Eystein Jansen

Paleoceanographic reconstructions of surface ocean conditions in the Greenland, Iceland and Norwegian seas through the last 14 ka based on diatoms

Prior reconstructions of the last glacial maximum and the Holocene did not include detailed paleoclimate data from the Greenland, Iceland and Norwegian (GIN) seas, which cover sensitive areas for oceanographic boundaries, sea ice distribution and deep water formation. In order to fill this gap, we studied nine well dated sediment cores from this area and reconstructed the paleoceanographic conditions of the surface GIN seas at 2000-year intervals since 13,400 BP based on the diatom record. Results show that a N.-S. extending sea-ice-free corridor had opened along Norway already at 13,400 BP, indicating a northward flow of a branch of the North Atlantic Drift, possibly caused by a change in the jet stream flow due to the decreased height of the North American ice sheet. A major change of climatic conditions occurred over the GIN seas as the insolation anomaly reached its maximum around 9000 BP, when the sea ice cover and the oceanic fronts retreated to a northwesterly position along Greenland, and the ocean temperatures rose. The first half of the Holocene is recorded as the warmest period during the last 13,400 BP in the GIN seas. The duration of this climatic optimum decreased both toward the north and toward the margins of the area. The second half of the Holocene is characterized by a cooling trend in step with the decreasing insolation. These results support the proposal of Imbrie et al. (1992) that the GIN seas region acts as an initial responder to insolation changes.

Rapid changes in the mechanism of ocean convection during the last glacial period

High-amplitude, rapid climate fluctuations are common features of glacial times. The prominent changes in air temperature recorded in the Greenland ice cores are coherent with shifts in the magnitude of the northward heat flux carried by the North Atlantic surface ocean; changes in the oceans thermohaline circulation are a key component in many explanations of this climate flickering. Here we use stable-isotope and other sedimentological data to reveal specific oceanic reorganizations during these rapid climate-change events. Deep water was generated more or less continuously in the Nordic Seas during the latter part of the last glacial period (60 to 10 thousand years ago), but by two different mechanisms. The deep-water formation occurred by convection in the open ocean during warmer periods (interstadials). But during colder phases (stadials), a freshening of the surface ocean reduced or stopped open-ocean convection, and deep-water formation was instead driven by brine-release during sea-ice freezing. These shifting magnitudes and modes nested within the overall continuity of deep-water formation were probably important for the structuring and rapidity of the prevailing climate changes.

A high-resolution diatom record of the last deglaciation from the SE Norwegian sea : documentation of rapid climatic changes

Although the pulsating nature and the abruptness of the last deglaciation are well documented in marine and land records, very few marine records have so far been able to capture the high-frequency climatic changes recorded in the Greenland ice core Dye 3. We studied high-resolution sediment cores from SE Norwegian Sea, which display a detailed climatic record during the last deglaciation comparable to that of Dye 3. Accelerator mass spectrometry age control of the cores enables us to correlate this record in detail with continental records. The results indicate that the surface waters of the SE Norwegian Sea were seasonally ice free after 13,400 B.P. The Bolling/Allerod interstadial complex (13,200–11,200 B.P.) was a climatically unstable period with changing Arctic-Subarctic conditions. This period was punctuated by four progressively more severe sea surface temperature (SST) minima: between 12,900–12,800 B.P. (BCP I); 12,500–12,400 B.P. (BCP II); 12,300–12,000 B.P. (OD I); and 11,800–11,500 B.P. (OD II). The Younger Dryas (YD) (11,200–10,200 B.P.) represents the severest and most prolonged cold episode of this series of climatic deteriorations. It was bounded by very rapid SST changes and characterized by Arctic-Polar conditions. The first true warm Atlantic water incursion to the SE Norwegian Sea took place around 10,100 B.P., followed by a brief cooler condition between 9900–9600 B.P. (YD II). The early Holocene climatic optimum occurred between 8000–5000 B.P. A conceptual model is proposed where meltwater fluxes are suggested to cause the observed instability in the SST record.

