fast and slow spreading ridges structure and hydrothermal activity ultramafic topographic highs and ch4 output different parts of the world ridge system have quite different morphologies which reflect different constructional processes it appears that hydrothermal circulation at all spreading centers is an important exchange process between the ocean and the newly formed oceanic crust this hydrothermal circulation may vary according to morphology and crustal composition and may also affect ridge constructional processes the tag 26 n and snake pit 23 n hydrothermal sites on the mid atlantic ridge mar display mn ch4 ratios in overlying seawater similar to those of nonsedimented east pacific rise sites i e about 0 2 mol l in contrast large methane anomalies with very low mn ch4 ratios of 0 005 mol l are associated with ultramafic topographic highs near 15 n close to the axis and on ultramafic walls of the rift valley the association of ultramafic bodies and ch4 anomalies in seawater indicates active serpentinization processes ch4 is produced during serpentinization according to the fisher tropsch reaction charlou et al 1991 many ultramafic rock assemblages have been sampled on the mar away from fracture zones in particular by the deep sea drilling project changes of mechanical properties and of density of uplifted deep material in the accreting plate boundary zone caused by serpentinization may play an important role in the construction of slow spreading ridges

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

magnetostratigraphic data from late quaternary sediments from the yermak plateau arctic ocean evidence for four geomagnetic polarity events within the last 170 ka of the brunhes chron summary palaeomagnetic investigations of two sediment cores recovered from rv polarstern near the eastern slope of the yermak plateau sites ps 1533 and ps 2212 reveal convincing evidence for four polarity events of the earth s magnetic field during the last 170 ka a comprehensive rock magnetic study of the sediments proved that fine grained magnetite is the principal carrier of the remanent magnetization no changes in magneto mineralogy across the polarity transitions in the sediments investigated were found calcareous nannofossil biostratigraphy ams 14c accelerated mass spectrometry and oxygen isotope data and 10be and 230th stratigraphies yielded age ranges of 24 29 ka for the mono lake event 34 43 ka for the laschamp event 72 86 ka for the norwegian greenland sea event and 118 128 ka for the blake event two reverse polarity samples at the base of core ps 2212 3 kal are interpreted as the termination of the biwa i event 171 181 ka the events exhibit full inversion of inclination in both cores the data suggest that the transition process of the earth s magnetic field during such polarity events requires some 1 ka

observations and modeling of alongshore variability in dune erosion at egmond aan zee the netherlands abstract dunes can erode within a few hours when exposed to high storm surge levels and large waves if the dunes are the primary defense such as in the netherlands this could result in flooding of the hinterland when dunes breach models are often used to analyze how dunes will respond to extreme conditions but they can also be applied to study milder events and with that gain more insight in the processes of dune erosion there is however growing demand for validating existing models under field conditions here we calibrate the xbeach dune erosion model with pre and post storm topography measurements of the dune erosion event in january 2012 at egmond aan zee the netherlands furthermore hydrodynamical data of the intertidal zone along a cross shore transect in the same area collected 3 months prior to the storm event are used to hydrodynamically calibrate and validate xbeach during the january 2012 storm the dune erosion was variable alongshore from the erosion of embryo dunes the forming of a dune scarp to considerable slumping of the entire dune face this caused the erosion volume v to range from 5 to 40 m3 m the calibrated model reproduces the alongshore variability in v with reasonable accuracy additional simulations show that the alongshore variation in v is due to variability on the initial dune topography primarily steepness and that variability in beach and nearshore bathymetry was of secondary importance

evaluation of xbeach performance for the erosion of a laboratory sand dune abstract a new set of laboratory data is used to investigate the bathymetry change of a steep sand dune exposed to waves and high water levels and subsequently compared to the results of numerical simulations using xbeach bichromatic wave boundary conditions are used to simulate a combined short wave and long wave field for two water level elevations corresponding to collision with the dune face and overwashing of the dune crest in the collision regime case episodic slumping due to the undercutting of the dune results in sudden erosional events followed by long periods of wave driven reshaping at the dune toe in the overwash regime case morphological changes are faster and sediment transport rates are higher the xbeach model was used to simulate wave driven erosion of the dune at the two water levels observed in the laboratory the model was not able to precisely recreate the cross shore spatial variability of significant wave height observed in the experiments however near bed wave orbital and mean current velocities were in good agreement with observations following rapid initial adjustment the model results were in agreement with measured dune morphology at successive times xbeach was sensitive to several parameters that control the rate of erosion including the critical avalanching slope under water the threshold water depth and the sediment transport formulation and performed well after careful selection of the best combination of these parameters overall the model predictions were in better agreement with laboratory observations for dune erosion in the overwash regime case than the collision regime case

