open system coral ages reveal persistent suborbital sea level cycles sea level is a sensitive index of global climate that has been linked to earth s orbital variations with a minimum periodicity of about 21 000 years although there is ample evidence for climate oscillations that are too frequent to be explained by orbital forcing suborbital frequency sea level change has been difficult to resolve primarily because of problems with uranium thorium coral dating here we use a new approach that corrects coral ages for the frequently observed open system behavior of uranium series nuclides substantially improving the resolution of sea level reconstruction this curve reveals persistent sea level oscillations that are too frequent to be explained exclusively by orbital forcing

relaxation oscillators in concert a framework for climate change at millennial timescales during the late pleistocene 1 using a box model of the north atlantic ocean and a parameterization of heinrich events we suggest that self sustained oscillations of the large scale oceanic circulation provide a framework to accommodate crucial elements of late pleistocene climate variability i dansgaard oeschger style oscillations with varying interstadial length ii synchronization between dansgaard oeschger stadials and heinrich events and iii younger dryas type events the latter result from the restart of the oscillations after a glacial maximum and can be regarded as dansgaard oeschger stadials overprinted by rapidly changing boundary conditions

absolute calibration of the greenland time scale implications for antarctic time scales and for δ14c abstract we propose a new age scale for the two ice cores grip and gisp2 that were drilled at greenland summit based on accelerator mass spectrometry 14c dating of foraminifera in core md95 2042 paleoceanography 15 2000 565 calibrated by means of recently obtained paired 14c and 230th measurements on pristine corals marine radiocarbon calibration curve spanning 10 500 to 50 000 years bp thousand years before present based on paired 230th 234u 238u and 14c dates on pristine corals geological society of america bulletin 2003 submitted for publication the record of core md95 2042 can be correlated very precisely to the greenland ice cores between 30 and 40 ka bp our scale is 1 4 ka older than the grip ss09sea time scale journal of quaternary science 16 2001 299 at the older end of marine isotope stage 3 we use published 230th dates from speleothems to calibrate the record using this scale we show a 14c record that is broadly consistent with the modelled record earth planet sci lett 200 2002 177 and with the data of hughen et al science 303 2004 202 but not consistent with the high values obtained by beck et al science 292 2001 2453 or by voelker et al radiocarbon 40 1998 517 we show how a set of age scales for the antarctic ice cores can be derived that are both fully consistent with the greenland scale and glaciologically reasonable

dansgaard oeschger and heinrich event imprints in alboran sea paleotemperatures past sea surface temperature sst evolution in the alboran sea western mediterranean during the last 50 000 years has been inferred from the study of c37 alkenones in international marine global change studies md952043 core this record has a time resolution of 200 years allowing the study of millennial scale and even shorter climatic changes the observed sst curve displays characteristic sequences of extremely rapid warming and cooling events along the glacial period comparison of this alboran record with 18o from greenland ice greenland ice sheet project 2 core shows a strong parallelism between these sst oscillations and the dansgaard oeschger events five prominent cooling episodes standing out in the sst profile are accompanied by an anomalous high abundance of neogloboquadrina pachyderma sinistral which is confined to the duration of these cold intervals these features and the isotopic record reflect drastic changes in the surface hydrography of the alboran sea in association with heinrich events hl 5

post lgm upward shift of the mediterranean outflow water recorded in a contourite drift off nw spain abstract the nw iberian continental margin is characterised by a complex morphology and by a sedimentary system which was highly dynamic over glacial to interglacial times the sedimentary history of the continental slope was strongly influenced by the interaction of bottom currents with topographic highs of structural origin leading to the accumulation of several sediment drifts a combined analysis of gravity cores from different water depth with hydroacoustic data reveals the vertical behaviour of the upper mediterranean outflow water mow core after the last glacial maximum lgm a coarser grained interval during deglacial and early holocene times 17 2 to 9 9 cal ka bp points to an increase in bottom current strength this increase in velocity was probably related to oceanic density fronts which migrated through the 300 m thick transition zone between the underlying labrador sea water and the overlying mow radiocarbon dates timed the current strengthening to 17 2 cal ka bp and a following weakening of the bottom current to 13 3 cal ka bp at 1965 m water depth and to 9 9 cal ka bp at 1885 m water depth the depth dependent current weakening suggests an upward shifting of the transition zone by 80 m that was related either to an overall shallowing of mow or a vertical contraction of this water mass the upward movement happened over a time interval of approximately 3 4 thousand years in addition sediment core analysis reveals significant lateral heterogeneities within cm to dm thick sediment layers in the contourite drift these heterogeneities suggest a need of a detailed core coverage across current influenced deposits for palaeoceanographic studies to minimize misinterpretations

the tempo of climate change during dansgaard oeschger interstadials and its potential to affect the manifestation of the 1470 year climate cycle 1 the oxygen isotope record from the gisp2 ice core serves a air temperature proxy to estimate cooling rates during dansgaard oeschger interstadials between 15 53 thousand years before present the duration of individual dansgaard oeschger interstadials scales inversely with the cooling rate and high cooling rates are associated with large continental ice volume and vice versa cooling rates determine the extent to which the postulated 1470 year pacing cycle of the dansgaard oeschger interstadials is visible in paleoclimatological records only cooling rates associated with intermediate ice volume approx 84 20 m relative sea level equivalent allow the climate system to respond to succeeding trigger events of the 1470 year cycle and result in stadials interstadials of nearly the same duration

exceptional fossil preservation during co2 greenhouse crises abstract exceptional fossil preservation may require not only exceptional places but exceptional times as demonstrated here by two distinct types of analysis first irregular stratigraphic spacing of horizons yielding articulated triassic fishes and cambrian trilobites is highly correlated in sequences in different parts of the world as if there were short temporal intervals of exceptional preservation globally second compilations of ages of well dated fossil localities show spikes of abundance which coincide with stage boundaries mass extinctions oceanic anoxic events carbon isotope anomalies spikes of high atmospheric carbon dioxide and transient warm wet paleoclimates exceptional fossil preservation may have been promoted during unusual times comparable with the present co 2 greenhouse crises of expanding marine dead zones oceanic acidification coral bleaching wetland eutrophication sea level rise ice cap melting and biotic invasions

deepwater carbonate deposition in response to re flooding of carbonate bank and atoll tops at glacial terminations the late quaternary has experienced large glacial interglacial climatic variations and related 10 s to 100 meters high amplitude sea level fluctuations at milankovich frequencies from 10 s to 100 kyr during which carbonate platform tops have been exposed and re flooded in many occasions this study focuses on the accumulation of calci turbidites the aragonite onset sharp increase in fine sediments and their timing in deep basins adjacent to carbonate platforms a particular emphasis is developed on the occurrence of the first gravity flow event and aragonite onset sharp increase and their linkage to the initial re flooding of the platform tops during deglaciations three basins adjacent to isolated platforms in the bahamas the northern nicaragua rise and the gulf of papua were selected to represent pure carbonate versus mixed systems in quiescent versus tectonically active settings and various carbonate bank top morphologies ranging from atoll to relatively deeply and narrowly flooded flat top banks in spite of these differences each record illustrates a clear relationship between the timing of platform top re flooding and initiation of significant carbonate export by gravity flows and low density plumes into the surrounding basins the concept of re flooding window is introduced to characterize the prolific period of time during which bank and atoll tops are flooded enough to produce large export of bank derived aragonite and of calci turbidites in adjacent basins according to our datasets the main re flooding windows have occurred mainly on the last part of the sea level rise at each glacial termination t those periods being marked by some of the highest rates of sea level rise the analysis of a long piston core from the earthquakes prone walton basin northern nicaragua rise demonstrates that sea level not seismic activities played a major role as trigger mechanism for the initiation of gravity flows since the last four glacial interglacial transitions and supports the existence of an extra glacial termination during the early portion of the transition from mis 7 4 to mis 7 3 tiiia

sea level and climatic controls on turbidite occurrence for the past 26 kyr on the flank of the gaoping canyon off sw taiwan submarine canyons are major conduits for delivery of sediments originating from the continental margin into the deep sea here we analyze the effects of changing sea levels and climate e g sediment supply on the frequency of turbidites over the last 26 kyr our study is based on high resolution age controls for turbidites in core md178 10 3291 from a water depth of 2070 m on the flank of the gaoping canyon offshore in sw taiwan unlike most other submarine canyons worldwide the head of the gaoping canyon has remained connected to the river mouth during the recent flooding of the continental shelf our results reveal that turbidity currents are less frequent during periods of sea level lowstand in contrast turbidity currents have been more frequent since 12 kyr bp during the final stage of rising sea level and sea level highstands this may be the result of increased terrestrial sediment delivery due to enhanced rainfall intensity moreover comparing to other source to sink systems the turbidite occurrence in the flank of gaoping canyon through the last glacial cycle might have been more strongly influenced by climatic changes due to short sediment storage response time

from source to sink a new perspective on the past dynamics of the murray canyon group from benthic foraminiferal communities abstract we present fossil benthic foraminiferal assemblage data from marine sediment core ss02 06 gc2 located in the abyssal plain of the murray canyon group offshore south australia the sedimentological characteristics indicate the presence of turbidite deposits showing classical bouma like sequences dated between 40 and 12 cal ka bp these results confirm the previous interpretation of the observed large deep water holes in the abyssal area where the core was sampled as being gouged by surges of high energy turbidity currents the presence of good indicator taxa and unique assemblages occupying specific bathymetric depths allows the determination of the source origin of the sediments making the turbidites three distinct faunal groups are found 1 mostly shelf species 2 mostly bathyal species and 3 mostly abyssal species in the sediment core these groups present a quasi systematic succession with nearly all bouma like sequences starting with the dominance of bathyal species in the coarse grained base followed by the dominance of shallow species in the silty part and finally with abyssal species in the clays to explain such phenomena turbidites triggered by mixed hyperpycnal hypopycnal flow processes and turbidity currents during periods of river floods are considered for the first time within the murray canyon group they are mostly related to periods of increased fluvial discharges during wet phases in the murray darling basin

