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

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

the late pleistocene palaeoclimatic record of north atlantic deep sea sediments revealed by mineral magnetic measurements eight cores of north atlantic deep sea sediment are correlated using simple rapid and non destructive measurements of magnetic susceptibility and isothermal remanence this stratigraphy is demonstrably related to late pleistocene palaeoclimatic variations by comparison with the oxygen isotope and microfossil records of these cores fluctuations in the concentration dependent magnetic properties of the cores largely reflect their lithological composition consisting of climatically modulated cycles of marl glacial horizons and nannofossil chalk interglacial horizons high magnetic concentration characterises glacial horizons which correspond to periods when carbonate productivity was severely restricted and ice rafted detrital inputs were prolific coincident with the migration south of the north atlantic polar front to within 3 of latitude north of the coring site in contrast interglacial horizons are characterised by low magnetic concentration and high carbonate content of the sediment consequent on the retreat of the polar front and the re establishment of coccolith floras in its wake during these periods ratios formed between individual magnetic parameters are independent of magnetic concentration and are thus free of any climatic influences wrought by carbonate oscillations in this study however such ratios exhibit a definite response to palaeoclimatic variations attributable to differences in the composition and grain size of the magnetic mineral assemblage of glacial vis a vis interglacial horizons of the cores these differences in mineral magnetic composition are substantially confirmed by mossbauer effect studies of selected samples representing peak glacial and interglacial horizons of one core such glacial interglacial contrasts in the concentration mineralogy and grain size of the magnetic assemblage most probably result from variations in the source and flux density of detritral inputs induced by changes in climate x ray diffraction studies of the same core indicate that variations in the mineralogy of the detrital fraction of the sediment are consistent with those of the magnetic assemblage together these data point to the conclusion that a subsidiary low latitude possibly aeolian detrital flux supplements high latitude ice rafted input in glacial horizons of the cores the low latitude flux however is also identifiable in interglacial horizons independent evidence suggests that this low latitude aeolian flux may be derived from the sahel region of north africa and transported to the coring site by harmattan winds associated with saharan dust outbreaks

apparent correlation of palaeomagnetic intensity and climatic records in deep sea sediments most reports of a correlation between pleistocene climate and geomagnetic field intensity rely strongly on the assumption that sediment natural remanent magnetic nrm intensity provides a record of geomagnetic field strength and is not sensitive to local changes in properties of the sediment critical assessment of relevant data presented here and elsewhere1 2 from deep sea sediment cores shows that a pronounced dependence of nrm intensity on sediment composition can occur which implies that this assumption is unlikely to be generally valid as sediment composition often reflects varying depositional conditions induced by climatic change the significance of correlations proposed between pleistocene palaeomagnetism and climatic indicators in deep sea sediments may be less dramatic than sometimes supposed

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

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

climatic change and caco3 preservation an 800 000 year bathymetric reconstruction from the central equatorial pacific ocean sixteen deep sea cores from the central equatorial pacific are used to reconstruct a continuous 800 000 year 800 kyr record of bathymetric variations in carbonate preservation as measured by calcium carbonate caco3 content the depth of the sedimentary lysocline has fluctuated markedly in conjunction with late pleistocene climate cycles while the carbonate critical depth ccrd which is the water depth where the sediments contain 10 caco3 has remained relatively constant as a result the depth of the lysocline controls the bathymetric position and thickness of the caco3 transition zone defined as the depth from the lysocline to the ccrd modern and interglacial aged sediments show poor caco3 preservation and thick caco3 transition zones glacial aged sediments show good preservation and deep thin zones due to the deepening of the lysocline detailed comparison of the caco3 preservation and oxygen isotope records from the central equatorial pacific confirms the observation that preservation maxima and minima tend to occur during the latter half of glacial and interglacial stages and on climate transitions rather than during the middle of climatic stages during the nine major glacial stages of the last 800 kyr the lysocline deepened by at least 400 to 800 m this deepening indicates an increase in the abyssal carbonate ion concentration co3 and a depression of the calcite saturation horizon best explained by the deeper presence of a more carbonate saturated water mass the bottom of the transition zone has remained at a relatively constant depth during the brunhes chron indicating a balance between caco3 sedimentation and dissolution in the deepest waters of the central equatorial pacific

