tree roots methodological review and new development in dating and quantifying erosive processes exposed roots have been used in dendrogeomorphology since the 1960s to determine erosion rates this was done by using the root axis as the relative position of the former soil layer and then relating this measure to the age of the root in order to quantify the amount of soil erosion over time recently wood anatomical research revealed the possibility of determining the first year of exposure of roots due to specific anatomical changes within the respective annual ring as a consequence using exposed roots that are still in contact to the soil surface it is possible to reconstruct the size and position of the root at the time of exposure consequently a new equation is presented to calculate the thickness of the eroded soil layer allowing a detailed reconstruction of erosion rates the technique also helps to determine if the root was exposed by erosional processes or just by its ongoing secondary growth without any erosion involved

rainfall runoff and erosion in napa valley vineyards effects of slope cover and surface roughness the effects of slope cover and surface roughness on rainfall runoff infiltration and erosion were determined at two sites on a hillside vineyard in napa county california using a portable rainfall simulator rainfall simulation experiments were carried out at two sites with five replications of three slope treatments 5 10 and 15 in a randomized block design at each site 0 bsol 64 m2 plots prior to initiation of the rainfall simulations detailed assessments not considered in previous vineyard studies of soil slope cover and surface roughness were conducted significant correlations at the 95 confidence level between the physical characteristics of slope cover and surface roughness with total infiltration runoff sediment discharge and average sediment concentration were obtained the extent of soil cracking a physical characteristic not directly measured also affected analysis of the rainfall runoff erosion process average cumulative runoff and cumulative sediment discharge from site a was 87 and 242 greater respectively than at site b this difference was linked to the greater cover extent of soil cracking and bulk density at site b than at site a the extent of soil cover was the dominant factor limiting soil loss when soil cracking was not present field slopes within the range of 4 16 although a statistically significant factor affecting soil losses had only a minor impact on the amount of soil loss the horton infiltration equation fit field data better than the modified philip s equation owing to the variability in the treatment parameters affecting the rainfall runoff erosion process use of anova methods were found to be inappropriate multiple factor regression analysis was more useful for identifying significant parameters overall we obtained similar values for soil erosion parameters as those obtained from vineyard erosion studies in europe in addition it appears that results from the small plot studies may be adequately scaled up one to two orders of magnitude in terms of land areas considered copyright 2000 john wiley sons ltd

paleogene igneous underplating and subsidence anomalies in the rockall faeroe shetland area abstract crustal thickening caused by igneous underplating related to continental break up in the northeast atlantic has been invoked as the cause of the apparent lack of thermal subsidence seen in many mesozoic basins in the northwest european area in this study we examine a set of commercial wells in the rockall faeroe shetland region and calculate tectonic subsidence histories the rate of post rift basement subsidence during the cretaceous cenozoic determined from backstripping calculations always implies a higher degree of extension than the total basement subsidence assuming uniform extension furthermore the size of this discrepancy increases northward along the rockall trough and westwards in the northern north sea towards scotland we interpret this depth anomaly to reflect the effect of igneous underplating rethickening the crust following mesozoic and cenozoic extension late paleocene extension is identified in subsidence reconstructions throughout the west of britain area and is greatest in the rockall trough assuming that the underplating is gabbroic in composition the subsidence depth anomalies indicate a regional pattern of 2 3 km underplated between the erris trough and porcupine basin increasing to 4 5 km in the west lewis basin and hebrides shelf area in the faeroe shetland basin underplating is 1 0 1 5 km in the east and increases to 4 5 km in the west the data support the hypothesis of a major asthenospheric thermal anomaly centered under the faeroes in the paleocene dynamic uplift of the region is often difficult to identify but is locally seen to develop rapidly around 63 ma implying rapid emplacement of the mantle anomaly the intrusion of gabbroic bodies to the base of the crust in the late paleocene may have had an impact on the thermal maturation of petroleum source rocks in the region however thermal maturity indicators show smaller peak heatflows than predicted from simple thermal models suggesting that underplating was a gradual process achieved through the accretion of several small rapidly cooled bodies

special collection on the june 1993 volcanic eruption on the coaxial segment juan de fuca ridge the mid ocean ridge system is the primary site of oceanic crustal accretion and a major source of chemical thermal flux from the earth s interior to the hydrosphere although geological mapping geophysical exploration and hydrographie chemical surveys have provided important insights into accretionary processes until recently there has been no method of monitoring the dynamics of these systems over long periods and large areas insights into spreading center dynamics have depended on serendipitous discoveries of evidence of activity based on water column anomalies baker et al 1987 bathymetric differences fox et al 1992 or direct field observations haymon et al 1993 in situ instrumentation has been used to monitor dynamics at a few selected sites for long periods but these instruments are generally unable to telemeter data in real time

the june july 1993 seismo acoustic event at coaxial segment juan de fuca ridge evidence for a lateral dike injection the coaxial segment juan de fuca ridge was the site of an intense swarm of earthquakes that began at 21 43 gmt on june 26 1993 julian day 177 the swarm started near 46 15 n and migrated northward over the next 40 hours to 46 36 n where the majority of 676 events occurred during the following 3 weeks of activity the earthquakes propagated nne at a velocity of 0 3 0 1 m s 1 from the southern to northern swarm sites the activity went undetected by land based seismic networks along the oregon and washington coasts suggesting that the earthquakes were all m 4 0 the character of this earthquake swarm is very similar to dike injections observed at krafla and kilauea volcanoes the earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the coaxial segment the reservoir acting as the dike s source likely resides beneath or to the south of the initial southern swarm of earthquakes the large magma supply at axial volcano cannot be ruled out as the source for the coaxial dike even though axial volcano exhibited no earthquake activity related to the coaxial swarm the t wave earthquake swarm reported here is the first deep ocean observation of volcanic seismicity associated with what most likely was a mid ocean ridge dike injection event

influences of human activities on sediment discharge from devastated weathered granite hills of southern china effects of 4 year elimination of human activities abstract sediment discharge is a serious problem in the weathered granite mountains of southern china human activities such as the harvesting of ferns and litter for fuel are the main source of sediment but the details of the processes and mechanisms leading to sediment discharge are unclear data on sediment yields percentages of peak storm runoff and fern undergrowth and litter cover extent in the two basins one a control from which people have been excluded since 1997 and the other where normal harvesting practices are permitted were used to evaluate how human activity affects sediment discharge the annual sediment discharge decreased immediately after implementing control practice despite no marked changes in the 4 years after 1997 in the percentages of peak storm turnoff and the fern and litter cover extent and density the 4 year period of control practice was too short to affect the relationship between peak rainfall and peak storm runoff and the fern and litter covers and their amounts but long enough to contribute to the reductions in sediment discharge in the control basin by decreasing erodibility thus sediment discharge was probably reduced to some extent by the elimination of human impacts even though a well developed vegetation layer did not develop

magnetic properties of zero age oceanic crust a new submarine lava flow on the juan de fuca ridge a volcanic eruption that occurred in october 1993 on the juan de fuca ridge shows unusually high magnetization when compared with older samples from the off axis juan de fuca ridge and other pacific sites these data plus data from other young oceanic basalts indicate that oceanic crustal magnetization may decay rapidly by 10 in the first year the considerably slower decrease in magnetization for rocks from older off axis sites agrees with previous models of progressive low temperature oxidation of the mineral titanomagnetite the magnetization for newly erupted submarine basalts however is not consistent with the exponential decay predicted from older sites indicating that oxidation may not be the sole process responsible for the short term decay similarly weak field magnetic susceptibility of the younger rocks does not show an obvious corresponding decrease with increasing age as expected for the oxidation of titanomagnetite to titanomaghemite these preliminary results indicate that decay of magnetization of oceanic crustal rocks with time may have two distinct stages with only the later stage being due to the progressive oxidation of the magnetic minerals

