Characteristics of Lignin-derived Phenolic Compounds in Arid Lake, Northeastern China and Climatic Implications

Lignin is widely distributed in vascular plants, and lignin-derived phenolic compounds generated by decomposition could provide information on the source of organic matter and the degradation degree of lignin. The conventional method for lignin deconstruction is complex and involves lignin hydrolysis via alkaline/acid chemical reagents. The analytical technique pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) breaks the chemical bonds of large molecule compounds by instantaneous high temperature to generate a series of small molecule compounds without introducing pretreatment methods such as chemical extractions, realizing the online analysis of complex organic matter that is not easy to be gasified. This technique is characterized by low sample volume, high organic matter extraction ratio, good reproducibility, and convenient operation. It has been shown that the high-temperature cracking products of peat and lake sediments are similar to the results of traditional CuO oxidative decomposition. The distribution characteristics of phenolic compounds indicate the vegetation type and organic matter degradation characteristics. However, the optimization of analytical methods, application of environmental indication significance, and comparative studies of different matrix samples are still needed.

(1) Investigate suitable analytical methods for decomposing lignin in lake sediment samples and identify pyrolytic phenolic compounds in the sediments of Yiheshariwusu Lake in the northeast semi-arid region of China (Fig.E.1A, B). (2)Discuss the distribution characteristics of phenolic compounds in the sediments of Yiheshariwusu Lake. (3) Reveal the correlation between pyrolytic lignin phenols and regional climate change in the study area by combining traditional climate proxies, and provide an effective indicator for interpreting the response of terrestrial ecosystems to global climate change.

(1) Analytical method: An optimized analytical method of Py-GC/MS was established and applied to evaluate lignin-derived phenolic compounds in typical arid lake sediment. Samples were heated to 650℃ for 20s (heating rate 20℃/ms) and pyrolysis products were injected into the gas chromatography (GC) system in split mode, then separated in a nonpolar, low-bleed fused silica column (DB-1MS, 60m, 0.25mm i.d., 0.25μm film thickness, J&W). The GC oven program was set to increase from 40 to 320℃ at a rate of 4℃/min, and left at 320℃ for 18min. With internal electron ionization and ion trapping, the compounds were fragmented and identified in full scan mode (40-450amu). Blank and duplicate samples were analysed for quality control.

(2) Establishment of climatic proxy: Yiheshariwusu was selected as a typical arid lake and pyrolytical phenolic compounds of sediment cores were analysed. Historical variation of phenolic compound combing with radiocarbon dating results were revealed. According to "demethyl/demethoxy" oxidation reaction pathway of microorganisms to lignin, indicator related to degradation degree of lignin was established, and by comparing the indicators with conventional climate proxies previously published in the region, correlations between the indicators and climate features such as effective precipitation can be explored.

(1) Py-GC/MS analysis method for phenolic compounds was optimized. Phenolic compounds in the total pyrolytic products of sediments were categorized into 2 groups according to the type of functional group: alkyl-phenols (phenol compounds, PHs) and methoxy-phenols (lignin monomer compounds, LGs), which are further divided into o-, m- and p-compounds according to the position of the substituent on the benzene ring structure. Based on fine characterization of organic matter composition in the sediments of Yiheshariwusu Lake in Inner Mongolia, 21 phenolic compounds were identified and analyzed, including 9 PHs and 12 LGs (Table 1). Pyrolysis temperature is the main factor affecting the results of Py-GC/MS analysis of sediment organic matter fingerprinting. By discussing the effect of different pyrolysis temperatures on the distribution characteristics of the total pyrolytic products at 450℃, 550℃ and 650℃, it was determined that the relative concentration of lignin phenolic compounds increased significantly with increasing pyrolysis temperature. The ether bond (C—O—C) connecting the lignin skeleton structure benzene propane structural unit was further broken as the temperature was increased from 450℃ to 650℃. The relative concentration of phenolic compounds in the pyrolysis compounds reached the highest proportion (16.46%), while the proportion of aromatic hydrocarbons and aliphatic hydrocarbons increased by 3.98% and 10.26%, respectively. The natural macromolecules, which are not easily vaporized, are gradually cleaved into smaller ionic fragments of phenolic compounds under high pyrolysis temperature. As the cleavage temperature increases to 650℃, the flux of phenolic compounds into the chromatographic system increases and the gas chromatographic response is gradually enhanced, with the phenolic compounds reaching the highest ionic intensity response. At the same time, the unit peak area, shape and signal-to-noise ratio were all improved, which improved the accuracy of phenolic compound identification and analysis.

