Seasonally ice free glacial nordic seas without deep water ventilation

At present the Nordic Seas are a key region of North Atlantic Deep Water (NADW) formation. Two alternative scenarios have been suggested by some authors for the Last Glacial Maximum: (i) the Nordic Seas were permanently covered by sea ice, preventing the formation of NADW, or (ii) that they were seasonally free of ice and that deep water formation did occur. A modified scenario is presented here based on parallel ocean circulation modelling results from the GFDL primitive equation model and a planetary geostrophic model. It is suggested that the glacial Nordic Seas were at least seasonally ice free, but it is observed that there was never deep water formation from the surface; rather it occurred only in the North Atlantic south of 40°–50°N. North of 40°N, the weaker LGM northward flowing thermohaline conveyor is subducted below a reverse conveyor which occurred to a depth of over 1000 m. Various modelling experiments presented here indicate that the reversed conveyor was primarily caused by the colder conditions of the glacial North Atlantic that led to far stronger zonality of glacial analogue of the North Atlantic Current.     

Late Quaternary changes in surface water and deep water masses of the Nordic Seas and north-eastern North Atlantic: a review

Quantitative and semiquantitative proxy data based on more than 200 core-top samples and 100 deep-sea cores lead to important new insights about late Quaternary changes in paleo-oceanography, climate and microfaunal habitats in the north-eastern North Atlantic and Nordic Seas, insights resulting from a detailed investigation by the Kiel research project SFB 313/132 summarized in this paper. Planktonic foraminifera species provide reliable tracers of past sea surface temperatures and currents. The genus Beella in particular was found to trace subtropical water masses up to the far north. Benthic foraminifera species served as sensors of bottom currents and local flux rates of organic matter. New orders of time resolution are reached via stable isotope stratigraphy and accelerator mass spectrometry carbon-14 dating, allowing the identification of meltwater events lasting a few hundred years and shorter, a time range where, however, the yet unquantified role of bioturbation presents a growing problem. Based on this high-resolution stratigraphy a number of time slices (synoptic time intervals) are defined to reconstruct the incursion of Atlantic water masses, to map paleocurrent patterns within the Nordic Seas and the north-eastern North Atlantic and to test alternative circulation models — for example, for the last glacial maximum (LGM) and various meltwater episodes. These are clearly coeval with Dansgaard-Oeschger events found in Greenland ice cores, with the actual cause of the flickering climate as yet unknown. Likewise, there is ongoing controversy about the extent of past sea-ice cover and about possible changes from the present anti-estuarine to estuarine mode of deep water exchange between the North Atlantic and the Nordic Seas during the LGM. South of Iceland, however, the history of deep water renewal over the last glacial cycle covering the last 30000 years was largely deciphered.     

An Overview of Hydrological Characteristics and Changing Mechanism of Modern Nordic Seas Overflows

As a connection region between Arctic and North Atlantic oceans, the Nordic seas play a critical role in global climate system. The density waters overflow through Greenland-Scotland Ridge from the Nordic seas, as the main source of the North Atlantic Deep Water (NADW), which plays a key role in global ocean conveyor. The causes and processes, which give some instruction of the overflow variation are still uncertain. Based on a review of current and historical research results of modern Nordic seas overflows, hydrological and flux characteristics and variation features of overflows through three channels, which are Faroe-Shetland Channel, Iceland-Faroe Ridge and Denmark Strait, from Nordic sea were addressed separately. The origins of overflows water and factors and physical processes that may have impact on the three overflows were also analyzed separately. Intense mixing in overflow through Faroe-Shetland Channel was discussed. At last, the changing mechanism of the whole overflow from Nordic seas and relationships among overflows through different channels were summed up. The aim of this paper is to give some instructions and research directions to the internal readers.     