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

oligocene paleoceanography of the south atlantic paleoclimatic implications of sediment accumulation rates and magnetic susceptibility measurements paleoceanographic changes during the oligocene in the south atlantic have been examined using paleomagnetic rock magnetic carbonate content and sediment accumulation rate sar data we show that magnetic susceptibility ms is largely controlled by carbonate content at deep sea drilling project site 522 which in turn is the primary control on sar at the site over 5000 ms measurements were averaged over magnetostratigraphic polarity intervals for use as indicators of carbonate content and sar proxies the study produced three primary results 1 a spectral analysis of the ms data from one upper oligocene core reveals two dominant periodicities 44 7 10 4 kyr and 26 4 3 6 kyr which approximate milankovitch obliquity 41 kyr and precession 23 kyr periods 2 four pulses of increased sar lasting 1 to 2 m y punctuate the section implying longer term climatic changes these events are also seen in the contessa quarry section of the italian tethyan oligocene and 3 two of the sar maxima correspond to times of global cooling and possible ice volume increases at 31 and 36 ma

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

a comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials some results from lake sediments abstract a new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility arm i e specific arm obtained in a 1 oe steady field which is particularly sensitive to the single domain sd and small pseudo single domain psd grains of the finer magnetite fraction a second parameter low field susceptibility is relatively more sensitive to the coarser magnetite fraction larger psd and smaller multidomain md grains we can then obtain a measure of the ratio of coarse to fine grain magnetite for large numbers of samples by plotting arversus a simple idealized model based on sized magnetite samples is proposed to explain the use of the armversus plot for detecting relative grain size changes in the magnetic content of natural materials the sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of arm and are used to test the model the model is used to interpret the magnetic variations observed and the interpretations are supported by high field hysteresis measurements of the same sediments the combination of the high field hysteresis method of day et al 1 and the arm vs method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite the armvs method should be used as an intial means of identifying distinct groups of samples the high field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation

paleoenvironmental implications of rock magnetic properties of late quaternary sediment cores from the eastern equatorial atlantic rapid and nondestructive rock magnetic measurements of late quaternary sediment cores from the senegal continental slope and the sierra leone rise in the eastern equatorial atlantic correlate strongly with the specmap oxygen isotope master curve and carbonate content the rock magnetic paleoclimatic relationship results from differences in the concentration accumulation rate grain size and composition of magnetic minerals between glacials and interglacials at both sites magnetic mineral assemblages of glacial episodes are characterized by higher concentrations and accumulation rates of magnetic minerals by coarser grained ferrimagnetic minerals e g magnetite and by a lower ratio of ferrimagnetic to antiferromagnetic e g goethite hematite minerals by serving as a sensitive tracer for various deep sea sedimentary components rock magnetic properties can provide insights into changing sediment sources and fluxes and therefore are a useful addition to more traditional chemically and mineralogically based paleoceanographic techniques

physical properties of reykjanes ridge sediments and their linkage to high resolution greenland ice sheet project 2 ice core data five gravity cores taken from the reykjanes ridge have been used to establish a link between sediment physical properties and atmospheric records documented by 18o variations in greenland ice cores over the last 45 000 calendar years marine gamma ray attenuation porosity evaluator density and magnetic susceptibility variations could be linked with the ice core dansgaard oeschger and bond cycles this is supported by ice rafted detritus ird grain size the quartz feldspar ratio and carbonate isotopic and foraminiferal records the covariation of the sediment physical properties and 18o in greenland ice indicates a coupling of atmospheric temperature and paleocirculation variations gradual reduced bottom currents iceland scotland overflow water and enhanced iceberg discharges have been reconstructed for cold atmospheric periods relative to interstadial times in the study area the magnetic susceptibility signal is not related to the ice rafted detritus input but most probably reflects the variations of the iceland scotland overflow water intensity transporting titanomagnetite into the reykjanes ridge region

ice rafted evidence of long term north atlantic circulation abstract the absolute and relative rates of deposition of ice rafted volcanic and terrigenous clastic material were determined for the interval from the base of oxygen isotope stage 5 127 000 yrs b p to the lower part of stage 3 50 000 yrs b p in five subpolar north atlantic deep sea cores during this time interval global climate changed from a full interglaciation to nearly a full glaciation for the sediment derived from the continents dominantly quartz and feldspar the eastward decrease in deposition rates during interglacial intervals and eastward increase during glacial intervals is best explained by a southeastward jump in the locus of iceberg melting during climates colder than that of today a similar model may explain the eastward increase in volcanogenic debris during glacial time intervals however the eastward increase of volcanogenic debris during interglacial stage 5 requires a stronger southeastward component of flow from greenland than is evident in the modern circulation

