| 其他摘要 | Carbon, oxygen and strontium isotopes in marine carbonate is an important proxy in eochemical research. It supplies the key evidences to realize the fatal scientific problems, such as climatic change; the original carbon, oxygen and
strontium isotopic signals in seawater; secular variation of life-form vicissitude in ocean and land; and exogenic cycles of carbon and oxygen isotope. The results showed that brachiopod fossil shells was considered one of perfect samples because of the resistant of diagenesis, widely distributed and convenient operation. In this study, brachiopod fossil shells from different sedimentary units in north and south China were analysed for δ13C, δ18O and 87Sr/86Sr. To investigate the differences of δ13C, δ18O, and 87Sr/86Sr records between north and south China, and interpret sea level change, marine life-form vicissitude, intensity of weather and tectonic action.
In China, different strata were distributed in different sedimentary units from north to south in Devonian: south China was belonged to stable platform depositon,Longmen Mountain Sichuan was formed in miogeosyncline and north China was the outcome of eugeosyncline. Tectonic movement and volcanic activity were different between north and south China, whether were the discrepancy existed for δ13C, δ18O,and 87Sr/86Sr records of brachiopod shells, and were the differences existed for the sea level change, the life-form vicissitude and the intensity of weather.
Devonian brachiopod shells from north and south China were analysed for δ13C,δ18O, and 87Sr/86Sr records. The samples were screened for diagenetic alteration by the means optical microscope, cathodoluminescence and trace elements(Mn, Sr and Fe)contents. Some conclusions have beed received.
1) In the preservation experiments of brachiopod fossil shells: the microstructure of brachiopod fossil shells from north and south China were both completely, and have obviously fibrous layer and prismatic layer. But there were more microfissure which were filled by late substances in brachiopod fossil shells from north China. On the cathoduluminecscence, the brachiopod fossil shells from south China were almost non-luminescent besides of some shells with weak light because the late substances filled in the microfissure. Compared with south China, brachiopod fossil shells from north China have more weak light, especially for Neimenggu. The trace element contents showed the brachiopod fossil shells were preserved well. The results of preservement experiment showed brachiopod fossil shells from south China preserved well and brachiopod fossil shells from north China preserved worse. It will be affect the study of original isotopic signature, especially for oxygen isotope.
2) Because of the resetting of diagenesis, the δ13C, δ18O, and 87Sr/86Sr records of carbonate always changed. The comparison between carbonate whole rocks and brachiopod fossil shells showed: the δ13C and δ18O records of carbornate whole rocks were more nagetive than those values of brachiopod fossil shells, and 87Sr/86Sr ratios were inverse. But on the condition that don’t refer to the quantity study, the δ13C and 87Sr/86Sr values of carbonate whole rocks can be utilized to study the secular variation.But as for oxygen isotope, the difference is -2‰ ～ -3‰ between carbonate whole rocks and brachiopod fossil shells, it is so large for study the paleotemperature.
3) The carbon isotopic trend is a good proxy for the change of organic carbon burial rate. The results of carbon isotope showed that δ13C value were -2‰ ～ 4‰,and north China is more negative. The locfit curve of carbon isotope displayed it was similar for organic carbon burial rate between north and south China. In south China, three high-speed organic carbon burial rate were ascertained which was the results of the sea level change and the life-form vicissitude. In north China, the lower δ13C value may be caused by the diagenesis and the CO2 which was released in the volcanic activity.
4) Because of the affection of diagenesis, the δ18O values (-11.75‰ ~ -21.13‰)which came from north China was more negative than south China’s(Sichuan: -5.14‰~ -7.20‰, South China: -4.35‰ ~ -10.31‰). This results couldn’t supplied the definite key to whether the water-rock interaction control the δ18O values of the sea water. But the comparison the δ18O values between Sichuan and South China(Emsian:-7.20‰、-7.58‰, Gevetian: -5.62‰、-4.60‰, Frasnian: -5.18‰、-4.35‰), the δ18O values of miogeosyncline is consistent or even less than the δ18O values of stable paltform, showed that water-rock interaction couldn’t control the δ18O values of seawater.
5) The δ18O values (-9.9‰～-4.5‰) of Sichuan Longmenshan was obviously lower than the δ18O values of other area in the world, the discrepany was so large(-3‰~-4‰) in Emsian. The trend of trace element Mn , Fe and the evolution of the δ18O values showed: the seawater circulation in the Longmenshan was unsmooth,and it’s δ18O value was desalinized by the input of continental fresh water. According to the oxygen isotope thermometer calculate, the temperature of Longmen ocean was 52℃ in Emsian, it is a fatal to life-form. It is evidence for the δ18O values in Emsian was affected by the input of continental fresh water. From Eifilian to Frasnian, the temperature was 21℃ ～ 33℃, was a typical weather of tropical.
6) In early Devonian, high 87Sr/86Sr ratio showed that weather was intense.Maybe it was affected by Caledonian orogeny. More land was exposed which was beneficial to the weathering. And then 87Sr/86Sr ratio was declined showed that weather was weak. The reasons of low 87Sr/86Sr ratio were1) seafloor spreading bring more strontiun from mantle which was rich 86Sr, and 2) More land was flood because of the rise of sea level caused weather weaken. From late early Devonian to middle Devonian, the constant 87Sr/86Sr ratio showed the low weather and tectonic activity was stable. Late middle Devonian, the increasing of 87Sr/86Sr ratio showed weather was enhanced because of the increasing of temperature.
7) The comparison of δ13C, δ18O, and 87Sr/86Sr records from different sedimentary units showed: δ13C, δ18O, and 87Sr/86Sr records was affected by local area environment. When the study utilize the data which is gotten from different section to compile the whole curve, some points should be cautioned: carbon isotope could be used when distinguished the secular change or local area affection in the same sedimentary units and couldn't be used in the different sedimentary unit. Oxygen isotope, no matter what the same sedimentary unit or different sedimentary unit, can’t used to construct the evolutional curve. And 87Sr/86Sr ratios which was not affected by local area environment could be utilized to construct the evolution curve. |
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