NOAA/DOE OACES, 36 months

The bulk of the proposed work centers on using the merged DIC¹³ data set to determine the spatial distribution and magnitude of the anthropogenic DIC¹³ signal and air-sea ¹³CO₂ disequilibrium in the ocean.  Two estimates of the oceanic CO₂ uptake rates will be determined by incorporating the ocean-wide estimates of the anthropogenic DIC¹³ inventory change and air-sea ¹³CO₂ disequilibrium into atmospheric CO₂ and ¹³CO₂ budgets.  This effort will improve significantly the previous estimates made, with a much more limited oceanic DIC¹³ data set, by Quay et al. (1992) and Tans et al. (1993).  A third estimate of the global oceanic CO₂ uptake rate will be determined based on the similarity between the oceanic penetration depths of the anthropogenic DIC and DIC¹³ signals, as described by Heimann and Maier-Reimer (1996).

The oceanic distribution of the anthropogenic DIC¹³ change will be determined using three approaches.

1. Comparing depth profiles of DIC¹³ measured in the 1970s with those measured at nearby locations in the 1990s.  About 25 such sites have been identified.
2. Calculating the preformed DIC¹³ distribution along isopycnal surfaces. Preformed DIC¹³ represents the DIC¹³ value the water parcel had when it was last at the isopycnal outcrop site.
3. Using a multiple linear regression to predict the DIC¹³ measured at a more recent time (1990s) based on the dependence of DIC¹³ on other ocean parameters measured at an earlier time (1970s).

We intend to determine the relationship between the anthropogenic DIC¹³, DIC and bomb 14C signals in the world's ocean. The objectives are twofold. The anthropogenic DIC¹³ and DIC perturbations in the ocean correlate well based on OGCM predictions (Heimann and Maier-Reimer, 1996). Thus variations of the relative change in the reconstructed anthropogenic DIC¹³ and DIC perturbations in the ocean provide a useful diagnostic with which to test OGCM predictions of CO₂ uptake.  We will examine the relationship between the DIC¹³ and DIC perturbations via our collaboration with R. Feely (NOAA/PMEL) and colleagues who are proposing to reconstruct the anthropogenic DIC perturbation based on the WOCE and OACES DIC data set.  We will also determine the correlation between the anthropogenic DIC¹³ and bomb ¹⁴C perturbations measured during WOCE via our collaboration with R. Key (Princeton).  An improved correlation between the distributions of the anthropogenic DIC¹³ change and bomb ¹⁴C in the 1990s (vs that observed during GEOSECS in the 1970s) will reduce the error in the ¹⁴C-based extrapolation of the ocean-wide anthropogenic DIC¹³ perturbation. This, in turn, would reduce the error in ¹³C-derived estimates of oceanic CO₂ uptake rates.

Ann P. McNichol
National Ocean Sciences AMS Facility
Woods Hole Oceanographic Institution
McLean Laboratory, Mail Stop #8
360 Woods Hole Road
Woods Hole, MA 02543-1539
tel: (508) 289-3394
fax: (508) 457-2183
amcnicnol@whoi.edu
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