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(?⁵³Cr_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,the?⁵³Cr_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 the?⁵³Cr_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.The?⁵³Cr_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 of?⁵³Cr profiles of paleosols and ancient sedimentary rocks throughout geological history reveals two major characteristics:(1)The?⁵³Cr of the mid-Proterozoic is significant fluctuations,and(2)The?⁵³Cr of early mid-Proterozoic(1.8–1.4 Ga)as a whole are within the range of BSE,while?⁵³Cr 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.