roles of sorption and tube dwelling benthos in the cycling of phosphorus in bering sea sediments adsorption of dissolved phosphate onto iron hydroxides has been shown to be one of the primary regulators of phosphorus cycling in sediments bioturbation and bioirrigation by benthic infauna modify this cycling by accelerating the transport of dissolved and particulate phosphorus and by changing rates of reactions that occur in the sediment such as the adsorption of phosphate by amorphous iron hydroxides hydrographic processes vary regionally in the bering sea and nutrient exchange between the sediments of the broad shallow shelf and overlying water may influence water column productivity these characteristics make the bering sea a good study site for examining the processes that influence sedimentary cycling of phosphorus to examine these processes we collected samples in four domains southern middle shelf southern outer shelf southern off shelf consisting of the continental slope and bering sea basin and northern bering shelf based on hydrographic regime at each station we directly measured phosphate flux and sediment oxygen consumption using whole core incubations we also measured infaunal burrow abundances amorphous iron hydroxide concen trations and phosphate sorption we found that three out of the four domains had a high affinity for trapping phosphate in the sediment as indicated by their adsorption coefficients 6 59 81 81 however the measured phosphate fluxes could not be explained by the adsorption capacity of the sediment alone the results indicated that on the middle shelf the phosphate flux positively co varied with infaunal burrow abundances the high number of organisms in this domain 10 32 burrows per 50 cm

endorheic or exorheic differential isostatic effects of cenozoic sediments on the elevations of the cratonic basins around the tibetan plateau based on isostasy this paper elegantly explains the differential modern elevations of the large scale cratonic basins around the tibetan plateau the qaidam and tarim basins are endorheic regions with thick cenozoic deposits and they uplifted dramatically during the cenozoic in contrast the sichuan basin is exorheic and most cenozoic sediments flowed out of the basin no visible cenozoic uplift is observed we suggest that whether a basin is endorheic or exorheic significantly influences its modern elevation illustrating how exogenic geological processes impact endogenic geological processes in addition a new method of estimating paleoelevation is proposed which predicts successively accelerating uplift of the qaidam basin during the cenozoic and provides good constraints on the geological evolution of the northern tibetan plateau

anisotropy of magnetic susceptibility of eocene and miocene sediments in the qaidam basin northwest china implication for cenozoic tectonic transition and depocenter migration the cenozoic evolution of the qaidam basin especially its paleostress field can provide a better understanding of the dynamistic process of the northern tibetan plateau under certain conditions anisotropy of magnetic susceptibility ams holds great potential for investigating early tectonic events even where macroscopic and microscopic evidence of deformation is invisible a basin scale ams study of the middle to late eocene xiaganchaigou formation and the early to middle miocene xiayoushashan formation from seven locations was conducted covering most outcrops of these two formations within the qaidam basin in the western qaidam basin principal stress directions inferred from ams ellipsoids consist with those inferred from fold axial traces while at eboliang and in the northern qaidam basin most principal stress directions reflected by ams ellipsoids are different from those reflected by fold axial traces two epochs of compressive strain have been identified an early n s strain no later than oligocene and a late ne sw strain since miocene the early n s compression is more intense in the northern qaidam basin than that in the western qaidam basin while the late ne sw compression which dominates the modern nw se trending fold axial traces is more intense in the western qaidam basin than that in the northern qaidam basin the stress transfer provides a reasonable explanation for the southeastward migration of the deposition center in the qaidam basin during cenozoic moreover the appearance of e component compression may be in close relationship with the beginning of the left lateral strike slip kunlun fault or the eastward channel flow to the south of the kunlun fault

pleistocene drainage reorganization driven by the isostatic response to deep incision into the northeastern tibetan plateau pleistocene drainage basin integration led to progressive excavation of tertiary quaternary sedimentary basins along the yellow river in the northeastern tibetan plateau cosmogenic burial dating of ancestral river deposits and basin fi ll from two key watershed divides confi rms a fl uvial connection between basins at 0 5 1 2 ma prior to excavation by the yellow river pres ervation of the relict depositional surface that represents the maximum height of basin fi ll allows reconstruction of the volume of eroded material across a broad region we quantify the iso static response to this erosional unloading using a two dimensional fl exural model calculated maximum vertical displacements for different effective elastic thicknesses vary from 160 m to 260 m near the pleistocene spillway from the qinghai paleo lake we suggest that the isostatic response to fl excavation along the yellow river defeated local tributaries isolated lake qinghai and led to the development of an internally drained basin in the past 0 5 1 2 m y

valley formation by fluvial and glacial erosion cross valley profiles from the west slope of the olympic peninsula washington are used to investigate the relative effects of fluvial and glacial erosion on valley formation unlike most ranges where glaciers and rivers sequentially occupied the same valleys neighboring valleys in the olympic mountains developed in similar lithologies but were subject to different degrees of glaciation allowing comparison of the net effect of glacial and fluvial processes integrated over many glacial cycles upslope drainage area was used to normalize comparisons of valley width ridge crest to valley bottom relief and valley cross sectional area as measures of net differences in the mass of rock excavated from below ridgelines for 131 valley spanning transects valley width relief and cross sectional areas are similar for glaciated partly glaciated and unglaciated fluvial valleys with drainage areas of 10 km 2 but diverge downslope glaciated valleys draining 50 km 2 reach two to four times the cross sectional area and have to 500 m greater relief than comparable fluvial valleys partly glaciated valleys have intermediate dimensions at distances of 5 km from valley heads the cumulative upstream volume of rock removed to form valleys is two to four times greater in glacially incised valleys than in fluvial valleys the finding of strong differences in the net result of valley excavation by fluvial and glacial erosion supports the interpretation that alpine glaciers are more effective erosional agents than are rivers and implies that large alpine valleys deepened and enlarged in response to pleistocene glaciation

the phosphorus cycle in coastal marine sediments the concentration of dissolved phosphate in sediment box cores from the laurentian trough in the gulf of st lawrence increased sharply across the sediment water interface from 2 0 mol po4 1 in the bottom water to 6 3 mot po4 1 in the top cm remained almost constant at this value down to 5 15 cm depth and then increased rapidly with further depth in the region of constant concentration phosphate is buffered by sorption equilibria with the sediment the production rate of phosphate the sorption capacity of the sediment and the thickness of the diffusive boundary layer at the sediment water interface appear to control the shape of the pore water profile even though the buffering places an upper limit on the concentration gradient across the sediment water interface and hence on the flux the phosphate flux to the overlying water is controlled by the production rate of phosphate within the sediment a model is proposed to relate sediment chemistry to phosphorus fluxes

long term fluvial incision rates and postglacial river relaxation time in the french western alps from 10be dating of alluvial terraces with assessment of inheritance soil development and wind ablation effects studying river long profile development as a response to tectonic and climatic controls requires reliable age dating of paleo profile remnants preserved as river terraces cosmic ray exposure cre dating often represents the only method available to date river terraces but the interpretation of cosmogenic nuclide concentrations is complicated by pre depositional inheritance and post depositional disturbance of the terrace deposits through pedogenesis and surface inflation or deflation here we use cosmogenic 10be measurements to date alluvial terraces in the french western alps in order to estimate river incision rates and to infer river response to climatic fluctuations and tectonic forcing we assess inheritance by constructing 10be concentration vs pebble depth profiles and use a monte carlo technique to estimate terrace ages we find that inheritance is negligible on all terraces enabling us to date terraces as young as 5 ky terraces that predate the last glaciation experienced intense pedogenesis and wind ablation which led to significant scatter in the 10be concentrations of surface samples we assess these effects using a model of 10be ingrowth and show that the oldest cre ages of surface clasts are close to the probable terrace age we study two catchments which have undergone varying degrees of glacial disruption the buech river experienced variations in runoff and sediment flux during the last glaciation as well as occasional ice dammings of its outlet its upper terraces record incision rates averaged over 190 ky of 0 8 mm yr consistent with denudation rates estimated in the surrounding areas and suggesting long term stability of river incision rates climatic forcing is well documented for the drac river which was repeatedly dammed by glaciers during cold periods its postglacial incision history was triggered by an 800 m drop in base level following ice dam disappearance long profile development by knickpoint propagation explains the time lag of 2 5 ky between this base level drop and terrace abandonment upstream as well as subsequent peak incision rates of 6 cm yr followed by a gradual decrease in incision rates the present knickpoint location 55 km upstream from the glacial damming site enables us to calculate a fluvial response time of 15 20 ky controlled by knickpoint propagation rates of several meters per year within the drac river

formation of internally drained contractional basins by aridity limited bedrock incision 1 low internal relief aridity and internal drainage characterize the puna altiplano plateau and the tibetan plateau and the tarim basin structurally these areas are reverse fault bounded terrains with intervening wedge top basins that store thick accumulations of sediment for millions of years orographic barriers along the margins of these basins are oriented normal to moisture bearing winds resulting in regional aridity the combination of this aridity rapid shortening and uplift rates and resistant exposed lithologies coincides with the location of these internally drained basins we hypothesize that these large internally drained areas persist indefinitely when uplift overwhelms the fluvial systems and defeats the channel network to test this hypothesis we developed models of channel defeat to examine the threshold conditions required to fragment the channel network the results of these models suggest that low erodibility rocks moderate to high uplift rates and low precipitation favor defeat of orogen traversing channels most coupled tectonic landscape development models prevent the development of internal drainage and so may overestimate the voracity of erosion in regimes that favor internal drainage profoundly influencing predicted orogenic growth patterns

