Release of CO
2 from surface ocean water owing to precipitation of CaCO
3 and the imbalance between biological production of organic matter and its respiration, and their net removal from surface
water to sedimentary storage was studied by means of a quotient θ = (CO
2 flux to the atmosphere)/(CaCO
3 precipitated). θ depends not only on water temperature and atmospheric CO
2 concentration but also on the CaCO
3 and organic carbon masses formed. In CO
2 generation by CaCO
3 precipitation, θ varies from a fraction of 0.44 to 0.79, increasing with decreasing temperature (25 to 5°C), increasing atmospheric
CO
2 concentration (195–375 ppmv), and increasing CaCO
3 precipitated mass (up to 45% of the initial DIC concentration in surface water). Primary production and net storage of organic
carbon counteracts the CO
2 production by carbonate precipitation and it results in lower CO
2 emissions from the surface layer. When atmospheric CO
2 increases due to the ocean-to-atmosphere flux rather than remaining constant, the amount of CO
2 transferred is a non-linear function of the surface layer thickness because of the back-pressure of the rising atmospheric
CO
2. For a surface ocean layer approximated by a 50-m-thick euphotic zone that receives input of inorganic and organic carbon
from land, the calculated CO
2 flux to the atmosphere is a function of the CaCO
3 and C
org net storage rates. In general, the carbonate storage rate has been greater than that of organic carbon. The CO
2 flux near the Last Glacial Maximum is 17 to 7×10
12 mol/yr (0.2–0.08 Gt C/yr), reflecting the range of organic carbon storage rates in sediments, and for pre-industrial time
it is 38–42×10
12 mol/yr (0.46–0.50 Gt C/yr). Within the imbalanced global carbon cycle, our estimates indicate that prior to anthropogenic
emissions of CO
2 to the atmosphere the land organic reservoir was gaining carbon and the surface ocean was losing carbon, calcium, and total
alkalinity owing to the CaCO
3 storage and consequent emission of CO
2. These results are in agreement with the conclusions of a number of other investigators. As the CO
2 uptake in mineral weathering is a major flux in the global carbon cycle, the CO
2 weathering pathway that originates in the CO
2 produced by remineralization of soil humus rather than by direct uptake from the atmosphere may reduce the relatively large
imbalances of the atmosphere and land organic reservoir at 10
2–10
4-year time scales.
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