Showing papers on "Antarctic sea ice published in 1995"

Iceberg discharges into the north atlantic on millennial time scales during the last glaciation.

Abstract: High-resolution studies of North Atlantic deep sea cores demonstrate that prominent increases in iceberg calving recurred at intervals of 2000 to 3000 years, much more frequently than the 7000-to 10,000-year pacing of massive ice discharges associated with Heinrich events. The calving cycles correlate with warm-cold oscillations, called Dansgaard-Oeschger events, in Greenland ice cores. Each cycle records synchronous discharges of ice from different sources, and the cycles are decoupled from sea-surface temperatures. These findings point to a mechanism operating within the atmosphere that caused rapid oscillations in air temperatures above Greenland and in calving from more than one ice sheet.

Sea Ice-Albedo Climate Feedback Mechanism

Abstract: The sea ice-albedo feedback mechanism over the Arctic Ocean multiyear sea ice is investigated by conducting a series of experiments using several one-dimensional models of the coupled sea ice-atmosphere system. In its simplest form, ice-albedo feedback is thought to be associated with a decrease in the areal cover of snow and ice and a corresponding increase in the surface temperature, further decreasing the areal cover of snow and ice. It is shown that the sea ice-albedo feedback can operate even in multiyear pack ice, without the disappearance of this ice, associated with internal processes occurring within the multiyear ice pack (e.g., duration of the snow cover, ice thickness, ice distribution, lead fraction, and melt pond characteristics). The strength of the ice-albedo feedback mechanism is compared for several different thermodynamic sea ice models: a new model that includes ice thickness distribution, the Ebert and Curry model, the Maykut and Untersteiner model, and the Semtner level-3 an...

Reef drowning during the last deglaciation: Evidence for catastrophic sea-level rise and ice-sheet collapse

Abstract: Elevations and ages of drowned Acropora palmata reefs from the Caribbean-Atlantic region document three catastrophic, metre-scale sea-level–rise events during the last deglaciation. These catastrophic rises were synchronous with (1) collapse of the Laurentide and Antarctic ice sheets, (2) dramatic reorganization of ocean-atmosphere circulation, and (3) releases of huge volumes of subglacial and proglacial meltwater. This correlation suggests that release of stored meltwater periodically destabilized ice sheets, causing them to collapse and send huge fleets of icebergs into the Atlantic. Massive inputs of ice not only produced catastrophic sea-level rise, drowning reefs and destabilizing other ice sheets, but also rapidly reduced the elevation of the Laurentide ice sheet, flipping atmospheric circulation patterns and forcing warm equatorial waters into the frigid North Atlantic. Such dramatic evidence of catastrophic climate and sea-level change during deglaciation has potentially disastrous implications for the future, especially as the stability of remaining ice sheets—such as in West Antarctica—is in question.

Late Devensian and Holocene shorelines of the British Isles and North Sea from models of glacio-hydro-isostatic rebound

Abstract: Sea-level change around the British Isles since the time of the last glacial maximum is largely due to of the crustal rebound from the glacial unloading of northern Britain and the concomitant melt-water loading of the adjacent seas and Atlantic Ocean. Minor, but not insignificant, contributions also result from the rebound caused by the unloading of the distant ice sheets, including Fennoscandia and North America. Observations of sea-level change for this period constrain the glacio-hydro-isostatic rebound model parameters describing the effective lithospheric thickness or rigidity and the effective mantle viscosity, as well as certain ice sheet characteristics such as the ice thickness at the time of the last glacial maximum. The models permit palaeobathymetry and palaeoshorelines to be predicted for the British Isles region, including the North Sea. The resulting evolution of the coastlines exhibits a complex behaviour through time, one that is quite different from the usual models in which sea-level change is assumed to be a function of time only. In part this is because of the delayed response of the mantle to the spatially variable and time-dependent ice and water loads, and in part because the unloading history of the British ice sheet is different from those of the major global ice sheets. Thus, maximum emergence of the North Sea occurred after deglaciation had started and lasted for an extended period from about 15 000 to 12 000 (radiocarbon) years BP. During this relative sea-level still-stand shoreline features could have formed, for example, along the western edge of the Norwegian Trough when access to the firths of eastern Scotland would have been via a long and shallow marine inlet. Shoreline retreat across the North Sea became relatively rapid after about 10000 years. The model predictions for the Irish and Celtic Seas also suggest a complex behaviour, with the formation of a wide land bridge between about 20000 and 13 000 years ago. The model also suggests that as long as the Scottish ice extended across the northern Irish Sea, until about 14 000 years ago, there would have been a large freshwater periglacial lake located further south. Both the predicted sea-level height-age relations and the shoreline positions are consistent with a large body of observational evidence but some discrepancies occur, particularly in northern Scotland and Ireland where the ice heights may have been somewhat greater than assumed in the model.

