The Fluctuation Of Atmospheric Oxygen In The Mid-Proterozoic

Although the origin of eukaryotes occurred in the mid-Proterozoic（1.8 Ga–0.8 Ga）,the evolution of them was delayed up to a billion years.Low oxygen concentration may limit the evolution of eukaryotes,but the oxygen level（p O₂）of the mid-Proterozoic atmosphere-ocean system is still very controversial.It exists 3 distinctive opinions:“low”（p O₂<0.1–1%PAL;modern atmospheric oxygen level）,“high”（>0.1–1%or 4–8%PAL）or frequent“fluctuations”（<0.1–1%to>4–8%PAL）.To constrain the levels of atmospheric oxygen in the mid-Proterozoic,this study took black shale or carbonate samples of the mid-Proterozoic Chuanlinggou,Tuanshanzi,Gaoyuzhuang,Hongshuizhuang,Tieling and Xiamaling Formations in North China platform as targets,operating sedimentology,elemental geochemistry and Cr isotope geochemistry studies for long-sequence stratigraphic analysis.The results show that the values of the authigenic Cr isotope component(δ⁵³Cr_auth)in the early mid-Proterozoic（1.8–1.4 Ga）are largely unfractionated relative to the Bulk Silicate Earth（BSE）reservoir,and the concentrations of redox sensitive elements（RSEs）are close to the post-Archean Australian shale（PAAS）.Exceptionally,theδ⁵³Cr_auth value of middle Gaoyuzhuang Formation（～1.56 Ga）was higher than the range of BSE,showing a positive fractionated up to+0.66±0.01‰,and accompanied by the enrichment of RSEs.The shifts of theδ⁵³Cr_auth and RSEs dataset may reveal that the existence of a pulsed oxygenation event under the background of the overall low oxygen atmosphere during 1.8–1.4 Ga.Theδ⁵³Cr_auth values of the middle Hongshuizhuang Formation to the upper of the Xiamaling Formation（～1.46–1.40 Ga）showed 3-pulse positive fractionation relative to the BSE value,associated with the enrichment of RSEs.It may indicate that the atmospheric oxygen has been increased during the deposition of the Hongshuizhuang Formation at～1.46 Ga,and fluctuated around the threshold of starting the Mn-Cr oxidation cycle（～0.1–1%PAL）.The compilation ofδ⁵³Cr profiles of paleosols and ancient sedimentary rocks throughout geological history reveals two major characteristics:（1）Theδ⁵³Cr of the mid-Proterozoic is significant fluctuations,and（2）Theδ⁵³Cr of early mid-Proterozoic（1.8–1.4 Ga）as a whole are within the range of BSE,whileδ⁵³Cr of the late mid-Proterozoic（1.4–0.8Ga）are generally higher than the BSE value.These characteristics suggest that the atmospheric oxygen level in the mid-Proterozoic are frequently fluctuated,which is commonly low in early mid-Proterozoic（<0.1–1%PAL;1.8–1.4 Ga）but relatively“high”during late mid-Proterozoic（>0.1–1%PAL;1.4–0.8 Ga）.Pulsed increase of oxygen may promote the short-term evolution of eukaryotes,but frequent fluctuations in oxygen levels may be the significant reason for the delayed evolution of eukaryotes.