In the earth’s crust calcium makes up about 3.4% of the mass, exceeded by iron, 4.7%, aluminum, 7.5%, silicon 25.8% and oxygen 49.5%. Calcium, one of the elements of the original crust of the earth, is today found in igneous rocks as calcium silicates and in sedimentary and metamorphic rocks as calcium carbonates. The processes involved in weathering rocks, especially where some acid is present, as carbon dioxide dissolved in water or from growing lichens, are able to free some calcium from its sequestered location and send it on its way as a cation attracted to a water molecule.
Water carries the calcium cations from the highlands to the oceans. Concentrations of Ca
++
in fresh water range from 0.01 to 0.1 millimolar.
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High concentrations of calcium and/or magnesium cations in fresh water create what is called hard water. In seawater Ca
++
concentrations are 100 to 1000 times higher at 10 millimolar, with slightly greater concentrations in the deeper, colder water. This calcium then spends, on the average, one million years in the ocean before it appears on land again. The calcium ion remains in the sea water until it is precipitated out as calcium carbonate or (more rarely) as calcium sulfate, gypsum, which when heated becomes plaster of paris.
The upper levels of the ocean are supersaturated with Ca
++
and carbonate, Ca
--3
, ions. This means that all of these species which can be held in solution are in solution. The amount varies with different locations and conditions with saturation being greatest in warm shallow water with lower levels of CO2, because of photosynthesis and temperature. In these locations CaCO3 precipitates readily either inorganically or with the help of organisms. Organisms can accomplish this by building shells. This is one of the processes called biomineralization. As the organisms die their hard or mineralized parts, shells, fall to the ocean floor and accumulate or dissolve depending on depth, temperature and pressure. Shells and such which fall to the bottom of the deep parts of the ocean are most often redissolved because the deeper waters can hold more CO2 and are colder. The division between where the CaCO3 dissolves and accumulates is called the lysocline.
The mechanisms which cause CaCO3 accumulation in water are various, fascinating and not fully understood. Microscopic life, heterotrophic and photoautotrophic, is responsible for much of the deposition. Simkiss says the oldest evidence for life on earth is probably the “algal limestones” of Rhodesia (now Zimbabwe) which are dated as 2.7 billion years old.
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Thomas H. Huxley, in a 1868 lecture to working men titled “On a Piece of Chalk,”
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argues that the tiny organism Globigerina is responsible for much of the limestone which underlies Europe. Kormondy
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states that some aquatic plants occurring in alkaline waters release CaCO3 as a by-product of photosynthetic assimilation. As an example he says that 100 kg of
Elodea canadensis
can precipitate 2 kg of CaCO3 in 10 hours of sunlight under natural conditions.
The deposited CaCO3 can be mixed with other sediments from the sea or washed from the land depending on its location.
How does the calcium get back on land after if has done its time in the ocean, Obviously some is brought back as birds, animals and man harvest seafood, especially shellfish, and eat it on land, discarding the shells. The Indians did this when they harvested fish and planted it under their corn. (The form of calcium is different in fish skeletons. See below.) The High School in the Community gardens make use of compost which is created by pigs from supermarket and resturant wastes and leaves. Seashells are one of the few recognizable items in the compost, slow release calcium sources.
The majority of calcium, however, takes a different route back to the land. It takes a ride on a major geological process. The movements of crustal plates and continental land masses with various upthrusts has brought many of these accumulated CaCO3 deposits to or near the surface as limestone or, if it has undergone metamorphosis by pressure and temperature, as marble. Much of Europe and the central part of the United States east of the Mississippi River are underlain with limestone. The white cliffs of Dover are limestone. The northwestern part of Connecticut has bands of marble sandwiched between layers of metamorphosed sandstone and shale, indicating past open ocean sedimentary environment in a warm climate.
Calcium can also make it back to land through the evaporation of brackish inland seas and the processes which produce reefs.