CHANGES IN EROSION RATE AND EROSION PROCESSES INDUCED BY THE PASSAGE OF HEADWARD MIGRATING KNICKPOINTS IN THE LUQUILLO CZO: INSIGHTS FROM DETRITAL COSMOGENIC 10BE
In the Luquillo Mountains, the most prominent knickpoints are found along streams that drain the Rio Blanco batholith, an Eocene stock of quartz diorite. The lips of these knickpoint all stand at the same elevation (~600m), which is also the elevation of one of the uplifted wave-cut platform. They do not correlate with lithological changes and likely nucleated at the coast when uplift of the wave-cut platform started. They separate a slowly-eroding relict upland region from faster eroding lower slopes. We used in situ 10Be concentrations in river-borne quartz to compare erosion rates above and below the knickpoints. We find that over timescales of 104-105 years the lower slopes are eroding three times faster.
The change in erosion rate is also associated with a change in the dominant erosional processes, as reflected by the grain-size dependency of 10Be concentration in quartz sediments and the hillslope morphology revealed by analysis of a recently acquired LiDAR DEM. The uplands exhibit a strong variation in the 10Be concentration as a function of sediment grain size, which reveals a deepening landscape with faster erosion of slopes than the intervening ridge tops. Erosion is focused into deep coves were seepage and mass-wasting progressively dismantle a deep saprolite. Downstream of the knickpoint lips, such coves are absent, hillslopes are straight, ridge crests are narrow and the 10Be dependency with grain size is less pronounced, indicating a better coupling between ridge, slope and river erosion rates. Collectively, these observations indicate a shift from chemical weathering dominated erosion upstream of the knickpoints to mechanical weathering dominated landscape below the knickpoints.