systematic occurrences of malformed teratological acritarchs in the run up of early palaeozoic δ13c isotope excursions abstract the late ordovician and silurian are characterised by several strong global positive 13 c carb excursions some of them exceed 5 and thus belong to the strongest perturbations of the carbon cycle in the phanerozoic the onset of the excursions is characterised by extinction and or turnover events of several groups of marine invertebrates the causal mechanisms of the carbon cycle perturbations however are still unknown and currently a matter of vigorous scientific debate our own investigations in the hirnantian latest ordovician have shown that the onset of the major 13 c excursion hice is characterised by very high abundances of acritarchs showing abnormal teratological forms a critical review of published reports of abnormal acritarchs from the late ordovician to early devonian and a correlation of their occurrences with the global stable carbon isotope curve show that high abundances of teratological forms of acritarchs are often coeval to the run up of 13 c excursions high abundances of teratological forms in modern marine protists are commonly observed in environments with a high degree of environmental stress in the fossil record the challenge is to attribute abnormal forms of organisms to specific environmental circumstances our study implies that they are somehow related to the global carbon cycle i e to carbon isotopic composition of the ambient sea water and that they share a common extrinsic cause with the contemporaneous extinction and or turnover events in other fossil groups

incised channels and gully erosion in northern iberian peninsula controls and geomorphic setting abstract gully erosion has scarcely been reported in the cantabrian cordillera northern iberian peninsula even though this phenomenon is relatively frequent in this mountain range a total of 133 permanent channels with evidences of gully erosion have been mapped in a 71 8 km2 study area on the northern slope of this mountain range the underlying ordovician unit comprises quartzite sandstone and slate which are partially mantled by different surficial deposits the relief is fairly steep with average slopes of 27 and hillsides with level drops of the order of 700 m and generally vegetated by atlantic heaths a result of the degradation suffered by the natural vegetation following forest fires the mapped channels present different morphological features shallow and deep channels some of them with associated ravines and are usually related to headcut in first and second order pre existing channels although some of them are located on unchanneled hillsides with no evidence of pre existing channels moreover lower reaches of first and second order channels are often incised into bedrock a field description and morphometric study has been carried out on channels affected by gully erosion based on their cartography digital terrain models and several layers of thematic mapping this study describes some of the relations between relief factors the type of substratum and the development of gully erosion i there are robust correlations between morphometry relief and erosion intensity and ii gullying is strongly influenced by the presence of a deep regolith the erosion forms observed and the described relations suggest that gully development is associated to landsliding processes which are ultimately controlled by ground water movement through a porous regolith regional uplift and deforestation are argued to be the most likely causes of channel incision and gullying

an oceanic model for lithological and faunal changes tested on the silurian record a model for oceanic cycles is presented the cycles are expressed as changes between more humid low latitude and cooler high latitude climates p episodes and dryer low latitude and warmer high latitude climates s episodes the cyclicity may be self regulating through changes in the co 2 storage capacity of the deep ocean due to temperature changes and through changes in the oceanic input and output of dissolved carbonate the model permits over 30 predictions regarding observable changes in both deep water and shelf lithologies and variations in the abundance diversity and extinction rate of planktic and benthic faunas sedimentary changes involve fluctuations in the rate of clay deposition and carbonate production the advance and retreat of carbonate deposition on the shelf edge the timing of oolite reef and black shale formation and changes between oxic and anoxic deep water faunal effects include both iterative evolution and intermittently widespread taxa lazarus taxa these predictions agree well with changes observed in well known sequence of wenlock and ludlow age as well as changes seen in selected older and younger sequences

geomorphic changes in a complex gully system measured from sequential digital elevation models and implications for management high resolution digital elevation models dems were derived from sequential aerial photography of an active uvio mass movement gully complex in new zealand s north island east coast region to measure geomorphic changes over approximately one year the gully showed a complex behaviour combining uvial and mass movement erosion deposition and reworking of materials stored in an active debris fan during the measurement period 5200 1700 m3 of material were eroded from the 8 7 ha gully complex and 670 180 m3 from the 0 8 ha depositional fan giving a total of 5870 1710 m3 for the entire gully complex fan system the results provide a high resolution description of gully behaviour over a short time period and also demonstrate that mass movement slumping and debris ows accounted for almost 90 per cent 4660 200 m3 of the sediment generated this erosional response is described in terms of gully evolution by comparing the gully complex to other systems in the region in various stages of development the effect of gully evolution on geomorphic coupling between the gully complex and channel system is described and coupling is also shown to vary with the magnitude and frequency of rainfall events from a land management perspective the success of strategies such as tree planting to mitigate against gully erosion depends on the stage of gully development particularly on whether or not mass movement erosion has begun in contrast to gully rehabilitation efforts elsewhere basin wide afforestation in the early stages of gully incision is favoured over riparian planting given that mass movement assisted by excessive groundwater pressure is the main process leading to uncontrollable gully expansion to protect land effectively against continuing gully erosion of headwater catchments and resulting downstream aggradation it is necessary for land managers to understand the spatial and temporal variability of gully development fully so that mitigation efforts can be targeted appropriately copyright 2003 john wiley sons ltd

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

gully morphology hillslope erosion and precipitation characteristics in the appalachian valley and ridge province southeastern usa this study investigates gully erosion on an east tennessee hillslope in a humid subtropical climate the study area is deeply gullied in ultisols acrisol according to the world reference base for soil with thirty years of undisturbed erosional history with no efforts to correct or halt the erosion the objectives are 1 to examine how different gully morphologies channel sidewall and interfluve behave in response to precipitation driven erosion and 2 to identify an appropriate temporal scale at which precipitation driven erosion can be measured to improve soil loss prediction precipitation parameters total accumulation duration average intensity maximum intensity extracted from data collected at an on site weather station were statistically correlated with erosion data erosion data were collected from erosion pins installed in four gully systems at 78 locations spanning three different morphological settings interfluves channels and sidewalls kruskal wallis non parametric tests and mann whitney u tests indicated that different morphological settings within the gully system responded differently to precipitation p 0 00 for channels and sidewalls regression models relating erosion and precipitation parameters retained antecedent precipitation and precipitation accumulation or duration r2 0 50 p 0 00 for channels r2 0 28 p 0 00 for sidewalls but precipitation intensity variables were not retained in the models for interfluves less than 20 of variability in erosion data could be explained by precipitation parameters precipitation duration and accumulation including antecedent precipitation accumulation were more important than precipitation intensity in initiating and propagating erosion in this geomorphic and climatic setting but other factors including mass wasting and eolian erosion are likely contributors to erosion high correlation coefficients between aggregate precipitation parameters and erosion indicate that a suitable temporal scale to relate precipitation to soil erosion is the synoptic time scale this scale captures natural precipitation cycles and corresponding measurable soil erosion

