Week 5- Refining Chronology

Question 1

Firn Layer 1 - Convective zone

Answer 1

Gases are freely mixed with the atmosphere
by wind pumping (no gas fractionation occurs
in this section)

Question 2

Firn layer 2 - diffusive zone

Answer 2

Gases are nearly in diffusive equi-
librium. In this section, gas movements are
governed primarily by molecular diffusion,
and gas fractionation occurs

Question 3

Firn layer 3

Answer 3

Non-diffusive or lock-in zone. In this zone (the vertical mixing of
the gas is inhibited by sealing dense
layers, but horizontal mixing still occurs in less- dense layers. Eventually, all the bubbles are closed at the bottom of thelock-in zone.

Question 4

Firn

Answer 4

The porous and partially consolidated layer of snow on top of polar ice sheets

Question 5

Isotopes of gases in stagnant column of air-firn

Answer 5

Isotopes separate as a result of temperature gradients and gravitational settling (air in the free troposphere does not separate in this manner because of vigorous convective mixing). A gas mixture subjected to a temperature gradient will unmix slightly, with heavier gases generally moving toward colder regions

Question 6

Gases in polar firn

Answer 6

Diffuse 10 times as fast as heat.
So the isotopic signal penetrates to the bottom of the firn long before the temperature equilibrates, in about a decade.
The bubbles thus record a signal of the climate event as an abrupt increase in d15N, slightly smoothed by the diffusion process in the firn, followed by a gradual decrease in d15N over several hundred
years as the firn becomes isothermal once again

Question 7

Measurements of nitrogen and argon isotope rations in trapped air

Answer 7

Allow us to separate the thermal and
gravitational isotope fractionation effects, and provide us
with the past temperature gradient

Question 8

Amount for time it takes for methane to mix across the globe

Answer 8

12 months