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Monday, February 20, 2012

carbon sinks

Glaciated periods (glacials) end with a sharp increase in Carbon dioxide.  It looks most likely, as odd as it seems, that the rise in CO2 is the result rather than the cause of the melting.  In a couple of former blogs, I suggested why this might happen.  The release of CO2 then accelerates melting, in a feed-back loop.  Over the subsequent  years of the interglacial, carbon sinks continually reduce the level of atmospheric Carbon dioxide.  When Carbon dioxide levels fall sufficiently, snow begins to accumulate at high latitudes and high altitudes in the summer and we edge into another glacial.  I'd like to try to catalog as many potential sinks as I can think of, regardless of their perceived importance.  Others can work on working out which ones are significant and which not on the road to the next glacial.

Note that you can reverse each of these arguments to see what we are doing to increase atmospheric Carbon dioxide.

Corals and all other marine organisms that have a Calcium carbonate shell are sinks for carbon dioxide.  Every molecule of CaCO3 contains one molecule of Carbon dioxide (60.6% by weight).   In addition to the building of reefs by oysters and corals, some phytoplankton have calcium carbonate shells.  There are globigerina, a type of foraminifera, pteropods, a swimming snail and so forth.  When these die, they sink to the bottom of the ocean and form calcareous oozes.

 Curiously, at great depth (below about 4500m) the calcium carbonate dissolves so these oozes only form in medium deep water.  The white cliffs of Dover are such deposits and give a visual indication of the extent of carbon sequestration in the form of calcium carbonate that phytoplankton can effect.

Note: In an item I just saw on Discovery Channel (Feb20,2012) it appears that corals are much more common in the deep ocean than previously believed and even form quite large "hills" of Calcium carbonate in some locations.   This sink would presumably stop if the ocean overturn ceased and the deep ocean became anaerobic but as long as the system is working, deep water corals are another significant sink.

Question:::  What happens when this water which is enriched in dissolved Calcium Carbonate upwells into  shallower water???

Of special interest with regard to corals in the photic zone is that they are limited in their growth by the surface of the oceans.  If you have visited coral reefs you may have seen some of the brain corals which show concentric rings on their tops.  These rings are caused by  very low tides which have killed off the top of the coral.  As the diameter increases a new ring is formed.  Rather than recording yearly growth, these rings record king tides.  Corals don't grow in air.  Following the end of the last (and every other) glacial (ice period), the sea rose.  At the maximum extent of the last glacial the sea was about 120m below its present level. The last glacial lasted for about 125,000 years since the end of the Eemian interglacial.  With various smallish ups and downs, the sea level steadily dropped to this lowest point of 120m below its present level.  Coral reefs, of course, died as they were exposed and then a hundred thousands  years of waves pounded on the shore, washing the corals away, leaving cuts into the land.  Around 20,000 years ago the ice began to melt but the melt only really got under way about 11,000 years ago.  Sea level rose  quickly (in geological terms) to its present level.   During part of the transition to our present Holothurian interglacial, sea level occasionally was rising as much as 56mm/year.  The over all rise during the melt was about 6mm per year, twice the present rise.  For reference, at present (2012), the rate of sea level rise is about 2.8mm.yr.   

In tropical areas, corals grew to fill this 120m gap up to the surface of the ocean, an over all growth rate of 6mm per year.  It is estimated that the total area of coral reefs, world wide is 284,300 Square Kilometers.  This figure times 1,000,000square meters per square km times 120 meters depth times 2.5sg (specific gravity) for limestone times 60.6% Carbon dioxide in limestone  divided by 1billion gives a figure of 5,174 gigatons of Carbon Dioxide sequestered in corals since the end of the last ice period. To put this into perspective the total Carbon dioxide in the atmosphere today is about 3000 gigatons.  Corals which have grown since the ice age have sequestered about 1.6 times as much carbon dioxide as the Carbon dioxide at present in the atmosphere.  As sea level rises, we have a potential carbon sink in corals as long as we don't kill them with acidification or thermal shock. massive coral bleaching could, of course,  shut down this carbon sink.

