the merensky reef bushveld complex mixing of minerals not mixing of magmas a major geochemical hiatus occurs in the bushveld complex at the level of the platiniferous merensky reef close to the critical main zone boundary the origin of this hiatus and its relationship to mineralization has not been fully resolved initial sr isotope ratios for plagioclase and cr mgo values for orthopyroxene from critical and main zone rocks are significantly different geochemical data through the merensky and bastard cyclic units indicate that orthopyroxene in the merensky and bastard units originated from magma with a composition like that of the critical zone magma it sometimes occurs together with plagioclase that originated from main zone magma in a detailed study of the eastern limb the pyroxenite at the base of the merensky unit contains plagioclase and orthopyroxene that have compositions typical of the critical zone but in the overlying part of the merensky unit plagioclase compositions are more typical of the main zone whereas orthopyroxene retains a critical zone signature similarly in the bastard pyroxenite plagioclase has a range of sr isotope values between that of the main and critical zones but orthopyroxene has values typical of the critical zone these observations of mineral disequilibrium clearly show that the two major minerals in the merensky and bastard units were formed from two different but coexisting magmas a model that accounts for this disequilibrium is proposed here it invokes the influx of main zone magma at the base of the merensky unit during which the critical zone magma was displaced upward but the two magmas did not mix the elevated critical zone liquid continued to crystallize orthopyroxene which sank through the influx of magma of the main zone due to the density contrast these grains collected on the crystal pile to form the merensky pyroxenite the main zone magma into which the orthopyroxene accumulated crystallized interstitial plagioclase that had an sr isotope ratio typical of the main zone whole rock major element geochemical data show that a variable proportion of the plagioclase in both the merensky and bastard pyroxenites is cumulus it is inferred to have accumulated with orthopyroxene and has an initial sr isotope ratio typical of the critical zone thus the two pyroxenites now have a mixed sr isotope signature of cumulus critical zone and intercumulus main zone plagioclase that varies along strike above the pyroxenites the sr signature of the norites and anorthosites of both cyclic units is dominated by cumulus plagioclase from the main zone magma but some plagioclase with a critical zone signature may be present we conclude that the variations in initial sr isotope ratios do not result from mixing of magmas but from variable accumulation of orthopyroxene and plagioclase from a higher isotopically distinct layer of magma into an underlying layer the merensky and bastard cyclic units therefore may have features of either critical or main zone magma depending on which chemical parameter cr mgo and sr isotopes is considered they are therefore classified as a transitional unit between the critical and main zones models that suggest magma mixing was responsible for sulfide saturation and the creation of an immiscible sulfide liquid are not supported by this study and alternative models must be sought to explain the sulfide saturation event in the merensky reef

persistent millennial scale climatic variability over the past 25 000 years in southern africa abstract data from stalagmites in the makapansgat valley south africa document regional climatic change in southern africa in the late pleistocene and holocene a new tims u series dated stalagmite indicates speleothem growth from 24 4 to 12 7 ka and from 10 2 to 0 ka interrupted by a 2 5 ka hiatus high resolution oxygen and carbon stable isotope data suggest that postglacial warming was first initiated 17 ka was interrupted by cooling probably associated with the antarctic cold reversal and was followed by strong warming after 13 5 ka the early holocene experienced warm evaporative conditions with fewer c4 grasses cooling is evident from 6 to 2 5 ka followed by warming between 1 5 and 2 5 ka and briefly at ad 1200 maximum holocene cooling occurred at ad 1700 the new stalagmite largely confirms results from shorter holocene stalagmites reported earlier the strongest variability superimposed on more general trends has a quasi periodicity between 2 5 and 4 0 ka also present are weaker 1 0 ka and 100 year oscillations the latter probably solar induced given similarities to the antarctic records the proximate driving force producing millennial and centennial scale changes in the makapansgat record is postulated to be atmospheric circulation changes associated with change in the southern hemisphere circumpolar westerly wind vortex

iron in plagioclase in the bushveld and skaergaard intrusions implications for iron contents in evolving basic magmas the evolved iron rich rocks of the tholeiitic bushveld and skaergaard intrusions are similar in containing cumulus magnetite ilmenite plagioclase clinopyroxene apatite and olivine and also orthopyroxenes pigeonite in bushveld here we evaluate their liquid evolution trends using the total iron content in plagioclase determined by electron microprobe analyses to aid this analysis a revised mass balance model for the liquid evolution of skaergaard is presented for plagioclase in the upper zone of skaergaard it was previously demonstrated that total feo increases from 0 25 to 0 45 wt with differentiation and correlates inversely with an 100 ca na ca the reverse trend is observed in two recently published datasets for bushveld showing that total feo in plagioclase decreases upward through the magnetite bearing upper zone from 0 30 to 0 15 and from 0 40 to 0 25 in the western and northern limbs respectively and correlates positively with an the partition coefficient of total iron between plagioclase and magma increases with oxidation and polymerisation in the liquid although bushveld formed under slightly more oxidizing conditions than skaergaard differences in the partition coefficients cannot explain the two observed trends we therefore conclude that the differentiation trends of the liquids subsequent to magnetite saturation were fundamentally different the inferred liquid composition for bushveld contained about 15 wt total feo at the level of magnetite in which is slightly less than the total feo content of the subsequent cumulates in contrast the skaergaard liquid contained more total feo than the ensuing cumulates as a result in bushveld residual liquids total feo decreased after magnetite saturation whereas in skaergaard the residual liquids continued to become enriched in iron this conclusion is corroborated by simple mass balance calculations between modelled residual liquids and extracted cumulate rocks despite the mineralogical similarities of evolved iron rich rocks of skaergaard and bushveld their liquid evolution trends were very different and generalizations about the extent of iron enrichment in tholeiitic magmas should be avoided

palynological evidence for late quaternary warming episodes in southern africa abstract polleniferous sediments from southern africa associated with past warm episodes before the last glacial maximum are rare interpretation of environmental conditions during these phases are complicated by difficulties of dating in sediments from the northern areas north of 28 s the prominence of tropical woodland pollen suggests that the period between 7000 and 6500 yr b p was associated with optimal temperatures during the holocene identifying the warmest phase in the southern high lying grasslands and semi arid karoo shrublands south of 28 s is difficult because unlike the savanna areas of the north they do not yield good pollen indicators for changes in temperature such as those of frost sensitive trees however some pollen sequences from further south along the southern coast of africa 34 s and at marion island in the southern ocean 47 s suggest that the holocene temperature optimum occurred at about the same time in the sub antarctic area in the south and the subtropical regions in the north the advent of moister conditions in southern africa during the early to middle holocene is generally recorded earlier ca 7500 6500 yr b p in the north at ca 26 s than around 31 s ca 5000 yr b p this change is provisionally associated with a relative shift in seasonality from a predominance of all season precipitation to a greater proportion of summer rainfall which apparently reached the southern karoo areas two thousand years later than the northern bushveld region

effect of tree thinning and skidding trails on hydrological connectivity in two japanese forest catchments abstract land use composition and patterns influence the hydrological response in mountainous and forest catchments in plantation forest management operations fmo modify the spatial and temporal dynamics of overland flow processes however we found a gap in the literature focussed on modelling hydrological connectivity hc in plantation forest under different fmo in this study we simulated hc in two steep paired forest subcatchments k2 and k3 33 2 ha composed of japanese cypress chamaecyparis obtusa endl and japanese cedar cryptomeria japonica d don plantations 59 of the total area against a tree thinning intensity of 50 at different time additionally construction of new skidding trails and vegetation recovery was simulated on five thinning based scenarios that covered a 40 month test period july 2010 october 2013 as a future scenario six check dams located in the main streams were proposed to reduce sediment and radionuclide delivery an updated version of borselli s index of runoff and sediment connectivity was run using the d infinity flow accumulation algorithm and exploiting three 0 5 m resolution digital elevation models on the basis of the pre fmo scenario hc increased at catchment scale owing to tree thinning and the new skidding trails this change was more noticeable within the area affected by the fmo where hc increased by 11 4 and 10 5 in the cypress and cedar plantations in k2 respectively and by 8 8 in the cedar plantation in k3 at hillslope plot and stream scales the evolution in the values of hc was less evident except the increment by 5 4 observed in the streams at k2 after the fmo progressive vegetation recovery after the fmo triggered a slight reduction of connectivity in all compartments of both subcatchments forest roads and especially skidding trails presented the highest values of hc appearing as the most efficient features connecting the different vegetation patches with the stream network the spatial and temporal evolution of hc over the five past scenarios correlated well with the observed changes in runoff yield as well as with the available values of rainfall interception and throughfall before during and after the fmo the simulation of the proposed scenario recommends the construction of check dams as effective landscape features to somewhat reduce hc and thus to decrease the sediment and radionuclide delivery rates from the two subcatchments

trapped liquid paleo porosity and formation time scale of a chromitite ortho pyroxenite cumulate section bushveld south africa to evaluate compaction and interstitial melt expulsion during cumulate formation a 20 m cumulate section including the ug2 and ug3 chromitites from a 264 m drill core through the upper critical zone of the bushveld complex south africa has been studied the cumulates in the studied section are as follows 3 m plagioclase pyroxenite to pyroxenite pegmatoid footwall pyroxenite at the lower contact to ug2 0 7 m ug2 chromitite 6 8 m pyroxenite 0 24 m ug3 chromitite 2 0 m plagioclase rich pyroxenite changing locally to norite the two 5 cm leader stringers ug3a and ug3b and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites all pyroxenites are dominated by orthopyroxene opx and the cumulate sequence is topped by mottled anorthosite grading into norite stratigraphic concentrations of major and trace elements of 52 bulk rock samples were determined bulk rock mg numbers are 0 79 0 81 throughout the silicate cumulate units and 0 40 0 46 in the chromitite layers the stratigraphic distribution of six incompatible trace elements k rb ba cs zr and th has been used to determine the amount of trapped liquid ftl or paleo porosity in the cumulate rocks final porosities volume fractions based on averages from the six trace elements are 0 06 0 33 in the pyroxenites in chromitite layers trapped melt fractions of 0 12 0 36 are calculated from incompatible trace element concentrations but bulk sio2 concentrations and x ray tomography yield 0 04 0 17 higher porosities hence the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction as poikilitic opx was crystallizing while the silicate melt still equilibrated using a previously derived experiment based model for compaction time scales gravitationally driven chemical compaction in the ug2 ug3 pyroxenite section is calculated to occur within 1 10 years this time frame corresponds to the times necessary to cool a 20 m layer by 10 50 c the temperature interval argued to encompass the liquidus and almost complete solidification compaction within a decade can in fact easily develop the paleo porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid contrary to previous assertions melt expulsion from the cumulate pile does not hinder compaction calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate the pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites their entrapment below the chromitite layers suggests that these act as permeability barriers this is in part due to their finer grain size compared with the pyroxenites but is mainly due to the crystallization of large poikilitic opx during compaction

geomorphic threshold conditions for ephemeral gully incision abstract ephemeral gully erosion in cultivated land is an important source of sediment that is frequently being overlooked and not accounted for in soil erosion studies furthermore little is known about the factors controlling ephemeral gully erosion in this paper the available information on the initiation and location of ephemeral gullies is summarised focusing on the relationship between the upslope drainage area a and the critical slope gradient s cr for ephemeral gully initiation by plotting the non linear relationship between critical slope gradient measured immediately upstream of the incision head and drainage area at the incision head for gullied sites it was possible to draw a straight line on log log paper through the lower most points for each of the available datasets representing a critical slope area relationship for incision consequently below this line no incision occurs this line or critical relation can also be written as a power function between critical slope and area although many factors vary between the different datasets the exponent of the drainage area relationship 0 4 showed very little variation the observed critical slope area relationship can be related to a simple model of channel initiation by overland flow furthermore this relationship can be used to identify potentially unstable sites where control measures should be undertaken

