Physical Geography, Institute of Geography, Friedrich Schiller University Jena, Jena, Germany.
Physical Geography, Institute of Geography and Geology, University of Greifswald, Germany.
Sci Total Environ. 2020 May 10;716:137045. doi: 10.1016/j.scitotenv.2020.137045. Epub 2020 Jan 31.
The hydrogen isotopic composition of leaf wax-derived n-alkanes (δH) is a widely applied proxy for (paleo)climatic changes. It has been suggested that the coupling with the oxygen isotopic composition of hemicellulose-derived sugars (δO) - an approach dubbed 'paleohygrometer' - might allow more robust and quantitative (paleo)hydrological reconstructions. However, the paleohygrometer remains to be evaluated and tested regionally. In this study, topsoil samples from South Africa, covering extensive environmental gradients, are analysed. δH correlates significantly with the isotopic composition of precipitation (δH), whereas no significant correlation exists between δO and δO. The apparent fractionation (ε) is the difference between δH and δH (ε) and δO and δO (ε), respectively, and integrates i) isotopic enrichment due to soil water evaporation, ii) leaf (and xylem) water transpiration and iii) biosynthetic fractionation. We find no correlation of ε nor for ε with temperature, and no correlation of ε with potential evapotranspiration and an aridity index. By contrast, ε correlates significantly with both potential evapotranspiration and the aridity index. This highlights the strong effect of evapotranspirative enrichment on δO. In study areas without plant predominance using Crassulacean Acid Metabolism (CAM), coupling δO and δH enables to reconstruct δH and δO with an offset of ΔδH = 6 ± 27‰ and ΔδO = 0.8 ± 3.7‰, respectively, as well as relative humidity (RH) with an offset of ΔRH = 6 ± 17%. The paleohygrometer does, however, not work well for our study areas where CAM plants prevail (reconstructed δO, δH and RH are off by 3.1‰, 27.2‰ and 31.7%). This probably reflects plant-specific (phenological) adaptations and/or post-photosynthetic exchange reactions related to CAM metabolism. Overall, our findings corroborate that δH and δO are valuable proxies, and the paleohygrometer is a promising approach for paleoclimate reconstructions in southern Africa.
叶蜡烷烃的氢同位素组成（δH）是广泛应用于气候变化的代用指标。有人提出，与半纤维素衍生糖的氧同位素组成（δO）的耦合——这种方法被称为“古湿度计”——可能允许更稳健和定量的（古）水文重建。然而，古湿度计仍有待在区域内进行评估和测试。在这项研究中，分析了南非的表层土壤样本，这些样本覆盖了广泛的环境梯度。δH 与降水的同位素组成（δH）显著相关，而 δO 与 δO 之间不存在显著相关性。明显分馏（ε）是 δH 与 δH（ε）和 δO 与 δO（ε）之间的差异，分别整合了 i）由于土壤水蒸发导致的同位素富集，ii）叶片（和木质部）水蒸腾和 iii）生物合成分馏。我们发现 ε 与温度之间没有相关性，也没有 ε 与潜在蒸散和干旱指数之间的相关性。相比之下，ε 与潜在蒸散和干旱指数显著相关。这突出了蒸散富集对 δO 的强烈影响。在没有以景天酸代谢（CAM）为主的植物的研究区域中，耦合 δO 和 δH 可以重建 δH 和 δO，其偏移量分别为 ΔδH = 6 ± 27‰和 ΔδO = 0.8 ± 3.7‰，以及相对湿度（RH），其偏移量为 ΔRH = 6 ± 17%。然而，古湿度计在我们以 CAM 植物为主的研究区域中效果不佳（重建的 δO、δH 和 RH 偏差分别为 3.1‰、27.2‰和 31.7%）。这可能反映了植物特有的（物候）适应和/或与 CAM 代谢相关的光合作用后交换反应。总的来说，我们的研究结果证实了 δH 和 δO 是有价值的代用指标，古湿度计是南部非洲古气候重建的一种有前途的方法。
