why are geomagnetic excursions not always recorded in sediments constraints from post depositional remanent magnetization lock in modelling abstract many geomagnetic excursions have been documented during the brunhes chron however high resolution paleomagnetic studies of sediments often provide evidence of few if any excursions we have investigated the reasons for this observation by modelling the post depositional remanent magnetization pdrm lock in process a cubic lock in function produces more rapid lock in compared to linear and exponential functions proposed in the literature we therefore used a cubic function to model best case scenarios for the quality of the paleomagnetic record when a high frequency geomagnetic input signal is convolved with the sediment lock in function for a wide range of sedimentation rates even for a lock in depth of 10 cm where 95 of the pdrm is locked in 5 cm below the surface mixed layer an input signal containing abundant excursions with 1 kyr duration is smoothed so drastically at low sedimentation rates 1 3 cm kyr that no excursions are recorded excursions are further attenuated by increasing the lock in depth as well as by sampling at discrete stratigraphic intervals the pdrm process acts as a low pass filter for high frequency secular variation signals even at moderate sedimentation rates 10 cm kyr we have also modelled the effects of modulation of sedimentation rate by a climatic forcing function e g insolation even though the average sedimentation rate might be substantial pdrm recording becomes much less reliable in intervals where sedimentation rates are lowest in these cases some excursions are reliably recorded in detail while others may not be recorded at all our modelling suggests that in order to consistently detect the presence of geomagnetic excursions it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm kyr if the normal spectrum of geomagnetic field behaviour contains abundant excursions failure to document excursions in many apparently high resolution analyses probably results from pdrm lock in within relatively slowly deposited sediments or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates

the san venanzo maar and tuff ring umbria italy eruptive behaviour of a carbonatite melilitite volcano the late pleistocene san venanzo maar and nearby pian di celle tuff ring in the san venanzo area of umbria central italy appear to represent different aspects of an eruptive cycle accompanied by diatreme formation approximately 6x106 m3 of mostly lapillisized juvenile ejecta with lesser amounts of lithics and 1x106 m3 of lava were erupted the stratigraphy indicates intense explosive activity followed by lava flows and subvolcanic intrusions the pyroclastic material includes lithic breccia derived from vent and diatreme wall erosion roughly stratified lapilli tuff deposited by concentrated pyroclastic surge chaotic scoriaceous pyroclastic flow and inverse graded grain flow deposits the key feature of the pyroclastics is the presence of concentric shelled lapilli generated by accretion around the lithics during magma ascent in the diatreme conduits the rock types range from kalsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite phonolite and calcitic melilitite pyroclasts juvenile ejecta contain essential calcite whose composition and texture indicate a magmatic origin pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds the san venanzo maar forming event is believed to have been trigered by fluid rich carbonatite phonolite magma the eruptive centre the moved to the pian di celle tuff ring where the eruption of degassed olivine melilititic magma and late intrusions ended magmatic activity in the area in both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of co2 which flowing through the diatremes produced the peculiar intrusive pyroclastic facies and triggered explosions

petrology and mineralogy of wollastonite and melilite bearing paralavas from the central apennines italy small outcrops of wollastonite and melilite bearing pyrometamorphic rocks paralavas are found along the apennine chain in central italy at the localities of colle fabbri and ricetto these rocks have coarse to fine grained crystalline spotted and glassy textures the colle fabbri rocks have abundant and ubiquitous wollastonite with plagioclase clinopyroxene melilite is found only in the most ca rich and silica poor samples garnet perovskite quartz leucite silica rich glassy mesostasis and ocelli filled by calcite are present locally the ricetto rock is made up of wollastonite and melilite with minor clinopyroxene set in a glassy matrix with a few crystals of restitic quartz these rocks have variable contents of sio2 43 to 64 wt and cao 37 to 3 7 wt the latter decreases with increasing mgo al2o3 fe2o3 tot tio2 k2o sio2 and most trace elements excluding sr peculiar parageneses e g coexisting melilite and anorthite in the most silica poor samples abundance of wollastonite mineral compositions gehlenite rich melilite al ti and fe3 rich clinopyroxene al ti rich but fe poor garnet and major and trace element geochemistry suggest that the colle fabbri and ricetto rocks are not ultimately mantle derived but are the result of melting devolatilization and recrystallization of marly sediments 50 60 to 5 10 calcite with clay minerals quartz likely due to coal fires the 87sr 86sr and 143nd 144nd range from 0 70772 to 0 71190 and from 0 51223 to 0 51219 respectively again suggesting sedimentary protoliths of a mixed nature substantial chemical and isotopic differences between these samples and those from the roman magmatic province are highlighted

criteria for interpreting kimberlite as coherent insights from the muskox and jericho kimberlites nunavut canada the jurassic muskox and jericho kimberlites northern slave province nunavut canada contain a variety of facies exhibiting different geometries contact relationships internal organisation country rock abundance and olivine shapes although many have similar matrix groundmass mineralogies and textures five facies are examined that either have characteristics consistent with coherent rocks in general i e intrusive and extrusive non fragmental rocks or are mineralogically and texturally similar to kimberlite described as coherent or apparent coherent three facies are interpreted as coherent on the basis of 1 geological setting 2 apparent porphyritic texture 3 sharp contacts with fragmental kimberlite 4 relative abundance of elongate and unbroken olivine crystals and 5 paucity of country rock xenoliths while the remaining two facies are interpreted as fragmental on the basis of 1 the gradational contacts with demonstrably fragmental kimberlite 2 relative abundance and range of sizes of country rock lithic clasts and 3 numerous broken olivine crystals comparisons are made with coherent and apparent coherent kimberlite from the literature our three coherent facies are similar to literature reported coherent kimberlite dykes hosted in country rock ckd in terms of internal organisation low abundance of country rock xenoliths and apparent porphyritic texture conversely our two fragmental facies share attributes with previously described pipe filling coherent and apparent coherent kimberlite ckpf in terms of geometry internal organisation and abundance of country rock xenoliths we conclude that ckd and most ckpf although similar in matrix groundmass mineralogy and texture can be distinguished on the basis of internal organisation country rock lithic clast abundance texture e g apparent porphyritic texture and possibly olivine crystal shapes and suggest that fragmental kimberlite is more common than reported

geomagnetic field for 0 3 ka 2 a new series of time varying global models 1 steadily increasing numbers of archeomagnetic and paleomagnetic data for the holocene have allowed development of temporally continuous global spherical harmonic models of the geomagnetic field extending present and historical global descriptions of magnetic field evolution the current work uses various subsets of improved data compilations details of which are given in a companion paper by donadini et al 2009 and minor modifications of standard modeling strategies using temporally and spatially regularized inversion of the data and cubic spline parameterizations for temporal variations to produce five models with enhanced spatial and temporal resolution for 0 3 ka spurious end effects present in earlier models are eliminated by enforcing large scale agreement with the gufm1 historical model for 1650 1990 a d and by extending the model range to accommodate data older than 3 ka age errors are not considered as a contribution to data uncertainties but are included along with data uncertainties in an investigation of statistical uncertainty estimates for the models using parametric bootstrap resampling techniques we find common features but also significant differences among the various models indicating intrinsic uncertainties in global models based on the currently available holocene data model cals3k 3 based on all available archeomagnetic and sediment data without a priori quality selection currently constitutes the best global representation of the past field the new models have slightly higher dipole moments than our previous models virtual axial dipole moments vadms calculated directly from the data are in good agreement with all corresponding model predictions of vadms these are always higher than the spherical harmonic dipole moment indicating the limitations of using vadms as a measure of geomagnetic dipole moments

detransformation of the british geomagnetic secular variation record for holocene times in this note we replot the british post glacial secular variation curve in terms of actual values of declination and inclination rather than the transformed declination and inclination system used in our original paper turner thompson 1981 this should facilitate the comparison of our record with contemporaneous european archaeomagnetic data our composite record was obtained by averaging the dated records from 10 cores of lake sediment the sequence of unit vectors describing the magnetic direction record of each core was rotated so as to place its mean direction the mean over the section of core dated between 0 and 7000 bp the time span of the shortest core at zero transformed declination and inclination dt o it 0 the transformed coordinate frame of each core is thus derived from its mean direction assumed to represent the mean geomagnetic field direction over the 7000 yr period rather than from the core tube and its orientation during coring hence the problem of not knowing the azimuthal orientation and inclination of the corer during core collection is avoided and the 10 records can be averaged in this coordinate system transformed coordinates then provide a convenient generalised method of plotting averaged sediment records however in the archaeomagnetic method sample orientation is straightforward and the results can readily be plotted as absolute declination and inclination values so for consistency when comparing the two we have attempted here to detransform our record the following procedures assumptions and approximations were taken to produce the detransformed magnetograms and bauer type plots of figs 1 and 2 1 amplitude correction although the combination of 10 data sets produces a good estimate of the signature of the geomagnetic variations the following factors contribute to an overall loss of amplitude in both declination and inclination a the quality of magnetic recording varies widely between different sediments depending on the porosity of the sediment the size and shape distributions and the nature of the magnetic carrier verosub 1979 the loch lomond sediments carry a good high amplitude pattern of remanent directions of which the most recent closely follow the london observatory records the much wetter and less compacted sediments of llyn geirionydd on the

fennostack and fennorpis varve dated holocene palaeomagnetic secular variation and relative palaeointensity stacks for fennoscandia a holocene palaeomagnetic secular variation master curve fennostack and a relative palaeointensity curve fennorpis for fennoscandia are presented these curves were produced by stacking palaeomagnetic data obtained from six annually laminated varved lake sediment sequences and one non laminated sediment sequence the six independent varve chronologies were combined to form a timescale which extends between the present year and 10 200 cal yr bp smoothed inclination and declination curves show trends that are similar to a palaeomagnetic secular variation curve for the united kingdom except that the ages of most of the statistically significant features are a few hundred years younger in the fennoscandian data set these differences are most likely due to excessively old ages of the united kingdom features produced by the radiocarbon dating of bulk sediments the standardized relative palaeointensity stack shows that the intensity of the geomagnetic field in fennoscandia was highest at 2300 cal yr bp and lowest at 7000 cal yr bp three palaeointensity peaks pips and three palaeointensity lows pils are identified in fennorpis and indicate that the intensity of geomagnetic field varied significantly on millennial scales in this region during the holocene but there is much scatter in the reconstruction prior to 8000 cal yr bp higher frequency palaeointensity features present in individual records are not significant in the stacked data set which implies that these are a result of local environmental bias significant discrepancies exist between the data and the output of the cals7k 2 geomagnetic field model for the last 7000 yr in particular the largest declination swing observed in the data at 2670 cal yr bp feature f is not produced by the model which is most likely due to the influence of an inaccurately dated icelandic secular variation curve on the model result short term trends in standardized relative palaeointensity do differ from the model output but it is difficult to make a comparison because of the relative nature of the sediment record better absolute palaeointensity data are needed to reduce uncertainties in reconstructions of past geomagnetic field strength

fluvial changes of the guadalquivir river during the holocene in cordoba southern spain this research was funded by a grant from proyectos de cordoba siglo xxi coordinated by prof r jose roldan canas universidad de cordoba

