Oceanic CO<sub>2</sub> outgassing and biological production hotspots induced by pre-industrial river loads of nutrients and carbon in a global modeling approach

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.

Reconstructing the Preindustrial Coastal Carbon Cycle Through a Global Ocean Circulation Model: Was the Global Continental Shelf Already Both Autotrophic and a CO2 Sink?

“…Oceanic inputs of riverine compounds were prescribed as described in Lacroix et al. (2020). The developed scheme considers DIC, Alk, DIP, DIN, DSi, DFe, tDOM, POM, and Fe‐P loads to the ocean.…”

“…Regional ocean circulation models have also been used to resolve heterogeneous coastal systems (Fennel & Wilkin, 2009; Frischknecht et al., 2018; Gruber et al., 2011), yet they do not provide a global perspective. GOBMs have, in the past, only provided a coarse representation of spatial features on continental shelves (Bernard et al., 2011; Lacroix et al., 2020) and/or relied on strongly simplified process representations of the biogeochemical cycling in these environments (Bourgeois et al., 2016), such as riverine inputs of carbon and nutrients.…”

Harmonization of global surface ocean pCO<sub>2</sub> mapped products and their flux calculations; an improved estimate of the ocean carbon sink

“…Open Access ) with the regional distribution of these inputs remaining unclear (Aumont et al 2001;Lacroix et al 2020). Quantification of non-steady state behavior of the natural carbon cycle has only recently been proposed and significant uncertainty remains, with a magnitude range of 0.05-0.4 PgC/yr for 1994-2007 (Gruber et al 2019, McKinley et al 2020.…”

A regional hindcast model simulating ecosystem dynamics, inorganic carbon chemistry, and ocean acidification in the Gulf of Alaska

“…After a model spin-up of 10 years, the hindcast simulation (1980 to 2013) was forced at the surface with 3-hourly winds, surface air temperature, pressure, humidity, precipitation, and radiation from the Japanese 55-year Reanalysis (JRA55-do) 1.3 project (Tsujino et al, 2018). The atmospheric fields were used to compute surface stresses and fluxes using a bulk flux algorithm (Large and Yeager, 2008). Precipitation was solely counted as a negative salt flux and did not change any volume or dilute any other tracers, such as DIC and TA.…”