does an asymmetric thermohaline ice sheet oscillator drive 100 000 yr glacial cycles a hypothesis is presented that late quaternary 100 000 yr glacial cycles are driven by an asymmetric thermohaline ice sheet oscillator that emerged in the global climate system 650 000 950 000 yr ago perhaps when the main source of northern hemisphere deep water production shifted south from the arctic into the nordic seas it is hypothesised that the asymmetry is due to the increasing difficulty after 950 000 years ago of resetting an interglacial mode of the critical nordic limb of the salinity conveyor once it switches off and an ensuing iceberg flux enters the areas of downwelling a possible reason for both a southward shift and the resulting asymmetry is uplift of the greenland scotland submarine ridge from activity of the iceland mantle plume in this hypothesis an individual 100 000 yr glacial cycle begins when the northernmost limb of the salinity conveyor in the nordic seas is curtailed or even switched off perhaps due to the growing strength of competing antarctic bottom water aabw generated by interglacial recession of the west antarctic ice sheet wais from the west antarctic rift system such recession produces southern marginal seas where dense shelf water can collect and overflow into the abyss when northern ice sheets nucleated by this circulation switch develop marine components that calve icebergs into the nordic seas the salinity conveyor can no longer revert to an interglacial mode from orbital forcing as it did prior to 950 000 yr ago in order to reset an interglacial circulation mode of the conveyor ice sheets must continue to grow for 100 000 years until they capture enough excess volume to produce a gravitational collapse of marine based components so massive that all grounded ice is flushed from north atlantic continental shelves the outburst of icebergs produced by this collapse cripples the glacial mode of overturning in the northern north atlantic once this collapse ends however the nordic seas become nearly free of icebergs for the first time in 100 000 years because of the depletion of adjacent marine based components as a consequence north atlantic salinity increases rapidly switching the conveyor into a vigorous interglacial mode of operation and hence terminating the glacial cycle by lowering sea level the prolonged growth of northern hemisphere ice sheets during each 100 000 yr cycle drives antarctic grounding lines seaward across continental shelves squeezing off the source of densified shelf waters that feed aabw sea level rise and increased basal melting however caused by the subsequent collapse of northern ice sheets and the reintroduction of north atlantic deep water into the southern ocean reverses the process forcing retreat of antarctic grounding lines from their advanced last glacial maximum positions this retreat opens marginal seas for renewed formation of dense shelf water by expanding marginal seas and hence the source of dense shelf water ongoing recession of the wais strengthens aabw during the course of an interglaciation eventually forcing a thermohaline circulation switch in the nordic seas and initiating yet another 100 000 yr glacial cycle the 100 000 yr duration of each cycle is set by two factors inertia is built into the system by the long time required for ice sheets to grow to the excess volume necessary for a marine collapse that resets the salinity conveyor into an interglacial mode eccentricity driven changes in the amplitude of the precession or tropical half precession signal give rise to warming events that trigger such a collapse of excess ice about each 100 000 yr

a low order model of the heinrich event cycle if heinrich events result from free oscillations in the size and basal melting conditions of the laurentide ice sheet a rigorous quantitative analysis of ice sheet physics should describe their dynamics to explore this possibility i exploit two characteristic timescales which arise from ice sheet physics to construct a relaxation oscillator model of the north atlantic s heinrich events the numerical implementation of this model confirms the notion that the periodicity of heinrich events approximately 7 000 years is determined by the gross properties of a steady glacial climate e g an annual average sea level temperature of 10 c and an adiabatic lapse rate

flux and provenance of ice rafted debris in the earliest pleistocene sub polar north atlantic ocean comparable to the last glacial maximum relatively little is known in detail about the locations of the early pleistocene ice sheets responsible for ice rafted debris ird inputs to the sub polar north atlantic ocean during intensification of northern hemisphere glaciation inhg to shed new light on this problem we present the first combined in depth analysis of ird flux and geochemical provenance of individual sand sized ird deposited in the sub polar north atlantic ocean during the earliest large amplitude pleistocene glacial marine isotope stage mis 100 2 52 ma arguably the key glacial during inhg ird provenance is assessed using laser ablation lead pb isotope analyses of single feldspar grains we find that the pb isotope composition 206pb 204pb 207pb 204pb and 208pb 204pb of individual ice rafted 150 m feldspars deposited at dsdp site 611a odp site 981 and iodp site u1308 during mis 100 records a shift from predominantly archaean aged circum north atlantic ocean continental sources during early glacial ice rafting events to dominantly palaeozoic and proterozoic aged sources during full glacial conditions the distribution of feldspars in pb pb space for full glacial mis 100 more closely resembles that documented for feldspars deposited at the centre of the last glacial ird belt at iodp dsdp site u1308 609 during ambient non heinrich event ice rafting episodes of mis 2 23 8 ka than that documented for mis 5d 106 ka comparison of our early pleistocene and last glacial cycle datasets suggests that mis 100 was characterised by abundant iceberg calving from large ice sheets on multiple continents in the high northern latitudes not just on greenland

sr and nd isotopes as tracers of north atlantic lithic particles paleoclimatic implications sr and nd isotopic compositions have been measured on the lithic fraction of last climatic cycle sediments from the north atlantic 40 n 60 n in order to identify the origins of the particles from the reconstruction of their transport pathways we deduce the mechanisms that explain their distributions the main source regions are the canadian shield mostly the area of baffin bay and western greenland the scandinavian shield the european region british isles and bay of biscay and iceland we observe a significant glacial interglacial contrast characterized by a dominant icelandic input via near bottom transport by north atlantic deep water nadw during the interglacials and a largely continent derived contribution of surface transported ice rafted detritus ird during the glacial period during the last glacial period the heinrich events abrupt massive discharges of ird originated not only from the laurentide ice sheet as heretofore envisioned but also from other sources three other major north atlantic ice sheets fennoscandian british isles and icelandic probably surged simultaneously discharging ice and ird into the north atlantic as opposed to theories implying a unique laurentide origin gwiazda et al 1995 driven by an internal mechanism mcayeal 1993 we confirm that the icelandic and the fennoscandian ice sheets also surged as recently proposed by other authors and we here also distinguish a possible detrital contribution from the british isles ice sheet this pan north atlantic phenomenon thus requires a common regional external forcing

potential links between surging ice sheets circulation changes and the dansgaard oeschger cycles in the irminger sea 60 18 kyr surface and deepwater paleoclimate records in irminger sea core so82 5 59on 31 ow and icelandic sea core ps2644 68on 22ow exhibit large fluctuations in thermohaline circulation thc from 60 to 18 calendar kyr b p with a dominant periodicity of 1460 years from 46 to 22 calendar kyr b p matching the dansgaard oeschger d o cycles in the greenland ice sheet project 2 gisp2 temperature record grootes and tuiver 1997 during interstadials summer sea surface temperatures sstsu in the irminger sea averaged to 8oc and sea surface salinities sss averaged to 36 5 recording a strong irminger current and atlantic thc during stadials sstsu dropped to 2o 4oc in phase with sss drops by 1 2 they reveal major meltwater injections along with the east greenland current which turned off the north atlantic deepwater convection and hence the heat advection to the north in harmony with various ocean circulation and ice models on the basis of the ird composition icebergs came from iceland east greenland and perhaps svalbard and other northern ice sheets however the southward drifting icebergs were initially jammed in the denmark strait reaching the irminger sea only with a lag of 155 195 years we also conclude that the abrupt stadial terminations the d o warming events were tied to iceberg melt via abundant seasonal sea ice and brine water formation in the meltwater covered northwestern north atlantic in the 1 1460 year frequency band benthic 15 80 brine water spikes led the temperature maxima above greenland and in the irminger sea by as little as 95 years thus abundant brine formation which was induced by seasonal freezing of large parts of the northwestern atlantic may have finally entrained a current of warm surface water from the subtropics and thereby triggered the sudden reactivation of the thc in summary the internal dynamics of the east greenland ice sheet may have formed the ultimate pacemaker of d o cycles

the use of the common pb isotope composition of detrital k feldspar grains as a provenance tool and its application to upper carboniferous paleodrainage northern england k feldspar is a common detrital mineral in sandstones it contains little u or th and hence the common pb isotope composition of unaltered k feldspar is that of the source rock pb isotope variations in igneous and metamorphic crustal rocks define broad spatial patterns that make the pb signature of detrital k feldspar grains a useful provenance tool however it is unclear how robust this signal is and to what extent it can be modified by weathering transport burial diagenesis and or exposure to hydrothermal fluids this study reports an evaluation of the technique using two granite arkose pairs from helmsdale northern scotland and shap northwest england where the composition of the detrital grains can be compared to feldspars in the granite from which they were derived the results of this test have informed a pilot study using the pb isotope composition of detrital k feldspar in upper carboniferous fluvial sedimentary rocks of northern england to constrain the sediment routing pb isotope compositions have been measured using an in situ high resolution single grain technique that can distinguish original composition heterogeneity and subsequent alteration within individual sand grains the results for the granite arkose pairs show that the common pb isotope signature of the detrital k feldspar grains matches the in situ feldspars in the source granites the signal is independent of grain size medium grained sand to granules but alteration along a paleo regolith at helmsdale has resulted in perturbations in pb composition and an increase in the errors on the analyses the application of the technique to arkosic carboniferous serpukhovian bashkirian millstone grit group sandstones in the pennine basin reveals two discrete populations of k feldspars that cannot be distinguished petrographically matching of the compositions to potential sourcelands to the north and northeast of the basin suggest a far traveled 500 km source of k feldspar from the archean paleoproterozoic lewisian rocks of northwest scotland or their extension to the north but also an equally important proximal source emanating from the southern uplands belt of scotland in what would have been the lower part of the hinterland drainage basin of the millstone grit

origin of the northern atlantic s heinrich events as first noted by heinrich 1988 glacial age sediments in the eastern part of the northern atlantic contain layers with unusually high ratios of ice rafted lithic fragments to foraminifera shells he estimated that these layers are spaced at intervals of roughly 10 000 years in this paper we present detailed information documenting the existence of the upper five of these layers in odp core 609 from 50 n and 24 w their ages are respectively 15 000 radiocarbon years 20 000 radiocarbon years 27 000 radiocarbon years about 40 000 years and about 50 000 years we also note that the high lithic fragment to foram ratio is the result of a near absence of shells in these layers although we are not of one mind regarding the origin of these layers we lean toward an explanation that the heinrich layers are debris released during the melting of massive influxes of icebergs into the northern atlantic these sudden inputs may be the result of surges along the eastern margin of the laurentide ice sheet

