Anoxic marine basins are among the best candidates for deep-sea carbon sequestration, scientists say

Anoxic marine basins could be among the most viable places to carry out large-scale carbon sequestration in the deep ocean, while minimizing negative impacts on marine life. That’s what UC Santa Barbara researchers say in an article published in the journal AGU progress.

As we explore ways to actively reduce carbon levels in the atmosphere, sending plant biomass to these arid, oxygen-free areas of the seafloor becomes an option to consider, they suggest.

“The big picture here is that all the best models we have say we need to achieve some form of net CO2 reduction.₂ elimination in order to meet climate goals,” said geochemist, geobiologist and lead author Morgan Raven, referring to the goal of limiting global warming to 1.5°C above pre-industrial levels, such as ‘established by the International Panel on Climate Change.

There are various ways to store carbon; a promising method is to sink carbon in the form of plant biomass to the seafloor, so that vegetation cannot release CO₂ and methane into the air as it degrades. Ideally, carbon would be stored for hundreds or even thousands of years.

Although this idea is not new, it remains surrounded by a lot of uncertainty. How does the introduction of loads of plant matter affect the chemistry and ecology of the areas where they would be dumped? How can we ensure that decomposition products don’t escape into sensitive habitats, or that carbon doesn’t simply rise up into the water column and be released to the surface anyway? These are just some of the unintended consequences that could further damage already fragile ocean ecosystems or fail to meet carbon sequestration goals.

“And so a lot of this project grew out of the initial question: What is the least bad version of this idea that we can imagine?” said Raven, assistant professor of earth sciences.

Anoxic marine basins emerged as the most likely candidates. Not only are they deep, but they are largely isolated from major oxygen-supplying currents by their geology. They cannot support animal life and are populated primarily by highly specialized microbes and fungi whose metabolism is different from that of creatures living in oxygen-rich environments. Above all, these conditions are ideal for the conservation – essentially the stripping – of plant matter.

Not all anoxic marine basins are the same. The researchers chose three to examine – basins with different properties – to determine where biomass storage could be best achieved: the Black Sea in Eastern Europe, the Cariaco Basin near Venezuela, and the Cariaco Basin near Venezuela. ‘Orca in the Gulf of Mexico (United States).

“The cool thing about the Black Sea is that it’s so small that it’s largely isolated from the rest of the ocean,” Raven said. “And so it has gradually become more and more anoxic, especially recently, since humans dumped a lot of fertilizer on it over the last century.”

They also examined the Cariaco basin, which has the same chemical properties as the Black basin, but is subject to more rapid renewal of its waters. The third site was the “extremely strange” Orca Basin, a hypersaline mini-basin nestled in the continental slope. The salt concentration in the basin is so high that it creates a drastic difference in density compared to the upper waters.

“That interface where the seawater goes from normal seawater to brine, if you try to put a submersible there, you’ll bounce off that layer,” Raven said. The material could hypothetically be enclosed in the hypersaline layer once it passes the interface of the two densities.

Ultimately, given its size and isolation, the Black Sea basin emerged as the best option of the three. With a depth of 2,300 meters (7,500 feet) and an area of ​​322,367 square kilometers (124,467 square miles), this anoxic basin has the capacity to contain biomass at scales relevant to the global climate.

“In reality, the Black Sea is the place to make a dent in the climate,” Raven said. “And its deep waters are so isolated from the rest of the ocean.”

The notion of sinking plant biomass has attracted the attention of private investment, which in recent years has increased the level of funding for projects exploring the possibility of deep-sea carbon sequestration. Several organizations have taken on the challenge of submerging plant matter in the depths of the ocean, collecting biomass from a variety of sources, including cultivated or harvested fast-growing algae such as giant kelp or sargassum, or from terrestrial vegetation such as agricultural or forestry waste.

Each strategy has potential advantages and disadvantages that require further investigation, said Raven, who is a scientific advisor to the companies Seafields (ocean plant biomass) and Carboniferous (terrestrial plant biomass). This study is a step in this direction.

“Given the situation we find ourselves in and the commitments we have made regarding the Paris Agreement and California’s climate goals,” she said, “each year, carbon sequestration strategies become more necessary.