sr and nd isotopes as tracers of north atlantic lithic particles paleoclimatic implications sr and nd isotopic compositions have been measured on the lithic fraction of last climatic cycle sediments from the north atlantic 40 n 60 n in order to identify the origins of the particles from the reconstruction of their transport pathways we deduce the mechanisms that explain their distributions the main source regions are the canadian shield mostly the area of baffin bay and western greenland the scandinavian shield the european region british isles and bay of biscay and iceland we observe a significant glacial interglacial contrast characterized by a dominant icelandic input via near bottom transport by north atlantic deep water nadw during the interglacials and a largely continent derived contribution of surface transported ice rafted detritus ird during the glacial period during the last glacial period the heinrich events abrupt massive discharges of ird originated not only from the laurentide ice sheet as heretofore envisioned but also from other sources three other major north atlantic ice sheets fennoscandian british isles and icelandic probably surged simultaneously discharging ice and ird into the north atlantic as opposed to theories implying a unique laurentide origin gwiazda et al 1995 driven by an internal mechanism mcayeal 1993 we confirm that the icelandic and the fennoscandian ice sheets also surged as recently proposed by other authors and we here also distinguish a possible detrital contribution from the british isles ice sheet this pan north atlantic phenomenon thus requires a common regional external forcing

living coccolithophore communities in the norwegian greenland sea and their record in sediments abstract investigation of epipelagic coccolithophore communities along a transect from the voring plateau off norway to scoresbysund east greenland and comparison with coccolith assemblages in underlying surface sediments reveal that sedimentation of coccoliths is dependent on the following factors 1 composition of the coccolithophore communities and their bloom phases 2 grazing and selective destruction by zooplanktonic organisms 3 type of vertical flux in fecal pellets and 4 varying carbonate dissolution in different water masses dissolution increases towards the boundaries of carbonate sedimentation as a result the original composition of the living communities becomes increasingly obscured in the impoverished taphocoenoses however differential resistance to dissolution of the two primary calcareous nannoplankton species emiliana huxleyi and coccolithus pelagicus is such that occurrence and ratio of the two species in surface sediments reflect the various water masses in the norwegian greenland sea

changes to the dispersive characteristics of soils along an evolutionary slope sequence in the vera badlands southeast spain implications for site stabilisation abstract in dispersive marls near vera badland gullies are eroding into a previously stable matorral covered landscape this relatively recent activity was probably triggered by minor climatic or land use change in this landscape calvo cases and harvey earth surface processes and landforms 21 1996 725 argued that new gully head slopes with differing aspect represent a sequence of progressive erosion using this interpretation as a basis slope surface materials were explored for evidence of material chemical stabilisation by sampling along profiles positioned centrally on those slopes the extent to which the inferred geomorphic sequence is paralleled by progressive changes in site geochemistry is considered crusts and subsoils differ significantly in terms of ph and sodium adsorption ratio sar but not calcium content the crust is thus distinctive but not calcic rather the ph differences suggest a process involving an organic agent probably algae slope position was an important discriminant taking sar alone differences between crusts and subsoils were not significant at the slope base but increased in significance with distance up the profile considering differences between the three profiles the middle profile had significantly higher sar values both in the crust and at depth than the other profiles whereas late slope subsoils and crusts taken alone are not dispersive sar the potential stabilisation of this type of badland soil and surface is finally considered whereas harvey harvey a m 1982 the role of piping in the development of badlands and gully systems in southeast spain in bryan r yair a eds badland geomorphology and piping geobooks norwich pp 317 335 argues that site morphology suggests that geomorphological instability increases through time the soil sequence suggests decreases in material dispersivity and consequently surface autostabilisation it is concluded that the concept of badland stabilisation needs further clarification

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

east pacific rise from siqueiros to orozco fracture zones along strike continuity of axial neovolcanic zone and structure and evolution of overlapping spreading centers we have used the sea beam multibeam echo sounding system to survey the east pacific rise epr from 8 20 n to 18 30 n obtaining complete coverage of the epr axial neovolcanic zone and all intervening transform faults here we focus on the epr neovolcanic zone and the definition of overlapping spreading centers ocs s between the orozco and siqueiros transform faults the neovolcanic zone is narrow 0 5 2 0 km and continuous along strike and occurs within and near a strike continuous axial summit graben the neovolcanic zone and summit graben reside along the crest of a volcanic axial high which is 2 10 km wide and which continues uninterrupted along strike for 40 140 km within 20 50 km of an intersection with a transform fault the axial neovolcanic zone narrows and deepens and within only 3 6 km of the intersection the neovolcanic zone turns sharply into the transform valley at seven locations between the siqueiros and orozco transform faults the axial neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast with a classic ridge transform ridge plate boundary however offset rise terminations overlap each other by a distance approximately 3 times greater than their offset they curve toward each other and often one merges into the other along strike separating the osc s is an elliptical overlap basin up to 600 m deep whose long axis is subparallel to the adjacent spreading centers the overlap basin is characterized by volcanic constructional edifices a key difference between these offsets and small transform faults in the atlantic is that there are no transform fault structures in the overlap basin which link the offset rise axes a continuous axial depth profile reveals a long wavelength 20 60 km undulation of the rise axis which may be associated with variations in the magmatic budget along strike the lowest points in the axial depth profile occur near transform faults and at osc s suggesting that these features are associated with along strike minima in the magmatic budget we suggest that major phases of volcanic activity propagate episodically along the axis away from centers of magmatic inflation and that osc s develop where magmatic pulses fail to meet due to misalignment of fracture systems sinuous bends in the rise axis or other heterogeneities transform faults fail to develop on fast spreading centers where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate ridge transform fault pattern the osc geometry is unstable and evolves rapidly and the overlapping rises may propagate at rates faster than the local spreading rate one of the two osc s prevails while the other is abandoned while the locations of osc s may not be fixed relative to the rise axis they may tend to recur near the same place creating scars in older lithosphere which resemble fracture zones but whose fine scale structure is quite different there is excellent agreement between the observed structure of osc s and laboratory studies of osc s using wax models photoelastic studies of overlapping cracks and boundary element modeling of overlapping cracks seismic refraction and reflection near two osc s suggests that large offset osc s have separate magma chambers beneath the offset rise axes while small offset osc s may share a single axial magma chamber three dimensional inversion of magnetic anomalies suggests that significant petrologic anomalies may occur near some osc tips waxing and waning of overlapping magma chambers may give rise to the eruption of highly fractionated feti basalts and associated magnetic anomalies

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

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

magnetic susceptibility patterns and provenance of ice rafted material at feni drift rockall trough implications for the history of the british irish ice sheet abstract piston core enam 97 09 from feni drift atlantic margin west of ireland provides a record of ird input variability and provenance during the last 250 000 yrs the ird patterns can be related to late devensian detrital carbonate and meta sedimentary rock fragments represent two major ird types which were dominantly derived from irish inland ice and the irish sea ice stream respectively additional evidence for the latter source is based on heavy mineral data input from other circum north atlantic ice sheets notably the laurentide ice sheet is minor compared to british irish ird at least during periods of maximum expansion of the biis this is consistent with the marginal location of the core site with reference to the main north atlantic ird belt on the other hand laurentide ird presumably accounts for minor ird peaks at times of reduced ice volume in the british isles the main magnetic susceptibility peak during late isotope stage 2 termination i can be related to ird input from the laurentide ice sheet following early decay of the biis a similar pattern is observed during oxygen isotope stage 6 these observations are consistent with recent provenance studies indicating that only laurentide ird carries a strong magnetic susceptibility signal

analysis of sedimentary organic matter of a glacial interglacial change oxygen isotope stage in the norwegian greenland sea organic petrographical investigations of norwegian sea sediments deposited during oxygen istope stages 6 and 5 show significant changes in the relative amounts of reactive vitrinite and liptinite and inert inertinite and graphite particles the delta 13c values of the total organic fraction possibly reflect the variable content of reactives i e the isotopic composition is more negative to 23 9 per mille sign when the reactive components increase the n alkane distribution shows that the soluble organic matter deposited during late stage 6 and 5e reveals relatively low odd even predominance indices oep associated with shorter chain lengths this indicates a relatively higher marine algal contribution during stage 6 and 5e compared with stage 5d to 5a

black shale formation in the late paleocene early eocene arctic ocean and paleoenvironmental conditions new results from a detailed organic petrological study abstract the study of particulate organic matter om in arctic ocean sediments from the late cretaceous to the eocene iodp expedition 302 has revealed detailed information about the aquatic marine om fluxes biological sources preservation and export of terrestrial material here we present detailed data from maceral analysis vitrinite reflectance measurements and organic geochemistry during the campanian paleocene fluxes of land derived om are indicated by reworked and oxidized macerals vitrinite inertinite and terrigenous liptinite cutinite sporinite in the early eocene drastic environmental changes are indicated by peaks in aquatic om up to 40 45 lamalginite telalginite liptodetrinite dinoflagellate cysts and amorphous om up to 50 bituminite these events of increased aquatic om flux similar to conditions favoring black shale deposition correlate with the global 13c events paleocene eocene thermal maximum petm and elmo event freshwater discharge and proximity of the source area are documented by freshwater algae material pediastrum botryococcus and immature land plant material corphuminite textinite we consider that erosion of coal bearing sediments during transgression time lead to humic acids release as a source for bituminite deposited in the early eocene black shales

structure and variability of oceanic crust on the flanks of the east pacific rise between 11 and 13 n we use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0 5 m y old crust in the rivera ocean seismic experiment rose area of the east pacific rise between 11 and 13 n there are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron amplitude patterns recorded on the individual lines however suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200 km segment of lithosphere on the basis of the modeling of amplitude patterns at ranges of 5 10 km and of comparisons with the upper crust sampled at deep sea drilling project hole 504b and in ophiolite complexes layer 2 can be divided into three units 2a corresponds to extrusive volcanics with a steep velocity gradient 2b to extrusives with a near zero velocity gradient and 2c to a complex region of transition from extrusives to sheeted dikes the total thickness of layer 2 varies by about 0 7 km over the length of the line another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3 the variability of these two amplitude patterns provides direct evidence that the along strike structure of oceanic crust in the rose area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information

behavior of the northwestern part of the fennoscandian ice sheet during the last glacial maximum a response to external forcing abstract a sediment core from the lofoten contourite drift on the continental slope off northern norway proximal to the former vestfjorden traenadjupet ice stream details the development variability and decline of marine margins of the northwestern fennoscandian ice sheet during the time interval 25 3 14 cal ka bp including the last glacial maximum and onset of the deglaciation based on high resolution ird records from the core interval between 25 3 and 17 7 cal ka bp we report data points with a mean time step of 10 years between 17 7 cal ka bp and the holocene time steps are typically 50 years the core is divided into 7 informal ice rafted debris ird zones based on the variations in ird including 7 major ird maxima a g inferred to represent periods of high iceberg production petrological identification reveals dominance of crystalline ird monocrystalline plutonic and metamorphic rock fragments accounting for 75 80 of total ird assemblages while sedimentary fragments generally account for 15 20 the crystalline fragments including eclogite and mangerite from a nearby terrestrial source increase across the ird peaks while the sedimentary fragments remain constant this points to the importance of erosional products from icebergs originating from fast flowing paleo ice streams including the vestfjorden traenadjupet ice stream draining from the fennoscandian mainland during the ird maxima periods increased temperature of the adjacent surface water masses was probably an important external forcing factor on the fennoscandian ice sheet behavior because some ird maxima and plumite deposition from meltwater plumes post date periods of increased sea surface temperatures the peak ird depositions occur in centennial and millennial time cycles 200 1030 and 3900 year indicating some external forcing by solar variation both mechanisms could explain the observed synchronous instability of the northwestern fennoscandian ice sheet to other european ice sheets

