GEOCHEMICAL ANALYSES OF ESTUARINE SEDIMENTS ALONG THE CASCADIAN SUBDUCTION ZONE AND THEIR UTILITY IN ESTIMATING COSEISMIC SUBSIDENCE

Bulk sedimentary δ¹³C and C/N values in salt marshes are indicative of the depositional environment and source of samples collected, as well as elevation based on vegetation zonation within the marsh. Through the construction of modern geochemical datasets collected along transects in coastal estuaries, quantitative paleoenvironmental analysis of past depositional environments and elevation within the tidal frame can be inferred. Estimates of past coseismic deformation along the Cascadian margin are currently reconstructed using microfossil assemblages. In stratigraphic sequences lacking microfossils, alternative proxies are needed and geochemistry has shown to have potential from initial studies in Oregon. This study uses the differing geochemical signatures of C₃ and C₄ plants, which have contrasting salinity tolerances and distributions in heterosaline habitats such as marshes, to reconstruct past changes in salinity as a function of tidal inundation. The result is a modern bulk sediment proxy database of δ¹³C, δ¹⁵N, TOC, and C/N signals from 13 saltmarshes along the the Cascadian margin, encompassing Northern (Vancouver Island), Central (Washington, USA), and Southern (Oregon, USA) regions of the subduction zone. A total of 269 bulk sediment samples and 14 saltmarsh plant species were collected and analyzed. Results revealed δ¹³C values ranging from -17.11 ± 0.14‰ to -32.25 ± 0.02‰ for sediments and -14.54 ± <0.02‰ to -31.51 ± 0.04 ‰ for plants. Bulk sedimentary δ¹³C values in the Northern region of Cascadia ranged from -19.57 ± 0.8‰ to -31.36 ± <0.02‰, with values ranging from -17.11 ± 0.15‰ to -30.66 ± 0.02‰ in the Central region and -19.09 ± 0.15‰ to -32.25 ± 0.02‰ for the Southern region. Station elevations (relative to MTL) ranged from 0.31m to 2.65m in the Northern region, from -0.80m to 2.74m in the Central region, and -0.31m to 2.54m in the Southern region. The range in isotopic signatures and elevations across the marsh surface enables the use of these data in paleoenvironmental models with a range of applications (tectonics, sea level, carbon cycling, etc). The development and utility of a local (Northern, Central, and Southern) versus regional database (entire suite of marshes) in paleoenvironmental modeling will be evaluated with applications to the reconstruction of coseismic deformation.