Shelf-specific physical and biogeochemical processes and their effects on carbon cycling

Contact: Jan Kossack, Moritz Mathis, Corinna Schrum

Current research suggests that continental shelves contribute disproportionally to global oceanic CO2 uptake from the atmosphere, in particular through the efficient CO2 sinks of biologically productive mid- and high latitude shelves (Cai et al., 2006; Chen & Borges, 2009; Laruell et al., 2010; Bauer et al., 2013; Legge et al., 2020). The quantification of their contribution to the global marine carbon cycle however still entails large uncertainties (Roobaert et al., 2019; Lacroix et al., 2021; Krumins et al., 2013). For the North Western European Shelf (NWES), contributions of different biological and hydrodynamic drivers of the shelf carbon cycle remain poorly constrained. Especially an improved understanding of the balance between off-shelf carbon transports and sequestration in shelf sediments, and their relative importance for CO2 uptake from the atmosphere, is of key interest in the light of future impacts of climate change and options for marine coastal management (Legge et al., 2020).

In our research, we use sensitivity experiments performed with the coupled hydrodynamic-biogeochemical modeling system SCHISM-ECOSMO to investigate how tidal forcing, as one of the dominant hydrodynamic features on the NWES, impacts the shelf carbon cycle. We show that tides substantially increase net primary productivity on the NWES and find a significant contribution by internal tides in the Celtic Sea (Fig. 5). Our results further demonstrate that the enhanced biological productivity and inorganic carbon sequestration in the tidal scenario translates into an increased oceanic CO2 uptake, even though tidal currents reduce particulate carbon deposition in the shelf sediments.

Fig 5: Difference in vertically integrated mean annual primary production between experiments including and excluding tidal currents (tides - no tides).

Our results suggest that tides play an important role for the efficiency of the shelf carbon pump by promoting net carbon export from the NWES to the adjacent North Atlantic. The simulated experiments further allow the investigation of mechanisms and pathways behind off-shelf carbon transport, its natural variability and its relative importance in driving atmospheric CO2 drawdown on the NWES. These aspects are addressed in ongoing work.

References:

Bauer, J. E., Cai, W. J., Raymond, P. A., Bianchi, T. S., Hopkinson, C. S., Regnier, P. A. G., 2013. The changing carbon cycle of the coastal ocean. Nature, 504(7478), 61–70, doi:10.1038/nature12857

Cai, W. J., Dai, M. H. Wang, Y. C. (2006). Air-sea exchange of carbon dioxide in ocean margins: A province-based synthesis. Geophysical Research Letters, 33, L12603, doi: 10.1029/2006gl026219

Chen, C. T. A., Borges, A. V. (2009). Reconciling opposing views on carbon cycling in the coastal ocean: Continental shelves as sinks and near-shore ecosystems as sources of atmospheric CO2. Deep-Sea Res Part II-Top Stud Oceanogr, 56, 578–590, doi: 10.1016/j.dsr2.2009.01.001

Krumins, V., Gehlen, M., Arndt, S., Cappellen, P. V., Regnier, P. (2013). Dissolved inorganic carbon and alkalinity fluxes from coastal marine sediments: model estimates for different shelf environments and sensitivity to global change. Biogeosciences, 10, 371-398, doi: 10.5194/bg-10-371-2013

Lacroix, F., Ilyina, T., Mathis, M., Laruelle, G. G., Regnier, P. (2021). Historical increases in land-derived nutrient inputs may alleviate effects of a changing physical climate on the oceanic carbon cycle. Global Change Biology, 27, 5491– 5513, doi: 10.1111/gcb.15822

Laruelle, G. G., Durr, H. H., Slomp, C. P. Borges, A. V. (2010). Evaluation of sinks and sources of CO2 in the global coastal ocean using a spatially-explicit typology of estuaries and continental shelves. Geophysical Research Letters, 37, doi: 10.1029/2010gl043691

Legge, O., Johnson, M., Hicks, N., Jickells, T., Diesing, M., Aldridge, J., Andrews, J., Artioli, Y., Bakker, D. C. E., Burrows, M. T., Carr, N., Cripps, G., Felgate, S. L., Fernand, L., Greenwood, N., Hartman, S., Kröger, S., Lessin, G., Mahaffey, C., Mayor, D. J., Parker, R., Queirós, A. M., Shutler, J. D., Silva, T., Stahl, H., Tinker, J., Underwood, G. J. C., van der Molen, J., Wakelin, S., Weston, K., and Williamson, P. (2020). Carbon on the Northwest European Shelf: Contemporary Budget and Future Influences. Frontiers in Marine Science, 7, 241, doi: 10.3389/fmars.2020.00143

Roobaert, A., Laruelle, G. G., Landschützer, P., Gruber, N., Chou, L., Regnier, P. (2019). The spatiotemporal dynamics of the sources and sinks of CO2 in the global coastal ocean. Global Biogeochemical Cycles, 33, 1693-1714, doi: 10.1029/2019GB006239