Climatic env of the past

tectonic processes.

rain and snow become progressively depleted in ¹⁸O as air moves poleward

δ¹⁸O , foraminifera

fast spreading rates mean high CO₂ emissions to the atmosphere.

Opening and closing of seaways

fast uplift means fast weathering, which draws CO₂ down from the atmosphere.

Angle between the polar axis and a line perpendicular to the ecliptic plane

41,000 yr

Shape of Earth's orbit

100 kyr and 413 kyr

Movement of solstices/equinoxes relative to aphelion/perihelion

19,000 and 23,000 yr

rotation on its polar axis and revolution around the sun.

increased seasonality means warmer summers, more ice melt and deglaciation; decreased seasonality means cooler summers, less melt and glaciation.

ice sheets grow when summers are cool

appears in both marine and ice δ¹⁸O records

give insight into both global ice changes and past temperature changes.

evaporation preferentially lifts H₂¹⁶O, so oceans have higher δ¹⁸O and ice sheets (snow) have lower δ¹⁸O; more ice on land means higher ocean δ¹⁸O.

precessional cycle

increases, strengthens

peak was ~9,000 years ago.

Pollen: grasslands
Lake-level records and freshwater diatoms in North Atlantic sediments
Hydrogen isotopes in leaf waxes indicate later drying
Archaeological records of pastoral societies, then their retreat
Increased Nile flow and sapropels in the Mediterranean

inked to monsoons and wetland extent. High-latitude ice cores capture low-latitude signals, precession dominates low-latitude variability, and CH₄ sources are unlikely to be high-latitude (permafrost).

~90 ppm lower: 190 ppm (LGM) vs 280 ppm (pre-industrial)

Greenland ~100 ka; Antarctica ~800 ka

Central domes of ice sheets and other large ice bodies

Greenhouse gases in bubbles; δ¹⁸O of ice; dust and tephra; physical properties of ice

Aridity and volcanic events

Layer counting, ice-flow modeling, orbital tuning, methane synchronization

Colder oceans; less saline oceans; stronger biological pump; changed deep-ocean chemistry

More CO₂ dissolves in them

Surface-ocean photosynthesis is nutrient-limited today; the glacial atmosphere was dustier; the glacial biological pump was stronger (larger vertical δ¹³C gradient)
Three points: (1) ocean photosynthesis is nutrient-limited today; (2) sediment records show a dustier glacial atmosphere; (3) the vertical gradient in ocean δ¹³C shows a stronger biological pump in glacial periods.

NADW shut down, so more corrosive Antarctic Bottom Water bathed the seafloor and dissolved carbonates
With NADW shut down at the LGM, AABW filled the deep ocean and dissolved seafloor CaCO₃, raising carbonate ion (CO₃²⁻) and increasing ocean carbon storage.

Pollen, carbon isotopes and mass-balance models
Pollen records show vegetation changes, carbon isotopes track carbon exchange, and mass-balance models calculate the land carbon change.

Major reservoirs (in GtC) and fluxes (in GtC/yr), with pre-industrial sizes
The lecture asks you to know the major reservoirs (GtC) and fluxes (GtC/yr) and their pre-industrial sizes. Check your slides for the actual numbers.

Differential heating of land and ocean; summer-wet, winter-dry
Monsoons are driven by land–ocean heating contrasts and bring rain to land in summer and dry conditions in winter.

~11,000 years ago, and every 23,000 years before that
Precessional change produced stronger NH summer insolation ~11,000 years ago (early Holocene), recurring on a 23,000-year beat.

Antiphased: when one hemisphere's monsoon strengthens, the other's weakens
Brazil and China speleothem records are clearly antiphased, each following its own summer insolation (Feb at 30°S vs July at 25°N).

Strong monsoons → extensive wetlands → more CH₄ emissions
Monsoon rain builds wetlands, which emit CH₄. Most global wetlands are in northern latitudes and the tropics/subtropics. High-latitude sources like permafrost are unlikely to explain the precessional CH₄ signal.

CO₂: strong 100 ka beat following ice volume. CH₄: 100 ka beat plus a strong 23 ka (precession) beat
CH₄ carries the low-latitude precession signal because wetlands respond to monsoons; CO₂ follows global ice volume.

Past rainfall intensity
Speleothem δ¹⁸O is a signal of past rainfall intensity (monsoon strength) and tracks summer insolation.

Eccentricity ~100 kyr; obliquity 41 kyr; precession 19 and 23 kyr
Orbital variations change insolation (seasonality and latitudinal gradients) with these periodicities.

Breaking a time series into component sine waves, each with its own period and power, to detect recurring cycles
Any time series can be modeled as a stack of sine waves; the power of each equals its amplitude.

Theory says precession is strongest at 65°N, but the data show eccentricity (100 kyr) and obliquity (41 kyr) strongest
The weakest orbital insolation signal (eccentricity) produces the largest climate response, and the expected strong precession signal is surprisingly weak.

Hemispheric effects are antiphased and cancel; total summer insolation matters more than peak insolation
Hypothesis 1 (Raymo & Huybers 2008): antiphased hemispheric effects cancel.
Hypothesis 2: total summer insolation, controlled mainly by tilt, matters more than peak insolation, which is controlled by precession.

Albedo (+), sea level (+), ice elevation (+), bedrock loading (−)
Three positive feedbacks amplify change; bedrock loading (ice weight depresses the land) is negative.

Ice grows slowly and irregularly but melts rapidly; ice sheets respond slowly and are easier to melt than to grow
Marine δ¹⁸O shows rapid ice ablation and slow, irregular growth, which is why the response depends on the direction of temperature change.

The dominant cycle shifted from 41 kyr to 100 kyr; either eccentricity paces deglaciation by modulating precession, or ice sheets grew large enough to strongly affect climate
Hypothesis 1: eccentricity is a pacemaker, with deglaciation only under the warmest summers. Hypothesis 2: after ~900 ka ice sheets were big enough to strongly affect climate. Larger sheets may reflect bedrock changes (sedimentary rock scraped away, leaving crystalline bedrock).

Precession's insolation effects are antiphased between hemispheres, yet the cycle appears in both; the answer may be circulation changes and greenhouse gases (CO₂)
CO₂ follows ice volume, likely through ocean carbon storage during ice ages and release at deglaciations; the lecture says we don't fully know.