Temporal And Spatial Variation Of Major Ion Chemistry And Dissolved Carbon In The Headwater Region Of The Yellow River:Impact By Landcover Types

The river is connected to several large carbon pools?atmosphere,ocean,terrestrial ecosystem,lithosphere?which is an important circulation route in the global carbon cycle.Chemical weathering is also involved in the global carbon cycle on short-term and long-term scales.Studying chemical weathering is important for revealing its impact on the carbon cycle.The Yellow River headwater region has a high altitude and is inaccessible which has special research significance in the Yellow River Basin.Our samples were collected during the period from April 2016 to October 2016 with the time interval of every two months in the headwater region of the Yellow River,which distributed in glacierss,permafrost,grasslands,peatlands.The spatial and temporal variation characteristics of river water chemistry and dissolved carbon under different land cover types were carried out.The order of cation content is Ca²⁺>Mg²⁺>Na⁺>K⁺,and the order of anion ions is HCO₃^->SO₄^2->Cl^->NO₃^-.The dominant cations are Ca²⁺and Mg²⁺,and the Na⁺and K⁺contents are extremely high in some samples?lake points in the frozen soil type?.The dominant anions are HCO₃^-and SO₄^2-ions,and the Cl-content of some samples?lake points in the frozen soil type?is extremely high.The SiO₂ concentration ranged from 0.47 to 4.29 mg/L with an average of 2.51 mg/L.Glacierss and grasslands types are controlled by rock weathering.Most of peatlands are controlled by rock weathering,some samples are controlled by atmosphericprecipitation.Permafrost are controlled by theevaporation crystallization and rock weathering.The cation weathering type is Mg²⁺-Ca²⁺-?Na⁺+K⁺?type,and the dominant cations Ca²⁺and Mg²⁺account for 80.37%of the cation composition,the Na⁺and K⁺contents are extremely small.The anion weathering type is HCO₃^--SO₄^2--Cl^-type,the sample component points are closely attached to the HCO₃^-axis distribution,and the lake samples are closer to the Cl^-concentration axis.The dominant anion HCO₃^-accounts for 79.72%of the anion composition,and SO₄^2-accounts for about 14.6%.The Na⁺and K⁺are mainly derived from the dissolution of evaporated salts and the effects of human agricultural activities.Ca²⁺and Mg²⁺are produced by carbonate weathering and sulfate evaporation.Silicate weathering contributes less to Ca²⁺and Mg²+.The source of HCO₃^-is mainly affected by the weathering of carbonate.The average DOC concentration in the headwater region of the Yellow River ranged from 1.99 to 11.42 mg/L,with an average of 4.72 mg/L and showed the trend of wetland>frozen soil>grasslands>glaciers.The average concentration of DIC is0.96-5.16mmol/L,with an average of 3.14mmol/L,and it is expressed as permafrost>grasslands>glaciers>peatlands.The average variation of pCO₂ value is532-1353.25 uatm,with an average of 785.68 uatm.Among the four land types,peatlands>grasslands>glaciers>permafrost.With the increase of the distance along the path,the pCO₂ value changes from a wide range of fluctuations to a more concentrated change.The pCO₂ value is stable in the high altitude glacierss and permafrost regions,concentrated at 400-800 uatm,and the fluctuations in the grasslands and peatlands areas increase.It is 400-1400uatm,due to factors such as increased climate change and enhanced human activities in grasslandss and peatlands.