direct determination of the timing of sea level change during termination ii an outcrop within the last interglacial terrace on barbados contains corals that grew during the penultimate deglaciation or termination ii we used combined 230th and231pa dating to determine that they grew 135 8 0 8 thousand years ago indicating that sea level was 18 3 meters below present sea level at the time this suggests that sea level had risen to within 20 of its peak last interglacial value by 136 thousand years ago in conflict with milankovitch theory predictions orbital forcing may have played a role in the deglaciation as may have isostatic adjustments due to large ice sheets other corals in the same outcrop grew during oxygen isotope 18o substage 6e indicating that sea level was 38 5 meters below present sea level about 168 0 thousand years ago when compared to the 18o signal in the benthic v19 30 v19 28 record at that time the coral data extend to the previous glacial cycle the conclusion that deep water temperatures were colder during glacial periods

initiation of the european deglaciation as recorded in the northwestern bay of biscay slope environments meriadzek terrace and trevelyan escarpment a multi proxy approach three cores retrieved on the northwestern slope of the bay of biscay are described and discussed in the light of the european last deglaciation history this integrated sedimentological and micropalaeontological study provides a detailed evolution scheme for the deep and sea surface conditions of the bay during the final deglacial step with a direct link with the continental palaeoenvironments as early as 15 ka 14c bp a european precursor melting event is recorded as a purge of the channel and irish sea palaeoriver systems pleni heinrich event conditions occurred in the bay of biscay between 14 4 and 13 ka 14c bp with a typical canadian signature only recorded at 14 ka 14c bp namely 1 ka later than the first evidence of melting of the british ice sheet our data demonstrate that following heinrich event 2 the last glacial maximum was characterised by a gradual warming accompanied by at least two pulses of the north atlantic drift these north atlantic drift heat northward penetrations are supposed to have primarily forced the heinrich event 1 collapse

evidence from u th dating against northern hemisphere forcing of the penultimate deglaciation milankovitch proposed that summer insolation at mid latitudes in the northern hemisphere directly causes the ice age climate cycles1 this would imply that times of ice sheet collapse should correspond to peaks in northern hemisphere june insolation but the penultimate deglaciation has proved controversial because june insolation peaks 127 kyr ago whereas several records of past climate suggest that change may have occurred up to 15 kyr earlier2 3 4 5 6 7 8 there is a clear signature of the penultimate deglaciation in marine oxygen isotope records but dating this event which is significantly before the 14c age range has not been possible here we date the penultimate deglaciation in a record from the bahamas using a new u th isochron technique after the necessary corrections for recoil mobility of 234u and 230th and a small age correction for sediment mixing the midpoint age for the penultimate deglaciation is determined to be 135 2 5 kyr ago this age is consistent with some coral based sea level estimates but it is difficult to reconcile with june northern hemisphere insolation as the trigger for the ice age cycles potential alternative driving mechanisms for the ice age cycles that are consistent with such an early date for the penultimate deglaciation are either the variability of the tropical ocean atmosphere system or changes in atmospheric co2 concentration controlled by a process in the southern hemisphere

the dole effect over the last two glacial interglacial cycles detailed measurements of 18 o of atmospheric oxygen performed on air trapped in the vostok ice cores antarctica are used to extend the record of the dole effect over two climatic cycles back to 240 kyr b p except for glacial terminations i and ii and for an unexpected minimum occurring around 175 kyr the dole effect shows small variations adole within 0 5 these small variations however show a well marked 23 kyr precessional periodicity thus confirming the results obtained by bender et al 1994a for the first climatic cycle to explain the minimum value reached around 175 kyr we invoke the possibility of a peak in the oceanic productivity linked to climatic events induced at low latitudes under glacial conditions

mineralogically mature sandstones in accretionary prisms abstract plate tectonic interpretations of sandstone composition are made in the hope that detrital framework modes of sandstone suites provide information about the tectonic setting of basins of deposition and associated provenances dickinson et al 1983 the underlying assumption that provenance tectonics are directly associated with tectonics of depositional and post depositional sedimentary settings is not however universally true as sandstones from two contemporary accretionary prism complexes nias island indonesia and barbados west indies illustrate mineralogically immature sandstones rich in feldspar and volcanic lithic fragments are expected in such tectonic settings but both nias island and barbadian sandstones are mineralogically very mature dominated by quartz in b th cases this discrepancy between inferred and actual tectonic setting has been explained by postulating that sands from a provenance petrogenetically unrelated to the arc trench system e g continental or recycled orogen sands have been introduced into the accretionary prism by sedimentary or tectonic processes sandstones which are transported sedimentologically or tectonically across the boundaries of sedimentary tectonic provinces and which therefore occur in tectonic settings petrogenetically unrelated to the tectonics of the sediment source area are important components of at least some active accretionary complexes the possibility of trans boundary sedimentary and or tectonic transport must be allowed for in making plate tectonic interpretations based on sandstone petrog aphy especially in reconstructions of ancient orogenic belts

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

palaeoclimatology and palaeohydrography of the glacial stages on celtic and armorican margins over the last 360000 yrs core md03 2692 was retrieved in a water depth of 4064 m on the celtic margin bay of biscay during the sedicar cruise onboard the rv marion dufresne ii it covers the last 360 ka in a total length of 39 m multidisciplinary analyses have been applied to this sequence with the aim of studying the palaeoclimatic and palaeoenvironmental signals of the last few climatic cycles the analyses undertaken include 1 non destructive logging with physical properties magnetic susceptibility sediment colour x ray radiography and measurement of the major elements by x ray fluorescence 2 analyses of planktonic and benthic foraminifera lithic grains and stable isotopic measurements oxygen and carbon we have focused on the longterm evolution of glacial stages with special attention to terminations and heinrich events the results obtained confirm the close correlation between deep sea sedimentation recorded on the celtic margin and changes in the terrestrial environment of the adjacent continent heinrich layers have been identified in mis 2 3 6 and 8 we note the occurrence of laminated facies within deglacial sequences deposited during termination i and mis 6 these facies are closely linked to disintegration phases of the british irish ice sheet bis the laminations contain lower ice rafted detritus ird concentrations than the equivalent heinrich layers and are linked to abrupt changes in sea surface palaeotemperatures we suggest that the laminations are formed by an annual cycle of meltwater and iceberg release from the disintegrating bis generating cascading plumes of dense turbid meltwater coeval with ird release d 2005 elsevier b v all rights reserved

