detrital carbonate peaks on the labrador shelf a 13 7 ka template for freshwater forcing from the hudson strait outlet of the laurentide ice sheet into the subpolar gyre abstract the laurentide ice sheet lis was a large dynamic ice sheet in the early holocene the glacial events through hudson strait leading to its eventual demise are recorded in the well dated labrador shelf core md99 2236 from the cartwright saddle we develop a detailed history of the timing of ice sheet discharge events from the hudson strait outlet of the lis during the holocene using high resolution detrital carbonate ice rafted detritus ird 18o and sediment color data eight detrital carbonate peaks dcps associated with ird peaks and light oxygen isotope events punctuate the md99 2236 record between 11 5 and 8 0 ka we use the stratigraphy of the dcps developed from md99 2236 to select the appropriate r to calibrate the ages of recorded glacial events in hudson bay and hudson strait such that they match the dcps in md99 2236 we associate the eight dcps with h0 gold cove advance noble inlet advance initial retreat of the hudson strait ice stream hsis from hudson strait opening of the tyrrell sea and drainage of glacial lakes agassiz and ojibway the opening of foxe channel and retreat of glacial ice from foxe basin are represented by a shoulder in the carbonate data r of 350 years applied to the radiocarbon ages constraining glacial events h0 through the opening of the tyrell sea provided the best match with the md99 2236 dcps r values and ages from the literature are used for the younger events a very close age match was achieved between the 8 2 ka cold event in the greenland ice cores dcp7 8 15 ka bp and the drainage of glacial lakes agassiz and ojibway our stratigraphic comparison between the dcps in md99 2236 and the calibrated ages of hudson strait bay deglacial events shows that the retreat of the hsis the opening of the tyrell sea and the catastrophic drainage of glacial lakes agassiz and ojibway at 8 2 ka are separate events that have been combined in previous estimates of the timing of the 8 2 ka event from marine records sw iceland shelf core md99 2256 documents freshwater entrainment into the subpolar gyre from the hudson strait outlet via the labrador north atlantic and irminger currents the timing of freshwater release from the lis hudson strait outlet in md99 2236 matches evidence for freshwater forcing and lis icebergs carrying foreign minerals to the sw iceland shelf between 11 5 and 8 2 ka the congruency of these records supports the conclusion of the entrainment of freshwater from the retreat of the lis through hudson strait into the subpolar gyre and provides specific time periods when pulses of lis freshwater were present to influence climate

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

holocene paleoceanography of the northwest passage canadian arctic archipelago abstract a series of cores box and piston were collected at 2 key locations in lancaster sound cores 2004 804 009 bc and pc and barrow strait cores 2005 804 004 bc and pc in the canadian arctic archipelago to document the evolution of sea surface conditions in the main axis of the northwest passage during the holocene time period reconstruction of sea surface parameters summer temperature and salinity duration of sea ice cover were estimated based on transfer functions using dinoflagellate cyst assemblages as proxy indicators the chronology of these cores is based on calibrated ams 14c dates 210pb analyses and correlations between paleomagnetic secular variations of the geomagnetic field and a predicted spherical harmonic model of the geomagnetic field cals7 k 2 our age models for both cores indicate that 009 pc spans the last 11 100 cal bp while 004 pc encompasses the last 10 800 cal bp calculated sedimentation rates are in the range of 43 140 cm kyr for core 009 pc and 15 118 cm kyr for core 004 pc allowing for a millennial time scale resolution in each core our results indicate relatively harsh conditions in lancaster sound between 10 800 and 9000 cal bp summer temperatures 2 c cooler than at present which we associate with the presence of active ice streams in northernmost baffin bay and a weak west greenland current this is followed by a warming trend up to 3 c higher than present that took place between 8500 and 5500 cal bp which we associate with the holocene thermal maximum and to a large scale atmospheric pattern such as the arctic oscillation operating at the millennial time scale this is concomitant with an increase in the relative abundance of phototrophic dinoflagellate cyst taxa a gradual cooling of sea surface temperature and increased sea ice follow this from 5500 cal bp until the modern period in barrow strait harsh sea surface conditions prevailed from 11 100 to 5500 cal bp with summer temperatures as low as 4 c cooler than at present the warming trend occurred later in this region between 5500 and 1000 cal bp after which a gradual cooling is observed until the modern period the apparent shift or opposite warming trends between lancaster sound and barrow strait after 5500 cal bp could be the result of a change in atmospheric circulation patterns related to a possible shift in the ao mode from ao to ao comparison of ice core 18o record from devon island ice cap and the reconstructed sea surface temperature from core 009 pc suggests a strong atmospheric oceanic coupling throughout the holocene in this area

chironomids can be reliable proxies for holocene temperatures a comment on velle et al 2010 velle et al 2010 discussed discrepancies between scandinavian holocene chironomid inferred temperature estimates which they attribute to the response of chironomids to environmental variables other than temperature and to taxonomic shortcomings they suggest ways in which the reliability of chironomid based paleotemperature reconstructions could be improved by taking into account ecological complexity while we agree with many of their recommendations based on the results of other work we think their paper is unnecessarily pessimistic regarding the ability of existing chironomid based temperature inference models to provide reliable estimates of past temperature we offer a critique of the main points discussed by velle et al 2010 and provide evidence that chironomid based temperature inference models can reliably reconstruct mean july air temperature in the lateglacial and holocene over millennial and centennial timescales

