multicentury scale records of carbonate hydrographic variability on the northern iceland margin over the last 5000 years the waters off northern iceland are subjected to extreme hydrographic variability on annual to decadal timescales in years when cold low salinity water moves coastward and sea ice is prevalent i e the late 1960s marine productivity of surface waters is low because the water column is well stratified in the opposite oceanographic mode warm salty atlantic water dominates the shelf and vertical mixing results in high productivity we track these two contrasting modes by measuring the carbonate content of marine sediments a proxy for productivity in three cores from northern iceland the fjord record b997 328pc is anchored by eleven 14c ams dates and has a resolution of one sample every 50 yr thirteen oscillations occurred over the last 4800 cal yr with an average length of 370 yr these are superimposed on a long term decrease in net carbonate accumulation the little ice age is marked by the largest decrease in carbonate content and in flux with smaller minima at 2300 and 3000 cal yr b p marked peaks in carbonate optima occurred at 2000 and 3800 cal yr b p the carbonate record from b997 328pc can be correlated with records on the inner shelf b997 330pc and midshelf b997 327pc indicating that significant regional changes in oceanography occurred at the southern margin of the norwegian greenland sea the marine carbonate fluctuations closely track temperature reconstructions from the greenland summit site for the last 2000 cal yr and show similar but slightly offset oscillations between 2000 and ca 4800 cal yr b p

geology and petroleum potential of upper cretaceous and tertiary strata beaufort mackenzie area northwest canada 1 upper cretaceous to holocene strata on the beaufort sea continental margin were deposited as a series of northward prograding delta complexes totaling 12 16 km thick the succession contains eleven regionally extensive transgressive regressive sequences that were deposited during four large scale tectonostratigraphic phases each phase terminated by a major tectonic event during the cenomanian to middle maastrichtian sedimentation was limited to relatively thin organic rich muds on an outer shelf to basinal environment from the late maastrichtian to middle eocene deltaic depocenters were located along the southwestern margin of the canadian beaufort sea during the third depositional phase middle eocene to late miocene sedimentation was concentrated in the central canadian beaufort sea depocenters shifted to the eastern beaufort sea during the fourth depositional phase pliocene to early pleistocene the modern mackenzie delta may represent a new phase of deposition considerable volumes of oil and gas have been discovered in tertiary rocks principally in eocene and oligocene delta front strata with lesser volumes in paleocene and miocene strata three families of oils have been recognized in tertiary reservoirs the most important of which appears to have been derived from the eocene richards sequence in general tertiary strata have modest toc total organic carbon values 1 2 and low thermal maturity to drilled depths 4000 4500 m in the more extensively explored central beaufort mackenzie area however thermal and subsidence modeling of tertiary strata suggest that maturity rapidly increases between 5000 and 6000 m

application of tephrochronology to the timing and correlation of palaeoceanographic events recorded in holocene and late glacial shelf sediments off north iceland abstract the north icelandic shelf is partly distinguished by the tjornes fracture zone featuring numerous active basins in a mud dominated shelf environment late glacial and holocene high resolution sedimentary records from this area have been studied with tephrochronology as the main tool for correlation and for exact timing of palaeoceanographic events in the area data from three new piston cores from the shelf demonstrate the importance of tephra markers for the first chronological evaluation and correlation of the cores the correlation is extended with lithological logs and with magnetic susceptibility records a detailed multidisciplinary study including biostratigraphy and tephrochronology of a late holocene record of predominantly muddy sediments at the same location demonstrates that marked variations in the distribution of water masses occurred repeatedly through the last 4500 cal yr of special interest is the exact timing of a marked drop in sea surface temperature in the area indicated by ice rafting debris concentration to about 50 years before the hekla 3 eruption which occurred at 2980 cal yr bp this appears to predate most records of a general cooling event in nw europe by a couple of centuries two different possible age models one based on 14c dates combined with tephra markers and one based on tephra markers alone are discussed in context with the problem of different marine reservoir ages of the water masses in the area

late quaternary sedimentary processes and ocean circulation changes at the southeast greenland margin a study has been made of late quaternary depositional processes and bottom current activity on the southeast greenland margin using seismic sub bottom profiling and deep tow side scan sonar data as well as sediment core information the seabed data demonstrate the occurrence of strong southerly bottom currents prevailing on the slope and rise well defined longitudinal bedforms indicate maximum mean near bottom current velocities of up to at least 1 0 m s at the depth stratum of labrador sea water 800 1500 m similarly strong currents occur in denmark strait overflow water dsow at the base of the slope whereas more basinward the maximum dsow flow speed is lower iceberg plow marks were found down to about 700 m water depth both at the shelf edge and on the lower slope and rise the seafloor morphology is indicative of downslope sediment transport and mass flow deposition which is concluded to be a typically glacial feature after generally more sluggish deep water circulation during the last glacial maximum dsow basin ventilation was re established shortly before 13 3 ka on the shelf in front of the retreating greenland ice margin permanent or semi permanent sea ice conditions prevailed until about 12 5 ka at that time increased irminger current activity had resulted in warming and east greenland current egc controlled iceberg drift increased no evidence was found for a return to extreme glacial conditions or a ceasing of dsow flow during the younger dryas abundant coarse ird collected at greater water depth from shallow sub seabed strata has a provenance mainly in the northern part of east greenland 68 73 n which demonstrates the existence of a pre holocene egc system initially extending to beyond the shelf edge

