8 THE CHEMISTRY OF GLOBAL CLIMATE
There are also Earth processes that act as natural sinks. Photosynthesis is one of these
and takes place when plants and some microorganisms grow on land and within both
oceans and fresh water. The overall reaction, again a summary of very complex steps, is
also given by eqn 8.9, this time reading from right to left. You can see then that synthesis
and degradation of organic matter describe a cycle in which carbon dioxide is taken up
(sink) and then released (source). Another uptake mechanism is dissolution in sea water
where it then circulates in the great ocean currents. An ‘ultimate’ sink for the oceanic
dissolved carbonate is its precipitation as calcite (limestone, CaCO₃) to form part of the
sedimentary material.