New ice cores retrieved from the Taylor Glacier (Antarctica) blue
ice area contain ice and air spanning the Marine Isotope Stage (MIS) 5–4
transition, a period of global cooling and ice sheet expansion. We determine
chronologies for the ice and air bubbles in the new ice cores by visually
matching variations in gas- and ice-phase tracers to preexisting ice core
records. The chronologies reveal an ice age–gas age difference (Δage) approaching 10 ka during MIS 4, implying very low snow accumulation in
the Taylor Glacier accumulation zone. A revised chronology for the analogous
section of the Taylor Dome ice core (84 to 55 ka), located to the south of
the Taylor Glacier accumulation zone, shows that Δage did not exceed
3 ka. The difference in Δage between the two records during MIS 4 is
similar in magnitude but opposite in direction to what is observed at the
Last Glacial Maximum. This relationship implies that a spatial gradient in
snow accumulation existed across the Taylor Dome region during MIS 4 that
was oriented in the opposite direction of the accumulation gradient during
the Last Glacial Maximum.
Introduction
Trapped air in ice cores provides a direct record of the Earth's past
atmospheric composition (e.g., Bauska et al., 2016; Petrenko et al.,
2017; Schilt et al., 2014). Measurements of trace gas species,
particularly their isotopic composition, create a demand for large-volume
glacial ice core samples. Blue ice areas, where a combination of glacier
flow and high ablation rates bring old ice layers to the surface, offer
relatively easy access to large samples and can supplement traditional ice
cores (Bintanja, 1999; Sinisalo and Moore, 2010). Blue ice areas often
have complex depth–age and distance–age relationships disrupted by folding
and thinning of stratigraphic layers (e.g., Petrenko et al., 2006;
Baggenstos et al., 2017). Taking full advantage of blue ice areas requires
precise age control and critical examination of the glaciological context in
which they form.
Effective techniques for dating ablation zone ice include matching
globally well-mixed atmospheric trace gas records (e.g., CH4, CO2,
δ18Oatm, N2O) and correlating glaciochemical
records (e.g., δ18Oice, Ca2+, insoluble particles) with
existing ice core records with precise chronologies (Bauska et al., 2016;
Schilt et al., 2014; Petrenko et al., 2008, 2016; Schaefer et al., 2009;
Baggenstos et al., 2017; Aarons et al., 2017). Other
useful techniques include 40Aratm dating (Bender et al., 2008;
Higgins et al., 2015) and radiometric 81Kr dating (Buizert et al.,
2014). Matching gas and glaciochemical records can provide high precision
with relatively small samples, and some measurements can be made in field
settings. In contrast, 40Aratm and 81Kr require complex
laboratory work and do not provide the level of age precision available from
correlation methods, although these techniques do provide independent age
information that can extend beyond the age range of existing records.
A number of blue ice areas have provided useful paleoclimate archives
including Pakitsoq, Greenland, for the Younger Dryas–Preboreal transition
(Petrenko et al., 2006, 2009; Schaefer et al., 2009, 2006), Allan Hills, Victoria Land, Antarctica, for ice
90–250 ka and >1 Ma (Spaulding et al., 2013; Higgins et al.,
2015), Mt. Moulton, Antarctica, for the last interglacial (Korotkikh et
al., 2011), the Patriot Hills, Horseshoe Valley, Antarctica, for ice from
the last glacial termination (Fogwill et al., 2017), and Taylor Glacier,
McMurdo Dry Valleys, Antarctica, for ice spanning the last glacial
termination and MIS 3 (Bauska et al., 2016; Schilt et al., 2014;
Baggenstos et al., 2017; Petrenko et al., 2017). Taylor Glacier is
particularly well suited for paleoclimate reconstructions because of
the excellent preservation of near-surface ice, large age span, and continuity
of the record (Buizert et al., 2014; Baggenstos, 2015; Baggenstos et al.,
2017). The proximity of the Taylor Dome ice core site to the probable
deposition site for Taylor Glacier ice provides a useful point of comparison
for the downstream blue ice area records (Fig. 1).
(a) The locations of ice core sites discussed in this text are
indicated with blue dots on the continent outline (EDC: EPICA Dome C,
EDML: EPICA Dronning Maud Land, TALDICE: Talos Dome ice core, TD: Taylor Dome, WDC: West Antarctic Ice Sheet Divide core). (b) Landsat
imagery of Taylor Valley (Bindschadler et al., 2008). Blue arrows
conceptually show the modern storm trajectory as well as the hypothesized
storm trajectories for the Last Glacial Maximum (LGM) and Marine Isotope
Stage (MIS) 4 discussed later in the text. (c) Simplified map of Taylor
Glacier showing the main transect (red line) containing ice spanning the
Holocene-MIS 3 time period and drill sites discussed in the text (red dots).
This study extends the Taylor Glacier blue ice area archive by developing
ice and gas chronologies spanning the MIS 5–4 transition (74–65 ka), a
period of global cooling and ice sheet expansion. In 2014–2016 several ice
cores were retrieved approximately 1 km down-glacier from the “main
transect”, the across-flow transect containing ice from Termination 1
through MIS 3 (Baggenstos et al., 2017) (Fig. 1). This paper describes
(1) the dating of the new ice cores via matching of variations in CH4, δ18Oatm, dust, and δ18Oice to preexisting
records and (2) the description of a new climate record from Taylor Glacier
across MIS 4, which was previously thought to be absent from the glacier
(Baggenstos et al., 2017). New measurements of CH4 and CO2 from
the Taylor Dome ice core are used to revise the Taylor Dome chronology
across the MIS 5–4 transition and MIS 4 to allow for a better comparison of the
glaciological conditions at Taylor Dome with those at the accumulation
region for Taylor Glacier. This comparison allows for inferences about the
climate history of the Taylor Dome region implied from the differences in
the delta age (Δage = ice age - gas age) between the two sites.
Field site and methodsField site
Taylor Glacier is an outlet glacier of the East Antarctic Ice Sheet that
flows from Taylor Dome and terminates in the McMurdo Dry Valleys (Fig. 1). The Taylor Glacier deposition zone is on the northern flank of Taylor
Dome, a peripheral ice dome of the East Antarctic Ice Sheet centered at
77.75∘ S, 159.00∘ E on the eastern margin of the Ross Sea (Fig. 1). The
Taylor Glacier deposition zone receives 3–5 cm of ice equivalent accumulation
annually in present-day climate conditions (Kavanaugh et al., 2009a;
Morse et al., 1999). The glacier flows through Taylor Valley at a rate of
∼10 m a-1 and terminates near Lake Bonney, approximately
30 km from the Ross Sea (Kavanaugh et al., 2009b; Aciego et al., 2007).
The ablation zone extends approximately 80 km from the terminus
(Kavanaugh et al., 2009b). The close proximity to McMurdo Station
provides excellent logistical access to the site (e.g., Fountain et al.,
2014; Petrenko et al., 2017; Baggenstos et al., 2017; Marchant et al., 1994;
Aarons et al., 2017).
