This is a good problem to practice with, because this passage doesn't seem to have an explicit cause and effect statement at first glance. You are going to see a lot of questions like this, with a scientist and a hypothesis. They are almost always going to involve cause and effect. Think about the scientific process. The scientist observes a phenomenon (effect) and wants to know why this phenomenon occurs (cause). I'll break down the argument to demonstrate.
Premise 1: Air bubbles from the last ice age contain large amounts of ferrous material and surprisingly small amounts of carbon dioxide. ('Surprisingly' is important, because it suggests that the scientist expected more carbon dioxide.) Therefore, what could cause this CO2 decline? The effect is less CO2. The scientist wants to know the cause.
Premise 2: Algae absorb carbon dioxide. The second premise gives a possible cause. More algae would mean less CO2.
Conclusion: Ferrous material caused more algae to grow. The scientist suggests a link between ferrous material, algae, and CO2. A large amount of ferrous material, caused an increase of algae, which in turn caused a decrease in CO2. This proposed cause and effect chain forms the hypothesis.
