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The influence of major Quaternary climatic changes on growth and decay of the Greenland Ice Sheet, and associated erosional impact on the landscapes, is virtually unknown beyond the last deglaciation. Here we quantify exposure and denudation histories in west Greenland by applying a novel Markov-Chain Monte Carlo modelling approach to all available paired cosmogenic 10Be-26Al bedrock data from Greenland. We find that long-term denudation rates in west Greenland range from >50 m/Myr in low-lying areas to ~2 m/Myr at high elevations, hereby quantifying systematic variations in denudation rate among different glacial landforms caused by variations in ice thickness across the landscape. We furthermore show that the present day ice-free areas only were ice covered ca. 45% of the past 1 million years, and even less at high-elevation sites, implying that the Greenland Ice Sheet for much of the time was of similar size or even smaller than today.
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ARTICLE
Received 19 May 2016 |Accepted 6 Dec 2016 |Published 18 Jan 2017
One million years of glaciation and denudation
history in west Greenland
Astrid Strunk1, Mads Faurschou Knudsen1, David L. Egholm1, John D. Jansen2, Laura B. Levy1, Bo H. Jacobsen1
& Nicolaj K. Larsen1,3
The influence of major Quaternary climatic changes on growth and decay of the Greenland
Ice Sheet, and associated erosional impact on the landscapes, is virtually unknown beyond the
last deglaciation. Here we quantify exposure and denudation histories in west Greenland by
applying a novel Markov-Chain Monte Carlo modelling approach to all available paired
cosmogenic 10Be-26Al bedrock data from Greenland. We find that long-term denudation rates
in west Greenland range from 450 m Myr 1in low-lying areas to B2 m Myr 1at high
elevations, hereby quantifying systematic variations in denudation rate among different
glacial landforms caused by variations in ice thickness across the landscape. We furthermore
show that the present day ice-free areas only were ice covered ca. 45% of the past 1 million
years, and even less at high-elevation sites, implying that the Greenland Ice Sheet for much of
the time was of similar size or even smaller than today.
DOI: 10.1038/ncomms14199 OPEN
1Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, Aarhus 8000, Denmark. 2Institute of Earth and Environmental Science, University
of Potsdam, Karl-Liebknecht-Str 24, Potsdam 14476, Germany. 3Centre for GeoGenetics, Natural History Museum of Denmark, University of Copenhagen,
Øster Voldgade 5-7, Copenhagen K 1350, Denmark. Correspondence and requests for materials should be addressed to A.S. (email: astrid@geo.au.dk).
NATURE COMMUNICATIONS | 8:14199 | DOI: 10.1038/ncomms14199 | www.nature.com/naturecommunications 1
The Greenland Ice Sheet (GrIS) has been waxing and waning
through multiple glacial-interglacial cycles over the past
few millions of years, thereby sculpting the landscape we
observe today. Little is known of the temporal and spatial extent
of the GrIS before the last deglaciation, because subsequent
erosion has removed most of the evidence. Indeed, the Greenland
ice-core records span only the last B100 kyr (ref. 1), hence the
glacial history predating the last glacial cycle remains elusive. As
the GrIS advanced onto the continental shelf during the
Last Glacial Maximum (LGM), it efficiently eroded deposits
from earlier interglacial and glacial periods, leaving only scattered
onshore2–4 and offshore5–8 fragments of the longer-term geolo-
gical record. Most Quaternary deposits preserved between the
outer coast and the present ice margin have a Lateglacial or
Holocene origin, pointing to efficient erosion and a rapid ice
retreat across this area during the last deglaciation9,10. Over
previous glacial cycles, the ice sheet presumably expanded to
cover the continental shelf and then retreated inboard of the
present ice margin, but well-dated geological evidence testing this
hypothesis is lacking.
The origin and age of the ice-sculpted landscapes in Greenland
may be studied via quantification of past denudation rates (that is,
the removal of mass via physical and chemical weathering), in
particular spatial variations in denudation rate. Bedrock erosion
rates averaged over the Quaternary in Greenland are relatively
unconstrained, but estimated at B40 m Myr 1based on
North Atlantic marine sediment volumes11,12. This bulk
estimate combines sediment yield from diverse erosion regimes,
including diffusive areal scouring and incision focused along
valley troughs and fjords. Large parts of the fjord landscapes cut
into the west Greenland coastal margin were shaped by selective
linear glacial erosion, characterized by large spatial differences in
erosion due to variations in the subglacial thermal regime13. Here,
deep troughs and/or fjords dissect low-relief plateaus that
presumably were preserved over multiple glacial cycles under
thin and weakly erosive, cold-based ice. The troughs, in contrast,
were carved by thick ice masses with basal ice at the pressure-
melting point, which facilitates sliding and efficient bedrock
erosion14. Identifying how and when the present topography of
Greenland was established entails quantifying spatial patterns of
past denudation rates, which presents a considerable challenge in
such landscape settings imprinted by differential thermal glacial
regimes15,16.
Cosmogenic nuclides provide a widely used tool for quantify-
ing landscape denudation rates and exposure history17. The
method utilizes the constant bombardment of Earth by cosmic
rays produced in supernova explosions, which gives rise to
secondary cosmic-ray cascades in the atmosphere. When the
secondary neutrons and muons penetrate an exposed rock
surface, nuclear reactions produce in situ terrestrial cosmogenic
nuclides (TCNs), of which common nuclides used for dating
purposes are 10Be and 26Al (ref. 17). Depending on the geological
setting, TCNs are commonly used to date the timing of exposure,
which in glacial landscapes typically means the last deglaciation,
or to constrain the denudation rate of a bedrock surface. Repeated
intervals of burial and exposure during successive glacial and
interglacial periods cause discontinuous TCN production. This
imposes additional difficulties for constraining denudation rates,
because the exposure/burial history at a given site is generally also
unknown.
In order to accommodate complex exposure histories, paired
10Be-26Al measurements can be used to estimate a minimum-
limiting total history and the relative proportion of exposure
versus burial, by utilizing that the two nuclides have different
half-lives. Under constant exposure, the two nuclides will
accumulate following a predictable ratio controlled by the
production rates and half-lives, whereas the ratio during periods
of burial is governed by the half-lives only18,19. This approach,
however, has two major shortcomings: It ignores the ongoing
erosion and resultant loss of nuclides back through time, and
secondly it fails to resolve the alternating nature of exposure and
burial through multiple glacial and interglacial periods.
