intensification of northern hemisphere glaciations in the circum atlantic region 3 5 2 4 ma ice rafted detritus evidence abstract records of ice rafted detritus ird from ocean drilling program sites 644 and 907 in the nordic seas and deep sea drilling program sites 610 and 607 in the north atlantic ocean are used to determine the growth and history of northern hemisphere glaciations spanning the interval from 3 6 to 2 4 ma the records document that the initiation of large scale glaciation in the circum atlantic region occurred stepwise between 3 5 and 2 4 ma with greenland leading the other areas the first major pulse of ird occurs at 3 3 ma suggesting a distinct expansion of the greenland ice sheet progressive increases in ird occur from 3 0 ma with a synchronous ice sheet development between the greenland scandinavian and north american regions starting around 2 72 2 75 ma the regional records of ird correspond to major trends in the oxygen isotope record suggesting a tight connection between the record of global ice volume and evidence for expanding glaciers in the northern hemisphere it appears as if widespread ird distribution occurs when the global ice volume surpasses certain thresholds similar to the late quaternary situation

the hafnium neodymium isotopic composition of atlantic seawater the first full water column hafnium isotopic compositions of atlantic seawater have been obtained at seven locations from the labrador sea to the drake passage despite subpicomolar concentrations in seawater a precision of the hf isotopic measurements of 03 epsilon hf units was achieved an overall epsilon hf range between 3 1 in the labrador sea and 4 4 in antarctic bottom water was determined the distribution of which broadly reflects continental weathering inputs within particular water column profiles significant differences of up to 4 epsilon hf units occur combined with nd isotope data of the same samples it is evident that the hf isotopic composition of seawater is too radiogenic for a given nd isotopic composition and that the largest difference between expected and measured hf isotopic compositions in seawater occurs near the oldest continental crust in the labrador sea this corroborates the previous proposition which was mainly based on ferromanganese crust data that the hf isotopic composition of seawater is controlled by incongruent weathering of continental crust and possibly to some extent by hydrothermal contributions hafnium concentrations in the ocean do not increase along the deep ocean conveyer indicating an oceanic residence time of only a few hundred years which is significantly shorter than previously assumed the hf isotopic composition of past seawater can therefore serve as a proxy for short distance basin scale mixing processes and the regime and intensity of nearby continental weathering processes c 2009 elsevier b v all rights reserved

pliocene and quaternary depositional model of the algarve margin contourite drifts gulf of cadiz sw iberia seismic architecture tectonic control and paleoceanographic insights abstract the contourite drifts off southwest iberia that formed as a result of the interaction of the mediterranean outflow water mow with the continental middle slope were studied in the algarve margin using multichannel reflection seismic lines oil wells piston cores and a bathymetric compilation of four datasets the seismostratigraphic interpretation of a dense array of oil industry seismic and stratigraphic correlation allowed the identification of five seismic units of early pliocene through holocene in the faro and albufeira drifts and four correlative seismic units in the lagos and sagres drifts and three in the portimao drift a three phased evolutionary model for the contourite formation is proposed firstly a precursory phase of pliocene age made up of sheeted drifts represents an initial phase of deposition under bottom current activity that is correlated with the first stages of an enhanced mow at about 3 5 ma secondly the building up phase of early pleistocene age is related to a strengthening of the mow close to the onset of the northern hemisphere glaciations at about 2 6 ma during which were deposited low mounded drifts thirdly the growing phase from middle pleistocene through holocene suggests the presence of a stronger mow since the beginning of the middle pleistocene transition at about 1 3 1 0 ma accounting for the deposition of mounded drifts and formation of the alvares cabral moat seismostratigraphic interpretation and isochron maps allowed for the establishment of the main oceanographic climatic morphologic and tectonic factors that controlled the drifts deposition i the pliocene and quaternary mow circulation forced by climate changes ii the sea bottom topography inherited from the late miocene mainly shaped by the portimao lagos and bartolomeu dias canyons system iii the interaction between along slope and down slope processes since the pliocene and iv pliocene quaternary fault activity and diapirism

a simple evaluation of cleaning procedures on fossil benthic foraminiferal mg ca 1 recent studies suggest that benthic foraminiferal mg ca may be useful in reconstructing deep water temperature changes during the quaternary and on longer timescales rosenthal et al 1997 lear et al 2000 martin et al 2002 because changes in deep water temperature may be small sample preparation could play an important role in reconstructing accurate temperatures from shell mg ca a simple examination of changes in measured shell chemistry of benthic foraminifera from two intervals subjected to increasingly rigorous cleaning reveals that fossil benthic foraminiferal samples cleaned by oxidation yield lower mg ca values than samples only rinsed and weak acid leached reductive cleaning further lowered the shell mg ca of one of the two benthic species examined lower mg ca values following oxidation and reduction appear to reflect removal of contaminant mg associated with remnant organic matter and adsorbed phases

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

reevaluation of the oxygen isotopic composition of planktonic foraminifera experimental results and revised paleotemperature equations cultured planktonic foraminifera orbulina universa symbiotic and globigerina bulloides nonsymbiotic are used to reexamine temperature 18o relationships at 15 25 c relationships for both species can be described by linear equations equations for o universa grown under low light ll and high light hl share a slope of 4 80 0 21 c 1 with a hl ll offset of 0 33 due to symbiont photosynthetic activity the effect of co32 on o universa is 0 002 mol 1 kg 1 and is insensitive to temperature for g bulloides ontogenetic effects produce size related trends in temperature 18o whereby larger shells are enriched in 18o relative to smaller specimens the o universa temperature 18o relationships are more accurate than previously published equations for describing plankton tow data our equations do not explain planktonic core top data with the same precision but provide a good fit to benthic cibicidoides data below 10 c temperature 18o relationships for g bulloides provide good agreement with field data for this species from the northeast pacific

tracing past ocean circulation to understand how ocean circulation and climate may change in the future we must first understand how the system operated in the past without man9s influence new inorganic isotopic tracers are now helping to elucidate past ocean circulation patterns but controversies remain regarding the relative influence of ocean circulation and weathering on the isotopic signals

nd isotopes in deep sea corals in the north eastern atlantic abstract neodymium nd concentrations and isotopic signatures of living and fossil deep sea coral species lophelia pertusa desmophyllum dianthus and madrepora oculata from the northeast atlantic ocean have been investigated in order to test the ability of deep sea corals to reconstruct the seawater nd isotopic signature and past changes of ocean circulation in the eastern north atlantic small quantities of nd less than 45 ng g are incorporated into the aragonite skeleton of living deep sea corals that dwell at upper intermediate depths throughout the northeast atlantic rigorous cleaning techniques are needed in order to avoid nd contamination from manganese oxide and iron hydroxide coatings moreover nd isotopic compositions have been measured using thermal ionization mass spectrometry tims by nd oxide method our data indicate that the isotopic signatures of modern corals are similar to those of adjacent water masses implying that deep sea corals can serve as an archive of the seawater nd isotopic compositions in the past the first results from few fully cleaned fossils corals collected within the porcupine seabight and the southwest rockall bank reveal significantly higher end for corals dated between 150 40 and 3060 90 yrs than those of the living corals located in similar areas this suggests rapid hydrological variations along the eastern margin of the north atlantic ocean at intermediate water depth with higher contribution of the mediterranean overflow waters mow or other temperate atlantic mid depth water masses enacw or nac in the past

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

changes in mid depth north atlantic and mediterranean circulation during the late pliocene isotopic and sedimentological evidence abstract presently the intermediate depths of the north atlantic ocean are occupied by a great lens of warm saline water whose source is the mediterranean sea this water flows both westward and northward finally entering the norwegian sea where it may contribute to the formation of north atlantic deep water the late neogene history of mediterranean outflow in the atlantic can be monitored at dsdp ipod site 548 on the continental slope southwest of ireland using benthic foraminifera oxygen isotope values isotopic data from 154 samples indicate that mediterranean water was absent from the mid depth north atlantic from 3 4 to 3 2 ma ago however at about 2 9 ma ago the isotopic values at site 548 diverge from those recorded from the deep north atlantic and they can be interpreted to indicate the appearance of a new water mass possibly mediterranean water in the north atlantic water column this appearance may be related to climatic changes that occurred around the mediterranean basin at about 2 9 ma ago the analysis of 189 samples for grain size distributions shows that a significant increase in the silt size fraction occurs at the same level that isotopic analysis indicates a change in bottom waters at site 548 the grainsize data support the hypothesis that mid depth water mass changes occurred at about 2 9 ma ago

late gauss to early matuyama invasions of neogloboquadrina atlantica in the mediterranean and associated record of climatic change a well dated high resolution record of climatic change is presented for the late pliocene mediterranean principal component analysis on abundances of 14 planktonic foraminiferal species reveals a series of late gauss to early matuyama surface water cooling events which can be correlated with north atlantic glacial isotope stages 108 to 94 the abundance record of the benthic foraminifer trifarina angulosa suggests lowered bottom water temperatures at times of surface water cooling the record of surface water oxygen and carbon isotopes shows an inverse pattern with 18o maxima and 13c minima at times of cool sea surface temperature conditions concomitant decrease in p b value and increased admixtures of clastic material suggest that late gauss to early matuyama surface water cooling events in the mediterranean are accompanied with glacioeustatic sea level low stands major surface water cooling events are associated with invasions of neogloboquadrina atlantica this north atlantic mid to high latitude species is at 2 55 ma close to the gibraltar portal whereafter it could invade the mediterranean repeatedly during periods of climatic cooling until it disappears from all over the north atlantic and mediterranean at the end of glacial stage 96

