A STRONTIUM ISOTOPIC INVESTIGATION OF THE ORIGIN OF SUSPENDED PARTICULATE MATTER (SPM) IN THE MURRAY-DARLING RIVER SYSTEM, AUSTRALIA

The strontium isotopic composition of suspended particulate matter (SP M) from the Murray-Darling River system (M-DRS) in eastern Australia v aries significantly as a function of particle size. Colloidal matter(< 1 mu m) defines a linear trace (colloidal mixing line) on both Sr-87/S r-86 vs. 1/Sr mixing and Sr-87/Sr-86 vs Rb-87/Sr-86 isochron diagrams indicating simple two component mixing; isotopic ratios decrease as a function of particle size (e.g., Murray River-Merbein, Sr-87/Sr-86 = 0 .7221-0.7160), This is the first time this type of mixing has been rec ognised in fluvial systems and highlights the influence of particle si ze on isotopic composition. The two components involved are detrital s ilicate grains (aggregates) and varying amounts of natural organic mat ter present as surface coatings. The proportion of natural organic mat ter increases from approximately 10-80% with decreasing particle size. The natural organic matter contains high concentrations of unradiogen ic Sr which has been complexed from solution (e.g., Murray River-Merbe in, Sr-87/Sr-86 = 0.7164). Particulate fractions (>1 mu m) are dominat ed by the silicate component and are isotopically heterogeneous and di splaced from the colloidal mixing lines, with distinctly more radiogen ic Sr (e.g., Murray River-Merbein, Sr-87/Sr-86 = 0.7316-0.7354). The v ariation in the strontium isotopic composition is due to the different ial weathering of minerals in the sediment source. Weathering of plagi oclase produces unradiogenic Sr which dominates the colloidal and diss olved (<0.003 mu m) fractions; the particulate fractions are mainly pr oduced by the weathering of relatively radiogenic K-feldspar and mica. The Murray and Darling River systems have dramatically different rang es of Sr-87/Sr-86; 0.7160- 0.7316 and 0.7076-0.7114 respectively, due to differences in the composition and ages of rocks in their catchment s, The total SPM (>0.003 mu m) and dissolved (<0.003 mu m) fractions c ontain much less radiogenic Sr and higher [Sr] and lower [Rb] concentr ations than mean estimates of the catchment rocks. Relative to ''World Average'' estimates, the M-DRS has similar Sr-87/Sr-86 and [Sr] conce ntration, but markedly lower [Rb] concentration and Rb/Sr.