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# Atmospheric composition 1 million years ago from blue ice in the Allan Hills, Antarctica

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Significance Bubbles of ancient air trapped in ice cores permit the direct reconstruction of atmospheric composition and allow us to link greenhouse gases and global climate over the last 800 ky. Here, we present new ice core records of atmospheric composition roughly 1 Ma from a shallow ice core drilled in the Allan Hills blue ice area, Antarctica. These records confirm that interglacial CO 2 concentrations decreased by 800 ka. They also show that the link between CO 2 and Antarctic temperature extended into the warmer world of the mid-Pleistocene.
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Atmospheric composition 1 million years ago from blue
ice in the Allan Hills, Antarctica
John A. Higgins
a,1
, Andrei V. Kurbatov
b,c
, Nicole E. Spaulding
b
, Ed Brook
d
, Douglas S. Introne
b
, Laura M. Chimiak
a
,
Yuzhen Yan
a
, Paul A. Mayewski
b,c
, and Michael L. Bender
a
a
Department of Geosciences, Princeton University, Princeton, NJ 08544;
b
Climate Change Institute and
c
School of Earth and Climate Sciences, University of
Maine, Orono, ME 04469; and
d
College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331
Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved April 16, 2015 (received for review December 1, 2014)
Here, we present direct measurements of atmospheric composi-
tion and Antarctic climate from the mid-Pleistocene (1 Ma) from
ice cores drilled in the Allan Hills blue ice area, Antarctica. The
1-Ma ice is dated from the deficit in
40
Ar relative to the modern
atmosphere and is present as a stratigraphically disturbed 12-m
section at the base of a 126-m ice core. The 1-Ma ice appears to
represent most of the amplitude of contemporaneous climate
cycles and CO
2
and CH
4
concentrations in the ice range from 221
to 277 ppm and 411 to 569 parts per billion (ppb), respectively.
These concentrations, together with measured δD of the ice, are
at the warm end of the field for glacialinterglacial cycles of the
last 800 ky and span only about one-half of the range. The
highest CO
2
values in the 1-Ma ice fall within the range of in-
terglacial values of the last 400 ka but are up to 7 ppm higher
than any interglacial values between 450 and 800 ka. The low-
est CO
2
values are 30 ppm higher than during any glacial period
between 450 and 800 ka. This study shows that the coupling of
Antarctic temperature and atmospheric CO
2
extended into the
mid-Pleistocene and demonstrates the feasibility of discontinu-
ously extending the current ice core record beyond 800 ka by
shallow coring in Antarctic blue ice areas.
climate change
|
glacial cycles
|
atmospheric CO
2
|
ice cores
|
greenhouse gases
Ice cores serve as a critical archive of past environmental con-
ditions, providing constraints on global atmospheric composi-
tion and the climate of polar regions (1). Reconstructions of
atmospheric CO
2
and CH
4
from air trapped in ice cores dating as
far back as 800 ka indicate a link between greenhouse gases and
global climate in the form of 100-ky glacial cycles (Fig. 1). These
climate cycles are recorded in proxy records from deep sea
sediments reflecting variations in ocean temperature and conti-
nental ice volume (2). Deep sea records indicate that the 100-ky
glacial cycle developed only 900,000 y ago [the mid-Pleistocene
transition (MPT)] (3). Before this time and going back to 2.8 Ma,
glacial cycles lasted, on average, 40 ky (4). The origins of both
the 100- and 40-ky glacial cycles, their links to orbital forcing,
and changes in atmospheric greenhouse gases are debated.
Extending ice core records to earlier times would advance our
understanding of links between greenhouse gases, climate, and
causes of the MPT.
One archive for extending ice core records beyond 800 ky is
blue ice areas (BIAs), outcrops of glacial ice brought to the
surface by ice flow guided by bedrock topography (5). These
records are likely to be stratigraphically complex because of
deformation associated with ice transport but may also contain
the oldest easily accessible ice on the planet. In the Allan Hills,
Antarctica, the antiquity of shallow ice is documented by ter-
restrial ages of englacial meteorites exposed on the surface by
ablation (6). These ages cluster between 100 and 400 ky, with a
small number that extend to 1 Ma and a single meteorite yielding
an age of 2.2 Ma (7).
Here, we present the first, to our knowledge, direct snapshots
of atmospheric composition during the MPT from an ice core
drilled at Site BIT-58 (8) in the Allan Hills BIA (Fig. S1). We
date ice and trapped gases directly, taking advantage of the slow
leak of
40
Ar into the atmosphere from the decay of
40
K in Earths
interior (the
40
Ar
atm
geochronometer). The observed increase in
the
40
Ar/
38
Ar ratio of the atmosphere is small, resulting in un-
certainties for a single sample of ±213 ky (9). Measurements of Ar
isotope ratios date a 12-m section at the base of Site BIT-58 to 1
Ma (Fig. 2). We report on measurements of CO
2
and CH
4
con-
centrations, the δ
18
OofpaleoatmosphericO
2
,δD, and deuterium
excess (d) in the 1-Ma ice from Site BIT-58. This work provides
a direct window into atmospheric composition and Antarctic
climate during the mid-Pleistocene.
Climate Records from Ice Cores in the Allan Hills BIA
Previous research using surface samples and vertical ice cores
from the Allan Hills Main Ice Field (MIF) has shown that
shallow coring (<200 m) can yield high-quality records of Ant-
arctic climate and atmospheric composition that are continuous
over long time intervals (10). In particular, results from surface
samples and cores drilled along the MIF flow line reveal a
continuous climate record spanning marine isotope stage (MIS)
5/6 (Fig. 1). It is exposed both at the surface along a 5-km
transect and in the long core (225 m) at Site 27 (10). The ice was
dated to 85250 ka by combining
40
Ar
atm
with the stratigraphy of
both δD of the ice [isotopic temperature (11)] and δ
18
Oof
paleoatmospheric O
2
[δ
18
O
atm
(12)]. Measurements of CO
2
and
CH
4
at Site 27 are also consistent with records from other
Antarctic ice cores for this time period (Fig. S2). The climate
record from Site 27 across MIS 5/6 provides an important ref-
erence frame for comparison with ice of greater antiquity found
in the Allan Hills BIA, because environmental conditions in the
source regions are expected to be similar.
