Biogenic aragonite

Grossman, E. and Ku, T.-L. 1986 Oxygen and carbon isotope fractionation in biogenic aragonite temperature effects. Chemical Geology (Isotope Geoscience Section) 59 59-74. 

White RMP, Dennis PF, Atkinson TC (1999) Experimental calibration and field investigation bof the oxygen isotopic fractionation between biogenic aragonite and water. Rapid Comm Mass Spec 13 1242-1247... 

Submarine lithification and precipitation of cements in deep sea carbonate sediments are relatively rare processes in typical major ocean basin sediments. Milliman and his associates have summarized much of the information on these processes (Milliman, 1974 Milliman and Muller, 1973,1977). The cements are of both aragonitic and magnesian calcite mineralogies, and are largely restricted to shallow seas such as the Mediterranean and Red seas, and sediments in the shallower parts of major ocean basins in which biogenic aragonite is also present. The formation of carbonate cements will be discussed in detail in subsequent chapters. 

Figure 12 Chemical composition of biogenic aragonites (a) Chemical composition of the biogenic aragonite of three taxa, showing the strong differences in Sr, Mg, and S contents, (b) Sr/Mg substitution ratio or L coefficient in the three taxa...

At shallow depths carbonate cements may cause sands to become brittle and hard. Carbonate which precipitates on the sea floor may also form hard grounds in dominantly clastic sequences. Sandstones may become calcite cemented due to dissolution of biogenic aragonite at relatively shallow depth (less than a few hundred meters). Calcareous sediments flushed by meteoric water at shallow depth or exposed during regression may become rapidly ce-... 

Figure 1. Histogram of measurements of molar U/Ca ratio in a number of samples of reef-building corals and one giant clam sample (after Edwards 1988). Also indicated is the U/Ca ratio of seawater. This illustrates the point that corals do not fractionate U from Ca by large amounts when they make their skeletons. U/Ca ratios of corals are similar to values from inorganically precipitated marine aragonite. Mollusks along with most other biogenic minerals exclude uranium. Note that the horizontal axis is on a log scale and that the U/Ca ratio of the clam is almost 5 orders of magnitude lower than that of the corals. This difference is the fundamental reason why there are difficulties with uranium-series dating of mollusks.

The degree of satiuation decreases with increasing depth for two reasons. First, the solubility of biogenic calcite and aragonite increases with depth due to increasing... 

All surface seawater is presently supersaturated with respect to biogenic calcite and aragonite with Cl ranging from 2.5 at high latitudes and 6.0 at low latitudes. The elevated supersaturations at low latitude reflect higher [COj ] due to (1) the effect of temperature on CO2 solubility and the for HCO3, and (2) density stratification. At low latitudes, enhanced stratification prevents the upwelling of C02-rich deep waters. 

Figure 11. Data documenting the calibration of the carbonate clumped-isotope thermometer for inorganic calcite grown in the laboratory (filled circles) and aragonitic corals grown in nature at known temperatures (an example of one of several biogenic materials we have also calibrated unfilled symbols). The large, gray circle shows the result of analyses of a modern soil carbonate collected from the Bolivian Altiplano plateau. The horizontal position of this data point is based on the mean annual surface temperature near the site of collection between 2004 and the present.

It will be shown later that biogenic magnesian calcites are structurally disordered and chemically heterogeneous aragonite also shows some evidence of structural disorder. Both these factors play a role in determining the "solubilities" of these phases and in their reactivity in natural systems. 

It should be kept in mind that, in spite of these major variations in the CO2-carbonic acid system, virtually all surface seawater is supersaturated with respect to calcite and aragonite. However, variations in the composition of surface waters can have a major influence on the depth at which deep seawater becomes undersaturated with respect to these minerals. The CO2 content of the water is the primary factor controlling its initial saturation state. The productivity and temperature of surface seawater also play major roles, in determining the types and amounts of biogenic carbonates that are produced. Later it will be shown that there is a definite relation between the saturation state of deep seawater, the rain rate of biogenic material and the accumulation of calcium carbonate in deep sea sediments. 

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