downfan transformation from turbidity currents to debris flows at a channel to lobe transitional zone the lower pleistocene otadai formation boso peninsula japan transformation of turbidity currents into debris flows across a channel to lobe transition zone was recognized on the basis of lateral mapping of a falling stage systems tract deposit in the lower pleistocene otadai formation submarine fan sequence on the boso peninsula japan the most proximal outcrops comprise 1 to 4 m thick turbidites which are laterally equivalent to debrites encased in turbidites in outcrops farther down flow such turbidites and encased debrites were deposited from single flow events the most distal outcrops comprise only thinner 2 to 40 cm thick turbidites debrites contain many siltstone clasts and finer grained sediments which are characterized by deep water faunas and clay mineral composition similar to those of interbedded hemipelagites therefore the transformation of turbidity currents into debris flows is interpreted to have occurred in response to the incorporation of many siltstone clasts and finer grained sediment particles into the precursor turbidity currents through erosion of muddy substrates subsequently turbulence in the precursor turbidity currents was suppressed and near bed flow with higher sediment concentration developed intense erosion of muddy substrates is interpreted to have occurred in response to the increase in intensity of turbulence in the precursor turbidity currents at the mouths of middle fan channels the precursor turbidity currents or turbidity currents generated by dilution of debris flows produced a turbidite deposit which was subsequently overrun by the debris flow in a proximal lobe environment the debris flows are interpreted to have subsequently been transformed into turbidity currents which are recorded by turbidite deposition in the more distal lobe environment debrites can characterize the proximal portion of an attached lobe deposit in a channel to lobe transition zone such debrites can be significant heterogeneities for fluid flows in sand prone successions from middle fan channel to attached lobe deposits

the multiple inverse method applied to meso scale faults in mid quaternary fore arc sediments near the triple trench junction off central japan the multiple inverse method for fault slip data is applied to meso scale faults observed in mid quaternary fore arc sediments near the triple trench junction oa the boso peninsula japan data from the otadai umegase and kokumoto formations were processed and three stresses were obtained as significant solutions vertical axial compression and triaxial stresses with the s3 axis in wnw ese and nne ssw directions the triaxial stresses were determined from the otadai and umegase data however the wnw ese tensile stress is not detected from the youngest kokumoto suggesting that the stress is older than the formation the area was subject to a wnw ese tensile stress 01 2 1 0 ma but tensile direction changed to a ne sw trend thereafter the succession was simultaneous with a tectonic event in the landward slope of the sagami trough suggesting that the subduction of the philippine sea plate aaected the stresses in the overriding plate the older tensile stress was probably a manifestation of the gravitational collapse of the hayama mineoka ridge which was growing parallel to the trough the inferred stress history is concordant with the variation of plate convergence at the trough 2000 elsevier science ltd all rights reserved

preferential mode of gas invasion in sediments grain scale mechanistic model of coupled multiphase fluid flow and sediment mechanics 1 we present a discrete element model for simulating at the grain scale gas migration in brine saturated deformable media we rigorously account for the presence of two fluids in the pore space by incorporating forces on grains due to pore fluid pressures and surface tension between fluids this model which couples multiphase fluid flow with sediment mechanics permits investigation of the upward migration of gas through a brine filled sediment column we elucidate the ways in which gas migration may take place 1 by capillary invasion in a rigid like medium and 2 by initiation and propagation of a fracture we find that grain size is the main factor controlling the mode of gas transport in the sediment and we show that coarse grain sediments favor capillary invasion whereas fracturing dominates in fine grain media the results have important implications for understanding vent sites and pockmarks in the ocean floor deep subseabed storage of carbon dioxide and gas hydrate accumulations in ocean sediments and permafrost regions our results predict that in fine sediments hydrate will likely form in veins following a fracture network pattern and the hydrate concentration will likely be quite low in coarse sediments the buoyant methane gas is likely to invade the pore space more uniformly in a process akin to invasion percolation and the overall pore occupancy is likely to be much higher than for a fracture dominated regime these implications are consistent with laboratory experiments and field observations of methane hydrates in natural systems

tidal height and frequency dependence of acoustic velocity and attenuation in shallow gassy marine sediments remote prediction of gassy marine sediment properties is important for geohazard assessment gas bubble resonance theory suggests that gassy sediments exhibit acoustic wave velocity frequency and attenuation frequency relationships that depend on gas bubble size gas content and sediment elastic properties an acoustic monitoring experiment to investigate gas bubble resonance effects was undertaken at an intertidal site at dibden bay southampton united kingdom a vertical hydrophone array was positioned to straddle the top of the gassy zone identified on acoustic reflection profiles at about 1 m below the seabed a miniboomer in the seabed above the array was used to generate broadband 600 hz to 3000 hz acoustic signals every 10 min during a 24 hour period with water depths varying between 0 m subaerial exposure at low tide and 2 35 m at high tide the calculated frequency spectra of compressional wave attenuation coefficient show an attenuation maximum over 200 db m that shifts in frequency from 1050 hz at low tide to 1250 hz at high tide thus for the first time providing direct evidence of in situ gas bubble resonance in marine sediments modeling suggests that effective gas bubble radii of 11 mm to 13 mm are responsible for the attenuation maximum supported by x ray computed tomography scan observations on a pressure core which also indicate that bubble shape depends on sediment type modeling of bubble size fluctuations due to pressure equilibration cannot reproduce the observed frequency shift of the attenuation maximum implying that gas diffusion and nonspherical bubbles are significant

high resolution diatom record of paleoceanographic variations across the early middle pleistocene boundary in the chiba section central japan abstract we conducted diatom analyses of a core from the pleistocene marine sequence drilled at the chiba section central japan to reveal detailed paleoceanographic variations across the early middle pleistocene boundary diatom valves are most abundant at 4 5 m below the matuyama brunhes magnetic polarity boundary followed by the second highest abundance at 18 6 m above the boundary which is consistent with the sea level changes inferred from planktonic 18 o data the highest abundance is correlated with the marine isotope stage mis 19 3 sea level highstand and the second highest with the mis 19 1 highstand the abundance of the extinct species actinocyclus ingens is quite low around these sea level highstands whereas it is greatest at the core depth with the minimum diatom total valves which is correlated to the mis 19 2 lowstand cold water diatom species that are characteristic of the oyashio current are dominant but coexist with warm water species of the kuroshio current below a core depth just postdating mis 19 2 above this depth warm diatom species of the kuroshio current become dominant the change in dominant diatoms suggests that the kuroshio front shifted northward just after mis 19 2 and remained in the northern area until at least early mis 18 this shift caused a rapid increase in sea surface temperature by 4 5 c within a sediment thickness of 1 3 m representing 300 years

inferred glacio eustatic control for high frequency depositional sequences of the plio pleistocene kazusa group a forearc basin fill in boso peninsula japan abstract the plio pleistocene forearc basin fill of the kazusa group as much as 3000 m thick is represented by a series of marine environments ranging from deep sea basin plain to shallow sea it developed during about 2 4 to 0 45 ma and is well exposed in the boso peninsula japan chronostratigraphic resolution of the kazusa group is partly within about 0 05 m y based on the combination of fission track dating of volcanic ash layers and bio and magneto stratigraphy many volcanic ash layers are intercalated in the kazusa group and provide distinct chronostratigraphic surfaces within the heterogeneous lithologies detailed outcrop studies on depositional facies discontinuities and lateral terminations of volcanic ash layers allow high resolution sequence stratigraphic interpretation of the kazusa group seventeen depositional sequences were identified in the kazusa group each depositional sequence was tentatively correlated with the stable oxygen isotope sea level index integrated with the age framework of the kazusa group this correlation indicates that frequencies of the depositional sequences can be estimated to be in the range of 0 2 0 02 m y and are within fourth and fifth order stratigraphic cyclicities depositional sequences constitute third order transgressive and highstand sequence sets and the kazusa group is interpreted to represent a third order composite sequence with a duration of about 1 95 m y development of the third order composite sequence was also controlled by an overall rise and fall of glacio eustatic sea levels overprinted by tectonic events related to fluctuations in oceanic plate motion which occurred at about 2 6 ma and 0 5 ma respectively

bubble growth and rise in soft sediments the mechanics of uncemented soft sediments during bubble growth are not widely understood and no rheological model has found wide acceptance we offer definitive evidence on the mode of bubble formation in the form of x ray computed tomographic images and comparison with theory natural and injected bubbles in muddy cohesive sediments are shown to be highly eccentric oblate spheroids disks that grow either by fracturing the sediment or by reopening preexisting fractures in contrast bubbles in soft sandy sediment tend to be spherical suggesting that sand acts fluidly or plastically in response to growth stresses we also present bubble rise results from gelatin a mechanically similar but transparent medium that suggest that initial rise is also accomplished by fracture given that muddy sediments are elastic and yield by fracture it becomes much easier to explain physically related phenomena such as seafloor pockmark formation animal burrowing and gas buildup during methane hydrate melting

correlation of shape and size of methane bubbles in fine grained muddy aquatic sediments with sediment fracture toughness abstract gassy sediments contribute to destabilization of aquatic infrastructure air pollution and global warming in the current study a precise shape and size of the buoyant mature methane bubble in fine grained muddy aquatic sediment is defined by numerical and analytical modeling their results are in a good agreement a closed form analytical solution defining the bubble parameters is developed it is found that the buoyant mature bubble is elliptical in its front view and resembles an inverted tear drop in its cross section the size and shape of the mature bubble strongly correlate with sediment fracture toughness bubbles formed in the weaker sediments are smaller and characterized by a larger surface to volume ratio that induces their faster growth and may lead to their faster dissolution below the sediment water interface this may prevent their release to the water column and to the atmosphere shapes of the bubbles in the weaker sediments deviate further from the spherical configuration than those in the stronger sediments modeled bubble characteristics important for the acoustic applications are in a good agreement with field observations and lab experiments

eustatic sea level changes interpreted from seismic stratigraphy a critique of the methodology with particular reference to the north sea jurassic record seismic stratigraphic methods for interpreting eustatic sea level change are based on generalized stratigraphic models and chronostratigraphic interpretations that have not been substantiated by publication of the necessary basic supportive data chronostratigraphic correlations of sea level change curves between basins are based partly on circular reasoning onlap and offlap seismic models may not indicate sea level change but flexural subsidence facies shifts or changes in sea level in the opposite direction to that proposed resolution of some of these problems may be achieved by publication of basic data now in confidential industry files the widely acclaimed vail curves should not be used as templates for basin correlation until such data have been provided and such use should be supported by appropriate conventional lithostratigraphic and biostratigraphic documentation from each new basin where such data have been made available as with the north sea jurassic serious questions of interpretation are raised

off shelf sedimentary record of recurring global sea level changes during the plio pleistocene evidence from the cyclic fills of exhumed slope systems in central italy the markedly cyclic sedimentary successions of four late pliocene to early pleistocene slope turbidite systems exposed in eastern central italy have been resolved into 31 high frequency sequences chronological constraints from biostratigraphy and magnetostratigraphy indicate that these successions form a composite partially overlapping stratigraphic record and sequence bounding surfaces can be convincingly correlated with glacial oxygen isotope stages g2 60 c 2 65 1 7 ma inclusive the studied successions therefore preserve an extraordinary and legible record of recurring orbitally dictated glacio eustatic sea level fluctuations and provide an unprecedented opportunity to examine the deep water sedimentary response to such high frequency changes from an outcrop perspective supplementary material sedimentological attributes of lithofacies identified in the studied successions are available at www geolsoc org uk sup18549

