Comment by dredmorbius

This is a mechanism by which some oil deposits are thought to have formed, and by which a large quantity of biospheric carbon was sequestered during earlier warm spells, refered to as the Eocene Azolla Event.

Essentially: arctic seas formed fresh-water "lenses" through meltwater, which promoted plant growth (in particular azolla, though likely also algae and plankton). This growth then sank to the sea-floor, depositing as oils (and much ultimately undergoing keroginisation to form petroleum).

<https://en.wikipedia.org/wiki/Azolla_event>

Similar mechanisms have been proposed for addressing carbon sequestration goals in the present, e.g., "CO2 sequestration by propagation of the fast-growing Azolla spp. " <https://pmc.ncbi.nlm.nih.gov/articles/PMC8520330/>.