quaternary history of the antarctic circumpolar current evidence from the scotia sea within the scotia sea the axis of the antarctic circumpolar current acc is geographically confined and sediments therefore contain a record of palaeo flow speed uncomplicated by acc axis migration we outline holocene and last glacial maximum lgm current controlled sedimentation using data from 3 5 khz profiles cores and current meter moorings geophysical surveys show areas of erosion and deposition controlled by neogene basement topography deposition occurs in mounded sediment drifts or flatter areas where 500 1000 m of sediment overlies acoustic basement 3 5 khz profiles show parallel continuous sub bottom reflectors with highest sedimentation rates in the centre of the drifts and reflectors converging towards marginal zones of non deposition locally on the flanks of continental blocks e g south georgia downslope processes are dominant the absence of mudwaves on the sediment drifts may result from the unsteadiness of acc flow a core transect from the acc axis south to the boundary with the weddell gyre shows a southward decrease in biogenic content controlled by the polar front and the spring sea ice edge both these features lay farther north at lgm the cores have been dated by relative abundance of the radiolarian cycladophora davisiana and by changes in the biogenic ba content a palaeoproductivity indicator sedimentation rates range from 3 to 17 cm ka the grain size of holocene sediments shows a coarsening trend from south to north consistent with strongest bottom current flow near the acc axis though interpretation is complicated by the presence of biogenic grains year long current meter records indicate mean speeds from 7 cm s in the south to 12 cm s in the north with benthic storm frequency increasing northwards lgm sediments are predominantly terrigenous and show a clearer northward coarsening trend with well sorted silts in the northern scotia sea assuming a constant terrigenous source this implies stronger acc flow at the lgm contrasting with weaker weddell gyre flow deduced from earlier work

oceanographic and climatic changes over the past 160 000 years at deep sea drilling project site 594 off southeastern new zealand southwest pacific ocean high resolution textural carbonate microfossil and oxygen 18o and carbon 13c stable isotopic analyses are presented for the late quaternary isotopic stages 1 to 6 interval of a core at deep sea drilling project site 594 situated just south of the present subtropical convergence in northernmost subantarctic surface waters on the southern flank of chatham rise in the southwest pacific downcore alternations of pelagic and hemipelagic oozes correspond to interglacial and glacial episodes respectively interglacial oozes contain a northern subantarctic assemblage of planktonic foraminifera with rare cool subtropical species while glacial oozes are characterized by species typical of southern most subantarctic waters and include radiolaria with affinity for antarctic waters the planktonic 18o record for the site supports a 3 6 c temperature change in near surface waters between interglacial and glacial stages and indicates that during stage 5e the near surface waters were about 1 c warmer than at present a pronounced cooling during stage 5d matches that of the vostok ice core d record and negative excursions at the end of stages 2 and 6 support a southern latitude warming preceding northern hemisphere deglaciation benthic foraminifera typical of cold intermediate to deep waters can increase dramatically in abundance in the hemipelagic ooze intervals when 18o results suggest a temperature drop in glacial stage bottom waters of 2 4 c possibly a result of upward displacement of antarctic intermediate water by circumpolar deep water at the site the foraminiferal 13c records support a reduced influence of north atlantic deep water in the southwest pacific during glacial stages when nutrient enhancement occurred in both bottom and surface waters despite the pronounced changes between interglacial and glacial conditions inferred at site 594 contributed to by a substantial northward shift by at least 5 of latitude in the position of the antarctic convergence south of new zealand during glacial episodes we conclude that the subtropical convergence remained locked to chatham rise approximately 44 s throughout stages 1 to 6 major compression of the intervening belt of subantarctic water during glacial episodes and the associated very steep thermal gradients and intensified atmospheric and oceanic circulation patterns that developed in this part of the southwest pacific account for the harsh frigid environment reported for on land southern new zealand at these times

the last deglaciation in the southern ocean the isotopic and micropaleontological deglacial records of three deep sea cores from 44 s to 55 s have been dated by accelerator mass spectrometry the available records did not allow accurate dating of the initiation of the deglaciation by 13 000 years b p sea surface temperatures reached values similar to the present values a cool oscillation abruptly interrupted this warm phase between 12 000 and 11 000 years b p initiation of this cooling therefore preceded the northern hemisphere younger dryas by approximately 1000 years complete warming was reached by 10 000 years b p more or less synchronous with the northeast atlantic ocean

changes in deep and intermediate water properties in the western north atlantic during marine isotope stages 11 12 evidence from odp leg 172 abstract the interval of time represented by marine isotope stages 11 and 12 360 470 ka contains what may be the most extreme glacial and interglacial climate conditions of the late pleistocene it has been suggested that sea level rose by 160 m at the termination of glacial stage 12 this is 30 greater than the sea level rise that followed the most recent glacial maximum there have been few detailed studies of the unique conditions that existed during the stage 11 12 time period because of the lack of high quality core material this problem has been addressed by the collection of high deposition rate cores from sediment drifts in the western north atlantic during ocean drilling project leg 172 benthic foraminiferal 13 c data from cores collected between 4600 and 1800 m were used to reconstruct bathymetric gradients in deep and intermediate water properties for selected time slices during this glacial interglacial cycle during glacial stage 12 the deep western north atlantic was filled by a water mass that was more nutrient enriched than modern antarctic bottom water above 2000 m a more nutrient depleted water mass existed during this glacial stage such an intermediate water mass has been described for more recent glacial periods and presumably forms in a more proximate region of the north atlantic interglacial stage 11 water mass properties closely resemble those of the present day western north atlantic a nutrient depleted water mass 13 c of 0 75 1 0 similar to modern north atlantic deep water existed between 3500 and 2000 m this was underlain by a water mass with lower 13 c values 13 c change of 0 95 from stage 12 to stage 11 this is twice the whole ocean 13 c change reported for the transition from the last glacial maximum to the holocene

a discussion of hunter and sparks contrib mineral petrol 95 451 461 silica poor iron rich liquids like those estimated for skaergaard by wager and brown are preserved in labrador as chilled pillows iron rich rocks in the kiglapait intrusion have likewise been produced by liquids poor in silica there is no reason to expect such liquids in volcanoes so their absence as lavas means nothing about their possible plutonic occurrence successful tests of the siliceous liquid hypothesis would include showing that the proposed rhyolitic liquids can produce the observed olivines in the range fo 10 40 they would require showing that the observed low f o 2 oxide mineral assemblages can occur at the high silica contents proposed and that the silicate mineral assemblages and compositions can represent high silica liquids i doubt very much that these critical tests can be met

origin of detrital fluxes in the southeast indian ocean during the last climatic cycles because of a close relationship between detrital flux variations and magnetic susceptibility ms flux ms cm 3 of bulk sediment multiplied by the linear sedimentation rate variations in the southeast indian basin of the southern ocean ms flux profiles have been used to examine the spatial and temporal detrital flux changes in this basin during the last climatic cycle results indicate a general increase in detrital material input during the coldest periods suggesting a widespread phenomenon at least on the basin scale mineralogical data geochemical data and 87sr 86sr isotopic ratios have been used to determine the origin and transport mechanisms responsible for increased detrital flux during glacial periods mineralogical and geochemical data show that these glacial highs are due to increases in both kerguelen crozet volcanic and antarctic detrital inputs the 87sr 86sr isotopic composition of the 45 m fraction indicates that the kerguelen crozet province contributes to at least 50 of the coarse particule input to the west this contribution decreases eastward to reach less than 10 these tracers clearly indicate that the crozet kerguelen province was a major source region of detrital in the western part of the basin during glacial times in contrast material of antarctic origin is well represented in the whole basin fine and coarse fractions because of the minor amount of coarse particles in the sediments volcanic particles from kerguelen and crustal particles from antarctica have most probably been transported by the antarctic bottom water current and or the circumpolar deepwater current during glacial periods as is the case today nevertheless the presence of coarse particles even in low amount suggests also a transport by ice rafting sea ice and icebergs originated from both kerguelen and antarctic sources however the relative importance of both hydrographic and ice rafting modes of transport cannot be identified accurately with our data during low sea level stands glacial maximum periods increasing instability and erosion of the continental platform and shallow plateaus could have resulted in a more efficient transfer of crustal and volcano detrital material to the southeast indian basin at the same time extension of the grounded ice shelves over the continental margins and increase in the erosion rate of the antarctic ice sheet could have induced a greater input of ice rafted detritus ird to southern ocean basins enhancement of the circumpolar deepwater current strength might have also carried a more important flux of detrital material from kerguelen however an increase in the bottom water flow is not necessarily required

radiocarbon reservoir age of high latitude north atlantic surface water during the last deglacial the radiocarbon reservoir age of high latitude north atlantic ocean surface water is essential for linking the continental and marine climate records and is expected to vary according to changes in north atlantic deep water nadw production measurements from this region also provide important input and or tests of oceanic radiocarbon using 3 d global ocean circulation models here we present a surface water radiocarbon reservoir age record of the high latitude western north atlantic for the deglacial period via the use of fossil cold water corals growing in waters that are rapidly exchanged with nearby surface waters the reservoir age of high latitude north atlantic surface waters was computed from the radiocarbon age difference between our radiocarbon calibration record http radiocarbon ldeo columbia edu and our marine radiocarbon data 230th 234u 238u dates provide the absolute coral ages our high latitude north atlantic ocean reservoir age data combined with recalculated reservoir ages based on published coexisting terrestrial and marine material and vedde ash radiocarbon dates from central and eastern north atlantic show modern values 380 140 year n 14 during the bolling and allerod warm period and a 200 year increase in reservoir age 590 130 year n 10 during the entire younger dryas yd cold episode the reservoir age then decreased to 270 20 year n 2 at the preboreal yd transition although the dates are too sparse for us to be confident in this estimate we are not able to resolve the timing of the transition to increased reservoir ages from the mid allerod to the yd due to the relatively small change and correspondingly large uncertainty in the estimates the atmospheric 14c record derived from our atmospheric radiocarbon record displays a 40 per mil increase from 12 900 to 12 650 cal years bp coincident with the shift to high reservoir ages in the early yd cold event intrusion of 14c depleted antarctic intermediate water aaiw to the high latitude north atlantic and reduction of nadw formation are possible causes for the coincident shift to high reservoir ages in the north atlantic surface ocean and increased atmospheric 14c during the beginning of the yd event

enhanced δ13c and δ18o differences between the south atlantic and south pacific during the last glaciation the deep gateway hypothesis enhanced vertical gradients in benthic foraminiferal 13c and 18o in the atlantic and pacific during the last glaciation have revealed that ocean overturning circulation was characterized by shoaling of north atlantic sourced interior waters nonetheless our understanding of the specific mechanisms driving these glacial isotope patterns remains incomplete here we compare high resolution depth transects of cibicidoides spp 13c and 18o from the southwest pacific and the southwest atlantic to examine relative changes in northern and southern sourced deep waters during the last glacial maximum lgm and deglaciation during the lgm our transects show that water mass properties and boundaries in the south atlantic and pacific were different from one another the atlantic between 1 0 and 2 5 km was more than 1 enriched in 13c relative to the pacific and remained more enriched through the deglaciation during the lgm atlantic 18o was 0 5 more enriched than the pacific particularly below 2 5 km this compositional difference between the deep portions of the basins implies independent deep water sources during the glaciation we attribute these changes to a deep gateway effect whereby northern sourced waters shallower than the drake passage sill were unable to flow southward into the southern ocean because a net meridional geostrophic transport cannot be supported in the absence of a net east west circumpolar pressure gradient above the sill depth we surmise that through the lgm and early deglaciation shoaled northern sourced waters were unable to escape the atlantic and contribute to deep water formation in the southern ocean

