double flood basalts and plume head separation at the 660 kilometer discontinuity several of the world s flood basalt provinces display two distinct times of major eruptions separated by between 20 million and 90 million years these double flood basalts may occur because a starting mantle plume head can separate from its trailing conduit upon passing the interface between the upper mantle and the lower mantle this detached plume head eventually triggers the first flood basalt event the remaining conduit forms a new plume head which causes the second eruptive event the second plume head is predicted to arrive at the lithosphere at least 10 million years after the first plume head in general agreement with observations regarding double flood basalts

using multi radiotracer techniques to better understand sedimentary dynamics of reworked muds in the changjiang river estuary and inner shelf of east china sea abstract reworked muds including mobile muds resuspended sediments and bio mixed muds are ephemeral layers in the benthic boundary of large river delta front estuaries ldes in this study multiple radiotracers 210pb 137cs 234th 7be 235u 228th 228ra 226ra and 40k were combined to evaluate the sedimentary dynamics of the mud belt on the inner shelf of the east china sea ecs adjacent to the changjiang river estuary we first categorized eight types of typical 210pb profiles in marine sediments and specified which ones could be used as possible evidence of reworked muds the types of 210pb profiles observed in previous studies and this study were then used to examine the depth distribution of reworked mud layers of the benthic boundary of ecs we found that reworked mud depth in the along shore mud belt of the inner shelf of ecs was deeper than those in the outer shelf and slope area reaching up to 150 cm deep in the changjiang river lde and up to 50 cm deep in the zhejiang fujian coast mud area sediment core tracer profiles also showed that the reworked mud layer could be further divided into several sub layers with distinct redox conditions and microbe sulfate reducing bacterial abundance srb abundance intensively fluctuated with changing redox environment and larger abundance values were usually observed in anoxic conditions based on 7be and 234th the top layer ca 2 4 cm thick was the most modern and active zone showing evidence of reworking on seasonal timescales the deeper sections of the reworked mud layer were generally modified over longer timescale years which were examined using the more long lived tracers e g 137cs 228thex and 228ra 226ra ratio we found for the first time in this region that age of the reworked layers was usually much younger than underlying accumulating burial layers we posit that this multi radionuclide approach is critical for evaluating the complex biogeochemical sediment record on delta front estuaries of large rivers subject to changing climate conditions and human impacts over the last several centuries

predictive mapping of seabed substrata using high resolution multibeam sonar data a case study from a shelf with complex geomorphology abstract this study explores the full potential of high resolution multibeam data for the automated and accurate mapping of complex seabed features under a predictive modelling framework for an area of seabed on the carnarvon shelf in western australia morphometric variables and textural measures were derived from multibeam bathymetry and backscatter data several feature extraction approaches were applied to backscatter angular response curves to obtain new features these derivatives and new features were used separately and in combination in the predictions despite the complex distribution of various hard substrata within the study area we achieved a nearly perfect prediction of hard vs soft seabed types with an auc area under curve close to 1 0 the predictions were also satisfactory for gravel sand and mud content with r 2 values that range from 0 55 to 0 73 this study demonstrates that using a full range of derivatives and new features from both multibeam bathymetry and backscatter data optimises the accuracy of seabed mapping from the modelled relationships between sediment properties and multibeam data we confirmed that coarser sediment generally generates stronger backscatter return importantly the results again highlight the advantages of applying proper feature extraction approaches over using original backscatter angular response curves

morphology and origin of the osbourn trough the osbourn trough is a 900 km long east west trending gravity low visible in satellite altimetry data which intersects the tonga trench at 25 30 s a recent survey collected gravity magnetic echo sounder and swath bathymetry data on three adjacent north south trending ship tracks centered on the trough the linear gravity low is 20 30 mgal less than the regional value and is accompanied by a flat lying 200 500 m deep sediment filled valley swath bathymetry images reveal several parallel east west trending linear ridges and valleys on either side of the main trough as well as other morphologic features characteristic of relict spreading centers including a prominent inside corner high and possible pseudo fault trace the presence of magnetic anomalies possibly anomalies 33 and 32 suggests that the seafloor here was formed after the end of the cretaceous normal superchron anomaly 34 these data support the conclusion that this trough is a spreading center which stopped spreading in late cretaceous time the existence of this feature has important implications for tectonic reconstructions in this region the osbourn trough could be part of the fossil spreading center between the pacific plate and a fragment of the phoenix plate the bellingshausen plate this would require the seafloor between the osbourn trough and the chatham rise to the south to be a remnant fragment of the bellingshausen plate this remnant may have joined to the pacific plate when the hikurangi plateau entered the gondwana subduction zone at the chatham rise possibly causing the cessation of spreading on the osbourn trough

high resolution paleomagnetic dating of sediments deposited in the central baltic sea during the last 3000 years abstract sediment cores from the main sedimentary basin in the baltic sea the gotland deep provide a high resolution record of changes in the holocene paleo environment and paleoclimate due to the inherent inaccuracy reservoir effect redeposited material of the 14 c dating method in dating marine sediments the known changes in the earth s magnetic field recorded in three sediment cores from the gotland deep were used to produce a more detailed and accurate timescale the variations in the inclination and declination were compared to the paleosecular variations over the last 3000 years recorded in annually laminated lake sediments from finland this correlation made it possible to construct for the late holocene gotland deep sediments a high resolution time scale with an estimated absolute accuracy of better than 50 years this new timescale made it possible to determine the mean annual rates of sedimentation with an unprecedented accuracy the sedimentation rates were found to have varied considerably both in time from 0 2 up to 4 3 mm year and space indicative of the patchy nature of accumulation in the gotland deep during the past 3000 years