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

changes in north atlantic deep water formation associated with the dansgaard oeschger temperature oscillations 60 10 ka we closely comparedhigh resolution d 13 c records of benthic foraminifera cibicides wuellerstorfi a proxy for deep water ventilation with the dansgaard oeschger temperature oscillations our results reveal different perturbations of deep water formation in the north atlantic ocean associated with the millennial scale climate oscillations during the last glacial period the cooling episodes associated with the drastic heinrich events are related to large reductions of deep water formation and a northward migration of 13 c depleted southern source deep waters to 621n in the north atlantic ocean the inter heinrich events which are correlatedto the other coldstad ials are markedby significant changes of sea surface temperature aroundthe rockall plateau variations in the flux of icebergs to the north atlantic ocean but are not associated with such important reduction of deep water formation if changes in the thermohaline circulation thc are associatedwith these millennial scale climatic oscillation they affect only the deeper water masses below 2000 m of the north atlantic ocean we thus show that equivalent degree of cooling over greenland is obtained with different perturbations of deep water formation our results either question the role of the thc as the unique explanation for these millennial scale climate oscillations or call upon an amplifying mechanism not yet taken into account r 2002 elsevier science ltd all rights reserved

time integrated 3d approach of late quaternary sediment depocenter migration in the tagus depositional system from river valley to abyssal plain quantification of sediment volumes in continental to deep ocean basins is key to understanding processes of sediment distribution in source to sink depositional systems using our own and published data we present the first quantification of sediment volume changes in basins along the course of a major southwest european river during the deglaciation the salient points of this quantitative record in the tagus and equivalent north atlantic basins show crucial roles for sea level climate and land use in the distribution of sediments the bypass of sediments starved the tagus basins and subsequently sedimentation mainly occurred on the tagus abyssal plain during the sea level lowstand of the last glacial maximum the main sediment depocenter rapidly shifted via the continental shelf to the lower tagus valley during sea level rise in the deglaciation period finally the main sediment depocenter shifted further landward into the lower tagus valley during sea level high stand in the holocene during the high stand phase last 7 ky sediment flux increased up to 2 5 times due to climate and land use changes the average catchment denudation rate during the last 12 ky 0 04 0 1 mm y is in agreement with those of other european catchments our study clearly demonstrates the added value of detailed knowledge of 3d depocenter distribution size and chronology this allowed us to identify an increased sediment flux during the last 7 ky which was not identified using local observations from boreholes alone the uniqueness of the tagus depositional system lies in the combination of a large accommodation space in the bedrock confined lower tagus valley the steep lowstand surface gradient and the narrow continental shelf with canyons indenting the shelf break

coupling of the hemispheres in observations and simulations of glacial climate change abstract we combine reconstructions climate model simulations and a conceptual model of glacial climate change on millennial time scales to examine the relation between the high latitudes of both hemispheres a lead lag analysis of synchronised proxy records indicates that temperature changes in greenland preceded changes of the opposite sign in antarctica by 400 500 yr a composite record of the dansgaard oeschger events shows that rapid warming cooling in greenland was followed by a slow cooling warming phase in antarctica the amplitudes rates of change and time lag of the interhemispheric temperature changes found in the reconstructions are in excellent agreement with climate model simulations in which the formation of north atlantic deep water is perturbed the simulated time lag between high northern and southern latitudes is mainly determined by the slow meridional propagation of the signal in the southern ocean our climate model simulations also show that increased deep water formation in the north atlantic leads to a reduction of the antarctic circumpolar current through diminishing meridional density gradients in the southern ocean we construct a simple conceptual model of interhemispheric dansgaard oeschger oscillations this model explains major features of the recorded temperature changes in antarctica as well as the general shape of the north south phase relation found in the observations including a broad peak of positive correlations for a lead of antarctica over greenland by 1000 2000 yr the existence of this peak is due to the regularity of the oscillations and does not imply a southern hemisphere trigger mechanism contrary to previous suggestions our findings thus further emphasise the role of the thermohaline circulation in millennial scale climate variability

the last glacial interglacial transition lgit in the western mid latitudes of the north atlantic abrupt sea surface temperature change and sea level implications abstract high resolution reconstructions of sea surface temperature uk 37 sst coccolithophore associations and continental input total organic carbon higher plant n alkanes n alkan 1 ols in core d13882 from the shallow tagus mud patch are compared to sst records from deep sea core md03 2699 and other western iberian margin cores results reveal millennial scale climate variability over the last deglaciation in particular during the lgit in the iberian margin heinrich event 1 h1 and the younger dryas yd represent two extreme episodes of cold sea surface condition separated by a marine warm phase that coincides with the bolling allerod interval b a on the neighboring continent following the yd event an abrupt sea surface warming marks the beginning of the holocene in this region ssts recorded in core d13882 changed however faster than those at deep sea site md03 2699 and at the other available palaeoclimate sequences from the region while the sst values from most deep sea cores reflect the latitudinal gradient detected in the iberian peninsula atmospheric temperature proxies during h1 and the b a the tagus mud patch core d13882 experienced colder ssts during both events this is most certainly related to a supplementary input of cold freshwater from the continent to the tagus mud patch a hypothesis supported by the high contents of terrigenous biomarkers and total organic carbon as well as by the dominance of tetra unsaturated alkenone c37 4 observed at this site the comparison of all western iberia sst records suggests that the sst increase that characterizes the b a event in this region started 1000 yr before meltwater pulse 1a mwp 1a and reached its maximum values during or slightly after this episode of substantial sea level rise in contrast during the yd holocene transition the sharp sst rise in the tagus mud patch is synchronous with meltwater pulse 1b the decrease of continental input to the mud patch confirms a sea level rise in the region thus the synchronism between the maximum warming in the mid latitudes off the western iberian margin the adjacent landmasses and greenland indicates that mwp 1b and the associated sea level rise probably initiated in the northern hemisphere rather than in the south

rapid fluctuations in the deep north atlantic heat budget during the last glacial period 1 our understanding of past ocean climate interactions has been hampered by a paucity of proxy reconstructions of physical ocean circulation parameters here we present the first detailed record of deepwater temperature variability that resolves dansgaard oeschger climate fluctuations during the last glaciation results indicate a positive coupling between abrupt temperature changes over greenland and in the deep north atlantic that was mediated by perturbations to the atlantic overturning circulation the occurrence of sharp warming events at the center of the heinrich stadials 4 and 5 further indicates that the response of the ocean interior was less static across these major climate events than has been previously apparent

