U N I V E R S I T Y
O F
B E R G E N
Geophysical Institute
Detection of excess CO2 in seawater caused by
hydrothermal vent release of CO2 gas and CO2 rich
water
Helle Augdal Botnen
Abdirahman Omar
Truls Johannessen
uib.no
Geophysical Institute
Outline
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Motivation
Back-calculation method
Data
Total inorganic carbon results
Total alkalinity results
pH results
Discussion
Summary
uib.no
Geophysical Institute
Motivation
• Transport and store large
amounts of CO2 on the
Norwegian continental shelf
– i.e. OED CO2 atlas
http://www.npd.no/en/Publications/Reports/CO2-Storage-Atlas-/
• Potential for leakage from
ocean transport and ocean
reservoir storage is present
• Need to be aware of the
consequences for the marine
environment should a leakage
or seepage occur
http://www.co2crc.com.au/
uib.no
Geophysical Institute
Motivation
• Investigate if the back-calculation method can be used to
detect excess CO2 in the sea water near the Jan Mayen
hydrothermal vent field
• Analyse the results from the back-calculation method in
regards of elevated concentration of CO2
• Discuss the potential consequences of CO2 escaping
from the hydrothermal vents, as a natural analogue to
leakage from CCS transportation and storage
uib.no
Geophysical Institute
Back-calculation method
• Difference between the
reference station and the
hydrothermal vent field for
CT
• For AT there is no
significant difference
• CT is influenced by:
1) biological activity
2) salinity variations
3) air-sea gas exchange
• AT is influenced by 1) and
2)
Measured profiles from the reference station (blue) and the hydrothermal
vent field (black)
• Difference explained by
natural variability?
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Geophysical Institute
Back-calculation method
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Method used to investigate the amount of anthropogenic CO2 in a sea water
sample
– Remove biology and salinity variations
– Left with background CT, which compared with historical measurements
give the changes in CT due to anthropogenic CO2
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Equations are as follows:
nCT ={CT − Corg −1/2∗(nAT−k)−b}∗Sref/S +b
Corg = RC:P ∗ δP
nAT = (corrAT −b)∗Sref/S +b
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Geophysical Institute
Data: Hydrothermal vent field
The data are from two separate hydrothermal
vent fields:
Trollveggen and Soria Moria
For comparison data from a reference station
is taken at a significant distance:
Same water masses
Same biology
Same atmospheric CO2
No influence from the hydrothermal vent field
uib.no
Geophysical Institute
Data: Hydrothermal vent field
• The hydrothermal vent field
is on a volcanic ridge
Pedersen et al., 2010
• Both consists of several
chimneys with hightemperature fluid and gas
escaping
Pedersen et al., 2005
• The gas consist of mainly
CO2 and presence of CO2
rich water
• Hydrates are also present
along the orifices and bottom
of the chimneys
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Geophysical Institute
Data: Reference station
• Important that the reference
station is stable
• Comparison to historical
data
• Carina 2001
• Cruise 2012
• Trends reveal reference
station is stable, and
phosphate correction
provides best fit
• Can assume same biology,
air-sea gas flux, and water
mass regime at both
locations
uib.no
Geophysical Institute
Total inorganic carbon (CT) results
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Significant difference between the
reference station and the
hydrothermal vent stations
– Significant CT values found at
several depths
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Dissolution of CO2 as it ascends
through the water column explain
the distribution
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Highest difference near surface
where there is a strong gradient in
temperature, salinity, and density
uib.no
Geophysical Institute
Total alkalinity (AT) result
• The AT show no
significant differences
between the reference
station and the
hydrothermal vent field,
except surface value at
Soria Moria
uib.no
Geophysical Institute
pH result
• The pH from the hydrothermal
vent field is lower than the
reference station
• Consequence of CO2 uptake
near the chimneys and bubble
plumes causing acidification
• The decrease in pH is quite
low compared to vent fluid pH
uib.no
Geophysical Institute
Discussion
• The back-calculation method provide significant
difference between the reference and the hydrothermal
vent profiles for CT
• The difference between the reference station is relatively
small at certain depths, still the method is sensitive
enough to pick up the differences
• No significant signal in AT does not mean the method is
not working. This is expected.
uib.no
Geophysical Institute
Discussion
CT = [CO2] + [HCO3-] + [CO32-]
AT = [HCO3-] + 2[CO32-] + [B(OH)4-] + other
CO2(g)  CO2(aq)
CO2(aq) + H2O  H+ + HCO3HCO3-  H+ + CO32• The CT is influenced by the CO2, while the AT is not
which means the uptake can be viewed as an air-sea
gas exchange
uib.no
Geophysical Institute
Summary
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Back-calculation method is sensitive enough to pick up the elevated CT
signal from the hydrothermal vent field. The method can be used to detect
small amounts of CO2 due to i.e. leakage or seepage from CCS
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CT is influenced by the escaped CO2 from the chimneys, while AT is not. In
regards of ocean carbon chemistry this is expected
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The pH is reduced in the hydrothermal vent area, as a consequence of the
dissolution of CO2
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Escaped CO2 from the hydrothermal vent field has a detectable impact on
the CT, and the pH in the surrounding sea water. For a CCS leakage or
seepage there could be impacts of similar nature. However, mass flux is
necessary to say how large impact
uib.no
Geophysical Institute