a study of cleaning procedures used for foraminiferal mg ca paleothermometry the various cleaning steps required for preparation of foraminiferal samples for mg ca and sr ca analysis are evaluated for their relative importance and effects on measured elemental ratios it is shown that the removal of silicate contamination is the most important step for the measurement of mg ca ratios in an example bulk sample mg ca decreases from 10 5 to 2 5 mmol mol 1 during clay removal oxidation of organic material causes a lowering of sample mg ca in the order of 10 or approximately 1 c when converted to temperature use of dilute acid leaching to remove adsorbed contaminants causes partial dissolution of the sample carbonate and a corresponding decrease in mg ca reductive treatment also causes dissolution of the sample and a decrease in the mg ca ratio of up to 10 15 sample preparation for sr ca analysis does not require the same degree of rigor as is necessary for mg ca work the within run reproducibility of the method described here for g ruber in a core top sample from the arabian sea was 1 8 mean sample ratio was 4 72 mmol mol 1 when converted to temperature this becomes 28 0 2 c the equivalent result for sr ca was 0 5 mean ratio 1 44 mmol mol 1

holocene environmental evolution of the se greenland shelf north and south of the denmark strait irminger and east greenland current interactions abstract holocene climatic and paleoceanographic development of the se greenland shelf is studied from cores md99 2317 and md99 2322 at sites north and south of the denmark strait respectively lithofacies ird counts calcium carbonate percentages benthic and planktic foraminiferal assemblages and oxygen isotope analyses and summer ssts reveal significant climate variations in the holocene driven by declining solar insolation and its interaction with waning continental ice sheets and changing atmospheric pressure patterns large changes in the east greenland and irminger currents and the greenland ice sheet are manifested as a 4 part division of the holocene an early holocene cold interval dominated by melting of the greenland ice sheet and polar front retreat extends from 11 8 to 9 5 cal kyr bp a cold interval from 9 5 to 8 1 cal kyr bp involved episodic cooling of the irminger current resulting from the last phases of laurentide ice sheet deglaciation and delayed the holocene optimum off east greenland by 3 kyr relative to peak summer solar insolation which likely helped to limit the early holocene melting of the greenland ice sheet the period 8 1 3 5 cal kyr bp represents a climatic optimum interval of maximum greenland ice sheet retreat and strong irminger current inflow to the denmark strait between 6 8 and 3 5 cal kyr bp the irminger current penetrated further north into the nordic seas than has been observed in recent decades this signal is consistent with diminished northerly winds a weaker greenland high and contracted subpolar gyre by 5 cal kyr bp periods of increased polar water and decreasing salinity in the irminger current suggest a transition toward expansion of the subpolar gyre and increased polar water in the egc the neoglacial interval from 3 5 to 0 2 cal kyr bp was cold and variable with increased freshwater forcing from the arctic ocean advance of the greenland ice sheet and southward advance of the polar front enhanced northerly winds and a strengthened greenland high are consistent with thicker and more extensive polar water and greatly diminished northward advection of irminger current in the denmark strait

reevaluation of the oxygen isotopic composition of planktonic foraminifera experimental results and revised paleotemperature equations cultured planktonic foraminifera orbulina universa symbiotic and globigerina bulloides nonsymbiotic are used to reexamine temperature 18o relationships at 15 25 c relationships for both species can be described by linear equations equations for o universa grown under low light ll and high light hl share a slope of 4 80 0 21 c 1 with a hl ll offset of 0 33 due to symbiont photosynthetic activity the effect of co32 on o universa is 0 002 mol 1 kg 1 and is insensitive to temperature for g bulloides ontogenetic effects produce size related trends in temperature 18o whereby larger shells are enriched in 18o relative to smaller specimens the o universa temperature 18o relationships are more accurate than previously published equations for describing plankton tow data our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic cibicidoides data below 10 c temperature 18o relationships for g bulloides provide good agreement with field data for this species from the northeast pacific

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

interaction between warm atlantic sourced waters and the east greenland current in northern denmark strait 68 n during the last 10 600 cal a bp changes in the holocene interaction of the i cold fresh east greenland current egc and ii warm saline irminger current ic in northern denmark strait have been reconstructed from benthic and planktic foraminifera assemblages ice rafted debris grain size analyses and quantitative x ray diffraction during the time from c 10 600 to 8000 cal a bp palaeoceanographic reconstructions reveal waning deglacial influence from the receding greenland ice sheet and presence of a strong egc caused low surface water productivity from c 8000 cal a bp a predominant influence of atlantic sourced ic waters on subsurface water conditions became established in northern denmark strait which accompanied low surface water productivity relatively warm surface and subsurface water conditions i e reduced egc and strong ic influence are found from c 6500 to 4500 cal a bp representing holocene optimum like conditions a mid to late holocene egc strengthening caused increased stratification and formation of a distinct halocline however we recognize millennial scale periods of reduced stratification by an enhanced influence of atlantic sourced ic water on surface water conditions i at c 2500 1400 cal a bp the time of the roman warm period and ii at c 300 cal a bp the later part of the little ice age these periods of oceanic warming probably relate to changes in the subpolar gyre dynamics that led to enhanced entrainment of atlantic sourced ic water into northern denmark strait