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

rapid rise of sea level 19 000 years ago and its global implications evidence from the irish sea basin supports the existence of an abrupt rise in sea level meltwater pulse at 19 000 years before the present b p climate records indicate a large reduction in the strength of north atlantic deep water formation and attendant cooling of the north atlantic at this time indicating a source of the meltwater pulse from one or more northern hemisphere ice sheets warming of the tropical atlantic and pacific oceans and the southern hemisphere also began at 19 000 years b p these responses identify mechanisms responsible for the propagation of deglacial climate signals to the southern hemisphere and tropics while maintaining a cold climate in the northern hemisphere

an ams radiocarbon method to determine the emplacement time of recent deep sea turbidites abstract this paper reports the evaluation of a novel method based on accelerator mass spectrometer ams radiocarbon dating of planktonic foraminifera to evaluate the times of emplacement of recent deep sea turbidites it assumes that the standing stock of relatively large 150 m planktonic foraminifera in the bioturbated surface mixed layer of box cores represents an integration of the flux from the water column since the turbidite was emplaced and exploits the potential of the ams method to analyse the resultant small samples which can be readily separated the method is evaluated by application to three atlantic turbidites which form the present day sediment surfaces on the horseshoe madeira and tagus abyssal plains the time of emplacement of madeira abyssal plain turbidite is estimated at 1120 730 and 870 years before present each 120 140 years in three different cores the resultant weighted mean emplacement time 930 76 years before present is somewhat older than previous estimates by geochemical methods the times of emplacement for the horseshoe 240 and 140 years before present each 120 years and tagus 300 120 years before present abyssal plains are both consistent with initiation by the lisbon earthquake of 1755

pleistocene δ13c variability of north atlantic intermediate water variations in the stable isotopic composition of benthic foraminifera from deep sea drilling project dsdp site 502b in the caribbean sea are used to reconstruct atlantic intermediate water circulation variability over the last 1 2 m y comparison of this record with other north atlantic benthic isotope records indicates that atlantic intermediate water circulation was relatively enhanced during glacial maxima when north atlantic deep water nadw production was reduced however a simple compensatory relationship between intermediate and deepwater circulation is not apparent geochemical models have shown that such changes in ocean circulation can affect atmospheric co2 levels by changing vertical nutrient and alkalinity profiles the 13c difference between caribbean site 502b and deep equatorial pacific site 677 is highly coherent and in phase with ice volume like the 18o record there is an increase in amplitude 40 and a large increase in 100 kyr power after 0 7 ma the 1 2 13c amplitude scales to 70 ppm v in atmospheric co2 using boyle s 1986 box model result the implied increase in co2 amplitude after 0 7 ma may suggest a positive feedback role in effecting the higher amplitude climatic fluctuations which characterize the last 0 7 m y

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

shallow marine sediment cores record climate variability and earthquake activity off lisbon portugal for the last 2000 years abstract sea surface temperature sst river discharge and biological productivity have been reconstructed from a multi proxy study of a high temporal resolution sedimentary sequence recovered from the tagus deposition center off lisbon portugal for the last 2000 years sst shows 2 c variability on a century scale that allows the identification of the medieval warm period mwp and the little ice age lia high iron fe and fine sediment deposition accompanied by high n alkane concentrations and presence of freshwater diatoms during the lia 1300 1900 ad science 292 2001 662 suggest augmented river discharge whereas higher total alkenone concentrations point to increased river induced productivity during the mwp 550 1300 ad science 292 2001 662 larger mean grain size and low values of magnetic susceptibility and concentrations of fe n alkanes and n alcohols are interpreted to reflect decreased runoff at the same time increased benthic and planktonic foraminifera abundances and presence of upwelling related diatoms point to increased oceanic productivity on the basis of the excellent match found between the negative phases of the north atlantic oscillation nao index and the intensified tagus river discharge observed for the last century it is hypothesized that the increased influx of terrigenous material during the lia reflects a negative nao like state or the occurrence of frequent extreme nao minima during the milder few centuries of the mwp stronger coastal upwelling conditions are attributed to a persistent positive nao like state or the frequent occurrence of extreme nao maxima the peak in magnetic susceptibility centered at 90 cm composite core depth ccd is interpreted as the result of the well known 1755 ad lisbon earthquake the lisbon earthquake and accompanying tsunami are estimated to have caused the loss of 39 cm of sediment 355 years of record most of the lia and the instantaneous deposition of a 19 cm sediment bed

coupled climate and sea level changes deduced from huon peninsula coral terraces of the last ice age abstract huon peninsula papua new guinea is a tectonically unstable uplifting shoreline ringed by emergent coral terraces the terraces were formed during episodes of rapid sea level rise when corals constructed large discrete coral platforms that were subsequently uplifted uranium series ages of four prominent huon peninsula last glacial ois 3 coral terraces coincide with the timing of major north atlantic climate reversals at intervals of 6000 7000 yr between 30 000 yr and 60 000 yr ago terrace elevations when combined with uplift indicate 10 15 m high sea level excursions at these times we attribute the growth of the terraces directly to sea level rises arising from ice calving episodes from major north atlantic ice sheets and the antarctic ice sheet that precipitated extremes of cold climate called heinrich events these periods are associated with major discharges of land based ice and enhanced concentrations of ice rafted debris in deep sea cores sea levels at this time were 60 90 m lower than present

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

recurrent decadal scale dust events over holocene western africa and their control on canyon turbidite activity mauritania abstract sediment records from continental shelves and slopes might provide paleoenvironmental information in the highest temporal resolution but are often hampered due to strong erosional and reworking processes here we present a holocene sedimentary record from an exceptional shelf mud belt depocenter off northern mauritania compared to a second sediment core located inside a large canyon system at the adjacent continental slope both records are of outstandingly continuous and highest temporal resolution 9 a cm and are investigated by sedimentological and geochemical methods a series of sharply defined recurrent dust peaks is preserved in the shelf archive each event has lasted for a single decade only and seems to coincide with an individual turbidite bed in the canyon a joint mechanism should thus be responsible for both of these deposits and we suggest a regional atmospheric trigger only short lasting trade wind strengthening would cause such pronounced aridity over western saharan africa the effect would be massive dust export to shelf and slope recently developed high resolution aridity and humidity records from western africa support the existence of these events over the holocene and identify them as being controlled by the atlantic system as far south as 19 n

identifying instrumental and historical earthquake records in the sw iberian margin using 210pb turbidite chronology 1 textural mineralogical and geochemical analyses of near surface sediments from six different stations offshore sw iberian peninsula revealed the presence of thin detrital intervals corresponding to very fine grained turbidites precise dating of the two most recent sedimentary layers based on 210pb and 137cs geochronology provides ages of 1971 3 ad and 1908 8 ad these ages correspond to high magnitude historical and instrumental earthquakes that occurred in the region the 1969 horseshoe earthquake mw 8 0 and the 1909 benavente earthquake mw 6 0 such a good correlation between turbidites and high magnitude mw 6 historical and instrumental seismic events suggests that the record of mass transport deposits can also be used as a paleoseismic indicator in low convergence margins

quaternary surface water stable isotope signal from calcareous nannofossils at dsdp site 593 southern tasman sea oxygen isotope values from calcareous nannofossils in four cores spanning the quaternary from dsdp site 593 in tasman sea are compared with the 18o signal of planktonic and benthic foraminifers from the same samples the classic mid late quaternary isotope stages are exhibited with stage 12 particularly well developed when 18o values of nannofossils are adjusted for coccolithophore vital effects they indicate larger by 1 6 c surface to bottom paleotemperature gradients and greater by 1 3 c changes in mean sea surface temperature between full glacial and interglacial conditions than do 18o values from planktonic foraminifers along with the foraminifers the nannofossils record a bimodal distribution of 18o between the early and mid late quaternary indicating a significant change in global ice budget the 13c of nannofossils also shows a bimodal distribution but is opposite to that for the foraminifers nannofossil 18o values record a shift of 0 8 at isotope stage 8 corresponding to a major reduction in abundance of the previously dominant gephyrocapsids a shift in 13c of 1 5 also occurs at stage 8 and a shift in 13c of 1 2 at around stage 14 the 18o shift in nannofossils is at least a pacific wide phenomenon the 13c shifts are possibly global the 13c signal of nannofossils exhibits an antipathetic relationship to that of benthic foraminifers back to isotope stage 18 but no significant correlation beyond this level to the base of the quaternary this is interpreted as reflecting local productivity dominating global influences on 13c since stage 18 at dsdp site 593 the difference between nannofossil and benthic foraminifer 13c signals 13c tends to be maximum during glacial stages and minimum during interglacials throughout the section showing a strong correlation with the nannofossil 18o signal the increased partitioning of 13c between surface and bottom waters during the glacial periods may indicate heightened productivity in surface waters in the southern tasman sea at these times

meltwater pulse 1a from antarctica as a trigger of the bolling allerod warm interval meltwater pulse 1a mwp 1a was a prominent feature of the last deglaciation which led to a sea level rise of 20 meters in less than 500 years concurrent with mwp 1a was the onset of the bolling allerod interstadial event 14 600 years before the present which marked the termination of the last glacial period previous studies have been unable to reconcile a warm northern hemisphere with mwp 1a originating from the laurentide or fennoscandian ice sheets with the use of a climate model of intermediate complexity we demonstrate that with mwp 1a originating from the antarctic ice sheet consistent with recent sea level fingerprinting inferences the strength of north atlantic deep water nadw formation increases thereby warming the north atlantic region and providing an explanation for the onset of the bolling allerod warm interval the established mode of active nadw formation is then able to respond to subsequent freshwater forcing from the laurentide and fennoscandian ice sheets setting the stage for the younger dryas cold period the melting of continental ice sheets during the last deglaciation provided a freshwater source to the ocean that affected global sea level and the strength of the thermohaline circulation an exceptionally large melting event inferred from far field relative sea level records occurred

