How about wood?

It grows on trees, and if we bury it under anerobic conditions it won’t rot and release the stored carbon. No need for sealants or preservatives, the wooden piles that the city of Venice was built on are still intact after hundreds of years buried in mud.

Bonus, we can drive them vertically into the earth to stabilize it. That will help us address the landslides that have become more common due to the changing climate.