GSA DATA REPOSITORY 2010224
Supplementary material
Balco, G., Rovey, C.W., II., ‘Absolute chronology for major Pleistocene advances of the
Laurentide Ice Sheet’
Materials and Methods.
Analytical methods. We disaggregated paleosol samples in water and isolated the 0.25-0.85 mm
grain size fraction by wet-sieving, then extracted and purified quartz using heavy-liquid separation and repeated etching in HF (S1). This yielded quartz samples with 20-100 ppm Al.
We extracted Be and Al from quartz by HF dissolution followed by ion exchange chromatography
(S1), then measured Be and Al isotope ratios by accelerator mass spectrometry at the Lawrence
Livermore National Laboratory, Center for Accelerator Mass Spectrometry (LLNL-CAMS). We
measured total Al in the samples by extracting aliquots from the dissolved quartz-HF solution,
evaporating HF in the presence of H2 SO4 , redissolving in dilute HNO3 /H2 SO4 , and measuring
Al concentrations by ICP optical emission spectrophotometry against gravimetrically prepared Al
standards. Each total Al determination reflects at least two duplicate analyses on each of at least
two separate aliquots.
Al isotope ratios were normalized to the ‘KNSTD’ standard series (S2). Be isotope ratios were
normalized at the time of measurement to a variety of Be standards; for this work we have renormalized all Be measurements to the ‘07KNSTD’ isotope ratio standards of (S3).
Total process blanks contained 65000 ± 40000 atoms 26 Al, 0.2-1.3 % of the total number of atoms
in any sample. We used two different 9 Be carriers. For high-nuclide-concentration samples, we
used a commercially available Be standard solution. Total process and carrier blanks using this
carrier had 175000 ± 20000 atoms 10 Be, 0.7-2% of the total number of atoms in any sample. For
low-concentration samples, we used several Be carrier solutions derived from deep-mined beryl.
Total process and carrier blanks using these carrier solutions had 3000-20000 atoms 10 Be, 0.1-1.3%
of the total number of atoms present in any sample.
Table S1 lists 26 Al and 10 Be concentrations.
Data reduction and age determinations. Calculations of both simple burial ages and isochron burial
ages use implicit solution methods to account for post-burial nuclide production by deeply penetrating muons. Basically, one i) begins by computing an apparent burial age assuming burial at
infinite depth, ii) calculates the post-burial nuclide production at the actual sample depth given
that burial age, iii) subtracts this nuclide inventory from the measured nuclide inventory, iv) recalculates the burial age, and v) iterates steps i-iv until the calculation converges on a solution.
Alternatively, this can be described as an optimization problem in which one finds the burial age
that best fits a forward model for nuclide accumulation and decay that includes a burial history
inferred from the site stratigraphy and any other relevant geologic evidence.
The methods for computing simple burial ages and isochron burial ages are described in detail
in (S4) and (S5), respectively. (S5) includes MATLAB code for computing isochron burial ages;
however, this study used a different Be isotope ratio standardization and 10 Be decay constant, so a
variety of constants in this code must be updated to yield the burial ages we report in the present
work. Both simple and isochron burial dating methods use a ‘sawtooth’ age/depth model in
which successive layers of overburden are instantaneously emplaced and then experience steady
erosion until emplacement of the next unit, as described in (S4). Thus, calculating a burial age
from any of our sites requires: i) the number, thickness, and density of all overburden units, ii)
an estimate for age of overburden units that overlie the unit to be dated, and iii) an estimate for
the surface erosion rate that prevails between the emplacment of one overburden unit and the
next. Thicknesses and densities of overburden units are tabulated in Table S2; our methods for
density measurement are described in (S1). To obtain age estimates for overburden units above
the one being dated, we began by calculating the ages of the uppermost tills in the section, and
then incorporated these results into the age/depth model used to compute the ages of older units
as we proceeded downward. When surficial loess was present, we assigned it an age of 0.1 ±
0.05 Ma. We took the surface erosion rate to be 10 ± 5 m Ma−1 always. These assumptions are
described and justified in detail in (S5).
An important aspect of the present work is that we calculated all burial ages using a common set
of isotope ratio standardizations, production rates, and decay constants, including 10 Be and 26 Al
production rates by muons from (S6) and (S7), 10 Be and 26 Al production rates by spallation from
the global calibration data set and ‘St’ scaling scheme of (S8) (restandardized to the Be isotope
ratio standards of (S3)), the 26 Al half-life from (S2) (0.705 ± 0.017 Ma), and the 10 Be half-life of (S9)
and (S10) (1.387 ± 0.012 Ma) . For this reason, some burial ages for published data reported here
differ from the originally published ages.
