The Response Of Microbial Lipids In Chinese Soils To The Modern Processes And Its Paleoenvironment Implications

Lipids as biomarkers can record environmental change on basis of variations of their structures and compositions, and therefore have been widely applied to reconstruct paleoenvironment in diverse geologic settings. However, the present is the key to the past. The feasibility of proxies for paleoenvironment should be validated in the modern processes prior to their use in paleoenvironment interpretation. The study on the response of lipids to environmental temperatures and precipitation can provide the prerequisite and basis for application of lipids on paleoenvironment reconstruction. Plants can only respond to the environmental change at a large time scale while the structures and compositions of microbial lipids can sensitively track the ambient environmental change such as temperatures etc., thus playing a more and more important role in the Quarternary paleoenvironment research. Soils, as a focus of global change research, are the ideal materials for studying the response of microbial lipids to environments because soils are rich in microbial lipids, readily collected and widely distributed in different climate zones.This dissertation will focus on microbial lipids in the surface soils across an alitude transect of Mt. Jianfengling and from different climate zones of China. The temperature and precipitation difference as a result of altitudes and climate zones can provide favorable conditions for validating and establishing novel organic geochemical proxies in terms of microbial lipids. So many kinds of microbial lipids including fatty acids, fatty alcohols and archaeal or bacterial glycerol dialkyl glycerol tetraethers (GDGTs) have been detected by gas chromatogram and mass spectrometry as well as liquid chromatogram and tandem mass spectrometry, and also used to establish new paleothermometers. The factors controlling branched GDGTs distribution in surface soils from different climate zones of China have been investigated and the equation for MBT/CBT index has been calibrated to cater for its use in Chinese regional environment. Besides, the environmental factors influencing the terrestrial input index, BIT has also been determined. We also simulate the impact of oxidation on paleoenvironmental proxies including TEX86, MBT/CBT and BIT index by hydrogen peroxide, which may provide the basis for accurate paleoenvironmental interpretation. Integrated with progress in above research, conclusions were drawn as follows.1) The microbes in the surface soils of Mt. Jianfengling can produce diverse biomarkers including (anteiso) iso fatty acids,(anteiso) iso fatty alcohols, unsaturated fatty alcohols, 3-hydroxy fatty acids,2-hydroxy fatty acids, archaeal and bacterial GDGTs. The branched fatty alcohols in the soils occur as wax and can only be released by saponification. The iso fatty alcohols have been found in pure-cultured gram-positive bacterium and AAPB, thus providing direct evidence for the sources of branched fatty alcohols. Four novel paleothermometers including the ratios of a15/i15 (acids), a15/i15 (alcohols), a 15/n-15(alcohols) and unsatured/saturated octadecanol were established based on microbial lipids in the soils of Mt. Jianfengling, where the ratios of a15/i15 (alcohols), a 15/n-15(alcohols) and unsaturated/saturated octadecanol correlate negatively with altitudes while the ratio of a15/i15 (acids) increase with increasing altitudes. The most significant correlation exists between the ratio of a15/n-15(alcohols), altitudes-derived mean annual temperature (MAT) and soil pH, yielding a calibration equation a15/n-15= 0.114*pH+ 0.0234* MAT-0.773 (R2=0.80). The response of microbial fatty acids and alcohols to the altitude in fact results from the adaptation of microbial membrane to ambient temperature. Moreover, archaeal and bacterial GDGTs both show great potential in reconstructing terrestrial paleoenvironment due to their response to temperature change in Mt. Jianfengling.2) The microbial branched GDGTs and archaeal GDGTs are widely distributed in soils from different climate zones of China and their relative abundance are primarily controlled by soil pH. The microbial branched GDGTs have higher abundance than archaeal GDGTs when soil pH is below 8.0, while in the alkaline soils with pH higher than 8.0 the achaeal GDGTs markedly exceed in abundance over bacterial GDGTs. Archaeal GDGT V without rings are known to primarily come from Euryarchaeota including methanogens and halophilic archaea whereas crenarchaeol are mainly derived from ammonia-oxidizing Crenarchaeota. On this point, archaeal commomunity index (ACI) was defined to describe the relative abundance of Euryarchaeota and Crenarchaeota. The ACI shows a close relationship with soil pH, i.e. the ACI values decrease with higher soil pH first and then increase when soil pH is higher than 8.0. The ACI shows that the Crenarchaeota dominate when soil pH is between 6.0 and 8.0 while the abundance of halophilic archaea increase when soil pH is higher than 8.0. The reliance of ACI on soil pH indicates that the most suitable environment for ammonia-oxidizing Crenarchaeota to flourish may have pH between 6.0 and 8.0.3) The MBT and CBT index of branched GDGTs in Chinese soils show good environmental significance. The correlation coefficient between MBT in Chinese soils and mean annual precipitation (MAP) or soil pH is much higher than MBT vs. MAT while in global calibration MBT is closely related to MAT rather than MAP, indicating that global calibration for MBT and MAT does not fit for the paleoenvironment reconstruction in Chinese geologic settings. However, CBT index in Chinese soils also shows good negative correlation with soil pH, in consistent with its global calibration. Integrated with above relations, two new calibrations for paleotemperature and paleoprecipitation reconstruction in China were established, i.e. MBT=-0.02+0.21*CBT+ 0.03* MAT (R2=0.83) and MBT=0.134+0.109*CBT+0.000318*MAP (R2=0.86), which improved the accuracy in Chinese regional paleoenvironment reconstruction.4) The terrestrial input index, BIT is remarkably influenced by pH of Chinese soils. The BIT values decrease with higher soil pH. Most BIT values in the soils with pH higher than 6.0 are lower than 1, which can not fit for the prerequisite to accurately and correctly estimate the terrestrial input, i.e. the BIT values of soils should approximate to 1. Therefore, soil pH in the river drainage basin should be considered before terrestrial input estimation via BIT index. If the BIT values of soils are much lower than 1.0, terrestrial input estimation should be based on BIT values of soils to avoid underestimate.5) The TEX86 values decrease while CBT values increase with increasing oxidation of GDGTs in soils, suggesting that ring-bearing GDGTs are more readily degraded than GDGTs without rings. The MBT of branched GDGTs in soils keep constant throughout the oxidation experiment, leading to neglectable change in MBT/CBT-derived temperatures. The BIT values increase with enhanced oxidation, implying that branched GDGTs are more resistant to oxidation than archaeal GDGTs. Therefore, the oxidation of GDGTs in sediments may result in overestimate of terrestrial input via BIT index. The results in the oxidation simulation experiments will provide basis for accurately reconstructing paleoenvironment based on TEX86, MBT/CBT and BIT.