the direct wave root in marine seismology abstract the direct wave root is an evanescent wave which is observed below the sea floor when the direct water wave is incident on the sea floor from above at supercritical angles for shallow borehole receivers it is the largest amplitude arrival on large offset seismograms the wave has been studied and observed in electromagnetics and acoustics but this paper is the first report of the direct wave root in marine seismology theoretically an expression for the direct wave root can be obtained from a branch line integral in the complex ray parameter plane for the transmission problem between two acoustic half spaces synthetic seismogram analysis based on the discrete wavenumber or reflectivity method can be applied to study the more realistic cases of elastic bottoms gradients beneath the sea floor and intrinsic attenuation steep gradients below the sea floor give amplitude depth relations which are not exponential attenuation q 40 affects amplitudes less than 1 db observations of the direct wave root have been made at the deep sea drilling project site 504b in the costa rica rift area and the observed properties are consistent with theory it is proposed that more detailed measurements of the decay of the direct wave root amplitude with depth be used to obtain high resolution measurements of upper crustal elastic properties at seismic frequencies

drilling deep into young oceanic crust hole 504b costa rica rift hole 504b is by far the deepest hole yet drilled into the oceanic crust in situ and it therefore provides the most complete ground truth now available to test our models of the structure and evolution of the upper oceanic crust cored in the eastern equatorial pacific ocean in 5 9 m y old crust that formed at the costa rica rift hole 504b now extends to a total depth of 1562 3 m below seafloor penetrating 274 5 m of sediments and 1287 8 m of basalts the site was located where the rapidly accumulating sediments impede active hydrothermal circulation in the crust as a result the conductive heat flow approaches the value of about 200 mw m predicted by plate tectonic theory and the in situ temperature at the total depth of the hole is about 165 c the igneous section was continuously cored but recovery was poor averaging about 20 the recovered core indicates that this section includes about 575 m of extrusive lavas underlain by about 200 m of transition into over 500 m of intrusive sheeted dikes the latter have been sampled in situ only in hole 504b the igneous section is composed predominantly of magnesium rich olivine tholeiites with marked depletions in incompatible trace elements nearly all of the basalts have been altered to some degree but the geochemistry of the freshest basalts is remarkably uniform throughout the hole successive stages of on axis and off axis alteration have produced three depth zones characterized by different assemblages of secondary minerals 1 the upper 310 m of extrusives characterized by oxidative seafloor weathering 2 the lower extrusive section characterized by smectite and pyrite and 3 the combined transition zone and sheeted dikes characterized by greenschist facies minerals a comprehensive suite of logs and downhole measurements generally indicate that the basalt section can be divided on the basis of lithology alteration and porosity into three zones that are analogous to layers 2a 2b and 2c described by marine seismologists on the basis of characteristic seismic velocities many of the logs and experiments suggest the presence of a 100 to 200 m thick layer 2a comprising the uppermost rubbly pillow lavas which is the only significantly permeable interval in the entire cored section layer 2b apparently corresponds to the lower section of extrusive lavas in which original porosity is partially sealed as a result of alteration nearly all of the logs and experiments showed significant changes in in situ physical properties at about 900 1000 m below seafloor within the transition between extrusives and sheeted dikes indicating that this lithostratigraphic transition corresponds closely to that between seismic layers 2b and 2c and confirming that layer 2c consists of intrusive sheeted dikes a vertical seismic profile conducted during leg 111 indicates that the next major transition deeper than the hole now extends that between the sheeted dikes of seismic layer 2c and the gabbros of seismic layer 3 which has never been sampled in situ may be within reach of the next drilling expedition to hole 504b therefore despite recent drilling problems deep in the hole current plans now include revisiting hole 504b for further drilling and experiments when the ocean drilling program returns to the eastern pacific in 1991

velocity structure in upper ocean crust at hole 504b from vertical seismic profiles hole 504b provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust providing a baseline for comparison with seismic measurements elsewhere we determine p and s velocities from vertical seismic profiles vsps obtained on ocean drilling program odp legs 111 and 148 four issues are considered the location of the seismic layer 2 3 boundary p to s wave conversion by scattering transverse isotropy and poisson s ratio as an indicator of lithology porosity and structure 1 in the p velocity profile the change in slope marking the layer 2 3 boundary coincides with the top of the sheeted dike unit seismic layer 2 is composed of the extrusives and the lithologic transition zone the layer in which flows and dikes interfinger 2 even in these normal incident vsps several second arrivals with velocities indicative of vertically polarized shear energy are observed p to s wave conversion within the upper 110 m of basement occurs by scattering from surface roughness and volume heterogeneities and does not depend on angle of incidence as predicted by plane boundary transmission coefficient analysis 3 vertical velocities determined from the vsp differ by 10 from horizontal velocities obtained from the oblique seismic experiment ose on deep sea drilling project dsdp leg 92 the p wave velocity structure is determined by small and intermediate 1 cm pore structure with no measurable anisotropy the large scale well oriented vertical fractures which are formed tectonically do not have a detectable effect on compressional wave velocities 4 high poisson s ratio in the upper 300 m of basement coincides with an extrusive layer composed of pillows and thin flows low poisson s ratio at 850 1150 m below seafloor mbsf coincides with the downhole decrease in bulk porosity caused by the transition from extrusives to dikes relatively large aspect ratio cracks are required to produce such low values of poisson s ratio the cracks were likely created by hydraulic fracturing when hot dikes encountered low temperature seawater

