Rapid Communication The marine 14 C age of the Vedde Ash Bed along the west coast of Norway

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1 JOURNAL OF QUATERNARY SCIENCE (2001) 16 (1) 3 7 Copyright 2001 John Wiley & Sons, Ltd. Rapid Communication The marine 14 C age of the Vedde Ash Bed along the west coast of Norway STEIN BONDEVIK 1, *, JAN MANGERUD 2 and STEINAR GULLIKSEN 3 1 Department of Geology, University of Tromsø, Dramsveien 201, N-9037 Tromsø, Norway 2 Department of Geology, University of Bergen, Allégaten 41, N-5007 Bergen, Norway 3 Radiological Dating Laboratory, NTNU, Sem S landsvei 5, N-7034 Trondheim, Norway Bondevik, S., Mangerud, J. and Gulliksen, G The marine 14 C age of the Vedde Ash Bed along the west coast of Norway. J. Quaternary Sci., Vol. 16, pp ISSN Received 14 April 2000; Revised 24 September 2000; Accepted 4 October 2000 ABSTRACT: Articulated molluscs, sea urchins and barnacle fragments close to the Vedde Ash Bed in a shallow marine deposit on the west coast of Norway have been 14 C dated. The weighted mean of four dates from a sediment slice 8 cm thick centred on the Vedde Ash Bed is C yr BP. The most accurate 14 C age of the Vedde Ash from terrestrial plant macrofossils is yr BP. The difference is the 14 C reservoir age for coastal water at the west coast of Norway during the mid-younger Dryas and equals yr. This is 230 yr older than the reservoir age for the Bølling/Allerød and for the present day in this area. The result supports earlier conclusions of a higher reservoir age for the Younger Dryas in the North Atlantic and Nordic Seas, although our reservoir age of yr is a few hundred years younger. This suggests that the 14 C reservoir age at Vedde Ash time may increase from coastal water towards the open North Atlantic and Nordic Seas. Copyright 2001 John Wiley & Sons, Ltd. KEYWORDS: Vedde Ash; marine 14 C reservoir age; shallow marine deposit; western Norway; chronology. Introduction The marine 14 C reservoir age is the difference between 14 C ages of samples grown contemporaneously in the atmosphere and in the sea (Stuiver et al., 1986). Today the inflow of ventilated Atlantic surface water to the North Sea and Norwegian Sea results in a 14 C reservoir age of yr along the coast of western Norway (Mangerud and Gulliksen, 1975; Bondevik et al., 1999). Bondevik et al. (1999) showed, by dating paired samples of terrestrial plant fragments and marine molluscs from a shallow marine deposit (Kulturmyra, Fig. 1), that the reservoir age during the Bølling/Allerød ( C yr BP) along the Norwegian coast was yr and thus not different to present-day values. However, this sequence did not contain any marine shells from the Younger Dryas. Here we report 14 C dates of articulated shells and other marine macrofossils adjacent to the Vedde Ash Bed from Kvaltjern, * Correspondence to: Stein Bondevik, Department of Geology, University of Tromsø, Dramsveien 201, N-9037 Tromsø, Norway. Stein ibg.uit.no Contract grant sponsor: Research Council of Norway located on the outer coast of western Norway, ca. 20 km west of Bergen (Fig. 1). Several studies have used the Vedde Ash (Mangerud et al., 1984) to estimate the reservoir age for the mid-younger Dryas in the North Atlantic and Nordic Seas (Bard et al., 1994; Austin et al., 1995; Sveinbjörnsdottir et al., 1998; Haflidason et al., 2000). The method has been to compare the 14 C age of terrestrial fossils from the ash bed in lakes with the 14 C age of marine shells or foraminifers from the ash zone in the open ocean. Based on these dates it has been concluded that the reservoir age was greater by yr during the Younger Dryas in the North Atlantic. In this paper we have used the same method. Setting and methods The studied site Kvaltjern, is an 8 10 m deep basin, located on the outer coast of western Norway (Fig. 1). It is filled with marine, lacustrine and peat deposits in stratigraphical order. The threshold of the basin was levelled to 17.8 m a.s.l. in 1975 (Indrelid et al., 1976) and at that time Kvaltjern was a small lake (tjern = small lake). In the 1980s a new

