The annual fallout of radionuclides 32Si, 7Be, 210Pb, and 137Cs in the shelf snow samples collected near the Dakshin Gangotri, East Antarctica, has been estimated. The polar fallout of cosmic-ray produced 32Si estimated to be 2.34 X10-5 dpm cm-2 y-1. The fallout of 7Be and 210Pb is estimated to be 4.2 and 1.86 X 10-2 dpm cm-2 y-1. The depth profiles of electrical conductance, 210Pb, δ18O, and cosmogenic radioisotopes 10Be and 36CI have been measured in a 60 m long ice core. Based on 210Pb and δ18O, the mean annual accumulation rates have been calculated. These rates are 0.20 and 0.23 m of ice equivalent per year during the past ~150 years. Based on electrical conductance measurements and using these accumulation rates, a volcanic event, 'Tambora' that occurred in 1815 AD, was identified. δ18O values suggested that the beginning of the 19th century was colder by about 2°C than the recent past and middle of the 18th century. The fallout of 36CI reported here agrees well with the mean global production rate estimated earlier.
TopIntroduction
Long ice cores from the Polar ice sheets provide information on continuous records of past volcanic eruptions and accumulation rates of ice, climatic and environmental changes, atmospheric and nuclear Fallout, and solar and terrestrial variability for more extended periods (Orheim et al., 1986; Delmas et al., 1992; Nijampurkar & Rao 1993a; Beer et al., 1994; Stuiver et al., 1995 and Hammer et al., 1997). Considerable interest is shown in studying high-resolution records during the last few centuries from different archives like glaciers, ice sheets, tree rings, and lake sediments (Orheim et al., 1986; Ramesh et al., 1989; Nijampurkar & Rao 1993a and Petterson et al., 1993). Such studies record interannual signals of variations in the worldwide surface air temperatures, sea surface temperatures, precipitation, which appear to correlate well with the 11-year solar cycle, heavy rainfall, and flood events (Seleshi et al., 1994 and Currie,1994).
Measurements of the D.C. (Electrical Conductivity[EC] of solid ice) is an essential tool for obtaining historical records of volcanic eruptions in terms of solid acids like H2SO4, HCI, and microparticles by estimation of actual accumulation rates of ice, giving accurate time index during past several thousands of years (Hammer et al., 1997). Stable isotopes of oxygen (δ18O) are excellent time markers and climatic indicators (Jouzel et al., 1987). Natural radioisotopes of different half-lives (210Pb, 10Be, and 36CI) give information about their Fallout and depositional history during different time scales. 210Pb (t1/2=22.3a) provides accumulation rates of ice during the last century. In contrast, the cosmogenic radioisotope 10Be (t1/2 =1.6Ma) with a much longer half-life gives long term accumulation rates of ice ~ during millions of years and information on climatic changes, solar cycles, and sunspot activities during different time scales (Beer et al., 1991). Another cosmogenic isotope 36CI (t1/2=0.3Ma), also produced during nuclear weapons testing from 1940-1980, is a valuable tracer to study the transport process in the atmosphere and hydrology (Synal et al., 1990). These two cosmogenic isotopes are the most suitable proxies to reconstruct solar and terrestrial variability on scales longer than a few centuries. The ratio of 10Be/36CI is advantageous in dating older ice (Nishiizumi et al., 1983). With this introduction, our scientific study is to obtain short term high-resolution records of climatic changes, identifying the volcanic events and annual Fallout of radioisotopes during the past few centuries based on electrical conductance, 32Si, 210Pb, δ18O, 10Be, and 36CI analysis in snow, ice and ice core samples from Antarctica near the Indian station “Dakshin Gangotri.”
We have collected snow, ice, and ice core samples since the 1st Indian expedition to Antarctica. A few surface snow and ice samples were collected in 1982 for isotopic and related studies as a preliminary study. In continuation of these studies, Physical Research Laboratory [PRL] participated in the 5th scientific expedition in 1985 and collected many snow samples ranging from 25 to 300 Kg from the shelf ice at Dakshin Gangotri station. These were used to study the present-day Fallout of different natural (32Si, 10Be, and 210Pb) and artificial (137Cs) radioisotopes. These polar fallout values help estimate ice accumulation rates in remote regions where direct measurements pose several problems. With this background, a 60 m long ice core was raised by the Geological Survey of India [GSI] in 1992, and samples were given to PRL to study the snow accumulation rates and climatic changes and identify volcanic events in the last 150 years. Samples were also collected systematically from the ice wall near Maitri's southwest to study variations during the 21st expedition in 2002. Here we discuss the results obtained from the initial measurement of radioisotopes, electrical conductivity, and stable isotopes on a 60 m long ice core.