observation of platelet-layer thickness and ice-volume fraction with multi-frequency EM

AGU Fall Meeting

14-18 December 2015 M. Hoppmann ¹ , P. A. Hunkeler ¹ , S. Hendricks ¹ , T. Kalscheuer ² , R. Gerdes ^1,3

A glimpse beneath Antarctic sea ice:

observation of platelet-layer thickness and ice-volume fraction with multi-frequency EM

Contact: Mario.Hoppmann@awi.de or Stefan.Hendricks@awi.de

Sea floor

20 km +200

0

-500

-1000

(m)

HSSW

Sub-ice platelet layer

Grounding

line Ice-shelf cavity

Overturning

Tidal flows

Melting

I S W

Precipitation/freezing

Brine rejection

Ice shelf

Ice sheet ^{Sea ice}

Marine ice

10 m

1) High Salinity Shelf Water (HSSW) enters the cavity and melts the base of the ice shelf. Very cold, less saline Ice Shelf Water (ISW) is formed.

2) The ISW rises and becomes supercooled (the freezing point depends on pressu- re!). Supercooling is relieved through formation of ice platelets.

3) The crystals float upwards, while continuing to grow. They eventually accumulate beneath coastal sea ice, forming a sub-ice platelet layer (red box).

I. The story in short

II. Background: platelet-layer formation and importance

1 2

3

III. Methods

The platelet layer

• consists of individual crystals (platelets) up to 20 cm in diameter.

• is unconsolidated and porous, with interstitial water between the platelets.

• hosts a unique ecosystem (phytoplankton, crustaceans, fish, anemones, ...).

• reflects ocean/ice-shelf interaction, which is difficult to observe directly.

• contributes to coastal sea-ice mass and energy balance, especially fast ice.

Sorasen

+

Ridge

Quar Ice Shelf

Ekström Ice Shelf

Jelbart Ice Shelf

Halvfarryggen Ridge

Unneruskollen Island

0 12,5 25 50 75 100

Kilometers

Weddell Sea

Atka Bay

1.

A study area featuring sea ice with an underlying platelet layer.

3.

A suitable geophysical inversion

+

which models the subsurface until it fits to the EM data. We used a laterally-constrained Marquardt- Levenberg inversion (c) from the EMILIA software package (Hunkeler, 2015).

The challenge: Find an efficient method to determine platelet-layer volume on large scales.

Results taken from P. A. Hunkeler, M. Hopp- mann, S. Hendricks, T. Kalscheuer, R. Gerdes:

A glimpse beneath Antarctic sea ice: platelet- layer volume from multi-frequency electro- magnetic induction sounding. Accepted for publication in Geophysical Research Letters

Seawater

Sub-ice platelet layer Sea ice and snow

Conductivity in S m^-1

0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7

Breakup area

Thickness in m

10 5

-10 -5 0

0 10 5

ATKA03

ATKA07

ATKA11

ATKA16 ATKA21 ATKA24

RMSE

ATKA03 ATKA07

ATKA11

ATKA16 ATKA21 ATKA24

#*

#*

#*

#*

#*

#*

7°30'W 7°40'W

7°50'W 8°0'W

8°10'W 8°20'W

70°30'S 70°28'S

70°32'S

70°34'S

70°36'S

70°38'S

70°40'S

0 2,5 5

Ü

10 km

Ekström Ice Shelf

Seasonal fast ice

TerraSAR-X 2012-11-15

Neumayer III

Atka Bay

Break-up edge in August 2012

0 1 2 3 4 5 6 7 8 9 10

Sub-ice platelet-layer thickness (m)

ATKA07 ATKA11 ATKA16 ATKA21 ATKA24 ATKA03

Transect 3

Transect 6

Transect 4 Transect 7

Transect 5

Transects 1 & 2

c)

d)

1

2

1 2

#

*

#

* #*

#

* #*

#*

0 2,5 5 10 km

Break-up edge in August 2012

Sub-ice platelet-layer el. conducivity (Sm^-1)

0.5 2.7

a)

b)

