Pressure Dependence of Gadolinium Carbonate Complexation in Seawater

The influence of pressure on Gd carbonate complexation has been investigated at t = 25 °C">t = 25 °C and P = 1–600 atm">P = 1–600 atm . in synthetic seawater using a tributyl phosphate solvent extraction technique. The pressure dependencies of Gd carbonate stability constants appropriate to seawater (t = 25°C">t = 25°C and S = 35">S = 35 ) can be described as follows: logCO3swβ1(Gd) = 5.19t1̄.30 × 10−3(P − 1)">logCO3swβ1(Gd) = 5.19t1̄.30 × 10−3(P − 1) ; logCO3swβ2(Gd) ∼- 9.17−1.5 × 10−3(P − 1)">logCO3swβ2(Gd) ∼- 9.17−1.5 × 10−3(P − 1) ; logCO3swβ1(Gd) = 1.58–3.73 × 10−4(P − 1)">logCO3swβ1(Gd) = 1.58–3.73 × 10−4(P − 1) , where Lswβn(Gd) = [GdLn][Gd3+]−1[L]T−n, [] and []T">Lswβn(Gd) = [GdLn][Gd3+]−1[L]T−n, [] and []T represent free and total concentrations of indicated species (molal scale), and P">P is pressure in atmospheres. In seawater with a total alkalinity of 2.4 mmol/kg and total CO ₂ of 2.1 mmol/kg, the fraction of total Gd present as free Gd ³⁺ increases by a factor of approximately five or more as pressure increases from 1 atm. to 500 atm. at 2°C. Gadolinium speciation calculations indicate that changes in [CO ₃ ²⁻ ] _T and pH are dominant factors controlling the distribution of Gd species in upper oceanic column (0–1000 m depth). Below 1000 m water depth, the effect of pressure on Gd carbonate complexation exerts a major influence on the distribution of Gd species.