| The loess-paleosol deposits in China are an important terrestrial records of paleoclimaticand paleoenvironmental changes, and can easily be correlated to glacial-interglacial cycles in theQuaternary period. The loess deposit in the Shandong area, located in the eastern margin of theChinese loess belt, is very similar to the loess of Chinese Loess Plateau (CLP). These depositsmust be very significant for comprehending paleoclimatic and paleoenvironmental fluctuation.The Shandong loess corresponds to the loess in the Liaodong Peninsula, Hebei Provenance to thenorth; the loess in the CLP to the west; and the Xiashu loess in the middle-lower reaches of theYangtze river to the south. The provenance, transport mechanism and depositional environmentsof the loess deposits in the Shandong area must have some commonalities with each other. Thecharacteristic geographical location near the transition of the land and sea makes Miaodao areavery sensitive to the continental and marine environmental change. The Miaodao loess not onlyhas the characteristics of the typical eolian loess，but also has some characteristics which typicaleolian loess does not have. In contrast to the intensively investigated loess deposits in the areassurrounding Shandong area, the Shandong loess is still insufficiently documented and poorlyunderstood. This limits the interpretation of palaeoclimatic and palaeoenvironmental informationand prevents the correlation of this record with loess deposits in the Chinese Loess Plateau. Inorder to expand our knowledge of eolian deposits chronology of the Miaodao Island loess, andexplore the provenance of the loess deposits, the representative eolian deposits are selected tostudy. Based on careful field investigation in the islands of Miaodao Islands, we usesedimentologic, geochemical and OSL methods to to establish a spatial and temporalchronological framework, and analysis of the source material of the loess of Miaodao Islands.We can get the following conclusions:(1) Based on detailed field investigation,9samples retrieved from Miaodao Island profilesand30samples retrieved from7other representative profiles of the Shandong area from east towest were dated using quartz optically stimulated luminescence (OSL) in order to establish aspatial and temporal chronological framework. The preliminary OSL dating results of Miaodao Island loess profiles were dated by Ris TL/OSL-DA-20luminescence measurement system,using the single aliquot regenerative-dose(SAR) protocol. It showed that the equivalent doses(De)of samples of Beihuangcheng Island profile from the depth100cm,270cm,420cm and620cmare49.8±5.8Gy,50.9±3.4Gy,106.4±3.7Gy and80.2±3.8Gy, and the OSL dates are11.8±1.4ka,12.1±0.8ka,24.6±0.9ka and19.4±0.9ka, respectively. The equivalent doses (De) of samples ofTuoji Island profile from the depth100cm and660cm are102.62±1.59Gy and299.05±9.75Gy,and the OSL dates are20.73±0.32ka and64.01±2.09ka, respectively. And the equivalent doses(De) of samples of Nanchangshan Island Xianjingyuan profile from the depth80cm,250cm and390cm are66.2±4.1Gy,92.2±4.4Gy and211.8±10.8Gy, and the OSL dates are14.4±1.0ka、21.4±1.1ka and51.8±2.6ka, respectively. Our dating results in combination with previouspublished ages on eolian deposits showed that:(1) The oldest eolian deposits in the Shandongarea are in excess of100ka, while most of the loess was deposited after100ka,(2) the majorityof Holocene loess has been eroded and (3) the sections also presented a varying loessaccumulation rate during different depositional periods. The varying loess accumulation rate alsoindicates that the source material of Shandong area are different from each other.The analysis ofhigher sedimentation rates is also an indicator of the continental shelf desertification during thelast glacier period.(2) Here we report an analyses of the grain-size parameters, the frequency and cumulativefrequency distribution curves, the statistical indices, discriminant analysis, upper continentalcrust (UCC)-normalized major elemental composition, the scatter diagram of CIA vs. Na/Kmolar ratio, the triangle diagram of A-CN-K, trace element, and rare-earth element (REE)patterns of the Miaodao Island loess, and compare them with those of the loess and other typicalloess deposits in northern China. The characteristics of grain size parameters and frequency andprobability cumulative distribution curves, statistical indices and discriminant analysis result allimply an eolian origin of Miaodao Island Loess. The variation in Md, sand content, clay contentand discriminant analysis of the sections in the northeast coastal region (in the following order:BHC'TJ'XJY'LGZ) are clearly associated with their geographical locations. There is ageneral pattern of coherent decreasing grain size from north to south (in the following order:BHC'TJ'XJY'LGZ), which indicates that the windward side of the Miaodao Islands waslikely closer in proximity to the dust source areas in the past. This indicates that during the last glacial period of the Late Pleistocene, the continental shelf of the Bohai Sea was exposed due tothe lower sea level and became part of the arid and semi-arid area. As a result, the exposed andweathered marine sediments were carried by strong winter monsoon and deposited on the thewindward side along the Miaodao Islands.The results of the geochemical characteristics of major elements indicate that the content ofmajor element oxide is mainly composed of SiO2, Al2O3and CaO. The compositional abundanceof major element are in an order of SiO2＞Al2O3＞CaO＞Fe2O3＞Na2O＞K2O＞MgO. Theproportion of Na2O of Miaodao loess is obvious higher than the Luochuan loess, while it is stillquite lower than that of upper continental crust (UCC). The characteristics of major, trace andrare earth element all imply the eolian origin of Miaodao Island Loess.The CIA value ofMiaodao loess sequence is between54.64and71.78, suggesting weak chemical weatheringintensity under cold and dry climatic conditions to median chemical weathering intensity. TheMiaodao loess and other aeolian deposits follow the similar trend line in the A-CN-K diagram,suggesting that the loess sources have finished multiple cycles of weathering, erosion, transportand deposition of crustal material, which are used to represent the average composition of theUCC. The changes of CIA, Na/K, the Eluvial Coefficient and the Coefficient of Weathering andEluviations suggested a progressively weakened trend of the chemical weathering intensity in inthe following order: UCC'Yili Loess'Beihuangcheng Loess'Xianjingyuan Loess'XifengLoess'Tuoji Loess'Luochuan Loess'Liaonan Loess'Luochuan Paleosol'CAS'XiashuLoess'Jiujiang Red Loess'Xuancheng Red Loess. This character also indicates the diffenenceprovenance between Miaodao Loess and the typical loess deposits.The results of chemicalweathering intensity inferred from CIA and Na/K shows that the Miaodao loess sequence shouldhave similar chemical weathering intensity to the zhoujiagou loess in the south of Liaoningprovince, which has the similar geographical location an natural environment, this may indicatesthat the provenance of Miaodao Loess must have the same transport mechanism and depositionalenvironments. |
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