Repeated Megafloods from Glacial Lake Vitim, Siberia, to the Arctic Ocean Over the Past 60,000 Years

University of Wollongong Research Online

Faculty of Science, Medicine and Health - Papers: part A Faculty of Science, Medicine and Health

1-1-2018

Repeated megafloods from glacial Lake Vitim, Siberia, to the Arctic Ocean over the past 60,000 years

Martin Margold Stockholm University, [email protected]

John D. Jansen Stockholm University, University of Glasgow, [email protected]

Alexandru Tiberiu Codilean University of Wollongong, [email protected]

Frank Preusser University of Bern, Stockholm University, University of Freiburg, [email protected]

Artem L. Gurinov Lomonosov Moscow State University

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Recommended Citation Margold, Martin; Jansen, John D.; Codilean, Alexandru Tiberiu; Preusser, Frank; Gurinov, Artem L.; Fujioka, Toshiyuki; and Fink, David, "Repeated megafloods from glacial Lake Vitim, Siberia, to the Arctic Ocean over the past 60,000 years" (2018). Faculty of Science, Medicine and Health - Papers: part A. 5329. https://ro.uow.edu.au/smhpapers/5329

Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] Repeated megafloods from glacial Lake Vitim, Siberia, to the Arctic Ocean over the past 60,000 years

Abstract Cataclysmic outburst floods transformed landscapes and caused abrupt climate change during the last deglaciation. Whether such events have also characterized previous deglaciations is not known. Arctic marine cores hint at megafloods prior ot Oxygen Isotope Stage (OIS) 2, but the overprint of successive glaciations means that geomorphological traces of ancient floods emainr scarce in Eurasia and North America. Here we present the first well-constrained terrestrial megaflood ecorr d to be linked with Arctic archives. Based on cosmogenic-nuclide exposure dating and optically stimulated luminescence dating applied to glacial-lake sediments, a 300-m deep bedrock spillway, and giant eddy-bars > 200-m high, we reconstruct a history of cataclysmic outburst floods from glacial Lake Vitim, Siberia, to the Arctic Ocean over the past 60,000-years. Three megafloods have reflected the rhythm of urE asian glaciations, leaving traces that stretch more than 3500 km to the Lena Delta. The first flood was coincident with deglaciation from OIS-4 and the largest meltwater spike in Arctic marine-cores within the past 100,000 years (isotope- event 3.31 at 55.5 ka). The second flood marked the lead up to the local Last Glacial Maximum, and the third flood occurred during the last deglaciation. This final 3000 km 3 megaflood stands as one of the largest freshwater floods ever documented, with peak discharge of 4.0-6.5 million m 3 s −1 , mean flow depths of 120-150 m, and average flow elocitiesv up to 21 m s −1 .

Disciplines Medicine and Health Sciences | Social and Behavioral Sciences

Publication Details Margold, M., Jansen, J. D., Codilean, A. T., Preusser, F., Gurinov, A. L., Fujioka, T. & Fink, D. (2018). Repeated megafloods from glacial Lake Vitim, Siberia, to the Arctic Ocean over the past 60,000 years. Quaternary Science Reviews: the international multidisciplinary research and review journal, 187 41-61.

Authors Martin Margold, John D. Jansen, Alexandru Tiberiu Codilean, Frank Preusser, Artem L. Gurinov, Toshiyuki Fujioka, and David Fink

This journal article is available at Research Online: https://ro.uow.edu.au/smhpapers/5329

1 Repeated megafloods from glacial Lake Vitim, Siberia, to 2 the Arctic Ocean over the past 60,000 years 3 4 Martin Margold1*, John D. Jansen2, Alexandru T. Codilean3, Frank Preusser4, Artem 5 L. Gurinov5, Toshiyuki Fujioka6, David Fink6 6 7 1Department of Physical Geography, Stockholm University, SE-10691, Stockholm, Sweden. 8 2Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, 8000 Aarhus, 9 Denmark. 10 3School of Earth & Environmental Sciences, University of Wollongong, Northfields Avenue, 11 Wollongong, NSW, 2522, Australia. 12 4Institute of Earth and Environmental Sciences, Albert-Ludwigs-Universität Freiburg, 13 Albertstr. 23-B, 79104 Freiburg, Germany. 14 5Department of Geomorphology & Palaeogeography, Faculty of Geography, Lomonosov 15 Moscow State University, Leninskie gory, GSP-1, Moscow, 119991, Russian Federation. 16 6Australian Nuclear Science & Technology Organisation, New Illawarra Road, Lucas 17 Heights, NSW, 2234, Australia. 18 19 *Correspondence to: [email protected] 20 21 Abstract 22 Cataclysmic outburst floods transformed landscapes and caused abrupt climate change during 23 the last deglaciation. Whether such events have also characterized previous deglaciations is 24 not known. Arctic marine cores hint at megafloods prior to Oxygen Isotope Stage (OIS) 2, 25 but the overprint of successive glaciations means that geomorphological traces of ancient 26 floods remain scarce in Eurasia and North America. Here we present the first well- 27 constrained terrestrial megaflood record to be linked with Arctic archives. Based on 28 cosmogenic-nuclide exposure dating and optically stimulated luminescence dating applied to 29 glacial-lake sediments, a 300-m deep bedrock spillway, and giant eddy-bars > 200-m high, 30 we reconstruct a history of cataclysmic outburst floods from glacial Lake Vitim, Siberia, to 31 the Arctic Ocean over the past 60,000-years. Three megafloods have reflected the rhythm of 32 Eurasian glaciations, leaving traces that stretch more than 3500 km to the Lena Delta. The 33 first flood was coincident with deglaciation from OIS-4 and the largest meltwater spike in 34 Arctic marine-cores within the past 100,000 years (isotope-event 3.31 at 55.5 ka). The second 35 flood marked the lead up to the local Last Glacial Maximum, and the third flood occurred 36 during the last deglaciation. This final 3000 km3 megaflood stands as one of the largest 37 freshwater floods ever documented, with peak discharge of 4.0–6.5 million m3s–1, mean flow 38 depths of 120–150 m, and average flow velocities up to 21 m s–1. 39 40 1. Introduction 41 Discoveries of cataclysmic glacial lake outburst floods have expanded the scale and 42 frequency of known mass-transport events on Earth and other planets (Baker, 2001). 43 Freshwater discharges from glacial lakes associated with waning ice sheets are thought to 44 have disrupted ocean circulation and sparked pronounced climate fluctuations (Broecker et 1

45 al., 1989; Barber et al., 1999) while transporting cubic kilometres of rock, soil, and biomass 46 (Korup and Tweed, 2007). During the last deglaciation in North America, the final collapse 47 of glacial Lake Agassiz-Ojibway triggered the 8.2 ka cold event (Barber et al., 1999), and 48 outbursts from Lake Missoula cut the spectacular Channeled Scabland (Baker and Bunker, 49 1985; Baker et al., 2016). However, records of outbursts prior to Oxygen Isotope Stage 2 50 (OIS-2) have remained elusive in both Eurasia and North America (e.g., Bjornstad et al., 51 2001), even though Arctic marine cores show that large Eurasian glacial lakes (Fig. 1; 52 Mangerud et al., 2004; Svendsen et al., 2004) were a major meltwater source (Spielhagen et 53 al., 2004). 54 55 Figure 1 here (column width) 56 57 1.1. Pleistocene glacial lake outburst floods in southern Siberia 58 Floods from glacial lakes dammed by the continental sectors of the Eurasian Ice Sheet are yet 59 to be reconstructed in detail (see Mangerud et al., 2004; Komatsu et al., 2016), but more 60 information is available on outbursts from glacial lakes in southern Siberia. Here, extensive 61 mountain ranges with alpine relief surround deep intermontane basins (Petit and Déverchère, 62 2006; Krivonogov and Safonova, 2017) that have hosted large glacial lakes repeatedly in the 63 Pleistocene when their drainage became blocked by glaciers (Grosswald and Rudoy, 1996; 64 Komatsu et al., 2016). The elevated positions of these basins within the continental interior 65 meant that sudden failure of the ice dams unleashed high-magnitude floods down the valleys 66 of the major Siberian rivers towards the Arctic Ocean (Grosswald and Rudoy, 1996; Carling 67 et al., 2010; Komatsu et al., 2016). The floods from basins in the Altai and Sayan mountains 68 (Fig. 1) have received considerable attention (e.g., Baker et al., 1993; Herget, 2005; Reuther 69 et al., 2006; Komatsu et al., 2009; Bohorquez et al., 2016; Batbaatar and Gillespie, 2016a, b): 70 the Lake Chuja-Kuray floods are the only outbursts comparable in discharge to the Missoula 71 floods (O'Connor and Costa, 2004) that have been reconstructed in detail. In comparison, the 72 Pleistocene glacial lakes and floods of Transbaikalia (Fig. 1) have thus far attracted little 73 interest. Building on earlier reconnaissance work (Margold and Jansson, 2011; Margold et al., 74 2011), here we reconstruct outburst floods of cataclysmic magnitude from glacial Lake Vitim 75 (Figs. 1, 2), based on their erosional and depositional legacy stretching from the headwaters 76 of the Vitim River down to the Lena River delta and Arctic Ocean sediment cores. 77 78 Figure 2 here (1.5 column width) 79 80 2. Glacial Lake Vitim 81 The Vitim River, one of the largest tributaries of the Lena, has its source in the mountains 82 east of Lake Baikal. After crossing the Vitim Plateau in its upper reaches, the Vitim drains 83 the Muya-Kuanda Depression (Fig. 2), a deep intermontane basin containing >1 km depth of 84 sediments associated with the north-east arm of the Baikal Rift System (Petit and Déverchère, 85 2006; Enikeev, 2008; Krivonogov and Safonova, 2017). North of the Muya-Kuanda 86 Depression, the Vitim cuts a narrow valley between the North Muya and Kodar ranges, which 87 are mostly composed of the crystalline rocks of the Baikal–Muya Orogen (Rytsk et al., 2011), 88 reaching elevations of ~3000 m above sea level. The Vitim then crosses the Baikal-Patom 89 Uplands before meeting the Lena at the north-western front of the uplands (Fig. 2). 2

90 91 The existence of an ice-dammed lake in the Muya-Kuanda Depression was first suggested by 92 Obruchev (1929) and was briefly noted by Grosswald and Rudoy (1996). While the origin of 93 the palaeo-lake (Fig. 2) was disputed at first (cf. Kulchitskiy and Pulyavskiy, 1988; 94 Kulchitskiy et al., 1995; Ufimtsev et al., 1998; Kolomiets, 2008), the damming of the Vitim 95 River by Pleistocene glaciers from the Kodar Mountains is now accepted as the primary 96 forming mechanism (Osadchiy, 1979, 1982; Bazarov, 1986; Krivonogov and Takahara, 2003; 97 Enikeev, 2009; Margold and Jansson, 2011; Margold et al., 2011). Margold et al. (2016) 98 established that glaciers were sufficiently extensive to dam the Vitim River most recently 99 during the Last Glacial Maximum at OIS-2 (Fig. 2). The geomorphological record of glacial 100 Lake Vitim (Fig. 2) was mapped by Margold and Jansson (2011) using Landsat imagery, 101 Shuttle Radar Topographic Mission (SRTM) 3 arc-sec data, and Google Earth. A relict 102 shoreline was traced along a substantial portion of the lake's perimeter at a constant 840 m 103 above sea level (Fig. 2; Kulchitskiy and Pulyavskiy, 1988; Margold et al., 2011). This 104 elevation matches an overflow to the Amur River, which drains to the Pacific Ocean, and so 105 marks the maximum possible lake level at which the lake covered an area of 23,500 km2 (Fig. 106 2; Margold et al., 2011). The lake volume was calculated at 3000 km3 using the 3 arc-sec 107 SRTM data (Margold et al., 2011), and based on present-day river discharge data for the 108 Vitim River in the Muya-Kuanda Depression, it was estimated that lake-filling would take 109 <100 years to reach the maximum impoundment depth of 490 m (Fig. 2; Margold et al., 110 2011). In their GIS analysis, Margold et al. (2011) identified geomorphological evidence for 111 a catastrophic release of the lake: a 6 km long, 2 km wide, and 300 m deep bedrock spillway 112 in the ice dam area ~ 15 km downstream from Lake Oron (Figs. 3, 4). In addition, the Vitim 113 valley-sides appear to have been trimmed by large-magnitude floods immediately 114 downstream of Lake Oron (Margold et al., 2011; Fig. 3). Yet there has thus far been no 115 attempt to reconstruct the outburst floods from glacial Lake Vitim based on evidence from 116 the field and the first such effort is reported here. 117 118 Figure 3 here (full-page width) 119 Figure 4 here (column width) 120 121 3. Methods 122 3.1. Remote sensing and field surveys 123 We refine the geomorphology mapped by Margold and Jansson (2011) with a resurvey of the 124 Vitim valley at higher resolution thanks to the release of SRTM 1 arc-sec data in 2014. 125 Mapping was carried out with ArcGIS 10.2 (Environmental Systems Research Institute, 126 2011). Two field expeditions (summer 2011 and 2012) aimed to verify the mapping and to 127 search for additional information about the lake and its cataclysmic drainage. The fieldwork 128 focused on two areas: (i) the former lake basin (the Muya-Kuanda Depression), and (ii) the 129 ice dam area and the Vitim valley downstream. 130 131 Relict shorelines were visited at two sites in the Muya-Kuanda Depression, but no dateable 132 materials were located. A well-rounded boulder interpreted as an ice-rafted dropstone was 133 found on the slope below one of the shorelines (56.431018° N, 115.467247° E), indicating

