Throughout the deep sea wonders series, we’ve explored the remote ecosystems and hidden
worlds of the abyss, and the weird and wonderful creatures that call it home.
We’ve uncovered the ecological interactions that drive biodiversity beneath the waves,
and touched upon the history of ocean exploration.
But by far the most important process to take place in the oceans is one that is often overlooked.
The marine carbon cycle.
Together, the creatures of the oceans, and the physical features of their environment
play an important role in sequestering carbon and taking it out of the atmosphere.
Through the biological processes of photosynthesis, predation, and decomposition, and the physical
movements of currents, the oceans take in more carbon than they release. Thus, they
can be called a carbon sink, and they help to protect our planet from the damaging effects
of climate change.
Let’s take a look at the incredible interactions that comprise the carbon cycle of the oceans.
It all begins at the surface. Here, the oceans absorb large amounts of carbon dioxide as
it diffuses into the surface, much of which then moves back and forth in a constant exchange
between the oceans and the atmosphere in what’s called the ‘fast carbon cycle’. But some
of this carbon will remain in the oceans for thousands of years, for a community of very
small but very important creatures form the foundation of the biological carbon pump.
The phytoplankton. Microscopic plant-like organisms that sequester the carbon by undergoing
the process of photosynthesis. A byproduct of this process is the oxygen that almost
all life on earth relies on, and incredibly, phytoplankton are responsible for producing
an estimated 80% of all oxygen in our atmosphere.
Photosynthesis takes the carbon out of the atmosphere and stores it in the plankton as
sugars, and from here it will be assimilated into the biomass of other organisms from higher
trophic levels that feed on the plankton. It begins to move throughout the food web.
Zooplankton and filter feeders primarily consume phytoplankton. Planktivores like krill filter
them out of the water. They in turn are predated by secondary consumers, as the carbon moves
through them into larger organisms like the great baleen whales.
An individual whale will take in krill equivalent to 4% of its body weight every day - and their
long lifespans mean they are able to keep much of this carbon from returning to the
atmosphere.
But even this is a short period of time relative to the overall cycle. In order to ensure the
carbon remains in the oceans for thousands of years, it needs to be transported the deep
sea, far from the surface waters. Let’s take a look at some of the mechanisms that
enable this transfer to occur.
Every night, a phenomenon known as diel vertical migration takes place. Jewels of bioluminescent
light emerge from the depths, for this process sees animals from the twilight zone, between
200 and 1,000 metres deep, follow the movements of plankton as they migrate to surface waters
at dusk. A huge diversity of organisms migrate up towards the surface as night begins to
fall, and migrate back down again come the dawn.
A mass movement so dense that it makes up the largest mass migration on the planet,
consisting of a vertical feast that fuels the ocean’s food web and causes a net transfer
of organic carbon from the sunlight zone at the surface, to the twilight zone of the deep.
From predatory squid to filter feeding sharks, the creatures that bring the carbon to deeper
waters act as temporary carbon stores, releasing some back into the atmosphere as they respire,
but storing much of it within their bodies and acquiring more as they hunt and grow.
And when they die or defecate, waste products containing carbon are released into the open
ocean. But unlike respiration which produces gases, these processes don’t return the
carbon to the atmosphere. Fundamentally, they move it downwards into even deeper waters.
A constant shower of organic material and debris falling from the more productive shallower
waters, called marine snow, which not only supplies the isolated deep with nutrients,
but also plays an important part in what’s known as the ‘slow carbon cycle’. On its
journey, most of the marine snow is consumed by detrivores.
Vampire squid catch the particles on filamentous tentacles that they extend into the water.
Polychaete worms build mucus nets around themselves to trap the marine snow.
About 50 gigatons of carbon is drawn down into the biological pump each year but only
a small fraction of this material actually makes it to the sea floor. But over a long
period of time, this sediment accumulates.
