| 摘要: |
| 墨脱地处喜马拉雅山东段，拥有中国最完整的垂直自然带谱，是研究植被、土壤随海拔和气候条件变化的理想场所，深入理解现代土壤理化性质与植被和气候要素的关系可为古气候重建提供重要依据。通过对喜马拉雅山东段南坡墨脱不同海拔（海拔跨度为800—3600 m）不同植被带（阔叶林带、混交林带和针叶林带）表土样品的粒度、磁化率、地球化学元素分析，探讨其理化性质组成垂直变化特征及其与温度、降水之间关系。结果表明：（1）墨脱地区不同海拔表土粒度以砂粒组分为主，约占53.2%，其次是粉砂、黏土。（2）在混交林带百分比频率磁化率与海拔关系明显，黏土对海拔及气候因子较为敏感。（3）表土地球化学组成以SiO2、Al2O3、Fe2O3为主，其次是MgO、CaO、Na2O、K2O，其主要成分在不同的海拔高度略有差异。不同海拔和植被带的元素对气候的响应敏感程度不同，可能与该地复杂的地形、植被等综合因素有关。（4）墨脱地区阔叶林带物理、生物风化较强，而针叶林带以化学风化为主，混交林带处于二者之间。 |
| 关键词: 喜马拉雅南坡 墨脱 表土 海拔 植被带 理化指标 |
| DOI：10.7515/JEE222085 |
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| 基金项目:第二次青藏高原综合科学考察研究（2019QZKK0101）；中国科学院战略性先导科技专项（XDB26020403）；黄土与第四纪地质国家重点实验室开放基金（SKLLQG1938） |
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| Physicochemical composition and climate response of surface sediments at different altitudes in Motuo on the southern slope of the Himalayas |
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XIE Mengping, SONG Yougui, LAN Minwen, ZHANG Mingyu, HAN Yixiao
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1. National Key Laboratory of Loess Science, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. Xi’an Institute for Innovative Earth Environment Research, Xi’an 710061, China
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| Abstract: |
| Background, aim, and scope Certain physicochemical indexes of topsoil are closely related to climatic factors including temperature, and precipitation. Understanding the relationship between modern topsoil properties and climatic factors is essential for quantitative paleoclimate reconstruction. Motuo located in the eastern Himalayas, exhibits a significant elevation gradient of over 7000 m from Nnamjagbarwa Peak (7782 m a.s.l.) to the Baxika (150 m a.s.l.). This region features a complete vertical zonation of vegetation, from alpine meadow to tropical forest, presenting an ideal place to investigate the relationships among vegetation, soil and climate conditions across altitudinal gradients. This study aims to explore the vertical variations in the physicochemical composition of topsoil and its relationship with temperature and precipitation. Materials and methods Twenty-seven topsoil samples were collected at 100 m intervals from 800 m to 3600 m a.s.l. along the southern slope of the Himalayas. Grain size, magnetic susceptibility and geochemical elements were measured to discuss the vertical variation characteristics of topsoil composition and their correlation with climatic factors. Results (1) The grain size of topsoil at different altitudes in Motuo is mainly composed of sand accounting for an averaged 53.2%, followed by silt and clay. (2) In the mixed forest zone, frequency dependent magnetic susceptibility (χfd%) shows a clear relationship with altitude, and clay is positively correlated with both altitude and climatic factors. (3) The oxides of topsoil in this area mainly consist of SiO2, Al2O3 and Fe2O3, followed by MgO, CaO, Na2O and K2O, with slight variations in the primary components at different altitudes. The sensitivity of elements to climate varies across different altitudes and vegetation zone, likely due to the region’s complex topography and vegetation. (4) Physical and biological weathering dominates in the broad-leaved forest zone of Motuo, while chemical weathering is more prominent in the coniferous forest zone, with the mixed forest zone falling in between. Discussion The formation of topsoil across the three vegetation zones is influenced by various factors, including parent material, vegetation, and climate. In the broad-leaved forest zone, physical weathering (precipitation, root wedging etc.) and pedogenesis dominate, resulting in finer grain size. The χfd% increases with altitude likely due to the high temperature and abundant precipitation in this zone, which facilitate the transformation of strong magnetic miners into weaker ones, particularly when the soil is oversaturated. Zirconium (Zr), primarily found in zircon, is depleted at lower altitudes by strong current erosion. Barium (Ba) is similarly reduced at low altitudes in this zone. In mixed forest zone, clay content is the lowest, indicating weaker physical weathering conditions than broad-leaved forest zone. The coarser grain size may result from the combined effects of topography and vegetation coverage. Magnetic susceptibility and organic matter show a positive correlation with altitude. Zr concentration is higher than that in the broad-leaved forest zone, likely resulting from decreased precipitation. In the coniferous zone, the clay content shows considerable fluctuations, with grain size generally becoming finer as altitude increases. This trend may be explained by intensified soil disintegration from seasonal freeze-thaw cycles. The χfd% values lack a clear trend or pattern, possibly due to soil erosion causing the migration of magnetic minerals or insufficient iron precipitation, which reduces the concentration of magnetic minerals in the soil. Additionally, a positive correlation is observed between altitude and organic matter content, with higher altitudes associated with greater organic matter accumulation. This may be attributed to lower microbial activity in colder conditions, which slows the decomposition and transformation of organic matter. Conclusions The variations in grain size, magnetic susceptibility, and geochemical elements differ across altitudes and vegetation zones, closely connected to the complex interplay of terrain, vegetation, and climate in Motuo. In the mixed forest, altitude has a significant impact on χfd%, and the clay component is particularly sensitive to changes in altitude, mean annual temperature, and precipitation. Zr shows a strong correlation with altitude and climate factors, making it a valuable indicator for assessing changes in atmospheric precipitation within specific altitude ranges. Recommendations and perspectives This study enhances our understanding of the relationships between the physicochemical properties of topsoil and climate conditions, offering valuable insights for paleoclimate reconstruction in Motuo. |
| Key words: southern slope of the Himalayas Motuo topsoil altitude difference vegetation zone physicochemical indexes |
