Canadian Journal of Earth Sciences
The Neoglacial History of Robson Glacier, British Columbia.
Journal: Canadian Journal of Earth Sciences
Manuscript ID cjes-2016-0187.R2
Manuscript Type: Article
Date Submitted by the Author: 02-Aug-2017
Complete List of Authors: Luckman, Brian; Dept of Geography Masiokas , Mariano ; Instituto Argentino de Nivologia Glaciologia y Ciencias Ambientales nicolussi, Kurt; Institute of Geography, University of Innsbruck, 6020 Innsbruck,Draft Austria. Is the invited manuscript for consideration in a Special N/A Issue? :
Keyword:
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1 The Neoglacial History of Robson Glacier, British Columbia.
2
3
4 B. H. Luckman 1, M. H. Masiokas 1, 2 and K. Nicolussi 3
5
6 1 Department of Geography, University of Western Ontario, London, Canada N6A 5C2
7 2 Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA),
8 CCT CONICET, Mendoza, Argentina 9 3 Institute of Geography, UniversityDraft of Innsbruck, 6020 Innsbruck, Austria. 10
11 Corresponding author: B. H. Luckman, e mail [email protected], Phone 1 519 661
12 2111 ext 85012.
13
14 Abstract
15
16 As glaciers in the Canadian Rockies recede glacier forefields continue to yield
17 subfossil wood from sites overridden by these glaciers during the Holocene. Robson
18 Glacier in British Columbia formerly extended below treeline and recession over the last
19 century has progressively revealed a number of buried forest sites which are providing
20 one of the more complete records of glacier history in the Canadian Rockies during the
21 latter half of the Holocene. The glacier was advancing ca 5.5km upvalley of the Little Ice
22 Age terminus ca. 5.26 cal ka BP; at sites ca. 2km upvalley ca. 4.02 and ca. 3.55 cal ka.BP
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23 and 0.5 1 km upvalley between 1140 and 1350 A.D. There is also limited evidence based
24 on detrital wood of an additional period of glacier advance ca 3.24 cal ka BP. This record
25 is more similar to glacier histories further west in British Columbia than elsewhere in the
26 Rockies and provides the first evidence for a post Hypsithermal glacier advance at ca.
27 5.26 cal ka BP in the Rockies. The utilization of the wiggle matching approach using
28 multiple 14C dates from sample locations determined by dendrochronological analyses
29 enabled the recognition of 14 C outliers and an increase in the precision and accuracy of
30 the dating of glacier advances.
31
32 Key Words: Robson Glacier, Canadian Rockies, Neoglacial, dendrochronology,
33 radiocarbon dating. Draft
34
35 Introduction
36
37 Most alpine glaciers in the Northern Hemisphere reached their maximum
38 Holocene extent during the “Little Ice Age” of the last few centuries (Grove 2004).
39 Subsequent recession during the last ca. 100 years at several sites has revealed subfossil
40 wood and/or other organic materials that were overidden by earlier glacier events.
41 Increasingly studies of this wood and buried, glacially overridden, forests are being used
42 to define earlier periods of glacier advance and link them to global scale climatic controls
43 (e.g. Solomina et al. 2016; Le Roy et al. 2015). Although increasing numbers of such
44 forefield records are becoming available, including many from British Columbia (e.g.
45 Koch et al. 2007; Menounos et al. 2009; Mood and Smith 2015; St. Hillaire and Smith
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46 2017), few such records are available for the Canadian Rockies (Osborn et al. 2001;
47 Wood and Smith 2004; Luckman 2006). These sites are relatively rare and, individually,
48 only provide a partial record of local glacier histories: regional history must be compiled
49 by the assembly, analysis and correlation of records from many sites.
50 Robson Glacier in British Columbia formerly extended below treeline and
51 recession over the last century has progressively revealed detrital wood and a number of
52 buried forest sites which are providing one of the more complete records of glacier
53 history in the Canadian Rockies during the latter half of the Holocene. This paper updates
54 and reviews new evidence from Robson Glacier and places it within the context of
55 regional glacier history. It also outlines some of the problems of reconstructing glacier
56 history from such evidence. Draft
57
58 Site description and overview of previous work
59
60 Robson Glacier is a ca. 6km long valley glacier (ca 13.9 km 2 in 2006) that drains
61 the eastern and northern flanks of Mount Robson, British Columbia (Figure 1). It has a
62 well developed series of lateral and terminal moraines that extend down to 1660m and
63 abut the Continental Divide. The oldest terminal moraine has a closed forest cover and is
64 about 2km downvalley of the present snout. Sheet 32A of the Inter Provincial Boundary
65 Commission (Cautley and Wheeler 1924) shows the glacier front within ca 100m of the
66 terminal moraine in 1922 1. Heusser (1956) used dendrochronology (ring counts) to date
67 the terminal to ca. 1783 based on the oldest of seven trees he cored on its surface,
68 applying a 12 year ecesis estimate. He also used ring counts to date several additional
1 The 1924 map is based on 1922 terrestrial photogrammetry .
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69 moraines upvalley as 1801, 1864, 1891, 1907 plus several smaller features in the 1920s
70 1940s (see Heusser 1956; Luckman 2000). A proglacial lake developed in front of the
71 glacier after ca. 1960 and has expanded upvalley as the glacier front has receded. Four
72 buried forest sites have been exposed during glacier recession which allowed extensive
73 sampling of wood remains. Each of these will be described briefly followed by the
74 description of dendrochronological analyses and radiocarbon dating of these materials
75 and a discussion of the chronological implications of these findings.
76
77 Heusser Buried forest site
78
79 Heusser (1956) identified a Draftburied forest site, overlain by ca 1 2m of till, that was
80 exposed in abandoned stream channels cut by proglacial streams between ca 1925 1950.
81 He obtained an early Libby 14 C date of 450±150 yr BP from an in-situ stump at this site.
82 Investigations at this locality by Luckman between 1980 and 1992 recovered over 45
83 detrital and in situ logs 2 and stumps up to 30cm in diameter (Luckman 1995). Tree ring
84 series from these logs were successfully calendar crossdated, initially using a master
85 chronology from a site at Bennington Glacier (ca. 75km to the south east) and later with
86 the chronology from the Columbia Icefield (Luckman 1995; Luckman and Wilson 2005).
87 The overridden trees at this site grew between 867 and 1350 A.D. and death dates from in
88 situ stumps indicated that the glacier advanced over this site between ca.1142 and 1350
89 A.D. at a net average rate of ca. 2 2.5m /year (Luckman 1995). This site provided the
2 In situ logs or stumps are preserved in growth position: detrital logs have been transported from their (usually unknown) growth location.
