Oceanic CO2 outgassing and biological production hotspots induced by pre-industrial river loads of nutrients and carbon in a global modelling approach

Abstract. Rivers are a major source of nutrients, carbon and alkalinity for the global ocean, where the delivered compounds strongly impact biogeochemical processes. In this study, we firstly estimate pre-industrial riverine fluxes of nutrients, carbon and alkalinity based on a hierarchy of weathering and land-ocean export models, while identifying regional hotspots of the land-ocean exports. Secondly, we implement the riverine loads into a global biogeochemical ocean model and describe their implications for oceanic nutrient concentrations, the net primary production (NPP) and CO2 fluxes globally, as well as in a regional shelf analysis. Thirdly, we quantify the terrestrial origins and the long-term oceanic fate of riverine carbon in the framework, while assessing the potential implementation of riverine carbon fluxes in a fully coupled land-atmosphere-ocean model. Our approach leads to annual pre-industrial riverine exports of 3.7 Tg P, 27 Tg N, 158 Tg Si and 603 Tg C, which were derived from weathering and non-weathering sources and were fractionated into organic and inorganic compounds. We thereby identify the tropical Atlantic catchments (20 % of global C), Arctic rivers (9 % of total C) and Southeast Asian rivers (15 % of total C) as dominant providers of carbon to the ocean. The riverine exports lead to a global oceanic source of CO2 to the atmosphere (231 Tg C yr−1), which is largely a result of a source from inorganic riverine carbon loads (183 Tg C yr−1), and from organic riverine carbon inputs (128 Tg C yr−1). Additionally, a sink of 80 Tg C yr−1 is caused by the enhancement of the biological carbon uptake by dissolved inorganic nutrient inputs, resulting alkalinity production and a slight model drift. While large outgassing fluxes are mostly found in proximity to major river mouths, substantial outgassing fluxes can also be observed further offshore, most prominently in the tropical Atlantic. Furthermore, we find evidence for the interhemispheric transfer of carbon in the model; we detect a stronger relative outgassing flux (49 % of global river induced outgassing) in the southern hemisphere in comparison to the hemisphere's relative riverine inputs (33 % of global river inputs), as well as an outgassing flux of 17 Tg C yr-1 in the Southern Ocean. Riverine exports lead to a strong increase in NPP in the tropical West Atlantic, Bay of Bengal and the East China Sea (166 %, 377 % and 71 % respectively). While the NPP is not strongly sensitive to riverine loads on the light limited Arctic shelves, the CO2 flux is strongly altered due to substantial dissolved carbon supplies to the region. While our study confirms that the ocean circulation is the main driver for open ocean biogeochemical distributions, it reveals the necessity to consider riverine exports for the representation of heterogeneous features of the coastal ocean, to represent riverine-induced carbon outgassing, as well as to consider the long-term volcanic CO2 flux to close the atmospheric carbon budget in a coupled land-ocean-atmosphere setting.