A modern analogue for plant colonization of palaeotsunami sands in Cascadia, British Columbia, Canada

Abstract

The sediment stratigraphy of a modern analogue for pioneer-plant colonization of palaeotsunami sands near Tofino, British Columbia, Canada, shows that a sand outcrop is continuous with a buried sand stratum deposited by tsunamis of the ad 1700 Cascadia earthquake. Plants growing on the sand outcrop form a low-marsh community that includes Triglochin maritimum, Salicornia virginica, Spergularia canadensis, Puccinellia pumila and Plantago maritima. In contrast, low marshes on muddy substrates include only Carex lyngbyei. Although Carex lyngbyei colonizes low marsh on mud, it is restricted to middle marsh or higher elevation communities on sand. With two-way indicator species analysis (TWINSPAN) and detrended corre spondence analysis (DCA), plant cover-abundance clearly distinguishes sandy from muddy low marsh, a finding that suggests that particle size influences plant colonization. TWINSPAN and DCA show that pollen spectra of the sandy marsh surface resemble fossil pollen of sediments that immediately overlie buried sand deposits. Grain size of intertidal deposits influences primary plant succession, and species on sand respond to relative sea-level change just as those on mud. Pollen assemblages in surface sediments reflect plants growing nearby, and taphonomic distortion of the pollen spectrum is minimal at this locality. These observations suggest that past co-seismic subsidence can be estimated by use of pollen assemblages as proxy indicators of elevation relative to sea level.