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

We assessed the utility of delta C-13 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 delta C-13/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 delta C-13 values from the groups defined with delta C-13/bulk geochemistry analyses decrease with an increasing elevation; -24.1 +/- 1.7 parts per thousand in the tidal flat and low marsh; -27.3 +/- 1.4 parts per thousand in the middle and high marsh; and -29.6 +/- 0.8 parts per thousand in the highest marsh samples. We applied the modern foraminiferal and delta C-13 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 +/- 0.39 m and 0.71 +/- 0.56 m) suggesting that delta C-13 has potential for identifying amounts of relative sea-level change due to tectonics. (C) 2013 Elsevier B.V. All rights reserved.