134 that glaciers were calving into Lake Vitim, consistent with earlier mapping (Margold and 135 Jansson, 2011). Travelling by boat along the Muya River (Fig. 2) led to the discovery of 136 glaciolacustrine sediments exposed in the river bank (Fig. 5). Despite two attempts in 2011 137 and 2012, we were not able to locate and re-examine a Muya riverbank exposure described 138 by Russian authors (Kulchitskiy et al., 1990; Krivonogov and Takahara, 2003) as comprising 139 two glaciolacustrine units separated by a palaeosol 14C-dated to ~45–37 ka. Other 140 sedimentary exposures were visited elsewhere in the Muya-Kuanda Depression to gain an 141 insight to the nature of sedimentation, reworking and erosion, as described in Section 4.4. A 142 giant eddy bar, located across the Vitim River from the former settlement of Nerpo (Figs. 2, 143 6), was discovered when boating down the Vitim. Other giant eddy bars at Bur (Figs. 2, 7) 144 and on a tributary, Bodaybo River (Figs. 2, 8), were located thanks to sand extraction 145 activities. 146 147 Figure 5 here (full-page width) 148 Figure 6 here (column width) 149 Figure 7 here (column width) 150 Figure 8 here (full-page width) 151 152 153 154 3.2. Geochronology 155 In order to establish a chronology for the existence of glacial Lake Vitim and for its outburst 156 events, we apply both optically stimulated luminescence (OSL) and cosmogenic nuclide 157 exposure dating to depositional and erosional landforms associated with the floods. Namely, 158 these are: the section with exposed glaciolacustrine sediments at the Muya River, the 159 bedrock-incised spillway and the giant eddy bars at Nerpo, Bur, and Bodaybo. The location 160 of these sites is marked in Fig. 2, the geomorphology of the ice-dam area with the location of 161 the samples for 10Be surface exposure dating is displayed in Fig. 3, and the sedimentary 162 stratigraphy at the sites from which we collected samples for OSL and for 10Be sediment 163 depth-profiles is displayed in Figs. 5-8. 164 165 3.2.1. Optically stimulated luminescence dating 166 All OSL samples were collected by hammering stainless steel tubes (30 x 8 cm) into freshly 167 cut excavations. At the Muya River sedimentary section (Figs. 2, 5) a total of eight OSL 168 samples were collected from fluvial sands and glaciolacustrine muds (samples GLV-11-1, 169 GLV-11-3, MS-12-01 to MS-12-06; Table 1), although only three of these were measured 170 successfully (Table 1). OSL samples were also collected from hand-excavated pits at the crest 171 of the giant eddy bars at Nerpo (NT-12-01) and Bur (GLV-11-9; Figs. 2, 6, 7, Table 1). A 172 total of three OSL samples (GLV-11-5, GLV-11-7, BUP-12-01; Table 1) were collected from 173 a sand extraction pit (the ‘Upper Pit’ site) cut into a giant eddy-bar in the Bodaybo River 174 valley, a major tributary of the Vitim River (Figs. 2, 8). Two samples were collected from the 175 eddy bar sediments (GLV-11-5, GLV-11-7), and one sample was collected from the 176 overlying thin aeolian sand sheet (BUP-12-01; Fig. 8). 177 Samples were prepared and measured at the Stockholm University luminescence laboratory. 178 Sediment from the outer parts from the tube was used for gamma spectrometric

179 measurements (Preusser and Kasper, 2001). Cosmic dose was calculated using present-day 180 depth. The average sediment moisture during burial was estimated based on present day 181 water content and grain size composition of the sample. For two samples (GLV-11-1, GLV- 182 11-3), we expect a significant change of sediment moisture in the past (due to submergence 183 by Lake Vitim) and used 20 ± 5 % from burial until 10 ka ago, and 8 ± 4 % for the remaining 184 time. 185 186 Material from the inner part of the sample was sieved (125-160 µm) and chemically pre- 187 treated (HCl, H2O2, Na-oxalate). The quartz fraction was isolated using heavy liquids and 188 then etched with 40 % HF (60 min), followed by 10 % HCl treatment (> 120 min) and re- 189 sieving to remove feldspar remains. Equivalent dose (De) was determined (Risø DA-20) 190 using a modified version of the SAR-protocol of Murray and Wintle (2000), with preheating 191 at 230°C for 10 s prior to all OSL measurements (blue diode stimulation for 60 s at 125°C, 192 Hoya U340 detection filter). Measurements were also conducted on small aliquots (2 mm, ~ 193 100 grains), but these turned out to be problematic as discussed below. 194 195 Table 1 here 196 197 3.2.2. Cosmogenic nuclide dating 198 All samples for 10Be dating of rock surfaces were collected with a hammer and chisel. Four 199 bedrock samples (GLV-1,3,5,7) were collected along the western rim of the spillway (Fig. 3) 200 and a single sample (GLV-9, Fig. 9) from a talus block on the spillway slopes. Two boulders 201 were sampled on the Oron moraine (GLV-11,-13; Fig. 3; their ages were reported in Margold 202 et al. [2016]) and one sample from a large detached block in a bedrock outcrop (GLV-15) in 203 the Oron delta (Fig. 3). All of the collected samples were of granitic lithology with the 204 exception of samples GLV-13 (mica schist) and GLV-15 (metasedimentary). Some of the 205 sampled surfaces were covered by a moss blanket but none of them displayed signs of any 206 substantial weathering. In addition, two 10Be sediment depth-profiles were collected; one at 207 the crest of Nerpo giant eddy-bar (6 samples, Fig. 6) and the other at ‘Upper Pit’ on the 208 Bodaybo River (11 samples, Fig. 8). 209 210 Figure 9 here (column width) 211 212 The GLV series of samples (Table 2), collected in 2011, were processed at the Deutsches 213 GeoForschungsZentrum GFZ, Potsdam. Quartz was separated and cleaned following 214 procedures based on Kohl and Nishiizumi (1992) and 10Be was extracted using ion 215 chromatography following procedures described by von Blanckenburg et al. (1996). 10Be/9Be 216 ratios were measured at the University of Cologne’s accelerator mass spectrometer (AMS) 217 laboratory (Dewald et al., 2013). The 10Be/9Be ratio of full procedural blanks, prepared from 218 a phenakite Be spike solution added to all samples, was (0.8 ± 0.3) × 10-15 and blank 219 corrected 10Be/9Be ratios ranged between (158 ± 6) x 10-15 and (792 ± 8) x 10-15. 10Be/9Be 220 ratios were normalised to the 2007 KNSTD standard KN01-5-3 with nominal 10Be/9Be ratio 221 of 6.32 × 10-12 (Nishiizumi et al., 2007). The Nerpo and Bodaybo depth-profile sediment 222 samples (BUP and NT series, Table 3), collected in 2012, were processed at the Australian 223 Nuclear Science and Technology Organisation (ANSTO) using an initial treatment of

224 successive etching by hot phosphoric acid (Mifsud et al., 2013) and a final clean via standard 10 225 HF/HNO3 methods. Be was extracted using ion-chromatography methods reported in Child 226 et al. (2000). Three full procedural blanks prepared from a similar phenakite spike solution 227 gave a final mean 10Be/9Be ratio of (3.5 ± 0.4) × 10-15 (n = 8, mean of 3 targets). Blank 228 corrected 10Be/9Be ratios ranged between (36 ± 2) x10-15 and (533 ± 9) x 10-15. The 10Be/9Be 229 ratios were measured at the ANSTO Antares AMS Facility (Fink and Smith, 2007) and were 230 normalised to the 2007 KNSTD standard KN01-5-2 with a nominal 10Be/9Be ratio of 8.56 × 231 10-12 (Nishiizumi et al., 2007). Errors for the final 10Be concentrations (atoms/g) for all 232 samples (GLV, BUP, and NT) were calculated by summing in quadrature the statistical error 233 for the AMS measurement, 2% for reproducibility, and 1% for uncertainty in the Be spike 234 concentration. 235 236 Table 2 here 237 Table 3 here 238 239 3.2.2.1. Monte Carlo-based modelling of depth profiles and exposure age calculations 240 10Be deposition ages for the depth profiles were calculated using the age calculator code of 241 Hidy et al. (2010; version 1.2) available from http://geochronology.earthsciences.dal.ca. The 242 modelled Bayesian 'most probable age' and associated uncertainties are obtained from relative 243 probability density functions constructed from all the ages generated from a customised 244 Monte Carlo simulation. The calculator uses a time-independent spallation production model 245 based on the St scaling scheme (Stone, 2000). Muon production is calculated using Heisinger 246 et al. (2002a, b) following the approach of Balco et al. (2008) and thus results are compatible 247 with the online calculators formerly known as the CRONUS online calculators. We use a 248 modified global average 10Be sea-level and high-latitude spallation production rate of 3.99 ± 249 0.22 atoms g-1 yr-1 (Heyman, 2014), which is derived from the St scaling scheme, and is 250 statistically identical to the recent 10Be spallation production rates reported in Borchers et al. 251 (2016). Bedrock exposure ages for the 8 GLV samples were calculated using the online 252 calculators formerly known as the CRONUS online calculators (Balco et al., 2008; version 253 2.2; constants 2.2.1) and, for consistency with our depth profile ages, are reported here using 254 the St production rate scaling, with the same reference spallation production rate of 3.99 ± 255 0.22 atoms g-1 yr-1. We adopt the 10Be half-life of 1.387 ± 0.012 Ma (Chmeleff et al., 2010; 256 Korschinek et al., 2010) in all our calculations. 257 258 3.3. Quantifying peak discharge of the outburst floods 259 Neither the details of the ice-dam breach mechanism, nor the size of the breach and its rate of 260 growth are known. Thus, we opted for a conservative estimate of peak discharge based on 261 unequivocal minimum floodstages indicated by a series of high eddy/expansion bars 262 developed along the Vitim valley (Fig. 2, Table 4). We applied the one-dimensional step- 263 backwater hydraulic model, HEC-RAS (Hydrologic Engineering Center, 2010, version 4.1), 264 in order to obtain a minimum estimate of the peak discharge of floods responsible for the 265 highest eddy/expansion bars. The widely applied HEC-RAS model produces an energy- 266 balanced water-surface profile as a function of discharge, roughness, and flow geometry. This 267 simple one-dimensional model is most appropriate for our reconstruction of the Vitim 268 megafloods, because floodstage is the only available source of palaeoflow data and in

