BOREAS. The first Holocene relative sea-level curve from the middle part of Hardangerfjorden, western Norway

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1 The first Holocene relative sea-level curve from the middle part of Hardangerfjorden, western Norway ANDERS ROMUNDSET, ØYSTEIN S. LOHNE, JAN MANGERUD AND JOHN INGE SVENDSEN BOREAS Romundset, A., Lohne, Ø. S., Mangerud, J. & Svendsen, J. I (January): The first Holocene relative sea-level curve from the middle part of Hardangerfjorden, western Norway. Boreas, Vol. 39, pp /j x. ISSN The first relative sea-level (RSL) curve from the mid-hardangerfjorden area covering the entire Holocene is presented. The curve is based on a series of AMS 14 C dates on terrestrial plant macrofossils across the isolation level in each of five lakes located between 3.5 and 74.5 m a.s.l. During the first 1200 years, the RSL fell very rapidly from the marine limit at 98 m a.s.l. to 33 m a.s.l., i.e. at a rate of 5.4 cm yr 1. The emergence rate then slowed considerably and was close to standstill cal. yr BP. However, an emergence of 16.5 m has taken place during the past 6000 years. Radiocarbon dates of terrestrial plant macrofossils from the basal strata in a lake above the marine limit and mollusc shells from glaciomarine silt in the isolation basins yielded a mean age for the local ice-margin retreat of cal. yr BP. This verifies that Hardangerfjorden was glaciated during the Younger Dryas an interpretation that has recently been disputed. The ice margin retreated at a rate of about 300 m yr 1 from the position of the Younger Dryas moraine to this site some 60 km further into the fjord. Anders Romundset ( anders.romundset@uit.no), Department of Geology, University of Tromsø, Dramsvegen 201, NO-9037 Tromsø, Norway; Øystein S. Lohne ( oystein.lohne@geo.uib.no), Jan Mangerud ( jan.mangerud@geo.uib.no) and John Inge Svendsen ( john.svendsen@geo.uib.no), Department of Earth Science, University of Bergen, Alle gaten 41, NO-5007 Bergen, Norway and The Bjerknes Centre for Climate Research, Alle gaten 55, NO-5007 Bergen, Norway; received 5th March 2009, accepted 29th April Raised Holocene marine terraces occurring along the 180 km long Hardangerfjorden in western Norway represent former shorelines reaching maximum elevations of 30 and 125 m a.s.l. near the mouth and head of the fjord, respectively (Fig. 1) (Hamborg 1983). The marine limit is considerably higher in the inner fjord region than at the coast, because the ice-sheet thickness increased inland during the last glaciation, resulting in differential rebound from the glacio-isostatic depression. Several 14 C-dated relative sea-level (RSL) curves have been constructed from the outer coast, i.e. west of the Younger Dryas Herdla Halsnøy moraines (Fig. 1) (Kaland 1984; Krzywinski & Stabell 1984; Anundsen 1985; Lohne et al. 2004, 2007; Vasskog 2006). However, no curve from areas inside the mouth of the fjord and covering the entire Holocene has been published previously. The main purpose of this study was therefore to construct a new RSL curve from an area well inside the mouth of Hardangerfjorden, the only curve thus far showing the Holocene sea-level changes from fjord areas in SW Norway. A curve from this area will improve our description and understanding of the shoreline configuration and uplift history along the fjord, and thus for the SW part of the Scandinavian Ice Sheet. We used the so-called isolation basin method, where lake basins below the marine limit are cored with the aim of identifying and dating the transition from marine to freshwater sediments, i.e. when the basins were isolated from the sea (Hafsten 1960). The study area, Tørvikbygd (Figs 1, 2), is the innermost site along Hardangerfjorden, where there is a series of isolation basins at different altitudes between the present sea level and the marine limit; this area was therefore selected for the study. A secondary objective was to improve the chronology of when this part of Hardangerfjorden became icefree. The classical interpretation is summarized by Mangerud (2000), who concludes that the ice margin retreated some distance inland during the Allerød, and that a subsequent re-advance during the mid- and late Younger Dryas filled the fjord basin and deposited the terminal moraine that crosses the outer part of Hardangerfjorden (Fig. 1). A reconstruction of the Younger Dryas glacier profile in Hardangerfjorden (Hamborg & Mangerud 1981) depicts about 1100 m ice above present-day sea level in our study area, in addition to the ice filling the m deep adjacent fjord basin. The final retreat of the ice margin, and thus the start of the sea-level history inside the Herdla Halsnøy moraines, accordingly took place during the early Holocene, a conclusion supported by our new observations. However, in the past decade, an alternative hypothesis has been advocated (Helle et al. 1997, 2000, 2007; Bakke et al. 2000, 2005; Bakke 2004; Helle 2004, 2008) postulating that the entire fjord up to the head at Eidfjord (Fig. 1) remained ice-free during the Younger Dryas. If this were correct, the sea-level history in our study area would at least date back to the Allerød. However, in our opinion, the alternative hypothesis was falsified and the classical interpretation verified by Mangerud (2000) and DOI /j x r 2009 The Authors, Journal compilation r 2009 The Boreas Collegium

2 88 Anders Romundset et al. BOREAS Fig. 1. Map of Hardangerfjorden and surrounding areas, SW Norway. The Herdla Halsnøy terminal moraines were deposited by the Scandinavian Ice Sheet at its maximum Younger Dryas position (Aarseth & Mangerud 1974), whereas the Eidfjord Osa terminal moraines near the head of Hardangerfjorden testify an advance or standstill of the ice margin during the Preboreal (Anundsen & Simonsen 1967). Dashed segments of the lines indicate correlations between various marginal deposits. The isobases of the Younger Dryas sea level are given for the area outside the Herdla Halsnøy moraines and the direction is extended on the map to Tørvikbygd. Lohne (2005); all the new data presented in this article also support the classical interpretation. Methods Fieldwork Lake elevations were acquired from digitalized maps with an uncertainty of maximum 10 cm on lake levels (M. Dalatun, pers. comm. 2004). The elevations of the two highest-lying basins were not given on the maps. Using a calibrated digital barometer, these were measured several times from the upper growth limit of bladder wrack (Fucus vesiculosus), which approximates the elevation of mean tide level. We used the lake altitudes, i.e. the elevation of the outlet