CO2 air-sea exchange due to calcium carbonate and organic matter storage: pre-industrial and Last Glacial Maximum estimates
Other literature type
Mackenzie, F. T.
Release of CO<sub>2</sub> from surface ocean water owing to
precipitation of CaCO<sub>3</sub> 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 model that gives the
quotient θ=(CO<sub>2</sub> released to the atmosphere)/(CaCO<sub>3</sub>
precipitated). The surface ocean layer is approximated by a euphotic zone,
50 m thick, that includes the shallower coastal area and open ocean. θ depends on water temperature, CaCO<sub>3</sub> and organic carbon mass formed,
and atmospheric CO<sub>2</sub> concentration. At temperatures between 5 and
25°C, and three atmospheric CO<sub>2</sub> pressures – 195 ppmv
corresponding to the Last Glacial Maximum, 280 ppmv for the end of
pre-industrial time, and 375 ppmv for the present – θ varies from
a fraction of 0.38 to 0.79, increasing with decreasing temperature,
increasing atmospheric CO<sub>2</sub> content, and increasing CaCO<sub>3</sub>
precipitated mass (up to 45% of the DIC concentration in surface water).
For a surface ocean layer that receives input of inorganic and organic
carbon from land, the calculated CO<sub>2</sub> flux to the atmosphere at the Last
Glacial Maximum is 20 to 22×10<sup>12</sup> mol/yr and in pre-industrial
time it is 45 to 49×10<sup>12</sup> mol/yr. In addition to the
environmental factors mentioned above, flux to the atmosphere and increase
of atmospheric CO<sub>2</sub> depend on the thickness of the surface ocean layer.
The significance of these fluxes and comparisons with the estimates of
other investigators are discussed. Within the imbalanced global carbon
cycle, our estimates are in agreement with the conclusions of others that
the global ocean prior to anthropogenic emissions of CO<sub>2</sub> to the
atmosphere was losing carbon, calcium, and total alkalinity owing to
precipitation of CaCO<sub>3</sub> and consequent emission of CO<sub>2</sub>. Other
pathways of CO<sub>2</sub> exchange between the atmosphere and land organic
reservoir and rock weathering may reduce the imbalances in the carbon cycle
on millenial time scales.