early preboreal cooling in the nordic seas region triggered by meltwater at the younger dryas preboreal transition a high resolution core from the northeastern norwegian sea reveals a two step warming of sea surface temperatures dated at respectively 10 200 10 000 and 9700 9500 14 c yr b p 11 450 11 350 and 11 150 11 000 cal yr b p warming was interrupted by a period having stable temperatures and a reduction in sea surface salinity and we suggest that this pause in warming was triggered by an increase in freshwater supply that may have hampered the north atlantic heat conveyor the freshwater influx correlates to an atmospheric cooling over both the greenland ice sheet and northwest europe and to cooling of surface temperatures in the nordic seas freshwater may have been supplied from the waning fennoscandian ice sheet

late holocene ca 4 ka marine and terrestrial environmental change in reykjarfjordur north iceland climate and or settlement core b997 328pc is from the landward side of a 100 m deep basin in reykjarfjordur a small fjord on the north coast of the northwest peninsula of iceland this is an area that can be severely affected by incursions of polar sea ice but has not suffered marked land erosion during the settlement period the core is 422 cm in length and consists principally of fine grained muds with in situ molluscs eleven ams 14c dates on molluscs indicate a constant sediment accumulation rate of ca 1 cm 10 yr 1 for the last 4280 50 yr bp we measured and derived a variety of proxies for indications of changes in the nearshore environment these included physical properties and grain size mass accumulation rates micropalaeontology pollen and foraminifera multivariate analysis and constrained clustering were used to define major changes in the proxies in several parameters the major change in the fjord environment occurred close to 1000 14c yr ago thus approximately coincident with the onset of the settlement of iceland other changes are noted at around 1600 and 3400 14c yr ago betula pollen disappears from the record ca 1500 yr bp and there are peaks in marine productivity occurring ca 2 and 3 4 ka a major decline in carbonate accumulation and a sharp increase in the cold water benthic foraminifer elphidium excavatum forma clavata 400 yr ago represents the local marine signature of the little ice age none of the changes in our data can be explained adequately by a simple call to land use practices and an increase in land to sea transport but both the pollen and foraminiferal records indicate a decrease and even loss of warm elements in both flora and fauna over the past 4 kyr copyright 2001 john wiley sons ltd

multi proxy records showing significant holocene environmental variability the inner n iceland shelf hunafloi abstract over the last 50 yr the n iceland margin has seen dramatic changes in temperature and salinity associated with the relative dominance of warm atlantic water versus cold arctic or polar waters we report a study of holocene environmental changes to assess the sensitivity of this area on a longer timescale we present sedimentologic isotopic and biotic coccolith proxies from a 5 5 m piston core b997 330pc that was retrieved from a small basin on the inner n iceland shelf seven ams dates indicate a relatively constant rate of sediment accumulation which averaged 55 cm kyr sampling resolution of our proxies varied between 16 and 90 yr sample our data indicate substantial variability in environmental conditions over the last 10 000 cal yr total carbonate content and coccolith concentrations track each other and indicate that the early holocene was a time of low net carbonate accumulation net accumulation of carbonate reached maximum values in the mid to late holocene 4 2 cal ka but there has since been a substantial decrease with a minima during the little ice age north atlantic drift coccolith species show peaks in accumulation rates at 2 and 8 cal ka and low influx between 3 5 and 6 cal ka reworked neogene coccoliths occur also 8 ka and coincide with light 18 o values in the epifaunal foraminifera cibicides lobatulus principal component analysis indicates considerable covariance between biotic isotopic and sedimentological parameters three axes explain 84 of the variance in the data set and define three groups of interrelated variables a major change occurs ca 5 cal ka after which conditions become more variable with a 1 5 kyr cyclicity

ash layers from iceland in the greenland grip ice core correlated with oceanic and land sediments four previously known ash layers ash zones i and ii saksunarvatn and the settlement layer all originating in iceland have been identified in the central greenland ice core grip this correlation of the ash between the different environments is achieved by comparison of the chemical composition of glass shards from the ash this establishes and confirms detailed correlations between the different types of depositional records and the absolute dating of the younger part of the ice core by counting annual layers dates the eruptions accurately a precise connection with dates obtained by14c beyond the range of dendrochronology is established which provides an excellent confirmation of230th 234u dates from corals four additional icelandic ash layers have also been identified in the core but not yet correlated with known ash deposits