seasonal production patterns of planktonic foraminifera in the ne atlantic ocean implications for paleotemperature and hydrographic reconstructions 1 sediment trap samples from omex 2 49 n 13 w provide a continuous record of the seasonal succession of planktonic foraminifera in the midlatitude north atlantic and reveal a complex relationship between periods of production and specific hydrographic conditions neogloboquadrina pachyderma dextral coiling d globigerina bulloides and globorotalia inflata are found in great numbers during both the spring and summer seasons whereas globigerina quinqueloba globorotalia hirsuta globorotalia scitula and globigerinita glutinata are associated predominantly with the increase in productivity during the spring bloom globigerinella aequilateralis orbulina universa and globigerinoides sacculifer are restricted to late summer conditions following the establishment of a warm well stratified surface ocean an annually integrated fauna from the sediment trap comprising 13 000 individuals is used to evaluate the accuracy of five faunal based statistical methods of paleotemperature estimation all of the temperature reconstruction techniques produce estimates of 16 c and 11 c for summer and winter surface temperature respectively which are in excellent agreement with regional hydrographic data and suggest that the sediment trap assemblage is well represented in the core top faunas analysis of the key species that dominate the omex 2 sediment trap fauna g bulloides g inflata and n pachyderma d based on 18o derived temperatures from north atlantic core top samples suggests that seasonal variations in planktonic foraminiferal production are nonuniform across the midlatitudes and that this is likely to complicate reconstructing past seasonal hydrographic dynamics using these taxa

north atlantic climate and deep ocean flow speed changes during the last 230 years variations in the near bottom flow speed of iceland scotland overflow water isow are documented in a 230 year long deep sea sediment record from the eastern flank of reykjanes ridge in the subpolar north atlantic at sub decadal time scales for recent decades the isow palaeocurrent reconstructions show similarities with observational hydrographic data furthermore recent isow flow changes fall mostly within the range of its variability of the past 230 years the record also reveals a hitherto unrecognized coupling of deep flow speeds in the subpolar north atlantic with the north atlantic oscillation nao index with more less vigorous isow flow during negative positive phases of the nao our results suggest that the changes in isow vigor are largely controlled by the transport and characteristics of labrador sea water rather than variations in the overflow itself with implications for the meridional overturning of the atlantic ocean and climate

deglacial sea level record from tahiti corals and the timing of global meltwater discharge the timing of the last deglaciation is important to our understanding of the dynamics of large ice sheets1 and their effects on the earth s surface2 3 moreover the disappearance of the glacial ice sheets was responsible for dramatic increases in freshwater fluxes to the oceans which probably disturbed the ocean s thermohaline circulation and hence global climate4 7 sea level increases bear witness to the melting of continental ice sheets but only two such records from barbados8 9 and new guinea10 11 corals have been accurately dated but these corals overlie active subduction zones where tectonic movements are large and often discontinuous especially in new guinea so the apparent sea level records may be contaminated by a complex tectonic component here we date fossil corals from tahiti which is far from plate boundaries and thus is likely to be tectonically relatively stable and remote from the locations of large former ice sheets the resulting record indicates a large sea level jump shortly before 13 800 calendar years bp which corresponds to meltwater pulse 1a in the barbados coral records8 9 the timing of this event is more accurately constrained in the tahiti record revealing that the meltwater pulse coincides with a short and intense climate cooling event12 15 that followed the initiation of the bolling allerod warm period12 16 but preceded the younger dryas cold event by about 1 000 years

denmark strait during the late glacial maximum and marine isotope stage 3 sediment sources and transport processes abstract the sediment records from north and south of denmark strait cores ps2644 and md99 2323 and on the kangerlussuaq trough mouth fan ktmf md99 2260 are evaluated for the period 12 to 60 cal ka bp with the goal of evaluating 1 the relative roles of the greenland and iceland ice sheets in sediment composition 2 the processes of sediment supply and 3 the relationship if any between the atmospheric derived ice core records and glacial marine sediment history these questions were addressed using counts of coarse 2 mm ice rafted debris ird magnetic susceptibility grain size spectra and sediment mineral composition a significant difference in these variables occurred between ms3 and mis2 with the latter showing small to modest abrupt changes in felsic and mafic minerals and in ird compared to larger and less variable changes during mis2 the dominant bedrock source archived in the cores is the cenozoic basaltic outcrops from east greenland and iceland 60 but there are frequent pulses of felsic rich sediments 17 36 which coincided with greenland stadials there is also a persistent detrital carbonate input the largest shift in most sediment proxies occurred 28 30 cal ka bp reflecting the sustained expansion of the ice sheets the grain size spectra on the snorri drift md99 2323 reflected complex depositional histories involving the supply of poorly sorted glacially derived sediment sand silt and clay and reworking by bottom currents

sortable silt records of intermediate depth circulation and sedimentation in the southwest labrador sea since the last glacial maximum abstract the labrador sea is a vital region for the atlantic meridional overturning circulation amoc where overflow waters from the nordic seas mix with locally produced labrador sea water lsw before exiting to the interior of the atlantic ocean the dynamical sedimentary proxy of mean sortable silt size ss can give information on past changes in deep water circulation speed and the strength of amoc we have produced ss records from two core sites at depths between 1500 and 2000 m on the continental slope east of newfoundland to reconstruct changes in intermediate depth water circulation speed including glacial north atlantic intermediate water and labrador sea water over the past 22 000 years increases in ss appear to coincide with much of the deglaciation as well as the mid late holocene end member modeling suggests that ice rafted debris ird is an important factor in interpreting ss during the deglaciation we find that a robust increase in ss is likely unrelated to ird during the past 5 ka and probably reflects increased flow at intermediate depths due to local production of lsw strengthening as nordic seas overflows weakened at this depth our results highlight both the complications of producing ss records in ird rich slope environments and the promise that this proxy nevertheless has for reconstructing dynamical changes in deep ocean currents

elusive isotopic properties of deglacial meltwater spikes into the north atlantic example of the final drainage of lake agassizthis article is one of a series of papers published in the special issue on the theme polar climate stability network the impact of the final drainage of lake agassiz some 8 4 ka ago on the atlantic meridional overturning circulation is still debated the lack of isotopic response in planktic foraminifer records at the outlet of the drainage channel constitutes a puzzling element in this debate to estimate the response of the 18o salinity relationship in ocean surface waters to drainage we use a core raised from southeast hudson bay to document 18o values of lake waters immediately prior to drainage valves of candona sp an ostracod from lacustrine varved sediments underlying marine clays yielded 18o values clustered around 20 5 versus vpdb vienna peedee belemnite assuming a lake bottom temperature of about 0 c an isotopic composition of 25 versus vsmow vienna standard mean ocean water is calculated for paleolake waters not unlike values reported for earlier stages of lake agassiz this value is only slightly more negative than those estimated for laurentide ice sheet lis meltwaters during the