benthic foraminiferal abundance and stable isotope changes in the indian ocean sector of the southern ocean during the last 20kyr paleoceanographic implications abstract benthic foraminiferal faunas and benthic stable isotopes during the last 20 kyr were investigated in core md02 2488 46 28 8 s 88 01 3 e 3420 m water depth from the southeast indian ocean sector of the southern ocean in order to document paleoceanographic changes from the last glacial through deglaciation and into the holocene variations occurred in the accumulation rates species composition diversity and stable isotopes during the last 20 kyr the benthic 18o and 13c records during the last 28 kyr show that no significant deep water circulation changes took place before 15 6 calendar kyr ka the 13c increase at 15 6 ka is interpreted as reflecting the arrival of better ventilated deep waters at the site and is associated with the progressive increase in of miliolids from 15 ka to 0 ka an interval 16 15 ka of low benthic foraminiferal accumulation rates bfars and an increase in the relative abundance of stainforthia complanata deep waters were 2 2 0 4 c colder during the last glacial than at present carbonate dissolution proxies indicate that carbonate dissolution was higher during the last glacial than during the last deglacial and holocene bfars and the percentage of species indicative of high productivity suggest that the flux of organic matter resulting from surface water productivity was generally higher between 20 and 13 ka than between 12 and 0 ka consistent with the observed higher proportion of biosiliceous sediment at 20 14 ka and published records the combined percentages of the two most abundant species alabaminella weddellensis and epistominella exigua phytodetritus species suggest that seasonality of productivity intermittency of primary productivity fluctuated over the last 20 kyr and was higher during the last glacial and deglaciation 20 10 ka compared with the holocene 10 0 ka the relative abundances of a weddellensis and e exigua do not covary over the last 20 kyr an observation also reported from other areas

geochemical decoupling of water masses in the variscan oceanic system during late devonian times abstract neodymium isotope data obtained from conodonts collected in morocco and poland attest a geochemical decoupling of seawater in the variscan realm during the late devonian the water masses on the shelves were characterized by unradiogenic e nd values from about 7 to 12 the surface water in the bordering oceans the rheic ocean and the variscan sea yielded more radiogenic e nd values ranging from about 1 to 6 a compilation of nd and c isotope signatures from the moroccan meseta suggests that the shelf and the surface oceanic water differed also in their carbon isotope evolution the geochemical decoupling constrains a restricted water exchange between shelves and the ocean oceanic waters entered far into the shelf areas only during the semichatovae transgression in the frasnian zone 11 sea level fluctuations constituted the major force that modified circulation and thus controlled the mixing of shelf and surface oceanic seawater the nd conodont data reveal that the surface water of the variscan oceanic water resembled the seawater of the modern pacific ocean rather than those of the atlantic and indian oceans the geochemical character of the variscan oceanic water is consistent with the geotectonic regime of the variscan realm in late devonian and carboniferous times

large fluctuations of dissolved oxygen in the indian and pacific oceans during dansgaard oeschger oscillations caused by variations of north atlantic deep water subduction a s and s h were supported by the paleoclimate program of the national science foundation as part of the paleovar project atm 0602395

unstable behavior of the laurentide ice sheet over deforming sediment and its implications for climate change geologic records of fluctuations of the laurentide ice sheet margin following the most recent glacial maximum ca 20 000 14c yr b p identify fundamental differences in ice sheet behavior depending on subglacial bed conditions rapid and irregular icemargin fluctuations occurred only over areas of deforming sediment indicating nonclimatic forcing controlled by the inherent instability of coupled ice sheet deforming sediment dynamics in contrast largely uninterrupted ice margin retreat with no evidence of significant readvance occurred over rigid bed areas indicating stable behavior unstable ice sheet behavior was most pronounced from 15 000 until 10 000 14c yr b p by which time most of the ice margin had retreated onto a rigid bed unstable ice sheet behavior would have been an integral component in controlling variable fluxes of icebergs and meltwater as well as meltwater routing to the north atlantic thus affecting thermohaline circulation the abrupt climate oscillations in the north atlantic region that ended at 10 000 14c yr b p may thus have their origin in the inherently unstable behavior of the laurentide ice sheet overriding deforming sediment

a global compilation of late holocene planktonic foraminiferal δ18o relationship between surface water temperature and δ18o we review the different sources of uncertainty affecting the oxygen isotopic composition of planktonic foraminifera and present a global planktonic foraminifera oxygen isotope data set that has been assembled within the margo project for the late holocene time slice the data set consists of over 2100 data from recent sediment with thorough age control that have been checked for internal consistency we further examine how the oxygen isotopic composition of fossil foraminifera is related to hydrological conditions based on published results on living foraminifera from plankton tows and cultures oxygen isotopic values 18o of margo recent fossil foraminifera are 0 2 0 8 higher than those of living foraminifera our results show that this discrepancy is related to the stratification of the upper water mass and generally increases at low latitudes therefore as stratification of surface waters and seasonality depends on climatic conditions the relationship between temperature and 18o established on fossil foraminifera from recent sediment must be used with caution in paleoceanographic studies before models predicting seasonal flux abundance and 18o composition of a foraminiferal population in the sediment are available we recommend studying relative changes in isotopic composition of fossil planktonic foraminifera these changes primarily record variations in temperature and oxygen isotopic composition of sea water although part of the changes might reflect modifications of planktonic foraminifera seasonality or depth habitat

analysis of analog and digital records of the 1982 arkansas earthquake swarm abstract the arkansas swarm is the largest earthquake swarm ever recorded in the central or eastern united states three component digital and single component analog recordings for 88 earthquakes occurring during a 12 day period provide a quality data set of manageable proportions for study of the swarm9s seismic properties these 88 earthquakes occurred within a small volume of about 45 km 3 at depths from 4 to 7 km composite focal mechanisms indicate predominantly strike slip motion with the p axis oriented ne sw seismic wave velocity studies indicate that significant temporal and spatial variations of seismic wave velocities are present the v p v s ratio ranges from as low as 1 59 to as high as 1 78 with the majority of values falling between 1 67 to 1 70 an anomalous region immediately north of the center of swarm activity is found where both the p and s wave velocities and v p v s ratio increase dramatically starting about 1 day before the 5 july m d 3 8 event two strong secondary arrivals between the direct p and s arrivals can be clearly identified especially at stations nnw of the activity these secondary arrivals must be converted p waves from s waves incident on two deep discontinuities the above observations are compatible with a small highly heterogeneous swarm source volume within which the ambient stress regime can vary rapidly in both time and space some identified clustering and v p v s changes are clearly precursory to the major events of the sequence but no simple repeating pattern is evident in this restricted data sample

dysaerobic conditions during heinrich events 4 and 5 evidence from phosphorus distribution in a north atlantic deep sea core abstract reactive phosphorus undergoes diagenetic transformation once transferred into marine sediments the degree of regeneration and redistribution of phosphorus depends on early diagenetic and environmental conditions which may be linked to larger scale phenomena such as bottom water circulation water column ventilation and organic carbon flux phosphorus phases of the

modeling the effect of freshwater pulses on the early holocene climate the influence of high frequency climate variability freshwater 4 67 10 14 m 3 into the labrador sea at three different constant rates 1 5 sv 1 sv 1 10 6 m 3 s 1 i n 10 years 0 75 sv in 20 years and 0 3 sv in 50 years for each rate five ensemble experiments have been performed varying in initial conditions the freshwater pulses produce a weakening of the thermohaline circulation the perturbed state is in agreement with proxy evidence for the 8 2 ka event two types of recovery of the thermohaline circulation occurred differing in time scale 1 200 years and 2 200 years in the experiments with 10 year and 20 year pulses both types of recovery were observed this suggests that the model response is unpredictable in the range of parameters studied here it is hypothesized that the unpredictability is associated with annual to decadal climate variability our results demonstrate that several types of recovery may exist with the same kind of perturbation the interpretation of events observed in proxy data may be thus more complex than realized until now since the magnitude and duration of climatic events caused by freshwater pulses is likely to depend strongly on nonlinear dynamics inside the coupled atmosphere sea ice ocean system index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4255 oceanography general numerical modeling 4532 oceanography physical general circulation 4215 oceanography general climate and interannual variability 3309 9325 information related to geographic region atlantic ocean keywords holocene thermohaline circulation predictability meltwater coupled climate model

sedimentary record of the mid pleistocene climate transition in the southeastern south atlantic odp site 1090 abstract one important goal of leg 177 of the ocean drilling program odp was to explore the nature of the mid pleistocene climate transition mpt on the southern hemisphere a suitable mpt record was encountered at site 1090 in the southeastern south atlantic where a 44 m thick sequence of quaternary diatom bearing foraminiferal muds and oozes was recovered on the agulhas ridge environmental responses to the mpt comprised changes in terrestrial climate biological productivity and regional ocean circulation as inferred from compositional sediment data and clay mineralogy a shift towards more arid conditions occurred between 900 and 800 ka in southern africa changes in palaeoceanography already started earlier since 1150 ka northward displacements of the polar front appeared during glacial periods and shifted the area of dominant diatom deposition towards site 1090 likewise glacial interglacial contrasts in regional conveyor circulation strengthened after 1200 ka and became most severe after 650 ka however while changes in regional conveyor circulation likely responded in tune with global ice volume changes and show the onset of 100 kyr cycles after 1200 ka an unusual 130 kyr pattern characterises the pattern of frontal movements between 1200 ka and 650 ka probably in response to imperfect adaptation of regional climate to the global 100 kyr climate cycles

reef drowning during the last deglaciation evidence for catastrophic sea level rise and ice sheet collapse elevations and ages of drowned acropora palmata reefs from the caribbean atlantic region document three catastrophic metre scale sea level rise events during the last deglaciation these catastrophic rises were synchronous with 1 collapse of the laurentide and antarctic ice sheets 2 dramatic reorganization of ocean atmosphere circulation and 3 releases of huge volumes of subglacial and proglacial meltwater this correlation suggests that release of stored meltwater periodically destabilized ice sheets causing them to collapse and send huge fleets of icebergs into the atlantic massive inputs of ice not only produced catastrophic sea level rise drowning reefs and destabilizing other ice sheets but also rapidly reduced the elevation of the laurentide ice sheet flipping atmospheric circulation patterns and forcing warm equatorial waters into the frigid north atlantic such dramatic evidence of catastrophic climate and sea level change during deglaciation has potentially disastrous implications for the future especially as the stability of remaining ice sheets such as in west antarctica is in question