the juan fernandez microplate north of the pacific nazca antarctic plate junction at 35 s abstract results of the r v thomas washington pascua 3 expedition provide evidence for the existence of the juan fernandez microplate just north of the junction between the east pacific rise epr and the chile fracture zone prior to pascua 3 the microplate in the region had been hypothesized from the pattern of seismicity the eastern and western boundaries of the juan fernandez microplate are well defined and represent north south trending spreading centers characterized by very slow and very fast rates of accretion respectively in agreement with the rates the eastern boundary is represented by a rift valley and the western boundary by an epr type axial ridge the northern boundary of the juan fernandez microplate is a 100 trending wide fracture zone complex which may have resulted from northward transform fault migration the fracture zone fails to meet the zone of accretion at the pacific nazca juan fernandez triple junction in this area the zone of accretion displays a double ridge with a large overlap the southern boundary of the juan fernandez microplate is still poorly constrained the plate geometry derived from seabeam differs from that derived by anderson fontana et al 1986 14 from a plate motion inversion scheme using primarily earthquake first motion solutions together with limited bathymetric and magnetic data

habitat characterization of the vema fracture zone and puerto rico trench abstract although many of the regions on and close to the mid ocean ridges have been extensively mapped and sampled the abyssal intraplate regions remain essentially unsampled and unmapped leaving huge gaps in our understanding of their geologic history and present activity prominent bathymetric features in these intraplate regions are fracture zones here we present bathymetric and sampling information from a transatlantic transect along the vema fracture zone ca 11 n covering crustal ages from 109 0 ma on the african plate and 0 62 ma on the south american plate the vema fracture zone is the intraplate trace of the active vema transform plate boundary which offsets the present day mid atlantic ridge by ca 300 km left laterally juxtaposing zero age crust with crust of 20 million years age our results show clear evidence of tectonic activity along most of the fracture zone in most places likely associated with active fluid flow within the active vema transform at crustal ages of ca 10 ma we found clear indications of fluid flow both in the sediments and the overlying water column this region is 120 km from the nearest spreading axis and increases by almost an order of magnitude the maximum off axis distance that active hydrothermal discharge has been found on the oceanic crust sampling of the igneous seafloor was possible at all crustal ages and the accretionary fabric imprinted on the plate during its production was prominent everywhere seafloor sediments show signs of extensive bioturbation in one area high concentrations of spherical mn nodules were also found and sampled at the end of the transect we also mapped and sampled the puerto rico trough a 8000 m deep basin north of the caribbean arc here the seafloor morphology is more complicated and strongly influenced by transpressive tectonics

glacigenic sediments on a passive continental margin as exemplified by the barents sea abstract reflection seismics data borehole data and gravity cores have been used to map the sedimentary facies of the epicontinental southwestern barents sea its continental margin and adjacent coastal areas an upper unconformity with a glacially eroded morphology extends over most of the continental shelf up to 300 m of stratiform glacigenic sediments overlie this unconformity these sediments can be grouped into seismic units which are separated by smooth or irregular erosional surfaces each sequence may be up to 150 m thick normally with a lateral extent of 100 300 km three dominant seismic signatures may be discerned semi transparent stratified and chaotic reflection patterns these sediments are partly glacifluvial in origin but on the whole they represent more or less tectonized glacimarine deposits the deeper troughs on the continental shelf contain an undeformed postglacial glacimarine marine trough fill the greatest thickness up to 1000 m of glacigenic sediments occurs at the shelf break and below the upper slope the depocentres are situated at trough mouth fans the sediments comprise several prograding sequences with a complex sigmoid oblique character indicating alternating upbuilding and depositional bypass erosion in the topset segment small and large scale slide scars are observed on the upper slope and debris lobes on the lower slope a marked progradation seems to have occurred during the plio pleistocene glacial periods this progradation also resulted in an unstable slope with various types of sediment gravity flows currents flowing along the slope were established during interglacials and gullying by cold dense shelf water and downslope transport occurred maximum net sedimentation rates are 0 2 m ka continental slope 0 4 m ka shelf break 0 1 m ka shelf stratiform diamictons 1 m ka shelf trough fills and 60 m ka fjord trough fills

interpreting rock eval pyrolysis data using graphs of pyrolizable hydrocarbons vs total organic carbon kerogen the major organic component of sedimentary rocks is commonly analyzed by rock eval pyrolysis plotting the data on a graph of s 2 vs total organic carbon toc and determining the regression equation is the best method for determining the true average hydrogen index and measuring the adsorption of hydrocarbon by the rock matrix such a plot also indicates the type of kerogen present and avoids the problem of increasing hydrogen index with total organic carbon content with the s 2 vs toc diagram the organic component of different suites of samples may be compared and their petroleum generation potentials established as an example the diagrams are used to evaluate the sedimentary environments and petroleum potential of the paleogene evaporitic sediments of the mulhouse basin alsace france

recent distribution and accumulation of organic carbon on the continental margin west off spitsbergen the study compiles the controlling factors for organic matter sedimentation patterns from a suite of organogeochemical parameters in surface sediments off spitsbergen and direct seabed observations using a remotely operated vehicle rov in addition we assess its storage rates as well as the potential of carbon sinks on the northwestern margin of the barents sea with short sediment cores from a selected fjord environment storfjord while sedimentation in the fjords is mainly controlled by river meltwater discharge and coastal erosion by sea ice glaciers resulting in high supply of terrigenous organic matter atlantic water inflow and thus enhanced marine organic matter supply characterizes the environment on the outer shelf and slope local deviations from this pattern particularly on the shelf are due to erosion and out washing of fine grained material by bottom currents spots dominated by marine productivity close to the island have been found at the outer isfjord and west off prins karls forland as well as off the kongsfjord krossfjord area and probably reflect local upwelling of nutrient rich atlantic water derived water masses accumulation rates of marine organic carbon as well as reconstructed primary productivities decreased since the middle of the last century negative correlation of the isfjord temperature record with reconstructed productivities in the storfjord could be explained by a reduced annual duration of the marginal ice zone in the area due to global warming extremely high accumulation rates of marine organic carbon between 5 4 and 17 2 g m 2 yr 1 mark the storfjord area and probably high latitude fjord environments in general as a sink for carbon dioxide

rift valley no rift valley transition at mid ocean ridges a simple mechanical model for the axial topographic and gravity variations with spreading rate is developed the model emphasizes the rapid transition from one mechanism to another with changing spreading rate our model has a brittle layer over a ductile layer which are being pulled by forces away from the ridge axis there exists a potential failure zone of width lf where the stresses within the brittle plate exceeds its yield strength if the yield strength of the brittle plate is nearly constant this width is largely determined by the strain rate and the viscosity of the strong mantle the high ductility of oceanic crust at high temperatures relative to that of mantle at the same temperatures leads to the concept of a decoupling region a critical feature in this model the size of the decoupling region ld is determined by the thickness of the oceanic crust and temperatures within the crust if the decoupling region is small ld lf as shown to be the case for fast spreading ridges then each of the two layers responds separately to the horizontal stretching the brittle plate will be broken only at the axis and any buoyant force of the region would lift it up to form the rise crest high as a result of local isostasy the changing size of the decoupling region gives rise to the transition from atlantic type axial topography to pacific type as well as the transition from a high amplitude highly variable gravity signature to a low amplitude very uniform gravity signature parameters can be chosen so that such a transition occurs at a half rate of 35 mm yr which is the critical half rate suggested by both topographic and gravity data

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

evolution of the norwegian current and the scandinavian ice sheets during the past 2 6 m y evidence from odp leg 104 biogenic carbonate and terrigenous records abstract records of biogenic and terrigenous components have been obtained from the interval corresponding to the last 2 6 m y of odp sites 643 and 644 in order to reconstruct surface and deep water regimes in the norwegian sea surface water regimes record long lasting moderate glacial conditions during the interval 2 6 1 0 ma small intrusions of atlantic water episodically penetrated into the norwegian sea forming a narrow tongue along the eastern margin which is documented at site 644 the polar front was most probably situated between the site 644 and 643 locations on the outer voring plateau during these time intervals deep water regimes reflect long term persistent corrosive bottom waters most probably due to a weakly undersaturated water column and a low rate of carbonate shell production in surface waters deep water production in the norwegian greenland sea may have operated in a different way e g brine formation during winter sea ice growth bottom waters were oxygenated throughout the entire period and deep water was exchanged persistently with the north atlantic increased glacial interglacial environmental contrasts are documented reflecting a strengthening of the norwegian current and intensified glaciations on the surrounding land masses during the interval 1 0 0 6 ma during this time a major shift in the mode of deep water production occurred the onset of large amplitudes in glacial interglacial environmental conditions with maximum contrasts in surface water regimes different modes of deep water production and intensified exchange with the north atlantic marks the last 0 6 ma a broad development of the norwegian current is observed during peak interglacials while during glacials seasonally variable sea ice cover and iceberg drift dominate surface water conditions

three dimensional magnetic modeling of a propagating rift galapagos 95 30 w a combined deep tow and seabeam investigation at 95 30 w on the cocos nazca spreading center has revealed the crustal contact between the propagating rift and the dying rift systems the observed 27 km offset between the axes creates an ideal situation for the application of magnetic methods the normally magnetized crust of the propagating rift tip penetrates into older crust which was created when the earth s main field was reversed as the v shaped structure of the rift clearly violates the conventional two dimensional assumptions a full three dimensional analysis is required in addition the distorting influence of over 1000 m bathymetric relief must be removed from the observed field inversions have been performed on the gridded representations of the observed magnetic field and bathymetry working in the fourier domain the result is a gridded rock magnetization distribution the inversion of the surface data covers a large area 6000 km2 and demonstrates close agreement with magnetization amplitudes of rock samples at existing dredge sites in general the propagating rift process appears to be much more orderly than the dying rift process the magnetic polarity transition widths are narrower and the boundaries have fewer undulations than the dying rift which appears to be quite episodic in behavior the average propagation rate is 52 mm yr compared to the average spreading half rate of 29 mm yr the locations of the boundaries suggest that the acceleration to the normal spreading rate on the propagating rift requires about 250 000 years the inversion of the deep tow data near the seafloor provides a high resolution definition of the location of the tip of the propagating rift at 2 38 1 n 95 30 0 w

