Early Mesoproterozoic Marine Environment And Eukaryotic Evolution In The North China Platform

The co-evolution of early eukaryote and environment during the Mesoproterozoic is a key scientific question in geolocial community.It is widely considered that the evolutionary stasis of eukaryotic life during the Mesoproterozoic was related to low oxgen level in atmosphere-ocean system and/or the low availability of important major or trace nutrient elements.However,Recent paleontological studies indicate apparent eukaryotic diversification during the onset of the Mesoproterzoic,including the occurrence of earliest multicellular macroscopic eukaryotic fossils in the Member ? of Gaoyuzhuang Formation in North China.This biological evolution event is probably related to the fluctuation of the redox environment and the cycle of nutrient elements in the ocean at that time.To addresss this question,the study performed integrated studies of stratigraphy,sedimentology,nitrogen isotopes compositions(?15N),organic and inorganic carbon isotope compositions(?13Corg,?13Ccartb),I/(Ca+Mg)ratios,major and tarce element contents for the Gaoyuzhuang Member ? in three sections in central North China Craton,including the Xinglong section in Chengde area,the Malugou and Gangou sections in Yanqing area.In all the three studied sections,carbonate I/(Ca+Mg)are dominated by nonzero low values,with most values<0.5 ?mol/mol,indicating suboxic-anoxic enviroments.In the Xinglong section,redox sensitive elements(RSEs,such as:Mo,U,V)are low in the lower part,whereas moderate-strong enrichments in Mo,U,V are observed in the upper part.This pattern,coupled with Mo-U co-variance,implies that Member ? of the Gaoyuzhuang Formation in the Xinglong section was mainly deposited in suboxic to anoxic environments with intermittent occurrence of euxinia.The 813Corg values in the Xinlong section vary between-33.87‰ and-31.23‰,with an average of-32.94±1.05‰(n=60),which is comparable to that from the Jixian and Pingquan sections,likely suggesting an increased contribution of secondary production(chemoautotrophic)in anoxic environment to TOC.These geochemical proxies,coupled with previous data from the equivalent strata,reinfore the reference that the Mesoproterozic ocean in North China was strongly stratified,but pulse of oxygenation event may have resulted in the deepening of chemocline.The ?15N values in the Xinlong section fall between 4.62‰ and 11.49‰,with an average values of 7.47±1.53‰.The persistenly high ?15N values are interpreted as the result of partial denitrification similar to modern ocean,indicating the operation of aerobic N cycling and the presence of a fairly stable nitrate pool in the surface oxic layer.The maintainance of nitrate pool in stratified Mesoproterozoic ocean may have benefited from the combined effects of oxygenation and low primary productivity.Further quantititive calculations based on P and TOC contents reveal that primary productivity during the depostion of the Gaoyuzhuang Formation was 1-2 orders magnitude lower than modern ocean.Increased availability of nitrate could have contributed to the expansion of eukaryotic life at this time.However,our data also suggest that nitrate alone was not the only trigger.The occurrence of multicellular eukaryotic fossils in the Gaoyuzhuang Member ? is also broadly coincident with the rise of P and some micro nutrient elements.Therefore,the ca.1.56 Ga evolutionary leap was likely facilitated by multiple environmental factors,including a rise in O2 levels and increasing supplies of phosphorus and other bio-essential trace elements.