Glaciers as records of climate

• Ice cores: – Detailed records of

temperature, precipitation, volcanic eruptions

– Go back hundred of thousands years

Past climate reconstructions

• Instrumental records– Global air temperature: limited records

• Proxy records of climate– “proxy” = a measure of climate conditions of

the past– clues such as temperature, precipitation– EX: ICE-CORES

Global temperature- instrumental record

Global temperature- reconstructed

Methods of Dating Ice Cores

• Counting of Annual Layers – Temperature Dependent – Marker: ratio of 18O to 16O– find number of years that the ice-core accumulated over– Very time consuming; some errors

• Using volcanic eruptions as Markers – Marker: volcanic ash and chemicals washed out of the

atmosphere by precipitation – use recorded volcanic eruptions to calibrate age of the ice-

core– must know date of the eruption

How do we reconstruct past climate from ice cores?

• Oxygen Isotope analysis:– Examining type of water isotopes contained

in ice– Gives clues about temperature at time of

deposition

Isotopes Defined

Name Electrons Protons Neutrons Abundance

16O 8 8 8 99.76%

18O 8 8 10 0.20%

Isotope = atoms of the same element with a different number of neutrons (different mass)

Example: Oxygen Isotopes

Stable Isotopes

16O (Light Element)

18O (Heavy Element)

Chemical and Biological processes can sort the light elements from the heavy elements

Fractionation

Change in 18O value

Oxygen isotopes

• Three isotopes: 18O,17O and 16O

• Important isotopes: 18O and 16O

• Modern 18O/16O ratio: • 1:500

Isotopic change• Evaporation: lighter 16O evaporates more easily

from a water body resulting atmospheric H2O vapor is poorer in 18O than oceanic water

• Condensation: heavier 18O are precipitated faster than lighter 16O;

• So: coldest snow is lightest (less heavy 18O

isotopes, more lighter 16O isotopes)

Evaporative Fractionation

Water vapor 18O?

Precipitation will be depleted in 18O relative to the standard (ocean water)

Negative 18O

18O/16O ratio: “delta 18O”

• Stable isotope ratios are expressed as parts per thousand (per mil – ‰) relative to a standard

• Ratio expressed as deviation of 18O from the Standard Mean Ocean Water ratio (SMOW)

Delta O18 and temperature

• Temperature affects 18O/16O ratio:

– colder temperatures more negative values for the delta 18O

– warmer temperatures delta 18O values that are less negative (closer to the standard ratio of ocean water)

Delta 18O and temperature

Seasonal variations

• Summer (warmer) and winter (colder) periods in glacial ice can be observed as long as the delta 18O ratio is locked into the ice

Continental effect

Altitudinal effect

• As water molecules travel up on an ice sheet water changes from vapor to liquid18O is precipitated first

• So: ice on the top of the glacier has less 18O than at the base of the mountain

How far back do records go?

• Greenland: the last 100,000 years

• Antarctica: record going back 400,000 years has been reconstructed

Temperature reconstructed from Greenland Ice core