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10 1 Cosmogenic Be exposure age dating across Early to Late Weichselian
2 ice-marginal zones in northwestern Russia
3 HENRIETTE LINGE, EILIV LARSEN, KURT H. KJÆR, IGOR DEMIDOV, EDWARD J. BROOK, GRANT M. RAISBECK 4 AND FRANCOISE YIOU
5 Linge, H., Larsen, E., Kjær, K. H., Demidov, I., Brook, E. J., Raisbeck, G. M. & Yiou, F. 2006 (August): 6 BOREAS Cosmogenic 10Be exposure age dating across Early to Late Weichselian ice-marginal zones in northwestern Russia. 7 Boreas, Vol. 35, 00 00. Oslo. ISSN 0300-9483. 8 Rock samples from the Kanin Peninsula and the Timan Ridge were analysed for in situ cosmogenic 10Be for 9 exposure age dating purposes. Crystalline rocks were sampled at four sites on the Kanin Peninsula, either from 10 bedrock outcrops or from glacial erratics, giving overall similar 10Be ages. Outcropping sandstone and crystalline 11 erratics were available from three sites at the Timan Ridge. The highly weathered sandstone gives substantially 12 younger 10Be ages than the adjacent erratics. The exposure ages from the Kanin Peninsula suggest that the last 13 deglaciation of this area took place between 55 and 37 10Be kyr ago, in agreement with a preceding Kara Sea 14 glaciation (55 45 kyr BP). The northwest coast of the peninsula was probably just outside the maximum limit of 15 the last Scandinavian glaciation (20 17 kyr BP). Glacial erratic exposure ages from the Timan Ridge suggest that 16 the 55 45 kyr BP Kara Sea glaciation reached the northern part of the ridge. The exposure dates do not show 17 conclusive evidence regarding the existence of a Timan Ridge ice cap. 18 Henriette Linge (e-mail: [email protected]), Bjerknes Centre for Climate Research, Alle´gaten 55, and 19 Department of Earth Science, University of Bergen, Alle´gaten 41, NO-5007 Bergen, Norway; Eiliv Larsen (e-mail: 20 [email protected]), Geological Survey of Norway, Leiv Erikssons v. 39, NO-7491 Trondheim, Norway; Kurt H. 21 Kjær (e-mail: [email protected]), Natural History Museum of Denmark, Geological Museum, Øster Voldgade 5 7, 22 DK-1350 Copenhagen K, Denmark; Igor Demidov (e-mail: [email protected]), Russian Academy of Sciences, 23 Karelian Research Centre, Institute of Geology, 11 Pushkinskaya Street, Petrozavodsk, 185610, Russia; Edward J. 24 Brook (e-mail: [email protected]), Department of Geosciences, Oregon State University, Wilkinson Hall, 25 Corvallis, OR 97331, USA; Grant Raisbeck (e-mail: [email protected]), Franc¸oise Yiou, Centre de 26 Spectrome´trie Nucle´aire et de Spectrome´trie de Masse, IN2P3-CNRS, Baˆtiment 108, F-91405 Orsay, France; 27 received 23rd June 2005, accepted 24th March 2006.
28 Elucidating glacial chronologies is often difficult, 2002; Kjær et al. 2003; Jensen et al. 2006), and absolute 29 especially beyond the range of radiocarbon dating. In ages are given by OSL dates (Larsen et al. 2006a; 30 northwest Russia, a major effort has been made on the Thomas et al. 2006). The lateral extent of the various 31 use of optically stimulated luminescence (OSL) dating glacial advances is obtained by mapping of landforms 32 (e.g. Svendsen et al. 2004; Larsen et al. 2006a). Four and spatial distribution of till sheets (Astakhov et al. 33 consecutive major glacial advances of Weichselian age 1999; Larsen et al. 1999, 2006a; Demidov et al. 2004; 34 have been recorded in the White Sea area of northwest Svendsen et al. 2004). 35 Russia (Larsen et al. 2006a). These were related to the There are, however, uncertainties both in terms of 36 Kara Sea (100 90 kyr BP, 55 45 kyr BP), the Barents ice-sheet spatial distribution and chronology, and there 37 Sea (70 65 kyr BP) and the Scandinavian (20 17 kyr are large area overlaps of reconstructed ice-disperse 38 BP) ice sheets. Additionally, the area was glaciated by a stages (Larsen et al. 2006a). Furthermore, there are 39 local ice cap (75 70 kyr BP) across the Timan Ridge some disagreements between these reconstructions 40 (Houmark-Nielsen et al. 2001; Kjær et al. 2003). (Larsen et al. 2006a) and those from the adjoining 41 Differentiation between individual ice sheets is based area to the east (Svendsen et al. 2004), mainly 42 on various ice-flow indicators (Larsen et al. 1999, concerning the existence, timing and extent of Early 43 2006a; Kjær et al. 2001, 2003). Between till beds there is Weichselian ice-dammed lakes and of the Timan Ridge 44 a variety of sediments representing ice-free periods ice cap. In the Pechora Lowland, east of the Timan 45 (Houmark-Nielsen et al. 2001; Lysa˚ et al. 2001; Kjær Ridge, the ice-dammed Lake Komi existed between 46 et al. 2003; Jensen et al. 2006; Larsen et al. 2006a, b), 100 and 80 kyr ago, and its shorelines have been 47 and a composite Weichselian stratigraphy is established mapped westwards through a lower point on the Timan 48 (Houmark-Nielsen et al. 2001; Kjær et al. 2003; Ridge (Astakhov et al. 1999). As such an extensive 49 Demidov et al. 2004; Larsen et al. 2006a). The relative shoreline setting is not found west of the Timan Ridge 50 chronology in this composite stratigraphy is based on (Houmark-Nielsen et al. 2001; Lysa˚ et al. 2001; Kjær 51 ice-flow indicators, Eemian and Middle Weichselian et al. 2003; Larsen et al. 2006a), it has been suggested 52 marine markerUNCORRECTED beds (Devyatova 1982; Funder et al. that the shorelines andPROOF sediments were destroyed by a
DOI 10.1080/03009480600781909 # 2006 Taylor & Francis C:/3B2WIN/temp files/SBOR41_S100.3d[x] Tuesday, 23rd May 2006 15:22:8
2 Henriette Linge et al. BOREAS 00 (2006)
