天山北麓黄土记录的30万年以来古气候演变

Paleoclimatic Changes Since 300 ka Recorded by Loess Deposits Along the North Pediment of Tianshan Mountains

对天山北麓黄土分布特点与形成年代的研究,以及以此为载体应用各代用指标的古气候重建已经取得了不少进展。在前人工作的基础上,选取黄土—古土壤序列分明,底部S3古土壤发育的鹿角湾剖面为主要研究对象,结合天山北麓的其他多个典型黄土剖面,运用色度、粒度、常量地球化学元素含量和环境磁学参数等指标,探讨大约30万年以来的古气候演变。鹿角湾黄土实验结果显示,各古气候代用指标随剖面深度变化与黄土—古土壤更替大概一致,剖面上部(L2及其以上)与下部区别明显。粒度分布揭示黄土物源输入主要受控于两股不同风系,随黄土古土壤的更替两者的主次地位发生变化。环境磁学实验结果显示,剖面下部磁性弱,频率磁化率低,但古土壤层稳定单畴含量较高,反映湿润成壤环境下超顺磁亚铁磁性矿物溶解或转化为弱磁性矿物;剖面上部比剖面下部的磁性要强,磁性矿物粒度更粗;S0为磁性成壤增强模式,与其他黄土古土壤层不同。再结合其他典型剖面的记录,可以推断研究区30万年以来有干旱化的趋势,但S2发育阶段总体上可能比S3发育阶段略为湿润,这一阶段之后气候明显变干,S1发育阶段明显比S2与S3发育阶段干旱,干旱化趋势一直保持至全新世开始之前。

Considerable progress has been made in studies of the distribution,age and paleoclimatic significance of loess deposits in arid central Asia. However,the focus has been on long-term drying trends(since 800 ka)or shortterm moisture evolution(since the last interglacial or during the Holocene). The paleosols are generally undeveloped in the pediment of the Tianshan Mountains,which makes detailed paleoclimate reconstruction difficult. Based on previous work,a number of methods were used in the present study to investigate loess deposit samples from different sections along the north pediment of the Tianshan Mountains. These include a typical loess section at Lujiaowan at relatively high elevation,and sections in low-lying areas along the southern margin of the deserts. The Lujiaowan section consists of four paleosols and intercalated loess dating back to 300 ka. According to the stratigraphic divisions on the Chinese loess plateau,the four down-section paleosols were designated S0,S1,S2(subdivided into S2S1,S2L1 and S2S2),and S3. The sections possess similar variations in color,grain size,geochemistry and rock magnetism with depth,consistent with loess/paleosol alternation. They also indicate a significant change in paleoclimatic proxies at the S2/L2 boundary. The grain size distribution of loess is unimodal,but is bimodal for the paleosols(or broadly unimodal),suggesting that the loess origins are related to two winds,the westerlies and near-surface winds. The variation in the dominance of the winds is consistent with the loess/paleosol alternation. Also,grain sizes are finer in the lower part of the section(S2,L3 and S3)than in the upper part. The chemical index of alteration(CIA)and(CaO+Na;O+MgO)/TiO;ratio indicate more intensive weathering in the paleosols than in adjacent loess units. Integrated rock magnetic analysis indicates that the loess–paleosol sequence experienced three stages in the relation between pedogenetic intensity and magnetic susceptibility in response to different paleoclimates. The lower paleosols(S3 and S2)show higher pedogenetic intensity but lower magnetic susceptibility,reflecting humid paleoclimates. The upper loess–paleosol sequence exhibits the dominance of wind strength(for L2-L1)or pedogenesis(for S0)in magnetic enhancement. Compared to the underlying or overlying loess,the paleosols have fine magnetic grain size. It is suggested that the SIRM/χ and χ;/SIRM ratios are sensitive to climate change. A study of the relationship between SIRM and χ of the loess from the Lujiaowan,Jingouhe,Dongwan and Midong sections indicated that the origins of magnetic variations along the northern piedmont of the Tianshan Mountains are section-specific,and differ between layers.However,a common drying trend following loess deposition was observed. Various proxies implied that S2S2 was more highly developed than S3. Likewise,S2b,one of the sub-units of the S2 paleosols,shows lower χ than S3 in the Dongwan town loess section. The time span in which S2S2 or S2b developed was possibly wetter than the interglacial period during which S3 developed. This is consistent with studies of the loess from Kunlun Mountains. For the paleoclimate evolution over the last interglacial/glacial period,the grain size parameters of the Dongwan loess demonstrate a drying trend,which is in line with the χ;/χ and SIRM/χ ratios of the Midong loess.This study provides further evidence for the contribution of the different winds to the loess deposits along the northern piedmont of the Tianshan Mountains. Paleoclimate evolution was affected by both westerly and near-surface winds. The westerlies dominated the moisture variation,and the near-surface winds were responsible to a greater extent for the origin of the loess deposits. The different magnetic mechanisms resulting from varying geographic exposure to paleoclimatic changes were revealed. The local pedogenetic signals revealed by rock magnetism are the result of changes in moisture conditions.Both the high-elevation Lujiaowan loess and the low-lying Dongwan loess show that the S2 paleosols experienced more humid conditions than the S3 paleosols. L2 loess was deposited during much drier periods;S1 is less developed than both S2 and S3 due to the presence of inadequate moisture. The aridity further intensified from the last glacial to the onset of the Holocene. The drying trend and major drying events are attributable to the uplift of the Tibet plateau.