Effects of Mass Wasting on Fluvial Sediments in Puerto Rico Following Hurricane Maria

Abstract

Mass wasting can supply abundant sediment to fluvial systems, but the effects on carbon cycling and chemical weathering signatures in fluvial sediments are ambiguous. Here we examine the impacts of mass wasting on sediment delivered to the fluvial systems in high relief and high precipitation watersheds in southeastern Puerto Rico to investigate the role of hurricane-induced mass wasting on 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 topsoil and saprolite in relatively shallow landslides as opposed to 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 variances in CIA values observed 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.