reactivation of map scale faults in response to changes in crustal stress examples from boso peninsula japan abstract we have investigated the reactivation of quaternary faults near the trench trench trench type triple junction off boso peninsula central japan on the pacific side of the boso peninsula psbp region map scale nne ssw trending faults developed during 1 2 0 7 ma although these faults are inactive at present we observed multiple and overlapped slickenlines on parts of map scale fault planes in the psbp region these slickenlines can be classified into two different directions which we consider to be the reactivation of the map scale faults in response to a change in crustal stress after 1 ma based on angular misfits between the predicted and observed slip directions we successfully compared the older and younger slip directions to dip slip normal faulting and oblique sinistral normal faulting respectively the reactivation of the map scale faults in response to a relatively short 105 yrs stress change has been verified based on the direct evidences on the map scale fault planes

archaeomagnetic results from southern italy and their bearing on geomagnetic secular variation archaeodirectional results from kilns and other baked structures in southern italy are presented they are generally compatible with the much larger data sets from france and bulgaria in particular a summary of all the results associated with the well known eruption of vesuvius that destroyed pompeii n 9 d 355 i 5 8 95 1 5 provides a reliable archaeomagnetic anchor point supporting the french and bulgarian master curves it is extremely well constrained in time and it comprises independent studies carried out in four different countries furthermore it is derived from a diverse set of features agreement amongst which argues strongly against significant perturbations due to magnetic refraction structural disturbance or depositional shallowing in terms of geomagnetic secular variation we interpret the western european archaeomagnetic data summarized here in terms of an open loop caused by westward drift followed by an inclination low spanning the first few centuries ce representing the signal of a static flux pulse that reaches a maximum magnetic moment of a few percent of the earth s main central dipole 2005 elsevier b v all rights reserved

oxygen isotopes and sea level from the time that detailed oxygen isotope records derived from foraminifera living in the constant temperature environment of the abyssal ocean became available there has been a discrepancy between the ice volume record that these records imply and that derived from the altitude of dated coral terraces around the world here we re examine the data and conclude that the temperature of the abyssal ocean has been an actively varying component of the climate system

sequence response to syndepositional regional uplift insights from high resolution sequence stratigraphy of late early pleistocene strata periadriatic basin central italy abstract this paper deals with the depositional sequences that occur within the uppermost part of the plio pleistocene periadriatic basin fill in the southern marche region central italy the succession is an early pleistocene easterly dipping clastic wedge showing an overall shallowing trend from slope clays to shallow marine and non marine deposits comprising two major sequences namely qmb and qmc analysis has provided new insights into i the nature of sedimentary facies and facies associations occurring within the upper part of qmb and qmc ii the gradual contact within qmb between regressive littoral deposits rld and underlying deep marine blue clays iii the composite origin of the emilian surface which is a widespread erosional unconformity separating qmb from qmc iv the cyclothemic pattern of qmc composed of downstepping small scale depositional sequences v the role played by synsedimentary uplift on the stacking pattern of small scale sequences and their internal architecture up to three small scale depositional sequences have been recognised within qmc qmc1 qmc2 and qmc3 they are up to 50 m thick and defined by previously unrecorded lower rank regionally extensive surfaces facies organization indicates that as a rule in ascending stratigraphic order each small scale sequence may includes a distinctive basal unconformity surface that can be traced from incised valleys to associated interfluves an incised valley fill a transgressive surface of marine erosion a transgressive systems tract a highstand systems tract a regressive surface of marine erosion and an attached falling stage systems tract in proximal positions the small scale sequences are dominated by transgressive and highstand systems tracts with incised valley fills and falling stage systems tracts absent or volumetrically much less significant in relatively basinward locations where the regressive surfaces of marine erosion converge with the lower sequence boundary the falling stage systems tract may represent the entire depositional sequence high frequency sequences display a distinctive stacking pattern and form a tectonically induced forced regressive sequence set underlain by a composite tectonically enhanced regressive surface of marine erosion formed by the lateral connection of lower rank sequence boundaries

sedimentology and high resolution sequence stratigraphy of the late middle to late miocene angostura formation western borbon basin northwestern ecuador an integrated analysis of facies and palaeontological content of the middle to late miocene c 12 4 9 1 ma angostura formation led to the identification of facies associations indicative of shoreface and inner shelf settings the angostura formation comprises eight high frequency sequences that are stacked to form a tectonically driven lowstand sequence set the most complete examples of sequences are bounded by transgressively modified unconformities and exhibit a threefold subdivision 1 a basal suite of deepening upward shoreface sediments transgressive systems tract including a base of cycle shell concentration 2 a mid cycle shell bed consisting of molluscan shells dispersed in a matrix of inner shelf muddy fine grained sandstones 3 a shallowing upward unit of inner shelf to lower shoreface sediments almost barren of mollusc fossils highstand systems tract biostratigraphic constraints allowed a reasonable correlation between sequence bounding unconformities and the late middle to late miocene high frequency glacio eustatic changes derived from recent 18 o studies this correlation has far reaching implications and leads to the following conclusions 1 glacio eustasy in tune with oxygen isotope changes at fourth order frequency 200 ka 1 ma duration may have been the principal factor regulating stratigraphic packaging in the angostura formation 2 these sequences provide an excellent shallow marine outcrop record of late middle to late miocene antarctic glaciations

estimation of maximum burial depth of neogene quaternary fore arc basin formation based on laboratory porosity measurements under pressure abstract estimating the maximum effective stress that rocks have experienced p e max or the maximum burial depth for sedimentary rocks d max is important for many types of research ranging from engineering applications to estimation of tectonic evolution we estimated p e max and d max for the kazusa fore arc basin formations the kazusa group in the boso peninsula of japan using a laboratory based method we carried out measurements of porosity n with siltstone specimens from the kazusa group formations the umegase otadai kiwada ohara and katsuura formations under various effective pressure p e conditions and estimated p e max from the inflection points of the log p e log n curve on the p e increasing path except for the specimens from the ohara formation estimated values of p e max ranged from approximately 13 24 mpa this range corresponded to approximately 1 3 3 2 km of d max differences in d max among the specimens were at least four times smaller than distances normal to bedding planes among the sampling locations this suggests that the formations were not deposited horizontally but that deposition proceeded as the subsidence center of the fore arc basin moved in a northwestward nw direction and that formations were then uplifted almost horizontally the p e max of the specimens from the ohara formation were 6 10 mpa smaller than the others thus it is possible that pore pressure at the sampling location was more than 6 mpa larger than the hydrostatic condition when the sediments were deposited and lithified previous studies reported the center of a high porosity zone at the ohara formation and this high porosity zone probably developed due to p p over pressurization these results support the applicability of this method to estimation of tectonic evolution of sedimentary basins and magnitude of over pressurization

methane bubble growth in fine grained muddy aquatic sediment insight from modeling abstract methane ch 4 is the most abundant hydrocarbon and one of the most important greenhouse gases in the atmosphere ch 4 bubble growth and migration within muddy aquatic sediments are closely associated with sediment fracturing in this paper we present the modeling of buoyancy driven ch 4 bubble growth in fine grained muddy aquatic sediment prior to the beginning of its rise we designed a coupled mechanical reaction transport numerical model that enables a differential fracturing over the bubble front as it occurs in nature when the fracturing increment stays constant at the bubble head and subsides towards bubble tail during bubble growth we show that this differential fracturing over the bubble front controls the bubble shape and size temporal evolution and is significantly affected by the critical stress intensity factor of the muddy sediment the intercalated stages of elastic expansion and fracturing during the bubble growth shorten with time as the bubble approaches its terminal size prior to its ascent our simulations reveal a high asymmetry in the bubble shape growing with time with respect to its initial symmetric penny shaped configuration it was found that the bubble grows allometrically while the importance of the bubble surface area growth with time we also confirmed the earlier predictions about the inverted tear drop bubble cross section just prior to the beginning of its rise modeling of the terminal bubble characteristics will permit prediction of the delivery of gaseous methane from the sediment to the atmosphere via the water column

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

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

sedimentary successions formed by fifth order glacio eustatic cycles in the middle to upper quaternary formations of the oga peninsula northeast japan abstract the northern coast of the oga peninsula is a type locality for quaternary sedimentary exposures on the sea of japan side of northeast japan middle to late pleistocene shallow marine to nonmarine sediments are continuously exposed on sea cliffs along the anden coast these sediments have been mapped and classified into nine sedimentary facies based on grain size sedimentary structures intensity of bioturbation and associated fossils the depositional environments and paleo water depths of each facies have been estimated based on comparison with the present sub bottom sedimentary features observed in coastal systems around the oga peninsula on the basis of sedimentary facies analysis variations in the depositional depth through time have been reconstructed and correlated with the specmap 18o curve using four marker tephras as time controls the two curves generally agree between approximately 80 and 450 ka suggesting that the sedimentary cycles at the anden coast faithfully record 5th and 6th order glacio eustatic sea level changes peak to peak comparison between the reconstructed depositional depth curve and the 18o curve reveals that 1 unconformities are formed during the late eustatic sea level falls and or the following lowstands 2 transgressive lag deposits are mainly formed below wave base 3 deepening upward successions which sometimes occur below the ravinement surfaces are formed in the inner part of estuary or barrier island environments where wave action was minor 4 shoaling upward successions formed during eustatic sea level falls are deposited under the influence of intense wave action in a strand plain coastal plain system and 5 the characteristic combination of thinner deepening upward successions and thicker shoaling upward successions reflects the asymmetric pattern of glacio eustatic sea level changes and or lower sediment supply during transgression

gas bubble cavities in deltaic muds lake powell delta glen canyon national recreation area hite utah abstract cavities produced by migrating gas mainly methane emanating from the near subsurface are a rarely reported sedimentary structure from either modern or ancient clastic sediments the delta of the colorado river in lake powell glen canyon national recreation area provides an excellent recently accessible locality for examining soft sediment deformation structures generated by escaping gases and fluids sub meter scale to tens of meter scale structures including craters salses and sand and mud volcanoes are present on the modern lake powell delta abundant sub centimeter scale cavities are found in excavations through crater sediments and exposed along cut bank exposures of the colorado river in cross section the cavities are subvertical irregular to sigmoidal to lenticular shaped 1 3 mm short axes up to 3 0 cm long axes and 1 cm intermediate axes cavities are developed in cm scale laminated silty clay and clay graded beds the tops of the cavities are wider than their bases the cavities observed in the lake powell delta are interpreted as mode 1 fractures and probably represent a clastic morphologic analog to molar tooth carbonate structures the characteristics of the cavities and the geometries and deformation of the surrounding delta mud host are consistent with bubbles and fracturing generated by gas migration experiments and theoretical calculations but vary because of inherent heterogeneity of the sediments fractures developed as gas migrated into pre existing cavities causing overpressure in cavities and leading to subvertical mode 1 fracture propagation recent sudden decompression related to rapid lake level drops of lake powell may have enhanced the release of gas primarily bacterially produced leading to the development of these soft sediment deformation structures

paleoclimatic and paleoceanographic records through marine isotope stage 19 at the chiba composite section central japan a key reference for the early middle pleistocene subseries boundary abstract marine isotope stage mis 19 is an important analogue for the present interglacial because of its similar orbital configuration especially the phasing of the obliquity maximum to precession minimum however sedimentary records suitable for capturing both terrestrial and marine environmental changes are limited and thus the climatic forcing mechanisms for mis 19 are still largely unknown the chiba composite section east central japanese archipelago is a continuous and expanded marine sedimentary succession well suited to capture terrestrial and marine environmental changes through mis 19 in this study a detailed oxygen isotope chronology is established from late mis 20 to early mis 18 supported by a u pb zircon age and the presence of the matuyama brunhes boundary new pollen marine microfossil and planktonic foraminiferal 18o and mg ca paleotemperature records reveal the complex interplay of climatic influences our pollen data suggest that the duration of full interglacial conditions during mis 19 extends from 785 0 to 775 1 ka 9 9 kyr which offers an important natural baseline in predicting the duration of the present interglacial a younger dryas type cooling event is present during termination ix suggesting that such events are linked to this orbital configuration millennial to multi millennial scale variations in our 18o and mg ca records imply that the subarctic front fluctuated in the northwestern pacific ocean during late mis 19 probably in response to east asian winter monsoon variability the climatic setting at this time appears to be related to less severe summer insolation minima at 65 n and or high winter insolation at 50 n our records do not support a recently hypothesized direct coupling between variations in the geomagnetic field intensity and global regional climate change our highly resolved paleoclimatic and paleoceanographic records coupled with a well defined matuyama brunhes boundary 772 9 ka duration 1 9 kyr establish the chiba composite section as an exceptional climatic and chronological reference section for the early middle pleistocene boundary

