Thursday, November 05, 2015

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--> CaCO+ 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.


Anonymous said...

Every time I read something about this, I get more and more confused. Where does this reaction come into this

CO2 + CaCO3 + H20 -> 2HCO3-

My understanding was that one of the ways of removing CO2 was the carbonate cycle. One reason it is ultimately slow is that a way to replenish CaCO3 is through silicate weathering of rocks, which is a slow process.

You seem to be showing an entirely different process. (My comment is a little incoherent, but that's because I'm confused :-) )

EliRabett said...

Notice that the reaction you wrote is the reverse of the reaction for shell forming. That's because there is an equilibrium (e.g. both forward and reverse reactions occur.

An important difference between the two is that Ca2+ + 2HCO3- --> CaCO3 + H2O + CO2, the formation of shells, is "catalyzed" by the critters that grow the shells. The biochemistry is much more complicated than the overall reaction.

The reaction you wrote CaCO3 + H2O + CO2 --> Ca2+ + 2HCO3 is part of the set of inorganic reactions that control the pH of the oceans, the area where Roger Revelle showed the way. Eli wrote about some of that Part I Part iI

In complex (bio)chemical systems there are many things happening simultaneously and the balance between them is what determines concentrations.

Anonymous said...

Eli, this bunny is confused by the first diagram. The 'Litterfall' flux - which I have seen on no other diagram like this - tips the balance strongly in favour of the Earth absorbing much more C each year than we can throw at the atmosphere. The numbers work out to:

271.8 (absorbed) - 215.9 (emitted) = 55.9 (absorbed)

And with a net flux into the oceans of 92 - 90 = 2Pg/yr, possibly an ocean acidification problem. But If you leave out the 'Litterfall' flux, then you wind up with a C in the atmosphere problem too, like we actually do have (and I'm sure which the diagram is meant to illustrate):

219.1 (absorbed) - 215.9 (emitted) = 3.2 (emitted)

Or am I interpreting the sign/usage of 'Literfall' correctly? Also, the numbers are 5 years out of date, and I believe we are now emitting in the neighbourhood of 9Gt C per year from fossil fuel burning alone :-\

Anonymous said...

Okay, thanks. Been a long time since I've done any chemistry.

Anonymous said...

Aha. Found another diagram with 'Litterfall', and as there is no interaction with the atmosphere concerning it, it does not contribute to the yearly flux. So perhaps it is wrong/misleading to represent it with a red arrow, as it is only notational.

Also, we emitted around 32Gt CO2 in 2014. The atomic weight of CO2 is:

12 (C) + 32 (O x 2) = 44

Using ratios:

32Gt/44 = x/12

x = 8.7 GtC emitted in 2014. And 2015 might well come out to ~9GtC.

Unknown said...

There are a few studies that suggest that warming can weaken the organic carbon pump. There is higher metabolic energy requirements for all life with higher temperatures, using up more organic carbon in the surface ocean and leaving less particulate organic carbon to fall to the bottom.

The carbon cycle is like climate--it can be described simply with a few equations, but interactions between climate, biology, and geology make changes difficult to quantify.

EliRabett said...

Notice that the litterfall does not include the CO2 created in the generation of the calcium carbonate by reactions with the hydrogen carbonate ions. What you want to track is the carbon atom fluxes, both organic and inorganic.

Hank Roberts said...

Points to Berkeley Earth for getting involved in source tracking, I think:

Aaron said...

This sounds like the first chapter in a great story, too terrible to be told by Homer.

We wait with bated breath for Chapter 2.