a field of volcanoes on the manihiki plateau mud or lava abstract a field of about 100 cones surrounding a 1900 m high seamount was discovered on the northeastern edge of manihiki plateau 10 18 5 s 161 27 5 w during a february 1987 cruise of the r v moana wave in the territorial waters of the cook islands the cones appear as acoustically transparent piercements on single channel seismic reflection profiles collected during this and earlier expeditions to the area seamarc ii sidescan images typically reveal nearly conical steep sided peaks of about 1 km diameter rising several hundred meters above the smooth and featureless plateau surface many of the cones appear to be formed by multiple flow units a few have summit depressions and moats partially surround some of the cones distributed like holes on a dart board individual cones become more densely spaced about a bulls eye located near the eastern edge of the manihiki plateau 50 km southwest of rakahanga island at that location about 40 cones coalesce to form the seamount which is about 25 km in diameter and rises from a plateau depth of 3200 m to a summit at a depth of 1310 m sidescan images show that incised channels trend away from the seamount toward the east where the irregular seafloor in the area truncates tilted e dipping sub bottom reflectors to the south and west the seafloor and sub bottom reflectors are smooth and dip gently to the southwest recent foraminiferal ooze was cored from a satellite cone dredging of the seamount summit recovered middle eocene planktonic foraminifera embedded in burrowed limestone some of the limestone cobbles had manganese crusts coralline fragments and recent foraminifera were also recovered in the same dredge despite the apparent absence of igneous rocks geopotential modelling indicates a basement high at this location if fluid escape rather than magma flow is involved in the genesis of these cones it is localized by faulting associated with basement uplift or the presence of an older igneous intrusion at this time we cannot say with certainty whether the cones are igneous or sedimentary features

the acoustic turbid layer in muddy sediments of eckernfoerde bay western baltic methane concentration saturation and bubble characteristics in eckernfoerde bay in the western baltic an enclosed basin with organic rich mud acoustic turbidity has been observed since 1952 to investigate the relationships between acoustic properties and free gas we developed a method for rapid sub sampling of gravity cores to determine the total methane content from temperature measurements determinations of the salinity of the pore water and data from the literature saturation limits were calculated for each core it is demonstrated that there is a certain amount of free gas in the mud of eckernfoerde bay but subbottom depths of the oversaturation and of the acoustic turbidity do not correspond the existence of free gas implies the existence of bubbles in the sediment which are regarded to be responsible for the acoustic turbidity bubble sizes observed by x ray computed tomography while still under in situ pressure and temperature vary between 1 and 10 mm equivalent diameter the distribution of bubbles is discontinuous occurring in depth zones from 2 to 20 cm thickness

age and isotopic relationships among the angrites lewis cliff 86010 and angra dos reis results of a wide ranging isotopic investigation of the unique antarctican angrite lew 86010 lew are presented together with a reassessment of the type angrite angra dos reis ador the principal objectives of this study are to obtain precise radiometric ages initial sr isotopic compositions and to search for the erstwhile presence of the short lived nuclei 146sm and 26al via their daughter products the isotopic compositions of sm u ca and ti were also measured this allows a detailed appraisal to be made of the relations between and the geneology of these two angrites lew proves to be severely contaminated with modern terrestrial pb which is shown to result from terrestrial weathering nevertheless concordant pb pb model ages of pyroxene separates were obtained 2 4 55784 52 ga for lew and 4 55780 42 ga for ador uranium isotopic compositions are normal within error the inferred initial pb isotopic composition is within error of primordial pb as defined by canon diablo troilite because of the extreme upb ratios of both angrites this places an upper limit of 2 ma on the time between volatile element loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates initial 87sr86sr ratios were found to be indistinguishable in lew ador and the cumulate eucrite moore county the derived initial 87sr86sr for all three meteorites is 0 698970 15 2 a 147sm 143nd isochron for lew was obtained yielding an age of 4 553 34 ga with an initial 143nd144nd of 0 506682 49 2 146sm142nd isotope systematics were also measured anomalies on 142nd arising from extinct 146sm were clearly resolved resulting in an initial 146sm144sm of 0 0071 17 with nd142 2 57 0 62 at the time of isotopic closure both of these sm nd methods imply derivation of lew from a reservoir with chondritic smnd ratio they are also quite consistent with the previously reported systematics for ador overall the age and isotopic similarities between lew and ador are striking it suggests almost simultaneous production on the same asteroid even though recent experimental studies imply that the two are not comagmatic calcium and titanium do not exhibit enrichments in the n rich isotopes in contrast to cais making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula no evidence was found for live 26al in lew when combined with the pb pb age and initial 87sr86sr data this firmly excludes 26al being an important heat source in the early solar system heat supplied for for example 1 the differentiation of the angrite and eucrite parent bodies and 2 metamorphism of the chondrite parent bodies must come from some other source the debate over live vs extinct 26al in the earliest solar system remains unresolved by this new data from lew however both the angrite and eucrite parent bodies have extremely low rbsr and high upb ratios compared to the solar nebula therefore the common initial 87sr86sr of 0 698970 can be firmly associated with the u pb angrite age above and the formation of both bodies this enables an absolute chronology of the early solar system to be established based upon 87sr86sr differences the oldest cais are a minimum of 11 4 ma older than the angrites i e 4 569 5 ga the age and isotopic constraints are discussed with respect to current collapse condensation and accretion timescales calculated for the solar nebula

