Saturation horizon

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.)... 

A significant fraction of the CO2 injected into the atmosphere as a result of fias-sil fuel burning is now dissolving into the surfece ocean. This is causing a decline in seawater pH and O,. A recent modeling effort, shown in Figure 15.13, predicts a precipitous rise in the aragonite saturation horizon by the year 2100, with surfece waters in... 

Aragonite saturation horizon prediotions for the year 2100. Vaiues mapped are A[C03 ] = [CO ] n situ - PO ]aragonite saturation hore positive A[C02-]a indicates supersaturation and... 

On the opposite end of the spectrum, thermodynamics cannot explain why some PIC can sink through undersaturated waters without dissolving to accumulate on the seafloor. This is a widespread phenomenon as evidenced by the spatial %CaCOj gradients seen in the surface sediments (Figure 15.5). If the saturation horizon dictated the survival of sinking and accumulating PIC, a sharp depth cutoff should exist below which calcium carbonate is absent from the surface sediment. The importance of this kinetic barrier to dissolution is also seen in the relatively high fraction of surfece-water PIC (20 to 25%) that accumulates in the sediments as compared to the low fraction of surfece-water POC (1%). 

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. 

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. 

Figure 11 A sketch of the theoretical relationships among the depths of the lysocline, the CCD, and the saturation horizon (Cl = 1). Dot density represents relative CaC03 content in the sediments.

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. 

The effect of organic matter driven dissolution is to raise the carbonate transition in sediments relative to the saturation horizon in the water column (Emerson and Archer, 1990 Figure 11), but there should be little change in the ACOa iys ccD necessary to create the transition in %CaCOa. Thus, the argument about the relationship between the dissolution rate constant and observed transition of %CaCOa in sediments (Appendix A) should not be affected. 

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. 

DEPTH OF SATURATION HORIZON DISSOLUTION MECHANISMS DISSOLUTION IN THE PAST SEDIMENT-BASED PROXIES SHELL WEIGHTS... 

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)... 

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