Uranium and plutonium in anoxic marine sediments of the Santiago River mouth (Eastern Pacific, Mexico)

摘要

The uranium (U) and plutonium (Pu) content with depth in a sediment core collected in the continental shelf off the mouth of the Santiago River in the Mexican Pacific was studied to evaluate the contamination effects of the effluent of the Santiago-Lerma River as it moves into the sea. The large mass of terrestrial detritus delivered by the river influences the physicochemical and geochemical processes in the seafloor. Abnormal concentrations of U and Pu in sediments were examined as indicative of the effects of anoxic conditions. One of the indicators of pollution of seawater is the bacterial activity of the shallow seabed layer; and among the prevailing bacteria, the magnetotactic ones induce the formation of euhedral and framboidal shapes (pyrite). These pyrite entities are by-products of anoxic environments loaded with decomposing detrital material and are very abundant in the surface layers of the sediment core analyzed. The pyrite formation is the result of a biochemical reaction between iron and organic sulphur reduced by bacteria, and the pyrite entities precipitate to the seafloor. In the same upper zone of the profile, U-238 is readily immobilized, while U-234 is oxidized and dissolved in seawater by the effect of hot atom chemistry. This may cause the activity ratio (AR) U-234/U-238 disequilibrium (near 0.41). Furthermore, in the shallow layer of the sediment core, an abnormally high concentration of Pu239+240 was detected. In this upper layer, the activity concentrations found were 3.19 Bq kg(-1) for U-238, 1.32 kg(-1) for U-234 and 2.78 Bq kg(-1) for (239+240) Pu. In the lower fractions of the sediment core, normal values of AR (234)u/U-238 (approximate to 1) were found, with traces of Pu239+240. (C) 2016 Elsevier Ltd. All rights reserved.