A combination of relatively high sublimation rates (∼ 10 cm a-1) and relatively slow flow creates an ablation zone where ancient
ice with a large range of ages is exposed at the surface of Taylor Glacier
(Kavanaugh et al., 2009a, b). An along-flow
transect of water stable isotopes from just below the equilibrium line to
the terminus revealed ice from the last glacial period outcropping at
sporadic places along the transect (Aciego et al., 2007). The sporadic
nature of the outcrops was later shown to be an artifact of sampling nearly
parallel to isochrones such that they were occasionally crossed
(Baggenstos et al., 2017). More recent across-flow profiles dated with
stratigraphic matching of well-mixed atmospheric gases revealed ice that
varies continuously in age from the Holocene to ∼50 ka
(Schilt et al., 2014; Bauska et al., 2016; Baggenstos et al., 2017), with
ice of last interglacial or older age found near the terminus of the glacier
(Baggenstos et al., 2017; Buizert et al., 2014). The most heavily sampled
archive is a 500 m section called the main transect, oriented
perpendicular to isochrones (Fig. 1) across a syncline–anticline pair
containing ice spanning from ∼50 ka before present (BP) to the mid
Holocene (7 ka) (Baggenstos et al., 2017). Ice stratigraphy in the main
transect dips approximately vertically so that it is possible to obtain
large quantities of ice of the same age by drilling vertical or
near-vertical ice cores (e.g., Baggenstos et al., 2017; Petrenko et al.,
2017, 2016; Schilt et al., 2014; Bauska et al., 2016, 2018). Ice containing the full MIS 5–4 transition was
formerly considered to be missing from the glacier (Baggenstos, 2015;
Baggenstos et al., 2017), but we show here that a new ice core near the main
transect contains an intact record with ice dating from 76.5–60.6 ka and air
dating from 74.0–57.7 ka.
Core retrieval
In the 2013–2014 season an exploratory core was drilled vertically using a
PICO hand auger 380 m away (-380 m by convention) from a benchmark
position (77.75891∘ S, 161.7178∘ E in January 2014) along
the main transect (Fig. 1). In the 2014–2015 field season another
exploratory core was drilled vertically using the PICO hand auger
approximately 1 km down-glacier from the main transect (77.7591∘ S, 161.7380∘ E in December 2014) where older ice near the surface was
suspected. This site is hereafter referred to as the MIS 5–4 site (Fig. 1). An ice core was drilled directly adjacent to the PICO borehole at the
MIS 5–4 site using the Blue Ice Drill (BID), a 24 cm diameter shallow coring
device designed for retrieving large-volume ice samples suitable for trace
gas and isotope analysis (Kuhl et al.,
2014). The section 9–17 m was sampled in the field for laboratory trace gas
analyses at Oregon State University (OSU) and at the Scripps Institution of
Oceanography (SIO).
In the 2015–2016 season a second large-volume core was drilled directly
adjacent to the previous MIS 5–4 boreholes using the BID, and the sections
0–9 and 17–19.8 m were sampled for trace gas analyses at OSU and at SIO.
The entire 0–19.8 m of this core was sampled for continuous-flow analysis
(CFA) in the field and at the Desert Research Institute (DRI). Samples for
all analyses were cut with a band saw on the glacier, stored in chest
freezers at <-20∘C in camp, and flown to McMurdo Station
within 2 weeks of retrieval, where they were stored at <-20∘C. Storage temperature was <-20∘C for the
remainder of their shipment to the USA and subsequent storage in
laboratories.
Analytical methods
A field laboratory at the Taylor Glacier field camp permitted continuous
measurements of CH4 and particle count on ice core samples within days
of drilling and recovery (Table 1). CH4 concentration was measured
using a Picarro laser spectrometer coupled to a continuous gas-extraction
line with a de-bubbler similar to that described in Rhodes et al. (2013).
The continuous CH4 data were calibrated by measuring standard air of
known CH4 concentration introduced into a stream of gas-free water to
simulate a bubble–liquid mixture similar to the melt stream from ice core
samples. The tests indicated 3.5 %–5.5 % loss of CH4 due to dissolution
in the melt stream. We adjusted the continuous CH4 data upwards by
5 % to account for the solubility effect, which resulted in a good
agreement between our measurements and other Antarctic CH4 records
(e.g., Schilt et al., 2010). Insoluble particle abundance was also
measured continuously in the field using an Abakus particle counter coupled
to the continuous meltwater stream. In order to obtain exploratory gas age
information and verify the continuous CH4 data, discrete ice core
samples were also measured for CH4 concentration in the field using a
Shimadzu gas chromatograph coupled to a custom melt–refreeze extraction
line, a manually operated version similar to the automated system used at
OSU (Mitchell et al., 2011, 2013).
Summary of new datasets. Gas chromatograph (GC) and mass
spectrometer (MS) measurements were made on discrete samples. Picarro,
Abakus, and ICP-MS measurements were made by continuous-flow analysis.
Analytical precision is from method reference or the pooled standard deviation
of replicate samples. OSU: Oregon State University, SIO: Scripps
Institution of Oceanography, DRI: Desert Research Institute.
* Superscripts denote references for analytical procedures: 1 Mitchell et al. (2013, 2011); b Ahn et al. (2009); c Severinghaus et al. (1998),
Petrenko et al. (2006); d Bauska et al. (2014); e Rhodes
et al. (2013); f Maselli et al. (2013); g McConnell (2002).
Laboratory analyses on recovered samples and archived Taylor Dome samples
included discrete CH4 and CO2 concentrations, δ15N of
atmospheric N2, δ18O of atmospheric oxygen
(δ18Oatm), continuous CH4 concentration, δ18Oice, major ion and elemental chemistry, and insoluble particle
counts (Table 1). Continuous chemistry, dust, δ18Oice, and
CH4 measurements were made at DRI by melting 3.5 cm × 3.5 cm
∼1 m longitudinal samples of ice and routing the melt stream
to in-line instruments (McConnell, 2002; Maselli et al., 2013). Insoluble
particles were measured using an Abakus particle counter, water isotopes
using a Picarro laser spectrometer (Maselli et al., 2013), and CH4
using a Picarro laser spectrometer and air extraction system similar to that
used in the field (Rhodes et al., 2013). Continuous CH4 data
measured at DRI were calibrated with air standards as described above. The
upward adjustment to account for dissolution in the melt stream was 8 % in
this case. Discrete CH4 and CO2 measurements were made at OSU.
CH4 was measured using an Agilent gas chromatograph equipped with a
flame ionization detector coupled to a custom melt–refreeze extraction
system (Mitchell et al., 2011). CO2 was measured (1) on an Agilent
gas chromatograph equipped with a Ni catalyst and a flame ionization
detector coupled to a custom dry-extraction “cheese grater” system for
carbon isotopic analyses (Bauska et al., 2014), as well as (2) on a similar
Agilent gas chromatograph coupled to a dry-extraction needle-crusher system
(Ahn et al., 2009). δ15N–N2 and δ18Oatm were measured at SIO using a Thermo Delta V mass
spectrometer coupled to a custom gas-extraction system (Severinghaus et
al., 1998; Petrenko et al., 2006).
Discrete measurements of CH4 and CO2 were made at OSU on archived
Taylor Dome ice core samples following the same procedures described above
(Table 1).
Data uncertainties
The analytical uncertainties associated with new data presented in this
paper are reported in Table 1. In addition to the uncertainties in
concentration and isotopic measurements, we address uncertainties related
to (1) smoothing of gas records due to dispersion and mixing in the CFA
system (Rhodes et al., 2013; Stowasser et al., 2012), (2) depth
uncertainty in gas and ice samples, and (3) artifacts due to contamination
of gas and dust in near-surface ice. The effect of analytical smoothing is
negligible, as demonstrated by close agreement of continuous CH4 with
high-resolution discrete CH4 data from 9 to 17 m in the 2014–2015 MIS 5–4 core (Fig. S1 in the Supplement). Depth uncertainties of up to 20 cm resulted from
unaligned, angled core breaks of up to 10 cm in length as well as small
depth logging errors. Contamination is only a concern in near-surface ice
where thermal expansion and contraction cause abundant cracks on the
surface of Taylor Glacier. The cracks rarely penetrate below 4 m and have
never been observed deeper than 7 m (Baggenstos et al., 2017). Gas
measurements may be sensitive to contamination from resealed cracks between
0 and 4 m of depth, and dust measurements may be affected by local dust deposition
between 0 and 40 cm of depth (Baggenstos et al., 2017, 2018).
