Nd-Sr isotopic and trace element geochemistry of river sediments and soils in a fertilized catchment, New South Wales, Australia

Abstract

Neodymium and strontium isotopes and a suite of trace elements have been used to distinguish between the various sources of particulate loads and soils in a major catchment of the Murray-Darling drainage system, the largest river in Australia. One of the goals was to estimate additions of Sr and rare earth elements of anthropogenic (fertilizer) origin to the natural catchment sources to the soils and streams. Among possible sediment sources, Tertiary basalts and Paleozoic metagraywackes have the lowest 87Sr/ 86Sr and highest ϵ Nd, whereas Paleozoic metapelitic rocks have negative ϵ Nd and the highest 87Sr/ 86Sr. Phosphate fertilizers have strongly negative ϵ Nd and 87Sr/ 86Sr similar to Tertiary seawater. Soils formed on basalt and metagraywacke have compositions that are shifted toward higher 87Sr/ 86Sr and lower ϵ Nd than their parent rocks. REE patterns and elemental ratios such as Nd/P are also distinctive between fertilizer and natural catchment sources. Reservoir sediment from the upper catchment have isotopic and trace element compositions that confirm that the dominant source of stream particulates is from basalt soils in the steep upland part of the catchment. Mixing calculations based on isotopic and elemental compositions for reservoir sediment in the upper catchment are consistent with less than 0.2% bulk addition by mass of fertilizer to the natural sediment source. The isotopic compositions of soils in less easily eroded portions of the upper catchment reflect the addition of a component to the soil which is interpreted to be wind-blown dust, derived either from Paleozoic granitoids that dominate the lower regions of the catchment or from more distal westerly sources. Sediment from the lower catchment requires that the dominant source below the reservoir is derived from Paleozoic granitoid and metapelitic rocks of the New England fold belt. The lower catchment sediments show no definitive evidence of either basalt or fertilizer input. Natural sources dominate the particulate loads in these streams.