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

diatom surface sediment assemblages around iceland and their relationships to oceanic environmental variables canonical correspondence analysis of diatoms from surface sediment samples and oceanographic environmental variables shows that summer and winter sea surface temperatures water depth and winter sea surface salinity are the main environmental factors affecting diatom distribution around iceland of these summer sea surface temperature is the most important five diatom assemblages are distinguished and the distribution of these assemblages is clearly correlated with oceanic current patterns in the region the sea ice diatom assemblage is limited to the area where the east greenland current polar water has its strongest influence and the cold diatom assemblage is basically controlled by the less cold east icelandic current modified polar water the mixing diatom assemblage results from the interaction between the cold east greenland and east icelandic currents and the warm irminger current the warm diatom assemblage is located in the area dominated by the irminger current and may be used as an indicator of warm water masses atlantic water the coastal diatom assemblage is the only one strongly influenced by both water depth and summer water temperatures

north atlantic nordic seas exchanges abstract the northeastern part of the north atlantic is unique in the sense that it is much warmer in the surface than other ocean areas at similar latitudes the main reason for this is the large northward transport of heat that extends to high latitudes and crosses the greenland scotland ridge to enter the nordic seas and the arctic there the warm atlantic water is converted to colder water masses that return southwards over the ridge partly as surface outflows and partly as overflows through the deep passages across the ridge in this paper the state of knowledge on the exchanges especially across the eastern part of the greenland scotland ridge is reviewed based on results from the ices nansen north atlantic norwegian sea exchanges project from the nordic woce project and from other sources the accumulated evidence allows us to describe the exchanges in fair detail the origins of the waters the patterns of their flow towards and over the ridge and their ultimate fate there is also increasing information on temporal variations of the exchanges although dynamical changes are still not well understood quantitative estimates for the volume transport of most of the overflow branches seem reasonably well established and transport measurements of the atlantic inflows to the nordic seas are approaching acceptable levels of confidence which allows preliminary budgets to be presented the deep overflows are driven by pressure gradients set up by the formation of deep and intermediate water the dominance of deep overflows over surface outflows in the water budget argues that this thermohaline forcing also dominates over direct wind stress and estuarine forcing in driving the atlantic water inflow across the greenland scotland ridge while wind stress seems to influence the characteristics and distribution of the atlantic water north of the ridge

late holocene summer sea surface temperatures based on a diatom record from the north icelandic shelf diatom assemblages from a high resolution core on the north icelandic shelf reveal a general late holocene cooling trend which is interrupted by three relatively warm periods diatom data from sur

metal binding to dissolved organic matter and adsorption to ferrihydrite in shallow peat groundwaters application to diamond exploration in the james bay lowlands canada the speciation and solubility of kimberlite pathfinder metals ni nd ba and k in shallow peat groundwaters is investigated over the yankee zulu and golf kimberlites in the attawapiskat region james bay lowlands canada the purpose of this study is to examine the relationship between dissolved organic matter dom complexation with kimberlite pathfinder metals and determine the spatial distribution of those metals in shallow peat groundwaters along sampling transects over subcropping kimberlites nickel nd ba and k complexation with dom and the adsorption of these metals onto ferrihydrite were calculated using visual minteq 3 0 and the nica donnan database calculations predict almost 100 of soluble nd ni and ba form complexes with dom at sampling sites with little to no contribution from upwelling groundwater i e dissolved organic c doc concentrations 40 132 mg l ph 3 9 5 5 and log ionic strength 6 3 in only the most ombrotrophic peat groundwater conditions does a majority fraction of k bind to dom by contrast under conditions with large contributions from upwelling groundwaters i e doc concentrations 640 mg l ph 5 5 6 5 and log ionic strength 3t o 2 as little as 10 of nd and ni and 0 k and ba are predicted to complex with dom the modeling calculations suggest the dominant control on metal dom complexation particularly with respect to ni and nd is competitive effects for dom binding sites due to elevated ionic strength where there is evidence of strong groundwater upwelling visual minteq modeling of metal adsorption on ferrihydrite surfaces predicts that under strong upwelling conditions ni and nd are scavenged from solution due to increased ferrihydrite precipitation and decreased fractions of metals complexed with dom analytical geochemical data are consistent with model predictions of metal adsorption on ferrihydrite total dissolved ni and nd concentrations at sites of strong upwelling are up to five times lower than waters with little to no upwelling and log ferrihydrite saturation indices logsi ferr indicate precipitation values up to 5 at sites of strong groundwater upwelling where the majority of ni and nd complex with dom and ferrihydrite is highly under saturated logsiferr 18 to 5 the concentrations of total ni and nd are elevated compared to other sites along sampling transects metal complexation with dom effectively inhibits metal scavenging from solution via adsorption and or from forming secondary mineral precipitates also because alkaline earth metals do not compete strongly with ni and nd for adsorption sites on ferrihydrite surfaces but do compete strongly for insoluble organic sites ni and nd are more likely to adsorb onto ferrihydrite

