How Oxygen Increased and How Carbon Dioxide Decreased

PAPER 2 • SPEC 4.9.1.3, 4.9.1.4

How Oxygen Increased & Carbon Dioxide Decreased

From a CO₂-rich early atmosphere to today's ~80%/20% mix. Photosynthesis, oceans, sedimentary rocks, and fossil fuels — the four ways carbon got removed.

Why this lesson matters

Lesson 9.01 ended with an atmosphere of mostly CO₂, water vapour, and very little oxygen. This lesson is the bridge to today's atmosphere — and it's the one place AQA expects you to know multiple parallel mechanisms.

What you'll be able to do

State when algae first produced oxygen and what happened next.

Write the photosynthesis equation and use it to explain BOTH the rise of oxygen and the fall of carbon dioxide.

Give multiple, distinct mechanisms for how CO₂ decreased.

Describe the formation of fossil fuels — particularly natural gas — using the precise mark-scheme phrasing.

Compare the rates of change of O₂ and CO₂ on a graph (the Q06.3 trap).

Why this is high-yield

Module 9 is reliably tested on Paper 2. The Jun 2022 paper alone had a 10-mark question entirely on this material — and on Q06.5 (natural gas formation), fewer than 1 in 20 students scored all 3 marks. That's where this lesson focuses.

Where We Left Off (Recap from 9.01)

By the time the oceans had formed, here's what the atmosphere looked like:

Atmosphere ~3.5 billion years ago

Mainly CO₂ (though already falling thanks to ocean dissolution)

Little or no O₂

Some N₂ building up from volcanic activity

Water vapour (most had condensed into oceans)

Small amounts of methane and ammonia

The two big questions for this lesson

1. How did oxygen get to ~20% of the atmosphere?

2. How did carbon dioxide drop from "mainly CO₂" to today's tiny ~0.04%?

2.7 Billion Years Ago — Algae Evolved

This is the key date the spec wants you to know. Everything about today's atmosphere starts here.

From the spec, 4.9.1.3 (verbatim)

"Algae first produced oxygen about 2.7 billion years ago and soon after this oxygen appeared in the atmosphere. Over the next billion years plants evolved and the percentage of oxygen gradually increased to a level that enabled animals to evolve."

The sequence to memorise

~2.7 billion years ago: algae evolve, start photosynthesising

Soon after: oxygen begins to appear in the atmosphere

Over the next ~1 billion years: plants evolve and spread

Eventually: O₂ reaches a level high enough to support animal life

Order matters

Algae came first. Plants came later. Saying "plants made the oxygen" misses the spec sequence. The spec word is "algae and plants" — algae are not optional.

The Photosynthesis Equation

One equation does both jobs at once: it explains why O₂ went up and why CO₂ went down. The spec calls this out specifically.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

(requires light energy, absorbed by chlorophyll)

What this equation tells you

Reactants (CO₂ + H₂O): CO₂ is removed from the atmosphere.

Products (glucose + O₂): oxygen is released into the atmosphere.

Net effect: photosynthesis simultaneously removes CO₂ and releases O₂.

Why this matters for Q06.3

Because both the rise of O₂ and the fall of CO₂ are driven by the same single process, their rates of change are similar. That's the gradient comparison AQA tested in P2 Jun22.

Photosynthesis — Visual

Algae and plants act like a conveyor belt: CO₂ goes in, O₂ comes out.

The O ₂ Build-Up Timeline

Why the spec stresses "gradually increased": this didn't happen overnight. Oxygen built up over more than a billion years.

Memory hook

2.7 → algae → photosynthesis. If you remember just one date for this whole lesson, make it 2.7 billion years ago.

How CO₂ Decreased — Four Mechanisms

This is the slide AQA tests most. The spec lists multiple causes — and Q06.4-style questions reward giving distinct mechanisms.

Photosynthesis

Algae and plants use CO₂ to make glucose, releasing O₂.

Dissolution in oceans

CO₂ dissolves in seawater (continued from 9.01).

Sedimentary rocks

Marine organisms use dissolved CO₂ to build calcium carbonate shells. Shells settle, form rocks like limestone.

Fossil fuels

Carbon from plants and marine organisms gets buried, locked underground as coal, crude oil, and natural gas.

Examiner Alert

When a question asks for "two reasons" CO₂ decreased, you must give two distinct mechanisms. "Photosynthesis happened in algae and then in plants" is ONE mechanism, not two. Pick from the four above.

