Jan Mangerud

Apparent radiocarbon ages of recent marine shells from Norway, Spitsbergen, and Arctic Canada

Abstract The mean apparent radiocarbon ages of marine shells, colleted alive before the initiation of atomic bomb testing, and also before the main input of dead carbon derived from fossil fuels, are found to be 440 yr for the coast of Norway, 510 yr for Spitsbergen, and 750 yr for Ellesmere Island, Arctic Canada. The relationship between these apparent ages and the oceanic circulation pattern, is discussed. Also possible variations of the apparent ages back in time are discussed.

The North Atlantic atmosphere-sea surface 14C gradient during the Younger Dryas climatic event

Abstract We attempt to quantify the 14C difference between the atmosphere and the North Atlantic surface during a prominent climatic period of the last deglaciation, the Younger Dryas event (YD). Our working hypothesis is that the North Atlantic may have experienced a measurable change in 14C reservoir age due to large changes of the polar front position and variations in the mode and rate of North Atlantic Deep Water (NADW) production. We dated contemporaneous samples of terrestrial plant remains and sea surface carbonates in order to evaluate the past atmosphere-sea surface 14C gradient. We selected terrestrial vegetal macrofossils and planktonic foraminifera (Neogloboquadrina pachyderma left coiling) mixed with the same volcanic tephra (the Vedde Ash Bed) which occurred during the YD and which can be recognized in North European lake sediments and North Atlantic deep-sea sediments. Based on AMS ages from two Norwegian sites, we obtained about 10,300 yr BP for the ‘atmospheric’ 14C age of the volcanic eruption. Foraminifera from four North Atlantic deep-sea cores selected for their high sedimentation rates ( > 10 cm kyr−1) were dated by AMS (21 samples). For each core the raw 14C ages assigned to the ash layer peak is significantly older than the 14C age obtained on land. Part of this discrepancy is due to bioturbation, which is shown by numerical modelling. Nevertheless, after correction of a bioturbation bias, the mean 14C age obtained on the planktonic foraminifera is still about 11,000–11,100 yr BP. The atmosphere-sea surface 14C difference was roughly 700–800 yr during the YD, whereas today it is 400–500 yr. A reduced advection of surface waters to the North Atlantic and the presence of sea ice are identified as potential causes of the high 14C reservoir age during the YD.

A Younger Dryas Ash Bed in western Norway, and its possible correlations with tephra in cores from the Norwegian Sea and the North Atlantic

Abstract A bed of volcanic ash up to 23 cm thick is found in lacustrine and marine sediments in western Norway. It is formally mamed the Vedde Ash Bed, and its age is approximately 10,600 yr B.P., i.e., mid-Younger Dryas. The bed consits of pure glass having a bimodal basaltic and rhyolitic somposition. The geochemistry of the glass shards suggests an Icelandic source. By means of stratigraphic position and geochemistry, the ash is correlated with ash zones found in cores from the continental shelf, the Norwegian Sea, and the North Atlatic.

FLUCTUATIONS OF THE SVALBARD–BARENTS SEA ICE SHEET DURING THE LAST 150 000 YEARS

Abstract On Spitsbergen, western Svalbard, three major glacial advances have been identified during the Weichselian. All three reached the continental shelf west of the Svalbard archipelago. Radiocarbon, luminescence and amino acid dating of interbedded interstadial and interglacial sediments indicate that these glacial advances have Early (Isotope Stage 5d), Middle (Stage 4), and Late Weichselian ages (Stage 2). An additional, more local, advance has been dated to Isotope Stage 5b. The Late Weichselian ice sheet expanded across the entire Barents Sea. However, in the south-western Barents Sea, the Late Weichselian till is the only till above Eemian sediments, indicating that the Early- and Middle Weichselian ice advances were restricted to the Svalbard archipelago and the northern Barents Sea. A major problem with the onshore sites is the dating of events beyond the range of the radiocarbon method. To overcome this, the onshore record has been correlated with marine cores from the continental slope and the deep-sea west of Svalbard, where a chronology has been established by oxygen isotope stratigraphy. Ice rafted detritus (IRD) was used as the main monitor of glaciation. The IRD record closely mirrors the glaciation history as interpreted from the onshore sections. During the Late Weichselian, the largest IRD peak occurred during deglaciation, a pattern also postulated for the earlier events. Given this, the results from the marine cores indicate that the ages for the first glacial advances during the Weichselian were a few thousand years older than interpreted from the onshore stratigraphy.