implications of bank failures and fluvial erosion for gully development field observations and modeling 1 gully erosion is most commonly triggered by fluvial erosion following natural and anthropogenic disturbances or as a response to changes in climate and tectonic forcing and base level drop field observations attribute the headward growth and widening of many gully systems to gravitational mass wasting processes of oversteepened sidewalls soil saturation groundwater sapping and tension crack development contribute to the instability recent landscape evolution models treat such mass failures as slope dependent continuous sediment transport processes sometimes conditioned on a slope threshold or with nonlinear dependence on slope gradient in this study we first present an explicit physically based theory for the stability analysis of gully heads and walls the theory is based on the force balance equation of an assumed planar failure geometry of a steep gully wall with a potential failure plane dipping into the incised gully bed and tension cracks developing behind the scarp face then we test the theory against field data collected in our field site in colorado and against other published data second the theory is implemented in a one dimensional hillslope profile development model and the three dimensional channel hillslope integrated landscape development child to study the effects of soil cohesion erosion thresholds and stochastic climate on the tempo of gully development and morphology preliminary results indicate that wider and shallower gullies develop and integrate forming wide valleys when soil cohesion is small as soil cohesion increases erosion slows down gullies become deeper with vertical walls and episodic mass failures occur differences in storm intensity duration characteristics and erosion thresholds are predicted to have a significant impact on gully development vertical gully walls develop rapidly and gullies enlarge by slab failures in a climate characterized by high intensity short duration storm pulses however under low intensity long duration storms gullies quickly stabilize and vertical walls are eliminated and rounded forming diffusion dominated hilltops erosion thresholds have a similar impact on the tempo of gully erosion but in the opposite direction lowering the erosion threshold enhances gully widening by slab failures gully walls stabilize when the erosion threshold is high due to a reduction in the erosion of the failure material on the toe of gully walls

prediction of concentrated flow width in ephemeral gully channels empirical prediction equations of the form w d aq b have been reported for rills and rivers but not for ephemeral gullies in this study six experimental data sets are used to establish a relationship between channel width w m and flow discharge q m 3 s 1 for ephemeral gullies formed on cropland the resulting regression equation w d 2 51 q 0 412 r 2 d 0 72 n d 67 predicts observed channel width reasonably well owing to logistic limitations related to the respective experimental set ups only relatively small runoff discharges i e q 0 02 m 3 s 1 were covered using field data where measured ephemeral gully channel width was attributed to a calculated peak runoff discharge on sealed cropland the application field of the regression equation was extended towards larger discharges i e 5 10 4 m 3 s 1 q 0 1m 3 s 1 comparing w q relationships for concentrated flow channels revealed that the discharge exponent b varies from 0 3 for rills over 0 4 for gullies to 0 5 for rivers this shift in b may be the result of i differences in flow shear stress distribution over the wetted perimeter between rills gullies and rivers ii a decrease in probability of a channel formed in soil material with uniform erosion resistance from rills over gullies to rivers and iii a decrease in average surface slope from rills over gullies to rivers the proposed w q equation for ephemeral gullies is valid for sealed cropland with no significant change in erosion resistance with depth two examples illustrate limitations of the w q approach in a first example vertical erosion is hindered by a frozen subsoil the second example relates to a typical summer situation where the soil moisture profile of an agricultural field makes the top 0 02 m five times more erodible than the underlying soil material for both cases observed w values are larger than those predicted by the established channel width equation for concentrated flow on cropland for the frozen soils the equation w d 3 17 q 0 368 r 2 d 0 78 n d 617 was established but for the summer soils no equation could be established copyright 2002 john wiley sons ltd

paleosecular variations since 9000 yr bp as recorded by sediments from maar lake shuangchiling hainan south china abstract high resolution paleosecular variation psv can play an important role in sediment dating and for understanding the temporal characteristics of the geodynamo of the earth s magnetic field in this article based on magnetic susceptibility correlation a composite sediment profile has been compiled from a 16 m long core hn 2 and a 8 m deep well hn 3 in shuangchiling maar lake a crater basin located on hainan island southern china fine and homogeneous silty clay corroborates with a stable continuous and undisturbed deposition a u channel based paleomagnetic study constrained by ams14c chronology derived from leaves plant debris and woods and supported by rock magnetic data indicates that the lake sediments document a reliable paleosecular variation psv record since 9000 cal year bp comparison of our psv features with other records from lakes biwa japan erhai china baikal russia as well as archaeomagnetic records demonstrate that the inclination pattern is generally synchronous within chronological uncertainties suggesting further psv dating may be valuable for stratigraphic correlation on a regional scale

development and controlling factors of gullies and gully complexes east coast new zealand sequential aerial photographs of a small headwater catchment in the waiapu basin east coast region north island new zealand were interpreted to measure and analyse temporal changes in active area of gullies and gully complexes for a longer time span 1939 2003 and with higher temporal resolution compared to previous studies we focus on the conditions leading to the development of gullies and gully complexes under pasture and forest by using topographic thresholds slope area relationships of catchments for the initiation of gullies and gully complexes in addition the influence of two different lithologies as well as the occurrence of major rainfall events was related to gully activity twenty gullies and four gully complexes occupying 62 5 ha or 12 5 per cent of the catchment area occurred in the study catchment between 1939 and 2003 however the majority of these were not active at all of the dates studied gullies developed in the sandstone dominated tapuwaeroa formation tended to attain their maximum size by 1957 with a mean catchment area of 2 1 ha gullies developed in mudstone of the whangai formation attained their maximum size in 1939 with a mean catchment area of 4 31 ha exceptions are gullies which developed into mass movement deposits or into an earth flow deposit as well as gullies developed under indigenous forest topographic threshold values for gullies under pasture and indigenous forest show that values for gullies under forest plot far above the threshold line of gullies under pasture indicating that the topographical threshold for gully development under forest is higher compared to under pasture a threshold value of 9 4 ha in catchment area is needed for the development of gully complexes under pasture all located in the whangai formation and with the same orientation as the strike of the mudstones gully complex area and dominance of mass movement erosion increased with larger catchment area a decreasing distance to the threshold line for gullies under pasture indicates a later development for gully complexes no gully complexes developed under indigenous forest indicating that the threshold value for gully complex development is higher than for gully complexes under pasture and was not reached in the study area a model of shifting topographical threshold for gully development for a given catchment is developed which depends on land use when a catchment has an indigenous forest cover the topographical threshold is very high after conversion to pasture threshold values decrease drastically with the invasion of scrub the threshold slowly increases and returns to a similar level to that under indigenous forest after reforestation development of gullies and gully complexes is a highly dynamic phenomenon and phases of expansion and inactivity indicate that models describing only unidirectional advancing stages without periods of inactivity are not suitable therefore this study adds more phases to models of gully and gully complex development in the east coast region the threshold line for gully initiation under pasture and a value of 9 4 ha in catchment area for gully complex initiation permits one to predict which catchments under similar environmental settings develop gullies and gully complexes on a physical basis this enables land managers to implement sustainable land use strategies to reduce erosion rates of gullies and gully complexes copyright 2006 john wiley sons ltd

archaeomagnetic secular variation in the uk during the past 4000 years and its application to archaeomagnetic dating this paper examines the limitations and deficiencies of the current british archaeomagnetic calibration curve and applies several mathematical approaches in an attempt to produce an improved secular variation curve for the uk for use in archaeomagnetic dating the dataset compiled is the most complete available in the uk incorporating published results phd theses and unpublished laboratory reports it comprises 620 archaeomagnetic directional data and 238 direct observations of the geomagnetic field and includes all relevant information available about the site the archaeomagnetic direction and the archaeological age a thorough examination of the data was performed to assess their quality and reliability various techniques were employed in order to use the data to construct a secular variation sv record moving window with averaging and median as well as bayesian statistical modelling the sv reference curve obtained for the past 4000 years is very similar to that from france most differences occurring during the early medieval period or dark ages two examples of dating of archaeological structures medieval and pre roman are presented based on the new sv curve for the uk and the implications for archaeomagnetic dating are discussed

gully development in the moldavian plateau of romania abstract gully erosion has been recognized as an important environmental threat in the moldavian plateau of eastern romania the main objective of this study was to define the process based gully development by providing quantitative information from long term field measurements in small catchments three main areas of monitoring gullies were explored aerial photographs of flights in 1960 and 1970 classical leveling and repeated survey through a particular close stakes grid after 1980 the caesium 137 technique has been used effectively in the areas of deposition of gully sediments to obtain reliable information on dating specific levels of sediments and to provide chronological measures of gully development most of the discontinuous gullies exhibit both proper gullying mainly if not exclusively by gully head advance and aggradation of the gully basin floor results indicated that the mean gully head retreat was 0 92 m year 1 and the mean areal gully growth was 17 0 m2 year 1 both values indicate a slow erosion rate for this area the average annual regime of gullying is pulsatory one that is best described by great fluctuations conventional measurements on sedimentation over the period 1987 1997 indicate a higher rate of aggradation in the upper half of the gully floor information on the caesium 137 depth profile was used to provide estimates of a mean sedimentation rate of 4 4 cm year 1 over the period 1963 1996 and 2 5 cm year 1 after 1986 for the short successive discontinuous gullies a new classification of the discontinuous gullies based on two criteria respectively the field patterns and the rate of aggradation within the gully basin floor was established for continuous gullies linear gully head retreat areal gully growth and eroded material rates were quantified for three periods 1961 1970 1971 1980 and 1981 1990 results indicate that gully erosion has decreased since 1960 this gullying decline is due to the rainfall distribution and the increased influence of soil conservation practices the mean gully head retreat of 12 5 m year 1 between 1961 1990 was accompanied by a mean areal gully growth of 366 8 m2 year 1 and a mean erosion rate of 4168 t year 1 as with discontinuous gullies continuous gullies showed pulsatory development the critical period for gullying over 1981 1996 covers 4 months from mid march to mid july in an area with mean annual precipitation around 500 mm another main finding of this 16 year stationary monitoring was that 57 of the total gullying occurred during the cold season with the remainder during the warm season of the total gully growth 66 results from only four years 1981 1988 1991 and 1996 when a greater amount of precipitation fell

depositional remanent magnetization toward an improved theoretical and experimental foundation the first theoretical predictions for the behavior of magnetic particles in water were that sedimentary magnetizations would be fully aligned with the ambient field yet redeposition experiments showed a strong and quasi linear dependence on the external field this empirically observed linearity has served as the fundamental assumption of sedimentary paleointensity studies for decades we present redeposition experiments which suggest instead that the relationship between depositional remanence drm and applied field may frequently be curved for magnetic fields in the range of the earth s numerical simulations using a flocculation model can explain the redeposition data and suggest that drm will be significantly non linear when the flocs are small several microns there is a strong dependence of floc size on salinity particularly in low salinity environments floc size has a profound influence on the efficiency of drm hence low salinity environment may give results with poor reproducibility the size of the floc in which magnetic particles are embedded is not accounted for in current methods of normalization yet is the most important parameter on the bright side however it now seems possible to quantitatively explain paleointensity in sedimentary systems opening the door to absolute paleointensity estimates from sediments whose key parameters of floc size distribution and settling times can be constrained

