Mertz Glacier Research Articles

Corrigendum to “Phytoplankton biomass and photosynthetic competency in the summertime Mertz Glacier Region of East Antarctica” [Deep-Sea Research II 50 (8–9), 1415–1440

Corrigendum to “Phytoplankton biomass and photosynthetic competency in the summertime Mertz Glacier Region of East Antarctica” [Deep-Sea Research II 50 (8–9), 1415–1440

Methyl chloride and methyl bromide degradation in the Southern Ocean

This study presents shipboard measurements of the loss rate constants of methyl bromide and methyl chloride in surface seawater in the Southern Ocean, using a 13C stable isotope incubation technique. The measurements were made during October–December, 2001, on a cruise track extending from Hobart, Tasmania to Buchanan Bay (Mertz Glacier) at the coast of Antarctica (46–67°S, 138–145°E). Significant loss rates were measured for both compounds, even in very cold waters where chemical loss rates were negligible. These observations are attributed to biological uptake, and they explain the tendency for high latitude waters to be undersaturated with respect to atmospheric methyl bromide and methyl chloride. These observations are the first open ocean measurements demonstrating the biological degradation of methyl chloride.

An anomalous late‐season change in the regional sea ice regime in the vicinity of the Mertz Glacier Polynya, East Antarctica

This paper examines a major late‐season shift in atmospheric circulation over the East Antarctic sea ice zone that occurred in October 1999. The mean wind direction at 65°S, 145°E changed from south‐southeasterly to west‐southwesterly, resulting in almost a complete reversal of the climatological East Wind Drift pattern in large‐scale sea ice advection over the Southern Ocean sector 135°–170°E. By comparison with National Centers for Environmental Prediction/National Center for Atmospheric Research Reanalysis time series from 1980 to 2000, this shift is shown to be unusual in terms of both its persistence and impact, although similar though less prolonged patterns occurred in 1984 and 1985. Lasting from October 1999 to February 2000, it had a profound impact on sea ice concentration and extent over a vast area. By coinciding with the period of maximum ice extent and persisting through to the springtime melt phase, it also contributed to the rapid annual meltback of sea ice. It also had a large and complex effect on the behavior of the globally important Mertz Glacier Polynya and on the areal extent of thick perennial sea ice to the east. While a midlatitude blocking high pattern provided the initial impetus for the “shift” in 1999, the persistence of west‐southwesterly winds through February 2000 appears to relate to an unusually persistent southward migration of the Antarctic Circumpolar Trough (ACT). In addition to the prolonged 1999 episode, a strong seasonal signal is apparent throughout the mean wind speed and direction time series, related to the ACT migration. Although less persistent than in 1999, this annual shift has implications for the observed dramatic annual meltback of East Antarctic sea ice.

Recent iceberg calving events in the Ninnis Glacier region, East Antarctica

This paper describes a major calving of the Ninnis Glacier tongue in January 2000. This event, which took 10 years to complete, produced a major change in the George V Land coastline and a large iceberg (∼800 km2). By grounding or becoming “locked in” by fast ice, bergs produced locally and drifting in from afar reside for long periods (10–20 years) in the region ∼146 to 154°E to have a profound impact on sea ice distribution, both locally and 100s of kilometres up- and downstream. They are responsible for the formation of a lens of thick perennial ice to the east (∼14 000 km2 in area), and polynyas. Iceberg movement is sporadic, with intermittent ungroundings of large bergs occurring every 5–13 years. “Escaping” bergs have a temporary impact on the Mertz Glacier polynya to the west. Although public attention focuses on vast bergs, assemblages of small bergs appear to be equally important in terms of their impact on regional fast and pack ice distribution. Possible global change scenarios are discussed. The need for field observations and improved bathymetric and oceanographic data is emphasized.

Summer plankton production and nutrient consumption patterns in the Mertz Glacier Region of East Antarctica

Abstract We report on a biological and nutrient survey conducted off the eastern Adelie and George V Coasts of East Antarctica in late December 2000 and January 2001. The region was dominated by the Mertz Glacier Tongue (MGT) and its associated polynya that controlled the amount of open water through the season and influenced stratification near forming and melting ice. As in other regions of the Southern Ocean, large concentrations of nitrate, phosphate and silicic acid were found. Chlorophyll levels varied from 0.3 mg m−3 in pack-ice and slope regions to greater than 10 mg m−3 in a Phaeocystis bloom encountered among grounded ice northeast of the MGT. Net utilization rates of nutrients were derived from the deficit between measured concentrations and estimated winter values. Nitrate (N+N) and silicic acid utilization rates ranged from 2 to 12 and 14 to 30 mmol m−2 d−1, respectively, and were greatest in Commonwealth Bay. The ratio of nitrate to phosphate utilization was low ( 10. Lower silicic acid to N+N ratios were found in regions with substantial populations of Phaeocystis sp. in the eastern, grounded ice region. However, there was significant variation in the silicic acid to N+N utilization that may be induced by mixed-layer iron levels that varied from

