PROJECT DESCRIPTION: COLLABORATIVE RESEARCH: RESOLVING AMBIGUOUS EXPOSUREAGE DEGLACIATION CHRONOLOGIES WITH MEASUREMENTS OF IN-SITU-PRODUCED
COSMOGENIC CARBON-14
This proposal aims to apply measurements of in-situ-produced cosmogenic 14C in Antarctic glacial
deposits to address important ambiguities in reconstructing past Antarctic ice sheet change that have not
been resolved by existing exposure-age data sets.
This is important because much of our understanding of Antarctic ice sheet change between the Last
Glacial Maximum (LGM; ca. 15,000-25,000 years ago) and the present stems from cosmogenic-nuclide
exposure-dating of glacial deposits in currently-ice-free areas. Ice sheets transport rock and sediment
eroded from their bed, where it has not been exposed to the surface cosmic-ray flux. They deposit it at ice
margins, at which point exposure to the cosmic-ray flux begins. Thus, the concentration of trace cosmicray-produced radionuclides in glacially transported clasts in ice-free areas is directly proportional to the
deglaciation age of a site.
Exposure-age deglaciation chronologies are simple to interpret, powerful, and effective in simple
situations where all the glacial sediment present on a nunatak was deposited during the most recent
deglaciation (e.g., Stone et al., 2003). However, many ice-free areas of Antarctica are covered by glacial
deposits that were emplaced during many previous glacial-interglacial cycles, and preserved during
periods of ice cover by frozen-based ice that did not transport or erode existing deposits (Sugden et al.,
2005; Balco, 2011). These areas display surficial glacial deposits with a wide range of exposure ages that
often do not directly represent the emplacement age of the deposits. Exposure-age data are extremely
complex to interpret in this situation, in particular when no samples with exposure ages postdating the
LGM are found. In this case, there is no way to distinguish between two incompatible possibilities: First,
the site was never covered by ice during the LGM; second, the site was ice-covered during the LGM, but
researchers failed to find a few glacially transported clasts deposited at or after the LGM among a large
abundance of similar-appearing but older clasts. In other words, absence of evidence is not evidence of
absence. A number of past exposure-dating studies, many in important locations from the perspective of
ice sheet reconstruction, are subject to this ambiguity. It presents a significant obstacle to accurate
reconstruction of LGM-to-present ice sheet change in Antarctica.
Here we describe a means of resolving this ambiguity by measuring the concentration of in-situ-produced
cosmogenic carbon-14 in quartz in samples collected during these previous studies. This is possible
because 14C has a two-order-of-magnitude shorter half-life (5730 yr) than other cosmic-ray-produced
radionuclides that were used in these past studies, specifically 10Be (1.5 Ma), 26Al (0.7 Ma), and 3He
(stable). A short half-life acts to i) limit or negate the importance of inherited nuclide concentrations
produced during previous glacial-interglacial cycles, and ii) maximize the effect of short periods of ice
cover on nuclide concentrations. Because of these features, the two scenarios described above -- no ice
cover during the LGM, or ice cover but no emplacement of glacial deposits -- can be resolved by 14C
measurements on either bedrock surfaces or erratic clasts that predate the LGM. Furthermore, in cases
where the LGM-to-present deglaciation history is already known from an unambiguous exposure-age
chronology, additional 14C measurements can also potentially provide a chronology for ice sheet
thickening prior to the LGM, information which cannot be obtained at all from existing data sets and would
be extremely valuable in attributing sea level change prior to the LGM.
Very few previous Antarctic exposure-dating studies (three that we are aware of) have taken advantage
of this possibility. The reason for this is that so far, in-situ-produced 14C measurements have been
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difficult, time-consuming, and only possible at a very small number of laboratories worldwide. Thus, these
studies have only included a handful of analyses. In this proposal we aim to take advantage of a new,
automated system for 14C extraction from quartz, currently under construction by PI Brent Goehring at
Tulane University, to apply this technique at the scale needed to make significant progress on
constraining LGM-to-present, and also potentially pre-LGM, ice thickness changes in important regions of
Antarctica where existing exposure-age data cannot provide these constraints.
In the rest of this proposal we will: i) outline the importance of accurate reconstructions of LGM-topresent Antarctic ice sheet change and highlight areas where past exposure-dating studies lead to
ambiguous reconstructions; ii) describe how these ambiguities could be resolved by targeted 14C
measurements; and iii) detail a number of sites where such measurements, carried out on existing
samples collected in previous field projects, would significantly improve reconstructions of past ice sheet
change.
This proposal does not involve new fieldwork in Antarctica.
Figure 1. Map of Antarctica
showing all sites mentioned in
text.
Importance of reconstructing LGM-to-present ice sheet change.
Reconstructing past Antarctic ice sheet change is important because an accurate understanding of past
ice sheet changes is necessary for determining the causes of past and future sea level changes.