2 4 JOURNAL OF QUATERNARY SCIENCE Figure 1 Kvaltjern is located on the outer coast of western Norway facing the open North Sea, approximately 20 km west of Bergen. The basin was a shallow depression on the sea floor from the time of deglaciation ca C yr BP, until ca C yr when it emerged above sea-level (Krzywinski and Stabell, 1984). At the time the Vedde Ash was deposited, relative sea-level was close to the marine limit, ca. 32 m above present sea-level. During the Younger Dryas there was an open connection between the North Sea and Hjeltefjorden through the narrow passage across Kvaltjern. road was built across the outlet, the lake drained, and the easternmost part of the basin damaged by landfill. The stratigraphy in the western and deepest part of the basin is still preserved. A Russian peat corer was used to map the lithostratigraphy across the basin. Based on this mapping a site was selected for piston coring, providing 2-m-long core sections of diameter 11 cm. To make sure we obtained enough material for dating close to the Vedde Ash Bed, three parallel cores were collected less than 1.2 m apart. The cores were X-ray photographed for examination of sedimentary structures and as an aid to locating articulated shells. The cores were easily correlated by matching sedimentary structures and the Vedde Ash Bed. For loss-on-ignition the samples were dried at 105 C for 24 h and ignited at 550 C for 2 h. Weight loss was calculated as a percentage of the dried sample weight. Both 18 O and 13 C were measured using a Finnigan MAT 251 mass spectrometer. See Table 1 for information about the 14 C measurements. Results The Vedde Ash and the Late Weichselian deposits in Kvaltjern The marine deposits found in the lower part of the basin are similar to deposits described from many other basins in this area (e.g. Mangerud, 1977). At the base ( cm) is a pebbly, grey silty sand deposited immediately after deglaciation (Fig. 2). This is overlain ( cm) by olive-grey organic silt with many fragments of Mytilus edulis, typical of the Allerød. In contrast, the overlying Younger Dryas deposits ( cm) have a pure grey colour with numerous matrix-supported pebbles, and also contain a colder water mollusc fauna, without e.g. Mytilus shells. The transition to the Holocene is marked by the return of the olive grey (later brownish grey) colour to the deposits, disappearance of pebbles, and reappearance of Mytilus shells. The transition to brown lacustrine gyttja is sharp at ca. 670 cm. According to the sea-level curve (Krzywinski and Stabell, 1984) Kvaltjern was isolated from the sea ca C yr ago. The Vedde Ash is present as a clear, visible, bed 1 2 cm thick (Fig. 3), at cm (Fig. 4). Counts of glass shards show a well-defined peak with more than rhyolitic and basaltic shards per cubic centimetre at 834 cm, followed by a tail, with decreasing amounts upwards, as described previously in both marine and lacustrine sequences (Mangerud et al., 1984; Birks et al., 1996). The sharp lower boundary of the ash bed indicates very little bioturbation. Between 860 and 825 cm there are many fine laminae of sea weed. From the Vedde Ash and upwards to 770 cm the deposit is filled with barnacle fragments, and mollusc species including Astarte borealis, Hiatella arctica, Chlamys islandica and the sea urchin Strongylocentrotus droebachiensis are frequent (Table 1). The upper boundary at 770 cm is clearly erosive. The marine 14 C age of the Vedde Ash at Kvaltjern Four of the samples dated are 5 cm from the Vedde Ash Bed. Three of these are AMS dates from 2 to 4 cm above the lower boundary of the Vedde Ash and show statistically identical ages: yr BP, yr BP and yr BP (Table 1). A conventional date on an

