evaporation from cultivated soils containing rock fragments abstract evaporation from bare soil surfaces represents an important water loss for agriculture in semi arid regions numerous efforts have been undertaken to modify the topsoil characteristics mulching tillage in order to create a thin dry topsoil that reduces evaporation however little attention was paid to the role of natural rock fragments in topsoils with respect to evaporation this paper presents the results of laboratory experiments simulating evaporation from initially wet and air dry soils containing a range of rock fragment contents and compares them to field conditions evaporation was stimulated by blowing fans at one high evaporative demand eo 7 7 9 2 mm day 1 time domain reflectometry tdr was used to investigate the relation between the water content of the topsoil and the actual evaporation rate for soils at field capacity initial fine earth water content decreases with rock fragment content and consequently evaporation rates decrease in the same order for air dry soils that received a limited amount of rain 10 and 20 mm an opposite behaviour was observed initial fine earth water content and evaporation rates increase with rock fragment content a strong positive sigmoidal relation between relative evaporation rate actual over open pan evaporation rates and fine earth water content in the centre of the wetted soil section was observed during the laboratory experiments except for the columns covered with a mulch there were no systematic differences in this relation between the treatments a rock fragment mulch reduces evaporation rate at a given soil water content significantly these trends explain the often ambivalent effects of rock fragments on evaporation rates in the field

thermal models of oceanic crustal accretion linking geophysical geological and petrological observations 1 models of oceanic crustal accretion should be able to reproduce both petrological and geological observations from ophiolites and geophysical observations from active spreading ridges we have developed thermal models of crustal accretion at fast spreading ridges that incorporate petrological variation and allow for a variable vertical distribution of crystallization including petrological variation has a major effect on the distribution of melt within the crust and the consequent interpretation of model results in terms of geophysical observables in particular the narrow melt lens imaged by seismic reflection surveys can be produced by the presence of large scale compositional stratification within the crust such as that observed in the oman ophiolite controlling the vertical distribution of crystallization allows a range of crustal accretion mechanisms to be tested including the many sills and gabbro glacier models both gabbro glacier and many sills models can broadly match the geophysical observations but the preferred model is a hybrid where the uppermost 25 or more of the lower crust forms in a gabbro glacier and the lowermost 25 or more of the lower crust forms by crystallization in situ the success of all models is dependent on the strength and distribution of hydrothermal cooling

eolian depositional phases during the past 50 ka and inferred climate variability for the pampean sand sea western pampas argentina abstract the pampean sand sea which occurs from the argentinian pampas to the eastern andean piedmont hosts presently stabilized dune fields spanning the late quaternary this study integrates previous results and presents new geomorphic stratigraphic sedimentological and chronologic data for nineteen 2 m thick eolian successions for the san luis paleo dune field western pampas to better constrain the depositional history six eolian depositional phases are identified spanning the past 50 ka interposed with paleosols and or bounded by erosive surfaces age control was from 61 osl ages of small aliquots of quartz grains from eolian stratigraphic units the inferred timing of eolian phases are at ca 70 10 yr 190 20 yr 12 to 1 ka 22 to 17 ka 29 to 24 ka and 40 to 32 ka a maximum span for periods of pedogenesis at ca 12 to 17 ka 22 to 24 ka and 29 to 32 ka was provided by bounding osl ages which broadly overlap with high stands of pluvial lakes and glacier advances in the central andes we infer that the added precipitation may reflect expansion of the southern hemisphere monsoon associated with northern hemisphere heinrich events leading to episodes of significantly wetter conditions 350 mm map to at least 35 s most of the holocene 12 ka to 0 8 ka was characterized by sand sheet deposit under drier than present conditions 100 450 mm map associated with monte type vegetation shrub steppe the latest two eolian depositional phases occurred at ca 190 and 70 yr ago during the historic period with european settlement and are related to anthropogenic landscape disturbance though the youngest phase was concomitant with 1930s drought wet conditions dominated since ca ad 1970 with new lakes and rivers forming across this eolian terrain an incongruous environmental response in reference to drier conditions for most of the holocene

late pleistocene holocene eolian deposits of the southern buenos aires province argentina a preliminary model abstract the aim is to reconstruct the sedimentological history of the late pleistocene holocene eolian deposits of southern buenos aires province hence a preliminary model is proposed in order to analyze the different stages of the sedimentological cycle the deposits include sandy loess fine sand sheets and dunefields the southern buenos aires province represents the depositional area whereas the source and transport environments are located in the northern patagonian andes and northern extra andean patagonia according to the mineralogical characteristics of the material explosive volcanism is considered the predominant process of particle formation particularly as generator of pyroclastic deposits which were reworked by different erosive processes glacial fluvial eolian during the late glacial maximum and the holocene alluvial deposits which were mainly generated by fluvial erosion of volcaniclastic sedimentary units were deposited along the floodplains of the colorado and negro rivers including those distal segments located in the emerged offshore platform the fluvial regime depended upon the regime of the glaciers hence it reflected the major glacial fluctuations which took place in the andes floodplains were deflated by southwest winds transporting the material towards the southern buenos aires province sandy loess and loessial sands and to la pampa province dunefields

biogeomorphic relationships between slope processes and globular grimmia mosses in haleakala s crater maui hawai i abstract globular mosses were found in haleakala s crater maui at five locations between 2175 and 2725 m the highest altitude site with abundant epilithic mosses growing on alkali olivine basalt outcrops and a large mossball population was studied mossballs form when moss cushions are dislodged from rocks but continue growing unattached to substrate detachment agents include rainsplash desiccation wind frost and disturbance by birds dark rumped petrels that burrow nests under outcrops or by goats when loosened moss polsters are transported down steep 26 34 slopes by different geomorphic processes including frost mainly needle ice activity runoff and wind mossballs contained two species grimmia trichophylla grev and grimmia torquata drumm growing separately or commingled weight size and various shape indices were determined for 260 specimens shape and size were correlated larger mosses become less spheroidal because heavier specimens are less disturbed by needle ice remaining immobile for increasingly longer time periods thus becoming flattened distance of downslope transport from source rockwalls was measured for 330 specimens 83 shifted 100 cm but only 5 had moved 200 839 cm heavier mossballs moved short distances thus 88 of all biomass remained within 200 cm from outcrops substrate soils were compared with those within globoids surface site soils were much coarser than mossball grains twelve substrate samples had on average 21 3 gravel 2 mm 6 1 fines 0 063 mm and 2 1 organic matter in contrast 12 mossballs contained this area is frequently subjected to freezing temperatures while fog interception from incoming tradewinds contributes significant moisture amounts to precipitation ground disturbance by frost activity seemed significant needle ice grows frequently on this slope as evidenced by widespread presence of miniature sorted stripes striated soils fine earth flags below stones gaps around stones and fine soil raised patches observed during multiple visits biogeomorphic interactions between mossballs and substrate at haleakala are briefly discussed from the perspective of self organized criticality and interpreted to represent a self replicating moss dispersal system dependent on slope geomorphic processes

the rootzone of an ancient hydrothermal system exposed in the troodos ophiolite cyprus abstract a well exposed oceanic hydrothermal rootzone in the troodos ophiolite cyprus is composed of the basal sheeted dikes the upper 50 100 m of the gabbroic sequence and a contact aureole interpreted as a fossil conductive boundary layer the basal sheeted dikes and upper gabbros were pervasively altered at high temperatures 400 775 c and cut by a series of broadly distributed mineralized faults that are oriented parallel to the strike of the sheeted dike complex fluid inclusion and mineralogical data for fault breccias show that circulating fluids were 350 400 c fairly oxidizing and relatively h2s rich with a range in salinity 1 5 7 8 wt nacl equivalent field and petrological evidence suggest that the rootzone migrated from the basal sheeted dikes into the upper gabbros and back again perhaps for several cycles these migrations and p t oscillations in the conductive boundary layer were likely caused by fluctuations in thermal conditions imposed by the magmatic system at a spreadin

