![The (modern) Arctic Ocean and Nordic Seas in Polar map projection for orientation (top), and the Greenland-Scotland Ridge, with shallow depths highlighted in grey (bottom). DS: Denmark Strait; FBC: Faroe Bank Channel; FSC: Faroe-Shetland Channel; IFG: Iceland-Faroe Gap. Areas depicted in grey could not be passed by icebergs of >470 m total thickness, areas in white were blocked by any ice thicker than 720 m. Ice sheets from Greenland, Iceland and Scandinavia calved into this region. Red arrows indicate examples of deep iceberg scourmarks (800-1200 m water depth) or ice shelf traces (Lomonossov Ridge, close to 1000 m) [Color figure can be viewed at wileyonlinelibrary.com].](https://www.researchgate.net/profile/Walter-Geibert/publication/360110925/figure/fig1/AS:1148319439032322@1650792097226/The-modern-Arctic-Ocean-and-Nordic-Seas-in-Polar-map-projection-for-orientation-top_Q320.jpg)
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JOURNAL OF QUATERNARY SCIENCE (2022) 1–4 ISSN 0267-8179. DOI: 10.1002/jqs.3431
Reply to ‘Challenging the hypothesis of an arctic ocean lake during recent
glacial episodes’by Hillaire‐Marcel, et al
W. GEIBERT,
1
* J. MATTHIESSEN,
1
J. WOLLENBURG,
1
and R. STEIN,
1,2
1
Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
2
MARUM–Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen, Bremen, Germany
Received 19 March 2022; Accepted 28 March 2022
ABSTRACT: Hillaire‐Marcel et al. bring forward several physical and geochemical arguments against our finding of
an Arctic glaciolacustrine system in the past. In brief, we find that a physical approach to further test our hypothesis
should additionally consider the actual bathymetry of the Greenland–Scotland Ridge (GSR), the density maximum of
freshwater at 3–4°C, the sensible heat flux from rivers, and the actual volumes that are being mixed and advected.
Their geochemical considerations acknowledge our original argument, but they also add a number of assumptions
that are neither required to explain the observations, nor do they correspond to the lithology of the sediments. Rather
than being additive in nature, their arguments of high particle flux, low particle flux, export of
230
Th and
accumulation of
230
Th, are mutually exclusive. We first address the arguments above, before commenting on some
misunderstandings of our original claim in their contribution, especially regarding our dating approach.
©2022 The Authors. Journal of Quaternary Science Published by John Wiley & Sons Ltd.
The physical viewpoint
We include here a complete map of the Greenland–Scotland
Ridge (GSR, Fig. 1), based on modern bathymetry. It shows that the
majority of the GSR is shallower than 500 m (ca.400–420 m in
peak glacials). Only small parts of the Denmark Strait, the
Iceland–Faroe Gap and Faroe Bank Channel are deeper than
500 m, up to approximately 750 m (ca.650–670 m in peak
glacials). Consequently, it is not necessary to cover the entire GSR
with an 800 m ice shelf to restrict saline water inflow; already
icebergs of more than 450 m thickness had to strand on the GSR.
Once a couple of larger icebergs also block the Faroe Bank
Channel and the Faroe–Shetland Channel, the GSR is blocked
almost entirely. There is evidence for sufficiently thick grounded
ice in the Nordic Seas (NS): iceberg scourmarks of 800 m and
more are known from both north (Blischke et al., 2019) and south
(Kuijpers and Werner, 2007) of the GSR (Fig. 1), which is no
surprise because the Greenland, Iceland and the Scandinavian ice
sheets were calving into the NS, presumably creating an iceberg
armada retained by the sill, immobilised by falling sea levels, and
waiting for sea‐level rise, their slow melting, or a salinity increase
that modified iceberg freeboard, for their release.
Hillaire‐Marcel et al. state that it was necessary to block all
inflow of saline water at the GSR to turn the Arctic Ocean (AO)
and NS fresh. We would think that it is simply necessary that
more salt is exported than imported over a prolonged period. If
saline water entering the NS at the GSR is diluted by the
freshwater leaving the system, as commonly seen in fjords with
a series of sills (Edwards and Edelsten, 1977), no fully saline
water is available to replenish NS waters and the AO. Stärz
et al. (2017), found that already free depths of around 50 m
across the entire width of the GSR lead to a freshening of the
AO, when other gateways are closed, although in much
warmer periods.
We also disagree with the statement that there is no
evidence of non‐marine periods in the NS, as there are
carbonate‐free,
230
Th
ex
‐free intervals at least for MIS6 docu-
mented throughout the NS (Geibert et al., 2022). The
occurrence of foraminifera and consequently reliable δ
18
O
signals in the NS is discontinuous.