Preservation of Miocene glacier ice in East Antarctica

Abstract: ANTARCTIC climate during the Pliocene has been the subject of considerable debate. One view holds that, during part of the Pliocene, East Antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a¤-4. An alternative viewpoint argues for the development of a stable polar ice sheet by the middle Miocene, which has persisted since then5a¤-10. Here we report the discovery of buried glacier ice in Beacon valley, East Antarctica, which appears to have survived for at least 8.1 million years. We have dated the ice by 40Ar/39Ar analysis of volcanic ash in the thin, overlying glacial till which, we argue, has undergone little (if any) reworking. Isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet. We suggest that stable polar conditions must have persisted in this region for at least 8.1 million years for this ice to have avoided sublimation.

Relationships between the Interannual Variability of Antarctic Sea Ice and the Southern Oscillation

Abstract: A climatology of Antarctic sea ice extent based on 20 years of data (1973–1992) is presented, including measures of interannual variability and extrema. In the first half of the year the greatest variability is found in the Western Hemisphere, particularly in the Weddell and Ross Sea regions, while in the second semester the variability displays a considerable degree of zonal symmetry. We have used this dataset to explore the possible links between Antarctic sea ice extent and the Southern Oscillation Index (SOI). To do this we have calculated their correlation for all pairings of calendar months, as well as with the SOI taken from the year before and subsequent to that of the time of the sea ice data. Most of the correlations assume their largest magnitude when the SOI leads the anomalies in the sea ice, but these differ considerably between the three ocean basins. The extent of Indian Ocean sea ice in the months April through July is positively correlated with the SOI during most of the previou...

Geology and seismic stratigraphy of the Antarctic margin

Abstract: The Antarctic Ice Sheet has greatly affected global climate, sea level, ocean circulation, and southern hemisphere biota during Cenozoic times Much of our understanding of the evolution of the ice sheet has been inferred from isotopic studies on distant deep-ocean sediments, because few Cenozoic rocks are exposed on the Antarctic continent Yet, large differences occur between past ice volumes inferred from isotopic studies and those inferred from low-latitude sea-level variation The massive quantities of glacially transported terrigenous sediments that lie beneath the Antarctic continental margin provide an additional, more direct record of the inferred ice sheet fluctuations Volume 68 addresses the history of ice sheet fluctuations as recorded by geological and geophysical investigations of selected areas of the Antarctic continental margin As described below, the volume gives data and results from on-going research by a major multinational project

The freshwater budget and under-ice spreading of Mackenzie River water in the Canadian Beaufort Sea based on salinity and 18O/16O measurements in water and ice

Abstract: Observations of salinity and oxygen isotope composition (δ18O) were made for the Beaufort shelf-Mackenzie estuary waters in September 1990, just prior to ice formation, and for both the water column and ice in April–May 1991, at the end of winter. These measurements are used to determine the apportioning of fresh water in the estuary between its two main sources, runoff and sea ice melt. Changes in disposition of water between seasons and amounts frozen into the growing ice sheet are also derived. Two domains are considered in order to construct a freshwater budget for the Mackenzie shelf, the nearshore within which landfast ice grows in winter and the outer shelf. Most of the winter inflow from the Mackenzie River appears to remain impounded as liquid under the ice within the landfast zone at the end of winter, and about 15% of it is incorporated into the landfast ice. Oxygen isotopes (δ18O) in ice cores collected from across the shelf record the progress beneath the ice of new Mackenzie inflow as it invades the nearshore throughout winter. Rates of spreading are about 0.2 cm s−1 away from the coast and 1.3 cm s−1 along the coast. As this inflow spreads across the shelf, it progressively shuts off convection driven by brine production at locations within the landfast ice. Salinity and δ18O in the offshore water column suggest that about 3 m of sea ice was formed in the outer shelf domain. Since both brine and newly formed sea ice can be advected off the shelf, a complete budget for brine or sea ice production cannot be established without first measuring the advection of one of these two components.

