新近系-第四系界面撞击事件研究的新进展

通过对中国黄土高原南缘的段家坡黄土-红粘土剖面Ga/M边界(2.48MaBP)上下地层中5种微颗粒和土样微量元素的研究,进一步确定了下更新统午城组黄土底部记录着两次撞击事件的信息。较早的一次记录于L38中,年代为2.53-2.48MaBP,以多种撞击成因或与之有关的微颗粒为标志,且有丰度为58.3×10-12g/g的Ir异常和14.3×10-9g/g的Au异常给与确证。这是一次真正的N/Q界面撞击事件,在气候地层上与从上新统红粘土向更新统黄土的过渡相一致,在磁性地层上与从高斯正极性向松山反极性的转变相联系。较晚的一次记录于S37中,年代为2.43-2.42MaBP,在多种微颗粒标志中,还包括含有Ir异常分别为3.89×10-6g/g的宇宙尘和2.15×10-8g/g的生物球粒。文中用综合成因模式解释了不同特性的撞击记录共存的现象,并指出中国黄土在天文地质研究方面尚蕴藏着大量信息。 

RECENT PROGRESS OF THE STUDY ON NEOGENE-QUATERNARY BOUNDARY IMPACT EVENT

Up to now, five kinds of micrograins, including cosmic dust, microtektites, volcanic spherules, impact volcanic ash and organic spherules, especially the iridium anomalies in bulk rock and micrograins have been discovered in the strata around the Gauss/Matuyama bound-ary in Duanjiapo loess/red clay section located at the south margin of the North China Loess Plateau.From these records, tow impact events with a time interval of ca.0.1Ma have been determined, having an age of 2.53-2.48MaBP and 2.43-2.42MaBP respectively.The early one, as can be correlated not only to the climate stratigraphic red clay/loess transition, but al-so to the magnetostratigraphic Gauss/Matuyama transition, is undoubtedly the Neogene/Quaternary boundary impact event.It is also shown in this paper that a comprehensive mode of origin can be used for explanation of the coexistence of impact records with different prop-erties.