sea level change along the french atlantic and channel coasts since the time of the last glacial maximum abstract holocene sea level change along the french atlantic and english channel coast is not uniform because of the response of the crust to the changing ice and water loads in late pleistocene and holocene time the pattern of the spatial variability is a north south trend of decreasing sea levels levels at any time this results primarily from the isostatic response to the melting of the fennoscandian ice sheet superimposed upon this is a mainly east west trend resulting from the isostatic response of the crust to the addition of the meltwater into the atlantic ocean the combined effect is that at any time sea levels vary spatially along this coastline by amounts that are predicted to reach 20 m from the time of the last glacial maximum to about 10 000 radiocarbon yr ago about 15 m at 8000 yr ago and about 10 m at 6000 yr ago thus observations of past sea levels should not be combined into a single sea level curve unless first corrected for the glacio hydro isostatic factors alternatively the data should only be used to construct regional curves for relatively short sections of the coastline such as pas de calais and picardie normandie cotes du nord finistere or the vendee and charente maritime region even within such constrained regions the spatial variability may still be of the order of a few meters analysis of the observational data by region indicates that major oscillations in sea level are unlikely to have occurred that levels at no location exceeded the present mean sea level and that there may have been a small 3 m increase in eustatic sea level over the past 6000 yr palaeoshorelines and palaeo water depths for the english channel and the atlantic coast shelf have been predicted using the isostatic and eustatic models and the results indicate that in situ marine deposits older than 6000 7000 yr are unlikely to be found along many sections of the present coastline

discrimination of biogenic and detrital magnetite through a double verwey transition temperature magnetite occurs widely in natural environments in both inorganic and biogenic forms discrimination of the origin of magnetite has important implications from searching for past microbial activity to interpreting paleomagnetic and environmental magnetic records in a wide range of settings in this study we present rock magnetic and electron microscopic analyses of marine sediments from the continental margin of oman low temperature magnetic data reveal two distinct verwey transition tv temperatures that are associated with the presence of biogenic and inorganic magnetite this interpretation is consistent with room temperature magnetic properties and is confirmed by electron microscopic analyses our study justifies the use of two distinct tv temperatures as a diagnostic signature for discriminating inorganic and biogenic magnetite simple low temperature magnetic measurements therefore provide a tool to recognize rapidly the origin of magnetite within natural samples in addition our analyses reveal progressive down core dissolution of detrital and biogenic magnetite but with preservation of significant amounts of fine grained magnetite within sediments that have been subjected to severe diagenetic alteration we demonstrate that preservation of magnetite in such environments is due to protection of fine grained magnetite inclusions within silicate hosts our results therefore also provide new insights into diagenetic processes in marine sediments

giant magnetofossils and hyperthermal events abstract magnetotactic bacteria biomineralize magnetic minerals with precisely controlled size morphology and stoichiometry these cosmopolitan bacteria are widely observed in aquatic environments if preserved after burial the inorganic remains of magnetotactic bacteria act as magnetofossils that record ancient geomagnetic field variations they also have potential to provide paleoenvironmental information in contrast to conventional magnetofossils giant magnetofossils most likely produced by eukaryotic organisms have only been reported once before from paleocene eocene thermal maximum petm 55 8 ma sediments on the new jersey coastal plain here using transmission electron microscopic observations we present evidence for abundant giant magnetofossils including previously reported elongated prisms and spindles and new giant bullet shaped magnetite crystals in the southern ocean near antarctica not only during the petm but also shortly before and after the petm moreover we have discovered giant bullet shaped magnetite crystals from the equatorial indian ocean during the mid eocene climatic optimum 40 ma our results indicate a more widespread geographic environmental and temporal distribution of giant magnetofossils in the geological record with a link to hyperthermal events enhanced global weathering during hyperthermals and expanded suboxic diagenetic environments probably provided more bioavailable iron that enabled biomineralization of giant magnetofossils our micromagnetic modelling indicates the presence of magnetic multi domain i e not ideal for navigation and single domain i e ideal for navigation structures in the giant magnetite particles depending on their size morphology and spatial arrangement different giant magnetite crystal morphologies appear to have had different biological functions including magnetotaxis and other non navigational purposes our observations suggest that hyperthermals provided ideal conditions for giant magnetofossils and that these organisms were globally distributed much more work is needed to understand the interplay between magnetofossil morphology climate nutrient availability and environmental variability

on the magnetocrystalline anisotropy of greigite fe3s4 the ferrimagnetic mineral greigite cubic fe3s4 is well known as an intracellular biomineralization product in magnetic bacteria and as a widely occurring authigenic mineral in anoxic sediments due to the lack of suitable single crystal specimens the magnetic anisotropy parameters of greigite have remained poorly constrained to the point where not even the easy axis of magnetization is known here we report on an effort to determine the anisotropy parameters on the basis of ferromagnetic resonance fmr powder spectroscopy on hydrothermally synthesized chemically pure greigite microcrystals dispersed in a nonmagnetic matrix in terms of easy axis orientations the fmr data are consistent with or or less likely a more general type with a g factor of 2 09 the anisotropy field is about 90 mt and in some samples may reach 125 mt compared to 30 mt for cubic magnetite this confirms the dominating role of cubic anisotropy on the magnetic properties of greigite which we show to be responsible for large sirm k values k1 is in the range 15 23 j m3 or 10 15 kj m3 yielding upper limits of 44 or 34 nm for the superparamagnetic grain size respectively

sedimentary structure and origin of a mud cored pseudo tidal sand ridge eastern yellow sea korea abstract using high resolution seismic profiles sidescan sonar images and piston cores we have examined an elongate ridge located off the tae an peninsula of korea in the eastern yellow sea this ridge 50 km long 30 m high and up to 12 km wide is one of many similar longitudinal ones which are approximately parallel to the direction ne sw of strong tidal currents bathymetric profiles across the ridge are all asymmetric the eastern ridge flank is narrow and dips steeply towards the adjoining rocky valley below the eastern ridge flank less steeply sloping subbottom reflectors are truncated by a seafloor parallel erosion surface on which is draped a 45 cm thick unit of muddy sand a piston core taken here indicates that the subbottom reflectors are due to interlayered coarse sediments the more gently sloping western ridge flank contains sand waves ca 100 m wavelength ene wsw trend on which megaripples 10 15 m wavelength wnw ese trend are superimposed indicating recent bottom current activity in particular the reflectors beneath the westernmost ridge show westward progradation over an acoustically transparent unit of thin sand sheets however subbottom profiles collected over the ridge crest do not show conspicuous reflectors only a few discontinuous faint reflectors are observed the lack of conspicuous reflectors below the ridge crest area may reflect more thorough bottom current generated sediment homogenization as well as poor penetration of acoustic energy due to the current scoured rippled seafloor we interpret the ridge as an early holocene tidal mud tongue blanketed by recently reworked relict palimpsest sands the pseudo tidal sand ridge has prograded westward the fine mud tongue sediments might have been derived mostly from the korean peninsula via the han river at a time when sea level was lower than present

magnetofossil spike during the paleocene eocene thermal maximum ferromagnetic resonance rock magnetic and electron microscopy evidence from ancora new jersey united states previous workers identified a magnetically anomalous clay layer deposited on the northern united states atlantic coastal plain during the paleocene eocene thermal maximum petm the finding inspired the highly controversial hypothesis that a cometary impact triggered the petm here we present ferromagnetic resonance fmr isothermal and anhysteretic remanent magnetization first order reversal curve and transmission electron microscopy analyses of late paleocene and early eocene sediments in drillcore from ancora new jersey a novel paleogeographic analysis applying a recent paleomagnetic pole from the faeroe islands indicates that new jersey during the initial eocene had a 6 9 degrees lower paleolatitude 27 3 degrees for ancora and a more zonal shoreline trace than in conventional reconstructions our investigations of the petm clay from ancora reveal abundant magnetite nanoparticles bearing signature traits of crystals produced by magnetotactic bacteria this result the first identification of ancient biogenic magnetite using fmr argues that the anomalous magnetic properties of the petm sediments are not produced by an impact they instead reflect environmental changes along the eastern margin of north america during the petm that led to enhanced production and or preservation of magnetofossils

an appalachian amazon magnetofossil evidence for the development of a tropical river like system in the mid atlantic united states during the paleocene eocene thermal maximum on the mid atlantic coastal plain of the united states paleocene sands and silts are replaced during the paleocene eocene thermal maximum petm by the kaolinite rich marlboro clay the clay preserves abundant magnetite produced by magnetotactic bacteria and novel presumptively eukaryotic iron biomineralizing microorganisms using ferromagnetic resonance spectroscopy and electron microscopy we map the magnetofossil distribution in the context of stratigraphy and carbon isotope data and identify three magnetic facies in the clay one characterized by a mix of detrital particles and magnetofossils a second with a higher magnetofossil to detrital ratio and a third with only transient magnetofossils the distribution of these facies suggests that suboxic conditions promoting magnetofossil production and preservation occurred throughout inner middle neritic sediments of the salisbury embayment but extended only transiently to outer neritic sediments and the flanks of the embayment such a distribution is consistent with the development of a system resembling a modern tropical river dominated shelf