mg ca in the benthic foraminifera cibicidoides wuellerstorfi and cibicidoides mundulus temperature versus carbonate ion saturation magnesium calcium mg ca ratios in two widely used benthic foraminiferal species cibicidoides wuellerstorfi and cibicidoides mundulus cibicidoides kullenbergi picked from global ocean core tops and from six cores in the north atlantic ocean have been measured to investigate mg ca variability cleaning effect and influences from bottom water temperature bwt and deep water carbonate ion saturation co32 replicate measurements of the two species reveal mg ca variability of 0 07 0 07 mmol mol compared with mg ca in samples cleaned by oxidative cleaning mg ca ratios are significantly lowered by 0 10 0 09 mmol mol in samples cleaned by reductive cleaning likely due to preferential leaching during the reductive cleaning step the cleaning influence is large relative to glacial interglacial mg ca changes 0 05 0 31 mmol mol by this study regression of core top data from a wide range of locations show that c wuellerstorfi mg ca is strongly affected by deep water co32 with a sensitivity of 0 009 mmol mol per mol kg when bwt is included in regressions the correlation is slightly more significant with a weak bwt sensitivity on mg ca of between 0 03 0 01 and 0 07 0 02 mmol mol per c because co32 and bwt co vary for most core top samples an alternate method of separating the two effects was made by comparing glacial to interglacial changes in mg ca from a depth transect of north atlantic cores where the bwt and carbonate ion histories since the last glacial period have been well constrained results from this transect suggest that the bwt effect is 0 03 mmol mol per c weaker than inferred from the core top study therefore core top and down core data suggest that c wuellerstorfi mg ca is insensitive to bwt changes the weak bwt effect when compared with natural variability complicates the use of mg ca in this species to reconstruct past bwt taking all measured samples from various environments into account c mundulus mg ca ratios show no correlation with bwt or deep water co32

directly measured mid depth circulation in the northeastern north atlantic ocean the circulation of water masses in the northeastern north atlantic ocean has a strong influence on global climate owing to the northward transport of warm subtropical water to high latitudes but the ocean circulation at depths below the reach of satellite observations is difficult to measure and only recently have comprehensive direct observations of whole ocean basins been possible here we present quantitative maps of the absolute velocities at two levels in the northeastern north atlantic as obtained from acoustically tracked floats we find that most of the mean flow transported northward by the gulf stream system at the thermocline level about 600 m depth remains within the subpolar region and only relatively little enters the rockall trough or the nordic seas contrary to previous work our data indicate that warm saline water from the mediterranean sea reaches the high latitudes through a combination of narrow slope currents and mixing processes at both depths under investigation currents cross the mid atlantic ridge preferentially over deep gaps in the ridge demonstrating that sea floor topography can constrain even upper ocean circulation patterns

deep sea temperature and ice volume changes across the pliocene pleistocene climate transitions earth has undergone profound changes since the late pliocene which led to the development 2 7 million years ago ma and intensification 0 9 ma of large scale northern hemisphere ice sheets recorded as transitions in the benthic foraminiferal oxygen isotope 18ob record here we present an orbitally resolved record of deep ocean temperature derived from benthic foraminiferal magnesium calcium ratios from the north atlantic which shows that temperature variations are a substantial portion of the global 18ob signal the record shows two distinct cooling events associated with the late pliocene lpt 2 5 to 3 ma and mid pleistocene mpt 1 2 to 0 85 ma climate transitions whereas the lpt increase in ice volume is attributed directly to global cooling the shift to 100 000 year cycles at the mpt is more likely to be a response to an additional change in ice sheet dynamics

palaeohydrology of the fazzan basin and surrounding regions the last 7 million years abstract we have integrated information on topography geology and geomorphology with the results of targeted fieldwork in order to develop a chronology for the development of lake megafazzan a giant lake that has periodically existed in the fazzan basin since the late miocene the development of the basin can be best understood by considering the main geological and geomorphological events that occurred thought libya during this period and thus an overview of the palaeohydrology of all libya is also presented the origin of the fazzan basin appears to lie in the late miocene at this time libya was dominated by two large rivers systems that flowed into the mediterranean sea the sahabi river draining central and eastern libya and the wadi nashu river draining much of western libya as the miocene progressed the region become increasingly affected by volcanic activity on its northern and eastern margin that appears to have blocked the river nashu in late miocene or early messinian times forming a sizeable closed basin in the fazzan within which proto lake megafazzan would have developed during humid periods the fall in base level associated with the messinian desiccation of the mediterranean sea promoted down cutting and extension of river systems throughout much of libya to the south of the proto fazzan basin the sahabi river tributary know as wadi barjuj appears to have expanded its headwaters westwards the channel now terminates at al haruj al aswad we interpret this as a suggestion that wadi barjuj was blocked by the progressive development of al haruj al aswad k ar dating of lava flows suggests that this occurred between 4 and 2 ma this event would have increased the size of the closed basin in the fazzan by about half producing a catchment close to its current size 350 000 km2 the fazzan basin contains a wealth of pleistocene to recent palaeolake sediment outcrops and shorelines dating of these features demonstrates evidence of lacustrine conditions during numerous interglacials spanning a period greater than 420 ka the middle to late pleistocene interglacials were humid enough to produce a giant lake of about 135 000 km2 that we have called lake megafazzan later lake phases were smaller the interglacials less humid developing lakes of a few thousand square kilometres in parallel with these palaeohydrological developments in the fazzan basin change was occurring in other parts of libya the lower pliocene sea level rise caused sediments to infill much of the messinian channel system as this was occurring subsidence in the al kufrah basin caused expansion of the al kufrah river system at the expense of the river sahabi by the pleistocene the al kufrah river dominated the palaeohydrology of eastern libya and had developed a very large inland delta in its northern reaches that exhibited a complex distributary channel network which at times fed substantial lakes in the sirt basin at this time libya was a veritable lake district during humid periods with about 10 of the country underwater

late pliocene to early pleistocene astronomically forced sea surface productivity and temperature variations in the mediterranean abstract principal component analysis of late pliocene to early pleistocene planktonic foraminiferal abundances from the mediterranean yielded two significant components which represent first order approximations for sea surface productivity and temperature this interpretation is confirmed by the good fit between the score plots and sea surface productivity and sea surface temperature proxy records based on selected eutrophic and temperature sensitive species using present day habitat characteristics spectral analysis revealed significant peaks in the orbital frequency bands of the spectrum suggesting that variations in sea surface productivity and temperature are controlled by the quasi periodic variations in the earth s orbit precession forced variations are reflected primarily in sea surface productivity and to a lesser degree in sea surface temperature maxima in sea surface productivity and temperature in this frequency band occurred at times of sapropel formation the sea surface productivity maxima can be explained by an increased input of river borne nutrients and by the shoaling of the pycnocline and the associated intensification of the deep chlorophyll maximum dcm layer precession forced variations in sea surface temperature are not related to glacial cycles since this frequency component is almost completely absent in ice volume proxy records for this time interval sapropel bound sea surface temperature maxima presumably reflect periods of maximum insolation during the northern hemisphere summer the contrasting winter signal corresponds to minimum insolation values which may be reflected by peak abundances ofgloborotalia inflata obliquity forced variations are reflected primarily in sea surface temperature and not in sea surface productivity these temperature fluctuations correspond with ice volume changes registered in the oxygen isotope record at the north atlantic dsdp site 607 this shows that obliquity forced sea surface temperature variations in the mediterranean are primarily driven by glacial cycles

