Timing of Abrupt Climate Change of the Younger Dryas

H. Merritt, I.S. Nurhati, A. WilliamsPaleoclimatology & Paleoceanography

Spring 2006

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.

The Younger Dryas Stadial

o Brief cold climate period (~1300 years)o Named for an Arctic Scandinavian flowero After Pleistocene and before warmer Holoceneo Debated spatial extension (hemispheric or global?)o Some believed to be caused by Lake Agassiz

freshwater influx (=hampered thermohaline circulation in the Atlantic)

Lake Agassiz

Evidences of Worldwide Impact

o Scandinavian forest turned to tundrao Higher snowfall and glaciation rates in the

mountains of the worldo Higher amounts of dust from Asian desertso Drought in the Middle East (which may

have inspired the creation of Agriculture)

GISP2o 3000m-deep ice core on the

summit of Greenland, drilled near the European GRIP core

o Back to >100,000 years, and are believed to be valid and agree down to a few meters above Greenland’s bedrock

o Have been used extensively in recreating the climate of the North Atlantic and the world

Greenland Ice Core Recordso Drastic change about 11.6 ky bp that is well preserved in

the ice coreo Change came at the end of the Younger Dryas o Due to the abrupt nature of change common methods of

climate reconstruction are not as effective as usual

Methane

o In the Greenland ice core, very high levels of methane were found along this time period

o Methane suggests high precipitation in methane producing regions

o In order to better understand what mechanisms are driving this, the chronology of these events is key

Limitationo 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

o Using δ18O, the temperature change leading into the Holocene is underestimated by a factor of 2

o Leads to search for independent paleothermometer

Limitation (contd.)o The air trapped in the ice is younger than the ice

30y (Law Dome, coastal)

7,000y (Vostok, interior)o 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

A New Way

o The way to confront the gas-age—ice-age issue is to compare the composition of gases to other gases

o By examining the thermal diffusion of stable isotopes of atmospheric gas trapped in ice, temperature can be found.

o This relies on the fact that gas mixtures will fractionate in a temperature gradient according to their mass

Obtaining Data

o Once ice core is drilled, the gases are extracted and their isotopic compositions are found through a melt-refreeze technique that releases gases

o 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

Analysis

o 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

o 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

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

Air-Ice Core Gas Fractionation1. Thermal Fractionation - Thermal gradient drives

diffusive molecular transport

Fractional deviationof R and Ro

Temp ratio

HEAVIER GAS IS ENRICHED IN COLDER REGION

Thermal diffusion

factor

Example: δ15N (15N and 14N)

2. Gravitational settling

Mass difference Depth

298K 308K

δ15N=+0.2‰ on the cold-end

15N

HEAVIER GAS IS ENRICHED ON THE BOTTOM

80m, 236K

ICE

AIR

δ15N=+0.4‰ relative to top

15N

o With a +5ºC step function

o Gas diffuses 10x faster than heat

o Diffusion rate depends on the mass, ~7% faster for heavier 15N14N

Heat & Molecular Diffusion in Firn 5ºC warming 15N

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

Inflection point:

1700.3m = 11.64 kyr bp,

with ±20 yr uncertainty

X : previous study

Bad data points excluded

Replicates pair of data

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 GRAVITY EFFECT Amplitude of: δ40Ar/4 = δ15N

IF ONLY THERMAL EFFECT Change in: 2 x δ40Ar = δ15N

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

The anomaly in Ar is less than N2 suggesting the thermal effect

Ar amplitude is about ¾ instead of ½, suggesting gravitation effect through deepening

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

www.aquatic.uoguelph.ca/wetlands/page1.htm

Methane and Warming at the End of the Younger Dryas

o Pre-industrial source of methane was wetlandso Heavy rainfall increases standing water in bogs, which

increases methane productiono Abrupt climate change at the end of the Younger Dryas was

thought to have been hemisphere wideo Amount of methane found was too high to be local; the

residence time of methane in the atmosphere is very shorto Wetlands that produce methane are found hemisphere wide.o Methane is not a very strong greenhouse gas.o Does methane cause climate change?

http://www.nasa.gov/centers/goddard/news/topstory/2005/methane.html

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

o 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.

Methane and the Tropical Hydrology-NADW Link

↑ 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

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

Conclusions

o 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

o The diffusion of gas in ice core can be modeled by the thermal and gravity gradient mechanisms

o 5-10°C (with revised=10±4°C is estimated for the increase in temperature)

o Methane has proven not be the cause of this abrupt warming event, rather a consequence

ReferencesGrachev, 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_Dryashttp://www.agu.org/revgeophys/mayews01/node6.htmlhttp://www.ldeo.columbia.edu/res/pi/arch/examples.shtml