Dorthe Klitgaard-Kristensen

The last 18 kyr fluctuations in Norwegian sea surface conditions and implications for the magnitude of climatic change: Evidence from the North Sea

A combined record of three cores spanning the last 18 kyr from the northern North Sea is investigated for content of benthic and planktonic foraminifera and stable oxygen isotopes. The paleoenvironmental development through this time period shows an early deglaciation (18–14.4 ka) and the Younger Dryas (12.7–11.5 ka) characterized by arctic/polar conditions and increased ice rafting in the Norwegian Channel. During the Bolling-Allerod period, warm sea surface temperature (9°C) conditions similar to present conditions are inferred, while bottom waters stayed cold (0–1°C) with normal salinity. The Bolling-Allerod period is interrupted twice at 13.9–13.6 ka (Older Dryas) and at 13.0–12.8 ka (Inter-Allerod Cooling Period) by reductions in sea surface temperatures and increased sea ice cover. The beginning of the Holocene period is marked by increases in surface and bottom water temperature. Superimposed on the broad climatic changes through the Holocene, a series of short-lived oscillations in the ocean circulation are recorded. The amplitude of these Holocene events appears larger in the early Holocene (prior to 8 ka) than compared with the remaining part of the Holocene. This amplification can possibly be attributed to a general increased freshwater budget in the North Atlantic at this time during the final stages of the deglaciation of the Laurentide and Scandinavian ice sheets.

The Faroe-Shetland Gateway: Late Quaternary water mass exchange between the Nordic seas and the northeastern Atlantic

Abstract Thirteen piston and gravity cores from the Faroe–Shetland area were investigated for their planktic and benthic foraminiferal and oxygen isotopic distributions. Eight time-slices between 18 ka BP and the present were reconstructed to study variations in surface and deep water exchange between the SE Norwegian Sea and the northeast Atlantic Ocean. Today, a relatively strong northward flow of warm North Atlantic surface water is counterbalanced by a southward outflow of newly convected cold bottom water, the Norwegian Sea Overflow Water. During the last glacial maximum at 18 ka BP both the surface and bottom flows were slow and the climate conditions were Arctic. The convection north of the Faroe area was weak and unstable. The first indication of the deglaciation is a decrease in the planktic oxygen isotope values discernible southwest of the Faroe Islands at 15.5 ka BP. The deglaciation proceeded northeast and eastward synchronous with a gradual intensification of northward flowing warmer Atlantic Intermediate Water along the sea bottom. Meltwater fluxes increased between 14 and 13 ka BP producing cold surface waters, and the climatic cooling was extreme. There was no southward overflow of cold bottom water during this time period and the exchange of water masses between the Nordic seas and the North Atlantic Ocean was essentially reversed, i.e. estuarine. During the Bolling Interstadial at 12.5 ka BP northward flowing warm surface water was present to the east of the Faroe–Shetland Channel, wedged below a tongue of polar water spreading from the northwest and reaching into the Faroe–Shetland Channel. Convection in the Nordic seas and overflow of cold deep water started during the Bolling Interstadial. The polar water spread more eastward and southward during the following cold spell, the Younger Dryas, around 10.3 ka BP. The polar water was overlying the warmer, but more saline Atlantic water, which flowed northward below the cold surface water. The overflow of cold bottom water was supposedly only slightly weaker than during the Bolling Interstadial. Strong inflow of warm surface water took place during the Early Holocene at 9.5 ka BP and relatively dense cold water flowed southward along the bottom. The rate of water mass exchange reached a maximum at 6.5 ka BP, when both the inflow of warm Atlantic surface water and the outflow of cold dense bottom water appear to have been stronger than today.

