| Yangzong Lake(North altitude 24?51?24?-24?57?58?,east longitude 102?58?47?-103?01?41?)is located in the plateau in middle Yunnan Province,where it belongs to a typical rift lake of faulted structure.In the recent decades,a great many industrial and mining enterprises and tourism facilities have been built around the lake,leading to substantially increasing amount of total pollutants as a result of intense human activities,the massive arsenic pollution in 2008 for instance,has fully deprived the lake of basic ecological function.In this article,current water quality at south bore(S1),middle bore(S2)and north bore(S3)is investigated,carbonate and grain size of sediments,organic matters and their carbon and nitrogen isotopes,total sulfur,sulfate,reduced sulfur and organic sulfur are analyzed,chronosequence of sedimentation in the last 200 years is established on the basis of 210 Pb and 137 Cs dating,together with comprehensive analysis of interrelation among environmental indicators,with following conclusions:(1)Electrode potential indicates anaerobic environment in the lakebed,the existence of thermocline contributes to some extent to the process of sedimentation.(2)Although element S continuously changes with oxidation-reduction of sediments in the core,the variation of stable reduced sulfur and organic sulfur in percentage to a certain degree distinctly indicates intensity variation and pollution sources of sulfuric pollutants,with sensitive response to the intensity of human activities.(3)Due consideration should be given to the difference of element S in spatial distribution in the bore of sediments,and oxidation-reduction state inside various bores,so as to restore the ecosystem in a more effective manner according to environmental change.The elements Fe and Mn in pore water will release into the water with the appearance of thermocline,but reduced sulfate can chemically combine with these elements to produce sulfide,thus release of Fe and Mn is suppressed to some extent.(4)The change of sedimentary environment of Yangzong Lake in the last 200 years can be roughly divided into three stages,both trend change of bores and value change of several environmental indicators indicate certain similarity,and difference among lake areas.Stage I(1808-1959 a A.D.,39-22 cm):Variation of all environmental indicators during 1808-1959 a A.D.(39-22cm)is stable as reflected by sediment sections,except 3 bores indicated with distinct difference in terms of detail variation.1872-1926 a A.D.(34-28cm)bore S3 probably reflects artificial forest or human activities of exploitation and reclamation of lake basin,logging and charcoaling,but with the sign of slight increase of C4 plants(e.g.gramineous plants like maize)during 1926-1960,resulting in decreased ?13Corg value of S3.Stage II(1960-1989 a A.D.,22-11 cm): Stage II a(1960-1979 a A.D.,22-16 cm)The detrital substances and plant residues encroached in the lake in this stage may have direct impact on the variation of sediments in surrounding lake basin,thus visibly reserved in the bores.Organic matters entering in the lake basin are mainly composed of C3 plants with slightly increase.The content of organic sulfur appears by minor fluctuation;as for occurrence of lake acidification phenomenon in this stage,organic sulfur and iron sulfide will occur alternately during reduction of sulfate,however,when the content of active iron is relatively low in the sediments,organic sulfur will prevail.Stage II b(1980-1989 a A.D.,16-11 cm)It's clearly found in this stage more evident sign of human activities,including certain difference between 1980-1985 a A.D.and 1986-1989 a A.D..According to meteorological data,precipitation increased during 1980-1985,corresponding sediments recorded higher water power.Precipitation decreased during 1986-1989,median diameter of bores near the lake outlet was likely to be more sensitive to the variation of precipitation than that in the middle of the lake.The variation of TS,SO42-S,reduced sulfur and organic sulfur increased notably during 1980-1990,reflecting intensified human activities,and especially accelerated in terms of growth rate from 1986 onward.The increase of nutrition level will further enhance primary productivity of the lake,and enable ?15N value to maintain enrichment trend.Gradually,Yangzong Lake got rid of being an acidified lake,and evolved to be a neutral and slightly alkaline lake.Stage III(1990-2015 a A.D.,11-1 cm):Since 1990s',industrial and mineral industries,tourism and fishery have developed around Yangzong Lake,leading to year-on-year increased amount of emission of industrial wastewater and domestic sewage.The sharp drop of water level in 1993 was clearly responded in all environmental indicators.The input of organic matters during 1997-1999 composed mainly of inflow of organic substances and nutrient salt by human activities.The peak of total sulfur during 1997-1998 was probably related to the tailings emitted by factories in the southwestern area from 2001 onward,and heavy tailings will migrate downstream together with the sediments.Bore S2 with higher content of SO42-would produce more FeS2 during 2007-2008,but not excluding the possibility of tailings emitted from a few factories in this period.The content of S tended to decrease during 2008-2015,which was likely to be connected with the spray of sediment adsorbent of polymerization ferric chloride(PFC)after arsenic pollution of the lake.Sulfur,as an ideal medium for the study,comprehensively expresses the sign of human activities and oxidation-reduction process of sediments. |
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