To minimize this problem we avoided analyses of ice with visible fractures.
Age models for Taylor Glacier and Taylor DomeTaylor Glacier MIS 5–4 cores
For the new MIS 5–4 cores the sections retrieved during the 2014–2015 season
(9–17 m) and 2015–2016 season (0–9 and 17–20 m) are hereafter treated as
one ice core record (unified depth and age scales), which is justified given
the close proximity of the boreholes (<2 m spacing at surface) and
the minimal depth uncertainty between the cores (≤∼20 cm). The depth uncertainty is the maximum offset due to angle breaks at the
ends of cores, which never exceeded 10 cm. Observable depth offsets between
replicate measurements also do not exceed 20 cm (discussed in more detail
below and in the Supplement). No depth adjustments were made to
the raw data from any of the ice cores.
A gas age model for the Taylor Glacier MIS 5–4 cores was constructed by
matching variations in CH4 and δ18Oatm to preexisting
ice core records synchronized to the Antarctic Ice Core Chronology (AICC)
2012 (Veres et al., 2013; Bazin et al., 2013) (Fig. S1). This approach
is valid for the gas age scale because CH4 and 18Oatm are
globally well mixed (Blunier et al., 2007; Blunier and Brook, 2001).
Variations in CH4 were tied to the EPICA Dronning Maud Land (EDML)
record (Schilt et al., 2010), and δ18Oatm was tied to
the North Greenland Ice Coring Project (NGRIP) record (Landais et al.,
2007). These datasets were chosen because they contain the
highest-resolution CH4 and δ18Oatm data available
on the AICC 2012 timescale for this time period. Tie points linking ages to
depths were manually chosen (Fig. S1 and Table 2). Ages between the tie
points were interpolated linearly.
Tie points relating Taylor Glacier depth to gas age on the AICC
2012 timescale. Bold font indicates tie points <4 m of depth where
abundant cracks in shallow ice may cause contamination of gas records (see
text). DO refers to a Dansgaard–Oeschger event.
DepthGas ageAge rangeDataDataFeature descriptionReferenceTie point(m)(ka)(ka)sourcerecordsource1.7458.2157.30–59.00CH4This studyPeak during DO 16–17EDMLCH4This study3.1559.1058.21–59.60CH4This studyPeak during DO 16–17EDMLCH4This study4.1959.6659.60–59.70CH4This studyMidpoint transition DO 16–17EDML CH4This study5.4059.9459.71–60.78CH4This studyLow before DO 16–17EDML CH4This study7.7964.9064.30–65.40CH4This studyPeak during DO 18EDML CH4This study11.2469.9269.00–70.36CH4This studySmall peak between DO 19 and DO 18EDML CH4This study12.4370.6270.25–71.10CH4This studyLow after DO 19EDML CH4This study13.2571.2170.94–71.42CH4This studyHigh before transition late DO 19EDML CH4This study16.2072.2772.10–72.45CH4This studyMidpoint transition DO 19EDML CH4This study17.4072.7072.20–73.30δ18OatmThis studyMidpoint transitionNGRIP δ18OatmThis study19.2773.7473.35–74.50δ18OatmThis studyLow before transitionNGRIP δ18OatmThis study
CO2 data were not used to tie Taylor Glacier to AICC 2012. An offset
between the Taylor Glacier data and the Antarctic composite record of
Bereiter et al. (2015) during the MIS 4–3 CO2 increase between 64
and 60 ka (Taylor Glacier lower by ∼13 ppm at 61.5 ka; Fig. 2) could bias our age model toward older ages. This offset may be real
(e.g., Luthi et al., 2008), and we note that CO2 offsets of even
larger magnitude exist between Taylor Glacier and the composite record in
the interval 68–64 ka (Fig. 2).
Measurements of trace gases (CH4 and CO2), stable
isotopes (ice and O2), insoluble particles, and nss Ca2+ from the
Taylor Glacier ice core on new gas and ice age scales. All ice core data are
synchronized to AICC 2012. CH4 data from <4 m of depth and dust
data from <40 cm of depth are colored dark gray to denote potential
contamination by surface cracks. NGRIP: North Greenland Ice Coring
Project, TG: Taylor Glacier MIS 5–4 BID cores, EDML: EPICA Dronning
Maud Land, EDC: EPICA Dome C, TALDICE: Talos Dome. *, †, and
^ denote smoothing with 5000-point, 100-point, and 50-point
LOESS algorithms, respectively.
Nonetheless, the general agreement with trends in preexisting CO2
measurements supports the chosen tie points for the new gas age scale
(Fig. 2). The resemblance of the Taylor Glacier δ18Oatm
record to NGRIP δ18Oatm between 72 and 63 ka also
supports the gas age scale because tie points younger than 72 ka were
picked only from CH4 data. This is particularly important because
CH4 variability is small between 70 and 60 ka, limiting potential tie point
selections. Good agreement between CH4 variability in the new MIS 5–4
cores and the independently dated δ18O–CaCO3 from Hulu
Cave speleothems (Wang et al., 2001) also suggests that the gas age scale
is accurate (Fig. S5). Agreement between atmospheric CH4
concentration (a global signal) and Hulu Cave speleothem δ18O–CaCO3 is expected because both parameters are sensitive to
shifts in the latitudinal position of the Intertropical Convergence Zone and
the delivery of moisture via the tropical rain belts (Rhodes et al.,
2015; Buizert et al., 2015).
An ice chronology was constructed for the new Taylor Glacier MIS 5–4 cores
by matching variations in Ca2+, insoluble particle count, and δ18Oice to preexisting EPICA Dome C (EDC) dust (Lambert et al.,
2008, 2012) and δ18Oice records (Jouzel
et al., 2007) synchronized to AICC 2012 (Fig. S2). This approach has been
used successfully at Taylor Glacier before (e.g., Baggenstos et al.,
2018), and it is possible because to first order the temporal patterns of
dust content and δ18Oice in Antarctic ice are highly
correlated at different ice core locations across the continent (Mulvaney
et al., 2000; Schüpbach et al., 2013). Tie points were chosen manually
(Fig. S2 and Table 3), and ages were interpolated linearly between them.
The synchronized records are displayed in Fig. 2. A more detailed
discussion and justification of tie point choices for the Taylor Glacier MIS 5–4 chronologies are provided in the Supplement.
Tie points relating Taylor Glacier depth to ice age on the AICC
2012 timescale. Bold font indicates tie points <0.4 m of depth
where abundant cracks in shallow ice may cause contamination of dust
measurements (see text). Ice-phase parameters (dust and δ18Oice) are unaffected by surface cracks below 0.4 m of depth.
AIM refers to Antarctic Isotope Maximum event, and MIS refers to
Marine Isotope Stage.