the univariant curve liquid forsterite anorthite diopside in the system cmas at 1 bar solid solutions and melt structure one atmosphere liquid present experiments were carried out in the cmas system using an ordinary quench furnace apparatus the runs including reversal and duplicate experiments describe the univariant curve l fo an di between the invariant points q l fo an di sp and f l fo an di oen located respectively at 1245 1 c and 1244 1 c the thermal divide on this curve m3 l fo an di is located at 1275 1 c and plots well within the silica saturated field in agreement with longhi s 1987 experiments along the univariant curve l fo an di liquid composition evolves away from the thermal divide either toward invariant points q or f and pierces the silica saturation plane i e the join di an en in the silica saturated field in this compositional range the al solubility in clinopyroxene changes drastically from one side of the thermal divide to the other with great increase of al solubility in the silica undersaturated field four endmembers must be used to describe the complex solid solution of anorthite caal2si2o8 camgsi3o8 mgal2si2o8 and si4o8 the last two of these are present only within the silica saturated field unlike clinopyroxene the mg content of anorthite is insensitive to the thermal barrier but is only sensitive to silica saturation plane olivine composition can be described by a binary solid solution of forsterite and monticellite with no ca in the m2 site as with anorthite olivine compositions exhibit a marked change with crossing of the silica saturation plane the above features imply that the solubility of minor elements in crystalline phases al in clinopyroxene ca in olivine and mg in anorthite selectively respond to only one or another of these particular plane results have many important consequences one is the likelihood of changes in melt speciation depending on position with respect to the thermal divide and the silica saturation plane

the effects of feo on the system cmas at low pressure and implications for basalt crystallization processes the common occurrence of olivine clinopyroxene and plagioclase in natural basalts may suggest that crystallization of these phases exerts important controls on the evolution of natural basalts at low pressure in order to understand the effects of such an assemblage on the evolution of basalt an experimental study of the divariant assemblage 1 an cpx ol in the system cmas feo was undertaken at 1 bar and under very reducing conditions experiments have been performed along three isothernal sections detailed electron microprobe data have been collected on all the coexisting phases combining these data with available data in the system cmas the compositions of all the coexisting phases have been described as functions of temperature and cao content in the melt by applying a multiple linear regression method this allows a quantitative characterization of the divariant assemblage 1 an epx ol in the system cmas feo from 1275 c to 1160 c with liquid compositions ranging from mg 1 to mg 0 28 knowing the composition temperature relationships the basic t x configuration of the assemblage 1 an cpx ol has been analysed and mass balance calculations have been performed to examine the feo effect on different crystallization processes addition of feo to the system cmas transforms the thermal divide in the assemblage 1 an cpx ol to a thermal ridge and shifts the spine of the thermal ridge towards sio2 poor compositions with decreasing temperature this indicates that tholeiitic basalts can be relatively silica poor and evolve towards slightly more silica poor compositions with increasing feo content pigeonite replaces ortheonstatite as a crystallizing phase along the silica rich boundary of the assemblage 1 an cpx ol in some iron and silica rich composition olivine possibly terminates its crystallization or even starts resorption prior to pigeonite precipitation this suppresses silica enrichment in the melt crystallization paths of the assemblage 1 an cpx ol are determined by the detailed t x relations of the thermal ridge with the melt evolving away from the spine the liquid evolution trend depends strongly on the initial compositions a tholeiitic liquid can follow a moderate silica enrichment path at the same very reducing condition of there is a high modal proportion of olivine a fenner mechanism therefore is at work at the same place as a bowen mechanism

transport of carbonate sediment in the western north atlantic evidence from oxygen and carbon isotopes abstract in most deep ocean environments the carbonate component is usually considered to be of pelagic origin deposited on the sea bed without extensive transport even when evidence exists that carbonate fractions have been transported the transport is usually viewed as being intrabasinal stable isotope analyses 18 o and 13 c of the carbonate contained in the fine fraction m of eight cores from the north american continental margin and the west flank of the mid atlantic ridge mar suggest that carbonate sedimentation patterns may be influenced by transport to a greater extent than previously recognized in holocene sections of all the cores the fine fraction and foraminiferal isotopic signals are comparable in cores from the flanks of the mar the fine fraction and foraminiferal isotopic signals remain comparable during the entire glacial interglacial cycle on the north american margin however the pattern is strikingly different starting about 10 ka and extending back into glacial times the fine fraction isotopic signal in margin cores becomes increasingly depleted reaching a value of 8 pdb the foraminiferal signal from these same cores follows the typical glacial interglacial pattern and gets heavier during glacial time the holocene correspondence of foraminiferal and fine fraction isotope curves throughout the north american basin suggests that both have the same intrabasinal pelagic source however in late glacial age sediments from the north american margin the divergence of foraminiferal and fine fraction isotope signals indicates different sources for each component based on the isotopic signals and the timing of changes in the signal a substantial proportion of the fine fraction carbonate on the north american margin appears to be derived from diagenetically altered carbonate rock this fine carbonate sediment probably resulted from physical erosion of carbonate rock by glacial processes and deposition in the north american basin by the retreating wisconsin glacier

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

differentiation of ferro basaltic magmas under conditions open and closed to oxygen implications for the skaergaard intrusion and other natural systems because processes such as fractional crystallization and crystallization under conditions closed to oxygen are difficult to simulate in the laboratory there is a need for quantitative models of magma crystallization behaviour comparison of experimental data on an iron rich basaltic composition with predictions of the melts free energy minimization algorithm shows that although liquidus temperatures and silicate mineral equilibria are predicted relatively well the saturation of fe ti oxides is not we have used the same experimental data to construct an alternative crystallization model based on known equilibrium phase relations mineral melt partitioning of major elements and mass balance constraints the model is used to explore the consequences of equilibrium and fractional crystallization in systems open and closed to oxygen liquid lines of descent for perfect equilibrium and perfect fractional crystallization are predicted to be very similar in a system open to oxygen the model predicts that magnetite saturation leads to strongly decreasing iron and increasing silica contents of residual liquids whereas systems closed to oxygen crystallize less abundant magnetite leading to a less pronounced iron depletion in the liquid predicted bulk solid compositions and variations offo with falling temperature agree well with those observed or inferred from the cumulates of the skaergaard intrusion but none of the predicted liquid lines of descent are consistent with the extreme iron enrichment proposed for this intrusion based on mass balance calculations compositional factors such as water and phosphorus are not thought to be the source of the discrepancy as the cumulates of the basistoppen sill which closely resemble those of skaergaard may be used to calculate a liquid line of descent in agreement with that predicted by the model for fractional crystallization closed to oxygen a comparison of the predicted t io paths and liquid lines of descent with those inferred from natural systems suggests that volcanic centres such as iceland and hawaii evolve under conditions open to oxygen whereas evidence from plutonic environments e g skaergaard and kiglapait layered intrusions suggests that they evolved under conditions more closed to oxygen the compositional evolution of the melt phase in volcanic and plutonic systems may therefore be different although the results of this study suggest that magnetite saturation will limit fe enrichment in all environments to 20wt feo consistent with enrichments reported for volcanic glasses

mg in plagioclase experimental calibration of a new geothermometer and diffusion coefficients abstract the temperature sensitive exchange of mg between plagioclase pl and clinopyroxene cpx has been studied experimentally accounting for different anorthite contents in plagioclase x an and various silica activities a sio 2 in the system the partitioning of mg between plagioclase and clinopyroxene was determined over a temperature range of 1100 1200 c using plagioclase single crystals of different compositions x an 0 5 0 8 surrounded by different clinopyroxene bearing matrix powders to account for different silica activities from 0 55 to 1 0 the experimental design also allows the diffusivity d mg pl of mg in plagioclase under these conditions to be determined both k mg pl cpx defined as k mg pl cpx c mg pl c mg cpx and d mg pl decrease with temperature and increase with a sio 2 isothermal data for different x an in plagioclase show a linear increase of ln k mg pl cpx with increasing x an but d mg pl appears to be insensitive to x an the partitioning data allow a new geothermometer to be calibrated which may be widely applicable to terrestrial and extraterrestrial rocks where plagioclase and clinopyroxene coexist t k 9219 2034 x an ln k mg pl cpx 1 6 ln a sio 2 application of this geothermometer to experimental data from this study reproduces the experimental temperatures within 20 c diffusion of mg in plagioclase is described by d mg pl m 2 s 1 1 25 10 4 m 2 s 1 exp 320 924 j mol 1 rt a sio 2 2 6

reduced solar activity as a trigger for the start of the younger dryas it is generally assumed that changes in ocean circulation forced the abrupt climate changes during the late pleistocene including the younger dryas event recently however it was proposed that variations in solar irradiance could have played a much more prominent role in forcing pleistocene climate changes furthermore two physical mechanisms were recently published that explain how relatively small changes in solar irradiance could have had a strong impact on the climate system we discuss the possibility that an abrupt reduction in solar irradiance triggered the start of the younger dryas and we argue that this is indeed supported by three observations 1 the abrupt and strong increase in residual c 14 at the start of the younger dryas that seems to be too sharp to be caused by ocean circulation changes alone 2 the younger dryas being part of an similar to 2500 year quasi cycle also found in the c 14 record that is supposedly of solar origin 3 the registration of the younger dryas in geological records in the tropics and the mid latitudes of the southern hemisphere moreover the proposed two physical mechanisms could possibly explain how the north atlantic thermohaline circulation was perturbed through an increase in precipitation together with iceberg influxes in addition the full magnitude of the younger dryas cooling as evidenced by terrestrial records in europe could be explained we conclude that a solar triggering of the younger dryas is a valid option that should be studied in detail with climate models c 2000 elsevier science ltd and inqua all rights reserved

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

the differentiation of the skaergaard intrusion mcbirney and naslund have missed the two central points that we reiterate for clarity first the composition of the cotectic cumulate extract for the lzc best estimated from the average composition of adcumulate rocks had a substantially lower silica content than any conceivable basaltic liquid so silica enrichment is unavoidable this is true of all the dyke compositions reported by brooks and nielsen even the anomalous rock e with sio2 46 second it is the normal behaviour of tholeiite basaltic magma to produce increasingly iron rich cumulate extracts with little change in silica until feti oxide precipitation occurs when silica enrichment and compositional divergence begins such divergence between equivalent cumulate plutonic and fractionated volcanic rocks hads been widely documented skaergaard magmas do have some uncommon features however similar assemblages exist in volcanic rocks as in the andesitic to rhyolitic glasses of the loch ba ring dyke mull sparks 1988 it is also a basic principle that elements enriched in cumulate rocks often become depleted in the corresponding liquids iron enrichment in the skaergaard cumulates is complementary to iron depletion in liquids

graphical analysis of the orthopyroxene pigeonite augite plagioclase equilibrium at liquidus temperatures and low pressure there are both natural and experimental observations of the coexistence of three pyroxenes orthopyroxene opx pigeonite pig augite aug with plagioclase pl commonly the assemblage occurs as an intermediate product in the following fractionation trend of a mafic magma opx aug pl opx aug pig pl aug pig pl to clarify the phase equilibria constraints on the existence of this mineral assemblage we have graphically analyzed the change in topology of an isobaric isoplethic section ol aug pl qtz with fe 25 50 where fe fe fe mg and an 50 arising from an increase in the fe value of silicate liquid the analysis shows that the stability field of the mineral assemblage opx aug pig pl is restricted and can onlycrystallize in the interval between two invariant points t 4 1 ol opx pig aug pl l and t 4 2 qtz opx pig aug pl l that emerge successively during expansion of a liquidus volume of pigeonite within the isobaric isoplethic section ol aug pl qtz at fe values lower than t 4 1 the 3 pyroxene assemblage cannot exist due to the absence of a contact surface between the primary volumes of plagioclase and pigeonite at fe values greater than t 4 2 the assemblage is unstable due to separation of the primary volumes of orthopyroxene and augite

