| The wide areas of Guangxi is covered by the carbonate rock strata.And the karstification uses widely.The background value of cadmium in Guangxi soil is 0.267mg/kg,it is 2.75 times higher than the background level of soil cadmium in China.It is only second to Guizhou Province and higher than other regions in China.It has obvious background geochemical characteristics of cadmium.According to the results of previous study and our previous work,most of the areas where soil Cd anomalies occur are in karst areas,and the relationship between carbonate rocks and Cd anomalies has become a hot topic in the current study.Therefore,the study of carbonate rocks in karst areas has been conducted.The distribution characteristics of cadmium element content is great significance for understanding the formation of cadmium-abnormal soil in karst regions.Taking the carbonatite rocks as study objects,which are from Devonian-Triassic system in the karst areas of Mashan County and Longzhou County in Guangxi.336 samples of rock and 32 samples of soil were collected respectively.Samples were pre-treated and microwave-digested.The Cd content of the samples was determined by graphite furnace atomic absorption spectrometry.On this basis,a detailed summary of the distribution of Cd elements in the carbonate rocks from the Devonian to the Triassic formations in the above area was carried out,and the distribution characteristics of the Cd elements in the soil and its relationship between the carbonate rocks were analyzed.The main conclusions are as follows:1.In the 37 carbonate rock profiles,the Cd content in the rock can be detected,and 95% of the carbonate samples exceed the Cd abundance value of the crust.2.From the sample analysis results obtained,there are differences in cadmium content in carbonate rocks formed at different stages of the same era.Among them,the Lower Devonian has the highest carbonate content.The content of Cd in Upper Devonian is secondary,and the Cd content in Middle Devonian is lowest.The content of cadmium in Lower Carboniferous carbonate rocks is lower than that in Middle Carboniferous.3.The highest values of Cd from the Devonian to Triassic in the study area occur to the Permian strata.Through analysis of variance,the results show that there is a significant difference in the content of Cd in carbonate rocks of different strata.The content is:Permian> Carboniferous> Devonian>Triassic,and the content of Cd element from Devonian to Triassic exceeds the average content of Cd elements in carbonate rocks in the earth's crust,keeping an abnormity of high Cd.The contents of Cd element in the Triassic is smaller and within the normal range.4.Among different carbonate rock types,the content of Cd in limestone is the highest,followed by dolomite,and the content of Cd in marl is small.The percentages of the three types of rocks exceeding the carbonate mean are 97%,94%,and 71%,respectively.5.The secondary cave carbonate rocks in the Devonian to Permian carbonate formations have higher Cd content,and their contents range from 0.373 to 2.419 mg/kg.Comparing the Cd content of the secondary carbonate with that of the carbonate Cd in the same strata,the results show that the Cd element content of the Devonian secondary carbonate is 3.760 times higher than that of the carbonate rock.The content of Cd in Carboniferous secondary carbonates is 7.417 times higher than the average value of Cd elements in Carboniferous carbonate rocks.The content of Cd in Permian carbonate rocks is 7.353 times higher than that of the double-stacks.6.By comparing the collected soil samples with the average Cd content of carbonates in the corresponding formations,it was found that the Cd contents in the Devonian to Permian strata exceeded the Cd content in the carbonates of the local strata by 58.669 times,36.612 times,and 12.827 times respectively.Single factor index method,geological accumulation index method and potential ecological risk index method were used to evaluate the pollution degree of Cd in the soil of the study area.The results showed that the soil in the study area was seriously polluted by Cd,and there was a strong potential ecological risk. |
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