surficial seabed sediment properties derived from seismic profiler responses abstract as a result of the complex and variable nature of sea floor sedimentary environments and the problems associated with obtaining representative samples of sea floor materials it has long been accepted that geophysical methods hold the greatest potential for rapid assessment of sea floor sediment physical property variability this paper presents results of two experimental geophysical studies aimed at developing in situ sea floor sediment classification methodologies applicable to siliciclastic environments at one of the two study sites irvine bay in the clyde sea an area of approximately 10 km2 was intensively surveyed using a boomer seismic profiler and digital data acquisition system three core samples recovered in the area were used to calibrate seabed reflection responses at spot locations and once calibrated the entire geophysical dataset was inverted to produce maps showing the spatial distribution of sediment physical properties porosity density and grain size distribution at the seabed surface very good agreement was found between seismic predictions of sediment properties and physical measurements made on independent core and grab samples similar levels of agreement were found for seismo acoustic predictions made after surveying a transect in the southern baltic sea on the basis of these experimental studies it appears that with only limited ground truth information it is possible to invert high resolution seismic reflection data to produce quantitative information on the spatial distribution of sea floor sediment physical properties

phreatomagmatic eruptions on the ontong java plateau chemical and isotopic relationship to ontong java plateau basalts the compositions of glass clasts in volcaniclastic rocks recovered from drilling at site 1184 on the eastern salient of the ontong java plateau ojp are investigated using microbeam analytical methods for major minor and trace elements these data are com pared with whole rock elemental and isotopic data for bulk tuff samples and with data from basalts on the high plateau of the ojp three subunits of hole 1184a contain blocky glass clasts thought to represent the juvenile magmatic component of the phreatomagmatic eruptions that generated the volcaniclastic rocks the glass clasts have unaltered centres and are all basaltic low k tholeiites with flat chondrite normalized rare earth element ree patterns their elemental compositions are very similar to the kwaimbaita type and kroenke type basalts sampled on the high plateau each subunit has a distinct glass com position and there is no intermixing of glass compositions between subunits indicating that each subunit is the result of one eruptive phase and that the volcaniclastic sequence has not experienced reworking the relative heterogeneity preserved at site 1184 contrasts with the uniformity of compositions recovered from individual sites on the high plateau and suggests that the eastern salient of the ojp had a different type of magma plumbing system our data support the hypothesis that the voluminous subaerially erupted volcani clastic rocks at site 1184 belong to the same magmatic event as the construction of the main ontong java plateau thus the ojp would have been responsible for volatile fluxes into the atmosphere in addition to chemical fluxes into the oceans and these factors may have influenced the contemporaneous oceanic anoxic event

new paleomagnetic results from basaltic drill cores of the nauru basin western pacific tectonic and magnetostratigraphic implications abstract the voluminous volcanic eruptions in the nauru basin western pacific have long been regarded as important research targets for tectonic history of the pacific plate and for the widespread cretaceous volcanic activity in the western pacific the nauru basin volcanic rocks were recovered at site 462 by deep sea drilling project dsdp legs 61 and 89 where more than 600 m of lavas and sills were drilled thereby making it the deepest penetration into crust of cretaceous age in the pacific ocean for paleomagnetism this section represents a unique possibility for averaging out secular variation to obtain a reliable paleolatitude estimate however previous paleomagnetic studies have only been subjected to alternating field af demagnetization on several core samples thus unable to provide comprehensive understanding on the paleolatitude of the basin the work reported here aims to determine the cretaceous paleomagnetic paleolatitude for the pacific plate and define the magnetostratigraphy for the basaltic sections drilled in the nauru basin a total of 391 basaltic rock samples were carefully re sampled from dsdp sites 462 and 462a stepwise thermal and af demagnetizations have isolated characteristic components in the majority of the samples the most important findings from this study include 1 two normal and one reversed polarity intervals are identified in site 462 and six normal and six reversed polarity intervals are found in site 462a although possible erroneous markings of the opposite azimuth for some reversed polarity cores during the dsdp coring cannot be completely ruled out 2 based on previous radiometric ages the magnetostratigraphic correlations with the geomagnetic polarity time scale gpts indicate that the lower basaltic flow unit in site 462a began to erupt at least before 130 ma no correlation is available for the upper sill unit 3 paleosecular variation for the lower flow unit has been sufficiently averaged out whereas bias may exist for that of the upper sill unit 4 the calculated mean inclination of 50 for the lower flow unit yields a paleolatitude of 30 8 s for the nauru basin at the time of emplacement this value is well to the north of suggested location in plate reconstruction models suggesting that there has been a significant amount of apparent polar wander of the nauru basin and pacific plate since 130 ma in addition the paleolatitude for the nauru basin is 7 further south and the basin s age is more than 10 my older than those of the ontong java plateau ojp which suggest that the volcanic eruptions of the lower flows in the nauru basin are unlikely related to the emplacement of the ontong java plateau

