The importance of sea ice biota for the ecosystem in the northwestern Weddell Sea
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<p>The western Weddell Sea along the northward branch of the Weddell Gyre is a region of major outflow of various water masses, thick sea ice, and biogeochemical matter, linking the Antarctic continent to the world oceans. It features a deep shelf and the second largest ice shelf (Larsen C) in the WS, and its perennial sea ice cover is among the thickest on earth. This region is undergoing dramatic changes due to the breakup of ice shelves along the Antarctic Peninsula, which results in oceanographic conditions unprecedented in the past 10,000 years. Since this region is difficult to access, comprehensive physical and biogeochemical information is still lacking. During the interdisciplinary Weddell Sea Ice (WedIce) expedition to the northwestern Weddell Sea on board the German icebreaker RV Polarstern in spring 2019, oceanographic and biogeochemical studies were conducted together with in-situ snow and ice sampling. Most stations visited contained second- and third-year ice. Additional airborne ice-thickness surveys revealed a mean ice thicknesses between 2.6 and 5.4&#8201;m, increasing from the Antarctic Sound towards the Larsen B region. Usually rotten ice was present below a solid, ~30&#8201;cm thick surface-ice layer, however, pronounced gap layers, typical for late summer ice in the marginal ice zone, were rare. The associated high algal biomass was only found north of the Antarctic Sound. Nevertheless, diatom-dominated standing stocks of integrated sea ice algae biomass were among the highest, previously described in Antarctic waters. In contrast, despite overall high macro-nutrient concentrations in the water, the biomass of the flagellate dominated phytoplankton was negligible for primary production in the entire region. Overall, it seems that despite changing light conditions for the phytoplankton due to the loss of ice shelves, the sea ice-derived carbon represents an important control variable for higher trophic levels in the western Weddell Sea.</p>Keywords:
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Ground and aerial surveys along the north coast of Ellesmere Island confirm that a considerable area of shelf ice remains, although it is not as extensive as it once was due to periodic ice island calvings. However, the lost ice shelf is quickly replaced by landfast sea ice. The sea ice often persists for many years and thickens sufficiently to be considered as the restoration of former ice shelf. The landfast ice quickly assumes an undulating topography, similar to the ice shelves, the development of which is encouraged by melt water and wind action. Even under the present conditions of negative mass balance, the sea ice reaches considerable, undeformed thicknesses. The thick sea ice forming today could be the precursor of an expansion of the ice shelves.
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Abstract Recent observations indicate that two cryospheric components, namely the Antarctic sea ice and ice shelf over the Southern Ocean, have been changing over the decades. Here we analyze results from an ocean–sea ice–ice shelf model to examine variability in the Antarctic sea-ice extent and ice-shelf basal melting. The model reproduces seasonal and interannual variability in the Antarctic sea-ice extent and demonstrates that summertime ice-shelf basal melting is closely anti-correlated with the sea-ice extent anomaly. For example, the unprecedented minimum of the Antarctic sea-ice extent in the 2016 spring was accompanied by a substantial increase in the Antarctic ice-shelf melting in the model. Detailed analysis of Antarctic coastal water masses flowing into the ice-shelf cavities illustrates the physical linkage in the strong anti-correlation. This study suggests that the Antarctic summer sea-ice extent in the regions where the sea-ice edge approaches the Antarctic coastline can be a proxy for Antarctic coastal water masses and subsequent ice-shelf basal melting.
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Ice shelves play an important role in determining regional ocean properties and in modulating ice flux from
land to sea. Their dynamics are complex, however, and localised rifts and zones of weakness can have a
significant but poorly understood effect on flow and ultimately on the integrity of the shelf.
The Brunt Ice Shelf (BIS)- Stancomb Wills Ice Tongue (SWIT) System, situated on the Caird Coast, Oates
Land, Antarctica, is characterised as a thin, unbounded ice shelf with a highly heterogeneous structure. In
contrast to most ice shelves, icebergs calve along much of the grounding line but are trapped and subsequently
bound together by sea ice. This calf-ice / sea-ice aggregate makes up a large part of the Brunt Ice Shelf in
particular, and this heterogeneity makes the BIS-SWIT a good test case for investigating the importance of
weak zones in shelf dynamics.
