Warm Atlantic Surface Water Research Articles

Deep ocean 14C ventilation age reconstructions from the Arctic Mediterranean reassessed

The present-day ocean ventilation in the Arctic Mediterranean (Nordic Seas and Arctic Ocean), via transformation of northward inflowing warm Atlantic surface water into cold deep water, affects regional climate, atmospheric circulation and carbon storage in the deep ocean. Here we study the glacial evolution of the Arctic Mediterranean circulation and its influence on glacial climate using radiocarbon reservoir-age reconstructions on deep-sea cores from the Fram Strait that cover the late glacial period (33,000–20,000 yr ago; 33–20 ka). Our results show high Benthic-Planktic 14C age differences of ∼1500 14C years 33–26.5 ka suggesting significant water column stratification between ∼100–2600 m water depth, and reduction and/or shoaling of deep-water formation. This phase was followed by break-up of the stratification during the Last Glacial Maximum (LGM; 26–20 ka), with Benthic-Planktic 14C age differences of ∼250 14C years, likely due to enhanced upwelling. These ocean circulation changes potentially contributed to the final intensification phase of glaciation via positive cryosphere-atmosphere-ocean circulation-carbon cycle feedbacks. Our data also do not support ‘extreme aging’ of >6000 14C years in the deep Arctic Mediterranean, and appear to rule out the proposed outflow of very old Arctic Ocean water to the Nordic Seas during the LGM and to the subpolar North Atlantic Ocean during the deglacial period.

Palaeoceanographic reconstruction of surface-ocean changes in the southern Norwegian Sea for the last ~130,000 years based on diatoms and with comparison to foraminiferal records

Fossil marine diatom assemblages in a sediment core from the central northern Faroe slope in the Norwegian Sea were used to reconstruct palaeoceanographic changes in the surface water mixed layer from the last ~130,000 years (Marine Isotope Stage (MIS) 6/5 transition to MIS 1 (including the Eemian and Holocene interglacials) and to compare the results with previously published results on planktic foraminifera representing the subsurface conditions of the thermocline. Diatom floras from MIS 5 of the Nordic seas have rarely been studied in detail before and never the entire period from pre-Eemian to present. The composition of diatom species together with maxima in absolute abundance of diatoms, indicate two periods of warmer sea surface temperatures correlating with the Eemian and Holocene interglacials, respectively. The Eemian differs from the Holocene in that the Iceland-Faroe Front never developed, suggesting the cold East Icelandic Current (originating from the East Greenland Current) running north of Iceland was reduced or more mixed with Atlantic water than during the Holocene and that the surface temperature and salinity gradients were weaker. The northern Faroe slope was in the early Weichselian of MIS 5d–5a mainly influenced by weaker inflow of Atlantic water and stronger influence of the East Icelandic Current and by seasonal sea-ice cover. During the later part of the Weichselian (MIS 4–MIS 2) cold conditions prevailed with extensive sea-ice cover except during the warmer interstadials. The diatom floras were more sensitive to climate changes than the planktic foraminifera and indicate longer periods with warm surface conditions, and increased influence of the warm Atlantic surface water from the Faroe Current in MIS 5, suggesting a shallower and much steeper thermocline during the transitional periods compared to today. The diatoms are good indicators for Atlantic water inflow and possible convection in the Nordic seas, not merely reflecting orbital variations in insolation as previously suggested.

A high-resolution multi-proxy lake record of Holocene environmental change in southern Iceland

Vestra Gislholtsvatn, a non-glacial lake in southern Iceland, provides a continuous high-resolution multi-proxy paleoenvironmental record spanning more than 10 ka. It is characterized by an interplay between volcanic and climatic controls on the lake sediment environmental proxies. Initial lake ontogeny was disrupted by deposition of the Saksunarvatn tephra (∼10.3 ka) followed by unusually slow ontogeny for 2 ka. Principal component analysis defines three climate zones between 8 and 1 ka. Quasi-stabilization and warm summer proxies between ∼8 and 5.7 ka define Holocene Thermal Maximum conditions. A subtle shift to cooler summers between ∼5.7 and ∼4.2 ka is interpreted as early Neoglaciation, intensifying after 4.2 ka. Following settlement of Iceland (∼1.1 ka) anthropogenic signals overprint the climate record. Throughout the record short-term variability is attributed to tephra fall events in the catchment from nearby explosive volcanism, whereas centennial- and millennial-scale changes are attributed to climate. When compared with other Icelandic lakes, environmental proxies from Vestra Gislholtsvatn indicate less variability albeit similar timing of environmental change, suggesting a relatively stable flow of the Irminger Current along the south coast of Iceland compared with a more variable flow of warm Atlantic surface water along the western and northern coasts of Iceland after 8 ka.