crustal feeding in the oman ophiolite from the top and from the bottom a thermal and mass balance model the stability of the basalt melt lens identified below fast spreading ridges has been tested by a thermal and mass balance model in order to investigate how the generation of the crust by basaltic melt can be affected by spreading rate the mpdel is constrained using field information from the oman ophiolite the main piece of evidence dealing with the nature and structure of the root zone of sheeted dikes which acts as a thermal boundary layer between the melt lens below and the hydrothermally cooled sheeted dike unit above for a mass balance calculation we first assume that the gabbro unit is entirely created by the melt issued from the lens we examine how the heat generated by crystallization of this mass can be evacuated through the boundary layer on top of the melt lens a steady state situation is found for a given spreading rate and ratio of the nusselt number to the field deduced thickness of the boundary layer if as suggested by geophysical data the melt lens exists only for spreading rates larger than 6 cm yr the boundary layer needs to be essentially conductive with increasing spreading rates and increasing heat supply the system thermally adjusts itself until it must work in a new mode geological evidence suggests that this could be achieved by developing an additional feeding of the gabbro unit by a large number of sills being emplaced below the melt lens into the lower gabbros

an experimental investigation of the influence of water and oxygen fugacity on differentiation of morb at 200 mpa crystallization experiments were performed at 200mpa in the temperature range 1150 950 c at oxygen fugacities corresponding to the quartz fayalite magnetite qfm and mno mn3o4 buffers to assess the role of water and fo2 on phase relations and differentiation trends in mid ocean ridge basalt morb systems starting from a primitive mgo 9 8 wt and an evolved morb mgo 6 49 wt crystallization paths with four different water contents 0 35 4 7 wt h2o have been investigated in primitive morb olivine is the liquidus phase followed by plagioclase clinopyroxene amphibole is present only at water saturated conditions below 1000 c but not all fluid saturated runs contain amphibole magnetite and orthopyroxene are not stable at low fo2 qfm buffer residual liquids obtained at low fo2 show a tholeiitic differentiation trend the crystallization of magnetite at high fo2 mno mn3o4 buffer results in a decrease of melt feo mgo ratio causing a calc alkaline differentiation trend because the magnetite crystallization temperature is nearly independent of the h2o content in contrast to silicate minerals the calc alkaline differentiation trend is more pronounced at high water contents residual melts at 950 c in a primitive morb system have compositions approaching those of oceanic plagiogranites in terms of sio2 and k2o but have ca na ratios and feo contents that are too high compared with the natural rocks implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites

the influence of surface volcaniclastic layers from haleakala maui hawaii on soil water conservation this study focuses on the soils and surficial volcaniclastic layers of haleakala s crater maui hawaii the main objective is to assess the effects of covers with fragments of various sizes ash cinder lapilli on soil water conservation soil and gravel samples were collected in haleakala national park from a site at 2505 m where the hawaiian silversword a giant rosette plant grows densely on pyroclastic materials an evaporation experiment lasting 22 days showed a gradual drop in seven pairs of soil samples initially at field capacity one set of samples was left bare the other was covered by gravel mulches gm these resulted in lengthening of t100 total desiccation time by a factor of 2 3 5 2 bare soils dried after 70 140 h but drying time under gravel was 246 509 h mean grain size mz and sorting had the greatest influence on evaporation rates coarse lapilli mz 13 8 mm were less effective than fine ash mz 2 9 3 8 mm in preventing water losses while medium grained cinder mz 4 5 2 mm produced the greatest water savings lapilli mean 0 48 and ash 0 6 were moderately to well sorted cinders with a broader grain size range 1 13 were poorly sorted this allowed infilling of large interstices between coarse fragments by smaller grains effectively reducing pore size and therefore evaporation rates a second experiment determined water storage and rates of water loss by mulches wetting occurred swiftly within 3 4 min all gravel types were dry within 26 h this drying process is important for water conservation as it effectively prevents further water loss from the mulch surface larger fragments stored less water this is related to their surface area weight ratio which increases for smaller particles thus fine ash or cinder grains with a high total area intercept more water than larger lapilli a 5 cm thick layer of ash intercepted an average of 6 8 mm of rain slightly more than the same depth of cinder 6 3 mm but lapilli retained only 4 7 mm therefore light rainfall events are more likely to contribute water to soil under lapilli than below finer pyroclastic material volcaniclastic covers serve an important ecological role in haleakala by prolonging periods of water availability to plants thus allowing hawaiian silverswords to grow for longer time spans

steady as a rock biogeomorphic influence of nurse rocks and slope processes on kūpaoa dubautia menziesii shrubs in haleakalā crater maui hawai i abstract this study examines biogeomorphic interactions between nurse rocks slope processes and 300 k paoa dubautia menziesii shrubs in haleakal crater maui hawai i research objectives were to assess the association of k paoa with substrates upslope and downslope of plants and proximity to the closest rock uphill contrast shrub substrate relationships with site frequency of sediment types measure surface soil shear strength and compressibility on 50 paired locations near boulders and investigate the aggregation characteristics and spatial patterns of k paoa in relation to rock and substrate variation data analyzed came from three 100 plant surveys at 3 sites a plant census at 2720 2975 m altitude and wandering quarter transects wqts across two areas 2610 2710 m ground sediment cover was estimated along four phototransects on these sites data for the three 100 plant surveys included substrate type outcrops blocks cobbles pebbles exposed soil organic litter upslope from each plant and distance to the largest rock upslope the two surveys examined along wqts included substrate type found downslope from k paoa plant height plant diameters across and along the slope and distance between successively censused plants most plants grew downslope of nurse rocks 74 were adjacent to blocks or outcrops and 17 near cobbles plants showed avoidance for finer substrates only 5 3 and 2 7 grew on near bare soils and pebbles respectively about 92 of k paoa were 10 cm downslope of rocks 89 grew 2 cm away and 83 in direct contact with a rock some seedlings also grew on pukiawe leptecophylla tameiameiae nurse plants several stable rock microsites protected plants from disturbance by slope processes causing debris shift site sediments were significantly finer than substrates near plants shrubs grew preferentially adjacent to boulders 20 cm wide which were more common near plants than across sites soils downslope of 50 boulders mean 41 3 cm showed higher shear strength and compressibility than soils along rock sides these boulder dams stop descending debris steadily deflecting them toward rock sides whereas protected soils downslope of blocks are infrequently disturbed and develop into downslope elongated clast free fine earth flags k paoa were uniformly dispersed across slopes showing a single phase mosaic arrangement without apparent aggregation slopes where k paoa grows had a relatively uniform distribution of surface sediments observed k paoa patterns may have resulted from the array of microsites generated by surface rocks and sediments as shrubs simply responded to such spatial structure

control of regional sea level by surface uplift and subsidence caused by magmatic underplating of earth s crust magmatic underplating of the crust is a common feature of major basalt provinces the emplacement of magma within the lithosphere leads to surface uplift with a magnitude of the order of 10 of the thickness of the underplated material the composition of associated igneous rock samples suggests that much of the underplated material is made of gabbro this gabbro is denser than the magma from which it crystallizes so uplift of earth s surface caused by the emplacement of magma at the crust mantle boundary must be followed by subsidence as the magma solidifies this subsidence is equal to approximately half the original uplift and takes place within 0 1 m y of injection of the magma the association of magmatic underplating with surface uplift followed by subsidence provides a mechanism for changes of sea level including previously unexplained episodic highstands of sea level

anatectic migmatites from the roof of an ocean ridge magma chamber a well preserved contact aureole at the base of the sheeted dyke is likely to be centred at or very close to the sheeted complex in the troodos ophiolite cyprus records the partial melting dyke gabbro boundary this region of the crust is a of the roof of a mid ocean ridge magma chamber hydrothermally dynamic environment that is subjected to rapid changes altered dykes within the lowermost 10 30 m of the sheeted dyke in rheology and thermal structure it is here that melt is complex were recrystallized to pyroxene and hornblende hornfels episodically extracted to build the upper crust e g deand closest to the underlying gabbros 30 at water undersaturated conditions and temperatures if not all of the lower crust e g phipps morgan chen 875 c geological considerations indicate that the duration of 1993 quick denlinger 1993 boudier et al 1996 melting events was probably on the time scale of eruptive cycles i e and magmatic heat is rapidly transferred across a thin years to centuries this resulted in disequilibrium partial melting 100 m conductive boundary layer e g lowell and highly imperfect melt segregation within the contact aureole burnell 1991 and advectively transported out of the we hypothesize that partial melting is a common process within crust by hydrothermal fluids e g cann et al 1985 the roof zones of axial magma chambers at fast spreading ridges geophysical surveys along the east pacific rise epr and that a component of geophysically imaged axial magma chambers provide indirect evidence that the depth and internal may be partially melted upper crust rather than partially crystallized properties e g melt volumes proportion of melt and magma field relations suggest that where the highest degrees of crystals of amcs are not static and change on the time melting occurred a mixture of anhydrous hornfels and hydrous scale of eruptive cycles years to centuries wilcock leucocratic melt disaggregrated and was assimilated into the magma delaney 1996 singh et al 1998 chamber it is well known from ophiolite studies that the roofs of amcs can be marked by polyphase magmatism and intense interactions between hydrothermal fluids and rocks particularly within the uppermost few hundred

