Svend Funder

The Preboreal oscillation around the Nordic Seas: terrestrial and lacustrine responses

The occurrence of an early Preboreal climatic cooling/oscillation (PBO) in lacustrine and glacial records from northwest Europe, Iceland and Greenland is reviewed and documented. The often subtle response of the proxy records to this oscillation, in combination with its short duration, make it difficult to detect. Owing to its chronostratigraphic position between the 10 000-9900 and 9600-9500 14 C plateaux (c. 11 300-11 150 calendar yr BP) it is also difficult to 14 C date with precision. We find that the vegetation response to the PBO varies between sites and regions. In contrast to the pioneer vegetation in Iceland and southern Sweden, the expanding birch-pine forest in Germany-Denmark was more susceptible to deteriorating growing conditions. The combined lacustrine, tree-ring and glacial records imply that the PBO was characterised by cool and humid conditions throughout northwestern and central Europe. This is documented by vegetation changes, decreased aquatic production, increased soil erosion, increased 2 H and 13 C content in tree-rings, readvances or stillstands of the ice sheet in Norway and Finland, and ingression of brackish water into the Baltic. Icelandic proxy records from lake sediments and glacial moraines imply cooler conditions than during the previous Preboreal period, but not as extreme as during the Younger Dryas. Greenland records suggest that the early Preboreal was characterised by ice readvances, as an effect of cool climate and increased precipitation (in relation to the Younger Dryas). It was not until the end of the PBO that climate was warm enough to melt the land-based ice sheet. This Preboreal oscillation, found on both sides of the Nordic Seas, is interpreted as an effect of increased freshwater forcing on the thermohaline circulation in the Nordic Seas, which is implied by a simultaneous and distinct rise in the atmospheric 14 C/ 12 C ratio. A slow-down of the thermohaline circulation may tempor- arily have pushed the Polar Front further south.

Forested Arctic: Evidence from North Greenland

Shallow-water marine sediments of the Kap Kabenhavn Formation at lat 82°30′ N, Perry Land, North Greenland, contain an abundance of well-preserved remains of terrestrial vegetation and invertebrate fauna indicative of a mosaic of forest tundra environments having similarities to present conditions in Labrador. The presence of foraminifera faunas, marine molluscs, and one mammal species (supported by studies of paleomagnetism and amino acid diagenesis) suggests an age of ∼2 Ma, at the Pliocene-Pleistocene transition. At this time, the arctic tree line was located 2500 km to the north of its present position in the northwest Atlantic region, and forest tundra vegetation existed in lowland areas bordering on an Arctic Ocean without perennial sea-ice cover.

Hydrography and mollusc faunas of the Baltic and the White Sea–North Sea seaway in the Eemian

Abstract Palynologically dated mollusc and cirriped faunas from a region extending from the North Sea through the Baltic and Karelia to the Arkhangelsk region show that the hydrography of the Baltic was very different from the Holocene. For 2–2.5 ka in the Early Eemian a seaway existed from the Barents to the North Sea through Karelia, until it was severed at the present continental watershed to the north of Lake Onega. After this the Ladoga–Onega trough remained an arm of the Baltic for several millennia. The benthic faunas are comparable to the Holocene, but their boundaries were displaced much further into the Baltic. Notable differences from the Holocene are the absence or rarity of the Macoma balthica biocoenosis, and the presence of cold Portlandia -dominated biocoenoses in Karelia. In the Belt Sea and western Baltic winter sea surface temperatures and salinity were higher than now by ca. 6°C and 15‰, and the distinctly brackish top layer was missing. At the same time cold bottom water (≈2.5°C) with a tendency to anoxia characterised the Karelian arm of the Baltic. Water exchange through this area was inhibited by the constriction to the north of Lake Onega, and the basin and threshold bathymetry. Water transport through the White Sea–Baltic seaway was too sparse to play an active role in the North Atlantic surface circulation or climatic change in the region. The high salinity and temperature in the Belt Sea and western Baltic persisted throughout the Eemian, and are explained by wider and deeper passage from the North Sea to Kattegat, wider straits through Denmark, higher salinity in the North Sea, higher evaporation, as well as more dispersed fresh water supply. The advection of oceanic water into the Baltic culminated in the Early Eemian, before the Carpinus zone, and probably resulted in an oceanic climate in the Baltic region, while at the same time cold winters produced cold bottom water in the Karelian arm.

Holocene boreal molluscs in Greenland — palaeoceanographic implications

Abstract Boreal molluscs, now extinct in Greenland waters, occur as subfossils in West Greenland between lat. 65°30′ and 68°30′N. A number of new localities for these warmth demanding molluscs are described, and the list of species now comprises the gastropod Emarqinula fissura and the bivalvalves Heteranomia squamula, Modiolus, Arctica islandica, Panopea norvegica, and Zirphaea crispata. During their time the molluscs formed an isolated northern outpost, located in the area which — as a result of the complex oceanic circulation — today has Greenlands highest ocean surface temperatures. The occurrences are 14C dated to the interval 8400–4900 yr B.P., and summer surface temperatures were 1–3°C higher than now. For its beginning this period coincides with similar “marine optimal periods” in East Greenland and on the Baffin Island shelf, whereas it took 1000 years before the beginning of warmer than present temperatures on land. On the basis of this evidence it is concluded that the rise of sea surface temperatures was afforded by a decrease in inflow of cold polar water and/or increase in inflow of warm atlantic water into Davis Strait, rather than climatic change.

Isotope stage 5 (130-74 ka) in Greenland, a review

Abstract Marine and glacigene deposits, dated with varying degrees of certainty to isotope stage 5 (130-74 ka) are exposed in coastal cliffs in 18 areas and localities in northern and eastern Greenland. Dating methods include thermoluminescence (TL), U-Th methods and correlation with the deep sea record. Amino-acid parameters are used for correlation between localities. Deposits from the last interglaciation (isotope substage 5e) are rare and have been found only at three localities in both West and East Greenland. Their mollusc and foraminifer faunas indicate more vigorous sea surface circulation and sea surface temperatures some degrees higher than known from the Holocene. During this warm period ice sheet growth began. In Northwest Greenland the ice sheet reached its maximum for the last ice age during the succeeding phase, provisionally correlated with isotope substage 5d. This was followed by a new warm period with increased influx of warm subarctic water, and high summer temperatures on land, provisionally correlated with isotope substage 5a. In East Greenland the observations indicate a more complex ice sheet history. The record from Greenland is very similar to that obtained in adjacent coastal areas of the high Arctic implying that the rapid shifts from warm to cold during isotope stage 5 were especially conducive to the formation of the embryonic North American and Scandinavian ice sheets at high northern latitudes, probably because the pulses of warm water were required to advect sufficient moisture into these areas to allow the growth of continental ice sheets.