holocene thinning of the greenland ice sheet the nature of the response of the greenland ice sheet gis to climate change is a fundamental question in climate science but the details remain unclear during the holocene climate optimum 6 000 to 9 000 years ago for example many locations in the northern hemisphere showed unusual warmth yet changes in greenland appeared inconsistent vinther et al now standardize the previously disparate seeming ice core records of climate history on the gis during the holocene and find consistent evidence for a stronger climate optimum than was previously recognized together with elevation reduction and marginal ice sheet thinning the response of the greenland ice sheet gis to changes in climate remains uncertain during the holocene climatic optimum an unusually warm period from about 9 000 to 6 000 years ago changes in greenland appear inconsistent with the rest of the northern hemisphere here the greenland holocene temperature history and the evolution of gis surface elevation are extracted from four gis locations the results may resolve this disparity on entering an era of global warming the stability of the greenland ice sheet gis is an important concern1 especially in the light of new evidence of rapidly changing flow and melt conditions at the gis margins2 studying the response of the gis to past climatic change may help to advance our understanding of gis dynamics the previous interpretation of evidence from stable isotopes 18o in water from gis ice cores was that holocene climate variability on the gis differed spatially3 and that a consistent holocene climate optimum the unusually warm period from about 9 000 to 6 000 years ago found in many northern latitude palaeoclimate records4 did not exist here we extract both the greenland holocene temperature history and the evolution of gis surface elevation at four gis locations we achieve this by comparing 18o from gis ice cores3 5 with 18o from ice cores from small marginal icecaps contrary to the earlier interpretation of 18o evidence from ice cores3 6 our new temperature history reveals a pronounced holocene climatic optimum in greenland coinciding with maximum thinning near the gis margins our 18o based results are corroborated by the air content of ice cores a proxy for surface elevation7 state of the art ice sheet models are generally found to be underestimating the extent and changes in gis elevation and area our findings may help to improve the ability of models to reproduce the gis response to holocene climate

mechanisms behind the temporary shutdown of deep convection in the labrador sea lessons from the great salinity anomaly years 1968 71 abstractfrom 1969 to 1971 convection in the labrador sea shut down thus interrupting the formation of the intermediate dense water masses the shutdown has been attributed to the surface freshening induced by the great salinity anomaly gsa a freshwater anomaly in the subpolar north atlantic the abrupt resumption of convection in 1972 in contrast is attributed to the extreme atmospheric forcing of that winter here oceanic and atmospheric data collected in the labrador sea at ocean weather station bravo and a one dimensional mixed layer model are used to examine the causes of the shutdown and resumption of convection in detail these results highlight the tight coupling of the ocean and atmosphere in convection regions and the need to resolve both components to correctly represent convective processes in the ocean they are also relevant to present day conditions given the increased ice melt in the arctic ocean and from the greenland ice sheet the analysis herein shows that the shutdown was initiat

preferential dissolution of benthic foraminiferal calcite during laboratory reductive cleaning we have investigated the effect of cleaning procedures on eight element ca ratios in three widely used benthic foraminifera species from core top and down core sediments two cleaning techniques were employed 1 comparison between mg cleaning and cd cleaning methods and 2 comparison between the various constituent reagents li ca b ca and sr ca ratios remained unchanged for samples subjected to different treatments but mg ca mn ca zn ca cd ca and u ca were substantially decreased when foraminifera shells were cleaned with reagents containing citrate in contrast no significant decreases in element ca ratios were observed for samples cleaned in n2h4 without citrate indicating that the role of n2h4 was insignificant during reductive cleaning and that citrate is responsible for decreases of mg ca mn ca zn ca cd ca and u ca decreases in these ratios are most likely due to 1 removal of contaminant particles enriched in mg mn zn cd and u and or 2 preferential leaching of caco3 by the formation of stable metal complexes through chelation between citrate and metals heterogeneously distributed in foraminiferal shells due to the negligible effect of n2h4 and preferential dissolution of caco3 during reductive cleaning it is suggested that the reductive cleaning step should be omitted and the mg cleaning method barker et al 2003 could be employed to clean foraminiferal shells for trace element measurements

seasonal mg ca variability of n pachyderma s and g bulloides implications for seawater temperature reconstruction given the importance of high latitude areas in the ocean climate system there is need for a paleothermometer that is reliable at low temperatures here we assess the applicability of the mg ca temperature proxy in colder waters 5 10 c by comparing for the first time the seasonal mg ca and 18o cycles of n pachyderma s and g bulloides using a sediment trap time series from the northern north atlantic while both species show indistinguishable seasonal 18o patterns that clearly track the near surface temperature cycle their mg ca are very different g bulloides mg ca is high 2 0 3 1 mmol mol but varies in concert with the seasonal temperature cycle the mg ca of n pachyderma s on the other hand is low 1 1 1 5 mmol mol and shows only a very weak seasonal cycle the 18o patterns indicate that both species calcify in the same depth zone consequently depth habitat differences cannot explain the contrasting mg ca patterns the elevated mg ca in pristine g bulloides might be due to the presence of high mg phases that are not preserved in fossil shells the contrasting absence of a seasonal trend in the mg ca of n pachyderma s confirms other studies where calcification temperatures were less well constrained the reason for this absence is not fully known but may include species specific vital effects the very different seasonal patterns of both species mg ca underscore the importance of parameters other than temperature in controlling planktonic foraminiferal mg ca our results therefore lend further caution in the interpretation of mg ca temperature reconstructions from high northern latitudes

