The first iron ore deposits in rocks appeared about 1.8 billion years ago. What took place to make this occur?

A. Some prokaryotic organisms began to absorb carbon dioxide from seawater.

B. Some eukaryotic organisms began to absorb carbon dioxide from seawater.

C. Some prokaryotic organisms began to release oxygen into seawater.

D. Some eukaryotic organisms began to release oxygen into seawater.

C

This question was one of the two most criticised by teachers, but even so there were only three comments about it. The question was thought to be too difficult and not based on understanding specified in the program. The statistics for the paper show the expected answer was the commonest chosen by candidates, with 45 % answering correctly and a moderately high discrimination index. The question was based on this Application: Changes to the Earth’s atmosphere, oceans and rock deposition due to photosynthesis. Candidates were expected to know that photosynthesis caused oxygen to appear in the atmosphere and increase in percentage and that oxygen levels in the oceans would also have increased. They were then expected to deduce that this oxygen will have caused dissolved iron to be oxidised and iron oxide to sediment and become part of rocks formed on the ocean floor. This indicates that either answer C or D is correct. Candidates are not expected to know dates for the origins of photosynthetic prokaryotes and eukaryotes, but they are expected to know that prokaryotes evolved first. Prokaryotes therefore must have produced oxygen before eukaryotes and caused the formation of iron ore deposits. Consequently, answer C is correct.