Phytoplankton biomass and photosynthetic competency in the summertime Mertz Glacier Region of East Antarctica

Abstract Vertical profiles of water temperature, salinity, beam transmission, density, pressure, wind speed, wind direction, phytoplankton biomass (chlorophyll a (Chl a) plus phaeophytin a), and photosynthetic competency (by fast repetition-rate fluorometry) are presented for the Mertz Glacier region, East Antarctica, for a 3-week period during the austral summer 2000–2001. Injection of low-salinity water from the melting of the ice pack formed a shallow (ca. 25 m) mixed layer offshore. Two distinct deep mixing features were observed associated to varying degrees with high chlorophyll levels and high photosynthetic competency. Along the Adelie Land coast, a deep mixing layer (1400 m) with Chl a concentrations of 386 mg m−2 and elevated Fv/Fm (>0.5) was observed. At the eastern end of the study area, along a seaward extension of fast-ice, a bloom of Phaeocystis antarctica bloom formed within a shallow (24 m) mixed layer, with surface Chl a concentrations of ca. 8 mg m−3 and elevated Fv/Fm (0.5). This feature also had high surface salinity (>34) and was contiguous with a deep-mixing feature (236 m). FRRF parameters Fv/Fm and σPSII were strongly suppressed in surface waters (

Wintertime oceanography of the Adélie Depression

Abstract The formation of high-salinity shelf waters beneath coastal polynyas from enhanced sea-ice production and brine rejection during wintertime is critical to the production of Antarctic Bottom Waters. We report on the first wintertime measurements of high-salinity shelf water formation over the Adelie Depression off the East Antarctic coast between 143° and 146°E, during the Mertz Glacier Polynya Experiment in July–August 1999. The general circulation and evolution of water masses in the Adelie Depression during winter are described, and in particular we quantify the rates of brine rejection (sea-ice growth), ocean heat flux, and latent heat flux, in the Mertz polynya central to the formation of high-salinity shelf waters. We find shelf waters above the minimum sill depth with sufficient density to become Adelie Land Bottom Water and suggest the highest production rates occur near Commonwealth Bay. Sea-ice growth and heat transfer rates are calculated from the divergence of heat and freshwater fields around a closed volume beneath the Mertz polynya. The sea-ice growth estimates and associated Monte Carlo errors are found to range from 4.8±1.7, 4.1±1.5, 8.4±1.7 cm d−1, respectively, over a 3-week period. The average growth rate is 5.8 cm d−1. Near the Mertz polynya the sensible heat from ocean transport is between 15±6 and 43±11 W m−2, with an average of 30 W m−2, and the latent heat budget is between 125±45 and 254±52 W m−2, with an average of 174 W m−2. We conclude that during winter the Mertz polynya is primarily a latent heat polynya.

Modelling the impacts of persistent Antarctic polynyas with an atmosphere–sea-ice general circulation model

Abstract A coupled atmosphere–sea-ice model is used to assess the effect of Antarctic polynyas on the heat exchange between the surface and the atmosphere, the ice production rate within the polynyas, and the possible mechanisms of formation and maintenance of the polynyas. Results are presented for sensitivity studies with the model using prescribed minimum open-water fractions (25% and 30%) at a number of locations around the coast of East Antarctica and in the Amundsen, Ross and Weddell Seas. Although to some extent unphysical for the ocean, this technique allows the heat fluxes and freezing rates to be computed for the constrained conditions compared with an unconstrained control simulation. The model simulations suggest that Antarctic coastal polynyas, including the Mertz Glacier polynya, are primarily latent heat polynyas (where the heat loss from open water is balanced by the latent heat released with ice production) rather than sensible heat ones (where the heat loss is balanced by heat from the deep ocean). When a polynya exists, the turbulent latent heat flux from the ocean surface to the atmosphere can increase several times in magnitude and the average latent heat flux (over open-water and sea-ice fractions) can be sustained at over 60 W m −2 . The sensible heat flux changes sign and increases by about 2 orders of magnitude. This loss of turbulent heat (sensible plus latent) is offset only slightly by increased absorbed short-wave radiation in winter, but this offset increases greatly in spring when more short-wave radiation is available. In the polynyas, the large heat loss to the atmosphere is mostly balanced by the heat released due to ice formation, and this is accompanied by a high rate of salt rejection. The average ice production simulated in the polynyas is more than 11 m/year, with 5 m formed during 3 months of winter. These results provide quantitative support for the sustained high ice-freezing rates in polynyas estimated in other studies, which have used satellite and in situ observational data.

