upper mantle electrical resistivity structure beneath the central mariana subduction system this paper reports on a magnetotelluric mt survey across the central mariana subduction system providing a comprehensive electrical resistivity image of the upper mantle to address issues of mantle dynamics in the mantle wedge and beneath the slow back arc spreading ridge after calculation of mt response functions and their correction for topographic distortion two dimensional electrical resistivity structures were generated using an inversion algorithm with a smoothness constraint and with additional restrictions imposed by the subducting slab the resultant isotropic electrical resistivity structure contains several key features there is an uppermost resistive layer with a thickness of up to 150 km beneath the pacific ocean basin 80 100 km beneath the mariana trough and 60 km beneath the parece vela basin along with a conductive mantle beneath the resistive layer a resistive region down to 60 km depth and a conductive region at greater depth are inferred beneath the volcanic arc in the mantle wedge there is no evidence for a conductive feature beneath the back arc spreading center sensitivity tests were applied to these features through inversion of synthetic data the uppermost resistive layer is the cool dry residual from the plate accretion process its thickness beneath the pacific ocean basin is controlled mainly by temperature whereas the roughly constant thickness beneath the mariana trough and beneath the parece vela basin regardless of seafloor age is controlled by composition the conductive mantle beneath the uppermost resistive layer requires hydration of olivine and or melting of the mantle the resistive region beneath the volcanic arc down to 60 km suggests that fluids such as melt or free water are not well connected or are highly three dimensional and of limited size in contrast the conductive region beneath the volcanic arc below 60 km depth reflects melting and hydration driven by water release from the subducting slab the resistive region beneath the back arc spreading center can be explained by dry mantle with typical temperatures suggesting that any melt present is either poorly connected or distributed discontinuously along the strike of the ridge evidence for electrical anisotropy in the central mariana upper mantle is weak

is the electrical conductivity of the northwestern pacific upper mantle normal 1 the normal oceanic mantle project based on ocean bottom geophysical observations has been underway since 2010 to investigate the physical state of the oceanic lithosphere and asthenosphere we have conducted electromagnetic surveys on old 130 ma seafloor in the northwestern pacific region where no active tectonic processes have been identified in order to image the electrical conductivity structure beneath the region so far data have been collected at four sites through a pilot survey conducted from june 2010 to august 2012 a one dimensional electrical conductivity model was obtained by preliminary analysis of the data by using the magnetotelluric method the model shows that the resistive 0 01 s m 1 lithospheric mantle is as thick as 80 km and that the asthenospheric mantle below has a conductivity of 0 03 s m 1 the resistive layer is slightly thicker than that beneath the philippine sea but significantly thinner than that beneath the area off the bonin trench in the pacific ocean there is a greater difference in age between the survey area and the philippine sea 0 60 ma than between the survey area and the area off the bonin trench 140 155 ma this comparison suggests that the relation between age and lithospheric thickness is not as simple as that predicted by the concept of lithospheric cooling it also suggests that the lithosphere beneath the area off the bonin trench in the pacific ocean is abnormally thick if the mantle beneath the survey area in this study is normal as expected from the plate cooling model

marine magnetotellurics imaged no distinct plume beneath the tristan da cunha hotspot in the southern atlantic ocean the tristan da cunha tdc is a volcanic island located above a prominent hotspot in the atlantic ocean many geological and geochemical evidences support a deep origin of the mantle material feeding the hotspot however the existence of a plume has not been confirmed as an anomalous structure in the mantle resolved by geophysical data because of lack of the observations in the area marine magnetotelluric and seismological observations were conducted in 2012 2013 to examine the upper mantle structure adjacent to tdc the electrical conductivity structure of the upper mantle beneath the area was investigated in this study three dimensional inversion analysis depicted a high conductive layer at 120 km depth but no distinct plume like vertical structure the conductive layer is mostly flat independently on seafloor age and bulges upward beneath the lithospheric segment where the tdc islands are located compared to younger segment south of the tdc fracture zone while the bathymetry is rather deeper than prediction for the northern segment the apparent inconsistency between the absence of vertical structure in this study and geochemical evidences on deep origin materials suggests that either the upwelling is too small and or weak to be resolved by the current data set or that the upwelling takes place elsewhere outside of the study area other observations suggest that 1 the conductivity of the upper mantle can be explained by the fact that the mantle above the high conductivity layer is depleted in volatiles as the result of partial melting beneath the spreading ridge 2 the potential temperature of the segments north of the tdc fracture zone is lower than that of the southern segment at least during the past 30 myr

coast effect distortion of marine magnetotelluric data insights from a pilot study offshore northeastern japan abstract we report on strong coast effect distortions observed for broadband marine magnetotelluric mt data collected on the forearc offshore northeastern japan eight days of horizontal electric and magnetic fields recorded at eight seafloor stations and the horizontal magnetic fields from a land remote station were processed with a robust multiple station algorithm yielding good mt responses and inter station transfer functions at periods of 7 10 000 s transverse electric te mode responses have cusps in apparent resistivity and negative phases at periods around 1000 s while the transverse magnetic tm mode responses are galvanically depressed below the te responses an analysis of inter station transfer functions confirms that the apparent resistivity cusps are a magnetic field rather than electric field phenomenon consisting of an amplitude minimum and rapid phase change around a characteristic frequency poynting vectors for a te coast effect model study illustrate that the anomalous phases are associated with energy diffusing back up to the seafloor from below after being turned around from its usual downward propagating trajectory by inductive coupling between the conductive ocean and the resistive seafloor along the continental margin we show that the characteristic frequency and position of the te mode apparent resistivity cusps are determined by a relatively simple combination of the electrical resistivity of the seafloor the depth of the ocean and the distance from the coastline by including coastlines and bathymetry in 2d inversion we recover the seafloor conductivity structure along the forearc demonstrating that broadband data can constrain the thickness of conductive forearc sediments and the underlying high resistivity associated with the mantle wedge and subducting oceanic lithosphere

sea effect correction in magnetotelluric mt data and its application to mt soundings carried out in jeju island korea summary when magnetotelluric mt data are obtained in the vicinity of the coast the surrounding sea makes it difficult to interpret subsurface structures especially at the deep parts of the subsurface we apply an iterative method to remove the sea effect the iterative method was originally developed to remove the distortion due to topographic changes from mt data recorded on the seafloor the iterative sea effect correction method is carried out in two steps the first corrects the sea effect whereas the second inverts the sea effect corrected responses the two steps are alternatively carried out until the criterion for either the inversion or the seaeffect correction is satisfied because the surrounding 3 d sea bathymetry is only incorporated into forward modelling for the sea effect correction it can be more robust than the method that directly incorporates 3 d sea bathymetry into a model space for inversion the synthetic examples show that the sea effect correction method yields an inverted model comparable to the true model by applying the sea effect correction method to real field data acquired in jeju island korea we also demonstrate that the sea effect correction method effectively removes the sea effects from the 1 d and 2 d real field data which helps enhance the inversion results on the basis of these results it may be concluded that the iterative sea effect correction method can be used as a promising technique for recovering the true response of the subsurface in mt data suffering from sea effects