Uncertainties in burial ages include i) 26 Al and 10 Be measurement uncertainties, ii) site-specific
uncertainties in the thicknesses, densities, ages, and estimated erosion rates of overburden units,
iii) uncertainties in nuclide production rates by both spallation and muons, and iv) uncertainties in
the 26 Al and 10 Be decay constants. Uncertainties are calculated by linearization, numerical partial
differentiation, and adding in quadrature. (S5) and (S11) discuss the importance of various sources
of uncertainty at length. For all except one of the sites discussed in this work, the tills immediately
overlying the paleosols we sampled are relatively thick (> 1000 g cm−2 ). In this situation, total
uncertainties in the age are dominated by i) measurement uncertainties, and ii) uncertainties in
the decay constants. Uncertainties in production rates and the burial history of the samples make
a negligible contribution. At a single site (site 11 in Table 1, the intra-Alburnett paleosol at Conklin
Quarry), the till overlying the samples was thinner (600 g cm−2 ) and uncertainties in the burial
history of the samples make a significant contribution to the total uncertainty in the burial age.
Figure S1 shows graphical solutions for burial ages at sites where we used the simple burial dating
method; Figure S2 shows burial isochrons for sites where we used the isochron method.
References.
S1. J. Stone et al., Methods and procedures, UW Cosmogenic Nuclide Lab.
(Available online: http://depts.washington.edu/cosmolab/chem.html)
S2. Nishiizumi, K. Preparation of 26 Al AMS standards. Nuclear Instruments and Methods in
Physics Research Section B 223-224, 388-392 (2004).
S3. Nishiisumi, K., et al.. Absolute calibration of 10 Be AMS standards. Nuclear Instruments and
Methods in Physics Research Section B 258, 403-413 (2007).
S4. Balco, G., Rovey, C.W., II., Stone, J.O.H. The first glacial maximum in North America. Science
307, 222 (2005).
S5. Balco, G., Rovey, C.W., II. An isochron method for cosmogenic-nuclide dating of buried soils
and sediments. American Journal of Science 308, 1083-1113 (2008).
S6. Heisinger, B. et al.. Production of selected cosmogenic radionuclides by muons 1. Fast
muons. Earth and Planetary Science Letters 200, 345 (2002).
S7. Heisinger, B. et al.. Production of selected cosmogenic radionuclides by muons 2. Capture of
negative muons. Earth and Planetary Science Letters 200, 357 (2002).
S8. Balco, G., Stone, J.O.H., Lifton, N.L., Dunai, T.J. A complete and easily accessible means of
calculating surface exposure ages or erosion rates from 10 Be and 26 Al measurements. Quaternary Geochronology 3, 174-195 (2008).
S9. Chmeleff, J., von Blanckenburg, F., Kossert, K., Jakob, D. Determination of the 10 Be half-life
by multi-collector ICP mass spectrometry and liquid scintillation counting. Nuclear Instruments and Methods in Physics Research Section B 268, 192-199 (2010).
S10. Korschinek. G. et al. Determination of the 10 Be half-life by Heavy-Ion Elastic Recoil Detection and liquid scintillation counting. Nuclear Instruments and Methods in Physics Research
Section B 268, 187-191 (2010)
S11. Balco, G., Stone, J., Mason J. Numerical ages for Plio-Pleistocene glacial sediemnt sequences
by 26 Al/10 Be dating of quartz in buried paleosols. Earth and Planetary Science Letters 232,
179-191 (2005)
S12. Granger, D.E. A review of burial dating methods using 26 Al and 10 Be. in Siame, L.L., Bourles,
D.L., Brown, E.T., eds, In-situ-produced Cosmogenic Nuclides and Quantification of Geological
Processes (Geological Society of America Special Paper 415), 1-16 (2006).
Steady erosion line
6
6
[26Al] / [10Be]
5 m/Ma
4
4
3
2
2
1
a. Musgrove clay pit
0
Steady erosion line
6
[26Al] / [10Be]
0.5 Ma burial
4
1 Ma
1.5 Ma
2 Ma
2.5 Ma
2
3 Ma
b. Pendleton clay pit
0
5
6
7
8
9
10
[10Be] (105 atoms g-1)
Figure S1a,b. 10 Be - 26 Al/10 Be diagram for samples from colluvium underlying the Atlanta Fm.
at the Musgrove (a) and Pendleton (b) clay pits. The burial age of these samples is the age the
Atlanta till was emplaced. See (S12) for a complete description of this diagram. The dashed lines
are contours of surface erosion rate prior to burial and are labeled in the upper panel; the solid
lines are contours of burial age and are labeled in the lower panel. In each diagram, the contours
of burial age are drawn for the present depth of the samples. The gray ellipses are 68% confidence
intervals reflecting measurement uncertainty.
