Introduction

1. Ice stream controls on Antarctic deglaciation Bethan Davies, Neil Glasser, Jonathan Carrivick, Mike Hambrey, John Smellie bdd@aber.ac.uk Funded by NERC grant AFI 9/01 (NE/F012942/1), awarded to Professor Neil Glasser. • Methods • 2.1 Remote-Sensing • Mapped geomorphology and glaciology of study region following standard procedures (Glasser et al, 2008; 2009). These maps were ‘ground-checked’ during fieldwork. • Data sources: ASTER and SPOT-5 satellite imagery, BAS aerial photographs, and DEMs created by the SPIRIT programme and by the Czech Geological Survey. • 2.2 Geomorphological mapping • 7-week onshore field campaign (January to March 2011) to Ulu Peninsula. Field techniques included geomorphological mapping, sedimentological analysis and logging of ice-cored facies. • 2.3 Cosmogenic Isotope Dating • Basalt and granite erratic rock samples were collected from boulders on moraine ridge crests or glacigenic surfaces. 50 samples were taken from Ulu Peninsula and 18 from Terrapin Hill. Samples were taken from boulders measuring > 1m b-axis, and sample depths were < 5 cm. • Analysed10Be and 26Al on quartz from granite samples and 36Cl on pyroxenes from basalt samples. Co-isotopic analyses were undertaken because complex exposure can be problematic in Antarctica, where many glaciers are cold-based. • Introduction • The northern Antarctic Peninsula is an area of considerable uncertainty regarding Last Glacial Maximum (LGM) ice sheet dynamics, elevation and extent. The onshore record is uncertain and chronostratigraphy is poor. There is a need for an independent dating programme. • Marine geological evidence indicates that the Antarctic Peninsula Ice Sheet (APIS) reached the continental shelf edge during the LGM, with ice streaming in Prince Gustav Channel (Fig. 1), forming mega-scale glacial lineations (MSGLs) (Evans et al., 2005). Ice began to recede at ~18 ka (Davies et al., 2012). • Key unknowns include: (i) whether the Mount Haddington Ice Cap remained as an independent ice dome; (ii) ice thickness at the LGM and rates of ice sheet thinning; (iii) how and when Prince Gustav Ice Stream developed. Figure 3. Sampling Trinity Peninsula granite boulders for cosmogenic nuclide exposure age dating. This boulder is at Kaa Bluff, a degraded moraine fragment with abundant granite erratics. • The aim of this project is to reconstruct ice dynamics and the thinning and wasting chronology of the NE Antarctic Peninsula during and after the LGM. • Our key hypotheses are: • During the LGM, James Ross Island was inundated by a thick APIS, depositing Trinity Peninsula erratics with older exposure ages at high-elevations across the island. • Prince Gustav Ice Stream developed during deglaciation, depositing erratics with younger exposure ages at low altitudes on coastal sites. Figure 2. Isolated granite boulders on a blockfield on Lachman Mesa on Ulu Peninsula A B Figure 6. Co-isotope plot of 26Al/10Be versus 10Be. Theoretical cosmogenic concentrations in a surface affected by no erosion and in a surface in erosion equilibrium are depicted by black lines according to CRONUS production rates. External uncertainties of these lines due to a 6% error in both 10Be and 26Al production rates are represented by the grey areas. This plot suggests that no burial and exhumation of samples has occurred since deposition. Figure 7. Conceptual figure showing ice stream initiation during deglaciation. A warming climate resulted in a more dynamic ice sheet, with ice streams penetrating further inland towards the centre of the ice sheet, drawing down and contributing to thinning. Figure 4. Situation of granite erratics on Ulu Peninsula. Isolated granite boulder on a basalt pebble-cobble gravel on the interior of Ulu Peninsula. B. Numerous granite boulders, pebbles and cobbles within the coastal erratic-rich drift at the neck of Cape Lachman. Figure 8. A. Ice core temperature record from the Mount Haddington ice core (Mulvaney et al., 2012). Temperature anomaly compared with 1961-1990 mean; 100 year average. B. Relative sea level record from Beak Island, Prince Gustav Channel (near James Ross Island; Roberts et al., 2012), with a suite of coupled sea-level models and marine microfossils from James Ross Island. C. Sample altitude and mean age showing ice-sheet thinning, and the cluster of 10Be ages related to the recession of Prince Gustav Ice Stream (this study). Triangles indicate high elevation granite boulders on basalt-rich lag surfaces at high elevations (>360 m a.s.l.) (4 samples) deposited by a thick, cold APIS. Diamonds indicate Trinity Peninsula erratic boulders on coastal areas of James Ross Island within the erratic-rich