The Neoproterozoic Red Bed Of The Jiuliqiao Formation In Huainan,Anhui And And Paleoenvironment Significance

The early Neoproterozoic is an important transitional period of biological evolution.After a billion-year evolutionary stasis in the middle Proterozoic,There is a significant increase in the diversity and quantity of eukaryotes in the early Neoproterozoic that are widely distributed around the world and can be used for biostratigraphic correlation.Earth surface environment during the early Neoproterozoic was also characterized by large perterbations in redex conditions and global carbon cycling as evidenced by large negative inorganic carbon isotope(?¹³C_carb)excursions(such as Bitter Springs anomaly and Islay negative anomaly).The internal relationship between these biological development events and environmental events,however,is a unresolved but an important scientific problem,largely due to the contradiction concerning the marine environment in the early Neoproterozoic.This paper focued on the marine red bed deposits from the early Neoproterozoic Jiuliqiao Formation in Huainan area,Anhui Province which is located in the Southern margin of North China Platform.Integrated studies of sedimentology,inorganic carbon isotope chemostratigraphy and ratios of iodine-to-calcium-magnesium(I/(Ca+Mg))were performed for the Jiuliqiao Formation to characterize the sedimentary environments and ocean redox conditions.Field investigation and microscopic observation indicate that the Jiuliqiao Formatino was mainly deposited in intertidal to shallow subtidal zone.Abundant hematites were unevenly distributed in the wackestone,including lamellar crystals and loosely aggregated neddle hematite.The small size and microstuctures suggest that most of hematites the primary precipitation and can record coeval seawater and/or porewater conditions.The carbon isotope compostions of study section show an overall decreasing trend upwards from mean value of?+1‰at base to-2‰at the top of section,reprensenting an obvious negative carbon isotope anomaly.Combining the carbon isotope data of the Jiuliqiao Formation in the previous article,paleontological fossil evidence and radiometric dating,the negative?¹³C_carbexcursion in the Jiuliqiao Formation can be correlated with that reported from the Gouhou Formation in Huaibei area.Both of them may be equivalent to the Bitter Springs anomaly in Svalbard.The I/(Ca+Mg)from study section are dominated by non-zero but low values,ranging from0.2 to 1.5?mol/mol.Most of samples have I/(Ca+Mg)values higher than the background baseline of Proterizoic carbonate(0.5?mol/mol).If recording the primary signal,the I/(Ca+Mg) data from the Jiuliqiao Formation,coupled with the occurrence of abundant primary hematites,point to dysoxic-suboxic conditions.Although with some fluctuation,the I/(Ca+Mg)show an overall increasing trend upsection,which is opposite to that of?13Ccarb profile.The coupled variations of I/(Ca+Mg)and?¹³C_carbare similar to their pattern observed in the Ediacran Shuram excursion,although the magnitudes of both proxies are smaller in study section.Such pattern likely implies an increase of oxygen level in seawater,providing the favorable environments for the evolutions of eukaryotes during the early Neoproterozoic.