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90 first evidence for a calendar dated early Little Ice Age (LIA) advance in the Canadian
91 Rockies.
92
93 Upper Extinguisher Tower Site
94
95 Luckman et al. (1993) reported on a large, possibly in situ , Pinus albicaulis stump
96 and rootstock, initially discovered by a climber, lying on the ground surface about 30m
97 from the lateral margin of the glacier adjacent to Extinguisher Tower, a 300m high
98 bedrock pinnacle on the north side of the glacier. The snag was located 20 30m from the
99 contemporary ice front on a relatively gently sloping thinly till covered bedrock surface,
100 about 3.5km upvalley of the 1990 snoutDraft and some 300 350m higher. Two small detrital
101 Abies logs were also found lying on the surface a short distance down glacier from the
102 stump. Four 14 C dates from these samples yield ages between ca 3130 and 3360 14 C yr
103 B.P. (E1 E4, Table 1, Luckman et al. 1993; Luckman 1995).
104
105 Glacier Toe area
106
107 In the late 1980s a low bedrock ridge emerged from beneath the northern flank of
108 the glacier snout as it receded upvalley. Proglacial drainage from the north flank of the
109 glacier cut a channel between this ridge and the northern valley side, creating a small
110 delta where it entered the proglacial lake. In 1992 three logs were recovered from the
111 west shore of the lake, a short distance upstream of the Heusser site. Tree ring series
112 from two of these logs (logs R9210 and R9212, see below) crossdated and provided a
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113 strong “floating” (undated) chronology but could not be matched with the existing
114 calendar dated tree ring chronology from the Heusser site. The source of these logs was a
115 buried forest site close to the 1993 glacier front. Luckman (1995) briefly described this
116 site and reported three radiocarbon dates between 3500 and 3710 14 C yr B.P. (G1 G3,
117 Table 1) that confirm that the material found in the lake (G1) was of similar age to in situ
118 material at the glacier front (G2, G3). The balance of this paper reports subsequent,
119 mainly dendrochronological, investigations at this and two other sites that provide a more
120 detailed history of glacier variations at this glacier.
121
122 New Field Investigations and Site descriptions
123 Glacier Toe site (Figure 2A D)Draft
124 The Glacier Toe site is approximately 2 300m upvalley of the lake and lies in a
125 saddle between a broad low bedrock ridge, ca. 40m higher than the lake, and the valley
126 side. The site was exposed in the 1990s by erosion of a proglacial stream originating from
127 the northern flank of the glacier. In 1993 the braided proglacial river channel and coarse
128 gravel bars in front of the glacier contained many wood fragments plus sheared and
129 shattered detrital logs up to 5m long (Figure 2C), including some protruding vertically
130 through the gravels. A small river bank section in one of the gravel bars, ca 50m
131 downstream of the icefront, revealed 20 30cm of compacted organic litter with needles,
132 cones, abundant roots and stunted trees overlying ca. 20 30cm of till (Figure 2D,
133 Luckman, 1995). A radiocarbon date of 3710±70 14 C yr B.P (G2, Table 1) was obtained
134 from one of these roots. Several large trunks, some partially impregnated with till, were
135 also associated with the paleosol and buried within the gravels (Figure 2B). A second
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136 radiocarbon date of 3650±50 14 C yr BP (G3, Table 1) was obtained from ca 31 rings of a
137 large, detrital, 2m high, whitebark pine rootstock. A similar date of 3500±60 14 C yr. B.P.
138 (G1, Table 1) from one of the logs recovered from the lake confirmed that the wood
139 recovered from the lake is of similar age and has been washed in by the proglacial
140 stream. Glacier recession after 1995 exposed the upvalley, downslope side of the ridge
141 and the proglacial drainage initially built a small sand and gravel delta at the ice front
142 graded to the col (possibly burying other material). With further recession and lowering
143 of the ice front the proglacial stream abandoned the col route (the “Creek site” Figure
144 2A) and flowed downslope, entering the ice and flowing to the lake via a sub glacial
145 conduit (Figure 2A, left). Between 1993 and 2000 54 cross sections were collected from
146 detrital logs lying in the former streamDraft channel, on the delta surface downstream or along
147 the shorelines of the proglacial lake. These logs were assumed to have been derived by
148 fluvial and possibly glacier erosion of the buried forest site associated with the paleosol at
149 this locality.
150
151 Ridge site (Figure 2E)
152
153 In 1995 a second site with buried wood and other organic remains was exposed
154 along the ridge crest approximately 1 200m southwest of the glacier toe site and 10 15m
155 higher. Many sheared and shattered stumps and logs up to 25cm in diameter were lying
156 on the surface or exposed in a series of annual moraines formed between ca 1992 and
157 2000 as the glacier margin receded back down the southern flank of the ridge. Clumps of
158 moss and organic material were also found on the surface and, although some tree stumps
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159 were possibly in growth position, no actual paleo surface was exposed. A radiocarbon
160 date of 3160±70 14 C yr. B.P. (R1, Table 1) was obtained from 31 rings in a small detrital
161 wood fragment from this site. This date was at least 400 years younger than dates from
162 the adjacent Glacier Toe site, raising the possibility that trees at these two sites were
163 killed by different glacier events. The initial radiocarbon date indicated that wood from
164 the ridge site was of similar age to the Upper Extinguisher Tower site upvalley.
165 Approximately 32 cross sections from log fragments up to 2m long were recovered from
166 this site between 1995 and 2000.
167
168 Lower Extinguisher Tower site (Figure 3)
169 Draft
170 In 2003 British Columbia Parks personnel reported a newly exposed site near
171 Extinguisher Tower (Figures 1 and 3). Two large logs ca 30 35cm diameter protruded
172 from a stream cut on the east side of the glacier. This site is some 400 500m downvalley
173 of the Upper Extinguisher Tower site and separated from it by a bedrock cliff (Figure
174 3A). A radiocarbon date of 4780±60 14 C yr B.P. (ET1 Table 1) was obtained from the
175 outer hundred rings of one of these logs (R0301) indicating that it is significantly older
176 than both the wood from the Upper Extinguisher site and the Glacier Toe and Ridge sites
177 over 3km downvalley (Luckman 2007). The large streambank exposure revealed that
178 these large logs outcropped about 1 2m above a partially buried paleosol surface,
179 developed on and buried by till, which could be traced for over 50m downstream.
180 Several other logs were recovered along this section together with an erect in situ stump
181 rooted in a rust coloured paleosol (Figure 3F and G). This colouration in the Canadian
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182 Rockies is usually associated with the weathering of volcanic tephra (King 1984;
183 Beaudoin and King 1994) although limited excavation did not reveal a discrete tephra
184 layer at this site. The most likely source is the Mazama tephra dated at ca 6900 14 C yr.
185 B.P. ( 7.63±0.15 ka, Zdanowicz et al. 1999).
186 Detrital logs and a second in situ stump were found in a small dry tributary
187 streambed some 60 100m upvalley of the main section (Figure 3B and C). Sixteen cross
188 sections were recovered from this new site, eight from each locality.
