Southeast Greenland snow accumulation rate variability and perennial firn aquifer

extent using Icebridge radar data

Clément MIEGE1, Richard R. FORSTER1, Lora S. KOENIG2 Jason E. BOX3 and Evan W. BURGESS4

1. Department of Geography, University of Utah2. NASA Goddard Space Flight Center3. Geological Survey of Denmark and Greenland (GEUS)4. Alaska Science Center, USGS, Anchorage

Outline of the presentation

• Introduction

• Methods

• Part 1: accumulation rates

• Part 2: Firn aquifer

• Conclusion and Future work

Introduction Methods Accumulation rates Firn aquifer Conclusions

Modeled snow accumulation rates and discrepancies (1960-2008)

Burgess et al., 2010 Box et al., 2013

Net snow accumulation = only net gain of mass for Greenland ice sheet

Accurate spatial and temporal accumulation rates -> important for ice sheet mass balance estimates from models

Southeast Greenland•14% of total area•High accumulation (>1 m w. eq)•1/3 of total accumulation•<10% of in-situ measurements

•> 1 m w. eq. difference between RACMO2 and PMM5

Introduction Methods Accumulation rates Firn aquifer Conclusions

Recent surface melt and increase of the percolation facies

1400m 1540m 1710m 1997m

Depth (m)

Harper et al., 2012

Humphrey et al., 2012

Vertical pipes + ice lenses complicate radar-layer

identification

Melt days for May-Aug 2007 (Van Angelen et al., 2012)

Introduction Methods Accumulation rates Firn aquifer Conclusions

NASA Operation IceBridge (OIB) data for 2009-2013in Southeast Greenland

Source NSIDC(http://nsidc.org/icebridge/portal/)

In recent years, important amount of airborne radar data collected over the Southeast portion of the ice sheet.

Introduction Methods Accumulation rates Firn aquifer Conclusions

Field and airborne data locations

Two ground traverses in 2010 and 2011 -> Arctic Circle Traverses (ACT)Operation Ice Bridge Accumulation and Snow radar for 2011Point measurements: shallow firn cores (50-60 m)

Introduction Methods Accumulation rates Firn aquifer Conclusions

Ground-penetrating radar (GPR) and firn cores

Commercial GPR from GSSICentral freq.: 400 MHz-> 30 cm (resolution)

4inch drill (US Ice drilling program)

ACT10-A firn core

Introduction Methods Accumulation rates Firn aquifer Conclusions

Accumulation and Snow radars during OIB 2011

Recent accumulation rates Firn aquifer

Accumulation radar: Freq. 600 - 900 MHz -> 65 cm vertical resolution (Rodriguez-Morales et al., 2013)

Snow radar: Freq. 2-8 GHz-> 5 cm vertical resolution(Panzer et al., 2013)

2010 surface

OIB radar data, Leuschen, 2011 NSIDC

Introduction Methods Accumulation rates Firn aquifer Conclusions

Isochrone tracking and dating

ACT11-C

1

2345

-> 2008

-> 1999-> 1995-> 1991-> 1983

ACT11-C density

Process: 1. Get depth-age scale from firn core(s)2. Convert radar TWT to depth using relationship between dielectric constant and firn-core density from Kovacs et al., 1995.3. Use spatially interpolated density to calculate accumulation rates in w. eq.

Errors:- Error in radar depth varies along the radar profile as density changes- Error in firn-core dating (1 year)

Accumulation rates derived from Accumulation Radar

Introduction Methods Accumulation rates Firn aquifer Conclusions

Averaged accumulation rates derived for 4 different time periods

Averaged accumulation rates compared with Calibrated Polar MM5

West East

Introduction Methods Accumulation rates Firn aquifer Conclusions

Calibrated Polar MM5 mean accumulation for 1958 - 2008

Southeast Greenland 2011 snow accumulation (Snow Radar)

Accumulation for 2010/2011, derived from OIB Snow Radar

Density = 400 kg m-3 is used to convert snow radar snow thickness to w. eq.

Polar MM5 acc. data from Burgesset al. JGR, 2010

Introduction Methods Accumulation rates Firn aquifer Conclusions

Field observations of the perennial firn aquifer (PFA): April 2011, 2013

Snapshots from borehole camera taken at PFA field work site (April 2013)

Forster et al., in review

Surface (0 m)

Firn and ice lenses(5-6m)

Water top(12 m)

Under water(13 m)Perennial firn aquifer (PFA):

- Extensive storage of liquid water at depth(> 25 km in this profile)- Follows local topography (unconfined aquifer)- Persists over the winter

Introduction Methods Accumulation rates Firn aquifer Conclusions

PFA shallowest spots are located into topographic depression

ATM data from Krabill, 2011 NSIDC

PFA also detected with OIB Accumulation Radar (AR)

Introduction Methods Accumulation rates Firn aquifer Conclusions

Regional mapping of the firn aquifer in April 2011

Perennial firn aquifer extent:

• Mean depth to PFA top: 23 m• Average elevation : 1600m

• Observations are limited to flight paths. Example: Southern tip, only 2000-m contour

• Only surface of the water is detected, no volume estimate

SE

S

Forster et al.

Introduction Methods Accumulation rates Firn aquifer Conclusions

PFA evolution in 2011 and 2012

Firn aquifer is found in high accumulation areas

RACMO data Van Angelen et al., 2012

OIB 2011 OIB 2012 OIB 2011 + 2012

Introduction Methods Accumulation rates Firn aquifer Conclusions

No significant increase of the aquifer surface between 2011 and 2012-> Firn aquifer should drain to readjust from summer melt

PFA temporal evolution over the ACT-11 traverse line

Introduction Methods Accumulation rates Firn aquifer Conclusions

Summary

Accumulation rates:• OIB Accumulation Radar:

• Capture recent multi-year internal layers (no annual signal)• Accumulation trend overall agrees with the model• Isochrone tracking is ambiguous due to surface melt/percolation features

• OIB Snow Radar:• Last year of accumulation -> improve spatial resolution of accumulation

Perennial Firn aquifer:• Extensive year-round liquid water storage• Associated with high melt rates and high accumulation• Will have implication for:

• Water storage + drainage mechanisms • Ice sheet energy (able to bring heat to depth)

• No water volume estimate yet

Introduction Methods Accumulation rates Firn aquifer Conclusions

Future work

1. Estimate water volume in the firn aquifer (-> field work in 2013, 2014)

2. Add the 2013 OIB data to the current accumulation/aquifer maps

3. Reduce density uncertainties along low to high accumulation transects

4. Map aquifer past extent: use of depth sounder radar data

Many thanks to the field teams, ice drilling program (IDDO), Polar Field Services and CReSIS team for making this study possible!

Thanks for your attention !