Massimo Frezzotti

Ground-based measurements of spatial and temporal variability of snow accumulation in East Antarctica

The East Antarctic Ice Sheet is the largest, highest, coldest, driest, and windiest ice sheet on Earth. Understanding of the surface mass balance (SMB) of Antarctica is necessary to determine the present state of the ice sheet, to make predictions of its potential contribution to sea level rise, and to determine its past history for paleoclimatic reconstructions. However, SMB values are poorly known because of logistic constraints in extreme polar environments, and they represent one of the biggest challenges of Antarctic science. Snow accumulation is the most important parameter for the SMB of ice sheets. SMB varies on a number of scales, from small-scale features (sastrugi) to ice-sheet-scale SMB patterns determined mainly by temperature, elevation, distance from the coast, and wind-driven processes. In situ measurements of SMB are performed at single points by stakes, ultrasonic sounders, snow pits, and firn and ice cores and laterally by continuous measurements using ground-penetrating radar. SMB for large regions can only be achieved practically by using remote sensing and/or numerical climate modeling. However, these techniques rely on ground truthing to improve the resolution and accuracy. The separation of spatial and temporal variations of SMB in transient regimes is necessary for accurate interpretation of ice core records. In this review we provide an overview of the various measurement techniques, related difficulties, and limitations of data interpretation; describe spatial characteristics of East Antarctic SMB and issues related to the spatial and temporal representativity of measurements; and provide recommendations on how to perform in situ measurements.

A Review of Antarctic Surface Snow Isotopic Composition: Observations, Atmospheric Circulation, and Isotopic Modeling*

A database of surface Antarctic snow isotopic composition is constructed using available measurements, with an estimate of data quality and local variability. Although more than 1000 locations are documented, the spatial coverage remains uneven with a majority of sites located in specific areas of East Antarctica. The database is used to analyze the spatial variations in snow isotopic composition with respect to geographical characteristics (elevation, distance to the coast) and climatic features (temperature, accumulation) and with a focus on deuterium excess. The capacity of theoretical isotopic, regional, and general circulation atmospheric models (including “isotopic” models) to reproduce the observed features and assess the role of moisture advection in spatial deuterium excess fluctuations is analyzed.

Spatial and temporal variability of snow accumulation in East Antarctica from traverse data

Recent snow accumulation rate is a key quantity for ice-core and mass-balance studies. Several accumulation measurement methods (stake farm, fin core, snow-radar profiling, surface morphology, remote sensing) were used, compared and integrated at eight sites along a transect from Terra Nova Bay to Dome C, East Antarctica, to provide information about the spatial and temporal variability of snow accumulation. Thirty-nine cores were dated by identifying tritium/b marker levels (1965-66) and non-sea-salt (nss) SO4 2- spikes of the Tambora (Indonesia) volcanic event (1816) in order to provide information on temporal variability. Cores were linked by snow radar and global positioning system surveys to provide detailed information on spatial variability in snow accumulation. Stake-farm and ice-core accumulation rates are observed to differ significantly, but isochrones (snow radar) correlate well with ice-core derived accumulation. The accumulation/ablation pattern from stake measurements suggests that the annual local noise (metre scale) in snow accumulation can approach 2 years of ablation and more than four times the average annual accumulation, with no accumulation or ablation for a 5 year period in up to 40% of cases. The spatial variability of snow accumulation at the kilometre scale is one order of magnitude higher than temporal variability at the multi-decadal/secular scale. Stake measurements and firn cores at Dome C confirm an approximate 30% increase in accumulation over the last two centuries, with respect to the average over the last 5000 years.

Rifted(?) crust at the East Antarctic Craton margin: gravity and magnetic interpretation along a traverse across the Wilkes Subglacial Basin region

Abstract Early geophysical studies hypothesized a continental rift structure beneath the Wilkes Subglacial Basin. Recent models favour a flexural origin for the basin linked to Transantarctic Mountains uplift and to East Antarctic Craton lithospheric rigidity. Flexural modelling predicts crustal thickening beneath the basin. Gravity modelling along the International Trans-Antarctic Scientific Expedition traverse (1998/99), however, reveals crustal thinning beneath the basin. At 75°S the crust thins from 37 km beneath the Transantarctic Mountains to 31±2 km beneath the Wilkes Basin. The western flank of the basin features a sharp magnetic break. This signature may arise from a fault separating highly magnetic Precambrian craton crust from weakly magnetic Neoproterozoic(?) crust. Much later crustal extension may have focussed along the craton margin. The eastern flank of the Wilkes Basin exhibits a prominent aeromagnetic signature. Potential field modelling predicts 1–4 km thick sedimentary infill within the Wilkes extended terrane, interpreted mainly as Beacon Supergroup intruded by Jurassic Ferrar tholeiites. The adjacent Adventure Subglacial Trench is a narrow rift basin with 25±5 km thick crust and a 10±4 km sedimentary infill.

