Morten Hald

Enhanced Modern Heat Transfer to the Arctic by Warm Atlantic Water

Water flow from the Atlantic Ocean into the Arctic through the Fram Strait is warmer than at any time in the past 2000 years. The Arctic is responding more rapidly to global warming than most other areas on our planet. Northward-flowing Atlantic Water is the major means of heat advection toward the Arctic and strongly affects the sea ice distribution. Records of its natural variability are critical for the understanding of feedback mechanisms and the future of the Arctic climate system, but continuous historical records reach back only ~150 years. Here, we present a multidecadal-scale record of ocean temperature variations during the past 2000 years, derived from marine sediments off Western Svalbard (79°N). We find that early–21st-century temperatures of Atlantic Water entering the Arctic Ocean are unprecedented over the past 2000 years and are presumably linked to the Arctic amplification of global warming.

Modelling the Eurasian ice sheet through a full (Weichselian) glacial cycle

Recently acquired glacial geological and oceanographic datasets provide information on the Weichselian glaciations of Scandinavia and the Eurasian Arctic. A numerical ice-sheet model, forced by global sea level and solar insolation changes, was run to reconstruct ice sheets compatible with these data. A ‘maximum’ reconstruction assumes that the modern-type temperature distribution across the Eurasian Arctic is reduced by 10 8C at three stages during the Weichselian, which are related to minimum levels of solar insolation. Conversely, a ‘minimum’ model incorporates a reduction in temperature of only 5 8C in Early and Middle Weichselian time. The ‘maximum’ reconstruction employs the relatively larger sea-level fall suggested by the d 18 O deep-sea record, while the ‘minimum’ run uses the more conservative sea-level estimate from New Guinea coral reef terraces. The maximum model predicts three major glacial advances in the Weichselian. These compare well to geological evidence for ice-sheet growth during the Early, Middle and Late Weichselian. Geological evidence for the Late Weichselian ice sheet is compatible with either reconstruction if ice growth across the Taymyr Peninsula is curtailed. The models show that ice-sheet advance caused by the interaction of sea level and solar insolation changes yields a time-dependent ice volume function similar to that established from the geological record. Periods of seasonally open water within the seas bordering the Eurasian Arctic generally occur prior to glaciation, and may provide a source of precipitation for ice-sheet growth. In contrast, periods of ice-rafted debris deposition and depletion in surface-ocean d 18 O in sea-floor sediments compare well with the model’s determination of ice-sheet decay and melting. q 2001 Elsevier Science B.V. All rights reserved.

BENTHIC FORAMINIFERAL ASSEMBLAGES FROM THE SOUTHERN KARA SEA, A RIVER-INFLUENCED ARCTIC MARINE ENVIRONMENT

Calcareous foraminifers and hydrographic parameters in 113 bottom samples from the southern Kara Sea were examined to improve the usage of foraminifers as paleoenvironmental proxies for river-dominated high-latitude continental shelves. Foraminiferal taxa form a succession from near-estuarine to distal open-sea locations, characterized by a gradual increase in salinities. Foraminiferal assemblages are discriminated into three groups: river-proximal, -intermediate, and -distal. This succession appears to be controlled by a combination of feeding conditions and bottom salinities, and are related to riverine fluxes of freshwater, organic matter, and sediments. Morphological and behavioral adaptations of foraminifers to specific environments are discussed.

SEASONAL DYNAMICS OF BENTHIC FORAMINIFERA IN A GLACIALLY FED FJORD OF SVALBARD, EUROPEAN ARCTIC

Seasonal variations in benthic foraminiferal populations from a sediment-laden fjord were analyzed in order to provide insights into arctic foraminiferal ecology and to improve the interpretation of the late-glacial record. The fjord is 25 km long and 100 m deep with a large tidewater glacier at the fjord head. A pilot transect of eight stations sampled in August 1995 revealed the typical off-glacier sequence of foraminiferal taxa. Unidentified allogromiids were abundant in the vicinity of the ice front. Further down the fjord Elphidium excavatum f. clavatum and Cassidulina reniforme co-dominated the glacier-proximal fauna. Nonionellina labradorica and Islandiella norcrossi characterized the glacier-distal setting. In 1996 three glacier-proximal stations were then sampled in March, May, July, August, September and November. Compared to the summer of 1995 the summer of 1996 was colder, resulting in weaker glacial meltwater discharge, and the foraminiferal fauna became less influenced by glaciers. This is portrayed by an increase in glacier-distal N. labradorica and a decrease in glacier-proximal C. reniforme and especially E. excavatum. Taxonomic diversity was higher in winter, possibly reflecting a more stable environment in the absence of the turbid meltwater plume, the source of ecological stress.

MODERN BENTHIC FORAMINIFERA OFF NOVAYA ZEMLYA TIDEWATER GLACIERS, RUSSIAN ARCTIC

Living and dead foraminifera have been investigated along three transects away from tidewater glaciers of northern Novaya Zemlya. There are three glacier-proximal dominant foraminiferal taxa: Allog...

