quantitative mineralogy of surface sediments on the iceland shelf and application to down core studies of holocene ice rafted sediments abstract quantitative x ray diffraction analyses on the 2 mm sediment fraction from the iceland shelves are reported for subglacial diamictons seafloor surface sediments and the last 2000 cal yr bp from two cores the overall goal of the paper is to characterize the spatial variability of the mineralogy of the present day surface sediments 18 non clay minerals and 7 clay minerals compare that with largely in situ erosional products typified by the composition of subglacial diamictons and finally examine the late holocene temporal variability in mineral composition using multi mineral compositions the subglacial diamictons are dominated in the non clay mineral fraction by the plagioclase feldspars and pyroxene with 36 7 6 1 and 17 9 3 5 wt respectively with smectites being the dominant clay minerals the surface seafloor sediments have similar compositions although there are substantial amounts of calcite plus there is a distinct band of sites from nw to n central iceland that contain 1 6 wt of quartz this latter distribution mimics the modern and historic pattern of drift ice in iceland waters principal component analysis of the transformed wt log ratio non clay minerals is used to compare the subglacial surface and down core mineral compositions fifty eight percent of the variance is explained by the first two axes with dolomite microcline and quartz being important foreign species these analyses indicate that today the nw n central iceland shelf is affected by the import of exotic minerals which are transported and released from drift ice the down core mineralogy indicates that this is a process that has varied over the last 2000 cal yr bp

climate and oceanographic variability in the sw barents sea during the holocene the holocene section of the marine sediment core psh 5159n located in the sw barents sea has been studied at high resolution with a multiproxy approach a well stratified water column occurred at the site 11 9 8 ka bp the stratification was probably a result of a winter sea ice cover and or fresh warm surface waters during summer stratification and resultant reduction in air sea interaction allowed for warmer bottom water temperatures the general situation 11 9 8 ka bp could have been associated with an anomalous high pressure system over the nordic seas and the arctic ocean during the 11 10 5 ka bp interval the polar front was located close to the barents sea margin the polar front moved towards the site from 10 5 ka bp and from 9 8 to 7 5 ka bp it was probably located close to the site at 7 5 ka bp the polar front retreated eastwards as the present day oceanographic pattern established the mid holocene was in general characterized by rather stable conditions in contrast highly variable cond

modern seasonal variability and deglacial holocene change of central arctic ocean sea ice cover new insights from biomarker proxy records for the reconstruction of sea ice variability a biomarker approach which is based on 1 the determination of sea ice diatom specific highly branched isoprenoid ip25 and 2 the coupling of phytoplankton biomarkers and ip25 has been used for the first time such a data set was obtained from an array of two sediment traps deployed at the southern lomonosov ridge in the central arctic ocean at water depth of 150 m and 1550 m and recording the seasonal variability of sea ice cover in 1995 1996 these data indicate a predominantly permanent sea ice cover at the trap location between november 1995 and june 1996 an ice edge situation with increased phytoplankton productivity and sea ice algae input in july august 1996 and the start of new ice formation in late september the record of modern sea ice variability is then used to better interpret data from sediment core ps2458 4 recovered at the laptev sea continental slope close to the interception with lomonosov ridge and recording the post glacial to holocene change in sea ice cover based on ip25 and phytoplankton biomarker data from core ps2458 4 minimum sea ice cover was reconstructed for the bolling allerod warm interval between about 14 5 and 13 calendar kyr bp followed by a rapid and distinct increase in sea ice cover at about 12 8 calendar kyr bp this sea ice event was directly preceded by a dramatic freshwater event and a collapse of phytoplankton productivity having started about 100 years earlier these data are the first direct evidence that enhanced freshwater flux caused enhanced sea ice formation in the arctic at the beginning of the younger dryas in combination with a contemporaneous abrupt and very prominent freshwater meltwater pulse in the yermak plateau fram strait area these data may furthermore support the hypothesis that strongly enhanced freshwater and ice export from the arctic into the north atlantic could have played an important trigger role for the onset of the younger dryas cold reversal during the early holocene sea ice cover steadily increased again ice edge situation reaching modern sea ice conditions more or less permanent sea ice cover probably

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

a deglacial middle holocene record of biogenic sedimentation and paleoproductivity changes from the northern norwegian continental shelf 1 the relative proportions of marine organic carbon and biogenic carbonate in a high resolution record from a glacial trough on the northern norwegian continental shelf were used to decipher changes in biogenic sedimentation and paleoproductivity from the last deglaciation to the middle holocene decadal scale to century scale oscillations in biogenic sedimentation and surface ocean productivity recorded in the andfjorden sediments are synchronous with abrupt climate changes in open oceanic and atmospheric regimes of the high northern latitudes following several fluctuations during the bolling allerod interstadial the most dramatic drop in marine organic carbon and biogenic carbonate proportions occurred during the younger dryas however many short term and low amplitude events during the preboreal and the holocene have also affected biogenic sedimentation in the outer andfjorden apparently these abrupt changes in biogenic sedimentation were caused by the suppression of a potential upwelling center in the outer andfjorden by variable atlantic water inflow over the past 14 000 years the adjustment of decadal high productivity coastal systems to freshwater forced multiple cooling events during the late glacial to middle holocene highlights the impact of global climate changes on the climate sensitive local ecosystems off coastal norway

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

alternative routing of lake agassiz overflow during the younger dryas new dates paleotopography and a re evaluation abstract overflow from glacial lake agassiz has been implicated in affecting late glacial ocean circulation and climate the timing of the diversion of lake agassiz overflow away from its southern routing to the gulf of mexico correlates closely with the younger dryas ca 11 10 14 c ka new models of paleotopography in the eastern outlet region of lake agassiz adjusted for differential isostatic rebound show that overflow to the superior basin would have occurred when the lake formed its herman norcross and tintah beaches just after 10 9 14 c ka if the laurentide ice sheet lis had retreated from the thunder bay ontario region by that time however the absence of coarse flood deposits normally associated with large overflows plus the morphology of possible overflow channels in the thunder bay region suggest that agassiz overflow may not have been routed into the great lakes during the younger dryas as previously believed in addition non carbonate radiocarbon dates in the thunder bay area are all younger than 10 14 c ka suggesting that this area may not have been deglaciated until after the younger dryas thus preventing lake agassiz overflow through this area an alternative route of overflow from lake agassiz during the younger dryas could have been through the northwestern clearwater outlet to the arctic ocean although all research to date shows that this region was covered by ice until 10 14 c ka on the other hand it is possible that older and coarser overflow sediment may now lie buried beneath finer deposits associated with the 10 14 c ka marquette readvance of the lis which ponded waters in valleys between lake agassiz and lake superior or that eastward overflow in this area was not catastrophic

coupled ocean and atmospheric changes during greenland stadial 1 in southwestern europe abstract paleoclimate reconstructions suggest that the complex variability within the greenland stadial 1 gs 1 over western europe was governed by coupled ocean and atmospheric changes however few works from the north atlantic mid latitudes document both the gs 1 onset and its termination which are often considered as single abrupt transition events here we present a direct comparison between marine alkenone based sea surface temperatures and terrestrial pollen data at very high resolution 28 years mean from the southwestern iberian shelf record d13882 our results reveal a rather complex climatic period with internally changing conditions the gs 1 onset gs 1a 12890 12720 yr bp is marked by a progressive cooling and drying gs 1b 12720 12390 yr bp is the coldest and driest phase gs 1c 12390 12030 yr bp is marked by a progressive warming and increase in moisture conditions gs 1 termination gs 1d 12030 11770 yr bp is marked by rapid switches between cool wet cold dry and cool wet conditions although hydroclimate response was very unsteady throughout the gs 1 and in particular during its termination phase the persistence of an open temperate and mediterranean forest in southwestern iberia during the entire episode suggests that at least some moisture was delivered via the westerlies we propose coupled ocean and atmospheric mechanisms to reproduce these scenaria changes in the strength of the atlantic meridional overturning circulation amoc as well as variations in the north atlantic sea ice growth have favoured the displacement of the polar jet stream s latitudinal position and contributed to a complex spatial pattern and strength of the westerlies across western europe

the impact of the north atlantic ocean on the younger dryas climate in northwestern and central europe the main results of multiproxy climate reconstructions for the younger dryas and experiments carried out with an atmospheric general circulation model are discussed quantitative temperature inferences for northwestern and central europe show that winter conditions were remarkably extreme with values of 20 30 c below that of today annual temperature ranges with a continental signature 30 34 c were reconstructed for the entire study area compared with today the greatest cooling was experienced in the seaboard area the comparison of the climate reconstructions and simulation experiments indicates that sea ice in the north atlantic ocean played a decisive role in the climate of the study area by cooling the surface air temperatures and by controlling the position of the storm track surface air circulation was westerly even during younger dryas winters a slight northward shift of the mean atlantic sea ice margin during winter may explain the proposed subdivision of the younger drays into a phase of maximum cold and humidity succeeded by a less cold and relatively dry phase 1998 john wiley sons ltd

semi quantitative estimates of paleo arctic sea ice concentration based on source specific highly branched isoprenoid alkenes a further development of the pip25 index abstract previously reported concentration data for highly branched isoprenoid hbi alkenes and the sterol 24 methylcholesta 5 22e dien 3 ol brassicasterol or epi brassicasterol obtained from 45 barents sea surface sediments reflecting contrasting sea ice conditions have been combined together in the form of the pip 25 index with outcomes compared with recent mean decadal satellite derived spring and summer sea ice concentrations reasonably good positive linear relationships were found between pip 25 indices and spring sea ice concentration with the strongest correlation found when using a tri unsaturated hbi hbi iii as the open water counterpart to the sea ice biomarker ip 25 likely as a result of the greater sensitivity and selectivity of hbi iii to the neighbouring sea ice conditions compared to epi brassicasterol the quality of the linear fits between p iii ip 25 i e pip 25 based on hbi iii and spring sea ice concentration together with their respective spatial distributions were also far less dependent on the balance factor c than for p b ip 25 i e pip 25 based on epi brassicasterol which may have important positive consequences for down core semi quantitative sea ice reconstruction and when making comparisons between outcomes from different arctic regions or climatic epochs in contrast consistently weak linear relationships were observed between individual biomarkers and spring or summer sea ice concentration and between pip 25 values and summer sea ice concentration however we provide evidence that a lower limit threshold for p iii ip 25 might represent a useful qualitative proxy for the past occurrence of summer sea ice surface sediment biomarker data from other regions will be needed before the significance of our findings can be fully evaluated

