新仙女木期黄河晋陕峡谷古风成沙层年代及其物质来源

Age and provenance of Younger Dryas paleo-aeolian sand layers in the Jin-Shaan Gorges of the Yellow River

通过对黄河晋陕峡谷壶口至龙门段开展广泛细致的野外考察,发现了典型的晚更新世以来黄土—土壤层夹古风成沙层剖面—北桑峪剖面(BSY)。结合不同类型沉积物的野外宏观特征、磁化率、粒度成分、石英颗粒表面特征和地球化学元素的对比分析,确定黄河晋陕峡谷壶口至龙门段BSY剖面中所夹古风成沙层的性质为中沙质细沙,是在干旱多风环境下形成的沙质沉积物。基于光释光(OSL)测年数据,确定其沉积年代为12.5-11.6 ka,记录了黄河中游发生在末次冰消期的极端干旱事件,其与相邻区域沙漠/黄土过渡带中湖沼沉积和风成黄土—古土壤剖面记录的新仙女木事件相对应。该剖面中古风成沙沉积物的物质来源,主要来自黄河晋陕峡谷的古河床相沙层物质。这是因为该河段处于干旱半干旱季风气候区,在新仙女木时期,黄河晋陕峡谷处于极端干冷的环境之中,冬季风强盛、夏季风衰弱,黄河水位下降明显,河漫滩和江心洲将大面积出露,大量的沙物质在风力作用下向岸边输移,成为河谷两侧缓坡台地上古风成沙沉积层出现的主要沙源地。这个研究成果对于进一步揭示黄河中游流域极端干旱事件发生的时间性规律及其与季风气候变化的关系,具有重要的科学意义。

Fieldwork investigations were carried out in the Hukou-Longmen reach in the Jin- Shaan （Shanxi- Shaanxi） Gorges of the Yellow River. Paleo- aeolian sand layers were found interbedded in the Late Pleistocene and Holocene loess-soil profile--Beisangyu profile （BSY）. Analytical results, including field investigation, magnetic susceptibility, grain-size distribution, surface textures of quartz sand, and geochemical elements, indicate that the paleo-aeolian sands are medium-sized sandy fine sands, which were formed in a dry and windy climate. And the extreme drought events recorded by paleo-aeolian sand layers were dated to 12.5-11.6 ka, with optically stimulated luminescence （OSL） dating method. The extreme drought event coincided with the Younger Dryas that was recorded by the lacustrine sediment profiles and the aeolian loess-soil profiles in the desert/loess transitional zone in the adjacent region. The paleo-aeolian sands were sourced mainly from ancient riverbed sands in the Jin- Shaan Gorges of the Yellow River. The Jin-Shaan Gorges is situated in a semi-arid zone with a temperate continental monsoonal climate. The region responds sensitively to global change. During the Younger Dryas period, the region is in an extremely dry and cold environment, with strong winter monsoon and weak summer monsoon. The water level of the Yellow River will decline obviously. Consequently, floodplains and channel bars in the river are exposed in a broad area, and a large quantity of sand material drifts to the banks of the river under the force of the wind, which becomes the main source of the paleo-aeolian sand layer in the platforms on the gentle slopes on both sides of the river valley. These results would be of great significance in further understanding the temporal regularities of the extreme drought event and the relations between extreme drought event and monsoonal climate change.