vertex manganese transport through oxygen minima abstract manganese transport through a well developed oxygen minimum was studied off central mexico 18 n 108 w in october november 1981 as part of the vertex vertical transport and exchange research program refractory leachable and dissolved mn fractions associated with particulates caught in traps set at eight depths 120 1950 m were analyzed particles entering the oxygen minimum had relatively large mn loads however as the particulates sank further into the minimum total mn fluxes steadily decreased from 190 nmol m 2 day 1 at 120 m to 36 nmol m 2 day 1 at 400 m manganese fluxes then steadily increased in the remaining 800 1950 m reaching rates of up to 230 nmol m 2 day 1 at 1950 m manganese concentrations were also measured in the water column dissolved mn levels 1 were consistently observed within the 150 600 m depth interval in contrast suspended particulate leachable mn amounts were especially low at those depths and never exceeded 0 04 nmol kg 1 the combined water column and particle trap data clearly indicate that mn is released from particles as they sink through the oxygen minimum rate of change estimates based on trap flux data yield regeneration rates of up to 0 44 nmol kg 1 yr 1 in the upper oxygen minimum 120 200 m however only 30 of the dissolved mn in the oxygen minimum appears to be from sinking particulate regeneration the other 70 probably results from continental slope release horizontal transport processes dissolved mn scavenges back onto particles as oxygen levels begin to increase with depth scavenging rates ranging from 0 03 to 0 09 nmol kg 1 yr 1 were observed at depths from 700 to 1950 m these scavenging rates result in mn residence times of 16 19 years and scavenging rate constants on the order of 0 057 yr 1 manganese removal via scavenging on sinking particles below the oxygen minimum is balanced by mn released along continental boundaries and transported horizontally via advective diffusive processes manganese appears to be very weakly associated with particulates nevertheless the amounts of mn involved with sinking biogenic particles are large and the resulting fluxes are on the same order of magnitude as those necessary to explain the excess mn accumulating on the sea floor the overall behavior of mn observed in this and other studies strongly suggests some type of equilibrium occurring between dissolved and particulate phases this equilibrium appears to shift in direct or indirect response to dissolved oxygen levels

profiles of dissolved and particulate thorium isotopes in the water column of coastal southern california abstract profiles of 234 th 232 th and 230 th in the santa monica basin were measured 234 th in the upper part of the water column suggests that the residence time of dissolved 234 th with respect to adsorption onto and removal by particles increases with depth from 3 4 weeks at the surface to 5 months below the euphotic zone the partition of 234 th between dissolved and particulate forms points to a residence time of 4 7 days for particulate 234 th in the surface euphotic zone rapid removal of particulate 234 th from the surface water precludes the possibility of an adsorption desorption equilibrium state between dissolved and particulate 234 th based on the 230 th data the overall residence time of dissolved th in the water column is estimated at 5 6 years both dissolved 230 th and 232 th profiles exhibit higher concentrations at the surface than immediately underneath suggesting a terrestrial input for them from the surface to the bottom of the water column the concentration of dissolved 232 th decreases by a factor of 3 4 and the dissolved 230 th 232 th activity ratio increases by a factor of about five from 2 3 to 9 12 this reflects the surface source of the terrestrial end member and its removal at depth on the average about 6 of the 234 th 25 of the 230 th and 50 of the 232 th were found in the particulate form which is as expected and relates to their respective sources to the difference in radioactive decay half life and to the kinetics of the scavenging process

onset of mid cretaceous volcanism by elevation of the 670 km thermal boundary layer a pulse of ocean crustal production during the mid cretaceous begins at the same time that magnetic reversal frequency drops to zero we suggest that slab penetration into the lower mantle caused a thermal boundary layer normally at 670 km to be rapidly advected upward as the slabs descended through the lower mantle the onset of near surface melting due to the rise of this boundary layer coincides with the slabs9 arrival at d which drove up the heat flux across the core mantle boundary and stabilized the magnetic reversal process the slab penetration event was coincident with the breakup of the gondwana supercontinent and triggered the subsequent formation of oceanic plateaus in the central pacific and indian ocean basins

geochemistry of cretaceous volcaniclastic sediments in the nauru and east mariana basins provides insights into the mantle sources of giant oceanic plateaus

acoustic discrimination of relatively homogeneous fine sediments using bayesian classification on mbes data highlights acoustically meaningful clustering correlating with ground truth data seascape scale acoustic mapping through classification per beam footprint bayesian approach for discriminating sedimentary units with low heterogeneity geo acoustic resolution a new measure of sedimentary acoustic class separability modern seafloor mapping is based on high resolution mbes systems that provide detailed bathymetric and acoustic intensity backscatter information we examine and validate the performance of two unsupervised mbes classification techniques for discriminating acoustic classes of sedimentary units with small grain size variability the first technique based on a principal components analysis pca is commonly used in literature and has been applied for comparison with the more recent approach of bayesian statistics by applying these techniques to a mbes dataset from an estuarine area in the netherlands we tested their ability to discriminate fine grained sediments at least 70 silt holding small percentages of coarser material such as sand shell hash or shells we focus on the bayesian technique as it outputs acoustically significant classes related to backscatter values this technique utilizes backscatter values averaged over scatter pixels projected pulse lengths inside the footprint of each beam the originality of our application lies in the fact that the optimal number of classes is derived by utilizing a number of beams simultaneously it is assumed that the backscatter values per beam vary relatively to the varying seafloor types by treating the beams separately across track variation in the seafloor type can also be accounted for thereby the classification is guided by outer more discriminative beams additionally we control the optimal number of classes by employing the quantitative criterion of goodness of fit 2 the bayesian acoustic classes show correlation with grain size parameters such as coarse fraction 500 m percentage and mean of the grain size 500 m when analyzed with multiple linear regression in order to examine the relative scale of the acoustic classification results we compare the bayesian acoustic classes with underwater video interpretation our results reveal that the bayesian approach enhances the sedimentological interpretation of mbes high resolution data by providing classification on seascape scale here meters to tens of meters hence we suggest that backscatter processing techniques are more commonly applied to produce classes that discriminate sediments with low grain size contrast to describe this ability we introduce the term geoacoustic resolution we want to encourage the use of the bayesian technique also in deep sea applications based on auv data where sediments express low variability but sampling would be time consuming and costly the advantages of this method would favor mapping of macro habitats which appear at meter scale and require datasets of sufficient resolution in order to be quantitatively described