40ar 39ar and pb pb study of individual hornblende and feldspar grains from southeastern baffin island glacial sediments implications for the provenance of the heinrich layers lead isotope analysis of individual feldspar grains and 40ar 39ar analysis of individual hornblende grains from three samples taken from baffin island ice proximal till deposits are consistent with a provenance recording a two stage geologic history including formation of continental crust in the late archean and partial isotopic resetting and or reequilibration during major paleoproterozoic regional metamorphism and magmatism significantly although the populations are small and the two stage archean paleoproterozoic history of their sources are similar the pb isotope compositions of samples from baffin island do not match the results from heinrich layer h2 in core hu87 033 009 from northeast of the hudson strait in the labrador sea specifically in h2 samples from hu87 033 009 the 207pb 204pb is high for a given value of 206pb 204pb additionally the range in 206pb 204pb is greater in hu87 033 009 samples and there are two distinct trajectories of 208pb 204pb the steeper of the 208pb 204pb versus 206pb 204pb trends is not seen in the studied baffin island glacial sediment samples samples of feldspar grains from heinrich layers h1 h2 h4 and h5 from core v28 82 in the eastern north atlantic have a 207pb 204pb versus 206pb 204pb relation similar to that of baffin island till samples but there is also a high 208pb 204pb that we have not captured in this suite of glacial sediment samples our results highlight the strength of a combined radiogenic isotope approach to characterizing the geological history of sediment sources and provide a first step in assessing the variability in isotope character in this region of dynamic glacial events during the late quaternary period

warming of surface waters in the mid latitude north atlantic during heinrich events 1 during the six heinrich events of the last 70 kyr episodic calving from the circum atlantic ice sheets released large numbers of icebergs into the north atlantic these icebergs and associated meltwater flux are hypothesized to have led to a shutdown of atlantic meridional overturning circulation and severe cooling in large parts of the northern hemisphere however due to the limited availability of high resolution records the magnitude of sea surface temperature sst changes related to the impact of heinrich events on the midlatitude north atlantic is poorly constrained here we present a record of u37k based ssts derived from sediments of integrated ocean drilling project site u1313 located at the southern end of the ice rafted debris ird belt in the midlatitude north atlantic 41 n we demonstrate that all six heinrich events are associated with a rapid warming of surface waters by 2 4 c in a few thousand years the presence of ird leaves no doubt about the simultaneous timing and correlation between rapid surface water warming and heinrich events we argue that this warming in the midlatitude north atlantic is related to a northward expansion of the subtropical gyre during heinrich events as a wide range of studies demonstrated that in the central ird belt heinrich events are associated with low ssts these results thus identify an antiphased seesaw pattern in ssts during heinrich events between the midlatitude warm and northern north atlantic cold this highlights the complex response of surface water characteristics in the north atlantic to heinrich events that is poorly reproduced by freshwater hosing experiments and challenges the widely accepted view that within the ird belt of the north atlantic heinrich events coincide with periods of low ssts

tracking the sources of icebergs with lead isotopes the provenance of ice rafted debris in heinrich layer 2 the provenance of ice rafted debris ird deposited in the north atlantic before during and after heinrich event 2 has been determined through measuring the lead isotopic composition of single feldspar grains and multiple grain composites from the larger than 150 m size fraction from cores from the eastern and western north atlantic and from the labrador sea single grain analyses are used to identify the specific continental sources of the ird whereas composite samples are used to assess the relative ird contributions from different sources all single grains from heinrich layer 2 h 2 as well as h 2 composites plot along a correlation line on a 207pb 204pb versus 206pb 204pb diagram characteristic of the churchill province of the canadian shield this is yet another strong piece of evidence that this heinrich event was dominated by a massive iceberg discharge of the laurentide ice sheet lobe located over hudson bay in contrast single grains from the ambient glacial sediment above and below h 2 have multiple sources many of them also lie along the correlation line with h 2 grains but many others have pb signatures consistent with derivation from the grenville province and the appalachian range in north america and possibly from scandinavia and greenland composites from the ambient sediment generally lie well to the right of the h 2 reference line in agreement with the results of the single grain analyses the evidence provided by lead isotopes regarding the dominant role played by the hudson bay lobe of the laurentide ice sheet in the development of the heinrich events lends support to the binge purge model advanced by macayeal 1993a b that invokes trapping of geothermal heat by the base of the icecap and subsequent basal melting as the mechanism that triggered the heinrich events

european contribution of ice rafted sand to heinrich layers h3 and h4 abstract this paper reports the sr nd isotopic composition of the carbonate free sand 63 m 1 mm fraction of heinrich layers 3 and 4 from about 17 cores from the northern atlantic ocean 38 n to 68 n the results indicate that the transporting icebergs were calved from many ice sheets from around the north atlantic at least the fennoscandian icelandic and laurentide our isotopic data show a more distinctly european origin for ice rafted debris ird deposited during the atypical heinrich event h3 than for background ird in contrast heinrich layer h4 a representative of typical heinrich events h1 h2 h4 and h5 presents a clear dominant laurentide signal this result confirms that these typical heinrich events are associated with a massive discharge of icebergs from the laurentide ice sheet however the theory that the laurentide ice sheet set the events into motion alley r b macayeal d r 1993 ice rafted debris associated with binge purge oscillations of the laurentide ice sheet paleoceanography 9 4 503 511 cannot explain the fact that a european signal of an early h4 sample has been observed prior to the laurentide signature of a late h4 sample in the same core along the european margin this observation indicates that the european ice sheets may have surged before the north american ones and suggests that the laurentide ice sheet was not the sole motor behind the heinrich events moreover in the norwegian sea ird has a scandinavian origin in both h3 and h4

isotope stratigraphy sedimentation rates deep circulation and carbonate events in the labrador sea during the last 200 ka high resolution stable isotope and accelerator mass spectrometry 14c measurements in 25 box and piston cores from the labrador sea allow the reconstruction of major paleoceanographical changes during the last 200 ka in this basin and also document the links between the interior of the laurentide ice sheet and the north atlantic ocean during this interval two deep circulation regimes are indicated by contrasting sedimentation rates on the slopes i the modern situation is characterized by a gyre of the north atlantic deep water nadw components into the basin which is responsible for the high energy western boundary undercurrent this regime also prevailed during the former true interglacials and possibly during shorter warm intervals e g bolling allerod ii a more frequent nadw free situation during glacial and interstadial intervals is marked by the presence of a vertically homogeneous water mass in the basin sedimentological records also indicate that carbonate events i e pulses of

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

catastrophic ice shelf breakup as the source of heinrich event icebergs 1 heinrich layers of the glacial north atlantic record abrupt widespread iceberg rafting of detrital carbonate and other lithic material at the extreme cold culminations of bond climate cycles both internal glaciologic and external climate forcings have been proposed here we suggest an explanation for the iceberg release that encompasses external climate forcing on the basis of a new glaciological process recently witnessed along the antarctic peninsula rapid disintegrations of fringing ice shelves induced by climate controlled meltwater infilling of surface crevasses we postulate that peripheral ice shelves formed along the eastern canadian seaboard during extreme cold conditions would be vulnerable to sudden climate driven disintegration during any climate amelioration ice shelf disintegration then would be the source of heinrich event icebergs

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

40ar 39ar ages and 40ar concentrations of fine grained sediment fractions from north atlantic heinrich layers abstract new k ar ages based on 40 ar 39 ar incremental heating of 40 ar 39 ar based k ar age of 970 4 from orphan knoll in the southern labrador sea within the range found in eastern north atlantic heinrich layers thus the k ar data are robust in their indication of a dominant labrador sea ice rafted source to even the finest sediment fraction in the eastern north atlantic during the massive detrital carbonate bearing heinrich events of the last glacial cycle h1 h2 h4 h5 close correspondence of the radiogenic argon concentration 40 ar from the de carbonated a 40 ar 39 ar based k ar age of 433 5 million years for h11 in v28 82 is within the range of published data from background sediments in the eastern north atlantic and is consistent with published results across this interval in the driezack seamounts in contrast the 40 ar 39 ar based k ar age of h11 in the western atlantic core ew9303 jpc37 is 614 5 million years a brick red sample from approximately the interval of h3 of core ew9303 ggc40 yielded a 40 ar 39 ar based k ar age of 567 1 million years comparable to the published range of 523 543 ma from the 2 16 m fractions from that interval on the dreizack seamounts both jpc37 and ggc40 are located in the path of the north atlantic drift the older ages from western samples of h3 and h11 may result from dilution of a hudson strait source or an elevated age from southeastern laurentide sources

iceberg discharges into the north atlantic on millennial time scales during the last glaciation high resolution studies of north atlantic deep sea cores demonstrate that prominent increases in iceberg calving recurred at intervals of 2000 to 3000 years much more frequently than the 7000 to 10 000 year pacing of massive ice discharges associated with heinrich events the calving cycles correlate with warm cold oscillations called dansgaard oeschger events in greenland ice cores each cycle records synchronous discharges of ice from different sources and the cycles are decoupled from sea surface temperatures these findings point to a mechanism operating within the atmosphere that caused rapid oscillations in air temperatures above greenland and in calving from more than one ice sheet

ice sheet action versus reaction distinguishing between heinrich events and dansgaard oeschger cycles in the north atlantic 1 glaciers and ice sheets play an active role in the climate system and the global hydrological cycle the stability of continental ice sheets must be better understood for assessments of future sea level rise and to uncover the causes of millennial scale climate variability that characterized the last glacial period ice rafted debris ird in the midlatitude oceans and subpolar seas tells of widespread calving of icebergs from the northern hemisphere ice sheets during the last glacial period but the climatic implications of this ird are unclear does the sediment record indicate repeated dynamical collapse of the ice sheets with ice sheets actively forcing the climate system alternatively were ice sheet margins simply advancing and retreating in response to climate vacillations on the basis of simulations of iceberg delivery to the ocean during the last glacial cycle we argue that the marine record exemplifies both of these phenomena heinrich events were clearly episodes of internal dynamical instability of the laurentide ice sheet while millennial scale ird is more simply interpreted as a response of the circum atlantic ice sheets to dansgaard oeschger climate cycles ice sheets in different coastal regions respond differently to climate fluctuations but overall iceberg fluxes increase in cold periods peaking within a few centuries of climatic cooling regions with relatively warm wet climates scandinavia western north america and svalbard are the most sensitive to millennial climate variability with rapid response times and large millennial variability in iceberg fluxes

