Platelet Ice Under Arctic Pack Ice in Winter

Abstract

Plain Language Summary
Platelet ice is a particular type of ice that consists of decimeter sized thin ice plates that grow and collect on the underside of sea ice. It is most often related to Antarctic ice shelves and forms from supercooled water with a temperature below the local freezing point. Here we present the first comprehensive observation of platelet ice formation in freely drifting pack ice in the Arctic in winter during the international drift expedition MOSAiC. We investigate its occurrence under the ice with a remotely controlled underice diving robot. Measurements of water temperature from automatic measurement devices distributed around the central MOSAiC ice floe show that supercooled water and thus platelet ice occur widely in the winter Arctic. This way of ice formation in the Arctic has been overlooked during the last century, as direct observations under winter sea ice were not available and contrary to typical Antarctic observations; manifestation of platelet ice in Arctic ice core stratigraphy has been more challenging to identify.

Full text

Platelet Ice Under Arctic Pack Ice in Winter
Christian Katlein
1
, Volker Mohrholz
2
, Igor Sheikin
3
, Polona Itkin
4
, Dmitry V. Divine
5
,
Julienne Stroeve
6,7
, Arttu Jutila
1
, Daniela Krampe
1
, Egor Shimanchuk
3
, Ian Raphael
8
,
Benjamin Rabe
1
, Ivan Kuznetsov
1
, Maria Mallet
1
, Hailong Liu
9
, Mario Hoppmann
1
,
Ying‐Chih Fang
1
, Adela Dumitrascu
10
, Stefanie Arndt
1
, Philipp Anhaus
1
,
Marcel Nicolaus
1
, Ilkka Matero
1
, Marc Oggier
11
, Hajo Eicken
11
, and Christian Haas
1
1
Alfred‐Wegener‐Institut Helmholtz‐Zentrum für Polar‐und Meeresforschung, Bremerhaven, Germany,
2
Leibniz
Institute for Baltic Sea Research, Rostock, Germany,
3
Arctic and Antarctic Research Institute, St. Petersburg, Russia,
4
UiT
University of Tromsø, Tromsø, Norway,
5
Norwegian Polar Institute, Tromsø, Norway,
6
University College of London,
London, UK,
7
Center for Earth Observation Science, Department of Environment and Geography, University of
Manitoba, Winnipeg, Manitoba, Canada,
8
Thayer School of Engineering, Dartmouth College, Hanover, NH, USA,
9
Shanghai Jiao Tong University, Shanghai, China,
10
University of Gothenburg, Gothenburg, Sweden,
11
International
Arctic Research Center, University of Alaska Fairbanks, Fairbanks, AK, USA
Abstract The formation of platelet ice is well known to occur under Antarctic sea ice, where subice
platelet layers form from supercooled ice shelf water. In the Arctic, however, platelet ice formation has
not been extensively observed, and its formation and morphology currently remain enigmatic. Here, we
present the first comprehensive, long‐term in situ observations of a decimeter thick subice platelet layer
under free‐drifting pack ice of the Central Arctic in winter. Observations carried out with a remotely
operated underwater vehicle (ROV) during the midwinter leg of the MOSAiC drift expedition provide clear
evidence of the growth of platelet ice layers from supercooled water present in the ocean mixed layer. This
platelet formation takes place under all ice types present during the surveys. Oceanographic data from
autonomous observing platforms lead us to the conclusion that platelet ice formation is a widespread but yet
overlooked feature of Arctic winter sea ice growth.
Plain Language Summary Platelet ice is a particular type of ice that consists of decimeter sized
thin ice plates that grow and collect on the underside of sea ice. It is most often related to Antarctic ice
shelves and forms from supercooled water with a temperature below the local freezing point. Here we
present the first comprehensive observation of platelet ice formation in freely drifting pack ice in the Arctic
in winter during the international drift expedition MOSAiC. We investigate its occurrence under the ice with
a remotely controlled underice diving robot. Measurements of water temperature from automatic
measurement devices distributed around the central MOSAiC ice floe show that supercooled water and thus
platelet ice occur widely in the winter Arctic. This way of ice formation in the Arctic has been overlooked
during the last century, as direct observations under winter sea ice were not available and contrary to typical
Antarctic observations; manifestation of platelet ice in Arctic ice core stratigraphy has been more
challenging to identify.
1. Introduction
Platelet ice is a characteristic feature of Antarctic landfast sea ice, where supercooled ice shelf waters lead to
the advection and growth of subice platelet layers (Hoppmann et al., 2020). They consist of loosely attached
decimeter sized plate‐shaped ice crystals (Hoppmann et al., 2017; Langhorne et al., 2015; Smith et al., 2001)
and can be up to several meters thick. These ice platelets form by nucleation in supercooled layers of sea-
water either at depth (Dieckmann et al., 1986) or directly at the ice underside (Leonard et al., 2006;
Mahoney et al., 2011) in the vicinity of large ice shelves, which provide supercooled water due to basal ice
shelf melt in the water circulation of ice shelf cavities. The porous structure provides shelter for a particular
ice associated ecosystem (Arrigo et al., 2010; Günther & Dieckmann, 2004; Vacchi et al., 2012) and is thus
important for biogeochemical cycles (Thomas & Dieckmann, 2002).
As ice shelves are much less common in the Arctic (Dowdeswell & Jeffries, 2017), observations of platelet ice
in the Arctic are rare, and the processes causing its formation are poorly understood. The availability of
©2020. The Authors.
This is an open access article under the
terms of the Creative Commons
Attribution License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.
RESEARCH LETTER
10.1029/2020GL088898
Key Points:
•Here we present extensive
observations of platelet ice
formation under Arctic winter sea
ice
•The subice platelet layer appears to
form locally due to seed crystals in
ocean surface supercooling
Supporting Information:
•Supporting Information S1
Correspondence to:
C. Katlein,
ckatlein@awi.de
Citation:
Katlein, C., Mohrholz, V., Sheikin, I.,
Itkin, P., Divine, D. V., Stroeve, J., et al.
