Constraining Global Marine Iron Sources and Ligand-Mediated Scavenging Fluxes With GEOTRACES Dissolved Iron Measurements in an Ocean Biogeochemical Model

Somes, C. J., Dale, A. W., Wallmann, K., Scholz, F., Yao, W., Oschlies, A., Muglia, J., Schmittner, A., & Achterberg, E. P. (2021). Constraining Global Marine Iron Sources and Ligand-Mediated Scavenging Fluxes With GEOTRACES Dissolved Iron Measurements in an Ocean Biogeochemical Model. Global Biogeochemical Cycles, 35(8), e2021GB006948. https://doi.org/10.1029/2021GB006948

Summary

Iron is a key, bio essential micronutrient controlling phytoplankton growth in vast regions of the global ocean. Despite its importance, uncertainties remain high regarding external iron source fluxes and internal marine cycling on a global scale, including removal (scavenging) rates and mechanisms. Iron concentrations in the ocean are affected not only by the source fluxes but also by the presence of ligands, compounds that maintain iron in a dissolved form (more bioavailable) and counteract removal mechanisms (transferring dissolved iron to particulate, less bioavailable form). In this study, the authors used a global dissolved iron (Fe) data set, including GEOTRACES measurements, to constrain source and scavenging fluxes in the marine iron component of a global ocean biogeochemical numerical model. The variable ligand parameterization improved the global model-data misfit the most, suggesting that bacteria are an important source of ligands to the ocean. Further parameterization of atmospheric deposition and release of iron from sediments further improved the model most notably in the surface ocean. High scavenging rates were then required to maintain the iron inventory. The model simulates a tight spatial coupling between source inputs and scavenging rates, which may be too strong due to underrepresented ligands near source inputs, contributing to large uncertainties when constraining individual fluxes with dissolved iron concentrations. Model biases remain high and are discussed to help improve global marine iron cycle models.

The Sensitivity of the Marine Carbonate System to Regional Ocean Alkalinity Enhancement

Burt, D. J., Fröb, F., & Ilyina, T. (2021). The Sensitivity of the Marine Carbonate System to Regional Ocean Alkalinity Enhancement. Frontiers in Climate, 3, 68. https://doi.org/10.3389/fclim.2021.624075

Summary

Ocean Alkalinity Enhancement (OAE) simultaneously counteracts atmospheric concentrations of CO2 and ocean acidification; however, no previous studies have investigated the response of the marine carbonate system response to alkalinity enhancement on regional scales. This is a first modelling study focusing on regional implementations of OAE that can sequester more atmospheric CO2 than a global implementation. The authors revealed that regional alkalinity enhancement has the capacity to exceed carbon uptake by global OAE. Additionally, while the marine carbonate system becomes less sensitive to alkalinity enhancement in all modelled experiments globally, regional responses to enhanced alkalinity vary depending upon the background concentrations of dissolved inorganic carbon and total alkalinity. Furthermore, the Subpolar North Atlantic displays a previously unexpected alkalinity sensitivity increase in response to high total alkalinity concentrations.

Contrasting drivers and trends of ocean acidification in the subarctic Atlantic

Pérez, F. F., Olafsson, J., Ólafsdóttir, S. R., Fontela, M., & Takahashi, T. (2021). Contrasting drivers and trends of ocean acidification in the subarctic Atlantic. Scientific Reports, 11(1), 13991. https://doi.org/10.1038/s41598-021-93324-3

Summary

The processes of warming and acidification (anthropogenic CO2 (Canth) accumulation, decreasing pHT (increasing [H+]T; concentration in total scale) and calcium carbonate saturation) in the subarctic zone of the North Atlantic are unequivocal in the time-series measurements of the Iceland (IS-TS, 1985–2003) and Irminger Sea (IRM-TS, 1983–2013) stations. Both stations show high rates of Canth accumulation with different rates of warming, salinification (water becoming more saline) and stratification (separation of the water column into layers with different densities caused by differences in temperature or salinity or both) linked to regional circulation and dynamics. At the IS-TS, advected and stratified waters of Arctic origin drive a strong increase in [H+]T, in the surface layer, which is nearly halved in the deep layer (44.7 ± 3.6 and 25.5 ± 1.0 pmol kg−1 yr−1, respectively). In contrast, the weak stratification at the IRM-TS allows warming, salinification and Canth uptake to reach the deep layer. The acidification trends are even stronger in the deep layer than in the surface layer (44.2 ± 1.0 pmol kg−1 yr−1 and 32.6 ± 3.4 pmol kg−1 yr−1 of [H+]T, respectively). The driver analysis detects that warming contributes up to 50% to the increase in [H+]T at the IRM-TS but has a small positive effect on calcium carbonate saturation. The Canth increase is the main driver of the observed acidification, but it is partially dampened by the northward transport of water with a relatively low natural CO2 content.

Anthropogenic CO2 and ocean acidification in Argentine Basin Water Masses over almost five decades of observations

Fontela, M., Velo, A., Gilcoto, M., & Pérez, F. F. (2021). Anthropogenic CO2 and ocean acidification in Argentine Basin Water Masses over almost five decades of observations. Science of The Total Environment, 779, 146570. https://doi.org/https://doi.org/10.1016/j.scitotenv.2021.146570

Summary:

This study evaluated chemical data from eleven hydrographic cruises conducted between 1972-2019 in the Argentine Basin, western South Atlantic Ocean. The aim was to quantify natural and human induced stressors in the carbon system. The authors reported an increase of the mean annual atmospheric carbon dioxide concentration (CO2atm) from 325 to 408 ppm of volume (ppm) (64%) in a 47 year time-span covered this study. This increase leads to an increase in anthropogenic carbon (Cant) across all the water column and consequently to ocean acidification (a decrease in excess carbonate), in particular in the upper and intermediate water masses, that in the Argentine Basin region are very sensitive to changes in carbon system. The large rate of intermediate water masses acidification is a combined effect of carbon uptake, deoxygenation, and increased remineralization of organic matter. If CO2 emissions follow the path of business-as-usual emissions (SSP 5.85), the upper water masses would become undersaturated with respect to carbonate ion concentrations at the end of the century. The undersaturation in the intermediate water masses in the region of the Argentine Basin is virtually unavoidable.

 

Policy relevant message:

The upper water masses in the Argentine Basin region will become undersaturated with respect to carbonate ion concentrations at the end of the century if CO2 emissions follow the path of business-as-usual emissions. The undersaturation in the intermediate water masses in the region is virtually unavoidable.

Reconciling the Size-Dependence of Marine Particle Sinking Speed

Cael, B. B., Cavan, E. L., & Britten, G. L. (2021). Reconciling the Size-Dependence of Marine Particle Sinking Speed. Geophysical Research Letters, 48(5), e2020GL091771. https://doi.org/10.1029/2020GL091771

Summary

Sinking particles in the ocean comprise a major flux within the global carbon, nutrient, and oxygen cycles. Particle flux is strongly influenced by sinking speed, which in turn is thought to be strongly influenced by particle size. However, observed variability in particle geometry and composition complicates this size-sinking relationship and has introduced significant uncertainty into ocean flux numerical models. This study revised flux models currently used in earth system model predictions and has implications for the predicted shrinking of marine particles with climate change.