Aerobic reoxidation in marine sediments
This exercise consists of two small experiments\demonstrations and you get your data
almost instantaneously. Thus you can already start data treatment during the exercise (see
“theoretical work” below). A full report contains answers to all 16 points listed below
and should be delivered no later than the 13. of Marts. Most of the time at 1. of Marts is
dedicated report-writing at MBL.
Microprofile experiment
Background
In coastal environments, only a minor part of the benthic oxygen consumption is related
to direct aerobic respiration. The major part of the oxygen consumption is caused by
reoxidation of products from the anaerobic heterotrophic activity in the deeper sediment
layers. These products include NH4+, Fe2+, Mn2+ and H2S. The latter is the result of
sulfate reduction and in sediments with low iron- and manganese- oxide concentrations
the H2S can diffuse up to the oxic sediment zone. H2S is a very potent poison that
inactivates many metal carrying enzymes and benthic H2S release often leads to fauna
death in coastal lagoons and isolated sedimentation basins. H2S-oxidation is an exergonic
process and many chemoautotrophic bacteria have specialized in gaining energy from
aerobic oxidation of H2S. In reduced sediments this becomes very apparent as the
bacteria form a massive white cover on the sediment surface optimizing their position to
the H2S - O2 interphase. In such instances the bacteria mat form the last barrier before
H2S emerges to the overlying water.
At present there is no simple technique to discriminate between the oxygen consumption
related to respiration or reoxidation. But from porewater microprofiles of H2S and O2 it is
possible to quantitatively evaluate the importance of H2S oxidation for the total oxygen
consumption rate. Oxygen and H2S profiles can be measured by microelectrodes.
However, “sulfide” actually exists in three forms S2-, HS-, H2S and the sensor applied in
the present exercise is only sensitive to the H2S fraction. The equilibrium between the
three forms is pH dependent as outlined by the diagram below:
Thus in order to recalculate H2S profiles to total H2S (called TH2S below), the pH has to
be measured in parallel and the pH-effect has to be accounted for.
Description of work:
We have collected sediment cores from two different locations in “Øresund”. The two
stations mainly differ by the water depth and thereby in the external supply of organic
material. The cores are labeled A (shallow water) and B (deep water). Each team has a
core from each site and should
1) describe the two cores (color, texture, fauna etc.) and any visual macroscopic
irregularities.
Both cores are submerged in well-mixed water baths (salinity 30) kept at room
temperature (20 oC). Three different calibrated microsensors H2S, pH and O2 are ready
for mounting in the micro-manipulator to obtain a number of individual profiles of each
chemical specimen (Fig 1).
Microsensor
Water
Strip-chart recorder
Air-pump
Picoamp meter
Profiling start
0%
214.3
100%
Sediment
Start profiling with the O2 microsensor in an area free of shells or hard substrata. When
the sensor has been positioned at the selected spot, you estimate the relative position of
the sediment surface. This is done by moving the sensor slowly downwards - using the
micromanipulator - until a signal change at the strip-chart recorder is observed – this
position is close to the sediment surface. The sensor is then moved a few hundred
microns backwards until the 100%-saturation value (equivalent to 238 M) is reached,
the sensor is now ready for microprofiling. This it is done by moving the sensor
downwards in increments of 0.1 mm. Each time the sensor is moved a small mark is
made on the recorder paper. The profiling is continued until a low constant value (signal
at 0% saturation) is reached. Subsequently the sensor is moved back up in the water
phase, moved horizontally to another position and the procedure is repeated at different
locations (a minimum of three profiles are measured in each of the two cores) - if times
allow you can make a small microtransect with multiple profiling to characterize the
small scale heterogeneity.
After this the H2S and the pH-sensor is mounted in the micromanipulator. The two
sensors are fixed together and are vertically aligned so that microprofiles of the two
chemical specimens are obtained at the same depth horizon simultaneously. Try to locate
the position of the sediment surface visually (for later alignment between the O2 and the
H2S/pH profiles) and perform 2-3 microprofiles with a reasonable vertical resolution in
the sediment corer from site A (the sediment from the deeper site, B, does not contain
H2S). If time allows you can try and add formalin to one of the cores and follow the
response in the benthic O2 distribution.
Slurry experiment
Background
Aerobic reoxidation of the reduced products from the anaerobic activity occur
spontaneously. However, most processes are catalyzed by chemoautotrophic bacteria that
increase the oxidation rates by a factor of 100-1000 and the bacteria harvest the energy.