threshold of each basin, to determine past sea levels. We consider that this represents the highest astronomically forced tide (HAT), or a slightly higher level due to storm surges, at the time of isolation. The HAT is presently 81 cm higher than the mean tide level at the nearest tide gauge in Norheimsund, 10 km away (Fig. 1) (Tidevannstabeller 2007). A potential source of error is erosion of the basin threshold at the outlet after lake isolation. All thresholds consist of either bedrock or compact blocky till, and fluvial incision by the outlet river is considered almost negligible. However, the thresholds of two basins have been slightly lowered by humans and correspondingly corrected, as described for each basin. Fieldwork took place during spring 2004 and autumn Lake coring was undertaken partly from ice and partly from a small raft held in place by several anchors. A Russian corer (Jowsey 1966), a modified Geonor-type piston corer with a rig and a wire-controlled piston corer (Nesje 1992) were used. A handheld GPS and an echo-sounder were used to map the bathymetry of each lake prior to coring, with readings taken every 2 m while rowing slowly along transects. Reconnaissance coring was conducted along one or several transects across each basin floor before selecting the site for the sample core. In many cases, cores within each basin could easily be correlated in the field based on visual characteristics. Sediments deposited during the brackish phase of isolation were readily identified as pronounced visual boundaries in the sediment columns. The cores that held the best developed and undisturbed sequences from each basin were used for further analyses. Core locations are marked on each basin map.

3 BOREAS Holocene sea-level curve from Hardangerfjorden, W Norway Storemyr 60 18'N Eidesvatn Høgstemyr Tangatjørn 60 15'N 200 Bergsvatn Tørvikvatn 6 10'E 6 10'E YD-isobase MAP LEGEND Lake Bog Outlet 6 13'E Hardangerfjorden 1 km Sediment analysis and identification of the isolation level Samples for loss-on-ignition (LOI) analysis were dried overnight at 1051C and burned at 5501C for1hbefore weight loss was calculated from the dried sample weight. The percentage LOI values are given in each core log. The approximate position of the marine freshwater transition was identified visually as a laminated zone (Kaland et al. 1984; Svendsen & Mangerud 1987). Diatom checks were made at levels below, within and above this zone in order to identify the precise position of the isolation contact. During the process of basin isolation, the marine diatom assemblages are completely replaced by freshwater diatoms. The diatomological isolation contact is therefore readily detected. It is defined as the lower boundary of solely lacustrine diatoms in the sediments (Kjemperud 1986), and thus reflects the time when the last influx of marine water occurred. The diatoms were checked mainly on smear slides, where a small portion of bulk sediment was mounted on the slide. However, some samples with high organic content were treated with 35% hydrogen peroxide for removal of organic matter prior to mounting. Under a microscope, 3 4 transects of the slides were checked to identify the dominant species and associated environment (marine/mixed/lacustrine) (Lohne et al. 2004). The results of the diatom analysis are given in Table 'E N 60 18'N 60 15'N Fig. 2. Tørvikbygd and the investigated basins; for location, see Fig. 1. Contour interval 100 m. The Younger Dryas isobase orientation 3511N (Lohne et al. 2007) used for construction of a shoreline diagram (Fig. 12) is marked. The short distance between the basins makes tilt adjustments unnecessary for construction of the relative sea-level (RSL) curve (Fig. 11). AMS 14 C-dating All dates used to construct the sea-level curve, as well as some others, were obtained from fragments of terrestrial plants. Slices of sediments were cut from levels of interest in the cores and sieved at 0.5 mm mesh width. Plant fragments were handpicked under a stereomicroscope, carefully cleaned and identified. Leaves from Betula pubescens were found frequently, and therefore in most cases were used for dating the isolation contacts. The samples were dried at room temperature, weighed, put into sample tubes and submitted for dating. Accelerator mass spectrometry (AMS) was radiocarbon-dated at the Poznan Radiocarbon Laboratory, and the resulting 14 C-dates calibrated using the computer software OxCal v4.0 (Bronk Ramsey 1995, 2001, 2006) with the IntCal04 data set (Reimer et al. 2004). The calibrated ages are given with the 2s interval in Table 2 and in all figures. Ages of the isolation events are substantiated by several dates across each isolation contact, and also with additional dates at levels above and below. The series of dates were subjected to Bayesian probability analysis using the P_Sequence deposition model (Bronk Ramsey 2008), which is included in the OxCal v4.0 software. We used a depositional unit granularity of 1 mm (k = 1000 m 1 ), which we consider to be representative of typical isolation sequence sediments (gyttja and silt). We also dated marine mollusc shells and shell fragments, mainly in order to obtain minimum ages for the ice-margin retreat. Filter feeders were preferred because these digest living matter with 14 C in equilibrium with the sea water (Mangerud et al. 2006), but in most levels we found only species or small shell fragments that could not be identified. A mean value for the present day 14 C marine reservoir age of North Atlantic water outside the coast is C years; the corresponding DR value is 2030 years (Mangerud et al. 2006). This water fills the deeper part of the fjord and dominates the surface waters, where it is mixed with fresh water from run-off. The reservoir age will therefore have considerable local variations in shallow water due to the influence of varying amounts of fresh water, and will usually be lower than in the open ocean (Mangerud et al. 2006). During the early Preboreal, the marine reservoir age along the coast was some 250 to C years (Bondevik et al. 2006). These latter values, and the above-mentioned freshwater influence, suggest that the current North Atlantic water reservoir ages are too high for the shallow water where our molluscs lived. However, as accurate values cannot be determined from the sparse amount of available data, we use the mean value years (Table 2), accepting that it is probably too high. Calibrated ages are again obtained with OxCal v4.0, based on the 04 data set (Hughen et al. 2004), using the cited DR value of years and given with the 2s interval (Table 2).

