The Spatial Change Character Of δ～(13)C Of Total Organic Matter And δ～(13)C, δD Of Long-chain N-alkanes Of The Surface Soils Across East China And Their Paleoenvironmental Significance

More than 100 surface soil samples spanning 18°N to 50°N within east China were systematically gathered, the content of total organic carbon (TOC) and its carbon isotopic composition (δ¹³C_TOC) of these samples were measured. The extracted n-alkanes from these samples were analyzed by a gas chromatogram (GC), and then their carbon (δ¹³C_n-alkanes) and hydrogen (δD_n-alkanes) isotopes were measured. Based on the detailed analysis of the obtained data, some new knowledge has been gained.The long-chain n-alkanes with a characteristic odd-over-even number predominance that extracted from these surface soil samples indicate they were derived from terrestrial high plants. To a certain degree, the distribution of the long-chain n-alkanes molecules is similar between the samples that gathered under different vegetation types (forest or grassland). That means it is unfeasible to determine the overlying vegetation type based on the distribution of the long-chain n-alkanes molecules of the surface soil samples. The GC analyses of the n-alkanes extracted from these soil samples are critical to the results of paleovegetation type reconstruction based on such a simple model that long-chain n-alkanes dominated by C₂₇ or C₂₉ were from trees and C31 were from grasses.In east China, bothδ¹³C_TOC andδ¹³C_n-alkanes of these surface soil samples have the same spatial change character. More importantly, there is a positive correlation relationship between them. Theδ¹³C_n-alkanes of the long-chain n-alkanes that derived from terrestrial high plants record the C₃/C₄ ratio of the overlying vegetation unquestionably, so theδ¹³C_TOC with same spatial change character should also record the same information. In our study region, in spite of the diversity of the climate and vegetation types, the organic matter derived from other sources do not disturbed the carbon isotopic information of the organic matter that derived from terrestrial high plants significantly. Such a result can partly demonstrate that the reconstructions of past C₃/C₄ change base onδ¹³C_TOC are reliable in terrestrial deposits that slightly influenced by aquatic biology and microorganism (e.g. the loess-paleosol sequences of Chinese Loess Plateau). However, the paleoenvironmental indicators that with opposite spatial and temporal change trends toδ¹³C_TOC (e.g.δ¹³C of carbonate in loess-paleosol sequences) can not record the past C₃/C₄ changes.Within east China, bothδ¹³C_TOC andδ¹³C_n-alkanes can indicate: in the high latitudinal region that above 40°N, the regional vegetation almost composed by pure C₃ plants; in the middle latitudinal region that between 31°N and 40°N, C₄ plants is a stable component in regional vegetation, and some vegetation are dominated by C₄ plants, however, pure C₃ vegetation is absent in this region; in the low latitudinal region that below 31°N, the contribution of C₄ plants to primary production is decreased relative to middle latitudinal region, no vegetation is dominated by C₄ plants in this region, regional vegetation are pure C₃ vegetation or C₃/C₄ mixed vegetation that dominated by C₃ plants. The analyses of the contribution of C₄ plants to primary production within east China and its relationship to climatic factors combine with the comparison with analogous international results strongly suggested that temperature has a dominant control on the growth of C₄ plants. However, if the temperature is high enough, the relative abundance of the C₄ plants will decrease when the precipitation increased to a certain degree. The result of our modern process study indicates that the predominated C₃ vegetation in the high latitudinal band throughout the whole last climatic cycle was caused by the low temperature; the increase of the relative abundance of C₄ plants in the middle latitudinal band from the last glacial to the Holocene mainly reflect the increase of the temperature; predominated C₄ vegetation in the low latitudinal band during the last glacial replaced by C₃ vegetation in the Holocene mainly attribute to the increase of precipitation. Our result of modern process study combine with the reconstructed results of global mean paleotemperature and paleoconcentration of atmospheric carbon dioxide demonstrate that the asynchronous expansion of C₄ plants since the Miocene in different regions may be the response to the changes of moisture supply controlled by the atmospheric circulations, not the response of the biosystem to the significant decline of the atmospheric carbon dioxide level.The comparison betweenδD_n-alkanes andδ¹³C_TOC andδ¹³C_n-alkanes and vegetation types suggest that both photosynthetic pathways and vegetation types are not the main factor that controls the hydrogen isotopic composition of long-chain n-alkanes that derived from terrestrial high plants. The same spatial change trend ofδD_n-alkanes and precipitationδD indicate that the latter may has a dominate control on the former, in other words, theδD_n-alkanes derived from terrestrial high plants can record theδD of precipitation. Even so, due to the complicacy of the factors that control the 8D of precipitation, the explanation of paleo-δD_n-alkanes records must be very cautious.Our results emphasized the importance of modern process study. The paleoenvironment reconstruction results can be explained correctly based on detailed and plentiful modern process study. On the other hand, the results of modern process study also need the test of paleoenvironment reconstruction. Only the well combination of these two aspects can promote the study of paleoenvironment reconstruction.