A new concept for the paleoceanographic evolution of Heinrich event 1 in the North Atlantic

New records of planktonic foraminiferal δ¹⁸O and lithic and foraminiferal counts from Eirik Drift are combined with published data from the Nordic Seas and the “Ice Rafted Debris (IRD) belt”, to portray a sequence of events through Heinrich event 1 (H1). These events progressed from an onset of meltwater release at ～19 ka BP, through the ‘conventional’ H1 IRD deposition phase in the IRD belt starting from ～17.5 ka BP, to a final phase between 16.5 and ～15 ka BP that was characterised by a pooling of freshwater in the Nordic Seas, which we suggest was hyperpycnally injected into that basin. After ～15 ka BP, this freshwater was purged from the Nordic Seas into the North Atlantic, which preconditioned the Nordic Seas for convective deep-water formation. This allowed an abrupt re-start of North Atlantic Deep Water (NADW) formation in the Nordic Seas at the Bølling warming (14.6 ka BP). In contrast to previous estimates for the duration of H1 (i.e., 1000 years to only a century or two), the total, combined composite H1 signal presented here had a duration of over 4000 yrs (～19–14.6 ka BP), which spanned the entire period of NADW collapse. It appears that deep-water formation and climate are not simply controlled by the magnitude or rate of meltwater addition. Instead the location of meltwater injections may be more important, with NADW formation being particularly sensitive to surface freshening in the Arctic/Nordic Seas.     

Freshwater pulses in the eastern Arctic Ocean during Saalian and Early Weichselian ice-sheet collapse

Improved multiparameter records from the northern Barents Sea margin show two prominent freshwater pulses into the Arctic Ocean during MIS 5 that significantly disturbed the regional oceanic regime and probably affected global climate. Both pulses are associated with major iceberg-rafted debris (IRD) events, revealing intensive iceberg/sea ice melting. The older meltwater pulse occurred near the MIS 5/6 boundary (∼131,000 yr ago); its ∼2000 year duration and high IRD input accompanied by high illite content suggest a collapse of large-scale Saalian Glaciation in the Arctic Ocean. Movement of this meltwater with the Transpolar Drift current into the Fram Strait probably promoted freshening of Nordic Seas surface water, which may have increased sea-ice formation and significantly reduced deep-water formation. A second pulse of freshwater occurred within MIS 5a (∼77,000 yr ago); its high smectite content and relatively short duration is possibly consistent with sudden discharge of Early Weichselian ice-dammed lakes in northern Siberia as suggested by terrestrial glacial geologic data. The influence of this MIS 5a meltwater pulse has been observed at a number of sites along the Transpolar Drift, through Fram Strait, and into the Nordic Seas; it may well have been a trigger for the North Atlantic cooling event C20.     

Variability of the Arctic front during the last climatic cycle: application of a novel molecular proxy

In the Nordic Seas, the Arctic front (AF) marks the boundary between the waters of the North Atlantic Drift/Norwegian Current and those of the Arctic domain. Long- or short-term shifts in the position of the AF may affect climate conditions in the northern hemisphere. Arctic water masses are also the loci of modern open ocean convection; hence, defining these areas in the past is important for reconstructing and modelling ocean circulation and its variability. C₃₇ alkenones are biomarkers for some algae of the Class Prymnesiophyceae (e.g. coccolitho-phorids such as Emiliania huxleyi). These alga occur in most parts of the oceans, in ice-free conditions, and are found nowadays throughout the Nordic Seas. We have related the sedimentary abundance of the tetraunsaturated C₃₇ alkenone (C_37:4) to two types of water masses in the Nordic seas. In locations affected by Atlantic water masses percentages of C_37:4 are less than 5%, whereas in Arctic type water masses these increase to more than 5%. We propose that this observation can be used as a modern analogue to reconstruct the position of the AF in North Atlantic Quaternary sediments. Using this novel molecular proxy we can infer that the southward migration of the AF in the NE Atlantic reached ≈ 50 °N during the last glacial maximum (LGM), but perhaps only 60 °N during the Younger Dryas, and that ocean conditions free of sea ice prevailed throughout the Northern North Atlantic in summer.     