4 90 Anders Romundset et al. BOREAS Table 1. Diatom indicator species from the investigated basins used to determine the isolation contacts. species are classified as polyhalobous (P) and mesohalobous (M), brackish species as oligohalobous halophilous (OH) and lacustrine species as oligohalobous indifferent (OI) and halophobous (H). Indicator diatoms Basin environment Høgstemyr, core , 359, Aulacoseira cf. tethera (OI), Frustulia rhomboids (H), Fragilaria virescens (OI), Anomoeoneis 358, 357 brachysira (H), Pinnularia interrupta (OI), Pinnularia streptoraphe (H) Tangatjørn, core Pinnularia nodosa (H), cf. Cyclotella sp. (?), Pinnularia interrupta (OI), Fragilaria virescens (OI), Tabellaria flocculosa (H) 1137 Paralia sulcata (P), Plagiogramma staurophorum (P), Pinnularia nodosa (H), Pinnularia interrupta (OI), Cocconeis placentula (OI), cf. Cyclotella sp. (?) 1140 Pinnularia quadratarea (P), Plagiogramma staurophorum (P), Paralia sulcata (P), Gomphonema acuminatum (OI), Diploneis smithii (P), Diploneis didyma (M), Pinnularia nodosa (H) 1142 Pinnularia quadratarea (P), Diploneis didyma (M), Plagiogramma staurophorum (P), Caloneis brevis (P), Stauroneis sp. (?) Eidesvatn, core Aulacoseira distans (OI), Cymbella minuta (OI), Cyclotella radiosa (OI), Frustulia rhomboids (H), Cocconeis placentula (OI), Anomoeoneis vitrea (OI), Epithemia sorex (OI) 5315 Pinnularia lundii (OI), Tabellaria fenestrate (H), Tabellaria flocculosa (H), Cymbella minuta (OI), Stephanodiscus alpinus (OI), Cyclotella radiosa (OI), Achnanthes sp. (OI/H) 5316 Cocconeis placentula (OI), Epithemia sorex (OI), Pinnularia lundii (OI), Cyclotella radiosa (OI), Stephanodiscus alpinus (OI), Fragilaria virescens (OI) Paralia sulcata (P), Rhabdonema minutum (P), Thalassiosira sp. (P), Navicula directa (P), Chaetoceros resting stages (P) 5319 Diploneis smithii (P), Navicula litoricola (P), Paralia sulcata (P), Navicula directa (P), Rhabdonema minutum (P), Thalassiosira sp. (P) 5320 Paralia sulcata (P), Diploneis smithii (P), Rhabdonema minutum (P), Cocconeis scutellum (P), Thalassiosira sp. (P) Storemyr, core Cyclotella antiqua (OI), Tabellaria flocculosa (H), Achnanthes minutissima (OI), Fragilaria construens (OI), Epithemia sorex (OI), Cymbella cymbiformis (OI), Cymbella sinuta (OI) 1078 Cymbella minuta (OI), Epithemia sorex (OI), Cocconeis placentula (OI), Gomphonema acuminatum (OI), Tabellaria flocculosa (H), Fragilaria virescens (OI) 1080 Navicula directa (P), Fallacia pygmaea (M), Cymbella minuta (OI), Biremis ambigua (P), Tabellaria flocculosa (H), Thalassiosira sp. (P), Mastogloia smithii (M) 1083 Paralia sulcata (P), Diploneis smithii (P), Nitzschia hungarica (M), Pinnularia quadratarea (P), Cocconeis scutellum (P), Biremis ambigua (P), Fallacia pygmaea (M) 1086 Paralia sulcata (P), Pinnularia quadratarea (P), Caloneis brevis (P), Diploneis bombus (P), Rhabdonema minutum (P), Nitzschia hungarica (M), Biremis ambigua (P) Bergsvatn, core Frustulia rhomboids (H), Tabellaria flocculosa (H), Achnanthes minutissima (OI), Anomoeoneis brachysira (H), Cymbella minuta (OI), Cymbella cymbiformis (OI), Achnanthes pusilla (OI) 1642 Tabellaria flocculosa (H), Achnanthes minutissima (OI), Pinnularia interrupta (OI), Navicula radiosa (OI), Pinnularia viridis (OI) 1644 Cocconeis placentula (OI), Navicula palpebralis (P), Tabellaria flocculosa (H), Diatoma tenue (OH), Paralia sulcata (P), Mastogloia smithii (M) Paralia sulcata (P), Navicula palpebralis (P), Thalassiosira nordenskiöldi (P), Mastogloia smithii (M), Amphora cf. gracialis (P/M), Cocconeis scutellum (P) 1652 Paralia sulcata (P), Fallacia pygmaea (M), Mastogloia pumila (M), Diploneis smithii (P), Navicula palpebralis (P), with marine influence, with lacustrine influence, with lacustrine influence, with lacustrine influence 1656 Paralia sulcata (P), Amphora cf. gracialis (P/M), Caloneis brevis (P), Cocconeis scutellum (P) 1660 Diploneis didyma (M), Paralia sulcata (P), Fallacia pygmaea (M), Diploneis smithii (P), Cocconeis scutellum (P), Thalassiosira sp. (P) Tørvikvatn, core , 3135, 3140, 3145, 3150, 3155, 3159, 3165 Cyclotella radiosa (OI), Tabellaria flocculosa (H), Achnanthes minutissima (OI), Frustulia rhomboids (H), Pinnularia interrupta (OI), Cymbella minuta (OI) 3170 Cyclotella radiosa (OI), Frustulia rhomboids (H), Cymbella minuta (OI), Achnanthes minutissima (OI), Tabellaria flocculosa (H), Cyclotella sp. (?)