clay mineral composition of pampean loess argentina abstract this paper summarizes current knowledge on clay minerals on the pleistocene and holocene pampean loess detrital illite is the predominant clay mineral of the vitroclastic loess with minor amounts of montmorillonite and kaolinite minerals the illite with a small k content corresponds to a highly illitic illite smectite component in paleosols its crystallinity index decreases and there are more expandable interlayers as in the chinese loess

mineralogy and cation exchange capacity of the fine silt fraction in two soils from the southern chaco region argentina the mineralogical composition of the 2 20 m silt fraction from two soil profiles developed upon loessial sediments in the southern chaco region of argentina was studied by means of x ray diffractometry scanning electron microscopy sem and transmission electron microscopy tem these fine silts from the chaco region contained more quartz and less feldspar and glass than those pointed oit for fine silts of loessial materials from the pampa region microscopic analyses in particular evidenced some complexity with regard to both the composition and the condition of constituents of this fraction even though a considerable quantity of silt size grains are quite fresh different processes of weathering transformation and formation of mineral constituents have also been evidenced cation exchange capacities of these fine silts ranged from 8 to 23 cmolc kg 1 and accounted for approximately 1 6 to 1 3 of the cec values of the soil samples the high cec of the silt may be due mainly to mica and mica grains undergoing a transformation process but contaminating clay particles clay aggregates weathered feldspars silt grains with neoformed fibrous clays and weathered biological silica particles would also contribute

deformation processes at fast to ultra fast oceanic spreading axes mechanical approach abstract the morphology of fast to ultra fast oceanic spreading ridges such as the east pacific rise epr is characterized by an axial dome 5 10 km wide culminating at 300 500 m above the surrounding seafloor this dome is bounded by lateral grabens that develop systematically 2 to 6 km apart from the spreading axis a large summit trough 200 m to 2 km wide locally notches the axial high only where the dome is inflated indicative of a time average robust magma supply this summit trough is thought to represent an elongated axial summit caldera asc created as a result of the subsidence of the top of the axial magma chamber amc such subsidence is likely caused by a temporary decrease in melt supply into the shallow magma reservoir suffering continuous regional extension analog experiments using small scale modeling have been performed in order to better constrain the tectonic evolution of the axial region the experimental apparatus includes an elongated balloon filled with water as an analog of the magma reservoir set in a central groove in a table it is capped with a silicone layer representing hot rocks below the brittle ductile transition and is covered by a sand layer representing the brittle crust the experiments integrate withdrawal of the balloon and extension at the boundary of the model by the mean of two mobile walls three experimental setups allowed us to study independently the mechanical parameters controlling the axial tectonic evolution extension without withdrawal withdrawal without extension withdrawal and synchronous extension we show that the morphology of the epr axis can be considered as the result of both horizontal and vertical movements two symmetrical lateral grabens develop on both sides of a non deformed axial dome when single extension is applied to a model with a thin silicone layer normal faults of the lateral grabens are rooted on two divergent velocity zones dvzs located on the edges of the groove this situation is regarded as an analog of the natural case where the top of the amc acts as a stress free boundary that fails to transmit the extensional stresses to the upper brittle layer an important deflation of the balloon without extension results in the creation of a central collapse trough limited by reverse faults during synchronous extension and withdrawal the initiation of the lateral grabens is favored by a balloon deflation even if such deflation is unable to generate a superficial collapse this last case is considered as representative of the evolution of epr segments showing little variations in melt supply into the amc higher deflation rates under continuous extension correspond to epr segments undergoing strong variations in melt supply in such experiments the lateral grabens are created together with a central collapse trough developing in a way similar to that of experiments involving only balloon deflation finally we show that dvzs located at the brittle ductile boundary are the key mechanical elements which may explain the structural evolution of the axial region of fast to ultra fast spreading ridges the distance from axis and the width of the dvzs directly control the location and the distribution of the lateral grabens

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

volcanic materials as mulches for water conservation abstract arid regions are characterised by a limited rainfall a circumstance that acts as a barrier to dryland farming lanzarote in the canary islands spain is one of the most arid regions in europe with less than 150 mm annual rainfall and potential evapotranspiration in excess of 2000 mm a traditional farming system developed on the island has led to a diversified and productive form of agriculture that uses no irrigation the system utilizes a layer of 10 20 cm of tephra covering the natural soil the tephra acts as mulch and is highly effective for soil water conservation in this work we present the results of a 3 year monitoring of soil moisture content in three plots covered with two types of basaltic tephra with different grain sizes the results are compared to those obtained on adjacent plots that were not covered with the mulch sampling was conducted once a month every 10 cm up to a depth of 1 m compared to the noncovered soils the tephra covered soils managed to retain eight times more water in the surface layer during the driest months at a depth of 1 m twice the amount of water was retained in the tephra covered plots this positive effect was explained by the tephra s physical properties particularly its low water retention capacity and high porosity enhancing water infiltration and reducing evaporation rates differences observed in the water conservation depending on the mulch type was explained largely by grain size the pyroclastic materials proved to be very effective for soil water conservation under arid conditions

superparamagnetism of two modern soils from the northeastern pampean region argentina and its paleoclimatic indications summary the magnetic susceptibility carried by pedogenic fine grained ferrimagnets has been widely used as paleoclimatic proxy to elucidate long term paleoclimatic variations for windblown terrestrial loess paleosol sequences however the magnetic properties of the lithogenic parentmaterialcanmaskthepedogenicsignals inthisstudy wesystematicallyinvestigatedthe origin of the superparamagnetism of two modern soils from the northeastern humid pampean region argentina developed on loess materials of different mineralogical composition the samples were treated with the citrate bicarbonate dithionite cbd reagent which is known to dissolve the submicron pedogenic ferrimagnets while leaving unaltered the coarse grained ones the magnetic material accounting for the frequency dependent magnetic susceptibility peak at about 50 k remained in the residuals and is independent of the pedogenic processes in addition pedogenic ferrimagnetic particles in the two soils have a magnetic signature comparable to that of the soils from the chinese loess plateau it is also suggested that the for the bulk samples does not seem to be a reliable paleoclimatic proxy for the pampean soils investigated in this study instead the cbd soluble magnetic signals could be more useful to detect paleoenvironmental variations in this region these new findings provide improved understanding of the magnetic assemblage in the pampean loess soils and make it feasible to retrieve the paleoclimatic signals carried by the pedogenic cbd soluble iron oxides after removing the effects of the lithogenic inputs

drilling to gabbro in intact ocean crust sampling an intact sequence of oceanic crust through lavas dikes and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid ocean ridges but it has been an elusive goal of scientific ocean drilling for decades recent drilling in the eastern pacific ocean in hole 1256d reached gabbro within seismic layer 2 1157 meters into crust formed at a superfast spreading rate the gabbros are the crystallized melt lenses that formed beneath a mid ocean ridge the depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid ocean ridges melt lenses occur at shallower depths at faster spreading rates the gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by hole 1256d

rock fragments in top soils significance and processes this introductory paper reviews various aspects of rock fragments in top soils such information is required for various reasons and in particular for predicting the impact of climatic or landuse changes on the response of these soils particular attention is paid to the definition and measurement of rock fragment content in top soils the density of soils containing rock fragments the spatial distribution and movement of rock fragments in top soils the effects of rock fragments on some key hydrological processes thermal properties of top soils physical soil degradation soil erosion and soil productivity

