Date of Award
2026
Degree Type
Thesis
Degree Name
Master of Science in Oceanography
Department
Oceanography
First Advisor
Erin Peck
Abstract
Coastal salt marshes on tectonically active margins face dual pressures from episodic seismic events and anthropogenic modifications. This study investigated 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 preserved in stratigraphy, (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 documented abrupt subsidence and widespread low marsh conditions after 1700 CE followed by heterogeneous recovery, consistent with regional studies. Density and sand content most clearly recorded rapid subsidence, whereas organic matter and d13C tracked long-term recovery with d15N supportive of these trends 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 showed 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 declined significantly over time, 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 ± 0.4 g cm³ (n = 79) to 0.55 ± 0.1 g cm³ (n = 70). These findings demonstrated that dune modification had significantly reduced mineral supply to the back-barrier salt marsh, but elevation recovery had already been underway through intrinsic post-seismic uplift and sediment-biogenic feedbacks rather than being initiated by anthropogenic stabilization.
Recommended Citation
Hall, Emily, "INVESTIGATING NETARTS BAY, OREGON COASTAL LANDSCAPE CHANGE FROM TECTONIC SUBSIDENCE THROUGH ISOTOPIC, STRATIGRAPHIC RECORDS AND QUANTIFYING DUNE INFLUENCE ON SALT MARSH MORPHODYNAMICS" (2026). Open Access Master's Theses. Paper 2729.
https://digitalcommons.uri.edu/theses/2729