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

final closure of panama and the onset of northern hemisphere glaciation abstract the greenland ice sheet is accepted as a key factor controlling the quaternary glacial scenario however the origin and mechanisms of major arctic glaciation starting at 3 15 ma and culminating at 2 74 ma are still controversial for this phase of intense cooling ravelo et al 1 a c ravelo d h andreasen m lyle a o lyle m w wara regional climate shifts caused by gradual global cooling in the pliocene epoch nature 429 2004 263 267 proposed a complex gradual forcing mechanism in contrast our new submillennial scale paleoceanographic records from the pliocene north atlantic suggest a far more precise timing and forcing for the initiation of northern hemisphere glaciation nhg since it was linked to a 2 3 c surface water warming during warm stages from 2 95 to 2 82 ma these records support previous models g h haug r tiedemann effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation nature 393 1998 673 676 2 claiming that the final closure of the panama isthmus 3 0 2 5 ma j groeneveld s steph r tiedemann d nurnberg d garbe schonberg the final closure of the central american seaway geology in prep 3 induced an increased poleward salt and heat transport associated strengthening of north atlantic thermohaline circulation and in turn an intensified moisture supply to northern high latitudes resulted in the build up of nhg finally culminating in the great irreversible climate crash at marine isotope stage g6 2 74 ma in summary there was a two step threshold mechanism that marked the onset of nhg with glacial to interglacial cycles quasi persistent until today

isotopic evidence for plio pleistocene environmental change at gona ethiopia abstract a 4 5 ma record of fluvial and lacustrine deposits is well exposed at gona in the afar depression of ethiopia we use isotopic values of pedogenic carbonate and fossil teeth to reconstruct plio pleistocene environmental change at gona an increase in 13c values of pedogenic carbonates since 4 5 ma points to a shift from woodlands to grassy woodlands in the early pliocene 10 4 to 3 9 vpdb to more open but still mixed environments in the late pleistocene 3 0 to 1 4 vpdb this pattern is also seen in isotopic records elsewhere in east africa however at 1 5 ma the higher proportion of c4 grasses at gona is largely a result of a local facies shift to more water limited environments the wide range of 13c values of pedogenic carbonate within single stratigraphic levels indicates a mosaic of vegetation for all time intervals at gona that depends on depositional environment elements of this mosaic are reflected in 13c values of both modern plants and soil organic matter and plio pleistocene soil carbonate indicating higher amounts of c4 grasses with greater distance from a river channel in both the modern and ancient awash river systems 18o values of pedogenic carbonates increase up section from 11 9 in the early pliocene to 6 4 vpdb in the late pleistocene the wide range of 18o values in paleovertisol carbonates from all stratigraphic levels probably reflects short term climate changes and periods of strong evaporation throughout the record based on the comparison between 18o values of plio pleistocene pedogenic carbonates and modern waters we estimate that there has been a 6 5 increase in mean annual 18o values of meteoric water since 4 5 ma 18o values of pedogenic carbonate from other east african records indicate a similar shift increasing aridity and fluctuations in the timing and source of rainfall are likely responsible for the changes in 18o values of east african pedogenic carbonates through the plio pleistocene

abrupt cold spells in the northwest mediterranean hitherto unknown abundance peaks of left coiling l c neogloboquadrina pachyderma from a gulf of lions piston core indicate that abrupt cold spells associated with atlantic heinrich events affected the mediterranean n pachyderma l c is typical of sub polar waters in the open ocean the southern edge of its glacial north atlantic bioprovince reached south portugal only trace abundances of n pachyderma l c are known from quaternary mediterranean sediments suggesting that no significant invasions occured via the strait of gibraltar the gulf of lions abundance peaks therefore seem to reflect area specific thriving of a normally rare but indigenous taxon in the western mediterranean through local favorable habitat development the general planktonic foraminiferal record suggests that the basic hydrographic regime in the gulf of lions with wintertime deep convective overturn was relatively stable over the past 60 kyr under these conditions high abundances of n pachyderma l c would essentially imply temperature reductions of the order of 5 8 relative to the present

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

testing the extraction of past seawater nd isotopic composition from north atlantic deep sea sediments and foraminifera neodymium isotopes provide a paleoceanographic proxy for past deep water circulation and local weathering changes and have been measured on various authigenic marine sediment components including fish teeth ferromanganese oxides extracted by acid reductive leaching cleaned foraminifera and foraminifera with fe mn oxide coatings here we compare nd isotopic measurements obtained from ferromanganese oxides leached from bulk sediments and planktonic foraminifera as well as from oxidatively reductively cleaned foraminiferal shells from sediment cores in the north atlantic sedimentary volcanic ash contributes a significant fraction of the nd when the ferro manganese fe mn oxide coatings are leached from bulk sediments reductive leachates of marine sediments from north atlantic core tops near iceland or directly downstream from iceland scotland overflow waters record nd values that are significantly higher than seawater indicating that volcanic material is easily leached by acid reductive methods the nd values from sites more distal to iceland are similar to modern seawater values showing little contamination from iceland derived volcanogenic material in all comparisons core top planktonic foraminifera nd values more closely approximate modern deep seawater than the bulk sediment reductive leached value suggesting that the foraminifera provide a route toward quantifying the nd isotopic signature of deep north atlantic water masses

closure of the panama seaway during the pliocene implications for climate and northern hemisphere glaciation the panama hypothesis states that the gradual closure of the panama seaway between 13 million years ago 13 ma and 2 6 ma led to decreased mixing of atlantic and pacific water masses the formation of north atlantic deep water and strengthening of the atlantic thermohaline circulation increased temperatures and evaporation in the north atlantic increased precipitation in northern hemisphere nh high latitudes culminating in the intensification of northern hemisphere glaciation nhg during the pliocene 3 2 2 7 ma here we test this hypothesis using a fully coupled fully dynamic ocean atmosphere general circulation model gcm with boundary conditions specific to the pliocene and a high resolution dynamic ice sheet model we carry out two gcm simulations with closed and open panama seaways and use the simulated climatologies to force the ice sheet model we find that the models support the panama hypothesis in as much as the closure of the seaway results in a more intense atlantic thermohaline circulation enhanced precipitation over greenland and north america and ultimately larger ice sheets however the volume difference between the ice sheets in the closed and open configurations is small equivalent to about 5 cm of sea level we conclude that although the closure of the panama seaway may have slightly enhanced or advanced the onset of nhg it was not a major forcing mechanism future work must fully couple the ice sheet model and gcm and investigate the role of orbital and co2 effects in controlling nhg

an intensity ratio calibration method for the accurate determination of mg ca and sr ca of marine carbonates by icp aes an inductively coupled plasma atomic emission spectroscopy icp aes method for the accurate and precise simultaneous measurement of the mg ca and sr ca content of carbonates was established while a precision of 0 3 1 standard deviation sd is easily obtainable for both mg ca and sr ca analysis a ca matrix effect complicates achieving similar levels of accuracy with conventional calibration procedures an alternative ratio calibration procedure is proposed which overcomes the ca matrix effects and ensures the accuracy of the mg ca and sr ca analysis of marine carbonates to 0 3 almost an order of magnitude better than conventional calibration methods the longer term precision is 0 1 if the batch run average values are corrected for longer term drift the method is suitable for analysis of foraminiferal calcite and coral aragonite and can easily be adjusted for the analysis of other carbonates or microsamples

enhanced circulation during a warm period the early pliocene is the most recent interval in which equilibrium oceanic conditions can be studied in the context of global warmth relative to today to characterize thermohaline circulation during this warm period 4 4 3 1 ma we combined new benthic foraminiferal isotopic data from the southeast atlantic with published data and reconstructed pacific and atlantic ocean nutrient distributions the data indicate enhanced ventilation of the deep atlantic and intermediate pacific during the early pliocene enhanced ventilation implies that delivery of surface water to high latitudes in western boundary currents of north pacific and atlantic subtropical gyres was probably also enhanced future modeling of this warm period needs to reconcile reduced meridional surface temperature gradients with enhanced subtropical gyre and thermohaline circulation

on the temporally varying northward penetration of mediterranean overflow water and eastward penetration of labrador sea water abstract historical hydrographic data in the eastern north atlantic are used to suggest a connection between the northward penetration of mediterranean overflow water mow and the location of the subpolar front the latter of which is shown to vary with the north atlantic oscillation nao during persistent high nao periods when the subpolar front moves eastward waters in the subpolar gyre essentially block the northward flowing mow preventing its entry into the subpolar gyre conversely during low nao periods the subpolar front moves westward allowing mow to penetrate past porcupine bank into the subpolar gyre the impacts of an intermittent penetration of mow into the subpolar gyre including the possible effect on water mass transformations remain to be investigated

atlantic mediterranean water exchange during the late neocene the nature of water exchange between the mediterranean and the atlantic is examined for three time periods during the late neogene in order to assess the impact of flow patterns between these basins on bottom water conditions in the oceans the large volume of carbonates and evaporites deposited in the mediterranean during the messinian salinity crisis appears to have caused the open ocean to become more undersaturated in calcium carbonate resulting in increased dissolution of deep sea carbonates during the latest miocene during the earliest pliocene an estuarine type of flow existed between the atlantic and mediterranean waters entered the mediterranean at depth and flowed out at the surface benthic foraminiferal stable isotopic evidence from the north atlantic indicates that north atlantic deep water nadw was produced throughout the early pliocene and that the absence of saline mediterranean outflow did not have a significant effect on bottom water production eustatic lowering of sea level during the last glacial maximum 18 000 years b p reduced water exchange between the atlantic and the mediterranean as a result salinities in the mediterranean increased substantially but it seems unlikely that diminished nadw production during the last glacial episode can be attributed to a reduction in mediterranean outflow

a short circuit in thermohaline circulation a cause for northern hemisphere glaciation the cause of northern hemisphere glaciation about 3 million years ago remains uncertain closing the panamanian isthmus increased thermohaline circulation and enhanced moisture supply to high latitudes but the accompanying heat would have inhibited ice growth one possible solution is that enhanced moisture transported to eurasia also enhanced freshwater delivery to the arctic via siberian rivers freshwater input to the arctic would facilitate sea ice formation increase the albedo and isolate the high heat capacity of the ocean from the atmosphere it would also act as a negative feedback on the efficiency of the conveyor belt heat pump