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

the causes of piping in a set of abandoned agricultural terraces in southeast spain this study aims to identify some of the factors contributing to the piping process in abandoned terraces in seven sites located in the murcia region southeast spain the local landscape has the appearance of badland geomorphology possessing deep valleys which until the 1970s were terraced and cultivated the terraces have now been abandoned to the piping process the research focuses on the relationship between the size and position of pipes and the physical and geochemistry properties of the local highly dispersive marl lithology and the extent to which the reworking of the materials for terrace cultivation has exacerbated piping erosion morphologically the terracing in the stream beds has resulted in a series of gentle benches separated by steeper risers with variable drop riser height a series of flat infiltration surfaces result and because the resculpting produces a reduced bulk density infill with enhanced throughflow infiltrating water is forced along steep hydraulic gradients to a low outfall at the base of the riser paralleling studies of land levelling and terrace abandonment in similar materials elsewhere piping in the study area is shown to be closely related to the physical and chemical characteristics of the materials its sodicity and dispersive character considerable differences in structure texture mineralogical composition and material s surface crust and subsoil geochemical properties emerged from site signature plots and these results suggest that agricultural processes affect the dispersive character of the crust materials presumably because agricultural activity increases the buffering effect of organic acids on the dispersed clay this is supported by the observation that on the one site that was levelled but not used for agriculture a stable organically distinct crust like the abandoned agricultural sites has not formed the changes to the material s aggregate stability and bulk density properties when terracing occurs is examined but insufficient evidence is available from the data collected to unpick the causes of aggregate stability changes in the study plots the way in which differences between crust and subsoil enhance infiltration is discussed and the role of the terrace drop in influencing hydraulic gradients and pipe development is illustrated a possible index to describe the relationship between evacuated material volumes and the controlling parameters discussed above is tested it is concluded that terracing which is intended as a conservation practice has in dispersive materials actually enhanced piping leading to the abandonment of farming

improvements to the dispersion status of piped gully soils following reworking and stabilisation by vegetation the physical and chemical properties of sodic gully bank soil material deposited as in channel sedimentary structures below a pipe system in alkaline marls colorado usa were re analysed for their dispersive status using a new methodology the original samples were taken from in gully sedimentary structures with differing densities of established vegetation cover taking the more fully established vegetation densities to suggest increasing time since the soil samples were reworked by hydraulic processes the samples were assigned to a three point scale fresh no vegetation stabilised some vegetation and stabilised revegetated using detailed site descriptions these groups were found to be significantly different from each other in terms of the sar1 and ec2 the material s ph was also significantly different following revegetation the site signatures for each class suggest that a model of stabilisation for the soils following revegetation can be usefully described by this approach

dynamics of atlantic water advection to the norwegian greenland sea a time slice record of carbonate distribution in the last 300 ky abstract short term variability in volume and configuration of atlantic water advection to the norwegian greenland sea ngs has recently been identified as a new and significant element for northern hemisphere glacial interglacial climatic fluctuations the amount of salt heat and moisture transferred by atlantic waters to the high arctic effectively influences northern hemisphere climates and is critical for the overall oceanic turnover rate in particular determining the extent and duration of atlantic water intrusions in the ngs is essential because of 1 its effect on the climate of the nordic continents and 2 its trigger function on the stability of the ice sheets with a weak inflows of atlantic water during glacials promoting an increase in ice volume by moisture supply to polar areas and b positive feed back mechanisms of atlantic water intrusions during ice sheet decay in this study the dynamics of surface water mass regimes in the ngs have been reconstructed for the past 300 ky by means of time slice compilations of calcium carbonate distribution and micropaleontologic proxies in addition carbonate preservation records are used to quantify deep water ventilation and production rates in contrast to the climap scenario but in accordance with many recent publications results of this study indicate that atlantic water advection was also a common feature during most glacial periods but varied strongly in volume and configuration significant modes recognized are 1 the full interglacial mode with a wide inflow of warm atlantic waters covering a broad area in the eastern sector of the ngs and extending far north combined with excellent carbonate preservation and very efficient deep water formation stages 1 and 5 5 1 2 the moderate interglacial mode with a rather small extension of atlantic waters along a narrow corridor in the southeastern sector of the ngs but still very efficient deep water exchange stages 9 isotopic events 7 5 7 3 7 1 5 3 and 5 1 3 pronounced cool phases during interglacials with only a very narrow inflow of atlantic waters into the southeastern ngs while the coastal areas off norway were still affected by iceberg drift substages 7 4 and 5 2 carbonate preservation records indicate ongoing deep water formation in the ngs 4 the normal glacial mode characterized by a variable but persistent inflow of cool temperate atlantic water lower stage 8 repetitively during stages 6 4 3 1 and 2 carbonate preservation records indicate ongoing deep water formation in the ngs 5 short glacial and deglacial periods characterized by episodic high inputs of ird evidenced by a wide coverage of diamicton deposition and strong carbonate dissolution e g episodically within stages 10 6 and 2 the 6 5 4 3 and 2 1 transitions some of these ngs diamicton layers correlate with heinrich layers in the north atlantic but generally ngs diamicton layers occur more frequently than heinrich layers the role of atlantic water influence during these events remains enigmatic but atlantic water influence is frequently found just above and below these sections a scenario with rapid ice sheet disintegration induced by atlantic water advection via increase of the thermal regime along the ice margin possibly triggered by small scale sea level rise might be envisaged more obviously the short term cessation of deep water formation in the ngs as evidenced by the dissolution spikes may be correlated with contemporaneous ventilation minima in the north atlantic

temporal evolution of overlapping spreading centers at 16 20 n on the east pacific rise abstract a compilation of sea beam data on the right stepping overlapping spreading center osc at 16 20 n on the east pacific rise epr and in the surrounding region is presented sea beam data from the rise flanks at 16 20 n reveal three deep basins at least one of which is flanked by a curved ridge whose morphology is similar to that at the offset the widths of these offaxis basins range from 8 to 13 km and allow an estimate to be made of the sizes of the offsets at which they were formed it is proposed that they represent traces on the rise flank which provide a record of the development of the offset and a basis for prediction of its future evolution the repeated pattern of abandoned overlap basins and cuspate ridges supports the notion that oscs are episodic on a time scale of approximately 200 000 years and are cyclic in nature it also reveals an alternation of dominance between the southern and northern segments which have shifted the location of the discontinuity along the axis so as to create zig zag traces of basins on the ridge flanks

interaction and linkage of extension fractures and normal faults examples from the rift zone of iceland abstract field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of iceland each zone consists of a pair of extension fractures or a pair of normal faults with lengths from metres to kilometres these zones evolve from an underlapping stage through an overlapping stage the most common configuration and finally to a linkage stage of all the zones only 7 are underlapping whereas 93 are overlapping with hook shaped fracture pairs the length width ratios of the overlapping zones have a mean value of 3 5 the preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures length overstep and the development of the interaction in the overlapping zones most fracture pairs show moderate shear strike slip components related to local variations in the extension opening directions vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase both in turn are proportional to the total lengths of the faults forming the pair the geometrical and kinematic features of overlapping spreading centres at mid ocean ridges show close similarities to those reported here these similarities indicate that the architecture and evolution of overlapping zones are scale independent

allochthonous versus autochthonous organic matter in cenozoic sediments of the norwegian sea evidence for the onset of glaciations in the northern hemisphere the cenozoic sediments sampled in odp leg 104 on the voring plateau show a distinct variability of the total organic carbon content toc and the accumulation rates of toc based on the geochemical and organic petrographic characterization of the sedimentary organic matter om the allochthonous and autochthonous proportion of the om could be quantified the results clearly demonstrate that high toc percentages and toc accumulation rates in cenozoic sediment sections display a generally high input of allochthonous organic matter oxidized and partly well rounded organic particles built up the main portion of om within the miocene toc rich sediments the most probable source of this oxidized om are reworked sediments from the scandinavian shelf changes in the input of these organic particles are to some degree correlative with sea level changes the cenozoic accumulation of autochthonous om is low and does not reveal a clear variation during the miocene and early pliocene in spite of a high accumulation rate of biogenic opal during the early miocene the accumulation rate of autochthonous toc is low the autochthonous particle assemblage is dominated by relatively inert om like dinoflagellate cysts this points to an intensive biological and or early diagenetic degradation of the marine om under well oxidized bottom water conditions during the last 23 myr nevertheless a continuation of marine om degradation during later stages of diagenesis cannot be excluded a prominent dominance of allochthonous om over autochthonous is documented with the beginning of the pliocene at 2 45 ma the episodic occurrence of ice rafted thermally mature om reflects the onset of the glacial erosion of mesozoic coal and black shale bearing sediments on the scandinavian and barents sea shelves the first occurrence of these in view of the actual burial depth thermally overmature om particles is therefore a marker for the beginning of the strong scandinavian glaciation and the advance of the glacial front toward the shelves

simulation of summer storms of differing recurrence intervals in a semiarid environment using a kinematic routing scheme a kinematic flood routing procedure has been devised for a small dendritic headwater gully network on the western slope of colorado the program is spatially distributed incorporating lateral inflows from 103 field sites on the network for which channel geometry variables are known this model in which a lateral inflow algorithm for the sideslopes between each channel site is convoluted into a freeze type 1978 numerical scheme is fully developed in this paper although the field basis of the lateral inflow algorithm has been tested elsewhere faulkner 1990 sensitivity tests were needed for the roughness and hillslope velocity estimates used in the routing procedure after these successful tests a suitably precalibrated run of the model was compared with a field monitored runoff event on the watershed and results again were encouraging however peak attentuation downstream was more pronounced in reality than on the simulation so the model was also modified by inclusion of allowances for transmission loss the tendency that the model had displayed for peak size attenuation downstream was considerably enhanced using the model the geomorphic role of the flashfloods which affect the watershed in the summer months is briefly considered by applying the model to existing records of local summer storm rainfall events as a basis for event simulation these simulations show that downstream attenuation of the flood wave on concave networks in steep semiarid terrain was likely to be a common occurrence possibly resulting in down net deposition and differences in geomorphic behaviour between upstream and downstream sites the discussion is finally broadened to consider the relative importance of common as compared to freak watershed events in maintaining these differences

mechanical decoupling and thermal structure at the east pacific rise axis 9 n constraints from axial magma chamber geometry and seafloor structures abstract we study the relationships between the seafloor structures and the axial magma chamber geometry in the 9 n overlapping spreading center osc area on the fast spreading east pacific rise epr our observations are based on a new high resolution bathymetric map of the 9 n osc area derived from picks of the seafloor arrival on 3d seismic data and on previously published data that constrain the presence and distribution of melt below the 9 n osc differences in the orientation of structures between the seafloor and the magma chamber indicate a sharp change in principal stress directions with depth suggesting that the brittle crust above the melt sill is decoupled from the melt sill itself and the ductile crust underlying it the stress field within the brittle upper crust results from a local interaction of the two overlapping spreading centers whereas the stress field in the crust below the melt sill corresponds to the regional stress field imposed by plate separation given this mechanical structure of the crust the melt sill shape and location appear to be controlled by the following factors the location of the deep melt source below the melt sill the ambient stress field at the depth of the melt sill and the stress field in the brittle upper crust above the melt sill which thermally shapes the roof of the melt sill through repeated eruptions

the role of some site geochemical processes in the development and stabilisation of three badland sites in almeria southern spain samples of both surface and subsurface soils were collected on various surfaces in three badland areas of differing ages tabernas vera and mocatan in almeria province southern spain data were explored with the aim of identifying any diagnostic geochemical relationships between site ec ph sediment size and sar which might constitute typical signatures for site materials with some success data for the three sites are further used to explore the hypothesis that changes in site geochemistry through time are involved in badland stabilisation it was found that 1 the physico chemical properties of the three sites can be differentiated by the functional relationship between ec and sar and this may represent a useful tool in characterising sites where piping predominates 2 there is some equivocal evidence that sediment size distribution may play an additional role in site diagnosis and evolution and 3 surfaces appear to lose sodium over time the suggestion being that sodium is either progressively buffered by calcium during leaching or exchanges with hydrogen on the cation exchange sites under a vegetation cover since other research suggests that smectite can also relocate down the profile on some marl sites reducing hydraulic conductivity and suppressing pipes we argue that the surface at tabernas may be capable of long term autostabilisation in which piping intensity progressively reduces however this possibility really applies only to fine grained densely structured materials and evidence from the silty triassic rich unit in the mocatan basin suggests that without a high clay content piping may be expected to increase through time from this standpoint contemporary badland morphology at mocatan does not represent an early stage of the forms seen in vera and tabernas

control of organic carbon accumulation in the late quaternary equatorial atlantic ocean drilling program sites 664 and 663 productivity versus terrigenous supply organic geochemical records of the last 940 kyr are presented for equatorial atlantic ocean drilling program odp sites 663 and 664 and discussed with regard to the development of ocean productivity and african paleoclimate proportions of marine and terrigenous organic matter om are estimated from elemental pyrolytic isotopic and petrologic data spectral analyses reveal a strong power at the eccentricity and obliquity band indicating a close response of tropical organic sedimentation to the climatic evolution at high latitudes the orbital covariance of organic carbon with biogenous opal and terrigenous records favor that glacially enhanced dust supply and surface water mixing were primary controls for deposition of organic carbon wind borne supply of terrigenous om contributes 26 to 55 and 0 to 39 to the bulk om based on microscopic and isotopic records respectively admixture of c4 plant matter was approximated to contribute up to 16 to the bulk organic fraction during peak glacial conditions