crustal structure at the blake spur fracture zone from expanding spread profiles we present results from wkbj and reflectivity synthetic seismogram modeling of 10 reversed expanding spread profiles esps along flow lines parallel to the blake spur fracture zone across 140 ma atlantic crust these profiles provide detailed constraints on variations in crustal structure at the fracture zone seven profiles on either side of the fracture zone show a normal structure for old oceanic crust a 2 3 km thick upper layer with a steep velocity gradient is underlain by 4 5 km of crust with velocities of 6 5 7 2 km s and low gradient and a sharp transition to upper mantle velocities of 8 km s several velocity discontinuities were detected within the upper 2 3 km but these do not generally coincide with intracrustal reflectors detected by simultaneous normal incidence reflection profiles this structure shows little regional variation toward the fracture zone despite the small magnetic anomaly offset 12 km and the indistinct topographic signature of the fracture zone three esps within a 10 20 km wide ribbon centered on the fracture zone trough show clearly anomalous crustal structure relative to normal oceanic crust a 2 4 km thick high gradient upper layer is underlain by a thick prism of material with a velocity of 7 2 7 6 km s and high poisson s ratio probably at least 0 29 which is consistent with 15 30 serpentinization of upper mantle peridotites this interpretation requires the action of off axis hydrothermal circulation penetrating the cracked and relatively permeable fracture zone lithosphere to a depth of at least 7 km the original igneous crust in a narrow region close to the fracture zone is thus inferred to have been much thinner than adjacent normal crust which may imply a sharp reduction in the magma budget at the ends of the adjacent spreading segments

the first direct measurements of upper oceanic crustal compressional wave attenuation the first direct measurement of compressional wave attenuation of the uppermost 650 m of oceanic crust was performed using data recorded by seafloor hydrophones and large 56 116 kg deep explosive sources the site was 13 km east of the southernmost juan de fuca ridge on crust 0 4 m y old spectral ratios were performed between bottom refracting waves and direct water waves adjusted for spreading losses and transmission coefficient losses several tests of the data were performed demonstrating that attenuation is linearly related to frequency between 15 and 140 hz but frequency independent components of attenuation are also evident values of compressional wave q cluster between 20 and 50 and do not show any systematic variation with depth over 650 m the attenuation results also indicate the presence of heterogeneities within the crust as the solutions for each receiver s data set are significantly different no evidence for azimuthal variations of attenuation are supported by the data although the data do not optimally sample a wide variation of azimuths our attenuation values are judged to be normal to higher than expected for the whole oceanic crust based upon comparisons to results from synthetic seismogram modeling by others and by modeling signal to noise ratios of typical seismic refraction profiles the results are consistent with recent laboratory measurements at ultrasonic frequencies for dry and saturated basalts at seafloor pressures and temperatures

seismic attenuation in upper ocean crust at hole 504b seismic attenuation and its relationship to borehole stratigraphy in the upper 1 8 km of ocean basement at hole 504b are determined from analysis of vertical seismic profile vsp data vsp data provide unambiguous measurements of seismic amplitude decay along a vertical propagation path through the crust and ancillary borehole measurements enable detailed modeling of the relative contributions from geometrical spreading scattering and intrinsic loss mechanisms to this decay about 60 of the total observed amplitude decay occurs in the pillow basalt section and is due mostly to geometrical spreading and scattering from impedance contrasts the remaining amplitude decrease is concentrated in two layers at 500 650 and 800 900 meters below seafloor mbsf 225 375 and 525 625 m below basement across which amplitude rapidly decays and the frequency characteristics of the downgoing wave field are significantly and permanently modified attenuation in these layers is not due to scattering but rather to an intrinsic mechanism that can be characterized by q of 10 and 8 respectively it is likely that the q structure of both of these intervals is formed with the crust near the ridge and thus related to fundamental ocean crust forming processes the shallow interval coincides with a change in alteration mineralogy deposited by late stage fluid flow and may separate lower lavas that were emplaced within the rift zone from upper lavas that were emplaced by off axis flow through large lava tubes intrinsic attenuation in the deeper horizon is probably due to an increase in porosity and cracking associated with either intracrustal deformation or subhorizontal faulting there is negligible attenuation of seismic frequencies in the dikes below 1000 mbsf 725 m subbasement

formation succession and landscape history of central european summit raised bogs a multiproxy study from the hrubý jesenik mountains central european raised bogs developed on mountain summits are specific ecosystems of high conservation importance but their history remains largely unknown pollen macrofossils testate amoebae and peat characteristics were analysed in a peat sequence of the vozka bog hrub jesenik mountains eastern sudetes czech republic past water chemistry and water table depths were reconstructed by transfer functions calibrated from recent testate amoeba data and long term environmental averages peat initiation started in the middle holocene approximately 4200 bc by the process of paludification resembling the development of atlantic blanket bogs around 100 bc the vegetation changed from eriophorum vaginatum poor fen to the ombrotrophic bog vegetation similar to the recent situation a hiatus in peat sequence was revealed between ad 1320 and 1954 it can be explained either by human activities or by an extreme drought causing decomposition of previously accumulated peat local bog development was larg