anatomy of labrador sea heinrich layers abstract heinrich layers h layers are distinct decimetre to centimetre thick layers of ice rafted debris ird that were deposited in the north atlantic during the late and middle pleistocene h layers h layers are characterized by high detrital carbonate and low foraminifera contents in the labrador sea h layers reach metre thickness in some proximal core sites near the iceberg source of the hudson strait ice stream and show five distinct depositional facies involving sediment lofting and low density turbidity currents as sediment delivery processes besides ice rafting thick massive ice rafted layers type i h layers occur in the most proximal parts of h layer 3 and older h layers within 300 km distance from the assumed hudson strait ice stream terminus h layers somewhat more distal than type i h layers consist predominantly of stacked thin layers of graded muds containing ird type ii h layers the graded muds that are spiked with ird resulted from the deposition of fine grained lofted sediment that collected dropstones and grains under the iceberg route at greater distance from the hudson strait outlet on the slope and rise south of the strait h layers on the levees of tributary canyons to the northwest atlantic mid ocean channel namoc consist of alternations of thin mud turbidites with intercalated laminae of ird type iii h layers on the levees of namoc type iv h layers consist of layers of ird alternating with fewer fine grained spill over turbidites because the spill over frequency from the deep channel was less than that from the less deep canyons on the slope type v is made up of bioturbated hemipelagic muds with coarser ird and occurs in regions between canyons not reached by spill over turbidity currents and in distal regions of the open ocean or on seamounts transport of significant portions of the sediment in h layers by suspended sediment columns lofted from sand carrying fresh water turbidity currents type ii and by low density turbidity currents types iii and iv explains the high percentage of detrital carbonate in the fine 80 of the total detrital carbonate of the bulk sediment which cannot be derived from icebergs alone it also explains the low magnetic susceptibility and low grey levels on the colour scale compared to h layers in the north atlantic east of greenland the anomalously great thickness of individual h layers on the slope and rise off the hudson strait as documented in isopach maps reflects the combined effect of the various processes involved in their deposition four hypotheses for the origin of h events are discussed i the binge purge model ii the subglacial outburst flow model iii the external forcing model and iv the catastrophic ice shelf break up model

onset of hudson strait heinrich events in the eastern north atlantic at the end of the middle pleistocene transition 640 ka 1 heinrich events are well documented for the last glaciation but little is known about their occurrence in older glacial periods of the pleistocene here we report scanning xrf and bulk carbonate 18o results from integrated ocean drilling program site u1308 reoccupation of deep sea drilling project site 609 that are used to develop proxy records of ice rafted detritus ird for the last 1 4 ma ca sr is used as an indicator of ird layers that are rich in detrital carbonate i e heinrich layers whereas si sr reflects layers that are poor in biogenic carbonate and relatively rich in detrital silicate minerals a pronounced change occurred in the composition and frequency of ird at 640 ka during marine isotope stage mis 16 coinciding with the end of the middle pleistocene transition at this time hudson strait heinrich layers suddenly appeared in the sedimentary record of site u1308 and the dominant period of the si sr proxy shifted from 41 ka prior to 640 ka to 100 ka afterward the onset of heinrich layers during mis 16 represents either the initiation of surging of the laurentide ice sheet lis off hudson strait or the first time icebergs produced by this process survived the transport to site u1308 we speculate that ice volume i e thickness and duration surpassed a critical threshold during mis 16 and activated the dynamical processes responsible for lis instability in the region of hudson strait we also observe a strong coupling between ird proxies and benthic 13c variation at site u1308 throughout the pleistocene supporting a link between iceberg discharge and weakening of thermohaline circulation in the north atlantic

provinciality of ice rafting in the north atlantic application of 40ar 39ar dating of individual ice rafted hornblende grains the history of the northern hemisphere ice sheets is an important aspect of the paleoclimate system the layered record of ice rafted detritus ird and other climate indicators preserved in deep sea sediment cores provides the potential to unravel the sequences of events surrounding important intervals during the last glacial cycle the work presented here is a first step in the goal to characterize the composition of glacially derived detritus in proximal locations near the laurentide greenland and scandinavian ice sheets the intent of this paper is to highlight the potential for using provenance studies of ird in marine sediments in particular 40 ar 39 ar ages of hornblende grains in this case to trace the history of ice sheets in the past published work on land and at the glacial marine margins has mapped the margins of important ice sheets including the locations where glacial ice extended to the shelf slope break at these shelf slope break locations large glacial marine fans termed trough mouth fans tmf are known to occur in about 25 locations around the north atlantic nordic arctic oceans by characterizing the sediment sources in these tmfs the major ird components for deposition in deep sea sediment cores are documented although there is substantial overlapin the geological histories of the continental sources around the north atlantic there are also some systematic variations that will allow distinction of different sources realization of the full potential awaits characterizing the sources with multiple tracers as well as sedimentological studies and documenting the geographic pattern of dispersal in marine sediments within small time windows r 2002 published by elsevier science ltd

considerations on the chronological and causal relationships between talus flatirons and palaeoclimatic changes in central and northeastern spain abstract talus flatirons are relatively common landforms in arid and semiarid areas they are frequent in the duero tajo and ebro tertiary basins in spain the generation of a flatiron requires the alternation of periods with a prevalence of accumulation processes followed by periods with a preponderance of erosion processes in the three tertiary basins considered here talus flatirons have been found in numerous places and up to five stages of slope evolution have been recognised in some locations the second youngest stage s 2 has been dated in several places in the ebro and duero basins through 14 c the age of this stage ranges from 2529 52 to 3590 40 14 c ybp the third youngest stage s 3 has yielded dates of 27 862 444 radiocarbon years bp in the ebro basin and 28 550 130 14 c ybp in the duero basin s 4 has been dated in the ebro basin to 35 570 490 14 c ybp the s 2 slope facets correspond to the iron and bronze age cold stages the s 3 and s 4 flatirons may be correlated with the heinrich events h 3 and h 4 these dates may indicate that the accumulation periods on the slopes correspond to cold global events the dates obtained for the stages s 3 and s 4 in the central and northeastern sectors of spain and their good correlation with heinrich events suggest that flatirons could be related to climatic sequences in the upper pleistocene and holocene

hydrological relationship between the north atlantic ocean and the mediterranean sea during the past 15 75 kyr the mediterranean sea hydrology at the time of the heinrich formation in the north atlantic ocean was analyzed by comparing sea surface temperatures ssts and oxygen isotope composition of seawater w changes during the past 75 kyr in two marine cores these were compared to the palynological variations derived in the mediterranean sea core during the last glacial the two oceanic sst records show similar and synchronous patterns with several long term cooling periods ending by abrupt sst increases at the time of the heinrich events cold ssts and low salinity prevailed in the mediterranean sea the freshwater budget was similar to the modern one permitting the presence of a mixed forest on the mediterranean borderlands the post heinrich periods are marked by a freshwater budget decrease limiting oak and fir tree growth in the mediterranean region increase of precipitation or reduction of evaporation is observed before the heinrich episode and is associated with a well developed mixed mediterranean forest

ice rafted debris associated with binge purge oscillations of the laurentide ice sheet the north atlantic sediment record suggests quasi periodic 7000 to 12 000 year period ice rafted debris ird depositions during at least the last glacial period the cause of these heinrich events as they are commonly known is not fully understood however they may point to surges of the ice stream that drained the hudson bay hudson strait region of the laurentide ice sheet we investigate a simple conceptual model of ice stream instability the binge purge model to suggest ways in which the ice stream could have entrained sufficient debris to account for the estimated mass of ird associated with a typical heinrich ird layer in the north atlantic 1 0 0 3 1015 kg we find that freezing of debris laden ice at the bed of the ice stream during the brief 750 years surge phase of the ice stream s hypothesized binge purge cycle can incorporate up to 5 1 1015 kg this amount is sufficient to meet the constraints of the north atlantic sediment record but by no means verifies the binge purge model as the cause of heinrich events

oxygen and carbon isotopes of detrital carbonate in north atlantic heinrich events abstract we report oxygen and carbon isotope results of detrital carbonate grains from heinrich layers at three sites in the north atlantic located along a transect from the labrador sea to the eastern north atlantic oxygen isotopic values of individual detrital carbonate grains from six heinrich layers at all sites average 5 6 1 5 1 n 166 reflecting values of dolomitic limestone derived from source areas in northeastern canada the 18o of bulk carbonate at integrated ocean drilling program iodp site u1308 re occupation of deep sea drilling project dsdp site 609 in the eastern north atlantic records the proportion of detrital to biogenic carbonate and 18o decreases to 5 during heinrich h events 1 2 4 and 5 relative to a background value of 1 to 2 for biogenic carbonate bulk 18o also decreases during h3 and h6 but only attains values of 1 indicating either a greater proportion of biogenic to detrital carbonate or a different source because the 18o of detrital carbonate is 9 lower than foraminifer carbonate any fine grained detrital carbonate not removed from the inner test chambers will lower foraminifer 18o we conclude bulk carbonate 18o is a sensitive proxy for detrital carbonate and may be useful for identifying heinrich layers in cores within and near the margins of the north atlantic ice rafted detritus ird belt

a diagnostic analysis of heinrich glacial surge events the episodes of ice rafted detritus ird accumulation identified in north atlantic sediment cores indicate periods of abnormal iceberg production mainly of the laurentide ice sheet in this paper we develop further a scaling theory of these so called heinrich events leading to a general criterion for basal melting of an ice sheet which we take as a necessary condition for surging and streaming this criterion depends not only on the physical properties of ice sheets and the magnitude of the geothermal and viscous heating but also on the history of changes in ice elevation atmospheric carbon dioxide and sea surface temperature the thermal effects of which at the top of an ice sheet are transmitted with a time delay to the basal boundary by thermal advection we apply this criterion to various past and present ice sheets showing their general vulnerability to surging in greater detail we use estimates of the time variations of ice volume specmap carbon dioxide vostok and north atlantic sea surface temperature site k708 1 to estimate when basal melting conditions prevailed for the laurentide ice sheet over the past 140 000 years thereby making it vulnerable to heinrich type surges a reasonable level of agreement is obtained with the observed record of north atlantic ird activity including the six numbered heinrich events as well as earlier activity of this kind that occurred when the ice volume was lower

