ZhanJu Lin ,FuJun Niu ,Jing Luo ,MingHao Liu ,GuoAn Yin

1.State Key Laboratory of Frozen Soil Engineering,Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

2.Key Laboratory of Highway Construction &Maintenance Technology in Permafrost Region,Ministry of Transport,CCCC First Highway Consultants Co.Ltd,Xi’an,Shaanxi 710075,China

1 Introduction

Thermokarst lakes are common features of ice-rich permafrost landscapes.Such lakes develop as a result of ice-rich permafrost thawing or melting of massive ground ice (Brown and Grave,1979;Hinzmanet al.,1997).The lakes are of irregular shape and depth,and when enlarged,frequently have collapsing shorelines(Burn and Smith,1990).Thermokarst lakes are widely distribution in ice-rich sediments throughout the permafrost zone along the Qinghai-Tibet Engineering Corrdor (QTEC),especially in the Chumaerhe high plain,Hoh Xil hill region,and Beiluhe Basin (Linet al.,2010,2011a;Niuet al.,2011).It has been ever reported that some thermokarst lakes have been initiated by terrain disturbance related to construction and exploration (French,1975,1987;Thomas and Ferrell,1983),while most are the result of natural events,such as climate change and forest fires (Thie,1974;Chatwin,1983;Harry and French,1983).For the Qinghai-Tibet Plateau (QTP),limited studies suggest that most thermokarst lakes were initiated due to terrain disturbance associated with construction (Linet al.,2011b) and recent climate change.

A thermokarst lake occupies a depression formed by thaw settlement (K??b and Haeberli,2001).Once thermokarst lakes occur on the ground surface in permafrost regions,the ground temperature beneath the lake is higher than that of the surrounding permafrost because year-round temperatures at the lake bottom is equal to or higher than 0 °C,except in shallow lakes (Niuet al.,2011).Most thermokarst lakes expand and deepen with the thermal process(Williams and Smith,1989) and promote geomorphic and sedimentary processes (Murton,2009).Numerous studies have focused on thermokarst lake features in Beiluhe Basin,QTP,such as distribution,size,water depth,water temperature,ground temperatures at and beneath the lake-bottom,and its influence on the permafrost (Linet al.,2010,2011a;Niuet al.,2011),however,their history is infrequently reported.Therefore,the purposes of this paper is to investigate ages of thermokarst lakes BLH-A and BLH-B and provide scientific evidence for the initiation of thermokarst lakes in Beiluhe Basin.

2 Regional conditions and thermokarst lake description

The Beiluhe Basin,a main part of the Hoh Xil Nature Reserve Region with an altitude of 4,600 m in elevation,lies in the interior of the QTP,China (Figure 1).The basin is 350 km away from Golmud City and is bounded by Chumarhe High Plain to the north and Fenghuo Mountains to the south.This region is influenced by a cold and arid climate.The basin’s mean annual air temperature is-3.8 °C,the highest value of 21.3 °C is in mid-July and lowest value of-21.4 °C is in late January.Mean annual precipitation is 368 mm,and potential evaporation of 1,538 mm from 2003 to 2006(data from the weather station of Permafrost Study Station in Beiluhe).The ground surface is generally sparse in vegetation,and covered by fine sand with gravel (0.5 to 2 m in thickness).The underlying layers include silty clay (up to 8 m in thickness in some places)and mudstone with a sandstone interlayer.Permafrost characterized by high ice-content is continuous in the basin.Mean annual ground temperature ranges from-1.8 to-0.5 °C.Permafrost thickness ranges from 20 to 80 m,with an active layer of 1.8 to 3.0 m in thickness.

Figure 1 Location of the study area in the Qinghai-Tibet Plateau.Permafrost underlies 75% of the total area of the Plateau and is relatively warm and in places,ice-rich.The study site is within the continuous permafrost zone (from figure 1 of Lin et al.,2011a)

Numerous thermokarst lakes can be found within the basin (Figure 2).The mean area is about 8,500 m2,with a maximum of over 60,000 m2,and a minimum of 1,200 m2.Water depth varies from 0.5 to 2.5 m (Linet al.,2010).Approximately 70% of the lakes are elliptical in shape and 13% are elongate.Field observations show that about 80% of the lakes do not freeze to the bottom (Niuet al.,2011).These thermokarst lakes,isolated or linked,are occurred in the lower terrains where ice-rich permafrost or massive ground ice exist.

In this paper we investigate the ages of thermokarst lakes BLH-A and BLH-B (Figure 2).BLH-A is located on the west side of the Qinghai-Tibet Railway(QTR) Test Section at DK1141+015 mileage(34°49.51′N,92°55.38′E).This lake has an approximate area of 15,000 m2,elliptical in shape with the major axis of about 150 m and a minor axis of about 120 m.This lake is a closed perennial type,with water depth up to 2 m,and average ice thickness of about 0.45 to 0.5 m over the entire cold season.BLH-B is located on the east side of the QTR at DK1130 mileage(34°54.25′N,92°57.30′E).The area is about 30,000 m2,circular in shape with a diameter of about 200 m.The water depth is 1.2 m and average ice thickness is about 0.4 m over the entire cold season.