a formation and evolution of overlapping spreading centers constrained on the basis of physical modelling abstract physical modelling and geodynamic analysis have been carried out concerning the formation and evolution of overlapping spreading centers oscs with fast spreading rates crust and underlying magma chamber were modelled by solidified and melted respectively a composite system composed of mineral oils solid hydrocarbons and surface active substances regularities of the model crust quasi brittle failure under tension were investigated obtained results permit to conclude that there are two classes of oscs small ones with widths d d 4 6 km differing in structure formation and mechanism of their evolution small oscs have a short life of the order of 10 3 10 4 years controlled by periodic jumps of spreading centers tectonic cycles large oscs exist for millions of years and represent quasi stationary structures a formation and evolution model of oscs is proposed on the basis of the experimental results

accumulation rates of coarse grained terrigenous sediment in the norwegian greenland sea signals of continental glaciation abstract the accumulation rates ar of coarse 63 m terrigenous material in the norwegian greenland sea ngs were investigated for the last ca 200 kyr comparisons of accumulation rates in the northern western and eastern ngs show different histories the highest ar peak in the eastern ngs occurs near the beginning of stage 6 183 ka this peak may signal the rapid growth and deterioration of the scandinavian ice sheet an ar peak in the northern ngs at the stage 6 5 transition termination ii 128 ka may signal the decay of the barents sea ice sheet the highest peak in the western ngs occurs during the stage 2 1 transition termination i 13 6 ka due to its location near the east greenland current correlation with ice sheets is difficult comparisons of the ar data from the eastern ngs with terrestrial data from scandinavia demonstrate a clear link between glacial advances retreats and accumulation rates of coarse grained material in the deep ocean the retreat of tidewater ice fronts correlates with high ars thus marine data can be used to determine the movement history of continental ice sheets

dynamic evolution of badlands in the roglio basin tuscany italy abstract the roglio basin in the relatively arid central tuscany italy is characterised by the presence of 191 badlands that have developed on pliocene fine grained blue clays a morphometric analysis on the badlands has highlighted that these landforms formation and distribution are primarily related to the steepness of the hill flanks which favour runoff and gravitational processes along with favourable aspects typically the southern ones that experience a stronger solar induced daily thermal variation a detailed physico chemical analysis of the clayey material involved has also been carried out with the specific aim of testing whether this plays a role in the various stages of activity shown by the roglio basin badlands the results indicate that there are no significant differences in the physico chemical properties of an active vs an inactive dormant badland observation and field data however indicate that the dynamic of the roglio basin badlands is strongly controlled by the occurrence of earth flows and the velocity of sediment removal at the bottom of the badlands these two processes variously combined at different intensity in relationship to many local factors are likely to be the main responsible for the different stages of activity shown by the roglio basin badlands

a multiproxy reconstruction of the evolution of deep and surface waters in the subarctic nordic seas over the last 30 000yr on the basis of various lithological mircopaleontological and isotopic proxy records covering the last 30 000 calendar years cal kyr the paleoenvironmental evolution of the deep and surface water circulation in the subarctic nordic seas was reconstructed for a climate interval characterized by intensive ice sheet growth and subsequent decay on the surrounding land masses the data reveal considerable temporal changes in the type of thermohaline circulation open water convection prevailed in the early record providing moisture for the fennoscandian barents ice sheets to grow until they reached the shelf break at 26 cal kyr and started to deliver high amounts of ice rafted debris ird into the ocean via melting icebergs low epibenthic 18o values and small sized subpolar foraminifera observed after 26 cal kyr may implicate that advection of atlantic water into the nordic seas occurred at the subsurface until 15 cal kyr although modern like surface and deep water conditions first developed at 13 5 cal kyr thermohaline circulation remained unstable switching between a subsurface and surface advection of atlantic water until 10 cal kyr when ird deposition and major input of meltwater ceased during this time two depletions in epibenthic 13c are recognized just before and after the younger dryas indicating a notable reduction in convectional processes despite an intermittent cooling at 8 cal kyr warmest surface conditions existed in the central nordic seas between 10 and 6 cal kyr however already after 7 cal kyr the present day situation gradually evolved verified by a strong water mass exchange with the arctic ocean and an intensifying deep convection as well as surface temperature decrease in the central nordic seas this process led to the development of the modern distribution of water masses and associated oceanographic fronts after 5 cal kyr and eventually to today s steep east west surface temperature gradient the time discrepancy between intensive vertical convection after 5 cal kyr but warmest surface temperatures already between 10 and 6 cal kyr strongly implicates that widespread postglacial surface warming in the nordic seas was not directly linked to the rates in deep water formation

organic matter deposition along the kara and laptev seas continental margin eastern arctic ocean during last deglaciation and holocene evidence from organic geochemical and petrographical data abstract organic petrologic maceral analysis and bulk organic geochemical studies were performed on five sediment cores from the eurasian continental margin to reconstruct the environmental changes during the last 13 000 yr the core stratigraphy is based on ams 14c dating and correlation by magnetic susceptibility and lithostratigraphic characteristics variations in terrigenous freshwater and marine organic matter deposition document paleoceanographic and paleoclimatic changes during the transition from the last deglaciation to the holocene glacigenic diamictons deposited in the st anna trough northern kara sea during the last glacial maximum lgm are characterized by reworked terrigenous organic matter in contrast the laptev sea shelf was not covered by an ice sheet but was exposed by the lowered sea level increased deposition of marine organic matter mom during deglaciation indicates enhanced surface water productivity possibly related to influence of atlantic waters the occurrence of freshwater alginite gives evidence for river discharge to the kara and laptev seas after the lgm at the eastern laptev sea slope the first influence of atlantic water masses is indicated by an increase in the contents of mom and dinoflagellate cysts with operculodinium centrocarpum prior to 10 000 yr bp high sedimentation rates in the kara and the laptev seas with the adjacent slope at the beginning of the holocene are presumably related to increased freshwater and sediment discharge from the siberian rivers evidence for elevated holocene freshwater discharge to the laptev sea has been found between 9 8 and 9 kyr bp at 5 kyr bp and at 2 5 kyr bp in the kara sea an increased freshwater signal is obvious at 8 5 kyr bp and at 5 kyr bp higher portions of mom were accumulated in the st anna trough and at the eurasian continental margin at several intervals during the holocene increased primary productivity during these intervals is explained by seasonally ice free conditions possibly associated with increased inflow of atlantic waters

petrology of organic matter in modern and late quaternary deposits of the equatorial atlantic climatic and oceanographic links abstract organic petrologic and geochemical analyses were performed on modern and quaternary organic carbon poor deep sea sediments from the equatorial atlantic the study area covers depositional settings from the west african margin odp site 959 through the equatorial divergence odp site 663 to the pelagic equatorial atlantic response of organic matter om deposition to quaternary climatic cycles is discussed for odp sites 959 and 663 the results are finally compared to a concept established for fossil deep sea environments littke r sachsenhofer r f 1994 organic petrology of deep sea sediments a compilation of results from the ocean drilling program and the deep sea drilling project energy and fuels 8 1498 1512 organic geochemical results obtained from equatorial atlantic deep sea deposits provide new aspects on the distribution of sedimentary om in response to continental distance atmospheric and oceanographic circulation and depositional processes controlling sedimentation under modern and past glacial interglacial conditions the inventory of macerals in deep sea deposits is limited due to mechanical breakdown of particles degree of oxidation and selective remineralization of labile mostly marine om nevertheless organic petrology has a great potential for paleoenvironmental studies especially as a proxy to assess quantitative information on the relative abundance of marine vs terrigenous om discrepancies between quantitative data obtained from microscopic and isotopic 13 c org analyses were observed depending on the stratigraphic level and depositional setting strongest offset between both records was found close to the continent and during glacial periods suggesting a coupling with wind born terrigenous om from central africa since african dust source areas are covered by c4 grass plants supply of isotopically heavy om is assumed to have caused the difference between microscopic and isotopic records

variations in surface water mass conditions in the norwegian sea evidence from holocene coccolith and dinoflagellate cyst assemblages coccolith and dinoflagellate cyst assemblages have been investigated in five sediment cores from the norwegian sea and fram strait both fossil groups are characterized by similar patterns of composition the assemblages contain high proportions of single species the coccolith flora is of low diversity and consists almost entirely of coccolithus pelagicus and emiliania huxleyi the dinoflagellate cysts are generally dominated by operculodinium centrocarpum and nematosphaeropsis labyrinthus other species especially bitectatodinium tepikiense peridinium faeroense and impagidinium pallidum sometimes contribute considerably to the assemblages based on the abundance of the assemblages and the ratio change between the dominating species it has been possible to establish three intervals of distinct major changes in surface water mass conditions sparse occurrences of coccoliths and dinoflagellate cysts have been observed before 10 000 yrs b p indicating harsh environmental conditions with a distinct influence of meltwater and temporarily very slight inflow of atlantic water the modern surface water circulation pattern was reinitiated during termination ib the assemblages suggest slightly cooler and probably less saline surface water conditions than are present today until 7500 yrs b p solar insolation may have caused a first temperature peak which is responsible for the early holocene productivity maximum a considerable change in the composition of dinocyst and coccolith assemblages occurs corresponding approximately to the onset of the holocene climatic optimum this change was most probably linked to an almost synchronous reorganization of the hydrographic properties in the entire north atlantic realm after the ice sheets had vanished since 6000 yrs b p the norwegian current with its modern oceanographic and ecological properties has been fully established

structural analysis of a transform fault rift zone junction in north iceland abstract on the north coast of iceland the rift zone in north iceland is shifted about 120 km to the west where it meets with and joins the mid ocean kolbeinsey ridge this shift occurs along the tjornes fracture zone an 80 km wide zone of high seismicity which is an oblique non perpendicular transform fault there are two main seismic lineaments within the tjornes fracture zone one of which continues on land as a 25 km long wnw trending strike slip fault this fault referred to as the husavik fault meets with and joins north trending normal faults of the theistareykir fissure swarm in the axial rift zone the most clear cut of these junctions occurs in a basaltic pahoehoe lava flow of holocene age where the husavik fault joins a large normal fault called gudfinnugja at this junction the husavik fault strikes n55 w whereas gudfinnugja strikes n5 e so that they meet at an angle of 60 the direction of the spreading vector in north iceland is about n73 w which is neither parallel with the strike of the husavik fault nor perpendicular to the strike of the gudfinnugja fault during rifting episodes there is thus a slight opening on the husavik fault as well as a considerable dextral strike slip movement along the gudfinnugja fault consequently in the holocene lava flow there are tension fractures collapse structures and pressure ridges along the husavik fault and pressure ridges and dextral pull apart structures subparallel with the gudfinnugja fault the 60 angle between the husavik strike slip fault and the gudfinnugja normal fault is the same as the angle between the tjornes fracture zone transform fault and the adjacent axial rift zones of north iceland and the kolbeinsey ridge the junction between the faults of husavik and gudfinnugja may thus be viewed as a smaller scale analogy to the junction between this transform fault and the nearby ridge segments using the results of photoelastic and finite element studies a model is provided for the tectonic development of these junctions the model is based on an analogy between two offset cuts mode i fractures loaded in tension and segments of the axial rift zones or parts thereof in the case of the husavik fault the results indicate that the tjornes fracture zone in general and the husavik fault in particular developed along zones of maximum shear stress furthermore the model suggests that as the ridge segments propagate towards a zero underlapping configuration the angle between them and the associated major strike slip faults gradually increases this conclusion is supported by the trends of the main seismic lineaments of the tjornes fracture zone