drainage evolution of the heihe river in western hexi corridor china derived from sedimentary and magnetostratigraphic results abstract the uplifting of the tibetan plateau has significantly changed the environment in surrounding areas by delivering abundant water and sediment the heihe river draining the qilian shan in the ne tibetan plateau acts as a dominant sediment routing system from the uplifted ne tibetan plateau to the hexi corridor as well as the badain jaran desert reconstructing the evolution of the heihe river could provide evidence for the birth of the badain jaran desert and enhance the understanding of sedimentary basin fill and the relationship between tectonism drainage evolution and environmental changes with this aim two parallel cores dwj and xkjd with depths of 140 and 68 2 m respectively were drilled in the floodplain of the heihe river the facies analysis of the sedimentary sequences from the drilling cores showed that the sedimentary environment changed from the lake system to a delta system and finally to a fluvial system at the depths of 133 3 and 68 m respectively the magnetostratigraphic results revealed ages of approximately 1 75 and 1 12 ma for the dwj and xkjd cores respectively and an age of approximately 1 1 ma for the transition from delta to fluvial environment in both the cores the change of the sedimentary environment at approximately 1 1 ma was caused by the formation of the integrated heihe river the integrated heihe river may have developed via mechanisms such as river capture and river diversion due to the uplifting of the north qilian shan and the longshou shan the present study suggested that the formation of large inland rivers such as the heihe river and the shiyang river in nw china played an important role in the evolution of the deserts and the loess plateau

the origin and flux of icebergs released into the last glacial maximum northern hemisphere oceans the impact of ice sheet topography the precipitation fields of a palaeoatmospheric general circulation model are used to derive estimates of the geographical distribution and flux of icebergs from the laurentide fennoscandinavian and eastern siberian ice sheets at the last glacial maximum lgm the atmospheric model fields from lgm simulations using climap or peltier ice 4g ice orography were studied to test the sensitivity of the predicted flux the estimated northern hemispheric lgm iceberg flux is 3500 4000 km3 yr 1 of which about 60 issued directly into the north atlantic the iceberg flux from the st lawrence area is of similar significance to that issuing from hudson strait in all estimates both the north pacific and the arctic received substantial iceberg fluxes ca 700 km3 yr 1 with relatively minor differences occurring between the two ice sheet reconstructions apparent discrepancies between arctic deep sea core samples of ice rafted debris and our estimates of mean glacial iceberg flux may be ascribed to coastal trapping of bergs the existence of floating ice tongues or a rapid exit of icebergs from the arctic basin into the greenland sea through the fram strait copyright 2001 john wiley sons ltd

the laurentide and innuitian ice sheets during the last glacial maximum the late wisconsinan advance of the laurentide ice sheet started from a middle wisconsinan interstadial minimum 27 30 14 c ka bp when the ice margin approximately followed the boundary of the canadian shield ice extent in the cordillera and in the high arctic at that time was probably similar to present ice advanced to its late wisconsinan stage 2 limit in the northwest south and northeast about 23 24 14 c ka bp and in the southwest and far north about 20 21 14 c ka bp in comparison to some previous reconstructions of ice extent our current reconstruction has substantially more late wisconsinan ice in the high arctic where an innuitian ice sheet is generally acknowledged to have existed in the atlantic provinces where ice is now thought to have extended to the continental shelf edge in most places and on eastern baffin island where ice probably extended to the fiord mouths rather than to the fiord heads around most of the ice margin the late wisconsinan maximum ice extent either exceeded the extent of earlier wisconsinan advances or it was similar to the early wisconsinan advance ice marginal recession prior to 14 14 ck a bp occurred mainly in deep water and along the southern terrestrial fringe however heinrich event 1 probably drew down the entire central ice surface at 14 5 14 c ka bp sufficiently to displace the labrador sector outflow centre 900 km eastward from the coast of hudson bay the onset of substantial ice marginal recession occurred about 14 14 c ka bp in the northwest southwest and south but not until about 10 11 14 c ka bp in the northeast and in the high arctic thus the period of maximum ice extent in north america generally encompasses the interval from b24 21 to 14 14 c ka bp or considerably longer than the duration of the lgm defined as occurring during a period of low global sea level as well as during a time of relative climate stability b18 14 c ka bp the interval of advance of much of the laurentide ice sheet to its maximum extent between b27 14 c ka bp and b24 14 c ka bp coincides with a suggested interval of rapid fall in global sea level to near lgmlevels r 2001 elsevier science ltd all rights reserved

millennial scale ice rafting events and hudson strait heinrich like events during the late pliocene and pleistocene a review abstract various types of abrupt millennial scale climate variability such as dansgaard oeschger and heinrich events characterized the last glacial period over the last decade a number of studies demonstrated that such millennial scale climate variability was not limited to the last glacial but inherent to quaternary climate here we review the occurrence and origin of millennial ice rafting events in the north atlantic during the late pliocene and pleistocene last 3 4 ma with a special focus on north atlantic hudson strait hs heinrich like events we show that heinrich layers 5 4 2 and 1 in marine sediment cores from across the north atlantic all bear the organic geochemical fingerprint of the hudson area using this framework and combining previously published results detailed investigations into the organic and inorganic chemistry of ice rafted debris ird found across the north atlantic demonstrate that prior to mis 16 650 ka ird in the north atlantic did not originate from the hudson area of northern canada the signature of this early ird is distinctly different compared to that of hs heinrich layers rather ice rafting events during the late pliocene and early pleistocene predominantly emanated from the calving of the greenland and fennoscandian ice sheets and possibly minor contributions from local ice streams from the north american and british ice sheets compared to north atlantic hs heinrich events these early pleistocene ird events had a limited impact on surface water characteristics in the north atlantic north atlantic hs heinrich like events first occurred during mis 16 at the same time the dominant frequency in silicate rich ird accumulation shifted from the obliquity 41 ka to a 100 ka frequency across the north atlantic iceberg survivability or a change in iceberg trajectory likely did not control this change in ird regime these results lend further support for the existing hypothesis that an increase in size thickness of the laurentide ice sheet controls the occurrence of north atlantic hs heinrich events favoring an internal dynamic mechanism for their occurrence

evidence for iceberg armadas from east antarctica in the southern ocean during the late miocene and early pliocene abstract sediments from ocean drilling program site 1165 in the indian ocean sector of the southern ocean off prydz bay contain a series of layers that are rich in ice rafted debris ird here we present evidence that ird rich layers at site 1165 at 7 4 8 and 3 5 ma record short lived massive discharges of icebergs from wilkes land and adelie land more than 1500 kilometers to the east of the depositional site this distant source of icebergs is clearly defined by the presence of ird hornblende grains with 40ar 39ar ages of 1200 1100 ma and 1550 1500 ma ages that are not found on the east antarctic continent in locations closer to site 1165 this observation requires enormous amounts of detritus carrying drifting icebergs most likely in the form of large icebergs these events probably reflect destabilization surge and break up of ice streams on the wilkes land and adelie land margins of the east antarctic ice sheet in the vicinity of the low lying aurora and wilkes basins they occurred under warming conditions but each coast seems to have produced ice rafting events independently at different times the data presented here constitute the first evidence of far traveled icebergs from specific source areas around the east antarctic perimeter launch of these icebergs may have happened during quite dramatic events perhaps analogous to heinrich events in the north atlantic

sedimentary evolution of the foreland basin in the ne tibetan plateau and the growth of the qilian shan since 7 ma we studied the magnetostratigraphy and sedimentary facies of a 550 m long drill core from the jiudong basin in the ne tibetan plateau our aims were to reconstruct the late cenozoic sedimentary evolution of this foreland basin and to determine the spatiotemporal pattern of growth of the qilian shan the magnetostratigraphy indicates that the sedimentary sequence was deposited during ca 7 0 ma from ca 6 7 3 0 ma the sediment accumulation rate increased gradually from similar to 30 mm k y to 120 mm k y which was associated with the gradual evolution of sedimentary facies from a shallow lake delta front to braided rivers the progradation of the depositional system from 7 ma to 3 ma probably reflects the growth of the relief of the qilian shan caused by tectonic uplift the occurrence of a continuous braided river environment from 3 ma to the present suggests that the high relief of the qilian shan developed before 3 ma an abrupt decrease of the sedimentation rate to similar to 46 mm k y during 3 0 1 8 ma and the deposition of coarse grained sediments indicates the uplift of the basin center we interpret this to reflect the propagation of the thrust system of the qilian shan into the basin along a southward dipping decollement from ca 3 ma climatic changes may have influenced the sedimentary sequence by introducing long distancetransported thin coarse sand gravel layers which are sandwiched within the sequence and likely were a response to cooling events or climatic transitions the widespread occurrence of deformation within the basin region in the ne tibetan plateau at ca 3 ma indicates that this date marks the basinward growth of the deformation system

patterns of ice rafted detritus in the glacial north atlantic 40 55 n the observation by heinrich 1988 that during the last glacial period much of the input of ice rafted detritus to the north atlantic sediments may have occurred as a succession of catastrophic events rekindled interest on the history of the northern ice sheets over the last glacial period in this paper we present a rapid method to study the distribution of these events both in space and time using whole core low field magnetic susceptibility we report on approximately 20 cores covering the last 150 to 250 kyr well defined patterns of ice rafted detritus appear during periods of large continental ice sheet extent although these are not always associated within their maxima most of the events may be traced across the north atlantic ocean for the six most recent heinrich layers hl two distinct patterns exist hl1 hl2 hl4 hl5 are distributed along the northern boundary of the glacial polar front over most of the north atlantic between 40 and 50 n hl3 is more restricted to the central and eastern part of the northern atlantic the nd sr isotopic composition of the material constituting different heinrich events indicates the different provenance of the two patterns hl3 has a typical scandinavia arctic icelandic young crust signature and the others have a large component of northern quebec and northern west greenland old crust material these isotopic results obtained on core su 9008 from the north american basin are in agreement with the study by jantschik and huon 1992 who used k ar dating of silt and clay size fractions of an eastern basin core me 68 89 these data confirm the large spatial scale of these events and the enormous amount of ice rafted detritus they represent

