What the flux? Assessing the global ocean biological carbon pump

Dr Stephanie HensonDr Nathan Briggs, Dr Anna Hickman

CO2 taken up from the atmosphere by biological processes in the ocean is transferred into the deep 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.  Despite its importance, our understanding of this pump is currently limited due to the scarcity of ship-board observations, in particular of how particulate organic carbon (POC) fluxes vary both spatially and seasonally. However, now a growing network of underwater robots are opening the door for long-term, high resolution observations of these sinking particles.  Biogeochemical-Argo floats (https://biogeochemical-argo.org/) are unmanned autonomous vehicles that make measurements over years, to depths of 1000 m, including optical backscatter data which can be used to estimate POC fluxes.  The global scale of the float network will allow important, currently unresolved, questions to be addressed, such as: What is the global spatial pattern of fluxes? (Marsay et al., 2015); How do fluxes vary seasonally? (Briggs et al., 2020); How might these patterns change in the future? (Canadell et al., 2021 – Section  Filling this knowledge gap using these new data will allow better predictions of how this important planetary carbon flux is affected by both natural variability and climate change.



The project will initially use data obtained from biogeochemical-Argo floats to determine the spatial and temporal patterns in POC fluxes and their attenuation with depth.  The global database of biogeochemical-Argo data will be initially be screened for suitable data.  You will then transform optical backscatter data profiles into estimates of POC concentration using established empirical relationships, benefitting from the expertise of supervisor Briggs who has established protocols for processing, calibrating and interpreting these datasets (Briggs et al., 2020).  POC concentration will be converted into flux by applying estimates of particle sinking speed, and different metrics to characterise the fraction of POC flux reaching the mesopelagic zone will be calculated and compared.  Your initial analysis will focus on establishing the global scale patterns of flux and attenuation, before moving on to assessing the characteristics of seasonal variability.  Depending on your interests, subsequent work may include exploiting additional autonomous technologies to study particular processes or locations in more detail, satellite data to broaden the study to larger spatial and temporal scales, or using global biogeochemical models (including climate change simulations) to explore the mechanisms and patterns of variability in POC fluxes.


NOC Southampton

The INSPIRE DTP programme provides comprehensive personal and professional development training alongside extensive opportunities for students to expand their multi-disciplinary outlook through interactions with a wide network of academic, research and industrial/policy partners. The student will be registered at the University of Southampton and hosted at the National Oceanography Centre (Southampton). Specific training will include: use of autonomous platform data for biogeochemical studies, analysis of satellite data and model output, relevant aspects of ocean biogeochemistry and its interaction with climate (e.g. IMBER ClimEco summer school). The student will be part of the Ocean BioGeosciences group at NOC, which is renowned globally as one of the leading centres of excellence in biological carbon pump research. The student may attend appropriate university Masters level lectures to gain relevant background knowledge. Presentation of the results at (inter)national conferences will be encouraged. There will also be the opportunity to participate in a research cruise.


Eligibility & Funding Details: 

Please see https://inspire-dtp.ac.uk/how-apply for details.

Background Reading: 

Briggs, N., G. Dall’Olmo, G., and H. Claustre (2020), Major role of particle fragmentation in regulating biological sequestration of CO 2 by the oceans, Science, 367(6479), 791–793. https://doi.org/10.1126/science.aay1790

Marsay, C, R. Sanders, S. Henson, K. Pabortsava, E. Achterberg and R. Lampitt (2015), Attenuation of sinking particulate organic carbon flux through the mesopelagic ocean, PNAS, 112(4), 1089-1094, doi: 10.1073/pnas.1415311112

Canadell, J. G., P. M.S. Monteiro, M. H. Costa, L. Cotrim da Cunha, P. M. Cox, A. V. Eliseev, S. Henson, M. Ishii, S. Jaccard, C. Koven, A. Lohila, P. K. Patra, S. Piao, J. Rogelj, S. Syampungani, S. Zaehle, K. Zickfeld, 2021, Global Carbon and other Biogeochemical Cycles and Feedbacks. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. https://www.ipcc.ch/report/ar6/wg1/