Supplementary Material
Rapid thinning of the late Pleistocene Patagonian Ice Sheet followed migration of
the Southern Westerlies
Jake Boex(1), Christopher Fogwill
(1,2),
Stephan Harrison*(1) Neil Glasser
(3),
Christoph
Schnabel(4), Andrew Hein(5), Sheng Xu (6)
1. Department of Geography, University of Exeter, Exeter, EX4 4RJ, UK.
2. Climate Change Research Centre, Faculty of Science, University of New South Wales,
Sydney, New South Wales, Australia.
3. Institute of Geography and Earth Science, Aberystwyth University, Aberystwyth, Wales.
4. NERC Cosmogenic Isotope Analysis Facility, SUERC, East Kilbride, G75 0QF, UK.
5. School of GeoSciences, University of Edinburgh, Edinburgh, Scotland, UK
6. Scottish Universities Environmental Research Centre (SUERC), Scottish Enterprise
Technology. Park, East Kilbride, UK G75 0QF
*corresponding author
Methods
Geomorphological mapping and sampling strategy
Detailed geomorphological analysis was undertaken in the field to ground-truth the
landform mapping from satellite imagery, maps from the Institute Geographical Militar of
Chile, Landsat imagery and aerial photographs. In order to establish a chronology of ice
sheet thinning glacial erratics and glacially eroded bedrock was sampled from key
landforms and altitudinal profiles for CRN exposure analysis using 10Be and 26Al.
Exposure analysis with in situ CRN’s provides a means of directly measuring the time
elapsed since the deposition of an erratic or erosion of a bedrock surface by glacial ice 1.
Samples were selected to constrain the age of moraines or ice sheet trimlines; these
consisted of glacially transported erratic boulders on depositional landform. Samples from
erratics and bedrock surfaces were also taken to record the time at which three mountains
became ice free through ice sheet thinning. Boulders were carefully selected from stable
exposed geomorphic surfaces to avoid the risk of rolling or excessive snow cover as this
would affect the measured exposure age. The erratics chosen were predominantly quartzrich granidiorites and around half a kilogram of whole rock was required for analysis.
Samples were returned to the UK and processed at the laboratories at the NERC CIAF at
SUERC and at the University of Exeter. Crushing was followed by mineral separation and
chemical etching to produce clean quartz 2. The quartz was then dissolved in order to
chemically extract and separate
10Be,
and for seven samples
were then measured by AMS at SUERC. Results for
10Be
26Al.
and
The extracted isotopes
26Al
concentrations were
converted to exposure ages using the CRONUS-Earth online calculator, version 2.2 using
the NZ Macaulay landslide, NZ calibration data set (Table S2 SOM3). The Dunai time
varying model4 is used for the data presented here to allow direct inter-comparison with
the study by Hein5 which have been recalibrated6. No correction was made for erosion,
snow cover or isostatic uplift in this study and therefore the exposure ages presented are
minimum ages.
Field sampling
Rock samples were collected during field campaigns to during January 2008 and 2009. A
mixed sampling strategy was employed taking rounded erratic boulders from depositional
glacial landforms including moraines and other ice contact features and boulders perched
on polished striated streamlined bedrock surfaces at exposed sites the transect.
Streamlined bedrock surfaces and subsamples of upper surfaces (upper 5-7 cm) of
boulder and large cobble erratics were removed manually using a 4 lb lump hammer and
chisel. In each case approximately 1 kg rock was obtained per sample. Care was taken to
avoid hollows and other potential areas of snow accumulation and/or drifting. Sample
location and elevation were recorded using handheld GPS receivers (±10 m). Topographic
shielding was measured using a sighting clinometer. Sample and location details are
recorded in Table S1.
Analytical methods
Samples were reduced to pure quartz at the University of Exeter Cosmogenic Nuclide
Laboratories following standard procedures7, 8. 9. Sample preparation was completed at the
NERC Cosmogenic Isotope Analysis Facility (CIAF) at The Scottish University
Environmental Research Centre (SUERC) and The University of Exeter Cosmogenic
Nuclide Laboratories. Resulting 10Be/9Be ratios after carrier addition were measured by the
AMS facility at SUERC10. Measurements were standardised to the NIST SRM-4325 Be
standard material with a revised nominal 10Be/9Be ratio of 2.79 x 10-11[11]. At CIAF,
sample preparation was carried out following the procedure described in detail in Wilson
2
with some modifications 12. The 250 – 500μm size fractions were used. The dissolution of
purified quartz samples in Exeter was carried out in batches of seven samples and one
blank. In that laboratory 350-500 μg 9Be carrier was used for all samples and blanks (n=4).
