USE OF DENDROCHRONOLOGY TO STUDY A SUBFOSSIL FOREST AT KENT, WASHINGTON USA THAT WAS BURIED BY LAHAR-DERIVED SEDIMENTS FROM MOUNT RAINIER ABOUT 530 CE

Ariel Q. Moran, Patrick T. Pringle, Beverly K. Luke, Centralia College Science Dept., 600 Centralia College Blvd., Centralia WA 98531

Abstract We are conducting a dendrochronological investigation of subfossil tree specimens found during excavations in 1996 along the Green River floodplain in Kent, Washington. The trees were buried in an andesite-rich, lahar-derived sand and gravel from Mount Rainier and dated at ~1500 yr BP by Vallance and Pringle (2008). The Kent buried forest is located about 120 km (72 mi) downstream of Mount Rainier. Two thin tephra layers at Mount Rainier associated with distinct small or moderate eruptive events ~1,600–1500 yr B.P. are inferred to be correlative with clay-poor lahar deposits found locally along the White River and the andesitic sediment that buried the trees at Kent (Sisson and Vallance, 2008). We studied wood samples from eleven of the trees, eight of which were Douglas-fir (Pseudotsuga menziesii). The samples were mounted, polished, and sanded before being scanned at 0.01mm resolution. Use of ImageJ software to measure the annual growth rings and Cofecha program for analysis of the measurements, as well as visual examination and cross-dating methods, revealed likely correlations among at least five of the subfossil trees. Four of the trees had begun growth of early wood before their death, indicating the trees likely died in the spring. We investigated undated lahar deposits upstream (southeast) of Enumclaw at two locations: Mud Mountain Dam and in a terrace along the White River near MP 32 of WA State Route 410. At the latter location, two seeds recovered from a peat deposit 2–4 cm (~ 1 in) below a clay-poor lahar yielded radiocarbon ages of ~2,400 yr B.P. Using rates of peat accumulation of about 2.5 cm per century measured lower at the same outcrop, we infer that the MP 32 deposits are likely from one of two earlier Summerland eruptive episodes of Mount Rainier at either 2,500–2,400 yr B.P. or 2,200 yr B.P. and thus not correlative with the deposits that buried the Kent forest at least a half millennium later. We plan to continue our tree-ring analysis of the Kent buried forest in hopes of assembling a provisional floating chronology for this cohort of subfossil trees.

Introduction Geologists Dwight “Rocky” Crandell (1963, 1971) and Donal Mullineaux (1970, 1974) conducted pioneering investigations on the postglacial deposits of the Puget Lowland and Mount Rainier. Crandell found evidence of enormous lahars that consist of mainly sediment and rock debris that flowed along the valleys from the volcano for many tens of kilometers – some as far as Puget Sound. Crandell noted that some lahar deposits had a high clay content and probably originated from landslides, whereas other low-clay-content deposits were the result of lahar triggered by interactions of hot rock, snow, and ice. Later studies by Scott and others (1992;1995), and Vallance and Scott (1997) showed that lahars or lahar-derived floods had reached as far as 100 km (60 mi) downstream of the volcano (Figs. 1 and 2). Further investigations and mapping efforts continued to uncover the volcanic history of Mount Rainier owing to discoveries of additional buried forests and geotechnical information about subsurface deposits of Mount Rainier origin in the 1990s and later (Dragovich and others, 1994; Palmer, 1997; Zehfuss and others, 2003a,b; Zehfuss, 2005). Furthermore, Vallance and Pringle (2008) and Sisson and Vallance (2008) reevaluated Holocene volcanism at Mount Rainier and attempted to link volcanism to the triggering of devastating lahars, showing that the massive lahar-derived gravelly sand deposits, such as those that buried trees at Auburn and Fife, and that extend as far as the Port of Seattle, were triggered by moderate-size eruptions at Mount Rainier.

References Bronk, Ramsey C., 1995 Radiocarbon Calibration and Analysis of Stratigraphy: The OxCal Program Radiocarbon, v. 37, no. 2, p.