seabeam depth changes associated with recent lava flows coaxial segment juan de fuca ridge evidence for multiple eruptions between 1981 1993 after a swarm of earthquakes was detected on the coaxial segment of the juan de fuca ridge in june july 1993 the area was resurveyed with seabeam multibeam sonar to search for depth changes associated with a submarine volcanic eruption quantitative comparison of the 1993 seabeam survey with surveys in 1981 82 and 1991 shows one area of seafloor depth change up to 29 m between 1991 93 exactly where a pristine lava flow was discovered in addition two other depth anomalies up to 37 m and 20 m are identified between 1981 91 evidence that other recent eruptions have occurred along this spreading ridge segment

detection of changes in ridge crest morphology using repeated multibeam sonar surveys a quantitative analysis method is developed to objectively identify significant bathymetric changes between multibeam sonar surveys of the same seafloor area the technique involves mathematical gridding of bathymetric surveys from different years co registration of grids based on prominent features covered in both surveys calculation of slope weighted depth difference grids and determination of the boundaries of statistically significant differences the resulting difference grids are used in interpreting geological processes and calculating the volume of new material added to the seafloor the method is applied to data from the southern juan de fuca ridge where a seafloor volcanic eruption of 0 05 km3 is inferred to have occurred between 1983 and 1987 camera tow data and submersible observations provide ground truth which confirms the validity of the quantitative analysis method and indicates empirically that the detection limit of the method allows new seafloor landforms measuring 1 105 m2 in area or about 200 300 m in diameter and 5 15 m in thickness to be detected through repeated sea beam bathymetric surveys

on isostasy at atlantic type continental margins cross spectral techniques have been used to analyze the relationship between free air gravity anomalies and topography of approximately 100 profiles over the continental margins of eastern north america southwest africa and the coral sea lord howe rise the resulting admittance functions for these margins have been interpreted in terms of the flexure model of isostasy the relatively young coral sea lord howe rise margin is associated with the lowest value of the effective elastic thickness te 5 km while the relatively old eastern north american margin is associated with the highest value 10 te 20 km thus te appears to increase with the age of the margin these results which are consistent with flexure studies from the major ocean basins have been interpreted in terms of a simple model in which the flexural strength of the lithosphere increases with age the model suggests that a low rigidity plate or airy type model is most applicable early in rifting history while a high rigidity plate is more applicable later in margin evolution thus sediments at a margin initially load a relatively thin and hot plate while sediments later in margin evolution load a relatively thick and cold plate the model explains a number of features of the tectonic stratigraphic evolution of margins these include a change in flexural style from a narrow and deep basin to a broad and shallow basin coastal plain onlap and uplift in the continental interior furthermore the flexure model of isostasy contributes in a major way to observed free air gravity anomalies at continental margins in particular the model explains the existance of airy isostatic gravity anomalies at margins the amplitude and wavelength of these anomalies are a function of the sediment supply the type of mechanisms controlling sediment deposition up building and or out building and the thermal structure history and age of the margin

alvin magnetic survey of zero age crust coaxial segment eruption juan de fuca ridge 1993 magnetic profiles obtained by deep submersible alvin across a newly erupted lava flow on the juan de fuca ridge jdfr show strong magnetic anomaly fields 20 000 nt and high crustal magnetization 50 a m contrast relative to the surrounding older lavas these results are consistent with previous ideas that extrusive crust is initially highly magnetized within the high amplitude magnetic anomaly over the center of the flow there is a clearly defined deep notch like magnetic low this notch may indicate thermal demagnetization associated with a subsurface feeder dike weakly magnetized intrusives or altered extrusive rock if the former mechanism is active the notch should disappear with time as the dike cools

relating cenozoic north sea sediments to topography in southern norway the interplay between tectonics and climate abstract about 482 000 km 3 of sediment ca 24 m ma accumulated in the north sea during the cenozoic early cenozoic sedimentation was likely due to uplift of the circum north atlantic landmasses related to continental break up kilometre scale transient uplift and in some areas permanent uplift generated sources for progradational influx of clastic sediments from scotland the shetland platform and to a lesser degree southwestern norway the eocene sedimentation pattern was similar to the palaeocene with lower rates of accumulation associated with flooding and tectonic quiescence sediment influx from the shetland platform continued throughout the cenozoic while supply from southern norway increased markedly around the eocene oligocene coeval with the greenhouse icehouse transition mass balance calculations of sediment and eroded rock volumes suggest that while some topography along the western margin of norway may be pre cenozoic significant uplift of the main paleic surface in southern norway occurred around the early oligocene sedimentation rates were almost ten fold higher than the cenozoic average in the plio pleistocene slightly higher than the global average mass balance calculations indicate that plio pleistocene erosion over deepened a pre existing topography

the existence of a thin low viscosity layer beneath the lithosphere the horizontal temperature gradient at the base of the lithosphere at an oceanic fracture zone where plate of different ages is juxtaposed is expected to drive a local circulation the characteristics of which can be constrained by the amplitude wavelength and age dependence of the geoid two dimensional numerical models of convection in a fluid layer overlain by a solid conducting lid have been used to generate theoretical geoid profiles at right angles to the fracture zone only a thin low viscosity layer provides a reasonable fit to the data the best model so far obtained has a fluid layer 150 km thick with viscosity 1 5 1019 pa s under a 75 km lid such a layer which is incapable of transmitting strong horizontal shear stresses could provide the decoupling mechanism between plate and deep mantle flow required to balance the forces on the plates

monochromatic t waves from underwater volcanoes in the pacific ocean ringing witnesses to geyser processes abstract we analyze swarms of exceptionally intense and sustained t waves recorded during 1991 through 1994 at the polynesian seismic array the strongest swarm lasted 15 months and originated southwest of the eltanin fracture zone in the immediate vicinity of a documented seamount associated with a recognized volcanic chain branching out of the local segment of the mid oceanic ridge the eltanin t waves are characterized by an exceptionally monochromatic spectrum featuring a single line of a few hertz frequency in the range 2 to 80 hz while no similar characteristics had previously been observed in 25 yr of recording in polynesia comparable spectra were recorded during a 1993 swarm at the revilla gigedo islands following by 4 months a documented eruption of socorro island in both cases we interpret the t waves as evidence of a major underwater volcanic process the existence of a single resonating frequency and in particular the absence of any overtones is generally not compatible with the now classical models of the resonance of a fluid filled crack and we speculate that the source of the phenomenon may be the oscillation of a bubbly liquid which could result from vaporization of seawater in the presence of a large lava lake

flexural response of passive margins to deep sea erosion and slope retreat implications for relative sea level change we suggest that the magnitude of the early late oligocene eustatic fall as predicted from sequence stratigraphy has been overestimated because of flexural rebound along margins that was induced by late eocene to early oligocene erosional unloading the unloading of sediments from a continental margin during periods of deep sea erosion and slope retreat will be accompanied by some form of flexural rebound the magnitude and form of this rebound are controlled by 1 the amount of sediment removed from the margin and 2 the strength of the lithosphere at the time of sediment removal along margins with narrow continental shelves this rebound will induce a relative sea level fall resulting in a basinward shift in the shoreline and the predicted systems tracts will be similar to those expected for a eustatic fall in contrast rebound along wide continental shelves will not alter the position of the shoreline even though a relative sea level fall may be recorded across the outer shelf deep sea drilling and seismic reflection data from many margins support the interpretation of a late eocene to early oligocene 38 34 ma deep sea erosional event best developed along the continental slope and rise seismic reflection and drilling data also document a prominent canyon cutting event across the shelf during the early late oligocene 32 29 ma this event has been interpreted in terms of a large eustatic fall 150 m nevertheless there appears to be a correlation between late eocene to early oligocene deep sea erosion and the development or at least enhancement of unconformities across the shelf during the early late oligocene

fate of event hydrothermal plumes on the juan de fuca ridge current meter moorings were deployed in august 1993 at the megaplume site of the june 1993 seismic event on the coaxial segment of the juan de fuca ridge and were recovered in april 1994 record averaged flow was about 2 cm s across axis which is sufficiently large to advect plumes off axis in a few days during july two megaplumes 1800 m level disappeared between repeated ctd tows a deeper plume 2150 m had a relatively strong signature during august and extended southeastward in agreement with the observed flow a ctd tow along the east flank of axial volcano did not locate remnants of the megaplumes but did find deeper plumes which could have come from off axis advection from the coaxial sources followed by southward advection along the east side of ridge

