1) Researchers have discovered a 750-km long subglacial canyon beneath the central Greenland ice sheet using ice-penetrating radar data. 2) The canyon has a depth of up to 800 m and width of up to 10 km near the coast, and likely influenced basal water flow beneath the ice sheet over past glacial cycles. 3) Modeling of hydraulic potential suggests the canyon provided a pathway for subglacial water flow both before and during ice sheet formation, influencing the basal hydrology of northern Greenland.

1. DOI: 10.1126/science.1239794
, 997 (2013);341Science
et al.Jonathan L. Bamber
Paleofluvial Mega-Canyon Beneath the Central Greenland Ice Sheet
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2. 11. P. S. Lykawka, T. Mukai, Icarus 192, 238–247 (2007).
12. L. Dones et al., Icarus 142, 509–524 (1999).
13. M. S. Tiscareno, R. Malhotra, Astron. J. 126, 3122–3131
(2003).
14. M. H. M. Morais, A. Morbidelli, Icarus 160, 1–9 (2002).
15. M. H. M. Morais, A. Morbidelli, Icarus 185, 29–38
(2006).
16. J.-M. Petit et al., Astron. J. 142, 131 (2011).
17. N. A. Kaib, R. Roskar, T. Quinn, Icarus 215, 491–507
(2011).
18. J. Horner, N. Wyn Evans, Mon. Not. R. Astron. Soc. 367,
L20–L23 (2006).
19. H. F. Levison, M. J. Duncan, Icarus 127, 13–32 (1997).
20. B. Gladman et al., Astron. J. 144, 23 (2012).
21. J. L. Elliot et al., Astron. J. 129, 1117–1162 (2005).
22. C. Shankman, B. J. Gladman, N. Kaib, J. J. Kavelaars,
J. M. Petit, Astrophys. J. Lett. 764, L2 (2013).
23. A. A. Christou, Icarus 144, 1–20 (2000).
Acknowledgments: The data are available at the IAU Minor
Planet Center’s online database under MPEC K13F19. We
thank N. Kaib for making the scattering object model available
to us. We thank S. Lawler, C. Shankman, and N. Kaib for
proofreading and constructive comments. M.A., S.G., and
B.G. were supported by the National Sciences and Engineering
Research Council of Canada. This work is based on observations
obtained at the Canada-France-Hawaii Telescope, operated
by the National Research Council of Canada, the Institut
National des Sciences de l’Univers of the Centre National de la
Recherche Scientifique of France, and the University of Hawaii.
Supplementary Materials
www.sciencemag.org/cgi/content/full/341/6149/994/DC1
Supplementary Text
Figs. S1 to S4
References (24–51)
20 March 2013; accepted 25 July 2013
10.1126/science.1238072
Paleofluvial Mega-Canyon Beneath
the Central Greenland Ice Sheet
Jonathan L. Bamber,1
* Martin J. Siegert,1
Jennifer A. Griggs,1
Shawn J. Marshall,2
Giorgio Spada3
Subglacial topography plays an important role in modulating the distribution and flow of basal
water. Where topography predates ice sheet inception, it can also reveal insights into former
tectonic and geomorphological processes. Although such associations are known in Antarctica, little
consideration has been given to them in Greenland, partly because much of the ice sheet bed is
thought to be relatively flat and smooth. Here, we present evidence from ice-penetrating radar
data for a 750-km-long subglacial canyon in northern Greenland that is likely to have influenced
basal water flow from the ice sheet interior to the margin. We suggest that the mega-canyon
predates ice sheet inception and will have influenced basal hydrology in Greenland over past
glacial cycles.