53 later ice advance (Mangerud et al. 2004). The direction commonly are 30 40 m thick or more. The bedrock 54 of this ice advance, however, is also debated. It is on the northern part of the Timan Ridge consists of 55 interpreted as originating on the Timan Ridge by sandstone and conglomerate, frequently exposed as 56 Houmark-Nielsen et al. (2001), Kjær et al. (2003), tors and cliffs along stream valleys (Fig. 2). Tors, 57 Demidov et al. (2004) and Larsen et al. (2006a), karst-like features and deflation surfaces are common, 58 whereas others believe it came from the Barents Sea and the tundra vegetation consists mainly of lichen, 59 as neither stratigraphic nor geomorphological traces of mosses and low shrubs. Erratic boulders of gneiss and 60 an ice cap have been found along the eastern flank of granite are found at all three sites on the northern 61 the Timan Ridge and on the Pechora Lowland Timan Ridge. Outcropping crystalline rock is found 62 (Astakhov et al. 1999; Mangerud et al. 1999; Svendsen at the Kola Peninsula, the Kanin Peninsula, Novaya 63 et al. 2004). Zemlya and the southern part of the Timan Ridge 64 With many uncertainties still being associated with (Fig. 1). 65 the ice-sheet reconstructions as above, a potential tool 66 for constraining the glacial chronology is surface 67 exposure dating using in situ cosmogenic nuclides. Material and methods 68 Glacial erosion of bedrock and entrained boulders 69 can remove previously accumulated cosmogenic nu- Rock samples were collected with hammer and chisel 70 clides; subsequent deglaciation allows accumulation of from bedrock outcrops and glacial erratics during 71 cosmogenic nuclides to recommence if the rock surface fieldwork at the Kanin Peninsula in 2000 and 2002 72 is subaerially exposed. The concentration of in situ (sites 0005, 0006, 0008, 0209), and at the Timan Ridge 73 produced cosmogenic nuclides therefore provides a in 2002 (sites 0206, 0207, 0208) (Fig. 1). The geometry 74 means of direct dating of glacially moulded surfaces 75 (e.g. Gosse & Phillips 2001). This is particularly of the surrounding topography and sampled rock 76 important for settings devoid of organic material surface was measured with clinometer and compass, 77 suitable for radiocarbon dating in environments where and positions were recorded by GPS. The collected 78 initial organic production significantly post-dates de- samples and their sites are described in Table 1. Figure 79 glaciation, or where the deposits are older than the 2 shows typical geomorphological features and settings 80 range of the radiocarbon method. The sampling from the Timan Ridge. 81 programme for this study was designed to capture the Samples were crushed and pulverized, and the 250 82 regional glacial history; implemented on the Timan 710 mm fraction was used for further processing 83 Ridge and on the Kanin Peninsula, exposure ages are following previously reported methods (Linge et al. 10 9 84 expected to decrease from south to north. Although it 2006) (see Table 1 for further details). Be/ Be ratios in 85 turned out that in some areas suitable material was not the prepared BeO targets were measured at the AMS 86 available, the overall aim is to independently test the Tandetron Accelerator Facility at Gif-sur-Yvette (Rais- 87 regional palaeogeographical reconstruction of Larsen beck et al. 1987, 1994), relative to the NIST standard 10 9 88 et al. (2006a). (SRM 4325) with a certified Be/ Be ratio of 2.68 / 10 11. 10Be concentrations (Table 2) were calculated from the known sample, 9Be carrier weights and the 89 Setting measured 10Be/9Be ratio. This was then corrected for procedural blanks, sample thickness and topographic 90 Extending from the Timan Ridge in the east to the shielding. The magnitude of the various corrections is 91 Varanger Peninsula in the west, the Timan Varanger small, and the total corrections (0.2 3.0%) are within 92 Belt comprises predominantly terrigenous Neoproter- the error of the combined counting statistics and long- 93 ozoic rock successions (Fig. 1), deformed and meta- term instrumental uncertainty. The results are reported 94 morphosed at comparatively low grade during the without attempting to correct for seasonal snow cover 95 Baikalian orogeny. The Timan Ridge is an important or long-term weathering loss. 96 regional element in the circum-Barentsian geological The exposure ages (Table 2) were calculated by 97 structure; it is a major anticline reaching an elevation adapting the sea-level, high-latitude (SLHL /608) 98 of about 300 m a.s.l., and extends northwest south- 10 99 east for 800 km (Olovyanisknikov 2000). The Timan Be production-rate estimate (5.1 atoms/g yr) and 100 Ridge turns to the west northwest across the northern scaling proportions (97.8% spallogenic, 2.2% muon 101 part of the Kanin Peninsula, where the maximum capture) from Stone (2000). The calculated site-specific 102 elevation is 241 m a.s.l. Our study area extends from production rates were reduced by a factor of 0.875 to 103 the northern Timan Ridge in the southeast to the account for the measured 14% difference (Middleton 10 9 104 northern part of the Kanin Peninsula in the northwest et al. 1993) between the NIST certified Be/ Be ratio 10 105 (Fig. 1). At the Kanin Peninsula, bedrock is frequently for the Be standard (SRM 4325) and the ratio 106 exposed along the heights, but outcrops are rare in obtained by Middleton et al. (1993) (see Table 2 for 107 theUNCORRECTED lowlands where Quaternary sediment successions further details).PROOF C:/3B2WIN/temp files/SBOR41_S100.3d[x] Tuesday, 23rd May 2006 15:22:8
BOREAS 00 (2006) Exposure age dating, NW Russia 3
Fig. 1. The location of the study area (grey box) in relation to Arkhangelsk is shown on the inset map with coordinates. Simplified distribution of the major bedrock lithologies in the central part of the Arkhangelsk region adopted from Kjær et al. (2001). Numbers on the bedrock map refer to sites where samples for 10Be exposure dating have been collected and analysed.
