National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2002

Global estimate of CaCO3 export from the upper ocean

Lee, K., R.A. Feely, C.L. Sabine, F.J. Millero, and R. Wanninkhof

In Sixth International Carbon Dioxide Conference, Sendai, Japan, 1–5 October 2001, 712–714 (2001)

Existing model-based global estimates of calcium carbonate (CaCO) export vary considerably ranging from 0.5 to 1.5 Gt C yr. Here we estimate global-scale CaCO export from the vertically integrated APOT (APOT = NAT + NO) decrease in the mixed layer during the seasonal warming period together with diffusive APOT flux from the upper thermocline in the following manner:

CaCO export |S = (A HS+1 [APOTS - APOTS+1] × 0.5) + A KV(dAT/dm) × 0.5 (1)

where S denotes season (time step); [APOTS - APOTS+1] is the change in the APOT concentration in the mixed layer, H; dAT/dm (mol m) is the vertical AT gradient between the mixed layer and the upper thermocline; and KV (m month) is vertical eddy diffusivity. The annual NAT cycle is deduced from regional algorithms relating NAT to sea surface temperature (SST), along with global records of seasonal mean SST. Estimated NAT fields are then combined with NO fields to calculate APOT fields. Changes in the mixed layer APOT inventory are estimated using a derived annual cycle of APOT and global records of the mixed layer depth. Mass balance of APOT in the mixed layer yields a global CaCO export production of 0.9 ± 0.3 Gt C yr, which represents extratropical waters with temperatures <20°C and the upwelled waters in the eastern equatorial Pacific. The average rate of CaCO export during the summer is approximately two times higher than that observed during the winter. Thus, estimates of net CaCO production for extratropical waters are sealed up by 25%. Global extrapolation of this scaling factor yields an annual rate of net CaCO production of 1.1 ± 0.3 Gt C yr for 1990. This data-based estimate will better constrain the budget of CaCO in the ocean. The method used to estimate global CaCO export is described in detail elsewhere (Lee, 2001).

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