(2020). Platelet ice under Arctic pack
ice in winter. Geophysical Research
Letters,47, e2020GL088898. https://doi.
org/10.1029/2020GL088898
Received 15 MAY 2020
Accepted 10 AUG 2020
Accepted article online 17 AUG 2020
KATLEIN ET AL. 1of10

supercooled water plays a central role for the growth of decimeter scale ice platelets (Lewis & Perkin, 1983,
1986; Weeks & Ackley, 1986). Jeffries et al. (1995) presented one of the few descriptions of platelet ice in the
Arctic Ocean. Their study identified platelet ice crystals in 22 out of 57 ice cores collected in the Beaufort Sea
during August and September 1992 and 1993. They suggest four different sources for supercooled water, two
of which require the presence of ice shelves and coastal interactions and are therefore not relevant for the
central Arctic Ocean. The other two include small scale “ice pump”mechanisms (Lewis & Perkin, 1983,
1986) and the interaction of summer meltwater with the underlying colder seawater, leading to the
formation of false bottoms in underice melt ponds and platelet ice crystals (Eicken, 1994; Martin &
Kauffman, 2006; Notz et al., 2003). They describe platelet ice as a widespread feature in the Beaufort Sea
based on their ice core analysis. Carnat et al. (2017) describe two cores with platelet ice signature. Early
observations from Lewis and Lake (1971) stay vague in the description but show that the phenomenon is
not new. The Russian drifting Station NP‐2015 also detected platelet formation caused by meltwater
percolation through the ice cover (personal communication I. Sheikin), and an indirect observation under
fast ice in summer was described by Kirillov et al. (2018).
Subice platelet layers can be separated from frazil ice in such way that the geometric size of the platelet ice
crystals is on the order of 1–10 cm. Frazil ice describes the crystal habit resulting from the initial stages of sea
ice growth, when small disk and needle‐like crystals smaller than 1 cm appear suspended in the upper water
column or at the ocean surface (Hoppmann et al., 2020; Weeks & Ackley, 1986; Zubov, 1963). Subice platelet
layers exhibit a rather random orientation of crystal axes. This is significantly different from the skeletal
layer at the bottom of growing sea ice, where parallel oriented ice lamellae are growing into a microscale
layer of constitutionally supercooled water caused by the brine expulsion during sea water freezing
(Lofgren & Weeks, 2017; Rutter & Chalmers, 1953; Shokr & Sinha, 2015).
No extensive direct in situ observations of platelet ice under Arctic sea ice particularly during winter are
available. Anecdotal reports from divers, such as during the Tara expedition (Ragobert & Roblin, 2008) or
the “Under the Pole”diving expedition (Bardout et al., 2011), allude that this feature has been mostly over-
looked in the Central Arctic. Figure S1 and Table S1 in the supporting information provide an overview of
previous observations.
Here, we present the first extensive, more systematic in situ observations of growing subice platelet layers
under Arctic sea ice in winter. Dives with a remotely operated vehicle (ROV) during the international
Arctic drift expedition “Multidisciplinary Observatory for the Study of Arctic Climate”(MOSAiC) from
Figure 1. (a) Drift track of MOSAiC floe in the Central Arctic Ocean from October 2019 to mid‐May 2020. Black dots denote start and end of Drift Legs 1, 2, and 3,
respectively. Platelet ice was observed between 30 December 2019 and 28 March 2020 (black highlighted track). (b) Map of ice draft derived from multibeam
sonar survey on 21 January 2020 with most prominent locations of the ubiquitous platelet ice observations (gray symbols), brinicles (light blue symbols), and ice
core samples (red stars). White letters indicate the position of the ROV access hole (RC) and the MSS deployment hole (OC). Red letters refer to ice cores taken at
the ROV site (R), the ice mechanics site (M), and the ridge site (F).
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January to March 2020 around 88°N (Figure 1) revealed a widespread coverage of decimeter scale platelet ice
crystals growing on and under the bottom of the ice.
2. Materials and Methods
2.1. Study Area
The ice floe of the MOSAiC drift experiment of the German research icebreaker Polarstern (Knust, 2017)
consisted of a conglomerate of various ice types, out of which deformed second year ice and relatively level
residual ice (first year ice grown into a remaining matrix of very rotten melted ice; WMO, 2014) were the
most abundant. Initial ice thicknesses during the mobilization of the drift station in the beginning of
October 2019 were as little as 20–30 cm for the residual ice and around 60–80 cm for the undeformed second
year ice (Krumpen et al., 2020). By March, ice growth had increased the level ice thickness to about 145 cm
for the residual ice and around 200 cm for the second year ice (Figure S2). Pressure ridges with typical keel
drafts of 5–7 m and maximum of 11 m characterized the deformed ice. More details about the composition
and history of the MOSAiC floe can be found in Krumpen et al. (2020).
2.2. ROV Operations
We carried out ROV dives from a hole through the ice covered by a heated tent. The M500 ROV (Ocean
Modules, Atvidaberg, Sweden) was equipped with a comprehensive sensor suite including cameras as well
as a 240 kHz multibeam sonar (Katlein et al., 2017) and provided an operating range of 300 m from the access
hole. We documented platelet ice occurrences mostly with four cameras: a high‐definition zoom video cam-
era (Surveyor HD, Teledyne Bowtech, Aberdeen, UK), two standard definition video cameras (L3C‐720,
Teledyne Bowtech, Aberdeen, UK), and a 12 megapixel still camera (Tiger Shark, Imenco AS, Haugesund,
Norway).
