suppose there's a flow of nutrients of type A from area one to area two. Currently, iron is the chokepoint in nutrient consumption of A, so that A is never completely consumed while going from one to two.

By adding excess iron into area one, or in the middle of the region between this flow from area one to two, you now have the possibility to consume A completely as iron no longer limits A's consumption.

So what happens to area two's consumption of A, if it became more scarce? May be nothing - or may be you now have another choke point of resources that wasn't there previously, leading to a change. If you weren't sure if this change would occur, or dont know, perhaps adding iron to area one is not a good idea, until such outcomes have been studied and acertained first.

  aren't we already doing large scale iron fertilization of the oceans

I'm no ocean ecologist, but it seems like there might be several consequences to adding billions of tons of plankton to the ocean

Carbon capture is for offsetting historical emissions, and the few niche ones that are so hard to replace that we'll need decades of research.

We’re seeing pretty dramatic decreases in emissions across any advanced society that took major regulatory action to decrease emissions. EU emissions in 2023 were 37% lower than in 1990, despite population increases. A good chunk of the early drops were due to fuel substitution, but now increasingly they’re due to the deployment of renewables. In 2024, about 47% of the EU’s total electricity generation for the year came from renewable sources! These are incredible results, especially if you compare them to a decade or two earlier. If you’d told me that in a matter of 15-20 years, the EU’s pro-renewable industrial policy would have achieved these results (say back in the late 2000s) I would have been very surprised. Yet here we are, and the numbers keep jumping every year. So I would not say “regulatory approaches haven’t worked,” I would say “wow, this is one of the most effective regulatory interventions that human beings have ever devised.”

The reason global emissions haven’t dropped is because China’s emissions kept rising during this time. However: now we’re seeing a similar process unfold in China, just at a much larger scale. There the central government is massively subsidizing and encouraging the deployment of renewables. Current estimates are that China’s emissions may have already plateaued and are entering a structural decline. Again, yes, a lot of this is due to technology being available. But the rapid deployment of the tech is a function of strong government intervention.

We are going to reach a point very soon where 90%+ of any modern nation’s energy can be supplied using renewables, nuclear and storage. It’s actually coming very fast. (And that 90% is really only capped by intermittency issues, not because there are major limits on what we can generate.) At that point we’ll be into the very substantial “mop up” phase where we try to work through the remaining high-emissions industries. This will require massive amounts of regulatory intervention, since most of the barriers will be due to the need to replace existing infrastructure on an expedited schedule.