Coastal salt marshes on tectonically active margins face dual pressures from episodic seismic movement and anthropogenic modifications. This study investigates how the 1700 CE Cascadia Subduction Zone earthquake and the 1900 CE introduction of invasive dune grass influenced back-barrier salt marsh morphodynamics at Netarts Bay, Oregon. It was hypothesized that (1) salt marsh habitats and elevation transitions would be detectable in geochemical proxies, (2) modern dune stabilization significantly limits mineral sediment delivery to the salt marsh, and (3) this reduction would be preserved in the stratigraphic record. Stratigraphic and isotopic analyses of sediment cores incorporating CT scans, dry bulk density, organic matter, stable carbon and nitrogen isotopes and ²¹⁰Pbₑₓ age dating were employed to reconstruct paleo-elevation habitats and accretionary trends. Comparison of the geochemical record with current elevation zones documents abrupt subsidence and widespread low marsh conditions after 1700 CE followed by heterogeneous recovery, consistent with regional studies. Density and sand content most clearly record rapid subsidence, whereas organic matter and δ¹³C track long-term recovery with δ¹⁵N supportive but less sensitive. While stable isotopes effectively identified broad habitat transitions, dry bulk density and organic matter provided more consistent indicators of long-term elevation recovery. Results show that following the 1700 CE subsidence event, the marsh maintained a stable vertical accretion rate of 2.0 ± 0.2 mm yr⁻¹. Sand content and dry bulk density decline significantly with age, with a statistically significant change point at 1930 CE. The fringing marsh region shows the strongest post-1900 CE mean dry bulk density reductions from 1.67 g cm³ to 0.55 g cm³. These findings demonstrate that dune modification has significantly reduced mineral supply to the back-barrier salt marsh, but elevation recovery was already underway through intrinsic post-seismic uplift and sediment–biogenic feedbacks rather than being initiated by anthropogenic stabilization.