El Niño–La Niña cycles and biogeochemical sampling: variability of element concentrations within E. camaldulensis leaves in semi-arid Australia

Abstract

We conducted the analysis of the chemistry of leaves from Eucalyptus camaldulensis (Australian River Red gums) growing in a creek system that crosscuts known Pb-Zn mineralization to assess the influence of cyclic El Niño and La Niña weather patterns on biogeochemical exploration. Samples were collected over two periods: in 2005 immediately after a severe El Niño event where the previous year’s rainfall was 188 mm; and in 2012 after a La Niña event where the previous year’s rainfall was 605 mm. During both periods, elemental components of the mineral system (Ag, Pb, Zn and Cu) were present within the samples at elevated concentrations. Following the El Niño years of low rainfall, the trees directly overlying mineralization contained higher values of both Pb (up to 323 ppm) and Ag (up to 821 ppb) than in the wet La Niña year (Pb up to 69.2 ppm and Ag up to 165 ppb). The spatial distribution for Pb and Ag in both the 2005 and 2012 samples was nearly identical, but with consistently higher concentrations (averaging four times higher) in the 2005 samples. Zinc and Cu values in leaves are also elevated adjacent to mineralization but show less contrast between the two sampling periods, presumably because the uptake of these trace nutrients is largely determined by biological factors. We interpret the large seasonal variations in Pb and Ag to reflect passive uptake of these non-nutrient elements, the concentrations of which in the plant tissues was controlled by the chemistry of the available stream base aquifer, which was comparatively diluted in the La Niña sampling period. These results demonstrate that changes in available water play a significant role in diluting the resulting metal concentration within the trees. These results have implications for mineral exploration sampling programs, particularly those that may take place over multiple field seasons. These implications include changes to anomaly/background cut-off for different years and changes to the dispersion halo.