geomorphometric assessment of spatial sediment connectivity in small alpine catchments abstract complex and rugged topography induces large variations in erosion and sediment delivery in the headwaters of alpine catchments an effective connection of hillslopes with the channel network results in highly efficient sediment transfer processes such as debris flows in contrast morphological conditions producing decoupling of hillslopes from channels e g glacial cirques may exclude large areas of the catchment from sediment delivery to its lower parts moreover an efficient connection between hillslopes and channel network does not always ensure an effective downstream transfer of sediment low slope channel reaches e g in hanging valleys cause sediment deposition which often results in changes of the sediment transport processes typically from debris flow to streamflow with low bedload and suspended load rates the availability of high resolution digital terrain models such as those derived from aerial lidar improves our capability to quantify the topographic controls on sediment connectivity a geomorphometric index based on the approach by borselli et al 2008 was developed and applied to assess spatial sediment connectivity in two small catchments of the italian alps featuring contrasting morphological characteristics the results of the geomorphometric analysis were checked against field evidences showing good performance and thus potential usefulness of the index

sediment discharges during storm flow from proximal urban and rural karst springs central kentucky usa summary since the mid 1990s various studies have addressed the timing of sediment transport to karst springs during storm flow or the composition and provenance of sediment discharged from springs however relatively few studies have focused on the flow thresholds at which sediment is mobilized or total sediment yields across various time scales we examined each of these topics for a mainly urban spring blue hole and a rural spring sp 2 in the inner bluegrass region of central kentucky usa suspended sediment consisted mostly of quartz silt and sand with lesser amounts of calcite and organic matter total suspended sediment tss values measured during storm flow were greater at sp 2 than at blue hole by aggregating data from four storms during 2 years we found that median suspended sediment size jumped as q exceeded 0 5 m 3 s for both springs at blue hole tss tended to vary with q and capacity approached 1 g l but no systematic relationship between tss and q was evident at sp 2 sediment fluxes from the blue hole basin were 2 orders of magnitude greater for storms in march 2002 and 2004 than september 2002 and 2003 in contrast sediment fluxes from the sp 2 basin were of similar magnitude in september 2003 and march 2004 the overall range of area normalized fluxes for both springs 9 16 10 3 4 45 10 2 kg ha h overlaps values reported for farm plots and a stream in the inner bluegrass region and for other spring basins in the eastern usa and western europe sediment compositions sizes and responses to storms in the basins may differ because of land use e g the extent of impervious cover in the blue hole basin basin size larger for blue hole conduit architecture which appears to be more complex in the blue hole basin and the impoundment of sp 2 which may have promoted decadal scale storage of sediment upgradient

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

geomorphic connectivity within abandoned small catchments stolowe mts sw poland abstract studying 5 small catchments 0 6 2 7 km2 located in the central part of the stolowe mountains sw poland which are partly or totally abandoned has revealed the substantial changes of connectivity within the catchments field investigation geomorphic mapping morphometric analyses and gis based analyses of historical data show that slope channel coupling is much less efficient than in the past mainly due to the decrease of unpaved road network especially roads linked directly with streams also land use change with considerable loss of arable land does not support erosion from slopes the general direction of geomorphic change due to depopulation and lack of maintenance of anthropogenic features tends towards higher stability and lower efficiency of morphological processes connected with a sustained decrease in slope channel coupling this tendency is evidenced by relict landscape features e g road gullies abandoned roads formed in the period of more intensive land use for which higher dynamics of the morphogenetic system may be inferred the evidence of contemporary erosion and mineral material transport is very local and spatially limited to a few roads which are still in use sediment transfer is additionally slowed down by the presence of numerous anthropogenic barriers within slope and valley floor domains even though their total impact may seem only local and negligible at the catchment scale their contribution to increase the diversity of morphological processes is significant especially the neglect of hydrotechnical constructions enhances trapping of material within the catchments

chromite in the platreef bushveld complex south africa occurrence and evolution of its chemical composition the regional distribution and chemical composition of massive and disseminated chromitites through a platreef sequence and along a strike distance of over 20 km were investigated to correlate them both within the framework of the northern limb and to the eastern and western limbs of the bushveld complex the chromitite layers and seams of the platreef form two main chromite bearing zones the upper chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at approximately 20 m below the platreef top contact and the lower chromitite that is composed of multiple seams in feldspathic harzburgite pyroxenite and norite close to the bottom contact of the platreef with footwall electron micro probe analyses reveal that the chemical composition of chromite depends on the host rock type norite and pyroxenite host chromite with the highest cr2o3 content while harzburgite hosted chromites are cr and mg poor the wide range in chromite compositions is explained by the influence of late magmatic processes including post cumulus growth and re equilibration interaction with fluid and sulphide saturated magmatic liquid and contact metamorphism each of these processes is characterised by its own distinct geochemical signature but generally they lead to a decrease in mg and al and an increase in both di and tri valent fe in the chromite the occurrence of chromitite locally on the different distance from the contact between the upper platreef sills and the overlying main zone magma suggests erosion of the upper platreef by the main zone as it was emplaced the localisation of chromitites supports an independent development of the northern limb during the critical zone emplacement although the chemical composition of chromite and co existing silicates from ultramafic rocks suggest a critical zone affinity with the eastern and western limbs of the bushveld complex

sediment carbon fate in phreatic karst part 1 conceptual model development abstract recent research has paid increased attention to quantifying the fate of carbon pools within fluvial networks but few if any studies consider the fate of sediment organic carbon in fluviokarst systems despite that karst landscapes cover 12 of the earth s land surface the authors develop a conceptual model of sediment carbon fate in karst terrain with specific emphasis upon phreatic karst conduits i e those located below the groundwater table that have the potential to trap surface derived sediment and turnover carbon to assist with their conceptual model development the authors study a phreatic system and apply a mixture of methods traditional and novel to karst studies including electrical resistivity imaging well drilling instantaneous velocimetry dye tracing stage recording discrete and continuous sediment and water quality sampling and elemental and stable carbon isotope fingerprinting results show that the sediment transport carrying capacity of the phreatic karst water is orders of magnitude less than surface streams during storm activated periods promoting deposition of fine sediments in the phreatic karst however the sediment transport carrying capacity is sustained long after the hydrologic event has ended leading to sediment resuspension and prolonged transport the surficial fine grained laminae occurs in the subsurface karst system but unlike surface streams the light limited conditions of the subsurface karst promotes constant heterotrophy leading to carbon turnover the coupling of the hydrological processes leads to a conceptual model that frames phreatic karst as a biologically active conveyor of sediment carbon that recharges degraded organic carbon back to surface streams for example fluvial sediment is estimated to lose 30 of its organic carbon by mass during a one year temporary residence within the phreatic karst it is recommended that scientists consider karst pathways when attempting to estimate organic matter stocks and carbon transformation in fluvial networks

coarse sediment connectivity in river channel systems a conceptual framework and methodology identification of coarse sediment connectivity down river channel systems is important for understanding the linkages between river reaches the influence of sediment sources on channel morphology and the mechanisms and liability to propagation of morphological change a conceptual framework is developed in which several types and degrees of connectivity are proposed it is hypothesised that sediment connectivity varies with sediment sources and with stream competence to transport the coarse fractions a method of identifying the status of reaches is developed based on field mapping of morphology and sedimentology the analysis is exemplified by two channels one an ephemeral channel in se spain the other a perennial channel in nw england questions arise over whether reaches that lack channel stores transfer coarse sediment three scenarios are suggested 1 lack of flux due to lack of competence of the reach 2 flushing through of sediment due to high competence and 3 potential transport of coarse sediment but exhaustion or lack of availability changes in connectivity will take place over different time scales depending on the nature of exchange and time scales of storage short term variations in connectivity can be identified by repeated mapping

multiple sulfur isotopes reveal a magmatic origin for the platreef platinum group element deposit bushveld complex south africa the platreef ore horizon of the bushveld complex south africa is the third largest platinum group element ore deposit in the world but the origin of its ore remains enigmatic a complex contact relationship between the igneous and footwall rocks of the bushveld complex coupled with evidence for widespread late stage hydrothermal processing obscures the original mineralization history of the platreef we constrain the parental magmatic origin of the platreef by exploiting multiple sulfur isotope contrasts across bushveld complex contact zones in order to see through the effects of postmineralization hydrothermal activity we report s isotope measurements made on samples collected along two profi les through the platreef into underlying metapelitic and metacarbonate footwall rocks in both profi les igneous rocks far from the contact have low 33 s values average 33 s 0 15 whereas metasedimentary rocks far from the contact have high 33 s values 33 s to 5 04 with a smoothly varying profi le between the two end members the midpoint in both isotope profi les is displaced into the footwall defi ning a classic advective dispersive tracer geometry this geometry is not present in the associated 34 s values the displacement of the 33 s front suggests fl uid transport and advection of s into the country rocks this was accompanied by back diffusion of the s isotope tracer into the platreef the platreef magma was apparently s saturated prior to emplacement and counterintuitively lost s during the formation of the present platreef ore horizon

pickup probability for sediment entrainment the study presents a theoretical derivation for a new pickup probability formulation for sediment transport sand particles are assumed to be subjected to a hydraulically rough flow the derived equation compares well with the experimental data found in published literature using the proposed relation the shields criterion for the definition of the threshold condition for sediment transport is equivalent to 0 6 of the pickup probability

the petrogenesis of platinum group element reefs in the upper main zone of the northern lobe of the bushveld complex on the farm moorddrift south africa boreholes have intersected the interval hosting the pyroxenite marker at the base of the upper main zone on the farm moorddrift in the northern bushveld complex the concentrations of the major and compatible trace elements in the rocks are in the range of other pyroxenite marker intervals elsewhere in the bushveld complex mgo up to 22 wt cr up to 300 ppm over a stratigraphic height of some 300 m the interval is characterized by a broad but irregular reversal in mg number and cr v analogous to stratigraphically equivalent horizons elsewhere in the bushveld complex the rocks have distinctly higher concentrations in incompatible trace elements particularly large ion lithophile elements lile and light rare earth elements lree than other main zone rocks and thus appear to contain an elevated crustal component the interval hosts several thick meters to 10s of meters but mostly relatively low grade and laterally discontinuous pge reefs that have been delineated over a strike length of ca 1 km pge contents locally reach approximately 16 ppm pt pd au in rocks containing up to approximately 4 to 5 percent sulfides the pge tenors of most sulfides are 100 ppm pt pd ratios are above unity and pd ir is 200 based on well defined positive correlations between pt and pd with ir we interpret the mineralization as being largely of primary magmatic origin most previous models interpreted the pyroxenite marker to have formed from a major replenishment to the bushveld magma chamber with relatively primitive magma the new magma would have been emplaced on top of an approximately 4 km thick cumulate package consisting of the ultramafic mafic lower and critical zones and the lower to central portions of the gabronoritic main zone here we propose that the upper main zone including the pyroxenite marker did not result from a magma replenishment instead it crystallized from the s saturated residual magma of the upper critical zone cumulates and originally directly overlaid the latter the approximately 1 to 2 km thick lower central main zone package formed from a crystal charged magma emplaced at a relatively late stage in the solidification history of the bushveld chamber between the largely consolidated upper critical and upper main zones we envisage that the emplacement of this large magma batch resulted in subsidence of the center of the layered suite perhaps in response to the evacuation of a putative lower staging chamber from which the bushveld complex was fed semiconsolidated portions of the cumulate package sagged and slumped toward the center of the complex and the flowing crystal mushes underwent mechanical unmixing of light relatively buoyant plagioclase from denser pyroxenes oxides and sulfides this resulted in the formation of an interlayered sequence of relatively melanocratic sulfide enriched with relatively leucocratic sulfide poor layers reef formation is thus partly the result of cumulate reorganization in response to the emplacement of the main zone magma