Variations in Atlantic surface ocean paleoceanography, 50°‐80°N: A time‐slice record of the last 30,000 years

Eight time slices of surface-water paleoceanography were reconstructed from stable isotope and paleotemperature data to evaluate late Quaternary changes in density, current directions, and sea-ice cover in the Nordic Seas and NE Atlantic. We used isotopic records from 110 deep-sea cores, 20 of which are accelerator mass spectrometry (AMS)-14C dated and 30 of which have high (>8 cm /kyr) sedimentation rates, enabling a resolution of about 120 years. Paleotemperature estimates are based on species counts of planktonic foraminifera in 18 cores. The δ18O and δ13C distributions depict three main modes of surface circulation: (1) The Holocene-style interglacial mode which largely persisted over the last 12.8 14C ka, and probably during large parts of stage 3. (2) The peak glacial mode showing a cyclonic gyre in the, at least, seasonally ice-free Nordic Seas and a meltwater lens west of Ireland. Based on geostrophic forcing, it possibly turned clockwise, blocked the S-N flow across the eastern Iceland-Shetland ridge, and enhanced the Irminger current around west Iceland. It remains unclear whether surface-water density was sufficient for deepwater formation west of Norway. (3) A meltwater regime culminating during early glacial Termination I, when a great meltwater lens off northern Norway probably induced a clockwise circulation reaching south up to Faeroe, the northward inflow of Irminger Current water dominated the Icelandic Sea, and deepwater convection was stopped. In contrast to circulation modes two and three, the Holocene-style circulation mode appears most stable, even unaffected by major meltwater pools originating from the Scandinavian ice sheet, such as during δ18O event 3.1 and the Bolling. Meltwater phases markedly influenced the European continental climate by suppressing the “heat pump” of the Atlantic salinity conveyor belt. During the peak glacial, melting icebergs blocked the eastward advection of warm surface water toward Great Britain, thus accelerating buildup of the great European ice sheets; in the early deglacial, meltwater probably induced a southward flow of cold water along Norway, which led to the Oldest Dryas cold spell. An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGUs FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95PA01453, Variations in Atlantic surface ocean paleoceanography, 50°-80°N: A time-slice record of the last 30,000 years, M. Sarnthein et al.) Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009;

A giant three-stage submarine slide off Norway

15.00. Payment must accompany order.

The distribution of living benthic foraminifera on the continental slope and rise off southwest Norway

One of the largest submarine slides known, The Storegga Slide, is located on the Norwegian continental margin. The slide is up to 450 m thick and has a total volume of about 5,600 km3. The headwall of the slide scar is 290 km long and the total run-out distance is about 800 km. The slide involved sediments of Quaternary to Early Tertiary age and occurred in three stages. Earthquakes combined with decomposition of gas hydrates are believed to be the main triggering agents for the slides. The first slide event is tentatively dated to be about 30,000 to 50,000 years B.P. and the two last major events are dated to be at 6,000 to 8,000 years B.P.

Apparent long-term cooling of the sea surface in the northeast Atlantic and Mediterranean during the Holocene.

Abstract Surface sediment samples taken by ☐ corer from 45 stations on the Norwegian continental margin and in the Norway Basin have been investigated for their benthic foraminiferal content. Unlike previous studies, the living benthic foraminiferal fauna was differentiated from empty tests comprising the foraminiferal death assemblage. Factor analysis of both the living and dead faunal data reveals six living species assemblages and five corresponding dead assemblages. The additional living assemblage is characterized by the arenaceous speciesCribrostomoides subglobosum that dominates between 1400 and 2000 m water depth, but is rare in the dead faunal data. Trifarina angulosa and, to a lesser extent,Cibicides lobatulus characterize the shallowest foraminiferal assemblage from 200 to 600 m water depth, where it is associated with strong bottom currents and warm, saline Atlantic water of the North Atlantic Drift. On the slope between 600 and 1200 m water depth, theMelonis zaandami Species Assemblage dominates, particularly in areas characterized today by rapid sedimentation of terrigeneous material. Between 1000 and 1400 m depth, where the slope is covered by fine grained, organic-rich, terrigeneous mud, the living foraminiferal assemblage is characterized byCassidulina teretis andPullenia bulloides. Below 1400 m, three foraminiferal assemblages are found:C. subglobosum is found from 1400 to 2000 m,Cibicidoides wuellerstorfi andEpistominella exigua predominantly live from 2000 to 3000 m water depth, and below 3000 m,Oridorsalis umbonatus andTriloculina frigida dominate the fauna. All of theElphidium excavatum tests found in this study and theCassidulina reniforme tests found above 500 m water depth were found to be reworked. Analysis of the sediment grain-size distribution and the organic carbon content in surface samples from the deepest stations suggest that the abundance ofC. wuellerstorfi andE. exigua is positively correlated to relatively coarse (caused by planktic foraminifera) and organic-rich sediments, whereas high frequencies ofO. umbonatus andT. frigida coincide with low organic carbon content. We suggest thatC. wuellerstorfi is adapted to deep-sea environments with relatively high food supply, tolerating relatively low interstitial water oxygen content, whereasO. umbonatus may tolerate lower food supply prefering well-oxygenated interstitial waters.