oligocene glacio eustasy and erosion on the margins of the north atlantic oligocene foraminiferal 18 o records suggest the development of ice caps inferred glacio eustatic falls at ca 36 31 and 25 ma biostratigraphic analyses of wells from the united states east coast and irish continental margins consistently show that upper oligocene sediments overlie a diachronous erosional surface underlain by lower oligocene to eocene strata at the minimum the hiatus extends between ca 34 and 30 ma we speculate that erosion during a glacio eustatic fall near the early late oligocene boundary ca 32 31 ma developed 1 an unconformity on the margins 2 numerous canyons noted in seismic profiles from the margins of the north atlantic and 3 a coastal offlap event using 18 o data we apply a model for eustatic changes and margin response that explains the relationships of sea level unconformities and coastal onlap offlap events

time clustering behavior of spreading center seismicity between 15 and 35 n on the mid atlantic ridge observations from hydroacoustic monitoring abstract an earthquake catalog derived from the detection of seismically generated t waves is used to study the time clustering behavior of moderate size 3 0 m earthquakes between 15 and 35 n along the mid atlantic ridge mar within this region the distribution of inter event times is consistent with a non periodic non random clustered process the highest degrees of clustering are associated temporally with large mainshock aftershock sequences however some swarm like activity also is evident temporal fluctuations characterized by a power spectral density p f that decays as 1 f are present within the time sequence with ranging from 0 12 to 0 55 for different regions of the spreading axis this behavior is negligible at time scales less than 5 103 s and earthquake occurrence becomes less clustered smaller as increasing size thresholds are applied to the catalog a power law size frequency scaling for mid atlantic ridge earthquakes also can be demonstrated using the distribution of acoustic magnitudes or source levels although fractal seismic behavior has been linked to the structure of the underlying fault population in other environments power law fault size distributions have not been observed widely in the mid ocean ridge setting

dynamic support by the iceland plume and its effect on the subsidence of the northern atlantic margins abstract a transect of drill sites across the east greenland margin made by odp leg 152 provides important new data on the subsidence history of this margin complementing earlier odp and dsdp drill sites on the conjugate hatton bank as well as the voring plateau fluvial sediments recovered from under the basalts of the dipping reflector series indicate uplift and subaerial exposure of the margin prior to basalt eruption the basaltic basement at site 918 located oceanward of the continent ocean transition shows anomalously slow subsidence during the first 15 20 ma after continental break up compared to normal ocean crust similar anomalous subsidence characterizes the early subsidence at dsdp sites 552 554 on the hatton bank here however the anomaly is of lower magnitude and of shorter duration at the voring plateau dynamic support is apparent on the landward side of the continent ocean transition but offshore the evidence for strong long duration support is lacking we interpret the subsidence results to reflect dynamic support of the north atlantic rifted margins by the iceland plume with the greater support at the east greenland sites reflecting their position closer to the plume centre during the initial post rift period than those on the hatton bank or voring plateau the subsidence data are in accord with models of a large plume head of 1000 km radius but cannot resolve between models proposing arrival of a pre existing plume into the area during the late palaeocene rather than the incubation of a new plume under the area at that time the low degree of dynamic support seen at the voring plateau and the marked difference between the support at the east greenland drill sites versus those at hatton bank does not support the plume centre being at kangerdlugssuaq at 55 ma but instead at a location further west under the greenland craton

cenozoic uplift and subsidence in the north atlantic region geological evidence revisited abstract the topographic evolution of the passive margins of the north atlantic during the last 65 myr is the subject of extensive debate due to inherent limitations of the geological geomorphological and geophysical methods used for studies of uplift and subsidence we have compiled a database of sign time and amplitude where possible of topographic changes in the north atlantic region during the cenozoic 65 0 ma our compilation is based on published results from reflection seismic studies aft apatite fission track studies vr vitrinite reflectance trends maximum burial sediment supply studies mass balance calculations and extrapolation of seismic profiles to onshore geomorphological features the integration of about 200 published results reveal a clear pattern of topographic changes in the north atlantic region during the cenozoic 1 the first major phase of cenozoic regional uplift occurred in the late palaeocene early eocene ca 60 50 ma probably related to the break up of the north atlantic between europe and greenland as indicated by the northward propagation of uplift it was preceded by middle palaeocene uplift and over deepening of some basins of the north sea and the surrounding areas 2 a regional increase in subsidence in the offshore marginal areas of norway the northern north sea the northern british isles and west greenland took place in the eocene ca 57 35 ma 3 the oligocene and miocene 35 5 ma were characterized by regional tectonic quiescence with only localised uplift probably related to changes in plate dynamics 4 the second major phase of regional uplift that affected all marginal areas of the north atlantic occurred in the plio pleistocene 5 0 ma its amplitude was enhanced by erosion driven glacio isostatic compensation despite inconclusive evidence this phase is likely to be ongoing at present

a hotspot model for iceland and the voring plateau the proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot this produces more basalt and thicker crust at these locations thus forming a plateau over time the geometric constraints of such a model predict a unique orientation location and age progression for a plateau formed by this mechanism the hotspot will channel material to the closest part of the rise therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis the plateau will be symmetric with respect to the location of the rise axis at the time of formation also the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor just with thicker crust a set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the norwegian greenland sea as a means of evaluating the model s predictions by locating the iceland hotspot reconstructing the relative positions of the greenland and european plates and then assuming material would be channeled from the hotspot to the closest section of the rise crest we can trace the tectonic evolution of the greenland faeroe and voring plateaus the model is able to locate the plateaus explain their orientations and predict an age progression that satisfies observed age determinations the analysis demonstrates that both plateaus could have been formed by the iceland hotspot with the greenland faeroe plateau being in effect a continuation of the voring plateau which was cut off due to transform motion between the northern and southern spreading centers

soil erodibility and processes of water erosion on hillslope abstract the importance of the inherent resistance of soil to erosional processes or soil erodibility is generally recognized in hillslope and fluvial geomorphology but the full implications of the dynamic soil properties that affect erodibility are seldom considered in canada a wide spectrum of soils and erosional processes has stimulated much research related to soil erodibility this paper aims to place this work in an international framework of research on water erosion processes and to identify critical emerging research questions it focuses particularly on experimental research on rill and interrill erosion using simulated rainfall and recently developed techniques that provide data at appropriate temporal and spatial scales essential for event based soil erosion prediction results show that many components of erosional response such as partitioning between rill and interrill or surface and subsurface processes threshold hydraulic conditions for rill incision rill network configuration and hillslope sediment delivery are strongly affected by spatially variable and temporally dynamic soil properties this agrees with other recent studies but contrasts markedly with long held concepts of soil credibility as an essentially constant property for any soil type properties that determine erodibility such as soil aggregation and shear strength are strongly affected by climatic factors such as rainfall distribution and frost action and show systematic seasonal variation they can also change significantly over much shorter time scales with subtle variations in soil water conditions organic composition microbiological activity age hardening and the structural effect of applied stresses property changes between and during rainstorms can dramatically affect the incidence and intensity of rill and interrill erosion and therefore both short and long term hillslope erosional response similar property changes linked to climatic conditions may also significantly influence the stability and resilience of plant species and vegetation systems full understanding of such changes is essential if current event based soil erosion models such as wepp and eurosem are to attain their full potential predictive precision the complexity of the interacting processes involved may however ultimately make stochastic modelling more effective than physically based modelling in predicting hillslope response to erodibility dynamics