field observations of slush ice generated during freeze up in arctic coastal waters abstract in some years large volumes of slush ice charged with sediment are generated from frazil crystals in the shallow beaufort sea during strong storms at the time of freeze up such events terminate the navigation season and because of accompanying hostile conditions little is known about the processes acting the water saturated slush ice which may reach a thickness of 4 m exists for only a few days before freezing from the surface downward arrests further wave motion or pancake ice forms movements of small vessels and divers in the slush ice occurs only in phase with passing waves producing compression and rarefaction and internal pressure pulses where in contact with the seafloor the agitated slush ice moves cobble size material generates large sediment ripples and may possibly produce a flat rampart observed on the arctic shoreface in some years processes charging the slush ice with as much as 1000 m 3 km 2 of sediment remain uncertain but our field observations rule out previously proposed filtration from turbid waters as a likely mechanism sedimentary particles apparently are only trapped in the interstices of the slush ice rather than being held by adhesion since wave related internal pressure oscillations result in downward particle movement and cleansing of the slush ice this loss of sediment explains the typical downward increase in sediment concentration in that part of the fast ice canopy composed largely of frazil ice the congealing slush ice in coastal water does not become fast ice until grounded ridges are formed in the stamukhi zone one to two months after freeze up begins during this period of new ice mobility long range sediment transport occurs the sediment load held by the fast ice canopy in the area between the colville and sagavanirktok river deltas in the winter of 1978 1979 was 16 times larger than the yearly river input to the same area this sediment most likely was rafted from canada more than 400 km to the east during a brief time period in the previous fall ocean turbulence is greatly reduced while the congealing slush ice drifts about therefore new ice then forming in intervening open water areas is clean these events explain the patchy appearance of the fast ice after the summer snowmelt more work on the important phenomena reported here is needed to close a major gap in the knowledge of the arctic marine environment

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

evidence for changes in the north atlantic deep water linked to meltwater surges during the heinrich events abstract deep sea sediment records from north atlantic cores 40 n 55 n provide evidence of several massive iceberg discharges known as heinrich events during the last glacial period high resolution benthic 18 o and 13 c records from north atlantic sediment cores were used to monitor the impact of heinrich events on thermohaline circulation and to estimate the sensitivity of deep oceanic circulation to changes in freshwater input to the north atlantic surface waters our data indicate that major rearrangements of deep water masses were directly associated with these massive iceberg discharges to trace in detail the deep water conditions in the north atlantic benthic 13 c values in several cores were used to generate time slices before during and after heinrich event 4 dated at 35 ka bp although north atlantic deep water continued to form during the oxygen isotope stage 3 at 37 ka bp deep circulation was characterized by an increased incursion of deep waters of southern origin which reduced the 13 c composition of north atlantic deep waters particularly in the eastern atlantic basin north atlantic deep water production was reduced during heinrich layer 4 hl4 at 35 ka bp synchronously with the changes in the surface water hydrology deep convection processes may have occurred in areas not affected by the salinity decrease soon after hl4 33 ka bp the 13 c distribution was similar to that before the event similarly a rapid return to initial 13 c values was observed at the end of the most clearly defined heinrich events hl5 hl4 and hl1 comparison between deep circulation patterns corresponding to hl4 the last glacial maximum and heinrich event 1 indicates that each of these periods was characterized by a different circulation state associated with changes in convection sites

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

land sea correlations for the last glaciation inferred from a pollen and dinocyst record from the portuguese margin pollen and dinoflagellate cyst assemblages from core su 81 18 recovered off portugal 37 46 n 10 11 w 3135 m water depth have been used to document the short term environmental changes that occurred in southwest europe since 25 000 yr b p the relationship between the oceanic and continental environments has been further examined by the use of other marine proxies coarse sedimentary fraction foraminifera and by comparison with proximal land pollen records heinrich 2 h2 and heinrich 1 h1 events were the most extreme parts of the highly variable last glacial period with the maximum extension of dry steppe on land and the occurrence of cool and dilute waters at the core site our study shows that h1 and h2 are divided in two distinct phases one with neogloboquadrina pachyderma left coiling associated with the maximum input of ice rafted debris reflecting the in situ release of icebergs and the occurrence of cool and dilute seawater at the core site the other with dinoflagellate cysts of subpolar affinity bitectatodinium tepikiense reflecting a seasonal control marked by warm summer sst and cold winter sst

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

millennial scale glacial variability versus holocene stability changes in planktic and benthic foraminifera faunas and ocean circulation in the north atlantic during the last 60 000 years two piston cores ds97 2p from the reykjanes ridge in the central north atlantic ocean 1685 m water depth and enam33 from southwest of the faeroe islands in the ne atlantic 1217 m water depth have been investigated for their planktic and benthic foraminiferal content ds97 2p is situated near the subarctic front and productivity measured by accumulation rates of benthic and planktic foraminifera has been generally high during the holocene the productivity shows a clear decrease from an early holocene maximum to a late holocene minimum coeval changes in the benthic faunas indicate that the food supply changed from large irregular pulses during the early holocene to a more sustained flux during the late holocene presumably in concert with decreasing bottom current activity oxygen conditions in the bottom water became poorer another feature of the late holocene is an increasing instability of the north atlantic thermohaline circulation regime nevertheless the changes in faunal composition and productivity during the holocene were gradual as compared to the discontinuous distribution patterns and abrupt productivity shifts during the glacial the glacial shifts were on a millennial time scale and correlate with the interstadial stadial phases of the dansgaard oeschger cycles in the greenland ice cores the faunas of the warm interstadial phases resembled the holocene faunas and both surface and bottom productivity was high the faunas suggest that the interstadial circulation pattern was very similar to the modern system with convection in the nordic seas and generation of north atlantic deep water the planktic faunas during the cold stadials and heinrich events were completely dominated by the polar species neogloboquadrina pachyderma s and surface conditions were cold and the productivity low the benthic faunas were dominated by species that presently thrive in areas with a low amount of food and reduced oxygen content the water column was probably stratified with low saline cold surface water overlying poorly aerated intermediate water masses