the geochemical cycle of 9be a reconnaissance an initial survey has been made of the geochemistry of 9be in natural waters with a view to utilizing the isotope with the cosmogenic species 10be t1 2 1 5 m y in geochronological studies of sedimentary accumulation the mobility of9be in continental waters is a strong function of ph with acid streams ph 6 being strongly enriched as compared to alkaline carbonate rivers large scale scavenging of beryllium occurs in the estuaries of rivers containing more than about 200 pm of the element in alkaline systems of lower concentration the element appears to behave conservatively the net flux of beryllium to the ocean is estimated as that equivalent to an average effective river concentration of about 150 pm beryllium is strongly enriched in ridge crest hydrothermal solutions over ambient seawater the flux is approximately 70 of that from the continents the deep water concentrations of 9be vary by only 20 between the north atlantic and north pacific demonstrating that the chemical saltation produced by scavenging from the deep water column and release from the sediments predominates over simple advective concentration along flow lines isotopic homogenization of 10be and 9be is probably achieved in the ocean the situation in continental waters appears much less promising

relocation of soft mud deposits an example from the archipelago sea northern baltic sea abstract active sedimentation is one of the distinguishing characteristics of the baltic sea this has led to the formation of fractally distributed soft sediment deposits muds that can be up to several meters thick in this study we examined for the first time stability of soft sediment deposits within a 3 km 2 sized sedimentary basin in the northern baltic sea eastern archipelago sea finland using acoustic profiling 28 khz and 3 5 khz and 200 khz multibeam surveys on three occasions from 2011 to 2013 sub bottom profile data and rasterized deposit thickness data revealed maximal annual thickness changes between 1 1 m erosion or 1 1 m deposition along primary survey lines and skimming of up to 9 of the soft deposit area the most notable finding was the significant instability of mud when examined at local scale there was substantial and very localized erosion and deposition apparent horizontal movement of unconsolidated mud 250 400 m per year between 2012 and 2013 maximum relocation of unconsolidated deposits was approximately 0 3 km 3 a 1 with 94 export to the exterior area 2011 2012 and was likely affected by abnormally high wave action in the northern baltic sea during the period of observation the combination of acoustic and mathematical techniques is shown to be highly suited to examination of temporal variations in sub bottom level environments

subsidence and growth of pacific cretaceous plateaus abstract the ontong java manihiki and shatsky oceanic plateaus are among the earth s largest igneous provinces and are commonly believed to have erupted rapidly during the surfacing of giant heads of initiating mantle plumes we investigate this hypothesis by using sediment descriptions of deep sea drilling project dsdp and ocean drilling program odp drill cores to constrain plateau subsidence histories which reflect mantle thermal and crustal accretionary processes we find that total plateau subsidence is comparable to that expected of normal seafloor but less than predictions of thermal models of hotspot affected lithosphere if crustal emplacement was rapid then uncertainties in paleo water depths allow for the anomalous subsidence predicted for plumes with only moderate temperature anomalies and volumes comparable to the sources of modern day hotspots such as hawaii and iceland rapid emplacement over a plume head of high temperature and volume however is difficult to reconcile with the subsidence reconstructions an alternative possibility that reconciles low subsidence over a high temperature high volume plume source is a scenario in which plateau subsidence is the superposition of 1 subsidence due to the cooling of the plume source and 2 uplift due to prolonged crustal growth in the form of magmatic underplating this prolonged crustal growth and uplift scenario may explain the low and thus submarine relief during plume initiation the late stage eruptions found on ontong java 90 ma and manihiki 70 ma a large portion of the high seismic velocity lower crust and the widespread normal faults observed throughout and along the margins of the three plateaus such late stage underplating may have occurred continuously or in discrete stages over 30 m y and implies lower magmatic fluxes than previously estimated

a low frequency multibeam assessment spatial mapping of shallow gas by enhanced penetration and angular response anomaly this study highlights the potential of using a low frequency multibeam echosounder for detection and visualization of shallow gas occurring several meters beneath the seafloor the presence of shallow gas was verified in the bornholm basin baltic sea at 80 m water depth with standard geochemical core analysis and hydroacoustic subbottom profiling successively this area was surveyed with a 95 khz and a 12 khz multibeam echosounder mbes the bathymetric measurements with 12 khz provided depth values systematically deeper by several meters compared to 95 khz data this observation was attributed to enhanced penetration of the low frequency signal energy into soft sediments consequently the subbottom geoacoustic properties contributed highly to the measured backscattered signals those appeared up to 17 db higher inside the shallow gas area compared to reference measurements outside and could be clearly linked to the shallow gas front depth down to 5 meter below seafloor no elevated backscatter was visible in 95 khz mbes data which in turn highlights the superior potential of low frequency mbes to image shallow sub seafloor features small gas pockets could be resolved even on the outer swath up to 65 strongly elevated backscattering from gassy areas occurred at large incidence angles and a high gas sensitivity of the mbes is further supported by an angular response analysis presented in this study we conclude that the mbes together with subbottom profiling can be used as an efficient tool for spatial subbottom mapping in soft sediment environments

