山东大黑山岛北庄黄土沉积特征及其环境演变

摘要

The loess study plays critical roles in the paleoclimatic research.The Chinese loess deposited in the East Asia has great significance in revealing climatic and paleoenvironmental changes during the Quaternary due to its unique geographical location and environmental conditions.Different with Loess Plateau,the island loess deposited in coastal area of Shandong Province,East China,records the climate and sea level changes on orbital and millennial scales.Although the island loess study could reveal regional paleoenvironmental changes,poor information could be found.The study chose the Beizhuang loess section (BZ:37°57′56″N,120°37′15″E;26m a.s.l.) in Daheishan Island,which is a typical island loess of the Miaodao Islands in Shandong Province.4 OSL samples at 50cm,100cm,150cm and 200cm deep were collected,and a 16.1-m deep core was also obtained from BZ section.Back to the lab,792 grain-size samples was collected at 2cm intervals and 404 magnetic susceptibility samples were obtained at 4cm intervals from the core.Results showed that the OSL age at the 50cm depth on BZ section is 26.23±4.92ka belongs to the Late Pleistocene.Thus,Holocene loess is not available on the very top of BZ section.Through the analysis with other two loess sections in the same area (Xianjingyuan section,37°55′51.7″N,120°44′47.3″E;46.7m a.s.1.and Miaodao stratigraphical section,37°56′31.9″N,120°40′35.9″N;17m a.s.l.),we conclude that BZ loess mainly accumulated during the last interglacial time.Grain size results showed that silt from 5μm to 63μm was the main component of the samples from BZ profile with an average content of 60.1％.The grain size frequency distribution curves of BZ loess showed a three-peak pattern.The comparison of grain size characteristic with Luochuan loess identified BZ loess as a typical eolian deposits which is also proved by Sahu discrimination formula,but its mean grain size is coarser.Analysis with other island loess in Miaodao Island area showed that the grain size of the island loess increases from south to north,which indicate that the provenance of this area is not only remote dust carried from northwest inland but also nearby source material from north direction.The BZ section has much lower magnetic susceptibility value than Luochuan loess averaged as 31.53×10-8m3/kg suggesting the grain size and provenance difference of BZ section.The fluctuant trend of the magnetic susceptibility curve is basically consistent with that of the median grain size curve of BZ section,which provides a significant basis for stratigraphic division.Based on the above results,we classified BZ loess as one paleosol layer,one loess layerand one paleosol layer,each corresponding to the MIS 3～5 cycles respectively.Thus,top layer of 50～380cm is paleosol corresponding to MIS 3;layer 380～1220cm is loess corresponding to MIS 4;layer 1220～161cm is another paleosol corresponding to MIS 5.The grain size difference among layers showed climate and environmental change in BZ section,which indicated the meaning of island loess study in Miaodao Island area.%发育于山东滨海岛屿区的黄土与黄土高原等地内陆黄土相比,是沿海地区轨道尺度气候变化和海平面变化的重要记录,对研究区域古气候古环境演变具有特殊价值.本文通过对山东庙岛群岛大黑山岛北庄黄土剖面钻孔岩芯样品进行光释光(OSL)测年、粒度和磁化率分析,并与黄土高原典型黄土以及山东其他地区黄土进行了对比.研究结果表明,北庄黄土剖面0.5m处OSL年代为26.23±4.92ka,处于晚更新世末期,说明剖面全新世黄土缺失,与区域内其他剖面进行年代对比,可推断北庄黄土主要为末次间冰期以来堆积的黄土.与黄土高原典型黄土相比,北庄黄土为典型风成沉积物,但其平均粒径比黄土高原典型黄土粗,且与同区域其他岛屿上黄土相比粒径具有由南向北逐渐增大的变化趋势,这可能与不同阶段气候变化引起的海面升降而导致的主要物源不同有关.根据北庄黄土粒度和磁化率变化趋势并结合OSL测年结果,将该剖面从上到下划分为古土壤层、黄土层和古土壤层,分别对应着MIS 3～MIS 5气候旋回.因此,大黑山岛北庄黄土剖面记录了末次间冰期以来山东东部沿海地区古气候环境变化进程.