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 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 Stream 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 Dome 994 m 0 m 100k BPpresent

Matching model with data from Siple Dome 994 m 0 m 115k BP present 115k BP present Layer-thickness profile Accumulation history Depth-age profile

Model results from Siple Dome thick-ice prescribed thin-ice prescribed inferred accumulation history - change from present (0.12 m/yr) inferred accumulation history - change from present (0.12 m/yr) 0.45x present 0.8x present 994 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 ); age is k BP Depth-age relationship (Hammer et al, 1994; Blunier and Brook, 2002) 2180m Old 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 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 present 25k 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 present 120k BP