189
190 Methods
191
192 Dendrochronological analysesDraft
193 Dendrochronological studies on the subfossil samples were used to develop
194 “floating chronologies” from this material at the three sites by measuring and crossdating
195 ring width series from these logs. Cross sections were prepared, sanded and polished
196 using standard procedures (Stokes and Smiley 1968). Measurements were carried out at
197 the University of Western Ontario Tree ring Laboratory using the TRIM or Velmex
198 systems with 0.001mm or 0.01mm precision or on a Velmex system at the Biogeography
199 Laboratory, Department of Earth Sciences, Brock University. Where practical, two or
200 more radii were measured for each sample and cross correlation was carried out between
201 radii to verify measurement accuracy. Master chronologies were developed for each log.
202 Crossdating trials were carried out using the program COFECHA (Holmes 1983;
203 Grissino Mayer 2001) as well as the program TSAP Win (Rinn 2001). For the
204 COFECHA analyses, each tree ring series was first high pass filtered, prewhitened and
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205 log transformed to enhance the year to year variability and facilitate the identification of
206 possible dating errors. The TSAP analyses high pass filter the data and use the sign test,
207 Gleichläufigkeit and t values to crossdate the samples.
208 The measured series averaged 160 years (range 42 405 years) in length at the
209 Glacier Toe and Ridge sites and 211 years (range 65 456) at the Lower Extinguisher Site.
210 At each site the longest, clearest and most complete series, were first correlated (in
211 COFECHA) against all other series using 50 yr moving segments lagged with 50%
212 overlap to identify provisional sets of well correlated series that could be grouped into
213 several “floating” master chronologies. Subsequently, the remaining “undated” log
214 chronologies were tested against these “master” series and, following numerous trials,
215 final master series were developedDraft for both the Lower Extinguisher and Glacier
216 Toe/Ridge sites combining series of variable length and with different periods of overlap.
217
218 Radiocarbon dating
219
220 Initial exploratory radiocarbon dates (G1 G3, R1 and ET1, Table 1) were
221 obtained to determine the approximate age of the logs at each site. These dates were from
222 relatively large samples, prior to any dendrochronological analyses and were not
223 anchored within the floating tree ring chronologies. Subsequently, a second set of dates
224 (G4, G5, R2 and ET2, Table 1) were obtained from crossdated ring sequences to clarify
225 temporal relationships between the dated samples. Three additional dates (G6, G7 and
226 ET3) were later obtained in an attempt to resolve dating inconsistencies between the
227 radiocarbon and tree ring dating from this material. These three later dates were included
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228 in an AMS calibration run at Beta Analytic. They are the mean of two (G6) or three (G7,
229 ET3) separate AMS determinations and are the most precisely constrained dates. An
230 additional high precision date (R3) was obtained from the Klaus Tschira Archäometrie
231 Zentrum laboratory in Germany. Further attempts to reconcile dating scenarios were
232 carried out by “wiggle matching” of selected 14 C dates using the programme OxCal
233 version 4.3 (Bronk Ramsay et al. 2001).
234 Results
235 Glacier Toe and Ridge sites
236 At the Glacier Toe/lake site 15 of the 55 logs crossdated compared with 15 of 32
237 from the Ridge site (Tables S1, S2). This modest success reflects that many samples
238 contained growth anomalies such asDraft reaction wood, radial asymmetry or very narrow ring
239 series. Most samples were Engelmann spruce ( Picea engelmanni ) or alpine fir ( Abies
240 lasiocarpa ) 3 although a few whitebark pine ( Pinus albicaulis ) may have been present.
241 Mean ring widths were 0.48mm (range 0.13 1.19mm) and the series showed relatively
242 low Mean Sensitivity (0.215, see Fritts 1976) and high first order autocorrelation (mean
243 0.799) that indicates relatively low interannual variability within ring series.
244 Rarely did contiguous ring series correlate over their entire length: poorer
245 correlations occured most frequently at the beginning or end of the series where tight
246 rings or growth distortions are usually greatest. Trees were considered to be crossdated
247 and assigned “floating years” if they crossdated strongly with the relevant master
248 chronology for at least 75 years or correlated strongly with other radii or trees that did
249 correlate with the master. Many radial samples were only partial i.e. the pith and/or,
250 more frequently, the outermost ring was missing or indistinguishable. The outermost
3 Ring series of these two species are strongly correlated in this region.
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251 rings are often very narrow or poorly preserved. However, outer ring dates are assumed
252 to approximate death dates when the outer surface of the sample clearly followed a ring
253 boundary over part of the circumference of the sample.
254 Ultimately two separate chronologies were developed from these series.
255 Chronology A is 319 years in length, assigned dating of 794 1112A 4 and includes 18 logs
256 crossdating with R9210 and R9212 (Figure 4). Chronology C is 287 years long, assigned
257 dating of 1731 2017C 4 and includes 9 logs that crossdate with sample R9532 (Figure 5)5.
258 The logs in Chronology A are predominantly from the Glacier Toe site (13/20) whereas
259 the majority of samples in Chronology C are from the Ridge site (8/10, see Tables S1 and
260 S2). The longest series from log R2327 (405 years), found on the delta, did not correlate
261 with either master chronology and isDraft treated as a distinct series (see below).
262
263 Samples from both the Ridge and Glacier Toe site crossdated into the Chronology
264 A and the youngest outermost dates from both sites are identical 1112A (Figure 4, Table
265 S1). This chronology indicates that mature trees had been growing for over 300 years at
266 or close to the Glacier Toe site prior to the glacier advance. Many of the logs were
267 sheared and/or had been snapped, probably during the glacier advance, and several had
268 rootstocks attached (e.g. Figure 2C). Although the eight youngest death dates are between
269 ca 1108 1112A, the full range of outer dates indicates that these trees could have been
270 killed or died over a 50 150 year period 6.
4 This is a floating chronology where the relative age of tree rings is correct but the absolute, calendar age, is undefined. The “dating ” applied to the chronology is in arbitrary “A” or “C” years 5 Each sampled log is considered to be from a separate tree although in some cases e.g. R0021 and R0019 (Table S1) they are possibly derived from the same tree. 6 However, not all outer ring dates are kill dates resulting from glacier activity: several sections are incomplete and it is also probable that some of samples are from trees that were dead prior to the arrival of the glacier.
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271 Chronology C is composed of tree ring series from 10 logs, mainly (8/10) from the Ridge
272 site. None of these logs cross date with the Chronology A and the two master series do
273 not crossdate. The end dates of the seven youngest ring series range between 2022 and
274 2002C (Figure 5, Table S1).
