黄河上游官亭盆地全新世山洪泥流盛行期及其与全球气候变化关系研究

A PREVAILING PERIOD OF FLASHFLOOD AND MUDFLOW IN THE GUANTING BASIN OF THE UPPER YELLOW RIVER AND ITS RELATIONSHIP WITH GLOBAL CLIMATE CHANGE

黄河上游青藏高原东北边缘的自然环境具有高度的敏感性和脆弱性,其对全球变化的响应受到国内外学术界的广泛关注。特别是关于官亭盆地喇家遗址及其周边区域的史前灾害与形成机制问题,国内外地学与考古学界存在激烈的学术争论。通过多年对官亭盆地黄河北岸古洪积扇前沿和第二级阶地及吕家沟、岗沟、马家沟等区域细致的野外考察,在该区域选取4个天然沉积剖面,对其进行高分辨率的系统性采样,并分析测定磁化率、粒度成分和砂级颗粒。结果表明:晚更新世晚期到全新世早期官亭盆地黄河北岸古洪积扇前沿和黄河第二级阶地区域沉积了风成黄土(L1-1和Lt),全新世中期形成了黑垆土古土壤(S0)。该区域沉积剖面结构普遍的特征是,全新世中期黑垆土古土壤被山洪泥流层和山洪砂土层分隔为多个层段,这是发生在OSL年龄3960~3650 a,对应14C年龄3850~3600 a B.P.期间的大规模暴雨山洪泥流灾害事件造成的结果。这期间官亭盆地发生了两场多次大地震,并且伴随短时段高强度暴雨诱发的大规模山洪泥流,其在古洪积扇前沿溢出沟槽,扩散到黄河第二级阶地沉积下来,淹没农田和聚落,以喇家遗址为代表的大型齐家文化聚落由此而毁灭。通过与全球气候变化对比分析,发现这个山洪泥流盛行期发生在全球气候变化的转折阶段。气候恶化造成青藏高原边缘大气环流不稳定,对流旺盛,极端性暴雨频发。在史前人类土地开发利用与多次大地震造成的坡面失稳背景条件之下,暴雨诱发大规模山洪和泥流、泥石流等灾害性地表过程,毁灭了喇家遗址的齐家文化聚落。这个研究结果对于深入理解黄河上游史前气候环境变化、自然灾害和人地关系演变具有重要科学价值。

The natural environment over the northeastern margin of the Tibetan Plateau in the upper Yellow River is highly sensitive and fragile because of the high elevation and deep incision of the river systems.The Holocene environmental evolution and its response to global change and human impact over this region have received widespread attention from scientists at home and abroad.In the Guanting Basin(35°49'~35°50'N,102°43'~102°50'E)along the Yellow River at the foot of the Lajishan and Jishishan Mountains,archaeological excavation has exposed the fearsome scene of pre-historical disasters that ruins a huge settlement of the Qijia Culture(4150~3850 a B.P.)of the Neolithic.However,due to a lack of overall investigation of the sediments,stratigraphy,pedogenensis,human disturbance and the surface processes,the causation and the mechanism of the prehistoric catastrophe are still unclear and thus open to debate and disagreement among the scholars in the field of earth sciences and archaeology.During our extensive field investigations during the last ten years,the complete and continuous pedo-stratigraphic profiles were identified at four sites on the second terrace of the Yellow River and the simple connected front edge of the palaeo-pluvial fans formed by the Lüjiagou,Ganggou and Majiagou gullies in the north part of the Guanting Basin.These include the Shanglajia profile(SLJ:35°51'52"N,102°48'20"E)about 4.5 m thick,the Lajiayizi profile(LJYZ-A:35°51'49"N,102°48'27"E;LJYZ-B:35°51'51"N,102°48'18"E;LJYZ-C:35°51'35"N,102°48'46"E)about 4.8 m thick,the Erfangcun profile(EFC:35°51'42"N,102°49'11"E)about 4.2 m thick and the Ganggou'an profile(GGA:35°51'58"N,102°48'31"E)about 6.0 m thick.After our detailed observation and survey in these profiles,high-resolution samples were taken every 5 cm down the GGA profile and every 4 cm down the other profiles from the ground surface continuously.In the laboratory,these soil and sediment samples were processes carefully and magnetic susceptibility and particle-size distribution were analyzed systematically.And especially,the non-dust fraction(particles>0.1 mm)were separated by wet-sieving and then examined under a binocular microscope for identification of pottery shards,charcoal and rock fragments brought in by flashflood and human activities since the Neolithic.The results show that eolian loess(L1-1 and Lt)were deposited and blanketed the front edge of the palaeo-pluvial fans and also the second terrace of the Yellow River during the Late Pleistocene to Early Holocene.And during the mid-Holocene Climatic Optimum,a thick and fertile palaeosol(S0)of Heilusol was formed over the flat ground.A common pedo-stratigraphic characteristic is that this palaeosol(S0)are split into several sub-layers by the inserted flashflood and mudflow deposits.And the famous Lajia Ruins was built on the ground consisting of the lower sub-layer(S0-lower)of the palaeosol(S0)and buried directly by the flashflood and mudflow deposits(RCs and FFD).These means that the huge settlement of the Qijia Culture was destroyed by the enormous flashfloods and mudflows overflowed the tributary gullies,which came from the hills of the Tertiary Red Formation and also form the Lajishan Mountains and the Jishishan Mountains of the old metamorphic rocks.Our OSL dating indicates that these large-scale catastrophic surface processes induced by intensive stormrains occurred during 3960~3650 a,equivalent to14C age 3850~3600 a B.P.More in-depth investigation discovered that the ground over the Lajia Ruins were badly broken by earthquake fissures.Our research shows that there were two large earthquakes occurred in the Guanting Basin at 3850 a B.P.and 3600 a B.P.respectively.It was the first earthquakes accompanied by stormrains,flashfloods and mudflows from the tributary gullies destroyed the prehistorical Lajia settlement and killed the people at home.With consideration of the Holocene climate change,it is found that this prehistorical catastrophe occurred just during the climatic decline since 4200 a B.P.that defined by various proxy-data over the world.Apart from the great earthquakes,the prevailing period of intensive stormrains,flashfloods and mudflows are obviously caused by the highly variable and unstable atmospheric circulation at the northeast margin of the Tibetan Plateau during the global climate decline.And also,extensive soil erosion over the gully and hilly regions induced by prehistoric human land reclamation and vegetation clearance over a thousand years may have intensified the flashfloods and mudflows to certain extend.The disastrous period was over at 3600 a B.P.and the palaeosol formation(S0-upper)was recovered over the front edge of the palaeo-pluvial fans and the second terrace of the Yellow River.Then,the people of Xindian/Kayue Cultures(3600~2600 a B.P.)reoccupied the land in these areas and developed mixed agriculture of stockbreeding with some millets cultivation.These results are of great importance in understanding prehistoric environment evolution,surface processes,natural disasters and human-land relationship over the upper Yellow River at the northeast margin of the Tibetan Plateau.