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

older crust underlies iceland summary the oldest rocks outcropping in northwest iceland are 16 myr old and in east iceland 13 myr the full plate spreading rate in this region during the cenozoic has been 2c m a 1 and thus these rocks are expected to be separated by 290 km they are however 500 km apart the conclusion is inescapable that an expanse of older crust 210 km wide underlies iceland submerged beneath younger lavas this conclusion is independent of any considerations regarding spreading ridge migrations jumps the simultaneous existence of multiple active ridges three dimensionality or subsidence of the lava pile such complexities bear on the distribution and age of the older crust but not on its existence or its width if it is entirely oceanic its maximum age is most likely 26 37 ma it is at least 150 km in north south extent but may taper and extend beneath south iceland part of it might be continental a southerly extension of the jan mayen microcontinent this older crust contributes significantly to crustal thickness beneath iceland and the 40 km local thickness measured seismically is thus probably an overestimate of present day steady state crustal production at iceland

threshold conditions for initiation of valley side gullies in the middle veld of swaziland abstract gullied and ungullied catchments in four areas of deeply weathered saprolite in the middle veld of swaziland can be successfully separated p

quantifying historical gully erosion in northern bavaria the wolfsgraben is one of many ravines that cut into the silty sandy material of the triassic benchlands of northern bavaria germany within the research area a gully several meters in depth has carved in to a bluff 500 m west of the upper main valley an analysis of 17 exposures and 30 drillings was conducted within the gully and its colluvial fan in order to reconstruct the soil formation extreme rainfall events and land use changes throughout history detailed field studies chemical soil analysis dating methods of charcoal and pottery as well as written documents were combined to produce a high resolution stratigraphy nine main phases of landscape evolution caused by extreme rainfall runoff and soil erosion in agriculturally used areas have been identified since the medieval period the highest amount of soil loss occurred during 14th 18th and 19th centuries a result of intensive land use climate changes affected the intensity of rainfall and the magnitude of soil loss during these periods d 2003 elsevier science b v all rights reserved

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

on the orientation of tensile dikes in a ridge centered plume abstract we present three dimensional numerical models of the ascent of a plume under a spreading ridge and the concomitant melt generation and crust formation using a mantle viscosity which depends on pressure temperature melt content and optionally water content from the velocity field of these convection models we compute the viscous stress tensor in the mantle assuming that melt rich channels or melt filled dikes are oriented parallel to the maximum compressional stress we calculate the orientation of such dikes in the partially molten zone of the plume head and beneath the ridge for the central part of the plume we find dike orientations parallel to the ridge in shallow parts of the plume head while they are ridge perpendicular at greater depth the boundary between these two regimes is shifted downwards for a water rich plume which dehydrates upon melting the two regimes of different dike or melt channel orientations seem to be in good agreement with seismic observations on depth dependent seismic anisotropy indicating a preference of our wet model we find a modest focusing of melt towards the spreading center beneath normal ridge in both models but also in the fully dehydrated parts of the plume head in the model where water was included those are regions where upwelling is essentially passive in contrast the plume in the water free model and the only partly dehydrated parts of the plume in the water bearing model tend to develop a dike orientation which moves melt away from the spreading center these regions are characterized by active upwelling the defocusing of melt in actively upwelling plume mantle has radial symmetry about the plume axis i e an outward orientation of the dikes is not only visible in planes perpendicular to the ridge but also along the ridge

controlling factors of the size and location of large gully systems a regression based exploration using reconstructed pre erosion topography abstract the role of different factors potentially affecting the size and location of large gully systems is explored in two tributary basins of the tagus river s portugal the hillslopes of these basins corresponding to tertiary sedimentary formations are affected by hundreds of large gully systems varying in extent from somewhat 2 to 3 ha a study set of 90 gully systems were vectorized from aerial ortophotos and analysed in relation to a set of potential control variables obtained by reconstructing the pre erosion topography multiple linear regression and logistic regression were combined in order to investigate controls over the general morphometry of these landforms area perimeter and sinuosity and their location in the respective basins results indicate that gully systems initiated as bank gullies in response to an incision by the drainage network in both basins and then evolved as a function of upslope contributing area developing progressively more sinuous outlines as a result of headcut bifurcation pre erosion topographic form was shown to have only a marginal effect with results suggesting gully systems have grown essentially through mass movements in headcuts and sidewalls influenced by subsurface water

iceberg hosted nanoparticulate fe in the southern ocean mineralogy origin dissolution kinetics and source of bioavailable fe abstract sediments from icebergs and glaciers contain nanopartculate fe iii oxyhydroxides including ferrihydrite which form in aqueous oxic micro environments where fe ii bearing rock minerals oxidise and high degrees of supersaturation are promoted by freezing and thawing an ascorbic acid extraction dissolves only labile fe present in fresh loosely aggregated ferrihydrite that is directly or indirectly bioavailable glacial and iceberg sediments contain ferrihydrite aggregates that provide 0 04 to 0 17 fe soluble in ascorbic acid rather larger than the concentrations in a limited suite of atmospheric dusts the dissolution behaviour of labile fe from glacial and iceberg sediments by ascorbic acid is controlled by the access of reactant or removal of solute through micropores to or from active sites in the interior of ferrihydrite aggregates a first order kinetic model is presented to examine the rates at which bioavailable fe can be supplied by melting icebergs in the weddell sea the model utilizes rate constants from the literature for the processes which solubilise fe from nanoparticulate ferrihydrite dissolution photochemical reduction and grazing and the processes that remove fe nanoparticulates sinking scavenging and incorporation in faecal material and render them less reactive transformation aging model results demonstrate that icebergs can supply bioavailable fe to the weddell sea by the dissolution of nanoparticulate ferrihydrite despite loss removal of nanoparticles by sinking aging transformation scavenging and incorporation into faecal pellets at rates that are comparable to atmospheric dust dissolution enhanced by photochemical reduction and grazing provides the most rapid rates of bioavailable fe production

a typical calanchi landscape on the eastern apennine margin atri central italy geomorphological features and evolution abstract the surroundings of the town of atri abruzzo region central italy situated about 10 km from the adriatic shoreline can be considered as representative of most part of the italian seaward peninsular landscape the subject of this research the calanchi landscape often generally considered as badlands by non italian authors is in fact made up of many singular landforms each of which calanco corresponds to a more or less extended hydrographic unit the dynamics are mainly the result of the action of the particularly aggressive climatic factors such as the mediterranean rainfall regime on weak substrata clay silt and sand of the plio pleistocene marine cycles the morphogenetic activity is not only limited to channel erosion but it is also due to repeated superficial slides so that it should in fact be better to consider the calanco as the result of a combined erosion process sensu zachar d 1982 soil erosion elsevier amsterdam since the mechanism regulating this process remains unclear this work wants to offer a contribution to this research field multitemporal geomorphological analysis has helped us to distinguish the different areas in which the calanchi evolution has been more or less important in the past in such a way it has been possible to characterize the landscape evolution as a function of the parameters that influence soil loss especially in areas such as that of atri where calanchi like landforms are highly developed moreover this analysis coupled with the processing of 40 years rainfall data also taking into account the concurrent land use changes has clearly shown that the areas affected by calanchi erosion on the eastern apennine margin still tectonically active are progressively diminishing

