Table 1.

CO₂, HCO₃⁻, and Mn²⁺ speciation and concentrations in the Archean ocean

Species	Contemporary	Archean (>2.5 billion years ago)
PCO2, atmosphere, kPa	0.03	0.9*, 9†, 900‡
HCO3− seawater, mM	2§ (0.5)	NA (15–15,000)
Mn2+, seawater (mean)¶, ng/kg	20	40–100
Mn2+ speciation seawater	Mn2+aq	[MnII2(HCO3)4]n

Numbers in parentheses indicate the calculated concentration at pH 7.5 based solely on equilibrium with atmospheric CO₂ in line 1 (terrestrial sources excluded). 

^*From ref. 36. 

^† From ref. 37. 

^‡ From ref. 38. 

^§ From ref. 39. 

^¶ Derived from Precambrian vs. Phanerozoic limestones (36).