Weak hydrothermal carbonation of the Ongeluk volcanics: evidence for low CO 2 concentrations in seawater and atmosphere during the Paleoproterozoic global glaciation

摘要: It was previously revealed that the total CO2 concentration in seawater decreased during Late Archean. In this paper, to assess secular change of seawater, we focused on Paleoproterozoic era when Earth experienced its first recorded global glaciation. The 2.4 Ga Ongeluk Formation outcrops Kaapvaal Craton, South Africa. formation consists mainly submarine volcanic rocks have erupted undeformed lavas are mostly carbonate-free but contain rare disseminated calcites. carbon isotope ratio calcite (δ13Ccc vs. VPDB) ranges from − 31.9 − 13.2 ‰. relatively low δ13Ccc values clearly indicate carbonation partially contributed by 13C-depleted derived decomposition organic matter beneath seafloor. absence higher than − 13.2‰ is consistent with exceptionally results suggest occurred subseafloor hydrothermal circulation just after eruption lavas. Previously, it reported carbonate content uppermost crust 3.2 2.6 Ga, indicating a decrease time. However, average (as carbonate) (< 0.001 wt%) much lower those 2.6 Ga representatives and even modern equivalents. This finding suggests further period between 2.6 2.4 Ga. Thus, very probable evidence for extremely Considering crusts also shows good correlation independently estimated atmospheric pCO2 levels through history, seem highly likely reflects at We conclude continuous seawater/atm. 3.2 Ga one contributing factors