the galapagos rift at 86 w 5 variations in volcanism structure and hydrothermal activity along a 30 kilometer segment of the rift valley a 30 km segment of the galapagos rift near 86 w has been mapped in detail using the angus towed camera system the submersible alvin and multi narrowbeam sonar data recent volcanic activity and active hydrothermal circulation are evident along the entire length of the segment mapped there are however clear along strike variations in these processes which render previous two dimensional models obsolete although alternate explanations are possible eruptive sequences appear to begin with the outpouring of surface fed sheet flows and end with more channelized pillow flows in the western portion of the rift studied sheet flows dominate with the entire valley floor covered by recent flows associated with a broad shield volcano the eastern portion on the other hand is narrower consisting primarily of less voluminous pillow flows of apparently the same youthful age three possible models for the volcanic evolution of this rift segment are presented according to the first model the extrusive portion of the crust is formed by a distinct volcanic episode followed by a long period of volcanic quiescence the volcanic phase begins with voluminous sheet flows emerging from numerous eruptive fissures which in time evolve into a narrow pillow ridge farther along strike where the flows are smaller and the extrusive zone narrow the marginal portions undergo continued fissuring and subsequent uplift to form marginal highs and lows this deformational activity also affects the extrusive zone once volcanic activity ends converting the distinctly lobate topography of the active period into highly lineated fault controlled terrain according to the second model extension and voleanism can be viewed as a continuous process without major periods of volcanic quiescence the initial lava flows of a new eruptive sequence fill low areas frequently spilling over local sills and flooding much of the rift valley these sheet flows are then capped by pillow ridges but from an areal view sheet flows dominate since they can flow over much greater distances in the third model along strike differences in flow type arise from ephemeral differences in spreading rate along adjacent segments of the rift separated by transient transform faults or adjacent segments of en echelon spreading segments in this third model eruptions dominated by sheet flows are associated with a faster spreading element than eruptions dominated by pillow flows active hydrothermal vent fields were found throughout the study area as well as numerous yellow and ocher accumulations of what appear to be massive sulfide deposits these latter deposits are not associated with active venting apparently reflecting previous active hydrothermal phases when the exiting temperatures were sufficently high to permit surficial sulfide deposition the active vent fields exhibit along strike variations in their exiting temperatures either due to ephemeral differences in spreading rate or variations in the volcanic flow sequences

evolution of the central nordic seas over the last 20 thousand years the deep and surface water paleoceanographic evolution of the central nordic seas over the last 20 thousand years was reconstructed using various micropaleontological isotopic and lithological proxy data these show a high spatial and temporal complexity of the oceanic circulation when compared with other records from the region during early deglaciation a collapse of ice sheets surrounding the nordic seas released large amounts of freshwater that affected both the surface and bottom water circulation and significantly contributed to heinrich stadial 1 during the younger dryas the central nordic seas were affected by a last major freshwater plume which probably originated from the arctic ocean when major ice rafting had ceased around 11ka subsurface temperatures started to rise however atlantic water advection and subsurface temperatures reached their maximum in the central nordic seas later than along the eastern continental margin that spatio temporal offset is explained by a gradual re routing and westward expansion of the atlantic water flow during times when the greenland sea gyre system became more steadily established in the greenland basin the holocene thermal maximum ended c 5 5ka and time coeveal with an increase in sea ice export from the arctic in the lofoten basin the cooling occurred later after 4ka and together with a weakening of the overturning processes the neoglacial cooling was reached c 3ka together with low solar irradiance expanding sea ice and a slight decrease in deep convection at c 2ka subsurface temperatures began to rise again due to an increasing influence of atlantic waters

quaternary glaciations of northern europe this paper presents the outcome of a workshop held in berlin in february 2009 concerned with current research on the glacial history of northern europe including the british isles the methodologies presently used to resolve this topic are outlined particular attention is given to new analytical methods deriving from high resolution remote imaging of glacial terrain both on land and on the sea bed key new stratigraphic sections higher resolution results from conventional geochronological methods like radiocarbon and more recently developed technologies such as luminescence and cosmogenic radionuclide dating the relationships between the results derived from these two methods are discussed in further detail along with possible explanations for these differences an outline of a most likely glacial history of the scandinavian and british and irish ice sheets is presented along with possible links to global climate change as represented by the marine isotope mis record tentative evidence for glaciation is identified in mis 22 16 12 10 8 6 4 and 2 and correlations of phases within the last glaciation are also explored for both the scandinavian and british and irish ice sheets the results show that the character and extent of glaciation in different parts of the region are not synchronous and much more geochronological work is required before regional correlations can be established with confidence

strain transfer between disconnected propagating rifts in afar we showed before that both the aden and red sea plate boundaries are currently rifting and propagating along two distinct paths into afar through the opening of a series of disconnected propagating rifts here we use new geochronological tectonic and paleomagnetic data that we acquired mostly in the southeastern part of afar to examine the geometry kinematics and time space evolution of faulting related to strain transfer processes it appears that transfer of strain is accommodated by a bookshelf faulting mechanism wherever rifts or plate boundaries happen to overlap without being connected this mechanism implies the rotation about a vertical axis of small rigid blocks along rift parallel faults that are shown to slip with a left lateral component which is as important as their normal component of slip rates of 2 3 mm yr by contrast where rifts do not overlap either a classic transform fault maskali or an oblique transfer zone mak arrasou kinematically connects them the length of the aden red sea overlap has increased in the last 0 9 myr as the aden plate boundary propagated northward into afar as a consequence the first order blocks that we identify within the overlap did not all rotate during the same time span nor by the same amounts similarly the major faults that bound them did not necessarily initiate and grow as their neighboring faults did despite these variations in strain distribution and kinematics the overlap kept accommodating a constant amount of strain 7 to 15 of the extension amount imposed by plate driving forces which remained distributed on a limited number seven or eight of major faults each one having slipped at constant rates 3 and 2 mm yr for vertical and lateral rates respectively the fault propagation rates and the block rotation rates that we either measure or deduce are so fast 30 130 mm yr and 15 38 myr respectively that they imply that strain transfer processes are transient as has been shown to be the case for the processes of tearing rift propagation and strain jumps in afar

volcanic development of small oceanic central volcanoes on the flanks of the east pacific rise inferred from narrow beam echo sounder surveys abstract narrow beam bathymetric surveys of about 30 volcanoes on young crust of the cocos plate were collected in order to help determine the origin and evolution of oceanic central volcanoes the volcanoes are located on ocean crust 1 5 7 5 m y in age and range in size from to 600 km 3 these volcanoes differ greatly in shape development of linear features small scale morphologic characteristics and crater development we conclude that these large differences in morphologic characteristics are due to the complex interaction of many factors in their development such as conduit geometry magma ascent rates eruption rate history tectonic activity crater formation and filling and others elongation directions of individual volcanoes and clear linear bathymetric features near these volcanoes are among the lines of evidence that strongly suggest these volcanoes originate and evolve on and near oceanic fracture zones their origin and morphologic evolution is thus closely tied to the tectonic processes and thermal regimes at fractures we speculate that these volcanoes on fast spreading lithosphere are generated by episodic pulses of melt production beneath ridge crests in the vicinity of fracture zones petrologic evidence shows that these volcanoes cannot be fed from sub axial magma chambers beneath the east pacific rise instead they tap the same mantle source as the ridge crest at mantle depth

evidence for a warm last glacial maximum in the nordic seas or an example of shortcomings in uk 37 and uk 37 to estimate low sea surface temperature the occurrence of ice free conditions in the nordic seas during the last glacial maximum has been demonstrated using microfossil and biomarker approaches an accurate picture of the prevailing oceanographic conditions may be more difficult to ascertain thus uncertainties for most proxies increase at the cold extreme of temperature calibrations and glacial sediments contain small amounts of microfossils and biomarkers here we discuss sea surface temperature estimates derived from alkenone indices for the last glacial maximum in the nordic seas that provide a surprising scenario where surface temperatures were warmer than at present we conclude that the uk37 and uk37 estimates may not be interpreted in terms of sea surface temperature as several factors may have conspired to bias the molecular signal however using an alternative index based on the abundance of the c37 4 alkenone we estimate a maximum temperature of 6 c

sensitivity of last glacial maximum climate to sea ice conditions in the nordic seas published reconstructions of last glacial maximum lgm sea surface temperatures and sea ice extent differ significantly we here test the sensitivity of simulated north atlantic climates to two different reconstructions by using these reconstructions as boundary conditions for model experiments an atmospheric general circulation model has been used to perform two simulations of the lgm and a modern day control simulation standard climap reconstructions of sea ice and sea surface temperatures have been used for the first simulation and a set of new reconstructions in the nordic seas northern atlantic have been used for the second experiment the new reconstruction is based on 158 core samples and represents ice free conditions during summer in the nordic seas with accordingly warmer sea surface temperatures and less extensive sea ice during winter as well the simulated glacial climate is globally 5 7 k colder than modern day with the largest changes at mid and high latitudes due to more intense hadley circulation the precipitation at lower latitudes has increased in the simulations of the lgm relative to the simulation with the standard climap reconstructions reduction of the sea ice in the north atlantic gives positive local responses in temperature precipitation and reduction of the sea level pressure only very weak signatures of the wintertime icelandic low occur when the standard climap sea surface temperature reconstruction is used as the lower boundary condition in lgm with reduced sea ice conditions in the nordic seas the icelandic low becomes more intense and closer to its present structure this indicates that thermal forcing is an important factor in determining the strength and position of the icelandic low the arctic oscillation is the most dominant large scale variability feature on the northern hemisphere in modern day winter climate in the simulation of the lgm with extensive sea ice this pattern is significantly changed and represents no systematic large scale variability over the north atlantic reduction of the north atlantic sea ice extent leads to stronger variability in monthly mean sea level pressure in winter the synoptic variability appears at a lower level in the simulation when standard reconstructions of the sea surface in the lgm are used a closer inspection of storm tracks in this model experiment shows that that the synoptic lows follow a narrow band along the ice edge during winter the trajectories of synoptic lows are not constrained to the sea ice edge to the same degree when the sea ice extent is reduced seasonally open waters in the nordic seas in the new reconstruction apparently act as a moisture source consistent with the current understanding of the rapid growth of the fennoscandian and barents ice sheets during the lgm the signal from the intensified thermal forcing in the north atlantic in boreal winter is carried zonally by upper tropospheric waves and thus generates non local responses to the changed sea ice cover

constraints on sst estimates for the northern north atlantic nordic seas during the lgm abstract a map of estimated calcification temperatures of the planktic foraminifer neogloboquadrina pachyderma sinistral tnps for the nordic seas and the northern north atlantic for the last glacial maximum was produced from oxygen isotopes with support of mg ca ratios to arrive at the reconstruction several constraints concerning the plausible salinity and 18 o fields were employed the reconstruction indicates inflow of temperate waters in a wedge along the eastern border of the nordic seas and at least seasonally ice free waters the reconstruction from oxygen isotopes shows similarities with mg ca based paleotemperatures in the southern and southeastern sector while unrealistically high mg ca values in the central nordic seas prevent the application of the method in this area the oxygen isotope based reconstruction shows some agreement with temperature reconstructions based on the modern analogue technique but with somewhat lower temperatures and a stronger internal gradient inside the nordic seas all told our results suggest a much more ice free and dynamic high latitude ocean than the climap reconstruction