the response of the southern greenland ice sheet to the holocene thermal maximum to determine the long term sensitivity of the greenland ice sheet to a warmer climate we explored how it responded to the holocene thermal maximum 8 5 cal kyr b p calibrated to calendar years before present i e a d 1950 when lake records show that local atmospheric temperatures in greenland were 2 4 c warmer than the present records from five new threshold lakes complemented with existing geological data from south of 70 n show that the ice margin was retracted behind its present day extent in all sectors for a limited period between ca 7 and 4 cal kyr b p and in most sectors from ca 1 5 to 1 cal kyr b p in response to higher atmospheric and ocean temperatures ice sheet simulations constrained by observations show good correlation with the timing of minimum ice volume indicated by the threshold lake observations the simulated volume reduction suggests a minimum contribution of 0 16 m sea level equivalent from the entire greenland ice sheet with a centennial ice loss rate of as much as 100 gt yr for several millennia during the holocene thermal maximum our results provide an estimate of the long term rates of volume loss that can be expected in the future as regional air and ocean temperatures approach those reconstructed for the holocene thermal maximum

diachronous retreat of the greenland ice sheet during the last deglaciation abstract the last deglaciation is the most recent interval of large scale climate change that drove the greenland ice sheet from continental shelf to within its present extent here we use a database of 645 published 10be ages from greenland to document the spatial and temporal patterns of retreat of the greenland ice sheet during the last deglaciation following initial retreat of its marine margins most land based deglaciation occurred in greenland following the end of the younger dryas cold period 12 9 11 7 ka however deglaciation in east greenland peaked significantly earlier 13 0 11 5 ka than that in south greenland 11 0 10 ka or west greenland 10 5 7 0 ka the terrestrial deglaciation of east and south greenland coincide with adjacent ocean warming 14c ages and a recent ice sheet model reconstruction do not capture this progression of terrestrial deglacial ages from east to west greenland showing deglaciation occurring later than observed in 10be ages this model data misfit likely reflects the absence of realistic ice ocean interactions we suggest that oceanic changes may have played an important role in driving the spatial temporal ice retreat pattern evident in the 10be data

sea surface conditions in northernmost baffin bay during the holocene palynological evidence the analysis of cores collected in northernmost baffin bay from within the area of the north water polynya permits definition of a composite sedimentary sequence ca 12 m thick spanning the last 10 000 14 c yr with only a few discontinuities palynological analyses were performed in order to reconstruct changes in surface water conditions and biogenic production transfer functions using dinocyst assemblages were applied to estimate sea surface temperature sst and salinity as well as the seasonal duration of sea ice cover at the base of the record prior to 9300 14 c yr bp dinocysts and organic linings of benthic foraminifers are sparse indicating harsh conditions and low productivity after ca 9300 14 c yr bp the increased concentration of benthic foraminifers up to 10 3 linings cm 3 and dinocyst fluxes 10 2 10 3 cysts cm 2 yr 1 reveals high biological productivity related to open water conditions the early to middle holocene from ca 9000 to ca 3600 14 c yr bp is marked by relatively high species diversity in dinocyst assemblages and the significant occurrence of autotrophic taxa such as spiniferites elongatus pentapharsodinium dalei and impagidinium pallidum this assemblage suggests conditions at least as warm as at present from ca 6400 to ca 3600 14 c yr bp transfer functions indicate warmer conditions than at present with sst in august fluctuating up to 5 5 c after 3600 14 c yr bp the dinocyst record suggests a trend of decreasing temperature toward modern values marked by recurrent cooling events copyright 2001 john wiley sons ltd

late holocene expansion of istorvet ice cap liverpool land east greenland abstract the greenland ice sheet is undergoing dynamic changes that will have global implications if they continue into the future in this regard an understanding of how the ice sheet responded to past climate changes affords a baseline for anticipating future behavior small independent ice caps adjacent to the greenland ice sheet hereinafter called local ice caps are sensitive indicators of the response of greenland ice marginal zones to climate change therefore we reconstructed late holocene ice marginal fluctuations of the local istorvet ice cap in east greenland using radiocarbon dates of subfossil plants 10 be dates of surface boulders and analyses of sediment cores from both threshold and control lakes during the last termination the istorvet ice cap had retreated close to its maximum holocene position by 11 730 cal yr bp radiocarbon dates of subfossil plants exposed by recent recession of the ice margin indicate that the istorvet cap was smaller than at present from ad 200 to ad 1025 sediments from a threshold lake show no glacial input until the ice cap advanced to within 365 m of its holocene maximum position by ad 1150 thereafter the ice cap remained at or close to this position until at least ad 1660 the timing of this the most extensive of the holocene expansion is similar to that recorded at some glaciers in the alps and in southern alaska however in contrast to these other regions the expansion in east greenland at ad 1150 appears to have been very close to if not at a maximum holocene value comparison of the istorvet ice cap fluctuations with holocene glacier extents in southern hemisphere middle to high latitude locations on the antarctic peninsula and in the andes and the southern alps suggests an out of phase relationship if correct this pattern supports the hypothesis that a bipolar see saw of oceanic and or atmospheric circulation during the holocene produced asynchronous glacier response at some localities in the two polar hemispheres

