3Ma以来黄土高原朝那黄土-红粘土序列赤铁矿记录及其古气候意义

HEMATITE RECORD OF 3Ma LOESS-RED CLAY SEQUENCES IN THE CENTRAL CHINESE LOESS PLATEAU 3Ma AND ITS PALEOCLIMATIC SIGNIFICANCE

土壤和前期研究表明黄土-红粘土序列的赤铁矿含量与环境密切相关,对气候的干湿变化非常敏感,气候越湿热,赤铁矿含量越高。本文研究表明黄土-红粘土样品漫反射光谱一阶导数曲线在555～575nm间的峰面积作为赤铁矿指数可以客观有效地指示赤铁矿含量的高低。通过黄土高原朝那剖面红粘土-黄土序列的系统漫反射光谱分析,建立了3Ma以来黄土高原赤铁矿含量变化记录,揭示赤铁矿含量在约2.5Ma,1.5Ma,0.6Ma和0.15Ma显著阶段性减小,指示了黄土高原阶段性变冷干的演化历史。

It has been confirmed that the aeolian loess-red clay sequence on the Chinese Loess Plateau（CLP）records the evolutionary information of East Asian Monsoon, which is still debated by various different views even with the same paleoclimatic indicators,such as the hematite content. Previous research on soil explores that the content of hematite is closely sensitive to humidity fluctuation, and has close relevance with the environmental evolution. However, there are conflicting views on the paleoclimatic significance of hematite proxy. The trace hematite concentration in loess- paleosol samples could be detected through the analysis of diffuse reflectance spectroscopy（DRS）. For the reason mentioned above, we focus on Chaona red clay-loess section in CLP to see the change of hematite content spanning the last 3 Ma. The study area is located near the town of Chaona（35 ~7＇N, 107 ~ 12＇E ） in the Central CLP. The loess deposit of the sequence is 175m thick, and contains 33 paleosols units. The paleosol within the loess are of brownish or reddish color with substantial clay skins developed. Carbonate nodules are scattered commonly below the soils. And there is a 125m thick red clay section underlying the loess-paleosol. Samples were chosen from the loess-paleosol sequence and the uppermost part of the red clay with a paleomagnetic age of 3Ma. We collected nearly one thousand samples for DRS analysis with an interval of ca. 20cm from the Chaona section. Reflectance spectra of samples were measured in Purkinje General TUI901 UV-VIS spectrophotometer with a diffuse reflectance attachment （reflectance sphere）from 360 nm to 850 nm at 1 nm intervals. Reflectance data were analyzed by first derivative. Typical loess and paleosol samples were firstly deferrated using the CBD procedure to produce a natural matrix material and secondly added hematite in known weight percentages. Comparison of first derivative spectral patterns for varying percentages of hematite mixed with the deferrated loess matrix shows that the area of shadow between 555nm and 575nm of first derivative of reflected spectra could be used as an effective index to represent the content of hematite in red clay-loess samples. The content of hematite in soils tends to be higher under warmer and more moisture climatic conditions ,while lower under cooler and drier conditions. Such viewpoint is supported by the correlation analysis between annual average precipitation, temperature and hematite content of several modern soil samples from East Asian monsoon region. The hematite content in red clay-loess section can be effectively indicated by hematite index. Through DRS analysis and the application of hematite index, we obtain continuous record of hematite in CLP since the past 3 Ma. It＇s possible that the excessive organic content in paleosol lays leads to the abnormal low hematite content and unusual high magnetism susceptibility in these layers, considering the fact that there is more organic matter in S1 - S5 than underlying section in CLP generally. And the trend of significant gradual decrease of hematite content explores the general evolution of paleoclimate in CLP tending to be cooler and drier, especially at the time of 2. 5Ma, 1.5Ma, 0. 6Ma and 0. 15Ma,which is agreed with records of particle size analysis,pollen and biomarker in C haona section.