SOILSCAPES OF THE ANTHROPOCENE IN THE CARIBBEAN: A VIEW FROM PUERTO RICO’S NORTHCENTRAL ALLUVIAL VALLEYS

In this presentation, I summarize preliminary stratigraphic, geochemical and radiocarbon results from site MN-4, a 5m cutbank profile from the Río Grande de Manatí (RGM), northcentral Puerto Rico. The RGM originates in the Central Mountain Range at an elevation >600 masl, and traverses north through the Northern Karst, characterized by haystack hills, sinkholes, extensive cave systems, and the island’s largest aquifers. An objective of this dissertation is to evaluate long-term anthropogenic disturbance and environmental change at the RGM over the last 5,000 years. Identifying soil sequences with alternating periods of paleosol formation (‘stability’) and high sedimentation (‘instability’) over floodplain catenas can elucidate long-term changes in catchment dynamics. At site MN-4, a sequence of five possible ‘paleosols’ buried under coarser material were identified in a broader stratigraphy study. Particle size analysis, organic matter and carbonate content, and magnetic susceptibility are used in an attempt to assess if these units are backswamp deposits or represent pedogenesis events. The lowermost unit of the MN-4 sequence dated to 6,400 cal yr, BP, at least 1,000 years prior to human colonization providing a baseline to assess human impacts. In addition to MN-4, we investigate the floodplain further in three sediment cores (MN-A, MN-C, MN-E) obtained following a cross-valley transect to compare horizontal, vertical, and chronological changes in floodplain stratigraphy.