identification of contrasting seasonal sea ice conditions during the younger dryas abstract a palaeo sea ice reconstruction for northern norway for the interval ca 13 8 7 2 cal ka bp has been performed using a combined organic geochemical proxy based study of a marine sediment core jm99 1200 obtained from andfjorden 69 15 95 n 16 25 09 e and the outcomes compared with a series of other palaeoclimate studies carried out previously within this study particular emphasis has been placed on the identification of the sea ice conditions during the younger dryas and the application of different biomarker based proxies to both identify and quantify seasonal sea ice conditions firstly the appearance of the specific sea ice diatom proxy ip 25 at ca 12 9 cal ka bp in the sedimentary record has provided an unambiguous but qualitative measure of seasonal sea ice and thus the onset of the younger dryas stadial the near continuous occurrence of ip 25 for the next ca 1400 yr demonstrates seasonal sea ice during this interval although variable abundances suggest that the recurrent conditions in the early mid younger dryas ca 12 9 11 9 cal ka bp changed significantly from stable to highly variable sea ice conditions at ca 11 9 cal ka bp and this instability in sea ice prevailed for the subsequent ca 400 yr during which an extremely short interval of permanent or near permanent sea ice was observed at ca 11 75 cal ka bp at ca 11 5 cal ka bp ip 25 disappeared from the record indicating ice free conditions that signified the beginning of the holocene the ip 25 concentrations were also combined with those of the open water phytoplankton biomarker brassicasterol to generate p b ip 25 data from which more quantitative measurements of sea ice were determined p b ip 25 data were consistent with consecutively seasonal then infrequent sea ice cover for the early mid and late younger dryas while further comparisons of the p b ip 25 data with the outcomes of previous correlations with modelled sea ice concentrations suggested that these intervals were characterised by ca 60 90 and ca 0 60 sea ice cover respectively both qualitative and quantitative sea ice records were also consistent with sea surface temperature and other palaeoclimate estimates derived previously from various other proxy studies the contrasting seasonal sea ice conditions during the younger dryas were further verified through a comparison of the concentrations of ip 25 with those of another highly branched isoprenoid hbi alkene that is di unsaturated and believed to also be produced by sea ice diatoms the ratio of the hbi diene to ip 25 termed dip 25 is believed to provide a useful indicator of stability or variability in sea ice conditions and complements the outcomes from the ip 25 and p b ip 25 index data the identification of contrasting seasonal sea ice conditions during the younger dryas will likely contribute to the debate regarding the climatic impacts that took place during this intriguing interval

strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation the climate of the last glacial period was extremely variable characterized by abrupt warming events in the northern hemisphere accompanied by slower temperature changes in antarctica and variations of global sea level it is generally accepted that this millennial scale climate variability was caused by abrupt changes in the ocean thermohaline circulation here we use a coupled ocean atmosphere sea ice model to show that freshwater discharge into the north atlantic ocean in addition to a reduction of the thermohaline circulation has a direct effect on southern ocean temperature the related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical north atlantic caused by a fast wave adjustment process we present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of greenland and antarctic temperature changes the slow changes in antarctic temperature and its similarity to sea level as well as a possible time lag of sea level with respect to antarctic temperature during marine isotope stage 3

reply to punctuated transgression comment on oliver t s n donaldson p sharples c roach m and woodroffe c d punctuated progradation of the seven mile beach holocene barrier system southeastern tasmania abstract our interpretation of the depositional history of the prograded barrier at seven mile beach in tasmania described in oliver et al 2017a was based on the morphology of ridges apparent in the lidar based digital elevation data and a sample of 14 optically stimulated luminescence osl ages dougherty 2018 has identified gaps in the chronology and speculated that progradation may have occurred as sea level fell from a mid holocene highstand inappropriately applying sea level curves from mainland australia despite a highstand being inferred by early research in tasmania glacio isostatic modelling and recent sea level studies adopted a prevailing view that excluded a highstand our observations led us to question this prevailing view and to suggest that it might be appropriate to reopen the debate on holocene sea level change in tasmania we welcome the renewed interest in the chronology and sea level history of this prograded barrier and look forward to further clarification based on new evidence the site may have the potential to become one of the more continuous and better constrained sea level records in southern australia

the flickering switch of late pleistocene climate change polar ice contains a unique record of past climate variations previous greenland ice cores have documented relatively warm interstadial periods during the last glaciation and short centuryscale returns to colder conditions during the glacial to interglacial warming see for example ref 1 these climate features have also been observed to varying degrees in ocean sediment cores2 4 and terrestrial pollen and insect records5 7 here we report electrical conductivity measurements from a new greenland ice core which confirm these previous observations and also reveal a hitherto unrecognized mode of rapid climate variation fluctuations in ice conductivity on the scales of 5 20 years reflect rapid oscillations in the dust content of the atmosphere this flickering between two preferred states would seem to require extremely rapid reorganizations in atmospheric circulation

sea ice variations in the central canadian arctic archipelago during the holocene abstract a sea ice record for barrow strait in the canadian arctic archipelago caa is presented for the interval 10 0 0 4 cal kyr bp this holocene record is based primarily on the occurrence of a sea ice biomarker chemical ip25 isolated from a marine sediment core obtained from barrow strait in 2005 a core chronology is based on 14c ams dating of mollusc shells obtained from ten horizons within the core the primary ip25 data are compared with complementary proxy data obtained from analysis of other organic biomarkers stable isotope composition of bulk organic matter benthic foraminifera particle size distributions and ratios of inorganic elements the combined proxy data show that the palaeo sea ice record can be grouped according to four intervals and these can be contextualised further with respect to the holocene thermal maximum htm spring sea ice occurrence was lowest during the early mid holocene 10 0 6 0 cal kyr bp and this was followed by a second phase 6 0 4 0 cal kyr bp where spring sea ice occurrence showed a small increase between 4 0 and 3 0 cal kyr bp spring sea ice occurrence increased abruptly to above the median and we associate this interval with the termination of the htm elevated spring sea ice occurrences continued from 3 0 to 0 4 cal kyr bp although they were more variable on shorter timescales within this fourth interval we also provide evidence for slightly lower and subsequently higher spring sea ice occurrence during the mediaeval warm period and the little ice age respectively comparisons are made between our proxy data with those obtained from other palaeo climate and sea ice studies for the caa

a calendar age estimate of the younger dryas holocene boundary at krakenes western norway the younger dryas holocene transition yd h in the sediments of krakenes lake western norway is well marked both lithologically and palaeobiologically at 756 5 cm in the investigated core a ser

holocene variability in sea ice cover primary production and pacific water inflow and climate change in the chukchi and east siberian seas arctic ocean in this study we present new detailed biomarker based sea ice records from two sediment cores recovered in the chukchi sea and the east siberian sea these new biomarker data may provide new insights on processes controlling recent and past sea ice changes the biomarker proxy records show i minimum sea ice extent during the early holocene ii a prominent mid holocene short term high amplitude variability in sea ice primary production and pacific water inflow and iii significantly increased sea ice extent during the last ca 4 5k cal a bp this late holocene trend in sea ice change in the chukchi and east siberian seas seems to be contemporaneous with similar changes in sea ice extent recorded from other arctic marginal seas the main factors controlling the millennial variability in sea ice and surface water productivity are probably changes in surface water and heat flow from the pacific into the arctic ocean as well as the long term decrease in summer insolation the short term centennial variability observed in the high resolution middle holocene record is probably related to solar forcing our new data on holocene sea ice variability may contribute to synoptic reconstructions of regional to global holocene climate change based on terrestrial and marine archives

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

complementary biomarker based methods for characterising arctic sea ice conditions a case study comparison between multivariate analysis and the pip 25 index abstract the discovery of ip25 as a qualitative biomarker proxy for arctic sea ice and subsequent introduction of the so called pip25 index for semi quantitative descriptions of sea ice conditions has significantly advanced our understanding of long term paleo arctic sea ice conditions over the past decade we investigated the potential for classification tree ct models to provide a further approach to paleo arctic sea ice reconstruction through analysis of a suite of highly branched isoprenoid hbi biomarkers in ca 200 surface sediments from the barents sea four ct models constructed using different hbi assemblages revealed ip25 and an hbi triene as the most appropriate classifiers of sea ice conditions achieving a 90 cross validated classification rate additionally lower model performance for locations in the marginal ice zone miz highlighted difficulties in characterisation of this climatically sensitive region ct model classification and semi quantitative pip25 derived estimates of spring sea ice concentration spsic for four downcore records from the region were consistent although agreement between proxy and satellite observational records was weaker for a core from the west svalbard margin likely due to the highly variable sea ice conditions the automatic selection of appropriate biomarkers for description of sea ice conditions quantitative model assessment and insensitivity to the c factor used in the calculation of the pip25 index are key attributes of the ct approach and we provide an initial comparative assessment between these potentially complementary methods the ct model should be capable of generating longer term temporal shifts in sea ice conditions for the climatically sensitive barents sea

reconstruction of sea surface conditions at middle to high latitudes of the northern hemisphere during the last glacial maximum lgm based on dinoflagellate cyst assemblages abstract a new calibration database of census counts of organic walled dinoflagellate cyst dinocyst assemblages has been developed from the analyses of surface sediment samples collected at middle to high latitudes of the northern hemisphere after standardisation of taxonomy and laboratory procedures the database comprises 940 reference data points from the north atlantic arctic and north pacific oceans and their adjacent seas including the mediterranean sea as well as epicontinental environments such as the estuary and gulf of st lawrence the bering sea and the hudson bay the relative abundance of taxa was analysed to describe the distribution of assemblages the best analogue technique was used for the reconstruction of last glacial maximum lgm sea surface temperature and salinity during summer and winter in addition to sea ice cover extent at sites from the north atlantic n 63 mediterranean sea n 1 and eastern north pacific n 1 three of the north atlantic cores from the continental margin of eastern canada revealed a barren lgm interval probably because of quasi permanent sea ice six other cores from the greenland and norwegian seas were excluded from the compilation because of too sparse assemblages and poor analogue situation at the remaining sites n 54 relatively close modern analogues were found for most lgm samples which allowed reconstructions the new lgm results are consistent with previous reconstructions based on dinocyst data which show much cooler conditions than at present along the continental margins of canada and europe but sharp gradients of increasing temperature offshore the results also suggest low salinity and larger than present contrasts in seasonal temperatures with colder winters and more extensive sea ice cover whereas relatively warm conditions may have prevailed offshore in summer from these data we hypothesise low thermal inertia in a shallow and low density surface water layer