425-430 Bronk, Ramsey C., van der Plicht, J.; Weninger, B, 2001, 'Wiggle Matching' radiocarbon dates, Radiocarbon, v. 43, no. 2a, p. 381-389. Cook, E. R.; Krusic, P .J., 2008, A tree-ring standardization program based on detrending and autoregressive time series modeling,

with interactive graphics (ARSTAN): Tree-Ring Laboratory, Lamont-Doherty Earth Observatory. Crandell, D. R., 1963, Surficial geology and geomorphology of the Lake Tapps quadrangle, Washington: U.S. Geological Survey Professional Paper 388-A, 84 p., 2 plates. Crandell, D. R., 1971, Postglacial lahars from Mount Rainier volcano, Washington: U.S. Geological Survey Professional Paper 677, 75 p., 3 plates. [accessed April 10, 2002, at http://vulcan.wr.usgs.gov/Volcanoes/Rainier/Publications/PP677/framework.html Dragovich, Joe D.; Pringle, Patrick T.; Walsh, Timothy J., 1994. "Extent and geometry of the mid-Holocene Osceola mudflow in the Puget Lowland–Implications for Holocene sedimentation and paleogeography." Washington Geology, v. 22, no. 3, p. 3-26. [Accessed at http://www.dnr.wa.gov/Publications/ger_washington_geology_1994_v22_no3.pdf Henri D. Grissino-Mayer, 2001. Assessing crossdating accuracy: A manual and tutorial for the computer program COFECHA. Tree-

Ring Research 57(2): 205-221. Holmes, R. L., 1994, Dendrochronology Program Library—User’s manual: Laboratory of Tree-Ring Research, University of Arizona,

Tucson, USA. Mullineaux, Donal R., 1996, Pre-1980 tephra-fall deposits erupted from Mount St. Helens, Washington: U.S. Geological Survey Professional Paper 1563, 99 p. [link: http://pubs.er.usgs.gov/usgspubs/pp/pp1563 Pringle, Patrick T.; Vallance, Jim; Magirl, Chris, 2013, Mount Rainier—Geologic Hazards, Geomorphology, and Engineering Geology, IN Reed, Patricia, (ed.), Field Guide volume for the Association of Engineering and Environmental Geoscientists 2013 Annual Meeting, Seattle WA: AEG. Poster link: http://www.centralia.edu/academics/earthscience/pubs/AEG_2013_Mt Rainier_fieldtrip_1_final.pdf DOI: 10.13140/RG.2.2.24553.90729 Scott, K. M.; Vallance, J. W.; Pringle, P. T., 1995, Sedimentology, behavior, and hazards of debris flows at Mount Rainier, Washington: U.S. Geological Survey Professional Paper 1547, 56 p., 1 plate. Sisson, T. W.; Vallance, J. W., 2008, Frequent eruptions of Mount Rainier over the last ~2,600 years: Bulletin of Volcanology online, DOI 10.1007/s00445-008-0245-7, [24 p.]. Stuiver, M., Reimer, P. J., and Reimer, R. W. 2017, CALIB 7.1 [WWW program] at http://calib.org. Steer, James, H., 2010, Fundamentals of tree ring research: The University of Arizona Press, Tucson, Arizona, p. 368. Stokes, Marvin A.; Smiley, Terah L., 1968, An introduction to tree-ring dating: University of Arizona Press, 73 p. Vallance, J. W.; Pringle, P. T., 2008, Lahars, tephra, and buried forests—The postglacial history of Mount Rainier, In Pringle, Patrick T., 2008, Roadside geology of Mount Rainier National Park and vicinity: Washington Division of Geology and Earth Resources Information Circular 107, 191 p. [URL: http://www.dnr.wa.gov/ResearchScience/Topics/GeologyPublicationsLibrary/Pages/pub_ic107.aspx] Yamaguchi, D.K., 1991, A simple method for cross-dating increment cores from living trees: Can. Jour. Forest Res., v. 21, p. 414–416. Zehfuss, P. H., 2005, Distal records of sandy Holocene lahars from Mount Rainier, Washington: University of Washington Doctor of Philosophy thesis, 141 p. Zehfuss, P. H.; Vallance, J. W.; Pringle, P.; Brown, T., 2003b, Holocene lahar-runout deposits as far as Seattle, Washington, from Mt. Rainier volcano [abstract]. In Cities on volcanoes 3, Abstract volume: International Association of Volcanology and Chemistry of the Earth’s Interior, p. 147.