the influence of organic matter addition by caulescent andean rosettes on surficial soil properties abstract this report examines the effects of addition of organic matter to soils below caulescent andean rosettes in the paramo de piedras blancas venezuelan andes at 4260 4300 m altitude two plant species coespeletia timotensis forma lutescens and espeletia semiglobulata were investigated in an area occupied by a series of small hills the soils here were lithic and typic cryochrepts developed on pegmatitic granite and mica schist paired surface soil samples were obtained beneath plant canopies and in bare areas outside of these 106 samples were gathered organic matter content was 36 to 52 higher under rosettes than in adjacent bare areas soils below plants were also finer but these differences were not as pronounced due to the variation in organic matter physical and chemical soil properties differed sharply between rosette and control areas soils were darker below plants bulk density decreased and porosity and water content at field capacity increased under rosettes cation exchange capacity total n content and exchangeable bases ca mg k and p levels were considerably higher beneath rosettes there soil acidity decreased and percentage base saturation and c n ratios increased simple and multiple regressions indicated that soil properties were closely dependent on organic matter content the percentage of fines 0 05 mm was also correlated with physical soil properties and cation exchange capacity but its effect on property variance was considerably less pronounced than that of organic matter the results indicate that pedogenesis in this harsh periglacial paramo is significantly affected by the addition of plant litter thus andean rosettes appear to exert considerable influence on the evolution of the coarse high altitude soils

magnesium isotopic variations in loess origins and implications abstract loess deposits serve as important continental archives for studying quaternary climatic variations and for estimating the average chemical composition of the upper continental crust here we report high precision mg isotopic data for 19 loess samples from china argentina and europe which were previously used to estimate the composition of the upper continental crust the results show that global loess samples have heterogeneous mg isotopic compositions with 26mg per mil deviation of the 26mg 24mg ratio from the dsm3 standard values ranging from 1 64 to 0 25 and a weighted average of 0 89 which is lighter than both crust and mantle silicates mgo content and 26mg of loess positively correlate with cao al2o3 ratio suggesting a two component mixing between carbonates and secondary silicate minerals the large variation in mg isotopic composition of loess results from a combination of factors including source heterogeneity eolian sorting during transport of the loess and chemical weathering during formation of the loess deposit our results suggest that mg isotopic composition of loess has potential to be a proxy indicator to characterize the paleoclimatic change but may not represent the average mg isotopic composition of the upper continental crust due to mixing sorting of isotopically distinct components and isotope fractionation during loess deposit formation

influence of substrate on the distribution of the hawaiian silversword argyroxiphium sandwicense dc in haleakala maui hi the spatial patterns of the hawaiian silversword argyroxiphium sandwicense dc were studied in haleakala maui hi the silversword is a giant rosette plant nearly brought to extinction by human impact and goat browsing during the 1920s but stern protection has resulted in the resurgence of plant populations silversword regeneration is occurring vigorously in soils with surficial layers of volcaniclastic fragments ten sites with sizable silversword populations and the associated substrates were examined in haleakala s crater between 2175 and 2755 m at each site the population structure of 100 plants was determined along wandering quarter transects which limit sampling bias substrates where rosettes were rooted and the size of the biggest stone fragments upslope and downslope from the plant s base were determined volcaniclastic substrates were examined with photographs along 12 m long transverse transects fine debris samples gravel and pebbles were analyzed by mechanical sifting of surficial fragments gathered from 15 15 cm miniplots volcaniclastic fragments in silversword areas have different sources i most were ejected during eruptions yielding pyroclastic materials ash to blocks that built cinder cones on haleakala s crater ii on steep slopes weathering of olivine alkali basalt outcrops and subsequent mass wasting of rock fragments contributed a mantle of clasts to slopes below iii on some cinder cones welded spatter was produced during late eruptive stages this coalesced into ruff like agglutinate layers around crater rims called pohaku o hanalei wreath of stones agglutinate deposits weather gradually supplying clasts that roll downhill and accumulate on cone flanks iv on weathered aa lava flows stone fragments and rocky crags abound along eroding lava flow ridges associated with vegetation but the intervening troughs covered with fine ash and cinder remain bare silversword population structures indicate healthy regeneration all populations display a typical pattern of decreasing silversword numbers with increasing size age nearly 65 of the rosettes are rooted near cobbles 5 10 cm long or pebbles 2 5 5 cm and 23 grow at the base of blocks 10 cm or outcrops 9 of the plants are on gravel 2 5 cm but only 1 have germinated on sandy soil without granules or stones 2 of the silverswords grow on organic litter of dead rosettes plants on six sites showed noticeable differences in clast size between their upslope and downslope sides particles are significantly larger upslope from rosettes this suggests that successful seedlings germinate below clasts which deflect finer downslope moving sediment thus protecting young plants from burial by descending debris statistical tests indicate a few blocks above plants may have been stopped by adult rosettes such a relationship underscores the dynamic nature of steep cinder and rubble talus slopes regularly affected by dry debris slides frost creep and other mass wasting processes slope instability is also evinced by i common bending and deformation of silversword roots particularly in seedlings which trail upslope ii pronounced downhill tilting and asymmetry of tall rosettes on steep gradients iii presence of accumulated fine debris steps 9 9 13 8 cm thick upslope from plants and blocks iv ubiquitous fine earth flags exposed below embedded blocks which intercept descending debris and deflect it laterally v widespread miniature sorted stripes

root zone of the sheeted dike complex in the oman ophiolite the root zone of the sheeted dike complex representing a thin zone hundred meters thick of extreme thermal gradient 5 c m is regarded as a thermal boundary between the convective magma chamber system below and the main convective hydrothermal circuit which closes above at the base of this root zone the root zone of the sheeted dike complex is located at the top of the high level foliated gabbro unit where the foliation steepens and where the first diabase dikes appears it is a complex zone characterized by mutual intrusions of microgabbros dikes that we call protodikes with brownish microgranular contacts against the gabbro matrix upward viscous flow in the protodikes and in the reheated enclosing gabbros generate a diffuse transition to the sheeted complex protodike margins stretched in the enclosing flowing doleritic gabbros form a complicated network which can be depicted thanks to microstructural analysis later diabase dikes cross cut the section these relationships are obscured by the hydrothermal circulation which has generated in particular isotropic amphibole gabbro veins these veins tend to propagate horizontally they may be interpreted as the downward closure of the main hydrothermal convective circuit

isotopic constraints on the source of argentinian loess with implications for atmospheric circulation and the provenance of antarctic dust during recent glacial maxima we present rare earth element ree and sr nd isotopic data for argentinian loess 28 38 s with two aims 1 to examine the source regions of argentinian loess and the constraints that these put on palaeo wind directions 2 to further investigate the source of antarctic ice core dust and to test the hypothesis that some of it could be derived from a region to the north of patagonia into which the dry dusty westerly dominated patagonian climate expanded during quaternary glacial maxima sr nd isotopic data for argentinian loess from north of 37 s are distinct from patagonian loess compositions in that they have more radiogenic sr 87sr 86sr 0 7059 0 7123 and less radiogenic nd nd 0 8 to 6 4 ree patterns and sr nd isotopic values are relatively homogeneous for multiple samples taken from single loess sections but show significant differences between sections in general there is a northward change from patagonia like ree patterns and isotopic values away from volcanogenic signatures and towards those that are more like the continental crust the latitudinal nd isotopic pattern is remarkably similar to that for andean volcanic rocks and suggests derivation of loess from the andes by more or less direct westerly transport for loess sections in the north the data imply a contribution from palaeozoic gneisses to the northwest in the chilean altiplano sr nd data for extra patagonian argentinian loess north of 37 s do not support a significant role for this source region in supplying dust to antarctica at the last glacial maximum this conclusion contrasts with previous studies that suggest a significant northward shift in the climatic belts and in particular the westerlies and the antarctic polar front during quaternary glacial maxima very systematic relations between the sr nd isotopic composition of loess and their andean source highlights shifts in the sr isotopic composition of loess to more radiogenic values and strongly suggests that the slight offset between patagonian loess and ice core dust identified by previous workers is due to grain size differentiation effects