5 BOREAS Holocene sea-level curve from Hardangerfjorden, W Norway Thalassiosira sp. (P), Tabellaria flocculosa (H), Cyclotella radiosa (OI), Cymbella minuta (OI), Diploneis smithii (P), Diploneis didyma (M), Pinnularia quadratarea (P) 3180 Navicula rhynchocephala (OI), Cymbella minuta (OI), Tabellaria flocculosa (H), Anomoeoneis vitrea (OI), Navicula radiosa (OI), Achnanthes minutissima (OI), Frustulia rhomboids (H) 3185 Cyclotella sp. (OI), Cymbella minuta (OI), Navicula rhynchocephala (OI), Thalassiosira eccentrica (P), Tabellaria flocculosa (H), Anomoeoneis vitrea (OI), Mastogloia smithii (M) 3190 Tabellaria flocculosa (H), Cocconeis scutellum (P), Navicula directa (P), Pinnularia interrupta (OI), Thalassiosira eccentrica (P), Cocconeis placentula (OI) 3196 Cocconeis placentula (OI), Mastogloia smithii (M), Cocconeis scutellum (P), Diploneis didyma (M), Paralia sulcata (P), Thalassiosira eccentrica (P), Frustulia rhomboids (H), Navicula digitoradiata (OI), Thalassiosira nordenskiöldi (P) 3200 Diploneis smithii (P), Mastogloia smithii (M), Cocconeis scutellum (P), Thalassiosira eccentrica (P), Rhabdonema minutum (P) 3207 Rhabdonema minutum (P), Mastogloia smithii (M), Thalassiosira eccentrica (P), Fallacia pygmaea (M) Paralia sulcata (P), with lacustrine influence and lacustrine and lacustrine and lacustrine Results Høgstemyr (100 m a.s.l.) Høgstemyr (Figs 2, 3, myr = mire) is a small (5030 m), completely in-filled basin with a bedrock sill only a few metres above the assumed marine limit in the area (Hamborg 1983). The altitude of the outlet was measured several times to sea level with a digital barometer, each time yielding 100 m a.s.l., which we consider to be a reasonably good estimate. The basin was cored in order to test whether sea level could have been higher than indicated by the highest marine terraces (usually represented by glaciofluvial deltas), and to search for terrestrial plant macrofossils for 14 C-dating the earliest postglacial vegetation, i.e. thus providing a minimum age for the establishment of ice-free conditions. The original pond has been filled with lacustrine, organic sediments (gyttja) and subsequently covered by peat. The drainage area is km 2 and consists mainly of bare bedrock with patches of till and other sediments. The bathymetry was surveyed with a depthmeasure stick, which showed that the mire is less than 2 m deep, except in a small bedrock depression where it is close to 4 m deep (Fig. 3). Two Russian cores were collected from this deepest part. The corer stopped in sand and gravel that we assume was deposited before the drainage area of the basin became vegetated. The basal strata in the core consist of a 5 cm thick layer of brownish-grey silt (Fig. 3) with an LOI of about 5%. There is a gradual transition from the silt to a homogeneous gyttja above. Only freshwater diatoms were found in the basal layers (Table 1), indicating that the basin is located above the marine limit. Two samples of terrestrial plant macrofossils near the base of the core were radiocarbon-dated and yielded the same age, i.e cal. yr BP (Table 2). Tangatjørn (74.5 m a.s.l.) Tangatjørn (Fig. 4, tjørn = tarn) is a small lake surrounded by partly floating peat, from which all the cores were obtained. The basin is located 74 m a.s.l. The outlet threshold is across a flat area with some till. We conservatively postulate a maximum of 1 m erosion and use an altitude of 74.5 m a.s.l. The basin (70130 m) was surveyed with a Russian corer, before 110 mm piston cores were collected from the deepest part reached from the quagmire (412 m, Fig. 4). A small brooklet enters the basin near this core site. Below 11.4 m of peat and lacustrine gyttja we sampled 1.3 m of grey, silty sediments with very low organic content and the occurrence of a few mollusc shells. The lowermost c. 70cm also contains frequent subangular dropstones of pebbles and cobbles. The corer was stopped by large stones or bedrock. Shell fragments show that the lower silt is of marine origin (Fig. 4). A large (75 mg) shell fragment of Mytilus edulis found near the base of the core was dated to cal. yr BP (Table 2). The upper 30 cm of the marine silt is brownish grey because of a slightly higher content of organic material; the final transition to the lacustrine gyttja above is expressed as a sharp increase in the percentage LOI values (from o5 to 440%). The lowermost few centimetres of the gyttja are laminated, and diatoms indicate that the top of the laminated zone at 1136 cm represents the isolation of Tangatjørn (Table 1). Five radiocarbon dates were obtained across the isolation level (Fig. 4). The OxCalgenerated age-depth model resulting from Bayesian P_Sequence analysis of the series is shown in Fig. 5. The sedimentology suggests that the deposition changes at 1138 cm, where a so-called boundary has been added to the model. One date was rejected due to poor agreement index (Bronk Ramsey 1995). According to the modelling, the isolation, identified at 1136 cm, occurred cal. yr BP (2s), with a weighted mean age of cal. yr BP (Table 3). Eidesvatn (33 m a.s.l.) Eidesvatn (vatn = lake) is the largest lake in this study, stretching c m, and has steep underwater slopes down to a relatively flat bottom at c. 50 m water

