CPClimate of the PastCPClim. Past1814-9332Copernicus PublicationsGöttingen, Germany10.5194/cp-12-115-2016Laurentide Ice Sheet basal temperatures during the last glacial cycle as inferred from borehole dataPicklerC.https://orcid.org/0000-0001-6847-3881BeltramiH.hugo@stfx.cahttps://orcid.org/0000-0001-9576-8933MareschalJ.-C.GEOTOP, Centre de Recherche en Géochimie et en Géodynamique, Université du Québec à Montréal, Québec, CanadaClimate & Atmospheric Sciences Institute and Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, CanadaCentre ESCER pour l'étude et la simulation du climat à l'échelle régionale, Université du Québec à Montréal, Québec, CanadaH. Beltrami (hugo@stfx.ca)22January201612111512725June201527August20159December20155January2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://cp.copernicus.org/articles/12/115/2016/cp-12-115-2016.htmlThe full text article is available as a PDF file from https://cp.copernicus.org/articles/12/115/2016/cp-12-115-2016.pdf
Thirteen temperature–depth profiles (≥ 1500 m) measured in boreholes in
eastern and central Canada were inverted to determine the ground surface
temperature histories during and after the last glacial cycle. The sites are
located in the southern part of the region that was covered by the Laurentide Ice
Sheet. The inversions yield ground surface temperatures ranging from -1.4 to
3.0 ∘C throughout the last glacial cycle. These temperatures, near
the pressure melting point of ice, allowed basal flow and fast flowing ice
streams at the base of the Laurentide Ice Sheet. Despite such conditions,
which have been inferred from geomorphological data, the ice sheet persisted
throughout the last glacial cycle. Our results suggest some regional trends
in basal temperatures with possible control by internal heat flow.
Introduction
The impact of future climate change on the stability of the
present-day ice sheets in Greenland and Antarctica is a major concern of the
scientific community e.g.,. Satellite
gravity measurements performed during the Gravity Recovery and Climate Experiment (GRACE) mission suggest that the mass
loss of the Greenland and Antarctic glaciers has accelerated during the
decade 2002–2012 . Over the past 2 decades, mass
loss from the Greenland Ice Sheet has quadrupled and contributed to a fourth
of global sea level rise from 1992 to 2011 .
In 2014, two teams of researchers noted that the collapse of the Thwaites
Glacier basin, an important component holding together the West Antarctic Ice
Sheet, was potentially underway . The
collapse of the entire West Antarctic Ice Sheet would lead to rise in sea
level by at least 3 m. The present observations of the ice sheet mass balance
are important. However, to predict the effects of future climate change on
the ice sheets, it is necessary to fully understand the mechanisms of ice
sheet growth, decay, and collapse throughout the past glacial cycles. The
models of ice sheet dynamics during past glacial cycles show that the
thickness and elevation of the ice sheets and the thermal conditions at their
base are key parameters controlling the basal flow regime and the evolution
of the ice volume .
During the last glacial cycle (LGC), ∼ 120 000–12 000 yr BP, large ice
sheets formed in the Northern Hemisphere, covering Scandinavia and almost all
of Canada . The
growth and decay of ice sheets are governed mainly by ice dynamics and ice-sheet–climate interactions .
Ice dynamics are also strongly controlled by the underlying geological
substrate and associated processes .
studied how the ice thickness and the nature of the
geological substrate control flow at the base of the glacier. They showed
that soft beds, beds of unconsolidated sediment at relatively low relief,
result in thin ice sheets that are predisposed to fast ice flow when
possible. Hard beds, beds of high-relief crystalline bedrock, on the other
hand, provide the ideal conditions for the formation of larger, thick ice
sheets that experience stronger bed–ice-sheet coupling and slow ice flow.
Basal flow rate is also affected by the basal temperatures, which play a key
role in determining the velocity. However, the ice sheet evolution models use
basal temperatures that are poorly constrained because of the lack of direct
data pertaining to thermal conditions at the base of ice sheets. The
objective of the present study is to use borehole temperature–depth data to
estimate how the temperatures varied at the base of the Laurentide Ice Sheet
during the last glacial cycle.
