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Plutonium bio-signature as tracer of climate changes in ocean transport

Periodic Report Summary - PLUTOTRACE (Plutonium bio-signature as tracer of climate changes in ocean transport)

The overall aim of PLUTOTRACE is to apply Pu isotopic records in corals and bivalve shells to the quantification of recent (the last 50 years) changes in ocean current fluxes. The specific objectives in this proposal are to:
A. optimise and validate a mass spectrometric method for the determination of Pu isotope ratio in corals and bivalve shells.
B. investigate the temporal variations of the Pu signature over the past 50 years in the tropical Pacific along the NEC and the KC to the marginal seas of the NW Pacific.
C. test the hypothesis that the total Pu concentrations in the surface water are related to the surface mixed layer depth.
D. reconstruct variations in the past surface mixed layer depths in the path of the NEC and KC, and study the relationship with past sea surface temperature.
E. construct a conceptual model of the alteration in water mass transport from the tropical Pacific to the marginal seas of the NW Pacific.
F. determine the past variation in the Pu signature in the outflow of water from the Irish Sea through the North Channel to the NE Atlantic.

A literature review has been published on Pu as tracer for ocean processes, including comprehensive summaries of the different sources of Pu in the oceans and the behaviour of Pu in the marine environment (see Plutonium isotopes as tracers for ocean processes: A review, P. Lindahl, S.-H. Lee, P.J. Worsfold and M.R. Keith-Roach, Marine Environmental Research, 69, 73-84 (2010). doi: 10.1016/j.marenvres.200908.002).

A mass spectrometric method using Neptune MC-ICP-MS for the ultra-trace determination of Pu isotopes in marine samples was optimised to maximise Pu recovery and removal of interfering elements. Separation strategies for marine calcium carbonate samples were compared. Validation was accomplished by determining Pu in certified marine reference materials. The analytical method allowed detection of 0.02 fg mL-1 Pu. Results from the study, including Pu determination in modern corals from the Chuuk Lagoon in the tropical Northwest Pacific and Geotraces intercalibration seawater samples (see Ultra-trace determination of plutonium in marine samples using multi-collector inductively coupled plasma mass spectrometry, P. Lindahl, M. Keith-Roach, P. Worsfold, M.-S. Choi, H.-S. Shin, S.-H. Lee, Analytica Chimica Acta 671, 61-69 (2010). doi:10.1016/j.aca.2010.05.012).

Modern coral cores from three sites (Guam, Chuuk and Iki) in the Northwest Pacific were obtained in collaboration with scientists from Japan and Korea. The sampling sites were located along the path of the North Equatorial Current and the Kuroshio Current to study the temporal and spatial alteration in Pu signal along the path from the source to the marginal seas of the Northwest Pacific. Seasonal growth layers of the coral cores were sampled and analysed for Pu according to the optimised analytical method. The coral bands were age determined using a combination of stable isotope data and visual observation of the specific seasonal growth bands aided by X-radiography photos. The results from the Guam coral indentified that the 240Pu/239Pu atom ratio from the 1952 Ivy Mike nuclear test was higher (0.46) than previously reported in the literature. The recent Guam coral bands (1981-1999) are dominated by transport of remobilised Pu from the former Pacific Proving Grounds in the Marshall Islands, which strengthen the hypothesis that Pu is transported large distances from its sources in the North Pacific (submitted to Geochimica et Cosmochimica Acta).

About 100 archived bivalve shells from the Korean east coast in the Sea of Japan were obtained from Chuncheon National University. To determine the age, the samples were analysed using low level gamma-ray spectrometry detectors for the potential disequilibrium between 210Pb and 226Ra. However no reliable dating was achieved. A modern coral core from Iki Island, with coral bands which covered the years 1962-2008, was therefore obtained. Iki Island is located at the inlet to the Sea of Japan and is therefore a suitable proxy for the historical Pu signal of water movement into the Sea of Japan.

Due to the opportunity to access and analyse a larger number of coral samples than was originally planned, coupled with limited access to a suitable research vessel for seawater sampling, the research plan was modified to focus on achieving a better spatial resolution of the historical Pu signal along the NEC and KC. This will provide improved parameterisation of the conceptual model of past changes in ocean movement in the Northwest Pacific. Work towards this objective will be undertaken during the second phase of the project at the University of Plymouth.