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

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

late pliocene variations of the mediterranean outflow late pliocene changes in the advection of mediterranean outflow water mow derivates were reconstructed at northeast atlantic dsdp odp sites 548 and 982 and compared to records of wmdw at west mediterranean site 978 neodymium isotope end values more positive than 10 5 11 reflect diluted mow derivates that spread almost continuously into the northeast atlantic from 3 7 to 2 55 ma reaching rockall plateau site 982 from 3 63 to 2 75 ma from 3 4 to 3 3 ma average mow temperature and salinity increased by 2 4 c and 1 psu both at proximal site 548 and distal site 982 the rise implies a rise in flow strength coeval with a long term rise in both west mediterranean sea surface salinity by almost 2 psu and average bottom water salinity bws by 1 psu despite inherent uncertainties in bws estimates the changes were linked with major mediterranean aridification and a drop in african monsoon humidity in contrast to model expectations the rise in mow salt discharge after 3 4 ma did not translate into improved ventilation of north atlantic deep water since it possibly was too small to significantly influence atlantic meridional overturning circulation right after 2 95 ma with the onset of major northern hemisphere glaciation long term average bottom water temperature bwt and bws at site 548 dropped abruptly by 5 c and 1 2 psu in contrast to more distal site 982 where bwt and bws continued to oscillate at estimates of 2 c and 1 5 2 5 psu higher than today until 2 6 ma we relate the small scale changes both to a reduced mow flow and to enhanced dilution by warmwaters of a strengthenednorth atlantic current temporarily replacingmowderivates at rockall plateau

temporal relationships of carbon cycling and ocean circulation at glacial boundaries evidence from high sedimentation rate south atlantic deep sea cores indicates that global and southern ocean carbon budget shifts preceded thermohaline circulation changes during the last ice age initiation and termination and that these were preceded by ice sheet growth and retreat respectively no consistent lead lag relationships are observed during abrupt millennial warming events during the last ice age allowing for the possibility that ocean circulation triggered some millenial climate changes at the major glacial interglacial transitions the global carbon budget and thermohaline ocean circulation responded sequentially to the climate changes that forced the growth and decline of continental ice sheets

quantitative reconstruction of holocene millennial scale precipitation in the asian monsoon margin of northwest china revealed by phytolith assemblages from calcareous root tubes in the tengger desert the spatio temporal characteristics and driving mechanisms of the asian summer monsoon asm during the early and middle holocene remain unclear moreover the timing of maximum monsoon precipitation in the asian monsoon margin of northwest china during this period is also a subject of considerable debate herein we analyzed phytolith assemblages within 55 calcareous root tube crt samples from the hinterland of the tengger desert to quantitatively reconstruct holocene millennial scale precipitation changes and discuss the potential forcing mechanisms our results revealed that the mean annual precipitation map was 138 53 to 149 18 mm during 10 0 7 0 cal kyr bp 179 26 to 192 26 mm during 7 0 5 0 cal kyr bp and 129 20 to 161 13 mm during 5 0 1 0 cal kyr bp the quantitative reconstruction results suggested that precipitation in this area was low in the early holocene then increased to a maximum in the middle holocene 30 higher precipitation than present and gradually decreased in the late holocene maximum monsoon precipitation did not occur until the middle holocene in this area because the northernmost margin of the asm reached its northernmost latitude lagging behind its maximum monsoon intensity suppressed by high northern latitude ice volume forcing during the early holocene despite high summer insolation in addition to quantifying the holocene millennial scale precipitation changes in the northwestern margin of the asm this study also provides new evidence for spatial and temporal variations of the asm during the holocene

shelf sandstones in the mowry shale evidence for deposition during cretaceous sea level falls two previously undescribed sandstone beds in the lower cretaceous mowry shale in north central wyoming form elongate retrogradational bodies meters thick and tens of kilometers in length that are encased in shale the sandstones were deposited under different depth and oxygen conditions than were the shales the sands are extensively bioturbated and contain wave induced features oscillation ripples and hummocky stratification whereas the surrounding muds contain few biogenic structures and lack evidence of wave motion mowry sandstone units are interpreted to have been deposited by storms in shallow 10 to 30 m oxygenated water the shales that encompass these sandstones were deposited largely from turbid bottom flows in dysaerobic and anaerobic water a drop in sea level is postulated to have promoted deposition of each sandstone bed on sections of the shelf where muds had accumulated previously below wave base in oxygen poor water during low water level this region was exposed to well oxygenated water and to the effects of storms subsequent sea level rise shifted the locus of sand deposition wave base and the intersection of oxygen boundaries with the seafloor in a shoreward direction the retrogradational geometry of each sand body is explained by the shoreward migration of sand deposition during sea level rise

124 000 year periodicity in terrestrial vegetation change during the late pliocene epoch the late pliocene 3 2 6 million years ago is an interval of exceptional interest for understanding the earth s climate system it was a time of progressive global cooling resulting in the growth of large terrestrial ice sheets and the initiation of extensive northern hemisphere glaciation1 12 the build up of the ice sheets was cyclical and apparently paced by the orbitally driven oscillations in incoming solar radiation milankovitch cycles at periods of approximately 41 kyr obliquity and 23 19 kyr precession here we present a high resolution continental record of late pliocene climate change detailing the response of terrestrial vegetation to this interval of dramatic global environmental change the annually laminated sequence of lake sediments from pula maar in hungary represents approximately 320 kyr of accumulation between 3 0 and 2 6 million years ago spectral analyses of the record indicate terrestrial responses to incoming solar radiation at obliquity and precession periodicities but the strongest response appears at a period of 124 kyr calculations indicate that variations in insolation forcing at this periodicity were negligible at this time the pula record thus demonstrates that internally driven nonlinear responses of the climate system at a period of 124 kyr were at least as important as external forcing at the orbital frequencies of precession and obliquity in driving late pliocene large scale environmental change

plio pleistocene high low latitude climate interplay a mediterranean point of view abstract the high low latitude climate interplay during the plio pleistocene global cooling is not yet well understood insight on the mediterranean region can provide some clues about past significant climate changes since the basin reflects the climate dynamics of both high latitude and low latitude regions being connected to the north atlantic and subjected to monsoon influence here we shade light on this connection problem by performing a spectral analysis on an eastern mediterranean stack of planktonic records spanning the last 5 ma and by further comparing it to north atlantic and pacific deep and surface water records our main conclusion is that the mediterranean detected the main global climate transitions over the last 5 myr although sapropel depositions indicate that it remained influenced by the african summer monsoon during the whole interval our analysis reveals that until 2 2 ma the mediterranean planktonic record is driven by regional processes dominated by precession the progressive emergence of the 41 kyr frequency in the mediterranean records around 2 8 ma suggests that since this date the mediterranean was more and more affected by the high latitude climate dynamics forcing than by the low latitude one moreover during the ongoing plio pleistocene cooling the 41 kyr frequency signal in the mediterranean records anticipated high latitude deep water response to the intensification of the northern hemisphere glaciations nhg and lagged the signal in tropical latitudes finally toward 1 2 ma the results suggest that the progressive shift from the 41 kyr to the 100 kyr frequency was led by the northern high latitudes overall our results confirm that the mediterranean is an ideal site to study the interplay between high and low latitude climates

high tide of the warm pliocene implications of global sea level for antarctic deglaciation we obtained global sea level eustatic estimates with a peak of 22 m higher than present for the pliocene interval 2 7 3 2 ma from backstripping in virginia united states new zealand and enewetak atoll north pacifi c ocean benthic foraminiferal 18 o values and mg ca 18 o estimates statistical analysis indicates that it is likely 68 confi dence interval that peak sea level was 22 5 m higher than modern and extremely likely 95 that it was 22 10 m higher than modern benthic foraminiferal 18 o values appear to require that the peak was 20 21 m our estimates imply loss of the equivalent of the greenland and west antarctic ice sheets and some volume loss from the east antarctic ice sheet and address the longstanding controversy concerning the pliocene stability of the east antarctic ice sheet

late quaternary variability of mediterranean outflow water from radiogenic nd and pb isotopes mediterranean outflow water mow is characterised by higher temperatures and salinities than other ambient water masses mow spreads at water depths between 500 and 1500 m in the eastern north atlantic and has been a source of salinity for the atlantic meridional overturning circulation in the north atlantic we used high resolution nd and pb isotope records of past ambient seawater obtained from authigenic ferromanganese coatings of sediments in three gravity cores at 577 1745 and 1974 m water depth in the gulf of cadiz and along the portuguese margin complemented by a selection of surface sediments to reconstruct the extent and pathways of mowover the past 23 000 years the surface and downcore nd isotope data from all water depths exhibit only a very small variability close to the present day composition of mowbut do not reflect the present day nd isotopic stratification of the water column as determined from a nearby open ocean hydrographic station in contrast the pb isotope records show significant and systematic variations which provide evidence for a significantly different pattern of the mow pathways between 20 000 and 12 000 years ago compared with the subsequent period of time