Temporal variations in ground surface temperatures (GST) are recorded by
Earth's subsurface as perturbations to the “steady-state” temperature
profile
see, e.g.,. With
no changes in GST, the thermal regime of the subsurface is governed by the
outflow of heat from Earth's interior resulting in a profile where
temperature increases with depth. In homogeneous rocks without heat sources,
the “equilibrium” temperature increases linearly with depth. When changes in
ground surface temperature occur and persist, they are diffused downward and
recorded as perturbations to the semi-equilibrium thermal regime. These
perturbations are superimposed on the temperature–depth profile associated
with the flow of heat from Earth's interior. For periodic oscillations of the
surface temperature, the temperature fluctuations decrease exponentially with
depth such that
T(z,t)=ΔTexp(iωt-zω2κ)exp(-zω2κ),
where z is the depth, t is time, κ is the thermal diffusivity of
the rock, ω is the frequency, and ΔT is the amplitude of the
temperature oscillation. Long time-persistent transients affect subsurface
temperature to great depths. were the first to attempt to infer past climate from such temperature–depth profiles;
specifically, they estimated the timing of the last glacial retreat from temperature
measurements in the Calumet copper mine in northern Michigan.
estimated the perturbations to the temperature gradient
caused by the last glaciation and suggested a correction to heat flux
determinations in regions that had been covered by ice during the LGC. A
correction including the glacial–interglacial cycles of the past 400 000 years
was proposed by to adjust the heat flow measurements
made in Canada. It was only in the 1970s that systematic studies were
undertaken to infer past climate from borehole temperature profiles
. The use of borehole temperature data
for estimating recent (< 300 years) climate changes became widespread in the
1980s because of concerns about increasing global temperatures
. High-precision borehole
temperature measurements have been mostly made for estimating heat flux in
relatively shallow (a few hundred meters) holes drilled for mining
exploration. Such shallow boreholes are suitable for studying recent (< 500 years)
climate variations and the available data have been interpreted in
many regional and global studies e.g.,and references
therein. However, very few deep (≥ 1500 m)
borehole temperature data are available to study climate variations on the
timescale of 10 to 100 kyr. Oil exploration wells are usually a few kilometers deep
but are not suitable because the temperature measurements are not made in
thermal equilibrium and lack the required precision. Nonetheless, a few deep
mining exploration holes have been drilled, mostly in Precambrian shields,
where temperature measurements can be used for climate studies on the timescale of the LGC. In Canada, measured temperatures in a deep
(3000 m) borehole near Flin Flon, Manitoba, and used direct models to show
that the surface temperature during the Last Glacial Maximum (LGM), ∼ 20 000 yr BP,
could not have been more than 5 K colder than present. Other
Canadian deep-borehole temperature profiles have since been measured
revealing regional differences in temperatures during the LGM. From a deep
borehole in Sept-Îles, Québec, found surface
temperatures to have been approximately 10 K colder than in the present day. This was confirmed
by who studied four deep holes and suggested that LGM
surface temperatures were colder in eastern Canada than in central Canada.
examined eight deep boreholes located in central
to eastern Canada and observed significant regional differences in heat
flux, temperature anomalies and ground surface temperature histories.
have studied a 2400 m deep well near Fort McMurray,
Alberta, penetrating the basement. They interpreted the variations in heat flux as being due to a 9.6 K
increase in surface temperature at 13 ka. have
revised this estimate to include the effect of the variations in heat
productions with depth which is non-negligible in granitic rocks with
high heat production. They concluded that temperatures at the base of
the ice sheet in Northern Alberta were about -3 ∘C during the LGM.
On the other hand, studies of deep boreholes in Europe lead to different
conclusions for the Fennoscandian Ice Sheet which covered parts of Eurasia
during the LGC see, e.g., .
analyzed heat flow variations with depth in several boreholes from the Baltic
Shield and the Russian Platform and found a 8±4.5 K temperature increase
following the LGM. studied a ∼ 5 km deep borehole
in the Urals, Russia, and found a postglacial warming of 12–13 K, with basal
temperatures below the melting point of ice during the LGM. This was
confirmed by recent work indicating that temperatures in the Urals were
∼-8∘C at the LGM .
In this study, we shall examine all the deep-borehole temperature profiles
measured in central and eastern Canada in order to determine the temperature
at the base of the Laurentide Ice Sheet, which covered the area during the
LGC. The geographical extent of the study is confined to the southern portion
of the Laurentide Ice Sheet because deep mining exploration boreholes have
only been drilled in the southernmost part of the Canadian Shield.
Theory
For calculating the temperature–depth profile, we assume that heat is
transported only by vertical conduction and that the temperature perturbation
is the result of a time-varying, horizontally uniform, surface temperature
boundary condition. The temperature at depth in the Earth, for a homogeneous
half-space with horizontally uniform variations in the surface temperature,
can be written as
T(z)=To+QoR(z)-∫0zdz′λ(z′)∫0z′H(z′′)dz′′+Tt(z),
where To is the reference ground surface temperature
(steady-state or long-term surface temperature), Qo is the reference heat
flux (steady-state heat flux from depth), λ(z) is the thermal
conductivity, z is depth, H is the rate of heat generation, and
Tt(z) is the temperature perturbation at depth z due to time-varying
changes to the surface boundary condition. R(z) is the thermal resistance
to depth z, which is defined as
R(z)=∫0zdz′λ(z′).
