467
Processes in the Oceans
eastern basin and the western basin. The vertical mixing below the mixed layer in the
Cariaco Trench is prevented by the presence of a strong pycnocline. The salinity decreases
with depth, and the stability of the basin water is caused by the temperature gradient.
Coastal upwelling causes the high productivity in the surface waters and contributes the
organic matter to the deep waters of the Cariaco Trench. Because the horizontal and vertical exchange is limited, the dissolved oxygen used to oxidize the organic matter is not
rapidly replenished, and the deep waters have become anoxic. The sedimentary record
suggests that anoxic conditions have prevailed in the deep waters for the past 1100 years,
probably punctuated by deep- water renewal (Richards, 1965).
The Cariaco Trench has been widely used as a natural laboratory for the study of anaerobic processes since its deep waters were found to be anoxic in 1954. From the oceanographic data accumulated over three decades to the 1990s, it became increasingly clear
that the anoxic portion of the Cariaco Trench is not in steady state, and that the values of a
number of properties have been changing systematically with time (Scranton et al., 1987).
Although uncertainty exists about the accuracy of each data set, increases in temperature
and the concentration of sulfide have been reported by several workers (Richards, 1965;
Bacon et al., 1980).
Zhang and Millero (1993a) measured a number of chemical parameters in the Cariaco
Trench. These results have been used to examine its temporal changes in the hydrography
and chemistry. Water samples were collected at two stations (see Figure 10.55). Temperature
and salinity data obtained for both the eastern and western basins of the Cariaco Trench are
presented in Figure 10.56. The maximum temperature occurs at the surface and decreases
smoothly with depth, approaching a nearly constant value of about 17.2°C below 600 m.
The temperature below 200 m shows no significant differences between the two basins.
The maximum salinities occurred at 60 m in both basins, 36.86 in the western basin and
36.90 in the eastern basin. These maxima disappeared during the dry season (January to
May), probably because of mixing brought about by the stronger winds and lower air temperatures typical of that season. Below the maxima, the salinities decrease smoothly with
depth and become almost constant at about 36.21 below 500 m in both basins. The changes
in temperature between the surface and deep waters result in the density gradient that
separates the oxic and anoxic waters.
0
20
40
60
80
100
Depth (m)
0
400
800
1200
1600
2000
[La] (pM)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Depth (m)
0
100
200
300
400
500
[Ce]/[Nd]
[Ce]/[Nd] shales ratio
Figure 10.52
Profiles of La 3+ and the ratio of Ce/Nd in the Black Sea.
Processes in the Oceans
eastern basin and the western basin. The vertical mixing below the mixed layer in the
Cariaco Trench is prevented by the presence of a strong pycnocline. The salinity decreases
with depth, and the stability of the basin water is caused by the temperature gradient.
Coastal upwelling causes the high productivity in the surface waters and contributes the
organic matter to the deep waters of the Cariaco Trench. Because the horizontal and vertical exchange is limited, the dissolved oxygen used to oxidize the organic matter is not
rapidly replenished, and the deep waters have become anoxic. The sedimentary record
suggests that anoxic conditions have prevailed in the deep waters for the past 1100 years,
probably punctuated by deep- water renewal (Richards, 1965).
The Cariaco Trench has been widely used as a natural laboratory for the study of anaerobic processes since its deep waters were found to be anoxic in 1954. From the oceanographic data accumulated over three decades to the 1990s, it became increasingly clear
that the anoxic portion of the Cariaco Trench is not in steady state, and that the values of a
number of properties have been changing systematically with time (Scranton et al., 1987).
Although uncertainty exists about the accuracy of each data set, increases in temperature
and the concentration of sulfide have been reported by several workers (Richards, 1965;
Bacon et al., 1980).
Zhang and Millero (1993a) measured a number of chemical parameters in the Cariaco
Trench. These results have been used to examine its temporal changes in the hydrography
and chemistry. Water samples were collected at two stations (see Figure 10.55). Temperature
and salinity data obtained for both the eastern and western basins of the Cariaco Trench are
presented in Figure 10.56. The maximum temperature occurs at the surface and decreases
smoothly with depth, approaching a nearly constant value of about 17.2°C below 600 m.
The temperature below 200 m shows no significant differences between the two basins.
The maximum salinities occurred at 60 m in both basins, 36.86 in the western basin and
36.90 in the eastern basin. These maxima disappeared during the dry season (January to
May), probably because of mixing brought about by the stronger winds and lower air temperatures typical of that season. Below the maxima, the salinities decrease smoothly with
depth and become almost constant at about 36.21 below 500 m in both basins. The changes
in temperature between the surface and deep waters result in the density gradient that
separates the oxic and anoxic waters.
0
20
40
60
80
100
Depth (m)
0
400
800
1200
1600
2000
[La] (pM)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Depth (m)
0
100
200
300
400
500
[Ce]/[Nd]
[Ce]/[Nd] shales ratio
Figure 10.52
Profiles of La 3+ and the ratio of Ce/Nd in the Black Sea.