new inferences of mantle viscosity from joint inversion of long wavelength mantle convection and post glacial rebound data we perform joint inversions for mantle viscosity of geophysical observables associated with both mantle convection and glacial isostatic adjustment gia our data include coefficients of the non hydrostatic geoid up to degree 8 and decay times associated with post glacial relative sea level rsl variations in hudson bay and fennoscandia we find that both data sets may be reconciled using a single profile of mantle viscosity which is characterized by a significant increase with depth across this region this result weakens previous arguments that the mantle rheology has significant transient effects the viscosity profiles we obtain are found to reconcile the observed free air gravity anomaly over hudson bay

modeling suspended sediment discharge from the waipaoa river system new zealand the last 3000 years hydrotrend a hydrologic transport model is used to simulate the water and suspended sediment discharge of the waipaoa river system over the last 3 kyr a time period in which a well documented sequence of natural events and anthropogenic activities that profoundly impacted drainage basin processes occurred comparisons between observed and simulated data show that the model output replicates the frequency and distribution of flow events and the suspended sediment concentration discharge relationship and the long term trends in suspended sediment discharge are consistent with the sediment record preserved on the middle shelf water discharge tracks precipitation and average annual discharge may have been up to 20 higher and 6 lower at different times in the past suspended sediment discharge changed from 2 3 4 5 to 14 9 8 7 mt yr 1 during the anthropocene increasing by 140 after polynesian arrival by 350 after european colonization and by 660 after the catchment headwaters were deforested

the adjustment of mantle plumes to changes in plate motion the relative motion of hotspots and lithospheric plates implies a velocity shear in the underlying mantle causing horizontal advection of mantle plumes as they rise toward the lithosphere consequent tilting of plumes parallel to the direction of plate motion indicates that plumes must undergo a period of readjustment after the velocity vector for plate motion is altered thus the shape of bends in the surface tracks of hotspots resulting from changes in plate motion will reflect the plume adjustment laboratory experiments as well as computations using a simple theory developed in richards griffiths 1988 for the dynamics of continuous plume conduits demonstrate that the bend in the surface track has a radius of curvature approximately equal to the maximum horizontal deflection of the conduit thus the sharpness of the bend at an age of 43ma in the hawaiian emperor volcanic chain implies that the deflection of the underlying plume in that case was small 200 km this small deflection is expected for plumes carrying large buoyancy fluxes and it indicates that tilting of the conduit is unlikely to be sufficient to cause diapiric instability

influence of storm related sediment storage on the sediment delivery from tributary catchments in the upper waipaoa river new zealand although much is known about overall sediment delivery ratios for catchments as components of sediment production and sediment yield little is known about the component of temporary sediment storage sediment delivery ratios focused on the influence of storm related sediment storage are measured at matakonekone and oil springs tributaries of the waipaoa river basin east coast of new zealand the terrace deposits of both tributaries show abundant evidence of storm related sedimentation especially sediment delivered from cyclone bola a 50 year return rainfall event which occurred in 1988 the sediment delivery ratio is calculated by dividing the volume of sediment transported from a tributary to the main stream by the volume of sediment generated at erosion sites in the tributary catchment because the sediment delivery volume is unknown it can be calculated as the difference between sediment generation volume and sediment storage volume in the channel reach of the tributary the volume of sediment generated from erosion sites in each tributary catchment was calculated from measurements made on aerial photographs dating from 1960 1 44 000 and 1988 1 27 000 the volume of sediment stored in the tributary can be calculated from measurements of cross sections located along the tributary channel which are accompanied by terrace deposits dated by counting annual growth rings of trees on terrace surfaces sediment delivery ratios are 0 93 for both matakonekone catchment and oil springs catchment results indicate that oil springs catchment has contributed more than twice the volume of sediment to the waipaoa river than the matakonekone catchment 2 75 106 m3 vs 1 22 106 m3 although large volumes of sediment are initially deposited during floods subsequent smaller flows scour away much of these deposits the sediment scouring rate from storage is 1 25 104 m3 a 1 for matakonekone stream and 0 83 104 m3 a 1 for oil springs stream matakonekone and oil springs channels respond to extreme storms by instantaneously aggrading then gradually excavating the temporarily stored sediment results from matakonekone and oil springs streams suggest a mechanism by which event recurrence interval can strongly influence the magnitude of a geomorphic change matakonekone stream with its higher stream power is expected to excavate sediment deposits more rapidly and allow more rapid re establishment of storage capacity copyright 1999 john wiley sons ltd

a low seismic wavespeed anomaly beneath northwestern india a seismic signature of the deccan plume abstract seismic tomography of the indian region using p wave arrival times reveals an approximately cylindrical region of lowered seismic velocities in the upper mantle to the north of the gulf of cambay and the present exposure of the deccan flood basalts this anomaly with low velocities interrupts the characteristically high seismic velocities in the lithosphere beneath peninsular india and extends from shallow depth down to a more extensive low velocity zone beneath 200 km the centre of the low velocity anomaly lies just below the earliest alkaline magmatism which has been directly associated with the deccan plume in the northern portion of the cambay graben these magmatic events precede the main deccan flood eruptions by at least 3 myr the lowered seismic wavespeeds link into the ancient narmada lineament and to some extent to the south but here unfortunately the tomographic image runs out of resolution synthetic tests indicate that such a zone of lowered wavespeeds can be recovered with some loss of amplitude towards the south the association of the current seismic anomaly with the deccan traps is intriguing since the cambay graben remains a place of raised heat flow one explanation would be that the seismic anomaly represents the signature of the initial conduit through which plume material forced its way through the lithosphere beneath the cambay rift the zone of lowered seismic velocities beneath the indian lithosphere might then represent the remains of the initial plume head which would need to have moved with india over the 65 myr since the deccan eruptions

a comparison of earthflow movement mechanisms on forested and grassed slopes raukumara peninsula north island new zealand abstract surface movement rates on forested earthflows are 2 3 orders of magnitude less than those on grassed earthflows subsurface deformation results largely from extension flow on grassed earthflows and from compression flow on forested earthflows a rafting mechanism in which blocks of roots from individual trees interact with those of neighbouring trees to retard surface movement is used to explain deformation profiles of forested earthflows the rheology of earthflow materials is changed by the presence of tree roots

supply and removal of sediment in a landslide dominated mountain belt central range taiwan abstract a strong coupling between hillslope and valley systems is often inferred for mountain landscapes dominated by bedrock landsliding we reveal the nature of this link using data sets on landsliding and sediment transport from two montane catchments draining the eastern central range of taiwan here the magnitude frequency distribution of landslides can be modeled by a robust power law but this scale invariance is not mirrored in the sediment discharge at the mountain front instead downstream sediment loads reflect a complex response to both sediment supply and ambient hydraulic conditions the rivers do not transport significant amounts of sediment unless it is provided by hillslope mass wasting in the catchment removal of landslide debris is a function of the transport capacity of the stream at the site of entry thus there is a dual supply and transport control on sediment loads in bedrock floored streams over a monitoring period of 25 yr the bulk of the sediment leaving the mountain bel

magnitude and frequency of landsliding in a large new zealand catchment knowledge of long term average rates of erosion is necessary if factors affecting sediment yields from catchments are to be understood without such information it is not possible to assess the potential influence of extreme storms and therefore to evaluate the relative importance of various components of a sediment budget a study of the sediment budget for the waipaoa catchment north island new zealand included evaluation of long term rates of landsliding for six landslide prone land systems in the catchment the number of landslides per unit area generated by each of several storms was counted on sequential aerial photographs and correlated with the magnitude of the corresponding storm the resulting relationships were combined with magnitude frequency relationships derived for storms from 70 to 100 year rainfall records in the area to estimate a long term magnitude frequency relationship for landsliding for each land system the long term average values of the areal landslide frequency number of slides per unit area per unit time were then calculated from these relationships the volumes of a sample of landslide scars were measured in the field and the proportion of slides that deliver sediment to channels was determined from aerial photographs these measurements then allowed calculation of the long term average rate of sediment production to streams from landslides for different land systems and types of vegetation results suggest that shallow landslides currently contribute about 15 5 of the suspended sediment load in the waipaoa river above the kanakanaia gauging station and that 75 of the sediment production from the landslides occurs during storms with recurrence intervals of less than 27 years reforestation of 6 3 93 km2 of the slide prone lands in the catchment between 1990 and 1995 resulted in a calculated decrease in slide derived sediment of 10 calculations suggest that reforestation of an additional 3 66 km2 of the catchment in areas with the most sensitive combinations of land system and storm regime could decrease the total sediment inputs from landsliding by about 20

the thin hot plume beneath iceland summary we present the results of a seismological investigation of the frequency dependent amplitude variations across iceland using data from the hotspot array currently deployed there the array is composed of 30 broad band passcal instruments we use the parameter t de ned in the usual manner from spectral ratios halderman davis1991 to compare observed s wave amplitude variations with those predicted due toboth anelastic attenuation and diiraction eiects four teleseismic events at a range of azimuths are used to measure t a 2 d vertical cylindrical plume model with a gaussian shaped velocity anomaly is used to model the variations that part of t caused by attenuation was estimated by tracing a ray through iasp91 then superimposing our plume model velocity anomaly and calculating the path integral of 1 oq that part of t caused by diiraction was estimated using a 2 d nite diierence code to generate synthetic seismograms the same spectral ratio technique used for the datawas then used to extract a predictedt thet variations caused by anelastic attenuation are unable to account for the variations we observe but those caused by diiraction do we calculate the t variations caused by diiraction for diierent plume models and obtain our best t plume which exhibits good agreement between the observed and measured t the best t plume model has a maximum s velocity anomaly of 12 per cent and falls to 1 e of its maximum at 100 km from the plume centre this is narrower than previous estimates from seismic tomography which are broadened and damped by the methods of tomography this velocity model would suggest greater ray theoretical traveltime delays than observed however we nd that for such a plume wave front healing eiects at frequencies of 0 03 0 175 hz the frequency range used to pick s wave arrivals causes a 40 per cent reduction in traveltime delay reducing the ray theoretical delay to that observed

catchment scale influences on riverine organic matter in southern new zealand abstract riverine particulate organic matter pom is a critical vector for nutrient cycling of carbon at both regional and global scales pom and suspended sediment ss are transported concomitantly through rivers and their concentrations change in association to landscape transformations and natural fluctuations the proportion of pom to ss mobilised in rivers under base flow is an important component of carbon flux to oceans and is a useful metric for understanding hillslope river coupling and hydraulic connectivity we quantified pom during base flow conditions in southern new zealand to assess the role of different catchment characteristics that control the spatial variability of organic matter in the fluvial environment the proportion of pom to total suspended material tsm can be

