Observing the biological carbon pump with autonomous underwater vehicles

CO2 taken up from the atmosphere by biological processes is transferred into the ocean’s “twilight zone” mostly in the form of sinking particles. Without this “biological carbon pump”, atmospheric CO2 would be 50% higher than it already is. Critically important is the depth at which particulate organic carbon (POC) is remineralised (i.e. converted back into inorganic carbon) as it plays a key role in setting the air-sea balance of CO2
. Despite its importance, our understanding of this pump is currently limited due to the scarcity of ship-board observations, in particular of how POC remineralisation varies over timescales of days to months. New autonomous technologies are opening the door for long-term, high resolution observations of these sinking particles. Gliders are unmanned autonomous vehicles that make measurements over weeks to months, several times per day and to depths of 1000 m, including optical backscatter data. This can be used to estimate particle properties, including their organic carbon content. The project will use data obtained from gliders and ship board measurements to determine remineralisation depth and its temporal variability. Key questions to be addressed include whether POC flux and remineralisation are always in phase, or whether they can become decoupled; the timescales of variability in POC and remineralisation, e.g. associated with transient pulses; and what factors might affect when and how variability in POC and remineralisation occurs. Filling this knowledge gap will allow better predictions of how this important planetary carbon flux is affected by both natural variability and climate change.

This project will use both existing glider datasets, and new data to be collected in the South Atlantic, to investigate the seasonal changes in carbon flux and remineralisation. The student will initially analyse an existing glider dataset from the Northeast Atlantic. The student will transform backscatter data into estimates of POC concentration, benefitting from the expertise of other group members who have established protocols for processing, calibrating and interpreting glider datasets. The focus will be on the seasonal changes in POC remineralisation and controlling factors, such as temperature or oxygen concentration. Additional datasets from the Southern Ocean and Benguela upwelling regions will also be available. Subsequent work may include exploiting additional autonomous technologies, such as the Bio-Argo float network, to broaden the study to larger scales, or using global biogeochemical models (including climate change simulations) to explore the mechanisms and patterns of variability in POC and remineralisation.

All doctoral candidates will enrol in the Graduate School of NOCS (GSNOCS), where they will receive specialist training in oral and written presentation skills, have the opportunity to participate in teaching activities, and have access to a full range of research and generic training opportunities. GSNOCS attracts students from all over the world and from all science and engineering backgrounds. There are currently around 200 full- and part-time PhD students enrolled (~60% UK and 40% EU & overseas).
The Ocean Biogeochemistry and Ecosystems group is renowned globally as one of the leading centres of excellence in biological carbon pump research with plankton ecologists, numerical modellers, remote sensing specialists, theoreticians and particle flux geochemists working together to address the most significant problems in biological oceanography. Specific training will include: concepts in biogeochemical oceanography, processing and analysing autonomous underwater vehicle data, bio-optics, data synthesis and statistical skills. The student can attend appropriate university Masters lecture courses to gain relevant background knowledge if needed. Presentation of results at national and international conferences will be encouraged. The student will also have the opportunity to participate in a research cruise.