how extreme was northern hemisphere seasonality during the younger dryas abstract in explaining the rapid transitions associated with the younger dryas cooling a reduced meridional overturning circulation has traditionally been invoked but such a scenario has been difficult to reproduce in model studies without adding excessive amounts of freshwater to the north atlantic more recent studies challenge this view and indicate that the role of an extensive sea ice cover may have been as important in promoting abrupt climate change as reorganisations of the north atlantic ocean gildor h tziperman e 2003 sea ice switches and abrupt climate change philosophical transactions of the royal society of london series a mathematical physical and engineering sciences 361 1935 1942 based on glacier evidence from eastern greenland denton g h alley r b comer g c broecker w s 2005 the role of seasonality in abrupt climate change quaternary science reviews 24 1159 1182 suggest that the seasonal temperature amplitude increased by about 20 c during the younger dryas such a switching of seasonality lends support to the idea of a fast expanding sea ice cover because it allows for extremely cold winters that are balanced by relatively mild summers however climatic interpretations based on the geometry and length of glaciers under such conditions as in scoresby sund is not well understood and equilibrium line altitude ela estimates should therefore be regarded as tentative here we discuss the absolute seasonal amplitude during the younger dryas by taking winter precipitation into account and show that the changes in seasonality may have been limited to 10 c we propose that by reducing the seasonal response in greenland compared with western europe we better understand the hinged door modus operandi cohmap 1988 climatic changes of the last 18 000 years observations and model simulations science 241 1043 1052 of the polar front and sea ice cover where the absolute southward migration of sea ice is highest in the east atlantic region

seeking a holocene drift ice proxy non clay mineral variations from the sw to n central iceland shelf trends regime shifts and periodicities quantitative x ray diffraction analysis of the 2 mm sediment fraction was carried out on 1257 samples from the seafloor and 16 cores from the iceland shelf west of 18 w all but one core b997 347pc were from transects along troughs on the nw to n central shelf an area that in modern and historic times has been affected by drift ice the paper focuses on the non clay mineralogy of the sediments excluding calcite and volcanic glass quartz and potassium feldspars occupy similar positions in an r mode principal component analysis and oligoclase feldspar tracks quartz these minerals are used as a proxy for ice rafted detritus ird accordingly the sum of these largely foreign minerals q k to icelandic bedrock is used as a proxy for drift ice a stacked equi spaced 100 a record is developed which shows both low frequency trends and higher frequency events the detrended stacked record compares well with the flux of quartz mg cm 2 a 1 at md99 2269 off n iceland the multi taper method indicated that there are three significant frequencies at the 95 confidence level with periods of ca 2500 445 and 304 a regime shift analysis pinpoints intervals when there was a statistically significant shift in the average q k weight and identifies four ird rich events separated by intervals with lower inputs there is some association between peaks of ird input less dense surface waters from 18o data on planktonic foraminifera and intervals of moraine building copyright 2009 john wiley sons ltd

climate induced changes in sedimentary regimes for organic matter supply on the continental shelf off northern norway abstract with this study i suggest that changes in biogeochemical processes in northern norwegian fjords during the last glacial interglacial transition 14 3 to 6 3 ka b p are predominantly climate induced variable strength in atlantic water inflow intrusion of arctic waters and sea ice coverage various nutrient supply as well as several re advances of continental ice sheets are recorded by both bulk organic and selected biomarkers in sediments of the andfjord stable isotope 13corg 15n and rock eval pyrolysis data indicate a strong supply of terrestrial organic matter during stadials e g younger dryas older dryas whereas enhanced input of marine organic matter prevailed during the interstadials bolling allerod profiles of selected biomarkers of marine origin show a characteristic climate induced variation during the whole time interval where relatively low concentrations of dinosterols and c37 alkenones may indicate similar to today a minor contribution of their respective plankton groups coccolithophorids dinoflagellates the prevalence of arctic waters and seasonal sea ice coverage during the younger dryas is indicated by a single maximum of 24 methylenecholesterol probably indicating the dominance of sea ice diatoms and or the prevalence of diatoms of thalassiosiraceae in surface waters the sediments of the allerod in contrast show a pronounced maximum in dinosterols indicating a better adaptation of dinoflagellates to an oceanic regime that is characterized by incipient intrusion of atlantic water highly variable sea surface temperatures and well stratified water masses the concentration of higher molecular weight n alkanes however follows the deglacial early holocene trend of climate amelioration from the last deglaciation to the middle holocene with maxima during ice advances stadials and minima during the middle holocene warm period these new results significantly improve the understanding of climate induced response on organic matter supply in fjords and provide a better knowledge of past variations of biogeochemical processes in high latitude coastal environments

a comparison between multiproxy and historical data ad 1990 1840 of drift ice conditions on the east greenland shelf 66 n the reduction in sea ice along the se greenland coast during the last century has severely impacted ice rafting to this area in order to reconstruct ice rafting and oceanographic conditions in the area of denmark strait during the last 150 years we conducted a multiproxy study on three short 20 cm sediment cores from outer kangerdlugssuaq trough 300 m water depth the proxy based data obtained have been compared with historical and instrumental data to gain a better understanding of the ice sheet ocean interactions in the area a robust chronology has been developed based on 210pb and 137cs measurements on core po175gkc 9 66 2 n 32 w and expanded to the two adjacent cores based on correlations between calcite weight percent records our proxy records include sea ice and phytoplankton biomarkers and a variety of mineralogical determinations based on the 2 mm sediment fraction including identification with quantitative x ray diffraction ice rafted debris counts on the 63 150 m sand fraction

younger dryas sea level and meltwater pulse 1b recorded in barbados reef crest coral acropora palmata the younger dryas climate event occurred during the middle of the last deglacial cycle and is marked by an abrupt shift in the north atlantic polar front almost to its former glacial position trending east to west using high precision and high accuracy u th dated barbados reef crest coral acropora palmata we generate a detailed sea level record from 13 9 to 9000 years before present kyr b p and reconstruct the ice volume response to the younger dryas cooling from the mid allerod 13 9 kyr b p to the end of the younger dryas 11 65 kyr b p rates of sea level rise decreased smoothly from 20 mm yr 1 to 4 mm yr 1 culminating in a 400 year slow stand before accelerating into meltwater pulse 1b mwp 1b the mwp 1b event at barbados is better constrained as beginning by 11 45 kyr b p and ending at 11 1 kyr b p during which time sea level rose 14 2 m and rates of sea level rise reached 40 mm yr 1 we propose that mwp 1b is the direct albeit lagged response of the northern hemisphere ice sheets to the rapid warming marking the end of the younger dryas coinciding with rapid warming in the circum north atlantic region and the polar front shift from its zonal to meridional position 11 65 kyr b p as predicted by glaciological models the ice sheet response to rapid north atlantic warming was lagged by 400 years due to the thermal inertia of large ice sheets the regional circum north atlantic younger dryas climate event is elevated to a global response through sea level changes starting with the global slowdown in sea level rise during the younger dryas and culminating with mwp 1b no meltwater pulses are evident at the initiation of the younger dryas climate event as is often speculated

abrupt shifts of productivity and sea ice regimes at the western barents sea slope from the last glacial maximum to the bolling allerod interstadial abstract advanced knowledge of spatio temporal constraints on the barents sea ice sheet during the late weichselian glaciation overshadows relatively limited understanding of seasonal sea ice experiencing an annual advance retreat cycle and primary productivity trends accompanying massive abrupt climate changes during glacial deglacial cycles such paleo reconstructions are crucial prerequisites for improved comprehension and prediction of current and future climate change here we investigate sea ice and phytoplankton biomarker distributions in a barents sea sediment core covering ca 25 8 15 4 cal kyr bp to elucidate abrupt shifts of spring summer sea ice concentrations and relative sympagic pelagic productivity trends at the southwestern continental slope despite significant presence of seasonal sea ice the last glacial maximum lgm and initial shelf edge deglaciation sedg at the core site are characterised by occurrence of productive coastal polynya adjacent to the maximum ice sheet extent the onset of perennial i e multi year ice cover and near zero productivity during heinrich stadial 1 hs1 ca 18 0 16 3 cal kyr bp accompanies significant meltwater fluxes from ice sheet debuttressing and the consequent stagnation of thermohaline circulation rapid sea ice retreat and unprecedented pelagic productivity observed after 16 3 cal kyr bp coincides with areal ice sheet deglaciation and is potentially linked to the release of sub surface heat and nutrient reservoirs together with reinvigorated deep water circulation following millennial heating of the deep ocean during hs1 we find that a multivariate fingerprinting approach involving assessment of both downcore and surface biomarker distributions is able to distinguish relative ice algal and pelagic diatom productivity driven by sea ice dynamics

rapid oceanic and atmospheric changes during the younger dryas cold period the younger dryas event which began approximately 12 900 years ago was a period of rapid cooling in the northern hemisphere driven by large scale reorganizations of patterns of atmospheric and oceanic circulation 1 3 environmental changes during this period have been documented by both proxy based reconstructions 3 and model simulations 4 but there is currently no consensus on the exact mechanisms of onset stabilization or termination of the younger dryas 5 8 here we present high resolution records from two sediment cores obtained from lake krakenes in western norway and the nordic seas multiple proxies from lake krakenes are indicative of rapid alternations between glacial growth and melting during the later younger dryas meanwhile reconstructed sea surface temperature and salinity from the nordic seas show an alternation between sea ice cover and the influx of warm salty north atlantic waters we suggest that the influx of warm water enabled the westerly wind systems to drift northward closer to their present day positions the winds thus brought relatively warm maritime air to northern europe resulting in rising temperatures and the melting of glaciers subsequent input of this fresh meltwater into the ocean spurred the formation of sea ice which forced the westerly winds back to the south cooling northern europe we conclude that rapid alternations between these two states immediately preceded the termination of the younger dryas and the permanent transition to an interglacial state the impact of freshwater pulses released from northern hemisphere ice sheets on the north atlantic meridional overturning circulation has been suggested as a likely mechanism for the younger dryas cooling 7 11 in addition a responsive sea ice cover has been proposed as a mechanism that could rapidly alter the heat exchange between the ocean and atmosphere during the last glacial period 12 13 the roles of the ocean and the atmosphere in triggering the climate shifts around the younger dryas are much discussed 9 these hypotheses are difficult to resolve partly because the temporal resolution of most palaeoclimate archives is too coarse to enable the anatomy of the younger dryas stadial to be adequately described 14 to address these questions we here present a lake sediment record at sub annual resolution together with new data on the sea surface conditions of the norwegian atlantic current nwac throughouttheyoungerdryasstadial fig 1

