1,721,251 research outputs found
Accumulation patterns around Dome C, East Antarctica, in the last 73 kyr
No full textWe reconstruct the pattern of surface accumulation in the region around Dome C, East Antarctica, since the last glacial. We use a set of 18 isochrones spanning all observable depths of the ice column, interpreted from various ice-penetrating radar surveys and a 1-D ice flow model to invert for accumulation rates in the region. The shallowest four isochrones are then used to calculate paleoaccumulation rates between isochrone pairs using a 1-D assumption where horizontal advection is negligible in the time interval of each layer. We observe that the large-scale (100sĝ€km) surface accumulation gradient is spatially stable through the last 73ĝ€kyr, which reflects current modeled and observed precipitation gradients in the region. We also observe small-scale (10ĝ€sĝ€km) accumulation variations linked to snow redistribution at the surface, due to changes in its slope and curvature in the prevailing wind direction that remain spatially stationary since the last glacial. © Author(s) 2018
High-resolution boundary conditions of an old ice target near Dome C, Antarctica
No full textA high-resolution (1 km line spacing) aerogeophysical survey was conducted over a region near the East Antarctic Ice Sheet's Dome C that may hold a 1.5 Myr climate record. We combined new ice thickness data derived from an airborne coherent radar sounder with unpublished data that was in part unavailable for earlier compilations, and we were able to remove older data with high positional uncertainties. We generated a revised high-resolution digital elevation model (DEM) to investigate the potential for an old ice record in this region, and used laser altimetry to confirm a Cryosat-2 derived DEM for inferring the glaciological state of the candidate area. By measuring the specularity content of the bed, we were able to find an additional 50 subglacial lakes near the candidate site, and by Doppler focusing the radar data, we were able to map out the roughness of the bed at length scales of hundreds of meters. We find that the primary candidate region contains elevated rough topography interspersed with scattered subglacial lakes and some regions of smoother bed. Free subglacial water appears to be restricted from bed overlain by ice thicknesses of less than 3000 m. A site near the ice divide was selected for further investigation. The high resolution of this ice thickness data set also allows us to explore the nature of ice thickness uncertainties in the context of radar geometry and processing. © Author(s) 2017
Regional Antarctic snow accumulation over the past 1000 years
No full textHere we present Antarctic snow accumulation variability at the regional scale over the past 1000 years. A total of 79 ice core snow accumulation records were gathered and assigned to seven geographical regions, separating the high-accumulation coastal zones below 2000 m of elevation from the dry central Antarctic Plateau. The regional composites of annual snow accumulation were evaluated against modelled surface mass balance (SMB) from RACMO2.3p2 and precipitation from ERA-Interim reanalysis. With the exception of the Weddell Sea coast, the low-elevation composites capture the regional precipitation and SMB variability as defined by the models. The central Antarctic sites lack coherency and either do not represent regional precipitation or indicate the model inability to capture relevant precipitation processes in the cold, dry central plateau. Our results show that SMB for the total Antarctic Ice Sheet (including ice shelves) has increased at a rate of 7 ± 0.13 Gt decadeg 1 since 1800 AD, representing a net reduction in sea level of ∼ 0.02 mm decadeg 1 since 1800 and ∼ 0.04 mm decadeg 1 since 1900 AD. The largest contribution is from the Antarctic Peninsula (∼ 75 %) where the annual average SMB during the most recent decade (2001-2010) is 123 ± 44 Gt yrg 1 higher than the annual average during the first decade of the 19th century. Only four ice core records cover the full 1000 years, and they suggest a decrease in snow accumulation during this period. However, our study emphasizes the importance of low-elevation coastal zones, which have been under-represented in previous investigations of temporal snow accumulation. © 2017 The Author(s)
Geothermal flux and basal melt rate in the Dome C region inferred from radar reflectivity and heat modelling
