Diatom evidence for tsunami inundation from Lagoon Creek, a coastal freshwater pond, Del Norte County, California

Diatoms preserved in sand layers deposited in a coastal freshwater pond in Del Norte County, California, record repeated inundation by tsunamis. The pond deposits at Lagoon Creek contain primarily peat interbedded with landward-thinning and landward-fining layers of sand. Diatom samples from six vibracores confirm that the peat formed in a primarily freshwater environment. The sand layers, however, contain allochthonous marine diatom species, and in some cases allochthonous brackish-marine diatom species in muddy rip-up clasts. Many of the marine diatoms found in the sand sheets are also found living in the surf zone of the modern beach adjacent to the site. The brackish-marine diatoms consist of many species that are also found living on modern intertidal mudflats. Marine diatoms have been found in sand layers as far inland as 1,400 m, allowing an estimate of landward inundation during tsunami events at the site. The brackish-marine mudflat species are interpreted to have lived in a short-lived saline layer at the bottom of the pond that was emplaced during tsunami inundation events. Accelerator mass spectrometry (AMS) radiocarbon dating of primarily twigs and spruce cones indicates that the pond sediment record spans approximately 3,000 years of deposition. Additional age control came from the identification of the 0.93 - 1.3 ka tephra from Little Glass Mountain, deposited between two of the sand layers. Ages from the sand layers agree well with ages documented for tsunamis and earthquakes in tidal wetland stratigraphy and lakes elsewhere along the Cascadia subduction zone.