The freshwater budget and under-ice spreading of

Abstract: Observations of salinity and oxygen isotope composition (5180) were made for the Beaufort shelf-Mackenzie estuary waters in September 1990, just prior to ice formation, and for both the water column and ice in April-May 1991, at the end of winter. These measurements are used to determine the apportioning of fresh water in the estuary between its two main souxces, runoff and sea ice xttex(. nanges in disposition of water between seasons and amounts frozen into the growing ice sheet are also derived. Two domains are considered in order to construct a freshwater budget for the Mackenzie shelf, the nearshere within which landfast ice grows in winter and the outer shelf. Most of the winter inflow from the Mackenzie River appears to remain impounded as liquid under the ice within the landfast zone at the end of winter, and about 15% of it is incorporated into the landfast ice. Oxygen isotopes (Slso) in ice cores collected from across the shelf record the progress beneath the ice of new Mackenzie inflow as it invades the nearshere throughout winter. Rates of spreading are about 0.2 cm s - away from the coast and 1.3 cm s -1 along the coast. As this inflow spreads across the shelf, it progressively shuts off convection driven by brine production at locations within the landfast ice. Salinity and 580 in the offshore water column suggest that about 3 m of sea ice was formed in the outer shelf domain. Since both brine and newly formed sea ice can be advected off the shelf, a complete budget for brine or sea ice production cannot be established without first measuring the advection of one of these two components.

Oceanic evidence for coherent fluctuations in Fennoscandian and Laurentide ice sheets on millennium timescales

Abstract: PROXY temperature records from Greenland ice cores1,2 and North Atlantic sediment cores3 have provided evidence for a high degree of climate instability during the last glacial period. Much of this variability seems to be linked with the dynamics of the Laurentide ice sheet that covered North America at this time3, which dis-charged iceberg flotillas into the North Atlantic that are now recorded in sediment cores as Heinrich events4. How (if at all) this variability was manifested on the other side of the Atlantic— in the Nordic seas and the ice sheets of northwest Europe and Scandinavia—has been unclear. Here we present sediment, micro-fossil and oxygen isotope data from a sediment core in the Norweg-ian sea, which reveal cooling events and iceberg discharges analogous to Heinrich events. We show that these climate fluctua-tions in the Norwegian Sea were in phase, or were phase-locked, with air temperatures over Greenland, suggesting that the rapid changes in heat fluxes in the North Atlantic recorded in previous records3 were felt in this high-latitude region. The iceberg dis-charges in our record seem to have come from the Fennoscandian ice sheet, implying that this and the Laurentide ice sheets fluctu-ated coherently on timescales shorter than those of Milankovitch orbital cycles.

The role of sea ice in 2 x CO2 climate model sensitivity. Part 1: The total influence of sea ice thickness and extent

Abstract: As a first step in investigating the effects of sea ice changes on the climate sensitivity to doubled atmospheric CO2, the authors use a standard simple sea ice model while varying the sea ice distributions and thicknesses in the control run. Thinner ice amplifies the atmospheric temperature senstivity in these experiments by about 15% (to a warming of 4.8 C), because it is easier for the thinner ice to be removed as the climate warms. Thus, its impact on sensitivity is similar to that of greater sea ice extent in the control run, which provides more opportunity for sea ice reduction. An experiment with sea ice not allowed to change between the control and doubled CO2 simulations illustrates that the total effect of sea ice on surface air temperature changes, including cloud cover and water vapor feedbacks that arise in response to sea ice variations, amounts to 37% of the temperature sensitivity to the CO2 doubling, accounting for 1.56 C of the 4.17 C global warming. This is about four times larger than the sea ice impact when no feedbacks are allowed. The different experiments produce a range of results for southern high latitudes with the hydrologic budget over Antarctica implying sea level increases of varying magnitude or no change. These results highlight the importance of properly constraining the sea ice response to climate perturbations, necessitating the use of more realistic sea ice and ocean models.