variability of surface water characteristics and heinrich like events in the pleistocene midlatitude north atlantic ocean biomarker and xrd records from iodp site u1313 mis 16 9 1 a reconstruction of milankovitch to millennial scale variability of sea surface temperature sst and sea surface productivity in the pleistocene midlatitude north atlantic ocean marine isotope stage mis 16 9 and its relationship to ice sheet instability was carried out on sediments from integrated ocean drilling program iodp site u1313 this reconstruction is based on alkenone and n alkane concentrations u37k index total organic carbon toc and carbonate contents x ray diffraction data magnetic susceptibility and accumulation rates increased input of ice rafted debris occurred during mis 16 12 and 10 characterized by high concentrations of dolomite quartz and feldspars and elevated accumulation rates of terrigenous matter minimum input values of terrigenous matter on the other hand were determined for mis 13 and 11 peak values of dolomite coinciding with quartz plagioclase and kalifeldspar peaks and maxima in long chain n alkanes indicative for land plants are interpreted as heinrich like events related to sudden instability of the laurentide ice sheet during early and late deglacial phases of the glacials the coincidence of increased toc values with elevated absolute concentrations of alkenones suggests increased glacial productivity probably due to a more southern position of the polar front alkenone based sst reached absolute maxima of about 19 c during mis 11 3 and absolute minima of 10 c during mis 12 and 10 within mis 11 prominent cooling events mis 11 22 and 11 24 occurred the absolute sst minima recorded directly before and after the glacial maxima mis 10 2 and 12 2 are related to heinrich like event meltwater pulses as supported by the coincidence of sst minima and maxima in c37 4 alkenones and dolomite these sudden meltwater pulses especially during terminations iv and v probably caused a collapse of phytoplankton productivity as indicated by the distinct drop in alkenone concentrations ice sheet disintegration and subsequent surges and outbursts of icebergs and meltwater discharge may have been triggered by increased insolation in the northern high latitudes

a continuum mixture model of ice stream thermomechanics in the laurentide ice sheet 2 application to the hudson strait ice stream episodic exportations of ice rafted debris to the north atlantic in the late pleistocene suggest quasiperiodic ice streaming or surging activity on the northeastern margin of the laurentide ice sheet much of this efflux of ice may originate from an ice stream issuing from hudson strait and tapping into core regions of the laurentide in hudson bay labrador and the foxe basin applying the continuum mixture theory outlined by marshall and clarke this issue we model the thermomechanical evolution of the hudson strait ice stream in a three dimensional finite difference model of the laurentide ice sheet our simulations focus on internal dynamics of the ice stream under thermal regulation of basal flow we find surge cycles of stream activity interspersed with quiescent periods where the ice stream is frozen to the bed modeled surge durations vary from 105 to 3260 years while surge periodicities range from 585 to 22 410 years with pervasively warm or cold internal temperature distributions in the ice ice streams can also establish modes of permanent activity or inactivity under thermal regulation our most vigorous ice streams produce peak values of approximately 0 03 sv of freshwater flux to the north atlantic from basal meltwater and iceberg production associated ice stream velocities in this maximum case approach 6700 m yr 1 the total ice volume mobilized in a single surge event is equivalent to a global sea level rise of 0 04m in the most tranquil surge and almost 0 6m in the most extreme case these velocities and sea level impacts are an order of magnitude less than those predicted by macayeal 1993a b and only our most exuberant streams approach the iceberg flux estimates of dowdeswell et al 1995 we propose that the sediment load of icebergs emanating from hudson strait in a surge event may exceed expectations from contemporary icebergs

pb isotope measurements of sanidine monitor standards implications for provenance analysis and tephrochronology abstract samples of fish canyon and cobb mountain sanidine monitor standards irradiated and fused with 7 w of co2 laser power for 40 ar 39 ar geochronology analyses give pb isotope composition and pb concentrations that are indistinguishable from unprocessed aliquots of the same monitors the absence of measurable change in pb isotope composition indicates the feasibility of measuring the pb isotope composition of feldspar grains that have been previously analyzed for their 40 ar 39 ar ages therefore it should be possible to fingerprint datable volcanic ash layers for purposes of correlating noncontiguous layers with similar ages application of this combined technique to detrital feldspar grains most detrital grains would not be sanidine so further tests are necessary should also improve the sensitivity of provenance analysis

magnetic susceptibility patterns and provenance of ice rafted material at feni drift rockall trough implications for the history of the british irish ice sheet abstract piston core enam 97 09 from feni drift atlantic margin west of ireland provides a record of ird input variability and provenance during the last 250 000 yrs the ird patterns can be related to late devensian detrital carbonate and meta sedimentary rock fragments represent two major ird types which were dominantly derived from irish inland ice and the irish sea ice stream respectively additional evidence for the latter source is based on heavy mineral data input from other circum north atlantic ice sheets notably the laurentide ice sheet is minor compared to british irish ird at least during periods of maximum expansion of the biis this is consistent with the marginal location of the core site with reference to the main north atlantic ird belt on the other hand laurentide ird presumably accounts for minor ird peaks at times of reduced ice volume in the british isles the main magnetic susceptibility peak during late isotope stage 2 termination i can be related to ird input from the laurentide ice sheet following early decay of the biis a similar pattern is observed during oxygen isotope stage 6 these observations are consistent with recent provenance studies indicating that only laurentide ird carries a strong magnetic susceptibility signal

sources of labrador sea sediments since the last glacial maximum inferred from nd pb isotopes pb isotopes have been measured in the clay size fraction of late glacial and holocene deep sea sediments recovered from two labrador sea piston cores that have been previously analyzed for nd isotopes the newly acquired pb isotopic data allow us to better constrain the different source areas that supplied clay size material during the last deglaciation until 8 6 kyr calendar ages nd pb data can be modeled mainly as a mixture between a precambrian crust and lower paleozoic material originating from the denudation of the pan african orogen the old material originates mainly from the archean lower proterozoic or both terranes of greenland and also probably corresponding terranes of labrador although minor input of other precambrian material is recorded in some detrital carbonate rich deglacial samples from orphan knoll the phanerozoic crustal end member consists of sediment material mainly originating from northwestern europe this source area is found to be the only significant source of young crustal material in early holocene sediments from the greenland rise no significant input from the mid atlantic volcanism is apparent this study puts further constraints on the deep circulation pattern during the last deglaciation it is concluded that at that time european phanerozoic material was carried from the norwegian sea through the wyville thompson ridge into the iceland basin by the north east atlantic deep water no evidence for an overflow is found either south of the iceland iceland scotland ridge or through the denmark strait

a 200 ka geomagnetic chronostratigraphy for the labrador sea indirect correlation of the sediment record to specmap abstract a chronostratigraphic framework for the last 200 ka based on relative variations of geomagnetic paleointensity has been derived for the labrador sea where 18o stratigraphies are compromised by lack of benthic foraminifera and environmental perturbations of the planktic record relative paleointensity records derived from three labrador sea piston cores are correlated to a recently developed 200 ka paleointensity composite sint 200 y guyodo j p valet earth planet sci lett 143 1996 23 36 the chronology for sint 200 was derived by correlating oxygen isotope stratigraphies from individual paleointensity records to the specmap chronostratigraphy the correlation of sint 200 to labrador sea paleointensity records allows the specmap chronology to be imported into the labrador sea although comparisons of the chronostratigraphy based on paleointensity correlations with those derived from ams 14c dates and 18o data indicate significant discrepancies support for the new chronostratigraphic framework is provided by 1 consistent correlation of labrador sea detrital layers to north atlantic heinrich layers 2 general consistency between planktic 18o values and the specmap reference curve and 3 feasible prediction for the age of ash layer 2 as the new chronostratigraphy is tied to specmap labrador sea sediment cores can now be directly compared with any specmap dated record

late quaternary ice rafting events in the sw pacific ocean off eastern new zealand new records of ice rafted debris ird in eight cores from campbell plateau reveal periodic incursions of icebergs over the last 200 kyr observations of modern iceberg trajectories and provenance studies show the icebergs originated mainly in antarctica and traveled east and north to the plateau margins via the antarctic circumpolar current there local currents and winds dispersed icebergs across the plateau and adjacent regions as far north as chatham rise at 43 s the shallow rise crest along rise circulation and strong thermal gradient associated with the nearby subtropical front inhibited further northward transport although ird concentrations are very low 26 grains g downcore profiles and a 18o chronology reveal distinct peaks during i the transition from marine isotope stages mis 7 to 6 ii late interglacial mis 5 and iii glacial mis 2 in addition smaller peaks occur in mis 4 and 3 this generalised pattern is also evident off antarctica indicating that periodic destabilisation of ice shelves was the main driving force behind the campbell plateau ird events however there are significant differences between antarctic and new zealand ird records that probably reflect palaeoceanographic influences on iceberg dispersal the mis 5 ird event is better represented off antarctica confirming that last interglacial changes in sea level and temperatures encouraged iceberg discharge and proximal melting conversely the mis 2 event is relatively better shown on campbell plateau implying that last glacial temperatures inhibited proximal iceberg melting and together with a more vigorous oceanic circulation allowed more icebergs to reach the distant plateau a tentative correlation with records from the southeast atlantic and southwest indian oceans suggests that the ird events were felt over much of the southern ocean for at least the last 70 kyr

modelling iceberg trajectories sedimentation rates and meltwater input to the ocean from the eurasian ice sheet at the last glacial maximum we model iceberg flow paths from the eurasian ice sheet and the associated meltwater production and sedimentation rates within the norwegian greenland sea during the last glaciation results from a numerical ice sheet model an atmospheric general circulation model and an ocean general circulation model are collated and used to provide iceberg production rate wind field and surface current forcings to an iceberg trajectory model the iceberg model then determines how icebergs issuing from the eurasian ice sheet travel across the ocean and eventually melt in addition the release of iceberg sediments is also predicted the results show that iceberg trajectories are complex and that common features of iceberg movement are clustering in zones of convergence and exit into the north atlantic through the iceland faeroes channel eurasian icebergs do not penetrate into the interior of the arctic ocean the gathering of icebergs produces a complex meltwater pattern that does not follow the conceptual idea of decreasing meltwater production with distance from the ice sheet margin sedimentation results are compared with the meltwater configuration and are found to be a poor indicator of past zones of iceberg melt with zones of sedimentation extending significantly less far