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

depositional regimes in the norwegian greenland sea the last two glacial to interglacial transitions various models of surface and deep water circulation in the norwegian greenland sea ngs have been proposed for the last two glacial to interglacial transitions although much progress has been made in understanding the sedimentary response to climatic and oceanographic changes conflicting interpretations have been developed to clarify some of these discrepancies and to test or modify the existing circulation concepts a multiparameter approach is applied combining sedimentological micropaleontological organic geochemical and isotopic methods on the basis of indicative properties a combined litho and organofacies concept is developed and calibrated with modern depositional settings beneath different surface water masses sedimentary regimes are then derived for glacial and deglacial settings

glacial history and paleoceanography of the southern yermak plateau since 132 ka bp abstract the southern yermak plateau yp is situated at the entrance to the arctic ocean in the narrow marginal ice zone miz between the polar and arctic fronts north west of svalbard a gravity core jm10 02gc has been analysed in order to reconstruct paleoceanographic conditions and the movement of the sea ice margin as well as the glacier ice conditions of the svalbard barents sea ice sheet sbis during the last interglacial glacial cycle the distribution of planktic and benthic foraminifera planktic and benthic oxygen and carbon isotopes and variations in ice rafted debris ird has been investigated the sediment core covers the time interval from the marine isotope stage mis 6 5e transition termination ii c 132 ka bp to the early holocene during termination ii tii the sbis retreated and the sea ice margin was in distal position whereas during mis 5 to mis 4 the sea ice margin was close to the core site several core intervals interpreted as representing mis 5e mis 5c mis 5a mis 3 and mis 1 were barren of calcareous microfossils whereas the intervals representing mis 4 and mis 2 were characterised by high productivity hp of planktic and benthic foraminifera these glacial hp zones were associated with the open water conditions resulting from the advection of atlantic water aw and retreat of the sea ice margin the barren zones during mis 5 mis 3 and mis 1 resulted from the proximity of the sea ice margin whereas during mis 2 the likely cause was an advance of the sbis

variations in soil dispersivity across a gully head displaying shallow sub surface pipes and the role of shallow pipes in rill initiation a small bifurcating gully head displaying shallow pipe development was surveyed to explore how far three dimensional patterns of geochemistry and sediment size can be related to hydraulic gradients in the local marl bedrock almeria se spain the crust sub crust and parent materials were sampled every 20 cm across a 2 m by 3 m grid and then analysed for dispersive and granulometric characteristics spatial patterns of sodium adsoption ratio sar for each layer were plotted separately in situ material at depths of 5 10 cm was only weakly dispersive and the thin 0 2 cm depth crust is also found to be mostly non dispersive paralleling ndings from other eld sites in almeria however the signature relating sar to electrical conductivity for each layer shows that in places the immediate sub crust layer 2 5 cm is highly dispersive the pattern is not random rather the sar of this sub crust layer follows inferred hydraulic gradients the dispersive hot spots being located in the most incised part of the small gully exacerbating the erodibility of that position patterns of sediment particle size and sorting do not correlate with inferred hydraulic gradients but surface material is slightly siltier than the sub crust clay fraction increased with depth and sar is shown to have a weak inverse relationship to particle size this association between sar and the increased clay fractions in the lower layers supports the inference that massive pipe enlargement in the messinian rich unit is suppressed by sub surface swelling since a reduction in in ltration capacity fc with depth can be inferred from these results in ltrating water must be de ected into the already vulnerable sub crust layer during rainfall events explaining the development of shallow pipe forms at preferential depths it is concluded that calcium replaces sodium in the crust during leaching leaving a calcic crust and a sub crust that is sodic and prone to subsequent pipe enlargement rill morphology in these materials also suggests that rills develop from these pipes when pipe roofs collapse i e rill discontinuity bridges steep headwalls and rills with excessively high depth to width ratios copyright 2004 john wiley sons ltd

organo and lithofacies of glacial interglacial deposits in the norwegian greenland sea responses to paleoceanographic and paleoclimatic changes abstract based on a multiparameter approach a combined litho and organofacies concept was developed for glacial and interglacial sediment sections along an e w transect through the central part of the norwegian greenland sea ngs modern and past surface water regimes are clearly displayed by specific litho and organofacies patterns interglacial conditions reveal specific atlantic water associated lithofacies a and b3 in the eastern and central sector of the norwegian greenland sea ngs corresponding interglacial organofacies in general are not well correlated to lithofacies due to strong diagenetic degradation of labile e g autochthonous organic matter om while in near surface sediments a marine dominated organofacies i 1 is preserved under atlantic water masses this correlation is not evident for lower holocene and isotope stage 5 deposits however during isotopic event 5 5 1 increased proportions of marine om are recorded in a high accumulating core on the voring plateau glaciomarine background lithofacies b and c indicate minor input of ice rafted debris ird and seasonal variable sea ice cover corresponding organofacies ii 1 non oxidized and ii 2 oxidized are dominated by allochthonous om most spectacular are glacial diamictons lithofacies e and f evidencing short term sediment pulses due to a sudden disintegration of far advanced tide water ice margins on the outer shelves these diamictons bear specific organofacies iii 1 and iii 2 with a clear predominance of terrigenous and reworked om some of the diamictons seem to occur contemporaneous with the so called heinrich layers h1 and h2 suggesting a common trigger mechanism for the almost simultaneous disintegration of huge continental ice masses along the shelves of north america and the eastern margin of the ngs application of a combined organo and lithofacies concept provides essential information on spatial and temporal variations of water mass characteristics the oceanic effects of ice sheet dynamics and circulation models

late quaternary calcium carbonate sedimentation and terrigenous input along the east greenland continental margin nine large box cores collected in the western greenland iceland norwegian gin sea were analyzed for calcium carbonate content and coarse fraction components stratigraphic control is based on oxygen isotope records performed on four of the cores all cores were correlated using oxygen and carbon isotope data fluctuations in calcium carbonate content coarse terrigenous particle content and volcanic ash beds glacial and interglacial cycles are documented by a number of terrigenous particle events and differentiated calcium carbonate production which can be correlated to major paleoceanographic and paleoclimatic shifts in the older core sections extensive deposition of ice rafted detritus ird persisted until about oxygen isotope stage 9 and document a relatively stable environment in this area strongly influenced by cold surface water masses the following interval was characterized by variations in the general surface water circulation pattern although high amounts of ird are still present in iceland sea sediments pronounced shifts in terrigenous input and pelagic carbonate records were identified close to the oxygen isotope stage full size image 1 k and full size image 1 k boundaries indicating massive melting of icebergs in the western gin sea these enormous inputs of debris were followed by an increase in calcium carbonate production caused by maximum intrusion of warm atlantic surface waters relatively high calcium carbonate contents also show that only little dilution by fine grained material and a minimum of ice melting occurred during these warm phases during oxygen isotope stage 5 conditions were more uniform indicating a less pronounced westward penetration of atlantic waters as compared to the holocene the last glacial stages 4 2 is characterized by the occurrence of heinrich like events although high ird contents were present throughout this interval a distinct contrast between the northernmost cores and the cores in the south is indicative of a rather strong westward penetration of atlantic water in the north and of an area dominated by cold water east of the kolbeinsey ridge during the holocene however conditions west of jan mayen seem to have remained constantly dominated by cold surface waters throughout the whole time investigated

long and short term evolution of several mediterranean denudation hot spots the role of rainfall variations and human impact abstract severe slope denudation is one of the most important environmental problems in the mediterranean region in particular badlands are widespread in central italy as a result of extensive clayey outcrops current climatic conditions and rapid quaternary uplift human activity may significantly affect the development and evolution of these denudation hot spots especially through changes in land use this work is focused on a badland site that has developed over the last 30 years on a gently sloping area of cropland in central italy the site named bargiano underwent cropland abandonment in 1980 and was selected to evaluate the effects of external forcing such as human impact and rainfall variations on the development of water erosion processes the remnants of the 1980 cropland surface and the present eroded surface were sampled using differential global positioning system techniques to perform a volumetric estimation of the eroded material and to calculate the average erosion rate for fairly large time spans the dgps survey was coupled with a 2 year monitoring with erosion pins in the region which permitted comparisons between long and short term average erosion rates in addition the analysis of the rainfall series recorded at the orvieto pluviometric station allowed us to assess and evaluate the role of critical rainfall events on denudation intensity finally a further comparison of the measured and estimated erosion rates from the bargiano site with the data recorded at other long lasting monitoring stations that are set up in certain key areas of central italy the tevere river and ombrone river basins revealed that the bargiano experimental hillslope experienced considerably higher erosion rates than were noted at the other sites our investigations demonstrated that cropland abandonment caused an increase in the erosion rate relative to what happened in natural areas characterized by similar physiographic conditions this finding is consistent with previously reported results for other badlands found in the mediterranean area moreover the results obtained suggest that after the abandonment of its cropland the bargiano site evolved in a cyclical manner that was driven by intrinsic thresholds to a greater extent than it was affected by external climatic changes

overlapping spreading centers implications from crack growth simulation by the displacement discontinuity method overlapping spreading centers osc s are a fundamental aspect of accretionary processes at intermediate and fast spreading centers and typically occur at deep points along the axial depth profile they have a characteristic geometry consisting of two en echelon overlapping curving ridges separated by an elongated depression the length to width ratio of this overlap basin is typically 3 1 we have been successful in reproducing the overlapping spreading center geometry by modelling the growth of two initially parallel elastic cracks of given length and offset in a tensile stress field at infinity a boundary element displacement discontinuity method was used to solve this problem our calculated results are compared with seafloor observations in terms of the size and shape of the overlap region for small osc s there is a very good agreement between calculations and observations but for large ones the overlap basin tends to be longer than our predicted results indicate this suggests that the assumptions made in the model i e perfectly elastic isotropic and homogeneous medium are perhaps valid for the brittle lid above the magma chamber that underlies osc s with small offsets 2 km but oversimplified for osc s with large offsets our modelling shows that the initial interaction of closely spaced surface ruptures along spreading centers is to deflect away from one another as they approach the deflection will be the greatest for small misalignments of the fracture systems thus even minor misalignments of the spreading centers may result in the development of osc s where the misalignment is less than the width of the cracking front the fracture systems may meet head on creating a saddle point along the axial depth profile our results support the hypothesis suggested by macdonald et al 1984 in which overlapping spreading centers develop where two magmatic pulses migrate toward each other along the strike of the spreading center following fracture systems and magmatic conduits which are imperfectly aligned

gloria survey of the east pacific rise near 3 5 s tectonic and volcanic characteristics of a fast spreading mid ocean rise abstract a long range sidescan sonar gloria survey of an approximately 100 km square area of the east pacific rise crest between 3 s and 4 s extends results obtained in the same area by lonsdale 1977 using deep tow the axial linear volcano was found to be continuous over a distance of 75 km the presence of major inward and outward facing fault scarps was confirmed but the gloria data show several distinct differences between the two fault sets the inward dipping faults are more numerous more closely spaced and longer than the outward dipping ones and their dip slopes backscatter sound more extensively than those of the outward dipping faults moreover most of them appear to be formed within 2 km of the axis whereas the majority of the outward dipping faults begin to develop between 5 and 8 km from the axis these differences suggest that the two sets of faults have different origins the horizontal pattern of inward dipping faults is similar to those observed on other mid ocean rises at all spreading rates though the lengths and separations of the scarps are slightly and their throws considerably less than on slower spreading rises this common horizontal pattern suggests that inward dipping faults on all rises have a common mode of formation regardless of spreading rate horizontal tension is probably the dominant factor but an additional mechanism is needed to explain the polarization of fault dips that occurs in the region 2 8 km from the axis the similarity of major fault spacings on the east pacific rise to those on slower spreading rises suggests that faulting is invariant in space rather than time and that the lithosphere where these faults are formed about 2 km from the spreading axis has a similar small thickness for all spreading rates this is attributed to the regulating effect of hydrothermal circulation and plate cooling

glacial interglacial records of the reflectance of sediments from the norwegian greenland iceland sea nordic seas the reflectance of sediments gray level were measured on 11 sediment cores from the norwegian greenland iceland sea nordic seas the analyzed time interval covers the past five glacial interglacial cycles although the results demonstrate that the gray level method has a potential for stratigraphic purposes it is indicated that gray level changes in the nordic seas are not necessarily driven by variations in the content of biogenic calcite a detailed comparison of gray level values with contents of total caco3 carbonate and total organic carbon toc reveals no overall causal link between these proxies however specific glacial core sections with layers containing organic rich sediment clasts as a consequence of iceberg rafting seem to correlate well with law gray level values of those cores which show relatively high and comparable carbonate values in the last three main interglacial intervals stages 11 5 5 and 1 stage 11 is always marked by the highest gray level values a close inspection of the surface structure of the foraminiferal tests as well as the conduction of reflectance measurements on these tests leads to the conclusion that enhanced carbonate corrosion occurred during stage 11 the test corrosion not only affected the reflectance of the tests by making them appear whiter it also seems responsible for the comparatively high gray level values of the total sediment in stage 11 in contrast the relatively low gray level values found in stages 5 5 and 1 are not associated with enhanced test corrosion this observation implies that variable degrees of carbonate corrosion can have a profound effect on total sediment reflectance

