RRS Discovery Cruise DY096, 28 November – 14 December 2018. 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. DY096 marked the start of one year of observations by CUSTARD. Cruise objectives included the deployment of a surface mooring and two gliders to make observations throughout the year until they would be retrieved on a cruise at the start of 2020. Immediately prior to the recovery cruise a process cruise is planned. For DY096 the surface mooring was provided and deployed by a team from WHOI as part of the NSF Ocean Observatories Initiative. The mooring had also been adapted by WHOI to integrate novel NOC lab-on-a-chip nitrate and silicate sensors. Another aim was to recover the surface mooring already at the site, now into its third year after recovery was not possible in 2017, and the lower parts of two subsurface moorings, all parts of the original OOI array. These deployments and recoveries were the priorities together with the collection of observations to calibrate the sensors on mooring and gliders. If further iron and optical particulate data could be obtained it would also be very useful to inform planning for the 2019 process cruise. Weather, swell and a ship issue restricted time on site to 102 hours of which conditions were suitable to deploy/recover equipment over the side for just 61 hours, out of a 16 day cruise. Nevertheless, all of the priorities were achieved. It was also possible to collect some limited samples for iron concentrations but not to collect any standalone optical particulate observations. CUSTARD (NE/K015613/1) is part of the NERC Role of the Southern Ocean in the Earth System (RoSES) programme.

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