青藏高原北部冰前湖沉积记录的中晚全新世冰川活动

Glacial activity since the mid-Late Holocene reconstructed by the proglacial Lake Buruo Co, northern Tibetan Plateau

青藏高原的冰川为亚洲地区大多数人口提供了重要的水资源,研究青藏高原的冰川活动历史有助于预估未来气候变化条件下的冰川响应。本文通过对羌塘腹地布若错沉积物岩芯的粒度、元素以及总有机碳等指标进行分析,重建了该流域中晚全新世以来的冰川活动历史。结果表明,布若错流域在5.2~4.0 cal.ka B.P.期间,冰川退缩,气候相对温暖;然而,4.0 cal.ka B.P.以来环境逐渐变干,印度季风持续减弱;随着3.2 cal.ka B.P.左右大规模的冰川前进,布若错流域进入新冰期,并受到较强西风的影响,气候寒冷干旱;1.3 cal.ka B.P.以来,流域内冰川仍然维持着较大规模,然而湖面却略有升高,可能反映了西风带来了一定的降水。此外,该流域存在4次显著的冰川前进事件,它们分别发生在3.6~3.4 cal.ka B.P.、3.2~2.3 cal.ka B.P.、1.9~1.7 cal.ka B.P.和0.4~0.1 cal.ka B.P.。这4次冷期与青藏高原其他古气候记录对应较好,这种频繁发生的冷事件可能与太阳辐射的长期衰减以及太阳活动的周期性变化有关,热带辐合带的持续南退是季风持续减弱的重要原因;此外,中晚全新世北大西洋的气候变化通过增强的西风环流对该流域产生影响。

Glaciers on the Tibetan Plateau(TP) are an important freshwater resource for majorities of populations in Asian. Therefore, it is important to understand past variations in glacier activity on the TP to assess their possible response to ongoing climatic warming. Buruo Co(34°20'~34°28'N, 85°42'~85°49'E, 5170 m a.s.l.) is situated between the the Buruogangri Mountain and Zangsegangri Mountain in the "cold and dry core" of the TP. Buruo Co is a closed proglacial mountain lake with a current surface area of 87.5 km2 and catchment area of 550.5 km2. The maximum water depth exceeds 100 m. There are many glacier tongues near the lake shoreline and the lake is mainly fed by water from melting snow and glacier ice. Thus, the sediment in Buruo Co is very sensitive to the glacier activity. A 435 cm long core of Buruo Co(BRLC13-1) was obtained by piston corer, which was used for AMS14C dating and analysis of climate proxies including grain size, total organic carbon, total inorganic carbon(TIC), particle shapes of Quartz grains and the geochemical element content. Besides, a 65 cm long core(BRGC13-5) was obtained by gravity corer and used for 210Pb and 137Cs dating. Based on the CRS model, the age at 1 cm depth was -56 a B. P. while at the same depth the radiocarbon age is 3800 a B. P., indicating a carbon reservoir age of 3856 years. The age-depth frame was established by baysian model using 210Pb and 137Cs measurement and AMS14C data and the basal age of the BRLC13-1 is estimated to be 5216 cal.ka B. P. In this study, we analyzed the high resolution record and reconstructed the history of glacial activity since 5.2 cal.ka B. P. in Buruo Co catchment. We suppose that the increase in fine franction of sidements indicate the glacier advancce, and the TIC reflect the changes in lake level. The result shows that the retreating glaciers released large amounts of freshwater during 5.2~4.0 cal.ka B. P. During 4.0~1.3 cal.ka B. P., the Westerlies were gradually enhanced and the climate became colder and drier, in response, the glaciers advanced. Subsequently, during 1.3~0 cal.ka B. P., under relatively cold conditions, a large volume of glacial ice was maintained. Besides, according to the changes in fine franction of sediments, 4 episodes of glacial advance at Buruo Co catchment are detected:at 3.6~3.4 cal.ka B. P., 3.2~2.3 cal.ka B. P., 1.9~1.7 cal.ka B. P. and 0.4~0.1 cal.ka B. P. Which corresponds to Bond events in the North Atlantic and these cold events in Buruo Co were influenced by climatic oscillations in the North Atlantic, via the Westerlies. We also suppose that the summer solar insolation, solar activity and the migrations of the Intertropical Convergence Zone influence the climate on the north of TP.