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr

dimensional threshold for fracture linkage and hooking fracture connectivity in rocks depends on spatial properties of the pattern including length abundance and orientation when fractures form a single strike set they hardly cross cut each other and the connectivity is limited linkage probability increases with increasing fracture abundance and length as small fractures connect to each other to form longer ones a process for parallel fracture linkage is the hooking where two converging fracture tips mutually deviate and then converge to connect due to the interaction of their crack tip stresses quantifying the processes and conditions for fracture linkage in single strike fracture sets is crucial to better predicting fluid flow in naturally fractured reservoirs for 1734 fractures in permian shales of the lodeve basin se france we measured geometrical parameters in 2d characterizing three stages of the hooking process underlapping overlapping and linkage we deciphered the threshold values shape ratios and limiting conditions to switch from one stage to another one the hook set up depends on the spacing s and fracture length lh with the relation s 0 15 lh once the hooking is initiated with the fracture deviation length l l 0 4 lh the fractures reaches the linkage stage only when the spacing is reduced to s 0 02 lh and the convergence c is 0 1 l these conditions apply to multi scale fractures with a shape ratio l s 10 and for fracture curvature of 10 20

segment scale variations in the crustal structure of 150 300 kyr old fast spreading oceanic crust east pacific rise 8 15 n 10 5 n from wide angle seismic refraction profiles summary we have simultaneously inverted seismic refraction and wide angle moho reflection traveltimes for the 2 d crustal thickness and velocity structure of 150 300 kyr old crust along the east pacific rise epr between the siqueiros and clipperton fracture zones fzs our results show a strong correlation between ridge segmentation and upper and mid crustal seismic velocities with higher velocities near segment centres and lower velocities near segment ends low crustal velocities at the clipperton and siqueiros fzs are interpreted as fracturing resulting from brittle deformation of the crust in the transform domain a relict overlap basin left on the pacific plate by the 9 03 n overlapping spreading centre osc as it propagated southward is associated with a large 1 km s 1 negative upper and mid crustal velocity anomaly this anomaly is consistent with the presence of an unusually thick extrusive section within the basin and with tectonic alteration fracturing and shearing arising from rotation of the basin as it was formed the discordant zone left by this osc on the cocos plate is characterized by moderately low crustal velocities probably because of crustal fracturing as the osc propagated into older crust higher crustal velocities near segment centres may reflect a higher ratio of dikes to extrusives in the upper crust and lower intensity tectonic alteration of the crust than near segment ends the mean crustal thickness along the epr between the siqueiros and clipperton fzs is 6 7 6 8 km the thickest crust is found beneath the lamont seamounts 9 km and in a southward pointing v shaped band located just north of the off axis trace of the 9 03 n osc 7 3 7 8 km the thinnest crust 6 km is found proximal to the clipperton and siqueiros fzs the crust associated with the off axis trace of the 9 03 n osc is not anomalously thin suggesting that magma supply beneath the osc is similar to that of the northern and southern segments we see a similar pattern of crustal thickness variation to that determined using multichannel reflection data including a gradual thickening of the crust from north to south along the northern ridge segment and the location of the thickest crust just north of the 9 03 n osc however the magnitude of the along axis crustal thickness variation we observe along the northern ridge segment between 9 50 n and 9 15 n 1 3 1 8 km excluding the lamont seamounts is significantly less than the 2 3 km of variation previously reported weakening the case for the existence of a low density mantle diapir at 9 50 n inferred from gravity data the band of thick crust located just north of the off axis trace of the 9 03 n osc suggests a close genetic link between this feature and the osc thus we attribute the pattern of crustal thickness variations along the northern segment to the kinematics of the southward propagating 9 03 n osc over the past 0 5 myr and not to along axis melt migration away from a mantle diapir as previously proposed

seismicity and active tectonic processes in the ultra slow spreading lena trough arctic ocean summary with its remote location in the ice covered fram strait lena trough is a poorly known segment of the global mid ocean ridge system it is a prominent member of the ultra slow spreading mid ocean ridges but its spreading mechanisms are not well understood we relocalized teleseismically recorded earthquakes from the past five decades to identify tectonic processes in lena trough and the adjacent spitsbergen fracture zone fz during two cruises with rv polarstern in 2008 and 2009 we deployed seismic arrays on ice floes to record the local seismicity of lena trough we could identify and localize microseismic events which we assume to be present in the entire rift valley in contrast our relocalization of teleseismically recorded earthquakes shows an asymmetric epicentre distribution along lena trough with earthquakes occurring predominately along the western valley flanks of lena trough in 2009 february march several high magnitude earthquakes peaking in an mb 6 6 event occurred in an outside corner setting of the spitsbergen fz this is the strongest earthquake which has ever been recorded in fram strait and its location at the outside corner high of the ultra slow spreading ridge is exceptional comparing the seismicity with the magnetic anomalies and high resolution multibeam bathymetry we divide lena trough in a symmetrically spreading northern part and an asymmetrically spreading southern part south of the south lena fz we propose that a complex interaction between the former de geer megashear zone which separated greenland from svalbard starting at late mesozoic early cenozoic times and the developing rift in the southern lena trough resulted an increasing eastward dislocation towards the spitsbergen fz between older spreading axes and the recent active spreading axis which we believe to be located west of the bathymetric rift valley flanks in a wide extensional plain

migration of mid ocean ridge volcanic segments the propagation of small offset volcanic spreading centre segments along mid ocean ridge crests may reflect absolute motion of the plate boundary relative to the underlying mesospheric frame such a relationship could be caused by a purely vertical flow of the mantle under spreading centres and would have value in constraining past plate motions from non transform trends generated during along ridge propagation and in linking the major element variability of oceanic crust and upper mantle to the bulk composition and temperatures of mantle ascending under mid ocean ridges

causes and consequences of arctic freshwater routing into the nordic seas during late marine isotope stage 5 sediment core ps1904 reveals continuous records of planktic and benthic foraminiferal stable isotopes 18o 13c from the north eastern greenland continental margin the data show good comparability with other records from the nordic seas allowing the stratigraphic range of ps1904 to be dated to marine isotope stage mis 6 focusing on mis 5 reveals light 18o values during mis 5a compared to the last interglacial peak mis 5e which indicates that surface and bottom water layers were strongly affected by freshwater during the former event we present two possible scenarios explaining the origin and routing of the freshwater i drainage of a eurasian proglacial lake coupled with the collapse of the kara sea ice sheet at the mis 5b a boundary and ii destabilization and melting of the nearby greenland ice sheet although both scenarios could have acted simultaneously sediment records from the eurasian sector of the arctic ocean hint at the proglacial lake system in north western siberia as the largest freshwater source regardless of the actual source the freshwater lowered the surface ocean salinity causing water column stratification and sea ice expansion increased sea ice abundance led to a higher albedo and probably contributed to the cooling and global ice sheet growth that occurred subsequently during mis 4

sedimentary environment and glacial history during the last 40 ka of the voring continental margin mid norway abstract this study is based on detailed investigation of sediment cores and high resolution seismics we identified and describe five lithofacies on the voring plateau and eight on the mid norwegian continental slope the various lithofacies are mainly related to the fluctuations of the fennoscandian ice sheet and the varying intensity of bottom currents and inflow of atlantic water masses ocean circulation was highly variable between 40 and 22 14 c ka bp being vigorous during interstadials and sluggish during stadials between ca 22 and 15 14 c ka bp the sedimentary environment was significantly influenced by fluctuations of the fennoscandian ice sheet repeatedly reaching the outermost shelf these fluctuations are reflected in the sedimentary record as ice rafted debris ird accumulation peaks deposition of stratified diamicton and glacigenic debris flows on the continental slope during this period the sediment accumulation rate increased bottom currents influenced the sedimentary pattern and surface waters were seasonally ice free indicating inflow of atlantic waters subsequent to ca 15 14 c ka bp the glacier influence decreased as the margin of the fennoscandian ice sheet retreated to reach the coast before 12 5 14 c ka bp the modern sedimentary environment is characterised by relatively strong bottom current action causing winnowing or non deposition down to approximately 1000 m water depth

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

alkenones and coccoliths in ice rafted debris during the last glacial maximum in the north atlantic implications for the use of uk37 as a sea surface temperature proxy the uk37 index has proven to be a robust proxy to estimate past sea surface temperatures ssts over a range of time scales but like any other proxy it has uncertainties for instance in reconstructions of the last glacial maximum lgm in the northern north atlantic uk37 indicates higher temperatures than those derived from foraminiferal proxies here we evaluate whether such warm glacial estimates are caused by the advection of reworked alkenones in ice rafted debris ird to deep sea sediments we have quantified both coccolith assemblages and alkenones in sediments from glaciogenic debris flows in the continental margins of the northern north atlantic and from a deep sea core from the reykjanes ridge certain debris flow deposits in the north atlantic were generated by the presence of massive ice sheets in the past and their associated ice streams such deposits are composed of the same materials that were present in the ird at the time they were generated we conclude that ice rafting from some locations was a transport pathway to the deep sea floor of reworked alkenones and pre quaternary coccolith species during glacial stages but that not all of the ird contained alkenones even when reworked coccoliths were present we speculate that the ratio of reworked coccoliths to alkenone concentration might be useful to infer whether significant reworked alkenone inputs from ird did occur at a particular site in the glacial north atlantic we also observe that alkenones in some of the debris flows contain a colder signal than estimated for lgm sediments in the northern north atlantic this is also clear in the deep sea core studied where the warmest intervals do not correspond to the intervals with large inputs of reworked coccoliths or ird we conclude that any possible bias to uk37 estimates associated with reworked alkenones is not necessarily towards higher values and that the high sst anomalies for the lgm are unlikely to be the result of a bias caused by ird inputs

the morphology of propagating spreading centers new and old recent seabeam investigations over the east pacific rise in the northeast pacific have shown the existence of a propagating spreading center at 18 n the propagating and dying spreading centers are joined by a zone of orthogonal lineations which appear to be the traces of a continuously migrating transform fault similar orthogonal lineations occur at 16 n 107 30 w on the western side of the trace of a 5 m y old propagating spreading center the amplitude and the complexity of the topographic signature over the earlier propagating spreading center are higher and greater than over the presently active center suggesting that the relief of the earlier center may owe its origin to differential subsidence of thermally contrasting segments of lithosphere analogous to fracture zone topography of similar origin traces of former propagating spreading centers are thus a combination of two sets of landforms those resulting from propagation and those resulting from the juxtaposition of two blocks of lithosphere of different thickness

