306
Chemical Oceanography, 4th Edition
The dissolution rates in the laboratory studies of Morse (1983) and coworkers resulted in
two equations. For Ω A ≤ 0.44,
R(% per day) = 110(1 – Ω) 2.39
(7.130)
and for Ω A ≤ 0.44,
R(% per day) = 1318(1 – Ω) 7.27
(7.131)
Although the equations are different, the calculated rates are in reasonable agreement.
Since aragonite production appears to be high in the surface waters of the Pacific, and the
deeper waters are undersaturated, the transport of the aragonite and dissolution could
result in transporting carbon to deep waters. Betzer and coworkers (1984) have estimated
that 90% of this aragonite flux would be dissolved in the upper 2.2 km of the water column. The values of aragonite precipitation of waters precipitating on the Bahamas Banks
are quite similar to those shown in Figure 7.45 (Morse et al., 2010). The higher solubility in
the beginning is thought to be high magnesium calcite that slowly precipitates to aragonite with time. Recent work has shown that fish produce a high magnesium calcite from
drinking seawater (Wilson et al., 2009). The solubility of this high magnesium calcite like
the material in the Bahamas is twice that of aragonite (Woosley, Millero, and Grosell, 2012).
Much of the fish- produced high magnesium calcite is soluble enough to dissolve above the
saturation horizon for aragonite.
Feely et al. (2004) examined the saturation state of calcite and aragonite in ocean waters
and the rates of dissolution of CaCO 3 in the oceans. They determined the aragonite and
calcite saturation states in the major oceans using the new carbonate data determined
over the 10 yr on the WOCE cruises. These results are shown in Figure 7.46. The values
ranged from 3000 and 4500 m in the North Atlantic to 500 and 700 m in the North Pacific,
respectively, for aragonite and calcite. The effects of the anthropogenic CO 2 on the saturation levels are shown in Figure 7.47. The added CO 2 to the oceans has decreased the saturation levels by as much as 500 m. As will be discussed further, over the next 200 yr the
surface waters of the oceans may be undersaturated with respect to aragonite. Feely et al.
(2004) also determined the rates of dissolution of aragonite and calcite in the oceans. They
determined the amount of CaCO 3 dissolved from the equation
ΔCaCO 3 = 0.5[TA Meas – TA 0 ] + 0.63(0.0941 AOU)
(7.132)
The value of TA 0 is the preformed value of TA at the surface, and the last term corrects
for changes in TA caused by the oxidation of plant material. By dating the waters using
chloro fluorocarbons (CFCs) or 14 C, Feely et al. were able to estimate the rates of dissolution
of CaCO 3 along constant density surfaces. The dissolution rates they found are presented
in Table 7.9. The maximum values ranged from 0.5 μmol kg –1 yr –1 in the Atlantic to 1.2 μmol
kg –1 yr –1 in the Pacific. Most of the dissolution occurred near the aragonite saturation. This
is above the lysocline level and indicates that the dissolution may be occurring in microorganisms or particulate flocs of low pH or that the mineral may be high- magnesium
calcite that is two times more soluble than aragonite. More work is needed to clarify these
issues. The results indicate that about 50 to 70% of the calcium carbonate made in surface
waters (0.8 to 1.4 Pg CaCO 3 yr –1 ) is dissolved in the upper water column. More recently,
Friis et al. (2006, 2007) suggested that the higher values of TA – TA 0 are due to ocean mixing. They also indicated that the normalization of TA should be relative to the extrapolated
value when S = 0. I find it hard to believe that this is the case.
Chemical Oceanography, 4th Edition
The dissolution rates in the laboratory studies of Morse (1983) and coworkers resulted in
two equations. For Ω A ≤ 0.44,
R(% per day) = 110(1 – Ω) 2.39
(7.130)
and for Ω A ≤ 0.44,
R(% per day) = 1318(1 – Ω) 7.27
(7.131)
Although the equations are different, the calculated rates are in reasonable agreement.
Since aragonite production appears to be high in the surface waters of the Pacific, and the
deeper waters are undersaturated, the transport of the aragonite and dissolution could
result in transporting carbon to deep waters. Betzer and coworkers (1984) have estimated
that 90% of this aragonite flux would be dissolved in the upper 2.2 km of the water column. The values of aragonite precipitation of waters precipitating on the Bahamas Banks
are quite similar to those shown in Figure 7.45 (Morse et al., 2010). The higher solubility in
the beginning is thought to be high magnesium calcite that slowly precipitates to aragonite with time. Recent work has shown that fish produce a high magnesium calcite from
drinking seawater (Wilson et al., 2009). The solubility of this high magnesium calcite like
the material in the Bahamas is twice that of aragonite (Woosley, Millero, and Grosell, 2012).
Much of the fish- produced high magnesium calcite is soluble enough to dissolve above the
saturation horizon for aragonite.
Feely et al. (2004) examined the saturation state of calcite and aragonite in ocean waters
and the rates of dissolution of CaCO 3 in the oceans. They determined the aragonite and
calcite saturation states in the major oceans using the new carbonate data determined
over the 10 yr on the WOCE cruises. These results are shown in Figure 7.46. The values
ranged from 3000 and 4500 m in the North Atlantic to 500 and 700 m in the North Pacific,
respectively, for aragonite and calcite. The effects of the anthropogenic CO 2 on the saturation levels are shown in Figure 7.47. The added CO 2 to the oceans has decreased the saturation levels by as much as 500 m. As will be discussed further, over the next 200 yr the
surface waters of the oceans may be undersaturated with respect to aragonite. Feely et al.
(2004) also determined the rates of dissolution of aragonite and calcite in the oceans. They
determined the amount of CaCO 3 dissolved from the equation
ΔCaCO 3 = 0.5[TA Meas – TA 0 ] + 0.63(0.0941 AOU)
(7.132)
The value of TA 0 is the preformed value of TA at the surface, and the last term corrects
for changes in TA caused by the oxidation of plant material. By dating the waters using
chloro fluorocarbons (CFCs) or 14 C, Feely et al. were able to estimate the rates of dissolution
of CaCO 3 along constant density surfaces. The dissolution rates they found are presented
in Table 7.9. The maximum values ranged from 0.5 μmol kg –1 yr –1 in the Atlantic to 1.2 μmol
kg –1 yr –1 in the Pacific. Most of the dissolution occurred near the aragonite saturation. This
is above the lysocline level and indicates that the dissolution may be occurring in microorganisms or particulate flocs of low pH or that the mineral may be high- magnesium
calcite that is two times more soluble than aragonite. More work is needed to clarify these
issues. The results indicate that about 50 to 70% of the calcium carbonate made in surface
waters (0.8 to 1.4 Pg CaCO 3 yr –1 ) is dissolved in the upper water column. More recently,
Friis et al. (2006, 2007) suggested that the higher values of TA – TA 0 are due to ocean mixing. They also indicated that the normalization of TA should be relative to the extrapolated
value when S = 0. I find it hard to believe that this is the case.