Mass wasting plays an important role on carbon cycling and sequestration by exposing fresh, weatherable bedrock and delivering hillslope sediments to lowlands and fluvial systems. Chemical weathering signatures of landslide-derived fluvial sediments can be used to understand linkages between hillslope and fluvial processes, and thus to characterize spatiotemporal dynamics of sediments. However, chemical signatures of fluvial sediments derived by landslides are yet to be fully understood. Here we compare the bulk chemistry, mineralogy, and grain size of fluvial sediments collected pre- and post-Hurricane Maria (landfall on Sept. 20, 2017) in the Rio Guayanés and Rio Guayabo River watersheds in southeastern Puerto Rico to help fill this knowledge gap.

Relative to fluvial muds collected before Hurricane Maria, mud samples collected after the storm exhibit higher weathering index values, but coarser grain size modes. We infer that small landslides triggered by Hurricane Maria transported slope materials from shallow depths, including weathered topsoil and saprolite, as opposed to previous deep-seated landslides which likely sampled regolith and bedrock. The variances in weathering indices observed pre- and post-hurricane do not reflect climate change, but rather subtle differences in transport mechanism which produce significant differences in weathering indices recorded by fluvial sediments. We propose that weathering indices provide a means to understand sediment dynamics in mountainous regions, particularly for sediment transported in the immediate aftermath of landslides triggered by extreme events, such as precipitation and earthquakes, and also provide important datasets required for mapping potential carbon sequestration across a landscape.