3 MARINE 14 C AGE OF THE VEDDE ASH 5 Table 1 Radiocarbon ages, carbon and oxygen isotopes of marine macrofossils from the Vedde Ash Bed in cores , and from Kvaltjern and from Lysevågvatnet Laboratory Core/depth Depth relative Material dated 14 C age 13 C 18 O number a (cm) to Vedde Ash (yr BP) c ( PDB) ( PDB) (cm) b Beta / Many parts of ± Strongylocentrotus droebachiensis, largest part is 3 4 cm in diameter. Spicules are missing T / Many barnacle fragments, ± possibly Balanus balanus g submitted to laboratory TUa /831 2 Articulated Astarte borealis, ± well preserved periostracum TUa / (2 4) Most/all parts of a ± Strongylocentrotus droebachiensis, including spicules, tooth and plates TUa / ( ) Barnacle fragment, possibly ± Not Balanus balanus, ca. 1 cm measured 2cm T / (14 21) One valve of Chlamys ± islandica, ca. 3 cm 5 cm. Seven grams submitted to laboratory Beta / Articulated Astarte borealis, ± well preserved periostracum a The T-samples are conventionally dated samples at the Radiological Dating Laboratory in Trondheim. The TUa-samples were prepared and targets produced at the Radiological Dating Laboratory in Trondheim and the measurements performed at the Svedberg Laboratory, Uppsala University, Sweden. The Beta-samples were measured at Beta Analytic Inc. b Depth is relative to the lower boundary of the Vedde Ash bed. + is below and is above. c Ages are conventional 14 C ages, not corrected for reservoir age. assemblage of broken barnacle fragments, 0 4 cm below the lower boundary, yielded yr BP (T-14492). The weighted mean of these four dates is yr BP. These dates were obtained from a sediment slice 8 cm thick centred on the Vedde Ash (Table 1). Assuming a constant sedimentation rate during the Younger Dryas, these 8 cm accumulated in ca. 80 yr. The maximum difference between the four dates is yr, i.e. of the same order as the calculated period of sedimentation. We conclude that yr BP is an accurate estimate of the 14 C age of marine organisms at this site at the Vedde Ash time. Also the other radiocarbon dates are compatible with this result. For example, the 14 C age for the Strongylocentrotus droebachiensis, cm below the lower boundary of the Vedde Ash, is well inside the expected range, and the dates above fit with the estimated sedimentation rates (Fig. 4 and Table 1). Discussion Determining a 14 C age for the Vedde Ash in the open ocean can be problematic because of bioturbation, low sedimentation rates, redeposition of foraminifers used for dating, and ice rafting of glass shards (Austin et al., 1995). After considering the effect of bioturbation, Bard et al. (1994) concluded that a mean marine 14 C age for the Vedde Ash is about yr BP for deep-sea cores from the North Atlantic. A similar age was obtained from the Norwegian Channel (11100 yr BP) (Haflidason et al., 1995). These estimates are yr older than ours. However, from a core on the Hebridean shelf, showing extremely high sedimentation rates (500 cm representing the Younger Dryas), Austin et al. (1995) concluded from an age depth model based on 14 C dated molluscs, that the earliest occurrence of Vedde Ash shards dates to yr BP, close to our estimate of yr BP. A terrestrial 14 C age of the Vedde Ash eruption has been obtained from many sites (Mangerud et al., 1984; Bard et al., 1994; Birks et al., 1996; Wastegard et al., 1998). Both Bard et al. (1994) and Birks et al. (1996) conclude that the age is ca yr BP, and the weighted mean from lake Krakenes, at yr BP (Birks et al., 1996), is considered here to be the most accurate age. This is further supported by dates from Madtjärn where the weighted mean of four dates is yr BP (Wastegard et al., 1998), and from the GRIP ice-core, where the Vedde Ash is ice-core years (Grönvold et al., 1995), equivalent to ca C yr BP (Stuiver et al., 1998).

