Incorrect when uncorrected: Reconstructing vegetation history using n-alkane biomarkers in loess-paleosol sequences - A case study from the Saxonian loess region, Germany
Incorrect when uncorrected: Reconstructing vegetation history using n-alkane biomarkers in loess-paleosol sequences - A case study from the Saxonian loess region, Germany
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DOI：
10.1016/j.quaint.2012.01.023
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发表时间：
2013-05
影响因子：
2.2
通讯作者：
M. Zech;Tobias Krause;Sascha Meszner;D. Faust
中科院分区：
文献类型：
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作者：
M. Zech;Tobias Krause;Sascha Meszner;D. Faust
本研究旨在通过研究两个黄土-古土壤序列(Gleina和Rottewitz LPSs)中的沉积正构烷烃作为植物叶蜡来源的生物标志物，为重建萨克逊黄土地区的古环境做出贡献。与黄土层相比，魏氏古土壤以正构烷烃浓度极大值为特征。当这与高的奇偶优势(OEPS)相吻合时，这不仅表明增加了正构烷烃的输入和/或产生，而且还增加了正构烷烃在土壤中的保存，即在土壤淹水的土壤中。相比之下，埋藏的菊花土表现出低的OEPS，表明在好氧土壤条件下加速降解。截断的间冰期古土壤中正构烷烃含量很低。这反映了以前富含正构烷烃的Eemian表层土壤的去除，低正构烷烃输入到BT-SD层(Eemian luvisol的底土层)和/或强烈的土壤有机质矿化。4个调查的正构烷烃比在魏氏时期表现出明显的变化，根据乔木/灌木(以nC27或nC29为主)和草(以nC31或nC33为主)正烷烃的不同贡献，试探性地推断植被变化。然而，使用OEP作为降解的指标，并考虑到正构烷烃比例容易产生降解影响，上述解释很可能是不正确的。降解可以解释正构烷烃比的大部分变异性。应用基于现代正构烷烃参考数据集的端元模型，可以半定量地估计树木/灌木与草衍生的正构烷烃的贡献。这些模拟结果表明，Gleina和Rottewitz LPS都是在盛行草原下形成的。乔木/灌木的次要贡献可能只出现在罗特维茨的Gleinaer土壤复合体、Gleina LPS的棕色Gley soll顶部和全新世土壤中。未经校正的正构烷烃比推断的魏氏植被变化并未得到证实。总体而言，正构烷烃结果支持Gleina和Rottewitz LPS区的Gleinaer土壤杂岩不是完全相同的观点，而是年代地层学上的等值。据推测，更为有利的区域气候条件出现在西南部裸露的罗特维茨地区，使乔木/灌木植被在魏克斯利山脉的有利时期得以生长。正构烷烃生物标志物方法的一个主要缺点是只能重建落叶林地到草地的植被变化，反之亦然，因为针叶树的正构烷烃浓度低两到三个数量级。
This study aims at contributing to the reconstruction of the paleoenvironment of the Saxonian loess region by investigating sedimentary n-alkanes as plant leaf-wax derived biomarkers in two loess-paleosol sequences (LPSs), namely the Gleina and the Rottewitz LPSs. The Weichselian paleosols are characterised by n-alkane concentration maxima when compared to the loess layers. When this coincides with high odd-over-even predominances (OEPs), this points not only to increased n-alkane input and/or production but also to increased n-alkane preservation in the soils, namely in gleysols with water-logged edaphic conditions. By contrast, the buried cambisols reveal low OEPs, indicating accelerated degradation under aerobic edaphic conditions. The n-alkane concentrations in the truncated Eemian Interglacial paleosol are very low. This reflects the removal of the formerly n-alkane-enriched Eemian topsoil, low n-alkane input into the Bt-Sd-horizon (subsoil horizon of the Eemian luvisol) and/or strong soil organic matter mineralization. Four investigated n-alkane ratios show distinct variations during the Weichselian, tempting inferences of vegetation changes in terms of varying contributions of tree/shrub-derived (dominated by nC27or nC29) versus grass-derived (dominated by nC31or nC33) n-alkanes. However, using the OEP as a proxy for degradation and considering that n-alkane ratios are prone to degradation effects, the above interpretation is very likely incorrect. Degradation can account for most variability of the n-alkane ratios. Applying an endmember model based on a modern n-alkane reference dataset allows estimating the contribution of tree/shrub- versus grass-derived n-alkanes semi-quantitatively. These modelling results suggest that both the Gleina and the Rottewitz LPSs were formed under prevailing grassland. Minor contributions of trees/shrubs are only likely for the Gleinaer Soil Complex in Rottewitz LPS, for the top of a Brown Gleysol in Gleina LPS and the Holocene soil. The Weichselian vegetation changes as inferred from the uncorrected n-alkane ratios are not confirmed. Overall, the n-alkane results support the idea that the Gleinaer Soil Complex in Gleina and Rottewitz LPS, respectively, are not identical but rather are chronostratigraphic equivalents. More favourable regional climatic conditions presumably occurred in the southwesterly exposed Rottewitz locality and allowed tree/shrub vegetation to grow during favourable periods of the Weichselian. A major shortcoming of the n-alkane biomarker method is that only vegetation changes of deciduous woodland to grassland and vice versa can be reconstructed, because coniferous trees exhibit, by two to three orders of magnitude, lower n-alkane concentrations.