geological controls on seismic sequence resolution a seismic sequence analysis of the canterbury basin eastern south island of new zealand has illustrated the roles of subsidence sediment supply and current activity as controls on sequence resolution and architecture during basin evolution the rates of sediment supply and subsidence determine the background depositional regime transgressive or regressive and effectively determine the frequency response of the continental margin sedimentary section to input signals with a broad range of frequencies including eustasy a regressive progradational depositional regime and minimal current erosion favor the preservation of high frequency sequences particularly at fourth order level under less favorable conditions the record of sequences is incomplete or ambiguous such frequency response characteristics must be considered when inverting sequence records to derive the frequency of the input cyclicity and when making global comparisons of regional sequence stratigraphic studies clastic basins with simple subsidence histories and uniform or increasing rates of sediment supply develop from a transgressive phase characterized by ramplike major sequence boundaries to a mature progradational shelf phase with clinoform sequences and optimum sequence resolution the mature phase constitutes the preferred setting for sequence stratigraphic analyses

marine and nonmarine deposition in a long term low accommodation setting an example from the middle pleistocene qm2 unit eastern central italy abstract facies anatomy and large scale stratigraphic organization of mid pleistocene strata qm2 unit exposed near the town of ortona eastern central italy have been defined through an integrated outcrop and subsurface dataset and record sedimentation within a long term low accommodation setting the studied interval can be subdivided into nine facies associations reflecting a range of continental marginal marine nearshore and innermost shelf environments and contains a well preserved record of high frequency relative sea level changes in the form of a stack of three unconformity bounded sequences each of the three sequences designated qm23 qm22 and qm21 from oldest to youngest consists of a specific subset of the nine identified facies associations as a function of preserved depositional systems and systems tracts typically the following architectural elements occur in ascending stratigraphic order i a basal unconformity molded by fluvial incision and subaerial exposure during sea level fall and shoreface erosion during subsequent rise ii a maximum regressive surface which is coincident with the sequence boundary iii a transgressive systems tract including braided river conglomerates and floodplain fines within the confines of an incised valley palustrine carbonates and back barrier lagoon mudstones and a deepening and fining upward shoreface to offshore transition facies succession bounded at the base by a wave ravinement surface iv a sharp regressive surface of marine erosion produced by shoreface incision during relative sea level fall and v a prograding downstepping and offlapping sharp based lithofacies assemblage of either beach conglomerates or sandstones ascribed to the falling stage systems tract overlain by a rooted paleosol marking subaerial exposure sequences were deposited on the uplifting palaeo adriatic shelf in response to repeated high frequency and high amplitude eustatic changes in sea level that would promote the creation of accommodation space by subtracting space to deposition the contemporaneous regional uplift of the basin margin may be accounted for the shingled downstepping configuration of individual sequences to form a falling stage sequence set of a larger composite sequence in dip view the erosional contact at the base of the qm2 falling stage sequence set is a composite regressive surface resulting from the downdip convergence and amalgamation of multiple erosional surfaces produced by the progressive basinward translation of successive sequences results from this study have direct implications for the oil and gas field development and exploration when making predictions of reservoir and seal geometries within shallow marine marginal marine and nonmarine sediments emplaced in similar long term limited accommodation settings

pleistocene unconformity bounded shelf sequences wanganui basin new zealand correlated with global isotope record abstract eleven successive unconformity bound sequences in the castlecliff section wanganui basin each typically composed of a lower variably conglomeratic 0 1 2 m thick shell bed and an upper 5 20 m thick barren siltstone are identified in the stratotype of the local new zealand middle pleistocene castlecliffian stage based on magnetostratigraphy the section is correlated with the published benthonic oxygen isotope curve for dsdp core 552a from the north atlantic each of the sequences accumulated during one of the odd numbered interglacial isotope stages in the interval 31 to 11 inclusive the unconformities developed during intervening glacial ages when the shelf was exposed through glacio eustatic lowering the correlation provides a geochronologic framework for assessment of the eustatic contribution to the generation of the sequences in a situation where the paleoshorelines and related shelf sequences are preserved and for which an independent approximation of the sea level history is available in the form of an oxygen isotope curve

continuous geomagnetic field models for the past 7 millennia 2 cals7k 1 we present two continuous global geomagnetic field models for recent millennia cals3k 2 covering the past 3000 years and cals7k 2 covering 7000 years from 5000 bc to 1950 ad the models were determined by regularized least squares inversion of archeomagnetic and paleomagnetic data using spherical harmonics in space and cubic b splines in time they are derived from a greatly increased number of paleomagnetic directional data compared to previous efforts and for the first time a significant amount of archeointensity data is used in this kind of global model allowing the determination of evolution of geomagnetic dipole strength while data accuracy and dating uncertainties remain a limitation reliable low resolution global models can be obtained the results agree well with previous results from virtual axial dipole moment vadm studies from archeomagnetic intensity data apart from a systematic offset in strength a comparison of model predictions with the previous 3000 year model cals3k 1 gives general agreement but also some significant differences particularly for the early epochs the new models suggest that the prominent two northern hemisphere flux lobes are more stationary than cals3k 1 implied extending considerably the time span of stationary flux lobes observed in historical models between 5000 bc and 2000 bc there are time intervals of weak dipole moment where dipole power is exceeded by low degree nondipole power at the core mantle boundary model coefficients and evaluation code can be obtained from the earthref digital archive erda together with animations and snapshots plots for every 100 years at http www earthref org detailed urls for the different material are listed in appendix a

high frequency depositional sequences of the upper part of the kazusa group a middle pleistocene forearc basin fill in boso peninsula japan abstract a middle pleistocene forearc basin fill of the upper part of the kazusa group is represented by slope to shelf sediments and is well exposed in the boso peninsula of japan the slope to shelf sediments are a northeasterly prograded clastic wedge as much as 700 m in maximum thickness and include many volcanic ash layers mapping of volcanic ash layers together with the interpretation of depositional facies permits identification of stratal terminations and spatial and temporal variations of depositional environments this allows sequence stratigraphic analysis of the slope to shelf sediments of the upper part of the kazusa group six depositional sequences were identified in the upper part of the kazusa group by correlating each depositional sequence with the stable oxygen isotope sea level index integrated with the age framework for the upper part of the kazusa group ca 0 72 0 45 ma periodicity of the depositional sequences of the upper part of the kazusa group can be estimated at about 20 000 to 90 000 years the high frequency depositional sequences are interpreted to be a result of interaction between high sedimentation rate in the forearc basin and high rate of glacio eustatic sea level changes during the middle pleistocene

methane production potential of subsurface microbes in pleistocene sediments from a natural gas field of the dissolved in water type central japan abstract despite the important contribution of biogenic methane to natural gas resources worldwide little is known about the methane production potential of microbes in deep subsurface environments we investigated the potential of microbes in pleistocene sediments from a natural gas field of the dissolved in water type in central japan core sediments were collected from 287 to 606 m below ground level and sludge sediments were collected from the settling ponds where sediment particulates of reservoir rocks associated with the production water had been deposited radiotracer experiments using the core samples suggested that microbial methanogenesis occurs in situ in all samples with carbonate reduction as the main pathway the methane production rate in the deepest sample was as high as those in the subseafloor sediments from the blake ridge region where methane hydrate formed with biogenic methane is known to exist long term tracer free incubation experiments showed that intense methane production started either immediately in the sludge samples or after a lag time in the core samples abundance of sedimentary organic carbon decreased during incubation implying that carbon in the produced methane was derived from sedimentary organic matter phylogenetic analysis of microbes in the incubated core samples and stable carbon isotopic compositions of the produced methane indicated that methane was produced mostly through the acetoclastic pathway implying that the pathway changed from the initial carbonate reduction pathway during the intense methane production our results indicate the potential of subsurface microbes to metabolize a substantial fraction of fossil recalcitrant sedimentary organic matter for methanogenesis

eustatic and regional tectonic controls on late pleistocene paleovalley morphology in the central kanto plain japan abstract this study analyzed the morphology of three late pleistocene buried paleovalleys in the central kanto plain japan by using data from sedimentary cores and boreholes the paleo arakawa valley and the paleo nakagawa valley merge to form the paleo tokyo valley which extends into tokyo bay although the last glacial sea level fall controlled the formation of these paleovalleys fluvial terraces developed differently in these paleovalleys reflecting regional differences in tectonic activity the paleo tokyo valley beneath the tokyo lowland and the paleo arakawa valley beneath the arakawa lowland are in relatively stable or uplifting regions and both paleovalleys have several distinct fluvial terraces forming stepped cross sectional profiles in contrast in the paleo nakagawa and the paleo arakawa valleys beneath the subsiding menuma lowland terrace surfaces are obscure and there is a smooth gradient from the valley margins to the valley axis the structure of the paleovalleys in the central kanto plain can thus be explained by the combination of eustatic sea level fall and concurrent regional tectonic movement

sedimentary processes and depositional environments of a continuous marine succession across the lower middle pleistocene boundary kokumoto formation kazusa group central japan abstract the chiba composite section of the kokumoto formation in central japan is a remarkable thick marine succession and is a candidate for formal recognition as the global boundary stratotype section and point gssp for the base of the middle pleistocene stage although there have been numerous studies of the kokumoto formation the sedimentology of the formation particularly in the chiba composite section has been unclear we use field observations and laboratory analyses to elucidate the sedimentary processes and depositional environments of the formation the chiba composite section includes the matuyama brunhes paleomagnetic reversal which is the primary guide for the lower middle pleistocene boundary along with multiple tephra deposits that provide isochronous correlational tie points one bed byk e also known as ontake byakubi tephra immediately underlies the paleomagnetic reversal and hence provides a useful widespread marker for the boundary the predominant silty beds of the chiba composite section are intensely bioturbated and lack evidence of episodic deposition such as slumps or muddy turbidites however minor sandy beds intercalated within the silty section particularly in its lower part are either 1 fining upward sequences with faint planar or ripple laminations and an erosional base or 2 sequences containing abundant mud clasts in a poorly sorted sand matrix with a sharp base and an irregular top surface the silty beds are interpreted to be hemipelagite formed by deposition of fine grained suspended material under stable and calm bottom water conditions the sandy deposits are interpreted to be sediment gravity flow deposits i e turbidites and debrites these sedimentary processes and the trace fossil assemblage indicate that the chiba composite section was formed in a continental slope environment the depositional environments of the chiba composite section together with tephrostratigraphy make it suitable for high resolution studies on the paleoenvironment paleoclimate and paleoceanography of the northwest pacific ocean across the lower middle pleistocene boundary it is therefore a suitable candidate to become the gssp for the middle pleistocene

