二十二万年以来东亚夏季风的千年尺度变化及其在不同时期的差异

本文采用游离铁和全铁的比值,建立了0．220MaB．P．以来长武黄土-古土壤序列的高分辨率古风化强度序列,揭示出0．220MaB．P．以来黄土高原古环境发生了一系列千年尺度的变化,且多数在同一剖面的磁化率记录上没有明显反映或反映较弱。由于该地区冬季土壤温度低于0℃,化学风化的强度主要受控于夏季的温度和降水。据此我们认为,这种千年尺度的波动是东亚古夏季风强度变化的信号。此指标与格陵兰GRIP冰芯δ¹⁸O记录对比发现,二者在末次冰期有较好的吻合性,而夏季风在末次间冰期(深海氧同位素阶段5e)及以前的变化要平缓得多。因此,本文的结果显示出：0．220MaB．P．以来,东亚夏季风在千年尺度上的大幅度变化在末次冰期最为显著,此前则表现为一种较为稳定的模式。

MILLENNIAL-SCALE OSCILLATIONS OF THE EAST-ASIAN SUMMER MONSOON OVER THE LAST 0.220MA

In this study, a high-resolution paleoweathering record of the loess-soil sequence from the central Loess Plateau covering the last 0.220Ma has been generated using the ratio between the CBD (citrate bicarbonate dithionite) extractable free Fe₂O₃ (Fed) andtotal Fe₂O₃(Fet). The new record shows a series of millennial oscillations, which are not documented by magnetic susceptibility changes at the same site. Since chemicalweathering in this region depends mainly upon the summer precipitation and temperatures, we interpret these changes as a result of millennial-scale variations in the strength of the East-Asian summer monsoon. Over the last glacial Period, general agreement can be observed between the loess weathering record and the GRIP-δ¹⁸O record.However, our loess weathering Timeseries seems to suggest a rather stable climatepattern for the typical last interglacial Period (marine δ¹⁸O stage 5e) although smallfluctuations can be observed. At least the three warm peaks and the two cold peaksshown in the GRIP data cannot be clearly defined in our weathering record. Thesummer monsoon may have not experienced such drastic changes or its amplitudeswere much smaller than in the Greenland region. This interpretation seems to besupported by the well-developed Luvisol within the S₁ soil complex in the easternand southern loess Plateau, which would have been formed under relatively stablewarm-humid conditions.Our data also show that high amplitude fluctuations were more characterishcs forthe last glacial period while the older portion (including the L₂ and S₂ units) showsrather smoothed variations. This seems to suggest that the instability of climate in theEast-Asian monsoon zone be significantly amplified since the termination of the lastinterglacial period. Moreover, the fluctuation pattern of Fed / Fet for the portion olderthan 140 000aB. P. is significantly different from that of the GRIP-δ¹⁸O record,suggesting that the millennial oscillations in the North Atlantic and northern Chinamay be less coupled for earlier glacial-interglacial cycles in comparison with the lastglacial Period.