an unmetasomatized source for the malaitan alnoite solomon islands petrogenesis involving zone refining megacryst fractionation and assimilation of oceanic lithosphere the ma tan alnijite contains a rich and varied megacryst suite of unprecedented compositional range we have undertaken trace element and isotope modeling in order to formulate a petrogenetic scheme which links the host alniiite to its entrained megacrysts this requires that a proto alntiite magma is the product of zone refining initiated by diapiric upwelling where the initial melt passes through 200 times its volume of mantle isotopic evidence indicates the source of the proto alniiite contains a time integrated lree depleted signature 143nd 44nd 0 51274 impingement upon the rigid lithosphere halts or dramatically slows the upward progress of the mantle diapir at this point the magma cools and megacryst fractionation begins with augites crystallizingjirst followed by subcalcic diopsides and finally phlogopites garnet probably crystallizes over the entire range of clinopyroxene fractionation estimated proportions of fractionating phases are 30 augite 24 5 subcalcic diopside 27 garnet 12 9 phlogopite 5 bronzite 0 5 ilmenite and 0 1 zircon as this proto alnoite magma crystallizes it assimilates a subducted component of seawater altered basalt which underplates the ontong java plateau this is witnessed in the isotopic composition of the megacrysts and alnoite

rare earth distributions with a positive ce anomaly in the western north atlantic ocean from the vertical distributions of the rare earth elements ree in the sargasso sea we now report the first profiles of pr tb ho tm and lu in seawater together with profiles for la ce sm eu and yb the first observations of positive ce anomalies in seawater are ascribed to reducing inshore sediments as a source for ce ref 1 all vertical profiles are consistent with adsorption of trivalent rare earths by settling particles possibly siliceous or calcareous followed by their release at or near the sea floor on dissolution of the carriers the very different ce profile demonstrates the additional effects of oxidation reduction reactions

isotopic evidence for the origin of the manihiki and ontong java oceanic plateaus abstract pb nd and sr isotopic results for lavas of the cretaceous ontong java and manihiki oceanic plateaus fall well within the modern day oceanic island or hotspot field the data provide no evidence of old continental basements but indicate a major involvement of kerguelen type or em i like mantle in the sources of both plateaus which appear to have probably been formed at least in part by hotspots however the presently active hotspots that pacific plate reconstructions suggest might have been possible plateau sources lack kerguelen type isotopic compositions either these hotspots did not participate in the formation of the two plateaus or if they did kerguelen type material must have been volumetrically much more important early in their existence two hypotheses for the origins of these plateaus which involve hotspot sources are consistent with the sparse available geochemical geochronological and geophysical data the first holds that the plateaus formed cataclysmically in association with surfacing plume heads the second posits a relatively steady but robust hotspot at or near a ridge crest and requires a much longer period of formation a near ridge origin appears to be indicated by evidence that most of the pacific plateaus were built largely on relatively young ocean crust however we suggest that a near ridge origin is also compatible with the plume head concept in that plume heads appear very likely to become associated with spreading axes through their influence on rift propagation which should be substantially greater than for ordinary hotspots in either case the lack of hotspot tracks seamount chains attached to the two plateaus would be a consequence of ridge migration or rift propagation in a near ridge setting

very high resolution side scan sonar mapping of biogenic reefs of the tube worm lanice conchilega reefs of the tube building polychaete lattice conchilega are known to represent hotspots of biodiversity within inter and subtidal soft sediments of the north sea however because of their patchy distribution point sampling does not appropriately map their subtidal spatial distribution this study evaluated the feasibility to detect l conchilega reefs by very high resolution side scan sonar imagery a subtidal very high resolution 410 khz side scan survey combined with grab sampling revealed high densities of l conchilega up to 1979 ind m 2 to coincide with a higher reflectivity patchy and grainy acoustic facies from the side scan sonar imagery individual reefs were estimated to reach a maximum size of 15 m 2 to ground truth the acoustic facies the distribution of intertidal l conchilega reefs was mapped at low tide and side scan sonar imagery was recorded during the following high tide intertidal l conchilega reefs had a patch size of 0 8 m 2 up to 11 6 m 2 elevated 7 5 to 11 5 cm above the surrounding seafloor and covered approximately 10 of the selected area the very high 445 khz resolution side scan sonar imagery revealed a similar acoustic facies as in the subtidal lower resolution 132 khz side scan sonar imagery was less efficient to detect physically less developed l conchilega reefs we conclude that 1 there are no major technical restrictions to map l conchilega reefs using side scan sonar 2 the developmental stage of l conchilega reefs impacts the detectability of the reefs and 3 very high resolution side scan sonar imagery is considered a necessity when mapping small scale structures such as l conchilega reefs c 2008 elsevier inc all rights reserved

removal of 230th and 231pa at ocean margins abstract uranium thorium and protactinium isotopes were measured in particulate matter collected by sediment traps deployed in the panama basin and by in situ filtration of large volumes of seawater in the panama and guatemala basins concentrations of dissolved th and pa isotopes were determined by extraction onto mno 2 adsorbers placed in line behind the filters in the in situ pumping systems concentrations of dissolved 230 th and 231 pa in the panama and guatemala basins are lower than in the open ocean whereas dissolved 230 th 231 pa ratios are equal to or slightly greater than ratios in the open ocean particulate 230 th 231 pa ratios in the sediment trap samples ranged from 4 to 8 in contrast to ratios of 30 or more at the open ocean sites previously studied particles collected by filtration in the panama basin and nearest to the continental margin in the guatemala basin contained 230 th 231 pa ratios similar to the ratios in the sediment trap samples the ratios increased with distance away from the continent suspended particles near the margin show no preference for adsorption of th or pa and therefore must be chemically different from particles in the open ocean which show a strong preference for adsorption of th ocean margins as typified by the panama and guatemala basins are preferential sinks for 231 pa relative to 230 th furthermore the margins are sinks for 230 th and to a greater extent 231 pa transported by horizontal mixing from the open ocean