10 108 Exposure age results and discussion distribution of Be exposure ages obtained at sites 0006 and 0008. 109 While awaiting consensus from the ongoing CRO- 110 NUS-EU joint work programme (www.cronus-eu.net) 10 Kanin Peninsula 111 regarding refinement of the Be decay constant, 10 112 calibration of the Be production rate, and modelling The sites at the Kanin Peninsula (0005, 0006, 0008, 113 of cosmic ray fluxes to improve the scaling factors, we 0209) are represented by either bedrock samples or 10 114 report uncorrected Be concentrations together with glacial erratic boulder samples. In the simplest 10 115 exposure ages as Be kyr, both with one-sigma scenario, with glacial erosion of bedrock/boulders 116 analytical errors (Table 2). Production rate variations and subsequent subaerial exposure of the eroded rock 117 due to geomagnetic changes are believed to be minor surfaces from deglaciation until today, the surficial
118 for geomagnetic latitudes /608 (Lal 1991; Masarik cosmogenic nuclide content should reflect the elapsed 119 et al. 2001; Pigati & Lifton 2004). In Fig. 3A, the time since deglaciation. This implies that internal 120 results from the Kanin Peninsula and Timan Ridge are agreement between surfaces analysed from the same 121 plotted separately versus sample elevation. In this site is expected, given similar weathering rates and 122 diagram, open circles represent ages obtained on degree of snow cover since deglaciation. In reality, the 123 erratics and solid squares represent ages obtained on low number of samples collected from each site at the 124 bedrock. Figure 3B shows a southwest northeast Kanin Peninsula shows variable degree of agreement 125 cross-sectionUNCORRECTED of western Kanin Peninsula and the (Fig. 3A, B). PROOF C:/3B2WIN/temp files/SBOR41_S100.3d[x] Tuesday, 23rd May 2006 15:22:10
4 Henriette Linge et al. BOREAS 00 (2006)
Fig. 2. Weathering features at the Timan Ridge sites: A. Sandstone cliffs at site 0208. Surfaces have weathering pits (arrows), grikes dissect the surfaces and corridors separate the cliffs. Samples 02-411 and 02-412 were collected from a horizontal surface in the distance (arrows). B. Site 0206 has numerous granite erratics resting on sandstone surfaces. Pedestal (arrow) height is around 5 8 cm underneath smaller boulders (not sampled for exposure dating). C. Sandstone sculptures (including tors) and deflation fields are common in sandstone areas; example is from downstream site 0207. Sample 02-408 was collected from the top surface of the tor to the right.
126 At site 0006, two bedrock surfaces give 529/3 and Two glacial erratic samples from site 0209 give 10 10 127 559/4 Be kyr and one bedrock surface from a lower overlapping exposure ages of 369/3and429/3 Be 10 128 elevation gives 189/2 Be kyr (Table 2, Fig. 3A, B). kyr (Table 2, Fig. 3A). The apparent close agreement 129 From these samples it is suggested that the last between them suggests that they share similar trans- 130 deglaciation at the site occurred around 50 to 60 port/erosion and exposure/weathering histories, i.e. 10 131 Be kyr ago. The low-elevation surface must have thorough erosion during glacial transport and stable 132 experienced events or undergone processes causing it to present positions throughout their exposure history. 133 be significantly younger than the higher-elevation Site 0005 has exposure ages from three bedrock 10 134 surfaces, which is in agreement with field observations surfaces of 399/3, 489/4 and 509/4 Be kyr (Table 2, 135 of an ice-contact terrace being situated between the Fig. 3A). The younger of these is a sample collected 136 sampled locations (Fig. 3B). from a lower elevation than the two older surfaces. 137 From site 0008 only glacial erratics were available; With the two higher-elevation samples giving over-
138 three samples yield exposure ages of 559/6, 819/6 and lapping exposure ages, it is suggested that they reflect 10 139 959/7 Be kyr (Table 2, Fig. 3A, B). The large internal the most likely age of the last deglaciation at the site. 140 variability at one site is problematic and makes the Apart from being an outlier compared to the two older 141 reliability of any of the ages questionable. As the two ages, there is, however, no obvious reason to reject the 142 older ages nearly overlap within their one-sigma younger age. 143 analytical errors, it is tempting to suggest that these Summarizing the results from the Kanin Peninsula; 10 144 boulders were sufficiently eroded during transport and five ages around 50 to 55 Be kyr are obtained 145 hence to regard the ages as realistic. On the other hand, from both bedrock and glacial erratic surfaces sampled 146 given the possibility of an inherited signal (for the older 100 150 m a.s.l. Sites 0005 and 0209 give three ages 10 147 surfaces), the youngest age can be claimed to be the around 40 Be kyr for intermediate-elevation bedrock 148 most reliable, provided that this erratic has been in a (60 74 m a.s.l.). This age-altitude trend may be 149 stableUNCORRECTED position since first exposed. indicative PROOF of vertical melting during deglaciation. The C:/3B2WIN/temp files/SBOR41_S100.3d[x] Tuesday, 23rd May 2006 15:22:11
BOREAS 00 (2006) Exposure age dating, NW Russia 5
Table 1. Description of visited sites and of samples collected and processed for 10Be AMS measurement.