the disturbance regime of an early holocene swamp forest in the czech republic as revealed by dendroecological pollen and macrofossil data abstract a unique remnant of forest dating back to the period 9733 7897 yr bc and consisting of hundreds of tree bases was discovered in the czech republic we aimed to reveal the complex disturbance history of this sub fossil forest using dendrochronology and to describe its detailed plant species composition changes using palaeobotanical techniques analysing such early holocene forest dynamics should help us understand the ability of the forest community to actively adapt to climate change and generally to understand the role of dynamic instability in ecosystem evolution we anatomically identified woody species in 488 samples and determined the ages growth suppressions releases and fire scars in 116 well preserved tree ring series using a modern boundary line approach this image of the forest structure and dynamics was supplemented with analyses of pollen spectra and plant macrofossils in excavated profiles in order to achieve accurate dating we dated 87 samples using 14c and synchronized tree ring series and compared them with an existing pinus sylvestris chronology the developmental trajectory of the forest was unique and did not match the general trend of postglacial pine growth in central europe palaeobotanical proxies indicated that during the circa 2000 years the forest persisted this early holocene ecosystem passed through several phases reflected in the species composition of the vegetation as well as in habitat conditions nevertheless the dominance of pine and the complex fine scale disturbance regime were relatively robust and did not change fundamentally low severity fires and short term changes in soil moisture regime were crucial disturbance agents in the ecosystem stand replacing disturbances were not found up to the gradual collapse of the forest around 8300 yr bc replaced by a swamp community the disturbance regime was relatively stable suggesting a mitigating effect of changing climate due to the predominance of pine in the forest

holocene peatland development and hydrological variability inferred from bog pine dendrochronology and peat stratigraphy a case study from southern sweden dendrochronological analysis was applied to subfossil remains of scots pine pinus sylvestris l buried in a south swedish peat deposit in combination with peat stratigraphy this approach was explored for its potential to provide information on the local hydrological and depositional history at the site forming the basis for a regional palaeohydrological analysis a 726 year ring width chronology was developed and assigned an absolute age of 7233 6508cal a bp 5284 4559 bc through cross dating with german bog pine chronologies whereas two short additional records of older ages were radiocarbon dated registration of growth positions of individual trees allowed assessment of the spatial dynamics of the pine population in response to hydrological changes and peatland ontogeny annually resolved growth variability patterns in the pine population reveal several establishment and degeneration phases probably reflecting fluctuations in bog surface wetness a major establishment phase at 7200 6900 cal a bp reflects the onset of a period of lowered groundwater level also indicated by increased peat humification and a development consistent with regional temperature and lake level reconstructions revealed from other proxies this study demonstrates that subfossil bog pine populations may provide annually to decadally resolved reconstructions of local groundwater variability which are highly relevant in a long term palaeoclimatic context copyright c 2012 john wiley sons ltd

lateral heterogeneity in the seismic structure of upper oceanic crust western north atlantic we carried out a borehole seismic experiment at deep sea drilling project hole 418a in order to study the vertical and lateral velocity variations in 100 m y old oceanic crust we clamped a seismometer successively at five depths between 41 m and 430 m within the extrusive basalt layer while air gun and explosive sources were fired on eight radial lines and four concentric circles to 8 km range our travel time data do not give consistent evidence of anisotropy however our experimental error may mask anisotropy of up to 0 2 0 3 km s velocities from inflection point analysis sonic log and reflectivity synthetic seismograms support results of inversion of radial line p arrival times velocity increases with an almost linear gradient of 1 3 s 1 from 4 6 km s at the top of basement to 1 8 km depth the velocity profile at 418a does not differ significantly at the 95 confidence level from that at 417d located 7 5 km away seismic velocities at shorter length scales do vary laterally travel time anomalies indicate that seismic velocity in the upper 0 5 km increases laterally northwest to 3 km range other data indicate that this velocity anomaly in the extrusive layer may be due to alteration controlled by topography or to primary porosity variations

three dimensional upper crustal heterogeneity and anisotropy around hole 504b from seismic tomography we have used over 16 000 compressional wave travel times recorded on 11 ocean bottom seismometers to determine the magnitude of p wave heterogeneity and anisotropy in the upper 2 km of the oceanic crust in an 150 km2 area around hole 504b the deepest hole drilled into the oceanic crust our best fitting one dimensional isotropic upper crustal velocity model for the 150 km2 area around the drill site is similar to velocity models at hole 504b determined using downhole sonic logs vertical seismic profiles oblique seismic experiments and conventional refraction profiling there is no evidence seismic layer 2 is anomalously thin at hole 504b suggesting that the occurrence of the seismic layer 2 3 boundary within the upper sheeted dike section which has been documented in hole 504b is a general feature of this 5 9 ma crust on the scale of a few kilometers and averaged over seismic wavelengths of a few hundred meters the heterogeneity in upper crustal p wave velocity around hole 504b is comparatively small within the 571 m thick extrusive section maximum p wave velocity differences are 300 400 m s 1 a 6 8 velocity anomaly these velocity differences decrease to 1 km into the crust the highest upper crustal velocities are found beneath basement ridges west and south of hole 504b while the lowest upper crustal velocities occur beneath the flanking basement troughs there is an azimuthal dependence to the travel time residuals slow direction n s which is consistent with 2 4 crack induced anisotropy in the upper crust the magnitude of upper crustal heterogeneity observed near hole 504b is significantly less than that found in zero age crust at the mid atlantic ridge but comparable to that observed off axis in crust formed at faster spreading ridges

seismic energy partitioning and scattering in laterally heterogeneous ocean crust we present finite difference forward models of elastic wave propagation through laterally heterogeneous upper oceanic crust the finite difference formulation is a 2 d solution to the elastic wave equation for heterogeneous media and implicitly calculates p and sv propagation compressional to shear conversion interference effects and interface phenomena random velocity perturbations with gaussian and self similar autocorrelation functions and different correlation lengths a are presented which show different characteristics of secondary scattering heterogeneities scatter primary energy into secondary body waves and secondary stoneley waves along the water solid interface the presence of a water solid interface in the model allows for the existence of secondary stoneley waves which account for much of the seafloor noise seen in the synthetic seismograms for the laterally heterogeneous models