Significance
Bubbles of ancient air trapped in ice cores permit the direct
reconstruction of atmospheric composition and allow us to link
greenhouse gases and global climate over the last 800 ky. Here,
we present new ice core records of atmospheric composition
roughly 1 Ma from a shallow ice core drilled in the Allan Hills
blue ice area, Antarctica. These records confirm that interglacial
CO
2
concentrations decreased by 800 ka. They also show that
2
and Antarctic temperature extended into
the warmer world of the mid-Pleistocene.
Author contributions: A.V.K., P.A.M., and M.L.B. designed research; J.A.H., A.V.K., N.E.S.,
L.M.C., and Y.Y. performed research; E.B., D.S.I., P.A.M., and M.L.B. contributed new re-
agents/analytic tools; J.A.H., A.V.K., N.E.S., E.B., L.M.C., P.A.M., and M.L.B. analyzed data;
and J.A.H. and M.L.B. wrote the paper.
The authors declare no conflict of interest.
1
To whom correspondence should be addressed. Email: jahiggin@princeton.edu.
1073/pnas.1420232112/-/DCSupplemental.
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Site BIT-58 sits near the crest of the northwest to southeast
trending ice ridge off the MIF in an area of Allan Hills where
terrestrial meteorite ages suggest ice older than 1 Ma (6, 7).
Although directions and magnitudes of ice flow at Site BIT-58
are not known, measurements from the nearby MIF are consis-
tent with flow to the east or northeast. Bedrock topography,
determined using ground penetrating radar, indicates that Site
BIT-58 sits on a local bedrock high with a total ice thickness of
130 m. Although drilling came to within 5 m of bedrock, re-
covered ice was clean, and there was no evidence for contami-
nation from bedrock from visual inspection or the chemistry of
trapped gases (δO
2
/N
2
and air content) (SI Text).
Measured
40
Ar
atm
ages from the 126-m core at Site BIT-58 are
shown in Fig. 2. The core can be separated into two distinct
sections. An upper unit extends from 25 to 113 m, with a
weighted mean
40
Ar
atm
age of 320 ±160 ky (1σ;n=19) (Table
S1). Below is a basal unit from 113 m to at least 126 m (the
bottom of the core), with an average
40
Ar
atm
age of 990 ±110 ky
(1σ;n=6). The transition between 320-ka and 1-Ma ice occurs
between Ar-dated samples at 112.8 and 117.4 m. The shift in age
occurs across an interval from 113- to 115-m depth that is also
associated with relatively large changes in the δD of the ice and
δ
18
O of paleoatmospheric O
2
(Fig. 2 and Table S2). Within the
1-Ma ice, the
40
Ar
atm
ages from different depths are in-
distinguishable. As a result, although the ice below 117 m at
Site BIT-58 likely reflects a range of ages around 1 Ma, the
range is less than our analytical uncertainty (±213 ky; 1σ).
Two independent lines of evidence support an
40
Ar
atm
age
>800 ky for the ice below 117.4 m at Site BIT-58. First, 3 of 15
CO
2
measurements yield concentrations that are higher than any
measured from European Project for Ice Coring in Antarctica
(EPICA) Dome C between 450 and 800 ky (Fig. 3Band Table
S3). Second, a subset of the paired measurements of CO
2
and
CH
4
shows no overlap with known atmospheric values between
450 and 800 ky (Fig. 3B).
In addition, water isotopes show that there is an abrupt tran-
sition in ice age at 113 m. Records of deuterium excess (d) in
the Site BIT-58 icea measure of temperature, relative hu-
midity, seasonality of precipitation, and wind speed in the pre-
cipitation source region (13)indicate two distinct populations
0300 600 900 1200
300
270
240
210
180
150
Age (kyr)
CO2
(ppm)
δDice
(‰)
800
700
600
500
400
300
CH 4
(ppb)
-370
-390
-410
-430
-450
-275
-295
-315
-335
-355
3
3.5
4
4.5
5
5.5
δ18Oforam
()
3
2
1
0
-1
-2
Mg/Catemp
C)
Allan Hills BIT-58
1 Ma ice
A
B
C
D
E
Fig. 1. Records of (A)CH
4
,(B)CO
2
, and (C)δD from the Allan Hills BIA (Site 27; black line and black symbols between 115 and 250 ka) compared with records
from Vostok/EPICA Dome C (green, red, and blue lines) (11, 1820). The range of gas and ice properties in the 1-Ma ice from Site BIT-58 is shown to the right
(Tables S1S4). Boxes around the 1-Ma data indicate an age uncertainty of ±89 ky (SE) for n=6 measurements of ice below 117 m assuming an external
reproducibility (1σ)of±213 ky (Materials and Methods has additional details). Dshows the stacked benthic foraminiferal δ
18
O record (4), and Eshows a record
of deep ocean temperature based on foraminiferal Mg/Ca (17). ppb, parts per billion.
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www.pnas.org/cgi/doi/10.1073/pnas.1420232112 Higgins et al.
above and below 113-m depth (Fig. S3 and Table S4). Measured
d values for both populations range from +0.9to 10.7.
Parenthetically, these values are extremely depleted relative to
those observed in the Antarctic interior [e.g., d +4to
+12at EPICA Dome C (14)], consistent with a local source
of precipitation for the 320-ka and 1-Ma ice in the Allan Hills
BIA (13).
The variability in δD and δ
18
O
atm
observed in the 320-ka unit
at Site BIT-58 is comparable in magnitude with the variability
associated with MIS 5/6 at nearby Site 27 (10). However, the
abrupt transitions in δD and δ
18
O
atm
suggest that the ice within
the 320-ka unit at Site BIT-58 is stratigraphically disturbed. δD
ice
varies on submeter scales within the 1-Ma ice (i.e., between 118
and 120 m) (Fig. 2), indicating that this unit is stratigraphically
complex as well (Fig. 2 and Fig. S4). Stratigraphic disturbance
associated with ice transport is not surprising in the Allan Hills
BIA given the subglacial topography of the region, which in-
cludes large and abrupt changes in ice thickness [from >1kmto
<200 m over a few kilometers (15)], and the distance over which
the ice has been transported from its source region [20 km at
present (16)]. As a result, the Allan Hills BIA is unlikely to
preserve long (>100 ky) continuous records of ice >800 ky in age.
