I. E. Tabacco

Eight glacial cycles from an Antarctic ice core

The Antarctic Vostok ice core provided compelling evidence of the nature of climate, and of climate feedbacks, over the past 420,000 years. Marine records suggest that the amplitude of climate variability was smaller before that time, but such records are often poorly resolved. Moreover, it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records. Here we report the recovery of a deep ice core from Dome C, Antarctica, that provides a climate record for the past 740,000 years. For the four most recent glacial cycles, the data agree well with the record from Vostok. The earlier period, between 740,000 and 430,000 years ago, was characterized by less pronounced warmth in interglacial periods in Antarctica, but a higher proportion of each cycle was spent in the warm mode. The transition from glacial to interglacial conditions about 430,000 years ago (Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long—28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.The Antarctic Vostok ice core provided compelling evidence of the nature of climate, and of climate feedbacks, over the past 420,000 years. Marine records suggest that the amplitude of climate variability was smaller before that time, but such records are often poorly resolved. Moreover, it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records. Here we report the recovery of a deep ice core from Dome C, Antarctica, that provides a climate record for the past 740,000 years. For the four most recent glacial cycles, the data agree well with the record from Vostok. The earlier period, between 740,000 and 430,000 years ago, was characterized by less pronounced warmth in interglacial periods in Antarctica, but a higher proportion of each cycle was spent in the warm mode. The transition from glacial to interglacial conditions about 430,000 years ago (Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long—28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.

A revised inventory of Antarctic subglacial lakes

The locations and details of 145 Antarctic subglacial lakes are presented. The inventory is based on a former catalogue of lake-type features, which has been subsequently reanalysed, and on the results from three additional datasets. The first is from Italian radio-echo sounding (RES) of the Dome C region of East Antarctica, from which 14 new lakes are identified. These data also show that, in a number of occasions, multiple lake-type reflectors thought previously to be individual lakes are in fact reflections from the same relatively large lake. This reduces the former total of lake-type reflectors by six, but also adds a significant level of information to these particular lakes. The second dataset is from a Russian survey of the Dome A and Dome F regions of East Antarctica, which provides evidence of 18 new lakes and extends the coverage of the inventory considerably. The third dataset comprises three airborne RES surveys undertaken by the US in East Antarctica over the last five years, from which forty three new lakes have been identified. Reference to information on Lake Vostok, from Italian and US surveys taken in the last few years, is now included.

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.

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.

An International Plan for Antarctic Subglacial Lake Exploration

Discovery of at least 100 subglacial lakes beneath the vast East Antarctic Ice Sheet has focused international attention on the challenges presented by the way we conduct science in such unique and inhospitable settings in an atmosphere of increasingly stringent environmental concerns. Exploration of subglacial environments will require careful and detailed planning, organization, and international cooperation. To this end, the Scientific Committee on Antarctic Research (SCAR) convened an international Group of Specialists (Subglacial Antarctic Lake Exploration Group of Specialists—SALEGOS) to develop a detailed assessment of the needs and critical milestones to be accomplished during the implementation of a subglacial exploration and research program. This paper surveys the progress and recommendations made by SALEGOS since its inception regarding the current state of knowledge of subglacial environments, technological needs and challenges, international management, the portfolio of scientific projects, and “clean” requirements for entry, observatory deployment, and sample retrieval.

Ice discharge of eastern Dome C drainage area, Antarctica, determined from airborne radar survey and satellite image analysis

Eastern Dome C, southern Talos Dome and northern Taylor Dome are drained by the Priestley, Reeves, David, Mawson and Mackay outlet glaciers, which flow into the Scott Coast on the west side of the Ross Sea, Antarctica. Airborne radar surveys were conducted on these glaciers to determine ice thickness and bed morphology along transverse and longitudinal profiles of the grounded and floating segments. A new analysis of a I,andsat Thematic Mapper satellite image using a tracking technique was used to measure ice velocity at grounding lines and along ice tongues. The integration of radar and satellite data helped to locate grounding lines and to calculate the ice discharge. Changes in ice fluxes of floating glaciers were used to determine basal melting and freezing rates. The ice discharge calculated is less than half that required for a zero net surface mass balance according to the inputs given by the accumulation estimates widely adopted at present. The basal melting rates of meteoric ice represent 50% of the net ablation rate.

