conceptual framework for assessing the response of delta channel networks to holocene sea level rise abstract recent research has identified two fundamental unit processes that build delta distributary channels the first is mouth bar deposition at the shoreline and subsequent channel bifurcation which is driven by progradation of the shoreline the second is avulsion to a new channel a result of aggradation of the delta topset the former creates relatively small branching networks such as wax lake delta the latter generates relatively few long distributaries such as the mississippi and atchafalaya channels on the mississippi delta the relative rate of progradation to aggradation and hence the creation of accommodation space emerges as a controlling parameter on channel network form field and experimental research has identified sea level as the dominant control on holocene delta growth worldwide and has empirically linked channel network changes to changes in the rate of sea level rise here i outline a simple modeling framework for distributary network evolution and use this to explore large scale changes in holocene channel pattern that have been observed in deltas such as the rhine meuse and mississippi rapid early to mid holocene sea level rise forced many deltas into an aggradational mode where i hypothesize that avulsion and the generation of large scale branches should dominate slowing of sea level rise in the last 6000 yr allowed partitioning of sediment into progradation facilitating the growth of smaller scale distributary trees at the shorelines of some deltas and a reduction in the number of large scale branches significant antecedent topography modulates delta response the filling of large incised valleys for example caused many deltas to bypass the aggradational phase human effects on deltas can be cast in terms of geologic controls affecting accommodation constriction of channels forces rapid local progradation and mouth bar bifurcation while accelerated sea level rise increases aggradation and induces more frequent channel avulsion

did the last sea level lowstand always lead to cross shelf valley formation and source to sink sediment flux 1 it is commonly believed that the efficiency of sediment flux from continents to oceans is maximized during relative sea level rsl lowstands by means of cross shelf valleys that are directly connected to the continental slope and deep marine environment while such conditions have been documented for the last rsl lowstand along several continental margins there is increasing evidence that radically different conditions persisted during the last glacial maximum lgm elsewhere with lowstand shorelines that remained on the continental shelf here we analyze the relationship between the lgm 21 ka shoreline and the shelf edge for the gulf of mexico off the united states and the bay of biscay off france a geophysical model is used to compute shoreline positions corrected for isostatic movements and the shelf edge position is quantified by means of curvature the conditions in the two study areas differed markedly throughout the gulf of mexico study area lgm sea level dropped to a point commonly 40 m below the shelf edge consistent with conventional sequence stratigraphic models while in the bay of biscay the modeled lgm shoreline remained well landward of the shelf edge in places separated by hundreds of kilometers these observations hint at potentially significant implications for 1 the source to sink sediment flux from continents to oceans and its variation in time and space 2 sequence stratigraphic models that predict deep marine sedimentation as being particularly prominent during rsl lowstands and 3 the occurrence of paleovalleys and related features on the continental shelf in addition our findings raise fundamental questions about the mechanics of shelf edge formation

planform dynamics of the lower mississippi river this paper presents an analysis of the planform behaviour of the lower mississippi river lmr using a series of maps and hydrographic surveys covering the period 1765 1975 data allow analysis at various time and space scales using fixed and statistically defined reaches both before and after extensive channel modification previous research has interpreted planform change in relation to geomorphological or engineering regime type analyses of channel length and width for the lmr as a single system the analysis here is broadly consistent with these approaches but highlights the importance of meander geometry in the form of the radius of curvature width ratio this neglected factor helps resolve paradoxes relating to observed changes in sediment transport and channel stability when viewed over smaller time and space scales analysis of dynamics using fixed reach boundaries reveals a downstream trend in the pattern of planform behaviour which is closely related to the distribution of valley floor deposits and which also reflects neotectonic influences analysis of changes using statistically determined reach boundaries shows that over shorter time scales meander trains are continually formed and modified over a period of approximately 120 years zones of more or less dynamic behaviour thus move through the lmr the research also provides a context for 20th century engineering interventions to the river these have constrained the magnitude of planform adjustment but also altered the kind of response that is now possible in relation to changes in discharge and sediment load and as a consequence of internal feedbacks within the lmr system copyright 2006 john wiley sons ltd

fluvial hydraulics of deltas and alluvial fans delta formation and its associated steam channel hydraulics have been studied using data obtained in a large laboratory flume the findings for deltas are generally applicable to alluvial fans delta or fan shape is directly related to the pattern of stream flow its loci and mode of deposition in response to a change in water discharge sediment feeding rate or base level stream channel adjustments on the delta are characterized by the formation of a single channel or multiple distributary channels the changes in channel pattern are associated with large changes in channel width and stream power expenditure stream channel changes are related to the stream s tendency to seek equal power expenditure along the channel they also provide delicate adjustments in velocity and sediment load to favor restoration of channel s equilibrium

how does a monzogranite turn into a trachydacitic extrusion mantled by basinal volcaniclastics and peperites the case of south ouessant armorican variscides france how many volcanic bodies are being confused with plutonic ones worldwide the purpose of this study is not to provide an answer to this question but rather to illustrate this issue through an example from the french armorican variscides it concerns a magmatic body cross cutting highly strained terranes in the ouessant island regarded for decades as a granitoid monzogranite on the basis of both its coarse grained texture and its mineralogy however the volcanic origin of the metamorphosed series flanking this foliated body is here recognized by pillow lavas deposit layers and fiamme bearing volcaniclastics all emplaced onto the soft substrate floor of a fault bounded basin among other things similarities between feldspar megacrysts porphyrocrysts in both the volcaniclastics and the adjoining formerly monzogranitic massive body lead us to reinterpret the latter as a trachydacitic extrusion in our model the corresponding viscous lava progressively flowed in the basin recovering the earlier volcanic formations and inducing load effects on the underlying soft sediments along with compaction of the previously deposited pumices to produce fiamme the interpretation of the south ouessant area as a visean transtensional volcano sedimentary basin provides a new perspective on the distribution of the variscan pull apart basins in the armorican massif