DepthIce ageAge rangeDataDataFeature descriptionReferenceTie point(m)(ka)(ka)sourcerecordsource0.3461.4759.50–63.93Insoluble particlesThis studyPeak near end of MIS 4EDC laser dustThis study1.2563.9363.00–64.70nssCa2+This studyPeak late MIS 4EDC nssCa2+This study1.8064.9164.00–65.65Insoluble particlesThis studyPeak late MIS 4EDC laser dustThis study2.4565.6565.00–66.30Insoluble particlesThis studyPeak mid MIS 4EDC laser dustThis study3.1066.7366.10–67.40nssCa2+This studyPeak mid MIS 4EDC nssCa2+This study4.4768.6367.86–69.60nssCa2+This studyPeak mid MIS 4EDC nssCa2+This study4.9469.7269.30–70.10nssCa2+This studyLow early MIS 4EDC nssCa2+This study5.6070.2069.70–70.65nssCa2+This studyPeak early MIS 4EDC nssCa2+This study7.7571.9571.00–73.00δ18OiceThis studyPeak AIM 19EDC δ18OiceThis study12.2073.6273.00–74.50δ18OiceThis studyLow between AIM 19 and AIM 20EDC δ18OiceThis study16.6275.7574.60–76.75δ18OiceThis studyPeak AIM 20EDC δ18OiceThis study19.7676.5075.75–77.00nssCa2+This studyEnd of record, loosely constrainedEDC nssCa2+This studyTaylor Glacier -380 m main transect core
To investigate continuity between the Taylor Glacier main transect and the
new MIS 5–4 site, we constructed a gas age scale for the ice core at -380 m
on the main transect collected during the 2013–2014 season (Fig. 3). Gas
ages were determined by matching CH4 data to EDML on AICC 2012 (Table 4). The chronology of the -380 m core is more uncertain than for the MIS 5–4
cores because there are fewer features to match in the gas records, but the
synchronous variability in CH4, CO2, and δ18Oatm
is unique to the late MIS 4 and MIS 4–3 transition. The observation of late
MIS 4 air (but not the full MIS 5–4 transition) was the basis for moving our
2014–2015 ice reconnaissance efforts down-glacier from the main transect
where older ice is closer to the surface.
Measurements of trace gases (CH4 and CO2) and stable
isotopes (O2 and N2) from the -380 m main transect Taylor Glacier
ice core and MIS 5–4 ice cores on new gas age scales. All ice core data are
synchronized to AICC 2012. CH4 data from <4 m of depth are
colored gray to denote potential contamination by surface cracks. NGRIP: North Greenland Ice Coring Project, TG: Taylor Glacier, EDML: EPICA
Dronning Maud Land, EDC: EPICA Dome C, TALDICE: Talos Dome.
Tie points relating -380 m main transect core depth to gas age on
the AICC 2012 timescale.
Depth (m)Gas age (ka)DataData sourceFeature descriptionReference recordTie point source3.75159.53CH4This studyHigh value at start of DO 16–17EDML CH4This study5.30159.83CH4This studyLow before DO 16–17EDML CH4This study9.92964.40CH4This studyLow after DO 18EDML CH4This study14.84966.00CH4This studyLow before DO 18EDML CH4This studyTaylor Dome core
The early Taylor Dome chronologies (e.g., Steig et al., 1998, 2000) were recently revised by Baggenstos et al. (2018) from 0 to 60 ka
in light of evidence that the original timescales were incorrect (e.g.,
Mulvaney et al., 2000; Morse et al., 2007). To investigate the new Taylor
Glacier MIS 5–4 climate archive in the context of the glaciological history
of the Taylor Dome region, we revised the Taylor Dome gas and ice age scales
for the period 84–55 ka (504–455 m). We adopted the recently published age
ties (Baggenstos et al., 2018) for the interval that overlaps our
new records (60–55 ka). We then extended the timescale to 84 ka using new
and preexisting data. Gas tie points were chosen by manually value matching
variations in Taylor Dome CH4 data to EDML CH4 on AICC 2012. One
of the new tie points matches the variability observed in a preexisting CH4
record from Taylor Dome (Brook et al., 2000) to the EDML CH4 record
(Supplement), and three tie points adopted from Baggenstos et
al. (2018) match variations observed in preexisting Taylor Dome CO2
data (Indermühle et al., 2000) to WD2014 (Buizert
et al., 2015) (Fig. S3 and Table 5). Ice tie points were chosen by
manually matching variations in the Taylor Dome Ca2+ record
(i.e., Mayewski et al., 1996) to
EDC dust (Lambert et al., 2012, 2008) on AICC 2012
(Fig. S4 and Table 6).
Tie points relating Taylor Dome depth to gas age on the AICC 2012
timescale.
DepthGas ageAge rangeDataDataFeatureReferenceTie point(m)(ka)(ka)sourcedescriptionrecordsource455.9554.66754.167–55.167CO2Indermühle et al. (2000)Midpoint transition A3WAIS CO2Baggenstos et al. (2018)460.9057.91357.413–58.413CO2Indermühle et al. (2000)Midpoint transition A4WAIS CO2Baggenstos et al. (2018)464.6259.9959.70–60.50CH4Brook et al. (2000)Low before DO 16–17EDML CH4This study467.1062.30361.803–62.803CO2Indermühle et al. (2000)Midpoint transition A4WAIS CO2Baggenstos et al. (2018)474.9565.5065.00–66.80CH4This studyLow before DO 18EDML CH4This study483.1070.4069.70–71.20CH4This studyLow CH4 after DO 19EDML CH4This study486.9572.2772.00–72.70CH4This studyMidpoint transition DO 19EDML CH4This study493.5076.0575.75–76.30CH4This studyMidpoint transition DO 20EDML CH4This study503.9083.9083.65–84.10CH4This studyHigh at DO 21 onsetEDML CH4This study
Tie points relating Taylor Dome depth to ice age on the AICC 2012
timescale.
DepthIce ageAge rangeDataData sourceFeature descriptionReferenceTie point(m)(ka)(ka)sourcerecordsource455.1055.8054.25–57.00Ca2+Mayewski et al. (1996)See original workWAIS Ca2+Baggenstos et al. (2018)457.6058.8557.50–60.10Ca2+Mayewski et al. (1996)See original workWAIS Ca2+Baggenstos et al. (2018)463.3061.4761.00–62.00Ca2+Mayewski et al. (1996)Peak late MIS 4EDC laser dustThis study466.4063.5062.80–63.75Ca2+Mayewski et al. (1996)See original workWAIS Ca2+Baggenstos et al. (2018)467.8064.3063.90–64.80Ca2+Mayewski et al. (1996)See original workWAIS Ca2+Baggenstos et al. (2018)468.1064.6664.20-65.40Ca2+Mayewski et al. (1996)Low late MIS 4EDC nssCa2+This study471.3765.5765.00–66.10Ca2+Mayewski et al. (1996)Peak mid MIS 4EDC laser dustThis study472.7066.7166.00–67.25Ca2+Mayewski et al. (1996)Peak mid MIS 4EDC nssCa2+This study475.1267.4767.00–68.00Ca2+Mayewski et al. (1996)Low mid MIS 4EDC nss Ca2+This study476.9068.6367.75–69.40Ca2+Mayewski et al. (1996)Peak early MIS 4EDC nssCa2+This study478.7069.7069.25–70.10Ca2+Mayewski et al. (1996)Low early MIS 4EDC nssCa2+This study479.9070.1569.70–70.60Ca2+Mayewski et al. (1996)Peak early MIS 4EDC nssCa2+This study484.3071.9571.60–72.30δ18OiceSteig et al. (1998)Peak AIM 19EDC δ18OiceThis study487.4073.6273.30–74.00δ18OiceSteig et al. (1998)Low between AIM 19 and AIM 20EDC δ18OiceThis study490.8075.7575.00–76.10δ18OiceSteig et al. (1998)Peak AIM 20EDC δ18OiceThis study493.4077.0876.65–77.50δ18OiceSteig et al. (1998)Low before AIM 20EDC δ18OiceThis study502.7583.983.00–84.90δ18OiceSteig et al. (1998)Peak AIM 21EDC δ18OiceThis study
The general agreement between the Taylor Dome CO2 record and
preexisting data from other ice cores supports our revised gas age scale
(Fig. 4), but we did not use the CO2 data in constructing the age
scale apart from the points mentioned above. The general resemblance between
Taylor Dome δ18Oatm and NGRIP δ18Oatm
also supports the gas age scale, although the Taylor Dome δ18Oatm data are somewhat scattered due to lower measurement precision
(Sucher, 1997). Taylor Dome CH4 data on the new timescale also
agree well with δ18O–CaCO3 variability in Hulu Cave
speleothems (Fig. S5). The Supplement provides further
justification for the tie point choices in our revised Taylor Dome chronology.