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

evidence for a change in the periodicity of tropical climate cycles at 2 4 myr from whole core magnetic susceptibility measurements the earth s orbit about the sun is influenced by gravitational interactions with the moon and other planets the resulting orbital perturbations give rise to cyclical variations in orbital eccentricity obliquity and precession with periods of 100 41 19 and 23 kyr respectively1 these variations are climatically important because they affect the seasonal and latitudinal distribution of solar radiation magnetic susceptibility measurements in deep sea sediment cores can be used as a sensitive indicator of temporal variations in the concentration of terrigenous material supplied to the sea bed here we use this approach to study the evolution of cycles of terrigenous sedimentation at ocean drilling program sites in the arabian sea and the eastern tropical atlantic we show from spectral analyses of the two susceptibility time series spanning the past 3 2 3 5 myr that the records are driven strongly by orbital forcing before 2 4 myr before present bp both records are strongly influenced by the 23 and 19 kyr periodicities but after 2 4 myr they are both significantly influenced by the 41 kyr periodicity this shift coincides with the beginning of major northern hemisphere glaciation which indicates for the indian ocean site that the supply of terrigenous sediment carried by monsoonal winds has responded to the rapid increase in ice cover at this time we cannot however exclude possible effects of changes in bottom water circulation on the susceptibility record of the atlantic site

gas hydrate systems at hydrate ridge offshore oregon inferred from molecular and isotopic properties of hydrate bound and void gases abstract we report and discuss molecular and isotopic properties of hydrate bound gases from 55 samples and void gases from 494 samples collected during ocean drilling program odp leg 204 at hydrate ridge offshore oregon gas hydrates appear to crystallize in sediments from two end member gas sources deep allochthonous and in situ as mixtures of different proportions in an area of high gas flux at the southern summit of the ridge sites 1248 1250 shallow 0 40 m below the seafloor mbsf gas hydrates are composed of mainly allochthonous mixed microbial and thermogenic methane and a small portion of thermogenic c2 gases which migrated vertically and laterally from as deep as 2 to 2 5 km depths in contrast deep 50 105 mbsf gas hydrates at the southern summit sites 1248 and 1250 and on the flanks of the ridge sites 1244 1247 crystallize mainly from microbial methane and ethane generated dominantly in situ a small contribution of allochthonous gas may also be present at sites where geologic and tectonic settings favor focused vertical gas migration from greater depth e g sites 1244 and 1245 non hydrocarbon gases such as co2 and h2s are not abundant in sampled hydrates the new gas geochemical data are inconsistent with earlier models suggesting that seafloor gas hydrates at hydrate ridge formed from gas derived from decomposition of deeper and older gas hydrates gas hydrate formation at the southern summit is explained by a model in which gas migrated from deep sediments and perhaps was trapped by a gas hydrate seal at the base of the gas hydrate stability zone ghsz free gas migrated into the ghsz when the overpressure in gas column exceeded sealing capacity of overlaying sediments and precipitated as gas hydrate mainly within shallow sediments the mushroom like 3d shape of gas hydrate accumulation at the summit is possibly defined by the gas diffusion aureole surrounding the main migration conduit the decrease of gas solubility in shallow sediment and refocusing of gas by carbonate and gas hydrate seals near the seafloor to the crest of the local anticline structure

modeling of the influence of gas hydrate content on the electrical properties of porous sediments the geophysical investigation of gas hydrate accumulations within marine sediments and the sediments of permafrost regions suffer from a lack of information on the influence of the hydrate content on the physical sediment properties the estimation of the hydrate content using downhole electrical measurements based on archie s law requires the knowledge of the saturation exponent the saturation exponent is an empirical parameter that includes influences from the internal rock structure such as pore shape connectivity and constrictivity of the pore network and the distribution of the conducting phase based on different models that account for the different morphological forms of gas hydrates found during gas hydrate sampling in various research wells the influence of gas hydrate content on the electrical properties of the hydrate bearing sediment was investigated for all studied forms of hydrate occurrence disseminated in the pore space nodular and layered the saturation exponent depends on the sediment properties and on saturation itself the growth of gas hydrate nodules lenses and layers is a process that is assumed to result in the displacement and compaction of the surrounding sediment because of this change of sediment properties during hydrate generation the saturation exponent for these forms of hydrate occurrence depends strongly on the relationship between porosity and formation resistivity factor expressed in the form of archie s cementation exponent for the case that hydrate occurs disseminated in the pore space and the assumption that capillary effects are important for hydrate generation the saturation exponent depends on grain size and grain size sorting for the parameters chosen for these model calculations the saturation exponent varies between 0 5 and 4 the use of a constant mean value for the saturation exponent of approximately 2 can result in both underestimation and overestimation of the hydrate content