the role of pedogenesis in palaeosols of mexico basin and its implication in the paleoenvironmental reconstruction abstract most of the paleoenvironmental information for the basin of mexico comes from sedimentary proxies which unfortunately are incomplete for the terminal pleistocene and the holocene in this paper we present a temporal and spatial reconstruction of past soil cover in the south and north of the basin of mexico during the mis 2 and 1 marine isotope stage 2 and 1 palaeosol profiles were selected from three locations cuicuilco copilco and avon in the south basin and compared with soils studied in the teotihuacan valley pachuca and texcoco at north of the basin the results show that the pleistocene soil cover is more homogeneous dominated by the presence of luvisol type soils with secondary carbonates dated to around 20 000 cal yr bp during the holocene the soil distribution changes with andosols developing in the south while vertisols and calcisols form in the north the new soil distribution reflects differing climate conditions during the holocene conditions which characterize the basin today the andosol cover in south basin however represents the result of short term pedogenesis andosols are considered high quality soils that were attractive to settlers and were a factor in the development of sedentary villages

autogenic attainment of large scale alluvial grade with steady sea level fall an analog tank flume experiment a graded river conveys its sediment load without net deposition or erosion the graded state is thought to represent the long term response of alluvial rivers to steady external forcing we show here that alluvial rivers building deltas can be in grade as an autogenic response to steady sea level fall consider an antecedent graded river profile the upstream end of which consists of an alluvial bedrock transition and the downstream end of which is a fixed overfall where constant sea level is maintained the antecedent graded profile is then drowned by a jump in sea level after which sea level drops the result is a new river profile ending in a prograding delta that deposits on top of the antecedent profile if the rate of sea level fall is constant and the length of the antecedent reach is sufficient the new profile eventually becomes parallel or quasi parallel to the antecedent profile maintaining grade as it progrades in the experiments reported here series of graded river profiles with prograding deltas are created by stacking fluviodeltaic systems each graded profile and its associated delta is stacked on its immediate predecessor for each fluviodeltaic system a graded alluvial profile is attained with any constant rate of sea level fall provided that the antecedent profile is of sufficient length experiments suggest that this autogenic approach to grade is more rapid for higher rates of sea level fall lower rates of sediment supply and higher water discharges

loess like and palaeosol sediments from lanzarote canary islands spain indicators of palaeoenvironmental change during the late quaternary abstract on lanzarote canary islands quaternary saharan dust and weathered local volcanic material were trapped in miocence to pliocene valleys dammed by younger volcanic edifices these sediments show sequences of alternating reddish clayey and loess like yellowish silty material in order to investigate if reddish clayey layers contain material derived from local pedogenesis and if so which pedogenetic processes were active we performed sedimentological micromorphological and environmental magnetic analyses the analyses demonstrate that these layers contain material derived from local soils these soils were characterised by clay formation rubefication and the formation of superparamagnetic particles during periods of enhanced soil moisture thus they can serve as natural archives in order to reconstruct the terrestrial palaeoclimatic history of lanzarote the distribution of soil material in the profiles shows that cold periods of the late quaternary were characterised by more humid conditions than today using palaeontological remains and a comparison with recent soils on tenerife we can roughly estimate maximal palaeoprecipitation values during more humid periods

ebb tide dynamics and spreading of a large river plume abstract momentum balances in the near field region of a large tidally pulsed river plume are examined the authors concentrate on a single ebb tide of the columbia river plume using the regional ocean modeling system roms configured to hindcast flow conditions on the washington and oregon shelves and in the columbia river estuary during ebb plume interior streamwise balances are largely between advection pressure gradient and frictional forces stream normal balances in this region reduce to centrifugal cross stream pressure gradient and coriolis terms i e the gradient wind balance commonly assumed in river plume bulge investigations temporal derivatives are most important at the plume front and as the ebb progresses winds were light and contributed little to the force balance midebb stress and vertical salt flux were largest at a midplume depth where stratification and vertical shear were also high consistent with shear induced mixing internal stress slows the spreading plume consi

age and archaeological implications of xitle volcano southwestern basin of mexico city the pedregal lavas are fresh well exposed basaltic flows erupted from the xitle scoria and cinder cone in the southwestern part of the basin of mexico these lavas cover an area of 70 km 2 and were emplaced over pyramids and other buildings e g cuicuilco and copilco archaeological sites today a part of mexico city including the national university is built on the flows initial strombolian activity produced an ash fallout layer which was immediately followed by effusive emplacement of lava flows the xitle cone grew on the north facing slope of ajusco volcano and lava flowed down to the n ne until it reached the basin floor more than 30 radiocarbon dates have been obtained by several workers on charcoal samples from beneath the lava and several ages for the eruption have been proposed from these dates most dated samples were not directly produced by xitle s eruption but instead are artifacts of human activity that predates the eruption thus these ages mostly about 2000 bp are older than the eruption a new age of 1670 35 years bp ad 245 315 obtained on charcoal samples collected just beneath the lavas is favored for the xitle eruption these samples originated by ignition of vegetation during the emplacement of hot scoriaceous tephra the new age is within the classic period of mesoamerican archaeology whereas the earlier reported ages are at the end of the preclassic the new age carries important implications for the timing of population shifts within the basin of mexico q 2000 elsevier science b v all rights reserved

rock magnetism and pedogenetic processes in luvisol profiles examples from central russia and central mexico abstract despite a vast literature concerning magnetic properties of loess sequences we still do not fully understand how magnetic components and properties are related to particular soil forming processes that can vary with each type of genetic horizon in order to establish the role of lithogenic factors in the link between magnetic properties and soil forming processes we carried out a study of two complete profiles of buried interglacial luvisols one formed in loess in russia alexandrovsky quarry and the other in volcaniclastics in mexico barranca tlalpan in both profiles soil genetic horizons have contrasting differences of their magnetic properties in the alexandrovsky profile the magnetic susceptibility is enhanced in the paleosol compared to parent material in the barranca tlalpan sequence enhancement is absent in the soil profile increase of fine grained magnetic components in the soil is attributed to neoformed minerals however this process cannot compensate for the loss of lithogenic magnetic minerals in any of the genetic horizons and the resulting trend is depletion in the whole soil profile the pedogenic environment of eluvial horizons in both luvisols is destructive to all magnetic components both primary and secondary higher concentrations of antiferromagnetic components hematite and goethite found in e horizons are related to redoximorphic processes

