100
Fig. 4.9. The temporal changes
of potential temperature of
water below 1200 m in the Cariaco Trench (Zhang and Millero
1993b)
t
~
.a
~
cu
c.
E
cu
....
~ c
~
F.J. Millero
17.1 '--'---'---'--~--'---'---r--'---'--~
17.0
16.9
16.8
16.7
16.6 '-----'----'------'------'-------'------'--------''-----'-------'-----'
1950
1960
1970
Time (yr)
1980
1990
The concentrations of ammonia, phosphate and silicate in the deep waters of the
Cariaco Trench have also increased with time since the first measurements were made
in the 1950S (see Fig. 4.lO). The rates of increase are 0.282 IlM yr- 1 for ammonia,
0.03721lM yr- 1 for phosphate and 0.8541lM yr- 1 for silicate. These increases are consistent with the increase of H2S with time in the deep waters of the Cariaco Trench. If
these compounds behave conservatively they should have accumulated in the anoxic
water column over the period in fixed ratios as predicted. The ratio of the rate increase
for ammonia to that of sulfide is equal to 0.25, which is close to the value of 0.30 predicted from the model. The ratio of the rate of increase for phosphate to sulfide is 0.034,
while the theoretical value is 0.019. The larger increase for phosphate may be due to
another source such as the incorporation of PO~- on Fe and Mn oxides that sink below the interface. The dissolution of phosphate minerals in the anoxic water or diffusion from sediment's pore waters might also provide the input of the phosphate in the
anoxic waters. The flux of silicate from the pore water of the sediment into the water
column has been suggested as the cause of the high concentrations of silicate in the
deep waters of the Cariaco Trench (Fanning and Pilson 1972; Scranton et al. 1987).
By extrapolating the concentrations of H 2 S and NH: to zero, it is possible to estimate the last time that the trench was oxic. The earlier rate of increase (1955 to 1969)
in the H 2 S with time extrapolated to zero gives a date of 1916. A similar extrapolation
for NH: gives a date of 1914. These estimations, from two independent chemical compounds, are in good agreement for the last occurrence of a complete turnover of the
trench waters. It is interesting to note that the 210Pb dating of the sediments indicates
that some disturbance of the sediment water interface occurred around 1932 and 1897
apparently due to earthquakes around 1900 and 1929. These events could also have
Fig. 4.9. The temporal changes
of potential temperature of
water below 1200 m in the Cariaco Trench (Zhang and Millero
1993b)
t
~
.a
~
cu
c.
E
cu
....
~ c
~
F.J. Millero
17.1 '--'---'---'--~--'---'---r--'---'--~
17.0
16.9
16.8
16.7
16.6 '-----'----'------'------'-------'------'--------''-----'-------'-----'
1950
1960
1970
Time (yr)
1980
1990
The concentrations of ammonia, phosphate and silicate in the deep waters of the
Cariaco Trench have also increased with time since the first measurements were made
in the 1950S (see Fig. 4.lO). The rates of increase are 0.282 IlM yr- 1 for ammonia,
0.03721lM yr- 1 for phosphate and 0.8541lM yr- 1 for silicate. These increases are consistent with the increase of H2S with time in the deep waters of the Cariaco Trench. If
these compounds behave conservatively they should have accumulated in the anoxic
water column over the period in fixed ratios as predicted. The ratio of the rate increase
for ammonia to that of sulfide is equal to 0.25, which is close to the value of 0.30 predicted from the model. The ratio of the rate of increase for phosphate to sulfide is 0.034,
while the theoretical value is 0.019. The larger increase for phosphate may be due to
another source such as the incorporation of PO~- on Fe and Mn oxides that sink below the interface. The dissolution of phosphate minerals in the anoxic water or diffusion from sediment's pore waters might also provide the input of the phosphate in the
anoxic waters. The flux of silicate from the pore water of the sediment into the water
column has been suggested as the cause of the high concentrations of silicate in the
deep waters of the Cariaco Trench (Fanning and Pilson 1972; Scranton et al. 1987).
By extrapolating the concentrations of H 2 S and NH: to zero, it is possible to estimate the last time that the trench was oxic. The earlier rate of increase (1955 to 1969)
in the H 2 S with time extrapolated to zero gives a date of 1916. A similar extrapolation
for NH: gives a date of 1914. These estimations, from two independent chemical compounds, are in good agreement for the last occurrence of a complete turnover of the
trench waters. It is interesting to note that the 210Pb dating of the sediments indicates
that some disturbance of the sediment water interface occurred around 1932 and 1897
apparently due to earthquakes around 1900 and 1929. These events could also have