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

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Abstract

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. © 2013 Elsevier B.V.

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Engelhart, S. E., Horton, B. P., Vane, C. H., Nelson, A. R., Witter, R. C., Brody, S. R., & Hawkes, A. D. (2013). Modern foraminifera, δ13C, and bulk geochemistry of central Oregon tidal marshes and their application in paleoseismology. Palaeogeography, Palaeoclimatology, Palaeoecology, 377, 13–27. https://doi.org/10.1016/j.palaeo.2013.02.032

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