comparison of varve and 14c chronologies from steel lake minnesota usa annually laminated sediments varves offer an effective means of acquiring high quality palaeoenvironmental records however the strength of a varve chronology can be compromised by a number of factors such as missing varves ambiguous laminations and human counting error we assess the quality of a varve chronology for the last three millennia from steel lake minnesota through comparisons with nine ams 14c dates on terrestrial plant macrofossils from the same core these comparisons revealed an overall 8 4 discrepancy primarily because of missing uncountable varves within two stratigraphic intervals characterized by low carbonate concentrations and obscure laminations application of appropriate correction factors to these two intervals results in excellent agreement between the varve and 14c chronologies these results together with other varve studies demonstrate that an independent age determination method such as 14c dating is usually necessary to verify and potentially correct varve chr

radiocarbon constraints on the history of the western irish ice sheet prior to the last glacial maximum accelerator mass spectrometer c 14 dating of in situ monospecific marine microfaunas and reworked shells records high relative sea level on the western coast of ireland between ca 40 and 19 call calibrated ka requiring substantial isostatic loading by the irish ice sheet iis during that interval glacial and marine deposits also record a rapid fluctuation of the us margin onto the continental shelf ca 28 cal ka evidence that the coast was not subsequently overridden by the iis indicates that at least for this sector of the ice sheet the ice margin during the last glacial maximum lgm was less extensive than the 28 cal ka event our data from western ireland support c 14 and cl 36 chronologies from the irish sea basin in identifying an us that was as extensive and thick as the lgm iis for much of the 20 k y interval prior to the lgm

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

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

west african monsoon variability during the last deglaciation and the holocene evidence from fresh water algae pollen and isotope data from core kw31 gulf of guinea abstract the study of aquatic and mangrove pollen grains and algae from freshwater environments from core kw31 recovered off the mouth of the niger river 3 31 1 n 05 34 1 e 1181 m water depth documents large scale changes in river discharge linked to variations of atlantic monsoon rainfall during the last glacial interglacial transition and the holocene two main taxa dominate the algal record pediastrum which records the strongest river input from 14 500 to 7100 cal b p and botryococcus which indicates surficial runoff in more arid periods particularly at the onset of the deglaciation and during the younger dryas several dry events punctuated the first and more intense phase of the so called african humid period 14 500 cal b p 9800 cal b p during the younger dryas 13 400 12 100 cal b p then around 11 400 11 200 cal b p indicating major climate instability freshwater discharges decreased after 7100 cal b p and definitely dropped to modern values after 3500 cal b p in agreement with land data the microfossil record of core kw31 shows that the increase in freshwater input to the ocean and correlative rainfall over western tropical africa were gradual in response to insolation forcing at the transition from glacial to interglacial situations

response of global deep water circulation to earth s climatic change 135 000 107 000 years ago the response of deep ocean circulation to major climatic events of the past 135 000 years is inferred using carbon 13 analyses of benthic foraminifera from the major ocean basins the results demonstrate that the circulation of the world ocean deep water is very sensitive to climate and that it changes drastically during climatic transitions

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

expansion and retreat of the jura ice sheet france during the last glacial maximum in continental environments while there is no uninterrupted record of glaciation glacier activity can be reconstructed from the nature and geometry of sediments trapped by lake filling examination of the combe d ain lake filling provides insight into the history of the jura mountains during the last glacial maximum lgm from the evidence of outcrops borehole cores and seismic reflection profiles of the lake filling we show that glaciation was marked by three stages of stabilization then recessional moraines marked five episodes of stabilization in the course of glacial retreat comparison with b 18 o data from ice cores of greenland makes possible to date the last glacial maximum at 25 500 22 000 cal bc and

the quaternary channel river seismic stratigraphy of its palaeo valleys and deeps based on the interpretation of geophysical data very high resolution seismic data combined with em1000 swath bathymetry this paper reviews the history of the channel river throughout the late cenozoic new evidence does illustrate how the interplay of tectonics eustacy and climate have influenced this northwest european drainage system the concepts of sequence stratigraphy allow the subdivision of the sedimentary record into depositional sequences bounded by unconformities resulting from globally synchronous sea level changes however the recognition of eustatic sea level changes in a cold climate environment is very difficult because of the interplay of additional phenomena such as glacio isostatic sea level changes with glacio eustatic changes for our interpretation it was necessary to adjust these concepts with the relative importance of geomorphological processes determining the behaviour of rivers and steering their evolution the foundations of the modern drainage system were laid after the oligocene channel inversion in general these early rivers occupied shallow valleys the pleistocene cold climates resulted in fluvial incision through time the channel river has adopted a drainage system that can be divided into three parts i the drainage basin comprising the southern bight the eastern channel and the london and paris basins ii the river zone itself which begins at the cotentin peninsula and passes through the hurd deep before reaching the continental shelf break in the celtic sea iii the depositional basin represented by the celtic banks complex and by the deep sea fans located at the foot of shamrock and black mud canyons during important lowstands marine oxygen isotope stages 22 16 10 6 and 2 the channel river seems to have reached the shelf break as indicated by the internal structure of the banks copyright 2003 john wiley sons ltd

distribution patterns of dinoflagellate cysts and other organic walled microfossils in recent norwegian greenland sea sediments dinoflagellate cysts and other organic walled microfossils have been studied in recent surface sediments from the entire norwegian greenland sea more than 30 taxa have been recognized of which only few show a distinct distribution pattern and allow description of four assemblages the occurrence of most taxa is related to the relatively warmer waters of the norwegian sea algidaspaeridium minutum s 1 brigantedinium simplex and impagidinium pallidum are the only species showing a preference for colder water masses two species i pallidum and nematosphaeropsis labyrinthus are mainly restricted to the oceanic environment whereas the other species have also been reported from neritic environments in previous studies due to the limited knowledge of the ecological and sedimentological factors influencing the occurrence of dinoflagellate cysts in oceanic environments their distribution in recent sediments can be only related to surface water masses in a broad sense although the distribution of assemblages correlates with specific surface water masses comparison with assemblages recovered from sediment traps deployed basinwide in the norwegian greenland sea dale and dale 1992 revealed some major discrepancies in species composition and percentage abundances the differences cannot be explained with certainty at the moment although there is some evidence that transport of dinoflagellate cysts and other fossilizable microplankton in water masses by currents in sea ice and sediments may modify the assemblages found in recent oceanic surface sediments from the norwegian greenland sea

