A strontium isotopic investigation of the origin of suspended particulate matter (SPM) in the Murray-Darling River system, Australia

Abstract

The strontium isotopic composition of suspended particulate matter (SPM) from the Murray-Darling River system (M-DRS) in eastern Australia varies significantly as a function of particle size. Colloidal matter ( < 1 μm) defines a linear trace (colloidal mixing line) on both 87Sr86Sr vs. 1/Sr mixing and 87Sr86Sr vs. 87Rb86Sr isochron diagrams indicating simple two component mixing; isotopic ratios decrease as a function of particle size (e.g., Murray River-Merbein, 87Sr86Sr = 0.7221−0.7160). This is the first time this type of mixing has been recognised in fluvial systems and highlights the influence of particle size on isotopic composition. The two components involved are detrital silicate grains (aggregates) and varying amounts of natural organic matter present as surface coatings. The proportion of natural organic matter increases from approximately 10–80% with decreasing particle size. The natural organic matter contains high concentrations of unradiogenic Sr which has been complexed from solution (e.g., Murray River-Merbein, 87Sr86Sr = 0.7164). Particulate fractions (>1 μm) are dominated by the silicate component and are isotopically heterogeneous and displaced from the colloidal mixing lines, with distinctly more radiogenic Sr (e.g., Murray River-Merbein, 87Sr86Sr = 0.7316−0.7354). The variation in the strontium isotopic composition is due to the differential weathering of minerals in the sediment source. Weathering of plagioclase produces unradiogenic Sr which dominates the colloidal and dissolved (<0.003 μm) fractions; the particulate fractions are mainly produced by the weathering of relatively radiogenic K-feldspar and mica. The Murray and Darling River systems have dramatically different ranges of 87Sr/86Sr; 0.7160–0.7316 and 0.7076–0.7114 respectively, due to differences in the composition and ages of rocks in their catchments. The total SPM (>0.003 μm) and dissolved (<0.003 μm) fractions contain much less radiogenic Sr and higher [Sr] and lower [Rb] concentrations than mean estimates of the catchment rocks. Relative to “World Average” estimates, the M-DRS has similar 87Sr86Sr and [Sr] concentration, but markedly lower [Rb] concentration and Rb/Sr.