dinocyst based reconstructions of sea ice cover concentration during the holocene in the arctic ocean the northern north atlantic ocean and its adjacent seas abstract sea ice cover extent expressed in terms of mean annual concentration was reconstructed from the application of the modern analogue technique to dinocyst assemblages the use of an updated database which includes 1492 sites and 66 taxa yields sea ice concentration estimates with an accuracy of 1 1 10 holocene reconstructions of sea ice cover were made from dinocyst counts in 35 cores of the northern north atlantic and arctic seas in the canadian arctic the results show high sea ice concentration 7 10 with little variations throughout the interval in contrast in arctic areas such as the chukchi sea and the barents sea the reconstructions show large amplitude variations of sea ice cover suggesting millennial type oscillations with a pacing almost opposite in western vs eastern arctic other records show tenuous changes with some regionalism either in trends or sea ice cover variability during the mid holocene and notably at 6 0 5 ka minimum sea ice concentration is recorded in the eastern fram strait northern baffin bay and labrador sea however this minimum cannot be extrapolated at the scale of the arctic and circum arctic the comparison of recent observations and reconstructions suggests larger variations in the arctic sea ice cover during the last decades than throughout the holocene

holocene sea ice variations in greenland onshore evidence the oldest dated driftwood log from northern greenland is c 9300 cal years old which is about 2000 years younger than the beginning of the last deglaciation and 2000 years younger than the oldes

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

landscape chronology and glacial history in thule northwest greenland abstract constraining past responses of ice sheets to changes in climate enhances our ability to predict future changes in ice extent and sea level here we employ surficial geologic mapping stratigraphic analysis radiocarbon dating of marine mollusks and analysis of cosmogenic 10 be in glacially deposited boulders to make inferences about past ice extents in thule northwest greenland our work shows the existence of two distinct diamict units in the thule area we infer that one diamict was deposited by the main greenland ice sheet while the second was deposited by a subsequent outlet glacier re advance boulders in the two diamicts n 13 and n 15 respectively have similar bimodal distributions of cosmogenic 10 be ages both diamicts have a group of younger 10 be ages as well as a more diffuse group of considerably older 10 be ages 20 30 ka many of the boulders likely contain nuclides inherited from previous periods of exposure suggesting that glacial ice in the thule region has been cold based and non erosive in the past the landscape chronology we develop suggests that deglaciation after the last glacial maximum occurred 10 8 ka and was later followed by a re advance of outlet glaciers possibly in concert with the 9 3 ka or 8 2 ka cold events

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

postglacial chironomid assemblage succession in northernmost ungava peninsula canada sediment cores spanning the postglacial period were recovered from two coastal freshwater basins located 300 km apart in northernmost ungava nunavik canada a basal date from the sequence recovered near wakeham bay suggests that the region was ice free by as early as 9600 cal a bp the initial development of both investigated lacustrine ecosystems was characterised by chironomid assemblages strongly dominated by the subtribe tanytarsina these early postglacial assemblages showed greatly reduced diversity but much greater abundance than during subsequent periods of lake evolution an abrupt and distinct turnover in sedimentary chironomid assemblage composition and productivity marked by a switch to a dominance of orthocladiinae occurred around 6 ka bp this turnover likely corresponds with a significant disturbance in the regional environment which could be linked to the final disappearance of the last remnants of the laurentide ice sheet these findings based on the first palaeolimnological investigations undertaken in northernmost ungava provide new insights into lateglacial and postglacial faunal recolonisation patterns in aquatic ecosystems of the eastern canadian arctic copyright 2009 john wiley sons ltd

holocene climate change in arctic canada and greenland abstract this synthesis paper summarizes published proxy climate evidence showing the spatial and temporal pattern of climate change through the holocene in arctic canada and greenland our synthesis includes 47 records from a recently published database of highly resolved holocene paleoclimate time series from the arctic sundqvist et al 2014 we analyze the temperature histories represented by the database and compare them with paleoclimate and environmental information from 54 additional published records mostly from datasets that did not fit the selection criteria for the arctic holocene database combined we review evidence from a variety of proxy archives including glaciers ice cores and glacial geomorphology lake sediments peat sequences and coastal and deep marine sediments the temperature sensitive records indicate more consistent and earlier holocene warmth in the north and east and a more diffuse and later holocene thermal maximum in the south and west principal components analysis reveals two dominant holocene trends one with early holocene warmth followed by cooling in the middle holocene the other with a broader period of warmth in the middle holocene followed by cooling in the late holocene the temperature decrease from the warmest to the coolest portions of the holocene is 3 0 1 0 c on average n 11 sites the greenland ice sheet retracted to its minimum extent between 5 and 3 ka consistent with many sites from around greenland depicting a switch from warm to cool conditions around that time the spatial pattern of temperature change through the holocene was likely driven by the decrease in northern latitude summer insolation through the holocene the varied influence of waning ice sheets in the early holocene and the variable influx of atlantic water into the study region

