The molecular organic compounds have been identified by gas chromatography (GC) and GC-mass spectrometry (GC-MS) from Mesoproterozoic rocks in the Xuanlong (宣龙) depression in North China. The main saturated compounds are n-alkanes, monomethylalkanes, n-alkylcyclohexanes, acyclic isoprenoids, and hopanes. The dominant lower-molecular-weight n-alkanes are indicative of the main contribution of microorganisms, in particular, the chemosynthetic bacteria. The presence of abundant monomethylalkanes (mid- and end-branched) and the long chained (>C20) acyclic isoprenoids indicates the existence of abundant bacteria and/or archaea in ancient oceans. The low abundance of pristane and phytane is suggestive of the relatively low abundance of photosynthetic autotrophs in comparison with chemosynthetic bacteria in the Mesoproterozoic oceans in North China. The sedimentary environmental condition is suboxic/anoxic, as indicated by the low value of the Pr/Ph ratio as well as the presence of abundant sulfur-bearing organic compounds, consistent with the other geochemical data in North China and elsewhere in the world. Both the composition of the primary producers and the sedimentary environmental conditions are favorable for the formation of hydrocarbon source rocks.
Because literatures about the lipid compositions of modern soils in karst areas are scarce,we have studied the soil horizons overlying the Heshang(和尚) Cave that has provided paleoclimate records from speleothem lipid contents.Our analysis reveals a series of n-alkanes,free fatty acids,n-alkanols and n-alkan-2-ones distribution and relative abundance changing with the depth,and in which the ratios of the lower molecular weight to higher molecular weight n-alkanes,free fatty acids,n-alkanols and n-alkan-2-ones have a peak at a subsurface depth of 5 to 10 cm.An accompanying peak in 17β(H),21β(H)-hop-22(29)-ene(diploptene)and a shift to less negative n-alkane carbon isotopic values also identify this layer in the karst soil.This pattern indicates the existence of a subsurface soil layer in which the microorganisms that produce these compounds are especially abundant.The carbon isotopic values of individual plant wax n-alkanes are about 3‰ greater at the base of the 30-to 40-cm soil profile than in the surface layer,probably as a result of selective microbial degradation of n-alkanes from different primary sources.The lipids and carbon isotopic values of individual plant wax n-alkanes study of the overlying soil show a strong microbial activity in this karst soil and help in interpreting the lipid compositions and specific carbon isotopic value of n-alkanes of the stalagmites of the Heshang Cave for paleoenvironmental reconstructions.
The trace fossil Zoophycos was enriched in the lower part of the Guadalupian Maokou Formation of Permian in Laibin, Guangxi of South China, and characterized by observable lamellae within its spreiten. Associated with these lamellae are the prolific coccus- and spirillum-shaped microbe fossils. These microbes morphologically identified are 0.4―3.0 μm in diameter or length. Molecular fossils, including normal alkanes (dominated by C18, without an odd-over-even predominance), acyclic isoprenoids (such as pristane (Pr) and phytane (Ph)), extended tricyclic terpanes, pentacyclic triterpanes, steranes, al-kylcyclohexanes, dibenzothiophenes, benzonaphthothiophenes, benzobisbenzothiophenes, were in-strumentally identified in the wackestone characterized by the occurrence of abundant Zoophycos (composite ichnofabric indices are 4) and crowded microbial fossils. The value of the Pr/Ph ratio is less than 1, indicative of a dysoxic condition. The identification of abundant sulfur compounds (the thio-phene series) related to the contribution of reductive sulfur favors the occurrence of sulphate-reducing bacteria. The association of abundant microbial colonies with microbial molecular fossils within the spreiten suggests that trace fossils Zoophycos would be a multifunctional garden carefully constructed by the Zoophycos-producer, where different microbial colonies were orderly and carefully planted and cultured in different minor lamellae within spreiten. Hence, it is proposed that the Zoophycos-producer symbiosed with microbial colonies on the mutual basis of food supply and the redox conditions.
GONG YiMingXU RanXIE ShuChengHUANG XianYuHU BinQI YongAnZHANG GuoCheng
On the basis of GC and GC-MS analyses, the seasonal variation of leaf lipids is observed in five plant species in Qingjiang in Hubei Province. The CPI values (carbon preference index) of n-alkanes decrease gradually from May to November, though the carbon number distributions and the predominant carbon number in an individual species keep unchanged. The declined CPI values might be caused by the leaf fading, which is further supported by a comparison of the defoliated leaves with the fresh leaves. This observation infers the CPI values of n-alkanes in Quaternary sediments can be used as a tool to index the humification, and thus the climatic and the environmental conditions. The plants analyzed here show a remarkable difference in n-alkanes abundance, suggesting their differential contribution to the n-alkanes identified in sediments. The abundance and the compound species of the n-alkenes detected in the plant leaves change greatly with the seasons due to the influence of temperature, with the least compounds being identified in August.