Records may also be difficult to interpret, because layers can be
overturned or oriented parallel to the drilling direction. However,
deformation and folding also confer a benefit by increasing the
likelihood that a given volume of ice samples a wider range of time
periods and climate states. In addition, the shallow burial depths
and cold temperatures that characterize ice core records from the
Allan Hills BIA should minimize diffusive exchange of gases and
the loss of paleoclimatic information in ice older than 1 My (17).
Given the evidence for stratigraphic disturbance, records of
ice and gas chemistry in the 1-Ma unit at Site BIT-58 represent
0
400
800
1200
40Aratm age (ka)
-330
-310
-290
-270
δDICE ()
1.6
1.2
0.8
0.4
0.0
δ18Oatm ()
Depth (meters)
120100806040
320 ka
990 ka
Fig. 2. Measured δ
18
O
atm
,δD
ice
,
40
Ar
atm
ages, CO
2
, and CH
4
concentrations of the 126-m ice core from Site BIT-58. Error bars on
40
Ar
atm
ages of ±213 ky
represent 1σuncertainties associated with repeat measurements of Princeton Air (n=67).
CO2
(ppm)
200 240 280
-300
-320
-340
δDICE ()
-280
CO2
(ppm)
300
220 260
700
600
400
CH
4 (ppb)
500
0-450 ka
450-800 ka
990 ka
180
Site 27 113-145 ka
Vostok 0-400 ka
990 ka
-420
-440
-460
-480
BA
Fig. 3. (A) Cross-plot of δD
ice
and CO
2
from Allan Hills Site 27 (114155 ka; open blue circles), 1-Ma ice from Site BIT-58 (filled black circles), and Vostok (0400 ka;
open red circles) (1) assuming a zero gas age/ice age difference. CO
2
concentrations are taken from the Vostok record (1) and paired with δD from Site 27
using the δ
18
O
atm
chronology from ref. 12. (B) Cross-plot of CO
2
and CH
4
concentrations for Site BIT-58 (filled black circles) compared with Vostok/EPICA Dome
C(1820) for 0800 ka [0450 ka (open blue circles) and 450800 ka (open red circles)]. ppb, parts per billion.
Higgins et al. PNAS
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snapshots or short intervals of mid-Pleistocene climate that span
some fraction of the full range of glacialinterglacial variability.
Despite these limitations, our data represent the oldest direct
measurements of atmospheric CO
2
,CH
4
,δ
18
OofO
2
, and Ant-
arctic δD
ice
and permit important preliminary conclusions to be
drawn about atmospheric composition, Antarctic climate, and
glacial cycles during the mid-Pleistocene.
Snapshots of Atmospheric Composition and Antarctic
Climate at 1 Ma
Given the disturbed nature of the 1-Ma ice from Site BIT-58, we
treat each of our data points as a snapshot of mid-Pleistocene
climate. We then compare the field of values in the 1-Ma ice with
those of younger ice in stratigraphically continuous cores at
Allan Hills Site 27, Vostok, and EPICA Dome C. Exactly how
many unique views exist in the 1-Ma ice is uncertain. Cross-plots
of the data do not show correlations that would reflect simple
mixing of two end members. To estimate the fraction of glacial
interglacial variability captured in our 1-Ma ice, we compare the
range of variability in the 1-Ma ice and the MIS 5/6 ice from Site
27 with the range of variability in marine climate proxies [benthic
foraminfera δ
18
O stack (4) and Mg/Ca records (18); Fig. 1] over
the same time periods.
Considering only the benthic δ
18
O stack, the amplitudes of
glacial cycles between MISs 19 and 38 average 1.1or 60%
of the amplitude of MIS 5/6 (1.9). In comparison, the range in
δD values that we observe in the 1-Ma ice is 4651% of the range
for MIS 5/6 at Site 27, suggesting that we have recovered 8088%
of the full mid-Pleistocene glacialinterglacial variability. Taking
into account the effects of changes in the partitioning of the δ
18
O
signal between deep ocean temperature and ice volume using
Mg/Ca-based temperature reconstructions (18) lowers this esti-
mate to 75% of the full glacialinterglacial range. Therefore, the
1-Ma ice at Site BIT-58 represents most (7588%) but probably
not all of the glacialinterglacial climate range during the MPT.
The missing ice most likely comes from glacial maxima, because
these intervals are generally underrepresented in ice cores due
to lower accumulation rates.
The ranges of measured CO
2
and CH
4
concentrations in the
1-Ma ice are also reduced compared with glacial cycles over the
last 800 ky (Fig. 1). Measured CO
2
concentrations range from
221 to 277 ppm or 4550% of the average glacial cycle since 800
ka (1, 19, 20). Measured CH
4
concentrations range from 411 to
569 parts per billion or 3540% of the average glacial cycle since
800 ka (21). The reduction in the range of measured CO
2
con-
centrations in the 1-Ma ice is comparable with that observed for
δD
ice
between the 1-Ma ice and MIS 5/6 at Site 27, whereas the
reduction in the range of measured CH
4
concentrations in the
1-Ma ice is slightly larger. Three of twelve measured CO
2
con-
centrations in the 1-Ma ice are higher than any CO
2
concen-
trations between 450 and 800 ka (Fig. 3B). Minimum CO
2
concentrations are also 30 ppm higher than any glacial maxima
values since 800 ka. Cross-plots of CO
2
and δD for the 1-Ma ice
indicate a relationship between East Antarctic temperature
change and CO
2
that is similar to that for MIS 5/6 at Site 27 and
other Antarctic sites (Fig. 3A). Cross-plots of CH
4
and CO
2
show
that 1-Ma samples fall within the envelope of glacialinterglacial
variability over the last 800 ky. However, for a given CO
2
con-
centration, CH
4
concentrations in the 1-Ma ice populate the
lower end of the range (Fig. 3B).
Implications for Glacial Cycles During the MPT
Our results support and reveal a number of features of mid-
Pleistocene atmospheric composition and Antarctic climate.
First, previous reconstructions of MPT CO
2
levels from mea-
surements of B isotopes in planktic foraminifera have shown that
interglacial CO
2
was similar to present values and that glacial
CO
2
was 30 ppm higher than 0- to 800-ka glacial values during
the MPT (22). In agreement with these reconstructions, Site
BIT-58 values indicate that maximum CO
2
concentrations were
slightly higher than interglacial values between 450 and 800 ka
and comparable with values between 0 and 450 ka. Our records
are also consistent with higher CO
2
concentrations at glacial max-
ima during the MPT. However, the Site BIT-58 record is probably
incomplete, and our minimum measured value of 221 ppm is an
upper bound.