New velocity map and mass-balance estimate of Mertz Glacier, East Antarctica, derived from Landsat sequential imagery

AbstractAutomatic feature tracking on two Landsat images (acquired inJanuary 2000 and December 2001) generates a complete and accurate velocity field of Mertz Glacier, East Antarctica. This velocity field shows two main tributaries to the ice stream. Between the tributaries, a likely obstruction feature in the bedrock results in a slow-down of the flow. A third Landsat image, acquired in 1989 and combined with the 2000 image, permits the determination of the glacier mean velocity during the 1990s. Although some parts of the Mertz Glacier system show evidence of slight speed increase, we conclude that the Mertz flow speed is constant within our uncertainty (35 m a−1). Using this complete velocity field, new estimates of the ice discharge flux, 17.8 km3 a−1 (16.4 Gt a−1), and of the basal melting of the tongue, 11 m a−1 of ice, are given. Our results lead to an apparent imbalance of the drainage basin (ice discharge 3.5 km3 a−1 lower than the accumulation). Considering previous studies in the Mertz Glacier area, we then discuss the uncertainty of this imbalance and the problems with accumulation mapping for this region.

Winter distribution and abundance of crabeater seals off George V Land, East Antarctica

This study is the first to describe the winter distribution of crabeater seals (Lobodon carcinophagus) in East Antarctica. The study was conducted in the Mertz Glacier Polynya region from July to August 1999. In total 89 crabeater seals were seen in 26 groups which ranged in size from 1 to 35 animals (mean = 3.2). The mean observed haulout density along a 200 m wide strip transect was 0.108 seals per km2, or 0.042 groups per km2. Crabeater seals were not uniformly distributed in the polynya but selected areas of stable ice over shallow (< 1000 m) waters. We used a generalized linear model to assess the relationship of seal distribution to the physical attributes of sea ice concentration, thickness, and ocean depth. We found that ice thickness and ocean depth were the most important determinants of seal distribution. Crabeater seals occurred in areas where the ice affords them a stable haulout platform while allowing them access to Antarctic krill that live directly beneath the ice.

Genetic differentiation in the Antarctic coastal krill Euphausia crystallorophias.

The population genetics of the Antarctic neritic krill species Euphausia crystallorophias was examined by nucleotide sequence variation in its mitochondrial DNA. A 616 base pair region of the cytochrome c oxidase subunit I (COI) gene was screened for mutations by single-strand conformational polymorphism (SSCP) combined with restriction digestion. E. crystallorophias caught in three different regions of the Antarctic coastline were used--two samples from the Mertz Glacier Polynya and one sample each from the western side of the Antarctic Peninsula and from the Davis Sea. Significant genetic differences between krill samples were identified. However, the extent of these differences did not correlate with the degree of geographic separation between the sampling sites. This suggests that the genetic structuring may be the result of small-scale differentiation rather than differentiation between resident populations in separate parts of the Southern Ocean. The possibility that genetic differences between samples within a region are as important as differences between regions has implications for other studies of krill population genetics.

Ice formation in the Mertz Glacier polynya, East Antarctica, during winter

AbstractDuring August 1999, detailed data were collected in the Mertz Glacier polynya along the coast of Antarctica on the drift of newly forming ice tracked with drifting buoys and the ice thickness in the vicinity of the buoys over time. Using these measurements, we estimate the ice-growth rate and the processes which are important in the early stages of ice formation. We find that although there is rapid frazil formation in the open-water areas near the coast and Mertz Glacier Tongue, this frazil ice can take several days to consolidate. A period of warmer weather, when temperatures reached as high as 0°C, delayed the consolidation of the frazil for > 4.5 days. The undeformed new-ice growth rate averaged about 4 cm d-1 for the first 5 days of formation. Ridging and rafting doubled the total growth rate to an average of 8 cm d-1. Blowing and falling snow was incorporated into the surface of the newly forming ice, with 16 of 22 ice cores having some snow in the top few centimeters.

