A RECORD OF LATE HOLOCENE COASTAL EVOLUTION AND RELATIVE SEA-LEVEL CHANGE FROM A COASTAL SALT POND, SOUTHWEST PUERTO RICO

Coastal lagoons and salt ponds are important natural resources that are ecologically significant and represent a critical buffer zone between terrestrial and marine environments. Additionally, they contain unique sedimentary archives of paleoenvironmental information including past climates, marine flooding events (due to tropical cyclones or tsunami), sea-level change, paleoecology and geoarchaeology. They are also delicate and dynamic environments at risk due to human activities and highly vulnerable to climate change. It is important to understand the paleoenvironmental history of these settings to establish records of recent changes and events and assess how they may respond to future changes. Cores recovered from a coastal salt pond in southwest Puerto Rico provide a record of coastal change over the last ~2800 years. Radiocarbon ages of mollusk shells, Halimeda plates and mangrove wood sampled from three primary facies provide a chronostratigraphic framework. The records indicate that the salt pond was previously an open marine lagoon connected to the adjacent Caribbean from at least ~2800 yr BP to ~1400-1700 yr BP. At this time the lagoon was abruptly isolated from adjacent coastal waters. Following isolation of the lagoon, deposition of mangrove peats began. Mangroves appear to have dominated up until ~400 yr BP when there was a transition to the current hypersaline pond conditions. Overall, this stratigraphy is reflective of a relative sea level fall and progradation of the shoreline. This scenario is somewhat problematic as most accepted Caribbean sea-level curves for this time period indicate sea level slowly rising towards its current position. Isolation of the pond may have been the result of changes in sedimentary dynamics and barrier development associated with slowly rising sea level. However, there are other emergent records on Puerto Rico that are consistent with relative sea-level fall and indicate recent and significant changes along the coast of western and southwestern Puerto Rico. Other possible mechanisms of relative sea-level and coastal change include tectonic uplift and glacial isostatic adjustment (GIA) effects. It may also be plausible that barrier formation and isolation of the pond was a result of a large-scale depositional event associated with a tropical cyclone or tsunami.