Accelerated rates of soil erosion threaten the stability of ecosystems, nutrient cycles, and global food supplies if the processes that produce soil cannot keep pace. Over millennial timescales, the rate of soil production is thought to keep pace with the rate of surface erosion. This paradigm in the Earth Sciences holds that an underlying mechanism lowers the rate of soil production when soil is thick and increases the rate of soil production when soils are thin. However, the coupling mechanism is uncertain. We require empirical data on soil production rates in landscapes where soil erosion has accelerated in response to environmental or tectonic perturbations.
Our study area is the Rio Blanco watershed in the Luquillo Mountains, Puerto Rico. Uplift of the Luquillo Mountains initiated a wave of river incision propagated by up-stream migrating knickpoints that has divided the topography into a patchwork of slowly and rapidly eroding terrain. We combined measurements of three independent isotopic systems: in situ 10Be, in situ 14C, and uranium-series nuclides to constrain the timing of geomorphic soil production and weathering rates across the soil-saprolite boundary on ridgetops spanning a ~200-800-meter elevational gradient, where the erosion rates at ridgetops vary from >500 m/My to 25 m/My. Each one of these isotope systems contains an internal clock that is active inside of the soil profile. Cosmogenic 14C and 10Be clocks record the amount of time that soil travels through the top few meters of Earth’s surface. Cosmogenic 14C adjusts more rapidly to short-term changes, while the 10Be clock records processes on the order of 103-104 years. The clock contained by uranium-series nuclides starts when the parent material is broken down by chemical weathering.
Results/Conclusions
Our data suggest that landscape evolution coupled to base-level fall impacts soil production in unexpected ways. In the relict topography, we found ubiquitously high rates of soil production, decoupled from soil erosion. In the adjusting topography, the soil-saprolite boundary lowers at rates comparable to soil erosion, but chemical weathering rate is invariant. We compared our results to a global compilation of soil production vs. soil thickness in sites that have likely experienced constant erosion rates and climate over geologic timescales with sites that may experience transient erosion responses to environmental change. We conclude that soil production resists self-arresting behavior in some locations and is uniformly slow in arid and semi-arid settings - independent of soil depth.