Changes in Atlantic Thermohaline Circulation under Different Atmospheric CO2 Scenarios in a Climate Model

INTRODUCTIONThermohaline circulation(THC)is normally de-fined as the density-driven global-scale oceanic circu-lation,which flows northwardin the upper layer andsouthward in the deeper layer in the Atlantic.Itplays ani mportant role in the global meridional heatand the freshwater transports(Marotzke,2000).Thus,changes in the THC alter the global oceanheat transport and affect the global cli mate(Broeck-er,1991).The increase in the concentration of greenhousegases will reduce the effici…     

Rare earth element transport in the western North Atlantic inferred from Nd isotopic observations

The isotopic composition of Nd in the water column from several western North Atlantic sites and formational areas for North Atlantic Deep Water shows extensive vertical structure at all locations. In regions where a thermocline is well-developed, large isotopic shifts (2 to 3 ϵ units) are observed across the base of the thermocline. Regions without a thermocline are characterized by much more gradual shifts in isotopic composition with depth. In general, the data reveal an excellent correlation between the Nd isotopic distribution in the western North Atlantic water column and the distribution of water masses identified from temperature and salinity characteristics. NADW, as identified from T-S properties, is also characterized by a well-defined isotopic composition having ϵ_Nd(0) = −13.5 ± 0.5. This signature is associated with waters identified as NADW from high latitudes near formational areas in the Labrador Sea down to the equatorial region. The isotopic signature of NADW would appear to be formed by a blend of more negative waters originating in the Labrador Sea (ϵ_Nd(0) < −18) and more positive waters originating in the overflows from the Norwegian and Greenland Seas (ϵ_Nd(0) ≈ −8 to −10) and is consistent with classical theories on the formation of NADW. The isotopic signature of NADW is propagated southward to the equator where it is gradually being thinned out by mixing from above and below with more radiogenic Nd associated with northward-spreading Antarctic Intermediate and Bottom Waters. The preservation of the isotopic signature of NADW over these large distances indicate that the REE undergo extensive lateral transport. The isotopic composition of Nd is largely conservative over the time scales of mixing within the Atlantic in spite of the intrinsic nonconservative behavior of neodymium. Nd concentration gradients generally show surface waters to be depleted in Nd relative to deep waters, which must require vertical transport processes. However, isotopic differences in the water column preclude the local downward transport of REE from the surface into underlying deep waters as a simple explanation of the concentration gradient. The apparent decoupling of REE in NADW from overlying (local) surface waters and the increasing concentration with depth provide a conflict with simple vertical transport mechanisms that is not yet resolved.     

Paleogeography and paleobathymetry of the Mesozoic and Cenozoic North Atlantic Ocean

The history of the North Atlantic Ocean has been traced quantitatively back in time using age and subsidence of the oceanic crust in an attempt to reconstruct the vertical and horizontal elements of its physiographic evolution. Special emphasis has been paid to the history of the Iceland-Faeroe Ridge and of the epicontinental seas around this young basin. The implications of this evolution for changes in the hydrographic regime and for temporal as well as spatial contraints of the surface and bottom water circulation of the North Atlantic are enormous. During its early history this part of the world ocean was connected to the circum-equatorial Tethys Ocean (Late Jurassic to mid-Cretaceous). However, the formation of a deep water pathway to the South Atlantic towards the end of the Mesozoic and the opening of the Norwegian-Greenland Seas during the Early Cenozoic caused the North Atlantic to become part a longitudinal basin allowing an exchange between the Artic and Antarctic polar water masses.Dedicated to Professor Dr. E. Seibold, President of the German Research Society, on the occasion of his 60th birthday.     

Non‐synchronous deposition of North Atlantic Ash Zone II in Greenland ice cores,and North Atlantic and Norwegian Sea sediments: an example of complex glacial‐stage tephra transport

Tephra provides regional chronostratigraphical marker horizons that can link different climate archives with highly needed accuracy and precision. The results presented in this work exemplify, however, that the intermittent storage of tephra in ice sheets and during its subsequent iceberg transport, especially during glacial stages, constitutes a potential source of serious error for the application of tephrochronology to Nordic Seas and North Atlantic sediment archives. The peak shard concentration of the rhyolitic component of the North Atlantic Ash Zone II (NAAZ‐II) tephra complex, often used to correlate marine and ice core records in Marine Isotope Stage (MIS) 3, is shown to lag the eruption event by ca. 100–400 years in some North Atlantic and Norwegian Sea cores. While still allowing for a correlation of archives on millennial timescales, this time delay in deposition is a major obstacle when addressing the lead–lag relationship on short timescales (years to centuries). A precise and accurate determination of lead–lag relationships between archives recording different parts of the climate system is crucial in order to test hypotheses about the processes leading to abrupt climate change and to evaluate results from climate models. Copyright © 2011 John Wiley & Sons, Ltd.