Carbon Isotope from Shallow Marine System in North China:Implications for Stratigraphical Correlation and Sea-Level Changes in Cambrian

Cambrian System at the Shatan Section in northern Henan,North China,consists of sedimentary successions of tidal flat and shallow-water carbonate platform facies.Data of carbon isotope(δ^(13)C_(carb))from this section reveals five positive δ^(13)C_(carb)(P_(st-1)-P_(st-5))and five negative δ^(13)C_(carb) excursions(N_(st-1)-N_(st-5)).In the positive excursions,δ^(13)C_(carb) rises to 0.88‰,1.05‰,2.04‰,1.00‰ and 2.97‰,respectively,while in the negative excursions δ^(13)C_(carb) drops to -8.00‰,-3.50‰,-1.00‰,-0.33‰ and around -2.00‰,respectively.On the basis of chronostratigraphy of Cambrian,the most remarkable carbon isotope excursions can be correlated regionally and globally.In addition,one second-order and ten third-order sequences have been recognized at this section.Correlating third-order sequences and chemostratigraphy indicates that carbonates from the basal part of the transgression system tract(TST)and the upper part of the high-water system(LHST)generally have lighter δ^(13)C_(carb) values,whereas massive carbonates with microbialite from the lower part of the high-water system tract(EHST)usually have heavier δ^(13)C_(carb) values.The association of δ^(13)C_(carb) values with sea-level fluctuations suggests that the positive carbon isotope excursions or high δ^(13)C_(carb) values may have been caused by an increase in the marine primary productivity in response to maximum seawater flooding during the transgression.

Carbon-Isotope Excursions Recorded in the Cambrian System,South China:Implications for Mass Extinctions and Sea-Level Fluctuations

Cambrian carbonates with abundant fossils of agnostoid trilobites deposited on the southern slope （Jiangnan slope belt） of the Yangtze Platform and in the Jiangnan deepwater basin are well exposed in the Wangeun Section of western Hunan, South China, and in the Duibian A Section of western Zhejiang, southeastern China, respectively. To better understand the response of carbonisotope excursions to depositional environment changes, mass extinctions and eustatic events, we collected 530 carbonate samples in fresh roadcut exposures of the two measured sections for analysis of carbon and oxygen isotopic compositions. Data of δ^13C from the Wangcun Section, western Hunan, South China, demonstrate that the Cambrian carbon-isotope profile includes three remarkable positive excursions CPEwc-1, 2, 3 in the Upper Series 2, in the Lower and in the Middle Furongian Series. Three distinctive negative excursions CNEwc,-1, 2, 3 were separately tested in the Lower Terreneuvian Series, Lower Series 3 and in the Upper Furongian Series. Similarly, in the corresponding horizons in the Duibian A Section, Zhejiang Province, southeastern China, three positive excursions CPEdb-1, 2, 3 and three negative excursions CNEdb-1, 2, 3 also have been discovered. We interpret these significant carbon-isotope excursions as being associated with enhanced biogenic prodnctivity, mass extinctions and eustatic events.