Robert B. Bacastow

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

A global synthesis of the 13C/12C 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, δ13C, 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 δ13C with relatively little variability. The most distinctive feature in the spatial distribution of δ13C seen in our data is a maximum of δ13C 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 13C (low δ13C) and air-sea exchange near the front and upwelling further south of waters with low δ13C resulting from the remineralization of organic matter. Additional features are maxima in δ13C 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 δ13C are observed, the Pacific data exhibiting only about half the amplitude of the Atlantic. Secular decreases in δ13C caused by the invasion of isotopically light anthropogenic CO2 into the ocean (the 13C 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 13C Suess effect of approximately −0.018‰ yr−1 from 1980 to 1995. This effect is nearly the same as the 13C 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 13C Suess effect more than the surface ocean effect.

Atmospheric Carbon Dioxide, the Southern Oscillation, and the Weak 1975 El Niño

The observed rate of change of the atmospheric carbon dioxide concentration at the South Pole, Fanning Island, Hawaii, and ocean weather station P correlates with an index of the southern oscillation and with El Ni�o occurrences. There are changes at all four stations that seem to be in response to the weak 1975 El Ni�o. Thus, even poorly developed El Ni�o events may affect the atmospheric carbon dioxide concentration.

The 13C Suess Effect in the world surface oceans and its implications for oceanic uptake of CO2: Analysis of observations at Bermuda

Surface ocean water δ13C measurements near Bermuda are examined in an attempt to find the annual decrease caused by the addition of anthropogenic CO2 to the atmosphere. We refer to this trend as the surface ocean 13C Suess effect. Interannual variability, which may be related to the El Nino - Southern Oscillation in the Atlantic Ocean, is apparent. We try to correct the data for this variability so as to better determine the trend. The trend has implications for the uptake of anthropogenic CO2 by the oceans. We employ a three-dimensional model of ocean chemistry to relate the trend at Bermuda to the average ocean trend, then use the average ocean trend to estimate the vertical diffusivity K in a one-dimensional ocean model, and finally use this model to calculate the oceanic uptake of CO2. Uncertainties associated with the estimation of the Suess effect at Bermuda and in the analysis procedure preclude a firm estimate of the oceanic uptake of CO2. Results are, in general, consistent with the low side of the Intergovernmental Panel on Climate Control estimation of 2.0 ± 0.8 GtC yr−1. With a longer record at Bermuda and δ13C observations at additional locations, we believe this approach will lead to a useful estimate of oceanic uptake.

The effect of temperature change of the warm surface waters of the oceans on atmospheric CO2

It is widely believed that the concentration of atmospheric CO2 is largely controlled by the high-latitude oceans, because these waters can be viewed as an outcropping of oceanic deep water. If this tenet is correct, cooling of the low-latitude surface oceans could contribute only marginally to an explanation of the observed decrease in atmospheric CO2 concentration of about 80 ppm during glacial periods. This tenet is examined with three-dimensional ocean chemical models based on the circulation fields from 10-level and 15-level ocean general circulation models developed at the Max-Planck-Institut fur Meteorologie, Germany. The three-dimensional models predict atmospheric CO2 changes that are much larger than those predicted by outcrop models, thereby casting doubt on the use of outcrop models to model atmospheric CO2 changes. The atmospheric response time to a change in sea surface temperature is about 120 years in the 15-level model, the more realistic of the two three-dimensional models. This response is fast enough, and the predicted amplitude is of the right size, for one to explain the CO2 increase associated with the Dansgaard-Oeschger warming events observed in ice core measurements as the effect of a warming of the surface oceans.