Thermal conductivity is measured on core samples, usually by the method of
divided bars . Radiogenic heat production is also
measured on core samples. The temperature perturbation at depth z resulting
from surface temperature variations can be written as Tt(z)=∫0∞z2πκt3exp(-z24κt)To(t)dt,
where t is time before present, κ is thermal diffusivity
and To(t) is the surface temperature at time t.
For a stepwise change ΔT in surface temperature at time t before
present, the temperature perturbation at depth z is given by
Tt(z)=ΔTerfc(z2κt),
where erfc is the complementary error function.
If the ground surface temperature variation is approximated by a series of
constant values ΔTk during K time intervals (tk-1,tk), the
temperature perturbation can be written as follows:
Tt(z)=∑k=1KΔTk(erfcz2κtk-erfcz2κtk-1).
The ΔTk represent the departure of the average GST during each
time interval from To.
First-order estimate of the GST history
We have used variations in the long-term surface temperature with
depth directly to obtain a first-order estimate of time variations in surface
temperature. To interpret subsurface anomalies as records of GST history
variations, we must separate the climate signal from the quasi steady-state
temperature profile. The quasi steady-state thermal regime is estimated by a
least-squares linear fit to the lowermost 100 m section of the
temperature–depth profile. The slope and the surface intercept of the fitted
line are interpreted as the steady-state temperature gradient Γo
and the long-term surface temperature To, respectively. The bottom part
of the profile is the section least affected by the recent changes in surface
temperature and dominated by the steady-state heat flow from Earth's
interior. When we are using a shallower section of the profile, the estimated
long-term temperature and gradient are more affected by recent perturbations
in the surface temperature. The shallower the section, the more recent the
perturbation. Thus, we determine how the estimated long-term surface
temperature varies with time by iterating through different sections of the
temperature–depth profile. Using the scaling of Eq. (), the
time it takes for the surface temperature variation to propagate into the
ground and the depth of the perturbations are related by
t≈z2/4κ,
where κ is thermal diffusivity, z is the depth, and t is
time.
This approach yields only a first-order estimate because the temperature
perturbations are attenuated as they are diffused downward. However, the
temporal variation in the long-term surface temperature is obtained by
extrapolating the semi-equilibrium profile to the surface. The effect of a
small change in gradient on the long-term surface temperature is proportional
to the average depth where the gradient is estimated. Variations in the
calculated long-term surface temperature are thus less attenuated than the
perturbations in the profile and may well provide a gross approximation of
the GST history but be independent of the model assumptions in an inversion
procedure.
Inversion
In order to obtain a more robust estimate of the GST histories, we have
inverted the temperature–depth profiles for each site, and whenever profiles
from nearby sites are available, we have inverted them jointly. The inversion
consists of determining To, Qo, and the ΔTk from the
temperature–depth profile (Eq. ). For each depth where
temperature is measured, Eq. () yields a linear equation in
ΔTk. For N temperature measurements, we obtain a system of N
linear equations with K+2 unknowns, To, Qo, and K values of
ΔTk. Even when N≥K+2, the system seldom yields a meaningful
solution because it is ill-conditioned. This means that the solution is
unstable and a small error in the data results in a very large error in the
solution . Different authors have proposed different
inversion techniques to obtain the GST history from the data see, e.g.,.
Here, to obtain a solution regardless of the number of equations and
unknowns and to reduce the impact of noise and errors on this solution,
singular-value decomposition is used . This technique is well documented, and further details can be
found in , , ,
and .
Simultaneous inversion
As meteorological trends remain correlated over a distance on the order of
500 km (Hansen and Lebedeff, 1987), boreholes within the same region are assumed to
have been affected by the same surface temperature variations and their
subsurface temperature anomalies are expected to be consistent. This holds
only if the surface conditions are identical for all boreholes. If these
conditions are met, jointly inverting different temperature–depth profiles
from the same region will increase the signal-to-noise ratio. Singular-value
decomposition was used to jointly invert the sites with multiple boreholes. A
detailed description and discussion of the methodology can be found in papers
by and .
