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Aragonite and The Carbon Cycle by Sam Gamble October 1997 Aquarium.Net

Sam Gamble writes on Aragonite and its role in the carbon cycle, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

Aragonite and The Carbon Cycle

by Sam Gamble

The sand bed system of aquariums has historically been relying on carbonate sand as surfaces for bacterial populations to attach themselves. First of all it seems obvious since this is the type of sand and rock that is found in reef areas. Essentially the reef itself produces it from a myriads of chemical and biological components and processes. There is a catch to understanding the subject of carbonate. You first have to understand that carbonate is a carbon atom joined by electron bonds to three oxygen atoms. This tricky foursome can do their bonding more than one way, forming more than one structure. The different structures have slightly different traits and abilities. The most preferable is aragonite.

Carbonates compose a group of sedimentary rocks and sand that are fundamentally similar. The materials of dissolved constituents can be deposited either by direct inorganic precipitation, or by biological intervention. Organisms can extract minerals from effected water to build skeletal structures, and those are deposited as discrete particles when the organism dies. This is a form of equilibrium; recruitment, utilization, and deposition, and back to recruitment.

Carbonates represent a different fraction of the products of typical weathering, and therefore have a different chemical composition. The carbonates concentrate CaO and MgO from weathering and the CO2 from the atmosphere. Carbonate rocks are excellent indicators of specific depositional environments, and are reservoirs for hydrocarbons and provide the host bodies for certain types of mineral deposits. They only comprise about 4% of the volume of rocks covering the Earth's surface, but carbonates make up about 35% of the rocks of the continents, and are important in modern oceans.

The difference between aragonite and calcite falls into a similar arrangement. Calcite is the most abundant mineral in limestone, i.e. ancient marine sediments. The foursome of carbon and oxygen have been joined by another comrade, calcium. Calcite occurs in two forms: High-magnesium calcite (CaCO3 containing more than 4% MgCO3) is more stable in seawater than low-magnesium calcite. In less dense aquatic environments it is not stable, and frequently it is converted to low-magnesium calcite after the death of the organism incorporating the substance.

Low-magnesium calcite (CaCO3 containing less than 4% MgCO3) is secreted by some modern organisms e.g. brachiopods, but more commonly it forms by recrystallization of aragonite and low-magnesium calcite during diagenesis.

Aragonite (CaCO3) is the most abundant mineral in modern carbonate sediments. It is a fairly stable variation of calcite. A lot of the modern carbonate-secreting organisms secrete aragonite rather than calcite, e.g. cnidarians (stony corals), depending on the specific amino-acid composition of the protein matrix in which the mineral is forming. It is not generally stable in diagenetic conditions, and is converted quickly to calcite after death of the organism.

The transitional transformations and storage capacity of aragonite makes it preferable to use as a substrate in a natural aquarium system. This doesn't mean that it dissolves quickly in a water. However, the components in aragonite will seemingly change to meet the needs of balance and equilibrium if the conditions are suitable.

The production and occurrence of limestone (aragonite is a form) is fundamentally more important than first realized. Surprisingly, limestone holds more calcium and more carbon than today's atmosphere, oceans, coal, and oil deposits, and living matter combined. As a source of CO2, limestone is the largest warehouse of the planet. When released more than 95% of the CO2 going into the atmosphere is natural, consisting of ocean release, and land release. Nature's release of CO2 is basically at equilibrium (carbon cycle). Together, the ocean uptake and plant uptake outweighs natural releases and absorbs some man made CO2.

The carbon cycle demonstrates how carbon atoms always play a part in both living and dead things, reacting to form millions of compounds with the same carbon atoms. Carbon is always changing bonds, and virtually never destroyed. The cycle will continue onward possibly forever

If aragonite is a source of calcium carbonate and carbon (carbon dioxide), with beneficial trace elements included, we need another mythical case history to see a way the cycle works. The most important illustration is the equilibria of the carbon cycle. - - A reef tank with a sand bed filter and plenum arrangement, has been built and stocked with invertebrates and corals (hard and soft). The sand bed was started with a culture of living sand. General maintenance protocol was followed during the first 3 months..

The general parameters look normal, but there is a situation with pH cycles being between 8.0 -8.1 until adding a buffer to get 8.3. The calcium level will not go above 280 - 300 ppm. There is a noticed trend that the coralline algae does not grow very fast. When the buffered pH finally rises the coralline turns whiter and appears to be diminishing. However, the coral and fish seem to be doing great.