core top calibration of mg ca in tropical foraminifera refining paleotemperature estimation 1 optimal use of mg ca as a paleotemperature proxy requires establishing calibrations for different species of foraminifera and quantifying the influence of dissolution to achieve this goal we have measured mg ca and 18o in a series of tropical and subtropical core tops including four depth transects the ceara rise the sierra leone rise and the rio grande plateau in the atlantic and the ontong java plateau in the pacific focusing on spinose mixed layer dwelling species globigerinoides ruber and globigerinoides sacculifer and nonspinose thermocline dwelling neogloboquadrina dutertrei shell mg ca in g sacculifer is 5 15 lower than in g ruber while n dutertrei mg ca is 49 55 lower than in g ruber this statistically significant offset has allowed us to establish different calibrations for each species multilinear regression analysis was used to develop calibration equations that include a correction term for the dissolution effect on mg ca in foraminiferal calcite presented in this paper are two sets of calibrations one set using core depth as a dissolution correction and another using co32 as a dissolution parameter the calibrations suggest that g ruber is the most accurate recorder of surface temperature while g sacculifer records temperatures below the surface at 20 30 m the depth habitat of n dutertrei is more uncertain owing to the wide range in habitat depths depending on hydrographic conditions but on average mg ca and 18o data suggest it is at 50 m of the three species n dutertrei is the most sensitive to dissolution up to 23 decrease in shell mg ca per km while g sacculifer is the most resistant

holocene climate variability in the labrador sea formation of labrador sea water proper commenced about 7000 years ago during the holocene interglacial to test whether fresher surface water conditions may have inhibited labrador sea water convection during the early holocene we measured planktonic foraminiferal globigerina bulloides oxygen isotopes 18 o and mg ca ratios at orphan knoll cores hu91 045 093 and md95 2024 3488 m in the labrador sea to reconstruct shallow subsurface summer conditions temperature and seawater 18 o lighter foraminiferal 18 o values are recorded during the early holocene between 11000 and 7000 years ago part of these lighter foraminiferal 18 o values can be explained by increased calcification temperatures reconstructed seawater 18 o values were however still on average 0 5 lighter compared with those of recent times confirming that fresher surface waters in the labrador sea were probably a limiting factor in labrador sea water formation during the early holocene

the spatial and temporal complexity of the holocene thermal maximum the holocene thermal maximum a period of relatively warm climate between 11 000 and 5 000 years ago1 2 is most clearly recorded in the middle and high latitudes2 3 of the northern hemisphere where it is generally associated with the local orbitally forced summer insolation maximum however proxy based reconstructions have shown that both the timing and magnitude of the warming vary substantially between different regions2 3 4 suggesting the involvement of extra feedbacks and forcings here we simulate the holocene thermal maximum in a coupled global ocean atmosphere vegetation model we find that before 7 000 years ago summers were substantially cooler over regions directly influenced by the presence of the laurentide ice sheet whereas other regions of the northern hemisphere were dominated by orbital forcing our simulations suggest that the cool conditions arose from a combination of the inhibition of labrador sea deep convection by the flux of meltwater from the ice sheet which weakened northward heat transport by the ocean and the high surface albedo of the ice sheet we thus conclude that interglacial climate is highly sensitive to relatively small changes in ice sheet configuration

rapid holocene deglaciation of the labrador sector of the laurentide ice sheet abstract retreat of the laurentide ice sheet lis following the last glacial maximum 21 000 yr bp affected regional to global climate and accounted for the largest proportion of sea level rise although the late pleistocene lis retreat chronology is relatively well constrained its holocene chronology remains poorly dated limiting our understanding of its role in holocene climate change and sea level rise here new 10be cosmogenic exposure ages on glacially deposited boulders are used to date the final disappearance of the labrador sector of the lis ls lis these data suggest that following the deglaciation of the southeastern hudson bay coastline at 8 0 0 2 cal ka bp the southwestern margin of the ls lis rapidly retreated 600 km in 140 yr and most likely in 600 yr at a rate of 900 m yr 1 with final deglaciation by 6 8 0 2 10be ka the disappearance of the ls lis 6 8 10be ka and attendant reduction in freshwater runoff may have induced the formation of labrador deep seawater while the loss

millennial scale variations of the holocene north atlantic mid depth gyre inferred from radiocarbon and neodymium isotopes in cold water corals abstract variations in north atlantic ocean mid depth circulation during the holocene are poorly understood it is believed that they had a significant influence on the properties of water entering the nordic sea by redistributing heat and freshwater potentially affecting deep water formation and climate to improve our knowledge of the ne atlantic mid depth circulation radiocarbon and neodymium isotope analyses have been then carried out on precisely dated u th l pertusa and m oculata coral fragments from two sediment cores taken at 750 m water depth on the sw rockall trough margin cold water coral cwc end values vary between 12 2 0 3 and 16 6 0 4 and result from variable contributions of unradiogenic mid depth subpolar gyre mid spg water 15 and more radiogenic eastern north atlantic water enaw 11 which is transported northward to the rockall trough by boundary currents along the european margin increased coral end reflects a westward contraction of the mid spg water and a higher proportion of enaw the mid holocene from 8 8 to 6 8 ka bp is marked by unradiogenic coral end from 16 6 0 4 to 14 6 0 5 indicating a greater eastward extension of the mid spg this is followed by a shift from 6 8 to 5 ka bp toward more radiogenic end values from 15 4 0 3 to 13 3 0 2 suggesting a westward contraction of the mid spg and a higher proportion of enaw the mid holocene long term change in end is characterized by millennial variations of up to 2 5 epsilon units well marked during the late holocene indicating that eastward extension of the mid spg coeval with warm periods in northern europe e g the medieval climatic anomaly and the roman warm period most of the cwc derived 14c values match the global ocean values indicating that the water masses bathing the corals were generally well ventilated the exceptions are a few short intervals of eastward extension of the mid spg which are characterized by lower 14c during the late holocene we propose that these minor 14c fluctuations in the rockall trough may be related to local changes in the mixed layer depth or to variability in the advection of water from the labrador sea where deep convection gives rise to thermocline waters that are relatively depleted in terms of 14c the eastward extension of the mid spg between 8 8 and 6 8 ka bp is associated with the holocene climatic optimum concurrent with a maximum strength of the iceland scotland overflow water isow which is indicative of an increase in the atlantic meridional overturning circulation amoc this period is followed by a transition in the north atlantic circulation which occurred around 6 8 ka bp and coincides with the onset of labrador sea water formation a decrease of the isow strength and a contraction of the mid spg leading to a greater intrusion of saline subtropical water into the subpolar atlantic

