| Heavy metals(HMs),such as Mercury(Hg)could migrate and enrichment in the environment which might pose extremely ecological risks.Therefore,researchers had paid lots of focus on tracing the source and geochemical cycle of toxic trace heavy metals in the environment.As a key area of the ecological barrier of the Yangtze River Basin,the subalpine ecosystem in western Sichuan is extremely vulnerable to suffer the external pollution input due to the increasing human disturbance and global climate change.The karst landscape in Minshan Mountain,a typical subalpine ecosystem of Western Sichuan,is easily be polluted by heavy metals with a series of external disturbance,such as the frequent geological disasters,intense bedrock weathering,heavy soil erosion,and the increasing inhuman emission.However,there are inadequate attentions to the distribution and source of heavy metals in these kinds of vulnerable subalpine ecosystem,especially for the HMs in those special travertine depositions.Based on this,we selected Jiuzhaigou National Nature Reserve(JNNR)in Aba Prefecture,Sichuan Province as the research area.We investigated and analyzed the content of heavy metals in the bedrock,soil and litterfalls in JNNR firstly.Then,we identified the source of HMs in water by combining the end-member model and APCS-MLR model.Thirdly,the factors influencing the distribution and seasonal variance of HMs in travertines was also verified through seasonal monitoring.Finally,we investigated the distribution and source of different travertines deposited in those typical travertine landscape of Western Sichuan.The main results are as following:(1)There is significant spatial difference of the distribution of HMs in JNNR soil.The average contents of As(18.98 mg/kg),Hg(107μg/kg)and Pb(13.86 mg/kg)in surface soil(0-5cm)were significantly higher than those of As(14.92 mg/kg),Hg(82.07)and Pb(9.734 mg/kg)in subsurface soil(10-20cm).The spatial heterogeneity of heavy metals in soil is not only influenced by the difference abilities of migration or enrichment among heavy metals during rock weathering,but also associated with the input of litterfall.The average content of Cd(0.10 mg/kg)in the coniferous litterfall is higher than that of mixed forest(0.07 mg/kg).The average contents of As(0.71 mg/kg),Co(0.69 mg/kg)and Ni(5.53 mg/kg)in former were significantly lower than those of As(1.54 mg/kg),Co(1.08 mg/kg)and Ni(6.95 mg/kg)in latter.However,there were significant differences in the migration and enrichment ability of heavy metals in different types of litterfalls,indicating that the sources of various heavy metals in litterfall and the contributions of litterfall to soil were different.The enrichment coefficient of Hg in different types of litterfall is nearly 30 times higher than that of bedrock,indicating that the input process of litter is the main source of Hg in surface soil.(2)The concentration of HMs in JNNR water is significantly lower than China surface water environmental quality standards I(GB3828-2002).The average concentration of Cr(3.48μg/L),Ni(1.20μg/L),Sr(548.36μg/L),U(1.51μg/L)and Pb(3.57μg/L)in water is 1.4～6.4 times that of the average river concentration in karst areas in southwest China.The concentrations of Zn(19.61μg/L),As(0.55μg/L),Rb(1.05μg/L),Ba(15.88μg/L)and Hg(2.45ng/L)are significantly lower than the average concentrations in karst rivers of southwest China.More than 75%of heavy metals in water are affected by weathering of bedrock,soil erosion,atmospheric precipitation and anthropogenic input.Although carbonate weathering is dominated the source of most HMs,such as As Cr,Ni,Zn,AS,Cd and Pb in different seasons,the change contribution of silicate rock weathering among the season has a greater impact on Cr,Ni,Cu and Zn in water bodies.Soil erosion is the most important source of Ni,Cu,Zn,As and Cd in water bodies.The contribution of anthropogenic input to the above heavy metals increased during the wet season,but was still smaller than that of other sources.The main source of Hg in water is atmospheric precipitation and soil erosion,and these two ways contributed more than 80%sources of Hg in water during the wet period.(3)The sedimentation of travertine is significantly influenced by biological processes.The travertine deposition in the Pearl Shoal(ZZT),a shoal flow system,was influenced by biological deposition,while that deposition of travertine in Nuorilang(NRL),the color pool system,was significantly influenced by hydrology-weather condition.The deposition rates of travertine in the Pearl Shoal were similar in the four seasons,ranging from 63.88-367.48g/m~2·yr.The deposition rate of travertine in the Nuorilang area has obvious seasonal differences,with the lowest deposition rate in spring(539.83±305.70g/m~2·yr)and the highest deposition rate in summer(1567.31±277.70g/m~2·yr),which is nearly three times that of spring.The deposition rates of travertine in different seasons in the two regions are in the order as summer>autumn>winter>spring while the deposition rate in Pearl Soal was significantly lower than that in Nuorilang area.Besides,the regional environment differences led to the variation response of mercury content and deposition rate of travertine to climate.As the travertine deposition in ZZT had more frequently exchange with atmosphere than in NRL,the Hg content in travertine deposited in ZZT was significantly positively correlated with the GEM concentration while the correlation did not exist in NRL.In wet seasons,the dilution effect caused by precipitation had more influenced in travertine deposition process than the influence algae development caused by warming up process.Meanwhile,dilution effect strongly inhibited in the content and deposition rate of Hg in shoal flow system but improved the deposition rate of Hg in those travertines deposited under forest canopy.The water disturbance caused by both the increase of precipitation and the physicochemical condition in wet seasons might be the potential reason for the decrease of Hg content but the increase of Hg deposition rate in NRL.(4)After identified the spatial distribution and source apportionment of three different development levels named Huanglong(5A scenic spot),Shenxianchi(4A scenic spot)and Kalonggou(3A scenic spot),it is found that human disturbance,geological background and regional climatic condition of different travertines landscapes were the potential reasons affecting the content of heavy metals in travertine of Western Sichuan.In the above three typical travertine landscapes with different development levels,the HMs contents in order from the largest to the smallest is Huanglong,Shenxianchi and Kalonggou,and the peak value of HMs in travertine of Huanglong and Kalonggou scenic spots all appeared in the entrance area with more intense human disturbance.According to the HMs contents of 173 travertine samples collected from 26 typical karst travertine landscape in Western Sichuan,the average contents of As(15.24mg/kg),Cd(0.17mg/kg),Cr(4.47mg/kg),Hg(11.83μg/kg),Tl(0.057mg/kg),Co(0.53mg/kg),Ni(2.83mg/kg),Rb(1.90mg/kg)in Northwest Sichuan travertine landscape are with 1.52～6.54 times higher than those in Ganzi Prefecture.It is indicated that the HMs in the travertine in Northwest Sichuan are more influenced by various source contributions,while the karst weathering processes of local geological bedrock are dominated in the HMs contents of travertine in Ganzi region.In this thesis,we found that the contribution of alpine karst processes to soil heavy metals is limited in the subalpine forest ecosystem,and the input of litterfall contributes more heavy metals to the surface soil through the analysis of HMs content and distribution in different environment components.The results of end-member analysis and source apportionments show that atmospheric precipitation and soil erosion are still the main sources of Hg in the water,even in the remote watershed dominated by karst recharge.At last,although travertine deposition with different types and in different regions have the potential to record changes of heavy metal contents,there are still lots of much works to quantify the relationship between travertine deposits and heavy metal sources. |
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