Enhanced pedogenic weathering of hard clay in the Changjiang (Yangtze River) Delta during the last-glacial sea-level lowstand

Deciphering lowland silicate weathering during glacial sea-level lowstands remains challenging because the exposure-related signal is superimposed on an inherited weathering background. The widespread hard clay in the Changjiang (Yangtze River) Delta provides a valuable archive of last-glacial lowstand exposure, recording parent-material accumulation, subaerial exposure and pedogenic weathering. Here, we investigated the hard clay and adjacent deposits in Core TY from the southern Changjiang Delta using chronological, sedimentological, clay-mineralogical, elemental geochemical, and Sr Nd isotopic data. Sr Nd isotopes indicate that the parent material was predominantly Changjiang-derived, with no evidence of a major contribution from distal aeolian dust. Compared with adjacent depositional units, the hard clay exhibits higher CIA and lower WIP values, with clay-mineral evidence also supporting stronger chemical weathering. Using the underlying floodplain deposits as a local depositional analogue, we established an empirical Quartz enrichment –CIA baseline and derived ΔCIA from the hard clay's departure from this trend as a first-order estimate of relative in situ pedogenic weathering. For Core TY, an empirical conversion of ΔCIA yields an illustrative profile-integrated pedogenic CO 2 -consumption equivalent of ∼43–49 mol/m 2 . The Core TY chronology, considered alongside regional sea-level and paleoclimate records, supports a broad two-stage evolution of pedogenic weathering. Stage I involved parent-material accumulation interspersed with relatively weak pedogenic modification during middle MIS 3, whereas the subsequent Stage II was characterized by prolonged subaerial exposure and cumulative pedogenic weathering under variable climatic conditions until Holocene burial. The ΔCIA profile further reveals clear vertical differentiation, with generally stronger alteration in the middle–upper hard clay, consistent with stronger pedogenic leaching nearer the paleosurface. Overall, the Core TY record shows that the Changjiang Delta hard clay can preserve a measurable pedogenic overprint on its inherited depositional background, while the depositional-baseline approach provides a means of resolving such signals in comparable coastal and deltaic deposits.