Question

How do we know sea level and ice volume during previous glaciations?

Short answer

Sea level and ice volume during past glaciations are determined using coral reefs, sediment core oxygen isotope ratios, ice cores, and climate models.

Step-by-step solution

Understanding Sea-Level Proxies

Scientists use proxies, which are indirect measures, to estimate past sea levels. Two common methods are studying coral reefs and analyzing sediment cores. Corals grow at known sea levels, so fossilized corals can indicate past sea levels. Sediment cores contain foraminifera, whose shell isotopic composition reflects past ocean temperatures and ice volumes.

Analyzing Oxygen Isotopes

The ratio of oxygen isotopes in foraminifera shells from sediment cores is crucial. Specifically, the ratio of oxygen-16 to oxygen-18 (O/O) varies with ice volume and sea levels. During glaciations, more O is trapped in ice sheets, leading to higher O/O ratios in seawater, which is recorded by foraminifera.

Using Ice Cores for Ice Volume

Ice cores drilled from glaciers and ice caps contain layers that represent annual snowfall and ice compaction. Gases trapped in these layers, like carbon dioxide and methane, as well as isotopic composition, provide clues about past temperatures and ice volumes. The greater the ice volume, the more pronounced these changes are.

Interpreting Data and Models

Scientists combine data from coral reefs, sediment cores, and ice cores with climate models to understand past sea levels and ice volumes. They compare these models with modern records to ensure accuracy and consistency. This combined approach allows for the reconstruction of glaciation periods and climate changes.

Key concepts

Sea-level proxies

Sea-level proxies are invaluable tools used by scientists to determine past sea levels and ice volumes. Proxies are indirect evidence or markers that help infer details about historical climates.
Two main types of sea-level proxies are coral reefs and sediment cores. Coral reefs are particularly helpful because they grow at specific sea levels, making them natural indicators of past sea positions. By studying fossilized corals, scientists can estimate how high or low the sea was at different times.
On the other hand, sediment cores, which are cylindrical sections extracted from undersea beds, contain microscopic organisms called foraminifera. These tiny creatures build shells using the minerals available from the surrounding water. The isotopic composition of these shells provides clues about past ocean temperatures and, subsequently, about sea levels and ice volumes.

Oxygen isotopes

Oxygen isotopes play a key role in understanding ancient climates. In particular, scientists study the ratio between two isotopes of oxygen: oxygen-16 ( O^{16} ) and oxygen-18 ( O^{18} ). This ratio is important because it changes with variations in global ice volume and sea levels.
During ice ages or periods of extensive glaciation, light oxygen ( O^{16} ) gets trapped in large volumes of ice. Consequently, the ocean becomes enriched in the heavier oxygen ( O^{18} ), leading to a higher O^{18}/O^{16} ratio.
Foraminifera, the microorganisms found in sediment cores, record this isotopic ratio in their shells. By analyzing these ratios, scientists can infer the ice volume and sea level changes over millions of years.

Ice core analysis

Ice cores are cylindrical samples taken from ice sheets and glaciers, revealing layer by layer information about Earth's climate history. These layers essentially act as a frozen timeline, recording yearly snowfall and atmospheric conditions.
When scientists examine ice cores, they look for two crucial types of data: gases trapped in bubbles and the isotopic composition of the ice. The gases, primarily carbon dioxide and methane, reveal information about past atmospheric conditions, including greenhouse gas concentrations.
The isotopic composition of the water in the ice provides insight into past temperatures and ice volumes. Just like with oxygen isotopes in ocean sediments, ice cores can also show changes in O^{16} and O^{18} ratios, allowing scientists to deduce periods of glaciation and temperature changes.

Climate modeling

Climate modeling is an essential tool for reconstructing past climates and predicting future changes. Scientists combine data from sea-level proxies, isotopic analyses, and ice cores with complex mathematical models to simulate climate systems. These models allow researchers to create a virtual world where they can manipulate variables and observe potential impacts on climates.
By integrating proxy data and ice core records into these models, researchers can verify their accuracy. They compare model predictions with known historical data to ensure the model reliably simulates climate processes.
This method gives us invaluable insights into how our planet’s climate has changed over millennia, helping scientists to not only map past glaciations and sea level changes but also to foresee future climate scenarios and prepare for potential impacts.