Deciphering anthropogenic uranium sources in the equatorial northwest Pacific margin

Jixin Qiao,Daniela Ransby,Peter Steier

doi:10.1016/j.scitotenv.2021.150482

Abstract

This work reports the first high-resolution deposition records of anthropogenic uranium (236U and 233U) in a sediment core taken at the continental slope of the Philippine Sea off Mindanao Island in the equatorial northwest Pacific Ocean. Two notable peaks were observed in both profiles of 236U and 233U concentrations, with a narrower peak in 1951–1957 corresponding to close-in Pacific Proving Grounds (PPG) signal, and a broader peak in 1960s–1980s corresponding to the global fallout from nuclear weapons testing. 236U and 233U areal cumulative inventories in the studied sediment core are (2.79 ± 0.20) ∙ 1012 atom ∙ m−2 and (3.12 ± 0.41) ∙ 1010 atom ∙ m−2, respectively, about 20–30% of reported 233U and 236U inventories from the direct global fallout deposition. The overall 233U/236U atomic ratios obtained in this work vary within (0.3–3.5) ∙ 10−2, with an integrated 233U/236U atomic ratio of (1.12 ± 0.17) ∙ 10−2. The contribution from global fallout and close-in PPG fallout to 236U in the sediment core is estimated to be about 69% and 31%, respectively. We believe the main driving process for anthropogenic uranium deposition in the Philippine sediment is continuous scavenging of dissolved 236U from the surface seawater by sinking particles.

Highlights

The long-lived 236U (t1⁄2 = 2.34 ∙ 107 y) has been increasingly applied as a promising oceanic tracer to investigate water circulation transport pathways and timescales
In the environment is from anthropogenic sources including 1) fallout of the atmospheric nuclear weapons testing estimated in the range of
This work presents the first dataset of anthropogenic uranium (236U and 233U) in a sediment core collected at the continental slope of the Philippine Sea

Summary

Introduction

Environmental concentrations of 233U have become accessible, which opens up the application of paired 233U–236U to identify the origination for 236U (Hain et al, 2020; Qiao et al, 2021) This is based on the fact that the 233U/236U ratios, only depend on the utilization history of the nuclear fuel, are not affected by the environmental pathways 233U(t1⁄2 = 1.59 ∙ 105 y) was mostly produced during nuclear weapons testing primarily via 235U (n,3n) 233U reaction by fast neutrons or directly by 233U-fueled devices, whereas almost no 233U is produced in thermal power reactors or reprocessing plants (Hain et al, 2020). Large temporal variations in 233U/236U atomic ratios ((0.2–15.3) ∙ 10−2) were recorded during 1950s–1970s in regional natural archives (Hain et al, 2020; Lin et al, 2021a) due to different source inputs of 233U and 236U from the atmospheric weapons testing

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