radiocarbon evidence for rates of lateral expansion in raised mires in southern finland radiocarbon ages from the bottom peat deposits and detailed morphometric measurements at five peatlands in the boreal region of southern finland were used to reconstruct the holocene patterns of raised mire development the results suggest that the areal expansion of the mires has been extremely rapid at times up to several hundreds hectares per millennium but the periods of more rapid expansion were not coincident among the mires the rate of lateral extension was shown to depend mainly on the topography of the mineral substrate horizontal expansion did not always correlate with vertical growth mires expanded slowly between ca 6000 and 4000 yr b p probably as a consequence of drier climatic conditions at that time the mires also expanded slowly during the two to three recent millennia the reason for this most probably lying in steepening of the bottom gradient toward the marginal areas of the present mires the results point to a substantial measure of local environmental control over the lateral extension in raised mire ecosystems

seismic amplitude modeling and the shallow crustal structure of the east pacific rise at 12 n velocity structures of the upper 2 km of the oceanic crust were used to model seismic refraction data collected on the eastern flank of the east pacific rise at 12 n during the rivera ocean seismic experiment rose data generated with a bolt 1500c air gun source show multiple narrow 1 km in range peaks in the amplitude of the first refracted arrival record sections separated by as much as 150 km indicate similar amplitude versus range patterns suggesting that the amplitude peaks are caused by consistent structural features and not by random crustal inhomogeneities these amplitude highs do not consistently correlate with seafloor bathymetry and hence cannot be explained solely by topographic focussing of seismic energy through travel time analysis and ray tracing we find a crustal velocity model with two vertical zones of concave gradients which focus rays at 6 and 8 km range synthetic seismograms and power versus range patterns computed for this model by the reflectivity algorithm are consistent with the two peak amplitude pattern observed on nearly all air gun record sections these results suggest that the vertical gradient zone structure of the uppermost 2 3 km of ocean crust is continuous to first order over the rose area

seismic velocity structure at deep sea drilling project site 504b panama basin evidence for thin oceanic crust we present an analysis of wide angle reflection refraction data collected in the immediate vicinity of deep sea drilling project hole 504b in the panama basin currently the deepest drill hole 1 288 km into oceanic crust the data were acquired with a 1785 inch3 air gun array and fixed gain sonobuoy receivers and consist of four intersecting profiles shot along three different azimuths near normal incidence multichannel seismic mcs reflection data were acquired simultaneously observed p and s wave arrivals out to maximum ranges of 30 km provide constraints on the velocity structure of the middle and lower crust and on total crustal thickness comparison of the travel times and amplitudes of the p and s wave arrivals on all four profiles revealed important similarities which were modeled using the reflectivity synthetic seismogram method forward modeling shows that in contrast to standard oceanic velocity models a velocity depth profile that better explains the observed data is characterized by high velocity gradients up to 0 6 km s km in the middle crust a 1 8 km thick low velocity zone vp 7 1 6 7 km s immediately above moho and a total crustal thickness of only 5 km interpretation of the high velocity gradients in the middle crust is constrained by the observation of p wave amplitude focusing at ranges of 16 19 km although not as well developed in comparison to the p wave arrivals s wave arrivals show similar focusing total crustal thickness is constrained by the combined interpretation of a p wave wide angle reflection event observed at a range of 16 28 km and an mcs reflection event with a crustal travel time of 1 4 1 5 s although these events cannot be directly correlated their travel times are consistent with the assumption that both have a common origin amplitude modeling of the wide angle event demonstrates that these events are generated at the moho

lateral heterogeneity in the upper oceanic crust at deep sea drilling project site 504 the lateral variability of the upper oceanic crust can be studied by mapping the amplitude or power content of arrivals from a dense array of identical sources to a single receiver in this paper we present the results from a borehole seismic experiment at deep sea drilling project site 504 in the eastern equatorial pacific seismic amplitude anomalies of 20 db and more are present with a horizontal correlation length of 1 3 km finite difference synthetic seismogram techniques are used to predict seismic wave propagation through realistic bathymetry basement relief and lateral variations within basement a three dimensional model of the velocity structure around the borehole can be constructed the resultant model has horizontal gradients greater than 2 0 s 1 in the upper crust these gradients are comparable to the vertical gradients normally associated with oceanic crust assuming that velocity anomalies in the upper crust reflect changes in porosity the resultant structure provides constraints on the hydrothermal circulation at the site in addition the finite difference modeling indicates that polarity reversals can be generated by lateral velocity variations near the source and receiver

late holocene expansion of a south swedish peatland and its impact on marginal ecosystems evidence from dendrochronology peat stratigraphy and palaeobotanical data in this study a reconstruction of the long term development and lateral expansion of a south swedish peat bog was performed using a multi proxy approach including dendrochronology peat stratigraphy and macrofossil and pollen analyses by combining mapping of cross dated subfossil trees with radiocarbon dated peat sequences an improved approach to reconstruction of lateral peat expansion was applied apart from providing approximate ages of tree burial episodes the ring width records offer information on hydrological variations prior to the bog expansion new bog oak pine and alder chronologies are presented and their potential as a dating tool for peatland expansion as well as for local to regional environmental interpretations is examined our tree replication records show that increased amounts of bog trees in the central parts can be linked to drier bog surface conditions whereas an increase in wood remains in the marginal zone is related to enhanced preservation due to lateral bog expansion our reconstructions of the development of the peat deposit and associated changes in the distribution of vegetation communities provide new insight into peatland responses to climate change at the end of the holocene thermal maximum 5000 4000 cal yr bp less