oceanic processes as potential trigger and amplifying mechanisms for heinrich events marine sediments recorded a series of heinrich events during the last glacial period massive ice surges thatdeposited prominent layers of ice rafted debris in the north atlantic here we explore oceanic mechanisms thatcan potentially trigger and amplify the observed ice calving events simulations of abrupt glacial climate changewith a coupled ocean atmosphere sea ice model show a substantial regional sea level rise in the north atlantic inresponse to a collapse of the atlantic meridional overturning circulation moc the increased heat uptake ofthe global ocean after the moc collapse leads to an additional rise in global sea level we hypothesize that thesesea level changes have the potential to destabilize northern hemisphere ice shelves and ice sheets and to triggerice surges sea level rise due to ice calving and subsurface ocean warming provides two positive feedbackmechanisms contributing to further destabilization of ice shelves and ice sheets

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

environmental processes of the ice age land oceans glaciers epilog knowledge of the state of the earth at the last glacial maximum lgm an interval around 21 000 years ago is an important benchmark for understanding the sensitivity of global environmental systems to change much progress in understanding climates of the lgm has occurred in the 20 years since the end of the climap project of the 1970s climate long range investigation mapping and prediction here we review this progress based on presentations and discussion at a rst open science meeting of the epilog project environmental processes of the ice age land oceans glaciers held in delmenhorst germany may 1999 we outline key controversies and document protocols for epilog contributions so that a new synthesis of the lgm earth can emerge as an open project of the world s community of scientists 2001 elsevier science ltd all rights reserved

duration and structure of the past four interglaciations reanalysis and additional dating of the devils hole 18o paleotemperature record confirm that the last interglaciation in the great basin the continental analog of marine isotopic substage 5e lasted 22 000 yr consistent with the vostok paleotemperature record which suggests a duration of 19 000 yr for this event in antarctica the three preceding interglaciations in the devils hole record analogs of marine isotopic substages 7e 9c and 11c range from 20 000 to 26 000 yr in duration a 20 000 yr duration for the last interglaciation is consistent with tims uranium series dated sea level high stands thus the widely held view that interglaciations were of 11 000 to 13 000 yr duration and constituted only about 10 of mid to late pleistocene climatic cycles needs reexamination the warmest portion of each interglaciation in the devils hole time series is marked by a 18o plateau signifying apparent climatic stability for periods of 10 000 to 15 000 yr duration

evidence for heinrich layers off portugal tore seamount 39 n 12 w abstract the tore seamount is a circular volcano like feature 100 km in diameter with its summit at 2200 m water depth and a small 5000 m deep basin in its interior it is situated approximately 300 km west of lisbon and is surrounded by deep abyssal plains this site with a standard pelagic stratigraphy is the southernmost point where the so called heinrich events have so far been recorded a succession of alternating interglacial glacial periods reveals a stratigraphic record back to the beginning of isotopic stage 7 225 kyr climatic changes are identifiable by coherent variations in colour carbonate content and distribution of ice rafted detritus in the carbonate free fraction inputs of ice rafted quartz are well defined characteristics in common with other sites showing heinrich layers include a high terrigenous to biogenic ratio a dramatic decrease in the accumulation rate of foraminifera shells an increase in dolomite abundance and the occurrence of polar foraminiferal species indicating southwards penetration of cold waters which lead us to consider a wider southeastern extent of the north atlantic ice rafted detritus belt than hitherto if the presently accepted position of the polar front is maintained icebergs must have been swept southwards from the southern boundary of the pack ice in a current merging into the ancestral canary current bringing ice rafted material to the tore seamount the coincidence of reddish feldspar probably derived from the northern appalachian triassic red facies with the transparent quartz suggests at least a partial labrador source for all the heinrich layers here including hl 3 in comparison to other sites in the entire north atlantic two exceptions stand out the absence of hl 5 and the low detritus to biogenics ratio for hl 3 the simultaneous occurrence of these two types of ice rafted minerals is a new piece in the puzzle of the origin of heinrich layers

simulation of intense monsoons under glacial conditions paleoenvironmental and paleoclimatological proxy data indicate that strong continental monsoons took place under glacial conditions during marine isotopic stage 6 5 175 ka bp so far no climate model has explored the possible coexistence of glacial conditions at mid and high latitudes and of interglacial monsoons in the tropics here we use an atmospheric general circulation model and clearly demonstrate that high insolation can generate increased monsoon activity even with surface glacial conditions our experiments show that indian and african monsoons at 175 ka were stronger than nowadays and induced an increase in outflow of the nile river to the mediterranean sea this freshwater supply combined with the local low glacial evaporation may explain the stagnation of the eastern mediterranean sea leading to the deposit of sapropel s6 our simulations show also increased surface winds in the northern atlantic and pacific oceans which may have affected the bioproductivity in these areas