Specifically, a significant body of research over the past several decades has sought to reconcile i)
estimates of changes in global ice volume over time inferred from inversion of a global data set of paleosea-level proxy data, with ii) direct reconstructions of changes in the size of glaciers and ice sheets
derived from geological evidence. To date, these have not yet been satisfactorily reconciled (e.g.,
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Lambeck et al., 2014; Clark and Tarasov, 2014, and references therein). In Antarctica, estimates of the
total excess LGM ice volume relative to present range from ca. 30 m (from relative sea level inversions,
see discussion in Lambeck et al., 2014) to ca. 10 m (from model interpolation of geological data; e.g., see
Whitehouse et al., 2012 and Briggs et al., 2014). A possible reason for this discrepancy, proposed by
several authors (e.g., see discussion in Carlson and Clark, 2012), is that the most significant changes in
ice volume in Antarctica between the LGM and present took place in the large Ross and Weddell marine
embayments, where a large expanse of continental shelf permits large changes in grounding line position
and, in addition, few nearby geological constraints exist. Although researchers involved in collecting
geological and geochronological constraints on past ice sheet change in Antarctica have for the most part
argued that the geological evidence is not consistent with large changes in ice volume in the Ross and
Weddell embayments (e.g., Bentley et al., 2011), this evidence, as we discuss below, is sparse and
ambiguous in several important locations. Thus, other authors have argued that geological and
geochronological data in Antarctica are at present not adequate to determine with high confidence
whether geologically-based LGM ice sheet reconstructions are or are not consistent with sea-level
inversion estimates of ice volume (e.g., Clark, 2011; Clark and Tarasov, 2014 and references therein). To
summarize, the primary reason the work we propose here is important is that it will contribute to resolving
ambiguities regarding the LGM configuration of the Antarctic ice sheets. In addition, although we will
focus primarily on LGM ice extent in this proposal, the results of this project will also contribute to creating
accurate reconstructions of transient LGM-to-present ice sheet change from geological data. Such
reconstructions, in turn, are critically important to interpreting satellite gravity data used to estimate
present annual- and decadal scale rates of Antarctic ice volume change (e.g., Ivins et al., 2013; Shepherd
et al., 2012).
Why frozen-based ice cover and cosmogenic-nuclide inheritance lead to ambiguous ice sheet
reconstructions from exposure-age data.
Elevation (m)
1400
1300
1200
Present ice surface
0
5
10
15
Exposure age (ka)
20
25 1k
10k
100k
1M
Exposure age (yr)
10M
Figure 2. 10Be exposure ages from the Quartz Hills (Todd et al., 2010, plotted on linear (left) and log
(right) axes. The upper limit of the stratigraphically youngest glacial drift mapped at this site is 1400 m.
Although this drift displays a large range of exposure ages, the majority are 7-20 ka. All exposure ages
from stratigraphically older drifts, exposed above 1400 m elevation, exceed 100 ka. Thus, geologic
mapping of the youngest drift combined with the exposure-age data allows unambiguous reconstruction
of LGM ice thickness at this site. Most age-elevation plots in this proposal were generated using the ICED database of cosmogenic-nuclide exposure ages for Antarctica (http://hess.ess.washington.edu/iced/).
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Figure 2 shows cosmogenic-nuclide exposure-age data (in this case, measured using 10Be) from the
youngest glacial deposit mapped at a site adjacent to the Reedy Glacier in the southernmost
Transantarctic Mountains (Todd et al., 2010; see Figure 1 for location). The mapped extent of the drift
(below 1400 m elevation) and a large number of exposure ages between 7-20 ka indicate that i) this drift
is LGM in age; ii) the elevation of the LGM ice surface was 1400 m at 16-19 ka; and iii) glacier thinning
took place between the LGM and ca. 7 ka. Thus, this is an example of a glacial-geological and exposuredating study where, although a particular drift unit contains a few pre-exposed or recycled clasts with preLGM apparent exposure ages, the preponderance of LGM and Holocene ages allows unambigous
reconstruction of the LGM ice surface elevation.
In contrast, Figure 3 shows exposure-age data for erratics from glacial drift on Mt. Skidmore in the
Shackleton Range, adjacent to the eastern Weddell Sea (Hein et al. 2010; see Figure 1 for location). At
this site, the youngest erratics, lying on the present ice surface, are 40 ka. All erratics on the nunatak
itself have apparent exposure ages > 100 ka. Hein et al. proposed that the best explanation for these data
was that the LGM ice surface elevation was at or below the present ice surface, that is, this portion of the
ice sheet did not thicken during the LGM. However, as they noted, it is also possible that the LGM ice
surface was higher than present, but either no glacial drift was deposited by LGM ice, or that they did not
identify any clasts emplaced at the LGM among a much larger number of older clasts emplaced during
previous glacial-interglacial cycles and covered by frozen-based ice at the LGM.
900
Figure 3. Apparent 10Be (red) and 26Al
(green) exposure ages from Mt. Skidmore,
Shackleton Range (Hein et al., 2010). Ages
< 100 ka are only observed in erratics on
the present ice surface. Thus, it is unclear
whether i) the site was not covered by ice
during the LGM or ii) the site was covered,
but no erratics were deposited and/or found.