travel time analysis of borehole seismic data a method is presented for reducing travel time data from multiple offset borehole seismic experiments to velocity depth structure the technique which treats simultaneously data from any number of depths in the borehole in addition to surface data is based on the linear inversion scheme of dorman and jacobson 1981 given the parameters p t p px p and p t p px p the method solves for depth to specified slownesses assuming linear gradients in velocity between slowness values a practical limitation on the use of surface travel time measurements to resolve velocity depth structure is the necessity of assuming a surface velocity this is not necessary for the case of borehole data for a borehole receiver the velocity at the depth of the receiver can be obtained from the slope of the inflection point of the travel time curve thus a direct measure of the uppermost velocity of a section can be obtained from the travel time data of a shallow borehole receiver estimation of velocities from the inflection points of deeper receivers improves the resolution of the velocity depth function which would be obtained from surface data alone the technique is applied to data from three borehole seismic experiments in oceanic crust the experiments were carried out in the western atlantic crustal age 110 m y the gulf of california crustal age 1 m y and the costa rica rift area crustal age 6 m y all three experiments show relatively high upper crustal velocities 4 0 km s suggesting that layer 2a is not present even in the very young crust all sites had over 100 m of sediment thickness and it is postulated that sediment thickness and sediment permeability not merely age govern the velocity of the upper crust by accelerating the cementation of fractures and cracks

palynological evidence for late holocene human occupation recorded in two wetlands in sw turkey pollen diagrams from mountain lakes and marshes in sw turkey show evidence of intensive anthropogenic influence in the landscape between 3500 and 1300 bp three cores from within the territory of the classical city sagalassos western taurus pisidia were palynologically analysed to make a reconstruction of the past vegetation in the territory of sagalassos and to estimate the impact of its inhabitants on the landscape in pisidia two cores originate from an intramontane marsh gravgaz elevation 1215 m and one from a seasonal intramontane lake canakh elevation 1030 m human acitivity is apparent from c 2530 bp in the pollen diagrams a deforestation phase disturbance phase from c 2530 bp c 2480 bp till c 2280 bp c 2270 bp anthropogenically or climatologically driven precedes a cultivation period arboricultural phase from c 2280 bp 2270 bp till c 1480 bp c 1270 bp the results of the pollen analyses of the three cores are compared and show many similarities there are also similarities with other cores from sw turkey although the arboricultural phase mainly olive cultivation occurs later in the cores from the sagalassos territory than in other cores from sw turkey

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

finite difference synthetic acoustic logs the finite difference method is a powerful technique for studying the propagation of elastic waves in boreholes even for the simple case of an open borehole with vertical homogeneity the snapshot format of the method displays clearly the interaction between the borehole and the rock and the origin and evolution of phases we present an outline of the finite difference method applied to the acoustic logging problem including a boundary condition formulation for liquid solid cylindrical interfaces which is correct to second order in the space increments absorbing boundaries based on the formulations of reynolds 1978 and clayton and engquist 1977 were used to reduce reflections from the grid boundaries results for a vertically homogeneous sharp interface model are compared with the discrete wavenumber method and excellent agreement is obtained the technique is also demonstrated by considering sharp and continuous transitions damaged zones at the borehole wall and by considering the effects of wa

finite difference seismograms for laterally varying marine models summary the method of finite differences is applied to the elastic wave equation to generate synthetic seismograms for laterally varying seafloor structures the results are compared with borehole seismic data from the gulf of california deep sea drilling project site 485 in which lines were shot over flat and rough topography the significant new phenomenon observed in both the synthetic seismograms and the field data is the generation of a double head wave due to the interaction of the incident wavefront with the side of a hill and the flat seafoor adjacent to the hill in these models the hills are on the order of a seismic wavelength in height and steep velocity gradients occur over distances comparable to wavelengths ray theoretical methods would not be suitable for studying such structures true amplitude record sections are obtained by the finite difference method which show for these models that the head wave generated at the flat seafloor adjacent to the hill is lower in amplitude than if the hill were not present and is lower in amplitude than the head wave generated at the hill a second feature which is important for borehole receivers is the existence of the direct wave root in the upper basement this energy occurs below the sharp interface when the direct wave impinges on the interface from above there is no corresponding snell s law ray path for this energy and the energy is evanescent with depth in the lower medium the properties of both the double head wave and the direct wave root are clearly demonstrated in the finite difference snapshot displays

a sensitive high resolution record of late holocene climatic change from a raised bog in northern england proxy climatic data in the form of plant macrofossils have been analysed from a 5 m core from bolton fell moss cumbria uk detailed analysis of peat from the upper 50 cm of this core is used to demonstrate a strong correlation between changes in the relative proportion of taxa and known climatic changes over the last 1000 years the record of changes in bog vegetation contained within the peat profile is used to reconstruct changes in bog surface wetness for the latter half of the holocene as bog surface wetness is directly controlled by the prevailing climatic conditions this reconstruction can be viewed as a proxy climate record twelve radiocarbon age estimates on the 5m core suggest that between 50 and 500 cm peat accumulated at a relatively constant rate of 12 4 yr cm 1 the regular sampling intervals thus provide a time series of past bog surface wetness spectral analyses of this series indicates that wetness changes are cyclic with a ca 800 year periodicity

spreading rate independence of oceanic seismic layer 2 comparison of travel time and amplitude data from two oblique seismic experiments located on 110 m y old oceanic crust in the western atlantic and northwestern pacific oceans shows that compressional velocities within layer 2 are within experimental error identical at the two sites this result implies that at least for 110 m y old crust the velocity structure in layer 2 where extrusive lavas grade downward into a sheeted dike complex is independent of the spreading rate at which the crust was created

