Use of pollen and vascular plants to estimate coseismic subsidence at a tidal marsh near Tofino, British Columbia

Abstract

We use botanical data to estimate coseismic subsidence produced by the 1700 Cascadia earthquake at a tidal marsh near Tofino, British Columbia. Association indices for Cyperaceae (0.3), Triglochin-type (0.4), Poaceae (0.7), Potentilla-type (0.6), Achillea-type (0.6), and Ericales (0.6) pollen reveal that high-marsh-taxa have greater fidelity and, therefore, greater value as indicators of elevation than low-marsh-taxa. Pollen taxa that are abundant and have narrow elevation ranges include Poaceae, Potentilla-type, Achillea-type and Triglochin-type. Vegetation and surface pollen define four marsh zones: low, middle, high, and forest-edge transition. Deposits beneath the marsh include a buried peat capped by tsunami sand. The sand is abruptly overlain by peaty mud, which grades into peaty soil of the modern marsh. The buried peat contains pollen indicative of a high to forest-edge transition marsh. Pollen assemblages from the top of the buried peat and the overlying tsunami sand are similar, probably because the tsunami entrained litter as it moved over the marsh surface. Sediments above the tsunami sand yielded pollen characteristic of a low-to middle-marsh-environment, including abundant Cyperaceae, Triglochin-type, Poaceae, and Chenopodiaceae. We calibrated fossil pollen data to elevation using partial least squares, weighted averaging partial least squares, weighted averaging, and weighted averaging with tolerance downweighting. The last of these methods explains the largest amount of variance (root mean square error of prediction=0.3, r 2=0.85) and yielded values of coseismic subsidence at three sites of 0.65±0.3, 0.69±0.3, and 0.50±0.3 m (average=0.6±0.3 m). These values agree with previously published estimates of coseismic subsidence near Tofino, based on foraminifera.