estimation of authigenic alteration of biogenic and reactive silica in pearl river estuarine sediments using wet chemical digestion methods the extent of authigenic alteration of biogenic and reactive silica in pearl river estuarine sediments has been estimated using wet chemical digestion methods results show relatively constant distributions of biogenic and reactive si horizontally and vertically based on three core measurements the biogenic and total reactive si average 77 91 and 264 77 mol si g 1 respectively their extents of authigenic alteration are correspondingly estimated as 55 6 and 70 6 the average biogenic si accumulation rate is calculated as 1 91 109 mol si year 1 which translates into storage of 7 15 of the annual riverine dissolved silica input by contrast the total reactive si accumulation rate is as high as 6 49 109 mol si year 1 improving annual riverine silicic acid storage to 24 19 detailed investigation is required for a good understanding of early diagenetic process of biogenic and reactive silica in this subtropical area

stratigraphic controls on fluid and solute fluxes across the sediment water interface of an estuary shallow stratigraphic features such as infilled paleovalleys modify fresh groundwater discharge to coastal waters and fluxes of saltwater and nutrients across the sediment water interface we quantify the spatial distribution of shallow surface water groundwater exchange and nitrogen fluxes near a paleovalley in indian river bay delaware using a hand resistivity probe conventional seepage meters and pore water samples in the interfluve region outside the paleovalley most nitrate rich fresh groundwater discharges rapidly near the coast with little mixing of saline pore water and nitrogen transport is largely conservative in the peat filled paleovalley fresh groundwater discharge is negligible and saltwater exchange is deep 1 m long pore water residence times and abundant sulfate and organic matter promote sulfate reduction and ammonium production in shallow sediment reducing iron rich fresh groundwater beneath paleovalley peat discharges diffusely around paleovalley margins offshore in this zone of diffuse fresh groundwater discharge saltwater exchange and dispersion are enhanced ammonium is produced in shallow sediments and fluxes of ammonium to surface water are large by modifying patterns of groundwater discharge and the nature of saltwater exchange in shallow sediments paleovalleys and other stratigraphic features influence the geochemistry of discharging groundwater redox reactions near the sediment water interface affect rates and patterns of geochemical fluxes to coastal surface waters for example at this site more than 99 of the groundwater borne nitrate flux to the delaware inland bays occurs within the interfluve portion of the coastline and more than 50 of the ammonium flux occurs at the paleovalley margin

influence of water column anoxia on the elemental fractionation of carbon and phosphorus during sediment diagenesis abstract organic carbon oxidation and benthic total phosphorus flux measurements from sites covering a range of bottom water oxygen concentrations have been compiled the data show that p regeneration relative to carbon oxidation is much more extensive in sediments overlain by oxygen depleted waters these results are consistent with studies showing that low oxygen bottom water concentrations correlate with reduced total phosphorus burial efficiencies and enhanced organic carbon burial efficiencies based on these observations a positive feedback is proposed between water column anoxia enhanced benthic phosphorus regeneration and marine productivity this feedback may help explain the widespread accumulation of organic rich marine sediments from anoxic waters observed in the geologic record

sedimentary iron phosphorus cycling under contrasting redox conditions in a eutrophic estuary abstract phosphorus p is often a limiting nutrient within freshwater and estuarine systems thus excess inputs of p from anthropogenic activities dominantly agriculture can induce eutrophication in receiving water bodies the sequestration of p within estuarine sediments is controlled by sorption and precipitation processes which are impacted by local redox conditions and burial environment despite the global spread of oxygen depletion in coastal marine systems p burial under reducing conditions remains poorly understood we investigated p cycling in relation to iron fe redox chemistry in sediments from the eutrophic peel harvey estuary in western australia using a combination of porewater analysis sequential chemical p extractions synchrotron based micro scale x ray fluorescence mapping and fe k edge x ray absorption spectroscopy and po43 sorption experiments the sediments represented redox regimes varying from strongly reducing organic rich sediments with little or no reactive fe iii oxyhydr oxides to oxygenated sediments that were enriched in reactive fe iii phases organic p and fe associated p were the main p burial phases and the latter was quantitatively important even in sediments with an overall strongly reducing character we attribute this to adsorption of p onto micro scale fe iii oxyhydroxide enrichments and or fe bearing clay minerals the organic rich sediments showed a strong decline in p contents with depth p was released from organic matter and fe phases but apatite precipitation was apparently inhibited in these sediments despite greater and stronger po43 sorption capacity the oxic sediments contained relatively little p due to a lack of the primary p source in marine sediments organic matter our results provide detailed insight into p burial in dynamic estuarine sediments and show that micro scale spectroscopic analyses greatly advance our understanding of p sequestration processes

space geodetic constraints on glacial isostatic adjustment in fennoscandia analysis of global positioning system gps data demonstrates that ongoing three dimensional crustal deformation in fennoscandia is dominated by glacial isostatic adjustment our comparison of these gps observations with numerical predictions yields an earth model that satis es independent geologic constraints and bounds both the average viscosity in the upper mantle 5 x 10 20 to 1 x 10 21 pascal seconds and the elastic thickness of the lithosphere 90 to 170 kilometers we combined gps derived radial motions with fennoscandian tide gauge records to estimate a regional sea surface rise of 2 1 0 3 mm year furthermore ongoing horizontal tectonic motions greater than 1 mm year are ruled out on the basis of the gps derived three dimensional crustal velocity field

deformation partitioning during folding of banded iron formation abstract domainal electron and optical microscopy and c axis fabric analysis are utilized to document microstructures and the associated deformation partitioning between crystal plastic brittle and fluid assisted deformation mechanisms during folding of proterozoic banded iron formation a hematite quartz calcite multilayer from the quadrilatero ferrifero granite greenstone terrain southeastern brazil the operation of different mechanisms was partially determined by the contrasting rheologic response of these three minerals at greenschist facies metamorphic conditions fracturing was the main deformation process in hematite while solution transfer accounted for part of the deformation in both calcite and quartz notably there is strong interaction among the different deformation mechanisms which have influenced each other in various ways fracturing of hematite in hinge zones of folds caused opening of gaps which were sealed by direct precipitation of silica from the fluid phase as a consequence a chemical potential gradient between crystals and fluid phase was produced and quartz was dissolved to restore the thermodynamical crystal fluid equilibrium thus brittle deformation of hematite partially controlled solution precipitation creep in quartz heterogeneous access of fluid into the deforming medium also affected the deformation processes inhomogeneous deformation in the quartz calcite aggregates generated intergranular porosity and increased fluid access with solution transfer becoming dominant in these domains in contrast the relatively more homogeneous deformation in the pure quartz aggregates served to maintain well adjusted grain boundaries and reduced fluid access into the intergranular space such that these domains deformed uniformly by crystal plastic processes at relatively dry conditions

a reappraisal of postglacial decay times from richmond gulf and james bay canada decay times inferred from relative sea level rsl histories of previously glaciated regions provide a potentially important constraint on mantle rheology we present a new compilation of rsl data from richmond gulf and james bay canada this recompilation reveals errors in previous compilations that led to inaccurate estimates for the richmond gulf decay time in a series of recently published articles we derive updated estimates for the decay time at richmond gulf and james bay using a methodology that incorporates errors in both the age and the height of the sea level markers this exercise is guided by a series of synthetic rsl calculations that show that decay time estimates in the region can be significantly biased if the rsl time series are not corrected for global eustatic sea level trends or if the estimates are based on composite rsl histories derived by combining data from both the richmond gulf and the james bay regions our decay time analysis for richmond gulf applies the pioneering approach of walcott 1980 to a large database and we derive a value of 4 0 6 6 kyr where the range is defined by a misfit tolerance 10 per cent higher than the minimum our analysis for james bay is based on the uplift curve derived by hardy 1976 and we estimate a decay time of about 2 0 2 8 kyr the difference between our estimates for richmond gulf and james bay may be due to errors in the observational record from these regions but could also be influenced by lateral variations in lithospheric structure associated with the assembly of laurentia

differential loading by prograding sedimentary wedges on continental margins an arch forming mechanism 1 rapid deposition of prograding sedimentary wedges on continental margins will cause excess pore pressure fluid flow and compaction as they load the substratum they may also cause faulting and structural deformation in the sedimentary succession below and in front of the load large depositional units are in addition associated with pronounced isostatic subsidence the magnitude and effects each component has on the basin as a whole is often debated to address this problem we use a quantitative approach with a coupled hydromechanical mathematical model based on linear isotropic elasticity the equations are solved by the finite element method fem in the modeling a wedge shaped load emplaced successively over a period of 1 5 myr is used to simulate the progradation of a thick sedimentary wedge results show that the differential load generates excess pressure below the prograding depocenter with lateral fluid flow in front and a tail of draining pore pressure behind the sediments are pushed downward and laterally in front of the wedge and vertical and horizontal compressional and shear stresses are generated below the wedge the formation of the helland hansen arch located in the voring basin off mid norway is genetically associated with differential loading our results indicate that the whole eastern flank of the arch is a result of differential loading whereas the western flank is mainly related to thermal subsidence since paleogene times thus alternative explanations like intraplate stress seem to be irrelevant as a genetic mechanism

sea bottom anoxia in the archipelago sea northern baltic sea implications for phosphorus remineralization at the sediment surface abstract the effects of the hydrophysical environment on oxygen conditions and p remineralization at sediment surface were studied in the archipelago sea northern baltic sea at 94 sites the water column was profiled for conductivity temperature and oxygen and the topmost sediment was collected for p and c fractions and oxy hydroxide fe analysis the near bottom water and sediment parameters as well as visual observations on the vertical colour distribution in the sediment cores were used to statistically classify the sediment surfaces into oxic anoxic and suboxic fluctuating oxygen conditions bottoms the anoxic bottoms occurred in the basins with water depths less than 47 m and the suboxic bottoms were most common in the basins with depths between 20 60 m while the oxic bottoms dominated in the depths over 60 m it was concluded that the preferential development of oxygen deficiency in the shallow basins results from the combined effects of complex topography and seasonal temperature stratification on both vertical and lateral bottom water exchange in the anoxic bottoms c p ratios suggest enhanced release of p compared to the oxic and suboxic bottoms in the suboxic bottoms high inorganic p concentrations and the strong coupling between p and fe suggest active transformation of organic p to the inorganic p phases probably due to the fluctuating oxygen conditions differences in the spatial distribution of the p c and fe were insignificant except for detrital p which decreased seawards