Measurements of trace gases (CH4 and CO2), stable
isotopes (ice and O2), and Ca2+ from the Taylor Dome ice core on
new gas age and ice age scales. All ice core data are synchronized to AICC
2012. NGRIP: North Greenland Ice Coring Project, TD: Taylor Dome, EDML: EPICA Dronning Maud Land, EDC: EPICA Dome C, TALDICE: Talos Dome.
* denotes smoothing with a 100-point LOESS algorithm.
Age model uncertainties
There are two types of uncertainty associated with the new gas and ice age
models: (1) absolute age uncertainty propagated from the reference age scale
(AICC 2012) and (2) relative age uncertainty arising from depth offsets and
the manual selection of tie points. The latter is a function of (a) choosing
the correct features to tie, (b) the resolution of the data that define the
tie point features, and (c) the measurement error. To estimate relative age
uncertainty we assigned a maximum and minimum age to each chosen tie point
(Figs. 2, 4, Tables 2–3 and 5–6). The age ranges were
determined by closely examining the matched features and estimating the
maximum and minimum possible ages based on our judgment of factors (a)–(c)
above. The resulting error ranges for our tie points are conservative.
Maximum and minimum age scales were determined for the MIS 5–4 cores and the
Taylor Dome ice core by interpolating linearly between the maximum and
minimum age assigned to each tie point (Fig. 5a and c).
(a) New Taylor Glacier MIS 5–4 gas and ice age models, as well as (b) Taylor Glacier Δage and δ15N–N2. Where age data
and Δage are plotted in red, gas data are from the top 4 m where contamination from surface cracks is possible. (c) Revised Taylor
Dome gas and ice age models, as well as (d) Taylor Dome Δage and δ15N–N2. Δage data are plotted on the gas age scale.
Depth errors contribute additional uncertainty to the total relative
uncertainty described above. Depth errors between the Taylor Glacier MIS 5–4
cores were estimated by observing the depth offsets in features resolved by
the continuous versus discrete CH4 measurements (Fig. S1). The
largest depth offset was at the CH4 rise at ∼16.0 m:
there is a 10 cm offset between the continuous field CH4 and the
discrete laboratory CH4 and a 20 cm offset between the continuous and discrete laboratory CH4. Approximate 20 cm offsets are also
apparent in the ice phase by comparing insoluble particle count data
measured in the field versus in the laboratory (Fig. S2); 20 cm equates to
420 years on the new gas age scale in the interval of the ice core where gas age changes most rapidly with
depth (65–60 ka; Fig. 5a), and it equates to 360 years on the ice age scale in the interval of the ice core where ice
age changes most rapidly with depth (70–61 ka; Fig. 5a). We adopted 420 and 360 years as conservative estimates of the relative gas age error
and ice age error, respectively, due to depth uncertainty. These errors were
propagated into the calculations of maximum and minimum Taylor Glacier age
scales. We are unaware of depth uncertainties in the archived Taylor Dome
samples used in this study, so no additional depth uncertainty was added to
the age error estimates for Taylor Dome.
The mean of the estimated age errors along the cores provides a reasonable
cumulative estimate of the relative uncertainty in the new Taylor Glacier
MIS 5–4 and revised Taylor Dome chronologies. For Taylor Glacier the mean
relative uncertainty is ±0.9 ka for the gas age and +1.3 ka to -1.2 ka for the ice age. For Taylor Dome the mean relative uncertainty is +0.7 ka to -0.5 ka for the gas age and ±0.6 ka for the ice age. The
relative uncertainty is larger in Taylor Glacier due to the depth errors
described above.
We did not explicitly account for errors associated with interpolation.
Given our conservative estimates of tie point error, we believe any
additional uncertainty is minor relative to our conclusions. Tie points were
not assigned to the end points of our records unless there was clearly a
feature to match (with the exception of the last Taylor Glacier ice age tie
point described in the Supplement). Age models are
extrapolated from the closest pair of tie points for the interval 0–0.31 m
for the ice age scale and the intervals 0–1.74 and 19.27–19.8 m for the gas age scale.
We suspect there are differences between Taylor Glacier and EDML due to gas
transport in the firn layer because the features resolved in the new Taylor
Glacier CH4 data are generally smoothed relative to the same features
in EDML (Figs. 2 and S1). However, we believe that the effect of firn
smoothing on our tie point selections is within the estimated relative error
for the chronology (Fig. 5a). In contrast, CH4 features in the Taylor
Dome record appear less smoothed (Figs. 4 and S3).
The absolute age uncertainty in the reference timescale (AICC 2012) is 2.5 ka for ice age and 1.5 ka for gas age (Veres et al., 2013). By nature,
these errors are inherited by the Taylor Glacier 5–4 chronology and the
revised Taylor Dome chronology, though the total error in our chronologies
should be less than the total propagated EDC and EDML 1σ
uncertainties because the uncertainties in gas age and ice age are
correlated with depth. The close match of our gas age scales to the
radiometrically dated Hulu Cave record (Wang et al., 2001) indicates that
the absolute age uncertainties in our gas age scales are equal to or lower
than the implied AICC 2012 error estimates (Fig. S5). We estimate an upper
absolute age uncertainty of 1.5 ka for our Taylor Glacier and Taylor Dome
gas age scales based on the phasing of features in the δ18O–CaCO3 record from Hulu Cave and our CH4 records.
ResultsData quality and initial observations
Preliminary observations of CH4 variability in the MIS 5–4 PICO core
revealed that the air likely contained the full MIS 5–4 transition and the
MIS 4–3 transition (Fig. S1). The new Taylor Glacier MIS 5–4 ice cores
provide a record of the atmospheric history spanning 74–57.7 ka, including
the ∼40 ppm CO2 concentration decrease at the MIS 5–4
transition and the ∼30 ppm CO2 concentration increase
near the MIS 4–3 transition (Fig. 2). The new ice cores also record
millennial-scale variability in CH4, CO2, and δ18Oatm, as well as δ18Oice and dust. Taylor
Glacier δ18Oice is more variable than other Antarctic
records, most likely recording local-scale changes in postdepositional
alteration (Baggenstos, 2015; Baggenstos et al., 2018; Neumann et al.,
2005). We note that large features seen in other Antarctic stable isotope
records are preserved (e.g., 2 ‰–3 ‰ changes at the Antarctica
Isotope Maximum (AIM) 19 and AIM 20).
Field measurements (continuous CH4 and insoluble particles) were
replicated in the laboratory at DRI (Figs. S1 and S2). Replication allowed
for the assessment of data quality and supports the original data acquired in the
2014–2015 and 2015–2016 field seasons. Offsets between laboratory and field
measurements are minor in the section 4–20 m and are due to the depth
offsets described above (Figs. S1 and S2). CH4 offsets between field
and DRI data in the section 0–4 m are much larger (Fig. S1) and may be
attributed to contamination of the gas signal due to resealed thermal cracks
near the glacier surface (Baggenstos et al., 2017). We report these
shallow CH4 data for completeness. We assign two gas age tie points at
1.74 m (58.21 ka) and 3.15 m (59.10 ka) to offer a plausible gas age scale
for the shallow ice, but the gas age scale for 0–4 m is not interpreted
further and does not influence the conclusions of this study. CH4 data
from the section 0–1 m were excluded due to very high amounts of
contamination in both laboratory and field samples (CH4>1000 ppb). Continuous laboratory CH4 data were also excluded between
14.57–15.0 and 17.55–17.95 m due to technical problems with
instrumentation. Variations in Ca2+ and insoluble particle counts
generally agree with each other, suggesting that both parameters are recorders of
dust variability. Particle count data measured at DRI were averaged every 1 cm, explaining why the record appears less noisy than insoluble particle
counts measured in the field (Fig. S2).