G
reenland is an ancient craton dominated
by crystalline rocks of the Precambrian
shield,composed largely of Archaean [3100
to 2600 million years (My) before the present (B.P.)]
and Proterozoic (2000 to 1750 My B.P.) forma-
tions, with more limited sedimentary deposits along
parts of the continental margins (1). These rocks
are resistant to glacial erosion compared with the
sedimentary deposits that are thought to underlie
substantial sectors of Antarctica (2, 3). Although
there is evidence for some glaciation dating back
to 38 My B.P. (4), the island has only been exten-
sively ice-covered for no more than ~3.5 My (5),
as compared with the Antarctic ice sheet that first
grew at ~34 My B.P., and has existed persistently
since ~14 My B.P. There are, therefore, important
geologic and glacio-morphological differences
between the subglacial environments of the two
ice sheets.
Ice-penetrating radar(IPR) iscapable ofimaging
the ice-sheet base and, if collected as a series of
transects, can reveal the landscape beneath the
ice. In Greenland, such data have been acquired
on numerous occasions over the past 40 years
(6, 7). Using these data in combination with ice-
surface topography measurements, a quasilinear
feature was previously identified in northern
Greenland from a combination of ice-surface
topography and IPR data, but with limited knowl-
edge on its extent and configuration (8). By in-
specting IPR-derived basal topography from these
and more recent data sources, we have discovered
this bed feature to be a major subglacial canyon
that extends from almost as far south as Summit
in central Greenland to the northern margin of
the ice sheet, terminating in the fjord that drains
Petermann Gletscher (Fig. 1). The extent of the
canyon is at least ~750 km, but its southern limit
may continue further than we are able to identify
because the density of IPR tracks below 74.3° N
is limited. However, two tracks at 73.8° N, 42° W
cross an over-deepening that appears likely to be
the southern extension of the canyon. There is no
obvious evidence of a geological boundary in this
part of Greenland (9). The southern limit of the
canyon, however, does lie close to a minimum in
the free-air gravity anomaly and a maximum in the
terrain-correlated gravity anomaly, which indi-
cates crust that is isostatically thin and subject to
compressive inflow of crustal material (9), which
could form an explanation for its genesis.
The IPR data indicate that the canyon has a
depth and width of up to ~800 m and ~10 km,
respectively, toward the coast and a slightly re-
duced width and substantially shallower depth of
~200 m further south (Fig. 2 and figs. S1 and S2).
The valley width-to-height ratio (used as a mea-
sure of river channel morphology and formation)
for the profile in Fig. 2C is 30, which is indicative
of a broad, shallow channel, subject to limited
stream erosion (10, 11). The ratio for the profile
in Fig. 2A is ~0.5, suggesting linear stream in-
cision was important in the development of the
channel at this location, with no evidence of gla-
cial erosion being the dominant process (12). In-
deed, none of the profiles are typical of glacially
eroded valleys (fig. S2). The canyon follows a
1
Bristol Glaciology Centre, University of Bristol, BS8 1SS Bristol,
UK. 2
Department of Geography, University of Calgary, T2N 1N4
Calgary, Canada. 3
Dipartimento di Scienze di Base e Fondamenti,
Urbino University, 61029 Urbino, Italy.
*Corresponding author. E-mail: j.bamber@bristol.ac.uk
Fig. 1. Bedelevation[between–900and900m
above sea level (asl)] for northern Greenland.
The area plotted is indicated by the red box in the
inset. Airborne ice penetrating radar flight lines are
shown in gray, and the three bed profiles plotted in
Fig. 2 are shown by the solid blue (Fig. 2A), black
(Fig. 2B), and red (Fig. 2C) lines. The mauve line
shows the approximate location of the grounding
line on Petermann Gletscher. Other places discussed
in the text are also labeled: NGRIP, the onset area
of the North East Greenland Ice Stream (NEGIS),
and the camp near the highest point of the ice sheet,
Summit. The red ellipse marks the approximate
location of a marked minimum in crustal thickness,
and hence maximum in geothermal heat flux, iden-
tified from gravity data (9).