Regarding the removal of salt from the AO, we do not debate
the correctness of the calculations of Hillaire‐Marcel et al. for a
saline, remote ocean, but we consider them to be incomplete
for an almost enclosed system consisting of thick meteoric ice,
freshwater and (initially) saline water, fed by rivers and
glaciers. We start our consideration regarding the removal of
salt from the AO at Fram Strait. When Fram Strait was blocked
at the surface by ice of a certain thickness during glacial sea‐
level lowstands, freshwater from the East Siberian rivers had to
accumulate in the AO, filling it from the surface down to the
depth of the ice barrier, be it 100 or 1000 m. Arndt et al. (2014)
report iceberg scourmarks down to 1200 m just south of Fram
Strait. A surface ice barrier here would lead to an initial (glacio)
lacustrine circulation in an upper part of the AO by simple
replacement of seawater, with no mixing required.
Once a surface freshwater layer is in place, a different
mechanism comes into play, which has been overlooked by
Hillaire‐Marcel et al.: lakes have a fundamentally different
circulation from marine systems because freshwater is densest
at 4°C (at atmospheric pressure), not at the temperature
minimum. In temperate and polar freshwater systems this is
known to cause seasonal vertical turnover events, with a
collapsing stratification when water warmer than 4°Cis
cooled. In very deep lake systems like Lake Baikal (1642 m
depth), water density peaks around 3.2°C at depth, due to
pressure effects. There, already subtle differences in particulate
or dissolved solids, even variations in Si(OH)
4
, lead to riverine
inputs reaching depths >1000 m within weeks (Hohmann
et al., 1997). Therefore, river discharge, once cooled to 3–4°C
by melting ice, will inject freshwater and sensible heat at the
bottom of a colder freshwater column. For comparison, just
©2022 The Authors. Journal of Quaternary Science Published by John Wiley & Sons Ltd.
This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.
*
Correspondence: W. Geibert, as above.
E‐mail: walter.geibert@awi.de
Yukon and McKenzie today, both in catchment areas with
average annual air temperatures below 0°C and mostly
permafrost, deliver peak discharge around 15–17°C, together
supplying >5.3 PWh (Petawatt hours) per year to the AO (Yang
et al., 2014). East Siberian rivers have similar peak tempera-
tures (Lammers et al., 2007). The AO is therefore not a closed
system with respect to energy, especially if one allows–in good
agreement with our previous conservative estimate–an annual
liquid freshwater input of 2700 km
3
per year.
Assuming very conservatively that the rivers delivered (on
average) water with a temperature difference of 4°C compared
with dense saline waters to depth, we already calculate more
than 12 PWh energy as sensible heat that is annually introduced
to the system. This equates to >1379 GW, as compared with the
25 GW Hillaire‐Marcel et al. allow for tidal mixing. Freshwater of
3–4°C must have been delivered quickly to depth, protected from
further energy loss to the atmosphere, then flowing or
accumulating along the deep freshwater/seawater boundary.
This aspect stands in contrast to Hillaire‐Marcel's statement,
‘After isolation of the AO&NS basins, freshwater water input,
which would necessarily be near the water column surface,
would result in even more stratified water masses, …’.
Upon contact with colder seawater, the densest freshwater
would expand and, counter‐intuitively, rise when cooled,
unless salinity is entrained, creating an efficient energy and
salt transfer at the boundary, accompanied by mixing. It is
also worth noting that if deep mixing happens at any site
within the AO to great depth, most likely at the ocean margin,
baroclinic exchange will supply salt laterally from the central
parts. It is therefore not only the mixing in the centre that
controls the events, but the mixing and advection at the
boundaries.
Furthermore, a glaciolacustrine system, in contrast to a lake,
is constrained by rough topography not just at its bottom, but
also at the upper boundary of the liquid water phase, with
strong currents evolving along glacier fronts, episodic surges of
water masses from below the glacier and strong tidal currents
(Makinson and Nicholls, 1999). The situation only gets more
complex when estuarine aspects, as present here, are to be
considered as well. A further simplification of their model
concerns the volumes mixed during freshening. Most of the
fresh upper water column will not interact with the saline layer
below, making the incremental full water column mixing an
unsuitable mathematical approach. In addition, advective
transport would need to be taken into account for this system
with a directed flow.
