Vertical distribution of ²³⁶U in the North Pacific Ocean

Highlights

• First extensive study on ²³⁶U in the Pacific Ocean.

• Description of vertical distribution of ²³⁶U.

• Identification of anthropogenic input sources of ²³⁶U in the Pacific Ocean.

• Discussion of seawater circulation in the North Pacific Ocean.

Abstract

The first extensive study on ²³⁶U in the North Pacific Ocean has been conducted. The vertical distribution of ²³⁶U/²³⁸U isotopic ratios and the ²³⁶U concentrations were analysed on seven depth profiles, and large variations with depth were found. The range of ²³⁶U/²³⁸U isotopic ratios was from (0.09 ± 0.03) × 10⁻¹⁰ to (14.1 ± 2.2) × 10⁻¹⁰, which corresponds to ²³⁶U concentrations of (0.69 ± 0.24) × 10⁵ atoms/kg and (119 ± 21) × 10⁵ atoms/kg, respectively. The variations in ²³⁶U concentrations could mainly be attributed to the different water masses in the North Pacific Ocean and their formation processes. Uranium-236 inventories on the water column of each sampling station were calculated and varied between (3.89 ± 0.08) × 10¹² atoms/m² and (7.03 ± 0.50) × 10¹² atoms/m², which is lower than in former studies on comparable latitudes in the North Atlantic Ocean and the Sea of Japan. The low inventories of ²³⁶U found for the North Pacific Ocean in this study can be explained by the lack of additional input sources of artificial radionuclides, apart from global and regional/local fallout. This study expands the use of ²³⁶U as oceanographic circulation tracer to yet another ocean basin and shows that this isotope can be used for tracing circulation patterns of water masses in the Pacific Ocean.

Introduction

Various radioisotopes (³H, ¹³⁷Cs, ^239,240Pu, ⁹⁹Tc) have been used over the last decades as tracers in oceanographic research to add to basic data of seawater (e.g. temperature, density, salinity, dissolved oxygen, nutrients etc.) in the description of water mixing processes and circulation patterns (e.g. Aarkrog et al., 1987, Aoyama et al., 2008, Bowen et al., 1980, Povinec et al., 2005, Sano et al., 1995). According to their specific environmental behaviour, their different half-lives and input sources, radionuclides have found a large range of applications.

In recent years, the uranium isotope ²³⁶U has come into focus regarding a possible use as oceanographic tracer isotope (Casacuberta et al., 2014, Sakaguchi et al., 2012a). The long-lived (t_1/2 = 23.4 My) ²³⁶U was pointed out to be well suited as oceanographic circulation tracer, because of its conservative behaviour and long residence time in the oceans (Christl et al., 2013, Sakaguchi et al., 2016). Furthermore, as ²³⁶U almost entirely originates from anthropogenic production, its sources are comparably well-defined or can be reconstructed from e.g. annual distributions of ²³⁶U concentrations in corals (Sakaguchi et al., 2016, Winkler et al., 2012, Nomura et al., 2016). The natural background is negligibly small compared to the anthropogenic input from the fallout of nuclear weapons tests and discharges from nuclear reprocessing facilities (Steier et al., 2008). The combination of these characteristics makes ²³⁶U a perfect candidate as oceanographic tracer isotope that, owing to its long half-life, might also be able to outperform some more established tracer isotopes like ¹³⁷Cs (half-life: 30.2 y).

The detection of the low ²³⁶U/²³⁸U ratio in environmental samples was made possible by the progress in measurement techniques and especially the development of accelerator mass spectrometry (AMS) (e.g. Lachner et al., 2013, Steier et al., 2008). AMS allows to obtain data on ²³⁶U as isotopic ratio to the naturally occurring, comparably abundant ²³⁸U (about 3 μg/L seawater) (Chen et al., 1986). In seawater, ²³⁸U can be regarded as occurring in steady state and only shows small variations with salinity (Pates and Muir, 2007), which is in contrast to ²³⁶U, which has only been introduced into the environment in recent decades and is still far from reaching steady state.

In a recent publication, we presented the possibility of measuring ²³⁶U concentrations in seawater samples of only 1 L volume during comparably short measurement times (about 30 min per sample) for a whole depth profile in the Northeast Pacific Ocean (Eigl et al., 2016). The downsizing of sample volume in this procedure was feasible mainly due to a new chemical preparation for AMS targets that led to higher ion currents in the measurement. The method was constructed with the idea of applying ²³⁶U in oceanographic studies on a large scale, which would mean that hundreds to thousands of seawater samples need to be analysed and small samples (e.g. ≤ 1 L) are desirable for easy handling. Also, with the large number of samples needed for environmental studies, the focus turns to reducing measurement times when using a comparably costly technique like AMS.

In this research, the new target preparation method was used and 77 seawater samples from seven depth profiles in the North Pacific Ocean were analysed regarding their ²³⁶U/²³⁸U isotopic ratios in order to clarify the distribution of anthropogenic ²³⁶U in this ocean area, where data on this radionuclide is still extremely scarce.