275
276 Seven 14 C dates have been obtained from the Glacier Toe site and three from the
277 Ridge site. Six of the dates cluster between 3500 and 3870 14 C yr BP (4.31 3.77 cal ka
278 BP 7) with four diverging dates between 3320 3160 14 C yr BP (G5, G6 , R1 and R3, range
279 3.56 3.21 cal ka BP). Unfortunately most of the older (i.e. first acquired) radiocarbon
280 dates have large error terms and inspection of the calibration curves indicates a relatively
281 low slope and often multiple and wideDraft calendar age estimates for a given 14 C
282 determination. Wiggle matching analyses were applied to improve the precision and
283 accuracy of the dating of the tree ring chronologies. However, inconsistencies between
284 some radiocarbon ages and the relative position of the dated tree rings in the floating
285 chronologies presented problems in determining approximate calendar age equivalents
286 for tree ring chronology A 8. Attempts to “wiggle match” the three radiocarbon dates from
287 Chronology A (G1, G4 and G7) were unsuccessful and the dating for Chronology A was
288 finally modelled using only the G4 and G7 radiocarbon dates (Table 2). This results in a
289 median dating of 4333 4015 cal yr BP or ca. 4.33 4.02 cal ka BP (extreme range 4447
290 3969 cal yr BP, Table 2, Figure 6) for Chronology A. Wiggle matched estimates for
7 Most of the 14 C dates at Robson were initially reported as 14 C yr BP. For clarity the following discussion will only use the calendar equivalent ages i.e. cal ka BP = calendar years prior to 1950, rounded to decades per millennium, or in full (cal yr BP). Both dating equivalents are given in Tables 1 and 2. 8 e.g. Radiocarbon dates GI (3.77 cal ka BP, rings 984 1029A) and G7 (4.31 cal ka BP, rings 835 876A) are both from R9210. Date G4 (ca 4.09 cal ka BP, rings 945 1002A) from R9212 is consistent with G7 but not with G1.
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291 radiocarbon dates R2 and R3 (from log R9532) were used to date Chronology C. Median
292 dates for this chronology are from 3836 3545 cal yr BP (full 2σ range 3874 3472 cal yr
293 BP, Table 2) and clearly do not overlap with Chronology A (Figure 6).
294 Radiocarbon date G2 (4.06 cal ka BP) was taken from the trunk of a large, 2m
295 high, detrital whitebark pine stump and rootstock in gravels at the Glacier Toe site and
296 G3 (3.97 cal ka BP) is from a complete cross section of a rootstock from the paleosol.
297 Both dates are close to and consistent with the end (youngest) date for Chronology A
298 (3976 cal yr BP) and confirm that the glacier overran this site ca 4.0 cal ka BP.
299 Log R2327 9 provided the longest ringwidth record (405 years) from samples at
300 the Glacier Toe and Ridge sites. However the initial radiocarbon date (G5, ca. 3.56 cal ka
301 BP) obtained from the innermostDraft rings of sample R0026 indicated this log was
302 substantially younger than other dated samples at this site and died ca 3.2 3.1 cal ka BP.
303 A second, AMS date of ca. 3.48 cal ka BP (G6) from the same log confirmed this
304 younger age. Wiggle matching of dates G5 and G6 yielded modeled ages of 3.56 and
305 3.48 cal ka BP, respectively (Table 2) and dates the floating 405 year chronology of this
306 log to 3647 3242 cal yr BP (full 2 σ range 3732 3178 cal yr BP, Table 2) indicating that
307 it died ca. 750 years after the trees in Chronology A and ca. 300 years after the youngest
308 tree in Chronology C. Although the wiggle matched dating indicates there is an overlap
309 of ca. 100 years between Chronology C and the ring series in log R2327, the ring series
310 from these trees do not crossdate. The radiocarbon age of a small piece (31 rings) of
311 detrital wood from the Ridge site (R1, 3.38 cal ka BP) is of similar age to R2327.
312
9 R2327 was a large detrital log lying on the delta surface. This ringwidth record is a composite floating chronology of 13 well—dated series from five pieces (samples R0023 0027) that is 405 years long. Both radiocarbon dates (G5 and G6) were from sample R0026.
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313 Summary for the Glacier Toe and Ridge sites
314 Tree ring and radiocarbon dating of logs from these two sites indicate a mixed
315 population of logs with three different ages. The majority of dated samples from the
316 Glacier Toe site crossdate in Chronology A and were killed over about a 100 year period.
317 Eight have outer tree ring determined dates between 1108 and 1112A and nine of the
318 remainder date between 1060 and 1105A. Based on the wiggle matching of Chronology
319 A, 19 of these trees died between ca. 3.98 and 4.10 cal ka BP. Although none of the tree
320 ring dated logs at the creek site is in situ (most logs sampled were detrital having been
321 eroded by the river or washed out of the glacier) the 14 C date of 3.97 cal ka BP (G3)
322 derived from the 123 rings in a cross section of a root from the paleosol confirms the
323 presence of the glacier at this site Draftca 4.00 cal ka BP. The tree ring results also indicate
324 that the valley floor at this site was forested for at least 300 years prior to that glacier
325 advance.
326 The majority of the dated wood samples (8/10) recovered from the Ridge site
327 crossdate into the 292 year long Chronology C with seven trees dying over a 21 year
328 period (2022 2002C). Wiggle matching of radiocarbon dates (R2 and R3) indicates this
329 chronology extends from ca 3.83 3.55 cal ka BP. The poorly constrained radiocarbon
330 date R1 (3.38, 2 σ range 3.56 3.21 cal ka. BP, Table 1) from a wood fragment on the
331 Ridge partially overlaps the C chronology. The earliest rings in the trees dating to
332 Chronology C are ca 3.83 cal ka BP, about 150 years younger than the logs dating to
333 Chronology A and they died ca. 450 years later. The logs recovered from the Ridge site
334 have been directly reworked by the glacier as it pushed upslope against the ridge: some
335 were lying on the surface and others were incorporated into a series of annual moraines
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336 and flutings 10 . Based on this topographic setting it is assumed that original source of the
337 wood is close to the surface and the wood is unlikely to have travelled any distance from
338 its original source. This suggests that most of the material exposed at the Ridge site is
339 from a younger forest bed with trees that began growth ca 3.83 cal ka. BP and were
340 overridden ca 3.54 cal ka BP. However, as both chronologies contain samples recovered
341 from both sites there is clearly some admixture of debris from the two sources. These
342 results indicate Robson Glacier was advancing to a position ca 2km upvalley of its LIA
343 maximum at ca. 4.02 cal ka BP (2 σ range 4.14 3.97 cal ka BP). The glacier subsequently
344 readvanced to a similar position ca. 3.55 cal ka BP (2 σ range 3.58 3.47 cal ka BP),
345 incorporating material from forest trees almost 300 years old.
346 Draft
347 The 14 C dating on R2327 (A5 and A6, Table 1) indicates that this log died ca.
348 3.24 cal ka BP (2 σ range 3.33 3.18 cal ka BP), at least 300 years after other dated
349 material recovered from the Glacier Toe and Ridge sites. It grew at a site that had been
350 ice free for over 400 years. As this sample was found on the delta and was not in situ the
351 most logical explanation is that this log was derived from an exposure at an unknown
352 locality upglacier and has been reworked and transported to the delta by the glacial
353 meltwater stream.