beach and dune erosion during storm surges this paper presents the set up and results of an extensive research programme concerning the erosion of coastal dunes during storm surges a large number of two dimensional and three dimensional mobile bed model tests has been carried out to investigate the process of dune erosion the state of art after a series of small scale tests is summarized attention is focussed on large scale tests carried out in the delta flume with random waves up to 2 m significant height sediment concentration and orbital velocity measurements are discussed the large scale tests have confirmed the validity of a modelling technique based on the dimensionless fall velocity parameter h tw the model results are being applied to check the safety of existing coastal dunes as a water retaining structure that has to protect the major part of the netherlands from inundation during storm surges

improving predictions of swash dynamics in xbeach the role of groupiness and incident band runup abstract in predicting storm impacts on sandy coasts possibly with structures accurate runup and overtopping simulation is an important aspect recent investigations stockdon et al 2014 palmsten and splinter 2016 show that despite accurate predictions of the morphodynamics of dissipative sandy beaches the xbeach model roelvink et al 2009 does not correctly simulate the individual contributions of set up and infragravity and incident band swash to the wave run up in this paper we describe an improved numerical scheme and a different way of simulating the propagation of directionally spread short wave groups in xbeach to better predict the groupiness of the short waves and the resulting infragravity waves the new approach is tested against field measurements from the delilah campaign at duck nc and against video derived runup measurements at praia de faro a relatively steep sandy beach compared to the empirical fit by vousdoukas et al 2012 the xbeach model performs much better for more extreme wave conditions which are severely underestimated by existing empirical formulations for relatively steep beaches incident band swash cannot be neglected and a wave resolving simulation mode is required therefore in this paper we also test the non hydrostatic wave resolving model within xbeach for runup and overtopping against three datasets results for a high quality flume test show non hydrostatic xbeach predicts the run up height with good accuracy maximum deviation 15 a case with a very shallow foreshore typical for the belgian coast at wenduine was compared against detailed measurements overall the model shows correct behavior for this case finally the model is tested against a large number 551 of physical model tests of overtopping from the clash database for relatively high overtopping discharges the non hydrostatic xbeach performs quite well with increasing accuracy for increasing overtopping rates however for relatively low overtopping rates of less than 10 20 l m s the model systematically underestimates measured overtopping rates

modelling storm impacts on beaches dunes and barrier islands a new nearshore numerical model approach to assess the natural coastal response during time varying storm and hurricane conditions including dune erosion overwash and breaching is validated with a series of analytical laboratory and field test cases innovations include a non stationary wave driver with directional spreading to account for wave group generated surf and swash motions and an avalanching mechanism providing a smooth and robust solution for slumping of sand during dune erosion the model performs well in different situations including dune erosion overwash and breaching with specific emphasis on swash dynamics avalanching and 2dh effects these situations are all modelled using a standard set of parameter settings the results show the importance of infragravity waves in extending the reach of the resolved processes to the dune front the simple approach to account for slumping of the dune face by avalanching makes the model easily applicable in two dimensions and applying the same settings good results are obtained both for dune erosion and breaching

modeling dune response to an east coast low coastal dunes can act as a method of soft coastal protection against inundation and direct impact of waves during storms if they are substantially large enough in volume to withstand erosion without breaching however the time evolution of sand dunes under direct wave impact is not well understood and many available models require site specific calibration and have had limited verification at field scales here we test three models of varying complexity in their ability to predict both dry beach erosion volumes and dune to a retreat for an east coast low storm event that occurred on the gold coast australia the process based model xbeach which models the entire profile was able to reproduce both dune toe retreat and dry beach volume however was sensitive to calibration parameters the two parametric models that only modeled erosion above the initial dune toe position were capable of accurately predicting dune toe retreat however under estimated dry beach erosion volumes with no calibration the parametric model proposed by palmsten and holman 2012 produced the smallest errors of dune toe retreat with mean error in final dune position of 6 6 m or 18 of the total measured dune retreat with minimal calibration estimated absolute error in average dune toe retreat was less than 13 of observed retreat for all three models

