Week 3 videos: Oxygen Isotopes, Proxies, and ENSO: Comprehensive Study Notes

Isotope basics

• Isotopes are different versions of the same element. They have the same number of protons (which defines the element) but a different number of neutrons. Neutrons add weight to an atom.

• We use a special way to write them: mass number A = protons + neutrons. For oxygen, there are always 8 protons:

  • ${}^{16}\text{O}$ has 8 protons and 8 neutrons (total weight 16).

  • ${}^{17}\text{O}$ has 8 protons and 9 neutrons (total weight 17).

  • ${}^{18}\text{O}$ has 8 protons and 10 neutrons (total weight 18).

• How common are they? ${}^{16}\text{O}$ is the most common kind of oxygen, making up about 99.8% of all oxygen on Earth. ${}^{18}\text{O}$ is also common and useful for studying Earth's past. ${}^{17}\text{O}$ is very rare and harder to study.

• When scientists study isotopes for environmental changes, they always look at the ratio of heavy to light isotopes, meaning more neutrons on top, fewer neutrons on the bottom. Never the other way around!

• Because isotopes have different weights (due to neutrons), they act slightly differently in nature. Lighter isotopes (like ${}^{16}\text{O}$) tend to be more easily separated or changed in natural processes than heavier ones.

• Think of it like this: Hydrogen (${}^{1}\text{H}$) has one proton and no neutrons. Deuterium (${}^{2}\text{H}$) has one proton and one neutron. Adding that one neutron makes deuterium twice as heavy as hydrogen, which changes how it behaves. For oxygen, ${}^{16}\text{O}$ has 8 protons and 8 neutrons, while ${}^{18}\text{O}$ has 8 protons and 10 neutrons (two extra neutrons).

• Why care about heavy over light? The different weights (because of different neutron numbers) affect how isotopes separate or 'fractionate' in natural processes, which gives us clues about Earth's past.

Oxygen isotopes and fractionation fundamentals

• We mainly focus on two stable oxygen isotopes for climate studies: ${}^{18}\text{O}$ (the heavier one) and ${}^{16}\text{O}$ (the lighter one).

• Isotopic fractionation is like a sorting process. During natural changes, like water evaporating or freezing, isotopes get separated. This changes the amount of each isotope in different places, like oceans, atmosphere, clouds, and rain.

• Evaporation: When water evaporates from the ocean, the lighter ${}^{16}\text{O}$ water evaporates more easily. This makes the water vapor in the air (which then forms clouds) richer in ${}^{16}\text{O}$ and leaves the ocean water with relatively more ${}^{18}\text{O}$. They're separated!

• Precipitation: When those clouds make rain, the heavier ${}^{18}\text{O}$ tends to fall first. So, the rain will have more ${}^{18}\text{O}$ compared to ${}^{16}\text{O}$.

• Basically, what happens in the atmosphere (vapor getting lighter) is the opposite of what stays in the ocean (water getting heavier). This separation helps us understand past temperatures and water cycles when we find these isotope patterns in old rocks, ice, or shells.

• Fractionation simply means that one isotope becomes more concentrated (or 'enriched') than another due to physical processes. These clues can tell us about past temperature and water conditions if they are preserved.

• We often talk about the heavy-to-light ratio for isotopes, remembering that heavier ones have more neutrons.

Climate controls on oxygen isotopes: temperature and elevation signals

• There are two main rules about how the ${}^{18}\text{O}$/$ ${}^{16}\text{O}$ ratio in water changes with climate:

  • Atmosphere rule: When it gets warmer, the air's moisture tends to have more ${}^{18}\text{O}$ (we say its δ${}^{18}\text{O}$ value goes up).

  • Ocean rule: When it gets warmer, the ocean water tends to have less ${}^{18}\text{O}$ (its δ${}^{18}\text{O}$ value goes down) compared to colder times.

• What this means for oceans and rain:

  • In a warm climate, more ${}^{18}\text{O}$ evaporates from the ocean into the air, making the air's moisture heavier in ${}^{18}\text{O}$, while the ocean water becomes lighter.

  • In a cold climate, evaporation mostly picks up ${}^{16}\text{O}$, leaving more ${}^{18}\text{O}$ in the ocean. The air's moisture then has less ${}^{18}\text{O}$ (it's lighter).

• Elevation effect (how high you are): As air masses rise over mountains, the heavier water vapor (${}^{18}\text{O}$) tends to condense and fall out as rain first. So, by the time clouds reach high mountain tops, they've lost a lot of their ${}^{18}\text{O}$. This means rain and surface water in high mountains will have very little ${}^{18}\text{O}$ (a 'lighter' isotope signal). So, higher places usually mean lower δ${}^{18}\text{O}$ values in precipitation.

• In short, temperature and elevation control these ${}^{18}\text{O}$/$ ${}^{16}\text{O}$ ratios, allowing scientists to figure out what past climates were like.

• Notation reference: We use the term δ${}^{18}\text{O}$ to show how the heavy-to-light isotope ratio in a sample compares to a standard reference.

How oxygen isotopes are used to study past environments

• We can find clues about past oxygen isotope signals in many places:

  • Skeletons: Many marine and freshwater organisms build their skeletons from calcium carbonate ($\text{CaCO}_3$). The isotopes in their skeletons reflect the water's isotope composition when they were alive. These 'skeleton producers' include:

    • Sponges and corals

    • Foraminifera (tiny sea organisms)

    • Mollusks (snails and clams)

    • Coccolithophores (microscopic algae)

  • These skeletons are usually made of minerals called aragonite or calcite.