Snow dunes and glazed surfaces in Antarctica: New field and remote-sensing data

Abstract As part of the International Trans-Antarctic Scientific Expedition project, the Italian Antarctic Programme undertook two traverses from the Terra Nova station to Talos Dome and to Dome C. Along the traverses, the party carried out several tasks (drilling, glaciological and geophysical exploration). The difference in spectral response between glazed surfaces and snow makes it simple to identify these areas on visible/near-infrared satellite images. Integration of field observation and remotely sensed data allows the description of different mega-morphologic features: wide glazed surfaces, sastrugi glazed surface fields, transverse dunes and megadunes. Topography global positioning system, ground penetrating radar and detailed snow-surface surveys have been carried out, providing new information about the formation and evolution of mega-morphologic features. The extensive presence, (up to 30%) of glazed surface caused by a long hiatus in accumulation, with an accumulation rate of nil or slightly negative, has a significant impact on the surface mass balance of a wide area of the interior part of East Antarctica. The aeolian processes creating these features have important implications for the selection of optimum sites for ice coring, because orographic variations of even a few metres per kilometre have a significant impact on the snow-accumulation process. Remote-sensing surveys of aeolian macro-morphology provide a proven, high-quality method for detailed mapping of the interior of the ice sheet’s prevalent wind direction and could provide a relative indication of wind intensity.

Palaeoseismicity in the Gran Sasso Massif (Abruzzo, Central Italy)

Abstract The upper Maone and Venacquaro valleys, on the northern side of the Gran Sasso Massif (Apennines, Central Italy), have been studied in detail from the geological and geomorphological standpoints to date the activity of the faults that produced spectacular fault scarps in the Late Glacial and Holocene sediments and morphological features. The faults that produced the studied scarps have been active at least four times during the last 18,000 years, and each time, on the basis of the indications emerging from the study of the scarp heights, displacements of similar size were probably produced. The first faulting seems datable to a period slightly more recent than 18,000 years ago. The second faulting occurred after the moraines of the second Apennine Stage. These, preceding a lacustrine deposit dated 11,760 ± 160 BP, could be put in the older portion of the period between 16,000 and 13,000 years ago. The third faulting must be more recent than 6000–7000 years ago, and slightly older than 5450 ± 170 and 6110 ± 180 BP. A fourth phase is appreciably later than 3490 ± 160 BP and older than the year 1000 A.D. For the Venacquaro valley, the size of the displacements (more recent than 18,000 years ago), indicated by the height of the fault scarps and by geophysical data, is about 16–18 m; for the Val Maone the size of the displacements (more recent than the moraines of the second Apennine Stage) is 10–11 m. The respective slip-rates are 0.88–1 mm/year for the Venacquaro valley and approximately 0.67–0.78 mm/year for the Maone valley. It is very probable that the fault activity occurred during a seismic event or events that took place during a short time. The intervals between the seismic events or seismic periods (although these cannot be defined with precision) appear longer than 2500–3000 years and shorter than 6000–7000 years. It is possible that also in the Gran Sasso, as in other parts of the Abruzzo Apennines, seismic activity is characterized by events of great magnitude with long return periods.