biogeomorphic influence of soil depth to bedrock volcanic ash soils and surface tephra on silversword distribution haleakalā crater maui hawai i abstract this research examines the spatial distribution of haleakal silverswords argyroxiphium sandwicense dc subsp macrocephalum over basaltic a lava flows at silversword loop haleakal crater maui hawai i the overall study goal is to situate an extensive field data set within an historical geomorphic framework to propose a conceptual biogeomorphic model of landscape development at silversword loop landscape features were investigated along two belt transects 111 120 m long crossing lava flow deposits at 2175 m the lava flows 4 5 km long originated 2700 ybp at pu u o m ui a large cinder cone transects consist of contiguous 3 3 m 9 m2 quadrats plot data include topographic profiles soil depth to bedrock size of surface tephra determined by photosieving and plant density 23 soil samples were analyzed within 10 profiles weathered lava outcrops tephra blocks 10 cm and cobbles 5 10 cm cover lava flow ridges the gently sloping flanks of lava flow deposits merge into intervenient concave troughs filled with volcanic ash and small tephra soils along ridges are thin 17 cm whereas profiles on troughs are deeper 110 cm soils are sandy volcanic inceptisols lithic typic and aridic lithic haplustepts averaging 21 6 gravel 96 8 sand and 3 2 fines silt clay mean organic matter is just 0 92 these soils provide low water retention 23 2 at field capacity and rapid infiltration deeper trough soils are largely devoid of plants in contrast silverswords are virtually restricted to shallow soils on lava flow crests where they densely cluster about outcrops and blocks 90 of plants grow on soils 40 cm deep and 71 3 occupy plots with 40 ash cover 84 4 of rosettes are rooted on outcrops or near stones only 4 4 grow on soil and 11 2 germinated on plant litter patterns of lava flow weathering and soil depth to bedrock control silversword distribution at silversword loop shallow basalt bedrock and outcrops restrict soil drainage providing a perched water table that silversword roots can exploit surface blocks also provide safe stable microhabitats thus rosettes grow preferentially near stones geomorphological pedological and ecological data are integrated into a biogeomorphic sequence of soil development and landscape evolution at silversword loop for the past three millennia first through the eruptions and ash deposits of pu u o m ui followed 970 940 bp by pyroclastic activity in hal li i and ka lu u o ka o o cinder cones

roof assimilation at fast spreading ridges an investigation combining geophysical geochemical and field evidence 1 the role of assimilation at the roof of the axial magma chamber amc at fast spreading mid ocean ridges is investigated using field geophysical and geochemical data field observations from ophiolites indicate that roof assimilation is a common process but provide little constraint on the amount of material assimilated arguments based on geophysical data that include the subsidence of the layer 2a 2b boundary across the width of the amc and the depth of large axial summit troughs require at least 50 100 m of assimilation however significant variations in the depth of the amc on the east pacific rise epr over short along axis distances suggest up to 500 m fluctuations in the depth of the amc may be common on short timescales geochemical modeling of the over enrichment of chlorine in epr basalts suggests that 20 of the oceanic crust may go through a cycle of crystallization alteration then assimilation this is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation assimilation most likely occurs on a number of different timescales ranging from decadal due to perturbations in the thermal regime related to diking to longer period fluctuations controlled by magma supply from the mantle large scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation and for the interpretation of the geochemistry of the oceanic crust

biogeomorphological influence of the hawaiian silversword argyroxiphium sandwicense dc on soil erosion in haleakala maui hawai i abstract soils were examined at 2505 m elevation in haleakala s crater maui hawai i beneath 50 adult hawaiian silversword plants argyroxiphium sandwicense dc mean canopy diameter was 42 0 cm exposed volcanic inceptisols andic humitropepts seem significantly eroded beyond the dense rosette crowns but remain unaffected below plants rosettes are perched on isolated basal soil mounds or pedestals 27 121 mm high mean 77 5 mm geomorphic field response of soils below rosettes and adjacent 100 cm apart bare soils differs infiltration rates are higher under plants mean 158 7 mm min than in exposed control soils 60 0 mm min soils below silverswords also show greater shear strength 146 1 g cm 2 and compressibility 2 795 kg cm 2 than unprotected soils 36 1 g cm 2 and 0 108 kg cm 2 respectively soil in the plant mounds contains more organic matter this has influenced other pedological properties which also differ substantially between sampling positions substrate under plants has a porosity 53 higher than exposed soil while bulk density is 62 higher in soil outside the plant crown the observed microtopographic differences are ascribed to greater soil erosion by rainsplash and runoff outside the silversword canopy the dense rosette crown effectively intercepts raindrops soils beneath plants also have a high surface cohesiveness provided by a dense network of fine plant rootlets and partially decomposed organic material higher runoff rates occur on the less permeable substrate beyond rosettes which is affected by soil crusting

the role of tephra covers on soil moisture conservation at haleakala s crater maui hawai i abstract the influence of tephra covers on soil water was studied in haleakala maui hawai i during two summers eight sites with tephra layers and silverswords argyroxiphium sandwicense dc were sampled at 2415 2755 m at each site eight paired sample sets were obtained in bare soils and under adjacent tephra at three depths tephra were sharply separated from underlying soils and showed prominent vertical stratification tephra clast size distribution was assessed by photosieving and on interstitial gravel samples stones included 45 6 cobbles 29 4 pebbles and 25 blocks moisture content increased with depth in both positions but soils below tephra had more water at all depths than exposed areas surface soils beneath tephra contained 83 more water than bare ground soils at 5 10 cm had 106 greater moisture under rocks but only 70 at 10 15 cm differences between plots were statistically significant p all soils were coarse with 20 gravel and 94 sand most fine material 0 063 mm was silt as clay content was negligible organic matter percentage was low 1 65 bulk density and porosity were associated with moisture variation both in tephra insulated and bare soils 80 of field moisture was statistically p

mid ocean ridge sulfide deposits evidence for heat extraction from magma chambers or cracking fronts numerous seafloor observations show that the sizes of high temperature hydrothermal sulfide edifices vary dramatically with spreading rate on fast spreading ridges venting occurs from spindly chimneys which reach heights of about 10 m while on slow spreading ridges vents are frequently located near the tops of large sulfide mounds whose volumes may reach 105 106 m3 we argue that such variations are the result of a profound difference in the nature of hydrothermal heat extraction between fast spreading ridges where spreading occurs predominantly by magmatism and slow spreading ridges where there is a significant component of tectonic extension along fast spreading ridges a steady state axial magma chamber can be insulated by a relatively thick conductive boundary layer because heat extraction is limited by low permeability in such systems episodes of vigorous venting are linked to diking events not only because these introduce a significant heat source into the upper crust but also because the intrusion of a dike is the primary source of increased permeability near the ridge axis over a time scale of the order of a decade mineral deposition tends to clog the reaction and upflow zones and there is a high probability that young sulfide structures will be buried by subsequent eruptions on slow spreading ridges tectonic extension maintains the fluid pathways necessary to support vigorous convection in the waning stages of magmatism hydrothermal circulation driven by a downward migrating cracking front can cool the entire crust leading to the formation of very large sulfide deposits as the depth of circulation increases overburden pressures reduce the permeability and the hydrothermal heat fluxes decrease progressively once hydrothermal fluids penetrate the moho serpentinization clogs the fluid pathways and high temperature venting ceases until it is reactivated by a fresh magmatic intrusion

a multi proxy study of argentina loess marine oxygen isotope stage 4 and 5 environmental record from pedogenic hematite abstract a 12 m loess paleosol section la angostura from tafi del valle tucuman argentina was studied using rock magnetic measurements geochemistry mossbauer effect particle size and nd isotopes in order to elucidate changes in paleoenvironment at the site combining the results of these experiments allowed us to separate the effects of the two major controls parent material and pedogenesis on the measured parameters two behavior regimes were identified in the sediment 1 in the lower half of the section pedogenic processes though weak dominate the variation of the sediment s physical properties and 2 for the upper half of the section subtle changes in parent material control the majority of the measured parameters even those typically controlled by pedogenesis e g clay fraction applying rock magnetic mossbauer and bulk geochemistry measurements in concert we calculated an iron budget for the loess 80 of the iron in the sediment is in paramagnetic minerals 20 of the iron is in hematite 1 6 of the sediment s mass and 1 of the total iron is present as magnetite maghemite 0 1 of the sediment s total mass neither the pedogenic formation nor the depletion of ferrimagnetic material with increasing pedogenesis was consistently observed in this sediment throughout the entire section the concentration of hematite is argued to be sensitive to in situ alteration and it is further surmised to be sensitive to the available moisture at the site finally we present a curiosity within the error of our chronology the concentration of hematite seems to correlate with the coeval climate record from antarctica vostok it is possible that the sedimentary magnetic record from la angostura is a proxy for the strength of the south american summer monsoon

structure of uppermost fast spread oceanic crust exposed at the hess deep rift implications for subaxial processes at the east pacific rise the uppermost 2 km of the oceanic crust created at the fast spreading 135 mm yr 1 full rate equatorial east pacific rise epr is exposed for tens of kilometers along escarpments bounding the hess deep rift mosaics of large scale digital images from the remotely operated vehicle rov argo ii and direct observations from the submersible alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units these rock units are characterized by extensive faulting fine scale fracturing and rotations of coherent crustal blocks meters to tens of meters across the uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces through most of the basaltic lava unit however lava flow contacts dip 20 70 w toward the epr and generally increase in dip downward in the section dikes cutting the lavas and in the underlying sheeted dike unit generally dip 90 40 e away from the epr deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units we interpret this upper crustal structure as the result of subaxial subsidence within 1 2 km of the epr that accommodated the thickening of the basaltic lava unit to 500 m variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply these results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the hess deep rift if this type of structure is typical of uppermost oceanic crust generated at the epr these processes may be common along fast spreading mid ocean ridges