No full textBasal melt rate is the most important physical quantity to be evaluated when looking for an old-ice drilling site, and it depends to a great extent on the geothermal flux (GF), which is poorly known under the East Antarctic ice sheet. Given that wet bedrock has higher reflectivity than dry bedrock, the wetness of the ice-bed interface can be assessed using radar echoes from the bedrock. But, since basal conditions depend on heat transfer forced by climate but lagged by the thick ice, the basal ice may currently be frozen whereas in the past it was generally melting. For that reason, the risk of bias between present and past conditions has to be evaluated. The objective of this study is to assess which locations in the Dome C area could have been protected from basal melting at any time in the past, which requires evaluating GF. We used an inverse approach to retrieve GF from radar-inferred distribution of wet and dry beds. A 1-D heat model is run over the last 800ĝ€ka to constrain the value of GF by assessing a critical ice thickness, i.e. the minimum ice thickness that would allow the present local distribution of basal melting. A regional map of the GF was then inferred over a 80g 130km area, with a N-S gradient and with values ranging from 48 to 60g2. The forward model was then emulated by a polynomial function to compute a time-averaged value of the spatially variable basal melt rate over the region. Three main subregions appear to be free of basal melting, two because of a thin overlying ice and one, north of Dome C, because of a low GF. © Author(s) 2017
Bromine, iodine and sodium in surface snow along the 2013 Talos Dome-GV7 traverse (northern Victoria Land, East Antarctica)
No full textHalogen chemistry in the polar regions occurs through the release of halogen elements from different sources. Bromine is primarily emitted from sea salt aerosols and other saline condensed phases associated with sea ice surfaces, while iodine is affected by the release of organic compounds from algae colonies living within the sea ice environment. Measurements of halogen species in polar snow samples are limited to a few sites although there is some evidence that they are related to sea ice extent. We examine here total bromine, iodine and sodium concentrations in a series of 2 m cores collected during a traverse from Talos Dome (72°48'S, 159°06'E) to GV7 (70°41'S, 158°51'E) analyzed by inductively coupled plasma-sector field mass spectrometry (ICP-SFMS) at a resolution of 5cm. We find a distinct seasonality of the bromine enrichment signal in most of the cores, with maxima during the austral spring. Iodine shows average concentrations of 0.04 ppb with little variability. No distinct seasonality is found for iodine and sodium. The transect reveals homogeneous air-to-snow fluxes for the three chemical species along the transect due to competing effects of air masses originating from the Ross Sea and the Southern Ocean. © 2017 Author(s)
Antarctic climate variability on regional and continental scales over the last 2000 years
No full textClimate trends in the Antarctic region remain poorly characterized, owing to the brevity and scarcity of direct climate observations and the large magnitude of interannual to decadal-scale climate variability. Here, within the framework of the PAGES Antarctica2k working group, we build an enlarged database of ice core water stable isotope records from Antarctica, consisting of 112 records. We produce both unweighted and weighted isotopic (18O) composites and temperature reconstructions since 0 CE, binned at 5-A nd 10-year resolution, for seven climatically distinct regions covering the Antarctic continent. Following earlier work of the Antarctica2k working group, we also produce composites and reconstructions for the broader regions of East Antarctica, West Antarctica and the whole continent. We use three methods for our temperature reconstructions: (i) a temperature scaling based on the 18O-temperature relationship output from an ECHAM5-wiso model simulation nudged to ERA-Interim atmospheric reanalyses from 1979 to 2013, and adjusted for the West Antarctic Ice Sheet region to borehole temperature data, (ii) a temperature scaling of the isotopic normalized anomalies to the variance of the regional reanalysis temperature and (iii) a composite-plusscaling approach used in a previous continent-scale reconstruction of Antarctic temperature since 1 CE but applied to the new Antarctic ice core database. Our new reconstructions confirm a significant cooling trend from 0 to 1900 CE across all Antarctic regions where records extend back into the 1st millennium, with the exception of the Wilkes Land coast and Weddell Sea coast regions. Within this long-term cooling