Vegetation evolution on the central Chinese Loess Plateau since late Quaternary evidenced by elemental carbon isotopic composition

There are many controversial issues in loess studies such as natural vegetation types on the Chinese Loess Plateau during the historical periods and the spatial and temporal evolution of C3/C4 plants. Elemental carbon isotopic composition (δ13Cec) in the loess section may offer new evidence for these problems. Elemental carbon (EC) is produced by incomplete combustion of vegetation, and its carbon isotopic composition has a very small difference from that of the formal vegetation, then δ13Cec can be used as a record to recover the changes of vegetation. Elemental carbon was extracted by applying the oxidation method from the loess-paleosol sequence in the central Chinese Loess Plateau, and its carbon isotope composition was analyzed by the isotope mass spectrometer. The results showed that the vegetation in this region was a mixed type of C3 and C4 plants, dominated with C3 plants in most of the time. Since late Quaternary, C3/C4 plants may not follow a simple glacial-interglacial cycle mode on the Chinese Loess Plateau, but showing fluctuations. C3 plants increased gradually in L4 period, and more C3 plants occurred during S3 period, and C4 plants increased again during L3―L2 periods, after that, C3 plants dominated again during S1―S0 periods. During periods of paleosol development, C3 plants were abundant in S3 and S1, and there were more C4 plants in S2 and S0. During periods of loess sedimen- tation, there were more C3 plants in L4 and L1, and there were more C4 plants in L3 and L2. On the or- bital timescale, the vegetation variations revealed by δ13Cec record are consistent with the results of pollen data and also similar to the results obtained by organic carbon isotopic composition since the last glacial period.

The elemental carbon record in Weinan loess section since the last 21 ka

We studied the records of elemental carbon (EC) of the last 21 ka in the Weinan loess section, Shanxi Province. The variations of EC abundance and δ13CEC value along with depth (or age) were presented. There are four large peaks of EC abundance around the following years: 20.16 ka, 17.76 ka, 11.97 ka and 4.49 ka. Climatic situation was changed rapidly during these periods. The peaks around 11.97 ka and 20.16 ka are particularly sharp, occurring over intervals of tens to hundreds of years, which could represent short-duration intense events. δ13CEc values in the upper 4 m of the Weinan loess section vary between -11.71‰ and -21.34‰, which suggests that the vegetation pattern of the last 21 ka on the Loess Plateau is G4-dominated grasses.

^(10)Be/~7Be implies the contribution of stratosphere-troposphere transport to the winter-spring surface O_3 variation observed on the Tibetan Plateau

10Be/7Be is a stratospheric sensitive tracer.In this paper,measurements of 10Be/7Be and surface O3 from October 2005 to May 2006 at Mt.Waliguan (hereafter WLG,100.898°E,39.287°N,3810 m,a.s.l.),China global atmospheric watch (GAW) observatory,are introduced and used to investigate the stratosphere-troposphere transport (STT) and its impact on surface O3 on the Tibetan Plateau.The results show that the magnitude of STT is weak in winter,followed by strengthening from the end of winter to the middle of spring (from mid February to mid April) with large increases in 10Be,7Be,10Be/7Be and surface O3.At the end of spring (from the end of April to mid May in this paper),the STT weakened,and the continuous increase of surface O3 at WLG is produced by tropospheric photochemistry reactions.

Carbon isotopic composition,turnover and origins of soil CO_2 in a monsoon evergreen broadleaf forest in the Dinghushan Biosphere Reservoir,South China

