40 research outputs found
Northward advection of Atlantic water in the eastern Nordic Seas over the last 3000 yr
Three marine sediment cores distributed along the
Norwegian (MD95-2011), Barents Sea (JM09-KA11-GC),
and Svalbard (HH11-134-BC) continental margins have been
investigated in order to reconstruct changes in the poleward
flow of Atlantic waters (AW) and in the nature of upper
surface water masses within the eastern Nordic Seas over
the last 3000 yr. These reconstructions are based on a limited
set of coccolith proxies: the abundance ratio between
Emiliania huxleyi and Coccolithus pelagicus, an index of Atlantic
vs. Polar/Arctic surface water masses; and Gephyrocapsa
muellerae, a drifted coccolith species from the temperate
North Atlantic, whose abundance changes are related
to variations in the strength of the North Atlantic Current.
The entire investigated area, from 66 to 77 N, was affected
by an overall increase in AWflow from 3000 cal yr BP
(before present) to the present. The long-term modulation
of westerliesâ strength and location, which are essentially
driven by the dominant mode of the North Atlantic Oscillation
(NAO), is thought to explain the observed dynamics of
poleward AW flow. The same mechanism also reconciles the
recorded opposite zonal shifts in the location of the Arctic
front between the area off western Norway and the western
Barents Seaâeastern Fram Strait region.
The Little Ice Age (LIA) was governed by deteriorating
conditions, with Arctic/Polar waters dominating in the surface
off western Svalbard and western Barents Sea, possibly
associated with both severe sea ice conditions and a strongly
reduced AW strength. A sudden short pulse of resumed high
WSC (West Spitsbergen Current) flow interrupted this cold
spell in eastern Fram Strait from 330 to 410 cal yr BP. Our
dataset not only confirms the high amplitude warming of
surface waters at the turn of the 19th century off western
Svalbard, it also shows that such a warming was primarily
induced by an excess flow of AW which stands as unprecedented
over the last 3000 yr
Synchronized proxy-based temperature reconstructions reveal mid- to late Holocene climate oscillations in High Arctic Svalbard
Existing paleoclimate data are exceedingly sparse from southern Spitsbergen, a High Arctic region predicted to experience significant environmental changes because of amplified warming. We analyzed biotic and isotopic paleolimnological proxies to reconstruct past climate from a lacustrine sediment core, with a basal age of similar to 5500aBP, in southern Spitsbergen (77 degrees N). We used fossil Chironomidae assemblages to quantitatively reconstruct past mean July air temperatures and stable oxygen isotope values (delta O-18) of these fossils to estimate changes in mean annual air temperature. These proxy records are strikingly similar and show that the coldest anomaly since the mid-Holocene occurred between 350 and 50calaBP, during the 'Little Ice Age', whereas the warmest period in the summer temperature record occurred between 5500-5000 and similar to 2000calaBP. Our findings indicate that the natural long-term air temperature dynamics in our study area are most likely connected to solar minima and positive feedback mechanisms from sea-surface temperature maxima. The results also highlight that the recent temperature increase is unprecedented in its rate with a similar to 2 degrees C increase in the summer temperatures during the past similar to 50 years. (C) 2017 John Wiley & Sons, Ltd.Peer reviewe
A new multi-proxy investigation of Late Quaternary palaeoenvironments along the north-western Barents Sea (Storfjorden Trough Mouth Fan)
A new integrated micropalaeontological study on planktonic and benthic foraminifera, calcareous
nannofossils and diatoms was performed on three sediment cores from the Storfjorden Trough Mouth Fan to
reconstruct the Late Quaternary palaeoenvironmental and climatic history. Two main intervals were discussed: the
last deglaciation (16.2\u201311.7 ka BP) and the Holocene. The age model relies on palaeomagnetic parameters together
with 10 radiocarbon dates. Deglacial sediments had largely diluted the biogenic content which was scarce and