towards a model for the in situ origin of pge reefs in layered intrusions insights from chromitite seams of the rum eastern layered intrusion scotland the current debate on the origin of platinum group element pge reefs in layered intrusions centres mostly on gravity settling of sulphide liquid from overlying magma versus its introduction with interstitial melt fluids migrating upward from the underlying cumulate pile here we show that pge rich chromitite seams of the rum eastern layered intrusion provide evidence for an alternative origin of such deposits in layered intrusions these laterally extensive 2 mm thick chromitite seams occur at the bases of several cyclic mafic ultramafic units and show lithological and textural relationships suggesting in situ growth directly at a crystal liquid interface this follows from chromitite development along the edges of steeply inclined culminations and depressions at unit boundaries even where these are vertically oriented or overhanging high concentrations of pge up to 2 3 ppm pd pt are controlled by fine grained base metal sulphides which are closely associated with chromitite seams the following sequence of events explains the origin of the pge rich chromitite seams a emplacement of picritic magma that caused thermal and mechanical erosion of underlying cumulate followed by in situ growth of chromite against the base b precipitation of sulphide droplets on chromite grains acting as favourable substrate or catalyst for sulphide nucleation c the scavenging of pge by sulphide droplets from fresh magma continuously brought towards the base by convection since the rate of magma convection is 105 107 times higher than that of the solidification km year to km day versus 0 5 1 0 cm year the in situ formed sulphide droplets can equilibrate with picritic magma of thousands to million times their own volume as a result the sulphide bearing rocks are able to reach economic concentrations of pge several ppm we tentatively suggest that the basic principles of our model may be used to explain the origin of pge rich chromitites and classical pge reefs in other layered mafic ultramafic intrusions

the role of viscous particle segregation in forming chromite layers from slumped crystal slurries insights from analogue experiments the question of how the mineral layering in layered intrusions forms has been extensively debated for many decades there are many types of layering and it is of course possible that a number of mechanisms are involved of particular interest is how chromite layers form because these may contain valuable metals such as platinum group elements in addition to cr one model for the formation of these layers is that they formed through slumping of semi consolidated cumulates from the margins of the intrusion into the magma chamber during this slumping the grains are sorted by density and or size differences this study examines the viability of this process using analogue modelling starting materials beads and glycerine were scaled to match the density and size of the minerals chromite orthopyroxene and plagioclase present in layered mafic ultramafic layered intrusions and to match the density and viscosity of the silicate magma a perspex flume tank divided by a removable partition at one end was fully filled with glycerine a homogenized mixture of the beads was placed in the smaller partition of the tank representing the margins of the magma chamber the tank was then inclined between 16 and 45 the partition was removed and the beads flowed into the main part of the box the experiments were recorded by video camera allowing us to follow the dynamics of the flow during each run segregation of the beads was observed in the final deposits the larger less dense beads representing plagioclase concentrated at the top of the flow with the intermediate sized and medium density beads representing orthopyroxene in the middle and the smaller denser beads representing chromite at the bottom thus mimicking natural examples in experiments where the angle of inclination was low long thin layers formed such as those found in the bushveld complex in experiments where the angle of inclination was high thick but short layers formed a dimensionless analysis allows better understanding of the dynamics of the flow at the macroscopic scale the flow regime is strongly influenced by the viscosity of the fluid and is considered macro viscous where the role of the interstitial liquid is non negligible

geomorphic coupling and sediment connectivity in an alpine catchment exploring sediment cascades using graph theory through their relevance for sediment budgets and the sensitivity of geomorphic systems geomorphic coupling and sediment connectivity represent important topics in geomorphology since the introduction of the systems perspective to physical geography by chorley and kennedy 1971 a catchment has been perceived as consisting of landscape elements e g landforms subcatchments that are coupled by geomorphic processes through sediment transport in this study we present a novel application of mathematical graph theory to explore the network structure of coarse sediment pathways in a central alpine catchment numerical simulation models for rockfall debris flows and hillslope and channel fluvial processes are used to establish a spatially explicit graph model of sediment sources pathways and sinks the raster cells of a digital elevation model form the nodes of this graph and simulated sediment trajectories represent the corresponding edges model results are validated by visual comparison with the field situation and aerial photos the interaction of sediment pathways i e where the deposits of a geomorphic process form the sources of another process forms sediment cascades represented by paths a succession of edges in the graph model we show how this graph can be used to explore upslope contributing area and downslope source to sink functional connectivity by analysing its nodes edges and paths the analysis of the spatial distribution composition and frequency of sediment cascades yields information on the relative importance of geomorphic processes and their interaction however regardless of their transport capacity in the study area the analysis stresses the importance of mass movements and their interaction e g the linkage of large rockfall source areas to debris flows that potentially enter the channel network moreover it is shown that only a small percentage of the study area is coupled to the channel network which itself is longitudinally disconnected by natural and anthropogenic barriers besides the case study we discuss the methodological framework and alternatives for node and edge representations of graph models in geomorphology we conclude that graph theory provides an excellent methodological framework for the analysis of geomorphic systems especially for the exploration of quantitative approaches towards sediment connectivity

characterization of the rill geometry over straight hillslopes through spatial scales abstract experimental evidence is presented which shows that there exists a spatial scale over which ergodic assumptions are applicable for rilled hillslope surfaces the property of expected spatial rill density esrd is defined and this property is shown to stabilize after the averaging interval is more than 20 ft over the hillslope the rilled hillslope geometry may be readily quantified in terms of this easily measurable quantity the rill geometry has a significant influence on the surface distribution of overland flow and sediment transport an analysis of the spatial scales suggests that a continuum representation may be utilized in modeling these hydrodynamic flow phenomena for a complete characterization of the rilled hillslope geometry the distributions of the rill widths and rill depths at various locations along the hillslope have been presented these preliminary results show that the mean rill widths the mean rill depths and the esrd increase with increasing slope lengths the distributions of the rill widths and rill depths suggest that these quantities may be modeled by truncated gaussian or gamma distributions at different spatial locations the rill geometry properties may be related to hydrologic and geomorphologic properties of the hillslope it is suggested that the esrd is a better descriptor of the rill geometry on the hillslope than the rill density because of the wide variations exhibited by the distributions of the rill widths

catchment scale dis connectivity in sediment flux in the upper hunter catchment new south wales australia abstract dis connectivity within and between landscape compartments affects the extent and rate of transfer of energy and matter through catchments various landforms may impede sediment conveyance in a river system whether laterally to the channel termed buffers or longitudinally along the channel itself termed barriers a generic approach to analysis of landscape dis connectivity using slope threshold analysis in gis tied to air photograph interpretation and field mapping of buffers and barriers is tested in the upper hunter catchment australia under simulated conditions effective catchment area which is a measure of the proportion of a catchment that has the potential to contribute sediment to the channel network varies from 73 to just 3 of the total catchment area for differing subcatchments in the upper hunter this variability can be explained by the spatial distribution and assemblage of buffers and barriers in each subcatchment multiple forms of disconnectivity are evident in some subcatchments such that when one buffer or barrier is breached other features still impede sediment transfer within the system the importance of the position of buffers and barriers within any given subcatchment is emphasised spatial variability in valley width exerts a critical control on catchment connectivity with more efficient sediment conveyance in narrow valleys relative to wider valleys characterised by piedmonts terraces fans and extensive floodplains in which conveyance is impeded this variability reflects the landscape history and geological setting of each subcatchment the framework developed in this paper can be used to assess the impact of natural or human induced buffers and barriers on catchment scale sediment flux in any landscape setting providing a physical template atop which other biogeochemical fluxes could be examined

the sediment delivery problem abstract the linking of on site rates of erosion and soil loss within a drainage basin to the sediment yield at the basin outlet and improved knowledge and representation of the associated processes of sediment delivery represent a major research need within the field of erosion and sedimentation and also an important scale problem in drainage basin studies this paper reviews the limitations of the sediment delivery ratio concept by considering the problems of temporal and spatial lumping and its blackbox nature some recent advances in our understanding of the sediment delivery system and its modelling are described and the lack of detailed empirical investigations is highlighted the significance of recent concern for the role of sediments in the transport of nutrients and contaminants to sediment delivery studies is introduced and the need for further work in this field is emphasised

significance of 87 sr 86 sr ratios in the merensky cyclic unit of the bushveld complex in a number of mafic layered intrusions layers with platinum group element pge mineralization coincide with breaks in cumulus mineral composition and isotopic variation trends through the layered sequence1 2 for example in the bushveld complex in south africa abrupt increases in initial 87sr 86sr ratios occur at the ug 2 and merensky pegmatoid or reef horizons both of which are exploited for pges we report here the results of a detailed sr isotope study across the merensky unit and its immediate foot and hangingwall rocks which when considered together with new ideas on processes operating in magma chambers3 5 suggest that magma mixing and post cumulus infiltration of liquids displaced from below were important during crystallization of the merensky pegmatoid

what do models tell us about water and sediment connectivity connectivity has been embraced by the geosciences community as a useful concept to understand and describe hydrological functioning and sediment movement through catchments mathematical modelling has been used for decades to quantify and predict erosion and transport of sediments e g in scenarios of land use change or conservation measures being intrigued by both models and the connectivity concept as a group of modellers we aimed at investigating what different models could tell us about connectivity therefore we evaluated the response of contrasted spatially distributed models to landscape connectivity features and explained the differences based on different model structures a total of 53 scenarios were built with varying field sizes and orientations as well as the implementation of soil conservation measures these scenarios were simulated for two rainfall intensities with five event and process based water and soil erosion models erosion3d fullswof 2d landsoil openlisem and watersed results showed that rainfall amount plays the most important role in determining relative export and connected area of runoff and sediment in all models indicating that functional aspects of connectivity were more important than structural connectivity as for the role of structural landscape elements there was no overall agreement between models regarding the effects of field sizes crop allocation pattern and conservation practices agreement was also low on the spatial patterns of connectivity this overall disagreement between models was unexpected the results of this exercise suggest that the correct parameterization of runoff and sediment production and of routing patterns may be an important issue thus incorporating connectivity functions based on routing would help modelling forward our results also suggest that structural connectivity indices may not suffice to represent connectivity in this type of catchment relatively simple and monotonous land cover and functional connectivity indices should be applied