Second, all of the climate indicatorsCO
2
,CH
4
, and δD
ice
exhibit smaller amplitude variability in the 1-Ma ice than in
glacial cycles from 0 to 800 ka. Nevertheless, the relationships
between climate properties are indistinguishable from those
observed for glacial cycles over the last 800 ky. Some of the re-
duced variability in the 1-Ma ice may come from the fact that the
record likely captures most but not all of the glacialinterglacial
range during the MPT. Mean δD
ice
and CO
2
concentrations in
the 1-Ma ice are also shifted toward higher (warmer) values,
consistent with warmer climates during the MPT.
Third, measured CH
4
concentrations in the 1-Ma ice are low
relative to coeval CO
2
concentrations in the context of the 800-ka
ice core record (Fig. 1). The CH
4
concentration range in the
1-Ma ice also accounts for a smaller fraction of the 0- to 800-ka
range than any other climate property. One possible explanation
is that variations in atmospheric CH
4
during the MPT lacked the
overshoots associated with rapid Northern Hemisphere warming
events (23). Although ice rafting in the North Atlantic predated
the MPT (24), millennial-scale climate variability was much
stronger after the MPT (25). In the absence of these millennial
events during the MPT, high CH
4
concentrations would not be
expected to co-occur with cooler Antarctic temperatures and
lower CO
2
concentrations.
Conclusions
Ice cores containing archives of 1-My air have been recovered
from shallow depths in Antarctic BIAs, providing the first, to our
knowledge, direct measurements of atmospheric composition
during glacial cycles of the mid-Pleistocene. Although these re-
cords are stratigraphically complex and likely incomplete, our
results are consistent with the prevailing views (4, 22) that the
amplitude of glacial cycles was diminished during this time, cli-
mates were warmer, interglacial CO
2
concentrations were greater
than between 450 and 800 ka, and glacial CO
2
concentrations, at
most, were 30 ppm higher than between 0 and 800 ka. Our re-
cord also provides reconstructions of atmospheric CH
4
>800 ka
and shows that CH
4
concentrations are both surprisingly low and
exhibit less variability than the other climate indicators compared
with glacial cycles over the last 800 ky.
Materials and Methods
40
Ar
atm
Geochronometer. In the solid earth,
40
K decays to stable
40
Ar. Because
40
Ar slowly leaks into the atmosphere, its concentration increases with time.
In contrast, because
38
Ar and
36
Ar are stable, primordial, and nonradiogenic,
their atmospheric concentrations are constant. Thus, the
40
Ar/
38
Ar ratio of
the atmosphere rises with time (toward the future) and decreases with age
(toward the past), providing a tool for dating. The term of merit is the
paleoatmospheric
40
Ar/
38
Ar ratio, which is defined as
40
Ar
atm
=δ
40
Ar/
38
Ar
1.002δ
38
Ar/
36
Ar.
The latter term corrects for gravitational fractionation, so that δ
40
Ar
atm
is a
measure of the paleoatmospheric
40
Ar/
38
Ar ratio. Studies of
40
Ar
atm
as a
function of age in the Dome C and Vostok cores characterize its rate of
change over the last 800 ka, enabling its use for dating ice (9). We linearly ex-
trapolate the rate of change over the last 800 ka to older periods (i.e., 12Ma),
although even for ice of this age, the signal is small and uncertainties are
large for a single measurement at 213246 ky (Table S1). Reported errors
(1σ) for each sample reflect both analytical errors and uncertainties in the
calibration slope of the
40
Ar
atm
geochronometer.
Procedures for Ar analyses are a modification of the methods used in ref. 9.
Trapped air was wet-extracted from an 500-g ice core sample. Two getters
were used to sequentially purify the Ar by removing more than 99.9999% of
N
2
,O
2
, and other nonnoble gases. Samples were then admitted to a Finnigan
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MAT 252 Mass Spectrometer, which simultaneously measures δ
40
Ar/
38
Ar and
δ
38
Ar/
36
Ar. The SD of a single measurement of δ
40
Ar
atm
in a sample of local
air (Princeton Air) is ±0.0143or ±213 ka. Reproducibility of natural ice core
samples from repeated measurements of Holocene age ice from Antarctica
from this study and the study in ref. 9 is 0.005 ±0.0100(n=16), corre-
sponding to an age uncertainty of ±149 ka. The reason for the difference is
unclear at present, and as a result, we adopt the more conservative estimate
of uncertainty (replicate measurements of Princeton Air or ±213 ka; 1σ)for
this study. When possible, samples were measured in replicate from the
same depth. Each ice sample was measured in conjunction with an aliquot of
an in-house Ar standard and Princeton Air that had been processed through
the same procedures as the ice core samples, with the exception of wet
extraction. Each sample was analyzed for 3 h on the mass spectrometer.
This long analysis period permits the precise measurement of δ
40
Ar
atm
, de-
spite the very low natural abundance of
38
Ar. Measured
38/36
Ar ratios in the
ice samples deviate from the air standard because of gravitational frac-
tionation in the firn, with the magnitude of the gravitational fractionation
in
38/36
Ar being roughly two times that expected for
15/14
N.
δ
18
O
atm
and δ
15
N. Analyses for the O
2
/N
2
/Ar ratio, δ
18
OofO
2
, and δ
15
NofN
2
were carried out as described in ref. 26. Briefly, ice was melted in vacuum,
the dried headspace gases were collected by condensation at liquid helium
temperatures, and the samples were admitted to the mass spectrometer
(Thermo Finnegan Delta Plus XP) for elemental and isotope ratio analyses. Ex-
ternal reproducibility is typically about ±0.02for δ
15
Nand±0.03 for δ
18
O
atm
.
Samples with poor reproducibility in δ
15
N(>0.1) were omitted (n=2).
CH
4
, Air Content, and CO
2
.CH
4
was analyzed using a meltrefreeze technique
most recently described in ref. 27. Samples (6070 g ice) were trimmed,
melted under vacuum, and then, refrozen at about 70 °C. Methane con-
centrations in released air were measured using a gas chromatograph and
referenced to air standards calibrated by National Oceanic and Atmospheric
Administration (NOAA) Global Monitoring Division (GMD) on the NOAA04
scale. Precision was generally better than ±4 parts per billion. Measured CH
4
concentrations were not corrected for gravitational fractionation, which had
an effect that is minor given the small gravitational fractionation observed
in δ
15
N at Site BIT-58. Individual sample uncertainties are reported in Table
S3. The measurement also quantifies the total air content of the sample, an
important parameter for evaluating sample quality and possibly, elevation
of the deposition site.