rock magnetic criteria for the detection of biogenic magnetite abstract we report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron reducing bacteria magnetotactic bacterial mtb strains ms1 mv1 and mv2 and dissimilatory iron reducing bacterium strain gs 15 grown in pure cultures were used in this study our results suggest that a combination of room temperature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain sd particles of magnetite from mtbs the most useful magnetic property measurements include 1 acquisition and demagnetization of isothermal remanent magnetization irm using static pulse and alternating fields 2 acquisition of anhysteretic remanent magnetization arm and 3 thermal dependence of low temperature 20 k saturation irm after cooling in zero field zfc or in a 2 5 t field fc from 300 k however potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero field and field cooled sirms on warming through the verwey transition t 100 k intact chains of unoxidized magnetite magnetosomes have ratios of fc zfc greater than 2 where the parameter is a measure of the amount of remanence lost by warming through the verwey transition disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the fc zfc ratio to around 1 similar to values observed for some inorganic magnetite maghemite greigite and gs 15 particles numerical simulations of fc zfc ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the fc zfc parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole sediment samples the strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made compared to techniques such as electron microscopy

field observations and modelling of holocene sea level changes in the southern bay of biscay implication for understanding current rates of relative sea level change and vertical land motion along the atlantic coast of sw europe abstract the absence of basal peat in the stratigraphic sequences of the southern bay of biscay has long precluded the development of holocene sea level curves we have approached this problem by combining the indicative depositional meaning derived from the micropalaeontological composition and sand content with radiocarbon ages of 55 borehole samples obtained from three estuarine areas of the southern bay of biscay these new sea level index points have produced the first complete holocene sea level curve from this area we further reviewed all available sea level data from sw europe to provide the regional trend and use these data to calibrate a recently developed isostatic model field data and model reconstructions present a good agreement for the region considered a north south trend is apparent in the data and this is shown to be dominated by the influence of the deglaciation of eurasian ice sheets as suggested by previous studies for this region however some data model discrepancy in the south of the iberian peninsula suggests that local factors tend to dominate during the late holocene on comparing our results to estimates of recent sea level rise obtained from tide gauges and high resolution proxy records it is clear that this region has experienced a significant acceleration in sea level during the past century or so

rock magnetic properties of uncultured magnetotactic bacteria abstract understanding the magnetic properties of magnetite crystals fe3o4 produced by magnetotactic bacteria mtbs is of fundamental interest in fields of geosciences biomineralization fine particle magnetism and planetary sciences the database of bulk magnetic measurements on mtbs is however still too sparse to allow for generalizations due to difficulties in obtaining bacteria cells in sufficient quantities from natural environments and the fact that only a few cultivable strains are available here we report the first series of magnetic measurements on two air dried samples containing solely mtbs wild type cocci and magnetobacterium bavaricum which were directly isolated from carbonaceous lake sediments systematic rock magnetic studies show that 1 the magnetosomes in cells are dominated by single domain sd magnetite 2 the samples have delta ratios fc zfc higher than 2 3 the measured low temperature remanence cycling curves as well as the first order reversal curve forc diagrams are significantly different to those measured on synthetic sd magnetite samples and 4 the verwey transition temperature tv 100 k of mtb cells is distinctly lower than that from stoichiometric magnetite tv 120 125 k our results provide new insights on the magnetic properties of bacterial magnetite and advance the use of magnetic proxies for decoding the paleomagnetic signals of sediments containing bacterial magnetite

eustatic and hydrodynamic controls on the architecture of a deep shelf sand bank celtic sea the architecture of a tidal sand bank in the south eastern celtic sea was examined using very high resolution seismic surveys the bank comprises four depositional units the lowest unit 1 is characterized by gently dipping 1 8 strata that strike parallel to the length of the bank unit 1 is erosionally overlain by unit 2 which forms the bulk of the bank this unit consists of stacked sets of downcurrent dipping 7 12 master bedding formed by climbing sinuouscrested tidal dunes that are up to 20 m high these deposits are locally incised by an anastomosed channel network unit 3 that may represent a buried swatchway system the upper part of the bank comprises wave related deposits that are mainly preserved on the bank flanks unit 4 the outer bank surface is erosional the bank is believed to have formed during the last postglacial sea level rise the facies evolution from unit 1 to unit 3 indicates an upward increase in tidal energy mainly characterized by the thickening of dune cross bed sets in unit 2 the majority of bank growth is inferred to have occurred in water depths of the order of 60 m this evolution was controlled by relative sea level rise which is likely to have caused an episode of tidal resonance with associated strong tidal currents that were responsible for the incision of the deep cross cutting channels of unit 3 the transition to wavedominated sedimentation in unit 4 is related to the decay of resonance with continued sea level rise

micromagnetic simulation of magnetofossils with realistic size and shape distributions linking magnetic proxies with nanoscale observations and implications for magnetofossil identification abstract we build micromagnetic models to investigate the magnetic properties of biogenic magnetite a common type of magnetic mineral that is responsible for recording a wide range of biological geophysical and geological processes on earth the geometry of modelled particles is based on realistic size and shape distributions from nanoscale observations systematic changes in microstructures of biogenic magnetite ensembles are built and their magnetic properties are calculated which enables a quantitative and separate assessment of the effect of crystal morphology and chain structures although the same particle size and shape distributions are used in all calculations simulation results document large variations in magnetic properties i e wide distributions of coercivity b c 10 60 mt coercivity of remanence b cr 14 81 mt dispersion parameter dp 0 1 0 5 and skewness values s 0 7 1 1 due to variable degree of anisotropy and magnetostatic interactions previously the commonly observed biogenic soft and biogenic hard components on biogenic magnetite bearing samples were often interpreted to reflect crystal morphologies and that the small dp values of coercivity distributions were an indication of narrow particle size distributions our simulations suggest that these speculations are not always the case and that magnetosome microstructures likely exert a dominant control over their magnetic properties our modelling results provide a new theoretical perspective on the magnetic properties of biogenic magnetite which is important for understanding magnetic proxy signals from magnetofossils in a wide range of environmental and geological settings and for the search for biogenic magnetite in terrestrial rocks and in extra terrestrial materials

volcanic iron fertilization of primary productivity at kerguelen plateau southern ocean through the middle miocene climate transition abstract odp site 747 central kerguelen plateau contains a nearly complete record of the middle miocene climate transition mmct its unusually well preserved and rich foraminiferal assemblages have resulted in the mmct interval at site 747 becoming a focus of high resolution paleoclimatic studies that have established this sequence as a southern ocean reference we combine x ray fluorescence xrf elemental counts of fe ti and al and concentration dependent magnetic parameters to trace volcanic ash input to sediments at odp hole 747a we observe systematic variations in al ti and fe ti ratios that are consistent with increased primary productivity at times of increased terrigenous input and therefore increased nutrient supply a spike in terrigenous material which we interpret as volcanic ash coincides with major changes in species abundance and stable oxygen and carbon isotopic shifts within the mmct interval this correlation suggests that nutrient flux associated with volcanic iron fertilization played a significant role in controlling phytoplankton communities at site 747 we demonstrate that rapidly obtained xrf and magnetic parameters can provide useful proxies for terrigenous input and therefore nutrient fluxes in pelagic marine settings that can help to resolve local versus global drivers of paleoenvironmental and paleoceanographic changes

properties of intracellular magnetite crystals produced by desulfovibrio magneticus strain rs 1 desuffiovibrio magneticus strain rs 1 is an anaerobic sulfate reducing bacterium cells form intracellular nanocrystals of magnetite but are only weakly magnetotactic in order to understand the unusual magnetic response of this strain we studied magnetite crystals within cells grown with fumarate and sulfate many cells grown under either condition did not form magnetic crystals while others contained only i to 18 small similar to 40 nm magnetite containing magnetosomes bulk magnetic measurements of whole cells showed a superparamagnetic like behavior indicating that many of the magnetite crystals are too small to have a permanent magnetic moment at ambient temperature the temperature of the verwey transition is lower similar to 86 k than of magnetite from other magnetotactic strains likely indicating partial oxidation of magnetite into maghemite as a result of the small size and small number of magnetite magnetosomes the magnetic moments of most cells grown anaerobically with fumarate or sulfate are insufficient for magnetotaxis in addition to intracellular magnetite in some cultures another iron oxide hematite formed on the surfaces of cells the hematite grains are embedded in an extracellular polymeric material indicating that the crystals likely resulted from a biologically induced mineralization process although the hematite particles appear to consist of aggregations of many small 5 to 10 nm grains the grains have a consensus orientation and thus the whole particle diffracts as a single crystal the aligned arrangement of nanoparticles within larger clusters may reflect either a templated nucleation of hematite crystallites in an extracellular organic matrix or result from a self assembling process during the crystallization of hematite from ferric gels or ferrihydrite

