Platelet ice and the land-fast sea ice of McMurdo Sound, Antarctica

AbstractDendritic crystals of platelet ice appear beneath the columnar land-fast sea ice of McMurdo Sound, Antarctica. These leaf-like crystals are frozen into place by the advancing columnar growth. The platelets most probably begin to appear during July although in some parts of the Sound they may not appear at all. In addition, the amount and extent of platelet ice within the Sound varies from year to year. Previous authors have suggested that the formation of platelet ice is linked to the presence of the nearby ice shelf. It is a matter of debate whether these platelets form at depth and then float upwards or whether they grow in slightly supercooled water at the ice/water interface. The phenomenon is similar to that observed in the Weddell Sea region, but previous authors have suggested the two regions may experience different processes. This paper presents the results of field-work conducted in McMurdo Sound in 1999. Ice-structure analysis, isotopic analysis and salinity and temperature measurements near the ice/water interface are presented. Freezing points are calculated, and the possible existence of supercooling is discussed in relation to existing conjectures about the origin of platelets.

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Observations of a rift in the Ronne Ice Shelf, Antarctica

Journal of Glaciology ◽

10.3189/s0022143000003968 ◽

1994 ◽

Vol 40 (134) ◽

pp. 187-189

Author(s):

E.C. King

Keyword(s):

Weddell Sea ◽

Surface Elevation ◽

Water Interface ◽

Ice Shelf ◽

Seismic Profiling ◽

Ice Water

AbstractDuring seismic profiling on the northwest Ronne Ice Shelf Antarctica, a rift in the ice shelf was encountered. The rift trends southeast to northwest and is located approximately 30 km inland from the present-day ice front The rift is 340 m wide and the surface elevation of the ice shelf drops by 14.65 m over the axis of the rift. The rift has an asymmetrical base with a near-vertical ice-water interface on its northeast flank and a more gently dipping ice-water interface forming its southeastern flank. The ice shelf thins from a thickness of 350 m away from the rift to a thickness of 225 m at the rift axis. The rift is the probable location of a future major calving event on this section of the Ronne Ice Shelf, an event which would release an iceberg of up to 30 km by 180 km into the Weddell Sea.

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Observations of a rift in the Ronne Ice Shelf, Antarctica

Journal of Glaciology ◽

10.1017/s0022143000003968 ◽

1994 ◽

Vol 40 (134) ◽

pp. 187-189 ◽

Cited By ~ 6

Author(s):

E.C. King

Keyword(s):

Weddell Sea ◽

Surface Elevation ◽

Water Interface ◽

Ice Shelf ◽

Seismic Profiling ◽

Ice Water

Abstract During seismic profiling on the northwest Ronne Ice Shelf Antarctica, a rift in the ice shelf was encountered. The rift trends southeast to northwest and is located approximately 30 km inland from the present-day ice front The rift is 340 m wide and the surface elevation of the ice shelf drops by 14.65 m over the axis of the rift. The rift has an asymmetrical base with a near-vertical ice-water interface on its northeast flank and a more gently dipping ice-water interface forming its southeastern flank. The ice shelf thins from a thickness of 350 m away from the rift to a thickness of 225 m at the rift axis. The rift is the probable location of a future major calving event on this section of the Ronne Ice Shelf, an event which would release an iceberg of up to 30 km by 180 km into the Weddell Sea.

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Sea-ice habitat minimizes grazing impact and predation risk for larval Antarctic krill

Polar Biology ◽

10.1007/s00300-021-02868-7 ◽

2021 ◽

Author(s):

Carmen L. David ◽

Fokje L. Schaafsma ◽

Jan A. van Franeker ◽

Evgeny A. Pakhomov ◽

Brian P. V. Hunt ◽

...

