Lateral dimension of single layers (sealing effect?)

Modeling Δ-age (distribu2ons) of enclosed air in layered firn

Johannes Freitag, Sepp Kipfstuhl, Thomas Laepple, Katharina Klein, Katja Instenberg

Alfred Wegener Ins2tute for Polar and Marine Research Bremerhaven, Germany

NSF PIRE Firn Workshop 9/ 2013

Densifica)on

Air diffusion within the open pore space

Cri)cal close-off density (percola)on threshold)

Lateral dimension of single layers (sealing effect of dense layers?)

Modeling Δages in ice from the Antarc)c plateau 4 problem areas for Δage-calcula2ons

Johannes Freitag Impurity controlled Densifica)on model

neglected EDML: 900 years (recent)

EDC: 2000 -5000 years (Glacial)

EDML: 0.82-0.84gcm-3: 6m -> 70years (recent) EDC -> 300 years (Glacial)

EDML: 0.82gcm-3: 16m -> 190years (recent) EDC -> 800 years (Glacial)

EDML, EDC: 30 -50 years (recent, Glacial, (Schwander et al., 1997))

Outline

Lateral dimension of single layers (sealing effect?)

measurements from the last field campaign COFI (Antarc2ca, DML) Cri2cal density – pore close-off

measurements and model

Modeling Δage for EDC and comparing it with LD2010 and AICC12 chronologies Modeling gas-age distribu2ons

case studies using a percola2on and impurity-densifica2on model Conclusions

Johannes Freitag

3 m 12m 1000 m

Antarc2ca, DML, 2013, COFI-CAMP: T≈-45°C, Acc≈70mm weq/a

B40 B41 B42 B50

The lateral dimension of firn layers

7500 m

B52

Lateral dimension Cri2cal density age distribu2ons dage EDC Johannes Freitag

Intercomparison: similar mean and density variability

7500m 1000m 12m 3m

Lateral dimension Cri2cal density age distribu2ons dage EDC Johannes Freitag

Comparison of density profiles measured in 3m distance

Shallow firn

Deep firn

3m

Lateral dimension Cri2cal density age distribu2ons dage EDC Johannes Freitag

Deep firn Shallow firn

12m

Comparison of density profiles measured in 12m distance

Lateral dimension Cri2cal density age distribu2ons dage EDC Johannes Freitag

Comparison with (1km) and (7.5km)

Deep firn

Deep firn

7500m 1000m

Lateral dimension Cri2cal density age distribu2ons dage EDC Johannes Freitag

Similari2es in density varia2ons in deep firn (10m ...1000m ....10000m), more differences in shallow firn

Explana2on (in terms of the impurity-densifica2on link):

Impurity concentra2ons seem to be laterally more homogeneous than the surface density at DML.

Implica2ons:

Sealing effect for pore close-off

Strengthen the represen2veness of an ice-core record Summary

Lateral dimension Cri2cal density age distribu2ons dage EDC Johannes Freitag

Densifica)on

EDML: 900 years (recent)

EDC: 2000 -5000 years (Glacial) Air diffusion within the open pore space

EDML, EDC: 30 -50 years (recent, Glacial) Cri)cal close-off density (percola)on threshold)

EDML: 0.82-0.84gcm-3: 6m -> 70years (recent) EDC -> 300 years (Glacial)

Lateral dimension of single layers (sealing effect of dense layers?) EDML: 0.82gcm-3: 16m -> 190years (recent)

EDC -> 800 years (Glacial)

Modeling Δages in ice from the Antarc)c plateau

Johannes Freitag Impurity controlled Densifica)on model

neglected

4 problem areas for Δage-calcula2ons

Infinite, sealing (>2cm)

The cri)cal density (percola)on threshold and pore close-off)

Lateral dimension Cri2cal density age distribu2ons dage EDC Johannes Freitag

(Stauffer et al. 1985 / 1995)

Gradual air enclosure: a percola)on problem

Model for sintered firn: Network of Tetrakaidecahedrons on a BCC-Lafce

Air filled pore chanels

Occupa2on probability

Open Pores Closed pores

Lafce at the threshold from permeability to impermeability (percola2on threshold p^crit~0.4)

Fully occupied lafce (p=1):

Z=4 Z≈1.6

Z(p) ≈ linear

Lateral dimension Cri2cal density age distribu2ons dage EDC Johannes Freitag

n=0.10 ~ ρ=830 kg/m³ n=0.208 ~ ρ=726 kg/m³

CT-measurements of pore connec)vity

GREENLAND ANTARCTICA

Conclusion:

Universal cri)cal porosity (for homogeneous firn, subcm-scale)

Densifica)on

EDML: 900 years (recent)

EDC: 2000 -5000 years (Glacial) Air diffusion within the open pore space

EDML, EDC: 30 -50 years (recent, Glacial) Cri)cal close-off density (percola)on threshold)

EDML: 0.82-0.84gcm-3: 6m -> 70years (recent) EDC -> 300 years (Glacial)

Lateral dimension of single layers (sealing effect of dense layers?) EDML: 0.82gcm-3: 16m -> 190years (recent)

EDC -> 800 years (Glacial)

Modeling Δages in ice from the Antarc)c plateau

Johannes Freitag Impurity controlled Densifica)on model

neglected

4 problem areas for Δage-calcula2ons

Infinite, sealing (>2cm) Constant!

S=0.1, Rho=0.83g/cm3

Modeling Δage for EDC

d15N

GLACIAL HOLOCENE

MODEL INPUT

MODEL OUTPUT Predic2on of

classical densifica2on models (HL, PB,..)

Predic2on of

new impurity-model

Lateral dimension Cri2cal density age distribu2ons dage EDC Johannes Freitag

Summary

Lateral dimension Cri2cal density age distribu2ons dage EDC Johannes Freitag

Mul2-core drilling suggest con2nuous density layers in deep firn Universal cri2cal density for pore close-off on the cm-scale

Extended impurity-firn model predict dages of the EDC-core similar to the AIC12-chronology

– the missing model for dage-calcula2ons?

3d-percola2on-model for calcula2ng closed porosity in layered firn

Narrow age-distribu2ons in holocene and glacial firn due to layering,

occurence of small-scale inversions on the gas age scale