Posted on 19 August 2011 by Doug Mackie

This post is number 17 in a series about ocean acidification. Other posts: Introduction, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.

Welcome to the 17^th post in our series giving the fundamental background chemistry required to understand ocean acidification. This post we discuss the two main processes can move CO₂ from the air-sea boundary into deep waters: the biological pump and ocean circulation.

1. Biological pump

We have already discussed the biological pathway where photosynthesis transforms the CO₂ into big organic particles (like fish poo) that sink rapidly. As the particles fall to the bottom, they get eaten by bacteria, producing CO₂ in deep waters. Collectively this pathway is called the biological pump. Full details of the impact of increased CO₂ on the biological pump are the subject of current research by many scientists, but we can give a brief overview here.

In most areas of the ocean the activity of the biological pump is not limited by lack of carbon. If you are a gardener you will know that the plants in your garden do not have unlimited growth. Something stops them. It can be lack of sunlight, water, or a soil nutrient, such as cobalt. If you can figure out what is limiting the growth of your plants, you can add whatever you are missing. But, you can only do this until the next thing runs out. For example, adding phosphate to a garden might help initially, but once phosphate levels are sufficiently high, plant growth will be limited by something else, such as nitrogen.

Similarly, since ocean biological pump activity is not limited by a lack of carbon, adding CO₂ to surface waters does not substantially change the amount photosynthesis in surface waters. This means that the amount of carbon transported to deep waters (i.e. exported from surface waters) by the biological pump does also not change substantially when CO₂ is absorbed by the ocean.

Extra carbon might increase the efficiency of the biological pump in some areas, but the effect would likely be transitory. (You may have heard of Ocean Iron Fertilisation. In about 20% of the world, ocean photosynthesis is limited by a lack of iron. If iron is added, the water practically turns green with growth overnight. BUT, the effect is transitory and involves major changes to the species makeup of the ecosystem.

2. Ocean circulation

The other pathway to transport CO₂ into deep water is ocean circulation. There are several systems that move water in the oceans. Here we will discuss the high volume but slow (100s-1000s of years) circulation called the thermohaline ocean circulation which eventually swaps surface water with deep water. Briefly, the deep water circulation starts in the North Atlantic Ocean, where atmospheric cooling near Greenland and Scandinavia generates a downward-moving deep water current. This current flows south through to the Antarctic polar regions where further cold water is added by the same atmospheric cooling process. From there, the current flows into the Indian and Pacific Ocean.

The details vary from ocean basin to ocean basin, but thermohaline circulation causes surface water to exchange with deep water over a period of to a few centuries to a thousand years.

The high altitude atomic bomb tests of the 1950s and 60s produced a radioactive isotope of hydrogen called tritium. Deep water profiles [including those of the WOCE project that we used to make the figures for pH (Figure 13 in post 14) and Ω (Figure 15 in post 16)] show that tritium penetrates into deep water over about 1600 years. CFCs (gases) show the same pattern, so we expect that CO₂ will be similar.

For this reason, most added CO₂ remains in surface waters until the slow ocean circulation turns the surface waters over. This means the surface ocean is undergoing acidification to a much greater extent than the deep ocean.

However, if increased CO₂ in surface waters causes an increase in the efficiency of the biological pump (exporting organic matter to deep water) then an unexpected side effect could be transporting CO₂ to deep water more quickly than the purely physical processes that move tritium and CFCs – thus leading to an increased acidification of deep water.

Over the next few hundred years, surface waters will continue to undergo acidification. However, as surface water is slowly exchanged with deep water this will help slow the acidification of surface water. These next few hundred years will be a trying time.

Written by Doug Mackie, Christina McGraw, and Keith Hunter. This post is number 17 in a series about ocean acidification. Other posts: Introduction, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, Summary 1 of 2, Summary 2 of 2.