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

short and long baseline tiltmeter measurements on axial seamount juan de fuca ridge abstract long term observations of seismic activity and ground deformation at mid ocean ridges and submarine volcanoes are required for an understanding of the spatial and temporal characteristics of magma transport and intrusion to make precise records of tilt on the seafloor we have installed short baseline tiltmeters in six ocean bottom seismometers tilt obs and developed a long baseline 100 500 m two fluid tiltmeter lbt in the tilt obs the seismometer platform is levelled to better than 1 after deployment the tiltmeter consists of a pair of electrolytic bubble sensors mounted on a secondary levelling stage on the seismometer platform the levelling stage uses two motor driven micrometers on a triangular mounting plate to bring the sensors to null the sensitivity of these tiltmeters is 0 05 rad at a dynamic range of 0 2 mrad a long baseline instrument was developed to achieve a better spatial average of deformation most approaches used on land to measure stable long baseline tilt cannot be applied to a submarine instrument but tiltmeters in which the pressure of a fluid in tubes is measured are amenable to installation on the seafloor the development resulted in a device that is essentially a center pressure instrument folded back on itself with fluids of different densities in the two tubes during july to september 1994 these instruments were deployed on axial seamount on the juan de fuca ridge off washington state for a test of their relative performance on volcanic terrain yielding 9 weeks of continuous data seismic tilt and temperature from five tilt obs and one long baseline instrument drift on all instruments was of the order of 1 rad day with higher frequency variations of order 5 10 rad initial drift on the tilt obs is shown to be associated with platform settling rather than with the sensor or its mounting high frequency noise is coherent across instruments and tidal in character and we conclude that tidal currents moving the sensors are responsible

a laboratory investigation into the effects of slope on lava flow morphology abstract in an attempt to model the effect of slope on the dynamics of lava flow emplacement four distinct morphologies were repeatedly produced in a series of laboratory simulations where polyethylene glycol peg was extruded at a constant rate beneath cold sucrose solution onto a uniform slope which could be varied from 1 through 60 the lowest extrusion rates and slopes and highest cooling rates produced flows that rapidly crusted over and advanced through bulbous toes or pillows similar to subaerial toey pahoehoe flows and to submarine pillowed flows as extrusion rate and slope increased and cooling rate decreased pillowed flows gave way to rifted flows linear zones of liquid wax separated by plates of solid crust similar to what is observed on the surface of convecting lava lakes then to folded flows with surface crusts buckled transversely to the flow direction and at the highest extrusion rates and slopes and lowest cooling rates to leveed flows which solidified only at their margins a dimensionless parameter primarily controlled by effusion rate cooling rate and flow viscosity quantifies these flow types increasing the underlying slope up to 30 allows the liquid wax to advance further before solidifying with an effect similar to that of increasing the effusion rate for example conditions that produce rifted flows on a 10 slope result in folded flows on a 30 slope for underlying slopes of 40 however this trend reverses slightly owing to increased gravitational forces relative to the strength of the solid wax because of its significant influence on heat advection and the disruption of a solid crust slope must be incorporated into any quantitative attempt to correlate eruption parameters and lava flow morphologies these experiments and subsequent scaling incorporate key physical parameters of both an extrusion and its environment allowing their results to be used to interpret lava flow morphologies on land on the sea floor and on other planets

recent eruptions on the coaxial segment of the juan de fuca ridge implications for mid ocean ridge accretion processes the 1993 seismic swarm and volcanic eruption on the coaxial segment of the juan de fuca ridge was the first verified mid ocean ridge accretion event monitored by the national oceanic and atmospheric administration u s navy sound surveillance system sosus ambiguity in the location of the initial seismicity resulted in an uncertainty as to whether the dike intruded from the summit and north rift zone of the adjacent axial volcano or whether it arose locally within the coaxial segment however analyses of multibeam side scan sonar towed camera submersible and geochemical data show that the coaxial segment is morphologically structurally and geochemically distinct from the north rift zone of axial volcano there is no geologic or geochemical evidence that dike injections from axial volcano have extended north of 46 18 n in the past whereas the 1993 eruption site is at 46 31 n furthermore all seafloor manifestations of the 1993 dike injection lie along the central neovolcanic zone of the coaxial segment analyses of repeat seabeam surveys combined with seafloor observations show that two other eruptions of approximately the same volume as the 1993 eruption occurred along the coaxial segment in the 1981 1991 interval these three diking events may have relieved decades of accumulated stress over 35 km or more of this segment the spatial pattern and hydrothermal history of the 1993 event is consistent with a dike with a significant lateral component of injection

magma sources and plumbing systems during break up of the se greenland margin preliminary results from odp leg 152 abstract odp leg 152 drilled a transect across the se greenland seaward dipping reflector sequence the earliest and most landward lava flows hole 917a lower and middle series represent a pre break up continental sequence showing an overall variation from olivine basalt to dacite evolution in deep crustal magma reservoirs with a dwindling supply of primitive magma and an increasing role of crustal contamination can account for the variation eruption of picrite and olivine basalt magmas 917a upper series marked a dramatic change in the style of magmatism to one of unrestrained passage of primitive magma from the mantle to the surface during the final phase of break up the younger parts of the seaward dipping reflector sequence 915a and 918d comprise compositionally uniform basalt 7 6 0 8 mgo suggesting that an effective magmatic filtering system was established soon after break up in magma chambers associated with a spreading axis the continental to oceanic magmatic transition is marked by clear changes in composition the continental magmas had a garnet lherzolite source indicated by low y zr and were contaminated with continental lithospheric mantle and crust high ba zr the majority of the break up and oceanic volcanic rocks show no evidence for lithospheric contamination low ba zr and appear to have had a source in progressively shallower depleted morb source asthenosphere increasing y zr low nb zr

erosion rates in badland areas recorded by collectors erosion pins and profilometer techniques ebro basin ne spain abstract in badland areas of the ebro basin in a semiarid climate two erosion plots 257 m2 5 slope and 128 m2 23 slope on exposed tertiary clays were monitored over two years nov 1991 nov 1993 this material is characterized by high sodium absorption ratios which lead to high soil dispersivity the dominant erosion processes in both plots are rilling and sheet erosion rainfall intensity was recorded at a weather station connected to a data logger sediment production for single events was collected in tanks and ground lowering was measured every six months by erosion pins and microtopographic profile gauge techniques significant runoff was produced only by rainfall events above 5 mm another threshold at 20 mm rain was noted for rainfalls higher than 20 mm the 23 slope plot shows a greater runoff response than the 5 one rainfall events exceeding this threshold showed a higher sediment production for the steeper slope in the relationship between precipitation and sediment concentration an envelope curve can be drawn indicating that any rainfall event of a given amount and intensity has a maximum sediment concentration which we speculate to be a function of the runoff sediment transport capacity runoff response and sediment yield in the studied plots are controlled by the rainfall and soil characteristics and their seasonal variations in both plots the erosion pins show that erosion rates in rill areas are 25 50 higher than in the interrill areas sediment yield recorded by collector devices was higher than the rates measured by erosion pins the erosion rates based on rill cross sections by profilometers were higher than the ones recorded by collectors

acoustic detection of a seafloor spreading episode on the juan de fuca ridge using military hydrophone arrays until recently no practical method has been available to continuously monitor seismicity of seafloor spreading centers the availability of the u s navy s sound surveillance system sosus for environmental research has allowed the continuous monitoring of low level seismicity of the juan de fuca ridge in the northeast pacific on june 22 1993 noaa installed a prototype system at u s naval facility whidbey island to allow real time acoustic monitoring of the juan de fuca ridge using sosus on june 26 2145 gmt a burst of low level seismic activity with accompanying harmonic tremor was observed and subsequently located near 46 15 n 129 53 w on the spreading axis of the juan de fuca ridge over the following 2 days the activity migrated to the nne along the spreading axis with the final locus of activity near 46 31 5 n 129 35 w the nature of the seismicity was interpreted to represent a lateral dike injection with the possibility of eruption on the seafloor based on this interpretation a response effort was initiated by u s and canadian research vessels and both warm water plumes and fresh lavas were subsequently identified at the site

