Chemical Weathering And Its Palaeoclimate Significance During The Nantuo Glaciation In Guizhou

The"Snowball Earth"event in the neoproterozoichad a profound impact on the earth's atmospheric composition,biological evolution and biogeochemical cycle of elements.There is a great debate about the Neoproterozoic global glacial climate,that is,whether the earth's surface was completely covered by ice.Contrast to primary Snowball Earth hypothesis,in which earth surface(including oceans)was covered for several millions of years,several lines of evidence of sedimentary petrology,palaeontology,palaeoclimate modeling,and geochemistry showed that there existed open waters during Neoproterozoic Glaciations.Chemical Index of Alteration(CIA)is often used to determine the degree of chemical weathering in the provenance and to reflect the glacial climate change.However,the CIA index is affected by many factors.As for the multi-source and multi-component clastic rocks in nantuo ice age,few people discuss whether they can correctly reflect the chemistry of the ice age,so the results may not be convincing.This article selects the located in the slope with complete sedimentary records of glaciers and Daotuo mangnese mine lot core drill(ZK102)and Dalu Mangnese mine lot core drill(ZK505)located in Songtao country,and Wuhe profile in Jianhe county,Guizhou province,The drill and the section of Nantuo Formation are complete and continuous without obvious structural damage.The drilling core is not affected by late weathering,and the section is also weak from late weathering.It is an ideal material for studying the chemical weathering of Nantuo glacial period.Previous studies on the Wuhe section suggest that the Nantuo Glciation in the section experienced four climatic changes of different scales.Based on the study of sedimentology,petrology and mineralogy,this paper holds that Nantuo Glaciation in Guizhou province experienced four times of glacier advance and retreat,which is consistent with previous studies.Through the analysis of geochemical characteristics and observation under the aggregate microscope,it is concluded that during the Nantuo Glaciation,there was weak chemical weathering,which was mainly plagioclase weathering,among which potassium feldspar weathering was weak.Weathering trends are commonly plotted on A-CN-K triplots and interpreted in relation to a“predicted weathering trend”running parallel to the A-CN axis,the trend is in the direction of the A-K axis,which is caused by the weak weathering of K-feldspar in Nantuo Glacial strata,rather than potassium metasomatism in the sedimentary diagenesies that leads to.In terms of the loss of major elements,Ca O shows the most loss,followed by Na₂O and P₂O₅.The loss of Ca O and Na₂O are result of the chemical weathering of plagioclase,the loss of P₂O₅ reflects the weathering of bearing-P mineral.K₂O enrichment shows that K-feldspar weathering is weak.The loss of Sr and relative enrichment of Rb also indicate the weathering of plagioclase and weak weathering of potash feldspar.The loss of Cu and the negative anomaly of Ce reflect the change of the reducing environment during the Nantuo Glacial.Th/U was mostly between 4 and 10.23,and Rb/Sr was mostly between 0.52 and 2.13,indicating that there was strong chemical weathering during the Nantuo Glacial.The CIA was verified by comparing and discussing with previous studies on chemical weathering at the end of the glacial period,combining with its rock assemblage and microscopic weathering characteristics.It is believed that the CIA value is between 58 and 72,consistent with the characteristics of chemical weathering during the glacial period.It is believed that the CIA index of fine grain components of nantuo formation can reflect the degree of chemical weathering of multi-source and multi-component in the Nantuo Glaciation.Both the?^AlNa and?^AlCa can reflect the weathering during the Nantuo Glaciation.Furthermore,they reflects the fluctuation of temperature in Nantuo Glaciation climate.