Tine L. Rasmussen

Synchronized terrestrial-atmospheric deglacial records around the North Atlantic

On the basis of synchronization of three carbon-14 (14C)-dated lacustrine sequences from Sweden with tree ring and ice core records, the absolute age of the Younger Dryas-Preboreal climatic shift was determined to be 11,450 to 11,390 ± 80 years before the present. A 150-year-long cooling in the early Preboreal, associated with rising Δ14C values, is evident in all records and indicates an ocean ventilation change. This cooling is similar to earlier deglacial coolings, and box-model calculations suggest that they all may have been the result of increased freshwater forcing that inhibited the strength of the North Atlantic heat conveyor, although the Younger Dryas may have begun as an anomalous meltwater event.

Rapid changes in surface and deep water conditions at the Faeroe Margin during the last 58,000 years

A high-resolution piston core, ENAM93-21, from a water depth of 1020 m near the Faeroe-Shetland Channel is investigated for variations in magnetic susceptibility, surface oxygen isotopes, grain size distribution, content of ice-rafted detritus (IRD), and distribution of planktonic and benthic foraminifera. The core, covering the last 58,000 years, is correlated with the Greenland ice cores and compared with paleorecords from the Norwegian Sea and the North Atlantic Ocean. All fifteen Dansgaard-Oeschger climatic cycles recognized from the investigated time period in the Greenland ice cores have been identified in the ENAM93-21 core. Each cycle is subdivided into three intervals on the basis of characteristic benthic and planktonic faunas. Interstadial intervals contain a relatively warm planktonic fauna and a benthic fauna similar to the modern fauna in the Norwegian Sea. This indicates thermohaline convection as at present, with a significant contribution of deep water to the North Atlantic Deep Water (NADW). Transitional cooling intervals are characterized by more cold water planktonic foraminfera and ice-related benthic species. The benthic fauna signifies restricted bottom water conditions and a reduced contribution to the NADW. The peak abundance of N. pachyderma (s.) and the coldest surface water conditions are found in the stadial intervals. The benthic fauna is dominated by species with an association to Atlantic Intermediate Water, suggesting an increased Atlantic influence in the Norwegian Sea, and there was probably no contribution to the NADW through the Faeroe-Shetland Channel. The three different modes of circulation can be correlated to paleoceanographic events in the Norwegian Sea and the North Atlantic Ocean.

Circulation changes in the Faeroe-Shetland Channel correlating with cold events during the last glacial period (58–10 ka)

A core from the margin of the Faeroe-Shetland Channel covering the last glacial period (58–10 ka) reveals a very detailed record of oscillations in benthic and planktic foraminifera, oxygen isotopes, and ice-rafted detritus. The oscillations correlate almost exactly with the Dansgaard-Oeschger cycles in the Greenland ice cores, including similar subdivision of the cycles into warmer interstadial and colder stadial sections. The remarkably close agreement between the benthic faunas and their δ 18 O data and the record of the ice cores provides new empirical evidence for a close relationship between the deep-ocean circulation and the abrupt climatic changes of the last glacial period.

Millennial-scale glacial variability versus Holocene stability: changes in planktic and benthic foraminifera faunas and ocean circulation in the North Atlantic during the last 60 000 years