the geomorphometry of endorheic drainage basins implications for interpreting and modelling their evolution endorheic basins are catchments with no hydrological connection with marine environments they cover 20 of the earth s surface and are mostly located in arid regions their drainage networks converge to lakes salt flats or alluvial plains whose dynamics are strongly driven by precipitation evapotranspiration and groundwater discharge among other factors integrated surface drainage and the creation of whole drainage systems typical of open basins are commonly restricted in these regions interestingly the fluvial basin morphology of endorheic basins has not been extensively studied and a variety of quantitative morphological descriptors used in open basins have not been utilized in the geomorphic analysis of endorheic basins the objective of this study is to better understand the basin morphology of endorheic river basins by using well known geomorphological properties and their variations across scales for three basins in northern chile we computed the following descriptors and the corresponding relevant scales the cumulative distribution of contributing area the horizontal shape of the basins i e hack s law normalized euclidean length and sinuosity of the streams slope area relationship horton s ratios and drainage density we detected several properties typically found in open basins but certain features which seem to be unique to closed basins were also identified in particular we found that horizontal and vertical geomorphic features seem to be linked which suggests that an independent treatment of these features may not be appropriate for closed basins similar results were found regardless of the basin area which illustrates the relevant effects of features that are specific to these particular regions it is expected that our findings will improve both the geomorphic assessment of these basins and hydrological modelling of surface water and groundwater copyright c 2013 john wiley sons ltd

dynamic interactions between the gulf of guinea passive margin and the congo river drainage basin 2 isostasy and uplift 1 the gulf of guinea continental margin is paralleled by a coastal relief that isolates the congo drainage basin and can play a subsequent role in interactions between margin deposits and continental denudation migration of depocenters from the ogooe and kwanza rivers to the congo fan in the oligocene low present day sediment delivery or pleistocene submarine channels instabilities are possible evidences for this interaction in order to test this hypothesis we examine successively margin isostasy regional uplift erosional unloading and flexural bulge created by sediment loads satellite free air gravity and two dimensional thermomechanical modeling including depth of necking are used to determine the structure of continental margin and strength conditions for two selected sections across south gabon and congo fan both verify similar conditions for extension geometry depth of necking 10 20 km and strength equivalent elastic thickness of 20 30 km differences in observed gravity are explained mainly by differences in sediment volumes regional uplift during miocene of at least 450 m is attributed for both continent and margin as evidenced by the elevated base level of the congo drainage basin and the uplift of shelf break and coastal plains topography corrected for regional uplift valley erosion below relics surfaces and corresponding erosion unloading has been determined as a proxy of oligocene topography it shows a typical rift shoulder topography isolating congo drainage basin from margin in such conditions small amplitude flexural uplift related to sediment loads can possibly have negative feedbacks for both oligocene and present time

characterizing the transient geomorphic response to base level fall in the northeastern tibetan plateau analysis of hillslope gradient landscape relief and channel steepness in the daxia river basin provides evidence of a transient geomorphic response to base level fall on the northeastern tibetan plateau low gradient channels and gentle hillslopes of the upper watershed are separated from a steeper high relief landscape by a series of convex knickzones along channel longitudinal profiles downstream projection of the relict portions of the profiles implies 800 850 m of incision consistent with geologic and geomorphic records of post 1 7 ma incision in the lower watershed we combine optically stimulated luminescence dating of fluvial terrace deposits to constrain incision rates downstream of knickpoints with catchment averaged 10be concentrations in modern sediment to estimate erosion rates in tributary basins both above and below knickpoints both sources of data imply landscape lowering rates of 300 m ma 1 below the knickpoint and 50 100 m ma 1 above field measurements of channel width n 48 and calculations of bankfull discharge n 9 allow determination of scaling relations among channel hydraulic geometry discharge and contributing area that we employ to estimate the patterns of basal shear stress unit stream power and bed load transport rate throughout the channel network our results imply a clear downstream increase of incision potential this result would appear to be consistent with a detachment limited response to imposed base level fall in which steepening of channels drives an increase in erosion rates in contrast however we do not observe apparent narrowing of channels across the transition from slowly eroding to rapidly eroding portions of the watershed rather the present day channel morphology as well as its scaling of hydraulic geometry imply that the river is primarily adjusted to transport its sediment load and suggest that channel morphology may not always reflect the presence of knickpoints and differences in landscape relief

phosphorus speciation and availability in intertidal sediments of the yangtze estuary china abstract in order to better understand p cycling and bioavailability in the intertidal system of the yangtze estuary both surface 0 5 cm and core 30 cm long sediments were collected and sequentially extracted to analyze the solid phase reservoirs of sedimentary p loosely sorbed p fe bound p authigenic p detrital p and organic p the total sedimentary p in surface and core sediments ranged from 14 58 36 81 mol g 1 and 17 11 24 55 mol g 1 respectively and was dominated by inorganic p the average percentage of each fraction of p in surface sediments followed the sequence detrital p 54 9 fe bound p 23 7 organic p 14 3 authigenic p 6 3 loosely sorbed p 0 8 whereas in core sediments it followed the sequence detrital p 61 7 fe bound p 17 0 authigenic p 13 1 organic p 7 5 loosely sorbed p 0 7 post depositional reorganization of p was observed in both surface and core sediments converting organic p and fe bound p to authigenic p the accumulation rates and burial efficiencies of the total p in the intertidal area ranged from 118 70 904 98 mol cm 2 a 1 and 80 29 88 11 respectively high burial efficiency of the total p is likely related to the high percentage of detrital p and the high sediment accumulation rate in addition the bioavailable p represented a significant proportion of the sedimentary p pool which on average accounted for 37 4 and 25 1 of the total p in surface and core sediments respectively this result indicates that the tidal sediment is a potential internal source of p for this p limiting estuarine ecosystem

landscape and tectonic evolution of bayin river watershed northeastern qaidam basin northern tibetan plateau implications for the role of river morphology in source analysis and low temperature thermochronology

topographic relief driven by variations in surface rock density earth s surface topography is generated by tectonically induced variations in crustal thickness combined with erosion and to a lesser degree by vertical stresses caused by convection in the underlying mantle rock hardness and resistance to erosion are also commonly thought to influence topography because hard rocks such as granites and basalts usually form topographic highs in the landscape here we use analytical and numerical models to simulate the erosion induced isostatic rebound of rocks we find that the isostatic rebound that accompanies erosion causes denser rocks to occupy higher elevations in the landscape thereby creating topographic relief that is proportional to surface rock density differences rather than rock hardness we quantify this effect taking into account the flexural strength of the continental lithosphere we show that in a steady state erosional setting density dependent isostatic rebound can cause the densest rocks to be exhumed at double the rate of surrounding less dense rocks and has a stronger effect than typical rock hardness variations the results imply that denser rock formations should erode faster and therefore be characterized by younger thermochronological ages thermochronological data sets from the kinabalu granite in borneo and the shakhdara alichur gneiss domes in pamir confirm this counter intuitive result our findings imply that lateral variations in surface rock density have significant control on the shaping of the large scale features of earth s surface the high elevation in earth s topography of hard rocks such as granites and basalts was thought to be caused by their inherent resistance to erosion numerical modelling now demonstrates counterintuitively that erosion induced isostatic rebound of rocks which is density dependent causes granites and basalts to occupy high elevations because they are more dense than surrounding rocks

crustal structure of the central qaidam basin imaged by seismic wide angle reflection refraction profiling abstract we present the results of a seismic wide angle reflection refraction profile across the central qaidam basin the largest basin within the qinghai tibetan plateau the 350 km long profile extends from the northern margin of the east kunlun shan to the southern margin of the qilian shan the p and s wave velocity structure and poisson s ratio data provide constraints on composition the crust here consists of a near surface sedimentary layer and a four layered crystalline crust having several significant features 1 the sedimentary fill of the qaidam basin reaches a maximum thickness of 8 km and the basin shape mirrors the uplifted moho 2 within the four layers of the crystalline crust p s wave velocities increase with depth and fall within the following velocity ranges 5 9 6 3 km s 3 45 3 65 km s 6 45 6 55 km s 3 7 km s 6 65 km s 3 8 km s and 6 7 6 9 km s 3 8 3 9 km s respectively 3 low velocity zones with a 3 5 reduction in seismic velocity are detected in the lower half of the crust beneath the qaidam basin and its transition to the qilian shan 4 the crystalline crust is thickest beneath the northern margin of the basin towards the qilian shan 58 62 km and thinnest beneath the center of the basin 52 km variations in crustal thickness are caused most pronouncedly by thickness variations in the lowermost layer of the crust 5 poisson s ratio and p wave velocity values suggest that the qaidam crust has an essentially felsic composition with an intermediate layer at its base based on the crustal structure reported here we suggest that late cenozoic convergence is accommodated by thick skinned tectonic deformation with thickening involving the entire crust across the kunlun qaidam qilian system

