THE DYNAMICS OF CO-2 FIXATION IN THE SOUTHERN-OCEAN AS INDICATED BY CARBOXYLASE ACTIVITIES AND ORGANIC-CARBON ISOTOPIC-RATIOS

Recent studies have suggested that a direct relationship exists between the dissolved CO2 concentration and the carbon isotopic composition of phytoplankton in the surface ocean. Thus, measurement of the delta-C-13 of planktonic organic matter in deep-sea cores can potentially yield a record of the past atmospheric CO2 variations. However, we present here results from three cruises in the Indian and Atlantic sectors of the Southern Ocean (between 40 and 66-degrees-S) in which biochemical and physiological factors associated with photosynthetic processes lead to carbon isotopic fractionation by phytoplankton which cannot be directly related to variations within the mineral carbon pool. Simultaneous measurements of the carboxylase activities and the C-13/C-12 ratio of particulate organic carbon show that there is a large variability in phytoplankton carbon metabolism, especially on a seasonal scale, in spite of a relative uniformity of the environmental conditions. Phytoplankton carbon metabolism is clearly a main factor governing variations in the stable isotopic composition of organic matter in the euphotic layer. Interrelationships between light, Rubisco activity and delta-C-13 are clearly shown by our data. Heterotrophic processes also may influence the carbon isotope mass balance, especially during the break-up of the ice pack. In addition to the influence of photosynthetic metabolism, the effect of the meridional temperature gradient is also verified by our data set.