Spatiotemporal patterns of carbon‐13 in the global surface oceans and the oceanic suess effect

Abstract

A global synthesis of the 13 C/12 C ratio of dissolved inorganic carbon (DIC) in the surface ocean is attempted by summarizing high‐precision data obtained from 1978 to 1997 in all major ocean basins. The data, mainly along transects but including three subtropical time series, are accompanied by simultaneous, precise measurements of DIC concentration and titration alkalinity. The reduced isotopic ratio, δ 13 C, in the surface ocean water is governed by a balance between biological and thermodynamic processes. These processes have strongly opposing tendencies, which result in a complex spatial pattern in δ 13 C with relatively little variability. The most distinctive feature in the spatial distribution of δ 13 C seen in our data is a maximum of δ 13 C near the subantarctic front with sharply falling values to the south. We attribute this feature to a combination of biological uptake of CO 2 depleted in 13 C (low δ 13 C) and air‐sea exchange near the front and upwelling further south of waters with low δ 13 C resulting from the remineralization of organic matter. Additional features are maxima in δ 13 C downstream of upwelling regions, reflecting biological uptake, and minima in the subtropical gyres caused by strongly temperature dependent thermodynamic isotopic fractionation. At the time series stations, two in the North Atlantic Ocean and one in the North Pacific, distinct seasonal cycles in δ 13 C are observed, the Pacific data exhibiting only about half the amplitude of the Atlantic. Secular decreases in δ 13 C caused by the invasion of isotopically light anthropogenic CO 2 into the ocean (the 13 C Suess effect) have been identified at these time series stations and also in data from repeated transects in the Indian Ocean and the tropical Pacific. A tentative global extrapolation of these secular decreases yields a surface oceanic 13 C Suess effect of approximately −0.018‰ yr−1 from 1980 to 1995. This effect is nearly the same as the 13 C Suess effect observed globally in the atmosphere over the same period. We attribute this response to a deceleration in the growth rate of anthropogenic CO 2 emissions after 1979, which subsequently has reduced the atmospheric 13 C Suess effect more than the surface ocean effect.