Carbon Sequestration - grow and sink seaweed

I came across a news article about this project just today. It's a fascinating idea.

Short version.

Grow huge HUGE areas of seaweed in the distant ocean. Harvest the seaweed mechanically - pop the airbladders and bale it up. The bales are negatively buoyant with the airbladders popped, so will sink to the deep ocean floor. The environment there is essentially oxygen free, so the carbon will stay trapped on the seafloor for potentially hundreds of year.

Added twists - they have a system designed, but not yet tested at scale, to syphon nutrient rich water from the deeper ocean to feed the seaweed.

Seafields

I see a whole bunch of potential issues in their future, but they also have a lot of huge advantages that other tech-based carbon capture projects do not.  It is comparatively low energy intensity - equipment to bale up the seaweed, infrastructure to setup the floating farms, but then uses biological processes as the driving factor. Photosynthesis is inevitably going to be the cheapest way to capture huge amounts of carbon, and a free floating plant that grows rapidly seems absolutely ideal.

It will be interesting to see where this ends up. They are hoping to start substantial field trials next year.

Hi Michael,
I think that is a really cool project, and I would love to see some more info about it... but I do wonder if it's going to be viable long term. I think of all the energy involved to have people and boats and monitoring equipment and bouys and nets, and mechanical equipment... I'm just wondering if its worth all of that just to sink some carbon into the ocean. I've got to imagine there are ways to trap carbon in the soil that are much less energy intensive.

Major problem: If the bottom of the ocean is not oxygen free beforehand, it certainly will be afterwards. The decomposition of the seaweed will consume any oxygen present, and very efficiently kill off any life that depends on oxygen. This is a huge problem in the Baltic Sea, and I seem to remember that it is in some other places as well. In the Baltic, the reason is eutrophication leading to massive algal growth, followed by the algae sinking to the bottom - pretty much exactly what these people want to do intentionally. The problems with the idea don't stop there, though. An even worse flaw is the notion that no decomposition takes place if there is no oxygen. Well, there is such a thing as anaerobic bacteria, and anaerobic decomposition tends to produce methane. So, if I understand this right, they would effectively take carbon dioxide out of the atmosphere (while expending some fossile fuels for boats and machinery) only to turn it into methane, which is a way more potent greenhouse gas. And also kill off massive tracts of seabed. Sorry, but I think the world would be better off if this never happened.

Eino Kenttä wrote:Major problem: If the bottom of the ocean is not oxygen free beforehand, it certainly will be afterwards. The decomposition of the seaweed will consume any oxygen present, and very efficiently kill off any life that depends on oxygen. This is a huge problem in the Baltic Sea, and I seem to remember that it is in some other places as well. In the Baltic, the reason is eutrophication leading to massive algal growth, followed by the algae sinking to the bottom - pretty much exactly what these people want to do intentionally. The problems with the idea don't stop there, though. An even worse flaw is the notion that no decomposition takes place if there is no oxygen. Well, there is such a thing as anaerobic bacteria, and anaerobic decomposition tends to produce methane. So, if I understand this right, they would effectively take carbon dioxide out of the atmosphere (while expending some fossile fuels for boats and machinery) only to turn it into methane, which is a way more potent greenhouse gas. And also kill off massive tracts of seabed. Sorry, but I think the world would be better off if this never happened.

As far as I can see they are looking at a specific region where the ocean floor is very very deep. These abyssal oceans behave very different than, say, the Baltic. The large distance from land means that these waters tend to have very low nutrient levels to start with, which is why we don't naturally see large amounts of floating seaweed form in these regions. This is a very different situation from the Baltic where the waters are nutrient rich, leading to algal blooms. I don't know much about the Baltic - how much of the excessive nutrient levels are due to human pollution (nitrogen run off from agriculture etc?).

I'm not an expert on deep ocean chemistry, but even if your concerns about anaerobic decomposition are right, methane behaves very differently in deep water. It forms methane hydrates which are stable in the ocean sediment.

I do agree that these are issues to be concerned about, and to monitor. But that is exactly the point of trials.

Matt McSpadden wrote:Hi Michael,
I think that is a really cool project, and I would love to see some more info about it... but I do wonder if it's going to be viable long term. I think of all the energy involved to have people and boats and monitoring equipment and bouys and nets, and mechanical equipment... I'm just wondering if its worth all of that just to sink some carbon into the ocean. I've got to imagine there are ways to trap carbon in the soil that are much less energy intensive.

I don't think it is an either/or situation. Even if we pivot the entire agricultural land to regenerative ag with a carbon sequestration focus, it will be insufficient to reverse the amount of CO2 that has already been released through fossil fuel burning in to the atmosphere. And soil carbon is notoriously fragile. It returns to the atmosphere readily, following soil disturbance. It sounds like they are hoping for much longer term carbon sequestration measured in hundreds of year.