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

time specific aspects of facies state of the art examples and possible causes abstract the term time specific facies tsf was introduced to the scientific community by the late otto h walliser to refer to unique facies typical of particular narrow intervals some of which were related to bioevents in some senses however the concept was recognized much earlier and is even engrained in the very names of some geologic periods the concept of time specific facies is expanded slightly herein to include distinctive or unique regional to global characteristics of the sedimentary record that characterize particular intervals of geologic time the recognition of time specific and widespread processes in the sedimentary record is a critical step in unraveling the interplay between processes of differing scale these range from very short term but widespread facies that overlap with event deposits to general facies types that may persist for intervals up to 10s of millions of years in duration this paper briefly summarizes the history of development of the tsf concept provides examples to illustrate a few of the key aspects of time specific facies and offers a few tentative explanations for this phenomenon among the factors that control tsfs abrupt changes in redox conditions and early diagenetic mineralization sedimentary condensation often associated with abrupt sea level change altered climate and paleoceanography and biotic evolution and extinction seem to be most critical and permit a preliminary genetic classification of tsfs inevitably however many tsfs reflect multiple effects and some remain largely unexplained

beyond black shales the sedimentary and stable isotope records of oceanic anoxic events in a dominantly oxic basin silurian appalachian basin usa abstract episodes of instability in the global climato oceanic system have become the hallmark of the silurian period these events are marked by six large positive carbon isotope excursions associated with major extinctions the widespread environmental consequences of these perturbations of the global carbon cycle remain poorly documented a high resolution chronostratigraphic reconstruction of the appalachian foreland basin afb provides a framework to evaluate environmental signatures of two of these carbon isotope excursions valgu and ireviken events telychian sheinwoodian at a regional scale integration of published biostratigraphic range data mapping of sequence stratigraphic surfaces and facies and generation of high resolution carbon isotope profiles for several transects provides precise correlation of the basin stratigraphy yielding a high resolution temporal framework this chronostratigraphic reconstruction of the afb highlights the importance of time specific facies tsf during these intervals for instance packages of red green and gray marine rocks separately occupied discrete time intervals cutting across local facies belts indicative of shallow to deep marine environments similarly widespread iron mineralization occurred during discreet time slices as bathtub rings rimming the margins of the basin changes from one time specific facies to the next are coincident with changes in carbon isotope values indicating a genetic link between fractionation of the global carbon reservoir and redox changes from generally oxic to anoxic oceanic conditions in parts of the afb the basal part of the study interval was deposited during the valgu event this first event begins with an unconformity marking global lowstand this fall in sea level was coincident with rising carbon isotope values as recorded in adjacent basins a slight sea level rise and flooding of the subaerial unconformity occurred near the peak in carbon isotope values oxidized iron minerals were deposited in a zone that extended from the lower shoreface into the basin center pinching out into shallower parts of the basin thin pyrite phosphorite glauconite layers associated with black shales in the basin center were deposited over these ferruginous strata as the event ended and carbon isotope values shifted negatively to a more stable baseline temperatures warmed sea level reached a silurian high and oxic conditions associated with very low rates of organic carbon oc burial were established these conditions were relatively stable for nearly four million years this long period of stasis was punctuated by the ireviken event which displays a strikingly similar progression of tsf patterns yet more intense and longer lived than the preceding valgu event with the first positive shift in carbon isotopes at the onset of the ireviken event the basin redox system changed from largely oxidizing to reducing coincident with a rapid drop in sea level and an abrupt reappearance of iron mineralization on the basin margins the sea level fall associated with the shift to peak values of the carbon isotope excursion is marked by the progradation of a significant body of sand well out into the basin during this sea level drop conditions in the basin center became highly reducing and periodically sulfidic widespread ironstones and ankeritic carbonates were deposited isochronously rimming the shallow basin margins where they alternated with light gray pyritic shales indicating fluctuating redox conditions a large negative carbon isotope excursion of 3 is coincident with a shift from regression to transgression suggesting glacial melting via methane release continued deposition of ironstones through this time interval suggests a primary link to shallow marine redox processes rather than large scale transgression or regression of the shoreline the ironstones are overlain by gray to black pyritic shales deposited across all environments revealing a final stage of redox progression into sustained sulfidic conditions as peak values of the carbon isotope excursion returned to a high stable baseline near 4 the similar expression of tsfs within the afb of these two global events demonstrates that fractionation of the global atmospheric and oceanic carbon reservoirs had a predictable cascading impact on shallow sea floor environments the associated redox processes reveal the dynamic regional interactions of a shallow oxic epicontinental water mass with a broader anoxic oceanic water mass providing valuable insight into the origin of some of the largest extinctions in the silurian period

badass gullies fluvio mass movement gully complexes in new zealand s east coast region and potential for remediation abstract this paper reviews gully erosion in the east coast region of new zealand s north island and conceptualises fluvio mass movement gully complexes as badass gully systems tectonic setting and lithological control with steep slopes and a climate influenced by tropical cyclones predispose hill country in the east coast region to gully erosion the clearance of indigenous forest since the late 1800s has dramatically increased catchment erosion and paved the way for development of large scale fluvio mass movement gully complexes these features are a composite of fluvial and mass movement processes they are conceptualised as badass by not conforming to any existing gully model and by generating disproportionate results in east coast catchment sediment cascades their remediation is discussed but their nature means that prevention is better than a cure

theoretical constraints to gully erosion research time for a re evaluation of concepts and assumptions gullies are conceptualized in the literature as essentially fluvial forms with dimensional boundaries arbitrarily defined between rills and river channels this notion is incompatible with the existing variability of form and process as mass movements frequently exert a fundamental control on gully initiation and expansion to the point of features outgrowing their original contributing area the inability of a conceptual framework to incorporate existing observations inevitably constrains methodologies and research results in this commentary several examples of published results are contrasted with the prevailing assumption of an essentially fluvial nature with the purpose of encouraging discussion on the need for a revised conceptual framework in gully erosion research copyright 2011 john wiley sons ltd