late glacial and holocene palaeoceanography of the north icelandic shelf high resolution gravity cores and box cores from the north icelandic shelf have been studied for palaeoceanographic history based on lithological and biostratigraphical foramin iferal data results from two outer shelf cores covering the last 13 6 k 14 c yr bp are presented in this paper the sediments accumulated in north south trending basins on each side of the kolbeinsey ridge at water depths of ca 400 m sedimentation rates up to 1 5 m kyr 1 are observed during the late glacial and holocene the vedde and saksunarvatn tephras are present in the cores as well as the hekla 1104 a new tephra kol gs 2 has been identified and dated to 13 4 k 14 c yr bp and another tephra geochemically identical to the borrobol tephra has been found at the same level at present the oceanographic polar front is located on the north icelandic shelf which experiences sharp oceanographic surface boundaries between the cold east icelandic current and the warmer irminger current past changes in sedimentological and biological processes in the study area are assumed to be related to fluctuations of the polar front the area was deglaciated before ca 14 kyr bp but there is evidence of ice rafting up to the end of the gs 1 greenland stadial 1 younger dryas period increasing again towards the end of the holocene foraminiferal studies show a relatively strong gs 2 pre 13 kyr bp palaeo irminger current followed by severe cooling and then by unstable conditions during the remainder of the gi 1 greenland interstadial 1 bolling allerod and gs 1 younger dryas another cooling event occurred during the preboreal before the holocene current system was established at about 9 kyr bp after a climatic optimum between 9 and 6 kyr bp the climate began to deteriorate and fluctuate copyright 2000 john wiley sons ltd

anomalously mild younger dryas summer conditions in southern greenland the first late glacial lake sediments found in greenland were analyzed with respect to a variety of environmental variables the analyzed sequence covers the time span between 14400 and 10500 calendar yr b p and the data imply that the conditions in southernmost greenland during the younger dryas stadial 12800 11550 calendar yr b p were characterized by an arid climate with cold winters and mild summers preceded by humid conditions with cooler summers climate models imply that such an anomaly may be explained by local climatic phenomenon caused by high insolation and fohn effects it shows that regional and local variations of younger dryas summer conditions in the north atlantic region may have been larger than previously found from proxy data and modeling experiments

oxygen isotope and palaeotemperature records from six greenland ice core stations camp century dye 3 grip gisp2 renland and northgrip oxygen isotope variations spanning the last glacial cycle and the holocene derived from ice core records for six sites in greenland camp century dye 3 grip gisp2 renland and northgrip show strong similarities this suggests that the dominant influence on oxygen isotope variations reflected in the ice sheet records was regional climatic change differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios palaeotemperature estimates have been obtained from the records using three approaches i inferences based on the measured relationship between mean annual 18o of snow and of mean annual surface temperature over greenland ii modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile or iii by using monte carlo simulation techniques the third of these approaches was adopted to reconstruct holocene temperature variations for the dye 3 and grip temperature profiles which yields remarkably compatible results a new record of holocene isotope variations obtained from the northgrip ice core matches the grip short term isotope record and also shows similar long term trends to the dye 3 and grip inverted temperature data the northgrip isotope record reflects i a generally stronger isotopic signal than is found in the grip record ii several short lived temperature fluctuations during the first 1500 yr of the holocene iii a marked cold event at ca 8 2 ka the 8 2 ka event iv optimum temperatures for the holocene between ca 8 6 and 4 3 ka a signal that is 0 6 stronger than for the grip profile v a clear signal for the little ice age and vi a clear signal of climate warming during the last century these data suggest that the northgrip stable isotope record responded in a sensitive manner to temperature fluctuations during the holocene copyright 2001 john wiley sons ltd

the last 18 kyr fluctuations in norwegian sea surface conditions and implications for the magnitude of climatic change evidence from the north sea a combined record of three cores spanning the last 18 kyr from the northern north sea is investigated for content of benthic and planktonic foraminifera and stable oxygen isotopes the paleoenvironmental development through this time period shows an early deglaciation 18 14 4 ka and the younger dryas 12 7 11 5 ka characterized by arctic polar conditions and increased ice rafting in the norwegian channel during the bolling allerod period warm sea surface temperature 9 c conditions similar to present conditions are inferred while bottom waters stayed cold 0 1 c with normal salinity the bolling allerod period is interrupted twice at 13 9 13 6 ka older dryas and at 13 0 12 8 ka inter allerod cooling period by reductions in sea surface temperatures and increased sea ice cover the beginning of the holocene period is marked by increases in surface and bottom water temperature superimposed on the broad climatic changes through the holocene a series of short lived oscillations in the ocean circulation are recorded the amplitude of these holocene events appears larger in the early holocene prior to 8 ka than compared with the remaining part of the holocene this amplification can possibly be attributed to a general increased freshwater budget in the north atlantic at this time during the final stages of the deglaciation of the laurentide and scandinavian ice sheets