the climate influence on the mid depth northeast atlantic gyres viewed by cold water corals 1 the neodymium nd isotopic composition expressed in epsilon units nd of reef framework forming cold water corals provides unique measures of water mass provenance and mixing within the northeast atlantic today and in the past a reconstruction of near thermocline water nd from cold water corals of the gulf of cadiz and porcupine seabight spanning over the past 300 000 years now revealed that climate cooling during marine isotope stages mis 7 2 and mis 8 9 led to a retraction of the mid depth subpolar gyre mspg to the west conversely northern hemisphere warming and increasing fresh water fluxes to the northwest labrador sea favor a stronger eastward extension of the mspg blocking the northward flow of temperate atlantic water as observed during the early mis 1 and the early stage mis 5 5 these changes are likely the result of large scale south north displacement of the westerlies similar to present day observations that the north atlantic oscillation nao is linked with mid depth ocean circulation based on these observations we hypothesize that further climate warming will also strengthen the mspg leading to a salt and temperature decrease in the northeast atlantic whereas salinity and temperature will increase in the temperate atlantic however the amplitude of such changes on north atlantic overturning remains to be tested

new evidence for changes in plio pleistocene deep water circulation from southern ocean odp leg 177 site 1090 abstract changes in atlantic deep water circulation were reconstructed by comparing the benthic foraminiferal 13c record at odp site 1090 in the south atlantic with similar records from the north atlantic sites 982 607 925 929 and deep pacific site 849 oceans important deep water circulation changes occurred in the early pleistocene at 1 55 myr and during the mid pleistocene transition at 0 9 myr at 1 55 myr glacial 13c values in the southern ocean became significantly lower than those in the deep pacific establishing a pattern that persisted throughout the late pleistocene we propose that the lowering of 13c values of southern component water scw at this time resulted from expansion of sea ice and reduced ventilation of deep water during glacial periods after marine isotope stage 52 accompanying this change in southern ocean deep water circulation was enhanced interhemispheric coupling between the north and south atlantic after 1 55 myr at 0 9 myr the magnitude of glacial to interglacial variability in 13c increased and shifted to a longer frequency 100 kyr along with oceanic 18o ice volume calculation of percent northern component water ncw using site 1090 as the scw end member yielded 20 30 less reduction of ncw during glacial periods of the late pleistocene also a trend toward reduced glacial suppression of ncw during the past 400 kyr is not evident the apparent decoupling of ice volume and deep water circulation reported previously may be an artifact of using a pacific rather than a southern ocean carbon isotopic record to calculate past mixing ratios of ncw and scw

abrupt temperature changes in the western mediterranean over the past 250 000 years a continuous high resolution western mediterranean sea surface temperature sst alkenone record spanning the past 250 000 years shows that abrupt changes were more common at warming than at cooling during marine isotope stage mis 6 sst oscillated following a stadial interstadial pattern but at lower intensities and rates of change than in the dansgaard oeschger events of mis 3 some of the most prominent events occurred over miss 5 and 7 after prolonged warm periods of high stability climate during the whole period was predominantly maintained in interglacial interstadial conditions whereas the duration of stadials was much shorter

a method for climatic reconstruction of the mediterranean pliocene using pollen data pollen data from numerous sites around the mediterranean sea indicate that several important vegetation and climatic changes occurred during the pliocene these data are in good agreement with pollen records from northwest europe and with 18o curves from mediterranean and atlantic deep sea cores quantitative palaeoclimatic reconstructions from pliocene pollen data of the mediterranean region cannot be based on conventional modern analogue techniques as individual pliocene pollen spectra contain taxa representing temperate warm temperate and subtropical plants that do not grow together today instead we propose a new method that uses a climatic amplitude method modified to take partially into account the relative abundances of the taxa we applied this method to the pliocene garraf 1 palynological sequence from catalonia which provides a long continuous record of climatic change from 5 3 ma to the early pleistocene we estimate that annual temperatures were 1 to 5 c higher than today and the annual precipitation 400 to 1000 mm higher than today prior to the beginning of the late cenozoic glacial interglacial cycles in contrast temperature and precipitation both fell sharply during the glacial phases of the earliest glacial interglacial cycles

the mediterranean outflow splitting a comparison between theoretical models and canigo data abstract the mediterranean outflow in the atlantic is vertically subdivided into two main cores that are identified by temperature and salinity maxima hydrological data collected west of the strait of gibraltar and off the south and south west coasts of portugal in september 1997 have been used in two different models to shed some light on the mechanisms that cause a splitting of the mediterranean outflow these models are i a steady one dimensional streamtube model and ii a local time dependent model that includes cross stream variations the streamtube model applied at the northern respectively the southern part of the outflow appears capable of reproducing the observed density difference between the two cores the vertical separation however was underestimated the results from the local model suggest that a bump in the bottom topography may be a determining factor for the splitting tendency of the flow

neodymium isotopic composition and rare earth element concentrations in the deep and intermediate nordic seas constraints on the iceland scotland overflow water signature neodymium isotopic composition and rare earth element concentrations were measured in seawater samples from eleven stations in the nordic seas these data allow us to study how the iceland scotland overflow water isow acquires its neodymium signature in the modern ocean the waters overflowing the faroe shetland channel are characterized by nd 8 2 0 6 in good agreement with the only other data point published 19 years ago in the greenland and iceland seas the water masses leading to the formation of the isow display lower neodymium isotopic composition with nd around 11 and 9 respectively since no water masses in the nordic seas are characterized by nd 8 the radiogenic signature of the isow likely reflects inputs from the highly radiogenic norwegian basin basaltic margins jan mayen iceland faroe with nd 7 in addition to the neodymium isotopic composition the rare earth element patterns suggest that these inputs occur via the remobilization which includes resuspension and dissolution of sediments deposited on the margins whereas the neodymium isotopic composition behaves conservatively in the oceans in the absence of lithogenic inputs and can be used as a water mass tracer these results emphasize the role of interactions between sediments deposited on margins and seawater in the acquisition of the neodymium isotopic composition of water masses these results should allow a better use of this parameter to trace the present and the past circulation in the north atlantic

variability of silt grain size and planktonic foraminiferal preservation in plio pleistocene sediments from the western equatorial atlantic and caribbean abstract records of mean sortable silt and planktonic foraminiferal preservation from the ceara rise western equatorial atlantic and from the caribbean are presented to analyze the pliocene 3 5 2 2 ma to pleistocene 1 6 0 3 ma evolution of near bottom current strength and the carbonate corrosiveness of deep water during the mid pleistocene climate transition 1 ma a drastic decrease in glacial bottom current strength and an increase in carbonate corrosiveness is registered demonstrating a substantial decrease in the glacial contribution of the lower north atlantic deep water lnadw to the atlantic ocean also an increased sensitivity to eccentricity orbital forcing is registered after the mpt by contrast carbonate preservation increases considerably in the deep caribbean in response to a strong and persistent stable contribution of upper north atlantic deep water unadw we found evidence for the strongest and most stable circulation within the lnadw cell during the northern hemisphere cooling period between 3 2 and 2 75 ma this is in agreement with the superconveyor model which postulates that the highest nadw production took place prior to 2 7 ma a considerable decrease in bottom current strength and planktonic foraminiferal preservation is observed synchronous with the first occurrence of large scale continental ice sheets in the northern hemisphere this documents the final termination of the superconveyor at ca 2 75 ma however our data do not support a superconveyor in the interval between 3 5 and 3 2 ma when high amplitude fluctuations in bottom current flow and preservation in planktonic foraminifera are observed because of the great sensitivity of nadw production to changes in surface water salinity we assume that the high amplitude fluctuations of lnadw circulation prior to 3 2 ma are linked to changes in the atlantic salinity budget after 2 75 ma they are primarily controlled by ice sheet forcing in contrast to the stepwise deterioration of planktonic foraminiferal preservation in the western deep atlantic a trend toward better preservation from the pliocene to pleistocene is observed in the deep caribbean this indicates a long term increase in the contribution of unadw to the atlantic ocean

high amplitude variations in north atlantic sea surface temperature during the early pliocene warm period 1 we provide the first continuous orbital resolution sea surface temperature sst record from the high latitude north atlantic a region critical to understanding the origin of the plio pleistocene ice ages and proximal to regions that became frequently glaciated after 2 7 ma we analyzed sediments from ocean drilling program site 982 over the last 4 ma for their alkenone unsaturation index and compared this surface water signal to a benthic 18o record obtained from the same section we find that while ocean surface temperatures were significantly warmer 6 c than modern temperatures during the early pliocene they were also as variable as those during the late pleistocene a surprising result in light of the subdued variance of oxygen isotopic time series during the interval of 3 5 ma we propose two possible explanations for the high orbital scale sst variability observed either that a strong high latitude feedback mechanism not involving large continental ice sheets alternately cooled and warmed a broad region of the northern high latitudes or that by virtue of its location near the northern margin of the north atlantic drift the site was unusually sensitive to obliquity driven climate shifts on supraorbital time scales a strong sustained cooling of north atlantic ssts 4 5 c occurred from 3 5 to 2 5 ma and was followed by an interval of more modest cooling an additional 1 5 c from 2 5 ma to the present evolutionary orbital scale phase relationships between north atlantic sst and benthic 18o show that sst began to lead 18o significantly coincident with the onset of strong cooling at site 982 3 5 ma we speculate that these changes were related to the growth and subsequent persistence of a greenland ice sheet of approximately modern size through interglacial states