pollen indications of holocene palaeoenvironments at florisbad spring in the central free state south africa mid to late holocene pollen data from florisbad in the central free state south africa reveal a number of moisture fluctuations commencing at c 6500 bp the pollen sequence indicates an arid climate but shows more availability of moisture under grassy conditions from summer rain at c 6300 bp and again at c 4420 bp the interval between these dates is characterized by a dry episode with karroid shrubs some time between 5500 and 4500 bp possibly as a result of less marked summer seasonality after 4220 bp moisture conditions gradually fluctuated in intensity or seasonal availability until c 2100 1700 bp when strong summer evaporation is indicated at the upper end of the sequence the data are complemented by previously published results on environmental change from the nearby deelpan site to the west although some distinct events correlate regionally over south africa summer rains appear to have been significant at florisbad at an earlier stage in the mid holocene than in the karoo to the south

holocene pollen from swamp cave and hyrax dung deposits at blydefontein kikvorsberge karoo south africa abstract pollen analysis of material from a variety of sediment types including those from ponds streams a rock shelter and hyrax dung accumulations in the blydefontein basin 31 09 s 25 05 e fig 1b provide a record of vegetation change in the eastern karoo over the last 10 000 years the pollen composition fluctuated as a result of different taphonomic processes inherent in the various sampled deposits results further demonstrate that long term vegetation changes alternated between karoo shrub and grassland plant communities karoo shrubs suggesting relatively dry conditions were generally prominent in the early holocene until ca 5400 yr bp when more grassy vegetation began to flourish presumably in response to increased summer rain conditions pollen representation of the early holocene is not detailed enough to trace millennial scale variations during this period but more detailed middle to late holocene data show millennial or shorter scale shifts between grassland and drier karroid veld

mercury in the bushveld complex south africa and the skaergaard intrusion greenland abstract we present mercury concentrations for layered mafic rocks of the bushveld complex the skaergaard intrusion morb and a karoo dolerite sill as determined by combustion atomic absorption spectroscopy combustion aas with gold amalgamation pre concentration mercury in the bushveld complex and the skaergaard intrusion is generally low with about 1 1 and 0 8 ppb hg on average respectively the karoo dolerite sill chill margins and morb have average hg concentrations of 2 5 and 6 4 ppb respectively the reason for the low whole rock hg concentrations in layered mafic intrusions is that the element is incompatible with respect to the cumulus mineral assemblages our results show far lower hg concentrations for the skaergaard cumulates than previous measurements and we find no evidence for preferential enrichment in plagioclase rich rocks as previously reported we attribute this difference to inaccurate analytical methodology of the early measurements there are hg enrichments in distinct levels in both intrusions mercury enrichments in the bushveld complex are observed near the top of the lower zone 2 4 ppb in the merensky reef at 8 6 ppb the platreef at 3 ppb average and in the upper zone near the main magnetitite layer 4 7 ppb these enrichments are due to the concentration of hg by sulfides as suggested by a positive correlation between cu and hg there is no upward increase in hg with stratigraphic height or positive correlation between hg and zr in the bushveld complex as would be expected if hg acted as an immobile incompatible element the lack of correlation is explained by periodic degassing of the bushveld magma chamber in which volatile elements including hg and s escaped in contrast the skaergaard intrusion probably did not degas and there is hg enrichment in the sandwich horizon 53 ppb because of the combination of the incompatibility and the presence of sulfides

the role of magma mixing in the genesis of pge mineralization in the bushveld complex thermodynamic calculations and new interpretations the elegant magma mixing model for the formation of the platinum group elements pge mineralization associated with chromite and sulfide in the bushveld complex proposed by naldrett and von gruenewaldt 1989 has recently been questioned by cawthorn 1999 based on his own calculation cawthorn concluded that the concave upward sulfur solubility curve of a fractionated basaltic magma which is central to the model does not exist cawthorn 1999 pointed out that both the hanging wall and footwall sequences of the merensky reef formed from sulfide undersaturated magmas instead of sulfide saturated magmas as predicted by the model cawthorn argued that there is a mass balance problem with the model if these rocks did not contribute pge to the sulfides in the reef our thermodynamic calculations indicate that the concave upward curve of s solubility though different in detail from the original is very much in evidence our calculations also indicate that the content of s dissolved in the residual magma may fall below the s solubility curve when the rate of s increase in the magma due to the removal of silicate cumulates is less than the rate of increase in its s solubility due to the change in its temperature and composition during fractional crystallization mixing of such fractionated and yet sulfide undersaturated magma with appropriate amounts of a primitive magma is still capable of achieving sulfide oversaturation in the hybrid once the immiscible sulfides settle out of the magma the content of s dissolved in the hybrid may fall below its solubility curve again during further crystallization thus no sulfide segregation is expected in the rocks crystallized subsequently r factor assessment needs to consider the consequence of sulfide suspension in magma small quantities of sulfide droplets formed during the normal course of fractional crystallization may remain suspended in magma until the next replenishment and in this case they can be redissolved in the hybrid when the proportion of a new input is large enough because there is no early sulfide liquid segregation both the residual and new magmas will not be depleted in pge and are equally important for contributing pge to the sulfide liquid segregated during a subsequent magma mixing event

electrochemical processes in a crystal mush cyclic units in the upper critical zone of the bushveld complex south africa the upper critical zone ucz of the bushveld igneous complex displays spectacular layering in the form of cyclic units comprising a basal chromitite layer overlain by a sequence of silicate cumulates in the order from bottom to top pyroxenite norite anorthosite electron microprobe and laser ablation inductively coupled plasma mass spectrometry analyses of chromite and silicate minerals in layers between the ug2 chromitite and the merensky reef reveal variations in major and trace element compositions that defy explanation with existing models of cumulate mineral melt evolution the anomalous features are best developed at sharp contacts of chromitite with adjacent anorthosite and pyroxenite cumulates here chromite compositions change abruptly from high and constant mg mg fe 2 and fe 2 fe 3 ratios in chromitite layers to variable and generally lower values in chromite disseminated in silicate layers furthermore the composition of disseminated chromites varies depending on the host silicate assemblage for example in ti v and zn contents importantly the abrupt change in chromite composition across the chromitite silicate layer contacts is independent of the thickness of the chromitite layer and the estimated mass proportions of chromite to intercumulus liquid chemical variations in plagioclase are also abrupt and some are hard to reconcile with conventional models of re equilibration with intercumulus liquid among those features is the decoupling of alkalis from other incompatible lithophile elements in comparison with cumulus plagioclase intercumulus poikilitic plagioclase in chromitite layers is enriched in rare earth elements but strongly depleted in equally incompatible li k and rb strong alkali depletion is also observed in intercumulus pyroxene from ultramafic cumulates and chromitite layers to explain these features we propose a new model of post cumulus recrystallization which intensifies the modal layering in the crystal liquid mush producing the observed sequence of nearly monomineralic layers of chromitite pyroxenite and anorthosite that define the cyclic units the crucial element of this model is the establishment of redox potential gradients at contacts between chromite rich cumulates and adjacent silicate layers owing to peritectic reactions between the crystals and intercumulus melt because basaltic melts are ionic electrolytes with na as the main charge carrier the redox potential gradient induces electrochemical migration of na and other alkali ions selective mobility of alkalis can explain the enigmatic features of plagioclase composition in the cyclic units sodium migration is expected to cause remelting of previously formed cumulates and major changes in modal mineral proportions which may eventually result in the formation of sharply divided monomineralic layers the observed variations in ferric ferrous iron ratios in chromite from the cyclic units and fe distribution in plagioclase imply a redox gradient of

stable width and depth of gravel bed rivers with cohesive banks an analytical model is developed to determine the influence of the bank stability on the stable width and depth of alluvial gravel bed rivers with cohesive banks the formulation of the model is based on the assumption that the stable width corresponds with an optimum condition that is equivalent to the maximum bed load transporting capacity the optimum condition develops when the channel banks are at their limiting stability with respect to either mass failure or fluvial erosion two basic channel types are identified bank height and bank shear constrained mass failure stability is estimated using a simple total stress approach a method for estimating the critical bank shear stress based on model calibration is proposed analysis of field data indicates that the effect of the bank vegetation on bank stability can be expressed in terms of the critical bank shear stress the average critical bank shear stress value calculated for riverbanks covered by vegetation with well developed root networks was fo

the distribution of some platinum group elements in the ug 2 chromitite layer of the bushveld complex samples representing consecutive 2 cm thick slabs of a drill core through the ug 2 chromitite layer were analyzed for platinum group elements the results when plotted against height reveal an interesting pattern with sequences that follow exponential curves of the type y zx k where y equals the concentration of the element in samples taken at a distance x from the base of the layer and z is a constant the data fit the curves with correlation coefficients of better than 0 90 and levels of significance less than 0 1 percent which shows that the fit cannot be due to chance but that a process following or approximating an exponential equation must have been involved the most probable such process is fractionation according to the rayleigh law it is postulated that these sequences that follow the rayleigh law formed by bottom crystallization when the magma was in motion calculations indicate that the elements supplied to the site of crystallization were withdrawn from a surprisingly small thickness of magma and that their supply was not limited by diffusion between the sequences that follow the rayleigh law are sequences that probably crystallized from stagnant magma so that the supply of elements to the site of crystallization was diffusion controlled in these sequences the ratios between the different platinum group elements change progressively it is argued that these results reveal that the platinum group minerals crystallized very early in ug 2 layer type deposits and show a pattern of element behavior conforming closely with that of a classical compatible element which has a very large bulk distribution coefficient therefore the well known processes of enrichment applicable to such elements during processes of melting and crystallization could have been operative this one example is not sufficient to disclaim the possibility that platinum group elements can also behave like incompatible elements under different conditions as is assumed by many authors in this field

the application of semi quantitative methods and reservoir sedimentation rates for the prediction of basin sediment yield in spain traditionally the problems in predicting sediment yield at the basin scale are related to model s high data requirements the focus on a limited amount of erosion and sediment transport processes and a general lack of systems knowledge here we explore the performance of semi quantitative approaches to predict mean annual area specific sediment yield ssy at the scale of small to medium sized basins the factorial scoring model fsm verstraeten g poesen j de vente j koninckx x 2003 sediment yield variability in spain a quantitative and semiqualitative analysis using reservoir sedimentation rates geomorphology 50 4 327 348 the psiac model psiac 1968 pacific southwest inter agency committee report of the water management subcommittee on factors affecting sediment yield in the pacific southwest area and selection and evaluation of measures for reduction of erosion and sediment yield and some variations of these models were applied to predict ssy for the 60 reservoirs using the reservoir sedimentation rates as published by avendano salas et al avendano salas c sanz montero e gomez montana j l 1997 sediment yield at spanish reservoirs and its relationship with the drainage basin area dix neuvieme congres des grands barrages commission internationale de grands barrages florence pp 863 874 the fsm explained 72 of the variability found in reservoir sedimentation rates which is significantly better than estimates based on basin area alone explaining only 22 the fsm uses five factors i e topography lithology vegetation cover gullies basin shape to characterise a drainage basin in the vicinity 5 km of the reservoir and the main tributaries feeding the reservoir by providing a score an index is calculated by multiplying the scores per factor together with basin area this index is used to predict ssy the psiac model works in a comparable way though some other factors are used i e topography climate runoff lithology soil vegetation land use upland erosion channel erosion basin area is not part of the relation and the index is calculated by summation of all scores psiac explained 58 of the variation in ssy advantages of both models over physically based and other empirical models are that less data are required gully erosion is included and the importance of connectivity in the landscape is recognised this study thus demonstrates that semi quantitative approaches can provide fairly accurate and reliable estimates of ssy at the scale of small to medium sized basins with relatively low data requirements

