RRS Discovery Cruise DY111, 2 December 2019 – 9 January 2020. Punta Arenas, Chile – Punta Arenas, Chile. CUSTARD: Carbon Uptake and Seasonal Traits in Antarctic Remineralisation Depth

Abstract

The CUSTARD project examines how seasonal changes in nutrient availability for phytoplankton, at a key junction of the global ocean circulation, influence how long carbon is trapped in the ocean rather than escaping to the atmosphere as carbon dioxide.
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; the ‘remineralisation depth’. The objective of CUSTARD is to make new observations of the remineralisation depth and its controls in an important, yet remote, region of the Southern Ocean, using a combination of gliders, a mooring, sophisticated new sensors and a process cruise. The observations will be combined with modelling to determine the key processes regulating carbon uptake in the Southern Ocean.
CUSTARD fieldwork began with DY096 in Nov-Dec 2019. A surface mooring and two gliders were deployed at the OOI site (54.42 S 89 W) to make observations throughout the year. One glider was lost early on and the second had to be recovered in November 2019 after it became trapped at the surface. The mooring was recovered on DY112.
DY111 was a process cruise immediately prior to the mooring recovery cruise (DY112), to allow a more detailed study of the biogeochemistry of the site at the key spring bloom period. Objectives were: to deploy two other gliders (just for the duration of the cruise); to deploy 6 BGC ARGO floats on behalf of the SOCCOM project; to do multiple visits to 3 sites along 89 W (OOI, TN at 57S and TS at 60S); and to carry out a full depth CTD transect between OOI and TS at 1 degree latitude resolution. All objectives were met, though one glider had to be recovered immediately after deployment due to a leak. Additionally, a modest spatial survey was carried out collecting underway data and samples along a grid-pattern extending 90km west of 89W, to assess upstream properties and gradients.
The cruise was exceptionally fortunate both in weather and in coinciding strongly with the spring bloom spanning the area.
CUSTARD (NE/K015613/1) is part of the NERC Role of the Southern Ocean in the Earth System (RoSES) programme. Additional work was funded by the NERC Bridging International Activity and Related Research Into the Twilight Zone (NE/ S00842X/1).

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