the cycle of instability stress release and fissure flow as controls on gully head retreat gully head and wall retreat has commonly been attributed to fluvial scour and head collapse as a result of soil saturation sapping or piping the empirical evidence to substantiate these conceptual models is sparse however and often contradictory this paper explores the hydrological and mechanical controls on gully head and wall stability by modelling the hydrology stability and elastic deformation of a marl gully complex in granada province south east spain the hydrological and slope stability simulations show that saturated conditions can be reached only where preferential fissure flow channels water from tension cracks into the base of the gully head and that vertical or subvertical heads will be stable unless saturation is achieved owing to the high unsaturated strengths of marl measured in this research failure in unsaturated conditions is possible only where the gully head wall is significantly undercut head retreat thus requires the formation of either a tension crack or an undercut hollow finite element stress analysis of eroding slopes reveals a build up of shear stress at the gully head base and a second stress anomaly just upslope of the head wall although tension cracks on gully heads have often been attributed to slope unloading this research provides strong evidence that the so called sapping hollow commonly found in the gully headwall base is also a function of stress release although further research is needed it seems possible that pop out failures in river channels may be caused by the same process the hydrological analysis shows that once a tension crack has developed throughflow velocity in the gully headwall will increase by an order of magnitude promoting piping and enlargement of this weakened area it is therefore possible to envisage a cycle of gully expansion in which erosion channel incision or human action unloads the slope below a gully head leading to stress patterns that account for the tension crack and a stress release hollow the tension crack promotes faster throughflow encouraging hollow enlargement and piping which undercut the gully head the tension crack permits the development of positive pore water pressures behind the gully head leading either to failure or contributing to toppling finally the debris may be eroded by fluvial action unloading a new section of slope and completing the cycle of gully head retreat copyright 2001 john wiley sons ltd

geomagnetic secular variation as recorded in british lake sediments and its application to archaeomagnetic studies abstract lake sediments can play an important role in understanding and reconstructing temporal characteristics of the geomagnetic field as they potentially offer near continuous high resolution archives of magnetic information extending throughout the holocene to date lake sediment geomagnetic data has typically been excluded from british archaeomagnetic secular variation curves svc due to uncertainties with the acquisition of magnetisation by lake sediments this paper presents the argument that with regards to british datasets the real problem lies with poor chronological control and sets out to illustrate that with british archaeomagnetic data some progress has been made the results indicate that it is not currently possible to resolve secular variation on a time scale of 100 years from published british lake sediment data but more success has been made with data from archaeological sediments this level of detail has been considered necessary for the incorporation of lake sediment data into the british archaeomagnetic dataset as the ability to resolve short term geomagnetic changes is critical for the integration of any dataset into the british svc as the latter is predominantly employed to date archaeological architecture and artefacts requires that it has the ability to resolve changes over timescales relevant to human lifetimes using currently available data this retrospective critique confirms that in archaeological sediments depositional and thermoremanent magnetisation can record the same direction over the same time interval

testing the ephemeral gully erosion model egem for two mediterranean environments few models can predict ephemeral gully erosion rates e g creams egem the ephemeral gully erosion model egem was specifically developed to predict soil loss by ephemeral gully erosion although egem claims to have a great potential in predicting soil losses by ephemeral gully erosion it has never been thoroughly tested the objective of this study was to evaluate the suitability of egem for predicting ephemeral gully erosion rates in mediterranean environments an egem input data set for 86 ephemeral gullies was collected detailed measurements of 46 ephemeral gullies were made in intensively cultivated land in southeast spain guadalentin study area and another 40 ephemeral gullies were measured in both intensively cultivated land and abandoned land in southeast portugal alentejo study area together with the assessment of all egem input parameters the actual eroded volume for each ephemeral gully was also determined in the field a very good relationship between predicted and measured ephemeral gully volumes was found r2 0 88 but as ephemeral gully length is an egem input parameter both predicted and measured ephemeral gully volumes have to be divided by this ephemeral gully length in order to test the predictive capability of egem the resulting relationship between predicted and measured ephemeral gully cross sections is rather weak r2 0 27 therefore it can be concluded that egem is not capable of predicting ephemeral gully erosion for the given mediterranean areas a second conclusion is that ephemeral gully length is a key parameter in determining the ephemeral gully volume regression analysis shows that a very significant relation between ephemeral gully length and ephemeral gully volume exists r2 0 91 accurate prediction of ephemeral gully length is therefore crucial for assessing ephemeral gully erosion rates copyright 2001 john wiley sons ltd

secular variation as recorded in lake sediments a discussion of north american and european results type curves depicting secular variations in declination and inclination through holocene time have been constructed by stacking data from individual cores of lake sediment from east central north america and the u k there is a strong positive correlation in declination with zero phase shift froiti 0 to 4500 14 c years b p but from 4500 to 8500 14 c years b p the correlation is strongly negative which we interpret in terms of fluctuations in intensity of standing geomagnetic sources a source located under the atlantic ocean appears to have been dominant before ca 4500 14c years b p and then another similar source located either to the east or to the west of both observation areas gained dominance however the inclination type curves show optimum positive correlation for a phase shift of ca 400 years suggesting that the geomagnetic sources that caused them were drifting westward at ca 0 2 degrees a year there is no way of adjusting the timescales so as to obviate this apparent paradox secular variations of the total geomagnetic vector have been investigated by noting the sense clockwise anticlockwise of looping of the v g p paths though this cannot be interpreted uniquely in terms of westward or eastward drifting sources in the core as demonstrated by specific radial dipole models

application of the egem model to predict ephemeral gully erosion in sicily italy abstract recent research has shown the importance of ephemeral gully eg erosion in the context of global erosion and at the same time the lack of adequate models to estimate it at present the ephemeral gully erosion model egem is the only conceptual model specifically developed for ephemeral gully erosion estimation the main aim of the paper is to evaluate the performance of egem model adapted to local conditions compared with that of simple empirical equations a data set of 92 ephemeral gullies formed between 1995 and 2000 in a wheat cultivated study area in sicily italy was used for the purpose in order to improve egem performance two important adaptations were made both the rain distribution type and the causative rainfall of the hydrological component of egem were modified to reproduce local conditions the egem assumption of a constant depth for the entire length of the ephemeral gully was also modified egem performance was tested with two different rain distribution types three different causative rainfalls and two different eg depths the adaptation of the egem hydrological component improves the volume prediction with respect to the standard version of egem the improvement is mainly due to the rain distribution type whereas causative rainfall seems to be less important the use of adjusted eg depths is however essential to minimise the errors between measured and predicted volumes the capability of egem to predict the ephemeral gully cross section is weak and non significant relationships were found between measured and estimated width a regression analysis between ephemeral gully eroded volume and different morphological topographical and hydrological parameters confirms the great importance of eg length to explain eroded volume r 2 0 64 for the simple correlation between volume and length considering that the adaptation of the hydrological component and the measurement of eg depths are necessary for the eg volumes estimated by egem to be reliable it seems more simple as suggested by other authors to use empirical relations between eroded volume and length performed in different environments until more precise physically based models are developed

palaeosecular variation of the earth s magnetic field during the last 10000 years based on the annually laminated sediment of lake nautajarvi central finland a palaeomagnetic study of the lake nautajarvi sediment sequence in central finland demonstrates that varved lake sediments can provideregionally applicable secular variations curves for holocene stratigraphic correlation and age control based on x ray radiography and digital image analysis techniques we constructed an inherent and continuous varve chronology for the lake nautajarvi varve sequence which covers nearly 10000 years with an estimated counting error of less than 1 and a mean sedimentation rate of 0 66 mm yr 1 we reconstructed a record of the palaeomagnetic secular variation psv of the earth s magnetic field in this nearly 7 m long section of clastic organic varves mineral magnetic measurements indicated that pseudosingle domain magnetite is the major carrier of the remanence psv shows many of the familiar features of declination and inclination that have previously been recorded in northern europe in addition the lake nautajarvi record contains a continuous record of relative palaeoi

reply to stoppa et al 2011 comment on development of spheroidal composite bombs by welding of juvenile spinning and isotropic droplets inside a mafic eruption column

holocene geocentric dipole tilt model constrained by sedimentary paleomagnetic data paleomagnetic data although undeniably limited have been used to constrain complex time varying geomagnetic field models here we present four different versions of a time varying geocentric dipole model for the past approximately 9000 years based on a limited selection of sedimentary paleomagnetic data with good spatial coverage the robustness of the modeling approach is evaluated using modern magnetic observation station data provided by the world data centre for geomagnetism in edinburgh and we discuss the magnitude of the error introduced by insufficient data coverage the prediction in terms of directional variations is tested against paleomagnetic data from 63 different sedimentary sites and compared to results from recently published spherical harmonic geomagnetic field models we find that the dipole estimate can explain the paleomagnetic data almost as well as the more complicated spherical harmonic models specific areas and records are identified where there are significant differences between the performance of the dipole estimate and the spherical harmonic models our results suggest that many of the differences between the dipole model predictions and the paleomagnetic data can be related to poor paleomagnetic quality and or the lack of accurate and precise time control inherent to some of the records these results agree with valet et al 2008 who suggested that the cals7k 2 model probably underestimates the dipole contribution in order to reduce the misfit between the inversion and the data we conclude that better paleomagnetic data and associated time control are needed to improve global geomagnetic field models less

age and backfill overfill stratigraphy of two tufa dams tigray highlands ethiopia evidence for late pleistocene and holocene wet conditions abstract geomorphological mapping of parts of the mekele outlier reveals the presence of numerous river tufa dams spring tufas and old landslides indicating high groundwater levels and karstic activity in the area during former times u th dating of river tufa dams indicates that wet conditions prevailed in the highlands as early as 15 000 bp this is 4 to 5000 years before the first signs of post last glacial maximum lgm humid conditions deduced from former lake extensions in the ethiopian and kenyan rift tufa build up and the corresponding wet conditions in the highlands gradually came to an end from 5000 bp onwards in the study area environmental deterioration started around 3000 bp and is explained as a result of forest recession evidence is presented showing that forest retreat and land degradation in the area is not climate driven but the result of human activities

factors controlling the morphology and volume v length l relations of permanent gullies in the northern ethiopian highlands small scale aerial photographs and high resolution satellite images available for ethiopia since the second half of the twentieth century as for most countries allow only the length of gullies to be determined understanding the development of gully volumes therefore requires that empirical relations between gully volume v and length l are established in the field so far such v l relations have been proposed for a limited number of gullies environments and were especially developed for ephemeral gullies in this study v l relations were established for permanent gullies in northern ethiopia having a total length of 152 km in order to take the regional variability in environmental characteristics into account factors that control gully cross sectional morphology were studied from 811 cross sections this indicated that the lithology and the presence of check dams or low active channels were the most important controls of gully cross sectional shape and size cross sectional size could be fairly well predicted by their drainage area the v l relation for the complete dataset was v 0 562 l 1 381 n 33 r2 0 94 with 34 9 of the network having check dams and or being low active producing such relations for the different lithologies and percentages of the gully network having check dams and or being low active allows historical gully development from historical remote sensing data to be assessed in addition gully volume was also related to its catchments area a and catchment slope gradient sc this study demonstrates that v l and v a sc relations can be very suitable for planners to assess gully volume but that the establishment of such relations is necessarily region specific copyright 2013 john wiley sons ltd

