Carbon Isotope Of Long-chain N-alkanes Of The Stalagmite And Their Paolevegetation Significance In Qingjiang, Hubei Province

The middle reaches of Yangtze River drainage as a key passage of the East Asian monsoon is an ideal pot for the global climate research. A lot of the studies has dominate the stalagmite contains lots of climate information. All the studies concentrate on the trace elements, stable isotopic of carbon and oxygen, laminas, gray scale, fluorescence intensity and wavelength change, and so on, which provide the details for the source of organic matters, paleoprecipitation, paleoclimate and paleoclimate studies, but there are rare report about the paleoecological.Stalagmite contains abundant biomarkers which provide some paleoecological information. Specific carbon isotope value of the biomarkers can fine this paleoecological information at the molecular level, especially about the vegetation change, which has been used in many types of sediment successfully. So using the compound specific carbon isotope analysis to study stalagmite has great significance.In this paper, we studied n-alkanes specific carbon isotopic values and get the specific carbon isotopic values from 38kaB.P to now. Combining with the n-alkanes specific carbon isotopic values of modern plants, soil and stalagmites, we get the percent of C₄ paint using the Actualism principle and also comparisons the paleovegetation with the paleomonsoon.The result of the investigation of the vegetation showed:(1) The quantity and the dominating peak of n-alkanes changed from species to species. Most woody plant maximize at n-C₂₇ or n-C₂₉ and herbage at n-C₃₁ or n-C₃₃ according with the previous studies. But some exceptional species existed.(2) Leaf waxes are mixtures of abundance lipids, such as alkanes, fatty acids, long-chain alcohols, n-alkan-2-ones and some esters. The specific carbon isotopic values of n-alkanes are distinct.(3) The distribution and contents of lipids in the same plant change as the growing season proceeds, especially the n-alkenes, which responding well to the temperature: high contents under high temperature while low contents under low temperature.The result of the investigation of the soil showed:(1) The specific carbon values of n-alkanes in the soil indicated that the ¹3C enriched with the depth increasing. That may be caused by the activities of the soil microorganisms. And the profile is characterized by variable and large (3‰) carbon isotopic offsets between C₂₇, C₂₉, C₃₁ and C₃₃ n-alkanes.(2) The values of specific carbon isotope of n-alkanes showed obvious positive shift in the depth of 5～10cm due to the decomposition of microorganisms. The content of lipids including fatty acids, alcohols and M-alkan-2-ones revealed that the maximum quantity of microorganisms in the soil about 5¹0 cm under the soil surface. That also was testified by the ratio of low-molecular-weight and high-molecular-weight of n-alkanes and the contents of 7β(H), 21β(H)-hop-22(29)-ene.The result of the investigation of the stalagmites showed:(1) In this study, sequence extraction was adopted. Acid digestion was employed to deal with the stalagmites following the Soxhlet extraction. The n-alkane fractions in the stalagmites were "free" and can extract by the Soxhlet extraction. Some biomarkers denoting specific biological source can be attained through this way, for example, 3-hydroxy acids which is characteristic of Gram negative bacteria.We can presume the change of species and the quantity of the biomarkers based on the variations of the change of dominating peak and the contents.(2) The n-alkane fraction was separated by silica gel column chromatography and urea adduction. The compound-specific carbon isotope of C₂₇, C₂₉, C₃₁ and C₃₃ n-alkanes varied as time growing. The variation probably evoked by the environment, mainly the vegetation change.On the basis of variations of carbon isotopic composition of long-chain n-alkanes in the vegetation-soil-stalagmite systems, we get the value of fraction of carbon isotope in the karsts system. The distribution of the C₃ and C₄ plants can be expressed by the following formula:δ¹³C_S0 is the stalagmites form newly;δ¹³C₃ andδ¹³C₄ are the specific carbon isotopic values;â–³T¹³C₃ and AT¹³C₄ are the carbon isotopic offsets of n-alkanes in whole system; a and b represent the percentage of the C3 and C₄ vegetation. In this study,â–³T¹³C₃=â–³T¹³C₄.During different geological time, the distribution of the paleovegetation can be gained by the formula above. The percentage of C4 vegetation was related to the monsoon from 38kaB.P to now. From Holocene, the C₄ vegetation dominated in dry period, such as the 1.4kaB.P, 6.0kaB.P, 6.6kaB.P and 8200 Event. In the late Pleistocene, the monsoon was low-intensity. In some cold periods, such as Younger Dryas and Herench 1, the C₄ vegetation was in higher percentage. But the C4 vegetation was less in Bolling/Allerod warm periods, and the percentage of C₄ vegetation in 36KaB.P is higher than 37KaB.P.In this study, the specific carbon isotope analysis was employed in stalagmites to help recover the paleovegetation in this area. The variation of C3 and C4 vegetation correlates with the monsoon.