Climate history near the divide between the Ross and Amundsen Seas by H. Conway, Ed Waddington, Tom Neumann, Ginny Catania, Erin Pettit, Felix Ng and Dave.

Climate history near the divide between the Ross and Amundsen Seas

byH. Conway, Ed Waddington, Tom Neumann, Ginny Catania,

Erin Pettit, Felix Ng and Dave Morse

Acknowledgements: - National Science Foundation

- Raytheon Polar Services (especially Curt LaBombard, Joni English and Jeanine Watkins), Air National Guard and Ken Borek Air for logistical support

- M. Conway for assistance in the field

Ridge AB

Ridge BC

Ice Stream A

Ice

Stre

am C

Photo by Erin

1. Probing for a new site for deep drilling

2. Constraining past climate, thickness and configuration of WAIS

thick-ice LGM reconstruction (from Denton and Hughes, 2001)

Evidence for big ice sheet comes from:

- elevated moraines beside outlet glaciers, some of which have been dated (e.g. Denton et al, 1989; Ackert et al, 1999; Stone

et al, 2003)

- sediment cores from the Ross Sea that indicate grounded ice about 1000km beyond its present position (e.g. Domack et al, 1999; Shipp et al, 1999; Licht et al, 1999; Anderson et al, 2002)

thin-ice LGM reconstruction (from Denton and Hughes, 2001)

Evidence for little ice (at least in the central Ross Sea Embayment) comes from:

- comparison of stable isotopes at Siple Dome and Taylor Dome suggests only modest thinning (O(100m)) of Siple Dome during the Holocene (Steig, 2003)

- thermo-mechanical modeling of the ice streams suggests thickness changes of O(100m) at the present-day grounding line (Parizek et al., 2003, 2002)

Depth and thickness of layer of age A depends on:

1. past climate (accumulation history)

2. past ice dynamics (strain-thinning of a layer)

Climate and thickness history from depth-age data (Waddington et al., 1999; 2001; 2003)

- a thin layer might be produced by a lot of strain-thinning and/or low accumulation in the past and vice versa

- we use the depth-age data to constrain a model of depth-age relationship

- a trade-off between accumulation and dynamics

Siple Dome994 m

0 m

100k BPpresent

Matching model with data from Siple Dome

994 m

0 m

115k BPpresent

115k BPpresent

Layer-thickness profile

Accumulation history

Depth-age profile

Model results from Siple Dome

thick-ice prescribed

thin-ice prescribedinferred accumulation history- change from present (0.12 m/yr)

inferred accumulation history- change from present (0.12 m/yr)

0.45x present

0.8x present994 m

0 m

120k BP present

1750 m

120k BP present

Appeal to data from Byrd

BS68 retrieved in 1968 (Gow et al., 1968)- accumulation = 0.11 m/yr of ice (Gow et al., 1972; Langway et

al, 1994)

- depth-age (Hammer et al., 1994; Blunier and Brook, 2001)- thickness = 2164m; thinned 200m during Holocene (Steig

et al., 2001)- chemistry and volcanics (Gow and Williamson, 1971; Kyle et

al., 1981; Palais, 1985; Palais et al., 1988; Wilch et al., 1999)

prescribe 200m thinning during Holocene

inferred accumulation history- change from present (0.11 m/yr)

0.7x present

2148m

0m

120k BP present 120k BP present

Ground-based 1.5 MHz radar (2003):- low-frequency radar-detected layers are probably isochrones (acidity contrast inhereted from

snow deposition)- bright layer (“old faithful”) at 1280m - corresponds to “off-scale acidity … due to excessive

volcanism” (Hammer et al. 1994); age is 17.5-18k BP

Depth-age relationship (Hammer et al, 1994; Blunier

and Brook, 2002)

Bed @ 2180m

Old faithful @ 1280m

Bed

Old faithful

Byrd core

Measurements (guided by SOAR data from Morse/Blankenship):

- radar at 1MHz, 1.5MHz, 7 MHZ, and 200MHz along flow lines

- GPS surveys of poles (strain grid comprised of 100 poles)

Results from Inland Site E:Measurements so far ….

- accumulation = 0.22 m/a ice - from tracking continuous near surface radar layers (200 MHz) back to an ITASE core where accumulation is 0.24 m/a (pers comm, Dan Dixon)

- depth-age relationship from tracking radar layers (1.5MHz) back to Byrd

bed @ 3460 m (from low frequency radar)

Old faithful

+

+

+

+

+

+

Model results from Inland site E

- site E is 30 km from present divide. Prescribe no thickness change in past 20 k yrs

- limited because the radar-derived time scale extends back only 17.5 kyrs

inferred accumulation history(change from the modern 0.22 m/yr)

0.6x present

Photo by Erin

Old faithful

Model results

present25k BP

3460m

0m

Summary

1. Inversion of depth-age data to estimate ice sheet thinning is complicated by the trade-off between accumulation and dynamics

2. data from Byrd and preliminary data from Inland WAIS indicate that caution is needed when adapting an accumulation history inferred from Vostok; results suggest that accumulation during the glacial in WAIS may have been

as much as 60-70% that of today

3. Need to look at all available data; the new Inland Wais core will add more pieces to the puzzle

Vostok:accumulation history derived from temperatures inferred from stable isotopes

0.45x present

present120k BP