Oceanic uptake of CO2 re-estimated through delta C-13 in WOCE samples

In addition to C-14, a large set of delta C-13 data was produced at NOSAMS as part of the World ocean circulation experiment (WOCE). In this paper, a subset of 973 delta C-13 results from 63 stations in the Pacific Ocean was compared to a total number of 219 corresponding results from 12 stations sa mpled during oceanographic programs in the early 1970s. The data were analy zed in light of recent work to estimate the uptake of CO2 derived from foss il fuel and biomass burning in the oceans by quantifying the delta C-13 Sue ss effect in the oceans. In principle, the delta C-13 value of dissolved in organic carbon (DIC) allows a quantitative estimate of how much of the anth ropogenic CO2 released into the atmosphere is taken up by the oceans, becau se the delta C-13 of CO2 derived from organic matter (similar to -27 parts per thousand) is significantly different from that of the atmosphere (simil ar to -8 parts per thousand) Our new analysis indicates an apparent discrep ancy between the old and the new data sets, possibly caused by a constant o ffset in delta C-13 values in a subset of the data. A similar offset was re ported in an earlier work by Paul Quay et al. for one station that was not included in their final analysis. We present an estimate for this assumed o ffset based on data from water depths below which little or no change in de lta C-13 over time would be expected. Such a correction leads to a signific antly reduced estimate of the CO2 uptake, possibly as low as one half of th e amount of 2.1 GtC yr(-1) (gigatons carbon per year) estimated previously. The present conclusion is based on a comparison with a relatively small da ta set from the 70s in the Pacific Ocean. The larger data set collected dur ing the GEOSECS program was not used because of problems reported with the data. This work suggests there may also be problems in comparing non-GEOSEC S data from the 1970s to the current data. The calculation of significantly lower uptake estimates based on an offset-related problem appears valid, b ut the exact figures are tentative because the data set is small and the ca use for an offset remains unknown. Therefore, it would be desirable to exte nd this comparison to the Indian Ocean where it is believed that better GEO SECS delta C-13 data are available. (C) 2000 Elsevier Science B.V. All righ ts reserved.