354
355 The wood at the Upper Extinguisher site died after 3.36 3.60 cal ka BP and
356 postdates the trees killed at the Glacier Toe site. The age range of these Upper
357 Extinguisher samples overlaps the dating of both Chronology C and R2327 (Figure 6).
358 The Upper Extinguisher samples were recovered close to the lateral ice margin at a time
10 Small blocks of moss and organic material were also incorporated into the till.
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359 (1989) when the main glacier tongue was forming annual moraines at the Ridge site over
360 three kilometers downvalley. It is possible therefore that these trees were overwhelmed
361 during the same glacier advance ca 3.55 cal ka BP. Alternatively they may have been
362 killed during a later event of unknown extent, possibly related to the death of R2327 ca
363 3.24 cal ka BP.
364
365 New investigations at the Lower Extinguisher Tower Site
366 Eighteen cross sections were recovered from seventeen logs 11 at this site, seven
367 from the till section and eight from the dry creek bed upstream of the section. Mean
368 ringwidth of the 13 measured sections was 0.45mm and the series averaged ca 250 years
369 in length (range 135 456 years). RingDraft series from eight of these cross sections were
370 crossdated into a 520 year long floating chronology B (113 632B) 12 , including trees from
371 both parts of the site (Table 3). Unfortunately, neither of the in situ samples could be
372 crossdated.
373 Three radiocarbon dates, all dating ca. 5.51 cal ka BP were obtained from three
374 logs at this site but the tree ring dating of the samples differ (Table 1). Wiggle matching
375 of the three dates provided consistent dates of 5.33, 5.51 and 5.60 cal ka BP resulting in a
376 519 year long chronology from 5260 5779 cal yr BP (2 σ range of the last ring is 5279
377 5239 cal yr BP, Table 2). The outer ring dates from the eight well dated samples indicate
378 these trees were killed or died over a period of almost 200 years and that ice overrode this
379 site by about 5.26 cal ka BP (2 σ range 5.28 5.24 cal ka BP). However, the downvalley
380 extent of the glacier snout at this time cannot be determined.
11 R0301 and R0401 are from the same log. 12 The dating of the “B” years is independent of both “A” and “C” years.
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381 This site is at approximately 2100 2200m, ca 150 200m above present treeline.
382 The presence of trees up to 450 years old at these elevations ca. 5300 years ago, together
383 with a relatively weathered soil horizon, indicates these trees were probably remnants
384 from a higher, Hypsithermal, treeline. This also suggests that the glacier advance that
385 destroyed this forest ca. 5.26 cal ka BP was the first Neoglacial advance to reach this
386 point in the Robson Valley.
387
388 Regional correlation and discussion
389 The evidence available from Robson Glacier indicates at least four major periods
390 when Robson Glacier was advancing prior to the LIA maximum, namely 1140 1350 A.D.
391 (the Heusser site), ca 3.55 cal ka BPDraft (Ridge site), ca 4.02 cal ka BP (Glacier Toe site) and
392 ca. 5.26 cal ka. BP (Lower Extinguisher Site). There is also possible evidence of glacier
393 advance based on a detrital log ca 3.24 cal ka BP at the Glacier Toe site. However, the
394 onset of these advances is not known but probably occurred at least several decades
395 (based on tree kill dates) or possibly centuries before the dates given. Moreover, in all
396 cases the maximum downvalley extent of these events is not known, nor is the precise
397 timing of their culmination, though it would appear from the relative positions of the
398 Heusser, Glacier Toe, Ridge and Lower Extinguisher sites that these advances were
399 progressively more extensive over time.
400 Evidence of a glacier advance ca 5.25 cal ka BP has not been recognized
401 elsewhere in the Canadian Rockies. However, two detrital logs dating between 5.88 and
402 5.33 ka 13 were found at the Castle Creek Glacier in the Cariboo Mountains approximately
13 Dates referred to as ka are reported as published
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403 75 km west of Mount Robson (Maurer et al. 2012) and there is evidence of glacier
404 advances of similar age at many other sites in the Coast Ranges of British Columbia
405 (Menounos et al 2009; Mood and Smith 2015). There is also limited evidence for
406 possible earlier Neoglacial glacier events in the Rockies. Two detrital wood samples were
407 recently recovered from the gravel outwash in front of Dome Glacier that dated between
408 5.99 and 6.19 cal ka BP (5290±30 14 C yr BP, Beta 326374 and 5310±30 14 C yr BP, Beta
409 326375, Luckman, this paper). This site had earlier yielded detrital whitebark pine snags
410 dating between 6.79 and 7.46 ka. yr BP (Luckman et al. 1993) and a branch of similar
411 age (6.79 7.16 ka yr BP, 6090±60 14 C yr BP., Beta 264007, Luckman, this paper) was
412 recently washed out of the adjacent Athabasca Glacier. These detrital logs indicate forest
413 cover upvalley of these present glacierDraft snouts at several periods in the first half of the
414 Holocene, coincident with palynological evidence of higher treelines at this time
415 (Kearney and Luckman 1983; Luckman and Kearney 1986). However, the absence of in
416 situ material and stratigraphic context at Dome and Athabasca glaciers does not
417 unequivocably indicate that these trees were killed by glacial activity (see Luckman
418 1988) and therefore evidence of earliest Holocene glacier events remains fragmentary
419 compared with the more extensive data being recovered from sites in British Columbia.
420 Nevertheless it is probable that this material is indirect evidence of several periods of
421 glacier advance ca 6.2 6.0 and 7.5 6.8 ka. in the Canadian Rockies.
422 Menounos et al. (2008) identified a regional period of glacier advance across
423 western North America ca 4.2 ka based on glacial and lacustrine evidence. In the
424 Canadian Rockies the evidence for this event was based on two 14 C dates from Robson
425 (G2 and G3, Table 1) and three dates between 4.8 4.1 ka at Boundary Glacier near the
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426 Columbia Icefield. Evidence for this event is also found at Castle Creek Glacier in the
427 Cariboo Mountains where 3 detrital logs, moss and an in situ stump dating 4.35 3.84 ka
428 (mostly 4.15 3.98 ka, Maurer et al. 2012) have been recovered. A second in-situ stump
429 dating 4.96 4.45 ka was also found at the Castle Creek glacier site. Detrital logs of
430 similar age were found at two adjacent glaciers (Chiqui Glacier, 3 logs 4.14 3.73 ka. and
431 Chap Glacier 4.97 4.85 ka, Maurer et al. 2012). The Glacier Toe site at Robson now
432 provides the strongest evidence for a regional glacier advance ca 4.2 4.0 ka in the
433 Rockies. However, with the exception of the site at Boundary Glacier, all of the other
434 evidence previously reported for this event is from sites located west of the Continental
435 Divide in British Columbia (Koch et al, 2007; Menounos et al. 2009; Mood and Smith
436 2015). Draft
437
438 The best documented Neoglacial glacier event in the Canadian Rockies is the “Peyto
439 Advance” (Luckman et al. 1993; Menounos et al. 2009) dating between 3000 and 2800
440 14 C yr BP (calibrated ca. 2.90 3.4 ka). This evidence is best seen at Saskatchewan Glacier
441 where in situ stumps and abundant detrital wood have been dated betweeen 2940±60 and
442 2760±60 14 C yr BP (3.32 2.78 ka, Wood and Smith 2004). Recently two large logs (ca.