Sample Altitude Site description Sample description Sample Topographic Weight (m a.s.l.) thickness shielding (g)1 (cm) correction
Site 0006, Kanin Peninsula (68831?18ƒN, 43837?01ƒE) 00-418 115 Bedrock outcrops in area with Quartz vein in mica schist 4.0 1.0000 40.87 00-419 100blockfields and patterned ground Quartz vein in mica schist 0.5 1.0000 42.73 00-420 50cut by river valley. Quartz lens in meta-sandstone 1.5 0.9996 40.50 Site 0008, Kanin Peninsula (68833?23ƒN, 43848?38ƒE) 00-424 130 Gently sloping plateau area above Granite, erratic boulder 3.0 1.0000 45.71 00-425 125main river valley. Blockfield and Granite, erratic boulder 1.5 1.0000 40.40 00-426 113patterned ground. Granite, erratic boulder 3.0 1.0000 43.34 Site 0209, Kanin Peninsula, zero-point at Madakha (Larsen et al., this issue b) (68833?04ƒN, 44844?11ƒE) 02-521 60 Low-relief tundra. Few erratics, Granite, erratic boulder 1.0 1.0000 31.21 02-522 60 sampled sites are 1.6 and 2.4 km Granite, erratic boulder 2.0 1.0000 40.82 southwest of Madakha. Site 0005, Kanin Peninsula (67851?31ƒN, 46825?48ƒE) 00-415 74 Bedrock hills with blockfield Quartz vein in mica schist 1.5 0.9999 40.90 00-416 138separated from the coast by bog Quartz vein in mica schist 4.0 1.0000 41.39 00-417 150land. No erratics found. Quartz lens in mica schist 1.0 0.9999 41.93 Site 0208. Timan Ridge (67832?48ƒN, 48826?24ƒE) 02-411 135 Weathered sandstone outcrops Sandstone, bedrock 1.5 1.0000 40.27 02-412 135exposed as a ridge with Sandstone, bedrock 3.0 1.0000 40.30 02-413 138escarpments, tors and deflation Granite, erratic boulder 1.0 1.0000 41.93 02-414 137surfaces. Occasional erratics. Sandstone, bedrock 1.5 1.0000 40.24 02-415 137 Gneiss, erratic boulder 1.5 1.0000 34.72 Site 0207, Timan Ridge (67815?44ƒN, 48852?35ƒE) 02-406 176 Weathered sandstone plateau cut Sandstone, bedrock 2.5 1.0000 40.16 02-407 179by river valleys. Cliffs, tors and Sandstone, bedrock 1.5 1.0000 42.06 02-408 171deflation surfaces are common. Sandstone, bedrock 1.5 1.0000 40.30 02-409 148Tors, sandstone sculptures and Granite, erratic boulder 1.5 1.0000 40.30 02-410a 148erratics are common along the Granite, erratic boulder 1.5 1.0000 39.15 02-410b 148main valley further upstream. Granite, erratic boulder, 1.5 1.0000 38.09 weathered Site 0206, Timan Ridge (67801?07ƒN, 48852?46ƒE) 02-400 245 Weathered sandstone outcrops Sandstone, bedrock 1.0 1.0000 42.18 02-401 246exposed as a dissected plateau Sandstone, bedrock 1.0 1.0000 40.10 02-402 235along a river valley. Karst-like Granite, erratic boulder 1.5 0.9994 40.36 02-403 240features and tors are common. Sandstone, bedrock 1.5 1.0000 40.67 02-404 240Erratics on bedrock are resting Sandstone, bedrock 3.5 1.0000 40.49 02-405 235on pedestals. Granite, erratic boulder 1.5 0.9994 40.91
1Amount of clean quartz dissolved in the laboratory for 10Be extraction. Isolation of quartz for 10Be extraction was accomplished by magnetic separation, phosphoric acid (H3PO4) treatment (crystalline rocks) and repeated 24 h leaches in 1% hydrofluoric acid (HF) (both crystalline and sedimentary rocks). 0.5 mg 9Be spike was added to the first batch of 00 samples (416, 418, 419, 420), whereas 0.25 mg 9Be spike was added to the remaining 00 samples and all 02 samples. Corresponding blank samples were treated identically to the purified quartz samples. Combined anion and cation exchange procedures produced high-yield, purified Be samples containing only residual amounts of Ti, K and Mg. Residual Ti was common for sandstone samples owing to inclusions of thurmaline in the quartz grains. Selective precipitation techniques removed Ti and isolated Be as beryllium hydroxide (Be(OH)2). Repeated dissolution and re-precipitation of Be(OH)2 ensured removal of boron (B). Conversion to BeO was done by oxidation with a Bunsen burner flame for the 00 samples, and with a later acquired rapid incinerator for the 02 samples.
150 lowest sampled elevation, 50 m a.s.l. at site 0006, gives sites (0206, 0207, 0208). In the simplest scenario, where 10 151 the youngest age from the peninsula, i.e. 189/2 Be kyr an overriding glacier erodes bedrock and transports 152 (Fig. 3B). However, site 0008 reveals two ages of 81 and and erodes boulders, any from deglaciation continu- 153 95 kyr ( 100/ m a.s.l.) that are outliers compared to the ously exposed rock surface should have an in situ 154 other retrieved ages. cosmogenic nuclide content reflecting the time elapsed since deglaciation. This implies that bedrock and erratic surface ages would agree if their post-deposi- 155 Timan Ridge tional weathering rates and snow cover were similar. 156 Surface rock samples from both bedrock and erratic This is not the case for any of the sampled sites at the 157 boulders wereUNCORRECTED collected from the three Timan Ridge Timan Ridge. The PROOF sandstone outcrops have very C:/3B2WIN/temp files/SBOR41_S100.3d[x] Tuesday, 23rd May 2006 15:22:11
6 Henriette Linge et al. BOREAS 00 (2006)
Table 2. AMS measured 10Be concentrations and calculated 10Be exposure ages of erratic boulders and bedrock surfaces from the Kanin Peninsula and the Timan Ridge.
