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

δ¹³C values of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate organic C (POC) together with δ¹⁸O and δ²H values of water, δ³⁴S 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 δ¹³C_DIC values in the Murray River vary between -9.5 and -4.7‰ with a range of <3‰ within any sampling round δ¹³C_DIC values of the tributaries are -11.0‰ to -5.1‰. DIC concentrations of the Murray River increase from ~25mg/L in the middle and upper reaches of the river to 45-55mg/L in the lower reaches. However, the mass ratio of DIC as a proportion of the total dissolved solids (TDS) decreases from ~0.6-0.7 in the headwaters 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 pCO₂ values that are in the range 550-11,200ppm volume (ppmv), which are far higher than those in equilibrium with the atmosphere (~360ppmv). The δ¹³C_DIC values are similar to those that would be produced by the weathering of marine limestone (δ¹³C~0‰). However, the lack of marine limestones cropping out in the Murray-Darling Basin and the relatively uniform δ¹³C_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 pCO₂ values and δ¹³C_DIC values are best explained by a combination of mineralisation of low δ¹³C organic C and evasion to the atmosphere. The rate of these two processes may attain near steady state and control both DIC concentrations and δ¹³C values.