ML20151P772

Preliminary Tree Ring Dating of Late Holocene Subsidence Along Washington Coast, Presented at 880506-08 Meeting in Seattle,Wa

ML20151P772
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Site:	Trojan File:Portland General Electric icon.png
Issue date:	05/06/1988
From:	Yamaguchi D COLORADO, UNIV. OF
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References
NUDOCS 8808100183
Download: ML20151P772 (1)

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l 8 Puf.TMINARY TREE-ElRG DATING OF IATE-BULDCENE SUBSIDDiCE AIDNG THE WASHINGTON COAST David K. Yamaguchi, Mountain Research Station, Univ. of Colorado, Nederland, CO 80466, (303) 492-8841 The annual growth rings of trees vary in width in response to yearly variations in climate. Such ring width variation can be used to determine the i

' death dates of dead trees by matching their ring width patterns with those of living trees (Fritts 1976).

I have been using this approach to determine limiting death dates for large n stern redcedar (Thuja plicata) snags rooted in buried wetland soils in estuaries along the southwestern Washington coast. The burial of the soils and the death of the cedars are hypothesized to have resulted from coseismic l

subsidence caused by a great earthquake on the Cascadia subduction zone (Atwa-ter 1987). Calibrated radiocarbon ages from the buried soils show that these areas subsided about 300 yr B.P. (Atwater et al. 1987). More precise ages I

would be useful for testing the great earthquake hypothesis by determining if l subsidence occurred synchronously throughout coastel southwest Washington at this time.

Thus far, I have built a preliminary 610-yr long, ring width chronology j from 6 modern old-growth cedars on Long Island in Villapa Bay. In turn, by I

matching the ring patterns of subsided cedar snags with those of the Long Island trees, I have determined A.D. 1613, 1642, and 1682 limiting death dates i for 3 snags at the head of the Grays Harbor estuary (70 km N of Long Island).

Similarly. I have established A.D. 1664 and 1684 limiting death dates for 2 snags in the Copalis River estuary (90 km N of Long Island). Statistical analyses of ring width data from these trees, which generally follow the methodology of Yamaguchi (1996), show that the snag dates are highly signifi-cant:

probabilities of obtaining similar levels of ring-pattern matching by chance range from .001 to .0005.

The A.D. 1618 84 dates provide only limiting dates for subsidence and associated tree death because some external wood is missing from all collected samples due to weathering. Other samples, not yet analyzed, have been obtained from other snags at these sites, as well as from snags in the Palix R. and Crays R. estuaries (15 km N and 25 km SE of Long Island, respectively).

Collectively, the 4 sites span 100 km of Washington coastline.

I plan to expand this study to include sampling of other cedar-snag- l bearing subsided wetlands on the southwest Washington and northern Oregon coasts.

To this end, I appeal to conference attendees to inform me of any observations of large snags rooted on subsided wetlands, and their specific locations. In return, I may be able to provide limiting dates for subsidence at these sites. More inportantly, such collaboration should permit us to:

1) rigorously test the hypothesis of synchronous subsidence in different estuaries about 300 yr ago; 2) closely date this inferred last great Cascadia earthquake and document the length of coastline it subsided; and (3) refine l current estimates of its probable magnitude.

Atwater, B. F. (1987). Science 236, 942-944.

Atwater, B. F., Hull, A. G., and Bevis, K. A.

Fritts, H. C.

(1987). EO_S 67, 1468.

(1976). "Tree Rings and Climate." Academic Press, New York.

Yamaguchi, D. K. (1986). Tree Ring Bulletin 46, 47 54.

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