The Deep Blue Sea

In a previous post, Eli explored the rapid equilibrium between the three surface reservoirs for carbon dioxide, the atmosphere, the biosphere and the surface of the oceans, maybe to a depth of 1 km.  Just to remind the bunnies, here is the overview of the carbon cycle,

and here is the animated cartoon showing the rapid equilibration of the three top level reservoirs when a pulse of CO2 enters the atmosphere

That equilibration takes about 5 years.

In this episode Eli will explore a bit of the slower coupling between the surface ocean and the deep ocean.  Inquiring rabett's want him to point out that the amount of carbon in the intermediate and deep ocean is about 30 times that in the three fast reservoirs, give or take a bit, so that the overall process will be a dilution of the excess CO2 that humans have pushed into the atmosphere over a considerable period of time.

Being not an artist the owner operator has sketched out how the interchange between the surface and the deeper oceans works

The most interesting thing about this is that while the surface ocean and the deep ocean are linked through over turning at their interface, there is a one way valve moving carbon from the upper to the lower ocean, the biological pump.

Simply put, this represents the settling of organic carbon from dead critters. But it is not simple.  It fills Eli with awe and reminds him of Einstein's remark that God is subtle, but not malicious.  Also very clever.

It turns out that there are two biological pumps, and the interplay between them controls the interchange with the deep ocean and the atmosphere.  As shown in this figure from Rost and Riebesell

The organic carbon pump moves carbon in the form of organic material (here represented by CH2O, but really meaning any dead animal or plant) down into the deep sea.  The second mechanism, the carbonate counter pump is a consequence of the formation of calcium carbonate shells from the reaction of two hydrogen carbonate ions (HCO3^-) with a calcium ion Ca²⁺.  This also  increases the amount of dissolved CO2  which, in turn is partially pushed into the atmosphere, increasing the concentration in the atmosphere.  The CaCO3 to organic carbon ratio, called the rain ratio, is a measure of which pump is strongest.

However, and this being Rabett Run, you did know there was going to be one of those we are confronted with the problem of how neutrally buoyant organic critters sink to the bottom of the sea. For that Eli looked at Particulate organic carbon fluxes to the ocean interior and factors controlling the biological pump: A synthesisof global sediment trap programs since 1983 Susumu Honjo, Steven J. Manganini, Richard A. Krishfield and Roger Francois.  Turns out that the organic carbon forms aggregates with the shells, other forms of CaCO3, opal and the other crap (literally) that is floating in the seas including sand.  The biological pump is indeed a dirty business.  The biological pump which is the driver of interchange from the surface to the deep ocean.  Honjo, et al find that the carbonate driven biological pump dominates in the in the open seas except in the far north.

While the formation of shells 

Ca²⁺ + 2HCO3^- --> CaCO3 + H2O + CO2 

is neutral by itself in moving inorganic carbon from the surface to the deep ocean, with one molecule of calcium carbonate balanced by a molecule of carbon dioxide, the calcuim carbonate provides significant ballast for the settling of organic carbon into the deep ocean.  To an extent other ballasting material (opal, fecal matter, sand) take over in areas where shells made of calcium carbonate are missing.

So the next question is what will increasing sea surface temperatures and decreasing pH do to the biological pump.