The ROV dives covered many different sites, but several places were revisited (Figure 1b) due to repeating
routine dive missions allowing for a temporal assessment of platelet ice evolution. On 15 February 2020,
we towed an underice zooplankton net (ROVnet) with the ROV directly along the ice underside
(Wollenburg et al., 2020) to brush off platelet ice samples for structural analysis. In the lab, platelets were
frozen into a solid block of ice by adding sea water to the sample container, in order to later analyze the pla-
telet ice crystal structure.
2.3. Ice Core Sampling and Analysis
We extracted ice cores in three locations (Figure 1b) where subice platelet coverage had been previously con-
firmed by ROV imagery. We analyzed them for ice texture by preparing thin sections using the Double
Microtoming Technique (Eicken & Salganek, 2010; Shokr & Sinha, 2015) in the lab on board. We photo-
graphed the thin sections between crossed polarizers to identify crystal geometric properties. To associate
an approximate date of ice formation to each ice sample along the core, we used a simple ice‐growth model
based on the number of freezing‐degree days (Pfirman et al., 2004), forced by air temperatures recorded by
the Polarstern weather station.
2.4. Physical Oceanographic Measurements
We measured vertical and horizontal profiles of seawater conductivity, temperature, and pressure (CTD)
using three independent different types of platforms. One CTD sensor was mounted on the ROV (GPCTD,
SeaBird Scientific, USA), while we performed recurring deployments of a free‐falling microstructure sonde
(MSS 90LM, Sea and Sun Technologies, Trappenkamp, Germany) through a nearby hole in the ice
(Figure 1b). In addition, several autonomous stations with CTD packages at a depth of 10 m (SBE37,
SeaBird Scientific, USA) were operational in the MOSAiC distributed network at distances of 10–40 km from
the central floe (Figure S3). All devices were calibrated by the manufacturers immediately before the expedi-
tion. The respective measurement uncertainties are discussed in Text S1.
3. Results and Discussion
3.1. Subice Platelet Layer Morphology
We observed a 5 to 30 cm‐thick subice platelet layer covering the ice bottom as shown in Figure 2. The ice
platelets are composed of blade‐or disc‐shaped single ice crystals with c‐axis alignment normal to the
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platelet surface. Most platelets were firmly attached to their substrate but fragile to physical impact by the
ROV. When observed on ropes or chains, platelet ice crystals were tightly grown through their structure
(Figures 2b and S4) and not just loosely attached to the respective surface. This indicates that these
platelets grew on site and have not been advected in from deeper waters or horizontally as already
suggested by Lewis and Lake (1971). Contrary to Antarctic fast ice, we did not find meter‐thick layers of
platelet ice accumulation (Hoppmann et al., 2017; Hunkeler et al., 2016), possibly due to slower platelet
or faster congelation growth. The freezing front of the congelation ice quickly progressed downward into
the subice platelet layer and incorporated it by congelation ice growth in between the platelet crystals
(Dempsey et al., 2010). A thickness difference between Arctic and Antarctic subice platelet layers was
already proposed by Lewis and Perkin (1986) based on different driving depths in the ice pump mechanism.
We identified crystal sizes up to approximately 15 cm from the ROV camera footage. Maximum crystal size
retrieved with the towed zooplankton net was 9 cm, while the thicknesses of platelet crystals ranged from
0.8–2.5 mm. However, due to the limited size of the sampling bottle with a diameter of 10 cm and the phy-
sical interaction of the ROVnet (0.4 by 0.6 m opening) and platelet ice structures, platelets may well have
been broken during the sampling process.
Platelet ice growth depends on available crystallization nuclei or seed crystals for secondary nucleation.
Probably due to this reason, we did not observe platelet growth on the polymer‐covered thermistor strings
hanging in the water column. The complex structure of core‐mantle polyamide rope or metal parts provided
sufficient crystallization nuclei for platelet formation (Figures 2d and S4). Another explanation could be
material‐dependent adhesion of seed crystals as described in Robinson et al. (2020). This was particularly
obvious on 15 February 2020, when the ROV had been hanging for 3 days in 2 m water depth and was cov-
ered in up to 30 mm large platelet crystals on edges and corners, while particularly smooth plastic surfaces
were unaffected by platelet growth (Figure S5).
3.2. Spatial Distribution of Platelets
Platelet ice coverage was ubiquitous in the entire observational range of the ROV. However, platelet ice
growth was almost exclusively observed in the uppermost part of the water column, above a depth of 2–
Figure 2. (a) Close‐up picture of platelet ice covering a ridge block. The ROV manipulator opening in the foreground is about 9 cm wide. (b) Rope sling next to a
pressure ridge: Both the rope and the ridge are vastly covered in ice platelets. (c) Upward growing platelet ice in a ridge cavity. (d) Platelet ice crystals
covering the rope and chain of underwater installations. Note the lack of platelet growth on the plastic marker stick and the coverage of small platelet crystals
underneath the level ice.
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3 m. Deeper lying ridge keels as well as deep hanging ropes and instrument installations were not covered in
platelet ice. Few installations exhibited a vertical gradient of platelet ice growth coverage, with the most
extensive occurrence at the ice water interface and diminishing platelet cover toward depth (Figure S6).
This has been observed similarly in the Antarctic (Dayton et al., 1969; Hoppmann et al., 2020; Mahoney
et al., 2011). Platelet crystals were largest (up to 15 cm) and most prominent on blocks, ridges, and edges pro-
truding from the level ice, but at close inspection, we found also smaller‐scale platelet ice crystals (1–2 cm)
throughout the bottom of level ice. Also, these smaller platelets appeared different from ice lamellae
expected in the skeletal layer. We identified no significant spatial difference in underice roughness (and thus
platelet coverage) from acoustic backscatter derived from the multibeam sonar measurements (Figure S7).