CH4 variations in Taylor Glacier are smoother than in EDML. The largest
difference appears at DO 18 (64.9 ka) where Taylor Glacier CH4 is
∼40 ppb lower than EDML (and Taylor Glacier δ18Oatm is ∼0.1 ‰ more enriched
than NGRIP) (Fig. 2). The CH4 rise associated with DO 19 is less
attenuated: ∼20 ppb lower in Taylor Glacier relative to EDML
(72.3 ka, Fig. 2). Some of these differences may be due to higher
analytical noise in the EDML record (mean of EDML CH41σ=10.25 ppb between 74 and 60 ka). New Taylor Dome CH4 data from OSU show
little or no attenuation relative to the EDML record. Taylor Dome CH4
at the onset of DO 19 (72.3 ka) is 14 ppb higher than in EDML and 10 ppb
lower at the onset of DO 20 (75.9 ka) (Fig. 4). These offsets are within
the combined 1σ error of the measurements. The smoothing in the
three ice cores reflects the firn conditions in which bubble trapping
occurred, with smoother variations resulting from a thicker lock-in zone
that traps bubbles with a larger age distribution. The new CH4 data
suggest that Taylor Dome and EDML records are similarly smoothed by the firn,
while Taylor Glacier bubbles have a larger gas age distribution.
One clear observation from the new ice core is that the ice from MIS 4 is
very thin at Taylor Glacier; indeed the entire MIS 4 period (70–60 ka)
appears to be contained in ∼6 m of ice (Fig. 5a). This
partially explains why the MIS 4 interval has been relatively difficult to
locate. Thin ice could occur due to either low snow accumulation or
mechanical thinning of ice layers due to glacier flow. The implications of
thin layers for the accumulation history are discussed in more detail below.
Taylor Dome, in contrast, does not show such a steep age–depth relationship
(Fig. 5c).
Our new data also show that the ice at the MIS 5–4 site is stratigraphically
linked to the main transect. The evidence for this is that the -380 m core
contains air from late MIS 4 and the MIS 4–3 transition (Fig. 3). The
existence of MIS 4 ice on the main transect suggests continuity between the
two archives, i.e., that both archives originated from the same accumulation
zone. This is important because it means that it is possible to compare
climate information from the new MIS 5–4 site to climate information from
different intervals (e.g., the LGM) in ice from the main transect. More
broadly speaking, it is important to note that geologic evidence from Taylor
Valley suggests that Taylor Glacier has not changed dramatically in terms of
its extent or thickness in the last ∼2.2 Myr and that
Taylor Dome has remained a peripheral dome of the East Antarctic Ice Sheet
through the last ice age (Marchant et al., 1994; Brook et al., 1993). It
is therefore unlikely that the location of the Taylor Glacier accumulation
zone drastically changed during the intervals preserved in the main transect
and the MIS 5–4 site (∼77 to 7 ka).
A final observation is that the MIS 5–4 ice cores from Taylor Glacier have
very low δ15N–N2 (Fig. 5b). The δ15N–N2 enclosed in ice core air bubbles is controlled primarily
by gravitational fractionation in the firn column (Sowers et al.,
1992) (Supplement). To first order the δ15N–N2 records the height of the diffusive air column
(Sowers et al., 1992), an estimate for total firn thickness.
δ15N–N2 is also influenced by convective mixing near the
top of the firn (Kawamura et al., 2006; Severinghaus et al., 2010) and
vertical gradients in firn temperature induced by rapid shifts in ambient
temperature (Severinghaus et al., 1998). Low δ15N–N2 (<0.1 ‰) has been previously
observed at Taylor Glacier (e.g., main transect position -125 m) and Taylor
Dome (e.g., 380–390 m) and could result from thin firn and/or deep air
convection (Baggenstos et al., 2018; Severinghaus et al., 2010; Sucher,
1997). The observation that δ15N–N2 in the -380 m core is
similarly low as δ15N–N2 in the MIS 5–4 core supports
our interpretation that the archives originated from the same deposition
site (Fig. 3).
Gas age–ice age difference (Δage)
Gas is trapped in air bubbles in firn at polar sites typically 50–120 m
below the surface, and thus ice core air is younger than the ice matrix that
encloses it (Schwander and Stauffer, 1984). The magnitude of
the difference between ice age and gas age (Δage) depends primarily
on temperature and accumulation rate, with accumulation having a stronger
control (Herron and Langway, 1980; Parrenin et al., 2012; Capron et al.,
2013). Δage ranges from 100 to 3000 years in polar ice cores under
modern conditions (Schwander and Stauffer, 1984), with
high-accumulation sites having the smallest Δage (e.g., Buizert
et al., 2015; Etheridge et al., 1996) due to fast advection of firn to the
lock-in depth at which gases no longer mix with the overlying pore space.
Extrema in Δage up to 6500 years (Vostok) and 12 000 years (Taylor
Dome) have been documented for cold, low-accumulation sites at the Last
Glacial Maximum (e.g., Veres et al., 2013; Bender et al., 2006;
Baggenstos et al., 2018), when slow grain metamorphism and slow advection
of firn increase the lock-in time. Other important factors may include ice
impurity content (Horhold et al., 2012; Freitag et al., 2013; Bréant et
al., 2017), surface wind stress, local summer insolation (Kawamura et
al., 2007), and firn thinning. These factors are of secondary importance for
polar ice cores compared to the effects of temperature and accumulation rate
(Buizert et al., 2015).
Δage was calculated for the new Taylor Glacier ice core by subtracting
the gas age at a given depth from the independently determined ice age at
the same depth (Δage = ice age - gas age). The Δage in
the Taylor Glacier MIS 5–4 core approaches ∼10 ka during late
MIS 4 (Fig. 5b), which exceeds Δage for typical modern polar ice
core sites even where ice accumulates very slowly. This finding is
unprecedented in ice from Taylor Glacier, as Δage in ice from the
main transect does not exceed ∼3 ka between 10 and 50 ka
(Baggenstos et al., 2018). Our large Δage values imply that
accumulation in the Taylor Glacier accumulation zone decreased significantly
through MIS 4, which could have been caused by low precipitation and/or
high wind scouring. This interpretation is supported by the following lines
of evidence: (1) the depth–age relationship suggests the ice during MIS 4 is
very thin (Fig. 5a). This is in contrast to ice from the Last Glacial
Maximum, which is found at the surface of Taylor Glacier in two thicker
(layer thickness is ∼50 m) outcrops that dip approximately
vertically and strike along the glacier longitudinally (Baggenstos et
al., 2017; Aciego et al., 2007). Thin MIS 4 layers could be due to
mechanical thinning of the ice rather than low accumulation rates. However,
we note that ice thinning does not alter Δage because Δage
is fixed at the bottom of the firn when the ice matrix encloses bubbles
(Parrenin et al., 2012). This is unlike Δdepth, the depth
difference between ice and gas of the same age, which evolves with thinning.