We applied a diagnostic, dynamic/thermodynamic ice-shelf model to simulate the present flow of the ice
shelf that results from the ice-thickness distribution, the influx at the grounding line and the surface and
bottom temperature. We then compared the model results with flow velocities measured by Synthetic
Aperture Radar feature tracking. We found that our simulations were clearly improved by the use of a
high- resolution ice thickness distribution on the heterogeneous ice shelf calculated from ICESat surface
elevation data using an assumption of hydrostatic equilibrium. We then assessed the model’s sensitivity to
ice thickness, inflow velocities and a flow enhancement factor that parameterises the role of sea ice, whose
mechanical properties are known to be significantly different from those of meteoric ice.
We found that the numerical simulations were improved by incorporating the detailed variations in shelf
structure. Simulated flow velocities on either side of rifts in the ice shelf became decoupled as we softened
the sea ice within the rifts. On a larger scale, we found that soft sea ice can lead to a decoupling of the
movement of the Stancomb-Wills Ice Tongue and the Brunt Ice Shelf. When we simulated a regime where
sea ice was absent, ice shelf flow speeds increased along the western edge of the SWIT ice front, in general
agreement with observations made in just such a sea- ice-free dynamic regime that occurred in
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As a result of their importance in the production of Antarctic Bottom Water, processes over the Antarctic continental shelf have a strong impact on the global ocean. Some of the ice shelves that cover much of the continental shelf are thought to play a significant role in the production of AABW, particularly in the southern Weddell Sea
[Foldvik and Gammelsrod, 1988]. The impact of the ocean on ice shelves is of interest from a glaciological perspective also, as ice shelves form the seaward boundary of much of the Antarctic Ice Sheet.
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Variations in the amplitude of radar echoes from the bottom of the grid western half of the Ross Ice Shelf have been analyzed. Contrary to the results of a similar analysis performed for the grid eastern sector of the ice shelf, bands of low signal strength downstream from both Crary Ice Rise and the Siple Coast do not correlate with modern flow lines. The difference in direction between the radar bands downstream of Crary Ice Rise and the present velocity vectors and the absence of a comparable trend farther east suggest to us that the grounding line around Crary Ice Rise retreated within the last 1000 years. This hypothesis is reinforced by the observation of several domes and hollows in ice thickness downstream of Crary Ice Rise which are similar to a hollow now located in the wake of the ice rise and a dome on its eastern flank. We interprete this as evidence for a rapid increase in flow around the ice rise which carried downstream the ice topography formed around the ice rise. Similar but less detailed evidence found downstream of the Siple Coast suggests that there was a regional retreat of the West Antarctic grounding line.
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Analysis of vertical air photographs taken in 1959 and 1974 reveals that a total of 48 sq km, involving 3.3 cubic km, of ice calved from Milne and Ayles ice shelves between July 1959 and July 1974. In addition, Ayles Ice Shelf moved to about 5 km out of Ayles Fiord. It still occupied this exposed position in July 1984. The ice losses and movements have allowed the growth of thick sea ice that has developed an undulating topography similar to but smaller scale than that of the ice shelves. It is suggested that regular monitoring of the coastal ice of northern Ellesmere Island would enable such changes to be registered and assessed, as they could be of concern to offshore operations in the Beaufort Sea.Key words: air photographs, Milne Ice Shelf, Ayles Ice Shelf, ice island calvings, thick sea ice growth
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Abstract Ponds that form on sea ice can cause it to thin or break-up, which can promote calving from an adjacent ice shelf. Studies of sea ice ponds have predominantly focused on Arctic ponds formed by in situ melting/ponding. Our study documents another mechanism for the formation of sea ice ponds. Using Landsat 8 and Sentinel-2 images from the 2015–16 to 2018–19 austral summers, we analyze the evolution of sea ice ponds that form adjacent to the McMurdo Ice Shelf, Antarctica. We find that each summer, meltwater flows from the ice shelf onto the sea ice and forms large (up to 9 km 2 ) ponds. These ponds decrease the sea ice's albedo, thinning it. We suggest the added mass of runoff causes the ice to flex, potentially promoting sea-ice instability by the ice-shelf front. As surface melting on ice shelves increases, we suggest that ice-shelf surface hydrology will have a greater effect on sea-ice stability.
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Ground and aerial surveys along the north coast of Ellesmere Island confirm that a considerable area of shelf ice remains, although it is not as extensive as it once was due to periodic ice island calvings. However, the lost ice shelf is quickly replaced by landfast sea ice. The sea ice often persists for many years and thickens sufficiently to be considered as the restoration of former ice shelf. The landfast ice quickly assumes an undulating topography, similar to the ice shelves, the development of which is encouraged by melt water and wind action. Even under the present conditions of negative mass balance, the sea ice reaches considerable, undeformed thicknesses. The thick sea ice forming today could be the precursor of an expansion of the ice shelves.
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