Keyword(s):

Sea Ice ◽

Predation Risk ◽

Water Column ◽

Antarctic Krill ◽

Standing Stock ◽

Euphausia Superba ◽

Grazing Impact ◽

Water Interface ◽

Winter Habitat ◽

Ice Water

AbstractSurvival of larval Antarctic krill (Euphausia superba) during winter is largely dependent upon the presence of sea ice as it provides an important source of food and shelter. We hypothesized that sea ice provides additional benefits because it hosts fewer competitors and provides reduced predation risk for krill larvae than the water column. To test our hypothesis, zooplankton were sampled in the Weddell-Scotia Confluence Zone at the ice-water interface (0–2 m) and in the water column (0–500 m) during August–October 2013. Grazing by mesozooplankton, expressed as a percentage of the phytoplankton standing stock, was higher in the water column (1.97 ± 1.84%) than at the ice-water interface (0.08 ± 0.09%), due to a high abundance of pelagic copepods. Predation risk by carnivorous macrozooplankton, expressed as a percentage of the mesozooplankton standing stock, was significantly lower at the ice-water interface (0.83 ± 0.57%; main predators amphipods, siphonophores and ctenophores) than in the water column (4.72 ± 5.85%; main predators chaetognaths and medusae). These results emphasize the important role of sea ice as a suitable winter habitat for larval krill with fewer competitors and lower predation risk. These benefits should be taken into account when considering the response of Antarctic krill to projected declines in sea ice. Whether reduced sea-ice algal production may be compensated for by increased water column production remains unclear, but the shelter provided by sea ice would be significantly reduced or disappear, thus increasing the predation risk on krill larvae.

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Tank study of physico-chemical controls on gas content and composition during growth of young sea ice

Journal of Glaciology ◽

10.3189/172756502781831377 ◽

2002 ◽

Vol 48 (161) ◽

pp. 177-191 ◽

Cited By ~ 64

Author(s):

Jean-Louis Tison ◽

Christian Haas ◽

Marcia M. Gowing ◽

Suzanne Sleewaegen ◽

Alain Bernard

Keyword(s):

Sea Ice ◽

Sea Water ◽

Bubble Nucleation ◽

Water Dynamics ◽

Gas Content ◽

First Year ◽

Water Interface ◽

Atmospheric Gases ◽

Physico Chemical ◽

Ice Water

AbstractDuring an ice-tank experiment, samples were taken to study the processes of acquisition and alteration of the gas properties in young first-year sea ice during a complete growth–warming–cooling cycle. The goal was to obtain reference levels for total gas content and concentrations of atmospheric gases (O2, N2, CO2) in the absence of significant biological activity. The range of total gas-content values obtained (3.5–18 mL STP kg−1) was similar to previous measurements or estimates. However, major differences occurred between current and quiet basins, showing the role of the water dynamics at the ice–water interface in controlling bubble nucleation processes. Extremely high CO2concentrations were observed in all the experiments (up to 57% in volume parts). It is argued that these could have resulted from two unexpected biases in the experimental settings. Concentrations in bubbles nucleated at the interface are controlled by diffusion both from the ice–water interface towards the well-mixed reservoir and between the interface water and the bubble itself. This double kinetic effect results in a transition of the gas composition in the bubbles from values close to solubility in sea water toward values close to atmospheric, as the ice cover builds up.

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Attenuated carbon sequestration by Weddell Sea dense waters over the 21st century – an assessment with FESOM-REcoM

10.5194/egusphere-egu21-6248 ◽

2021 ◽

Author(s):

Cara Nissen ◽

Ralph Timmermann ◽

Mario Hoppema ◽

Judith Hauck

Keyword(s):