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

sulfur isotope variations within the platreef ni cu pge deposit genetic implications for the origin of sulfide mineralization the platreef the world s third largest platinum group element pge deposit is a 10 400 m thick pyroxenitic unit at the base of the northern limb of the mafic bushveld complex hosting pge mineralization in association with base metal sulfides the sulfide mineralization is thought to have been either largely magmatic in origin with contamination by assimilation of local floor rocks considered an ore modifying process or a product of assimilation of country rock s and silicic contamination which caused s saturation in the magma with the contamination being fundamental to the ore forming process we have performed an extensive and detailed sulfur isotope study of the platreef which indicates that magmatic signatures 34s 0 to 2 are preserved in early formed sulfide droplets within the platreef pyroxenites in the area from sandsloot to witrivier these values are comparable to sulfide inclusions in diamonds in the nearby klipspringer kimberlite and are considered to have a primary mantle origin there is no indication of any significant external s in any of the primary sulfides in the sandsloot witrivier area however later sulfides found in calc silicate floor rocks at sandsloot and zwartfontein and in xenoliths of calc silicate throughout the section indicate an input of country rock s anhydrite bearing horizons in the malmani dolomites may have exchanged s with magmatic platreef sulfide during platreef intrusion imparting a heavier s isotope signature in the fluid affected rocks than those in the early sulfides the archean basement although containing minor amounts of sulfide with a negative 34s value is not a significant contributor to the s budget of the platreef previous studies have indicated that in areas where the sedimentary floor rocks contain appreciable sulfides rather than sulfates such as at turfspruit the platreef sulfides are extensively contaminated with country rock s assimilation of pyrite bearing shales in the turfspruit area has locally upgraded the s content of the platreef and given the basal platreef sulfides a heavier isotopic signature sulfur saturation in the platreef magma took place before contamination probably in a staging chamber prior to intrusion a major pulse of magma entrained the preformed pge rich sulfides and was injected to form the platreef where assimilation of country rock sulfides upgraded the s content on a strictly local scale and hydrothermal leaching introduced s from country rock sulfates into later stage sulfides again on a local scale south of zwartfontein it is proposed that the platreef was intruded into sediments of the transvaal supergroup which formed the floor and the roof of the platreef north of zwartfontein the platreef intruded the boundary between the malmani supergroup sediments and the archean basement this could have potentially mineralized the roof calc silicates north of zwartfontein in a manner similar to the floor in the sandsloot area the later intrusion of the main zone magma formed a magmatic unconformity on top of the cold platreef and entrained large rafts of mineralized calc silicate for tens of kilometers north of the last footwall outcrop of calc silicate

a tale from three regions the archaeology of the pleistocene holocene transition in the western cape the caledon valley and the lesotho highlands southern africa abstract the archaeological and palaeoenvironmental records of the western cape the caledon valley and the lesotho highlands are examined across the pleistocene holocene boundary using data from three key long sequence sites elands bay cave rose cottage cave and sehonghong the settlement histories of all three regions are highly pulsed but there is some suggestion that the western cape was out of synch with the two other regions a relationship possibly linked to their different rainfall regimes in all three regions however people used the landscape quite differently in the period reviewed compared to the late holocene an increased use of r selected food resources is most visible at elands bay but in all three regions the early holocene witnessed a shift toward greater exploitation of smaller browsing bovids increased localization of social alliance networks suggested by the distributions of non local items and by patterns of stone raw material usage in the lesotho highlands caledon valley area may reflect increasing population densities as environmental productivity rose the importance is stressed of developing explanations of these phenomena at a range of appropriate spatio temporal scales and suggestions are made as to where future research may most profitably be directed

sediment carbon fate in phreatic karst part 2 numerical model development and application abstract the authors develop a numerical model to elucidate time distributed processes controlling sediment carbon fate in phreatic karst sediment carbon processes simulated in the new numerical model include in conduit erosion and deposition sediment carbon transport surficial fine grained laminae evolution carbon pool mixing microbial oxidation and the understudied process of sediment carbon exchange during equilibrium transport the authors perform a model evaluation procedure that includes generalized likelihood uncertainty estimation to quantify uncertainty of the model results modeling results suggest that phreatic karst conduits sustain sediment transport activity long after surface storm events cease the sustained sediment transport has the potential to shift the baseflow sediment yield of the phreatic karst to be on par with stormflow sediment yield the sustained activity is suggested to promote the exchange of sediment carbon between the water column and subsurface karst deposits during equilibrium sediment transport conditions in turn the sediment carbon exchange impacts the mixing of new and old carbon pools and the flux of carbon from phreatic karst integrated numerical model results from this study support the concept that phreatic karst act as a biologically active conveyor of sediment carbon that temporarily stores sediment turns over carbon at higher rates than surface streams and recharges degraded carbon back to the fluvial system the numerical modeling method adopted in this paper shows the efficacy of coupling carbon isotope fingerprinting with water quality modeling to study sediment carbon in phreatic karst

sedimentary sequences at sani top lesotho highlands southern africa detailed examination of sediments retained in two north facing cirques south of sani top latitude 29 30 s longitude 29 2 e has been undertaken as a contribution to the late quaternary history of highland lesotho the sediments are exposed by gullies incised through the deposits nine sections are discussed in detail inorganic sediments matrix supported diamictons and derived orange gravels as well as organic dark clays and peats are present eight dated organic samples range in age from 13490bp currently the oldest organic carbon date from highland lesotho to 2310 bp the sedimentary sequences are interpreted to present a sequence of late quaternary events with climatic implications

pge tenor and metal ratios within and below the merensky reef bushveld complex implications for its genesis the merensky reef is the consequence of several major influxes of magma into the bushveld magma chamber that gave rise to at least two cyclic units including the pre merensky and merensky cyclic units analysis of the s ni cu platinum group element pge and au contents of 1140 samples from 24 profiles through the rocks forming these units and two profiles extending downward from the reef to close to the ug 2 horizon of the western bushveld complex provide important petrogenetic constraints on the origin of the reef particular attention is paid to variations in the calculated pge tenors and cu pd ratios of sulfides both vertically and laterally all of the profiles show a decrease in pt tenor and concomitant increase in cu pd ratio upward across the pyroxenite of the merensky unit and laterally away from certain centres that are interpreted as feeder zones the data also document that more mafic horizons within the merensky to ug 2 interval of the southwestern bushveld are characterized by much lower cu pd 100 400 than the ambient ratio 4000 this is interpreted as demonstrating that these horizons are the consequence of incursions of magma whose composition has been modified in a staging chamber by reaction with sulfide resident there the favoured interpretation for the merensky reef is that the bushveld chamber s was occupied by a resident magma crystallizing orthopyroxene and plagioclase pre merensky and subsequently merensky magmas entered the chamber s intermittently from localized feeder structures mixing or mingling with the resident magma and giving rise to the more mafic cumulates at specific horizons including the merensky reef the mixing or mingling caused sulfide immiscibility in these cases as the new magma pulses flowed laterally away from the entry zones they deposited sulfide becoming depleted in pge on coming to rest the magma continued to segregate sulfide and crystallize pyroxene so that the sulfides became progressively depleted in pge and acquired a higher cu pd upward through the pyroxenite this model requires that the merensky magmas could dissolve up to two orders of magnitude more pge than current measurements on dry diopside anorthite melts have indicated it is suggested that very pge rich magmas can develop as a result of sulfide unsaturated magma reacting with early formed sulfide in a staging chamber dissolving fes and enriching the remaining sulfide in pge later magmas equilibrating with this enriched sulfide will themselves become highly enriched in pge

characteristics and palaeoenvironmental significance of relict sorted patterned ground drakensberg plateau southern africa abstract relict large sorted periglacial circles are described from the drakensberg plateau of southern africa morphological and sedimentological data for three populations of patterns occurring on high summit 3400 m a s l interfluves are presented the patterns range between 85 and 904 cm in diameter with centres encircled by primary and in some cases secondary borders patterns are sorted to maximum depths of 85 cm or more with borders occupying trough like depressions to a depth of 26 cm although it is found that contemporary freeze to 40 cm depth still permits some cryogenic activity within smaller patterns it is argued that the larger patterns are degrading owing to limited moisture and freeze duration and intensity the paper provides a literature summary of contemporary environmental parameters from 10 northern hemisphere localities where active relict large periglacial patterns occur findings suggest that patterns over 80 cm in diameter develop where the maat is 1 6 c or less it is demonstrated that the drakensberg palaeo maat reached such low temperatures during the lgm it is thus argued that the patterns developed during the lgm and that smaller varieties became reactivated during holocene cold phases the relict sorted circles provide confirmation for late quaternary permafrost above 3400 m a s l in southern africa the results further suggest pronounced palaeo climatic environmental variation over short spacial scales over southern africa s high plateau

river adjustment to changes in sediment load the effects of tin mining on the ringarooma river tasmania 1875 1984 the mining of alluvial tin in the ringarooma basin began in 1875 reached a peak in 1900 20 and had virtually ceased by 1982 during that time 40 million m3 of mining waste were supplied to the main river quickly replacing the natural bed material and requiring major adjustments to the channel based on estimates of sediment supply from more than 50 widely scattered mines and the frequency of flows capable of transporting the introduced load the river s transport history is reconstructed using a mass conservation model because of the lengthy time period 110 years and river distance 75 km involved the model cannot predict detailed change but it does reproduce the main pattern of sediment movement in which successive phases of aggradation and degradation progress downstream peak storage is predicted in that part of the river where braiding and anastomosis are best developed aggradation was most rapid in the upper reaches close to major supply points becoming slower and later with distance downstream channel width increased by up to 300 per cent where the valley floor was broad and braiding became relatively common bridges had frequently to be replaced while bed levels were still rising in lower reaches degradation began in upper ones notably after 1950 and by 1984 had progressed downriver over 30 km rates of incision reached 0 5 m yr 1 especially in the early 1970s when record high flows occurred as a result of degradation the bed material became gravelly through either reexposure of the original bed or lag concentration of coarser fractions also a narrower unbraided channel has developed the river is beginning to heal itself and upper reaches now have reasonably stable beds but at least another 50 years will be required for the river to cleanse its channel of mining debris

pressure fluctuations and the formation of the pge rich merensky and chromitite reefs bushveld complex the merensky reef and the underlying upper group 2 chromitite layer in the critical zone of the bushveld complex host much of the world s platinum group element pge mineralization the genesis is still debated a number of features of the merensky reef are not consistent with the hypotheses involving mixing of magmas uniform mixing between two magmas over an area of 150 by 300 km and a thickness of 3 30 km seems implausible the merensky reef occurs at the interval where main zone magma is added but the relative proportions of the pge in the merensky reef are comparable to those of the critical zone magma mineral and isotopic evidence in certain profiles through the merensky unit suggest either mixing of minerals not magmas and in one case the lack of any chemical evidence for the presence of the second magma the absence of cumulus sulphides immediately above the merensky reef is not predicted by this model an alternative model is proposed here that depends upon pressure changes not chemical processes to produce the mineralization in chromite rich and sulphide rich reefs magma was added at these levels but did not mix this addition caused a temporary increase in the pressure in the extant critical zone magma immiscible sulphide liquid and or chromite formed sinking sulphide liquid and or chromite scavenged pge as clusters nanoparticles or platinum group minerals from the magma and accumulated at the floor rupturing of the roof resulted in a pressure decrease and a return to sulphur undersaturation of the magma