iron sulfur phosphorus cycling in the sediments of a shallow coastal bay implications for sediment nutrient release and benthic macroalgal blooms we conducted a study to determine the seasonal relationship between iron sulfur and phosphorus in the upper sediments and pore waters of a shallow intercoastal bay from april 1999 to september 2000 sediment cores were collected from rehoboth bay delaware analyses of the sediments in the upper 4 cm revealed that redox conditions controlled fe s p concentrations in the sediments pore waters and overlying water monthly sampling showed a marked decrease in the reactive solid phase p pool ascorbate leachable fraction asc p and sharp increases in soluble p measured as po in pore waters and overlying waters as the conditions became more reducing through32 4 out the summer months these changes were paralleled by decreases in the amorphous fe iii ascorbate leachable fraction asc fe and total fe iii oxyhydroxide pools dithionite extracted fraction fe iii oxide and increases in solid fes fes2 the release of soluble p from sulfidic sediments to oxygenated overlying waters only occurred during periods of solid fes fes2 production which indicates that fe iii oxides act as a barrier to diffusive p flux during these anoxic conditions the regenerative p appears to induce secondary benthic algal blooms and promotes eutrophication in these inland bays through late summer by the late fall and into early spring sulfide production diminished and oxic conditions were reestablished as indicated by increases in solid amorphous and crystalline fe iii oxides and decreases in fes fes2 concentrations during this period increasing asc fe concentrations correlated with increases in asc p concentrations and decreases in pore water po the seasonal correlations 32 4 between fe s p indicate that fe redox chemistry controls sediment p flux to the overlying water column

cenozoic tectonic evolution of the qaidam basin and its surrounding regions part 3 structural geology sedimentation and regional tectonic reconstruction the qaidam basin is the largest topographic depression inside the tibetan plateau because of its central position understanding the tectonic origin of the qaidam basin has important implications for unraveling the formation mechanism and growth history of the tibetan plateau in order to achieve this goal we analyzed regional seismic reflection profiles across the basin and a series of thickness distribution patterns of cenozoic strata at different time slices the first order structure of the basin is a broad cenozoic synclinorium which has an amplitude ranging from 16 km in the west to 48 in the west to 15 s 1 to 1 3 10 17 s 1 the eastward decrease in upper crustal shortening requires a progressive shift in crustal thickening mechanisms across qaidam basin from dominantly upper crustal shortening in the west to dominantly lower crustal shortening in the east although sedimentation began synchronously at 65 50 ma across the entire basin the initiation ages of the southern and northern basin bounding structures are significantly different deformation started at 65 50 ma in the north and at 29 24 ma in the south this information and the existing inference that the uplift of the eastern kunlun range south of qaidam basin began after 30 20 ma imply that the paleogene 65 24 ma qaidam and hoh xil basins on both sides of the eastern kunlun range may have been parts of a single topographic depression 500 km wide in the north south direction between the qilian shan and fenghuo shan thrust belts in the north and south the development of this large paleogene basin in central tibet and its subsequent destruction and partitioning by the neogene uplift of the eastern kunlun range requires a highly irregular sequence of deformation possibly controlled by preexist ing weakness in the tibetan lithosphere

temporal and spatial patterns of cenozoic deformation across the qaidam basin northern tibetan plateau we conducted a systematic analysis of the temporal and spatial patterns of cenozoic deformation of the northeastern ne and southwestern sw parts of the qaidam basin the largest and thickest cenozoic sedimentary basin within the tibetan plateau by constructing balanced cross sections traversing the whole basin using high resolution seismic reflection data we found that that these two parts deformed synchronously with similar temporal patterns involving nearly constant ne sw contraction before c 2 5 ma and much more intense shortening possibly enhanced by fierce wind erosion since then however the spatial deformation patterns of these two areas differ markedly an eastward weakening trend in the sw part is not observed in the ne part we believe that this difference is due to their different boundary conditions this result places new constraints on the formation mechanisms of the northern tibetan plateau

mesozoic source rocks and petroleum systems of the northeastern qaidam basin northwest china the qaidam basin is a continental petroliferous basin located in northwest china its northern sector is an important hydrocarbon province where one of the earliest oil discoveries in china was made although mesozoic source rocks are understood to be important parts of qaidam petroleum systems the identification and distribution of mesozoic source rocks in the subsurface are poorly understood middle jurassic coal measures and associated shales have long been considered to be the primary mesozoic source rocks for the northeastern qaidam basin without any support from subsurface geochemical and geological data however new data have been gathered from earlier mesozoic sediments that were penetrated by the lengke 1 well a deep scientific well with a total depth of 5200 m 17 060 ft the well was drilled on the lenghu structural belt in 1997 and documented other effective source rocks geochemical analyses indicate that lower jurassic mudstones are good to excellent source rocks for the commercial oil wells in the lenghu area the upper middle jurassic shales are very good source rocks for the commercial oil wells in the yuka area two source rock intervals in the northeastern qaidam basin allow for two petroleum systems to be distinguished the lower jurassic lower jurassic tertiary and middle jurassic upper jurassic the lower jurassic lower jurassic tertiary petroleum system with a geographical extent of 15 000 km2 5800 mi2 and a cumulative amount of source rocks between 200 and 700 m 650 and 2300 ft represents a favorable target for future exploration the middle jurassic upper jurassic petroleum system however may have a lower exploration potential because of a smaller area 1500 km2 580 mi2 and a thinner cumulative amount of source rocks 100 200 m 330 650 ft

crustal structure of hubei province of china from teleseismic receiver functions evidence for lower crust delamination abstract western hubei province is at the southern end of the 3000 km long north south oriented xing anling taihangshan wulingshan topographic step in china which separates high rising plateaus and mountain ranges in the west from low elevation plains in the east we calculated teleseismic p receiver functions of 32 permanent broadband seismic stations in hubei province and estimated crustal thicknesses under them using the h method we also obtained detailed crustal structural images along three profiles using the ccp stacking method the results show an east west crustal thickness increase in the study area from 30 35 km to 45 50 km in less than 20 km of horizontal distance most likely in a step wise fashion the thin crust beneath the nanxiang and jianghan basins in eastern hubei extends into the interior of the wuling uplift and the huangling massif in western hubei the lack of mirror symmetry between the moho and surface topography suggests that part of the mountain ranges in western hubei is either compensated by non airy type isostasy models or is not in isostatic equilibrium but supported by the strength of the lithosphere the brittle or localized ductile deformation in the lower crust uppermost mantle as indicated by the abrupt moho steps seems to be decoupled with brittle deformation in the upper crust the ccp images also reveal an apparent double moho beneath the wudang mts which is interpreted to be due to a partially eclogitized lower crust after the original cratonic mantle lithosphere was replaced by warm and hydrated mantle materials in eastern china in the late mesozoic the moho steps were formed when a segment of eclogitized lower crust became gravitationally unstable and foundered into the mantle

the role of adsorption in sediment water exchange of phosphate in north sea continental margin sediments the effect of adsorption on the sediment water exchange of po was investj gated in sediments from four different types of sedimentary environments in the southern and eastern north sea in august 1991 and february 1992 nonlinear adsorption isotherms for oxidized sediment from eight stations indicate that north sea sediments differ widely in their capacity to adsorb po a good correlation between the value of the adsorption coefficient and nh oxalate extractable fe was observed a combination of the adsorption data with porewater po profiles solid phase results and measured and calculated rates of sediment water exchange of po for 15 stations in both august and february indicates that adsorption plays an important role in controlling sediment water exchange of po during at least a part of the year in three of the four north sea environments at most stations po adsorption constrains the flux of po to the overlying water at one station in the depositional environment of the skagcrrak however desorption is responsible for the maintenance of a flux of po to the overlying water a one dimensional reactiondiffusion model describing porewater po and solid phase p profiles was de deloped and applied to results for two stations the model results show that both enhanced retention and enhanced release of po can be adequately described when simultaneous equilibrium and first order kinetic reversible adsorptive reactions are assumed

flexural response of a continental margin to sedimentary loading and lithospheric rupturing the mountain ridge between the levant basin and the dead sea transform during the past 37 myr the levant basin subsided under a 6 km sedimentary section while the adjacent area israel s mountainous backbone judea hills emerged to an elevation of 1 km above sea level what was the flexural contribution of offshore loading to the inland uplift that was also affected at that time by widespread geodynamic heating and uplifting this question becomes particularly challenging considering that in the middle of this period the lithosphere adjacent to the basin 100 200 km east ruptured and a new plate boundary was formed along the dead sea transform dst how did this influence the strength of the lithosphere and how did it change the shape of the flexure we model the flexure caused by offshore sedimentary loading in two phases the pre dst phase applies a flexural solution for an elastic plate without lithospheric rupturing infinite boundaries the post dst phase applies a flexural solution of an elastic plate with a free edge set to resemble a lithosphere with no friction and free to move vertically the apparently unrealistic free edge simplification for a plate boundary provides an upper limit for the uplift it is shown that before the initiation of the dst the flexural effect of the offshore loading on the inland region was negligible and thus most of the observed inland uplift should be explained in terms of deep seated tectonic processes on the contrary the last phase of uplift postdating the formation of the dst was influenced by flexural loading offshore that was intensified near the plate edge

the diagenesis of phosphorus in a nearshore marine sediment abstract chemical analyses have been made for total inorganic phosphorus total organic phosphorus and organic carbon in plankton and in anoxic sediments from long island sound u s a when combined with laboratory experiments the measurements of other studies on the same sediments and mathematical modelling the results indicate that 1 1 a large proportion of dissolved phosphate in the upper 10 cm of sediment zone of bioturbation is provided by the release of adsorbed phosphate during the reduction of ferric oxyhydroxides 2 2 below the zone of bioturbation phosphate is liberated to solution solely via organic matter decomposition and the release is stoichiometrically coupled to bacterial sulfate reduction and ammonia formation 3 3 a steady state diagenetic model can be used to predict the profile of organic phosphorus from that of dissolved phosphate at depths below the zone of bioturbation 4 4 the flux of dissolved phosphate out of the sediment must arise largely from phosphate liberation occurring very close 5 5 there is a large preferential loss of phosphorus relative to carbon from organic matter at the time of or prior to burial whereas after burial phosphorus is not lost preferentially at this location