Paired 10Be-26Al bedrock data from high-elevation sites in west
Greenland and the Baffin Bay area indicate long and complex
exposure histories with significant periods of burial, suggesting
preservation under weakly erosive, cold-based ice over
several glacial cycles18,19. In contrast, recently published results
from high-elevation surfaces elsewhere in west Greenland
(Uummannaq), suggest near-continuous exposure throughout
much of the middle and late Quaternary—possibly as nunataks
during the LGM and prior glacial maxima20.
Recent advances in Monte Carlo modelling techniques make it
possible to constrain the history of long-term erosion and
exposure-burial periods by exploiting more efficiently the
differences in production and radioactive decay rates of paired
TCNs21,22. The Markov-Chain Monte Carlo (MCMC) model
approach developed by Knudsen et al. (ref. 21) is based on the
assumption that the exposure/burial history can be divided into
two distinct regimes: (i) glacial intervals with subglacial erosion
and, due to shielding by the overlying ice sheet, no exposure, and
(ii) interglacial intervals experiencing active subaerial erosion and
full exposure, assuming no significant shielding by for example,
snow, till, or vegetation (see ‘Methods’ section). The rates of
glacial and interglacial erosion may differ and vary spatially, but
for any particular bedrock sample the two erosion rates are
uniform throughout all glacials and interglacials, respectively. The
MCMC model does not include sudden individual erosion events,
such as subglacial plucking, but integrates the effects of plucking
events over time. By integrating the glacial and interglacial
erosion rates, it is possible to compute a robust, long-term
denudation rate for each sample. The exposure/burial history is
determined by applying a threshold value to a stacked benthic
marine d18O record23, which is a proxy for past global land-ice
volume.
In this study, we apply the new MCMC inversion model21 to
all available 10Be-26Al bedrock data from west Greenland,
encompassing 49 samples altogether. The most realistic and
up-to-date landscape information is integrated as boundary
conditions in the model set-up, which enable us to quantify past
denudation rates combined with exposure/burial histories. We
show that the denudation rate decreases with increasing elevation,
from 450 m Myr 1in low-lying areas to 1–5 m Myr 1at high-
elevation summit flats (4850 m a.s.l.). We also find that the
majority of samples are consistent with presence of an ice cover
ca. 45% of the past 1 Myr, whereas the fraction of ice-covered
periods was smaller (10–20%) at many high-elevation sites.
Results
Denudation rates and landscape evolution in west Greenland.
The paired 10Be-26Al bedrock data derive from four study sites in
west Greenland. We apply the MCMC approach to all samples
displaying a simple exposure 10Be age of 20 kyr or more. The
simple exposure ages are calculated based on the assumption of
continuous exposure, no inherited TCNs, and no post-glacial
erosion. The samples with an exposure 10Be age of 20 kyr or more
are believed to violate these preconditions, based on comparisons
with ages from boulders and radiocarbon ages of proglacial lake
sediments. The relevant samples include 11 samples from
Upernavik19, 19 samples from Uummannaq20,24,25, 10 samples
from Itilleq26 and 9 samples from Sukkertoppen20, the two latter
sites both belonging to the Sisimiut area (Fig. 1). The
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14199
2NATURE COMMUNICATIONS | 8:14199 | DOI: 10.1038/ncomms14199 | www.nature.com/naturecommunications
Uummannaq and Upernavik areas have high relief with deep
fjords that stretch from the present ice margin to the coast. The
samples located close to Sisimiut are from (i) a low-relief area
between the fjords (Itilleq), and (ii) from summit plateaus around
a local ice cap (Sukkertoppen).
All four sites were sampled across a wide range of elevations
from valleys to summits and show an overall trend of increasing
simple exposure 10Be age with altitude (Fig. 2a). From this we
identify an elevation threshold, above which all samples display
non-negligible inheritance, which must derive from periods of
exposure associated with earlier interglacials or ice-free periods.
Inheritance is defined by a nuclide inventory that exceeds
post-glacial production and implies inefficient bedrock erosion
(o2–3 m) over the last glacial period. In reality, however, it is not
possible to define a specific threshold below which there with
certainty is no inheritance. Our results show that high-elevation
summit flats (4850 m a.s.l.) yield the slowest long-term
denudation, typically B1–5 m Myr 1, whereas sites at lower
elevations have denudation rates of 15–20 m Myr 1(Fig. 2b).
There are exceptions to the general trend, as some low-elevation
samples have denudation rates o10 m Myr 1, demonstrating
some spatial variations in the erosion processes shaping the
landforms. The overall pattern of decreasing denudation rates
with increasing elevation is nonetheless consistent with the
distribution of minimum-limiting exposure and burial ages in
such landscapes19,27–29. This trend reflects that glacial erosion is
more efficient in fjords and valleys where the ice is thick enough
to be warm-based and reach the pressure-melting point at the
ice-bedrock interface. The low erosion rates found at inter-fjord
uplands are consistent with the presence of cold-based ice,
which is frozen to the ground and only moves due to internal
deformation, thereby preserving the high-elevation areas30. The
differential erosion has a major influence on the overall evolution
of the landscape and is key to understanding the age of landforms
and their development through the latter half of the Quaternary.
Our analyses further reveal that samples with non-detec-
table TCN inheritance must have total denudation rates
450 m Myr 1(See Supplementary Data 1). The denudation
rates in the fjord troughs (where TCN inheritance is negligible)
are therefore in excess of 50 m Myr 1and potentially one or two
orders of magnitude higher, as reported by previous studies based
on sediment yields31 .
Overall, the denudation rates modelled over the past 1 Myr
support the notion of selective linear erosion along the GrIS
margins as denudation rates drop more than an order of
magnitude with increasing elevation. We cannot exclude
the possibility that the spatial patterns of differential erosion
are somewhat influenced by bedrock erodibility, caused by
for example, variations in bedrock fracture density or orienta-
tion32,33. The linear appearance and the fairly uniform
orientation of the fjords certainly support the notion of a
structural inheritance within these glacially eroded landscapes15.