mineral magnetic study of lacustrine sediments from lake pumoyum co southern tibet over the last 19 ka and paleoenvironmental significance abstract high resolution proxy records from tibetan plateau are essential to understand the past climatic and environmental changes in this work we conducted systematic environmental magnetic studies on lacustrine sediments from lake pumoyum co southern tibet spanning from the last deglaciation 19 ka to present we correlated the magnetic proxies in the lake core with environmental changes the magnetic proxies from the 3 units were studied both unit 3 19 13 2 ka and unit 2 13 2 8 4 ka contain evidences for an anoxic or sub anoxic environment fine grained greigite dominates the bulk magnetic properties of unit 3 while both magnetite and greigite coexist in unit 2 overall this reveals a transition from an anoxic environment to an oxic environment in contrast unit 1 8 4 ka present spans the holocene when the lake environment was oxic there is an increasing trend in the concentration of magnetic minerals which reflects an increasing transport of magnetic minerals from the catchment into the lake due to the increased post glacial melt water flow although different mechanisms affect the magnetic assemblage and associated magnetic properties in the different units our results suggest that the magnetic proxies of the local lake environment on southern tibet are sensitive to environmental changes

morphodynamic behavior and seismostratigraphy of a sandbank bahia blanca estuary argentina abstract sediment dynamics in bahia blanca estuary argentina permit the formation of longitudinal banks one of which is la lista bank located at the mouth of the la lista tidal channel the purposes of the present research were to study the processes and mechanisms involved in the formation and maintenance of this bank and to identify seismic sequences which could be indicative of its origin and current morpho evolutionary trend to this end different methods and techniques such as side scan sonar 3 5 khz profiler and bottom sediment sampling were followed the analysis of seismic acoustic records revealed bottom features sand dunes sand ribbons current marks escarpment which based on our analysis of current measurements and our estimations of transport as bedload have contributed to tracing a sediment circulation pattern in the study area taken together these findings allowed us to identify the areas dominated by erosion sediment transport or depositional processes bedload material was found to be exported to the outer sector and held on the flanks of the bank particularly in its southern part inference on the morpho evolutionary trend in the study area was based on evidence related to migration patterns and changes observed on the flanks and inner structure of the bank the morphological evolution of la lista bank is associated to a residual sediment transport pattern with trajectories opposite on its two flanks which results from the action of ebb tides in the northern flank and flood tides on the southern flank this process induces a gradual increase in the bank height and longitudinal growth the la lista bank evolution agrees with the motion sequence and development pattern proposed by caston 1972 for a linear bank except that this model is repeated as the bank grows the seismostratigraphic data presented here indicates that the origin of this bank is associated with the holocene marine regression process

giant dune morphologies and dynamics in a deep continental shelf environment example of the banc du four western brittany france the aim of this paper is to improve the knowledge of spatio temporal variability of very large to giant dunes in deep tide dominated environment their growth mechanisms and dynamic evolution still remain in debate and constitute a significant scientific challenge in regard to difficulties to achieve accurate measures of migrations this motivated the realization of three recent swath bathymetry surveys across the wide dune field of the banc du four located offshore the western brittany the banc du four field is composed of more than 500 large dunes exhibiting a great diversity of morphologies ranging from 2d to 3d shapes and reaching the largest sizes of such sedimentary structure ever described wavelength over 1000 m and height over 30 m the analysis of the entire ensemble of dune parameters offers the opportunity to discuss the relative influence of forcings and the validity of the equations predicting the bedform geometry in shallow waters our results show a good height spacing power law correlation but do not strike the usual statistical regression observed in previous studies the steeper positive slope of the equation and the outstanding values of the height parameter reveal the complexity of local hydrodynamic regime interacting with bed load transport and sediment supply no clear relationship has been observed between asymmetry and size parameters water depth can not be considered as a major factor controlling the size of dunes otherwise the vertical sedimentary variation suggests the action of storm waves on dune crests and the horizontal migration rates of dunes in the eastern field of the banc du four were found to range from 3 to 20 m yr 1 such velocities were only mentioned before by van landeghem et al 2012 on deep continental shelves 70 m deep and attest of the still present morphodynamical equilibrium of the large dunes furthermore as was previously found for similar dunes in the irish sea no power law relationship between dune migration rates and dune size can be deduced similarly no strong correlation is established between migration rates and the degree of asymmetry nevertheless the dune asymmetry can be used to predict the migration direction as for dimension parameters the evolution asymmetry depends on the study area and migration rates more precisely when the crest lines of dunes form straight lines and their migration rates remain below 8 10 m yr 1 the degree of dune asymmetry increases with the migration rate this trend is reversed for the dunes with higher migration velocities and crests which are sinuous these results relativize the migratory predictions based on punctual geometrical observations

magnetotactic bacteria and their magnetofossils in sediments abstract living magnetotactic bacteria from freshwater environments show under natural and laboratory conditions a great variety of morphological appearances their magnetosomes vary in number shape and size one species of bacteria yields up to 1000 magnetosomes per cell individual particles reach a size of up to 200 nm as a rule they form elongated chains in bacteria which are held under laboratory conditions loop shaped arrangements and nearly unordered clusters are also found during the laboratory experiments different species of bacteria periodically dominated freshwater samples always exhibited a great variety of different magnetosomes on the other hand samples from marine environment clearly showed the preponderance of one type of particle these observations may reflect specific properties of the physico chemical milieu in which the bacteria grew at the end of our experiments north and south seeking bacteria co existed in roughly equal quantities quaternary unconsolidated sediments from the ammersee bavaria and the antarctic quaternary to tertiary deep sea sediments from the atlantic and pacific and jurassic limestones from the sonnwendgebirge tyrol austria were analyzed for fossil magnetosomes we found that fossil magnetosomes were morphologically similar to those from living bacteria but seemed to be corroded in some cases the observed differences in quantity size and shape of magnetosomes from various sediments may be influenced by variations in bacterial viability and particle stability at different sites but are not determined by geographical latitude

flume experiments on the alignment of transverse oblique and longitudinal dunes in directionally varying flows for more than a century geologists have wondered why some bedforms are orientated roughly transverse to flow whereas others are parallel or oblique to flow this problem of bedform alignment was studied experimentally using subaqueous dunes on a 3 6 m diameter sand covered turntable on the floor of a 4 m wide flume in each experiment two flow directions relative to the bed were produced by alternating the turntable between two orientations the turntable was held in each orientation for a short time relative to the reconstitution time of the bedforms the resulting bedforms were in equilibrium with the time averaged conditions of the bimodal flows dune alignment was studied for five divergence angles the angle between the two flow directions 45 67 5 90 112 5 and 135 the flow depth during all experiments was approximately 30 cm mean velocity was approximately 50 cm s 1 and mean grain diameter was 0 6 mm each experiment continued for 30 75 min during which time the flume flow was steady and the turntable position changed every 2 min at the end of each experiment water was slowly drained from the flume and dune alignment was measured transverse dunes defined relative to the resultant transport direction were created when the divergence angle was 45 and 67 5 and longitudinal dunes were created when the divergence angle was 135 at intermediate divergence angles dunes with both orientations were produced but transverse dunes were dominant at 90 and longitudinal dunes were dominant at 112 5 one experiment was conducted with a divergence angle of 135 and with unequal amounts of transport in the two flow directions this was achieved by changing the orientation of the turntable at unequal time intervals thereby causing the amount of transport to be unequal in the two directions the dunes formed during this experiment were oblique to the resultant transport direction these experimental dunes follow the same rule of alignment as wind ripples studied in previous turntable experiments in both sets of experiments the bedforms developed with the orientation having the maximum gross bedform normal transport the orientation at which the sum of the bedform normal components of the two transport vectors reaches its maximum value in other words the bedforms develop with an orientation that is as transverse as possible to the two flows in those cases where the two flows diverge by more than 90 and transport equal amounts of sand bedforms that are as transverse as possible to the two separate flows will be parallel to the resultant of the two flow vectors although such bedforms have been defined by previous work as longitudinal bedforms they are intrinsically the same kind of bedform as transverse bedforms

biomineralization crystallography and magnetic properties of bullet shaped magnetite magnetosomes in giant rod magnetotactic bacteria article i nfo magnetosomes produced by magnetotactic bacteria are of great interest for understanding bacterial biomineralization along with sedimentary magnetism and environmental magnetism one of the most intriguing species magnetobacterium bavaricum can synthesize hundreds of bullet shaped magnetite magnetosomes per cell which contribute significantly to magnetic properties of sediments however the biomineralization mechanism and magnetic properties of such magnetosomes remain unknown in this paper we have conducted a comprehensive study of the crystallography and magnetic properties of bullet shaped magnetosomes formed by uncultivated giant rod magnetotactic bacteria referred to as myr 1 recently discovered in lake miyun beijing china transmission electron microscopy observations reveal that each myr 1 cell contains hundreds of bullet shaped magnetite magnetosomes which are arranged into 3 5 braid like bundles of chains the formation of the bullet shaped magnetosomes can be divided into two stages initial isotropic growth up to 20 nm followed by elongation along the 100 direction which is unusual compared with the expected 111 magnetic easy axis although the 100 orientation is the hard axis of the face centered cubic magnetite the shape anisotropy of bullet shaped magnetosomes and intra bundle magnetostatic interactions confine the magnetization direction of the chain along the long axis of the cell bundle due to each bundle of magnetosome chains effectively behaving as an elongated single domain particle the myr 1 cells show high coercivity weak intra bundle magnetostatic interaction and high ratio these results provide new insights into the biomineralization process and magnetic properties of bullet shaped magnetite magnetosomes