Data description
We have used 13 deep boreholes (≥ 1500 m) across eastern and
central Canada to determine the temperature throughout and after the LGC. All
the sites are located in the southern portion of the Canadian Shield, which
was covered by the Laurentide Ice Sheet, which extended over most of Canada
during the LGC (Fig. ). Borehole locations and depths are
summarized in Table . All the holes that we logged were
drilled for mining exploration. The only exception is Flin Flon that was
drilled to be instrumented with low-noise seismometers for monitoring nuclear
tests. Detailed descriptions of the measurement techniques as well as the
relevant geological information can be obtained from heat flow
publications
.
We shall briefly summarize the main steps of the measurement technique, which has
been described in several papers
e.g.,. Temperature is
measured at 10 m intervals along the hole by lowering a calibrated probe
with a thermistor. The precision of the measurements is better than 0.005 K
with an overall accuracy estimated to be of the order 0.02 K Thermal
conductivity is measured on core samples. Samples are collected in every
lithology with an average of one sample per 80 m. The thermal conductivity
is measured with a divided bar apparatus on five cylinders of the core with
thickness varying between 0.2 and 1.0 cm. This method based on five
measurements on relatively large core samples provides the best estimate of
the thermal conductivity of the bulk rock and is unaffected by small
heterogeneities. Samples of the core have also been analyzed for heat
production following the method described by .
Technical information concerning the boreholes used in this study.
Map of central and eastern Canada and adjoining USA showing the
location of sampled boreholes. Thompson (Owl and Pipe), Manitouwadge (0610
and 0611) and Sudbury (Falconbridge, Lockerby, Craig Mine, and Victor Mine)
have several boreholes present within a small region. The number of profiles
available at locations with multiple holes is enclosed in parenthesis.
A description of eight sites – Flin Flon, Pipe, Manitouwadge 0610,
Manitouwadge 0611, Balmertown, Falconbridge, Lockerby, and Sept-Îles – can be
found in and . Five additional
profiles (Owl, near Thompson, Manitoba; Victor and Craig Mines, both near
Sudbury, Ontario; Matagami and Val d'Or, Québec) were analyzed. The Val
d'Or borehole was logged in 2010 to a depth of ∼ 1750 m. It is situated
15 km east of the mining camp of Val d'Or, Québec, in a flat forested
area. The Matagami borehole is located near the mining camp of Matagami, some
300 km north of Val d'Or. The Owl borehole, which was logged in 1999 and
2001, is located ∼5 km from the Birchtree Mine and ∼ 8 km south of
the city of Thompson, Manitoba. The two other new boreholes, Craig Mine and
Victor Mine, are located within the Sudbury structure, northeast of Lake
Huron, in Ontario. The Craig Mine borehole, near the town of Levack,
northwest of Sudbury, was logged in 2004. The deep mine was in operation
when measurements were made and pumping activity was continuous to keep the
deep mine galleries from flooding. The Victor Mine site was sampled in 2013,
close to the community of Skead, northeast of Sudbury, Ontario. Victor Mine
operated in 1959 and 1960, but exploration and engineering work is presently
underway to prepare for reopening the mine at greater depth.
We found systematic variations in thermal conductivity at Flin Flon, Thompson
(Owl), and Matagami, and we corrected them accordingly . We
calculated the thermal resistance and obtained a temperature vs.
thermal resistance profile that is almost linear. We calculated the heat flux
as the slope of the temperature resistance and found no discontinuity in heat
flux along the profile (Table ). For all the other sites
that show no systematic variations in conductivity, we have used the mean
thermal conductivity to calculate heat flux. Heat production was measured and
found to only be significant at two sites – Lockerby (3 µW m-3)
and Victor Mine (0.9 µW m-3) – and was therefore only
taken into account at these sites. Variations in heat production with depth
may affect the temperature profiles and the GST .
Because systematic variations in heat production with depth were not present
at these sites, they were not included in the inversion. In the absence of heat
production and in steady state, the heat flux does not vary with thermal
resistance. Variations in heat flux with thermal resistance (or depth) are thus a diagnostic of departure from a 1-D steady-state thermal regime. A
decrease in heat flux toward the surface is associated with surface warming, and enhanced heat flux is due to cooling. The heat flux profiles that we have
calculated for all the sites (Fig. ) exhibit clear departures
from the 1-D steady-state condition. The heat flux was calculated as the product
of the temperature gradient and the thermal conductivity within each
interval. No smoothing was applied. As expected, the gradient profile contains
high-frequency variations as the gradient always amplifies noise and errors
in the temperature measurements. Some holes appear to be noisier at depth
near the exploration targets because of small-scale conductivity variations
due to the presence of mineralization. Furthermore, the inclination of some
of the holes decreases markedly at depth resulting in larger errors in the
gradient. For example, the inclination at Val d'Or, 85∘ at the collar,
was only 20∘ near the end of the hole. Most of the profiles show a
very pronounced increase in heat flux with depth at shallow depth (< 200 m)
and a clear trend of increasing heat flux between 500 and 1500 m. The
increase at shallow depth is related to very recent (< 300 years) climate
warming. The trend between 500 and 1500 m is the result of the surface
warming that followed the glacial retreat at ca. 10 ka.
