Abstract. The ocean carbon cycle is a key player in the climate system through its role in regulating the atmospheric carbon dioxide concentration and other processes that alter the Earth's radiative balance. In the second version of the Norwegian Earth System Model (NorESM2), the oceanic carbon cycle component has gone through numerous updates that include, amongst others, improved process representations, increased interactions with the atmosphere, and additional new tracers. Oceanic dimethyl sulfide (DMS) is now prognostically simulated and its fluxes are directly coupled with the atmospheric component, leading to a direct feedback to the climate. Atmospheric nitrogen deposition and additional riverine inputs of other biogeochemical tracers have recently been included in the model. The implementation of new tracers such as “preformed” and “natural” tracers enables a separation of physical from biogeochemical drivers as well as of internal from external forcings and hence a better diagnostic of the simulated biogeochemical variability. Carbon isotope tracers have been implemented and will be relevant for studying long-term past climate changes. Here, we describe these new model implementations and present an evaluation of the model's performance in simulating the observed climatological states of water-column biogeochemistry and in simulating transient evolution over the historical period. Compared to its predecessor NorESM1, the new model's performance has improved considerably in many aspects. In the interior, the observed spatial patterns of nutrients, oxygen, and carbon chemistry are better reproduced, reducing the overall model biases. A new set of ecosystem parameters and improved mixed layer dynamics improve the representation of upper-ocean processes (biological production and air–sea CO2 fluxes) at seasonal timescale. Transient warming and air–sea CO2 fluxes over the historical period are also in good agreement with observation-based estimates. NorESM2 participates in the Coupled Model Intercomparison Project phase 6 (CMIP6) through DECK (Diagnostic, Evaluation and Characterization of Klima) and several endorsed MIP simulations.

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The 30 Si of biogenic silica ( 30 Si BSi ) in marine sediments is a promising proxy for the reconstruction of silicic acid utilization by diatoms in the geological past. The application of this proxy, however, requires an understanding of the modern 30 Si distributions and their controlling mechanisms. Here we present results from a modern climate simulation with a coupled ocean-sediment model that includes a prognostic formulation of biogenic silica production with concurrent silicon isotopic fractionation. In agreement with previous studies, biological fractionation combined with physical transport and mixing determines the oceanic distribution of simulated 30 Si. A new finding is a distinct seasonal cycle of 30 Si in the surface ocean, which is inversely related to that of silicic acid concentration and mixed layer depth. We also provide the first simulation results of sedimentary 30 Si, which reveal that (1) the 30 Si BSi distribution in the surface sediment reflects the exported 30 Si BSi signal from the euphotic zone and (2) the dissolution of biogenic silica in the sediment acts as a source of relatively light 30 Si into the bottom waters of the polar oceans, while it is a source of heavier 30 Si to the subtropical South Atlantic and South Pacific.