holocene sea surface conditions in the north atlantic contrasted trends and regimes in the western and eastern sectors labrador sea vs iceland basin abstract two sediment cores from the labrador sea and one from the iceland basin were analysed in order to compare holocene sea surface conditions across the northern north atlantic 18o 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 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 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 long term and high frequency fluctuations

using proglacial threshold lakes to constrain fluctuations of the jakobshavn isbrae ice margin western greenland during the holocene abstract the future response of the greenland ice sheet gis and its potential contribution to sea level rise are uncertain rapid changes of greenland s outlet glaciers over the past decade have made it difficult to extrapolate ice sheet change into the future this significant short term variability highlights the need for longer term geologic e g holocene records of ice margin fluctuations however a major challenge with reconstructing the gis during the holocene stems from it having been smaller than it is at present thus traditional glacial geologic approaches are not suitable we use radiocarbon dated sediment sequences from seven proglacial threshold lakes spanning 50 km of the western gis margin near jakobshavn isbrae to constrain the timing of early holocene deglaciation the duration that this sector of the western gis was smaller than its present configuration and the timing of its advance during neoglaciation our reconstructions suggest deglaciation 7300 cal yr bp minimum ice extent 6000 5000 cal yr bp and smaller than present ice configuration until at least 2300 cal yr bp for the ice margin south of jakobshavn isbrae and until 400 cal yr bp for the ice margin north of jakobshavn isbrae one relatively large proglacial lake that became briefly ice free during the middle holocene lies in a catchment that likely extends 10s of km inland beneath the gis suggesting significant middle holocene retreat of this portion of the gis the overall pattern of ice sheet change is inconsistent with existing ice sheet model reconstructions for this region but is consistent with numerous paleoclimate proxy and relative sea level data these continuous lacustrine records corroborate but provide closer age control than existing non continuous records of radiocarbon dated reworked bivalves from historical moraines in the region reconstructing ice margin change from proglacial threshold lakes is one of few approaches with the potential to constrain smaller than present ice sheet extent

rapid early holocene deglaciation of the laurentide ice sheet the demise of the laurentide ice sheet during the early holocene epoch is the most recent and best constrained disappearance of a large ice sheet in the northern hemisphere and thus allows an assessment of rates of ice sheet decay as well as attendant contributions to sea level rise here we use terrestrial and marine records of the deglaciation to identify two periods of rapid melting during the final demise of the laurentide ice sheet when melting ice contributed about 1 3 and 0 7cm of sea level rise per year respectively our simulations with a fully coupled ocean atmosphere model suggest that increased ablation due to enhanced early holocene boreal summer insolation was the predominant cause of laurentide ice sheet retreat although the surface radiative forcing in boreal summer during the early holocene is twice as large as the greenhouse gas forcing expected by the year 2100 the associated increase in summer surface air temperatures is very similar we conclude that our geologic evidence for a rapid retreat of the laurentide ice sheet may therefore describe a prehistoric precedent for mass balance changes of the greenland ice sheet over the coming century

amplitude and timing of temperature and salinity variability in the subpolar north atlantic over the past 10 k y paired planktic foraminiferal 18 o and mg ca data reveal trends of increasing temperatures 3 c and salinities in the subpolar north atlantic over the course of the holocene which were punctuated by abrupt events the trends likely refl ect an insolation forced northward retreat of the boundary between polar and north atlantic subsurface waters the superimposed variability does not appear to be periodic but tends to recur within a broad millennial band the records provide convincing evidence of open ocean cooling nearly 2 c and freshening during the 8 2 ka event and suggest similar conditions at 9 3 ka however the two largest temperature oscillations in our record 2 c occurred during the past 4 k y suggesting a recent increase in temperature variability relative to the mid holocene perhaps in response to neoglaciation which began at about this time

refining globigerinoides ruber mg ca paleothermometry in the atlantic ocean abstract the mg ca ratio of the planktonic foraminifera globigerinoides ruber white has become a widely used proxy for reconstructing sea surface temperature sst as numerous studies have shown that temperature is the primary control on mg ca ratios in foraminiferal calcite however a recent study of core top sediments across an atlantic meridional transect suggests that salinity might have a stronger control on foraminiferal mg ca ratios than previously thought by analyzing mg ca ratios and o 18 values in g ruber white arbuszewski et al 2010 found a 27 increase in g ruber mg ca ratios per 1 salinity unit increase for seawater salinities above 35 5 here we use shell weight analyses and sem images from a subset of the core tops used in the arbuszewski et al 2010 study across a narrow depth range 3197 3733 m to show that g ruber shells from the equatorial region are highly dissolved compared to those from the subtropical north and south atlantic gyres significantly impacting their mg ca ssts shell weights from the higher productivity equatorial region of the atlantic are on average 20 and 15 lower than those from the oligotrophic north and south atlantic gyres respectively given the large preservation gradient along the transect studied by arbuszewski et al 2010 application of a single dissolution corrected mg ca sst calibration equation dekens et al 2002 on cores from the subtropical gyres and the equatorial region is not appropriate when regional differences in preservation are considered as well as realistic habitat depths and calcification seasons for g ruber in temperate latitudes we find a strong correlation between observational ssts and calculated g ruber mg ca ssts in core top samples spanning 43 n to 25 s in the atlantic in addition our re calibrated mg ca ssts are more strongly correlated with isotopic calcification temperatures across the transect than originally reported by arbuszewski et al 2010 this study provides evidence that regional differences in preservation rather than salinity significantly affects g ruber mg ca ssts in the atlantic and supports culturing experiments that found salinity only has a minor influence on foraminiferal mg ca ratios