Steady erosion line
6
6
5 m/Ma
[26Al] / [10Be]
0.5 Ma burial
4
4
1 Ma
3
1.5 Ma
2
2 Ma
2
2.5 Ma
3 Ma
1
c. WB19
0
5
6
7
8
9
10
[10Be] (105 atoms g-1)
Figure S1c. 10 Be - 26 Al/10 Be diagram for samples from bedrock residuum underlying the Moberly
Formation till in the WB19 borehole. The burial age of these samples is the age the Moberly
Formation Formation till was emplaced. The diagram is constructed as described in the caption
to Figure S1a.
1.5
26Al (106 atoms g-1)
Production ratio
1.31 ± 0.13 (0.15) Ma
1
0.5
As measured
Corrected for post-burial
nuclide production
0
0
0.1
0.2
0.3
10Be (106 atoms g-1)
Figure S2a. 26 Al - 10 Be isochron for Atlanta Formation paleosol at the Musgrove clay pit. The
burial age of these samples is the age of the overlying Moberly Formation till. In this and subsequent figures, the light gray ellipses are 68% confidence regions on measured 26 Al and 10 Be
concentrations. The black ellipses reflect correction of the measured data for nuclide production
after burial, as described in (S5). The dark line is the isochron fit to the corrected data. The light
line has a slope given by the 26 Al/10 Be production ratio for comparison.
1.5
26Al (106 atoms g-1)
Production ratio
1
0.5
0.839 ± 0.096 (0.106) Ma
As measured
Corrected for post-burial
nuclide production
0
0
0.1
0.2
0.3
10Be (106 atoms g-1)
Figure S2b. 26 Al - 10 Be isochron for Moberly Formation paleosol in borehole FU02. The burial age
of these samples is the age of the overlying Fulton till.
1.5
26Al (106 atoms g-1)
Production ratio
1
0.5
0.74 ± 0.12 (0.13) Ma
As measured
Corrected for post-burial
nuclide production
0
0
0.1
0.2
0.3
10Be (106 atoms g-1)
Figure S2c. 26 Al - 10 Be isochron for Moberly Formation paleosol in borehole NF06. The burial age
of these samples is the age of the overlying Fulton till.
1.5
26Al (106 atoms g-1)
Production ratio
1
0.5
0.798 ± 0.044 (0.061) Ma
As measured
Corrected for post-burial
nuclide production
0
0
0.1
0.2
0.3
10Be (106 atoms g-1)
Figure S2d. 26 Al - 10 Be isochron for Moberly Formation paleosol in borehole WL3. The burial age
of these samples is the age of the overlying Fulton till.
1.5
26Al (106 atoms g-1)
Production ratio
1
0.21 ± 0.16 (0.016) Ma
0.5
As measured
Corrected for post-burial
nuclide production
0
0
0.1
0.2
0.3
10Be (106 atoms g-1)
Figure S2e. 26 Al - 10 Be isochron for Fulton paleosol in borehole PF2. The burial age of these
samples is the age of the overlying Columbia till.
1.5
26Al (106 atoms g-1)
Production ratio
1
0.31 ± 0.50 (0.50) Ma
0.5
As measured
Corrected for post-burial
nuclide production
0
0
0.1
0.2
0.3
10Be (106 atoms g-1)
Figure S2f. 26 Al - 10 Be isochron for Fulton paleosol in borehole SMS92A. The burial age of these
samples is the age of the overlying Columbia till.
1.5
26Al (106 atoms g-1)
Production ratio
1
0.21 ± 0.17 (0.18) Ma
0.5
As measured
Corrected for post-burial
nuclide production
0
0
0.1
0.2
0.3
10Be (106 atoms g-1)
Figure S2g. 26 Al - 10 Be isochron for Columbia paleosol at the Sieger pit. The burial age of these
samples is the age of the overlying Macon till.
1.5
26Al (106 atoms g-1)
Production ratio
1
0.5
0.87 ± 0.43 (0.43) Ma
As measured
Corrected for post-burial
nuclide production
0
0
0.1
0.2
0.3
10Be (106 atoms g-1)
Figure S2h. 26 Al - 10 Be isochron for paleosol within the Alburnett Formation till at Conklin Quarry.
The burial age of these samples is the age of the overlying portion of the Alburnett Fm. till.