10 10 3 Sample Altitude scaling factors Be production Be uncorrected Surface and topography Exposure age9/1s 1 10 2 10 rate at site 9/1s normalized Be9/1s ( Be kyr) (atoms/g yr) (105 atoms/g) (105 atoms/g) Neutrons Muons
Site 0006, Kanin Peninsula (68831?18ƒN, 43837?01ƒE) 00-418 1.131 1.058 5.76 2.509/0.16 2.579/0.16 51.59/3.2 00-419 1.114 1.050 5.67 2.719/0.19 2.709/0.19 55.09/4.0 00-420 1.056 1.025 5.38 0.869/0.07 0.849/0.07 18.09/1.6 Site 0008, Kanin Peninsula (68833?23ƒN, 43848?38ƒE) 00-424 1.149 1.066 5.85 4.649/0.36 4.769/0.37 95.09/7.5 00-425 1.143 1.063 5.82 2.749/0.31 2.769/0.31 54.99/6.2 00-426 1.129 1.057 5.75 3.919/0.27 4.009/0.28 81.09/5.6 Site 0209, Kanin Peninsula (68833?04ƒN, 44844?11ƒE) 02-521 1.067 1.030 5.44 1.729/0.14 1.729/0.14 36.59/3.0 02-522 1.067 1.030 5.44 1.959/0.14 1.979/0.14 41.89/3.0 Site 0005, Kanin Peninsula (67851?31ƒN, 46825?48ƒE) 00-415 1.083 1.037 5.52 1.889/0.16 1.899/0.16 39.59/3.4 00-416 1.159 1.070 5.90 2.419/0.19 2.489/0.20 48.59/3.8 00-417 1.173 1.076 5.97 2.609/0.21 2.619/0.21 50.59/4.1 Site 0208, Timan Ridge (67832?48ƒN, 48826?24ƒE) 02-411 1.155 1.068 5.88 1.169/0.08 1.169/0.08 22.79/1.6 02-412 1.155 1.068 5.88 1.229/0.11 1.249/0.11 24.29/2.1 02-413 1.159 1.070 5.90 2.169/0.16 2.179/0.17 42.49/3.3 02-414 1.157 1.070 5.89 0.749/0.06 0.739/0.06 14.29/1.3 02-415 1.175 1.070 5.89 2.339/0.16 2.359/0.17 46.09/3.3 Site 0207, Timan Ridge (67815?44ƒN, 48852?35ƒE) 02-406 1.205 1.090 6.13 1.429/0.11 1.449/0.11 26.99/2.1 02-407 1.208 1.092 6.15 1.269/0.09 1.279/0.09 23.79/1.7 02-408 1.199 1.087 6.10 1.819/0.20 1.829/0.20 34.39/3.8 02-409 1.171 1.075 5.96 5.439/0.37 5.499/0.37 107.99/7.3 02-410a 1.171 1.075 5.96 7.039/0.46 7.119/0.46 140.89/9.2 02-410b 1.171 1.075 5.96 6.519/0.41 6.589/0.43 130.09/8.5 Site 0206, Timan Ridge (67801?07ƒN, 48852?46ƒE) 02-400 1.291 1.127 6.56 3.049/0.22 3.059/0.22 53.89/3.9 02-401 1.293 1.128 6.57 2.019/0.14 2.029/0.14 35.39/2.4 02-402 1.279 1.122 6.50 4.029/0.27 4.069/0.20 72.59/4.9 02-403 1.285 1.124 6.53 3.019/0.19 3.049/0.37 53.79/3.5 02-404 1.285 1.124 6.53 2.599/0.17 2.669/0.46 47.09/3.2 02-405 1.279 1.122 6.50 4.529/0.32 4.579/0.43 81.89/5.8
1The 10Be concentrations are calculated from the known sample and 9Be carrier weights and the measured 10Be/9Be ratio. One-sigma errors in the 10Be concentrations are calculated from the number of 10Be counts, an additional 5% instrumental uncertainty based on long-term measurements of standards. 2The 10Be concentrations are corrected for procedural blank values, sample thickness and topographic shielding. Procedural blanks account 10 4 10 4 10 for a 0.21 2.9% reduction in Be, and amount to (9.989/4.49) /10 atoms Be (n /1) and (5.299/0.80) /10 atoms Be (n /2) for the 00 4 10 samples (0.5 and 0.25 mg spike, respectively), and (5.779/3.17) /10 atoms Be (n /5) for the 02 samples. One-sigma errors in the normalized 10Be concentrations combine the uncertainties from the uncorrected concentrations and the procedural blanks in quadrature. 10 Further corrections are made for sample thickness (0.5 4.0 cm, B/3.5% increase in Be) by assuming an exponential depth dependence of 2 10 the in situ production rate and an attenuation factor of 150 g/cm . The overall topographic shielding effect (B/0.1% increase in Be) on each sampled surface is calculated from the vertical shielding angle of the surrounding topography in 108 azimuth increments, following the description by Dunne et al. (1999). 3The exposure ages are calculated using a 10Be half-life of 1.51 Ma, and the SLHL 10Be production-rate estimate (5.1 atoms/g yr) and scaling proportions (97.8% spallogenic, 2.2% muon capture) from Stone (2000). Latitudinal and altitudinal scaling for neutron-induced production (spallation) of 10Be was done according to Lal (1991). Muon induced-production was scaled latitudinally as for neutrons, whereas altitudinal scaling assumes an exponential dependence on atmospheric pressure using an attenuation factor of 247 g/cm2 1- (Nishiizumi et al. 1989). A standard relationship between atmospheric pressure and altitude is used (Lal 1991). The calculated site-specific production rates are reduced by a factor of 0.875 to account for the measured difference (Middleton et al. 1993) between the 10Be/9Be ratio for the NIST 10Be standard (SRM 4325) used at Gif-sur-Yvette and the ICN standard used in Stone’s (2000) 10Be production rate estimate. Note that the 1 s errors only include AMS counting statistics and a long-term AMS instrumental uncertainty. UNCORRECTED PROOF 58 C:/3B2WIN/temp files/SBOR41_S100.3d[x] Tuesday, 23rd May 2006 15:22:12
BOREAS 00 (2006) Exposure age dating, NW Russia 7
Fig. 3. A. Graphic presentation of the surface exposure age results from the Kanin Peninsula (left panel) and the Timan Ridge (right panel). Open circles represent ages obtained on erratic boulders; solid squares represent bedrock exposure ages. Error bars are 1 s errors. Note that dates from sandstone bedrock (right panel) give younger ages than their corresponding crystalline erratics. B. A southwest northeast cross- section of the northwestern ridge of the Kanin Peninsula showing the relative position of the surface exposure samples for sites 0006 and 0008. Note the location of the ice-contact terrace at site 0006. Bedrock samples are represented by black triangles, erratic boulder samples are represented by grey triangles.
