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

Simon E. Engelhart·Benajamin P. Horton·Christopher H. Vane·Alan R. Nelson·Robert C. Witter·Sarah R. Brody·Andrea D. Hawkes
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Abstract

We assessed the utility of δ 13 C 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 δ 13 C/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 δ 13 C values from the groups defined with δ 13 C/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 δ 13 C 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 δ 13 C has potential for identifying amounts of relative sea-level change due to tectonics.

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Simon E. Engelhart, Benajamin P. Horton, Christopher H. Vane, Alan R. Nelson, Robert C. Witter, Sarah R. Brody, Andrea D. Hawkes. 2013. Modern foraminifera, δ13C, and bulk geochemistry of central Oregon tidal marshes and their application in paleoseismology. https://doi.org/10.1016/j.palaeo.2013.02.032

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