| dc.description.abstract | If climate change is left unchecked it will lead to unprecedented deterioration of human health, economy
and ecology. According to the IPCC, in order to avoid severe consequences, global warming will need to
be limited to 1.5°C. However, the 1.5°C warming will be exceeded if current trends continue, which is
why the need for Carbon Dioxide Removal (CDR) has become increasingly apparent. Ocean afforestation
is currently one of the most promising CDR approaches, with the least competition for space, high carbon
sequestration potential and high technical feasibility. Ocean afforestation approaches attempt to sequester
carbon by sinking seaweed to deep-sea areas. This research looks at the consequences of the seaweed
input to deep-seafloor. An early diagenetic model called RADI is used to predict the fate of the carbon
and the effect on biogeochemistry. The model was adapted to include new sources of sedimentary organic
matter, such as seaweed (Sargassum, Saccharina, Macrocystis) and Sugarcane bagasse, which are currently
considered potential candidates for ocean afforestation purposes.
Sargassum, an invasive free-floating species, has a large sequestration potential and is readily available.
Sinking Sargassum in pulse, large amounts over short times, leads to high carbon retention in the sediment
(up to 25% after two years) but leads to hypoxic conditions in the sediment for at least two years after
addition. Continuous Sargassum sinking also leads to carbon sequestration but with a much less invasive
impact on the seafloor. The carbon from continuous sinking does not remain in the sediment but is
remineralized and flows out to the bottom water as inorganic carbon. Saccharina, an edible coastal species,
could be used to grow on free floating organic buoy. Having the additional sequestration benefit from the
carbon fixed in the organics. Carbon retention is highest for the pulse addition of this seaweed (33% after
two years), compared to a continuous approach (30%) in which the seaweed is added over longer
timescales in small amounts. Since this pulse input also leads to hypoxic conditions in the sediment, the
continuous approach is more favourable for this approach. Macrocystis, the giant kelp known for forming
ecosystems, is a fast-growing coastal species. This species requires harvesting and baling for use in carbon
sequestration. Carbon retention is much higher for pulse addition (30%). Sugar cane bagasse is an
agricultural residue with high carbon content. Sinking this residue to anoxic basins, has been proven to
retain more carbon than in oxygenated bottom waters. This can be confirmed with the results which
showed a carbon retention of up to 50% after two years. The effect on the benthic biome is also less
intense since the low oxygen conditions already necessitate a specialized microbiome. Sugarcane bagasse
is furthermore the only addition capable of increasing bottom water pH. Whereas all seaweed approaches
had higher dissolved inorganic carbon than alkalinity flow to the bottom water, resulting in net
acidification. This research provides a first look into the effects of ocean afforestation on deep sea
biogeochemistry, and illustrates the importance of the composition, quantity and input duration of the
seaweed used. | |