6 92 Anders Romundset et al. BOREAS Table 2. Radiocarbon dates in this study. Core numbers 4100 refer to Russian peat cores, whereas core numbers o100 refer to piston cores. The ages have been calibrated with OxCal v4.0 (Bronk Ramsey 2001) and rounded off to the nearest 10. Core Material dated Comments Weight of submitted sample (mg) Laboratory no. 14 C age (yr BP) reservoir corrected age (38030 yr) Calibrated age (yr BP, 2s interval) Høgstemyr m a.s.l N E Twigs unidentified, Viola seed Lowermost macrofossils Poz Mosses unidentified, Dryas leaf Lowermost macrofossils Poz Tangatjørn m a.s.l N E Leaves, seeds, unidentified 18 cm above laminated sediment 6.43 Poz Leaves, seeds, unidentified 6 cm above laminated sediment Poz Leaves, seeds, unidentified Laminated sediments Poz Leaves, seeds, unidentified 7 cm below laminated sediment 7.70 Poz Twigs, unidentified 19 cm below laminated sediment Poz Mytilus edulis Glaciomarine sediments Poz Eidesvatn 33 m a.s.l N E Leaves, twigs, unidentified Disturbed stratigraphy 5.82 Poz Leaves, twigs, unidentified Disturbed stratigraphy Poz Grass, unidentified Disturbed stratigraphy Poz Piece of wood Disturbed stratigraphy Poz Betula leaves Laminated sediments 8.76 Poz Betula leaves Laminated sediments Poz Betula leaves Laminated sediments Poz Betula leaves Laminated sediments 8.40 Poz Betula leaves Laminated sediments 5.07 Poz Littorina littorea, whole shell Pre-isolation sediments Poz Nuculana pernula, one half Glaciomarine sediments Poz Macoma calcarea, one half Pre-isolation sediments Poz Shell fragments, unidentified Glaciomarine sediments Poz Shell fragments, unidentified Glaciomarine sediments Poz Shell fragments, unidentified Glaciomarine sediments 6.50 Poz Mya truncata, one half Disturbed stratigraphy Poz Betula leaves Disturbed stratigraphy Poz Betula leaves Disturbed stratigraphy Poz Storemyr 30 m a.s.l N E Leaves, unidentified Disturbed stratigraphy 7.81 Poz Leaves, unidentified Disturbed stratigraphy Poz Leaves, unidentified Disturbed stratigraphy Poz Leaves, unidentified 5 cm above laminated sediment Poz Leaves, unidentified Laminated sediments Poz Leaves, unidentified Laminated sediments 9.10 Poz Leaves, unidentified Laminated sediments 8.65 Poz Leaves, unidentified 5 cm below laminated sediment 5.99 Poz

7 BOREAS Holocene sea-level curve from Hardangerfjorden, W Norway 93 Leaves, unidentified Laminated sediments Poz Russian cores Macoma calcarea, paired shell Glaciomarine sediments Poz Littorina littorea, whole shell Glaciomarine sediments Poz Macoma calcarea fragment Glaciomarine sediments 8.32 Poz Bergsvatn m a.s.l N E Betula leaves 10 cm above laminated sediment 6.27 Poz Betula leaves Uppermost laminated sediment Poz Betula leaves, Pinus needle Uppermost laminated sediment Poz Betula leaves 10 cm below laminated sediment Poz Mya truncata, paired Pre-isolation sediments Poz Yoldiella, one half Glaciomarine sediments Poz Tørvikvatn 3 4 m a.s.l N E Betula leaves Laminated sediments 7.04 Poz Betula leaves Laminated sediments Poz Betula leaves Laminated sediments Poz Betula leaves Laminated sediments Poz Betula leaves Laminated sediments Poz Twig, unidentified Laminated sediments Poz Betula leaves Lowermost cored sediment Poz depth (Fig. 6A). The lake threshold is located 32.9 m a.s.l. and consists of till with large boulders where no or minimal erosion would be expected judging by the small outlet stream. We use an altitude of 33 m a.s.l. West of the lake, the terrain is cragged and steep. Only minor brooks enter the lake. The stratigraphy is heavily disturbed throughout most of the sediment column (Fig. 6), probably caused by subaqueous sliding. Two cores (Fig. 6B, C) from the deepest part of the lake, however, did sample about 1.5 m of apparently undisturbed sediments below the disturbed strata. The sediments in this part of the sequence consist of grey, slightly organic silt with frequent occurrence of dropstones in the lower part. The silt contains frequent intact mollusc shells and changes gradually into a 6 cm thick layer of finely laminated silty gyttja in Core 36, starting at 5319 cm depth (photo in Fig. 6C). From to 5316 cm depth, in the middle part of the laminated sequence, the diatom composition changes completely from fully marine to lacustrine (Table 1). Five samples of Betula leaves were picked from 1 cm thick levels through the laminated zone (Fig. 6C, Table 2). Bayesian analysis indicates that the isolation phase occurred cal. yr BP, with a weighted mean of cal. yr BP (Table 3). The upper boundary of the laminated zone is sharp and is overlain by coarsegrained sand interpreted as the basal part of the disturbed sequence. Four samples of mollusc shells from the lowermost dropstone-rich marine silt were dated (Table 2, Fig. 6B, C). One date was obtained from the sediment feeder Nuculana pernula, the others of unidentified shell fragments (see below). There is no apparent correlation between the disturbed sediment sequences in the different cores. The thickness of the disturbed deposits varies from a few decimetres to more than 3 m (core 36). Heavily deformed gyttja alternates with coarse sand layers, marine silt with shell fragments, gyttja clasts and terrestrial plant remains. Core 36, however, holds apparently undisturbed lacustrine gyttja in at least two cm thick segments (Fig. 6C). These could be periods of normal lacustrine sedimentation, and thus may indicate that there has been more than one sliding event. A wood fragment found in the lowermost part of the disturbed sequence in core 36 was dated to cal. yr BP (Fig. 6), i.e. much younger than the Storegga tsunami event (Bondevik et al. 1997). This discounts the possibility of a causal connection with the tsunami. The only remaining explicable process responsible for the disturbance is subaqueous sliding. Such slides are likely to have been triggered by rock falls from the steep slopes and cliffs near the lake. The apparent low lacustrine sedimentation rate in Eidesvatn can be explained by very little inflow of minerogenic matter and that organic matter has been oxidized through the deep water column. However, it is surprising that although all sediments deposited in cores

8 Anders Romundset et al Høgstemyr 100 m a.s.l Har d ang e rfjo 100 C rd e n A BOREAS 200m Man B Core site 10 m Photo 2 m 3 m Outflow D cal. yr BP (Viola seed, twigs) cal. yr BP (Dryas leaf, mosses) Diatoms L L L ttja gy t sil Loss on ignition (%) Fig. 3. A. Map of Høgstemyr at 100 m a.s.l. B. Detailed map with dotted line outlining the mire and 1 m depth contours illustrating the depression in the middle of the basin. C. Photograph of the site, view as indicated in B. Hardangerfjorden can be seen behind the pines in the background. The two dots indicate core sites. A man is encircled for scale. D. Photograph of the lower part of the Russian core and a log of the same interval. Ages are given in calibrated 14C years. For legend, see Fig and 37 between the isolation at and the mentioned date of cal. yr BP must have been eroded and re-deposited in the basin, sediments of this age have not been identified in any of the cores. Storemyr (30 m a.s.l.) Storemyr is a mire occupying the middle part of an oblong, m wide, basin (Fig. 7A). The outlet

9 BOREAS Holocene sea-level curve from Hardangerfjorden, W Norway m cal. yr BP cal. yr BP cal. yr BP (Leaves, seeds) cal. yr BP cal. yr BP (Twigs) Tangatjørn 74.5 m a.s.l Diatoms L X 1200 Outflow 1140 X M Loss on ignition (%) clay/gyttja sand silt boulder cal. yr BP (Mytilus edulis) Fig. 4. Tangatjørn at 74.5 m a.s.l. Core locations and some contours (irregular intervals) are shown on the vertical air photograph. For legend, see Fig. 6. Ages are given in calibrated 14 C years. threshold is a narrow bedrock pass where we assume minimal erosion. We use an altitude of 30 m a.s.l. for this basin, based on results of barometer measurements and the fact that the 30 m contour line on the map runs right through the basin threshold. A small stream entering the basin from the north has deposited a large alluvial fan in the basin; this part is therefore stable ground and was cultivated in the 1990s (Fig. 7A). The cores were obtained from the m wide mire in the central part of the basin. In all boreholes, we cored m of peat and gyttja above marine sediments. The marine sequence is characterized by a very low content of organic material, but the percentage LOI values reveal an increase in the uppermost few decimetres. At core site no. 146, we extracted more than 4 m of nearly massive, marine silt with the Russian corer (Fig. 7C). Dropstones are frequent below c cm. The core was investigated from the bottom upwards in order to detect mollusc shells. No shells were found in the lowermost metre of the sequence. The first occurrence of small fragments is at 1350 cm depth; intact, paired specimens become gradually more abundant from c cm upwards. Three samples from this zone were dated to the period yr BP (Table 2, Fig. 7C). The grey silt grades into a 3 4 cm thick laminated organic gyttja overlain by a homogeneous, brown gyttja. The diatom check showed that the isolation contact is located in the laminated zone (Fig. 7B). The isolation contact was dated in piston core (Fig. 7B), where it is located at 1079 cm depth (Table 1). Five radiocarbon dates were obtained on terrestrial leaves from the isolation sequence, and Bayesian modelling of the ages suggests that the basin was isolated within the period cal. yr BP, with a weighted mean age of 9790 cal. yr BP (Table 3). A characteristic bed was found 5 10 cm above the isolation contact in all cores. It consists of several thin layers fining upwards from fine sand to silt, and containing scattered plant macrofossils. The thickness and characteristics of the bed show almost no lateral variations through the cored part of the basin. The radiocarbon dates show that the bed accumulated shortly after the isolation of the lake (Table 2, Fig. 7B), and only freshwater diatom species were found, indicating no marine influence. The most likely explanation for this bed, which is unique in the basin, is that it was deposited by flooding of the in-flow brook, perhaps caused by a local slide that created a temporary dam further upstream.