443 10m and 7m long) at an adjacent site have yielded similar dates of 2860±30 yr BP (ca.
444 3.03 cal ka BP, Beta 326376) and 3010±30 yr BP (ca. 3.21 cal ka BP, Beta326377,
445 Luckman, this paper). At Robson the material recovered from the Upper Extinguisher
446 Site (3.25 3.69 ka.) was originally tentatively correlated with the Peyto Advance
447 (Luckman et al. 1993). However, these samples are 200 300 years older than the classic
448 material found at the Saskatchewan and Peyto Glacier sites and may be related to the
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449 glacier advance ca 3.55 cal ka BP identified by the tree ring and radiocarbon dated logs
450 at the Ridge site downvalley. The 3.24 cal ka death date for log R2327 provides the only
451 evidence at Robson that corresponds with the timing of the Peyto Advance. This date is
452 300 years younger than the death date for trees at the Ridge site and similar to dates
453 from detrital logs killed by the Peyto Advance at the Saskatchewan Glacier. However,
454 stronger evidence is needed to document the presence of the Peyto Advance at Robson
455 Glacier.
456 Given the relatively large error terms on some, especially older, radiocarbon
457 dates, the limited availability of dates and the absence of a stratigraphic context (the
458 samples are mainly detrital) it is challenging to disentangle the complex history of the
459 period between ca. 4.0 3.0 ka at Robson.Draft It seems likely that the glacial history of this
460 period is as complicated as that of the Little Ice Age and the last two millennia
461 documented in Europe and elsewhere (e.g. Barclay et al. 2013; Nicolussi et al. 2014; Le
462 Roy et al. 2015). New stratigraphic sections and better radiocarbon dating control will be
463 needed to resolve this complex history. There is no evidence at Robson for glacier events
464 between ca 3.24 cal ka BP and the early Little Ice Age advance ca. 1140 1350 A.D. at
465 the Heusser site downvalley.
466
467 Summary and Conclusions
468
469 Based on studies of subfossil wood, Robson Glacier was advancing at the Lower
470 Extinguisher Site, ca 5.5km upvalley of the LIA terminus ca. 5.26 cal yr BP.; at the
471 Glacier Toe/Ridge sites (ca. 2km upvalley) ca. 4.02 cal ka BP and ca 3.55 cal ka BP; and
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472 at the Heusser site (0.5 1 km upvalley) between 1140 and 1350 A.D. prior to the LIA
473 maximum ca 1783 A.D. A single, 400 year old detriatal log that died ca 3.24 cal ka BP
474 may also provide evidence of the the regional ”Peyto Advance” at Robson Glacier. The
475 utilization of the wiggle matching approach using multiple 14C dates from sample
476 locations determined by dendrochronological analyses enabled the recognition of 14 C
477 outliers as well as increasing the precision and accuracy of the dating of glacier advances.
478 The Robson record includes periods of glacier advance not previously well documented
479 from the Canadian Rockies and is more similar to that from sites further west in British
480 Columbia than other sites in the Rockies. These new results confirm multiple periods of
481 glacier advance in the Rockies in the late Holocene and provide the most securely dated
482 post Hypsithermal glacier advance identifiedDraft to date.
483
484 The Robson record also demonstrates some of the difficulties in identifying
485 discrete glacier events based on wood preserved in glacier forefields (see Ryder and
486 Thomson, 1986). The dated material at Robson comes mainly from valley floor sites
487 lacking stratigraphic context and may contain mixed asssemblages of detrital wood
488 derived from deposits of different ages. At sites with abundant material, preferably with
489 some in situ trees or a paleosol, evidence of the minimal extent and timing of glacier
490 events can be derived (see Le Roy et al. 2015). However, results from the Glacier Toe,
491 Ridge and Lower Extinguisher sites at Robson indicate death dates from trees buried
492 during the same event may have a range of 1 200 years. This combined with radiocarbon
493 error terms suggests that it is difficult to distinguish between (or correlate) inferred
494 glacier events that may be 200 300 years apart from small amounts of detrital materials.
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495 Studies from lateral moraine sections and historical materials in British Columbia,
496 Alaska and the Alps (e.g. Le Roy et al. 2015; St Hillaire and Smith 2017; Barclay et al.
497 2013) indicate several periods with multiple late Neoglacial glacier advances that may be
498 only a few centuries apart. It is therefore likely that the late Neoglacial history of the
499 Rockies is more complex than the limited story presented here and that continued work in
500 glacier forefields is needed to elucidate a more complete history.
501
502 Acknowledgements
503
504 Many individuals have been involved in the studies at Mount Robson. We would
505 like to thank Fred Dalley, Les Jozsa,Draft Gordon Frazer, Jessica Lusted, Scott St.George, Jim
506 Hamilton, Chris Somr, Martin Groleau, Emma Watson, Chris Zimmermann (B.C. Parks),
507 Carla Aruani and David, Heather and Helen Luckman plus Yellowhead Helicopters
508 (Valemount) for assistance in the field and laboratory. Wayne van Velzen and his staff
509 for permission to work in Mount Robson Provincial Park; Dan McCarthy for access to
510 the Velmex system at the Biogeography Laboratory of Brock University; Ron Hatfield at
511 Beta Analytic for assistance with radiocarbon dating; Karen vanKerkoerle (UWO) for
512 Figures 1, 4 7; David Barclay and an anonymous reviewer for comments on an earlier
513 version of this manuscript; and the Natural Sciences and Engineering Research Council
514 of Canada for financial support via Operating/Discovery Grants to Luckman over the
515 years
516
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Draft
Figure 1: Location of Robson Glacier. The Google image is from the summer of 2011. Notes: LIA= Little Ice Age terminal moraine (1783); H=Heusser site; R=Ridge site: GT= Glacier Toe site: E= Upper Extinguisher Tower site: ET= Lower Extinguisher Tower site.
132x183mm (158 x 158 DPI)
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Draft
Figure 2; The Glacier Toe site. A: Aerial oblique view, looking downvalley August 2004 showing the “Ridge” and “Creek” sites, the former stream delta (upper left) and proglacial lake beyond. In 2004 the proglacial stream along the east margin of the glacier flowed into the ice along the foot of the ridge (bottom left). B: Sheared and splintered log in a gravel bar near the ice front, June 1993. C: Sample R9501 in the creek, June 1995. This log had 315 rings dating between 794 and 1108 in Chronology A. D: Paleosol site exposed in the stream cut, June 1993. Till and the overlying paleosol were exposed in the stream bank immediately upstream of the large boulder (far left). Small trees, roots and organic debris were recovered from this section and a cross section of 123 rings from a root in this paleosol yielded a 3710±70 14C yr BP date (G2, Table 1). E: Splintered log and wood lying on the surface and partially buried in till of the 1995 annual moraine immediately outside the glacier front on the Ridge crest, June 1995.