monitoring pacific ocean seismicity from an autonomous hydrophone array since may 1996 an array of autonomous hydrophone moorings has been continuously deployed in the eastern equatorial pacific to provide long term monitoring of seismic activity including low level volcanic signals along the east pacific rise between 20 n and 20 s and the galapagos ridge the instruments and moorings were designed to continuously record low frequency acoustic energy in the sofar channel for extended periods and produce results comparable to those previously derived by using the u s navy sound surveillance system sosus in the northeast pacific the technology and methodology developed for this experiment including instrument design mooring configuration analysis software location algorithms with an analysis of errors and a predicted error field are described in detail volcanic activity is observed throughout the pacific along with seismicity along transform faults subduction zones and intraplate regions comparison data sets indicate detection thresholds and accuracy better than the land networks for open ocean areas and results comparable to or better than sosus volcanic seismicity along the fast spreading east pacific rise appears similar to documented examples in the northeast pacific but with much shorter durations one example from the intermediate spreading galapagos ridge is comparable to northeast pacific examples and several episodes of activity were observed in the wilkes transform fault zone a site of continuing off axis seismicity is located near 18 s and 116 w isolated intraplate earthquakes are observed throughout the study area earthquake information from this experiment and future observations will be provided through the world wide web and earthquake data centers

uplift and strength evolution of passive margins inferred from 2 d conductive modelling summary post breakup vertical motions of passive margins are seen here as a result of the post rift 2 d thermal evolution a 2 d finite element numerical model is performed to evaluate both the vertical and horizontal conduction that drive the thermal evolution of continental passive margins from breakup to post breakup states initial temperature configurations corresponding to non volcanic and volcanic margins are tested and lead to different thermal evolution of the lithosphere for both margins a thermal thickening is observed in the stretched lithosphere whereas the unstretched lithosphere undergoes first 0 80 ma a thermal thinning and secondly after 80 ma a thermal thickening in comparison with non volcanic margins volcanic margins show a slower thermal thickening and a greater thermal thinning in stretched and unstretched lithosphere respectively the variations with time of lithosphere thickness are then translated into isostatic vertical movements and reveal seaward thermal induced subsidence and landward thermal induced uplift the estimated uplift reaches up to 250 m in volcanic margins and 120 m in non volcanic margins the modelled timing and amount of uplift in both margins are consistent with present day topography of volcanic passive margins that stand two to three times higher than non volcanic margins using these thermal models we finally show that the 2 d strength of the margins drastically evolves with time from a seeward dominant strength 0 80 ma toward a landward dominant strength time larger than 80 ma these lateral strength evolution could have strong effect on the flexural response of the margin through time

sedimentology of magmatically and structurally controlled outburst valleys along rifted volcanic margins examples from the nuussuaq basin west greenland after a period of early palaeocene faulting and uplift of the nuussuaq basin west greenland two valley systems were incised into the underlying sediments incision of the older tupaasat valley took place during a single drainage event of large water masses which resulted in catastrophic deposition the valley was cut along early palaeocene nw to se trending normal faults clearly showing that the trend and the relief of the valley were structurally controlled the valley fill is up to 120 m thick and consists of a lower part of sandstones and conglomerates deposited from catastrophic flows characterized by very high concentrations of suspended coarse grained sediment load catastrophic deposition was followed by rapid decrease in flow discharge and the establishment of a lacustrine environment within the valley characterized by the deposition of heterolithic sediments the younger paatuutkloften valley system was mainly cut into the tupaasat valley fill which was completely or nearly completely eroded away in many places the younger valley is 1 2 km wide and up to 190 m deep incision of the paatuutkloften valley probably reflected renewed tectonic activity and uplift of the basin this phase was shortly followed by rapid major subsidence the valley fill deposits comprise a uniform succession of fluvial and estuarine sandstones the valley fill is topped by shoreface sandstones which are succeeded abruptly by offshore mudstones deposited shortly before and during the initial extrusion of a thick hyaloclastite succession the paatuutkloften valley fill is attributed to a very rapid rise in relative sea level contemporary with extensive volcanism it is suggested that this sequence of events coincided with the arrival of the north atlantic mantle plume in several respects the early palaeocene valley fill deposits of the nuussuaq basin are different from idealized facies models for incised valley systems and represent very special cases of incised valleys major differences from published examples include the dominance of catastrophic deposits and indications of large changes in relative sea level of several hundreds of metres taking place rapidly in less than 1 myr these changes were governed by the rise of the north atlantic mantle plume

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

the role of vegetation in the retardation of rill erosion abstract large rills with catchment areas measuring tens to hundreds of square metres pose serious soil erosion problems to south china after deforestation has exposed the deeply weathered granitic slopes to the impacts of tropical rainstorms runoff and sediment discharges of rills respond rapidly to rainfall but compared with bare rills vegetated rills show considerably lower amounts of flow and sediment yield from unit rainfall input rainfall interception is one main reason for reduced runoff and enhanced infiltration also lowers surface flow from vegetated areas potential sediment yield from unit runoff is greatly reduced when the rills are grown with fern and this provides a far more effective mechanism for soil erosion control than the more temporary measures of installing sediment traps along the rills

erosion rates and sediment budgets in vineyards at 1 m resolution based on stock unearthing burgundy france abstract a new and simple method is developed to efficiently quantify erosion and deposition rates based on stock unearthing measurements this is applicable to spatial scales ranging from plot to hillslopes and to time scales ranging from single hydrologic events to centennial scales the method is applied to a plot area on vineyard hillslopes in burgundy monthelie france with measurement of 4328 vine plants a sediment budget established at the plot scale shows a mean soil lowering of 3 44 1 cm over 20 years involving a minimal erosion rate of 1 7 0 5 mm yr 1 locally erosion rates can reach up to 8 2 0 5 mm yr 1 this approach allows the sediment redistribution to be mapped and analyzed at 1 m resolution it provides novel insights into the characterization of erosion patterns on pluri decennial scales and into the analysis of spatial distribution of erosion processes on cultivated hillslopes

observations of manganese and iron at the coaxial seafloor eruption site juan de fuca ridge magmatic injection during june july 1993 on the coaxial segment of the juan de fuca ridge resulted in an axial lava eruption an overlying chronic style hydrothermal plume and at least three megaplume like event plumes a comprehensive view of mn and fe liberated as hydrothermal discharge during this volcano hydrothermal event was obtained as part of a rapid field response that included continuous in situ measurements of mn and fe distribution maps for mn fe and temperature anomaly within the chronic and event plumes reveal significant thermochemical diversity that we ascribe to variable extraction efficiencies for heat and metals by source fluids and to plume age differences the mn heat data suggest that diffuse fluids could be the primary source of the chronic plume at northern coaxial segment but make only a minor contribution to event plumes measurements of ephemeral fe ii were used to estimate that the event plumes were 2 4 and 6 8 days old when first sampled corresponding to formation times during peak seismic activity the inventories of mn and fe within the three coaxial event plumes were similar to values previously reported for megaplumes

a regional scale study of multi decennial erosion of vineyard fields using vine stock unearthing burying measurements abstract erosion and deposition rates were quantified in a vineyard context languedoc france using a method based on vine stock bio markers the within field spatial structure of erosion field erosive state was described based on maps of unearthing height of vine stock the factors affecting variability of soil erosion slope gradient slope length soil type and field boundaries characteristics were analysed in relation to quantitative and spatial erosion data when this method was applied to a sample of 49 fields the intensity of erosion in the study area mean erosion rate 10 5 t ha 1 year 1 showed a high variability between fields the field erosive states fes were classified into five categories based on the intensity and spatial structure of the erosion such as uniform erosion of the field transfer of sediments from upslope to downslope regions and concentrated erosion on rills this study highlights the influence of the connectivity between fields on erosion intensity and variability