Late glacial palaeoceanography of Hinlopen Strait, northern Svalbard

Timing and structure of the Late and post-glacial development of the northern Svalbard margin, together with the initial influx of Atlantic water into the Arctic Ocean are still very poorly constrained. We investigated a sediment core (NP94-51) from a high accumulation area on the continental shelf north of Hinlopen Strait with the purpose of resolving the timing and structure of the last deglaciation. Detailed analyses of ice-rafted detritus, benthic and planktonic foraminiferal fauna, diatom flora, grain size and radiocarbon dates are used to reconstruct the palaeoceanographic evolution of the area. Our results indicate that the disintegration of Hinlopen Strait ice and possibly the northern margin of the Svalbard Ice Sheet commenced between 13.7 and 13.9 14C Ky BP. Influx of subsurface Atlantic waters into the area (12.6 14C Ky BP) and the retreat of the sea ice cover, with the accompanying opening of the surface waters (10.8 14C Ky BP), happened at different times and both much later than the disintegration of the ice sheets. The transition into the Holocene shows a two-step warming.

Late Holocene coastal hydrographic and climate changes in the eastern North Sea

We present a high-resolution palaeoenvironmental reconstruction covering the late Holocene from the Skagerrak and other sites in the North Sea area. The data, which are based on the analyses of marine sediment cores, reveal a marked environmental shift that took place between AD 700 and AD 1100, with the most pronounced changes occurring at AD 900. Both surface and bottom waters in the Skagerrak were subject to major circulation and productivity changes at this time due to an enhanced advection of Atlantic waters to the North Sea marking the beginning of the ‘Mediaeval Warm Period’ (MWP). The observed increase in bottom current strength is especially remarkable as there is hardly any comparable signal in the older part of the record going back to 1000 BC. At the transition to the ‘Little Ice Age’ (LIA) the bottom current strength remains at a high level, now probably forced by atmospheric circulation. Thus, despite opposite temperature forcing, these two consecutive climate scenarios are apparently able to generate distinctly stronger bottom currents in the Skagerrak than observed in the preceding 2000 years, and demonstrate the significance of climatic forcing in shaping the marine environment. Indeed, both the MWP and the LIA are reported as strong climatic signals in northwest Europe, being the warmest (except the late twentieth century) and coldest periods, respectively, during at least the last 2000 years.

Distribution of recent calcareous benthic foraminifera in the northern North Sea and relation to the environment

This paper compiles new and previously published data on recent calcareous benthic foraminifera (dead and living assemblages) in surface sediment samples from the northern North Sea area, focussing on the dead benthic foraminifera and their relation to the environment. Five dead benthic foraminiferal assemblages have been identified. In Scottish coastal areas Cibicides lobatulus and Rosalina sp. dominate in areas with strong current activity and coarse-grained sediments, whereas C. lobatulus and Trifarina angulosa dominate at similar conditions in the Norwegian coastal areas. Cassidulina laevigata assemblages occur in areas influenced by inflow of Atlantic water into the northern North Sea. In the central part of the Norwegian Channel Uvigerina mediterranea prevails in fine-grained sediments with high organic content and possibly low oxygen content. This species’ restricted distribution to the Norwegian Channel could possibly be related to the availability of food. Bulimina marginata and Hyalinea balthica dominate on the Fladen Ground where seasonal stratification is pronounced. This presumably leads to a decrease in the oxygen content in the bottom-waters during part of the year.

RAPID CHANGES IN THE OCEANIC FRONTS IN THE NORWEGIAN SEA DURING THE LAST DEGLACIATION : IMPLICATIONS FOR THE YOUNGER DRYAS COOLING EVENT

Abstract Two high-resolution cores, ENAM93-21 north of the Faeroe Islands and Troll 89.03 off southwest Norway, from the SE Norwegian Sea have been investigated for downcore variations in planktic foraminifera, IRD content and oxygen isotopes in a period from 15 to 9 ka. The results exhibit frequent and large amplitude changes in sea surface temperature during the last deglaciation. A sudden and profound increase in SST at 12.8 ka is followed by three abrupt and short-lived cooling events terminated by the Younger Dryas cooling. The presence of strong temperature gradients in the Norwegian Sea region during the Bolling–Younger Dryas period have implications for the heat flux towards the north. A possible time lag may exist between ENAM93-21 and Troll 89.03 at the beginning of the Younger Dryas cooling. These findings indicate that polar waters migrated eastwards at the beginning of the Younger Dryas period.