a new perspective for the sediment provenance evolution of the middle okinawa trough since the last deglaciation based on integrated methods abstract the sediment in the okinawa trough ot preserves considerable information regarding land ocean interactions and palaeoenvironmental evolution in the east asian marginal seas more specifically the sediment provenance in the ot must be studied to recover this information however arguments about this remain especially regarding the contribution of yellow river yr derived materials to middle ot depositions to resolve this problem we systematically investigated a gravity core m063 05 3 87 m in length since the last deglaciation collected from the middle ot by integrating environmental magnetism diffuse reflectance spectroscopy and transmission electron microscopy as well as analyses of sediment grain size clay mineralogy and major elements our results indicated that sediment from the yr has contributed dominantly to this core since the last deglaciation limited contributions were also made from the yangtze river and taiwan rivers during the holocene terrigenous inputs derived from the yr could be well characterized by the ratio of hematite hematite goethite additionally the kuroshio current kc activity since the last deglaciation which is an important influencing factor for sediment provenance changes was well deciphered using a new proxy reldm relative contribution of detrital ferrimagnetic minerals to bulk magnetic properties based on comprehensive analyses of our results and previously published studies we hypothesised that during the last deglaciation abundant resuspended sediment from the palaeo yr mouth was transported to the east china sea ecs continental shelf edge and the northern ot through seaward bottom flow and further spread into the middle ot since the middle to late holocene the central mud area of the south yellow sea and the mud area southwest off cheju island may have become the main sources to provide yr derived materials to the middle ot however the mud area derived sediment could only be restricted to a small range owing to the hindrance of the strengthened kc this study provides a new perspective for sediment provenance evolution of the middle ot since the last deglaciation and proposes new methods and parameters for future research on palaeoenvironmental evolution in the ecs

baroclinicity influences on storm divergence and stratiform rain subtropical upper level disturbances abstract divergence structures associated with the spectrum of precipitating systems in the subtropics and midlatitudes are not well documented a mesoscale model is used to quantify the relative importance different baroclinic environments have on divergence profiles for storms primarily caused by upper level disturbances in southeastern texas a subtropical region the divergence profiles simulated for a subset of the modeled storms are consistent with those calculated from an s band doppler radar realistic convective and stratiform divergence signals are also generated when applying a two dimensional convective stratiform separation algorithm to reflectivities derived from the mesoscale model although the model appears to underestimate stratiform rain area divergence profiles from the modeled precipitating systems vary in magnitude and structure across the wide range of baroclinicities common in southeastern texas barotropic storms more characteristic of the tropics generate the most elevated diver

a strong angular dependence of magnetic properties of magnetosome chains implications for rock magnetism and paleomagnetism 1 single domain magnetite particles produced by magnetotactic bacteria magnetosomes and aligned in chains are of great interest in the biosciences and geosciences here we investigated angular variation of magnetic properties of aligned magnetospirillum magneticum amb 1 cells each of which contains one single fragmental chain of magnetosomes with measurements at increasing angles from the chain direction we observed that i the hysteresis loop gradually changes from nearly rectangular to a ramp like shape e g bc and remanence decrease ii the acquisition and demagnetization curves of irm shift toward higher fields e g bcr increases and iii the forc diagram shifts toward higher coercivity fields e g bc forc increases for low temperature results compared to unoriented samples the samples containing aligned chains have a much lower remanence loss of field cooled fc and zero field cooled zfc remanence upon warming through the verwey transition higher ratio fc zfc for the measurement parallel to the chain direction and lower ratio larger fc and zfc values for the perpendicular measurement micromagnetic simulations confirm the experimental observations and reveal that the magnetization reversal of magnetosome chain appears to be noncoherent at low angles and coherent at high angles the simulations also demonstrate that the angular dependence of magnetic properties is related to the dispersion degree of individual chains indicating that effects of anisotropy need to be accounted for when using rock magnetism to identify magnetosomes or magnetofossils once they have been preserved in aligned chains additionally this study experimentally demonstrates an empirical correspondence of the parameter bc forc to bcr rather than bc at least for magnetite chains with strong shape anisotropy this suggests forc analysis is a good discriminant of magnetofossils in sediments and rocks

tidal evolution of the northwest european shelf seas from the last glacial maximum to the present 1 two dimensional paleotidal simulations have been undertaken to investigate tidal and tide dependent changes tidal amplitudes tidal current velocities seasonal stratification peak bed stress vectors that have occurred in the nw european shelf seas during the last 20 ka the simulations test the effect of shelf wide isostatic changes of sea level by incorporating results from two different crustal rebound models and the effect of the ocean tide variability by setting open boundary values either fixed to the present state or variable according to the results of a global paleotidal model the use of the different crustal rebound models does not affect the overall changes in tidal patterns but the timing of the changes is sensitive to the local isostatic effects that differ between the models the incorporation of ocean tide changes greatly augments the amplitude of tides and tidal currents in the celtic and malin seas before 10 ka bp and has a large impact on the distribution of seasonally stratified conditions magnitude of peak bed stress vectors and tidal dissipation in the shelf seas the predictions on seasonal stratification are supported by well dated evidence on tidal mixing front migration in the celtic sea additional experiments using the global model suggest that the variability of offshore tides has been caused mainly by changes of eustatic sea level and ice sheet extent in particular a large decrease observed at 10 8 ka bp is attributed to the opening of hudson strait accompanied by the retreat of the laurentide ice sheet

tide and wave dynamics on a sand bank from the deep shelf of the western channel approaches abstract the sedimentary response of deep shelf sediments to the interplay of tides and waves based on the example of the celtic sea sand banks is described grab samples and 1500 km of multibeam and side scan data have been collected from the 300 km2 densely surveyed kaiser bank area the bank is 140 170 m deep 30 m high and 60 km long oriented perpendicular to the shelf edge and nearly parallel to the major axis of the tidal ellipse surficial sediments of the bank consist of medium to gravely biolithoclastic sands they are swept by tidal currents that reach 0 9 m s 1 m above the seabed and under the occasional influence of waves the mobile sands are commonly bedformed and rest on a highly backscattering lag the flanks below 140 m mainly exhibit transverse tidal dunes and sand ribbons whereas the top of the bank mostly displays discontinuous sand patches and wave ripples the sand patches are interpreted as the remnants of tidal bedforms reworked by waves calculations of the threshold of motion of four modal grain sizes under various conditions at the seabed show that tidal bedforms are active during spring tides sometimes associated with waves whereas only the largest annual waves may explain the observed wave ripples as in active tidal banks there is a reversal of tidal bedload transport from one bank side to the other the sedimentation rate is very low on the kaiser bank in addition there is a loss of sediment at both bank extremities and the tidal bedload cross bank transfer is very small probably controlled by shelf residual currents and long term drift of sediment resuspended by waves however the present day action of waves at the bank top is less intense than it was at lower sea levels during the holocene as evidenced by seismic studies

pleistocene forced regressions and tidal sand ridges in the east china sea abstract tidal sand ridges are common features on modern shelves but only few examples of such preserved sand bodies are described in pleistocene deposits in the stratigraphic record some sand bodies encased in shales previously interpreted as sand ridges have been reinterpreted as shoreface deposits more than 5000 km of high resolution seismic data from the east china sea correlated to geotechnical boreholes and shallow cores demonstrate the potential of sand ridge preservation and allow reconstruction of the depositional history of pleistocene fourth order 100 kyr depositional sequences a high subsidence rate of about 300 m myr allows the preservation of three elementary sedimentary facies constitutive of a motif which was repeated during glacio eustatic cycles they consist of 1 regressive marine prodeltaic prograding wedges 2 estuarine and continental deltaic facies and 3 transgressive sand ridges similar in shape and orientation to modern sand ridges major discontinuities traceable over the entire outer continental shelf along distances of hundreds of kilometers are transgressive and regressive surfaces of marine erosion whereas sequence boundaries formed by fluvial erosion are difficult to identify on this low gradient shelf because of the asymmetry of the pleistocene glacio eustatic cycles most of the preserved sedimentary record with the exception of sand ridges corresponds to forced regressive deposits deposits that formed during a seaward shift of the shoreline due to relative sea level lowering

