"The ocean at the heart of humanity"
"The ocean's biological carbon pump"
-Hello, I am going to talk to you
about the ocean's biological carbon pump.
As the name suggests, it is a biological mechanism
which leads to the storage of carbon within the ocean.
We use the term "pump" because it explains almost 90%
of the ocean's CO₂ vertical gradient,
that is to say the fact that there is more CO₂ at the bottom of the ocean
than near the surface.
The other 10% of this gradient are due to a different mechanism
which is based on chemicophysical processes.
It is the solubility pump,
which depends on the ability of cold waters
to contain more CO₂.
The functioning of this carbon pump
consists of three steps that I will show you now.
What drives the pump is photosynthesis, which requires light.
That is why it takes place in the ocean's sunlit surface layer,
called the euphotic zone,
which consists of the first 150 meters of the ocean.
It is in these first 150 meters
that microalgae called phytoplankton perform photosynthesis.
And this process of photosynthesis
converts CO₂ and other nutrients, such as nitrogen, phosphorus and iron,
into organic matter.
This first step is what actually drives the pump.
Step 2: this organic matter leaves the euphotic zone.
We used to think that this organic matter
sank from the euphotic zone because of gravitation,
that it was responsible for the formation of aggregates.
But recently,
other injection mechanisms were discovered,
as you can see on this picture.
I will talk about two of them.
The first one is a physical mechanism
which consists of the transport of organic matter
by ocean currents flowing down vertically,
which are related to submesoscale currents.
So these strong, vertical currents carry organic matter
from the surface to the bottom of the ocean,
over distances that can go up to 10 kilometers.
The second mechanism is biological.
It consists of the vertical migration of organisms
that take refuge in the deeper parts of the ocean during the day,
where there is no light, to avoid predators.
Once the organic matter has left the surface
and has reached the bottom,
it undergoes a myriad of transformations,
and the flow progressively decreases.
First of all, it serves as food
for the living organisms of the mesopelagic zone.
Then, as it sinks, it is remineralized by bacteria
with CO₂ and nutrients.
So this process is part of a natural, biogeochemical cycle,
since this CO₂ and these nutrients
will go back to the surface thanks to ocean currents
and will fuel photosynthesis.
The biological pump is a process
which takes place at the surface of every ocean,
but whose efficacy varies depending on the region.
Behind me, you can see a map
showing the intensity of photosynthesis
at the surface of the oceans.
In some regions with temperate latitudes,
it is very intense.
And regions with tropical latitudes are oceanic deserts.
So the intensity of photosynthesis,
which is what drives the biological pump,
depends on many factors,
such as temperature, illumination, or availability of nutrients,
which explains why it varies that much from one region to another.
The second factor which influences this efficacy
is the type of phytoplankton that perform photosynthesis.
You can see different species of phytoplankton behind me.
Another factor is the attenuation coefficient,
which is an exponential coefficient
which decreases as you go down the 3 000 meters of the ocean
and which varies from one basin to another.
And one last factor which influences efficacy
is the physical scales of sequestration time.
These scales of sequestration time are related to ocean currents
and also vary from one part of the ocean to another.
Here, you can see sequestration times
that range from a hundred years to hundreds of years.
The darker the color, the longer the sequestration time.
How is the biological pump measured?
We used to set up sediment traps,
that is to say large cones that are about 1 meter long
and that can be placed at a fixed location
or that can simply drift,
and that allow to measure the gravitational flux.
But we recently started using
more comprehensive, three-dimensional observation methods
that give data on the different vertical injection modes
I told you about,
such as BioArgo's drifting, biogeochemical floats,
or gliders that are autonomous, remotely-operated robots.
On top of these in situ observations,
satellite observations are carried out and digital models are used
to get more comprehensive data.
To conclude, I would like to talk to you
about the relation between the biological pump
and the sequestration, by the ocean, of the CO₂ emitted by human activities
since the beginning of the industrial era.
The ocean has stored about 25% of the carbon
emitted by human activities.
But the storage of carbon mostly depends on chemicophysical processes,
not on the biological pump.
However, and this will be my last point,
climate change is altering the biological pump's functioning.
And consequently, the storage of anthropogenic carbon by the ocean
is expected to become less efficient.