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

autogenic incision and terrace formation resulting from abrupt late glacial base level fall lower chippewa river wisconsin usa abstract a paucity of research exists regarding the millennial scale response of inland alluvial streams to abrupt base level fall studies of modern systems indicate that over short time scales the response is a diffusion like process of upstream propagating incision in contrast evidence from the lower chippewa river lcr located in the upper midwest of the usa suggests that autogenic controls operating over time scales of several millennia can overwhelm diffusion resulting in incision that is prolonged and episodic during the last glacial maximum the lcr drained the chippewa lobe of the laurentide ice sheet to the glacial upper mississippi river umr as a meltwater stream it aggraded and filled its valley with glacial outwash as did its largest tributaries which were also meltwater streams its nonglacial tributaries aggraded too filling their valleys with locally derived sediment during deglaciation the umr incised at least twice abruptly lowering the lcr s base level 15 m at 16 ka or earlier and an additional 40 m at ca 13 4 ka each of these base level falls initiated incision of the lcr led by upstream migrating knickpoints the propagation of incision has however been a lengthy process the optically stimulated luminescence osl ages of terrace alluvium indicate that by 13 5 ka incision had advanced up the lcr only 15 km and by 9 ka only 55 km the process has also been episodic resulting in the formation of fill cut terraces inferred from gpr surveys and exposures of terrace alluvium that are younger and more numerous in the upstream direction autogenic increases in sediment load and autogenic bed armoring the result of periodic tributary stream rejuvenation and preferential winnowing of fines by the incising river may have periodically caused knickpoint migration and incision to slow and possibly stop allowing lateral erosion and floodplain formation to dominate a decline in sediment flux from stabilizing incised tributary stream systems would have led to renewed knickpoint migration and incision when floods of sufficient magnitude to breach the channel armor occurred minimal floodplain development along the upper section of the present day lcr along with the channel morphology of an unstable wandering gravel bed river immediately downstream from it suggest that the river is still responding to the base level falls that happened many millennia ago the autogenic controls on the lcr s response to umr incision are a direct consequence of the thick fills of noncohesive sediment that accumulated in its valley and the valleys of its tributary streams during the late wisconsinan making the lcr a prime example of a former proglacial river that remains a paraglacial fluvial system

phosphorus speciation in the sediment and mass balance for the central region of the great barrier reef continental shelf australia abstract solid phase p speciation has been determined in sediments from a transect across the central section of the continental shelf and slope of the great barrier reef gbr lagoon this region is characterized by a gradient of riverine aluminosilicate clay and silt nearshore seawards of which biogenic carbonate sediment predominates phosphorus speciation results show large variations along this transect organic p and authigenic apatite p are the major chemical forms of phosphorus in the central gbr continental shelf sediments post depositional reorganization of p was also observed converting organic p and iron bound p fe p to authigenic apatite p phosphorus burial rate was estimated from measurements of total p concentration and excess 210pb sediment mass accumulation rates burial efficiency varies significantly over the shelf inshore areas showed significant p remobilization from sediments to the water column up to 50 the mid and the outer shelf showed little evidence for remobilization except for coral reef platform sediments with more of the sediment p being in the less reactive authigenic apatite phases an appreciable fraction of this non labile authigenic apatite phase was identified as fish bone p sources and sinks over the central part of the gbr shelf were quantified using a mass balance approach this showed that coral sea shelf edge upwelling events are essential to satisfy the large p nutrient demand of the whole gbr lagoon p inputs due to upwelling events were greater than those contributed by local rivers over an average year

breakdown of feldspar volume gain and lateral mass transfer during mylonitization of granitoid in a low metamorphic grade shear zone abstract in a low metamorphic grade shear zone from quadrilatero ferrifero southeastern brazil granitoid rocks have been transformed into phyllonites and mylonites via activation of crystal plastic processes in quartz and fluid assisted reaction softening in feldspars quartz deformed by basal a and prism a slip with concurrent subgrain rotation recrystallization breakdown of matrix plagioclase an 22 35 via mica producing softening reactions required cationic exchanges between ca na and k forming replacement perthites in the k feldspar megacrysts which occupy around 60 of the original rock volume subsequent breakdown of the perthitic plagioclase an 12 15 led to disruption of the megacrysts enabling strain accommodation that localized the mylonite and phyllonite zones domainal volume and mass balance calculations based on major and minor elements indicate only small positive changes in mass and volume around 15 in the whole system however accentuated differences ranging from mass and volume losses of 25 and 35 at the margins to gains of 85 and 95 in the centre respectively exist between the individual longitudinal subzones occupied by the different tectonite types this scenario indicates that deformation proceeded as a nearly isochemical process although with lateral mobility of major elements principally si al and k within the shear zone these results are different from those previously reported for other granitic mylonites most of them indicating volume losses in compressional tectonic settings it is suggested that the extensional tectonic framework attributed to the moeda bonfim shear zone where mass volume losses are in principle not required enhanced the access of fluid and consequent transport of components into the shear zone principally fe si and k producing the small gains of mass and volume observed

the flexural slip mechanism abstract detailed field studies of turbiditic sequences from south georgia south atlantic north devon england and cardigan bay wales show that flexural slip occurs on discrete movement horizons between rock packets in which the beds have welded contacts stair stepping displacements of sedimentary dykes and of early quartz veins show that the movement horizons generally have a decimetre to metre spacing and are marked by bedding parallel quartz veins these veins are from 1 mm to several cm thick and can be used to identify movement horizons in the absence of displaced markers they consist of several sheets of quartz fibres which each carry a slickenfibre lineation and which together preserve a record of the displacements on an individual surface complex slickenfibre patterns and departures from ideal behaviour in which slip occurs orthogonal to the fold hinge probably result from changes in slip vector on the limbs of growing non cylindrical folds movement horizons show many of the features associated with large scale thrusting such as ramps duplexes and imbricate structures and the shear sense given by fibre steps on these surfaces and by duplexes within stratigraphically restricted packages changes across fold hinges chevron folds are thought to develop mainly by flexural flow in the early stages with flexural slip becoming dominant later as the beds become lithified new slip surfaces are generated as the dip of the fold limbs increases

interpreting the stratigraphic architecture of fluvial systems in internally drained basins models for fluvial architecture tend to focus on the effects of sea level oscillation and syndepositional tectonics in creating and removing accommodation space and determining the relative distribution of channel and overbank deposits in the stratigraphic record however in basins of internal drainage sea level change does not exert any influence and in settings where basin centre lakes are very shallow fluctuations in lake level have little impact on the fluvial system if there is variation in tectonic subsidence across the area this will affect depositional patterns but otherwise subsidence does not play a role in affecting sediment distribution in an endorheic setting the luna and huesca systems were deposited in the endorheic ebro basin at a time when deformation in the adjacent pyrenees had largely ceased hundreds of metres of fluvial and lacustrine sediment accumulated raising the level of the basin centre by stratigraphic aggradation and back filling of hinterland valleys during aggradation the systems appear to have maintained a constant slope across the fluvial profile the resulting stratigraphic architecture is well displayed in the ebro basin and provides a case study of architectural patterns in endorheic settings which is in marked contrast to those described from fluvial successions formed in externally draining systems

a correction for sediment load upon the ocean floor uniform versus varying sediment density estimations implications for isostatic correction abstract the isostatic correction for the sediment load on oceanic basement has often been calculated using a uniform sediment density irrespective of either water depth or total sediment thickness an alternative method is proposed here in which the sediment density is calculated using the density depth equations of hamilton 1976 for calcareous clay and terrigenous sediment sequences this alternative method takes into account the water depth and total sediment thickness when estimating the sediment density for example deep water 4000 m sediment sequences are likely to be clay rich thick sediment sequences 2000 m usually have a high terrigenous component and shallow sequences are likely to be calcareous rich sediment thicknesses in excess of 1500 m are generally close to a continental margin these density estimations are compared with measurements from 9 ocean drilling program drill sites 1 deep sea drilling project drill site and from densities derived from seismic interval velocities the most commonly used estimates of uniform sediment density were used to calculate isostatic corrections for a 12 km thick sediment sequence these results were then compared with the results determined using the proposed alternative method of estimating sediment densities the proposed method produces isostatic corrections similar to those derived by published methods for sediment sequences

simultaneous measurement of o2 mn fe i and s ii in marine pore waters with a solid state voltammetric microelectrode a solid state au hg voltammetric microelectrode was used to measure simultaneously and with millimeter spatial resolution the vertical distributions of o 2 mn 2 fe 2 hs and i in the pore water of sediments from the canadian continental shelf and slope the electrode was used shipboard to analyze undisturbed sediment cores and in a sediment mesocosm to determine the three dimensional distribution of redox species in the sediment surrounding an actively irrigated worm burrow in cores from emerald basin and cabot strait o 2 disappeared and mn 2 fe 2 and hs appeared in the vertical sequence predicted on the basis of thermodynamics in a core from the scotian slope o 2 disappeared at 14 mm depth but mn 2 was not detected over the 50 mm depth examined with the probe in all three cores i was detected in the pore water below the oxygen penetration depth in none of the cores did the distributions of o 2 and mn 2 overlap the thickness of the layer within which neither o 2 nor mn 24 could be detected ranged from 8 to 40 mm we suggest that nitrate not oxygen is used to oxidize mn 2 the upward directed mn 2 gradients of 3 5 and 10 mol m 4 in the emerald basin and cabot strait sediments respectively drive manganese fluxes that are morc than one order of magnitude lower than the o 2 fluxes which are estimated at 4 5 mmol m 2 d 1 the three dimensional distributions of solutes in the pore water in the sediment surrounding an irrigated worm burrow demonstrate that both the o 2 penetration depth and the depth where mn 2 is first detected are deeper in the vicinity of the worm burrow than away from it this is consistent with the notion that irrigation brings oxygenated water into the worm tube allowing oxygen to diffuse across the burrow wall into the sediment and react with mn 2 and other reduced porewater constituents three dimensional heterogeneity in porewater composition cannot be detected by conventional one dimensional coring and slicing techniques because these latter techniques average the porewater composition in a large volume of sediment they may indicate that the distributions of individual species overlap when in fact they do not

