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 (Sr-87/Sr-86 and delta Sr-88) from river water at Gulping, 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 Sr-87/Sr-86 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 Sr-87/Sr-86 values (>0.7092) are generally accompanied by high Rb/Sr ratios, and low pH and delta C-13 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 delta Sr-88 is large, from 0.147 parts per thousand to 0.661 parts per thousand, with higher delta Sr-88 values generally corresponding to higher Sr-87/Sr-86 values. Again, low delta Sr-88 values (<0.3 parts per thousand) suggest a dominant contribution from the weathering of carbonate rocks, whereas the high delta Sr-88 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, Sr-87/Sr-86, and delta Sr-88 for the Xijiang River to be 1.00 mu mol/L, 0.70960, and 038 parts per thousand, respectively. The Sr concentration and Sr-87/Sr-86 results are close to previous estimates, while the delta Sr-88 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. (C) 2013 Elsevier B.V. All rights reserved.