269 principle the quality of data input should dictate the level of modelling complexity (Carling et 270 al., 2003). Accordingly, the modelling outputs provide approximations only. In addition, we 271 applied three simple empirical equations for estimating peak discharge based on dimensions 272 of the impoundment and the breach – these are introduced together with the discharge values 273 in Section 4.3.2. 274 275 Table 4 here 276 277 3.3.1. HEC-RAS methods 278 The flood modelling was applied to the ~520 km reach of the Vitim River from the area of 279 the ice dam (near Lake Oron) to the Lena River junction (Fig. 2). Flow geometry was 280 reconstructed following standard procedures using Shuttle Radar Topographic Mission 1-arc- 281 sec digital elevation data coupled to the HEC-GeoRAS (Hydrologic Engineering Center, 282 2011) extension in ArcGIS 10.2 (Environmental Systems Research Institute, 2011). 283 284 We manually defined 253 valley cross-sections (spaced roughly every 2 km) and then 285 exported the data to HEC-RAS. We assumed a fixed channel boundary for the flow 286 modelling based on present-day channel morphology. Although we anticipate that the 287 megafloods locally scoured the former valley fill to bedrock, scour depth is a small fraction 288 of the full flow cross-section and therefore any resulting discharge underestimation is minor 289 relative to other sources of error. 290 291 We ignored tributary inputs along the modelled reach, as any contributing discharge would be 292 comparatively negligible. The tributaries and their junctions act primarily as discharge 293 storage zones during megafloods: Vitim floodwaters moved up tributaries and then re-entered 294 the Vitim mainstem during waning stages. In this way, flood deposits were emplaced at 295 junctions and up some tributary valleys; e.g., Bodaybo River ‘Upper Pit’ ~15 km upstream of 296 the Vitim junction (Fig. 2). Tributary mouths and other areas of flow expansion likely to 297 develop flow separation eddies were defined as ‘ineffective flow’ zones (in 78 of the 253 298 cross-sections); tributaries close to the former ice-dam (e.g., Amalyk River; Fig. 3) were very 299 likely blocked with ice and so were defined as ‘blocked obstructions’ (in 9 cross-sections). 300 301 The ~520 km longitudinal profile of the modelled Vitim River reach is smoothly concave 302 (Fig. 10a). Average bed slope at the upstream end is ~ 0.50 m/km, declining to ~ 0.15 m/km 303 just above the Lena River junction. Outbursts from glacial Lake Vitim resulted in deep flows 304 that were confined within the narrow Vitim valley in a setting comparable to megafloods 305 from Lake Chuja-Kuray (Herget, 2005). Yet, the Vitim River is considerably less steep than 306 the Chuja-Katun River (Chuja River ~ 6 m/km, Katun River ~ 2.2 m/km; [Herget, 2005]); 307 hence, flow velocities and energy losses in general are expected to be somewhat lower. 308 Energy losses in HEC-RAS are accommodated via coefficients for flow expansion and 309 contraction and by the roughness coefficient, Manning’s n. The history of glacial erosion 310 along the Vitim (Margold and Jansson, 2011) has left a remarkably uniform valley width 311 downstream; hence we retained the HEC-RAS default values for expansion and contraction at 312 0.1 and 0.3, respectively. To account for uncertainties with predicting the roughness of flows

313 > 200 m deep we used a range of Manning’s n from 0.03 to 0.05, following other megaflood 314 reconstructions (Herget, 2005; Carling et al., 2010). 315 316 Figure 10 here (1.5 column width) 317 318 Consistent with previous megaflood modelling (O’Connor and Beebee, 2009; Bohorquez et 319 al., 2016), we anticipated generally subcritical flow conditions possibly with local 320 transcritical flow zones. Accordingly, the HEC-RAS model runs were conducted under 321 conditions of ‘mixed regime’ steady uniform flow initiated by ‘normal depth’ floodstage at 322 the downstream and upstream cross-sections (using bed slopes given above). We consider the 323 giant flood bars were constructed rapidly during the steady flow conditions that followed the 324 initial flood wave after the ice-dam breach. 325 326 4. Results and interpretations 327 The new evidence we present for the existence of glacial Lake Vitim and its outburst floods 328 consists principally of the thick unit of glaciolacustrine muds exposed in the bank of the 329 Muya River (Fig. 5) and of the giant eddy bars strewn along the Vitim valley downstream 330 from the ice-dam area (Figs. 2, 6-8). These are massive, coarse-gravel and sand deposits 331 perched up to ~ 230 m above the Vitim River and their position at inner bends and tributary 332 junctions suggests they developed via flow-separation during very deep flows. Sedimentary 333 exposures at Bodaybo (Fig. 8) reveal ~ 30 m-thick, rhythmically-bedded sand, and up to 5 m 334 of open-work coarse gravels deposited rapidly from suspension—structures diagnostic of 335 extreme floods (Carling, 2013). Besides those giant bars investigated in the field, we mapped 336 a number of others from the SRTM 1 arc-sec data (Figs. 2, 11). In the following, we first 337 summarise the dating results (Section 4.1.) and proceed to reconstruct the chronology of the 338 outburst floods (Section 4.2.) as well as their peak discharge (Section 4.3.). We then return 339 upstream to the Muya-Kuanda Depression to reconstruct its sedimentary environment 340 (Section 4.4.), which is important not only for gaining better knowledge of the Quaternary 341 history of the region but also in order to allow for a comparison with the geological and 342 geomorphological record of high-magnitude glacial lake outburst floods elsewhere. 343 344 Figure 11 here (column width) 345 346 4.1. Summary of the geochronology 347 4.1.1. OSL ages 348 The OSL ages are summarised in Table 1 and Fig. 12. For most samples, we found a 349 prominent contamination of the quartz signal by feldspar that could not be removed by 350 additional etching and sieving. Four samples were abandoned as an insufficient number of 351 aliquots delivered pure quartz OSL signals (Fig. 13a). In sample NT-12-01 most aliquots 352 exhibit no OSL signal or a signal dominated by an unstable medium component (see Steffen 353 et al., 2008). In five of the remaining samples, Equivalent Dose values are mainly tightly and 354 normally distributed (Fig. 13b), indicating the presence of a large population of aliquots in 355 which the OSL signal was likely zero at the time of deposition. We applied the Central Age 356 Model (CAM) for all samples except one in which we applied the Minimum Age Model

357 (Galbraith et al., 1999) to identify the well-bleached population and the resulting value for 358 age calculation. 359 360 Figure 12 here (1.5 column width) 361 Figure 13 here (column width) 362 363 4.1.2. Cosmogenic 10Be ages 364 CRONUS-derived bedrock exposure ages are summarised in Table 2 and depth profile 10Be 365 data in Table 3; all are displayed in Fig. 12. Monte Carlo model parameters are summarised 366 in Table 5 and model results in Fig. 14. The Bayesian most probable age obtained for the 367 Nerpo depth profile is 17.7 ka (with ± 1 σ limits of 22.3–14.6 ka). These uncertainty bounds 368 are calculated directly from the probability density function constructed from the age results 369 (Fig. 14c). The sedimentary section in Bodaybo ‘Upper Pit’ consists of > 10 m of flood- 370 deposited sands and gravels capped by a unit of aeolian origin ~ 1 m thick (Fig. 8c). Thus, the 371 measured 10Be inventory of the lower portion of Bodaybo ‘Upper Pit’ section is composed of 372 (i) 10Be accumulated prior to the emplacement of the upper aeolian unit, and (ii) 10Be 373 accumulated after aeolian deposition at a substantially lower production rate. Accordingly, to 374 calculate an age for the lower portion of the Bodaybo ‘Upper Pit’ section, the 10Be 375 accumulated after the emplacement of the aeolian unit needs to be calculated and subtracted 376 from the measured 10Be values. In the case of the Bodaybo samples, this 10Be component 377 represents between 12 and 1% of the total measured 10Be concentration in the top and bottom 378 lower portion samples, respectively. 379 380 Figure 14 here (full-page width) 381 Table 5 here 382 383 The Bayesian most probable age obtained for the aeolian unit of Bodaybo ‘Upper Pit’ section 384 is 15.2 ka (with ± 1 σ limits of 19.6–11.3 ka). We obtain a Bayesian most probable age for 385 the lower portion of the Bodaybo ‘Upper Pit’ section (immediately before the emplacement 386 of the upper aeolian unit) of 19.7 ka (with ± 1 σ limits of 24.1–16.8 ka). Summing the two 387 ages we obtain a final age for the flood deposit in the lower portion of the Bodaybo ‘Upper 388 Pit’ section of 34.9 ka (with ± 1 σ limits of 41.1–30.0 ka). 389 390 The four 10Be samples (GLV-1,3,5,7, Figs. 3, 12, Table 2) taken from the rim of the spillway 391 yield a weighted-mean of 56.0 ± 1.8 ka (standard mean error; Fig. 12, Table 2), but given the 392 possibility of some surface weathering this should be regarded as a minimum. Although 393 relevant rock weathering data could not be located for Siberia, in order to quantify the 394 potential effect of surface erosion on the spillway exposure ages, we model a range of erosion 395 rates based on published weathering data from similar granitic rock surfaces in Arctic 396 northern Sweden (André, 2002). Reported surface weathering rates range between 0.2–1.0 397 mm ka-1, and these yield weighted-mean surface exposure ages between 56.5 ± 1.9 and 61.8 398 ± 2.3 ka, respectively. 399 400 4.2. Reconstructed flood chronology

401 Our reconstruction identifies at least three outburst floods from glacial Lake Vitim over the 402 past ~ 60 ka (Fig. 15). The oldest, Flood-I, formed the 300 m deep spillway when meltwater 403 overtopped and incised the ice-dam to its base, eroding ~3 km3 of underlying bedrock, 404 although we cannot rule out the cumulative effect of earlier overspills (Bjornstad et al., 2001; 405 Larsen and Lamb, 2016). Carving of the spillway reflected the configuration of the ice dam 406 and we surmise that an ice-cap downstream of Lake Oron probably blocked the Vitim valley 407 and buried the surrounding lower mountains in ice (Figs. 2, 16). The four 10Be samples from 408 the rim of the spillway yield a minimum (zero-erosion) weighted mean exposure age of 56.0 409 ± 1.8 ka and provide an estimated timing of Flood I (Fig. 15). 410 411 Figure 15 here (1.5 column width) 412 Figure 16 here (column width) 413 414 Flood-II is dated via OSL and Bayesian Monte Carlo analysis (Hidy et al., 2010) of a 10Be 415 depth-profile on a giant bar ~ 130 m above Vitim River at the Bodaybo ‘Upper Pit’ site (Fig. 416 12). Two statistically equivalent OSL dates: 33.3 ± 4.2 ka and 34.1 ± 4.0 ka (GLV-11-5 and - 417 7; Fig. 12, Table 1) demonstrate rapid deposition of a 13 m stack of rhythmically-bedded 418 sands and gravels. The 10Be depth-profile yields a Bayesian most probable age of 34.9 ka 419 (with ± 1 σ limits of 41.1–30.0 ka; Tables 3, 5, Figs. 12, 14). We traced the Flood-II source ~ 420 400 km upstream to the Muya-Kuanda Depression (as for all floods) where Lake Vitim 421 deposited 11 m of laminated glaciolacustrine muds now exposed in the banks of the Muya 422 River (Figs. 2, 5). Fluvial sands predating the lake yield an OSL age of 34.6 ± 3.7 ka (GLV- 423 11-3), whereas fluvial sands capping the muds yield 21.8 ± 1.5 ka (GLV-11-1; Fig. 12, Table 424 1). These two ages provide maximum and minimum-limiting bounds. 425 426 Three sites constrain the timing of Flood-III: an OSL age of 14.9 ± 2.0 ka from the giant eddy 427 bar at Bur (GLV-11-9; Fig. 12, Table 1), a Bayesian most probable 10Be depth-profile age of +4.6 10 428 17.7 /–3.1 ka on the giant eddy bar at Nerpo (Tables 3, 5, Fig. 12), and two Be exposure 429 ages of 19.5 ± 1.3 and 19.9 ± 1.4 ka from boulders topping the Oron moraine (Margold et al., 430 2016; Table 2 and Fig. 12), which postdate the megafloods. 431 432 Overall, our outburst chronology accords with the regional glacial history (Margold et al., 433 2016): Flood-I occurred during deglaciation following OIS-4; Flood-II, in the build-up to the 434 local Last Glacial Maximum; and Flood-III, during the last deglaciation (Fig. 15 and Table 435 6). The notion that Flood I carved the giant spillway is consistent with the fact that earlier 436 glaciations are thought to have been more extensive than the local Last Glacial Maximum 437 (Zolotarev, 1974, Bazarov et al., 1981; Bazarov, 1986; Enikeev, 2008, 2009; Margold et al., 438 2016). Whereas Flood-I cut through an extensive ice dam (Figs. 12, 16a) and incised the 439 spectacular bedrock spillway, Floods II and III were largely limited to the Vitim valley and 440 may have been responsible for the trimming of the valley slopes immediately downstream of 441 Lake Oron (Figs. 3, 12, 16b). 442 443 Table 6 here 444