Heat flux variation as a function of depth. Heat flux is calculated
as the product of thermal conductivity by the temperature gradient calculated
over three points. The Flin Flon, Owl and Matagami profiles have been corrected
to account for thermal conductivity variations with depth as shown in
Table .
Analysis and resultsLong-term surface temperatures
Estimated long-term surface temperatures as a function of time and depth
(i.e., ∝ depth squared) were determined for each borehole (Fig. ).
The time in these plots represents the time it took the signal
to propagate and not the time that the surface temperature perturbation
occurred. The range of long-term surface temperatures for each borehole was
estimated over its sampled depth and revealed the persistent long-term climate
trends. These trends are the mirror image of the heat flow trends.
Long-term surface temperature variations over time (left y axis)
and depth (right y axis) for all the boreholes. Time is determined from
depth by Eq. ().
A decreasing temperature trend with time and depth is apparent in all the
boreholes in Manitoba except Pipe, which does not show a clear trend.
Variations in surface temperature are not consistent, with almost no trend at
Pipe and a weak signal at Owl. The Flin Flon borehole is the deepest
available for this study and provides a history 4 times longer than that
of the two shallower boreholes at Owl and Pipe. It is consistent with a
colder period coinciding with the LGM. In western Ontario, Manitouwadge 0611
exhibits very strong oscillations at depth. The source of noise is difficult
to ascertain because of the complicated geological structure and the absence
of thermal conductivity data. Borehole 0610 at Manitouwadge is consistent
with colder temperatures during the LGM, but the Balmertown hole does not show
any variation in long-term surface temperature. Four profiles at Sudbury, are
consistent with colder temperature during the LGM, but the amplitude of the
trend varies between sites. Similar trends are observed for the three
easternmost holes in Québec with long-term temperatures on average 5 K
lower near the bottom of the holes than near the surface.
These first-order estimates suggest colder surface temperatures during the
LGM at most of the sites. In order to better quantify the surface temperature
changes, we must turn to inversion and obtain the GST histories.
Individual inversions
The GST histories at all the studied sites for the time period of 100 to
100 000 yr BP were inverted from the temperature–depth profiles. The time
span of the GST history model consists of 16 intervals whose distribution
varies logarithmically because the resolution decreases with time. A singular-value cutoff of 0.08 was used for all the individual profile inversions
(Figs. –). The singular-value cutoff
eliminates the part of the solution that is affected by noise and effectively
introduces a smoothing constraint on the solution . A
summary of the inversion results can be found in Table . Two
main episodes can be recognized in the GST histories: one is associated with
a minimum temperature that occurred around the LGM at ca. 20 ka. The
second is a warming observed at ca. 2–6 ka coinciding with the
Holocene climatic optimum (HCO), a warm period that followed the deglaciation
.
Summary of GST history results where To is the long-term
surface temperature, Qo is the quasi-equilibrium heat flow,
Tmin is the minimal temperature, Tpgw is the maximum
temperature attained during the postglacial warming, and tmin and
tpgw are the occurrence of the minimal temperature and maximum
postglacial warming temperature. Parentheses indicate sites where the GST
history is not reliable.
1 The temperature profile at this site may
be distorted by horizontal contrasts in thermal conductivity. 2 The
temperature profile in the lowermost part of the hole may be affected by
subvertical layering and thermal conductivity contrasts. 3 The
temperature profile may be affected by water flow caused by pumping in the
nearby mine.
For the purpose of the discussion, we have grouped the sites that are
from the same geographical region. We shall thus distinguish between
Manitoba, western Ontario, the Sudbury area, and Québec.