Scharlau Be carrier solution (982mg/l, density 1.02 g/ml) was used. Blanks were spiked
with 194-245μg 9Be carrier (CIAF) and 354-505 μg 9Be (Exeter). The corresponding
combined process and carrier blanks 10Be/9Be ratios ranged from 3.1 – 6.6 x 10-15 (Exeter
and CIAF). Sample and blank 10Be/9Be analytical uncertainties and a 2.5% carrier addition
uncertainty propagated into the 1σ analytical uncertainty for nuclide concentrations. These
blanks at CIAF were prepared from BeSO4 * 4 H2O (Merck, kindly provided by F. v.
Blanckenburg). Blank-corrected 10Be/9Be ratios ranged from 4.2 * 10-14 to 1.8 * 10-12.
For 26Al analysis at CIAF, Al carrier was added only to some samples to bring total Al to 2 mg per
sample [Merck, 985 μg Al /g solution]. Full chemistry blanks (n=2) had an average 26Al/27Al ratio of
1.3 * 10-15. Blank-corrected 26Al/27Al ratios of samples ranged from 1.5 to 5.2 * 10-13.
Carrier addition ranged from no carrier added to 1.8 mg
27
Al. Sample and blank 26Al/27Al analytical
uncertainties and a 2.5-4.5% uncertainty for the determination of total stable Al propagated into
the 1σ analytical uncertainty for nuclide concentrations.
Age determinations
Exposure ages were calculated using a version of the CRONUS-Earth online age
calculator. In light of recent CRN calibration rate studies in Patagonia we apply the
production rate for the isotopes 10Be and 26Al derived from New Zealand (NZ) that overlap
at 1σ with an independently derived production rate from Lago Argentino, Patagonia 3, 6
(http://hess.ess.washington.edu/math/al_be_v22/Age_input_NZ_calib.html) (Wrapper
script 2.2; Main calculator 2.1; Constants 2.2.1; Muons 1.1). We apply the recently revised
10Be
half-life (1.387 Ma) 13, 14 and Be isotope ratio standardization of Nishiizumi
Exposure ages are reported based on the Dunai scaling model
15
11.
using the same
calibration data set, ages differ by 2-4% depending on the choice of alternative scaling
model. The calculator uses sample thickness (Table S2) and density to standardise
nuclide concentrations to the rock surface. We used an assumed density of 2.7 g cm -3
(equivalent to the density of pure quartz). We include no correction for periodic snow cover
which is thought to be negligible in this setting for these samples. No erosion and no
correction for post-glacial uplift has been applied.
Choice of production rate model and scaling is often a pragmatic one and is an on going
subject of debate. For this reason, and to facilitate comparison with other datasets and
earlier work, we also report age results using the four other most commonly used scaling
schemes and the globally averaged production rate model (Table S2). The latter ages
were generated using the original CRONUS-Earth calculator Version 2.2 March 2009
(http://hess.ess.washington.edu/math/al_be_v22/al_be_multiple_v22.php) 13. If the global
production rate model is implemented without any calibration, ages are up to 11% younger
depending on the scaling scheme used (Table S2). We consider the ages based on the
regional production rate calibration dataset for NZ to be more reliable than those based on
the globally averaged production rate. At our low altitude, mid latitude location this
distinction is not trivial. Validation of the use of the NZ calibration data was demonstrated
with independent minimum and maximum radiocarbon control in Patagonia at Lago
Argentino at 50°S 6.
Table S1. Sample details
Sample details include SUERC AMS id, sample name, description, latitude / longitude,
elevation in a format that can be entered directly into the CRONUS-Earth calculator should
new site specific production rates become available16.