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

impact of biomineralization on the preservation of microorganisms during fossilization an experimental perspective abstract the biogenicity of fossil microbial biomorphs is often debated because their morphologies are poorly informative and the chemical structural and isotopic signatures of putative biogenic organic molecules have been altered during their incorporation into the sediments and the geological history of the host rock here we investigated the effect of encrustation by biominerals on the morphological and chemical degradation of escherichia coli cells during experimental thermal treatments non calcified e coli cells and e coli cells encrusted by calcium phosphates were exposed to heating under an argon atmosphere at two different temperatures 300 c 600 c for 20 h two additional experiments were performed on non calcified e coli cells at 300 c for 2 h and 100 h to discuss the influence of experiment duration organic residues of all experiments were characterized at a multiple length scale using a combination of scanning electron microscopy transmission electron microscopy raman microspectroscopy electron paramagnetic resonance epr spectroscopy fourier transform infrared ft ir spectroscopy and x ray absorption near edge structure spectroscopy xanes at the carbon k edge in the absence of encrusting biominerals the morphological structure of the organic residues of e coli cells was completely lost after heating at 300 or 600 c even after short 2 h long heating experiments the content of aromatic functional groups of the organic residues of non calcified e coli cells increased during heat treatment at 600 c for 20 h while the amide functional groups were lost as indicated by ft ir spectroscopy consistently the epr spectrum of these organic residues indicated important transformation as a comparison this spectrum appeared similar to epr spectra of ancient organic carbons such as carbons from the apex chert ca 3460 myr indicating a similar concentration of aromatic moieties in contrast calcified e coli exposed to the same conditions showed only limited morphological alteration as observed by electron microscopy as well as lower chemical transformation as detected by ft ir and epr spectroscopies despite the difficulties to relate experimental conditions directly to geological conditions these experiments evidence the influence of cell encrustation by minerals on their chemical and morphological preservation potential during fossilization processes

structure and morphology of magnetite anaerobically produced by a marine magnetotactic bacterium and a dissimilatory iron reducing bacterium intracellular crystals of magnetite synthesized by cells of the magnetotactic vibroid organism mv 1 and extracellular crystals of magnetite produced by the non magnetotactic dissimilatory iron reducing bacterium strain gs 15 were examined using high resolution transmission electron microscopy electron diffraction and57fe mo ssbauer spectroscopy the magnetotactic bacterium contained a single chain of approximately 10 crystals aligned along the long axis of the cell the crystals were essentially pure stoichiometric magnetite when viewed along the crystal long axis the particles had a hexagonal cross section whereas side on they appeared as rectangules or truncated rectangles of average dimension 53 35 nm these findings are explained in terms of a three dimensional morphology comprising a hexagonal prism of 110 faces which are capped and truncated by 111 end faces electron diffraction and lattice imaging studies indicated that the particles were structurally well defined single crystals in contrast magnetite particles produced by the strain gs 15 were irregular in shape and had smaller mean dimensions 14 nm single crystals were imaged but these were not of high structural perfection these results highlight the influence of intracellular control on the crystallochemical specificity of bacterial magnetites the characterization of these crystals is important in aiding the identification of biogenic magnetic materials in paleomagnetism and in studies of sediment magnetization

morphological features of elongated anisotropic magnetosome crystals in magnetotactic bacteria of the nitrospirae phylum and the deltaproteobacteria class high resolution transmission electron microscopy was used to study the crystallographic habits of the elongated magnetite crystals variously described as bullet tooth or arrowhead shaped in two recently described uncultured magnetotactic bacteria belonging to the nitrospirae phylum designated candidatus magnetoovum mohavensis strain lo 1 and candidatus thermomagnetovibrio paiutensis strain hsmv 1 and a cultured sulfate reducing magnetotactic bacterium of the deltaproteobacteria class of the proteobacteria phylum designated strain av 1 the elongation axes of the magnetosomes do not coincide with the easy magnetization axis which is 111 but they are parallel to 100 in lo 1 and av 1 and parallel to 110 in hsmv 1 in all three strains magnetosome magnetite crystals appear to elongate at constant width resulting in asymmetric shapes idealized crystal morphologies are proposed neither the control mechanism over crystal growth nor the adaptiveness if any of such unusual crystal habits are known at the moment since similar elongated and asymmetric morphologies are unknown in inorganically formed magnetite crystals these forms of magnetosome magnetite appear to be excellent biomarkers

magnetic detection and characterization of biogenic magnetic minerals a comparison of ferromagnetic resonance and first order reversal curve diagrams australian research council grant number dp120103952 australia new zealand iodp consortium anzic grant number le0882854

the detection of bacterial magnetite in recent sediments of lake chiemsee southern germany the sediments of lake chiemsee located in the alpine foreland in southern germany host a variety of magnetotactic bacteria mtb which contain intracellular crystals of magnetite arranged in linear chains to detect bacterial magnetite in the carbonate dominated surface sediments and further quantify its contribution to the magnetic signal of the sediments we conducted detailed rock magnetic measurements as well as complimentary non magnetic analyses electron microscopy powder x ray diffraction and sediment pore water analysis our results demonstrate that biogenic single domain magnetite characterized by bullet and truncated hexagonal prismatic shapes is the dominant ferrimagnetic component in the topmost few centimetres of the sediment the changes of magnetic properties with depth are due to the occurrence of live mtb and the downward increasing dissolution of biogenic magnetite moreover the ratios of remanence loss on warming through the verwey transition after field cooling and zero field cooling of saturation isothermal remanence dfc dzfc were determined as 1 47 and 1 25 for freeze dried and air dried sediment samples containing mtb respectively these low ratios suggest that the bacterial magnetite chains were disrupted to a large extent and or that the bacterial magnetosomes might have undergone partial

searching for single domain magnetite in the pseudo single domain sedimentary haystack implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations 1 magnetic hysteresis measurements of sediments have resulted in widespread reporting of pseudo single domain like magnetic properties in contrast the ideal single domain sd properties that would be expected to be responsible for high quality paleomagnetic records are rare determining whether sd particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment we apply a range of such techniques and find that sd particles are much more common than has been reported in the literature and that magnetite magnetofossils the inorganic remains of magnetotactic bacteria are widely preserved at depth in a range of sediment types including biogenic pelagic carbonates lacustrine and marine clays and possibly even in glaci marine sediments thus instead of being rarely preserved in the geological record we find that magnetofossils are widespread this observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments magnetofossils also have grain sizes that are substantially smaller than the 1 15 m size range for which there is reasonable empirical support for relative paleointensity studies the different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies this issue needs much closer attention despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years much remains to be understood about how they become magnetized

on the formation of sand banks of finite extent we consider a model where the fluid depth depends on both horizontal coordinates quasi steady depth uniform non divergent fluid flow is governed by inertial pressure and bottom frictional forces sand transport is proportional to the cube of the instantaneous current but augmented by a down slope component and by wind wave action and sand is conserved it is found that low parallel banks grow fastest so that in an extensive spatially uniform sea previous calculations for linear banks are appropriate the inclination of banks to the tidal currents can be interpreted in terms of similarly inclined deposition bands resulting from vorticity generation and advection in flow over a small isolated hump a small bump can evolve to an equilibrium bank typically after an initial rapid extension across the tidal currents provided that sand is sufficiently restricted and particularly if some wind wave action prevents growth up to the sea surface sand banks are likely to be in a late stage of evolution when the main change is a slow lengthening as the net current and transport along the bank side slows and turns around the bank end with net deposition the equilibrium is apparently stable except when there is an overall bed slope in the direction of the tidal currents or when sand is abundant

migration and internal architecture of marine dunes in the eastern english channel over 14 and 56 year intervals the influence of tides and decennial storms submarine dune dynamics are controlled by tidal currents and wind forces according to the relative in uence of these forces and the nature of dune sediment different bedform behaviors can be observed the footprint of the different hydrodynamic agents is recorded into the internal architecture of dunes this paper is concerned with bedforms that compose the thick sediment wedge located in the eastern english channel off the bay of somme this sedimentary archive constitutes an interesting feature to achieve a better understanding of seabed sediment dynamics and its timeline building stages the dynamics of large submarine dunes which are organized in elds are studied thanks to bathymetric and seismic data over the periods 1937 1993 and 1993 2007 dune morphology presents low lee and stoss side slopes on average 8 and 3 respectively and dune migration rate is not very high dune movements are in the direction of residual tidal currents i e toward the east with mean migration rates around 0 8 to 5 0 25 m yr 1 and up to 6 6 0 7 m yr 1 respectively at multi decennial and decennial time scales the dune internal archi tecture is complex with superimposed eastward prograding units displaying locally opposite progradation second order discon tinuities dip of 0 5 4 perpendicular to dune crests constitute dune master bedding by counting the number of second order re ectors between 1937 1993 and 1993 2007 the formation periodicity of these bounding surfaces is estimated to range from 4 to 18 years these time intervals coincide with the long term tidal cyclicities and also with the inter annual to decennial variability of storm activity in northern europe two theories were made to interpret the dune internal structures the second order surfaces are interpreted either as the depositional surfaces corresponding to the marks of weak energy periods weak tidal and storm action or as erosive surfaces due to an opposite direction of dune migration provoked temporarily by exceptional storms from the northeast

sedimentology and stratigraphy of a tidal sand bank in the southern north sea abstract a set of 125 vibrocores collected on the basis of a dense grid of high and very high resolution seismic coverage over a tidal sand bank the middelkerke bank provides a unique opportunity to reconstruct the stratigraphy and sedimentary facies succession of such a large tidal sand body five sedimentary facies are distinguished they range from clay to pebble and display various types of bedding and lithologies corresponding to different depositional environments a succession of four late quaternary units are defined on the basis of the stratigraphic framework given by the seismic grid the first unit represents the infill of a palaeovalley system by tidally influenced sedimentary facies it is followed by extensive and thick lagoonal or sub tidal deposits the third unit reveals the development of a landward prograding storm dominated sand bar this unit is dominated by clean fine sand the last unit corresponds to the present day tidal sand bank it shows the typical offshore tidal sand bank facies cross bedded sand prograding in an offshore direction like other banks located in a more offshore position the middelkerke bank is a composite bank incorporating facies which do not occur in classic tidal sand banks