midges as quantitative temperature indicator species lessons for palaeoecology calibration data sets give a unique opportunity to establish patterns of biological existence and their statistical associations with environmental variables by use of calibration data sets environmental variables can be inferred quantitatively the resulting long time series may assist in distinguishing natural environmental variability from human induced variability both in terms of climate change and biotic turnover however the validity of the palaeoenvironmental reconstructions depends on their accuracy precision and sensibility before performing palaeoenvironmental inferences key mechanisms controlling contemporary species distribution abundances and dynamics should be identified and understood an inference model is developed to produce reconstructions a major challenge lies in validating and interpreting the reconstructions calibration data sets involving midges diptera chironomidae suggest that climate has a broad scale regional control over midge existence and abundance often ove

holocene boreal molluscs in greenland palaeoceanographic implications abstract boreal molluscs now extinct in greenland waters occur as subfossils in west greenland between lat 65 30 and 68 30 n a number of new localities for these warmth demanding molluscs are described and the list of species now comprises the gastropod emarqinula fissura and the bivalvalves heteranomia squamula modiolus arctica islandica panopea norvegica and zirphaea crispata during their time the molluscs formed an isolated northern outpost located in the area which as a result of the complex oceanic circulation today has greenland s highest ocean surface temperatures the occurrences are 14c dated to the interval 8400 4900 yr b p and summer surface temperatures were 1 3 c higher than now for its beginning this period coincides with similar marine optimal periods in east greenland and on the baffin island shelf whereas it took 1000 years before the beginning of warmer than present temperatures on land on the basis of this evidence it is concluded that the rise of sea surface temperatures was afforded by a decrease in inflow of cold polar water and or increase in inflow of warm atlantic water into davis strait rather than climatic change

interglacial and holocene temperature reconstructions based on midge remains in sediments of two lakes from baffin island nunavut arctic canada abstract midge remains insecta diptera chironomidae and chaoboridae from two upland lake basins on the cumberland peninsula of baffin island were analyzed and the results used in a transfer function to reconstruct paleotemperatures this upland region remained ice free throughout the last glacial cycle and the two lake basins retain sediments deposited during the previous interglacial period as well as during the holocene period but they did not accumulate sediment during most of the glacial period midge remains are abundant and well preserved at both sites midge inferred summer surface water temperatures and mean july air temperatures were estimated using an inference model built on modern samples spanning a geographic range extending from devon island canada to maine usa data from twenty nine new surface samples from baffin island were added to an existing inference model the new weighted averaging wa model for summer surface water temperature yields r 2 jack 0 88 and rmsep 2 22 c for summer water temperatures and r 2 jack 0 88 and rmsep 1 53 c for mean july air temperatures reconstructions at both sites indicate that summer temperatures during the last interglacial were higher than at any time in the holocene and 5 to 10 c higher than present peak holocene temperatures occurred in the first half of the period and have decreased since about the mid holocene

holocene temperature history of northwest greenland with new ice cap constraints and chironomid assemblages from deltaso abstract arctic temperature shifts drive changes in carbon cycling sea ice extent and greenland ice sheet mass balance all of which have global ramifications paleoclimate data from past warm periods provide a unique means for assessing the sensitivity of these systems to warming climate but the magnitude and timing of past temperature changes in many parts of the arctic are poorly known here we assess orbital scale holocene temperature change in northwest greenland near the margin of the ice sheet using subfossil insect assemblages from lake deltaso based upon sedimentation history in this currently proglacial lake we also place constraints on holocene extents of the adjacent north ice cap a large independent ice cap reconstructed summer temperatures were warmer than present at the onset of lacustrine sedimentation following regional deglaciation by the greenland ice sheet sometime between 10 8 and 10 1 ka bp deltaso experienced the warmest summer temperatures of the holocene between 10 and 6 2 ka bp followed by progressive cooling that continued through the late holocene as summer insolation declined culminating in the lowest temperatures during the pre industrial last millennium deltaso chironomids indicate peak early holocene summer temperatures at least 2 5 3 c warmer than modern and at least 3 5 4 c warmer than the pre industrial last millennium we infer based upon lake sediment organic and biogenic content that in response to declining temperatures north ice cap reached its present day size 1850 ad having been smaller than present through most of the preceding holocene our synthesis of paleoclimate evidence from northwest greenland ellesmere island and northern baffin bay supports the timing of temperature trends inferred at deltaso and suggests that quantitative temperature reconstructions from deltaso may represent a minimum bound on regional early holocene warming collectively records from the region indicate 4 c summer cooling through the holocene intense early holocene warmth around northwest greenland argues against delayed onset of warmer than present conditions due to the influence of the nearby waning laurentide ice sheet and has implications for understanding the greenland ice sheet s sensitivity to climate change

