Effects of a tectonically-triggered wave of incision on riverine exports and soil mineralogy in the Luquillo Mountains of Puerto Rico

We document the long-term response of a tropical mountain stream to tectonics and show how this response exerts a first-order spatial control on the delivery of weatherable minerals to forest soils in its catchment. These minerals, in turn, affect nutrient availability and biogeochemical cycles. The Luquillo Mountains is a rainforest-covered isolated massif on Puerto Rico Island. Instead of displaying typical concave-up long profiles of most mountain streams, the rivers draining the southern flank of these mountains display a systematic downstream increase in gradient. Using concentrations of in situ-produced cosmogenic Be-10 in quartz from river sediments, we find that the downstream steepening is due to the propagation of an erosion wave along the rivers from the coastal plain upstream, toward the headwaters. Decreased Be-10 concentration along the steepened reaches (knickpoint faces or knickzones) results from faster denudation of the valley sides around the steepened reaches. The upper portions of the steepened reaches (knickpoint lips) cluster in elevation around the altitude of a flat-lying, dissected surface traceable around the Luquillo Mountains. Inspection of similar platforms over other parts of Puerto Rico and in the surrounding seas suggests that the dissected surface is an uplifted shore platform formed in the Early Pliocene (similar to 4 My ago). Upstream of the knickpoints, rivers possess alluvial reaches still graded to this uplifted platform. They represent relict profiles of the rivers that once drained an island that was surrounded by the now-uplifted platform. We conclude that the knickpoints initiated when the platform started to rise above the Caribbean Sea. The knickpoints then started propagating upstream, defining the front of a slow-moving wave of erosion. Be-10-derived catchment-scale denudation rates measured above and below the knickpoints indicate a 30-210% increase in denudation associated to the passage of the knickpoints. Mineralogical analysis of the soils show that soils upstream of the knickpoints overlie deeply depleted saprolite and only contain recalcitrant minerals, whereas soils downstream of the knickpoints experience an input of fresh, weatherable minerals. This influx considerably increases nutrient availability (Porder et al., 2015) in a forest where atmospheric inputs are otherwise the main source of nutrients (McDowell et al., 1990; Zarin and Johnson, 1995; Pett-Ridge, 2009). (C) 2015 Elsevier Ltd. All rights reserved.