poorly preserved. The first occurrence of Cibicidoides wuellerstorfi (benthic foraminifer), together with
Turborotalita quinqueloba (planktonic foraminifer) and Coscinodiscus spp. (diatoms) at 11.3 ka BP followed the end
of the Younger Dryas cold event and marked the beginning of the early Holocene warm period. Diatoms and
planktonic foraminifers indicated a warming of the surface water from 10.5 to 9.2 ka BP, identifying the Holocene
Thermal Maximum event. Bottom water fauna registered these warming conditions less clearly. Cooling events
were identified during the Holocene, in particular the 8.2 ka BP event and the Neoglacial between 3.2 and 2 ka BP,
as shown by the presence of cold-water taxa such as Gephyrocapsa muellerae (nannoplankton) and Neogloboquadrina
pachyderma (planktonic foraminifer). These events were influenced by sea ice extent, cold or
relatively warm current influxes
An abrupt weakening of the subpolar gyre as trigger of Little Ice Age-type episodes
We investigate the mechanism of a decadal-scale weakening shift in the strength of the subpolar gyre (SPG) that is found in one among three last millennium simulations with a state-of-the-art Earth system model. The SPG shift triggers multicentennial anomalies in the North Atlantic climate driven by long-lasting internal feedbacks relating anomalous oceanic and atmospheric circulation, sea ice extent, and upper-ocean salinity in the Labrador Sea. Yet changes throughout or after the shift are not associated with a persistent weakening of the Atlantic Meridional Overturning Circulation or shifts in the North Atlantic Oscillation. The anomalous climate state of the North Atlantic simulated after the shift agrees well with climate reconstructions from within the area, which describe a transition between a stronger and weaker SPG during the relatively warm medieval climate and the cold Little Ice Age respectively. However, model and data differ in the timing of the onset. The simulated SPG shift is caused by a rapid increase in the freshwater export from the Arctic and associated freshening in the upper Labrador Sea. Such freshwater anomaly relates to prominent thickening of the Arctic sea ice, following the cluster of relatively small-magnitude volcanic eruptions by 1600 CE. Sensitivity experiments without volcanic forcing can nonetheless produce similar abrupt events; a necessary causal link between the volcanic cluster and the SPG shift can therefore be excluded. Instead, preconditioning by internal variability explains discrepancies in the timing between the simulated SPG shift and the reconstructed estimates for the Little Ice Age onset
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Assessing reconstruction techniques of the Atlantic Ocean circulation variability during the last millennium
We assess the use of the meridional thermal-wind transport estimated from zonal density gradients to reconstruct the oceanic circulation variability during the last millennium in a forced simulation with the ECHO-G coupled climate model. Following a perfect-model approach, model-based pseudo-reconstructions of the Atlantic meridional overturning circulation (AMOC) and the Florida Current volume transport (FCT) are evaluated against their true simulated variability. The pseudo-FCT is additionally verified as proxy for AMOC strength and compared with the available proxy-based reconstruction. The thermal-wind component reproduces most of the simulated AMOC variability, which is mostly driven by internal climate dynamics during the preindustrial period and by increasing greenhouse gases afterwards. The pseudo-reconstructed FCT reproduces well the simulated FCT and reasonably well the variability of the AMOC strength, including the response to external forcing. The pseudo-reconstructed FCT, however, underestimates/overestimates the simulated variability at deep/shallow levels. Density changes responsible for the pseudo-reconstructed FCT are mainly driven by zonal temperature differences; salinity differences oppose but play a minor role. These results thus support the use of the thermal-wind relationship to reconstruct the oceanic circulation past variability, in particular at multidecadal timescales. Yet model-data comparison highlights important differences between the simulated and the proxy-based FCT variability. ECHO-G simulates a prominent weakening in the North Atlantic circulation that contrasts with the reconstructed enhancement. Our model results thus do not support the reconstructed FC minimum during the Little Ice Age. This points to a failure in the reconstruction, misrepresented processes in the model, or an important role of internal ocean dynamics