linking lithology and land use to sources of dissolved and particulate organic matter in headwaters of a temperate passive margin river system a number of rivers have been found to transport highly aged organic matter om however the sources of this aged material remain a matter of debate one potential source may be erosion and weathering of headwater lithologies rich in ancient sedimentary om in this study waters suspended particulates streambed sediments rocks and soils from fourteen small headwater watersheds of a mid size temperate passive margin river were sampled and characterized by d 14 c d 13 c and poc tpn ratios to identify sources of particulate and dissolved om delivered to the river mainstem these headwater sites encompass a range in lithology om rich shales om lean carbonate mudstone facies and om free crystalline rocks and land use types forested and agricultural and allow investigation of the influence of agriculture and bedrock types on stream om characteristics streams draining large areas of both agricultural land use and om rich lithology contain particulate om pom that is more 14 c depleted than streams draining forested shale free watersheds however this is not sufficient to account for the significantly lower d 14 c poc measured in the river mainstem dissolved om dom d 14 c are in all cases enriched compared to pom from the same stream but are otherwise highly variable and unrelated to either land use or lithology poc tpn ratios were likewise highly variable poc and doc d 13 c signatures were similar across all watersheds based on isotope mass balance 14 c free fossil om sources contribute 0 12 of total stream pom although these results do not unequivocally separate the influences of land use and lithology watershed coverage by shale and agriculture are both important controls on stream d 14 c poc thus export of aged particle associated om may be a feature of river systems along both

thermomechanical erosion of the lithosphere by mantle plumes a simplified semianalytic theory is presented that describes the process of lithosphere thinning through thermomechanical erosion by a hot mantle upwelling or plume the theory compares well with published numerical models but predicts that the rate of thinning is very dependent on the lateral scale of the upwelling and on the viscosity through the lithosphere asthenosphere transition a plume tail is likely to cause rapid local thinning reducing the lithosphere thickness substantially within a few million years plume heads or broadly spreading plume tail material are unlikely to cause significant thinning by this mechanism alone these results are consistent with geophysical evidence for localized thinning under the hawaiian volcanic chain but not under the broader hawaiian swell they are also qualitatively consistent with the distribution of basaltic volcanism in oceanic and continental hotspot tracks the theory predicts that thin continental lithosphere may be further thinned sufficiently by a plume tail to cause crustal melting within about 20 m y thus this mechanism may account for some delayed silicic magmatism on some slow continental plates higher thinning rates would have occurred in the archean

stirring geochemistry in mantle convection models with stiff plates and slabs abstract numerical models of mantle convection are presented that readily yield midocean ridge basalt morb and oceanic island basalt oib ages equaling or exceeding the apparent 1 8 ga lead isotopic ages of trace element heterogeneities in the mantle these models feature high viscosity surface plates and subducting lithosphere and higher viscosities in the lower mantle the formation and subduction of oceanic crust are simulated by means of tracers that represent a basaltic component the models are run at the full mantle rayleigh number and take account of faster mantle overturning and deeper melting in the past more than 97 of the mantle is processed in these models including the expected excess density of former oceanic crust readily accounts for the depletion of morb source relative to oib sources a novel finding is of gravitational settling of dense tracers within the low viscosity upper mantle as well as at the base of the mantle the models suggest as well that the seismological observation of a change in tomographic character in the deep mantle might be explained without the need to postulate a separate layer in the deep mantle these results expand the range of models with the potential to reconcile geochemical and geophysical observations of the mantle

a source to sink perspective of the waipaoa river margin abstract a fundamental goal of the earth science community is to understand how perturbations on earth s surface are preserved in the stratigraphic record recent source to sink s2s studies of the waipaoa sedimentary system wss new zealand are synthesized herein to provide a holistic perspective of the processes that generate transport and preserve sedimentary strata and organic carbon on the waipaoa margin in the late quaternary rapid uplift associated with subduction processes and weak sedimentary units have conspired to generate rapid rates of incision and erosion in the waipaoa catchment since the last glacial maximum lgm we show that although much of the sediment exported offshore during this time interval originated from valley excavation a substantial portion emanated from hillslopes mostly through deep seated landslide and earthflow processes that were vigorous during periods of rapid fluvial incision just prior to the pleistocene holocene transition lacustrine sediments deposited in naturally dammed 7 ky old lake tutira provide a record of holocene environmental controls on upper catchment sedimentation in the wss with 1400 storms identified storm frequency is modulated by the waxing and waning of atmospheric teleconnections between the tropics and antarctica furthermore clear long term changes in sediment yield are evident from the lake tutira record following human settlement as conversion to pasture is accompanied by a 3 fold increase in the long term lake sediment accumulation rate whereas there is ample evidence that waipaoa river flood deposits are routinely deposited offshore in the sheltered confines of poverty bay over the longer term waves and currents subsequently resuspend and transport these deposits both landward sandy fraction and seaward finer fraction thus the timing of sediment supply to areas of net sediment accumulation is more often driven by wave events that are not associated with river flooding therefore we conclude that asynchronicity of river sediment delivery and of wave resuspension in most instances precludes the direct preservation of flood events in the stratigraphic record of the waipaoa shelf over the longer term the sediment package preserved on the shelf and slope since the lgm can be explained in large measure by sequence stratigraphic models forced by varying sea level and ongoing tectonic deformation of the margin as sea level rose sediment supply to the slope was reduced by about a factor of 5 due to shelf trapping despite this reduction turbidites are found at similar frequency throughout the lgm present as the dominant trigger appears to be subduction earthquakes with large ones having a return interval of about 200 100 years sediment budget exercises that consider both modern river discharge versus centennial accumulation rates and post lgm terrestrial production versus offshore isopachs mass balances indicate that about half of the total sediment production from the waipaoa escapes the study area moreover a coupled sediment transport hydrodynamic model and observations of textural trends on the shelf indicate that a large fraction of the sediment is carried outside the study area along the shelf to the northeast by the river plume or by combined current wave activity therefore we conclude that the wss is an open system with sediment escape from the present day through the lgm the organic matter associated with sediment as it moves from upland source to marine sink is a product of particle history and provides a record of materials that have cycled over timescales of days to millions of years the ubiquity of fossil organic carbon oc in both the terrestrial and marine realms of the waipaoa attests both to the chronic nature of its source crumbling mudstones further destabilized by land use and its biogeochemical recalcitrance modern oc persists by virtue of its continual production along the s2s transit and is buried and preserved within the adjacent marine depocenters the waipaoa contrasts with dispersal systems on wide energetic shelves e g the amazon and fly rivers where sediment is extensively refluxed in oxygenated overlying water resulting in the biogeochemical incineration of particulate oc the waipaoa like other small mountainous rivers on active margins exhibits a high riverine oc preservation efficiency 50 in its marine depocenters because of the relatively rapid event driven accumulation of sediment

dispersal and transformation of organic carbon across an episodic high sediment discharge continental margin waipaoa sedimentary system new zealand abstract the rivers that drain active collisional margins of the southwest pacific deliver up to 35 of particulate organic carbon poc to the world ocean and are a key component of the global organic carbon flux however knowledge of the fate of terrestrial poc in the ocean is both limited and necessary for quantifying terrestrial and coastal ocean carbon budgets here the fate of terrestrial poc is determined off the high discharge waipaoa river sediment yield 15 mt y 1 based on a transect of seven cores from the river floodplain to the adjacent continental shelf and slope total organic carbon toc 13 c 14 c c n ratios and lipid biomarker compounds were used to determine biogeochemical characteristics of surface sediments from terrestrial source to marine sink and how these characteristics vary with river discharge complementary to surface sediments down core characteristics of three multi cores covering the shelf and slope regions were used to identify perturbations in sediment supply via major floods the presence of flood deposits allows us to compare their oc characteristics with non flood sediment thereby helping address the question of how flood events in the river catchment affect the transfer and fate of terrestrial oc through the marine environment results from this study show that as surface sediments are physically and biologically processed across the continental margin they gain a marine signature biomarker analyses of surface samples show decreases in terrigenous vascular plant sources with increasing distance offshore biomarkers also demonstrate that terrestrial oc is being transferred across the continental margin with plant sterols long chain alcohols and long chain fatty acids biomarkers indicative of vascular plants persisting as far offshore as the mid continental slope in contrast to ambient conditions represented by surface sediments rapid delivery by floods allows for more complete transfer of terrestrial carbon to the marine environment a 1 10 cm thick flood layer preserved from cyclone bola march 1988 contains a significant amount of terrestrially sourced oc which subsequently was rapidly buried by sediments delivered during less extreme conditions

estimation of temporally averaged sediment delivery ratio using aggradational terraces in headwater catchments of the waipaoa river north island new zealand the sediment delivery ratio was estimated for two periods 28 years and eight years following reforestation of seven tributary catchments 0 33 to 0 49 km2 in the headwaters of the waipaoa river basin north island new zealand in these catchments gully erosion which largely resulted from clearance of the natural forest between 1880 and 1920 is the main source of sediment to streams reforestation commenced in the early 1960s in an attempt to stabilize hillslopes and reduce sediment supply efforts have been partially successful and channels are now degrading though gully erosion continues to supply sediment at accelerated rates in parts of the catchment data from the area indicate that the sediment delivery ratio sdr can be estimated as a function of two variables the product of catchment area and channel slope and ag the temporally averaged gully area for the period sediment input from gullies was determined from a well defined relationship between sediment yield and gully area sediment scoured from channels was estimated from dated terrace remnants and the current channel bed terrace remnants represent aggradation during major floods this technique provides estimates of sdr averaged over periods between large magnitude terrace forming events and with the present channel bed the technique averages out short term variability in sediment flux comparison of gully area and sediment transport between two periods 1960 1988 and 1988 1996 indicates that the annual rate of sediment yield from gullies for the later period has decreased by 77 per cent sediment scouring in channels has increased by 124 per cent and sediment delivered from catchments has decreased by 78 per cent however average sdr for the tributaries was found to be not significantly different between these periods this may reflect the small number of catchments examined it is also due to the fact that the volume of sediment scoured from channels was very small relative to that produced by gullies according to the equation for sdr determined for the waipaoa headwaters sdr increases with increasing catchment area in the case where ag and channel slope are fixed this is because the amount of sediment produced from a channel by scouring increases with increasing catchment area however this relationship does not hold for the main stem of the study catchments because sediment delivered from its tributaries still continues to accumulate in the channel higher order channels are in effect at a different stage in the aggradation degradation cycle and it will take some time until a main channel reflects the effects of reforestation and its bed adjusts to net degradation results demonstrate significant differences among even low order catchments and such differences will need to be taken into consideration when using sdr to estimate sediment yields copyright 2001 john wiley sons ltd