While sheltered areas between ridge keels with low currents seemed to provide best conditions for platelet
growth, we observed significant platelet growth of similar size also at locations that were completely exposed
to the ice‐relative currents (Figure S4) and more than 100 m away from any significant ice feature. Lewis and
Milne (1977) attribute the presence of subice platelet layers to cracks or pressure ridges. While this seems to
coincide with the locations of our most prominent observations, we also observed platelet ice far away from
such features and can thus neither prove nor rule out the ridge associated ice‐pump mechanism of platelet
formation as predicted by Lewis and Perkin (1986).
We found no direct link between platelet ice distribution and brine drainage features. Despite the occasional
observation of brinicles—ice stalactites forming from the contact of descending, cold brine with seawater
(Lewis & Milne, 1977)—we encountered them both with and without intense platelet ice cover (Figure S8).
3.3. Temporal Variability
During MOSAiC, the ROV diving schedule only allowed for a weekly cycle of repeated visits (Figure S9).
Therefore, our information on the temporal variability of platelet ice occurrence is limited and less objective.
However, we could identify clear differences in the amount of new platelet ice formation between different
periods. These periods were characterized either by new crystal growth, the lack of such, or even a perceived
reduction in platelet ice cover. They are identified in Figure 3 to investigate a link between oceanographic
conditions and platelet ice formation. As the ROV sampling in the described location only started on 31
December 2019, we cannot provide a detailed assessment of the situation before. However, we observed
no platelet ice during ROV dives before 6 December 2019 in a different location approximately 1 km away.
We observed platelet ice for the last time during an ROV dive on 28 March 2020, after the floe had been
affected by deformation and the return of sunlight. This coincides with the time, when water temperatures
under the ice climbed above the local freezing point again (Figure 3c).
3.4. Supercooling
We found supercooled water, the basis for platelet ice formation, well below the ice water interface, which
we confirmed using three different independent measurement platforms. Temperature and salinity data
from the ROV, a free‐falling Microstructure Sonde (MSS), and several autonomous CTDs deployed at
10 m depth in 10–40 km distance from the ROV site all revealed water temperatures around 0.01–0.02 K
below the respective seawater freezing point in the uppermost mixed layer (Figure 3a). This degree of super-
cooling is similar to observations from the Antarctic (Mahoney et al., 2011) and larger than the calibration
uncertainty and uncertainties in the calculation of the local freezing point of seawater. Hence, we can con-
firm the existence of supercooled water several meters thick as prerequisite for platelet ice formation (Smith
et al., 2001). Measurement uncertainties might however obscure the absolute magnitude and depth of ocean
surface supercooling.
Within the mixed layer, the local seawater freezing point is pressure and therefore depth dependent, while
temperature and salinity values are approximately constant. Thus, freezing‐point departure increases
toward the surface with a higher level of supercooling in the uppermost mixed layer right under the ice
(Figures 3a and 3b). This can explain the observed decrease in platelet ice abundance below 2 m depth.
A simple hypothesis for platelet ice growth might thus be that water molecules attach to existing crystalliza-
tion nuclei, for example, at the ice underside as soon as they are in a strong enough state of supercooling.
Considering the turbulent nature of the mixed layer, where water particles get mixed up and down through
the entire mixed layer at a time scale of 30 min (Denman & Gargett, 1983), they oscillate between super-
cooled and nonsupercooled states. Thus, we hypothesize that platelet ice formation is only possible as
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soon as the temperature in the complete mixed layer lies below the vertically averaged seawater freezing
point. This can be either achieved by excessive atmospheric cooling during the Arctic winter (Danielson
et al., 2006; Skogseth et al., 2017) or due to a sudden shoaling of the mixed layer, cutting off mixing
beyond a certain depth, so that suddenly, most of the surface mixed layer has a temperature below the
freezing point causing respective formation of platelet ice. Platelet ice could also originate from frazil
crystals generated in the water column (Robinson et al., 2020; Skogseth et al., 2017) that rise up and
attach to the surface. If present, free‐floating frazil ice crystals should have been easily detected in light
beams used for ROV surveys or Secchi‐disk casts. No such enhanced light scattering by ice crystals was
observed, but we might have missed it particularly due to temporal limitations of the sampling schedule.
Another plausible explanation for platelet formation lies in the “ice‐pump”mechanism (Lewis &
Perkin, 1983, 1986): Descending salty brines generated by strong atmospheric cooling in leads or even
under a completely closed ice cover can melt deep lying ridge keels and thus supercool the water column
and respectively generate platelet ice. Determining the exact nature of the processes involved in the
temporally varying strength of platelet ice formation would require more targeted high temporal
resolution investigations of platelet growth than could be accomplished during the rigid observational
plan for MOSAiC.
Time series of MSS and autonomous observations show that the detected levels of platelet ice were only
apparent after a more temporally stable mixed layer with a depth of ~30 m had established in mid‐
December. Furthermore, the perceived decrease in platelet ice coverage observed in mid‐February was likely
linked to a passing eddy, decreasing the freezing‐point departure in the upper mixed layer (Figure 3b).
Observations of autonomous CTD sensors deployed in the distributed network at 10 to 40 km distance from
the central MOSAiC floe (Figure S3) consistently show similar amounts of ocean surface supercooling
(Figure 3c). This allows the conclusion that platelet ice formation under Arctic winter sea ice is not a local
curiosity, but a widespread, overlooked feature in the Central Arctic Ocean.
Figure 3. (a) Salinity, temperature, and freezing point departure observed by the ROV on 22 February 2020. (b) MSS time series of water temperature above the
surface freezing point. Note the consistent deepening of the supercooled layer indicated in blue color. (c) Time series of freezing‐point departure measured in 10 m
depth (and adjusted to 2 m depth in gray) from the autonomous observation stations. Vertical lines indicate platelet ice intensity observations classified as high
(solid lines), normal (thick dotted lines), and low intensity (dashed lines) based on visual ROV observations. Thin dotted lines indicate ROV surveys without
platelet ice observation. See Figure S3 for geometric location of stations relative to the central observatory.