Kent Trees The buried subfossil trees at Kent were discovered in 1996 during excavations by the City of Kent to enhance a wetland adjacent to the Green River (Fig 3). Most of the samples were Douglas-fir (Pseudotsuga menziesii). One of us, Patrick Pringle, with assistance from Brian Atwater and David Yamaguchi took samples from about 20 subfossil trees using chain saws and hand saws. Disk samples, or “cookies” were trimmed into wood strips that could be mounted and sanded.

Field Investigation of Deposits We visited Mud Mountain Dam along the White River and another site along a forest road about two miles farther upstream we call MP32 on State Route 410 to search for woody material that could help determine the age of the lahar deposits at each location (Pringle and others, 2013). We cleaned off a section of an outcrop and discovered evidence of lahar deposits, tephra, and peat. We were unable to find any wood samples at the Mud Mountain Dam location, however at the MP32 location we discovered datable seeds in a peat layer underneath a clay-poor lahar deposit from Mount Rainier. We collected seeds from the peat layer ~2–4in below the clay poor lahar (Fig.7).

Radiocarbon Dating The two large seeds (Fig. 7) were identified by Cynthia Updegrave as Oemelaria cerasiformis (written commun., 2017). These seeds were submitted to Direct AMS in Bothell, Washington for analysis. We calibrated the raw radiocarbon lab results using program Calib 7.1 (Table 1). Using the radiocarbon dating information, the maximum age of the lahar deposit above the seeds is around 2,400 years ago.

Sample Preparation and Analysis We used basic tree-ring analysis methods as noted in Stokes and Smiley (1968), Steer (2010), and Yamaguchi (1991). The samples were polished to 2000 grit abrasive paper, cleaned using compressed air, and scanned on an Epson 10000XL scanner at 2540 DPI (0.01 mm precision)(Fig. 4). After measuring the annual growth rings, we analyzed the samples using visual techniques including skeleton plotting with a stereo microscope and processing of the annual growth ring measurements using DPL (Dendrochronology Program Library) and program Cofecha (Holmes, 1994; Grissino-Mayer, 2001). We then used program Arstan to create indices of samples we could cross correlate (Cook and Krusic, 2008)(Fig. 5).

Pumice Layer We also discovered a thick pumice layer at the outcrop, which we sampled for further analysis. We analyzed this pumice by grounding up the sample with a mortar and pestle before adding water to allow us to sift through the sample to concentrate the higher density crystals. We then dried the sample before viewing it under a microscope. We discovered slender green crystals of the mineral Cummingtonite in the pumice (Fig 8), which highly suggests it is the Yn pumice layer from Mount Saint Helens (~3,500 yr B.P.).

Acknowledgements The authors thank the Centralia College Foundation for an undergraduate research grant that allowed us to conduct our research. We thank Brian Atwater, US Geological Survey (retired) and David Yamaguchi, dendrochronologist, for assisting in the original retrieval of these subfossil tree specimens. We would also thank Cynthia Updegrave, lecturer at University of Washington, for identifying the seeds discovered during the field investigation as Oemelaria cerasiformis. We thank Antonio Cano, a visiting Fulbright scholar from Spain, and Jarod Johnson, a student of the Evergreen State College, for assisting during our field investigation. Email communications with James Vallance of the US Geological Survey Cascades Volcano Observatory were valuable for assessing which eruptions might have produced the lahar deposits.