1.5
26Al (106 atoms g-1)
Production ratio
1
0.5
0.72 ± 0.37 (0.37) Ma
As measured
Corrected for post-burial
nuclide production
0
0
0.1
0.2
0.3
10Be (106 atoms g-1)
Figure S2i. 26 Al - 10 Be isochron for paleosol atop the Alburnett Formation till at Conklin Quarry.
The burial age of these samples is the age of the overlying Winthrop till.
Table S1. 26Al and 10Be measurements from Missouri and Iowa paleosols.
Boreholes
[10Be]1
103 atoms g-1
[26Al]2
103 atoms g-1
12.73
12.88
13.11
13.34
652.3 ± 11.7
652.5 ± 12.3
653.4 ± 11.1
616.5 ± 12
2196.3 ± 78.9
2286.2 ± 74
2159.5 ± 75
2260.4 ± 103.9
This paper
13.87
14.63
15.24
15.85
19.21
24.70
14.18
14.86
15.55
16.16
19.51
25.00
228.3 ± 7.5
264.3 ± 7
96.6 ± 3
76.5 ± 1.9
62.1 ± 1.5
55.9 ± 1.8
973.7 ± 46.9
1185.3 ± 54.5
407.5 ± 23.4
316.5 ± 17.1
232 ± 16.5
211.3 ± 17.6
Balco and Rovey (2008)
FU02-32-33.1
FU02-33.1-34
FU02-34-35
FU02-39-40
9.76
10.09
10.37
11.89
10.09
10.37
10.67
12.20
190.4 ± 4.2
233.5 ± 5.2
229.5 ± 4.1
89.1 ± 1.8
897.1 ± 30.1
1076.4 ± 43.1
995.3 ± 40.7
412.3 ± 19.8
This paper
9.3
NF06-31-31.75
NF06-34.25-35
NF06-38-38.5
9.45
10.44
11.59
9.68
10.67
11.74
87.8 ± 2.3
68.7 ± 1.9
62.5 ± 1.7
423.6 ± 29.8
324.5 ± 19.9
292.3 ± 30
This paper
Fulton
10.8
SMS-92A-36
SMS-92A-38
10.98
11.59
11.28
11.89
244.4 ± 4.6
194.6 ± 4.3
1354.6 ± 47.5
1051.7 ± 38.2
This paper
Fulton
13.9
PF2-45.75-48
PF2-48.75-52
PF2-52-53.5
PF2-53.5-55
13.95
14.86
15.85
16.31
14.63
15.85
16.31
16.77
215.2 ± 5.5
205 ± 4.5
208.9 ± 4.2
160.4 ± 3.9
927.5 ± 44.1
899.2 ± 45
927.9 ± 47.4
618.9 ± 42.1
Balco and Rovey (2008)
Borehole name
Latitude
Longitude
Surface
elevation (m)
Paleosol name
Paleosol top
depth (m)
WB19
38.964
-91.687
230
Atlanta
12.3
WB19-40.5-41.75
WB19-41.75-42.25
WB19-42.25-43
WB19-43-43.75
12.35
12.73
12.88
13.11
WL3
38.745
-90.99
256
Moberly
13.9
WL3-45.5-46.5
WL3-48-48.75
WL3-50-51
WL3-52-53
WL3-63-64
WL3-81-82
FU02
38.837
-91.997
267
Moberly
9.8
NF06
38.904
-91.443
265
Moberly
SMS92A
39.802
-92.474
271
PF2
38.904
-91.743
255
Sample name
Sample depth in core (m)
Top
Bottom
Reference
Table S1, continued
Quarries
Quarry name
Latitude
Longitude
Surface
elevation (m)
Musgrove Pit
38.862
-91.437
253
Paleosol name
Whippoorwill
Atlanta
Sample name
Sample depth below
paleosol surface (m)
Top
Bottom
[10Be]1
103 atoms g-1
[26Al]2
103 atoms g-1
837 ± 16.9
836.5 ± 17.3
840.4 ± 11.8
839.2 ± 17.2
1517 ± 42.8
1574.6 ± 48
1444.4 ± 44.3
1532.7 ± 47.3
Balco et al. (2005)
Balco et al. (2005)
Reference
MO-MP-1
0.00
(replicate measurement)
MO-MP-5
0.00
(replicate measurement)
0.20
MO-MP-2
0.00
(replicate measurement)
MO-MP-04-0
0.00
MO-MP-04-40
0.40
MO-MP-04-80
0.80
MO-MP-04-150
1.50
MO-MP-04-250
2.50
0.30
0.20
0.60
1.00
1.70
2.70
237.4 ± 4.8
233.7 ± 6.8
194.3 ± 4.9
110.8 ± 3.7
89 ± 2.3
94.6 ± 3.6
115.5 ± 3.2
780.7 ± 35.1
790.4 ± 61.2
684.7 ± 32.4
370.7 ± 25.1
290.4 ± 25.9
304.5 ± 19.9
311.6 ± 21.6
PP-WH-0
PP-WH-0.5
PP-WH-1
PP-WH-1.75