A high resolution opal and radiolarian record from the subpolar North Atlantic during the Mid-Pleistocene Transition (1069–779 ka): Palaeoceanographic implications

A high-resolution record of radiolarian assemblages from Site U1314 was studied to reconstruct hydrographic and climatic changes in the North Atlantic Ocean during the Mid-Pleistocene Transition period (1069–779ka). Besides the faunal record, absolute accumulation of radiolarians and total biogenic opal are used to determine changes in surface water productivity. Results show that the North Atlantic Arctic Front shifted back and forth repeatedly at a glacial/interglacial timescale, bringing the site under the influence of both cold Arctic (glacial), and much warmer Atlantic (interglacial) waters. During glacial intervals and “ice-rafted debris” (IRD) events, the deep-dwelling taxon Cycladophora davisiana was the greatest contributor of the radiolarian assemblage, suggesting cold surface conditions, melt-water discharge and changes in deep and intermediate circulation. Interglacial intervals were characterized by abundant shallow-dwelling taxa like Pseudodyctiophimus gracilipes and Lithomelissa setosa, increased opal accumulation, and higher radiolarian diversities, indicating a northward flow of warm Atlantic surface waters to the Site U1314 area. A marked change in the structure of the radiolarian assemblage occurred after MIS 22 (~860ka), with large taxa differences between warm and cold periods, probably in response to changing ocean conditions due to the higher amplitude of glacial/interglacial changes as the dominant periodicity of high-latitude climate oscillations shifted from 41-kyr to 100-kyr. Thus, we conclude that the radiolarian assemblage from the North Atlantic have changed drastically along with variations in ocean circulation in response to orbital and millennial-scale climatic variations that occurred in the Early and Mid-Pleistocene.

Warm Atlantic surface water inflow to the Nordic seas 34–10 calibrated ka B.P.

A number of short‐lasting warm periods (interstadials) interrupted the otherwise cold climate of the last glacial period. These events are supposedly linked to the inflow of the warm Atlantic surface water to the Nordic seas. However, previous investigations of planktonic foraminifera from the Nordic seas have not been able to resolve any significant difference between the interstadials and intervening cold stadials, as the faunas are continuously dominated by the polar species Neogloboquadrina pachyderma s. Here we examine the planktonic foraminifera assemblages from a high‐resolution core, LINK17, taken at 1500 m water depth off northern Scotland below the warmest part of the inflowing Atlantic water. The core comprises the time period 34–10 calibrated ka B.P., the coldest period of the last glaciation and the deglaciation. The results reveal a hitherto unknown faunistic variability indicating significant fluctuations in both surface water inflow and in summer sea surface temperatures. During the interstadials, relatively warm Atlantic surface water (4–7°C) flowed north into the eastern Norwegian Sea. During the stadials and Heinrich events the surface inflow stopped and the temperatures in the study area dropped to <2°C. The Last Glacial Maximum was nearly as warm as the interstadials, but the inflow was much more unstable. The data reveal two previously unrecognized warming events each lasting more than 1600 years and preceding Heinrich events HE3 and HE2, respectively. By destabilizing the ice sheets on the shelves the warmings may have played a crucial role for the development of Heinrich events HE2 and HE3.

Timing and mechanisms of surface and intermediate water circulation changes in the Nordic Seas over the last 10,000 cal years: a view from the North Iceland shelf

