The Carbon Isotope Excursions And Paleo-oceanography Of The Ordovician-silurian Carbonate Rocks From The Xainza Area,Tibet

The carbon isotope compostion of carbonate rocks in ancient times could reveal the carbon isotope compostition of paleo-seawater,approximatively.Therefore,the study of the carbon isotope value evolution is to research the carbon isotope value variation of ancient carbonate rocks.Based on the study of carbon isotope values of carbonate rocks,which come from different paleo-plates or different paleo-geographic locations,carbon isotope value characteristics,such as the range of variation,the magnitude of excursion and the style of excursion,can be recognitioned by cruves.And that,the carbon isotope value cruves can trace the carbon isotope events that have profound highly significance.What's more,it is very important for exploring the origin of carbon isotope excursions.In addition,the study of carbon isotopes is not only the effective supplement on lithostratigraphy and biostragraphy,but also is the direct measures to study global changes,such as paleo-ocean environment variations and paleo-climatic environment variations.The main research object of this thesis is carbonate rocks of the 5118 section in Xainza area,Tibet.Based on the outcrop observation,the section identification,element analysis,the cathodoluminescence analysis,carbon and oxygen isotope analysis,some information have been obtained,including petrologic features,mineralogical features,sedimentary facies features and elements geochemical features.And some results and conclusions have summarized as below:?1?During the Ordovician-Silurian period,the Xainza area in Tibet is belongs to the Lhasa block of the Yunnan Tibet block group.There has been abroadly developed shallow sea carbonate rocks.The 5118 section of Xainza area in Tibet has developed succession strata,which contains the Gangmusang Formation of the upper Ordivician,the Dewukaxia Formation of the lower-Middle Silurian and the Zhanongema Formation of the middle-upper Silurian.The lithology contains nodular limestone,bioclastic micrite limestone,mud shale,crystalline dolomite.There are a large number of biological fossils,such as conodonts,brachiopoda,spirocrinus,sponge spicule and ostracoda.Especially,plenties of graptolites and brachiopodas have hosted in shale,which formed in Hirnantian stage.?2?The original information has been preserved well,which have been accessed by 6 aspects as followed:?1?the intensity of crystallization and the degree of alteration is low,and it has apparent original sedimentary tectonics.?2?the correlation between carbon isotopes and oxygen isotopes is low,and only a few oxygen isotope values of carbonates shows lower values?<-10‰?.?3?Mn/Sr ratio values are lower than 2,and most of ratios are spread from 0.4 to 1.2,and the cathodoluminescence characterstics shows dark red light or non-luminance.?4?TOC values are lower than1mg/g,and the??values are distributed in 28 to 30.?5?H/C ratio values are more than 0.5,and the correlation between H/C ratio values and?¹³C_org values is low.?6?the rare element compostion has reveal that the carbonates influenced by terrigenous matter and diagenesis is limited.?3?4 times carbon isotope excursions have been observed from the carbon isotope values curves of carbonate rocks in Xainza area,Tibet.They are HICE,AICE,SICE and LICE,and the amplitude are+1.8‰,+1.6‰,+5.1‰,+6.3‰,respectively.By the contrastive analysis of carbon isotope excursions in different areas,it can be known that the carbon isotope excursions during the Ordovician-Silurian period are ubiquitous all over the world,and they are global carbon isotope excursion events.?4?By analog computing the parameter of paleo-ocean environment,it can infer that:?1?the tempertature of paleo-seawater is vary from+9.97⁺27.87?during the Ordovician-Silurian period.The tempertature of paleo-seawater during the Ordovician period is lower than that during the Silurian period,and it shows the lowest value at the last Ordovician period,and rise up gradually at the middle-late Silurian period.?2?5times organic carbon burial coefficient values excursions have been observed,and the maximum amplitude is 0.36,the minimum amplitude is 0.11,and the average value is0.22.?3?the main carbon ion in the paleo-seawater is HCO₃^-,and there are 3 low values in the[DIC]value curves.The amplitude of excursion can reach 300?M.?4?the PH value of the paleo-seawater is vary from 7.80 to 7.98 during the Ordovician-Silurian period,and which indicate that it is a relative stable period about power of hydrogen.?5?By researching the relationship between inorganic carbon isotope,organic carbon isotope and paleo-seawater parameter,we can learn that:?1?the correlation between the carbon isotope variation and[DIC],temperarure,PH values are low,While the correlation between the carbon isotope variation and organic carbon burial coefficient is obvious.?6?On one hand,GOBE during the Ordovician period has cased biological repid prosperity,which enhanced the photosynthesis production rate,and leaded to ¹³C enrich,relatively.On the other hand,the developed Antarctic ice sheet increased the strength of weathering,which also made ¹³C enrichment,relatively.So the two aspects combined action has speed up the plankton blooms,the reduction of pCO₂,climatic cooling quickly,the sea level falling,minification of creatures living space,and all of those lead to ¹³C enrich,tremendously.So the fundamental reason of carbon isotope excursion is the increasement of organic carbon burial coefficient values,while the mass extinction is likely to the eutrophication and biological overly propagate.?7?During the Ordovician-Silurian period,the temperature is rise up gradually,and pCO₂ has increased,and the ice sheet is melting,and the sea level is rising up,the land area is midishing,and the weathering has weakened.The melted ice sheet would lead to ¹²C enrich in paleo-seawater,while the biotic recovery could make ¹³C enrich in paleo-seawater.Those two factors give rise to the carbon isotope excursion amplitude is small.The pCO₂,temperature,the ice sheet and the land area are stable during middle-late Silurian period.Therefore,the 3 carbon isotope excursions during Silurian period are relate to bioproductivity,and the contribution of weathering is limited,while glaciers melting weakened the carbon isotope excursion amplitude.?8?Based on the systematic stable isotope analysis of carbonate rocks from the5118 section in the Xainza Area,Tibet,there are 4 carbon isotope excursions have been recognized firstly from the carbon isotope values curves of Ordovician-Silurian period.And the maximum amplitude peak has been founded from the carbon isotope values curves of Silurian period.?9?By comparison and analysis of the contemporaneous carbon isotope values curves from different areas,it has been confirmed that the carbon isotope values curves of the carbonate rocks from the Lhasa block has the characteristics of synchronism.And it can be as a reference section of carbon isotope for the research on the evolution process of paleoceanography in paleo-Tethys.