Influx of meltwater to subglacial Lake Concordia, East Antarctica

We present evidence for melting at the base of the ice that overlies Lake Concordia, an 800 km2 subglacial lake near Dome Concordia, East Antarctica, via a combination of glaciohydraulic melting (associated with the tilted ice ceiling and its influence on lake circulation/melting temperature) and melting by extreme strain heating (where the ice sheet is grounded). An influx of water is necessary to provide nutrients, material and biota to support subglacial lake ecosystems but has not been detected previously. Freezing is the dominant observed basal process at over 60% of the surface area above the lake. The total volume of accreted ice above the lake surface is estimated as 50-60 km3, roughly 25-30% of the 200 ± 40 km3 estimated lake volume. Estimated rates of melting and freezing are very similar, ±2-6 mm a−1. The apparent net freezing may reflect the present-day response of Lake Concordia to cooling associated with the Last Glacial Maximum, or a large influx of water either via a subglacial hydrological system or from additional melting of the ice sheet. Lake Concordia is an excellent candidate for subglacial exploration given active basal processes, proximity to the Dome Concordia ice core and traverse resupply route.

Surface topography of Dome Concordia (Antarctica) from kinematic interferential GPS and bedrock topography

Abstract A new plano-altimetric map of the Dome Concordia (Dome C) area was drawn up from 1995/96 kinematic double-frequency global-positioning-system (GPS) data of two different projects, as well as static GPS data from a geodetic net for deformation analysis and ice-flow velocity measurements covering an area of about 2000 km2. The GPS surveys were carried out for EPICA during the 10th and 11th Italian Expeditions to choose the optimal location for deep ice-core drilling at Dome C. The accuracy of the kinematic survey was tested by analysing the height-value differences at intersections between different profiles; values ranged from 50–150 mm. The new map was compared with the 1993 kinematic interferential GPS data and residuals between the 1993 and 1995 data were calculated. The surface topographic values were used to calculate the elevation of the Dome C area bedrock, obtained from the reference ground-based and airborne radio-echo-sounding surveys.

New bedrock map of Dome C, Antarctica, and morphostructural interpretation of the area

Abstract A new bedrock map of the Dome C area is presented, based on all radar data collected during Italian Antarctic Expeditions in 1995, 1997, 1999 and 2001. The map clearly distinguishes the Dome C plateau, along with valleys and ridges. The plateau develops at three different altimetric levels, and its morphology is characterized by hills and closed depressions. There are no visible features which can be ascribed to glacial erosion or deposition. The major valley is 15 km wide and 500 m deep; its axis is parallel to that of other valleys and ridges in the plateau. The valley bottom is not flat, but contains a saddle at its centre. The morphology of the major valley may be considered a relict one which was not modified by the overlying ice cap. Two large ridges, characterized by hills, saddles and depressions, lie near the boundaries of the area. The map is used to recalculate ice thickness below the European Project for Ice Coring in Antarctica (EPICA) borehole. The new thickness is 3300 m, 50m greater than before, implying that the expected palaeoclimate record from the ice core could extend back >800 kyr.

Five subglacial lakes and one of Antarctica's thickest ice covers newly determined by radio echo sounding over the Vostok-Dome C region

Radio echo sounding (RES) measurements were collected from 1995 to 2003 during Italian Antarctic expeditions over the Vostok–Dome C region. The data collected allow for the reconstruction of a bedrock elevation map between the Belgica Highlands and the Aurora Subglacial Basin (112.0° - 124.0° E; 74.0° - 78.0° S). Moreover, analysis of the RES data has revealed one of the thickest ice covers in Antarctica (4755 ± 16 m; 118.321° E, 76.059° S) as well as fi ve new subglacial lakes.