- Quantification and comparison of morphological changes over the last ~300 y in Oregon salt marshes provide valuable insights into the tectonic, hydroclimatic, and anthropogenic processes shaping this important intertidal zone. Understanding of the rates and drivers of salt marsh change contextualizes intertidal habitats within the sediment routing system (i.e., source to sink); informs models of local and global biogeochemical cycles, important for global climate predictions; and guides ecosystem service protection and restoration efforts. The estimation of rates and drivers of sediment accumulation is made possible by analysis of sediment archives, and though complex, comparison of histories of sediment accumulation between systems with spatiotemporally variable drivers reveals relationships, feedbacks, and thresholds. In particular, the Oregon margin provides an opportunity to compare a number of important drivers of centennial salt marsh morphologic change, including relative sea level rise, suspended sediment supply, basin area, bay morphology, and coseismic subsidence related to Cascadia Subduction Zone (CSZ) earthquakes, all of which will be discussed in this dissertation.
To quantify sediment and carbon accumulation rates within Oregon intertidal areas (mudflat, low marsh, high marsh, and scrub-shrub wetland) and to assess the relative importance of elevation, relative sea level rise, and fluvial suspended sediment supply on vertical accretion, excess 210Pb derived accretion rates were measured within seven Oregon estuaries (Youngs, Nehalem, Tillamook, Netarts, Salmon, Alsea, & Coquille). Accretion rates combined with downcore measurements of dry bulk density and organic carbon content produce rates of carbon burial, therefore further highlighting the importance of determining the factors controlling sediment accretion. In a subset of estuaries (Nehalem, Netarts, Salmon, Alsea, & Coquille), excess 210Pb profiles were reanalyzed to estimate time-varying rates of accumulation and lateral growth rates were assessed by comparing georeferenced and digitized aerial salt marsh imagery over the last ~80 y at roughly decadal resolution. Further, to establish the impact of coseismic subsidence during CSZ earthquakes on the evolution of Oregon intertidal areas, an age-depth method of combining excess 210Pb and high-resolution 14C dates with Bayesian modeling was tested as proof of concept in Netarts Bay stratigraphy following the 1700 CE earthquake.
Taken together, results of these investigations provide both much needed data on and insights into drivers of salt marsh morphodynamic change. For instance, two (Salmon & Alsea) of the seven estuaries exhibit mean rates of accretion less than relative sea level rise over the last century and thus are drowning, possibly linked to comparatively high rates of relative sea level rise, though mean annual sediment is not apparently limited. Further assessment of the temporal changes in rates of vertical accretion reveals that only Alsea is accreting at a pace significantly lower than the rate of relative sea level rise (the uncertainty range associated with the rate of accretion in Salmon overlaps with the uncertainty range for relative sea level rise). As Alsea experiences one of the fastest sea level rise rates along the coast (2.8 ± 0.8 mm y-1), it is possible that inundation stress and resulting loss of marsh plants could be the cause of drowning through decreased sediment retention, and therefore the threshold rate of drowning is relatively low. Further, high marshes that experience neutral or negative rates of sea level rise (Youngs & Coquille, respectively) have been accreting over the last century, even though the conceptual morphodynamic model requires the creation of accommodation space (volume of space created by long-term relative sea level rise) for growth. Unexpectedly high rates of accretion in systems with low or negative rates of sea level rise (Young, Nehalem, & Coquille) could be linked to large sediment fluxes, but time varying rates of accretion are not significantly higher during a period of elevated sediment loads (1944 to 1977), as a result of timber harvest and the wet phase of the Pacific Decadal Oscillation (PDO), indicating another cause. Comparison of storm hydrographs between Alsea and Nehalem reveals that Nehalem’s larger basin area results in longer flood durations and likely increased inundation by turbid-water. This relationship between large basins, long storm hydrographs, and increased inundation when sediment concentrations are likely high appears to explain the observed patterns of sediment accretion. Systems inundated with sediment-rich water will accrete (perhaps the case in Nehalem), systems inundated without sediment-rich water will not accrete (perhaps the case in Alsea as sea level rise is fast), and systems with sediment-rich water that does not inundate the platform will not accrete (perhaps the case in Coquille as sea level is falling). Further, vertical accretion appears linked to lateral expansion, as only salt marshes accreting at the same pace or greater as sea level rise expanded over the last century. However, comparisons between a period of enhanced fluvial sediment discharge (1944 to 1977) to more recent decades reveal that Oregon salt marshes expanded under elevated sediment fluxes, though the amount was limited in areas without available lateral accommodation space (Salmon & Coquille) or without filled vertical accommodation spaces (Alsea). The earthquake deformation cycle is additionally important in controlling marsh growth. Indeed, high resolution (~decadal) age-depth modeling of the last 300 y assessed with downcore facies indicators (e.g., dry bulk density) indicates that Netarts Bay has not yet reestablished the pre-1700 CE CSZ earthquake elevation and scrub-shrub habitat, despite having experienced comparatively moderate coseismic subsidence (~0.5 m). The filling of Netarts salt marsh accommodation space may be limited by a lack of suspended sediment, but comparison with the timing of accommodation space filling in other Oregon salt marshes and along elevation transects is required to better elucidate the mechanisms of filling. Clearly, in addition to shedding light on important rates and drivers of salt marsh growth, the results of this dissertation highlight multiple questions that warrant further investigation.