弱的早更新世间冰期东亚夏季风——来自黄土高原古土壤层中酸溶相元素的证据

在过去的暖期,东亚夏季风的强弱变化受到广泛关注,例如暖的早更新世间冰期时期(1. 6~2. 6 Ma).本文利用黄土高原洛川和赵家川剖面古土壤层中酸溶相元素的分布及其比值去重建早更新世间冰期时期东亚夏季风强度变化.结果显示酸溶相中以Ca元素(平均2. 6%)为主, Mg元素(平均1116. 5 mg/kg)次之, Sr元素(平均49. 5 mg/kg)和Mn元素(平均47. 6 mg/kg)最低.它们的含量在早更新世较高,中晚更新世时期较低,而Mg/Ca、Sr/Ca和Mn/Ca摩尔比值则呈现相反的变化.这些元素的分布及相应比值的变化受东亚夏季风控制,主要反应季风降水的变化.两个剖面古土壤层中元素及其比值均指示暖的早更新世间冰期时期黄土高原地区东亚夏季风较弱,中晚更新世间冰期时期夏季风较强,该结果与过去许多东亚夏季风重建记录接近.我们的研究结果也支持早更新世弱的赤道太平洋沃克环流导致弱的东亚夏季风这一假说.

WEAK EAST ASIAN SUMMER MONSOON DURING THE EARLY PLEISTOCENE INTERGLACIALS: EVIDENCE FROM ACETIC ACID-LEACHABLE ELEMENTS IN PALEOSOL LAYERS ON THE CHINESE LOESS PLATEAU

During the warm interglacial periods, the change of East Asian summer monsoon ( EASM ) intensity has been widely concerned, such as the warm Early Pleistocene interglacial periods ( 1.6-2.6 Ma). Here, we present the distributions of acetic acid-leachable elements( Ca, Mg, Sr and Mn) and their ratios( Mg/Ca, Sr/Ca and Mn/Ca) in the paleosol samples on the Chinese Loess Plateau ( CLP ) to reconstruct the interglacial EASM changes since 2. 6 Ma. Total of 205 paleosol samples were collected from typical Luochuan ( 35. 76°N , 109. 42°E;136 min depth) and Zhaojiachuan ( 35. 73°N , 107. 73°E;178 m in depth ) loess-paleosol sequences with average 7 ka sampling interval. These elements were extracted by 0. 1 mol/L acetic acid reacting with 4 hours. The results show that the acid-soluble elemental components of paleosol layers in both sections are dominated by Ca ( average 2. 6%), and then followed by Mg ( average 1116. 5 mg/kg) as well as Sr ( average 49. 5 mg/kg) and Mn ( average 47. 6 mg/kg) at minimum. Their distributions and related ratios mainly reflect monsoon precipitation changes, e.g., higher elemental concentrations and lower elemental ratios indicate stronger EASM intensity and vice versa. Three elemental concentrations during interglacial periods are higher during the Early Pleistocene than during the mid-Late Pleistocene. In contrast, the molar ratios of Mg/Ca, Sr/Ca and Mn/Ca increase generally from the Early Pleistocene to the mid-Late Pleistocene interglacial periods. Our records document that the weak interglacial EASM during the Early Pleistocene and hereafter strong EASM , generally consistent to many previously published EASM records from CLP and South China Sea. Our results also support the hypothesis presented recently that weak Equatorial Pacific Walker Circulation as a result of weak EASM during the Early Pleistocene interglacial periods.