environmental changes off north iceland during the deglaciation and the holocene foraminifera diatoms and stable isotopes abstract a combined study of foraminifera diatoms and stable isotopes in marine sediments off north iceland records major changes in sea surface conditions since about 15 800 cal years yr bp results are presented from two gravity cores obtained at about 400 m water depth from two separate sedimentary basins on each side of the submarine kolbeinsey ridge the chronology of the sedimentary record is based partly on ams 14c dates partly on the vedde and the saksunarvatn tephra markers as well as the historical hekla ad 1104 tephra during the regional deglaciation the planktonic foraminiferal assemblages are characterised by consistently high percentages of sinistrally coiled neogloboquadrina pachyderma however major environmental variability is reflected by changes in stable isotope values and diatom assemblages low 18o values indicate a strong freshwater peak as well as possible brine formation by sea ice freezing during a pre bolling interval greenland stadial 2 corresponding to the heinrich 1 event the foraminifera suggest a strong concurrent influence of relatively warm and saline atlantic water and both the foraminifera and the diatoms suggest mixing of cold and warm water masses similar but weaker environmental signals are observed during the younger dryas greenland stadial 1 around the level of the vedde ash each freshwater peak is succeeded by an interval of severe cooling both at the beginning of the bolling allerod interstadial complex greenland interstadial 1 and during the preboreal presumably associated with the onset of intense deep water formatiom in the nordic seas the holocene thermal optimum between 10 200 and about 7000 cal years yr bp is interrupted by a marked cooling of the surface waters around 8200 cal yr bp this cold event is clearly expressed by a pronounced increase in the percentages of sinistrally coiled n pachyderma corresponding to a temperature decrease of about 3 c a general cooling in the area is indicated after 7000 6000 cal yr bp both by the diatom data and by the planktonic foraminiferal data after a severe cooling around 6000 cal yr bp the planktonic foraminiferal assemblages suggest a warmer interval between 5500 and 4500 cal yr bp minor temperature fluctuations are reflected both in the foraminiferal and in the diatom data in the upper part of the record but the time resolution of the present data is not high enough to pick up details in environmental changes through the late holocene

surface sediment diatom distribution and holocene paleotemperature variations in the greenland iceland and norwegian sea one hundred and four surface sediment samples and two piston cores from the greenland iceland and the norwegian gin sea hopkins 1988 were analyzed diatoms were common to abundant in most surface sediment samples and throughout the holocene q mode factor analysis allowed the definition of six significantly different floral assemblages which explained 91 7 of the total variance mapping of the six factors showed their close affinity to hydrographic regimes these are the norwegian atlantic current assemblage thalassionema nitzschioides paralia sulcata proboscia alata arctic water assemblage thalassiosira gravidaspores sea ice assemblage nitzschia grunowii nitzschia cylindra thalassiosira hyalina arctic norwegian waters mixing assemblage rhizosolenia hebetata f semispina rhizosolenia borealis atlantic assemblage thalassiosira oestrupii and norwegian arctic waters mixing assemblage thalassiosira gravida vegetative cells transfer functions gint2 relating factor distributions to winter february and summer august surface water temperatures were generated the transfer functions have a standard error of estimate of 1 5 c august and 1 0 c february downcore studies of core 52 43 from the norwegian basin and core 57 5 from the iceland plateau revealed diatom abundance and sea surface paleotemperature trends in the area during the holocene the first appearance of diatoms in cores 52 43 and 57 5 occurs within the vedde ash layer 10 600 years b p and the saksunarvatn ash layer 9100 years b p respectively diatom abundances increase steadily throughout the pre boreal atlantic time to 25 and 35 million valves per gram dry sediment in the norwegian basin and iceland plateau cores respectively application of the temperature transfer functions gint2 to the two holocene records revealed higher than present sea surface temperatures during the boreal atlantic chronozones for the iceland plateau and during the middle younger dryas sub boreal chronozones for the norwegian basin core temperatures decreased steadily since then with a slight increase to the present values both the iceland and the norwegian seas experienced a strong influx of temperate atlantic surface waters in the beginning of the holocene sea surface temperatures dropped steadily in the latter part of the holocene as the influx of the atlantic surface waters diminished