(2) Distribution characteristics of phenolic compounds in Yiheshariwusu Lake were discussed. According to AMS ¹⁴C age data, historical variation of total phenolic compounds, PHs and LGs in lake sediment are generally consistent since 6.7ka, showing the characteristics of high relative concentration of 6.7-4.0ka and low concentration since 4.0ka. The variation characteristics of o-PHs, m-PHs, and p-PHs are consistent with total PHs, yet the change characteristics of p-LGs and LGs are different, the relative concentration of p-LGs decreased significantly near 5.4ka and remained at a low level since 3.8ka (average relative concentration of 0.29%). Combined with the lithological characteristics of sediment cores, relative concentration of total phenolic compounds and PHs decreased significantly around 4.0ka, probably due to the change of sedimentary lithology from sand to clay with smaller grain sizes during 4.0-3.8ka, where compounds with smaller molecular weights were preserved in the sediments, and the relative concentration of aliphatic hydrocarbons (e.g., n-alkanes, n-alkenes) and aromatic hydrocarbons significantly increased.

(3) Environmental indication significance of phenolic compounds was studied. According to previous studies of free n-alkanes distribution proxies (e.g., ACL_23-33), higher relative concentration in lake sediments of p-PHs indicated major herbaceous source of lignin, however, significant differences in the mean relative concentration of p-PHs and LGs indicated significant microbial degradation of organic matter. The mean value of p-PHs/p-LGs for Yiheshariwusu Lake sediments was 16.41 (n=31, range of 4.36-37.31), showing an overall increasing trend since 6.7ka, reflecting a gradual increase in microbial activities. p-PHs/p-LGs showed a consistent trend and negative correlation with δ¹³C_27-33 (n=31, R=-0.77, p < 0.01), meanwhile, variation of p-PHs/p-LGs positively correlated with the trend of increasing pollen of Chenopodiaceae and Poaceae in Hulun Lake sediment (n=31, R=0.54, p < 0.01), and on a larger scale, p-PHs/p-LGs are consistent and positively correlated with the gradual increase in the standardized precipitation index since 6.7ka in the northern hemisphere mid-latitudes (n=31, R=0.62, p < 0.01) (Fig.E.1C).

The suitable pyrolysis temperature for Py-GC/MS analysis of phenolic compounds in the sediments of Yiheshariwusu Lake is 650℃. The value of degradation index p-PHs/p-LGs corresponds to the degree of lignin degradation, and the larger the value, the stronger the microbial degradation. Applying the p-PHs/p-LGs index to the sediment samples of Yiheshariwusu Lake, the result show that degradation index (p-PHs/p-LGs) and the carbon isotope of free n-alkanes δ¹³C_27-33 has solid correlation since 6.7ka, indirectly indicating the change of effective precipitation, as the climate turned wet generally since 6.7ka, with terrestrial higher plants dominant, humid climate and sufficient organic matter provided a suitable living environment and relatively stable nutrient source for microorganisms, and the degradation generally increased. Since effective precipitation decreased during 6.3-5.5ka and 4.1-3.6ka, the degradation of lignin by microorganisms was relatively weakened. The p-PHs/p-LGs index corresponds to the characteristics of effective precipitation in the Hulun Buir region, revealing the correlation between microbial degradation and humidity change in arid and semi-arid regions. These findings provide a scientific basis for exploring the response of terrestrial ecosystems to climate change in the northern marginal region of the East Asian monsoon.