seasonal changes in the sediment flux on steep hillslopes in a humid diurnal frost environment mountain hillslopes are generally shaped by a combination of various types of sediment transport processes e g surface erosion soil creep and dry ravel which can occur simultaneously in the same area since sediment transport is generally affected by multiple microclimatic factors such as heavy rainfall and changes in ground temperature the types of predominant sediment transport processes vary by season we conducted field observations in the southern japanese alps in the period from 2009 to 2013 using sediment traps and time lapse cameras to investigate the seasonal changes in the type and flux of hillslope sediment transport on steep mountains in which both rainfall and diurnal freeze thaw triggered sediment transport in winter and early spring sediment transport via diurnal freeze thaw soil creep and dry ravel was highly active whereas rainfall induced soil creep and selective transport by overland flow were active in summer and autumn when precipitation is abundant average sediment flux of 0 033 and 0 074 kg m 1 day 1 respectively sediment flux was spatially variable and was affected by the form of the slope sediment flux on the concave slope was higher than on the ridge shaped slope during both freeze thaw and rainfall periods sediment flux on an old landslide exceeded that in second growth forest regardless of the slope shape temporal changes in the sediment flux were not completely synchronized among monitoring plots and were affected by slope shape grain size and episodic sediment supply events such as release of sediment from woody debris copyright 2016 john wiley sons ltd

erosion control in south china abstract severe soil erosion is common in south china where torrential rainfall erodes the deeply weathered and denuded granitic hills locally developed control measures have been applied successfully to reduce erosion including the benching of steep slopes the construction of check dams and reforestation of particular importance is the treatment of large gullies locally called broken hills these have led to the disintegration of hillslopes and the deposition of colluvium and fines on the agricultural land gully treatment includes the stabilization of the headwalls the containment of sediments in the gully floor zone and the reclamation of flatland below the gully mouth re vegetation of the slopes is essential and the planting of economic crops provides incentive to the farmers to preserve the vegetation cover

a geophysical study of the hatton bank volcanic margin a summary of the results from a combined seismic gravity and magnetic experiment abstract this paper summarizes the results of a geophysical study of the hatton bank volcanic margin in the north atlantic normal incidence and synthetic aperture profiles saps show seaward dipping reflectors interpreted as subaerial basalt flows on the continental slope and in the adjacent ocean basin velocity depth profiles derived from expanding spread profiles esps shot along strike show elevated velocities in the lower crust on the continental slope the high velocities are interpreted as evidence for igneous underplating or intrusion gravity modelling shows high densities in the lower crust supporting a model of igneous accretion an ocean bottom seismometer obs and multichannel refraction line shot perpendicularly to strike through the midpoints of the esps further constrained the velocity structure across the margin the velocity model was refined as each dataset was processed clearly showing that multiple datasets were required in order to constrain the velocity structure tightly in particular esps were found to produce misleading results in places where the assumption that the sub surface structure is one dimensional inherent in the esp technique was not valid the final velocity model is used to infer the thickness of igneous material accreted across the hatton margin which is estimated to increase westwards from zero at the top of the bank to 18 km at the bottom of the continental slope and then thin rapidly to 11 km in the adjacent iceland basin as the oceanic crust thins its seismic velocity decreases suggesting a decrease in percentage partial melting these observations strongly support a model in which volcanic margins are produced by elevated asthenospheric temperatures

aftershock sequences in the mid ocean ridge environment an analysis using hydroacoustic data abstract hydroacoustic data from autonomous arrays and the u s navy s sound surveillance system sosus provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow spreading mid atlantic ridge mar intermediate spreading juan de fuca ridge jdfr and fast spreading east pacific rise epr aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single link cluster slc analysis with a combined space time metric in the regions examined hydroacoustic data improve the completeness level of the earthquake catalog by 1 5 2 0 orders of magnitude allowing the decay constant p of the modified omori law mol to be determined for individual sequences a non parametric goodness of fit test indicates six of the seven sequences examined are described well by a mol model the p values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data for three sequences along the siqueiros discovery and western blanco transforms p values are estimated to be 0 94 1 29 the spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra transform spreading centers at these locations an aftershock sequence following a 7 1 m s thrust event near the northern edge of the easter microplate exhibits p 1 02 0 11 within the sequence aftershocks are located to the north of a large topographic ridge which may represent the surface expression of the shallow dipping fault that ruptured during the mainshock two aftershock sequences near 24 25 n and 16 35 n on the mar exhibit higher p values 1 74 0 23 and 2 37 1 65 although the latter estimate is not well constrained because of the small number of aftershocks larger p values along the ridge crest might reflect a hotter thermal regime in this setting additional monitoring however will be needed to determine if p value differences between the ridge and transform sequences are robust a 1999 sequence on the endeavour segment of the jdfr which has been correlated with changes in the hydrothermal system is described poorly by the mol model the failure of the mol model the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence

sediment yield on a devastated hill in southern china effects of microbiotic crust on surface erosion process in southern china most of the forests consist of pinus massoniana but they are usually poor thin and deteriorated due to human intervention such as cutting removal of litter for fuel and over grazing therefore exposed bare soil is found in many devastated hills as a consequence both the direct runoff percentage and the sediment yield are very high in order to understand the surface erosion processes in this environment we measured sediment yield and surface erosion depth in terms of catchment scale and carried out in situ sprinkling experiments focusing on the spatial variability of surface conditions and sediment sources the results of sediment yield measurements at the end of headwater catchment suggest the following characteristic 1 the surface water is capable of transporting more material than is supplied by detachment that is the sediment discharge process is detachment limited in situ sprinkling experiments indicate that 2 in the lower part of the hillslope where the surface is covered by microbiotic crust the ratio of surface runoff to rainfall is high but sediment yield is small 3 in the upper part of the hillslope where there is no microbiotic crust the ratio of surface runoff to rainfall is low but sediment yield is large measurements of surface erosion depths indicate that 4 microbiotic crusts provide a high degree of protection against soil erosion even though the surface runoff ratio is large at the lower hillslope consequently the surface erosion depth is almost zero at the lower part of hillslope 5 the main source of sediment was the upper part of hillslope based on these indications it can be concluded that when the sediment discharge process is detachment limited the effect of the surface crust and or the vegetation cover on the sediment discharge process is great that is the spatial variation of the surface crust and or the vegetation cover controls the source of sediment

the structural evolution of the northern viking graben and its bearing upon extensional modes of basin formation recent advances in the understanding of rift basin formation coupled with the increasing public availability of seismic and well data across the northern viking graben 60 61 n have enabled a detailed analysis of its development two rifting episodes can be recognized late permian early triassic and bathonian ryazanian major regional unconformities thought to be primarily tectonic rather than eustatic in origin separate and subdivide the rifting episodes the earlier episode involved extension about a n s axis ensuing triassic mid jurassic thermal subsidence was accommodated on steep faults during the later episode a new ne sw fault trend was superimposed on pre existing patterns major block rotation marking active rifting ceased at the end of the ryazanian during the second post rift episode there was a progressive migration of active faulting towards the basin margins and as a result a widening with time of the area undergoing subsidence asymmetric subsidence of the central part of the basin was hinged at the western margin of the horda platform and accommodated to the nw on major faults within the tampen spur factors for the second rifting episode were calculated both by relating subsidence to extension and by measuring observed extension values calculated by both methods increase consistently towards the basin axis for both rifting and thermal subsidence phases but are greater for the latter phase subsidence patterns are similar for both rifting and thermal subsidence episodes so that there is vertical stacking of relatively thick sequences in the axis of the northern viking graben these factors preclude the application of models involving uniform and non uniform stretching and also preclude oblique extension depth dependent stretching is preferred

microbial mineral floc associated with nascent hydrothermal activity on coaxial segment juan de fuca ridge following a june 1993 seafloor eruption at coaxial segment juan de fuca ridge extensive accumulation of microbial mineral floc was observed in association with abundant diffuse venting on new lava flows and around less extensive new venting on older basalts microscopic and sem examination revealed a diverse assemblage of microbial forms contained within a poorly organized organo mineral matrix samples more closely resembled microbial floc described from other hydrothermal areas than the layered bacterial mats observed after the eruption at 9 n on the east pacific rise iron and silica were the major compositional elements in all samples with the latter likely important in preserving microbial remains organic carbon content was on the order of 3 4 enzyme assays indicated the presence of autotrophic co2 fixation and potential for sulfide oxidation floc covered 7 14 of the seafloor on the hydrothermally active part of the new lava flow we discuss the possibility that the coaxial floc originated as ejecta from the sub seafloor hydrothermal system rather than from in situ production around vent openings

