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

Principal Investigator: Dr Adrian Martin, National Oceanography Centre

The Southern Ocean plays a pivotal role in the global carbon cycle via the upwelling of waters rich in CO2 which then exchange carbon with the atmosphere through a variety of processes. The sum total of these processes contribute to regulating the future evolution of our climate yet are poorly understood and quantified – factors which contribute to significant uncertainty in our ability to predict future climate.

CUSTARD (Carbon Uptake and Seasonal Traits of Antarctic Remineralisation Depth) is a large and multidisciplinary project focussed on understanding a key element of this problem. The overall objective is to understand how surface biogeochemistry (particularly of iron and silicate), physical circulation and plankton stoichiometry control the remineralisation depth of sinking organic material; a parameter which we know regulates air sea CO2 partitioning yet one for which we still have a poor understanding.

Preliminary model experiments conducted for this proposal suggest that it is particularly important to understand this interplay between circulation and remineralisation in the northern part of the Southern Ocean, where water upwelled at the Polar Front is advected north in the surface before being subducted at the northern boundary of the Southern Ocean. Sinking material produced in this region can, depending on its depth of remineralisation, either be retained within this so-called ‘upper limb’ of the Southern Ocean circulation or alternatively, if remineralised deep, enter the ‘lower cell’ and exit the Southern Ocean at depth. A key difference between the two pathways is the length of time that the carbon they retain is locked away from the atmosphere; hence understanding what regulates the fate of this carbon and how it may change into the future is critical to predicting the future evolution of our climate.

We will address our overall objective by focussing on an important region of the upper limb in the southeast Pacific which is a key area for the formation of water masses subducted northwards and a major route out of the Southern Ocean for carbon. We will tackle this using an innovative combination of modelling and fieldwork, using gliders and novel nutrient sensors, and forging a collaboration with the US Ocean Observatories Initiative.

In particular, we will address the following objectives:
1. To obtain an accurate picture of the seasonal air-sea CO2 flux and macronutrient drawdown
2. To quantify the link between iron and silicate availability and remineralisation depth
3. To observationally determine the seasonal cycle in remineralisation depth
4. To examine the link between seasonality in remineralisation depth and the trajectory of carbon from the surface out of the upper limb.

By delivering these objectives we will address the following hypotheses:
H1: There is a strong seasonal cycle in remineralisation depth driven by seasonal variability in surface silicate:carbon:nitrogen uptake as a function of iron availability
H2: This seasonal cycle in remineralisation depth leads to seasonal variability in the fate of organic carbon leaving the Southern Ocean via the upper limb

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