Late Quaternary tephra-derived paleosols in central Italy's carbonate Apennine Range: Stratigraphical and paleoclimatological implications

Abstract Detailed stratigraphical, pedological, and geochronological (14C dating) research was carried out on numerous sections located in the carbonate Apennine Range of central Italy. Two paleosols, Late Pleistocene (Pedomarker A) and Holocene (Pedomarker B) in age, were recognized as having developed from air-fall pyroclastic material. Given their special properties and widespread distribution, both soils represent valuable markers in the Late Quaternary stratigraphy of central Italy. Information about the climatic conditions that have influenced their pedogenesis through time has been obtained. Pedomarker A formed under temperate-wet conditions related to the last interstadial fluctuations of the Pleniglacial; its development ceased after ca. 30 ka because of the onset of Last Glacial Maximum dry-cold conditions. Pedomarker B formed at the beginning of the Holocene under udic and perudic soil moisture regimes; its development ceased in some localities because of a change towards a drier climate after ca. 4.5 ka. Petrographic and geochemical characteristics of the parent materials are discussed, and correlations with the Campanian Ignimbrite (Pedomarker A) and Neapolitan Yellow Tuff (Pedomarker B), both related to activity in the Phlegrean Fields, are proposed.

Climate variability along latitudinal and longitudinal transects in East Antarctica

Abstract In the framework of the International Trans-Antarctic Scientific Expedition (ITASE) programme, France and Italy carried out a traverse along one west–east and two north–south transects in East Antarctica from November 2001 to January 2002. Eighteen shallow snow–firn cores were drilled, and surface snow samples were collected every 5km along the traverse. Firn temperatures were measured in boreholes down to 30 m. The cores were analyzed for β radioactivity to obtain snow accumulation-rate data. The surface snow samples were analyzed for δ18O to correlate isotopic values with borehole temperatures. Multiple regression analysis shows a global near-dry-adiabatic lapse rate and a latitudinal lapse rate of 1.05˚C(˚ lat. S)–1, in the Dome C drainage area. Analysis of firn temperatures reveals a super-adiabatic lapse rate along the ice divide between Talos Dome and the Southern Ocean coast, and in some sectors along the ice divide between the Astrolabe Basin and D59. Snow accumulation rates and firn temperatures show warmer temperatures and higher accumulation values close to the ice divides extending from Talos Dome and Dome C to the Southern Ocean. The spatial pattern of data is linked with a katabatic-wind-source basin and moisture-source region.

Geophysical survey at Talos Dome, East Antarctica: the search for a new deep-drilling site

Abstract Talos Dome is an ice dome on the edge of the East Antarctic plateau; because accumulation is higher here than in other domes of East Antarctica, the ice preserves a good geochemical and palaeoclimatic record. A new map of the Talos Dome area locates the dome summit using the global positioning system (GPS) (72˚47’ 14’’S, 159˚04’ 2’’ E; 2318.5m elevation (WGS84)). A surface strain network of nine stakes was measured using GPS. Data indicate that the stake closest to the summit moves south-southeast at a few cm a–1. The other stakes, located 8 km away, move up to 0.33ma–1. Airborne radar measurements indicate that the bedrock at the Talos Dome summit is about 400m in elevation, and that it is covered by about 1900 m of ice. Snow radar and GPS surveys show that internal layering is continuous and horizontal in the summit area (15 km radius). The depth distribution analysis of snow radar layers reveals that accumulation decreases downwind of the dome (north-northeast) and increases upwind (south-southwest). The palaeomorphology of the dome has changed during the past 500 years, probably due to variation in spatial distribution of snow accumulation, driven by wind sublimation. In order to calculate a preliminary age vs depth profile for Talos Dome, a simple one-dimensional steady-state model was formulated. This model predicts that the ice 100m above the bedrock may cover one glacial–interglacial period.

Snow grain-size measurements in Antarctica

Grain size is an important but not well known characteristics of snow at the surface of Antarctica. In the past, grain size has been reported using various methods, the reliability, reproducibility and inter comparability of which is not warranted. In this paper, we present and recommend, depending on available logistical support, 3 techniques of snow grain sampling and /or imaging in the field as well as an ariginal digital image processing method, which we have proved to provide reproducible and intercomparable measures of a snow grain size parameter, the mean convex radius. Results from more than 500 samples and 3000 images of snow grains presented, which yield a still spatially limited, yet unprecendtedly wide picture of near-surface snow grain size distribution from field work in Antarctica. In particular, except at sites affected by a very particular meteorology, surface grains in the interior of the ice sheet are uniformly small (o.1 to 0.2 mm). The climate-related increase of grain size with depth through metamorphism is, as expected, not spatially uniform. Our Antarctic snow grain size database will continue to grow as field investigators bring new samples, images and measures of snow grain size.