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

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

erosion thresholds and suspended sediment yields waipaoa river basin new zealand characteristics of the suspended sediment load in the waipaoa river new zealand and in two of its tributaries mangatu and te arai rivers are examined for evidence of different dominant erosion processes in the basins upstream different shapes of the suspended sediment concentration water discharge relations lead to differences in long term average yield event yield magnitude frequency relations and relative importance of large flows and rare events in the mangatu river frequent runoff events 2 1 are relatively more important to the long term yield 11 540 t km yr half of the long term average load is transported by events with return periods less than 1 year and there is little evidence of an erosion threshold limitation on sediment supply this is consistent with the predominance in the mangatu basin of hillslope erosion processes that involve scour by surface and channelized runoff particularly gully erosion this contrasts with the te arai river where sediment concentration tends to be much lower at low and moderate flows frequent runoff events transport less of the long term yield 4600 t km 2 yr than do rarer large magnitude flood events event sediment yields are an order of magnitude lower during events with subannual return periods and half of the long term average load is transported during events with a return period of 2 years some of these characteristics appear to result in part from two populations of runoff events in the te arai basin nonetheless they are consistent with field evidence that most of the sediment supplied to the te arai stream network is generated by shallow landslides which are activated once a rainfall threshold is exceeded

crustal accretion and dynamic feedback on mantle melting of a ridge centred plume the iceland case abstract a major consequence of the interaction of a plume with an oceanic ridge is the enhanced melt production and associated crust generation in the case of iceland crustal thickness as large as 20 to 40 km has been reported crustal seismic velocities are high and have to be explained by thermal or chemical effects in the first part of the paper we address the question whether extraction of melt out of the plume beneath a slowly spreading ridge and deposition of extracted basalt volumes at the surface produces a dynamic feedback mechanism on mantle melting to study this question we solve the convection equations for a ridge centred plume with non newtonian rheology including melting melt extraction associated with deposition of cold crust at the surface of the model and using a simplified approach for compaction the assumption of cold crust is justified if the thickness of each deposited basaltic layer is less than roughly 1 km depending on the buoyancy flux of the plume crustal thicknesses between 10 and 40 km are modelled showing characteristic dipping structures resembling the rift ward dipping basaltic layers of east and western iceland comparing the resulting crustal thickness and magma generation rate with models in which the dynamic effect of crust deposition has been suppressed indicates that melt generation beneath a slowly spreading ridge is considerably damped by the dynamic feedback mechanism if the plume buoyancy flux exceeds 400 to 600 kg s based on the observed crustal thickness of iceland our models predict a plume buoyancy flux of 1140 kg s in the second part we study the accretion of the icelandic crust by a thermo mechanical model in more detail based on the navier stokes the heat transport and the mass conservation equations including volumetric sources hot 1200 c molten crustal material is injected into the newly forming crust with a constant rate at different crustal source regions a deep widespread emplacement of dykes and sills including crustal underplating b magma chambers at shallow to mid crustal level and c surface extrusions and intrusions in fissure swarms at shallow depth connected to volcanic centres we identify the material from the different source regions by a marker approach varying the relative dominance of these source regions characteristic crustal structures evolve showing shallow dipping upper crustal layers with dip angles between 10 and 15 the thermal structure of the crust varies between cold crust shallow source region dominating and hot crust deep source region dominating we use observations of maximum depth of seismicity to constrain the depth of the 650 c isotherm and seismological inferences on the lower crust to constrain temperatures in that region the best agreement with our models is achieved for crust formation dominated by deep dykes and underplating with a considerable influence of magma chamber accretion

accumulation of a ferric mineral in the biofilm of montacuta ferruginosa mollusca bivalvia biomineralization bioaccumulation and inference of paleoenvironments abstract infrared absorption spectra of the amorphous and phosphorus rich ferric mineral associated with the biofilm of montacuta ferruginosa a marine bivalve were obtained by applying the kbr disc method phosphate absorption bands due to p o stretching vibrations were observed at 1100 and 1020 cm 1 this result as well as the similarity of spectra with other well known bio minerals suggests that the mineral is an amorphous iron oxyhydroxide gel with phosphate sorbed on its surface rather than a pure ferric phosphate it is suggested that phosphate ions are of microbial origin because phosphate producing enzymes were detected in vivo apart from iron the mineral phase is devoid of other heavy metals despite its similarity with other abiotically precipitated phases it is argued that the mineral is the result of in situ microbial biomineralization processes in the biofilm and not the result of a simple bioaccumulation process this is supported by microscopic observations a geological implication is that the simple presence of such iron minerals in ancient sedimentary environments where microbes have not been fossilized might be considered as an indicator of microorganisms performing biomineralization by comparison with the present day environment of m ferruginosa precise paleoenvironmental conditions may be inferred

iron framboids in the hydrocarbon related middle devonian hollard mound of the anti atlas mountain range in morocco evidence of potential microbial biosignatures iron framboids in the context of a middle devonian hydrocarbon seep conical mound anti atlas morocco have been thoroughly investigated through a combination of techniques including optical and high resolution scanning electron microscopy x ray diffraction energy dispersive x ray spectroscopy raman microscopy and time of flight secondary ion mass spectroscopy framboids occur embedded in laminated carbonates of the mound and are precipitated along the veins that crosscut it and served as the conduits for channeled reduced hydrocarbon rich fluids the morphology structure and composition of the framboids contribute to the reconstruction of the complex interplay between the advected fluids and the associated biological and or abiological processes a comparison of our dataset with that from modern framboids formed under similar conditions indicates that the fe rich framboids represent replaced primary pyrite framboids produced as a consequence of bacterial sulfate reduction and the anaerobic oxidation of methane

iron bacterial and fungal mats bajocian stratotype mid jurassic northern normandy france abstract the oolithe ferrugineuse de bayeux formation is located at the historical bajocian stratotype of sainte honorine des pertes north of bayeux normandy the condensed formation ranges from the base of the humphriesianum zone to the parkinsoni zone and is divided into four beds of decimetric scale three main microfacies are present 1 oncoid rudstones 2 ooid bioclastic packstones and 3 silty burrowed wackestones packstones sedimentation took place in a very quiet environment below the photic zone and below or near the storm wave base the general setting is a distal carbonate ramp its lower part characterized by hemipelagic sedimentation indicated by the presence of planktonic foraminifers the inferred depth is around 100 m free oxygen concentration was low dysaerobic conditions are indicated by a scarcity of benthic macrofauna ferruginous structures are numerous in the first two microfacies and absent in the last hematite staining is not uniform and follows many sedimentary patterns among the more widespread fe structures are perforation infillings with endolithic micro organisms microstromatolites oncoids ooids blisters coatings and hardgrounds these structures can be associated and none are mutually exclusive hematite coated filaments of different sizes and shapes are observed in the micrite matrix the walls of various organisms the calcite crystals associated with the fe cortical laminations the perforations and burrow the hardgrounds and microstromatolites petrographical and sem examinations suggest that the laminated crusts oncoids and hardgrounds are formed by microbial iron mats dominated by filamentous bacteria and fungi seven types of microbes are recognized filaments five morphotypes spheroidal bodies and stalked bodies filamentous microfossils of type 1 to 4 resemble the present day filamentous bacteria beggiatoales and cytophagaceae because of their large diameter and their branching nature filaments of type 5 are possibly filamentous fungi another argument in favor of fungi is the presence of stalked and spheroidal bodies that resemble zoosporangia and oogonia of some oomycota in deep calm and dysaerobic waters many interfaces e g between aerobic and dysaerobic waters are present in the sediments the stability of the soluble reduced state of iron fe 2 is higher at such interfaces and many ferric iron encrusted microbial fossils are observed iron could thus serve as an electron donor for microbial iron oxidation processes other microbial iron deposition pathways are also possible it appears that regardless of geological age paleozoic mesozoic and geographical location the same microbiological mechanisms are probably responsible for the red color in calcareous stratified or unstratified bodies the presence of fossilized iron encrusted bacteria and fungi at interfaces may therefore serve as an indicator of anoxic to dysaerobic conditions in various paleo micro environments