3 Field sampling acqusition and dating technique

In March 2006,a 50 cm length sediment core was obtained by drilling from an ice platform at the center of BLH-A.In April 2012,a 25 cm length sediment core was obtained through a gravity sampler from an ice platform at the center of BLH-B.The samples were hermetically sealed within an 8 cm diameter plexiglass tube,and then were separated into 1 cm thick slices at the State Key Laboratory of Frozen Soil Engineering,Cold and Arid Regions Environmental and Engineering Research Institute,CAS.In May 2006,samples from BLH-A were sent to the Environmental Change and Surface Processes Lab,Tibetan Plateau Research Institute,CAS for age determination.In May 2012,samples from BLH-B (two slices at top and bottom)were mailed to Beta Analytic Inc.(4985 SW 74 Court Miami,Florida 33155,USA) and finished the determination of age.

The Environmental137Cs and210Pb dating technique was applied to BLH-A.This method uses210Pb,excess210Pbexe(half-life 2,213 years),and137Cs (half-life 3,012 years) contained in modern sediments to infer lake age,and widely used in lake sedimentology (Wan and Santschi,1987;Wanet al.,1990;Xu and Wan,2001).This method also calculates lake deposition rate.The radiocarbon dating technique was applied to BLH-B.According to the Report of Radiocarbon Dating Analyses,the method used,material type,applied pretreatment and two-sigma calibration are presented in table 1.Calibrations are calculated using the newest(2009) calibration database of INTCAL09.

Figure 2 Aerial view of Beiluhe Basin and lakes BLH-A and B.Landscape image source:SPOT5 2010

Table 1 Method used,material type,applied pretreatment and two-sigma calibration result for each sample

4 Results and analysis

4.1 Sediment dating at BLH-A

137Cs,produced by nuclear tests of artificial radionuclides,is spread by atmospheric circulation and accumulates within lake-bottom sediment (Wan,1999;Zhanget al.,2005).Therefore,based on137Cs characteristics in the sediment,average deposition rate of the sediment can be calculated which infers lake age.Figure 3a describes the specific activity change in the137Cs core.137Cs appears in the 3.67-Bq/kg-residue at a depth of 3.5 cm,however no peak value was found.Thus,lake age can only be calculated by the appearance of the residue.According to137Cs distribution in lake sediment profiles in the Northern Hemisphere,it is believed that residue horizon of137Cs corresponds to the year 1947,when global nuclear test began.The calculated deposition rate of lake sediment based on a time scale of 1947 is as follows:

Whereγindicates deposition rate;drepresents residue depth;tis time range;yis lake age;andhis sediment thickness.Calaculated result shows that this thermokarst lake may have initiated before 843 a.

Atmospheric210Pb results from the radioactive decay of222Rn,and accumulates within lake or sea sediment.Since lakes are relatively a closed system,the specific activity of210Pb in sediment decays exponentially at its half-life.Environmental210Pb dating technique has two commonly computing models,Constant Rate of Supply (CRS) and Constant Initial Concentration (CIC).In this study,the CIC model was used because the sediment beneath the thermokarst lake originated from surface erosion in the Beiluhe Basin.

In figures 3c and 3d,a consistent trend can be observed for the specific activity change of226Ra and210Pb from the BLH-A lake core.The specific activity of210Pbexeand210Pb decays along with sediment depth,and an approximate balance value appears in the 6 cm depth.Therefore,lake age can be calculated based on lake sediment depositing rate using the CIC model formula.The CIC model is as follows:

whereC(h) indicateshdepth specific activity;C0is surface specific activity;λis the decay constant of210Pb of 0.0311/a;andtis deposition age.

The specific activity decay of210Pbexewith depth at an equilibrium fits an exponential curve (see figure 4).Thus,the formula to calculate deposition rate is-λt=-0.55341x,then,deposition rateγis

The calculated lake age according to the210Pb CIC model is as follows:

Through the aforementioned analysis and calculations,BLH-A formed before 800–900 a.

Figure 3 Vertical distribution of specific activities in a borehole core from lake BLH-A.(a) to (d) reflect the specific activity variations of 137Cs,210Pbexe,226Ra,and 210Pb,respectively

Figure 4 Vertical distribution of 210Pb specific activities in upper 6 cm depth of lake BLH-A,and its trend fits an exponential function

4.2 Radiocarbon age at BLH-B

Lake samples from BLH-B provided ample carbon for an accurate measurement.Figure 5a presents the result of radiocarbon dating and calibration of radiocarbon age to calendar years at the topmost (Figure 5a,B1) and bottommost (Figure 5b,B2) samples.For B1,13C/12C variables equal to-25.5‰,and the conventional radiocarbon age is 1,450±30 a B.P.Two Sigma calibrated result (95% probability) is Cal.AD 560 to 650 (Cal.a B.P.1,390 to 1,300).Intercept of radiocarbon age with calibration curve is Cal.AD 610 (Cal.a B.P.1,340) and one Sigma calibrated result (68%probability) is Cal.AD 600 to 640 (Cal.a B.P.1,360 to 1,310) (Heatonet al.,2009;Reimeret al.,2009).

