1. Ice Cores, Stable Isotopes, and Paleoclimate
Ice sheets provide a long term record of water and air that have
been compressed and stored over hundreds of thousands of years
Similar to Fig in text
Trapped air has lag time in
age with ice, from hundreds
to thousands of years

2. Stable isotope ratios of oxygen in water are especially useful as a proxy for past temperatures

3. Isotopes are radioactive or stable
Stable Isotopes
Isotopes are radioactive or stable
Radioactive Isotope: changes atomic number and/or mass over time (decay)
Stable Isotope: do not change atomic number, only mass, and remain stable over time

4. Many different isotopes
Stable Isotopes
Many different isotopes
10Be produced by cosmic radiation, a proxy for solar insolation in the ice core record, plus flux in this element is a proxy for magnetic reversals and can provide a chronological marker in the ice core

6. Permil or per thousand is indicated by the symbol ‰
Stable Isotope Terminology
Permil or per thousand is indicated by the symbol ‰
The delta symbol, δ, refers to the ratio of heavy to
light isotopes compared to a standard, times 1000
Hence, 10 ‰ is the same as saying 1%
As the ratio of a heavy to light isotopes increase, so does the value of δ, e.g.,
18O/16O in water can change with temperature:
δ18O ratio will decrease if there is less 18O or more 16O in the water

7. Oxygen Isotopes
Oxygen has three isotopic states: 16O, 17O and the heavier 18O
>99% of all oxygen is 16O, but fluctuations in the ratio of 18/16O
result from changes in climate
The concentration of 18O in precipitation decreases with temperature. This graph shows the difference in 18O concentration in annual precipitation compared to the average annual temperature at each site. The coldest sites, in locations such as Antarctica and Greenland, have about 5 percent less 18O than ocean water. (Graph adapted from Jouzel et al., 1994)

8. How the oxygen proxy works

9. < δ18O δ
> δ18O

10. Cold periods result in more of the lighter 16O isotopes in the ice
with a corresponding increase in 18O isotopes in the ocean
Warming periods are the reverse

11. Antarctic Ice Cores and Paleoclimate

12. Where to place an ice-coring site

13. The gradually increasing weight of overlying layers compresses deeply buried snow into ice, but annual bands remain. Relatively young and shallow snow becomes packed into coarse and granular crystals called firn (top: 53 meters deep). Older and deeper snow is compacted further (middle: 1,836 meters). At the bottom of a core (lower: 3,050 meters), rocks, sand, and silt discolor the ice. (Photographs courtesy U.S. National Ice Core Laboratory)

14. How an ice core drill works
How are ice cores stored and used?

15. How to determine the age of an ice core
Count annual rings
2. Find known volcanic ash, dust, or other signatures that have
been dated elsewhere
3. Dust with radioactive isotopes (e.g., Uranium) with known
decay rate
This 19 cm long of GISP2 ice core from 1855 m depth shows annual layers in the ice. This section contains 11 annual layers with summer layers (arrowed) sandwiched between darker winter layers. From the US National Oceanic and Atmospheric Administration, Wikimedia Commons.

16. Currently, longest (3.2 km) and oldest ice core in Antarctica is at Dome C,
where ice sheet is very thick
Goes back ~800,000 yrs based on annual layers, Uranium dating
These layers become more compacted with depth, heat generated
after that melts the layers so record ends
However, in some sections of the East Antarctic Ice Sheet, there may
be layers going back 1.5 my and current research is attempting to
find these older layers
Cores provide δ18O and δD (Hydrogen) in water, CO2, methane, and
nitrous oxide (N2O) concentrations in trapped air (trace gases)
% CO2 relates to changes in plant biomass
% CH4 relates to total wetland area and decay
% N2O relates to soil microbial activities

17. Using CO2 from air trapped in ice cores
Atmosphere: 78% N2
21% O2
0.035% CO2, H20, CH4 (methane)
--- trace gases

18. CO2 in Earth’s History

19. CO2 with higher resolution
Thousands of years ago

20. Mauna Loa, Hawaii

21. Daily and annual changes in atmospheric CO2
Data since 1958, longest modern record of CO2, with diurnal changes from plants using CO2 in photosynthesis, releasing in respiration. Mauna Loa ideal for minimizing contaminations from human activities, high elevation.

22. Dome C

23. We measure the concentrations of the greenhouse gases carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) as well as the isotopic composition (e.g. d13C of CO2 and d18O of O2) of the air which is enclosed in bubbles or in air hydrates in the ice. To measure the CO2 concentration an ice sample is broken into small pieces in vacuum; the extracted air is measured with Infrared Laser Spectroscopy.

25. Solar Insolation See also Fig. 3.19 in text
Changes in orbit plus wobbling of Earth on its axis results in cyclic
changes in insolation and climate: 41,000, 100,000, and 400,000 yr cycles

26. Fig. 3.18 in text Methane (CH4) varies with CO2
Temperature line shows glacial
periods in the past in blue while
red indicates interglacial periods
that were warmer than today
Sea level drops with glacial
periods
Insolation with dashed line
indicating threshold for start of
an Ice Age. Note the long inter-
glacial we are now in.

27. Vostok ice core agrees with Dome C data, with some divergence within variations from likely errors

28. Bi-polar See Saw in Climate Change
Divergence of northern vs. southern hemisphere data due to differences in ocean circulation, more freshwater discharge from melting glaciers in the north. Global warming is complex and some regions will cool while most areas warm.
Landais et al. (2015)

29. Readings for next week:
Text Ch. 4: pp
Ch. 7
Exam I on Thur., will not include
lecture material on Tuesday