The Study On Geochemistry Of Paleoproterozoic Carbonate Rocks In Hutuo Group, Wutai Mountain

Carbon isotope is always related to biology evolution and atmospheric O2 inventory. Carbon cycle is directly related to rebuild the evolution history of paleoatmosphere, paleo-ocean, palaeoclimate and palaeoenvironment on the earth. The earth experienced many important transformation during the Precambrian Era,such as biology evolution, the rise of atmospheric oxygen et al. Palaeoproterozoic formation lacks variety of standard fossils, thus it become difficult to compare the whole world formations or large area formations. So it is very necessary to use carbon isotope to study formation of the Precambrian Era especially Palaeoproterozoic. So far, a Palaeoproterozoic positiveδ13Ccarb excursion has been discovered from the carbonate sequences in North America, South America, Africa, Australia, Asia and Europe during 2.20~2.06Ga, and it'll be a good way to compare large area formations by studying positiveδ13Ccarb excursion.This dissertation arrange paper of the Palaeoproterozoic positiveδ13Ccarb excursion from the carbonate sequences discovered all around the world, and analyse the degree of post-depositional impact to those data by evaluating and analyzing data from those paper. And we analyze post-depositional impact to Palaeoproterozoic carbonate rocks in Hutuo Group(mainly in Dongye subgroup), Wutai Mountain by petrologic and geochemical tests such as microscope researches, microcoring, main and trace element analysis, carbon and oxygen isotope analysis, based on the deep understanding of strata, lithostratigraphy, upper and lower limit of Hutuo Group, and previous research on Paleoproterozoic carbonate rocks on the earth understanding of the direction of the theory of sedimental petroleum geology, isotope geochemistry and geological chronology and so on. All conclusions are shown as follow:A Palaeoproterozoic positiveδ13Ccarb excursion has been discovered from the carbonate sequences in North America, South America, Africa, Australia, Asia and Europe during 2.20~2.06 Ga, which are usually named Lomagundi event. Previous studies have revealed that the earth surface system had significant variations in North America, South America, Africa, Australia, Asia and Europe during the period of Lomagundi Event, which is the Palaeoproterozoic positiveδ13Ccarb excursion from the carbonate sequences during 2.20~2.06 Ga. Hypotheses including biology evolvement, the rise of atmospheric oxygen, the Palaeoproterozoic ice age, supercontinent break-up and evaporation, have been proposed to interpret the Lomagundi Event. Based on consulting abundant articles about positiveδ13Ccarb excursion around world, we make a comment on those hypotheses including geology evidence, but different impotance. Based on the carbon isotope fractionation theory, the developing magnitude of organic matter buried will increase carbon isotope ratio of carbonate carbon reservoir. Biology evolvement raised the ratio of atmospheric oxygen, warm environment after the Palaeoproterozoic ice age and Kenorland supercontinent break-up all create good conditions for biology developing, thus increase carbon isotope ratio of carbonate carbon reservoir. Otherwise, evaporates distribute all over the world, it may cause positiveδ13Ccarb excursion of local occlusive environment, and scientists should study depositional environment of all places discovered with positiveδ13Ccarb excursion.Choosing original data from North America and Fennoscandian Shield to discuss whether samples used had been alterated by weathering, metamorphism and diagenesis effort. Connecting with Mn/Sr,δ13Ccarb andδ18Ocarb, we findδ13Ccarbandδ18Ocarb data of samples are reduced by weathering, metamorphism and diagenesis effort. After comparing with isotope data of microcoring and whole rock powder samples, we find the post-depositional impact plays a more important role on oxygen isotope than carbon isotope. The oxygen isotope data of samples from vein is very different from samples from dolomicrite and dolospar, which makes data of microcoring dolomcrite samples different from whole rock analysis samples.Comparing our data with ZhongHua's data by marking data on bar map of sedimentary formation in Dongye subgroup, we find that Our carbon isotope data consistent with ZhongHua's data, a few oxygen isotope data of microcoring dolomcrite samples are bigger than ZhongHua's data. It suggests post-depositional impact plays an important role on oxygen isotope. From analyzing the carbon and oxygen isotope on the bar map, we can see a turning belt on the boundary of Daguandong formation and Huaiyincun formation. It suggest that since the deposition of Doucun subgroup, the atmosphere become oxidated from reductive because there wass new species blooming which can make photosynthesis. At the medium depositional term of Dongye subgroup, the total organic matter quantity was big, organic carbon quantity buried prosperous.