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3. meandering path more typical of a large river
system. The dimensions of the Greenland canyon
are comparable with parts of the Grand Canyon
in the United States in terms of width and length
and, in its deepest region, are about half of the
Grand Canyon’s depth.
To assess the canyon’s influence on the flow
of subglacial water, and the extent to which it was
a mega-channel for surface water, we calculated
the hydraulic potential for both the present-day,
ice-free, and last glacial maximum (LGM) con-
figurations of the Greenland ice sheet (Fig. 3),
accounting where necessary for isostatically com-
pensated ice-free bedrock. We used the methods
described in Shreve (1972) (13), which have been
commonly applied to determine subglacial hydro-
logical pathways (14). Because of its size, the
canyon provides a hydraulic pathway in all cases
tested, suggesting that it has influenced basal
water flow in northern Greenland since ice sheet
inception and, indeed, before this. For the ice-free
(preglacial) state (Fig. 3A and fig. S3B), water is
predicted to flow south-to-north along the ma-
jority of the canyon. The isostatically compensated
channel gradient, without the ice sheet load, is
0.3 m km−1
, which is similar to the southern 500 km
of the Colorado River and around three times
larger than the average for the Mississippi River.
Regions where water diverts away from the canyon
(such as at 79.3° N, 48° W) are coincident with
limited IPR data coverage (Fig. 1), implying poor
resolution of the bed topography in such places.
Hence, we believe water will likely have routed
Fig. 2. Ice-penetrating radargram profiles across the canyon at three locations. The locations are
indicated in Fig. 1 by (A) the solid blue line, (B) the black line, and (C) the red line. There is an
exaggeration in the vertical by a factor of 26. The depth of the canyon in meters below sea level is
indicated in each profile alongside internal reflections within the ice and the bed return. North is
roughly into the page.
Fig. 3. Subaerial and subglacial hydraulic potential flowpaths. (A) An
isostatically compensated bed without an ice sheet. (B) An ice sheet con-
figuration at LGM. (C) The present-day ice sheet and bed configuration.
H, Helheim; P, Petermann; R, Ryder. The bed elevations shown are all for
present day. The isostatically compensated bed elevation is illustrated in
fig. S3B.
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4. continually along this feature before extensive ice
sheet formation ~3.5 My B.P. (5).
In full glacial conditions [based on LGM
(supplementary materials)], the flow of water is
influenced by the ice overburden pressure, which
forces water out of the canyon in parts of the
upper Petermann catchment and along the direc-
tion of ice flow (Fig. 3B). Nonetheless, the canyon
remains a conduit for the flow of basal water
toward the coast even under LGM conditions, es-
pecially northward of ~79° N. For the present ice-
sheet configuration, which is small and short-lived
compared with its expanded full glacial state,
steeper ice surface slopes (relative to LGM) force
more water from the canyon to the west at certain
locations (Fig. 3C and fig. S3A). In all cases,
above ~76° N and within the entire length of the
Petermann catchment, the canyon exerts a control
on basal water flow. For ~200 km, it provides an
uninterrupted hydraulic pathway (Fig. 3 and fig.
S3A) that ends at the terminus of Petermann
Gletscher. However, although extensive parts of
northern Greenland have been identified as having
a wet bed (fig. S4) (15), water currently may not
be ubiquitous throughout its length. Under full
glacial conditions, the ice sheet was larger and
thicker than at present day (4), and therefore, a
greater proportion of the bed was likely melt-
ing during the longer lasting LGM ice sheet
configuration.
The lack of substantial subglacial lakes in the
main Greenland ice sheet may be partly explained
by generally steeper ice surface slopes as com-
pared with those of Antarctica, but subglacial
water must be evacuated by some means or it will
pool. The subglacial canyon provides an efficient
pathway for this water routing over an otherwise
relatively flat bed (Fig. 3C). Given that water
beneath former ice sheets in Greenland was likely
to have been influenced by the canyon, it seems
probable that conditions have never been well suited
to substantial long-term basal water storage in
this sector of Greenland.
