现代植物叶蜡氢和纤维素氧同位素耦合的相对湿度重建方法验证

Validation of the relative humidity reconstruction method of coupled leaf wax hydrogen with cellulose oxygen isotopes by using modern plants

关键词:  相对湿度  叶水  叶蜡  α-纤维素

Key words:  relative humidity  leaf water  leaf wax  α-cellulose

摘要:

耦合叶蜡正构烷烃的δ²H值（δ²H_wax）与α-纤维素的δ¹⁸O值（δ¹⁸O_cell）定量重建相对湿度（RH）是一种新兴的研究方法，已被广泛用于定量重建地质历史时期的RH值。然而，部分针对现代植物和土壤的调查表明，该方法需更多验证研究进行校正。沿太白山海拔横断面，采集不同海拔的典型植物叶片，从中抽提叶水并测量其δ²H和δ¹⁸O，提取正构烷烃和α-纤维素并测量δ²H_wax和δ¹⁸O_cell。通过耦合δ²H_wax与δ¹⁸O _cell重建RH及植物叶水、源水的δ²H和δ¹⁸O，并将重建值与实测值进行相关分析。为提高模型精度，投入不同方法产生的参数进行运算并对比。结果表明：（1）δ²H_wax—δ¹⁸O_cell耦合方法能有效重建RH及叶水、植物源水的δ²H和δ¹⁸O。（2）生物合成分馏系数的取值对重建结果存在影响，更准确的取值能提升重建结果与实测值的相关性。（3）叶水δ²H和δ¹⁸O重建值与实测值相关性较高，源水δ²H和δ¹⁸O及RH的重建值与实测值相关性偏低。δ²H_wax—δ¹⁸O_cell耦合方法还需进一步完善和优化。

Abstract:

Background, aim, and scope The hydrogen isotope composition of leaf wax n-alkanes (δ²H_wax) exhibits a strong correlation with the hydrogen isotopic composition of precipitation, making it a valuable proxy for reconstructing past hydrological, climatic, and elevation changes. Similarly, the oxygen isotope composition of α-cellulose (δ¹⁸O_cell), primarily archived in tree rings, serves as an indicator of monsoon intensity and temperature variations. Recent studies proposed a coupling method utilizing δ²H_wax and δ¹⁸O_cell to reconstruct relative humidity (RH) in paleoenvironments. Although promising, the robustness and accuracy of this method require validation using modern plant archives. While successful under controlled climate experiments, field applications have shown inconsistent results. This study aims to test the effectiveness of the δ²H_wax—δ¹⁸O_cell coupling method in reconstructing RH across an altitudinal gradient on the Chinese Loess Plateau (CLP) using modern vegetation. Materials and methods Leaf samples from representative plant species across various elevations on the CLP were collected for the extraction of leaf water, α-cellulose, and n-alkanes, and the δ²H and δ¹⁸O values of these components were measured. RH was reconstructed using four methods: method 1 employed average biosynthetic fractionation factors for the biosynthetic fractionation factors of hydrogen isotope (ε_bio²) and the biosynthetic fractionation factors of oxygen isotope (ε_bio¹⁸) across the entire year; method 2 used seasonal average values for ε_bio² and ε_bio¹⁸; method 3 applied biosynthetic fractionation values fitted for ε_bio² and ε_bio¹⁸ along the altitudinal gradient; and method 4 substituted measured δ²H and δ¹⁸O values of leaf water into an empirical equation to reconstruct RH, serving as a benchmark for evaluating the performance of the other three methods. The reconstructed values of leaf water and source water isotopes, as well as RH, were then compared against instrumental observations to assess the accuracy and reliability of the coupling approach. Results Method 3 yielded the strongest correlation between reconstructed and measured leaf water isotope values (δ¹⁸O: n=37, r=0.71, P＜0.05; δ²H: n=37, r=0.86, P＜0.05). Method 2 showed significant correlation between reconstructed and actual source water isotope values (δ¹⁸O: n=33, r=0.64, P＜0.05; δ²H: n=33, r=0.64, P＜0.05). However, RH reconstructions using method 3 showed weaker correlations (annual RH: n=7, r=0.41, P=0.357; multi-year RH: n=9, r=0.03, P=0.931). Discussion The leaf water reconstructions demonstrated a strong agreement with observed values, while RH reconstructions remain less accurate and require further refinement. Potential sources of uncertainty include: (1) n-alkane hydrogen may not solely derive from leaf water; (2) spatial heterogeneity in leaf water isotopes; (3) partial recording of isotopic signals due to plant phenology and growth timing; (4) contributions of both precipitation and soil water to plant uptake, particularly in arid zones; (5) isotopic fractionation during root water uptake; and (6) inaccuracies introduced by model assumptions such as equilibrium vapor pressure inside and outside leaves. Additionally, canopy structure may bias RH recording due to differential microclimate exposure. Conclusions The δ²H_wax—δ¹⁸O_cell coupling method shows promise for reconstructing RH on the CLP, particularly in estimating δ²H and δ¹⁸O values of leaf and source waters. The accuracy of reconstructions depends heavily on the precision of biosynthetic fractionation values. RH estimates still exhibit relatively low correlation with measured data. Recommendations and perspectives Further research is necessary to enhance the accuracy and applicability of this method. Future validation should involve a broader range of plant functional types, climatic settings, and high-resolution temporal sampling to account for seasonal dynamics and plant water sources.