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3.5. Persistence in Ice Core Analysis
Despite the ubiquitous occurrence of platelet ice shown in our study, there is a general lack of extensive signs
of platelet ice formation in the texture of Arctic sea ice cores of the Transpolar Drift (Tucker et al., 1999). To
further investigate, we retrieved ice cores at three locations (Figure 1b) where we had documented platelet
ice beforehand with the ROV cameras. In contrast to most Antarctic landfast ice cores, all of the investigated
ice core bottom thin sections (Figure S10) showed only weak signs of incorporated platelet ice. Rapid conge-
lation ice growth of 5–9 cm per week might have concealed a more obvious signature of platelets (Dempsey
et al., 2010; Gough et al., 2012). However, in various places we found a few large, inclined crystals interpreted
as originating from platelet crystals. Moreover, during the first leg of MOSAiC at the end of November 2019,
an ice core retrieved at the second‐year ice site contained more clearly identifiable sections of platelet ice
(Figure S11). Thin section analysis indicates substantial microstructural and textural similarities with litera-
ture reports of Antarctic platelet ice (Jeffries et al., 1995; Langhorne et al., 2015; Leonard et al., 2006; Smith
et al., 2001).
To investigate this more closely, we analyzed the texture of collected platelet crystals refrozen into seawater.
The resulting texture (Figure S12) looks significantly different from the one described for freshwater‐derived
platelet ice by Jeffries et al. (1995). In particular, platelet ice crystals seen from the side have a rectangular
rather than triangular shape, and also, many platelet crystals exhibit subgrain boundaries, which are
described as absent in the work of Jeffries et al. (1995).
We thus have two hypotheses why these ubiquitous platelet ice crystals under Arctic winter sea ice do not
leave a strong record in the texture of ice cores. First, despite their spectacular voluminous appearance,
the ice platelets actually only take up a small volume fraction, so that it is unlikely to observe multiple plate-
let crystals in a submillimeter thick ice core thin section. This has been found also for Antarctic platelet ice
incorporated into fast growing congelation ice (Dempsey et al., 2010; Gough et al., 2012). Second, the platelet
crystals may serve as primary nucleation surfaces also for the congelation growth in a way that obscures
their initial origin. Both hypotheses could explain why such a widespread cover of subice platelet layers in
the winter Arctic has been overlooked in the last decades of sea ice texture investigations.
3.6. Physical, Ecological, and Biogeochemical Implications
Considering large‐scale energy fluxes and the thermodynamics of sea ice growth, platelet ice formation
under Arctic sea ice in winter does likely not affect the thermodynamics of sea ice growth significantly.
This is particularly due to Arctic platelet ice being a local seasonal phenomenon maintaining a closed energy
budget. In contrast, Antarctic platelet ice is often derived from water masses with spatially different origin
and thus disrupting the local energy budget. Even though the impact may be small for ice‐ocean physics,
the porous, ragged structure of the platelet ice interface does affect small‐scale roughness of the ice underside
and will in particular affect the entrainment of water constituents, such as sediments, nutrients, or biological
assemblages. One sample of subice platelets from the ROVnet showed elevated levels of halocarbons com-
pared to the general ice column, meaning this subice platelet layer could play a role also in different biogeo-
chemical cycles. Despite the assumed inactivity of the underice ecosystem during polar night, platelet ice
might still serve as a substrate for algal growth and protection for underice macrofauna, as we observed
amphipods maneuvering through the maze of crystal blades (Figure S13).
Platelet ice could also play a significant role in the poorly understood consolidation of voids, for example, in
sea ice ridges, where it would be able to close large gaps faster than by pure congelation ice growth. This
could explain why voids in ridge keels often appeared slushy when drilled through during MOSAiC
(Figure S14).
While platelet ice observations in the Arctic date back to the 1970s (Lewis & Milne, 1977), the thinner (Haas
et al., 2008; Kwok & Rothrock, 2009) and more dynamic sea ice (Kwok et al., 2013) of recent years might
increase rapid cooling of Arctic surface waters and thus promote platelet ice formation.
4. Summary
During the polar night of the international drift expedition MOSAiC in 2019–2020, we observed a wide-
spread coverage of the ice underside with a subice platelet layer. These up to 15 cm large platelet ice crystals
grew in situ from supercooled water of the uppermost mixed layer, both on exposed ice features and level ice.
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This is the first comprehensive in situ observation of subice platelet layer formation during Arctic winter in
the free‐drifting ice of the Central Arctic. As historic observations show, this is not a new phenomenon, but
only modern robotic equipment at a winter drift ice station allowed for its detailed observation.
Platelet ice formation has been overlooked so far as a widespread feature of ice growth during Arctic winter.
Our study provides the first observational evidence for a link between platelet growth intensity, mixed layer
stability, and supercooling, but the detailed processes with respect to their seasonal impacts on ice‐ocean
interactions are yet to be understood. In particular, we were able to show that this subice platelet layer does
not always leave a clear imprint on sea ice texture and was hence easily overlooked in past ice core analyses
(Figure S15).
The potential importance of subice platelet layers for the ice‐associated ecosystem and biogeochemical
fluxes during Arctic winter should be investigated more closely in the future. To improve our understanding
of the involved physical processes, we suggest a more targeted investigation during future Arctic winter cam-
paigns with the goal to achieve higher temporal resolution and more objective observations of platelet crystal
growth. This could be achieved by fixed underwater cameras in relation to water dynamics, potential ridge
keel melting, and thermodynamics in the mixed layer.