Carbon isotopic compositions of soil CO2 in rainy season (July) from two natural soil profiles (DHLS & DHS) in the monsoon evergreen broadleaf forest in the Dinghushan Biosphere Reservoir (DBR),South China,are presented.Turnover and origins of soil CO2 are preliminarily discussed in this paper.Results show that the content of soil CO2 varies between 6120 and 18718 ppmv,and increases with increasing depth until 75 cm,and then it declines.In DHLS,soil CO2 δ 13C ranges from -24.71‰ to -24.03‰,showing a significant inverse correlation (R2=0.91) with the soil CO2 content in the same layer.According to a model related to soil CO2 δ 13C,the soil CO2 is mainly derived from the root respiration (>80%) in DHLS.While in DHS,where soil CO2 δ 13C ranges from -25.19‰ to -22.82‰,soil CO2 is primarily originated from the decomposition of organic matter (51%–94%),excluding the surface layer (20 cm,90%).Radiocarbon data suggest that the carbon in soil CO2 is modern carbon in both DHLS and DHS.Differences in 14C ages between the "oldest" and "youngest" soil CO2 in DHLS and DHS are 8 months and 14 months,respectively,indicating that soil CO2 in DHLS has a faster turnover rate than that in DHS.The Δ14C values of soil CO2,which range between 100.0‰ and 107.2‰ and between 102.5‰ and 112.1‰ in DHLS and DHS,respectively,are obviously higher than those of current atmospheric CO2 and SOC in the same layer,suggesting that soil CO2 is likely an important reservoir for Bomb-14C in the atmosphere.

Apparent ages of suspended sediment and soil erosion of the Pearl River (Zhujiang) drainage basin

The carbon isotopic composition (Δ 14 C,δ 13 C) and apparent ages of suspended sediment were determined in the Pearl River in the years 1998,2000 and 2005.These results indicate that suspended POC consists mostly of young carbon and some "old carbon".Apparent ages of suspended POC range from 540 to 2050 a BP.The apparent ages are older in the Xijiang and Beijiang Rivers,while these values are variable in the Dongjiang River,including old and young samples.The suspended POCδ 13 C values increase with increasing Δ 14 C in the Pearl River,indicating that its source is soil and bedrock from depths under 15 30 cm.Since the organic carbon of the surface soil is quickly decomposed when it enters the rivers,its carbon isotopic characteristics are insignificant in suspended sediment.In the Pearl River drainage basin,there are areas with more severe soil erosion than others.For example,erosion is much more severe in the Xijiang River drainage basin when compared to the light soil erosion in the Dongjiang River drainage basin.

A biomass burning record from the Lingtai Loess Section during the last 370 ka and implication for climate and environment

The history of natural fire and its rela- tionship with climate and vegetation are revealed from the content of elemental carbon and associated pollen data and paleoclimatic substitutive indicators for the loess of Lingtai Section in the last 370 ka BP. The study indicates that intense episodes of vegeta- tion fires occurred during the interim especially when the climate was changing from wet to drought. The average content of elemental carbon in the intergla- ciers is higher than that in the glaciers, which coin- cides with the biomass change locally (or globally). The content of elemental carbon increases in the stage around 130 ka BP, indicating that the vegeta- tion and climate pattern have changed, which may contribute to the variation of CO2. As a whole, the content of elemental carbon increasing with the time reflects the increasing aridity trend to some degree. In addition, the occurrence of the maximum peak and the highest average content of elemental carbon in the Holocene reflects the occurrence of a rapid cli- mate event in 5900 a BP and more frequent fires caused by anthropic activities.

Concentration and seasonal variation of ^(10)Be in surface aerosols of Lhasa,Tibet

Thirty samples of total suspended particles were collected at a site in western part of Lhasa,Tibet from August 2006 to July 2007.The 10Be concentrations were determined by Accelerator Mass Spectrometer (AMS).Analysis of correlation between 10Be concentrations and meteorological factors revealed that the wet scavenging has little effect on 10Be.10Be can be used as a proxy of transport processes of the upper atmosphere over the Tibetan Plateau.Analysis of correlation between 10Be concentrations and NCEP reanalysis data demonstrated that higher 10Be concentrations in spring were probably caused by the atmosphere exchange from stratosphere to troposphere during February to June.Lower 10Be concentrations during August to September were consistent with the synchronous lower O3 values,suggestive of both 10Be and O3 were probably influenced by the atmosphere exchange from troposphere to stratosphere.