What are the spherical deposits found on the Arctic floor called?

Answer 1

Those are manganese (Mn) nodules not magnesium (Mg) as suggested by the previous answer.

The mining proposal was a big hoax designed by the CIA to cover up their recovery of a top secret sunken Russian sub in the middle of the Pacific. Mn is quite plentiful on land and not worth enough to make it economically recoverable from the sea floor.

Answer 2

Those are magnesium nodules and they're not unique to the Arctic floor. In fact, they're more common in proximity to the seafloor spreading regions. Several countries had plans to mine the nodules in the 70's but then dropped it because of international law.