Data Availability Statement
Data used in this manuscript were produced as part of the international Multidisciplinary drifting
Observatory for the Study of the Arctic Climate (MOSAiC) with the tag MOSAiC20192020. All data are
archived in the MOSAiC Central Storage (MCS) and will be available on PANGAEA after finalization of
the respective data sets according to the MOSAiC data policy. Screenshots from ROV video (Katlein,
Krampe, & Nicolaus, 2020), acoustic backscatter (Katlein, Anhaus, et al., 2020b), ice core data (Katlein,
Itkin, & Divine, 2020), and ROV CTD data (Katlein, Anhaus, et al., 2020a) are already available on
PANGAEA. Oceanographic data from autonomous platforms 2019O1–201908 can be accessed online (at
seaiceportal.de). Ice and snow thickness data were kindly provided by Stefan Hendricks.
References
Arrigo, K. R., Mock, T., & Lizotte, M. P. (2010). Primary producers and sea ice. In G. S. Dieckmann, & N. D. Thomas (Eds.), Sea ice
(pp. 283–325). Wiley. https://doi.org/10.1002/9781444317145.ch8
Bardout, G., Périé, E., & Poyelle, B. (2011). On a marché sous le pôle: Deepsea under the Pole. Paris: Chêne.
Carnat, G., Papakyriakou, T., Geilfus, N. X., Brabant, F., Delille, B., Vancoppenolle, M., et al. (2017). Investigations on physical and textural
properties of Arctic first‐year sea ice in the Amundsen Gulf, Canada, November 2007–June 2008 (IPY‐CFL system study). Journal of
Glaciology,59(217), 819–837. https://doi.org/10.3189/2013JoG12J148
Danielson, S., Aagaard, K., Weingartner, T., Martin, S., Winsor, P., Gawarkiewicz, G., & Quadfasel, D. (2006). The St. Lawrence polynya
and the Bering shelf circulation: New observations and a model comparison. Journal of Geophysical Research,111, C09023. https://doi.
org/10.1029/2005JC003268
Dayton, P. K., Robilliard, G. A., & Devries, A. L. (1969). Anchor ice formation in McMurdo Sound, Antarctica, and its biological effects.
Science,163(3864), 273–274. https://doi.org/10.1126/science.163.3864.273
Dempsey, D. E., Langhorne, P. J., Robinson, N. J., Williams, M. J. M., Haskell, T. G., & Frew, R. D. (2010). Observation and modeling of
platelet ice fabric in McMurdo Sound, Antarctica. Journal of Geophysical Research,115, C01007. https://doi.org/10.1029/2008JC005264
Denman, K. L., & Gargett, A. E. (1983). Time and space scales of vertical mixing and advection of phytoplankton in the upper ocean.
Limnology and Oceanography,28(5), 801–815. https://doi.org/10.4319/lo.1983.28.5.0801
Dieckmann, G., Rohardt, G., Hellmer, H., & Kipfstuhl, J. (1986). The occurrence of ice platelets at 250 m depth near the Filchner Ice Shelf
and its significance for sea ice biology. Deep Sea Research Part A. Oceanographic Research Papers,33(2), 141–148. https://doi.org/
10.1016/0198-0149(86)90114-7
Dowdeswell, J. A., & Jeffries, M. O. (2017). Arctic ice shelves: An introduction. In L. Copland, & D. Mueller (Eds.), Arctic ice shelves and ice
islands (pp. 3–21). Dordrecht: Springer Netherlands. https://doi.org/10.1007/978-94-024-1101-0_1
Eicken, H. (1994). Structure of under‐ice melt ponds in the central Arctic and their effect on the sea‐ice cover. Limnology and
Oceanography,39(3), 682–693. https://doi.org/10.4319/lo.1994.39.3.0682
Eicken, H., & Salganek, M. (2010). Field techniques for sea‐ice research, Fairbanks, Alaska: University of Alaska Press.
Gough, A. J., Mahoney, A. R., Langhorne, P. J., Williams, M. J. M., Robinson, N. J., & Haskell, T. G. (2012). Signatures of supercooling:
McMurdo Sound platelet ice. Journal of Glaciology,58(207), 38–50. https://doi.org/10.3189/2012JoG10J218
Günther, S., & Dieckmann, G. S. (2004). Seasonal development of algal biomass in snow‐covered fast ice and the underlying platelet layer in
the Weddell Sea, Antarctica. Antarctic Science,11(3), 305–315. https://doi.org/10.1017/S0954102099000395
Haas, C., Pfaffling, A., Hendricks, S., Rabenstein, L., Etienne, J.‐L., & Rigor, I. (2008). Reduced ice thickness in Arctic Transpolar Drift
favors rapid ice retreat. Geophysical Research Letters,35, L17501. https://doi.org/10.1029/2008gl034457
Hoppmann, M., Nicolaus, M., Paul, S., Hunkeler, P. A., Heinemann, G., Willmes, S., et al. (2017). Ice platelets below Weddell Sea landfast
sea ice. Annals of Glaciology,56(69), 175–190. https://doi.org/10.3189/2015AoG69A678
Hoppmann, M., Richter, M. E., Smith, I. J., Jendersie, S., Langhorne, P. J., Thomas, N. D., & Dieckmann, G. S. (2020). Platelet ice, the
Southern Ocean's hidden ice: A review. Annals of Glaciology.
10.1029/2020GL088898
Geophysical Research Letters
KATLEIN ET AL. 8of10
Acknowledgments
We are thankful to all members of the
MOSAiC collaboration who made this
unique expedition possible. We want to
thank all people enabling the MOSAiC
ROV and buoy programs at AWI, in
particular Julia Regnery, Kathrin
Riemann‐Campe, Martin Schiller, Anja
Nicolaus, and Dirk Kalmbach.