depositional regimes in the norwegian greenland sea the last two glacial to interglacial transitions various models of surface and deep water circulation in the norwegian greenland sea ngs have been proposed for the last two glacial to interglacial transitions although much progress has been made in understanding the sedimentary response to climatic and oceanographic changes conflicting interpretations have been developed to clarify some of these discrepancies and to test or modify the existing circulation concepts a multiparameter approach is applied combining sedimentological micropaleontological organic geochemical and isotopic methods on the basis of indicative properties a combined litho and organofacies concept is developed and calibrated with modern depositional settings beneath different surface water masses sedimentary regimes are then derived for glacial and deglacial settings

holocene periodicity in north atlantic climate and deep ocean flow south of iceland climate fluctuations during the past millennium are relatively well documented1 on a longer timescale there is growing evidence of millennial scale variability of holocene climate at periodicities of 2 500 and 950 years possibly caused by changes in solar flux 2 3 and 1 500 years maybe related to an internal oscillation of the climate system 4 5 6 but the involvement of deep water masses in these holocene climate changes has yet to be established here we use sediment grain size data from the iceland basin to reconstruct past changes in the speed of deep water flow the study site is under the influence of iceland scotland overflow water isow the flow of which is an important component of the thermohaline circulation that modulates european climate flow changes coincide with some known climate events the little ice age and the mediaeval warm period and extend over the entire holocene epoch with aquasi periodicity of 1 500 years the grain size data indicate afaster isow flow when the climate of northern europe is warmer however a second mode of operation is observed in the early holocene when warm climate intervals are associated with slower isow flow at that time the melting remnant of land based glacial age ice may have provided a sufficient source of fresh water to the ocean to reduce isow flow south of iceland

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

a regional 8200 cal yr bp cooling event in northwest europe induced by final stages of the laurentide ice sheet deglaciation the most notable change in 18o in greenland ice cores during the holocene occurs at 8200 cal yr bp here we present a new high resolution marine record from the northern north sea along with tree ring data from germany which contain evidence of a pronounced temperature drop 2 c contemporaneous with that of the greenland ice core records the synchronous timing of the cooling event in the greenland ice cores marine record and tree ring data from northwest europe reflects a regional influence on the north atlantic ocean atmospheric system suggesting a prominent role of the north atlantic thermohaline circulation the operation of the north atlantic ocean circulation is sensitive to variation in the freshwater budget implying that any change in freshwater flux is capable of altering the north atlantic circulation system we hypothesise minor but long term freshwater fluxes in the final stages of the deglaciation of the laurentide ice sheet as a forcing mechanism 1998 john wiley sons ltd

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

rapid changes in the mechanism of ocean convection during the last glacial period high amplitude rapid climate fluctuations are common features of glacial times the prominent changes in air temperature recorded in the greenland ice cores1 2 are coherent with shifts in the magnitude of the northward heat flux carried by the north atlantic surface ocean3 4 changes in the ocean s thermohaline circulation are a key component in many explanations of this climate flickering5 here we use stable isotope and other sedimentological data to reveal specific oceanic reorganizations during these rapid climate change events deep water was generated more or less continuously in the nordic seas during the latter part of the last glacial period 60 to 10 thousand years ago but by two different mechanisms the deep water formation occurred by convection in the open ocean during warmer periods interstadials but during colder phases stadials a freshening of the surface ocean reduced or stopped open ocean convection and deep water formation was instead driven by brine release during sea ice freezing these shifting magnitudes and modes nested within the overall continuity of deep water formation were probably important for the structuring and rapidity of the prevailing climate changes

chronology of late holocene climatic events in the northern north atlantic based on ams 14c dates and tephra markers from the volcano hekla iceland a combination of ams14c dating and tephrochronology has been used to date late holocene oceanographic events in a 335 cm marine record covering about 4600 cal yr with sedimentation rates exceeding 80 cm 1000 yr 1 the core site is located 50 km offshore on the northern icelandic shelf tephra markers from iceland serve to correlate the marine and terrestrial records especially notable is the presence of three geochemically correlated tephra markers from the icelandic volcano hekla hekla 4 hekla 3 and hekla 1104 benthic and planktonic foraminiferal abundance and distribution as well as the petrography of the sand fraction of the muddy shelf sediments are used as palaeoceanographic proxies the foraminiferal assemblages reflect a general cooling trend during the last 4600 yr a marked drop in sea surface temperatures is registered at about 3000 cal yr bp corresponding to the level of the hekla 3 tephra there is faunal indication of temperature amelioration during the medieval warm period and a cooling again during the little ice age periods of ice rafting events are indicated by ice rafted debris ird concentrations e g at around 3000 cal yr bp and during the little ice age the former event occurred just prior to the deposition of the hekla 3 tephra marker the largest holocene hekla eruption a correlation with terrestrial climatic events in iceland is presented a standard marine reservoir correction of 400 14c yr appears to be reasonable at least during periods with high influence of water masses from the irminger current on the northern icelandic shelf an increase to ca 530 14c yr may have occurred however when water masses derived from the east greenland current were dominant in the area copyright 2000 john wiley sons ltd