Elevation (m)
800
700
600
500
400
300
200
10k
Present ice
surface
100k
1M
Exposure age (yr)
10M
Sites such as this one where a large number of erratics with pre-LGM ages were found, but erratics with
LGM to Holocene ages were either completely absent or only found very close to the present ice surface,
are common at ice-free areas surrounding the Weddell Sea embayment (Figure 1). Besides the
Shackleton Range, these include several sites in the Pensacola Mountains (specifically, the Schmidt Hills,
discussed below), nunataks in the Lassiter Coast region (also discussed below), and some sites in the
Ellsworth Mountains (Fogwill et al., 2014 and references therein). The inherent ambiguity of these
exposure-age data sets leads to significant uncertainty in reconstructing LGM-to-present ice sheet
change in the Weddell embayment, as detailed by Hillenbrand and others (2014). They showed that it
was not possible on the basis of terrestrial or marine geological data to distinguish between a scenario in
which i) the Weddell Sea grounding line uniformly advanced to the outer continental shelf (in which case
the sites in the Shackleton Range would have been covered by thicker LGM ice), or ii) significant
grounding line advance only occurred in the central Weddell Sea, and only minimal grounding line
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advance took place in the eastern and western sides of the embayment (in which case they would not
have been covered by ice). Although it is important to note that the difference in these reconstructions in
terms of total ice volume could not by itself account for the difference between geological and sea-levelinversion estimates of Antarctic ice volume discussed above, the inability to distinguish between these
two possible scenarios for the Weddell embayment highlights the fact that the challenge in
unambiguously determining LGM ice thickness from existing exposure-age data presents a fundamental
obstacle to accurately reconstructing LGM Antarctic ice volume.
To summarize, the ambiguous interpretation of exposure-age data created by the fact that erratics with
apparent exposure ages older than the LGM do not exclude cover by frozen-based ice at the LGM, both
in the specific example of the Weddell Sea and in some other locations, leads to a large uncertainty in
the LGM ice sheet configuration over a significant fraction of the continent. In the next section we
describe how in-situ 14C measurements at specific sites can resolve the ambiguity presented by existing
exposure-age data sets as to whether the sites were covered by ice at the LGM or not. We then propose
to apply this technique synoptically at a number of sites in Antarctica, mainly focused on the Weddell
embayment.
In-situ-produced carbon-14 in quartz.
Like the commonly used cosmic-ray-produced radionuclides 10Be and 26Al, 14C is produced in-situ in the
quartz mineral lattice by high-energy neutron spallation (Lifton et al., 2001 and references therein).
However, its half-life is two orders of magnitude shorter (5730 years compared to 0.7 and 1.4 Ma for 26Al
and 10Be respectively). This is important because cosmogenic 14C concentrations in bedrock surfaces
retain less “memory” of past periods of exposure and burial than do longer-lived nuclides. In many cases,
the majority of the 10Be inventory in bedrock surfaces or erratics that have been repeatedly exposed and
covered by frozen-based ice reflects periods of exposure prior to the current one. Because of its short
half-life, this is not true for the 14C inventory in nearly all cases (e.g., Miller et al., 2006; Briner et al.,
2014).
Why LGM ice cover can be clearly detected or excluded by 14C measurements where it cannot
using existing data.
Figure 4 shows how measurements of cosmogenic 14C in quartz in samples, from either bedrock or erratic
clasts that were emplaced prior to the LGM and were not disturbed by LGM ice cover, spanning a range
of elevations, can be used to determine to what extent a site was covered by ice at the LGM. Given
continuous exposure of a rock surface for several times the 14C half-life, cosmogenic 14C concentrations
in quartz in this surface will reach an equilibrium concentration where production is balanced by decay
and loss to surface erosion (specifically, after 30,000 years the 14C concentration reaches 98% of
saturation). At the low rock surface erosion rates observed in many inland regions of Antarctica (ca. 0.2-2
m/Myr; see Balco and Shuster, 2009), this equilibrium concentration is only weakly dependent on the
erosion rate (varying the erosion rate within these bounds has only a 3% effect on the concentration).
In this project, we are interested in determining whether ice-free sites were ice-covered at the LGM. If
they were not, they would have experienced continuous exposure for at least the last ~30,000 years, so
quartz in surfaces that have been stable for at least this amount of time -- bedrock or pre-LGM erratics
that were not disturbed by LGM ice cover -- would display cosmogenic 14C concentrations at steady state
with respect to the production rate at the sample elevation (Figure 4). Note that the elevation dependence
of the steady state 14C concentration in quartz in Antarctica has been directly measured in the Antarctic
Dry Valleys (N. Lifton and CRONUS-Earth project, personal communication; see data on Figure 4), so
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potentially uncertain scaling extrapolations to estimate this value are minimized. If the samples did
experience ice cover for a time at the LGM, even if no subglacial erosion took place, ice cover exceeding
several meters thickness would halt nuclide production, and the 14C concentration would be reduced by
radioactive decay. If deglaciation occurred at ca. 15 ka, the subsequent time would not be sufficient to reestablish steady state, so an elevation transect of 14C concentrations in quartz would show a
discontinuity at the maximum elevation reached by the LGM ice surface (Figure 5). White et al. (2011)
described a similar approach, although it is important to note that our approach involves identifying a
discontinuity in a 14C elevation transect rather than comparing a measured 14C / 10Be ratio to the
(relatively uncertain) production ratio. Thus, their statement that one cannot detect periods of ice cover
prior to ~15 ka does not apply to our proposed approach.
800
Hypothetical ice surface elevation change
Resulting predicted C−14 concentration
700
Elevation (m)
600
Discontinuity at
LGM ice surface
elevation
500
400
300
200
Steady-state concentration in absence of LGM
ice cover
Data from Antarctic rock
surfaces that were not
ice-covered at the LGM
100
0
35
30
25
20
15
10
Time (thousands of years BP)
5
0 0
0.5
1
1.5
2
2.5
3
3.5
4
Cosmogenic 14C concentration in quartz (10 5 atoms/g)
Figure 4. Predicted elevation dependence of the cosmogenic 14C-in-quartz concentration in surface
samples from a hypothetical nunatak that was partially ice covered at the LGM. The ice thickness change
history at left implies the 14C concentrations at the present time given by the solid line in right panel. A
discontinuity between saturated and sub-saturated concentrations marks the upper limit of LGM ice
cover. Data plotted in right-hand figure are 14C concentrations measured at surfaces in Antarctica that are
independently known to have not been covered by ice at the LGM (unpublished CRONUS-Earth
calibration data analyzed by N. Lifton); these show typical measurement uncertainties for low-elevation
samples. Uncertainties would be lower at higher elevations due to the higher production rate.
