【摘要】 黄土有机质的主要来源是决定其稳定碳同位素(δ¹³C_org)能否应用于重建过去C₃/C₄相对丰度变化及相应的古气候变化的关键基础科学问题。如若黄土地层当中的有机质主要由粉尘携带而来,而非当地植被,显然其δ¹³C₍org不是一个局地植被过去C₃/C₄相对丰度变化及古气候变化的良好指示器。本文对黄土高原西部地区（六盘山以西）新获得和已发表的相关数据进行综合分析,尝试对该地区黄土有机质的主要来源进行定量的分析。结果表明,总体而言,该地区黄土地层有机质主要来源于当地植被,由粉尘携带而来的有机质贡献量不超过8%,所造成的有机碳同位素变化幅度不超过1.7‰。这些结果说明该地区的黄土地层有机质δ¹³C₍org是可以用来重建过去C₃/C₄植物相对丰度及相应的古气候变化的。为了更精确的重建,后续的工作应当更多考虑粉尘搬运过程当中的有机质输入以及粉尘沉积之后的微生物活动的可能潜在影响。更多还原

【Abstract】 Stable isotopic compositions of total organic carbon(δ¹³C_org),as a paleovegetation and paleoclimate indicator,have shown great potential in Chinese loess/paleosol sequences,especially in the western Chinese Loess Plateau（WCLP,west to Liupan Mounts.）,due to the high resolution signal resulted from high accumulation rate of loess in this area.Until now,δ¹³C_org has been applied in several loess/paleosol sequences in WCLP for paleoclimate reconstructions.At the same time,the primary provenance of the organic matter in the loess/paleosol sequences in WCLP has been concerned,because of the wind-borne nature of the Chinese loess which may result in the input of allothogenic organic matter to the loess/paleosol sequences in WCLP.If the organic matter in the loess/paleosol sequences in WCLP was mainly derived from the bedrocks in dust source areas of loess,then the loess δ¹³C_org in WCLP is not a good indicator of paleovegetation and paleoclimate apparently.In this paper,TOC concentrations and δ¹³C_org data of 13 surface soil samples distributed in WCLP and 48 low resolution loess/paleosol samples from 4 typical loess profiles in WCLP have been measured,combining with previously reported TOC concentrations and δ¹³C_org data from 11 bedrock samples from dust source regions andδ¹³C_org data of 10 plant litter samples in surface soils from the WCLP,for the assessment on the primary provenance of organic matter in loess/paleosol sequences in WCLP.Our results indicate that,in WCLP,average TOC concentration of surface soils is much higher than that of the rock samples in dust source regions,and averageδ¹³C_org of surface soils is close to that of the plant litters in the surface soils in WCLP,and both of them are much more negative than average δ¹³C_org of rock samples in dust source regions.That means,after the deposition,the dust in WCLP received a significant amount of organic matter with negative carbon isotopes Relatively and averagely,the TOC concentration of loess/paleosol samples in WCLP is much lower than that of surface soils in WCLP,and the δ¹³C_org of loess/paleosol samples in WCLP is much more positive than that of surface soils in WCLP.That means,during the burial process,the organic matter in loess of WCLP undergone significant decomposition accompany with the δ¹³C_org became more positive.Our quantitative estimation indicated that the contribution of organic matter from rock samples in dust source regions to the surface soils in WCLP is less than 8%（or 7.69%）.At the same time,both TOC concentrations and δ¹³C_org data from the low resolution loess/paleosol samples of the typical loess profiles in WCLP showed consistently regular variations along loess/paleosol layers.Concretely speaking,the TOC concentrations were relatively higher in paleosol layers developed during the Holocene（S₀）and the last glacial interstadial（L₁S₁）.The δ¹³C_org were more positive in the Holocene paleosol layer relative to the last glacial loess layer.During the last glacial,the δ¹³C_org were more negative in the paleosol layer developed during the last glacial interstadial relative to loess layers accumulated during the early and later last glacial（L₁L₂ and L₁L₁）.Altogether,our results demonstrate that organic matter in loess/paleosol sequences in WCLP was derived from local vegetation mainly,rather than bedrocks in dust source regions.Therefore,our results provided fundamental evidence for the validity of stable carbon isotopic composition of organic matter in loess/paleosol sequences in WCLP as proxy for paleovegetation and paleoclimate reconstruction.更多还原