The distribution of radiocesium and plutonium in sea ice-entrained arctic sediments in relation to potential sources and sinks

Abstract

Gamma counting of a range of grain size fractions of sediments entrained in Arctic Ocean sea ice indicate that the wide range of radiocesium activities that are observed in bulk samples are primarily a function of the geographical origin of the sediment, rather than mineral composition, or physical processes that increase the content of fine clays in sediments. Plutonium isotope ratios ( 240Pu: 239Pu) of sea ice sediments are consistent with an ultimate origin of the plutonium from bomb fallout ( 240Pu: 239Pu=∼0·18), and these sediment ratios differ significantly in plutonium isotope ratios from deep sea sediments of the Arctic Ocean. Much lower plutonium activities were observed in deep sea sediments relative to the sea ice entrained sediments. These differences in isotopic ratios indicate that on decadal scales, sedimentation of bomb fallout plutonium is not the sole source of plutonium to deep Arctic Ocean sediments. The large differences in total plutonium activity between some of the sea ice entrained sediments and all of the deep Arctic Ocean sediments also suggest that the total flux of plutonium from sea ice entrained sediments to the deep sea may be relatively small. Radiocesium activity in the sea ice entrained sediments is well correlated with total plutonium abundance, but the best-fit regression line does not pass through the origin, indicating that a small secondary source of cesium (3 to 9 Bq kg -1 dry weight) that is free of plutonium may contribute to the radiocesium activity observed in sediments entrained in Arctic Ocean sea ice. Based upon observations of carbon:nitrogen weight/weight ratios in excess of 20 in the organic carbon fraction, together with δ 13 C values less than −23%, several of the sea ice entrained sediments show indications of estuarine origin. However, these specific samples typically have low radionuclide burdens. Consideration of the low smectite content (<∼20% by weight) in all of the sea ice sediments and prevailing sea ice transport patterns suggest that a Siberian shelf origin west of the Lena River is improbable for any of the sea ice sediment samples. Nevertheless, in the absence of clear mechanisms for significantly increasing the radionuclide burden in sediments incorporated into sea ice, the radioactivity in the sediment source area appears to be the most crucial determinant of the ultimate radionuclide burden in sea ice sediments.