milankovitch cycles in neocene deep sea sediment pelagic carbonate sediments from the world ocean basins commonly show cyclic variations in amount and or degree of preservation of biogenic calcite with periodicities of several tens to several hundreds of thousands of years the direct causes of these cycles are fluctuations in noncarbonate dilution carbonate production carbonate dissolution and or current winnowing the overall driving force however is variation in the earth s orbital characteristics milankovitch cycles through their influence on global climate and depositional processes the main inferred climatic effects of orbital perturbations are on global ice volume global temperature ocean circulation and distribution of climatic patterns eustatic sea level is directly related to ice volume as is the rate of erosion of clastic material from continental margins changes in volume of sea ice affect the volume and intensity of bottom water flow which in turn may cause changes in the intensity of sediment winnowing by bottom currents in the intensity of upwelling of nutrient rich bottom waters and in the depth of carbonate dissolution change in productivity of calcareous plankton are difficult to prove as a cause of carbonate cycles but may have contributed to the formation of carbonate cycles off northwest and southwest africa fluctuations in winnowing of fine grained components have been demonstrated as a cause of cyclic variations in the coarse fraction component of carbonate sediments in the southwest pacific dilution of carbonate by clastic material probably was a major cause of fluctuations in carbonate content of deep sea sediments off northwest and southwest africa carbonate dissolution cycles probably are the most common manifestation of fluctuations in bottom water flow dissolution cycles are common in quaternary and neogene sediments of the north atlantic caribbean and eastern equatorial pacific the main cause of the carbonate dissolution was shoaling of the carbonate compensation depth during the early neogene in response to climatically induced fluctuations in the thickness of antarctic bottom water

development test and evaluation of exploitation technologies for the application of gas production from natural gas hydrate reservoirs and their potential application in the danube delta black sea abstract one important scientific objective of the national research project sugar submarine gas hydrate reservoirs was the development improvement and test of innovative concepts for the production of methane from natural gas hydrate reservoirs therefore different production methods such as the thermal stimulation using in situ combustion the chemical stimulation via injection of co2 as a gaseous liquid or supercritical phase and depressurization were tested alone or in combination at different scales in the laboratory experiments these ranged from pore and hydrate grain scale to 425 l reactor volume whereas numerical models were applied to describe the related processes from molecular to reservoir scale in addition the numerical simulations also evaluated the feasibility and efficiency of the application of these methods in selected areas such as the danube paleodelta in the black sea addressing the two dominant methane hydrate reservoir settings buried channel levee and turbidite systems it turned out that the injection of co2 or a co2 n2 gas mixture is not applicable for the danube paleodelta in the black sea because the local pressure and temperature conditions are too close to the equilibrium conditions of both the co2 hydrate and a co2 n2 mixed hydrate stability fields experiments using thermal stimulation and depressurization showed promising results but also some issues such as sufficient heat transfer in summary it can be said that the applicability and efficiency of each method has to be proven for each specific hydrate reservoir conditions based on the results obtained by numerical simulations the most promising and safe method for the production of ch4 from hydrate bearing sediments in the danube paleodelta would be the depressurization technique this study summarizes the main experimental and modeling results

origin of the sedimentary magnetic record at ocean drilling program sites on the owen ridge western arabian sea the 3 2 m y whole core magnetic susceptibility record obtained during ocean drilling program leg 117 from the owen ridge in the western arabian sea represents one of the most convincing demonstrations of the ability of rock magnetic measurements to yield paleoceanographically significant information the salient features of this record are that 1 it correlates strongly with variations in the concentration and flux of eolian dust 2 it is driven strongly at earth orbital periodicities and 3 there is a significant change in the spectral character at 2 4 m y which may reflect the effect of the initiation of major northern hemisphere glaciation on aridity cycles within the eolian source areas in view of its potential paleoclimatic significance we have examined the origin of the rock magnetic signal from the owen ridge in more detail we find that despite the strong relationship throughout the record between magnetic susceptibility and percent terrigenous content there has been significant postdepositional alteration of the magnetic minerals via the process of reductive diagenesis we ascribe two steplike shifts in magnetic properties in the uppermost part of the section to this process the first occurs at about 1 5 m depth and is characterized by a loss of fine grained ferrimagnetic material the second stepshift in magnetic properties occurs at a depth of 6 8 m and is characterized by changes in rock magnetic parameters which indicate significant loss of both low coercivity magnetite and high coercivity hematite and or goethite magnetic mineral components scanning electron microscope and energy dispersive xray analyses show that the bulk of the coarse grained strongly magnetic fraction is detrital titanomagnetite and that there is a significant reduction in the fe ti ratio of these grains across the lower diagenetic front this suggests that titanomagnetite grains with higher ti contents may be more resistant to the process of reductive diagenesis and this may provide a mechanism for the relative stability of the ferrimagnetic fraction below the second diagenetic front