4 6 JOURNAL OF QUATERNARY SCIENCE The difference between the marine age at Kvaltjern ( yr BP) and the terrestrial age at Kråkenes ( yr BP) yields yr as the reservoir age along the west coast of Norway at the time of the Vedde Ash. This is 230 yr greater than the present reservoir age and the reservoir age during the Bølling/Allerød (Bondevik et al., 1999) in this area. From a 14 C date of the Vedde Ash in the Troll core in the Norwegian Channel, Haflidason et al. (1995) obtained a reservoir age of 800 yr. We would expect that the reservoir age in the Norwegian Channel should be equal to the open Norwegian coast, especially as the Troll core is situated only ca. 70 km from Kvaltjern. The only explanantion we can suggest, if the difference is real, is water stratification; Haflidason et al. (1995) dated benthic foraminifers from a water depth of 300 m and the shells from Kvaltjern must have lived in a water depth of less than m. Possibly the inflow of Atlantic water was shallow, and the deeper part of the Norwegian Channel is influenced by water from the Norwegian Sea having a higher reservoir age, as indicated by the results from the open North Atlantic and Nordic Seas (Bard et al., 1994; Haflidason et al., 2000). During the Younger Dryas there was a reduced convection of surface waters to the North Atlantic and an increase of the sea-ice cover. Both these mechanisms contribute to higher reservoir ages (Bard et al., 1994). Numerical models suggest that the cover of sea-ice is the factor that contributes most to higher reservoir ages during the Younger Dryas (Stocker and Wright, 1996). In the northern North Sea a change from ice-free conditions during Bølling Allerød to as much as 7 months yr 1 of sea-ice cover at Vedde Ash time has been indicated (Rochon et al., 1998). The exchange of carbon between the atmosphere and the ocean is likely to be better along the Norwegian coast because of longer periods of seasonal open waters than farther west (Koç et al., 1993). This might explain why the reservoir age at Vedde Ash time seems to increase from the Norwegian coast towards the Norwegian Greenland Sea. Figure 2 Sedimentology and loss-on-ignition for cores , -23 and -25 from Kvaltjern. Conclusion The marine reservoir age for marine shells close to the Vedde Ash in a shallow marine basin at the west coast of Norway is yr. The setting of the locality is similar to most uplifted marine deposits of Younger Dryas age in Norway. Therefore, we consider yr as a useful estimate for the marine reservoir age correction of 14 C dates of Younger Dryas age along the Norwegian coast. However, variations along the coast must be expected, even larger than those observed for recent material (Mangerud and Gulliksen, 1975). Figure 4 Number of glass shards (Vedde Ash) and radiocarbon dates of marine macrofossils close to the Vedde Ash. The weighted mean of the four dates closest to the Vedde Ash are indicated by dashed lines: * indicates sample TUa-2049 from Lysevågvatnet, a basin similar to Kvaltjern, located ca. 30 km to the southwest. Acknowledgements ystein Lohne was a great help coring the basins. A new hydraulic device for operating the piston corer constructed by Dagfinn Bondevik and Roald Dyrdal, facilitated the coring operation. Mary Raste made the X-ray images of the cores that were used for locating the macrofossils. Tove Midtun drew the diagrams, Edel Ellingsen provided the loss-on-ignition data, and Rune Søraas made the 13 C/ 12 C and 18 O/ 16 O measurements. The work is a contribution to the Strategic University Programme at the University of Bergen, Rapid Sedimentation and High Resolution Stratigraphy: Processes and Application, funded by the Research Council of Norway. We are also grateful to Stefan Wastegård and an anonymous reviewer for comments that improved the manuscript, and to Geoffrey Corner who corrected the English.

5 Figure 3. The Vedde Ash appears as a greyish black bed 1 to 2 cm thick in the shallow marine deposits in Kvaltjern. Note the presence of marine macrofossils close to the ash bed. Here from a Russian peat corer sampled July 1999, downcore down ( photograph by Stein Bondevik). Copyright 2001 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 16 (2001)

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