AMS id
AMS id Sample
Sample
Latitude
Be (b)
Al (a) name
description
(DD)
Sierra Colorado upper limit
b3889
SC1
Erratic
-47.364250
b3981
a1091
SC2
Erratic
-47.363611
b3982
a1092
SC3
Erratic
-47.364183
Sierra Colorado lower limt
b3883
SC4
Erratic
-47.376650
b3983
a1093
SC8
Erratic
-47.356806
b3888
SC9
Erratic
-47.356806
Columna Moraine and highest summits of Cerro Oportus
b3882
COL2
Erratic
-47.223942
b3884
OW2
Erratic
-47.108833
b3986
OW3
Erratic
-47.109106
Lower Summit of Cerro Oportus
b3980
CO5
Erratic
-47.100556
b3253
a837
COC4
Erratic
-47.101433
Cerro Tramango altitudinal profile
b3252
a836
CTCB3
Bedrock
-47.161683
b3251
a835
CTC5
Erratic
-47.157700
a838
CTC11
Erratic
-47.126333
Maria Elena Moraine
b3989
LL460-2
Erratic
-47.071056
b3880
LL460-1
Erratic
-47.111472
b4786
LL586-1
Erratic
-47.080531
Palaeolake shoreline
b3877
LL145-1
Erratic
-47.125306
Longitude
(DD)
Elevation
(m)
Elvation/
pressure
Thickness
(cm)
Density
(g cm-2)
Shielding
correction
Erosion rate
(cm yr-1)
[Be-10]
atoms g-1
+/atoms g-1
Be AMS
standard
[Al-26]
atoms g-1
+/atoms g-1
Al AMS
standard
-71.663417
-71.664167
-71.643017
1368
1368
1407
std
std
std
3
3
3
2.7
2.7
2.7
0.9997
0.9997
0.9996
0.00E+00
0.00E+00
0.00E+00
1.43E+06
1.49E+06
2.37E+06
1.31E+05
4.77E+04
7.75E+04
NIST_27900
NIST_27900
NIST_27900
0.00E+00
9.113E+06
1.49E+07
0.00E+00
3.23E+05
5.12E+05
KNSTD
KNSTD
KNSTD
-71.676800
-71.690417
-71.690417
1178
1177
1175
std
std
std
3
2
3
2.7
2.7
2.7
0.9998
0.9997
0.9997
0.00E+00
0.00E+00
0.00E+00
2.66E+05
3.19E+05
2.69E+05
4.30E+04
1.13E+04
5.82E+04
NIST_27900
NIST_27900
NIST_27900
0.00E+00
2.116E+06
0.00E+00
0.00E+00
8.65E+04
0.00E+00
KNSTD
KNSTD
KNSTD
-71.786514
-72.208861
-72.208544
630
1895
1894
std
std
std
4
4
4
2.7
2.7
2.7
1.0000
0.9999
0.9999
0.00E+00
0.00E+00
0.00E+00
1.35E+05
3.63E+05
3.43E+05
1.61E+04
4.88E+04
1.16E+04
NIST_27900
NIST_27900
NIST_27900
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
0.00E+00
KNSTD
KNSTD
KNSTD
-72.115861
-72.114400
1302
1305
std
std
2
3
2.7
2.7
0.9932
0.9998
0.00E+00
0.00E+00
2.22E+05
2.12E+05
7.75E+03
6.02E+03
NIST_27900
NIST_27900
0.00E+00
1.402E+06
0.00E+00
5.25E+04
KNSTD
KNSTD
-72.558317
-72.553139
-72.568150
1521
1259
520
std
std
std
3
3
3
2.7
2.7
2.7
0.9993
0.9957
0.9933
0.00E+00
0.00E+00
0.00E+00
2.38E+05
2.00E+05
1.04E+05
6.14E+03
5.85E+03
3.03E+03
NIST_27900
NIST_27900
NIST_27900
1.626E+06
1.438E+06
7.092E+05
5.60E+04
5.30E+04
4.008E+04
KNSTD
KNSTD
KNSTD
-72.366694
-72.459833
-72.364953
463
493
586
std
std
std
2
2.5
4
2.7
2.7
2.7
0.9978
0.9956
0.9956
0.00E+00
0.00E+00
0.00E+00
9.82E+04
1.01E+05
1.09E+05
3.64E+03
19184.9464
2.49E+03
NIST_27900
NIST_27900
NIST_27900
0.000E+00
0.00E+00
0.00E+00
0.000E+00
0.00E+00
0.00E+00
KNSTD
KNSTD
KNSTD
-72.604444
231
std
3
2.7
0.9974
0.00E+00
7.37E+04
1.07E+04
NIST_27900
0.00E+00
0.00E+00
KNSTD
Table S2. Exposure age comparison
Comparison between CRN exposure age calculated with the standard global production
rate (Global PR Lal/Stone 10Be yrs) and the New Zealand production rate using the Dunai
scaling system (NZ PR Dunai 10Be yrs) as discussed 3,15. Our accepted ages are based on
the 10Be isotope exposure ages and are either arithmetic means (in italics) or based upon
the oldest sample from an individual feature (*) as discussed in the text.
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