the role of sidewall processes in gully development some n s w examples little attention has been paid to the role of sidewall processes in gully development simple estimates of linear incision sidewall erosion ratios at two localities in new south wales australia suggest that sidewall erosion is responsible for more than half the gully volume studies at three localities where overland flow and throughflow are limited to only one sidewall indicate that these processes are responsible for 10 30 per cent of gully volume these observations have important implications for gully management

ultramafic intrusion triggers hydrothermal explosions at colle fabbri spoleto umbria italy abstract the eroded colle fabbri volcano comprises intrusive and extrusive rocks which cover an area of about 10 000 m 2 in the umbria region central italy the outcrop is located at the sw boundary of the umbria valley graben umbria region on an n140 normal fault inside the intramountain ultra alkaline province of carbonatite and melilitite rocks of central southern italy a field survey of the outcrop allowed a reconstruction of igneous activity events of this unusual small scale volcano it is younger than 0 7 m y and rests on lower pleistocene conglomerates with intercalated clay beds a palaeosoil marks the base of the volcanic sequence it follows several metres of extrusive breccias composed of fragments of thermometamorphosed clay and travertine key features of these breccias are mud shells on blocks plastic mud lumps slumps and mechanical injection from one layer into another as well as plastic and vesiculated micro brecciated matrix the breccias are cemented by a variety of silicate sulphate and sulphate carbonate minerals deposited by intense hydrothermal circulation the breccias are related to phreatic explosions triggered by the eruption of a superficial hydrothermal system up to 1 m thick sill and a dykelet swarm intrude the breccias in the southern part of the outcrop there is a plug of melilitolitic composition which intrudes the breccias and deforms them upward a variety of contact breccias is also scattered around the sill and the plug in some places hyaloclastite formed when melt invaded water pockets contained in the encasing rocks under hydrostatic pressure conditions a thermal aureole which moves up to 10 m away from the contact is characterised by cordierite trydimite association thus indicating high temperature 1000 c contact phenomena

geomagnetic secular variation from recent lake sediments ancient fireplaces and historical measurements in southeastern australia abstract compilations of historical observations archaeomagnetic data from ancient fireplaces and palaemagetic results from short cores of sediment from lakes in southeastern australia particularly lake keilambete provide a detailed record of the geomagnetic secular variation during the last 3000 years the independent sets of data are in good agreement if the radiocarbon time scale for the lacustrine record is about 450 years too old the error is attributed to systematic incorporation of ancient carbon into the lake floor sediments mainly through erosion of sediment on the crater walls at times of low water level a significant lag between deposition and the acquisition of stable magnetic remanence is ruled out inclination has been abnormally steep during the last 500 years but remained fairly close to the axial dipole field value prior to that during the last 1000 years the predominant sense of looping of the magnetic vector corresponds to westward drift of the nondipole field secular variations on a time scale of 100 years can be resolved by the lacustrine record

dating environmental change using magnetic bacteria in archaeological soils from the upper thames valley uk an archaeomagnetic date has been recovered from an organic rich sediment at the base of an alluviated ditch section found during archaeological excavations near the village of yarnton oxford uk the magnetic minerals carrying this remanence include greigite and biogenic magnetite derived from magnetotactic bacteria that both indicate the presence of a micro aerobic environment it is proposed that the archaeomagnetic date from this material indicates the onset of floodplain conditions at the site due to increased population pressure and intensification of land clearance in the late british iron age

the tectonic significance of italian magmatism an alternative view to the popular interpretation the tyrrhenian rift zone has been the site of widespread magmatism since late tortonian times a pronounced asymmetrical distribution reflecting the tectonic structure characterizes italian magmatism sodic basalts occur on the western tyrrhenian flank and transitional morb basalts occur in the tyrhenian sea the eastern flank however is characterized by k alkaline and hk to ultra alkaline e g carbonatites and melilitites rocks major trace elements and isotopic compositions allow two major magmatic lineages to be identified one relating to a non radiogenic basaltic end member and the other to a mantle end member enriched in ca with high lile hfse ratio and high sr isotopic ratios their mantle sources are located within the lithosphere thermal boundary layer tbl and the metasomatized phlogopite carbonate asthenosphere at the base of the tbl respectively the composition and spatial distribution of volcanism and relative mantle sources tend to map the geometry of the lithospheric mantle and to define a pronounced increase in depth from less than 60 km to about 100 km across the boundary between the thinned tyrrhenian lithosphere and the adriatic lithosphere a mechanism of intra continental passive rifting which drives mantle upwelling is sufficient to satisfy the petrological and geochemical constraints and the observed tectonic environment without requiring a subduction plane

the matuyama brunhes boundary interval 500 900 ka in north atlantic drift sediments summary ocean drilling program odp sites 984 and 983 located on north atlantic sediment drifts provide high resolution records across the matuyama brunhes boundary at odp site 984 bjorn drift iceland basin the mean sedimentation rate in the 500 900 ka interval is 12 cm kyr 1 based on an age model derived by matching the planktonic and benthic oxygen isotope records to an ice volume model the matuyama brunhes polarity transition at site 984 as defined by virtual geomagnetic polar vgp latitudes 50 has an apparent duration of 7 kyr with a mid point at 773 5 ka compared with 772 5 ka at neighbouring site 983 outside the polarity transition at both sites 984 and 983 excursions in vgp latitudes to values 20 at 540 590 and 670 ka correspond to troughs in the palaeointensity record new u channel palaeomagnetic data across the matuyama brunhes boundary for working and archive halves of core sections from three holes at both sites 984 and 983 augment data published by channell lehman 1997 and are compared with back to back 1 cm 3 discrete samples clusters of vgps in the south atlantic and northeast asia in both u channel and discrete sample records imply that polarity transition fields have characteristics similar to the modern non axial dipole nad field

a delayed climatic response to solar forcing at 2800 cal bp multiproxy evidence from three irish peatlands multiproxy palaeohydrological records from three raised bogs in northern ireland indicate that a major shift to wetter cooler climatic conditions postdated the rapid decrease in solar activity at 2800 cal bp by 100 years this event is bracketed by two wiggle match radiocarbon dated cryptotephra layers in each profile enabling a high degree of chronological precision these replicated data corroborate previous findings based on irish peat humification profiles and may indicate spatial complexity in the climatic response to solar activity between oceanic and continental areas

linking long term gully and river channel dynamics to environmental change using repeat photography northern ethiopia abstract in the highlands of northern ethiopia gully occurrence is linked to poverty driven unsustainable use of the land in a vulnerable semi arid and mountainous environment where intensive rainfall challenges the physical integrity of the landscape trends in gully and river channel erosion and their relation to triggering environmental changes can proffer valuable insights into sustainable development in northern ethiopia in order to assess the region wide change in gully and river channel morphology over 140 years a set of 57 historical photographs taken in tigray and clearly displaying gully cross sections were precisely repeated from 2006 till 2009 ninety two percent of the gully and river sections n 38 increased in cross sectional area during the studied period especially after 1975 two repeatedly photographed catchments of lake ashenge and atsela allowed a detailed study of gully development from 1936 until 2009 a conceptual hydrogeomorphic model was devised for these catchments and validated for the northern ethiopian highlands three major phases can be distinguished in the hydrological regime of the catchments in the first phase between 1868 or earlier and ca 1965 the relatively stable channels showed an oversized morphology inherited from a previous period when external forcing in environmental conditions had caused the channels to shape in the second phase ca 1965 ca 2000 increased aridity and continued vegetation clearance accelerated the channel dynamics of the gully and river system the third phase ca 2000 present started after the large scale implementation of soil and water conservation measures in 2009 23 of the gully and river sections were stabilizing this paper validates previous research indicating severe land degradation in the second half of the 20th century additionally it demonstrates that the recent erosive cycle started around 1965 and that at the present time improved land management stabilizes headwater streams

pleistocene mass flow deposits of basaltic hyaloclastite on a shallow submarine shelf south iceland a pleistocene subaqueous volcanic sequence in south iceland consists of flows of basaltic hyaloclastite and lava with interbedded sedimentary diamictite units emplacement occurred on a distal submarine shelf in drowned valleys along the southern coast of iceland the higher sea level was caused by eustatic sea level change probably towards the end of a glaciation this sequence nearly 700 m thick rests unconformably on eroded flatlying lavas and sedimentary rocks of likely tertiary age a standard depositional unit describing the flows of hyaloclastite starts with compact columnar jointed basalt overlain by cubejointed basalt and or pillow lava this in turn is overlain by thick unstructured hyaloclastite containing aligned basalt lobes and bedded hyaloclastite at the top a similar lithofacies succession is valid for proximal to distal locations the flows were produced by repeated voluminous extrusions of basaltic lava from subaquatic fissures on the eastern rift zone of iceland the fissures are assumed to lie in the same general area as the 1783 laki fissure which produced 12 km3 of basaltic lava due to very high extrusion rates the effective water melt ratio was low preventing optimal fragmentation of the melt the result was a heterogeneous mass of hyaloclastite and fluid melt which moved en masse downslope with the melt at the bottom of the flow and increasingly vesicular hyaloclastite fragments above the upper and distal parts of the flow moved as low concentration turbulent suspensions that deposited bedded hyaloclastite

monitoring of gully erosion in the central ebro basin by large scale aerial photography taken from a remotely controlled blimp large deep gullies span barrancos are some of the most important sediment sources in the semi arid environment of the central ebro basin they are incised into the quaternary valley bottoms span vales which are characteristic landforms in this area in the research project eprodesert evaluation of processes leading to land degradation and desertification under extensified farming systems the development of a large barranco system is being investigated by different methods including documentation and monitoring by aerial photography geomorphological forms and processes such as sheet wash rill and gully erosion cannot be documented sufficiently by conventional remote sensing methods spatial and temporal resolution of satellite sensors as well as of conventional aerial photography do not correspond to the scale and dynamics of geomorphological processes with a specially designed hot air blimp as a sensor platform large scale aerial photographs were obtained from the barranco de las lenas specifically aimed at the scientific demands very high spatial and temporal resolution the development of the gully is documented and its dynamics are evaluated by a sequence of six aerial photographs taken between 1995 and 1998 d 2003 elsevier science b v all rights reserved

geomagnetic palaeosecular variation 0 13 000 yr bp as recorded in sediments from three maar lakes from the west eifel germany summary geomagnetic palaeosecular variation psv profiles have been obtained from 12 cores raised from three maar lakes in the west eifel germany age control is provided by varve chronology analysis of mean inclinations suggests that a post depositional remanent magnetization pdrm was acquired in all three lakes a lock in time of 150 yr corresponding to an average lock in depth of 24 cm has been determined from a comparison with archaeomagnetic sv data stacked psv results demonstrate directional geomagnetic field behaviour through the late and post glacial in central europe and show good agreement with the holocene british master curve turner thompson 1982 psv results suggest that westward drift of the non dipole field was not persistent over the last 13 000 yr spectral analyses yield declination variations with durations of about 1800 yr in the younger holocene 0 5000 yr bp and 4300 yr between 5000 and 13 000 yr bp the inclination exhibits swings of 1000 yr duration in the younger holocene and about 2000 yr in the older part moreover inclination spectra suggest the presence of one inclination variation with a very long duration approaching the length of the time span investigated 8600 yr