Figure 5 IntCal09 calibration curves of radiocarbon age to calendar years at top sample (a,B1),and bottom sample (b,B2) in BLH-B

For B2,13C/12C variables equal to-23.3‰,and the conventional radiocarbon age is 2,230±30 a B.P.Two Sigma calibrated result (95% probability) is Cal.BC 390 to 200 (Cal.a B.P.2,340 to 2,150).Intercept of radiocarbon age with calibration curve is Cal.BC 280(Cal.a B.P.2,230),Cal.BC 260 (Cal.a B.P.2,210),Cal.BC 240 (Cal.a B.P.2,190),and Cal.BC 240 (Cal.a B.P.2,180).One Sigma calibrated result (68%probability) is Cal.BC 380 to 350 (Cal.a B.P.2,330 to 2,300),Cal.BC 300 to 220 (Cal.a B.P.2,250 to 2,180),and Cal.BC 220 to 210 (Cal.a B.P.2,170 to 2,160)(Heatonet al.,2009;Reimeret al.,2009).

Dates are reported as radiocarbon years before present (present = AD 1950).By international convention,the modern references standard is 95%14C activity of the National Institute of Standard and Technology (NIST) Oxalic Acid (SRM 4990) and calculated using the Libby14C half-life (5,568 years).Quoted errors represent one relative standard deviation statistics (68% probability) counting errors based on the combined measurements of the sample,background,and modern reference standards.Measured13C/12C ratios were calculated relative to the PDB-1 standard.

The Conventional Radiocarbon Age represents the Measured Radiocarbon Age corrected for isotopic fractionation,calculated using delta13C.The Conventional Radiocarbon Age is not calendar calibrated.When available,the Calendar Calibrated result is calculated from the Conventional Radiocarbon Age and is listed as the "Two Sigma Calibrated Result" for two samples.

5 Discussions

Sediment samples (organic lacustrine silt) were extracted from two thermokarst lake bottoms at 0.5 and 0.25 m depth.For thermokarst lake BLH-A,environmental137Cs and210Pb dating of approximately 800–900 a B.P.has been obtained.This shortened calculated age is due to a lack of specific activities peak in the vertical distribution of137Cs.Recently,the annual mean ground temperature beneath the lake-bottom is considerably warm (close to 5 °C),and the actual time that the permafrost beneath the lake-bottom completely thawed may be earlier than the calculated value.The radiocarbon age for thermokarst lake BLH-B is 1,450 ±30 a B.P.at top and 2,230±30 a B.P.at bottom samples.The measured data of these two lakes demonstrate that thermokarst lake ages in Beiluhe Basin is approximately between 1,000 a B.P.and 3,000 a B.P.,providing evidence for the origin and growth of numerous thermokarst lakes in this area.

Based on the dating to two thermokarst lakes,we suggest that there were several periods of thermokarst activity in Beiluhe Basin.Thermokarst lake development is not solely associated with changing climatic conditions in this region,since those that formed around 2,000 a B.P.do not appear to be directly linked to climatic warming (Burn and Smith,1990).We speculate that the initiation of most thermokarst activity is related to earth events which lead to ground warming,such as geological activites or ponding of surface water on the Beiluhe Basin.In central Manitoba and Siberia,evidence shows that more recent thermokarst development is associated with climate warming during the last 130 years (Thie,1974).Mackay (1975) also noted that some thermokarst lakes in the upper Mackenzie Valley may be the result of climatic warming during the twentieth century,which may coincide with a phase of thermokarst lakes in Beiluhe Basin.However,some younger thermokarst lakes,e.g.,those that formed around 50–60 years,are directly related to anthropogenic activity (Linet al.,2011b).

6 Conclusions

1) Environmental137Cs and210Pb dating technique calculations suggest that lake BLH-A may have formed around 800–900 a B.P.,and radiocarbon age of lake BLH-B is approximately 1,450±30 a B.P.at top and 2,230±30 a B.P.at bottom samples.

2) We provide evidence for the initiation of thermokarst lakes in Beiluhe Basin.The possibilities are that initiation of most thermokarst activity around 2,000 a B.P.is related to earth events,and recent thermokarst development is associated with climate warming during the last 130 years.Some younger thermokarst lakes initiated around 50–60 years ago are directly related to anthropogenic activity in Beiluhe Basin.

This work was supported by the State Key Development Program of Basic Research of China (973 Plan,2012CB026101),the Open Foundation of Key Laboratory of Highway Construction &Maintenance Technology in Permafrost Region,CCCC First Highway Consultants Co.Ltd.,and the Independent Project of State Key Laboratory of Frozen Soil Engineering,CAS (Grant No.SKLFSE-ZY-14).The authors would like to express their gratitude to the editors and the unidentified reviewers who provided insightful suggestions,which significantly benefited the authors for revisions,and the improvement in English.

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