two dimensional time dependent hurricane overwash and erosion modeling at santa rosa island abstract a 2dh numerical model which is capable of computing nearshore circulation and morphodynamics including dune erosion breaching and overwash is used to simulate overwash caused by hurricane ivan 2004 on a barrier island the model is forced using parametric wave and surge time series based on field data and large scale numerical model results the model predicted beach face and dune erosion reasonably well as well as the development of washover fans furthermore the model demonstrated considerable quantitative skill upwards of 66 of variance explained maximum bias 0 21 m in hindcasting the post storm shape and elevation of the subaerial barrier island when a sheet flow sediment transport limiter was applied the prediction skill ranged between 0 66 and 0 77 in a series of sensitivity tests in which several hydraulic forcing parameters were varied the sensitivity studies showed that the variations in the incident wave height and wave period affected the entire simulated island morphology while variations in the surge level gradient between the ocean and back barrier bay affected the amount of deposition on the back barrier and in the back barrier bay the model sensitivity to the sheet flow sediment transport limiter which served as a proxy for unknown factors controlling the resistance to erosion was significantly greater than the sensitivity to the hydraulic forcing parameters if no limiter was applied the simulated morphological response of the barrier island was an order of magnitude greater than the measured morphological response

spatial and temporal evaluation of storm induced erosion modelling based on a two dimensional field case including an artificial unvegetated research dune abstract the objective of this study was to evaluate the temporal and spatial performance of the xbeach model predicting storm surge induced erosion and breaching on the basis of a two dimensional field case within the scope of a physical experiment a large scale artificial unvegetated research dune was built at the german baltic sea coastline and surveyed before during and after a surge event to generate high resolution digital elevation models of the temporal and spatial failure process the measured data was used to set up and calibrate two dimensional xbeach models the numerical results showed that the default settings of the latest xbeach x release led to a very good agreement bss 0 82 between the simulated and measured post storm bed levels in the area of the research dune while a higher agreement was observed on the overwash and inundation dominant eastern side than on the collision dominant western side evaluating the model accuracy during the storm surge using 44 digital elevation models of the seaside slope showed a spatially accurate prediction of the two observed breaches 5 m but also the occurrence of additional breaches with respect to timing and breach width evolution however the results highly deviated from the observation and were found to be very sensitive to changes in grid resolution and to changes of default parameters exchanging the nearshore bathymetry with measurements one and two months prior to the physical experiment where a 49 and 51 lower subaerial beach volume was observed resulted in a significantly overestimated dune erosion 31 and 33 and hence in a lower accuracy of the default model bss 0 69 overall the xbeach model is capable of accurately predicting the evolution of storm induced erosion during a sufficiently long and overwash inundation dominant surge event when current high resolution input data is available

foredunes and blowouts initiation geomorphology and dynamics this paper reviews the initiation dynamics geomorphology and evolution of incipient foredunes established foredunes and blowouts incipient foredunes may be initiated in a variety of ways leading to the formation of ramps terraces or ridges depending on progradation rates vegetation type and cover sediment transport rates and scale of erosional processes vegetation cover morphology growth rates and types combine with aerodynamic processes to determine dune and swale morphology established foredunes are classified into five morpho ecological types and the flow behaviour over two types is detailed a new model of foredune evolutionary paths at various possible time scales is presented blowout initiation dynamics and sediment transport and evolution is examined throughout the review the gaps in present understanding of dune processes and a range of ideas for new research possibilities are provided

