RATE OF SALT MARSH REEMERGENCE FOLLOWING THE 1700 CASCADIA SUBDUCTION ZONE EARTHQUAKE

Cascadia Subduction Zone (CSZ) megathrust earthquakes produce up to 3 m of co-seismic subsidence along the Pacific NW coast. This instantaneous relative sea level rise (RSLR) increases accommodation space (AS) of intertidal zones, thereby producing a first-order perturbation to coastal geomorphology and associated sediment routing systems. The most recent CSZ earthquake occurred in 1700 CE and is well preserved in the salt marsh stratigraphic record. Though AS has now been filled, with re-established high mashes accreting at approximately RSLR, the timeframe and mechanics are unclear. Loss of vegetation-enhanced sediment trapping combined with increased wave and tidal stresses may have created a negative feedback, preventing reestablishment of high marsh for over a century. Alternatively, AS may have quickly filled through above-average sediment fluxes and post-seismic rebound, and the high marsh was re-established in ~10 y, as has been observed after the Alaskan 1964 earthquake. We investigated the timescale of AS filling of a salt marsh in Netarts Bay, Oregon following the 1700 CSZ earthquake. To construct an approximately decadal-resolution ~300-y-long chronology we combined an excess ²¹⁰Pb chronology for ~150 y BP with a radiocarbon record measured at ~1-cm sampling intervals from ~150 – 300 y BP. To determine landscape evolution from mudflat to low marsh to high marsh, we measured down-core biogeochemical proxies for habitat type, including density; organic carbon and nitrogen contents; stable carbon and nitrogen isotopes; and element ratios derived from XRF core scanning. Results from numerous synthetic stratigraphies created using possible extreme AS filling scenarios indicate that the combination of high-density ¹⁴C sampling and Bayesian age-depth modeling (Bacon) can produce the desired decadal accuracy for reconstructing the history of AS filling following the 1700 CSZ earthquake. Results will inform our understanding of the ecogeomorphic response of intertidal areas to large perturbations, critical to predicting the vulnerability of these systems and their ecosystem services into an uncertain future.