445 We propose a coherent chronological model of glacial Lake Vitim megaflooding, while 446 acknowledging that not all ages in our dataset conform perfectly. In particular, two 10Be ages 447 cannot be accommodated in our flood chronology: the large block (7 x 7 x 2 m; Figs. 3, 9) 448 sampled midway down the talus slope in the bedrock spillway yielded an exposure age of 449 24.7 ± 1.7 ka (Table 2), and another large talus block (6 x 4 x 2 m) sampled on a bedrock 450 island in the Oron delta (Fig. 3) yielded an exposure age of 14.3 ± 1.0 ka (Table 2). We note 451 that the talus block in the spillway falls just outside the 1σ bounds with Flood III. Indeed, it is 452 likely that some of the discharge of Flood III were routed through the spillway and might 453 have caused erosion at the foot of its steep slopes. The age of the talus block in the Oron delta 454 corresponds with the dating of a potential Late Glacial ice re-advance in the area (Margold et 455 al., 2016) and, being just a few metres above the delta surface, possibly relates to glacial 456 events further up-valley, such as ice calving into Lake Oron, or flooding at a smaller scale 457 than the glacial Lake Vitim outbursts. 458 459 4.3. Reconstructed peak discharge 460 4.3.1. Peak discharge modelled with HEC-RAS 461 The minimum peak discharge accordant with the highest flood bars at Nerpo and Bur (dated 462 to Flood-III, Fig. 15) is estimated at 4.0–6.5 million m3/s assuming a plausible range of 463 Manning’s n (0.03 to 0.05; Tables 7, 8). This stands as one of the largest freshwater floods 464 ever documented on Earth (O'Connor and Costa, 2004). Our modelling predicts that the 465 Vitim valley was flooded to depths of ~120–150 m (locally up to 200 m), with average flow 466 velocities of up to 21 m s–1 (Table 8). By assuming a simple triangular flood hydrograph 467 (Alho et al., 2010), based on the drainage volume and peak discharge, we estimate that Lake 468 Vitim drained over a period of about 10 to 16 days. Flood-III was up to ~ 34-times larger 469 than the most extreme historical floods on the Lena River associated with ice-jams (O'Connor 470 and Costa, 2004). Owing to the effects of flow storage, the heights of the giant 471 eddy/expansion bars listed in Table 4 inevitably reflect some attenuation of the flood peak 472 downstream of the breached ice-dam. Lower flood bars at Bodaybo and at the nearby town of 473 Mamakan reflect flood discharges of no more than 1 million m3/s and were presumably 474 formed during the falling stages. 475 476 Table 7 here 477 Table 8 here 478 479 4.3.2. Predicting peak flood discharge with empirical equations 480 In addition to modelling the flood propagation with HEC-RAS, we apply three simple 481 empirical equations for estimating peak discharge (Q) based on dimensions of the 482 impoundment and the breach. Relevant parameters are: 483 V, total impoundment volume = 3 x 1012 m3; 484 D, maximum impoundment depth (below the shoreline at 840 m a.s.l.) = 490 m; 485 h, the change in impoundment level at the breach = 414 m (assuming that the bedrock 486 spillway was the sole outflow point), or 490 m (assuming that the main Vitim valley was 487 rapidly cleared of ice during the flood); and 488 b, the breach width. 489

490 Following O’Connor and Beebee (2009), peak discharge from ‘large’ impoundments such as 491 glacial Lake Vitim can be estimated by 492 493 Q ≈ g0.5 h2.5 494 495 where g is gravitational acceleration and h approximates the maximum specific energy 496 at the outflow. The two different h values yield peak discharge estimates of 10.9 and 16.6 497 million m3 s–1, where b/h ~ 1, but for wider breaches the estimates increase. An alternative, 498 regression-derived function from moraine-dammed lakes is proposed by Cenderelli (2000): 499 500 Q ≈ 0.3 (VD)0.49 501 502 which yields peak discharge estimates of 7.5 and 8.1 million m3 s–1 (using D = 414 and 503 490 m, respectively, as described above). A third regression-derived function from large- 504 scale outburst floods (V > 1 km3) is proposed by Herget (2005): 505 506 Q ≈ 6645 V0.98 507 508 and yields a peak discharge estimate of 17 million m3 s–1. While acknowledging that we 509 lack any information about the ice-dam breach, the range of peak discharges returned by the 510 three simple empirical equations, 7.5–17 million m3 s–1, is a similar order to that obtained 511 from the HEC-RAS flood modelling and suggests that the latter may point to a conservative 512 estimate. 513 514 4.4. Pre- and post-flood sedimentary environments of the Muya-Kuanda Depression 515 Sedimentary exposures in the Muya-Kuanda Depression reveal predominantly fluvial facies 516 locally reworked at the surface into aeolian dunefields, now stabilised by vegetation. The 517 Muya River is a sand-bed, meandering river with well-developed point-bars and numerous ~ 518 30 m high, concave-bank exposures formed where the eroding bank abuts fragments of a high 519 alluvial terrace. The 11 m glaciolacustrine mud unit deposited by glacial Lake Vitim is 520 located at one such terrace exposure. Sample MS-12-01 collected from 1.0 m below the top 521 of the terrace section yields an OSL age of 9.4 ± 2.0 ka and 11.6 ± 1.6 ka using Minimum and 522 Central age models (Galbraith et al., 1999), respectively (Figs. 5, 12, Table 1). This indicates 523 that the Muya River has incised ~ 30 m since ~ 11 ka, possibly in response to declining 524 postglacial sediment supply. The Muya River floodplain contains well-developed ridge-and- 525 swale topography, palaeochannel traces, oxbow lakes, and backplain swamps spanning a 526 channel belt 1–2 km wide. Such landforms indicate that channel incision has been 527 accompanied by considerable lateral channel migration over the Holocene. We surmise that 528 active reworking of the valley fill may have removed part of the recent glaciolacustrine 529 record of glacial Lake Vitim and, conversely, older (pre-Flood II) glaciolacustrine units 530 presumably lie buried and unexposed. The vigour of Holocene sedimentary dynamics along 531 the Muya and Vitim rivers may also explain the lack of clear evidence of abrupt lake- 532 draining, such as fluvial sand and gravel dunes observed in glacial lake basins elsewhere 533 (Waitt, 1980; Carling, 1996; Carling et al., 2009; Wiedmer et al., 2010). 534

535 The glaciolacustrine muds exposed in the Muya River section are bracketed by an OSL age 536 of 34.6 ± 3.7 ka (GLV-11-3; Fig. 12, Table 1) on fluvial sands just prior to Vitim-damming, 537 and an OSL age of 21.8 ± 1.5 ka (GLV-11-1; Fig. 12, Table 1) marking the local resumption 538 of fluvial sedimentation some time later. Although we observed no internal breaks in 539 glaciolacustrine sedimentation, we cannot exclude the possibility that the 11 m sequence 540 spans both lake phases that preceded floods II and III (by comparison, an ice-dammed lake in 541 the Chara Depression [~200 km north-east of the Muya-Kuanda Depression, Fig. 2] produced 542 a 17 m stack of varved glaciolacustrine muds over ~ 1200 y [Enikeev, 1986]). This suggests a 543 couple of possible scenarios: 1) the Muya River section hosts an incomplete record of one or 544 more lakes with an unidentified erosional break, or 2) post-flood fluvial sedimentation was 545 displaced elsewhere in the Muya valley for possibly several thousand years, meaning that the 546 OSL date of 21.8 ± 1.5 ka is very much a minimum age of the return of fluvial conditions at 547 the section we sampled. 548 549 5. Discussion 550 5.1. Correlation of the glacial Lake Vitim floods with sediments along the Lena River and in 551 the Lena Delta 552 Although the downstream extent of our field investigations was limited to the Vitim 553 confluence with its tributary Mamakan (Fig. 2), we identify possible traces of the Vitim 554 megafloods in previous descriptions of fluvial landforms and sediments along the Lena River 555 and in the Lena Delta. In the middle reaches of the Lena River just above the Aldan River 556 junction, the Bestyakh Terrace (Fig. 10a) stands 15–73 m above the Lena River and up to 70 557 km wide (Kamaletdinov and Minjuk, 1991). The Lena here becomes considerably less 558 laterally-confined; hence a megaflood would be expected to deposit its sediment load over a 559 wide area. The terrace is composed of four units of which the Mavrian unit of parallel and 560 cross-bedded sands is interpreted by previous workers (Kamaletdinov and Minjuk, 1991; 561 Grinenko et al., 1995) as comprising reworked glacifluvial material from the Baikal-Patom 562 Uplands (i.e., the Vitim River between the ice-dam site and the Lena confluence; Fig. 2). The 563 Mavrian unit has been correlated to the Muorin unit in the Lena Delta (Galabala, 1987), later 564 termed the Arga Complex (Schwamborn et al., 2002; Schirrmeister et al., 2011). 565 566 As the Lena River nears the coast, it passes between the Chekanovsky and Kharaulakh 567 mountain ridges, where the passage of extreme floods would be constricted from > 50 km 568 upstream to 2–10 km, focusing erosive power at the delta apex (Fig. 10b). We anticipate 569 catastrophic erosion, avulsion of primary distributaries, and abrupt delta-lobe switching 570 resulting in a mosaic of cut-and-fill across the delta. West of the active late Holocene-modern 571 accumulation zone lie erosional remnants of two terraces: the Arga Complex and Ice 572 Complex (Fig. 10b; Schwamborn et al., 2002; Schirrmeister et al., 2011). The Arga Complex, 573 correlated with the Bestyakh Terrace described above (Grinenko et al., 1995; Schirrmeister et 574 al., 2003; Fig. 10a), is noted as the thickest, most widespread sedimentary unit in the delta, 575 comprising medium-grained, well-sorted, weakly bedded, and organic-free sands. 576 Schirrmeister et al. (2003) suggest that the massive sandy deposits “accumulated on the 577 bottom and sides of large meltwater streams when large, extended areas of the periglacial 578 plain were flooded in the spring” (see also Schwamborn et al., 2002). The OSL chronology 579 ascribes these sandy deposits to > 100 ka to ~ 50 ka (Schirrmeister et al., 2011). However, we

