EFFECT OF RAINFOREST DEVELOPMENT ON THE PACE OF KNICKPOINT RETREAT DURING THE GROWTH OF A TROPICAL MOUNTAIN. INSIGHTS FROM THE LUQUILLO CZO, PUERTO RICO

The Luquillo Mountains at the northeastern tip of Puerto Rico stand slightly above 1,000 meters high and receive up to five meters of precipitation annually. Ancient uplifted shore platforms found around the mountain tops show that the Luquillo Mountains were once a small island a few hundreds of meters tall, which supported drier vegetation. We dated caves formed following the emergence of these shore platforms using ¹⁰Be-²⁶Al burial of cave sediments and established that uplift started 4 Ma ago, consistent with geological constraints, and with cladogenetic studies of species diversification in the mountain fauna. Forest succession theory maintains that trees drape existing landscapes as passive niche optimizers. However in the Luquillo Mountains forest exerts a powerful control on erosion. Most of the runoff is conveyed as quickflow through the soil macropores, preventing runoff and erosion of the mountain. As a result physical erosion is kept low and a thick saprolite has formed below even the sheerest mountain slopes. We present here one consequence of this tight coupling between mountain growth, climate change, forest evolution and river incision. We investigated the propagation of a wave of erosion on the southern side of the mountains. There, river knickpoints formed in response to shore platform emergence and have since migrated very slowly upstream. Modern detrital ¹⁰Be erosion rates in these catchments are consistent with slow long-term rates of knickpoint retreat. Analysis of knickpoints distribution and visual observations of stream beds show that the most common processes of river incision, namely abrasion and plucking, contribute little to knickpoint retreat. The studied rivers flow over a massive stock of quartz diorite surrounded by a hornfels-facies aureole. Slow erosion under the rainforest has reduced the bedload fluxes delivered to the knickpoints, and saprolitization of the quartz diorite has reduced the grain size of the bedload. Both processes have limited abrasion, while the massive nature of the quartz diorite has limited the efficacy of plucking. Mountain uplift, by altering its climate and its forest cover, has thus slowed down the wave of erosion and generated the steep, slowly propagating knickpoints that we observe today.