The low denudation rates at high elevations imply as little as
3–15 m of summit lowering, if extrapolated over the entire
*
Upernavik
Sisimiut
Dye 3
ODP 646
Qulleq-1
Uummannaq
a
13-GROR-71
326.5 ±33.4 kyr
13-GROR-70
305.5 ± 31.2 kyr
13-GROR-69
73.3 ± 7.4 kyr
13-GROR-59
54.5 ± 5.3 kyr
13-GROR-46
21.5 ± 2.1 kyr
13-GROR-39
60.7 ± 5.8 kyr
13-GROR-36
35.5 ± 3.5 kyr
13-GROR-37
71.0 ± 6.8 kyr
13-GROR-57
104.0±10.1 kyr
13-GROR-45
25.6 ± 2.5 kyr
UBE22
93.2 ±9.5 kyr
14-GROR-17
51.7 ± 4.9 kyr
14-GROR-16
112.1 ±10.7 kyr
14-GROR-15
49.0 ± 4.7 kyr
14-GROR-43
63.0 ± 6.1 kyr
14-GROR-03
42.6 ± 4.1 kyr 14-GROR-02
25.1 ± 2.4 kyr
14-GROR-39
55.5 ± 5.3 kyr
14-GROR-40
50.2 ± 4.9 kyr
14-GROR-41
85.1 ± 8.2 kyr
KA5
91.3 ±9.2 kyr
13-GROR-72
238.3 ± 24.0 kyr
IKE17
117.6 ±11.9 kyr
GU096
103.9 ±10.4 kyr
GU106
33.7 ± 3.3 kyr
GU103
83.9 ± 8.4 kyr
GU105
42.7 ± 4.2 kyr
GU111
73.7 ± 7.3 kyr
GU041
GU042
GU043
GU044
GU045
81.0 ± 8.1 kyr
53.0 ± 5.2 kyr
52.3 ± 5.2 kyr
64.2 ± 6.4 kyr
43.4 ± 4.3 kyr
GU113
52.4 ± 5.2 kyr
NAG01
NAG02
NAG03
NAG04
NAG05
NAG06
NAG07
NAG08
NAG09
NAG12
152.0 ±15.2 kyr
181.8 ±18.4 kyr
70.0 ± 7.1 kyr
128.7 ±12.9 kyr
37.2 ± 3.7 kyr
26.5 ± 2.7 kyr
25.7 ± 2.6 kyr
33.8 ± 3.3 kyr
23.3 ± 2.3 kyr
22.9 ± 2.2 kyr
IKE8
IKE9
87.3 ±8.8 kyr
36.5 ±4.6 kyr
13-GROR-61
13-GROR-63
13-GROR-64
*52.8 ±5.1 kyr
50.1 ±4.8 kyr
34.6 ±3.5 kyr
N
bc
d
N
N
Figure 1 | Overview maps of sample sites and simple exposure 10Be ages. (a) South Greenland overview with coloured boxes marking the four TCN
sample sites and black circles marking points of pre-LGM Quaternary data5,7,36.(b) Uummannaq area with sample sites containing 10Be and 26Al bedrock
data19. The point marked with an asterisk represents 3 sample sites, which are shown in the top left box. The scale bar is 50 km wide. (c) Sample sites with
10Be and 26Al bedrock data20,24,25 from the Upernavik area. The scale bar is 25 km wide. (d) Sisimiut area, covering samples from two sites; the inter-fjord
site Itilleq26 (red) and samples from the margins of the local ice cap Sukkertoppen20 (yellow). The scale bar is 50 km wide. The data points shown in
(bd) meet the criteria of being applicable to the MCMC model approach, by having simple exposure 10Be ages above 20 kyr and a 26Al/10Be ratio below
7.5. (bd) Simple exposure 10Be ages are calculated using Cronus Version 2.3 (ref. 45) and the Lal (1991)/Stone (2000) scaling scheme40,41. Figure 1 was
created using QGIS software46. All satellite images are from Landsat8, August 2016, courtesy of the U.S. Geological Survey.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14199 ARTICLE
NATURE COMMUNICATIONS | 8:14199 | DOI: 10.1038/ncomms14199 | www.nature.com/naturecommunications 3
Quaternary period. However, we consider this to be a minimum
estimate, as the patterns and pace of glacial erosion may have
changed systematically over time. Specifically, we envisage that
the erosional contrast between high surfaces and glacial troughs
has grown over time in response to the topographic change, as the
increasing relief around emerging fjords steered more and more
ice into the fjords and away from the inter-fjord uplands, which
furthermore experienced isostatic uplift14. Erosion may therefore
have been more uniformly distributed (that is, faster at high
surfaces and slower in troughs) during the early glaciations and
before the deep fjords were formed. Our MCMC model cannot
capture such transitions, as it assumes that rates of glacial and
interglacial denudation are uniform in time. The MCMC model
does, however, illuminate a robust and systematic elevation-
dependence of denudation rates within the most recent
glaciations.
Glaciation history in West Greenland. The quantification of
denudation rates is tied to an estimate of the most likely exposure
history, defined by the d18O threshold for each sample. The
exposure histories make it possible to calculate the cumulative
sum of exposure time over the alternating ice-free and ice-cov-
ered periods over the past 1 Myr. This cumulative, simulated
exposure/burial history is conceptually more advanced than the
minimum-limiting exposure and burial durations defined by the
simplest pathway to explain a point on the two-isotope diagram
(Fig. 3a), because it takes into account the most likely timing of
ice-free and ice-covered periods based on a proxy for past global
ice volume. For all samples, it is possible to define an exposure-
burial history that is consistent with the measured 10Be and 26Al
data. The proportion of cumulative exposure during the last
1 Myr, defined by the d18O threshold value, varies from o15% to
490%. There is no obvious relationship between the proportion
of cumulative exposure time and elevation, but samples char-
acterized by a relatively low degree of cumulative exposure
(o25%), or by a high degree of cumulative exposure (475%),
tend to derive from sites at high elevations relative to the
surrounding topography (Fig. 3b). As expected, the cumulative
exposure time is closely linked to the 26Al/10Be ratio, with high
26Al/10Be ratios corresponding to high proportions of cumulative
exposure (Fig. 3c). In general, the uncertainties associated with
estimates of cumulative exposure proportions are relatively high,
but they are considerably smaller for samples with a high degree
and, in particular, low degree of exposure. For samples with a low
degree of cumulative exposure, it is only possible to simulate
26Al/10Be ratios as low as the measured ratios if the exposure is
limited to short intervals during the warmest interglacial periods.