The Surface Energy Balance of Antarctic Snow and Blue Ice

Abstract: Little is known about the surface energy balance of Antartic blue-ice areas although there have been some studies of the surface energy balance of snow surfaces. Therefore, a detailed meteorological experiment was carded out in the vicinity of a blue-ice area in the Heimefrontfjella, Dronning Maud Land, Antarctica, during the austral summer of 1992/93. Since not all the surface fluxes could be measured directly, the use of a model was necessary. The main purpose of the model is to calculate the surface and subsurface temperatures from which the emitted longwave radiation and the turbulent fluxes can be calculated. The surface energy balance was evaluated at four locations: one on blue ice, and three on snow. Differences are due mainly to the fact that ice has a lower albedo (0.56) than snow (0.80). To compensate for the larger solar absorption of ice, upward fluxes of longwave radiation and turbulent fluxes are larger over ice. Moreover, the energy flux into the ice is larger than into snow due t...

Coupled response of the late glacial climatic shifts of northwest Europe reflected in Greenland ice cores: Evidence from the northern North Sea

Abstract: The climatic regimes of the land areas adjacent to the North Sea are controlled by the influx of Atlantic water north of the British Isles and into the North Sea. A high-resolution record from the Norwegian Channel off western Norway covering the past 15 ka ( 14 C) shows that prior to the Younger Dryas, there were three periods of sea-surface conditions there similar to those of the present, interspersed with arctic conditions. These changes, corresponding to an increase of (Approx.)5 °C in summer sea-surface temperatures, took place on the scale of

Bacteria in sea ice and underlying water of the eastern Weddell Sea in midwinter

Abstract: Bacteria in the water beneath the sea ice of the eastern Weddell Sea were homogeneously distributed. Direct counts resembled values from sprmg and autumn, whereas viable cell counts, total ATP concentrations, as well as heterotrophic assimilation and extracellular enzymatic activities were very low, implying a metabolic inactive bactenoplankton The consolidated sea ice had a very heterogeneous horizontal distribution of microbes on large as well as small scales but vertical profiles in low and densely populated ice cores exhibited similar patterns. A close relation between bacterial colonization of sea ice and genetic ice classes was revealed. Sea ice of the 'predominantly congelation ice' had the lowest bacterial biomass and displayed very low heterotrophic activities which were comparable to those of the water column. Samples of older sea ice belonging to the 'mainly frazil' and 'mixed ice' had maximal numbers of bacteria. They often included high proportions of culturable cells and dividing cells as well as large bacteria The bacteria of these ice classes were active and contributed significantly to the productivity in the Weddell Sea during winter 'Predominantly frazil ice' was less colonized; however, selective bacterial growth was also indicated in this typical winter ice by an increase m the proportions of culturable and psychrophihc bacteria with advancing age of the ice Psychrophilic bacteria dominated in consohdated sea-ice whereas facultative psychrophiles prevailed in young sea-ice and water, corroborating a strict partitiomng m a microbial sea-ice and a seawater regime. Generally, temperature does not appear to be the significant factor for the development of bacterial communities in the surface layer of the eastern Weddell Sea m winter smce the metabolically active bacterial flora develops in the very cold sea-ice environment. The organic matter supply and its unproved usabihty obviously controls bacterial activity as well as the selective enrichment of psychrophiles.

