The Characteristics Of ?¹³C In The Third Section Of Doushantuo Formation In South China And The Indication Of The Paleo-ocean Redox Conditions

The Ediacaran Doushantuo period(635-55 1Ma)was an important period of earth surface oxidation and biological evolution since the "Snowball Earth" event.During the sedimentary period of member ? of Doushantuo formation,the largest carbon isotope negative excursion(EN3)in geological history was observed,which reflected the highly dynamic carbon cycle characteristics.Due to closed relationships between the carbon cycle and the redox state of the atmosphere-ocean system and the biological evolution,it is significant to find out the characteristics and cycle mechanism of carbon for understanding the ancient atmosphere-ocean redox conditions and the co-evolution mechanism between life and environment in this important geologic time.Previous studies showed that the paleo-ocean redox conditions during the Doushantuo period was largely affected by regional geological background,which resulted in differences in the spatial redox structure between the shelf margin and the open sea areas.At present,the researches on the paleo-ocean in the sedimentary period of member ? Doushantuo formation in South China mainly focus on the inner shelf,shelf lagoon,the middle part of slope and basin.Because the preferential oxidation of the surface water,the shallow water is more sensitive to the changes of redox conditions of the paleo-ocean-atmosphere system.Based on the reconstruction of paleogeography,Xiushan is located in the upper slope,which is belong to the shallow water area in open ocean.We systematically collect the drilling core samples in this area and reconstruct the redox conditions of ancient ocean by using trace elements and inorganic carbon and oxygen isotope methods.In combined with the carbon isotopic data obtained from different sedimentary facies,we further discuss the evolution of water redox conditions in South China.The carbon isotope data obtained from of Doushantuo formation in xiushan area shows that all the values of 13Ccarb are negative and no significant negative carbon isotope excursion(EN3)can be identified.Thus,we suggest that the Xiushan area is located above the chemocline in open ocean during this period.Moreover,the consistent negative values of 13Ccarb imply the process of continuously oxidizing the dissolved ocean organic carbon pool(DOC).Comparison of carbon isotope data obtained from other areas in South China shows that during the sedimentary period of member III of Doushantuo formation the main oxidant in open ocean is atmospheric oxygen,which may suggest an oxidation event.With the continuous oxidation process of the ancient ocean,the chemocline in open sea continued to move down,and the chemocline at the end of the period of member III of Doushantuo formation may be located at the middle part of the slope(siduping area).Furthermore,we suggest that is should be more cautious to use carbon isotope curve as a regional comparison tool due to the significant differences in the change patterns of carbon isotope in South China.