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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