UC San Diego

The processes that shape Earth’s surface are constantly in flux. Continuous movements of soil and water and growth of ecosystems are what create the perfect stillness in Ansel Adam’s landscapes. In this dissertation, we investigate how a landscape responds to changes in the world around it. The tectonic uplift of Puerto Rico continues to shape the critical zone of the Luquillo Mountains today. We find that it has modified patterns of groundwater emergence, such that the river network is entrenching into ridgelines, in part, via the flux of groundwater. We demonstrate this with measurements of groundwater discharge and the grain size of the sediment it carries, compared to long-term channel incision rates derived with the cosmogenic radionuclide 10Be. Rock uplift enhances erosion rates, and a long-held belief in Earth Science is that soil production processes are linked and will respond to an enhanced erosive flux. By applying 10Be dating to measure soil production rates in the evolving landscape of the Luquillo Mountains, we determined that there is no such functional relationship. We re-evaluated the methods and assumptions that have been applied to this question elsewhere, and determined that in fact, soil production is independent of soil depth across the globe. This implies that no safety net will save us from the massive loss of soil due to anthropogenically accelerated erosion. Even outside of managed landscapes, accelerated erosion under climate change is an imminent possibility. We demonstrate this with a record of physical erosion over 26 ky recorded by 10Be concentrations in the layers of floodplain sediments. Erosion rates are greatly accelerated by the predominance of landsliding that occurs due to the onset of intense, frequent tropical storms in the Caribbean. The floodplain sediments contain a contemporaneous record of ecosystem dynamics in the δ13C isotope ratios of buried organic material. Similar to physical erosion in the watershed, ecological dynamics shift to a disturbance-dominated state due to coupling with a more volatile climate.