G02_02

Evolution of radiocarbon in the North Atlantic during 1990s-2020 inferred from in-situ observations and model simulations

Castrillejo M^1,2,  Wacker L², Lester J¹, Graven H¹

¹Department of Physics, Imperial College London, London, United Kingdom, ²Laboratory of Ion Beam Physics, ETH-Zurich, Zurich, Switzerland

Radiocarbon in dissolved inorganic carbon (DI¹⁴C) is an important tracer of the carbon cycle and of the processes involved in its uptake and redistribution such as the ocean circulation. In this work, we assess the spatial and temporal evolution of DI¹⁴C in the North Atlantic to better understand the processes driving its distribution in the water column. To that end, we use new Δ¹⁴C observations and model simulations obtained from the ‘Nucleus for European Modelling of the Ocean’ model, as well as other model outputs from the Coupled Model Intercomparison Project Phase 6. We focus on GO-SHIP A25 and A05 lines that cover the subpolar gates to the Arctic Sea and subtropical latitudes, respectively. The region between the two oceanic sections hosts major water mass transformation processes involved in the shallow to deep sequestration of climate properties. We collected 400 seawater samples in 2018-2020 to determine DI¹⁴C using the new ETH-LIP analytical method. The new Δ¹⁴C data are used to infer the water masses involved in major radiocarbon changes since the 1990s by comparison to previously reported data available in GLODAPv2 and CCHDO. Then Δ¹⁴C observations are compared to model outputs sampled at nearby geographical locations to identify the strengths and weaknesses of the various models in simulating Δ14C. Some model simulations were conducted using different wind forcing and atmospheric Δ¹⁴C and CO₂ boundary conditions, therefore allowing us to evaluate the role of ocean circulation (stationary vs time variable) and bomb-¹⁴C in shaping the water column distribution of DI¹⁴C.