TRANSPORT VS. SOURCE: QUANTIFYING SEASONAL CONTROLS on BEACH SEDIMENT CHARACTERISTICS IN VIEQUES, P.R., USA

As global climate warms, coastal systems will respond to changes in precipitation and evaporation, wind and wave direction and intensity, and changing storm patterns. Future beach stability depends on grain size, mineral composition, beach profile, wave energy, rates of longshore transport (LST), and source potential. Beach grain size has long been used as a proxy for wave climate in both quartz- and carbonate-dominated beaches, with varied success. Less attention had been paid to variable transport with regards to wave energy in mixed siliciclastic-carbonate settings. The island of Vieques, Puerto Rico, is located within a complex volcanic arc terrain in the Caribbean Sea. Varied bedrock geology includes plutonic and volcanic igneous rocks, limestone, marine sandstone, and modern reefs. The southern shoreline is characterized by a series of deep embayments and headlands, while the northern shoreline is comprised of poorly developed semi-continuous beaches. Relatively short transport pathways and distinct source regions make this an ideal place to assess the relative influences of two key factors controlling beach characteristics: sediment source and longshore transport rate. Wave climate around Vieques was estimated using a SWAN model to determine annual and seasonal wave energy and LST at nine selected beaches around the island. Surface sediment samples were collected and analyzed for grain size and mineralogical composition at each site. Quantification of the quartz-carbonate ratio of the sediment by XRD analysis yields relative magnitudes of terrigenous versus offshore sediment inputs for the beach locations, after accounting for carbonate bedrock contribution. Heavy liquid and manual separation yielded single-mineral subsamples for laser diffraction particle size analysis (LDPSA), to model transport rates for distinct mineral and size fractions. Data from these analyses shed light on the relative influence of grain size and grain density in consideration of sediment provenance, wave energy, longshore sediment transport, and beach morphology in the coastal zone. Results indicate that mean sediment transport around Vieques occurs during storm events, while fair-weather conditions serve to winnow fines and redistribute lighter carbonates around the island.