Between 3.5 and 1.5 billion years ago, what happened to iron in Earth's oceans?

Between 3.5 and 1.5 billion years ago, what happened to iron in Earth's oceans?

• A. Iron was removed from the oceans as it oxidized
• B. Iron levels increased due to reduced conditions
• C. Iron dissolved to form metallic iron layers
• D. Iron was depleted by biological uptake only

Answer: (A)

Iron in early oceans was kept in solution as Fe2+ under largely reducing conditions. As oxygen built up in the atmosphere and dissolved oxygen entered the oceans, that Fe2+ was rapidly oxidized to Fe3+. The ferric form is not very soluble in seawater and tends to hydrolyze and precipitate as iron oxides and hydroxides, which then settled to form sediments. This oxidation-driven precipitation removes iron from seawater, so the ocean becomes depleted of dissolved iron during this interval. That shift is also recorded in banded iron formations, which are the geological archives of episodic iron deposition tied to oxidation events. In contrast, conditions becoming more reduced would not drive iron out of solution; metallic iron layers would not form under normal oxidizing ocean chemistry; and while biology can use iron, biological uptake alone cannot explain the large-scale removal recorded in the rocks during that time.

Iron in early oceans was kept in solution as Fe2+ under largely reducing conditions. As oxygen built up in the atmosphere and dissolved oxygen entered the oceans, that Fe2+ was rapidly oxidized to Fe3+. The ferric form is not very soluble in seawater and tends to hydrolyze and precipitate as iron oxides and hydroxides, which then settled to form sediments. This oxidation-driven precipitation removes iron from seawater, so the ocean becomes depleted of dissolved iron during this interval. That shift is also recorded in banded iron formations, which are the geological archives of episodic iron deposition tied to oxidation events.

In contrast, conditions becoming more reduced would not drive iron out of solution; metallic iron layers would not form under normal oxidizing ocean chemistry; and while biology can use iron, biological uptake alone cannot explain the large-scale removal recorded in the rocks during that time.