So even if increased thinning caused the steep depth–age curve observed
during MIS 4, one would still need to invoke an explanation for the high Δage. (2) There is some degree of smoothing in the Taylor Glacier CH4
data relative to EDML, which can result from the expected longer gas
trapping duration in firn where accumulation rates are relatively low
(Köhler et al., 2011; Fourteau et al., 2017; Spahni et al., 2003). (3) As
Δage increased at the onset of MIS 4, the δ15N–N2 progressively decreased (Fig. 5b), which is consistent with thinning of
the firn column in response to decreased net accumulation. Inspection of
Fig. 5b reveals that the change in δ15N–N2 is not linear
with Δage, potentially due to nongravitational effects like thermal
fractionation (Severinghaus et al., 1998) or convective
mixing near the top of the firn (Kawamura et al., 2006). A very low
accumulation rate is known to be associated with deep convective mixing in
the firn (Severinghaus et al., 2010).
In contrast to Taylor Glacier, Δage at Taylor Dome reaches a maximum
of 3 ka at ∼56 ka and does not rise above 2.5 ka throughout
MIS 4 (Fig. 5d). The implication of the relatively “normal” Δage
is that net accumulation at Taylor Dome did not dramatically change
throughout MIS 4, while Δage in the Taylor Glacier accumulation
region did.
Δage uncertainty was determined by propagating the error reported for
the age models described above (Fig. 5a and c). The maximum and minimum
Δage curves were calculated by subtracting the oldest gas age scale
from the youngest ice age scale and vice versa. The mean Δage
uncertainty is ±2.2 ka for the Taylor Glacier MIS 5–4 cores and +1.0 ka to -1.3 ka for the Taylor Dome core. The larger uncertainty for Taylor
Glacier is due to the larger age uncertainties arising from the depth error.
The uncertainties we estimate for Δage are of similar magnitude as
the Δage uncertainty in other Taylor Glacier chronologies
(Baggenstos et al., 2018).
Accumulation rate estimates
Given mean annual temperature and Δage, it is possible to use models
of firn densification to estimate the accumulation rate at the Taylor
Glacier accumulation zone. We used an empirical firn densification model
(Herron and Langway, 1980) to compute firn density profiles for a
range of temperatures and mean accumulation rates (Supplement). Δage in the model is estimated by calculating the age
of the firn when it has reached the close-off depth (when the density is 0.83 g cm-3). The estimated accumulation rate comes from a simple
lookup function that scans the full range of temperature and Δage
and picks the corresponding accumulation rate (similar to work by Parrenin
et al., 2012). For a Δage of 10 ka and a temperature of -46∘C the estimated accumulation rate for the Taylor Glacier
MIS 5–4 cores is 1.9 mm yr-1 of ice equivalent. The temperature -46∘C is derived from the average δ18Oice
for the period of firn densification (70–60 ka) using the relationship
Δδ18Oice=0.5∘C-1 calibrated
using modern δ18Oice=-41 ‰ and
modern temperature at -43∘C (Waddington and Morse, 1994;
Steig et al., 2000), similar to Baggenstos et al. (2018). We used the
average δ18Oice from the Taylor Dome record because it is
less noisy and avoids the question of whether Taylor Glacier δ18Oice accurately records temperature (Baggenstos et al.,
2018). Since the close-off depth is estimated from the modeled firn density
profile (30 m), it is possible to estimate the expected δ15N–N2 assuming that the close-off depth is an approximation of
the height of the diffusive air column (Supplement). Assuming
a 3 m lock-in zone height and a 0 m convective zone height (see
the Supplement), the predicted δ15N–N2 (0.14 ‰) is enriched by a factor of 2 relative to measured
values (∼0.07 ‰ at 60 ka; Fig. 5b). The
difference in expected versus measured δ15N–N2 may imply
the influence of deep air convection in the Taylor Glacier firn column
(Kawamura et al., 2006; Severinghaus et al., 2010). To bring the
predicted δ15N–N2 into closer agreement we introduced a
convective zone height of 13.5 m (Fig. S7). The apparent influence of air
convection could be due to cracks that penetrate the surface of the firn
(e.g., Severinghaus et al., 2010), which only occur in firn with a low
mean accumulation rate.
A similar estimate was performed for the Taylor Dome core. Running the
models with a Δage of 2.3 ka (the Taylor Dome Δage at
∼60 ka when Taylor Glacier Δage is maximum, Fig. 5)
and a temperature of -46∘C yields an estimated mean
accumulation rate of 1.6 cm yr-1 of ice equivalent, almost a factor of 10
larger than Taylor Glacier. The estimated diffusive column height (53 m)
with a 3 m lock-in zone height and 0 m convective zone height predicts
δ15N–N2 of 0.26 ‰ (Fig. S8), in
somewhat better agreement with measured δ15N–N2 (Fig. 5d), implying less influence of deep air convection. The δ15N–N2 data from Taylor Dome are lower resolution and less
precise than the new Taylor Glacier data; in fact, there is not actually a
δ15N–N2 measurement at 60 ka (Fig. 5d). Still, we think
the closer agreement between modeled δ15N–N2 and the
nearest measured δ15N–N2 suggests a shallower convective
zone, consistent with higher mean accumulation rate.
These accumulation rate and firn thickness calculations estimate how low the
accumulation at Taylor Glacier may have been relative to Taylor Dome in late
MIS 4. We caution that these estimates are uncertain given that we
extrapolated below the empirical calibration range of the firn densification
model (lowest accumulation 2.4 cm yr-1 of ice equivalent at Vostok)
(Herron and Langway, 1980). We are unaware of firn densification
models that are specifically tailored to very-low-accumulation sites.
Another potential uncertainty in our estimates is that we did not account
for geothermal heat transfer through the firn, which is relatively close to
bedrock at Taylor Dome (the depth to bedrock is ∼550 m). The
effect of excess geothermal heat would drive firn temperatures higher,
decreasing Δage (Goujon et al., 2003). Higher firn temperatures
could also cause lower δ15N–N2, perhaps partially
explaining low values of δ15N–N2 observed at Taylor
Glacier and Taylor Dome.
Discussion
Despite the model uncertainties, we conclude that the simplest explanation
for the Δage patterns described above is markedly different
accumulation rates in the Taylor Dome versus Taylor Glacier accumulation
zones during MIS 4. Today the Taylor Glacier accumulation zone is on the
northern flank of Taylor Dome, whereas the Taylor Dome ice core site is on
the south flank (Fig. 1). The difference between the estimated
accumulation rate at Taylor Glacier versus Taylor Dome implies a gradient in
precipitation and/or wind scouring between the two locations. This
implication is perhaps not surprising given that a modern accumulation
gradient is observed in the same direction, with accumulation decreasing
from 14 to 2 cm yr-1 going from south to north (Morse
et al., 1999, 2007; Kavanaugh et al., 2009b). Moisture
delivery to Taylor Dome primarily occurs during storms that penetrate the
Transantarctic Mountains south of the Royal Society Range and reach Taylor
Dome from the south (Morse et al., 1998);
therefore, the modern-day accumulation rate decreases orographically from
south to north. The Taylor Glacier accumulation zone is effectively situated
on the lee side of Taylor Dome with respect to the modern prevailing storm
tracks (Morse et al., 1999) (Fig. 1). The difference between Δage at Taylor Glacier versus Taylor Dome is too large to be explained by
temperature contrasts between the two sites, which are on the order of 1–3 ∘C in the present day (Waddington and Morse, 1994).
A temporal change in the accumulation gradient across Taylor Dome (and hence
between Taylor Dome and the Taylor Glacier accumulation zone) has already
been suggested by other work for the Last Glacial Maximum. Morse et al. (1998) calculated the accumulation rate history
for the Taylor Dome ice core site using modern accumulation data, a
calculated ice flow field, and an age scale determined by the correlation of
isotope and chemical data with Vostok ice core records (Fig. 6). By mapping
the Taylor Dome age scale to ice layers resolved in radar stratigraphy,
Morse et al. (1998) also inferred the accumulation
rate history for a virtual ice core situated in the lee of the modern
prevailing storm trajectory, ∼7 km to the north of the Taylor
Dome drill site and likely near the hypothesized Taylor Glacier accumulation
zone (Fig. 1).