weathered appearances (Fig. 2), with solution pits, tors anomalously young due to weathering. Weathering 159 and various geomorphological features commonly losses occur both as release of 1 to 3-cm-thick slabs 160 found in karst landscapes. The erratic boulders, con- following the subhorizontal sandstone bedding, and as 161 sisting of granite and gneiss, have a fresher appearance, granular disaggregation. Wind action can probably although still weathered. effectively remove the resulting sand grains over time. 162 At site 0208, three surface exposure ages from The lowest age is retrieved from a top surface of a tor 163 10 bedrock give 149/1, 239/2 and 249/2 Be kyr (Table situated in a deflation field, a feature possibly more 164 2, Fig. 3A),UNCORRECTED where the first age is believed to be prone to weathering PROOF losses than the morphologically C:/3B2WIN/temp files/SBOR41_S100.3d[x] Tuesday, 23rd May 2006 15:22:15
8 Henriette Linge et al. BOREAS 00 (2006)
Fig. 4. Exposure age results in relation to the ice-sheet limits according to Larsen et al. (2006a: fig. 8). Plain numbers are 10Be exposure ages obtained from bedrock surfaces; underlined numbers are 10Be exposure ages obtained from glacial erratic boulder surfaces. Base of the map comes from the DEM model Gtopo03.
10 165 more extensive surfaces, where the two older ages were 829/6 Be kyr (Table 2, Fig. 3A). As seen from the 166 obtained (Fig. 2A). Two erratics give 429/3and469/3 other two Timan Ridge sites, the results obtained on 10 167 Be kyr (Table 2, Fig. 3A). A reasonably good bedrock surfaces agree reasonably well, as do the ages 168 agreement exists between ages obtained on similar from erratic boulder surfaces, but bedrock and erratic 169 material at this site; however, a clear disagreement is exposure ages do not overlap. At this site, lithological 170 evident when comparing bedrock versus erratics. This differences in weathering resistance were apparent from 171 discrepancy between bedrock and erratic exposure ages medium-sized granite erratics frequently resting on
172 is also found at the other two Timan Ridge sites, as is B/10 cm tall sandstone pedestals (Fig. 2B). 173 the difference in weathering resistance. Considering the results from the Timan Ridge, there 174 Site 0207 provides exposure ages from three bedrock is a clear disagreement between surface exposure ages 10 175 surfaces of 249/2, 279/2 and 349/4 Be kyr (Table 2, obtained on sandstone outcrops and those from 176 Fig. 3A). The highest of the bedrock surface exposure granite/gneiss erratic boulders. The difference in age
177 ages is obtained on the top surface of a /2 m tall tor can primarily be explained as a difference in weathering 178 (Fig. 2C), whereas the two younger ages are from resistance, and the low-grade metamorphosed sand- 179 surfaces protruding c. 0.5 m above their adjacent stone appears inadequate as a target material for 180 deflation surfaces. From a geomorphological point of surface exposure dating. Assuming that the granite 181 view, the surface of the tors should provide the oldest erratics give realistic exposure ages, pedestals of 5 8 182 age and it should be the least affected by snow cover. cm height (Fig. 2B) at the southernmost site suggest a 183 Three samples from two erratics situated upstream sandstone-weathering rate of at least 1 mm/kyr, since
184 from the bedrock outcrops give 1089/7, 1309/9 and the denudation rate beneath a boulder is lower than for 10 185 1419/9 Be kyr (Table 2, Fig. 3A). The two older ages unshielded bedrock (Lauritzen 2005). Explaining the 186 originate from the same boulder but from different age discrepancy between bedrock and erratics by 187 positions, where the younger age is from a weathered differences in weathering resistance would, however,
188 rock piece on the top surface. Sandstone surface ages require sandstone erosion rates /5 mm/kyr. This 189 agree reasonably, as do the erratic surface ages, whereas should theoretically produce pedestals taller than 190 ages obtained from bedrock disagree with those observed, but boulders are likely to slide off if the 191 obtained from erratics. pedestal diameter gets too small. 192 At the southernmost site 0206, four bedrock samples The bedrock surfaces do, however, show an increas- 10 193 give exposure ages of 359/2, 479/3, 549/3 and 549/4 ing trend in Be concentrations from north to south. 10 194 BeUNCORRECTED kyr, whereas two granite boulders give 739/5 and In the field,PROOF the sandstone at sites 0207 and 0208 C:/3B2WIN/temp files/SBOR41_S100.3d[x] Tuesday, 23rd May 2006 15:22:17
BOREAS 00 (2006) Exposure age dating, NW Russia 9