changes in north atlantic radiocarbon reservoir ages during the allerod and younger dryas estimates of the radiocarbon age of seawater are required in correlations between marine and terrestrial records of the late quaternary climate we radiocarbon dated marine shells and terrestrial plant remains deposited in two bays on norway s west coast between 11 000 and 14 000 years ago a time of large and abrupt climatic changes that included the younger dryas yd cold episode the radiocarbon age difference between the shells and the plants showed that sea surface reservoir ages increased from 400 to 600 years in the early yd stabilized for 900 years and dropped by 300 years within a century across the yd holocene transition

reconstruction of sea surface temperature salinity and sea ice cover in the northern north atlantic during the last glacial maximum based on dinocyst assemblages past sea surface conditions over the northern north atlantic during the last glacial maximum were examined from the study of 61 deep sea cores the last glacial maximum time slice studied here corresponds to an interval between heinrich layers h2 and h1 and spanningabout 20 16 ka on a 14c time scale transfer functions based on dinocyst assemblages were used to reconstruct sea surface temperature salinity and sea ice cover the results illustrate extensive sea ice cover along the eastern canadian margins and sea ice spreading only during winter over most of the northern north atlantic on the whole much colder winter prevailed despite relatively mild conditions in august 10 15 c at most offshore sites thus suggesting a larger seasonal contrast of temperatures than today lower salinity than at present is reconstructed especially along the eastern canadian and scandinavian margins likely because of meltwater supply from the surrounding ice sheets these reconstructions contrast with those established by climap on the basis of planktonic foraminifera these differences are discussed with reference to the stratigraphical frame of the last glacial maximum which was not the coldest phase of the last glacial stage the respective significance of dinocyst and foraminifer records is also examined in terms of the thermohaline characteristics of surface waters and the vertical structure of upper water masses which was apparently much more stratified than at present in the northern north atlantic thus preventing deep water formation

late weichselian marine 14c reservoir ages at the western coast of norway abstract a shallow marine late weichselian deposit on the outer coast of western norway contains both terrestrial plant material and articulated marine shells we have 14 c dated both types of material from eight different stratigraphic levels covering the time interval 12 300 to 11 100 14 c yr b p the difference between 14 c dated terrestrial plant material and marine shell material the marine reservoir age ranges from 200 to 525 yr with a weighted average of 380 32 yr this is almost identical to the present reservoir age of 379 20 yr for southern norway in the mid younger dryas yd interval the reservoir age in the north atlantic 55 n 65 n was 700 800 yr considerably greater than the present reservoir age and the age we have measured for the bolling allerod interval the reason for this increase during the yd is probably a combination of reduced inflow of surface waters to the north atlantic and more extensive sea ice cover evidence from marine cores show that the southeastern norwegian sea experienced rapid fluctuations in the inflow of warm atlantic surface water during the period 12 300 to 11 000 yr b p however the reservoir age apparently did not increase during these colder periods older dryas i and ii the reason is probably that in contrast to the yd these colder periods did not last long enough and or were of too limited extent to alter the reservoir age of the ocean a comparison of the obtained 14 c dates with the varve 14 c chronology from lake suigetsu indicates an age of 12 770 cal yr b p for the al yd boundary

the gin sea a synthesis of its physical oceanography and literature review 1972 1985 abstract a synthesis and review of the physical oceanography of the gin sea greenland iceland and norwegian seas is presented an accompanying bibliography is included from 1972 through 1985 some 1986 works are also noted emphasis is placed on describing the gin sea as a major semi enclosed basin that plays a role unique among the world s oceans by providing a strong two way advective exchange between the ice covered polar sea and the north atlantic ocean and at the same time acting as the primary site for northern hemisphere bottom water formation the water masses for the gin sea are defined in a manner as consistent as possible with the literature the large scale oceanographic and meteorological factors influencing the water mass formation and circulation are described a sample calculation of the various horizontal and vertical thermohaline driven exchanges is given

unstable younger dryas climate in the northeast north atlantic the younger dryas stadial greenland stadial 1 has long been recognized in proxy records throughout the north atlantic region as a stable cool period that started and ended abruptly however modeling experiments in which the younger dryas cooling was forced by meltwater predict much more unstable climatic conditions compared to the proxy records here we present evidence of a climatically unstable younger dryas based on a decadal resolution marine proxy record from the norwegian continental shelf this record shows that the north atlantic was characterized in the early younger dryas by arctic water and an increasing accumulation of fresh surface water that peaked at 12 500 calendar yr b p for 300 yr during the late younger dryas the sea surface temperatures fluctuated between glacial values 2 c and interglacial values 10 c on a subdecadal to multidecadal time scale this period of extreme instability preceded the more stable and warm holocene starting ca 11 500 calendar yr b p these results are consistent with modeling experiments that link the younger dryas climate changes to fluctuations of the north atlantic thermohaline circulation

holocene sea ice conditions and circulation at the chukchi alaskan margin arctic ocean inferred from biomarker proxies two sediment cores from the chukchi sea margin were investigated for the arctic sea ice biomarker ip25 along with marine and terrestrial sterols and glycerol dialkyl glycerol tetraethers gdgts this is the first paleoclimatic application of ip25 in the chukchi alaskan region of the arctic which is key for understanding arctic pacific interactions and is experiencing rapid sea ice retreat under present warming sea ice and related circulation conditions were characterized in this study with a multicentury resolution for the long term holocene record to multidecadal for the last several centuries sea ice was found to be present during the entire record but with considerable spatial and temporal variability after very low deglacial ip25 values possibly related to permanent sea ice and or an iceberg dominated environment cores from the upper slope and shelf show ip25 maxima interpreted as representing a relative proximity to the sea ice margin in the early ca 8 9 ka and middle ca 5 6 ka holoce

sea ice as the glacial cycles climate switch role of seasonal and orbital forcing a box model of the coupled ocean atmosphere sea ice and land ice climate system is used to study glacial interglacial oscillations under seasonally and orbitally varying solar forcing the dominant 100 kyr oscillation in land ice volume has the familiar sawtooth shape of climate proxy records and to zeroth order it does not depend on the seasonal and milankovitch forcing the sea ice controls via its albedo and insulating effects the atmospheric moisture fluxes and precipitation that enable the land ice sheet growth this control and the rapid growth and melting of the sea ice allow the sea ice to rapidly switch the climate system from a growing ice sheet phase to a retreating ice sheet phase and to shape the oscillation s sawtooth structure a specific physical mechanism is proposed by which the insolation changes act as a pacemaker setting the phase of the oscillation by directly controlling summer melting of ice sheets this mechanism is shown to induce deglaciations during periods of lower summer insolation superimposed on the 100 kyr are the linear milankovitch forced frequencies of 19 23 and 41 kyr the transition from 41 kyr glacial cycles to 100 kyr cycles one million years ago may be explained as being due to the activation of the sea ice switch at that time this would be the case if sea ice extent was more limited during the warmer climate of the early pleistocene

past extent of sea ice in the northern north atlantic inferred from foraminiferal paleotemperature estimates 1 using 150 core top samples we developed a conservative but fairly robust new measure to reconstruct past changes in maximum seasonal sea ice distribution in the northern north atlantic hitherto a major unknown the proxy is based on similarity maximum modern analog technique simmax estimates of threshold temperatures near the sea surface sst today almost 100 of all sites with sst 2 5 c during summer 0 4 for winters 1978 1987 and 0 75 c for little ice age winters lie seaward of the sea ice margin when applied to 60 sediment records of the last glacial maximum this proxy shows that peak glacial sea ice was far more restricted than in the classic climap project members 1981 reconstruction during glacial summer sea ice only covered the arctic ocean and western fram strait the northern north atlantic and nordic seas were largely ice free and thus formed a high latitude moisture source for the continued buildup of continental ice sheets in contrast sea ice spread far south across the iceland faeroe ridge during glacial winter with an inferred patch of sea ice also in the central east atlantic near the center of the azores high a broad ice free channel extended from 50 to 60 n forming an ideal site for large scale convection of glacial upper north atlantic deep water the extreme seasonality in glacial sea ice formation and melt in the nordic seas implies a major consumption of the regional energy income

the arctic sea ice biomarker ip25 a review of current understanding recommendations for future research and applications in palaeo sea ice reconstructions in recent years a novel proxy for the past occurrence of arctic sea ice has been proposed that is based on the variable marine sedimentary abundance of an organic geochemical lipid derived from sea ice diatoms in the spring this lipid termed ip25 ice proxy with 25 carbon atoms is a highly branched isoprenoid mono unsaturated alkene that appears to be sufficiently stable in sediments to permit meaningful palaeo sea ice reconstructions to be carried out over short to long term timescales since the first proposed use of ip25 as a proxy for palaeo sea ice by belt et al 2007 a number of laboratories have measured this biomarker in arctic sediments and it is anticipated that research activity in this area will increase further in the future the content of this review is divided into a number of sections firstly we describe the scientific basis for the ip25 proxy and its initial discovery in arctic sea ice sedimenting particles and sediments secondly we summarise the relatively few studies that have to date concentrated on examining the factors that influence the production and fate of ip25 and we identify some areas of future research that need to be addressed in order to improve our understanding of ip25 data obtained from sedimentary analyses what is clear at this stage however it that the presence of ip25 in arctic marine sediments appears to represent a proxy measure of past seasonal sea ice rather than permanent or multi year ice conditions thirdly we highlight the importance of rigorous analytical identification and quantification of ip25 especially if measurements of this biomarker are going to be used for quantitative sea ice reconstructions rather than qualitative analyses alone presence absence fourthly we review some recent attempts to make the interpretations of ip25 biomarker data more detailed and quantitative by combining sedimentary abundances with those of phytoplankton and other sea ice derived biomarkers thus the bases for the so called pip25 and dip25 indices are described together with an overview of potential limitations concluding that investigations into the use of these indices needs further research before their full potential can be realised in the final section we provide a summary of ip25 based palaeo sea ice reconstruction case studies performed to date these case studies cover different arctic regions and timescales spanning decades to tens of thousands of years

