Pumping Biologically?

Today I will talk about the other mechanism through which atmospheric carbon dioxide enters oceans, and is sequestrated within them. It is called the biological pump and can be decomposed into 3 steps:

Figure 1 Schematic showing the biological pump mechanism. Source: De La Rocha 2007

- Inorganic carbon is fixed into organic particles through photosynthesis. Light in the surface water, or euphotic zone enables phytoplankton to absorb CO2 which gets converted into organic matter.

CO2+H2O + light ----> CH2O + O2

- Most of this primary production (the process through which inorganic carbon is converted into organic carbon) decomposes in the upper layer. Some of it, though, reaches the deep sea where it faces the same fate: it is recycled and becomes CO₂ once again. This happens through decomposition by bacteria (De La Rocha 2007).

 

- Fixed carbon gets remineralized as part of the nutrient cycle. It will be used in primary production again. However, a portion of sinking primary production escapes decomposition and reaches the ocean floor. Particles aggregate forming marine snow, thereby increasing the sinking rate. Upon reaching the bottom, primary production is fixed as sediments and it can be sequestrated for thousands of years. Out of the 50–60 Pg C fixed into organic matter per year in the upper layer, only 0.3% (0.16 Pg C) becomes sequestrated in sediments (De La Rocha, 2007).

It is that sequestrated carbon that is ultimately responsible for lowering atmospheric CO₂ (De La Rocha 2007). Supposedly, levels of pre-industrial atmospheric carbon would have increased by two thirds without the biological pump (Broecker, 1982).