An energy-balance model of lake-ice evolution

Abstract: A physically based mathematical model of the coupled lake, lake ice, snow and atmosphere system is developed for studying terrestrial-atmospheric interactions in high-elevation and high-latitude regions. The ability to model lake-ice freeze-up, break-up, total ice thickness and ice type offers the potential to describe the effects of climate change in these regions. Model output is validated against lake-ice observations made during the winter of 1992-93 in Glacier National Park, Montana, U.S.A. The model is driven with observed daily atmospheric forcing of precipitation, wind speed and air temprrature. In addition to simulating complete energy-balance components over the annual cycle, model output includes ice freeze-up and break-up dates, and the end-of-season clear ice snow-ice and total ice deptlis for two nearby lakes in Glacier National Park, each in a different topographic setting. Modeled ice features are found to agree closely with the lake-ice observations. Model simulations illustrate the key role that the wind component of the local climatic regime plays on the growth and decay of lake ice. The wind speed affects both the surface temperature and the accumulation of snow on the lake-ice surface. Higher snow accumulations on the lake ice depress the ice surface below the water line, causing the snow to become saturated and leading to the formation of snow-ice deposits. In regions having higher wind speeds, significantly less snow accumulates on the lake-ice surface, thus limiting snow-ice formation. The ice produced by these two different mechanisms has distinctly different optical and radiative properties, and affects the monitoring of frozen lakes using remote-sensing techniques.

Nitrate in Polar Ice

Abstract: Nitrate is one of the major impurities in polar snow, and is relatively easy to analyse. Large amounts of data therefore exist, including some from cores extending into the last glaciation. However, the data are not easy to interpret, and we do not yet have an adequate knowledge of even the present-day sources of nitrate to polar snow, nor of the deposition processes that control the concentrations seen. It is clear that anthropogenic pollution has increased the concentrations in Greenland snow by a factor of two in recent decades, and that no similar increase is seen in Antarctica. In pre-industrial Greenland ice, a clear seasonality allows annual layer counting. The sources in pre-industrial ice are probably lightning and/or the stratosphere, while soil exhalation may be an additional major component in Greenland. Whereas nitrate in Holocene ice is present as nitric acid, in ice from the last glaciation it is present as neutral salt, associated with terrestrial cations.

Halocline water formation in the Barents Sea

Abstract: Hydrographic data from the first phase of the Coordinated Eastern Arctic Experiment (CEAREX) are analyzed. The data consist of temperature and salinity measurements made by a ship-based conductivity-temperature-depth (CTD) instrument and by a drifting SALARGOS buoy. These data were collected in the autumn and early winter of 1988-1989 in the northern Barents Sea, mostly in ice-covered conditions and then across the marginal ice zone (MIZ). The data show that relatively warm, salty water of Atlantic origin is modified by air cooling and ice melting to produce lighter water that has properties identical to (lower) halocline water found in the Arctic Ocean. This occurs mostly at the MIZ and to a lesser degree within the ice pack itself. At the MIZ the halocline water subjects underneath the lighter meltwater that resides directly under the ice pack; geostrophic velocity calculations indicate that it then turns eastward and flows toward the Kara Sea, in keeping with previous chemical tracer analyses. A rough calculation reveals that the amount of halocline water formed in this way in the Barents Sea and Fram Strait is 30-50% of that formed by ice growth in eastern Arctic polynyas.

Late Holocene advance of the Müller Ice Shelf, Antarctic Peninsula: sedimentological, geochemical and palaeontological evidence

Abstract: Marine sediment cores were obtained from in front of the Muller Ice Shelfin Lallemand Fjord, Antarctic Peninsula in the austral summer of 199&91. Sedimentological and geochemical data from these cores document a warm period that preceded the advance of the Miiller Ice Shelf into Lallemand Fjord. The advance of the ice shelf is inferred from a reduction in the total organiccarboncontent and an increase in well-sorted, aeolian, sand in cores proximal to the present calving line. This sedimentological change is paralleled by a change in the foraminifera1 assemblages within the cores. Advance of the ice shelf is indicated by a shift from assemblages dominated by calcareous benthic and planktonic forms to those dominated by agglutinated forms. A I4C chronology for the cores indicates that the advance of the Miiller Ice Shelf took place c. 400 years ago, coincident with glacier advances in other high southern latitude sites during the onset of the Little Ice Age. Ice core evidence, however, documents this period as one of warmer temperatures for the Antarctic Peninsula. We suggest that the ice shelf advance was linked to the exclusion of circumpolar deep water from the fjord. This contributed to increased mass balance of the ice shelf system by preventing the rapid undermelt that is today associated with warm circumpolar deep water within the fjord. We also document the recent retreat of the calving line of the Miiller Ice Shelf that is apparently in response to a recent (four decade long) warming trend along the western side of the Antarctic Peninsula.