constraints on crustal and mantle structure of the oceanic plate south of iceland from ocean bottom recorded rayleigh waves abstract from april to july 2002 we carried out a deployment of 6 ocean bottom seismometers and 4 ocean bottom hydrophones in the north atlantic south of iceland during the deployment period we recorded clear rayleigh waves from 2 regional and 14 teleseismic earthquakes this corresponds to a rayleigh wave detection rate of nearly 92 for events with mw 6 06 0 and epicentral distance less than 110 close to detection rate estimates based on noise level variability we measured rayleigh wave event station group dispersion and inter station phase dispersion for one mid atlantic ridge event the group dispersion curve is sensitive to the structure of the north east atlantic with an average age of 39 myr the phase dispersion curve is sensitive to the structure just south of iceland average plate age 33 myr both dispersion curves indicate faster velocities than previously postulated for oceanic plate generated at the reykjanes ridge a grid search approach was used to constrain the range of models fitting the data the high velocity seismic lid just south of iceland in the model for the phase dispersion path is slower or thinner than in the group dispersion model which averages over a larger area and a somewhat older plate age but the velocities in the low velocity half space are similar we further consider the residual bathymetry in the experimental area the residual anomaly decreases by 300 400 m from the reykjanes ridge to the 30 myr old plate south of iceland this decrease can be explained by the disappearance of a mantle thermal anomaly associated with the iceland plume both the residual bathymetry and the surface wave data are thus consistent with the notion that the southward spreading of the icelandic plume is channelised underneath the reykjanes ridge and does not spread far outside this channel based on the experience from the pilot experiment we estimate that a minimum recording time of 13 15 months in favourable weather conditions april september is required to record enough data to map the spreading plume with surface waves and to produce a tomographic image to a depth of 1000 km using body waves this can be achieved by a continuous deployment of at least 20 months or by two or three deployments during the spring and summer of consecutive years

from black and white to colour in the silurian abstract ironstones and iron rich limestones regularly occur as components of time specific intervals of the palaeozoic as well as in younger times brett et al 2012 this issue silurian sediments deposited at high latitudes along the peri gondwana border are characterized by black and white limestone and graptolitic shale sequences those in the carnic alps southern austria additionally contain colourful pink to red limestones and ironstones laminated structures such as the ferruginous coatings around skeletal fragments mostly trilobites and some cephalopods and echinoderms and stromatolitic features along discontinuity surfaces display dark red green white and brownish colours due to the presence of goethite magnetite hematite chamosite calcite and subordinate apatite confocal laser raman microscopy and complementary microscopic analysis of these ferruginous laminated structures document the presence of carbonaceous matter associated with fossilized microbial structures in the form of stromatolites filaments and coccoids suggesting a microbial role in the colouring of the silurian world of the carnic alps iron concentrations up to 30 that of matrix and surrounding non ferruginous rocks suggest blooms of iron microbe activity in response to the time specific occurrence of chemically charged sea water during global biotic events

iron bacterial mediation in phanerozoic red limestones state of the art different pathways can be proposed to explain the origin of the red pigmentation present in many phanerozoic limestones we investigate three possible models 1 telogenetic alteration 2 detrital input and 3 role of iron bacteria the third hypothesis is supported by our data for a number of phanerozoic series by this hypothesis not only can we highlight the importance of the microenvironments at the dysoxic anoxic water sediment interfaces for the bacterial fe oxidation but also can conclude that the dred limestonest have no particular paleogeographic meaning the seven studied red successions are rather poor in iron 350 ppm in the ammonitico rosso without large fluctuations the pigmentation is due to the dispersion of submicrometric hydro oxides now hematite formed by bacterial mediation during early diagenesis in various microaerophilitic environments different ferruginous morphs e g filaments coccoids microstromatolites suggest the presence of iron bacteria the limiting factor is the oxygen content which was low in these very quiet and relatively deep environments the presence of iron bacteria is also confirmed by iron isotope analysis on the ammonitico rosso fe isotopic compositions have been analyzed on residues as well as on leachates of red and grey facies of the same interval d 56 fe values are systematically lower in the red residues varying from 1 19x to 0 34x compared to the grey ones 0 27x to 0 09x this indicates a significant accumulation of lighter isotopes in the red parts suggesting that hematite have been formed via a biological pathway d 2005 elsevier b v all rights reserved

palaeozoic low oxygen high latitude carbonates silurian and lower devonian nautiloid and scyphocrinoid limestones of the anti atlas morocco abstract high latitude silurian and lowermost devonian basinal limestones of northern gondwana are characterised by mass accumulations of orthocone nautiloids bivalves and planktonic crinoids scyphocrinoids extensive exposures in the anti atlas of morocco reveal this basinal facies passes up gradient into an iron rich bioclastic facies associated with distal shelf siliciclastics and into crinoid brachiopod dominated bioclastic blankets and shoals the limestones formed in a cool water setting in which upwelling of nutrient rich waters during sea level lowstands is interpreted to have triggered the abundance of nautiloids and scyphocrinoids preservation of organic matter in under and overlying graptolite shales as well as iron precipitation in the neighbouring shallower water sub oxic zone the nautiloid limestones proved to be good correlation markers on a regional to global scale contemporaneous siliciclastic supply into the neighbouring facies belt as well as new biostratigraphic data are taken as evidence that these limestones can be correlated to 2nd order eustatic sea level lowstands and the following transgression the low oxygen basinal facies disappeared more or less abruptly during the early devonian pragian and was replaced by a well oxygenated basinal lowstand facies suggesting the end of upwelling at the same time the first reefal build ups coral biostromes and mud mounds appear in north africa indicating a significant rise in water temperatures the change from low oxygen lowstand mass accumulations to well oxygenated basinal facies with a concomitant rise in water temperature is in accordance with recent models of a cool silurian to earliest devonian lochkovian ocean with oscillating oxic to anoxic ventilated to poorly circulated oceanic states and a warm ocean with anoxic to euxinic poorly ventilated to stagnant ocean waters from the beginning of the pragian onwards