effects of land use changes in vegetation cover and sidewall erosion in a gully head of the penedes region northeast spain the present research addresses the effects of land use changes in the evolution of vegetation cover on gully erosion in the penedes region of northeast spain the contribution of vegetation cover to gully wall stabilization is also discussed the catchment area of the gully is 60 2 ha and is mainly composed of vineyards in this area large gullies have developed on tertiary marls and unconsolidated sandstones detailed aerial photographs 1 5000 and 1 7000 scale of three dates 1975 1995 and 2002 were ortho rectified to derive land use information in the catchment gully vegetation cover and height information to construct detailed digital elevation models land use and gully vegetation cover mapping of the most recent period was assisted by means of fieldwork and a photo interpretation key constructed to help map the older dates soil loss in the catchment was estimated for the 1975 and 2002 scenarios to determine the relations between land use changes to vegetation cover changes and gully sidewall erosion the revised universal soil loss equation was applied for that purpose sidewall mass movements were determined by subtraction of digital elevation models 1 m resolution of each period the results show important land use changes in the catchment due to field reorganization for the plantation of mechanized vineyards it increased soil loss from agricultural fields by 26 5 which influenced vegetation cover dynamics on gully walls with an important development of scrubland to forest and gully erosion with sidewall retreat at rates of 0 025 to 0 50 ha y 1 and mass movements at rates of 16 to 63 2 mg ha 1 y 1 these results indicate that vegetation cover on gully walls although suggesting an improvement with respect to wall protection against splash sheet and rill erosion did not affect sidewall retreat and mass movements copyright 2009 john wiley sons ltd

geomagnetic paleointensity between 1300 and 1750 a d derived from a bread oven floor sequence in lubeck germany geomagnetic paleointensities have been determined from a single archaeological site in lubeck germany where a sequence of 25 bread oven floors has been preserved in a bakery from medieval times until today age dating confines the time interval from about 1300 a d to about 1750 a d paleomagnetic directions have been published from each oven floor and are updated here the specimens have very stable directions and no or only weak secondary components the oven floor material was characterized rock magnetically using thellier viscosity indices median destructive field values curie point determinations and hysteresis measurements magnetic carriers are mixtures of sd psd and minor md magnetite and or maghemite together with small amounts of hematite paleointensity was measured from selected specimens with the double heating thellier method including ptrm checks and determination of trm anisotropy tensors corrections for anisotropy as well as for cooling rate turned out to be unnecessary ninety two percent of the thellier experiments passed the assigned acceptance criteria and provided four to six reliable paleointensity estimates per oven floor mean paleointensity values derived from 22 oven floors show maxima in the 15th and early 17th centuries a d followed by a decrease of paleointensity of about 20 until 1750 a d together with the directions the record represents about 450 years of full vector secular variation the results compare well with historical models of the earth s magnetic field as well as with a selected high quality paleointensity data set for western and central europe

holocene paleomagnetic secular variation from east china sea and a psv stack of east asia abstract paleomagnetic secular variation psv provides information on the mechanisms of the geodynamo and can also be used for stratigraphic correlation on a regional scale in this study we constructed psv curves of relative rapidly deposited holocene marine sequence from east china sea results show that the dominant magnetic carriers are pseudo single domain psd to multidomain md magnetites paleomagnetic directions are preserved by coarse grained magnetite even affected by post depositional diagenetic alteration the psv record of core md06 3040 exhibits six major inclination peaks and five declination swings during the holocene the periodicities for inclination are about 2000 and 550 years and for declination are about 1400 and 500 years the psv of md06 3040 is comparable to archeomagnetic and historic results for the last 2500 years and to psv results from lake sediments in southern china and japan during the holocene a psv stack of east asia is constructed by sedimentary and archeological psv results which can be used as an important reference curve for a large region and the psv correlation between sites can be applied for relative dating of east asia

comment on melluso et al 2003 reported data and interpretation of some wollastonite and melilite bearing rocks from the central apennines of italy two distinct occurrences of wollastonite and melilite bearing rocks from ricetto and colle fabbri which are located in the central italy apennine range are referred to as paralavas generated by melting and recrystallization of marly sediments likely due to coal fires by melluso et al 2003 we submit data demonstrating that these conclusions are incorrect ricetto is clearly pyrometamorphic as described by capitanio et al 2001 2004 colle fabbri is clearly an igneous outcrop as described in stoppa 1988 it is quite different in scale field relationships and chemistry and thus ricetto is irrelevant to its petrogenesis the sediment mixing model of melluso et al 2003 does not relate to the colle fabbri field data furthermore the ricetto miocene flysch country rock and the colle fabbri enclosing pliocene clays were not analyzed by melluso et al 2003 a limestone shale mixing calculation using sr and nd isotopic data in melluso et al 2003 tables 3 4 and a 1 4 3nd 1 4 4 nd ratio of 0 51213 for apennine shales op cit p 1297 fails to account for the colle fabbri samples which remain well clear of the hypothetical mixing line rocks of mantle origin including micaceous kimberlites and western australia lamproites plot in the variation field of colle fabbri and regionally associated igneous rocks colle fabbri is but one of a series of similar igneous melilite bearing and carbonatite occurrences that constitute the intramontane ultra alkaline province of central italy iup although colle fabbri does have unique features as do other of these occurrences stoppa et al 2003

the direction of the earth s magnetic field at london 1570 1975 the observations of magnetic declination and dip made in london over the past 400 years comprise one of the longest and most complete series of such data for anywhere in the world they give an indication of secular change on a time scale intermediate between that of recent observatory measurements and of archaeomagnetic measurements we have attempted to catalogue all direct measurements of declination and dip paying particular attention to completeness in the earlier years and tracing the data to their original source wherever possible sufficient details of the observations are given for an assessment of their reliability and accuracy the data are then corrected to a single site fitted with a continuous smooth curve and discussed

human impact on the environment in the ethiopian and eritrean highlands a state of the art abstract this review analyses the environmental evolution of the ethiopian highlands in the late quaternary the late pleistocene 20 000 12 000 14c years bp was cold and dry with 1 low lake levels in the rift valley 2 large debris fans on the flanks of lake abhe basin and 3 the blue nile transporting coarse bedload then a period with abundant and less seasonal rains existed between 11 500 and 4800 14c years bp as suggested by increased arboreal pollen high river and lake levels low river turbidities and soil formation around 5000 4800 14c years bp there was a shift to more arid conditions and more soil erosion many phenomena that were previously interpreted as climate driven might however well be of anthropic origin thick sediment deposits on pediments as well as an increase of secondary forest scrub and ruderal species in pollen diagrams are witnesses of this human impact one important aspect of the late quaternary palaeoenvironment is unclear changes in nile flow discharges and rift valley lake levels have been linked to changes in precipitation depth most authors do not take into account changes in land use in the highlands nor changes in the seasonality of rain both of which can lead to a change in runoff coefficients tufa and speleothem deposition around 14 000 years ago tend to indicate that at the end of the last glacial maximum lgm conditions might have been wetter than generally accepted the most important present day geomorphic processes are sheet and rill erosion throughout the country gullying in the highlands and wind erosion in the rift valley and the peripheral lowlands based on existing sediment yield data for catchments draining the central and northern ethiopian highlands an equation was developed allowing to assess area specific sediment yields sy 2595a 0 29 n 20 r 2 0 59 where sy area specific sediment yield t km 2 year 1 and a drainage area km2 with respect to recent environmental changes temporal rain patterns apart from the catastrophic impact of dry years on the degraded environment cannot explain the current desertification in the driest parts of the country and the accompanying land degradation elsewhere causes are changing land use and land cover which are expressions of human impact on the environment deforestation over the last 2000 3000 years was probably not a linear process in ethiopia studies on land use and land cover change show however a tendency over the last decades of increasing removal of remnant vegetation which is slowed down or reversed in northern ethiopia by a set aside policy ongoing land degradation requires urgent action at different levels of society soil and water conservation swc structures are now widely implemented local knowledge and farmer s initiatives are integrated with introduced swc techniques at various degrees impact assessments show clear benefits of the soil conservation measures in controlling runoff and soil erosion in high rain areas runoff management requires greater emphasis during the design of soil conservation structures in such areas investment in swc might not be profitable at farm level although benefits for society are positive this pleads in favour of public support the present land degradation in the ethiopian highlands has a particular origin which includes poverty and lack of agricultural intensification causes of these are to be found in the nature of past and present regional social relations as well as in international unequal development this review strengthens our belief that under improved socio economic conditions land husbandry can be made sustainable leading to a reversal of the present desertification and land degradation of the ethiopian highlands

measurement of the degree of smoothing in sediment paleomagnetic secular variation records an example from late quaternary deep sea sediments of the bermuda rise western north atlantic ocean abstract replicate paleomagnetic secular variation psv records for the last 15 000 yr have been recovered from two deep sea sediment gravity cores on the bermuda rise western north atlantic ocean the records have been correlated using several different parameters and dated by correlation to a nearby piston core which has a high resolution 14 c chronology the psv records are systematically subdued with respect to the expected local magnetic field variability during the holocene when sedimentation rates were about 10 cm kyr during the late pleistocene when sedimentation rates were a factor of 2 3 higher the psv variability was significantly larger we attribute this to smoothing of the psv records by a depositional post depositional remanence drm pdrm acquisition process with a 10 20 cm lock in interval the degree of psv smoothing was evaluated by comparing the vgp angular dispersion and vector spectral content of the bermuda rise cores with high resolution psv records from north america and great britain these results suggest a 50 reduction in the drm pdrm recording of vector variability during the holocene on the bermuda rise with most of the psv reduction occurring for features of less than 2000 yr in duration we can reproduce the subdued psv pattern with a mathematical model that simulates the site drm pdrm remanence acquisition process finally the replicate gravity cores document that psv records may be reproducible on a local scale even though they only retain a low pass filtered record of true magnetic field variability