oxygenation changes in the deep western arabian sea during the last 190 000 years productivity versus deepwater circulation 1 fluctuations in the abundance of selected foraminiferal indicator species and diversity allowed the reconstruction of changes in deepwater oxygenation and monsoon driven organic matter fluxes in the deep western arabian sea during the last 190 kyr times of maximum surface production coincide with periods of intensified sw monsoon as shown by the abundance of globigerina bulloides and enhanced carbonate corrosion benthic ecosystem variability in the deep arabian sea is not exclusively driven by variations in monsoonal upwelling and related organic matter supply to the seafloor but also by changes in deepwater ventilation deepening of the base of the oxygen minimum zone omz below 1800 m water depth is strongly coherent on the precessional band but lags proxies of sw monsoon strength by 4 to 6 kyr the out of phase relationship between omz deepening and maximum sw monsoon strength is explained by temporal changes in the advection of oxygen rich deepwater masses of north atlantic and antarctic origin this process affected the remineralization and burial efficiency of organic matter in the deep arabian sea resulting in the observed phase lag between maximum monsoon strength and organic carbon preservation

phase equilibrium constraints on angrite petrogenesis parameterizations of liquidus boundaries and solid solution in the cmas fe system shi 1992 have been employed to depict the liquidus equilibria relevant to the petrogenesis of angrites angrites are basaltic achondrites characterized by highly aluminous augite fassaite intermediate mg fe olivine and late stage cafe olivine kirschsteinite two important features of the equilibria on the olivine liquidus surface relevant to angrite petrogenesis are 1 the presence of a thermal divide on the ol aug plag liq boundary curve which separates the compositions of source materials that produce low silica angritic melts that crystallize highly aluminous augite from those that produce higher silica melts with tholeiitic to eucritic crystallization patterns and 2 the change in the pseudo invariant point on the low silica side of the thermal divide from a plagioclase peritectic involving spinel ol aug plag sp liq at high to intermediate mg mg mg fe to two pseudo eutectics involving kirschsteinite ol aug plag kir liq and ol kir plag sp liq at low mg the fassaitic aluminous augite pyroxene composition in angra dos reis ador the presence of minor green spinel and the absence of primary kirschsteinite prinz et al 1977 indicate that crystallization of the ador parental liquid was governed by the intermediate mg set of equilibria such that following crystallization of ol aug plag the plagioclase reacted completely at the plagioclase peritectic with the interstitial liquid which subsequently crystallized beyond the plagioclase peritectic onto the ol aug sp liquidus boundary curve the ador bulk composition is consistent with trapping 10 of the parental liquid in a cumulate with cotectic proportions of fassaite and olivine lewis cliff lew 86010 crystallized from a liquid with mg similar to that of ador but on the ol plag cotectic closer to the thermal divide such that the first pyroxene to crystallize had much lower al content than that of ador in the late stages of crystallization the 86010 residual liquid and that of lew87051 encountered the low mg set of equilibria involving kirschsteinite these relationships require either a higher degree of melting for the 86010 parent magma or source region different than ador s these relationships are also consistent with compositionally dependent ree partition coefficients between fassaite and the ador liquid being as much as 1 5 2 times higher than those for the 86010 liquid at the onset of pyroxene crystallization the combination of a trapped liquid component higher partition coefficients and smaller degrees of melting help to explain the observation that ador an apparent cumulate has ree concentrations twice as high as those in 86010 mittlefehdlt and lindstrom 1990 an apparent chilled liquid the absence of a strong negative eu anomaly in the ador parent liquid however requires relatively high degrees of partial melting to eliminate plagioclase in the source region resorption of plagioclase at the peritectic eliminates the eu anomaly that develops during crystallization so ultimately different source regions are required progressive iron loss from devolatilized primitive chondrites allende murchison produces source regions capable of producing a wide range of melt compositions with angritic to eucritic crystallization behavior the compositions of carbonaceous and ordinary chondrite provide a similar range of potential source region compositions however primitive chondrite fe source regions that produce angrite like melts have mg that is too low whereas chondrite fe sources that have mg sufficiently high to yield the mg in angrite minerals have too much silica or orthopyroxene component to yield angrite like liquids no single group of meteorites fe simultaneously satisfies the constraints of mg and silica component however mixtures of fe depleted chondrite plus a low silica component similar to ca al rich inclusions cais can satisfy the constraints the absence in angrites of 48ca and 50ti anomalies typical of cais lugmair and galer 1992 suggests that the low silica component was not simply an enrichment of cais but was the result of direct accretion of high temperature condensate grossman 1972 into sizable thermally shielded planetesimals thus angrites cryptically record mixing of planetesimal sized heterogeneities in the early solar system

experimental investigation of the stability of clinopyroxene in mid ocean ridge basalts the role of cr and ca al abstract the change in the stability field of clinopyroxene in mid ocean ridge basalt morb as a function of pressure has been used widely as a geobarometer based on results from crystallization experiments using morb like compositions it has been suggested that morb differentiation occurs at relatively high pressures at ultraslow and slow spreading ridges however differentiation requires the loss of substantial heat and it is unclear how this is possible at elevated pressures to better understand the controls on the stability field of clinopyroxene in morb like compositions we report a series of experiments performed at 0 1 mpa in which the temperature of clinopyroxene saturation was determined in melts with variable cr ca al and f o 2 the results show that increased cr and ca al lead to an expansion of the clinopyroxene stability field incorporating these results into a new model of morb differentiation shows that realistic parental melt cr contents can increase the temperature at which clinopyroxene saturation occurs relative to assuming a cr free melt as is commonly the case likewise high ca al melts will saturate clinopyroxene earlier than low ca al melts and their crystallization may provide an explanation for high mg clinopyroxene in oceanic gabbros the newly calibrated geobarometer gives lower crystallization pressures for morb at the slow spreading swir than previous calibrations but still suggests relatively higher pressures of crystallization with decreasing spreading rate

paleoceanographic reconstructions of surface ocean conditions in the greenland iceland and norwegian seas through the last 14 ka based on diatoms prior reconstructions of the last glacial maximum and the holocene did not include detailed paleoclimate data from the greenland iceland and norwegian gin seas which cover sensitive areas for oceanographic boundaries sea ice distribution and deep water formation in order to fill this gap we studied nine well dated sediment cores from this area and reconstructed the paleoceanographic conditions of the surface gin seas at 2000 year intervals since 13 400 bp based on the diatom record results show that a n s extending sea ice free corridor had opened along norway already at 13 400 bp indicating a northward flow of a branch of the north atlantic drift possibly caused by a change in the jet stream flow due to the decreased height of the north american ice sheet a major change of climatic conditions occurred over the gin seas as the insolation anomaly reached its maximum around 9000 bp when the sea ice cover and the oceanic fronts retreated to a northwesterly position along greenland and the ocean temperatures rose the first half of the holocene is recorded as the warmest period during the last 13 400 bp in the gin seas the duration of this climatic optimum decreased both toward the north and toward the margins of the area the second half of the holocene is characterized by a cooling trend in step with the decreasing insolation these results support the proposal of imbrie et al 1992 that the gin seas region acts as an initial responder to insolation changes

anhydrous partial melting of an iron rich mantle ii primary melt compositions at 15 kbar primary melt and coexisting mineral compositions at increasing degrees of partial melting at 15 kbar were determined for an iron rich martian mantle composition dw the composition of primary melts near the solidus was determined with basalt peridotite sandwich experiments in order to evaluate the approach of the liquids to equilibrium with a dw mantle assemblage experiments were also performed to establish the liquidus mineralogy of the primary melts primary melt compositions produced from an iron rich mantle are more picritic than those produced from an iron poor mantle by increasing the iron content of a model mantle composition decreasing the mg where mg atomic mg mg fe2 100 picritic and komatiitic magmas result at lower percentages of melting and at temperatures closer to the solidus than in an iron poor mantle terrestrial iron rich primitive volcanics may be the partial melting products of iron rich mg 80 source regions the dw partial melting results support the conclusion of previous authors that the parent magmas of the snc shergottites nakhlites chassignites meteorites were derived from a source region that had been previously depleted in an aluminous phase

synchronous high frequency oscillations in tropical sea surface temperatures and north atlantic deep water production during the last glacial cycle stable isotopic measurements of g sacculifer and c wuellerstorfi in a core from the western equatorial atlantic imply that there are parallel suborbital oscillations in surface water hydrography and deep water circulation occurring during oxygen isotope stages 2 and 3 low values of g sacculifer 18o accompany high values of c wuellerstorfi 13c linking warmer sea surface temperatures ssts in the tropics with increased production of lower north atlantic deep water nadw the amplitude of the 18o oscillations is 0 6 or 2 3 c which is superimposed on a glacial interglacial amplitude of about 2 1 using the g sacculifer 18o data we calculate that surface waters were colder during stage 2 than calculated by climap 1976 1981 the longer period 2 kyr oscillations in air temperature recorded in the greenland and antarctic ice cores appear to correlate with oscillations in sea surface temperature in the equatorial atlantic the magnitude of these oscillations in tropical sst is too large to have resulted from changes in meridional heat transport caused by the global conveyor alone the apparent synchroneity of equatorial sst and polar air temperature changes as well as the amplitude of the sst changes at the equator are consistent with the climate effects expected from changes in the atmosphere s greenhouse gas content h2ovapor co2 and ch4

deepwater source variations during the last climatic cycle and their impact on the global deepwater circulation the degree of similarity of the 13c records of the planktonic foraminiferal species n pachyderma and of the benthic foraminiferal genus cibicides in the high latitude basins of the world ocean is used as an indicator of the presence of deepwater sources during the last climatic cycle whereas continuous formation of deep water is recognized in the southern ocean the norwegian sea stopped acting as a sink for surface water during isotope stage 4 and the remainder of the last glaciation however deep water formed in the north atlantic south of the norwegian sea during the last climatic cycle as early as isotope substage 5d and this area was also the only active northern source during stages 4 2 a detailed reconstruction of the geographic distribution of 13c in benthic foraminifera in the atlantic ocean during the last glacial maximum shows that the most important deepwater mass originated from the southern ocean whereas the glacial north atlantic deep water cannot be traced south of 40 n at shallower depth an oxygenated 13c rich intermediate water mass extended from 45 n to 15 s in the pacific ocean a ventilation higher than the modern one was also found in open ocean in the depth range 700 2600 m and is best explained by stronger formation of intermediate water in high northern latitudes

water mass conversion in the glacial subarctic pacific 54 n 148 w physical constraints and the benthic planktonic stable isotope record benthic uvigerina spp cibicidoides spp gyroidinoides spp and planktonic n pachyderma sinistral g bulloides stable isotope records from three core sites in the central gulf of alaska are used to infer mixed layer and deepwater properties of the late glacial subarctic pacific glacial interglacial amplitudes of the planktonic 18o records are 1 1 1 3 less than half the amplitude observed at core sites at similar latitudes in the north atlantic these data imply that a strong negative w anomaly existed in the glacial subarctic mixed layer during the summer which points to a much stronger low salinity anomaly than exists today if true the upper water column in the north pacific would have been statically more stable than today thus suppressing convection even more efficiently this scenario is further supported by vertical i e planktic versus benthic 18o and 13c gradients of 1 which suggest that a thermohaline link between pacific deep waters and the subarctic pacific mixed layer did not exist during the late glacial epibenthic 13c in the subarctic pacific is more negative than at tropical subtropical pacific sites but similar to that recorded at southern ocean sites suggesting ventilation of the deep central pacific from mid latitude sources e g from the sea of japan and sea of okhotsk still convection to intermediate depths could have occurred in the subarctic during the winter months when heat loss to the atmosphere sea ice formation and wind driven upwelling of saline deep waters would have been most intense this would be beyond the grasp of our planktonic records which only document mixed layer temperature salinity fields extant during the warmer seasons also we do not have benthic isotope records from true intermediate water depths of the subarctic pacific