multiple merensky reef of the bushveld complex south africa the merensky reef mr of the bushveld complex is known for containing the lion s share of the world s reserves of platinum group elements pge it is commonly described as a single relatively thin layer or package of chromite and sulphide bearing orthopyroxenite hosted by almost barren norite and or anorthosite here we present a drill core section that contains however four separate mr orthopyroxenite layers hosted by mottled anorthosites each of these orthopyroxenites is distinguished through occurrence absence of marginal chromitite seams grain size petrographic composition and pge abundance we interpret them as multiple undercutting mr that occurs as sill like apophyses extending laterally from a pothole margin into the footwall the undercutting mr is attributed to in situ crystallization within sheet like melt filled cavities at potholes margins which are produced through thermal and or chemical erosion of the footwall rocks by replenishing magmas this finding indicates that the mr forming event involved several distinct pulses of replenishing magmas whose crystallization products were mostly eroded away in the stratiform portions but remained locally preserved in apophyses within footwall rocks the mineralized mr formed a few meters below a temporary chamber floor is indicative of strong thermal compositional convection in the basal layer of the bushveld s chamber the convection was able to deliver large volumes of fresh magma for the scavenging of pge by sulphide and chromite growing within the sheet like cavities of the mr potholes

channel floodplain connectivity during an extreme flood event implications for sediment erosion deposition and delivery the term connectivity has emerged as a powerful concept in hydrology and geomorphology and is emerging as an innovative component of catchment erosion modeling studies however considerable confusion remains regarding its definition and quantification especially as it relates to fluvial systems this confusion is exacerbated by a lack of detailed case studies and by the tendency to treat water and sediment separately extreme flood events provide a useful framework to assess variability in connectivity particularly the connection between channels and floodplains the catastrophic flood of january 2011 in the lockyer valley southeast queensland australia provides an opportunity to examine this dimension in some detail and to determine how these dynamics operate under high flow regimes high resolution aerial photographs and multi temporal lidar digital elevation models dems coupled with hydrological modeling are used to assess both the nature of hydrologic and sedimentological connectivity and their dominant controls longitudinal variations in flood inundation extent led to the identification of nine reaches which displayed varying channel floodplain connectivity the major control on connectivity was significant non linear changes in channel capacity due to the presence of notable macrochannels which contained a 3000 average recurrence interval ari event at mid catchment locations the spatial pattern of hydrological connectivity was not straight forward in spite of bankfull discharges for selected reaches exceeding 5600m 3 s 1 data indicate that the main channel boundary was the dominant source of sediment while the floodplains where inundated were the dominant sinks spatial variability in channel floodplain hydrological connectivity leads to dis connectivity in the downstream transfer of sediments between reaches and affected sediment storage on adjacent floodplains consideration of such variability for even the most extreme flood events highlights the need to carefully consider non linear changes in key variables such as channel capacity and flood conveyance in the development of a quantitative connectivity index copyright c 2013 john wiley sons ltd

erodibility of cohesive streambeds in the loess area of the midwestern usa excess stress parameters critical shear stress c and erodibility coefficient k d for degrading channels in the loess areas of the midwestern usa are presented based on in situ jet testing measurements critical shear stress and k d are used to define the erosion resistance of the streambed the jet testing apparatus applies hydraulic stresses to the bed and the resulting scour due to the impinging jet is related to the excess stress parameters streams tested were primarily silt bedded in texture with low densities which is typical of loess soils results indicate that there is a wide variation in the erosion resistance of streambeds spanning six orders of magnitude for c and four orders of magnitude for k d erosion resistance was observed to vary within a streambed from streambed to streambed and from region to region an example of the diversity of materials within a river system is the yalobusha river basin in mississippi the median value of c for the two primary bed materials naheola and porters creek clay formations was 1 31 and 256 pa respectively streambeds composed of the naheola formation are readily eroded over the entire range of shear stresses whereas only the deepest flows generate boundary stresses great enough to erode streambeds composed of the porters creek clay formation therefore assessing material resistance and location is essential in classifying and modelling streambed erosion processes of these streams

the role of magma mixing in the genesis of pge mineralization in the bushveld complex thermodynamic calculations and new interpretations a discussion sir the concept of magma mixing producing sulfide mineralization especially any enriched in platinum group elements such as the merensky reef has become well entrenched naldrett and von gruenewaldt 1989 presented calculations for the saturation limit of sulfur in the presumed parental magma of the bushveld complex and derivative liquids based on well established phase relationships for that magma as with most phase boundaries it is curved with a convexity such that mixing between two points straddling the curved boundary could lead to a mixed magma that was sulfur oversaturated a revised version was presented by li et al 2001a plotting data from the bushveld complex on a zr versus s diagram however whatever two compositional axes are used in a binary diagram they cannot completely define the sulfur solubility limits for multicomponent magmas such solubility limits on a binary diagram are only valid for the very specific compositions for which the curve was constructed but will not be applicable if different compositions are projected onto the diagram in both models it is assumed that the magma evolution trend and hence sulfur solubility defined by fractionation is exactly the same as that produced during magma mixing the first issue discussed here is why such an assumption is incorrect it was also suggested by li et al 2001a that during norite fractionation the magma may become sulfur undersaturated this interpretation of their phase diagram also requires reexamination finally the actual composition of the magmas calculated by li et al 2001a must be questioned in an erratum li et al 2001b published in response to a draft of this manuscript these errors were recognized however the brief retraction does not explain the reasons why such models calculations and interpretations are not applicable there are differences between magma series produced by fractionation and those produced

stochastic theory of particle trajectories through alluvial valley floors abstract temporary storage of sediment within alluvial valley floors modulates the long term transport of sediment through landscapes the fate of weathering minerals or sediment bound constituents in fluvial environments depends on the relative time scales of constituent degradation and particle residence time within valleys particles follow a set of trajectories through valley floors some particles pass directly through the channel reaching the basin outlet rapidly after being introduced to the fluvial system others remain for long periods in deposits such as flood plains traditional sediment routing theory based on the principle of sediment mass conservation along reaches of channel does not account for exchanges of sediment with temporary sediment storage reservoirs outside the channel such as flood plains deltas and alluvial fans this article formalizes a theory that incorporates the role of such exchanges in the migration of sediment through river systems by computing the probabilistic s

stochastic incipient motion criterion for spheres under various bed packing conditions incipient motion criteria based solely on time averaged bed shear stress may underpredict sediment transport the focus of this study is on the stochastic aspect of the problem of incipient motion specifically the role of near bed turbulent structures and bed packing density on the commencement of sediment motion is investigated the cornerstone of the proposed model is based on the concept that the particle motion is governed by the intermittent nature of near bed turbulence based on this mechanism in this article we provide a quantitative model for predicting the commencement of sediment entrainment for the first time under three representative bed packing densities corresponding to the isolated wake interference and skimming flow regimes the performance of the proposed model is compared to published experimental data and the conventional approach that is based on the consideration that flow parameters are statistically well represented by a normal distribution

origin of extremely pge rich mafic magma system an example from the jinbaoshan ultramafic sill emeishan large igneous province sw china abstract the 260 ma jinbaoshan sill is part of the permian emeishan large igneous province in sw china and is mainly composed of wehrlite and hosts a pge deposit containing 45 tonnes of pt and pd with ore grades ranging from 1 to 5 ppm pt pd the sill is composed of several pge rich horizons at different stratigraphic heights a 5 m thick pge rich horizon at the base of the sill which is underlain by a chromite rich pge poor layer accounts for 44 of the total ore reserve the jinbaoshan sill is rich in pges with ir ranging from 1 7 to 138 ppb ru from 0 9 to 39 ppb rh from 3 1 to 133 ppb and a total pt and pd from 76 to 5371 ppb all the rocks from the sill display primitive mantle normalized chacophile element patterns enriched in pges relative to ni and cu the low cu pd ratios 20 to 1938 of the rocks relative to the primitive mantle cu pd 7000 imply that there was no sulfide fractionation of the magmas the pge enrichment of the sill indicates that it formed from extremely pge rich magmas modeling results show that such pge rich magmas likely resulted from a multistage dissolution upgrading process in an open magma conduit system large amounts of chromite crystallization triggered sulfide saturation when pge rich magmas emplaced into the upper magma chamber of the system settling and sorting of chromite and most sulfide liquids along with olivine resulted in the formation of the largest pge rich horizon at the base of the sill the rest minor sulfide liquid accumulated to form pge rich horizons in the middle part of the sill aided by turbulence and convection within the magma chamber

the penikat intrusion finland geochemistry geochronology and origin of platinum palladium reefs the palaeoproterozoic penikat layered ultramafic mafic intrusion in northern finland is one of the most richly mineralized layered intrusions on earth containing at least six platinum group element pge enriched horizons exposed along 20 km of strike amongst them the sj reef which at 3 7 ppm pt pd over a width of 1 2 m is surpassed by few other pge reefs globally in terms of its endowment in pge important pge enrichments also occur in the pv reef average 2 6 ppm pd 4 ppm pt over 1 1 m and ap1 reef average 6 2 ppm pd 1 7 ppm pt over 0 7 m here we present new major and high precision trace element and nd isotope data from a traverse across the intrusion and a new u pb age of 2444 8 ma for the intrusion we show that the pge reefs formed by predominantly orthomagmatic processes as for example reflected by well defined positive correlations between pt pd and os ir ru contents late magmatic fluids played no significant role in concentrating pge there are at least six cyclic units in the intrusion displaying a progressive upward decrease in differentiation indices mg and cr v subdued stratigraphic variations in incompatible trace element ratios ce sm mostly 5 10 and nd isotope compositions end 3 to 1 indicate that mixing of magmas of distinct lineage or in situ contamination with country rocks was not required to form the pge reefs there is also no evidence for addition of external sulphur to the magma based on s se ratios at or below primitive mantle levels instead we propose that sulphide melt saturation at penikat was reached in response to fractionation of a siliceous high magnesium basalt and that the sulphides were concentrated through hydrodynamic phase sorting consistent with bonanza style pge grades in large potholes

flow parameters in a shallow conduit flow carbonate aquifer inner bluegrass karst region kentucky usa abstract in the carbonate aquifers which underlie most karst terrains groundwater flow is through a dendritic system of solution conduits in such aquifers termed shallow conduit flow aquifers the methods used to model granular and fracture aquifers are not generally applicable investigations were conducted in the inner bluegrass karst region of central kentucky with the objective of developing methods of modeling shallow conduit flow aquifers as well as obtaining quantitative information on a specific portion of the aquifer to assist in its management for water supply purposes in the inner bluegrass karst region groundwater basins are developed in each of which there is an integrated system of solution conduits which conducts recharge to a major spring one of the largest of these groundwater basins feeds royal spring which serves as the principal water supply for the town of georgetown the basin extends over 15 km to the southeast and most of its flow is furnished by underground diversions of cane run a surface stream with headwaters near the center of the city of lexington the principal objectives of the field investigation were to determine discharges at the spring and travel times to the spring from discrete recharge points within the basin termed swallets the spring is ungaged and an attempt was made to obtain a continuous discharge record by the dilution of dye introduced at a swallet comparison of the dye dilution discharge record with stage discharges at the spring revealed substantial discrepancies which are believed to be caused by as much as five sixths of the low flow discharge from the upper portion of the basin bypassing the spring the dye dilution method therefore provided significant insights into the geometry of the conduit system of the groundwater basin although it proved unsatisfactory as a method of determining discharges at the spring analysis of the travel times and stage discharges provided information on the conduit geometry by modeling the flow as open channel flow in a rectangular channel flow in the system is rapid ranging from 140 to 590 m h 1 although the flow rate increases with discharge the relationship is not simple owing to substantial increases in conduit cross sectional area at higher discharges flow is turbulent and subcritical under all conditions the most surprising result was the very low depth of flow calculated less than 17 cm at even the highest discharge although this must be considered an equivalent depth it is believed to indicate that active flow in shallow conduit flow aquifers is generally in a thin zone just beneath the water table

