Abyssal carbon and radiocarbon in the Pacific

Abstract

Vertical profiles of dissolved inorganic carbon in the Pacific show a well-defined concentration maximum at about 2.5-km depth due to oxidation and solution of particulate carbon from the surface. The ΣCO2 concentrations are inversely correlated with C14/C12 ratios (ΔC14 values), so that ΔC14 profiles have a mid-depth minimum but the absolute C14 concentration below 1.3 km is essentially constant. The ΣCO2 maximum is associated with a corresponding carbonate alkalinity maximum, and, in the South Pacific, with the deep oxygen minimum. In the North Pacific the O2 minimum lies above 1 km but the ΣCO2 and alkalinity maximum concentrations remain at about 2.5 km. The vertical profiles below the advective core of Intermediate Water are treated as stationary in the Eulerian sense, in a ‘z-diffusion water mass’ with vertical diffusion and advection, production from particulate flux, and radioactive decay. For a radioisotope, the mixing parameter K/w is obtained from salinity and/or potential temperature; the production or consumption ratio J/w is obtained from the stable isotope profile, and the upwelling velocity w from the profiles of absolute concentration of radioisotope. The ΣCO2 profiles in conjunction with geochemical estimates of the particulate flux also give a direct estimate of w ∼ 6 m/yr, consistent with estimates from various physical considerations. The CO2–O2 relationship approaches linearity only in the lowermost few hundred meters of the section because of the boundary-value effects. C14 profiles are consistent with w = 2 to 20 m/yr because of the scatter of individual measurements, but the observation that the concentration is essentially constant with depth gives w = 7 m/yr, K = 2 cm2/sec. Horizontal variations of C14 in bottom water are treated for a flowing open system affected by vertical diffusion from above, particulate flux or consumption, and radioactive decay. From 60°S to 40°N, at 3.5-km depth the absolute C14 concentration is constant, and the observed 50‰ decrease in ΔC14 is due to the increase in ΣCO2. The C14 concentration at all depths below 1.3 km and at all latitudes is constant because the production of C14 by particulate transport is approximately equal to the radioactive decay rate. Thus the vertical C14 gradient vanishes, and there is no vertical diffusive flux of radiocarbon. However, downward diffusion of stable carbon from the ΣCO2 maximum dilutes the specific activity of bottom water and constitutes a source of ‘dead carbon’ in the sea, which is continually added from above along any horizontal trajectory. The specific activity changes are therefore not the result of a ‘closed-system’ flow time as has previously been assumed in deriving bottom water flow velocities and do not give a direct record of elapsed flow time.