Surface And Subsurface Chemical Weathering Research In Hainan Island

Chemical weathering of rocks consumes atmospheric CO₂and plays an important role in regulating the global climate.The weathering contribution of tropical volcanic island/island arc areas is far greater than its area proportion,and influences the Sr isotope evolution of seawater through dissolved and particulate forms.In order to evaluate the surface and subsurface chemical weathering rates and their ability to consume atmospheric CO₂in Hainan Island,we conducted a comprehensive study on river water,groundwater,rainwater,bedrock and riverbed sediments of small"mono-lithology"watersheds that flowing through basalt and granite and three major large river basins(Nandu River,Changhua River and Wanquan River)in Hainan Island,China.The contents of major elements,radiogenic Sr isotope ratio(⁸⁷Sr/⁸⁶Sr)and stable Ca isotope of water and solid samples were analyzed.The results show that there is a positive correlation between the contents of silicate and bicarbonate in river water of both large and small river basins,which indicates that the chemical weathering flux in the study area mainly comes from silicate weathering,and preliminarily excludes the important contribution of trace carbonate minerals.Since the three large rivers all originate from the central part of Hainan Province,where granite is dominant,and the middle/lower reaches flow through the basalt distribution area,the major ions are mainly from the mixing of granite and basalt endmembers.There is a certain relationship between the chemical alteration index(CIA)and ⁸⁷Sr/⁸⁶Sr of the riverbed sediments,which may be attributed to the stage differential weathering of sediments with the deepening of weathering degree:in the early stage of weathering,biotite weathering is dominant,and⁸⁷Sr/⁸⁶Sr is higher.Then plagioclase begins to be weathered and ⁸⁷Sr/⁸⁶Sr decreases gradually.After a large amount of decomposition of plagioclase,the weathering of various potassium-rich minerals is intensified,and the ⁸⁷Sr/⁸⁶Sr value rises to the highest point at this time.In the stage of high weathering degree,with the gradual decrease of potassium-rich minerals,the stable weathering residual materials dominate,and the ⁸⁷Sr/⁸⁶Sr value gradually decreases to a low value and then stay steady.Therefore,the ⁸⁷Sr/⁸⁶Sr value of particulate matter transported to the estuary/coastal zone by river is controlled by lithology,topography,transportation time,climatic conditions and other factors,which will directly affect the evolution of ⁸⁷Sr/⁸⁶Sr value of seawater after the reaction of particulate matter with seawater.The preliminary result of Ca stable isotope shows that?⁴⁴Ca of river water in the small basaltic watersheds of Hainan Island varies from 0.80‰to 1.28‰(2SE=0.06).Since the Ca isotopic compositions of different basic minerals are relatively uniform,the changes of Ca isotope may not be caused by weathering of different endmembers,but are mainly controlled by surface processes that can cause quality fractionation,such as plant absorption and carbonate deposition.After the correction of atmospheric input,the average surface and underground chemical weathering rates of Hainan Island is 25.8 t/km²y and 17.8 t/km²y,respectively.Compared with the young active volcanic islands,namely,the Lesser Antilles Islands in the Caribbean Sea and the Reunion Island in the Indian Ocean(the underground chemical weathering rate of which is 2-5 times higher than the surface weathering rate),the underground water flow and total dissolved solid(TDS)in Hainan Island are much lower due to the relatively low precipitation and the low porosity of the old dead volcano,resulting in lower underground chemical weathering rate than that of the surface.The underground chemical weathering rate of Hainan Island is only close to that of Kamchatka active volcanic island in high latitude,which is about 6-25%of that of the Lesser Antilles and Reunion Islands,being one of the regions with the lowest contribution of underground weathering in the world.The surface chemical weathering and CO₂consumption rates of basalt area in Hainan Island are relatively low compared with other tropical volcanic islands,slightly lower than those of Hawaii and Deccan,and significantly lower than those of Java and Luzon in Southeast Asia.Global data in basaltic areas show that runoff has a very obvious control on chemical weathering rate under similar temperature conditions.Due to the low annual runoff,the CO₂consumption capacity of Hainan Island is only in a global average range.