'Marine Snow' – Episode 3

Chelsey Baker, National Oceanography Centre, UK

Cover photograph by Sofia Alexiou, NOC

06/01/2020

EPISODE 3 – Sampling the Sinking Marine Snowstorm

Onboard the RRS Discovery for the past 6 weeks we’ve been waiting for a snowstorm… a marine snowstorm. But what is marine snow? Marine snow is made up of dead algae and plankton poo as discussed in Episodes 1 and 2 (links).

Marine snow, which contains organic carbon, sinks down the water column from the upper ocean. The particles race towards the deep ocean and the faster they sink the less time there is for their carbon to be utilised by marine microbes. This process is similar to leaf litter falling in a forest, except instead of a few metres to fall to the forest floor, marine snow falls for several kilometres before reaching the seafloor. Although, less than 1 % of the algae produced in the upper ocean reaches the seafloor. Marine snow that sinks deeper than 1 kilometre stores carbon in the deep ocean for more than 1000 years, which prevents the carbon returning to the atmosphere. This is an important natural oceanic process that moderates atmospheric carbon dioxide levels.

Deep ocean marine snow samples from 2000 m deep in the North Atlantic after the spring bloom of algae which is similar to plants blooming on land in spring. Photograph by Chelsey Baker.

During the CUSTARD cruise Emmy McGarry and I, Chelsey Baker, from the National Oceanography Centre in Southampton, UK sampled sinking particles using Marine Snow Catchers. Marine Snow Catchers are 95 litre settling columns that we deploy to different depths in the water column to measure the particles sinking out of the upper ocean. The snow catchers are deployed one at a time and a heavy weight, called a messenger, is sent down the wire to trigger the marine snow catcher to sample seawater at the chosen depth.

A Marine Snow Catcher being deployed in the Southern Ocean. A messenger is sent down the wire to close the snow catcher at the chosen depth. Photograph by Sofia Alexiou and Chelsey Baker.

Marine snow catchers have been used since the 1980’s to capture fast sinking particles that are responsible for the majority of the carbon that sinks deeper than 1 kilometre. More recently marine snow catchers have been used to sample all the non-sinking and sinking particles by allowing the particles to settle in the snow catchers on the ships deck for 2 hours. This settling period allows the particles to be separated into suspended particles, i.e. non-sinking, slow-sinking particles and fast-sinking particles.

A diagram of how particles sink out during the two hour settling period whilst the marine snow catcher is on the ships deck.

Measuring the amount of carbon in the sinking particle pools is important as the faster particles sink the more carbon they will transport deeper than 1 kilometre for long term carbon storage. We will also be measuring other chemical properties of the sinking particles when we are back on dry land. This will aid us in identifying which types of algae contribute heavily to carbon storage. By measuring the chemical content and sinking rates of the particles we can estimate how much carbon is stored at the three stations that we visited during the cruise and understand how this changes as the spring bloom of algae progresses. It’s been a really successful cruise with calm weather and over 60 marine snow catcher deployments! Now to get back into the onshore labs to analyse all our samples…

Setting up a marine snow catcher before deployment. Literally all hands on deck! Photograph by Hugh Venables.
Chelsey sampling the base of the marine snow catcher for slow-sinking and fast-sinking particles. Photograph by Emmy McGarry.

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