Impact of climate on landscape form, sediment transfer and the sedimentary record

The relationship between climate, landscape connectivity and sediment export from mountain ranges is key to understanding the propagation of erosion signals downstream into sedimentary basins. We explore the role of connectivity in modulating the composition of sediment exported from the Frontal Cordillera of the south-central Argentine Andes by comparing three adjacent and apparently similar semi-glaciated catchment-fan systems within the context of an along-strike precipitation gradient. We first identify that the bedrock exposed in the upper, previously glaciated reaches of the cordillera is under-represented in the lithological composition of gravels on each of three alluvial fans. There is little evidence for abrasion or preferential weathering of sediment sourced from the upper cordillera, suggesting that the observed bias can only be explained by sediment storage in these glacially widened and flattened valleys of the upper cordillera (as revealed by channel steepness mapping). A detailed analysis of the morphology of sedimentary deposits within the catchments reveals catchment-wide trends in either main valley incision or aggradation, linked to differences in hillslope-channel connectivity and precipitation. We observe that drier catchments have poor hillslope-channel connectivity and that gravels exported from dry catchments have a lithological composition depleted in clasts sourced from the upper cordillera. Conversely, the catchment with the highest maximum precipitation rate exhibits a high degree of connectivity between its sediment sources and the main river network, leading to the export of a greater proportion of upper cordillera gravel as well as a greater volume of sand. Finally, given a clear spatial correlation between the resistance of bedrock to erosion, mountain range elevation and its covariant, precipitation, we highlight how connectivity in these semi-glaciated landscapes can be preconditioned by the spatial distribution of bedrock lithology. These findings give insight into the extent to which sedimentary archives record source erosion patterns through time.

Automated summary

The study finds that the proportion of bedrock in the sedimentary fan systems is underestimated due to sediment storage in widened glacial valleys. Precipitation leads to catchment-wide trends in either main valley incision or aggradation, and poor hillslope-channel connectivity in dry catchments results in export of gravel depleted in clasts sourced from the upper cordillera.