CO
2
concentrations were measured using the dry extraction (crushing)
method described in ref. 28; 8- to 15-g samples were crushed under vacuum,
and the sample air was condensed in steel tubes at 11 K. CO
2
concentrations
were measured after equilibration to room temperature using gas chro-
matography and referenced to air standards calibrated by NOAA GMD on
the World Meteorological Organization (WMO) scale. Generally, several
replicate ice samples were analyzed for each depth, and results were aver-
aged to obtain final CO
2
concentration. Typical SEMs are <1 ppm for four to
six replicates. Measured CO
2
concentrations were also not corrected for
gravitational fractionation. Individual sample uncertainties are reported in
Table S3.
δD
ice
and δ
18
O
ice
.When possible, samples for Site BIT-58 were subsampled
from cut slabs at 15-cm resolution in a 20 °C working freezer at the Climate
Change Institute, University of Maine. Because some portions of the core
were heavily fractured, continuous sections were not available at all depths.
For δD
ice
and δ
18
O
ice
measurements, ice samples were melted to liquid H
2
O
at room temperature, and 1.6-mL aliquots were transferred to glass vials for
analysis. Samples and laboratory standards, which span a wide range of
naturally occurring isotopic values previously calibrated against standard
mean ocean water, standard light Antarctic precipitation, and Greenland ice
sheet precipitation, were measured by cavity ring-down spectroscopy using a
Picarro Model L2130-i Ultra High-Precision Isotopic Water Analyzer coupled
with a High-Precision Vaporizer and liquid autosampler module; δ
18
O
ice
and
δD
ice
were measured simultaneously with an internal precision at 2σ=
±0.05for δ
18
Oand±0.10for δD
ice
.
ACKNOWLEDGMENTS. We would like to acknowledge US Ice Drilling Design
and Operations (IDDO), driller Mike Waszkiewicz, Kristin Schild, Melissa
Rohde, and Ken Borek Air for assistance with the field work. Mike Kalk and
Haun Marcott assisted with the CO
2
measurements. This work was funded by
National Science Foundation Grants ANT-0838843 (University of Maine) and
ANT-0838849 (Princeton University).
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Higgins et al. PNAS
|
June 2, 2015
|
vol. 112
|
no. 22
|
6891
EARTH, ATMOSPHERIC,
AND PLANETARY SCIENCES
Supporting Information
Higgins et al. 10.1073/pnas.1420232112
SI Text
Glaciology of BIAs and the Allan Hills BIA. BIAs are inland regions
where net ablation leads to the presence of ancient glacial ice at
the surface of an ice sheet. BIAs cover about 1% of the Antarctic
continent. Most are associated with nunataks, which lead to
upward flow of deeply buried ice. A number of forcings and
feedbacks maintain BIAs (1). Katabatic winds enhance ablation
and erode shallow snow patches. Smooth surfaces lead to espe-
cially high wind speeds at the ice surface, further promoting
ablation. Ice albedo is lower than snow albedo, leading to
warming and enhanced evaporation. Finally, ice flow may be very
slow. All of these factors are operating in the Allan Hills BIA.
Three BIAs are located to the west of the Allan Hills nunatak:
the Mid-Western Ice Field, the Near-Western Ice Field, and
the MIF. The latter is the focus of our work. Details on eleva-
tion, bedrock topography, and ice from the MIF can be found in
refs. 2 and 3. The basement is a mesa with a subsurface depth of
100200 m incised by deeper valleys, where ice flow is concen-
trated. Recent modeling and field studies show that ice is locally
sourced. The MIF at the Allan Hills is isolated from large inputs
of ice from the west by a deep valley through which ice drains to
the north, the south by an ice divide, and the east by the Allan
Hills themselves (25). Ice from the MIF flows very slowly to the
north (4). The local provenance of Allan Hills ice, within a 5-km-
long bedrock basin, simplifies the interpretation of the paleo-
climate data.
Additional Chemistry of Ice and Trapped Gases from the Allan Hills
BIA. Measured δ
15
N values of trapped N
2
average +0.098 ±
0.037(1σ), consistent with enrichment in
15
Ncausedby
gravitational fractionation in the diffusive zone of the firn at
the site of accumulation. The magnitude of the gravitational
enrichment in δ
15
N is small compared with a number of other
polar sites (e.g., +0.30.5at Byrd, Vostok, and EPICA
Dome C) (6), which we tentatively attribute to a thin diffusive
zone in the firn at the site of accumulation.
Total air contents [milliliters of air at standard temperature and
pressure (STD) per 1 g ice] for Sites BIT-58 and 27 samples
measured for CH
4
concentrations were indistinguishable be-
tween the two sites and ranged from 0.056 to 0.101, with an
average of 0.089 ±0.013 ml STP/g ice. These values agree well
with polar sites, with similar mean annual temperature and
elevation (7).
O
2
/N
2
ratios of trapped gases in ice cores are typically de-
pleted relative atmosphere by 1%, a consequence of the smaller
molecular diameter of O
2
compared with N
2
, resulting in pref-
erential loss of O
2
during the close off of bubbles at the firnice
transition (8, 9). Fractionation of O
2
/N
2
ratios during bubble close
off does seem to depend strongly on local summertime insola-
tion, an observation that has enabled O
2
/N
2
ratios of trapped gases
to be used to construct accurate ice core chronologies (10, 11).
Average O
2
/N
2
ratios for the ice from Site BIT-58 are 11.9 ±
5.3(1σ;320-kyice)and12.4 ±4.2(1σ; 1-Ma ice), indis-
tinguishable from measured O
2
/N
2
ratios from 0 to 800 ka from
Vostok and EPICA Dome C (11).
1. Bintanja R (1999) On the glaciological, meteorological, and climatological significance
of Antarctic blue ice areas. Rev Geophys 37(3):337359.
2. Delisle G, Sievers J (1991) Sub-ice topography and meteorite finds near the Allan Hills
and the Near Western Ice Field, Victoria Land, Antarctica. J Geophys Res 96(15):
577587.
