Timing of Abrupt Climate Change of the Younger Dryas

1. Timing of Abrupt Climate Change of the Younger Dryas H. Merritt, I.S. Nurhati, A. Williams Paleoclimatology & Paleoceanography Spring 2006

2. Overview Severinghaus, J.P., Sowers, T., Brook, E.J., Alley, R.B., and M.L. Benders. 1998. Timing of abrupt changes at the end of the Younger Dryas interval from thermally fractionated gases in polar ice. Nature 391:141-146. • The Younger Dryas • GISP2 • Gases in ice cores • Climate Implications

3. The Younger Dryas Stadial • Brief cold climate period (~1300 years) • Named for an Arctic Scandinavian flower • After Pleistocene and before warmer Holocene • Debated spatial extension (hemispheric or global?) • Some believed to be caused by Lake Agassiz freshwater influx (=hampered thermohaline circulation in the Atlantic)

4. Lake Agassiz

5. Evidences of Worldwide Impact • Scandinavian forest turned to tundra • Higher snowfall and glaciation rates in the mountains of the world • Higher amounts of dust from Asian deserts • Drought in the Middle East (which may have inspired the creation of Agriculture)

6. GISP2 • 3000m-deep ice core on the summit of Greenland, drilled near the European GRIP core • Back to >100,000 years, and are believed to be valid and agree down to a few meters above Greenland’s bedrock • Have been used extensively in recreating the climate of the North Atlantic and the world

7. Greenland Ice Core Records • Drastic change about 11.6 ky bp that is well preserved in the ice core • Change came at the end of the Younger Dryas • Due to the abrupt nature of change common methods of climate reconstruction are not as effective as usual

8. Methane • In the Greenland ice core, very high levels of methane were found along this time period • Methane suggests high precipitation in methane producing regions • In order to better understand what mechanisms are driving this, the chronology of these events is key

9. Limitation • The relationship of the δ18O ratio of ice and the paleotemperature has been shown to change over time, and may not be useful in certain situations of abrupt temperature change • Using δ18O, the temperature change leading into the Holocene is underestimated by a factor of 2 • Leads to search for independent paleothermometer

10. Limitation (contd.) • The air trapped in the ice is younger than the ice 30y (Law Dome, coastal) 7,000y (Vostok, interior) • In times of rapid change like the end of the Younger Dryas, this becomes an issue because the slight difference in age of the air compared to the age of the ice can make them have very significant differences in composition

11. A New Way • The way to confront the gas-age—ice-age issue is to compare the composition of gases to other gases • By examining the thermal diffusion of stable isotopes of atmospheric gas trapped in ice, temperature can be found. • This relies on the fact that gas mixtures will fractionate in a temperature gradient according to their mass

12. Obtaining Data • Once ice core is drilled, the gases are extracted and their isotopic compositions are found through a melt-refreeze technique that releases gases • Mainly the center of these cores are used to minimize the effect of the loss of gas during retrieval and the handling of ice samples

13. Analysis • Once gas is collected, it is isolated from other elements/molecules and then analyzed with a mass spectrometer to determine how much of each isotope is present in the sample • For gases such as argon, which are much less abundant than nitrogen, other gases may be added to create a “solution” much like a chemical in water so the sample has an appropriate volume for the analytical apparatus

14. Air-Ice Core Gas Fractionation Mixing with the atmosphere (~10m) Thermal Diffusion Fern (unconsolidated snow) Diffusion and compaction occurs Gravity Settling ice bubbles sealed off ~70m in Greenland ~96m at Vostok

15. Air-Ice Core Gas Fractionation 2. Gravitational settling 1. Thermal Fractionation - Thermal gradient drives diffusive molecular transport HEAVIER GAS IS ENRICHED ON THE BOTTOM HEAVIER GAS IS ENRICHED IN COLDER REGION Mass difference Depth AIR Fractional deviationof R and Ro Temp ratio Thermal diffusion factor ICE 80m, 236K Example: δ15N (15N and 14N) 15N 298K 15N 308K δ15N=+0.4‰ relative to top δ15N=+0.2‰ on the cold-end

16. Heat & Molecular Diffusion in Firn 5ºC warming 15N • With a +5ºC step function • Gas diffuses 10x faster than heat • Diffusion rate depends on the mass, ~7% faster for heavier 15N14N