0.00
0.15
0.31
0.53
0.15
0.31
0.53
0.69
704.8 ± 13.4
699.5 ± 17.7
708.8 ± 17.9
700 ± 17.7
1633.9 ± 61.2
1458.6 ± 103.8
1567.8 ± 75.4
1566.3 ± 77.3
Rovey and Balco (2010)
SP-COL-0
SP-COL-1
SP-COL-2
SP-COL-3
SP-COL-5
0.00
0.30
0.61
0.91
1.52
0.30
0.61
0.91
1.22
1.83
153.9 ± 3
79.8 ± 2.1
75.1 ± 1.9
73.6 ± 1.9
87.1 ± 2.5
883.5 ± 37
420.3 ± 22.4
392 ± 20.2
430.9 ± 24.8
452.9 ± 25.1
Balco and Rovey (2008)
0.20
Balco and Rovey (2008)
Pendleton Pit
38.786
-91.255
239
Whippoorwill
Sieger Pit
39.248
-91.806
233
Columbia
Conklin Quarry
41.699
-91.556
210
Westburg
(top Alburnett Fm.)
CQ-AU-0-20
CQ-AU-42-62
CQ-AU-90-110
CQ-AU-160-180
0.00
0.42
0.90
1.60
0.20
0.62
1.10
1.80
96.4 ± 1.9
130.9 ± 2.5
114.5 ± 2.3
91 ± 1.7
433.5 ± 20.3
563 ± 18.9
531 ± 20.2
375.7 ± 15.4
This paper
Unnamed
(intra-Alburnett Fm.)
CQ-AL-0-20
CQ-AL-66-86
CQ-AL-105-125
CQ-AL-145-155
0.00
0.66
1.05
1.45
0.20
0.86
1.25
1.55
84.5 ± 1.8
74.1 ± 1.4
50.4 ± 1
52.2 ± 1.3
393.9 ± 18
289.5 ± 15.9
217.6 ± 11.1
212.7 ± 11.3
This paper
Notes:
Normalized to the Be isotope ratio standards of Nishiizumi et al. (2007)
Normalized to the Al isotope ratio standards of Nishiizumi (2004)
1
2
References for Table S1:
Balco, G., Rovey, C.W., II, Stone, J.O.H., 2005. The First Glacial Maximum in North America. Science 307, p. 222.
Balco, G., Rovey C.W. II, 2008. An isochron method for cosmogenic-nuclide dating of buried soils. American Journal of Science 308, pp. 1083-1114.
Rovey, C.W., II, Balco, G., 2010. Periglacial climate at the 2.5 Ma onset of Northern Hemisphere glaciation inferred from the Whippoorwill formation, northern Missouri, USA. Quaternary Research 73, pp. 151-161.
Nishiizumi, K., 2004 Preparation of 26Al AMS standards. Nuclear Instruments and Methods in Physics Research Section B 223-224, 388-392.
Nishiizumi, K., Imamura, M., Caffee, M., Southon, J., Finkel, R., McAnich, J., 2007. Absolute calibration of 10Be AMS standards. Nuclear Instruments and Methods in Physics Research Section B , 258, 403–413.
Table S2. Mass thickness of stratigraphic units overlying the sampled paleosols. Not all units are present at all sites. Uncertainties in
overburden mass thickness are 6% and are derived from the reproducibility of density measurements for a given till unit.
Site name
Paleosol
Overlying till
Sites in Missouri
Musgrove Pit
Pendleton Pit
Musgrove Pit
WB19
WL3
FU02
NF06
SMS92A
PF2
Sieger Pit
Whippoorwill
Whippoorwill
Atlanta
Atlanta
Moberly
Moberly
Moberly
Fulton
Fulton
Columbia
Atlanta
Atlanta
Moberly
Moberly
Fulton
Fulton
Fulton
Columbia
Columbia
Macon
Sites in Iowa
Conklin Quarry
Conklin Quarry
Unnamed
Westburg
Upper Alburnett
Winthrop
Mass thickness of overburden units (g cm-2)
Atlanta
Moberly
1341
1100
2540
329
2540
1774
Fulton
Columbia
Macon
525
210
525
564
970
2166
1647
1331
Loess
1013
1784
1299
1248
1084
420
595
473
333
105
Alburnett
Winthrop
Aurora
Loess
679
1193
1193
1410
1410
805
805