祁连山疏勒河源多年冻土区沼泽草甸土释光年代学研究

Optically stimulated luminescence chronology of marsh meadow soils in the permafrost region of the Shule River headwaters

关键词:  祁连山  沼泽草甸土  多年冻土  光释光测年

Key words:  Qilian Mountains  marsh meadow soil  permafrost  OSL dating

摘要:

沼泽化草甸是青藏高原多年冻土区重要的自然湿地生态系统，也是高原过去高寒环境变化的良好记录载体。已有研究对沼泽草甸生态系统土壤的形成时代和演变过程关注较少，系统的年代学工作开展也较为缺乏。选取青海省祁连山疏勒河源多年冻土区的5个样地（SLZ1、SLZ2、SLZ3、SLZ4、SLZ5），采用粗颗粒（63—90 μm）石英单片再生剂量法测年，建立高寒沼泽草甸土壤年龄框架；结合土壤粒度、有机质特征、区域古气候记录探讨祁连山疏勒河源多年冻土区沼泽草甸土发育过程。结果表明：（1）高盖度沼泽草甸土的光释光测年结果集中于小冰期（0.6—0.3 ka），平均堆积速率可达4.47 mm∙a⁻¹；（2）中低盖度干涸退化的土壤和沼泽草甸土壤下部发育有棕褐色土壤，年代集中在全新世暖湿期（约7—2 ka）；（3）沼泽草甸土壤母质以风尘为主，形成发育模式为风尘加积型。

Abstract:

Background, aim, and scope The Qilian Mountains, located at the northeastern margin of the Qinghai-Xizang Plateau, are crucial for maintaining ecological stability in the Hexi Corridor and hosting the plateau’s major permafrost zone. Marsh meadows are important wetland ecosystems within these permafrost regions, providing vital services such as water retention, carbon storage, and habitat for alpine biodiversity. Despite their ecological importance, the formation processes and environmental controls of marsh meadow soils in permafrost areas remain poorly understood. This study aims to investigate the formation mechanisms and environmental controls of marsh meadow soils. Materials and methods Two representative marsh meadow soil sites (SLZ1 and SLZ5) were investigated in the permafrost region of the Shule River headwaters (Suli, Tianjun County, Qilian Mountains), from which 8 OSL samples were collected. Given that marsh meadow soils are highly sensitive to local microtopography, some areas have transitioned to alpine meadow vegetation. To capture this variability, three adjacent alpine meadow sites (SLZ2, SLZ3, and SLZ4) were also sampled to provide a comparison with marsh meadow soils, and 6 OSL samples were collected. In total, 14 OSL samples were obtained across the five sites. Grain size and total organic carbon (TOC) were analyzed, and 14 samples were dated using optically stimulated luminescence (OSL) with the single-aliquot regenerative-dose (SAR) protocol. Results The mean grain sizes of SLZ soils ranged from 24.62 to 57.27 μm. Marsh meadow soils showed finer peaks (30—67 μm) compared with other soils (59—88 μm). In marsh meadow soils, clay accounted for 6%—10%, silt 50%—73%, and sand 20%—45%, whereas in other soils, clay was around 10%, silt 40%—55%, and sand 30%—54%; all samples were identified as silty soils. TOC contents were higher in marsh meadow soils, reaching 2.0%—2.5% in the upper 20 cm and decreasing to around 0.5% below 80 cm. Quartz OSL ages ranged from 7.4 to 0.3 ka. High-coverage marsh meadow soils mainly developed during the Little Ice Age (0.6—0.3 ka). In SLZ1, ages at 8 and 60 cm were (0.3±0.03) ka, indicating rapid accumulation of the upper 60 cm. Other soils formed during the Middle Holocene (around 7—2 ka). Discussion An accumulation hiatus around 6.1 ka, spanning approximately 6.4—0.3 ka, was observed at multiple SLZ marsh meadow soil sites. The upper strata of SLZ1 and SLZ5 formed during the Little Ice Age, when cooler conditions enhanced permafrost thickness, reduced infiltration, and increased soil water retention, promoting marsh meadow development. In contrast, the lower strata of SLZ1 and SLZ5, along with soils at SLZ2 and SLZ3, developed during the warmer Middle Holocene, when thinner permafrost and stronger vertical water infiltration created less favorable conditions for marsh meadow formation. Grain size analysis shows that marsh meadow soils closely resemble atmospheric dust deposits from Xining and Duoba, highlight the substantial contribution of eolian dust to marsh meadow soil development. Large amounts of dust are released from the relatively low-elevation Qaidam Basin, providing abundant material for marsh meadow soil formation. As high-elevation soils, marsh meadow soils are particularly efficient at capturing and accumulating eolian dust, resulting in faster material accumulation rates. Under the combined influence of local topography, alpine vegetation, and the cooler climate of the Little Ice Age, marsh meadow soils experienced rapid deposition during the late Holocene. Conclusions (1) Marsh meadow soils with high vegetation cover primarily developed during the Little Ice Age, with an average accumulation rate of 4.47 mm·a⁻¹. (2) Soils with low vegetation cover and underlying brown soils mainly formed during the Middle Holocene. (3) Eolian dust deposition played a dominant role in marsh meadow soil formation. Recommendations and perspectives The development and persistence of marsh meadow ecosystems are strongly controlled by the interactions among climate, topography, vegetation, and hydrology. Local environmental conditions play a particularly critical role in maintaining these ecosystems. From a management and conservation perspective, monitoring soil moisture, permafrost stability, and sediment fluxes could provide early indicators of ecosystem vulnerability under future climate change. In addition, it is necessary to acquire additional samples to better constrain the development patterns of plateau marsh meadow soils.