The floating ice shelf in front of Petermann
Gletscher has been found to possess sub–ice-
shelf channels 1 to 2 km wide and 200 to 400 m
deep incised upward into the ice (16). These chan-
nels are associated with high basal melt rates,
exceeding 30 m year−1
. Their existence has been
attributed entirely to the action of the ocean (16).
We believe the efficient routing of basal water via
the canyon to the northern ice sheet margin is also
important in explaining these features. Analysis
of IPR echo-strength data has identified extensive
regions of the northern half of the Greenland ice
sheet that have water at the bed (15). Subglacial
melt rates in the vicinity of the North Greenland
Ice Core Project (NGRIP) drill site have been
estimated to exceed 1 cm year−1
(17) and are as
high as 15 cm year−1
near the onset of the North-
east Greenland Ice Stream (18). A study exam-
ining crustal thickness and geothermal heat flow
in Greenland found a minimum in the former (and
hence, a maximum in the latter) close to the north-
ern limit of the canyon (Fig. 1) (9). Thus, sub-
stantial volumes of subglacial meltwater are
generated at the bed, and additional surface melt-
water may also penetrate to the bed near the coast
(19). An effective means by which this basal water
is routed to the ice-sheet margin must exist, else
subglacial lakes would form. Basal water gen-
erated in the Petermann catchment is highly likely
to be routed through the Canyon to the ice sheet
terminus (Fig. 3C). Thus, basal water is likely to
be delivered at a point source to the ice-shelf cav-
ity, which is likely to influence the local subshelf
water circulation (20). Similar sub-shelf channels
have been identified underneath Pine Island
glacier and elsewhere in Antarctica (21), and we
suggest that well-organized subglacial meltwater
flow is likely to play a similar role in the de-
velopment of these features.
References and Notes
1. N. Henriksen, A. K. Higgins, F. Kalsbeek, T. C. R. Pulvertaft,
Geol. Surv. Denmark Greenland Copenhagen 18, 126 (2009).
2. D. J. Drewry, Tectonophysics 36, 301 (1976).
3. M. Studinger et al., Geophys. Res. Lett. 28, 3493–3496
(2001).
4. R. B. Alley et al., Quat. Sci. Rev. 29, 1728–1756 (2010).
5. G. Bartoli et al., Earth Planet. Sci. Lett. 237, 33–44 (2005).
6. J. L. Bamber et al., Cryosphere 7, 499–510 (2013).
7. Data set for (6) is available at https://docs.google.com/
file/d/0BylqEEvDu_qtWWdIYTFVcVpkd2s/edit?usp=
sharing).
8. S. Ekholm, K. Keller, J. L. Bamber, S. P. Gogineni,
Geophys. Res. Lett. 25, 3623–3626 (1998).
9. A. Braun, H. R. Kim, B. Csatho, R. R. B. von Frese, Earth
Planet. Sci. Lett. 262, 138–158 (2007).
10. W. B. Bull, L. D. McFadden, in Geomorphology in Arid
Regions, D. O. Doehring, Ed. (State University of New
York, New York, 1977), pp. 115–139.
11. The valley height-to-width ratio is Vf = 2Vfw/[(Eld – Esc) +
(Erd – Esc)], where Vfw is the valley floor width, Esc is the
valley floor elevation, Eld is the elevation of the left river
divide, and Erd is elevation of the right river divide.
12. M. A. Summerfield, Global Geomorphology. An
Introduction to the Study of Landforms (Longman,
Harlow, UK, 1991).
13. R. Shreve, J. Glaciol. 11, 205 (1972).
14. G. W. Evatt, A. C. Fowler, C. D. Clark, N. R. J. Hulton,
Phil. Trans. R. Soc. A 364, 1769–1794 (2006).
