By Sid Perkins
Pesticides, organic carbon, and other nutrients may be reaching the seafloor near continents via an unexpected route. They’re carried along the ocean bottom by dense, sediment-rich plumes of fresh water dumped into the sea by rivers, says a team of scientists.
Seawater typically is about 2.5 percent more dense than fresh water, so a river’s flow usually spills across the top of the ocean as it travels away from land. Normally, the river waters mix with the ocean to form a brackish blend, and any sediments carried by the flow settle out in shallow water to form the river’s delta. However, scientists studying California’s Monterey Bay have recorded five instances in the past dozen years when mud-choked swirls of unusually warm river water swept along the bottom of the bay at depths of nearly 1 kilometer.