complexity of holocene climate as reconstructed from a greenland ice core glaciochemical time series developed from summit greenland indicate that the chemical composition of the atmosphere was dynamic during the holocene epoch concentrations of sea salt and terrestrial dusts increased in summit snow during the periods 0 to 600 2400 to 3100 5000 to 6100 7800 to 8800 and more than 11 300 years ago the most recent increase and also the most abrupt coincides with the little ice age these changes imply that either the north polar vortex expanded or the meridional air flow intensified during these periods and that temperatures in the mid to high northern latitudes were potentially the coldest since the younger dryas event

transfer function between surface sediment diatom assemblages and sea surface temperature and salinity of the labrador sea abstract q mode factor analysis cabfac of 38 diatom species in 66 surface sediment samples from the labrador sea allowed the definition of six assemblages explaining 95 69 of the total variance the first assemblage 40 12 of the total variance is represented by thalassiothrix longissima a species characteristic of the irminger current and associated with the west greenland current the second assemblage 34 52 of the total variance is represented by thalassiosira gravida resting spores a cold water species associated with the labrador current a third assemblage 11 43 of the variance represented by chaetoceros spp is concentrated in the northeastern section of the labrador sea and associated with upwelling and high productivity the fourth the fifth and the sixth assemblages represented respectively by coscinodiscus marginatus coscinodiscus oculus iridis and coscinodiscus divisus and nitzschia frigida are less significant and account respectively for 4 95 2 62 and 2 04 of the total variance transfer functions derived by relating factor assemblages to surface water temperature and salinity august and february give standard errors of estimate of 0 93 c august and 0 64 c february for temperature and of 0 58 august and 0 45 february for salinity high correlations between the measured and estimated parameters confirm the validity of the paleoecological equations for the reconstruction of the labrador sea paleoclimate

a multiproxy reconstruction of the evolution of deep and surface waters in the subarctic nordic seas over the last 30 000yr on the basis of various lithological mircopaleontological and isotopic proxy records covering the last 30 000 calendar years cal kyr the paleoenvironmental evolution of the deep and surface water circulation in the subarctic nordic seas was reconstructed for a climate interval characterized by intensive ice sheet growth and subsequent decay on the surrounding land masses the data reveal considerable temporal changes in the type of thermohaline circulation open water convection prevailed in the early record providing moisture for the fennoscandian barents ice sheets to grow until they reached the shelf break at 26 cal kyr and started to deliver high amounts of ice rafted debris ird into the ocean via melting icebergs low epibenthic 18o values and small sized subpolar foraminifera observed after 26 cal kyr may implicate that advection of atlantic water into the nordic seas occurred at the subsurface until 15 cal kyr although modern like surface and deep water conditions first developed at 13 5 cal kyr thermohaline circulation remained unstable switching between a subsurface and surface advection of atlantic water until 10 cal kyr when ird deposition and major input of meltwater ceased during this time two depletions in epibenthic 13c are recognized just before and after the younger dryas indicating a notable reduction in convectional processes despite an intermittent cooling at 8 cal kyr warmest surface conditions existed in the central nordic seas between 10 and 6 cal kyr however already after 7 cal kyr the present day situation gradually evolved verified by a strong water mass exchange with the arctic ocean and an intensifying deep convection as well as surface temperature decrease in the central nordic seas this process led to the development of the modern distribution of water masses and associated oceanographic fronts after 5 cal kyr and eventually to today s steep east west surface temperature gradient the time discrepancy between intensive vertical convection after 5 cal kyr but warmest surface temperatures already between 10 and 6 cal kyr strongly implicates that widespread postglacial surface warming in the nordic seas was not directly linked to the rates in deep water formation

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

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

the marine 14c age of the vedde ash bed along the west coast of norway articulated molluscs sea urchins and barnacle fragments close to the vedde ash bed in a shallow marine deposit on the west coast of norway have been 14c dated the weighted mean of four dates from a sediment slice 8 cm thick centred on the vedde ash bed is 10920 24 14c yr bp the most accurate 14c age of the vedde ash from terrestrial plant macrofossils is 10310 50 yr bp the difference is the 14c reservoir age for coastal water at the west coast of norway during the mid younger dryas and equals 610 55 yr this is 230 yr older than the reservoir age for the bolling allerod and for the present day in this area the result supports earlier conclusions of a higher reservoir age for the younger dryas in the north atlantic and nordic seas although our reservoir age of 610 55 yr is a few hundred years younger this suggests that the 14c reservoir age at vedde ash time may increase from coastal water towards the open north atlantic and nordic seas copyright 2001 john wiley sons ltd