drift (5 samples), deposited by Prince Gustav Ice Stream. Squares (2 samples) indicate lower elevation samples deposited by the thinning ice sheet in the interior of Ulu Peninsula. Note that the period of rapid ice-sheet thinning and the onset of Prince Gustav Ice Stream observed on Ulu Peninsula coincides with rapid regional temperature increases and rapid eustatic sea level rise; the youngest deglaciation ages coincide with a period of rapid isostatic uplift on nearby Beak Island. Figure 1. Geological map of James Ross Island. Granite erratics from Trinity Peninsula are found on Ulu Peninsula. Note the glacial lineations mapped in Prince Gustav Channel. Previously published ages are provided. • Results • Ulu Peninsula is smooth and sculpted with widespread glacial drift. Granite erratics were found in three different situations (i) Isolated granite and basalt erratics on basalt-rich lag surfaces (Fig. 2); (ii) Isolated granite and basalt erratics on basalt-rich lag mesas (Fig. 4A); (iii) Coastal areas with numerous granite boulders and high percentages of erratics (Fig. 4B), including a moraine fragment at Kaa Bluff (144 m; Fig. 3). These areas are hatched on Fig. 5. • Co-isotope analysis (Be and Al ratios) shows no evidence for complex exposure histories (Fig. 6). Older exposure ages of granite boulders on tops of mesas (Fig. 5) indicate that the area was overrun by Trinity Peninsula ice during LGM glaciation, with the ice-sheet crossing the deep Prince Gustav Channel. Some mesas retained small ice caps until ~8 to ~6.6 ka. • Glacial drift with abundant Trinity Peninsula erratics in coastal areas (Fig. 4D, E) was deposited by Prince Gustav Ice Stream (Fig. 5). The northernmost promontory became ice-free around ~13 ka, western Brandy Bay ~9 ka and the interior of Ulu Peninsula at ~6 ka. • Conclusions • Prince Gustav Ice Stream was activated during deglaciation and thinning, (Fig 7), during a period of rapid warming and relative sea level rise (Fig. 8). The maximum altitude of the ice stream was 144 m. • Ice streams exerted a strong control on the deglaciation of the Antarctic Peninsula Ice Sheet. Dynamical ice-sheet behaviour during deglaciation is predicted from empirical and modelling studies, which show that rapid changes in ice-sheet reorganisation occur during deglaciation and that these changes influence rates of deglaciation and meltwater discharge. • Although deglacial ice-stream initiation has been inferred for former mid-latitude ice sheets, this is the first robustly dated example of ice-stream initiation, duration, thickness and thinning from Antarctica. Figure 5. Geomorphological map of Ulu Peninsula, James Ross Island. There are large, prominent ice-cored moraines around modern cirque glaciers and a large moraine flanks Brandy Bay. Red circles indicate granite boulders > 1m b axis. Previously published cosmogenic nuclide exposure ages are shown. Cosmogenic ages from this study are shown in blue labels. Note the cross-hatched regions, which indicate high percentages of granite erratics. Moraine fragments associated with granite erratics are found at Kaa Bluff and St. Martha Cove. Note the high erratics on the top of Lachman Mesa have older ages, while the ages of boulders within the coastal erratic-rich drift are younger, and represent the lateral margins of Prince Gustav Ice Stream. • Acknowledgements • We gratefully acknowledge the logistical support provided by BAS and the Royal Navy, particularly the captains and crew of RRS Ernest Shackleton,RRS James Clark Ross, and HMS Protector. This project is funded by NERC grant AFI 9/01 (NE/F012942/1) awarded to Professor Neil Glasser. The team acknowledge support from Alpkit, who provided down jackets and other clothing. • References • Davies, B.J. et al., 2012. Antarctic Peninsula Ice Sheet Evolution during the Cenozoic Era. Quat. Sci. Rev. 31, 30-66. • Evans, J., et al. 2005. Late Quaternary glacial history, flow dynamics and sedimentation along the eastern margin of the Antarctic Peninsula Ice Sheet. Quat. Sci. Rev. 24, 741-774. • Glasser, N.F. et al., 2008. The glacial geomorphology and Pleistocene history of South America between 38°S and 56°S. Quat. Sci. Rev., 27: 365-390. • Glasser, N.F., et al., 2009. Surface structure and stability of the Larsen C Ice Shelf, Antarctic Peninsula. J. Glac. 55: 400-410. • Roberts, S.J. et al., 2011. Geological constraints on glacio-isostatic adjustment models of relative sea-level change during deglaciation of Prince Gustav Channel, Antarctic Peninsula. Quat. Sci. Rev. 30: 3603-3617. • Mulvaney, R. et al., 2012. Recent Antarctic Peninsula warming relative to Holocene climate and ice-shelf history. Nature.