Evidence for Atmosphere Changes

Fossil and Rock Evidence

Sedimentary rocks provide a timeline of Earth's history. These rocks form from layers of sediment deposited over millions of years. By studying these layers, scientists can infer changes in the atmosphere over time.

• Fossilised air bubbles: Some sedimentary rocks, especially ancient volcanic rocks, trap tiny bubbles of ancient air. By analysing the gases in these bubbles, scientists can estimate the composition of the atmosphere at the time the rock formed.
• Changes in rock composition: The presence of certain minerals in rock layers indicates the atmospheric conditions when they formed. For example, iron-rich rocks can show evidence of oxygen levels. When iron reacts with oxygen, it forms iron oxides (rust), so layers rich in iron oxides suggest oxygen was present.

For instance, banded iron formations (layers of iron-rich minerals alternating with silica) formed around 2.5 billion years ago, indicating the rise of oxygen in the atmosphere during the Great Oxygenation Event.

Ice Core Data

Ice cores drilled from polar ice sheets contain trapped gas bubbles that preserve samples of ancient atmosphere. These cores provide detailed records of atmospheric composition and temperature over hundreds of thousands of years.

• Trapped gas bubbles: Air bubbles trapped in ice layers reveal the amounts of gases like carbon dioxide (CO9 ) and methane (CH4 ) in the past atmosphere.
• Historical CO9 levels: By measuring CO9 concentration in ice cores, scientists observe fluctuations linked to natural climate cycles such as ice ages and warm periods.
• Temperature correlations: The ratio of different oxygen isotopes in the ice itself helps estimate past temperatures, allowing comparison between temperature changes and greenhouse gas levels.

For example, ice core data from Antarctica shows CO9 levels rising and falling in cycles over the last 800,000 years, closely matching global temperature changes.

Learning example: If an ice core sample from 100,000 years ago shows a CO9 concentration of 200 ppm (parts per million) and a more recent sample shows 280 ppm, this indicates a significant increase in atmospheric CO9 over that period, linked to natural climate variations.

Tree Rings and Sediments

Tree rings and sediment layers provide indirect evidence of past atmospheric and climate conditions.

• Growth ring variations: Trees add a ring for each year of growth. The thickness of rings varies with climate conditions such as temperature and rainfall, which are influenced by atmospheric changes.
• Sediment deposits: Layers of sediment in lakes and oceans contain pollen, microfossils, and chemical signatures that reflect past climates and atmospheric composition.
• Climate indicators: Changes in tree ring patterns and sediment composition can indicate periods of warming or cooling, droughts, or increased greenhouse gases.

For example, narrow tree rings often indicate colder or drier years, while wider rings suggest warmer, wetter conditions.

Chemical Analysis of Ancient Atmosphere

Chemical evidence from minerals and isotopes helps reconstruct the composition of Earth's early atmosphere.

• Isotopic ratios: Ratios of isotopes (atoms of the same element with different numbers of neutrons) such as oxygen-16 and oxygen-18 in rocks and ice provide clues about past temperatures and atmospheric processes.
• Gas composition changes: Analysis of gases trapped in ancient minerals shows how levels of gases like oxygen, carbon dioxide, and nitrogen have changed over time.
• Evidence from mineral oxidation: The presence of oxidised minerals (like iron oxides) in ancient rocks indicates when oxygen started to accumulate in the atmosphere.

For example, before oxygen was abundant, iron in oceans stayed dissolved in water. When oxygen levels rose, iron oxidised and precipitated out, forming iron-rich rocks.

Learning example: Scientists measure the ratio of oxygen isotopes in a rock sample and find a higher proportion of oxygen-18, which suggests cooler global temperatures at the time the rock formed.