Sea-ice growth and water-mass modification in the Mertz Glacier polynya, East Antarctica, during winter

AbstractIn July-September 1999, an extensive oceanographic survey (87 conductivity-, temperature-and depth-measuring stations) was conducted in the Mertz Glacier polynya over the Adélie Depression off the Antarctic coast between 145° and 150° E. We identify and describe four key water masses in this polynya: highly modified circumpolar deep water (HMCDW), winter water (WW), ice-shelf water (ISW) and high-salinity shelf water (HSSW). Combining surface velocity data (from an acoustic Doppler current-profiler) with three hydrographic sections, we found the HMCDW to be flowing westward along the shelf break (0.7 Sv), the WW and HSSW flowing eastwards underneath Mertz Glacier (2.0 Sv) and that there was a westward return flow of ISW against the continent (1.2 Sv). Using a simple box model for the exchanges of heat and fresh water between the principal water masses, we find that the polynya was primarily a latent-heat polynya with 95% of the total heat flux caused by sea-ice formation. This heat flux results from a fresh-water-equivalent sea-ice growth rate of 4.9−7.7 cm d−1 and a mass exchange between HMCDW and WW of 1.45 Sv The inferred ocean heat flux is 8−14 W m−2 and compares well with other indirect estimates.

Sensible- and latent-heat-flux estimates over the Mertz Glacier polynya, East Antarctica, from in-flight measurements

AbstractCoastal polynyas can be regions of intense ocean-atmosphere heat transfer. In these polynyas, relatively warm ocean is exposed to cold, dry continental air, resulting in high sea-ice production rates. We have calculated ocean-atmosphere heat fluxes over an area of the Mertz Glacier polynya, East Antarctica, from atmospheric data collected in August 1999. Air-temperature and humidity data were measured using a probe extending from a helicopter undercarriage. Flights were made over the most vigorously ice-producing part of the polynya, within 20 km of the coast in southeast Buchanan Bay, both in very strong katabatic winds (20 m s–1) and during calmer conditions (5 m s–1). The total turbulent heat loss from the surface was nearly 575 W m–2 for the windy case and 250 W m–2 for the calmer one, with a ratio of sensible to latent heat in both cases of slightly more than four. These fluxes are in general agreement with other estimates of heat loss from Antarctic polynyas. Strong katabatic winds, sometimes exceeding 40 m s–1, were common in Buchanan Bay and the heat losses during the strong-wind cafe are probably typical of the region. We suggest that this inner part of the Mertz Glacier polynya has a very high ice-production rate.

Brine percolation and the transport properties of sea ice

AbstractSea ice is distinguished from many other porous composites, such as sandstones or bone, in that its microstructure and bulk material properties can vary dramatically over a small temperature range. For brine-volume fractions below a critical value of about 5%, which corresponds to a critical temperature of about −5°C for salinity of 5 ppt, columnar sea ice is effectively impermeable to fluid transport. For higher brine volumes, the brine phase becomes connected and the sea ice is permeable, allowing transport of brine, sea water, nutrients, biomass and heat through the ice. Over the past several years it has been found that brine transport is fundamental to such processes as sea-ice production through freezing of flooded ice surfaces, the enhancement of thermal and salt fluxes through sea ice, nutrient replenishment for sea-ice algal communities, and to sea-ice remote sensing. Motivated by these observations, recently we have shown how percolation theory can be used to understand the critical behavior of fluid transport in sea ice. We applied a percolation model developed for compressed powders of large polymer particles with much smaller metal particles, which explains the observed behavior of the fluid permeability in the critical temperature regime, as well as Antarctic data on surface flooding and algal growth rates. Moreover, the connectedness properties of the brine phase play a significant role in the microwave signature of sea ice through its effective complex permittivity and surface flooding. Here we review our recent results on brine percolation and its role in understanding the fluid and electromagnetic transport properties of sea ice. We also briefly report on measurements of percolation we made on first-year sea ice during the winter 1999 Mertz Glacier Polynya Experiment.

