Vol. 48, 01, 05ok.qxd 129 ANNALS OF GEOPHYSICS, VOL. 48, N. 1, February 2005 Key words Antarctica – radon – uranium – thori- um – spectrometry 1. Introduction The area under investigation (fig. 1) is a part of the Shetland microplate which originated at the subducting Pacific margin of Gondwana and contains Cretaceous-Early Miocene island arc extrusives (mainly basalts and andesites) and in- trusives (gabros, diorites, monzonites) (Birken- majer et al., 1991). Oceanic basalts are normally of low uranium and thorium content. and atmos- pheric radon activity is extremly low over ocean- ic island areas. In such a situation any migration of air from the continents, where radon activities are ten times higher can be easily detected, mak- ing radon a useful tracer of atmospheric circula- tion (see Lambert et al., 1970; Turekian et al., 1977; Reiter, 1978; Polian et al., 1986; Balkans- ki and Jacob, 1990; Jacob et al., 1997). However, locally, even within oceanic is- lands, magmatic differentiates of andesitic and granodioritic character and hydrothermal zones occur. They can be enriched in uranium and tho- rium making them a significant local source of radon emmanation. This possibility should be taken into account in the case of any meteoro- logical interpretation. A pioneering paper on radon activity in the King George Island Area published by Evangelista, Pereira (2002) did not contain elements of radiometric mapping, in spite of the consolidated geophysical experience of both authors probably because of logistic problems. Because this, mapping was performed during austral summer 2002/2003 by the present Mailing address: Dr. Andrzej Tomasz Solecki, Institute of Geological Sciences Wrocl⁄ aw Univeristy, pl. M. Borna 9, 50-204 Wrocl⁄ aw, Poland; e-mail asol@ing.uni.wroc.pl Radon and thoron daughter activities in the environment of the King George Island (West Antarctica) Andrzej Tomasz Solecki Institute of Geological Sciences Wrocl⁄ aw Univeristy, Wrocl⁄ aw, Poland Department of Antarctic Biology Polish Academy of Sciences, Warsaw, Poland Abstract Results of 253 gamma spectrometric analyses of radon daughters in soil and bedrocks of the King George Island (West Antarctica) are presented. Measured values range from 0.1 to 58.4 Bq/kg, and from 4.9 to 75.5 Bq/kg for 214Bi and 208Tl respectively, but most measurements fall in the lower part of this range due to predominantly basaltic character of the geological basement. Obtained gamma spectrometric data correspond well to very low soil gas radon content measured by means of Kodak LR115 being below 454 Bqm–3. Low soil gas radon content and char- acteristic type of architecture is responsible for low indoor radon activity in Arctowski Station being as low as 10- 15 Bqm–3. The highest 105 Bqm–3 indoor Rn activity has been measured in the greenhouse bungalow of the station. This increased value was probably connected with the presence of about 1000 kg of imported soil material in the greenhouse room of the bungalow. Local zones of increased uranium and thorium content, discovered as a result of radiometric mapping, can potentaily influence atmospheric radon used for meteorological interpretation. 130 Andrzej Tomasz Solecki author by means of a portable gammaspectrom- eter GR-320. 2. Analytical method Measurements of bedrock radon daughter ac- tivity were performed using an Exploranium GR-320 gamma radiation spectrometer with the standard NaI (Tl) GPX-21A detector of 0.35 L volume. Calibration of the detector was per- formed by the manufacturer using traceable test pads. Impulses supplied by the detector units were classified using channels 70-204 of the 256 channels of the spectrometer covering the energy window 850-2810 keV. Three bands (Regions of Interests ROI) corresponding to energy windows of radionu- Fig. 1. Localisation of the study area, three northernmost sites are located in the Keller Peninsula close to Fer- raz station. Table I. Gamma ray spectrometer GR-320 energy window characteristics. Band Radionuclide peak Channels Energy window (keV) Sensitvity RO/2 40K 1460 keV 109-122 1370-1570 0.661 cps/% RO/3 214Bi 1760 keV 129-142 1660-1860 0.067 cps/ppm RO/4 208Tl 2620 keV 179-204 2410-2810 0.025 cps/ppm 131 Radon and thoron daughter activities in the environment of the King George Island (West Antarctica) clide peaks 40K, 214Bi and 208Tl were set up (table I). The problem of stabilization of energy win- dows of channels was solved by means of con- tinous measurement of caesium 662 keV pho- tons from internal source in the band RO/1 cov- ering channels 51-60 (600-730 keV). Gain parameter responsible for fitting channels to energy windows was continously updated using the least-squares fit of a Gauss- ian caesium peak shape every time the 5000 level of Cs counts was exceeded. This ensured that system gain was always correct and se- lected channels corresponded to the desired energy windows. Measured activities of radon and thoron daughters were recalculated into uranium and tho- rium concentrations (in ppm) assuming existence of equilibrium in uranium and thorium decay se- ries, a common practice in this kind of measure- ments marked by eU and eTh notation instead of U and Th. For each measurement the detector was deployed in the field using a geometry as close as possible to 2Π. Sampling time ranged from 15 to 30 min depending on local radionuclide content to reach statistically significant counts in all ROI’s. Ambient temperature during measurements was in the range –2 to + 6°C, well above the recom- mended limit of –10°C Radon activity was measured by means of the Kodak LR115 solid state nuclear track detec- tors. In the case of indoor measurements detec- tors were placed on the wall at a height of 2 m. Soil-gas radon activity was measured in five sites at a depth of 20 cm and the detectors were fixed inside a plastic cap of 15 cm internal diameter. In both cases free air space around the detector was greater than 7.5 cm to avoid irradiation by plate- out particles. Real soil with organic matter hori- zon (3 cm thick) existed only in one case in oth- er cases physically weathered fine grained mate- rial of bedrock prevailed. 