It is possible, however, that the low-ratio samples were exposed
for longer periods of time before the last 3–4 glacial cycles, but
such a scenario is beyond the present modelling capability of the
MCMC approach, as it requires non-uniform erosion rates
and/or a time-dependent d18O-threshold level.
Discussion
The notion of high-elevation surfaces around Uummannaq that
remained free of ice during the LGM and earlier glacial maxima
was born from high 26Al/10Be ratios indistinguishable from the
production ratio of B6.75 (ref. 20). Here, we demonstrate that
the paired nuclide data from Uummannaq and Sukkertoppen
displaying high 26Al/10Be ratios are fully consistent with burial
during glacial maxima, including an LGM ice cover lasting
15–20 kyr (Fig. 3c). Due to the uncertainty on the measured TCN
concentration, it is not possible to firmly establish whether these
high-elevation surfaces were ice covered or ice free during the
LGM and earlier glacial maxima, as ice-free conditions also
remain a possibility for samples with 26Al/10Be ratios indis-
tinguishable from the production ratio. However, the majority
(39 out of 49) of the samples point to all three areas being ice
covered during glacial maxima. On the basis of the cumulative
exposure histories of the samples from Sisimiut, Uummannaq,
and Upernavik, we constrain the most likely exposure-burial
history associated with the waxing-waning GrIS in these three
Upernavik
Uummannaq
Itilleq
Sukkertoppen
Bedrock
Boulder
Only 10Be data
ab
Simple exposure 10Be age (kyr)
0 50 100 150 200 250 300
Altitude (m a.s.l.)
5 101520253035
Denudation rate (m Myr–1)
0
200
400
600
800
1,000
1,200
1,400
1,600
1,800
2,000
0
Altitude (m a.s.l.)
200
400
600
800
1,000
1,200
1,400
1,600
1,800
2,000
0
Figure 2 | Simple exposure 10Be ages and long-term denudation rates as a function of altitude. (a) Simple exposure 10Be ages tend to increase with
elevation. Above a certain threshold (dashed line), all bedrock samples contain a cosmogenic signal inherited from periods before the most recent
glaciation. The grey zone marks the limit used to constrain the timing of the Holocene deglaciation (10–16 kyr) for all sites (see ‘Methods’ section). 10Be
ages are calculated using Cronus Version 2.3 (ref. 45) and the Lal (1991)/Stone (2000) scaling scheme40,41.(b) Total denudation rates over the last 1 Myr
based on application of the MCMC approach to samples with a simple exposure 10Be age above 20 kyr and a 26Al/10Be ratio below 7.5. All three sites in
west Greenland show a clear trend of decreasing denudation rate with increasing elevation. Diamonds represent samples from the same previous studies,
but with information from 10Be only (that is, no 26Al data were available). The rates of denudation associated with these samples have larger uncertainties.
Error bars are defined as the first and third quartiles of the 200,000 iterations per sample.
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14199
4NATURE COMMUNICATIONS | 8:14199 | DOI: 10.1038/ncomms14199 | www.nature.com/naturecommunications
areas. We first compute the most likely exposure-burial history
for samples with a simple exposure 10Be age above 20 kyr
and 26Al/10Be ratios lower than the production ratio (See
Supplementary Data 1), and we apply Chauvenet’s criterion34
to exclude outliers (for example, samples GU110 and NAG11).
This provides an estimate of the broad-scale behaviour of the
GrIS at Sisimiut, Uummannaq and Upernavik (Fig. 4),
respectively, which on average suggests ice-free conditions for
B55% of the last 1 Myr. On the basis of our analysis of exposure/
burial histories, we propose that the expanding GrIS did not
engulf these areas in west Greenland when Marine Isotope Stage
(MIS) 5e terminated and was succeeded by the subsequent
Weichselian/Wisconsin glacial period. The GrIS appears to have
expanded across these three areas at around the onset of MIS 4
(B72 kyr ago). The average exposure/burial histories, based on
samples with 26Al/10Be ratios indistinguishable from the
production ratio (Fig. 4), suggest that most of the
high-elevation surfaces around Uummannaq were ice free for
almost 90 kyr before the LGM, during which the high surfaces on
average may have been ice covered for B18 kyr. Considerable
spatial variation is indicated by a few sites only experiencing
ice-free conditions during peak interglacial periods (for example,
GU110). However, in light of uncertainties, we advise some care:
the notion of ice advance and retreat occurring everywhere in the
landscape simultaneously is overly simplistic. At some sites, the
exposure-burial histories were possibly further complicated by
partial shielding associated with thin ice covers, meaning that the
TCN production was not completely halted during glacial
periods. The presence of such thin overlying ice covers would
prolong the estimated duration of ice-covered periods, but exert
negligible effect on the estimated erosion rates.
Our results reveal a glaciation history for west Greenland that
is in accordance with the sparse geological evidence available,
which suggests ice-free conditions in the fjord area for the
Holocene, the early Weichselian/Wisconsin, and during MIS 5e
(ref. 2). We find that ice-free periods associated with interglacials
MIS 5e, MIS 11 and MIS 21 stand out in all three areas as longer
than other ice-free intervals. Based on a reconstruction of Arctic
temperatures from a Siberian lake, MIS 11 is considered
significantly warmer than other interglacials35. Therefore, if the
Upernavik
Uummannaq
Itilleq
Sukkertoppen
a
26
Al/
10
Be
10Be concentration (atoms g–1)
3.5
4.5
5.5
6.5
7.5
104107
106
105
bc
26
Al/
10
Be
20 40 60 80 100
4.5
5.5
6.5
7.5
Altitude (m a.s.l.)
200
600
1,000
1,400
1,800
Proportion of cumulative
exposure over 1 Myr (%)
Proportion of cumulative
exposure over 1 Myr (%)
20 40 60 80 100
Figure 3 | Linking nuclide concentrations to exposure history. (a) Plot of 26Al/10Be ratio against 10Be concentration for all samples used in the MCMC-
based quantification of complex exposure histories. The upper black line marks the development of the ratio under constant exposure and the lower black
line marks the end points of an infinite number of different steady-state erosion rate scenarios with constant exposure. Error bars are based on
uncertainties of the 26Al and 10Be concentrations. (b) Sample altitude plotted against percentage of time exposed over the last 1 Myr for each sample.
