ASSESSING THE UTILITY OF δ13C AND BULK GEOCHEMISTRY IN ESTUARIES ALONG THE CASCADIA SUBDUCTION ZONE FOR COASTAL PALEOSEISMOLOGY

Holocene megathrust earthquakes from the Cascadia subduction zone are recorded by mud-over-peat contacts in multiple estuaries from northern California to British Columbia. Preliminary analyses at the estuary scale suggest that bulk δ¹³C sediment geochemistry can be used as a sea-level indicator to reconstruct the magnitude of coseismic deformation, specifically in the absence of more traditional microfossil data. For a sea-level indicator to be useful it must show a consistent relationship to tidal levels across large spatial scales (100’s-1000’s km). To test this we developed a database of bulk sediment geochemical variables (δ¹³C, δ¹⁵N, TOC, C:N) from 249 surface sediment and 20 living plant samples from 14 salt marshes in Oregon (N=2), Washington (N=9) and British Columbia (N=3). On Vancouver Island bulk sediment δ¹³C values ranged from -19.57 to -31.36 ± <0.08‰, in Washington from -17.11 to -30.66 ± <0.15‰, and in Oregon from -19.09 to -32.25 ±<0.15‰. At the estuary scale, bulk sediment δ¹³C values exhibit a significant correlation with elevation (e.g., R²=0.70; P=<0.002 at 8 sites and R²=0.50; P=<0.002 at 10 sites), whereby δ¹³C values become more positive from the upland to the tidal flat. Although, a margin-wide (BC, WA, OR) correlation between bulk sediment geochemistry and elevation (δ¹⁵N R²=0.5; P=<0.038 at 7 sites, C:N R²=0.5; P=<0.03 at 12 sites) is not always observed, in combination (δ¹³C, δ¹⁵N, TOC, C:N ) it has potential as a complementary method for reconstructing paleoseismology at Cascadia. This dataset may be particularly useful in determining bulk sediment geochemistry priors for a new suite of Bayesian transfer functions. Informative priors do not set hard limits on the paleo-marsh elevations (PME) that can be reconstructed, and are necessary to estimate coseismic deformation, rather they increase the probability that PMEs fall within the prior range. Priors in this instance would be bulk sediment geochemical values that fall above or below certain tidal datums.