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

what forced the collapse of european ice sheets during the last two glacial periods 150 ka b p and 18 ka cal b p palynological evidence article i nfo we present a new marine palynological record from the temperate north eastern atlantic core md03 2692 celtic armorican margin in the area of influence of the fleuve manche paleoriver during glacial times the runoff of this paleoriver was connected to the glacial history of european ice sheets including the british irish ice sheet biis our study conducted on dinoflagellate cysts assemblages over the last 200 ka associated with quantification of palynological remains reflecting continental influence pre quaternary dinocysts and micro algae pediastrum spp coenobia yields detailed insights into the sea surface paleoenvironments of this sector more specifically mechanisms responsible for the origin of unusual deposits of laminated sequences found at the end of marine isotope stage mis 2 and within mid mis 6 are here constrained we provide evidence of genetic similarities between fluvial discharges occurring before and during times of deposition of the laminated deposits our study clarifies the causes of the collapse of european glaciated systems at the end of mis 2 prior to the deposition of the laminated deposits pleni oceanic influence characterized by high concentrations of quaternary dinocysts prevailed and a dinocyst species tracing the penetration of the warm north atlantic drift into the bay of biscay is recorded however this scenario did not recur prior to the deposition of laminated sediments of mid mis 6 in addition contrary to the laminated deposits found at the end of mis 2 and directly associated with termination i mis 6 laminae appeared 20 ka earlier than termination ii our work shows that during the penultimate glacial stage the collapse of mid latitudes ice sheets around 170 ka b p may have immediately followed the mis 6 5 warming phase then the most important melting event around 150 ka b p may be linked to a peak in insolation at 65 n though less important than those during mis 6 5 and termination ii

dinoflagellate cyst evidence of heinrich like events off portugal during the marine isotopic stage 5 abstract dinoflagellate cysts were analysed from images core md952042 37 48 n 10 01 w retrieved from the tagus abyssal plain previous results of stable isotope and magnetic susceptibility measurements as well as of planktonic foraminiferal temperature reconstruction from this core suggest the occurrence of heinrich like events i e large ice sheet decay during marine isotopic stage 5 mis 5 dinoflagellate assemblages of this time period have revealed six dinocyst events that are characterised by peaks in bitectatodinium tepikiense percentages these events occur synchronously with heinrich like events previously identified they are coeval with major retreats of the forest on land indicating therefore drastic changes in the regional climate however results from the ice rafted detritus ird analysis of the 150 m lithic fraction shows that mis 5 of md952042 has only recorded one significant input of iceberg discharge located at the mis 6 mis 5 transition it seems therefore that it is the only event that could be called a true heinrich event

climatic conditions during marine oxygen isotope stage 6 in the eastern mediterranean region from the isotopic composition of speleothems of soreq cave israel at several times during marine oxygen isotope stage 6 the eastern mediterranean region was influenced by two extreme climatic systems the large ice sheet over northern europe and the wet tropics associated with african monsoons during this interval two major climatic events occurred in the region the sapropel s6 layer formed ca 176 ka in the eastern mediterranean basin owing to the increase in the african monsoon and another event although not large enough to form sapropel occurred ca 151 ka the isotopic composition of soreq cave speleothems seems to record these events as very low 18 o 13 c values dated as ca 178 and 152 ka the very low 18 o 13 c values of 6 and 11 to 12 respectively are typical of interglacial intervals but here they were recorded during a glacial interval such low peaks indicate that in this part of the eastern mediterranean region i e israel the rainfall amount increased dramatically moreover the isotopic record of the speleothems also shows that during the entire stage 6 although the climate was as cold as much of the last glacial the conditions were never as dry

the relationship of heinrich events and their european precursors over the past 60 ka bp a multi proxy ice rafted debris provenance study in the north east atlantic high resolution multi proxy records of ice rafted debris ird flux and provenance in the ne atlantic detail the development variability and decline of marine margins of the last glacial circum north atlantic ice sheets coupled lithological identification sr and nd isotopic composition and 40ar 39ar ages of individual hornblende grains reduce ambiguity as to ird potential source region allowing clear differentiation between laurentide lis icelandic and british bis ice sheet sources the icelandic and bis are collectively referred to as the nw european ice sheet nweis a step wise increase in the flux of ird to the core site at 26 5 ka bp documents bis advance and glaciation of ireland millennial scale variability of the bis at a 2 ka periodicity is inferred through clusters of pulsed ird fluxes throughout the late glacial 26 5 10 ka bp combination of these european ird events and the 7 ka periodicity of lis instability is thought to account for quasi synchronicity of the nweis and lis ird pulses at heinrich event h 2 and h1 previously suggested to represent the possible involvement of the nweis in the initiation of h events furthermore the lack of extensive nweis marine margin is inferred prior to h3 31 5 ka bp such that no european precursor event is associated with either h5 or h4 this suggests that precursor events were not directly implicated in the collapse of the lis and the persistent instabilities of the bis that are clustered at a 2 ka periodicity are incompatible with the concept that both h events and their precursors are independent responses to a common underlying trigger

phase lag between intertropical convergence zone migration and subtropical monsoon onset over the northwestern indian ocean during marine isotopic substage 6 5 mis 6 5 high resolution faunal and isotopic analyses of foraminifera were performed on core md96 2073 10 94 n 52 62 e 3142 m depth located close to socotra island in the upwelling area of the somali basin nw indian ocean this work focuses on marine isotopic stage 6 5 in order to reconstruct paleo upwelling changes and their links with the arabian sea summer monsoon and the migration of the intertropical convergence zone itcz correspondence and cluster analyses of planktonic foraminiferal abundances partly controlled by temperature and water mass productivity together with an upwelling intensification index show the occurrence of a strong upwelling between 176 and 165 ka this upwelling intensification responds to a northward migration of the itcz an isotopic depletion in the planktonic foraminifera 18o records occurring between 180 and 167 ka is interpreted as proof of a large salinity decrease in the surface waters probably linked to a strong input of fresh rainfall waters induced by an intense monsoon activity the lag between the onset of upwelling intensification and the strong monsoonal impact over the same area suggests a decoupling between both phenomena the migration of the itcz is influenced by obliquity and precessional forcing while the arabian sea summer monsoon precipitation depends only on precessional forcing

coherent patterns of ice rafted debris deposits in the nordic regions during the last glacial 10 60 ka we have synchronized records of ice rafted rock debris deposits of three sediment cores from the norwegian sea and the irminger basin during the last glacial period from 10 to 50 ka by combining the use of radiocarbon dates and adjustments of physical properties our synchronized records indicate that layers rich in ice rafted debris were deposited throughout the nordic regions at times near to synchronous with the major collapses of the laurentide ice sheet during the heinrich events there are also millennial scale coherent and near to synchronous deposits of ice rafted rock debris into the norwegian sea related to repetitive changes of the flux of icebergs from the fenno scandinavian the correlation with the cold phases of the dansgaard oeschger temperature record points to a close coupling between atmospheric temperature oscillations and variations of iceberg fluxes into the norwegian sea during the last glacial variations in atmospheric circulation patterns bringing moisture supply to high latitudes and the distribution of this moisture over the different northern hemisphere coastal ice sheets and ice shelves could be controlling both the timing of ice sheet advances and the flux of iceberg to the open ocean