195 appeared similar, i.e. showing equally low degree of ‘too old’ exposure ages. If the ice-marginal positions 196 metamorphosis and strong degree of weathering. Site are correct and soundly dated, exposure ages that are 197 0206 differs from these by not having large deflation ‘too old’ must be explained by re-deposition and 198 surfaces and by the occurrence of sandstone pedestals, insufficient erosion during glacial transport to remove 199 possibly indicating a higher degree of metamorphosis previously accumulated nuclides (‘inherited’ compo- 200 and/or a lower weathering rate (in agreement with nent or prior-exposure signal). 10 201 overall higher sandstone Be concentrations). Erratics on the Kanin Peninsula can be of both local 202 The bedrock surface ages suggest that the two and remote origin. Site 0209 is situated northeast of the 203 northern sites experienced a glaciation at a later stage local source, whereas the other sites are located within 204 than the southernmost site. The erratic boulder ages do or very close to it. From the reconstructed ice sheets, 205 not, however, support this. The erratic surface ages on with ice flowing from north and northeast, site 0209 206 the northernmost Timan Ridge site agree with ages most likely has boulders of remote provenance, 207 retrieved from the Kanin sites, whereas ages from the whereas site 0008 may have boulders of both local 208 middle site suggest ice-free conditions since the onset and remote origin. This makes the geomorphic setting 209 of the Eemian, and ages from the southern site suggest at site 0209 more reliable than at site 0008, with regard 10 210 ice-free conditions during the past 85 70 Be kyr. to sufficient erosion and avoiding an inherited compo- nent in the cosmogenic nuclide signal. The only ‘local’ provenance area for crystalline rocks 211 Comparison with the current on the Timan Ridge is southeast of the river Pyoza palaeogeographical reconstruction (Fig. 1). Depending on the ice-flow directions, a Timan-based glaciation is unlikely to be the primary 212 A composite lithostratigraphical model has been transport agent of boulders deposited on the northern 213 established for the region independently of absolute part of the ridge. This implies that the Timan glaciation 214 dating methods, using marine marker beds, ice-flow could only re-deposit already available erratics, and 215 directions from tills, and the stratigraphic relationships short transportation increases the possibility of preser- 216 between tills and interbedded sediments (Larsen et al. ving a prior-exposure signal. The erratic boulders from 217 2006a). OSL and radiocarbon dates made on inter-till site 0207 have 10Be concentrations too high to have 218 sediments constrain the age spans of glacial events, been freshly exposed after the Timan glaciation (Table 219 and the age spans for interstadial events have been 2), and from this it is suggested that the erratics from 220 assigned. Figure 4 shows the position of the cosmo- site 0207 indicate first exposure after the Saalian 221 genic exposure-dated sites in relation to the recon- glaciation. This area also displays spectacular sand- 222 structed ice margins with age constraints according to stone sculptures compared to the other two Timan 223 Larsen et al. (2006a). These reconstructions indicate Ridge sites. The 1 3 m relief of tor-like forms suggests 224 that all exposure-dated sites were ice covered at least (a) prolonged ice-free conditions, (b) high weathering 225 once during the Weichselian. Correlation of ice-mar- rates, or (c) preservation of relief by cold-based ice 226 ginal positions across the Timan Ridge is difficult, cover. Only the southernmost site 0206 on the Timan 227 however, particularly for the Kara Sea glaciations. Ridge gives exposure ages that can possibly be taken as 228 Because only the time elapsed since deglaciation should evidence of a Timan Ridge-based glaciation; two 229 be reflected by the exposure dates, the reconstruction erratic boulders here give exposure ages of 73 and 82 230 predicts the following: all sites at the Kanin Peninsula 10Be kyr. If these ages represent a continuous exposure 231 except the north-westernmost should give ages younger period, the implications for the Timan ice cap are that 232 than 45 kyr. The exception is where the Scandinavian it must have developed earlier than inferred from the 233 Ice Sheet (Fig. 3B) has partly influenced site 0006 reconstruction and/or that it wasted away around 75 234 et al. 10 (Demidov 2006). Similarly, the Timan Ridge sites Be kyr at the site. If this is not compatible with 235 should provide exposure ages younger than 70 kyr and geological evidence, the boulders must have a compo- 236 possibly younger than 45 kyr from the northernmost site exposure history (on-site or prior to deposition). site.