on the age of the laschamp geomagnetic excursion abstract the laschamp geomagnetic excursion is a critical tie point found directly in deep sea sediment cores and revealed in polar ice as an abrupt change in the rate of cosmogenic nuclide flux despite the importance of this excursion to quantifying paleoclimate proxy records archived in sediment and ice and to providing an independent calibration of the radiocarbon calendar its timing remains poorly known previous k ar 40 ar 39 ar and u th isochron determinations from lava flows at the type locality in the massif central france vary widely are imprecise and suggest a mean age of about 46 2 2 5 ka 2 results of 6 new unspiked k ar and 13 40 ar 39 ar incremental heating experiments on subsamples from three sites on the laschamp and olby flows are concordant and give a weighted mean age of 40 4 1 1 ka 2 uncertainty including analytical sources only that is 10 younger than the previous estimates considering that the 40 k 40 ar decay constant is not known to a precision better than 2 4 the most probable radioisotopic age for the laschamp excursion is 40 4 2 0 ka 2 analytical plus decay constant uncertainties this new age for the laschamp excursion agrees precisely with that deduced from the napis 75 deep sea sediment paleointensity stack when calibrated against the gisp2 ice core chronology using the o isotopes in ice and the magnetic properties of the marine cores

rapid climatic variations during marine isotopic stage 3 magnetic analysis of sediments from nordic seas and north atlantic abstract the bulk magnetic parameters of seven deep sea cores distributed from the nordic seas 67 n to the north atlantic as far south as the bermuda rise 33 n exhibit short term variations which correlate with rapid climatic changes during marine isotopic stage 3 mis3 the magnetic mineralogy is uniformly dominated by well sorted low ti content magnetites indicating that these variations are due to variations in the relative amount of magnetic minerals because the magnetic minerals predominantly originate from one common source area the nordic basaltic province these changes arise from changes in the efficiency of the transport of the magnetic particles by deep currents from the source to the site of deposition these results therefore show that the fast climatic changes are related to coeval fast changes in the strength of the deep sea circulation the latter was active reduced during the interstadials stadials and heinrich events transporting the magnetic particles from the norwegian sea into the north atlantic ocean along a path similar to the present path of the nadw it is tentatively suggested that the faeroe shetland channel and the denmark strait were the only two active paths for the overflow water during mis3 the presence of magnetic oscillations in the bermuda rise core in phase with those from the north atlantic indicates that the activity of the southern newfoundland basin gyre was linked to that of the nadw during mis3

14c activity and global carbon cycle changes over the past 50 000 years a series of 14c measurements in ocean drilling program cores from the tropical cariaco basin which have been correlated to the annual layer counted chronology for the greenland ice sheet project 2 gisp2 ice core provides a high resolution calibration of the radiocarbon time scale back to 50 000 years before the present independent radiometric dating of events correlated to gisp2 suggests that the calibration is accurate reconstructed 14c activities varied substantially during the last glacial period including sharp peaks synchronous with the laschamp and mono lake geomagnetic field intensity minimal and cosmogenic nuclide peaks in ice cores and marine sediments simulations with a geochemical box model suggest that much of the variability can be explained by geomagnetically modulated changes in 14c production rate together with plausible changes in deep ocean ventilation and the global carbon cycle during glaciation

phase relationships between millennial scale events 64 000 24 000 years ago a core recovered on the iberian margin off southern portugal can be correlated with greenland ice cores using oxygen isotope variability in planktonic foraminifera which closely matches the ice core records of temperature over greenland our age model identifies the base of every interstadial between 64 000 and 24 000 years ago and uses the greenland ice core project grip timescale the oxygen isotope signal in benthic foraminifera on this grip based timescale is quite different from the planktonic record and resembles the temperature record over antarctica when this is synchronized with greenland using the record of methane in the atmospheric air in the polar ice cores we interpret the benthic record as indicating significant fluctuations in ice volume during millennial events and we suggest that antarctic temperature changed as a function of ice volume

geomagnetic moment variation and paleomagnetic excursions since 400 kyr bp a stacked record from sedimentary sequences of the portuguese margin abstract a paleomagnetic study was performed in clayey carbonate sedimentary sequences deposited during the last 400 kyr on the portuguese margin northeast atlantic ocean declination and inclination of the stable remanent magnetization present recurrent deviations from the mean geomagnetic field direction the normalized intensity documents a series of relative paleointensity rpi lows recognized in other reference records three directional anomalies occurring during rpi lows chronologically correspond to the laschamp excursion 42 kyr bp the blake event 115 122 kyr bp and the icelandic basin excursion 190 kyr bp a fourth directional anomaly recorded at 290 kyr bp during another rpi low defines the portuguese margin excursion four non excursional rpi lows are recorded at the ages of the jamaica pringle falls mamaku calabrian ridge 1 and levantine excursions the rpi record is characterized by a periodicity of 100 kyr paleointensity lows often coinciding with the end of interglacial stages this record sets the basis of the construction of an authigenic 10be 9be record from the same sedimentary sequences carcaillet et al this issue

calcium carbonate preservation patterns in the central north atlantic during the last 150 000 years abstract the dissolution record of the canary and eastern north american basins was examined in 15 cores spanning the last 150 000 years stratigraphic control was based on fluctuations in percent carbonate and 18 0 16 0 dissolution changes were determined based on variations in percent planktonic foraminiferal fragments and benthonic foraminifera results indicate three main dissolution events coincident with 18 0 stages 2 4 and 6 the lysocline apparently shallowed about 300 m in the canary basin at these times there is no simple linear relationship between dissolution intensity and ice volume dissolution was more intense during stage 4 than during stage 2 even though 18 0 records indicate the reverse is true for ice volume also carbonate preservation during the lower part of stage 3 is comparable to stages 1 and 5e a comparison of atlantic pacific carbonate dissolution patterns indicates a tendency for out of phase preservation patterns in 18 0 stages 1 and 2 and an in phase dissolution event during stage 4 the out of phase pattern may partially reflect variations in the production rate of north atlantic deep water the in phase dissolution peak correlates with a major cooling regression transport of organic carbon from transient shallow reservoirs into the deep sea could have contributed to formation of such a pattern however additional evidence from longer atlantic pacific records is needed before the shelf basin model can be more fully evaluated

glacial water mass geometry and the distribution of δ13c of σco2 in the western atlantic ocean 1 oxygen and carbon isotopic data were produced on the benthic foraminiferal taxa cibicidoides and planulina from 25 new piston cores gravity cores and multicores from the brazil margin the cores span water depths from about 400 to 3000 m and intersect the major water masses in this region these new data fill a critical gap in the south atlantic ocean and provide the motivation for updating the classic glacial western atlantic 13c transect of duplessy et al 1988 the distribution of 13c of co2 requires the presence of three distinct water masses in the glacial atlantic ocean a shallow 1000 m southern source water mass with an end member 13c value of about 0 3 0 5 vpdb a middepth 1500 m northern source water mass with an end member value of about 1 5 and a deep 2000 m southern source water with an end member value of less than 0 2 and perhaps as low as the 0 9 values observed in the south atlantic sector of the southern ocean ninnemann and charles 2002 the origins of the water masses are supported by the meridional gradients in benthic foraminiferal 18o a revised glacial section of deep water 13c documents the positions and gradients among these end member intermediate and deep water masses the large property gradients in the presence of strong vertical mixing can only be maintained by a vigorous overturning circulation

late quaternary turbidite emplacement on the horseshoe abyssal plain iberian margin abstract the horseshoe abyssal plain hap is located on the iberian margin at approximately 4800 m water depth and is confined within topographic elevations with a relief of about 3000 m with the exception of gorringe bank which rises to only 20 m below sealevel the plain is built up by an alternation of turbidites of the order of meter thick beds numbered h2 h3 h8 and h13 contrasting with sets of thinner beds of the order of decimeters thick h4 5 6 7 and h9 10 11 12 and pelagites centimeters in thickness an intensive study of eight piston cores using visual observation color and thickness relative stratigraphic position of units magnetic susceptibility logs calcium carbonate content and mineralogy of turbid te bases concludes in a bed by bed correlation of all individual turbidites the major source of terrigenous material feeding the hap is the sao vicente canyon which incises the portuguese shelf while minor sources are the surrounding seamounts the elongate geometry of the abyssal plain with its single dominant source of sediments produced laterally continuous deposits from both large and small flows these covered the entire 356 km length of the plain grain size analysis of the four thickest turbidites h2 h3 h8 and h9 demonstrates only slight downcurrent fining and vertical grading expressed best in the ratio of coarse silt to fine silt plus clay a small amount of sand is carried the length of the plain the thicker units all show the same pattern of thickness with a maximum in the middle of the plain around a topographic constriction and bend this is most plausibly explained as due to reduction in flow speed of an initially supercritical flow causing enhanced deposition some of the beds appear to have a double coarse layer in the base which may indicate partial reflection of flows from the side of the basin it is suggested by application of equations for flow behavior that both thick 3 m and thin 0 3 m beds are due to supercritical flows a few tens of meters high however the thick beds resulted from high concentration flows cv 4 by volume whereas thinner beds require low concentration cv 1 to run out over the full length of the basin the stratigraphy is tied into the dated oceanic pelagic record by analysis of foraminifera in the pelagic layers above the turbidites and through recognition of two heinrich layers h 1 and h 2 ages 14 3 and 21 ka the resulting age framework shows higher turbidite frequency in the glacial 2 7 kyr than interglacial holocene 1 0 kyr this also gives higher mass flux during the glacial emplacement of turbidites cannot be clearly related to sea level changes but may well be due to seismic activity however one of the largest earthquakes in human experience lisbon in 1755 triggered only a thin turbidite invalidating the term seismite for thick turbidites