3. Spaulding NE, et al. (2012) Ice motion and mass balance at the Allan Hills blue-ice area,
Antarctica, with implications for paleoclimate reconstructions. JGlaciol58(208 ):399406.
4. Coren F, Delisle G, Sterzai P (2003) Ice dynamics of the Allan Hills meteorite concen-
tration sites revealed by satellite aperture radar interferometry. Meteorit Planet Sci
38(9):13191330.
5. Grinsted A, Moore JC, Spikes VB, Sinisalo A (2003) Dating Antarctic blue ice areas
using a novel ice flow model. Geophys Res Lett 30(19):15.
6. Sowers T, Bender M, Raynaud D, Korotkevitch YS (1992) The d15N of N2 in air trapped
in polar ice: A tracer of gas transport in the firn and a possible constraint on ice age-
gas age differences. J Geophys Res 97(D14):1568315697.
7. Martinerie P, Raynaud D, Etheridge DM, Barnola JM, Mazaudier D (1992) Physical and
climatic parameters which influence the air content in polar ice. Earth Planet Sci Lett
112(1-4):113.
8. Battle M, et al. (1996) Atmospheric gas concentrations over the past century in air
from firn at the South Pole. Nature 383(6597):231235.
9. Bender ML (2002) Orbital tuning chronology for the Vostok climate record supported
by trapped gas composition. Earth Planet Sci Lett 204(1-2):275289.
10. Suwa M, Bender M (2008) Chronology of the Vostok ice core constrained by O2/N2
ratios of occluded air, and its implication for the Vostok climate records. Quat Sci Rev
27(11-12):10931106.
11. Landais A, et al. (2012) Towards orbital dating of the EPICA Dome C ice core using
delta O-2/N-2. Climate of the Past 8(1):191203.
Higgins et al. www.pnas.org/cgi/content/short/1420232112 1of9
Fig. S1. Map of the Allan Hills and the MIF, Allan Hills, Antarctica (Advanced Spaceborne Thermal Emission and Reflection Radiometers image from November
of 2005). Blue bands denote areas with glacial blue ice outcrops at the surface. White contours are elevation (meters). Light blue dots denote shallow drill sites
(15- to 20-m depth). Dark blue dots denote deep drill sites 27 (225 m) and BIT-58 (126 m). The shaded red band reflects the flow line in the MIF as determinedin
the work in ref. 1.
1. Spaulding NE, et al. (2012) Ice motion and mass balance at the Allan Hills blue-ice area, Antarctica, with implications for paleoclimate reconstructions. J Glaciol 58(208):399406.
Higgins et al. www.pnas.org/cgi/content/short/1420232112 2of9
132128124120
Age (kyr)
300
285
270
255
240
550
650
750
CO
2
(ppm)
CH
4
(ppb)
Fig. S2. CO
2
[red symbols; in parts per million (ppm)] and CH
4
[green symbols; in parts per billion (ppb)] measurements from Site 27 compared with Vostok
using the δ
18
O
atm
chronology for Site 27 (1, 2). The mismatch between CO
2
and CH
4
values at 130 ky reflects uncertainties in the δ
18
O
atm
chronology (1).
1. Spaulding NE, et al. (2013) Climate archives from 80-250 ka in horizontal and vertical ice cores from the Allan Hills Blue Ice Area, Antarctica. Quat Res 80(3):562574.
2. Petit JR, et al. (1999) Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399(6735):429436.
-350 -330 -310 -290 -270
d-excess ()
-12
-10
-8
-6
-4
-2
0
+2
30-113 m
115-126 m
113-115 m
Fig. S3. δD
ice
and deuterium excess (details are in the text) for the Site BIT-58 ice core. Black, 115126 m (1-Ma ice); red =113115 m (transitional); white, 30
113 m (320-ka ice).
Higgins et al. www.pnas.org/cgi/content/short/1420232112 3of9
280
260
240
220
CO2 (ppm)
DICE ()
-290
-310
110 114 118 122 126
-330
600
500
CH4 (ppb)
400
Depth (meters)
320 ka 990 ka
Fig. S4. Measured δD
ice
,CO
2
,andCH
4
concentrations between 110 and 128 m atSite BIT-58. Dashed lines indicate depth ranges where samples were analyzed for
40
Ar
atm
. The hatched area is the depth range where we place the transition between 320- and 990-ka ice based on measurements of d excess (Fig. S3). ppb, parts per billion.
Table S1. Ar isotope measurements for standards and Site
BIT-58 ice
Depth (m)
38
Ar/
36
Ar ()
40
Ar
atm
() Age (ky) 1σ(ky)
Princeton Air* 0.000 0.000 0 213
Vostok
0.920 0.0002 3142
14.00
0.022 0.029 432 218
22.00
0.050 0.005 74 213
25.60
0.117 0.020 298 215
25.60
0.049 0.016 238 214
26.00
0.107 0.009 134 213
35.40
0.038 0.036 536 221
41.45 0.235 0.021 313 216
52.60 0.201 0.011 164 214
52.60 0.193 0.033 491 220
56.00 0.168 0.035 521 220
62.80 0.197 0.032 476 219
80.20 0.140 0.016 238 214
89.30 0.083 0.014 208 214
97.00 0.126 0.017 253 215
102.00 0.116 0.025 372 217
107.00 0.094 0.020 298 215
108.00 0.069 0.036 536 221
112.80 0.285 0.019 283 215
112.80 0.251 0.023 342 216
117.40 0.152 0.056 833 232
117.40 0.134 0.075 1,116 246
120.10 0.153 0.060 893 234
121.00 0.136 0.063 938 237
124.75 0.154 0.065 967 238
124.75 0.131 0.071 1,057 243
All samples normalized to air were collected at Princeton University. Un-
certainties correspond to 1σ(SD) of replicate analysis of Princeton Air or
Holocene ice from Vostok. The
40
Ar
atm
ages are calculated following the
work by Bender et al. (1) using the measured
38
Ar/
36
Ar ratio to correct for
gravitational fractionation and calibrated using ice of known age from Vostok
and EPICA Dome C. Uncertainties (1σ)foreach
40
Ar
atm
age are estimated
by propagating errors in the slope of the
40
Ar
atm
vs. ice age relationship
(m=0.00672 ±0.00074; 1σ) using our Holocene data, the data from the
work by Bender et al. (1), and the analytical uncertainty associated with each
samplehere, it is assumed to be equal to the long-term external reproduc-
ibility of Princeton Air or ±213 ky (1σ).
