PhD Candidate, Oregon State University, during AGeS project

Dating During the Pandemic: A Radiocarbon Success Story

Intertidal accommodation space filling following the 1700 Cascadia Subduction Zone Earthquake

Image 0: Me collecting sediment cores in Netarts Bay, Oregon.

The Cascadia Subduction Zone, which spans from British Columbia to Northern California, periodically experiences megathrust earthquakes M >8. During these ruptures, the last of which occurred in 1700, areas of the coast experience co-seismic subsidence of up to 3 m. This subsidence results in instantaneous sea level rise, the creation of accommodation space (i.e., volume below sea level that may be filled with sediment), and destructive tsunami that may simultaneously erode coastal areas and/or emplace a sandy deposit. These earthquake and tsunami deposits are well preserved in estuarine stratigraphy along the Pacific Northwest coast (as Illustrated in Image 1).  Much research has rightfully focused on predicting recurrence intervals, magnitudes, co-seismic subsidence rates, tsunami inundation, and landslide hazards. Yet there remains uncertainty related to the long-term impacts of earthquakes on coastal habitats – namely the timeframe of accommodation space filling following the co-seismic subsidence. We know that accommodation space fills and salt marshes grow back following earthquakes, but does it take less than a decade? Much longer, up to 150 years?

Image 1: Cascadia Subduction Zone earthquake cycle recorded in salt marsh stratigraphy and computed tomography (CT) scan of a 80 cm sediment core collected from an Oregon salt marsh displaying a characteristic tsunami deposit.

This is an important question to answer for a number of reasons. From a hazards perspective, our understanding of earthquake recurrence stems, at least in part, from statistics related to the number and timing of earthquake/tsunami deposits recorded in coastal stratigraphy. These deposits can only be preserved and identified if the proceeding earthquake occurs after the salt marsh has been reestablished. Thus, if accommodation space filling requires many decades to centuries, it is possibly that intermediate earthquakes may have occurred but went unrecorded. From a biogeochemical perspective, intertidal areas are located along the sediment routing system in a unique position – either functioning as a sediment sink or source to the coastal ocean depending on the regime. Thus, whether these habitats are accumulating, bypassing, or eroding sediment influences marine sediment sinks and records, and coastal biogeochemical cycling and productivity. From an ecogeomorphic perspective, understanding the rate and mechanisms of salt marsh reestablishment will better inform how these systems respond to large perturbation events beyond just earthquakes (e.g., storms, sea level rise, wildfire, landslides), and thus may improve our ability to predict how they will evolve into the future. From a social perspective, salt marshes provide a number of culturally and economically important ecosystem services including flood protection, habitat, and carbon burial. The next major Cascadia Subduction Zone earthquake will undoubtedly occur during a period of accelerated sea level rise, increasing the possibility that these habitats and the services they provide may be lost for many decades to centuries.


Below - Image 2: CT scan of the Netarts sediment core chosen for analysis. Darker areas are less dense and whiter areas are more dense. The core was originally 3 m long but was cut in half for transportation and the half on the left is the upper section with a clear 1700 tsunami deposit (dense, sandy layer with a sharp contact with buried marsh below) at around 70 cm depth. More tsunami deposits from past earthquakes are visible throughout the core.