Response of deep ocean circulation to initiation of northern hemisphere glaciation (3–2 MA)

Reconstructions of upper ocean temperature (T) during the Holocene (10–0 ka B.P.) were established using the alkenone method from seven, high accumulation sediment cores raised from the northeast Atlantic and the Mediterranean Sea (361N–751N). All these paleo-T records document an apparent long-term cooling during the last 10 kyr. In records with indication of a constant trend, the apparent cooling ranges from � 0.27 to � 0.151C kyr � 1 . Records with indication of time-variable trend show peak-to-peak amplitudes in apparent temperatures of 1.2–2.91C. A principal component analysis shows that there is one factor which accounts for a very large fraction (67%) of the total variance in the biomarker paleo-T records and which dominates these records over other potential secondary influences. Two possible contributions are (1) a widespread surface cooling, which may be associated with the transition fromthe Hypsithermal interval ( B9–5.7 ka B.P.) to the Neoglaciation (B5.7–0 ka B.P.); and (2) a change in the seasonal timing and/or duration of the growth period of alkenone producers (prymnesiophyte algae). The first contribution is consistent with many climate proxy records from the northeast Atlantic area and with climate model simulations including Milankovitch forcing. The second contribution is consistent with the divergence between biomarker and summer faunal paleo-T fromearly to late Holocene observed in two cores. Further work is necessary, and in particular the apparent discordance between biomarker and faunal T records for the relative stable Holocene period must be understood, to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records. r 2002 Elsevier Science Ltd. All rights reserved.

Intensification of Northern Hemisphere glaciations in the circum Atlantic region (3.5–2.4 Ma) – ice-rafted detritus evidence

Carbon isotopic records from benthic foraminifera are used to map patterns of deep ocean circulation between 3 and 2 million years ago, the interval when significant northern hemisphere glaciation began. The δ18O and δ13C data from four Atlantic sites (552, 607, 610, and 704) and one Pacific site (677) show that global cooling over this interval was associated with increased suppression of North Atlantic Deep Water (NADW) formation. However, the relative strength of NADW production was always greater than is observed during late Pleistocene glaciations when extreme decreases in NADW are observed in the deep North Atlantic. Our data indicate that an increase in the equator-to-pole temperature gradient associated with the onset of northern hemisphere glaciation did not intensify deepwater production in the North Atlantic but rather the opposite occurred. This is not unexpected as it is the “warm high-salinity” characteristic, rather than the “low temperature,” of thermocline waters that is critical to the deepwater formation process in this region today.

Pliocene-Pleistocene ice rafting history and cyclicity in the Nordic Seas during the last 3.5 Myr

Abstract Records of ice-rafted detritus (IRD) from Ocean Drilling Program Sites 644 and 907 in the Nordic Seas and Deep Sea Drilling Program Sites 610 and 607 in the North Atlantic Ocean are used to determine the growth and history of Northern Hemisphere glaciations spanning the interval from 3.6 to 2.4 Ma. The records document that the initiation of large-scale glaciation in the circum Atlantic region occurred stepwise between 3.5 and 2.4 Ma, with Greenland leading the other areas. The first major pulse of IRD occurs at 3.3 Ma suggesting a distinct expansion of the Greenland ice sheet. Progressive increases in IRD occur from 3.0 Ma, with a synchronous ice sheet development between the Greenland, Scandinavian and North American regions starting around 2.72–2.75 Ma. The regional records of IRD correspond to major trends in the oxygen isotope record, suggesting a tight connection between the record of global ice volume and evidence for expanding glaciers in the Northern Hemisphere. It appears as if widespread IRD distribution occurs when the global ice volume surpasses certain thresholds, similar to the late Quaternary situation.