³Corgs are different between the western and eastern Loess Plateau.Moreover,the temperature may influence the change of the totalδ¹³Corgs,although it is not the main controlling factor.3,There are three sub-stages in MIS3 in the western Loess Plateau according to the records of the totalδ¹³Corgs,grain size,and magnetic susceptibility from Caoxian, Dadiwan and Yuanbao sections.But the precipitation decreases in the later period compared to the earlier period during MIS3,Which is coincident with SPECMAP,but different from Guliya ice core records.High temperature and high precipitation in late MIS3 is reflected in the records of lake sediments from Tibetan Plateau and West China,and records of Guliya ice core.respectively,but not showed distinctively in loess sections in the western Loess Plateau.4,The research of modern plants and organic compounds lipid in surface soil, namely,the distribution model of n-alkanes in the western Loess Plateau showed:10 wood samples are the heavy-molecular-weight（HMW） homologues of the n-alkanes distribution with the main peak of C₂₇ or C₂₉,while in 3 kinds of herb n-alkanes,the HMW homologues distributions of C₃₁ or C₃₃ are the main peaks,which show a completely different distribution feature with woody plants.For the modern study of surface soil,surface soil n-alkanes distribution is directly related to the vegetation cover.Study shows that there is a direct relationship between HMW homologues distributions of the surface of the vegetation.The wood vegetation except for the highly mixed forest by herbaceous shrub has a distribution of n-alkanes C₂₇ and C₃₁ as the main peak.The highly mixed forest by herbaceous shrub has a distribution of n-alkanes C₂₇ or C₂₉ as the main peak,It is indicated that mixed herbs in the forest leads to the difference in this distribution,because of its high volume of n-alkanes. Grassland surface soil shows the same characteristics.When there are fallen leaves around grassland,the input of deciduous plants would be greater than the herbal input. The n-alkanes from grass show different main peak.As to the surface soil of large grass land,the n-alkanes distribution reflects the characteristic of herbaceous plants with the main peak of C₃₁.This study for conifer shows the characteristics of wood plants with the main peak of C₂₉ or C₂₇.5,The results of lipid organic compounds n-alkanes from three sections in the western Loess Plateau during MIS3 shows the heavy-molecular-weight（HMW） homologues of the n-alkanes distribution pattern:except for two samples from Caoxian section,C₂₉ is the main peak,while the other samples showing the performance of the C₃₁ as the main peak,and the C₂₇ content of the lowest(C₂₇＜C₂₉＜C₃₁),which presents the same distribution as a pure herbal vegetation types of modern surface soil samples but the different distribution from the wood plants and modern wood surface soil distribution of n-alkanes.It is preliminarily proposed that the vegetation of western part of Loess Plateau in MIS3 is dominated in grass but not forest.In combination with the analysis of totalδ¹³Corgs,all results suggest that C₃ grass is dominant in the vegetation of the western Loess Plateau during MIS3.6.Two Holocene sections,a swamp-alluvial loess section and a typical loess-paleosol section in Qin’an area,are used to reconstruct the history of past regional vegetation changes.It is found that the heavy-molecular-weight（HMW） homologues of the n-alkanes in all samples exhibit a pronounced odd-over-even predominance,maximizing at C₃₁ and the abundance of the nC₂₇-alkanes is the lowest, nC₂₇,nC₂₉,and nC₃₁ in turns(that is C₂₇＜C₂₉＜C₃₁),which is a typical grass n-alkanes model.The results are the same as those of modern surface soil samples under grass cover but different from forest cover and modern pine leaves,which yield preferentially nC₂₉-alkanes peak.Therefore,it is preliminary inferred that the area is dominated by grass cover but not forest cover during the whole Holocene.更多还原