Biomarkers Of The Typical Paleosols In Luochuan Section And Their Significance On Paleovegetation-environments

Biomarker is the remnant of vegetation and animal in stratum, its distribution and geochemistry characters record bio-formation or geo-history environment. The component and configuration of bio-remnant could change with geohistory environment, but fossil molecules have original C-structure of biology. So far, there are lots of study works on paleo-vegetation and environment which have been done, the research areas have reached aerosol, marine sediments, lacustrine and peat sediments, cave stalagmite, loess, paleo-vegetation, glacier, and estuarine sediments (rocks), and lots of achievements are obtained.The loess is widely distributed in the Northwestern China and has served as one of the best archives for paleoenvironment reconstruction. Together with deep sea sediments and ice core studies, they were considered mainstays of the global changes and great achievements have been documented in the recent decades. Using the GC-MS, biomarkers of samples taken from the weak developed paleosol (L1SS1) and loess layers (L₁LL₁ and L₁LL₂) in Luochuan loess section, which is one of the standard Quaternary typical section in NW China, were measured continuously, it was found that all the samples are rich in n-alkanes and other biomarkers. The indices, such as W_g/v = nC₃₁/(nC₂₇+nC₂₉+nC₃₁), W_w/v = (nC₂₇+ nC₂₉)/(nC₂₇+nC₂₉+nC₃₁), and W_w/g = (nC₂₇+ nC₂₉)/(nC₃₁+nC₃₃) recorded the ratios between grass and total vegetation, trees and total vegetation, trees and grass, which further represents the vegetation change history. The CPI of all the tested paleosol (L₁SS₁) samples change from 5.60 to 11.50, and much larger than 1, which indicate that the influences unrelated to the climate during the soil formation are neglectable. Based on the relative relationship analyses between magnetic susceptibility, grain size and biomarker indices, it proved that the magnetic susceptibility and grain size, the relativity reaches as high as -0.81, recorded the environmental change and climatic events, while the biomarkers related to the paleovegetation changes. When the climate changed from cold-dry to warm-humid, that means the improvements of the temperature and precipitation, the vegetation changes very quickly. While the environment deteriorated, that means when the climate changed from warm-humid to cold-dry, the vegetation changes lagged the climate changes and/or resistant to the changes, this phenomena might attribute to the stability of the vegetation system itself. Biomarker of all the samples analyzed characterized by a high carbon number majority of C₃₁, therefore, indicate that grass predominant the vegetation during interstadial of last glacier in Luochuan area and there was no forest developed during that time. We emphasize that this statement is limited in the study area and could be applied to situation all over the whole Loess Plateau as the climatic environment condition might be very different from studied area to the southern at that time, as that of today. Using gas chromatography-mass spectrometry (GC-MS) technique, we identified a series of biomarkers, including n-alkane, n-alkane-2-ones, isoprenoid etc from the loess-paleosol samples collected from the S₄ and adjacent L5, L4 of the Luochuan loess section, Northwestern China. Based on these data, especially n-alkanes and high-resolution magnetic susceptibility and grain size data, the paleoenvironment and paleovegetation history during S₄ was reconstructed. The CPI (Carbon Predominance Index) and correlation between n-alkanes and magnetic susceptibility and grain size data demonstrated that the molecular fossils in paleosol and loess layers can reflect the vegetation condition during the loess-paleosol formation, if the allochthonous organic inputs could be excluded reasonably. The ACL (average chain length) index is correlated well with paleomagnetic susceptibility and grain size variations, displaying their good synchrony with warm and humid climate. However, it relatively lagged behind the paleomagnetic susceptibility and the grain size variations when the climate began to deteriorate. During the formation period of paleosol, the n-alkanes were dominated by C₃₁ homologue, indicating that the primary organic input originated from herbs. Our study also demonstrated that the herbs were more flourish than wood plants in Loess Plateau, especially in the Luochuan area during the warm and humid phase, and there was no typical forest vegetation developed in the studied period. The n-alkanes of S₅ palaeosol have odd-carbon preference, the carbon distribution in all samples peaked at C₃₁ homologue, the abundance of C₂₇2931, which show that the biomarkers origin from vegetation dominated by grass. By comprision of palaeomag-netic susceptibility and grainsize, the ratio of n-alkane changes with the section depth, the palaeomagnetic susceptibility shows three subunits of S₅ palaeosol, the ratio of alkane can show the first subunit clearly, but the second and third are not clear, but it records the vegetation fluctuation of three subunts.The biomarkers of Xiaohushan lake sediments (MIS3) show that n-alkanes are dominated by C₂₉, but n-alkanes of Luochuan section (Sm) is dominated by C31, which shows the biomarkers are influenced by those of original region, but it record Paleovegetation and environment. Our study provides the most important information of the paleovegetation during the Late Pleistocene in Luochuan area, northern of the Loess Plateau. It proved that biomarkers could provide useful information on the vegetation development in the past, and such kind of results can be useful for the reforestation in the area, which bears practical significance to improve the modern environment.