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Figure 3: The Extinguisher Tower sites. A: View of the east lateral margin of the glacier in 2003 showing the Upper (E) and Lower (ET) Extinguisher Tower sites (photo by Chris Zimmermann, BC Parks). B: log in the abandoned tributary stream channel. C: Aerial view of the lower site (ET). The stars show critical locations; (a) in situ stump, (b) Location of Fig 3B, (c) location of Fig 3D. D: Large trunks in the lower section. R0301/401 has been cut and R0402 is being sampled. N ote the orange surface material adjacent to the logs. E: Detail of the lower section; (a) location of R0301/401, (b) location of R0403 (being cut), (c) location of R0407/8. The paleosol can be traced downslope from R0301/401 passing below R0403 to R0408. F: In situ stump R0408 and the cut log R0407 protruding horizontally higher up the section. Neither of these could be cross dated. G: Orange stained paleosol adjacent to and below R0408.
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Figure 4: Chronology A samples. The age distribution and record of the 20 logs comprising Chronology A at the Glacier Toe and Ridge sites (For details see Table S1). Notes. Ages are given in Chronology A years. The letters preceding the sample number indicate the collection site; C= creek bed or delta, L= lake shore, R=ridge site. Logs with approximate death date (outer surface follows ring boundaries) are identified by the short vertical lines.
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Figure 5: Chronology C. The age distribution and record of the 10 logs in the Chronology C. For details see Table S1. Notes. Ages are given in Chronology C years. The letters preceding the sample number indicate the collection site; C= creek bed or delta, L= lake shore, R=ridge site. Logs with approximate death date (outer surface follows ring boundaries) are identified by the short vertical lines.
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Figure 6 Age distribution of radiocarbon dates and tree-ring chronologies from the Robson Glacier sites. Data for the single 14C dates are as given in TableDraft 1. The wiggle matched tree-ring chronologies ar e based on the 14C dates indicated in brackets. Data for the “wiggle matched” chronologies are given in Table 2.
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Figure 7: Location of the main glacier sites referred to in the text.
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Table 1. Radiocarbon dates from the Robson sites. cal 14C 14C date cal 14C date 14C date date Ref a laboratory Log ID setting ring position c median cal ka source yr BP 2 σ range number b BP d cal ka BP
Upper Extinguisher site Luckman E1 Beta-28439 GDO In situ stump outer 3300±70 3.53 3.70-3.38 et al. 1993 Luckman E2 Beta-33012 R8902 In situ stump not known 3230±70 3.46 3.64-3.27 et al. 1993 Small detrital Luckman E3 Beta-35010 R8902x not known 3130±70 3.34 3.56-3.16 log et al. 1993 Small detrital Luckman E4 Beta-38309 R8905 not known 3360±60 3.60 3.82-3.45 log et al. 1993
Glacier Toe site Chronology A 794-1112A Detrital wood Luckman G1 Beta-62064 R9210 984-1029A 3500±60 3.77 3.96-3.62 (lake) 1995 Luckman G2 Beta-65381 R9311 Root in paleosol 123 rings 3710±70 4.06 4.29-3.85 1995 Large snag in Luckman G3 Beta-65382 R9314 not known 3650±60 3.97 4.15-3.83 gravels 1995 Detrital wood Menounos G4 Beta-200485 R9212 945-1002A 3730±70 4.09 4.35-3.88 (lake) et al. 2009 Detrital wood Draft Menounos G5 Beta-200484 R0026A 51-71 of 405 3320±80 3.56 3.82-3.38 (delta) et al. 2009 Detrital wood G6 Beta-378910 R0026A2 151-192 of 405 3260±21 3.48 3.56-3.41 this paper (delta) Detrital wood G7 Beta-378911 R921014 827-847A 3870±17 4.31 4.41-4.28 this paper (lake)
Ridge Site Chronology C 1731-2017C Luckman R1 Beta-84817 R95 Detrital wood 31 rings 3160±70 3.38 3.56-3.21 et al. 1996 Menounos R2 Beta-200486 R9532 Detrital wood 1811-1901C 3550±60 3.84 4.06-3.65 et al. 2009 R3 MAMS-31064 R9532b Detrital wood 2009-2017C 3300±23 3.52 3.58-3.46 this paper
Lower Extinguisher site Chronology B 113-632B Log in till Luckman ET1 Beta-187090 R0301 510-610B 4780±60 5.51 5.61-5.32 section 2007 Log in till ET2 Beta-200483 R0403 275-325B 4770±60 5.51 5.60-5.33 this paper section Log in till ET3 Beta-378912 R0402 356-396B 4803±17 5.51 5.59-5.48 this paper section
a reference number of sample. b The MAMS date is from the Klaus-Tschira-Archäometrie-Zentrum lab in Mannheim, Germany. c Ring position of the dated sample (when known) is given based on their position in the respective “floating” tree-ring chronology. Note that the youngest ring (closest to the pith) provides the oldest calendar date. d The median calendar date and two sigma range are based on calibration results established by using OxCal 4.3 (Bronk Ramsey 2009) and the IntCal13 calibration curve (Reimer et al. 2013) and rounded to the nearest decade
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Table 2. Wiggle matching results for radiocarbon dates. The table lists the calibration results of selected radiocarbon dates and related tree-ring series and chronologies derived from grouping of dates using OxCal 4.3 (Bronk Ramsey 2009). cal 14C cal 14C date 14C date ring position agreement ref a 14C date laboratory number log ID median 2 σ range yr BP (mid point) (%) b cal yr BP cal yr BP Chronology A (Glacier Toe site) 794-1112A G4 Beta-200485 R9212 3730±70 4085 4348-3880 945-1002A (974A)
a wiggle matched (G4,G7) 4154 4278-4108 102 Beta 378911 R921014 3870±17 827-867A (847A) G7 4311 4407-4280
a wiggle matched (G4,G7) 4280 4404-4234 105.2 b Chronology A (G4,G7) start 4333 4457-4287 b Chronology A (G4,G7) end 4015 4139-3969 105.1 G1 Beta-62064 R9210 3500± 60 3773 3959-3616 984-1029A (1007A)
a wiggle matched (G1,G4,G7) 4081 4144-3997 0.5
chronology A (G1,G4,G7) start 4294 4357-4210
chronology A (G1,G4,G7) end 3976 4039-3892 4.1 Chronology C (Ridge Site) 1731-2017C R2 Beta-200486 R9532 3550 ±60 3840 4062-3646 1810-1900C (1855C)
a wiggle matched (R2, R3) 3712 3750-3639 61.2 R3 MAMS-31064 R9532B 3300 ±23 3521 3577-3462 2009-2017C (2013C)
a wiggle matched (R2, R3) Draft3554 3592-3481 89.4 b Chronology C (R2,R3) start 3836 3874-3763 b Chronology C (R2,R3) end 3545 3583-3472 65.2 R2327 1-405 (R2327) G5 Beta-200484 R0026A 3320±80 3556 3819-3378 41-71 of 405 (57)
a wiggle matched (G5,G6) 3590 3675-3526 115.6 Beta-378910 R0026A2 3260±21 151-192 of 405 (172) G6 3482 3562-3414
a wiggle matched (G5,G6) 3475 3560-3411 103.1
b chronology R2327 (G5,G6) start 3647 3732-3583 1-405 b chronology R2327 (G5,G6) end 3242 3327-3178 113.2 Chronology B (Lower Extinguisher site (ET)) 113-632B ET1 Beta-187090 R0301 4780±60 5511 5609-5324 510-610B (560B)
a wiggle matched (ET1,2,3) 5331 5350-5310 43.2 ET2 Beta 200483 R0403 4770±60 5505 5602-5325 275-325B (300B)
a wiggle matched (ET1,2,3) 5591 5610-5570 75.5 ET3 Beta 378912 R0402 4803±17 5505 5592-5481 356-396B (376B) a wiggle matched (ET1,2,3) 5515 5534-5494 61.5 b chronology B start 5778 5797-5757 b chronology B end 5259 5278-5238 39.5
a G4, G5, etc. =radiocarbon date identification ; a = revised calibrated 14C date (median and 2 σ range) based on the combined calibration (wiggle matching) of the 14C dates in brackets; b= wiggle matched start and end dates of the related chronology (median and 2 σ range of these start and end dates);
b The suggested threshold for a satisfactory wiggle matching agreement of a single date is 60%, the overall agreement threshold (bold) for a wiggle matching analysis depends on the number of dates included, i.e., it is 50% for 2 combined dates and 40.8% for 3 dates (Bronk Ramsey 1995, 2009). Note: For other abbreviations see Table 1. https://mc06.manuscriptcentral.com/cjes-pubs
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Table 3. Cross sections recovered from the Lower Extinguisher Site.