mg ca paleothermometry in high salinity environments planktonic foraminiferal mg ca ratios have become a fundamental temperature proxy in past climate reconstructions however in the highly evaporative seas of the tropics and subtropics anomalously high planktonic foraminiferal mg ca ratios arise possibly linked to high salinities the extent to which salinity affects mg uptake into foraminiferal calcite remains disputed some studies suggest only minor salinity effects whereas others suggest a dominant role here we present new data from the highly saline 40 red sea which separate pure foraminiferal calcite from other phases the results show that high mg ca ratios 7 to 13 mmol mol found by conventional analysis of planktonic foraminifera from a red sea sediment core are not caused by increased mg uptake into foraminiferal calcite in a high salinity setting e g beyond those predicted by culturing studies but instead result from secondary high mg calcite overgrowths the overgrowths likely formed near the sediment seawater interface from caco3 supersaturated interstitial seawater

dynamics of north atlantic deep water masses during the holocene high resolution flow speed reconstructions of two core sites located on gardar drift in the northeast atlantic basin and orphan knoll in the northwest atlantic basin reveal a long term decrease in flow speed of northeast atlantic deep water neadw after 6 500 years benthic foraminiferal oxygen isotopes of sites currently bathed in neadw show a 0 2 depletion after 6 500 years shortly after the start of the development of a carbon isotope gradient between neadw and norwegian sea deep water we consider these changes in near bottom flow vigor and benthic foraminiferal isotope records to mark a significant reorganization of the holocene deep ocean circulation and attribute the changes to a weakening of neadw flow during the mid to late holocene that allowed the shoaling of lower deep water and deeper eastward advection of labrador sea water into the northeast atlantic basin

imprint of holocene climate variability on cold water coral reef growth at the sw rockall trough margin ne atlantic u th ages and temperatures derived from li mg have been measured on coral fragments of lophelia pertusa and madrepora oculata collected from two sediment cores which were taken from cold water coral cwc mounds at 700 790 m water depth at the sw rockall trough margin our data combined with previous published data have allowed us to first estimate the occurrence of cwc at the sw rockall trough margin during the holocene and second to better constrain the environmental conditions driving variability in cwc growth cwc abundance is marked by a pronounced increase in the mid holocene similar to 6 ka and is modulated by millennial scale variability throughout the late holocene the mid holocene proliferation of cwc coincides with lowest ird abundances and a major reorganization of the circulation at thermocline depth in the rockall trough marked by the progressive replacement of the fresh cold sub arctic intermediate water saiw by the saltier and nutrient rich eastern north atlantic water enaw this event must have established a modern like winter mixed layer and thermocline structure generating suitable conditions for enhanced surface productivity downslope transport of food particles bottom current acceleration at mound depth and thus cwc growth several short time intervals of decreased cwc occurrences closely match prominent increases in north atlantic drift ice and storminess in northern europe we therefore propose that high detrital supply and or changes in the vertical density gradient associated with millennial scale ice rafted detritus ird events are the likely controlling factors for cwc growth and subsequent mound formation on the sw rockall trough margin

centennial to millennial scale holocene climate deep water linkage in the north atlantic the stability of the earth s climate is strongly linked to the steadiness of the atmospheric and oceanic transport of heat from low to high latitudes high resolution holocene sedimentary records from the subpolar north atlantic demonstrate a persistent link between centennial to millennial scale fluctuations in the flow speed of iceland scotland overflow water inferred from granulometric proxy data and both the surface north atlantic current component of the meridional overturning circulation moc and published fennoscandian atmospheric temperature variability however these data also highlight the difficulties associated with the interpretation of geochemical and sedimentological deep water circulation proxies as they do not provide a wholly consistent picture the similarity between surface and deep ocean 13c records suggests a preformed component in the deep ocean 13c signal on the other hand the sedimentological results display a strong linkage to changing conditions in the overlying surface ocean waters suggesting this proxy better reflects the activity of the moc at least during the holocene of this region the most widespread oceanic perturbation coincides with an interval of possibly global climatic deterioration around 2700 years ago these results firmly establish deep water production in the nordic seas as a key component in the assortment of variables affecting climate during the holocene

introduction to holocene climate change new perspectives we live in the holocene its variability is our variability to fully understand modern climate change we need to accurately record how climate has been changing over the holocene s 11700 years as that is the backdrop to what is happening now on 4 5 april 2013 the geological society examined a range of evidence for changes in the holocene climate in a meeting entitled holocene climate change and organized by colin summerhayes nick mccave paul valdes graeme barker eric wolff and dan charman we hoped to find out more about what governed the occurrence of past cool events such as those that occurred at 8200 years ago and in the little ice age between roughly ad 1300 and 1850 and warm periods such as the holocene climate optimum the roman warm period and the medieval climate anomaly to what extent were these events global rather than regional what drove them what produced roughly periodic changes at intervals of about 1500 years seen in marine sediment records and speleothems was it ever warmer in the past 10000 years than it has been in the past 20 years if so when and by how much recent improvements in physical stratigraphy chemical stratigraphy and numerical modelling now allow us to examine in much more detail than before the effects of a wide variety of natural causes natural cycles and greenhouse gases and to integrate results from different land ice and ocean areas in ways not formerly possible to improve our understanding of holocene climate change the meeting was deliberately made broad in scope covering changes in ocean circulation and sea level terrestrial change change in the polar regions the modelling of any or all of those and the interaction between climate and humans as is often the case many of the 33 papers and

