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