factors controlling sediment yield from small intensively cultivated catchments in a temperate humid climate abstract data on sediment yield from 26 small cultivated catchments 10 10 000 ha in central belgium were collected based on sedimentation rates in small flood retention ponds area specific sediment yield varies from 0 4 to 20 6 t ha 1 year 1 for catchments ranging from 7 to 4873 ha various catchment properties were analysed in order to understand this large variation in sediment yield catchment area alone explains 64 of the observed variance in area specific sediment yield although no other parameter could explain more it is concluded that catchment area is not a valid parameter to link with sediment yield overall catchment geomorphology e g presented by the hypsometric integral is much more important in controlling sediment yield the lack of data however made this not very clear a multiple regression model including three morphological catchment properties is a valuable tool to predict total sediment yield t year 1 from catchments in central belgium this model has an efficiency of 0 81 furthermore it was concluded that the within catchment spatial distribution of parameters for instance slope steepness is very important but difficult to obtain one single representative value the use of spatially distributed erosion and sediment delivery models is therefore necessary accurate sediment yield data like the one presented in this study however are needed to validate these models

fluvial sediment budgets in the north carolina piedmont phillips j d 1991 fluvial sediment budgets in the north carolina piedmont geomorphology 4 231 241 sediment budgets for four large 1000 km 2 drainage basins in the piedmont physiographic province of north carolina were estimated from data compiled during erosion and sedimentation surveys budgets for the upper tar upper neuse haw and deep river basins showed broadly similar trends in allocation of eroded sediment among yield and storage sediment yield as a percentage of mean annual gross erosion within the basins averaged 10 this was less than the rate of alluvial storage which averaged 14 of annual gross erosion about 76 of the mean annual erosion was stored as colluvium on hillslopes there are relatively small differences in sediment delivery and storage ratios among the study basins this suggests that the relative order of magnitude of the allocation of eroded sediment for southern piedmont rivers is colluvial storage alluvial storage and yield of the sediment that does reach the streams more than half is stored as alluvium on an average annual basis aggrading channels and floodplains siltation of benthic habitats and sediment pollution problems are thus likely to persist unless dramatic reductions in upland erosion or in sediment delivery to streams are realized with respect to longer term basin evolution results point to the need for an improved understanding of the temporal scales of colluvial and alluvial sediment storage

bounded influence magnetotelluric response function estimation author posting blackwell 2004 this article is posted here by permission of blackwell for personal use not for redistribution the definitive version was published in geophysical journal international 157 2004 988 1006 doi 10 1111 j 1365 246x 2004 02203 x

soil erosion in small catchments of the quercy region southwester france during the holocene erosion rates in two small catchments of the quercy region southwestern france calculated from colluvial deposits trapped in karstic depressions show a progressive increase during the holocene rates lower than 0 2 t ha 1 yr 1 have been found for the period that covers the beginning of the holocene to the early iron age the soils buried in the depressions testify to the permanence of a forest cover until the late neolithic and to its replacement by a herbaceous vegetation during protohistorical times probably as a consequence of forest clearance and increased grazing by cattle after the early iron age erosion increased significantly and the mean rate reached 0 8 t ha 1 yr 1 before mediaeval times the associated coarse grained crudely stratified colluvium is thought to reflect ploughing of the surrounding fields erosion rate increased up to c 1 3t ha 1 yr 1 during mediaeval and modem times

constraints on the configuration of andaman nicobar subduction zone from em modeling abstract the electrical conductivity structures using long period magneto telluric data at three sites nbg nabagram hav havelock in andaman and cby campbell bay in nicobar region are investigated after obtaining 1d responses 2d forward models of electrical structure are constructed along profiles across north andaman middle andaman and nicobar islands the initial configuration along each profile is constrained from models based on seismic reflection data the responses obtained after iterative forward computations match the observed data at respective sites reasonably well new structures within the forearc high are discerned from north to south we delineate a moderately conductive body beneath north andaman which is associated with the presence of ophiolites around this location in the middle andaman a highly conductive body below the island is interpreted as presence of fluids along the active diligent fault facilitated by the barrier of the invisible bank a complex geometry of resistive material embedded in a highly conductive body inferred beneath at great nicobar at cby is commensurate with the presence of a layered magmatic body possibly linked to the volcanic seamount to the east near the tear of the subducting plate

approximations for the 2 d coast effect on marine magnetotelluric data marine natural source electromagnetic data acquired on continental margins are often of considerable scientific and commercial interest however the large conductivity contrast between the ocean and coast causes this type of data to be severely distorted for a 2 d coastal model this distortion is most pronounced for the marine magnetotelluric and geomagnetic response function derived from induced currents flowing parallel to the coast a maximal distortion occurs for a given period at a specific distance from the coast and causes severe anomalies in the magnitude and phase of the response functions based on a modelling study we empirically relate the characteristic period and characteristic distance to physical parameters such as the ocean depth and the host resistivity based on a simple analytical approach we test these approximations and show that maximum distortion occurs when destructive interference between the ocean and host response is at its highest while the coast effect causes a large distortion in the marine responses we show through a resolution analysis that it does not mask subsurface conductivity anomalies but in fact increases the sensitivity to the seafloor

inversion of magnetotelluric data in the presence of strong bathymetry topography we describe an algorithm for inverting magnetotelluric mt data in the presence of strong bathymetry or topography instead of correcting distortions due to bathymetry or topography we incorporate them directly into the inversion to achieve a high accuracy in computing mt responses we use finite difference approximations that permit sloping discrete boundaries inside model elements the same approach is applicable to any seafloor electromagnetic calculation and can also be used to incorporate steep topography on land we test our approach on various topographic features and compare our results to that of a finite element approach finally we present inversion examples that illustrate the effectiveness of our inversion algorithm in recovering true subsurface structures in the presence of strong bathymetry and topography

methods used in the u s s r to reduce near surface inhomogeneity effects on deep magnetotelluric sounding abstract the effects of near surface inhomogeneities on deep magnetotelluric sounding are considered methods are suggested allowing for and ruling out the near surface effects examples for practical application of these methods are given

prehistoric and roman gullying in the european oess belt a case study from central belgium in contrast with the understanding of present day soil erosion processes knowledge on past soil erosion phenomena is still rather limited although some studies report on severe gully erosion phases during the fourteenth and eighteenth centuries almost no evidence is available that documents earlier gully erosion phases this study investigates the development and age of two old permanent gullies that are conserved in the ancient meerdaal forest in central belgium the development history of both gullies is very similar in the first gully archaeological evidence was found indicating an erosion phase during roman times followed by a partial infilling of the gully in the second gully radiocarbon dating provided evidence of the same roman activity phase cal yr 46 bc ad 78 but also of an earlier incision phase during the middle bronze age cal yr 1743 1602 1568 1533 bc also here the erosion phase was followed by a partial infilling this limited infilling indicates that the catchment of the gullies was reforested quite rapidly hereby cutting off all runoff and sediment production this has led to a unique situation in the meerdaal forest with the conservation of about 43 similar large gullies in an area of about 17 km 2 this area has a high geovalue as the studied gullies are among the oldest and best conserved gullies in northwestern europe

