Study on the intensity of boreal karstification under different geological conditions:A case study at the recharge area of Baotu Spring drainage area,Jinan,Northern China

LIU Wen; XU Congcong; YU Lingqin; LI Haixiang; LU Qianqian; ZHU Qinfeng; LIU Haoran

doi:10.11932/karst20230502

Abstract：It is an indisputable fact that the process of karstification can form carbon sink,and the research on this process is in the ascendant.However,the factors affecting karstification process are very complicated,and the geological background is one of the most important aspects.The effect mechanism of different geological backgrounds on karstification and the data of dissolution rate under different geological backgrounds can provide a basis for understanding the karst process and its mechanism under sub-humid climate and improving the estimation accuracy of karst carbon sink.The recharge area of Baotu Spring area in Jinan,a typical boreal karst spring area,is selected as the research area,and the standard dissolution tablet test of carbonate rock is used for field measurement. At the northern edge of the mountainous region in central Shandong,the research area is located in the Baotu Spring area of Jinan,bordering Mount Tai in the south and the Yellow River in the north(116°40′30″-117°14′10″E;36°14′50″-36°46′10″N).The spring area borders the Mashan fault and Dongwu fault to the west and east,respectively.These two faults are permeable in the north and weakly permeable in the south.The southern boundary is the ridgeline of Mount Tai,and the igneous body in the north is called"Jinan Rock".Constrained by the special topography and geological structure,precipitation infiltration in the southern mountainous area recharges the Cambrian-Ordovician karst aquifer which presents strong karst development,and then moves from south to north.According to the statistics of Jinan Meteorological Bureau,Jinan City is characterized by a warm temperate continental climate with four distinct seasons at an annual average temperature of 14.7 ℃.Its average precipitation per year is 647.9 mm,more than 70％of which is concentrated in July,August and September with less from December to March.During the four-year test,pH values of precipitation in Jinan(Guishan)National Basic Meteorological Station averaged between 6.96 and 7.61,which means there was no acid rain in this period. The standard dissolution tablet method is widely used in the study of karstification process because of its advantages such as short-term monitoring,simple preparation of sampling,mature embedding method and the condition closer to the natural state than that in a laboratory.The study area is located in the low mountainous but hilly area of the north wing of Mount Tai and the southern Jinan far away from the urban area,which is also the recharge area of Baotu Spring area.The area is forest and shrub land with less interference from human activities.We buried standard dissolution carbonate tablets around the end of November 2017,and retrieved them around the end of November 2021,with a total test period of four years.When burying these tablets,we selected test sites with different basement lithology in the same small watershed where the meteorological conditions are not significantly different,to ensure the relative consistency of test backgrounds such as precipitation and temperature.Meanwhile,we collected layered soil samples for testing physical and chemical indexes,atmospheric and soil CO2,soil water content,etc. Results show that the average dissolution rates of granite,carbonate,loess and shale are 3.49 mg.cm-2·a-1,0.26 mg·cm-2·a-1,0.11 mg·cm-2·a-1 and 0.09 mg·cm-2·a-1,respectively.In other words,the influence of basement lithology on dissolution rate is significant.The dissolution rate of granite area is much larger than that of the other ones.The dissolution rate of carbonate area is larger than that of loess and shale,and the lowest in shale area.From a vertical profile perspective,the dissolution rate of carbonate rock area,loess area and shale area is atmospheric＞surface＞soil,and the rate in soil decreases with the increase of depth,while the dissolution rate in granite area shows a completely opposite trend. It is found that there is no obvious correlation between soil CO2 and dissolution rate.The variation trend of soil organic matter content in the profiles of carbonate rock,loess and shale is consistent with that of dissolution rate,indicating that the former may have a direct effect on the latter,but the consistency does not show in the granite area.There is an exponential correlation between soil pH and dissolution rate at all sites,and the dissolution rate decreased significantly with the increase of pH.Soil water content has an obvious linear relationship with the dissolution rate,and is the limiting factor of karstification process in boreal karst area compared with soil CO2. This study is a useful exploration in the boreal karst area,the results of which help to deepen the understanding of the karstification process under the semi-humid and semi-arid climate conditions,and provide a comparative example for other karst areas in the north and even the whole karst area of the same type.