post glacial variability of sea ice cover river run off and biological production in the western laptev sea arctic ocean a high resolution biomarker study abstract multi proxy biomarker measurements were applied on two sediment cores ps51 154 ps51 159 to reconstruct sea ice cover ip25 biological production brassicasterol dinosterol and river run off campesterol sitosterol in the western laptev sea over the last 17 ka with unprecedented temporal resolution the absence of ip25 from 17 2 to 15 5 ka in combination with minimum concentration of phytoplankton biomarkers suggests that the western laptev sea shelf was mostly covered with permanent sea ice very minor river run off and restricted biological production occurred during this cold interval from 16 ka until 7 5 ka a long term decrease of terrigenous riverine organic matter and a coeval increase of marine organic matter reflect the gradual establishment of fully marine conditions in the western laptev sea caused by the onset of the post glacial transgression intensified river run off and reduced sea ice cover characterized the time interval between 15 2 and 12 9 ka including the bolling allerod warm period 14 7 12 9 ka prominent peaks of the dip25 index coinciding with maximum abundances of subpolar foraminifers are interpreted as pulses of atlantic water inflow on the western laptev sea shelf after the warm period a sudden return to severe sea ice conditions with strongest ice coverage between 11 9 and 11 ka coincided with the younger dryas 12 9 11 6 ka at the onset of the younger dryas a distinct alteration of the ecosystem reflected in a distinct drop in terrigenous and phytoplankton biomarkers was detected during the last 7 ka the sea ice proxies reflect a cooling of the laptev sea spring summer season this cooling trend was superimposed by a short term variability in sea ice coverage probably representing bond cycles 1500 500 ka that are related to solar activity changes hence atmospheric circulation changes were apparently able to affect the sea ice conditions on the laptev sea shelf under modern sea level conditions

freshwater outbursts to the oceans from glacial lake agassiz and their role in climate change during the last deglaciation abstract lake agassiz was the largest lake in north america during the last deglaciation as the laurentide ice sheet lis retreated large volumes of water stored in this proglacial lake were episodically released into the oceans these waters were variably routed to the gulf of mexico arctic ocean north atlantic ocean and hudson bay during this period the three largest cooling events in the northern hemisphere closely followed 4 of the 5 largest outbursts from lake agassiz 1 the younger dryas which was preceded by a release of 9500 km 3 2 the preboreal oscillation preceded by releases of 9300 km 3 and 5900 km 3 and 3 the 8 2 ka cold event preceded by a 163 000 km 3 outburst these are respectively fluxes of 0 30 sv 0 29 sv 0 19 sv and 5 2 sv if released in 1 year because the influx of freshwater reaching the north atlantic ocean can inhibit thermohaline circulation partly depending on whether the ocean was in a glacial interglacial or transitional mode of circulation we believe that at least these large outbursts from lake agassiz may have provided the triggers for changes in ocean circulation and in turn for widespread climate change

high resolution record of late glacial and deglacial sea ice changes in fram strait corroborates ice ocean interactions during abrupt climate shifts the transition from last glacial to deglacial and subsequently to modern interglacial climate conditions was accompanied by abrupt shifts in the palaeoceanographic setting in the subpolar north atlantic knowledge about the role that sea ice coverage played during these rapid climate reversals is limited since most marine sediment cores from the higher latitudes provide only a coarse temporal resolution and often poorly preserved microfossils here we present a highly resolved reconstruction of sea ice conditions that characterised the eastern fram strait a key area for water mass exchange between the arctic ocean and the north atlantic for the past 30 ka bp this reconstruction is based on the distribution of the sea ice biomarker ip25 and phytoplankton derived biomarkers in a sediment core from the continental slope of western svalbard during the late glacial 30 ka to 19 ka bp recurrent advances and retreats of sea ice characterised the study area and point to a hitherto less considered oceanic and or atmospheric variability a long lasting perennial sea ice coverage in eastern fram strait persisted only at the very end of the last glacial maximum i e from 19 2 to 17 6 ka bp and was abruptly reduced at the onset of heinrich event 1 coincident with or possibly even inducing the collapse of the atlantic meridional overturning circulation amoc further maximum sea ice conditions prevailed during the younger dryas cooling event and support the assumption of an amoc reduction due to increased formation and export of arctic sea ice through fram strait a significant retreat of sea ice and sea surface warming are observed for the early holocene

holocene sea subsurface and surface water masses in the fram strait comparisons of temperature and sea ice reconstructions highlights holocene sea subsurface temperatures after husum hald 2012 estimated from planktic foraminifer fauna in e fram strait biomarkers and ip25 derived indices including dip25 indicate surface water variability delayed onset of early holocene conditions in subsurface 10 6 ka compared to surface 11 7 ka water conditions warm atlantic layer likely occupied uppermost 200 m in eastern fram strait between 10 and 9 ka diverging late holocene trends in surface and subsurface conditions linked to presence of strong pycnocline stratification abstract two high resolution sediment cores from eastern fram strait have been investigated for sea subsurface and surface temperature variability during the holocene the past ca 12 000 years the transfer function developed by husum and hald 2012 has been applied to sediment cores in order to reconstruct fluctuations of sea subsurface temperatures throughout the period additional biomarker and foraminiferal proxy data are used to elucidate variability between surface and subsurface water mass conditions and to conclude on the holocene climate and oceanographic variability on the west spitsbergen continental margin results consistently reveal warm sea surface to subsurface temperatures of up to 6 c until ca 5 cal ka bp with maximum seawater temperatures around 10 cal ka bp likely related to maximum july insolation occurring at that time maximum atlantic water aw advection occurred at surface and subsurface between 10 6 and 8 5 cal ka bp based on both foraminiferal and dinocyst temperature reconstructions probably a less stratified ice free nutrient rich surface ocean with strong aw advection prevailed in the eastern fram strait between 10 and 9 cal ka bp weakened aw contribution is found after ca 5 cal ka bp when subsurface temperatures strongly decrease with minimum values between ca 4 and 3 cal ka bp cold late holocene conditions are furthermore supported by high planktic foraminifer shell fragmentation and high 18o values of the subpolar planktic foraminifer species turborotalita quinqueloba while ip25 associated indices as well as dinocyst data suggest a sustained cooling due to a decrease in early summer insolation and consequently sea ice increase since about 7 cal ka bp in surface waters planktic foraminiferal data including stable isotopes indicate a slight return of stronger subsurface aw influx since ca 3 cal ka bp the observed decoupling of surface and subsurface waters during the later holocene is most likely attributed to a strong pycnocline layer separating cold sea ice fed surface waters from enhanced subsurface aw advection this may be related to changes in north atlantic subpolar versus subtropical gyre activity

do paleoclimate proxies agree a test comparing 19 late holocene climate and sea ice reconstructions from icelandic marine and lake sediments geochemical mineralogical and biological indicators preserved in sediments are widely used to reconstruct past climate change but proxies differ in the degree to which their utility as climate indicators has been validated via laboratory experiments modern spatial calibrations or down core comparisons with instrumental climate data multi proxy studies provide another means of evaluating interpretations of proxies this paper presents a multi proxy assessment comparing 19 sub centennially resolved late holocene proxy records covering the period 300 1900 ad from seven icelandic marine and lacustrine core sites we employ simple statistical comparisons between proxy reconstructions to evaluate their correlations over time and ultimately their utility as proxies for regional climate proxies examined include oxygen isotopic composition of benthic and planktonic foraminifera abundance of the sea ice biomarker ip25 allochthonous quartz in marine sediments a proxy for drift ice around iceland marine carbonate abundance total organic carbon concentration chironomid assemblages lacustrine biogenic silica and carbon nitrogen ratios in lake sediments most of the examined proxy records including temperature and sea ice proxies correlate strongly with each other over multi centennial timescales and thus do appear to record changes in regional climate copyright 2011 john wiley sons ltd

effective shear viscosity and effective bulk viscosity of firn of a temperate glacier kesselwandferner otztal alps 1967 1978

glacial north atlantic sea surface conditions reconstructed by glamap 2000 the response of the tropical ocean to global climate change and the extent of sea ice in the glacial nordic seas belong to the great controversies in paleoclimatology our new reconstruction of peak glacial sea surface temperatures ssts in the atlantic is based on census counts of planktic foraminifera using the maximum similarity technique version 28 simmax 28 modern analog technique with 947 modern analog samples and 119 well dated sediment cores our study compares two slightly different scenarios of the last glacial maximum lgm the environmental processes of the ice age land oceans glaciers epilog and glacial atlantic ocean mapping glamap 2000 time slices the comparison shows that the maximum lgm cooling in the southern hemisphere slightly preceeded that in the north in both time slices sea ice was restricted to the north western margin of the nordic seas during glacial northern summer while the central and eastern parts were ice free during northern glacial winter sea ice advanced to the south of iceland and faeroe in the central northern north atlantic an anticyclonic gyre formed between 45degrees and 60degreesn with a cool water mass centered west of ireland where glacial cooling reached a maximum of 12degreesc in the subtropical ocean gyres the new reconstruction supports the glacial to interglacial stability of sst as shown by climap project members climap 1981 the zonal belt of minimum sst seasonality between 2degrees and 6degreesn suggests that the lgm caloric equator occupied the same latitude as today in contrast to the climap reconstruction the glacial cooling of the tropical east atlantic upwelling belt reached up to 6degrees 8degreesc during northern hemisphere summer differences between these simmax based and published u37 k and mg ca based equatorial sst records are ascribed to strong sst seasonalities and sst signals that were produced by different planktic species groups during different seasons

the younger dryas cold interval as viewed from central greenland abstract greenland ice core records provide an exceptionally clear picture of many aspects of abrupt climate changes and particularly of those associated with the younger dryas event as reviewed here well preserved annual layers can be counted confidently with only 1 errors for the age of the end of the younger dryas 11 500 years before present ice flow corrections allow reconstruction of snow accumulation rates over tens of thousands of years with little additional uncertainty glaciochemical and particulate data record atmospheric loading changes with little uncertainty introduced by changes in snow accumulation confident paleothermometry is provided by site specific calibrations using ice isotopic ratios borehole temperatures and gas isotopic ratios near simultaneous changes in ice core paleoclimatic indicators of local regional and more widespread climate conditions demonstrate that much of the earth experienced abrupt climate changes synchronous with greenland within thirty years or less post younger dryas changes have not duplicated the size extent and rapidity of these paleoclimatic changes

