Environmental Engineering Reference
In-Depth Information
taken about one week apart in January 2004 (cruise 96). On the regular monthly
cruise (solid symbols in figure), nitrite values are very low below 250 m, nitrate
is at blank values, but ammonium begins to increase at about 200 or 250 m.
No sulfide measurements were made, so we do not know exactly where the
interface was on the regular monthly cruise, but continuous oxygen data from
that cruise indicated that oxygen values were less than 2 µM at about 250 m. In
contrast, during a second cruise taken about one week later, a secondary nitrite
maximum was observed in the deepest samples containing nitrate (270-290 m).
Ammonium was also present in these samples, although at relatively low levels.
Based on these profiles it appears that nitrite and ammonium may coexist just
above the first appearance of sulfide, but that this coexistence may not occur at
all times.
5.3 Anoxic Waters
The previous discussion has emphasized relatively short-term (annual to
interannual) variability at a single site. Data are also available to examine longer
trends in the deep water. Previous studies, over almost 50 years, have shown
that conditions at the bottom of the Basin (water depths greater than 1200 m)
are fairly (but not completely) stable. Scranton [29, 30], and Zhang and Millero
[37] reported steady changes in the chemistry of the bottom few hundred meters.
Scranton [29] demonstrated that these long term trends continued through the
next decade, until an earthquake in July of 1997 apparently resulted in sulfide
scavenging from deep water.
With the current data set we can look at the bottom waters from several
additional perspectives. As in the previous long term studies for the Cariaco,
we are restricting our discussion to samples taken below 1200 m. We have
also limited this analysis to dates between 1998 to present (and for silicate,
after December 2001) for reasons of analytical quality. We now have a long
enough data set to directly examine trends at one site for parameters measured
by a single lab. Fig. 7 shows increases in ammonium, phosphate and silica
over the course of the Cariaco program. Trends are highly significant (at the
p
<
0.01 level) because of the large number of observations (68 for phosphate
and ammonium; 20 for silica) being plotted. In spite of some cruise-to-cruise
variability, the trends are clear. Phosphate increases at a rate of 0.035 µMy
−
1
,
ammonium increases at 0.62 µMy
−
1
and silicate increases at 1.3 µMy
−
1
.The
time series for high quality silicate data is quite short, and it is most difficult
to distinguish trends in this data. However, the observed rates are very similar
to estimates by Zhang and Millero [37] who, using the quite small historical
data set available at that time, found increases of 0.0372
0.0069 µMy
−
1
±
0.096 µMy
−
1
for silicate. If we merge all data, we obtain overall rates of increase of 0.034
0.061 µMy
−
1
for phosphate, 0.28
±
for ammonium and 0.85
±