For all the reasons outlined above, we believe that
transferring diapycnal mixing values from the deep open
ocean with smooth topography to a glaciolacustrine (and
partly estuarine system), surrounded by near margins, with
relatively warm riverine freshwater input flowing at depth, is
inappropriate. Given that the physical considerations neglect
such important sources of energy and processes for the system,
it is reassuring that Hillaire‐Marcel et al. come up with
estimates for mixing of the entire AO basin that are already not
too far from what we proposed.
Regarding the potential sources of freshwater, the direction of
the arguments is not entirely clear to us. First, the authors
introduce a refined estimate of the precipitation that may reach
the AO in glacials, all of which needs to be removed via the GSR:
6320 km
3
/year, and they give an upper limit of 2700 km
3
/year
liquid water input (compared with 3220 km
3
/year today). Below,
however, they indirectly also dispute significant riverine inputs –
due to summer temperatures below freezing point, citing
‘Kageyama et al.’–a statement which we find neither supported
there, nor by modern river equivalents. For detailed calculations
of the substantial riverine input from East Siberia and Beringia we
refer to Alkama et al. (2006). Summer air temperatures were
typically just 0–4°C lower than today in peak glacials at this
latitude band (Kageyama et al., 2019), as confirmed in the
©2022 The Authors. Journal of Quaternary Science Published by John Wiley & Sons Ltd. J. Quaternary Sci., 1–4 (2022)
Figure 1. The (modern) Arctic Ocean and Nordic Seas in Polar map projection for orientation (top), and the Greenland–Scotland Ridge, with
shallow depths highlighted in grey (bottom). DS: Denmark Strait; FBC: Faroe Bank Channel; FSC: Faroe–Shetland Channel; IFG: Iceland–Faroe Gap.
Areas depicted in grey could not be passed by icebergs of >470 m total thickness, areas in white were blocked by any ice thicker than 720 m. Ice
sheets from Greenland, Iceland and Scandinavia calved into this region. Red arrows indicate examples of deep iceberg scourmarks (800–1200 m
water depth) or ice shelf traces (Lomonossov Ridge, close to 1000 m) [Color figure can be viewed at wileyonlinelibrary.com].
2 JOURNAL OF QUATERNARY SCIENCE
palaeobotanical records from Lake El'gygytgyn (Melles et al.,
2012). Today, substantial liquid (and warm) discharge is seen
even for subzero annual mean temperatures (Yang et al., 2014).
In addition, extensive subglacial sources need to be considered
(Montelli et al., 2020). Taking the total input of liquid meteoric
water as described here and ice together, because both displace
saline water, the mean residence time of water in the AO is
around just 2000 years.
The geochemical viewpoint
Hillaire‐Marcel et al. include in their explanations for low
230
Th
ex
,‘Under full glacial conditions with a ~800 m‐thick ice
shelf, as estimated above, possibly underlain by a low salinity
layer,
230
Th
xs
was close to nil at such sites.’This is exactly our
original argument, so we find the physical plausibility of our
finding in principle supported by their comment. The debate
then would condense to a discussion about the possible
thickness of a freshwater layer under an ice shelf in the
glacial AO.
Yet, they also introduce a number of other arguments. In
brief, we argued in the original publication that:
(1) Sufficient dilution to suppress
230
Th
ex
to nil would require
much higher sedimentation rates.
(2) Low sedimentation rates as seen in the AO are globally
associated with the highest
230
Th
ex
signals in saline systems
(Yang et al., 1986), even for sedimentation rates lower than
in the AO.
Figure 2 illustrates the expected
230
Th
ex
activities from
production in a saline water column, for variable water depths
and sedimentation rates. These values agree well with surface
sediment data from the Pacific Ocean (Yang et al., 1986). Hillaire‐
Marcel et al. now invoke a scenario in which both high and
extremely low particle fluxes occur during low
230
Th
ex
intervals,
pulsed and regionally distributed. We consider this scenario to be
inconsistent with the lithology of the
230
Th
ex
‐free intervals, which
are characterised by a mixture of coarse and fine particles. Even a
short particle pulse would be enough to remove
230
Th from the
water column. Even fine particle fluxes generating sedimentation
rates <1 mm/1000 years lead to >100‐fold
230
Th concentrations
elsewhere (Yang et al., 1986). Even an absence of sedimentation –
a hiatus –would not be visible as an absence of
230
Th
ex
activities
in sediment because then no sediment was deposited at all.
Another possibility to lose
230
Th would be water export from the
AO down, limited by the sill depth of Fram Strait; or one could
also build it up in the water column of the AO and remove it later
by particle flux. Again, both cannot happen simultaneously as
Hillaire‐Marcel et al. suggest in their conceptual figure.
