DIATOM VARIABILITY IN TIDAL-MARSH ENVIRONMENTS: IMPLICATIONS FOR CONSTRAINING LAND-LEVEL CHANGE FROM CASCADIA SUBDUCTION ZONE EARTHQUAKES

Coseismic subsidence during past earthquakes has been estimated from diatom assemblages across mud-over-peat contacts in tidal-marsh stratigraphic sequences in Cascadia. But the application of diatoms to coseismic subsidence studies from the tidal marshes of Washington is restricted by the absence of a local, quantitative modern diatom dataset to serve as a modern analogue.

In the process of building a modern diatom dataset from the tidal marshes of Washington, we assess the accuracy of diatom-based reconstruction of past land-level change by considering the influence of spatial variability and the environmental controls of diatom assemblages. Here, we test the spatial variability of diatom assemblages by comparing the modern diatom assemblages found at two tidal marshes located ~ 2 km apart: (1) the Niawiakum River and (2) the Bone River in Willapa Bay, WA. At each site, we collected modern surface samples of diatoms at stations along a shore-perpendicular elevation transect that transitions from an intertidal mudflat to freshwater wetlands. The transects capture the variations in elevations, substrates, and vegetation recorded in sedimentary sequences from repeated subduction zone earthquakes. We determined the location and relative elevations of the modern sampling stations using a Real Time Kinematic satellite navigation system tied to local tidal benchmarks to obtain vertical errors of < 1 cm. Measured elevations cover tidal datum from 0.5 m mean tide level to above highest astronomical tide. We identified diatoms to species level and classified the species by salinity and substrate. We compare the results of over 170 species across 60 genera. Our results will guide recommendations for the development of a regional modern diatom dataset in diatom-based reconstructions of earthquake subsidence in Cascadia.