tephrochronological study in the quaternary val d agri intermontane basin southern apennines italy in the south eastern depocentre of the val d agri basin southern apennines a volcanic ash layer crops out interbedded within poorly structured alluvial fan deposits of late pleistocene age textural depositional and pedological features of this weathered layer suggest a primary deposition from a pyroclastic fall out of volcanic ash chemical analyses of feldspars show an alkali trachytic composition and accessory minerals association allow to correlate this tephra layer with the regionally dispersed y 7 marine tephra layer tufo verde epomeo eruption ischia volcano dated at 56 4 ka the val d agri tephra here described for the first time was deposited during mis stage 3 its recovery and characterization permit to contribute to regional correlation of the mediterranean climatic and volcanic events from marine to continental successions and to describe landscape evolution of the southern apennines during glacial interglacial cycles

the lowermost mississippi river a mixed bedrock alluvial channel in this study the distribution of channel bed sediment facies in the lowermost mississippi river is analysed using multibeam data complemented by sidescan sonar and compressed high intensity radar pulse seismic data as well as grab and core samples of bed material the channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls the consolidated substratum is made up of latest pleistocene and early holocene fluvio deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends the exposed substratum commonly displays a suite of erosional features including flutes that are quantitatively similar in form to those produced under known laboratory conditions a total of five bed facies are mapped three of which include modern alluvial deposits and two facies that are associated with the relict substratum a radius of curvature analysis applied to the mississippi river centreline demonstrates that the reach scale distribution of channel bed facies is related to river planform from a broader perspective the distribution of channel bed facies is related to channel sinuosity higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment for example the ratio of alluvial cover to substratum is ca 1 5 1 for a 45 km segment of the river that has a sinuosity of 1 76 and this ratio increases to ca 3 1 for a 120 km segment of the river that has a sinuosity of 1 21 the exposed substratum is interpreted as bedrock and given the relative coverage of alluvial sediment in the channel the lowermost mississippi river can be classified as a mixed bedrock alluvial channel the analyses demonstrate that a mixed bedrock alluvial channel boundary can be associated with low gradient and sand bed rivers near their marine outlet

forced regressions in a sequence stratigraphic framework concepts examples and exploration significance sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in relative sea level on the shelf lowstand deposits which form during falls and subsequent stillstands of relative sea level can be recognized by the presence of an unconformity at the base the isolated and basinward position relative to the previous shoreline and the abrupt seaward translation of shallow water and shoreline facies into the basin across an unconformity surface this seaward translation of facies and shoreline regression in response to relative sea level lowering is termed a forced regression a forced regression is independent of variations of sediment flux and is in contrast with normal regressions that occur in response to excess sediment flux relative to space available on the shelf i e accommodation forced regressions commonly are associated with a zone of sedimentary bypass subaerial exposure and possible fluvial erosion between the newly formed and preceding shorelines certain shelf sands previously interpreted as offshore or mid shelf sand bodies thus can be reinterpreted as stranded lowstand shorelines associated with forced regressions this alternative interpretation has economic significance insofar as it suggests different subsurface correlations and reservoir geometries with the potential for development of new play types and enhanced recovery in o der fields examples of forced regression can be observed at a variety of scales and ages several such examples include the modern east coulee fan delta and the lower cretaceous viking formation in alberta canada the quaternary rhone delta and the quaternary hudson valley system

late holocene paleopedological records contained in tephra from el chichon volcano chiapas mexico abstract high rates of pedogenesis on unconsolidated volcanic materials imply the accelerated generation of the soil memory the purpose of this research was to extract the high resolution centennial scale paleopedological records from the late holocene paleosols of the el chichon volcano formed during the last 2000 years we studied buried monogenetic andosols on the dated tephras and a pedocomplex formed from a set of thin ash layers deposited on older acrisol derived from shales particle size distribution dcb extractable fe oxalate extractable fe al and si sand and clay mineralogy organic carbon content and stable c isotope composition in humus were evaluated and interpreted as the elements of the soil memory however some differences were found in properties of two buried soils especially in the kind of a horizons we attribute such differences of individual andosols to be caused by ancient human impact because it has been demonstrated that humid conditions persisted during the last 1300 years thus the drought that was supposed to provoke the collapse of the maya civilization at the end of i millennium a d should have been a short term event below the resolution of this paleopedological record the andic acrisol pedocomplex demonstrated the domination of the humid tropical climate over longer time intervals probably major part of the holocene with possible minor dry episodes

secondary ferrimagnetic minerals in welsh soils a comparison of mineral magnetic detection methods and implications for mineral formation summary a range of low temperature and isothermal magnetic measurements are used to identify the secondary ferrimagnetic mineral sfm grain sizes in 10 representative soil samples from wales a comparison of percentage frequency dependent susceptibility fd percentage and low temperature remanence measurements shows that they are sensitive to different ranges of superparamagnetic sp grains the relative distributions of sp grains and stable single domain ssd grains are similar in nine of the samples typical distributions for soils dominated by sfms are 20 30 per cent ssd and 70 80 per cent sp multidomain md grains were not detected in the samples studied there is evidence that some soils contain significant numbers of ultrafine sp grains 0 010 m that are not detected by low temperature remanence measurements at 20 k and which will have the effect of depressing values of low field susceptibility lf and fd percentage a mixing model suggests that fd percentage may be used semi quantitatively to estimate the proportion of sp grains in a sample the positively skewed grain size distributions strongly suggest a mechanism of sfm formation that is driven by processes at the 10 8 m scale thus supporting weathering and fermentation as controlling processes rather than the degradation of ssd bacterial magnetosomes and primary minerals

magnetic fabric and flow direction in basaltic pahoehoe lava of xitle volcano mexico abstract we sampled five basaltic lava flow units from xitle volcano mexico city to study the variation of anisotropy of magnetic susceptibility within their cooling boundaries we find that the mean maximum susceptibility parallels the geologically inferred flow direction in the units that were emplaced on a steeper slope whereas for those on a negligible slope the mean intermediate susceptibility points in the flow direction we propose however that the maximum susceptibility always points in the direction of local movement and that a change in slope produces a deviation of the local motion from that of the unit as a whole the axis of susceptibility closest to the geologically inferred flow direction usually plunges upflow in the basal part of the flow unit comprising an imbrication which clearly marks the flow azimuth of the lava thus the scenario of emplacement may influence the results in a predictable way we suggest that the degree of anisotropy could bear a direct relationship to either the viscosity of the lava the morphology of the flows or both based on a comparison with lavas from azufre argentina and ko olau o ahu volcanoes also we suggest that the shape of the susceptibility ellipsoid may be related to the degree of internal deformation of the lava flows we also compare the two methods currently available to calculate regions of confidence around the mean principal susceptibilities