tropical cyclone driven erosion of the terrestrial biosphere from mountains the transfer of organic carbon from the terrestrial biosphere to the oceans via erosion and riverine transport constitutes an important component of the global carbon cycle measurements of particulate organic carbon load and composition in the liwu river taiwan during cyclone triggered floods suggest that tropical cyclones may facilitate the delivery of non fossil particulate organic carbon to the ocean and its subsequent burial the transfer of organic carbon from the terrestrial biosphere to the oceans via erosion and riverine transport constitutes an important component of the global carbon cycle1 2 3 4 more than one third of this organic carbon flux comes from sediment laden rivers that drain the mountains in the western pacific region3 5 this region is prone to tropical cyclones but their role in sourcing and transferring vegetation and soil is not well constrained here we measure particulate organic carbon load and composition in the liwu river taiwan during cyclone triggered floods we correct for fossil particulate organic carbon using radiocarbon and find that the concentration of particulate organic carbon from vegetation and soils is positively correlated with water discharge floods have been shown to carry large amounts of clastic sediment6 non fossil particulate organic carbon transported at the same time may be buried offshore under high rates of sediment accumulation7 8 9 we estimate that on decadal timescales 77 92 of non fossil particulate organic carbon eroded from the liwu catchment is transported during large cyclone induced floods we suggest that tropical cyclones which affect many forested mountains within the intertropical convergence zone10 may provide optimum conditions for the delivery and burial of non fossil particulate organic carbon in the ocean this carbon transfer is moderated by the frequency intensity and duration of tropical cyclones

signals of watershed change preserved in organic carbon buried on the continental margin seaward of the waipaoa river new zealand abstract holocene sediments buried on the continental shelf seaward of the waipaoa river northeastern new zealand preserve a stratigraphic record of terrestrial environmental change the well characterized long term record of storms volcanism and human disturbance in this region provides an opportunity to examine how such changes are reflected in the character of organic carbon oc buried on the continental margin complimentary evidence obtained from analyses of the 14 c content and the elemental and stable carbon isotopic composition of different sedimentary fractions including charcoal wood and clay sized isolates indicates that these perturbations led to mobilization of oc components with variable storage histories in the watershed charcoal transported in the modern waipaoa river and buried in offshore depocenters includes a highly aged component that has apparently been slowly released from storage in soil or alluvial terraces the charcoal fraction ages become dramatically younger in sediments deposited after the taupo volcanic eruption 1717 cal yr bp and then polynesian settlement ca 700 cal yr bp both signaling biomass burning the mean age of woody plant fragments and clay bound oc deposited on the shelf also varies over time with older material having accumulated in the middle holocene and since human occupation deposition of older than average wood fragments and clay bound oc between about 5 and 3 6 kyr bp correlates with evidence for a period of increased storm frequency in the region and may reflect the enhanced delivery of aged soil and sedimentary rock mobilized from hill slopes via earthflows and or deep seated landslides similarly the deposition of older wood and clay bound oc commensurate with the first anthropogenic disturbance ca 700 years ago is consistent with accelerated mass wasting due to deforestation at the same time a change in the elemental and stable isotopic composition of bulk and clay bound oc buried on the shelf may reflect increased marine primary productivity and or mobilization of oc from deep levels in soil profiles deforestation of the waipaoa headwaters by european settlers beginning in the middle 19th century is evinced by a sharper rise in the age of clay associated oc buried offshore today deep gully incision into fractured sedimentary bedrock is a major source of sediment and kerogen to the river and this process has left its mark on the age of sedimentary oc delivered to the adjacent margin

seismic evidence for a fossil mantle plume beneath south america and implications for plate driving forces a teleseismic travel time study reveals the presence of a fossil plume in the deep upper mantle beneath brazil which has apparently remained geographically fixed with respect to the overly ing continent despite thousands of kilometres of plate motion this result implies that the upper mantle and lithosphere beneath south america have remained coupled since the breakup of gondwanaland and may provide an answer to the long standing question of what forces drive continental plates

mineral chemistry and density of subducted basaltic crust at lower mantle pressures subducted slabs are less dense than the surrounding mantle near the base of the transition zone 660 km depth because of the survival of garnet in former basaltic crust by this depth mantle peridotite has transformed to denser perovskitite1 2 the buoyancy of the former basaltic crust may contribute to the observed accu mulation or horizontal displacement of many slabs at the base of the transition zone3 here we report experimental confirmation of the widely held belief that the basaltic crust of slabs eventually transforms to a dense perovskititic lithology stable in the lower mantle synthetic mid ocean ridge basalt more glass subjected to pressures of 45 80 and 100 gpa in a laser heated diamond anvil cell transforms to an assemblage of aluminous mg fe silicate perovskite non quenchable casio3 perovskite stishovite and a sodic aluminous phase with the ca ferrite structure fig 1 per ovskititic more is about 0 06 g cm 3 more dense than a model lower mantle prem derived from seismological data thus even thermally equilibrated perovskititic slabs should encounter no sig nificant hindrance to subduction and convection in the lower mantle

numerical investigations of the mantle plume initiation model for flood basalt events recent studies have linked both the eruption of continental flood basalts and the emplacement of large oceanic plateaus to the initial phase of mantle plume activity i e the plume head model here we develop this concept further through numerical models of a large thermal diapir rising through the mantle and impinging upon the base of the lithosphere a finite element analysis in axisymmetric geometry is used to model the dynamics of solid state convective flow and heat transport in the mantle and an anhydrous batch melting model is used to estimate melt volumes and compositions obtained from partial melting in the shallow mantle we explore the effects of a number of model variables including mantle viscosity structure plume temperature and diameter and the mechanical response of the lithosphere our study emphasizes cases in which the lithosphere does not undergo significant extension prior to large scale melting a standard model is found to be consistent with some observed characteristics of flood basalt plume initiation events this model includes an initial diapir of radius 400 km and initial excess temperature 350oc rising through a mantle of viscosity 102 pa s and impinging beneath a model oceanic lithosphere 100 ma in age if lithospheric extension does not occur melting is almost entirely sublithospheric the melt volume produced is 3 5x106 km 3 and its composition has high mgo content 15 20 wt a time sequence of precursory central axial uplift of several kilometers followed by major melting volcanism is a robust feature of all the models regardless of mantle viscosity structure or lithospheric response low viscosity in the upper mantle compresses the timescale for these events and results in a constriction of the plume head initially impinging on the lithosphere the radial extent of the melting region about 500 km at the time of maximum melt production is only slightly affected by the upper mantle viscosity for nonrifting lithosphere no significant melt generation occurs for initial plume excess temperatures less than about 300oc the initial plume radius has only a modest effect upon the volume of melt obtained the most important aspect of lithospheric response is whether or not the uppermost crust and lithosphere above 40 km depth undergo extension otherwise the ductile strength of the lower lithosphere has little effect if the lithosphere is allowed to extend freely the melt volume increases by about an order of magnitude and it is comparable with some of the largest oceanic plateaus such as ontong java if continental or oceanic flood basalt events occur on old nonrifting lithosphere the primary melts are likely to be much more mgo rich than the basalts erupted at the surface this is consistent with abundant evidence for olivine fractionation in many flood basalt provinces along with the occurrence of picrite lavas and we hypothesize that fractionation occurs at a density trap at the crust mantle boundary moho at this stage and following it is likely that significant lower crustal and mantle lithospheric contamination of mantle plume derived magmas may occur both isotopically and in the trace elements the sequence and timing of both precursory and post magmatic events in the vast well exposed oceanic flood basalt terrane known as wrangellia se alaska british columbia are consistent with the predictions of our model the uplift predicted by our models is of order 2 4 km if we require that mantle plumes are hot enough to melt beneath unrifted lithosphere effects not modeled here such as small scale convection and hydrous melting will tend to reduce the model excess temperature and uplift in order to better understand the relation between mantle plumes and large igneous provinces several important advances are required implementation of a fractional melting model some realistic assessment of melt transport through the mantle and crust and a fully three dimensional convection model

quantifying sediment connectivity in an actively eroding gully complex waipaoa catchment new zealand abstract using a combination of airborne lidar 2005 and terrestrial laser scanning 2007 2008 2010 2011 sediment delivery processes and sediment connectivity in an 20 ha gully complex which significantly contributes to the waipaoa sediment cascade are quantified over a 6 year period the acquisition of terrain data from high resolution surveys of the whole gully fan system provides new insights into slope processes and slope channel linkages operating in the complex raw terrain data from the airborne and ground based laser scans were converted into raster dems with a vertical accuracy between surveys of 3 of sediment annually with a standard deviation of 20 806 m 3 the volumes of debris eroded from those areas dominated by surface erosion processes were higher than in areas dominated by landslide processes over the six year study period sediment delivery from the source zones to the fan was a factor of 1 4 times larger than the volume of debris exported from the fan into te weraroa stream the average annual volume of sediment exported to te weraroa stream varies widely from 23 195 to 102 796 m 3 fluctuations in the volume of stored sediment within the fan rather than external forcing by rainstorms or earthquakes account for this annual variation no large rainfall events occurred during the monitoring period therefore sediment volumes and transfer processes captured by this study are representative of the background conditions that operate in this geomorphic system

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

effects of temperature and mantle dynamics on estimates of the thermal conductivity in the deep mantle abstract estimates are made of the thermal conductivity ki of the deep mantle as a function of the temperature jump t across a thermal boundary layer at the core mantle boundary cmb three types of boundary layer corresponding to differing physical processes at the cmb are investigated and the effect of temperature dependent thermal conductivity is calculated for each the consequences of lateral heterogeneities are considered results indicate that if the temperature jump t seen by most of the mantle at the cmb is large 1350 k then ki cannot be more than 6 w m 1k 1 if the conductivity is constant or 8 w m 1k 1 if it is temperature dependent if t is small 350 k then ki may be 16 w m 1k 1 or higher the dynamics of the boundary layer is important results suggest that hot spot plumes are not the only mechanism for removing heat from the core

noble gas isotopic systematics of fe ti v oxide ore related mafic ultramafic layered intrusions in the panxi area china the role of recycled oceanic crust in their petrogenesis olivine and clinopyroxene grains have been separated from four large fe ti v oxide ore bearing intrusions panzhihua hongge baima and taihe in the panxi area emeishan large igneous province southwest china for he and ar isotope studies the samples examined revealed extremely low 3 he 4 he ratios 0 078 4 34 ra with the mean value 0 78 ra for gases extracted by stepwise heating this feature combined with low 40 ar 36 ar ratios can be interpreted as due to addition of subduction related fluids and melts that had been stored in the lithospheric mantle for long periods considering the regional geologic history such addition can be attributed to the paleo subduction that occurred along the western margin of the yangtze block during the neoproterozoic the subducted oceanic crust beneath the panxi area underwent eclogite facies metamorphism and subsequent exhumation the infiltration of subduction related melts and fluids into the lithospheric mantle led to enriched isotopic signatures from that of the slightly depleted asthenopheric mantle which has been suggested by the sr nd and pb isotopic data of the emeishan basalts and picrites in addition considerable amounts of eclogitic melts produced by partial melting of eclogite facies oceanic crust extensively contaminated the lithospheric mantle during the late permian partial melting of an upwelling mantle plume that contained an eclogite or pyroxenite component generated the parental fe rich magma that supplied the ore bearing intrusions the combination of these factors may have been the crucial reason that many world class fe ti v oxides deposits are clustered in the panxi area

