EVSC10001 · The Global Environment
Climate Through Deep Time & Glacial Cycles
This chapter shows how Earth scientists reconstruct climate across deep time when no thermometers existed, using calibrated palaeoclimate proxies such as oxygen-isotope (δ18O) records in deep-sea carbonate and Antarctic ice cores, and how the three Milankovitch orbital cycles pace the ~100 kyr glacial–interglacial sawtooth. It is a high-yield Part-A topic: examiners reliably ask you to explain proxies, separate the slow carbon–silicate thermostat from fast positive feedbacks, and draw a labelled diagram of the orbital cycles and the sawtooth they pace.
What this chapter covers
- 01Palaeoclimate proxies (why we read indirect records)
- 02Oxygen-isotope (δ¹⁸O) palaeothermometry and its sign
- 03Ice cores: trapped CO₂, CH₄ and δD temperature
- 04Carbon–silicate thermostat (slow negative feedback)
- 05Milankovitch cycles: eccentricity, obliquity, precession
- 06Orbital pacing rule: high-latitude summer insolation
- 07The ~100 kyr glacial–interglacial sawtooth
- 08Fast positive feedbacks: ice-albedo and CO₂ solubility
Part-A: read a δ¹⁸O record and draw the glacial–interglacial sawtooth
- +1Read the proxy sign: light ¹⁶O evaporates preferentially and is locked into ice during glacials, so the ocean (and the CaCO₃ shells recording it) becomes ¹⁸O-enriched — therefore the HIGH δ¹⁸O extreme (~+4.8‰) = cold, high-ice GLACIAL; low δ¹⁸O (~+3.2‰) = warm interglacial.
- +1Draw the labelled diagram: x-axis = time (older → younger), y-axis = δ¹⁸O with the heavy (glacial) end up or ice volume up; plot a repeating curve with ~100 kyr spacing between glacial peaks.
- +1Label the asymmetry: a SLOW build-up limb (gradual cooling/ice growth into each glacial) and a near-vertical FAST deglaciation, marked as the 'termination'; label one full glacial peak and one interglacial trough.
- +1Explain the pacing: the ~100 kyr rhythm matches the eccentricity Milankovitch cycle (with 41 kyr obliquity and 23 kyr precession beating within it), which sets high-latitude summer insolation and thus the TIMING of glaciations.
- +1Explain the amplitude/shape: small orbital nudges are amplified by FAST positive feedbacks (ice-albedo and CO₂ solubility releasing CO₂ on warming), producing the large swing and the sharp termination — add an annotation 'orbit times it, feedbacks amplify it'.
Key terms
- Palaeoclimate proxy
- A measurable feature of a natural archive (ice, carbonate, speleothem) that varies systematically with a past climate variable, so it can stand in for a direct measurement once calibrated and dated.
- Oxygen-isotope (δ¹⁸O) palaeothermometry
- Using the ¹⁸O/¹⁶O ratio of marine carbonate or ice as a temperature/ice-volume proxy: high δ¹⁸O in deep-sea carbonate signals a cold, high-ice glacial because light ¹⁶O is preferentially locked into ice sheets.
- Milankovitch cycles
- Three slow, predictable variations in Earth's orbit that redistribute incoming sunlight: eccentricity (~100 kyr orbit shape), obliquity (~41 kyr axial tilt of ~22–24.5°) and precession (~23 kyr axis wobble).
- Glacial–interglacial cycle
- The repeating Quaternary alternation between cold, high-ice glacials and warm interglacials, dominated by a ~100 kyr 'sawtooth' of slow ice build-up followed by rapid deglaciation (termination).
- Carbon–silicate thermostat
- A slow negative feedback that stabilises deep-time climate: warming speeds up silicate weathering, which draws down atmospheric CO₂ and cools the planet (and the reverse), holding the long-term temperature dial.
- Positive feedback (ice-albedo / CO₂ solubility)
- A fast amplifying loop in glacial cycles: ice and snow raise albedo and warming oceans release CO₂, so an initial cooling or warming reinforces itself — these set the amplitude of the swings the orbit merely paces.
Climate Through Deep Time & Glacial Cycles FAQ
Why does high δ¹⁸O in deep-sea carbonate mean a glacial?
Evaporation favours light ¹⁶O, so vapour and the snow that builds ice sheets are ¹⁶O-rich. During a glacial, huge volumes of ¹⁶O are stored in ice on land, leaving the ocean enriched in heavy ¹⁸O. Carbonate shells inherit that ratio, so a high δ¹⁸O records a cold, high-ice-volume glacial.
What are the three Milankovitch cycles and their periods?
Eccentricity (the shape of the orbit, ~100 kyr), obliquity (axial tilt swinging ~22–24.5°, ~41 kyr) and precession (the wobble of the tilt direction, ~23 kyr). They change where and when sunlight falls, especially high-latitude summer insolation, rather than the total solar input.
Do Milankovitch cycles cause the ice ages?
They PACE them, not POWER them. The orbital forcing only redistributes sunlight by latitude and season, which sets the timing. The large magnitude of a glacial (~−5 °C globally, low CO₂, high albedo) comes from fast positive feedbacks like ice-albedo and CO₂ solubility that amplify the small nudge.
Why is the glacial cycle a 'sawtooth' shape?
Ice builds up slowly over tens of thousands of years (a gradual cooling ramp) but deglaciation is fast and near-vertical (a 'termination'). The asymmetry comes from fast positive feedbacks that accelerate once warming and CO₂ release begin, rapidly collapsing the ice sheets.
What does the Antarctic ice-core record show over ~800 kyr?
CO₂ and temperature rise and fall together through repeated glacial–interglacial cycles, with similar total solar input each time. This co-variation is the clinching evidence that small orbital nudges are amplified by fast carbon-cycle and albedo feedbacks.
Exam move
Build every answer on the mantra "orbit times it, feedbacks amplify it," and never write prose without the scored diagram. Memorise the proxy sign (high δ¹⁸O = cold glacial), the three cycle periods (100/41/23 kyr), and the one distinction examiners love: the SLOW NEGATIVE carbon–silicate thermostat (deep-time stability, hundreds of kyr to Myr) versus the FAST POSITIVE ice-albedo and CO₂ feedbacks (rapid glacial swings). For any Part-A part here, sketch the labelled sawtooth (axes, glacial peak, interglacial trough, slow limb, fast termination, '~100 kyr') first, then add the orbital cycles cartoon, then write three or four mark-earning sentences linking summer insolation → snow survival → ice growth → feedback amplification.