middle holocene to present sedimentary environment in the easternmost gulf of finland baltic sea and the birth of the neva river abstract the easternmost gulf of finland is the recipient of the neva river the largest river draining to the baltic sea the depositional history and endozoobenthic activity in the area have not been studied before based long sediment cores although such studies are desirable to provide a paleoenvironmental context for the recent environmental changes and the ecosystem projections for the future baltic sea two replicate long sediment cores from the easternmost gulf of finland record brackish water mud accumulation beginning at 5900 60 calendar years before present cal bp the brackish water muds are characterized by thin beds with erosional bases fining upward grain sizes and increasing upward organic contents a storm induced flow origin is inferred for the thin mud beds contrasting with previous interpretations that these muds accumulate by quiescent suspension settling the bed primary microstructures are overprinted by biodeformation and burrow mottling by incipient small planolites and arenicolites polykladichnus ichnofossils of the continental skolithos ichnofacies left behind by endobenthic oligochaetes and chironomid larvae the depositional succession is cut by a strong erosional hyperpycnal flow event at 3320 40 cal bp caused by the birth of the neva river which is now documented offshore for the first time mineral magnetic properties show pseudosingle domain magnetite as the main magnetic mineral present increased magnetic grain sizes and reduced loi values indicate a strong lithic influx immediately after the neva river birth the dominance of riverborne material in the accumulating sediments ever since highlights the strong influence of reworking in the shallow sea area increased diameters and the deeper penetration of biogenic sedimentary structures demonstrate improved seafloor oxygenation after the river birth during the medieval warm period ca 1000 700 cal bp and the last century increased sedimentary lithic contents and magnetic grain sizes indicate an elevated riverine influx due to increased precipitation and or the increased reworking of sediments by storms the results provide a paleoenvironmental context for the study area and deepen the understanding of storm induced sediment dispersal and the formation of sedimentary fabric in shallow wave dominated sea areas

wave modelling in archipelagos abstract the archipelago sea located in the baltic sea between the gulf of bothnia and the baltic proper comprises thousands of small islands and shoals the sheltering effects depth induced wave breaking and refraction make the modelling of waves challenging in this area ten different high resolution grids were generated based on coastal nautical charts with horizontal resolutions of 0 1 nmi and 0 5 nmi using two different methods to compile land sea mask and additional grid obstructions the wave model wam was used to model the wave field in the archipelago sea the modelled wave field was compared against measurements made during a research cruise of r v aranda in september 2010 the 0 1 nmi grids had the highest accuracy in modelling the local wave field however they overestimated the attenuation of the open sea waves propagating into the archipelago sea the 0 5 nmi grids overestimated the energy of the local wind waves and underestimated the attenuation of the open sea waves when grid obstructions were used in the 0 5 nmi grids the attenuation of wave energy was modelled with good accuracy but the energy of the local wind waves was still slightly overestimated the accuracy of the forcing wind field had significant effect on the accuracy of the locally generated wave field inside the archipelago the depth induced wave breaking and wave refraction were shown to have a prominent effect on the modelled wave field at the southern edge of the archipelago sea the 0 5 nmi grids were unable to describe these features in as much detail as the 0 1 nmi grids

southern california inner basin sediment trap calibration the deployment of particle interceptor traps pits in the three inner basins of the southern california bight santa barbara santa monica and san pedro where preserved laminated bottom sediments occur provides a natural calibration between the parameters determined with the pits and those derived from the historical sedimentary deposits the accumulation rates and chemical composition of the pit materials compare favorably with these recently deposited bottom sediments the ratio of the measured particle collection rate to the sediment accumulation rate for these three basins averaged0 93 0 20 radionuclide 210pb 228th 232th and trace element fe cu zn cd pb compositions of the pit materials and surface sediments agree within generally 30 in each basin an application of pits as a sampling system in an outer basin san nicolas where the sedimentary record has been obscured by bioturbation is also presented to further demonstrate the utility of pits for spatial and temporal studies of various aspects of particle transport and sediment deposition based on the calibration of the pits in the three inner basins we conclude from the comparison of the radionuclide and trace element compositions between the pit materials and surface sediments from the san nicolas basin that recently deposited sedimentary material has been mixed by bioturbation with older material from the last several hundred years excess210pb and the228th 232th activity ratio in the pit material are 6 and 15 times higher than in the surface sediment total pb and1n hno3 leachable pb are 4 and 8 fold higher in the pit material

pin pricking the elephant evidence on the origin of the ontong java plateau from pb sr hf nd isotopic characteristics of odp leg 192 basalts age corrected pb sr and nd isotope ratios early aptian basalt from four widely separated sites on the ontong java plateau that were sampled during ocean drilling program leg 192 cluster within the small range reported for three earlier drill sites for outcrops in the solomon islands and for the nauru and east mariana basins hf isotope ratios also display only a small spread of values a vitric tuff with end t 4 5 that lies immediately above basement at site 1183 represents the only probable example from leg 192 of the singgalo magma type flows of which comprise the upper 46 750 m of sections in the solomon islands and at leg 130 site 807 on the northern flank of the plateau all of the leg 192 lavas including the high mgo 8 10 wt kroenke type basalts found at sites 1185 and 1187 have end t between 5 8 and 6 5 they are isotopically indistinguishable from the abundant kwaimbaita basalt type in the solomon islands and at previous plateau nauru basin and east mariana basin drill sites the little fractionated kroenke type flows thus indicate that the uniform isotopic signature of the more evolved kwaimbaita type basalt with 5 8 wt mgo is not simply a result of homogenization of isotopically variable magmas in extensive magma chambers but instead must reflect the signature of an inherently rather homogeneous relative to the scale of melting mantle source in the context of a plume head model the kwaimbaita type magmas previously have been inferred to represent mantle derived largely from the plume source region our isotopic modelling suggests that such mantle could correspond to originally primitive mantle that experienced a rather minor fractionation event e g a small amount of partial melting approximately 3 ga or earlier and subsequently evolved in nearly closed system fashion until being tapped by plateau magmatism in the early aptian these results are consistent with current models of a compositionally distinct lower mantle and a plume head origin for the plateau however several other key aspects of the plateau are not easily explained by the plume head model the plateau also poses significant challenges for asteroid impact icelandic type and plate separation perisphere models at present no simple model appears to account satisfactorily for all of the observed first order features of the ontong java plateau the geological society of london 2004