a precursor to the matuyama brunhes transition field instability as recorded in pelagic sediments abstract the period some 20 25 kyr just prior to the most recent generally recognized geomagnetic field polarity transition the matuyama to brunhes reversal appears to be marked by significant geomagnetic variability manifested as pronounced oscillations in intensity we compare several previously published paleomagnetic records with new high resolution paleomagnetic data obtained from five pelagic marine sites north atlantic dsdp hole 609b equatorial atlantic odp hole 665a and western equatorial pacific odp holes 803b 804c and 805b using standard rock magnetic normalization for all of the samples as well as a thellier thellier method on the sediments of hole 804c we consistently find a decrease in paleointensity dip of 1 approximately 15 kyr prior to the matuyama to brunhes transition in the five new records as well as in the previously published records despite sedimentation accumulation rates sar that range from 11 cm kyr to 1 cm kyr these sequences yield paleointensity curves that are broadly similar in form even at the lowest sars the intensity of the pre reversal low dip1 appears to be of the same magnitude as that of the transition itself dip2 of 1 in some of the records a directional excursion to nearly full normal polarity accompanies dip1 and remains after alternating field af and or thermal demagnetization whereas in other records the directional changes vanish with demagnetization and appear to be caused by overprinting a viscous remanent magnetization vrm contribution to nrm was identified in two of the records and until removed by af or thermal demagnetization was found to blur the double dip nature of the paleointensity profiles into an apparent single dip and also resulted in an apparent but erroneous sawtooth like post transitional sudden increase in paleointensity after appropriate normalization the magnitude of the post transitional recovery was much reduced the magnetic directions of three of the new records after cleaning and adjusting the stable declinations to either 0 normal or 180 reverse map to vgp positions lying in the pacific the directional variations however are far less consistent than the intensity variations the confirmed global existence of this dip so closely preceding a major reversal invites questions about its relation to the reversal itself the apparent normal character of this interval can also present problems for magnetostratigraphical interpretations based on coarse or incomplete sampling by mimicking the brunhes matuyama reversal some 15 kyr earlier in the section than its true location

modeling cross shore sandbar behavior on the timescale of weeks we compare predictions of a coupled wave averaged cross shore waves currents bathymetric evolution model to observations of onshore and offshore nearshore sandbar migration the observations span a 10 and 44 day period with onshore offshore bar migration at duck north carolina and at hasaki kashima coast japan respectively a 3 5 month period of onshore bar migration at duck and a 22 day period of offshore bar migration at egmond netherlands with best fit parameter values the modeled temporal evolution of the cross shore bed profiles agrees well with the observations model skill defined as 1 minus the ratio of prediction to no change error variances ranges from 0 50 at egmond to 0 88 for the prolonged onshore bar migration at duck localized in time and space reductions in model skill coincide with alongshore variations in the observed morphology consistent with earlier observations simulated offshore bar migration takes place during storms when large waves break on the bar and is due to the feedback between waves undertow suspended sediment transport and the sandbar simulated onshore bar migration is predicted for energetic weakly to nonbreaking conditions and is due to the feedback between near bed wave skewness bedload transport and the sandbar with negligible to small effects of bound infragravity waves and near bed streaming under small waves and conditions when breaking and nonbreaking conditions alternate with the tide the sandbar is predicted to remain stationary the intersite differences in the optimum parameter values are at least partly induced by insensitivity to parameter variations parameter interdependence and errors in the offshore wave forcing

an example of a recent accretion on the mid atlantic ridge the snake pit neovolcanic ridge mark area 23 22 n abstract the inner rift vailey of the mark area mid atlantic ridge at the kane fracture zone was investigated with the submersible nautile at the latitude of the snake pit hydrothermal field 23 22 n a transect across the axial valley shows a stepwise increase in the sediment thickness away from the axis these steps probably reflect major magmatic and tectonic events in the evolution of the inner floor dives on the summit of the snake pit neovolcanic ridge constrain a generalized geological map and document relations between hydrothermal activity magmatism and faulting the neovolcanic ridge was constructed by the build up of several hundred metres of pillow lava flows its summit is intensely fissured and cut by an axial graben about 100 m wide three hydrothermal sites were recognized within the axial graben the eastern site the most active is structurally related to the eastern fault scarp the central site is located within the graben the western site associated with the western fault scarp is more complex and was constructed by two successive hydrothermal events the new data suggest a model of evolution for the inner floor of the rift valley involving alternating stages of magmatism and volcanism which produce ridges and troughs preserved outside of the axial domain these volcano tectonic cycles have an approximate duration of 100 000 years we discuss variations in the morphology of the mar along strike in the light of this model of evolution