Carbon Sequestration ◽

Sea Ice ◽

Continental Shelf ◽

Bottom Water ◽

Water Mass ◽

Weddell Sea ◽

Ice Shelf ◽

Deep Waters ◽

Water Formation ◽

Water Mass Transformation

Deep and bottom water formation regions have long been recognized to be efficient vectors for carbon transfer to depth, leading to carbon sequestration on time scales of centuries or more. Precursors of Antarctic Bottom Water (AABW) are formed on the Weddell Sea continental shelf as a consequence of buoyancy loss of surface waters at the ice-ocean or atmosphere-ocean interface, which suggests that any change in water mass transformation rates in this area affects global carbon cycling and hence climate. Many of the models previously used to assess AABW formation in present and future climates contained only crude representations of ocean-ice shelf interaction. Numerical simulations often featured spurious deep convection in the open ocean, and changes in carbon sequestration have not yet been assessed at all. Here, we present results from the global model FESOM-REcoM, which was run on a mesh with elevated grid resolution in the Weddell Sea and which includes an explicit representation of sea ice and ice shelves. Forcing this model with ssp585 scenario output from the AWI Climate Model, we assess changes over the 21st century in the formation and northward export of dense waters and the associated carbon fluxes within and out of the Weddell Sea. We find that the northward transport of dense deep waters (&#963;2>37.2 kg m-3 below 2000 m) across the SR4 transect, which connects the tip of the Antarctic Peninsula with the eastern Weddell Sea, declines from 4 Sv to 2.9 Sv by the year 2100. Concurrently, despite the simulated continuous increase in surface ocean CO2 uptake in the Weddell Sea over the 21st century, the carbon transported northward with dense deep waters declines from 3.5 Pg C yr-1 to 2.5 Pg C yr-1, demonstrating the dominant role of dense water formation rates for carbon sequestration. Using the water mass transformation framework, we find that south of SR4, the formation of downwelling dense waters declines from 3.5 Sv in the 1990s to 1.6 Sv in the 2090s, a direct result of the 18% lower sea-ice formation in the area, the increased presence of modified Warm Deep Water on the continental shelf, and 50% higher ice shelf basal melt rates. Given that the reduced formation of downwelling water masses additionally occurs at lighter densities in FESOM-REcoM in the 2090s, this will directly impact the depth at which any additional oceanic carbon uptake is stored, with consequences for long-term carbon sequestration.

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Variability Of Thermohaline Circulation Under An Ice Shelf

Annals of Glaciology ◽

10.1017/s0260305500009009 ◽

1990 ◽

Vol 14 ◽

pp. 338

Author(s):

H.H. Hellmer

Keyword(s):

Sea Ice ◽

Boundary Conditions ◽

Thermohaline Circulation ◽

Circulation Model ◽

Melting Zone ◽

Weddell Sea ◽

Shelf Water ◽

Ice Shelf ◽

Grounding Line ◽

Ice Shelf Water

The production of Antarctic Bottom Water is mainly influenced by Ice Shelf Water, which is formed through the modification of shelf water masses under huge ice shelves. To simulate this modification a two-dimensional thermohaline circulation model has been developed for a section perpendicular to the ice-shelf edge. Hydrographic data from the Filchner Depression enter into the model as boundary conditions. In the outflow region they also serve as a verification of model results. The standard solution reveals two circulation cells. The dominant one transports shelf water near the bottom toward the grounding line, where it begins to ascend along the inclined ice shelf. The contact with the ice shelf causes melting with a maximum rate of 1.5 m a−1 at the grounding line. Freezing and therefore the accumulation of “sea ice” at the bottom of the ice shelf occurs at the end of the melting zone at a rate on the order of 0.1 ma−1. Both rates are comparable with values estimated or predicted by models concerning ice-shelf dynamics. As one example of model sensitivity to changing boundary conditions, a higher sea-ice production in the southern Weddell Sea, as might be expected for a general climatic cooling event, is assumed. The resultant decrease/ increase in temperature/salinity of the inflow (Western Shelf Water) reduces the circulation under the ice shelf and therefore the outflow of Ice Shelf Water by 40%. The maximum melting and freezing rate decreases by 0.1 ma−1 and 0.01 m a−1, respectively. and the freezing zone shifts toward the grounding line by 100 km. In general the intensity of the circulation cells, the characteristics of Ice Shelf Water, the distribution of melting and freezing zones and the melting and freezing rates differ from the standard results with changing boundary conditions. These are the temperature and salinity of the inflow, the surface temperature at the top, and the extension and morphology of the ice shelf.