three dimensional inversion of seafloor magnetotelluric data collected in the philippine sea and the western margin of the northwest pacific ocean we report a result of three dimensional 3 d upper mantle electrical conductivity inversion of seafloor magnetotelluric data we used existing data at 25 sites in the philippine sea and the western margin of the pacific ocean in order to obtain a reliable model by 3 d inversion we evaluated the large and small scale topographic effects we also conducted a comprehensive search of the one dimensional 1 d profiles of the study area in order to determine the best initial and prior models a two phase inversion method was applied so that the error floors for the diagonal and off diagonal elements of the impedance tensor could be separately controlled through this first attempt at inverting real data we obtained basic knowledge about tuning the inversion parameters and conditions we also proposed a procedure to evaluate the reliability of the 3 d conductivity anomalies imaged by the inversion by conducting checkerboard and sensitivity tests after the iterations converged 13 distinct anomalies were found in the inverted 3 d conductivity model four conductive and two resistive anomalies were confirmed to be resolved enough by the data through the checkerboard test then the sensitivity tests were conducted to quantify how each anomaly was required by the observed data and we confirmed that the intensities of three conductive anomalies and one resistive anomaly were statistically significant this paper presented an example of possible approach in 3 d seafloor electromagnetic inversion procedure for imaging reliable electrical conductivity structure of the oceanic mantle which will be useful in understanding dynamics and evolution of solid earth

electrical conductivity imaging of the philippine sea upper mantle using seafloor magnetotelluric data abstract we performed a three year seafloor electromagnetic survey in the philippine sea including the western edge of the pacific ocean to image electrical features of a deep mantle slab stagnating in the transition zone and the surrounding mantle in three dimensions 3 d the project iterated one year deployment of ocean bottom electromagnetometers obems using a total of 37 instruments installed at 18 sites the data obtained have been analyzed in the order of their recovery based on a magnetotelluric mt method in this study we attempt to obtain a one dimensional 1 d electrical conductivity model beneath the philippine sea and the pacific region separately that can be used as a reference model in the first step toward the 3 d analysis the resultant 1 d models show three main features 1 the conductivity in the shallower 200 km of the upper mantle depths of the two regions contrasts sharply which is qualitatively consistent with the large difference in lithospheric age 2 the conductivity at 200 300 km depth in both regions is more or less the same at approximately 0 3 s m 1 3 the conductivity just below 400 km depth is higher for the philippine sea mantle than for the pacific mantle the conductivity structure can be interpreted in terms of the thermal structure mantle hydration and existence of partial melt using experimental results for the conductivity of mantle minerals if the conductivity is interpreted simply as the effect of temperature the mantle beneath the philippine sea could be hotter than the dry solidus of mantle peridotite and thus partially molten however beneath the pacific region the present analysis suggests that the partial melting is not required under the assumed peridotitic composition even if we consider mantle hydration

paleoenvironmental implications of two relative indicator rodent taxa during the pleistocene to holocene transition in south eastern washington state usa local pollen data diminution of body size of three ungulate species and decreased mammalian richness and evenness all indicate grass decreased in abundance during the pleistocene holocene transition pht in eastern washington state usa this paleoenvironmental history suggests that the abundance of remains of microtus sp should decrease as remains of peromyscus maniculatus increase in abundance during the pht these two taxa have well known ecologies relative to one another pht sediments at the marmes archeological site in south eastern washington state are sorted into three chronologically sequent isotaphonomic analytical units relative abundances of remains of microtus sp and p maniculatus shift across the analytical units in the predicted manner whether raw abundances or rarified abundances are considered these results suggest relative abundances of remains of microtus sp and p maniculatus recovered from other sites may be used as relative indicators of paleohabitats their abundances at homestead cave utah shift as predicted from the late pleistocene through the early holocene copyright 2014 john wiley sons ltd

shipboard geophysical indications of asymmetry and melt production beneath the east pacific rise near the melt experiment near the mantle electromagnetic and tomography melt experiment seamounts form and off axis lava flows occur in a zone that extends farther to the west of the east pacific rise than to the east indicating a broad asymmetric region of melt production more seamounts slower subsidence and less dense mantle on the western flank suggest transport of hotter mantle toward the axis from the west variations in axial ridge shape axial magma chamber continuity off axis volcanism and apparent mantle density indicate that upwelling is probably faster and more melt is produced beneath 17 degrees15 s than beneath 15 degrees55 s recent volcanism occurs above mantle with the lowest seismic velocities

an application of thin sheet electromagnetic modelling to the tasman sea abstract the method of thin sheet approximation is invoked for the ocean layer to model and interpret magnetotelluric mt data observed on the floor of the tasman sea between australia and new zealand a technique to remove 3d distortion from the observed seafloor mt data is developed from the thin sheet model mt responses the de distorted data then appear isotropic and are reinverted using 1d procedures this approach thus assists interpretation through a combination of 3d forward modelling procedures and 1d inversion a seafloor sediment layer is shown to have a strong effect on observed seafloor mt data also an exercise is carried out to compare the results given by the vertical gradient sounding vgs method with seafloor mt data the electrical conductivity structure beneath the tasman sea is analysed in terms of an upper mantle lithosphere and asthenosphere and a highly conducting lower mantle the observed anisotropy in seafloor mt data is diagnostic of a lithospheric conductivity of less than 10 4 s m 1 conductivity rises by two orders of magnitude below 80 km to greater than 10 2 s m 1 and is probably related to an asthenospheric layer in the upper mantle seismic interpretations similarly place a low velocity zone below a depth of 70 km at a depth of approximately 400 km the conductivity is approximately 1 s m 1 consistent with global estimates for the lower mantle there is little evidence for major change with age in the structure across the tasman sea

soil erosion and sediment deposition in the belgian oess belt during the holocene establishing a sediment budget for a small agricultural catchment a method to establish a holocene sediment budget for a 103 ha agricultural catchment representative for the belgian loess belt is presented soil erosion and sediment deposition were determined based on 185 coring locations and a large excavation in the valley bottom results were integrated in a gis and interpolation techniques applied to derive spatial patterns of erosion and sedimentation total soil erosion sediment deposition and sediment export were calculated and the results show that volumes are highly dependent on the interpolation technique used sediment delivery ratios between 20 and 42 are derived and are consistent with data reported in previous studies this clearly shows that the majority of the sediments produced during the holocene have been stored near their source area and have not been delivered to the downstream rivers the spatial distribution of soil erosion and sediment deposition within the catchment is strongly dependent on slope gradient and position within the catchment which suggests that since human impact began topography has been the main factor determining long term soil erosion and sedimentation