eocene paleosols on king george island maritime antarctica macromorphology micromorphology and mineralogy abstract we investigated the origin of reddish layers between basalt flows in an early eocene volcanic succession in king george island maritime antarctica previous studies in the area classified these layers as regolith surfaces baked zones and paleosols but the properties of these layers and the reasons for their classification are not clarified since the early eocene epoch was one of the warmest phases on earth during the cenozoic a proper investigation of these layers is crucial to obtain additional regional information about the paleoenvironment the focus of this study is on three profiles located in the admiralty bay on king george island they were classified as paleosols based on field micromorphological and mineralogical properties main soil forming processes identified by micromorphological analyses were clay neoformation pedo bioturbation and a weak redoximorphosis the occurrence of plant residues in all profiles was a reliable indicator that these soils formed a land surface during the early eocene abundant weathered volcanic glass and pyroclasts may indicate andosolization although the main diagnostic features for andosols which are the presence of nanocrystalline minerals and organic matter accumulation were not detected or only in small amounts respectively the long term burial of the paleosol could be a reason for lacking of these diagnostic features clay minerals associated with moderately weathered volcanic soils smectite were detected in the x ray diffraction analysis a predominantly reddish colour of most of the soil horizons is given by hematite confirmed by the determination of pedogenic oxides nevertheless rubification was not solely a result of pedogenesis but also of reddening of oxyhydroxides by dehydration during diagenesis other diagenetic processes were burial compaction loss of organic matter and induration in the horizons directly buried by a basalt flow voids were filled by zeolite crystals an effect related to lava cooling another possible diagenetic effect was the geochemical rejuvenation and homogenization of the profiles despite the occurrence of these features the results are in agreement with previous studies indicating a humid temperate paleoclimate for king george island during the early eocene

quantitative models of sedimentary basin filling summary quantitative modelling of the filling of sedimentary basins was begun in earnest in the 1960s dozens of themes and variations have been proposed since then and have yielded an abundance of idealized stratigraphic patterns as functions of both imposed changes and basin properties post plate tectonic modelling began with rigid lid models which show the stratigraphic signature of subsidence variation this work introduced the connection between stratigraphy and the rheology of the lithosphere rigid lid models are the simplest type of geometric model in which the sediment surface is assigned prescribed geometries usually corresponding to different depositional environments these can reproduce many aspects of overall stratal geometry but are formally restricted to relatively long timescales for which quasi steady surface topography can be assumed so called dynamic models attempt to represent the morphodynamics of the sediment surface by abstracting and averaging short term transport processes most of the dynamic models proposed to date can be seen as special cases of a single general morpho dynamic equation the most important result of the first wave of quantitative basin filling models is that even relatively simple models can produce reasonable stratal patterns we now have a wide array of tools for exploring scenarios searching for general behaviours and effects and making initial quantitative predictions we have also learned that basin response to external forcing as recorded in stratigraphy can be as sensitive to the characteristics of the basin as to the forcing the main brake on the development of basin modelling is not computing power but lack of methods and data for testing the models we have already developed physical experiments which are only just beginning are one means of doing this experimental stratigraphy is a bridge to quantitative field tests which will require collaboration among academic researchers from a wide range of areas and between academia and industry on projects of greater scale and degree of integration than we have seen so far the advancement of quantitative sedimentary geology will also require significant changes in the way the subject is taught at all levels

punctuated sand transport in the lowermost mississippi river 1 measurements of sand flux and water flow in the mississippi river are presented for a portion of the system 35 50 km upstream from the head of its subaerial delta these data are used to provide insight into how nonuniform flow conditions present in the lower reaches of large alluvial rivers affect the timing and magnitude of sand transport near the river outlet field surveys during both low and high water discharge include 1 sequential digital bathymetric maps defining mobile river bottom topography which were used to estimate bed material flux 2 multiple water velocity profiles and 3 multiple suspended sediment profiles collected using a point integrated sampler these data show that total sand transport increases by two orders of magnitude over the measured range in water discharge 11 300 to 38 400 m3 s 1 during low water discharge no sand is measured in suspension and sand discharge via bed form migration is minimal during high water discharge 54 of the sand discharge is measured in suspension while 46 of the sand discharge is part of bed form migration the component of boundary shear stress associated with moving this sediment is estimated using a set of established sediment transport algorithms and values for the total boundary shear stress are predicted by fitting logarithmic velocity functions to the measured profiles the estimates of boundary shear stress using measurements of suspended sand transport bed form transport and downstream oriented velocity profiles are internally consistent moreover the analyses show that boundary shear stress increases by nearly 10 fold over the measured water discharge range we show how this increase in shear stress is consistent with backwater flow arising where the river approaches its outlet the hydrodynamic properties of backwater flow affect the timing and magnitude of sand flux and produce punctuated sand transport through the lowermost mississippi river our field data are used to evaluate the influence of this sand transport style on development of the mixed bedrock alluvial channel for the lowermost mississippi river

palaeolandforms and volcanic impact on the environment of prehistoric cuicuilco southern mexico city abstract the pedregal lava field covers an area of 80 km 2 in the southern basin of mexico and includes the cuicuilco archaeological site cuicuilco is considered an early urban centre in the basin of mexico constructed during the late preclassic a few centuries bc and abandoned at the beginning of the first millenium ad three new radiocarbon ages together with reinterpretation of previous ages and detailed analyses of the slopes of the cuicuilco s pyramid support the hypothesis that cuicuilco was abandoned long before the xitli eruption xitli lavas followed several river valleys damming some and causing them to silt up magdalena xocotitla puente volado and chicuautitla rivers subsequently deflecting the old drainage from lake xochimilco to lake texcoco upper and lower piedmont and river landscapes are reconstructed on the basis of soil characteristics and lithostratigraphic units palaeolandforms suggest an agricultural environment prior to the xitli eruption