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

the value of a physically based model versus an empirical approach in the prediction of ephemeral gully erosion for loess derived soils abstract a data set on soil losses and controlling factors for 58 ephemeral gullies has been collected in the belgian loess belt from march 1997 to march 1999 of the observed ephemeral gullies 32 developed at the end of winter or in early spring winter gullies and 26 ephemeral gullies developed during summer summer gullies the assessed data have been used to test the physically based ephemeral gully erosion model egem and to compare its performance with the value of simple topographical and morphological indices in the prediction of ephemeral gully erosion analysis shows that egem is not capable of predicting ephemeral gully cross sections well although conditions for input parameter assessment were ideal some parameters such as channel erodibility critical flow shear stress and local rainfall depth showed great uncertainty rather than revealing egem s inability of predicting ephemeral gully erosion this analysis stresses the problematic nature of physically based models since they often require input parameters that are not available or can hardly be obtained with respect to the value of simple topographical and morphological indices in predicting ephemeral gully erosion this study shows that for winter gullies and summer gullies respectively over 80 and about 75 of the variation in ephemeral gully volume can be explained when ephemeral gully length is known moreover when previously collected data for ephemeral gullies in two mediterranean study areas and the data for summer gullies formed in the belgian loess belt are pooled it appears that one single length l volume v relation exists v 0 048 l 1 29 r 2 0 91 these findings imply that predicting ephemeral gully length is a valuable alternative for the prediction of ephemeral gully volume a simple procedure to predict ephemeral gully length based on topographical thresholds is presented here secondly the empirical length volume relation can also be used to convert ephemeral gully length data extracted from aerial photos into ephemeral gully volumes

holocene sediment dynamics in an upland temperate lake catchment climatic and land use impacts in the english lake district accelerated erosion and transport of fine sediment from upland temperate catchments can reflect increased erosivity and or erodibility due in turn to climatic and or human forcing identification of sediment fluxes and sources over holocene timescales can both enable understanding of the relative impacts of these forcings and provide perspective on recent sediment fluxes here we present a 5500 yr record of sediment fluxes and sources from lake bassenthwaite in the english lake district utilizing magnetic measurements and fuzzy clustering coupled with independent pollen and archaeological records to identify the timing and impact of catchment disturbance this record shows that recent sediment flux increases ie within the last 150 years are unprecedented in scale throughout the mid late holocene and appear to be in response to specific human changes occurring within the catchment earlier episodes of human activity from the mid holocene onwards show no link with increased lake sediment fluxes

palaeomagnetic dating of late holocene sediments in fennoscandia abstract palaeomagnetic secular variation curves that have been dated by varve analysis counting annual laminations are used to date four holocene lacustrine sediment sequences from central finland and northwest russia lat 62 66 n several fine scale geomagnetic declination and inclination fluctuations can be identified during the last 3200 yr which is the most accurately dated interval in the present data relative palaeomagnetic dating is a powerful dating tool for sediments of large and deep lakes in northern europe which are difficult to date using the 14 c method because of low organic content contamination by older carbon and or scarcity of terrestrial macrofossils suitable for ams 14 c dating the dating error during the last 3200yr is estimated to be less than 50 yr such accuracy is not readily attainable using other methods

correlation of paleointensity variation records in the brunhes matuyama polarity transition interval abstract paleomagnetic records of the 0 78 ma brunhes matuyama b m polarity reversal interval described from high resolution 8 11 cm kyr deep sea sediments from western equatorial pacific odp sites 767b and 769a show two successive and marked decreases in paleointensity of about 15 kyr wavelength each and a shift from full reversed to fuli normal polarity directions in only 2 kyr associated with the uppermost paleointensity decrease in contrast transition records from deep sea sediments with lower sedimentation rates tend to show simpler paleointensity variation with more complex patterns of directional change for example the b m transition record described from deep sea sediment core v16 58 from the southern indian ocean we suggest that the twin 10 be peaks reported in v16 58 correspond to increased production of this cosmogenic isotope associated with the double paleointensity dips recorded in odp 767b 769a the low resolution v16 58 paleomagnetic record can be accounted for by modeling of the geomagnetic field history in odp 767b at a nominal recording rate of 1 cm kyr and using an exponential lock in process filter with a half fixing depth of 16 cm the double paleointensity dips become a single broad magnetization low and the change in paleomagnetic directions is offset by about 16 cm below the actual level of the polarity transition similar to what we infer for v16 58 the low resultant magnetizations will be especially prone to overprinting which if in the present field will tend to give an americas vgp path the main features of the 10 be profile will be unaffected by the remanence lock in process and consequently retain their usefulness for correlation as a proxy of geomagnetic dipole intensity variation

geomagnetic field for 0 3 ka 1 new data sets for global modeling 1 paleomagnetic and archeomagnetic records are used in both regional and global studies of earth s magnetic field we present a description and assessment of five newly compiled data sets also used in the companion paper by korte et al 2009 to produce a series of time varying spherical harmonic models of the geomagnetic field for the last 3000 years data are drawn from our compilation of lake sediment records and from the online database geomagia50v2 the five selections are available from the earthref digital archive at http earthref org cgi bin erda cgi n 944 data are grouped according to the source of material and we conducted separate assessments of reliability for archeomagnetic artifacts and lava flows the arch3k dat data set and for sediments sed3k dat the overall number of data is 55 greater than in previous compilations constrained data sets were selected using different criteria for each group winnowing of archeological data was based on uncertainties supplied by the original data providers the lake sediment data assessment relied on preassigned age uncertainties and one or more of the following comparisons with archeomagnetic data from the same region regional consistency among several lakes and consistency with global archeomagnetic models we discuss relative merits of a larger unconstrained data set or a smaller possibly more reliable one the constrained data sets eliminate a priori up to 35 of the available data in each case and rely on potentially subjective assessments of data quality given the limited data available our analyses indicate that iterative rejection of a small number 1 1 5 of outlying data during global field modeling is a preferable approach specific regional comparisons among the models and data support the conclusion that korte et al s outlier free cals3k 3 model based on all available measurements from sediments and archeological artifacts currently provides the best global representation of the 0 3 ka field the arch3k 1 model provides a better fit to the denser european archeomagnetic data and may be better in that region

limitations on stratigraphic analyses due to incomplete age control and their relevance to sedimentary paleomagnetism abstract a major limitation in the analysis of physical quantities measured from a stratigraphic core is incomplete knowledge of the depth to age relationship for the core records derived from diverse locations are often compared or combined to construct records that represent a global signal time series analysis of individual or combined records is commonly employed to seek quasi periodic components or characterize the timescales of relevant physical processes assumptions that are frequently made in the approximation of depth to age relationships can have a dramatic and harmful effect on the spectral content of records from stratigraphic cores a common procedure for estimating ages in a set of samples from a stratigraphic core is to assign based on complementary data the ages at a number of depths tie points and then assume a uniform accumulation rate between the tie points imprecisely dated or misidentified tie points and naturally varying accumulation rates give rise to discrepancies between the inferred and the actual ages of a sample we develop a statistical model for age uncertainties in stratigraphic cores that treats the true but in practice unknown ages of core samples as random variables for inaccuracies in the ages of tie points we draw the error from a zero mean normal distribution for a variable accumulation rate we require the actual ages of a sequence of samples to be monotonically increasing and the age errors to have the form of a brownian bridge that is the errors are zero at the tie points the actual ages are modeled by integrating a piecewise constant randomly varying accumulation rate in each case our analysis yields closed form expressions for the expected value and variance of resulting errors in age at any depth in the core by monte carlo simulation with plausible parameters we find that age errors across a paleomagnetic record due to misdated tie points are likely of the same order as the tie point discrepancies those due to accumulation rate variations can be as large as 30 kyr but are probably less than 10 kyr we provide a method by which error estimates like these can be made for similar stratigraphic dating problems and apply our statistical model to an idealized marine sedimentary paleomagnetic record both types of errors severely degrade the spectral content of the inferred record we quantify these effects using realistic tie point ages their uncertainties and depositional parameters

geomagnetic secular variations in greece through the last 6000 years obtained from lake sediment studies summary geomagnetic declination and inclination secular variation curves have been constructed back to 6000 yr bp from palaeomagnetic measurements of sediments cored in lakes volvi trikhonis and begoritis greece they have been dated by palynological studies and by comparison with observatory and archaeomagnetic records radiocarbon ages are all affected by the old carbon factor cores from all three lakes contain a thin ash layer which we associate with the pompeii eruption of vesuvius in ad 79 averaged curves of secular variations of declination and inclination have been computed for the whole study the broader scale pattern of secular variations is basically similar to lake sediment records from the uk poland and switzerland though finer detail in the structure of some of the principal features is resolved

hydrothermal eruptions of nisyros dodecanese greece past events and present hazard abstract the detailed analysis of the craters of hydrothermal eruptions and related products present on nisyros island demonstrates the ephemerality of these morphological forms in other words the mere recognizable existence of the craters and associated deposits implies recency of hydrothermal activity the minimum temperature required to cause the explosive phenomenon and possibly the depth of the reservoir which can be evaluated on the basis of the correlation between the diameter of the crater and the depth of explosion as proposed by fytikas and marinelli 1976 are therefore closely representative of the current hydrothermal circulation both field evidence and historical records indicate that all the deposits of hydrothermal eruption recognized on nisyros island were emplaced as debris flows almost all the ballistic ejecta were entrained in these debris flows and either redeposited far from their landing sites or involved in later crater collapse and erosion this emplacing mechanism implies that the original products were characterized by a water content higher than about 5 by weight steam driven hydrothermal eruptions one of which took place in 1871 originated deposits of limited dispersion as no sign of these erodible products can be found in the field today surface geology and fluid geochemistry together with subsurface information e g primary and hydrothermal lithologies distribution of temperature with depth physical chemical characteristics of deep water bearing zones indicate that two distinct hydrothermal aquifers are present underneath the southeastern part of the caldera floor both aquifers were probably involved in the most important historically documented hydrothermal eruptions which occurred in 1873 at that time violent earthquakes fractured the brittle aquiclude separating the two aquifers and caused a sudden transfer of fluids from the deep to the shallow aquifer thus triggering the hydrothermal eruptions hydrothermal eruptions will probably occur in future and this hazard must be taken into serious consideration the southern half of lakki plain where all past eruptions took place and active fumaroles are concentrated is the zone at highest risk at present gas geochemistry represents an effective tool to detect changes in the p t conditions of the shallow aquifer and particularly the phenomena of pressure build up that may lead to a hydrothermal eruption

a comparison of geomagnetic secular variation as recorded by historical archaeomagnetic and palaeomagnetic measurements palaeomagnetic methods can extend the documentary record of changes in the earth s magnetic field far into the past tolerable agreement is found between various methods demonstrating the geophysical value of palaeomagnetic experiments combining results from the different approaches of investigating secular change can lead to a better perspective and to superior models of geomagnetic field behaviour lake sediments have recently been found to hold remarkably detailed signatures of past field changes a mathematical approach to formulating an empirical description of global geomagnetic field behaviour is proposed and applied to palaeomagnetic data spanning the last 10 ka

a matlab tool for archaeomagnetic dating a matlab tool for archaeomagnetic dating has been developed in this work well dated palaeosecular variation curves psvcs can be used to date archaeological artefacts with unknown ages the archaeomagnetic direction declination and or inclination and the archaeointensity obtained from the archaeological artefact are compared with a master psvc in addition historical lava flows with controversial ages can be dated using this methodology the dating process follows the descriptions given by lanos 2004 which is based on the combination of temporal probability density functions of the three geomagnetic field elements here we develop an interactive tool in matlab code to carry out archaeomagnetic dating by comparing the undated archaeomagnetic or lava flow data with a master psvc the master psvcs included with the matlab tool are the different european bayesian curves and those generated using both regional and global geomagnetic field models a case study using all the psvcs available in europe and some undated archaeomagnetic data has been carried out to analyze how the different psvcs affect the dating process in addition the dating uncertainty and the relocation error have been analyzed in the european region results show that some regional bayesian psvcs and the regional scha dif 3k archaeomagnetic model are the best choices to obtain an accurate date in europe moreover when it is available the full geomagnetic field vector must be used for archaeomagnetic dating