(c) Plot of 26Al/10Be ratio against percentage of time exposed over the last 1 Myr for each sample. There is a clear correlation between 26Al/10Be ratios and
the percentage of exposure over the last 1Myr based on the MCMC-modelled exposure history. The horizontal dashed line marks the production ratio of
26Al/10Be and the vertical dashed line marks 87% of exposure in 1 Myr, which corresponds to 15kyr of ice cover during the LGM. Error bars are defined as
the first and third quartiles of the 200,000 iterations per sample.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14199 ARTICLE
NATURE COMMUNICATIONS | 8:14199 | DOI: 10.1038/ncomms14199 | www.nature.com/naturecommunications 5
interior of southern Greenland was completely deglaciated, as
geological findings suggest7,36, it most likely happened during
MIS 11, which appears to have been both warm and relatively
long-lasting. Our results also indicate that the ice-covered periods
with the longest duration coincide with deposits of marine glacial
debris flows in the Davis Strait5,37 (Fig. 4). Four of these five long-
lasting ice-covered periods, where the ice extent reached the
Greenland shelf, most likely experienced 450 kyr of continuous
ice cover, except for most high-elevation plateau locations that
probably were ice-covered only during glacial maxima (Fig. 4c,
red line).
In summary, we illustrate how multiple paired cosmogenic
nuclides can be used to shed light on the response of the GrIS to
climate change during the last 1 Myr. We constrain the most
likely glaciation histories and differential denudation rates in the
fjord areas of west Greenland, and hereby quantify rates of
lowering associated with various glacial landforms. Our applica-
tion of the MCMC modelling approach to paired cosmogenic
nuclides opens new avenues for quantifying glacial and
interglacial denudation rates, which are essential to understand
long-term landscape evolution and the origin of the glacial
landscape we observe today.
Methods
Markov-Chain Monte Carlo inversion approach and model set-up.The novel
MCMC inversion model21 used in this study to quantify past denudation rates and
exposure/burial histories includes four model parameters; the interglacial and
glacial erosion rates (in m Myr 1), as well as the timing of the last deglaciation
(in kyr) and the d18O-threshold level (in %). Application of the d18O-threshold
level to the global benthic marine d18O record23, which is a proxy for changes in
past global ice volume, is used to define the exposure/burial history in the model
simulations, following the idea presented by Stroeven et al.38. The stacked d18O
record is smoothed using a 5-kyr running window so it reflects the major marine
isotope stages (MISs) and sub-stages. The smoothed d18O record thus allows
changes in exposure/burial regimes in the model set-up that are consistent with the
available knowledge of large-scale glacial advances and retreats in Scandinavia30.
The Holocene deglaciation is a free parameter in the model and we apply a set of
wide boundary values, allowing the deglaciation to take place between 10–16 kyr in
all three areas. This is a rather conservative estimate based on bedrock-boulder
pairs from each of the three areas10,19,24–26 and allows considerable variation due
to elevation and distance to present ice margin. The interval of the Holocene
GDF 5
GDF 4
GDF 3
GDF
LR04
benthic δ18O
(‰)
Sisimiut
(67 °N)
Upernavik
(73 °N)
Uummannaq
(71 °N)
5.0 4.4 3.8 3.2
0
Modeled burial and exposure history
Age (kyr)
100
200
300
400
500
600
700
800
900
1,000
b
ac
Elgygytgyn
diatom productivity
(Si/Ti)
7
9
1
1
13
17
19
21
23
25
15
27
2
4
6
8
10
12
14
16
18
20
22
24
26
5e
0.5 1
Shelf
edge
Present
ice margin
Fjord
area
Outer
coast
GDF 2
Colde
r
Warmer
Bur Exp Bur Exp Bur Exp
Ice free Ice cover
Figure 4 | Climate and modeled glaciation history in west Greenland throughout the last 1 Myr. (a) The stacked benthic marine d18O record23 is a proxy
for global ice volume and is divided into numbered marine isotope stages (MIS). We determine the exposure history by applying a threshold to this global
climate record. (b) The diatom productivity curve from Lake El’gygytgyn35, which is a temperature proxy based on limnic Arctic data, indicating very warm
conditions during MIS 11. (c) Quantification of the most likely periods of exposure (‘Exp’ and green coloration) and burial by ice cover (‘Bur’ and blue
coloration) based on 39 bedrock samples (black lines) from four sites in the Fjord Area of west Greenland19,20,24–26 (the Sisimiut exposure/burial history
covers the sample sites Itilleq and Sukkertoppen). Red lines mark the glaciation history of samples that only have experienced very short durations of burial
and therefore have 26Al/10Be ratios at or above the production ratio of 6.75. The red lines are based on three samples from Sisimiut and seven samples
from Uummannaq. It is likely that the ice sheet extended to the shelf edge and deposited glacial debris flows during the five most prolonged ice-covered
periods5,37.
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14199
6NATURE COMMUNICATIONS | 8:14199 | DOI: 10.1038/ncomms14199 | www.nature.com/naturecommunications
deglaciation also frames the possible range of d18O-threshold values, which are
allowed to vary between 3.6%and 4.7%. For samples with 26Al/10Be ratios
indistinguishable from the production ratio of ca. 6.75, we expand the d18O
boundary values to 3.6–5.0%, allowing for the possibility of continuous exposure
throughout the last 1 Myr. The glacial and interglacial erosion rates have broad
boundary values of 0.1–1,000 m Myr 1. We used all available paired 26Al/10Be
data for bedrock samples in Greenland with a simple exposure 10Be age above
20 kyr and a 26Al/10Be ratio below 7.5. Ratios above this level are unlikely to form
in any burial and/or exposure scenario and they cannot be simulated based on the
currently known muonic and spallogenic production rates.
We follow the procedure demonstrated by Knudsen et al.21 and compute the
production and decay/erosion of 10Be and 26Al throughout the Quaternary for
different combinations of the model parameters, and compare the results to the
measured concentrations between each simulation. We use a sea level high-latitude
10Be production rate of 4.01 atoms g 1per year and 26Al production rate of 27.07
atoms g 1per year, based on the calibration set by Borchers et al.39 for 10Be and
26Al surface production rates and the Lal (1991)/Stone (2000) scaling scheme40,41
for all 49 samples from the three areas in west Greenland (see Supplementary
Data 1).