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

holocene evolution of the western greenland ice sheet assessing geophysical ice sheet models with geological reconstructions of ice margin change abstract geophysical ice sheet models are used to predict future ice sheet dimensions and in turn these projections help estimate the magnitude of eustatic sea level rise before models can confidently predict ice sheet behavior they must be validated by being able to duplicate the geological record of ice sheet change here we review geological records of greenland ice sheet gris change with emphasis on the warmer than present middle holocene and compare these records to published studies that numerically simulate gris behavior through the holocene geological records are concentrated in west and southwest greenland which are also the regions where the gris margin likely experienced the greatest distance of inland retreat during the middle holocene several records spanning from melville bugt to jakobshavn isfjord in western greenland indicate the gris achieved its minimum extent between 5 and 3 ka and farther south in the kangerlussuaq region new data presented here indicate the ice margin reached its minimum extent between 4 2 and 1 8 ka in the narsarsuaq region in southern greenland the gris likely achieved its minimum configuration between 7 and 4 ka we highlight key similarities and discrepancies between these reconstructions and model results and finally we suggest that despite some degree of inland retreat the west and southwest gris margin remained relatively stable and close to its current position through the holocene thermal maximum

holocene temperature history at the western greenland ice sheet margin reconstructed from lake sediments abstract predicting the response of the greenland ice sheet to future climate change presents a major challenge to climate science paleoclimate data from greenland can provide empirical constraints on past cryospheric responses to climate change complementing insights from contemporary observations and from modeling here we examine sedimentary records from five lakes near jakobshavn isbrae in central west greenland to investigate the timing and magnitude of major holocene climate changes for comparison with glacial geologic reconstructions from the region a primary objective of this study is to constrain the timing and magnitude of maximum warmth during the early to middle holocene positive anomaly in summer insolation temperature reconstructions from subfossil insect chironomid assemblages suggest that summer temperatures were warmer than present by at least 7 1 ka the beginning of the north lake record ka thousands of years before present and that the warmest millennia of the holocene occurred in the study area between 6 and 4 ka previous studies in the jakobshavn region have found that the local greenland ice sheet margin was most retracted behind its present position between 6 and 5 ka and here we use chironomids to estimate that local summer temperatures were 2 3 c warmer than present during that time of minimum ice sheet extent as summer insolation declined through the late holocene summer temperatures cooled and the local ice sheet margin expanded gradual insolation driven millennial scale temperature trends in the study area were punctuated by several abrupt climate changes including a major transient event recorded in all five lakes between 4 3 and 3 2 ka which overlaps in timing with abrupt climate changes previously documented around the north atlantic region and farther afield at 4 2 ka

holocene climate changes in eastern beringia nw north america a systematic review of multi proxy evidence abstract reconstructing climates of the past relies on a variety of evidence from a large number of sites to capture the varied features of climate and the spatial heterogeneity of climate change this review summarizes available information from diverse holocene paleoenvironmental records across eastern beringia alaska westernmost canada and adjacent seas and it quantifies the primary trends of temperature and moisture sensitive records based in part on midges pollen and biogeochemical indicators compiled in the recently published arctic holocene database and updated here to v2 1 the composite time series from these proxy records are compared with new summaries of mountain glacier and lake level fluctuations terrestrial water isotope records sea ice and sea surface temperature analyses and peatland and thaw lake initiation frequencies to clarify multi centennial to millennial scale trends in holocene climate change to focus the synthesis the paleo data are used to frame specific questions that can be addressed with simulations by earth system models to investigate the causes and dynamics of past and future climate change this systematic review shows that during the early holocene 11 7 8 2 ka 1 ka 1000 cal yr bp rather than a prominent thermal maximum as suggested previously temperatures were highly variable at times both higher and lower than present approximate mid 20th century average with no clear spatial pattern composited pollen midge and other proxy records average out the variability and show the overall lowest summer and mean annual temperatures across the study region during the earliest holocene followed by warming over the early holocene the sparse data available on early holocene glaciation show that glaciers in southern alaska were as extensive then as they were during the late holocene early holocene lake levels were low in interior alaska but moisture indicators show pronounced differences across the region the highest frequency of both peatland and thaw lake initiation ages also occurred during the early holocene during the middle holocene 8 2 4 2 ka glaciers retreated as the regional average temperature increased to a maximum between 7 and 5 ka as reflected in most proxy types following the middle holocene thermal maximum temperatures decreased starting between 4 and 3 ka signaling the onset of neoglacial cooling glaciers in the brooks and alaska ranges advanced to their maximum holocene extent as lakes generally rose to modern levels temperature differences for averaged 500 year time steps typically ranged by 1 2 c for individual records in the arctic holocene database with a transition to a cooler late holocene that was neither abrupt nor spatially coherent the longest and highest resolution terrestrial water isotope records previously interpreted to represent changes in the aleutian low pressure system around this time are here shown to be largely contradictory furthermore there are too few records with sufficient resolution to identify sub centennial scale climate anomalies such as the 8 2 ka event the review concludes by suggesting some priorities for future paleoclimate research in the region

paleoecological evidence for abrupt cold reversals during peak holocene warmth on baffin island arctic canada a continuous record of insect chironomidae remains preserved in lake sediments is used to infer temperature changes at a small lake in arctic canada through the holocene early holocene summers at the study site were characterized by more thermophilous assemblages and warmer inferred temperatures than today presumably in response to the positive anomaly in northern hemisphere summer insolation peak early holocene warmth was interrupted by two cold reversals between 9 5 and 8 cal ka bp during which multiple cold stenothermous chironomid taxa appeared in the lake the earlier reversal appears to correlate with widespread climate anomalies around 9 2 cal ka bp the age of the younger reversal is equivocal but it may correlate with the 8 2 cal ka bp cold event documented elsewhere widespread abrupt climate shifts in the early holocene illustrate the susceptibility of the climate system to perturbations even during periods of enhanced warmth in the northern hemisphere