Sedimentary Rocks — Marine Shells Lock Up Carbon

This is the mechanism most students miss. It's how limestone formed — and it's a permanent CO₂ store.

The four-step process

1. CO₂ dissolved in the oceans.

2. Marine organisms (e.g. plankton, shellfish, corals) used the dissolved CO₂ to build calcium carbonate (CaCO₃) shells.

3. When organisms died, their shells sank and built up as sediment on the seabed.

4. Over millions of years, this sediment compacted into sedimentary rock — limestone.

Why this counts as "decreasing CO₂"

Carbon that was once CO₂ in the air is now locked up as solid CaCO₃ in rocks. It's removed from the atmosphere and stored on a geological timescale.

Fossil Fuels — Carbon Buried Underground

Spec 4.9.1.4 explicitly names the deposits you must be able to describe: limestone, coal, crude oil, and natural gas. Three of these are fossil fuels.

The two formation pathways

Fossil fuelFormed fromWhereCoalDead plants (mostly land plants)Buried in swamps, then undergroundCrude oil & natural gasDead plankton (tiny marine organisms)Buried in marine sediments

P2 Jun22 Q06.5 — Examiner Report

"Some students were insufficiently precise about the size of the organisms involved in natural gas formation, and there was some confusion with the formation of coal."

Natural Gas Formation — The 3-Mark Recipe

The Jun 2022 examiner report says fewer than 1 in 20 students got all 3 marks on this. Here's the precise mark-scheme phrasing.

Mark scheme — three marks available

Plankton (died) — accept "tiny marine organisms"

Covered by sediments — accept "buried", or "in anaerobic conditions"

Subjected to high temperature AND high pressure — over millions of years

Three traps that lose the mark

"Heat" instead of "high temperature" — examiner report says this lost the third mark for many students.

"Trapped under rocks" — examiner report explicitly rejects this. It describes the outcome, not the process. Use "covered by sediments".

"Plants" for natural gas — wrong. Plants form coal. Plankton (marine) form natural gas and crude oil.

Coal Formation — Land Plants

Coal has a different story to crude oil and gas. Different organisms, different setting.

Coal — three steps

Land plants died in swampy regions (e.g. ancient forests).

Buried under sediment, in oxygen-poor (anaerobic) conditions, so they didn't rot away completely.

Compressed under high pressure and high temperature over millions of years, slowly turning into coal.

Quick comparison

Coal: plants → swamps → underground.

Crude oil & natural gas: plankton → seabed → buried.

The mechanism (anaerobic + high P + high T over millions of years) is the same — the organisms and setting differ.

Limestone — Not a Fossil Fuel

Limestone is in the same spec list as coal, oil, and gas — but it's the only one that isn't a fossil fuel. It still locks up carbon, just in a different way.

Limestone formation

Marine organisms (corals, shellfish, plankton) extracted dissolved CO₂ from seawater to build calcium carbonate shells.

When they died, their shells accumulated on the seabed.

Over millions of years, the shells were compressed into sedimentary rock — limestone.

Limestone vs fossil fuels

Limestone stores carbon as CaCO₃ (a salt). It's not burned for energy.

Fossil fuels store carbon in hydrocarbons. Burning them releases that carbon back as CO₂.

Comparing Rates — The Q06.3 Skill

If you're shown a graph of how O₂ rose and CO₂ fell over time, AQA wants you to spot one specific connection.

Mark scheme phrasing (P2 Jun22 Q06.3)

"the percentage of carbon dioxide decreased and the percentage of oxygen increased"

"the increase and decrease (in percentage) occur at the same / similar rate"

Why the rates are similar

Look at the photosynthesis equation again: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Six molecules of CO₂ are removed for every six molecules of O₂ produced. Same single process, opposite directions — so the gradients of the two curves on a graph mirror each other.

P2 Jun22 Q06.3 — Examiner Report

"Having mentioned carbon dioxide in 06.2, some students only felt the need to refer to an increase in oxygen, and omitted any reference to a further fall in carbon dioxide. The idea that the rates of change were the same or similar was much less often seen."

Mark Scheme Language to Use

These phrases are taken verbatim from P2 Jun22 mark schemes — they're the wording AQA accepts for full credit.