The Metropolis-Hastings MCMC technique42,43 is used to map the family of
model parameters that provides the best, weighted least-squares fit to the measured
data. For each sample, we use four ‘random walks’, which start at different places in
the model space, to ensure that the result does not depend on the starting position
of the search through the model space. A burn-in phase of 1,000 iterations is used
to make a crude initial search of the model space, whereas 50,000 iterations and an
acceptance ratio of 0.4 are used in the main MCMC phase, when finding the most
probable scenarios amongst the 4 50,000 iterations. To estimate the model
parameters for each walker, we use the median of the 200,000 simulations, whereas
the associated uncertainties are based on the 25% and 75% quartiles.
Compilation of the model output.As the results obtained with the four different
walkers are very similar, we use the average of all simulations for each sample
(200,000) to estimate the model parameters, which, in turn, makes it possible to
estimate the total denudation rate as well as the exposure/burial history. The
exposure/burial history associated with each sample emerges by applying the
median d18O threshold value (Supplementary Fig. 1) to the global marine benthic
d18O record23, hereby defining periods of exposure and burial as the intervals
below/above this threshold, respectively. Supplementary Fig. 2 shows an example of
the d18O threshold value for one sample. The exhumation history and total
denudation rate over the last 1 Myr are estimated for each sample by integrating
the glacial and interglacial erosion rates (Supplementary Fig. 1) with the burial/
exposure history, and subsequently computing the median exhumation history
based on the 200,000 simulations (example of exhumation history is shown in
Supplementary Fig. 3). The minimum denudation rate of 450 m Myr1
computed for low-lying areas and glacial troughs are based on 25% quartiles from
samples with uncertainties that overlap the timing of the Holocene deglaciation.
We compile the exposure/burial history for each of the three areas (Supplementary
Fig. 4) by taking the average median d18O threshold value of all samples with a
simple exposure 10Be age exceeding 20 kyr. The samples within each area are
additionally grouped into samples with 26Al/10Be ratios below the production ratio
(6.75) and samples with ratios above, or indistinguishable from, 6.75.
Till-cover sensitivity.We test the effect of a till cover during exposure periods,
which would dampen the nuclide production and could cause too-young exposure
ages if not taken into account. To understand how the reduced nuclide production
due to the presence of a till cover with variable thickness affects the estimate of the
denudation rate and d18O-threshold value, we apply the correction factor for till
using equation (1) (ref. 17):
ftill ¼ezrL1ð1Þ
to the nuclide production rates, using a till density of 2,200 kg m 3(ref. 44).
Supplementary Fig. 5a–f shows how four different samples (GU041, GU113,
14-GROR-40 and 13-GROR-70) respond to till with a thickness varying from 0.1 m
to 1.0 m covering the bedrock during 25, 50 or 100% of the ice-free periods. The
samples derive from three different sites and represent different elevations and
landscape settings. The lowermost points in each subplot show the modelled
denudation rate (Supplementary Fig. 5a–c) and d18O-threshold value
(Supplementary Fig. 5d–f) without till cover.
In general, the d18O-threshold value is less affected by till cover than the
denudation rate, but the effect is indistinguishable with regard to both denudation
rate and exposure history (determined by d18O-threshold value) for most of the
samples. Except for one sample, it is only in the most extreme cases of 0.5–1.0 m of
till cover during 100% of the ice-free periods that the results are significantly
affected by the presence of till. The presence of a till cover would result in
denudation rate estimates that are slightly too high and d18O-threshold value
estimates slightly too low, if the till cover is not accounted for. The magnitude of
these effects depends on the duration and thickness of the till cover. In general, till
covers are rarely observed today in the sampled regions, and we do therefore not
expect till cover to present a significant problem when estimating the landscape
history in west Greenland based on TCNs.
Data availability.The authors declare that the main data supporting the findings
of this study are available within the paper and its Supplementary Information files.
Extra data are available from the corresponding author on request.
References
1. Seierstad, I. K. et al. Consistently dated records from the Greenland GRIP,
GISP2 and NGRIP ice cores for the past 104 ka reveal regional millennial-scale
d18O gradients with possible Heinrich event imprint. Quat. Sci. Rev. 106, 29–46
(2014).
2. Funder, S., Hjort, C. & Kelly, M. Isotope stage 5 (130-74 ka) in Greenland, a
review. Quat. Int 10–12, 107–122 (1991).
3. Funder, S., Abrahamsen, N. & Feyling-Hanssen, R. W. Forested Arctic:
evidence from North Greenland. Geology 13, 542–546 (1985).
4. Funder, S., Jennings, A. E. & Kelly, M. in Quaternary Glaciations-Extent and
Chronology, Part II: North America, Vol. 2 (eds Ehlers, J. and Gibbard, P. L)
425–430 (Elsevier, 2004).
5. Nielsen, T. & Kuijpers, A. Only 5 southern Greenland shelf edge glaciations
since the early Pliocene. Sci. Rep. 3, 1875 (2013).
6. Carlson, A. E., Stoner, J. S., Donnelly, J. P. & Hillaire-Marcel, C. Response of
the southern Greenland Ice Sheet during the last two deglaciations. Geology 36,
359–362 (2008).
7. Reyes, A. V. et al. South Greenland ice-sheet collapse during Marine Isotope
Stage 11. Nature 510, 525–528 (2014).
8. Larsen, H. C. et al. Seven million years of glaciation in Greenland. Science 264,
952–955 (1994).
9. Larsen, N. K. et al. Rapid early Holocene ice retreat in West Greenland. Quat.
Sci. Rev. 92, 310–323 (2014).
10. Winsor, K., Carlson, A. E., Caffee, M. W. & Rood, D. H. Rapid last-deglacial
thinning and retreat of the marine-terminating southwestern Greenland ice
sheet. Earth Planet. Sci. Lett. 426, 1–12 (2015).
11. Laine, E. P. New evidence from beneath the western North Atlantic for the
depth of glacial erosion in Greenland and North America. Quat. Res. 14,
188–198 (1980).
12. Bierman, P. R. et al. Preservation of a preglacial landscape under the center of
the Greenland Ice Sheet. Science 344, 402–405 (2014).