10 96 Anders Romundset et al. BOREAS Bergsvatn (16.5 m a.s.l.) Today, this lake has a bedrock threshold at 16.3 m a.s.l. In the past, the outlet brook has supplied water to a mill, and, based on our observations, we believe that the outlet has been deepened (by man) by some tens of OxCal v4.05 Bronk Ramsey (2007): r:5 IntCal04 atmospheric curve (Reimer et al. 2004) R_Date (8990 ±50) R_Date (9350 ±50) R_Date (rejected) Isolation Boundary Change R_Date (9620 ±50) Diatomological isolation contact centimetres, maximum 0.5 m. We therefore use the altitude 16.5 m a.s.l. The basin is m wide and exceeds 5 m water depth only in a trough in the SW part (Fig. 8). The shallow depths allowed us to use the Russian corer for compiling detailed profiles of the lateral variations of lake sediments. The sedimentary units vary widely in thickness, but the stratigraphy in the different parts of the basin is the same. A greyish marine silt in the lower part grades upwards into an up to 4 m thick, more organic-rich, marine mud. This is overlain by a c. 10 cm thick and distinct bed of laminated gyttja, capped by 1 2 m of homogeneous lacustrine gyttja up to the lake floor. A Yoldiella shell sample from the lowermost sediments in Russian core no. 134 yielded an age of cal. yr BP (Table 2). Isolation of the basin was dated in core (Fig. 8), which is a 110 mm piston core taken at c. 15 m water depth (Fig. 8). The core contained the best-developed laminated sequence and seems to reflect a continuous sedimentation during isolation. The diatom check showed that the isolation contact is located at 1643 cm depth (Table 1). Four dates were performed on terrestrial plant remains, and Bayesian modelling indicates that the lake was isolated between 6120 and 5960 cal. yr BP, with a weighted mean age of 6040 cal. yr BP (Table 3) R_Date (9800 ±50) Modelled date (cal. yr BP) Fig. 5. The dating results from Tangatjørn, with a modelled age of the isolation event, using the P_Sequence deposition model (Bronk Ramsey 2008) in OxCal v4.0 (Bronk Ramsey 2001) for fine-grained gyttja with an estimated depositional unit granularity of 1 mm (k = 1000 m 1 ). The sedimentology suggests that the deposition changes at a depth of 1138 cm, where we have inserted a boundary in the model which allows for deposition change. This is also marked as a change in colour of the probability distribution area. One date had an agreement index o60% and was consequently rejected, but is still marked on the figure. The diatomological isolation contact is placed at a depth of 1136 cm, at which level the OxCal model produces an age estimate of cal. yr BP. Tørvikvatn (3 4 m a.s.l.) Tørvikvatn is presently located 2.5 m a.s.l. A seasonally large stream drains the lake across a threshold which was lowered about 1 m in the early 20th century for agricultural purposes (J. Drage, pers. comm. 2004). We examined the outlet during fieldwork, but were unable to estimate the lowering more precisely. The altitude of Tørvikvatn is much more sensitive for sea-level reconstruction than the basins located at higher elevations, since it was isolated during a period of slower emergence. We therefore introduce uncertainty to the report of lowering, assuming it was between 0.5 and 1.5 m, and thus that the original threshold was 3 4 m a.s.l. In the lower parts of all cores there is olive-grey, relatively organic-rich, marine mud (Fig. 9). Dating Betula leaves from the bottom of a 6 m long core from Table 3. Calendar year estimates for ages of the isolation events, rounded off to the nearest 10. The Bayesian probabilities were calculated using the P_Sequence deposition model (Bronk Ramsey 2008) in OxCal v4.0 (Bronk Ramsey 2001) for fine-grained gyttja with an estimated depositional unit granularity of 1 mm (k = 1000 m 1 ). Event Depth (cm) No. of dates used in the Bayesian model OxCal posterior range 95.4% (cal. yr BP) OxCal weighted average (m) of posterior distribution (cal. yr BP) Tangatjørn: isolation dates, 1 omitted Eidesvatn: isolation dates, 1 omitted Storemyr: isolation dates, 1 omitted Bergsvatn: isolation dates, 1 omitted Tørvikvatn: isolation dates, 1 omitted

11 BOREAS Holocene sea-level curve from Hardangerfjorden, W Norway 97 A Eidesvatn 33 m a.s.l. a b C yr BP (Leaves and twigs) D yr BP (Betula leaves) yr BP (Betula leaves) Depth (m) Distance from outlet (m) yr BP (Leaves and twigs) B gyttja/clay Gyttja Silt Organic mud Sand Gravel/pebble silt sand yr BP (Macoma calcarea) gravel yr BP (Shell fragm.) Lamination Disturbance Numerous mollusc shells yr BP (Shell fragm.) yr BP (Shell fragm.) Diatom dominance species Mixed species species Russian core Piston core gyttja/clay silt sand gravel yr BP (Grass) yr BP (Wood) yr BP (Littorina littorea) yr BP (Nuculana pernula) gyttja/clay Diatoms L L L M M M silt sand gravel yr BP yr BP yr BP yr BP yr BP (all Betula leaves) Fig. 6. A. Vertical air photograph of Eidesvatn at 33 m a.s.l., with a longitudinal depth profile from echo-sounding. Numbered core sites are marked on both photograph and profile. B D. Logs of three cores. Note that an undisturbed record of the basin isolation was found only in core , the details of which are given with the photograph in C. There is apparently no correlation between the cores other than for the lowermost, undisturbed strata. The disturbances were caused by several slides. Ages are given in calibrated 14 Cyears. the deepest site yielded the age cal. yr BP (Table 2), indicating that the thickness of Holocene marine mud is far greater in this basin than in the others, which is reasonable since this basin remained longest below sea level. Core held the best-developed sequence and was used for further analysis (Fig. 9). A massive, olive-grey marine mud found from the bottom at 3300 to 3200 cm is overlain by a more than 50 cm thick laminated mud and capped by a less than 1 m thick homogeneous, dark brown lacustrine gyttja. A few silt layers in the laminated zone, also reflected in the percentage LOI values, were most likely caused by the river flooding when entering the lake. The densely laminated zone is considerably thicker (55 cm) than in the other basins in this study. The diatom check revealed only marine species below the laminated sequence (Table 1). In the laminated sequence, there is a mixed flora of both marine and lacustrine species up to 3175 cm depth, with the exception of a diatom slide at 3180 cm, where only lacustrine species were found (Table 1). The return to a mixed flora at 3175 cm was probably caused by a storm surge some time after the highest astronomically forced tide (HAT) fell below the lake threshold. In 1990, the port in Bergen experienced a spring tide 61 cm above HAT (Tidevannstabeller 2007); similar events can be expected to have occurred

