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

Abstract

<p><strong>Abstract.</strong> 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 CO<sub>2</sub> 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 CO<sub>2</sub> to the atmosphere (231 Tg C yr<sup>−1</sup>), which is largely a result of a source from inorganic riverine carbon loads (183 Tg C yr<sup>−1</sup>), and from organic riverine carbon inputs (128 Tg C yr<sup>−1</sup>). Additionally, a sink of 80 Tg C yr<sup>−1</sup> 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 CO<sub>2</sub> 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 CO<sub>2</sub> flux to close the atmospheric carbon budget in a coupled land-ocean-atmosphere setting.</p>

Global Carbon Budget 2021

“…12) and highlights the need for model improvement. Further uncertainty stems from the regional distribution of the river flux adjustment term being based on one model study yielding the largest riverine outgassing flux south of 20 • S (Aumont et al, 2001), with a recent study questioning this distribution (Lacroix et al, 2020). The diverging trends in S OCEAN from different methods is a matter of concern, which is unresolved.…”

“…The river flux adjustment was distributed over the latitudinal bands using the regional distribution of Aumont et al (2001;north: 0.16 GtC yr −1 , tropics: 0.15 GtC yr −1 , south: 0.30 GtC yr −1 ), acknowledging that the boundaries of Aumont et al (2001; namely 20 • S and 20 • N) are not consistent with the boundaries otherwise used in the GCB (30 • S and 30 • N). A recent modelling study (Lacroix et al, 2020) suggests that more of the riverine outgassing is located in the tropics than in the Southern Ocean, and hence this regional distribution is associated with a major uncertainty. Anthropogenic perturbations of river carbon and nutrient transport to the ocean are not considered (see Sect.…”

“…12, and the larger model-data mismatch, Fig. B2), or uncertainties in the regional river flux adjustment (Hauck et al, 2020;Lacroix et al, 2020).…”

“…12, bottom row). This might be explained by the data products potentially underestimating the winter CO 2 outgassing south of the Polar Front (Bushinsky et al, 2019), by model biases, or by the uncertainty in the regional distribution of the river flux adjustment (Aumont et al, 2001;Lacroix et al, 2020) applied to f CO 2 -based data products and inverse models to isolate the anthropogenic S OCEAN flux. CO 2 fluxes from this region are more sparsely sampled by all methods, especially in wintertime (Fig.…”

Carbon Fluxes in the Coastal Ocean: Synthesis, Boundary Processes, and Future Trends

“…Global ocean biogeochemical models (GOBMs) (Dunne et al 2020, Tjiputra et al 2020, such as those used in the United Nations Intergovernmental Panel on Climate Change (Ciais et al 2013) or the Global Carbon Project (Friedlingstein et al 2020), have resolutions that are typically too coarse to provide a reliable basis for the analysis of coastal ocean carbon fluxes (Holt et al 2009, Mathis et al 2017). An equally important limitation is that these models do not include or only provide simplistic representations of processes relevant to the coastal ocean interface, such as time-varying riverine fluxes (Lacroix et al 2020), their modulation by the estuary-coastal vegetation continuum (Bauer et al 2013), and benthic carbon processing (Bianchi et al 2021, Krumins et al 2013. However, improved model resolution, afforded by advances in computational capabilities, allows GOBMs to represent important features of the coastal ocean, such as three-dimensional material and substance transports, residence times (Lacroix et al 2021a, Liu & Gan 2017, air-sea CO 2 exchange (Bourgeois et al 2016), and riverine fluxes (Dunne et al 2020;Lacroix et al 2020Lacroix et al , 2021a representation of specific coastal processes (e.g., heterotrophic respiration and burial of organic carbon, benthic calcification) greatly lags behind improvements in model resolution.…”