Supplementary material

Holocene ecosystem change in Little Llangothlin Lagoon, Australia: implications for the management of a Ramsar listed wetland.

Craig Woodward, James Shulmeister, Atun Zawadzki, David Child, Linda Barry, Michael Hotchkis

Supplementary methods:

Plutonium accelerator mass spectrometry analysis method

Stratospheric fallout plutonium is associated with fine particles and aerosols in the atmosphere and therefore also in soils and sediments where it eventually settles. Pu extracted by first combusting samples to remove organics from the matrix, followed by acid leaching of the sorbed Pu. A NIST 4334H 242Pu spike is added prior to dissolution to allow the use of isotope dilution. Actinides are concentrated from solution by iron hydroxide co-precipitation, and then separated from matrix components by ion exchange chromatography using the actinide specific Eichrom® resin TEVA®. The purified plutonium component is then dispersed into an iron oxide matrix for AMS analysis. Further detail on this method is given in Child et al. (2008).

Iron oxide pellets containing the Pu are loaded into a sample holder (cathode) and placed into the ion source of the ANTARES HVEE 9MV accelerator. A negative ion beam is produced by use of a Cs sputter ion source, and the resultant Pu ion beam formed is mass analysed and then accelerated to 4MV. This high energy beam is then energy analysed and the 4+ charge state selected, followed by a second stage of mass analysis. This process eliminates all molecular interferences and yields a separated beam of Pu4+ ions to be counted in a gas ionisation chamber. The beam is cycled rapidly between isotopes by use of a fast cycling electrostatic bouncing system Zorko et al. (2010).

ANSTO 210Pb Alpha spectrometry method

Lead-210 (210Pb) (half-life 22.2 years), a member of the uranium decay series, accumulates in sediment as a result of fall-out from the atmosphere and in situ production. 210Pb activity derived from the atmosphere fall-out, the fraction used in the 210Pb dating calculations, is referred to as unsupported 210Pb, whereas 210Pb activity derived from in-situ production is referred to as supported 210Pb. The unsupported 210Pb activity of a sample is calculated by subtracting the supported 210Pb activity from the total activity. Total 210Pb activity in a sample can be measured directly by gamma spectrometry, or indirectly from its progeny, polonium-210 (210Po), by alpha spectrometry. Supported 210Pb activity can be measured from one of its grandparents, radium-226 (226Ra), by either gamma or alpha spectrometry.

Due to the limited amount of sample available, it was not possible to analyse the core samples by gamma spectrometry, where at least 30g of sample is required. The alpha spectrometry method which only required 2g of sample was chosen to analyse the samples from the core.

Each dried sediment sample was spiked with polonium-209 (209Po) and barium-133 (133Ba) tracers. Each sediment sample was subsequently leached with hot concentrated acids to release polonium and radium. Polonium was autoplated onto silver disks after adding the reducing agent hydroxylammonium chloride. Radium and barium were isolated by co-precipitation and collected as colloidal micro-precipitates on fine membrane filter papers. The activities of 210Po on the silver disks and 226Ra on the membrane filters were determined by alpha spectrometry. Each membrane filter was also counted by gamma spectrometry to measure the 133Ba tracer activity. Chemical yield recoveries of 210Po and 226Ra were calculated using the recoveries of 209Po and 133Ba tracers, respectively. The unsupported 210Pb activity on each sediment sample was calculated by subtracting the 226Ra activity from the 210Po activity.

Supplementary References

Child, D.P., Hotchkis, M.A.C., Williams, M.L. (2008) High sensitivity analysis of plutonium isotopes in environmental samples using accelerator mass spectrometry (AMS). Journal of Analytical Atomic Spectrometry 23: 765-768.

Zorko, B., Child, D. P., Hotchkis, M. A. C. (2010) Fast switching electrostatic deflector system for actinide isotopic ratio measurements. 11th International Conference on Accelerator Mass Spectrometry (AMS-11), 14th – 19th September 2008. Spazio Etoile, Rome. In Nuclear Instruments & Methods in Physics Research Section b-Beam Interactions with Materials and Atoms: Proceedings of the Eleventh International Conference on Accelerator Mass Spectrometry, 268(7-8), 827-829.