| Pollen aid in quantifying change in sea level and vegetation associated with the 1700 Cascadia earthquake at tidal mashes near Tofino, British Columbia. Deposits beneath marshes include a buried peat capped by tsunami-deposited sand. A broad platform of sand at Jensen's Bay includes six vegetation zones determined with two-way indicator-species analysis (TWINSPAN): muddy low (2.7--3.3 m), sandy low (3.0--3.5 m), middle (2.9--3.6 m), high (3.4--4.0 m), forest-edge transition (3.9--4.1 m), and alder-sedge carr (3.9--4.3 m). Marshes at English Cove lack abundant sand, fringe the forest edge in narrow strips, and include low (2.0--2.9 m), middle (2.8--3.5 m), high (3.2--3.9 m), and forest-edge transition (3.7--4.2 m). Pollen assemblages collected from surface sediments at English Cove reflect vegetation variation along transects from mudflats to forest edge. Surface pollen define five marsh zones: mudflat and low (1.7--2.7 m), low (2.6--3.0 m), middle (3.3--3.6 m), high (3.6--3.9 m), and forest-edge transition (4.0--4.1 m). Euclidean distance is low (≤0.2) for pollen samples collected from the same zone and high (>0.2) for samples from different zones. High marsh pollen such as Achillea-type, Poaceae, and Potentilla-type have narrow elevation tolerances (<0.5 m) and high association indices (>0.5), which shows they are good proxy indicators of relative sea level and vegetation. Paleoelevations of fossil pollen assemblages at English Cove determined with weighted averaging (inference error = 0.3, r2 = 0.85) accommodate estimates of coseismic subsidence of 0.65 +/- 0.3, 0.69 +/- 0.3, and 0.50 +/- 0.3 m (average = 0.6 +/- 0.3 m). Euclidean distance between surface and subsurface pollen samples shows that fossil pollen assemblages of the buried peat have good analogues in surface pollen but those that overlie the tsunami sand do not, which suggests that coseismic subsidence and tsunami deposition allow atypical plant communities to establish.; Sandy low and middle marshes at Jensen's Bay provide a modern analogue for vegetation colonization of tsunami sand. TWINSPAN and detrended correspondence analysis (DCA) distinguish sandy low and muddy low marsh communities, which indicates that sand influences species composition. (Abstract shortened by UMI.)... |