the celtic sea banks an example of sand body analysis from very high resolution seismic data very high resolution seismic data from the kaiser i hind sand bank southern celtic sea recently highlighted the internal structure of the enigmatic celtic banks which are among the deepest and largest shelf sand ridges the main body of the bank is made up of 4 seismic depositional units which reflect a transgressive evolution new data on the detailed architecture of two of these units allow discussion of bank growth in terms of either 1 a channel levee system preserved both by lateral migration and aggradation of the channels or 2 a package of large offshore tidal sediment bodies bar chains and or giant dunes careful geometrical observations of seismic discontinuities make the second hypothesis more likely the unit architecture is analysed in terms of long to short term processes of build up long term processes are evinced by the landward stacking of erosive sub units in response of the last post glacial sea level rise whereas short term processes control the seaward progradation of sand bodies and fills due to the ebb predominance of the western channel approaches

middle to late holocene storminess in brittany nw france part ii the chronology of events and climate forcing this study focuses on the recurring climate conditions required for the largest storms occurring in nw france brittany it is based on the analysed records of storm events along western brittany coast see part i in this manuscript part ii storm recurrence is explored along with forcing mechanisms periods of more frequent storm events over the two last centuries are analysed first in order to link these events with possible forcing mechanisms north atlantic oscillation nao and atlantic multidecadal oscillation amo modes triggering the most destructive storms then palaeostorm events are discussed at the holocene scale from 6000 yr bp to present to verify the forcing mechanisms most recorded events appear to be linked with cooling episodes mostly in winter a transition to or from a negative winter nao mode a positive amo mode extreme storms occur immediately prior to the medieval warm period mwp maximum effects are reached prior to the onset of the mwp and during the maunder and dalton solar minima low storm activity occurred during the sporer minimum linked to an acceleration of the atlantic meridional overturning circulation amoc main storm triggers seem to correspond to a positive amo mode with an unstable jetstream configuration driving a negative nao in this study four specific weather configurations were defined to explain each type of recorded storminess the strongest storms correspond to low amo and decennial negative nao modes e g little ice age or high amo in association with dominant low nao modes as during the early middle age and present day period fresh or warm oceans in association with a positive nao mode are stormy but with very low sting storms frequency although in agreement with the orbital forcing and the holocene glacial history increasing storm frequency and intensity is most probably partly biased by continuous sea level rise and resulting erosion

asteroid impact vs deccan eruptions the origin of low magnetic susceptibility beds below the cretaceous paleogene boundary revisited abstract the respective roles of an asteroid impact and deccan traps eruptions in biotic changes at the cretaceous paleogene k pg boundary are still debated in many shallow marine sediments from around the world the k pg boundary is marked by a distinct clay layer that is often underlain by a several decimeter thick low susceptibility zone a previous study of the gubbio section italy lowrie et al 1990 attributed low magnetization intensity in this interval to post depositional dissolution of ferrimagnetic minerals dissolution was thought to be a consequence of downward infiltration of reducing waters that resulted from rapid accumulation of organic matter produced by mass extinctions after the k pg event we compare the magnetic properties of sediments from the gubbio section with those of the bidart section in southern france the two sections are similar in their carbonate lithology and the presence of a boundary clay and low susceptibility zone when compared to background cretaceous sediments the low susceptibility zone in both sections is marked by an absence of biogenic magnetite a decrease in total ferrimagnetic mineral content and a preferential loss of magnetite with respect to hematite features that are consistent with reductive dissolution however unlike the gubbio section where the low susceptibility zone starts immediately below the boundary clay the low susceptibility zone and the clay layer at bidart are separated by a 4 cm carbonate interval that contains abundant biogenic magnetite such separation casts doubt on a causal link between the impact and sediment bleaching more likely the low susceptibility layer marks a different environmental event that preceded the impact an episode of increased atmospheric and oceanic acidity associated with deccan traps volcanism that occurred well before the k pg impact is argued here to account for the distinct magnetic properties of the low susceptibility intervals

the superficial sediments of the english channel and its western approaches the english channel and its western approaches constitute a 700 km long epicontinental sea located in a temperate environment and in a tectonic setting where subsidence is minimal a sedimentological survey of this region reveals quite distinct provinces a central sector western channel where predominantly bioclastic sediments are widely represented bounded by two mainly terrigenous rich zones the outer terrace of the celtic sea and the eastern channel lateral sedimentary variations of decreasing grain size are interpreted in terms of current velocity patterns the various types of such sequences may relate to the degree of mixing of older lithoclastic sediments with the holocene bioclastic supply a scenario for the evolution of the recent sedimentation of this epicontinental sea is presented starting from a permanent marine zone at least since the wurm period that bounds the bay of biscay the holocene transgression progressed over the english channel in the western channel earlier terrigenous deposits were gradually overlapped by bioclastic sediments that originated on the pre mesozoic rocky substratum and which were particularly extensive off the armorican massif in the subsequently submerged eastern channel the pre holocene clastic source has undergone comparatively less modification and still crops out in most of the area with the opening of the straits of dover at about 9000 yr bp submersion was complete and new hydrodynamic conditions developed as the eustatic level stabilized the present sediment distribution is in harmony with the hydrodynamic setting except on the southern rise of the celtic sea where both morphology and sedimentary patterns still largely reflect pre holocene conditions resume la mer de la manche et ses approches occidentales constituent une plateforme epicontinentale qui s etend suivant un axe ne sw d environ 700 km depuis le pas de calais jusqu au rebord continental c est un milieu sedimentaire de climat tempere a subsidence negligeable et a faible taux de sedimentation la couverture de depots meubles est mince quelques metres en general sauf au niveau des bancs sableux et des paleovallees souvent comblees s alluvions anciennes la repartition des sediments superficiels est essentiellement controlee par les courants de maree particulierement forts le long des cotes armoricaines et en manche centrale en manche occidentale un important gradient de vitesses decroissantes se developpe du sud est au nord ouest les houles s exercent principalement sur la frange cotiere ou leur action devient dominante dans l ensemble les depbts fins sont rares et concentres dans les baies et les estuaires ou les suspensions se decantent par contre les sediments graveleux a caillouteux sont largement repartis une opposition nette se dessine entre une province orientale surtout terrigene et une province occidentale au materiel bioclastique abondant presentant un gradient de dispersion en direction du sud ouest ces caracteres morphologiques et sedimentaires du milieu sont fortement marques par son histoire quaternaire notamment par la derniere oscillation marine un schema de mise en place et d evolution du modele sedimentaire au cours du quatenaire recent est propose partant de la zone en permanence submergee durant le wurm la transgression holocene progressa sur la manche le pas de calais s ouvrit vers 9000 bp et peu a peu les conditions hydrodynamiques actuelles se mirent en place sous un climat devenu tempere progressivement les depots terrigenes anciens furent contamines par du materiel bioclastique holocene

the identification and biogeochemical interpretation of fossil magnetotactic bacteria magnetotactic bacteria which most commonly live within the oxic anoxic transition zone oatz of aquatic environments produce intracellular crystals of magnetic minerals specifically magnetite or greigite the crystals cause the bacteria to orient themselves passively with respect to the geomagnetic field and thereby facilitate the bacteria s search for optimal conditions within the sharp chemical gradients of the oatz the bacteria may also gain energy from the redox cycling of their crystals because magnetotactic bacteria benefit from their magnetic moments natural selection has promoted the development of traits that increase the efficiency with which the intracellular crystals impart magnetic moments to cells these traits also allow crystals produced by magnetotactic bacteria called magnetofossils when preserved in sediments to be distinguished from abiogenic particles and particles produced as extracellular byproducts of bacterial metabolism magnetofossils are recognizable based on their narrow size and shape distributions distinctive morphologies with blunt crystal edges chain arrangement chemical purity and crystallographic perfection this article presents a scheme for rating magnetofossil robustness based on these traits the magnetofossil record extends robustly to the cretaceous and with lesser certainty to the late archean because magnetotactic bacteria predominantly live in the oatz the abundance and character of their fossils can reflect environmental changes that alter the chemical stratification of sediments and the water column the magnetofossil record therefore provides an underutilized archive of paleoenvironmental information several studies have demonstrated a relationship between magnetofossil abundance and glacial interglacial cycles likely mediated by changes in pore water oxygen levels more speculatively a better developed magnetofossil record might provide constraints on the long term evolution of marine redox stratification more work in modern and ancient settings is necessary to explicate the mechanisms linking the abundance and character of magnetofossils to ancient biogeochemistry

lidar offshore structural mapping and u pb zircon monazite dating of variscan strain in the leon metamorphic domain nw brittany an exceptional structural picture of the immerged variscan basement offshore the leon metamorphic domain is supplied by high resolution lidar and echosounder data recorded in the molene archipelago western brittany france various types of fabrics are identified and from in situ rock sample analyses further combined with field structural data are interpreted on a lineament trajectory map as the trace of magmatic and tectonic structures our onshore offshore study leads us to propose a two phase kinematic model that emphasizes the role of a strike slip duplex in an ew trending relay zone linking the north armorican and pierres noires ductile shear zones nasz pnsz dextral shearing occurred within a transtensional setting synchronously with magmatic intrusions st renan granite and an offshore gabbro diorite complex dated at 314 320 ma by new u th pb ages it post dated an early regional foliation related to top to the ne ductile transpressional shearing our study emphasizes the key role of strike slip tectonics in the nw part of the armorica variscan belt