orbital forcing of bottom current enhanced sedimentation on feni drift ne atlantic during the mid pleistocene feni ridge is a plastered sediment drift deposited along the northwestern flank of rockall trough ne atlantic under the influence of deep geostrophic currents formed by intermittent overflows of arctic intermediate water from the norwegian sea across the iceland scotland ridge nsow lower deep water ldw a derivative of antarctic bottom water from the south atlantic mixed with old north atlantic deep water nadw also bathes the lower flanks of the drift below 2500 m deep sea drilling project site 610 was drilled in a sediment wave field on the surface of feni drift 2417 m though the sediments recovered proved to be typically pelagic preserving a high resolution 5 cm kyr 1 quaternary record of orbitally modulated ice rafting and biogenic productivity cycles but showing little evidence of current affected deposition from particle size and compositional analyses of the biogenic and lithogenic 10 m silt which we then compared with records of the percentage and flux of ice rafted detrital ird sand foraminifera and coccoliths for the same interval cross spectral comparisons were then made between these records and variations in a model of northern hemisphere ice volume ice sheet growth and decay in response to orbitally forced insolation changes at 65 n variations in the relative proportions and fluxes of ird sand foraminifera and coccoliths all covary closely with orbitally forced ice volume fluctuations but with differing amplitude coherency and phase relationships at each milankovitch frequency however there is a significant component of variance in pelagic and ice rafted sedimentation which is controlled by bottom currents and this shows excellent phase and coherency agreement with the ice volume model at all orbital frequencies the relative enrichment coarseness and flux of the contourite silt fraction of feni drift sediments all increase during glacial or cold stages of the mid pleistocene relative to warm stages suggesting that currents were stronger during these periods possible explanations for such changes in current activity include 1 an increased flux of nsow via the wyville thompson ridge into the rockall trough during glacials but not due to increased production of nsow 2 ldw presently lying below 2500 m rises during glacials by at least 50 60 m thereby flooding over the surface of feni ridge at site 610 3 the path of a major recirculation gyre in the lower nadw ldw system presently situated marginally south of site 610 encroaches upon the southern tip of feni ridge during glacials and 4 the rockall trough itself is a site of deep water formation during glacials analogous to labrador sea deep water formation today

evaluation of gas hydrate deposits in an active seep area using marine controlled source electromagnetics results from opouawe bank hikurangi margin new zealand several known gas seep sites along the hikurangi margin off the east coast of new zealand were surveyed by marine controlled source electromagnetic csem experiments a bottom towed electric dipole dipole system was used to reveal the occurrence of gas hydrate and methane related to the seeps the experiments were part of the international multidisciplinary research program new vents carried out on german r v sonne in 2007 cruise so191 to study key parameters controlling the release and transformation of methane from marine cold vents and shallow gas hydrate deposits two csem lines have been surveyed over known seep sites on opouawe bank in the wairarapa region off the se corner of the north island the data have been inverted to sub seafloor apparent resistivity profiles and one dimensional layered models clearly anomalous resistivities are coincident with the location of two gas seep sites north tower and south tower on opouawe bank a layer of concentrated gas hydrate within the uppermost 100 m below the seafloor is likely to cause the anomalous resistivities but free gas and thick carbonate crusts may also play a role seismic data show evidence of fault related venting which may also indicate the distribution of gas hydrates and or authigenic carbonate geochemical profiles indicate an increase of methane flux and the formation of gas hydrate in the shallow sediment section around the seep sites takahe is another seep site in the area where active venting higher heat flow shallow gas hydrate recovered from cores and seismic fault planes but only moderately elevated resistivities have been observed the reasons could be a the gas hydrate concentration is too low even though methane venting is evident b strong temporal or spatial variation of the seep activity and c the thermal anomaly indicates rather temperature driven fluid expulsion that hampers the formation of gas hydrate beneath the vent

