Clay mineral, geochemical and Sr–Nd isotopic fingerprinting of sediments in the Murray–Darling fluvial system, southeast Australia

Abstract

Clay minerals, trace elements and isotopic signatures (87Sr/86Sr; 143Nd/144Nd) were investigated to fingerprint fine-grained sediments (<2 μm) from the major tributaries of the Murray–Darling fluvial system. Mineralogical, chemical and isotopic signatures in the river clays are clearly inherited from the assemblage of source rocks and soils in the upper catchments of each river. As Murray and Darling tributaries originate in geological formations of different age and composition, significant differences in the respective mineralogical, chemical and isotopic signatures occur between the two river systems. The Darling River system is characterised by abundant smectite, low Ba, K, Rb, Cs, Sn, and high Hf and Zr concentrations, low relatively constant 87Sr/86Sr ratios (0.708 – 0.717) and wide range of εNd(0) values (1.4 to −6.2). In contrast, the Murray River system shows high illite values, high Ba, K, Rb, Cs and Sn concentrations, low Hf and Zr concentrations, a wide range of 87Sr/86Sr ratios (0.726 – 0.775) and relatively constant εNd(0) values (−8.9 to −10.3). Thus, it is possible to trace provenance of suspended material in individual river systems and, by applying simple mixing equations, budget percentage contributions of tributaries at river junctions can be calculated. Based on Sr and Nd isotopic ratios and concentrations, a contribution of 36% for the Darling and 64% for the Murray to the fine-grained sediment (<2 μm) downriver was calculated for the main Darling–Murray junction at Wentworth/Mildura.