If asked about...Mark-scoring phrasing

How O₂ increased"algae / plants evolved" + "photosynthesis produced oxygen" — accept a description of photosynthesis

What process caused changes in C (Q06.4)"photosynthesis" — single word answer for 1 mark

Comparing O₂ & CO₂ rates (Q06.3)"CO₂ decreased AND O₂ increased" + "same / similar rate"

Natural gas formation (Q06.5)"plankton (died)" + "covered by sediments" + "high temperature AND high pressure"

Two reasons CO₂ decreasedAny TWO of: photosynthesis / dissolved in oceans / shell formation (carbonate sediments) / fossil fuel formation

Common Errors to Avoid

Wrong / weak answerWhy it loses marksCorrect version

"Plants made all the oxygen."Algae came first. Spec says "algae and plants"."Algae first produced oxygen ~2.7 billion years ago; plants evolved later."

"CO₂ decreased because of photosynthesis." (when asked for two reasons)One mechanism, not two.Two distinct: e.g. "photosynthesis used CO₂" + "CO₂ dissolved in oceans / formed sedimentary rocks".

"Natural gas formed from plants under heat."Plants make coal, not gas. "Heat" is not "high temperature"."Plankton died, were covered by sediments, and subjected to high temperature and high pressure."

"O₂ went up." (when asked to compare with CO₂)Missed the second mark — needs CO₂ mention AND rate."CO₂ decreased and O₂ increased at the same / similar rate."

Practice Question 1

Explain how the percentage of oxygen in Earth's atmosphere increased to a level that allowed animals to evolve.3 MARKS

Mark 1

Algae evolved about 2.7 billion years ago and started producing oxygen.

Mark 2

They produced oxygen by photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. (A description of photosynthesis is also accepted.)

Mark 3

Over the next billion years, plants evolved and the percentage of oxygen gradually increased until it was high enough to support animal life.

Saying just "plants made oxygen" misses the algae step and won't get full marks. The spec uses "algae and plants" as a unit — both must appear.

Practice Question 2

Carbon dioxide levels in the atmosphere fell sharply after life evolved on Earth.

Give two different processes that decreased the amount of carbon dioxide in the atmosphere.2 MARKS

Any TWO distinct mechanisms score the marks. Example pair:

Mark 1

Photosynthesis — algae and plants used carbon dioxide to make glucose, removing CO₂ from the atmosphere.

Mark 2

Carbon dioxide dissolved in the oceans and was used by marine organisms to make calcium carbonate shells, which formed sedimentary rocks (e.g. limestone).

Photosynthesis + fossil fuel formation

Dissolution in oceans + sedimentary rock formation

Photosynthesis + sedimentary rock (shells of marine organisms)

"Photosynthesis by algae" and "photosynthesis by plants" count as one mechanism, not two. Pick mechanisms with different chemistry.

Practice Question 3

Crude oil is a fossil fuel.

Describe how deposits of crude oil were formed.3 MARKS

Mark 1

Plankton (tiny marine organisms) died.

Mark 2

Their remains were covered by sediments (also accepted: "buried" or "in anaerobic conditions").

Mark 3

They were subjected to high temperature AND high pressure over millions of years.

"Plants" — wrong for crude oil. Plants make coal. Plankton make crude oil and natural gas.

"Heat" — examiner report rejects this; needs "high temperature".

"Trapped under rocks" — examiner report explicitly rejects this; use "covered by sediments".

Inspired by P2 Jun22 Q06.5 (which asked about natural gas). Mark scheme phrasing taken verbatim from AQA.

Lesson Summary

How Oxygen Increased

~2.7 billion years ago: algae evolved, began photosynthesising

Soon after: oxygen appeared in the atmosphere

Over the next ~1 billion years: plants evolved

Oxygen gradually increased to a level that allowed animals to evolve

Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

How CO₂ Decreased — Four Mechanisms

1

Photosynthesis — algae and plants used CO₂ for glucose

2

Dissolution — CO₂ dissolved in oceans

3

Sedimentary rocks — marine shells (CaCO₃) → limestone

4

Fossil fuels — coal (plants), crude oil & natural gas (plankton)

Fossil Fuel Formation Recipe

Coal: dead plants + sediment burial + high T + high P

Crude oil & natural gas: dead plankton + sediment burial + high T + high P + (anaerobic conditions)

"Heat" ≠ "high temperature"; "trapped under rocks" is explicitly rejected

Three Examiner Traps

Plants vs algae: always credit algae first.

Two reasons ≠ one: distinct mechanisms only.

Rate comparison: O₂ rises and CO₂ falls at the same / similar rate — single photosynthesis equation drives both.

Up Next: Lesson 9.03

Greenhouse Gases & Human Activities — three named greenhouse gases, the IR-trapping mechanism, and how human activities have reversed the long CO₂ downtrend that this lesson described.