noble metal enrichment processes in the merensky reef bushveld complex we have analysed sulphides silicates and chromites of the merensky introduction reef for platinum group elements pges re and au using laser the enrichment of noble metals in layered gabbroic ablation inductively coupled plasma mass spectrometry and synthetic intrusions to form stratiform merensky type ore horizons pyrrhotite standards annealed with known quantities of noble metals is an extremely efficient yet poorly understood process os ir and ru reside in solid solution in pyrrhotite and pentlandite metal enrichment must be related to the formation of rh and part of the reef s pd in pentlandite whereas pt au re magmatic layering as the ore horizons form an integral and some pd form discrete phases olivine and chromite often part of the magmatic stratigraphies of their host insuspected to carry os ir and ru are pge free all phases analysed trusions metal concentration is also related to the prescontain noble metals as discrete micro inclusions with diameters ence of magmatic sulphides because the noble metals are either dissolved in base metal sulphides or occur as typically 100 nm inclusions in sulphides commonly have the discrete noble mineral phases intergrown with sulphides element combinations os ir pt and pt pd au inclusions in vermaak hendriks 1976 ballhaus ryan 1995 olivine and chromite are dominated by pt au pd few inclusion ballhaus ulmer 1995 spectra can be related to discrete noble metal phases and few there is considerable controversy as to where the inclusions have formed by sub solidus exsolution rather some metals came from and how metal enrichment worked pge inclusions notably those in olivine and chromite are earlyfor a stratiform platinum group element pge deposit magmatic nuggets trapped when their host phases crystallized we inside the magmatic stratigraphy of a layered intrusion suggest that the silicate melt layer that preceded the merensky reef there are two possibilities either the metals were derived was pge oversaturated at early cumulus times experiments from the melt column above the ore horizon or they came combined with available sulphide silicate partition coefficients sugfrom the magma equivalent to the cumulate package gest that a silicate melt in equilibrium with a sulphide melt underlying the ore owing to the large density contrast containing the pge spectrum of the merensky ore would indeed be between silicate and sulphide melt a popular view is that oversaturated with respect to the least soluble noble metals sulphide the metals were enriched by gravitational settling of melt apparently played little role in enriching the noble metals in sulphide melt i e out of the magma column above the the merensky reef rather its role was to immobilize a pre existing ore campbell et al 1983 barnes naldrett 1985 in situ stratiform pge anomaly in the liquid stratified magma naldrett et al 1987 on the other hand it is well chamber established that gravitational crystal settling plays little role in producing magmatic layering and magmatic stratigraphies mcbirney noyes 1979 morse 1986 campbell 1987 and this cannot remain without consequence for stratiform noble metal deposits a third alternative is noble metal enrichment by late

contrasting sulphide contents of the bushveld and sudbury igneous complexes the sudbury igneous complex sic contains abundant sulphides especially near the base and hosts one of the world s largest nickel and copper deposits the bushveld complex bc contains relatively little sulphide but hosts the world s largest platinum group element deposits the most recent calculations of the sulphur solubility in magmas that produced the bc are based on the sulphur solubility of mid ocean ridge basalts that have less sio2 than bushveld magmas such a difference may lead to an overestimation of sulphur solubility by as much as 25 the revised sulphur solubility curve presented here for bushveld magmas may also have relevance to the sic in view of its siliceous nature sulphur solubility curves can be used to determine the proportion of sulphide expected in cumulate rocks once sulphur saturation is attained these models are tested using observed sulphide contents in both intrusions the observed decreasing sulphur contents 0 3 0 05 s from the base of the sic upward are broadly consistent with these sulphur solubility curves and are consistent with sulphide saturation through the entire mafic portion in contrast the lower half of the bc contains extremely little sulphur generally 0 02 s except for two thin layers which is not consistent with sustained sulphide saturation at any level previous interpretations of the sulphur content of bushveld rocks have suggested that the lower and critical zones were sulphide saturated but that they had then lost some of the sulphide due to various processes the present sulphide content of the cumulates of the bc is so low that if they had once been saturated over 90 of all the sulphide must have been removed mass balance calculations indicate that these large amounts of displaced sulphur remain unaccounted for in such models instead the observed sulphur contents are in reasonable agreement with that expected in a cumulate sequence forming from a sulphur undersaturated magma whereas the merensky reef and bastard pyroxenite contain minor sulphides the compositions of the immediate hanging wall rocks indicate sulphide undersaturation such an abrupt return to sulphide undersaturation is not consistent with models involving sulphide formation from large volumes of magma one possible explanation for these two observations is that intermittent sulphur degassing occurred through a fractured roof of the bc so that the magma was never continuously sulphur saturated with respect to an immiscible sulphide liquid

origin of the pegmatitic pyroxenite in the merensky unit bushveld complex south africa the genesis of the pegmatitic pyroxenite that often forms the base of the merensky unit in the bushveld complex is re examined large 1 cm orthopyroxene grains contain tricuspidate inclusions of plagioclase and chains and rings of chromite grains which are interpreted to have grown by reaction between small primary orthopyroxene grains and superheated liquid this superheated liquid may have been an added magma or be due to a pressure reduction as a result of lateral expansion of the chamber there would then have been a period of non accumulation of grains permitting prolonged interaction with the crystal mush at the crystal liquid interface crystal ageing and grain enlargement of original orthopyroxene grains would ensue only after the pegmatitic pyroxenite had developed did another layer of chromite and pyroxenite with normal grain size accumulate above it immiscible sulphide liquids formed with the second pyroxenite but percolated down as a result of their density contrast even as far as the footwall anorthosite in some cases whole rock abundances of incompatible trace elements in the pegmatitic pyroxenite are comparable with or lower than those of the overlying pyroxenite and so there is no evidence for addition and or trapping of large proportions of interstitial liquid or of an incompatible element enriched liquid or fluid in the production of the pegmatitic rock because of the coarse grained nature of the rock modal analysis especially for minor minerals is unreliable annealing has destroyed primary textures such that petrographic studies should not be used in isolation to distinguish cumulus and intercumulus components geochemical data suggest that the merensky pyroxenite both pegmatitic and non pegmatitic typically consists of about 70 80 cumulus orthopyroxene and 10 20 cumulus plagioclase with a further 10 of intercumulus minerals and could be considered to be a heteradcumulate

sr and pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the bushveld complex south africa microdrilling and progressive leaching evidence for sub liquidus contamination within a crystal mush progressive leaching of plagioclase for sr iso topes and microdrilling for sr and pb isotopes from grains of plagioclase and orthopyroxene from the critical zone and the lower zone indicates that these minerals are not in isotopic equilibrium leaching suggests critical zone plagioclase either lost rb or had a more radiogenic sri rim relative to the core whereas plagioclase from an upper zone sample is isotopically homogeneous for sri microdrilling analyses of plagioclase from the lower and critical zones consistently have a higher initial 87 sr 86 sr sri and a less radiogenic modeled 238 u 204 pb composition l2 than coexisting ortho pyroxene the range of calculated sri for plagioclase and orthopyroxene is 0 70506 0 70662 34 and 0 70290 0 70654 36 respectively the average difference in sri between mineral pairs was 0 00095 the range of calculated l2 for plagioclase and orthopyroxene is 9 42 10 30 average 9 7 and 9 83 15 75 average 10 1 respectively the range of measured 208 pb 206 pb for plagioclase and orthopyroxene is 34 757 36 439 33 and 36 669 41 845 85 respectively one orthopyroxenite without evidence for more than one population of crystal size distribution nonetheless had sri 0 70654 36 with calculated l2 of 10 32 for larger grains as compared with sri 0 70290 32 and calculated l2 of 9 97 for smaller grain size fractions isotopic results from this study demonstrate that whole rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the bushveld complex however systematic changes have the potential to elucidate the timing of contamination with regard to other processes crystal aging compaction driven recrystallization and mineral exsolution occurring within a slowly cooled crystal liquid vapor mush system

sediment connectivity in the high alpine valley of val muschauns swiss national park linking geomorphic field mapping with geomorphometric modelling abstract the efficiency of sediment routing through mountain sediment cascades is controlled by the connectivity of hillslopes to the main river system a lack of connectivity may cause long term sediment storage and exclude large fractions of a basin from the sediment routing for several thousand years here we studied sediment dynamics in a small formerly glaciated valley in the swiss alps to characterise the sediment connectivity to the stream we calculated a morphometric index using a gis algorithm the modelling results were tested against a field based geomorphic mapping of sediment storages which were evaluated with respect to their state of de coupling in accordance to the field diagnostics the modelling results indicate very well that the present day sediment flux is conditioned by the glacial valley morphometry inherited through pleistocene glaciation especially in the upper hanging valleys the connectivity index is reduced noticeably due to the glacial cirque morphology based on the field mapping 30 of the hillslope sediment cascades are interrupted and 20 of the storage boundaries are currently affected by a lack of material transfer as a consequence 29 of the basin surface is currently disconnected from the main river nevertheless the gis algorithm overestimates the connectivity within the basin because it fails to calculate decoupling between neighbouring pixels in digital terrain models dtms around 35 of the basin surface which has been mapped in the field as being decoupled is related to relative high connectivity our study highlights the potential of morphometric gis modelling for studying sediment connectivity but additionally emphasises the indispensability of geomorphic field mapping for a holistic understanding of mountain cascading systems

holocene palaeoenvironments inferred from a sedimentary sequence in the tsoaing river basin western lesotho abstract the paper presents a sedimentological palynological and phytolith record from a 13 m deep holocene sedimentary sequence located in the tsoaing river valley southwestern lesotho six conventional radiocarbon and two ams dates provide a relatively high resolution holocene record for the sedimentary sequence ranging from ca 12 000 to 4000 years bp pollen is absent in the upper section but present in the lower 2 m confirming terminal pleistocene holocene conditions reported in previous published pollen and charcoal records from the region the absence of pollen in the upper layers of ca 9000 years bp and younger suggests that conditions over southwestern lesotho throughout much of the holocene was typified by a seasonal climate that prevented long term preservation of plant remains although other plant material like robust spores microscopic charcoal and phytoliths withstood oxidation sedimentological and phytolith results suggest that the period from ca 8600 to 8450 years bp experienced rapid environmental change towards drier conditions phases of chemical disintegration with organic input including local swamp phytoliths are suggested at ca 7000 years bp and again after 4500 years bp

the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber platinum group element pge deposits in the bushveld complex and other layered intrusions form when large incompletely solidified magma chambers undergo central subsidence in response to crustal loading resulting in slumping of semi consolidated cumulate slurries to the centres of the intrusions and hydrodynamic unmixing of the slurries to form dense layers enriched in sulfides oxides olivine and pyroxene and less dense layers enriched in plagioclase the most economic pge cr and v reefs form in large multiple replenished intrusions because these cool relatively slowly and their central portions subside prior to termination of magmatism and complete cumulate solidification the depth of emplacement has to be relatively shallow as otherwise ductile crust would not be able to flex and collapse in smaller intrusions cooling rates are faster subsidence is less pronounced and where it occurs the cumulate may be largely solidified resulting in insignificant mush mobility and mineral sorting layering is thus less pronounced and less regular and continuous and the grades of the reefs are lower but the reefs can be relatively thicker an additional factor controlling the pge cr and v prospectivity of intrusions is their location within cratons intra cratonic environments offer more stable emplacement conditions that are more amenable to the formation of large layered igneous bodies furthermore intrusions sited within cratons are more readily preserved because cratons are underlain by thick buoyant keels of harzburgite that prevent plate tectonic recycling and destruction of crust

