Seasonal changes in the radiogenic and stable strontium isotopic composition of Xijiang River water: Implications for chemical weathering

Seasonal changes in strontium (Sr) isotopic systematics in large rivers can effectively track temporal variations in regional chemical weathering, and help to better constrain the Sr isotopic budget of global oceans. However, such records are scarce. Here we present a year-long time-series of both radiogenic and stable Sr isotope measurements (87Sr/86Sr and δ88Sr) from river water at Guiping, in the middle reaches of the Xijiang River, South China. Temporal changes in chemical weathering in the drainage basin were investigated by recording seasonal changes in the composition of both Sr isotopes and major ions in the river water. River water 87Sr/86Sr values range from 0.708487 to 0.710336, with most values being < 0.7092, indicating a dominant contribution from the weathering of carbonate rocks. High 87Sr/86Sr values (> 0.7092) are generally accompanied by high Rb/Sr ratios, and low pH and δ13C of dissolved inorganic carbo (DIC) and occur during periods of large river flux. This indicates that the contribution to the chemistry of river water from the weathering of silicate rocks is enhanced during rainy seasons. Seasonal variation in river water δ88Sr is large, from 0.147‰ to 0.661‰, with higher δ88Sr values generally corresponding to higher 87Sr/86Sr values. Again, low δ88Sr values (< 0.3‰) suggest a dominant contribution from the weathering of carbonate rocks, whereas the high δ88Sr values may represent a contribution from intensive weathering of silicate rocks. Using the time-series data, we estimated the flux-weighted averages of Sr concentration, 87Sr/86Sr, and δ88Sr for the Xijiang River to be 1.00 μmol/L, 0.70960, and 0.38‰, respectively. The Sr concentration and 87Sr/86Sr results are close to previous estimates, while the δ88Sr value is amongst the highest recorded for the large rivers of the world, and is close to that of seawater. Our results indicate that large rivers draining through tropical/sub-tropical regions could play an important role in regulating the stable Sr isotopic budget of global oceans.