magnetic susceptibility variations in upper pleistocene deep sea sediments of the ne atlantic implications for ice rafting and paleocirculation at the last glacial maximum magnetic susceptibility ms variations are used to intercorrelate 17 upper pleistocene sediment cores taken from the ne atlantic between 40 degrees and 60 degrees n the ms based correlation depends on regionally consistent patterns of variation in the deposition of ice rafted detritus ird in response to pleistocene glaciations and especially to high frequency ice rafting episodes referred to in recent studies as heinrich events the sedimentological and rock magnetic basis for the apparent relationship between the ms signal and ird content of ne atlantic sediments is examined by 1 comparing the ms profiles of selected cores with their records of coarse fraction 150 mu m lithic fragment abundance and neogloboquadrina pachyderma sin percentages and 2 normalizing ms by expressing it both on a carbonate free basis and as a quotient with anhysteretic remanent magnetization a parameter sensitive to magnetic mineral grain size variations these comparisons show that variations in bulk sediment ms are only partly driven by simple carbonate dilution productivity and dissolution effects changes in both the concentration and grain size of magnetic minerals within the lithogenic noncarbonate fraction also impose a significant influence on bulk ms values in particular horizons rich in ird are associated with significant increases in the relative proportion of coarse grained multidomain ferrimagnetic particles in the sediment this is because ice rafting in contrast to most other mechanisms capable of transporting detrital magnetic minerals to pelagic environments has a high potential for delivering large ferrimagnetic grains as components of sand sized polycrystalline lithic fragments this fundamental linkage between the ird content and ms signal of ne atlantic sediments is used to reconstruct the patterns of variation in ird deposition and by inference surface currents of the last glacial maximum lgm similar to 18 19 ka relative to the present day ne atlantic using the time slice mapping approach developed by the climap project group our lgm holocene ms ratio map based on sample pairs from over 80 deep sea cores confirms that there was a weak cyclonic gyre north of the polar front in the lgm north atlantic the gyre comprised a sluggish warm current in the ne atlantic flowing north between latitudes 47 degrees and 62 degrees n partly fed by subtropical waters from south of the polar front and carrying large numbers of icebergs derived from several sources most of which melted between latitudes 45 degrees and 52 degrees n the warm current probably continued its flow into the iceland basin where it fed into a south flowing current which transported melting icebergs from iceland and scandinavia along the western flank of the reykjanes ridge

a rapid method for magnetic granulometry with applications to environmental studies we have developed a rapid method of characterizing the fine scale variation sub micron to hundreds of microns m in the grain size of magnetite in long sediment cores this method is a new rock magnetic tool and can be readily applied to the following disciplines 1 environmental studies 2 hydrology 3 petrology and 4 geomagnetism the method consists of a comparative plot of the depth variation of two magnetic parameters the first parameter low field less than 0 1 millitesla mt or 1 oe susceptibility in a steady field is plotted on the abscissa the second parameter anhysteretic remanent magnetization arm imparted by a smoothly decreasing af of peak value 100 mt 1 000 oe in the presence of a weak steady field 0 01 0 1 mt or 0 1 1 oe is plotted on the ordinate for a sediment column in which the magnetic mineral is magnetite a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points arm

trench investigation on the main strand of the bocono fault in its central section at mesa del caballo merida andes venezuela the mesa del caballo trench assessment confirms the holocene activity of the main strand of the bocono fault at the apartaderos pull apart basin fifteen earthquakes of which fourteen have been radiocarbon dated have been recognized spanning the last 20 500 yr recurrence intervals of these 7 magnitude events are variable the dominant mode of recurrence is 400 450 yr and the second one is 900 yr eventually some events are 1400 1800 yr apart we suspect that our seismic record may be incomplete this could be easily justified by several conditions most of the earthquake recognitions is based on open crack filling and they superpose spatially eventually masking or destroying older fills trenching may miss some events because the fault is made of en echelon riedel shears and a short return period may lead to faint differences between paleosoils few hundreds years of age apart this trench also images an older activity of the fault as evidenced by plentiful earthquake triggered liquefaction features as well as slumping and rotational sliding by comparing paleoseismic results between the morro de los hoyos and mesa del caballo trenches it appears that both fault strands bounding the apartaderos pull apart basin move simultaneously besides the main strand also coseismically slips twice in between those common events in other words the seismic scenario could be that the northern strand recurs every 1200 1350 yr while the southern does every 400 450 yr this is also in agreement with a respective slip share of 25 and 75 of the 9 10 mm yr average slip of the bocono fault in the merida andes central sector

rock magnetism of late neogene and pleistocene deep sea sediments relationship to sediment source diagenetic processes and sediment lithology we use bivariate scatter plots to illustrate variations in selected rock magnetic properties low field susceptibility anhysteretic and isothermal remanence of late neogene and pleistocene deep sea sediments from 16 sites in the arctic ocean north atlantic equatorial atlantic and north pacific ocean and the arabian sea our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology geographical area and the dominant mode of terrigenous sedimentation which at these sites is via ice rafting via bottom currents or via eolian processes overall correlations between sediment magnetic properties and gross lithology is poor and factors such as the source and transport path of terrigenous sediment and detrital magnetic minerals together with the action of reductive diagenetic processes are the major controls on the magnetic properties on the bivariate scatter plots sites with major ice rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations relatively coarse ferrimagnetic grain sizes and scattered sample point distributions in contrast sites where we infer significant bottom current supply of terrigenous material have tightly grouped sample point distributions carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration caused by glacial interglacial fluctuations in carbonate accumulation and eolian activity sediments containing significant volcanogenic material have high concentrations of relatively coarse grained ferrimagnetic material reductive diagenesis is a significant determinant of sediment magnetic properties in high productivity areas and has the effect of preferentially removing the fine grained ferrimagnetic fraction causing a coarsening of the ferrimagnetic grain size distribution and a rise in the antiferromagnetic ferrimagnetic ratio

