EFFECTS OF HURRICANE-INDUCED MASS WASTING ON SOIL TRANSPORT AND CHEMICAL WEATHERING IN A TROPICAL FLUVIAL SYSTEM IN PUERTO RICO

Mass wasting can supply abundant sediment to fluvial systems, but the effects of mass wasting on carbon cycling and chemical weathering in fluvial systems are ambiguous. Here we examine the impacts of mass wasting on sediment delivered to two fluvial systems in high-relief and high-precipitation watersheds in southeastern Puerto Rico to investigate the role of hurricane-induced mass wasting on soils, chemical weathering, carbon cycling, and fluvial sediment properties. Previous work hypothesized that anomalously low chemical index of alteration (CIA) values observed in fluvial sediment in these watersheds were a result of landslides delivering less weathered regolith to the Rio Guayanés and Rio Guayabo. On September 20, 2019, Hurricane Maria made landfall in Puerto Rico, resulting in thousands of landslides across the island and an opportunity to examine this hypothesis. However, the sediments collected after the hurricane-induced landslides have higher weathering index values and coarser-grain sizes than those collected in 2014. We infer that the heavy precipitation associated with Hurricane Maria led to the mass movement of weathered soil and saprolite in relatively shallow landslides rather than in deep-seated landslides sampling bedrock. The exposure of fresh bedrock via landslide scarps, and the deposition of sediment on hillslopes and within the fluvial system may play an important role in carbon sequestration. The variation in CIA values observed in the fluvial sediments are not indicative of the degree of climate change, but instead are likely due to subtle differences in transport mechanism which can lead to significant differences in the weathering index values.