Evolution of subpolar North Atlantic surface circulation since the early Holocene inferred from planktic foraminifera faunal and stable isotope records (original) (raw)
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Paleoceanography, 2012
1] At the peak of the previous interglacial period, North Atlantic and subpolar climate shared many features in common with projections of our future climate, including warmer-than-present conditions and a diminished Greenland Ice Sheet (GIS). Here we portray changes in North Atlantic hydrography linked with Greenland climate during Marine Isotope Stage (MIS) 5e using (sub)centennially sampled records of planktonic foraminiferal isotopes and assemblage counts and ice-rafted debris counts, as well as modern analog technique and Mg/Ca-based paleothermometry. We use the core MD03-2664 recovered from a high accumulation rate site ($34 cm/kyr) on the Eirik sediment drift (57 26.34′N, 48 36.35′W). The results indicate that surface waters off southern Greenland were 3−5CwarmerthantodayduringearlyMIS5e.Theseanomalouslywarmseasurfacetemperatures(SSTs)prevaileduntiltheisotopicpeakofMIS5ewhentheywereinterruptedbyacoolingeventbeginningat3-5 C warmer than today during early MIS 5e. These anomalously warm sea surface temperatures (SSTs) prevailed until the isotopic peak of MIS 5e when they were interrupted by a cooling event beginning at 3−5CwarmerthantodayduringearlyMIS5e.Theseanomalouslywarmseasurfacetemperatures(SSTs)prevaileduntiltheisotopicpeakofMIS5ewhentheywereinterruptedbyacoolingeventbeginningat126 kyr BP. This interglacial cooling event is followed by a gradual warming with SSTs subsequently plateauing just below early MIS 5e values. A planktonic d 18 O minimum during the cooling event indicates that marked freshening of the surface waters accompanied the cooling. We suggest that switches in the subpolar gyre hydrography occurred during a warmer climate, involving regional changes in freshwater fluxes/balance and East Greenland Current influence in the study area. The nature of these hydrographic transitions suggests that they are most likely related to large-scale circulation dynamics, potentially amplified by GIS meltwater influences.
Quaternary Science Reviews, 2004
Two sediment cores from the Labrador Sea and one from the Iceland Basin were analysed in order to compare Holocene seasurface conditions across the northern North Atlantic. d 18 O measurements on meso-and epi-planktonic foraminifera (Neogloboquadrina pachyderma left-coiling and Globigerina bulloides), along with sea-surface condition reconstructions using transfer functions based on dinoflagellate cyst assemblages, were used to document thermohaline properties of the upper water column. At the Labrador Sea sites, a decreasing trend in sea-surface temperature following an early Holocene maximum, is observed. At the Iceland Basin site, a hiatus between 13 and 8 ka BP prevents us from documenting the entire Holocene, but a stable to slightly decreasing trend is observed from 8 ka BP onwards. Sea-surface salinity and potential density (s y ) show little variation at the Orphan Knoll site in the Labrador Sea since 7 ka BP, whereas they depict a decreasing trend at the two other sites since ca 8 ka BP. This shift in s y values suggests a progressive enhancement of Labrador Sea Water formation and a relative decrease in the Nordic seas components of the North Atlantic Deep Water (i.e., Denmark Strait Overflow Water and Norwegian Sea Overflow Water). Discrepancies are also observed with regard to millennial-frequency oscillations in sea-surface conditions. At the easternmost site, they likely match previously documented cold water pulses in the north-eastern Atlantic, whereas they do not show any clear pattern at the westernmost sites. Hence, the western and eastern areas show different behaviours with respect to both longterm and high-frequency fluctuations. r
"Surface water conditions at the Integrated Ocean Drilling Program (IODP) Site U1314 (Southern Gardar Drift, 56º 21.8’ N, 27º 53.3’ W, 2820 m depth) were inferred using planktic foraminifer assemblages between Marine Isotope Stage (MIS) 19 and 11 (ca. 800–400 ka). Factor analysis of the planktic foraminifer assemblages suggests that the assemblage was controlled by three factors. The first factor (which explained 49% of the variance) is dominated by transitional and subpolar species and points to warm and salty surface water conditions (Atlantic water). The second factor (37%) is dominated by Neogloboquadrina pachyderma sin and has been associated with the presence of cold and low saline surface waters (Arctic water). Finally, the third factor (9%), linked to a significant presence of Turborotalita quinqueloba, reflects the closeness of the Arctic front (the boundary between Atlantic and Arctic water). The position of the Arctic and Polar fronts has been estimated across the glacial-interglacial cycles studied according to planktic foraminifer abundances from Site U1314 (and their factor analysis) combined with a synthesis of planktic foraminifer and diatom data from other North Atlantic sites. Regarding at the migrations of the Arctic front and the surface water masses distribution across each climatic cycle we determined five phases of development. Furthermore, deep ocean circulation changes observed in glacial-interglacial cycles have been associated with each phase. The high abundance of transitional-subpolar foraminifers (above 65% at Site U1314) during the early interglacial phase indicated that the Arctic front position and surface water masses distribution were similar to present conditions. During the late interglacial phase, N. pachyderma sin and T. quinqueloba slightly increased indicating that winter sea ice slightly expanded southwestwards whereas the ice volume remained stable or was still decreasing. N. pachyderma sin increased rapidly (above 65% at Site U1314) at the first phase of glacial periods indicating the expansion of the Arctic waters in the western subpolar North Atlantic. During the second phase of glacial periods the transitional-subpolar assemblage throve again in the central subpolar North Atlantic associated with strong warming events that followed ice-rafting events. The third phase of glacial periods corresponds to full glacial conditions in which N. pachyderma sin dominated the assemblage for the whole subpolar North Atlantic. This division in phases may be applied to the last four climatic cycles."
