汉江辽瓦店全新世黄土-古土壤序列风化过程及古洪水事件记录

对汉江上游谷地包含古洪水滞流沉积层的辽瓦店(LWD)全新世黄土-古土壤剖面的常量元素含量、粘粒含量、磁化率和Rb/Sr比值等指标进行了对比分析。结果发现:在LWD剖面的风化成壤过程中,元素Na、Mg和Al表现为较强烈的迁移淋失,虽然元素K、Fe和Ca有一定程度的迁移淋失,但比较微弱,呈现NaMgAlKFeCaSi的迁移序列;磁化率、Rb/Sr、CIA值、钾纳比(K2O/N2O)和淋溶系数的分布随地层的变化而同步变化。上述变化指示古土壤S0形成时期,气候温暖湿润,成壤作用强烈,马兰黄土L1堆积时期气候寒冷干旱,风化成壤作用最弱,黄土L0堆积时期气候有所好转,风化成壤作用强于马兰黄土堆积时期,但弱于古土壤形成时期。古土壤S0形成的全新世中期可能出现过一次较为暖湿的次级环境变化,在剖面中上部夹有一层古洪水滞流沉积层(SWD),呈灰白色(10YR6/1),为十分均匀的细粉砂,与上下相邻地层呈突变接触关系且界线清晰,向垂直河岸方向快速尖灭,是一次剧烈的气候突变事件的产物。

WEATHERING INTENSITY VARIATIONS IN A HOLOCENE LOESS-PALEOSOL PROFILE FROM UPPER-HANJIANG CATCHMENT IN CENTRAL CHINA

The Holocene sediment profile was surveyed in detail and sampled systematically at the Liaowadian site on the first river terrace in the upper Hanjiang valley. The stratigraphic and sedimentological characteristics, pedogenic modification characteristics and the profile structure were investigated in the field. Magnetic susceptibility, particle-size distribution, geochemical characteristics and optical simulated luminescence (OSL) age were analyzed in laboratory. Based on the stratigraphic correlation and OSL dating, the chronologic framework of the LWD profile was established as follows: Malan loess L₁ was accumulated in the last glacial period; the transitional loess L_t was accumulated in the early Holocene (11 500 ~ 8 500 a B.P.);the palaeosols (S₀) was formed in the middle Holocene(8 500 ~ 3 100 a B.P.);Holocene loess L₀ was accumulated in the late Holocene (3 100 a B.P. so far). A detailed study comparing the contents of major elements, content of clay, magnetic susceptibility, Rb/Sr ratio, CIA, (K₂O/N₂O), (CaO+K₂O+N₂O)/Al₂O₃ was conducted in this paper to explore the environmental features in the Holocene. The analytical results show that, Na, Mg and Al are relatively intensive leached, K, Fe and Ca are leached in a certain extent, and the element mobility and migration ability of the major elements are ranked in the following order: Na>Mg>Al>K>Fe>Ca>Si. The distributions of content of clay, magnetic susceptibility, Rb/Sr ratio, CIA, (K₂O/N₂O), (CaO+K₂O+N₂O)/Al₂O₃ imply that during the palaeosols (S₀) formation, chemical weathering was intense, and the climate was warmer and wetter, the leaching and migrating was intense, the stage of Malan loess L₁ accumulation, weathering intensity was weakened, and the climate turned to be relatively dry, the leaching and migrating was not conspicuous. During the stage of Holocene loess L₀ accumulation, the climate turned to be improved, the weathering intensity was between those during palaeosols (S₀) formation and Malan loess L₁ accumulation, the leaching and migrating was weaker. The CIA, (K₂O/N₂O) and (CaO+K₂O+N₂O)/Al₂O₃ of 205-165 cm layer during palaeosols (S₀) formation occurred significant changes: the CIA and(K₂O/N₂O) have an obvious peak area, and the(CaO+K₂O+N₂O)/Al₂O₃ have an obvious low ebb area, which indicate the weathering and pedogenic modification was intense and there may be a secondary environmental change that the climate became relatively warm and wet. The palaeoflood slackwater deposits (SWD) was lied in the middle-upper of the profile, presenting gray-white(10 YR6/1), homogeneous texture, loose, very uniform fine slit, the stratigraphic boundary of its upper and lower adjacent showed a mutation relationship and clear boundaries. To direction of vertical bank of river tip off quickly, the palaeoflood SWD samples was OSL dated to 1 810-1 710 a B.P. (AD 200-300), which is equal to the Eastern Han dynasty to Wei Jin era (AD 25-316), indicating that a dramatic climate change event for the upper-Hangjiang catchment occurred in the end of Eastern Han dynasty during the late Holocene, which caused catastrophic floods in the upper-Hangjiang catchment.