WEATHERING SIGNATURES IN TERRESTRIAL MUDS: PROVENANCE MASKS PALEOCLIMATE SIGNALS

Siliciclastic muds concentrate physical and chemical weathering products. However, both primary rock composition and climate can affect the mineralogy and geochemistry of these sediments. In this study we used several statistical techniques to test the influence of provenance (primary rock composition and texture) and climate (temperature and precipitation) on these muds to further elucidate potential climate interpretations based on chemical and physical weathering signatures. Quantitative analyses of granulometric, mineralogic, and geochemical attributes of muds collected from drainage basins underlain by crystalline basement in endmember climates indicate that provenance and mineral sorting tend to mask paleoclimate signals. These effects permeate CIA (Chemical Index of Alteration) values and MFW (mafic-felsic-weathering) plots, where high CaO content is heavily weighted within the calculations, resulting in even felsic-sourced sediment often plotting as mafic, due to relative enrichment of CaO from preferential sorting of plagioclase and other Ca-rich minerals into the finer size fractions during transport. These results cast doubt on the indiscriminate use of CIA values and MFW plots for interpreting chemical weathering and paleoclimate within muds particularly from glacial systems, although CIA can usefully discriminate hot-humid climates. Additionally, CIA values from fluvial sediments correlate more strongly to mean annual precipitation than to mean annual temperature over a range of climatic conditions. These results are counter to recent interpretations from datasets composed primarily of sediment analyses from tropical and temperate watersheds where CIA values correlated with mean annual temperature. However, removing sediments formed in hot-humid climates from the datasets significantly reduced correlations between CIA and climate. This implies that CIA may be a useful metric for interpreting paleoclimate in systems that receive high amounts of precipitation, where the composition of the primary source material is well constrained—such as soil/paleosol profiles. However, there is significant variability within field areas, leading to overlapping values across wide ranges of climate conditions.