In summary, 14C measurements on either bedrock surfaces or surface clasts that were emplaced prior to
the LGM allow one to unambiguously determine whether or not a site was covered by ice during the LGM.
While there is a lower limit on the duration of ice cover that would be detectable using this approach
(approximately 1-3 ka, depending on site elevation), existing data from Antarctica that pertain to the
duration of LGM ice cover show that the Antarctic ice sheets were most likely near their maximum extent
for more than 10 ka (summarized in Carlson and Clark, 2012, p. 25-27).
Theoretically, this approach to detecting periods of ice sheet cover could also be addressed with 10Be and
26Al concentrations (Lal, 1991). The much longer half-lives of 10Be and 26Al, however, require orders-ofmagnitude longer periods of ice cover to detectably affect concentrations of these nuclides. For example,
173,000 years is required for the 26Al/10Be ratio to decrease to 92% of its equilibrium value (that is, to be
detectably different from equilibrium given 8% measurement uncertainty on the 26Al/10Be ratio), whereas
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only 700 years is required for the 14C concentration to decrease to 8% below its saturation value.
Measurement of 14C, therefore, has enormously greater sensitivity than any other cosmogenic-nuclide
system for detecting whether ice-free sites were covered by ice during the LGM, even if only for a short
period of time (< 1 kyr).
New information obtainable from 14C measurements: an ice advance and retreat chronology.
14C
Figure 5. Predicted
concentrations at
the present time given a surface that
begins with a saturated 14C concentration
and then experiences ice cover at the
LGM and subsequent deglaciation. If the
deglaciation age is known, the 14C
concentration is uniquely related to the
duration of ice cover. Note that saturation
with respect to cosmogenic 14C
production is ~98% reached after only 30
kyr of exposure. Similarly, 14C
concentrations after 30 kyr of ice cover
are nearly zero due to radioactive decay.
[C−14] at present time (105 atoms/g)
An additional capability of 14C measurements is that they permit one to determine the duration of ice
cover at a site where the deglaciation age is already unambiguously known from 10Be exposure ages of
glacial erratics (or from other independent chronological evidence). This approach is essentially a form of
cosmogenic-nuclide burial dating (e.g., Lal, 1991; Granger, 2006), first applied using the 14C/10Be pair on
proglacial bedrock at the Rhone Glacier, Switzerland (Goehring et al., 2011). In the Rhone Glacier
example, deglaciation following the LGM is known from nearby erratics with 10Be exposure ages, and the
present 14C concentration in bedrock reflects burial and erosion of the bedrock surfaces during a late
Holocene glacier advance-retreat cycle. We propose that this approach can be applied to Antarctic sites
because i) it is likely that a bedrock or pre-LGM erratic surface is at or near its saturation concentration
with respect to 14C at initiation of LGM ice cover; and ii) the potential complicating factor of subglacial
erosion of the bedrock surfaces is not relevant at Antarctic sites that were covered by cold-based, nonerosive ice at the LGM. These assumptions leave the 14C concentration measured at the present time a
function of two parameters of interest, the age of first ice cover at the LGM and the deglaciation age. At
many sites in Antarctica (such as the Ford Ranges discussed below), the latter is known from 10Be
exposure ages of erratics (see Figure 8), so the former can be inferred from the present 14C
concentration. Figure 5 shows this relationship: given a known deglaciation age, there is a unique
relationship between the measured 14C concentration and the onset (or duration) of LGM ice cover.
1.6
Individual lines correspond to
deglaciation age of site:
1.4
12 ka
11 ka
10 ka
9 ka
1.2
8 ka
1
7 ka
6 ka
0.8
15 ka
5 ka
20 ka
25 ka
30 ka
Time LGM ice cover began (yr BP)
35 ka
There are two potential difficulties with this approach. First, it is difficult to verify the assumption that
bedrock surfaces displayed 14C saturation at the time LGM ice cover began. If this assumption is relaxed,
however, even though the measured 14C concentration no longer implies a unique age for ice advance, it
continues to provide a limiting age, which would still be more information than we have at present about
the duration of LGM ice cover in much of Antarctica. Second, as shown in Figure 5, because of the short
half-life of 14C, the longer the duration of LGM ice cover, the less sensitive the relationship between ice
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cover duration and 14C concentration. This is evident in the decreasing slope with LGM onset age in
Figure 5; in this example, LGM onset ages older than ca. 30-35 ka would not be uniquely resolvable.
However, we emphasize that even establishing limits on the duration of LGM ice cover would, at nearly all
locations in Antarctica, provide new information that we do not now have. Thus, we argue that exploring
this potential use of cosmogenic 14C measurements, in addition to our primary goal, described above, of
better establishing the LGM configuration of the Antarctic ice sheets, is likely to yield potentially valuable
information.
Research plan and specific goals.