The North Iceland shelf bears essential components of the present surface and intermediate circulation of the northern North Atlantic. Instrumental and historical data give evidence of the sensitivity of this domain to broad, regional-scale oceanic and atmospheric anomalies. Our investigation of the paleohydrological variability off Northern Iceland throughout the last 10 000 cal yr suggests that atmospheric forcing alone, through combined changes in strength of the wind stress curl and sea-level atmospheric pressure pattern over the Nordic Seas, is sufficient to explain the recorded changes in origins and dynamics of surface and intermediate water masses. Our biotic proxies, coccoliths and benthic foraminifera, were extracted from a giant piston core (MD99-2269) collected in a shelf trough where sediment accumulated at an excess rate of 2 m/kyr. The mid-Holocene from 6.5 to 3.5 cal kyr BP was a time of peaked carbonate production and subsequent sedimentation, and strong water-column stratification with a thick layer of cold-fresh Arctic surface water overlapping an enhanced flow of Irminger/Atlantic Intermediate water. Applying conditions triggering present-time carbonate plankton blooms in the studied area, we infer that a lowered cyclonic activity associated with decreased winter storms and reduced production of Arctic Intermediate Water in the Iceland Sea were conductive of the recorded mid-Holocene water column structure. The opposite situation (warm Atlantic surface water, low vertically-integrated inflow of Irminger water, abutment of Arctic Intermediate water in deep shelf troughs) characterized the early Holocene as well as a shorter late Holocene period centred at 2 cal kyr BP. The Little Ice Age (ca. 0.2–0.6 cal kyr BP) and a short event at around 3 cal kyr BP stand as times of extreme advection of polar waters and extended sea–ice development. A comparison of the recorded long-term Holocene evolution of water column structure off Northern Iceland with climate and hydrological changes in the north-eastern Atlantic suggests that the strength of Atlantic inflow into the Nordic Seas was subjected to a balance between the Irminger and the Norwegian branches. This balance is thought to be mostly related to changes in the intensity and location of westerly winds and associated atmospheric pressure gradients in the North Atlantic.

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

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 Bølling 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 Bølling 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 Bølling 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.

Recent and Pleistocene carbonate dissolution in sediments of the Norwegian–Greenland Sea

Abstract Surface sediment samples from the Norwegian–Greenland Sea were investigated to reconstruct the spatial distribution of recent carbonate dissolution on the seafloor. Additionally, carbonate dissolution records of Ocean Drilling Program sites 985 and 987 are presented to outline the development of Pleistocene carbonate preservation. Today, well-preserved carbonate tests can be observed along the inflow of warm Atlantic surface water, extending as far as into the northernmost Norwegian–Greenland Sea. Increased dissolution is indicated along the continental margins and in the deepest parts of the Greenland Basin. Factors favoring carbonate preservation were found to be supersaturation of the water column with respect to calcium carbonate, high carbonate rain and probably excess alkalinity of bottom waters supplied by the arctic river discharge. Supralysoklinal dissolution is most important for recent carbonate dissolution in the Norwegian–Greenland Sea, whereas the deepest parts of the Greenland Basin reaches the calcite saturation horizon. Pleistocene dissolution records show some prominent peaks of extreme carbonate dissolution. During the Brunhes chron, carbonate dissolution maxima can be related to meltwater pulses, which probably inhibited deep-water formation in the Norwegian–Greenland Sea during deglaciation events. Long-term severe carbonate dissolution is evident during the late Matuyama chron. This can be probably related to low carbonate rain, due to a more eastwards located East Greenland Current and the nearly absence of the not yet polar adapted Neogloboquadrina pachyderma sin. during that period. Extreme dissolution events during the late Matuyama indicate strongly reduced deep-water formation.

Late Weichselian Marine 14C Reservoir Ages at the Western Coast of Norway

A shallow marine Late Weichselian deposit on the outer coast of western Norway contains both terrestrial plant material and articulated marine shells. We have 14C dated both types of material from eight different stratigraphic levels covering the time interval 12,300 to 11,100 14C yr B.P. The difference between 14C-dated terrestrial plant material and marine shell material (the marine reservoir age) ranges from 200 to 525 yr, with a weighted average of 380 ± 32 yr. This is almost identical to the present reservoir age of 379 ± 20 yr for southern Norway. In the mid-Younger Dryas (YD) interval the reservoir age in the North Atlantic (55°N–65°N) was 700–800 yr, considerably greater than the present reservoir age and the age we have measured for the Bølling–Allerød interval. The reason for this increase during the YD is probably a combination of reduced inflow of surface waters to the North Atlantic and more extensive sea-ice cover. Evidence from marine cores show that the southeastern Norwegian Sea experienced rapid fluctuations in the inflow of warm Atlantic surface water during the period 12,300 to 11,000 yr B.P. However, the reservoir age apparently did not increase during these colder periods (Older Dryas I and II). The reason is probably that, in contrast to the YD, these colder periods did not last long enough and/or were of too limited extent to alter the reservoir age of the ocean. A comparison of the obtained 14C dates with the varve 14C chronology from Lake Suigetsu indicates an age of 12,770 cal yr B.P. for the AL/YD boundary.

