PII S0016-7037(98)00308-1 Nd-Sr isotopic and trace element geochemistry of river sediments and soils in a fertilized catchment, New South Wales, Australia CANDACE E. MARTIN 1,2, * and MALCOLM T. MCCULLOCH 1 1 Research School of Earth Sciences , The Australian National University, Canberra, ACT 0200 Australia 2 Department of Geography, The Australian National University, Canberra, ACT 0200 Australia (Received October 5, 1998; accepted in revised form November 3, 1998) 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 87 Sr/ 86 Sr and highest  Nd , whereas Paleozoic metapelitic rocks have negative  Nd and the highest 87 Sr/ 86 Sr. Phosphate fertilizers have strongly negative  Nd and 87 Sr/ 86 Sr similar to Tertiary seawater. Soils formed on basalt and metagraywacke have compositions that are shifted toward higher 87 Sr/ 86 Sr and lower  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. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION Phosphate fertilizers have been implicated in the occurrence of toxic blue-green algal (cyanobacteria) blooms in many Austra- lian rivers during low flow conditions. Phosphorus is consid- ered to be a limiting nutrient to algal growth, and circumstances leading to elevated levels of P are considered to be a major contributing cause of algal blooms (Hecky and Kilham, 1988). In turbid rivers such as those which predominate in Australia, P is overwhelmingly associated with the particulate load (Cait- cheon et al., 1995). Understanding the possible sources of P to rivers is consequently a problem that involves tracing the sources of the sediment itself. Because P is monoisotopic, direct stable isotope tracing using this element is impossible. In this paper, radiogenic isotope and trace element geochemical techniques have been used as tracers of the particulate loads of rivers. The isotopic abundances of naturally occurring radiogenic neodymium and strontium varies between different rock, sed- iment and soil types. Because of the long half-lives of their parent nuclides 87 Rb (4.89  10 10 years) and 147 Sm (1.06  10 11 years), the isotopic ratios 87 Sr/ 86 Sr and 143 Nd/ 144 Nd are effectively constant over the time scale of recent surficial processes. Nd and Sr isotopes are therefore powerful tracers of the sources of the particulate and dissolved loads of rivers (Goldstein and Jacobsen, 1987, 1988a; Palmer and Edmond, 1992; Andersson et al., 1994; Alle ´gre et al., 1996; Dupre ´ et al., 1996; Edmond et al., 1996). In igneous rocks, much of the Nd and Sr is partitioned into the same phases that contain P, particularly the mineral apatite or volcanic glass (Hofmann and White, 1982; McDonough et al., 1985; Sun and McDonough, 1989; Price et al., 1991). Chemical weathering may serve to change the Sr and Nd isotopic compositions of weathered relative to unweathered materials by preferential weathering of primary minerals with different Rb/Sr and Sm/Nd, such as plagioclase, pyroxene or amphibole (Graustein and Armstrong, 1983; Aberg et al., 1989; Graustein, 1990; Price et al., 1991; Blum et al., 1993; Miller et al., 1993; Douglas et al., 1995; Taylor and Blum, 1995; Borg and Banner, 1996; Bullen et al., 1996, 1997). When liberated during chemical weathering, these elements are either released to solution, taken up in new min- erals, or trapped by adsorption onto the surface of particles. Phosphate fertilizers are widely applied in agricultural areas of Australia to counteract the naturally low available P content of most soils. Most phosphate fertilizers are produced from phosphorites or from guano deposits from a variety of locations around the world, such as continental margin deposits in Jor- dan, west Africa and the southeastern U.S.A., or from ocean islands in the Pacific Ocean (Nauru Island) or Indian Ocean (Christmas Island), and therefore represent an additional pos- sible source of Nd and Sr in catchments. These fertilizers should retain isotopic and elemental abundances characteristic of their sources, which may be distinct from the natural sources in the catchment. Thus, geochemical tracing using Sr and Nd * Author to whom correspondence should be addressed. Pergamon Geochimica et Cosmochimica Acta, Vol. 63, No. 2, pp. 287–305, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/-1801 $20.00 + .00 287