17. 0.4‰ during a stable cold period +0.15‰ at 11.6kyr bp followed by a decline (recall +0.2‰ for our 10K example) ~70m in Greenland ~96m at Vostok

18. X : previous study Bad data points excluded Replicates pair of data Inflection point: 1700.3m = 11.64 kyr bp, with ±20 yr uncertainty

19. Separating the thermal vs. gravity effects A dynamic densification model predict a 6m deepening in fern column = ↑ gravity settling  ↑ δ15N by 0.03% Use δ40Ar (40Ar/ 36Ar) • δ40Ar is not affected by glacial-interglacial change (unlike δ18O) • Ar is half sensitive to thermal diffusion than N2 • δ ~ Δm δ15N (15N/ 14N), Δm N=1, δ40Ar (40Ar/ 36Ar), Δm Ar=4 Hence, δ40Ar/4=δ15N IF ONLY THERMAL EFFECT  Change in: 2 x δ40Ar = δ15N IF ONLY GRAVITY EFFECT  Amplitude of: δ40Ar/4 = δ15N

20. Separating the thermal vs. gravity effects IF ONLY GRAVITY EFFECT  Amplitude of: δ40Ar/4 = δ15N IF ONLY THERMAL EFFECT  Change in: 2 x δ40Ar =δ15N Ar amplitude is about ¾ instead of ½, suggesting gravitation effect through deepening The anomaly in Ar is less than N2 suggesting the thermal effect

21. Abrupt warming temp (& corrected) Severinghaus et al. (1998) 5-10°C of abrupt warming (highly tentative) ~ high analytical uncertainties ~ unknown thermal diffusion factor for N2 and Ar at -40°C Grachev & Severinghaus (2004) Revised to 10±4°C ~ acquiring the thermal diffusion factor ~ three different approaches involving δ15Nexcess,δ15N, δ40Ar, and δ18O

22. www.aquatic.uoguelph.ca/wetlands/page1.htm Methane and Warming at the End of the Younger Dryas

23. Pre-industrial source of methane was wetlands • Heavy rainfall increases standing water in bogs, which increases methane production • Abrupt climate change at the end of the Younger Dryas was thought to have been hemisphere wide • Amount of methane found was too high to be local; the residence time of methane in the atmosphere is very short • Wetlands that produce methane are found hemisphere wide. • Methane is not a very strong greenhouse gas. • Does methane cause climate change? http://www.nasa.gov/centers/goddard/news/topstory/2005/methane.html

24. Methane seems to RESPOND to climate change, not CAUSE climate change

25. Methane and the Tropical Hydrology-NADW Link • There is a proposed link between changes in the tropical hydrological cycle and North Atlantic deep water (NADW) Theory: Increased evaporation over the tropical Atlantic would produce methane rise shown in core, followed years later by an increase NADW formation and Greenland temperature shown in δ18O.

26. ↑ Evaporation over tropical Atlantic (or increased precipitation in tropics) Hemisphere increase in methane atmospheric concentration Increase salinity of water, saltier warm water gets to poles decades later & is cooled Salty water sinks Increase in NADW formation Increased heat budget More precipitation Increase temperature in Greenland

27. According to this theory, the methane rise would precede the increase in temperature indicated by δ18O by several decades.

28. Conclusions • Abrupt warming at the end of the YD (11.6 ky bp) can be shown using δ15N and δ40Ar, because δ18O is less useful for rapid change • The diffusion of gas in ice core can be modeled by the thermal and gravity gradient mechanisms • 5-10°C (with revised=10±4°C is estimated for the increase in temperature) • Methane has proven not be the cause of this abrupt warming event, rather a consequence

29. References Grachev, A. M., and J.F. Severinghaus. 2004. A revised +10±4°C magnitude of the abrupt change in Greenland temperature at the Younger Dryas termination using published GISP2 gas isotope data and air thermal diffusion constants. Quaternary Science Reviews 24: 513-519. http://en.wikipedia.org/wiki/Younger_Dryas http://www.agu.org/revgeophys/mayews01/node6.html http://www.ldeo.columbia.edu/res/pi/arch/examples.shtml