geophysical constraints on the crustal architecture of the troodos ophiolite results from the iangass project summary we present results from modelling a combined 160 km seismic wide angle reflection refraction and gravity profile sited across the sheeted dykes lavas and sediments of the northern part of the troodos ophiolite cyprus the p wave seismic velocity and density model including five separate layers is defined to a depth of 10 km this shows the ophiolite sequence dipping to the east beneath the central well resolved 85 km section of the line between kambos in the west and kelia in the east the upper layer average velocity 2 83 km s 1 density 2 21 g cm 3 is interpreted as consisting of sediments and the upper extrusive group beneath this accepting that layer boundaries may be gradational beyond the experiment resolution the layers are identified as the lower extrusive and basal groups 3 57 km s 1 2 31 gc m 3 the sheeted dyke complex sdc 4 57 km s 1 2 72 g cm 3 a gabbroic layer 6 31 km s 1 2 92 g cm 3 and an ultramafic cumulate layer and or serpentinized peridotite 7 00 km s 1 3 00 g cm 3 while the thickness 6 km of this layer beneath the proposed gabbroic layer is defined positive identification of its composition is not possible owing to the lack of discriminatory s wave velocity information the underlying material provides an unreversed velocity of 8 0 km s 1 beneath the western end of the profile these modelled velocities and densities are compared with seismic models of oceanic crust and their implications for models of the tectonic evolution of the troodos ophiolite are discussed several major faults are interpreted from the models with throws of up to 500 m offsetting both the extrusive sdc and the sdc gabbroic layer boundaries these are all downthrown to the west there being no evidence from the data to suggest any major faults downthrown to the east our results therefore do not support existing tectonic models for a multiple graben structure within the ophiolite recording either the progress of an eastward stepping spreading centre or off axis graben formation instead assuming that the faults are downthrown towards the ridge axis the data are consistent with a single spreading centre located to the west of the recording profile

impact of crustal evolution on changes of the seismic properties of the uppermost ocean crust one of the most important observations of oceanic crustal evolution is that the uppermost part of the igneous crust layer 2a appears to thin and eventually disappear as its seismic velocity doubles from approximately 2 2 to 4 5 km s 1 in the first 40 m y the most likely cause is decreasing porosity in extruded basalts that compose the uppermost igneous crust by filling cracks with hydrothermally generated minerals a by product of off axis hydrothermal circulation and alteration of basalts new insights from two recent papers on crustal hydrothermal circulation patterns and modeling of the effects of crack geometry distributions upon seismic velocities provide guidance to relate filling of cracks within the basement to seismic velocity increases a qualitative model of layer 2a crustal evolution is outlined that satisfies many geophysical and geochemical observations some seemingly unrelated somewhat surprisingly the crustal age at which layer 2a seismic velocities reach their maximum value is primarily controlled by the integrated permeability of the overlying sediment which in turn controls the hydrologic regime necessary for secondary mineral precipitation speculative predictions can be used to verify refine or invalidate this dynamic model of crustal evolution major scientific and technical obstacles must be overcome before a definitive model can be quantified with such a rigorous model of crustal upper crustal evolution in hand remote geophysical sensing tools can begin to fulfill the promise of interpreting the physical state composition and past history of oceanic crust

quantitative comparison of seismic data sets with waveform inversion testing the age dependent evolution of crustal structure studies of seismic variability play a more important role in the understanding of processes that form oceanic crust than individual velocity depth solutions found in isolation for such variability studies solutions must be contrasted with one another this is normally not straightforward owing to the lack of rigorous uncertainty estimates available for solutions here i present a method for the direct comparison of such solutions by hypothesis testing a null hypothesis is proposed that a single velocity solution is consistent with two sets of data the hypothesis is tested by seeking such a model using waveform inversion and comparing its misfit against the misfit from fully independent solutions the degree of similarity of the velocity structures can be further quantified by solving for the pair of solutions that are as similar as possible while consistent with each set of data the f test is used to evaluate the significance of each additional effective model parameter as it lends independence to the two solutions the method possesses advantages of a hypothetical monte carlo search of all models because the complete forward problem is carried out at each step yet it is tractable because the search direction is well defined the method is applied to two data sets collected near the mid atlantic ridge on 0 and 7 m y crust in order to quantify the differences between the solutions the comparison finds quantifiable differences in both layers 2 and 3 between these two regions suggesting that significant evolution of the entire crustal section has taken place in the first 7 m y

estimation of seismic velocities of upper oceanic crust from ocean bottom reflection loss data this paper describes a bayesian inversion of acoustic reflection loss versus angle measurements to estimate the compressional and shear wave velocities in young uppermost oceanic crust layer 2a the data were obtained in an experiment on the thinly sedimented western flank of the endeavor segment of the juan de fuca ridge using a towed horizontal hydrophone array and small explosive charges as sound sources measurements were made at three sites at increasing distance from the ridge spreading center to determine the effect of age of the crust on seismic velocities the inversion used reflection loss data in a 1 3 octave band centered at 16 hz the compressional and shear wave velocities of the basalt were highly sensitive parameters in the inversion the compressional wave velocity increased from 2547 30 to 2710 18 m s over an age span of 1 4 million years ma from the spreading center an increase of 4 5 1 0 ma the basalt shear wave velocity increased by nearly a factor of 2 from 725 to 1320 m s o