calibration of stable isotopic data an enriched δ18o standard used for source gas mixing detection and correction we present empirically based calibrations of our measurements made on a finnigan mat252 equipped with a kiel device to vienna pee dee belemnite using an enriched 18o standard calibrations include corrections for biases caused by the differences in isotopic composition of carbonate standards measured on the two parallel extraction lines of the kiel device and for decreases in the isotopic difference between the reference and sample gas caused by mixing in the source after correcting for these biases the precision of 2200 nbs19 analyses 10 300 g is 0 07 for 18o and 0 03 for 13c we have shared our standard enriched in 18o with 18 laboratories engaged in paleoceanographic research producing the first large scale interlaboratory calibrations for this community using correction procedures reported here water mass reconstructions using data produced on multiple mass spectrometers may now be possible with a precision approaching the level necessary to reconstruct temperature salinity and density variability in the deep ocean

on the origin and pathway of the saline inflow to the nordic seas insights from models the behaviours of three high resolution ocean circulation models of the north atlantic differing chiefly in their description of the vertical coordinate are investigated in order to elucidate the routes and mechanisms by which saline water masses of southern origin provide inflows to the nordic seas an existing hypothesis is that mediterranean overflow water mow is carried polewards in an eastern boundary undercurrent and provides a deep source for these inflows this study however provides an alternative view that the inflows are derived from shallow sources and are comprised of water masses of western origin carried by branches of the north atlantic current nac and also more saline eastern north atlantic water enaw transported northwards from the bay of biscay region via a shelf edge current sec flowing around the continental margins in two of the models the mow flows northwards but reaches only as far as the porcupine bank 53 n in third model the mow also invades the rockall trough extending to 60 n however none of the models allows the mow to flow northwards into the nordic seas instead they all support the hyporthesis of there being shallow pathways and that the saline inflows to the nordic seas result from nac derived and enaw water masses which meet and partially mix in the rockall trough volume and salinity transports into the southern rockall trough via the sec are in the various models between 25 and 100 of those imported by the nac and are also a similarly significant proportion 20 75 of the transports into the nordic seas moreover the highest salinities are carried northwards by the sec these being between 0 13 and 0 19 psu more saline at the southern entrance to the trough than those in the nac derived waters this reveals for the first time the importance of the sec in carrying saline water masses through the rockall trough and into the nordic seas furthermore the high salinities found on density surfaces appropriate to the mow in the nordic seas are shown to result from the wintertime mixing of the saline near surface waters advected northwards by the sec nac system throughout we have attempted to demonstrate the extent to which the models agree or disagree with interpretations derived from observations so that the study also contributes to an ongoing community effort to assess the realism of our current generation of ocean models

late pliocene variations in the carbon isotope values of north atlantic benthic foraminifera biotic control of the isotopic record abstract analyses of benthic foraminifera carbon isotope values were made in 172 samples from the late pliocene at dsdp site 548 which is located in the northeast atlantic on the continental slope southwest of ireland 1253 m water depth the carbon isotope record is quite stable from about 3 4 to 2 5 ma and contrasts with the record at site 552a in the north atlantic 2311 m water depth which is much more variable for this time period the carbon record at site 548 is also consistently more negative than that of site 552a after 2 5 ma the benthic carbon isotope record of uvigerina at site 548 becomes extremely variable with large negative deviations these isotopic variations might be associated with paleoceanic circulation changes caused by glaciation between 2 5 and 2 ma however paired analyses of uvigerina and cibicidoides species for 38 samples within the glacial interval yield carbon isotopic records which are quite different from one another the uvigerina record contains larger negative deviations than does the cibicidoides record also the isotopic difference between uvigerina and cibicidoides increases during glaciations and in a long term trend from 2 5 to 2 0 ma the lack of covariance between the uvigerina and cibicidoides carbon isotope records indicates that after 2 5 ma at site 548 biotic factors influenced the benthic isotopic record

seven ambiguities in the mediterranean palaeoenvironmental narrative abstract a review of seven outstanding issues on mediterranean palaeoenvironments is presented these are related to the dominant orbital pacing of climate variability the length of the interglacial vegetation succession the influence of the african summer monsoon the seasonality of precipitation during boreal insolation maxima the moisture balance during glacial maxima and the appearance of the mediterranean type climate rhythm and evolution of mediterranean sclerophyllous plants what emerges is that 1 marine 18oplanktonic and sst records show that precession has been a fundamental tempo of mediterranean climate change representing both a low latitude signal runoff from north africa and the direct influence of insolation at mediterranean latitudes but high latitude glacial effects 41 kyr and 100 kyr cycles became superimposed after 2 8 ma sapropel and dust deposition patterns in marine cores reveal that obliquity also has an effect on mediterranean climate through dry wet oscillations which are independent of glacial interglacial variability 2 the temperate part of interglacial vegetation succession has a duration of approximately half a precession cycle this persisted during the interval of obliquity dominated glacial cycles 2 8 1 ma with distinct forest successions following the precessional cycles however these are not always separated by an open vegetation phase because of minimal ice growth producing an impression of a prolonged interglacial forest interval 3 the effect of an enhanced african monsoon during summer insolation maxima has been mainly indirect in terms of nile discharge and runoff along the north african coast leading to increased freshwater input into the mediterranean sea reduced deep water ventilation and sapropel deposition 4 the notion of an accentuated summer rain regime in the northern mediterranean borderlands also contributing to a freshening of the mediterranean sea during boreal insolation maxima is not supported by the available evidence which suggests increased summer aridity 5 recent improvements in chronological precision and data resolution point to an increase in aridity and decreased temperatures during the last glacial maximum 21 2 ka but suggest an increase in effective moisture during the immediately preceding interval of 24 27 ka 6 the mediterranean type climate is not exclusively a post 3 6 ma phenomenon but may have appeared intermittently during the course of the tertiary or before 7 if that is the case then the paradigm that the sclerophyllous evergreen habit represents a pre adaptation to summer drought may need re evaluation

departures from eustasy in pliocene sea level records estimates for sea level three million years ago a period with similar atmospheric co2 levels to today vary from 10 to 40 m above present glacial isostatic adjustment modelling suggests that variations in the height of palaeoshorelines result from the residual adjustment of continental flexure following recent glaciations

evidence for enhanced mediterranean thermohaline circulation during rapid climatic coolings molecular biomarkers c37 alkenones n nonacosane and n hexacosanol and toc are used together with benthic 18o and 13c data to document the hydrographic response of the western mediterranean sea to rapid climatic variability these proxies are recorded in core md 95 2043 alboran sea affording the study of the dansgaard oeschger d o and heinrich he variability during the last glacial period the results suggest that rapid changes in the western mediterranean thermohaline circulation occurred in parallel to sea surface temperature oscillations enhanced deep water ventilation occurred during cold intervals he and d o stadials probably driven by a strengthening of north westerly wind over the north western mediterranean sea in contrast decreased intensity of the thermohaline circulation is detected during warm intervals d o interstadials which led to low oxygenated deep water masses and better preservation of the organic matter in the sediment

mediterranean functioning and sapropel formation respective influences of climate and hydrological changes in the atlantic and the mediterranean abstract during ocean drilling program odp legs 160 and 161 sapropels were recovered both in the western and eastern mediterranean this obliges to a reassessment of the previous studies focused on sapropels from only the eastern mediterranean and to consider the changes which occurred in the mediterranean climate but also in the water characteristics both in the atlantic and in the western mediterranean in the north atlantic the position of the polar front which migrated southwards during glacial times and the melting of northern ice caps during interglacial periods together with the convection in the labrador and norwegian seas appear essential to control the salinities of the waters facing the strait of gibraltar the salinities of the surface and intermediate layers constitute the first driving force of the mediterranean dynamics the second driving force being the mediterranean climate the stagnation of deep waters leading to sapropel deposition in the western mediterranean may be explained by a drastic weakening of the density difference between mediterranean outflow and atlantic intermediate waters facing the strait of gibraltar this weakening was induced primarily by the salinity decrease of atlantic surface water and secondly by a rather high salinity in the atlantic intermediate layer rather than by a drastic deterioration of the mediterranean climate this scenario probably concerns most of the sapropel events and it may be used for the knowledge of atlantic and mediterranean functioning over climatic changes

