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

Engelhart, S.E., Horton, B.P., Vane, C.H., Nelson, A.R., Witter, R.C., Brody, S.R., Hawkes, A.D.
2 013
Palaeogeography, Palaeoclimatology, Palaeoecology
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 ± 0.39 m and 0.71 ± 0.56 m) suggesting that δ13C has potential for identifying amounts of relative sea-level change due to tectonics.
EES Authors: 
Simon Engelhart (2010)
Andrea Hawkes (2008)
Research Track Category: 

Department of Earth and Environmental Science / University of Pennsylvania, 251 Hayden Hall, 240 South 33rd Street, Philadelphia, PA 19104-6316