Rapid climatic shifts during isotope stages 2–4 in the Polar North Atlantic

Sediment cores from the Polar North Atlantic provide evidence of six periods of sea-ice breakup during isotope stages 4, 3, and 2, probably caused by inflow of North Atlantic surface water into the Polar North Atlantic. These periods are characterized by having a high number of foraminifera/g; they last from 2000 to >10000 yr, and constitute ≈ 50% of the total time span. These periods of sea-ice breakup correlate temporarily to the Heinrich events and the early temperature maximum in the Bond cycles of the North Atlantic and Greenland ice record. Our hypothesis is that massive iceberg discharges that flooded the North Atlantic during each Heinrich event probably triggered an oceanographic regime that gave a much more vigorous surface circulation pattern in the Polar North Atlantic, which contributed to the breakup of the sea-ice cover. Open-water conditions in the Polar North Atlantic are inversely related to terrestrial interstadials of coastal Norway, suggesting ice-sheet starvation during the cold periods and ice-sheet growth when an open-water surface circulation existed in the Polar North Atlantic. Our data document synchronous variations on a October 1900 yr time scale between Arctic oceanic climate, Northern Hemisphere ice-sheet dynamics, and ocean-atmosphere temperature changes.

Quaternary sediments and environments on the continental shelf off northern Norway

Abstract Based on lithologic, palaeontologic and chronostratigraphic investigations of close to 200 gravity cores from troughs, deep banks and shallow banks the following late Quaternary environment can be outlined: In the Weichselian, deposition of basal tills was followed by deposition of laminated clay in a sea-ice environment. Later a pebbly pelite was deposited in the troughs at the same time as the banks were iceberg ploughed. Then (13,000 yrs B.P.) a period with incipient winnowing occurred on the deep banks and deposition of sandy pelite took place in the troughs. The Holocene commenced with a marked environmental change due to intrusion of Atlantic water, the fauna changed from arctic to boreal, high-energy winnowing forming a lag deposit took place on the banks, and high accumulation rates in the troughs occurred due to the winnowing and sediment influx from the downwasting continental icesheet. During the later part of the Holocene the winnowing diminished on the deeper banks, on the shallow banks and on the shelf break it still prevails, and in the troughs calcareous sandy mud is being deposited. The surface sediments comprise three main facies, bouldery and pebbly sand, sand, and sandy mud, whose distribution mainly depends on the prevailing bottom current regime. The composition of the older Quaternary sediments is demonstrated by some selected seismic profiles.

Early Preboreal cooling in the Nordic seas region triggered by meltwater

At the Younger Dryas–Preboreal transition, a high-resolution core from the northeastern Norwegian Sea reveals a two-step warming of sea surface temperatures dated at respectively 10 200–10 000 and 9700–9500 14 C yr B.P. (11 450–11 350 and 11 150–11 000 cal. yr B.P.). Warming was interrupted by a period having stable temperatures and a reduction in sea surface salinity, and we suggest that this pause in warming was triggered by an increase in freshwater supply that may have hampered the North Atlantic heat conveyor. The freshwater influx correlates to an atmospheric cooling over both the Greenland ice sheet and northwest Europe and to cooling of surface temperatures in the Nordic seas. Freshwater may have been supplied from the waning Fennoscandian ice sheet.

Recent calcium carbonate dissolution in the Barents Sea: Paleoceanographic applications

Abstract Studies of benthic foraminifera in surface sediments of the Arctic, epicontinental Barents Sea, show several indications of calcium carbonate dissolution. Low values of calcium carbonate correspond to low ratios of calcareous/agglutinating foraminifera, planktic/benthic foraminifera, high ratios of living/dead foraminifera, corroded calcareous foraminifera, and high numbers of exposed organic linings of foraminifera. Dissolution increases eastwards and northwards in the study area. We suggest that the dissolution is caused by the presence of dense, cold, saline and CO 2 -rich bottom water, which is linked to sea-ice production and the position of the Oceanic Polar Front. We also think calcium carbonate dissolution in this area indicates a sink for atmospheric CO 2 .

Late-glacial and Holocene paleoceanography and sedimentary environments in the St. Anna Trough, Eurasian Arctic Ocean margin

Based on stratigraphical analysis of twelve sediment cores from the Saint Anna Trough, we reconstruct changes in paleoceanography and sedimentary environment during the last deglaciation and the Holocene. Detailed analysis of benthic and planktic foraminiferal fauna, stable oxygen and carbon isotope analysis, lithostratigraphy and radiocarbon dates, are used to reconstruct the following evolution: After the deglaciation of the Saint Anna Trough >13,300 yr B.P. until 9500 yr B.P., the environment was mainly characterised by low biogenic production, carbonate dissolution and deposition of proximal to distal glaciomarine sediments. Intervals with high abundance of the benthic foraminifer Cassidulina teretis, may indicate influx of Atlantic Water at bottom. The transition into the present interglacial started at 9500 yr B.P. reflected by increased production of foraminifera and bivalves. After 8000 yr B.P. there was a marked drop in planktic δ18O followed by a rise in planktic foraminifera and subsequently an increase of C. teretis. These paleoceanographic changes reflect increased heat transport into the area and are coupled to changes in Nordic seas. The early Holocene warming of the Saint Anna Trough were delayed by ca. 2000 years relative to the northeast Nordic seas. Early Holocene sedimentation rates were relatively high (>100 cm/1000 yr), declining drastically after 8000 yr B.P. (<50 cm/1000 yr). This was presumably caused by reduction in winnowing and/or riverine input.