Deciphering the uranium isotopic signature of coastal water and sediments from Tokyo Bay using a multi-collector inductively coupled plasma mass spectrometer

Abstract

We have measured ²³⁶U/²³⁸U and ²³⁵U/²³⁸U values in JMS-1 (geochemical reference material of Tokyo Bay sediments) and coastal seawater using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) equipped with both the retarding potential quadrupole lens and desolvating nebuliser system. For the accurate measurement of ²³⁶U/²³⁸U values (e.g., in the range of 10⁻⁹), mass spectrometric interferences on ²³⁶U isotope from both the peak tailing of ²³⁸U and polyatomic ion of ²³⁵UH were carefully corrected. With the sequential extraction experiments for JMS-1, whose U isotopic signatures were characteristic of isotopically-depleted U with industrial uses, authigenic U was extracted into a soluble fraction, and lithogenic U and anthropogenic U were gathered in an insoluble fraction. The anthropogenic U is likely to have been provided in insoluble forms and have deposited on the bay floor. Absence in differences of ²³⁶U/²³⁸U and ²³⁵U/²³⁸U values for seawater observed between inside and outside Tokyo Bay implies the negligibly small contribution of the anthropogenic U to U in the seawater. The data obtained here demonstrate the effective reduction of the interferences on ²³⁶U and the versatility of the isotopic signatures of U as an effective tracer for environmental circulation of U in nature.