Variability in surface and deep water conditions in the nordic seas during the last interglacial period

Stable isotope, foraminifera and ice rafted detritus (IRD) records covering the last interglacial (the Eemian) from 7 sediment cores in a transect from the Norwegian to the Greenland Sea are presented. The percentages of Neogloboquadrina pachyderma (s.) and Globigerina quinqueloba, foraminiferal content, and to some extent planktonic stable isotope records, demonstrate marked, regional changes in surface water conditions. Importantly, the variability in the abundances of subpolar foraminifera and foraminiferal content are not coherent, implying that these two types of proxies fluctuated independently of each other and most likely reflect changes in sea surface temperature and surface water carbonate productivity, respectively. Paleoceanographic reconstructions demonstrate significant movements of the oceanographic fronts. At the warmest periods, the Arctic front was located far west of the present-day location, at least within the Iceland Sea region. At 126–125 ka, this was most probably due to a stronger or more westerly located Norwegian current. Within the later warm intervals, higher heat flux to the western part of the basin reflects a combination of a stronger Irminger current and/or a weaker east Greenland current. During the main cold spell at ∼124 ka, a diffuse Arctic front had a more southeasterly location than today, and intrusion of Atlantic surface waters was probably limited to a narrow corridor in the Eastern Norwegian Sea. A general correspondence between minima in sea surface temperatures and light benthic δ 18O may indicate enhanced influx of freshwater to the basin within the cold events. At least in the Norwegian Sea, we find some evidence that the changes in surface water conditions are associated with changes in deep water ventilation. The majority of the fluctuations may be related to occasional breakdown or reduction of the thermohaline circulation within the Nordic seas. In the earliest Eemian, this could result from meltwater forcing. During the remaining part of the last interglacial the fine balance between temperature and salinity, which the deep water formation is depending on, may have been disturbed by periodic increases in fresh water supply or variable influx of warm Atlantic surface waters.

A palaeoclimatic and palaeoceanographic record of the last 11 000 14C years from the Skagerrak-Kattegat, northeastern Atlantic margin

Marine sediments of about 116 m drilled onshore at Skagen near the northernmost tip of Jutland, Denmark, provides a continuous and detailed palaeoceanographic and palaeoenvironmental record of the last 11 000 14C years in the Skagerrak-Kattegat. The whole Younger Dryas was characterized by an arctic-subarctic environment where the North Jutland Current flows today, instead of being succeeded by a minor climate warming at about 10 600 BP as recorded in the southern Kattegat and in the areas strongly influenced by the Baltic Current and currents along the Swedish west coast. The changes in components of diatom assemblages, combined with previously published benthic foraminiferal data from the present core, reflect a two-step climatic warming and consequently environmental change during the transition from the Younger Dryas to the Preboreal. An abrupt warming of the sea surface waters took place at about 10 100 BP, which resulted from the inflow of warm Atlantic surface water. Summer sea-surface temperature rose about 8°C. It had a great influence on the surface water of the Skagerrak-Kattegat and shallow areas, but had only little effect on the bottom water in the deep parts of the region. Warming of bottom water occurred a few hundred years after the first warming, and is most likely simultaneous with the end of the Preboreal oscillation. A warm period at 7700-5100 BP is registered by diatom assemblages that resulted from the opening of the English Channel because of eustatic sea-level rise, and by larger influence of the Norwegian Coastal Current and currents along the Swedish west coast. A climate cooling was documented at about 5100 BP by acceleration of the South Jutland Current.

The Eemian and Weichselian stratigraphy of the Langelandselv area, Jameson Land, East Greenland

Coastal Jameson Land is characterized by thick Quaternary deposits from the last interglacial/glacial cycle. The successions at the mouth of Langelandselv exhibit a key stratigraphy where sediments from the Langelandselv interglaciation (Eemian) are overlain by three till units interbedded with glacimarine and deltaic interstadial successions. Immediately after the retreat of glaciers after the extensive Scoresby Sund glaciation (Saalian). advection of warm Atlantic surface water surpassed what is known from the Holocene. The two lowermost Weichselian tills, deposited during the Aucellaelv and Jyllandselv stades (Early Weichselian), reflect short‐lasting readvances of fjord glaciers. Luminescence dates and correlation with adjacent areas suggest ages of 110–80 ka and 70–60 ka for the Hugin Sø and the Møselv interstades, respectively.

