Early Holocene Evolution of San Francisco Estuary, Northern California

Abstract

Abstract Fossil foraminifers (Elphidium excavatum forma selseyensis) recovered from a sediment core from the San Francisco Bay are analyzed for their oxygen isotopic composition. About 8 m of core believed to represent the first 600 years of the current (Holocene) bay's existence at the sample site (from approximately 7650–7050 cal YBP) are highlighted here. This record, deposited at a high, uniform accumulation rate, shows pronounced variation in measured carbonate δ18O, including some of the highest values in the entire Holocene record at the site. It is proposed that this interval records a period during which the fundamental mixing dynamics of the modern estuary became established. We interpret the extremely high δ18O values at about 7400 cal YBP as being indicative of a strongly stratified estuary sampled below its halocline and argue that subsequent details of the isotope record suggest a gradual decrease in water column stratification until conditions qualitatively characteristic of the modern estu...