白洋淀地区晚冰期以来的气候和环境演变

华北平原晚冰期以来气候环境演变研究对该地区社会发展、灾害风险评估和科学应对未来全球增温背景下极端降水和洪涝事件具有重要意义。本文以华北平原中部白洋淀地区高阳剖面（BG-2019）为研究对象,通过高精度AMS¹⁴C、OSL定年技术和高分辨率孢粉组合、粒度组分分析,恢复和重建了白洋淀地区晚冰期以来（距今13710 a—今）区域植被演替和气候环境变化历史。结果显示：BG-2019剖面在距今10270~13710 a和距今4630~5400 a发育湖相沉积,距今3470~3700 a发育沼泽相沉积;距今7130~8000 a发育河流—入湖三角洲相沉积,距今3700~4630 a和距今3230~3470 a发育河流相沉积;距今8000~10270 a和距今5400~7130 a存在明显的沉积间断/地层缺失;表明采样剖面所在位置缺乏连续的湖相地层。晚冰期白洋淀为局地小湖沼;中全新世湖沼较发育、范围广,但也不是连续广袤的湖相沉积;晚全新世湖泊范围收缩。晚冰期和全新世白洋淀流域植被景观存在显著差异;晚冰期气候寒冷干燥,平原发育以蒿属、藜亚科、禾本科和菊科等为主的草地,周围山地森林覆盖度低;中全新世气候温暖湿润,平原大部仍发育以蒿属、藜亚科和禾本科为主的草地,湖区水蕨和水生植物繁盛,周围山地生长松属、栎属为主的针阔混交林,森林覆盖度增高;晚全新世气候温和偏干,平原仍是以蒿属、藜亚科和禾本科等为主的草地,西部山地生长以松属为主的针阔混交林,森林覆盖度较高。

Climatic and environmental evolution of the Baiyangdian area since the Lateglacial

The North China Plain is a typical diluvial-alluvial plain with an unstable depositional environment and poor sedimentary continuity. However, the effects of changes in sedimentary phases or hiatuses were not considered in previous studies of the sedimentary record of the North China Plain, which limits our understanding of the environmental and climatic evolution of the region during the historical period. Baiyangdian, the largest freshwater lake in the plain, is a potentially valuable archive of regional paleoenvironmental information since the late Pleistocene. We investigated a sedimentary profile (BG-2019; 38°46'2.55"N, 115°49'0.41"E; 7 m a.s.l) in Gaoyang County, in the southwestern part of Baiyangdian Lake region; the profile is ~6 m long. The results of AMS ¹⁴C and OSL dating indicate that profile BG-2019 has two intervals of lacustrine facies, two intervals of fluvial facies, one interval of swamp facies, one interval of fluvial-lacustrine delta facies, and two intervals of non-deposition or stratigraphic lacunae. The ages of the various facies are as follows. Lacustrine facies: 10270-13710 cal a BP and 4630-5400 cal a BP; swamp facies: 3470-3700 cal a BP; fluvial-lacustrine delta facies: 7130-8000 cal a BP; fluvial facies: 3700-4630 cal a BP and 3230-3470 cal a BP;; intervals of interrupted deposition: 8000-10270 cal a BP and 5400-7130 cal a BP. Therefore, no continuous lacustrine facies exists in the Baiyangdian region since the Lateglacial. Baiyangdian was a small lake during the Lateglacial, but was more extensive during the middle Holocene, although with discontinuous lacustrine facies. During the late Holocene the lake began to shrink under the influence of climate change and human activity. No other than human activity around the modern Baiyangdian Lake was reserved. Pollen analysis shows that the vegetation landscape of the Baiyangdian Basin differed substantially between the Lateglacial and the middle Holocene. During the Lateglacial, the climate was cold and dry, and vegetation dominated by Artemisia, Chenopodioideae, Poaceae and Asteraceae developed in the plain, while the western mountains had a limited forest cover. During the middle Holocene, when the climate was warm and humid, grassland vegetation dominated by Artemisia, Chenopodioideae and Poaceae developed in the plain; Ceratopteris and aquatic plants flourished in and around the lake; and coniferous-broadleaved mixed forest dominated by Pinus and Quercus developed in the western mountains, with an increased forest cover. During the middle-late Holocene, when the climate was relatively mild and dry, grassland vegetation dominated by Artemisia, Chenopodioideae and Poaceae persisted in the plain; and coniferous-broadleaved mixed forest dominated by Pinus developed in the western mountains, with a high forest cover age.