This same process can occur on a much larger scale in the form of carrion. Whale carcasses
support complex and diverse whale-fall communities, consisting of deep sea scavengers and decomposers
that assimilate the carbon into their own bodies when they feed upon the flesh. As many
deep sea denizens are not adapted to a life in shallower waters, this carbon will tend
to remain in the depths, either in the bodies of creatures like the sleeper sharks and hagfish,
or in the sea floor mud itself.
The amount of carbon stored in a typical 40-tonne whale carcass is equivalent to about 2000
years of material that falls as marine snow, making it a significant event in the biological
carbon pump as a flux of organic material from the surface ocean to the deep sea.
But to understand why the transfer of carbon to the depths is so important, we need to
examine what occurs once it gets there.
Material that collects on the sea floor from marine snow, jelly-falls and whale falls gets
buried beneath layers of sediment. When compacted over millions of years, these deposits are
transformed into hard limestone rock and carbonate structures. This can lock the carbon away
for millions of years, taking it out of the biological pump and making it a part of the
carbonate pump. Raised from the seabed by tectonic processes, the White Cliffs of Dover
in England were formed by this very process. The chalk cliffs are composed of millions
of years worth of sunken organic material, and for this reason, abundant fossils can
be found here. Evidence that the biological pump has been driving the marine carbon cycle
since the dawn of life on Earth.
In addition to the food web leading to carbon being stored on the sea floor, carbon is also
sequestered by corals and shell-building organisms which use dissolved carbon to create shells,
skeletons and plates. Thus, ecosystems like coral reefs contribute to the ability of the
ocean to become a carbon sink, as even when the polyps die they leave behind large carbonate
structures that store carbon for extended periods of time. Rocky carbonate reefs form
from dead coral, locking away the carbon, and providing a solid surface from which newer
corals grow and sequester even more carbon.
When the shell-builders, like crabs and sea urchins, die and sink, the carbon in their
shells is transported down to the deep ocean where the carbon can become part of seafloor
sediments. Many shells dissolve before reaching the bottom, in a process that releases carbon
dioxide into deep ocean currents. But those that do not dissolve build up slowly on the
sea floor, where they too are eventually transformed into limestone, sequestering carbon for far
longer than any other carbon store on Earth.
The movement of carbon via the processes of respiration, consumption, excretion, death
and decay are not the only ways in which carbon is cycled in the oceans. For in addition to
marine animals, physical processes also help to make the oceans an overall carbon sink.
While the biological pump consists of animals and their interactions transporting carbon,
the physical pump is instead based on ocean circulation. Upwelling brings carbon from
deep waters up towards the surface, releasing it into the atmosphere. At the same time,
the opposing action of downwelling takes carbon out of the atmosphere and carries it away
into the deep. A process called the thermohaline circulation demonstrates the role of temperature
and salt content in controlling this movement of carbon.
When warm water in oceanic surface currents move away from the equator, they are cooled
and often become saltier. The cooler, saltier seawater is important for two reasons. Firstly,
it is able to take up more carbon dioxide, and secondly, it is denser than warmer water.
This causes it to sink and transport the dissolved carbon downwards - in some places all the
way to the deep sea floor, where the carbon remains in the waters for hundreds of years,
far from the atmosphere.
The marine carbon cycle consists of many processes that allow carbon to be transferred
between non-living and living matter. Together, these processes allow the oceans to take in
more carbon than they release, making the marine world a carbon sink overall. This helps
to mitigate the effects of human-driven carbon emissions, but it isn’t enough, and it is
not without a cost. As increased carbon is released into the atmosphere, the oceans absorb
more, and become more acidic. This leads to the bleaching of corals, and causes a major
loss of biodiversity. The marine carbon cycle can only do so much, and with many of its
inhabitants and interactions threatened by human activity, it may not be able to maintain
this for much longer. The issue of carbon emissions is a problem that we as humans must
work to solve, otherwise we risk causing irreversible damage to marine ecosystems, food webs, and
fundamentally, the carbon cycle of the oceans.