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

Significance of climate conditions in the variability of eolian dust deposition since the last deglaciation in the Zoigêregion recorded by the Hongyuan peat

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。

There are many debates on the mechanism of abrupt climate change in East Asia since the late deglacial period,due to the shortage of high-resolution climate reconstruction records.Meanwhile,few studies have been conducted on the eolian dust deposition in peat and little attention has been paid to long-term evolution of dust flux and its climate background in Zoigêregion from the northeastern Qinghai-Tibetan Plateau.In this study,a 450-cm peat profile HY2014(32°46'49.850''N,102°30'57.047''E;3509m a.s.l.)from Hongyuan peatland in Zoigêregion,western Sichuan Province was selected as the research carrier.Here we analyzed the grainsize compositions and geochemical element contents(including K,Ti,Zr,Rb,Sr and V and others)of ash from 225 samples at 2-cm interval in HY2014 profile.Based on high-resolution AMS 14C dating results measured by 13 samples from different depth of this profile,the evolutionary history of dust flux since the late deglacial period is reconstructed and its dynamics is revealed.In combination of the previous studies that obtained from Hongyuan peat and other records,this study also aims to discuss synthetically the control mechanism of dust deposition in Hongyuan area.The results show that:(1)The geochemical elements such as Ti,V,Sr,EF_(Zr),EF_(Rb) and EF_(K) in Hongyuan peat are mainly aeolian inputs,which can be used to assist to the explanation of dust flux in this area and to reveal the evolutionary history of atmospheric circulation.(2)The dust flux in Hongyuan area varied at 0.17~40.74μg/(cm2·a)and was abnormally high during the last glacial period(14000~11600cal.a B.P.),with the mean value of 21.52μg/(cm2·a).The high value of dust flux during 14000~12800cal.a B.P.was caused by the extraordinary palaeoflood events.During 12800~11600cal.a B.P.,the climate was cold and dry when the intensity of both the East Asian winter monsoon and the westerly wind were very strong.And a large area of exposed loose sediments were transported into the Hongyuan peatland by these two circulations.During 11600~3100cal.a B.P.,the dust flux showed an overall downward trend,in which fluctuated significantly during 11600~6400cal.a B.P.,and was increased and much stable between 6400cal.a B.P.and 3100cal.a B.P.The mean value of dust flux was 1.60μg/(cm2·a)during 11600~6400cal.a B.P.,and slightly increase to 1.78μg/(cm2·a)during 6400~3100cal.a B.P.However,the dust flux increased again to 3.55μg/(cm2·a)after 3100cal.a B.P.The climate was generally warm and humid and the vegetation coverage in the dust source areas was high during the Early and Middle Holocene;meanwhile,East Asian winter monsoon and the westerly wind were weaker.Owing to these factors,the dust fluxes declined clearly.However,the westerly wind weakened,while the East Asian winter monsoon was stronger during the Late Holocene.The variations of ash content and its characteristics of geochemical element and particle size compositions in Hongyuan peat,were in good agreement with the evolution of dust flux along the timeseries.(3)The dust deposition flux of Hongyuan peatland clearly recorded the Younger Drays event and seven dust enhancement events,which can be also explored in other records from Northwest and Northern China.When the dust event occurred,the East Asian winter monsoon intensified,the climate condition in the dust source area deteriorated and the vegetation coverage reduced,so that more dust can be released into the Hongyuan peat.