580 propose that the high-energy deposits observed along the Lena's middle reaches and at the 581 delta are consistent with the expected long-range effects of cataclysmic outburst floods from 582 Lake Vitim. This suggestion finds support in the available chronology for the stratigraphy of 583 the delta: the deposition of the lower Arga Complex (Schwamborn et al., 2002; Schirrmeister 584 et al., 2011) is coeval with Flood-I, and the timing of floods II and III matches erosion gaps 585 in the Ice Complex (32–17 ka and 17–8 ka) at Kurungnakh-Sise Island (Wetterich et al., 586 2008), a site especially vulnerable to megaflood erosion (Fig. 10b). 587 588 5.2. Megaflood traces in Arctic Ocean sediment cores 589 A large meltwater influx to the Arctic Ocean marks the penultimate deglaciation at ~ 57–52 590 ka (Nørgaard-Pedersen et al., 1998; Spielhagen et al., 2004; Stein, 2008). Drainage of glacial 591 lakes from the Eurasian ice sheet is cited as the meltwater source (Fig. 1; Mangerud et al., 592 2004; Spielhagen et al., 2004), although details of volume and timing remain unresolved. The 593 most pronounced meltwater pulse, labelled isotope event 3.31 at 55.5 ka (Martinson et al., 594 1987; Nørgaard-Pedersen et al., 1998), is identified in circum-Arctic cores via peaks in 595 isotopic freshwater indicators and ice-rafted debris (Martinson et al., 1987; Nørgaard- 596 Pedersen et al., 1998). At the continental margin north-west of the Laptev Sea (core PS2741- 597 1; Fig. 1), close to where the Lena River met the former coast at lower sea-level, isotope 598 event 3.31 is shown to be the most prominent peak in the sand fraction throughout the entire 599 core length—significantly more pronounced than Termination II, although comparable after 600 accounting for ice rafted debris (Knies et al., 2000; Stein, 2008). Our Flood I is dated by the 601 incision of the bedrock spillway to 56.0 ± 1.8 ka. The coincident timing and the sedimentary 602 signature lead us to connect Flood-I with event 3.31, although it is likely that deglaciation 603 from OIS-4 released multiple coeval meltwater sources around the Arctic (Mangerud et al., 604 2004; Stein, 2008). Accounting for some bedrock surface weathering on the spillway, the 605 weighted mean surface exposure ages for Flood-I range between 56.5 ± 1.9 and 61.8 ± 2.3 ka 606 (see Section 4.1.2.), which is still within 1σ uncertainty of the timing of isotope event 3.31 607 (55.5 ± 5 ka; Martinson et al., 1987). 608 609 5.3. Possible climate implications of cataclysmic glacial outburst floods 610 Abrupt meltwater influx to the Arctic Ocean can have important implications for climate. 611 Much attention has been directed at freshwater influx disruption to the Atlantic Meridional 612 Overturning Circulation (AMOC) during the last deglaciation (Broecker et al., 1989; Barber 613 et al., 1999; Condron and Winsor, 2012), but the impact of similar events prior to OIS-2 has 614 not drawn a similar level of interest (Oppo and Lehman, 1995), as so little is known of these 615 older episodes. Massive pre-OIS-2 outbursts from Eurasia produced Arctic freshwater pulses 616 that the Transpolar Drift (Fig. 1) could have conveyed to regions critical for AMOC 617 perturbation (Condron and Winsor, 2012). Indeed, modelling experiments indicate that the 618 AMOC is highly sensitive to freshwater export from the Arctic Ocean (Rennermalm et al., 619 2006; Zhang et al., 2014). While the floods documented here for glacial Lake Vitim likely did 620 not have sufficient volume to cause hemispheric-wide impact on climate, influx of water 621 from the lakes dammed by the Eurasian Ice Sheet could possibly have triggered climate 622 feedbacks as with North American meltwater discharges during the last deglaciation 623 (Broecker et al., 1989; Barber et al., 1999). The climate dynamics underlying the OIS-4–3 624 transition (Rasmussen et al., 2014) stands as a prime candidate for further investigation.

625 626 6. Conclusions 627 We reconstruct cataclysmic outburst floods from glacial Lake Vitim in Siberia that followed 628 the Vitim and Lena rivers into the Arctic Ocean. Using a combination of 10Be exposure 629 dating and OSL, we establish a chronology for these floods that distinguishes three flood 630 events over the past 60,000 years. The timing of the floods is consistent with the regional 631 glacial history: Flood I occurred during deglaciation following OIS-4 and its timing at ~ 56 632 ka is coincident with isotope event 3.31 documented in Arctic Ocean sediment cores; Flood- 633 II occurred during the build-up to the local Last Glacial Maximum; and Flood-III during the 634 last deglaciation. The glacial Lake Vitim outbursts are among the largest freshwater floods 635 ever documented. We estimate the peak discharge of Flood-III using the HEC-RAS model at 636 4.0–6.5 million m3s–1, with mean flow depths of 120–150 m and average flow velocities up to 637 21 m s–1. These results offer a new record of outburst floods spanning OIS-3 and 2, and one 638 of the largest megaflood discharges for which timing and geomorphic impacts are reasonably 639 well resolved. 640 641 7. Acknowledgments 642 We thank Anton Brichevskiy for partaking in fieldwork, Larisa Chechetkina and staff of the 643 Vitim Nature Reserve for logistical support, and Charles Mifsud and Steven Kotevski for 644 their assistance with measurement of 10Be samples at ANSTO. This research was funded by 645 the Swedish Society for Anthropology and Geography, the Bolin Centre for Climate 646 Research, the Royal Swedish Academy of Sciences (FOA12Althin- 034), Stockholm 647 University, and Stiftelse YMER-80 to Martin Margold; and by the Australian Institute of 648 Nuclear Science and Engineering (ALNGRA12021P), the Australian Research Council 649 (DP130104023), and a Marie Skłodowska-Curie fellowship to John Jansen. We heartily 650 thank Victor Baker, Paul Carling, Tim Cohen, Jürgen Herget, Oliver Korup, Wolfgang 651 Schwanghart, and Chris Stokes for comments on early drafts of the manuscript. We thank 652 two anonymous reviewers who provided constructive comments on the submitted version of 653 the manuscript. 654 655 8. References 656 Alho, P., Baker, V.R., Smith, L.N., 2010. Paleohydraulic reconstruction of the largest Glacial 657 Lake Missoula draining(s). Quaternary Science Reviews 29, 3067–3078. 658 André, M.-F., 2002. Rates of postglacial rock weathering on glacially scoured outcrops 659 (Abisko-Riksgränsen area, 68° N). Geografiska Annaler A: Physical Geography 84, 660 139–150. 661 Baker, V.R., 2001. Water and the martian landscape. Nature 412, 228–236. 662 Baker, V.R., Benito, G., Rudoy, A.N., 1993. Paleohydrology of late Pleistocene 663 superflooding, Altay mountains, Siberia. Science 259, 348–350. 664 Baker, V.R., Bjornstad, B.N., Gaylord, D.R., Smith, G.A., Meyer, S.E., Alho, P., 665 Breckenridge, R.M., Sweeney, M.R., Zreda, M., 2016. Pleistocene megaflood 666 landscapes of the Channeled Scabland. In: Lewis, R.S., Schmidt, K.L. (Eds.), 667 Exploring the Geology of the Inland Northwest. Geological Society of America 668 Field Guide 41, pp. 1–73.

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888 Reuther, A.U., Herget, J., Ivy-Ochs, S., Borodavko, P., Kubik, P.W., Heine, K., 2006. 889 Constraining the timing of the most recent cataclysmic flood event from ice- 890 dammed lakes in the Russian Altai Mountains, Siberia, using cosmogenic in situ 891 10Be. Geology 34, 913–916. 892 Rytsk, E.Yu., Kovach, V.P., Yarmolyuk, V.V., Kovalenko, V.I., Bogomolov, E.S., Kotov, 893 A.B., 2011. Isotopic structure and evolution of the continental crust in the East 894 Transbaikalian segment of the Central Asian Foldbelt. Geotectonics 45, 349–377. 895 Schirrmeister, L., Grosse, G., Schnelle, M., Fuchs, M., Krbetschek, M., Ulrich, M., Kunitsky, 896 V., Grigoriev, M., Andreev, A., Kienast, F., Meyer, H., Babiy, O., Klimova, I., 897 Bobrov, A., Wetterich, S., Schwamborn, G., 2011. Late Quaternary 898 paleoenvironmental records from the western Lena Delta, Arctic Siberia. 899 Palaeogeography, Palaeoclimatology, Palaeoecology 299, 175–196. 900 Schirrmeister, L., Grosse, G., Schwamborn, G., Andreev, A., Meyer, H., Kunitsky, V.V., 901 Kuznetsova, T.V., Dorozhkina, M., Pavlova, E., Bobrov, A., 2003. Late Quaternary 902 history of the accumulation plain north of the Chekanovsky Ridge (Lena Delta, 903 Russia): a multidisciplinary approach. Polar Geography 27, 277–319. 904 Schwamborn, G., Rachold, V., Grigoriev, M.N., 2002. Late Quaternary sedimentation history 905 of the Lena Delta. Quaternary International 89, 119–134. 906 Spielhagen, R., Baumann, K.H., Erlenkeuser, H., Nowaczyk, N.R., Nørgaard-Pedersen, N., 907 Vogt, C., Weiel, D., 2004. Arctic Ocean deep-sea record of northern Eurasian ice 908 sheet history. Quaternary Science Reviews 23, 1455–1483. 909 Steffen, D., Preusser, F., Schlunegger, F., 2009. OSL quartz age underestimation due to 910 unstable signal components. Quaternary Geochronology 4, 353–362. 911 Stein, R., 2008. Chapter Six Quaternary Variability of Palaeoenvironment and Its 912 Sedimentary Record. In: Stein, R. (Ed.), Arctic Ocean Sediments: Processes, 913 Proxies, and Paleoenvironment. Elsevier, Amsterdam, 287–437. 914 Stone, J.O., 2000. Air pressure and cosmogenic isotope production. Journal of Geophysical 915 Research: Solid Earth 105, 23753–23759. 916 Svendsen, J.I., Alexanderson, H., Astakhov, V.I., Demidov, I., Dowdeswell, J.A., Funder, S., 917 Gataullin, V., Henriksen, M., Hjort, C., Houmark-Nielsen, M., Hubberten, H.W., 918 Ingolfsson, O., Jakobsson, M., Kjaer, K.H., Larsen, E., Lokrantz, H., Lunkka, J.P., 919 Lysa, A., Mangerud, J., Matiouchkov, A., Murray, A., Moller, P., Niessen, F., 920 Nikolskaya, O., Polyak, L., Saarnisto, M., Siegert, C., Siegert, M.J., Spielhagen, 921 R.F., Stein, R., 2004. Late quaternary ice sheet history of northern Eurasia. 922 Quaternary Science Reviews 23, 1229–1271. 923 Ufimtsev, G. F., Skovitina, T. M. and Kulchitskiy, A. A., 1998. Rockfall-dammed lakes in 924 the Baikal region: Evidence from the past and prospects for the future. Natural 925 Hazards 18, 167–183. 926 von Blanckenburg, F., Belshaw, N.S., and O'Nions, R.K., 1996. Separation of 9Be and 927 cosmogenic 10Be from environmental materials and SIMS isotope dilution analysis. 928 Chemical Geology 129, 93–99. 929 Waitt Jr, R.B., 1980. About forty last-glacial Lake Missoula jökulhlaups through southern 930 Washington. The Journal of Geology 88, 653–679. 931 Wetterich, S., Kuzmina, S., Andreev, A.A., Kienast, F., Meyer, H., Schirrmeister, L., 932 Kuznetsova, T., Sierralta, M., 2008. Palaeoenvironmental dynamics inferred from