digital image processing of seabeam bathymetric data for structural studies of seamounts near the east pacific rise digital marine bathymetric data are rapidly increasing in both volume and variety these data provide new perspectives on the morphology of the seafloor they also present a number of problems with regard to developing display methods that convey the large amount of information contained in these data we have used digital image processing techniques to reduce and display seabeam multibeam data for three young less than 5 ma seamounts on the eastern flank of the east pacific rise navigational errors in location were corrected by identifying offsets between features observed in crossing tracks and then translating the tracks relative to a control track images of the seamounts were then generated using spatial filtering techniques the images reveal more information than contour maps because 1 more depth intervals can be displayed and 2 the continuous tone nature of the images shown in shaded relief or color coded form more readily conveys morphologic information the images suggest a significant degree of structural control on seamount shape more so than can be delineated by examination of contour maps alone in one case a chain of seamounts strikes at an angle perpendicular to the abyssal hill direction although each seamount within the chain is elongate in a direction parallel to the hills in another case a polygonally shaped flat topped seamount overlies a region of the crust where the abyssal hills display two strike directions images of the third seamount demonstrate a complicated structural control on morphology including a control on the distribution of satellite constructs finally the image presentations clearly depict regions where cross tracks could not be registered because of 1 residual navigation errors and 2 the presence of steep slopes where the depth and location estimates can vary depending on the angular relationship between the ship track azimuth and the strike of the slope

late quaternary sedimentation and glacial history of the western svalbard continental margin abstract glacigenic sediments recovered in shallow cores from the western svalbard continental slope are subdivided into five facies associations based on grain size sedimentary structure mineralogy petrography and geochemistry two diamicton facies are recognised one of which is interpreted as hemipelagic mud with variable amounts of ice rafted debris ird and the other as a product of mass movement the diamictons are associated with melting of icebergs during glacial melt events and debris flow deposition on the submarine fans during peak glaciation respectively laminated to layered mud and turbidites seem to be closely related to eustatic fall in sea level and erosion of banks located on the continental shelf resulting in accumulation of fine grained organic rich deposits and thin silt and sand layers on the continental slope middle weichselian was characterised by several phases of extensive iceberg production and input of ird the first phase 60 55 ka is correlated with a glacier advance on svalbard but the following phases 54 44 ka can not be correlated with glacier advances on land this demonstrates that there may have been more glaciations than recorded on svalbard and that ice sheets at high northern latitudes fluctuated more frequently than previously assumed the composition of the ird deposited after 54 ka suggests that the ice sheets during these advances were located in the eastern svalbard barents sea area the late weichselian growth of the svalbard barents sea ice sheet occurred probably in two steps nucleation of a continental based ice sheet between 27 and 22 5 ka followed by a rapid advance to the shelf edge when the ice margin reached the soft clay rich sediments on the continental shelf eustatic lowering of the sea level during the initial phase led to bank exposure and erosion bringing large quantities of fine grained organic rich sediments to the deep sea during peak glaciation glaciers provided a localised supply of sediments to the shelf troughs and the upper slope which were redistributed by debris flows the marine based svalbard barents sea ice sheet started to retreat around 14 5 ka through massive iceberg discharge the retreat was interrupted by a short lived advance accompanied by erosion of the shelf banks and redistribution of organic rich sediments to the continental slope the final ice recession started around 12 ka and ended close to 9 ka when the fjords of svalbard were essentially ice free large amounts of meltwater released during the removal of the svalbard barents sea ice sheet cooled down the surface waters of the norwegian greenland sea and formed a widespread sea ice cover between 12 and 8 ka input of terrigenous material was greatly reduced during the early part of holocene the present interglacial is characterised by erosion of the shelf and upper slope by bottom currents leaving a prominent lag deposit

the iceland plate boundary zone propagating rifts migrating transforms and rift parallel strike slip faults unlike most of the mid atlantic ridge the north america eurasia plate boundary in iceland lies above sea level where magmatic and tectonic processes can be directly investigated in subaerial exposures accordingly geologic processes in iceland have long been recognized as possible analogs for seafloor spreading in the submerged parts of the mid ocean ridge system combining existing and new data from across iceland provides an integrated view of this active mostly subaerial plate boundary the broad iceland plate boundary zone includes segmented rift zones linked by transform fault zones rift propagation and transform fault migration away from the iceland hotspot rearrange the plate boundary configuration resulting in widespread deformation of older crust and reactivation of spreading related structures rift propagation results in block rotations that are accommodated by widespread rift parallel strike slip faulting the geometry and kinematics of faulting in iceland may have implications for spreading processes elsewhere on the mid ocean ridge system where rift propagation and transform migration occur

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

a multiproxy approach to reconstruct the environmental changes along the eurasian continental margin over the last 150 000 years abstract sediment cores located along the eurasian continental margin arctic ocean have been studied to reconstruct the environmental changes in terms of waxing and waning of the barents kara sea ice sheets atlantic water inflow and sea ice distribution over the last 150 kyr the stratigraphy of the cores is based on stable oxygen isotopes ams 14 c and paleomagnetic data we studied variations in marine and terrigenous input by a multiproxy approach involving direct comparison of sedimentological and organo geochemical data extensive episodes of northern barents sea ice sheet growth during marine isotope stages mis 6 and 2 have been supported by at least subsurface atlantic water inflow moisture bearing storms low summer insolation and minimal calving of ice ice advance during mis 4 was probably restricted to the shallow shelf between mis 4 and mis 2 large ice sheet fluctuations correspond to contemporary laurentide surging events and indicate short term climatic changes in the arctic ocean as has been recorded in lower latitudes in contrast in low precipitation areas in eastern eurasia glacial activity was rather limited only distinct ice rafted debris ird input during termination ii and early mis 3 reflects severe glaciations on the northern severnaya semlya margin during mis 6 and mis 4 we conclude that 1 oscillations of ice sheets are less frequent along the eastern eurasian margin than in areas with continuous moisture supply like the western eurasian margins and that 2 major fluctuations of the kara sea ice sheet during the last 150 kyr apparently followed the major interglacial glacial mis 5 4 and mis 7 6 transitions rather than the precession 23 kyr and the tilt 41 kyr cyclicity of the earth s orbit as observed for the scandinavian sis and the svalbard ice sheets respectively mangerud j jansen e landvik j y 1996 late cenozoic history of the scandinavian and barents sea ice sheets in solheim a riis f elverhoi a faleide j j jensen l n cloetingh s eds impact of glaciations on basin evolution data and models from the norwegian margins and adjacent basins global and planetary chance special issue 12 pp 11 26 surface and or subsurface atlantic water masses coupled with seasonally ice free conditions penetrated continuously to at least the franz victoria trough during the last 150 kyr however sustained periods of open water were largely restricted to substages 5 5 5 1 and the holocene as indicated by distinct carbonate dissolution and higher accumulation of marine organic matter mom signals of periodic open water conditions along the northern margin of severnaya semlya are of less importance higher production of foraminifera probably due to atlantic water inflow occurred between 38 and 12 14 c kyr and corresponds to periodic atlantic water advection penetrating into the arctic ocean however marine organic proxies indicate a continuous decrease of surface water productivity from the western to the eastern eurasian continental margin due to a more extensive sea ice cover over the last 150 kyr

new aspects of organic carbon deposition and its paleoceanographic implications along the northern barents sea margin during the last 30 000 years we studied variations in terrigenous tom and marine organic matter mom input in a sediment core on the northern barents sea margin over the last 30 ka using a multiproxy approach we reconstructed processes controlling organic carbon deposition and investigated their paleoceanographic significance in the north atlantic arctic gateways variations in paleo surface water productivity are not documented in amount and composition of organic carbon the highest level of mom was deposited during 25 23 ka as a result of scavenging on fine grained reworked and tom rich material released by the retreating svalbard barents sea ice sheet during the late weichselian a second peak of mom is preserved because of sorptive protection by detrital and terrigenous organic matter higher surface water productivity due to permanent intrusion of atlantic water and high suspension load release by melting sea ice during 15 9 11 2 ka

sedimentation history of the northern north sea margin during the last 150 ka the norwegian channel ice stream ncis is one the defining features of the fennoscandian icesheet still little is known of the detailed dynamics of this ice stream in relation to regional changes in ice cover paleoceanographic and climatic changes sedimentological data from core md99 2283 in combination with seismic data allow a detailed chronological reconstruction of the outbuilding of the margin and the ice extent in southern scandinavia through the last 150 ka an integrated stratigraphy of the margin is presented and compared to the glacial history changes in the regional ice cover are reflected in the accumulation rates the clay mineralogy the coarse chalk fraction and the number of ird 2 mm in core md99 2283 while the sedimentation on the north sea fan as derived from seismic data provides direct evidence for the glacial activity at the shelf edge tentative evidence was found for two early weichselian glacial advances in southern scandinavia and possibly scotland at around 110 and 80 ka bp from 42 cal ka bp the ice cover expanded in southern fennoscandia and led to increased deposition on the margin and the occurrence of local melt water outbursts significantly increased accumulation rates coarse chalk local meltwater output and smectite occur during the ice expansion in the north sea from around 34 cal ka bp the main outbuilding phase of the nsf during the last glacial cycle occurred after 30 cal ka bp from around 24 cal ka bp the ncis became highly active and advanced at least three times prior to the final retreat from the shelf edge around 19 0 cal ka bp

overlapping spreading centres new accretion geometry on the east pacific rise in a detailed seabeam investigation of the east pacific rise epr from 8 n to 18 n a new kind of volcano tectonic geometry associated with fast spreading centres has been discovered figs 1 2 at several locations along the rise axis the neovolcanic zone is discontinuous and is laterally offset a short distance 1 15 km in contrast to a classic ridge transform ridge plate boundary however the offset ridge terminations overlap each other by a distance approximately equal to or greater than the offset they curve sharply towards each other and often merge into one another along strike separating the overlapping spreading centres oscs is a closed contour depression up to several hundred metres deep which is sub parallel to the trend of the oscs the region between the oscs is a complex zone of both shear and rotational deformation with no obvious transform parallel structures based on wax model studies of spreading centres we suggest here that transform faults fail to develop at fast spreading centres where the lateral offsets are small 15 km because the lithosphere is too thin and weak to maintain a classic rigid plate spreading centre transform fault pattern the osc geometry is unstable and evolves rapidly one of the two oscs prevails while the other is abandoned a significant area of sea floor created at fast spreading rates may bear the imprint of this newly observed process

sedimentary facies of glacial interglacial cycles in the norwegian sea during the last 350 ka sediment fluxes were highest in the norwegian sea during late glacial early deglacial periods i e at oxygen isotope transition 43 below transition 65 at various levels within stage 6 and below stage 9 dark diamictons deposited at these times reflect intense iceberg rafting in surface waters fed by surges along the front of the marine based parts of the continental ice sheets in the southeastern sector of the norwegian sea the high organic carbon content 0 5 1 3 in these layers reflects input from erosion of terrigenious matter rich sediments outcropping on the shelves partial oxidation of organic matter and decreased deep water renewal may explain the strong carbonate dissolution observed during these periods interglacial environments were strongly variable throughout the last 350 ka circulation patterns of stage 5e best resemble modern conditions while stage 7 and 9 sediments record a much weaker norwegian current

early diagenesis of marine organic matter alteration of the carbon isotopic composition in two distal carbonate turbidites labelled a and e from the madeira abyssal plain oxidation of organic matter om has been accompanied by a reduction in hydrogen index hi and change in the isotopic composition of carbon since the emplacement of turbidite 0 5 ka ago 35 of the om in the upper part has been oxidised by oxygen and nitrate diffusing from sea water into the sediment there is an accompanying decrease of 0 3 in 13c om and a decrease of 25 in hi turbidite e was emplaced about 73 ka and was exposed to bottom water for 12 3 ka before being buried beneath later turbidites during exposure oxygen and nitrate diffused into its upper part and oxidised 50 of the toc without an accompanying change in isotopic composition further oxidation of the more refactory organic components decreased toc by a further 25 and decreased 13c by 2 5 oxidation also reduced hi by 50 from originally low values unless corrected for the isotopic changes accompanying extended oxic and suboxic degradation of organic matter in sediments will give rise to erroneous estimates of marine terrestrial om ratios