dynamic support by the icelandic plume and vertical tectonics of the northeast atlantic continental margins late paleocene early eocene continental rifting in the northeast atlantic differs significantly from earlier episodes of margin formation in the central atlantic at a nonvolcanic margin rifting occurs over a wide area with little associated magmatism postrift subsidence decreases in a predictable exponential pattern with time in contrast subsidence analysis of ocean drilling program and deep sea drilling project drill sites from the voring plateau hatton bank and east greenland margin show that in these areas the continent ocean transition cot is very sharp with s increasing over a horizontal distance of 30 50 km from values of 1 1 1 50 on the continent side to at the cot drilling penetrated the thick seaward dipping basaltic sequences that typify the east greenland margin at site 917 fluvial sandstones underlying the basalts show that the area was subaerially exposed prior to continental breakup but the amount and timing of any uplift are presently unconstrained sediment backstripping techniques allow a comparison between the reconstructed and predicted subsidence histories and thus an estimate of the thermal anomaly through time anomalous slow subsidence in the early postrift period at 63 n offshore east greenland is attributed to support by the icelandic plume dynamic support is weaker on the hatton bank and on the voring plateau variations in the strength and duration of the support suggest that the plume was a large 1000 km radius structure that lay under the greenland craton at the time of breakup current data suggest that it may have crossed the east greenland coast at 40 ma discrepancies between uniform stretching models and the reconstructed subsidence for sites on the east greenland shelf and the voring plateau allow the amount of igneous underplating at the time of breakup to be estimated calculations suggest a maximum of around 8 8 km of gabbroic underplating occurred at the time of breakup under the east greenland shelf and 2 4 km under the voring plateau although both these bodies thin rapidly away from the cot

volcanic eruption of the mid ocean ridge along the east pacific rise crest at 9 45 52 n direct submersible observations of seafloor phenomena associated with an eruption event in april 1991 in april 1991 we witnessed from the submersible alvin a suite of previously undocumented seafloor phenomena accompanying an in progress eruption of the mid ocean ridge on the east pacific rise crest at 9 45 n 52 n the volume of the eruption could not be precisely determined although comparison of pre and post eruption seabeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were 10 m high effects of the eruption included 1 increased abundance and redistribution of hydrothermal vents disappearance of numerous vent communities and changes in characteristics of vent fauna and mineral deposits within the eruption area since december 1989 2 murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates 3 destruction of a vent community by lava flows mass wasting and possible hydrovolcanic explosion at a site known as tubeworm barbecue in the axial summit caldera asc at 9 50 6 n 4 near critical temperatures of hydrothermal vent fluids ranging up to 403 c 5 temporal variations over a 2 week interval in both temperatures and chemical isotopic compositions of hydrothermal fluids 6 unusual compositions of end member vent fluids with ph values ranging to a record low of 2 5 salinities ranging as low as 0 3 wt nacl one twelfth that of seawater and dissolved gases reaching high concentrations 65 mmol l for both co2 and h2s 7 venting at temperatures above 380 c of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices 8 disorganized venting of both high temperature fluids black and gray smoke and large volumes of cooler diffuse hydrothermal fluids directly from the basaltic seafloor rather than from hydrothermal mineral constructions 9 rapid and extensive growth of flocculent white bacterial mats species unknown on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid and 10 subseafloor downslope migration of magma normal to the ridge axis in a network of small scale 1 5 m diameter lava tubes and channels to distances at least 100 200 m outside the asc we suggest that in april 1991 intrusion of dikes in the eruption area to 200 m beneath the asc floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor rich hydrothermal fluids through the overlying rubbly cavernous lavas low salinities and gas rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid vapor phase separation boiling and phase segregation hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and or hydrovolcanic explosions these fissures and pits were lined with white mats of a unique fast growing bacteria that was the only life associated with the brand new vents the prolific bacteria which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids may thrive on high levels of gases in the vapor rich hydrothermal fluids initially escaping the hydrothermal system white bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic blizzard up to 50 m above the bottom the bacterial proliferation of april 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling and or by grazing as other organisms are re established in the biologically devastated area

a detailed comparison of repeated bathymetric surveys along a 300 km long section of the southern east pacific rise little is known about the frequency magnitude relationship for volcano tectonic events over long sections of fast spreading ridges a technique that can both detect and characterize the size shape and approximate time of a volcanic or tectonic event is to survey an area of seafloor on multiple occasions using multibeam sonar systems we have performed a detailed comparison of repeated bathymetric surveys of a 300 km long section of the southern east pacific rise with the objective of detecting and characterizing any changes in seafloor morphology associated with eruptions and perhaps tectonic deformation repeat bathymetry data were collected as part of three cruises that surveyed the axial high between 16 s and 18 40 s latitude in late 1995 in late 1996 and in mid 1997 during the latter two surveys the way points of the 1995 survey were repeated with the same survey vessel thus closely approximating the acquisition geometry of the first survey we have developed new methods for removing systematic errors and suppressing random noise in the bathymetry data which reduce errors between surveys to a root mean square value of 8 m or with areas 0 2 km 2 and depth differences 4 m we estimate that any seafloor depth changes that occurred between the survey times must have dimensions less than these values or they would have been detected for comparison lava flows detected along the juan de fuca and gorda ridges using similar techniques are 10 75 m in thickness and have estimated total areas of 0 52 2 8 km 2

the magnetic quiet zone in the eastern gulf of aden implications for the early development of the continental margin summary the magnetic quiet zone in the eastern gulf of aden is located between the oceanic crust of sheba ridge and the continental crust of arabia and somalia and is separated from both by important structural boundaries the seaward boundary is marked by the end of the seafloor spreading magnetic anomaly sequence and by a basement depth discontinuity the landward boundary is marked by escarpments made up of a series of normal faults these escarpments extend from 2 3 km below sea level to 1500 m above sea level and are equivalent of the hinge zone found at mature continental margins the magnetic field in the quiet zone is flat in some areas and in others is characterized by anomalies of up to several hundred gammas which are correlatable over distances of up to about 20 km and which appear related to basement topography the basement lacks the topographic slope characteristic of mid ocean ridge flanks and is characterized by moderately rough relief the crustal structure appears quite heterogeneous and where the crustal thicknesses have been determined they are slightly greater than those of oceanic crust new heat flow measurements show high values 95 7 123 3 mw m 2 in the quiet zone with values decreasing from sheba ridge toward the coast the unusual structure of the quiet zone and the observations that more opening appears to have occurred between arabia and somalia than can be accounted for by the oceanic crust of sheba ridge leads to the suggestion that the magnetic quiet zone was generated by diffuse extension of continental crust through a combination of rotational listric faulting and dyke injection this possibility is investigated using both a stretching or lithospheric attenuation model and a model in which a portion of the extension occurs through dyke injection it is found that these models can adequately match the observed heat flow and basement depths although very large amounts of extension 4 6 are required in the deep seaward portion of the quiet zone this results in more extension than is compatible with the documented motion between arabia and africa however formation of the magnetic quiet zone occurred over a period of 10 15 myr rather than instantaneously as assumed in the simple models when the effects of a finite length rifting episode are considered less extension is required and the observed geophysical data are consistent with a diffuse extension origin for the magnetic quiet zone