geomagnetic field variations in western europe from 1500bc to 200ad part i directional secular variation curve abstract to improve the geomagnetic field secular variation curve svc of western europe during protohistoric times archaeomagnetic directions of 39 archaeological kilns or hearths from france were investigated the dating of each archaeological structure was established with archaeological or chronometric methods thirty seven of these structures are dated from the first millennium bc one from the end of the second millennium bc and the last one from the fourth millennium bc thermomagnetic curves unblocking temperatures and coercivities suggest that the main carrier of the remanent magnetization is a ti poor titanomagnetite archaeodirections were obtained by alternating field and thermal demagnetizations on almost 900 specimens the anisotropy tensor of thermoremanent magnetization was determined for 35 structures and 22 mean archaeodirections were corrected for anisotropy the new archaeodirections are very consistent with previously published data a new directional svc was built using bivariate statistics with selected western europe data located within 1000 km of paris selection criteria include the number of samples the dating reliability and the accuracy of the mean archaeodirection resulting secular variation between 1500bc and 0ad mainly shows large changes in declination while inclinations are bracketed between 65 and 75 the declinations show a strong maximum with values 30 35 around 800 750bc followed by a sharp decrease to values around 0 at 500bc and close to 7 around 250bc the main features of the secular variation from 1500bc to 0ad appear to be a dominant westward drift and two major changes around 800 and 250bc compared to the global and regional geomagnetic models the new reference data are better fitted by arch3k cst 1 and scha dif 3k than by arch3k 1 and cals3k 4 models the strong variation of the archaeodirection between 1000 and 500bc makes archaeomagnetism very useful for dating purposes

compositional variation in depositional sequences of the upper part of the kazusa group a middle pleistocene forearc basin fill in the boso peninsula japan abstract shelf to slope successions of the upper part of the kazusa group as much as 700 m thick constitute a northeasterly prograded clastic wedge of middle pleistocene age and consist of six depositional sequences fundamental component building blocks of depositional sequences i e lowstand transgressive and highstand systems tract sediments are characterized by unique combinations of depositional processes and petrofacies transgressive systems tract sediments are characterized by a high frequency of volcanic ash layers and volcanogenic petrofacies indicating a substantial contribution of pyroclastic material to marine environments this corresponds to the storage of coarse grained terrigenous clastic sediments at a river mouth area during transgressions highstand systems tract sediments are characterized by a lower frequency of volcanic ash layers and more quartzose petrofacies and may have been affected by abrasion and destruction of unstable lithic fragments by nearshore dynamic processes such as reworking by waves tides and longshore currents in response to active progradation of a coastal depositional system in general progradation of coastal depositional systems is associated with active supply of coarse grained terrigenous sediments by a fluvial system the increase in supply of coarse grained terrigenous sediments reduces the frequency of volcanic ash layers in the upper part of the highstand systems tract sediments lowstand systems tract sediments show more variable petrofacies than that of highstand systems tract sediments and may have been affected much more by the direct fluvial supply of coarse grained terrigenous sediments which were not subjected to abrasion and destruction of unstable lithic fragments by nearshore dynamic processes these results demonstrate that the development of the upper part of the kazusa group can best be interpreted in terms of the interaction between marine and fluvial processes and fluctuation in the relative rate of coarse grained terrigenous sediment supply the frequency of volcanic ash layers and the relative contents of unstable volcanic rock fragments in marine sediments may be a sensitive indicator of the dynamic processes and may provide an additional opportunity for characterizing systems tracts within a depositional sequence in a volcanic arc setting

archaeomagnetic secular variation in germany during the past 2500 years summary the german archaeomagnetic data set was supplemented with 35 new directions from ger man sites mainly dating from the past 3000 yr together with archaeomagnetic data from the neighbouring countries of germany a database of 166 reliable archaeomagnetic directions has been compiled for the reference area which is defined by a 500 km radius circle around gottingen the retrieved directions come from well dated archaeological structures and about 40 per cent of the dating relay on natural science methods such as radiocarbon thermolumines cence dendrochronology dating or historical documents from this data set a secular variation sv reference curve has been calculated using a bivariate algorithm which fits a natural cubic spline based on roughness penalty to declination inclination and time simultaneously the error tube surrounding this curve was obtained from bayesian modelling of the experimental errors which can also take stratigraphic information into account the obtained sv reference curve for the past 2500 yr is similar to that from france but also significant differences are seen comparison of the curves does not show a simple westward drift of the sv pattern the german reference curve allows archaeomagnetic dating in the reference area and extends this dating technique to sites situated in middle europe

a conceptual model for soils containing large gas bubbles offshore site investigations have indicated the presence of undissolved gas bubbles in the soils forming the sea bed at various locations throughout the world the gas bubbles are typically much larger than the normal void spaces so they cannot be considered as occluded bubbles within the pore water which simply change the compressibility of the pore fluid the effect of these large bubbles on the engineering properties of the soil must be understood if offshore construction is to take place in areas of gas bearing sediments this paper describes a conceptual model for soils containing large gas bubbles the model consists of a matrix of saturated soil surrounding isolated gas filled cavities the cavities forming the bubble sites are fixed within the soil so that gas drainage occurs only by the movement of dissolved gas the behaviour of the model is complicated by the processes of localized consolidation and bubble flooding des recherches conduites en mer ont indique qu il y a partout dans le monde d

fate of rising methane bubbles in stratified waters how much methane reaches the atmosphere there is growing concern about the transfer of methane originating from water bodies to the atmosphere methane from sediments can reach the atmosphere directly via bubbles or indirectly via vertical turbulent transport this work quantifies methane gas bubble dissolution using a combination of bubble modeling and acoustic observations of rising bubbles to determine what fraction of the methane transported by bubbles will reach the atmosphere the bubble model predicts the evolving bubble size gas composition and rise distance and is suitable for almost all aquatic environments the model was validated using methane and argon bubble dissolution measurements obtained from the literature for deep oxic saline water with excellent results methane bubbles from within the hydrate stability zone typically below 500 m water depth in the ocean are believed to form an outer hydrate rim to explain the subsequent slow dissolution a model calibration was performed using bubble dissolution data from the literature measured within the hydrate stability zone the calibrated model explains the impressively tall flares 1300 m observed in the hydrate stability zone of the black sea this study suggests that only a small amount of methane reaches the surface at active seep sites in the black sea and this only from very shallow water areas 100 m clearly the black sea and the ocean are rather effective barriers against the transfer of bubble methane to the atmosphere although substantial amounts of methane may reach the surface in shallow lakes and reservoirs

sediment damage caused by gas exsolution a key mechanism for mud volcano formation abstract gassy sediments are common in marine environments and are characterized by a specific mechanical behavior significantly different from that of water saturated sediments it is shown that gas causes damage and initiates fractures in sediments to define the controlling parameters dominating the damage process during gas exsolution and its consequences in terms of hydro mechanical behavior we developed a specific consolidation apparatus to test sediments collected from the active absheron mud volcano indeed mud volcano formation is initiated by gas exsolution and subsequent mud generation at depth from stratified sediments gas was generated in the samples by circulating carbonated water through the fine grained sediments then decreasing the total pressure particular attention was given to the impact of gas saturation and associated damage and fractures on p wave velocity sediment compressibility permeability and preconsolidation pressure results show that fracture geometry is mainly controlled by the degree of gas saturation and preconsolidation pressure the damage level increases with the degree of gas saturation while the elastic modulus of sediments is degraded experimental data show that sediments do not entirely recover their original mechanical behavior when gas dissipates finally the experimental data confirm that gas exsolution and expansion is a key mechanism for mud generation

a review of the milankovitch climatic beat template for plio pleistocene sea level changes and sequence stratigraphy the milankovitch theory of climate change predicts that global ice volume and hence sea level changes were controlled by long term quasi periodic variations in the earth s orbital parameters obliquity precession and eccentricity d 18 o records from deep sea cores are a proxy for sea level changes and have an orbitally tuned chronology covering the last 5 ma the sea level signal in d 18 o data from east equatorial pacific core v19 30 is well calibrated with sea level data from coral terraces on huon peninsula new guinea over the last 140 ka and with less certainty back to 340 ka over the last 140 ka the sea level contribution to benthic glacial interglacial isotopic variation is about 1 2 1 3 or 0 011 m 1 and for core v19 30 the glacial age temperature contribution from deep ocean cooling of 1 7oc is 0 4 independent constraints on late pliocene sea level changes interpreted from shallow marine continental margin records indicate that the sea level d 18 o calibration may not have been the same over the last 2 6 ma and the temperature correction is unlikely to have been the same in all glacial periods and all ocean settings nevertheless the astronomically tuned isotopic records from deep sea cores provide an accurate chronology and approximate the magnitudes of sea level changes over the last 2 6 ma against which the facies architecture of stratigraphic sequences can be analysed and the concepts of sequence stratigraphy properly evaluated 1998 elsevier science b v all rights reserved

non uniform across shelf variations in thickness grain size and frequency of turbidites in a transgressive outer shelf the middle pleistocene kakinokidai formation boso peninsula japan abstract across shelf variations in thickness grain size and frequency of sandstone beds in a transgressive outer shelf succession were investigated from the middle pleistocene ca 0 7 ma kakinokidai formation on the boso peninsula japan the transgressive deposits are generally muddy and contain slumps and slump scars the intercalated sandstone beds are interpreted to have been formed from turbidity currents as a response to erosion and resuspension of sandridge complex deposits in the southwestern upslope area during storm events mapping of volcanic ash beds and a transgressive surface in the base of the formation permits detailed bed by bed correlation of the outer shelf sandstone beds although overall thickness grain size and frequency of sandstone beds decrease in the downslope direction some sandstone beds locally thin out and coarsen in association with slump scars in the surrounding muddy deposits these sandstone beds subsequently thicken and fine and finally thin out in the farther downslope area in addition to the local thinning of sandstone beds the frequency of sandstone beds first decreases and then increases in the farther offshore direction from this evidence we concluded that these non uniform patterns of across outer shelf variations in thickness grain size and frequency of sandstone beds were caused by the local increases in flow speeds and subsequent expansion and reduced speeds of turbidity currents along with a local increase in the seafloor gradient that was induced by the development of slump scars in the transgressive outer shelf floor these physiographic features in the outer shelf are interpreted not to have permitted monotonous downslope thinning and fining of sandstone beds compared with the bed shape models of depletive turbidity currents and with the proximality trend of shelf sandstones from modern and ancient highstand stage shelf systems

sedimentary cyclicity in the marine pliocene pleistocene of the wanganui basin new zealand sequence stratigraphic motifs characteristic of the past 2 5 m y earth s climatic history since 2 5 ma has been controlled by milankovitch variations in the planetary orbit comprising alternate periods of glaciation and interglaciation with a dominant frequency of 41 000 yr concomitantly eustatic sea level has fluctuated 70 to 130 m causing rapid transgressions and regressions of the shoreline across the world s continental shelves the resulting sedimentary record is cyclothemic each cyclothem corresponding to a single climate and sea level cycle the wanganui basin new zealand contains a 2 km thick almost complete composite record since isotope stage 100 ca 2 5 ma in the form of 47 superposed cyclothems of shelf origin each cyclothem corresponds to an unconformity bound stratigraphic sequence and typically contains a transgressive systems tract sometimes a mid cycle shell bed a highstand systems tract and sometimes a regressive systems tract no advantage accrues from using transgressive regressive units rather than cyclothems and or sequences in description of the succession six basic sequence motifs represent deposition in locations between the shoreline and offshore shelf i e the hawera birdgrove turakina seafield castlecliff and rangitikei motifs a seventh the nukumaru motif which includes dominant coquina limestone represents deposition in shallow water areas of reduced terrigenous sediment on the flank of the basin the sequence motifs represented in any section change systematically in sympathy with basin scale changes in subsidence and sediment supply in contrast with the 41000 year length of individual glacioeustatic sequences these basin wide tectonic cycles have a periodicity of many hundreds of thousands to a few million years i e that of third or fourth order sequences of the exxon type this coupled with the restriction of strongly cyclothemic sediments to geological periods of known glacio eustasy permiancarboniferous pliocene pleistocene suggests that tectonic subsidence cycles rather than glacio eustasy are the driving forces behind the development of the third and fourth order unconformity bound sequences that are reported to occur throughout the stratigraphic record