12 98 Anders Romundset et al. BOREAS A B C Cultivated area Storemyr 30 m a.s.l Outflow Basin outline m 1050 Flood deposit Isolation gyttja/clay Diatoms silt fine sand Core image Loss on ignition (%) yr BP yr BP yr BP yr BP yr BP yr BP yr BP yr BP Leaves yr BP (Littorina) yr BP (paired Macoma) Numerous paired Bathyarca, one Nuculana pernula 122 mg no shells found yr BP (Macoma fragment 8.32 mg) Flood deposit Isolation 1000 Fig. 7. A. Vertical air photograph of Storemyr 30 m a.s.l. The outline of the basin is marked with a dotted line and the main in-flow and out-flow brooks with blue arrows. B. Log of core showing both the isolation sequence and the flood deposit shortly above. C. Log of core , where we managed to penetrate almost 5 m below the isolation sequence. The ages of the lowermost occurring mollusc shells represent minimum ages for the ice-margin retreat. For legend, see Fig. 6. Ages are given in calibrated 14 C years. no shells found Frequent shells gyttja/clay silt fine sand in Tørvikbygd, although of smaller magnitude due to the more sheltered location. From the slide at 3170 cm depth and upwards, only lacustrine diatoms were found, and the isolation contact has been inferred to be at 3173 cm depth. Five dates were performed on terrestrial plant remains, and Bayesian modelling indicates that isolation of the lake took place between 1540 and 1450 cal. yr BP, with a weighted mean age of 1500 cal. yr BP or AD 450 (Table 3). Timing of the ice-margin retreat We have two sets of 14 C dates that constrain the age of the ice-margin retreat, i.e. of plant macrofossils and molluscs. The two radiocarbon dates of terrestrial plant macrofossils from Høgstemyr (Fig. 3, Table 2), both yielding cal. yr BP, provide a minimum age for the ice-margin retreat in this area (Fig. 10). They were obtained from basal lacustrine sediments located just above bedrock in the deepest part of the basin. Considering that sediments tend to be routed to deeper parts of a basin it seems likely that continuous sedimentation prevailed soon after the site became ice-free. Accordingly, these dates indicate that the area became ice-free only shortly before c cal. yr BP. We put much effort into sampling the deepest marine sediments from the isolation basins, and collected several cores in each basin in order to detect lateral stratigraphic variations. In most cases the corer was stopped by coarse sediments, which we assume were till or glacial gravel resting directly on bedrock. We dated shell fragments picked from the overlying laminated, dropstone-rich bluish silt, interpreted as a glaciomarine deposit. The high concentration of pebbles probably reflects iceberg rafting from the calving ice-front further up-fjord. The ages of the shell samples (Table 2, Fig. 10) indicate that the sedimentation rates in this part of the sequence were so rapid that all dates represent a time shortly after the site became ice-free. Most of the calibrated ages overlap within the 2s interval (Fig. 10). The spread can be due to variation in the marine reservoir age caused by meltwater influence. Some of the ages may also be slightly too old, due to dating of sediment feeders such as Nuculana and Yoldiella (Mangerud et al. 2006) and unidentified fragments, since carbonate rocks that could have yielded old carbon to the pore water are found in the area. Agreement with the dates of the terrestrial plant material from Høgstemyr (Fig. 10), however, indicates that the shell dates are reliable. By taking into account the dates conducted on terrestrial plant remains as well as mollusc shells we conclude a minimum age for ice-free conditions of about cal. yr BP. As argued above, we assume

13 BOREAS Holocene sea-level curve from Hardangerfjorden, W Norway 99 Bergsvatn 16.5 m a.s.l. Tørvikvatn 3.5 m a.s.l m Outflow m Outflow Diatoms yr BP (Betula leaves) yr BP (Betula leaves) yr BP (Betula leaves, Pinus needle) yr BP (Betula leaves) 1650 Diatoms Betula leaves cal. yr BP cal. yr BP cal. yr BP cal. yr BP cal. yr BP cal. yr BP (Twig) gyttja silt Loss on ignition (%) Fig. 8. Bergsvatn at 16.5 m a.s.l. A log is given for core from which we dated the basin isolation. For legend, see Fig. 6. Ages are given in calibrated 14 C years that the dated sediments accumulated immediately after the site became ice-free. We therefore conclude that the dates approximate the real age of when Tørvikbygd was ice-free. A radiocarbon-dated sample of terrestrial plant macrofossils collected from basal lake sediments in Skorsvatn, about 12 km further up-fjord (Fig. 1), yielded cal. yr BP (Fig. 10) (Poz-590, J. Bakke pers. comm. 2008, also cited in Helle 2008), and thus also supports our suggested minimum age of the ice-free conditions. Lohne (2005) concluded that the terminal moraine at Halsnøy (Fig. 1) was deposited during the mid- and late Younger Dryas, and that the ice front started to retreat at the very onset of the Holocene, as dated at Os just north of Hardangerfjorden (Fig. 1) (Bondevik & Mangerud 2002). In order to calculate the retreat rate between Halsnøy and Tørvikbygd it is necessary to know when the recession commenced. The accepted stratotype for the Pleistocene/Holocene (= Younger Dryas/ Holocene) boundary is now defined at a depth of m in the NGRIP ice core (Walker et al. 2008). The age at this depth was found to be yr before the year 2000 with an estimated 2s uncertainty of 99 years in the GICC05 time scale (Walker et al. 2008), corresponding to yr BP when using the conventional 14 C zero year AD However, in the present context we believe it more appropriate to use the slightly younger age of with a 1s uncertainty of 3300 gyttja 0 30 Loss on ignition (%) Fig. 9. Tørvikvatn at 3.5 m a.s.l. A log is given for the isolation sequence in core For legend, see Fig. 6. Ages are given in calibrated 14 C years. 140/ 60 yr BP, which was obtained for the Younger Dryas/Holocene transition in the lake record from Krakenes in western Norway (Fig. 1) (Gulliksen et al. 1998). There are two reasons for using this age rather than the ice-core chronology. One is that the Younger Dryas Holocene boundary from the Krakenes record was determined after calibration of the 14 C ages to the tree-ring time scale in the same way as our dates. The second is that the boundary at Krakenes gives the timing when the local glacier next to the lake melted away due to significant regional climatic warming. We consider it likely that also the ice-sheet margin in Hardangerfjorden began to withdraw at around the same time (i.e yr BP). The Krakenes lake sequence provides a better age estimate for the initial recession than the existing 14 C dates from Halsnøy (Lohne 2005) due to the large uncertainty for individual dates introduced by the plateau in the calibration curve at silt