The sites from Manitoba – Flin Flon, Owl and Pipe – have not recorded a
very strong signal and do not exhibit common regional trends as observed in
(Fig. ). The lack of regional trends is expected as Thompson
(Owl and Pipe) and Flin Flon sites are ∼ 300 km apart and the
present-day ground surface temperatures differ by 3 K. The weak signal
recorded may in part be due to the present ground surface temperature being
very low; it is close to 0 ∘C in Thompson where intermittent
permafrost is found. At Flin Flon, where the present ground temperature is
near 3 ∘C, we found that ground surface temperature variations were
small, confirming previous studies by . The surface
temperature was at a minimum around the LGM and was near the melting point of
ice (-0.3 ∘C). For Pipe, little to no signal was recorded. We
found that there was minimal change in the ground surface temperature
(∼ 2.5 K in amplitude) over the past 100 000 years. For Owl, the
amplitude of the temperature changes is double (∼ 5 K), with the
minimum temperature (-2.4 ∘C) around the LGM and a warming around
the HCO. Although this result is plausible, some uncertainty remains as
noted that high heat flux is correlated with high thermal
conductivity in the Thompson Belt. The elevated thermal conductivity is due
to the presence of vertical slices of quartzites, which increase thermal
conductivity by a factor of 1.7. Furthermore, the site is affected by a
poorly resolved conductivity structure as thermal conductivity measurements
vary in the deepest part of the borehole, between 2.21 and
5.14 W m-1 K-1. The lateral heat refraction effects due to the
thermal conductivity contrast affect the temperature profiles and alter the
GST history. For these reasons, we have little confidence in the robustness
of the Owl GST history reconstruction. In western Ontario (Balmertown and
Manitouwadge 0610 and 0611), results at all sites show minimum temperatures
around the LGM, with a very weak minimum at Balmertown
(Fig. ). However, the amplitude of the temperature change
is much larger at Manitouwadge 0611 (∼ 10 K) than at Balmertown
(∼ 2 K) and Manitouwadge 0610 (∼ 3 K). As the two Manitouwadge
sites are only ∼ 40 km apart, this difference is surprising. While
Manitouwadge 0611 yields a plausible GST history, it appears to have an
amplified signal. The site is located in a complex geological structure and
lacks thermal conductivity data. There is also a change in the temperature
gradient at 500 m, which cannot be explained. In the absence of thermal
conductivity data, we cannot consider the GST history for Manitouwadge 0611
as reliable.
Ground surface temperature history from the Manitoba boreholes at
Flin Flon and Thompson (Pipe and Owl). The temperatures have been shifted
with respect to the reference surface temperature of the site, To, as
shown in Table .
Ground surface temperature history for the western Ontario
boreholes: Balmertown and Manitouwadge 0610 and 0611. The temperatures have
been shifted with respect to the reference surface temperature of the site,
To, as shown in Table .
Ground surface temperature history for all the boreholes around
Sudbury, Ontario (Victor Mine, Falconbridge, Lockerby, and Craig Mine).The
temperatures have been shifted with respect to the reference surface
temperature of the site, To, as shown in Table .
Ground surface temperature history for the boreholes in Québec,
Matagami, Val d'Or and Sept-Îles. The temperatures have been shifted with
respect to the reference surface temperature of the site, To, as shown
in Table .
All four sites in the Sudbury region – Craig Mine, Falconbridge, Lockerby,
and Victor Mine – have recorded minimal temperatures around the LGM
(Fig ). However, the minimum past temperatures for the region
do vary between sites. The coldest minimum temperature was found at Craig
Mine. The amplitude of the temperature changes for the site (∼ 12 K)
is much larger than those of the other sites, which vary between ∼ 5
and 7 K. This difference is unexpected as these sites are all within the
Sudbury structure and should have recorded similar histories. The Craig Mine
signal appears amplified, which could be the result of water flows induced by
pumping at levels below 2000 m in the mine. We thus believe that the GST
history for Craig Mine is not reliable. The minimum temperatures at Lockerby
and Victor Mine, 2.8 and 3.0 ∘C, are also the highest of the study.
Moreover, these are the only two sites with non-negligible heat production: 3
and 0.9 µW m-3, respectively. The corrections for heat
production produce an increase in the temperature gradient proportional to
depth and result in an amplification of the warming signal in the profile.
Consequently, the minimum and maximum temperatures would be higher at these
sites – Lockerby and Victor Mine – than at those with negligible heat
production.
GST changes from simultaneous inversion with respect to the
long-term temperature at 100 ka. The Sudbury GST changes include (black) and
exclude (red) Craig Mine.
The GST histories from the three boreholes in Québec, Matagami, Val d'Or
and Sept-Îles, display regional differences (Fig. ).
However, for all three sites, the minimum temperatures are synchronous and
occurred around the LGM. For all the sites used in this study, the lowest
minimum temperature occurred in this region (-1.4 ∘C at Sept-Îles).
At all sites, excluding Pipe, we found that the ice retreat was followed
by a warm episode that can be associated with the HCO, a warm period whose
maximal temperatures have been dated to 4.4–6.8 ka with palynological
reconstructions from northern Ontario and northern Michigan
.