Winter amplification of the European Little Ice Age cooling by the subpolar gyre
Climate reconstructions reveal a strong winter amplification of the cooling over central and northern continental Europe during the Little Ice Age period (LIA, here defined as c. 16th-18th centuries) via persistent, blocked atmospheric conditions. Although various potential drivers have been suggested to explain the LIA cooling, no coherent mechanism has yet been proposed for this seasonal contrast. Here we demonstrate that such exceptional wintertime conditions arose from sea ice expansion and reduced ocean heat losses in the Nordic and Barents seas, driven by a multicentennial reduction in the northward heat transport by the subpolar gyre (SPG). However, these anomalous oceanic conditions were largely decoupled from the European atmospheric variability in summer. Our novel dynamical explanation is derived from analysis of an ensemble of last millennium climate simulations, and is supported by reconstructions of European temperatures and atmospheric circulation variability and North Atlantic/Arctic paleoceanographic conditions. We conclude that SPG-related internal climate feedbacks were responsible for the winter amplification of the European LIA cooling. Thus, characterization of SPG dynamics is essential for understanding multicentennial variations of the seasonal cycle in the European/North Atlantic sector
Late Holocene surface water changes in the eastern Nordic Seas : the message from carbonate and organic-walled phytoplankton microfossils
La variabilitĂ© de lâintensitĂ© du flux dâeaux atlantiques et de la nature des masses dâeau de surface le long des marges occidentales de la NorvĂšge, de la mer de Barents et du Svalbard a Ă©tĂ© reconstituĂ©e sur la base des assemblages de coccolithes et dinokystes prĂ©sents dans cinq carottes sĂ©dimentaires marines reprĂ©sentatives de lâHolocĂšne terminal. Les rĂ©sultats sont prĂ©sentĂ©s sous la forme de reconstructions qualitatives et quantitatives (fonctions de transfert MAT) Ă haute rĂ©solution temporelle (Ă©chelle dĂ©cennale Ă sub-sĂ©culaire). Un travail visantĂ valider les traceurs micropalĂ©ontologiques utilisĂ©s a Ă©tĂ© rĂ©alisĂ© en parallĂšle Ă lâobjectif principal, et sâest en particulier nourri de la collecte et de lâexamen de populations vivantes distribuĂ©es le long de plusieurs transectszonaux en mer de NorvĂšge, mer dâIslande et Ă travers le dĂ©troit de Fram.Nos rĂ©sultats indiquent que la partie orientale des mers Nordiques (66 Ă 77°N) a Ă©tĂ© sujette Ă une tendance globale Ă lâaugmentation du flux dâeaux atlantiques (AW) au cours des derniers 3000 ans. La dynamique rĂ©cente de ce flux mĂ©ridien est supposĂ©e rĂ©pondre Ă la modulation long-terme de la force et de la localisation de la ceinture des vents dâouest qui est essentiellement pilotĂ©e par lâOscillation Nord Atlantique. Ce mĂȘme mĂ©canisme atmosphĂ©rique rĂ©concilie le dĂ©placement zonal et contradictoire du front arctique entre le domaine ouest-norvĂ©gien, et les façades occidentales de la mer de Barents et du dĂ©troit de Fram. La variabilitĂ© rapide du flux dâAW reproduit la succession des phases climatiques historiques classiques chaudes (PĂ©riode Chaude Romaine, PĂ©riode Chaude MĂ©diĂ©vale, PĂ©riode Moderne : flux accentuĂ© dâAW) et froides (PĂ©riode Sombre, Petit Age Glaciaire : flux rĂ©duit dâAW) des derniers 2500 ans. Un Ă©vĂ©nĂšnement rapide de renforcement du flux dâAW en mers Nordiques a Ă©tĂ© identifiĂ© pendant le Petit Age Glaciaire entre 330 et 410 ans BP (cal.). Nos rĂ©sultats indiquent que les variations dâintensitĂ© du flux dâAW vers lâOcĂ©an Arctique ont eu un impact majeur sur la