Two piston cores, DS97-2P from the Reykjanes Ridge in the central North Atlantic Ocean (1685 m water depth) and ENAM33 from southwest of the Faeroe Islands in the NE Atlantic (1217 m water depth), have been investigated for their planktic and benthic foraminiferal content. DS97-2P is situated near the Subarctic Front and productivity measured by accumulation rates of benthic and planktic foraminifera has been generally high during the Holocene. The productivity shows a clear decrease from an early Holocene maximum to a late Holocene minimum. Coeval changes in the benthic faunas indicate that the food supply changed from large, irregular pulses during the early Holocene to a more sustained flux during the late Holocene. Presumably in concert with decreasing bottom current activity oxygen conditions in the bottom water became poorer. Another feature of the late Holocene is an increasing instability of the North Atlantic thermohaline circulation regime. Nevertheless, the changes in faunal composition and productivity during the Holocene were gradual as compared to the discontinuous distribution patterns and abrupt productivity shifts during the glacial. The glacial shifts were on a millennial time scale and correlate with the interstadial-stadial phases of the Dansgaard-Oeschger cycles in the Greenland ice cores. The faunas of the warm interstadial phases resembled the Holocene faunas, and both surface and bottom productivity was high. The faunas suggest that the interstadial circulation pattern was very similar to the modern system with convection in the Nordic seas and generation of North Atlantic Deep Water. The planktic faunas during the cold stadials and Heinrich events were completely dominated by the polar species Neogloboquadrina pachyderma s, and surface conditions were cold and the productivity low. The benthic faunas were dominated by species that presently thrive in areas with a low amount of food and reduced oxygen content. The water column was probably stratified with low saline, cold surface water overlying poorly aerated, intermediate water masses.

Deep sea records from the southeast Labrador Sea: Ocean circulation changes and ice-rafting events during the last 160,000 years

[1] Results from two deep sea cores from northeast of Newfoundland at 1251 and 2527 m water depth, respectively, indicate that during the time period from 160,000 to 10,000 years BP, ice rafting events in the Labrador Sea were accompanied by rapid variations in deep and surface water circulation. Twelve ice-rafting events occurred, each coinciding with high concentrations of detrital carbonate and oxygen isotopic depletion of both surface and bottom waters. Eleven of these can be correlated with the North Atlantic Heinrich events H1-H11. The remaining very conspicuous ice-rafting event took place early in MIS substage 5e, at a time when the planktic faunal assemblage suggests marked warming of the sea surface. In the shallower core, benthic delta(13)C values rise from a minimum during the deglaciation to peak substage 5e values following the last ice-rafting event, indicating that the ventilation of intermediate depths was renewed after the deglaciation was complete and continued throughout substage 5e. The benthic foraminifera suggest that this well-ventilated water mass was comparable to the modern Labrador Sea Water (LSW). The benthic faunas suggest that a relatively warm intermediate water mass entered the SE Labrador Sea during Heinrich events. Generally low benthic delta(13)C values indicate that this water mass was poorly ventilated and rich in inorganic nutrients. Isotope data and benthic faunal distributions indicate that North Atlantic Deep Water (NADW) formed in the Norwegian-Greenland Sea reached the SE Labrador Sea between the Heinrich events.

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.

Climatic instability, ice sheets and ocean dynamics at high northern latitudes during the last glacial period (58-10 KA BP)

Abstract Oxygen isotope,magnetic susceptibility and foraminiferal distribution data are presented for a high resolution core (ENAM93-21) located at the northeast Faeroe Margin. This core recorded a rapid succession of faunistic, sedimentologic and isotopic variations which paralleled the Greenland ice core isotopic records with their typical succession of abrupt temperature rises and gradual coolings (the Dansgaard-Oeschger cycles). The most notable feature was the contemporaneous changes in surface and bottom water conditions and circulation that appeared tightly coupled with the rapid climate fluctuations. Warm episodes (‘interstadials’) were appeared with higher sea surface temperatures and thermohaline convection in the Norwegian-Greenland Sea. The Polar Front was located north of the ENAM93-21 site. Cold episodes (‘stadials’) were associated with an increase in the input of melting Fennoscandian icebergs, low sea surface temperature and salinity, and no thermohaline convection in the Norwegian-Greenland Sea. Intermediate waters changed to an estuarine mode at the Faeroe Margin with a reversed flow pattern through the Faeroe-Shetland Channel. The Polar Front was located south of the ENAM93-21 site.