*N=67.
N=5 (Holocene age).
Vertical near-surface fractures present.
1. Bender ML, Barnett B, Dreyfus G, Jouzel J, Porcelli D (2008) The contemporary degassing rate of 40Ar from the solid Earth. Proc Natl Acad Sci USA 105(24):82328237.
Higgins et al. www.pnas.org/cgi/content/short/1420232112 4of9
Table S2. δ
15
NofN
2
,δ
18
O
atm
, and O
2
/N
2
ratios of Site BIT-58 ice
Depth (m) δ
15
N()1σ()δ
18
O
atm
()1σ()O
2
/N
2
()1σ()
32.87* 0.115 0.004 1.350 0.033 18.7 1.8
35.40* 0.094 0.011 1.120 0.055 11.9 6.1
37.92 0.079 0.005 0.804 0.017 8.8 1.1
41.45 0.156 0.025 1.426 0.026 12.0 0.5
44.09 0.082 0.008 0.988 0.043 8.3 3.8
50.32 0.143 0.001 1.206 0.017 10.8 3.6
52.64 0.129 0.018 1.194 0.009 11.3 2.9
57.78 0.062 0.001 0.346 0.053 8.9 3.1
62.80 0.065 0.003 0.375 0.073 10.7 4.6
64.39 0.136 0.017 0.903 0.106 7.3 3.5
67.64 0.114 0.001 1.025 0.011 8.0 1.7
71.62 0.129 0.053 0.265 0.075 8.8 3.3
77.45 0.075 0.021 0.157 0.076 0.8 4.7
80.17 0.125 0.042 0.155 0.031 12.5 1.2
83.50 0.062 0.006 0.489 0.019 12.7 0.3
86.86 0.042 0.055 0.287 0.035 7.7 0.5
91.60 0.031 0.019 0.365 0.004 15.0 0.5
97.12 0.072 0.009 0.177 0.047 13.0 2.2
102.56 0.087 0.019 0.598 0.085 8.4 1.0
108.35 0.073 0.016 0.832 0.041 7.6 4.5
110.59 0.101 0.016 1.262 0.026 9.2 2.0
112.78 ——1.293 0.067 8.7 0.9
113.40 0.178 0.025 1.299 0.027 9.2 1.2
114.28 0.061 0.015 0.481 0.008 4.5 1.8
114.90 0.046 0.041 0.621 0.001 5.7 0.3
116.82 0.067 0.132 0.019 9.7 0.4
117.96 0.065 0.005 0.265 0.064 3.6 1.0
120.17 0.122 0.039 0.210 0.038 10.0 1.0
120.78 0.148 0.053 0.018 0.064 4.5 0.4
121.86 0.146 0.011 0.202 0.035 1.8 0.6
122.28 0.086 0.024 0.506 0.047 4.4 3.6
124.75 0.089 0.008 0.193 0.093 10.0 0.4
The δ
18
O
atm
values and O
2
/N
2
ratios were corrected for gravitational fractionation using
the δ
15
NofN
2
(1). Uncertainties reflect 1σof replicate analyses of ice from the same depth.
For δ
15
NofN
2
, two samples were excluded because of poor reproducibility, although the
individual δ
15
N measurements were still used to correct δ
18
O
atm
and O
2
/N
2
ratios for
gravitational fractionation.
*Vertical near-surface fractures present.
1. Naafs BDA, Hefter J, Stein R (2013) Millennial-scale ice rafting events and Hudson Strait Heinrich(-like) Events during the late Pliocene and Pleistocene: A review. Quat Sci Rev 80(1):128.
Higgins et al. www.pnas.org/cgi/content/short/1420232112 5of9
Table S3. CH
4
,CO
2
, and air content of 1-Ma Site BIT-58 ice
Depth (m) CO
2
(ppm) SE (ppm) CH
4
(ppb) SE (ppb) Total air (ml STP/g) Age* (ky)
Site 27
46.97 272.68 0.78 602.64 0.095 119.9
53.42 276.12 0.35 605.09 0.098 121.5
57.31 274.80 0.19 613.92 0.098 122.5
63.95 276.02 0.94 633.43 0.095 124.2
79.00 278.91 0.08 690.33 0.100 128.0
108.32 269.58 1.22 629.74 0.072 130.4
112.04 266.69 2.07 607.49 0.067 130.6
Site BIT-58
114.17 221.10 0.89 433.90 3.38 0.087
115.50 221.65 0.61 453.66 0.72 0.099
116.17 227.75 0.19 473.33 12.44 0.090
116.82 271.60 0.36 518.43 0.101
117.38 255.52 0.63
118.36 230.37
118.54 240.66
118.81 241.44 0.82 501.68 0.89 0.056
118.98 230.11 411.38
119.43 246.60 523.60 1.62 0.093
120.17 245.64 0.22 569.16 0.088
121.75 274.35 3.74 504.13 2.01 0.094
122.34 277.36 0.42 529.69 1.41 0.101
122.80 246.48 520.26
123.42 240.78 540.96
124.09 269.49 0.70 512.31 0.50 0.100
124.75 263.87 1.25 494.89 0.080
126.37 236.68 0.00 464.43 0.16 0.073
Uncertainties reflect replicate analyses of ice from the same depth. ppb, parts per billion; STP, standard
temperature and pressure.
*Age for Site 27 CO
2
and CH
4
measurements obtained from the δ
18
O
atm
age model (linear interpolation be-
tween tie points) developed in ref. 1.
1. Spaulding NE, et al. (2013) Climate archives from 80-250 ka in horizontal and vertical ice cores from the Allan Hills Blue Ice Area, Antarctica. Quat Res 80(3):562574.