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

petrography of the dike gabbro transition at iodp site 1256 equatorial pacific the evolution of the granoblastic dikes 1 the ocean drilling program odp integrated ocean drilling program iodp three leg campaign at site 1256 leg 206 expeditions 309 and 312 provides the first continuous in situ sampling of fast spread ocean crust from the extrusive lavas through the sheeted dikes and down into the uppermost gabbros cocos plate east pacific rise eastern equatorial pacific the lowest 60 m of the dikes above the gabbros were transformed to granoblastic dikes through a metamorphic overprint characterized by two pyroxene domains formed under granulite facies conditions equilibrium temperatures estimated by the two pyroxene thermometer range between 930 c and 1050 c implying that conditions within the granoblastic zone were appropriate for hydrous anatexis with the potential to generate partial melts of trondhjemitic composition the downhole evolution of the granoblastic overprint is expressed by systematic changes in texture phase composition and calculated equilibrium temperature consistent with thermal metamorphism by a deeper heat source thermal modeling implies a long lasting heat source located beneath the granoblastic dikes providing thermal energy over several thousands of years the most likely such source is a steady state high level axial magma chamber amc located at the base of the sheeted dike section we interpret the interval of granoblastic dikes as part of a dynamic conductive boundary overlying the amc

first data on volume and type of deflated sediment from southern puna plateau and its role as source of the chaco pampean loess abstract this paper presents the first attempt at qualitatively and quantitatively evaluating the sedimentary contribution of the puna plateau to the loess and soils of the argentinean pampas the origin of the pampean sediment has been traditionally ascribed to northern patagonia but new evidence supports a dual supply western central andes north patagonia and puna altiplano plateau three different lines of evidence show here that puna dust supply is active and produces significant sedimentary deposition up to places like buenos aires city the first evidence is meteorological and we show here one event related to an abnormal rainfall that deposited large quantities of a reddish brownish silt eroded out from the puna calling the attention of the environmental agency of buenos aires city the second evidence is the sampling of active wind transport collecting extremely coarse material carried out in the saltation layer suggesting the fine grained material escapes the area as fugitive dust this also demonstrates that observed gravel bearing bedforms of the puna are active and that the transport system seems to be activated by polar fronts only results show that the saltation curtain is abnormally thick and coarse suggesting that the kinetic energy peaks ca 7 10 times higher than for normal saltation while the transported grain size coincides with the lagged sediment in this basin composing different bedforms varying between sandy gravel and gravelly sand the third evidence is the quantification of the exported material by defining a reference surface as the initial time of evolution and the present day surface the initial surface is an alluvial surface of an estimated age of 11 12 ka bp volume balance was achieved by a detailed topographic stratigraphic and geophysical survey reconstructing old alluvial and present surfaces two gridding methods were used and more conservative results of the volume balance indicate a total erosion of ca 3 386 hm 3 of sediment in 72 ha spliced in 0 963 hm 3 of sediment that was left within the place forming gravel dunes while the remaining volume of ca 2 423 hm 3 is sediment truly exported from the surveyed area results imply a continuous blanket of ca 4 m thick of sediment exported out of this basin in the holocene these findings prove that the puna contribution to the chaco pampean plain is significant although not unique thus true proportions of sediment contribution to soils and loess from these multiple sources should be better evaluated in future studies

formation of large summit troughs along the east pacific rise as collapse calderas an evolutionary model summit troughs wider than 500 m and 30 110 m deep are present along 15 20 of the fast spreading east pacific rise epr they occur only along ridge segments with large cross sectional areas indicative of a time averaged robust magma supply where available seismic data confirm that these troughs are underlain by an axial magma chamber amc 1 2 1 6 km below the seafloor furthermore detailed investigation of the large summit troughs which notch the epr between 17 56 and 18 35 s indicates that the vertical relief of the troughs tends to be maximum where the amc is shallowest both of these observations are inconsistent with the predictions from a model in which large summit troughs form by rifting of the brittle upper crust during phases of amagmatic extension rather we propose that they represent elongated collapsed calderas that form when the melt supply to formerly inflated amcs wanes or nearly ceases because seismic studies constrain the melt lens along the epr to be only 30 80 m thick the proposed existence of collapsed calderas 40 110 m deep implies that the entire magma reservoir comprising the melt lens and the underlying crystal mush zone deforms and compacts during periods of waning magma supply in particular we suggest that a voluminous crystal mush zone will stretch in response to steady state seafloor spreading when the magma supply temporarily decreases the resulting caldera will further widen with each subsequent dike intrusion when an abundant melt supply finally resumes the associated tumescence of the neovolcanic zone and profuse lava flows will combine to smooth out the caldera

loess of southern south america abstract loess and loessoid loess like reworked loess sediments extend across the chaco pampean plains and the nw mountain environments of argentina and in neighbouring countries paraguay brazil uruguay bolivia loessoid sediments are much more abundant than primary loess the beginning of the loessoid sedimentation cycle was related to a phase of late miocene orogeny in the andes the plio pleistocene record is mostly composed of loessoid sediments modified by pedogenesis which produced welded palaeosols the late pleistocene holocene loess record reveals a heterogeneous composition across the region coarse textures and an andean derived volcanic composition prevail in the southern pampas finer textures and material coming from the andes and the other two sources sierras pampeanas and the parana basin characterize the loess deposits of the northern pampas and the eastern chaco a southern patagonian source is also suggested for the mountain valley loess of tucuman although a western andean provenance has been proposed it is believed that the material in the western chaco was derived from the bolivian andes loess deposition was related to a multistage transport mechanism involving fluvial and aeolian processes inferred westerly and southwesterly wind directions as dominant carriers of the aeolian deposits are in agreement with westerly palaeowind simulations using climate models however the role played by westerly tropospheric winds and northerly winds remains to be established

seismic structure and indicators of magma budget along the southern east pacific rise in this paper we re examine the relationship between seismically constrained variations in crustal structure along the southern east pacific rise sepr and the segment scale variations in axial depth morphology basalt geochemistry and hydrothermal activity that have often been attributed to along axis differences in the supply of magma to the mid ocean ridge along 800 km of the fast spreading sepr good correlations exist between axial depth ridge cross sectional area mantle bouguer anomaly and the mgo weight percent of basalts recovered from the rise axis these correlations indicate along axis changes in crustal thickness and temperature consistent with variations in magma supply on time scales of 100 000 years in contrast we show that the depth and width of the midcrustal magma sill the thickness of seismic layer 2a and the intensity of hydrothermal venting are poorly correlated with regional variations in ridge depth and cross sectional area we suggest that the emplacement geometry width of the intrusion zone and flow lengths not magma supply controls extrusive layer seismic layer 2a thickness we hypothesize that magma lens properties and hydrothermal activity are closely linked to spreading events dike intrusion eruptions faulting which occur on much shorter timescales 10 100 years than the longer term variations in magma supply reflected in along axis changes in the shape and depth of the ridge axis