13. Sugden, D. E. Landscapes of glacial erosion in Greenland and their relationship
to ice, topographic and bedrock conditions. Inst. Br. Geogr. Spec. Publ. 7,
177–195 (1974).
14. Kessler, M. A., Anderson, R. S. & Briner, J. P. Fjord insertion into continental
margins driven by topographic steering of ice. Nat. Geosci. 1, 365–369 (2008).
15. Swift, D. A., Persano, C., Stuart, F. M., Gallagher, K. & Whitham, A.
A reassessment of the role of ice sheet glaciation in the long-term evolution of
the East Greenland fjord region. Geomorphology 97, 109–125 (2008).
16. Medvedev, S., Hartz, E. H. & Podladchikov, Y. Y. Vertical motions of the fjord
regions of central East Greenland: impact of glacial erosion, deposition, and
isostasy. Geology 36, 539–542 (2008).
17. Dunai, T. J. Cosmogenic Nuclides - Principles , Concepts and Applications in the
Earth Surface Sciences (Cambridge University Press, 2010).
18. Bierman, P. R., Marsella, K. A., Patterson, C., Davis, P. T. & Caffee, M.
Mid-Pleistocene cosmogenic minimum-age limits for pre-Wisconsinan glacial
surfaces in southwestern Minnesota and southern Baffin Island: a multiple
nuclide approach. Geomorphology 27, 25–39 (1999).
19. Corbett, L. B., Bierman, P. R., Graly, J. A., Neumann, T. A. & Rood, D. H.
Constraining landscape history and glacial erosivity using paired cosmogenic
nuclides in Upernavik, Northwest Greenland. Bull. Geol. Soc. Am 125,
1539–1553 (2013).
20. Beel, C. R., Lifton, N. A., Briner, J. P. & Goehring, B. M. Quaternary evolution
and ice sheet history of contrasting landscapes in Uummannaq and
Sukkertoppen, western Greenland. Quat. Sci. Rev. 149, 248–258 (2016).
21. Knudsen, M. F. et al. A multi-nuclide approach to constrain landscape
evolution and past erosion rates in previously glaciated terrains. Quat.
Geochronol. 30, 100–113 (2015).
22. Margreth, A., Gosse, J. C. & Dyke, A. S. Quantification of subaerial and episodic
subglacial erosion rates on high latitude upland plateaus: Cumberland
Peninsula, Baffin Island, Arctic Canada. Quat. Sci. Rev. 133, 108–129 (2016).
23. Lisiecki, L. E. & Raymo, M. E. A Pliocene-Pleistocene stack of 57 globally
distributed benthic d18O records. Paleoceanography 20, 1–17 (2005).
24. Roberts, D. H., Rea, B. R., Lane, T. P., Schnabel, C. & Rode
´s, A. New constraints
on Greenland ice sheet dynamics during the last glacial cycle: Evidence from
the Uummannaq ice stream system. J. Geophys. Res. Earth Surf. 118, 519–541
(2013).
25. Lane, T. P. et al. Controls upon the last glacial maximum deglaciation of the
northern Uummannaq ice stream system, west Greenland. Quat. Sci. Rev. 92,
324–344 (2014).
26. Roberts, D. H. et al. Ice sheet extent and early deglacial history of the
southwestern sector of the Greenland Ice Sheet. Quat. Sci. Rev. 28, 2760–2773
(2009).
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14199 ARTICLE
NATURE COMMUNICATIONS | 8:14199 | DOI: 10.1038/ncomms14199 | www.nature.com/naturecommunications 7
27. Fabel, D. et al. Spatial patterns of glacial erosion at a valley scale derived from
terrestrial cosmogenic 10Be and 26Al concentrations in rock. Ann. Assoc. Am.
Geogr 94, 241–255 (2004).
28. Gjermundsen, E. F. et al. Minimal erosion of Arctic alpine topography during
late Quaternary glaciation. Nat. Geosci. 8, 789–792 (2015).
29. Briner, J. P. et al. Using in situ cosmogenic 10Be, 14C, and 26Al to decipher the
history of polythermal ice sheets on Baffin Island, Arctic Canada. Quat.
Geochronol. 19, 4–13 (2014).
30. Fabel, D. et al. Landscape preservation under Fennoscandian ice sheets
determined from in situ produced 10Be and 26Al. Earth Planet. Sci. Lett. 201,
397–406 (2002).
31. Koppes, M. et al. Observed latitudinal variations in erosion as a function of
glacier dynamics. Nature 526, 100–103 (2015).
32. Du¨hnforth, M., Anderson, R. S., Ward, D. & Stock, G. M. Bedrock fracture
control of glacial erosion processes and rates. Geology 38, 423–426 (2010).
33. Iverson, N. R. A theory of glacial quarrying for landscape evolution models.
Geology 40, 679–682 (2012).
34. Bevington, P. R. & Robinson, D. K. Data Reduction and Error Analysis for the
Physical Sciences, 2nd ed., 313p (McGraw-Hill, New York, 1992).
35. Melles, M. et al. 2.8 million years of Arctic climate change from Lake
El’gygytgyn, NE Russia. Science 337, 315–320 (2012).
36. Willerslev, E. et al. Ancient biomolecules from deep ice cores reveal a forested
southern Greenland. Science 317, 111–114 (2007).
37. O
´Cofaigh, C. et al. An extensive and dynamic ice sheet on the west Greenland
shelf during the last glacial cycle. Geology 41, 219–222 (2013).
38. Stroeven, A. P., Fabel, D., Harbor, J., Ha
¨ttestrand, C. & Kleman, J. Quantifying
the erosional impact of the Fennoscandian ice sheet in the Tornetra
¨sk-Narvik
corridor, northern Sweden, based on cosmogenic radionuclide data. Geogr.
Ann. Ser. A Phys. Geogr. 84, 275–287 (2002).
39. Borchers, B. et al. Geological calibration of spallation production rates in the
CRONUS-Earth project. Quat. Geochronol. 31, 188–198 (2016).
40. Lal, D. Cosmic ray labeling of erosion surfaces: in situ nuclide production rates
and erosion models. Earth Planet. Sci. Lett. 104, 424–439 (1991).
41. Stone, J. O. Air pressure and cosmogenic isotope production. J. Geophys. Res.
105, 23753–23759 (2000).