a rock magnetic study of giant piston core ll44 gpc3 from the central north pacific and its paleoceanographic implications previous magnetostratigraphic studies of nonfossiliferous pre pliocene age pelagic clay cores from the central north pacific have proven unsuccessful because of the unstable behavior of the natural remanent magnetization the inability to obtain a reliable magnetostratigraphy has severely limited the temporal resolution that can be achieved in paleoceanographic studies of these nonfossiliferous pelagic clays we carried out a rock magnetic study of core ll44 gpc3 which spans the interval 0 70 m y ago to determine whether rock magnetic parameters can be used to construct a high resolution stratigraphic framework for paleoceanographic studies of central north pacific pelagic clays despite the instability of the natural remanence in addition we tested the ability of rock magnetic methods to detect and characterize the paleoceanographic changes that are recorded in the sediments of ll44 gpc3 stratigraphic variations in rapid and nondestructive rock magnetic parameters and related ratios reflect changes in the concentration mineralogy and grain size of the magnetic minerals within the sediments rock magnetic parameters exhibit coherent fluctuations within both the stable and unstable sections of ll44 gpc3 this result suggests the potential use of these parameters for regional correlation and relative dating of the nonfossil bearing pelagic clays of the central north pacific gyre major fluctuations in rock magnetic parameters plotted as accumulation rates correspond to intervals of paleoenvironmental change that have been previously detected by other paleoceanographic methods in addition variations in a parameter proportional to the concentration of goethite hematite correlate remarkably well with the mass accumulation rate of the total eolian component suggesting that in some situations it can be used as a proxy indicator for eolian activity

calcareous nannofloral response to termination ii at 45 n 25 w northeast atlantic abstract termination ii glacial to interglacial transition from oxygen isotope stage 6 to stage 5 is considered to be one of the most rapid and abrupt climatic changes during the late quaternary our high resolution geochemical and nannofloral study of the interval from 140 to 100 ka in the northeast atlantic core t90 9p 45 17 5 n 25 41 3 w shows that termination ii 135 125 ka occurred in two steps of drastic changes interrupted by a short period of stability and followed by at least 25 ka of relatively stable interglacial conditions traditionally referred to as the eemian age substage 5 5 a primary productivity proxy based on coccolith accumulation rate number cm 2 ka 1 shows higher values during the eemian than during glacial stage 6 and interglacial substage 5 4 with the maximum values 70 10 9 coccoliths cm 2 ka 1 at 123 122 ka the coccolith assemblage was dominated by two morphotypes of gephyrocapsa alternating in dominance under different climatic conditions gephyrocapsa muellerae was more abundant in colder periods whereas small gephyrocapsa dominated in the eemian emiliania huxleyi syracosphaera pulchra and florisphaera profunda increased in the eemian whereas coccolithus pelagicus and helicosphaera carteri decreased small gephyrocapsa together with emiliania huxleyi are associated with periods of higher primary productivity the pelagic sedimentary record of termination ii in core t90 9p includes a 5 cm thick layer enriched in ice rafted material a heinrich layer in contrast to termination i where such a heinrich layer h1 is associated with the first meltwater pulse of the deglaciation the record of core t90 9p suggests that the heinrich event of termination ii h11 occurred during the second stage of deglaciation 129 4 127 1 ka it is characterised by accumulation of ice rafted debris and low coccolithophore diversity the coincident apparent drop in coccolith accumulation rate could be explained either by changes in primary productivity due to reduction in salinity and light or by dilution by ice rafted material five models are applied for the deposition of the heinrich layer with changing thickness 3 cm and duration 1000 500 and 250 yr these scenarios result in a lowered constant or even slightly increased primary productivity during this event

