303
The Carbonate System
and CaCO 3 compensation depth in sediments are frequently at the same depth. Thus, the
causes of the compensation depth being deeper than the saturation depth are the variable
rates of dissolution of various forms of CaCO 3 . If the sedimentation rates are high, it is possible that CaCO 3 (s) could be preserved before it dissolves. This would cause the calcium
CCD to be below the lysocline.
Values of the lysocline in the Atlantic and Pacific are shown in Figure 7.41. The lysocline
is higher in the Pacific because of the greater undersaturation at lower depths. A comparison of the saturation horizon with the lysocline and calcium CCDs is shown in Figure 7.42.
The values of the lysocline and the CCD are not affected by the saturation states. The CCD
is close to the lysocline except in the equatorial region. This is due to the higher productivity of these waters. The higher the supply rate of CaCO 3 (s), the deeper the CCD will be (see
Figure 7.43).
Morse (1983) attempted to understand the causes of these increases in rates using laboratory studies. He has shown that when the saturation of waters gets to a critical value,
CaCO 3 (s) starts to dissolve. This critical value is about 30% undersaturation, or at ΔCO 3
2– –
CO 3
2– (sat) = –10 μmol kg –1 (i.e., the solution could absorb another 10 μmol kg –1 of CaCO 3 ).
Broecker and Peng (1982) showed this critical ΔCaCO 3 by the percentage of CaCO 3 (s) in
sediment core tops (see Figure 7.44). This so- called critical value is strongly dependent on
the value selected for the solubility product of the CaCO 3 (s). More recent studies by Byrne
(University of South Florida) have concentrated on measuring solubility dissolution rates
using minerals and waters collected in the oceans. He and his coworkers have found that
the rates of dissolution of aragonitic CaCO 3 (s) measured at sea can be described by
R = 130 {1 – [Ca 2+ ][CO 3
2– ]/1.78 K SP (cal)} 3.1
(7.129)
where Ksp (aragonite) = 1.78 Ksp (calcite). The factor of ρ = 1.78 is slightly higher than the
theoretical value of 1.5. This is related to the changes in Ksp (aragonite) and Ω A for aragonite solubility as a function of time (see Figure 7.45).
Latitude
–60 –50 –40 –30 –20 –10 0
10 20 30 40 50 60
Depth (m)
0
1000
2000
3000
4000
5000
6000
Pacific Ocean
West Atlantic Ocean
East Atlantic Ocean
Figure 7.41
Depths of the lysocline in various oceans.
The Carbonate System
and CaCO 3 compensation depth in sediments are frequently at the same depth. Thus, the
causes of the compensation depth being deeper than the saturation depth are the variable
rates of dissolution of various forms of CaCO 3 . If the sedimentation rates are high, it is possible that CaCO 3 (s) could be preserved before it dissolves. This would cause the calcium
CCD to be below the lysocline.
Values of the lysocline in the Atlantic and Pacific are shown in Figure 7.41. The lysocline
is higher in the Pacific because of the greater undersaturation at lower depths. A comparison of the saturation horizon with the lysocline and calcium CCDs is shown in Figure 7.42.
The values of the lysocline and the CCD are not affected by the saturation states. The CCD
is close to the lysocline except in the equatorial region. This is due to the higher productivity of these waters. The higher the supply rate of CaCO 3 (s), the deeper the CCD will be (see
Figure 7.43).
Morse (1983) attempted to understand the causes of these increases in rates using laboratory studies. He has shown that when the saturation of waters gets to a critical value,
CaCO 3 (s) starts to dissolve. This critical value is about 30% undersaturation, or at ΔCO 3
2– –
CO 3
2– (sat) = –10 μmol kg –1 (i.e., the solution could absorb another 10 μmol kg –1 of CaCO 3 ).
Broecker and Peng (1982) showed this critical ΔCaCO 3 by the percentage of CaCO 3 (s) in
sediment core tops (see Figure 7.44). This so- called critical value is strongly dependent on
the value selected for the solubility product of the CaCO 3 (s). More recent studies by Byrne
(University of South Florida) have concentrated on measuring solubility dissolution rates
using minerals and waters collected in the oceans. He and his coworkers have found that
the rates of dissolution of aragonitic CaCO 3 (s) measured at sea can be described by
R = 130 {1 – [Ca 2+ ][CO 3
2– ]/1.78 K SP (cal)} 3.1
(7.129)
where Ksp (aragonite) = 1.78 Ksp (calcite). The factor of ρ = 1.78 is slightly higher than the
theoretical value of 1.5. This is related to the changes in Ksp (aragonite) and Ω A for aragonite solubility as a function of time (see Figure 7.45).
Latitude
–60 –50 –40 –30 –20 –10 0
10 20 30 40 50 60
Depth (m)
0
1000
2000
3000
4000
5000
6000
Pacific Ocean
West Atlantic Ocean
East Atlantic Ocean
Figure 7.41
Depths of the lysocline in various oceans.