storage and release of fossil organic carbon related to weathering of sedimentary rocks abstract the biogeochemical carbon cycle which plays an undeniable role in global climate change is defined both by the size of carbon reservoirs such as the atmosphere biomass soil and bedrock and the exchange between them of various mineral and organic carbon forms among these carbon forms fossil organic carbon foc i e the ancient organic matter stored in sedimentary rocks is widely observed in modern environments but is not included in the supergene carbon budget using a digitized map of the world and an existing model of co 2 consumption associated with rock weathering we establish the global distribution of foc stored in the first meter of sedimentary rocks and a first estimation of annual foc delivery to the modern environment resulting from chemical weathering of these rocks results are given for the world s 40 major river basins and extended to the entire continental surface with a mean value of 1100 10 9 t mainly controlled by shale distribution the global foc stock is significant and comparable to that of soil organic carbon 1500 10 9 t the annual chemical delivery of foc estimated at 43 10 6 t yr 1 and controlled by the areal distribution of shales and runoff is of the same order of magnitude as the foc output flux to oceans chemical weathering of bedrock within the amazon basin produces one quarter of the total global flux of foc derived from chemical weathering and thus is expected to govern foc release on a global scale these results raise important questions concerning the role of foc in the modern carbon cycle as well as the origin and the budget of carbon in soils and rivers

the role of effective discharge in the ocean delivery of particulate organic carbon by small mountainous river systems recent research has shown that small mountainous river systems smrs account for a significant fraction of the global flux of sediment and particulate organic carbon poc to the ocean the enormous number of smrs precludes intensive studies of the sort conducted on large systems necessitating development of a conceptual framework that permits cross system comparison and scaling up herein we introduce the geomorphic concept of effective discharge to the problem of source to sink poc transport this idea recognizes that transport effectiveness is the product of discharge frequency and magnitude wherein the latter is quantified as a power law relationship between discharge and load the rating curve an analytical solution for effective discharge qe identifies two key variables the standard deviation of the natural logarithm of discharge sq and the rating exponent of constituent i bi data from selected smrs are used to show that for a given river qe poc qesediment qe for different poc constituents e g pocfossil vs pocmodern differs in predictable ways and qe for a particular constituent can vary seasonally when coupled with the idea that discharge peaks of small rivers may be coincident with specific oceanic conditions e g large waves wind from a certain direction that determine dispersal and burial these findings have potentially important implications for poc fate on continental margins future studies of poc transport in smrs should exploit the conceptual framework provided herein and seek to identify how constituent specific effective discharges vary between rivers and respond to perturbations delivery of particulate organic carbon poc from the land to the sea by rivers and its eventual burial in ocean sediments are critical processes in the global carbon cycle sequestration of poc in marine sediments is a major long

organic carbon in floodplain alluvium signature of historic variations in erosion processes associated with deforestation waipaoa river basin new zealand 1 we use c org and 13c to trace the origin of the alluvium deposited on the waipaoa river floodplain at mcphail s bend between 1853 and 2002 the overbank deposits exhibit a more positive range of 13c values 26 3 0 6 than contemporary suspended sediment in transport at intermediate flows 27 8 0 2 when gully erosion releases most material to stream channels reflecting the greater contribution made by shallow landsliding in the headwaters during large precipitation events overbank sediment associated with recent floods contains a higher 50 amount of organic carbon than alluvium deposited by pre 1927 floods which was derived from outcrops of weathered bedrock on steep riparian hillslopes that were the primary sediment source before the soil mantled hillslopes elsewhere in the headwaters of the basin were destabilized by deforestation the recent alluvium resembles the bw horizon of soils present on these hillslopes sediment deposited during floods generated by localized storms and an overbank event that did not feature widespread shallow landsliding also has a distinctive signature that provides an indication of its provenance the organic carbon associated with the alluvium appears to be old 4031 40 b p in the case of the carbon associated with sediment deposited in 2002 and to be derived from weathered cretaceous and tertiary sedimentary rocks

the timing of crustal extension and the eruption of continental flood basalts the association between mantle plumes continental flood basalt eruption and the extension and rifting of the crust is well recognized1 3 but there has always been controversy as to whether the plume or the rifting was the initiating factor4 5 was rifting triggered by doming above the thermal plume2 3 6 or was rifting the driving force preceding the associated volcanicity7 8 here i show that recent flow by flow mapping of two continental flood basalt provinces the columbia river basalt group and the deccan basalt group allows one to infer the relative timing of volcanicity and crustal extension in both provinces eruption of the main tholeiitic phase preceded significant extension and crustal thinning this suggests that the primary event in the formation of these provinces was the upwelling of a mantle plume and that the subsequent crustal extension rifting and decompression was in part a consequence of the initiation of that plume

sedimentology of tufa facies and continental microbialites from the palaeogene of mallorca island spain abstract the middle eocene and oligocene basins in northwestern half of mallorca are filled by detrital rocks conglomerates calcarenites and lutites and carbonates deposited in alluvial fluvial palustrine and lacustrine settings the sediment supplied to these continental basins was transported by alkaline bicarbonate rich waters as a consequence these non marine carbonate deposits display extensive developments of diverse kinds of organosedimentary facies the sedimentary record from three localities peguera alaro and sineu enabled one to characterize well preserved eocene tufa deposits as well as eocene and oligocene stromatolites and oncolites a total of ten carbonate facies are described and interpreted the facies related to tufa deposits are diverse types of boundstones rudstones and packstones of bryophytes in situ stems s l phytoclasts etc and associated oncolite rudstones and bioclastic mudstones to packstones these facies deposited in littoral lacustrine and fluvio lacustrine environments oncolites and stromatolites present a great variety of shapes and sizes oncolites from centimetres to several meters in diameter that somehow were conditioned by environmental conditions within shallow lacustrine and fluvial settings three types of lamination apparently dependent on the type of microorganisms responsible for calcite precipitation are distinguished fan shaped and bush shaped grouped filaments and isolated to loosely grouped filaments the 13c and 18o composition of these deposits agrees with carbonate precipitation in freshwater hydrologically open systems with similar carbon sources throughout the eocene and oligocene however eocene and oligocene samples are set apart by the 18o oligocene facies present lower 18o values which might indicate deposition from isotopically less evolved waters perhaps under slightly greater precipitation evaporation conditions tufa facies have the highest 13c composition and show some positive covariation of 13c and 18o primarily reflecting the effects of physical and biological co2 outgassing and maybe also minor evaporation

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

deflection of plumes by mantle shear flow experimental results and a simple theory summary linear hotspot tracks indicate relative motion between some hotspot sources and the overlying lithosphere in order to better understand the effects of plate motions and mantle convection on mantle plumes and associated hotspots we consider the path of a continuous plume of buoyant material in a shear flow laboratory experiments show that steady plumes bent over by a linear velocity profile follow parabolic trajectories plume trajectories are also shown to evolve quadratically in time toward a new steady shape after a change in the ambient flow a remarkably simple description of plume dynamics provides good agreement with the experimental data the plume trajectory is determined by the combination of horizontal advection by mantle flow and vertical rise according to a modified stokes law these results are applied to some idealised examples which demonstrate effects of depth dependent mantle viscosity and return flow at least some mantle plumes are likely to be sufficiently robust that they remain close to vertical and therefore stable

production storage and output of particulate organic carbon waipaoa river basin new zealand 1 we compute the particulate organic carbon poc yield of the waipaoa river new zealand using sediment rating curves in conjunction with measurements of the carbon content of the suspended sediment to ascertain the source of the carbon and the extent to which the poc flux is tied to different erosion processes we determined the stable isotopic carbon composition 13c and carbon to nitrogen c n ratio of weathered bedrock soil and regolith most poc is derived from suspended sediment generated by gully erosion incision into weathered bedrock supplemented by landsliding during extreme events the specific yield of poc from the headwaters drainage area 1580 km2 is 55 g m 2 yr 1 which is very high by global standards and by comparison with other turbid steep land rivers the annual loss to floodplain storage is 4 3 6 kt of the mean annual poc yield 86 7 kt thus the waipaoa river is a very effective conduit for transporting poc to the ocean

experimental and crystal chemical study of the basalt eclogite transition in mars and implications for martian magmatism abstract this paper represents the start of a campaign to identify possible source regions for pyroxene phyric enriched and depleted martian basalts using high pressure studies on martian meteorite compositions that represent liquids or near liquids our first experiments focus upon mineralogical and crystal chemical aspects of the basalt eclogite transformation on mars it is anticipated that like earth eclogites are not the dominant upper mantle assemblage however like earth they may be important hosts for p cl f oh ti ree sr y high field strength elements hf and zr in the upper mantle this initial experimental study evaluates the major and trace element crystal chemistry of potential martian eclogite assemblages using a martian melt composition que 94201 this composition is appropriate for this study because it is evolved so it is pyroxene phyric contains abundant phosphate which is important for storage of ree and is very well studied in the high pressure experiments garnet and omphacitic pyroxene are the dominant phases the garnet has a compositional range from al 56 9 gr an 24 6 py 18 5 to al 41 5 gr an 28 1 py 30 4 to al 40 3 gr an 22 6 py 37 1 where al almandine gr an grossular andradite and py pyrope and the pyroxene has jadeite and acmite components of up to 26 0 and 9 9 respectively the garnet is enriched in ree over omphacite this is especially true for the hree which likely remain sequestered in residual garnet during melting this observation suggests that garnet may play a key role in producing the relatively flat ree patterns of enriched martian basalts the prime substitution couple that incorporates ree into garnet is ree 3 x site mg 2 y site r 2 x site al 3 y site where r 2 ca mn 2 fe 2 and mg this is a very effective couple that accounts for both charge balance and ionic size restrictions in individual sites the prime substitution couple that incorporates ree into omphacite is ree 3 m2 site na 1 m2 site for 2 ca 2 m2 site this is a less effective couple than that for garnet because na is pulled in two directions in omphacite it must balance the al in the m1 site in the jadeite component and it also has to provide a charge deficiency for each ree 3 cation in the m2 site the potential for the presence of eclogite in the martian mantle has implications for bulk trace element partitioning differences between eclogite and peridotite assemblages that are closely linked to garnet and pyroxene compositions further the occurrence of eclogites may have implications for the capability of the martian mantle to produce magmas and the major element composition of those magmas