benthic δ18o records in the north atlantic over the last glacial period 60 10 kyr evidence for brine formation four high resolution benthic isotope records from north atlantic and south norwegian sea cores are compared for the past 60 kyr the benthic records show light 18o peaks simultaneously to or following the heinrich events the amplitude of these light peaks varies between 1 for the most northern core enam93 21 and 0 35 for the deep north atlantic cores the high 18o gradient between the north atlantic cores and the pacific core v19 30 suggests a regional influence of 18o depleted meltwater to deeper depth during periods of iceberg discharges the most likely process for transferring 18o depleted meltwater to depth is sea ice formation induced by brine formation during heinrich events 4 and 3 this mecanism was of major importance compared to open ocean convection in the southern norwegian sea and at the northern boundaries of the atlantic leading to intermediate to deep water formation other 18o excursions appearing in all the records point to the contribution of both brine formation and global changes in the 18o of seawater associated with the heinrich event periods

sortable silt and fine sediment size composition slicing parameters for palaeocurrent speed and palaeoceanography fine sediment size 63 ixm is best measured by a sedimentation technique which records the whole size distribution repeated size measurement with intermediate steps of removal of components by dissolution allows inference of the size distribution of the removed component as well as the residue in this way the size of the biogenic and lithogenic noncarbonate fractions can be determined observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 ixm the dynamics of sediment erosion deposition and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10 1xm grain size and noncohesive above that size thus silt coarser than 10 ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed silt that is f mer than 10 ixm behaves in the same way as clay 2 xm useful parameters of the distribution are the 10 63 ixm mean size and the percentage 10 63 ixm in the fine fraction we cannot use size distributions to distinguish the nature of the currents therefore to infer water mass advection speeds i e the mean kinetic energy of the flow ku regions of high eddy kinetic energy ke must be avoided at the present such abyssal regions lie under the high surface k e of major current systems gulf stream kuroshio agulhas antarctic circmpolar current and brazil falldand currents in the argenthe basin this is probably a satisfactory guide for the pleistocene with regard to the carbonate subfraction of the size spectrum size modes due to both cocco iths and foramlnlferal fragments can be recognized and analyzed with the boundary between them again at about 10 lm the flux of less than 10 lm carbonate at pelagic sites above the lysocline is another candidate for a productivity indicator

paleoceanographic reconstructions of surface ocean conditions in the greenland iceland and norwegian seas through the last 14 ka based on diatoms prior reconstructions of the last glacial maximum and the holocene did not include detailed paleoclimate data from the greenland iceland and norwegian gin seas which cover sensitive areas for oceanographic boundaries sea ice distribution and deep water formation in order to fill this gap we studied nine well dated sediment cores from this area and reconstructed the paleoceanographic conditions of the surface gin seas at 2000 year intervals since 13 400 bp based on the diatom record results show that a n s extending sea ice free corridor had opened along norway already at 13 400 bp indicating a northward flow of a branch of the north atlantic drift possibly caused by a change in the jet stream flow due to the decreased height of the north american ice sheet a major change of climatic conditions occurred over the gin seas as the insolation anomaly reached its maximum around 9000 bp when the sea ice cover and the oceanic fronts retreated to a northwesterly position along greenland and the ocean temperatures rose the first half of the holocene is recorded as the warmest period during the last 13 400 bp in the gin seas the duration of this climatic optimum decreased both toward the north and toward the margins of the area the second half of the holocene is characterized by a cooling trend in step with the decreasing insolation these results support the proposal of imbrie et al 1992 that the gin seas region acts as an initial responder to insolation changes

zooming in on heinrich layers theories explaining the origin of the abrupt massive discharges of ice rafted detritus ird into the glacial north atlantic the heinrich layers hls generally point to the laurentide ice sheet as the sole source of these events until it was found that the irds also originated from icelandic and european ice sheets bond and lotti 1995 snoeckx et al 1999 grousset et al 2000 this apparent contradiction must be reconciled as it raises fundamental questions about the mechanism s of hl origin we have analyzed two 12 cm thick hls in an ultrahigh resolution mode 1 2 century intervals in a mid atlantic ridge piston core the 18o record n pachyderma left coiling reveals strong excursions induced by the melting of the icebergs these excursions are associated with a strong decrease in the amount of planktic foraminafersand with a 3 c cooling of the surface waters counts of coarse detrital grains reveal that ird are deposited according to a typical sequence 1 volcanic glass 2 quartz and feldspars 3 detrital carbonate that implies a chronology in the melting of the differentpan atlantic ice sheets sr and nd isotopic composition confirm that in both heinrich layers h1 and h2 precursor ird came from first europe iceland followed then by laurentide derived ird an internal cyclicity can be identified during h1 and h2 about four to six major abrupt discharges occurred roughly on a century timescale the 13c and 15n records reveal that dominant inputs of continent derived organic matter are associated with ird within the hls hiding the plankton productivity signal