We have compared the ranges of temperature in the inverted GST histories with
those of the long-term surface temperature variations for all the sites
(Table ). Although the total range varies between sites from
∼ 2 to ∼ 8 K, the two methods yield consistent values that
differ by less than 1 K at most of the sites. The warming trend in the
long-term surface temperature variations is consistent with the inverted GST
histories. This correlation between the long-term surface temperature ranges
and the persistent long-term GST history trends suggests that our results are
robust. The inversion of borehole profiles has a very limited resolving
power, and short-period oscillations can seldom be recovered. In practice,
the duration of an episode must be proportional (1/3 to 1/2) to the time
when it occurred. The last glacial period is easily identified, but
shorter-period events such as the HCO that lasted for 1–2 kyr around 5 ka
are just beyond the threshold of resolution. It is thus possible that
constraining the GST to include the HCO would result in colder GST during the
LGM, as suggested by Will Gosnold in his comment on the discussion paper and
in Gosnold et al. (2011).
This would require obtaining proxy data from sites close to Flin Flon and
Thompson and requires documenting the temperature conditions at the bottom of
Lake Agassiz, which covered most of Manitoba after the glacial retreat.
Summary of GST history results for simultaneous inversions where
tmin and tpgw are the occurrence of the minimal
temperature and maximal temperature associated with postglacial warming and
ΔT is the temperature range.
We have inverted the boreholes of Thompson (Owl and Pipe), Manitouwadge (0610
and 0611) and Sudbury (Craig Mine, Falconbridge, Lockerby, and Victor Mine)
simultaneously to observe regional trends and to improve the signal-to-noise
ratio (Fig. ). The simultaneous inversion results are
summarized in Table . For simultaneous inversion, the
temperature–depth profiles were truncated to ensure a common depth for all
the boreholes. This ensures consistency and facilitates comparison because we
are examining the subsurface temperature anomalies for the same time period.
For all inversions, the minimum temperature occurs around the LGM and the
deglaciation is followed by a warming associated with the HCO. Although Owl
and Manitouwadge 0611 have questionable individual inversions, we have still
performed simultaneous inversions for the regions of Thompson and
Manitouwadge in an attempt to decrease the signal-to-noise ratio. For
Thompson, the width of the GST history temperature range is ∼ 4 K. As
Pipe did not appear to record a signal, the simultaneous inversion appears to
have damped the questionable signal recorded at Owl slightly. The amplitude
difference for the Manitouwadge GST history is ∼ 8 K, which lies
between that of 0610 (∼ 3 K) and 0611 (∼ 10 K).
The Sudbury simultaneous inversion was performed with and without Craig Mine.
This was done to check whether the inclusion of Craig Mine, a site which we
suspect to have been affected by water flow, affects the reconstructions.
Both inversions display similar trends; however, there is a difference in the
range of temperature variations. The inversion excluding Craig Mine yielded a
ΔT of 7 K, similar to those of the individual inversions of
Falconbridge (∼ 7 K), Lockerby (∼ 7 K) and Victor Mine (∼ 5 K).
Upon inclusion of Craig Mine, ΔT was increased to 11 K, demonstrating
that the presence of a signal due to water flow in the temperature profile at
this site has a strong effect on the results of the simultaneous inversion.
Discussion
The minimum temperatures of the GST histories occur around the LGM,
representing the basal temperatures of the Laurentide Ice Sheet. These
temperatures vary spatially and range from -1.4 to 3.0 ∘C, near or above
the pressure melting point of ice. Such spatial variation is expected as
numerous studies have demonstrated present-day spatial basal temperature
variability beneath the Antarctic and Greenland Ice Sheets
see, e.g.,. The highest
basal temperatures occur within the Sudbury basin at Lockerby and Victor
Mine. The Sudbury region has the highest average heat flux of the Canadian
Shield, ∼ 54 mW m-2, because crustal heat production is
higher than average . The lowest basal temperature is
recorded at Sept-Îles, where the heat flux is the lowest of the studied
regions, ∼ 34 mW m-2. These correlations suggest a link
between heat flux and basal temperatures. This is further supported by
modeling work showing that heat flux influences the thermal structure and
properties of ice sheets, including basal temperatures and ice flow
. However, the Sept-Îles basal
temperature has been also been linked to its proximity to the edge of the ice
sheet and to an area of thinner ice . Our study is consistent
with a possible link between basal temperature and heat flux along with ice
dynamics in the Laurentide Ice Sheet during the LGC, but further modeling
work is necessary to confirm such a relationship. Variations in these
parameters (heat flux and ice dynamics) could account for the differences
observed in the basal temperatures beneath the Fennoscandian and Laurentide
ice sheets.