holocene palaeoecology and climate history of south georgia sub antarctica based on a macrofossil record of bryophytes and seeds the results are presented of a detailed plant macrofossil analysis of a c 14 dated holocene organic sequence from a sub antarctic island south georgia in the peat infilling of a rock basin 31 bryophyte species and many of the principal species of the present day angiosperm flora associated with bog communities have been identified the zonation of the bryophytes and the seeds and fruits shows a striking correspondence with the sediment stratigraphy and indicates five ecological phases the first four reflect a succession from a minerotrophic shallow pool to an ombrotrophic warnstorfia polytrichum bog after the deposition of a greyish brown layer c 2200 c 14 years bp a wet deschampsia warnstofia sanionia rostkovia bog came into existence which is still present today the ecological phases are radiocarbon dated they can be interpreted in terms of climate development during the holocene climate ameliorated rapidly at about 10 000 c 14 years bp followed by a drier period around 7000 c 14 years bp the climate became wetter and the following c 2500 years probably represent the climatic optimum between 4500 and 2600 c 14 years bp drier conditions prevailed at the study site the most striking change occurred at 2600 c 14 years bp when a marked change in vegetation and ecology indicates much wetter conditions less

abrupt climatic changes and an unstable transition into a late holocene thermal decline a multiproxy lacustrine record from southern sweden the transition from a middle holocene relatively warm and stable climate to a cooler and unstable late holocene climate is reconstructed using sediments from lake igelsjon south central sweden this multiproxy study illustrates local regional and global scale responses to climatic change by focusing on a previously identified abrupt hydrological shift to cooler and or wetter conditions around 4000 cal yr bp the results suggest that between ca 4600 and ca 3400 cal yr bp the environment around and within the lake responded in two major well defined steps the first between 4450 and 4350 cal yr bp and the second between 4000 and 3800 cal yr bp a series of rapid fluctuations of short duration were superimposed on the general cooling trend with the most severe aquatic response peaking at ca 3800 cal yr bp pollen percentage and influx values show forest composition and pollen productivity changes and a distinct decline in total and corylus pollen influx in the period of 4000 3500 cal yr bp stomatal based reconstruction of atmospheric co2 concentration produced a tenuous decrease with a minimum between 3650 and 3500 cal yr bp copyright 2005 john wiley sons ltd

mid to late holocene climate change an overview the last 6000 years are of particular interest to the understanding of the earth system because the boundary conditions of the climate system did not change dramatically in comparison to larger glacial interglacial changes and because abundant detailed regional palaeoclimatic proxy records cover this period we use selected proxy based reconstructions of different climate variables together with state of the art time series of natural forcings orbital variations solar activity variations large tropical volcanic eruptions land cover and greenhouse gases underpinned by results from general circulation models gcms and earth system models of intermediate complexity emics to establish a comprehensive explanatory framework for climate changes from the mid holocene mh to pre industrial time the redistribution of solar energy due to orbital forcing on a millennial timescale was the cause of a progressive southward shift of the northern hemisphere nh summer position of the intertropical convergence zone itcz this was accompanied by a pronounced weakening of the monsoon systems in africa and asia and increasing dryness and desertification on both continents the associated summertime cooling of the nh combined with changing temperature gradients in the world oceans likely led to an increasing amplitude of the el nin o southern oscillation enso and possibly increasingly negative north atlantic oscillation nao indices up to the beginning of the last millennium on decadal to multi century timescales a worldwide coincidence between solar irradiance minima tropical volcanic eruptions and decadal to multi century scale cooling events was not found however reconstructions show that widespread decadal to multi century scale cooling events accompanied by advances of mountain glaciers occurred in the nh e g in scandinavia and the european alps this occurred namely during the little ice age lia between ad w1350 and 1850 when the lower summer insolation in the nh due to orbital forcing coincided with solar activity minima and several strong tropical volcanic eruptions the role of orbital forcing in the nh cooling the southward itcz shift and the desertifi cation of the sahara are supported by numerous model simulations other simulations have suggested that the fingerprint of solar activity variations should be strongest in the tropics but there is also evidence that changes in the ocean heat transport took place during the lia at high northern latitudes with possible additional implications for climates of the southern hemisphere sh

low frequency and high frequency changes in temperature and effective humidity during the holocene in south central sweden implications for atmospheric and oceanic forcings of climate an integrated use of independent palaeoclimatological proxy techniques that reflect different components of the climate system provides a potential key for functional analysis of past climate changes here we report a 10 000 year quantitative record of annual mean temperature tann based on pollen climate transfer functions and pollen stratigraphical data from lake flarken south central sweden the pollen based temperature reconstruction is compared with a reconstruction of effective humidity as reflected by a 18o record obtained on stratigraphy of lacustrine carbonates from lake igelsjon c 10 km from lake flarken which gives evidence of pronounced changes in effective humidity the relatively low tann and high effective humidity as reflected by a low evaporation inflow ratio suggest a maritime early holocene climate 10 000 8 300 cal year bp seemingly incompatible with the highly seasonal solar insolation configuration we argue that the maritime climate was due to the stronger than present zonal flow enhanced by the high early holocene sea surface temperatures in the north atlantic the maritime climate mode was disrupted by the abrupt cold event at 8 200 cal year bp followed at 8 000 cal year bp by a stable holocene thermal maximum the latter was characterized by tann values about 2 5 c higher than at present and markedly dry conditions indicative of stable summer time anti cyclonic circulation possibly corresponding with modern blocking anticyclonic conditions the last 4 300 year period is characterized by an increasingly cold moist and unstable climate the results demonstrate the value of combining two independent palaeoclimatic proxies in enhancing the reliability generality and interpretability of the palaeoclimatic results further methodological refinements especially in resolving past seasonal climatic contrasts are needed to better understand the role of different forcing factors in driving millennial scale climate dynamics

