The Late Quaternary Evolution of the Southern California Coast: Sea-Level Change, Storms, and Subsidence

The highly developed and populated coast of southern California is at risk for inundation due to storms or tsunamis; coastal erosion; wildfires; earthquake-driven shaking, liquefaction, and subsidence; sea-level rise; and other hazards. Estuaries formed during periods of sea-level transgressions often record environmental changes that have affected the region over time. These records of past climate, sea level, inundation events, and other changes can provide a baseline with which to compare current changes. Past recurrence intervals of hazard events also provide an estimate for how often and at what magnitude we can expect events to recur in the future. Here we compile relative sea-level indicators from other studies on southern California estuaries to show that relative sea level in southern California has risen 0.8 ± 0.3 mm/yr over the last 4000 years, less than modern rates and predicted future rates of sea-level rise. Predicted acceleration of sea-level rise could exacerbate the effects of storm and tsunami inundation along the coast. We use a variety of biological, sedimentological, and geochemical measurements on sediment cores from Carpinteria Marsh, Carpinteria, California, to document environmental changes that have occurred along the Santa Barbara Channel over the last 10,000 years. We show evidence for at least 37 episodes of alluvial fan progradation due to large floods over the past 7 ka, clusters of which correspond to regional records of wet climate conditions. We also show evidence for one coseismic subsidence event at 1.0 ± 0.1 ka, which may correlate with records of other earthquakes in the region. Finally, the topmost sediments within the marsh record environmental changes within the historical period. Within these sediments, we use a multi-proxy approach to document the history of the marsh over the last 200 years including the appearance of European pollen species, makers of industrialization, and an overwash deposit related to the 1861-2 winter storm season in California