Higgins et al. www.pnas.org/cgi/content/short/1420232112 6of9
Table S4. δD, δ
18
O, and deuterium excess of Site BIT-58 ice
Depth (m) δD()δ
18
O()d()
32.87 292.7 35.9 5.3
34.79 288.5 35.4 5.4
35.40 291.7 35.8 5.5
36.75 290.7 35.7 5.1
37.72 293.0 35.8 6.3
37.92 290.8 35.7 5.2
39.48 289.8 35.5 5.8
40.38 295.6 36.2 6.3
41.11 293.1 35.9 5.6
41.45 297.9 36.3 7.8
41.77 294.2 35.9 7.1
43.72 289.5 35.4 5.9
44.09 288.2 35.2 6.3
44.78 289.1 35.4 5.5
45.64 288.5 35.3 6.4
46.54 288.3 35.4 4.8
49.11 332.8 41.3 2.2
50.10 320.1 39.0 7.9
50.32 303.1 36.8 8.3
52.64 325.0 39.6 8.4
54.17 323.2 39.1 10.1
55.80 301.1 36.5 8.9
56.61 286.9 35.1 6.5
57.78 280.2 34.5 3.9
59.09 292.8 36.4 1.7
61.51 277.3 34.2 3.5
62.53 284.1 35.1 3.3
62.80 282.1 34.7 4.2
63.82 285.6 35.0 5.4
64.39 286.1 35.0 5.9
67.64 302.0 36.7 8.4
69.49 313.5 38.7 4.2
71.62 281.5 35.0 1.3
71.63 311.3 38.2 5.7
71.69 313.4 39.3 0.9
74.13 301.8 37.2 4.3
75.02 281.0 34.7 3.7
80.17 311.1 38.0 7.2
81.77 310.8 37.7 9.1
83.01 299.8 36.6 7.1
83.50 310.9 38.3 4.6
86.86 313.4 38.3 6.8
88.88 295.2 36.5 3.2
89.96 301.4 37.6 0.3
91.60 320.2 39.5 3.9
93.32 294.0 35.9 6.8
95.75 296.9 36.3 6.9
97.12 295.3 36.1 6.7
99.99 313.0 37.9 9.6
100.76 310.7 37.6 9.8
102.56 285.5 35.1 5.1
103.01 296.0 36.1 7.2
104.52 293.2 35.8 7.1
106.65 295.7 36.1 7.1
107.62 292.6 35.7 7.2
108.35 297.3 36.4 6.3
109.07 293.0 35.9 6.2
110.59 293.9 35.9 7.0
110.86 292.1 35.6 7.4
111.98 295.2 35.9 8.1
112.78 293.8 35.8 7.5
113.20 294.0 35.9 6.6
113.32 294.9 36.0 7.0
Higgins et al. www.pnas.org/cgi/content/short/1420232112 7of9
Table S4. Cont.
Depth (m) δD()δ
18
O()d()
113.40 293.6 35.8 6.8
113.94 303.0 37.3 4.6
114.28 309.4 38.0 5.6
114.38 311.0 38.0 7.1
114.52 311.7 37.9 8.8
114.64 312.1 38.3 6.0
114.87 320.4 39.1 7.9
114.90 316.1 38.4 8.5
115.00 316.9 38.9 5.7
115.10 309.4 37.9 6.5
115.20 316.8 38.4 9.2
115.30 310.0 37.9 6.6
115.41 308.6 37.7 7.1
115.50 309.0 38.0 5.0
115.64 318.6 38.7 8.7
115.73 309.6 37.9 6.6
115.82 310.0 38.0 6.1
115.91 311.2 38.0 7.3
116.00 310.6 38.1 6.2
116.09 306.4 37.7 5.2
116.09 308.7 37.7 7.2
116.17 312.8 38.4 5.9
116.43 306.5 37.5 6.9
116.54 309.1 37.9 6.1
116.63 308.5 37.7 6.6
116.72 304.6 37.4 5.6
116.82 299.9 37.0 3.6
117.13 302.2 37.3 3.7
117.21 302.5 37.4 3.6
117.29 302.7 37.4 3.7
117.36 307.4 38.0 3.4
117.71 301.5 37.1 4.6
117.76 303.0 37.2 5.3
117.82 301.8 37.3 3.6
117.90 303.1 37.2 5.7
118.03 304.0 36.9 8.6
118.11 307.4 37.6 6.7
118.12 307.1 37.3 8.8
118.27 315.5 38.2 9.9
118.36 321.5 39.3 7.4
118.48 332.3 40.2 10.4
118.54 323.2 39.4 8.2
118.69 302.5 36.9 7.2
118.81 316.5 38.5 8.5
118.98 312.6 37.7 10.7
119.06 327.8 39.8 9.4
119.16 314.9 38.3 8.9
119.26 301.2 37.2 3.7
119.43 299.3 36.9 4.4
119.74 316.0 38.2 10.4
119.83 320.1 38.8 10.1
119.93 308.2 37.7 6.8
119.94 299.2 36.9 3.8
120.03 299.9 37.2 2.5
120.17 306.7 37.5 6.9
120.41 309.7 37.6 8.6
120.50 307.8 37.6 7.0
120.59 308.3 38.0 4.7
120.69 308.7 37.7 6.8
120.79 308.0 37.8 5.6
120.89 306.6 37.7 4.7
120.99 300.6 37.2 2.7
Higgins et al. www.pnas.org/cgi/content/short/1420232112 8of9
Table S4. Cont.
Depth (m) δD()δ
18
O()d()
121.22 302.2 37.1 5.3
121.68 304.8 37.2 6.9
121.75 304.5 37.4 5.4
121.86 300.2 36.8 5.5
121.93 301.6 36.8 6.9
122.01 302.2 37.0 6.6
122.13 302.3 37.0 6.6
122.21 299.7 36.6 6.7
122.34 302.9 37.2 5.5
122.45 298.6 36.6 5.8
122.56 299.6 36.7 5.7
122.69 301.3 37.0 5.0
122.80 315.0 38.4 7.8
122.92 310.8 38.0 7.0
123.03 312.7 38.2 7.5
123.34 314.4 38.3 8.2
123.42 314.9 38.4 7.9
123.54 305.1 37.5 5.0
123.68 298.2 36.6 5.2
123.76 298.8 36.6 6.2
123.84 296.6 36.5 4.6
123.93 296.9 36.6 4.4
124.07 296.3 36.5 4.1
124.18 297.2 36.5 5.5
124.28 298.5 36.8 3.9
124.38 298.0 36.8 3.8
124.48 298.3 36.8 4.3
124.58 301.8 37.0 5.7
124.70 298.1 36.8 3.8
125.12 295.4 36.4 3.8
125.75 303.9 37.0 7.8
126.37 304.1 37.0 7.9
Higgins et al. www.pnas.org/cgi/content/short/1420232112 9of9
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