Clay Li and Nd isotopes response to hydroclimate changes in the Changjiang (Yangtze) basin over the past 14,000 years

Abstract In large river basins, the relationship between silicate weathering and climate variability on various temporal scales is still highly debated. This study presents clay Li-Nd isotopes and elemental compositions of bank sediments from the Changjiang (Yangtze) River, and of sediments from core CM97 recovered from its delta. In the modern basin, clay δ 7 Li and eNd values of bank sediments range from −4.1‰ to 0.9‰ and from −13.1 to −10.7, respectively. In core CM97, clay δ 7 Li and eNd values vary from −2.5‰ to −1.1‰ and from −12.7 to −11.9. We infer that the variations of clay Li isotope compositions are mostly indicative of weathering intensity changes in the Changjiang basin, and are little influenced by mineralogical sorting, lithology and early diagenesis. Over the last 13 kyr, three periods with distinct clay mineralogy and geochemical compositions can be identified. During the Younger Dryas, the positive excursion of clay δ 7 Li (by ∼1.4‰) is consistent with enhanced incongruent weathering of silicates (i.e., weakened weathering intensity) in the mid-lower Changjiang basin, likely due to climate cooling. The minor variation of clay δ 7 Li (average −1.6 ± 0.2‰) at 11–2 ka confirms the small changes of weathering conditions, which is consistent with mild climatic variations. Over the last 2 kyr, isotopic fluctuations are best explained by sediment source changes, which might be caused by the intensification of human activities in the mid-lower basin. Overall, this study supports a rapid response of silicate weathering to hydroclimate changes on millennium timescale in large catchments. It also provides new insights on Li isotope fractionations at the continental scale during the Quaternary period.