Direct observation of the oceanic CO2 increase revisited

Abstract

We show, from recent data obtained at specimen North Pacific stations, that the fossil fuel CO2 signal is strongly present in the upper 400 m, and that we may consider areal extrapolations from geochemical surveys to determine the magnitude of ocean fossil fuel CO2 uptake. The debate surrounding this topic is illustrated by contrasting reports which suggest, based upon atmospheric observations and models, that the oceanic CO2 sink is small at these latitudes; or that the oceanic CO2 sink, based upon oceanic data and models, is large. The difference between these two estimates is at least a factor of two. There are contradictions arising from estimates based on surface partial pressures of CO2 alone, where the signal sought is small compared with regional and seasonal variability; and estimates of the accumulated subsurface burden, which correlates well other oceanic tracers. Ocean surface waters today contain about 45 micromol.kg-1 excess CO2 compared with those of the preindustrial era, and the signal is rising rapidly. What limits should we place on such calculations? The answer lies in the scientific questions to be asked. Recovery of the fossil fuel CO2 contamination signal from analysis of ocean water masses is robust enough to permit reasonable budget estimates. However, because we do not have sufficient data from the preindustrial ocean, the estimation of the required Redfield oxidation ratio in the upper several hundred meters is already blurred by the very fossil fuel CO2 signal we seek to resolve.

Figure 2

Correlation between measured values of alkalinity and salinity for surface waters along the WOCE P17N transect. The strong linear correlation is to be expected, and the standard deviation is only ±6 μmol⋅kg⁻¹.

Figure 1

Cruise tracks and station locations for WOCE Leg P17N.

Figure 3

Specimen calculation of a vertical profile of the partial pressure of CO₂ at WOCE P17N Station 13, corrected for the effects of oxidative organic matter decomposition and carbonate dissolution so as to reveal an approximation to the original water mass surface conditions. The corrections resulting from the use of the original (1934) Redfield ratios (17, 18) and the more recent coefficients (1994) of Anderson and Sarmiento (25) are shown: both profiles are computed utilizing the thermodynamic constants of Goyet and Poisson (30). The deep water value of 280 ± 6 ppm given by the Anderson and Sarmiento coefficients is close to the established preindustrial atmospheric level.

Figure 4

Comparison of the calculated pCO₂ profile from WOCE P17N stations utilizing the thermodynamic constants from several sources. The deep water values are in agreement to about ±10 ppm, but the warmer surface waters show a spread of about ±25 ppm. It is not necessary to use these constants to derive a ΔTCO₂ value, but they are required to link calculated changes in oceanic chemical composition to the atmospheric driving signal.

Figure 5

Image of the calculated mixed layer depth for the North Pacific Ocean for the month of March 1995 from data supplied by the Fleet Numerical Meteorological and Oceanography Center model (37).