Asked by caleb
Give an example of how fossil evidence can be used to learn about the climate of past times on Earth.
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Example: oxygen isotopes in marine microfossils (foraminifera)
- What is measured: tiny calcium‑carbonate shells of fossil foraminifera recovered from ocean sediment cores are analyzed for the ratio of oxygen isotopes O‑18 to O‑16 (reported as δ18O).
- Why it works: the O‑18/O‑16 ratio incorporated into a shell depends on water temperature and the isotopic composition of seawater. When large amounts of water are locked up in ice sheets, seawater becomes enriched in the heavier O‑18; colder water also produces higher δ18O in shells.
- What we learn: by measuring δ18O through a dated sediment core you can track cycles of colder (glacial) and warmer (interglacial) climates and estimate past ocean temperatures and global ice volume over hundreds of thousands to millions of years.
- Example result: the δ18O records from foraminifera were key to establishing the timing and magnitude of past ice ages (glacial–interglacial cycles).
(Other fossil clues: fossil leaves and stomatal density can indicate past CO2 and climate; finding tropical plant or coral fossils at high latitudes indicates a much warmer past climate.)
- What is measured: tiny calcium‑carbonate shells of fossil foraminifera recovered from ocean sediment cores are analyzed for the ratio of oxygen isotopes O‑18 to O‑16 (reported as δ18O).
- Why it works: the O‑18/O‑16 ratio incorporated into a shell depends on water temperature and the isotopic composition of seawater. When large amounts of water are locked up in ice sheets, seawater becomes enriched in the heavier O‑18; colder water also produces higher δ18O in shells.
- What we learn: by measuring δ18O through a dated sediment core you can track cycles of colder (glacial) and warmer (interglacial) climates and estimate past ocean temperatures and global ice volume over hundreds of thousands to millions of years.
- Example result: the δ18O records from foraminifera were key to establishing the timing and magnitude of past ice ages (glacial–interglacial cycles).
(Other fossil clues: fossil leaves and stomatal density can indicate past CO2 and climate; finding tropical plant or coral fossils at high latitudes indicates a much warmer past climate.)
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