genesis of flood basalts from eclogite bearing mantle plumes on the basis of the geochemistry of flood basalts we investigate the hypothesis that the source material for these magmas includes a significant eclogite component derived from ancient subducted oceanic crust entrained into mantle plumes at the core mantle boundary we present finite element models of the melting in starting plumes plume heads in both continental and oceanic regions starting plume models in which the plume heads are composed solely of pyrolite mantle are incapable of generating sufficient volumes of melt or any melt at all for reasonable plume head temperatures 1400 1600 c or lithosphere ages 6 25 100 m y this result is in contrast to the results of farnetani and richards 1994 whose models require higher plume head temperatures 1600 1700 c and a weak mobile lithosphere in order to generate volumes of melt similar to flood basalts plumes composed of 15 wt eclogite yield volumes of melt comparable to flood basalts in both oceanic and continental settings the maximum extent of melting in these models is about 10 30 this agrees with the model of hofmann and white 1982 who argued that ocean island basalts are derived from subducted oceanic crust if the embedded eclogite thermally reequilibrates with the surrounding plume material then melt volumes and total extents of melting increase by a additional factor of 2 3 a simple two dimensional rifting model for a slow spreading ocean basin underlain by a lens of hot plume material shows that plume related melting occurs over 50 60 m y a much longer period than for flood basalts

effect of lithospheric root on decompression melting in plume lithosphere interaction models summary global distribution of large igneous provinces lips shows that volcanic events associated with diapiric plume heads have often occurred near the margins of cratonic lithosphere previous numerical models of plume lithosphere interaction and decompression melting assumed the lithosphere as a rigid lid of mostly uniform thickness here we present a 3 d numerical model of a starting mantle plume consisting of a diapiric head and a trailing conduit which interacts with a moving lithospheric plate of non uniform thickness we study in particular the effect of a lithospheric root on plume flow and partial melting the plume is deflected away from the root preferentially in the upstream direction the presence of a root facilitates more than three fold enhancement in the melting rate by restricting the horizontal spreading of plume material and the diffusion of thermal anomaly in comparison to the model having no root the large melting rate is sustained for longer duration in the presence of depletion buoyancy of the residue our model does not require an excessively high plume temperature or compositionally heterogeneous plume head and reproduces the characteristics of melting rates and melt volumes seen in flood basalt volcanic provinces and the trailing hotspot tracks the model also supports occurrence of volcanism of different ages at a given location as a result of initial melting in the upstream region and its subsequent passage over the plume conduit due to plate motion inferences for the occurrence of such late stage volcanism are discussed for the deccan volcanic province cenozoic volcanism of eastern australia is also discussed as an example of deflection of a plume due to the presence of thick lithosphere

floodplain construction by recent rapid vertical accretion waipaoa river new zealand the rate of vertical accretion typically 14 18 mm h 1 during eight floods in the waipaoa river basin with recurrence intervals of 5 to 60 years was determined by relating the floodplain stratigraphy at mcphail s bend to the 1948 1995 flood history overbank deposits remaining after a flood that occurred in march 1996 suggest a rate of vertical accretion of 15 mm h 1 by contrast because the flow velocity across the floodplain was too high to permit deposition from suspension during the record flood of march 1988 the rate of vertical accretion was only 6 mm h 1 the sequence of deposition is highly discontinuous and the rapid vertical accretion is a response to a late 19th to early 20th century phase of deforestation in the headwaters that probably initiated a far greater change in suspended sediment yield than in discharge cross section surveys conducted since 1948 indicate that the high suspended sediment load of the waipaoa river also promoted in channel deposition which effected a progressive reduction in bankfull channel width although due to the overbank deposition channel capacity remained constant 1998 john wiley sons ltd

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

watershed control on the carbon loading of marine sedimentary particles abstract previous investigations of the factors governing organic carbon burial on continental margins have pointed toward the important apparently protective association of carbon with mineral particles these studies have also revealed dramatic transformations of carbon particle relationships at the land sea interface riverine particles in some settings lose a large portion of their loads of sorbed terrestrial carbon upon discharge to the ocean and gradually reload to similar levels with marine carbon the eel river in northern california and the adjacent continental shelf were selected as an ideal system to investigate the rates of these processes the river is episodically subject to large floods and the shelf stratigraphy preserves a record of the resultant large pulses of sediment and carbon input to the marine environment carbon isotopic carbon to nitrogen and carbon to surface area ratios of particles in flood deposits were expected to reflect the rapid unloading of terrestrial carbon from discharged particles whereas nonflood sediments that have accumulated at slower rates on the shelf were expected to carry higher loads of marine carbon our results indicate however that particles on the eel shelf have retained their loads of terrigenous carbon and that a significant portion of the particle sorbed carbon buried on the shelf is kerogen derived from the mesozoic tertiary franciscan complex we hypothesize that rates of uplift and mass wasting in the eel watershed and rates of particle delivery to and burial on the continental shelf are so rapid that kerogen is not completely oxidized and is recycled instead the loading of carbon on clay sized particles delivered to the shelf moreover is dependent on river discharge and may reflect the relative importance of different mass wasting processes during precipitation events of varying intensity the eel river system is likely to be representative of other small mountainous rivers and indicates that processes on land may play an important role in governing the amount and character of carbon being buried on the continental margins

rheological control of oceanic crust separation in the transition zone mineral physics observations suggest that distinct density and rheological differences exist between the crustal component of oceanic lithosphere and the underlying mantle we have conducted numerical experiments to investigate the influence of both density and viscosity on the effectiveness of recycling of oceanic crust into the lower mantle confirming previous results the density inversion at 670 km depth alone is not sufficient to prevent crustal recycling however a soft layer may exist between the strong garnet crust and cold slab interior models employing a simplified newtonian sandwich model show that this thin weak layer can effectively decouple the crust and slab once entrained into the lower mantle the then lighter crust can rise sufficiently fast as a rayleigh taylor instability to avoid further entrainment these results suggest that the crustal component of slabs may be trapped at 670 km depth leading to a garnet enriched transition zone

landslide impact on organic carbon cycling in a temperate montane forest in humid forested mountain belts bedrock landslides can harvest organic carbon from above ground biomass and soil ocmodern while acting to refresh the landscape surface and turnover forest ecosystems here the impact of landslides on organic carbon cycling in 13 river catchments spanning the length of the western southern alps new zealand is assessed over four decades spatial and temporal landslide maps are combined with the observed distribution and measured variability of hillslope ocmodern stocks on average it is estimated that landslides mobilized 7 6 2 9 tc km 2 yr 1 of ocmodern 30 of which was delivered to river channels comparison with published estimates of ocmodern export in river suspended load suggests additional erosion of ocmodern by small shallow landslides or overland flow in catchments the exported ocmodern may contribute to geological carbon sequestration if buried in sedimentary deposits landslides may have also contributed to carbon sequestration over shorter timescales 100 years 5 4 3 0 tc km 2 yr 1 of the eroded ocmodern was retained on hillslopes representing a net carbon sink following re vegetation of scar surfaces in addition it was found that landslides caused rapid turnover of the landscape with rates of 0 3 of the surface area per decade high rates of net ecosystem productivity were measured in this forest of 94 11 tc km 2 yr 1 which is consistent with rapid landscape turnover suppressing ecosystem retrogression landslide ocmodern yields and rates of turnover vary between river catchments and appear to be controlled by gradients in climate precipitation and geomorphology rock exhumation rate topographic slope copyright 2011 john wiley sons ltd

mineral chemistry of mildly alkalic basalts from the 25 ma mont crozier section kerguelen archipelago constraints on phenocryst crystallization environments phenocryst compositions and mineral melt equilibria in the mildly alkalic basalts from the 25 ma mont crozier section on the kerguelen archipelago are used to estimate the depths at which magmas stalled and crystallized and to constrain the rule of crustal structure in the evolution of magmas produced by the kerguelen mantle plume the crozier section of nearly 1000m height consists of variably porphyritic flows up to 21 vol phenocrysts dominated by plagioclase clinopyroxene olivine fe ti oxides feldspars show an extreme range of compositions from high ca plagioclase an to sanidine and variable textures that are related to extensive fractionation degassing and mixing in relatively low pressure sub volcanic magma chambers although clinopyroxene is a minor phenocryst type 0 3 vol its non quadrilateral components principally al 1 9 8 6 wt al o vary widely the results of clinopyroxene liquid thermobarometry and clinopyroxene structural barometry indicate that the crozier magmas crystallized at pressures ranging from 1 kbar to 11 12 kbar with high al clinopyroxene recording the highest pressures of crystallization high al clinopyroxene rich cumulates may represent an important component of the seismic crust to mantle transition zone at 14 16 km depth high pressure high al clinopyroxene crystallization became important in the mildly alkalic basaltic magmas from the kerguelen archipelago as ascending magmas stalled and fractionated at or near the crust mantle interface which became deeper as a result of progressive crustal thickening as the archipelago moved from a ridge centered setting at 40 ma to an intraplate position by 25 ma

organic carbon fluxes to the ocean from high standing islands the transfer of terrestrially derived yet poorly quantified organic carbon to the oceans is an important yet unagreed upon proportion of the world s carbon budget the few data that exist suggest that high standing oceanic islands in the southwest pacific may make important contributions to the overall terrestrial flux of particulate organic carbon poc to the oceans we have determined the poc flux from several streams in new zealand with those data and previous measurements from taiwan and papua new guinea we estimate the poc flux from high standing islands in the southwest pacific to be 48 1012 g c yr 1 a value slightly lower than previous estimates these islands which make up only 3 of earth s landmass contribute 17 35 of the estimated poc entering the world s oceans annually and thus may be important sources of terrestrial carbon to the ocean anthropogenic activities especially deforestation and forest harvesting have probably exacerbated the natural flux few to no data exist for many of these islands and thus a more detailed assessment awaits further measurements

mantle regulation of core cooling a geodynamo without core radioactivity abstract the case for radioactivity in the core based on the power requirements of the geodynamo is re evaluated previous calculations of mantle regulation of core thermal evolution have used an inappropriate formula new calculations with a more appropriate formula yield lower core heat loss in the past thus mitigating the implication of unreasonably high past core and mantle temperatures multiple thermal evolutions leading to present heat flows are also demonstrated depending on the efficiency of mantle removal of core heat some with moderately high past core heat loss and some with low and steady core heat loss the latter would permit a low or moderate power dynamo without core radioactivity key uncertainties are the efficiency of core cooling by the mantle the thermal conductivity of the core and the energy or entropy flow required to maintain the dynamo the present rate of heat loss from the core is argued to be still rather uncertain and a commonly used estimate of the thermal conductivity of the core is shown plausibly to be too high and in any case to be uncertain by perhaps a factor of 2 the geochemical difficulties associated with postulating radioactive heat sources in the core are stressed