fission track ages of tuff layers related to the pliocene pleistocene boundary on the boso peninsula japan abstract fission track ages of zircon crystals from four tuff layers in the late cenozoic sediment sequence of the boso peninsula japan are 1 6 0 2 myr the kurotaki formation 5 5 0 6 and 5 2 0 5 myr the uppermost part of the amatsu formation and 11 5 0 8 myr the middle part of the amatsu formation these ages provide numerical age constraints on magneto biostratigraphy the normal polarity interval in the lower part of the kiwada formation corresponds to the olduvai polarity subzone the boundary between the pliocene and pleistocene lies slightly above the olduvai polarity subzone

archaeomagnetic investigations in greece and their bearing on geomagnetic secular variation archaeomagnetic results are presented from kilns and other heated structures on mainland greece and on the islands of crete delos euboea and thasos they span the interval from the mid third millennium bce to the mid second millennium ce the results obtained are in good agreement with the comprehensive dataset from bulgaria and reveal secular variation with amplitudes of 25 in inclination and 30 in declination the sharp inclination low in the first few centuries ce now recognized at sites ranging from crete to britain is a robust geomagnetic feature reflecting a static flux pulse due to a magnetic source in the outermost core another much more pronounced inclination low early in the second millennium bce is tentatively identified the emerging greek secular variation curve provides archaeomagnetic dates for five kilns of hitherto uncertain age 2006 elsevier b v all rights reserved

paleomagnetic intensity data a window on the dynamics of earth s fluid core abstract we consider paleomagnetic intensity data obtained from sedimentary cores as a time sequence related to fluctuations of the fluid velocity in earth s core although this is a gross oversimplification of the relationship between the geomagnetic and velocity fields some aspects of their possible interaction might be obtained through inversion of paleomagnetic intensity data in particular a signature for the onset and development of turbulence as seen in laboratory experiments on rotational parametric instabilities in rotating fluids may be contained in a paleointensity time sequence just as in laboratory experiments changes in flow regimes can be traced using these observations the paleomagnetic intensity itself is subject to several factors which contribute to uncertainty including the determination of the intensity as well as errors in timing due to offsets in age determination non linear inversions of several records from the odp 983 data set of paleomagnetic amplitudes are presented growth and decay with time of paleomagnetic amplitudes are modelled as the sum of an exponential plus two harmonics this simple model accounts for most of the paleomagnetic amplitude signal yielding geophysically plausible growth and decay rates for a proposed rotational parametric instability in earth s core this model provides a new interpretation for observed paleomagnetic intensity changes and thus an improved understanding of the dynamical processes producing the geomagnetic field

orogeny and relative plate motions example of the japanese islands abstract the paleogeography of the eastern margin of asia near japan for the past 250 ma has been reconstructed on the basis of the relative motions between the oceanic plates and the eurasian plate engebretson et al 1982 and on land geology the relationships between the plate kinematics and orogeny are as follows calc alkaline magmatism is related to the presence of subduction when a plate boundary is a transform fault there is no volcanism very slow convergence rates do not give rise to magmatism migration of rft type triple junctions or ridge subduction produces forearc volcanism near the trench and also a wider volcanic zone uplift of low p t type regional metamorphic belts is related to collision episodes the uplift of the high p t blueschist belts on the contrary is not contemporaneous with them they appear to be related to the start of the subduction of a young plate after the migration of an rft type triple junction because the formation of a voluminous accretionary complex is contemporaneous with this and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high p t type metamorphic belt the subduction of a young plate would thus be an important factor causing these orogenic events highly oblique convergence also results in formation of a serpentinite melange zone with fragments of blueschists presumably along a mechanical plate boundary in the forearc

mechanical response of sediments to bubble growth modeling the process of bubble growth in sediments requires an understanding of the physics that controls bubble shape and the interaction of the growing bubble with the sediment to acquire this understanding we have conducted experiments in which we have injected gas through a fine capillary into natural and surrogate sediment samples and have monitored pressure during bubble growth to provide information about stress and strain in gas injection studies with natural sediment samples we have observed two modes of bubble growth behavior one of these modes characterized by a saw tooth record of pressure as the bubble grows is consistent with fracture of the medium observations indicate that bubble growth by fracture should correspond to bubbles that are coin or disk shaped this shape is confirmed in observations of bubbles in natural sediments and in our studies of bubble injection into gelatin a surrogate sediment material interpretation of the stress strain results for bubble growth also required that we measure young s modulus e the measurements show e to be near 0 14 mn m2 which differs by more than 4 orders of magnitude from values that have been reported in the literature our measurements of e give substantially better estimates of bubble shape than are predicted using the literature values our data are interpreted with linear elastic fracture mechanics lefm which predicts that the critical pressure for bubble growth will depend on the bubble volume v raised to the 1 5 power while evidence of substantial heterogeneity in sediment properties is apparent in our results this v 1 5 dependence is confirmed through application of lefm theory we have determined the critical stress intensity factor k1c a material property and the principal determinant of bubble shape and growth by fracture our values of k1c range from 2 8 10 4 mn m 3 2 to 4 9 10 4 mn m 3 2 for our natural sediment samples from cole harbor nova scotia we have also estimated the critical stress intensity factor for eckernforde bay samples by analyzing published images of natural bubbles the k1c obtained in this way is similar to our cole harbor results and is 5 5 10 4 mn m 3 2

x ray computed tomography imaging of gas migration in water saturated sediments from capillary invasion to conduit opening 1 the strong coupling between multiphase flow and sediment mechanics determines the spatial distribution and migration dynamics of gas percolating through liquid filled soft granular media here we investigate by means of controlled experiments and computed tomography ct imaging the preferential mode of gas migration in three dimensional samples of water saturated silica sand and silica silt sediments our experimental system allowed us to independently control radial and axial confining stresses and pore pressure while performing continuous x ray ct scanning the ct image analysis of the three dimensional gas migration provides the first experimental confirmation that capillary invasion preferentially occurs in coarse grained sediments whereas grain displacement and conduit openings are dominant in fine grained sediments our findings allow us to rationalize prior field observations and pore scale modeling results and provide critical experimental evidence to explain the means by which conduits for the transit of methane gas may be established through the gas hydrate stability zone in oceanic sediments and cause large episodic releases of carbon into the deep ocean

first order description of the mechanical fracture behavior of fine grained surficial marine sediments during gas bubble growth 1 bubbles in sediments imaged via computed tomography ct scanning and in surrogate transparent material gelatin are well described geometrically as eccentric oblate spheroids while sediments are undoubtedly visco elasto plastic solids only part of that complex behavior appears to influence significantly the formation and shape of gas bubbles specifically the shape of these bubbles can be explained if the mechanical response of fine grained sediment is approximated by linear elastic fracture mechanics lefm to determine the adequacy of the lefm approximation for gas bubble growth in fine grained sediments a number of gas bubbles were injected and grown in natural sediments while monitoring the size and shape using an industrial ct scanner a comparison of measured inverse aspect ratios iars of the injected bubbles with calculated iars from pressure records provides support for the lefm theory deviations from lefm are observable in the data but as bubbles grow larger they trend more closely toward the theory the use of lefm has been shown to describe gas bubble growth in shallow coastal sediments to first order

insights on the geomagnetic secular variation in the eastern mediterranean first directional data from cyprus abstract the first directional archaeomagnetic results from cyprus are presented based on the study of seven baked clay structures excavated at the copper smelting site of agia varvara almyras situated some 20 km south of nicosia this ancient copper working site represents a unique example of complete primary production of the copper metal in cyprus in total five furnaces one ore roasting pit and an oven were sampled in situ for an archaeomagnetic investigation all the studied structures are well dated based on pottery finds and radiocarbon dates with ages ranging from 600 bce to 50 bce several rock magnetic experiments were performed to determine the main magnetic carrier and to investigate the thermal stability of the studied material the direction of the characteristic remanent magnetization chrm for each structure has been determined by demagnetization of 7 15 specimens in alternating magnetic fields up to 100 120 mt mean directions were calculated assuming a fisherian distribution and are well defined for five out of the seven structures studied the new directions are compared with the directional data and the secular variation curves available from the surrounding area and with the predictions of global geomagnetic field models they are an important contribution to improving our knowledge of the geomagnetic field variations in the eastern mediterranean and offer new insights about fast geomagnetic directional changes during the first millennium bce that were not previously identified by the available geomagnetic field models predictions in this area

a method for correcting geographically separated remanence directions for the purpose of archaeomagnetic dating summary spatial variations in the geomagnetic field must be taken into account if secular variation master curves and directional magnetic dates are to be optimized two methods for relocating remanence vectors have been proposed and in this paper their relative accuracies are compared using a numerical model based on the present day field a method which converts archaeomagnetic directions via a virtual geomagnetic pole is shown to be the more efficient transformation for an archaeomagnetic region the size of the british isles 900 km radius the maximum error in relocating vectors to a central location is predicted to be of the order of 1 2

sediment undulations induced by free gas in muddy marine sediments a modeling approach 1 sediment undulations observed on the continental shelf in muddy prodeltas are often marked by the presence of gas in the upslope location the growth and migration of bubbles within fine grained muddy sediments are associated with sediment fracturing that damages the sediment and changes its effective mechanical properties the presented theoretical investigation demonstrates by modeling that undulations may be induced by creep deformations downslope from the region with gas presence comparison of the dimensions of the modeled undulations with the observed ones suggests that the undulations may be enhanced over time by different processes e g by the different types of the sediment deposition

relation of sequence stratigraphy to modern sedimentary environments one method of testing the concept of sequence stratigraphy is to compare it to quaternary sediments in which chronology stratigraphic relations and facies geometry are more clearly understood than in older rocks rapid deposition rates during quaternary glacial eustatic cycles in large deltaic depocenters generate sequences comparable to those in the ancient stratigraphic record in the northern gulf of mexico the late wisconsinan holocene mississippi river has deposited a type 1 sequence that includes lowstand transgressive and high stand systems tracts characteristics of modern mississippi river sedimentary environments support the methodology used in sequence analysis but the short time taken for sequence generation here raises important questions about sequence time scales correlation and driving mechanisms

