Chemical weathering plays an important role in sequestering atmospheric CO 2 , but its potential influence on global climate over geological timescales remains debated. To some extent, this uncertainty arises from the difficulty in separating the respective contribution of sedimentary and crystalline silicate rocks to past weathering rates in the geological record; two types of rocks having presumably different impact on the long-term carbon cycle. In this study, we investigate the use of rare earth element (REE) and neodymium isotopes ( Nd ) in leached iron oxide fractions of river sediments for tracing the origin of weathered rocks on continents. A new index, called 'concavity index' (CI), is defined for measuring the degree of mid-REE enrichment in geological samples, which enables the determination of the source of iron oxides in sediments, such as seawater-derived Fe-oxyhydroxide phases, ancient marine Fe oxides derived from the erosion of sedimentary rocks, and recent secondary oxides formed in soils via alteration of crystalline silicate rocks or pyrite oxidation. Using this index, we demonstrate that the  Nd difference between paired Fe-oxide and detrital fractions in river sediments (defined here as  Nd Feox-Det ) directly reflects the relative contribution of sedimentary versus crystalline silicate rocks during weathering. While rivers draining old cratons and volcanic provinces display near-zero  Nd Feox-Det values indicative of dominant silicate weathering (0.5 ± 1.1; n=30), multi-lithological catchments hosting sedimentary formations yield systematically higher values (2.7 ± 1.2; n=44), showing that sedimentary rock weathering can be traced by the occurrence of riverine Fe oxides having more radiogenic Nd isotope signatures compared to detrital fractions. This assumption is reinforced by the evidence that calculated  Nd Feox-Det values agree well with previous estimates for carbonate and silicate weathering rates in large river basins.

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