prehistoric earthquake deformations near masada dead sea graben earthquake inducedfluidizations and suspensions of lake sediments associated with syndepositionalfaults formapaleoseismicrecordinthedeadseagraben theassociation offluidized beds with surface faulting supports the recognition of mixed layers as reliable earthquake indicators and provides a tool for the study of very long term 70 kar seismicity along the dead sea transform the faults compose a fault zone that offsets laminated sediments of the late pleistocene lake lisan they exhibit displacements of as much as 2 m layers of massive mixtures of laminated fragments are interpreted as disturbed beds each formed by an earthquake the undisturbed laminated layers between these mixed layers represent the interseismic interval a typical vertical slip of about 0 5 m per event is separated by several hundred years of quiescence the fault zone lies within the dead sea graben 2 km east of masada where archaeology and historical accounts indicate repeated strong earthquake damage the distribution of strikes in the fault zone resembles that of the faults exposed in and around the graben including the seismogenic ones the excellent exposures over hundreds of metres allow an unprecedented temporal and spatial resolution of slip events on faults

the predictability of cross shore bed evolution of sandy beaches at the time scale of storms and seasons using process based profile models deterministic and probabilistic profile models have been compared with hydrodynamic and morphodynamic data of laboratory and field experiments on the time scale of storms and seasons the large scale laboratory experiment is a pure 2d case and offers and ideal test case for cross shore profile models as disturbing alongshore non uniformities are absent the field experiments are performed at the egmond site the netherlands during the eu coast3d project and represent storm time scale oct nov 1998 as well as seasonal time scale conditions may 1998 sep 1999 the objective of the paper is to present information of coastal processes on these time scales and to assess the predictive capabilities coastal process based profile models with respect to hydrodynamics and morphodynamics at sandy beaches on the time scales of storms and seasons profile models can quite accurately errors smaller than 10 represent the cross shore significant wave height distribution in the surf zone if the wave breaking model is properly calibrated the wave breaking coefficient should be a function of local wave steepness and bottom slope for most accurate results profile models can reasonably represent the cross shore and longshore currents undertow in a pure 2d case and in 3d field conditions profile models including cross shore mixing effects and breaker delay effects do not produce better predictions of the longshore and cross shore current velocities profile models using default settings can quite reasonably simulate the behaviour of the outer and inner bars on the storm time scale the behaviour of the beach cannot be modelled with sufficient accuracy on the storm time scale profile models can reasonably simulate the post storm onshore bar migration provided that the near bed orbital velocities and wave asymmetry related sand transport are represented in a sufficiently accurate way using non linear wave theories profile models cannot simulate the beach recovery processes on the post storm time scale because these essentially 3d processes are not sufficiently known to be included in the models profile models using default settings cannot simulate the behaviour of the outer and inner bars and the beach on the seasonal time scale the behaviour of the outer bar on the seasonal time scale can only be represented properly after tuning using measured bed profiles the simulation of the inner bar and beach morphology on the seasonal time scale could not be improved by tuning

large scale dune erosion tests to study the influence of wave periods article i nfo large scale physical model tests were performed to quantify the effects of the wave period on dune erosion attention was focussed on 2d cross shore effects in a situation with sandy dunes and extreme water levels and wave conditions besides profile measurements detailed measurements in time and space of water pressure flow velocities and sediment concentrations were performed in the near near shore area it was concluded that a longer wave period leads to a larger dune erosion volume and to a larger landward retreat of the dune face tests with double peaked wave spectra showed that the influence of the spectral shape on dune erosion was best represented by the tm 1 0 spectral mean wave period better than the peak wave period tp the effect of the wave period on dune erosion was implemented in a dune erosion prediction method that estimates erosion volumes during normative storm conditions for the dutch coast more details of the measurements and additional analyses of physical processes are described in an accompanying paper by van thiel de vries et al van thiel de vries j s m van gent m r a reniers a j h m and walstra d j r submitted for publication analysis of dune erosion processes in large scale flume experiments in this volume of coastal engineering