the formation of sio2 rich melts within the deep oceanic crust by hydrous partial melting of gabbros small amounts of felsic evolved plutonic rocks often called oceanic plagiogranites always occur as veins or small stocks within the gabbroic section of the oceanic crust four major models are under debate to explain the formation of these rocks 1 late stage differentiation of a parental morb melt 2 partial melting of gabbroic rocks 3 immiscibility in an evolved tholeiitic liquid and 4 assimilation and partial melting of previously altered dikes recent experimental data in hydrous morb type systems are used to evaluate the petrogenesis of oceanic plagiogranites within the deep oceanic crust experiments show that tio2 is a key parameter for the discrimination between different processes tio2 is relatively low in melts generated by anatexis of gabbros which is a consequence of the low tio2 contents of the protolith due to the depleted nature of typical cumulate gabbros formed in the oceanic crust on the other hand tio2 is relatively high in those melts generated by morb differentiation or liquid immiscibility since the tio2 content of many oceanic plagiogranites is far below that expected in case of a generation by simple morb differentiation or immiscibility these rocks may be regarded as products of anatexis this may indicate that partial melting processes triggered by water rich fluids are more common in the deep oceanic crust than believed up to now at slow spreading ridges seawater may be transported via high temperature shear zones deeply into the crust and thus made available for melting processes

biogeomorphological influence of slope processes and sedimentology on vascular talus vegetation in the southern cascades california abstract the vascular vegetation of alpine talus slopes between 2035 and 3095 m altitude was studied at lassen volcanic national park california in the cascade range taluses show a diverse flora with 79 plant species growth forms include coniferous trees shrubs suffrutices herbs graminoids and ferns spatial patterns of plant distribution were studied along 40 point intercept transects plant cover was low 0 32 7 on all slopes spatially variable and showed no consistent trends sedimentological characteristics were determined by photosieving next to 1500 plants this census indicated preferential plant growth on blocks and cobbles with 43 2 and 23 3 of the plants growing on these stones respectively fewer specimens were rooted on pebbles 13 or on stone free gravel areas 20 5 growth forms displayed different substrate preferences 92 5 of the shrubs and 83 of the suffrutices colonized blocks or cobbles but only 57 2 of the herbs and 59 8 of the graminoids grew on large stones plants are associated with large clasts because 1 coarse talus is more stable than fine sediment areas which are more frequently disturbed by various geomorphic processes and 2 large stones help conserve substrate water beneath them while moisture quickly evaporates from fine debris root patterns were studied for 30 plant species 10 specimens for each species were excavated and inspected and several root growth ratios calculated all species exhibited pronounced root asymmetry as roots for most plants grew upslope from their shoot base for 23 species all specimens had 100 of their roots growing upslope for the other 7 species 92 2 99 3 of below ground biomass extended uphill this uneven root distribution is ascribed to continual substrate instability and resulting talus shift as cascading debris progressively buries roots and stems plants are gradually pushed and or stretched downhill various disturbance events affect root development slope erosion following rubble removal often exposes plant roots debris deposition can completely bury plants some may survive sedimentation producing new shoots that grow through accumulated debris shrubs may propagate by layering as adventitious roots develop along buried stems or produce new clones along their roots slope processes may damage and transport plant pieces downhill some species can sprout from severed displaced root or stem fragments vegetation interacts with several geomorphic processes including debris flows grain flows rockfall snow avalanches frost creep and runoff larger plants may alter local patterns of debris movement and deposition damming cascading debris on their upslope side and deflecting sediments laterally to plant margins where they form narrow elongated stone stripes

geochemistry of a late quaternary loess paleosol sequence in central argentina implications for weathering sedimentary recycling and provenance abstract a loess paleosol sequence in the pampean plain central argentina was analyzed geochemically in order to evaluate the weathering signature and sedimentary recycling as well as to analyze the origin of these loessic sediments corralito i sequence is composed of three paleosols and a buried soil separated by loess layers the loess paleosol sequence shows a similar geochemical composition throughout suggesting that pedogenesis has not been intense enough to mask the chemical signature of the parent material weathering indices cia ciw pia and lwi elemental ratios rb sr cao tio2 and na2o tio2 and the a cn k ternary diagram indicate incipient chemical alteration for this sequence compatible with a weathering limited regime as expected paleosols cias 60 exhibit a slightly higher chemical alteration than loess layers cias 58 mass balance calculations reveal the losses of some major oxides i e cao na2o and sr in the buried soil and paleosols compared to its loess mantles reflecting the weathering of plagioclase besides the losses of k2o and ba in the buried soil and paleosols i and ii when compared to its loess layers suggest the chemical alteration of k feldspar the chemical alteration of volcanic glass is evidenced by the relative losses of as in almost all levels conversely the relative gains of fe2o3 and mgo reveal that pyroxenes and amphiboles or fe oxy hydroxides are concentrated in paleosols and the buried soil compared to its loess mantles the geochemical approach used to constrain the origin of these pampean plain loessic sediments indicates they derive mainly from the andean region conversely the pampean ranges the parana river basin and uruguayan precambrian outcrops which were pointed out as minor local sources do not contribute significantly to the loess deposits this work also shows that corralito i loess paleosol sequence and other loess samples from argentina do not evidence sedimentary recycling suggesting that these pampean plain loess deposits are mainly composed by young materials derived from undifferentiated volcanic rocks

pedostratigraphy and pedological and geochemical characterization of las carreras loess paleosol sequence valle de tafi nw argentina abstract the las carreras loess paleosol sequence is considered the most complete quaternary pedostratigraphic record in south america it consists of 32 paleosols formed during the past 1 15 ma chronological control is provided by magnetostratigraphy sediment and paleosol characterization is based on a variety of methods including soil chemistry geochemistry and micromorphology the main pedogenic processes identified are carbonate leaching and reprecipitation ped formation and translocation of clay and organic matter in many cases the boundaries between individual paleosols are not clearly defined due to indistinct transitional zones based on both pedological and sedimentological characteristics the sequence was subdivided into three units with paleosols in units i and iii designated as luvisols and paleosols in unit ii tentatively classified as luvic kastanozems this subdivision reflects environmental changes associated with the mid pleistocene transition overall geochemical variability is low and largely related to pedogenesis the homogeneity suggests a rather constant provenance that seems to be linked to the central andes pursuant to the chaco model of iriondo 1997 models of deposition of loess and loessoids in the upper quaternary of south america journal of south american earth sciences 10 71 79 and includes direct volcanic ash falls and local sources in a subordinate manner in a paleoclimatic context loess paleosol alternation is interpreted to reflect climatic cycles that were primarily caused by the variability of the south american monsoon system

patterns of rock fragment cover generated by tillage erosion abstract intensively cultivated areas in the upper part of the guadalentin catchment southeast spain show a systematic spatial pattern of surface rock fragment cover rc the objective of this paper is to quantify and to explain this spatial rock fragment cover pattern therefore a map of an intensively cultivated area of 5 km2 was digitised and for each pixel total topographic curvature was calculated next rock fragment cover was determined photographically at 35 sites with a range of total slope curvatures a linear relation between total curvature and rock fragment cover was found except for narrow concavities it was hypothesised that this pattern can be explained by a significant net downslope movement of rock fragments and fine earth by tillage the displacement distances of rock fragments by tillage with a duckfoot chisel were measured by monitoring the displacement of tracers painted rock fragments and aluminium cubes on 5 sites having different slopes the rare of tillage erosion for one tillage pass with a duckfoot chisel expressed by the diffusion constant k equals 282 kg m for up and downslope tillage and only 139 kg m for contour tillage nomograms indicate that mean denudation rates in almond groves due to tillage erosion 3 to 5 tillage passes per year can easily amount to 1 5 2 6 mm year for contour tillage and up to 3 6 5 9 mm year for up and downslope tillage for a field 50 m long and having a slope of 20 these figures are at least one order of magnitude larger than reported denudation rates caused by water erosion in similar environments hence tillage erosion contributes significantly to land degradation the downslope soil flux induced by tillage not only causes considerable denudation on topographic convexities hill tops and spurs and upper field boundaries but also an important sediment accumulation in topographic concavities hollows and valley bottoms and at lower field boundaries kinetic sieving i e the upward migration of rock fragments by the tines of the duckfoot chisel also concentrates the largest rock fragments in the topsoil in such a way that a rock fragment mulch develops in narrow valleys and at the foot of the slopes these results clearly indicate that tillage erosion is the main process responsible for the observed rock fragment cover pattern in the study area since the study area is representative for many parts of southern spain where almond groves have expanded since 1970 the results have a wider application they show to what extent intensive tillage of steep slopes has contributed to the increase in soil degradation to changes in hillslope morphology i e strong denudation of convexities development of lynchets and rapid infilling of narrow valley bottoms and to the development of rock fragment cover patterns which control the spatial variability of the hydrological and water erosion response within such landscapes