Late Quaternary calcium carbonate sedimentation and terrigenous input along the east Greenland continental margin

Nine large box cores collected in the western Greenland-Iceland-Norwegian (GIN) Sea were analyzed for calcium carbonate content and coarse fraction components. Stratigraphic control is based on oxygen isotope records performed on four of the cores. All cores were correlated using oxygen and carbon isotope data, fluctuations in calcium carbonate content, coarse terrigenous particle content and volcanic ash beds. Glacial and interglacial cycles are documented by a number of terrigenous particle events and differentiated calcium carbonate production which can be correlated to major paleoceanographic and paleoclimatic shifts. In the older core sections, extensive deposition of ice-rafted detritus (IRD) persisted until about oxygen isotope stage 9 and document a relatively stable environment in this area, strongly influenced by cold surface water masses. The following interval was characterized by variations in the general surface water circulation pattern, although high amounts of IRD are still present in Iceland Sea sediments. Pronounced shifts in terrigenous input and pelagic carbonate records were identified close to the oxygen isotope stage 9 8 and 6 5 boundaries, indicating massive melting of icebergs in the western GIN Sea. These enormous inputs of debris were followed by an increase in calcium carbonate production caused by maximum intrusion of warm Atlantic surface waters. Relatively high calcium carbonate contents also show that only little dilution by fine-grained material and a minimum of ice melting occurred during these warm phases. During oxygen isotope stage 5, conditions were more uniform indicating a less pronounced westward penetration of Atlantic waters as compared to the Holocene. The last glacial (stages 4-2) is characterized by the occurrence of “Heinrich”-like events, although high IRD contents were present throughout this interval. A distinct contrast between the northernmost cores and the cores in the south is indicative of a rather strong westward penetration of Atlantic water in the north and of an area dominated by cold water east of the Kolbeinsey Ridge during the Holocene. However, conditions west of Jan Mayen seem to have remained constantly dominated by cold surface waters throughout the whole time investigated.

Sudden changes in North Atlantic circulation during the last deglaciation

Sudden changes in the flow of warm Atlantic surface waters into the Norwegian Sea occurred frequently during the last deglaciation, typically involving shifts in sea surface temperature of ⩾5 °C in fewer than 40 years. These led to equally large and rapid changes in atmospheric temperatures, and to shifts in Atlantic deep thermo-haline circulation and ice-sheet melting rates.

Upper Cenozoic history of the Labrador Sea, Baffin Bay, and the Arctic Ocean: A paleoclimatic and paleoceanographic summary

Foraminifera, palynomorphs, and stable isotopes in planktonic foraminifera have been studied in piston cores from the northern Labrador Sea, southern Baffin Bay, and the Alpha Ridge region of the central Arctic Ocean. Foraminifera in cores from the Labrador Sea and southern Baffin Bay show glacial‐interglacial δ18O values similar to those reported from the Norwegian and Greenland seas. In contrast to the eastern North Atlantic Ocean, however, peaks of Subarctic foraminifera and dinoflagellates in the western North Atlantic cores show that relatively warm Atlantic surface water continued to flow into the northern Labrador Sea during the early glaciation growth phases of isotopic stages 2, 4, 6, and 8. Boreal and Subarctic pollen suggest that warm Atlantic air flowed into the northern Labrador Sea and Baffin Bay during the ice sheet growth phases. The advection of warm air masses and the presence of open waters during the summer probably provided moisture for the growth of the Laurentide, Innuitian, and Greenland ice sheets. Isotopic and microfossil records in Arctic cores can also be tentatively correlated with major high latitude north Atlantic glacial‐interglacial events. Magnetostratigraphic, palynological and amino acid dating, however, shows that sedimentation on the Alpha Ridge has been very slow during the past 0.73 m.y.; therefore, events of less than 20,000 years duration cannot be clearly discerned. Interpretation of pre‐Pleistocene paleoenvironments is further limited by the sparseness of calcareous and siliceous microfossils.

Some Factors Affecting the Extent of Ice in the Barents Sea Area

Variations in flow of the Gulf Stream system and in transport of warm Atlantic water across the Wyville-Thomson Ridge northeastwards (via the North Cape Current) affect the areal extent of ice in the Barents Sea. Theoretical considerations and data are presented relating variations in the Florida Current (southern Gulf Stream system), as indicated by mean sea level changes at Charleston, S.C., and Miami, Fla., to ice conditions in the Barents Sea three years later. Low sea level at Charleston and Miami means strong flow of the Florida Current, contraction of the North Atlantic eddy, little warm Atlantic surface water discharged into the Barents Sea and thus more ice. Conversely, high sea level at Charleston and Miami results in more Atlantic water in Barents Sea and less ice. Relationship of ice to winds in Barents Sea area is also briefly discussed. In the period 1925-1938, good agreement between sea level, ice, and wind curves occurs in 12 of the 14 years.