thermohaline instability in the north atlantic during meltwater events stable isotope and ice rafted detritus records from core so75 26kl portuguese margin a benthic isotope record has been measured for core so75 26kl from the upper portuguese margin 1099 m water depth to monitor the response of thermohaline overturn in the north atlantic during heinrich events evaluating benthic 18o in ts diagrams in conjunction with equilibrium c fractionation implies that advection of mediterranean outflow water mow to the upper portuguese margin was significantly reduced during the last glacial 15 compared to 30 today the benthic isotope record along core so75 26kl therefore primarily monitors variability of glacial north atlantic conveyor circulation the 14c accelerator mass spectrometry ages of 13 54 07 and 20 46 12 ka for two ice rafted detritus ird layers in the upper core section and an interpolated age of 36 1 ka for a third ird layer deeper in the core are in the range of published 14c ages for heinrich events h1 h2 and h4 marked depletion of benthic 13c by 0 7 1 1 during the heinrich events suggests reduced thermohaline overturn in the north atlantic during these events close similarity between meltwater patterns inferred from planktonic 18o at site 609 and ventilation patterns inferred from benthic 13c in core so75 26kl implies coupling between thermohaline overturn and surface forcing as is also suggested by ocean circulation models benthic 13c starts to decrease 1 5 2 5 kyr before heinrich events hl and h4 fully increased values are reached 1 5 3 kyr after the events indicating a successive slowdown of thermohaline circulation well before the events and resumption of the conveyor s full strength well after the events benthic 13c changes in the course of the heinrich events show subtle maxima and minima suggesting oscillatory behavior of thermohaline circulation a distinct feature of thermohaline instability in numerical models inferrred gradual spin up of thermohaline circulation after hl and h4 is in contrast to abrupt wanning in the north atlantic region that is indicated by sudden increases in greenland ice core 18o and in marine faunal records from the northern north atlantic from this we infer that thermohaline circulation can explain only in part the rapid climatic oscillations seen in glacial sections of the greenland ice core record

re advance of the fennoscandian ice sheet during heinrich event 1 abstract five sediment cores bathymetric and 2d seismic reflection data from vestfjorden northern norway were studied in order to constrain the timing and extent of a major ice stream draining the northwestern fennoscandian ice sheet during the last deglaciation 15 10 14 c ka b p the bathymetric and seismic data indicate that vestfjorden traenadjupet was covered by grounded ice reaching the shelf break during the last glacial maximum lgm after the lgm break up and recession towards outer inner vestfjorden at 13 6 14 c ka b p the ice re advanced towards the outer part of vestfjorden prior to 13 1 14 c ka b p this is inferred from glaciomarine sediments overlain by diamicton at the base of the sediment cores interpreted as basal tills and associated contemporaneous build up of grounding zone wedges in the central vestfjorden this sequence of events is coherent with the late weichselian ice sheet dynamics both along the western norwegian continental shelf and in the barents sea possibly indicating a direct response to meltwater injection and accompanied cooling during disruption of the thermohaline circulation associated with heinrich event 1 enhanced supply of suspension load resulted in deposition of laminated sediments at 13 14 c ka b p reflecting the retreat of the ice stream towards the inner fjord in response to global climate amelioration and increased atlantic water inflow two additional ice re advances towards the norwegian coastline are inferred from distinct input of ice rafted debris ird with a typical mineralogical fingerprint derived from neoproterozoic early silurian calcite and dolomite marbles in the ice sheet catchment area thus the re advances may be concurrent with the older dryas 12 2 14 c ka b p skarpnes and younger dryas 10 7 14 c ka b p tromso lyngen cooling events

sea surface temperature variability in the north atlantic during the last two glacial interglacial cycles comparison of faunal oxygen isotopic and mg ca derived records climate variability in the northeast atlantic was investigated on glacial interglacial and millennial time scales during the last 200 000 years using sea surface temperature sst records derived from planktonic foraminiferal diversities and from mg ca measurements on globigerina bulloides paleoceanographical interpretations are supported by species composition analyses benthic and planktonic isotopic data as well as records of iceberg rafted debris ird differences of climate development are recognized for both interglacial and glacial periods temperature estimates indicate slightly warmer conditions up to 2 c during marine oxygen isotope stage mis 5e than during the holocene in contrast to the last glaciation when the sst minimum coincided with a minimum in solar insolation immediately before termination i during the penultimate glaciation a long sst minimum occurred at times of intermediate solar insolation well preceding the onset of termination ii this discrepancy between two glacial terminations may be explained by an inherently different orbital configuration characteristic for each glacial interval despite these differences between the two glacial trends the superimposed shorter lived climatic events reveal the same order of principal steps implying their common causal nature a direct comparison of faunal ssts with those retrieved from mg ca analysis shows that mg ca derived temperatures follow the general glacial interglacial trend however the latter appear to be largely overestimated supported by 18o data in g bulloides which show little response to millennial scale variability there seems to be a need for species dependent calibration experiments that also consider the different oceanographic settings this particular species can live in

late quaternary stage 2 and 3 meltwater and heinrich events northwest labrador sea two major meltwater events are documented in cores from the nw labrador sea one occurred ca 20 000 14c yr b p in association with deposition of a major detrital carbonate unit both prior to and after this event 18o values of near surface planktonic foraminifera were 4 5 indicating fully enriched glacial values a younger event ca 14 000 14 c yr b p is characterized by a dramatic change in 18o from 4 5 to 2 0 and coincided with the retreat of ice from the outer se baffin shelf possibly into hudson strait these meltwater events coincide with heinrich h layers 1 and 2 from north atlantic sediments the 14 000 14c yr b p meltwater event indicates that the eastern margin of the laurentide ice sheet also underwent rapid retreat at approximately the same time as other ice sheet margins around the ne north atlantic a third major detrital carbonate event at the base of hu87 033 009 possibly correlative with heinrich layer 3 occurred ca 33 960 675 14 c yr b p however this is older than the accepted date for h 3 of 27 000 14c yr b p and may be h 4

the deglaciation of the northern hemisphere a global perspective orbitally induced increase in northern summer insolation after growth of a large ice sheet triggered deglaciation and associated global warming ice albedo sealevel and greenhouse gas feedbacks together with tropical warming from weakening winds in response to polar amplification of warming caused regionalto global near synchronization of deglaciation effects were larger at orbital rather than millennial frequencies because ice sheets and carbon dioxide vary slowly ice sheet linked changes in freshwater delivery to the north atlantic and possibly free oscillations in the climate system forced millennial climate oscillations associated with changes in north atlantic deep water nadw flow the north atlantic typically operates in one of three modes modern glacial and heinrich deglaciation occurred from a glacial mode ocean that in comparison to modern had shallower depth of penetration of nadw formed further south causing strong northern cooling and the widespread cold dry and windy conditions associated with the glacial maximum and the cold phases of the millennial dansgaard oeschger oscillations the glacial mode was punctuated by meltwater forced heinrich conditions that caused only small additional cooling at high northern latitudes but greatly reduced the formation of nadw and triggered an oceanic seesaw that warmed some high latitude southern regions centered in the south atlantic

a methodology for determining orientations in unscribed core abstract this paper describes a simple methodology for determining paleocurrent directions or fracture orientations from core that has traditionally been considered unoriented provided the core does not intersect bedding at a 90 angle and that the attitude of bedding strike and dip and the drill hole azimuth and inclination are known paleocurrent directions and fracture orientations can be determined in present day geographic coordinates without the need for scribed core this technique supplements dip meter and borehole image logs and allows the use of structures that are too small to be imaged geophysically

sources of fine sized organic matter in north atlantic heinrich layers δ13c and δ15n tracers organic carbon oc and total nitrogen tn concentrations and stable isotope ratios d13c d15n of fine 50 m size fractions of deep sea sediments from the central north atlantic were employed to identify changes in sources of organic matter over the past 50 ka bp ambient glacial sediments are characterised by values that reflect mixtures of marine and terrestrial inputs averages 1s oc tn 7 6 0 8 d13c 22 8 1 0 d15n 5 5 0 6 d13c oc and tn concentrations shift to higher values during the holocene indicating a gradual decrease of fine terrigenous supply to the north atlantic the unchanged d15n record between last glacial and holocene stages indicates that the central north atlantic region remained oligotrophic at least during the past 50 ka bp but additional studies are required to support this result in terms of nitrogen oceanic budget during the phases of enhanced ice rafted detrital supply corresponding to prominent heinrich events hl1 hl2 hl4 and hl5 fine sized sedimentary organic matter has lower oc and tn concentrations contrasting sharply with those of ambient glacial sediments lower d13c down to 28 and d15n down to 1 6 values and high oc tn ratios up to 14 7 1 1 are found for hl1 hl2 and with lesser extent for hl4 these values reflect enhanced detrital supply originating from poorly differentiated soil horizons that characterise periglacial climate conditions and from organic matter bearing rock sources of the underlying geological basement during hl5 only the d13c offset records the input of fine size ice rafted organic matter gradually changing soil development conditions during the time interval covering hl5 to hl1 marine isotope stages 5 to 2 as well as varying erosion levels have been hypothesized on the basis of constant d13c increasing oc tn and decreasing d15n values

morphology and evolution of bars in a wandering gravel bed river lower fraser river british columbia canada a hierarchical typology for the channels and bars within aggradational wandering gravel bed rivers is developed from an examination of a 50 km reach of lower fraser river british columbia canada unit bars built by stacking of gravelly bedload sheets are the key dynamic element of the sediment transfer system linking sediment transport during individual freshets to the creation development and remoulding of compound bar platforms that have either a lateral or medial style primary and secondary unit bars are identified respectively as those that deliver sediment to compound bars from the principal channel and those that redistribute sediment across the compound bar via seasonal anabranches and smaller channels the record of bar accretion evident in ground penetrating radar sequences is consistent with the long term development of bar complexes derived from historical aerial photographs for two compound bars inter annual changes associated with individual sediment transport episodes are measured using detailed topographic surveys and longer term changes are quantified using sediment budgets derived for individual bars from periodic channel surveys annual sediment turnover on the bars is comparable with the bed material transfer rate along the channel indicating that relatively little bed material bypasses the bars bar construction and change are mainly accomplished by lateral accretion since the river has limited capacity to raise bed load onto higher surfaces styles of accretion and erosion and therefore the major bar form morphologies on fraser river are familiar and consistent with those in gravelly braided channels but the wandering style does exhibit some distinctive features for example 65 year histories reveal the potential for long sequences of uninterrupted accretion in relatively stable wandering rivers that are unlikely in braided rivers