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

dissolution and pyritization of magnetite in anoxie marine sediments concentrations of magnetite were determined with depth in sediments of varying h2s content taken from long island sound foam nwc and sachem sites and from a site in the rapidly depositing subaqueous portion of the mississippi delta for the three long island sound sites dissolution of magnetite during burial is clearly demonstrated and for all four sites the rate of magnetite dissolution is proportional to the concentration of dissolved pore water sulfide by combining magnetite depth distributions with sedimentation rate data and measurements of dissolved sulfide we find that magnetite dissolution is well described by the following rate law dcmagdt 1 1 10 5c0 5bcmagamag where cmag is the concentration of magnetite grams per gram of sediment cs is the concentration of dissolved sulfide mm and amag is the specific surface area of magnetite in the sediment cm2 per gram of magnetite this equation means that for a typical range of magnetite surface areas and dissolved sulfide concentrations the half life of magnetite in anoxic marine sediments ranges from about 50 to 1000 years sem observations of magnetite grains from the different sediment sites reveal that magnetite dissolution may be accompanied by extensive replacement by pyrite this only occurs if magnetite is in contact with high concentrations of h2s 1 mm for relatively long periods of time several hundred years with low concentrations of h2s 1 mm magnetite dissolution occurs without identifiable pyrite replacement we suggest that this feature of magnetite diagenesis may prove to be a useful paleosulfide indicator

late pleistocene paleoclimatology of the central equatorial pacific a quantitative record of eolian and carbonate deposition detailed records of vso 6i3c percentage and mass accumulation rate of caco and eolian percentage mass accumulation rate and grainsize generated for core rcl l 210 from the equatorial pacific reveal the timing of paleoclimatic events over the past 950 000 yr the caco percentage record shows the standard pacific correlation of high caco content with glacial periods but displays a marked change of character about 490 000 yr ago with older stages showing much less variability the carbonate mass flux record however does not show such a noticeable change sedimentation rates vary from about 0 5 to 3 0 cm l000 yr and during the past 490 000 yr sections with enhanced sedimentation rates correspond to periods of high caco percentage eolian mass accumulation rates an indication of the aridity of the source region are usually higher during glacial times eolian grainsize an indication of the intensity of atmospheric circulation generally fluctuates at a higher frequency than the loo ooo yr glacial cycle the mid brunhes climatic event centered at 300 000 yr ago appears as a 50 000 yr interval of low intensity and reduced variability of atmospheric circulation furthermore the nature of this entire record changes then with the younger portion indicating less variation in wind intensity than the older part of the record the late matuyama increase in amplitude of paleoclimatic signals begins 875 000 yr ago in the eolian record 25 000 yr before the 6i80 and caco percentage amplitude increases about

a 420 000 year record of cyclicity in oceanic and atmospheric processes from the eastern equatorial pacific samples from the upper portion of a cyclic pelagic carbonate sediment sequence in deep sea drilling project dsdp hole 503b 4 0 n 95 6 w are the first group to be analyzed for paleoceanographic and paleoclimatic proxy indicators of ice volume deep ocean and surface water circulation and atmospheric circulation in order to resolve the complex origin of the cyclicity temporal resolution is taken from the 18o time scale most other parameters are calculated in terms of their mass flux to the seafloor caco3 percent in the sediments fluctuates in the well known pacific pattern and is higher during glacial times the fluxes of opal and organic carbon have patterns similar to each other and show a variability of a factor of 2 5 to 4 the longer organic carbon record shows flux maxima during both glacial and interglacial times the accumulation patterns of both opal and organic carbon suggest that the variability in surface water productivity and or seafloor preservation of those materials is not simply correlated to glacial or interglacial periods eolian dust fluxes are greater during interglacial periods by factors of 2 to 5 indicating that eolian source regions in central and northern south america were more arid during interglacial periods the record of eolian grain size provides a semiquantitative estimation of the intensity of the transporting winds the eolian data suggest more intense atmospheric circulation during interglacial periods opposite to the anticipated results we interpret this observation as recording the southerly shift of the intertropical convergence zone to the latitude of hole 503b during glaciations

physical properties and gas hydrate at a near seafloor thrust fault hikurangi margin new zealand the p paku fault zone drilled at iodp site u1518 is an active splay fault in the frontal accretionary wedge of the hikurangi margin in logging while drilling data the 33 m thick fault zone exhi

formation of gas hydrate from dissolved gas in natural porous media abstract migration of methane bearing fluids from the depth is commonly proposed to explain the formation of marine gas hydrate this formation mechanism is viable only if the gas hydrate can nucleate and grow directly from an aqueous solution while phase equilibrium calculations suggest that the gas hydrate should crystallize from the dissolved gas barriers associated with nucleation may impede or even prevent hydrate formation at present experimental support for the phase equilibrium calculations is insufficient to confirm whether the hydrate can form in the sea floor from the dissolved gas here we report on a set of experiments in which the hydrate is crystallized from dissolved co 2 in natural porous media the experiments demonstrate that the hydrate crystals can nucleate in the absence of free gas providing important experimental justification for models of hydrate formation that depend on fluid migration our results are also relevant for other formation models that depend on in situ methane generation from biogenic sources experimental evidence for hydrate formation at low gas concentrations implies less stringent requirements on the biogenic sources of methane compared with some previous estimates