southern ocean origin for the resumption of atlantic thermohaline circulation during deglaciation during the two most recent deglaciations the southern hemisphere warmed before greenland at the same time the northern atlantic ocean was exposed to meltwater discharge which is generally assumed to reduce the formation of north atlantic deep water yet during deglaciation the atlantic thermohaline circulation became more vigorous in the transition from a weak glacial to a strong interglacial mode here we use a three dimensional ocean circulation model to investigate the impact of southern ocean warming and the associated sea ice retreat on the atlantic thermohaline circulation we find that a gradual warming in the southern ocean during deglaciation induces an abrupt resumption of the interglacial mode of the thermohaline circulation triggered by increased mass transport into the atlantic ocean via the warm indian ocean and cold pacific ocean water route this effect prevails over the influence of meltwater discharge which would oppose a strengthening of the thermohaline circulation a southern ocean trigger for the transition into an interglacial mode of circulation provides a consistent picture of southern and northern hemispheric climate change at times of deglaciation in agreement with the available proxy records

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

sediment instability on the portuguese continental margin under abrupt glacial climate changes last 60 kyr abstract it is well established that orbital scale sea level changes generated larger transport of sediments into the deep sea during the last glacial maximum than the holocene however the response of sedimentary processes to abrupt millennial scale climate variability is rather unknown frequency of distal turbidites and amounts of advected detrital carbonate are estimated off the lisbon setubal canyons core md03 2698 at 4602 mwd within a chronostratigraphy based on radiometric ages oxygen isotopes and paleomagnetic key global anomalies we found that 1 higher frequency of turbidites concurred with northern hemisphere coldest temperatures greenland stadials gs including heinrich h events but more than that an escalating frequency of turbidites starts with the onset of global sea level rising and warming in antarctica and culminates during h events at the time when rising is still in its early mid stage and the atlantic meridional overturning circulation amoc is re starting this short time span coincides with maximum gradients of ocean surface and bottom temperatures between gs and antarctic warmings antarctic isotope maximum aim 17 14 12 8 4 2 and rapid sea level rises 2 trigger of turbidity currents is not the only sedimentary process responding to millennial variability land detrital carbonate with a very negative bulk 18o signature enters the deep sea by density driven slope lateral advection accordingly during gs 3 possible mechanisms to create slope instability on the portuguese continental margin are sea level variations as small as 20 m and slope friction by rapid deep and intermediate re accommodation of water masses circulation 4 common forcing mechanisms appear to drive slope instability at both millennial and orbital scales

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

regional and global benthic δ18o stacks for the last glacial cycle although detailed age models exist for some marine sediment records of the last glacial cycle 0 150 ka age models for many cores rely on the stratigraphic correlation of benthic 18o which measures ice volume and deep ocean temperature change the large amount of data available for the last glacial cycle offers the opportunity to improve upon previous benthic 18o compilations such as the lr04 global stack not only are the age constraints for the lr04 stack now outdated but a single global alignment target neglects regional differences of several thousand years in the timing of benthic 18o change during glacial terminations here we present regional stacks that characterize mean benthic 18o change for 8 ocean regions and a volume weighted global stack of data from 263 cores age models for these stacks are based on radiocarbon data from 0 to 40 ka correlation to a layer counted greenland ice core from 40 to 56 ka and correlation to radiometrically dated speleothems from 56 to 150 ka the regional 18o stacks offer better stratigraphic alignment targets than the lr04 global stack and furthermore suggest that the lr04 stack is biased 1 2 kyr too young throughout the pleistocene finally we compare global and regional benthic 18o responses with sea level estimates for the last glacial cycle

south dials north climate is greatly influenced by ocean circulation in the north atlantic but warming episodes as glacial conditions turned into interglacials may have been triggered by events far to the south

palaeocurrent reconstruction sediment and thorium focussing on the iberian margin over the last 140 ka sedimentological data in particular sediment grain size from core md95 2040 2465 m water depth and a nearby core omii 9k 1833 m water depth presently under the influence of lower north atlantic deep water and deep intermediate water respectively have been used to reconstruct changes in the vigour of near bottom water flow on the iberian margin the pattern of the palaeocurrent index indicates faster flow during the warm periods and slower in cold this also holds in the younger dryas but there is an added effect of brief collapse of the circulation in termination 1b due to meltwater input the cold periods are generally characterised by elevated clay percentage terrigenous material 2 m diameter and overall sediment accumulation fluxes in comparison warm periods of higher flow have lower depositional flux and a tendency to silt terrigenous material 2 63 m dominance consistent with the higher flow speed and suppression of clay deposition this is in contrast with a recent 230thxs based study of two cores md95 2040 and 2039 j thomson et al earth planet sci lett 165 1999 255 270 which concluded that increased glacial sediment and th focussing was caused by stronger currents the crucial factor is that the controlling speed is that of the flow at the depositional site rather than that of the erosional or transporting flow to the site high glacial maximum fluxes of carbonate 6 g cm 2 ka 1 are largely composed of fine 63 m non coccolith material suggesting a significant detrital component it is proposed that the sediment and th arrive on the lower slope via different routes sediment down slope in bottom nepheloid layers and turbidity currents th scavenged from the water column by organic matter and are rapidly mixed on the bed removal of this material from a large area and its deposition in a small one adjacent to oporto seamount together with a small amount of direct down slope supply accounts for the properties of the deposits allowance for erroneous fluxes due to possible coring expansion stretching of the upper part of md95 2040 does not invalidate these conclusions

climatic and oceanographic changes in the northeast atlantic reflected by magnetic properties of sediments deposited on the portuguese margin during the last 340 ka abstract rock magnetic parameters measured along two giant piston cores md95 2040 40 34 n 9 51 w and md95 2042 37 47 n 10 09 w collected off the portuguese margin related to other proxy climatic data have been used to reconstruct magnetic mineralogical changes of in relation to environmental and climatic conditions over the north atlantic western europe and northwest africa during the last three climatic cycles since isotope stage 10 thin discrete layers containing coarse grains of titano magnetite are associated with events of iceberg discharge during heinrich events 1 6 heinrich quat res 29 1988 142 that have equivalent events in isotope stages 5 8 concentrations of fine grained ti magnetite and hematite goethite varying in phase opposition are directly linked with alternations of cold and warm climatic periods spectral analyses of the rock magnetic signals reveal milankovitch periods at 100 and 41 ka confirming the relationship with long term climatic changes at high latitudes the nature ti magnetite and size range of the finest ferrimagnetic fraction as well as its variation suggest a control by deep currents carrying a colloidal clayey fraction from remote sources iceland faeroes mid atlantic ridge variation of hematite goethite contents is linked with transport by rivers and winds from the neighbouring continent a tight correlation with the d o cycles in greenland ice records confirms that north atlantic oceanic regimes and continental wind regimes were strongly influenced by millennial scale climatic changes throughout the last 350 ka

paleocean circulation during the last deglaciation a bipolar seesaw hughen et al 1998 have documented that during the first 200 years of younger dryas time the 14c content of atmospheric co2 increased by 50 and that during the remainder of this 1200 year duration cold event it steadily declined the initial increase in 14c c was likely the result of a reduction in the atlantic s conveyor circulation however were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump then it is difficult to understand why the climate conditions in the northern atlantic basin remained cold throughout the younger dryas modeling exercises by stocker and wright 1996 mikolajewicz 1998 and schiller et al 1998 show that if the conveyor is terminated the transfer of radiocarbon into the deep sea shifts to the southern ocean thereby stabilizing the atmospheric 14c c ratio paleoclimatic evidence from the antarctic continent suggests that this model based scenario might have been played out in the real world while the younger dryas cooling has been documented in many places around the world including new zealand denton and hendy 1994 sowers and bender 1995 using their 18o in o2 based correlation between the ice core 18o in ice records for antarctica and greenland have demonstrated that in antarctica the younger dryas was a time of maximum warming the point of this paper is that the steep rise in 18o rise in antarctic ice which commenced close to the onset of the younger dryas might have been caused by heat released to the atmosphere in response to an increase in deep sea ventilation in the southern ocean