the influence of salinity on mg ca in planktic foraminifers evidence from cultures core top sediments and complementary δ18o the mg ca ratio in foraminiferal calcite is one of the principal proxies used for paleoceanographic temperature reconstructions but recent core top sediment observations suggest that salinity may exert a significant secondary control on planktic foraminifers this study compiles new and published laboratory culture experiment data from the planktic foraminifers orbulina universa globigerinoides sacculifer and globigerinoides ruber in which salinity was varied but temperature ph and light were held constant combining new data with results from previous culture studies yields a mg ca sensitivity to salinity of 4 4 2 3 4 7 1 2 and 3 3 1 7 per salinity unit 95 confidence respectively for the three foraminifer species studied here comparison of these sensitivities with core top data suggests that the much larger sensitivity 27 4 per salinity unit derived from atlantic core top sediments in previous studies is not a direct effect of salinity rather we suggest that the dissolution correction often applied to mg ca data can lead to significant overestimation of temperatures we are able to reconcile culture calibrations with core top observations by combining evidence for seasonal occurrence and latitude specific habitat depth preferences with corresponding variations in physico chemical environmental parameters although both mg ca and 18o yield temperature estimates that fall within the bounds of hydrographic observations discrepancies between the two proxies highlight unresolved challenges with the use of paired mg ca and 18o analyses to reconstruct paleo salinity patterns across ocean basins the first step towards resolving these challenges requires a better spatially and seasonally resolved 18osw archive than is currently available nonetheless site specific reconstructions of salinity change through time may be valid

seasonal patterns of shell flux δ18o and δ13c of small and large n pachyderma s and g bulloides in the subpolar north atlantic past water column stratification can be assessed through comparison of the delta o 18 of different planktonic foraminiferal species the underlying assumption is that different species form their shells simultaneously but at different depths in the water column we evaluate this assumption using a sediment trap time series of neogloboquadrina pachyderma s and globigerina bulloides from the nw north atlantic we determined fluxes delta o 18 and delta c 13 of shells from two size fractions to assess size related effects on shell chemistry and to better constrain the underlying causes of isotopic differences between foraminifera in deep sea sediments our data indicate that in the subpolar north atlantic differences in the seasonality of the shell flux and not in depth habitat or test size determine the interspecies delta delta o 18 n pachyderma s preferentially forms from early spring to late summer whereas the flux of g bulloides peaks later in the season and is sustained until autumn likewise seasonality influences large and small specimens differently with large shells settling earlier in the season the similarity of the seasonal delta o 18 patterns between the two species indicates that they calcify in an overlapping depth zone close to the surface however their delta c 13 patterns are markedly different 1 parts per thousand both species have a seasonally variable offset from delta c 13 dic that appears to be governed primarily by temperature with larger offsets associated with higher temperatures the variable offset from delta c 13 dic implies that seasonality of the flux affects the fossil delta c 13 signal which has implications for reconstruction of the past oceanic carbon cycle citation jonkers l s van heuven r zahn and f j c peeters 2013 seasonal patterns of shell flux delta o 18 and delta c 13 of small and large n pachyderma s and g bulloides in the subpolar north atlantic paleoceanography 28 164 174 doi 10 1002 palo 20018

observations of the polarization of light reflected from sea ice as part of a large interdisciplinary program investigating the electromagnetic properties of sea ice we made spectral measurements of the albedo reflectance and stokes vector of the reflected radiance field the overall program encompassed observations of sea ice physical properties optical properties and microwave properties plus an extensive modeling effort measurements were made of an evolution sequence including young sea ice pancake ice snow covered ice first year ice and ponded ice the effects of surface roughness were investigated by artificially roughening part of a 31 cm thick smooth young ice sheet spectral and total albedos were sensitive to surface conditions consistent with earlier studies stokes vector observations exhibited the greatest variability in the plane of incidence of the solar beam reflectances at 0 azimuth increased sharply with zenith angle smoother surfaces such as melt ponds pancakes and bare ice exhibited a larger increase than the snow covered cases with their rougher surfaces since the solar zenith angle was close to brewster s angle specularly reflected light was highly polarized in the smooth ice cases there was significant polarization associated with the increase in reflectance implying a substantial contribution from specular reflection this contribution was greater at longer wavelengths where specular reflection was a larger component of the reflected radiance away from the plane of incidence the stokes vector showed little angular variation and the reflected light at these angles was largely unpolarized

holocene water mass changes in the labrador current the labrador current is part of the anticlockwise subpolar gyre and plays a major role in the formation of north atlantic deep water it is influenced by the west greenland and baffin currents supp

the muddy bottom of lake pannon a challenge for dreissenid settlement late miocene bivalvia abstract the late miocene dreissenids of lake pannon a long existing eastern central european lake bear witness to two very different modes of life which allowed these bivalves to successfully settle in what appears to be an inhospitable environment one guild is restricted to the genera mytilopsis conrad 1858 and sinucongeria lorenthey 1894 these representatives are extreme r strategists which tend to produce temporal boom and bust populations during time windows of improved ecological conditions these opportunities are interpreted here to correlate with cyclic shifts in the epilimnion hypolimnion relation and the corresponding oxygenation of bottom waters events which are probably triggered by low frequency orbital forcing the second guild is composed solely of congeria partsch 1836 the paradox of the proliferation of a single species characterised by extreme size and weight in a muddy oxygen deficient lake environment avoided by nearly all other molluscs points to a hidden mechanism not accessible to other species we tentatively link this mechanism with chemosymbiosis functional morphology in interplay with a highly sulphidic over saturated interstitial water support an interpretation of the large congeria subglobosa as a successful h2s pump model

spreading of the labrador sea water to the irminger and iceland basins 1 labrador sea water lsw is a principal convectively formed water mass of the subpolar north atlantic spna using extensive oceanographic archives we demonstrate striking changes in spna caused by massive lsw production during 1987 1994 and document recent salinification and warming imminently bringing spna to the state last time seen in the 1960s two prominent lsw classes are spreading across spna since the 1980s the first record dense deep and voluminous class has been progressively built by intense winter convection through 1987 1994 even though most of this lsw has left spna its remnants are still present there the second shallower class strengthens in 2000 over subsequent years its core becomes slightly thicker and deepens the anomalous signals acquired by these lsw classes in their formation region arrive in the irminger and iceland basins with the characteristic delays of two and five years for deeper lsw and a year and four years for shallower lsw