volcanic impact on the southern basin of mexico during the holocene the interstratification of volcanic and glacial deposits in central mexico has been helpful in correlating the time sequence of these events two glacial advances and many eruptions have occurred during the last 10 000 years although conditions have changed drastically since the last eruptions in the 1920s popocatepetl the origin and evolution of several lakes basin of mexico are related to vulcanism and periods of warmer and colder climatic conditions fifty monogenetic volcanoes have erupted near mexico city during the holocene probably during 20 22 different eruptive periods since some are superimposed vents similar to the jorullo vents which erupted 200 years ago in michoacan mexico the youngest of these volcanoes are chichinautzin and xitle 2030 60 years bp whose lavas covered approximately 80 km each the xitle eruption began with ash emission mainly towards the nw and was later dominated by lava flow fed by tubes this lava covered the cuicuilco prehispanic urban center and the agricultural areas near it the lava dammed and caused silting of the magdalena xicotitla and chicuautitla rivers and deflected the drainage pattern in general the eruption of chichinautzin and the other holocene volcanoes have drastically changed the topography and caused fires as can be seen in the carbonized branches at the base of the flows chichinautzin volcano was formed by successive thin lavas which flowed to the south and by thicker flows to the east west and north older holocene volcanoes are responsible for the formation of the zempoala and xochimico lakes popocatepetl is a 5450 m high stratovolcano to the east of mexico city which has been subject to glacial processes its eruptions have been documented since the 14th century the last being in the 1920s most of these eruptions caused ash fall mainly to the east and northeast but lava emissions and flooding have also been reported larger eruptions in the earlier history produced pyroclastic flows and surges lahars plinian pumice falls and two debris avalanches a preliminary map of the type of volcanism which has affected the different areas and its implications for future hazards is presented

paleosols beneath a lava flow in the southern basin of mexico the effect of heat on the paleopedological record abstract volcanic activity affects past and present landscapes here we present a case study related to the heat impact of the xitle lava flow on the former soil cover the eruption occurred 1670 years b p in south mexico city the evaluated properties include those related to the soil memory used as tools of paleoenvironmental interpretation in volcanic paleosols macro and micromorphology color grain size distribution magnetic susceptibility and carbon and nitrogen contents changes of these properties by the heat influence are relevant for an adequate paleoenvironmental reconstruction two sites and three soil profiles are selected to show soil modifications by the volcanic activity the two sites represent the following 1 slightly transformed paleosol buried by the lava flow but separated from it by a 10 cm thick volcanic ash layer site cuicuilco enah 2 moderately and strongly transformed paleosols two profiles inside the copilco archeological zone one of the copilco profiles represents a buried paleosol with a very thin 3 cm ash layer on top at the contact with the lava the second profile is the most affected by the lava flow as it is directly beneath the lava all studied paleosols are sandy have a low amount of organic matter narrow c n ratios and very high values of magnetic susceptibility the strongest modifications are observed in the upper horizons which are in contact with the lava however the presence of the ash layer minimizes the heat effect despite the observed modifications it is possible to establish three cycles of soil formation in the paleolandscape the first and the oldest related to the development of a luvisol type soil indicates a longer time of landscape stability during the second cycle a pedosediment with a high concentration of artifacts was formed the last and the youngest cycle includes the ab and the aeb horizons present only in copilco they form by an andosol type pedogenesis which reflects a shorter time of soil development the presence of the aeb horizon is interpreted as the result of the degradation of the ab horizon either due to the heat effect of the lava or to cultivation and irrigation practiced by the pre hispanic communities

magnetic stratigraphy of chinese loess as a record of natural fires magnetic susceptibility records of paleosols and loess show high correlation with oxygen isotope stratigraphy of ocean sediments kukla 1987 providing a global paleoclimatic record different models have been put forth to explain the nature and cause of susceptibility variations but consensus has not yet been achieved our low temperature studies reveal a secondary magnetite component in paleosols that is characterized by a higher verwey transition 115k than that for the magnetite 100 k in unaltered loess the same shift in the verwey transition can be achieved by heating and cooling loess samples this is consistent with a new hypothesis that the magnetic signal from paleosols may be produced by natural fires in the past natural fire intensity is sensitive to the amount of annual precipitation so that increased fire induced susceptibilities should reflect an increase in the humidity of regional climate

linking river flood dynamics to hyperpycnal plume deposits experiments theory and geological implications turbid river plumes entering ocean or lake water of lesser density i e hyperpycnal plumes can plunge to form turbidity currents providing an important link between terrestrial sediment sources and marine depositional sinks a leading hypothesis suggests that hyperpycnal plume deposits accurately record the rising and falling discharge of a flooding river in terms of sediment size grading bedform sequence and deposit thickness which if correct has significant implications for unraveling river dynamics reservoir potential and earth history from marine event beds herein we present one of the first experimental flume studies aimed at testing this hypothesis results indicate that depth averaged hyperpycnal plume velocities can be uncorrelated or even anti correlated with river discharge at certain seabed locations because of translation of the plunge point resulting from temporal variations in discharge and sediment concentration through the duration of a river flood an advection length scale of settling sediment is found to be an important control on hyperpycnal plume deposits where coarse sediment sand is most likely to record multiple flow accelerations and decelerations related to plunge point translation even for a river flood with a single peaked hydrograph in contrast fine sediment mud is relatively insensitive to local plunge point dynamics and is most likely to preserve directly rising and falling river discharge finally it was found that the necessary fluvial sediment concentration to form a plunging plume can be much larger than the concentration typically used assuming density equivalence because of deposition upstream of the plunge point

morphodynamics of subaqueous levee formation insights into river mouth morphologies arising from experiments 1 levee formation by deposition from sediment laden flows debouching into open waters has been conceptually and analytically linked to jet hydrodynamics since the middle of the 20th century despite this long standing association between jets and levees the morphodynamics controlling subaqueous levee development remain largely unquantified and poorly understood here we report the results of physical experiments on subaqueous levee development modeled on floodplain tie channel processes in the laboratory we created subaqueous levees from a sediment laden jet entering a basin of still water levee formation occurred where the local shear velocity of the jet declined below the critical shear velocity for entrainment of sediment into suspension the rate of levee deposition depended on the rate of lateral sediment transfer to the jet margins and the settling velocity of suspended sediment in our friction dominated flows lateral sediment flux to the levees was primarily due to dispersive lateral transport driven by turbulence high rates of sediment transfer from the jet core to the margins led to levee growth that outpaced deposition along the flow centerline under hypopycnal conditions we failed to produce levees we observed distinct differences in levee morphology due to changes in the size and density of suspended sediment under identical hydrodynamic conditions our experimental results and review of published data on hypopycnal river mouths suggest that suspended sediment characteristics relative magnitude of bed friction and channel outlet aspect ratios may have a more significant influence on river mouth morphology than the buoyancy of discharging waters as envisioned in existing conceptual models