sequence stratigraphy of sixth order 41 k y pliocene pleistocene cyclothems wanganui basin new zealand a case for the regressive systems tract this study is based on a late pliocene and early pleistocene approximately 2 6 1 7 ma succession about 1 km thick of 20 sixth order 41 k y duration cyclothems of shelf origin exposed in the rangitikei river valley in the eastern part of wanganui basin the cyclothems correlate with 18 o isotope stages 100 58 and each 41 k y glacialinterglacial stage couplet is represented by an individual depositional sequence comprising transgressive highstand and regressive systems tracts unlike most examples inferred from the stratigraphic record these systems tracts were deposited during phases of known sea level cycles indicated by the contemporary oxygen isotope ice volume curve because of the high rate of subsidence of wanganui basin glacioeustatic sea level falls during most cycles were not of sufficient magnitude to expose the outer shelf thus the rangitikei section provides an exceptional example of regressive strata deposited landward of the contemporary shelf break simple one dimensional modeling shows that moderate to high rates of basin subsidence 1 2 mm yr and low rates of sedimentation 0 2 mm yr during transgressions combined to produce an accommodation surplus at the relative highstand this surplus accommodation was infilled during the late highstand and ensuing fall partly by aggradational highstand systems tract shelf siltstone and chiefly by strongly progradational shoreface sediments of the regressive systems tract rangitikei regressive systems tracts are distinguished from forced regressive systems tracts sensu hunt and tucker 1992 by their different stratal geometry by definition forced regressive systems tracts display an erosional contact with the underlying highstand systems tracts and typically occur as a series of downstepped disjunct shoreline wedges stranded on the shelf and or slope in contrast regressive systems tracts exhibit a gradational lower contact above which parasequences are stacked in a strongly progradational pattern terminated by the superjacent sequence boundary cyclothems display two types of motif termed rangitikei dt depositional transgression and rangitikei nt nondepositional transgression which include the following architectural elements in ascending stratigraphic order 1 a basal sequence boundary that is coincident with either the transgressive surface of erosion which displays small scale up to 50 cm erosional relief and may be penetrated by the ichnofossil ophiomorpha or its deeper water correlative conformity 2 either a thick 5 30 m transgressive systems tract comprising a deepening upward nearshore to inner shelf mixed carbonate siliciclastic lithofacies succession depositional transgression or a thin 2 m transgressive systems tract comprising condensed fossiliferous facies deposited on the sediment starved offshore shelf nondepositional transgression 3 a sharp downlap surface separating condensed fossiliferous facies of the transgressive systems tract from terrigenous siltstone of the superjacent highstand systems tract 4 a highstand systems tract comprising a 10 20 m thick interval of aggradational shelf siltstone and 5 a thick up to 45 m progradational inner shelf to shoreface lithofacies assemblage ascribed to the regressive systems tract condensed shell beds are associated with intrasequence and sequence bounding discontinuities and together with the sedimentological and stratal characteristics of the sequences are important indicators of stratigraphic architecture four types of shell bed are associated with surfaces formed by four different types of stratal termination onlap backlap downlap and flooding surface shell beds cf kidwell 1991 are associated respectively with the transgressive surface of erosion apparent truncation at the top of the transgressive systems tract the downlap surface and local marine flooding surfaces a fifth shell bed type termed a compound shell bed forms in offshore environments where the downlap surface converges with the sequence boundary and elements of both the downlap and the backlap shell beds become mixed or superposed the shell beds mark zones of stratal attenuation and can be used as surrogates for seismic discontinuities when applying sequence stratigraphic concepts at outcrop scale

three millennia of directional variation of the earth s magnetic field in western europe as revealed by archeological artefacts directional secular variation of the geomagnetic field over the last 2000 years has been defined in western europe from numerous archeomagnetic studies however the number of archeomagnetic results for older periods is much more limited for this reason we present new data obtained from fired archeological structures found in two french sites loupiac and aspiran dated within the first millennium b c latest bronze earliest iron age transition 850 700 b c and iron age 525 475 b c respectively from a compilation of archeomagnetic results from western europe great britain italy and france and northern africa tunisia we propose a directional secular variation curve for western europe that covers the entire first millennium b c this curve exhibits a large clockwise motion with rapid changes during the first half of the millennium while the last four centuries b c are characterized by weak variations

two models global sea level change and sequence stratigraphic architecture abstract two different conceptual models underlie the application of sequence stratigraphy by exxon stratigraphers and later researchers one the global sea level model gsm relates to presumed sea level behaviour through time the other the sequence stratigraphic model ssm relates to the stratigraphic record produced during a single cycle of sea level change though the two models are inter related they are logically distinct and it is important to test them separately a summary is presented of the nature of the two models and of the nomenclature that is used in their description it is concluded a that the global sea level model comprises an assembly of local relative sea level events which are widely recognisable within their parent sedimentary basin and b that the sequence stratigraphic model is robust and its application is therefore an insightful way to approach the interpretation of sedimentary rocks

large scale stratigraphic architecture eustatic variation and unsteady tectonism a theoretical evaluation we incorporate a process based depositional model with basin subsidence models to predict stratigraphic records this allows us to investigate the importance of subsidence geometry on coastal stratigraphy and thus to characterize and compare the stratigraphic architecture of two categories of tectonic basins the models demonstrate that the correlation of stratigraphic sequences to eustatic cycles is not the same in passive margin basins as in foreland basins and that in a foreland basin the record of episodic tectonism is distinct from that of eustatic sea level change in the model sediment transport is approximated by slope controlled diffusion nonmarine transport is treated as more efficient diffusion than marine transport three different subsidence and sediment supply models are examined a simple passive margin basin a simple foreland basin and then a foreland basin for which vertical motions are driven by thrust shortening that is compensated flexurally and for which sediment supply is related to relief predicted passive margin stratigraphies for cases of varying eustatic sea level are similar to those of natural basins and include progradational packages and subaerial unconformities which are used to define sequence boundaries that form during sea level fall we explore the timing relationships between stratigraphic features and a sinusoidal sea level history showing that the phase relationship depends on subsidence sediment flux efficiency of sediment transport and period and amplitude of sea level when the basic geometry of the basin is inverted placing the sediment supply on the side with maximum subsidence as is the case in foreland basins the sequence character changes markedly subaerial erosion does not generate unconformities in the models of a dynamic foreland basin sediment supply and subsidence are linked to the structure of the flanking thrust belt and are not necessarily constant for steady thrusting and variable sea level unconformities that define sequence boundaries form only on the distal or forebulge side of the basin and the ages of the sequence boundaries correlate to limes of rising sea level in cases of constant sea level but variable thrusting subaerial unconformities are cut locally on both the proximal margin of the basin and the distal margin of the basin yet the ages of the proximal margin and distal margin unconformities are out of phase in the tectonic cycle erosion is most pronounced during quiescence on the proximal side and during thrusting on the distal side

the role of sediment structure in gas bubble storage and release ebullition is an important pathway for methane emission from inland waters however the mechanisms controlling methane bubble formation and release in aquatic sediments remain unclear a laboratory incubation experiment was conducted to investigate the dynamics of methane bubble formation storage and release in response to hydrostatic head drops in three different types of natural sediment homogenized clayey silty and sandy sediments initially quasi uniform through the depth of the columns were incubated in chambers for 3 weeks we observed three distinct stages of methane bubble formation and release stage i microbubble formation displacing mobile water from sediment pores with negligible ebullition stage ii formation of large bubbles displacing the surrounding sediment with concurrent increase in ebullition and stage iii formation of conduits with relatively steady ebullition the maximum depth averaged volumetric gas content at steady state varied from 18 8 in clayey to 12 0 in silty and 13 2 in sandy sediment gas storage in the sediment columns showed strong vertical stratification most of the free gas was stored in an upper layer whose thickness varied with sediment grain size the magnitude of individual ebullition episodes was linearly correlated to hydrostatic head drop and decreased from clayey to sandy to silty sediment and was in excess of that estimated from gas expansion alone indicating the release of pore water methane these findings combined with a hydrodynamic model capable of determining dominant sediment type and depositional zones could help resolve spatial heterogeneities in methane ebullition at medium to larger scales in inland waters

tectonics global changes in sea level and their relationship to stratigraphical sequences at the us atlantic continental margin of geological the principal factors that control the extent of seas through geological time are vertical movements of the lithosphere and global changes in sea level the relative height of the sea surface determines the facies and the thickness of sediments that can accumulate in a sedimentary basin backstripping studies show that the primary factors affecting the subsidence of rifted sedimentary basins are thermal contraction following heating and thinning of the lithosphere at the time of rifting and sedimentary loading factors such as compaction palaeobathymetry erosion and global sea level changes also contribute but their combined affects are small compared to those of thermal contraction and sedimentary loading simple models have been constructed which combine the effects of sedimentary loading and thermal contraction with those of compaction sub aerial erosion and global changes in sea level in the models it was assumed that the lithosphere was heated and thinned by stretching at the time of rifting sedimentary loading occurs by flexure of a lithosphere that progressively increases its flexural rigidity with age following rifting and that sediment compaction and bathymetry change across a basin but do not vary significantly with geological time furthermore different assumptions were made on the magnitude of curves of global sea level changes and the relationship between denudation rate and regional elevation the models show that tectonics in the form of thermal contraction of the lithosphere and flexure and slowly varying global changes in sea level can explain a number of the stratigraphic features of the us atlantic continental margin in this paper some of the implications of these results are examined for studies of a sea level changes through geological time and b the maturation history of continental margin basins

eustatic signals in deep marine sedimentary sequences recovered at odp site 978 alboran basin western mediterranean sea abstract a continuous section of pliocene marine sediments was recovered at ocean drilling program site 978 located in the alboran sea between spain and morocco three pliocene subunits have been defined at site 978 the lowermost subunit ic 129 2 m thick is characterized by alternating beds of lighter more calcareous and darker less calcareous claystone with bioturbated upper and lower contacts type 1 cycles the middle subunit ib 67 1 m thick is composed of relatively homogeneous nannofossil claystone and the uppermost subunit ia 211 6 m thick contains abrupt based darker terrigenous layers interpreted as turbidites that are interstratified with lighter nannofossil claystone type 2 cycles the rhythmically bedded light and dark layers in subunit ic correlate with those in the rosello composite section of sicily a global reference standard for the pliocene time scale these sedimentary cycles are products of variations in precession and resulting continental runoff missing cycles occur during eustatic highstands the shift to more homogeneous sedimentation in subunit ib is represented in similar aged sequences throughout the mediterranean which display evidence of submarine mass wasting mediterranean wide slope degradation was likely a response to rapid sea level change at approximately 3 ma this change in sedimentation style was accompanied by an upsection increase in sediment accumulation rates associated with turbidite influx in subunit ia turbidite frequency throughout the pliocene section can be linked to eustatic changes in sea level with turbidite maxima corresponding with mid sequence downlap surfaces and their associated condensed sections

sequence stratigraphy of mississippi fan related to oxygen isotope sea level index 1 the techniques of sequence stratigraphy were applied to the analysis of stratal patterns on seismic data from the mississippi fan to outline a method for 1 understanding the processes and growth patterns of modern submarine fans and 2 predicting reservoir and seal for exploration in deep water settings we developed a sea level index from the oxygen isotope record for the last 3 5 m y tied to limited biostratigraphic control to provide a time framework within which the depositional patterns were analyzed analysis of the seismic data reveals that at least thirteen depositional sequences comprise the bulk of the mississippi fan section the base of each sequence is defined by an erosional unconformity in a proximal position that can be traced regionally across the fan complex with its correlative conformity seismic facies analysis of individual sequences suggests that the mississippi fan is characterized by the cyclic stacking of individual lowstand systems tract deposits consisting dominantly of the lowstand basin floor and lowstand slope fans lowstand slope fans consisting of leveed channel complexes are volumetrically the greatest component of the mississippi fan lowstand basin floor fans commonly are sandy and thin and are associated with the base of each depositional sequence these fans are characterized by a mounded stratal geometry

is northern honshu a microplate abstract seismic slip vectors along the japan trench the eastern margin of the japan sea and the sagami trough are compared with global relative plate motions rm2 minster and jordan 1978 to test a new hypothesis that northern honshu japan is part of the north american plate this hypothesis also claims that the eastern margin of the japan sea is a nascent convergent plate boundary kobayashi 1983 nakamura 1983 seismic slip vectors along the japan trench are more parallel to the direction of the pacific north american relative motion than that of the pacific eurasian relative motion however the difference in calculated relative motions is too small avoid to the possibility that a systematic bias in seismic slip vectors due to anomalous velocity structure beneath island arcs causes this apparent coincidence seismic slip vectors and rates of shortening along the eastern margin of the japan sea for the past 400 years are also consistent with the relative motion between the north american and eurasian plates calculated there seismic slip vectors and horizontal crustal strain patterns revealed by geodetic surveys in south kanto beneath which the philippine sea plate is subducting indicate two major directions one is the relative motion between the north american and philippine sea plates and the other that between the eurasian and philippine sea plates one possible interpretation of this is that the eastern margin of the japan sea may be in an embryonic stage of plate convergence and the jump of the north american eurasian plate boundary from sakhalin central hokkaido to the eastern margin of the japan sea has not yet been accomplished in this case northern honshu is a microplate which does not have a driving force itself and its motion is affected by the surrounding major plates behaving as part of either the eurasian or north american plate another possibility is that the seismic slip vectors and crustal deformations in south kanto do not correctly represent the relative motion between plates but represent the stresses due to non rigid behaviors of part of northern honshu