links between early holocene ice sheet decay sea level rise and abrupt climate change the beginning of the holocene interglacial was marked by ice sheet melting and sea level rise a review of sea level and climate records identifies two sea level jumps associated with the final drainage of glacial lake agassiz and links them to an abrupt cooling event

cosmogenic 10be ages of the saglek moraines torngat mountains labrador cosmogenic 10 be ages on boulders from the saglek moraines of the torngat mountains labrador suggest that the moraines were deposited in their type area 13 4 6 1 5 ka the 10 be ages on boulders from similar moraines in a valley system 100 km north of the type area yield a mean age of 12 0 6 2 1 10 be ka these data support the hypothesis that the saglek moraines were deposited by a regional system of outlet glaciers that drained the laurentide ice sheet and left extensive areas of the torngat mountains ice free as nunataks weighted mean ages for erratic boulders and bedrock 5 50 m higher in elevation than the saglek moraines at both field areas are indistinguishable at 1s from the weighted mean ages of moraines in each area these data either indicate that thicker late wisconsinan ice receded to the level of the saglek moraines at 12 13 10 be ka or that the saglek ice margin represents the maximum late wisconsinan extent but the ice margin fluctuated sufficiently to create a vertical zone of 50 m with essentially the same exposure history

nd and pb isotope signatures of the clay size fraction of labrador sea sediments during the holocene implications for the inception of the modern deep circulation pattern holocene on the basis of sediment supply variations for the last 12 kyr three sources have contributed to the sediment mixture the north american shield the pan african and variscan crusts and the mid atlantic ridge clay isotope signatures indicate two mixtures of sediment sources the first mixture 12 2 6 5 ka is composed of material derived from the north american shield and from a young crustal source from 6 5 ka onward the mixture is characterized by a young crustal component and by a volcanic component characteristic of the midatlantic ridge since the significant decrease in proximal deglacial supplies the evolution of the relative contributions of the sediment sources suggests major changes in the relative contributions of the deep water masses carried by the western boundary undercurrent over the past 8 4 kyr the progressive intensification of the western boundary undercurrent was initially associated mainly with the transport of the northeast atlantic deep water mass until 6 5 ka and with the denmark strait overflow water thereafter the establishment of the modern circulation at 3 ka suggests a reduced influence of the denmark strait overflow water synchronous with the full appearance of the labrador seawater mass our isotopic data set emphasizes several changes in the relative contribution of the two major components of north atlantic deep water throughout the holocene index terms 4267 oceanography general paleoceanography 4558 oceanography physical sediment transport 1040 geochemistry isotopic composition chemistry 9325 information related to geographic region atlantic ocean 9604 information related to geologic time cenozoic keywords clay size fraction sedimentary mixings deep circulation nd and pb isotopes north atlantic labrador sea

variations in the strength of the north atlantic bottom water during holocene abstract we report here on a multi proxy study of the changes in the dynamics and the properties of bottom water mass in the subpolar north altantic during holocene magnetic properties coupled with sortable silt and benthic carbon isotopes are investigated for holocene marine sedimentary sequences located in the charlie gibbs fracture zone 53 n and in central 57 n and southern gardar drift 59 n all the cores are located at water depths bathed by the iceland scotland overflow water isow mixed at the southernmost locality with southern sourced water masses the long term variations in measured proxies are fitted with similar polynomial curves an early holocene event characterized by a shutdown shoaling of the bottom circulation at the deepest sites is most likely related to the main deglacial freshwater inputs it is followed by a progressive strengthening deepening of the overflow water which culminates around 6 kyr in coincidence with the holocene thermal maximum after 6 kyr corresponding to a drastic hydrological reorganization in the north atlantic a general decline in the bottom flow strength is observed until about 2 kyr b p when it reached its present day state after detrending several short periods of reduced bottom flow strength and sedimentary transport from the northern detrital sources are observed with a periodicity of around 600 yr with no clear relationship at this time scale between surface and deep ocean

geomorphological reconstruction of the labrador sector of the laurentide ice sheet abstract the labrador sector formed one of three major domes of the laurentide ice sheet during the last wisconsinan glaciation reconstruction of its evolution is important because it modulated atmospheric and oceanic circulation over 1 100 ka time scales this paper reports new geomorphological evidence of ice flow which is used to reconstruct glimpses of the behaviour of the labrador sector during its evolution flow evidence indicates a southern position 51 n of the labrador ice divide presumed to be the usual location when the ice sheet was close to its maximum extent a previously inferred dramatic surge of labrador ice across hudson strait and onto baffin island at 9 9 9 4 ka is verified seemingly contradictory evidence regarding deglaciation in northern quebec has been analysed and a reconstruction developed that solves this problem deglacial retreat of the labrador ice sheet was highly asymmetric with rapid retreat of the southern margin northwards migration of the centre of mass by 500 km and slow retreat along the northern margin as it shrank its thermal regime changed from cold centred to entirely cold based and it finally fragmented into many residual ice caps and glaciers this dynamic and asymmetric behaviour may be due to destabilisation of the labrador ice sheet triggered by heinrich event ice streaming through hudson strait

freshwater forcing of abrupt climate change during the last glaciation large millennial scale fluctuations of the southern margin of the north american laurentide ice sheet occurred during the last deglaciation when the margin was located between about 43 and 49 n fluctuations of the ice margin triggered episodic increases in the flux of freshwater to the north atlantic by rerouting continental runoff from the mississippi river drainage to the hudson or st lawrence rivers we found that periods of increased freshwater flow to the north atlantic occurred at the same time as reductions in the formation of north atlantic deep water thus providing a mechanism for observed climate variability that may be generally characteristic of times of intermediate global ice volume