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Simulation of the crystal growth of platelet sea ice with diffusive heat and mass transfer

Annals of Glaciology ◽

10.3189/2015aog69a777 ◽

2015 ◽

Vol 56 (69) ◽

pp. 127-136 ◽

Cited By ~ 8

Author(s):

Pat Wongpan ◽

Patricia J. Langhorne ◽

David E. Dempsey ◽

Lisa Hahn-Woernle ◽

Zhifa Sun

Keyword(s):

Mass Transfer ◽

Crystal Growth ◽

Sea Ice ◽

Heat And Mass Transfer ◽

Ice Cover ◽

Three Dimensions ◽

Crystallographic Structure ◽

Ice Shelf ◽

Coastal Sea ◽

Platelet Ice

AbstractAntarctic coastal sea ice often grows in water that has been supercooled by interaction with an ice shelf. In these situations, ice crystals can form at depth, rise and deposit under the sea-ice cover to form a porous layer that eventually consolidates near the base of the existing sea ice. The least consolidated portion is called the sub-ice platelet layer. Congelation growth eventually causes the sub-ice platelet layer to become frozen into the sea-ice cover as incorporated platelet ice. In this study, we simulate these processes in three dimensions using Voronoi dynamics to govern crystal growth kinetics. Platelet deposition, in situ growth and incorporation into the sea-ice cover are integrated into the model. Heat and mass transfer are controlled by diffusion. We extract and compare spatial-temporal distributions of porosity, salinity, temperature and crystallographic c-axes with observations from McMurdo Sound, Antarctica. The model captures the crystallographic structure of incorporated platelet ice as well as the topology of the sub-ice platelet layer. The solid fraction, which has previously been poorly constrained, is simulated to be ∼0.22, in good agreement with an earlier estimate of 0.25 ± 0.06. This property of the sub-ice platelet layer is important for biological processes, and for the freeboard-thickness relationship around Antarctica.

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Evolution of supercooling under coastal Antarctic sea ice during winter

Antarctic Science ◽

10.1017/s0954102011000265 ◽

2011 ◽

Vol 23 (4) ◽

pp. 399-409 ◽

Cited By ~ 30

Author(s):

Gregory H. Leonard ◽

Patricia J. Langhorne ◽

Michael J.M. Williams ◽

Ross Vennell ◽

Craig R. Purdie ◽

...

Keyword(s):

Sea Ice ◽

Freezing Point ◽

Circulation Pattern ◽

Late Winter ◽

Ice Shelf ◽

Mcmurdo Sound ◽

The North ◽

Antarctic Sea Ice ◽

Ice Production

AbstractHere we describe the evolution through winter of a layer of in situ supercooled water beneath the sea ice at a site close to the McMurdo Ice Shelf. From early winter (May), the temperature of the upper water column was below its surface freezing point, implying contact with an ice shelf at depth. By late winter the supercooled layer was c. 40 m deep with a maximum supercooling of c. 25 mK located 1–2 m below the sea ice-water interface. Transitory in situ supercooling events were also observed, one lasting c. 17 hours and reaching a depth of 70 m. In spite of these very low temperatures the isotopic composition of the water was relatively heavy, suggesting little glacial melt. Further, the water's temperature-salinity signature indicates contributions to water mass properties from High Salinity Shelf Water produced in areas of high sea ice production to the north of McMurdo Sound. Our measurements imply the existence of a heat sink beneath the supercooled layer that extracts heat from the ocean to thicken and cool this layer and contributes to the thickness of the sea ice cover. This sink is linked to the circulation pattern of the McMurdo Sound.