quaternary carbonate record from the equatorial indian ocean and its relationship with productivity changes abstract the caco3 content in quaternary deep sea sediments from pacific and atlantic oceans have been suggested to respond differently to glacial interglacial cycles caco3 contents are highest during glacials in the pacific but highest during interglacials in the atlantic ocean it is not yet clear as to whether a pacific or an atlantic pattern of caco3 fluctuations dominates the indian ocean we have analyzed the ocean drilling program odp site 709a from the western equatorial indian ocean for the last 1370 ka to determine the relationships between percentages and fluxes of caco3 and quaternary paleoclimatic changes we also analyzed the coarse 25 m and fine 18 o of the planktonic foraminifera globigerinoides ruber this indicates that caco3 fluctuations at this site do not show either a pacific or an atlantic pattern of caco3 fluctuations fluxes of caco3 0 38 to 2 46 g cm 2 ka 1 in total sediment and ba al ratios 0 58 to 3 93 g cm 2 ka 1 show six fold variability through the last 1370 ka which points out that productivity changes are significant at this site fluxes of the fine caco3 component demonstrate a 26 fold change 0 02 to 0 52 g cm 2 ka 1 whereas the coarse caco3 component exhibit eight fold change 0 13 to 1 07 g cm 2 ka 1 this suggests that productivity variations of calcareous nannofossils are greater in comparison with the foraminifera on the other hand mean values of coarse caco3 fluxes are higher compared to those of fine caco3 which reveals that the foraminifera contribute more to the bulk caco3 flux than the calcareous nannofossils in the equatorial indian ocean

resolving the younger dryas event through borehole thermometry meltwater influxes may partly explain the low oxygen isotope values measured in the dye 3 and camp century ice cores this has led to speculation that greenland may not have cooled during the younger dryas and underlined the need for independent checks of the oxygen isotope record using optimal control methods and heat flow modeling the author makes a valiant but ultimately futile attempt to distinguish the younger dryas event in the ice sheet temperatures measured at dye 3 south greenland the author discusses the prospects for attempting the same in the new summit boreholes in central greenland how that will require more accurate temperature measurements a coupled thermo mechanical model and a refined uncertainty analysis he concludes by discussing how borehole temperature analysis may improve the climate histories determined from ice cores

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

extreme iron enrichment and liquid immiscibility in mafic intrusions experimental evidence revisited abstract this paper examines phase equilibria and mass balance constraints on fe enrichment in tholeiitic liquids in plutonic environments the peak of fe enrichment is thought to roughly coincide with magma saturation in fe ti oxides and olivine starts to react with the liquid at about the same time this crucial stage of crystallization is examined in detail using a compilation of chemical analyses of 64 experimental charges that comprise liquids quenched glasses equilibrated with the liquidus assemblage of olivine plagioclase high ca pyroxene and low ca pyroxene some samples also contain fe ti oxides it is shown that the 4 phase liquidus assemblage does not constrain a narrow range of liquid compositions the concentrations of sio 2 in the selection of experimental glasses vary broadly from 42 to 66 wt silica content shows strong negative correlations with feo and cao al 2 o 3 and strong positive correlation with alkalis extreme fe enrichment above 22 wt feo is observed only in alkali free or alkali poor liquids broad compositional variations for the multiply saturated liquids are attributed to strong non ideality and complex concentration activity relationships in ferrobasaltic melts liquid immiscibility characteristic of fe rich silicate liquids is the ultimate consequence of non ideality petrogenetic implications of phase equilibria and mass balance constraints are discussed for a classical example of the skaergaard intrusion in east greenland where the trend of extreme fe enrichment has been in contention it is proposed that seemingly conflicting results of experiments on skaergaard natural cumulate rocks and model melt compositions can be reconciled if it is assumed that silicate liquid immiscibility in skaergaard started not at the very end of crystallization but earlier soon after the start of ilmenite and magnetite crystallization

neogene overflow of northern component water at the greenland scotland ridge in the north atlantic ocean flow of north atlantic deep water nadw and of its ancient counterpart northern component water ncw across the greenland scotland ridge gsr is thought to have played an important role in ocean circulation over the last 60 ma the iceland plume has dynamically supported an area which encompasses the gsr consequently bathymetry of the gsr has varied with time due to a combination of lithospheric plate cooling and fluctuations in the temperature and buoyancy within the underlying convecting mantle here we reassess the importance of plate cooling and convective control on this northern gateway for ncw flow during the neogene period following wright and miller 1996 to tackle the problem benthic foraminiferal isotope data sets have been assembled to examine 13c gradients between the three major deep water masses i e northern component water southern ocean water and pacific ocean water composite records are reported on an astronomical timescale and a nonparametric curve fitting technique is used to produce regional estimates of 13c for each water mass confidence bands were calculated and error propagation techniques used to estimate ncw and its uncertainty despite obvious reservations about using long term variations of 13c from disparate analyses and settings and despite considerable uncertainties in our understanding of ancient oceanic transport pathways the variation of ncw through time is consistent with independent estimates of the temporal variation of dynamical support associated with the iceland plume prior to 12 ma 13c patterns overlap and ncw cannot be isolated significant long period variations are evident which are consistent with previously published work from 12 ma when lithospheric cooling probably caused the gsr to submerge completely long period 13c patterns diverge significantly and allow reasonable ncw estimates to be made our most robust result is a dramatic increase in ncw overflow between 6 and 2 ma when dynamical support generated by the iceland plume was weakest between 6 and 12 ma a series of variations in ncw overflow have been resolved

deep south atlantic carbonate chemistry and increased interocean deep water exchange during last deglaciation abstract carbon release from the deep ocean at glacial terminations is a critical component of past climate change but the underlying mechanisms remain poorly understood we present a 28 000 year high resolution record of carbonate ion concentration a key parameter of the global carbon cycle at 5 km water depth in the south atlantic we observe similar carbonate ion concentrations between the last glacial maximum and the late holocene despite elevated concentrations in the glacial surface ocean this strongly supports the importance of respiratory carbon accumulation in a stratified deep ocean for atmospheric co 2 reduction during the last ice age after 9 mol kg decline during heinrich stadial 1 deep south atlantic carbonate ion concentration rose by 24 mol kg from the onset of bolling to pre boreal likely caused by strengthening north atlantic deep water formation bolling or increased ventilation in the southern ocean younger drays or both pre boreal the 15 mol kg decline in deep water carbonate ion since 10 ka is consistent with extraction of alkalinity from seawater by deep sea caco 3 compensation and coral reef growth on continental shelves during the holocene between 16 600 and 15 000 years ago deep south atlantic carbonate ion values converged with those at 3 4 km water depth in the western equatorial pacific as did carbon isotope and radiocarbon values these observations suggest a period of enhanced lateral exchange of carbon between the deep south atlantic and pacific oceans probably due to an increased transfer of momentum from southern westerlies to the southern ocean by spreading carbon rich deep pacific waters around antarctica for upwelling invigorated interocean deep water exchange would lead to more efficient co 2 degassing from the southern ocean and thus to an atmospheric co 2 rise during the early deglaciation

the differentiation of the skaergaard intrusion we find no support for the claim that the skaergaard magma followed the trend of common tholeiitic volcanic magmas such as those of iceland and the scottish tertiary the end product of differentiation was not a large mass of rhyolite but an iron rich silica poor liquid not unlike that deduced by wager in 1960 the proposal that a large mass of rhyolitic liquid occupied the upper levels of the intrusion finds no support in the field the thick series of ferrogabbos which became richer in iron and poorer in silica until they reached a field of immiscibility cannot be reconciled with crystallization of a large mass of felsic magma mass balance calculations that indicate otherwise are invalid because they fail to take into account large volumes of rocks that differ in composition from those assumed in the calculations while ignoring the existence of major units of the intrusion hunter and sparks propose that lavas in scotland and iceland are more relevant to the liquid compositions than rocks that are intimately associated with the intrusion their argument that the skaergaard intrusion followed a trend of silica enrichment that is universal to tholeiitic magmas is based on an incomplete knowledge of the rocks and faulty calculations of mass balance relations we agree that much remains to be learned about the skaergaard intrusion and the basic mechanisms of magmatic differentiation in this case however we are ready to hang our case on well established field relations and a mass of laboratory data for what must be the most intensely studied body of rock on earth

glacial deep water geometry south atlantic benthic foraminiferal cd ca and δ13c evidence a depth transect of cores from 1268 to 3909 m water depth in the western south atlantic are ideally situated to monitor the interocean exchange of deep water and variations in the relative strength of northern and southern sources of deep water production benthic foraminiferal cd ca and 13c data suggest that glacial north atlantic intermediate water gnaiw extended at least as far south as 28 s in the western south atlantic the core of nutrient depleted water was situated at 1500 m above and below water masses with higher nutrient concentrations when examined in conjunction with published paired cd ca and 13c from intermediate depth cores from other basins it appears that the extent of gnaiw influence on the intermediate waters of the world s oceans was less than suggested previously differentiating among possible pathways for the glacial deep ocean 3 km requires a better understanding of the controls on cd ca and 13c values of benthic foraminifera

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

changes in the distribution of δ13c of deep water σco2 between the last glaciation and the holocene carbon isotopic measurements on the benthic foraminiferal genus cibicidoides document that mean deep ocean 13c values were 0 46 lower during the last glacial maximum than during the late holocene the geographic distribution of 13c was altered by changes in the production rate of nutrient depleted deep water in the north atlantic during the late holocene north atlantic deep water with high 13c values and low nutrient values can be found throughout the atlantic ocean and its effects can be traced into the southern ocean where it mixes with recirculated pacific deep water during the glaciation decreased production of north atlantic deep water allowed southern ocean deep water to penetrate farther into the north atlantic and across low latitude fracture zones into the eastern atlantic mean southern ocean 13c values during the glaciation are lower than both north atlantic and pacific 13c values suggesting that production of nutrient depleted water occurred in both oceans during the glaciation enriched 13c values in shallow cores within the atlantic ocean indicate the existence of a nutrient depleted water mass above 2000 m in this ocean

the age and origin of the younger dryas climate event in greenland ice cores 230th 234u and 14c dating of barbados corals has extended the calibration of 14c years b p to calendar years b p beyond the 9200 year tree ring series bard et al 1990 this now permits the conversion of 14c chronozones which delimit major climate shifts in western europe to calendar years the younger dryas chronozone defined as 11 000 to 10 000 14c years b p corresponds to 13 000 to 11 700 calendar years b p this calibration affects the interpretation of an intensely studied example of the younger dryas climate event the 18o anomaly between 1785 and 1793 m in dye 3 ice core the end of the 18o anomaly in dye 3 ice core has been dated by measurements of 14c in air bubbles andree et al 1984 1986 and by annual layer counting hammer et al 1986 the older 14c dates fall out of the range of the tree ring calibration series but can now be calibrated to calendar years using the barbados 230th 234u calibration the 14ccorrected age for the end of the 18o event is 10 300 400 calendar years b p compared to the annual layer counting age of 10 720 150 years b p thus the younger dryas event in the dye 3 ice core ends in the preboreal chronozone 11 700 to 10 000 calendar years b p and is not correlative with the end of the younger dryas event identified in pollen records marking european vegetation changes the end of the dye 3 18o event is however correlative with the end of meltwater pulse ib fairbanks 1989 marking a period of intense deglaciation with meltwater discharge rates exceeding 13 000 km yr