terrestrial sources and export of particulate organic carbon in the waipaoa sedimentary system problems progress and processes abstract the composition of particulate organic carbon in sedimentary systems can provide information concerning sources of materials and their history the nsf margins source to sink study of the waipaoa sedimentary system in new zealand provides a platform to calibrate the particulate organic carbon sedimentary record by linking chemical characteristics to terrestrial and marine sedimentary processes this report is a preliminary evaluation of the available organic carbon concentration elemental c n ratio and c isotopic composition data for the terrestrial portion of the system samples analyzed were rocks soils floodplain sediments and riverine suspended sediments measurements confirm previous conclusions that a portion of the riverine particulate organic carbon is derived from sedimentary rocks the rock c kerogen concentration is relatively invariant 0 25 from rock source to riverine and floodplain sediments the non rock c is derived from c3 vegetation in most of the watershed the exception being the floodplain in which local c4 plant inputs are detected the non rock organic carbon has a mean 14c age of approximately 1000 years which reflects a mixture of modern and aged components the presence of aged non rock c which is chronic in its supply to the river indicates storage of particulates within the watershed remobilized colluvial and alluvial deposits are among the possible sources systematic differences are found between 13c databases generated by various groups while there may be natural explanations for these differences methodological artifacts have to be considered standardization of methods that cross source to sink boundaries is recommended

a plume head melting under a rifting margin abstract a large igneous province lip in the form of a long narrow band of thickened oceanic crust runs along the atlantic margin of north america abutting the rifted continental shelf we propose that this like many other lips has a mantle plume origin there is evidence that when the central atlantic ocean opened the rift was underlain by the flattened head of a mantle plume and that the rift site had drifted away from the plume tail by the time of the rifting so that the tail took little part in the formation of the lip we carried out numerical simulations in which we rifted the lithosphere over various model plume heads and calculated the volumes of melt produced we found that the thickness and width of the resulting thickened oceanic crust is very sensitive to the thermal structure directly under the rift and the structure of the lithosphere to fit observations of the lip a thin flat plume head is required such a plume head results when a mantle plume with temperature dependent viscosity passes through a significant step reduction in the background mantle viscosity at 660 km depth however an extensive layer of low viscosity under the rift results in a region of thickened crust much wider than the layer is deep by decoupling the mantle flow from the lithosphere to avoid decoupling we propose that there must be significant topography on the lithosphere and the rift site is a region of thinned lithosphere very thick crust next to the margin can be explained by lithospheric necking and the resulting fast initial upflow under the rift

numerical modelling of a mantle plume the plume head lithosphere interaction in the formation of an oceanic large igneous province abstract the thermomechanical processes associated with formation of large igneous provinces lips remain poorly understood owing to fundamental difficulties in simulating plume lithosphere interactions in current numerical models these models which aim to simulate the rise of mantle plume and the spread of plume head material imply a mechanically over simplified lithosphere and commonly a flat lithosphere zero vertical displacement as the upper boundary condition we propose a new numerical model derived from lithospheric scale models it has a high numerical resolution in the lithospheric domain and explicitly accounts for 1 free upper surface boundary condition 2 elastic plastic ductile lithospheric rheology including surface faulting and 3 vertical strength variations in the lithosphere we study the final stages of plume ascent and we focus on surface and lithospheric evolution and intra plate strain localisations the experiments predict that the first surface elevation occurs in less than 0 2 ma after plume initiation at 400 km depth variation of rheological parameters results in different surface elevations 500 2500 m ascent 2 10 m yr and base plate strain rates 10 12 10 15 s 1 fast 0 2 0 3 m yr plume head flattening starts at the moment when the plume head reaches the base of the lithosphere it leads to large scale extension and deep normal faulting at the centre of the plateau and to strong thermomechanical erosion at its base the erosion is maximal not under the plume centre as was predicted before but in two large bordering zones our study locally is the igneous province of the caribbean plate where the pre existing farallon lithosphere has been affected by the galapagos hotspot activity that generated thermal perturbations and crustal thickening with two main episodes of volcanism and underplating

annual fossil organic carbon delivery due to mechanical and chemical weathering of marly badlands areas a key issue in the study of the carbon cycle is constraining the stocks and fluxes in and between c reservoirs among these the role and importance of fossil organic carbon foc release by weathering of outcropping sedimentary rocks on continental surfaces is still debated and remains poorly constrained our work focuses on foc fluxes due to chemical and mechanical weathering of marls in two experimental watersheds with typical badlands geomorphology draix watersheds laval and moulin alpes de haute provence france organic matter from bedrock soil litter and riverine particles are characterized by rock eval 6 pyrolysis foc fluxes due to mechanical weathering are then estimated by monitoring the annual particulate solid exports at the outlets of the watersheds 1985 2005 period foc fluxes from chemical weathering were calculated using ca2 concentrations in dissolved loads year 2002 to assess the amount of foc released by the dissolution of the carbonate matrix results show that foc delivery is mainly driven by mechanical weathering with a yield ranging from 30 to 59 t km 2 yr 1 in the moulin 0 08 km2 and laval 0 86 km2 catchments respectively 1985 2005 average the release of foc attributed to chemical weathering was 2 2 to 4 2 t km 2 for the year 2002 these high foc fluxes from badlands are similar to those observed in tectonically active mountain catchments at a regional scale badland outcropping within the durance watershed does not exceed 0 25 in area of the rhone catchment but could annually deliver 12 000 t yr 1 of foc this flux could correspond to 27 of the total particulate organic carbon poc load exported by the rhone river to the mediterranean sea at a global scale our findings suggest that erosion of badlands may contribute significantly to the transfer of foc from continental surfaces to depositional environments copyright 2012 john wiley sons ltd

river sediment discharge to the oceans present day controls and global budgets empirical relationships between river sediment yields and a large number of hydroclimatic biological and geomorphological parameters are investigated at the global scale for a set of 60 major world river basins the environmental characteristics were extracted from various global computer data sets and natural river sediment fluxes were taken from the literature it is found that sediment yields can be best correlated by forming the products of hydroclimatic and geomorphological factors that is runoff intensity basin slope an index characterizing rock hardness and an index characterizing rainfall variability over the year the best parameter combination varies to some extent when the rivers are grouped according to their average climatic situation but it is always a combination of the above mentioned parameters that yields the most significant models in arid climates however regression coefficients are greater than in humid climates indicating that erodibility is much greater than in arid climates when extrapolated to the total continental area on the basis of the corresponding data sets the empirical models result in a total sediment flux of about 16 gigatons discharged to the oceans every year which is in good agreement with previous estimates derived from compilations of world wide river data consequently two global maps are proposed showing the spatial distributions of river sediment yields on the continents as well as the inputs of these sediments to the oceans in a grid point scale by far most of the sediments originate from the young orogenic belts of the continents mainly due to a combination of steep morphologies and high runoff intensities

riverine particulate organic carbon from an active mountain belt importance of landslides 1 we investigate the routing and transfer of particulate organic carbon poc from the western southern alps new zealand using organic carbon corg and nitrogen norg concentrations and stable carbon isotopes 13corg in this active mountain belt sediment discharge is dominated by landslide derived material landsliding acts to homogenize the geochemically diverse hillslope poc mixing poc from the standing biomass and soil with the fossil poc from bedrock as a result the poc in river sediment at the mountain front is a binary mixture of fossil and nonfossil carbon sourced from many landslide deposits we calculate that nonfossil biogenic poc makes up 63 7 of the total poc in the suspended load of rivers draining the western southern alps the erosional flux of biogenic poc from these catchments represents a transfer of 39 tc km 2 a 1 of atmospheric co2 averaged over the west flank of the mountain belt if more than 10 of this poc is preserved in sediments on geological timescales then this process is the most significant way in which the southern alps and similar tectonically active mountain belts with restricted alluvial aprons consume atmospheric co2

sediment flux from a mountain belt derived by landslide mapping in humid uplands landsliding is the dominant mass wasting process in the western southern alps of new zealand landslides are scale invariant and have a power law magnitude frequency distribution independent studies from other regions suggest that this is a general property of landsliding this observation is of critical importance to the evaluation of the impact of events of different length scales over different time intervals on landscape evolution it is particularly useful when estimating regional geomorphic rates because it constrains the frequency and overall significance of extreme events which cannot otherwise be evaluated by integrating the complete response of the system we estimate the regional denudation rate due to landsliding to be 9 4 mm yr 1 sediment discharge from the western southern alps is dominated by landslide derived material

a benchmark comparison for mantle convection codes summary we have carried out a comparison study for a set of benchmark problems which are relevant for convection in the earth s mantle the cases comprise steady isoviscous convection variable viscosity convection and time dependent convection with internal heating we compare nusselt numbers velocity temperature heat flow topography and geoid data among the applied codes are finite difference finite element and spectral methods in a synthesis we give best estimates of the true solutions and ranges of uncertainty we recommend these data for the validation of convection codes in the future

the difference between oceanic and continental tholeiites a fluid dynamic explanation continental tholeiites have higher sio2 k2o and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts these differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface although there is little doubt that most continental tholeiites have assimilated crustal material the lcoation and mechanism of assimilation remain uncertain longhi 1981 has argued that magmas derived directly from the mantle should crystallize little orthopyroxene the abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust since orthopyroxene is an early crystallizing phase in layered intrusions this assimilation must occur early before the magma enters the chamber assimilation can occur at the margins of the dykes which feed magma chambers depending on the nature of the flow if the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls if the flow is laminar a conductive profile develops at the margin and the flowing magma chills against the walls protecting them from thermal erosion the nature of flow in a dyke can be predicted from the reynolds number the criteria for turbulence reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and if this is the case assimilation of low melting point components in the walls of the dyke is inevitable it is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

rapid channel response to variability in sediment supply cutting and filling of the tarndale fan waipaoa catchment new zealand abstract in the headwaters of the waipaoa catchment fluvio mass movement gully complexes are currently significant contributors of sediment delivery of which is buffered by fans which have developed in the small tributaries that lead from the gully complexes biannual channel cross section surveys made since 1983 show seasonal aggradation incision cycles channels generally incise in summer and infill during the winter months this seasonal pattern of cutting and filling is driven by variations in sediment supply from the gully complexes in this paper to better understand patterns and processes of sediment delivery at the top of the sediment cascade we examine cut and fill cycles on the 11 ha tarndale fan over a two and a half year period analysis of sequential dems compiled from survey data obtained between december 2004 and august 2007 indicates cutting and filling may involve up to 47 000 tonnes of sediment over as little as three months during discrete severe rainstorm and or wet weather periods mass movements in the gully complex are enhanced and contribute large quantities of sediment to the fan via debris flows and landslides which infill channels during drier periods mass movement activity is inhibited and runoff incises the feeder channels