We find that Hillaire‐Marcel et al. combined all theoretically
possible explanations for reduced
230
Th fluxes (high particle
fluxes, low particle fluxes, export from the Arctic water
column, build‐up in the Arctic water column, and a specula-
tion on the role of organic compounds) into one scenario,
irrespective of internal consistency. None of the proposed
processes has the potential to reduce
230
Th to nil, as actually
observed, for the observed lithology, and in total, the proposed
effects are not additive, but they neutralise each other. We also
point to recent studies highlighting the role of terrigenous
material and hydrothermal particles for
230
Th scavenging in
the AO (Gdaniec et al., 2020, Valk et al., 2018, 2019).
Misunderstandings of our study
Right in the title, Hillaire‐Marcel et al. introduce our finding of a
largely freshwater‐filled AO&NS basin as the ‘Arctic lake’
hypothesis–a term we never used. Instead, we called it a
‘glaciolacustrine system which has no modern equivalent’for
good reason, as outlined above. This is not the only example of a
deviation from the original description. The authors summarise
our dating approach inadequately when stating, ‘This [broad
interpolation from
230
Th
ex
decay] has been the approach used by
Geibert and colleagues (2021)…’. In fact, we transferred the age
model from core PS1533 dated by a variety of constraints
(Spielhagen et al. (2004) and references therein) to other cores for
which only radiocarbon or
230
Th
ex
were available (Geibert et al.,
2021). Interpolation played no important role for our age
boundaries, but correlation of similar
230
Th
ex
profiles between
cores to a well‐dated core. We also took oxygen isotope curves
from outside the Arctic into account. It is also worth noting that in
the original paper, we had restricted the definitive occurrence of
freshwater to 2500 m, the deepest cores investigated, when we
noted, ‘…its almost complete absence, even below 2500 m water
depth. This implies that the water mass underlying the ice shelf
was fresh, not saline, at least to this depth.’
Conclusions
‘Occam's razor’favours simple explanations over complicated
ones. We need just one simple factor –the temporary absence of
sea salt–to allow a synthesis of many seemingly contrasting
observations from different disciplines within and outside the
Arctic. The alternative scenario of Hillaire‐Marcel et al. adds
several mutually counteracting mechanisms, and it neglects
evidence from within and beyond the Arctic sedimentary record.
Adopting our explanation, the behaviour of thorium in the Arctic
is no longer peculiar.
In summary, we therefore believe it would be premature to
close the discussion about a hypothesis that can offer additional
aspects to some long‐standing problems of Quaternary science
like the origin of Heinrich events, Dansgaard–Oeschger events,
or the tipping points for rapid melting at terminations, just based
on the simplified physical model of Hillaire‐Marcel et al.
Acknowledgements. We thank Hartmut Hellmer for a plausibility
check and advice regarding our energy calculations. Ingrid Stimac,
who is part of the original team of authors, is not part of the authors
here because her contribution was not discussed. She still supports our
finding. WG wrote this text in consultation with all coauthors. Open
Access funding enabled and organized by Projekt DEAL.
©2022 The Authors. Journal of Quaternary Science Published by John Wiley & Sons Ltd. J. Quaternary Sci., 1–4 (2022)
Figure 2. Modelled expected initial
230
Th
ex
activity in sediments for
selected (saline) water depths, assuming a
230
Th production rate of
0.0267 dpm m
‐3
a
‐1
, and a dry bulk density of 1 g cm
‐3
. Maximum
230
Th
ex
seen in the Arctic Ocean is well within the expected range and
does not indicate excessive accumulation of
230
Th in the Arctic
Ocean. The
230
Th
ex
minima <0.1 dpm g
‐1
, however, cannot be
explained with dilution as they would require excessive sedimentation
rates.
REPLY TO 'CHALLENGING THE HYPOTHESIS OF AN ARCTIC OCEAN LAKE DURING RECENT GLACIAL EPISODES' 3
Ethics statement
We state that questions of data availablility, conflicts of interest,
ethics approval, patient consent, permission to reproduce
material from other sources and clinical trial registration do not
apply here. Only public funding was received.
Abbreviations.
230
Th, thorium‐230;
230
Th
ex
(synonymous to
230
Th
xs
),
excess of the radioisotope thorium‐230 over its progenitor
234
U;
AO, Arctic Ocean; DS, Denmark Strait; FBC, Faroe Bank Channel;
FSC, Faroe–Shetland Channel; GSR, Greenland–Scotland Ridge;
IFG, Iceland–Faroe Gap; NS, Nordic Seas; PWh, Petawatt hours.
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4 JOURNAL OF QUATERNARY SCIENCE