Site a sample # N Rings b inner c outer death d mean RW e Master f 14C g TS R0401 290 343 632 x 0.59 M TS R0301 355 255 609 0.52 M ET1 TS R0404 201 396 596 x 0.42 M TS R0403 456 100 556 sapwood 0.33 M ET2 SB R0416 270 279 548 x 0.38 M SB R0413 216 306 522 0.27 tentative TS R0402 228 366 487 x 0.59 M ET3 TS R0418 303 113 415 x 0.53 M SB R0414 233 116 348 x 0.26 M SB R0417 174 119 294 x 0.51 poor xd SB R0419 193 90 282 0.57 poor xd SB R0408 174 in situ assymetric TS R0406 155 complex SB R0411 135 x 0.37 TS R0407 112 nm SB R0412 110 Draft nm TS R0405 79 nm
SB R0415 65 nm a TS= till section, SB= dry former stream bed. b N rings = total rings in the sample (including unmeasured or poorly dated inner or outer rings); c Inner and Outer rings for the crossdated record (chronology B years); d Mean rw= mean ringwidth; nm= not measured e x- outer complete ring present f M = included in chronology, Poor xd = weak crossdate; g 14C identifies the 14C dates from the sample.
Note: R0301 and R0401 are different cross sections from the same tree.
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Table S1. Listing of tree ring-dated cross sections recovered from the Glacier Toe and Ridge sites Chronology A cross-dated series Site a Sample # N Ring b Inner c Outer mean rw d Youngest e Death f 14C g C R9308 118 995 1112 0.51 1112 X R R9533 158 955 1112 0.77 1112 X L R9211 137 974 1110 0.55 1110 X C R9403 137 974 1110 0.79 1110 X L R9210 303 807 1109 0.54 1109 X G1, G7 L R9410 152 958 1109 0.86 1109 X R R0008 228 882 1092 0.20 1108 X C R9501 315 794 1108 0.61 1108 X C R9404 137 967 1105 0.66 1105 X L R9408 122 967 1103 0.61 1103 X L R9302 159 942 1100 0.67 1100 C R9504 151 944 1094 0.85 1094 R R9529 89 997 1085 1.08 1085 X L R0030 122 964 1085 0.69 1085 X L R9212 261 925 1066 0.56 1084 G4 R R9526 120 960Draft 1079 0.76 1079 L R9414 120 954 1073 0.63 1073 X R R9522 194 867 1060 0.39 1060 R R0006 146 857 1002 0.43 1002 X R R0010 124 809 932 0.29 932 X
Chronology C cross-dated series 14 Site Sample # N Rings Inner Outer Mean rw Youngest Death C C R9402 138 1885 2022 1.08 2022 X R R9532 287 1731 2017 0.40 2017 X R2, R3 R R0016 233 1782 2014 0.33 2014 R R0021 182 1829 2010 0.41 2010 X R R0019 161 1844 2004 0.45 2004 X R R9524 242 1763 2004 0.45 2004 X L R9318 68 1935 2002 0.59 2002 R R0017 169 1811 1979 0.34 1979 R R0018 146 1832 1977 0.30 1977 X R R9530 148 1777 1924 0.47 1924
a Site location, C= creek (includes delta), L=lake, R= ridge; b N rings = total rings in the sample (including unmeasured or poorly dated inner or outer rings); c Inner and Outer rings for the crossdated record only; d Mean rw= mean ringwidth (measured rings only); youngest = outermost ring of the cross section (may include unmeasured outer rings); e youngest = outermost ring of the cross section (may include unmeasured outer rings); f probable outer ring or bark (rarely) present; g 14C shows number of 14C dates from this sample.
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Table S2. Samples not crossdating with the A or C chronologies. Mean Mean site sample # N rings RW 14C site sample # N rings RW 14C C R2327 405 0.38 G5, G7 L R9323 126 0.85 C R9506 302 1.74 L R9411 124 0.35 R R9523 269 0.29 C R9311 123 nm G2 C R9309 256 0.15 L R9510 123 0.85 C R9415 244 0.45 C R9311 123 0.13 L R9320 231 0.76 L R9322 122 0.42 R R9531 231 0.31 R R0015 117 0.55 R R0009 216 0.84 C R9310 116 0.41 C R9306 214 0.57 R R0011 115 0.58 R R9528 212 0.66 L R0031 113 0.42 L R9508 205 0.55 L R0031 113 0.42 C R0002 197 0.57 C R9507 106 0.86 R R0013 191 0.19 L R9321 104 0.51 L R9405 190 0.49 L R9413 104 0.79 L R9317 181 0.49 C R9307 102 0.22 L R9319 167 0.72 Draft R R0022 97 0.56 R R0012 165 0.17 R R9301 93 0.61 C R9401 163 0.62 L R9406 93 0.57 R R0014 152 0.32 C R9304 91 0.76 L R9412 151 0.67 L R9511 87 0.31 L R9316 149 0.45 C R9305 82 1.09 R R9530 148 0.47 C R9312 81 0.62 C R0005 147 0.70 R R9520 81 0.71 L R9505 147 0.38 L R9407 72 0.82 L R9409 142 0.62 L R9318 68 0.59 C R0003 139 0.33 C R0004 42 0.87 R1 R R0020 134 0.43 R R95 31 nm G3 R R9525 126 0.45 C R9314 na nm Notes: Nm= not measured; For other abbreviations see Table S1
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