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 high resolution authigenic 10be 9be record of geomagnetic moment variations over the last 300 ka from sedimentary cores of the portuguese margin a high resolution study of authigenic be isotopes 10be and 9be combined with continuous relative paleointensity records has been performed along the same marine sedimentary sequences from the portuguese margin n e atlantic covering the past 300 ka in order to assess relationships between geomagnetic moment variations and 10be production rate variations a carefull examination of the various ways of taking into account environmental disturbing effects on the authigenic 10be concentration leads to the conclusion that the most reliable proxy of cosmonuclide production rates is presently the authigenic 10be 9be ratio eight intervals of significant authigenic 10be 9be enhancement evidence geomagnetic moment drops related to global paleomagnetic excursions some being already admitted others being proposed as new geomagnetic features since contrarily to sedimentary magnetic remanence the authigenic 10be 9be records dipole moment variations without significant acquisition delay it provides better constraints on their timing comparison of 10be 9be and benthic 18o records from the same cores suggests that dipole moment lows preferentially occured during or at the end of interglacial episodes with a quasi period of 100 ka

low sea level stand emplacement of megaturbidites in the western and eastern mediterranean sea piston cores from the balearic and herodotus abyssal plains in the mediterranean sea show that the late pleistocene to holocene sedimentary sequence is dominated by turbidite muds on each plain one turbidite bed is conspicuous by its thickness and this bed can be correlated basinwide on the basis of geochemical compositional analysis and its apparent correspondence with a distinct acoustically transparent layer on high resolution seismic records these megabeds on the two plains represent megaturbidites of very large volume 300 600 km3 each and are shown by ams radiocarbon dating to have been emplaced during the last low stand of sea level at the height of the last glacial maximum the megabed on the balearic abyssal plain is derived from the southern european margin and is the main sedimentation event over the last 120 ka it emplaced as much material as was deposited by smaller flows during the previous 25 ka sedimentation rate curves for the balearic abyssal plain show that falling sea level correlates with increased terrigenous deposition and that gross sedimentation rates in the basin increased as sea level fell from 120 18 ka due to more frequent emplacement of distal turbidites the herodotus abyssal plain megabed is derived from the libyan egyptian continental shelf west of the nile delta and was the dominant sedimentation event in this basin during the past 60 kyr high resolution seismic profiles from the ionian and sirte abyssal plains in the central mediterranean also suggest possible low sea level emplacement of megabeds in these regions available evidence suggests widespread emplacement of megaturbidites throughout the mediterranean at the last glacial maximum although the triggering mechanisms for these events remain speculative catastrophic destabilisation of the margin after a long period of accumulation with an increased rate of sediment supply is suggested

millennial scale variability of deep water temperature and δ18odw indicating deep water source variations in the northeast atlantic 0 34 cal ka bp 1 paired measurements of mg ca and 18occ calcite 18o in benthic foraminifera from a deep sea core recovered on the iberian margin md99 2334k 37 48 n 10 10 w 3 146 m have been performed in parallel with planktonic 18occ analyses and counts of ice rafted debris ird the synchrony of temperature changes recorded in the greenland ice cores and in north atlantic planktonic 18occ allows the proxy records from md99 2334k to be placed confidently on the gisp2 time scale this correlation is further corroborated by ams 14c dates benthic mg ca measurements in md99 2334k permit the reconstruction of past deep water temperature tdw changes since 34 cal ka bp calendar kiloyears before present using these tdw estimates and parallel benthic 18occ measurements a record of deep water 18o 18odw has been calculated results indicate greatly reduced tdw in the deep northeast atlantic during the last glaciation until 15 cal ka bp when tdw warmed abruptly to near modern values in parallel with the onset of the bolling allerod interstadial subsequently tdw reverted to cold glacial values between 13 4 and 11 4 cal ka bp in parallel with the younger dryas cold reversal and the h0 ice rafting event similar millennial scale tdw changes also occurred during the last glaciation indeed throughout the last 34 cal ka millennial 18odw and tdw changes have remained well coupled and are linked with ird pulses coincident with heinrich events 3 2 1 and the younger dryas when transitions to lower tdw and 18odw conditions occurred in general millennial tdw and 18odw variations recorded in md99 2334k describe an alternation between colder low 18odw and warmer high 18odw conditions which suggests the changing local dominance of northern sourced north atlantic deep water nadw versus southern sourced antarctic bottom water aabw the observed similarity of the tdw and gisp2 18oice records would therefore suggest a common component of variability resulting from the coupling of nadw formation and greenland climate a link between greenland stadials and the incursion of cold low 18odw aabw in the deep northeast atlantic is thus implied which contributes to the relationship between greenland climate and the millennial benthic 18occ signal since 34 cal ka bp

the greenland ice core chronology 2005 15 42 ka part 2 comparison to other records a new greenland ice core chronology gicc05 based on multi parameter counting of annual layers has been obtained for the last 42 ka here we compare the glacial part of the new time scale which is based entirely on records from the northgrip ice core to existing time scales and reference horizons covering the same period these include the grip and northgrip modelled time scales the meese sowers gisp2 counted time scale the shackleton fairbanks grip time scale sfcp04 based on 14c calibration of a marine core the hulu cave record three volcanic reference horizons and the laschamp geomagnetic excursion event occurring around greenland interstadial 10 gicc05 is generally in good long term agreement with the existing greenland ice core chronologies and with the hulu cave record but on shorter time scales there are significant discrepancies around the last glacial maximum there is a more than 1 ka age difference between gicc05 and sfcp04 and a more than 0 5 ka discrepancy in the same direction between gicc05 and the age of a recently identified tephra layer in the northgrip ice core both sfcp04 and the tephra age are based on 14c dated marine cores and fixed marine reservoir ages for the laschamp event gicc05 agrees with a recent independent dating within the uncertainties

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

characterization of the micrites in the late miocene vermetid carbonate bioconstructions salento peninsula italy record of a microbial metazoan association abstract small carbonate bioconstructions composed of micrite and vermetids occur in the salento peninsula southern italy at the base of the reefal early messinian novaglie formation these lens shaped bioconstructions are tens of meters in length and up to 2 5 m in thickness and crop out patchily along the south eastern salento coast line micrite is the most abundant component its origin and role in this association have been investigated by means of microfacies epifluorescence and organic matter analyses three different types of micrite were recognized i not weakly fluorescent detrital micrite with a few fine bioclasts ii fluorescent micrite rich in fine bioclasts iii fluorescent clotted peloidal micrite automicrite the first type of micrite mainly present in the basal layer of the carbonate bioconstructions surrounds vermetids in life position the type ii and type iii micrites occur in the overlying deposits characterized by sub horizontally isooriented vermetid shells the high fluorescence of the type ii and iii micrite can be related to organic matter derived respectively from decaying metazoan and microbial organisms twofold organic matter origin was supported by ft ir and gc ms analyses carried out on the extracted organic matter micrite i shows very low intensity of organic matter functional groups confirming its abiotic origin the spectra of the automicrite iii are characterized by the presence of stretching c c vibrations attributable to alkene and or unsaturated carboxylic acids that may be synthesized by microbes gc ms investigations indicate the presence of extended hopane series straight chain saturated c14 cl6 monounsaturated c16 c18 and diunsaturated cl8 acids diagnostic of microbial activity microbial communities appear to have played a prominent role in the deposition and stabilization of salento micrite vermetid carbonate bioconstructions the type iii micrite classifiable as microbialite or automicrite can be regarded a sort of primary framework of these small bioconstructions

age of the mono lake excursion and associated tephra abstract the mono lake excursion mle is an important time marker that has been found in lake and marine sediments across much of the northern hemisphere dating of this event at its type locality the mono basin of california has yielded controversial results with the most recent effort concluding that the mle may actually be the laschamp excursion earth planet sci lett 197 2002 151 we show that a volcanic tephra ash 15 that occurs near the midpoint of the mle has a date not corrected for reservoir effect of 28 620 300 14c yr bp 32 400 gisp2 yr bp in the pyramid lake basin of nevada given the location of ash 15 and the duration of the mle in the mono basin the event occurred between 31 500 and 33 300 gisp2 yr bp an age range consistent with the position and age of the uppermost of two paleointensity minima in the napis 75 stack that has been associated with the mle philos trans r soc london ser a 358 2000 1009 the lower paleointensity minimum in the napis 75 stack is considered to be the laschamp excursion philos trans r soc london ser a 358 2000 1009

geomagnetic field intensity during the last 60 000 years based on 10be and 36cl from the summit ice cores and 14c abstract based on 10 be 36 cl and 14 c records we reconstruct past changes in the geomagnetic field intensity under the assumption that radionuclide records are mainly influenced by changes in their production rates subsequent comparison of these records with independent geomagnetic field reconstructions allows us to assess the reliability of our approach and to identify periods where our method or the geomagnetic field reconstructions might be problematic the geomagnetic field derived from the 10 be flux to summit in central greenland agrees well with independent geomagnetic field intensity reconstructions however compared to independent palaeomagnetic data the radionuclide record suggests increased geomagnetic field intensities especially for the period from 20 000 to 30 000 years bp 36 cl from the grip ice core agrees well with the 10 be record for the last ice age and hence also the reconstructed geomagnetic field based on the tree ring radiocarbon record it is possible to reconstruct geomagnetic field intensity changes during the holocene a comparison of radiocarbon based and independent geomagnetic field reconstructions indicates that changes in the carbon cycle significantly influenced the atmospheric radiocarbon concentration before 10 000 years bp

