DOCUMENTING THE EVOLUTION OF A SOUTHERN CALIFORNIA COASTAL LAGOON DURING THE MID- TO LATE HOLOCENE

Coastal wetlands are declining globally, and while wetland restoration looks to offset these losses, its success relies on anticipating environmental response to external forces. The purpose of this study is to investigate the sedimentological record of Los Peñasquitos Lagoon to determine the processes that can drive environmental transitions in a Southern California coastal wetland. For this project we analyzed three cores from the wetland for grain size, loss on ignition, and shell assemblages to reconstruct environmental change over the past ~4,000 years. From the results, we identified four distinct facies: open embayment, fluvial delta, immature marsh, and mature marsh. The lagoon was initially characterized as an open embayment that persisted for >2,000 years. Approximately 1200 cal yrs BP, a short lived wet climatic period during an overall drier climate state triggered a fluvial deltaic progradation at the head of the lagoon. As the wet period ended and drier conditions returned, the delta began to retreat. The lagoon infilled as fluvial sediment was displaced, and the mouth bar was permanently established. These changes resulted in a transition to an immature marsh by ~500 cal yrs BP that developed into a mature marsh around the end of the Little Ice Age (~150 cal yrs BP), driven by increased sediment delivery that facilitated elevation building in the marsh. Ultimately, these environmental transitions were driven by climate variability, although evidence of human impacts was observed more recently in the record. Therefore, future restoration efforts must consider both natural climatic variability (e.g., droughts) and anthropogenic influences (e.g., sea level rise) to have maximum impact in preserving coastal wetlands.