地球の海水組成と生命の進化４６億年

Abstract

The redox state of the surface environment of early Earth is still controversial (e.g. Ohmoto, 1997). Many previous papers suggest that oxygen was free before 2.7 Ga, and then gradually increased due to oxygen-producing photosynthesis (e.g. Holland, 1999; Farquhar et al., 2000). But, a detailed and quantitative estimate is still lacking. It is well known that deposited carbonate minerals are equilibrated with ambient seawater in a microbial or abiotic environment. The composition and mineralogy allow us to estimate the physical and chemical properties of pale minerals with primary sedimentary structures in shallow and deep-sea deposits, in order to eliminate secondary carbonate and contamination of detrital materials, and to estimate the redox condition of seawater over time.We estimated the depositional environments from the field occurrence of coexisting basaltic lava and sedimentary rocks and the fabric of the carbonates themselves. The shallow marine deposits have included sedimentary carbonates with a stromatolite structure and elastic layers, and amygdaloidal and matrix carbonates of hot-spot basaltic lava since 3.5 Ga. Deep-sea carbonates have included interstitial carbonate minerals in a matrix of hyaloclastite and amygdaloidal carbonate minerals accompanied by MORB-type basalts since 3.5 Ga. In addition, we excavated at three localities in South China, and obtained the complete sequence from the Marinoan tillite to early Cambrian rocks. The carbonate rocks belong to shallow marine deposits.Deep-sea carbonates have only faint Ce and Eu anomalies between 3.5 and 1.9 Ga. The negative Ce anomaly of shallow carbonates has frequently deviated from those of deep-sea carbonate since 2.78 Ga. It fluctuated greatly, and was very large at 2.5 and 2.3 Ga. We calculated the oxygen activity of shallow and deep seawater respectively, based on Ce content and anomalies of carbonate minerals at given parameters of atmospheric carbon dioxide content (pCO2) and Ca. content of seawater. The results show that the oxygen content of the deep sea was low and constant until at least 1.9 Ga. The oxygen content of shallow seawater increased from 2.7 Ga, but fluctuated. In particular, it was at a minimum during and after Snowball Earth events. It became quite high at 2.5 and 2.3 Ga, but eventually increased after the Phanerozoic. We also calculated it under another condition of high pCO2 to show that the seawater was more oxic even in the Archean than at present. The calculation suggests a relatively low pCO2 through geologic time.