late quaternary palaeoceanography of the circumpolar deep water from the south tasman rise we use sediment cores from the south tasman rise str to reconstruct deepwater circulation in the southwest pacific sector of the southern ocean sediment cores md972106 45 09 s 146 17 e 3310 m water depth and gc34 45 06 s 147 45 e 4002 m water depth preserve records covering the last 160 k yr with chronology controlled by calibrated accelerator mass spectrometry radiocarbon dates and benthic foraminiferal 18o tied to specmap the stir benthic foraminiferal 13c records provide new 13c values for southern ocean deep water spanning the last 160 kyr at sites unlikely to be affected by variations in productivity the records establish that glacial benthic foraminifera cibicidoides spp 13c values are lower relative to interglacial values and are comparable to previous glacial benthic 13c records in the indian and pacific sectors of the southern ocean comparisons of the benthic foraminiferal 13c time series at the str are made with the equatorial pacific v19 30 and site 846 and the equatorial atlantic geob1115 the similarity of benthic 13c records at the str to the equatorial pacific suggest the southern ocean deep water mass closely tracked those of the deep pacific and the presence of a 13c gradient between the str and the equatorial atlantic suggests there was continual production of northern source deep water over the past 160 k yr

core top evaluation of benthic foraminiferal isotopic ratios for paleo oceanographic interpretations abstract the benthic foraminifera cibicides wuellerstrofi oridforsalis tener and pyrgo rotalaria from twelve core tops in the norwegian greenland sea and c wuellerstorfi o tener and o umbonatus from four core tops on the rio grande rise were analysed for oxygen and carbon isotopes the norwegian greenland sea data yield statistically discrete means for both 18o and 13c for each species the interregional comparison of our data to published data from the east pacific rise shows that c wuellerstorfi 18o and 13c variations reflect oceanographic differences between sites whereas the data on oridorsalis and pyrgo do not we conclude that c wuellerstorfi is a reliable sensor of bottom water conditions whereas other taxa are of unproven utility supportive biological studies and the lack of positive covariance of 18o to 13c among deep sea benthic taxa lead us to reject models calling for incorporation of metabolic co2 to explain the isotopic variation among deep sea benthic foraminifera we propose a within sediment microhabitat hypothesis to explain the 13c variations among taxa

the effect of cao on the partitioning behavior of ree y and sc between olivine and melt implications for basalt carbonate interaction processes abstract the partitioning of ree y and sc r3 between olivine and melt has been investigated experimentally during basalt carbonate interaction three synthetic basalts meltmg 72 meltmg 75 and meltmg 78 were doped with 0 10 and 20 wt caco3 and then equilibrated for 72 h at 1 atm 1 150 1 200 and 1 250 c and the qfm oxygen buffer the thermal decomposition of caco3 produced cao contents in the melt up to 22 wt regular relationships are found between the ionic radius and the partition coefficient dr3 showing typical near parabolic patterns dr3 is weakly dependent on temperature but decreases with increasing caco3 in the starting material e g dsc decreases from 0 20 to 0 13 from the point of view of the lattice strain theory dr3 is described in terms of the radius of the crystal site r0 the young modulus e due to the elastic response of that site to lattice strain caused by cation insertion and the strain free partition coefficient d 0 3 the value of r0 decreases as ca cations are accommodated into the more distorted m2 site of olivine via progressive ca fe substitutions this mechanism is accompanied by a higher proportion of mg cations entering into the smaller m1 site making the optimum ionic radius smaller and favoring the crystallization of more forsteritic olivines from decarbonated melts the enrichment of ca in the crystal lattice is also proportional to the number of si and ca cations available in the melt this causes e to be anticorrelated either with ca in olivine or the activity of cao in the melt r3 cations behave as network modifiers and during basalt carbonate interaction the increasing abundance of non bridging oxygens enhances the solubility of ree y and sc in the melt as a consequence d 0 3 is negatively correlated with the degree of melt depolymerization additionally the strain of the crystal lattice dominates the dr3 parabolic patterns and d 0 3 is strongly controlled by forsterite and aluminium concentrations in olivine the accommodation of ree y and sc in the crystal lattice requires maintenance of local charge balance by the generation of vacancies in accord with a paired substitution of r3 and a vacancy for mg in octahedral sites

glacial water mass structure and rapid δ18o and δ13c changes during the last glacial termination in the southwest pacific abstract changes in ocean circulation are thought to have contributed to lowering glacial atmospheric co 2 levels by enhancing deep ocean sequestration of carbon that was returned to the atmosphere during glacial terminations high resolution benthic foraminiferal 13 c and 18 o records from a depth transect of cores in the southwest pacific ocean presented here provide evidence that both wind and thermohaline driven circulation drove co 2 from the ocean during the last deglaciation shallow geochemical stratification in the glacial southern ocean was followed by a short pulse of rapid 13 c enrichment to intermediate water depths during heinrich stadial 1 indicative of better ventilated intermediate waters co occurring with documented wind driven upwelling in the southern ocean intermediate depth 13 c enrichment paused at the start of the antarctic cold reversal 14 7 ka implying a brief shallow restratification while deeper layers were progressively flushed of 13 c depleted and 18 o enriched waters likely caused by the increasing influence of deep waters sourced from the north atlantic the coincidence of atmospheric co 2 increases with these geochemical shifts in both shallow and deep cores suggests that shifts in both atmospheric and oceanic circulation contributed to the deglacial rise of co 2

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

disequilibrium dihedral angles in layered intrusions a microstructural record of fractionation the dihedral angle formed at junctions between two plagioclase grains and a grain of augite is only very rarely in textural equilibrium in gabbros from kilometre scale crustal layered intrusions the median of a population of these disequilibrium angles cpp varies systematically within a single layered intrusion remaining constant over large stretches of stratigraphy with significant increases and decreases associated with the addition or reduction respectively of the number of phases on the liquidus of the bulk magma the stepwise changes in cpp are present in the upper zone of the bushveld complex the megacyclic unit i of the sept iles intrusion and the layered series of the skaergaard intrusion the plagioclase bearing cumulates of rum have a bimodal distribution of cpp dependent on whether the cumulus assemblage includes clinopyroxene the presence of the stepwise changes is independent of the order of arrival of cumulus phases and of the composition of either the cumulus phases or the inferred composition of the interstitial liquid the only parameter that behaves in an exactly analogous manner to cpp is the rate of change in enthalpy with temperature h t during crystallization both h t and cpp increase with the addition of a liquidus phase and decrease with the removal of a liquidus phase the replacement of one phase by another has little effect on h t and no discernible effect on cpp an increase of h t results in an increase in the fraction of the total enthalpy budget that is the latent heat of crystallization the fractional latent heat it also increases the mass crystallized in each incremental temperature drop the crystal productivity these increases of both fractional latent heat and crystal productivity are likely to cause an increase in the time taken to form three grain junctions in the mush via thermal buffering of a thickened mushy layer we suggest these are the underlying causes of stepwise increases in cpp stepwise changes in the geometry of three grain junctions in fully solidified gabbros thus provide a clear microstructural marker for the progress of fractionation down the liquid line of descent in layered intrusions

petrogenesis of early permian olivine bearing cumulates and associated basalt dykes from bocca di tenda northern corsica implications for post collisional variscan evolution abstract the early permian gabbroic sequence of bocca di tenda northern corsica intrudes a plutonic suite made of hornblende granitoids and is crosscut by basalt dykes with chilled margins the gabbroic sequence includes olivine gabbronorite cumulates that consist mainly of euhedral olivine fo78 72 and plagioclase an68 61 and poikilitic clinopyroxene mg 0 85 0 80 orthopyroxene and ti pargasite pyroxenes from the olivine gabbronorites have 18 o values of 6 3 to 6 7 which are evidence for the involvement of a crustal component in the petrogenesis of these rocks the basalts have mg values of 57 to 53 and chondrite normalised patterns characterised by a steady decrease from lree to hree relative to modern n morb they show also moderate enrichment of nb and ta over zr and hf which are in turn slightly enriched over y and hree the olivine gabbronorites and basalts show overlapping nd isotopic compositions initial nd of 2 4 to 1 4 and of 3 0 to 1 5 respectively and have clinopyroxenes with nearly parallel incompatible trace element patterns thus revealing that these two rock types formed by melts with similar compositions most likely the primary melts of bocca di tenda gabbroic sequence were slightly enriched in most incompatible elements and nd isotopic compositions relative to modern n morb

evidence for two step deglaciation and its impact on north atlantic deep water circulation oxygen and carbon isotope records from benthic and planktonic foraminifera are presented for the past 35 000 years in the northeastern atlantic the results support the idea that the last deglaci ation took place in two major steps1 4 and conflict with theories calling for a strong reduction in north atlantic deep water formation to explain the abrupt cooling of the younger dryas cold period5 7

late holocene paleoenvironments of northwest iceland from lake sediments abstract frozen sediment cores from two lakes in northwest iceland vatnsdalsvatn and thi riksvallavatn provide a nearly 1000 year decadal scale record of past environmental changes based on environmental magnetism inductively coupled plasma optical emission spectroscopy geochemistry carbon content and c n analyses accelerator mass spectrometry 14 c and 210 pb results provide chronologies for both sites covering the late 20th century to 960 bp at vatnsdalsvatn and to 480 bp at thi riksvallavatn these analyses reveal higher levels of sedimentary carbon during the little ice age between 600 and 50 bp at vatnsdalsvatn c n analyses suggest that some of this carbon enrichment came from terrestrial plants higher erosion rates during the little ice age may also have increased the influx of limiting nutrients enhancing algal productivity erosion is the dominant mechanism for sedimentary changes at both sites and seems to occur at predictable intervals geochemical indicators of erosion oscillate with a distinct 80 100 yr periodicity for nine centuries at vatnsdalsvatn and for four centuries at thi riksvallavatn these erosion cycles are consistent with long term stability in the positive mode of the north atlantic oscillation but may also be attributed to local climatic effects associated with great salinity anomalies