buried palaeosols of the nevado de toluca an alternative record of late quaternary environmental change in central mexico buried palaeosols of central mexico not previously analysed from a palaeopedological standpoint have helped to develop a reliable regional model of late quaternary climatic change this paper focuses upon morphological and micromorphological properties particle size distribution and extractable fe al and si of seven palaeosols named pt1 pt7 in the vicinity of the nevado de toluca volcano the characteristics of andic cambisol pt1 similar to those of modern soils in semi arid environments indicate a drier climate in the first half of the holocene humic andosols pt2 pt4 are analogous to modern volcanic soils of humid forest environments they show evidence that a moist palaeoclimate prevailed during marine oxygen isotope stages ois 2 and 3 luvisols pt5 and pt6 which are assumed to have formed at the end of marine ois 5 to marine ois 4 also indicate humid conditions we attribute the differences between andosols pt2 pt4 and luvisols pt5 pt7 to variations in the duration in the pedogenesis rather than to changes in palaeoclimate micromorphological features of andosol to luvisol transition confirm that these soils form an evolutionary sequence being for the most part consistent with lacustrine records the palaeosol properties do not reflect the episodes of drier climate during the last glacial maximum however as shown by diatom and palynological data from lake sediments mesoclimate variations and or incompleteness of soil memory reflecting mostly periods of humid pedogenesis probably provide the reason for this disagreement copyright 2001 john wiley sons ltd

diagenetic reddening of early eocene paleosols on king george island antarctica abstract the objective of this study was to determine if the early eocene paleosols on king george island maritime antarctica have acquired their reddish color during the paleopedogenesis by burial diagenesis and or by heating of a covering lava flow we used micromorphology diffuse reflectance spectroscopy x ray diffraction and mineral magnetic properties to identify the iron oxides in these paleosols these are weakly moderately developed paleosols formed on a basaltic tephra under a cool humid paleoclimate therefore prone to ferrihydrite and goethite rather than hematite formation however the iron oxide assembly was dominated by maghemite and hematite nevertheless the large grain size and high crystallinity of hematites suggested they are rather diagenetic than pedogenic the clustering content and distribution of hematites finely dispersed on the groundmass coating replacing primary minerals and as mottles and or nodules rather than their crystal size were responsible for the color differences these properties were likely the result of past pedogenesis processes forming originally ferrihydrite linked with paleodrainage conditions heating by the covering lava flow affected the magnetic properties of two profiles but without affecting their color taken together all these results suggest that the burial reddening took place by dehydration and transformation of ferrihydrite to hematite our findings highlight the importance of detailed mineralogical analysis to identify iron oxides present in reddish paleosols because identification based solely on morphology e g yellowish color gradual horizon transitions can lead to misinterpretation of paleoenvironmental conditions

andosol to luvisol evolution in central mexico timing mechanisms and environmental setting abstract surface and buried andosols and buried luvisols of the nevado de toluca late quaternary tephra paleosol sequence central mexico were studied to show whether these soils present an evolutionary sequence and to determine the pedogenic mechanisms and environmental factors involved in the evolutionary process micromorphological observations and mineralogical composition of fine sand and clay fractions were used to detect type and succession of soil forming process some of the buried andosols defined as intergrade andosols have a predominantly blocky structure humus depleted areas redoximorphic features and thin clay coatings in ah horizons clay fractions of buried andosols contain halloysite besides amorphous components whereas in modern andosols allophane is dominant luvisols have micro areas with granular structure and abundant phytoliths in the groundmass of bt horizons assumed to be the relict andosol features luvisol clay fractions are dominated by halloysite and kaolinite primary minerals show micromorphological weathering features in all studied soils being stronger in luvisols however even in luvisols sand fractions consist mostly of unstable volcanic silicates we hypothesise that the studied profiles form an evolutionary sequence andosols intergrade andosols luvisols the soil transformation is supposed to be linked to progressive crystallisation of 1 1 clay minerals comparing the nevado de toluca paleosol properties with the existing data on volcanic soil climo and chronesequences and assessing the regional paleopedological and lacustrine records of quaternary paleoclimates we concluded that wet dry climatic oscillations took place during the formation of the studied paleosols rapid crystallisation of 1 1 minerals occurred during dry phases which speeded up the andosol to luvisol transformation and made it independent from the primary mineral weathering status the andosol to luvisol transformation accelerated by climatic fluctuations is thought to be a common soil evolutionary pathway in the subtropical and tropical regions of recent volcanism which suffered contrasting precipitation oscillations in the quaternary

backwater controls of avulsion location on deltas river delta complexes are built in part through repeated river channel avulsions which often occur about a persistent spatial node creating delta lobes that form a fan like morphology predicting the location of avulsions is poorly understood but it is essential for wetland restoration hazard mitigation reservoir characterization and delta morphodynamics following previous work we show that the upstream distance from the river mouth where avulsions occur is coincident with the backwater length i e the upstream extent of river flow that is affected by hydrodynamic processes in the receiving basin to explain this observation we formulate a fluvial morphodynamic model that is coupled to an offshore spreading river plume and subject it to a range of river discharges results show that avulsion is less likely in the downstream portion of the backwater zone because during high flow events the water surface is drawn down near the river mouth to match that of the offshore plume resulting in river bed scour and a reduced likelihood of overbank flow furthermore during low discharge events flow deceleration near the upstream extent of backwater causes enhanced deposition locally and a reduced channel fill timescale there both mechanisms favor preferential avulsion in the upstream part of the backwater zone these dynamics are fundamentally due to variable river discharges and a coupled offshore river plume with implications for predicting delta response to climate and sea level change and fluvio deltaic stratigraphy