Furthermore, we thank Johannes
Lemburg from the AWI workshop and
Hauke Flores for providing the
ROVnet. We also thank the captain,
crew, and chief scientists of RV
Polarstern and support icebreakers IB
Kapitan Dranitsyn and RV Akademik
Fedorov for their support (Project ID:
AWI_PS122_00). The participation of
Dmitry V. Divine in the MOSAiC
expedition was supported by Research
Council of Norway project HAVOC
(No. 280292) and project DEARice
supported by EU ARICE program (EU
Grant Agreement No. 730965).
Participation of Ilkka Matero was sup-
ported by the Diatom ARCTIC project
(BMBF Grant 03F0810A), part of the
Changing Arctic Ocean Programme,
jointly funded by the UKRI Natural
Environment Research Council
(NERC) and the German Federal
Ministry of Education and Research
(BMBF). Stefanie Arndt was funded by
the German Research Council (DFG) in
the framework of the priority program
“Antarctic Research with comparative
investigations in Arctic ice areas”by
grant to SPP1158. We thank one anon-
ymous reviewer and Pat Langhorne for
improving this manuscript during the
peer‐review process. This study was
funded by the Alfred‐Wegener‐Institut
Helmholtz‐Zentrum für Polar‐und
Meeresforschung and the Helmholtz
Research program PACES II. Operation
and development of the ROV were
supported by the Helmholtz
Infrastructure Initiative “Frontiers in
Arctic Marine Monitoring (FRAM).”

Hunkeler, P. A., Hoppmann, M., Hendricks, S., Kalscheuer, T., & Gerdes, R. (2016). A glimpse beneath Antarctic sea ice: Platelet layer
volume from multifrequency electromagnetic induction sounding. Geophysical Research Letters,43, 222–231. https://doi.org/10.1002/
2015GL065074
Jeffries, M. O., Schwartz, K., Morris, K., Veazey, A. D., Krouse, H. R., & Gushing, S. (1995). Evidence for platelet ice accretion in Arctic sea
ice development. Journal of Geophysical Research,100(C6), 10,905–10,914. https://doi.org/10.1029/95JC00804
Katlein, C., Anhaus, P., Matero, I., & Nicolaus, M. (2020a). Pressure, temperature, conductivity and dissolved oxygen raw data during ROV
dives from POLARSTERN cruise PS122/2. PANGAEA.
Katlein, C., Anhaus, P. M., Matero, I., & Nicolaus, M. (2020b). Sea ice draft and acoustic backscatter (240 kHz) raw data derived from
multibeam sonar during the ROV BEAST dive PS122/2_18‐19. PANGAEA. https://doi.org/10.1594/PANGAEA.917498
Katlein, C., Itkin, P., & Divine, D. V. (2020). Salinity measured on sea ice core PS122/2_24‐114 during MOSAiC Leg 2. PANGAEA. https://
doi.org/10.1594/PANGAEA.919474
Katlein, C., Krampe, D., & Nicolaus, M. (2020). Extracted frames from main ROV camera videos during MOSAiC Leg 2. PANGAEA. https://
doi.org/10.1594/PANGAEA.919398
Katlein, C., Schiller, M., Belter, H. J., Coppolaro, V., Wenslandt, D., & Nicolaus, M. (2017). A new remotely operated sensor platform for
interdisciplinary observations under sea ice. Frontiers in Marine Science,4(281). https://doi.org/10.3389/fmars.2017.00281
Kirillov, S., Dmitrenko, I., Rysgaard, S., Babb, D., Ehn, J., Bendtsen, J., et al. (2018). The inferred formation of a subice platelet layer below
the multiyear landfast sea ice in the Wandel Sea (NE Greenland) induced by meltwater drainage. Journal of Geophysical Research:
Oceans,123, 3489–3506. https://doi.org/10.1029/2017JC013672
Knust, R. (2017). Polar research and supply vessel POLARSTERN operated by the Alfred‐Wegener‐Institute. Journal of large‐scale research
facilities JLSRF,3. https://doi.org/10.17815/jlsrf-3-163
Krumpen, T., Birrien, F., Kauker, F., Rackow, T., von Albedyll, L., Angelopoulos, M., et al. (2020). The MOSAiC ice floe: Sediment‐laden
survivor from the Siberian shelf. The Cryosphere Discussion,2020,1–20. https://doi.org/10.5194/tc-2020-64
Kwok, R., & Rothrock, D. A. (2009). Decline in Arctic sea ice thickness from submarine and ICESat records: 1958‐2008. Geophysical
Research Letters,36, L15501. https://doi.org/10.1029/2009GL039035
Kwok, R., Spreen, G., & Pang, S. (2013). Arctic sea ice circulation and drift speed: Decadal trends and ocean currents. Journal of Geophysical
Research: Oceans,118, 2408–2425. https://doi.org/10.1002/jgrc.20191
Langhorne, P. J., Hughes, K. G., Gough, A. J., Smith, I. J., Williams, M. J. M., Robinson, N. J., et al. (2015). Observed platelet ice distri-
butions in Antarctic sea ice: An index for ocean‐ice shelf heat flux. Geophysical Research Letters,42, 5442–5451. https://doi.or g/10.1002/
2015GL064508
Leonard, G. H., Purdie, C. R., Langhorne, P. J., Haskell, T. G., Williams, M. J. M., & Frew, R. D. (2006). Observations of platelet ice growth
and oceanographic conditions during the winter of 2003 in McMurdo Sound, Antarctica. Journal of Geophysical Research,111, C04012.
https://doi.org/10.1029/2005JC002952
Lewis, E. L., & Lake, R. A. (1971). Sea ice and supercooled water. Journal of Geophysical Research,76(24), 5836–5841. https://doi.org/
10.1029/JC076i024p05836
Lewis, E. L., & Milne, A. R. (1977). Underwater sea ice formation in “Polar Oceans”. Paper presented at Proceedings of the SCOR/SCAR
Polar Oceans Conference, Arctic Institute North America.