The technical aspects of extracting and measuring in-situ 14C have developed relatively recently when
compared to other cosmogenic nuclides such as 10Be, 26Al, or 3He. Recent progress on technical issues
has overcome many of the early extraction challenges (Lifton et al., 2001; Pigati et al., 2010; Hippe et al.,
2013; Goehring et al., 2014; Lifton et al., in press); the main limitation on its use is now the timeconsuming nature of the analysis. Cosmogenic 14C uses a pure quartz separate prepared in the same
way as for 10Be or 26Al measurements; the sample is heated at high temperature in-vacuo, evolved CO2
purified, and its 14C/12C ratio determined by accelerator mass spectrometry (AMS; Lifton et al., 2001).
Sample extraction in this project will take place in PI Goehring’s newly constructed laboratory at Tulane
University. All aspects of 14C extraction, purification, and graphitization will be automated, thereby
increasing both sample throughput (~1 per day) and repeatability, as evidenced by results from a similar
laboratory at Purdue University’s PRIME Laboratory (Lifton et al., in press). AMS analysis for 14C will be
carried out at the Purdue Rare Isotope Measurement Laboratory. Predicted 14C concentrations in the
proposed work are similar to concentrations routinely measured (e.g., Lifton et al., 2001; Miller et al.,
2006; Goehring et al., 2011; White et al., 2011), and therefore should yield typical measurement precision
of 3-5%.
The research plan is directed at two main goals. The first is is to better constrain the LGM ice sheet
configuration at sites where existing 10Be data are ambiguous. As discussed above, many such sites
occur at the margins of the Weddell embayment, so we are focusing on these sites with the goal of
improving the overall understanding of LGM ice sheet configuration in this region. This will involve
analysing samples from the Schmidt Hills and Thomas Hills (collected and already analyzed for 10Be by
co-PI Balco and his collaborator Claire Todd), the Shackleton Range (collected and previously analyzed
for 10Be by Andy Hein, University of Edinburgh; see attached letter of collaboration), and the Lassiter
Coast (collected in recent fieldwork and already analysed for 10Be by Joanne Johnson, British Antarctic
Survey; see attached letter of collaboration). The second goal is to attempt to apply 14C measurements at
sites where the Holocene deglaciation history is already independently well constrained by a LGM-toHolocene 10Be exposure age chronology; the aim here is to complement these deglaciation chronologies
with new information about i) advance of the Antarctic ice sheets to their LGM configuration, and ii) a
potential middle Holocene ice thickness minimum and late Holocene thickening in the Antarctic Peninsula
region.
Research goal 1: ambiguous 10Be data sets around the Weddell embayment.
Shackleton Range. Figure 3 and associated discussion above, as well as a review by Hillenbrand et al.
(2014), highlight the importance of determining unambiguously whether or not nunataks in the Shackleton
Range sampled by Hein et al. (2010) were or were not covered by ice at the LGM. To determine this, we
will measure 14C concentrations in samples, including pre-LGM erratics and bedrock, from the full range
of elevations available at several sites in the western Shackleton Range. At present, the samples are held
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by Andy Hein (University of Edinburgh), who has agreed to make them available for this project (see
attached letter of collaboration).
Lassiter Coast. 10Be ages from erratics and bedrock at sites on the Bowman Peninsula of the Lassiter
Coast are all in the range 100-800 ka, much older than the LGM, even when just 20 m above the modern
ice surface (Figure 7; unpublished data supplied by Joanne Johnson, British Antarctic Survey). These
sites are located at the present western edge of the Ronne-Filchner ice shelf edge. At present, the only
constraint on timing of LGM-to-present grounding line retreat along the Lassiter Coast comes from a
radiocarbon age (5398 yr BP) from till collected from a marine core on the inner shelf of the Ronne
Trough near the sample sites (Hillebrand et al., 2014). Thus, the 10Be dataset as it stands at present does
not allow us to either confirm or rule out that the LGM ice surface was any higher during the LGM than at
the present day. Thus, we propose to measure 14C in bedrock and/or erratic samples with pre-LGM
apparent 10Be exposure ages from the full elevation range available at this site. These samples are held
by Joanne Johnson (British Antarctic Survey), who has agreed to make them available for this project
(see attached letter of collaboration).
Figure 7. Apparent Be-10 exposure ages
on erratics from nunataks at the Lassiter
Coast, adjacent to the western edge of
the Ronne-Filchner ice shelf.
Unpublished data collected by Joanne
Johnson (BAS).
Elevation (m)
1000
800
600
Present ice surface
400
10k
100k
1M
10M
Exposure age (yr)
Schmidt and Thomas Hills, Pensacola Mountains. These sites are located in the southernmost Weddell
embayment (see Figure 1), adjacent to the Foundation Ice Stream. co-PI of this proposal Greg Balco and
collaborator Claire Todd collected exposure-age data from these sites in 2011 and 2012. The Schmidt
Hills are near the grounding line of the Foundation Ice Stream; the Thomas Hills are 100 km upstream.
Samples from the Schmidt and Thomas Hills are currently held by Balco at BGC.
At the Schmidt Hills, a large number of 10Be exposure ages are uniformly greater than 100 ka, providing
no evidence for ice thickening, or by implication grounding line advance, at the LGM (Figure 6, left panel).