medium term gully headcut retreat rates in southeast spain determined from aerial photographs and ground measurements this paper deals with gully retreat rates at different time scales whereby the short term time scale may span a time interval of 1 5 years the medium term time scale a time interval of 5 50 years and the long term time scale a time interval of more than 50 years an analysis of high altitude aerial photographs in combination with ground measurements allowed us to quantify volumetric gully head retreat rates for 12 permanent gullies in southeast spain guadalentin and guadix study areas over a 40 43 year time interval medium term time scale this resulted in an average retreat rate ve of 17 4 m3 year 1 a power relationship between drainage basin area a and medium term volumetric gully head retreat rate ve 0 069a0 380 r2 0 51 n 21 was found by combining the gully head retreat rates obtained in this study with those obtained by a dendrochronological method the exponent b and the coefficient of determination r2 of the power relationships ve aab increases from the short term to the long term time scale expressing the increasing importance of drainage basin area in gully development with time considerable differences between gully head retreat rates measured at the short and medium term time scales at individual gullies showed the importance of land use changes and unsuccessful management practices on gully head retreat and the episodic nature of gully head retreat when piping and tension cracking are involved higher gully head retreat rates are obtained at the medium term time scale compared to the short term time scale but the differences are not significantly different at 10 the medium term methods tend to measure proportionally more high gully retreat rates but less extreme values compared to the short term method this is explained by a more equal distribution of extreme rainfall events both in space and time at a longer term time scale and hence a higher probability of measuring the average effect of both small and extreme rainfall events at each gully

chamosite siderite and the environmental conditions of their formation in chamosite type phanerozoic ooidal ironstones abstract two characteristic ironstone minerals occurring in chamosite type ironstone deposits were investigated these are the minerals of the chlorite and the carbonate series the samples are from 31 localities and 8 countries czech republic france germany united kingdom egypt nigeria and usa these deposits are identical to those presented by mucke and farshad mucke a farshad f in press whole rock and mineralogical composition of phanerozoic ooidal ironstones comparison and differentiation of subtypes ore geology reviews with ordovician to late cretaceous ages electron microprobe analyses of 197 carbonates and 64 chlorites were carried out the analytical points of the carbonates in a magnesite calcite siderite rhodochrosite 100 mol diagram are distributed within a limited area close to the siderite end member the area can be subdivided into two fields 1 the carbonates of the ordovician ironstones of the welsh prague and thuringian basins have high siderite concentrations 87 1 to 90 6 mol magnesite contents from 6 7 to 8 3 mol and calcite concentrations from 1 6 to 2 4 mol in the ordovician ironstone of krusna hora prague basin the siderite concentrations are lower 78 3 mol and those of magnesite 15 6 mol and calcite 5 7 mol are higher 2 the carbonates of the other ironstones jurassic to late cretaceous have lower siderite 72 5 to 79 mol and higher magnesite 12 6 to 15 7 mol and calcite 6 9 to 12 2 mol contents the average values of the rhodochrosite end member lie in the range between 0 13 and 2 31 mol and the siderite rhodochrosite ratio 158 on average the analytical points of the chlorite analyses are presented in the fetot mg alvi 100 atom diagram one field containing exclusively chlorites with green internal reflections can be subdivided into two areas one area contains the chlorites of the ordovician ironstones consisting of 62 8 to 77 7 atom fetot 17 5 to 30 9 atom alvi and 4 1 to 7 4 atom mg and the other those of the jurassic to late cretaceous ironstones consisting of 55 7 to 62 5 atom fetot 26 to 33 8 atom alvi and 9 to 13 atom mg the second field contains altered chlorites of ferruginized ironstones aswan and red mountain formation these chlorites present brown internal reflections and have compositions 34 9 to 46 7 atom fetot 24 8 to 42 2 atom alvi and 16 0 to 28 5 atom mg that do not reflect the original diagenetic environment within the al al fe mg mg mg fe atom diagram the chlorite analyses are concentrated in a limited field which is distinctly separated from chlorites formed in other environments the occurrence of carbonate and chlorite both with slightly varying compositions dominated by their iron end members which occur characteristically in association with framboidal pyrite and organic matter are indications that three prerequisites for the formation of the ironstones are comparable in narrow limits these are 1 the composition of the protoliths 2 the depositional environment of the protoliths and their rates of deposition and 3 the environmental conditions that occurred during the lithification of the protoliths due to submarine diagenesis the protoliths consist of mixtures of kaolinite fe3 oxide hydroxide and organic matter the depositional environments are marine basins in which fully marine conditions occur and thus the availability of mg and ca not contained in the protoliths exists the rates of deposition control the amount of mg in the chloritess and that of mg and ca in the carbonatess and the environmental conditions during submarine diagenesis depend on the availability of organic matter creating reducing conditions due to bacterial oxidation the resulting conditions are reducing and depending on the amount of organic matter pass successively from post oxic formation of chamosite and authigenic rice grain shaped siderite via sulphidic formation of framboidal pyrite into methanic formation of abundant xenomorphic siderite that replaces chamosite

numerical modelling of dyke interaction and its influence on oceanic crust formation abstract dykes are an essential element in building oceanic crust most prominent in sheeted dyke complexes in the upper crust since dykes alter the magnitude and orientation of the local stress field they cannot be treated as passive infillings of extensional fractures we use a quasi static iterative 2 d boundary element method allowing for a wholesale movement of fluid filled fractures effects of stress and pressure gradients buoyancy and enclosed fluid mass are considered the implications of the dyke induced stress field are analysed combining the simulation of fracture propagation with computation of dyke interaction dyke interaction occurs by the adaptation of ascending dykes to the stress field caused by previous fractures arrested in the crust and leads to focussing and crossing of dykes examples for applications are introduced concerning e g the generation of a magma chamber and the formation of the sheeted dyke complex our main results are that the interaction between dykes can be considerable and that the most important controlling factor is stress the interaction is small when the horizontal tensional stress is large compared to the pressure in the dyke head otherwise dykes tend to attract each other and to form centres of high dyke density or sill layers

gully position characteristics and geomorphic thresholds in an undisturbed catchment in northern australia gullying is a significant process in the long term dynamics and evolution of both natural and rehabilitated i e post mining landscapes from a landscape management perspective it is important that we understand gully initiation and development as it is well recognized that catchment disturbance can result in the development of gullies that can be very difficult to rehabilitate this study examines gully position using geomorphic statistics relating to features such as depth width and length in a catchment undisturbed by european activity in the northern territory australia the results demonstrate that gullying occurs throughout the catchment and that a slope area threshold does not exist and that gully position broadly follows the catchment area slope relationship simple relationships relating catchment area and slope to gully depth width and length provide poor results despite these relationships having been found to apply for ephemeral gullies in cropland the results suggest that gully initiation thresholds are low as a result of an enhanced fire regime a threshold model for gully position that uses catchment area and slope to switch between gully and hillslope was evaluated and found broadly to capture gully position copyright 2006 john wiley sons ltd