occurrence of fossil organic matter in modern environments optical geochemical and isotopic evidence abstract this study relates to the input and fate of fossil organic matter fom in the modern environment and focuses on two experimental watersheds overlying jurassic marls le laval and le brusquet 1 km2 in area located near digne alpes de haute provence france considering that fom delivery is mainly a result of different processes affecting sedimentary rocks bio chemical and mechanical weathering samples from different pools were collected bedrocks weathering profiles soils and riverine particles the samples were examined using complementary techniques optical palynofacies methods geochemical rock eval 6 pyrolysis c n ratio molecular gas chromatography mass spectrometry and isotopic stable c isotopic composition as a result fom markers were identified and tracked through the different pools the results highlight the contribution of fom which can be found in all the studied pools transfer of fom between bedrocks weathering profiles governed mainly by chemical weathering and bedrocks riverine particles governed mainly by mechanical weathering reveals any qualitative change weathering profiles soils transition governed by bio chemical weathering is characterized by alteration of the fom which is difficult to describe because fossil material is mixed with recent organic matter rom despite this latter point the study provides evidence for a relative resistance of fom to weathering processes and points to its contribution to the modern geosystem and the c cycle

residence time of alluvium in an aggrading fluvial system relationships between the surface area and age of alluvial deposits were used to estimate the residence time of alluvium in the 2205 km2 waipaoa river basin new zealand the contemporary waipaoa river is an efficient transporter of sediment to the continental shelf but the basin has been characterized by rapid channel and valley aggradation in the historic period and by extensive mid to late holocene alluvial storage in the lower reaches the area weighted mean age of alluvial deposits in the lower part of the river basin is 4400 yr these deposits comprise terrace remnants isolated by downcutting and holocene to recent sediments that are potentially remobilizable by the modern river even though the amount of storage is small relative to downstream transport the majority of the potentially remobilizable alluvium is likely to remain in storage for 100 yr and its half life time for 50 per cent removal is 2000 yr within the confines of the flfloodplain the apparent loss of older deposits is due primarily to burial but losses of the most recent deposits are due almost entirely to remobilization 30 40 per cent with the remainder preserved in the alluvial record for at least 104 yr most of this sediment is likely to remain in storage until there is a shift to a degradational state copyright 2006 john wiley sons ltd

geomorphologic controls on the age of particulate organic carbon from small mountainous and upland rivers 1 to assess the role that erosion processes play in governing the character of particulate organic carbon poc discharged from small mountainous and upland rivers a suite of watersheds from oregon california and new zealand was investigated the rivers share similar geology tectonic setting and climate but have sediment yields that range over 3 orders of magnitude the 14c age of the poc loads is highly correlated with sediment yield carbon isotope mass balances reveal that the rivers carry bimodal mixtures of modern plant and ancient rock derived oc at lower yields modern plant oc dominates the material delivered to the river by sheetwash and shallow landsliding with increasing yield a progressively larger part of the poc is contributed directly from bedrock erosion via deep gully incision our results support the inference that active margin watersheds are important sources of aged poc to the ocean

the isotopic composition of particulate organic carbon in mountain rivers of taiwan small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon poc to the oceans this input has implications for the geochemical stratigraphic record we have investigated the stable isotopic composition of poc 13corg in rivers draining the mountains of taiwan in 15 rivers the suspended load has a mean 13corg that ranges from 28 1 0 8 to 22 0 0 2 on average 37 samples per river over the interval of our study to investigate this variability we have supplemented suspended load data with measurements of poc in bedrock and river bed materials and constraints on the composition of the terrestrial biomass fossil poc in bedrock has a range in 13corg from 25 4 1 5 to 19 7 2 3 between the major geological formations using coupled 13corg and n c we have found evidence in the suspended load for mixing of fossil poc with non fossil poc from the biosphere in two rivers outside the taiwan central range anthropogenic land use appears to influence 13corg resulting in more variable and lower values than elsewhere in all other catchments we have found that 5 variability in 13corg is not controlled by the variable composition of the biomass but instead by heterogeneous fossil poc in order to quantify the fraction of suspended load poc derived from non fossil sources fnf as well as the isotopic composition of fossil poc 13cfossil carried by rivers we adapt an end member mixing model river suspended sediments and bed sediments indicate that mixing of fossil poc results in a negative trend between n c and 13corg that is distinct from the addition of non fossil poc collapsing multiple fossil poc end members onto a single mixing trend as an independent test of the model fnf reproduces the fraction modern fmod in our samples determined from 14c measurements to within 0 09 at the 95 confidence level over the sampling period the mean fnf of suspended load poc was low 0 29 0 02 n 459 in agreement with observations from other mountain rivers where physical erosion rates are high and fossil poc enters river channels the mean 13cfossil in suspended poc varied between 25 2 0 5 and 20 2 0 6 from catchment to catchment this variability is primarily controlled by the distribution of the major geological formations it also covers entirely the range of 13corg found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous poc if land sourced poc is preserved in marine sediments then changes in the bulk 13corg observed offshore taiwan could instead be explained by changes in the onshore provenance of sediment the range in 13corg of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans care should be taken in accounting for fossil poc in marine deposits supplied by active mountain belts

geochemistry and tectonic significance of the stony mountain gabbro north carolina implications for the early paleozoic evolution of carolinia abstract carolinia comprises a collection of neoproterozoic early paleozoic magmatic arc and sedimentary terranes that were amalgamated and accreted to laurentia in the early to middle paleozoic in central north carolina mafic rocks of the stony mountain gabbro intrude sub aqueous volcanic and sedimentary rocks and submarine epiclastic sedimentary rocks of the albemarle group the age of the stony mountain gabbro is constrained to the early cambrian middle ordovician field relations indicate that the gabbro represents the final phase of magmatism following the eruption and deposition of the neoproterozoic earliest cambrian albemarle group yet the gabbro pre dates regional metamorphism and tectonism related to the late ordovician accretion of carolinia to laurentia the stony mountain gabbro has a sub alkaline basaltic composition variable tio 2 mgo and ni cr values the rocks have a geochemical signature typical of island arcs the degree of lree enrichment prominent negative nb anomalies and nb th ratios are all features of low k to medium k tholeiitic basalts in modern island arc subduction related lavas isotope data are dominated by juvenile compositions that are consistent with derivation from lithospheric and asthenospheric sources during decompression melting of the mantle the stony mountain gabbro records subduction zone magmatism in a rifted island arc setting and can be modeled as the product of 10 15 hydrous partial melting of variable mixtures of morb and oib like mantle sources overprinted by a minor subducted slab derived hydrous fluid component by analogy with modern settings the rocks of the stony mountain gabbro are comparable to morb like to oib type enriched rocks from the lau island and sumisu rift and are interpreted to have formed within an evolving early paleozoic island arc back arc rift basin system the presence of an early cambrian arc back arc rift system in carolinia is broadly coeval with arc back arc volcanism in other peri gondwanan blocks of the appalachians and may be related to the early paleozoic opening of the rheic ocean

mantle mixing and continental breakup magmatism abstract the frequent formation of large igneous provinces during the opening of the atlantic ocean is a surface manifestation of the thermal and chemical state of convecting mantle beneath the supercontinent pangea recent geochemical and geophysical findings from the north atlantic igneous province all point to the significant role of incomplete mantle mixing in igneous petrogenesis on the basis of a whole mantle convection model with chemical tracers i demonstrate that sublithospheric convection driven by surface cooling can bring up dense fertile mantle without a thermal anomaly when small scale convection in the upper mantle breaks down into the lower mantle strong counter upwelling takes place entraining a large amount of dense crustal fragments accumulated at the base of the mantle transition zone this multi scale mantle mixing could potentially explain a variety of hotspot phenomenology as well as the formation of both volcanic and non volcanic rifted margins with a spatially and temporally varying distribution of fertile mantle

terrestrial sedimentation and the carbon cycle coupling weathering and erosion to carbon burial this paper examines the linkages between the carbon cycle and sedimentary processes on land available data suggest that sedimentation on land can bury vast quantities of organic carbon roughly 1015 g c yr 1 to evaluate the relative roles of various classes of processes in the burial of carbon on land terrestrial sedimentation was modeled as a series of 864 scenarios each scenario represents a unique choice of intensities for seven classes of processes and two different global wetland distributions comparison was made with presumed preagricultural conditions the classes of processes were divided into two major component parts clastic sedimentation of soil derived carbon and organic sedimentation of autochthonous carbon for clastic sedimentation masses of sediment were considered for burial as reservoir sediment lake sediment and combined colluvium alluvium and aeolian deposits when the ensemble of models is examined the human induced burial of 0 6 1 5 1015 g yr 1 of carbon on land is entirely plausible this sink reaches its maximum strength between 30 and 50 n paddy lands stand out as a type of land use that warrants future study but the many faces of rice agriculture limit generalization in an extreme scenario paddy lands alone could be made to bury about 1 1015 g c yr 1 arguing that terrestrial sedimentation processes could be much of the sink for the so called missing carbon is reasonable such a hypothesis however requires major redesign of how the carbon cycle is modeled unlike ecosystem processes that are amenable to satellite monitoring and parallel modeling many aspects of terrestrial sedimentation are hidden from space

evidence from magnetic fabric for the flow pattern of magma in the mackenzie giant radiating dyke swarm continental flood basalts and their associated swarms of mafic dykes are thought to result from melting in mantle plumes1 3 but the processes by which the resulting magma makes its way from mantle to crust remain obscure here we use magnetic anisotropy measurements to reconstruct the pattern of magma flow in the proterozoic mackenzie dyke swarm our results indicate that magma was injected vertically within 500 km of the focal point of the dyke swarm and then travelled horizontally at all further distances out to at least 2 100 km the sharp transition from vertical to horizontal flow at between 500 and 600 km may correspond to the outer boundary of melt generation in the mantle plume believed to be responsible for the mackenzie igneous events

controls on carbon and nitrogen export in an eroding catchment of south eastern queensland australia effects of gully and channel erosion on the export of sediments are in general well understood but the effects on carbon c and nitrogen n export remain an open question we examined these effects and the role of flow magnitude total wet season rainfall catchment size and the c and n content and solubility of most probable sediment sources in a subtropical catchment we sampled the baseflow and the rising stage of high flow events for one wet season and analysed 5 years of water quality data from event sampling stations total suspended sediment was the main variable explaining n export followed by rainfall flow and catchment size n was exported mainly in particulate organic form and c in dissolved form the quality of most probable sediment source fractions explains these results and points to fractionation during transport into c and n richer and c and n poorer fractions with travel distance ultimately determining the average quality of transported sediment for different flow magnitudes erosion would have caused a lower c and n concentration in sediments a lower proportion of mineralized n a larger proportion of dissolved organic c and a larger c n ratios of the soluble fraction as compared with unincised grassed valleys this would alter the rates of nutrient cycling and energy flow within and across ecosystem compartments in streams receiving this export copyright 2014 john wiley sons ltd