regional modeling of the geomagnetic field in europe from 6000 to 1000 b c we have developed a first low degree regional geomagnetic model for the european continent valid for the period 6000 1000 b c from a selected compilation of sedimentary and archeomagnetic data the scha dif 8k model this model provides information about both direction declination and inclination and intensity of the earth s magnetic field by connecting it with our previous model scha dif 3k valid from 1000 b c to 1900 a d and the igrf we furnish continuous geomagnetic field information for the last 8000 years in europe it has been developed using the revised spherical cap harmonic analysis in 2 dimensions technique r scha2d and using the norm of the earth s magnetic field to constrain the inversion problem the size of the cap is 22 and the maximum degree of the expansion is 2 the linearization problem was solved by using the truncated taylor s series applied to the expressions representing the relationship between the declination inclination and intensity data and the cartesian components of the geomagnetic field we used the geocentric axial dipole gad field as our initial or reference field for time we used the classical sliding overlapping window method the size of the window was set to 100 years shifted by 50 years we compared the model s prediction with the input data with the global cals7k 2 model and with new independent data the regional model shows a better fit to the input and to the independent data than the global model especially in terms of intensity and agrees with the virtual axial dipole moment given by other studies for the last 8000 years the european geomagnetic field has recorded rapid changes or archeomagnetic jerks the average field for the last 8000 years in europe is indistinguishable from the gad field

dating the glen garry tephra a widespread late holocene marker horizon in the peatlands of northern britain the distal icelandic tephra known as the glen garry layer has been dated by ams 14c at eight ombrotrophic peat bogs from northern england and scotland the geochemistry of all the tephra layers is consistent with there being only one glen garry tephra rather than two or more from eruptions of the same volcanic source at seven of the sites all in scotland the tephra layer was bracketed by 1 cm contiguous samples below at and above the horizon at the one english site walton moss cumbria a suite of 16 ams dates was applied to a 116 cm section of a peat core containing the tephra the resulting dates were calibrated and wiggle matched using the bcal and bpeat programmes to give an estimated age for the glen garry tephra of 2176 cal bp with a 2 range of 2210 1966 cal bp this 244 year range is an improvement on the 410 years calibrated date range for the date without the wiggle match this age estimate will allow future work on peat profiles and other sediments containing the tephra to use the date as a pinning point in age depth models and obviate the need for radiocarbon dating at this period

comment on melluso et al 2003 the ricetto and colle fabbri wollastonite and melilite bearing rocks of the central apennines italy the bimodal rock of ricetto consists of a felsic glassy component which prevails volumetrically at the top of the rock and mafic spherules of acicular diopside anorthite wollastonite pseudomorph after melilite which are concentrated at the bottom of the rock this texture clearly indicates liquid immiscibility which can be modeled in the compositional space sio 2 al 2 o 3 mgo cao and al 2 o 3 cao feo t mgo the spinifex and dendritic forms are due to quenching from a melt at temperature of 1000 1100 c melilite was the first phase to crystallize and the subsequent rapid cooling caused its breakdown to a more stable assemblage the strict chemical similarity between the rock under study and the surrounding siliciclastic sandstones together with the results of 1 4 c radiometric analysis on coal fragments found at bottom of the convex central part of the body demonstrate that the rock of ricetto is a man made pyrometamorphic product the genetic attribution of the colle fabbri occurrence to pyrometamorphic phenomena somewhat similar to ricetto is difficult to be asserted as the crystallization history at colle fabbri has been much more complicated than at ricetto the common presence of melilite and wollastonite cannot account for a direct comparison between the ricetto and colic fabbri rocks as conspicuous mineralogic petrologic and geologic differences exist between these two occurrences

predicting gully width and widening rates from upstream contribution area and rainfall a case study in sw spain abstract gully width w is usually characterized by having a power relation with discharge however calculating runoff discharge in ungauged basins where these gullies typically form is inherently difficult and associated with large uncertainties this paper examines a simplified approach to relating gully width directly to the runoff contributing area a in a cultivated area in sw spain the effect of rainfall and land use on this w a relation was also analysed gully width was measured in 46 sections through the analysis of 10 historical orthophotos between 1956 and 2013 in a 21 km2 catchment in s spain these were validated by field measurements in 2013 widening rates varied strongly over time at between 0 and 2 19 m year 1 no significant differences in the gully widening rates were found between the two land uses present in the study zone olive groves and herbaceous crops the data obtained show a significant power relation of the form w a for all time periods analysed except for 1980 when many gullies were filled in artificially due to a change in land use in the study area the power relation coefficient varied at between 0 2 and 0 3 a good correlation was obtained between the number of days with daily rainfall of above 13 mm and the increase in the coefficient over time the results of this study on gully width dynamics provide new insights into improving the estimation of gully volume for example in combination with gully headcut retreat models and including the effects of different climate scenarios

continuous global geomagnetic field models for the past 3000 years abstract several global geomagnetic field models exist for recent decades but due to limited data availability models for several centuries to millennia are rare we present a continuous spherical harmonic model for almost 3 millennia from 1000 b c to 1800 a d based on a dataset of directional archaeo and paleomagnetic data and axial dipole constraints the model named continuous archaeomagnetic and lake sediment geomagnetic model for the last 3k years cals3k 1 can be used to predict both the field and secular variation comparisons and tests with synthetic data lead to the conclusion that cals3k 1 gives a good general large scale representation of the geomagnetic field but lacks small scale structure due to the limited resolution of the sparse dataset in future applications the model can be used for comparisons with additional new data for that time span for better resolved regions the agreement of data with cals3k 1 will provide an idea about the general compatibility of the data with the field and secular variation in that region of the world for poorly covered regions and time intervals we hope to iteratively improve the model by comparisons with and inclusion of new data animations and additional snapshot plots of model predictions as well as the model coefficients and a fortran code to evaluate them for any time can be accessed under http www mahi ucsd edu cathy holocene holocene html the whole package is also stored in the earthref digital archive at http www earthref org

post depositional remanent magnetization lock in and the location of the matuyama brunhes geomagnetic reversal boundary in marine and chinese loess sequences article i nfo pdrm lock in loess paleosol bioturbation disturbs detrital magnetic particles after deposition so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post depositional remanent magnetization pdrm whereby the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle the relationship between the matuyama brunhes boundary mbb and oxygen isotope age tie points in marine sediments has been widely used to determine the depth at which the paleomagnetic signal is locked in however such analyses can be badly affected by age discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location it is therefore necessary to separately compare paleomagnetic data with respect to either benthic or planktonic foraminiferal oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis when the global data set is subjected to such a rigorous analysis few reliable data remain for the mbb using two complementary approaches we estimate that the mbb is on average shifted 20 cm below its true position in marine sediments this offset is the sum of the thickness of the bioturbated surface mixed layer which is possibly dominant and the pdrm lock in depth there is also controversy concerning observed differences in the position of the mbb relative to paleoclimatic proxies in marine sediments and chinese loess deposits for the chinese loess quartz grain size is insensitive to pedogenic alteration and is a useful parameter for determining the true position of the mbb with respect to paleoclimatic boundaries we conclude that the mbb occurs late in marine oxygen isotope stage 19 and in the upper part of chinese paleosol unit s8 rather than at the mid or lower part of loess unit l8 our results require adjustment of the generally accepted positions for the mbb and resolve a longstanding chronological conundrum for marine and chinese loess sequences

factors related to gully erosion in woody encroachment in south eastern australia abstract gullies initiate when the equilibrium within a drainage line is upset by increased discharge or decreased soil resistance to detachment and transport of soil particles and is triggered and accelerated by human induced changes to vegetation cover woody encroachment is the increase in density cover extent and biomass of woody plants and can result in low ground cover which increases the potential of soil erosion this paper reports the relationships between gully erosion and site characteristics such as topography vegetation cover and road infrastructure in a 120 km study area of semi arid woodland subject to woody encroachment in south eastern australia this study characterised gully morphology and activity identified the factors related to gully volume and estimated the topographic thresholds for sub catchments subject to woody encroachment firstly relationships between gully volume activity topography vegetation cover and road infrastructure were explored secondly site characteristics of 32 sub catchments with and without gullies were compared finally the topographic thresholds for unstable and stable sub catchments with and without gullies respectively were estimated using the slope drainage area relationship s aa b all gullies were active and high gully volume was related to large sub catchments long gullies and short distances to roads stable sub catchments without gullies had gentler slopes higher foliage projective cover and ground cover than unstable sub catchments with gullies when data for sub catchments with and without gullies were combined the main factors related to gully volume were drainage area at the gully or stream heads and their local slope and ground cover the mean topographic threshold for gully development provided insights into the drainage area and slope conditions needed for gully development gully erosion in the study area is most likely the result of interactions among topography vegetation and human made structures roads in space and time the results of the study advance our understanding of gully erosion dynamics in areas of woody encroachment and can be used to identify areas susceptible to gullying

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

depth of post depositional remanence acquisition in deep sea sediments a case study of the brunhes matuyama reversal and oxygen isotopic stage 19 1 although post depositional magnetization pdrm in deep sea sediments appears to be acquired during the earliest stages of sediment compaction the natural variability of the pdrm lock in depth in deep sea sediments is poorly understood and as yet unquantified here we consider variations in the relative stratigraphic positions of oxygen isotopic interglacial stage 19 1 and the brunhes matuyama b m chronozone reversal for eight deep sea sediment cores results from a similar study of the displacement between a widespread microtektite layer and the b m boundary are also included 1 the pdrm lock in depth and the temporal relationships between the b m and stage 19 1 datums can be determined from the offsets between the paleomagnetic and the isotopic and microtektite stratigraphies for cores with sedimentation rates greater than 1 cm kyr a the depth offset between the paleomagnetic and isotopic datums is a linear function of sedimentation rate a simple model r 2 0 77 demonstrates that 1 pdrm acquisition occurs 16 cm below the sediment surface and 2 the b m reversal occurs 6 kyr 4 2 kyr after the stage 19 1 datum and the stage 19 1 datum occurs 9 kyr 4 3 kyr after the deposition of the australasian microtektite strewnfield this example argues against simple geophysical models linking geomagnetic field reversals to climate change or impact events the b m boundary is anomalously deep 30 50 cm in very low accumulation rate sediments 1 cm kyr 1 and this may reflect the unusual physical properties of these sediments a review of the geotechnical literature suggests that very low accumulation rate sediments have sufficient time to develop enhanced interparticle rigidity structural strength which inhibits early compaction and hence pdrm acquisition

near east paleomagnetic secular variation recorded in sediments from the sea of galilee lake kinneret abstract paleomagnetic records of declination and inclination from sediments recovered from the bed of lake kinneret 32 4 n 35 7 e have been dated by radiocarbon techniques the sediments span the last 5000 yr the changes in inclination down the sediment cores are more pronounced than the declination fluctuations and are repeatable between the three coring sites which are several kilometers apart magnetic susceptibility logs display 13 maxima in the 5 m long sequences with a pronounced susceptibility minimum about 1000 yr b p many of the susceptibility maxima and minima can be easily correlated between coring sites and are shown to be dominantly related to changes in sediment carbonate content the natural remanent magnetization intensity follows a similar pattern to that of susceptibility and the natural remanence of the kinneret sediments is presumed to reside in detrital magnetite grains carried into the lake by the river jordan from the basalt rich bedrock of the rift floor and the golan heights the 14c chronology is strongly supported by a pollen study in which pronounced changes in the proportion of olive pollen were interpreted as being due to extensive cultivation of olives around galilee in the hellenistic and byzantine periods the kinneret paleosecular variation records if accurately dated point to a complex spatial pattern of holocene secular variation with significant variations over distances as small as 1000 2000 km

