Boron isotope constraints on Neoarchean ocean pH and climate

Ocean pH is a fundamental environmental parameter because it is linked to atmospheric CO₂ and ocean chemistry. Constraining the evolution of seawater pH on early Earth is thus key for paleoclimate reconstruction. However, disagreement exists between different estimates of Precambrian seawater pH, which range from 5.7 to as high as ~10. The boron (B) isotope compositions of marine carbonates are potential proxies for seawater pH but data from Precambrian carbonates are scarce.

Here, we examined B isotopes, major and trace element compositions of samples from a drill-core of the ~2.6 Ga Carawine Dolomite from Western Australia to constrain environmental conditions. The Carawine Dolomite contains sedimentological features indicating deposition in a shallow marine environment. The δ¹¹B values range from +8.8 to +12.5‰ with an average of +10.6 ± 1.8‰. Published data on dolomitic carbonates formed during the Neoproterozoic Snowball Earth indicate that dolomitization does not change the δ¹¹B signature, but meteoric diagenesis and metamorphism typically reduce the B content and decrease δ¹¹B. However, the Carawine Dolomite was likely dolomitized during early diagenesis, shows little evidence of meteoric alteration and only underwent a low degree of metamorphism. No correlations were observed between δ¹¹B and B/Ca, Na/Ca, Sr/Ca, or Mn/Sr ratios, suggesting primary δ¹¹B signals were preserved.

Reconstruction of ocean pH requires knowledge of the δ¹¹B value of seawater. We calculated seawater pH under a wide range of seawater δ¹¹B values (+15 to +36‰) using the measured δ¹¹B of the Carawine Dolomite. The results suggest that Neoarchean ocean pH was between 7.4 and 9.3. The observed δ¹¹B values of the Carawine Dolomite are significantly higher than those reported for Neoproterozoic carbonates around "Snowball Earth" events, likely indicating more benign climatic conditions. Seawater B isotopic composition may have varied considerably through geologic history, mainly due to changes in the global boron cycle. During the Neoarchean, the seafloor weathering flux could have been much higher, which would shift seawater towards higher δ¹¹B values. Thus, it is most likely that ocean pH during the Neoarchean was less alkaline than today. This finding is consistent with recent estimates derived from carbon cycle models.