Calcite saturation depth

Calculations such as those by Morse and Mackenzie (1990) indicate that the calcite saturation depth is generally —1 km greater than proposed by Berger (1977) and that it is much greater than R. It appears only loosely related to the FL. In the equatorial eastern Atlantic Ocean, FL is —600 m shallower than the saturation depth. If these observations are close to correct, the long cherished idea of a tight relation between seawater chemistry and carbonate depositional facies must be reconsidered (Mekik et al., 2002). The influence of near interfacial diagenetic processes on these relationships is discussed in the next section. 

Figure 7.15 A simple ocean-atmosphere-continent system. Pressure of C02 enhances Ca release from the continental crust (which is assumed to be made of CaSi03) and controls the depth of calcite saturation. Calcite precipitation is therefore controlled by the hypsometric curve, equation (7.4.8), and Pco2-...

Depth in meters of the (a) aragonite and (b) calcite saturation horizons (fi = 1) in the global ooeans. Source-. After Feeley, R. A., et al. (2004). Science 305(5682), 362-366. (See oompanion website for oolor version.)... 

Figure 4.6. Variation in the saturation depth of seawater with respect to calcite as a function of potential Pc02 f°r seawater at 2°C, S=35, and At=2400 ieq kg-1 seawater.

Typical vertical saturation profiles for the North Atlantic, North Pacific, and Central Indian oceans are presented in Figure 4.10. The profiles in the Atlantic and Indian oceans are similar in shape, but Indian Ocean waters at these GEOSECS sites are definitely more undersaturated than the Atlantic Ocean. The saturation profile in the Pacific Ocean is complex. The water column between 1 and 4 km depth is close to equilibrium with calcite. This finding is primarily the result of a broad oxygen minimum-C02 maximum in mid-water and makes choosing the saturation depth (SD) where Oc = 1 difficult (the saturation depth is also often referred to as the saturation level SL). 

We have calculated the saturation depth (SD) with respect to calcite for various regions of the Atlantic, Pacific and Indian oceans as shown in Figure 4.12. The saturation depth is deepest in the Eastern Atlantic ocean. In both the eastern and western Atlantic Ocean, the saturation depth becomes shallower to the south,... 

Figure 4.12. Saturation depths with respect to calcite in different areas of the ocean calculated from corrected GEOSECS data (see text).

A detailed study of the chemistry of pore waters near the sediment-water interface of sediments from the equatorial Atlantic was conducted by Archer et al. (1989) using microelectrodes that were slowly lowered into the sediment. By modeling the resulting data they were able to confirm that calcite was dissolving above the saturation depth as a result of benthic oxidation of organic matter. The estimated in situ rate constant for calcite dissoluton was 1-100% day1. This rate constant is 10 to 100 times slower than the one used in previous models, which was based on experimental data. If the slower rate constant proves to be correct, then dissolution of calcite by benthic metabolic processes will be of major importance. 

Figure 7. The depth distribution of the Ro and calcite saturation levels, the foraminiferal lysocline and the calcium carbonate compensation depth in the Western and Eastern Atlantic Ocean (after Ref. 40)...

Figure 5 Results of in situ dissolution experiments. Peterson (1966) re-weighed polished calcite spheres after a 250 d deployment on a mooring in the North Pacific. Honjo and Erez (1978) observed the weight loss for calcitic samples (coccoliths, foraminifera and reagent calcite) and an aragonitic sample (pteropods) held at depth for a period of 79 d. While Peterson hung his spheres directly in seawater, the Honjo-Erez samples were held in containers through which water was pumped. The results suggest that the calcite saturation horizon lies at 4,800 200 m in the North Atlantic and at about 3,800 200 m in the North Pacific. For aragonite, which is 1.4 times more soluble than calcite, the saturation horizon in the North Atlantic is estimated to be in the range 3,400 200 m.

The other two processes involve dissolution of calcite after it reaches the seafloor. A distinction is made between dissolution that occurs before burial (i.e., interface dissolution) and dissolution that takes place after burial (i.e., pore-water dissolution). The former presumably occurs only at water depths greater than that of the saturation horizon. But the latter has been documented to occur above the calcite saturation horizon. It is driven by respiration CO2 released to the pore waters. 

Table 1 The calcite-saturation carbonate ion concentration in cold seawater and the slope of this solubility as a function of water depth based on a 1 atm solubility of 45 pimol CO kg and a AT of 40 cm mol . ...

Surface water temperature is approximately 32°C throughout the year. Water temperature is 34.25°C at a depth of 51 meters (R.J. Hoffman, U.S. Geological Survey, 1990, personal communication). Water in Devils Hole is slightly supersaturated with respect to calcite (saturation index averages about 0.18) with calculated Pco2 values from 0.0123 to 0.0141 atm (Plummer et al., 2000). The water has been supersaturated with calcite for at least 500,000 years. 

While errors in evaluating the depths of both the saturation horizon and the onset of CaCOs dissolution complicate field tests of the importance of chemical equilibrium, the change in the degree of calcite saturation (AC03= over the depth... 

As soon as the calcite saturation level is reached, biogeochemical carbonate crystals accumulate, mixed with remains of ostracods sediments prograding towards the lake centre form foresets. Foresets can include turbidite beds a few centimetres thick due to turbid underflow. Calcium carbonate precipitates only when Characeae are present. Along the talus, and with depth, the water cools down increasing calcite dissolution. 