Effects of regional fast-ice and iceberg distributions on the behaviour of the Mertz Glacier polynya, East Antarctica

AbstractAn observational account of research carried out in July-August 1999 shows that grounded iceberg and related fast-ice distributions, and periodic "break-outs" of fast ice (in winter as well as at other times), have an important impact on the size and behaviour of the Mertz Glacier polynya, East Antarctica, and a smaller polynya to the east. Analysis of satellite and in situ data shows that a semi-constant "stream" of thick broken-out fast ice and other large floes from the east extends westwards from north of the glacier terminus to form a compact barrier to the net west-northwesterly export of ice formed in the polynya. An annual fast-ice promontory to the west further narrows the outlet path. As a result of this and high ice-production rates, the polynya periodically "back-fills", significantly reducing the open-water area present. Intervening "flush-outs" by synoptic storm events clear the polynya region to some extent before it back-fills again. This cycle continued from mid-March until early October in 1999, when a significant change in the regional ice drift occurred. A preliminary comparison with data from 1998 indicates that the timing and magnitude of the processes may vary interannually. Similar morphological features were also observed in 1963 (on a declassified photoreconnaissance satellite image).

Mathematician on Ice

A winter expedition into frigid Antarctic waters no pleasure cruise. Screaming winds, bone-chilling temperatures, high seas, driving snow, and crunching ice create conditions that try the hardiest of souls. It's an unlikely place to find a mathematician. Last year's 8-week voyage of the icebreaking research vessel Aurora Australis from Tasmania wasn't Kenneth M. Golden's first venture into the Antarctic ice pack. An applied mathematician at the University of Utah in Salt Lake City, he had also studied sea ice on three previous Antarctic outings. Foul weather never kept Golden from relishing an Antarctic voyage. I loved watching the waves, he says. When the sun squeaked through storm clouds during the few hours of daylight, he could also enjoy spectacular views of giant icebergs, crumpled ice sheets, and penguins waddling across snow or skimming the water. About four dozen researchers were aboard the icebreaker on its 1999 voyage. Their destination was a sizable region, called a polynya, of open water and thin ice that had appeared in the ice-bound waters near the toe of the Mertz Glacier off the Antarctic coast. The sea-ice pack insulates the ocean from the atmosphere. A hole in this lid permits a significant amount of heat to escape from the water to the much chillier air. At the same time, surface waters cool, become denser, and sink, contributing to ocean circulation (SN: 7/15/00, p. 42). New ice begins to form, filling in the hole. As the boundary layer between ocean and atmosphere in polar regions, sea ice has a significant impact on global climate, Golden notes. Polynyas play a crucial role in energy exchange and ice formation, so the occurrence of one inevitably draws the attention of researchers. On a broader front, sea ice is most important in the context of climate variability and both as an indicator and an agent of climate change, says geophysicist Hajo Eicken of the University of Alaska in Fairbanks. The need to understand better the physical properties of sea ice, track its movements, and remotely monitor its thickness, temperature, and roughness has prompted a number of recent studies of its structure. In May at a Society for Industrial and Applied Mathematics meeting in Philadelphia on mathematical aspects of materials science, Golden described how new mathematical models have provided insights into the way brine seeps through ice, carrying heat and nutrients. A vast expanse of sea ice off Antarctica's coast.

On Mertz and Ninnis Glaciers, East Antarctica

AbstractTwo large glacier tongues, which extend substantially across the coastline of King George V Land in East Antarctica, have been studied by remote sensing (synthetic aperture radar, JERS-1). The tongue of Mertz Glacier is in a state of advance, while the Ninnis Glacier tongue is retreating. Some more specific points are: The distinctive surface structure and the form of the glacier tongues indicates that they are floating.While the tongue of Ninnis Glacier has lost about two-thirds of its area since 1913, the Mertz Glacier tongue has advanced substantially and has about doubled its areal extent over the same time period.The annual movement of the tongue of Mertz Glacier was determined as about 1.2 km. This is close to the value of the advance of the tip of the tongue since 1963, which was determined as 0.9 km year−1.

On Mertz and Ninnis Glaciers, East Antarctica

AbstractTwo large glacier tongues, which extend substantially across the coastline of King George V Land in East Antarctica, have been studied by remote sensing (synthetic aperture radar, JERS-1). The tongue of Mertz Glacier is in a state of advance, while the Ninnis Glacier tongue is retreating. Some more specific points are:The distinctive surface structure and the form of the glacier tongues indicates that they are floating.While the tongue of Ninnis Glacier has lost about two-thirds of its area since 1913, the Mertz Glacier tongue has advanced substantially and has about doubled its areal extent over the same time period.The annual movement of the tongue of Mertz Glacier was determined as about 1.2 km. This is close to the value of the advance of the tip of the tongue since 1963, which was determined as 0.9 km year−1.