Eukaryotic Fossils From The Gaoyuzhuang Formation(Mesoproterozoic) Of North China And Environmental Significance

The mid-Proterozoic,which is in between the two major oxygenation events?GOE,².4 Ga and NOE,⁰.8 Ga?,has witnessed the origin of eukaryotes and their early diversification,and thus provided an important window into the interaction between early life evolution and ocean environment changes.Recent study shows that some multicellular eukaryote-bearing?such as the“Gaoyuzhuang fossils”reported by Zhu et al.,2017,Grypania and Chuaria?horizons are approximately correlated with negative shifts in?¹³C_org and Ce/Ce*anomalys in the Gaoyuzhuang Formation of Mesoproterozoic,and likely indicate that the increase of dissolved oxygen in seawater may have played a vital role in the early eukaryote evolution.To further reveal the eukaryote assemblage and its relations with redox condition in marine encironments,this thesis mainly focues the eukaryotes from the second and third members of the Gaoyuzhuang Formations in several sections within the Yanshan Basin,North China.After analyzing 300 acid-digested black shale samples and microfossils in 216 thin sections,abundant microfossils were found from the basal part of the Gaoyuzhuang Formation in several sections.Based on a detailed comparison on fossil morphology and micro-structures,14 species were identified,including spinose acritarches,Vase-shaped organic bodies and filamentous fungi.By observaions of fossil morphology,surface ornament and altra-structures,and analyses on element compositions of the microfossils using microscopic,SEM and EDS technology,it was found that the microfossils are commonly large?10–350 um in diameter?,with various surface ornaments and complex shell structures which were previously rarely known in Precambrian prokaryotes.In addition,Dictyosphaera and Shuiyousphaeridium with well-preserved wall surface structures were studied in details using SEM,which provided some key information for futher defining the classification criteria in separating eukaryotes from prokaryotes.In addition,an analysis on previously published data of the early Mesoproterozoic shows that eukaryote fossils were widely distributed in different continents and regions during this age,and likely suggests that eukaryotes have evolved to a certain level during middle Mesoproterozoic,rather than in a so-called“evolutionary stasis”.Furthermore,the co-occurrence of eukaryotes and negative shift in?¹³C_org,lower Ce/Ce*anomaly and Fe speciation variations indicate that the proliferation of eukaryotes in the third member of Gaoyuzhuang Formation was most likely facilitated by an increased dissolved oxygen in seawater.