• Foraminifera skeletons are especially useful because they can preserve water signatures for millions of years (up to 66 million years or more), helping us track long-term climate changes.

• Sometimes, we can learn from the shape of organisms, not just their chemistry:

  • The way Foraminifera shells coil can tell us about past water temperatures.

  • Mollusk shells grow in layers, like tree rings, which can show climate changes season by season or year by year.

• Besides skeletons, other materials also preserve isotope clues:

  • Soil carbon nodules: Lumps of calcium carbonate that form in soil can also hold carbon and oxygen isotope signals, giving insights into past temperatures and water sources.

• Other materials in the natural record:

  • Apatite: A mineral found in teeth and bones can also store oxygen isotope information.

  • Fossil ice and snow layers (ice cores): These layers directly capture water isotopes from past precipitation and temperatures. Ice cores also trap ancient air bubbles, letting us study the history of Earth's atmosphere.

  • Amber: Tree resin that hardens into amber can also trap tiny air bubbles, similar to ice cores, giving us clues about ancient air.

• Layers: Layers in sediments and ice cores work like a timeline, helping us match isotope data to specific seasons or years.

• We also use other clues (proxies) to get a full picture:

  • Palynology: Studying fossil pollen and tiny algae tells us about past plants and climates.

  • Dendrochronology: Tree rings not only help date things but also show atmospheric carbon signals from when the tree grew. We can check this with carbon isotopes in the rings.

  • Leaves and plant shapes: The shape of ancient leaves and plants gives clues about climate, especially when combined with carbon isotopes.

  • By combining many clues (like oxygen, hydrogen, and carbon isotopes; shell shapes; strontium and sulfur isotopes), we can build a strong picture of ancient temperatures, rain patterns, and other environmental conditions.

• The main goal is to reconstruct past environments by bringing together all these different pieces of evidence and using what we know about today's climate to understand signals from the past.

Hydrogen isotopes and additional proxies

• Besides oxygen isotopes, hydrogen isotopes are also very useful for understanding past climates.

• The two main hydrogen isotopes are protium (${}^{1}\text{H}$) and deuterium (${}^{2}\text{H}$ often called D). The ratio of ${}^{2}\text{H}$/$ ${}^{1}\text{H}$ in water is a key clue for understanding temperature and how water moves in weather systems and past climate records.

Summary of the multi-proxy approach and relevance

• Scientists use a variety of clues, not just one, to understand climate: oxygen and hydrogen isotopes, carbon isotopes, strontium and sulfur isotopes, shell shapes, layers in shells, pollen, air in amber, ice cores, and tree rings.

• Understanding how things work now helps us interpret ancient records, showing how Earth's climate has changed over time and how different natural processes affect isotopes.

• This approach, combining all these different clues, allows researchers to figure out past temperatures, rain patterns, how high land was, where water came from, and even what seasons were like over vast periods of Earth's history.

El Niño and La Niña: climate patterns in the Pacific

• El Niño is a major climate pattern in the Pacific Ocean that affects weather all over the world. It's not just a storm.

• Where the name comes from: South American fishermen noticed warmer ocean waters in the Pacific around December, which often brought more fish. They called it El Niño (Spanish for "the Christ Child") because it happened around Christmas time.

• How often it happens and why: El Niño usually occurs every 2 to 7 years. The exact reasons it starts are complicated, but scientists watch for signs to predict it.

• Its effects: El Niño can cause both very heavy rainfall in some areas and severe droughts in others, depending on where you are.

• A big example: The El Niño event in 1997–1998 caused huge floods and droughts globally.

  • The damage from this event was estimated to be about $36 billion.

Normal Pacific climate pattern (baseline for comparison)

• Imagine a map of the Pacific Ocean. Under normal conditions:

  • Strong trade winds blow from east to west, pushing warm surface water toward the western Pacific (towards Asia and the Pacific Islands).

  • As warm water moves west, cool water from deep in the ocean rises up along the eastern Pacific (near Central and South America). This is called upwelling.

  • This normal pattern usually means less rain on the western side and more rain on the eastern side.

El Niño year dynamics (weakened trade winds)

• During an El Niño year, the trade winds become weaker. They still push some water west, but not as much as usual.

• What happens to the water: This means less warm water gets pushed toward Asia and the Pacific Islands. Instead, cooler water builds up in the western Pacific, while warmer water collects near Central and South America.

• Changes in weather:

  • The Pacific jet stream (a fast-moving air current that guides storms) moves south from its usual path, changing where storms go.

  • In the United States, typically, northern regions (like the US/Canada border) experience warmer and drier weather.

  • The Gulf Coast and Southwest US usually get more rain and wetter conditions.

  • South Asia and the Pacific Islands often become warmer and drier, which can lead to droughts there.

• So, the same El Niño event can cause very different weather in different parts of the world due to these changes in air and ocean currents.

La Niña: the opposite climate pattern

• La Niña is the opposite of El Niño! During La Niña, the trade winds are stronger than usual.

• What happens: These strong winds push even more warm water towards the western Pacific and cause even more cool water to rise to the surface in the eastern Pacific. This makes the eastern Pacific even cooler than normal.

• Impacts: The weather impacts are often the opposite of El Niño in many places, though they can vary by region. This shows that these Pacific climate patterns affect global weather but in different ways depending on the area.

• Main idea: El Niño and La Niña are major global climate patterns driven by changes in the Pacific Ocean. They change weather and climate worldwide, not just cause local storms.