a 10 000 year record of arctic ocean sea ice variability view from the beach we present a sea ice record from northern greenland covering the past 10 000 years multiyear sea ice reached a minimum between 8500 and 6000 years ago when the limit of year round sea ice at the coast of greenland was located 1000 kilometers to the north of its present position the subsequent increase in multiyear sea ice culminated during the past 2500 years and is linked to an increase in ice export from the western arctic and higher variability of ice drift routes when the ice was at its minimum in northern greenland it greatly increased at ellesmere island to the west the lack of uniformity in past sea ice changes which is probably related to large scale atmospheric anomalies such as the arctic oscillation is not well reproduced in models this needs to be further explored as it is likely to have an impact on predictions of future sea ice distribution

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

measurement of 7be in soils and sediments by gamma spectroscopy abstract measurement of cosmogenic 7 be in soils and sediments by gamma spectroscopy has been routine for three decades however the analytical constraints of this method have yet to be adequately described natural abundance of 7 be in bulk soils and sediments is typically low 10 6 to 10 8 atoms kg 1 resulting in very low signal to noise ratio in the gamma spectrum 7 be quantification is further complicated by numerous interferences from ubiquitous 238 u and 232 th series radionuclides we investigated the environmental gamma spectrum in the region of 7 be emission using spectra obtained from hundreds of natural stream sediment samples one well characterized sediment naturally enriched in 7 be and pure standard materials enriched in both 238 u series and 232 th series decay chains in typical samples no fewer than three discrete photopeaks directly interfere with 7 be measurement one of which the 228 ac 478 3 kev photon yields significant positive bias risks of 7 be false positives due to this overlapping photopeak approach 50 for typical soils or sediments detection limits for 7 be in these materials using conventional high purity germanium instruments are as low as 0 1 bq but increase by up to a factor of four in the presence of modest activities of 238 u and 232 th series nuclides here we demonstrate a methodology for the measurement of low 7 be abundances in soils and sediments that is unbiased and that optimizes precision

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

high arctic holocene temperature record from the agassiz ice cap and greenland ice sheet evolution we present a revised and extended high arctic air temperature reconstruction from a single proxy that spans the past 12 000 y up to 2009 ce our reconstruction from the agassiz ice cap ellesmere island canada indicates an earlier and warmer holocene thermal maximum with early holocene temperatures that are 4 5 c warmer compared with a previous reconstruction and regularly exceed contemporary values for a period of 3 000 y our results show that air temperatures in this region are now at their warmest in the past 6 800 7 800 y and that the recent rate of temperature change is unprecedented over the entire holocene the warmer early holocene inferred from the agassiz ice core leads to an estimated 1 km of ice thinning in northwest greenland during the early holocene using the camp century ice core ice modeling results show that this large thinning is consistent with our air temperature reconstruction the modeling results also demonstrate the broader significance of the enhanced warming with a retreat of the northern ice margin behind its present position in the mid holocene and a 25 increase in total greenland ice sheet mass loss 1 4 m sea level equivalent during the last deglaciation both of which have implications for interpreting geodetic measurements of land uplift and gravity changes in northern greenland

influence of the water content on x ray fluorescence core scanning measurements in soft marine sediments the x ray fluorescence xrf core scanner provides bulk sediment chemistry data measured nondestructively at the split core sediment surface although this method is widely accepted there is little known about the effects of physical properties such as density and water content on xrf core scanner data comparison of xrf scanner measurements from the sediment surface and dry powder samples of sediment core geob7920 indicates strongly reduced element intensities for the lighter elements al and si we relate the lower element intensities of the measurements taken at the sediment surface to the amount of water in the sample volume analyzed by the xrf core scanner the heavier elements k ca ti and fe remain relatively unaffected by the variation of any physical property within sediment core geob7920 additionally we successfully use the elemental intensity of cl as a proxy for the seawater content in the sample volume analyzed by the xrf core scanner this enables the establishment of a correction function for the elements al and si that corrects for the radiation absorption of the water content in sediment core geob7920 off cape blanc nw africa

the deglaciation and neoglaciation of upernavik isstrom greenland abstract we constrain the history of the greenland ice sheet margin during the holocene at upernavik isstrom a major ice stream in northwestern greenland radiocarbon dated sediment sequences from proglacial threshold lakes adjacent to the present ice margin constrain deglaciation of the sites to older than 9 6 0 1 ka this age of deglaciation is confirmed with 10 be ages of 9 9 0 1 ka from an island adjacent to the historical ice position the lake sediment sequences also constrain the ice margin to have been less extensive than it is today for the remainder of the holocene until 1100 to 700 yr ago when it advanced into two lake catchments the ice margin retreated back out of these lake catchments in the last decade the early holocene deglaciation in melville bugt one of few locations around greenland where a vast stretch of the current ice margin is marine based preceded deglaciation in most other parts of greenland earlier deglaciation in this ice sheet sector may have been caused by additional ablation mechanisms that apply to marine based ice margins furthermore despite ice sheet models depicting this sector of greenland as relatively stable throughout the holocene our data indicate a 20 km advance retreat cycle within the last millennium