trend from 0 to 1900 CE, we find that the warmest period occurs between 300 and 1000 CE, and the coldest interval occurs from 1200 to 1900 CE. Since 1900 CE, significant warming trends are identified for the West Antarctic Ice Sheet, the Dronning Maud Land coast and the Antarctic Peninsula regions, and these trends are robust across the distribution of records that contribute to the unweighted isotopic composites and also significant in the weighted temperature reconstructions. Only for the Antarctic Peninsula is this most recent century-scale trend unusual in the context of natural variability over the last 2000 years. However, projected warming of the Antarctic continent during the 21st century may soon see significant and unusual warming develop across other parts of the Antarctic continent. The extended Antarctica2k ice core isotope database developed by this working group opens up many avenues for developing a deeper understanding of the response of Antarctic climate to natural and anthropogenic climate forcings. The first long-term quantification of regional climate in Antarctica presented herein is a basis for data-model comparison and assessments of past, present and future driving factors of Antarctic climate. © 2017 Author(s)
Antarctic ice shelf potentially stabilized by export of meltwater in surface river
No full textMeltwater stored in ponds and crevasses can weaken and fracture ice shelves, triggering their rapid disintegration. This ice-shelf collapse results in an increased flux of ice from adjacent glaciers and ice streams, thereby raising sea level globally. However, surface rivers forming on ice shelves could potentially export stored meltwater and prevent its destructive effects. Here we present evidence for persistent active drainage networks - interconnected streams, ponds and rivers - on the Nansen Ice Shelf in Antarctica that export a large fraction of the ice shelf's meltwater into the ocean. We find that active drainage has exported water off the ice surface through waterfalls and dolines for more than a century. The surface river terminates in a 130-metre-wide waterfall that can export the entire annual surface melt over the course of seven days. During warmer melt seasons, these drainage networks adapt to changing environmental conditions by remaining active for longer and exporting more water. Similar networks are present on the ice shelf in front of Petermann Glacier, Greenland, but other systems, such as on the Larsen C and Amery Ice Shelves, retain surface water at present. The underlying reasons for export versus retention remain unclear. Nonetheless our results suggest that, in a future warming climate, surface rivers could export melt off the large ice shelves surrounding Antarctica - contrary to present Antarctic ice-sheet models, which assume that meltwater is stored on the ice surface where it triggers ice-shelf disintegration. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved
Ice front fluctuation, iceberg calving flux and mass balance of Victoria Land glaciers
No full textThe coast of Victoria Land extends from Williamson Head (69°11'S, 158°E) to McMurdo Sound (77°S, 163°E). A comparison of various documents and images spanning several decades has allowed the ice front fluctuation and the iceberg calving flux during this century to be estimated. During the periods from 1956-65 to 1972-73 the floating glaciers underwent a reduction of 978 km2with an iceberg calving flux of about 134 km2yr-1. After this, during the periods from 1972-73 to 1989-91, the floating glaciers underwent an advance of 272 km2with an iceberg calving flux of about 53 km2yr-1. Glacier tongues with bottom accretion calve less often than those with bottom melting. Most floating glaciers have shown cyclic behaviour without a strong trend. Exceptions to this general style are Hells Gate ice shelf, McMurdo Ice Shelf and floating glaciers of Cape Adare which have undergone a significant retreat since the beginning of the 20th century. The different behaviour of these floating glaciers has been hypothesized as being due to: increased energy available for meltwater production of marine ice that progressively warmed these thin ice shelves and then increased iceberg calving (Hells Gate and McMurdo), or to increased melting at the ice-ocean interface related to a major intrusion of Circumpolar Deep Water from the nearby continental slope (Cape Adare). An estimate of the mass balance of East Antarctica from which these glaciers are fed shows a positive value, that is significant despite all the uncertainties of balance measurements
