Paleoclimatic Information Recorded By Microbes And Organic Matter In Chinese Loess ---studies On The Contributions To The Magnetic Susceptibility Of Magnetotactic Bacteria And The Characteristic Biomarkers

Characteristics of magnetotactic bacteria (MB) and the contained magnetosomes(MS) in the Quatemary Chinese loess-paleosol sequences were studied systemicallythrough selecting sectinn Xifeng in Gansu Province in the central Loess Plateau andsection Duanjiapo in Shaanxi Province, the southeast of the Plateau on the basis ofthe modem climatic and geologic backgrounds. The MB growth and MS formationunder differeni temperatures were simulated for the first time in China. Primarystudies on the contributions of MB and MS to the magnetic suscghbility of loess-paleosol sequences were also carried out to try to reveal the mechanism of themagnetic suscePtibility as a climatic proxy Moreovef, characteristics of MB and MSin the peat, Southeast of China were compared with those in Loess. Besides,characteristic Biomarkers of loess-paleosol sequences and the MB were analyzed toshow the "fossil behavior" of MB in loess-paleosol sequences and the paleo-climaticsignificance. The major results can be drawn as fOllowing:1. Influences of MB on the suscoptibility of loess-Paleosol sequences can berepresented in both the contributions of MB to the susceptibility and those of MS"fossils" to the frequent susceptibility. It is shown by the expetimental resultsthat the susceptibility of 10g Chinese loess (paleosol) sarnples can be increased116.4% at most after MB cultured fOr 10-60 days.2. MBs are widely diStribllted in various environmenis. Howevet, the amount,morphologies and sizes of MB vary somewhat in different environments.Nevertheless, MS formation may be related more closely with the environment.Conditions are favorable for MS formation with temperature varying in a certainrange (10℃ or above), neutral to alkaline pH and enough Fe which can be usedby MB. The EDAX results show that MS in section Xifeng may contain elementS, somewhat different from that in section Duanjiapo.w~l3. The Total Organic Carbon content (TOC) of the Chinese loess can reflect thetrend of the paleo-climatic changes. The paleo-humidity of S,-, paleosolformation of section Xifeng was as high as 80% calculated by the T.^.' thelargest of layers from L, down to L,. The paleo-environment when S,-, formedwas quite stable shown by the OI and HI.4. It is shown by CPI value, C,./C,,, C,,lC,, of n-alkane and CPI^ value, C,e/ C,, ofn-fatty acid and the unsaturated components of layers from L, to S, of bothsections that it was relatively cold then in the Loess Plateau, and colder in Xifengthan in Duanjiapo. Furthermore, there might exist an abruPt humidity variationfrom S, formation to Ls accumulation in section Xifeng. MB perhaPs is animportant origin of the fatty acid in loess-paleosol sequences.