nd isotopic composition of water masses and dilution of the mediterranean outflow along the southwest european margin nd isotopic compositions epsilon nd of seawater profiles and deep sea corals collected off the coast of iberia and from the bay of biscay were measured 1 to constrain the nd isotopic composition of water masses along the southwest european margin 2 to track the mediterranean outflow water mow during its northward propagation and 3 to establish hydrological changes during the last 1500 years the eastern north atlantic central water enacw is characterized by nd isotopic composition of around 12 0 mediterranean sea water msw is collected from 800 and 1200 m depth and is characterized by epsilon nd values ranging from 10 9 off the coast of iberia to 11 6 in the bay of biscay these epsilon nd results suggest a strong dilution of the pure mow at the strait of gibraltar epsilon nd 9 4 of approximately 40 and 30 along its northward circulation pathway essentially with a contribution from enacw at around 2000 m depth epsilon nd water profiles display the occurrence of a nonradiogenic water mass epsilon nd 13 originating from the labrador sea labrador sea water fossil deep sea corals dated between 84 and 1500 years display nd isotopic compositions that vary moderately from present day seawater values suggesting a weaker influence of mow in the formation of msw during the dark ages and the little ice age these recent cold events seem to be associated with a reduction in the northward penetration of msw which may result from a greater eastward extension of the middepth subpolar gyre and or a reduction of msw formation likely tied to a variation in deep mediterranean water production

high frequency climate linkages between the north atlantic and the mediterranean during marine oxygen isotope stage 100 mis100 high resolution records of mediterranean and north atlantic deep sea sediments indicate that rapid changes in hydrology and climate occurred during marine oxygen isotope stage 100 mis100 at 2 52 ma which exhibits characteristics similar to late pleistocene dansgaard oeschger bond cycles and heinrich events as in the late pleistocene north atlantic oceanographic and atmospheric changes were probably transmitted into the mediterranean by some combination of surface water inflow and atlantic mediterranean atmospheric pressure gradients our data suggest that the mechanism s responsible for sub milankovitch and millennial scale climate oscillations were the same over at least the past 2 6 ma notwithstanding the transition from the 41 kyr dominated glacial cycles of the late pliocene to the 100 kyr cycles of the late pleistocene

neodymium isotopes in the mediterranean sea comparison between seawater and sediment signals nine depth profiles of dissolved nd concentrations and isotopic ratios epsilon nd were obtained in the levantine basin the ionian the aegean the alboran seas and the strait of gibraltar thirteen core top sediments and nile river particle samples were also analyzed leached with 1 n hcl acetic acid or hydroxylamine hydrochloride the seawater epsilon nd values become more radiogenic during the eastward circulation in the mediterranean sea the relationship between salinity and the seawater epsilon nd shows that the nd isotopic signature is more conservative than salinity in the mediterranean sea the water mass with the highest epsilon nd 4 8 is found at about 200 m in the easternmost levantine basin the average epsilon nd value for deep waters is 7 0 in the eastern basin 2 5 epsilon units higher than in the western basin by examining the sensitivity of seawater epsilon nd to nd inputs from the nile we conclude that the most significant radiogenic nd source is partially dissolved nile river particles the nd flux from the nile river water has a minor influence on the mediterranean seawater end except for the easternmost levantine basin the leachate epsilon nd values are consistent with the seawater values in the easternmost levantine basin the leachate end values obtained with hcl leaching are systematically higher than the seawater values the relationship between leachate and residual epsilon nd values indicates that the hcl leaching partially dissolves lithogenic nd so the dissolution of nile river particles is the cause of the observed shift some end values obtained with hydroxylamine hydrochloride leaching are higher than those obtained with hcl leaching although the reason for this shift is not clear sr 87 sr 86 successfully detects the presence of a nonmarine component in the leachate our results suggest that leaching performance may vary with the mineralogy of marine sediments at least in the case of the mediterranean sea copyright c 2004 elsevier ltd

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

north atlantic deep water export to the southern ocean over the past 14 myr evidence from nd and pb isotopes in ferromanganese crusts the intensity of north atlantic deep water nadw production has been one of the most important parameters controlling the global thermohaline ocean circulation system and climate here we present a new approach to reconstruct the overall strength of nadw export from the north atlantic to the southern ocean over the past 14 myr applying the deep water nd and pb isotope composition as recorded by ferromanganese crusts and nodules we present the first long term nd and pb isotope time series for deep southern ocean water masses which are compared with previously published time series for nadw from the nw atlantic ocean these data suggest a continuous and strong export of nadw or a precursor of it into the southern ocean between 14 and 3 ma an increasing difference in nd and pb isotope compositions between the nw atlantic and the southern ocean over the past 3 myr gives evidence for a progressive overall reduction of nadw export since the onset of northern hemisphere glaciation nhg the nd isotope data allow us to assess at least semiquantitatively that the amount of this reduction has been in the range between 14 and 37 depending on location

late pliocene northern hemisphere glaciations the continental and marine responses in the central mediterranean abstract detailed pollen analyses and oxygen isotope records of three foraminiferal species globigerina bulloides uvigerina peregrina and cibicides pachyderma from the semaforo and vrica composite sections crotone southern italy have been compared to the global climatic changes depicted by late pliocene early pleistocene foraminiferal 18o records of site 607 in the north atlantic and hole 653a in the tyrrhenian basin west mediterranean major overturns in the mid altitude vegetation are shown near isotopic stages 82 60 58 and 50 at about 2 03 ma 1 6 ma and 1 37 ma according to the raymo et al 1989 and ruddiman et al 1989 timescales at the same dates glacial 18o maxima either became higher or display step increases in the western mediterranean or in the open ocean as well this suggests that size increases of northern hemisphere ice sheets were the driving factor for regional or local marine and continental environmental changes within the mediterranean basin near isotopic stages 62 60 close to the conventional plio pleistocene boundary the climatic conditions severed enough within the mediterranean basin to modify the continental environment as depicted by a sudden increase of artemisia percentages while the first significant southward migration of the north polar front may have been recorded by an influx of left coiling neogloboquadrina pachyderma in the central mediterranean it also appears that boreal guests entered the mediterranean during phases of 18o enrichment of foraminiferal calcite there does not seem to be any discrepancy between the climatic concept of the pliocene pleistocene boundary and its chronostratigraphic definition

new constraints on the pb and nd isotopic evolution of ne atlantic water masses 1 time series of lead pb and neodymium nd isotope compositions were measured on three ferromanganese crusts recording the evolution of ne atlantic water masses over the past 15 ma the crusts are distributed along a depth profile 700 4600 m comprising the present day depths of mediterranean outflow water and north east atlantic deep water a pronounced increase of the 206pb 204pb in the two deeper crusts starting at 4 ma and a decrease in 143nd 144nd in all three crusts took place between 6 4 ma and the present these patterns are similar to isotope time series in the western north atlantic basin and are consistent with efficient mixing between the two basins however the changes occurred 1 3 ma earlier in the eastern basin indicating that the northeastern atlantic led the major change in pb and nd isotope composition probably due to a direct supply of labrador seawater via a northern route the pb isotope evolution during the pliocene pleistocene can generally be explained by mixing between two end members corresponding to mediterranean outflow water and north east atlantic deep water but external sources such as saharan dust are likely to have played a role as well the pb isotope composition of the shallowest crust that grew within the present day mediterranean outflow water does not show significant pb isotope changes indicating that it was controlled by the same pb sources throughout the past 15 ma

paleoceanography of the atlantic mediterranean exchange overview and first quantitative assessment of climatic forcing 1 the mediterranean sea provides a major route for heat and freshwater loss from the north atlantic and thus is an important cause of the high density of atlantic waters in addition to the traditional view that loss of fresh water via the mediterranean enhances the general salinity of the north atlantic and the interior of the eastern north atlantic in particular it should be noted that mediterranean water outflowing at gibraltar is in fact cooler than compensating inflowing water the consequence is that the mediterranean is also a region of heat loss from the atlantic and contributes to its large scale cooling uniquely this system can be understood physically via the constraints placed on it by a single hydraulic structure the gibraltar exchange here we review the existing knowledge about the physical structure of the gibraltar exchange today and the evidential basis for arguments that it has been different in the past using a series of quantitative experiments we then test prevailing concepts regarding the potential causes of these past changes we find that 1 changes in the vertical position of the plume of mediterranean water in the atlantic are controlled by the vertical density structure of the atlantic 2 a prominent early holocene contourite gap within the gulf of cadiz is a response to reduced buoyancy loss in the eastern mediterranean during the time of sapropel 1 deposition 3 changes in buoyancy loss from the mediterranean during mis3 caused changes in the bottom velocity field in the gulf of cadiz but we note that the likely cause is reduced freshwater loss and not enhanced heat loss and 4 strong exchange at gibraltar during atlantic freshening phases implies that the gibraltar exchange provides a strong negative feedback to reduced atlantic meridional overturning given the very counterintuitive way in which the strait of gibraltar system behaves we recommend that without quantitative supporting work qualitative interpretations of how the system has responded to past external forcing are unlikely to be robust

