Late Holocene paleoenvironmental changes in the Seal Beach wetland (California, USA): A micropaleontological perspective

Abstract

Pollen, benthic foraminifera, diatoms, sediment grain size, and organic matter content from a 230-cm long-AMS dated sediment core (Core SB-51B) were used to reconstruct paleoenvironmental changes and paleoclimatic evolution within the Seal Beach wetland (southern California, USA), during the last ~ 2000 years. A Q-mode cluster analysis based on diatom and foraminiferal data identified three distinct units: 1) the lowest sandy to silty-sand unit deposited before 1838 cal years BP (230 –140 cm in depth) is devoid of microfossils; 2) a clay rich intermediate unit dated between 1838 and 513 cal years BP (140–52 cm) is dominated by brackish and brackish-marine diatoms and foraminifera; and 3) a clay-rich zone deposited after 513 cal years BP(52 cm to the surface) contains high salt marsh adapted microfossils. Fresh-water and salt-tolerant diatoms from Unit 2 indicated several fresh-water influx events at 105 cm, 82 cm, and 75 cm possibly from a nearby stream. A shift from saltmarsh plants to terrestrial plants was inferred by pollen analysis. Throughout the record, the pine pollen increases suggest a shift towards cooler and wetter conditions during the last 300 years. Regional pollen records support our findings indicating a trend from drier conditions during the last 1800 years to the cooler, more mesic conditions of the Little Ice Age. The analyzed sedimentary sequence indicated the possible occurrence of three seismic events during the Late Holocene: E3 and E2 before 1760 cal years BP and E1, which took place just prior to 390 cal years BP.