Saturation horizon calcite

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. 

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... 

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.)... 

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.)... 

Feely R.A. and Chen C.-T. A. (1982) The effect of excess CO2 on the calculated calcite and aragonite saturation horizons in the northeast Pacific. Geophys. Res. Lett. 9, 1294-1297. 

How well can we presently determine the saturation-horizon depth (where D = 1) for calcite in the sea If we assume that we know the calcium concentration exactly, then the error in D is determined by the errors in and the measured carbonate ion concentration, [CO ]. Mucci (1983) was able to determine repeated laboratory measurements of the apparent solubility product, p, at 1 atm pressure to — 5%, and the pressure dependence at 4 km is known to 10%. These errors compound to 11% in the value of K sp (4 km). Carbonate ion concentrations in the sea are almost always calculated from Ax and Die. Being slightly conservative about accuracy of these values in ocean surveys ( 4p.eqkg for Ax and 2p.molkg for DIG they can be determined with errors about half these values if conditions are perfect), and assuming we know exactly the value of the... 

Organic matter degradation within the sediments creates a microenvironment that is corrosive to CaCOa even if the bottom waters are not, because addition of DIC and no At to the pore water causes it to have a lower pH and smaller [COa ]. Using a simple analytical model and first-order dissolution rate kinetics, Emerson and Bender (1981) predicted that this effect should result in up to 50% of the CaCOa that rains to the seafloor being dissolved even at the saturation horizon, where the bottom waters are saturated with respect to calcite. Because the percent CaCOa in sediments is so insensitive to dissolution and the saturation-horizon depth so uncertain, this suggestion was well within the constraints of environmental observations. 

Recent measurements of calcium and alkalinity in the ocean above the calcite saturation horizon (Milliman et ai, 1999 Chen, 2002) suggest dissolution in supersaturated waters. The proposed mechanisms are variations of the organic matter driven CaC03 dissolution mechanism. In these cases the authors suggest that microenvironments in falling particulate material (Milliman et al., 1999) or anerobic dissolution in sediments of the continental shelves and marginal seas (Chen, 2002) are locations of CaC03 dissolution. As the details and accuracy of measurements improve, thermodynamic and kinetic mechanisms required to interpret the results become more and more complex. 

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. 

Following the suggestion of Emerson and Bender (1981) that the release of respiration CO2 in pore waters likely drives calcite dissolution above the saturation horizon, a number of investigators took the bait and set out to explore this possibility. David Archer, as part of his PhD thesis research with Emerson, developed pH microelectrodes that could be slowly ratcheted into the upper few centimeters of the sediment from a bottom lander. He deployed these pH microelectrodes along with the O2 microelectrodes and was able to show that the release of respiration CO2 (as indicated by a reduction in pore-water O2) was accompanied by a drop in pH (and hence also of CO ion concentration). Through modeling the combined results, Archer et al. (1989) showed that much of the CO2 released by respiration reacted with CaCOs before it had a chance to escape (by molecular diffusion)... 

Table 6 CaCOs dissolution rates at sites above the calcite saturation horizon (i) in situ microelectrode profiling (NW Atlantic and Ceara Rise) and (ii) in situ whole-core squeezer (Ceara Rise and Cape Verde Plateau).

The balance between dissolution and preservation of CaCOs in the oceans has important implications for the marine alkalinity balance and therefore for the atmospheric CO2 concentration. On the order of half the CaCOs dissolution in the oceans occurs in sediments. Studies of the sedimentary dissolution process are leading to a quantitative understanding of the role of sediments in the marine CaCOs cycle. Most importantly, they have highlighted the potential importance of the role of dissolution in sediments above the calcite lysocline, which is driven by neutralization of acids produced by oxic metabolism. Dissolution above the saturation horizon cannot be ignored in the marine CaCOs cycle, and possible temporal variations in its extent must be considered in interpreting the temporal record of sedimentary CaCOs accumulation. 

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... 

Observations from benthic flux experiments in which Ai and Ca fluxes have been measured both below and above the calcite saturation horizon confirm the effect of organic matter degradation on the dissolution of CaCOs in most but not all situations. Jahnke and Jahnke... 

While calcite dissolution below saturation horizons in the water colunm can be described by the equations reported in section 9.3.2., dissolution due to COj release from organic carbon respiration is expressed by the following equations ... 

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).

Event II was the main plankton extinction and productivity crisis coimected to the rapid collapse of the surface-to-deep water carbon isotope gradient and drops in barium and carbonate accumulation rates. Curiously, there was a hundredfold increase in the concentration of foraminifera relative to total carbonate. It could be due to intensified deep circulation with winnowing of the fine fraction. Or possibly to better the preservation of the dissolution-prone planktonic forms through deepening of the CCD and/or lowered rates of in situ dissolution caused by decreased decay of organic carbon in sediment pore waters. There is support for this idea from the fact that coccoHths tend to be more dissolution-resistant than foraminifera, also from calcite dissolution above the calcite saturation horizon is driven mainly by titration by metabolic carbon dioxide derived from organic carbon decay at or near the sediment-water interface. 

Two attempts have also been made to remove the other mcer-tainty. Calcite solubility measurements have been made by Ingle et al. (1973) and by Berner (1976). Unfortunately these estimates disagree by 35 per cent at 2 C and I6 per cent at 25 C. As we shall see the Ingle et al. value supports the contention that saturation horizon and lysocline axe nearly coincident while the Berner value supports the contention that the saturation horizon lies a kilometer or two above the lysocline. 

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