933 late Quaternary permafrost deposits on Kurungnakh Island, Lena Delta, Northeast 934 Siberia, Russia. Quaternary Science Reviews 27, 1523–1540. 935 Wiedmer, M., Montgomery, D.R., Gillespie, A.R., Greenberg, H., 2010. Late Quaternary 936 megafloods from Glacial Lake Atna, Southcentral Alaska, U.S.A. Quaternary 937 Research 73, 413–424. 938 Zhang, X., Prange, M., Merkel, U., Schulz, M., 2014. Instability of the Atlantic overturning 939 circulation during Marine Isotope Stage 3. Geophysical Research Letters 41, 940 GL060321. 941 Zolotarev, A.G., 1974. The Baikal-Patom Upland, In: Florensov, N.A. (Ed.), The Highlands 942 of Cisbaikalia and Transbaikalia. Nauka, Moscow. (In Russian) 943 944 Figure captions 945 Fig. 1. Regional setting. Glacial Lake Vitim and the flood route to the Arctic Ocean along 946 the Vitim and Lena rivers are drawn in magenta, glacial lakes dammed by the Eurasian Ice 947 Sheet during OIS-4 (Mangerud et al., 2004) are in green. The study area of Transbaikalia and 948 the location of Fig. 2 are marked in black, as well as the Altai and Sayan mountain ranges 949 discussed in the text. 950 951 Fig. 2. Glacial Lake Vitim and the surrounding region. Reconstructed minimum LGM ice 952 extent in the Kodar Mountains is drawn in white shading; a more extensive glaciation 953 indicated by the glacial geomorphology is shown schematically (white dashes; Margold et al., 954 2016). Also shown: ice-dam area (green), Muya River glaciolacustrine section (red dot), 955 dated moraines at Oron and spillway (pink), giant bars dated (purple) and undated (black), 956 relict shorelines (red), and overflow to the River Nercha-Amur (yellow). 957 958 Fig. 3. Glacial Lake Vitim ice dam area. DEM-derived hill-shaded image (SRTM 1 arc-sec 959 data), including glacially modified, interconnected valleys surrounding the spillway. Also 960 shown: location of 10Be exposure dating samples taken from the spillway rim (red dots), 961 spillway talus slope (light green dot), and a talus block on a bedrock island within Oron delta 962 (dark green dot); 10Be-dated moraine boulders (yellow); and flood-trimmed slopes of the 963 Vitim valley (black arrows) downstream of Lake Oron. Note view direction and transect 964 location for Fig. 4a, b. 965 966 Fig. 4. Spillway and Vitim valley transect. (a) View from the south-west rim of the 967 spillway (see Fig. 3 for the direction of view). Note the Vitim River visible in the distance. 968 (b) Vitim valley and the spillway transect measured via SRTM 1-arc-sec data (see Fig. 3 for 969 location). 970 971 972 Fig. 5. Muya-left bank main lacustrine section [56.421°N, 115.536°E]; location shown in 973 Fig. 2. (a) Stratigraphic stack of the 27 m-high river bank section as logged in 2012. The 974 section is topped by a flattish and forested terrace (~505 m a.s.l.), which ramps up to ~ 4-5 m 975 higher along the upstream part of the exposure. Abbreviations in the grain-size key: M – silt, 976 fS – fine sand, mS – medium sand, cS – coarse sand, gG – granule, pG – pebble, and cG – 977 cobble. Locations of OSL samples collected in 2012 are indicated (samples that did not yield

978 ages are in grey). (b) View of the section in 2012 – see person for scale (white ellipse). The 979 unit of glaciolacustrine muds is marked by arrows. Locations of all OSL samples yielding an 980 age is indicated. (c) Laminations of silt and minor fine-sand (light-grey) and clays (dark-grey) 981 in the glaciolacustrine unit, with occasional cross-bedding and no identifiable traces of 982 organic matter. (d) View of the position of OSL sample GLV-11-1 collected in 2011 from 983 fluvial sands above the glaciolacustrine muds – note the sharp contact between the units at 984 the bottom of the pit. (e) View of the position of OSL sample GLV-11-3 collected in 2011 985 from fluvial sands below the glaciolacustrine muds – note the sharp contact between the 986 units. See Fig. 12 for the derived ages. 987 988 Fig. 6. Giant eddy bar at Nerpo [57.521 °N, 115.296 °E]; location shown in Fig. 2. (a) 989 Stratigraphic stack logged in a pit excavated on the crest of the bar (location marked by a 990 black dot in panel b). Samples constituting the 10Be depth profile are drawn in blue (see Fig. 991 12 for the derived age), location of the OSL sample is also indicated. Grain-size attributes are 992 drawn as in Fig. 5. (b) Hillshade image derived from SRTM 1-arc-sec elevation data draped 993 with colour hypsometry. Outline of the bar is marked by arrows. 994 995 Fig. 7. Giant eddy bar at Bur [57.882 °N, 114.351 °E]; location shown in Fig. 2. 996 Stratigraphic stack logged in a sand extraction pit in the giant bar at Bur. Position of OSL 997 sample GLV-11-9 is indicated (see Fig. 12 for the derived age). Grain-size attributes are 998 drawn as in Fig. 5. 999 1000 Fig. 8. Giant eddy-bar exposures at Bodaybo. Proximal portion of an eddy-bar partially 1001 blocking the Bodaybo River tributary to the Vitim River (see Fig. 2), with (a), possibly ice- 1002 rafted block (1 x 2 x 2.5 m) and boulder cluster [57.844 °N, 114.152 °E]. (b) High-energy, 1003 suspension-load deposit (~ 7 m thick) of planar coarse sands (1–3 cm thick) interbedded with 1004 matrix-supported gravels (up to 8 cm diameter) [57.846 °N, 114.149 °E]. (c) Eddy-bar 1005 deposits at ‘Upper Pit’ [57.922 °N, 114.219 °E]: ~ 13 m of rhythmic, planar interlaminated 1006 coarse and fine sands, coarsening-downwards to ~ 30 cm beds of coarse sands and angular, 1007 matrix-supported gravels (up to 60 cm diameter); all overlain by a ~1 m aeolian sand sheet 1008 (white dash boundary). Shown are OSL samples (black circles) and samples (blue dots) 1009 constituting the 10Be depth profile (see Fig. 12 for the derived ages). Stratigraphic stack 1010 (right-panel) indicates grain-size attributes for the section seen in panel c (see Fig. 5 caption 1011 for abbreviations in the grain-size key): (d) Close-up of a 5 m thick unit of mostly open-work 1012 gravels (5–15 cm diameter) indicating rapid deposition from high-energy flow [57.851 °N, 1013 114.159 °E]. 1014 1015 Fig. 9. Spillway talus slope. (a) View looking up to the spillway rim from near the base of 1016 the talus slope, note person for scale. (b) Sampling a large talus block (GLV-9; 7 x 7 x 2 m, 1017 57.3917°N, 116.4917°E) midway down the talus slope for 10Be exposure dating (24.7 ± 1.7 1018 ka). The block is pinned between other large blocks (~3 x 2 m & ~1 x 1 m). See Fig. 3 for 1019 location. 1020 1021 Fig. 10. (a) Vitim-Lena river long profile. Valley profile extending from the Muya-Kuanda 1022 Depression to the Lena Delta (derived from SRTM 1-arc-sec data). (b) Lena Delta

1023 schematic map. At the Arctic coast and just before meeting its delta, the Lena River passes 1024 between the steep Chekanovsky and Kharaulakh ridges. The bedrock constriction focuses 1025 erosive power at the delta apex and drastically lowers the preservation potential of sediments 1026 on the proximal Kurungnakh-Sise Island (K; Wetterich et al., 2008). The two major erosional 1027 terrace remnants on the delta are shown: the Arga Complex, 20–30 m a.s.l. (green) and Ice 1028 Complex, 30–55 m a.s.l. (orange), in addition to the modern active accumulation zone, 1–12 1029 m a.s.l. (grey; Schwamborn et al., 2002; Schirrmeister et al., 2011). This map is redrawn from 1030 Wetterich et al. (2008). 1031 1032 Fig. 11. Plan view of the best-developed giant eddy bar on the Vitim [58.9409°N, 1033 112.8342°E] in a hillshade image derived from SRTM 1-arc-sec elevation data draped with 1034 colour hypsometry. The giant bar is developed on the inside convex-bend of the lower Vitim 1035 (see Fig. 2 for location) and is the largest known eddy bar on the Vitim River: ~ 5 km long, ~ 1036 2 km wide and with a crest standing ~ 120 m above river level. Outline of the bar is marked 1037 by arrows. 1038 1039 Fig. 12. Summary of geochronology for glacial Lake Vitim and its floods. Chronology is 1040 shown ± 1σ, weighted mean ages (wma) are calculated for sites with multiple dates. See 1041 Table 2 for samples GLV-9 and GLV-15, which are not directly relevant for the 1042 reconstructed flood chronology. 1043 1044 Fig. 13. Optically stimulated luminescence (OSL) dating. (a) Example of a natural and an 1045 artificial OSL decay curve of an aliquot of sample GLV-11-1. The almost identical shape 1046 indicates the absence of unstable signal components. (b) The spread of individual Equivalent 1047 Dose values for sample GLV-11-1 shows a narrow distribution around 80 Gy, which is 1048 interpreted to represent the aliquots that were zeroed at the time of deposition. 1049 1050 Fig. 14. Age modelling from cosmogenic 10Be depth profiles at Nerpo and Bodaybo 1051 ‘Upper Pit’. (a) Cosmogenic 10Be depth profiles: red circles are measured 10Be 1052 concentrations (see Table 3); dashed black curve represents the theoretical depth profile with 1053 the lowest chi-squared fit to the data; grey shading comprises multiple curves representing all 1054 depth profiles fitting the 10Be data. Uncertainties are shown at 3σ level for Nerpo and 2σ level 1055 for Bodaybo ‘Upper Pit’ upper and lower units. Note that uncertainties for Nerpo 10Be 1056 concentrations are shown at 3- rather than 2σ level because no solutions were found for this 1057 depth profile for a confidence limit < 3σ. (b) Age versus chi-squared plot for the depth 1058 profiles in (a). Dashed line shows age with minimum chi-squared value. (c) to (e) Smoothed 1059 chi-squared (black curves) and Bayesian probability density functions (red curves) for the 1060 depositional age, erosion rate, and 10Be inheritance of the Nerpo and Bodaybo units based on 1061 the 10Be depth profiles. All calculations performed using age calculator code of Hidy et al. 1062 (2010; version 1.2). See also Table 5 for details on model parameters used. 1063 1064 Fig. 15. Outburst flood magnitude and chronology over the past 60 ka. Flood volumes 1065 from Lake Vitim (blue columns), Lake Missoula (Baker et al., 2016; Gombiner et al., 2016) 1066 and Lake Chuja-Kuray (outlined brown columns; Herget, 2005). Note that only Flood-III was 1067 modelled directly; volumes shown for floods I and II are speculative, but shorelines at ~840

1068 m a.s.l. match an overflow to the adjoining catchment and restrict glacial Lake Vitim’s 1069 maximum volume to ~ 3000 km3 (Margold et al., 2011). Schematic ice extent is shown in 1070 grey (Svendsen et al., 2004; Margold et al., 2016) along with Arctic meltwater events 1071 recorded in core PS2185 (Spielhagen et al., 2004). Chronology is shown via coloured error 1072 bands (±1σ) and limiting ages (red arrows; see the text and Fig. 12, Table 6 for more 1073 information). Age intervals of Vitim floods (blue columns) are defined by the span of 1σ 1074 error bands. 1075 1076 Fig. 16. Hypothesised ice dam configurations. Schematic depiction of two hypothesised 1077 configurations of the ice dam. (a) Vitim valley blocked by an ice cap covering the Kodar 1078 Mountains and burying the surrounding lower ground with ice. In this configuration, the 1079 Vitim valley was filled with ice for several tens of kilometres. Flood I (dashed blue line, 1080 though note we refrain from inferring the exact mechanism of the ice dam collapse), carving 1081 the bedrock spillway, is inferred to have occurred under this ice-dam configuration. The 1082 regional glacial geomorphology indicates that at least one previous glaciation was even more 1083 extensive and most of the landscape visible here was covered by ice (Margold et al., 2016; 1084 see Fig. 2 here). (b) Vitim valley blocked by a single ice lobe emanating from the valley 1085 occupied at present by Lake Oron. In this configuration, the length of the ice dam was only a 1086 few kilometres and a minor retreat of the valley glacier would trigger release of the 1087 impounded glacial Lake Vitim; note spillway, which must have formed under more extensive 1088 ice cover, in background. 1089 1090 Tables 1091 Table 1. Summary table of OSL data. 10 1092 Table 2. Summary of bedrock sample Be results and exposure ages. 1093 Table 3. Summary of 10Be results in depth profile samples. 1094 Table 4. Giant-bar floodstage indicators along the Vitim valley 1095 Table 5. Summary of Monte Carlo model parameters. 1096 Table 6. List of floods & available evidence 1097 Table 7. Predicted minimum peak discharge for Flood-III using different Manning’s n. 1098 Table 8. Predicted flood variables for Flood-III using different Manning’s n. 1099