distribution de la glace de mer arctique au cours du dernier millier dâannĂ©es, les variations reconstruites de lâextension du couvert de glace Ă lâechelle de lâocĂ©an arctique Ă©tant parfaitement corrĂ©lĂ©es (Ă©chelle subsĂ©culaire) avec nos reconstructions qualitatives de la dynamique de lâAW au large du Svalbard et de la mer de Barents. La diminution importante de lâextension de la banquise durant le 20Ăšme siĂšcle est synchrone dâun flux record dâAW Ă travers le dĂ©troit de Fram, flux qui, dâaprĂšs nos donnĂ©es, est sans prĂ©cĂ©dent pour les derniers 3000 ans.Five marine sediment cores distributed along the Norwegian, western Barents Sea, and Svalbard continental margins have been investigated in order to reconstruct late Holocene changes in the poleward flow of the Norwegian Atlantic Current (NwAC) and West Spitsbergen Current (WSC) and the nature of the upper surface water masses within the eastern Nordic Seas. This research project is based on the use of dinocyst and coccolith assemblages for qualitative and quantitative reconstructions of surface water conditions from high resolution sediment cores, and involve upstream investigations on proxy reliabilities. The investigated area (66 to 77°N) was affected by an overall increase in the strength of the AW flow from 3000 cal. yrs BP to the Present. The long-term modulation of westerlies strength and location which are essentially driven by the dominant mode of the North Atlantic Oscillation (NAO), is thought to explain the observed dynamics of the AW flow. The same mechanism also reconciles the recorded opposite zonal shifts in the location of the Arctic Front between the area off western Norway and the western Barents Sea-eastern Fram Strait region. Submillenial changes in AW flow are organised according to known pre-Anthropocene warm (RWP, MCA and the Modern period: strong poleward flow) and cold (LIA, DA: weak poleward flow) climatic spells. A sudden short pulse of resumed high WSC flow interrupted the LIA in the eastern Nordic Seas from 330 to 410 cal. yrs BP. Our results are indicative of a major impact of AW flow dynamics on the Arctic sea ice distribution during the last millenium, when changes in reconstructed sea-ice extent are negatively correlated with the strength of the WSC flow off western Barents Sea and western Svalbard. The extensive decrease in sea ice extent during the last century is synchronous with an exceptional increase in AW flow. The previously reconstructed high amplitude warming of surface waters in eastern Fram Strait at the turn of the 19th century was therefore primarily induced by an excess flow of AW which stands as unprecedented over the last 3000 years
Late Holocene surface water changes in the eastern Nordic Seas : the message from carbonate and organic-walled phytoplankton microfossils
La variabilitĂ© de lâintensitĂ© du flux dâeaux atlantiques et de la nature des masses dâeau de surface le long des marges occidentales de la NorvĂšge, de la mer de Barents et du Svalbard a Ă©tĂ© reconstituĂ©e sur la base des assemblages de coccolithes et dinokystes prĂ©sents dans cinq carottes sĂ©dimentaires marines reprĂ©sentatives de lâHolocĂšne terminal. Les rĂ©sultats sont prĂ©sentĂ©s sous la forme de reconstructions qualitatives et quantitatives (fonctions de transfert MAT) Ă haute rĂ©solution temporelle (Ă©chelle dĂ©cennale Ă sub-sĂ©culaire). Un travail visantĂ valider les traceurs micropalĂ©ontologiques utilisĂ©s a Ă©tĂ© rĂ©alisĂ© en parallĂšle Ă lâobjectif principal, et sâest en particulier nourri de la collecte et de lâexamen de populations vivantes distribuĂ©es le long de plusieurs transectszonaux en mer de NorvĂšge, mer dâIslande et Ă travers le dĂ©troit de Fram.Nos rĂ©sultats indiquent que la partie orientale des mers Nordiques (66 Ă 77°N) a Ă©tĂ© sujette Ă une tendance globale Ă lâaugmentation du flux dâeaux atlantiques (AW) au cours des derniers 3000 ans. La dynamique rĂ©cente de ce flux mĂ©ridien est supposĂ©e rĂ©pondre Ă la modulation long-terme de la force et de la localisation de la ceinture des vents dâouest qui est essentiellement pilotĂ©e par lâOscillation Nord Atlantique. Ce mĂȘme mĂ©canisme atmosphĂ©rique rĂ©concilie le