Lewis, E. L., & Perkin, R. G. (1983). Supercooling and energy exchange near the Arctic Ocean surface. Journal of Geophysical Research,
88(C12), 7681–7685. https://doi.org/10.1029/JC088iC12p07681
Lewis, E. L., & Perkin, R. G. (1986). Ice pumps and their rates. Journal of Geophysical Research,91(C10), 11,756–11,762. https://doi.org/
10.1029/JC091iC10p11756
Lofgren, G., & Weeks, W. F. (2017). Effect of growth parameters on substructure spacing in NaCl ice crystals. Journal of Glaciology,8(52),
153–164. https://doi.org/10.3189/S0022143000020827
Mahoney, A. R., Gough, A. J., Langhorne, P. J., Robinson, N. J., Stevens, C. L., Williams, M. M. J., & Haskell, T. G. (2011). The seasonal
appearance of ice shelf water in coastal Antarctica and its effect on sea ice growth. Journal of Geophysical Research,116, C11032. https://
doi.org/10.1029/2011JC007060
Martin, S., & Kauffman, P. (2006). The evolution of under‐ice melt ponds, or double diffusion at the freezing point. Journal of Fluid
Mechanics,64(3), 507–528. https://doi.org/10.1017/S0022112074002527
Notz, D., McPhee, M. G., Worster, M. G., Maykut, G. A., Schlünzen, K. H., & Eicken, H. (2003). Impact of underwater‐ice evolution on
Arctic summer sea ice. Journal of Geophysical Research,108(C7), 3223. https://doi.org/10.1029/2001JC001173
Pfirman, S., Haxby, W., Eicken, H., Jeffries, M., & Bauch, D. (2004). Drifting Arctic sea ice archives changes in ocean surface conditions.
Geophysical Research Letters,31, L19401. https://doi.org/10.1029/2004GL020666
Ragobert, T., & Roblin, E. (2008). Tara: Journey to the heart of the climate machine. Retrieved from https://oceans.taraexpeditions.org/en/
media-library/films/tara-journey-to-the-heart-of-the-climate-machine/
Robinson, N. J., Grant, B. S., Stevens, C. L., Stewart, C. L., & Williams, M. J. M. (2020). Oceanographic observations in supercooled water:
Protocols for mitigation of measurement errors in profiling and moored sampling. Cold Regions Science and Technology,170, 102954.
https://doi.org/10.1016/j.coldregions.2019.102954
Rutter, J. W., & Chalmers, B. (1953). A prismatic substructure formed during solidification of metals. Canadian Journal of Physics,31(1),
15–39. https://doi.org/10.1139/p53-003
Shokr, M., & Sinha, N. (2015). Sea ice: Physics and remote sensing (pp. 1–579). John Wiley & Sons. https://doi.org/10.1002/9781119028000
Skogseth, R., Nilsen, F., & Smedsrud, L. H. (2017). Supercooled water in an Arctic polynya: Observations and modeling. Journal of
Glaciology,55(189), 43–52. https://doi.org/10.3189/002214309788608840
Smith, I., Langhorne, P., Haskell, T., Trodahl, H., Frew, R., & Vennell, R. (2001). Platelet ice and the land‐fast sea ice of McMurdo Sound,
Antartica. Annals of Glaciology,33,21–27. https://doi.org/10.3189/172756401781818365
Thomas, D. N., & Dieckmann, G. S. (2002). Biogeochemistry of Antarctic sea ice. Oceanography and Marine Biology,40, 143–170.
Tucker, W. B., Gow, A. J., Meese, D. A., Bosworth, H. W., & Reimnitz, E. (1999). Physical characteristics of summer sea ice across the Arctic
Ocean. Journal of Geophysical Research,104(C1), 1489–1504. https://doi.org/10.1029/98JC02607
Vacchi, M., DeVries, A. L., Evans, C. W., Bottaro, M., Ghigliotti, L., Cutroneo, L., & Pisano, E. (2012). A nursery area for the Antarctic
silverfish Pleuragramma antarcticum at Terra Nova Bay (Ross Sea): First estimate of distribution and abundance of eggs and larvae
under the seasonal sea‐ice. Polar Biology,35(10), 1573–1585. https://doi.org/10.1007/s00300-012-1199-y
Weeks, W. F., & Ackley, S. F. (1986). The growth, structure, and properties of sea ice. In N. Untersteiner (Ed.), The Geophysics of Sea Ice
(pp. 9–164). Boston, MA: Springer US. https://doi.org/10.1007/978-1-4899-5352-0_2
10.1029/2020GL088898
Geophysical Research Letters
KATLEIN ET AL. 9of10

WMO (2014). JCOMM expert team on sea ice: Sea‐ice nomenclature: Snapshot of the WMO sea ice nomenclature WMO No. 259, Volume 1
—Terminology and codes; Volume II—Illustrated glossary and III—International system of sea‐ice symbols, WMO‐JCOMM, Geneva,
Switzerland, doi:http://hdl.handle.net/11329/328
Wollenburg, J. E., Iversen, M., Katlein, C., Krumpen, T., Nicolaus, M., Castellani, G., et al. (2020). New observations of the distribution,
morphology and dissolution dynamics of cryogenic gypsum in the Arctic Ocean. The Cryosphere,14(6), 1795–1808. https://doi.org/
10.5194/tc-14-1795-2020
Zubov, N. N. (1963). Arctic ice. San Diego, CA: U.S. Navy Electronics Laboratory. https://archive.org/details/arcticice00zubo/page/n1/
mode/2up
10.1029/2020GL088898
Geophysical Research Letters
KATLEIN ET AL. 10 of 10