magnetic properties of pelagic marine carbonates abstract pelagic carbonates are deposited far from continents usually at water depths of 3000 6000 m at rates below 10 cm kyr and are a globally important sediment type recent advances with recognition of widespread preservation of biogenic magnetite the inorganic remains of magnetotactic bacteria have fundamentally changed our understanding of the magnetic properties of pelagic carbonates we review evidence for the magnetic minerals typically preserved in pelagic carbonates the effects of magnetic mineral diagenesis on paleomagnetic and environmental magnetic records of pelagic carbonates and what magnetic properties can tell us about the open ocean environments in which pelagic carbonates are deposited we also discuss briefly late diagenetic remagnetisations recorded by some carbonates despite recent advances in our knowledge of these phenomena much remains undiscovered we are only at early stages of understanding how biogenic magnetite gives rise to paleomagnetic signals in sediments and whether it carries a poorly understood biogeochemical remanent magnetisation recently developed techniques have potential for testing how different magnetotactic bacterial species which produce different magnetite morphologies respond to changing nutrient and oxygenation conditions future work needs to test whether it is possible to develop proxies for ancient nutrient conditions from well calibrated modern magnetotactic bacterial occurrences a tantalizing link between giant magnetofossils and paleogene hyperthermal events needs to be tested much remains to be learned about the relationship between climate and the organisms that biomineralised these large and novel magnetite morphologies rather than being a well worn subject that has been studied for over 60 years the magnetic properties of pelagic carbonates hold many secrets that await discovery

morphology internal structure and reversal of asymmetry of large subtidal dunes in the entrance to gironde estuary france abstract the gironde is a macrotidal estuary characterized by important changes in tidal current velocity and river discharge two surveys were made in the lower estuary in june 1987 and october 1989 in an area with large dunes the symmetry of these dunes 1 5 6 7 m high changes from flood dominated in upstream areas to ebb dominated in downstream areas a transition zone consists of symmetrical dunes with a very low vertical form index the internal structure of the dunes revealed by seismic records shows a hierarchy of bounding surfaces related to fluctuations in movement speed and asymmetry of the dunes the lowest part of symmetrical dunes consists of ebb oriented reflectors while the upper part has flood oriented reflectors the same profile lines run in 1989 show that each large dune can be identified despite an upstream shift of the transition zone of about 1000 m comparison of seismic records shows that some lower reflectors are preserved between the two surveys and that the dunes moved downstream a mean distance of 30 m in 1989 the internal structure of symmetrical dunes was reversed with flood oriented reflectors in the lower part overlain by ebb oriented reflectors both seasonal changes in river discharge and fortnightly oscillations of the tidal range control the magnitude and orientation of the net bed load transport in the study area because the response time of the large dunes is longer than that of the superimposed small dunes the reversal of asymmetry of the large dunes requires a long period process while fortnightly oscillation or even semi iurnal reversal of the tidal current during spring tides could cause reversal of small dunes

architecture and long term evolution of a tidal sandbank the middelkerke bank southern north sea abstract the internal structure of the middelkerke bank one of the flemish banks located in the southern north sea off the coast of oostende belgium has been studied in the framework of the marine science and technology mast program co funded by the european community a dense grid of high and very high resolution seismic profiles has been used as well as several vibrocorings seven major seismic units can be identified in the quaternary sediments bounded by major discontinuities correlated across the whole study area the lower units clearly appear as being deposited during periods of relative low sea level channel infillings shoreface estuarine and or ebb tidal delta deposits the present shape of the bank results partly from recent erosional processes reworking the underlying deposits thus the lower part of the bank as a morphological feature does not consist of offshore tidal sands the master bedding of the upper part of the bank consists of inclined reflectors dipping at an angle of about 5 in the same direction as the bank s steep face these reflectors very similar to those described by houbolt 1968 are interpreted as being the result of alternating periods of deposition and erosion related to the episodic combination of tidal currents and storms

size distributions of nanoparticles from magnetotactic bacteria as signatures of biologically controlled mineralization this paper addresses the problem of magnetite nanoparticle size distributions in magnetotactic bacteria the methods described in the paper can be used to determine the origin of natural magnetite nanoparticle samples we analyzed 36 histograms related to bacterial inorganic and biomimetic nanoparticle sizes using statistical software we concluded that the size of the nanoparticles in cultured magnetotactic bacteria follows an extreme value distribution magnetite in uncultured samples can be treated as a two component mixture containing extreme value and or log normally distributed nanoparticles analysis of the time dependent formation of bacterial magnetite revealed that the magnetite size distribution transforms from the initial log normal young bacterial culture through normal like toward the extreme value distribution evolved culture it seems that at a certain moment during bacterial magnetite formation the bacterial system starts to behave as a closed system the closing of the system must be followed by another unknown process because restriction of the nutrient supply into magnetosomes is insufficient for the generation of the extreme value distribution based on our analysis approximately 50 of the magnetite particles in the martian meteorite alh 84001 follow an extreme value distribution

long term evolution and internal architecture of a high energy banner ridge from seismic survey of banc du four western brittany france the recent completion of a coupled seismic and swath bathymetric survey conducted across the sand ridge system of the banc du four located on the atlantic continental shelf of brittany mer d iroise france provided new data for the study of the long term evolution of deep tidal sand ridges five seismic units are distinguished within the ridge separated by pronounced major bounding surfaces the basal unit is interpreted to be shoreface deposits forming the core of the ridge it is overlaid by a succession of marine sand dunes fields forming the upper units sandwave climbing which combines progradation and accretion is the major process controlling the growth of the ridge the elevation of the preserved dune foresets reaches values of about 20 30 m within the ridge the foresets indicate a combination of giant dunes characterized by numerous steep up to 20 clinoforms corresponding to a high energy depositional environment moreover the presence of scour pits linked to the 3d geometries of giant dunes allow the growth of bedforms migrating oblique to the orientation of giant dune crest lines all of the radiocarbon ages of the biogenic surficial deposits of the banc du four range from 10 036 to 2748 cal years b p and suggest the banc du four has grown during the last sea level rise the apparent absence of recent surface deposits could be caused by a change in benthic biogenic productivity or the non conservation of recent deposits in contrast the presence of relatively old sands at the top of the ridge could be explained by the reworking and leakage of the lower units that outcrop locally at the seabed across the ridge moreover the long term evolution of the ridge appears strongly controlled by the morphology of the igneous basement the multiphase accretion of the ridge is closely linked to the presence of a residual tidal current eddy consecutive with the progressive flooding of the coastal promontories and straits that structured the igneous basement therefore the banc du four should be thought of as a representative example of a large scale high energy banner bank

review of the low temperature magnetic properties of magnetite from a rock magnetic perspective the low temperature magnetic properties of magnetite are reviewed and implications for rock magnetism considered the behaviour of fundamental properties of magnetite at low temperatures near the verwey transition tv are documented and attention is given to various verwey transition theories the low temperature behaviour of the magnetic energies that control domain structure is reviewed in detail for the first time in rock magnetic literature the low temperature anomaly in spontaneous magnetization ms is documented and the differences between the saturation magnetization and ms near the verwey transition are discussed it is argued that the low temperature behaviour of the magnetocrystalline anisotropy and in particular the anomaly at tv is most likely to affect multidomain remanence during low temperature cycling for multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through tv is likely to reduce the stability of closure domains

low temperature magnetic properties of pelagic carbonates oxidation of biogenic magnetite and identification of magnetosome chains 1 pelagic marine carbonates provide important records of past environmental change we carried out detailed low temperature magnetic measurements on biogenic magnetite bearing sediments from the southern ocean ocean drilling program odp holes 738b 738c 689d and 690c and on samples containing whole magnetotactic bacteria cells we document a range of low temperature magnetic properties including reversible humped low temperature cycling ltc curves different degrees of magnetite oxidation are considered to be responsible for the observed variable shapes of ltc curves a dipole spring mechanism in magnetosome chains is introduced to explain reversible ltc curves this dipole spring mechanism is proposed to result from the uniaxial anisotropy that originates from the chain arrangement of biogenic magnetite similar to published results for uniaxial stable single domain sd particles the dipole spring mechanism reversibly restores the remanence during warming in ltc measurements this supports a previous idea that remanence of magnetosome chains is completely reversible during ltc experiments we suggest that this magnetic fingerprint is a diagnostic indicator for intact magnetosome chains although the presence of isolated uniaxial stable sd particles and magnetically interacting particles can complicate this test magnetic measurements through the eocene section of odp hole 738b reveal an interval with distinct magnetic properties that we interpret to originate from less oxidized biogenic magnetite and enrichment of a biogenic hard component co occurrence of these two magnetic fingerprints during the late eocene in the southern ocean indicates less oxic conditions probably due to increased oceanic primary productivity and organic carbon burial