delta progradation driven by an advancing sediment source coupled theory and experiment describing the evolution of elongated deltas 1 most of the models of alluvial fan and delta morphodynamics to date have used one of two assumptions concerning the upstream boundary condition 1 a fixed sediment feed point at the vertex or 2 a feed point corresponding to a bedrock alluvial transition that e g migrates upstream under conditions of constant base level here however we present both experimental and numerical results pertaining to a new configuration i e one in which the sediment feed point migrates downstream the research is motivated by the operation of the tailings pond of a mine in which a delta forms the mine operators must frequently move the outfalls of their slurry pipelines downstream to avoid their burial under sediment as the delta aggrades this downstream migration results in a characteristic delta configuration under the right conditions as the tailings delta progrades downstream it does not have time to fill all the available space in the lateral direction thus leaving unfilled open water on either side the result is the formation of an elongated delta here we use both experiments and numerical modeling to characterize the control of the feed point migration rate on the lateral extent of the prograding deltas we use these results to obtain a partial explanation of natural elongated deltas in the case of such deltas the downstream progradation of natural levees provides an analog to the downstream migration of the outfall of a slurry pipeline

palaeomagnetic study of the xitle pedregal de san angel lava flow southern basin of mexico abstract a detailed study of the 2000 years old xitle pedregal de san angel volcanic field in the southern basin of mexico was undertaken to assess the reliability of the palaeomagnetic record as derived from fresh well preserved and exposed lava flows the xitle vent is on the slope of the ajusco volcano which results in a topographic difference of over 800 m in less than 12 km of horizontal distance most sites present a mean direction with a small within site dispersion around the dipolar direction for the locality but some sites particularly in the basin sector of flat relief away from the vent show shallow inclinations anisotropy of magnetic susceptibility ams shows normal flow fabrics with horizontal foliation planes and small anisotropy degree ams lineations correlate with observed flow directions the magnetic properties vary systematically across flow units but directions do not show a consistent pattern secular variation effects do not apparently contribute to the shallow inclinations or directional scatter some lava structures like pressure crests and blocky fronts give shallow inclinations and scattered directions respectively the resulting overall mean direction is well defined and close to the dipolar direction b 26 dec 359 8 inc 32 8 k 167 95 2 2 but this excludes two apparent directional groups the mean direction for one group with b 19 dec 359 0 inc 35 1 k 247 95 2 1 may be the representative estimate for the field shallow inclinations are considered anomalous and associated to a characteristic specific to given sectors of lava flows palaeointensity determinations have been obtained for six samples from five sites by using the thellier and shaw methods results agree well with previous studies however the standard deviation calculated for the mean value remains high after incorporation of the new data mean palaeointensity based on five new determinations and eight early data is n 13 56 50 6 16 mt and the mean for the overall data set is n 20 60 50 9 21 mt results illustrate some practical considerations and limitations in methodology and the reliability of the palaeomagnetic record in volcanic rocks

late pleistocene buried humic soils within a tephra soil sequence near unzen volcano kyushu japan abstract relatively few late pleistocene buried humic soils in volcanic areas have been studied to date because they are not common worldwide the present study examined the origin of soil organic matter som in mainly late pleistocene buried humic soils within a tephra soil sequence near unzen volcano northeastern shimabara peninsula kyushu sw japan the buried humic soil horizons were divided into seven horizons in a tephra soil sequence the total carbon contents were very high for late pleistocene soils with a value of 60 g kg 1 obtained for the oldest buried humic soil 8ab horizon the main phytolith types in the buried humic soils are pleioblastus sect nezasa in a and 2ab and sasa sect crassinodi in 3ab 4ab 5ab 6ab 7ab c and 8ab som was derived mainly from gramineae in the oldest buried humic soil 8ab horizon the 13c values indicate that c3 plants contributed more than 50 of the plant derived carbon in the som and the phytoliths indicate the dominance of c3 grasses e g sasa sect crassinodi in the ne and se of the peninsula the late pleistocene humic soils formed due to an abundant supply of organic matter from mainly grass vegetation such as sasa sect crassinodi regardless of climate change at 30 ka or earlier in the unzen volcano area the present study contributes to the paleoenvironmental reconstruction for the late pleistocene

weathering characteristics of interflow volcanic boles from mandla lobe eastern deccan volcanic province abstract the deccan volcanism straddling the cretaceous paleogene boundary resulted into one of the most extensive continental flood basalts in the western and central peninsular india the interflow sedimentary units boles often cited as paleosols are the rare archives of paleoenvironmental conditions existed during successive lava flows a debate on the origin of boles as true paleosols exists therefore in order to trace out regional extent of such conditions pedogenic non pedogenic in the context of their paleoenvironmental significance during the deccan volcanism we studied here mineral magnetic characteristics of 36 color differentiated bole beds from the mandla lobe in the eastern deccan volcanic province edvp classified into four types gray yellow brown and red based on their munsell notations the gray yellow and brown boles show dominantly ferrimagnetic composition chiefly magnetite and titanomagnetite while red boles characterize mixed ferri and antiferromagnetic mainly hematite mineralogy the granulometric domain parameters suggest larger single domain sd and the multidomain md ferrimagnets in the majority of cases whereas superparamagnetic sp particles with frequency dependent susceptibility fd 2 6 to 20 3 10 8 m 3 kg 1 are limited to the few red and brown boles only the significant presence of primary parent magnetic minerals i e coarse sd md titanomagnetite and magnetite in the majority of boles depict incipient to moderate weathering owing to limited time intervals during successive eruptions the combination of hematite and sp particles present only in the few red and brown boles depicts baking effect rather than pedogenesis the comparison of magnetic properties of boles from edvp and western deccan volcanic province wdvp shows significant differences in the concentration of sp ferrimagnetic particles and hematite attributed to variable weathering intensity duration and heating related alterations from the upper flows both edvp and wdvp boles indicate similar first order paleoenvironmental conditions without any major pedogenesis that can be regionally correlated

