Modern foraminifera, δ¹³C, and bulk geochemistry of central Oregon tidal marshes and their application in paleoseismology

We assessed the utility of δ¹³C and bulk geochemistry (total organic content and C:N) to reconstruct relative sea-level changes on the Cascadia subduction zone through comparison with an established sea-level indicator (benthic foraminifera). Four modern transects collected from three tidal environments at Siletz Bay, Oregon, USA, produced three elevation-dependent groups in both the foraminiferal and δ¹³C/bulk geochemistry datasets. Foraminiferal samples from the tidal flat and low marsh are identified by Miliammina fusca abundances of >45%, middle and high marsh by M. fusca abundances of <45% and the highest marsh by Trochamminita irregularis abundances >25%. The δ¹³C values from the groups defined with δ¹³C/bulk geochemistry analyses decrease with an increasing elevation; -24.1±1.7‰ in the tidal flat and low marsh; -27.3±1.4‰ in the middle and high marsh; and -29.6±0.8‰ in the highest marsh samples. We applied the modern foraminiferal and δ¹³C distributions to a core that contained a stratigraphic contact marking the great Cascadia earthquake of AD 1700. Both techniques gave similar values for coseismic subsidence across the contact (0.88±0.39m and 0.71±0.56m) suggesting that δ¹³C has potential for identifying amounts of relative sea-level change due to tectonics.