correlation of paleointensity variation records in the brunhes matuyama polarity transition interval abstract paleomagnetic records of the 0 78 ma brunhes matuyama b m polarity reversal interval described from high resolution 8 11 cm kyr deep sea sediments from western equatorial pacific odp sites 767b and 769a show two successive and marked decreases in paleointensity of about 15 kyr wavelength each and a shift from full reversed to fuli normal polarity directions in only 2 kyr associated with the uppermost paleointensity decrease in contrast transition records from deep sea sediments with lower sedimentation rates tend to show simpler paleointensity variation with more complex patterns of directional change for example the b m transition record described from deep sea sediment core v16 58 from the southern indian ocean we suggest that the twin 10 be peaks reported in v16 58 correspond to increased production of this cosmogenic isotope associated with the double paleointensity dips recorded in odp 767b 769a the low resolution v16 58 paleomagnetic record can be accounted for by modeling of the geomagnetic field history in odp 767b at a nominal recording rate of 1 cm kyr and using an exponential lock in process filter with a half fixing depth of 16 cm the double paleointensity dips become a single broad magnetization low and the change in paleomagnetic directions is offset by about 16 cm below the actual level of the polarity transition similar to what we infer for v16 58 the low resultant magnetizations will be especially prone to overprinting which if in the present field will tend to give an americas vgp path the main features of the 10 be profile will be unaffected by the remanence lock in process and consequently retain their usefulness for correlation as a proxy of geomagnetic dipole intensity variation

famous and amar segments on the mid atlantic ridge ubiquitous hydrothermal mn ch4 δ3he signals along the rift valley walls and rift offsets dynamic hydrocast experiments enabled mn tdm ch4 concentrations and 3he ratio to be recorded through vertical cross sections of hydrothermal plumes along the famous segment and the southern part of the amar segment on the mid atlantic ridge between 36 n and 37 n mn ch4 and 3he figures all along both segments are well above the seawater background in the open ocean they are interpreted to be the result of time integrated hydrothermal discharges dispersed and mixed in a closed basin delineated by the rift valley and the segment ends hydrothermal activity along the famous and amar segments appears to be similar a comparison of the residence times of the three tracers from the dispersed time integrated signals is proposed although the background values in these closed basin are high some proximal and or large hydrothermal inputs overprinted on the general time integrated plume can be detected i e the rainbow site south of amar based on the depth and the location of plumes hydrothermal activity is not by far limited to the neo volcanic inner floor of the valley and should involve the walls and complex offsets of the rift valley considering the mn and ch4 concentrations in these plumes two types of ocean mantle interaction may be represented hot focused discharges on ultramafic exposures rainbow site and low temperature diffuse serpentinisation

a novel eye in eyeless shrimp from hydrothermal vents of the mid atlantic ridge rimicaris exoculata1 is a shrimp that swarms over high temperature 350 c sulphide chimneys at mid atlantic ridge hydrothermal fields 3 600 m 1 7 this shrimp lacks an externally differentiated eye1 having instead a pair of large organs within the cephalothorax immediately beneath the dorsal surface of the transparent carapace connected by large nerve tracts to the supraesophageal ganglion these organs contain a visual pigment with an absorption spectrum characteristic of rhodopsin ultra structural evidence for degraded rhabdomeral material suggests the presence of photoreceptors no image forming optics are associated with the organs we interpret these organs as being eyes adapted for detection of low level illumination and suggest that they evolved in response to a source of radiation associated with the environment of hydrothermal vents

modelling storm induced beach dune evolution sefton coast liverpool bay uk storm induced dune evolution on a sandy coastal system is investigated using a nested modelling approach applied to the sefton coast liverpool bay uk real time offshore water levels and waves were used as model boundary forcings a delft3d coarse grid setup is used to simulate time and space varying sea surface elevations on which offshore waves are transformed by applying the swan model to establish the wave boundary for the high resolution morphological model xbeach statistical comparisons between model predicted and measured post storm profiles at a number of locations along the coast suggest that xbeach successfully captures storm induced beach change along the sefton coast predicted bed evolution of the beach dune system shows alternate erosion and sedimentation areas in the nearshore strong bed level changes are found at the northern part of the sefton coast when north westerly nw extreme waves and winds coincide with spring high tide morphological changes in the southern part are significantly lower than that in the north as a result of nw wave dissipation on the shoals located to the north of the crosby channel which creates low wave actions in that area in addition erosion of the dune foot is observed at some locations along the beach temporal simulation of beach dune evolution as a result of variable forcing conditions during storms provides useful insight into the morphodynamic processes of beach dune systems during storms using sefton as an example which is very useful for developing coastal management strategies over the existing conceptual tools