identification of paleo arctic winter sea ice limits and the marginal ice zone optimised biomarker based reconstructions of late quaternary arctic sea ice abstract analysis of 100 surface sediments from across the barents sea has shown that the relative abundances of the mono unsaturated sea ice diatom derived biomarker ip 25 and a tri unsaturated highly branched isoprenoid hbi lipid hbi iii are characteristic of the overlying surface oceanographic conditions most notably the location of the seasonal sea ice edge thus while ip 25 is generally limited to locations experiencing seasonal sea ice with higher abundances found for locations with longer periods of ice cover hbi iii is found in sediments from all sampling locations but is significantly enhanced in sediments within the vicinity of the retreating sea ice edge or marginal ice zone miz the response of hbi iii to this well defined sea ice scenario also appears to be more selective than that of the more generic phytoplankton biomarker brassicasterol the potential for the combined analysis of ip 25 and hbi iii to provide more detailed assessments of past sea ice conditions than ip 25 alone has been investigated by quantifying both biomarkers in three marine downcore records from locations with contrasting modern sea ice settings for sediment cores from the western barents sea intermittent seasonal sea ice and the northern norwegian sea ice free high ip 25 and low hbi iii during the younger dryas ca 12 9 11 9 cal kyr bp is consistent with extensive sea cover with relatively short periods of ice free conditions resulting from late summer retreat towards the end of the yd ca 11 9 11 5 cal kyr bp a general amelioration of conditions resulted in a near winter maximum ice edge scenario for both locations although this was somewhat variable and the eventual transition to predominantly ice free conditions was later for the western barents sea site ca 9 9 cal kyr bp compared to nw norway ca 11 5 cal kyr bp for both locations coeval elevated hbi iii but absent ip 25 potentially provides further evidence for increased atlantic water inflow during the early holocene but this interpretation requires further investigation in contrast ip 25 and hbi iii data obtained from a core from the northern barents sea demonstrate that seasonal sea ice prevailed throughout the holocene but with a gradual shift from winter ice edge conditions during the early holocene to more sustained ice cover in the neoglacial a directional shift that has undergone a reverse in the last ca 150 yr according to observational records our combined surface and downcore datasets suggest that combined analysis of ip 25 and hbi iii can provide information on temporal variations in the position of the maximum winter arctic sea ice extent together with insights into sea ice seasonality by characterisation of the miz combining ip 25 with hbi iii in the form of the previously proposed pip 25 index yields similar outcomes to those obtained using brassicasterol as the phytoplankton marker importantly however some problems associated with use of a variable balance factor employed in the pip 25 calculation are potentially alleviated using hbi iii

holocene dynamics of the arctic s largest ice shelf ice shelves in the arctic lost more than 90 of their total surface area during the 20th century and are continuing to disintegrate rapidly the significance of these changes however is obscured by the poorly constrained ontogeny of arctic ice shelves here we use the sedimentary record behind the largest remaining ice shelf in the arctic the ward hunt ice shelf ellesmere island canada to establish a long term context in which to evaluate recent ice shelf deterioration multiproxy analysis of sediment cores revealed pronounced biological and geochemical changes in disraeli fiord in response to the formation of the ward hunt ice shelf and its fluctuations through time our results show that the ice shelf was absent during the early holocene and formed 4 000 years ago in response to climate cooling paleoecological data then indicate that the ward hunt ice shelf remained stable for almost three millennia before a major fracturing event that occurred 1 400 years ago after reformation 800 years ago freshwater was a constant feature of disraeli fiord until the catastrophic drainage of its epishelf lake in the early 21st century

freshwater forcing from the greenland ice sheet during the younger dryas evidence from southeastern greenland shelf cores abstract we present evidence for freshwater forcing from the greenland ice sheet during the last deglaciation explore its potential involvement in the younger dryas cooling and investigate the role of atlantic intermediate water in the retreat of the greenland ice sheet paleoceanographic conditions and ice sheet retreat history are interpreted from ird stable isotopes and foraminiferal assemblages in sediment cores retrieved along the length of the kangerlussuaq trough a cross shelf trough on the southeast greenland shelf the vedde ash 11 980 cal yr bp which is a marker bed within the younger dryas and the saksunarvatn tephra 10 180 cal yr bp an early holocene marker horizon have been located in these cores the tephra markers and calibrated 14c dates on foraminifers and molluscs provide the basis for the age models in the cores the stable isotope records indicate a 0 5 depletion of the 18o between 13 4 and 12 8 cal ka bp a pronounced light isotope event exceeding 1 5 on the inner shelf and 0 8 on the outer shelf occurs at the beginning of the younger dryas cold period 12 8 cal ka bp and ends by 12 cal ka bp immediately before deposition of the vedde ash the light isotope spikes are interpreted as glacial meltwater spikes associated with retreat of the adjacent greenland ice sheet associated with the light isotope events are episodic high percentages of the atlantic intermediate water benthic foraminiferal species cassidulina neoteretis which continue from the bolling allerod into the younger dryas high percentages of this species suggest that atlantic intermediate water entered the kangerlussuaq trough during the allerod and the younger dryas via the irminger current as it does today we conclude that advection of atlantic intermediate water into the kangerlussuaq trough south of the denmark strait promoted melting and retreat of the marine portions of the greenland ice sheet explaining the observed glacial meltwater spikes

a new greenland ice core chronology for the last glacial termination 1 we present a new common stratigraphic timescale for the north greenland ice core project ngrip and grip ice cores the timescale covers the period 7 9 14 8 kyr before present and includes the bolling allerod younger dryas and early holocene periods we use a combination of new and previously published data the most prominent being new high resolution continuous flow analysis cfa impurity records from the ngrip ice core several investigators have identified and counted annual layers using a multiparameter approach and the maximum counting error is estimated to be up to 2 in the holocene part and about 3 for the older parts these counting error estimates reflect the number of annual layers that were hard to interpret but not a possible bias in the set of rules used for annual layer identification as the grip and ngrip ice cores are not optimal for annual layer counting in the middle and late holocene the timescale is tied to a prominent volcanic event inside the 8 2 kyr cold event recently dated in the dye 3 ice core to 8236 years before a d 2000 b2k with a maximum counting error of 47 years the new timescale dates the younger dryas preboreal transition to 11 703 b2k which is 100 150 years older than according to the present grip and ngrip timescales the age of the transition matches the gisp2 timescale within a few years but viewed over the entire 7 9 14 8 kyr section there are significant differences between the new timescale and the gisp2 timescale the transition from the glacial into the bolling interstadial is dated to 14 692 b2k the presented timescale is a part of a new greenland ice core chronology common to the dye 3 grip and ngrip ice cores named the greenland ice core chronology 2005 gicc05 the annual layer thicknesses are observed to be log normally distributed with good approximation and compared to the early holocene the mean accumulation rates in the younger dryas and bolling periods are found to be 47 2 and 88 2 respectively

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

paleoceanographic development in the sw barents sea during the late weichselian early holocene transition abstract the late weichselian early holocene variability of the north atlantic current has been studied with focus on the zonal component of this meridional transport during the transition from glacial to interglacial conditions the investigated sediment core is from 409 m water depth in the sw barents sea eight accelerator mass spectrometry ams 14c dates show that the core covers the last 20 000 cal yr b p with a centennial scale resolution during late weichselian early holocene planktic foraminiferal assemblages were analyzed using the 100 m size fraction and foraminiferal planktic and benthic 13c and 18o isotopes were measured furthermore a range of physical and chemical analyses has been carried out on the bulk sediment samples four time periods have been identified which represent the varying oceanographic conditions in ingoydjupet a glacial trough located off the north coast of norway in the sw barents sea 1 the late glacial before ca 15 000 cal yr b p influenced by the nearby ice sheets with high amounts of sea ice or iceberg transported detritus 2 the late oldest dryas stadial and the bolling allerod interstadial ca 15 000 12 700 cal yr b p with cold surface water conditions influenced by the collapse of the nearby ice sheets high amounts of sea ice or iceberg transported detritus and melt water and weak subsurface inflow of atlantic water 3 the younger dryas cold stadial 12 700 11 650 cal yr b p with low primary productivity and extensive sea ice cover and 4 the preboreal and early holocene 11 650 6800 cal yr b p cal yr b p with strong influx of atlantic water into the area near absence of ice rafted debris and generally ameliorated conditions in both surface and bottom water masses as seen from a high flux of foraminifera and increased marine primary production

variability of sea ice conditions in the fram strait over the past 30 000 years sea ice is a critical component of the climate and oceanographic system in the north atlantic ocean a biomarker record reveals millennial scale and glacial interglacial fluctuations in sea ice coverage in the northernmost atlantic ocean over the past 30 000 years

evolution of the irminger and east icelandic current systems through the holocene revealed by diatom based sea surface temperature reconstructions abstract sediment core md99 2269 from the northern shelf of iceland has been studied to assess the stability of the east icelandic irminger current system during the holocene the 25 4 m long holocene section was studied with a 40 years average resolution the chronology is based on the combination of 24 14c ams dates and two tephra layers diatoms are utilized to quantify the development and variability of the august sea surface temperature sst for the last 11 5 cal kyr bp three different transfer function methods have been used to reconstruct august sst at the site weighted average partial least squares wa pls maximum likelihood ml and imbrie and kipp i k the combined use of the three methods allows the august sst reconstruction with a higher confidence level than with using only one statistical technique these three reconstructions generate very similar august sst trends through the holocene the onset of the holocene climate optimum is characterized at the site by a rapid august sst increase at 10 4 cal kyr bp of almost 4 c the holocene climate optimum is recorded between 10 4 and 4 7 cal kyr bp the august ssts were around 2 c warmer during the major part of the holocene compared with the august ssts for the later half of the holocene combining the diatom record with the benthic foraminifera records from the same core enables us to reconstruct the evolution of both surface and bottom water masses during the holocene this reconstruction shows dominance of the arctic waters throughout the water column at the onset of the holocene and subsequent increase of the influence of the atlantic water especially during the holocene climate optimum this strong dominance of atlantic water tends to decrease between 4 7 and 2 2 cal kyr bp especially in the upper part of the water column since 2 2 cal kyr bp the trend is reversed suggesting an increased influence of atlantic water at least in the upper water column

does the trigger for abrupt climate change reside in the ocean or in the atmosphere two hypotheses have been put forward to explain the large and abrupt climate changes that punctuated glacial time one attributes such changes to reorganizations of the ocean s thermohaline circulation and the other to changes in tropical atmosphere ocean dynamics in an attempt to distinguish between these hypotheses two lines of evidence are examined the first involves the timing of the freshwater injections to the northern atlantic that have been suggested as triggers for the global impacts associated with the younger dryas and heinrich events the second has to do with evidence for precursory events associated with the heinrich ice rafted debris layers in the northern atlantic and with the abrupt dansgaard oeschger warmings recorded in the santa barbara basin