Ocean Overturn
At the poles, sea ice freezes out fresh water ice leaving salty brine behind.  This brine is heavier than open ocean sea water and hence sinks.  As it moves across the ocean bottom, it picks up nutrients from the mineralization of the constant rain of organic material from the surface.  Where wind conditions are suitable, surface water is pushed away from the shoreline to be replaced by this upwelling, nutrient rich water.  As this water comes into the photic zone, productivity is immense.  The ocean off Peru is an example and the Anchovi produced there provides a significant percent of the fish meal for world animal production.  The Atlantic overturn is another example with the Gulf Stream being the surface manifestation of this overturn.  The shut down of such ocean systems would cause Carbon dioxide to rise more quickly in the atmosphere

If the water in a swamp is sufficiently stagnant and there is sufficient organic loading to make the bottom of the swamp anaerobic, then all cellulose that falls into it is preserved.  This includes all plant material and probably explains the formation of many of our coal measures.   If the area is sinking, the organic material is buried, heated, gives up its volatile fraction and what remains is mostly carbon with some mineral material which forms the ash when coal is burnt.  This process removes carbon from the atmosphere and also contributes to sending us toward another ice age.  

Growing Forests
A mature tropical forest, at first glance, would seem to be a huge sink for carbon dioxide.  After all, the rate of photosynthesis is huge.  This is an illusion.   Once a tropical forest has truly reached maturity, the rate at which vegetation dies and is oxidized equals photosynthesis.  Mature tropical forests, however, do represent a large sequestering of Carbon as long as they remain untouched but they have no net effect on removing more carbon from the atmosphere.

The carbon content of organic material, wood included, is about 50% of the dry weight of that material.  If an plant is 80% water, 20% dry matter, then it's carbon content is about 10% (of its wet weight). The cutting down of the forests of the world has released a large amount of carbon into the atmosphere and this carbon would be rapidly taken up by letting the forests grow again.  In this way, tropical forests could be a huge carbon sink if allowed to regrow.  The only way a mature tropical forest will continue to sequester carbon is if it contains swamps as detailed above.  Of course another way is to selectively log tropical forests (at a rate that doesn't imperil their survival) and to use the wood in long lasting buildings, furniture etc.  New trees grow where the old ones are harvested and a mature tropical forest then becomes a carbon sink.

New forests are a whole different ball game.  At the maximum extent of the last period of glaciation ('glacial' as opposed to 'interglacial'), ice, estimated to have reached a depth of 3km, covered most of Canada and a strip of America down to and extending beyond New York.  Much of Europe, Germany, Poland and  Russia and the UK were covered.  Smaller ice sheets were found on high land right down to the equator (Mt Kilimanjaro, for instance).  When the continental glaciers melted all the scraped-clean land was open for colonization (primary and secondary succession).  Much of this land ended up clad in forests of giant trees with their sequestered carbon.  As the tree line moves north with global warming, we have another potential carbon sink.

While we are at it, temperate forests are a different situation from tropical forests.  When Tropical forests are clear felled, it is found that the soil is thin and is exhausted very quickly.  This is due to the characteristics of humus.  Humus is the final break-down product of organic material and contains a lot of carbon.  Amongst it's other characteristics, it holds nutrients in a form that can be used by plants.  Above about 25 degrees C, Humus breaks down.  

In temperate forests, the humus builds and builds, depositing more and more organic material in the ground.  When temperate forests are clear felled, agriculture can carry on for far longer than in the tropics, before the soil is exhausted. Temperate forests can continue to sequester carbon even after they are mature (with respect to the total mass of live vegetation they contain). The re-establishment of temperate forests not only would sequester considerable carbon but continue to sequester carbon when the forest is mature.  It is a bit rich for northern hemisphere countries to complain about the destruction of tropical forest when they have decimated their own forests.

