U-Pb ages of zircon grains reveal a proximal dust source of the Xiashu loess, Lower Yangtze River region, China

Deciphering the eolian sources is critical to understand the paleo-significance of the Quaternary eolian deposits(the Xiashu loess)in the lower reaches of the Yangtze River.Both a local source from the fluvial sediments of the Yangtze River and a distal source from the northern deserts similar to that of the loess on the Chinese Loess Plateau(CLP)have been proposed.Recent researches show great advantages of detrital zircon ages as a source tracer for Asian dust based on the laser ablation U-Pb isotopic dating technique.This work presents the U-Pb ages of zircon grains extracted from the Xiashu loess.The results indicate that the Xiashu loess has a very different age distribution of zircon grains from that of the loess on the CLP as well as the materials in the arid lands of the Asian Interior.Instead,the zircon ages of the Xiashu loess are indistinguishable from the fluvial sediments of the Yangtze River,indicating the dominance of proximal dust source.Proximal source of the Xiashu loess implies that extensive eolian processes might have existed in the currently wet South China,possibly in response to the full glacial conditions after the middle Pleistocene transition of global climate.

红原泥炭记录的晚冰期以来若尔盖地区粉尘通量变率及其气候影响

由于高分辨率的气候重建记录,尤其是粉尘通量重建结果仍然较少,晚冰期以来东亚地区的气候突变机制存在很多争论。本研究对青藏高原东北部若尔盖红原泥炭HY2014剖面(深度450cm)样品的灰分含量及其粒度组成和K、Ti、Zr、Rb、Sr、V等元素含量进行高分辨率测定,基于AMS14C测年结果,重建了晚冰期以来该地区粉尘通量的变化动态,并结合过去的研究结果综合探讨粉尘通量的影响机制。结果表明:1)红原泥炭中的Ti、V、Sr、EF_(Zr)、EF_(Rb)、EF_(K)等元素主要为风成输入,可辅助粉尘通量来共同反映研究区大气粉尘沉积的变化历史。2)14000~11600cal.a B.P.,红原地区粉尘通量总体偏高,其中14000~12800cal.a B.P.期间由于红原地区发生特大古洪水事件使大气粉尘通量数值明显增加,而12800~11600cal.a B.P.期间气候冷干,强劲的东亚冬季风和北半球西风急流将裸露的松散沉积物搬运沉积到红原泥炭地中,导致红原大气粉尘通量增加;11600~3100cal.a B.P.粉尘通量低,其中在11600~6400cal.a B.P.期间波动较为明显,而在6400~3100cal.a B.P.粉尘通量波动更为稳定。早中全新世气候总体暖湿,受东亚冬季风和北半球西风急流影响较小且植被覆盖度较高,粉尘通量总体较低;3100cal.a B.P.以来东亚冬季风增强,红原地区粉尘通量再次升高。3)红原泥炭粉尘沉积通量记录了新仙女木事件,还清晰地记录了此后发生的7次粉尘增强事件,这在中国西北和北部的其他记录中也多有体现。粉尘增强事件发生时,东亚冬季风增强,粉尘源区干旱化,植被覆盖面积减小,粉尘释放增多,红原泥炭中粉尘含量升高。

Preface

Deciphering the eolian sources is critical to understand the paleo-significance of the Quaternary eolian deposits(the Xiashu loess)in the lower reaches of the Yangtze River.Both a local source from the fluvial sediments of the Yangtze River and a distal source from the northern deserts similar to that of the loess on the Chinese Loess Plateau(CLP)have been proposed.Recent researches show great advantages of detrital zircon ages as a source tracer for Asian dust based on the laser ablation U-Pb isotopic dating technique.This work presents the U-Pb ages of zircon grains extracted from the Xiashu loess.The results indicate that the Xiashu loess has a very different age distribution of zircon grains from that of the loess on the CLP as well as the materials in the arid lands of the Asian Interior.Instead,the zircon ages of the Xiashu loess are indistinguishable from the fluvial sediments of the Yangtze River,indicating the dominance of proximal dust source.Proximal source of the Xiashu loess implies that extensive eolian processes might have existed in the currently wet South China,possibly in response to the full glacial conditions after the middle Pleistocene transition of global climate.