Lecture 14 – Orbital Variations and Ice Sheets (p.2)

Q: What are the three orbital parameters that cause variations in insolation?

Tilt/obliquity, eccentricity, and precession.

Q: Under what conditions is ice sheet growth maximized?

Summer insolation is at its minimum AND mean annual temperature at the ice sheet site is below −10°C, allowing up to 30 cm/year of growth.

Q: At what temperature do ice sheets begin melting?

When mean annual temperature exceeds −8°C and summer insolation is high; melt rate increases at higher temperatures.

Q: What is the key rule linking summer insolation to ice sheet behaviour?

Low summer insolation → ice sheets grow; high summer insolation → ice sheets shrink.

Q: Which latitudes are most relevant for testing Milankovitch's ice sheet predictions?

~65°N — the northern edge of North America, Scandinavia, and Russia/Siberia.

Q: What is the climate point (P)?

The point where the equilibrium line intercepts the Earth's surface — accumulation equals ablation, so the ice sheet is in balance.

Q: How does summer insolation move the climate point?

Low summer insolation shifts it southward onto northern landmasses (ice growth); high summer insolation shifts it northward into the Arctic Ocean (no ice sheets).

Q: What happens when the climate point shifts north into the Arctic Ocean?

All northern landmasses fall below the equilibrium line into the ablation regime, so no ice sheets can form.

Q: What happens when the climate point shifts south onto the northern edge of a landmass?

Permanent ice begins to accumulate on northern landmasses above the equilibrium line.

Q: What were the major North American ice sheets during the Last Glacial Maximum (~20,000 years ago)?

The Laurentide and Cordilleran ice sheets, which extended much farther south than today.

Q: What is the ice elevation feedback and why is it a positive feedback?

As ice sheets grow vertically (exceeding 2,000 m), temperatures at the top decrease (~6.5°C per km), expanding the accumulation area and promoting further growth.

Q: How much does temperature decrease at the peak of a 2 km ice sheet?

About 13°C (2 km × 6.5°C/km lapse rate).

Q: What limits the ice elevation positive feedback at very high elevations?

At extremely cold temperatures, moisture availability decreases so snowfall drops — reducing the accumulation rate.

Q: Why does mass balance tilt with latitude on diagrams?

Temperature decreases with both increasing latitude (northward) and increasing elevation, so ice growth is enhanced toward the north and at higher elevations.

Q: What is the smallest insolation change among the three orbital parameters, and which parameter causes it?

Eccentricity, at only ~0.2% variation in solar radiation from maximum to minimum.

Q: Which orbital parameter most affects seasonality in polar regions, and on what cycle?

Obliquity (axial tilt), on a ~41,000-year cycle.

Q: What determines whether an ice sheet has a negative mass balance?

When ablation exceeds accumulation — the ice sheet is shrinking.

Q: Why do ice sheets lag behind solar insolation changes?

Max ice growth is only ~0.3 m/year, and ice sheets can exceed 3,000 m thick — requiring over 10,000 years to reach full size.

Q: What is the phase lag for each orbital parameter?

~¼ of each cycle: obliquity ~8,000–10,000 years, precession ~5,000–6,000 years, eccentricity ~25,000 years after the solar insolation minimum.

Q: How does bedrock respond to ice sheet loading?

Ice (0.97 g/cm³) depresses continental crust (2.7 g/cm³) — 3.3 km of ice equals the weight of 1 km of rock, causing the land surface to sink.

Q: How does bedrock depression create a feedback on ice sheets?

Sinking by 1 km causes a 6.5°C temperature increase at the ice base (lower elevation = warmer), which can trigger decay conditions.

Q: What is the bedrock depression time lag?

Up to 15,000 years. During ice growth, delayed depression provides positive feedback; during melting, delayed rebound also provides positive feedback.

Q: What are the three sequential factors controlling ice sheet growth and decay?

(1) Changes in summer insolation, (2) lag of ice volume behind insolation, (3) lag of bedrock depression/rebound behind ice volume.

Q: Describe the full interglacial-to-glacial-to-interglacial cycle.

Interglacial (P in Arctic Ocean, no ice) → insolation decreases, P shifts south, ice forms → ice grows rapidly at minimum insolation → insolation rises but ice still grows (lag) while bedrock sinks → P shifts far north, ice at low elevation melts rapidly.

Q: What are moraines, and what are their limitations as glacial evidence?

Poorly sorted deposits (boulders mixed with fine grains) left by glaciers. Problems: not continuous, hard to date, often removed by the next ice advance.

Q: What are tills/tillites?

Solidified glacial sediment containing mixed rock sizes. Same limitations as moraines: not continuous, hard to date, often destroyed by subsequent glaciation.

Q: Why are ocean sediments preferred over continental evidence for reconstructing ice sheet history?

Ocean sediments (ice-rafted debris, dropstones, foraminifera shells) provide continuous records that are easily age-dated, unlike moraines and tills.

Q: How do foraminifera shells record glacial cycles?

δ¹⁸O values in their CaCO₃ shells reflect temperature and ice volume changes. The records show an asymmetric sawtooth pattern — gradual cooling in stages followed by rapid warming — due to ice sheet growth/bedrock lag times.

Q: What does the sawtooth pattern in δ¹⁸O records indicate?

Slow, stepwise ice sheet growth (due to lags) followed by rapid collapse, reflecting the asymmetric dynamics of ice accumulation vs. melting.