Sensitivity Testing of Marine Turbidite Age Estimates along the Cascadia Subduction Zone

Abstract

ABSTRACT Marine turbidite studies infer that 19–20 ∼Mw 9 earthquakes ruptured the full Cascadia subduction zone (CSZ) in the past 10 kyr, a hypothesis that relies on concurrent turbidite deposition generated from seismogenic strong ground motion along the ∼1100 km margin. Correlation of marine turbidite deposits is based on petrophysical characteristics and radiocarbon geochronology, the latter of which relies on a series of age corrections and calibrations for marine radiocarbon age and sedimentological parameters. In this work, I isolate several key variables in turbidite age assessment and systematically test how previous assumptions and new calibration curves affect estimated ages, and thus whether geochronologic analyses independently support coeval turbidite deposition. For radiocarbon age calibration, I test the impact of (1) updating global marine reservoir age corrections; (2) updating local marine reservoir age estimates; and (3) selectively applied marine reservoir age excursions. From the calibrated radiocarbon ages, I calculate turbidite age and uncertainty using a Monte Carlo approach with a broad range of sedimentation rates and substratal erosion. By simply updating the global marine radiocarbon calibration, individual radiocarbon ages differ from published estimates by several hundred years. Updates to the local reservoir age corrections are minimal because existing data remain limited yet have potential for great impact on turbidite ages. Of the sedimentological parameters tested, sedimentation rate has the largest impact on estimated turbidite age, with individual ages changing up to 500 yr from published estimates. For radiocarbon samples of turbidites previously inferred to correlate, the individual ages typically show increased scatter and overall uncertainty, even for models that only update the global marine reservoir calibration. These results highlight the major age uncertainty associated with current coseismic turbidite age analyses in Cascadia and how independent constraints on local reservoir corrections and sedimentation rate are critical for accurate turbidite age estimates in the Pacific Northwest.