the physics of bubbles in surficial soft cohesive sediments abstract this paper reviews current understanding of the physics of bubbles in soft unlithified cohesive sediments certainly in the top 100 m from the sediment water interface but likely much deeper experimental evidence primarily ct scans and internal pressure records indicate that near surface bubbles grow by elastically compressing and fracturing these sediments which results in thin irregular disks of gas these data do not suggest either plastic or fluid behavior on the part of the bulk sediment in addition there is no hint of capillary invasion of gas into the pores of fine grained sediments the growth rate of an elastic fracturing disk shaped bubble can vastly exceed that of a spherical bubble depending on the eccentricity of the bubble this effect results from the far more favorable surface to volume ratio of flattened bubbles the initial rate of rise of bubbles in cohesive sediments also appears to be governed by a fracture process which is driven by pseudo buoyancy the rate of bubble rise could be controlled by various possible mechanisms but only the predictions from a visco elastic fracture propagation model seem to produce rise velocities that are not ballistic experimental evidence demonstrates that a rising bubble can deviate from its path to the surface if it encounters a zone of sediment with lower lateral fracture toughness such a bubble will begin to move in a lateral direction but stall because pseudo buoyancy is no longer operative subsequent bubbles will do the same and accumulate in the low fracture toughness zone this explains the occurrence of shallow sub seafloor domes of gas and modeling of this process predicts only modest over pressures in such structures

long term enso like events represented in the middle pleistocene shelf successions boso peninsula japan abstract millennial century and decadal scale stratigraphic cyclicities are described from transgressive shelf successions of the middle pleistocene ichijiku and kakinokidai formations ca 0 7 ma in the boso peninsula japan the ichijiku formation ca 400 m thick represents a shelf sandridge complex developed under the influence of the large meanders of the paleo kuroshio current and intertongues with outer shelf muddy deposits of the kakinokidai formation ca 170 m thick to the northeast most of the two formations are classified as a transgressive systems tract formed during the rise in glacioeustasy from oxygen isotope stages 18 to 17 three different types of cyclicities are identified in the transgressive shelf deposits the first cyclicity ca 3000 years duration is represented by coarsening upward units of the kakinokidai formation associated with the northeastward extension of the sandridge complex lower parts of each coarsening upward unit are characterized by intercalations of tempestites more than the upper parts the second cyclicity ca 250 years duration is defined by master bedding surfaces within sandridge deposits interpreted as formed in response to lateral migration of foresets of the sandridge complex the third cyclicity ca a few decades or shorter duration is defined as the units bounded by reactivation surfaces within large scale trough cross bedded sandstone units the three different types of cyclicities are interpreted to document fluctuations in the paths and speeds of the paleo kuroshio current in response to the large meanders oscillations of the large meanders of the modern kuroshio current every few to 10 years are interpreted to be in response to enso el nino southern oscillation events and are also associated with the decrease in frequency of typhoons in the western pacific if our stratigraphic interpretation is true the paleo kuroshio current controlled shelf successions document high frequency oscillations in paleoclimates responding to the long term enso like events in the western pacific during the middle pleistocene

stability of bubbles in a linear elastic medium implications for bubble growth in marine sediments 1 methane bubbles in muddy fine grained sediments grow initially through a process of elastic expansion punctuated by discrete fracture events lefm growth the ability of the surrounding sediments to support a stress and actively resist expansion can under conditions of low gas production or high sediment toughness result in the cessation of growth and the presence of stable bubbles thus it is possible for a bubble to stop growing despite the presence of a source in the sediments that continues to produce gas this contrasts with growth of bubbles in a fluid medium which cannot support a stress and so will continue to grow as long as a surrounding source provides gas this no growth condition is the result of the coupling between gas supply methane production or supersaturation and the sediment mechanics here we quantify this condition and present a criterion for the switch between no growth and the lefm growth regimes we apply this theory to the sediments of eckernforde bay in the kiel bight germany and despite the absence of measurements for the key sediment mechanical properties we can provide a qualitative explanation for the sizes and shapes of the observed bubble population with depth in the sediment we also show how the release of hydrostatic pressure can stimulate growth by pushing otherwise stable bubbles into the lefm growth regime this could provide a mechanism for the release of bubbles during periods of low water such as during low tide or wave events

archaeomagnetic dating of santorini volcanic eruptions and fired destruction levels of late minoan civilization archaeomagnetic dating on the minoan ash horizons of the santorini volcano and on fired destruction levels at late minoan sites on crete demonstrates that the basal plinian air fall ash of the first minoan pumice is contemporaneous with the destruction levels on central crete while the higher minoan ashes are contemporaneous with the destruction levels in extreme eastern crete these destruction levels were almost certainly caused by seismic activity rather than the ash fall the determination of a time gap between these events lead to a reappraisal of the archaeological evidence and is important volcanologically

diachronous evolution of systems tracts in a depositional sequence from the middle pleistocene palaeo tokyo bay japan palaeo tokyo bay is a relic of the plio pleistocene kazusa forearc basin in the boso peninsula of japan the sedimentary infill of palaeo tokyo bay is characterized by shallow marine to paralic sediments of the middle to upper pleistocene shimosa group sequence stratigraphical analysis has been used to describe spatial and temporal variations in the depositional systems of the lowest units of the shimosa group deposited during the early stage of development of palaeo tokyo bay three different type of depositional systems were recognized sand ridge to shelf srs shelf to delta sdl and shelf to non deltaic nearshore sns systems they overlie early transgressive estuarine deposits infilling lowstand valleys incised in the south eastern margin of palaeo tokyo bay these systems were developed during late transgressive through highstand stages of a relative sea level cycle which may have been controlled by a glacio eustatic sea level change at about 0 4 ma spatial variation in depositional systems is largely identical to that in modern tokyo bay environmental conditions similar to those prevailing at the present day probably characterized the early history of palaeo tokyo bay the timing of highstand systems tracts within a high frequency depositional sequence was analysed in terms of the effect of sedimentation rate based on the mapping of a chronostratigraphical surface marked by the hy4 volcanic ash layer from spatial variations in sedimentation rate it was possible to identify the diachronous evolution of highstand systems tracts from the sdl system through the sns system to the srs system time lag is indicated by major bounding surfaces such as maximum flooding or downlap surfaces associated with a condensed section which developed immediately above or below the hy4 volcanic ash layer the lag may be of the order of a few thousands to tens of thousands of years within a depositional sequence with a total of duration of about 100 000 years

lower pleistocene to present structural deformation and sequence stratigraphy of the continental shelf offshore eel river basin northern california seismic stratigraphic sequences and deformational features are mapped beneath the continental shelf of the offshore eel river basin of northern california using a closely spaced high resolution multichannel seismic grid geometries of sequences and morphologies of bounding unconformities reflect competing tectonic and glacioeustatic influences producing shifting sedimentation patterns in the offshore basin during the last v1 myr estimated timing of unconformity formation correlates generally with a deep ocean global n 18 o curve both before and after a v500kyr transition from 41 kyr to 100 kyr dominated cycles we suggest that glacioeustatic fluctuations are the dominant control on unconformity formation basinwide however regional tectonic influences on sequence development are also observed folding associated with gorda north america plate convergence ended progressively from south to north between v1 0 ma and v500 ka in response to northward migration of the mendocino triple junction mtj since v500 ka continued encroachment of the mtj produces rotation of preexisting structures uplift of the table bluff anticline tba related periods of channel incision south of the tba and generally reduced sediment preservation across the shelf mtj related uplift may also have induced formation of eel canyon over the past v1 0 myr a dominant northern sediment source becomes progressively less important a southern source probably the paleo eel river has become dominant since v750 ka seismic unconformities fall into two categories 1 irregular surfaces of limited mappable extent interpreted as incision exposure surfaces forming during relative sea level lowstands and 2 smooth laterally extensive regional unconformities interpreted as ravinement surfaces that erode lowstand surfaces in all but deeply incised areas during sea level transgressions a sequence stratigraphic model for sediment deposition and preservation on the eel river shelf predicts that preserved sediment on the eel margin is dominated by fluvially derived highstand silts and muds deposited by longshore directed currents and waves lowstand sediments are preserved only in fluvial channels infilled during late lowstand early transgression preserved transgressive sediments are likely limited to a thin veneer of well sorted coarse sediment or shell debris deposited above the transgressive ravinement future deep coring will be required to confirm these predictions 2002 elsevier science b v all rights reserved

stratigraphy of the kazusa group boso peninsula an expanded and highly resolved marine sedimentary record from the lower and middle pleistocene of central japan major climatic reorganizations including changes in the nature of glacial interglacial cycles through the pleistocene are a key issue for improving the understanding of earth s climate system highly resolved marine sedimentary records are essential to reconstruct the details of these past climatic changes and investigate the mechanisms responsible for them the kazusa group located in the central part of the japanese island chain is well exposed and contains a remarkably continuous and thick deep and shallow water marine sedimentary succession this group also contains well preserved marine microfossils pollen paleomagnetic reversal events geochemical signatures and a large number of tephra beds these features allow us to establish a robust chronological and stratigraphic framework for the kazusa group and provide a rare opportunity to study oceanic and terrestrial climatic and environmental changes at high resolution especially through the lower and lower middle pleistocene in the boso peninsula chiba prefecture the kazusa group is deeply incised yielding spectacular river cut exposures the matuyama brunhes paleomagnetic reversal is observed immediately above the widespread byakubi e byk e tephra in the chiba composite section within the kokumoto formation kazusa group because the matuyama brunhes boundary customarily serves as the primary guide for the lower middle pleistocene subseries boundary the chiba composite section is considered an excellent candidate for its global boundary stratotype section and point gssp this study reviews the published data for the kazusa group hitherto almost exclusively in japanese in order to place the chiba composite section within its broader depositional and chronostratigraphic context

new sedimentary structures in seismites from sw tanzania evaluating gas vs water escape mechanisms of soft sediment deformation seismite horizons are abundant in cretaceous sandstones of the rukwa rift basin southwestern tanzania diverse morphologies of soft sediment deformation are preserved including two new unusual sedimentary structures herein referred to as 1 balloon shaped inflation structures and 2 surface fractures with linked sandstone splays this original description of new sedimentary structures contributes to the growing catalogue of seismically induced deformation features their unusual morphologies bring to the forefront an important though seldom touched upon question of how to differentiate between gas and water escape in soft sediment deformation features the recognition of the spectrum of soft sediment deformation structures contained in strata and their deformational mechanisms is important because it permits the differentiation between triggering mechanisms particularly seismic activity and can constrain such events spatially and temporally we interpret the surface fractures and linked sandstone splays to reflect a high gas effusion rate formed by the release of high pressure gas followed by a limited expulsion of water the balloon shaped inflation structures reflect lower gas effusion rates due to expulsion of lower pressure gas that did not breach the cretaceous surface seal when these gas pulses did breach the paleosurface they formed gas discharge pits these seismogenic structures are consistent with deposition of cretaceous strata in the rukwa rift during periods of active carbonatite volcanism seismicity and possibly hot spring activity this documentation serves as an important data point for the re examination of the classification scheme of soft sediment deformation structures as primarily water escape structures to accommodate for the genesis of some secondary sedimentary features by gas escape

