Cosmogenic radionuclide constraints on active margin coastline uplift and geomorphic rates, Santa Cruz, California, United States of America

Tracing sediment movement between linked geomorphic systems represents a large challenge for the geomorphic community. In this dissertation I address one such system, the actively uplifting coastline near Santa Cruz, California, in which the generation, storage, and transport of sediment must all be documented. The terrain is characterized by broad flat marine terraces, steep fluvial basins, and narrow discontinuous beaches. I use in-situ cosmogenic radionuclides to constrain the timing and linkages between processes involved in long-term landscape evolution. Cosmogenic radionuclides are used to date surfaces that are hundreds of thousands of years old, and to determine erosion rates and track sediment inputs over timescales that are much longer than historic records. Their longer record integrates over long timespans that both include rare large events and reduce the importance of recent anthropogenic modifications. In this study I demonstrate the use of cosmogenic radionuclides in simultaneously probing the terrace, fluvial and littoral systems along the Santa Cruz coastline. Specifically, I use 10Be and 26Al concentrations measured in sand-sized quartz to examine rates of sediment production in fluvial basins, to determine beach concentrations from past sea-level highstands and the duration of sediment storage on marine terraces, and to constrain fluvial and cliff-derived sediment inputs to the littoral cell. The cosmogenic radionuclide dates of the five major Santa Cruz terraces correlate to marine oxygen isotope stages 3, 5a, 5c, 5e, and 7. These are considerably younger than suggested in any previous work; the implied uplift rate of 1.1 mm yr−1 is correspondingly faster than previously proposed. The cosmogenic radionuclide-derived fluvial erosion rates in the five largest coastal basins are typically around 0.20 to 0.25 mm yr−1 and correspond well with the historical record. In the littoral cell, the ratio of fluvial sediment to sediment derived from wave-driven cliff backwearing is roughly equal along the northern coastline, and is about 75% fluvial sediment in Monterey Bay, downdrift from the mouth of the San Lorenzo River. I have therefore demonstrated that cosmogenic radionuclides can be successfully employed to probe issues of large-scale landscape evolution, and should prove of great value in addressing other linked geomorphic systems.