The Oxygenation Events And The Multicellularization Of Eukaryote In Mesoproterozoic Gaoyuzhuang Formation In Jixian North China

Between the two major atmospheric oxygenation events,the GOE and NOE,Mesoproterozoic?1.8-0.8 Ga?is believed to be one of the critical periods for origin of eukaryotes and of early life evolution in the Earth history.Eukaryotes occurred in this period generally show delayed evolution with a very low diversity,which are commonly attributed to low oxygen concentration in the atmosphere and,therefore,low availability of bio-essential elements in the ocean.Recent study found some macroscopic fossils of potential multicellular eukaryotes,including Grypania and Chuaria,fromthe¹.56 Ga Gaoyuzhuang carbonates.This may indicate that during Mesoproterozoic,redox state of the atmosphere-ocean system of was not in a persistent low oxygenation,but with marked fluctuations,with the occurrence of macroscopic eukaryotes consistent with the short-term events of oxygen increase.This paper chooses the second to third members of the Gaoyuzhuang Formation at Jixian as the research target,using multiple geochemical proxies obtained from the carbonate rocks to analysis paleo-redox conditions of the ocean where the carbonates were deposited from.In this paper,various geochemical analyses were conducted on the carbonate samples collected from the Gaoyuzhuang Formation at Tianjin,including main elements,rare earth elements and the redox sensitive elements,with a focus particularly on the proxies of Ce/Ce*,Y/Ho,and Pr(_SN)/Yb(_SN)ratios.The present study shows that though most of the Gaoyuzhuang formation was formed in an anoxic environmental conditions,the water oxidation degree was apparently fluctuated.Two major episodes of increased oxygenation can be recognized in the second and third member of the Gaoyuzhuang formation,respectively,which likely reflected the oxygen concentration increase in atmosphere and ocean.Importantly,the present study shows that the occurrence of macroscopic eukaryotic fossils including Grypania and Chuaria were in good accordance with the two increased oxygenation intervals identified in in this paper,likely suggesting that increased oxygenation of sea-water has facilitated evolution of these macroscopic eukaryotes.Using sedimentary facies,multiple-proxy geochemical analyses,and bombined with the occurrence data of potential multicellular eukaryotic fossils,the present study also argues that the oxidation state of middle Proterozoic atmosphere-ocean system state may have not kept in a stable platform shape,and the ocean redox conditions may have fluctuated markedly in time.Increased oxygenation episodes of the ocean promoted the eukaryote evolution and diversification,which may have explained the resoons why these fossils occurred only in some specific intervals.