西风区昭苏黄土剖面中碳酸盐矿物组成及其古环境意义辨识

碳酸盐矿物是黄土的重要组成部分,是古环境重建的重要指标之一。通过对地处西风区的新疆伊犁盆地昭苏黄土剖面的全岩样品和分粒级样品的Ⅹ射线衍射测试,分析了其碳酸盐矿物组成,利用麦夸特算法计算了碳酸盐矿物的含量。综合碳酸盐矿物的含量、粒径分布及风化特征等因素,对其古环境意义进行了辨识,从而建立了该剖面末次冰期以来碳酸盐矿物记录的古气候环境变化序列。结果表明: 昭苏剖面中原生碳酸盐矿物(方解石和白云石)含量占主导地位,次生方解石对样品中总方解石含量的贡献较少; 白云石含量可能与亚轨道尺度冷暖交替导致的西风环流强度变化有关; 方解石含量的变化可能指示了西风区降水量的变化; 碳酸盐矿物含量揭示了昭苏地区在末次冰期时气候冷湿,在全新世时气候暖干。

CARBONATE MINERALS OF ZHAOSU LOESS SECTION IN WESTERLY AREA AND THEIR PALEOENVIRONMENTAL SIGNIFICANCE

The carbonate minerals are important components of loess sediments,and they are regarded as the effective proxy indexes for paleoclimate reconstruction in the Chinese Loess Plateau.The loess-paleosol sequence in the Ili Basin dominated by westerly circulation provide a good archive for the research of the history of paleoenvironment and paleoclimate and westerly circulation.However,paleoclimatic significance and adaptability of carbonate minerals of eolian loess sediments in the Ili baisn are still unknown.Here,the authors report carbonate minerals results of a 690cm thickness loess-paleosol sequence(named Zhaosu Poma(ZSP)section).The ZSP loess section(42.69°N,80.25°E; elevation 1875m)is situated on a second terrace of the Tekes river in the South Ili Basin.The section can be divided into five pedostratigraphic units with bottom of the upper S1 overlying on fluvial gravel sediments.According to the extrapolation of OSL dating,the age of the bottom of this section is nearly 80ka.Based on X-ray diffraction analysis,the authors report the carbonate minerals composition both in 138 bulk samples and 45 different grain size fractions of ZSP.Levenberg-Marquardt(LM) method is applied to determine the quantitative analysis of carbonate minerals.Besides,the paleoenvironmental significances of calcite and dolomite are discussed according to their content,grain size distribution and weathering characteristics.Then climatic changes since the last glacial period have been constructed in term of carbonate minerals.The results show that the carbonate minerals are dominated by detrital carbonate such as calcite and dolomite.Pedogenic carbonate(mainly calcite)has little contribution to total carbonate.The content of dolomite is related with westerly circulation intensity variations caused by alternation of sub-orbit scale cold and warm climate.The content variations of calcite maybe reflect the precipitation changes in westerly area.Therefore,the contents of carbonate minerals in some extent are good indicators of the strength of westerly circulation and temperature and precipitation,and they can be used to reconstruct paleoclimate and paleoenvironment.Based on the data of carbonate minerals in ZSP, paleoclomatic change from the last glacial period can be divided into two stages:(1)low temperature and high precipitation and strong westerly circulation period at 80.45~12.40ka,rapidly dropping in precipitation and rapidly increasing in temperature and weakening in westerly circulation period at 12.4ka~0.(2)Climate change in Ili Basin is well correlated with Greenland ice core records(GISP2)and other environmental records in Northwest China,most of cold events are clearly correlated with Heinrich events and warm events(Dansgaard-Oeschger events),demonstrates that the westerly circulation experienced a millennial variation similar to high latitude northern Hemisphere.(3)The climate change model of ZSP representing obvious alternation of warm-dry and cool-humid,and the tendency of paleoenvironment evolution becomes to warmer and dryer since Holocene.(4)The period of time between 26.04ka and 59.45ka corresponds to the marine oxygen isotope 3 stage(MIS 3) is dominated by a cold and humid climate with stronger westerly circulation, lower temperature and higher precipitation, especially during MIS 3b substage.