Twentieth century behaviour of the ice fronts in Antarctica: environmental change evidence
No full textA comparison of various documents, dated several years apart, has allowed the surficial ice discharge, the ice front fluctuation and the iceberg calving flux of Victoria Land coast (Antarctica) during this century to be estimated. The Hells Gate and McMurdo Sound ice shelves and the floating glaciers of Cape Adare have undergone a significant retreat since the beginning of the 20th century. The different behaviour of these floating glaciers with respect to others has been presumed to be due to increased energy available for meltwater production and to increased melting at the ice-ocean interface related to Circumpolar Deep Water. A first estimate of the mass balance of glaciers that fringe the Victoria Land Coast shows a significantly positive value, despite all the uncertainties of balance measurements
Glaciers and ice sheets: Current status and trends
No full textAbout 10 % of the land surface on Earth is covered by glacier ice, with an estimated total volume equivalent to about 66 m of potential sea-level rise. Almost the totality (99 %) of this volume is locked in the polar ice sheets, while less than 1 % forms all the other mountain glaciers and ice caps. In the last three decades, the general retreat of the mountain glaciers and the accelerated flow and ice loss from several outlet glaciers draining the Greenland and the Antarctic ice sheets came to general attention as a major evidence of climate warming and as a potential contribution to the sea-level rise, to local shortage of water resources and to other environmental risks. Here, we present a short review of the most recent data and assessments on the present status and on trends of glaciers and polar ice sheets. The Greenland ice sheet (12 % of total glacier ice volume) over the last three decades showed an increase of the extent of the surface melt area and an acceleration of many marine-terminating glaciers; as a consequence, the ice sheet is losing ice at an increasing rate that reached -263 ± 30 Gt/year in the 2005-2010 time interval, equivalent to a sea-level rise of 0.72 ± 0.08 mm/year. The much larger, higher and colder composite Antarctic ice sheet (87 % of total glacier ice volume), in the same 2005-2010 time interval, had an ice loss of -81 ± 37 Gt/year. Mountain glaciers and ice caps are retreating in all the major glacierized regions, with the exception of a few mountain areas where contrasting patterns have been observed. Although containing less than 1 % of the total glacier ice, mountain glaciers and ice caps suffered a total ice loss of -259 ± 28 Gt/year in the period 2003-2009, equivalent to a sea-level rise of 0.71 ± 0.08 mm/year. The overall contribution of glaciers and ice sheets is estimated equivalent to a sea-level rise of 1.50 ± 0.16 mm/year for the period 2003-2009, or about 60 % of the total sea-level rise in the same period. Various estimates of the total glacier contribution to the sea-level rise by the end of the twenty-first century have been recently proposed, ranging from a few decimeters to 2 m, with most plausible projections at about 0.5 m. Most probably Greenland, the Antarctic Peninsula and the West Antarctic ice sheet will continue to lose ice, while the sign of the East Antarctica contribution is uncertain. Mountain glaciers will most likely continue to lose ice, although at different rates in the various mountain regions. For the European Alps and the Southern Alps (New Zealand), a loss of more than 70 % of their present volume is expected by the end of the twenty-first century. The glaciers' contraction in the mountain areas may cause slope failures, debris mobilization, outburst floods from glacial lakes, and water deficits, particularly in the summer season, in the arid zones in the coming decades. Together with other changes occurring in the cryosphere such as the Arctic sea-ice reduction, the snow cover decline and the permafrost degradation, the glacier retreat is considered part of a larger picture of environmental changes, directly or indirectly caused or increased by the human impact, leading to new environmental conditions, thus deserving to be indicated as the Anthropocene. Still more open to future responses is the consideration if the ongoing glacier reduction and the rise of the sea level will contribute to leave such a footprint in the geologic record as to require a new stratigraphic unit, a new time epoch in the billion years long history of the Earth. © 2013 Accademia Nazionale dei Lincei
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