Calcium carbonate with depth

Parameters influencing the distribution of calcium carbonate with increasing water depth in equatorial Pacific sediment. Note that fi is reported as a percentage (%). Source From van Andel, Tj. H., et al. (1975). Cenozoic History and Paleoceanography of the Central Equatorial Pacific Ocean, Geological Society of America, Boulder, CO, p. 40. 

The distribution of calcium carbonate in sediments with ocean depth shows wide variations. In open ocean basins, where rates of detrital sedimentation are moderate to low, sediments above 3000 meters water depth are generally high in calcium carbonate, whereas sediments below 6000 meters generally have very low calcium carbonate content. Between these depths there is a poor correlation between the weight % calcium carbonate and depth (Smith et al., 1968). Turekian... 

Figure 3. General distribution of calcium carbonate with increasing water depth in deep ocean basins (after Ref. 9)...

The carbonate compensation depth (CCD) occurs where the rate of calcium carbonate dissolution is balanced by the rate of infall, and the calcium carbonate content of surface sediments is close to Owt.% (e.g., Bramlette, 1961). The CCD has been confused with the calcium carbonate critical depth (sometimes used interchangeably with the lysocline discussed next), where the carbonate content of the surface sediment drops below 10 wt.%. A similar marker level in deep-sea sediments is the ACD, below... 

The solubility of calcite and aragonite increases with increasing pressure and decreasing temperature in such a way that deep waters are undersaturated with respect to calcium carbonate, while surface waters are supersaturated. The level at which the effects of dissolution are first seen on carbonate shells in the sediments is termed the lysocline and coincides fairly well with the depth of the carbonate saturation horizon. The lysocline commonly lies between 3 and 4 km depth in today s oceans. Below the lysocline is the level where no carbonate remains in the sediment this level is termed the carbonate compensation depth. 

Calcium carbonate solubility is also temperature and pressure dependent. Pressure is a 6r more important fector than temperature in influencing solubility. As illustrated in Table 15.1, a 20°C drop in temperature boosts solubility 4%, whereas the pressure increase associated with a 4-km increase in water depth increases solubility 200-fold. The large pressure effect arises from the susceptibility of the fully hydrated divalent Ca and CO ions to electrostriction. Calcite and aragonite are examples of minerals whose solubility increases with decreasing temperature. This unusual behavior is referred to as retrograde solubility. Because of the pressure and temperature effects, calcium carbonate is fer more soluble in the deep sea than in the surfece waters (See the online appendix on the companion website). 

Although surfece waters are supersaturated with respect to calcium carbonate, abiogenic precipitation is imcommon, probably because of unfevorable kinetics. (The relatively rare formation of abiogenic calcite is discussed further in Chapter 18.) Marine organisms are able to overcome this kinetic barrier because they have enzymes that catalyze the precipitation reaction. Because fl declines with depth, organisms that deposit calcareous shells in deep waters, such as benthic foraminiferans, must expend more energy to create their hard parts as compared to surfece dwellers. 

In contrast to calcium carbonate, all seawater is undersaturated with respect to BSi. As shown in Table 16.1, the imdersaturation is very large and increases with depth because the solubility of BSi increases with pressure. Thus, all siliceous hard parts are subject to dissolution. Nevertheless, about 25% of the BSi created in the surfece waters survives the trip to the seafloor via pelagic sedimentation. Direct observations of this transport... 

Saturation horizon The depth range over which seawater is saturated with respect to calcium carbonate, i.e., D = 1. At depths below the saturation horizon (D < 1), calcium carbonate will spontaneously dissolve if exposed to the seawater for a sufficient period of time. 

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. 

As previously mentioned, the primary processes responsible for variations in the deep sea C02-carbonic acid system are oxidative degradation of organic matter, dissolution of calcium carbonate, the chemistry of source waters and oceanic circulation patterns. Temperature and salinity variations in deep seawaters are small and of secondary importance compared to the major variations in pressure with depth. Our primary interest is in how these processes influence the saturation state of seawater and, consequently, the accumulation of CaC03 in deep sea sediments. Variations of alkalinity in deep sea waters are relatively small and contribute little to differences in the saturation state of deep seawater. 

One of the most controversial areas of carbonate geochemistry has been the relation between calcium carbonate accumulation in deep sea sediments and the saturation state of the overlying water. The CCD, FL, R0, and ACD have been carefully mapped in many areas. However, with the exception of complete dissolution at the CCD and ACD, the extent of dissolution that has occurred in most sediments is difficult to determine. Consequently, it is generally not possible to make reasonably precise plots of percent dissolution versus depth. In addition, the analytical chemistry of the carbonate system (e.g., GEOSECS data) and constants used to calculate the saturation states of seawater have been a source of almost constant contention (see earlier discussions). Even our own calculations have resulted in differences for the saturation depth in the Atlantic of close to 1 km (e.g., Morse and Berner, 1979 this book). 

More recent calculations such as those in this book indicate substantially lower saturation depths. Those calculated here are plotted in Figure 4.21. The SD is generally about 1 km deeper than that presented by Berger (1977). Clearly the new SD is much deeper than the R0 and appears only loosely related to the FL. Indeed, in the equatorial eastern Atlantic Ocean, the FL is about 600 m shallower than the SD. If these new calculations are even close to correct, the long cherished idea of a "tight" relation between seawater chemistry and carbonate depositional facies must be reconsidered. However, the major control of calcium carbonate accumulation in deep sea sediments, with the exceptions of high latitude and continental slope sediments, generally remains the chemistry of the water. This fact is clearly shown by the differences between the accumulation of calcium carbonate in Atlantic and Pacific ocean sediments, and the major differences in the saturation states of their deep waters. 

Other reactions of probable less importance than those above leading to undersaturated conditions with respect to calcium carbonate near the sediment-water interface include nitrate reduction and fermentation (e.g., Aller, 1980). Such reactions may also be important near the sediment-water interface of continental shelf and slope sediments, where bioturbation and bioirrigation can result in enhanced transport of reactants. Generally, as water depth increases over continental slope sediments, the depth within the sediment at which significant sulfate reduction commences also increases. It is probable that the influence of reactions other than sulfate reduction on carbonate chemistry may increase with increasing water depth. 

One major paper attacking the problem of the relationship between the preservation of calcium carbonate in shallow anoxic marine sediments and their chemistry was by Aller (1982). The study was conducted at sites in Long Island Sound. The calcium carbonate content of the sediments decreased with increasing water depth. At the shallow FOAM (Friends of Anaerobic Muds) site shell layers associated with storms resulted in irregular variations in the carbonate content of the sediment. Ca2+ loss from the pore waters, indicative of calcium carbonate precipitation, was found only at the FOAM site below -20 cm depth. During the winter, elevated Ca2+ to CL ratios were observed near the sediment-water interface... 

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