Investigating weathering signatures in terrestrial muds: Can climatic signatures be separated from provenance?

Abstract

Siliciclastic muds (clay- and silt-sized sediment) concentrate physical and chemical weathering products. However, both rock composition and climate can affect the mineralogy and geochemistry of these sediments. We quantitatively assessed the influence of provenance and climate on muds collected from end-member climates to test, which, if any, of these potential weathering signatures are indicative of climate in fine-grained, fluvial sediments. Granulometry, mineralogy, and geochemistry of the studied muds indicated that provenance and mineral sorting hinder interpretation of (paleo)climate signals. These issues also affect chemical index of alteration (CIA) values, as well as mafic-felsic-weathering (MFW), Al2O3−(CaO* + Na2O)−K2O (A-CN-K), and Al2O3−(CaO* + Na2O + K2O)−(FeOT + MgO) (A-CNK-FM) ternary plots, decreasing their utility as paleoclimate proxies. CaO content is heavily weighted within the calculations, resulting in even felsic-sourced sediment commonly plotting as mafic owing to the relative enrichment in CaO from preferential sorting of Ca-rich minerals into the mud-sized fraction during transport. These results cast doubt on the indiscriminate use of CIA values and ternary plots for interpreting chemical weathering and paleoclimate within muds, particularly from glacial systems. Most notably, the positive correlations between CIA and climatic parameters (mean annual temperature and mean annual precipitation) diminished when sediments that had formed in nonglacial settings were filtered out from the data sets. This implies that CIA may only be applicable when used in nonglacial systems in which the composition of the primary source material is well constrained—such as soil/paleosol profiles. Within this end-member climate data set, CIA was only useful in discriminating hot-humid climates.