thermal infrared spectral character of hawaiian basaltic glasses multispectral thermal infrared remote sensing of hawaiian basalts younger than a few hundred years old can be used to discriminate basalt lava flows that are difficult to map with visible and near infrared images laboratory thermal infrared reflectance spectra of the exposed surfaces of mauna loa and kilauea basalts show systematic changes with age fresh glass collected from active lava flows shows a broad reflectance feature between 8 and 12 m indicative of a strong degree of disorder after a few weeks of exposure to the environment the spectra of the top surfaces of samples begin to exhibit spectral features suggestive of ordering into silicate chainlike and sheetlike units with progressive aging features of apparent sheetlike structures become the preferred mode as soon as 3 years after emplacement a silica rich coating can begin to develop on the surfaces adding another spectral feature to the laboratory reflectance measurements the thermal infrared multispectral scanner tims with six channels between 8 and 12 m has been used to take advantage of these spectral differences in basalts to map flows in hawaii remote sensing instruments with increased spectral resolution in the thermal infrared would aid in further discrimination of basalts with more subtle differences in age quenching and weathering histories

abrupt changes heinrich events in late quaternary north atlantic marine environments a history and review of data and concepts in the last several years evidence has mounted that a series of abrupt changes in sediment delivery and palaeoceanography effected the north atlantic they are recorded in a variety of proxy records but most dramatically in changes of grain size and mineralogy associated with postulated iceberg rafting events specifically heinrich h events this review paper examines the evidence for such evidence prior to the development of the heinrich event concept 1988 1992 and then examines the explosion of data and correlations that stemmed from the acceptance and use of this paradigm specific attention is focused on ideas pertaining to the cause s of glaciological mechanisms sediment delivery and sediment source and major gaps in our understanding of the underlying glaciological glacial geological and glacial marine processes are stressed abrupt changes across boundaries associated with h 2 in the labrador sea are illustrated from core hu87033 009lcf the case is made that correlations between records need to be more rigorous especially within the range of radiocarbon dating but it appears that abrupt changes in the delivery of iceberg rafted debris which characterize these intervals are multisourced it is unclear how global climate can simultaneously influence the abrupt behaviour of ice margins in different parts of the world for marine based margins changes in relative sea level are one possibility a qualitative model is presented for triggering h events which includes global changes in mass balance changes in relative sea level at ice margins due to glacial isostasy and changes in the basal thermal regime of tidewater ice streams 1998 john wiley sons ltd

provenance of icebergs during heinrich event 3 and the contrast to their sources during other heinrich episodes the sources of ice rafted debris ird from heinrich layer 3 h 3 in the north atlantic were studied by means of pb isotopic analyses of single and multiple grain samples of ice rafted feldspars from core v28 82 49 27 n 22 16 w pb ratios of h 3 grains point to multiple continental sources and are similar to the isotopic composition of the background sediment this similarity as well as previous observations about the age and the sr and nd isotopic composition of the fine fraction of h 3 in cores of the same area are all consistent with the hypothesis that in the eastern north atlantic along the latitude band of maximum ird deposition 40 50 n h 3 is a product of foraminifera dissolution and or low productivity in contrast analyses of composite samples of h 1 h 2 h 4 and h 5 plot in 206pb 204pb versus 206pb 204pb coordinates along the churchill province line supporting the premise that the hudson strait was the major pathway of iceberg drainage during these four events since an advance of the laurentide ice sheet during h 3 is demonstrated by increases in detrital carbonate at the level of h 3 in cores from the western north atlantic the results obtained here suggest that if indeed that advance took place the iceberg discharge melted mostly in the western basin

signatures and significance of aeolian fluvial bacterial and diagenetic magnetic mineral fractions in late quaternary marine sediments off gambia nw africa 1 two gravity cores retrieved off nw africa at the border of arid and subtropical environments geob 13602 1 and geob 13601 4 were analyzed to extract records of late quaternary climate change and sediment export we apply end member em unmixing to 350 acquisition curves of isothermal remanent magnetization irm our approach enables to discriminate rock magnetic signatures of aeolian and fluvial material to determine biomineralization and reductive diagenesis based on the occurrence of pedogenically formed magnetic minerals in the fluvial and aeolian ems we can infer that goethite formed in favor to hematite in more humid climate zones the diagenetic em dominates in the lower parts of the cores and within a thin near surface layer probably representing the modern fe2 fe3 redox boundary up to 60 of the irm signal is allocated to a biogenic em underlining the importance of bacterial magnetite even in siliciclastic sediments magnetosomes are found well preserved over most of the record indicating suboxic conditions temporal variations of the aeolian and fluvial ems appear to faithfully reproduce and support trends of dry and humid conditions on the continent the proportion of aeolian to fluvial material was dramatically higher during heinrich stadials especially during heinrich stadial 1 dust export from the arabian asian corridor appears to vary contemporaneous to increased dust fluxes at the continental margin of nw africa emphasizing that meltwater discharge in the north atlantic had an enormous impact on atmospheric dynamics

iceberg production debris rafting and the extent and thickness of heinrich layers h 1 h 2 in north atlantic sediments the pattern of heinrich layer distribution for the last two events h 1 14 5 and h 2 21 1 ka mapped from magnetic susceptibility analysis of more than 50 north atlantic ocean cores provides the most detailed information to date on their extent and thickness an integrated spatial average thickness for the layers is 10 15 cm and there is a strong distance decay eastward the pattern of deposition over the north atlantic is similar for events h 1 and h 2 indicating that icebergs followed similar drift tracks rates of iceberg production and sediment flux from the hudson strait drainage basin of the north american laurentide ice sheet the major iceberg source for the events were calculated by using a mass balance approach this provides an envelope of sedimentation rates and the prediction that it would take between 50 and 1250 yr of iceberg sediment delivery to accumulate a heinrich layer averaging 10 cm thick over the north atlantic depending on the model assumptions used the most likely duration of heinrich events is 250 1250 yr

millennial scale iceberg discharges in the irminger basin during the last glacial period relationship with the heinrich events and environmental settings high resolution records of coarse lithic content and oxygen isotope have been obtained in a piston core from the irminger basin the last glacial period is characterized by numerous periods of increased iceberg discharges originating partly from iceland and corresponding to millennial scale instabilities of the coastal ice sheets and ice shelves in the nordic area a comparison with midlatitude sediment cores shows that ice rafted material corresponding to the heinrich events was deposited synchronously from 40 to 60 n there are thus two oscillating systems every 5 10 kyr massive iceberg armadas are released from large continental ice caps whereas more frequent instabilities of the coastal ice sheets in the high latitude regions occur every 1 2 3 8 kyr at the time of the heinrich events the synchroneity of the response from all the northern hemisphere ice sheets attests the existence of strong interactions between the two systems

terrigenous fluxes at the celtic margin during the last glacial cycle the sedimentary sections of three cores from the celtic margin provide high resolution records of the terrigenous fluxes during the last glacial cycle a total of 21 14 c ams dates allow us to define age models with a resolution better than 100 yr during critical periods such as heinrich events 1 and 2 maximum sedimentary fluxes occurred at the meriadzek terrace site during the last glacial maximum lgm detailed x ray imagery of core md95 2002 from the meriadzek terrace shows no sedimentary structures suggestive of either deposition from high density turbidity currents or significant erosion two paroxysmal terrigenous flux episodes have been identified the first occurred after the deposition of heinrich event 2 canadian ice rafted debris ird and includes ird from european sources we suggest that the second represents an episode of deposition from turbid plumes which precedes ird deposition associated with heinrich event 1 at the end of marine isotopic stage 2 mis 2 and the beginning of mis 1 the highest fluxes are recorded on the whittard ridge where they correspond to deposition from turbidity current overflows canadian icebergs have rafted debris at the celtic margin during heinrich events 1 2 4 and 5 the highresolution records of heinrich events 1 and 2 show that in both cases the arrival of the canadian icebergs was preceded by a european ice rafting precursor event which took place about 1 1 5 kyr before two rafting episodes of european ird also occurred immediately after heinrich event 2 and just before heinrich event 1 the terrigenous fluxes recorded in core md95 2002 during the lgm are the highest reported at hemipelagic sites from the northwestern european margin the magnitude of the canadian ird fluxes at meriadzek terrace is similar to those from oceanic sites 2002 elsevier science b v all rights reserved

evidence for an additional heinrich event between h5 and h6 in the labrador sea 1 an additional heinrich ice rafting event is identified between heinrich events 5 and 6 in eight cores from the labrador sea and the northwest atlantic ocean it is characterized by sediment rich in detrital carbonate 40 caco3 with high concentration of floating dropstones high coarse fraction 150 m content and has a sharp contact with the underlying but grades into the overlying hemipelagic sediment it also shows lighter 18onpl values indicating freshening due to iceberg rafting and or meltwater discharge this event is correlated with dansgaard oeschger event 14 and interpreted as an additional heinrich event h5a the thickness of h5a in the labrador sea reaches up to 220 cm this additional heinrich event has also been reported in cores ps2644 and so82 5 from the northern north atlantic with the recognition of h5a the temporal spacing between heinrich events 1 to 6 becomes more uniform 7 ka

