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.
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.
1. Introduction
2. Methods
•
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.
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.
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.
•
Analysed 10Be and 26Al on quartz from granite samples and 36Cl on pyroxenes from basalt samples. Coisotopic analyses were undertaken because complex exposure can be problematic in Antarctica, where
many glaciers are cold-based.
•
•
Our key hypotheses are:
1. 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.
2. 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 4. Situation of granite erratics on Ulu
Peninsula.
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.
A. Isolated granite boulder on a basalt pebblecobble gravel on the interior of Ulu
Peninsula.
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.
B. B. Numerous granite boulders, pebbles and
cobbles within the coastal erratic-rich drift at
the neck of Cape Lachman.
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.
3. 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.
4. 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 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.
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.
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.
Note that the period of rapid icesheet 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.