Water column and sea-ice primary production during Austral spring in the Bellingshausen Sea

Abstract: The findings of a cruise to study the phytoplankton bloom dynamics associated with the marginal ice zone (MIZ) in the Bellingshausen Sea during Austral spring (November-December) 1992 are reported. Biomass and rate process measurements were carried out at stations located in the ice, ice edge and open water along the 85°W meridian in order to establish the productivity of the microalgae associated with sea-ice and in the water column. In addition, a series of transects along 85°W from sea-ice to open water conditions enabled an assessment of the development of phytoplankton populations. Low phytoplankton biomass and production were noted at ice-covered and ice-edge stations and in the open water close to the ice edge. Observations from the transects indicated no development of a classical ice edge bloom despite evidence that sea-ice had retreated more than 100 km during the study period. Survey data along the 85°W line revealed a region of high chlorophyll, centred on 67.5°S, which was initially observed during brash ice conditions. This feature, which remained geographically consistent, persisted for at least 25 days and was thought to be associated with a frontal region. Water column primary production ( 14 C) in this high chlorophyll region was ca 0.8 g C m −2 day − , more than 8 times higher than noted in the MIZ. Phytoplankton photosynthetic characteristics within this region indicated that cells were adapted to a low light regime. A critical depth of 80 m, estimated directly from oxygen flux measurements, was sufficient to permit the initiation and net growth of phytoplankton standing stocks in a mixed layer of ca 70 m. A modelling approach using 14 C observations suggested that phytoplankton growth was less than the sum of the algal loss terms within this feature. An advective supply of cells therefore would be required to sustain the observed high and constant algal biomass. In addition, although this high chlorophyll feature was initially observed during brash-ice conditions, the available data suggest that it was initiated under open water conditions.

Effect of Sea Ice on the Salinity of Antarctic Bottom Waters

Abstract: Brine rejection during the formation of Antarctic sea ice is known to enhance the salinity of dense shelf waters in the Weddell and Ross Seas As these shelf waters flow off the shelves and descend to the bottom, they entrain ambient deep water to create new bottom water It is not uncommon for ocean modelers to modify salinity boundary conditions around Antarctica in an attempt to include a “sea ice effect” in their models However, the degree to which Antarctic salinities are enhanced is usually not quantified or defended In this paper, studies of shelf hydrography and δ 18O are reviewed to assess the level of salinity enhancement appropriate for ocean general circulation models The relevant quantities are 1) the salinity difference between the water masses modified on the shelves and the final offshelf flow and 2) the flux of salt (or freshwater) that gives rise to this salinity difference Onshelf/offshelf salinity changes in the Weddell and Ross Seas appear to be fairly small, 015–020 sa

Thermomechanical modelling of Northern Hemisphere ice sheets with a two-level mass-balance parameterization

Abstract: A 3-D time-dependent thermomechanical ice sheet model was used together with a two-level (snow accumulation/ runoff) mass-balance model to investigatethe Quaternary ice sheets of the northern hemisphere The model freely generates the ice sheet geometry in response to specified changes of surfacetemperature and mass balance, and includes bedrock adjustment, basal sliding and a full temperature calculation within the ice The mass balanceparameterisation makes a distinction between snowfall and melting Yearly snow-fall rates depend on the present precipitation distribution, and are variedproportional to changes of surface temperature and the moisture content of the air The ablation model is based on the positive degree-day-method, anddistinguishes between ice and snow melting This paper discusses steady-state characteristics, conditions for growth and retreat and response time scales ofice sheets as a function of a prescribed lowering of summer temperature Most notably, the modelled extents of the Eurasian ice sheet for a summertemperature lowering of 6-7 K and of the Laurentide ice sheet for a cooling of 9-10 K are in reasonable agreement with most reconstructions based ongeological evidence, except for the presence of a large ice sheet stretching from Alaska across the Bering Strait to most of eastern Siberia In addition, wetbasal conditions turned out to be always confined to the margin, whereas central areas in these reconstructions remained always cold-based This is ofrelevance for processes involving reduced basal traction