Lecture 16 – Age Dating with Radioactive Isotopes

Q: What do increasing δ¹⁸O values of CaCO₃ in benthic foraminifera indicate?

Decreasing temperatures. The records are also partially influenced by the ice volume effect.

Q: What are the three phases of Northern Hemisphere Quaternary glaciations?

Pre-glacial phase (before 2.75 Ma), small glaciation phase (2.75–0.9 Ma), and large glaciation phase (last 900,000 years).

Q: What are the four key observations from the 2.75 Myr benthic foram δ¹⁸O record, and what explains each?

(1) A gradual drift toward more positive δ¹⁸O values (first ice-rafted debris at 2.75 Ma), reflecting long-term cooling, deep-ocean cooling, and growing ice volume. (2) Cyclic oscillations at 41,000-year (obliquity) and 23,000-year (precession) periodicities — ice survived during low summer insolation and melted during high insolation, producing up to 50 advance/retreat cycles (small glaciation phase, 2.75–0.9 Ma). (3) In the last 900,000 years, higher δ¹⁸O maxima on ~100,000-year cycles (eccentricity) — ice sheets persisted longer and grew larger, interrupted by rapid melting (large glaciation phase). (4) Within the 100,000-year cycles, the 41,000- and 23,000-year cycles persist as secondary oscillations.

Q: How do coral reefs help quantify the ice volume effect?

20,000-year-old coral reefs indicate sea level 110 m below today (coincident with the LGM). Reefs dated at 82,000 and 104,000 years grew 15–20 m below current sea level, confirming an ice volume effect on ocean δ¹⁸O.

Q: What is the relationship between sea level change and ocean δ¹⁸O?

A 10 m sea level change equals approximately +0.1‰ change in δ¹⁸O of ocean water.

Q: How do glacial–interglacial temperature swings compare to Holocene variability?

Glacial cycles show ~10°C variation between interglacials and glacial maxima, compared to only ~1°C during the Holocene.

Q: How does the rate of recent warming compare to natural glacial cycles?

The rate of temperature change in the last 100 years is much faster than during natural glacial cycles.

Q: What is the Quaternary?

The last 2.6 million years, when glaciations occurred in the Northern Hemisphere. It is subdivided by marine isotope stages.

Q: How are marine isotope stages numbered?

Odd numbers represent interglacials; even numbers represent glacial periods.

Q: What is the Pleistocene?

The interval from 2.6 Ma (onset of major N Hemisphere glaciations) to 11,700 years BP (end of the last glaciation).

Q: What causes natural climate variations on timescales of hundreds of thousands of years?

Orbital variations (Milankovitch cycles).

Q: Under natural conditions, what do orbital calculations predict for Earth’s future climate?

Earth is heading toward a new glacial period in the next 20,000 to 40,000 years.

Q: How many stable isotopes exist, and how do they differ from radioactive isotopes?

There are 260 stable isotopes that do not decay. Radioactive isotopes (over 80 naturally occurring) are unstable and emit particles or radiation to reach a more stable state.

Q: Why does radioactive decay occur?

Because a radioisotope has an imbalance of protons and neutrons, causing it to emit alpha particles, beta particles, or gamma radiation to reach a more stable configuration.

Q: What is alpha decay?

Emission of an alpha particle (a helium-4 nucleus: 2 protons + 2 neutrons), reducing mass number by 4 and atomic number by 2.

Q: What is beta-minus (negatron) decay?

A neutron converts to a proton, emitting an electron. The mass number stays the same but the element changes. Occurs in isotopes below the valley of stability.

Q: What is beta-plus (positron) decay?

A proton converts to a neutron, emitting a positron. The mass number stays the same but the element changes. Occurs in isotopes above the valley of stability.

Q: What is electron capture decay?

A proton absorbs an inner orbital electron and converts into a neutron. Mass number stays the same. Occurs in isotopes above the valley of stability.

Q: What is the valley of stability?

The region on the chart of nuclides where stable isotopes lie; radioactive isotopes decay toward this valley.

Q: What does the radioactive decay equation describe?

How the number of parent atoms N(t) decreases over time: N(t) = N₀ · e(−λt), where N₀ is the initial number of parent atoms, λ is the decay constant, and t is time.

Q: What does “minerals closed” mean in age dating?

The point when isotopes can no longer enter or leave a mineral, marking the start of the radiometric clock (t = 0).

Q: What is the decay constant (λ)?

λ = ln(2) / t½. It describes the constant rate at which a parent isotope decays.

Q: What is a half-life (t½)?

The time needed for half of the parent isotope to decay into the daughter product. After 1 half-life 50% remains, after 2 half-lives 25%, after 3 half-lives 12.5%.

Q: What is the useful dating range for a radioactive decay system?

Typically 5–10 times the half-life, because beyond that the remaining parent material is too small to measure accurately.

Q: For age dating of old rocks, what is typically measured?

Both the decaying parent isotope and the newly formed daughter products.

Q: What is an example of a decay system where only the parent isotope is measured?

Carbon-14 (¹⁴C) dating.