Late warming and early cooling of the sea surface in the Nordic seas during MIS 5e (Eemian Interglacial)

Abstract Geochemical identification of a tephra layer found in two cores from the NE Atlantic Ocean and the SE Norwegian Sea, respectively, and dated to 127 ka BP has enabled us to obtain a precise correlation across the Iceland–Scotland Ridge at the Marine Isotope Stage (MIS) 6/5 transition. The direct distance between the two cores is only about 200 km . South of the Iceland–Scotland Ridge, sea surface temperatures rose abruptly at 130 ka BP at the onset of MIS 5e and at least 2–3000 years earlier than north of the ridge. Maximum sea surface temperatures south of the ridge occurred during this initial phase of MIS 5e, when temperatures in the Nordic Seas were still low. North of the ridge, the sea surface warmed rapidly at 127 ka BP. Correlations between the North Atlantic records and the Eemian of Northwest Europe tentatively indicate that the initial phase of MIS 5e correlates with the early part of the Eemian characterised by a warm, continental type of climate. The period after the warming of the Nordic seas corresponds to the slightly cooler and more oceanic middle Eemian interval in Europe. The sea surface temperatures fell gradually north of the ridge during the later part of MIS 5e and they were low during MIS 5d–5a. South of the ridge the temperatures remained relatively high. The data shows that there was no outflow of deep water from the Norwegian Sea during the later part of MIS 6. Outflow began at the MIS 6/5 transition simultaneous with the sea surface warming south of the ridge.

Multidecadal ocean variability and NW European ice sheet surges during the last deglaciation

A multiproxy paleoceanographic record from the Atlantic margin off the British Isles reveals in unprecedented detail discharges of icebergs and meltwater in response to sea surface temperature increases across the last deglaciation. We observe the earliest signal of deglaciation as a moderate elevation of sea surface temperatures that commenced with a weakly developed thermocline and the presence of highly ventilated intermediate waters in the Rockall Trough. This warming pulse triggered a series of multidecadal ice-rafted debris peaks that culminated with a major meltwater discharge at 17,500 years before present related to ice sheet disintegration across the NW European region. The impact of meltwater caused a progressive reduction in deep water ventilation and a sea surface cooling phase that preceded the collapse of the Laurentide Ice Sheet during Heinrich event 1 by 500–1000 years. A similar sequence of rapid ocean-ice sheet interaction across the European continental margin is identified during the Bolling-Allerod to Younger Dryas transition. The strategic location of our sediment core suggests a sensitive and rapid response of ice sheets in NW Europe to transient increases in thermohaline heat transport.

Holocene climate variations at the entrance to a warm Arctic fjord: evidence from Kongsfjorden trough, Svalbard

Abstract The North Atlantic Current transports warm and salty water into the Nordic Seas and continues northwards into the Arctic Ocean as the West Spitsbergen Current. This current flows along the west coast of the Svalbard archipelago and into the fjords and troughs on the Svalbard shelf. We have investigated a core (NP05-11-21GC) from the Kongsfjorden trough, which spans the last 12 ka. The core site presently experiences seasonal inflow of Atlantic Water masses, and may therefore provide a record of past variations in Atlantic Water inflow to the Arctic. Lithological analysis and benthic foraminifera have been used to reconstruct the palaeoceanographic development in the area. The results show that cold and harsh conditions prevailed during the late part of the Younger Dryas and that the site was in proximity to glaciers. After 11.8 ka BP the first influence of Atlantic Water is seen in the fauna. This was followed by changes in glacial activity at 11.5 ka BP. During the period 11.5–10.6 ka BP the fauna indicate increased influence of Atlantic Water and the final deglaciation of the fjord after the Younger Dryas period. These initial ameliorated conditions were interrupted by a 250-year long cooling starting at 11.3 ka BP, corresponding to the Preboreal Oscillation. After 10.6 ka BP a marked change from an ice proximal to ice distal environment occurred accompanied by the strong influence of Atlantic Water masses. In the Mid-Holocene at 7 ka BP, the influence of Atlantic Water diminished but no sign of an immediate response of the glaciers to this are found in the core. The evidence suggests that intensification of glacial activity started as late as c. 3.5 ka BP. Comparing the core from Kongsfjorden trough with two shelf records from Svalbard generally shows similar faunal development although some differences exist. These are assumed to be related to the distance of each record to the position of the Arctic front.