Lithium isotope behaviour during weathering in the Ganges Alluvial Plain

摘要

The Ganges river system is responsible for the transportation of a large flux of dissolved materials derived from Himalayan weathering to the oceans. Silicate weathering-driven cooling resulting from uplift of the Himalayas has been proposed to be a key player in Cenozoic climate variation. This study has analysed Li isotope (delta Li-7) ratios from over 50 Ganges river waters and sediments, in order to trace silicate weathering processes. Sediments have delta Li-7 of similar to 0%, identical to bulk continental crust, however suspended sediment depth profiles do not display variations associated with grain size that have been observed in other large river systems. Dissolved delta Li-7 are low (similar to 11%) in the Ganges headwaters, but reach a constant value of 21 +/- 1.6% within a relatively short distance downstream, which is then maintained for almost 2000 km to the Ganges mouth. Given that Li isotopes are controlled by the ratio of primary mineral dissolution to secondary mineral formation, this suggests that the Ganges floodplain is at steady-state in terms of these processes for most of its length. Low delta Li-7 in the mountainous regions suggest silicate weathering is therefore at its most congruent where uplift and fresh silicate exposure rates are high. However, there is no correlation between delta Li-7 and the silicate weathering rate in these rivers, suggesting that Li isotopes cannot be used as a weathering-rate tracer, although they do inform on weathering congruency and intensity. The close-toconstant delta Li-7 values for the final 2000 km of Ganges flow also suggest that once the size of the alluvial plain reached more than similar to 500 km (the flow distance after which riverine delta Li-7 stops varying), the Ganges exerted little influence on the changing Cenozoic seawater d 7 Li, because riverine delta Li-7 attained a near steady-state composition. (C) 2016 Elsevier Ltd. All rights reserved.