What's the fuss about carbon flux?

Today started as any other; a 2300 alarm, an apple from the galley, and a rendezvous in the main lab to prep our CTD cast (if you missed it, here’s a CTD 101). But today was an exciting CTD as I fired my own bottles! By which I mean, I pressed the button at the correct time at the correct depth 24 times, but it was a big moment for me.

I again took all the samples I need - another 50 litres (10 litres per depth from 5 different depths) filtered through 0.2 micron mesh for genetics analysis, and flow cytometry samples taken for cell enumeration, fluorescence and size analysis back on dry land. The process in all, from preparing the CTD to the last samples in the freezer takes about 4-5 hours, after which nothing looks more appealing than a ship bunk bed!

After my sleep, I woke just in time for lunch (which is, in my defence, at 11:30), which was, as always, delicious. The afternoon’s operation was recovering sediment traps which we’d deployed a few days ago and had left sitting, gathering samples in the water column ever since.

A sediment trap is essentially a way we can measure actual sinking of organic matter out of the water column. This is one of the most fundamental metrics we need in order to relate ocean observations back to practical change in the capacity of our ocean to store carbon. It’s all very well knowing which and how much phytoplankton are in the water, because they are the ones who carry out biological transformation of dissolved carbon dioxide to solid forms, but those solid forms can only store carbon in a useful way if they can somehow get out of the upper layer of the ocean, to store in the deep ocean.

Left: the traps are left out in the ocean for a few days, attached to the mooring. The officer on watch then locates the mooring by the 'high flyer', the buoy with the flag attached, which has an array of communication devices to ping its location back to us. Right: these are the sediment traps, which are here being recovered after sitting at 150-300 m for a few days.

Our current definition for considering carbon ‘stored’, is 100 years below 1,000 metres. Much of the organic carbon that is produced in the surface ocean never makes it out of the euphotic zone due to what we term ‘remineralisation’, the process by which sinking organic matter is turned back into dissolved organic carbon by microbes. Some good ways for carbon to escape this cycle and make it to the deep ocean are a) simple sinking (the slowest option) b) being eaten by zooplankton and excreted in larger, heavier particles, c) forming aggregates which are faster sinking than the sum of their parts, and d) calcifying – becoming part of inorganic carbon ‘shells’ which sink when the organisms that make them die.

Together, all these forms of sinking organic detritus are termed ‘marine snow’ – a rather romantic term for what is mostly dead cells and plankton poo! But tracking the sinking of these particles is essential if we want to understand how much of the carbon in the surface is actually stored. Which is where sediment traps come in.

Left: once recovered, the top layer of seawater is siphoned off, so the brine solution can be filtered. Right: particulate organic matter on the filter, which will be analysed for its organic matter composition.

Sediment traps are open-topped columns filled with a very dense brine solution (much denser than seawater), which are attached in arrays to racks and lowered into the ocean, in situ. We put them at different depths, from 150 to 300 metres, and leave them out there for a few days, attached to a simple mooring. Particles that sink into the brine are therefore trapped there and stored, ready for us to sample when we recover the traps. After a few days we pull the traps back out, siphon off the seawater that has settled on top of the brine, and then pull the brine down through a fine mesh filter.

The biomass collected on that filter, then, can be weighed and analysed for organic and inorganic carbon, phosphorus and nitrogen. And finally, this can be scaled up to calculate rates of carbon storage, per unit area, per unit time, for this part of the ocean at least.