During the summer, the top foot or two of permafrost melts and a range of dwarf, ground hugging trees, lichens and mosses grow.  In the winter this freezes.  Each year another small layer is added and the layer of organic material deepens.  Permafrost areas are carbon sinks.  Of course, the flip side is that thawing them with global warming is a source of carbon. It is estimated that the carbon stored in permafrost today is greater than all the carbon of all living things and is twice the carbon in the atmosphere.     If the layer is deep enough (about 300m) the pressure is great enough that a portion of this carbon is in the form of methane clathrate which only needs a little warming to be released. In addition, any geological methane seeping up under permafrost can also be stored as clathrate as it comes into contact with moisture.  The depth of the frozen soil is not the only factor for the creation of methane clathrate.  The frozen permafrost can act as an impermeable layer, like the lid of a pressure cooker and clathrate can form at shallower depths.

It is interesting to note (and counter-intuitive) that by covering an area of permafrost with a deep layer of insulating  ice during a glacial, the permafrost will be melted by the heat coming up from the earth.  This deep organic soil would break down anaerobically giving up methane.  This would likely collect at the bottom of the ice sheet as a methane clathrate, ready to be suddenly released when the ice sheet melted.  Of course, the clathrate already stored in the permafrost would give up it's methane too which would seep up and be stored in the ice sheet. This may be part of the explanation of how Carbon dioxide rises so quickly at the end of glacials.  Released methane trapped under the ice oxidizes rapidly into Carbon dioxide and shows up in bubbles in the ice cores from Greenland and Antarctica as Carbon dioxide rather than as Methane.

Note that the Firn layer (permeable top of an ice sheet) is about 70m and gas can diffuse between this layer and the atmosphere so the signal of sudden methane or for that matter Carbon Dioxide would be smeared out throughout the layers.

Native grasslands have most of their biomass underground.  This is an evolutionary adaptation to fire.  Grass fires are intense but short and the heat doesn't penetrate far underground.  With the introduction of nutrients from wind-imported-dust, grasslands in their natural state continue to grow upward and add more and more organic material.  Organic material is ultimately stored in deep rich humus containing soil.  This rich accumulation of nutrients was mined by farmers planting wheat and other crops, especially in the great plains of America, and the carbon returned to the atmosphere.  In pre-agricultural times, grasslands were carbon sinks. Now they are sources of carbon.  If returned to their native state, they once more would sequester carbon.  Some research suggests that the north American Buffalo can produce more meat per hectare on native North American prairie than our much vaunted beef cattle.  At the same time they allow the sequestration of carbon to once more occur in the grasslands. Have a look at this TED talk on how to restore our grasslands.  Direct seed drilling without plowing has also been shown to increase the Carbon content of soils.

Calcareous Oozes 
Many plankton animals such as foramanifera have calcareous tests.  These sink  form layers of calcium rich deposits on the ocean bottom up to about 4500m.  Below this depth, calcium carbonate is soluble.  These oozes get buried and are carried toward subduction zones by the ocean bottom conveyor system.  Over geological time, these deposits of lime are recycled by volcanoes as Carbon dioxide. 60.6% of calcium carbonate is Carbon dioxide. When you think of deposits like the chalk cliffs of Dover it is apparent that calcareous deposits are a large sink for Carbon dioxide.

Silicate rocks
As erosion or volcanic activity exposes new silicate rocks to the atmosphere, they are attacked by Carbon dioxide, producing carbonates.  This is also a sink for Carbon dioxide but on a geological time scale.

As cement is produced a large amount of CO2 is released.  As it cures the Carbon dioxide is re-absorbed.  All cement structures around the world are slowly absorbing some of the Carbon dioxide that was released when they were produced.

 Carbon sinks are slow but inevitable.  As they proceed, the Carbon dioxide content of the air decreases, the climate cools until snow can begin to accumulate over the summer.  It would appear that with the amount of sequestered carbon we have introduced into the atmosphere by burning fossil fuels, the next descent into a glacial  will be much delayed.  It is interesting to think that we will have used up the world supply of sequestered carbon in the blink of an eye.  We would expect the present interglacial age to last another 10 or 20 thousand years.  With our Carbon output, let's say we have pushed the end of the present interglacial out to 30,000 years.  If we still exist then as a species, which seems unlikely, our descendants will be faced with the onset of a glacial age with no large resources of sequestered carbon left to counter it.