GEOCHEMISTRY AND WEATHERING FLUXES OF THE YANGTZE HEADWATERS IN CHINA

The headwaters of the Yangtze (Chang Jiang) in China/Eastern Tibet drain the tectonically active Himalayan-Tibet orogeny. Riverine fluxes of the Yangtze along with other Himalayan/Tibetan river systems make up a significant fraction of the global weathering budget. To interpret the geochemistry of rivers in terms of lithology, tectonics, weathering rates and global climate implications, we use major ion and trace element concentrations along with Sr isotopes. The fraction of the total flux released by weathering of silicate rocks, a sink for atmospheric CO₂, is important and contributes to changing global climate on geological time-scales.

We sampled over 150 pristine locations on various tributaries spanning a period of 5 years from 1999 through 2004. Major element chemistry (along with Sr, Rb, Ba, Cs, U and Th) is comparable to those of other large rivers draining the Himalayas. Total dissolve solids range from 0.9 mmol/kg to 10 mmol/kg with dissolved fluxes ranging from 1 x 10⁴ mol km^-2 yr^-1 to 2 x 10⁶ mol km^-2 yr^-1.

Using major ion concentrations we evaluate contributions from weathering sources (carbonates, silicates etc). Ternary cation (Ca, Mg, Na+K) and anion (Alkalinity, Si, Cl+SO₄) diagrams provide a quick qualitative view of relative contributions from different sources. The Upper Yangtze basin largely consists of Trassic flysch and carbonates and, as expected, carbonate weathering dominates the dissolved load. We quantify weathering fluxes using major element concentrations along with Sr isotope ratios, which provide additional constraints on the source of the weathering flux (e.g. radiogenic silicates v/s carbonate). Dissolved ⁸⁷Sr/⁸⁶Sr values range from 0.7097 to 0.715, reflecting simple mixing between silicate, carbonate and evaporite end-members. Annual net CO₂ drawdown is estimated at 2x10² mol km^-2 yr^-1 to 1x10⁶ mol km^-2 yr^-1 and is comparable to the range of other global rivers.