As for the infrastructure required; yes there is an overhead to get setup. But it also sounds like this seaweed is insanely productive and easily grown that there will be a very high return on investment. The economics will be based on the global market for CO2 credits... provided they can sell the carbon credits for more than they cost to produce it will be a viable business. The carbon credit market is well established, they just need to ensure that their business model works. I'm sure that this is already pretty well costed out.

What I like about this project is that it is genuinely a form of carbon sequestration. So many carbon credit schemes are very dubious. People get paid for not chopping down forests... but those forests were not in danger of being felled anyway. Or they sell credits for planting trees... but no one checks on the start of those trees in 5 years time, and most of the planting has failed. By comparison this model is transparent, accountable, effective etc... I can see environmentally concerns businesses choosing to buy from a scheme like this, rather than others, because it is demonstrably sinking thousands of tonnes of carbon that would otherwise not have been sunk.

I'd like to see more of what Bren Smith is doing, near-shore seaweed and shellfish farming, for human food and also for soil amendments. The eco-system benefits are huge, fish habitat, storm surge mitigation...
The rate that we are losing topsoil, mining nutrients such as phosphorus, and feeding ever more humans and our livestock; it seems short-sighted to dump seaweed into the deepest part of the sea.

I don't have any science to back it up, but I think if you took the same amount of space on land, as would be used in the ocean, and planted trees or ran some sort of cattle in a rotation (or better yet both), I have a feeling you would sequester just as much carbon.

You don't have to get as many people on board to do something that scale in the ocean, as on land, and it is certainly easier to disturb the carbon on land than it is at the bottom of the ocean. I still say it would be easier on land :)

Michael Cox wrote:
As far as I can see they are looking at a specific region where the ocean floor is very very deep. These abyssal oceans behave very different than, say, the Baltic. The large distance from land means that these waters tend to have very low nutrient levels to start with, which is why we don't naturally see large amounts of floating seaweed form in these regions. This is a very different situation from the Baltic where the waters are nutrient rich, leading to algal blooms. I don't know much about the Baltic - how much of the excessive nutrient levels are due to human pollution (nitrogen run off from agriculture etc?).

I'm not an expert on deep ocean chemistry, but even if your concerns about anaerobic decomposition are right, methane behaves very differently in deep water. It forms methane hydrates which are stable in the ocean sediment.

I do agree that these are issues to be concerned about, and to monitor. But that is exactly the point of trials.

Oh, right, forgot about the clathrate formation. My bad.
However, I'm still not convinced this is a good idea. From what I understand, any oxygen present would still be consumed, and there are fish and other animals that need oxygen even on those depths.
Regarding the Baltic, basically all the eutrophication comes from nutrient runoff from agriculture and forestry, but the immediate problem is large amounts of organic matter sinking into water with low oxygen levels, and depleting the oxygen. I can't see that this would be any different in deep ocean. The only factor that's fundamentally different would be the pressure, and I think there are aerobic bacteria that tolerate high pressures. If there are fish, it would be weird if there were no bacteria. The temperature is probably not that much different, since the density of water is highest at 4 degrees Celsius, so...

Matt McSpadden wrote:I don't have any science to back it up, but I think if you took the same amount of space on land, as would be used in the ocean, and planted trees or ran some sort of cattle in a rotation (or better yet both), I have a feeling you would sequester just as much carbon.

You don't have to get as many people on board to do something that scale in the ocean, as on land, and it is certainly easier to disturb the carbon on land than it is at the bottom of the ocean. I still say it would be easier on land :)

The sargassum they intend to use has a potential doubling time of 9 to 20 days. With the technology they have bringing up nutrients from deep in the ocean using a syphon - no energy input needed once it is up and running - the productivity of these farms will be utterly bonkers. It will far outstrip the productivity of any attempt at land based CO2 sequestration... and has the added convenience that you can sink bales in situ to the seafloor, with no need to do further processing.

The technical difficulties that I foresee are less to do with the growing, baling, and sinking process... it will be more to do with durability of the equipment on high seas, in potentially extreme storms. Again, surmountable problems, and worth overcoming given the massive potential.

Eino Kenttä wrote:
However, I'm still not convinced this is a good idea. From what I understand, any oxygen present would still be consumed, and there are fish and other animals that need oxygen even on those depths.
Regarding the Baltic, basically all the eutrophication comes from nutrient runoff from agriculture and forestry, but the immediate problem is large amounts of organic matter sinking into water with low oxygen levels, and depleting the oxygen. I can't see that this would be any different in deep ocean.

I did some digging. Found a study from the 90s that looked at carbon sequestration and oxygen levels on the ocean floor.