patterns of development of lavaka madagascar s unusual gullies lavaka madagascar s abundant gullies show a complex pattern of development fourteen independent measures of size and shape in 93 lavaka were investigated by principal components analysis and discriminant function analysis typical lavaka start either as wounds and raw patches on mid hillside that develop step like head scarps or as toe slopes collapsed by seepage slumping or undercutting they become deep raw vertical sided teardrop to heart shaped gashes that may be deepest and broadest uphill as the walls retreat slump and become overgrown lavaka become longer broader gentler and partly filled concavities they can grow to reach from the valley floor to the hill crest or even beyond the crest before healing over unless excessive erosion creates tors and inselbergs they heal over and end up as unremarkable hillside reentrants and side valleys

temperature of the icelandic crust inferred from electrical conductivity temperature surface gradient and maximum depth of earthquakes abstract two different models of the structure of the icelandic crust have been presented one is the thin crust model with a 10 15 km thick crust beneath the axial rift zones with an intermediate layer of partially molten basalt at the base of the crust and on the top of an up domed asthenosphere the thick crust model assumes a 40 km thick and relatively cold crust beneath central iceland the most important and crucial parameter to distinguish between these different models is the temperature distribution with depth three methods are used to estimate the temperature distribution with depth first the surface temperature gradient measured in shallow wells drilled outside geothermal areas second the thickness of the seismogenic zone which is associated with a 750 c isothermal surface third the depth to a layer with high electrical conductivity which is associated with partially molten basalt with temperature around 1100 c at the base of the crust combination of these data shows that the temperature gradient can be assumed to be nearly linear from the surface down to the base of the crust these results are strongly in favour of the thin crust model the scattered deep seismic reflectors interpreted as moho in the thick crust model could be caused by phase transitions or reflections from melt pockets in the mantle

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

medium term evolution of a gully developed in a loess derived soil field surveys in the belgian loess belt revealed the presence in many forested areas of large permanent gully systems most of which are currently inactive in cultivated areas such gullies can only be observed in cross sectional soil profiles through hollows as virtually all such large gullies are currently infilled with colluvium little is known about the spatial distribution initiation and temporal evolution of these large permanent gully systems on loess derived soils therefore the medium term evolution of a gully initiated in a cultivated area on loess derived soils southwest of leuven belgium in may june 1986 was studied over 13 years two intense rainfall events created this ephemeral gully which was not erased by subsequent tillage between june 1986 and the december 1999 eight field surveys were conducted to measure gully dimensions during two surveys topographic indices e g slope and drainage area were also measured daily rainfall for the measuring period were obtained from a rainfall station located some 10 km southwest of the gully analysis of rainfall data showed that no extreme rainfall event was required to initiate such large permanent gullies as observed in forested areas and through cross sectional profiles in cultivated fields in the belgian loess belt return periods of the event that caused the gully varied between 1 year and 25 years depending on the assumptions used for defining event rain intensity once established length surface area and volume of the studied gully evolved with time cumulative rainfall or cumulative runoff following a negative exponential relation this accords with observations reported for gullies in australia and the usa this study shows that a degressive increase of gully extension can be largely explained by the evolution of a slope drainage area factor s a which is proportional to stream power with time while gully length and gully surface area asymptotically evolve towards a final value gully volume decreased at a given point in time from this it is inferred that sediment deposition will potentially infill the gully to such an extent that the farmer can drive across it from this moment on the combined effect of water and tillage erosion in the gully drainage area will lead towards rapid infilling this expected evolution of a gully in cultivated fields accords with observations of large infilled gully systems in cultivated areas in eastern belgium the permanent gullies observed under forest are attributed to the fact that after severe gully erosion this area was reforested or abandoned therefore the sediment source was cut off and the gully was not filled in by sediment deposition

the effect of the large scale mantle flow field on the iceland hotspot track abstract fluid dynamical simulations were carried out in order to investigate the effect of the large scale mantle flow field and the depth of the plume source on the structure of the iceland plume through time the time dependent location and shape of the plume in the earth s mantle was calculated in a global model and it was refined in the upper mantle using a 3d cartesian model box global flow was computed based on density heterogeneities derived from seismic tomography plate motion history served as a velocity boundary condition in both models hotspot tracks of the plume conduits and the plume head were calculated and compared to actual bathymetry of the north atlantic if a plume source in the lowermost mantle is assumed the calculated surface position of the plume conduit has a southward component of motion due to southward flow in the lower mantle depending on tomography model assumed plume age and buoyancy the southward component is more or less dominating plume models having a source at the 660 km discontinuity are only influenced by flow in the upper mantle and transition zone and hence rather yield westward hotspot motion many whole mantle plume models result in a v shaped track which does not match the straight greenland iceland faroe ridge models without strong southward motion such as for a plume source at 660 km depth match actual bathymetry better plume tracks were calculated from both plume conduits and plume heads a plume head of 120 k anomalous temperature gives the best match between plume head track and bathymetry

gully erosion impacts factors and control gully erosion attracts increasing attention from scientists as reflected by two recent international meetings poesen and valentin eds catena 50 2 4 87 564 li et al 2004 gully erosion under global change sichuan science technology press chengu china 354 pp this growing interest is associated with the increasing concern over off site impacts caused by soil erosion at larger spatial scales than the cultivated plots the objective of this paper is to review recent studies on impacts factors and control of gully erosion and update the review on dgully erosion and environmental change importance and research needst poesen et al 2003 catena 50 2 4 91 134 for the farmers the development of gullies leads to a loss of crop yields and available land as well as an increase of workload i e labour necessary to cultivate the land gullies can also change the mosaic patterns between fallow and cultivated fields enhancing hillslope erosion in a feedback loop in addition gullies tend to enhance drainage and accelerate aridification processes in the semi arid zones fingerprinting the origin of sediments within catchments to determine the relative contributions of potential sediment sources has become essential to identify sources of potential pollution and to develop management strategies to combat soil erosion in this respect tracers such as carbon nitrogen the nuclear bomb derived radionuclide 137 cs magnetics and the strontium isotopic ratio are increasingly used to fingerprint sediment recent studies conducted in australia china ethiopia and usa showed that the major part of the sediment in reservoirs might have come from gully erosion gullies not only occur in marly badlands and mountainous or hilly regions but also more globally in soils subjected to soil crusting such as loess european belt chinese loess plateau north america and sandy soils sahelian zone north east thailand or in soils prone to piping and tunnelling such as dispersive soils most of the time the gullying processes are triggered by