a review of the australian new zealand sector of the southern ocean over the last 30ka aus intimate project the australia new zealand region of the southern ocean is influenced by several of the major global water masses of the oceans and is the prime entry point for cold deep waters into the pacific basin during the last glacial there was increased sea ice extent around antarctica as far north as 55 s as well as increased iceberg presence inferred from ice rafted debris evidence from microfossil assemblages suggests that sea surface temperatures sst were up to 7 c cooler consistent with recent estimates of cooling for new zealand derived from glacier modelling and other terrestrial proxies the subtropical front stf subantarctic front saf and polar front pf had migrated north except where the position of the fronts were controlled bathymetrically despite the potential for iron fertilisation by increased dust input into the ocean during the glacial there is limited evidence for higher total biological productivity in the pacific sector of the southern ocean the altered oceanic circulation during the glacial also decreased nutrients in the surface waters and affected the outgassing of co 2 this contributed to an increased storage of co 2 in the deep waters and lowering of the carbonate lysocline during the deglaciation sea ice retreat and sst increased rapidly at 18 ka roughly synchronous with the reinvigoration of deep water circulation in the southern ocean and the release of co 2 stored in the deep waters the gradient in carbon isotopes 13 o benthic between antarctic intermediate water aaiw and lower circumpolar deep water lcdw was greatest at the start of the deglaciation suggesting that the aaiw ventilation preceded lcdw ventilation or there was a significant change in air sea fractionation of 13 o benthic there was a slight enrichment in 18 o planktic decrease in ssts and a reduction in intermediate and deep water circulation between 14 and 12 5 ka bp during the antarctic cold reversal acr coincident with glacier advances in the new zealand southern alps and other terrestrial records of cooling following the acr there was a second more minor release of deep water co 2 most likely related to reinvigoration of north atlantic deep water nadw from 10 ka onwards the modern intermediate and deep water circulation was established air temperatures and ssts reached a maximum in the early holocene with the stf located at its most southerly position and there was a widespread retreat of antarctic ice sheets to their modern position a decline to modern sst and air temperatures in the mid to late holocene followed while millennial cycles overprinted on the sst and 18 o planktic records may be the result of subtle changes in the position and strength of the westerly winds during the holocene

similar rates of modern and last glacial ocean thermohaline circulation inferred from radiochemical data today the ocean thermohaline circulation transports half of the 231pa produced by radioactive decay in the atlantic ocean water column to the southern ocean this export respectively imparts low and high 231pa 230th ratios to the surface sediments of these oceans ocean sediments from the last glacial maximum bear a similar isotopic fingerprint implying that advection of north atlantic deep intermediate water into the circumpolar deep water of the southern ocean occurred at a similar or slightly higher rate during the last glacial period

late glacial and holocene palaeocurrents around rockall bank ne atlantic ocean core samples and hydrographic data from 50 to 60 n and 15 to 25 w in the northeast atlantic ocean are used to study changes in current activity from the last glacial to present we establish a new method to distinguish between the effects of changing bottom current speed and varying size input by drawing upon sediment flux data and detailed grain size analysis changes in current speed are recorded by the mean size of the sortable coarse noncarbonate silt component which increases with current vigor by winnowing away of fines our method involves the definition of an ideal sortable silt input function recorded at a site accumulating unmodified pelagic flux only the ideal profile is then compared with records from other sites to determine the history of current speed at a range of water depths the upper surface of southern source water ssw probably shoaled during the glacial as indicated by the covariation of sortable silt records from sites presently covered by ssw and lower north atlantic deep water lnadw the data suggest that production of lnadw was suppressed during the glacial increased following the last glacial maximum and declined at the start of termination 1a a second pulse of production occurred immediately before the younger dryas event intermediate waters were generally faster flowing during the glacial and may have expanded their depth domain such that a single glacial intermediate water mass covered depths from 1100 to more than 2000 m

experimental study of a kiglapait marginal rock and implications for trace element partitioning in layered intrusions abstract experiments on a chilled marginal rock from the kiglapait intrusion labrador have been performed in order to constrain the partitioning behaviour of major and trace elements during formation of kiglapait lower zone lz cumulates at 0 1 mpa and f o 2 close to fmq the liquidus is bracketed between 1225 and 1212 c and the only liquidus phases are plagioclase an 73 74 and olivine fo 82 85 in contrast lz troctolites contain fo 67 5 olivine and an 63 4 plagioclase plagioclase melt partition coefficients d s for li ti rb sr y zr ba la and eu determined by ion microprobe analysis of run products are similar to those for other basaltic systems but differ systematically from apparent d s calculated from bulk analyses of lz plagioclases and modelled liquid compositions obtained by summation the discrepancies in d s and phase compositions are inconsistent with diffusion controlled boundary layer growth crystallisation of trapped intercumulus liquid is quantitatively plausible however a the volume of liquid required to reconcile experimental and model d s 20 1 is larger than that estimated from observed proportions of intercumulus phases 11 2 and b this mechanism alone cannot reduce x an in lz plagioclases due to very slow al si interdiffusion the discrepancy between experiment and model is best resolved if the parental melt to lz cumulates was more evolved higher fe mg na ca ree and alkalis lower sr than the summed bulk composition in accord with a recent revision of the kiglapait fe mg based on olivine compositions

a salt oscillator in the glacial atlantic 1 the concept as shown by the work of dansgaard and his colleagues climate oscillations of one or so millennia duration punctuate much of glacial section of the greenland ice cores these oscillations are characterized by 5 c air temperature changes severalfold dust content changes and 50 ppm co2 changes both the temperature and co2 change are best explained by changes in the mode of operation of the ocean in this paper we provide evidence which suggests that oscillations in surface water conditions of similar duration are present in the record from a deep sea core at 50 n based on this finding we suggest that the greenland climate changes are driven by oscillations in the salinity of the atlantic ocean which modulate the strength of the atlantic s conveyor circulation

seasonal variability of stable carbon isotopes δ13cdic in the skagerrak and the baltic sea distinguishing between mixing and biological productivity abstract we documented the annual cycle of the carbon isotopic composition of dissolved inorganic carbon 13cdic in the water columns of the skagerrak and baltic sea to obtain an increased understanding of the processes involved controlling the carbon isotopic distribution in shelf seas the lowest 13cdic values 4 9 were found in the low oxygen brackish baltic bottom water whereas the highest values 1 8 were observed in the surface water of the skagerrak during late summer photosynthesis drove the high 13cdic values between 1 0 and 1 8 noted in the surface waters of both the skagerrak and the baltic the 13cdic values below the halocline in the baltic reflect mixing of brackish water and the more saline water from the skagerrak and foremost organic matter remineralization processes that release significant amounts of low 13c co2 similarly in the stagnant fjord basins little deep water exchange and the degradation of terrestrial and marine organic matter set the 13c composition deep water renewal in the fjord basins resulted in rapid increases of the 13cdic on the order of 1 whereas remineralization processes caused a decrease in 13cdic of 0 1 0 3 per month depending on location the combined effects of water mixing and remineralization processes estimated using apparent oxygen utilization aou values yielded the expression 13cdic 0 032 s 0 01 aou 0 12 for the baltic skagerrak region at water depths below the halocline

constraints on the ocean oxygen isotopic enrichment between the last glacial maximum and the holocene paleoceanographic implications abstract we review the various methods which have been applied to estimate the change of seawater 18 o w between the last glacial maximum lgm and the holocene the most accurate constraints on these estimates are provided by the measurement of pore waters 18 o and by high resolution records of benthic foraminifer 18 o in the high latitude oceans of both hemispheres they show that the 18 o of seawater in the deep ocean during the lgm was 1 05 0 20 heavier than today with significant regional variations constraints resulting from ice sheet models are less accurate because both the volume and isotopic composition of each ice sheet are still poorly known the amplitude of the benthic 18 o change between the lgm and the holocene together with the 18 o and 13 c values of the benthic foraminifera genus cibicides during the lgm show that the southern ocean deep waters were extremely cold close to the freezing point during this time deep waters of the south atlantic and the pacific oceans were at least 1 3 c warmer than those of the southern ocean overall the glacial deep ocean below 2500 m was characterized by extremely cold temperatures everywhere lower than 0 c 18 o values of benthic foraminifer from the north atlantic are highly variable this variability suggests that deep atlantic waters were not homogeneous probably because they resulted from the sinking of different surface water masses at various locations during winter the deep waters in the north atlantic were at most 2 c warmer than in southern ocean alternatively they could have been nearer the freezing point with a 18 o value lighter than the mean ocean water brine formation during winter would preserve such light 18 o values of the northern north atlantic surface water

a benthic foraminiferal record of middle to late pliocene 3 15 2 85 ma deep water change in the north atlantic abstract records of benthic foraminifera from north atlantic dsdp site 607 and hole 610a indicate changes in deep water conditions through the middle to late pliocene 3 15 to 2 85 ma quantitative analyses of modern associations in the north atlantic indicate that seven species fontbotia wuellerstorfi cibicidoides kullenbergi uvigerina peregrina nuttallides umboniferus melonis pompilioides globocassidulina subglobosa and epistominella exigua are useful for paleoenvironmental interpretation the western north atlantic basin site 607 was occupied by north atlantic deep water nadw until 2 88 ma at that time n umboniferus increased indicating an influx of southern ocean water sow the eastern north atlantic basin hole 610a was occupied by a relatively warm water mass possibly northeastern atlantic deep water neadw through 2 94 ma when sow more strongly influenced the site these interpretations are consistent with benthic 18o and 13c records from 607 and 610a raymo et al 1992 the results presented in this paper suggest that the north atlantic was strongly influenced by northern component deep water circulation until 2 90 2 95 ma after that there was a transition toward a glacially driven north atlantic circulation more strongly influenced by sow associated with the onset of northern hemisphere glaciation the circulation change follows the last significant sst and atmospheric warming prior to 2 6 ma

prediction of plagioclase melt equilibria in anhydrous silicate melts at 1 atm many models for plagioclase melt equilibria have been proposed over the past 30 years but the focus is increasingly on the effects of water content and pressure however many geological and petrological applications concern low pressure and low water systems such as the differentiation of large terrestrial basaltic magma chambers and lunar and asteroidal magmatism there is therefore a justified need to quantify the influence of anhydrous liquid composition on the composition of equilibrium plagioclase at 1 atm with this in mind a database of over 500 experimentally determined plagioclase liquid pairs has been created the selected low pressure anhydrous experiments include both natural and synthetic liquids whose compositions range from basalt to rhyolite four equations are proposed derived from this data the first is based on a thermodynamically inspired formalism explicitly integrating the effect of temperature this equation uses free energies and activities of crystalline anorthite available from the literature for the activity of anorthite in the liquid phase it is found that current models of the activity of individual oxides are insufficient to account for the experimental results we have therefore derived an empirical expression for the variation of anorthite activity in the liquid as a function of melt composition based upon inversion of the experimental data using this expression allows the calculation of plagioclase composition with a relative error less than 10 however in light of the fact that temperature is not necessarily known for many petrological applications an alternative set of t independent equations is also proposed for this entirely empirical approach the database has been divided into three compositional groups treated independently for regression purposes mafic ultramafic alkali rich mafic ultramafic and intermediate felsic this separation into distinct subgroups was found to be necessary to maintain errors below acceptable limits but results across group boundaries were found to be comparable overall 50 of plagioclase compositions are predicted to within 2 of the experimentally derived value and 90 to within 5 representing a significant improvement over existing models

a 40 000 yr pollen and diatom record from lake tritrivakely madagascar in the southern tropics abstract links between southern and northern hemisphere climates during the late quaternary are poorly known partly due to the scarcity of continuous climatic records in the southern tropics pollen and diatom evidence from lake tritrivakely 19 47 s provides information on vegetational and hydrological changes in the central highlands of madagascar over the past 40 000 yr most of the record reflects natural environmental variability since humans arrived on the island ca 2000 yr b p during glacial times the migration of mountain plants toward lower altitudes is consistent with a temperature decrease and with reduced atmospheric co2levels in the lake a positive mean annual hydrologic balance from 38 000 to 36 000 and from 17 500 to 9800 cal yr b p coincided with periods of decreasing summer insolation and preceded by several millennia lake rises in the northern tropics a negative hydrologic budget during periods of maximum seasonal contrast in solar radiation is partly attributed to high summer evaporation rate the last glacial maximum was cool and dry the deglacial warming occurred in two steps the first step accompanied by an increase in wetness occurred abruptly at ca 17 000 cal yr b p about two millennia earlier than in the northern hemisphere it is abundantly documented in southern terrestrial data the second step at 15 000 cal yr b p was in phase with the first major temperature change in the northern hemisphere

evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate evidence from southern ocean sediments for the effect of north atlantic deep water flux on climate

glacial and holocene stable isotope distributions in the southeastern indian ocean we have reconstructed glacial interglacial variations in cibicidoides wuellerstorfi 13c and 18o in the southeastern indian ocean using depth transects of cores from off western and southern australia we compare these new results with published stable isotope records to constrain glacial interglacial changes in indian ocean chemistry and circulation the strong last glacial maximum lgm deep water 13c depletions previously observed for the northeastern indian ocean did not extend throughout the low latitude glacial indian ocean we observe only a small lgm 13c decrease off southern australia in marked contrast to the strong lgm 13c depletions observed in many cores from the atlantic and western indian sectors of the southern ocean it is not yet clear whether the very low lgm 13c values in the deep northern and southwestern indian ocean are an artifact of surface ocean productivity or whether they reflect real gradients in deep water chemistry

thermodynamic analysis of fe and mg partitioning between plagioclase and silicate liquid thermodynamic analysis of fe and mg bearing plagioclase and silicate liquid was carried out based on reported element partitioning data between plagioclase and silicate liquid in reduced conditions solution properties of ternary feldspar standard state properties of plagioclase endmembers and solution properties of multicomponent silicate liquid derived mixing properties of fe and mg bearing plagioclase are in harmony with estimated results from synthetic experiments in the systems caal2si2o8 cafesi3o8 and caal2si2o8 camgsi3o8 based on the determined solution properties of the plagioclase a computer program to calculate the element partition relationships between fe and mg bearing plagioclase and multicomponent silicate liquid was developed the feo mgo and mgo mgo feo in plagioclase predicted from known liquid compositions and pressure are in agreement with measurements within 0 2 wt 0 1 wt and 0 1 mol ratio respectively the fe3 content in plagioclase crystallized at high oxygen fugacity can be estimated with this program the fe3 total fe ratio in plagioclase crystallized near the quartz fayalite magnetite buffer ranges from 0 to 0 5 which is consistent with previous study on natural plagioclase in submarine basalt derived solution properties of the fe and mg bearing plagioclase are also used to calculate equilibrium composition relationship between olivine and plagioclase change of xfo in olivine coexisting with plagioclase affects mgo and feo contents in plagioclase greatly the present model predicts xfo of coexisting olivine from the chemical composition of plagioclase to 0 1 accuracy at given pressure and temperature

the impact of cold north atlantic sea surface temperatures on climate implications for the younger dryas cooling 11 10 k the sensitivity of global climate to colder north atlantic sea surface temperatures is in vestigated with the use of the giss general circulation model north atlantic ocean temperatures 18 000 b p resembling those prevalent during the younger dryas were incorporated into the model of the present climate and also into an experiment using orbital parameters and land ice characteristic of 11 000 b p the results show that with both 11 000 b p and present conditions the colder ocean temperatures produce cooling over western and central europe in good agreement with younger dryas paleoclimatic evidence cooling also occurs over extreme eastern north america although the precise magnitude and location depends upon the specification of ocean temperature change in the western atlantic despite the presence of increased land ice and colder ocean temperatures the younger dryas summer air temperatures at northern hemisphere midlatitudes in the model are warmer than those of today due to changes in the orbital parameters chiefly precession and atmospheric subsidence at the perimeter of the ice sheets

the role of deep ocean circulation in setting glacial climates the glacial cycles of the pleistocene involve changes in the circulation of the deep ocean in important ways this review seeks to establish what were the robust patterns of deep sea water mass changes and how they might have influenced important parts of the last glacial cycle after a brief review of how tracers in the modern ocean can be used to understand the distribution of water masses i examine the data for biogeochemical circulation rate and conservative tracers during glacial climates some of the robust results from the literature of the last 30 years include a shoaled version of northern source deep water in the atlantic expanded southern source water in the abyss and deep ocean salt rather than heat stratification of the last glacial maximum lgm deep sea and several lines of evidence for slower overturning circulation in the southern deep cell we combine these observations into a new idea for how the ocean atmosphere system moves from interglacial to glacial periods across a single cycle by virtue of its influence on the melting of land based ice around antarctica cooling north atlantic deep water nadw leads to a cold and salty version of antarctic bottom water aabw this previously underappreciated feedback can lead to a more stratified deep ocean that operates as a more effective carbon trap than the modern helping to lower atmospheric co 2 and providing a mechanism for the deep ocean to synchronize the hemispheres in a positive feedback that drives the system to further cooling

co 2 record in the byrd ice core 50 000 5 000 years bp the analysis of air in polar ice cores revealed 30 lower co2 values during glacial periods than during interglacial periods at present this is confirmed by results from six different ice cores two from greenland and four from antarctica1 5 in all cores the co2 change coincides with the change in the isotopic composition of the ice expressed as either the 18o or d ratio both are indicators for the mean annual surface temperature6 to investigate the relationship between atmospheric co2 concentration changes with changes in climate the atmospheric co2 concentration during and at the end of the last glaciation has to be known in detail to achieve this we have studied a great number of samples from the deep ice core from byrd station westantarctica drilled in 1968 these measurements allow us to reconstruct the atmospheric co02 concentration in the time period 50 000 15 000 yr bp in great detail

variability in the deep and intermediate water circulation of the atlantic ocean during the past 25 000 years northern hemisphere modulation of the southern ocean the relative flux of north atlantic deep water nadw out of the atlantic can be monitored in the southern ocean where high 13c nadw mixes rapidly with low 13c recirculated pacific water before forming antarctic bottom water aabw by using 13c measured in benthic foraminifera as a water mass tracer it is shown that the southern ocean was dominated by pacific water with little nadw contribution during the last glaciation these tracer results document the direct physical connection between insolation controlled northern hemisphere oscillations and changes in the chemical and physical properties of southern ocean water the 13c records from the deep tropical atlantic document the transition from an atlantic dominated by southern ocean water during the last glaciation to an atlantic dominated by nadw during the holocene with more southern ocean water in the deep atlantic during the last glaciation the boundary between nadw and southern ocean water was north of its modern position allowing more southern ocean water to enter the eastern atlantic through the romanche fracture zone carbon isotope records from the caribbean and mediterranean seas suggest that the influence of mediterranean overflow water mow extended to the caribbean during the last glaciation and therefore was a volumetrically much more important water mass in the intermediate depth atlantic than it is today

hydrospheric transports the oxygen isotope record and tropical sea surface temperatures during the last glacial maximum a model is developed to consider explicitly the effect of glacial to holocene changes in transports of fresh and salt water on the distribution of oxygen isotopes in seawater and therefore on the calibration of the foraminiferal 18o paleothermometer the published foraminiferal isotopic record is shown to be consistent with glacial surface temperatures at low latitudes cooler than today by about 4 c in the atlantic and indian oceans and about 2 c in the pacific given glacial transport patterns supported by the geologic record if in addition net vapor transport from the indian and atlantic oceans to the pacific was enhanced significantly during glacial times relative to today the isotopic record would be consistent with a sea surface cooler by up to 5 c throughout the tropics at the last glacial maximum as expected based on proxies for continental surface temperatures

near synchroneity of new zealand alpine glaciations and northern hemisphere continental glaciations during the past 750 kyr the brunhes magnetochron 0 730 kyr at deep sea drilling project site 594 off eastern south island new zealand comprises up to 100 m of alternating units of pelagic and hemipelagic ooze formed during interglacial and glacial periods respectively the youngest hemipelagic interval corresponds to glacial isotope stage 2 and can be correlated with extensive glacial and glacio fluvial deposits of the late last glaciation 27 13 kyr bp on south island inferring the same genetic link for the other hemipelagic sediment units down core we show here that as many as 12 major episodes of expanded alpine glaciation have occurred on south island in the past 730 kyr each of these glacial episodes coincided with a period of significant 18o enrichment 2 in the core resulting from changes in ocean isotope composition associated with major growth phases of the northern hemisphere ice sheets the inferred history of alpine glaciations on this remote south pacific island appears to be closely phase locked 3 kyr to world wide climate changes

deep sea sedimentary analogs for the vostok ice core 1 many applications of the vostok ice core depend critically on the ability to make stratigraphic ties to marine records in the adjacent southern ocean here we present oxygen isotopic records from high accumulation rate sites in the south atlantic sector of the southern ocean collected for the purpose of complementing the recently extended d record from the vostok ice core the combination of several planktonic foraminiferal 18o records from northern subantarctic piston cores demonstrates that all of the millennial scale oscillations expressed in the vostok ice core over the last 60 ky are also present in marine records the observations also support the assumption that the millennial scale oscillations common to both marine and ice archives are synchronous thus providing a rationale for extending the marine ice core comparison through the last 400 000 years making use of a marine drilled core odp site 1089 by aligning the phase of these common abrupt events we anchor the vostok chronology to an orbitally tuned marine sediment chronology a refinement that allows examination of a variety of paleoclimatological issues such as the relationship between deep ocean variability and antarctic polar climate for example this exercise suggests that over at least the 4 major deglaciation events the primary orbital scale changes in the chemistry and most likely the temperature of the deep southern ocean were synchronous with changes in atmospheric pco2 and polar air temperatures we also find that the deuterium excess in the ice core resembles marine foraminiferal 13c records and that the deuterium excess is synchronous with an anomalous foraminiferal 18o signal the residual between normalized versions of vostok d and foraminiferal 18o these observations demand a tight link between the vostok isotopic record and the air sea interaction of the subantarctic zone

the intensification of northern component deepwater formation during the mid pleistocene climate transition we reconstruct mid pleistocene marine isotope stages miss 13 18 deepwater hydrography at ocean drilling program site 1063 4583 m water depth subtropical north atlantic using benthic foraminiferal stable isotope records these new records complete an 900 kyr long stratigraphy spanning miss 8 29 250 1030 ka when combined with previously published records from site 1063 the results indicate a change in the circulation regime of the abyssal subtropical north atlantic during mis 17 prior to mis 17 no significant glacial or interglacial 13c gradients are evident between site 1063 and the deep south atlantic after mis 17 interglacial intervals at site 1063 are characterized by 13c values that consistently approach those recorded in the deep north atlantic comparing site 1063 13c values to 26 additional published records throughout the entire atlantic basin supports the idea that this 13c increase is unique to the deep north atlantic after mis 17 the basin wide influence of higher 13c values suggests an increased relative flux of northern sourced bottom waters during interglacial periods the timing of northern sourced water influence at site 1063 is consistent with the timing of a shift in the orientation of the arctic front thus this shift may signify a link between the northward penetration of relatively warm saline surface waters into the norwegian greenland seas stimulating deep convection our findings fit well with the model of imbrie et al 1993 for the importance of the nordic heat pump in establishing strong 100 kyr cyclicity in late pleistocene glacial cycles