1100

A Transpolar Drift r c t i c O c e a n

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L r L e ib t S es R W u s s i a Fig. 2 50°N lia a ik a b s l n a k a K a y a i r a S n a T z a k B h s t A l a n t a 500 km 40°N i M o n g o l i a 60°E 80°E 100°E 120°E

Fig. 112°E 114°E 116°E 118°E

Lena ti m 59°N Vi s a n d p l Fig. 11 U m o t a x t e n t P a p e - e c 58°N i c l ‘Upper Pit’ M L G a e - p r Bodaybo (Fig. 8) k i Bur n c a (Fig. 7) a i k t a Nerpo a m

B m a (Fig. 6) M e M h c Vitim Fig. 3 S G l a Spillway c i r a t 57°N ( n e d o t a t L G M a d O r a w n ) ro t tim n d n Vi t e e x a o e r i c a K M h g e L G C c i a t e d a R a n l a t L G M a G ( n o t d r a u y w n ) M h Fig. 5 r t o ya u a N M M u y a - K u a n d 56°N

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Fig. (a)

Vitim

(b) m a.s.l. XY 900 800 700 600 Spillway 500 Vitim R. 400

0 1 2 3 4 5 6 7 km

Fig. 4 (a) fS cS pG Depth (m) M mS gG cG 0 Brown to orange to light-brown soil, massive silty fine sand; diffuse transition to basal lens (1cm) of medium sand to coarse sand.

1 MS-12-01 Interlaminated fine & medium sand with ripples (1-3 cm); some crossbed foresets (0.5-3 cm) dipping 30° to channel.

Rhythmic planar beds (dipping 30° upstream) of fine sand (<0.5 cm) capping medium sand (1-3 cm); contains a diagonal fault (20 cm offset).

2 Rhythmic planar beds (1-3 cm) of silty fine sand and capping medium sand (with small-scale erosional contacts).

Rhythmic planar & wavy beds (0.5-2 cm) of medium sand interlaminated with mixed silty fine & medium sand; occasional thicker fine sand 3 beds; rare small-scale troughs containing coarse sand to very coarse sand; cryoturbation convolutions at base.

4 Strongly cryoturbated beds of very coarse sand & granules with ‘cryoturbation intrusions’ of medium & silty fine sand. FLUVIAL FLUVIAL

5 Rhythmic planar beds (1-5 cm) & frequently trough-crossbeds of silty medium sand (0.5-1 cm) capping medium sand to coarse sand to granules.

Channel-fill containing subrounded gravel (up to 5 cm) & ?colluvial subangular clasts (up to 12 cm); matrix & interbeds (1-5 cm) of crossbedded silty medium sand capping coarse sand & granules. 6

Two rhythmic beds of medium sand capping coarse sand & granules. 7 Gravel (1-2 cm, max 8 cm) in matrix of coarse sand & granules. Massive bed of coarse & medium sand. Brown massive clayey silt; pedogenically altered Mainly horizontal, finely laminated clayey silt (varve-like dark/light grey banding), with some wavy beds & small-scale crossbedding & flame 8 MS-12-02 structures (up to 4 cm high); lamination thickness varies from 1-2 mm up to 50 mm. Displayed in panel (c).

9 MS-12-03 (b) (c)

MS-12-04 11 MS-12-01

12 GLV-11-1 Disturbed/slumped massive clayey silt

LACUSTRINE with no observable lamination & some blocks with vertical laminations.

13 GLV-11-3

MS-12-06 MS-12-05 Planar bed of coarse & medium sand 15 (rare granules) fining-up to medium sand SLUMPED capped by a muddy drape (3-5 cm). MATERIAL (d) (e) 16 Dark-grey clayey silt (organic-rich) with a small medium sand lens (2 cm).

17 Matrix-supported gravel (mainly <1 cm, max 30 cm) among massive coarse to medium sand. GLV-11-1 18 Planar beds of coarse sand capped with organic-rich fine sand; occasional angular ?colluvial gravel (max 10 cm).

FLUVIAL FLUVIAL 19 SLUMPED MATERIAL GLV-11-3 24 Coarse gravel bar-platform (up to boulder-size) sloping down to water level. 25

Fig. (a) fS cS pG Depth (m) M mS gG cG 0 Light-brown grading down to dark-orange soil, poorly- sorted medium sand & muds.

Rhythmic beds containing well-sorted medium sand to 0.5 well- sorted coarse sand (1-2 cm); lightly cryoturbated (up to 10 cm of displacement).

1.0 NT-12-01 Rhythmic beds containing medium to coarse sand, strongly cryoturbated with irregular blobs of muddier matrix & 1.5 cm frost vein at 135 cm. 1.5

Uniform, well-sorted medium sand with slightly muddy 2.0 matrix; permafrost boundary at 230 cm.

2.5

(b) 57°32' N

Elevation 1115 m asl

590 57°30' N 470 365 310 265 Vitim 1km 115°16' E 115°20' E

Fig. fS cS pG Depth (m) M mS gG cG 0 Uniform, rhythmic beds containing coarse to very coarse sand to granule beds (1-3 cm). 0.5

1.0 GLV-11-9

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Rounded gravels up to cobble-size. Grey fine sandy mud. 5.0 Weathered bedrock

Fig. fS cS pG (a) (c)

metres M mS gG cG BUP-12-01 0 GLV-11-5

4 (b)

(d) 10

GLV-11-7 12

Fig. (a)

(b)

Fig. Shoreline (a) (b) at 840 m asl

Arga Island

100 km

450

400 Lake Oron / ice dam

350 a Elevation (m a.s.l.)

300 n e L 250 Vitim-Lena confluence 200

150

Bestyakh terrace 100

50 Lena delta

0500 1000 1500 2000 2500 3000 Distance (km) Vitim Lena

Fig. 112°40'E 112°50'E 113°E

59°N

Elevation 1080 m asl 325 58°55'N 245 210 195 180 2km

Fig. BUP-12-01 19.6 ± 2.5 ka (OSL) Aeolian unit GLV-11-5 33.3 ± 4.2 ka (OSL) Flood deposits Bodaybo Upper Pit giant bar GLV-11-7 33.7 ± 2.9 ka (OSL; GLV-11-5 and 7 wma), 34.1 ± 4.0 ka (OSL) 34.9 (+6.2, –4.9) ka (Be-10 depth profile)

V itim Bur giant bar GLV-11-9 14.9 ± 2.0 ka (OSL) Nerpo giant bar Spillway 17.7 (+4.6, –3.1) ka 56.0 ± 1.8 ka (Be-10 wma) (Be(Be-10 10 depthpth profile) GLV-1 59.1 ± 3.9 GLV-3 54.1 ± 3.6 GLV-5 55.9 ± 3.7 GLV-7 55.2 ± 3.6

ve . ensi Ice dam ext s MS-12-01 um minim 9.4 ± 2.0 ka (OSL, MAM) Ice dam t 11.6 ± 1.6 ka (OSL, CAM) Soil Oron moraine Fluvial sands Oron moraine M 19.7 ± 1.0 ka (Be-10) GLV-11-1 19.7 ± 1.0 ka (Be-10 wma) 21.8 ± 1.5 ka (OSL) GLV-11 19.5 ± 1.3 GLV-13 19.9 ± 1.4 r Glaciolacustrine a muds GLV-11-3 d 34.6 ± 3.7 ka (OSL) Fluvial sands o K

Glaciolacustrine sediments Mu m ya 34.6 ± 3.7 to 21.8t i ± 1.5 ka (OSL) V i V i e t k i a L m G l a c i a l 50 km

Fig. (a)

600

Natural ca. 80 Gy irraditation 500

400

300

OSl signal (a.u.) signal OSl 200

100

0 0102030405060 Illumination time (s)

(b)

35 0.5

30 0.4

25 Relative frequency Relative

0.3 20

Aliquot 15 0.2

0.1 5

0 0.0 0 50 100 150 200 250 Equivalent Dose (Gy)

Fig. 13 Nerpo

(a) (b) (d)

Bodaybo ‘Upper Pit’ upper unit

(a) (b) (d)

Bodaybo ‘Upper Pit’ lower unit

(a) (b) (d)

Fig. 14 ARCTIC MELTWATER EVENTS ) 3 Flood III Flood II Flood I

3000 Ice cap

Missoula All Spillway Bodaybo 2000 Muya R. Bodaybo Ice extent (schematic) Bodaybo Valley glaciers Oron

Glacial lake volume (km Oron Nerpo Bur 1000 Chuja-Kuray —isotope event 3.31— 10Be exposure 10Be depth pr. OSL No ice

10 20 30 40 50 60 Age (ka)

Fig. 15 (a)

y a lw il p S

e k a l

l i a a c G l

(b)

V i t i m

y a lw il p S

e k a l

l i a a c G l

Fig. Table 1. Summary table of OSL data with burial depth and elevation as used for calculation of cosmic dose rate, assumed average sediment moisture during burial (mod. = modelled, see text), the concentration of dose rate relevant elements (K, Th, U), total dose rate (D), number of accepted aliquots (n), the overdispersion value (od.), mean Equivalent dose (De) and the resulting OSL age. For six of the samples no OSL age was calculated because of [1] all aliquots being contaminated by feldspar signals, [2] only three aliquots passing rejection criteria, [3] complex distribution of Equivalent dose (Fig. 13) with ambiguous application of the Minimum Age Model, or [4] low OSL signal level and potential contamination by medium component.