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

eustatic control of turbidites and winnowed turbidites global changes in sea level primarily the results of tectonism and glaciation control deep sea sedimentation during periods of low sea level the frequency of turbidity currents is greatly increased episodes of low sea level also cause vigorous contour currents which winnow away the fines of turbidites in the rock record the occurrence of most turbidites and winnowed turbidites closely corresponds to global lowstands of paleo sea level this observation may be useful in predicting the occurrence of deep sea reservoir facies in the geologic record

recent sediment transport and deposition in the nazare canyon portuguese continental margin processes pathways and fluxes of sediment transport and deposition in the nazare submarine canyon portuguese continental margin were investigated by water column profiling of suspended particulate matter recording of near bottom currents and suspended particulate matter fluxes with benthic landers and by analysis of surface sediment cores the results show that fine grained suspended sediment from the adjacent shelf is captured in the upper canyon and actively transported through the canyon to the abyssal plain under influence of tidal currents and intermittent sediment gravity flows tidal currents are actively resuspending and transporting sediment in the upper to middle canyon as reflected by high concentration of suspended particles in bottom waters high horizontal and vertical sediment fluxes in the bottom water layer and high sediment accumulation rate on the seabed diminishing strength of tidal currents leads to focussed sediment deposition in the middle canyon further transport of sediments toward the lower canyon takes place predominantly by sediment gravity flows occurring on yearly or longer timescale turbidity currents strong enough to transport sand sized material to the lower canyon occur only on centennial or longer timescales

oxygen 18 16 variability in greenland snow and ice with 10 3 to 105 year time resolution the 3 km long greenland ice sheet project 2 gisp2 ice core presents a 100 000 year detailed oxygen isotope profile covering almost a full glacial interglacial cycle measuranents of isotopic fluctuations in snow frost and atmospheric water vapor samples collected during summer field seasons up to 20 0 are compatible with the large and abrupt 80 160 changes observed in accumulated tim snow pit 15 80 profiles from the gisp2 summit area however show rapid smoothing of the 180 160 signal near the surface beyond about 2 m depth the smoothedi5180 signal is fairly well preserved and can be interpreted in terms of average local weather conditions and climate the longer climate fluctuations also have regional and often global significance in the older part of the record corresponding to marine isotope stages mis 5a to 5d the effect of orbital climate forcing via the 19 and 23 kyr precession cycles and the 41 kyr obliquity cycle is obvious from the end ofmis 5a at about 75 000 years b p till the end of the glacial at the younger dryas preboreal transition at 11 650 years b p the o180 160 record shows frequent rapid switches between intermediate interstadial and low stadial values fourier spectra of the oscillations that are superimposed on the orbitally induced changes contain a strong periodicity at 1 5 kyr a broad peak at 4 0 kyr and additional shorter periods detailed comparison of the gisp2 180 160 record with the vostok antarctica 15d record pacific ocean foraminiferal 180 160 grande pile france tree pollen and insolation indicates that a counterpart to many of the rapid 180 160 fluctuations of gisp2 can be found in the other records and that the gisp2 isotopic changes clearly are the local expression of climate changes of worldwide extent correlation of events on the independent gisp2 and specmap time scales for the interval 10 000 50 000 years b p shows excellent chronometric agreement except possibly for the event labeled 3 1 the glacial to interglacial transition evidently started simultaneously in the arctic and the antarctic but its development and its expression in greenland isotopes was later suppressed by the influence of meltwater especially from the barents sea ice sheet on deep water formation and ocean circulation meltwaters from different ice sheets bordering the north atlantic also influenced ocean circulation during the bolling allerod interstadial complex and the younger dryas and led to a distinct development of european climate and greenland 180 160 values the holocene interval with long term stable mean isotopic values contains several fluauations with periods from years to millennia dominant is a 6 3 year oscillation with amplitude up to 3 to 4 0 periodicities of 11 and 210 years also found in the solar modulated records of the cosmogenic isotopes 1obe and 14c suggest solar processes as the cause of these cycles depression of180 160 values cooling by volcanic eruptions is observed in stacked gisp215 80 records but the effect is small and not likely to trigger major climate changes

evidence from the high latitude north atlantic for variations in antarctic intermediate water flow during the last deglaciation we present evidence that the characteristic chemical signature based on coupled benthic foraminiferal cd ca and d13c of antarctic intermediate waters aaiw penetrated throughout the intermediate depths of the atlantic basin to the high latitude north atlantic during the abrupt cooling events of the last deglaciation heinrich 1 and the younger dryas aaiw may play the dynamic counterpart to the bipolar seesaw when near freezing salty bottom waters from the antarctic aabw sluggishly ventilate the deep ocean our data reinforce the concept that interglacial circulation is stabilized by salinity feedbacks between salty northern sourced deep waters nadw and fresh southern sourced waters aabw and aaiw further the glacial ocean may be susceptible to the more finely balanced relative densities of nadw and aaiw due to either freshwater input or a reversal of the salinity gradient such that the ocean is poised for nadw collapse via a negative salinity feedback the unstable climate of the glacial period and its termination may arise from the closer competition for ubiquity at intermediate depths between northern and southern sourced intermediate waters

cortex a shipboard xrf scanner for element analyses in split sediment cores abstract the cortex corescanner texel is a non destructive xrf system for the logging of split sediment cores it is a seaworthy instrument that remains active during heavy weather conditions the scanner produces reliable counts for k to sr highly correlated with concentrations measured with atomic absorption spectrometry aas the option to scan al si and elements heavier than sr is in preparation a core section of 1 m can be scanned at 2 cm intervals within an hour a maximum resolution of 1 mm can be achieved at present each region and sedimentary environment requires separate calibration in the nearby future the generator settings will be standardized which may result in the development of a standard calibration procedure because of the high resolution of its semi quantitative records the cortex is a useful tool for detailed time series analysis inter core correlation and shipboard analysis

antarctic temperature and global sea level closely coupled over the past five glacial cycles sea level has varied by over one hundred metres across glacial interglacial cycles over the past 520 000 years an extended sea level reconstruction shows a strong coupling between these sea level changes and antarctic surface temperatures over the past five glacial cycles

climatic control of turbidite deposition on the madeira abyssal plain studies of sedimentation in the southwestern part of the madeira abyssal plain have revealed sequences of turbidites correlatable over thousands of square kilometres turbidites are sediments transported by turbidity currents and represent rapidly accumulated sedimentary units1 in this case in the distal part of the flow they are very fine grained and homogeneous occasionally with silty bases individual turbidites are separated by thin pelagic units which alternate from oozes to pelagic clays a stratigraphy for these pelagic units has been produced from combining coccolith distributions and lithological changes which enables comparison to cores containing only pelagic sediments from outside the turbidite province these comparisons show that turbidity currents cause no significant erosion in the area the major turbidites occur between lithological alternations which reflect global climate changes associated with glaciations and related eustatic sea level fluctuations turbidites have been linked with eustacy on previous occasions but only with low stands of sea level2 3 our work shows that turbidites are more likely to occur during regressions and transgressions rather than in the quiet periods between

rock magnetic detection of distal ice rafted debries clue for the identification of heinrich layers on the portuguese margin abstract high resolution measurements of concentration and grain size dependent magnetic parameters along a sedimentary sequence deposited on the portuguese margin 38 n during the last 120 ka allowed the detection of multiple thin layers slightly enriched by a coarse lithogenic fraction sizes mineralogy shapes and mode of deposition identify the material as ice rafted detritus ird the four most recent events dated by calibrated radiocarbon ages are contemporaneous with heinrich events he1 he4 older events can be identified as he5 he6 or with minor ird that occurred during stage 5 the resolution and sensitivity obtained with rock magnetic methods enable the detection of multiple secondary events showing that iceberg delivery was a frequent phenomenon directly linked to temperature variations over ice caps heinrich events representing the extreme cases