However, data from the nearby Williams Hills, adjacent to the Academy Glacier, a tributary of the
Foundation Ice Stream, show that the LGM ice surface was several hundred meters higher than present
in the early Holocene (Balco and Todd, unpublished data; Tremblay et al., 2014). Significant thickening at
this site is difficult to reconcile glaciologically with the hypothesis that no ice thickening took place at the
Schmidt Hills and thus that the grounding line of the Foundation Ice Stream did not advance. 14C
measurements from the Schmidt Hills would resolve this conflict and, we propose, provide unambiguous
evidence for whether the grounding line in the inner Weddell Sea significantly advanced at the LGM.
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In preparation for this proposal, we (Goehring) measured the 14C concentration in quartz in three erratics
with apparent 10Be ages > 100 ka from elevations above 500 m at the Schmidt Hills. Measured 14C
concentrations in all three were equal to or greater than predicted saturation concentrations at their
respective elevations, which would tend to indicate that the LGM ice surface at the Schmidt Hills was not
above 500 m. However, measured concentrations in two samples were significantly greater than
expected saturation concentrations. Although there exist complicated geomorphic scenarios, involving
transport of pre-exposed erratics from much higher elevations by LGM ice, that could in part account for
this observation, these are not consistent with other geomorphic evidence, which suggests the possibility
of unrecognized measurement errors. Thus, these measurements require independent replication, in this
case by Goehring in his Tulane laboratory. In addition, they are from relatively high elevations at the site
and provide no information about LGM ice cover at lower elevations. Thus, at the Schmidt Hills, we
propose to make 14C measurements on samples from the full elevation range available at this site (220950 m). In addition, because bedrock at this site (a friable phyllite-grade metapelite with metabasalt
intrusions) is not suitable for cosmogenic-nuclide measurements, we will make replicate measurements
on multiple erratics at several elevations to ensure that LGM-to-present disturbance of erratics is not a
possible explanation for anomalous results. Finally, 10Be results from the Schmidt Hills are sparse at
middle elevations (Fig. 6). Samples from these elevations were collected, so we will make several
additional 10Be measurements from poorly represented elevations to identify additional samples with preLGM 10Be exposure ages for 14C analysis and archive similar data density across the elevation range
available at this site.
1000
Schmidt Hills
Thomas Hills
Elevation (m)
800
600
Present ice
surface
400
200
10k
Present ice
surface
100k
1M
Exposure age (yr)
10M
1k
10k
100k
1M
Exposure age (yr)
Figure 6. Apparent 10Be exposure ages at the Schmidt (left) and Thomas Hills (right), Pensacola
Mountains, Antarctica. Unpublished data collected by Greg Balco and Claire Todd; also available via the
ICE-D database (http://hess.ess.washington.edu/iced/).
Apparent 10Be exposure ages from the Thomas Hills are, for the most part, well in excess of LGM age in
the range 50-200 ka. However, six samples between 680-820 m elevation display early Holocene
apparent exposure ages. This would appear to suggest LGM ice cover of the site followed by Holocene
deglaciation. However, because these samples occur in a limited elevation range and do not form a clear
age-elevation relationship, these data could potentially be explained by other geomorphic processes and
the conclusion that they represent early Holocene deglaciation is weak. In addition, even if these data do
record early Holocene deglaciation, they do not by themselves constrain the maximum elevation reached
by LGM ice. Thus, at this site, as at the Schmidt Hills, we propose to measure 14C concentrations in
erratics with pre-LGM exposure ages (again, metapelite and metabasalt bedrock is unsuitable for
analysis) across the full range of elevations available at this site. We have samples at elevations higher
10
than represented by the data in Figure 6, so we will make additional 10Be as well as 14C measurements as
needed to extend the elevation range available.
Research goal 2: a potential chronology of ice sheet advances as well as retreats.
A secondary goal of this project is to attempt to apply the concept of “burial dating” with in-situ-produced
14C in quartz, as described above, to quantify the duration of past periods of ice cover in addition to when
they ended. We propose two trial applications of this approach: one to determine the duration of LGM ice
cover at sites in the Ford Ranges of West Antarctica where a detailed and unambiguous exposure-age
chronology of LGM-to-present deglaciation already exists (Stone et al., 2003); another to potentially
investigate the timing of repeated Holocene ice shelf formation and breakup in the Larsen embayment of
the Antarctic Peninsula.
Ford Ranges of West Antarctica. Stone et al. (2003) collected 10Be exposure ages from several nunataks
in this region (see Figure 1 for location; Figure 8 shows an example from one of these nunataks, Mt.
Darling). All these data showed clear and consistent Holocene age-elevation relationships indicating ice
sheet thinning between 10 ka and the present. At most elevations, erratic clasts with Holocene exposure
ages lie on weathered bedrock with apparent exposure ages well in excess of the LGM and typically >
100 ka (Sugden et al., 2005). This juxtaposition indicates that we can use the relationship shown in
Figure 5 above to relate 14C concentrations in bedrock samples to the duration of LGM ice cover. This, in
turn, would potentially provide information about the growth of the West Antarctic Ice Sheet prior to the
LGM, information that we do not now have. We note that our approach is not applicable to some bedrock
samples at low elevations that have Holocene apparent 10Be exposure ages as well as geomorphic
evidence that significant subglacial erosion took place during LGM ice cover. Thus, in this part of the
project we propose to measure 14C in bedrock samples from relatively high elevations, with the goal of
determining whether it is possible to reconstruct ice advance to its LGM position. These samples are
currently held by John Stone, who has agreed to provide them to this project (see attached letter of
collaboration).
Figure 8. 10Be data from Mt. Darling in
the Ford Ranges (Stone et al., 2003).