three dimensional topography corrections of magnetotelluric data summary topographic effects due to irregular surface terrain may prevent accurate interpretation of magnetotelluric mt data three dimensional 3 d topographic effects have been investigated for a trapezoidal hill model using an edge finite element method the 3 d topography generates significant mt anomalies and has both galvanic and inductive effects in any polarization this paper presents two different correction algorithms which are applied to the impedance tensor and to both electric and magnetic fields respectively to reduce topographic effects on mt data the correction procedures using a homogeneous background resistivity derived from a simple averaging method effectively decrease distortions caused by surface topography and improve the quality of subsurface interpretation nonlinear least squares inversion of topography corrected data successfully recovers most of structures including a conductive or resistive dyke

establishing sediment budgets for two small lowland agricultural catchments in the uk sediment budgets have been established for two small 4 km2 lowland agricultural catchments by using 137cs measurements sediment source fingerprinting and more traditional monitoring techniques to quantify the individual components of the budget the gross and net erosion rates for the fields on the catchment slopes were estimated using 137cs measurements within selected fields which encompassed a representative range of slope angles slope lengths and land use these estimates were extrapolated over the entire catchment using a simple topographically driven soil erosion model terrain based gis tbgis superimposed on a dem to derive catchment average gross and net erosion rates suspended sediment yields were measured at the catchment outlets and sediment source fingerprinting techniques were used to establish the relative contributions from the catchment surface subsurface tile drains and eroding channel banks to the sediment yields in channel and wetland storage were quantified using both direct measurements and 137cs measurements the sediment budgets established for the catchments highlighted the importance of subsurface tile drains as a pathway for sediment transfer accounting for ca 60 and 30 of the sediment output from the two catchments erosion from channel banks contributed ca 10 and 6 of the sediment output from the two catchments although the suspended sediment yields from these catchments were considered high by uk standards ca 90 t km 2 year 1 the sediment delivery ratios ranged between 14 and 27 indicating that a major proportion of the mobilised sediment was stored within the catchments in field and field to channel storage were shown to be of similar magnitude but storage of sediment in the channel system and associated wetlands was relatively small representing 5 of the annual suspended sediment yield

rapid development and infilling of a buried gully under cropland central belgium abstract analysis of contemporary and past gully erosion and infilling processes allowed to reconstruct the long term evolution of a permanent gully system under cropland an active and a buried gully under cropland were investigated the recent sediment deposits within the active gully adjacent to the buried gully showed that the recent gully was filling in at a mean rate of 6 4 cm a 1 in the buried gully several erosion and deposition phases could be identified and the type of deposited sediments revealed a complex infilling history charcoal pottery and brick fragments of different sizes were found at all depths of the gully infilling their age indicates that the first gully incised after the midst of the 17th century most probably in the second half of the 18th century or the early 19th century gully morphology and analogy with the processes in the recent gully indicate that the buried gully filled in rapidly overall five cycles of cut and fill occurred in 350 years or less and four cycles even within little less than a few decades indicating that gully development and infilling under cropland can be very rapid processes

the nature of small scale flooding muddy floods and retention pond sedimentation in central belgium abstract this study investigates the spatial variation of small scale flooding and muddy floods in rural areas in a medium sized study area 5516 km2 and the linkage with controlling factors a questionnaire set up in central belgium indicates that 43 of the municipalities have to deal from time to time with muddy floods generated from direct runoff from arable land and 36 with flooding of permanent streams a strong relation exists between the nature of the problem and the site in relation to topography and landuse areas suffering from muddy floods have significantly steeper cultivated slopes compared to areas suffering only from small scale flooding the high spatial and temporal frequency of small scale flooding and muddy floods results in emotional and significant economic damage to private households as a control measure more than 100 retention ponds have been constructed with 50 more to be built in the near future the mean cost for the construction of a retention pond amounts to 380 000 euro these retention ponds store large quantities of sediment from runoff events and must thus be dredged regularly with costs of the order of 1 5 million euro yearly the dredged sediment volumes can be used to assess and predict sediment yield for these drainage basins values vary between 0 19 and 6 m3 ha 1 year 1 for basins ranging from 25 to 5000 ha

age and source of colluvial sediments at vaihingen enz germany abstract colluvial sediments accumulated on the lower slopes of a ridge as a result of past and present soil erosion were dated by optical 14 c and archaeological methods this allowed us to reconstruct the depositional history of the colluvium and also to identify temporary sedimentary sinks in its transportational pathway a cascade model of colluvium formation is developed colluvial sediments resulting from early soil erosion in the neolithic to iron age periods were mainly deposited on the upper slopes significant deposition on the lower slope occurred for the first time during the iron age and roman period since then deposition rates have increased because of more intensive land use

pleistocene irvingtonian cudahyan vertebrates from the texas panhandle and their geographic and paleoecologic significance abstract the cudahy fauna was named by c w hibbard for a fossil assemblage recovered from silty clays beneath the pearlette volcanic ash at the type cudahy ash pit and other sites in meade county kansas the pearlette ash comprises several variously dated yellowstone ashes the cudahy lava creek b ash is most recently dated at 0 639 0 002 ma and is an important geochronologic marker in the great plains the term cudahy ash pit local fauna is now used for that of the type locality although other cudahy type cudahyan local faunas have been described from 1968 to 1971 30 tons of silty clay and diatomite were collected from below outcrops of lava creek b ash at elevations of 988 1058 m 3240 3470 ft in the high plains of the texas panhandle screenwashing yielded about 30 mammal taxa and numerous small amphibians reptiles birds and mollusks four local faunas woody draw bull draw deadman s creek and mayfield ranch are recognized these faunas dominated by shrews and microtine arvicoline rodents are compositionally similar to each other and to the cudahy and sunbrite ash pit local faunas of meade county even though the panhandle sites are 300 340 km 186 211 miles southwest of the kansas sites and are 213 280 m 700 920 ft higher in elevation the panhandle faunas contain two new species of sorex as well as sorex cf cinereus sorex cudahyensis sorex lacustris sorex microsorex pratensis sorex megapalustris blarina cynomys spermophilus geomys tobinensis thomomys cf talpoides peromyscus cragini reithrodontomys moorei ondatra annectens mictomys meltoni microtus llanensis rare microtus paroperarius and microtus meadensis the large sample size of the latter two taxa permits statistical comparisons of teeth with those from kansas the texas faunas further substantiate the cudahyan as a subage of the irvingtonian between the older sappan and younger sheridanian subages and equivalent to irvingtonian ii based on microtine arvicoline rodents local habitats consisted of ponds or playas with marshy areas suitable for shrews muskrats and bog lemmings moist grassy meadows nearby would have been inhabited by voles mice and some of the shrews the less abundant gophers ground squirrels and prairie dog would have preferred drier grassy uplands with well drained soils several mammals e g bog lemming show affinities with extant boreal species that range through canada and the northern united states the collective evidence suggests that the texas panhandle enjoyed cooler summers with more effective rainfall during cudahy time