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

north atlantic deep water circulation collapse during heinrich events paleoceanographic proxy data and ocean general circulation models have been combined to investigate the response of the north atlantic ocean to heinrich type meltwater episodes because of the uncertainties over the origin of the heinrich events three different scenarios have been modeled 1 a heinrich event caused by a dramatic increase in the iceberg discharge from the laurentide ice sheet or labrador ice shelf 2 a heinrich event driven by enhanced iceberg discharge from the barents shelf which was transported into the northern north atlantic and 3 a meltwater episode that was confined to the nordic seas the ocean regional and global circulation models cannot distinguish between the laurentide and barents shelf heinrich event scenarios because both led to a complete cessation of the deep water thermohaline conveyor belt the meltwater episode confined to the nordic seas only weakened the overturning and may represent circulation changes that may occur during the dansgaard oeschger events

hydrological impact of heinrich events in the subtropical northeast atlantic reconstructing the impact of heinrich events outside the main belt of ice rafting is crucial to understanding the underlying causes of these abrupt climatic events a high resolution study of a marine sediment core from the iberian margin demonstrates that this midlatitude area was strongly affected both by cooling and advection of low salinity arctic water masses during the last three heinrich events these paleoclimatic time series reveal the internal complexity of each of the last three heinrich events and illustrate the value of parallel studies of the organic and inorganic fractions of the sediments

precambrian sequence stratigraphy abstract sequence stratigraphy studies the change in depositional trends in response to the interplay of accommodation and sediment supply from the scale of individual depositional systems to entire sedimentary basin fills as accommodation is controlled by allogenic mechanisms that operate at basinal to global scales the change in depositional trends is commonly synchronized among all environments established within a basin thus providing the basis for the definition of systems tracts and the development of models of facies predictability all classical sequence stratigraphic models assume the presence of an interior seaway within the basin under analysis and are centered around the direction and types of shoreline shifts which control the timing of all systems tracts and sequence stratigraphic surfaces in overfilled basins dominated by nonmarine sedimentation the definition of systems tracts is based on changes in fluvial accommodation as inferred from the shifting balance between the various fluvial architectural elements the method of sequence stratigraphy requires the application of the same set of core principles irrespective of the age of strata under analysis from precambrian to phanerozoic the study of precambrian basins is often hampered by poorer stratal preservation and by a general lack of time control however where sedimentary facies are well preserved the lack of time control may be partially compensated by a good knowledge of facies architecture and relationships as well as by paleocurrent data the latter are particularly important to understand the stratigraphic record of tectonically active basins where abrupt shifts in paleoflow directions allow one to infer tectonic events and map the corresponding sequence bounding unconformities arguably the most important contribution of precambrian research to sequence stratigraphy is the better understanding of the mechanisms controlling stratigraphic cyclicity in the rock record and hence of the criteria that should be employed in a system of sequence stratigraphic hierarchy there is increasing evidence that the tectonic regimes which controlled the formation and evolution of sedimentary basins in the more distant geological past were much more erratic in terms of origin and rates than formerly inferred solely from the study of the phanerozoic record in this context time is largely irrelevant as a parameter in the classification of stratigraphic sequences and it is rather the stratigraphic record of changes in the tectonic setting that provides the key criteria for the basic subdivision of the rock record into basin fill successions separated by first order sequence boundaries these first order basin fill successions are in turn subdivided into second and lower order sequences that result from shifts in the balance between accommodation and sedimentation at various scales of observation irrespective of the time span between two same order consecutive events sequences identified in any particular basin are not expected to correlate to other first and lower order sequences of other basins that may be similar in age but may have different timing and duration

tracing the source of ancient reworked organic matter delivered to the north atlantic ocean during heinrich events a major effort of the geochemical and paleoclimate community has been to identify the specific sources of the ice rafted debris ird in heinrich layers hls although the general consensus is that the majority of the ird originated from the hudson area of northern canada the specific sources are not well constrained here we compare the diagnostic organic geochemical signature of hls to that of a number of paleozoic outcrops across the former margin of the laurentide ice sheet we show that the biomarker signature of upper ordovician strata from southampton and baffin island is compatible with that found in hls in the labrador sea and north atlantic while the biomarker signature of other paleozoic formations from the former margin of the laurentide ice sheet is not in addition to the biomarker signature key inorganic characteristics 18o end and 87sr 86sr ratios of these formations from southampton and baffin island are consistent with those reported from hls the location of these formations in and around the hudson strait is compatible with palaeo ice flow regimes through the hudson strait allowing for easy entrainment and rapid transport to the ocean based on these results we propose that these specific upper ordovician formations form a main source of ird in hls and hence infer an active role of the hudson strait paleo ice flow in these events

an alternative suggestion for the pliocene onset of major northern hemisphere glaciation based on the geochemical provenance of north atlantic ocean ice rafted debris abstract the onset of abundant ice rafted debris ird deposition in the nordic seas and subpolar north atlantic ocean 2 72 millions of years ago ma is thought to record the pliocene onset of major northern hemisphere glaciation nhg due to a synchronous advance of north american laurentide scandinavian and greenland ice sheets to their marine calving margins during marine isotope stage mis g6 numerous marine and terrestrial records from the nordic seas region indicate that extensive ice sheets on greenland and scandinavia increased ird inputs to these seas from 2 72 ma the timing of ice sheet expansion on north america as tracked by ird deposition in the subpolar north atlantic ocean however is less clear because both europe and north america are potential sources for icebergs in this region moreover cosmogenic dating of terrestrial tills on north america indicate that the laurentide ice sheet did not extend to 39 n until 2 4 0 14 ma at least 180 ka after the onset of major ird deposition at 2 72 ma to address this problem we present the first detailed analysis of the geochemical provenance of individual sand sized ird deposited in the subpolar north atlantic ocean between mis g6 and 100 2 72 2 52 ma ird provenance is assessed using laser ablation lead pb isotope analyses of single ice rafted 150 m feldspar grains to track when an ice rafting setting consistent with major nhg first occurred in the north atlantic ocean during the pliocene intensification of nhg inhg we investigate when the pb isotope composition 206pb 204pb 207pb 204pb 208pb 204pb of feldspars deposited at dsdp site 611 first resembles that determined for ird deposited at this site during mis 100 2 52 ma the oldest glacial for which there exists convincing evidence for widespread glaciation of north america whilst quaternary magnitude ird fluxes exist at site 611 during glacials from 2 72 ma we find that the provenance of this ird is not constant instead we find that the pb isotope composition of ird at our study site is not consistent with major nhg until mis g2 2 64 ma we hypothesise that ird deposition in the north atlantic ocean prior to mis g2 was dominated by iceberg calving from greenland and scandinavia we further suggest that the grounding line of continental ice on northeast america may not have extended onto the continental shelf and calved significant numbers of icebergs to the north atlantic ocean during glacials until 2 64 ma

weathering fluxes and sediment provenance on the sw scottish shelf during the last deglaciation the reconstruction of past ice sheet dynamics can shed a light on long term ice stream activity and in turn provide constraints on the response of modern ice sheets to climate change the hebrides ice stream his flowed across part of the western scottish shelf to the shelf break during the last glacial cycle and drained a large portion of the northern sector of the british irish ice sheet to investigate the deglacial dynamics of the his following the last glacial maximum lead pb isotope records were extracted from the femn oxyhydroxide and detrital fractions of recovered laminated glacimarine mud sequences to monitor the changing activity of his during its retreat these provide timing and some source information of glacially weathered inputs to the marine environment the femn oxyhydroxide fraction in the samples is dominated by allochthonous particles pre formed and shows a marked decrease from radiogenic 20 05 206pb 204pb at 21 cal ka bp to less radiogenic pb isotope compositions 19 48 towards the windermere interstadial 15 4 13 ka this decrease represents a reduction in the flux of subglacially derived radiogenic pb to the continental shelf is interpreted as being associated with the break up of the ice stream in western scotland around that time the pb sr and nd isotopic signatures of the detrital fraction indicate a preponderance of moine sourced fine sediments originated from the nw highlands in the core locations from 21 to 15 cal ka bp dimlington stadial windermere interstadial most likely dictated by the orientation of ice flow tidal and oceanic current directions and sediment delivery in contrast ice rafted debris in a 21 cal ka bp old basal diamicton contains volcanic derived material suggesting different provenance for different grain sizes the femn oxyhydroxide 208pb 204pb ratio shows an unusual inversion relative to the other pb isotope ratios and is attributed to the introduction of secondary weathering phases from a source with contrasting 208pb 204pb but similar 206pb 204pb and 207pb 204pb in the detrital fraction the inversions are constrained to periodic spikes which may indicate an increased contribution from a high th u source potentially the neighbouring archaean amphibolitic lewisian basement in the outer hebrides this study demonstrates how geochemical investigation on continental shelves can be used to constrain the timing activity and flow sources of palaeo ice streams

landscape preservation under fennoscandian ice sheets determined from in situ produced 10be and 26al some areas within ice sheet boundaries retain pre existing landforms and thus either remained as ice free islands nunataks during glaciation or were preserved under ice differentiating between these alternatives has significant implications for paleoenvironment ice sheet surface elevation and ice volume reconstructions in the northern swedish mountains in situ cosmogenic 10be and 26al concentrations from glacial erratics on relict surfaces as well as glacially eroded bedrock adjacent to these surfaces provide consistent last deglaciation exposure ages 8 13 kyr confirming ice sheet overriding as opposed to ice free conditions however these ages contrast with exposure ages of 34 61 kyr on bedrock surfaces in these same relict areas demonstrating that relict areas were preserved with little erosion through multiple glacial cycles based on the difference in radioactive decay between 26al and 10be the measured nuclide concentration in one of these bedrock surfaces suggests that it remained largely unmodified for a minimum period of 845 418 461 kyr these results indicate that relict areas need to be accounted for as frozen bed patches in basal boundary conditions for ice sheet models and in landscape development models subglacial preservation also implies that source areas for glacial sediments in ocean cores are considerably smaller than the total area covered by ice sheets these relict areas also have significance as potential long term subglacial biologic refugia

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

european climatic response to millennial scale changes in the atmosphere ocean system during the last glacial period abstract pollen foraminifer dinocyst and coarse lithic high resolution analyses and 18 o measurements have been carried out for the last glacial section of marine core md95 2042 located near the southwestern margin of the iberian peninsula the pollen data indicate a high frequency of vegetational changes on the adjacent continent during this period suggesting a climatic variability very similar to that of the dansgaard oeschger cycles recorded by the greenland ice cores the detailed direct correlation of the terrestrial and marine proxy data from core md95 2042 indicates a three phase pattern of heinrich events in land and ocean environments the first and last phases of the h5 and h4 events are characterized by a mild and humid climate in southwestern europe probably associated with the european origin of the ice rafted detritus the middle phase exhibits a cold and dry climate in iberia linked with the maximum input of ice rafted detritus this phase seems to correspond with the laurentide ice sheet surges between the heinrich events several cold and dry periods on land are correlated with stades of the dansgaard oeschger cycles the impact of the dansgaard oeschger stades in southwestern europe seems to be preferentially connected to the cold winter air masses reaching this mid latitude region

southernmost evidence of large european ice sheet derived freshwater discharges during the heinrich stadials of the last glacial period galician interior basin northwest iberian continental margin abstract reconstruction of circum atlantic ice sheet motion and instabilities is crucial to understanding the mechanisms that triggered and or enhanced abrupt climate changes using enviromagnetic and geochemical data we provide a continuous and well dated record of the evolution of glacial interglacial sedimentation on the northwest iberian margin during the last glacial period covering the last six heinrich stadials the record shows european sediments that were related to meltwater pre events during the initial stages of hs1 hs2 and hs4 that corroborate the channel river depositional history the record also includes ird from the laurentide ice sheet and the european ice sheet during the final stages of these stadials i e heinrich events therefore this study provides insight into one of the potential forcing mechanisms for heinrich events and by inference for heinrich stadials

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