237 Kara Sea glaciation, 100 90 kyr BP, and Timan Ridge Barents Sea glaciation, 70 65 kyr BP, and Kara Sea 238 glaciation, 75 70 kyr BP glaciation, 55 45 kyr BP
239 According to the palaeogeographic reconstruction, the According to the palaeogeographic reconstruction, the 240 Kanin Peninsula was ice covered 100 90 kyr ago Kanin Peninsula was ice-covered both at 70 65 kyr 241 (Fig. 4), hence excluding the existence of exposure and 55 45 kyr ago (Fig. 4), excluding the existence of 242 ages older than 90 kyr BP. Likewise, the exposure sites exposure dates older than 45 kyr. The Timan Ridge, on 243 at the Timan Ridge were ice covered 75 70 kyr ago the other hand, was overall ice-free (Fig. 4), suggesting 244 (Fig. 4), excluding the existence of exposure ages older that exposure dates younger than 70 kyr should be 245 than 70 kyr. Nevertheless,UNCORRECTED in both areas we obtained common. PROOF C:/3B2WIN/temp files/SBOR41_S100.3d[x] Tuesday, 23rd May 2006 15:22:17
10 Henriette Linge et al. BOREAS 00 (2006)
246 Evidence of the older of the two reconstructed ering resistant lithologies, and (ii) partly confirm the 247 glaciations is not represented in the exposure data general aspects of the reconstructed palaeogeography 248 set, as the younger of the two is likely to have for the region. 249 obliterated the traces of it (with possible exceptions From the Kanin Peninsula, exposure ages around 10 250 being the erratics at site 0008). Exposure age results 50 55 Be kyr are found for both bedrock and glacial 251 from the Kanin Peninsula largely agree with the erratic surfaces sampled at 100 150 m a.s.l., whereas 252 glaciation model, suggesting it was last glaciated 55 bedrock and erratics at intermediate elevations (60 74 10 253 45 kyr ago (Larsen et al. 2006a). Although the total m a.s.l.) give ages around 40 Be kyr. The lowest 254 number of samples is limited, the decrease in ages with sampled elevation, 50 m a.s.l. at site 0006, gives the 10 255 lower altitudes (Fig. 3A) could reflect a vertical melting youngest age from the peninsula of 189/2 Be kyr. 256 of stagnant ice after the 55 45 kyr glacial advance. From the Timan Ridge a clear disagreement exists 10 257 From the Kanin Peninsula, ages around 55 50 Be between surface exposure ages obtained on sandstone 258 kyr are found for both bedrock and glacial erratic outcrops and those from granite/gneiss erratic 259 surfaces sampled above 100 m a.s.l., whereas sites 0005 boulders. A notable difference in weathering resistance 10 260 and 0209 give ages around 40 Be kyr for intermedi- can account for this, and low-grade metamorphosed 261 ate-elevation bedrock. The lowest sampled elevation, sandstone appears to be inadequate as a target material 262 50 m a.s.l. at site 0006, is physically separated from the for exposure dating. The erratic boulder ages, however, 263 other samples by an ice-contact terrace and gives the do not clarify the picture; from north to south the 10 10 264 youngest age from the peninsula, i.e. 189/2 Be kyr sites give around 39 49, 100 150 and 68 88 Be kyr 265 (Fig. 3B). (1 s age ranges). In this respect, the northernmost 266 For the Timan Ridge, the glaciation model suggests Timan site agrees with the Kanin sites, whereas the two 267 ice-free conditions after 70 kyr (Fig. 4). The exposure sites to the south suggest ice-free conditions through- 268 age results, however, suggest that the 55 to 45 kyr Kara out the Weichselian and ice-free conditions during the 10 269 Sea Ice Sheet extended further to the south, and that past ca 75 Be kyr, respectively. 270 the ice margin was situated somewhere between sites As an independent test of the palaeogeographical 271 0208 and 0207 (Fig. 4). reconstruction of Larsen et al. (2006a), the exposure dates (i) do not provide conclusive information on the Timan ice cap, (ii) agree with the reconstruction of the 272 Scandinavian glaciation, 20 17 kyr BP Kanin Peninsula being glaciated around 55 45 kyr BP 273 A till bed found in coastal sections on northwestern and, furthermore, suggest a vertical downwasting of 274 Kanin Peninsula is interpreted to belong to the Last this ice sheet, (iii) suggest that the 55 45 kyr BP Kara 275 Glacial Maximum and to have been deposited by a Sea Ice Sheet also reached as far south as to the 276 Scandinavian ice sheet (Demidov et al. 2006). As northernmost site at the Timan Ridge, and (iv) support 277 discussed above, most Kanin Peninsula exposure sites field evidence of the Late Weichselian Scandinavian Ice 10 278 suggest subaerial conditions since 55 40 Be kyr, Sheet reaching the northwestern tip of the Kanin 279 supporting the reconstruction (Fig. 4) and leaving the Peninsula. 280 exposure sites east of this ice margin. According to 281 these reconstructions the western tip of the Kanin Acknowledgements. This work is a contribution to the ‘QUEEN’ 282 Peninsula was glaciated from the west by the LGM project (Quaternary Environment of the Eurasian North) a 283 Scandinavian Ice Sheet. The westernmost and lowest network project within the European Science Foundation. The 284 site on the Kanin Peninsula gives a bedrock surface Norwegian Research Council supported HL (grant no. 138537/432) 10 through a postdoctoral research stay at Washington State University, 285 exposure age of 18 Be kyr, supporting this recon- Vancouver. Lena Ha˚kansson assisted in the fieldwork and Leah 286 struction (Fig. 4). The sample position is separated Bjerkelund, Jason Dzurisin and Hanne Linge assisted with the 287 from the higher sampled positions by an ice-contact laboratory work. Tandetron operation was supported by Institut 288 terrace (Fig. 3B), but unfortunately the site did not National de Physique Nucle´aire et de Physique des Particules (IN2P3) and Institut National des Sciences de l’Univers (INSU) 289 provide other suitable surfaces to be sampled. divisions of Centre National de la Recherche Scientifique (CNRS). Svend Funder kindly provided Fig. 3B. Comments by Per Mo¨ller, Vincent Rinterknecht and Jan A. Piotrowski improved the manu- Conclusions script. This is publication no. A104 of the Bjerknes Centre for Climate Research. 290 The sampling programme for this study aimed at 291 capturing the regional glacial history, and exposure 292 ages were expected to decrease from south to north. 293 Interpretation of the resulting exposure ages proved to References 294 be less straightforward, but the results bring useful Astakhov, V. I., Svendsen, J. I., Matiouchkov, A., Mangerud, J., 295 information both on methodology and glacial history Maslenikova, O. & Tveranger, J. 1999: Marginal formations of the 296 as they (i) demonstrate both suitable and unsuitable last Kara and Barents ice sheets in northern European Russia. 297 settingsUNCORRECTED of bedrock and erratics with regard to weath- Boreas 28PROOF,23 45. C:/3B2WIN/temp files/SBOR41_S100.3d[x] Tuesday, 23rd May 2006 15:22:18
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