late pleistocene geomagnetic excursion in icelandic lavas confirmation of the laschamp excursion in 1980 kristjansson and gudmundsson 1 reported a late glacial geomagnetic excursion in three hills in the reykjanes peninsula iceland with shallow negative inclinations and westerly declinations they named it the skalamaelifell excursion more extensive field work has identified the same excursional paleomagnetic direction declination 258 inclination 15 at four additional outcrops in a 10 10 km area in the reykjanes peninsula the excursion lavas are olivine tholeiites with similar petrography and chemical compositions paleointensity determinations by the thellier method average 4 2 0 2 t for 8 samples more than an order of magnitude weaker than the present geomagnetic field in iceland together these results suggest extrusion of the excursion lavas in a very brief span of time probably less than a few hundred years kar dating of the excursion lavas gives a mean age for 19 determinations of 42 9 7 8ka 2 compilation of thirty kar ages of the laschamp and olby flows by three laboratories yield a new age for the laschamp excursion in france of 46 6 2 4ka 2 the age of the excursion in southwestern iceland is statistically indistinguishable from the laschamp excursion at the 95 confidence level and both have very low paleointensities therefore we suggest that the laschamp and olby flows in france and the skalamaelifell units of iceland recorded essentially the same geomagnetic excursion differences in the virtual paleomagnetic poles vgps of these excursions may be due to 1 the probable non dipole character of the geomagnetic field during the excursion 2 rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in france and iceland and or 3 crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations

numerical simulation of a low lying barrier island s morphological response to hurricane katrina abstract tropical cyclones that enter or form in the gulf of mexico generate storm surge and large waves that impact low lying coastlines along the gulf coast the chandeleur islands located 161 km east of new orleans louisiana have endured numerous hurricanes that have passed nearby hurricane katrina landfall near waveland ms 29 aug 2005 caused dramatic changes to the island elevation and shape in this paper the predictability of hurricane induced barrier island erosion and accretion is evaluated using a coupled hydrodynamic and morphodynamic model known as xbeach pre and post storm island topography was surveyed with an airborne lidar system numerical simulations utilized realistic surge and wave conditions determined from larger scale hydrodynamic models simulations included model sensitivity tests with varying grid size and temporal resolutions model predicted bathymetry topography and post storm survey data both showed similar patterns of island erosion such as increased dissection by channels however the model under predicted the magnitude of erosion potential causes for under prediction include 1 errors in the initial conditions the initial bathymetry topography was measured three years prior to katrina 2 errors in the forcing conditions a result of our omission of storms prior to katrina and or errors in katrina storm conditions and or 3 physical processes that were omitted from the model e g inclusion of sediment variations and bio physical processes

long term earthquake clustering a 50 000 year paleoseismic record in the dead sea graben the temporal distribution of earthquakes in the dead sea graben is studied through a 50 000 year paleoseismic record recovered in laminated sediments of the late pleistocene lake lisan paleo dead sea the lisan represents more than 10 times the 4000 years of historical earthquake records it is the longest and most complete paleoseismic record along the dead sea transform and possibly the longest continuous record on earth it includes unique exposures of seismite beds earthquake induced structures associated with slip events on syndepositional faults the seismites are layers consisting of mixtures of fragmented and pulverized laminae the places where the seismites abut syndepositional faults are interpreted as evidence for their formation at the sediment water interface during slip events on these faults thicker sediment accumulation above the seismites in the downthrown blocks indicates that a seismite formed at the water sediment interface on both sides of the fault scarps modern analogs and the association with surface ruptures suggest that each seismite formed during a m l 5 5 earthquake the 230 th 234 u ages of a columnar section obtained by thermal ionization mass spectrometry give a mean recurrence time of 1600 years of m l 5 5 earthquakes in the dead sea graben the earthquakes cluster in 10 000 year periods separated by quiet periods of similar length this distribution implies that a long term behavior of the dead sea transform should be represented by a mean recurrence of at least 20 000 year record this observation has ramifications for seismic hazard assessment based on shorter records