Δage, δ15N–N2, and estimated
accumulation rate for Taylor Glacier and Taylor Dome from 75 to 7 ka. Δage and δ15N–N2 data between 55 and 7 ka are from
Baggenstos et al. (2018) and 80–55 ka are from this study, except all
Taylor Dome δ15N–N2 data, which are from Sucher (1997). Δage data are plotted on the gas age scale. TD: Taylor Dome, TG: Taylor Glacier, HL: Herron and Langway (1980).
The accumulation histories inferred from the layer thicknesses revealed
differences for the two sites but not in the direction expected from the
modern south-to-north storm trajectory. The Last Glacial Maximum
accumulation histories were characterized by extremely low accumulation at
the Taylor Dome ice core site relative to higher accumulation at the
northern virtual ice core site. The possibility that different layer
thicknesses (and inferred accumulation histories) were a result of
differential ice flow was rejected because deeper layers did not show the
same effect (Morse et al., 1998). The reversed
accumulation gradient inferred from ice layer thicknesses was qualitatively
confirmed by independent Δage determinations on Taylor Glacier and
Taylor Dome ice made by Baggenstos et al. (2018), which revealed a Taylor
Glacier Δage of ∼3000 years and a Taylor Dome Δage of ∼ 12 000 years at the Last Glacial Maximum.
Accumulation rate estimates from a firn densification model (Fig. 6)
confirmed that the orientation of the accumulation gradient was
north to south, in the opposite direction of the gradient observed today
(Fig. 1).
Our new Δage data and accumulation rate estimates indicate an
accumulation gradient in the same direction as the modern one but opposite to that
of the Last Glacial Maximum. The accumulation rate estimates by Morse et al. (1998) qualitatively agree with this pattern:
>60 ka (Fig. 6). It is hypothesized that the reversed
accumulation gradient at the Last Glacial Maximum resulted from a shift in
the trajectory of storm systems that delivered moisture to Taylor Dome,
possibly in response to the extension of grounded ice far into the Ross Sea
(Morse et al., 1998). If indeed the Antarctic ice
sheet extended far enough into the Ross Sea to alter the atmospheric
circulation during the Last Glacial Maximum, the implication of our new data
is that a similar situation did not exist during MIS 4. This hypothesis
seems at odds with independent evidence that the Southern Hemisphere
experienced full glacial conditions during MIS 4 (Schaefer et al., 2015;
Barker and Diz, 2014). A possible explanation is that the sea level minimum
at MIS 4 was 25 m higher than during the Last Glacial Maximum due to the
lack of extensive Northern Hemisphere ice sheets (Shakun et al., 2015;
Siddall et al., 2003; Cutler et al., 2003), which limited how far grounded
ice from the West Antarctic Ice Sheet could extend into the Ross Embayment.
This suggestion is consistent with (1) data suggesting that the maximum Ross Ice
Shelf extent occurred during the last glacial termination (Hall et al.,
2015; Denton and Hughes, 2000) rather than MIS 4 and (2) the notion that
the grounding line position in the Ross Sea is set by the balance between marine
forcing (basal melting) and accumulation on the Antarctic ice sheets
(Hall et al., 2015).
A second hypothesis arises from the notion that broad differences in
regional atmospheric dynamics between MIS 4 and the Last Glacial Maximum
might occur, without invoking changes in the extent of the Ross Ice Shelf as
a mechanism for disrupting the atmospheric circulation. The Amundsen Sea
Low, a low-pressure center that influences the Ross Sea and Amundsen Sea
sectors of Antarctica, responds strongly to changes in tropical climate
(Raphael et al., 2016; Turner et al., 2013) and exhibits cyclonic
behavior that likely controls the path of storms that enter the Ross
Embayment and reach Taylor Dome, as implied by Morse et al. (1998) and explored by Bertler et al. (2006).
An intensified or shifted Amundsen Sea Low during MIS 4 relative to the Last
Glacial Maximum might result in strong meridional flow across Taylor Dome
that maintained a south-to-north orographic precipitation gradient.
Interestingly, variability in the Amundsen Sea Low has been linked to the
extent of Northern Hemisphere ice sheets (Jones et al., 2018), which were
smaller in extent at MIS 4 relative to the Last Glacial Maximum. In summary,
the anomalous accumulation gradients we document on Taylor Dome in MIS 4 may
have their origin in the modest Northern Hemisphere ice volume at that time.
Conclusions
We obtained the first ice core from the Taylor Glacier blue ice area that
contains air with ages unambiguously spanning the MIS 5–4 transition and the
MIS 4–3 transition (74.0–57.7 ka). The ice core also contains ice spanning
the MIS 5–4 transition and MIS 4 (76.5–60.6 ka). The gas age–ice age
difference (Δage) in the cores approaches 10 000 years during MIS 4,
implying extremely arid conditions with very low net accumulation at the
site of snow deposition. To the south of the Taylor Glacier accumulation
zone, the Taylor Dome ice core exhibits lower Δage (1000–2500 years)
during the same time interval. This implies a steep accumulation rate
gradient across the Taylor Dome region with precipitation decreasing toward
the north and/or extreme wind scouring affecting the northern flank. The
direction of the gradient suggests that the trajectory of storms was
south to north during MIS 4 and that storm paths were not disrupted by
Antarctic ice protruding into the Ross Sea or by changes in the strength
and/or position of the Amundsen Sea Low, as occurred at the Last Glacial
Maximum.
Data availability
Data will be made available through the US Antarctic Program Data Center and
the National Center for Environmental Information.
The supplement related to this article is available online at: https://doi.org/10.5194/cp-15-1537-2019-supplement.
Author contributions
JAM made measurements at OSU on Taylor Glacier samples, developed
chronologies, and prepared the paper; JAM, EJB, and JRM made
measurements in the field; TKB made measurements at OSU on the -380 m Taylor
Glacier core; SB and SM made measurements at OSU on Taylor Dome samples; SAS
made measurements on all new Taylor Glacier samples at SIO except the -380 m
core, which were made by DB; JRM made measurements on Taylor Glacier samples
at DRI; all authors provided valuable feedback and made helpful
contributions to writing the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank Mike Jayred for maintaining and operating the blue ice drill and
Kathy Schroeder and Chandra Llewellyn for managing the Taylor Glacier field
camp; both tasks were Herculean. We thank Peter Sperlich, Isaac Vimont,
Peter Neff, Heidi Roop, Bernhard Bereiter, Jake Ward, and Andrew M. Smith
for help with field logistics and drilling, sampling, and packing ice cores.
We thank Howard Conway and Ed Waddington for feedback on an early version of
the paper, as well as Christo Buizert and Justin Wettstein for helpful
conversations about the Amundsen Sea Low. We thank Michael Kalk, Aron Buffen, and Michael Rebarchik for laboratory assistance at Oregon State
University and Monica Arienzo and Nathan Chellman for operation of the
continuous melter and other instrumentation at the Desert Research Institute. We
thank the United States Antarctic Program, with particular thanks to Science
Cargo, the BFC, and Helicopter Operations. We also thank Brian Eisenstatt
and Duncan May for ensuring the delivery of critical supplies to the glacier
at critical times during both Antarctic field seasons.
Financial support
This research has been supported by the National Science Foundation, Office of Polar Programs (grant nos. PLR-1245821, PLR-1245659, and PLR-1246148) and the UK National Environmental Research Council (grant no. 502625).
Review statement
This paper was edited by Denis-Didier Rousseau and reviewed by four anonymous referees.
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