15. G. K. A. Oswald, S. P. Gogineni, IEEE Trans. Geosci.
Rem. Sens. 50, 585–592 (2012).
16. E. Rignot, K. Steffen, Geophys. Res. Lett. 35, L02503
(2008).
17. D. Dahl-Jensen, N. Gundestrup, S. P. Gogineni,
H. Miller, Ann. Glaciol. 37, 207–212 (2003).
18. M. Fahnestock, W. Abdalati, I. Joughin, J. Brozena,
P. Gogineni, Science 294, 2338–2342 (2001).
19. I. Joughin, S. Tulaczyk, M. Fahnestock, R. Kwok, Science
274, 228–230 (1996).
20. F. Straneo et al., Nat. Geosci. 4, 322–327 (2011).
21. D. G. Vaughan et al., J. Geophys. Res. 117, F03012 (2012).
Acknowledgments: J.L.B., J.A.G., and G.S. were supported by
funding from the ice2sea program from the European Union
7th Framework Programme, grant 226375. This work is ice2sea
contribution number 158. J.L.B. and M.J.S. were supported by
Natural Environment Research Council grant NE/H021078/1.
We acknowledge the use of data products from CReSIS
generated with support from NSF grant ANT-0424589, NASA
grant NNX10AT68G, and the NASA Operation IceBridge
project. J.L.B. wrote the main text with substantial contribu-
tions from M.J.S. and input from all authors. J.A.G. carried out
the hydraulic potential calculations. G.S. calculated the
isostatically adjusted bed, and S.J.M. produced the LGM ice
sheet configuration.
Supplementary Materials
www.sciencemag.org/cgi/content/full/341/6149/997/DC1
Materials and Methods
Figs. S1 to S5
References (22–30)
29 April 2013; accepted 31 July 2013
10.1126/science.1239794
Social Learning of Migratory Performance
Thomas Mueller,1,2,3
* Robert B. O’Hara,2
Sarah J. Converse,4
Richard P. Urbanek,5
William F. Fagan1
Successful bird migration can depend on individual learning, social learning, and innate navigation
programs. Using 8 years of data on migrating whooping cranes, we were able to partition
genetic and socially learned aspects of migration. Specifically, we analyzed data from a reintroduced
population wherein all birds were captive bred and artificially trained by ultralight aircraft on
their first lifetime migration. For subsequent migrations, in which birds fly individually or in
groups but without ultralight escort, we found evidence of long-term social learning, but no effect
of genetic relatedness on migratory performance. Social learning from older birds reduced
deviations from a straight-line path, with 7 years of experience yielding a 38% improvement
in migratory accuracy.
M
echanisms underlying the complex phe-
nomenon of animal migration have been
particularly well studied in birds (1–5).
In some taxa, individuals may migrate alone, un-
aided by conspecifics and relying instead mostly
on endogenous, genetically inherited navigation
programs (6). In other species, innate programs
alone are not sufficient, and experiential learning
is critical to successful navigation, as adult ani-
mals often have markedly better navigational ca-
pabilities than juveniles (7–9). Information transfer
from more experienced individuals to inexperi-
enced ones can be essential to navigational success,
1
Department of Biology, University of Maryland, College Park,
MD 20742, USA. 2
Biodiversity and Climate Research Centre
(BiK-F), Senckenberg Gesellschaft für Naturforschung, and Goethe
Universität Frankfurt, Senckenberganlage 25, 60325 Frankfurt
(Main), Germany. 3
Smithsonian Conservation Biology Institute,
National Zoological Park, 1500 Remount Road, Front Royal, VA
22630, USA. 4
U.S. Geological Survey, Patuxent Wildlife Research
Center, 12100Beech Forest Road,Laurel, MD 20708,USA. 5
U.S.
Fish and Wildlife Service, Necedah National Wildlife Refuge,
N11385 Headquarters Road, Necedah, WI 54646, USA.
*Corresponding author. E-mail: muellert@gmail.com
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