CATCHING CLAYS: NATURAL ARCHIVED RECORDS OF CLIMATE AND COLONIAL DISTURBANCE IN THE FLOODPLAIN OF THE RIO FAJARDO, NORTHEASTERN PUERTO RICO (Invited Presentation)

Climate change is projected to bring extreme seasonality and more frequent, intense tropical storms to the Caribbean. How will these changes impact the terrestrial environment? In this study, we collected co-spatial, natural archives of biotic and abiotic terrestrial processes operating over the past ~25 ky within the Rio Fajardo watershed, in northeastern Puerto Rico. The proxy records were derived from a five-meter-thick stratigraphic section exposed by cut bank incision. This stratigraphic sequence captures major shifts in the Caribbean climate, the intensification of the El Nino-Southern Oscillation, and the arrival of humans on the island. Looking at contemporaneous changes in the living and mineral worlds gives us greater insight into the dynamics between these two interrelated systems. We constrained the sediment chronology with the radiocarbon ages of organic deposits and interpreted ecosystem dynamics from changes in the stable carbon isotopic ratio of sedimentary organic material compared to δ¹³C ratios of contemporary carbon sources. Introducing a new application for meteoric ¹⁰Be, we generated a record of paleo-erosion rates in the catchment from the concentration of meteoric ¹⁰Be in layers of the floodplain sediments. Grain size data, clay mineralogies, and geochemical indices down profile were also used to provide context for our interpretations. Our findings show that terrestrial systems in this watershed responded similarly to external forcing from climate and human activities. During the last glacial and early Holocene epochs both biotic (δ¹³C) and abiotic proxies (¹⁰Be_met and geochemical data) indicated dynamic equilibrium with climate. The past five thousand years of record are characterized instead by pulsed responses to disturbances in both systems. Colonial-era land use drove changes that significantly exceeded natural variability in any proxy over the period of record.