quantifying the post tectonic topographic evolution of closed basins the ebro basin northeast iberia closed endorheic sedimentary basins are key recorders of the climatic erosional and tectonic history of their surrounding topography playing an active role in its evolution by changing the local geomorphological base level when these basins become exorheic the accelerated incision along the new fluvial network can excavate excellent stratigraphic outcrops but this often removes the uppermost infill and essential information about the late basin history is lost here we propose estimating the opening age and past elevation of captured closed basins by combining the flexural isostatic compensation of the eroded volume with available constraints on sediment age we use this method to constrain the post tectonic evolution of the cenozoic ebro basin in northeast iberia the similar results obtained for 4 dated stratigraphic columns show the robustness of the model and date the basin opening as 12 0 7 5 ma with a maximum paleoelevation of the basin of 535 750 m the isostatic rebound associated with basin erosion as much as 630 m in the center of the basin may explain the absence of a canyon excavated by the ebro river during the mediterranean sea level fall associated with the messinian salinity crisis

iron limitation in the western interior seaway during the late cretaceous oae 3 and its role in phosphorus recycling and enhanced organic matter preservation the sedimentary record of the coniacian santonian oceanic anoxic event 3 oae 3 in the north american western interior seaway is characterized by a prolonged period of enhanced organic carbon oc burial this study investigates the role of fe in enhancing organic matter preservation and maintaining elevated primary productivity to sustain black shale deposition within the coniacian santonian aged niobrara formation in the usgs 1 portland core iron speciation results indicate the development of a reactive fe limitation coeval with reduced bioturbation and increased organic matter preservation suggesting that decreased sulfide buffering by reactive fe may have promoted enhanced organic matter preservation at the onset of oae 3 an fe limitation would also provide a feedback mechanism to sustain elevated primary productivity through enhanced phosphorus recycling additionally our results demonstrate inconsistencies between fe based and trace metal redox reconstructions iron indices from the portland core indicate a single stepwise change whereas the trace metal redox proxies indicate fluctuating redox conditions during and after oae 3 using fe speciation to reconstruct past redox conditions may be complicated by a number of factors including fe sequestration in diagenetic carbonate phases and efficient sedimentary pyrite formation in a system with limited fe supply and high levels of export production

quaternary erosion induced isostatic rebound in the western alps the high elevation and deep incision of the alps have traditionally been used as an argument for recent tectonic activity that has elevated the belt and increased erosion rates normal faulting and horizontal extension however dominate current tectonic activity and isostatic compensation of thinning crust should lead not to increased but to decreased mean elevations here we test the idea that enhanced quaternary erosion of the alps and isostatic compensation of the mass removed can account for the distribution of present day geodetically measured rates of vertical movement in the western alps using geophysical relief and kuhlemann s estimated average erosion rate for the alps we quantify the spatial distribution of erosion and the volume of eroded rock respectively from these we obtain a map of rock eroded within a given time span the calculated isostatic response of the alpine lithosphere to erosional unloading for a variety of values of the flexural rigidity of the alpine lithosphere reaches a maximum of 500 m since 1 ma in the inner swiss alps and vertical movement extends across the entire belt including peri alpine basins assuming a steady erosion rate since 1 ma this rebound accounts for half of the measured vertical motion of 1 1 mm yr in the southern valais thus a significant fraction 50 of the present day vertical movement results from the isostatic response to enhanced erosion during plio quaternary time

the role of base level watershed attribute and sediment accumulation in the landscape and tectonic evolution of the circum tibetan plateau basin and orogen system abstract the india asia collision produces the outstanding circum tibetan plateau basin and orogen system in this study we systemically analyze and discuss the relationship between isostasy base level and sedimentary process in the landscape and tectonic evolution of the circum tibetan plateau basin and orogen system considering of isostatic effects the basin filling processes can individually lead to the subsidence of the sedimentary floor bringing numerous geological implications on the conditions of ideal geological models and isostatic equilibrium the sedimentary surface of the exorheic basins with strong hydrodynamic condition is constrained by sea level through the river longitudinal profile and crustal thinning processes of the non flexural exorheic basins can be sculpted by sedimentary records when the source supply keeps sufficient long term and large scale rising and dropping of the sea level can influence the subsidence rate of the extensional exorheic basins on the other hand the endorheic basins are of individual base levels and isostatic effects of the sedimentary loading can lead to the uplift of the sedimentary surface in this situation the uplift process of the endorheic basins can be estimated by sedimentary records regional reorganization of river watersheds can lead to a series of geological impacts such as the changing of base level the redistribution of loose deposits beginning or end of depositional discontinuity and fluctuation of the sedimentary floor thus the geological implications for the changing of the sedimentary rates and the depositional discontinuity should be carefully identified in the landscape and tectonic evolution of the circum tibetan plateau basin and orogen system

flexural deformation due to amazon fan loading a feedback mechanism affecting sediment delivery to margins the uplift associated with the peripheral bulge engendered by sediment loads such as the amazon fan may provide a potential tectonic feedback mechanism that affects both coastal and fluvial depositional processes for example topographic and geologic maps of the brazilian margin document the existence of a drainage divide affecting coastal river networks that is spatially coincident with the location of the peripheral bulge predicted by our flexural modeling of the fan load while regions in close proximity to the fan load are depressed regions farther from the load are lifted i e peripheral bulge by 25 50 m which is sufficient to cause subaerial exposure of large portions of the shelf and modification of existing fluvial drainage networks furthermore our modeling results suggest that the development of the peripheral bulge may have deflected the sediment laden waters of the amazon river progressively southward through time this prediction is consistent with the existence of canyons off the amapa coast north of the present day amazon canyon and with the distribution of amazon fan sediments both of which suggest that the proto amazon river delivered sediment farther north along the coast however changing the location of sediment input into a basin also changes the shape of the cumulative sediment load and thus the resultant deformation this feedback between sediment loading and the peripheral bulge uplift modifies the spatial and temporal history of sediment delivery to the margin and provides a viable way to change base level without having to invoke glacial eustasy the efficiency of this feedback is dependent on the flexural wavelength and thus the strength of the lithosphere at the time of loading and the relative position of the sediment load with respect to the continental margin

cable bacteria generate a firewall against euxinia in seasonally hypoxic basins seasonal oxygen depletion hypoxia in coastal bottom waters can lead to the release and persistence of free sulfide euxinia which is highly detrimental to marine life although coastal hypoxia is relatively common reports of euxinia are less frequent which suggests that certain environmental controls can delay the onset of euxinia however these controls and their prevalence are poorly understood here we present field observations from a seasonally hypoxic marine basin grevelingen the netherlands which suggest that the activity of cable bacteria a recently discovered group of sulfur oxidizing microorganisms inducing long distance electron transport can delay the onset of euxinia in coastal waters our results reveal a remarkable seasonal succession of sulfur cycling pathways which was observed over multiple years cable bacteria dominate the sediment geochemistry in winter whereas after the summer hypoxia beggiatoaceae mats colonize the sediment the specific electrogenic metabolism of cable bacteria generates a large buffer of sedimentary iron oxides before the onset of summer hypoxia which captures free sulfide in the surface sediment thus likely preventing the development of bottom water euxinia as cable bacteria are present in many seasonally hypoxic systems this euxinia preventing firewall mechanism could be widely active and may explain why euxinia is relatively infrequently observed in the coastal ocean

the tectonic evolution of the quadrilatero ferrifero minas gerais brazil abstract the structural framework of the quadrilatero ferrifero is the result of two main deformational events the first is an extensional event of palaeoproterozoic age between 2 100 and 1700 ma the transamazonian event which resulted in the formation of archaean granite gneiss domes and the nucleation of regional synclines in the overlying rio das velhas and minas supergroups strata such regional synclines are continuous with each other and are bordered by structural granite gneiss highs at the contact between these units ductile brittle to ductile extensional shear zones are developed representing the dislocation surfaces of archaean blocks this tectonism is probably related to the evolution of a metamorphic core complex formed in the hinterland to the west of a transamazonian collision zone which structured the paramirim craton the second event is compressive and associated with the closure of the pan african brasiliano proto ocean 650 500 ma situated to the east of quadrilatero ferrffero qf during this event a west verging fold and thrust belt ftb developed causing inversion amplification translation and rotation of the basinal synclines the ftb affected mainly the eastern portion of qf and obliterated many of the tectonic features of the extensional event

normal faulting driven by denudational isostatic rebound fluvial piracy in the guadix baza basin southeast spain promoted erosion of a high volume of sediments during the late quaternary after the former internal drainage of this basin changed to external due to headward erosion by the guadalquivir river as a response to load release this basin underwent uplift which in turn enhanced fluvial incision and erosional processes differential erosion within the guadix baza basin resulted in dissimilar isostatic rebound we numerically model the lithospheric response to sediment unloading and show a total rising of 15 m in the guadix subbasin compared to 2 m in the baza subbasin this differential uplift is likely to have been accommodated along the baza normal fault which has been active throughout the quaternary modeling results suggest that 13 of the total quaternary fault throw directly arises from the differential isostatic readjustment between subbasins this example is one of the first estimates of fault slip partitioning between tectonic and isostatic effects due to unloading driven by river incision

