EVOLUTION OF SOUTHERN CALIFORNIA COASTAL WETLAND

Coastal wetlands such as lagoons, marshes, and estuaries comprise between 10-20% of the Pacific shoreline. Southern California wetlands in particular are dynamic and complex environments that formed much later than their Atlantic, Gulf, and Pacific Northwest counterparts. The goal of this study was to characterize the evolution of one such Southern California coastal wetland. We collected sediment cores from Los Penasquitos Lagoon in San Diego County, one core from a freshwater marsh section of the wetland, and 2 others from the estuarine marsh. These cores were supplemented with surface sediment samples from local tidal channel, fluvial, and beach environments to facilitate paleo-environmental characterization. All sediment (core and surface) samples were analyzed for grain size distributions, and total organic matter was also measured on core samples via Loss-on-Ignition. Sedimentary ages in the cores were determined from radiocarbon dating. Collectively from the grain size data we characterized the cores into six facies: estuarine wetland, freshwater wetland, fluvial, estuarine subtidal, low energy beach, and high energy beach. The results show a basal beach deposit, followed by a transition into an estuarine subtidal facies. This transition reflects the filling of the estuary with littoral sediment and the formation of a mouth bar that likely occurred sometime between 4-5 kya. The estuarine subtidal facies may represent a back-barrier lagoon environment that persisted for a couple of thousand years. During this time, estuarine communities including oysters proliferated within the lagoon. At ~1 kya a large storm or series of storms caused a breach in the mouth bar returning the system to a depositional setting where beach/littoral sediment accumulated. Evidence for this storminess can be seen as a shell hash layer at depths between ~165-190 cm. Over the last 1,000 years as the mouth bar stabilized the lagoon transitioned into a wetland, and landward portions of the wetland continued to build upward into a freshwater wetland. The results of this study provide insight into the dynamic evolution of Southern California wetlands as they evolved over the past several millennia.