climate regulates the erosional carbon export from the terrestrial biosphere erosion drives the export of particulate organic carbon from the terrestrial biosphere pocbiosphere and its delivery to rivers the carbon transfer is globally significant and can result in drawdown of atmospheric carbon dioxide co2 if the eroded pocbiosphere escapes degradation during river transfer and sedimentary deposition despite this recognition we lack a global perspective on how the tectonic and climatic factors which govern physical erosion regulate pocbiosphere discharge obscuring linkages between mountain building climate and co2 drawdown to fill this deficit geochemical 13c 14c and c n hydrometric water discharge suspended sediment concentration and geomorphic slope measurements are combined from 33 globally distributed forested mountain catchments radiocarbon activity is used to account for rock derived organic carbon and reveals that pocbiosphere eroded from mountain forests is mostly 1300 14c years old annual pocbiosphere yields are positively correlated with suspended sediment yields confirming results from taiwan and a recent global analysis and are high in catchments with the steepest slopes based on these relationships and the global distribution of slope angles 3 arc second it is suggested that topography steeper than 10 16 of the continental area may contribute 40 of global pocbiosphere erosional flux climate is shown to regulate pocbiosphere discharge by mountain rivers by controlling hydrologically driven erosion processes in catchments where discharge measurements are available 8 of the 33 a significant relationship exists between daily runoff mm day 1 and pocbiosphere concentration mg l 1 r 0 53 p 0 0001 the relationship can be described by a single power law and suggests a high connectivity between forested hillslopes and mountain river channels as a result annual pocbiosphere yields are significantly correlated with mean annual runoff r 0 64 p 0 0001 a shear stress pocbiosphere erosion model is proposed which can explain the patterns in the data the model allows the climate sensitivity of this carbon flux to be assessed for the first time for a 1 increase in annual runoff pocbiosphere discharge is predicted to increase by 4 in steeper catchments pocbiosphere discharge increases more rapidly with an increase in annual runoff for comparison a 1 increase in annual runoff is predicted to increase carbon transfers by silicate weathering solute fluxes in mountains by 0 4 0 7 depending on the fate of the eroded pocbiosphere river export of pocbiosphere from mountains may act as an important negative feedback on rising atmospheric co2 and increased global temperature erosion of carbon from the terrestrial biosphere links mountain building and climate to the geological evolution of atmospheric co2 while the carbon fluxes are sensitive to predicted changes in runoff over the coming century

multiple volcanic episodes of flood basalts caused by thermochemical mantle plumes the hypothesis that a single mushroom like mantle plume head can generate a large igneous province within a few million years has been widely accepted the siberian traps at the permian triassic boundary and the deccan traps at the cretaceous tertiary boundary were probably erupted within one million years these large eruptions have been linked to mass extinctions but recent geochronological data reveal more than one pulse of major eruptions with diverse magma flux within several flood basalts extending over tens of million years this observation indicates that the processes leading to large igneous provinces are more complicated than the purely thermal single stage plume model suggests here we present numerical experiments to demonstrate that the entrainment of a dense eclogite derived material at the base of the mantle by thermal plumes can develop secondary instabilities due to the interaction between thermal and compositional buoyancy forces the characteristic timescales of the development of the secondary instabilities and the variation of the plume strength are compatible with the observations such a process may contribute to multiple episodes of large igneous provinces

the transfer of modern organic carbon by landslide activity in tropical montane ecosystems 1 geomorphic processes play an important role in the transfer and storage of carbon within steep mountainous terrain among these mass wasting stands out because of its impact on above and below ground carbon pools and its potential for releasing or sequestering carbon a combined remote sensing and gis modeling approach was used to quantify the amount and spatial redistribution of modern organic carbon mobilized by mass wasting activity in a tropical mountain setting the study focused on a population of hundreds of shallow translational landslides triggered by hurricane mitch 1998 on seven watersheds draining the southern flank of the sierra de las minas mountain range slm in central eastern guatemala results illustrate that mass wasting contributed to the transfer of 43 104mgc or 3 of the pre event c in above ground vegetation and soils for an equivalent carbon flux rate of 0 08 0 33 mgc ha 1 y 1 depending on whether we consider hurricane mitch to be a landslide triggering event with a 20 year or an 80 year recurrence interval while 30 of this carbon was delivered to hillslopes or first order streams with a presumed high potential for long term sequestration the remaining 70 was delivered to higher order streams with unknown carbon retention capabilities therefore the ultimate fate of the carbon released by landsliding is very uncertain but depending on the proportion sequestered by colluvial deposits the recurrence interval of landslide triggering events and the rate of ecosystem recuperation at the landslide failure sites mass wasting could be either a net source or sink of carbon in a simulated setting based on the slm study results in which all carbon transferred by landslides from all tropical mountains of the globe is released to the atmosphere it would represent an amount equivalent to 1 11 of the global carbon currently being released by the burning of fossil fuels meanwhile in a projected scenario where a significant proportion of the carbon transferred by landslides is retained within sedimentary deposits sequestration rates would equal 2 19 of the residual land sink

a magmatic loading model for coronae on venus 1 models for the formation of coronae quasi circular volcanotectonic features on venus must explain four critical characteristics coronae display 1 a wide range of diameters 2 complex varied topography 3 fracture annuli and 4 sometimes extensive volcanism previous models have difficulty simultaneously satisfying all four constraints on the basis of observations and interpretations of features on venus and earth and experiments in geophysical fluid dynamics we propose that coronae form in response to magmatic loading of the crust over zones of partial melting at the tops of thermally buoyant heads of transient mantle plumes that impinge on the base of the thermal lithosphere by tying corona formation to a melt zone and not directly to an impinging upwelling our conceptual model can account for the wide range in corona diameters lateral crustal flow facilitated by magmatic heating may lead to central depressions characteristic of many coronae a thermomechanical finite element simulation with a dissipating thermal anomaly leads to crustal thinning and predicts an elevation profile consistent with one of the more populous topographic classes of coronae deformation concentrated at the transition from thicker to thinner lithosphere above the thermal anomaly yields a narrow annulus of enhanced differential stress consistent with formation of fracture annuli estimates of excess heat in an upwelling exceed the thermal energy in the melt that comprises the corona band pass filtered maps of the topography and gravity fields in the beta atla themis region are consistent with upwellings now impinging on the lithosphere beneath seven coronae previously inferred to be active

mantle plumes and flood basalts enhanced melting from plume ascent and an eclogite component new numerical models of starting plumes reproduce the observed volumes and rates of flood basalt eruptions even for a plume of moderate temperature arriving under thick lithosphere these models follow the growth of a new plume from a thermal boundary layer and its subsequent rise through the mantle viscosity structure they show that as a plume head rises into the lower viscosity upper mantle it narrows and it is thus able to penetrate rapidly right to the base of lithosphere where it spreads as a thin layer this behavior also brings the hottest plume material to the shallowest depths both factors enhance melt production compared with previous plume models the model plumes are also assumed to contain eclogite bodies inferred from geochemistry to be recycled oceanic crust previous numerical models have shown that the presence of nonreacting eclogite bodies may greatly enhance melt production it has been argued that the eclogite derived melt would react with surrounding peridotite and refreeze however recent experimental studies indicate that eclogite derived melts may have reached the earth s surface with only moderate or even minor modification combined with an assumed 15 vol component of eclogite our models yield a sharp peak in melting of about 1 3 myr duration and volumes of melt that encompass those observed in flood basalt provinces

carbon isotope geochemistry of the santa clara river the santa clara river is a prototypical small mountainous river with a headwater height greater than 1000 m and a basin area smaller than 10 000 m 2 although individual small mountainous rivers export trivial amounts of sediment and carbon to the ocean as a group these rivers may export a major fraction as much as 50 of the total global river sediment flux milliman and syvitski 1992 making their geochemistry relevant the study of the ocean s carbon cycle in addition many small rivers export sediment in a few high flux events causing massive sporadic discharge of carbon onto coastal shelves discharge conditions very different from those of large rivers this class of rivers is an end member of the river ocean carbon exchange system opposite the earth s largest river the amazon the carbon mass and isotopic properties of the santa clara river are significantly different from previously studied large rivers during the 1997 1998 winter all santa clara carbon pools were old with flux weighted average l4c values of 428 76 for particulate organic carbon 73 31 for dissolved organic carbon and 644 58 for black carbon the age of exported carbon is primarily due to the deep erosion of old soils and not to inclusion of fossil fuel carbon additionally the 13c signatures of exported carbon pools were high relative to terrestrial carbon bearing a signature quite similar to marine carbon average particulate organic carbon poc 13c 22 2 0 8 the santa clara s estuary is small and drains onto the narrow eastern pacific coastal margin exporting this old soil organic matter directly into the ocean if the santa clara export patterns are representative of this class of rivers they may be a significant source of refractory terrestrial carbon to the ocean

geothermal constraints on emeishan mantle plume magmatism paleotemperature reconstruction of the sichuan basin sw china the middle late permian emeishan large igneous province elip in southwestern china represents a classic example of a mantle plume origin to constrain the thermal regime of the elip and contemporaneous magmatic activity in the northeastern sichuan basin maximum paleotemperature profiles of deep boreholes were reconstructed using vitrinite reflectance ro and apatite fission track data two heating patterns were identified 1 heating of the overlying lithosphere by magma storage regions and or magmatic activity related to the mantle plume which resulted in a relatively strong geothermal field and 2 direct heating of country rock by stock or basalt borehole ro data and reconstructed maximum paleotemperature profiles near the elip exhibit abrupt tectonothermal unconformities between the middle and late permian the profiles in the lower subsections i e pre middle permian exhibited significantly higher gradients than those in the upper subsections distal to the basalt province high paleo geotemperatures hereafter paleotemperatures were inferred despite deformation of the paleogeothermal curve due to deep faults and igneous rocks within the boreholes in contrast ro profiles from boreholes without igneous rocks i e late permian contained no break at the unconformity paleotemperature gradients of the upper and the lower subsections and erosion at the middle late permian unconformity revealed variations in the thermal regime the inferred spatial distribution of the paleothermal regime and the erosion magnitudes record the magmatic and tectonic thermal response to the emeishan mantle plume

gully erosion and sediment production te weraroa stream new zealand conversion to pasture early in the twentieth century was ameliorated by reforestation that commenced in 1962 estimates of sediment production were made using the change in gully area observed in sequential aerial photographs channel storage was assessed from stream cross section surveys at its peak gully erosion affected 6 of the total catchment area the amount of sediment contributed from gullies declined by 62 as the forest became established but of the 28 7 mt of sediment generated by gully erosion between 1950 and 1988 48 was stored in the channel along the lower 8 km of te weraroa stream even if the amount of sediment generated by gully erosion continues to decline it likely will be many decades before the gravel is released from storage index terms 1815 hydrology erosion and sedimentation 1824 hydrology geomorphology 1625 1878 hydrology water energy interactions keywords channel storage gully erosion sediment production citation gomez b k banbury m marden n a trustrum d h peacock and p j hoskin gully erosion and sediment production te weraroa stream new zealand water resour res 39 7 1187 doi 10 1029 2002wr001342 2003