Change in the North Atlantic circulation associated with the mid-Pleistocene transition
Climate of the Past, 2018
The southwestern Iberian margin is highly sensitive to changes in the distribution of North Atlantic currents and to the position of oceanic fronts. In this work, the evolution of oceanographic parameters from 812 to 530 ka (MIS20-MIS14) is studied based on the analysis of planktonic foraminifer assemblages from site IODP-U1385 (37 • 34.285 N, 10 • 7.562 W; 2585 m b.s.l.). By comparing the obtained results with published records from other North Atlantic sites between 41 and 55 • N, basin-wide paleoceanographic conditions are reconstructed. Variations of assemblages dwelling in different water masses indicate a major change in the general North Atlantic circulation during MIS16, coinciding with the definite establishment of the 100 ky cyclicity associated with the mid-Pleistocene transition. At the surface, this change consisted in the redistribution of water masses, with the subsequent thermal variation, and occurred linked to the northwestward migration of the Arctic Front (AF), and the increase in the North Atlantic Deep Water (NADW) formation with respect to previous glacials. During glacials prior to MIS16, the NADW formation was very weak, which drastically slowed down the surface circulation; the AF was at a southerly position and the North Atlantic Current (NAC) diverted southeastwards, developing steep south-north, and east-west, thermal gradients and blocking the arrival of warm water, with associated moisture, to high latitudes. During MIS16, the increase in the meridional overturning circulation, in combination with the northwestward AF shift, allowed the arrival of the NAC to subpolar latitudes, multiplying the moisture availability for ice-sheet growth, which could have worked as a positive feedback to prolong the glacials towards 100 ky cycles.
1995
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 •5180 and •513C distributions depict three main modes of surface circulation: (1) The Holocene-style interglacial mode which largely persisted over the last 12.8 •4C 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 •80 event 3.1 and the B611ing. 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. Introduction The circulation regime in the Nordic Seas has a strong impact on the climate of the adjacent continents. Today, the warm waters of the North Atlantic Drift, the Irminger Current, Paper number 95PA01453. 0883-8305/95/95PA-01453 $10.00
Canadian Journal of Earth Sciences, 2008
A micropaleontological investigation was conducted on two sediment cores from the Reykjanes Ridge (RR; core LO09-14; 59°12.30′N, 31°05.94′W) and the Faroe–Shetland Channel (FSC; core HM03-133-25; 60°06.55′N, 06°04.18′W) to document hydrographical changes of the North Atlantic Current (NAC) during the Holocene. Dinocyst and coccolith assemblages were analyzed, and quantitative reconstructions of sea surface temperatures (SSTs) and sea surface salinities (SSSs) were conducted based on dinocyst assemblages. Both proxies suggest a major reorganization of surface circulation patterns in the northeastern North Atlantic between 7 and 5.4 ka BP. At both sites, SSSs before 6.5–7 ka BP were lower than during the mid-late Holocene, suggesting dispersal of meltwater through the NAC. Long term trends of SSTs, however, show higher than present summer SSTs on the RR from 9.3 to ∼6 ka BP, and lower than present SSTs in the FSC until ca. 5.4 ka BP. The contrasted SST trends at the two sites suggest ...
Epic3paleoceanography 15 Pp 76 84 Issn 0883 8305, 2000
To reconstruct the history of water mass exchange between the NE Atlantic and the Nordic seas, sediment cores from ˜2 km water depth were studied across Termination II (TII) and through the last interglaciation (MIS5e). During early TII the sudden appearance of the low-latitude planktonic foraminifera Beella megastoma is noted in both regions along with a steep decrease in benthic foraminiferal δ18O. Since other proxies indicate that surface waters were cold and stratified because of meltwater, conditions which prevented near-surface thermohaline circulation and vertical convection in the Nordic seas, water mass exchange between the two areas occurred at the subsurface. During later TII, surface conditions changed, and this subsurface circulation style was eventually replaced by vertical convection. In the Nordic seas, B. megastoma vanished from the record together with ice-rafted debris (IRD) at the end of TII, while subpolar foraminiferal abundance rose. Peak interglacial conditions with intensive vertical convection now fully developed, generating a bottom water temperature gradient of ˜4°C between the two areas. However, surface water temperatures deteriorated in the Nordic seas already notably before IRD recurred, and δ18O increased at the end of MIS5e.
Paleoceanography, 1995
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 •5180 and •513C distributions depict three main modes of surface circulation: (1) The Holocene-style interglacial mode which largely persisted over the last 12.8 •4C 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 •80 event 3.1 and the B611ing. 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. Introduction The circulation regime in the Nordic Seas has a strong impact on the climate of the adjacent continents. Today, the warm waters of the North Atlantic Drift, the Irminger Current, Paper number 95PA01453. 0883-8305/95/95PA-01453 $10.00