天山乌鲁木齐河源1号冰川冰芯记录形成过程及年代划分

Chronology and record formation process of an ice core from Glacier No.1 at the Urumqi Riverhead in eastern Tianshan,China

冰川冰芯记录了历史时期的气候环境信息。天山乌鲁木齐河源1号冰川位于亚洲大陆干旱半干旱地区的腹地,1991-1998年在1号冰川钻取的4根深冰芯(T0～T3),用于探究该地区历史时期各项气候环境信息记录。通过化学离子季节变化特征以及δ18O和Mg2+的剖面与树轮重建的温度记录对比,分别对冰芯T2(80.1 m)的32 m以上和底部一段(33～77.25 m)定年,确定了整个冰芯年代与深度之间的关系,并建立了采样点雪冰中污化层所代表的年代和深度之间的关系方程式。结果表明:T2这根冰芯大约有658 a的时间;小冰期冰碛的年龄与冰芯中记录的冷湿时期相一致。在定年的过程中,冰芯形成过程的探讨提供了理论依据。

Glaciers ice core records the environmental information of the history.A suite of ice cores(T0-T3) were recovered from 1991 to 1998 at Glacier No.1 at the Urumqi Riverhead in eastern Tianshan,China,which are in the arid and semi-arid region in Central Asia,to explore the environmental information in the historical period in this area.As basic knowledge for the ice core dating,annual net accumulation(28.4 cm ieq) was determined by identifying overlaps in chemical profiles of two ice cores drilled in different year,which is higher than on site mass balance budget,suggesting a potion of ice mass came from higher reaches of the glacier through ice flow.The study of formation process of light stable isotopic ratio and dust layer in the ice core during the past several years made a great contribution to the ice core dating.Annual net accumulation at drilling site is a very useful reference parameter in ice core dating.The annual net accumulation of surface snow at drilling site can be obtained from specific mass balance measurement at this site or through ice formation process observation.In order to resolve the uncertainties in the ice core dating and better interpret the records,a research program was conducted during the past several years under PGPI,which specifically focused on the effects of meltwater involved post-depositional processes on chemical species in snow-firn pack and the formation process of ice core records.This study has resulted in a number of outcomes with respect to the formation processes of various records in ice core and the effects of post depositional process on the snow-firn chemistry,etc.,which,particularly the results of evolution processes of dust layer and δ18O,has been proven to be very useful for the ice core dating and the records interpretation.Retrieved in October 1991,the 91.64 m T0 was the only core reached to bedrock.Both T1 and T2 were recovered in May 1996 with the lengths of 6.09 m and 80.1 m respectively.T3 was 14.08 m long and extracted in October 1998.All of the ice cores retrieved from a relatively flat site in percolation zone of Glacier No.1 with elevations of 4 050 m for T0,T1 and T2,4 040 m for T3.This paper presents chronology establishment process of T2(80.1 m).Cross counting of seasonal variations in chemical species profiles was employed for dating the top 32 m section.By comparing δ18O and Mg2+ profiles to temperature record reconstructed from tree rings,obtained were dating results in lower part of the core(33-77.25 m),which presents the age-depth relationship for the entire core,meanwhile,vertical formation velocity of visible dust horizons at drilling site leads to another age-depth equation.The result indicates that T2 contained 658 years.In addition,the ages of LIA moraines are coincident with cold or wet eras in the ice core records.And the study on the formation process of the ice core records provides with a theoretic base for the ice core dating.