formal subdivision of the holocene series epoch a discussion paper by a working group of intimate integration of ice core marine and terrestrial records and the subcommission on quaternary stratigraphy international commission on stratigraphy this discussion paper by a working group of intimate integration of ice core marine and terrestrial records and the subcommision on quaternary stratigraphy sqs of the international commission on stratigraphy ics considers the prospects for a formal subdivision of the holocene series epoch although previous attempts to subdivide the holocene have proved inconclusive recent developments in quaternary stratigraphy notably the definition of the pleistocene holocene boundary and the emergence of formal subdivisions of the pleistocene series epoch mean that it may be timely to revisit this matter the quaternary literature reveals a widespread but variable informal usage of a tripartite division of the holocene early middle or mid and late and we argue that this de facto subdivision should now be formalized to ensure consistency in stratigraphic terminology we propose an early middle holocene boundary at 8200 a bp and a middle late holocene boundary at 4200 a bp each of which is linked to a global stratotype section and point gssp should the proposal find a broad measure of support from the quaternary community a submission will be made to the international union of geological sciences iugs via the sqs and the ics for formal ratification of this subdivision of the holocene series epoch copyright 2012 john wiley sons ltd

spatial and temporal variability of holocene temperature in the north atlantic region the early holocene climate of the north atlantic region was influenced by two boundary conditions that were fundamentally different from the present the presence of the decaying laurentide ice sheet lis and higher than present summer solar insolation in order to assess spatial and temporal patterns of holocene climate evolution across this region we collated quantitative paleotemperature records at sub millennial resolution and synthesized their temporal variability using principal components analysis pca the analysis reveals considerable spatial variability most notably in the time transgressive expression of the holocene thermal maximum htm most of the region but especially areas peripheral to the labrador sea and hence closest to the locus of lis disintegration experienced maximum holocene temperatures that lagged peak summer insolation by 1000 3000 years many sites from the northeastern north atlantic sector including the nordic seas and scandinavia either warmed in phase with maximum summer insolation 11 000 9000 years ago or were less strongly lagged than the baffin bay labrador sea region these spatially complex patterns of holocene climate development which are defined by the pca resulted from the interplay between final decay of the lis and solar insolation forcing

holocene thermal maximum in the western arctic 0 180 w the spatio temporal pattern of peak holocene warmth holocene thermal maximum htm is traced over 140 sites across the western hemisphere of the arctic 0 180 w north of 60 n paleoclimate inferences based on a wide variety of proxy indicators provide clear evidence for warmer than present conditions at 120 of these sites at the 16 terrestrial sites where quantitative estimates have been obtained local htm temperatures primarily summer estimates were on average 1 6 0 8 c higher than present approximate average of the 20th century but the warming was time transgressive across the western arctic as the precession driven summer insolation anomaly peaked 12 10 ka thousands of calendar years ago warming was concentrated in northwest north america while cool conditions lingered in the northeast alaska and northwest canada experienced the htm between ca 11 and 9 ka about 4000 yr prior to the htm in northeast canada the delayed warming in quebec and labrador was linked to the residual laurentide ice sheet which chilled the region through its impact on surface energy balance and ocean circulation the lingering ice also attests to the inherent asymmetry of atmospheric and oceanic circulation that predisposes the region to glaciation and modulates the pattern of climatic change the spatial asymmetry of warming during the htm resembles the pattern of warming observed in the arctic over the last several decades although the two warmings are described at different temporal scales and the htm was additionally affected by the residual laurentide ice the similarities suggest there might be a preferred mode of variability in the atmospheric circulation that generates a recurrent pattern of warming under positive radiative forcing unlike the htm however future warming will not be counterbalanced by the cooling effect of a residual north american ice sheet

a model of greenland ice sheet deglaciation constrained by observations of relative sea level and ice extent an ice sheet model was constrained to reconstruct the evolution of the greenland ice sheet gris from the last glacial maximum lgm to present to improve our understanding of its response to climate change the study involved applying a glaciological model in series with a glacial isostatic adjustment and relative sea level rsl model the model reconstruction builds upon the work of simpson et al 2009 through four main extensions 1 a larger constraint database consisting of rsl and ice extent data model improvements to the 2 climate and 3 sea level forcing components 4 accounting for uncertainties in non greenland ice the research was conducted primarily to address data model misfits and to quantify inherent model uncertainties with the earth structure and non greenland ice our new model termed huy3 fits the majority of observations and is characterised by a number of defining features during the lgm the ice sheet had an excess of 4 7 m ice equivalent sea level iesl which reached a maximum volume of 5 1 m iesl at 16 5 cal ka bp modelled retreat of ice from the continental shelf progressed at different rates and timings in different sectors southwest and southeast greenland began to retreat from the continental shelf by 16 to 14 cal ka bp thus responding in part to the bolling allerod warm event c 14 5 cal ka bp subsequently ice at the southern tip of greenland readvanced during the younger dryas cold event in northern greenland the ice retreated rapidly from the continental shelf upon the climatic recovery out of the younger dryas to present day conditions upon entering the holocene 11 7 cal ka bp the ice sheet soon became land based during the holocene thermal maximum htm 9 5 cal ka bp air temperatures across greenland were marginally higher than those at present and the gris margin retreated inland of its present day southwest position by 40 60 km at 4 cal ka bp which produced a deficit volume of 0 16 m iesl relative to present in response to the htm warmth our optimal model reconstruction lost mass at a maximum centennial rate of c 103 4 gt yr our results suggest that remaining data model discrepancies are affiliated with missing physics and sub grid processes of the glaciological model uncertainties in the climate forcing lateral earth structure and non greenland ice particularly the north american component finally applying the huy3 greenland reconstruction with our optimal earth model we generate present day uplift rates across greenland due to past changes in the ocean and ice loads with explicit error bars due to uncertainties in the earth structure present day uplift rates due to past changes are spatially variable and range from 3 5 to 7 mm a including earth model uncertainty