Name Calib & cor age (yr BP)(2σ, k=1)

Mean probability

14C Date (raw) Uncertainty ± offset Lab number

LargeseedQ cal BP 2348–2506 2439 2411 34 0

D-AMS 022685

Largeseed J cal BP 2351–2542 2477 2426 43 0

D-AMS 022686

Tinyseed Erroneous data 785 30 0

D-AMS 022684

Figure 1: Location map showing Mount Rainier and paths of ancient lahars (Vallance and Pringle, 2008; modified from Crandell, 1963)

Figure 7: Two seeds found in peat layer Oemelaria cerasiformis

Figure 6: Outcrop of peat, pumice, and lahar deposits about 2 mi upstream of Mud Mountain Dam along White River. We collected the seeds shown in Fig. 7 whose radiocarbon ages are noted in Table 1 from the middle peat layer 2–4 in below the overlying clay-poor lahar deposit.

Table 1: Calibrated radiocarbon ages on seeds collected 2-–4 inches below a clay-poor lahar deposit at MP32 peats location about two miles upstream of Mud Mountain Dam along the White River. The two large seeds help provide a maximum age for the overlying lahar deposit (see text for details). The sample “tinyseed” proved too small for analysis and gave an erroneous age. Results of lab testing were calibrated using program Calib 7.1 (Stuiver and others, 2017).

Figure 2: Map of Washington State showing location of Figure 1 lahars map.

Figure 8: Greenish cummingtonite crystal (center) helps identify Mount St Helens Yn pumice (~3,500 yr BP).

Figure 3: Subfossil tree excavation in Kent, Washington. View to the East.

Figure 4: Kent11a tree sample scanned at 2540 DPI (0.01 mm resolution).

Table 2: Wigglematch radiocarbon plot for five samples from tree 15 at Kent sampled by James Vallance (USGS) and Patrick Pringle. The results show the lahar dates vs. year and dark area shows wiggle match results and ~95% confidence level (518–550 CE) using program Oxcal (Bronk, 1995; Bronk and others, 2005).

Summary Provisional analysis of buried trees at Kent in the Puget Lowland records burial by lahars or lahar-derived floods that resulted from a moderately explosive eruption of Mount Rainier volcano in the early 6th century. More analysis is needed, but thus far we have determined that perhaps as many as four of the trees died at the same time during early spring. For future research we will need to continue analysis of the subfossil tree rings and attempt to cross date, which could allow for precise dating of the lahar from Mount Rainier. We will also need to reexamine the outcrop we visited near Mud Mountain Dam. This would allow us to sample the sedimentary deposits located at the outcrop, including the other pumice layers. We will also be able to search for other carbon samples in peat layers above the lahar.

Discussion Preliminary comparisons of the tree rings using program Cofecha as well as visual techniques noted previously reveals the internal crossdating of samples from trees 11, 18, and 15. Furthermore, we found that tree Knt20 matches samples 11, 12, and 15. Of these trees at least three likely died in the early spring, possibly May, because partial earlywood (formed at the beginning of the growing season) is visible under bark (Fig. 9).We could crossdate more of the exhumed trees, but with less confidence. Further investigations at upstream locations such as those mentioned and shown in Fig. 6 could yield datable carbon and tie upstream deposits with these trees or allow crossdating to the year.

Figure 9: Photomicrographs of outermost rings of polished subfossil wood samples Kent 15b (left) and Kent 11b (right). Arrows show earlywood after last complete ring. The Kent 15b image is scanned and has been measured using ImageJ, whereas photo of Kent 11b is of the wood sample.

Figure 5. Left top: Iindices (floating chronology) of annual growth rings of subfossil wood samples 12, 15, and 20 from Kent, Washington, generated using program Arstan. The bottom time series at left shows a composite set of indices made from averaging those of trees 12, 15, and 20, which we crossdated. X axis shows range of approximate years CE, and blue bar at top shows time of tree mortality based on radiocarbon wiggle matching results shown in Table 2 that have Sigma 2 (~95.4%) confidence of 518–550 CE.

bark