Tradeoff between climate and thickness histories at Siple Dome

during the past 25 ka

Ed Waddington, Howard Conway, Eric Steig, Richard Alley, Ed Brook, Ken Taylor, Jim White

From R. Bindschadler

- Has the retreat stopped or is it ongoing?- How thick was the WAIS at LGM?

LGM Reconstruction based on geological

data at outlet glaciers and a flow model for

ice sheet

Denton and Hughes (2000)

• 800 m thicker over Siple Dome• Can we test this with data?

What are the issues?

WAIS ice volume and sea level at LGM

Ice-core stable isotopes: climate change, or source-elevation change?

How do ice streams work? • Are they always the same length, and onsets migrate with grounding line?• Are onsets always at the same places, and

ice streams lengthen at LGM?

Where are the dipsticks?

• The only mountain dipsticks are far away …

•Ross Embayment has no outcrops in 2x106 km2

• We can’t use trimlines, cosmogenic isotopes, etc …

• What are we going to do?

Use the stratigraphy of the ice …?

Accumulation rate and depth-age

Higher accumulation rate means• Thicker annual layers• Younger ice at a given depth

• Depth-age scale records accumulation history

• Ice deforms• Ice moves away from site of deposition• layers get thinner

Steady-state layer thinning

Steady-state depth-age

Steady-state layers

Layers in a thinning ice sheet

• To make the ice sheet get thinner, the layers must be strained even more heavily.• With thinner layers, more layers can get packed into any given depth interval.

•The measured depth-age relation also contains information about past thickness.

Thinning ice cap

Thinning depth-age

Thinning ice sheet with constant accumulation

Steady state

Thinning layers

Steady state

Thinning ice sheet with constant accumulation

If ice sheet has always been thinning …Then infinitely old ice is high off the bed

Infinite age

Event Horizon

Depth-age with “false bottom”

South North

Asymmetric arch indicates that divide is moving to the right at 0.5 meters per year.

Siple Dome

A 1-D ice-flow model

•Dansgaard-Johnsen (1969)

•Particle paths in steady state

•Particle paths in transient states

Ice velocity at divide and flank

h as proxy for divide-migration history

Divide always nearby

Divide just passing through

Siple Dome depth-age data

• Annual layer counts from the surface to 514 m depth

• Occluded gases, correlated with GISP2 record (CH4, 18O of O2) Sparse data points for ice 8 ka and older

Steady state with today’s conditions – age (mis)match

Divide passing through

Divide hanging around

o CH4 dates

Layer counts

Steady state with today’s conditions – layer (mis)match

Divide passing through

Divide hanging around

Adjust accumulation to match age

•Steady accumulation, divide nearby•Steady accumulation, divide passing through•Accumulation adjusted to match data

Ice thickness held constant

… and layer thicknesses

•Steady accumulation, divide nearby•Steady accumulation, divide passing through•Accumulation adjusted to match data

Ice thickness held constant

Then, we must accept this accumulation history

•Divide always near

•Divide just passing through

But is constant thickness reasonable?

•How did ice get to edge of continental shelf at LGM, if the ice sheet was no thicker than today?

•Even for SDM ice 200 m thicker, slope to grounding line had to be 10-4, more typical of ice plain or ice shelf, than of ice stream.

Range of possible thickness histories

Divide just passing through

Implied accumulation histories

Sensitivity to thinning rate and timing

0

1

2

Required accumulation histories

Black – thinning from 16 ka to 2 ka

gray – thinning from 8 ka to 2 ka

200 m

400 m

1200 m

Guidance from Byrd and Taylor Dome ice cores

x Taylor Dome

So what is “reasonable”?Byrd ice core shows •~1/3 modern accumulation rate throughout ice age• Regular rise to modern values 20-10 ka BP• Relatively uniform through Holocene• Proximity of SDM and NBY and mid-tropospheric heights even at LGM, suggest that they saw similar storm patterns, and similar accumulation changes.

Taylor Dome shows •slowly rising accumulation over past 6 ka, along with little or no thickness change• SDM and Taylor Dome should track climate together after thick ice left western Ross Embayment at ~6 ka

Taylor Dome

Accumulation increased ~20% since 6 ka BP

Monnin et al. (2004) EPSL

Let’s revisit accumulation histories

Trend comparison of 18O at Taylor Dome and Siple Dome

Differences are due to elevation change at Siple Dome.

Siple Dome was 200 m -300 m thicker before 6 ka

(Steig and White)

•Shows slow rise in accumulation over past 6 ka•Suggests higher(?) accumulation in early Holocene• Suggests our preferred pattern is appropriate

Matt Spencer’s accumulation from bubble

number density

Hatherton-Darwin Glaciers

Anderson, Hindmarsh and Lawson (2004) Global and Planetary Change 42 143-153.

Hatherton Glacier moraines

•Re-interpretation based on transient valley-glacier flow model•200 m thinner than D&H (2000) at Hatherton•SDM only 500 m thicker than present

Anderson, Hindmarsh and Lawson (2004) Global and Planetary Change 42 143-153.

Conclusions - 1

•Our glaciological reconstruction uses data from the center of the Ross Embayment for the first time

•Reconstructions based on geological data from the margins are converging with reconstructions based on glaciological data• ~400 - 500 m thinning at Siple Dome is the common ground.

Conclusions - 2• If the ice divide was always near the core site, SDM was probably at most 200 m thicker at LGM• Even if site was not near divide, the very high accumulation required at 12-10 ka with thick LGM ice looks improbable, particularly in comparison with Byrd core.• Holocene pattern consistent with stable isotopes and Taylor Dome accumulation suggests ~200-300 m of thinning.

Conclusions - 3

•We think that ~400 m or less of thinning is most probable.

•Thinning ended at least several ka ago.

•This contrasts with current thinning at Roosevelt Island and Salzberger Bay

Conclusions - 4Grounded ice reached continental margin around LGM

• If it came from central Ross Embayment (near SDM), then it moved over a very slippery bed

• If bed was not slippery, but SDM was thin, then that ice may have originated farther north and closer to the continental margin, i.e. came through Transantarctic Mountains or from Marie Byrd Land.

Antarctica

NASA-Goddard Space Flight CenterScientific Visualization Studio

Thanks!

Antarctic Ice Sheet

NASA-Goddard Space Flight CenterScientific Visualization Studio