rapid fluctuations in sea level recorded at huon peninsula during the penultimate deglaciation about 140 000 years ago the breakup of large continental ice sheets initiated the last interglacial period sea level rose and peaked around 135 000 years ago about 14 meters below present levels a record of last interglacial sea levels between 116 000 years to 136 000 years ago is preserved at reef vii of the uplifted coral terraces of huon peninsula in papua new guinea however corals from a cave situated about 90 meters below the crest of reef vii are 130 000 2000 years old and appear to have grown in conditions that were 6 c cooler than those at present these observations imply a drop in sea level of 60 to 80 meters after 130 000 years sea level began rising again in response to the major insolation maximum at 126 000 to 128 000 years ago the early about 140 000 years ago start of the penultimate deglaciation well before the peak in insolation is consistent with the devils hole chronology

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

variation of labrador sea water formation over the last glacial cycle in a climate model of intermediate complexity abstract the variation of north atlantic deep water nadw formation over the last glacial cycle from oxygen isotopic substage 5e ois 5e the eemian to future global warming projections is investigated using the uvic earth system climate model the results are compared with available micropaleontological and stable isotope proxy paleo reconstructions equilibrium simulations for the eemian 125 kyr bp and the last glacial maximum lgm 21 kyr bp both reveal the absence of labrador sea water lsw formation although nadw formation still occurs albeit at a reduced rate relative to the modern times for the eemian the location of convection in the eastern north atlantic is similar to the present although it is generally shallower and less extensive in the case of the lgm deep convection has moved southward to the western coast of europe and is much more localised the inferred inception of a modern like circulation slightly before 7 kyr bp revealed by proxy reconstructions is not captured by the model unless the meltwater forcing from the laurentide ice sheet is applied in a long transient simulation this raises questions concerning the applicability of equilibrium simulations in capturing the early holocene climate in all global warming projections the lsw formation initially ceases as atmospheric co 2 rises but recovers once the level is held fixed in the atmosphere convection in the north extends further into the nordic seas as the sea ice edge retreats in all simulations convection remains active in the eastern north atlantic with its latitude depending on the position of the sea ice edge suggesting that the formation of lower nadw is a robust feature of late quaternary climate as the labrador sea is found to be very sensitive to the freshwater forcing it suggests that this region represents an ideal location for the concentration of observational studies to monitor a possible oceanic response to anthropogenic climate change

high resolution evidence for linkages between nw european ice sheet instability and atlantic meridional overturning circulation published studies show that ice rafted debris ird deposition preceding heinrich h events h1 and h2 in the ne atlantic was derived from the nw european ice sheets nweis possibly offering clues about ice sheet sensitivity and stability and the mechanisms that caused periodic collapse of the laurentide ice sheet lis we present detailed lithological and geochemical records including radiogenic isotope fingerprinting of ird deposits from core md01 2461 proximal to the last glacial british ice sheet bis demonstrating persistent instability of the bis with significant destabilisation occurring 1 5 1 9 kyr prior to both h1 and h2 dated at 16 9 and 24 1 kyr bp respectively in the ne atlantic paired mg ca and 18o data from the surface dwelling globigerina bulloides and subsurface dwelling neogloboquadrina pachyderma sinistral are used to determine late glacial variability of temperature salinity and stratification of the upper water column a picture emerges that the bis was in a continuing state of readjustment and never fully reached steady state increased sea surface temperatures appear to have triggered the episode of nweis instability preceding h1 it seems most probable that the so called precursor events were not linked to the h events however if response to a common thermal forcing is considered an increased response time of the lis up to 2 kyr longer than the nweis may be inferred negative salinity excursions of up to 2 6 indicate significant incursions of melt water associated with peaks in nweis instability decreased surface density led to a more stable stratification of the upper water column and is associated with reduced ventilation of intermediate waters recorded in depleted epibenthic 13c cibicidoides wuellerstorfi we suggest that instability and meltwater forcing of the nweis temporarily weakened atlantic meridional overturning circulation allowing transient advance of southern sourced waters to this site prior to h events 1 and 2

sr isotope record of quaternary marine terraces on the california coast and off hawaii abstract strontium isotopic ratios of dated corals have been obtained from submerged reefs formed during quaternary glacial periods off the hawaiian islands these data combined with data from deep sea sediments tightly constrain the secular variation of marine 87 sr 86 sr for the past 800 000 yr although long term trends are apparent no significant 0 02 rapid 87 sr 86 sr were resolved nor did we observe any samples with 87 sr 86 sr greater than that of modern seawater strontium in mollusks from elevated marine terraces formed during interglacial periods on the southern california coast show resolvable and consistent variations in 87 sr 86 sr which when compared to the trend of quaternary marine 87 sr 86 sr can be used to infer uplift rates and define approximate ages for the higher terraces the sr isotope age estimates indicate that uplift rates vary among crustal blocks and were not necessarily constant with time no contrast in sr isotopic ratios between similar age hawaiian and california fossils was observed confirming that any change in marine 87 sr 86 sr from glacial to interglacial periods must be small a realistic appraisal of the potential of sr isotope stratigraphy for chronometric applications in the quaternary suggests that the technique will be limited to relatively coarse distinctions in age