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

plume tectonics as a cause of mass wasting on the southeast greenland continental margin deposition of sediments on the southeast greenland shelf is limited by its narrow width low degrees of tectonic subsidence and lack of a block faulted basement topography a marked hiatus on the continental slope removing the eocene and oligocene together with a late eocene early oligocene hiatus on the continental rise can be related to mass wasting along the continental slope erosion correlates with a phase of rapid tectonic subsidence caused by relative retreat of the iceland plume from the area affecting the oceanic crust more than the continental shelf relative deepening of 1 km over 10 my steepened the continental slope and triggered erosion following the cessation of rapid subsidence a submarine fan progressively onlapped the continental slope the lack of a major mid oligocene erosion surface on the shelf despite shallow water depths indicates that the magnitude of the mid oligocene eustatic sea level fall is much less than the 200m predicted by the exxon sea level curve

connectivity assessment in mediterranean vineyards using improved stock unearthing method lidar and soil erosion field surveys the interaction between processes and landforms is accelerated in vineyards due to bare tilled soils that enhance splash runoff and erosion traditionally in order to assess these processes the stock unearthing method sum is considered a useful methodology that uses the graft union of the vine plant as passive bio indicator of the topsoil level changes however sum assumes that the topography between the rows is planar when development of a model of the current micro topography is performed thus we consider that there is a need to develop a new methodology isum improved sum that by adding new measurements in the inter row areas at least 3 allows inclusion of the absent micro topographical information of sum in this way the main aim of this research was to estimate the micro topographical changes and soil transport at three different slope positions upper middle and lower parts in a 25 years old vineyard in eastern spain lidar data were used to compare the connectivity processes at the catchment with the pedon scale we obtained maps that showed topsoil level changes and detected soil depletion and accumulation signals we estimated soil displacement rates of 1 6mghayr 1 depletion 2 8mghayr 1 and 13 3mghayr 1 accumulation at upper middle and lower slope positions respectively the total average soil erosion rates in the inter rows were 5 7mgha 1 yr 1 in the row areas deposition of 10 5mghayr 1 was measured field erosion marks surveyed after each rainfall event gave insights to the connectivity at the inter row and the slope scale it was found that at the row scale there were sinks of sediments underneath the vines and sources in the inter row parts although they are the highest and lowest local topographical terrain respectively this is due to tillage erosion that redistributed the sediments at the slope scale the lower part of the slope collected sediments from the upper slope positions which were transported along the middle part of the slope as the measurement of isum and the field survey after intense rainfall events demonstrated connectivity of the flows along the whole slope were rare twice and along individual slope parts connectivity was found eight times and 19 at the inter row scale after 8years of field observations

spectra and magnitudes of t waves from the 1993 earthquake swarm on the juan de fuca ridge a swarm of earthquakes on the crest of the juan de fuca ridge was detected in june and july 1993 by a network of hydrophones the activity migrated 60 km along the crest suggesting a lateral dike injection and the possibility of a volcanic eruption subsequent geologic and oceanographic investigations confirmed that an eruption had taken place examination of the individual acoustic arrivals shows changes in the character of the signal that are consistent with an injection of magma a reduction in the rise time of the wave packet and a proportional increase in high frequency energy was observed and is interpreted to result from a shoaling of the earthquake source region second the source magnitudes were largest at the onset of the swarm and became smaller over time also consistent with shoaling of the dike the appearance of the t wave arrivals changed significantly 5 days after the beginning of the swarm potentially indicating the onset of a surface eruption

episodic cenozoic tectonism and the development of the nw european passive continental margin the north atlantic margins are archetypally passive yet they have experienced post rift vertical movements of up to kilometre scale the cenozoic history of such movements along the nw european margin from ireland to mid norway is examined by integrating published analyses of uplift and subsidence with higher resolution tectono stratigraphic indicators of relative movements including results from the stratagem project three episodes of epeirogenic movement are identified in the early mid and late cenozoic distinct from at least one phase of compressive tectonism two forms of epeirogenic movement are recognised referred to as tilting coeval subsidence and uplift rotations 1 over distances of 100s of kilometres and sagging strongly differential subsidence rotations up to 4 over distances 100 km each epeirogenic episode involved relatively rapid 10 ma km scale tectonic movements that drove major changes in patterns of sedimentation to find expression in regional unconformity bounded stratigraphic units early cenozoic tilting late paleocene to early eocene c 60 50 ma caused the basinward progradation of shelf slope wedges from elongate uplifts along the inner continental margin and from offshore highs mid cenozoic sagging late eocene to early oligocene c 35 25 ma ended wedge progradation and caused the onset of contourite deposition in deep water basins late cenozoic tilting early pliocene to present 4 0 5 ma again caused the basinward progradation of shelf slope wedges from uplifts along the inner margin including broad dome like features and from offshore highs the early mid and late cenozoic epeirogenic episodes coincided with atlantic plate reorganisations but the observed km scale tectonic movements are too large to be accounted for as flexural deflections due to intra plate stress variations mantle lithosphere interactions are implied but the succession of epeirogenic episodes of differing form are difficult to reconcile with the various syn to post rift mechanisms of permanent and or transient movements proposed in the hypothetical context of a plume beneath iceland the epeirogenic movements can be explained as dynamic topographic responses to changing forms of small scale convective flow in the upper mantle tilting as coeval upwelling and downwelling above an edge driven convection cell sagging as a loss of dynamic support above a former upwelling the inferred cenozoic succession of epeirogenic tilting sagging and tilting is proposed to record the episodic evolution of upper mantle convection during ocean opening a process that may also be the underlying cause of plate reorganisations the postulated episodes of flow reorganisation in the ne atlantic region have testable implications for epeirogenic movements along the adjacent oceanic spreading ridge and conjugate continental margin as well as on other atlantic type passive margins

seismicity during lateral dike propagation insights from new data in the recent manda hararo dabbahu rifting episode afar ethiopia seismicity released during lateral dike intrusions in the manda hararo dabbahu rift afar ethiopia provides indirect insight into the distribution and evolution of tensile stress along this magma assisted divergent plate boundary in this paper 5 dike intrusions among the 14 that form the 2005 present rifting episode are analyzed with local and regional seismic data during dike intrusions seismicity migrates over distances of 10 15 km at velocities of 0 5 3 0 km h away from a single reservoir in the center of the rift segment confirming the analogy with a slow spreading mid ocean ridge segment comparison with geodetic data shows that the reservoir is located 7 km down rift from the topographic summit of the axial depression dikes emplaced toward the north are observed to migrate faster and to be more voluminous than those migrating southward suggesting an asymmetry of tension in the brittle elastic lithosphere seismicity during dike injections is concentrated near the propagating crack front in contrast faults and fissures in the subsurface appear to slip or open aseismically coeval with the intrusions the seismic energy released during dike intrusions in the manda hararo rift appears to be primarily modulated by the local magnitude of differential tensile stress and marginally by the rate of stress change induced by the intrusion the low level of seismic energy accompanying dike intrusions despite their significant volumes is likely an indicator of an overall low level of tension in the lithosphere of this nascent plate boundary

the influence of organic litter on the erosive effects of raindrops and of gravity drops released from desert shrubs abstract this paper reports on a laboratory study of the effects of water drop impacts on litter and sand splash beneath desert shrubs individual drops of 5 7 mm diameter were released from heights of 0 5 1 0 and 1 5 m selected to encompass the height range of typical desert shrubs onto targets of bare or partially litter covered saturated fine sand the natural litter largely derived from the saltbush atriplex vesicaria was collected from desert shrubland sites in western new south wales nsw the drop impacts caused both sand and litter particles to undergo splash displacement the mass of sand splashed was found to increase with drop fall height while mass of litter particles splashed did not vary significantly with fall height weights of sand moved by airsplash were significantly diminished by surface litter applied at the rate of 200 g m 2 these findings indicate that gravity drops released from desert shrubs may provide both an erosive force beneath these plants and a means for dispersing litter from the plant base into the surrounding landscape where litter may continue to affect hydrologic and erosional processes by restricting splash of mineral particles litter acts to limit soil splash from beneath shrubs and in this way may contribute to the persistence of plant mound microtopography that is common in desert shrublands under open field conditions large raindrops delivered in convective showers must cause similar airsplash transport of litter particles thus playing a role in the distribution of litter within the landscape