watershed erosion modeling using the probability of sediment connectivity in a gently rolling system abstract sediment connectivity has been shown in recent years to explain how the watershed configuration controls sediment transport however we find no studies develop a watershed erosion modeling framework based on sediment connectivity and few if any studies have quantified sediment connectivity for gently rolling systems we develop a new predictive sediment connectivity model that relies on the intersecting probabilities for sediment supply detachment transport and buffers to sediment transport which is integrated in a watershed erosion model framework the model predicts sediment flux temporally and spatially across a watershed using field reconnaissance results a high resolution digital elevation models a hydrologic model and shear based erosion formulae model results validate the capability of the model to predict erosion pathways causing sediment connectivity more notably disconnectivity dominates the gently rolling watershed across all morphologic levels of the uplands including microtopography from low energy undulating surfaces across the landscape swales and gullies only active in the highest events karst sinkholes that disconnect drainage areas and floodplains that de couple the hillslopes from the stream corridor results show that sediment connectivity is predicted for about 2 or more the watershed s area 37 days of the year with the remaining days showing very little or no connectivity only 12 8 0 7 of the gently rolling watershed shows sediment connectivity on the wettest day of the study year results also highlight the importance of urban suburban sediment pathways in gently rolling watersheds and dynamic and longitudinal distributions of sediment connectivity might be further investigated in future work we suggest the method herein provides the modeler with an added tool to account for sediment transport criteria and has the potential to reduce computational costs in watershed erosion modeling

stochastic sediment transport in soil erosion a stochastic model governing the downslope transport of a sediment particle during an erosion event is described particles alternate between resting on the soil bed and being transported by the overland flow probability densities and moments are constructed for the distribution of a particle s position at a given time and also for time of passage to a given location we show that a suitable averaging of the stochastic particle motions in our model gives rise to the deterministic erosion differential of hairsine and rose hairsine p rose c 1991 soil sci soc am j 55 2 320 324 the model generalized h einstein s stochastic model der geschiebebetrieb als wahrscheinlichkeitsproblem verlag rascher zurich 1937 for bedload transport in streams

sediment connectivity a framework for understanding sediment transfer at multiple scales a major challenge for geomorphologists is to scale up small magnitude processes to produce landscape form yet existing approaches have been found to be severely limited new ways to scale erosion and transfer of sediment are thus needed this paper evaluates the concept of sediment connectivity as a framework for understanding processes involved in sediment transfer across multiple scales we propose that the concept of sediment connectivity can be used to explain the connected transfer of sediment from a source to a sink in a catchment and movement of sediment between different zones within a catchment over hillslopes between hillslopes and channels and within channels using fluvial systems as an example we explore four scenarios of sediment connectivity which represent end members of behaviour from fully linked to fully unlinked hydrological and sediment connectivity sediment travel distance when combined with an entrainment parameter reflecting the frequency magnitude response of the system maps onto these end members providing a coherent conceptual model for the upscaling of erosion predictions this conceptual model could be readily expanded to other process domains to provide a more comprehensive underpinning of landscape evolution models thus further research on the controls and dynamics of travel distances under different modes of transport is fundamental copyright 2014 john wiley sons ltd

the platinum group element and gold contents of the marginal rocks and sills of the bushveld complex previous work has defined three suites of sills and marginal rocks adjacent to and below the rustenburg layered suite of the bushveld complex namely an ultramafic mgo approximately 30 a magnesian basaltic mgo approximately 12 and a tholeiitic suite mgo approximately 7 these three suites are thought to represent magmas which have intruded and been incorporated into the rustenburg layered suite at various times during its formation the elements ir ru rh pd pt and au have been analyzed in 17 samples representative of the three suites the ultramafic sills have the highest total noble metal content 46 ppb followed by the magnesian basaltic suite 37 ppb and the tholeiitic suite 21 ppb the ultramafic sills are characterized by a relatively fiat chondrite normalized platinum group element pattern with an average pt ir ratio of 3 52 and a pronounced rh anomaly the magnesian basaltic and tholeiitic suites both have steep positive chondrite normalized patterns with average pt ir ratios of 16 14 and 40 5 respectively and no rh anomaly modeling done using incompatible trace elements and petrographic mixing suggests the ultramafic sills represent crystal mushes of cumulate olivine and chromite that were intruded into the floor during the crystallization of the lower portion of the rustenburg layered suite the enrichment of ir ru and rh in the ultramafic sills appears to be associated with the cumulate enrichment of olivine or chromite in these sills the chondrite normalized patterns found for the merensky reef and the platreef are similar to those of the magnesian basaltic magma the pattern found for the ug 2 is much flatter and in some respects very similar to that found for the ultramafic sills on a worldwide basis the platinum group element contents of the magnesian basaltic and tholeiitic magmas generally fall in the range of basalts and chill zones of other layered suites the magnesian basaltic magma does have one of the highest pt contents measured for a magma

domination of hillslope denudation by tree uprooting in an old growth forest abstract razula forest preserve in the carpathian mountains of the czech republic is an unmanaged forest that has not been logged or otherwise anthropically disturbed for at least 83 years preceded by only infrequent selective logging we examined this 25 ha area to determine the dominant geomorphological processes on the hillslope tree uprooting displaces about 2 9 m 3 of soil and regolith per year representing about 1 5 uprooted trees ha 1 yr 1 based on forest inventory records dating back to 1972 and contemporary measurements of displaced soil and pit mound topography resulting from uprooting pits and mounds occupy 14 of the ground surface despite typical slope gradients of 0 05 mm 1 and up to 0 41 little evidence of mass wasting e g slump or flow scars or deposits colluvial deposits was noted in the field except in association with pit mound pairs small avalanche and ravel features are common on the upslope side of uproot pits surface runoff features were rare and poorly connected but do include stemwash erosion associated with stemflow no rills or channels were found above the valley bottom area and only small localized areas of erosion and forest litter debris indicating overland flow where these features occurred they either disappeared a short distance downslope indicating infiltration or indicate flow into tree throw pits surface erosion is also inhibited by surface armoring of coarse rock fragments associated with uprooting as well as by the nearly complete vegetation and litter cover these results show that the combination of direct and indirect impacts of tree uprooting can dominate slope processes in old growth unmanaged forests the greater observed expression of different hillslope processes in adjacent managed forests where tree uprooting dynamics are blocked by management activities suggests that human interventions can change the slope process regime in forest ecosystems

surface deformation in volcanic rift zones abstract the principal conduits for magma transport within rift zones of basaltic volcanoes are steeply dipping dikes some of which feed fissure eruptions elastic displacements accompanying a single dike emplacement elevate the flanks of the rift relative to a central depression concomitant normal faulting may transform the depression into a graben thus accentuating the topographic features of the rift if eruption occurs the characteristic ridge trough ridge displacement profile changes to a single ridge centered at the fissure and the erupted lava alters the local topography a well developed rift zone owes its structure and topography to the integrated effects of many magmatic rifting events to investigate this process we compute the elastic displacements and stresses in a homogeneous two dimensional half space driven by a pressurized crack that may breach the surface a derivative graphical method permits one to estimate the three geometric parameters of the dike height inclination and depth to center and the mechanical parameter driving pressure rock stiffness from a smoothly varying displacement profile direct comparison of measured and theoretical profiles may be used to estimate these parameters even if inelastic deformation notably normal faulting creates discontinuities in the profile geological structures open cracks normal faults buckles and thrust faults form because of stresses induced by dike emplacement and fissure eruption theoretical stress states associated with dilation of a pressurized crack are used to interpret the distribution and orientation of these structures and their role in rift formation

platinum group elements in silicate rocks of the lower critical and main zones at union section western bushveld complex concentration patterns of platinum group elements pge in the lower critical and main zones of the bushveld complex are modelled by a process of metal collection by segregating sulphide melt followed by fractional crystallization of monosulphide solid solution mss separation of os ir ru rh enriched mss cumulate ore and pt pd enriched residual sulphide melt is inferred to have occurred by upward percolation of interstitial silicate melt entraining sulphide melt whereas mss crystals are entrapped by early crystallizing chromite this process resulted in os ir ru rh enrichment of the chromitites and an upward increasing trend of pt pd os ir ru crystallization of platinum group minerals pgm from s undersaturated sulphide melt or from coalescing noble metal clusters are alternative possibilities to explain the observed pge patterns but remain difficult to model at present remobilization of pt and pd by percolating late magmatic hydrous fluids could have resulted in metal enrichment of pre existing magmatic sulphides but this model is not supported by the metal budget of the reefs and the field data there is no evidence for solid substitution of pge into oxides and silicates

conceptual assessment of dis connectivity and its application to the ganga river dispersal system abstract connectivity in river dispersal systems is an important evolving concept and helps in understanding their dynamics in this study the types of connectivity have been defined on the basis of two components namely physical contact and transfer of material based on this four types of connectivity are recognized these are a active connected system b inactive connected system c partially active connected system and d disconnected system connectivity indices have been proposed based on the process understanding of different types of connectivity this conceptual understanding of connectivity has been applied to the ganga dispersal system a large river system results of the connectivity analysis on the ganga dispersal system reveal that different sets of data such as sediment budgets geochemical isotopic data and radiogenic 238 u 234 u 230 th data are noncoherent therefore developing rigorous and well constrained quantitative multidisciplinary data sets is needed to have a better understanding of connectivity at multiple spatio temporal scales in large river dispersal systems this would ultimately help model the dynamics of such systems

evidence for nonlinear diffusive sediment transport on hillslopes and implications for landscape morphology steep soil mantled hillslopes evolve through the downslope movement of soil driven largely by slope dependent transport processes most landscape evolution models represent hillslope transport by linear diffusion in which rates of sediment transport are proportional to slope such that equilibrium hillslopes should have constant curvature between divides and channels on many soil mantled hillslopes however curvature appears to vary systematically such that slopes are typically convex near the divide and become increasingly planar downslope this suggests that linear diffusion is not an adequate model to describe the entire morphology of soil mantled hillslopes here we show that the interaction between local disturbances such as rainsplash and biogenic activity and frictional and gravitational forces results in a diffusive transport law that depends nonlinearly on hillslope gradient our proposed transport law 1 approximates linear diffusion at low gradients and 2 indicates that sediment flux increases rapidly as gradient approaches a critical value we calibrated and tested this transport law using high resolution topographic data from the oregon coast range these data obtained by airborne laser altimetry allow us to characterize hillslope morphology at 2 m scale at five small basins in our study area hillslope curvature approaches zero with increasing gradient consistent with our proposed nonlinear diffusive transport law hillslope gradients tend to cluster near values for which sediment flux increases rapidly with slope such that large changes in erosion rate will correspond to small changes in gradient therefore average hillslope gradient is unlikely to be a reliable indicator of rates of tectonic forcing or baselevel lowering where hillslope erosion is dominated by nonlinear diffusion rates of tectonic forcing will be more reliably reflected in hillslope curvature near the divide rather than average hillslope gradient