calibrating a glaciological model of the greenland ice sheet from the last glacial maximum to present day using field observations of relative sea level and ice extent we constrain a three dimensional thermomechanical model of greenland ice sheet gris evolution from the last glacial maximum lgm 21 ka bp to the present day using primarily observations of relative sea level rsl as well as field data on past ice extent our new model huy2 fits a majority of the observations and is characterised by a number of key features i the ice sheet had an excess volume relative to present of 4 1 m ice equivalent sea level at the lgm which increased to reach a maximum value of 4 6 m at 16 5 ka bp ii retreat from the continental shelf was not continuous around the entire margin as there was a younger dryas readvance in some areas the final episode of marine retreat was rapid and relatively late c 12 ka bp leaving the ice sheet land based by 10 ka bp iii in response to the holocene thermal maximum htm the ice margin retreated behind its present day position by up to 80 km in the southwest 20 km in the south and 80 km in a small area of the northeast as a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka bp which corresponds to a deficit volume relative to present of 0 17 m ice equivalent sea level our results suggest that remaining discrepancies between the model and the observations are likely associated with non greenland ice load differences between modelled and observed present day ice elevation around the margin lateral variations in earth structure and or the pattern of ice margin retreat

holocene fluctuations of bregne ice cap scoresby sund east greenland a proxy for climate along the greenland ice sheet margin abstract the greenland ice sheet is a major component of the arctic cryosphere and the magnitude of its response to future climate changes remains uncertain longer term records of climate near the ice sheet margin provide information about natural climate variability and can be used to understand the causes of past changes in the greenland ice sheet as a proxy for holocene climate near the ice sheet margin we reconstruct the fluctuations of bregne ice cap in the scoresby sund region of central east greenland bregne is a small ice cap 2 5 km 2 in area and responds sensitively to summer temperatures we employ a multi proxy approach to reconstruct the ice cap fluctuations using geomorphic mapping 10 be ages of boulders and bedrock and lake sediment records past extents of bregne ice cap are marked by moraines and registered by sediments in downvalley lakes 10 be ages of bedrock and boulders outboard of the moraines indicate that bregne ice cap was within 250 m of its present day limit by at least 10 7 ka multi proxy data from sediments in two move lake located downvalley from bregne ice cap indicate that the ice cap likely completely disappeared during early and middle holocene time increasing magnetic susceptibility and percent clastic material from 6 5 to 1 9 cal ka bp in two move lake sediments suggest progressively colder conditions and increased snow accumulation on the highlands west of the lake laminated silt deposited at 2 6 cal ka bp and 1 9 cal ka bp to present registers the onset and persistence of bregne ice cap during the late holocene 10 be ages of boulders on an unweathered unvegetated moraine in the bregne ice cap forefield range from 0 74 to 9 60 ka the youngest 10 be age 0 74 ka likely represents the age of the moraine whereas older ages may be due to 10 be inherited from prior periods of exposure this late holocene moraine marks the second largest advance of the ice cap since deglaciation of the region at the end of the last ice age the oldest moraine in the forefield dates to 2 6 cal ka bp the fluctuations of bregne ice cap were likely influenced by northern hemisphere summer insolation throughout the holocene and abrupt late holocene cold events

north south roughness anisotropy on venus from the magellan radar altimeter correlation with geology 1 analysis of the doppler spectrum of magellan radar altimeter echoes has revealed the spatially coherent deflection of the echo center from the nadir point tyler et al 1992 we present and analyze a global map of the doppler centroid of the radar altimeter echo which for the plains of venus characterizes the degree of anisotropy of the surface roughness in the north south direction at scales from decimeters to kilometers we found a global hemispherical trend of roughness anisotropy on average equator facing slopes are steeper than pole facing slopes we searched globally for correlations between contrasts in roughness anisotropy and surface geology we found that sharp contrasts in anisotropy often coincide with boundaries of geological units although not consistently we found a number of examples where radar bright rough lava flows have higher asymmetry than their surroundings diffuse radar dark crater related parabolic features and halos often have a symmetric roughness signature while typical regional plains away from these diffuse features often have an anisotropic roughness there is not however any direct and uniform correlation between the roughness anisotropy signature and geomorphologic terrain type or appearance in the radar images we believe that asymmetric topography of deposits of windblown material e g microdunes or smallscale wind streaks in the wind shadow of obstacles is the main contributor to the observed roughness anisotropy