The Biological Carbon Pump
How important is the biological pump overall? It turns out, it is very important. For instance, if the biological pump were turned off, atmospheric CO2 would rise to about 550 ppm (compared to the current 360 ppm). If the pump were operating at maximum capacity (that is, if all the ocean’s nutrients were used up) atmospheric CO2 would drop to a low of 140 ppm.
Interesting estimate by the University of California, San Diego.
The Marine Carbon Cycle
Altering this ratio of carbon atoms can be done, for example, by changing the amount of silicate (SiO4) in seawater. If there is plenty of silicate, marine organisms called “diatoms” will grow more happily. They fix carbon into organic matter, and they take much of it down to deep waters because many diatoms, at the end of their life cycle, tend to settle out of the water where they grew. If there is very little silicate available, organisms called “coccolithophores” grow more readily than diatoms.
Let us remember at least one element concerning the carbonate cycle:
Unusually intense blooms of carbonate-fixing plankton, like coccolithophores, would have the effect of bringing carbon dioxide from surface waters to the air above it – that is, increasing the atmospheric CO2 concentration. The same is true for coral and shell growth in shallow waters. We would like to know, then, what precisely causes the blooms of coccolithophores that can be seen on satellite surveys, and whether their intensity is increasing or decreasing as the planet warms. Unfortunately, this is not known at present.
During the overall cooling of the planet, in the last 40 million years, more and more silicate has been removed from ocean in the upwelling regions around the continents (due to stronger mixing from stronger winds). We know this because radiolarians (plankton organisms using silicate to make their skeletons) have been getting thinner and more delicate through time. In the last 3 million years this process of silicate extraction has enormously accelerated, as the Antarctic Ocean started to deposit vast amounts of diatom shells.
Diatoms sequester carbon but Coccoliths do not.