mapping recent sea ice conditions in the barents sea using the proxy biomarker ip25 implications for palaeo sea ice reconstructions abstract an analysis of the sea ice proxy ip 25 and the open water phytoplankton indicator brassicasterol has been carried out for 93 surface sediments collected from the barents sea and some additional data from previous studies have been included to augment the analysis further consistent with a specific and selective sea ice diatom origin for ip 25 this biomarker was identified in virtually all sediments from seasonally ice covered locations but was absent from the majority of sites beyond the maximum sea ice extent msie for the period 1983 2002 in contrast brassicasterol was detected in virtually all sediments from the region and concentrations were not at all associated with the msie instead more localised regions of brassicasterol concentrations were identified and these likely reflect different influences on production and deposition when the ip 25 and brassicasterol concentrations were combined together in the form of the previously proposed p b ip 25 index the aforementioned association between ip 25 occurrence and the msie was clarified further and in addition provided more detailed descriptions of sea ice cover in particular p b ip 25 data indicated seasonal sea ice or marginal ice zone conditions for the region of the barents sea within the msie according to previously defined p b ip 25 ranges we also amalgamated the biomarker data from the current study with those derived previously by muller et al 2011 and compared these larger datasets with satellite derived mean spring sea ice concentrations significantly although the correlation between the combined ip 25 concentration dataset with the satellite data was very similar to that reported previously by muller et al 2011 the corresponding correlation using the p b ip 25 values was poorer with the larger dataset and some possible reasons for this are discussed finally in a small number of cases ip 25 was identified in sediments somewhat distal from the msie and it is hypothesised that the occurrence of ip 25 in these sediments results from allochthonous input following sediment advection between shelf and trough environments this is the first reported example that provides evidence for the allochthonous input of ip 25 into marine sediments and it is recommended that this should be considered carefully when carrying out palaeo sea ice reconstructions based on the occurrence of this biomarker in the future

a novel chemical fossil of palaeo sea ice ip25 a unique and novel organic compound has been detected in sea ice samples from three locations in the canadian arctic thousands of kilometers apart it is likely that this biomarker is produced by diatoms living in the sea ice and we provide evidence which suggests that the compound a c25 monounsaturated hydrocarbon ip25 may be a specific sensitive and stable proxy for sea ice in sediments over at least the holocene since it has not been reported before we confirmed its identity by synthesis and used the synthesized compound as a reference for quantifying ip25 in a range of sediments from an east west transect in the canadian arctic the recent sediments are either seasonally covered with ice or have permanent ice cover older sediments containing ip25 have been dated using radiocarbon methods to at least 9000 yr if the concept proves generally applicable monitoring ip25 in further sediment cores along with other accepted proxies should allow movements in the position of the ice edge throughout the holocene at least to be better determined which is essential for accurate calibration of climate prediction models a similar approach for antarctic samples of ice and sediments would also seem worthwhile

a biomarker based reconstruction of sea ice conditions for the barents sea in recent centuries variations in sea ice occurrence for the barents sea since c ad 1700 have been determined by analysis of the abundance of the sea ice biomarker ip25 in three marine sediment box cores obtained from locations in the north southeast and southwest parts of the region depth age models for each core were established using excess 210pb activity profiles comparisons between these depth age models with those reported previously for the barents sea suggest that the proxy sea ice record may alternatively extend back to c ad 1500 1600 sedimentation accumulation rates and bulk densities were combined with ip 25 concentrations resulting in temporal changes to ip25 fluxes which have been interpreted in terms of sea ice variability the ip25 sea ice proxy data are also compared with other bulk organic geochemical parameters total organic carbon and c n ratios and with sediment particle size distributions the data indicate ice free conditions for the southwest barents sea for the past c 300 yr consistent

towards quantitative sea ice reconstructions in the northern north atlantic a combined biomarker and numerical modelling approach article i nfo organic geochemical analyses of marine surface sediments from the continental margins of east greenland and west spitsbergen provide for a biomarker based estimate of recent sea ice conditions in the northern north atlantic by means of the sea ice proxy ip25 and phytoplankton derived biomarkers e g brassicasterol and dinosterol we reconstruct sea ice and sea surface conditions respectively the combination of ip25 with a phytoplankton marker in terms of a phytoplankton marker ip25 index pip25 proves highly valuable to properly interpret the sea ice proxy signal as an under or overestimation of sea ice coverage can be circumvented a comparison of this biomarker based assessment of the sea ice distribution in the study area with 1 modern remote sensing data and 2 numerical modelling results reveal a good agreement between organic geochemical satellite and modelling observations the reasonable simulation of modern sea ice conditions by means of a regional ocean sea ice model demonstrates the feasibility to effectively integrate the complex atmospheric and oceanic circulation features as they prevail in the study area the good correlation between modelled sea ice parameters and the biomarker based estimate of sea ice coverage substantiates that linking proxy and model data occurs to be a promising concept in terms of a cross evaluation this combinatory approach may provide a first step towards quantitative sea ice reconstructions by means of ip25 future ip25 studies on marine surface sediments from the arctic realm however are recommended to extend and validate this new attempt of using ip25 in combination with a phytoplankton marker as a quantitative measure for sea ice reconstructions

variations in temperature and extent of atlantic water in the northern north atlantic during the holocene we compare six high resolution holocene sediment cores along a s n transect on the norwegian svalbard continental margin from ca 601n to 77 41n northern north atlantic planktonic foraminifera in the cores were investigated to show the changes in upper surface and subsurface water mass distribution and properties including summer sea surface temperatures sst the cores are located below the axis of the norwegian current and the west spitsbergen current which today transport warm atlantic water to the arctic sediment accumulation rates are generally high at all the core sites allowing for a temporal resolution of 10 10 2 years sst is reconstructed using different types of transfer functions resulting in very similar sst trends with deviations of no more than 71 0 1 51c a transfer function based on the maximum likelihood statistical approach is found to be most relevant the reconstruction documents an abrupt change in planktonic foraminiferal faunal composition and an associated warming at the younger dryas preboreal transition the earliest part of the holocene was characterized by large temperature variability including the preboreal oscillations and the 8 2 k event in general the early holocene was characterized by ssts similar to those of today in the south and warmer than today in the north and a smaller s n temperature gradient 0 231c 1n compared to the present temperature gradient 0 461c 1n the southern proxy records 60 691n were more strongly influenced by slightly cooler subsurface water probably due to the seasonality of the orbital forcing and increased stratification due to freshening the northern records 72 77 41n display a millennial scale change associated with reduced insolation and a gradual weakening of the north atlantic thermohaline circulation thc the observed northwards amplification of the early holocene warming is comparable to the pattern of recent global warming and future climate modelling which predicts greater warming at higher latitudes the overall trend during mid and late holocene was a cooling in the north stable or weak warming in the south and a maximum s n sst gradient of ca 0 71c 1n at 5000 cal years bp superimposed on this trend were several abrupt temperature shifts four of these shifts dated to 9000 8000 5500 3000 and 1000 and 400 cal years bp appear to be global as they correlate with periods of global climate change in general there is a good correlation between the northern north atlantic temperature records and climate records from norway and svalbard r 2007 elsevier ltd all rights reserved

the holocene younger dryas transition recorded at summit greenland analysis of ice from dye 3 greenland has demonstrated that the transition between the younger dryas and holocene climate periods occurred over a 40 year period a near annually resolved multiparameter record of the transition recorded in the gisp2 core from summit greenland shows that most of the transition occurred in a series of steps with durations of about 5 years some climate proxies associated with mid latitude sources appear to have changed about 15 years before climate proxies associated with more northern regions changes in atmospheric water vapor are likely to have played a large role in the climate transition

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

source specific biomarkers as proxies for arctic and antarctic sea ice abstract over the last decade or so certain source specific c25 highly branched isoprenoid hbi lipid biomarkers have emerged as useful proxies for arctic and antarctic sea ice thus ip25 ice proxy with 25 carbon atoms and ipso25 ice proxy for the southern ocean with 25 carbon atoms represent binary measures of past seasonal sea ice in the arctic and antarctic respectively a further tri unsaturated hbi generally referred to as hbi iii appears to provide proxy evidence for the region of open water found adjacent to sea ice i e the marginal ice zone miz in both polar regions this review provides an update on current knowledge pertaining to each proxy the first section focuses on describing those studies that have aimed to establish the underlying features of each proxy including source identification and spatial distribution characteristics the second section presents some important analytical considerations pertinent to the accurate identification and quantification of hbi biomarkers the third section describes how each hbi proxy is normally interpreted within the sedimentary record for palaeo sea ice reconstruction purposes this includes the interpretation of individual and combined biomarker profiles such as the pip25 index and multivariate decision tree models a summary of all previous palaeo sea ice reconstructions based on hbis is also given which includes examples that clarify or reinforce our understanding of the individual or combined biomarker signatures some knowledge gaps and areas for future research are also briefly described

sea ice switches and abrupt climate change we propose that past abrupt climate changes were probably a result of rapid and extensive variations in sea ice cover we explain why this seems a perhaps more likely explanation than a purely thermohaline circulation mechanism we emphasize that because of the significant influence of sea ice on the climate system it seems that high priority should be given to developing ways for reconstructing high resolution in space and time sea ice extent for past climate change events if proxy data can confirm that sea ice was indeed the major player in past abrupt climate change events it seems less likely that such dramatic abrupt changes will occur due to global warming when extensive sea ice cover will not be present

distributions of highly branched isoprenoid alkenes and other algal lipids in surface waters from east antarctica further insights for biomarker based paleo sea ice reconstruction the occurrence and variable abundance of certain di and tri unsaturated c25 highly branched isoprenoid hbi biomarkers in antarctic marine sediments has previously been proposed as a possible proxy measure of paleo sea ice extent in the southern ocean in the current study we obtained 47 near surface ca 0 10 m water samples taken from locations in east antarctica with different sea ice settings and analysed them for their hbi sterol and fatty acid content sampling locations ranged from the permanently open ocean zone pooz with no seasonal sea ice cover the near shore summer sea ice zone siz where sea ice remains long into the summer melt season and the marginal ice zone miz located between the pooz and the siz and with a highly variable latitudinal sea ice edge throughout the season a di unsaturated c25 hbi diene ii was only identified in surface waters from the miz and the siz consistent with a sea ice diatom origin for this biomarker in contrast a tri unsaturated c25 hbi triene iii was detected in all samples from the pooz the miz and the siz and with a stable isotopic composition 13c 35 1 5 consistent with a phytoplankton source the highest concentrations of diene ii and triene iii were in samples from the siz and the miz respectively thus providing further insights into the sea ice conditions likely favourable for their production and how their relative abundances the ii iii ratio in underlying sediments might be better interpreted for paleo sea ice reconstruction in this respect relatively high ii iii might be a good indicator of extended into summer seasonal sea ice cover while lower ii iii may provide a better indicator of the miz however the observation of highly variable ii iii within the polynya setting of the siz may also have significant impacts on sedimentary values distributions of diatom sterols and fatty acids were also variable between the three sampling zones but these were not as distinctive as those observed for the hbis

two possible routings for overflow from lake agassiz during the younger dryas a reply to comment by t fisher t lowell h loope on alternative routing of lake agassiz overflow during the younger dryas new dates paleotopography a re evaluation