Aminostratigraphy of European marine interglacial deposits

Molluscan fossils collected from shallow water marine sediment across NW Europe and nearby Arctic regions have been analysed for the extent of isoleucine epimerization (DL ratio) in indigenous protein residues. The DL ratios confirm that essentially all ‘classical’ Eemian sites from NW Europe are of the same age, and are correlative with the type locality near Amersfoort in the Netherlands; shells from interglacial marine sediment beneath the type Weichselian till in Poland also correlate with the type Eemian site. DL ratios in Holsteinian marine shells (0.29) are substantially higher than in their Eemian counterparts (0.17); ‘Late Cromerian’ shells yield even higher ratios (0.46). DL ratios in late glacial shells (0.06) and Middle Weichselian shells (0.09) permit differentiation from modern (0.01) and last interglacial material. Based on the position of the Matuyama-Brunhes boundary and the differences in DL ratios, the Eemian must correlate with isotope substage 5e, whereas the Holsteinian is most likely substage 7c, possibly stage 9 but certainly younger than stage 11. Intra-Saalian warm periods may be terrestrial equivalents of the younger substages of stage 7. Extensive pre-Eemian marine sediments along the SW coast of Denmark previously correlated with the Holsteinian are shown to be of ‘Late Cromerian’ age. The underlying till there is the first widespread evidence of a pre-Elsterian till in NW Europe. DL ratios in molluscs from last interglacial sites along the Arctic coast of the USSR, the Arctic Islands and eastern Greenland are substantially lower than in their European counterparts due to their low thermal histories. The combined mid- and high-latitude data are used to develop a predictive model for the expected DL ratio in any of several moderate epimerization-rate taxa for last interglacial sites with mean temperatures between −20 and +15°C. Not all sites could be unambiguously assigned to an established interglacial. The Fjosanger (Norway) and Margareteberg (Sweden) sites previously thought to be Eemian, yield DL ratios higher than in secure nearby Eemian material. It is yet unresolved whether these are aberrant sites or if they predate the last interglacial. In situ shoreline deposits encountered in borings in SW Belgium and in exposures on the Belgium coastal plain contain molluscs that yield DL ratios intermediate between secure Eemian and Late Weichselian ratios, raising the possibility that a late stage 5 high-sea-level event attained near-modern levels in the southern North Sea basin. Resolution of these uncertainties is the focus of future work.

Changes in North Atlantic Radiocarbon Reservoir Ages During the Allerød and Younger Dryas

Estimates of the radiocarbon age of seawater are required in correlations between marine and terrestrial records of the late Quaternary climate. We radiocarbon-dated marine shells and terrestrial plant remains deposited in two bays on Norways west coast between 11,000 and 14,000 years ago, a time of large and abrupt climatic changes that included the Younger Dryas (YD) cold episode. The radiocarbon age difference between the shells and the plants showed that sea surface reservoir ages increased from 400 to 600 years in the early YD, stabilized for 900 years, and dropped by 300 years within a century across the YD-Holocene transition.

The last glacial maximum on Spitsbergen, Svalbard

Most scientists have concluded previously that the west coast of Spitsbergen, Svalbard, remained ice-free during the late Weichselian, between 25,000 and 10,000 yr B.P. We conclude that the glaciation was more extensive. Terraces that were postulated to have been ice-free are covered by a thin, late Weichselian till. Sudden drop in the marine limit and basal radiocarbon dates of raised glaciomarine sediments demonstrates that the glaciers in the main fjords, Isfjorden and Van Mijenfjorden, terminated west (outside) of the fjord mouths. Basal radiocarbon dates from glaciomarine clay above till in cores from the continental shelf west of Spitsbergen yielded ages of about 12,500 yr B.P., from which we conclude that the ice extended to the shelf edge. Based on the extent of amino acid diagenesis in radiocarbon-dated molluscs, the duration of the maximum extension of the late Weichselian glaciation was short, certainly less than 10,000 years. During the ice-free period preceding that glaciation, at least back to 40,000 yr B.P., the glaciers on Svalbard were not significantly larger than at present, as shown by marine deposits close to the glacier snouts. Many radiocarbon dates place deglaciation of the outer coast at about 12,500 yr B.P. At about 10,000 yr B.P., the rest of the archipelago rapidly became ice-free.