magnetic properties of modern soils and quaternary loessic paleosols paleoclimatic implications the magnetic properties of paleosols developed in quaternary sequences of loess have been used for stratigraphic definition correlation with other terrestrial and deep sea sequences and paleoclimatic paleorainfall reconstruction in some loess paleosol sequences including those of the chinese loess plateau tajikistan and the czech republic maxima in magnetic susceptibility values correspond with the paleosol horizons and minima with the least weathered loess layers in other loess soil sequences including those of siberia alaska and argentina the relationship is completely opposite with susceptibility minima associated with the most developed paleosols to account for these opposite relationships the respective roles of 1 magnetic enhancement and 2 magnetic depletion and or dilution in determining soil magnetic properties are investigated for a range of modern soil types most magnetic enhancement is seen in the upper horizons of well drained cambisols absence or loss of magnetic iron oxides is apparent in acid podsol profiles and waterlogged soils for the cambisol profiles significant correlation is found between susceptibility and organic carbon cation exchange capacity and clay content the mineralogy morphology and grain size of soil magnetic carriers extracted from three modern enhanced soils and a paleosol from the chinese loess plateau are also identified by independent petrographic means microscopy and x ray diffraction of magnetic extracts magnetite and maghemite of ultrafine grain size from 0 4 to 0 001 i m are the major contributors to the magnetically enhanced soils weathering can concentrate detrital magnetic grains especially in the fine silt size fractions of soils the magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site specific basis taking into account the possibilities of pedogenic enhancement pedogenic dilution or depletion and allochthonous inputs of magnetic minerals excessively arid wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets well drained intermittently wet dry soils with reasonable buffering capacity and a substrate source of fe show most magnetic enhancement for those soils which favour magnetic enhancement processes correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall the almost unique pedogenic system in the chinese loess plateau where variation in soil forming factors other than climate is reduced to a global minimum allows use of the paleo susceptibility values as proxy paleorainfall values to identify the mechanism of any link between magnetic properties for example susceptibility and climate change the mineralogy and grain size of soil magnetic carriers requires detailed investigation given the prospect of 1 quantitative paleorainfall records from continental loess records 2 predictions of present and future climate change and 3 the value of paleoclimate data for testing of numerical climate models such magnetic investigations can contribute significantly to our understanding of past and future environmental change

backwater and river plume controls on scour upstream of river mouths implications for fluvio deltaic morphodynamics sediment flux from rivers to oceans is the fundamental driver of fluvio deltaic morphodynamics and continental margin sedimentation yet sediment transport across the river to marine boundary is poorly understood coastal rivers typically are affected by backwater a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume flow deceleration in the backwater zone as well as spreading of the offshore plume should render rivers highly depositional near their mouths leading to sedimentation and eventual elimination of the backwater zone at steady state this reasoning is counter to observations of riverbed scour erosional bed forms and long lived backwater zones near the mouths of some coastal rivers e g mississippi river united states to explain these observations we present a quasi 2 d model of a coupled fluvial backwater and offshore river plume system and apply it to the mississippi river results show that during high discharge events the normal flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface spatial acceleration of flow and erosion of the riverbed as proposed by lane 1957 the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume this points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks

quaternary paleosols tephra deposits and landscape history in south kamchatka russia abstract paleosols developed in various glacial and glaciofluvial deposits of potentially different ages in the southern kamchatka peninsula of russia are covered by multiple layers of volcanic ash and have been affected by permafrost weathering indices and iron fractionation data indicate slight differences in soil development between the paleosols on terminal moraines and associated terraces of two different ages in the central part of the region suggesting that the two depositional phases were separated by an interstadial rather than an interglacial according to the extent of weathering and the age of the oldest tephra overlying the paleosols ca 8000 bp both sets of deposits are of late pleistocene age glacial deposits on the west coast of the peninsula differ from those on the east in containing ice wedge casts also the paleosol here is less developed and is covered by lake sediments as well as tephra units the oldest tephra dated to ca 40 000 bp is present only in the west suggesting that the western glacial deposits date from an older middle pleistocene glaciation

a potential vorticity theory for the formation of elongate channels in river deltas and lakes 1 rivers empty into oceans and lakes as turbulent sediment laden jets which can be characterized by a gaussian horizontal velocity profile that spreads and decays downstream because of shearing and lateral mixing at the jet margins recent experiments demonstrate that this velocity field controls river mouth sedimentation patterns in nature diffuse jets are associated with mouth bar deposition forming bifurcating distributary networks while focused jets are associated with levee deposition and the growth of elongate channels that do not bifurcate river outflows from elongate channels are similar in structure to cold filaments observed in ocean currents where high potential vorticity helps to preserve coherent structure over large distances motivated by these observations we propose a hydrodynamic theory that seeks to predict the conditions under which elongate channels form our approach models jet velocity patterns using the flow vorticity both shearing and lateral spreading are directly related to the vertical component of vorticity we introduce a new kind of potential vorticity that incorporates sediment concentration and thus allows study of jet sedimentation patterns the potential vorticity equation reduces the number of fluid momentum equations to one without losing generality this results in a compact analytical solution capable of describing the streamwise evolution of the potential vorticity of a sediment laden jet from initial conditions at the river mouth our theory predicts that high potential vorticity is a necessary condition for focused levee deposition and the creation of elongate channels comparison to numerical laboratory and field studies indicates that potential vorticity is a primary control on channel morphology our results may be useful for designing river delta restoration schemes such as the proposed mississippi delta diversion

soil organic carbon stocks and forest productivity in volcanic ash soils of different age 1835 30 500 years b p in mexico abstract volcanic soils have a large potential for organic carbon storage and for forest growth in particular under moist temperate climate documenting both potentials may offer incentives for forest conservation and exploitation and for the allocation of bonds for environmental services we studied sites on lava flows of different ages 1835 4200 6200 8000 10 000 and 30 500 years b p covered by volcanic ashes within the sierra del chichinautzin volcanic field under pine or fir forest we analyzed the accumulation of organic carbon in the soils and its relation with the evolution of site quality forest productivity indicators and c storage in tree biomass we also studied the relation between soil c accumulation and different soil properties as soil ph clay contents and pyrophosphate extractable al and oxalate extractable al and si contents results show that carbon accumulation in the soil increases linearly with age within 1835 and 10 000 years old sites with a mean accumulation rate of 4 33 kg m 2 each 1000 years however between 10 000 and 30 500 years the amount of c stored decreases again by about 30 of the maximum soil c accumulation during the first 8000 years of the chronosequence site quality improves with soil development tree growth indexes approach stabilization at sites of ages between 10 000 and 30 500 years adult pine trees reach an accumulation of 27 kg c per tree each year during the first 10 000 years carbon accumulation in mineral surface horizons is associated with the formation of al bearing organo mineral complexes characterized by pyrophosphate extractions which seem to compete with the neoformation of short range order clay minerals as clay and al oxide formation progresses these soil variables contribute also to stabilize soil c although they are by far not as efficient as the organo mineral complexes