controlling factors of gullying in the maracuja catchment southeastern brazil hundreds of gullies vocorocas of huge dimensions up to 400 500 m long 150 m wide and 50 m deep are very common in the small maracuja catchment in southeastern brazil these erosional features which occur with an uneven intensity throughout the area started due to bad soil management practices at the beginning of european settlement at the end of the 17th century and nowadays are still evolving but at a slower rate as surface soils are usually very resistant to erosion the outcrop of the more erodible basement saprolites seems to be an essential condition for their beginning an analysis of well known erosion controlling factors was performed aiming to explain the beginning and evolution of these gullies and to understand the reasons for their spatial distribution data shows that geology and mainly geomorphology are the main controlling factors since gullies tend to be concentrated in basement rock areas with lower relief domain 2 of maracuja catchment mainly at the fringes of broad and flat interfluves at the detailed scale 1 10 000 gullies are more common in amphitheatre like headwater hollows that frequently represent upper quaternary gullies paleogullies which demonstrate the recurrence of channel erosion so gullies occur in areas of thicker saprolites domain 2 in places with a natural concentration of surface and underground water hollows saprolites of the preserved non eroded hollows are usually pressurized confined aquifer due to a thick seal of quaternary clay layer in a similar configuration to the ones found in hollows of mass movement mudflow sites in southeastern brazil therefore the erosion of the resistant soils by human activities such as road cuts and trenches valos or their mobilization by mudflow movements seem to be likely mechanisms of gullying initiation afterwards gullies evolve by a combination of surface and underground processes such as wash and tunnel erosion and falls and slumps of gully walls copyright 2005 john wiley sons ltd

the continental margin in iceland a snapshot derived from combined gps networks abstract the tectonical setting in iceland is quite complex due to the interaction of the iceland hot spot and the mid atlantic ridge while in the north of the island one active spreading zone exists the divergent motion in the centre and the south is distributed over at least two volcanic rift zones the spreading rate increases linearly along the western volcanic zone from north to south up to 8 mm yr at the hengill triple junction on the contrary the spreading rate of the parallel eastern volcanic zone decreases from 16 mm yr down to 6 mm yr at the island s southern coast the hreppar microplate between the two predominant rift zones has an independent motion which is distinct from that of the eurasian and north american plates a new detected feature is the spreading activity around the hofsjokull volcanic zone located in the centre of iceland with a significant rate of 6 mm yr during this investigation the coordinate sets of nearly 20 years of gps data acquisition on iceland were combined to get a velocity field for the surface of iceland this velocity field is based on a linear kinematic model with the consideration of local non linear effects like volcano up doming and displacements due to major earthquakes

development of badlands and gullies in the sneeuberg great karoo south africa abstract the study aims to examine the origin and development of land degradation with particular emphasis on badland and gully systems in the sneeuberg uplands of the great karoo this is an area of semiarid extensive stock farming where land degradation in the form of rill and gully erosion has accompanied the replacement of grassland by shrub vegetation species diversity has declined and ground cover has been reduced leading to a positive feedback loop which exacerbates the degradation many foot slopes developed in shales clays and colluvium have extensive incipient badland development with closely spaced gullying up to 1 5 m deep in valley bottom and valley side depression locations gullies up to 8 m deep have developed usually cut to bedrock through valley fills of mainly holocene colluvium both badlands and gullies appear to have developed since european settlement and to be part of the same hydrological system with extensive areas of bare ground badlands feeding water to incising gullies experiments using simulated rainfall throw some light on current processes badland areas are active under high frequency low magnitude rainfall events major gullies are likely to be the result of occasional high magnitude events but these have not been observed overgrazing in the past is the most likely cause of the degradation

seismotectonics of a newly formed transform zone near a hotspot earthquake mechanisms and regional stress in the south iceland seismic zone abstract we carry out mass inversion of focal mechanisms of earthquakes from 1991 to 2004 along the south iceland seismic zone within a quadrangle about 37 km wide and 65 km long with 33 878 earthquakes illuminating a crust volume of nearly 35 000 km3 our inversion aims at identifying the seismotectonic stress regimes that best account for the whole set of data not at examining local variations and stress perturbations in space and time regardless of magnitude selection the inversion reveals consistent orientations for both a primary regime with ne sw compression and nw se extension accounting for about three fourths of the data and a secondary regime with nw se compression and ne sw extension the corresponding earthquakes are highly intricate in space and time suggesting that the secondary regime which shows larger dispersion and slightly lower magnitudes as compared with the primary regime mainly results from the effects of elastic rebound stress drop and fluid migration because the data acquisition and our inversion technique are independent this analysis provides confirmation that even the negative magnitude earthquakes recorded by the icelandic seismic network sil are reliable our analysis reveals high levels of consistency between the seismotectonic stress pattern and the transform kinematics of the south iceland seismic zone the proximity of the icelandic mantle plume with a positive thermal anomaly modified the rheology of the upper lithosphere and hence influenced structural development simple shear probably prevails within a 20 km wide corridor in the underlying viscous layer inducing particular block faulting at the surface both the fault distribution and the effective block dimensions are consistent with a simple structural evolution model

alluvial gully erosion an example from the mitchell fluvial megafan queensland australia considerable attention has been focused on the role of gullies as a contributor to contemporary sediment loads of rivers in australia in southern australia rapid acceleration of hillslope gully erosion has been widely documented in the post european period last 200 years in the northern australian tropics however gully erosion processes operating along alluvial plains have not been well documented and can differ substantially from those gullies eroding into colluvium on hillslopes aerial reconnaissance surveys in 2004 along 13 500 km of the main stem rivers that drain into the gulf of carpentaria goc identified extensive areas of alluvial lands that have been impacted by a pervasive form of gully erosion more detailed remote sensing based mapping within the 31 000 km2 mitchell river fluvial megafan has identified that active gullying into alluvium occupies 0 4 129 km2 of the lower mitchell catchment these alluvial gullies are concentrated along main drainage channels and their scarp heights are highly correlated to the local relief between the floodplain and river thalweg while river incision into the megafan since the pleistocene has developed the relief potential for erosion other factors such as floodplain hydrology soil dispersibility and vegetation also influence the distribution of gullies in this paper we present a conceptual model of alluvial gullies and contend that they represent a distinct end member in the continuum of gully forms that have been described in the geomorphic literature an understanding of the processes driving this form of alluvial gullying can only be gained when they are differentiated from widely described colluvial hillslope gully models and theories we present evidence of type examples of alluvial gullying in the mitchell and through analysis of their distribution and morphology at different scales highlight some of the key mechanisms that are potentially initiating these features and driving their expansion copyright 2009 john wiley sons ltd

elastic and visco elastic stress triggering in the south iceland seismic zone due to large earthquakes since 1706 abstract damaging earthquakes in the south iceland seismic zone sisz occur fairly regularly and often as a series of events with a few days only between individual events tolerably reliable information on epicentre locations and mechanisms are available for 13 m 6 events between 1706 and 2000 for these events we computed the co and post seismic stress fields hereby approximating the sisz by a mixed elastic visco elastic layered half space the horizontal shear stress and the coulomb stress changes were analysed to detect possible trigger mechanisms which may aid future earthquake mitigation efforts we tested several criteria but must conclude that the start of an earthquake series in the sisz cannot be explained by triggering through previous events inside an individual series however one may infer triggering our results are in contradiction with the findings in other regions of the world the reason might be related to the fact that the sisz is not a mature fault zone in which old faults are re activated if a certain stress level threshold is passed in addition uncertainties in the model parameters as well as the neglect of horizontal variations in the model and of possible stress transfer due to volcanic activity further complicate the evaluation of our results and need to be taken into account in future studies