What happened to all the iron that was in Earth's oceans?

• A. It sank to the core
• B. It combined with oxygen to form rust
• C. It dissolved completely
• D. It formed iron sulfide

Answer: (B)

The main idea tested here is how oxygen changes what happens to dissolved iron in seawater. In ancient oceans, iron existed mainly as Fe2+ in reducing conditions. When oxygen began to accumulate in the atmosphere and dissolved in the oceans, that iron was oxidized to Fe3+ and combined with water to form insoluble iron oxides and hydroxides—essentially rust. This rust precipitated out of the water and settled to the seafloor, removing iron from the ocean and building up iron-rich sediments (the banded iron formations seen in the geologic record).

This fate fits the global history of iron in the oceans better than sinking to the core, dissolving completely, or forming iron sulfide, which would require different conditions. So the iron ends up as rust that precipitates and settles on the seafloor.