Radioisotope Dating And Environmental Record Of The Past Two Centuries From Tibetan Plateau And Its Surrounding Ice Cores

Mid-or low-latitude glaciers represent natural archives,well suited for studying past environmental and climatic conditions on the regional scale.Because of their close vicinity to human settlements,they are more important sources of information about the anthropogenic influences on climate and environmental change than polar ice cores.A reliable chronology is a crucial step for the interpretation of any climate archive.Dating by annual layer counting becomes difficult with increasing depth in the ice core due to the thinning effect.The reference horizons can offer very limited well known ages.Radioisotope 210Pb dating with a short half-life of 22.3 years is restricted to the last 150 years.For the older and deeper parts of ice cores,alternative dating method must be applied for the retrieval of an absolute age.In this paper we applied a new AMS14C dating method on the bottom parts of ice cores from Chongce ice cap?216.61 m long,35°14'56.58"N,81°5'27.7"E,6105 m a.s.l.;135.8 m long 35014'6.11"N,81°6'50.62"E,6010 m a.s.l.?,Miaoergou glacier?57.6 m long,43°03'19"N,94°19'21"E,4512 m a.s.l.?and East Rongbuk glacier?28°01'55"N,86057'52"E,6518 m a.s.l.?,which is based on extracting the jig-amounts of water-insoluble organic carbon fraction of carbonaceous aerosols embedded in the ice matrix.For the upper 0-44 m part of 216.61 m Chongce ice core,3H and 210Pb dating methods were applied to establish the age-depth relationship.Besides dating of ice cores from Tibetan Plateau,we reconstructed a high-resolution atmospheric cadmium record for 1776-2004 AD and 239,240Pu&236U records for 1940-1975 AD during the atmospheric nuclear weapons testing period in the Miaoergou ice core.The main conclusions can be summarized as follows.?1?In total 22 AMS14C ages were retrieved for the depth of 30-216 m,suggesting a no more than 4600 a BP record preserved in 216.6 m Chongce ice core.The very bottom ice calibrated 14C age of 4590±241 cal.a BP at a depth of 215.08-216.04 m from 210.61 m Chongce ice core is consistent with the bottom ice calibrated 14C age of 6253±227 cal.a BP at a depth of 134.32-135.04 m from 135.8 m Chongce ice core.The calibrated 14C ages closed to bedrock are 5388±359 cal.a BP at a depth of 56.2-57.1 m in Miaoergou ice core,6724±430 cal.a BP at a depth of 95.4₉5.8 m in East Rongbuk ice core,respectively.By Comparing with two published bottom,14C ages from Puruogangri ice core and Dunde ice core,the results imply that Tibetan Plateau glacier archive covering a period no more than 7000 a BP within the Holocene Climate Optimum.?2?The 3H peak in 1963 from nuclear test fallout with an activity of 3237±87 TU was detected at a depth of 21.4 m in 216.6 m Chongce ice core.The high 210Pb surface activity at the Chongce ice cap of 236±33 mBq/kg allowed reconstructing the past 122 years?1891-2013 AD?record for the upper 0-44 m in 216.6 m Chongce ice core.The 210Pb dating result is consistent with the 3H peak result.?3?Natural contributions dominated the atmospheric cycles of cadmium until 1940s,after that anthropogenic input is main contribution in the Miaoergou ice core.Soil and mineral dust input are major natural contribution to cadmium concentration.The concentration of cadmium increased sharply since 1940s is due to the increasing human activities?e.g.non-ferrous metal and steel production?from the East Europe and central Asia.The fast and comprehensive industrialization at the beginning of the 21st century in Xinjiang and neighboring regions may have caused the large increase of cadmium in the Miaoergou ice core during 2000-2004 AD.?4?The total deposition rate during NWT for 239Pu amounts to 1.55 x 109 atoms·cm-2 in the Miaoergou ice core,showing a little higher undisturbed fallout of 239Pu record compared to those from Alps and polar glaciers,which is consistent with the average latitudinal distribution of global inventory of 239Pu fallout.The total deposition rates of 239Pu from different ice cores in the north hemisphere show no obvious trend with the latitude,suggesting different deposition process and source contributions in different drilling sites.The observed 240Pu/239Pu atom ratios were 01810.02,except for one sample,which represents the global fallout ratio of 0.18.No indication for emission from local source?Lop Nor?was found.The observed 236U deposition flux of 3.5×108 atoms·cm-2 at the drill site agrees within variability of such measurements with another measurement at an Arctic site?Svalbard,79.83°N;24.02°E;750 m a.s.l.?of 1.6x108 atoms·cm-2.The 236U/239Pu ratios were between 0.12 and 0.43?average 0.27?,in good agreement with values from the literature.