Fig. 6.11 Schematic diagram showing depth relationship between degree of saturation for calcite in seawater and rate of CaC03 dissolution. At 4km depth, as seawater approaches undersaturation with respect to calcite, rate of dissolution of sinking calcite skeletons increases. The lysocline marks this increased rate of dissolution. Below the lysocline only large grains (foraminifera) survive dissolution if buried in the seabed sediment. Below the calcite compensation depth (CCD see text) all CaC03 dissolves, leaving red clays.

Fig. 11. Calcite saturation index vs depth for Crooks Gap. The water analysis used for the calculations was the original water analysis plus the amount of added calcium and bicarbonate determined from Fig. 7.

The association of marine barite with organic matter complicates the interpretation of occurrence of increased barite deposition found along midocean ridges because greater preservation of organic matter also occurs with increased carbonate sedimentation above the calcite compensation depth. Additionally, basin-scale dispersal of hydrothermal particles appears limited, especially for the relatively dense barite. Studies of marine barite saturation show that barite is below saturation in the water column but rapidly approaches saturation in the pore waters of deep-sea sediments. Discrete barite particles are nevertheless found in the microenvironments of suspended... 

Figure 3 Bathymetric profiles of calcium carbonate (calcite) saturation for hydrographic stations in the Atlantic and Pacific Oceans (data from Takahashi etai 1980). Carbonate saturation here is expressed as ACOa ", defined as the difference between the in situ carbonate ion concentration and the saturation carbonate ion concentration at each depth ACOa " = [C03 ]seawater - [COa Jsaturation)-The saturation horizon corresponds to the transition from waters oversaturated to waters undersaturated with respect to calcite (A 003 = 0). This level is deeper in the Atlantic than in the Pacific because Pacific waters are COa-enriched and [C03 ]-depleted as a result of thermohaline circulation patterns and their longer isolation from the surface. The Atlantic data are from GEOSECS Station 59 (30°12 S, 39°18 W) Pacific data come from GEOSECS Station 235 (16°45 N,161°23 W).

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. 

Solubility of calcite increases significantly with increasing pressure. In the present ocean which contains 10.3 x 10 3molkg I Ca2+, the depth zs of CaC03 saturation is given by... 

Broecker and Takahashi, 1978). zs is the depth in km and carbonate concentrations are in mol kg" . Since seawater Ca2+ concentration is allowed to change as a result of calcite precipitation and river input, it is more general to state that the saturation product Ks changes as... 

Saturation state of seawater, Cl, with respeot to (a) calcite and (b) aragonite as a function of depth. The dashed vertical line marks the saturation horizon. North Pacific profile is from 27.5°N 179.0°E (July 1993) and North Atlantio profile is from 24.5°N 66.0°W (August 1982) from CDIAC/WOCE database http //cdiac.esd.oml.gov/oceans/CDIACmap.html) Section P14N, Stn 70 and Section A05, Stn 84. Source From Zeebe, R.E. and D. Wolf-Gladrow (2001) Elsevier Oceanography Series, 65, Elsevier, p. 26. 

More detail on the horizontal variations in is provided in Figure 15.12, which illustrates that the depth of the saturation horizon for calcite rises from 4500 m in the... 

North Atlantic to 500 m in the North Pacific. This reflects an increasing addition of CO2 to deep waters as meridional overturning circulation moves them from the Atlantic to the Indian and then to the Pacific Ocean. Thus, as a water mass ages, it becomes more corrosive to calcium carbonate. Since aragonite is more soluble than calcite, its saturation horizon lies at shallower depths, rising from 3000 m in the North Atlantic to 200 m in the North Pacific. 

Based on thermodynamic considerations, sediments that lie at depths below the saturation horizon should have 0% CaCOj. This then explains why calcareous oozes are restricted to sediments lying on top of the mid-ocean ridges and rises and why the sediments of the North Pacific are nearly devoid of calcite and aragonite. (The low %CaCOj in the sediments of the continental margin is a result of dilution by terrestrial clay minerals.)... 

As with the calcareous tests, BSi dissolution rates depend on (1) the susceptibility of a particular shell type to dissolution and (2) the degree to which a water mass is undersaturated with respect to opaline silica. Susceptibility to dissolution is related to chemical and physical factors. For example, various trace metals lower the solubility of BSi. (See Table 11.6 for the trace metal composition of siliceous shells.) From the physical perspective, denser shells sink fester. They also tend to have thicker walls and lower surface-area-to-volume ratios, all of which contribute to slower dissolution rates. As with calcivun carbonate, the degree of saturation of seawater with respect to BSi decreases with depth. The greater the thermodynamic driving force for dissolution, the fester the dissolution rate. As shown in Table 16.1, vertical and horizontal segregation of DSi does not significantly coimter the effect of pressure in increasing the saturation concentration DSi. Thus, unlike calcite, there is no deep water that is more thermodynamically favorable for BSi preservation they are all corrosive to BSi. 

Increase in water depth will shift the equilibrium and between 3.000 and 4.000 m the ocean starts to become unsaturated with respect to aragonite and calcite due to pressure effects122-124. In contrast, the majority of river and lake waters are unsaturated at all depths, because precipitation of CaC03 will readily take place the moment saturation is exceeded. The solubility products are92 ... 

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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