accelerated subsidence and sedimentation in the levant basin during the late tertiary and concurrent uplift of the arabian platform tectonic versus counteracting sedimentary loading effects 1 since the middle eocene the northwest arabian platform has been emerging from the water and rising above sea level whereas the adjacent levant basin has been subsiding and accumulating a thick sedimentary section this study investigates these opposing vertical motions and the relative roles of tectonic driven versus isostatic adjustment forces involved such a distinction is strongly dependent on a reliable estimation of the paleobathymetry which in pelagic environments may vary substantially we use two different methods to estimate the paleo water depth in the deep levant basin in the tertiary time isostatic balancing calculations with the adjacent inland region are employed to estimate the paleo water depth in the deep levant basin in the middle eocene just before the commencement of the late tertiary tectonic phase for later periods we use morpho structural elements such as abrasion surfaces and incised canyons to build laterally changing topo bathymetry profiles our results indicate that in the mid eocene water depth in the deep levant basin was 2 3 5 km which gradually decreased to 1 5 km today based on these results our analysis shows that the enhanced subsidence of the levant basin reflects an isostatic response to sedimentary filling of a pre existing deep water basin with no involvement of a downward tectonic force on the contrary we suggest that the regional tectonic force was upward counteracting a sedimentary loading effect further regional implication of this understanding is that the cause for uplifting and exposure of the nw arabian platform in the late tertiary extended far westward beyond the inland region

benthic iron and phosphorus fluxes across the peruvian oxygen minimum zone benthic fluxes of dissolved ferrous iron fe2 and phosphate tpo4 were quantified by in situ benthic chamber incubations and pore water profiles along a depth transect 11 s 80 1000 m across the peruvian oxygen minimum zone omz bottom water o2 levels were 2 mol l 1 down to 500 m water depth and increased to 40 mol l 1 at 1000 m fe2 fluxes were highest on the shallow shelf maximum 316 mmol m 2 yr 1 moderate 15 4 mmol m 2 yr 1 between 250 m and 600 m and negligible at deeper stations in the persistent omz core continuous reduction of fe oxyhydroxides results in depletion of sedimentary fe al ratios tpo4 fluxes were high maximum 292 mmol m 2 yr 1 throughout the shelf and the omz core in association with high organic carbon degradation rates ratios between organic carbon degradation and tpo4 flux indicate excess release of p over c when compared to redfield stoichiometry most likely this is caused by preferential p release from organic matter dissolution of fish debris and or p release from microbial mat communities while fe oxyhydroxides can only be inferred as a major p source on the shallow shelf the benthic fluxes presented here are among the highest reported from similar oxygen depleted environments and highlight the importance of sediments underlying anoxic water bodies as nutrient sources to the ocean the shelf is particularly important as the periodic passage of coastal trapped waves and associated bottom water oxygenation events can be expected to induce a transient biogeochemical environment with highly variable release of fe2 and tpo4

high resolution magnetostratigraphic study of the paleogene neogene strata in the northern qaidam basin implications for the growth of the northeastern tibetan plateau abstract the cenozoic terrestrial intermontane qaidam basin on the northeastern edge of the tibetan plateau contains 12 km of sedimentary rocks that potentially document the accommodation of india asia convergence and the growth of the plateau the chronology remains incomplete hindering cross basin correlation between lithostratigraphic units and their further interpretation here we present a high resolution magnetostratigraphy spanning 5 km of paleogene neogene sequence at dahonggou in the northern qaidam basin based on correlation with the geomagnetic polarity time scale gpts we have dated the section to being between 52 and 7 ma the bottom conglomeratic unit ranging from 52 ma to 44 ma was deposited in high energy environments e g alluvial fan or braided river reflecting the earliest deformation and uplift of the basin bounding qilian shan fold thrust belt in response to india asia collision in addition we identified two major increases in sedimentation rate at 25 16 ma and after 9 5 ma and three phases of lesser increases at 52 44 ma 38 33 ma and 14 6 12 0 ma these increases in sedimentation rate are consistent with regional thermochronology and basin analysis studies which revealed enhanced motion on basin bounding thrust faults we argue that these accelerated sedimentation rates indicate pulsed tectonism in the northeastern tibetan plateau the pulse at 25 16 ma may further relate to phases of strong rainfall linked to an intense monsoon at that time

deformation mechanisms in the mylonite ultramylonite transition abstract the deformation mechanisms and controls that operate in the mylonite ultramylonite transition are interpreted from microstructural observation the investigated mylonites and ultramylonites were derived from a granitic protolith which was deformed under greenschist facies conditions and in the presence of fluid in a regional scale shear zone from northwest argentina several deformation mechanisms were recognized to operate simultaneously in different domains of the microstructure at each particular stage of the microstructural evolution this continuously mobile deformation partitioning present throughout the microstructural evolution ceases abruptly in the ultramylonite stage where a stable state microstructure is achieved domainal quartz c axis fabrics indicate that quartz deforms by crystal plastic processes at the initial and intermediate stages of deformation but solution transfer processes become predominant in the ultramylonite stage plagioclase is progressively transformed into muscovite through retrograde softening reactions k feldspar is progressively transformed into fine grade aggregates via cataclastic flow and incipient recrystallization mica deforms by kinking and basal slip with progressive development of fine grained morphologically oriented aggregates plagioclase disappearance as well as the development of intrafolial microfolds characterize the transition between the mylonitic and ultramylonitic domains disruption of these microfolds is interpreted to represent the ultimate control on the localization of the ultramylonite bands

reactive iron in marine sediments abstract a combined field laboratory study has been undertaken to explore the mineralogy concentrations and reactivity towards sulfide of iron in marine sediments also considered is the importance of bacterial fe liberation in regulating pore water chemistry two contrasting marine environments are included one is the relatively fe poor foam site and the other is the fe rich sediment of the subaqueous mississippi delta results show that oxide minerals are the most important fe phases in early diagenetic pyrite formation however viewed separately lepidocrocite and ferrihydrite are more reactive towards sulfide than goethite and hematite when fe oxides are present in relatively high concentrations dissolved sulfide is nearly absent from sediment pore waters with concomitant high concentrations of dissolved fe even in the presence of active sulfide production by sulfate reduction a combination of experimental results and diagenetic modelling shows that in some sediments pore water fe finds it origin in the bacterial reduction of iron oxides this seems the case even though greater amounts of fe are reduced by reaction of sulfide with iron oxides it appears that distinct microenvironments may exist in marine sediments where in one microenvironment sulfide reacts with fe oxides locally precipitating fe sulfide minerals in another fe reduced and solubilized by microorganisms migrates freely into solution

tibetan plateau river incision inhibited by glacial stabilization of the tsangpo gorge much attention has focused on how and when the tibetan plateau formed but far less research has been done on the controls on river incision into the plateau itself oliver korup and david montgomery address this issue which also relates to the question of why the tibetan plateau is still there at all despite hosting one of the deepest gorges on earth the plateau edge is extremely well defined and according to widely accepted stream power theory should be heavily dissected by one of asia s most powerful rivers the yarlung tsangp or yarlung zangbo korup and montgomery propose that the southeastern edge of the plateau was preserved in part by the presence of glaciers that were large enough during the holocene to stall aggressive river downcutting along the plateau margin a considerable amount of research has focused on how and when the tibetan plateau formed in the wake of tectonic convergence between india and asia1 although far less enquiry has addressed the controls on river incision into the plateau itself2 widely accepted theory3 predicts that steep fluvial knick points river reaches with very steep gradients in the eastern himalayan syntaxis at the southeastern plateau margin should erode rapidly4 driving a wave of incision back into the plateau preservation of the plateau edge thus presents something of a conundrum that may be resolved by invoking either differential rock uplift matching erosional decay5 6 7 or other mechanisms for retarding bedrock river incision8 9 in this region where high stream power excludes the potential for aridity as a simple limit to dissection of the plateau10 here we report morphologic evidence showing that quaternary depression of the regional equilibrium line altitude where long term glacier mass gain equals mass loss was sufficient to repeatedly form moraine dams on major rivers such damming substantially impeded river incision into the southeastern edge of the tibetan plateau through the coupled effects of upstream impoundment and interglacial aggradation such glacial stabilization of the resulting highly focused river incision centred on the tsangpo gorge could further contribute to initiating and accentuating a locus of rapid exhumation known as tectonic anaeurysm6

speciation and spatial distribution of solid phase iron in surface sediments of the east china sea continental shelf abstract speciation and reactivity characterization of solid phase fe in marine sediments are of significance to understanding its heterogeneous mineralogy and crystallinity the diagenetic cycling of fe and its regulating roles on many other elements in sediments in this study a combination of sequential and single step extractions was used for the determination of seven fe pools in surface sediments of the east china sea ecs continental shelf 1 carbonate associated fe fe ii carb plus acid volatile sulfide fe fe ii avs 2 easily reducible amorphous poorly crystalline fe oxides fe ox1 3 reducible crystalline fe oxides fe ox2 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u total fe fe t in the sediments is largely determined by terrestrial aluminosilicate particles as indicated by a great similarity of the fe t with that of the yangtze river and global riverine particulates the size of fe prs is found to be the largest pool followed by fe u fe ox2 fe mag fe ii avs carb fe ox1 and fe py the large fe prs may result from neoformation of fe rich clay minerals via reverse weathering and subsequent ageing the small sizes of fe ii avs carb and fe py pools is believed to be the result of low so 4 reduction due to generally low labile organic matter together with the oxic suboxic dynamic environments of the surface sediments the occurrence of fe ox1 fe ox2 and fe prs in the sediments is closely associated with the clay fraction as indicated by a high spatial correlation between the former and the latter highly reactive fe fe hr in the sediments is comparable to that in global marine sediments but apparently lower than in the yangtze river and global riverine particulates due probably to sequestration in the yangtze estuary the ratios of fe hr fe t fe pr fe t and fe u fe t in the ecs surface sediments consistently show more similarity to those in the yangtze river particulates than in the global continental margin or deep sea sediments the surface sediments maintain a high level of buffering capacity toward sulfidation suggested by a large fraction of highly reactive fe iii oxides fe iii hr in fe hr

