Low Carbon Dioxide Concentration During The "Boring Billion":Evidence Of Boron Isotopes And Calcified Sheaths From Carbonates Of The Tieling Formation In The North China Platform

Early studies suggested that the atmospheric CO₂ concentration during mid Proterozoic?1.8–0.8 Ga,e.g.,the Boring Billion?was about 10–200 times higher than present atmospheric level?PAL?,which may have maintained a long-lasting greenhouse climate under low solar irradiance in the Mesoproterozoic?1.6–1.0 Ga?.However,recent study shows that the microsparry calcite in Mesoproterozoic stromatolites was larely derived from the calcite precipitation induced by cyanobacterial photosynthesis in water column.This likely implies that the carbonate concentration mechanism?CCM?of cyanobacteria was already activated,and the atmospheric pCO₂ level should be lower than 10×PAL during the deposition of the Mesoproterozoic Tieling Formation?1.45–1.40 Ga?in North China.To further constrain the potential atmospheric pCO₂ and seawater pH of this period,an integrated investigation of cyanobacteria calcified sheath and whole rock boron?B?isotope was carried out in this study.In addition,REE and C isotope analyses were also conduscted on the carbonates,in order to reveal the potential relationship between the pH fluctuation and redox state of the seawater and the volcanic activity during deposition of the Tieling Formation.The results show that there existed were two periods of cyanobacteria sheath calcification in the lower part of Member?and Member?of the Tieling Formation,respectively,in well coincidence with the high?¹¹B value intervals of 31.3‰–31.6‰and 22.3‰–47.8‰.However,no calcified cyanobacteria sheath was found in the upper part of Member?to the bottom of Member?and in the upper part of Member?of Tieling Formation,in which low B isotopevalues were identified,with?¹¹B values of10.6‰–16.0‰and 1.8‰–16.9‰,respectively.These characteristics likely imply that the CCM of cyanobacteria was already onseted,though with an intermittent pattern,and atmospheric pCO₂ may have fluctuated around the critical value?10×PAL?required for CCM initiation.The causes for the occurrence or absence of cyanobacteria calcified sheaths and B isotope fluctuations are not fully understand at the moment,and worth to be further studied.The two negative excursions of B isotopes identified in the Tieling Formation are well consistent with the enhancement of negative Ce anomaly in time,likely indicating that the increase of seawater oxygenation lead to the oxidation of dissolved organic carbon,which in turn elevated the CO₂ concentration in surface seawater.To further clarify the mechanism of CO₂ fluctuation in surface seawater,more and detailed research work on the intensity of volcanic activities?such as Hg/TOC?and seawater redox state?such as I/?Ca+Mg??of this period are urgently needed.