Influences of oceanic rheostats and amplifiers on atmospheric CO 2 content during the Late Quaternary

Abstract

Although a clear explanation has yet to emerge for the observed decline in atmospheric CO 2 content during glacial episodes, numerous hypotheses have been offered. Selected examples are reviewed or evaluated here. A strengthened biological pump resulting from intensified upwelling at low latitudes was suggested early on as a likely direct cause of the glacial CO 2 drawdown. Two problems have since arisen with this hypothesis. First, on some upwelling continental margins, for example off California, Oregon, northwestern Mexico, Peru, and parts of NW Africa, organic matter accumulation decreased, in some areas markedly, during the Last Glacial Maximum (LGM). Second, upwelled cool CO 2-rich water degasses carbon dioxide in warm regions, and for a key area such as the modern equatorial Pacific to become a net sink for atmospheric CO 2, rather than a source as it is today, the rate of downward organic carbon export must exceed the rate of degassing. Recent sedimentary carbon and nitrogen isotopic data suggest however that the equatorial Pacific has remained a strong CO 2 source for at least the last 40,000 yr and probably much longer. Although the direct extraction of carbon from surface waters and burial in the sediments in productive regions now appears unlikely to provide the sought-for explanation, indirect effects related to export productivity may hold the key. These can modulate the chemical character of the ocean so as to increase its uptake of CO 2 (a rheostat effect) or they could increase indirectly the nutrient inventory of the sea (an amplifier effect). For example, it is now recognized that denitrification was greatly reduced during glacial maxima in all three principal oxygen minimum zones in the oceans, i.e. the subtropical north and south Pacific and the Arabian Sea. The implication is that nitrate, the limiting nutrient in the modern ocean, must have been more abundant during glacial periods, and that this surfeit would have supported increased export production in the meso or oligotrophic areas that are today nitrate-limited. Dust may represent another rheostat. Increases in atmospheric turbidity during glacials are clearly recorded by ice and marine sediment cores in both hemispheres. In addition to fostering enhanced export production by adding needed iron to nitrate-rich areas, it has been suggested recently that increased dust inputs to nitrate-depleted regions during glacials might have encouraged growth of iron-hungry N 2 fixing cyanobacteria, thus alleviating nitrate limitation. No sedimentary δ 15 N data yet exist to support this hypothesis, but it remains viable. Finally, regional changes in physical hydrography may have played a major and hitherto underestimated role. For example, in the glacial Southern Ocean south of the Polar Front, recent nitrogen isotope and other data imply that the upper water column was well stratified during the LGM. By limiting upwelling, this would have reduced the ocean-atmosphere CO 2 `leak’ in the area. This could have made a significant contribution to the pCO 2 drawdown.