Depth Elevation W K Th U D De Age Sample (cm) (m) (%) (%) (ppm) (ppm) (Gy ka-1) n od. (Gy) (ka) Muya River sedimentary section (56.421 °N / 115.536 °E)

GLV-11-1 700 495 mod. 2.89 ± 0.09 6.80 ± 0.30 1.62 ± 0.20 3.26 ± 0.21 31 0.10 71.1 ± 1.6 21.8 ± 1.5

GLV-11-3 1800 484 mod. 2.94 ± 0.15 5.82 ± 0.24 1.24 ± 0.18 3.04 ± 0.28 28 0.25 105.3 ± 5.7 34.6 ± 3.7

MS-12-01 100 501 8 ± 4 2.68 ± 0.15 5.42 ± 0.24 1.25 ± 0.20 3.17 ± 0.26 15 0.42 36.79 ± 4.07 11.6 ± 1.6

MS-12-01 100 501 8 ± 4 2.68 ± 0.15 5.42 ± 0.24 1.25 ± 0.20 3.17 ± 0.26 15 MAM 29.82 ± 5.89 9.4 ± 2.0

MS-12-02 810 494 15 ± 5 3.24 ± 0.16 11.10 ± 0.50 2.51 ± 0.21 - - - - [1]

MS-12-03 940 493 15 ± 5 3.07 ± 0.16 8.50 ± 0.40 2.22 ± 0.18 - 3 0.16 47.7 ± 4.8 [2]

MS-12-04 1070 491 25 ± 5 3.01 ± 0.10 7.00 ± 0.30 1.76 ± 0.22 - 34 0.59 49.1 ± 5.2 [3]

MS-12-05 1490 487 15 ± 5 2.74 ± 0.14 5.90 ± 0.30 1.64 ± 0.23 - - - - [1]

MS-12-06 1450 487 15 ± 5 3.03 ± 0.15 7.10 ± 0.30 1.86 ± 0.15 - 3 0.21 30.9 ± 3.9 [2]

Nerpo (57.521 °N / 115.296 °E)

NT-12-01 470 470 20 ± 10 2.52 ± 0.13 6.30 ± 0.30 1.76 ± 0.18 - 12 - - [4]

Bur (57.881 °N / 114.351 °E)

GLV-11-9 120 367 20 ± 10 2.60 ± 0.13 6.60 ± 0.30 1.33 ± 0.16 2.87 ± 0.34 18 0.24 42.8 ± 2.8 14.9 ± 2.0

Bodaybo ‘Upper Pit’ (57.922 °N / 114.219 °E;)

GLV-11-5 100 341 20 ± 10 2.37 ± 0.12 6.90 ± 0.30 1.71 ± 0.19 2.80 ± 0.32 21 0.20 93.3 ± 4.7 33.3 ± 4.2

GLV-11-7 1400 328 20 ± 10 2.50 ± 0.08 10.40 ± 0.40 2.14 ± 0.21 3.05 ± 0.34 24 0.12 103.9 ± 4.0 34.1 ± 4.0

BUP-12-01 25 342 20 ± 10 2.34 ± 0.12 7.60 ± 0.30 1.86 ± 0.18 2.81 ± 0.32 16 0.19 55.1 ± 3.1 19.6 ± 2.5 Table 2. Summary of bedrock sample 10Be results and exposure ages.

b Sample Topographic Quartz 10 9 c 10 c c,d Latitude / Longitude Elevation a Be spike Be/ Be Be Concentration Exposure age Sample Code thickness shielding Mass -15 3 -1 (°N/°E) (m a.s.l.) (Pg) (x10 ) (x10 atoms g SiO2) (ka) (cm) factor (g) Flood I spillway GLV-1 57.39436 / 116.49013 478 2 0.99922 37.49 273.7 772.4 ± 25.3 376.86 ± 12.82 59.1 ± 3.9 GLV-3 57.40051 / 116.49645 578 2.5 0.99927 37.54 268.1 792.1 ± 26.4 377.98 ± 13.08 54.1 ± 3.6 GLV-5 57.39310 / 116.49051 487 2 0.99917 37.30 272.1 738.3 ± 24.4 359.86 ± 12.35 55.9 ± 3.7 GLV-7 57.39114 / 116.49090 480 3 0.99904 37.67 273.6 721.1 ± 24.0 349.98 ± 12.11 55.2 ± 3.6

Spillway talus block GLV-9 57.39172 / 116.49166 452 2 0.97792 37.29 273.7 309.1 ± 11.4 151.60 ± 5.79 24.7 ± 1.7

Oron delta talus block GLV-15 57.2218 / 116.40675 363 3 0.98063 36.22 274.3 158.3 ± 6.4 80.11 ± 3.32 14.3 ± 1.0

Oron delta moraine GLV-11 57.15510 / 116.33270 436 2 0.998 37.62 274.0 248.0 ± 9.9 120.62 ± 4.93 19.5 ± 1.3 GLV-13 57.15567 / 116.33258 427 3 0.998 36.03 273.4 238.0 ± 9.3 120.79 ± 4.83 19.9 ± 1.4 a. The tops of all samples were exposed at the surface. b. A density of 2.7 g cm-3 was used based on the granitic composition of the samples. c. All uncertainties are reported at the 1-sigma level. Blank corrected 10/9 ratios. See text for details on level of blanks. d. Exposure ages were calculated with the online calculator formerly known as CRONUS (Balco et al., 2008), version 2.2; constants 2.2.1; (http://hess.ess.washington.edu/), using 10Be SLHL production rate of Heyman (2014). See text for full details of the cosmogenic 10Be analyses and exposure age calculations. Table 3. Summary of 10Be results in depth profile samples.

Sample Depth below Quartz Be spike 10Be/ 9Bea 10Be Concentrationa Sample Code thickness -15 3 -1 surface (cm) Mass (g) (Pg) (x10 ) (x10 atoms g SiO2) (cm) Nerpo (57.521 °N / 115.296 °E; 471 m a.s.l.) NT-345 25 4 15.78 327.3 133.3 ± 4.5 184.80 ± 7.49 NT-470 50 4 17.31 327.6 121.5 ± 4.1 153.76 ± 6.24 NT-595 75 4 28.09 331.8 149.4 ± 3.5 117.90 ± 3.80 NT-6-120 100 4 20.70 327.3 108.7 ± 3.4 114.90 ± 4.37 NT-8-212 192 4 17.32 326.8 85.6 ± 2.9 107.95 ± 4.38 NT-1-255 235 4 7.93 328.5 35.8 ± 2.2 99.00 ± 6.42

Bodaybo ‘Upper Pit’ upper section (57.922 °N / 114.219 °E; 351 m a.s.l.) BUP-01 7.5 5 61.57 332.0 533.1 ± 8.9 192.30 ± 5.37 BUP-03 25 4 17.18 326.5 155.1 ± 6.9 196.98 ± 9.82 BUP-04 45 4 19.30 325.5 152.1 ± 4.9 171.45 ± 6.74 BUP-05 65 4 22.12 324.9 166.0 ± 4.5 162.91 ± 5.75 BUP-06 85 4 30.17 331.0 203.1 ± 5.0 148.90 ± 4.95

Bodaybo ‘Upper Pit’ lower section (57.922 °N / 114.219 °E; 351 m a.s.l.) BUP-07 105 4 24.23 327.8 218.8 ± 7.0 198.00 ± 7.74 BUP-08 145 4 32.85 326.4 196.0 ± 5.0 130.16 ± 4.41 BUP-09 185 4 21.71 326.7 106.2 ± 3.7 106.75 ± 4.39 BUP-11 265 4 38.78 327.5 132.2 ± 3.3 74.62 ± 2.52 BUP-13 445 4 26.39 327.2 76.0 ± 3.3 62.97 ± 3.05 BUP-15 845 4 29.10 327.1 85.0 ± 3.5 63.89 ± 3.00

Bodaybo ‘Upper Pit’ lower section corrected for deposition age of Bodaybo upper section BUP-07 105 4 173.33 ± 10.45 BUP-08 145 4 114.57 ± 6.25 BUP-09 185 4 96.71 ± 5.24 BUP-11 265 4 70.09 ± 2.83 BUP-13 445 4 61.56 ± 3.08 BUP-15 845 4 63.24 ± 3.01

a. All uncertainties are reported at the 1-sigma level. Blank corrected 10/9 ratios. See text for details on level of blanks. Table 4. Giant-bar floodstage indicators along the Vitim valley (upstream to downstream)a.

Distance Elevation Bar-crest downstream of Latitude Longitude above Bar type & position elevation Lake Oron (dd) (dd) river level (m asl) (ice-dam area) (m) (km) Expansion bar, upstream of constrictionb 57.7024 116.2698 436 129 70 Eddy bar at Nerpo, inside-bend 57.5210 115.2958 471 204 153 Eddy bar complex at Bur (crest), inside-bend 57.8832 114.4073 466 233 232 Eddy-bar complex at Bodaybo R tributary-mouth 57.8473 114.1629 328 99 259 Eddy bar at Mamakan, inside-bend 57.8140 113.9811 307 80 270 Eddy bar, inside-bendb 57.9091 113.5863 371 152 302 Expansion bar, downstream of constrictionb,c 58.9409 112.8342 302 120 451 Eddy bar, tributary-mouthb 59.2696 112.8281 303 129 505 a Geographical coordinates & elevations taken from Google Earth in decimal degrees and metres above sea level, respectively. b Sites not visited in the field —interpreted from DEM and remote imagery. c The largest known flood bar on the Vitim: ~5 x 2 km (see Fig. 11). Table 5. Summary of Monte Carlo model parameters.

Site specific information Nerpo Bodaybo Bodaybo Upper Lower Latitude (decimal deg.) 57.521 57.922 57.922 Longitude (decimal deg.) 115.296 114.219 114.219 Altitude (m a.s.l.) 471 351 351

Shielding Topographic shielding 0.999 0.999 0.999 Cover 1 1 1

10Be production rate (atoms/g/yr) Spallation mean 6.39 5.72 5.72 Spallation standard dev. 0.35 0.32 0.32

Bulk material density (g/cm3) Mean 2.0 2.0 2.0 Standard deviation 0.2 0.2 0.2

Monte Carlo parameters Sigma confidence 3* 2 2 Number of profiles 105 105 105 Min age (a) 5000 0 5000 Max age (a) 35000 40000 35000 Min erosion rate (cm/ka) 0 0 0 Max erosion rate (cm/ka) 5 10 5 Min total erosion threshold (cm) 0 0 0 Max total erosion threshold (cm) 50 50 50 Min inheritance (atoms g-1) 85000 50000 50000 Max inheritance (atoms g-1) 105000 175000 70000 Neutrons (min value) 150 150 150 Neutrons (max value) 160 160 160 * No solutions were found for 2-sigma confidence limit. Table 6. List of floods & available evidence

Relevant Flood Evidence Dating figures 1 Spillway 56.0 ± 1.8 ka (10Be, this study) 2, 3, 4 Bestyakh Terracea,b 10a Arga Sandsc,d >100 ka to ~50 kad 10b Sand peak in core PS2741-1e Isotope-event 3.31f,g 55.5 kag 15 2 Glacio-lacustrine sediments Muya River 34.6 ± 3.7 to 21.8 ± 1.5 ka (OSL, this study) 2, 5, 12 33.7 ± 2.9 ka (OSL), 34.9 (+6.2, –4.9) ka (10Be, this Bodaybo 'Upper Pit' eddy bar 2, 8, 12 study) Bestyakh Terrace a,b 10a Erosion gap in Ice Complex (Lena Delta)h 32–17 kah 10b 3 Oron moraine (must postdate Flood 3) 19.7 ± 1.0 ka (Be-10)i 2, 3, 12 Eddy bar Nerpo 17.7 (+4.6, –3.1) ka (Be-10, this study) 2, 6, 12 Eddy bar Bur 14.9 ± 2.0 ka (OSL, this study) 2, 7, 12 Undated giant bars 2, 11 Bestyakh Terrace a,b 10a Erosion gap in Ice Complex (Lena Delta)h 17–8 ka h 10b a – Kamaletdinov and Minjuk (1991) b – Galabala (1987) c – Schwamborn et al. (2002) d – Schirrmeister et al. (2011) e – Knies et al. (2000) f – Nørgaard-Pedersen et al. (1998) g – Martinson et al. (1987) h – Wetterich et al. (2008) i – Margold et al. (2016) Table 7. Predicted minimum peak discharge for Flood-III using different Manning’s n.

Minimum peak dischargea Bar type & position x106 m3 s–1

n=0.03 n=0.04 n=0.05 Eddy bar at Nerpo 5.5 4.5 4.0 Eddy bar complex at Bur (crest) 6.5 5.0 4.5 a Discharge (rounded to nearest 0.5 million m3 s–1) at which giant eddy bars are submerged. Table 8. Predicted flood variables for Flood-III using different Manning’s n.

Flood variable n=0.03 n=0.04 n=0.05

Average flow velocitya (m s–1) 11–17 (21) 12–17 (21) 8–12 (14) Average Froude numbera 0.29–0.45 (0.54) 0.33–0.50 (0.61) 0.21–0.30 (0.36) Average shear stressa (N m–2) 212–485 (717) 165–373 (566) 294–615 (916) Average hydraulic deptha (m) 135–169 (193) 110–139 (159) 138–167 (189) Average top flow widtha (m) 4992–2102 2046–4286 2036–5079 a Averaged for total cross-sections given as 1-sigma range, with maximum value in parentheses.