establishing fine grained sediment budgets for the pang and lambourn locar catchments uk summary an integrated approach to data collection combining the use of 137cs measurements sediment source fingerprinting bed sediment surveys and conventional river monitoring has been successfully employed to establish the fine grained sediment budgets of two lowland groundwater fed catchments in the uk gross surface erosion is higher on cultivated land pang 55 263 t yr 1 or 507 t km 2 yr 1 lambourn 79 997 t yr 1 or 437 t km 2 yr 1 than on pasture pang 1960 t yr 1 or 140 t km 2 yr 1 lambourn 1425 t yr 1 or 95 t km 2 yr 1 in both study areas and a substantial proportion of the mobilized sediment is sequestered within the fields pang 28 058 t yr 1 or 228 t km 2 yr 1 lambourn 55 575 t yr 1 or 281 t km 2 yr 1 and between the individual fields and the river channel network pang 28 672 t yr 1 or 233 t km 2 yr 1 lambourn 24 782 t yr 1 or 125 t km 2 yr 1 the sediment contribution from banks and subsurface sources is relatively low and typically ca 5 t yr 1 in the pang and ca 11 t yr 1 in the lambourn representing only about 1 of the suspended sediment output from each study catchment the mean level of fine grained sediment storage in the main channel system is equivalent to 38 pang and 21 lambourn of the respective mean annual suspended sediment yields of the two catchments the estimated sediment delivery ratio for both study catchments is ca 1

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

correction of seafloor magnetotelluric data for topographic effects during inversion 1 the large contrast in electrical conductivity between seawater and the underlying seafloor accumulates boundary electric charges which can severely distort observed electric and magnetic fields for marine magnetotelluric mt studies correcting this topographic effect is critical to obtaining accurate conductivity models for the mantle previously correction for topography was based on the thin sheet approximation which breaks down at periods under 1000 s in the deep ocean this paper introduces an analysis method for seafloor mt data which combines removal of three dimensional 3 d topographic effects with inversion of the data for 2 d structure the observed mt impedance is first corrected to a flat lying seafloor datum using the observed bathymetry without invoking the thin sheet approximation the corrected mt response is then inverted in a flat seafloor model space because of coupling between topographic effects and deeper structure the correction and inversion steps are iterated until changes in each become small the procedure is verified using synthetic and real data tests for synthetic 3 d topography over a half space show that the method closely recovers the true half space model after a few iterations the procedure is also applied to real data collected in the mantle electromagnetic and tomography melt experiment on the east pacific rise at 17 s

mapping offshore sedimentary structure using electromagnetic methods and terrain effects in marine magnetotelluric data summary marine magnetotelluric mt and marine controlled source electromagnetic csem soundings can be used to study sedimentary structure offshore in an example of this application we collected mt and csem data in the 1 km deep water of the san diego trough california the trough is a pull apart basin and part of the complex pacific north american tectonic plate boundary and is flanked by the thirtymile bank to the west and the coronado bank to the east our mt data are highly distorted by seafloor topography and the coast effect which is largely 2 d and can be modelled using 2 d finite element codes the distortion includes a strong several orders of magnitude static depression of tm mode resistivities electric field perpendicular to structure upward cusps in the te mode resistivities electric field parallel to structure and negative te mode phases the depressed tm mode resistivity is a well known consequence of galvanic interruption of coastperpendicular electric fields the te mode distortion is an inductive effect associated with currents flowing along the edge of the deep ocean basins steepening the magnetic field and even causing a phase reversal in the horizontal field used for mt impedance calculations and thus generating negative phases the land side enhanced vertical magnetic field is well known as the geomagnetic coast effect but the ocean side consequences have been less well documented although the mt data are dominated by coast effect and topographic distortion inclusion of accurate bathymetry in the inversion model s finite element mesh allows the subseafloor geological structure to be recovered this shows the trough sediments to be about 3 km thick bounded to the west by resistive basement but to the east by conductive clastic sediments forming coronado bank amplitudes and phases of five frequencies of csem data from 0 1 to 1 0 hz collected along the axis of the trough are well fit with a simple 1 d layered model indicating that sediment resistivities increase with depth from 1 5 to 2 3 m and are no more than 3300 m thick thinning to the north in good agreement with the mt model an existing density model generated by fitting surface and deep towed gravity is in good agreement with the em interpretations in particular combining sediment densities and csem resistivities allows us to estimate pore water conductivity and temperature which follows a geothermal gradient of 25 4 8kk m 1

heavy metal contamination in overbank sediments of the geul river east belgium its relation to former pb zn mining activities overbank sediments of the geul river east belgium are highly contaminated by the heavy metals pb zn and cd due to former pb zn mining activities in the drainage basin geochemical variations in vertical overbank sediment profiles sampled 1 km north of the mine tailings of plombieres allow metal fluxes back to the 17th century to be reconstructed the vertical profiles are subdivided into three major units corresponding to different industrial periods based on sedimentological criteria as well as on the distribution of contaminants alluvial sediments with the highest heavy metal concentrations correspond to the major period of mining activity of the 19th century the fact that zn mining at the la calamine open mine started before large scale mining of the pbs zns subsurface exploitations is reflected in the vertical profiles by an increase in zn content before a marked increase in pb and cu the regional extent of contamination in the alluvial deposits was evaluated on the basis of the geochemical analysis of sediments at depths of the 0 20 cm and 80 100 cm most of the upper samples are extremely contaminated significant local variations in heavy metal concentration in the lower samples are interpreted in terms of which overbank sediment horizon has been sampled at a depth of 80 100 cm this indicates that blind sampling of overbank sediments to characterize the degree of contamination in shallow boreholes can give very erratic results

revised 1 d mantle electrical conductivity structure beneath the north pacific summary the 1 d electrical conductivity model in the mid mantle beneath the northern pacific was revised in order to discuss the mean state of the mantle and to obtain a credible starting model for 3 d inversions semi global geomagnetic depth sounding gds responses obtained at 13 stations and submarine cable magnetotelluric mt responses for eight cables in a period range of 1 7 113 d were used to obtain the revised structure it is well known that surface conductivity heterogeneity due to the ocean land conductivity contrast has a large influence on the responses up to a period of 20 d the effect of surface ocean land distribution was removed by performing 3 d electromagnetic induction modelling including a surface conductivity heterogeneity distribution laid above a 1 d mantle structure the corrected responses were averaged in the logarithmic space and the resulting responses quasi 1 d response were inverted to obtain 1 d conductivity models correction of the surface heterogeneity effect requires a 1 d conductivity model and the 1 d model must be similar to the resultant 1 d model for a consistent correction we attained this by making iterations of the surface layer correction and 1 d inversion synthetic tests revealed that the iterative scheme could recover the supposed structure even for a 3 d heterogeneous mantle since electromagnetic sounding is not sensitive to the presence of a sharp discontinuity we examined three different cases of conductivity variation with depth 1 a model with no discontinuity 2 a model with two jumps at 400 and 650 km depths and 3 a model with three jumps at 400 500 and 650 km depths the conductivity of the two jump model in the transition zone is higher than the experimentally determined conductivity of dry wadsleyite and ringwoodite if the difference is entirely due to the effect of water in the transition zone then the conductivity is consistent with a water content in the region of 0 5 wt per cent however if an additional discontinuity of electrical conductivity is allowed at the 500 km depth the obtained conductivity for the upper 100 km of the transition zone is lower and can be explained by a less water content 0 1 wt per cent in wadsleyite