0 0.05 0.1 0.15Ice-volume fraction ȕ 0.2 0.25 0.3 0.35 0.4

Probability density

0 2 4 6 8 10

Platelet-layer electrical conductivity ıp in Sm^-1

2.7 2.31 1.97 1.66 1.38 1.14 0.93 0.74 0.58

ȕ

_m=3

Mean: 0.26 Median: 0.28 StdDev: 0.09

0 5

1

m

0.36ȕ

2

1 3 4

IV. Results

Archie‘s Law:

Left: Platelet-layer thickness a) and conductivity b) resulting from inversion from multi-frequency EM data recorded on the landfast sea ice at Atka Bay, eastern Weddell Sea, in December 2012.

c) Inversion results of sea-ice and platelet-layer thickness and conductivity of a profile between points 1 and 2 indicated in a), along with results from drillhole measurements. The RMSE, a measu- re of the fit quality between inversion and measu- red data, is given in d) (lower values are better).

e) shows the distribution of ice-volume fractions β in the platelet layer, which is needed for calculation of the ice volume. It is derived from electrical con- ductivities via Archie‘s Law, here shown for a ce- mentation factor m = 3.

1. We present the first EM-based platelet-layer thickness and conductivity dataset.

2. Multi-frequency EM data inversion enables platelet-layer volume estimates:

platelets contribute about 50% to total first-year sea-ice volume at Atka Bay.

3. Results also allow conclusions about ice and ocean processes, such as ice melt, currents, primary productivity estimates, ...

V. Take-home messages

• The inversion results for thickness show good ag- reement with drillhole measurements (c), and show a generally plausible pattern (a).

• The inversion results for conductivity are also in a plausible range (b), with a possible indication of surface flooding (red colors in breakup area in c).

• A thinner (younger) platelet layer is less consoli- dated and has a lower ice-volume fraction (a,b).

• A cementation factor m>2 yields plausible values for ice-volume fractions (β<0.36) (e).

• The RMSE is suffiently low for an inversion with four free parameters (sea-ice as well as platelet layer thickness and conductivity) (d).

e)

1Alfred-Wegener-Institute, Bremerhaven, Germany , ²Uppsala University, Sweden, ³Jacobs University Bremen, Germany

Inversion results Drillhole measurements

C23B-0783

a) b) c)

s₁ s₂ s_n s₁ s₂ s_n s₁ s₂ s₃ s_n

Hoppmann, 2015

In Antarctica, ice crystals (platelets) form and grow in supercooled waters below ice shelves. These platelets rise, accumulate beneath nearby sea ice, and form a several meter thick sub-ice platelet layer. This special ice type is a unique habitat, influences sea-ice mass and energy balance, and its volume can be interpreted as an indicator for ice – ocean interactions.

Although progress has been made in determining and understanding its spatio-temporal variabili- ty based on point measurements, an investigation of this phenomenon on a larger scale remains a challenge due to logistical constraints and a lack of suitable methodology.

In the present study, we applied a lateral constrained Marquardt-Levenberg inversion to a unique multi-frequency electromagnetic (EM) induction sounding dataset obtained on the ice-shelf in- fluenced fast-ice regime of Atka Bay, eastern Weddell Sea. We adapted the inversion algorithm to incorporate a sensor specific signal bias, and confirmed the reliability of the algorithm by perfor- ming a sensitivity study using synthetic data. We inverted the field data for sea-ice and sub-ice platelet-layer thickness and electrical conductivity, and calculated ice-volume fractions using Archie’s Law. The thickness results agreed well with drillhole validation datasets within the uncer- tainty range, and the ice-volume fraction also yielded plausible results.

Our findings imply that multi-frequency EM induction sounding is a suitable approach to efficiently map sea-ice and platelet-layer properties. A successful application of this technique requires a break with traditional EM sensor calibration strategies due to the need of absolute calibration with respect to a physical forward model.

The authors thank Stephan Paul, Uwe Baltes, Johan Hermansson and the Neu- mayer III wintering crew of 2012 for their support. This work was funded by the POLMAR graduate school and the German Research Council by grants to SPP1158, NI 1092/2 and HE2740/12.

sea ice

platelet layer seawater

Primary field

Secondary field

2.

A multi-frequency EM instrument, a) mounted in a kayak, b) pulled over sea ice

Stations

Cell thickness may change

Grid cells of fixed thickness

Lateral constraints

Cell thickness may change

σ^b