The molecular distribution of long-chain n-alkanes in 62 soil samples collected from diverse locations across eastern China was analyzed.The long-chain n-alkanes were mostly dominated by n-C29 or n-C31,regardless of the overlying vegetation type at each site.The results were compared with those summarized from the literature,covering more than 100 soil samples within China and more than 300 genera of modern plants distributed worldwide.There were similar n-alkane distribution patterns for most genera, with no clear differences among grasses,shrubs,and trees.The evidence from analyses of surface soils and modern plants indicates that the relationship between the molecular distribution of long-chain n-alkanes of surface soils and source vegetation is highly complex,and is influenced by many factors.Further,it is suggested that source vegetation types should not be simply inferred from distribution patterns of long-chain n-alkanes in sediments.
RAO ZhiGuoWU YiZHU ZhaoYuJIA GuoDongHENDERSON Andrew
Laschamp地磁场漂移事件发生的确切年代(~42 ka BP)已通过相对独立的同位素定年手段确定,并被证实具有全球性特征。但这一事件在我国马兰黄土地层中的记录存在着明显的空间差异。本文初步对比了最近发表的黄土高原洛川剖面^(10)Be记录的Laschamp漂移信息和前人根据该剖面古地磁参数异常结合Heinrich冷事件所确定的Laschamp记录层位结果,进一步证实了这一地磁漂移事件在马兰黄土地层中记录的真实性.同时初步对比也显示根据旧Be产率异常增高对应的磁场强度低值确定的Lashcamp层位偏上,而古地磁异常确定的Laschamp层位相对偏下约76 cm。排除因采样剖面分辨率不同的可能影响,两种手段记录同一地磁漂移事件的显著层位差异具有指示意义,可能为未来研究黄土-古土壤序列沉积剩磁获得过程提供新的途径。
The southeastern Chinese Loess Plateau is the terminal deposition area of dusts transported by the East Asian winter monsoon and the frontal area penetrated by the East Asian summer monsoon,and thus a climate sensitive region.This paper reports a rock magnetic study of a Quaternary loess-paleosol section in such a region.We tried to reconstruct the paleoclimate evolution history in the region during 1.95–0.40 Ma with magnetic parameters.The results show a general up-section decreasing trend of the ratio of HIRM/(SIRM–IRM100 mT),indicating a long-term decreasing trend of hematite coercivities in the deposits,which can be mainly related to the cooling and aridification trend of the environment in interglacial depositional area and glacial dust source region.The ratio,lf/ARM,widely used to reflect the variations of magnetic mineral grain size,manifests a long-term increasing trend of the magnetic mineral grain size and tends to indicate an overall weakening trend of the East Asian summer monsoon that controlled the pedogenic intensity.Although the regional multi-segmented paleoclimatic records revealed by several magnetic parameters in our study,the long-term Asian cooling and aridification trend inferred here is of global correlation significance.
The application of rock magnetism methods to investigating the variations of magnetic minerals in the sediments is an important approach to the reestablishment of paleoclimate evolution. Thus we performed fine magnetic measurements on the loess-paleosol sequence (from L15 upwards to S5, in which L is short for Loess, S is short for Paleosol, the same hereinafter) of Yushan strati-graphic section, which is on the southeastern margin of Chinese Loess Plateau, in Lantian County of China's Shaanxi Province, and the thickness of which is ca. 40 m. Our study shows that the primary magnetic carriers of loess and paleosol in this section are magnetite, maghemite, hematite and goethite. Thermomagnetic analyses on the samples of representative horizons show that the higher pedogenesis degree of the sediments, the smaller variations of magnetization there will be before and after heating, probably related to the pedogenic alteration of loess sediments. Analyses of several magnetic parameters show a significant discrepancy between the paleoclimatic conditions recorded in the strata from the loess unit L15 upward to the paleosol unit S5 in the study area and those recorded in the relative strata of other sections on the Chinese Loess Plateau, and those recorded in marine sediments, indicating the great impact of regional geological background. Similarly, the rapid and intensive change recorded in the segment from L15 to S9-1, and the significant difference between the paleoclimate evolutions of the two periods before and after the change (from L15 to S9-1, and from L9 to the base of S6) indicate the strong alteration of magnetic carriers in the study area as a result of the alternations of summer and winter monsoons in East Asia.
通过对广东省湛江市湖光岩玛珥湖晚全新世沉积物中叶蜡烷烃(n-C29,n-C31,n-C33)及其碳同位素(δ13C)进行分析,探讨该湖区约3.5 ka BP以来的植被变化历史,重点讨论了人类活动对该区域植被变化可能带来的影响。叶蜡烷烃δ13C在1.7 ka BP以前偏轻,例如长链烷烃碳同位素在1.78 ka BP为-33.27‰,表明湖区C3植物占绝对优势,表现为典型的C3型生态系统;在1.7 ka BP之后碳同位素明显快速偏重,在1.63 ka BP为-28.26‰,指示了C4植物出现和C4比例上升的过程。C4植物的出现通常指示气候的干旱化,但是烷烃Paq和ACL指数(平均链长)则显示湖区气候可能在1.7 ka BP后变湿润。通过与研究区甘蔗的同位素和ACL指数的比较分析,推测这一矛盾现象可能与湖区开始有一定规模的甘蔗种植活动有关。