magnetotelluric imaging of the society islands hotspot in april june 1989 seafloor magnetotelluric data across and along the leading edge of the tahiti hotspot were obtained the magnetotelluric response functions were found to be strongly influenced by bathymetric and island effects and a new procedure for modeling and removing this distortion using a thin sheet approach combined with the measured water depths is introduced the corrected response functions are consistent with a two dimensional structure inversion of the data shows a slightly higher conductivity relative to a reference site located away from the hotspot down to 130 km depth beneath the active area southeast of tahiti underlain by a more resistive structure there is a suggestion for a change in conductivity in the 400 450 km depth range which is consistent with elevated temperatures this result is consistent with a mantle plume of limited extent less than 150 km radius located near the leading edge of the tahiti hotspot the magnetotelluric data provide no evidence for lithospheric thinning or for a strong thermal influence over a large area as would be required by a superswell model

middle pleistocene climate change in the colorado rocky mountains indicated by fossil mammals from porcupine cave fossiliferous sediments from the pit locality in porcupine cave park county colorado span multiple glacial and interglacial cycles between ca 800 000 and 400 000 yr b p at 2900 m elevation the uppermost cycle probably represents oxygenisotope glacial stage 12 stratigraphic levels 4 and 3 and interglacial stage 11 levels 3 2 and 1 as determined by arvicoline biostratigraphic data that place the sequence between 487 000 and 365 000 yr b p a no analog assemblage comprising the bog lemming mictomys meltoni of the m borealis lineage and the sagebrush vole lemmiscus curtatus lagurus curtatus characterizes both the glacial and the interglacial deposits climatographs for these two species suggest that compared to present conditions the following local climatic limits can be imposed on oxygenisotope stages 12 and 11 1 january effective moisture was greater resulting from average temperatures of at least 5 c cooler and or at least 15 mm more precipitation 2 january solar radiation was lower by at least 75 langleys 3 effective july solar radiation might have been lower by at least 50 langleys seasonal temperature extremes were not necessarily different from those of the present but seasonal precipitation was more equable relative abundance of mammal taxa and sedimentological data indicate that aridity increased at the onset of the interglacial

a new technique for the incorporation of seafloor topography in electromagnetic modelling summary we describe a new technique for incorporating seafloor topography in electromagnetic modelling it is based on a transformation of the topographic relief into a change in electrical conductivity and magnetic permeability within a flat seafloor since the technique allows us to model arbitrary topographic changes without extra grid cells and any restriction by vertical discretization we can model very precise topographic changes easily without an extra burden in terms of computer memory or calculation time the reliability and stability of the technique are tested by comparing the magnetotelluric responses from two synthetic seafloor topography models with three different approaches these incorporate the topographic change in terms of 1 the change in conductance using the thin sheet approximation 2 a series of rectangular block like steps and 3 triangular finite elements the technique is easily applied to any modelling method including 3d modelling allowing us to model complex structure in the earth while taking full account of the 3d seafloor topography

a cellular model of holocene upland river basin and alluvial fan evolution coulthard t j macklin m g kirkby m j 2002 a cellular model of holocene upland river basin and alluvial fan evolution earth surface processes and landforms 27 3 269 288

alluvial and colluvial sediment storage in the geul river catchment the netherlands combining field and modelling data to construct a late holocene sediment budget abstract we used a combined approach of a two dimensional erosion and hillslope sediment delivery model watem sedem and detailed geomorphological reconstructions to quantify the different components in a sediment budget for the geul river catchment southern netherlands since the high middle ages hillslope erosion and colluvium deposition were calculated using the model while floodplain storage was estimated using field data our results show that more than 80 of the total sediment production in the catchment has been stored as colluvium mostly generated by hillslope erosion while almost 13 is stored in the floodplain since the high middle ages this situation resembles a capacity limited system model results for the period prior to the high middle ages with a nearly completely forested catchment show that far less sediment was generated and that most of the sediments were directly transported to the main river valleys or out of the catchment a supply limited system geomorphological analysis of a large alluvial fan shows the sensitivity of the study area to changes in the percentage of arable land our combined field data modeling study presents an elegant method to calculate a catchment sediment budget for a longer period and is able to identify and quantify the most important sediment storage elements furthermore it provides a valuable tool to calculate a sediment budget while only limited dated fluvial sediment sequences are available

mantle dynamics beneath the east pacific rise at 17 s insights from the mantle electromagnetic and tomography melt experiment 1 the electromagnetic data from the mantle electromagnetic and tomography melt experiment are inverted for a two dimensional transversely anisotropic conductivity structure that incorporates a correction for three dimensional topographic effects on the magnetotelluric responses the model space allows for different conductivity values in the along strike cross strike and vertical directions along with imposed constraints of model smoothness and closeness among the three directions anisotropic models provide a slightly better fit to the data for a given level of model smoothness and are more consistent with other geophysical and laboratory data the preferred anisotropic model displays a resistive uppermost 60 km thick mantle independent of plate age except in the vicinity of the ridge crest in most inversions a vertically aligned sheet like conductor at the ridge crest is especially prominent in the vertical conductivity its presence suggests that the melt is more highly concentrated and connected in the vertical direction immediately beneath the rise axis the melt zone is at least 100 km wide and is asymmetric having a greater extent to the west off axis and to the east of the ridge the mantle is more conductive in the direction of plate spreading at depths greater than 60 km the flat resistive conductive boundary at 60 km agrees well with the inferred depth of the dry solidus of peridotite and the deeper conductive region is consistent with the preferred orientation of olivine inferred from seismic observations this suggests that the uppermost 60 km represents the region of mantle that has undergone melting at the ridge and has been depleted of water dissolved hydrogen by contrast the underlying mantle has retained a significant amount of water

structure of the mantle beneath the alboran basin from magnetotelluric soundings we present results of marine mt acquisition in the alboran sea that also incorporates previously acquired land mt from southern spain into our analysis the marine data show complex mt response functions with strong distortion due to seafloor topography and the coastline but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3d inversion model has allowed us to image the structure in the underlying mantle the resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath gibraltar which plunges nearly vertically beneath the alboran our model contains three primary features of interest a resistive body beneath the central alboran which extends to a depth of 150 km at this depth the mantle resistivity decreases to values of 100 ohm m slightly higher than those seen in typical asthenosphere at the same depth this transition suggests a change in slab properties with depth perhaps reflecting a change in the nature of the seafloor subducted in the past two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle or both of these the one in the center of the alboran basin in the uppermost mantle 20 30km depth beneath neogene volcanics and west of the termination of the nekkor fault is consistent with geochemical models which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics

electromagnetic evidence for volatile rich upwelling beneath the society hotspot french polynesia we have conducted a seafloor magnetotelluric survey that images for the first time three dimensional electrical conductivity structure in the upper mantle beneath the society hotspot a striking feature in our model is a high conductivity anomaly a few hundred kilometers in diameter which is continuous from the lowest part of the upper mantle to a depth of approximately 50 km below sea level using theoretical and experimental results from mineral physics we interpret the high conductivity anomaly as evidence of the melt fraction up to 2 2 vol which is robust regardless of assumed temperature and the existence of carbonated silicate melt beneath the hotspot our results suggest that the society hotspot is a pathway for ascending volatiles from the deeper part of the upper mantle to the surface

late quaternary environments and biogeography in the great basin abstract plant and animal remains found in packrat neotoma spp middens and cave fill from the eastern and southern great basin region reveal the presence of subalpine conifers and boreal mammals at relatively low elevations during the late wisconsin limber pine pinus flexilis and bristlecone pine p longaeva were important in the late pleistocene plant communities throughout this region spruce picea cf engelmannii and common juniper juniperus communis were present in some of the more northerly localities and douglas fir pseudotsuga menziesii and white fir abies concolor were present in southern and eastern localities single needle pinyon pine pinus monophylla common across this region today was apparently not present north of the sheep range of southern nevada during the late wisconsin pikas ochotona cf princeps small boreal mammals present in only a few great basin mountain ranges today were common throughout the region heather voles phenacomys cf intermedius have been found in two cave fill deposits in nevada though they are unknown in the great basin today limber and bristlecone pines are generally restricted to rocky substrates in modern subalpine habitats in the great basin and this may also have been the case when these plants grew at lower elevations during the late wisconsin subalpine conifers were present on the rock outcrops sampled by the packrat middens but shrub communities perhaps dominated by sagebrush artemisia spp may have been present on alluvial valley bottom substrates forested habitats would thus have been isolated habitat islands as they are today boreal small mammals including pikas and heather voles were able to colonize the great basin mountain ranges during the late pleistocene we suggest that these mammals were able to survive in the intervening valley bottoms under a cool summer climatic regime and that continuous forest or woodland corridors were not necessary for migration

asymmetric electrical structure in the mantle beneath the east pacific rise at 17 s the magnetotelluric component of the mantle electromagnetic and tomography melt experiment measured the electrical resistivity structure of the mantle beneath the fast spreading southern east pacific rise epr the data reveal an asymmetric resistivity structure with lower resistivity to the west of the ridge the uppermost 100 kilometers of mantle immediately to the east of the ridge is consistent with a dry olivine resistivity structure indicating a mantle depleted of melt and volatiles mantle resistivities to the west of the ridge are consistent with a low melt fraction about 1 to 2 percent interconnected melt distributed over a broad region and extending to depths of about 150 kilometers the asymmetry in resistivity structure may be the result of asymmetric spreading rates and a westward migration of the ridge axis and suggests distinct styles of melt formation and delivery in the mantle beneath the two plates

mictomys borealis northern bog lemming and the wisconsin paleoecology of the east central great basin abstract teeth of northern bog lemming mictomys borealis are reported from cathedral and smith creek caves and represent the first wisconsin remains of the genus from the great basin specimens from cathedral cave snake range are associated with u series ages of 24 000 to 15 000 yr b p previous work with pollen and packrat middens dating to the same age as the mictomys indicate that smith creek canyon contained a riparian locally mesic community including picea engelmannii spruce betula sp birch cercocarpus sp mountain mahogany and artemisia sp sagebrush among other species exposed canyon slopes and the adjacent valley apparently contained a more xeric steppe community including sagebrush and chenopodiineae species rocky outcrop permitted pinus flexilis limber pine and p longaeva bristlecone pine to grow adjacent to lake bonneville or low in the canyon the region apparently experienced a dry climate not necessarily drier than today however smith creek canyon was fed by glacial meltwater from mt moriah the northern bog lemming probably lived only in the riparian community and possibly on the north facing slope below cathedral cave few canyons of the snake range would have had the unusually mesic conditions found in smith creek canyon

land use and climatic impacts on the rhine system rheinlucifs quantifying sediment fluxes and human impact with available data abstract the rheinlucif project investigates the effects of land use and climatic impacts on sediment fluxes in the river rhine catchment among the world s large rivers the rhine system has been particularly affected by land use changes for 7500 years resulting in its transformation to an entirely man controlled environment geomorphologically this transformation is performed by largely man induced sediment fluxes since the mid holocene as the large scale of the rhine catchment inhibits a renewed recording of stratigraphic records the main goal is to synthesize quantitative data on sediment flux climatic and human impacts from already available data from various fields of earth surface and social sciences particularly history and archaeology this paper presents the first steps and results of methodological groundwork exploring how to derive sediment budgets from available spatially distributed data soil maps geological maps dems furthermore a new basically statistical approach utilising archaeological data is outlined this approach provides time series of quantified human impact i e information on population densities and open land woodland ratios the first results derived from four different sediment budget approaches are helpful in order to assess the quality of available data for budget calculation purposes although some estimates on erosion and alluvial deposition seem to broadly agree a considerable disparity of holocene erosion and colluviation values occurs when these values are compared to the outcome of an independently recorded soil mapping campaign in a pilot area however in general the results suggest that all applied approaches chiefly require an improved estimation of sediment storage on the one hand this accounts for the need of more precise information on holocene floodplain deposits on the other hand as the larger portion of man induced sediments still resides in the sloping parts of catchments realistic estimates on man induced colluvial deposition on slopes are essential apart from budgeting issues all data on holocene erosion and sedimentation in the rhine catchment lacked a sufficient dating control therefore further investigations on fluxes between budget components necessitate additional datings quantified data on human impact is indispensable to the modelling of sediment fluxes because human impact is deemed to be the major driving force of sediment production and re distribution although the archaeological approach was developed and tested by utilising well documented data about the neolithic era the promising statistical evaluation suggests that this method can be applied to even less well documented proto prehistoric and historic periods

adaptive unstructured grid finite element simulation of two dimensional magnetotelluric fields for arbitrary surface and seafloor topography summary we present an adaptive unstructured triangular grid finite element approach for effectively simulating plane wave diffusive electromagnetic fields in 2 d conductivity structures the most striking advantage of irregular grids is their potential to incorporate arbitrary geometries including surface and seafloor topography adaptive mesh refinement strategies using an a posteriori error estimator yield most efficient numerical solutions since meshes are only refined where required we demonstrate the robustness of this approach by comparison with analytical solutions and previously published numerical simulations maximum errors may systematically be reduced to for example 0 8 per cent for the apparent resistivity and 0 2 in the phase an additional accuracy study of the thickness of the air layer in e polarization suggests to keep a minimum thickness depending on lateral conductivity contrasts within the earth furthermore we point out the new quality and flexibility of our simulation technique by addressing two marine magnetotelluric applications in the first case we discuss topographic effects associated with a synthetic sinusoidal sea bottom model and in the second case we show a close to reality scenario using real bathymetry data from the east pacific rise at 17 s

