REE and Nd isotopes in sedimentary Fe oxides as proxies for shale weathering
Résumé
Chemical weathering plays an important role in sequestering atmospheric CO2, 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. Here, we present a novel method for tracing the origin of weathered rocks on continents, based on the measurement of REE and Nd isotopes (eNd) in leached iron oxide fractions of river sediments [1,2]. We show that the degree of mid-REE enrichment in leached sediment phases provides information on the source of Fe oxides, indicating the presence of ancient marine Fe oxides derived from the erosion of sedimentary rocks or more recent secondary oxides formed in soils via silicate weathering. We also demonstrate that the e Nd difference between paired Fe-oxide and detrital fractions in river sediments (DeNd Feox-Det) reflects the relative contribution of sedimentary vs crystalline silicate rocks during weathering. Rivers draining old cratons and volcanic provinces display near-zero DeNd Feox-Det values indicative of dominant silicate weathering (0.5 ± 1.1), while multi-lithological catchments hosting sedimentary formations
yield systematically higher values (2.7 ± 1.2). Taken together, these findings show that sedimentary rock weathering can be traced by the occurrence of riverine Fe oxides having more radiogenic Nd isotope signatures compared to detrital fractions. Finally, the influence of climate and geomorphic parameters on the Nd isotopic composition of sedimentary Fe oxides will be discussed, together with future perspectives.