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

2. Overview The Younger Dryas GISP2 Gases in ice cores Climate Implications 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.

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

4. Lake Agassiz

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

6. GISP2 o3000m-deep ice core on the summit of Greenland, drilled near the European GRIP core oBack to >100,000 years, and are believed to be valid and agree down to a few meters above Greenland’s bedrock oHave been used extensively in recreating the climate of the North Atlantic and the world

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

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

9. Limitation oThe relationship of the δ 18 O 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 oUsing δ 18 O, the temperature change leading into the Holocene is underestimated by a factor of 2 oLeads to search for independent paleothermometer

10. Limitation (contd.) oThe air trapped in the ice is younger than the ice 30y (Law Dome, coastal) 7,000y (Vostok, interior) oIn 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 oThe way to confront the gas-age—ice-age issue is to compare the composition of gases to other gases oBy examining the thermal diffusion of stable isotopes of atmospheric gas trapped in ice, temperature can be found. oThis relies on the fact that gas mixtures will fractionate in a temperature gradient according to their mass

12. Obtaining Data oOnce ice core is drilled, the gases are extracted and their isotopic compositions are found through a melt-refreeze technique that releases gases oMainly 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 oOnce 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 oFor 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 ice bubbles sealed off ~70m in Greenland ~96m at Vostok Mixing with the atmosphere (~10m) Fern (unconsolidated snow) Diffusion and compaction occurs Thermal Diffusion Gravity Settling

15. Air-Ice Core Gas Fractionation 1. Thermal Fractionation - Thermal gradient drives diffusive molecular transport Fractional deviation of R and Ro Temp ratio HEAVIER GAS IS ENRICHED IN COLDER REGION Thermal diffusion factor Example: δ 15 N ( 15 N and 14 N) 2. Gravitational settling Mass differenceDepth 298K308K δ 15 N=+0.2‰ on the cold-end 15 N HEAVIER GAS IS ENRICHED ON THE BOTTOM 80m, 236K ICE AIR δ 15 N=+0.4‰ relative to top 15 N

16. o With a +5ºC step function o Gas diffuses 10x faster than heat o Diffusion rate depends on the mass, ~7% faster for heavier 15 N 14 N Heat & Molecular Diffusion in Firn 5ºC warming 15 N

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

19. Separating the thermal vs. gravity effects A dynamic densification model predict a 6m deepening in fern column = ↑ gravity settling  ↑ δ 15 N by 0.03% Use δ 40 Ar ( 40 Ar/ 36 Ar) -δ 40 Ar is not affected by glacial-interglacial change (unlike δ 18 O) -Ar is half sensitive to thermal diffusion than N 2 -δ ~ Δm δ 15 N ( 15 N/ 14 N), Δm N=1, δ 40 Ar ( 40 Ar/ 36 Ar), Δm Ar=4 Hence, δ 40 Ar/4=δ 15 N IF ONLY GRAVITY EFFECT  Amplitude of: δ 40 Ar/4 = δ 15 N IF ONLY THERMAL EFFECT  Change in: 2 x δ 40 Ar = δ 15 N

20. Separating the thermal vs. gravity effects IF ONLY GRAVITY EFFECT  Amplitude of: δ 40 Ar/4 = δ 15 N IF ONLY THERMAL EFFECT  Change in: 2 x δ40Ar =δ15N The anomaly in Ar is less than N 2 suggesting the thermal effect Ar amplitude is about ¾ instead of ½, suggesting gravitation effect through deepening

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 N 2 and Ar at -40°C Grachev & Severinghaus (2004) Revised to 10±4°C ~ acquiring the thermal diffusion factor ~ three different approaches involving δ 15 N excess, δ 15 N, δ 40 Ar, and δ 18 O

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

23. oPre-industrial source of methane was wetlands oHeavy rainfall increases standing water in bogs, which increases methane production oAbrupt climate change at the end of the Younger Dryas was thought to have been hemisphere wide oAmount of methane found was too high to be local; the residence time of methane in the atmosphere is very short oWetlands that produce methane are found hemisphere wide. oMethane is not a very strong greenhouse gas. oDoes 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. oThere 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 δ 18 O. Methane and the Tropical Hydrology- NADW Link

26. ↑ Evaporation over tropical Atlantic (or increased precipitation in tropics) 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 Hemisphere increase in methane atmospheric concentration

27. According to this theory, the methane rise would precede the increase in temperature indicated by δ 18 O by several decades.

28. Conclusions oAbrupt warming at the end of the YD (11.6 ky bp) can be shown using δ 15 N and δ 40 Ar, because δ 18 O is less useful for rapid change oThe diffusion of gas in ice core can be modeled by the thermal and gravity gradient mechanisms o5-10°C (with revised=10±4°C is estimated for the increase in temperature) oMethane 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