Carbon Uptake and Seasonal Traits in Antarctic Remineralisation Depth (CUSTARD)

Principal Investigator: Dr Adrian Martin, National Oceanography Centre

The CUSTARD project will examine how seasonal changes in food availability for phytoplankton at a key junction of the global ocean circulation influences how long carbon is trapped in the ocean rather than escape to the atmosphere as carbon dioxide.

The surface ocean is home to billions of microscopic plants called phytoplankton which use sunlight and carbon dioxide to reproduce. When they die many of them sink, taking carbon that was originally in the atmosphere into the deep ocean, where it may be stored for hundreds to thousands of years. This helps keep our climate the way it is today.

The impact that this sinking ‘marine snow’ has on our climate is linked to how deep it sinks before it is respired by deeper dwelling ocean organisms such as bacteria, zooplankton and fish. This depth is referred to as the “remineralisation depth”. The deeper the snow sinks, the longer carbon is stored.  This effect is particularly important in the northern part of the Southern Ocean because the area is a junction in the ocean circulation. Stacked upon each other, from the surface to the seafloor almost 5km below, are four oceanic ‘motorways’ taking water to different far-flung parts of the global ocean. The motorway that the carbon is on when it is respired determines how long it will be kept away from the atmosphere.

Hence, if we want to understand the role of the Southern Ocean in regulating global climate we need to understand both how much carbon is used to make phytoplankton at the ocean surface and how deep this material penetrates into the ocean interior. Unfortunately we don’t understand either well; data are scarce in the Southern Ocean as it is remote, the weather is poor and few ships go there. As a result the models that we use for predicting future climate have large uncertainty in this region. The evidence that we do have suggests that the remineralisation depth and how it varies across the year are key to understanding how the system works.

In CUSTARD we will make new observations in an important, yet remote region of the Southern Ocean using an exciting combination of robotic vehicles and sophisticated new sensors. We will examine the processes that control the uptake of carbon and its fate at this key ocean junction. In particular we will explore how seasonal availability of nutrients can affect the type and health of the local phytoplankton, both of which change the depth to which the carbon penetrates. Modelling will allow us to determine where the carbon ending up on each motorway goes and how this varies through the year. Together the observations and model will allow us to determine the key processes regulating carbon uptake in this important area.  This will provide important information to those building the UK’s climate model that will be used to provide input to future high profile studies into the state of the world’s climate such as IPCC.