TEMPERATURE-DEPENDENCE OF CO2 FUGACITY IN SEAWATER

Ideally, the correction of the measured CO2 fugacity (fCO(2)) at temperature T-m to fCO(2) at the in-situ temperature T-in should be made by using at least 2 known parameters (pH-A(T), C-T-A(T),...) and the reliable constants for carbonic acid. In practice however, a measured CO2 property pair is not always available. When fCO(2) is measured alone, one must make an estimate of the effect of temperature on seawater fCO(2) from the accurate knowledge of seawater salinity and temperature and the approximate knowledge of the carbonate parameters. In this paper we present an empirical relationship that can be used to estimate the effect of temperature on fCO(2). The equation is of the form: fCO(2[l]) - fCO(2[20]) = A + Bt + Ct(2) + Dt(3) + Et(4) where fCO(2[l]) and fCO(2[20]) represent fCO(2) at temperatures t degrees C and 20 degrees C, respectively; the parameters A, B, etc. are functions of the ratio X = C-T/A(T): A = (a(0) + a(2)X + a(4)X(2))/(1 + a(1)X + a(3)X(2)) B = (b(0) + b(2)X + b(4)X(2))/(1 + b(1)X + b(3)X(2)) C = (c(0) + c(2)X + C(4)X(2))/(1 + C(1)X + C(3)X(2)) D = (d(0) + d(2)X + d(4)X(2))/(1 + d(1)X + d(3)X(2)) E = e(0) + e(l)X + e(2)X(2)ln(X) + e(3)exp(X) + e(4)/ln(X) where the parameters a(i), b(i), etc. are functions of salinity. The 25-parameter equation is fitted by the values of fCO(2) calculated using the constants of Goyet and Poisson (1989), when X varies from 0.8 to 1.0, t varies from -1 degrees C to 40 degrees C, and S varies from 30 to 40. For T-m - T-in within +/-10 degrees C, direct measurements of fCO(2) as a function of the temperature (from -1 to 30 degrees C) verify this equation within less than +/-5 mu atm.