东北平原杜蒙钻孔的磁性地层学和磁化率对松嫩古湖演化的指示

湖相沉积是过去气候环境变化研究的重要地质记录.东北平原在早-中更新世时期曾发育一个巨型湖泊(松嫩古湖), 对这一古湖的演化历史和机制仍需多钻孔的进一步研究.本研究针对东北平原杜蒙(HL)钻孔沉积物开展了²⁶Al/¹⁰Be同位素年代学、古地磁年代学、磁化率地层学和岩石磁学研究, 结果表明: (1)令字组和林甸组湖相沉积的界线为约900 ka; (2)林甸组的磁化率明显高于令字组, 令字组的高矫顽力弱磁性矿物贡献相对较大; (3)林甸组磁化率的升高可能与湖平面降低、水动力增强、入湖的强磁性矿物增多或者氧化还原条件改变, 从而生成了强磁性铁的硫化物有关; (4)约900 ka前后, 松嫩古湖萎缩, 可能与中更新世气候转型期间大西洋深层翻转流(Atlantic meridional overturning circulation, 简称AMOC)减弱致使热带辐合带(Intertropical Convergence Zone, 简称ITCZ)南移, 以及海平面下降造成更多陆地暴露地表, 水汽输送距离增加, 进而共同导致东亚夏季风降水减少有关.

Magnetostratigraphy and magnetic susceptibility of the Dumeng borehole sequence from Northeast China Plain and implications for sedimentary evolution of the Songnen paleo-lake

Lacustrine sediments are ideal archives for investigation of the evolution of climate and environment. During the early-mid Pleistocene, there was a huge lake (Songnen paleo-lake) in the Northeast China Plain. The evolutionary history and mechanism of this ancient lake need to be further studied with more boreholes. Here we present the results of 26Al/10Be and paleomagnetic geochronology, magnetic susceptibility stratigraphy and rock magnetism obtained from the HL core (A borehole in Dumeng county of the Northeast China Plain). The boundary between the lacustrine sediments of Lingzi Formation and Lindian Formation was dated to be about 900 ka. The magnetic susceptibility of Lindian Formation was evidently higher than that of Lingzi Formation, and hysteresis loops and χ-T curves of these representative samples show that the contribution of paramagnetic minerals and high coercivity weak magnetic minerals of Lingzi Formation was relatively large. The increase of magnetic susceptibility may be related to the decrease of lake level, the enhancement of hydrodynamic force, the increase of strong magnetic minerals entering the lake or the change of redox conditions, resulting in the formation of strong magnetic iron sulfide. At about 900 ka, the Songnen paleo-lake shrank, which may be related to the less East Asian summer monsoon precipitation. The weakening of the Atlantic meridional overturning circulation (AMOC) and the southward movement of the Intertropical Convergence Zone (ITCZ) in the tropical Pacific might be the main reason for the reduction of East Asian summer monsoon precipitation. In addition, the decreasing of the global sea level, caused by the expansion of ice sheet in high latitude, which led to more land exposure and longer transport distance of ocean water vapor, might be another cause of the East Asian summer monsoon precipitation reduction.