distribution of methane in the water column of the baltic sea the distribution of dissolved methane in the water column of the baltic sea was extensively investigated a strong correlation between the vertical density stratification the distribution of oxygen hydrogen sulfide and methane has been identified a widespread release of methane from the seafloor is indicated by increasing methane concentrations with water depth the deep basins in the central baltic sea show the strongest methane enrichments in stagnant anoxic water bodies max 1086 nm and 504 nm respectively with a pronounced decrease towards the pelagic redoxcline and slightly elevated surface water concentrations saturation values of 206 and 120 respectively in general the more limnic basins in the northern part of the baltic are characterized by lower water column methane concentrations and surface water saturation values close to the atmospheric equilibrium between 106 and 116 in contrast the shallow western baltic sea is characterized by high saturation values up to 746

evaluation of image based multibeam sonar backscatter classification for benthic habitat discrimination and mapping at stanton banks uk in recent years efforts have increased to develop quantitative computer directed methods for segmentation of multibeam mbes backscatter data this study utilises mbes backscatter data acquired at stanton banks uk and subsequently processed through the qtc multiview software environment in a bid to evaluate the program s ability to perform unsupervised classification statistical comparison with ground truth data grab stills and video enabled cross validation of acoustic segmentation and biological assemblages observed at the site 132 unspecified variables were extracted from user specified rectangular patches of the backscatter image reduced to three vectors by pca then clustered and classified by the software multivariate analyses of ground truth data were conducted on 75 stills images and 51 grab samples video footage coincident with the stills was divided into 30 s segments and coded by dominant substrate and species cross tabulation determined the interrelationship between software classifications multivariate analysis of the biological assemblages and coded video segments multiview optimally identified 19 classes using the automated clustering engine these were revised to 6 habitats a posteriori using combined analysis of ground truth data and multiview data products these habitats broadly correspond to major physiographic provinces within the region multivariate statistical analysis reveals low levels of assemblage similarity 35 for samples occurring within multiview classes irrespective of the mode of acquisition coded video data is more spatially appropriate than the other methods of ground truthing investigated although it is less well suited to the extraction of truly quantitative data multivariate analysis indicates assemblages within physiographically distinct multiview classes have a low degree of biological similarity supporting the notion that abiotic proxies may be contraindicative of benthic assemblage variations qtc multiview performs well as a mechanism for computer assisted segmentation of mbes backscatter imagery into acoustic provinces however a degree of caution is required prior to ascribing ecological significance to these classifications c 2008 elsevier ltd all rights reserved

improving riverbed sediment classification using backscatter and depth residual features of multi beam echo sounder systems riverbed and seafloor sediment classification using acoustic remote sensing techniques is of high interest due to their high coverage capabilities at limited cost this contribution presents the results of riverbed sediment classification using multi beam echo sounder data based on an empirical method two data sets are considered both taken at the waal river namely sint andries and nijmegen this work is a follow up to the work carried out by amiri simkooei et al j acoust soc am 126 4 1724 1738 2009 the empirical method bases the classification on features of the backscatter strength and depth residuals a principal component analysis is used to identify the most appropriate and informative features clustering is then applied to the principal components resulting from this set of features to assign a sediment class to each measurement the results show that the backscatter strength features discriminate between different classes based on the sediment properties whereas the depth residual features discriminate classes based on riverbed forms such as the fixed layer stone having riprap structure and riverbed ripples combination of these two sets of features is highly recommended because they provide complementary information on both the composition and the structure of the riverbed

on the role of sedimentological processes controlling phosphorus burial in the coastal zone of the baltic sea on the role of sedimentological processes controlling phosphorus burial in the coastal zone of the baltic sea

magnetic anomaly lineations from late jurassic to early cretaceous in the west central pacific ocean summary late jurassic to early cretaceous mesozoic magnetic anomaly lineations the phoenix magellan mid pacific mountains hawaiian and japanese lineation sets with fracture zones in the west central pacific ocean were identified more comprehensively than in any previous studies we fixed 2100 positions of identified magnetic anomalies based on magnetic data of 283 cruise tracks two remarkable fracture zones the phoenix and central pacific fracture zones were mapped and newly named our newly identified lineations from mlon to mo around the mid pacific mountains which belong to the hawaiian lineation set illustrated that the sea floor south of the mid pacific mountains has the same age as that of the north 132 118 ma our analysis of skewness parameters revealed that the older part of the phoenix set m17 m29 has skewness different from that of the younger part ml m14 implying an effect of magnetic overprints by the cretaceous volcanism it was confirmed that the spreading rate of the mesozoic pacific spreading system was the fastest in the world in the mesozoic a drastic change in spreading rates occurred simultaneously at the period between chrons m21 and m20 149 5 148 5 ma in all the mesozoic pacific spreading systems the event appears to be synchronous with events in other oceans such as the mesozoic atlantic and indian oceans