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Visualizing brine channel development and convective processes during artificial sea-ice growth using Schlieren optical methods

Journal of Glaciology ◽

10.1017/jog.2015.1 ◽

2016 ◽

Vol 62 (231) ◽

pp. 1-17 ◽

Cited By ~ 9

Author(s):

C. A. MIDDLETON ◽

C. THOMAS ◽

A. DE WIT ◽

J.-L. TISON

Keyword(s):

Sea Ice ◽

Growth Period ◽

Transport Processes ◽

Optical Methods ◽

Water Interface ◽

Ice Growth ◽

Convective Processes ◽

Ice Water ◽

Brine Channel ◽

Channel Development

ABSTRACTTwo non-invasive optical Schlieren methods have been adapted to visualize brine channel development and convective processes in experimentally grown sea ice obtained when a NaCl aqueous solution is cooled from above in a quasi-two-dimensional Hele–Shaw cell. The two different visualization methods, i.e. traditional and synthetic Schlieren optical imaging, produce high spatial resolution images of transport processes during ice growth, without any external perturbation. These images allow observations of the flow dynamics simultaneously within the ice layer, around the ice/water interface, and in the liquid water layer, revealing connections between the processes occurring within the two phases. Results from these methods show that desalination of the growing ice layer occurs by two concurrent, yet independent, mechanisms: (1) boundary layer convection persisting throughout the ice growth period, with short fingers present just below the ice/water interface, and (2) gravity-driven drainage from the brine channels producing deep penetrating convective streamers, which appear after a given time from the beginning of ice growth. The improved visualization and qualitative characterization of these processes show that Schlieren optical methods have exciting potential applications for future study of convective processes during sea-ice growth.

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The Weddell Sea Expedition 2019

10.5194/egusphere-egu2020-21896 ◽

2020 ◽

Author(s):

John Shears ◽

Julian Dowdeswell ◽

Freddie Ligthelm ◽

Paul Wachter

Keyword(s):

Sea Ice ◽

Autonomous Underwater Vehicles ◽

Remote Sensing Data ◽

Open Water ◽

Weddell Sea ◽

Scientific Programme ◽

Ice Shelf ◽

Remotely Operated Vehicle ◽

Radar Images ◽

Ice Conditions

The Weddell Sea Expedition 2019 (WSE) was conceived with dual aims: (i) to undertake a comprehensive international inter-disciplinary programme of science centred in the waters around Larsen C Ice Shelf, western Weddell Sea; and (ii) to search for, survey and image the wreck of Sir Ernest Shackleton&#8217;s Endurance, which sank in the Weddell Sea in 1915.&#160;The 6-week long expedition, funded by the Flotilla Foundation, required the use of a substantial ice-strengthened vessel given the very difficult sea-ice conditions encountered in the Weddell Sea, and especially in its central and western parts. The South African ship SA Agulhas II was chartered for its Polar Class 5 icebreaking capability and design as a scientific research vessel. The expedition was equipped with state-of-the-art Autonomous Underwater Vehicles (AUVs) and a Remotely Operated Vehicle (ROV) which were capable of deployment to waters more than 3,000 m deep, thus making the Larsen C continental shelf and slope, and the Endurance wreck site, accessible. During the expedition, a suite of passive and active remote-sensing data, including TerraSAR-X radar images delivered in near real-time, was provided to the ice-pilot onboard the SA Agulhas II. These data were instrumental for safe vessel navigation in sea ice and the detection and tracking of icebergs and ice floes of scientific interest.The scientific programme undertaken by the WSE was very successful and produced many new geological, geophysical, marine biological and oceanographic observations from a part of the Weddell Sea that has been little studied previously, particularly the area east of Larsen C Ice Shelf. The expedition also reached the sinking location of Shackleton&#8217;s Endurance, where the presence of open-water sea ice leads allowed the deployment of an AUV to the ocean floor to try and locate and survey the wreck. Unfortunately, SA Agulhas II later lost communication with the AUV, and deteriorating weather and sea ice conditions meant that the search had to be called off.

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