Glaciers as records of climate Ice cores Detailed

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 18 O to 16 O 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 icecore – 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 Isotope = atoms of the same element with a different number of neutrons (different mass) Example: Oxygen Isotopes Neutr Nam Electr Proto on s e ons ns Abund anc e 16 O 8 8 8 99. 76% 18 O 8 8 10 0. 20%

Stable Isotopes 16 O (Light Element) 18 O (Heavy Element) Chemical and Biological processes can sort the light elements from the heavy elements Fractionation Change in d 18 O value

Oxygen isotopes • Three isotopes: 18 O, 17 O and 16 O • Important isotopes: 18 O and 16 O • Modern 18 O/16 O ratio: • 1: 500

Isotopic change • Evaporation: lighter 16 O evaporates more easily from a water body resulting atmospheric H 2 O vapor is poorer in 18 O than oceanic water • Condensation: heavier 18 O are precipitated faster than lighter 16 O; • So: coldest snow is lightest (less heavy 16 isotopes, more lighter O isotopes) 18 O

Evaporative Fractionation Water vapor d 18 O? Precipitation will be depleted in 18 O relative to the standard (ocean water) Negative d 18 O

18 O/ O ratio: “delta 18 O” 16 • Stable isotope ratios are expressed as parts per thousand (per mil – ‰) relative to a standard 18 • Ratio expressed as deviation of O from the Standard Mean Ocean Water ratio (SMOW)

Delta O 18 and temperature • Temperature affects 18 O/16 O ratio: – colder temperatures more negative values for the delta 18 O – warmer temperatures delta 18 O values that are less negative (closer to the standard ratio of ocean water)

Delta 18 O and temperature

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

Continental effect

Altitudinal effect • As water molecules travel up on an ice sheet water changes from vapor to liquid 18 O is precipitated first • So: ice on the top of the glacier has less 18 O 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