The Biological Carbon Pump

The ocean gets a disproportionate share of the carbon dioxide available to the ocean-atmosphere system. The ratio is about 50 molecules of CO2 in the ocean for every one in the atmosphere. Why is this so? The main reason is that carbon dioxide readily reacts with water to make soluble species of ions, “bicarbonate” (formula: HCO3-), rather than trying to fit between the water molecules as a gas. Another reason is the physical pump described in the last section: cold water holds more carbon dioxide in solution than warm water. This cold, carbon dioxide-rich water is then pumped down by vertical mixing to lower depths.

The last reason for the ocean’s big share of carbon is its “biological pump.” The biological pump, in essence, removes carbon dioxide from the surface water of the ocean, changing it into living matter and distributing it to the deeper water layers, where it is out of contact with the atmosphere. Thus, when the ocean shares carbon dioxide with the atmosphere, it does so by not only simply taking on carbon dioxide into solution but also by incorporating the carbon dioxide into living organisms.

The Biological Pump: a Thought Experiment

An oxygen profile at an oceanic location with an oxygen minimum zone (OMZ), like the ones found in the Arabian Sea and off the California Coast, SW Africa, and the Peru. It shows high oxygen near the sea surface, low oxygen in the OMZ at mid-depth, and the increase in oxygen in the water column below the OMZ. Typical depths for the OMZ are 1 km water below sea level, although the precise depths of these features vary with geographic location and over time at a particular site.
To appreciate how this works let us do a simple thought experiment. We t with a well-mixed ocean, dark and quite cold throughout. We then turn on the Sun and heat the ocean from above. A warm-water layer develops on top of the ocean, and since it is "euphotic" (Greek for "well-lit") green algae will now grow in this layer. That algae are growing is just another way of saying that carbon dioxide is being fixed into carbon compounds (that is, carbon dioxide is being removed from solution in the water and incorporated into living things by photosynthesis). Some of these particles of the algae (dead organic stuff) sink out of the euphotic zone into the deeper cold waters. Others are “re-mineralized,” that is, they decay by the action of bacteria, releasing carbon dioxide to the water in the process.

How long can this process of carbon fixation, carbon particle settling, and carbon particle recycling continue in our experiment? It can continue until all the nutrients that are necessary for photosynthesis have been used up, and the surface water no longer contains the nutrients necessary to support the growth of algae.

What about the recycling of nutrients (like phosphorous, sulfur, and nitrogen) through decay of organic matter? Yes, the decay of the organic particles not only recycles carbon, but also the nutrients locked within. However, the amount that is being recycled is diminished all the time, as the export of particles to deeper and deeper layers continues. At some point in our thought experiment, the recycling becomes negligible because all the nutrients have been exported to the cold layers below and nothing can grow anymore. At this point, if we draw a depth profile of the concentrations of nutrients in the ocean waters, we should find practically nothing in the warm layer, a maximum below the warm layer, where bacteria have re-mineralized many of the particles received from above, and an exponential decay with depth, as there is less and less left for the bacteria to remineralize and as the settling organic matter becomes selected for those types which are hard to oxidize. At the point of the nutrient maximum, right below the upper warm layer, there would also be an oxygen minimum. If we now add a slow upward movement of the water, to simulate the process of deep circulation, we have a basic, first-order model of the oxygen minimum in the oceans.

Diagram illustrating the ocean’s biological pump. (1) Carbon dioxide is fixed by photosynthesis, (2) this organic matter sinks into deeper waters, (3) bacterial decay releases carbon dioxide and other nutrients, making them available to be used again by phytoplankton, until (4) ultimately deposition locks away the carbon in ocean sediments.

The Redfield Ratio
In the process of removing the nutrients from the surface layer, carbon also is being removed, and so the content of total dissolved carbon in the surface layer decreases. At the same depth as the nutrient maximum there is a maximum in total dissolved carbon as well. How much carbon is exported from the surface layer in the process of losing all the nutrients? To estimate this amount, one must know the ratio of nutrient atoms to carbon atoms within the organic matter settling out of the euphotic zone. Typical numbers describing the composition of phytoplankton are C:N:P = 106:16:1. In other words, whenever 106 carbon atoms are fixed into organic matter by photosynthesis, 16 nitrogen atoms are fixed (taken from nitrate and ammonia in the water), as well as one phosphorus atom. This sequence of numbers is called the "Redfield Ratio" after Alfred Redfield, the oceanographer who first determined and explained its importance in understanding the productivity of the ocean.

In our primitive model of the oxygen minimum layer, the upwelling seawater attempts to bring both carbon and nutrients back to the surface of the ocean. However, the biologic activity in the surface layer (aided by sunlight) keeps removing the nutrients and causing them to settle back down, together with the appropriate amount of carbon (determined by the Redfield Ratio). This is a way of pumping nutrients and carbon down, against the upward movement of upwelling, and hence the term "biological pump."

The biological pump in effect puts some of the carbon into a hidden reservoir, where the atmosphere cannot reach it. Thus, the atmosphere gets less than its share of carbon than it would otherwise. How much more carbon would be hidden away if we increase the upward motion of the deep water? The answer is that as long as the upward motion and the pumping action are balanced — that is, as long as the nutrients coming up from below are sent back down quickly enough by phytoplankton to remove them from the surface layer — there is no effect.

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.Thus, if we change the overall concentration of nutrients in the ocean there is a net effect on the carbon cycle.