42. Metropolis, N., Rosenbluth, A. W., Rosenbluth, M. N., Teller, A. H. & Teller, E.
Equation of state calculations by fast computing machines. J. Chem. Phys. 21,
1087–1092 (1953).
43. Hastings, W. K. Monte carlo sampling methods using Markov chains and their
applications. Biometrika 57, 97–109 (1970).
44. Balco, G. A. The sedimentary record of subglacial erosion beneath the Laurentide
Ice Sheet (PhD Thesis, University of Washington, 2004).
45. Balco, G. 10Be - 26Al exposure age calculator Available at http://
hess.ess.washington.edu/math/al_be_v23/al_be_multiple_v23.php (2008).
46. QGIS Development Team. QGIS Geographic Information System
http://www.qgis.org/ (2016).
Acknowledgements
M.F.K. and N.K.L thank the Villum Foundation’s Young Investigator Programme for
their support. J.D.J. was supported by a Marie Skłodowska-Curie Fellowship. D.L.E. was
supported by the Danish Council for Independent Research. We thank Lee Corbett and
Arjen Stroeven for highly constructive and helpful reviews.
Author contributions
A.S., N.K.L. and M.F.K. designed the study and A.S. performed the calculations. D.L.E.,
B.H.J. and L.B.L. helped interpreting the results. A.S., N.K.L. and M.F.K. wrote the paper
with contributions from D.L.E. and J.D.J.
Additional information
Supplementary Information accompanies this paper at http://www.nature.com/
naturecommunications
Competing financial interests: The authors declare no competing financial interests.
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How to cite this article: Strunk, A. et al. One million years of glaciation and denudation
history in west Greenland. Nat. Commun. 8, 14199 doi: 10.1038/ncomms14199 (2017).
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rThe Author(s) 2017
ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms14199
8NATURE COMMUNICATIONS | 8:14199 | DOI: 10.1038/ncomms14199 | www.nature.com/naturecommunications
... Currently available AHe data from the Maniitsoq region do not provide any record of Cenozoic processes that may have affected the (south-)western margin of Greenland, such as Paleogene rifting and break-up 26,42 , incision and erosion leading to formation of planation surfaces during Paleogene-Neogene times 10,11 , or glacial erosion and fjord incision in the Quaternary 43,44 . This lack of record is either because the magnitude of erosion or the thermal effects of these processes were insufficient to affect the AHe system or some of these processes were absent in the study area. ...
... The magnitude of an erosional event potentially leading to the formation of a planation surface was ≤3.5 km. The magnitude of glacial erosion onshore has not exceeded~3.5 km even at the water level of the fjords, where the greatest amount of attrition is expected as documented by cosmogenic isotopes 44 . ...
... This crystalline basement with its peneplain was then buried by Paleozoic (and potentially Mesozoic) sediments, reaching depths of <8 km in some places as constrained by Paleozoic ZHe data and thermal modelling results, and then re-exhumed from below this sedimentary cover during the Mesozoic. Block-faulted crystalline basement with peneplain remnants were then modified by glacial erosion in the Quaternary 43,44 . A Neoproterozoic peneplain finds support in the occurrences of Paleozoic cover sequences that were deposited on low-relief basement further north in Greenland 53 and more generally in the Canadian 54 and Baltic shields 55 . ...
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Accurate estimates of past topography are required to reliably reconstruct past ice sheets to infer paleoclimate. For this reason, understanding erosion rates across East Greenland is crucial to constrain landscape evolution driven by tectonics and climate-dependent erosion rates. Here we analyse published apatite fission track (AFT) data to constrain the spatial pattern of AFT bedrock ages across the landscape. We compare these bedrock ages with published detrital distribution to highlight ambiguity in the pattern of erosion. In contrast to earlier work, we regress a simple model of exhumation pace through the bedrock ages such that age can vary both as a function of elevation and position. The resulting iso-age surfaces enable us to determine potential source areas for detrital AFT ages distributions. We find that old ages observed in detrital distributions are just as likely to be sourced from low-elevation locations that are far from the coast, as high elevation locations close to the coast. Additional data from lower temperature systems are thus required to make firm conclusions on landscape evolution in the region and distinguish between the two landscape-forming scenarios.
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Erosion beneath glaciers and ice sheets is a fundamental Earth-surface process dictating landscape development, which in turn influences ice-flow dynamics and the climate sensitivity of ice masses. The rate at which subglacial erosion takes place, however, is notoriously difficult to observe because it occurs beneath modern glaciers in a largely inaccessible environment. Here, we present 1) cosmogenic-nuclide measurements from bedrock surfaces with well constrained exposure and burial histories fronting Jakobshavn Isbræ in western Greenland to constrain centennial-scale erosion rates, and 2) a new method combining cosmogenic nuclide measurements in a shallow bedrock core with cosmogenic-nuclide modelling to constrain orbital-scale erosion rates across the same landscape. Twenty-six 10Be measurements in surficial bedrock constrain the erosion rate during historical times to 0.4–0.8 mm yr-1. Seventeen 10Be measurements in a 4-m-long bedrock core corroborate this centennial-scale erosion rate, and reveal that 10Be concentrations below ~2 m depth are greater than what is predicted by an idealized production-rate depth profile. We utilize this excess 10Be at depth to constrain orbital-scale erosion rates at Jakobshavn Isbræ to 0.1–0.3 mm yr-1. The broad similarity between centennial- and orbital-scale erosion rates suggests that subglacial erosion rates have remained relatively uniform throughout the Pleistocene at Jakobshavn Isbræ.
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This is the first book to provide a comprehensive and state-of-the-art introduction to the novel and fast-evolving topic of in-situ produced cosmogenic nuclides. It presents an accessible introduction to the theoretical foundations, with explanations of relevant concepts starting at a basic level and building in sophistication. It incorporates, and draws on, methodological discussions and advances achieved within the international CRONUS (Cosmic-Ray Produced Nuclide Systematics) networks. Practical aspects such as sampling, analytical methods and data-interpretation are discussed in detail and an essential sampling checklist is provided. The full range of cosmogenic isotopes is covered and a wide spectrum of in-situ applications are described and illustrated with specific and generic examples of exposure dating, burial dating, erosion and uplift rates and process model verification. Graduate students and experienced practitioners will find this book a vital source of information on the background concepts and practical applications in geomorphology, geography, soil-science, and geology.
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