Historical Geology/Calcareous ooze
In this article we shall discuss the formation of calcareous ooze by carbonate-secreting organisms; the mechanisms which control the distribution of this ooze (such as the supply of nutrients and the carbonate compensation depth); and we shall look at how rocks are formed from the ooze and how we can recognize such rocks when we see them.
Calcareous ooze: what is it?
[edit | edit source]Calcareous ooze is a calcium carbonate mud formed from the hard parts (tests) of the bodies of free-floating organisms. Once this mud has been deposited, it can be converted into stone by processes of compaction, cementation, and recrystallization.
The main contributors to the ooze are coccolithophores and foraminifera. Coccolithophores are tiny single-celled organisms which cover themselves with tiny plates of calcite known as coccoliths. Foraminifera are also single-celled organisms. In some species this single cell will grow to be several centimeters in diameter, but most species of foraminifera are less than 1mm in diameter. While some will produce shells by gluing available sediment together, or by secreting shells from silica dissolved in seawater, most produce shells of calcite.
The deposition of foraminifera is generally more common today and in most regions; however, this varies from place to place and from time to time; coccoliths, for example, are more common in the ooze on the floor of the Mediterranean, and are also more common in rocks dating from the early Tertiary period.
Excepting a few odd species of foraminifera, these organisms float or swim near the surface of the ocean. When they die, they sink. Perhaps "sink" is too strong a word: they are sufficiently small that they drift gently down like dustmotes through air, and can take months to hit bottom. The ooze composed of their hard parts accumulates at a rate of about 10mm - 50mm per thousand years, varying from location to location; which doesn't sound like much, but is actually a faster rate than other marine sediments such as siliceous ooze or pelagic clay.
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The images to the right show, on the left, a photomicrograph of a coccolithophore, and on the right a photomicrograph of a foraminiferan.
Since calcareous ooze is formed from the hard parts of the bodies of free-floating organisms, this means that unlike ooids, which are nearshore sediments, and unlike reefs, which require shallow water, calcareous ooze can be deposited over vast swathes of the deep ocean floor.
However, calcareous ooze will not accumulate in the very deepest parts of the ocean, even if the surface is teeming with the right sort of organism. The reason for this will be discussed in the next section of this article.
The CCD
[edit | edit source]Calcium carbonate will dissolve in the presence of carbon dioxide and water, as follows:
i.e.
Readers familiar with chemistry will not be surprised to learn that this is the reverse of the process by which calcium carbonate precipitates. The question of which will happen, dissolution or precipitation, depends on the relative abundance of calcium ions, bicarbonate ions, calcium carbonate, and carbon dioxide present. To cut the chemistry short, we may say that where carbon dioxide is scarce, precipitation will take place, and when it is abundant, calcium carbonate will dissolve.
Now, colder and deeper water contains more carbon dioxide than shallower and warmer water. The calcium carbonate compensation depth (or carbonate compensation depth, or CCD) is the depth at which the concentration of carbon dioxide is sufficiently high that calcium carbonate is dissolved faster than it can settle.
To speak of "the" CCD as though it was one specific depth in the ocean is rather misleading: there are other factors besides depth which affect this issue. First, there is temperature: cold water will hold more carbon dioxide than warm water, and so the CCD will be deeper in warm water. And secondly, there is the fertility of the water. For the reader should always bear in mind that the carbonate compensation depth is not the depth at which calcium carbonate dissolves; rather, it is the depth at which calcium carbonate dissolves faster than it is deposited. We should also note that as aragonite is more unstable than calcite, it dissolves rather more readily, so the type of calcium carbonate being deposited also plays a role, and we should properly distinguish between the calcite compensation depth and the aragonite compensation depth.
These caveats aside, we may say that the CCD is about 4500 meters down, give or take a few hundred meters either way.
The fact that this form of chemical weathering takes place has been confirmed experimentally, by scientists who took perfectly machined spheres of calcium carbonate and left them for a year at various depths on the ocean floor. Those in shallow waters showed no signs of weathering; those that were left in deeper waters were found to be pitted and corroded as a result.
The existence of the CCD helps to explain the rather curious pattern of deposition of calcareous ooze on the ocean floor, as seen in the map to the right, where areas where calcareous ooze predominates are marked in yellow. Three things are required for this to be the main sediment: first, there must be sufficient nutrients for calcite-forming organisms to flourish; second, the ocean floor must be above the carbonate compensation depth; third, there must not be the right conditions for other sediments to swamp the deposition of calcareous ooze.
There is one point that we should emphasize: calcium carbonate below the CCD will not dissolve immediately, like an Alka-Selzer tablet fizzing away in water. The rate at which it dissolves is rather slow. It doesn't need to be fast, it just needs to be faster than the rate at which calcium carbonate is deposited. This point will be significant when we consider the evidence for plate tectonics in a later article.
Rocks from calcareous ooze: how do we know?
[edit | edit source]We should first sort out a small matter of vocabulary, Chalk might be defined as a stone which is, under a microscope, visibly composed of the tests of microorganisms. It differs from calcareous ooze itself by a degree of compaction and cementation that converts it from ooze to rock.
From the nature of its composition, it is by definition limestone. However, many experts on marine carbonate sediments will distinguish between chalk and what they are pleased to call "limestone", by which they mean a rock which has undergone more extensive recrystallization so that its origin as tests has been largely or completely obscured. We shall continue to regard chalk as a form of limestone, but anyone who wishes to read further on the subject should be aware that the distinction may be made.
If we ask, then, how we know that marine limestone is formed from calcareous ooze, half the question is already answered: we know that chalk is formed from calcareous ooze because it is still visibly formed from tests.
In more completely recrystallized limestone, however, such visible tests may be few and far between. Are we really entitled to say in such cases that the parent material was chiefly calcareous ooze?
The answer is yes. First of all, consider the question of mechanism: we expect time and burial at depth to produce recrystallization in chalk; and we have no alternative mechanism that would explain the production of such limestone.
This is somewhat of a negative argument. A more positive argument is produced by deep sea drilling. Geologists have taken core samples which progress from loose calcareous ooze at the top through stiff, compacted ooze, to chalk, with progressively greater dissolution, recrystallization and filling of pore spaces, to limestone in which "all detailed nanofossil morphology is lost near the base as sediment becomes almost totally recrystallized". (See here for further details.)
We can therefore suppose either that the transition upwards from limestone to ooze represents a gradual change in the process of deposition, from a process as yet undiscovered to the observable deposition of tests; or that the process of deposition was the same throughout but that the more deeply buried sediments have been affected to a greater degree by known processes, namely compaction, dissolution, and recrystallization. The latter hypothesis, being more parsimonious, is clearly to be preferred.