Adjacent nunataks show similar ageelevation relationships, clearly defining
early and middle Holocene thinning of
this part of the West Antarctic Ice
Sheet.
Elevation (m)
1100
1000
900
Present ice surface
800
0
2000
4000
6000
8000
10000 12000
Exposure age (yr)
Larsen embayment, northern Antarctic Peninsula. Here we propose to measure in-situ 14C in a set of
samples collected from nunataks adjacent to major glaciers draining the east side of the northern
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Antarctic Peninsula into the Larsen embayment. The 10Be exposure-age data set from this site (Balco et
al., 2013; Figure 9) is unusual in Antarctica because at elevations below 150 m above sea level, erratics
with middle Holocene (4-7 ka) apparent exposure ages occur adjacent to erratics of similar appearance
but with much younger apparent exposure ages of 100-500 years. Balco et al. (2013), based on evidence
from nearby marine sediment cores that the Larsen Ice Shelf collapsed in the early Holocene and
reformed in the late Holocene prior to its final collapse in recent decades, hypothesized that this
juxtaposition reflects early Holocene ice surface lowering due to overall regional deglaciation, followed by
glacier thickening caused by ice shelf formation and growth in the late Holocene and then complete
deglaciation of the sites in recent decades. This hypothesis would imply that i) the low-elevation erratics
with mid-Holocene exposure ages were emplaced in the early Holocene and then covered by thin, frozenbased ice during late Holocene glacier thickening; and ii) the erratics with very young exposure ages were
emplaced during final deglaciation of the sites in recent decades. This, in turn, is relevant to the present
proposal because it would imply that erratics with early Holocene ages above 150 m elevation will display
concordant 14C and 10Be ages, but erratics with early Holocene ages below 150 m elevation will display
significantly lower 14C concentrations due to a period of burial during the middle to late Holocene. This
situation is closely analogous to the Rhone Glacier example of Goehring et al. (2012), and comparison of
14C and 10Be concentrations in low-elevation samples would permit estimating the timing of initial thinning
caused by early Holocene ice shelf collapse as well as thickening caused by subsequent ice shelf
formation. Thus, we propose to measure 14C concentrations in samples with early to middle Holocene
apparent exposure ages at the two sites shown in Figure 9. These samples are held by co-PI Balco at
BGC.
Figure 9. 10Be data from the Sjogren-Boydell fjord, northeastern Antarctic Peninsula (left) and Drygalski
Glacier, same general area (right), from Balco et al. (2013). These data are unusual in that erratics with
both early-middle Holocene (5-7 ka) and extremely young (<500 yr) apparent exposure ages co-occur
at low elevations. This observation, with independent evidence for a mid-Holocene collapse and later
regrowth of adjacent parts of the Larsen Ice Shelf (see Balco et al. 2013 and references therein), leads
to the hypothesis that these data record complete deglaciation of the sites in the early Holocene
followed by glacier thickening due to ice shelf formation. Thus, lowest-elevation samples with early
Holocene ages should show disequilibrium 14C/10Be, and 14C measurements should enable an estimate
of the duration of the ice-free period.
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Research plan - summary.
We propose to measure 14C in 15 samples from each of the six target areas described above. The 15
sample total includes replicate measurements of selected samples. Replicate measurements with 14C are
particularly critical due to its slightly higher level of sample to sample scatter (6% vs 4%), compared to
more common cosmogenic nuclides (e.g, 10Be, 3He), based on replicate measurements of the same
material as part of CRONUS-Earth (Jull et al., 2013). We note however, that advances in the automation
of 14C extraction and increases in the amount of quartz used has lowered intra-laboratory variability to
~1% (Lifton et al., in press). We expect similar performance by the Tulane laboratory; however, we
propose to critically evaluate whether this is true by rigorous measurement of replicate samples. In
addition, because at some sites we propose to analyse glacially transported erratics that were covered by
ice at the LGM rather than bedrock samples, analysis of different erratics from the same locations is
needed to test the hypothesis that the erratics were not disturbed by LGM ice cover. If this hypothesis is
true, erratics with different pre-LGM apparent 10Be exposure ages will show indistinguishable 14C
concentrations, indicating that they have experienced the same exposure history during the ca. 30,000year lifetime of the 14C inventory. Measurement of samples will be spread out over the three year project
duration, 30 samples per year.
While the main thrust of the proposed work is the measurement of in situ 14C, additional 10Be
measurements are needed, as discussed above, at some of the sites, in particular the Thomas and
Schmidt Hills. We propose measurement of 10Be in approximately 20 samples over the three year project
period.
Division of responsibilities.
Responsibilities for this research will be divided between PI Goehring, co-PI Balco, and graduate students
supervised by Goehring at Tulane University. Both Balco and Goehring are specialists in geologic
applications of cosmogenic-nuclide geochemistry and have extensive field and analytical experience in
this area. Goehring, in particular, is a specialist in analysis of in-situ-produced 14C in quartz, expertise
which is only held by several researchers worldwide. Although this project does not involve Antarctic
fieldwork, both Balco and Goehring are experienced in glacial-geologic mapping and exposure-dating in
Antarctica; Balco has seven field seasons experience and Goehring one.
Goehring, with assistance from Balco, will lead the project and will be responsible for supervising and/or
carrying out all 14C measurements. Student participation in this research will be critical and students at
Tulane will perform much of the analytical work under Goehring’s instruction and supervision. This is
expected to form the basis of a PhD at Tulane. Balco will be primarily responsible for supplying samples
from his own collections and acquiring them from other contacts and collaborators (detailed above in the
site descriptions), and in addition will carry out the small number of additional 10Be measurements
described above. Goehring, Goehring’s students, and Balco will collaborate on data management,
analysis, and synthesis.
