Michel Pourchet

A compilation of data on lead 210 concentration in surface air and fluxes at the air‐surface and water‐sediment interfaces

A 210Pb database has recently been constructed and is now available. This base includes approximately 800 210Pb measurements of concentration in air at the Earths surface, together with deposition flux both at the atmosphere-Earth and water-sediment interfaces. Here the data are analyzed and summarized. The atmospheric data (concentration and deposition flux) are presented for the different geographical areas when sufficient measurements are available. The trends are discussed in terms of mechanisms (sources, atmospheric circulation, and climate]. Water-sediment data are classified into four types of water reservoirs which differ in their sedimentation mechanisms or in their in situ 210Pb production. The corresponding histograms are compared with the air-surface flux histogram. This database points out the complete lack of information in some large areas of the planet.

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.

PAST ANTARCTIC PENINSULA CLIMATE (1850-1980) DEDUCED FROM AN ICE CORE ISOTOPE RECORD

A detailed climatic study of the Antarctic Peninsula from 1850 to 1980 has been carried out through the analysis of deuterium content in the snow layers of Dalinger Dome (James Ross Island, Antarctic Peninsula). It is based on the high correlation found between mean deuterium contents at this site and temperature data from stations within this region going back as far as April 1903 for the Argentine Orcadas station.The strong correlation between isotopes and temperatures first reveals a 1956 isotope reference for the region considered. Secondly, the isotope-temperature gradient is estimated at 4.5%. °C−1 for deuterium.After checking that the major temperature anomalies on the Antarctic Peninsula recorded since 1904 (according to available data) correspond to annual mean stable isotope peaks at Dalinger Dome, the amplitude of four prior anomalies are estimated in °C. Finally, a cooling of about 2 °C since 1850 is suggested for the region.

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.

Natural and artificial radioactivity in the Svalbard glaciers

Abstract Natural and artificial radioactivity in the snow of 10 Svalbard glaciers has been measured from 31 ice core samples, drilled between 1981 and 1993. Of these ice cores, seven exhibit the well-known level arising from the fallout of the 1961–1962 atmospheric thermonuclear tests. The second level, due to the Chernobyl accident (26 April 1986), has been detected in all the studied glaciers; the maximum 137 Cs fallout reaches 22 Bq kg −1 and shows a high variability. The natural radioactivity, mostly due to 210 Pb, shows an in-depth variation which is not governed by its half-life (22.2 years). These measurements serve many glaciological purposes: absolute dating of the snow layers; air-snow transger and fallout studies; the determination of mean annual mass balances in the accumulation area of glaciers and their associated spatio-temporal variations.

Mercury in canopy leaves of French Guiana in remote areas.

A study of total Hg concentration in the foliage of the canopy was carried out in two remote areas in French Guiana. The sampled canopy is representative of the French Guiana canopy. The concentration in the foliage, 64+/-14 ngg(-1) (dry wt.), is used to estimate the annual input of total Hg to the soil through the litterfall, found to be 45+/-10 microgm(-2)y(-1). As translocation is negligible, mercury in the canopy originates mainly from atmospheric uptake by the leaves and this litterfall deposit represents a direct atmospheric input from the background atmospheric load into the soil.

Accumulation variability over a small area in east Dronning Maud Land, Antarctica, as determined from shallow firn cores and snow pits: some implications for ice-core records

We investigate and quantify the variability of snow accumulation rate around a medium-depth firn core (160 m) drilled in east Dronning Maud Land, Antarctica (75°00′S, 15°00′E; 3470 m h.a.e. (ellipsoidal height)). We present accumulation data from five snow pits and five shallow (20 m) firn cores distributed within a 3.5-7 km distance, retrieved during the 2000/01 Nordic EPICA (European Project for Ice Coring in Antarctica) traverse. Snow accumulation rates estimated for shorter periods show higher spatial variance than for longer periods. Accumulation variability as recorded from the firn cores and snow pits cannot explain all the variation in the ion and isotope time series; other depositional and post-depositional processes need to be accounted for. Through simple statistical analysis we show that there are differences in sensitivity to these processes between the analyzed species. Oxygen isotopes and sulphate are more conservative in their post-depositional behaviour than the more volatile acids, such as nitrate and to some degree chloride and methanesulphonic acid. We discuss the possible causes for the accumulation variability and the implications for the interpretation of ice-core records.

Radionuclides deposition over Antarctica

A detailed and comprehensive map of the distribution patterns for both natural and artificial radionuclides over Antarctica has been established. This work integrates the results of several decades of international programs focusing on the analysis of natural and artificial radionuclides in snow and ice cores from this polar region. The mean value (37+/-20 Bq m(-2)) of (241)Pu total deposition over 28 stations is determined from the gamma emissions of its daughter (241)Am, presenting a long half-life (432.7 yrs). Detailed profiles and distributions of (241)Pu in ice cores make it possible to clearly distinguish between the atmospheric thermonuclear tests of the fifties and sixties. Strong relationships are also found between radionuclide data ((137)Cs with respect to (241)Pu and (210)Pb with respect to (137)Cs), make it possible to estimate the total deposition or natural fluxes of these radionuclides. Total deposition of (137)Cs over Antarctica is estimated at 760 TBq, based on results from the 90-180 degrees East sector. Given the irregular distribution of sampling sites, more ice cores and snow samples must be analyzed in other sectors of Antarctica to check the validity of this figure.

Mass-balance estimates on the glacier complex Kongsvegen and Sveabreen, Spitsbergen, Svalbard, using radioactive layers

Analyses of total p and y radioactivity have been carried out on ten shallow ice cores collected in 1989 and 1990 on Kongsvegen and Sveabreen, Spitsbergen. No peak of total P radioactivity, corresponding to the Chernobyl accident (1986), can be identified. Chernobyl layers were identified by 137 Cs and 134Cs activities, and a signal from the nuclear tests in Novaya Zemlya (1961-62), was detected at one location by 137Cs activity. The mean net accumulation for the periods 1986-89 and 1962-88 was estimated for both glaciers. Using topographic data, the mean net ablation on Kongsvegen was estimated for the period 1964-90 and the mean net balances were calculated. The results agree with recent direct glaciological balance measurements. For the period 1986-89, the net accumulation was higher on Sveabreen than on Kongsvegen, and the equilibrium-line altitudes (ELA) were around 450 and 520 m a.s.!., respectively. Kongsvegen had a positive balance of 0.11 m w.eq. and Sveabreen was in equilibrium, whereas for the last 26 years the balance of Kongsvegen was slightly negative (-O.IOmw.eq.) and the ELA was around 560 m a.s.!.

Surface air concentration and deposition of lead-210 in French Guiana: two years of continuous monitoring.

To make up for the lack of data on (210)Pb aerosol deposition in tropical regions and to use this radionuclide as an aerosol tracer, a monitoring station was run for two years at Petit-Saut, French Guiana. Lead-210 concentration in air at ground level was monitored continuously together with atmospheric total deposition. The air concentration has a mean value of 0.23+/-0.02 mBq m(-3) during both wet and dry seasons, and it is only weakly affected by the precipitation mechanism. This result was unexpected in a wet tropical region, with a high precipitation rate. In contrast, deposition clearly correlates with precipitation for low/moderate rainfall (<15 cm per 15-day), while this correlation is masked by strong fluctuations at high rainfall. The estimated mean annual deposition over the last ten years is 163+/-75 Bq m(-2) y(-1). This provides a procedure for estimating this mean flux at other sites in French Guiana.