calibration of radiocarbon results pertaining to related archaeological events with the advent of the high precision radiocarbon calibration curve there is an increasing demand from archaeologists for results previously reported on the radiocarbon scale to be expressed on the calendar scale furthermore there is a general realization within the archaeological community that much more information can be derived from radiocarbon dating if the samples are taken from coherent well chosen contexts and if any available substantive archaeological information is included in the data analysis this paper describes how the bayesian approach to statistical data analysis can be used for the calibration of groups of radiocarbon results in situations where archaeological information is available a priori about the relationships between the contexts being dated recent innovations in the implementation of the bayesian paradigm are used and the methodology is illustrated by the analysis of data arising from two archaeological excavations

a spatial information technology approach for the mapping and quantification of gully erosion abstract in contrast to the effort during the last decades to investigate sheet interrill and rill soil erosion processes relatively few studies have been focused on quantifying and or predicting gully erosion the extension of the use of modern spatial information technologies such as geographical information systems gis digital elevation modelling dem and remote sensing have created new possibilities for research in this field a key issue to be addressed as the basis for predicting the effects of global changes such as land use and climate changes is the mapping and quantification of gully erosion rates including rate of retreat of gully walls and rate of sediment production this research work presents a method to compute the rate of retreat of gully walls and the associated rate of sediment production caused by gully erosion the proposed method uses multitemporal aerial photographs and multitemporal digital elevation models both of which have been processed using gis techniques the research was applied to a sample catchment of 25 km2 located in catalonia spain aerial photographs at scale of 1 30 000 from 1957 and orthophotos from 1993 were used to map gully erosion and determine erosion rates between 1957 and 1993 the rates of channel incision and sediment production were computed from the subtraction of multitemporal digital elevation models the rate of gully walls retreat was 0 2 m year 1 representing a rate of 0 9 m2 year 1 the maximum rate of channel incision 0 7 0 8 m year 1 occurred at the head of the gully and at meandering zones the rate of sediment production caused by gully erosion was 1322 142 ton ha 1 year 1 in comparison with other methods to compute sediment production caused by gully erosion processes the proposed method integrates the losses due to overland flow mass movements and gully deepening and supposes an improvement to locate the areas within the gullies with higher erosional activity

eastward and westward drift of the earth s magnetic field for the last three millennia abstract we analyse the secular variation captured by the archaeomagnetic field model cals7k 2 in an effort to determine episodes of eastward and westward motions of earth s magnetic field at the core mantle boundary cmb over the past 3000 yr the direction amplitude and geographical distribution of these motions are described we find that the clearest azimuthal motions are observed at mid to high latitudes in the northern hemisphere where both eastward and westward motions occur these azimuthal motions correspond to displacements and distortions of the two main quasi stationary high latitude magnetic flux patches similar motions are not observed in the southern hemisphere although this may be a consequence of the poorer data coverage there the globally averaged drift for the past 1000 yr has been westward since 1400 ad but eastward between 1000 ad and 1400 ad in the broad region of the cmb under europe the times of transition in the direction of the mean azimuthal motion coincide with the times at which archaeomagnetic jerks have been reported our results suggest that these are caused by a relatively rapid

gully head erosion processes on a semi arid valley floor in kenya a case study into temporal variation and sediment budgeting a three year monitoring programme of gully head retreat was established to assess the significance of sediment production in a drainage network that expanded rapidly by gully head erosion on the low angled alluvio lacustrine njemps flats in semi arid baringo district kenya this paper discusses the factors controlling the large observed spatial and temporal variation in gully head retreat rates ranging from 0 to 15 m a 1 the selected gullies differed in planform and in runoff contributing catchment area but soil material and land use were similar the data were analysed at event and annual timescales the results show that at annual timescale rainfall amount appears to be a good indicator of gully head retreat while at storm event timescale rainfall distribution has to be taken into account a model is proposed including only rainfall p and the number of dry days dd between storms which explains 56 per cent of the variation in retreat rate of the single headed gully of lam1 a detailed sediment budget has been established for lam1 and its runoff contributing area rca by measuring sediment input from the rca the sediment output by channelized flow and linear retreat of the gully head for nine storms it can be seen that erosion shifts between different components of the budget depending on the duration of the dry period dd between storms sediment input from the rca was usually the largest component for the smaller storms the erosion of the gully head occurred as a direct effect of runoff falling over the edge ghwaterfall and of the indirect destabilization of the adjacent walls by the waterfall erosion and by saturation ghmass storage the latter component ghmass storage was usually much larger that the former ghwaterfall the sediment output from the gully was strongly related to the runoff volume while the linear retreat because of its complex behaviour was not overall the results show that the annual retreat is the optimal timescale to predict retreat patterns more detailed knowledge about relevant processes and interactions is necessary if gully head erosion is to be included in event based soil erosion models copyright 2001 john wiley sons ltd

geomagnetic field variations in northern sweden during the holocene quantified from varved lake sediments and their implications for cosmogenic nuclide production rates palaeomagnetic analyses were conducted on two varved lake sediment sequences in northern sweden the magnetic properties of the sediment sequences are dominated by stable single domain magnetite with characteristics typical of bacterial magnetosomes alternating field demagnetization measurements indicate that the single domain magnetite is the dominant carrier of a stable natural remanent magnetization temporal variations in inclination and declination were obtained from a total of four cores and the data points were stacked according to their independent calendar year varve ages statistically significant patterns in inclination and declination form a regional palaeomagnetic secular variation psv curve which possesses features that are identical in form to the uk holocene psv master curve however the calibrated radiocarbon ages of uk features identified prior to 1500 bc are approximately 500 years older than their swedish varve dated equivalents which points to dating errors and or drifting of the geomagnetic field the sediments meet the uniformity criteria proposed for palaeointensity reconstruction and estimates of relative geomagnetic field intensity are calibrated against global dipole moment compilations a calculated nuclide production curve is derived from the reconstructed geomagnetic field intensity which empirically demonstrates the dominant modulation of cosmogenic nuclide production by dipole moment between 5000 bc and ad 1500

lake sediment record of the geomagnetic secular variation in britain during holocene times summary a palaeomagnetic record of the geomagnetic secular variation during the last 10000 years has been obtained from 10 cores of sediment from loch lomond scotland lake windermere north england and llyn geirionydd north wales a time scale is provided by 30 radiocarbon age determinations and pollen analyses on several of the cores the main swings and much fine detail of both declination and inclination records repeat well between cores and between lakes and the overall record is much more detailed than previous european records the new record shows that neither declination nor inclination swings have been periodic over the past 10000 years but that the main swings have become progressively shorter in duration during that time each swing is characterized by fine detail which enables use of the record as a secondary method of dating other european sediments the motion of the geomagnetic vector has been predominantly clockwise throughout the time period spanned but confirms a period of anticlockwise motion from 1100 to 600 bp first discovered by british archaeomagnetic investigations the record agrees with british and czechoslovakian archaeomagnetic records but not with japanese archaeomagnetic or north american lake sediment records this suggests that the secular changes are controlled by local growing and decaying drifting sources rather than by wobbling of the main geomagnetic dipole

the effect of some watershed soil characteristics and morphometric factors on the relationship between the gully volume and length in fars province iran abstract a survey consisting of 146 first order gullies selected from five sites with different land use soil and watershed properties was carried out to estimate the parameters of the empirical power function in order to describe the relationship between the gully volume and length in fars province iran subsequently the gullies were clustered into seven groups a to g based on nine morphometric factors the linearized functions were fitted to the natural logarithms of the volume and length in each group and the parameters a and b were determined the obtained a and b parameters are in the ranges of 1 0 10 8 and 0 8 1 4 respectively comparison of a and b parameters in each group with the mean of nine factors and the mean soil watershed and land use characteristics in each group showed that the gully volume and average cross section are correlated with the gully depths in different sections and the gully head slopes also it was shown that the parameters of fitted functions to the plots of gully volumes versus gully lengths are proportional to width at the gully heads the gully widths at the top and bottom of the five meter sections are correlated with the average upstream watershed area of the gullies

archaeomagnetic dating and palaeochannels sediments data from the mediaeval channel fills at hemington leicestershire this paper illustrates how archaeomagnetic dating can be used for palaeochannel fills within floodplain sequences the sites investigated are palaeochannels associated with the mediaeval braided anastomosing channels of an unstable reach of the river trent at hemington north west leicestershire this reach has revealed abundant archaeological remains including fish weirs anchor stones a norman mill and three mediaeval bridges the studies used here show good agreement between archaeomagnetic dating and other evidence from archaeology dendrochronology and radiocarbon dating fabric and mineral analyses of the sediments are used to identify the source of the remanent magnetization and it is believed that this is predominantly the result of the biogenic synthesis of magnetic minerals in high ph but anoxic conditions this study illustrates the type of alluvial sediments that are most suitable for archaeomagnetic dating

an empirical investigation of gully widening rates in upland concentrated flows abstract soil erosion in its myriad forms devastates arable land and infrastructure as an integral landscape feature gully erosion is a complex system as its evolution is controlled by upstream migration of a gully head headcut face incision of the gully bed plunge pool and gravitational mass movement widening on gully channel sidewalls bed incision is often limited by the presence of a less erodible soil layer when erosion reaches such a layer the gully typically widens creating a wide shallow cross section once a gully is initiated transport and deposition of the eroded soil and widening of the channel further govern its evolution our knowledge of these processes within agricultural regions however is still quite limited with theory largely borrowed from river hydraulics experiments were conducted to examine gully expansion widening as a function of channel slope and overland flow discharge packed soil beds were subjected to simulated rainstorms followed by clear water overland flow as bed slope and discharge increased channel widening increased an empirical equation based upon measured channel width from constant discharge experiments was developed in terms of slope and discharge by further quantifying key aspects of gully erosion processes results from this experimental campaign will significantly improve soil erosion prediction technology for use in agricultural regions

the mai maikden sedimentary sequence a reference point for the environmental evolution of the highlands of northern ethiopia abstract this paper presents a geomorphologic stratigraphic analysis of a travertine dammed lacustrine swampy sedimentary sequence composed of clay peaty layers and phytoclastic travertine sands deeply incised by the mai maikden river on the highlands of tigray northern ethiopia radiocarbon datings of peaty deposits allow us to establish that the travertine dams have developed at least between 7310 90 yr b p and 5160 80 yr b p in the upper part of the sequence palaeochannels filled with travertine and limestone coarse gravels and blocks indicate the activation of concentrated erosion on the surrounding slopes later both the travertine dam and the lacustrine swampy deposits were buried by alluvial and colluvial sediments still mixed with large amounts of organic matter testifying to generalized slope erosion processes the end of travertine deposition may be connected with a progressive reduction of vegetation cover and the subsequent decrease of co2 in groundwater the occurrence of the latter phenomena may be related to the onset of drier climatic conditions even though the finding of a large prehistoric settlement in the area also indicates some influence of human impact

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