Apparently the ocean floor goes through variable cycles of oxygen deficit. When in deficit, fish and other large animal move away. Small animals - worms, shellfish etc... die off. But when the area in utterly anoxic, the material that deposits essentially stops breaking down. These variable cycles account for periods of high carbon deposition in the marine sediments. So it looks like, at worst, this will exaggerate an already existing phenomenon. And i presume that the intention is that these bales will be rapidly buried, and so end up separated from any oxygen in the ocean water. I guess that simply stopping dropping bales in an area will rather rapidly allow the situation to return to normal on the ocean floor.

And then at the surface you get an absolute abundance of life. These floating seaweed mats will be oases in the ocean - shelter, nutrients, habitat etc...

        I think all the human effort required in that endeavor might be better used to bolster the population of whales, who are the true masters of carbon sequestration at sea. When they die naturally, at least, their tons and tons of mass either feed other creatures or sink to those anaerobic depths.

       I've long been fascinated by seaweed farming, both for food and ecological remediation. Wonder whether it would make sense for the bales of seaweed, grown as in OP, to be brought inland to arid places, in a sort of reverse-desertification effort (there must be better words!)? Of course the initial composting would release carbon, but if the result turned sand to soil... It must be far more practical to use biomass from terrestrial sources, but the post made me ponder. Thanks for that!

C. Lee Greentree wrote:         I think all the human effort required in that endeavor might be better used to bolster the population of whales, who are the true masters of carbon sequestration at sea. When they die naturally, at least, their tons and tons of mass either feed other creatures or sink to those anaerobic depths.
!

I’m not disagreeing with you about whale corpses significance, but the scale of the operations planned here utterly dwarfs anything you might gain from improved Whale populations.

They are talking about circular “farm modules” 15km in diameter, each sinking hundreds of tonnes of carbon per day. Each module is a floating barrier to contain the weed, and then boats to harvest and make bales.

I’m not discounting the idea, we need to look at everything, but there is a lot of money flying around right now for anything that has the remote possibility of sequestering carbon, thus even marginal ideas will get funding.  The last thing I want to see is a “solution” that ends up being more of a problem than a solution.  There will be lots of ideas that technically work, but just don’t scale up well.
I’ve heard of other strategies that entail sinking logs to the bottom of the ocean.

Kenneth Elwell wrote:I'd like to see more of what Bren Smith is doing, near-shore seaweed and shellfish farming, for human food and also for soil amendments. The eco-system benefits are huge, fish habitat, storm surge mitigation...

I'd even be happy a little further out, so long as they focus on the "dead zones" at the mouths of many North American rivers caused by nitrogen runoff. More artificial wetlands along said river shores, coupled with seaweed farming further out, may not sequester as much as the Seafields proposal claims it will, but I agree with the concern that those plants won't contain "just carbon" and the book I'm reading is very telling in its concern about phosphorus levels in the soil which are much slower to recover than nitrogen is.

As for whales, I don't know if this lovely video will work, but I'll try...


edited to find a version of the video that would embed...

Michael Cox wrote:
I did some digging. Found a study from the 90s that looked at carbon sequestration and oxygen levels on the ocean floor.

Apparently the ocean floor goes through variable cycles of oxygen deficit. When in deficit, fish and other large animal move away. Small animals - worms, shellfish etc... die off. But when the area in utterly anoxic, the material that deposits essentially stops breaking down. These variable cycles account for periods of high carbon deposition in the marine sediments. So it looks like, at worst, this will exaggerate an already existing phenomenon. And i presume that the intention is that these bales will be rapidly buried, and so end up separated from any oxygen in the ocean water. I guess that simply stopping dropping bales in an area will rather rapidly allow the situation to return to normal on the ocean floor.

Interesting! Do you have a link to that study, by any chance? What time-scale are we talking about for these cycles?
Wonder how rapidly the bales can be buried, given that the sedimentation that far from land (correct me if I'm wrong) is probably not super fast. But I suppose if the entire farm is floating, you could just move it continuously, so you never have more than one layer of bales in any given place.
Still a bit concerned about oxygen depletion, but I suppose part of the reason it's such a massive issue in the Baltic is that it's essentially landlocked. No new, oxygenated water coming in from elsewhere.
A  thought occurs to me though: If you have efficient means of pumping nutrient-rich water from the bottom to the surface, an alternative route would be to grow plankton with a carbonate shell. This would not need the boats or harvesting machinery, as the plankton sinks by itself as it dies, and the carbon being locked up as carbonate rather than carbohydrates would sidestep the oxygen depletion issue.

Aside from the technical and ecological questions, I'm a bit leery of, once again, using the oceans as humankind's toilet. Got something you want to get rid of? Just chuck it into the deep ocean and it's gone!