Imprint of an extreme rainfall event on landscape erosion traced by feldspar single-grain luminescence (Río Ñuble, Chile).

Single-grain post-infrared luminescence (SG-pIRIR) is able to trace river sediment dynamics stored in fluvial deposits through the interpretation of scatter in equivalent dose (D_e) distribution caused by heterogeneous bleaching (zeroing) of  grains by sunlight exposure prior to deposition.  Despite the challenge of heterogeneous bleaching, studies have observed that, in such settings though, luminescence signals measured in modern deposits tend to be better bleached downstream. It thus suggests that the study of alongstream luminescence signals may allow the quantification of fluvial transport processes and the transient storage of particles in floodplains.

 

This study explores SG-pIRIR D_e distribution from feldspars in modern floodplain deposits of the Río Ñuble (Chile) before and after a major rainfall and discharge event, to investigate whether SG-pIRIR luminescence can be used to trace the impact of such an extreme hydrological event on landscape erosion. This event took place in austral winter 2023, with cumulative rain exceeding 700 mm over 72 hours in the foothill regions, causing large-scale flooding of Andean rivers including adjacent lowlands. The comparison of SG-pIRIR D_e distribution before and after the event reveals a systematic increase in SG-pIRIR D_e values, with post-flood data exhibiting a pronounced increase in SG-pIRIR D_e, enhanced by a factor of 200–300 compared to the pre-flood data. Moreover, the increase of D_e values varies longitudinally being most pronounced at the front of the Andean Cordillera. We show that this pattern likely reflects the influx of newly eroded material in areas of the most intense rainfall and thus discharge during the flood. It indicates that longitudinal variation of luminescence are set by sediment input from landscape erosion with minor alongstream bleaching due to transport.