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

B. 5.2 Microscale analysis of the surface sediments
De Beer, D., Eickert, G.
The aim of the study was to determine the vertical distribution of microbial activities
and mass transport phenomena in the 3 main segments of the Håkon Mosby Mud
Volcano. The main microbial processes are anaerobic methane oxidation (AOM)
coupled to sulfate reduction and aerobic or anaerobic oxidation of sulfide (SO). The
net equations for the main carbon and sulfur cycling are:
AOM CH 4 + SO 4 -2 HCO 3 - + HS - + H 2 O Eq.1
SO HS - + 2O 2 SO 4 -2 + H + Eq.2
Due to AOM a slight pH increase can occur, which in turn could stimulate calcification.
It was hypothesized as a possible explanation of the calcite deposits often found in
and near sites of AOM. This should be accompanied by a calcium flux towards the
AOM zone. The second reaction is a simplification of the reality, as sulfide oxidation
occurs in steps, thus all intermediates can be expected. Also, these intermediates will
be used for sulfate reduction, actually are preferred the e-acceptors due to the low
activation energy. Especially in the low sediment temperatures, -1 o C was reported for
the site, the use of less oxidised sulfur species for e-acceptance would be favorable.
However, if SO is complete, with sulfate as final product, a strong pH decrease will
occur in the zone where sulfide and oxygen overlap.
To investigate these processes, we measured the distribution of oxygen, sulfide,
protons, carbonate and calcium. To study transport phenomena, finescale
temperature distributions were measured in the top 10 cm of the seafloor (in situ).
Heat is not a significant product of microbial processes, however, is produced at
depths in the volcano. Thus, the finescale temperature profiles can be used to
determine the occurrence of advection in the investigated sediment layer.
Methods
Both laboratory measurements on retrieved sediments and in situ measurements with
a profiling unit were performed. The profiling unit was mounted on a chamber lander
operated by Ursula Witte. The profiling unit has been described previously (Glud et
al., 1994). The lander was deployed as free-falling unit, occasionally the position was
- 200 -