Environmental Engineering Reference
In-Depth Information
Another chemical species for which temporal trends have been assessed is
sulfide. Scranton [30] reported an increase in sulfide of 14 µM in 9 years (1.56
µMy
−
1
). Zhang and Millero updated this number based on additional field work
and obtained a value of about 1.5 µMy
−
1
. As reported in Scranton [29] the rate
of increase of sulfide consistent with the literature and the first few CARIACO
cruises was about 1.58 µMy
−
1
. However, following the earthquake in 1997,
sulfide concentrations at the bottom of the basin water suddenly decreased by
about 20-40 µM [29]. Sulfide values measured for deep water since 1997 have
all been much lower than the previously predicted trend line, although recently,
sulfide concentrations in the bottom water have started to increase again. We
are unsure whether the large cruise-to-cruise variability is natural (perhaps due
to heterogeneity in deep basin sulfide accompanied by circulation at depth
in the basin) or whether it is related to the fact that our sulfide analyses are
made on preserved samples run weeks to occasionally months following the
cruise. We run sulfides on triplicate samples taken without bubbles into VICI
Precision gas tight syringes and preserved immediately by addition directly to
a solution of zinc chloride in a sample vial. Samples are stored in a cooler until
return to the lab, and then refrigerated except during shipping until analysis by
a modification of the Cline method [8]. The time between sample collection
and mixture with the preservative is less than 1 minute. The “within cruise”
variation is much smaller than the cruise-to-cruise variability. The most recent
data (cruises in January and May 2004) indicate that sulfide is again increasing,
with values below 1200 m of 54 µM in January and about 68 µMinMay
(Meredith Hayes, unpublished data). One possibility is that there may be more
spatial variability in the deep water than has been reported in the past and that
the mixing time of the basin is on the order of a few years. Unfortunately, we
do not yet have data from the western basin to assess this question.
In addition to long term trends in bottom water, there is information in
the ratios of the different nutrient species as a function of depth. Zhang and
Millero [37] made this comparison and concluded that the products of organic
matter remineralization were those predicted by sulfate reduction coupled to
oxidation of organic matter containing the Redfield ratio amounts of N and P.
Based on our detailed nutrient profile from January 2004, ratios of ammonium
to sulfide (0.374), phosphate to sulfide (0.0216) and silica to sulfide (0.788) are
all 13% higher than previously reported by Zhang and Millero [37] (Fig. 8).
This is qualitatively consistent with the fact that maximum sulfide decreased
at the time of the earthquake from 75 to about 50 µM. We have previously
attributed this decrease to formation of FeS solids [29], which would remove
sulfide without removing CH
4
, N or P. Other possible removal mechanisms
could include conversion of sulfide to elemental sulfur as has been seen in the
Black Sea (for example, [20]) or conversion to sulfite and thiosulfate as seen in