A calendar age estimate of the Younger Dryas-Holocene boundary at Kråkenes, western Norway

The Younger Dryas/Holocene transition (YD/H) in the sediments of Kråkenes Lake, western Nor way, is well marked both lithologically and palaeobiologically at 756.5 cm in the investigated core. A series of 70 AMS radiocarbon dates on terrestrial plant macrofossils and the NaOH-soluble fraction of lake sediment was measured between 585 and 840 cm, covering the time span c. 10 440 to 7915 BP on the radiocarbon timescale. Forty-three of these dates above 760 cm were wiggle-matched against the German oak-pine dendro calibration curve (IntCal 93) with recent corrections in both the oak and the pine sections. With an increase in age of the pine dendro-series of 200 6 20 yr, the calendar age of the YD/H lithostratigraphic boundary at Kråkenes is estimated to 11 530+40 -60 cal. BP. By using a date of 9750 BP (11 170 cal. BP) on the transition between the 10 000 and 9600 14C plateaux as a time marker, this result is compared with recent results from other archives. It is consistent with many of them, including the GRIP ice core, German pine series, Lake Gościaz, south Swedish lakes, and Baltic varves, suggesting that the Younger Dryas-Holocene transition in the North Atlantic region occurred within the range 11 500–11 600 cal. BP.

Late Cenozoic history of the Scandinavian and Barents Sea ice sheets

Abstract The oldest ice rafted material (IRD) on the Voring Plateau is dated to about 11 Ma, and is regarded as evidence that glaciers extended to sea level somewhere around the Nordic Sea at this time. We estimate that the major glaciations of Scandinavia and the Barents Sea-Svalbard area started at 2.5–2.8 Ma, when the amount of IRD on the Voring Plateau increased strongly, the deep sea δ18O curves indicate that onset of Northern Hemisphere glaciations, and the climate in The Netherlands was so cold that a major glaciation of Scandinavia is inferred. Most of the time until 0.9 Ma, the ice sheets were of intermediate size; they probably reached the coastal zone of western Norway for long periods. The center of glaciation is inferred to have been further north than during the later glaciations, and we speculate that this was the period of maximum glacial erosion of the Barents Sea. The largest glaciations, and also the warmest interglacials occurred during the last 900 kyrs, when the 100 kyr astronomic cycle became important. For the last glaciation, the Weichselian, the glacial fluctuations are known in greater detail both for Scandinavia and the Svalbard-Barents Sea region. In Scandinavia the glacial fluctuations apparently followed the 23 kyr precession cycle, whereas in the Barents Sea they followed the 41 kyr tilt cycle. In both areas more than one advance reached beyond the coast. We use the Weichselian record to “calibrate” the interpretation of more indirect evidences of glacial fluctuations, and apply the latter to the Scandinavian and Barents Sea glacial history since 2.5 Ma.

A 9000-Year-old Ash Bed on the Faroe Islands

Abstract Radiocarbon datings of the Saksunarvatn ash bed on the Faroe Islands indicate an age of 9000–9100 yr B.P. The Saksunarvatn ash bed differs geochemically from both ash zone 1 in the North Atlantic and the Vedde Ash Bed of Norway and the Norwegian Sea. All mentioned ashes are assumed to originate from Iceland. The Vedde Ash has been dated at 10,600 ± 60 yr B.P. Consequently, the Saksunarvatn and Vedde ash beds provide an opportunity for precise dating of events around the Pleistocene/Holocene boundary in marine cores, especially from the region where the two plumes overlap.