mediterranean outflow strengthening during northern hemisphere coolings a salt source for the glacial atlantic abstract high resolution grain size and magnetic susceptibility records from the eastern gulf of cadiz site md99 2339 1170 m water depth reveal contourites formed by the mediterranean outflow mow during the last 47 kyr bp oscillations in the mow s intensity occurred in phase with greenland temperature variations with a stronger outflow during northern hemisphere coolings such as dansgaard oeschger stadials heinrich events and the younger dryas benthic 13c data implies the western mediterranean deep water as one of the main sources feeding the outflow current while differential changes in the properties of the mediterranean source and entrained north atlantic central water largely control the mow s strength detailed studies for heinrich events 1 4 and 5 show that the flow strength peaked only when subtropical surface waters prevailed in the eastern gulf of cadiz while incursions of icebergs and subpolar surface water were not favorable for the mow s intensification as the mow was strong when the atlantic s thermohaline circulation thc was weakened the heat and salt injected by the mow into the intermediate north atlantic waters might have preconditioned the thc to switch from the stadial to the interstadial mode

the greenland ice core chronology 2005 15 42 ka part 1 constructing the time scale the greenland ice core chronology 2005 gicc05 is extended back to 42 ka b2k before 2000 ad i e to the end of greenland stadial 11 the chronology is based on independent multi parameter counting of annual layers using comprehensive high resolution measurements available from the north greenland ice core project ngrip these are measurements of visual stratigraphy conductivity of the solid ice electrolytical melt water conductivity and the concentration of na c a 2 s o 4 n o 3 n h 4 a n uncertainty estimate of the time scale is obtained from identification of uncertain annual layers which are counted as 0 570 5 years the sum of the uncertain annual layers the so called maximum counting error of the presented chronology ranges from 4 in the warm interstadial periods to 7 in the cold stadials the annual accumulation rates of the stadials and interstadials are on average one third and half of the present day values respectively and the onset of the greenland interstadials 2 3 and 8 based on 20 year averaged d 18 o

the late pleistocene holocene sedimentary evolution of the sines contourite drift sw portuguese margin a multiproxy approach abstract the sines contourite drift scd located in the alentejo margin southwest iberian continental margin has been through many depositional phases in result of climatic variations and bottom current oscillations which determined a variable depositional pattern and an irregular sedimentary evolution since the late pleistocene the scd being in the main path of the mediterranean outflow water mow which greatly constrains the sedimentary building of this drift constitutes the distal part of the gulf of cadiz contourite depositional system and its sedimentary evolution therefore reflects the history of mow variations in order to investigate this we report on a multiproxy analysis of grain size carbon content and physical geochemical and environmental magnetic properties on the ams 14c dated 350 cm long gravity core co14 gc 07 retrieved in the scd at 1425 meters water depth mwd the main objective is to reconstruct the evolution of the sedimentary package of the scd from 43 cal kyr bp to present and bring new insights about the impact of bottom currents activity on the morphosedimentary evolution of the margin results show the existence of four distinct depositional phases in response to climate variations and bottom current oscillations during the late pleistocene holocene phase 1 350 322 cm 42 9 cal kyr bp occurred in marine isotope stage mis 3 and presents silty muddy sediments with presence of shell fragments and moderate bioturbation phase 2 322 176 cm 42 9 30 5 cal kyr bp which lasted from middle mis 3 to the onset of mis 2 reveals sediment coarsening towards the top limit suggesting climate cooling and strong bottom current winnowing phase 3 176 144 cm 30 5 17 1 cal kyr bp covers most of mis 2 prior to the last deglaciation and shows the sharpest variations on all sediment properties and the coarsest sediment facies with fe rich layers associated with the increase of terrigenous input and sea level regression that coincide with the enhancement of mow phase 4 144 0 cm

sea level changes forced ice breakouts in the last glacial cycle new results from coral terraces abstract sea level cycles recorded by coral terraces at huon peninsula hp in papua new guinea match rapid climate changes that occurred between 30 000 and 65 000 years ago seen in greenland ice cores and marine sediments each cycle of 6000 7000 years ended with a sea level rise of 10 15 m lasting 1000 2000 years following a longer period of falling sea level precise dating shows that each rise corresponds to a heinrich episode of ice rafted detritus in north atlantic signalling massive ice outbreak from north america sea level may be the trigger that forced near synchronous breakouts of ice from north america and eastern greenland the hp sea level changes also correspond to similar cycles in benthic oxygen isotopes reported from the north atlantic which are too large to be explained entirely in terms of ice volumes jointly the sea level and isotope records suggest that the north atlantic deep ocean cooled as sea level fell and warmed as sea level rose in each 6000 year cycle

ice sheets by volume how big were the ice sheets at the height of the last glaciation around 21 000 years ago the best estimate yet comes from an analysis of the lower sea levels of the time and will further understanding of how ice sheet volume influences climate

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

palaeoenvironmental forcing during the middle upper palaeolithic transition in central western portugal article i nfo geoarchaeological analysis of the middle and upper palaeolithic record preserved in cave rock shelter and open air sites in the northern sector of the meso cenozoic of the western iberian peninsula margin portugal reveals several disconformities erosive unconformities hiatuses and surface stabilization phases a recurrent disconformity dated to ca 29 500 32 000 cal yr bp in the time range of heinrich event 3 must correspond to a main erosive event related to the impacts of climate change on the landscape including a reduction in vegetation cover and altered precipitation patterns with the consequent accelerated down cutting by stream systems slope reactivation and endokarstic reorganisation causing the erosion of sediments and soils accumulated in cave rock shelter and open air sites these processes create a preservation bias that may explain why early upper palaeolithic finds in primary deposition context remains exceptional in the carbonate areas of central western portugal and possibly elsewhere in the other places of iberia the impact of such site formation processes must therefore be duly considered in interpretations of the current patchy and scarce archaeological record of the middle upper palaeolithic transition in south western

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

timing of millennial scale climate change in antarctica and greenland during the last glacial period a precise relative chronology for greenland and west antarctic paleotemperature is extended to 90 000 years ago based on correlation of atmospheric methane records from the greenland ice sheet project 2 and byrd ice cores over this period the onset of seven major millennial scale warmings in antarctica preceded the onset of greenland warmings by 1500 to 3000 years in general antarctic temperatures increased gradually while greenland temperatures were decreasing or constant and the termination of antarctic warming was apparently coincident with the onset of rapid warming in greenland this pattern provides further evidence for the operation of a bipolar see saw in air temperatures and an oceanic teleconnection between the hemispheres on millennial time scales

recent sediment transport and deposition in the lisbon setubal and cascais submarine canyons portuguese continental margin abstract recent sediment transport and deposition in the lisbon setubal and cascais submarine canyons portuguese continental margin were investigated on the basis of water column profiles of suspended particulate matter records of near bottom currents and settling fluxes of particulate matter obtained with benthic landers and analysis of surface sediments the results show that fine grained predominantly lithogenic sediment derived from adjacent shelf areas accumulates in the upper reaches of the canyons sediment deposited further down in the middle and lower reaches of the canyons is essentially similar to the hemipelagic sediment found on the adjacent continental slope indicating that down canyon transport of sediment from the upper to the lower canyon is limited tidal currents measured at various depths in the lisbon setubal canyon appear sufficiently strong to resuspend and transport sediment but net up canyon flow of the bottom water may retain shelf derived lithogenic sediment in the upper canyon sediment gravity flows which in the nearby nazare canyon are an effective mechanism for down canyon sediment transport appear rare in the lisbon setubal canyon and probably also in the cascais canyon turbidity current events recorded in the sedimentary record of the canyons may correspond to seismic events of 1969 and 1755 ad

what caused the great lisbon earthquake in 1755 the great lisbon earthquake estimated magnitude 8 7 killed 60 000 people in southern iberia and northwest morocco and generated a 5 to 10 m high tsunami wave in his perspective gutscher discusses the current research on the tectonic source of the earthquake recent evidence suggests that an eastward dipping subduction zone beneath gibraltar may have caused the earthquake though no interplate earthquakes have been recorded here evidence from mud volcanoes seismic images of deformed sediments and heat flow data suggest that subduction is active and that the seismogenic fault zone may be locked numerous cruises planned in 2004 and 2005 may help to provide a definitive answer

dominant northern hemisphere climate control over millennial scale glacial sea level variability abstract based on a radiocarbon and paleomagnetically dated sediment record from the northern red sea and the exceptional sensitivity of the regional changes in the oxygen isotope composition of sea water to the sea level dependent water exchange with the indian ocean we provide a new global sea level reconstruction spanning the last glacial period the sea level record has been extracted from the temperature corrected benthic stable oxygen isotopes using coral based sea level data as constraints for the sea level oxygen isotope relationship although the general features of this millennial scale sea level records have strong similarities to the rather symmetric and gradual southern hemisphere climate patterns we observe in constrast to previous findings pronounced sea level rises of up to 25 m to generally correspond with northern hemisphere warmings as recorded in greenland ice core interstadial intervals whereas sea level lowstands mostly occur during cold phases corroborated by climber 2 model results the close connection of millennial scale sea level changes to northern hemisphere temperature variations indicates a primary climatic control on the mass balance of the major northern hemisphere ice sheets and does not require a considerable antarctic contribution