[Accujack]

Mines or potential mining sites for extracting Olivine on this scale don't exist within short distances of these tropical beaches. Many of the beaches are on islands which are essentially coral heads above the water. Digging down to any deposits of other rock are going to require either significant effort in terms of dewatering, caissons, and equipment or else mining underwater.

This is a good idea that's probably not practical. It sounds ok if you accept that that much rock can be mined, processed, and moved within the constraints given, but they're probably not even close with these estimates.

On top of that, you're talking about completely changing the composition of the beaches in much of the tropics, displacing the existing beach materials and probably causing mass die-offs and disruption in the ecosystems attached to them.

Plus, you'll have to keep hauling sand. Beaches in active areas like the tropics aren't static. The sand migrates down them and out into the ocean. Sometimes islands like the Bahamas even have to dredge sand back out of the water to rebuild beaches.

Underwater sand isn't going to absorb much atmospheric CO2.

[dentalnanobot]

Actually I think underwater sand can absorb dissolved CO2 in water - that’s how it’s alkalising effect on oceans works. I think I remember Vesta’s docs saying there are places in the ocean (between the UK and Europe IIRC) with enough current flow that you could just put the Olivine directly on the sea-bed.

[tom_mellior]

> Many of the beaches are on islands which are essentially coral heads above the water.

Afaik most Caribbean islands are of volcanic origin. Barbados is a notable exception, it's a coral island.

[Accujack]

Actually, the caribbean is the area I'm most familiar with. Lots of small islands in the Bahamas chain with sand and coral beaches.

Perhaps islands in deeper water are volcanic, but I still doubt that mining that much olivine anywhere near the place it would be used is possible.