geochronology of tag and snakepit hydrothermal fields mid atlantic ridge witness to a long and complex hydrothermal history geochronological studies of a large number of precipitates from the tag hydrothermal field and of few samples from snakepit hydrothermal field of the mid atlantic ridge show intermittent repeated hydrothermal events at both sites 210pb pb and 230th 234u measurements of sulfides iron and manganese oxides and 14c measurements of carbonates combined with observations of hydrothermal events recorded as discrete layers in sediment cores provide the basis for unravelling the temporal history of the fields the tag field shows intermittent activity over the past 120 000 years as evidenced by ages of low temperature mn oxides the presently active black smoker mound first formed about 40 000 50 000 years ago with precipitation of massive sulfides it has had intermittent pulsed high temperature activity every 5000 6000 years over the past 20 000 years which may reflect renewed magmatic activity at the ridge axis fluid flow is focussed at the mound site by structural and tectonic control suggested by the intersection of n s ridge parallel lystric normal faults and an e w transform fault periods of inactivity are marked by covering of the mound with pelagic carbonate ooze which is probably partially dissolved and reprecipitated as aragonite at the end of each high temperature event the snakepit field had an initial event about 4000 years ago probably shortly after the eruption of the volcanic ridge on which it sits a recent renewal still presently active was probably initiated by recent fissuring of the volcanic pile

a sixty thousand year paleomagnetic record from gulf of california sediments secular variation late quaternary excursions and geomagnetic implications a unique inclination record of geomagnetic secular variation for the past 60 000 years 60 ka has been obtained from continuously deposited sediments in the gulf of california recovered during deep sea drilling project dsdp leg 64 by hydraulic piston coring at site 480 the chronology of hole 480 was determined by 18o stratigraphy and varve counts indicating sedimentation rates approaching 1 m ka the paleomagnetic results of the upper 49 m show 1 the average inclination over this interval is identical to the geocentric axial dipole value at the sampling site 2 excursion directions occur between about 53 and 22 ka before present bp during this time the geomagnetic field was generally noisier than in the overlying and underlying sections with greater dispersion of the inclination 3 the laschamp excursion was apparently recorded at hole 480 between about 51 and 49 ka bp and the mono lake excursion between about 29 and 26 ka bp in addition a narrow 0 3 0 4 m zone near 23 ka bp has a very similar paleomagnetic signature as the excursion observed at summer lake oregon 1 and we suggest that the summer lake is distinct from and younger than the mono lake excursion by 3 5 ka and of considerably shorter duration lasting no more than a few hundred years 4 recurring inclination fluctuations were identified at site 480 characterized by end points with steep inclinations and shallow intermediate value s as compared with the geocentric axial dipole the inclination cycles are particularly apparent from 54 to 24 ka bp with a characteristic period of about 4 4 ka 5 the noisier inclination record between 54 and 24 ka bp might be related to the generally reduced dipole moment between about 20 and 50 ka and particularly low paleointensities for the laschamp and mono lake excursions

greigite detected as dominating remanence carrier in late pleistocene sediments lisan formation from lake kinneret sea of galilee israel summary a rock magnetic investigation of three sedimentary cores of lisan formation of late pleistocene age from lake kinneret sea of galilee northern israel demonstrates that the magnetization of these sediments is controlled by various degrees of a secondary chemical remanent magnetization crm carried by greigite fe3s4 this crm is superimposed on a primary detrital remanent magnetization drm that resides in ti magnetite this finding is independently confirmed by x ray diffraction xrd measurements performed on magnetic extracts of the sediments the domain state of the greigite is largely single domain behaviour sd thus dominating the magnetization therefore the magnetic record retrieve from of these sediments is not reflecting geomagnetic variations but rather chemical rock magnetic properties resulting from diagenetic processes the results of our study suggest that paleomagnetic record of greigite bearing sediments should be interpreted with caution because of the following reasons 1 geomagnetic secular variations can be biased due to large coercivity overlap between magnetite and greigite 2 alternating field af demagnetization can produce erroneous directions due to vector distortion by acquisition of a gyro remnant magnetization grm 3 estimation of relative paleointensity can be hampered by large scale variations in natural remnant magnetization nrm intensity caused by the acquisition of a secondary crm of unknown age and unknown extent superimposed on the drm the precipitation of greigite requires reducing conditions at the sediment water interface and or interstitial water associated with the presence of sulphur and iron oxides reducing conditions are typical of stratified lakes and other stratified water bodies such as the dead sea paleo lake lisan temporarily the eastern mediterranean sea the caspian sea the black sea and many other marine basins according to our findings paleomagnetic records from such environments carried dominantly by greigite therefore should be treated with care