Spatiotemporal patterns of carbon-13 in the global surface oceans and the oceanic Suess effect

A global synthesis of the C-13/C-12 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, delta(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 delta(13)C with relatively little variability. The most distinctive feature in the spatial distribution of delta(13)C seen in our data is a maximum of delta(13)C near the subantarctic front with sharply falling values to the south. We attribute this feature to a combination of biological uptake of CO2 depleted in C-13 (low delta(13)C) and air-sea exchange near the front and upwelling further south of waters with low delta(13)C resulting from the remineralization of organic matter. Additional features are maxima in delta(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 delta(13)C are observed, the Pacific data exhibiting only about half the amplitude of the Atlantic. Secular decreases in delta(13)C caused by the invasion of isotopically light anthropogenic CO2 into the ocean (the C-13 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 C-13 Suess effect of approximately -0.018 parts per thousand yr(-1) from 1980 to 1995. This effect is nearly the same as the C-13 Suess effect observed globally in the atmosphere over the same period. We attribute this response to a deceleration in the growth rate of anthropogenic CO2 emissions after 1979, which subsequently has reduced the atmospheric C-13 Suess effect more than the surface ocean effect.