Then shortly afterwards, coral was seen sprouting new colonies. The calcium reading was 280 ppm and pH tested at 8.3, and total alkalinity measured 11 meq/l. A balance seems to have become a formal condition for reproduction in the stony coral. What happened?

Sand bed tanks with balanced populations of filtering bacteria operate a little differently than the "Berlin Method' rock tanks. One difference is the buffering. The people who get the big blooms of coralline also supersaturate their systems with calcium hydroxide. However, there are reasons to believe this also unbalances the carbon cycle. Carbon dioxide, CO2, is a source of organic carbon. Some experts call it inorganic until it forms with other carbon molecule fragments. When calcium is supersaturated it causes precipitation as well as a building block for coralline algae. When this happens you not only get lots of coralline algae, but on the down side you are removing carbon from the carbonization cycle which yields energy for your system. It becomes stored in calcium carbonate, as calcite.

It is a release and storage balancing act when the tank is acting "naturally". In the rock style tank this wasn't a big concern. In the sand tank based on ecological balance it is important. Out on the reef you often don't see amounts of coralline that you do in some of the reef tank aquariums. Equilibrium of the environment prevents it just a bit. Calcium in the ocean is thought to be saturated, but not to the extent Kalkwasser additions have for some reef keepers.

When you see the coralline recede it is supplying back to the system or responding to that mechanism. In addition to mechanisms of the carbon cycle are causing desorption. You are perhaps witnessing equilibrium in action. If it all the coralline algae disappeared and stayed that way, then the system needs help. The figures look very good, if you were a biologist studying a real reef. They generally do the same thing, but do it more gradually, under less stressing conditions, over a longer time line.

As the system matures even more then the coralline will most likely become more prevalent. There will be enough energy (carbon) cycling going on to desorb the CaCO3 to plug carbon back into the carbon cycle. The CO2 chained up in the calcium carbonate molecule has helped fight metabolic by-products.

As stated earlier, aragonite in simplest terms is limestone, but with some important differences. It is calcium carbonate (calcite) that has been made from ancient sedimentation (diagenesis). However, aragonite has a more dense crystal structure and contains the trace elements Strontium (Sr), and Manganese (Mg). This facilitates pathways involved with energy cycling better. It takes more than water and hydrolysis to desorb energy yielding components. And better yet, most of this aragonite comes from areas that were once reefs and over time have become fossilized limestone deposits. A real plus for reef aquariums, a natural supply of important trace elements. Natural bacterial metabolism does much of the releasing.

When you use it to build a sand bed style reef tank, it will produce things you need. When carbon compounds break down and / or are metabolized by bacteria, a mild acid is a by product. This is normal - this is good. pH will drop a little and a natural release of buffering carbonate will result. Associated with it will be some of the trace elements that have been locked up in the crystalline structure of the sand.

To follow the carbon atom through this pathway; aragonite plus organic acids emit CO2. The CO2 is taken up by plants and in turn emit O2. The O2 links carbon to form carbonic acid, reducing aragonite again to CaCO3 (calcium carbonate). If calcium is made to precipitate, the carbon attracts O2 to form CO2 and we are back to square one.

CO2 (carbon dioxide) is a tremendous source of organic carbon. You can get it by changing the CaCO3 molecule with this natural acid or other metabolic processes. Our planet Earth as a whole, depends on it for a source and the storage of life giving carbon. From the calcium carbonate in the aragonite you have a stored, and huge supply of carbon waiting to be released from storage. This is not guaranteed, but occurs when the conditions are right.

Changing the natural checks and balances will change the character of the carbon cycle. The above is a good example we can see in our aquarium. In or out of our aquarium the same basic principles and mechanisms apply. Nature's release of CO2 is basically at equilibrium, bound to the carbon cycle. The carbon cycle has functioned successfully for millions of years. The carbon cycle is still occurring, but not at the equilibrium intended. Because of the carbon cycle, human's breathing does not increase CO2 emissions or collective levels. Also because of the carbon cycle all of the CO2 released from fossil fuel has not magically vanished. Balance is interrupted.

Apply this concept to your reef tank . . . . or apply it to your neighborhood while looking at your reef tank.

Questions or comments;

Sam Gamble 102170,3150@compusere.com

Created by liquid
Aquarium.Net
Last modified 2006-11-20 03:21
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