reconstruction of sea surface temperatures from assemblages of planktonic foraminifera multi technique approach based on geographically constrained calibration data sets and its application to glacial atlantic and pacific oceans abstract we present a conceptual framework for a new approach to environmental calibration of planktonic foraminifer census counts this approach is based on simultaneous application of a variety of transfer function techniques which are trained on geographically constrained calibration data sets it serves to minimise bias associated with the presence of cryptic species of planktonic foraminifera and provides an objective tool for assessing reliability of environmental estimates in fossil samples allowing identification of adverse effects of no analog faunas and technique specific bias we have compiled new calibration data sets for the north n 862 and south n 321 atlantic and the pacific ocean n 1111 we show evidence that these data sets offer adequate coverage of the sea surface temperature sst and faunal variation range and that they are not affected by the presence of pre holocene samples and or calcite dissolution we have applied four transfer function techniques including artificial neural networks revised analog method and simmax with and without distance weighting on faunal counts in a last glacial maximum lgm data set for the atlantic ocean 748 samples in 167 cores based on the glamap 2000 compilation and a new data set for the pacific ocean 265 samples in 82 cores and show that three of these techniques provide adequate degree of independence for the advantage of a multi technique approach to be realised the application of our new approach to the glacial pacific lends support to the contraction and perhaps even a cooling of the western pacific warm pool and a substantial 3 c cooling of the eastern equatorial pacific and the eastern boundary currents our results do not provide conclusive evidence for lgm warming anywhere in the pacific the atlantic reconstruction shows a number of robust patterns including substantial cooling of eastern boundary currents with considerable advection of subpolar waters into the benguela current a cooling of the equatorial atlantic by 5 c and steep sst gradients in the mid latitude north atlantic the transfer function techniques generally agree that subtropical gyre areas in both hemispheres did not change significantly since the lgm although the ann technique produced glacial sst in the southern gyre 1 2 c warmer than today we have revisited the issue of sea ice occurrence in the nordic seas and using the distribution of subpolar species of planktonic foraminifera in glacial samples we conclude that the norwegian sea must have been ice free during the summer

catastrophic flooding origin of shelf valley systems in the english channel britain became geographically isolated from continental europe when high interglacial seas flooded the shallow english channel and north sea shelf areas but a tenuous link remained the weald artois chalk ridge between southeast england and northwest france just how this isthmus was breached making britain an island has been a matter of conjecture a new bathymetric map of the sea floor may solve the mystery it reveals a large bedrock floored valley containing landforms including grooves and streamlined islands consistent with a megaflood event caused when a rock dam at the dover strait breached draining a large pro glacial lake in the north sea basin a hypothesis for the permanent isolation of britain from mainland europe during interglacial high sea levels has been confirmed gupta et al analysed a new regional bathymetric map of part of the area finding a valley with landforms indicating large scale subaerial erosion by high magnitude water discharges they suggest breaching of a rock dam at the dover strait instigated drainage of a large lake in the north sea basin megaflood events involving sudden discharges of exceptionally large volumes of water are rare but can significantly affect landscape evolution continental scale drainage patterns and climate change1 it has been proposed that a significant flood event eroded a network of large ancient valleys on the floor of the english channel the narrow seaway between england and france2 3 4 this hypothesis has remained untested through lack of direct evidence and alternative non catastrophist ideas have been entertained for valley formation5 6 here we analyse a new regional bathymetric map of part of the english channel derived from high resolution sonar data which shows the morphology of the valley in unprecedented detail we observe a large bedrock floored valley that contains a distinct assemblage of landforms including streamlined islands and longitudinal erosional grooves which are indicative of large scale subaerial erosion by high magnitude water discharges our observations support the megaflood model in which breaching of a rock dam at the dover strait instigated catastrophic drainage of a large pro glacial lake in the southern north sea basin2 we suggest that megaflooding provides an explanation for the permanent isolation of britain from mainland europe during interglacial high sea level stands7 and consequently for patterns of early human colonisation of britain together with the large scale reorganization of palaeodrainage in northwest europe4

enhanced aridity and atmospheric high pressure stability over the western mediterranean during the north atlantic cold events of the past 50 k y multiproxy paleoenvironmental records pollen and planktonic isotope from ocean drilling program site 976 alboran sea document rapid ocean and climate variations during the last glacial that follow the dansgaard oeschger climate oscillations seen in the greenland ice core records thus suggesting a close link of the mediterranean climate swings with north atlantic climates continental conditions rapidly oscillated through cold arid and warm wet conditions in the course of stadial interstadial climate jumps at the time of heinrich events i e maximum meltwater flux to the north atlantic western mediterranean marine microflora and microfauna show rapid cooling correlated with increasing continental dryness enhanced aridity conceivably points to prolonged wintertime stability of atmospheric high pressure systems over the southwestern mediterranean in conjunction with cooling of the north atlantic

geographical distributions of organic walled dinoflagellate cysts in surficial sediments of the benguela upwelling region and their relationship to upper ocean conditions the organic walled cyst content of 41 surface sediment samples from the south eastern south atlantic ocean have been studied to create a dataset that can be used for palaeoceanographic reconstructions in order to obtain insight into which environmental factors influence the distribution of individual cyst species the cyst associations have been compared with oceanographic characteristics of the overlying water masses i e temperature salinity density and stratification gradients the associations and relationships have been established by visual examination of the dataset and the multivariate ordination techniques detrended correspondence analysis and canonical correspondence analysis special attention has been given to the factors of transport and preservation of the cysts five associations have been recognised as being characteristic of 1 areas influenced by coastal upwelling and or river outflow 2 open ocean 3 agulhas current and southern benguela current 4 benguela current and 5 walvis bay shelf break area the factors dominant in influencing either directly or indirectly the cyst distributions appear to be the stratification in the upper 50 m of the water column nutrient concentration and seasonality variations in sea surface temperatures and salinities have only minor effect on cyst distribution

were glacial iceberg surges in the north atlantic triggered by climatic warming high resolution physical mineralogical sedimentological and micropalaeontological studies were carried out on north atlantic cores from the reykjanes ridge at 59degreesn and from the region southwest of the faeroe islands all core sites are situated along the pathway of iceland scotland overflow water isow and the various parameters measured display similar features previously identified carbonate oscillations keigwin and jones 1994 j geophys res 99 12397 12410 in the time span back to the marine isotope stage 5 4 transition and late glacial lithic events bond and lotti 1995 science 267 1005 1010 such as the heinrich ice rafting events are all represented in the core records long term trends and higher frequency changes in isow intensity were reconstructed on the basis of various independent proxy records the long term trends in circulation match theoretical orbitally forced insolation changes our observed links between ice rafted detritus ird input variations in sea surface temperature sst and circulation at greater depth point to the need to re examine the origin of ird events we suggest that these events may have been triggered by enhanced partly sub surface heat transport to the north enhanced northward heat transport may have caused bottom melting of floating outlet glaciers and ice shelves leading to increased iceberg discharge and ice sheet destabilization this discharge resulted in lower sst s and a lower temperature over greenland thus as shown by our records this scenario implies a temporary de coupling of surface processes and circulation at greater depth a key feature is the occurrence of a saw tooth pattern in the marine data which is similar to the greenland ice core records moreover the warming theory of ird events would explain the observed out of phase relationship between the greenland and antarctic ice core records and also the rapid establishment of higher temperatures over greenland immediately after the cold phases stadials of the dansgaard oeschger cycles less

