The presence of gas hydrates and free gas in oceanic sediments along the northeastern European Margin is documented in high-frequency near-vertical and wide-angle seismic reflection data. Shallow-water and deep-water gas hydrate instabilities can cause free gas to escape from oceanic sediments. Particularly, methane from shallow-water gas hydrate destabilization may then get transferred from the sediments into the water column, and eventually into the atmosphere. Deep-water gas hydrates are coincident with areas and depths of slope failures in continental margin sediments. Comparisons between seismicity and the potential hydrate distributions suggest a correlation between hydrate instability and margin instabilities along the north-eastern Atlantic Margin.

Reduction of anthropogenic CO2 emissions alone will not sufficiently restrict global warming and enable the 1.5°C goal of the Paris agreement to be met. To effectively counteract climate change, measures to actively remove carbon dioxide from the atmosphere are required. Artificial upwelling has been proposed as one such carbon dioxide removal technique. By fueling primary productivity in the surface ocean with nutrient-rich deep water, it could potentially enhance downward fluxes of particulate organic carbon (POC) and carbon sequestration. In this study we investigated the effect of different intensities of artificial upwelling combined with two upwelling modes (recurring additions vs. one singular addition) on POC export, sinking matter stoichiometry and remineralization depth. We carried out a 39 day-long mesocosm experiment in the subtropical North Atlantic, where we fertilized oligotrophic surface waters with different amounts of deep water. The total nutrient inputs ranged from 1.6 to 11.0 μmol NO3– L–1. We found that on the one hand POC export under artificial upwelling more than doubled, and the molar C:N ratios of sinking organic matter increased from values around Redfield (6.6) to ∼8–13, which is beneficial for potential carbon dioxide removal. On the other hand, sinking matter was remineralized at faster rates and showed lower sinking velocities, which led to shallower remineralization depths. Particle properties were more favorable for deep carbon export in the recurring upwelling mode, while in the singular mode the C:N increase of sinking matter was more pronounced. In both upwelling modes roughly half of the produced organic carbon was retained in the water column until the end of the experiment. This suggests that the plankton communities were still in the process of adjustment, possibly due to the different response times of producers and consumers. There is thus a need for studies with longer experimental durations to quantify the responses of fully adjusted communities. Finally, our results revealed that artificial upwelling affects a variety of sinking particle properties, and that the intensity and mode with which it is applied control the strength of the effects.

<p>To counteract climate change, measures to actively remove carbon dioxide from the atmosphere are required, since the reduction of global CO<sub>2</sub> emissions alone will not suffice to meet the 1.5 °C goal of the Paris agreement. Artificial upwelling in the ocean has been discussed as one such carbon dioxide removal technique, by fueling primary production in the surface ocean with nutrient-rich deep water and thereby potentially enhancing downward fluxes of organic matter and carbon sequestration. In this study we tested the effect of different rates and modes of artificial upwelling on carbon export and its potential attenuation with depth in a five-week mesocosm experiment in the subtropical Northeast Atlantic. We fertilized oligotrophic surface waters with different amounts of deep water in a pulsed (deep water fertilization once at the beginning) and a continuous manner (deep water fertilization every four days) and measured the resulting export flux as well as sinking velocities and respiration rates of sinking particles. Based on this, we applied a simple one-dimensional model to calculate flux attenuation. We found that the export flux more than doubled when fertilizing with deep water, while the C:N ratios of produced organic matter increased from values around Redfield (6.6) to ~8-13. The pulsed form of upwelling resulted in a single export event, while the continuous mode led to a persistently elevated export flux. Particle sinking velocity and remineralization rates were highly variable over time and showed differences between upwelling modes. We stress the importance of experiments with a prolonged application of artificial upwelling and studies including real world open water application to validate the CO<sub>2</sub> sequestration potential of artificial upwelling.</p>

Artificial upwelling brings nutrient-rich deep water to the sun-lit surface to boost fisheries or carbon sequestration. Deep water sources under consideration range widely in inorganic silicon (Si) relative to nitrogen (N). Yet, little is known about how such differences in nutrient composition may influence the effectiveness of the fertilization. Si is essential primarily for diatoms that may increase food web and export efficiency via their large size and ballasting mineral shells, respectively. With a month-long mesocosm study in the subtropical North Atlantic, we tested the biological response to artificial upwelling with varying Si:N ratios (0.07-1.33). Community biomass increased 10-fold across all mesocosms, indicating that basic bloom dynamics were upheld despite the wide range in nutrient composition. Key properties of these blooms, however, were influenced by Si. Photosynthetic capacity and nutrient-use efficiency doubled from Si-poor to Si-rich upwelling, leading to C:N ratios as high as 17, well beyond Redfield. Si-rich upwelling also resulted in 6-fold higher diatom abundance and mineralized Si and a corresponding shift from smaller towards larger phytoplankton. The pronounced change in both plankton quantity (biomass) and quality (C:N ratio, size and mineral ballast) for trophic transfer and export underlines the pivotal role of Si in shaping the response of oligotrophic regions to upwelled nutrients. Our findings indicate a benefit of active Si management during artificial upwelling with the potential to optimize fisheries production and CO2 removal.

Sinking marine aggregates have been studied for a long time to understand their role in carbon sequestration. Traditionally, sinking speed and respiration rates have been treated as independent variables, but two recent papers suggest that there is a connection albeit in contrasting directions. Here we collected recently formed (<2 days old) aggregates from sediment traps mounted underneath mesocosms during two different experiments. The mesocosms were moored off Gran Canaria, Spain (~ 27.9 N; 15.4 E) in a coastal, sub-tropical and oligotrophic ecosystem. We determined the respiration rates of organisms (mainly heterotrophic prokaryotes) attached to aggregates sinking at different velocities. The average respiration rate of fast sinking aggregates (>100 m d-1) was 0.12 d-1 ± 0.08 d-1 (SD). Slower sinking aggregates (<50 m d-1) had on average higher (p <0.001) and more variable respiration rates (average 0.31 d-1 ± 0.16 d-1, SD). There was evidence that slower sinking aggregates had higher porosity than fast sinking aggregates, and we hypothesize that higher porosity increase the settlement area for bacteria and the respiration rate. These findings provide insights into the efficiency of the biological carbon pump and help resolve the apparent discrepancy in the recent studies of the correlation between respiration and sinking speed.