In addition, we will collaborate closely with other specialists in Antarctic exposure-dating research Andy
Hein (U. of Edinburgh) and Joanne Johnson (British Antarctic Survey). As described above and in
attached letters of support, we rely on this collaboration to obtain samples from sites in the Weddell Sea
embayment collected by these researchers. We will also collaborate closely with them during subsequent
data analysis and synthesis.
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Summary: intellectual merit.
The primary goal of this proposal is to use in-situ cosmogenic 14C measurements on existing samples
collected from several sites in Antarctica to resolve major ambiguities in present LGM-to-present ice sheet
reconstructions. This, in turn, is important because of the critical nature of accurate reconstructions of ice
sheet change in constraining reconstructions of past sea level change. We aim to achieve this goal by
targeted 14C measurements in the region of Antarctica where the greatest ambiguity in existing exposureage reconstructions of past ice sheet change exist, specifically the Weddell Sea embayment.
In addition, radiocarbon measurements applied where independently constrained deglaciation
chronologies already exist can potentially yield new chronological information for past periods of ice sheet
thickening, which is important in sea-level reconstructions and ice sheet-climate model validation. We
propose to realize this potential by targeted measurements at sites chosen for the quality of the existing
deglaciation chronology.
Summary: broader impacts.
The proposed work will greatly clarify the behavior of the West Antarctic ice sheet, particularly in the
Weddell Sea sector, between the LGM and present by resolving ambiguous exposure age chronologies
of ice sheet thickness changes using in-situ-produced carbon-14. Resolving ambiguities in Antarctic ice
sheet behavior are key to fully understand reconstructions of paleo-sea level change, particularly the
ability to identify the ice sheet that contributed towards a change in sea level. Furthermore, constraints on
ice sheet thickness from the LGM-to-present allow one to place modern changes in ice sheet dimensions
within the context of past changes and refine predictions of future ice sheet behavior.
The proposed work will form the basis of graduate student research at Tulane University (one PhD). The
students will be integrated into the larger Antarctic ice sheet research community through attendance of
national and/or specialized conferences, such as the West Antarctic Ice Sheet meeting. During the course
of the proposed work, the student will gain valuable experience in extraction of in situ 14C from quartz,
making them him/her one of the few with experience in in situ 14C techniques, in addition to other
laboratory procedures and data processing. The students will be expected to participate and lead
manuscript development. In addition to training of a PhD student, the proposed work represents a
cornerstone of new faculty Goehring’s early career, including development of mentoring skills.
During completion of the proposed work, Goehring will develop a new course on sea level change, both
past and present, at Tulane University as part of Tides Program (http://tulane.edu/college/tides/). A
course of this nature is particularly germane in Louisiana and New Orleans. In the course students will be
exposed to both modern sea level changes and its consequences, as well as perspectives that paleo sea
level reconstructions provide. An understanding of the vital role that ice sheets play in controlling sea
level will be a cornerstone of the course, with knowledge gained in the course of the proposed work
continuously presented by Goehring and his student, given the student valuable teaching experience.
Results of previous work.
COLLABORATIVE RESEARCH: Terrestrial Exposure-Age Constraints on the last Glacial Maximum
Extent of the Antarctic Ice Sheet in the Western Ross Sea. PIs Balco, Goehring, and Claire Todd (Pacific
Lutheran University). $420,708 among three institutions; 8/1/2014 - 7/31/2016.
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This project aims to develop a cosmogenic-nuclide exposure-age chronology for changes in ice thickness
at the Last Glacial Maximum in the outer Ross Sea embayment. This is important because changes in ice
sheet thickness and grounding line position have been hypothesized to be an important contributor to
rapid global sea level change during so-called Meltwater Pulse 1A 14,500 years ago. Thus, the purpose
of this project is to obtain geological evidence from the closest available ice-free areas to the central and
outer Ross Sea that would permit evaluating this hypothesis.
We completed fieldwork for this project in the 2014-15 Antarctic field season. We identified and mapped a
range of geological evidence, including moraines and glacial drift, for past higher-than-present ice surface
elevations in northern Victoria Land, adjacent to the hypothesized LGM position of the grounding line of
the West Antarctic Ice Sheet in the Ross Sea (Figure 10). Geomorphic characteristics of these deposits
are consistent with an LGM age, although this cannot be established without exposure-age data. At
present, we are beginning sample preparation for cosmogenic-nuclide measurements planned to be
conducted in summer and fall 2015.
Figure 10. Results of fieldwork in the Tucker Glacier region of northern Victoria Land in the 2014-15
Antarctic field season. The Tucker Glacier flows from right (W) to left (E) in this image; its calving margin
is several km to the left (E) of the image extent. A tributary, the Whitehall Glacier, flows in from the north.
Green circles on the west side of the confluence represent granite bedrock samples collected from a
range of elevations for the purpose of establishing the LGM ice surface elevation in this region using the
approach described in Figure 4 above. On the other side of the Whitehall Glacier, a series of moraines
and erratics composed of the same granite record westward transport of granitic erratics at a time when
the surface of the Tucker Glacier was up to 200 m higher than present. Red circles denote locations of
samples collected to establish an exposure-age chronology for these deposits.
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