new insights on sources contributing dust to the loess record of the western edge of the pampean plain during the transition from the late mis 2 to the early holocene high resolution studies of palaeorecords located closer to the dust source areas of south america are relevant for increasing the knowledge on past climatic conditions in the southern hemisphere i

hot soils and cooler stones geoecological influence of volcanic tephra and boulders on soil temperature and significance for plant distribution in haleakalā crater maui hawai i abstract this project i investigates the geoecological influence of boulders and their dimensions thickness length on maximum daily soil temperatures beneath them at two sites 2175 and 2685 m in haleakal crater maui ii contrasts temperatures in bare volcanic ash soils with adjacent substrates covered by reddish tephra at 7 study sites 2170 2735 m these encompass most of the altitudinal range of k paoa and silversword two common plants in this mountain area and iii analyzes effects of tephra depth on soil maxima midday soil temperatures were measured with thermocouple thermometers 5 cm recording thermographs also monitored temperatures continuously in various contrasting substrates for 8 day periods both boulders and tephra substantially decreased soil maxima bare soil highs n 100 reached 47 6 c mean 37 2 4 4 c maxima below boulders n 40 4 109 cm thick 6 265 cm long peaked at 20 5 c mean 16 0 2 54 c whereas 80 readings under tephra 9 95 mm depth reached 32 0 c mean 25 2 5 2 c larger boulders were most effective in lowering temperatures spearman and kendall correlation tests indicate block thickness was slightly more important p p p values caused greater albedo higher tephra reflectivity helped reduce absorption of solar radiation and consequently soils remained cooler below stones such temperature reduction is ecologically significant for plant survival on stone covered areas as soil maxima 35 c may be deleterious to seed and or seedling survival

geochemical differentiation of quaternary sediments from the pampean region based on soil phosphorus contents as detected in the early 20th century abstract data obtained in the early 20th century on the total phosphorus tp content in soil materials from the pampean region have been processed and plotted on maps of the region approximately 1200 subsoil data and 1400 surface horizon data obtained using the same analytical method have been processed according to differences in phosphorus content several areas have been demarcated limits and characteristics of these areas have been correlated with data from the more recent literature regarding the p distribution and with both mineralogical and sedimentological information on surface materials results suggest that a confluence of sediments from different provenances i e the andean range the pampean hills and the brazilian shield is found in the argentine pampean region

paleosols in loess deposits of the argentine pampas abstract paleosol studies are in their first stages in argentina as far as we know the soil and paleosols correspond to buried alfisols and mollisols with hydromorphic and carbonatic features the paleosol sequences on loess are made up of a succession of b or c horizons which furthermore are polygenetic as a result of encroachment of each pedogenesis on the underlying loess mantle differences in mineral composition are scarce with the exception of increases in glass shards and phytoliths in some loess mantles the b fabric is mostly stipple speckled or mosaic speckled and microlaminated clay coatings are very common there are also amorphous features related to voids or to the soil matrix evidences of biological activity are present as an excremental fabric on which a fissure microstructure is superimposed the paleosol sequences consist of several loessoid mantles each corresponding to a depositional pulse followed by a quiescent pulse during which pedogenesis took place truncation is considered to be due to eolian fluviatile erosion also there probably are a horizons masked by alteration and pedogenesis a pedogenetic origin of the calcretes with redeposition in the buried paleosols is assumed

conservation of soil moisture by different stone covers on alpine talus slopes lassen california abstract this study reports on the influence of stone covers with different clast sizes on the soil moisture of alpine talus slopes in lassen california fifteen four plot sets were sampled in the dry season july 1990 in sandy areas and in talus covered with pebbles cobbles or blocks between 2740 and 2775 m three depths 0 5 5 10 10 15 cm were sampled field moisture content increased gradually with depth in all soil profiles and also in plots covered by increasingly larger rocks surface soils in sand areas were very dry but under rocks had water contents 6 to 14 times greater differences among plots decreased with depth but subsoil samples in sand were still drier than those beneath any stone cover at similar depths blocks were most effective in conserving moisture water content below them was higher than even in deep 10 15 cm sand soils soil temperatures were recorded in sand and under blocks for an 11 day period minima were not significantly different but average maxima were 5 6 c lower under blocks than in sand which reached highs 4 4 c lower than the air differences in soil moisture among talus types are ascribed to lower evaporation losses under stones due to both disruption of capillarity by the coarse particles which prevented water flow to the talus surface and to their efficient reduction of maximum temperatures an irrigation experiment was conducted at 2110 m on a steep talus on the chaos crags from july 18 to aug 2 1993 four 100 75 cm plots with the same surface types than at lassen received 22 5 mm water moisture content was then periodically sampled watering produced similar water distributions among soil depths and talus types to those in lassen evaporation occurred quickly in bare soils due to high air and soil temperatures the sand surface was already dry 2 days after watering but stone covered plots remained moist until day 15 when soils under blocks still retained 77 97 of the water content percent by weight at the start of the test

near solidus evolution of oceanic gabbros insights from amphibole geochemistry the near solidus evolution of plutonic rocks formed at slow spreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the mid atlantic ridge these new data allow unambiguous geochemical discrimination between amphiboles of magmatic and hydrothermal origin in turn this allows the gabbro solidus to be constrained to 860 30 c using amphibole plagioclase thermometry this is consistent with temperatures from associated secondary clinopyroxene magmatic amphibole which can be identified in almost all samples formed during metasomatism of a low porosity crystal mush by an evolved hydrous silicate melt these amphiboles are characterised by high f nb and f cl and low cl contents the amphibole forming reaction involved melt plagioclase and clinopyroxene amphibole blebs with a geochemically magmatic signature are found enclosed in the cores of some primitive clinopyroxene crystals there is no evidence for a seawater component in the magmatic amphibole as would be expected if high temperature seawater ingress leads to flux melting as has recently been suggested however the ingress of seawater derived fluids did occur at temperatures within error of the gabbro solidus forming amphibole in veins and replacing igneous phases these amphiboles are characterised by high cl b and cl f and low nb f and nb la the fluids involved in the formation of these amphiboles had compositions unlike seawater or hydrothermal vent fluids

variability in oceanic crustal thickness and structure multichannel seismic reflection results from the northern east pacific rise multichannel seismic reflection data acquired between 8 50 and 9 50 n and between 12 30 and 13 30 n along the east pacific rise provide a three dimensional view of the young oceanic crust seafloor to moho reflection travel times vary by up to 0 9 s within our study areas the total range of crustal travel times in the 9 n area is 1 55 to 2 45 s the total range in the 13 n area is 1 60 to 2 05 s the variation is systematic indicating thinner crust locally associated with overlapping spreading centers oscs and in the 9 n area segment scale variation along crustal isochrons crustal travel time is found to be a valid proxy for oceanic crustal thickness outside of the axial low velocity volume thickness can be calculated from time to 500 m even in the axial region thickness can be calculated to 1 km if low velocity zone position is known crustal thicknesses calculated from travel times vary by 2 6 km in the 9 n area and by 1 5 km in the 13 n area the majority of this variation is attributed to seismic layer 3 the lower crust segment scale variation of 1 8 km 5 5 to 7 3 km thickness is observed in the 9 n area with thinnest crust formed between 9 40 and 9 50 n and thickest formed between 9 15 and 9 20 n results imply a three dimensional pattern of magma supply to the 9 n segment the osc at 9 03 n is associated with major disruptions of the segment scale pattern in the form of local thin areas within the discordant zone the smaller osc at 12 54 n is not associated with dramatic changes in thickness of the surrounding crust in the absence of oscs the process of crustal formation displays more temporal uniformity along flow lines than spatial uniformity along isochrons within a segment thicker crust does not always correlate with shallower ridge bathymetry broader axial cross section or more negative mantle bouguer or subcrustal gravity anomaly variable thickness of the crust mantle transition region as well as crustal flow in the axial region may be responsible for this unexpected result we hypothesize that the geophysical signature of diapiric mantle upwelling beneath a fast spreading ridge is relatively thin crust associated with a thick moho transition zone and a subcrustal gravity low such a diapiric upwelling center appears to be now located beneath the east pacific rise near 9 40 to 9 50 n