About twenty-five billion tons of sediment are deposited in the ocean each
year, but plate tectonic subduction only removes about one billion tons
per year. Currently, ocean sediment thickness averages 400 meters. At
the observed deposition rate, it would accumulate in only twelve million
years, not the hundreds of millions of years that the oceans have been
around.
Yes, more sediment is deposited in the oceans than is removed by
subduction. However, subduction is not the only fate of sediment
deposited into the oceans. Some sediment deposited on the continental
margin can become part of the continent itself if the sea level falls
or the land is uplifted. Some calcium and organic sediments become
biomass or ultimately dissolve. Some sediment becomes compacted as it
deepens, so its volume is not indicative of the original sediment
volume. Some sediment is "scraped" off of subducting plates and
becomes coastal rocks.
The uniformitarian assumption in the claim is not valid. Tectonics
involves ocean basins forming and spreading, but it also involves them
closing up again (the Wilson cycle). When the basins close, the
sediment in the oceans is piled up on the edges of continents or
returned to the mantle. Much of British Columbia was produced when the
Pacific Ocean closed a few hundred million years ago and land in the
ocean accreted to the continent.
