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

Neodymium and strontium isotopes and a suite of trace elements have been us ed to distinguish between the various sources of particulate loads and soil s in a major catchment of the Murray-Darling drainage system, the largest r iver in Australia. One of the goals was to estimate additions of Sr and rar e earth elements of anthropogenic (fertilizer) origin to the natural catchm ent sources to the soils and streams. Among possible sediment sources, Tert iary basalts and Paleozoic metagraywackes have the lowest Sr-87/Sr-86 and h ighest epsilon(Nd), whereas Paleozoic metapelitic rocks have negative epsil on(Nd) and the highest Sr-87/Sr-86. Phosphate fertilizers have strongly neg ative epsilon(Nd) and Sr-87/Sr-86 similar to Tertiary seawater. Soils forme d on basalt and metagraywacke have compositions that are shifted toward hig her Sr-87/Sr-86 and lower epsilon(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 calc ulations based on isotopic and elemental compositions for reservoir sedimen t ill the upper catchment are consistent with less than 0.2% bulk addition by mass of fertilizer to the natural sediment source. The isotopic composit ions 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-bl own dust, derived either from Paleozoic granitoids that dominate the lower regions of the catchment or from more distal westerly sources. Sediment fro m the lower catchment requires that the dominant source below the reservoir is derived from Paleozoic granitoid and metapelitic rocks of the New Engla nd 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. Copyright (C) 1999 Elsevier Science Ltd.