International research team discovers previously unknown
processes in the Atlantic
In 2014, an international research team led by the Kiel Cluster
of Excellence “The Future Ocean” and the GEOMAR Helmholtz Centre for
Ocean Research Kiel was able to investigate in detail eddies in the
Atlantic Ocean which were characterized by extremely low oxygen
concentrations. The interdisciplinary analysis of the data and
samples has revealed processes which were not previously known to
occur in the Atlantic. This also includes the natural production of
considerable amounts of greenhouse gases, as the team has now
published in the international scientific journal Scientific
Reports.
Oxygen in the seawater is not only vital to most marine
organisms, its concentrations also affect the chemistry of the ocean
and that of the atmosphere above. In oceanic regions with very
little oxygen, for example, large amounts of the potent greenhouse
gas nitrous oxide, also called laughing gas, are produced via
biogeochemical processes and can then be released to the atmosphere.
Even though a natural moderate oxygen minimum zone (OMZ) exists
along some of the eastern boundaries of the Atlantic Ocean, the
Atlantic OMZ, unlike the OMZs of the Indian and Pacific oceans, was
not considered to be a region of extremely low oxygen
concentrations. New findings by an international research team led
by the Kiel Excellence Cluster “Future Ocean” and the GEOMAR
Helmholtz Center for Ocean Research Kiel, however, now imply that
this picture has to be corrected. This study was published yesterday
in the Nature Publishing Group journal Scientific Reports.
“The reason why the extremely low-oxygen regions in the Atlantic
have so far escaped research is simple: they are relatively small
and mobile in contrast to the well-known, large and stationary
oxygen minimum zones”, explains Dr. Damian Grundle from the Bermuda
Institute of Ocean Sciences, first author of the current study and,
until recently, a scientist at the ”Future Ocean”/GEOMAR. Extreme
low oxygen concentrations occur in the Atlantic in ocean eddies of
up to 100 kilometers in diameter, which migrate westward across the
ocean from the West African coast. Such eddies are difficult to
detect with conventional observation methods, and require a
combination of satellite, glider and ship-based observations.
The first observation of a low oxygen eddy in the Atlantic Ocean,
however, was detected by the Cape Verde Ocean Observatory, a project
which includes an ocean observation mooring north of the Cape
Verdean island of São Vicente. “We had a first indication of the
existence of these special eddies but still no exact information
from their interior,” reports the marine chemist Dr. Björn Fiedler
from GEOMAR, who led the project.
With the financial support of the Kiel Cluster of Excellence “The
Future Ocean”, an interdisciplinary group of scientists waited for
their next chance. It came in 2014: by means of satellite
observation they discovered a potential low oxygen eddy, which
formed off the coast of Mauritania and migrated towards Cape Verde.
From there, the team deployed autonomous sensor carriers, so-called
gliders, to intercept the eddy. As it approached the islands, the
researchers were also able to take water samples directly from the
eddy’s center with the Cape Verdean research vessel ISLANDIA.
“Chance also helped us. At the time we investigated the eddy the
German research vessel METEOR conducted a long planned expedition
for the Collaborative Research Centre 754 Kiel off Cape Verde.
We quickly convinced our colleagues to sample the eddy, too,” says
Dr. Fiedler. He adds: “Without the good infrastructure on the Cape
Verde Islands and the long-term cooperation with our colleagues
there, this campaign would not have been possible.”
Afterwards the obtained data and water samples were evaluated
physically, biogeochemically and biologically. “In a whole series of
publications, we were able to gain exciting new insights into the
hitherto unknown phenomenon in the Atlantic,” says Dr. Fiedler.
The latest study, now published in Scientific Reports, demonstrates
that at the core of the eddy, the highest levels of the greenhouse
gas nitrous oxide ever measured in the open Atlantic were found in
only 100 meters of water depth. This can be attributed to processes
that can deplete the plant nutrient nitrogen from the ocean,
producing, among other things, nitrous oxide in large quantities,
and further points out that scientists should now consider revising
our understanding of the chemical cycles in the Atlantic.