1. 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

2. From R. Bindschadler - Has the retreat stopped or is it ongoing? - How thick was the WAIS at LGM?

3. 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?

4. 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?

5. Where are the dipsticks? The only mountain dipsticks are far away … Ross Embayment has no outcrops in 2x10 6 km 2 We can’t use trimlines, cosmogenic isotopes, etc … What are we going to do?

6. Use the stratigraphy of the ice …?

7. Accumulation rate and depth-age Higher accumulation rate means Thicker annual layers Younger ice at a given depth Depth-age scale records accumulation history

8. Ice deforms Ice moves away from site of deposition layers get thinner Steady-state layer thinning

9. Steady-state depth-age

10. Steady-state layers

11. 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.

12. Thinning ice cap

13. Thinning depth-age Thinning ice sheet with constant accumulation Steady state

14. Thinning layers Steady state Thinning ice sheet with constant accumulation

15. If ice sheet has always been thinning … Then infinitely old ice is high off the bed Infinite age Event Horizon

16. Depth-age with “false bottom”

17. South North Asymmetric arch indicates that divide is moving to the right at 0.5 meters per year. Siple Dome

18. A 1-D ice-flow model Dansgaard-Johnsen (1969) Particle paths in steady state Particle paths in transient states

19. Ice velocity at divide and flank

20. h as proxy for divide-migration history Divide always nearby Divide just passing through

21. Siple Dome depth-age data Annual layer counts from the surface to 514 m depth Occluded gases, correlated with GISP2 record (CH 4,  18 O of O 2 ) Sparse data points for ice 8 ka and older

22. Steady state with today’s conditions – age (mis)match Divide passing through Divide hanging around o CH 4 dates Layer counts

23. Steady state with today’s conditions – layer (mis)match Divide passing through Divide hanging around

24. Adjust accumulation to match age Steady accumulation, divide nearby Steady accumulation, divide passing through Accumulation adjusted to match data Ice thickness held constant

25. … and layer thicknesses Steady accumulation, divide nearby Steady accumulation, divide passing through Accumulation adjusted to match data Ice thickness held constant

26. Then, we must accept this accumulation history Divide always near Divide just passing through

27. 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.

28. Range of possible thickness histories Divide just passing through

29. Implied accumulation histories

30. Sensitivity to thinning rate and timing 0 1 2

31. 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

32. Guidance from Byrd and Taylor Dome ice cores x Taylor Dome

33. 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

34. Taylor Dome Accumulation increased ~20% since 6 ka BP Monnin et al. (2004) EPSL

35. Let’s revisit accumulation histories

36. Trend comparison of  18 O 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)

37. 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

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

39. 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.

40. 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.

41. 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.

42. 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

43. Conclusions - 4 Grounded 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.

44. Antarctica NASA-Goddard Space Flight Center Scientific Visualization Studio

45. Thanks!

46. Antarctic Ice Sheet NASA-Goddard Space Flight Center Scientific Visualization Studio