The origins and behaviour of carbon in a major semi-arid river, the Murray River, Australia, as constrained by carbon isotopes and hydrochemistry

delta C-13 values of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate organic C (POC) together with delta O-18 and delta H-2 values of water, delta S-34 values of dissolved SO4, and major ion concentrations were measured in the Murray River and its tributaries between November 2005 and April 2007 to constrain the origins and behaviour of riverine C. delta C-13(DIC) values in the Murray River vary between -9.5 and -4.7 parts per thousand with a range of <37 parts per thousand within any sampling round. delta C-13(DIC) values of the tributaries are -11.0 parts per thousand to -5.1 parts per thousand. DIC concentrations of the Murray River increase from similar to 25 mg/L in the middle and upper reaches of the river to 45-55 mg/L in the lower reaches. However, the mass ratio of MC as a proportion of the total dissolved solids (TDS) decreases from similar to 0.6-0.7 in the headwaters to similar to 0.2-0.3 in the lower reaches of the river, with similar downstream changes in DIC/Cl ratios. This precludes simple evaporative concentration of DIC and is interpreted as the river evading CO2; this interpretation is consistent with pCO(2) values that are in the range 550-11,200 ppm volume (ppmv), which are far higher than those in equilibrium with the atmosphere (similar to 360 ppmv). The delta C-13(DIC) values are similar to those that would be produced by the weathering of marine limestone (delta C-13 similar to 0 parts per thousand). However, the lack of marine limestones cropping out in the Murray-Darling Basin and the relatively uniform delta C-13(DIC) values of the Murray River (even in upland reaches where the dominant rock types are metamorphosed silicates and granites) make this unlikely. Rather the high pCO(2) values and delta C-13(DIC) values are best explained by a combination of mineralisation of low delta C-13 organic C and evasion to the atmosphere. The rate of these two processes may attain near steady state and control both DIC concentrations and delta C-13 values. (C) 2010 Elsevier Ltd. All rights reserved.