We’ve all heard the news.  We have been – and continue to – pump too much carbon dioxide into the atmosphere.  Just like its ‘naturally produced’ counterpart, not all of the human-generated carbon dioxide stays in the atmosphere.  In particular, plants on the land are what we call a carbon sink.  They take up the carbon dioxide and use it for photosynthesis (which also produces oxygen as a waste product – hurrah for plants!).  Plants do a pretty nifty job but holding an estimated 85% of the active carbon on the planet and 25% of annual human carbon dioxide emissions, the oceans are also an extremely important carbon sink.

Carbon dioxide enters the ocean through one of two pathways.  Atmospheric carbon dioxide is dissolved through the surface layers of the ocean and converted to carbonate, whilst dissolved organic carbon can enter from terrestrial runoff – such as from rivers.  The carbon cycle is an entirely natural and vital biogeochemical cycle for ocean life.  Because it is one big cycle, it’s not a one-way streak - carbon is released from the ocean too.  Warm waters and upwelling zones tend to throw out carbon, whereas cold water tends to hold it in.  It seems that with anthropogenic carbon dioxide emissions, the ocean has been taking in a little more carbon than it is giving back.  Back in 2004, Christopher Sabine of NOAA and a host of international collaborators estimated that the ocean has taken in around 48% of fossil fuel and cement-manufacturing emissions.  This increase in carbon dioxide levels in the ocean has been causing issues – ocean acidification.  I’ll keep the problems of ocean acidification for another post, but just as one example, one of my earlier posts highlighted the problem from the point of view of Chilean abalone.

We still have a lot to learn about the ocean’s role in the carbon cycle, and its potential role in mitigating our continued insistence to pump carbon dioxide into the atmosphere.  Here are just a few open access pieces highlighting what we know so far.

Even the smallest creatures have a role to play

Part of the carbon cycle is based in living organisms.  Through this ‘biological pump’ photosynthetic organisms take in carbon through photosynthesis.  When these organisms die, they sink down in the ocean taking that carbon with them.  Some of that carbon is released again, some stays in the deep ocean for a very, very long time.  Nianzhi Jiao from Xiamen University and a team of international researchers have written a fantastic piece looking at what we know about the biological pump and particularly the role of bacteria and archaea.  Don’t be fooled by their small size, bacteria and archaea are actually ”responsible for the respiration of most of the carbon that sinks into the ocean’s depths” !  We still have a lot to learn about these tiny critters, so the team have put together a conceptual framework called the microbial carbon pump to help explain some of the complexities.  Check it out here 

Not all parts of the ocean are equal in carbon storage

So the ocean is one big carbon sink, but that doesn’t mean that every part of the ocean is the same.  Take a look at the Southern Ocean for example.  Despite being far from any sizable human populations the Southern Ocean is thought to hold around 40% of the global human derived carbon dioxide.  In this paper by Jean-Baptiste Sallée of the British Antarctic Survey and fellow researchers from CSIRO and the Antarctic Climate and Ecosystems Cooperative Research Centre, it is revealed how current, eddies and winds all play a vital role in drawing carbon dioxide down to the depths of the Southern Ocean.  You can check out a preprint of their paper here 

The thing about sperm whale poo…

Hang on – whale poo?  Yes really – I haven’t gone mad!  Trish Lavery of Flinders and colleagues have been taking a good look at sperm whales (Physeter macrocephalus) in the Southern Ocean.  Sperm whales are deep ocean feeders… really deep, with records of dives down to over 3 km!  That’s some way to go, but down in the depths are some of their tastiest squid and fish prey.  Of course what goes in must come out, which they like to do when they are in what is known as the photic zone – basically the upper layers of the ocean which still receive sunlight.   The things about whale poo is that its rich in iron.  Why is this important?  Well the Southern Ocean is notoriously low in iron – which is a problem for phytoplankton because they use the iron to create chlorophyll and undergo photosynthesis.  The whale’s poo is a veritable feast of iron, stimulating phytoplankton growth.  When they die, their remains – and the carbon dioxide stored in them from photosynthesis – sinks to the bottom.  You can read the paper here 

To learn more about the carbon cycle, why not have a look at this 4 minute video from Nathaniel Manning, courtesy of TED Ed

Afterwards, check out this 6 minute video from NASA Goddard taking a look carbon and the oceans

If you’re hungry for even more information on the role of the ocean in carbon storage, I highly recommend reading the IPCC special report on ‘Ocean Storage’.