dĂ©placement zonal et contradictoire du front arctique entre le domaine ouest-norvĂ©gien, et les façades occidentales de la mer de Barents et du dĂ©troit de Fram. La variabilitĂ© rapide du flux dâAW reproduit la succession des phases climatiques historiques classiques chaudes (PĂ©riode Chaude Romaine, PĂ©riode Chaude MĂ©diĂ©vale, PĂ©riode Moderne : flux accentuĂ© dâAW) et froides (PĂ©riode Sombre, Petit Age Glaciaire : flux rĂ©duit dâAW) des derniers 2500 ans. Un Ă©vĂ©nĂšnement rapide de renforcement du flux dâAW en mers Nordiques a Ă©tĂ© identifiĂ© pendant le Petit Age Glaciaire entre 330 et 410 ans BP (cal.). Nos rĂ©sultats indiquent que les variations dâintensitĂ© du flux dâAW vers lâOcĂ©an Arctique ont eu un impact majeur sur la distribution de la glace de mer arctique au cours du dernier millier dâannĂ©es, les variations reconstruites de lâextension du couvert de glace Ă lâechelle de lâocĂ©an arctique Ă©tant parfaitement corrĂ©lĂ©es (Ă©chelle subsĂ©culaire) avec nos reconstructions qualitatives de la dynamique de lâAW au large du Svalbard et de la mer de Barents. La diminution importante de lâextension de la banquise durant le 20Ăšme siĂšcle est synchrone dâun flux record dâAW Ă travers le dĂ©troit de Fram, flux qui, dâaprĂšs nos donnĂ©es, est sans prĂ©cĂ©dent pour les derniers 3000 ans.Five marine sediment cores distributed along the Norwegian, western Barents Sea, and Svalbard continental margins have been investigated in order to reconstruct late Holocene changes in the poleward flow of the Norwegian Atlantic Current (NwAC) and West Spitsbergen Current (WSC) and the nature of the upper surface water masses within the eastern Nordic Seas. This research project is based on the use of dinocyst and coccolith assemblages for qualitative and quantitative reconstructions of surface water conditions from high resolution sediment cores, and involve upstream investigations on proxy reliabilities. The investigated area (66 to 77°N) was affected by an overall increase in the strength of the AW flow from 3000 cal. yrs BP to the Present. The long-term modulation of westerlies strength and location which are essentially driven by the dominant mode of the North Atlantic Oscillation (NAO), is thought to explain the observed dynamics of the AW flow. The same mechanism also reconciles the recorded opposite zonal shifts in the location of the Arctic Front between the area off western Norway and the western Barents Sea-eastern Fram Strait region. Submillenial changes in AW flow are organised according to known pre-Anthropocene warm (RWP, MCA and the Modern period: strong poleward flow) and cold (LIA, DA: weak poleward flow) climatic spells. A sudden short pulse of resumed high WSC flow interrupted the LIA in the eastern Nordic Seas from 330 to 410 cal. yrs BP. Our results are indicative of a major impact of AW flow dynamics on the Arctic sea ice distribution during the last millenium, when changes in reconstructed sea-ice extent are negatively correlated with the strength of the WSC flow off western Barents Sea and western Svalbard. The extensive decrease in sea ice extent during the last century is synchronous with an exceptional increase in AW flow. The previously reconstructed high amplitude warming of surface waters in eastern Fram Strait at the turn of the 19th century was therefore primarily induced by an excess flow of AW which stands as unprecedented over the last 3000 years
PalĂ©ohydrologie de surface des mers nordiques Ă lâHolocĂšne terminal (derniers 3000 ans) : le message du phytoplancton Ă squelette calcaire et organique