sea surface temperatures and ice rafting in the holocene north atlantic climate influences on northern europe and greenland abstract the oceanographic conditions in the high latitude north atlantic ocean during the holocene were reconstructed through analyses of sea surface temperature sst alkenone unsaturation ratios and ice rafting mineralogy and grain size from two sediment sequences one recovered from the reykjanes ridge at 59 n and the other from the norwegian sea at 68 n comparison of our records to published ice core and terrestrial proxy climate data sets suggests that atmospheric temperature changes over northern europe and greenland were coupled to sst variability and ice rafting the records outline four major climatic phases i an early holocene thermal maximum that lasted until approximately 6 7 kyr bp ii a distinctly cooler phase associated with increased ice rafting between 6 5 and 3 7 kyr bp iii a transition to generally warmer but relatively unstable climate conditions between 3 7 and 2 kyr bp and iv a second distinct sst decline that took place between 2 and 0 5 kyr bp in contrast to the dominant control of northern hemisphere summer insolation on early holocene climate development via strong seasonality the trigger for the onset of relatively unstable climatic conditions in the north atlantic at 3 7 kyr bp is not straightforward however it is possible that this change was triggered by late holocene winter insolation increase at high northern latitude and or by inter hemispheric changes in orbital forcing the late holocene neoglaciation trend which is characteristic of numerous terrestrial archives in northern europe may not only be attributed to a gradual decrease in orbitally forced summer temperature but also to increase snow precipitation at high northern latitudes during generally milder winters

sea ice and marine climate variability for nw iceland denmark strait over the last 2000 cal yr bp md99 2263 is a 46 cm box core collected from djupall a trough that cuts across the nw iceland shelf and ends above denmark strait we provide a multiproxy record that documents changes in the regional marine climate over the last 1700 yr the depth age model is based on seven calibrated radiocarbon dates on mollusk shells and on 210pb and 137cs sediment accumulation rates were variable 0 2 0 8 mm yr but increased dramatically ad 1500 grain size magnetic properties quantitative mineral composition of the 2 mm sediment fraction benthic foraminiferal composition benthic and planktic 18o ratios and abundances fluxes of the sea ice biomarker ip25 were determined to better compare the various proxies 12 of the critical climate proxies were co ordinated into 100 yr sample time series which were examined by principal component analysis the 1st axis explained 49 of the variance and the 2nd axis explained an additional 17 the variables most strongly associated with the 1st axis were sediment pr

abrupt climate changes for iceland during the last millennium evidence from high resolution sea ice reconstructions a high resolution account of icelandic sea ice over the last millennium has been constructed using a novel proxy based on the presence in sediments of a biomarker ip25 produced by sea ice algae comparison with historical sea ice records demonstrates a strong correlation between documented sea ice occurrences and the ip25 proxy an excellent agreement is also observed between the ip25 record and a diatom based sea surface temperature reconstruction obtained from the same core and the crowley and lowery northern hemisphere temperature reconstruction using this approach we provide new historical sea ice data for periods where records are scarce or absent and evidence for abrupt changes to sea ice and or climate conditions around iceland during the little ice age

abrupt onset of the little ice age triggered by volcanism and sustained by sea ice ocean feedbacks 1 northern hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes however the causes of superposed century scale cold summer anomalies of which the little ice age lia is the most extreme remain debated largely because the natural forcings are either weak or in the case of volcanism short lived here we present precisely dated records of ice cap growth from arctic canada and iceland showing that lia summer cold and ice growth began abruptly between 1275 and 1300 ad followed by a substantial intensification 1430 1455 ad intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium a transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times and that cold summers can be maintained by sea ice ocean feedbacks long after volcanic aerosols are removed our results suggest that the onset of the lia can be linked to an unusual 50 year long episode with four large sulfur rich explosive eruptions each with global sulfate loading 60 tg the persistence of cold summers is best explained by consequent sea ice ocean feedbacks during a hemispheric summer insolation minimum large changes in solar irradiance are not required

an abrupt wind shift in western europe at the onset of the younger dryas cold period the younger dryas cold reversal during the last glacial termination is one of the most abrupt climate change events observed in the northern hemisphere analyses of varved lake sediments from western germany suggest that storminess in the north atlantic region increased within a single year at this time providing a mechanistic link between a decrease in the meridional overturning circulation and western european cooling the younger dryas cooling 12 700 years ago is one of the most abrupt climate changes observed in northern hemisphere palaeoclimate records1 2 3 4 annually laminated lake sediments are ideally suited to record the dynamics of such abrupt changes as the seasonal deposition responds immediately to climate and the varve counts provide an accurate estimate of the timing of the change here we present sub annual records of varve microfacies and geochemistry from lake meerfelder maar in western germany providing one of the best dated records of this climate transition5 our data indicate an abrupt increase in storminess during the autumn to spring seasons occurring from one year to the next at 12 679 yr bp broadly coincident with other changes in this region we suggest that this shift in wind strength represents an abrupt change in the north atlantic westerlies towards a stronger and more zonal jet changes in meridional overturning circulation alone cannot fully explain the changes in european climate6 7 we suggest the observed wind shift provides the mechanism for the strong temporal link between north atlantic ocean overturning circulation and european climate during deglaciation

striking similarities in temporal changes to spring sea ice occurrence across the central canadian arctic archipelago over the last 7000 years abstract a 7000 year spring sea ice record for victoria strait arc 4 and dease strait arc 5 in the canadian arctic archipelago caa has been determined by quantification of the sea ice diatom derived biomarker ip 25 in two marine sediment piston cores obtained in 2005 the chronologies of the arc 4 and arc 5 cores were determined using a combination of 14 c ams dates obtained from macrobenthic fossils and magnetic susceptibility measurements the ages of the tops of the piston cores were estimated by matching chemical and physical parameters with those obtained from corresponding box cores these analyses revealed that while the top of the arc 4 piston core was estimated to be essentially modern ca 60 cal yr bp a few hundred years of sediment appeared to be absent from the arc 5 piston core downcore changes to ip 25 fluxes for both cores were interpreted in terms of variations in spring sea ice occurrence and correlations between the individual ip 25 flux profiles for victoria strait dease strait and barrow strait reported previously were shown to be statistically significant at both 50 and 100 year resolutions the ip 25 data indicate lower spring sea ice occurrences during the early part of the record ca 7 0 3 0 cal kyr bp and for parts of the late holocene ca 1 5 0 8 cal kyr bp especially for the two lower latitude study locations in contrast higher spring sea ice occurrences existed during ca 3 0 1 5 cal kyr bp and after ca 800 cal yr bp the observation of consecutively lower and higher spring sea ice occurrence during two periods of the late holocene coincides broadly with the medieval warm period and little ice age epochs respectively the ip 25 data are complemented by particle size and mineralogical data although these may alternatively reflect changes in sea level at the study sites the ip 25 data are also compared to previous proxy based determinations of palaeo sea ice and palaeoclimate for the caa including those based on bowhead whale remains and dinocyst assemblages the spatial consistency in the proxy data which most notably indicates an increase in spring sea ice occurrence around 3 cal kyr bp provides a potentially useful benchmark for the termination of the holocene thermal maximum for the central caa

the role of seasonality in abrupt climate change abstract a case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the north atlantic region during the last glaciation and into the holocene the primary evidence comes from mismatches between mean annual temperatures from greenland ice cores in comparison with snowline changes in east greenland northern europe and north america the most likely explanation is a shutdown or reduction in strength of the conveyor this allows the spread of winter sea ice across the north atlantic thus causing the northern region to experience much colder winters because they mimic the greenland temperature rather than the snowline signal changes in the atlantic intertropical convergence zone and the asian monsoon may also share a winter linkage with greenland thus the paleoclimate record is consistent with the notion that a huge continental sector of the northern hemisphere stretching from greenland to asia was close to an extreme winter threshold during much of the last glaciation winter climate crossed this threshold repeatedly with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics

early holocene temperature variability in the nordic seas the role of oceanic heat advection versus changes in orbital forcing received 7 january 2011 revised 15 july 2011 accepted 21 july 2011 published 22 october 2011 1 the separate roles of oceanic heat advection and orbital forcing on influencing early holocene temperature variability in the eastern nordic seas is investigated the effect of changing orbital forcing on the ocean temperatures is tested using the 1dice model and the 1dice results are compared with new and previously published temperature reconstructions from a transect of five cores located underneath the pathway of atlantic water from the faroe shetland channel in the south to the barents sea in the north the stronger early holocene summer insolation at high northern latitudes increased the summer mixed layer temperatures however ocean temperatures underneath the summer mixed layer did not increase significantly the absolute maximum in summer mixed layer temperatures occurred between 9 and 6 ka bp representing the holocene thermal maximum in the eastern nordic seas in contrast maximum in northward oceanic heat transport through the norwegian atlantic current occurred approximately 10 ka bp the maximum in oceanic heat transport at 10 ka bp occurred due to a major reorganization of the atlantic ocean circulation entailing strong and deep rejuvenation of the atlantic meridional overturning circulation combined with changes in the north atlantic gyre dynamic causing enhanced transport of heat and salt into the nordic seas

mountain glacier evolution in the iberian peninsula during the younger dryas abstract we review the evolution of glaciers in the iberian mountains during the younger dryas 12 9 11 7 ka following the chronology of greenland stadial 1 gs1 and compare with available environmental and climate data to investigate glacier dynamics during cold stadial episodes the best examples of younger dryas moraines are found in the central pyrenees involving short ice tongues up to 4 km in length in the highest massifs above 3000 m a s l of the southern versant small cirque glaciers and rock glaciers formed during the yd occurred in other pyrenean massifs in the cantabrian range and in the gredos and guadarrama sierras central range as indicated by several rocky polished thresholds that were ice free at the beginning of the holocene although some former rock glaciers were re activated during the younger dryas glacial activity was limited in the southernmost part of the iberian peninsula sierra nevada most iberian records show vegetation changes during the yd characterized by a forest decline and an expansion of shrubs mainly juniperus and steppe herbs although the vegetation response was not homogeneous because of variable resilience among ecosystems available records also document a variable lake response in terms of hydrology and productivity with a decrease in sedimentation rates and organic productivity in most high altitude lakes and increases in salinity and relatively lower lake levels at lower altitudes the impact of the younger dryas on the coastal environment was almost negligible but it was responsible for a brief cessation in sea level rise high resolution analyses of new speleothem records have documented a double structure for the yd with an earlier drier phase followed by a relatively more humid period the review of geomorphological evidence demonstrates a strong latitudinal control of glacial activity during the yd with more intense development in the northern than in the southern regions the increase in humidity during the second phase of the yd in southern latitudes could have been a decisive factor for glacier advance in the northern iberian mountains

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