Preface The expedition ARK XIX/3 with the German icebreaking RV ...

Laboratory measurements
Central site
The sediments in this site were very soft and probably still fluidized. During retrieval
strong out-gassing occurred disturbing the structure of the sediments. The sediments
were left for settling for several hours before measurements commenced.
Central segment, 28/6/2003
Central segment, 28/6/2003
O2, H2S (mM)
0.6
7.9
0.5
H2S
7.8
pH 0.4
O2 0.3
7.7
0.2
7.6
0.1
7.5
0
-50 -0.1 0 50 1007.4
depth (mm)
pH
o2
h2s
pH
Su
lfi
de
(m
M)
2.5
0.6
2
0.5
1.5
0.4
0.3
1
0.2
0.5
0.1
0
0
-50 0
-0.5
50 100
-0.1
depth (mm)
H2
S
(m
M)
sulfide
H2S
Profiles in a fresh core from the central segment.
In the first day directly after sampling (location, time), the sediments showed a very
high oxygen uptake and penetration of only a few mm. Sulfide was detected in an
irregular pattern. The pH was always lower than in SW. No pH dip was found at the
oxic-anoxic interface where sulfide was consumed. The SW pH was cross-checked
with values obtained from fresh bottom-water samples (7.93). In a second core less
sulfide was found and the pattern was more regular. Also here no pH dip at the oxicanoxic
interface indicated the oxidation of sulfide to sulfate. Carbonate measurements
showed a strong increase with depth in the sediments. The sensors are not sensitive
to sulfide, indeed the profiles show a different pattern. The carbonate increase cannot
be exclusively caused by biological activity. Whereas the sulfide peaks in the top 5
cm, the pH and carbonate showed a shoulder at 2 cm, DIC continuously increases
with depth. In a duplicate experiment, similar data were obtained, showing microbial
activity at 4 cm contributing to the carbonate and DIC profile, but a flux from deep
sediments was obvious. Other, non-biological, processes must contribute to the
carbonate pool.
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