Five marine sediment cores distributed along the Norwegian, western Barents Sea, and Svalbard continental margins have been investigated in order to reconstruct late Holocene changes in the poleward flow of the Norwegian Atlantic Current (NwAC) and West Spitsbergen Current (WSC) and the nature of the upper surface water masses within the eastern Nordic Seas. This research project is based on the use of dinocyst and coccolith assemblages for qualitative and quantitative reconstructions of surface water conditions from high resolution sediment cores, and involve upstream investigations on proxy reliabilities. The investigated area (66 to 77°N) was affected by an overall increase in the strength of the AW flow from 3000 cal. yrs BP to the Present. The long-term modulation of westerlies strength and location which are essentially driven by the dominant mode of the North Atlantic Oscillation (NAO), is thought to explain the observed dynamics of the AW flow. The same mechanism also reconciles the recorded opposite zonal shifts in the location of the Arctic Front between the area off western Norway and the western Barents Sea-eastern Fram Strait region. Submillenial changes in AW flow are organised according to known pre-Anthropocene warm (RWP, MCA and the Modern period: strong poleward flow) and cold (LIA, DA: weak poleward flow) climatic spells. A sudden short pulse of resumed high WSC flow interrupted the LIA in the eastern Nordic Seas from 330 to 410 cal. yrs BP. Our results are indicative of a major impact of AW flow dynamics on the Arctic sea ice distribution during the last millenium, when changes in reconstructed sea-ice extent are negatively correlated with the strength of the WSC flow off western Barents Sea and western Svalbard. The extensive decrease in sea ice extent during the last century is synchronous with an exceptional increase in AW flow. The previously reconstructed high amplitude warming of surface waters in eastern Fram Strait at the turn of the 19th century was therefore primarily induced by an excess flow of AW which stands as unprecedented over the last 3000 years.La variabilitĂ© de lâintensitĂ© du flux dâeaux atlantiques et de la nature des masses dâeau de surface le long des marges occidentales de la NorvĂšge, de la mer de Barents et du Svalbard a Ă©tĂ© reconstituĂ©e sur la base des assemblages de coccolithes et dinokystes prĂ©sents dans cinq carottes sĂ©dimentaires marines reprĂ©sentatives de lâHolocĂšne terminal. Les rĂ©sultats sont prĂ©sentĂ©s sous la forme de reconstructions qualitatives et quantitatives (fonctions de transfert MAT) Ă haute rĂ©solution temporelle (Ă©chelle dĂ©cennale Ă sub-sĂ©culaire). Un travail visantĂ valider les traceurs micropalĂ©ontologiques utilisĂ©s a Ă©tĂ© rĂ©alisĂ© en parallĂšle Ă lâobjectif principal, et sâest en particulier nourri de la collecte et de lâexamen de populations vivantes distribuĂ©es le long de plusieurs transectszonaux en mer de NorvĂšge, mer dâIslande et Ă travers le dĂ©troit de Fram.Nos rĂ©sultats indiquent que la partie orientale des mers Nordiques (66 Ă 77°N) a Ă©tĂ© sujette Ă une tendance globale Ă lâaugmentation du flux dâeaux atlantiques (AW) au cours des derniers 3000 ans. La dynamique rĂ©cente de ce flux mĂ©ridien est supposĂ©e rĂ©pondre Ă la modulation long-terme de la force et de la localisation de la ceinture des vents dâouest qui est essentiellement pilotĂ©e par lâOscillation Nord Atlantique. Ce mĂȘme mĂ©canisme atmosphĂ©rique rĂ©concilie le dĂ©placement zonal et contradictoire du front arctique entre le domaine ouest-norvĂ©gien, et les façades occidentales de la mer de Barents et du dĂ©troit de Fram. La variabilitĂ© rapide du flux dâAW reproduit la succession des phases climatiques historiques classiques chaudes (PĂ©riode Chaude Romaine, PĂ©riode Chaude MĂ©diĂ©vale, PĂ©riode Moderne : flux accentuĂ© dâAW) et froides (PĂ©riode Sombre, Petit Age Glaciaire : flux rĂ©duit dâAW) des derniers 2500 ans. Un Ă©vĂ©nĂšnement rapide de renforcement du flux dâAW en mers Nordiques a Ă©tĂ© identifiĂ© pendant le Petit Age Glaciaire entre 330 et 410 ans BP (cal.). Nos rĂ©sultats indiquent que les variations dâintensitĂ© du flux dâAW vers lâOcĂ©an Arctique ont eu un impact majeur sur la distribution de la glace de mer arctique au cours du dernier millier dâannĂ©es, les variations reconstruites de lâextension du couvert de glace Ă lâechelle de lâocĂ©an arctique Ă©tant parfaitement corrĂ©lĂ©es (Ă©chelle subsĂ©culaire) avec nos reconstructions qualitatives de la dynamique de lâAW au large du Svalbard et de la mer de Barents. La diminution importante de lâextension de la banquise durant le 20Ăšme siĂšcle est synchrone dâun flux record dâAW Ă travers le dĂ©troit de Fram, flux qui, dâaprĂšs nos donnĂ©es, est sans prĂ©cĂ©dent pour les derniers 3000 ans