Quantifying carbon uptake in the North Atlantic Ocean using Biogeochemical-Argo floats

Located at: SAMS UHI, Oban, Scotland

Project description: The ocean provides a crucial uptake of carbon from the atmosphere, acting to moderate the rise in atmospheric CO2 levels by absorbing over a quarter of human-derived carbon emissions. The North Atlantic is a key region in this, accounting for 25% of the global ocean anthropogenic carbon inventory, despite comprising only 15% of its area. Its continued ability to fulfil this role is uncertain as climate change progresses.

The successful candidate will investigate how the North Atlantic carbon sink is evolving, and will evolve, in the future, by focussing on the mechanisms controlling nutrient delivery and biological carbon drawdown from the Florida Straits to the subpolar North Atlantic. This will be done using the emerging fleet of available Biogeochemical-Argo float profiles from the North Atlantic to quantify the drawdown of natural and anthropogenic CO₂ through the ocean solubility and biological carbon pumps. This PhD will form a link between the recently funded NERC Large Grant, C-Streams, where the overall aim is to address the importance of the Gulf Stream in the North Atlantic Carbon sink and the OSNAP programme, which continuously observes the transport of heat and freshwater in the subpolar North Atlantic and is supported by large group of SAMS Scientists.

The Gulf Stream is widely accepted as playing a central role in redistributing heat over the North Atlantic and moderating NW European climate. However, the role of the Gulf Stream is much less certain for North Atlantic carbon uptake. The C-Streams project aims to enhance our understanding of the downstream evolution of nutrient and carbon concentrations along the length of the Gulf Stream and into the subpolar North Atlantic by gathering targeted physical and biogeochemical observations. This PhD will draw on this new knowledge to investigate and quantify the role of biological drawdown in sequestering CO₂ from the atmosphere.

The successful candidate will use Biogeochemical-Argo float data from the North Atlantic, a relatively novel technology that will be extensively expanded during the lifetime of this project through targeted North Atlantic deployment (~20 floats are being deployed and 3 specifically for the C-Streams project). New observations of pH, oxygen, nitrate, chlorophyll, backscatter and PAR from floats downstream of the Florida Straits will be employed to reveal vertical and along- and across-stream variability in biogeochemical properties, as well as the along-stream evolution of water mass properties and the contribution of high-nutrient Southern Ocean sourced waters. These observations will further be used to link biogeochemical variability to physical processes and biological productivity in the subpolar North Atlantic.

The controlling biogeochemical processes will be inferred by diagnosing additional variables:

• preformed and regenerated nutrients to be derived from nitrate and oxygen [Carter et al., 2021],

o dissolved inorganic carbon concentrations and pCO2 [Williams et al., 2016, 2017] will be diagnosed from pH, and alkalinity derived from parameterisations [Carter et al., 2018; Bittig et al., 2018].

o A full decomposition of the carbon system will be derived in terms of anthropogenic, soft-tissue, saturated, carbonate, disequilibrium components [Williams & Follows, 2011; Sarmiento & Gruber, 2006].

o Air-sea CO2 fluxes [Gray et al., 2018].

o The impact of biological activity: using chlorophyll, integrated nitrate and vertical oxygen variability [Bushinsky & Emerson, 2015; Johnson et al., 2017; von Berg et al., 2020] and backscatter to estimate particulate organic carbon concentrations

• the changing structure and carbon drawdown efficiency of ecosystems in the North Atlantic

The student will collaborate with scientists at SAMS, UHI Shetland and through the wider C-Streams and OSNAP programmes (particularly at University of Liverpool, University of Southampton and NOC Southampton).

The project will primarily be desk-based and will involve extensive training to build up experience in MATLAB or Python. Depending on the candidate there may also be fieldwork opportunities, including on the forthcoming OSNAP cruise to the subpolar North Atlantic in late summer 2024, or the C-Streams mooring/biogeochemical sensor turnaround cruises in the Florida Straits.

Training opportunities:

• computer skills – MATLAB, python courses
• Sensor calibration
• meetings/workshops with the Biogeochemical-Argo community;
• fieldwork skills – opportunity to develop analytical chemistry/biology skills through the OSNAP/Florida Straits cruises
• Extra opportunities will be sought to support any training or development needs.

Float being deployed from RRS James Cook

Project supervisors:

Dr Robyn Tuerena, (Project Lead): SAMS UHI

Dr Clare Johnson, SAMS UHI

Dr Pete Brown, NOC Southampton

Prof Stuart Cunningham, SAMS UHI

Dr Shaun Fraser, UHI Shetland