Abstract
Abstract. Rivers are a major source of nutrients, carbon and alkalinity to the global ocean. In this study, we firstly estimate pre-industrial riverine loads of nutrients, carbon and alkalinity based on a hierarchy of weathering and terrestrial organic matter export models, while identifying regional hotspots of the riverine exports. Secondly, we implement the riverine loads into a global ocean biogeochemical model to describe their implications for oceanic nutrient concentrations, net primary production (NPP) and air–sea CO2 fluxes globally, as well as in an analysis of coastal regions. Thirdly, we quantitatively assess the terrestrial origins and the long-term fate of riverine carbon in the ocean. We quantify annual bioavailable pre-industrial riverine loads of 3.7 Tg P, 27 Tg N, 158 Tg Si and 603 Tg C delivered to the ocean globally. We thereby identify the tropical Atlantic catchments (20 % of global C), Arctic rivers (9 % of global C) and Southeast Asian rivers (15 % of global C) as dominant suppliers of carbon for the ocean. The riverine exports lead to a simulated net global oceanic CO2 source of 231 Tg C yr−1 to the atmosphere, which is mainly caused by inorganic carbon (source of 183 Tg C yr−1) and by organic carbon (source of 128 Tg C yr−1) riverine loads. Additionally, a sink of 80 Tg C yr−1 is caused by the enhancement of the biological carbon uptake from dissolved inorganic nutrient inputs from rivers and the resulting alkalinity production. While large outgassing fluxes are simulated mostly in proximity to major river mouths, substantial outgassing fluxes can be found further offshore, most prominently in the tropical Atlantic. Furthermore, we find evidence for the interhemispheric transfer of carbon in the model; we detect a larger relative outgassing flux (49 % of global riverine-induced outgassing) in the Southern Hemisphere in comparison to the hemisphere's relative riverine inputs (33 % of global C inputs), as well as an outgassing flux of 17 Tg C yr−1 in the Southern Ocean. The addition of riverine loads in the model leads to a strong NPP increase in the tropical west Atlantic, Bay of Bengal and the East China Sea (+166 %, +377 % and +71 %, respectively). On the light-limited Arctic shelves, the NPP is not strongly sensitive to riverine loads, but the CO2 flux is strongly altered regionally due to substantial dissolved inorganic and organic carbon supplies to the region. While our study confirms that the ocean circulation remains the main driver for biogeochemical distributions in the open ocean, it reveals the necessity to consider riverine inputs for the representation of heterogeneous features in the coastal ocean and to represent riverine-induced pre-industrial carbon outgassing in the ocean. It also underlines the need to consider long-term CO2 sources from volcanic and shale oxidation fluxes in order to close the framework's atmospheric carbon budget.
Highlights
Rivers deliver substantial amounts of carbon (C), phosphorus (P), nitrogen (N), silicon (Si), iron (Fe) and alkalinity (Alk) to the ocean (Seitzinger et al, 2005, 2010; Dürr et al, 2011; Beusen et al, 2009; Tréguer and De La Rocha, 2013; Beusen et al, 2016)
For terrestrial dissolved organic matter (tDOM), we extended Hamburg Ocean Carbon Cycle model (HAMOCC) with a new model tracer that was characterized by the described C : N : P : Fe mole ratio. tDOM was mineralized as a function of the tDOM concentration at a rate constant krem,tDOM and of an oxygen limitation factor ( O2 ), which decreases the maximum potential remineralization rate depending on the O2 concentration: dtDOM/dt = krem,tDOM · tDOM · O2
This was done with the reasoning that in HAMOCC, biogeochemical inputs are needed in order to maintain a stable ocean state, since the burial loss in the sediment induces a loss of CaCO3, opal and particulate organic matter (POM)
Summary
Rivers deliver substantial amounts of carbon (C), phosphorus (P), nitrogen (N), silicon (Si), iron (Fe) and alkalinity (Alk) to the ocean (Seitzinger et al, 2005, 2010; Dürr et al, 2011; Beusen et al, 2009; Tréguer and De La Rocha, 2013; Beusen et al, 2016). Riverine point sources of biogeochemical compounds have been omitted or poorly represented in global ocean biogeochemical models, despite being suggested to strongly impact the biogeochemistry of coastal regions (Froelich, 1988; Stepanauskas et al, 2002; Dagg et al, 2004) and being suggested to cause a pre-industrial source of atmospheric CO2 in the ocean (Sarmiento and Sundquist, 1992; Aumont et al, 2001; Gruber et al, 2009; Resplandy et al, 2018) This background CO2 outgassing flux of 0.2 to 0.8 Gt C yr−1 is significant in the context of the present-day oceanic C uptake of around 2.3 Gt C yr−1 (IPCC, 2013). This balance of the land C uptake and its oceanic outgassing is used to assess the potential implementation of riverine fluxes in a fully coupled land–atmosphere–ocean setting
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