Salt shakers, egg beaters, and scouring pads--

There seem to be several ways in which iron enters the ocean water.

Salt shakers
Dust that is blown off continents settles on the ocean surface. This dust contains iron and other minerals. This process occurs near continents and the continental shelves. So, picture this process as a giant salt shaker, sprinkling iron-rich dust onto the ocean from Earth’s deserts and other land areas.

Eggbeaters
As phytoplankton are eaten by zooplankton and other marine life, the nutrients within them are absorbed into the ocean food chain. These animals die, decay, and produce waste matter. All these particles sink toward the bottom of the ocean. That doesn’t help the living phytoplankton, which need to be near the ocean surface in order to find the sunlight they need to grow. The nutrients in the deeper water must be stirred up and lifted toward the surface by ocean currents and upwelling. Think of this process as a giant eggbeater, mixing up the water and sending iron upwards into the lighted zone.

Scouring pads
Much of the iron and other nutrients sink to the ocean floor, where they become part of the sediment. Some iron can sink to the bottom out of river runoff and melting ice. Some iron from hot, hydrothermal vents also sinks to the ocean floor. In shallow areas where there are swift currents, the ocean bottom is swept and scoured by the rushing water. Iron can be lifted from the sediment. Think of this process as a giant scouring pad, scraping up iron particles from the ocean floor and sending them up into the water.