changes in east atlantic deepwater circulation over the last 30 000 years eight time slice reconstructions using 95 epibenthic 13c records eight time slices were reconstructed to trace the distribution of east atlantic deepwater and intermediate water masses over the last 30 000 years our results show that there have been three distinct modes of deepwater circulation near the stage 3 2 boundary the origin of north atlantic deep water nadw was similar to today mode 1 however after late stage 3 the source region of the nadw end member shifted from the norwegian greenland sea to areas south of iceland mode 2 a reduced nadw flow persisted during the last glacial maximum with constant preformed 13c values the nutrient content of nadw increased markedly near the azores fracture zone from north to south probably because of the mixing of upwelled antarctic bottom water aabw from below which then advected with much higher flux rates into the northeast atlantic later the spread of glacial meltwater over the north atlantic led to a marked short term ventilation minimum below 1800 m about 13 500 14c years ago mode 3 the formation of nadw recommenced abruptly north of iceland 12 800 12 500 years ago and reached a volume approaching that of the holocene in the younger dryas 10 800 10 350 years b p another short term shutdown of deepwater formation followed between 10 200 and 9 600 years b p linked to a further major meltwater pulse into the atlantic each renewal of deepwater formation led to a marked release of fossil co2 from the ocean the likely cause of the contemporaneous 14c plateaus over the last 9000 years deepwater circulation varied little from today apart from a slight increase in aabw about 7000 14c years ago it is also shown that the oxygenated mediterranean outflow varied largely independent of the variations in deepwater circulation over the last 30 000 years

the mid pliocene 4 3 2 6 ma benthic stable isotope record of the southern ocean odp sites 1092 and 704 meteor rise abstract we present mid pliocene 4 3 2 6 ma benthic stable oxygen and carbon isotope data from ocean drilling program site 1092 odp leg 177 drilled in the sub antarctic sector of the southern ocean the results are compared with the stable isotope results from nearby site 704 odp leg 114 oxygen isotope data show that minimum values are about 0 5 less than those of the holocene which is consistent with the results from site 704 indicating only minor deglaciation of antarctica during the studied interval oxygen isotope data from both site 1092 and site 704 are slightly higher relative to pacific values during several intervals which could be related to the contribution of warm saline north atlantic deep water nadw comparisons of benthic carbon isotope gradients between sites located in the north atlantic sub antarctic sector of the southern ocean and pacific indicate that at times the gradient between the southern ocean and the pacific evolved differently than the atlantic pacific gradient this suggests that variations in nadw strength alone might not be responsible for the observed carbon isotope values in the southern ocean

neodymium isotopic composition of the mediterranean outflow and the eastern north atlantic the neodymium concentration and isotopic composition of the mediterranean outflow in the strait of gibraltar has been directly determined the outflow has a dissolved nd concentration of 23 pmoles kg with nd 0 9 4 these results are in clear disagreement with previous values inferred for the mediterranean this concentration and nd 0 do not sharply contrast with north atlantic deep water nadw values and can produce isotopic anomalies no larger than 1 unit in the north atlantic a new profile in the eastern north atlantic does not exhibit an nd 0 anomaly at the depth of the mediterranean water core in accordance with the above inferences while concentrations and nd 0 at this site indicate that nd is conservative in the deep eastern north atlantic a previously reported nd 0 anomaly in the eastern north atlantic originally associated with the mediterranean outflow has a different source the direct contribution of the mediterranean outflow to the nadw nd budget is small 10 of the high latitude sources however the outflow plus entrained water supply 30 of the total nd associated with northern latitude deep water sources concentrations of dissolved nd in the outflow are enriched compared to the atlantic inflow which has 16 pmoles kg with nd 0 11 8 the principal source of the excess dissolved nd is inferred to be partial dissolution of detrital particles simple mass balance calculations suggest that 0 6 of the nd associated with detrital river transported sediments and or 3 5 of the nd associated with eolian particles delivered to the mediterranean is mobilized during passage into the marine environment if general these mobilization efficiencies imply that the dissolution of detrital particles is a significant source of dissolved nd and other rare earth elements in seawater

modelling nd isotopes with a coarse resolution ocean circulation model sensitivities to model parameters and source sink distributions the neodymium nd isotopic composition nd of seawater is a quasi conservative tracer of water mass mixing and is assumed to hold great potential for paleoceanographic studies here we present a comprehensive approach for the simulation of the two neodymium isotopes 143nd and 144nd using the bern3d model a low resolution ocean model the high computational efficiency of the bern3d model in conjunction with our comprehensive approach allows us to systematically and extensively explore the sensitivity of nd concentrations and nd to the parametrisation of sources and sinks previous studies have been restricted in doing so either by the chosen approach or by computational costs our study thus presents the most comprehensive survey of the marine nd cycle to date our model simulates both nd concentrations as well as nd in good agreement with observations nd covaries with salinity thus underlining its potential as a water mass proxy results confirm that the continental margins are required as a nd source to simulate nd concentrations and nd consistent with observations we estimate this source to be slightly smaller than reported in previous studies and find that above a certain magnitude its magnitude affects nd only to a small extent on the other hand the parametrisation of the reversible scavenging considerably affects the ability of the model to simulate both nd concentrations and nd furthermore despite their small contribution we find dust and rivers to be important components of the nd cycle in additional experiments we systematically varied the diapycnal diffusivity as well as the atlantic to pacific freshwater flux to explore the sensitivity of nd concentrations and its isotopic signature to the strength and geometry of the overturning circulation these experiments reveal that nd concentrations and nd are comparatively little affected by variations in diapycnal diffusivity and the atlantic to pacific freshwater flux in contrast an adequate representation of nd sources and sinks is crucial to simulate nd concentrations and nd consistent with observations the good agreement of our results with observations paves the way for the evaluation of the paleoceanographic potential of nd in further model studies

late pliocene millennial scale climate variability in the northern north atlantic prior to and after the onset of northern hemisphere glaciation 1 sediments recovered at ocean drilling program site 984 on the reykjanes ridge provided multicentennial scale records of late pliocene climate change over the onset of northern hemisphere glaciation nhg 2 95 2 82 ma short term climate variations prior to and after the onset of continent wide glaciation were compared to test the hypothesis of whether dansgaard oeschger do cycles may have been triggered by continental ice breakouts during two selected interglacial stages prior to and after nhg g15 and g1 climate variability resembled that found in the holocene and the mid pliocene warm period in contrast do like periodicities of 1470 2900 and 4400 years indeed only occurred in glacial stages after the onset of nhg g14 g6 and 104 but not in stage g20 prior to the onset these results suggest a causal link between do cycles and the late pliocene onset of major nhg and ice breakouts in the north atlantic

a review of the physical oceanography of the mediterranean outflow abstract the physical oceanography of the mediterranean sea is reviewed with particular emphasis on the mediterranean outflow in the gulf of cadiz in this region the dense mediterranean water forms a high velocity bottom current that interacts strongly with the sea floor the major energy source for the plume comes from the release of potential energy as the plume descends the continental slope and the major energy sink is work against bottom stress which is as large as 4 pa where the plume begins to descend the continental slope in this region the current makes a nearly inertial turn that would otherwise appear to be steered by the underlying topography the mediterranean plume entrains the overlying north atlantic central water and thereby loses much of its density anomaly the mixed mediterranean water becomes neutrally buoyant in the lower portion of the north atlantic thermocline near cape st vincent there are then two preferred transport modes having somewhat different temperature and salinity whose distinct characteristics can be found far into the open north atlantic the temperature salinity and volume of the mediterranean water in the strait of gibraltar and in the gulf of cadiz appear to be roughly constant since modern measurements have been made the estimated westward transport of mediterranean water has gone down considerably as direct measurement techniques have been applied a recent estimate is that the westward transport of pure mediterranean water is only about a half a sv 1 sv 106 m3 s 1 the transport of mixed mediterranean water in the western gulf of cadiz is larger by about a factor of three or four because of the entrainment of north atlantic water

the oxygen and carbon isotope distribution in the mediterranean water masses abstract the oxygen and carbon stable isotope compositions of the present day mediterranean waters have been measured in order to evaluate their variability which is related to the specific climatic and hydrological conditions within the basin the experimental equation between the 18 o value and the salinity of water based on 300 measurements on surface intermediate and deep waters sampled during the vicomed 2 and 3 cruises in the western central and eastern mediterranean has a slope of 0 27 a value which is significantly lower than the slope of 0 45 as defined in the northeast atlantic ocean this difference in the 18 o salinity relationship which occurs immediately in the alboran basin is basically a characteristic of the climatic regime of the mediterranean i e of an excess evaporation over fresh water input the largest variations of these two parameters 18 o of water and 13 c of co 2 are observed in the surface waters mostly in the western mediterranean this evolution mirrors the progressive eastward restriction which separates the less evaporated and more productive western basins from the more evaporated and less productive eastern basins the intermediate waters constitute a homogeneous layer however their 18 o values decrease eastward by 0 35 at maximum due to progressive dilution by mixing with overlying and underlying water masses their 13 c values decrease also eastward by 0 35 at maximum due to an increasing input of nutrients issued from the regeneration of sinking organic particles the deep waters have similar 18 o values but slightly higher 13 c values often by less than 0 1 than the overlying intermediate waters indicating generally well ventilated conditions due to active winter convection