LONG-TERM CLIMATIC AND TOPOGRAPHIC CONTROLS ON SEDIMENT EXPORT IN THE***FIG1 LUQUILLO CRITICAL ZONE OBSERVATORY, PUERTO RICO**FIG1

In tropical landscapes, abundant moisture and high temperatures accelerate weathering processes and can result in the production of thick, clay-rich soils and saprolites. The erosion processes acting in these landscapes to remove these soils and the thick, woven vegetation cap that protects them set the pace for rejuvenation of fresh, unweathered minerals to the surface. These erosional processes can have important implications for nutrient availability and vegetation zonation in these landscapes where mobile cations and critical nutrients have been flushed from soils over many years and may limit net primary production and decomposition. Although much about the environmental controls on sediment transport and hysteresis patterns in streams can be learned from stream monitoring and sediment gaging data, the modern and historical record is short and may not capture the high magnitude, low frequency events that effectively shape the landscape over millennial timescales.

We will present recent work focused on understanding the spatial and temporal changes in erosion in and around the Luquillo Critical Zone Observatory – a site 15-km-wide, 1100-m-tall on the isolated massif at the eastern tip of Puerto Rico. The island was uplifted millions of years ago and lies in the path of seasonal hurricanes, yet it still contains upland relict landscapes formed prior to the formation of the high relief that is visible in the mountains today. The upland streams are separated from the coastal rivers by steep knickpoints that are transient geomorphic features associated with the uplift of the island. These knickpoints slowly cut back into the old, saprolite relict surfaces, and excavate fresh mineral mass. We quantified the gradient in erosion rates induced by the passage of these waves of erosion using cosmogenic ¹⁰Be produced in river borne quartz. Catchment-averaged erosion rates are 2-3 times higher below the knickpoints, which attests to the important role of topography in nutrient and sediment delivery to a system. A floodplain system in the Rio Fajardo with 19 kyr of stored sediment provides an archive for past hurricane conditions. Detrital ¹⁰Be and sedimentation rates in the floodplain system indicate 2-3 times higher rates of erosion and floodplain sedimentation during hurricane episodes during the Holocene.