3. Results Average measured bedrock radon and thoron daughter activity was 14.6 and 18.9 Bq/kg for 214Bi and 208Tl, respectively, reaching its maximum values 58.4 and 75.5 Bq/kg for granodioritc dropstone (table II). In the case of basalts, 214Bi activity was often below detection limit, while for 208Tl the lowest recorded value was 4.9 Bq/kg. Trimodal distribution of the ob- tained data was especially well visible in the case of thoron activity (fig. 3). Three groups of radon activities (214Bi): 4, 16 and 36 Bq/kg vis- ible in the fig. 2 corresponded to basement composed of: basalts, andesites and granodi- oritic quarzt lode, respectively. The same rocks in the case of thoron activity (208Tl) data yield- ed three maximas: 4, 20 and 52 Bq/kg (fig. 3). Quartz lodes of 2.8 ppm eU and 12.6 ppm eTh seem to be a potentialy high source of radon, and especially thoron emanation. Its outcrop tens of meters wide was covered with high Table II. Mean values of Rn daughter activities for various rocks types. 214 Bi activity 208 Tl activity eU ppm calculated eTh ppm calculated Number of Bq/kg Bq/kg from 214 Bi activity from 208 Tl activity measurements Basalt 5.6 8.7 0.45 ± 0.07 2.12 ± 0.29 35 Lahar (tillite) 13.7 18.5 1.10 ± 0.07 4.50 ± 0.15 24 Cobble beach 18.2 20.0 1.46 ± 0.11 4.87 ± 0.27 16 Andesite 21.0 25.4 1.69 ± 0.12 6.19 ± 0.43 11 Tuffite-zeolite 21.9 26.5 1.76 ± 0.13 6.45 ± 0.32 20 Granodiorite 58.4 75.5 4.70 18.4 1 Ezcurra fault zone 19.4 23.3 1.56 ± 0.15 5.68 ± 0.46 8 Keller peninsula 35.2 51.9 2.83 ± 0.12 12.65 ± 0.36 12 mineral vein 132 Andrzej Tomasz Solecki amounts of crushed material which resulted due to physical weathering in polar climate. Locally significant chemical weathering was superimposed due to abundance of pyrite. All these factors resulting in a strong increase in porosity and permeability can be responsi- ble for significant radon flux towards Brazil- ian Ferraz Station located few tens meters down slope. The highest result of five measurements of Rn soil gas activity 454 Bqm–3 was measured in the weathered volcanoclastic-zeolitic mate- rial of 21 and 26 Bq/kg for 214Bi and 208Tl re- spectively. Indoor radon activity values ob- tained were as low as 10-15 Bqm–3 (five meas- urements) in social rooms of Polish Henryk Arctowski station. The highest 105 Bqm–3 indoor Rn activity was measured in the greenhouse bungalow of the station. This increased value was probably connected with the presence of about 1000 kg of imported soil material in greenhouse room of the bungalow. 4. Conclusions Obtained results indicate that King George Is- land area is of extremely low Rn potential due to low uranium and thorium content and low emana- tion coefficient of the chemically unweathered bedrock material. Measured values are slightly lower than those based on 22 gamma-spectromet- ric measurements published by Evangelista and Pereira (2002), but a much better fit to Godoy et al. (1998) data and world scale data for areas of such a geology. However, the local increase in uranium and thorium concentration observed lo- cally can result in significant radon flux in favourable meteorogical conditions. Rapid pres- sure changes connected with cyclones can pro- mote radon flux from such zones (see Schery and Gaeddert, 1982), which can be interpreted as in- flux of remote continental air. From this point of view, results obtained at Brasilian Ferraz Station (Evangelista and Pereira, 2002) should be inter- preted very carefully since it is located downslope of the quartz lode outcrop covered with a high dump of crushed and weathered rocky material of relatively high uranium and thorium content. Acknowledgements The author would like to thank Prof. S. Rakusa-Suszczewski and other colleagues from the Department of Antarctic Biology Polish Academy of Sciences for their kind support, help and companionship in the remote area of West Antarctica. The author is also grateful to Brazil- ian Colleagues form Ferraz Station for their kind hospitality. Research was partly funded by grant No. 2022/W/ING of the Institute of Geological Sciences Wroclaw University. REFERENCES BALKANSKI, Y.J. and D.J. JACOB (1990): Transport of conti- nental air to the Subantarctic Indian Ocean, Tellus, 42B (1), 62-75. BIRKENMAJER, K., L. FRANCALANCI and A. PECCERILLO (1991): Petrological and geochemical constraints on the genesis of Mesozoic-Cenozoic magmatism of King George Island, South Shetland Islands, Antarctica, Antarctic Sci., 3 (3), 293-308. Fig. 2. Histogram of radon activity distribution. Fig. 3. 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