seismic anisotropy in the upper oceanic crust seismic anisotropy in the upper oceanic crust is observed in borehole data obtained at deep sea drilling project dsdp site 504b on dsdp leg 92 particle motion analysis of converted shear wave arrivals from explosive sources at various azimuths reveals a set of patterns which is indicative of hexagonally isotropic structure with a horizontal symmetry axis there are four diagnostic patterns 1 along symmetry axes where vertically polarized shear waves sv are generated but horizontally polarized shear waves sh are not generated the particle motions are purely vertical 2 for azimuths at which both sv and sh are generated and the sh velocity is significantly faster than sv a cruciform pattern with horizontal first motion is observed 3 for azimuths at which both are generated and the sv velocity is significantly faster than sh a cruciform pattern with vertical first motion is observed and 4 for azimuths at which both are generated and sv and sh velocities are similar elliptical particle motions are observed the shear wave particle motions and compressional wave travel times from a 2 km radius circle are consistent with an anisotropic model with hexagonal symmetry the compressional wave velocity has a two theta azimuthal variation between 4 0 and 5 0 km s the symmetry axis is horizontal with an azimuth of n20 w 10 the spreading direction at the site 6 m y age is north south the observed seismic anisotropy is most probably caused by the preferred orientation of large scale fractures and fissures in upper layer 2 which were created in the early stages of crustal development by near axis extensional processes and normal block faulting

age variations of oceanic crust poisson s ratio inversion and a porosity evolution model porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities porosity is particularly important in the uppermost young crust where rapid variations in velocities with depth and crustal age are observed knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of poisson s ratio are available from observations of compressional and shear seismic velocities p and s in this paper i present a joint seismic waveform inversion for p and s velocities are found while maximizing or minimizing poisson s ratio using a hypothesis testing mechanism i apply this method to ocean bottom hydrophone data in 140 ma atlantic crust the resulting solution corridor agrees with laboratory measurements without the low poisson s ratio anomalies at depths of 0 8 1 5 km found by spudich and orcutt 1980 and au and clowes 1984 on younger 15 ma pacific crust compiling other published p and s solutions an age dependent pattern emerges none of the solutions for crust older than 60 ma display the poisson s ratio anomaly i propose a simple crustal evolution model using thin and thick cracks to explain these observations thin cracks preferentially close at shallow depths in the crust producing the localized poisson s ratio anomaly sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements this model is consistent with similar models of crack populations and their evolution from shallow measurements

attenuation differences in layer 2a in intermediate and slow spreading oceanic crust in situ seismic attenuationq 1logs are derived from borehole velocity profiles and reveal sharp boundaries between morphologies of the extrusive volcanic layers in intermediate and slow spreading oceanic crust q 1logs are calculated from the scattering attenuation associated with vertical velocity heterogeneity in ocean drilling program holes 504b and 896a and in hole 395a located in 5 9 7 3 ma crust on the pacific and atlantic plates respectively our results strongly tie crustal properties to seismic measurables and observed geological structures we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to changes in the degree of vertical heterogeneity we interpret a distinct decrease in the q 1log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2a 2b boundary the boundary is located at 465 m depth below the sea floor in both hole 395a and 504b although this is likely to be a coincidence of the sediment thickness at these sites layer 2a is estimated to be approximately 150 m thick in hole 504b and 300 m thick in hole 395a cyclic sequences of high porosity pillows and low porosity massive units in the uppermost 100 m of volcanics in hole 395a result in large velocity heterogeneities which cause 5 times more attenuation in this layer than in hole 504b in hole 896a by contrast fewer pillows more massive flows and a greater volume of carbonate veins decrease the velocity heterogeneity and attenuation significantly over only 1 km distance from hole 504b we conclude that the attenuation in the extrusive volcanics of the ocean crust is largely controlled by variation in local heterogeneity and morphology as well as by subsequent hydrothermal alteration the observed differences inq 1profiles and layer 2a thickness at these sites may be attributed to variations in the volume and duration of volcanic activity at mid ocean spreading centers for these pacific and atlantic ridge segments

geoacoustic scattering from seafloor features in the rose area a strong refraction branch diffraction has been observed on ocean bottom hydrophone data from the rivera ocean seismic experiment rose modeling of the interaction between seismic acoustic energy and the ocean bottom has shown that this arrival could be caused by scattering from common seafloor features seismic acoustic wave propagation was computed for models of marine seismic refraction lines using a 2d heterogeneous formulation of the elastic wave equation these models demonstrate that a significant amount of energy is diffracted from seafloor structures such as hills and valleys when both the direct water wave and the refracted compressional wave traveling through the upper crust are incident upon a structure since much of the diffracted energy travels through the crust the diffracted arrivals appear on the refraction branch of the seismograms at large ranges energy partitioning at the seafloor of the two incident wave types produce both compressional p and shear s diffracted phases in

deposition models for chronological records abstract long term sedimentary sequences provide a wealth of useful information for research into the palaeo environment especially in relation to past climate change shorter records provide similar information in many archaeological contexts however if such records are to be used to provide precise timing of events and more critically the relative timing between different records methods are needed to provide accurate and precise age depth models for these sequences given the imprecision of individual calibrated radiocarbon determinations it is necessary to use the information we have about the deposition process to refine our chronologies and also to provide interpolation between dated levels in any sequence even with layer counted chronologies the uncertainties are sometimes hard to quantify this paper outlines a range of bayesian models for deposition which have been implemented in the computer programme oxcal these models can be used to combine information from the sediments themselves with radiocarbon or other direct dating information such models have the potential to integrate information between different records and provide a coherent chronology on which to base environmental or archaeological research