The accumulated reduced products can represent a significant “oxygen dept” that can be
re-paid over short time during storm events. Here sediment is resuspended up into the
overlying oxic water column. This can be visualized by incubating reduced sediment in
slurries and compare the rates to the results of the microprofile data above. Biological
inactivation (by formalin) can indicate to what extent the oxidation during resuspension is
biological or chemical mediated and to what extent the O2 consumption rate is constant
with decreasing O2 concentration provide information on the oxidation kinetics.
Description of work
Surface sediment (0-5 mm) from a core from each station (A and B) is transferred to each
their weighing boat and the sediment is gently homogenized. Subsequently 4x 3.0g (write
down the exact weight) of sediment from each weighing-boat is transferred to 4 Winklerbottles that subsequently are filled with 100% air-saturated sea-water (corresponding to
238 M). Each team now has 4 resuspension-bottles (remember to label them with teamnumber and the respective station index). One ml of formalin (37%) is added to one
bottle from station A and B respectively (use gloves). A small magnet is added to each
bottle and all bottles are placed on two central stirring plates that are runnig at a speed
sufficient to induce resuspension. The O2 concentration in each bottle is now followed by
successively transferring an O2 microelectrode between the bottles at a reasonable time
interval (measurements should preferentially be done in each bottle every 10 min). The
sensor current of the calibrated sensor at each time and for each bottle is noted when the
signal has stabilized along with the time of measurement. If time allows the O2
concentration is followed until 0 uM is reached. After each measurement a small amount
of water is added to avoid bubbles inside the bottles.
Theoretical work
Microelectrode experiment
2) Plot the 2-3 sets of O2, H2S and pH data versus sediment depth for the shallow site and
the 3 O2 microprofiles from the deeper site - indicate the estimated position of the
sediment surface as (Y=0). Use the units M and mm.
3) Plot the two TH2S values versus depth for the shallow site (indicate the estimated
position of the sediment surface as (Y=0)). For pH <9 the TH2S can be estimated from:
TH2S = H2S (1 + 10-7/10-x), where x equals the pH at the respective sediment depths. Use
the unit’s M and mm.
4) Describe the oxygen profiles from each site and calculate the average DBL-thickness,
the Diffusive O2 Uptake (DOU) and the volume specific respiration (R) for each site in
the following units mm, mmol m-2d-1, and mol cm-3 d-1, respectively
DOU (mmol m-2 d-1)  * D * 8640, where  is the slope of the concentration profile
within the DBL (unit M mm-1) and D is the molecular diffusion coefficient of oxygen in
water (unit 10-5cm2s-1). D equals 1.98 *10-5cm2s-1 at salinity 30 and 20oC. The value 8640
is a conversion factor to get the right units.
R (molcm-3d-1) = (DOU/OP) * 0.1, where OP is the oxygen penetration depth (unit cm)
and DOU the diffusive O2 uptake (unit mmol m-2 d-1). The value 0.1 is a conversion
factor to get the right units. This calculation assumes constant activity in the oxic zone
and zero-order kinetics.
5) Compare and comment upon the values from the two sites. How close are the values to
the theoretical possible DOU at the given conditions?
6) Explain the shape of the TH2S profiles and define the zones of sulfide production and
sulfide consumption.
7) Calculate the diffusive flux of TH2S into the oxic zone using a D-value of 1.4 10-5
cm2s-1 and calculate the fraction of O2 consumption used for TH2S oxidation assuming
complete aerobic sulfide oxidation to sulfate: H2S + 2O2 -> SO42- + 2H+. Is that a
reasonable assumption? Calculate the volume specific consumption rate of TH2S in the
oxic zone.
8) Calculate the average concentration of O2 and TH2S in the overlap zone. With these
concentrations and the rates calculated above what is the turn overtime (in sec), meaning
how long time does it take to consume the available O2 and TH2S in the reaction zone ?
In a pure un-catalyzed chemical oxidation of H2S with O2 it takes approximately 60 min
to remove 50% of the sulfide. How much faster is the sulfide oxidation in the sediment of
the shallow water station?
Slurry experiment
1) Describe how the O2 concentration in the Winkler bottles decrease. Give an
explanation for the respective phases.
2) Calculate the O2 consumption rate for the two respective stations during the initial and
the subsequent oxidation phase.
Rslurry (mol cm-3d-1) =  0.05 /(y/2.0), where  is the rate of O2 decrease in the bottles
(uM d-1), 0.05 (l) the bottle volume, y the number of gram sediment added and 2.0 the
estimated density of the sediment.
Comment upon potential differences between the incubations – how do they compare to
your expectations?
3) How does the values compare to the volume specific respiration (R) calculated from
the O2 microprofiles ? Comment and give suggestions for any potential differences.
4) How many days of diffusive mediated O2 consumption does a storm induced
resuspension event lasting for 1 day of the upper 5 mm of sediment compare to for each
station?