Ranges in surface temperature variations estimated from the
iteration of the long-term surface temperature as a function of depth
(column 1) and from inversion of the GST history (column 2), along with the
difference between the two (column 3).
1 The temperature profile at this site may
be disturbed by horizontal contrasts in thermal conductivity. 2 The
temperature profile in the lowermost part of the hole may be affected by
subvertical layering and thermal conductivity contrasts. 3 The
temperature profile may be affected by water flow caused by pumping in the
nearby mine.
The basal temperatures recorded, near the pressure melting point of ice,
indicate the possibility of basal flow and ice streams, two important factors
affecting ice sheet evolution. These processes have been suggested by
geomorphological evidence presented by and predicted by the
ICE-5G model . Basal flow has the ability to transport
large amounts of water from the interior of the ice sheet, leading to
thinning of the ice sheet and climatically vulnerable ice. It is postulated
to be a key factor in glacial terminations .
Furthermore, these temperatures demonstrate that the southern portion of the
Laurentide Ice Sheet was not frozen to the bed, suggesting basal sliding.
These conditions can lead to instability. Widespread basal sliding and
increased surface meltwater could have been a factor resulting in the rapid
collapse of the Laurentide Ice Sheet . However, these
basal temperatures and the associated melt persisted prior to and throughout
the LGM for more than 30 000 years with deglaciation only occurring rapidly
during the early Holocene . While this indicates that
basal temperatures near the pressure melting point of ice cannot be solely
responsible for ice sheet instability and collapse, it demonstrates that they
are a key parameter in ice sheet evolution and one that ice sheet evolution
models must take into account. Elevated basal temperatures, near or above the
pressure melting point of ice, have been recorded in the present-day ice
sheets . Our results indicate that this
alone cannot be considered as an indication of ice sheet collapse. However,
combined with other processes it could lead to instability and collapse.
Conclusions
Thirteen deep boreholes from eastern and central Canada were
analyzed to determine the GST histories for the last 100 kyr. The long-term
trends are consistent between sites. A warm period following the retreat of
the ice sheet is inferred at ∼ 2–6 ka in the inverted GST histories,
correlated to the Holocene climatic optimum.
The surface temperatures reached their minima during the LGM and post glacial
warming started at ca. 10 ka. The corresponding temperatures at the base of the Laurentide Ice Sheet
range from -1.4 to 3.0 ∘C and are all near or above the pressure
melting point of ice. Such temperatures allow for basal flow and fast-flowing
ice streams, two important factors affecting ice sheet evolution,
illustrating the need for models of ice sheet evolution to account for such a
key parameter as basal temperature. Despite the suggestion that melting took
place at its base, the Laurentide ice sheet persisted throughout the LGM for over
more than 30 000 years. This demonstrates that basal temperatures near the
melting point of ice do not indicate that an ice sheet is on the verge of
collapse. However, combined with other processes it could lead to instability
and collapse.
The differences between GST histories at different sites raise other
questions concerning the controls on temperatures at the base of ice sheets.
The equilibrium between heat flow from the Earth's interior and heat advection by
glacial flow determines the temperature at the boundary between the ice and
the bedrock. The correlation between higher heat flux and higher basal
temperatures in the Sudbury region suggests that variations in crustal heat
flux may account for some of the regional differences in basal temperatures
along with the dynamics of ice thickness controlled by the accumulation rate
and the distance to the edge of the ice sheet.
It is also noteworthy that similar deep-borehole studies in Europe suggest
that basal temperatures beneath the Fennoscandian Ice Sheet and in the Urals
during the LGC were much colder than
those observed in Canada . Because of the geological
similarities between the two regions, this contrast is likely to be due to
differences in climate and ice dynamics between Europe and North America
during the LGC.
Acknowledgements
We appreciate the constructive review from Dmitry Demezhko and comments by
reviewer V. M. Hamza. Thanks to W. Gosnold and J. Majorowicz for their interest
and useful comments. This work was supported by grants from the Natural
Sciences and Engineering Research Council of Canada Discovery Grant
(NSERC-DG, 140576948), a NSERC-CREATE award Training Program in Climate
Sciences, the Atlantic Computational Excellence Network (ACEnet), and the
Atlantic Canada Opportunities Agency (AIF-ACOA) to H. Beltrami.
H. Beltrami holds a Canada Research Chair. C. Pickler is funded by a NSERC-CREATE Training
Program in Climate Sciences based at St. Francis Xavier
University.
Edited by: C. Barbante
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