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

denan depression controlled by northeast directed olongbulak thrust zone in northeastern qaidam basin implications for growth of northern tibetan plateau abstract the denan depression is a unique depression in the northeastern qaidam basin with a maximum cenozoic sedimentary thickness of 5 km detailed field work interpretation of seismic profiles and analyzation of well data were conducted to define the cenozoic tectonic evolution of the northeastern qaidam basin all geological evidences indicate that the denan depression is controlled by the northeast directed olongbulak thrust at its southern boundary the denan depression grew in concert with the development of the northeast directed olongbulak thrust at least since it began to accept the xiaganchaigou formation supporting the early cenozoic growth of the northern tibetan plateau surface and subsurface data both point to enhanced tectonic activity since the quaternary in the northeastern qaidam basin leading to a more individual denan depression relative to the main qaidam basin the northern boundary of the denan depression is a passive boundary and no foreland developed at the northern slope of the denan depression

two phases of mesozoic north south extension in the eastern altyn tagh range northern tibetan plateau 1 the 300 km long east striking lapeiquan fault lies in the eastern altyn tagh range along the northern margin of the tibetan plateau and was interpreted as a north dipping thrust in early studies however our mapping shows that the fault is a south dipping normal fault juxtaposing archean proterozoic gneisses beneath an early paleozoic volcanic and sedimentary sequence its dip angle varies from 30 to 60 the central fault segment is expressed as a 30 50 m thick ductile shear zone with well developed mylonitic fabrics and stretching mineral lineations where the eastern and western segments are characterized by cataclastic deformation kinematic indicators such as asymmetric boudinage asymmetric folds and minor brittle and ductile faults within the fault zone consistently indicate a top south normal slip sense of shear the age of the lapeiquan fault is constrained by two types of information first a sequence of early middle jurassic sediments is locally present in the hanging wall of the lapeiquan fault the clasts of the jurassic strata particularly the stromatolite bearing cherty limestone and purple quartzite can be correlated uniquely with those in the footwall of the fault we interpret that the early middle jurassic strata were deposited in an extensional basin related to motion along the lapeiquan fault second 40ar 39ar thermochronologic analyses indicate two prominent cooling events in the lapeiquan footwall the older event occurred in the latest triassic earliest jurassic between 220 and 187 ma while the younger event occurred in the latest early cretaceous at 100 ma because the 220 187 ma cooling ages are widespread in the lapeiquan footwall we suggest it to represent the main stage of faulting we interpreted the younger phase of fault motion at 100 ma to have been related to fault reactivation the deformation was aided by motion along the south dipping qiashikan normal fault that merges with the eastern lapeiqaun fault from the regional tectonic setting it appears that mesozoic extension in northern tibet to have occurred in a back arc setting during northward subduction of the tethyan oceanic plate the findings of mesozoic extensional structures in northern tibet suggest that compressive stress induced by collision of the qiangtang and lhasa terranes with asia was not transmitted beyond northern tibet this in turn implies that the popularly inferred contractional setting for mesozoic evolution of the tian shan north of tibet needs a reevaluation based on a combination of both structural and sedimentological observations

speciation bioavailability and preservation of phosphorus in surface sediments of the changjiang estuary and adjacent east china sea inner shelf abstract the speciation potential bioavailability and preservation of phosphorus p in surface sediments of the changjiang yangtze river estuary and adjacent east china sea ecs inner shelf were investigated through the analyses of p fractions and sediment bulk properties a sequential extraction method sedex was used to separate and quantify the following six sedimentary p reservoirs exchangeable p ex p authigenic p au p detrital p de p organic p or p refractory p re p and fe bound p fe p total p tp in surface sediments ranged from 15 0 to 21 4 mol g 1 and was highest near the changjiang river mouth the average contribution of each form of p to tp was 55 6 de p 17 8 re p 16 1 or p 5 5 au p 2 5 ex p and 2 5 fe p respectively de p showed relatively higher concentrations in the river mouth and the ecs shelf region off the changjiang estuary high concentrations of or p were found mainly in mud areas showing a similar distribution pattern with silt sediment surface area ssa and total organic carbon toc re p was mainly distributed near the estuarine area and the zhe min coast bioavailable p bap accounted for 9 5 32 0 of tp with a mean of 21 2 and showed a similar distribution pattern to that of or p de p ssa and toc ssa loadings both decreased with increasing of ssa while or p ssa loadings varied little with ssa indicating that or p may have reached an adsorption desorption equilibrium on mineral surfaces toc to total organic p top sum of re p and or p ratios less than the redfield ratio 84 in average may have indicated efficient remineralization of organic matter in mobile muds of the changjiang estuary and adjacent ecs inner shelf furthermore the relatively high toc or p ratios 72 422 with a mean of 188 likely suggest a higher degree of preferential regeneration of labile or p over toc in sediments

quantifying landscape differences across the tibetan plateau implications for topographic relief evolution 1 we quantify the bulk topographic characteristics of the tibet qinghai plateau with specific focus on three representative regions northern central and southeastern tibet quantitative landscape information is extracted from shuttle radar topography mission digital elevation models we find that the morphology of the tibetan plateau is nonuniform with systematic regional differences the northern and central parts of the plateau are characterized by what we suggest to call positive topography i e a topography in which elevation is positively correlated with relief and mean slope a major change from the internally drained central part of tibet to the externally drained part of eastern tibet is accompanied by a transition from low to high relief and from positive to negative topography i e a topography where there is an inverse or negative correlation between elevation and relief and between elevation and mean slope relief in eastern tibet is largest along rivers as they cross an ancient eroded plateau margin at high angle to the major strike slip faults the yalong yulong thrust belt implying strong structural control of regional topography we propose that the evolution of river systems and drainage efficiency the ability of rivers to transport sediments out of the orogen coupled with tectonic uplift is the simplest mechanism to explain systematic regional differences in tibetan landscapes basin filling due to inefficient drainage played a major role in smoothing out the tectonically generated structural relief this mode of smoothing started concurrently with tectonic construction of the relief as most clearly illustrated today in the qilian shan qaidam region of the northeastern plateau in the interior of tibet further passive filling due to internal drainage only continued to smooth the local relief millions of years after the cessation of major phases of surface uplift due to crustal shortening thus diachronous beveling at high elevation produced the low relief surface of the high plateau in southeast tibet headward retreat of external drainages brought back in the global ocean base level first disrupting then interrupting the relief reduction process it produced a transitional topography by dissecting the old remnant plateau surface which introduced younger and steeper incision of this hitherto preserved high base level this provides a unifying mechanism for the formation of the low relief plateau interior and for the origin of the high elevation low relief relict surface in southeastern tibet our analysis brings forth the importance of surface processes in particular drainage efficiency in shaping plateau morphology and landscape relief such key processes appear to have been mostly ignored in numerical models of plateau deformation our results also cast doubt on and provide a more realistic alternative to the fashionable contention that a continuous preuplift low relief surface first formed at low elevation extending all the way to the south china sea shore before being warped upward in the late miocene pliocene by lower crustal channel flow

simultaneous estimation of global present day water transport and glacial isostatic adjustment global water transport between oceans and continents during the transition from glacial to interglacial times has been enormous the viscoelastic solid earth has been responding to this unloading of large ice masses with a rise of the land masses in a process termed glacial isostatic adjustment in addition significant changes in the land ocean water distribution occur at present as both present day changes in the ice water thickness and glacial isostatic adjustment affect space geodetic measurements it is difficult to untangle the relative contributions of these two processes here we combine gravity measurements and geodetic data of surface movement with a data assimilating model of ocean bottom pressure to simultaneously estimate present day water transport and glacial isostatic adjustment we determine their separate contributions to movements in the geocentre which occur in response to changes in the earth s mass distribution with uncertainties below 0 1 mm yr 1 according to our estimates mass losses between 2002 and 2008 in greenland alaska yukon and west antarctica are 104 23 101 23 and 64 32 gt yr 1 respectively our estimates of glacial isostatic adjustment indicate a large geocentre velocity of 0 72 0 06 mm yr 1 in the polar direction we conclude that a significant revision of the present estimates of glacial isostatic adjustments and land ocean water exchange is required present day changes in the thickness of ice sheets and glacial isostatic adjustment both affect space geodetic measurements a combination of gravity measurements and geodetic data of surface movement with a data assimilating model of ocean bottom pressure allows the simultaneous estimation of present day water transport and glacial isostatic adjustment

crustal strength in central tibet determined from holocene shoreline deflection around siling co abstract controversial end member models for the growth and evolution of the tibetan plateau demand quantitative constraints of the lithospheric rheology direct determinations of bulk crustal rheology however remain relatively sparse here we use the flexural rebound of lacustrine shorelines developed during the lingtong highstand around siling co in central tibet to place bounds on the effective elastic thickness t e and viscosity of tibetan crust shoreline features associated with the lingtong highstand complex 60 m above present lake level are deflected from horizontal by 2 4 m over wavelengths of 200 km optically stimulated luminescence dating of aggradational shoreline deposits indicates that these lake levels were reached at 6 4 ka assuming that surface loads were entirely supported by an elastic layer overlying an inviscid fluid the range and spatial distribution of variations in shoreline elevation are consistent with deflections predicted by a uniform elastic plate with thickness t e of 20 30 km if viscoelastic relaxation in response to lake withdrawal is complete our data suggest an average viscosity 10 19 pa s these results imply that the apparent viscosity of the lower crust inferred over millennial timescales is comparable with that estimated from post seismic relaxation over decadal timescale