sediment yield and storage variations in the negron river catchment south western parisian basin france during the holocene period in the negron river catchment area 162 km2 surface sediment stores are composed of periglacial calcareous greze 5 106 t and loess 21 106 t and holocene alluvium 12 6 106 t peat 0 6 106 t and colluvium 18 5 106 t seventy five per cent of the holocene sediments is stored along the thalwegs present net sediment yield calculated from solid discharge at the negron outlet is low 0 6 t km 2 a 1 due to the dominance of carbonate rocks in the catchment mean sediment yield during the holocene period is 7 0 t km 2 a 1 from alluvium stores and 7 6 t km 2 a 1 from colluvium stores thus the gross sediment yield during the holocene period is about 18 7 t km 2 a 1 and the sediment delivery ratio 3 per cent the yield considerably varies from one sub basin to another 3 9 to 24 5 t km 2 a 1 according to lithology about 25 per cent and 50 per cent of initial stores of periglacial greze and loess respectively were reworked during the holocene period sediment yield has increased by a factor of 6 in the last 1000 years due to the development of agriculture the very high rate of sediment storage on the slope during that period 88 per cent of the yield can be accounted for by the formation of cultivation steps rideaux it is predicted that the current destruction of these steps will result in a sediment wave reaching the valley floors in the coming decades subboreal and subatlantic sediments and pollen assemblages in the taligny marsh where one third of the alluvium is stored show the predominant influence of human activity during these periods in the negron catchment copyright 2002 john wiley sons ltd

inversion of ocean bottom magnetotelluric data revisited three ocean bottom magnetotelluric data sets from sites on the pacific plate are reinterpreted the initial analysis found a correlation between the lithospheric age and the depth to a conductive zone beneath each site that work also suggested that the resistivity increased below the conductor this analysis which includes new methods for constructing one dimensional conductivity models shows that the postulated increase in resistivity is not demanded by the data it also reveals an unexpectedly large nonuniqueness inherent in the interpretation of these data the previously reported trends with lithospheric age still exist but they are not as strong as initially believed finally it is shown rigorously that the different age sites are distinct in that no one dimensional model can account for all three data sets

northeastern pacific mantle conductivity profile from long period magnetotelluric sounding using hawaii to california submarine cable data we present results of a long period magnetotelluric mt investigation of the electrical structure beneath the eastern north pacific the electric field data consist of 2 years of continuously recorded voltages across an unpowered 4000 km long submarine telephone cable haw 1 extending from point arena california to oahu hawaii the electric field measurements are coherent to some degree with magnetic field measurements from honolulu observatory at periods of 0 1 to 45 days this coherence is enhanced at long periods over that observed with point electric field sensors due to horizontal averaging of the motional electric fields of spatial scale smaller than the cable length significantly diminishing their effect robust controlled leverage mt response estimates and their jacknife confidence limits are computed for the haw 1 to honolulu data an equivalent scalar mt response obtained from honolulu magnetic variations data is used to correct the haw 1 mt response for static shift and to extend the mt response estimate to periods of 100 days the composite response function satisfies necessary and sufficient conditions for consistency with a one dimensional conductivity structure and is most sensitive to structure between 150 and 1000 km inversion of the mt response reveals a conductive zone 0 05 0 1 s m between 150 and 400 km depth and a positive gradient below 500 km these observations are consistent with previous mt studies in the north pacific this upper mantle conductivity is too high to be explained by solid state conduction in dry olivine using reasonable mantle geotherms calculations based on measurements of hydrogen solubility and diffusivity in olivine indicate that h dissolved in olivine possibly combined with a lattice preferred orientation consistent with measured seismic anisotropy provide sufficient conductivity enhancement to explain the inversion results the high conductivity may also be explained by the presence of gravitationally stable partial melt comparison of the haw 1 results with long period mt studies conducted on land reveals differences in upper mantle conductivity between different tectonic regimes in particular the upper mantle beneath the pacific ocean is considerably more conductive than that beneath the canadian shield and similar in conductivity to that beneath the basin and range

electrical structure beneath the northern melt line on the east pacific rise at 15 45 s 1 the electrical structure of the upper mantle beneath the east pacific rise epr at 15 45 s is imaged by inverting seafloor magnetotelluric data obtained during the mantle electromagnetic and tomography melt experiment the electrical conductivity model shows no evidence for a conductive region immediately beneath the ridge in contrast to the model previously obtained beneath the epr at 17 s this observation can be explained by differences in current melt production along the ridge consistent with other observations the mantle to the east of the ridge at 60 100 km depth is anisotropic with higher conductivity in the spreading direction compared to the along strike direction similar to the 17 s region the high conductivity in the spreading direction can be explained by a hydrated mantle with strain induced lattice preferred orientation of olivine or by partial melt preferentially connected in the spreading direction

human and climate impact on catchment development during the holocene geul river the netherlands interest in the holocene development of small to medium sized river catchments in western and central europe in relation to changes in land use and climate has increased over the past years in this study we reconstruct the holocene landscape development and fluvial dynamics of the geul river the netherlands and the main forcing mechanisms of environmental change field studies were carried out and we used osl and 14c dating methods to reconstruct the holocene valley development our study shows that 2 periods of deforestation during the roman period and the high middle ages led to severe soil erosion and increased floodplain sedimentation in the catchment of the geul river possibly combined with periods of increased wetness during the high middle ages alluvial fans have been active since the roman deforestation phase our results show that the geul catchment is highly sensitive to changes in land use

the effect of seafloor topography on magnetotelluric fields an analytical formulation confirmed with numerical results summary magnetotelluric fields and impedances are distorted at undulating interfaces an analytical formulation is presented to calculate magnetotelluric effects in the presence of a sinusoidal interface in contrast to previous analytical approaches this formulation is not based on perturbation theory it is applicable to observations both on land and on the seafloor electric and magnetic fields as well as apparent resistivities and phases are calculated on the interface the topographic distortion on land mainly influences the tm mode data where the electric field is perpendicular to the geological strike both the tm mode and the orthogonal te mode data are distorted on the seafloor systematic parameter tests indicate which modes are independent of the period and the conductivity contrast provided that the induction depths are large relative to the amplitude of the topography the differing physics of the seafloor and land surface is illustrated by plotting current streamlines for the land model contour lines diverge below a hill and converge below a valley for the seafloor model electric currents mainly flow in the conductive sea water contour lines converge above a hill and diverge above a valley numerical results derived from finite element modelling support the analytical solutions streamlines of the electric current derived from a model for the central andes illustrate the connection between a graphical display of electromagnetic fields and an algebraic sensitivity analysis of the magnetotelluric impedance tensor