Radiometric studies of Lake Nyos (Cameroon) sediments: evidence of strong mixing and excess 210Pb

Abstract

Abstract A one-meter-long sediment core was extracted from the middle of Lake Nyos six months after the August 1986 gas burst disaster. Major and trace elements have been determined in twenty samples, including alpha and gamma spectrometry for the radioisotopes. Preliminary results show that: • - 137 Cs is present in uniform concentrations throughout the core. The measured activity (7.3 Bq kg −1 ) corresponds to a total deposit of approximately 1800 Bq m −2 in the one-meter-thick sediment layer, similar in magnitude to the average amounts deposited at these latitudes by atmospheric thermonuclear tests. • - 210 Pb, with a half-life of 22 years, is also uniformly distributed, showing a measured activity of 233 Bq kg −1 , three to four times higher than the values observed in other Cameroon lakes. The activity ratio of 210 Pb to 226 Ra is about 4. These data are interpreted as follows: • - The uniformly distributed 137 Cs and 210 Pb concentrations suggest complete mixing of the top of sediments at the sample location.; • - From the amount of deposited 137 Cs found in the core, this sediment mixing process does not appear to have involved significantly more than that top layer. • - The 210 Pb excess, not supported by 226 Ra, can be explained by an upwards release of radon gas ( 222 Rn) prior to the CO 2 burst.