Changes in the High-latitude Ocean as Possible Causes 
of Atmospheric CO, Variations 
U. Siegenthaler 
Physics Institute, University of Bern, Sidlerstrasse 5, 3000 Bern, SWITZERLAND 
Measurements on air enclosed in old polar ice have indicated that the atmospheric 
CO,g concentration was ca. 50-70 ppm lower in late glacial times than during the Holocene (e.g., 
Neftel et al., 1982). 
Similar measurements have been performed on samples from a Greenland ice core, dating 
ca. 30,000-40,000 B.P., and have yielded evidence of several CO» oscillations with an amplitude 
of ca. 50 ppm. Each change lasted on the order of a few centuries (Stauffer et al., 1984). A 
mechanism by which circulation changes in the high-latitude ocean could lead to rapid variations 
in atmospheric COg is proposed (Siegenthaler and Wenk, 1984; Wenk and Siegenthaler, 1985). 
The atmospheric COg level is controlled mainly by the physico-chemical properties of the surface 
ocean. Modification of the chemistry of the whole ocean seems to be too slow to explain these 
rapid variations. Marine biology exerts a strong influence on the {COg and alkalinity of surface 
water, thus influencing the biological productivity. This is not the case in the Antarctic Ocean 
where nutrients are abundant. A slowing down of the vertical mixing would imply a smaller 
upward flux of {CO, and nutrients. Assuming constant productivity, CO, and nutrients would be 
more completely used which would imply lower pCOg in these high-latitude surface waters. In 
areas with a warm surface, a slowing down of the circulation would not have a direct impact on 
pCOg because productivity would automatically decrease by the same factor as the upwelling rate 
of nutrients. Studies with a simple box model of the ocean-atmosphere system suggest that a 
sudden decrease by a factor of 2 of the water exchange between the surface and deep sea in 
high latitudes could lead to a COg decrease of ca. 40-50 ppm with a time constant of ca. 200 
years. Deep-sea sediment studies indicate rapid changes in the high-latitude surface conditions 
of the North Atlantic and the Antarctic Oceans at the end of the last glaciation. Studies of car- 
bon isotope ratios should help us to ascertain whether this proposed mechanism was indeed 
responsible for the CO 9 variation. 
The model calculations indicate that the high-latitude ocean is generally important for the 
atmospheric CO g since it is in rapid exchange with the large reservoir of the deep sea. COpg is 
transferred via the atmosphere between the different oceanic regions, so that pCOg in low lati- 
tudes tends to be similar to that of high latitudes. 
Topics of Research 
COg; climate: 
- Vertical circulation and mixing between surface and deeper waters: mean rate of 
exchange; frequency of deep water formation events; 
- Modification of surface properties in high latitudes by interaction with the atmosphere 
(heat and gas exchange) and with the marine biology (carbonate chemistry, nutrients); 
- Possible applications of remote sensing: wind stress (important for gas exchange); 
biological productivity. 
Ocean modeling: 
- Effect of changed surface conditions (temperature, sea ice, wind stress) during Glacial 
on deep water formation; 
- Effect of changed surface condition on thermocline processes, e.g. relation between sta- 
bility and apparent vertical eddy diffusivity; 
- Effect of changes in the North Atlantic on the other oceans, especially the Antarctic 
Ocean. 
Paleoclimatic studies: 
- Antarctic sediment studies: changes of surface conditions and of deep water formation; 
synchroneity with North Atlantic changes; 
- Rapid variations during glacial periods: continental evidence from Europe and other con- 
tinents, comparison with Greenland ice cores and deep-sea sediments. 