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

millennial scale oceanic climate variability off the western iberian margin during the last two glacial periods high resolution palaeoclimate records recovered from the iberian margin in core md95 2040 exhibit large fluctuations in oceanographic conditions over the last 190 ka large scale cooling of the surface ocean is indicated by the presence of the polar planktonic foraminifer neogloboquadrina pachyderma sinistral and in some instances the occurrence of ice rafted debris ird ice rafting episodes were prevalent in both of the last two glacials with greater intensity in stages 2 through 4 than in stage 6 the six youngest heinrich events are well defined during the last glacial but detrital carbonate is absent from heinrich layers hl6 hl5 and hl3 dansgaard oeschger stadial equivalent sub millennial ird deposition events have been detected in particular during stage 3 allowing a good match with the cooling displayed in the greenland ice core gisp2 sea surface temperature off portugal in stage 6 was in general warmer than during the last glacial pointing towards a weaker southward influence of polar water masses ice rafting occurred mainly in mid mis marine isotope stage 6 between 173 and 153 kyr as a group of poorly differentiated short duration quasi continuous events mainly marked by the high abundance of sinistral n pachyderma differences exist in ird composition relative to the last glacial with a reduced canadian derived detrital carbonate component combined with an important contribution of volcanic particles the lower magnitude and higher frequency of these events suggest that the higher temperatures would have induced iceberg waning closer to the source areas

the fleuve manche the submarine sedimentary features from the outer shelf to the deep sea fans bathymetric seismic and coring surveys have been carried out since the beginning of the 1990s on the outer shelf the slope and the continental rise of the western approaches margin most of the sedimentary features studied belong to a sedimentary system built up since the oligocene and fed by a palaeoriver the fleuve manche that flowed in the english channel during eustatic lowstands a geomorphological map details the spatial distributions of the palaeovalleys on the outer shelf the drainage basin like organised canyons on the slope and the channel levee and lobe complexes on the rise the obvious channel canyon connections and the assessed palaeovalley canyon links demonstrate that the fleuve manche was the single source for terrigeneous fluxes of the armorican deep sea fan when it was one of two sources of the celtic deep sea fan amongst parameters controlling the sedimentation two factors play a major role i subsidence controlled the possibly messinian riverine incision the turbidite activities during lowstands as well as marine infilling of the seawards part of the fleuve manche ii quaternary sedimentary supplies were influenced by the melting ice sheets and the connections with the onshore catchment basins copyright 2003 john wiley sons ltd

the origin of heinrich layers evidence from h2 for european precursor events recent well dated isotopic sr nd fingerprinting of heinrich layer ice rafted detritus ird on the european margin indicates supply from european ice sheets as precursors to laurentide ice sheet lis supply f e grousset et al geology 28 2000 123 126 h snoeckx et al mar geol 158 1999 197 208 these precursorevents lead lis input by up to 1 5 ka f e grousset et al geology 28 2000 123 126 and have been interpreted to indicate lis collapse during heinrichevents stimulated by events originating on the european side of the atlantic f e grousset et al geology 28 2000 123 126 such phasing of ird supply from different sources within heinrich layers therefore has implications for the origin and mechanics of heinrichevents we present evidence here that the ird comprising heinrich layer 2 h2 20 21 14c ka bp on the european continental margin contains detrital campanian upper chalk deriving from bedrock sources eroded on the celtic shelf by the british ice sheet bis in addition to lithic material sourced from the lis high resolution radiocarbon chronology indicates chalk grain deposition as discrete pulses both before and coincident with supply of lis sourced detritus the specificity of the chalk fingerprint to the bis enables a 700 1000 yr lag between the bis and lis events to be identified this phasing indicates a more rapid response of the outlet lobes draining the smaller bis than those draining the lis and implicates external climatic forcing of heinrichevents it is unlikely that this precursorevent represents ird event 18 the recently identified 1 2 ka ird cycle event which immediately precedes h2 because the lag between precursor and main event is here less than 1 5 ka and because such pervasive periodicity is not apparent in european continental margin ird records the later synchroneity between the bis and lis input in h2 identifies glacio eustatic sea level rise associated with lis discharges as a possible feedback mechanism causing destabilisation of ice streams elsewhere during heinrichevents

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

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

african monsoon enhancement during the penultimate glacial period mis 6 5 170 ka and its atmospheric impact 1 a reconstruction of northwest african summer monsoon strength during the cold marine isotopic stage mis 6 indicates a link to the seasonal migration of the intertropical convergence zone itcz high resolution studies of eolian dust supply and sea surface temperature recorded in marine core md03 2705 on the mauritanian margin provide a better understanding about the penultimate glacial history of northwestern african aridity humidity and upwelling coastal activity today site md03 2705 experiences increased upwelling and dust flux during the winter months when the itcz is in a southerly position analyses of foraminifera isotopic composition suggest that during mis 6 5 180 168 ka the average position of the itcz migrated north marked by an increase in the strength of the summer monsoon which decreased eolian dust transport and the coastal upwelling activity the northward migration is in phase with a specific orbital combination of a low precessional index with a high obliquity signal high resolution analysis of stable isotopes 18o and 13c and microscale resolution geochemical ti al and quartz grain counts determinations reveal that the transition between monsoonal humid mis 6 5 and dry mis 6 4 conditions has occurred in less than 1 3 ka such rapid changes suggest a nonlinear link between the african monsoonal rainfall system and environmental changes over the continent this study provides new insights about the influence of vegetation and oceanic temperature feedbacks on the onset of african summer monsoon and demonstrates that during the penultimate glacial period changes in tropical dynamics had regional and global impacts