millennial scale hydrographic changes in the northwestern pacific during marine isotope stage 19 teleconnections with ice melt in the north atlantic abstract marine isotope stage mis 19 has been suggested as the best orbital analogue of the present interglacial period thus its paleoenvironmental reconstruction will provide valuable information on future climate change excluding anthropogenic impacts the kuroshio current the largest western boundary current in the northwestern pacific transports large amounts of heat and water poleward and strongly influences both east asian and global climates however the responses of the kuroshio current to the variable intensity of the atlantic meridional overturning circulation amoc under this orbital configuration remain unknown here we provide new centennial scale resolution oxygen isotope 18 o records throughout mis 19 using benthic as well as subsurface and surface planktonic foraminifera from the chiba composite section a marine succession sequence in central japan to reconstruct detailed paleoceanographic variations in the northwestern pacific and reveal the response of the kuroshio current to climate variability in the north atlantic under this orbital configuration the vertical water temperature structure and its gradient t based on 18 o profiles elucidates the latitudinal oscillations of the kuroshio extension front kef on a millennial scale during the late mis 19 and mis 20 19 transition in association with amoc disruption reactivation due to freshwater discharges into the north atlantic the power spectra of the surface 18 o and t show half precession periodicity throughout mis 19 which probably originated from equatorial insolation at equinoxes co occurring with the orbital perihelion this periodicity was especially strong during mis 19c during which paleoceanographic records from the north atlantic show minor variations in the amoc and freshwater discharge our results suggest that millennial scale oceanographic variation and stability along the kef under this orbital configuration are influenced by both north atlantic climate variability and tropical insolation forcing

volcanic ash layers facilitate high resolution sequence stratigraphy at convergent plate margins an example from the plio pleistocene forearc basin fill in the boso peninsula japan abstract in this paper a method is presented for the definition of high resolution siliciclastic depositional sequences in onshore outcrop belts in a forearc basin setting and the development of high frequency depositional sequences in the infill of the plio pleistocene kazusa forearc basin in the boso peninsula of japan in terms of the interaction between forearc tectonics and glacio eustasy is discussed volcanic ash layers are commonly intercalated in siliciclastic successions at convergent plate margins and provide physically defined chronostratigraphic markers the mapping of lateral termination of volcanic ash layers together with the interpretation of depositional facies discontinuities permits the identification of onlap offlap and downlap stratal termination patterns in onshore outcrop belts in particular this type of analysis is useful to identify bounding surfaces in offshore muddy sediments the relative abundance of intercalated volcanic ash layers and their associated pyroclastic volcaniclastic fragments in marine siliciclastic successions is interpreted to correspond to temporal variation in the relative rate of supply of coarse grained terrigenous sediments to a marine environment during a relative sea level cycle in particular transgressive stages are characterized by the reduction of active supply of coarse grained terrigenous sediments to a marine elastic environment and this results in the increase of relative abundance of intercalated volcanic ash layers and their associated volcanogenic lithofacies this kind of sediment is hierarchically identified in response to the development of marine flooding surfaces transgressive systems tracts and transgressive sequence sets in general boundaries of depositional sequences and their component building blocks are interpreted to provide a chronostratigraphic framework for local and regional correlations of siliciclastic successions the mapping of volcanic ash layers can identify diachronous evolution of bounding surfaces and component building blocks of a high frequency depositional sequence in relation to spatial variations in sedimentation rates and basin subsidence which are superimposed by eustatic sea level changes

reconstruction of altitudinal transportation range of leaves based on stomatal evidence an example of the early pleistocene fagus leaf fossils from central japan abstract fossil plant assemblages play a critical role in paleoenvironment estimation especially for the cenozoic however the extent that the fossil assemblage reflects the source forest is difficult to estimate the comparison between modern assemblages and their subfossil assemblages shows a significant difference indicating that the transportation of plant organs can affect the accuracy of paleoenvironment analysis in this study we aim to detect leaf transport based on stomatal frequency stomatal index and stomatal density variation of 30 fossilized fagus crenata leaves from the sayama formation 1 54 1 66 ma we distinguished the sun and shade morphotype of fossil leaves by using undulation index and calculated stomatal frequency variation of only sun fossil leaves considering the stomatal index variation of fossil fagus crenata leaves the paleo pco2 variation in a warm period between 1 54 ma and 1 66 ma was estimated as ca 35 0 to 39 6 pa based on this paleo pco2 variation altitudinal transportation range of the early pleistocene fossil fagus crenata leaves was estimated at over 1000 m we presume that leaves of fagus crenata in the fossil assemblage possibly had come from a wide range of altitudes near the fossil site during the early pleistocene this provides an example of altitudinal transportation of plants before their deposition which suggests a consideration when reconstructing the palaeovegetation and palaeoclimate from fossil assemblages

transient growth of an isolated bubble in muddy fine grained sediments abstract methane bubbles in fine grained sediments have been shown to grow initially by elastic expansion and fracture a previous growth model assumed quasi steady state diffusion in which the methane porewater concentration quickly adjusted to changes in bubble geometry gardiner b s boudreau b p and johnson b d 2003a growth of disk shaped bubbles in sediments geochim cosmochim acta 67 8 1485 1494 here we present a finite element model that solves the transient form of the reaction diffusion equation and the coupled linear elastic fracture mechanics lefm in so doing we also employ a new theory for the post fracture bubble sizes based upon the full principles of lefm our findings indicate that the quasi steady state assumption is flawed due to violation of conservation of mass during fracture events when the new model is applied to sediment conditions found at cape lookout bight nc usa it is found that bubbles grow somewhat faster than previously thought a reference bubble of 0 5 cm 3 will form in about 6 days 2 5 days quicker than the old model predicted moreover typical bubbles of 0 04 cm 3 for this site can grow in as little as a day and a half we examined the sensitively of the finite element model to the various parameters in order to gain an understanding of how bubbles may behave under different sediment conditions the influence of tides on bubble growth through the process of rectified diffusion was also examined and it was found that this had little influence upon growth

chronostratigraphy of the pliocene pleistocene boundary in forearc basin fill on the pacific side of central japan constraints on the spatial distribution of an unconformity resulting from a widespread tectonic event the upper ikego formation pliocene and the urago formation pliocene to lower pleistocene have been examined for their lithostratigraphy calcareous nannofossil biostratigraphy and magnetostratigraphy both formations are forearc basin fills exposed on the miura peninsula on the pacific side of central japan the boundary between these formations has been previously treated as the western extension of the kurotaki unconformity which was considered to be related to widespread tectonic change at ca 3 ma in the japanese forearc region however the presence of the unconformity on the miura peninsula remains unclear due to sparse lithostratigraphic and chronostratigraphic data around the boundary between the both formations in the study area the ikego formation consists mainly of sandy mudstone and the urago formation of sandstone and muddy sandstone both formations contain numerous tuff beds key tuff beds and the last appearance of discoaster tamalis have been identified in the upper ikego formation and the lower urago formation allowing these formations to be considered conformable the following continuous stratigraphic markers were identified in ascending order top of the mammoth subchronozone 3 21 ma base 3 13 ma and top 3 05 ma of the kaena subchronozone last appearance of discoaster tamalis 2 76 ma the widespread tuff bed kgp ca 2 5 ma and the last appearance of discoaster pentaradiatus 2 41 ma sedimentation rates are almost constant among these age controlled horizons which indicates continuous sedimentation from 3 2 to 2 4 ma the results indicate that the kazusa group conformably overlies the miura group on the miura peninsula which provides a well established continuous stratigraphic section for analyses of regional paleoenvironmental and tectonic evolution in the japanese forearc region

rising methane gas bubbles form massive hydrate layers at the seafloor extensive methane hydrate layers are formed in the near surface sediments of the cascadia margin an undissociated section of such a layer was recovered at the base of a gravity core i e at a sediment depth of 120 cm at the southern summit of hydrate ridge as a result of salt exclusion during methane hydrate formation the associated pore waters show a highly elevated chloride concentration of 809 mm in comparison the average background value is 543 mm a simple transport reaction model was developed to reproduce the cl observations and quantify processes such as hydrate formation methane demand and fluid flow from this first field observation of a positive cl anomaly high hydrate formation rates 0 15 1 08 mol cm 2 a 1 were calculated our model results also suggest that the fluid flow rate at the cascadia accretionary margin is constrained to 45 300 cm a 1 the amount of methane needed to build up enough methane hydrate to produce the observed chloride enrichment exceeds the methane solubility in pore water thus most of the gas hydrate was most likely formed from ascending methane gas bubbles rather than solely from ch4 dissolved in the pore water

high resolution stratigraphy across the early middle pleistocene boundary from a core of the kokumoto formation at tabuchi chiba prefecture japan abstract we present a detailed stratigraphy of the lithology sedimentary facies magnetic susceptibility and chemical composition of a 54 m thick core sequence drilled adjacent to the chiba section a candidate for the global boundary stratotype section and point of the early middle pleistocene boundary siltstones rich in trace fossils are dominant throughout the core except for the lowermost sand beds the byakubi tephra is divided into three units of which the uppermost 39 54 39 49 m core depth is identified as a primary bed showing evidence of fallout the preliminary matuyama brunhes magnetic polarity boundary lies 1 m above the top of the fallout unit maximum and minimum susceptibility axes for anisotropy of magnetic susceptibility have shallow 73 inclinations respectively throughout the core the results suggest that sedimentary structures formed in calm environments remain undisturbed in the sediment clustered maximum axis directions indicate the influence of a persistent nw se contour current the homogeneous depositional environment for the siltstone interval is supported by relatively smooth variations in magnetic susceptibility and chemical composition stratigraphic correlation with published oxygen isotope data indicates that the depositional age of the core may extend from the end of marine oxygen isotope stage mis 20 to just after mid mis 19 from which an average sedimentation rate of 2 2 5 m ky is estimated these results suggest that the chiba section has a high potential to provide an excellent stratotype for the early middle pleistocene boundary

initial rise of bubbles in cohesive sediments by a process of viscoelastic fracture 1 an understanding of the mechanics of bubble rise in sediments is essential because of the role of bubbles in releasing methane to the atmosphere and the formation and melting of gas hydrates past models to describe and predict the rise of other buoyant geological bodies through a surrounding solid e g magmas and hydrofractures appear not to be applicable to bubbles in soft sediments and this paper presents a new model for gas bubble rise in soft fine grained cohesive sediments bubbles in such sediments are essentially dry little if any free water and grow through a process of elastic expansion and fracture that can be described using the principles of linear elastic fracture mechanics which assume the existence of a spectrum of flaws within the sediment fabric by extending this theory we predict that bubbles initially rise by preferential propagation of a fracture in a sub vertical direction we present a criterion for initial bubble rise once rise is initiated the speed of rise is controlled by the viscoelastic response of the sediments to stress using this new bubble rise model we estimate rise velocities to be of the order of centimeters per second we again show that capillary pressure plays no substantive role in controlling bubble growth or rise

predictions of the acoustic scattering response of free methane bubbles in muddy sediments the response of the sediments of eckernfoerde bay germany to acoustic remote sensing previously has been attributed qualitatively to gas features which were postulated to exist in situ within the sediment the existence of features as small as 0 5 mm equivalent spherical radius has now been confirmed by x ray computed tomography of cores taken and scanned at in situ pressures the interaction of an acoustic pulse from the acoustic sediment classification system ascs with this type of gassy sediment was modeled and the results are presented here the bubble scattering response model includes both the effects of nonspherical bubbles and of sediment resistance to shear deformation as expressed by the dynamic shear modulus model predictions made using the observed gas bubble distribution agree with normal incidence ascs data exhibiting extended returns i e greater than the source pulse length from the seafloor bubble layers as well as high attenuation within the bubble layers spectrograms of ascs and modeled return pulses were also similar these results suggest that the acoustic return characteristics of the near surface sediments of eckernfoerde bay are dominated by scattering from a distribution of gas bubbles with depth