the link between biomineralization and fossilization of bacteria insights from field and experimental studies abstract fossil biominerals and fossils of microorganisms in ancient rocks contain important biogeochemical signals decoding this record may reveal crucial information about the evolution of life on earth and terrestrial paleoenvironments however the identification of traces of life especially in very old rocks is extremely challenging because the morphological and chemical signatures of biominerals and microfossils are subtle of microscopic size and inevitably altered with aging in this review we stress on the fact that biomineralization is often the first step of fossilization and produces particular chemical structural and morphological features that can be preserved in fossil biominerals or microfossils with a special focus on fe biomineralization the taphonomic processes affecting biominerals and microfossils and altering their morphology and or chemical composition over time are then discussed in light of experimental fossilization simulations and field sample analyses we suggest that taxonomic biases observed in the fossil record may be related to differential abilities of species to trigger biomineralization this calls for studies of the effective biomineralizing activity and fossilization potential of each species present in highly diverse natural microbial communities finally recent analytical developments leave little doubt that very substantial progress in the study of biomineralization processes and ancient biostructures will be achieved in the near future

spatial arrangement of chains of magnetosomes in magnetotactic bacteria scanning and transmission electron microscopy sem tem were used to investigate the spatial arrangement of chains of magnetosomes in cells of two morphologically different types of magnetotactic bacteria which possess at least two chains magnetobacterium bavaricum and wildtype magnetic cocci the tem pictures show apparently very different arrangements of the chains within the cells stereo micrographs obtained by tilting individual bacteria in the tem however revealed that magnetosome chains in magnetic cocci always lie on opposite sides of the cell body and in close proximity to the cell envelope the rod shaped cells of m bavaricum contain up to 1000 bullet shaped magnetosomes forming 3 5 rope shaped bundles of magnetosomes arranged such that they are separated by the maximum possible distance from each other and positioned adjacent to the cell envelope these observations can be understood in terms of repulsion forces between parallel magnetic dipoles driving the chains apart from each other and forcing them to be in direct mechanical contact to the cell envelope thus the magnetic torque acting on the chains under the influence of the geomagnetic field can be transferred very effectively to the whole cell body moreover the formation of two or more chains inside the cell body is an effective means of achieving a magnetic moment large enough for a given cell size to overcome the viscous resistance with respect to the surrounding medium which is required to ensure an alignment with the geomagnetic field as fast as possible based on these two arguments and on the electron optical observations we hypothesize that all coccoid magnetotactic bacteria contain at least two chains of magnetosomes

sea level and deep water temperature changes derived from benthic foraminifera isotopic records we show that robust regressions can be established between relative sea level rsl data and benthic foraminifera oxygen isotopic ratios from the north atlantic and equatorial pacific ocean over the last climatic cycle we then apply these regressions to long benthic isotopic records retrieved at one north atlantic and one equatorial pacific site to build a composite rsl curve as well as the associated confidence interval over the last four climatic cycles our proposed reconstruction of rsl is in good agreement with the sparse rsl data available prior to the last climatic cycle we compute bottom water temperature changes at the two sites and at one southern indian ocean site taking into account potential variations in north atlantic local deep water 18o our results indicate that a last glacial maximum lgm enrichment of the ocean mean oxygen isotopic ratio of 0 95 is the lowest value compatible with unfrozen deep waters in the southern indian ocean if local deep water 18o did not increase during glacials with respect to present such a value of the lgm mean ocean isotopic enrichment would impose a maximum decrease in local bottom water 18o at the north atlantic site of 0 30 during glacials

magnetic anisotropy magnetostatic interactions and identification of magnetofossils 1 single domain magnetite particles produced by magnetotactic bacteria mtb and aligned in chains called magnetosomes are potentially important recorders of paleomagnetic paleoenvironmental and paleolife signals rock magnetic properties related to the anisotropy of magnetosome chains have been widely used to identify fossilized magnetosomes magnetofossils preserved in geological materials however ambiguities exist when linking magnetic properties to the chain structure because of the complexity of chain integrity and magnetostatic interactions among magnetofossils that results from chain collapse during post depositional diagenesis in this paper magnetic properties of three sets of samples containing extracted magnetosomes of the culturedmagnetospirillum magneticumstrain amb 1 were analyzed to determine how chain integrity and particle concentration influence magnetic properties intact mtb and well dispersed magnetosome chains are characterized by strong magnetic anisotropy and weak magnetostatic interactions but progressive chain breakup and particle clumping significantly increase the degree of magnetostatic interaction this results in a change of the magnetic signature toward properties typical of interacting single domain particles i e a decrease of the ratio of anhysteretic remanent magnetization to the saturation isothermal remanent magnetization decreasing in the crossing point of the wohlfarth cisowski test and in the delta ratio between losses of field and zero field cooled remanent magnetization across the verwey transition as well as vertical broadening of the first order reversal curve distribution we propose a new diagram that summarizes the verwey transition properties with diagnostic limits for intact and collapsed chains of magnetosomes this diagram can be used in conjunction with other parameters to identify unoxidized magnetofossils in sediments and rocks

island wakes in shallow coastal waters rattray island northeast australia is 1 5 km long 300 m wide and lies in well mixed water approximately 25 m deep its long axis is inclined at about 60 into the direction of the dominant semidiurnal tidal current the length of the wake in the lee of the island as documented by aerial photographs and satellite imagery appears to equal that of the wake behind a flat plate in a two dimensional flow at a reynolds number of about 10 however current metering drogues measurements and temperature mapping indicate internal wake velocities much greater than would be consistent with such a simple low reynolds number model further estimates of the turbulent eddy coefficient suggest an effective reynolds number more in the vicinity of 103 to reconcile these observational differences and to explain the observed upwelling in the core of the wake an ekman pumping model is proposed it is postulated that the ekman benthic boundary layer driven by rotation in the wake allows the vertical vorticity introduced into the water at the tip of the island at the point of separation to be negated by the vorticity of opposite sign introduced at the bottom further it is shown that a large fraction of the kinetic energy of the upstream flow facing the island is used to drive the wake eddies leading to the conclusions that the trapping of water in the lee of islands greatly increases head losses on continental shelves with numerous islands coral reefs and rock outcrops

types of internal structure and external morphology of submarine dunes under the influence of tide and wind driven processes dover strait northern france abstract the expression of tide and wind driven processes in the internal architecture and external morphology of very large submarine dunes is investigated at a dune field located in the central part of the dover strait southern north sea current measurements and sediment transport calculations enable quantification of the dominant tide and wind driven processes and very high resolution seismic recordings are used to analyse internal structure and external morphology of dunes in the study area moderate to storm winds induce temporary currents that can lead to the reversal of sediment transport direction and induced erosive processes on the seabed dune master bedding consists of extended erosive surfaces with 9 11 slopes that bound storm generated sets of cross beds 1 5 4 85 m thick 2 65 m on average three types of architectures are observed within the dune field and occasionally within single dunes these types which display a strong linkage between internal structure and external morphology consist of 1 asymmetric dunes with cosets which dip in the direction of the marked external asymmetry of the dunes 2 dunes with a complex external morphology composed of mega herringbone structures each displaying an opposing progradation direction and 3 dunes with a symmetric megaripple bedding unit at their summit the diversity of the internal architecture and external morphology results from the variable combinations of tide and wind driven processes occurring in the study area the asymmetry of tidal peak current velocity and the relative strength of wind driven currents are the parameters that have the most important influence on dune architecture asymmetric dunes with cosets result from a strong asymmetry in tidal peak current velocity which is reversed only slightly during storms dunes with mega herringbone structures and dunes with symmetric megaripple units at their summit are observed in areas where the asymmetry of the tidal peak current velocity is negligible allowing reversal of the sediment transport and dune migration directions each time moderate winds blow the latter architectural type is specific to sand rich areas the different dune architecture types display strong geometrical similarities to structures observed in dunes from other shelf environments where driving processes are different this implies that a study based only on a geometric description of internal structures will not enable the reconstitution of ancient depositional environments a better discriminant criteria seems to be the thickness of the second order sets of cross beds which is correlated with specific hydro sedimentary processes dune height also probably has an important influence on the thickness of second order sets of cross beds as dune height and set thickness display a clear relationship in the present study

chains clumps and strings magnetofossil taphonomy with ferromagnetic resonance spectroscopy magnetotactic bacteria produce intracellular crystals of magnetite or greigite the properties of which have been shaped by evolution to maximize the magnetic moment per atom of iron intracellular bacterial magnetite therefore possesses traits amenable to detection by physical techniques typically narrow size and shape distributions single domain size and arrangement in linear chains and often crystal elongation past strategies for searching for bacterial magnetofossils using physical techniques have focused on identifying samples containing significant amounts of single domain magnetite or with narrow coercivity distributions searching for additional of traits would however increase the likelihood that candidate magnetofossils are truly of biological origin ferromagnetic resonance spectroscopy fmr is in theory capable of detecting the distinctive magnetic anisotropy produced by chain arrangement and crystal elongation here we present analyses of intact and lysed magnetotactic bacteria dilutions of synthetic magnetite and sedimentary samples of modern carbonates from the great bahama bank oligocene miocene deep sea muds from the south atlantic and pleistocene lacustrine deposits from mono basin california we demonstrate that fmr can distinguish between intact bacterial magnetite chains collapsed chains and linear strings of magnetite formed by physical processes we also show that sediments in which the magnetization is likely carried by bacterial magnetite have fmr spectra resembling those of intact or altered bacterial magnetite chains