The biogeochemical research of microbial hopanoids in Dajiuhu peatland, central China

<p>Peatlands are a vast store of organic carbon and play a significant role in the global carbon cycle. The abundance of lipids, especially microbial hopanoids are, directly or indirectly, participating in the carbon cycle in peatlands. Although the hopanoids and their compound-specific carbon isotope composition have been applied to some extent in the study of paleoenvironment and paleoecology, it is still misty about the diagenetic transformation process and the main controlling factors of the early diagenetic transformation of hopanoids. The potential of paleo-ecological application of hopanoids is still lack of systematic understanding. Previously we investigated several typical peatlands in China and found pH plays an important role in the early diagenetic transformation of bacteriohopanepolyols (BHPs) into geohopanoids. We also found the pH value promotes the isomerization of geohopanoids. Here, we focus on the biogeochemical research of microbial hopanoids in Dajiuhu peatland. We  first carried out a series of modern process monitoring on a seasonal scale, such as monthly investigating the climate factors (air temperature, air humidity, rainfall), water, soil temperature, soil moisture, water chemistry (pH, ORP, conductivity, dissolved oxygen), the main nutrient (nitrate, phosphate, etc.), dissolved organic carbon (content, composition and isotopic composition). On this basis, we discussed the relationship between those compounds (composition, carbon and hydrogen isotopes) and the climate-environmental conditions as well as the carbon dynamics. Further on, we examined the response of the carbon cycle based on hopanoid index in a peat core (18ka BP) in Dajiuhu peatland. Our results showed that in acidic peat deposits in Dajiuhu peatland, the carbon isotopes of hopane are generally more than 5‰ positive compared with the carbon isotopes of n-alkanes from higher plants both the surface peat samples and core peat samples, which indicates that in acidic peat environment, the hopanoid-produced bacteria mainly take carbohydrate as carbon source. We also showed that the difference of the carbon isotopes between hopane and n-alkanes is not stable, suggesting that these hopanoids may use different carbon sources.</p>

Phytolith-inferred transfer function for paleohydrological reconstruction of Dajiuhu peatland, central China

Depth to water table (DWT, the depth from the water surface to the top of the peat surface) is one of the most important environmental variables related to the habitat types and distribution of vegetation within a subalpine peatland. The distribution of phytolith assemblages and basic environmental data from 43 surface soil samples with significant ecological and hydrological gradients were investigated to generate transfer functions for quantitative reconstruction of paleoenvironmental changes in Dajiuhu peatland, central China. Detrended correspondence analysis (DCA) and redundancy analysis (RDA) were employed to explore the relationship between main environmental variables and phytolith morphotypes and distributions. Our results indicate that the spatial distribution of phytoliths was significantly correlated with the DWT (25% variance), total organic carbon (TOC, 10% variance) and magnetic susceptibility (χ, 7% variance). We established the transfer functions for the significant variables based on modern analogue technique (MAT), weighted averaging techniques (WA) and weighted averaging partial least squares (WA-PLS), and model performance was assessed using bootstrap cross-validation. The high correspondence of the predicted DWT results based on phytolith-environment calibration data with observed data reflects that the phytolith-based WA-PLS is a reliable effective calibration method for the quantitative DWT reconstruction of ombrotrophic (rain-fed) subalpine peatland.

Peatland evolution and associated environmental changes in central China over the past 40,000 years

Central China has experienced stronger summer monsoon during warm periods such as Marine Isotope Stages (MIS) 1 and 3, and weaker summer monsoon during cool periods such as MIS 2. The evolution history of Dajiuhu subalpine peatland in central China can help investigate how the expansion and shrinkage of peatland were associated with monsoonal strength over the last glacial–interglacial cycle. Here we apply bulk organic carbon and molecular biomarkers (hopane and n-alkane) to reconstruct the evolution history for the Dajiuhu peatland over the past 40,000 yr. The results indicate fluctuations between lacustrine and peat-like deposition during MIS 3, steady lacustrine deposition during MIS 2, and peatland initiation and expansion during MIS 1 in the Dajiuhu peatland. Therefore, at the glacial–interglacial scale, warmer summer and cooler winter conditions in interglacial periods are crucial to trigger peat deposition, whereas reduced evaporation in glacial period instead of decreased monsoonal-driven precipitation would have played a predominant role in the regional effective moisture balance. However, within the Holocene (MIS 1), monsoonal precipitation changes appear to be the main controller on millennial-scale variations of water-table level of the Dajiuhu peatland.