tracking exhumation of andean ranges bounding the middle magdalena valley basin colombia the shortening history of the andes is important for understanding retroarc deformation along convergent margins and forcing mechanisms of cenozoic climate however the timing of uplift in the northern andes is poorly constrained with estimates ranging from cretaceous to pliocene detrital zircon u pb ages from the middle magdalena valley basin in colombia reveal two provenance shifts during cenozoic time the fi rst shift occurs between early and late paleocene strata where u pb results show a switch from proterozoic dominated to phanerozoic dominated age spectra we attribute this change to uplift related exhumation of the central cordillera the second shift occurs between middle late eocene and late oligocene strata where increased grenville ages and diminished mesozoic ages can be linked to uplift of the eastern cordillera our results show that signifi cant pre neogene deformation affected the northern andes underscoring the potential importance of andean uplift on the dynamics of paleogene climate

authigenic uranium relationship to oxygen penetration depth and organic carbon rain we have measured u in benthic incubation chambers sediment pore waters and in sediments along the california continental margin sedimentary u uptake rates based on a combination of sediment pore water profiles and benthic incubation chambers generally agree with those predicted from sediment accumulation rate data this agreement supports the view that most of the continental margin sedimentary u is delivered by diffusion across the seawater sediment boundary the average rate of authigenic u accumulation for all the sites examined here is 0 2 nmol cm 2 y 1 which is consistent with published global estimates of sedimentary u uptake in addition the accumulation rate of u in sediments exhibits a nonlinear relationship with the oxygen penetration depth and a linear relationship with the organic carbon rain rate these relationships highlight the potential utility for the u accumulation rate as a proxy for these processes

controls on back arc basin formation the relationship between subduction and back arc spreading has been well known since the early days of plate tectonics however the reasons why back arc basins are associated with some subduction systems but not all has remained elusive we examine the kinematic controls on subduction and back arc basins for both the present day and cenozoic to differentiate between the major competing hypotheses for back arc basin formation and to explain their temporal and spatial distribution our new data set of subduction and back arc basin parameters uses a new set of paleo oceanic age grids muller et al 2005 associated with a moving atlantic indian ocean hot spot reference frame o neill et al 2005 the plate model includes detailed reconstructed spreading histories of back arc basins based on marine geophysical and satellite gravity data our combined rotation and oceanic paleo age model provides the age distribution of subducting lithosphere through space and time convergence rates and the absolute motion of the downgoing and overriding plates we find that back arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years additionally we establish an age dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 with back arc spreading our results suggest that back arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge thereby creating accommodation space between the overriding and subducting plates once back arc extension is established it continues regardless of overriding plate motion indicating back arc spreading is not a simple consequence of overriding plate behaviour the landward migration of the overriding plate as a precursor to back arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle however once back arc extension is established rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space our analysis indicates the driving mechanism for back arc extension is a combination of surface kinematics properties of the downgoing slab the effect of lateral mantle flow on the slab and mantle wedge dynamics

back arc strain in subduction zones statistical observations versus numerical modeling 1 recent statistical analysis by lallemand et al 2008 of subduction zone parameters revealed that the back arc deformation mode depends on the combination between the subducting vsub and upper vup plate velocities no significant strain is recorded in the arc area if plate kinematics verifies vup 0 5 vsub 2 3 cm a in the hs3 reference frame arc spreading shortening occurs if vup is greater lower than the preceding relationship we test this statistical law with numerical models of subduction by applying constant plate velocities far away from the subduction zone the subducting lithosphere is free to deform at all depths we quantify the force applied on the two converging plates to sustain constant surface velocities the simulated rheology combined viscous non newtonian and brittle behaviors and depends on water content the influence of subduction rate vs is first studied for a fixed upper plate after 950 km of convergence steady state slab pull the transition from extensional to compressive stresses in the upper plate occurs for vs 1 4 cm a the effect of upper plate velocity is then tested at constant subduction rate upper plate retreat advance with respect to the trench increases extension compression in the arc lithosphere and increases decreases the subducting plate dip our modeling confirms the statistical kinematic relationship between vsub and vup that describes the transition from extensional to compressive stresses in the arc lithosphere even if the modeled law is shifted toward higher rates of upper plate retreat using our set of physical parameters e g 100 km thick subducting oceanic plate and short term simulations our results make valid the choice of the hs3 reference frame for assessing plate velocity influence on arc tectonic regime the subduction model suggests that friction along the interplate contact and the mantle stokes reaction could be the two main forces competing against slab pull for upper mantle subductions besides our simulations show that the arc deformation mode is strongly time dependent

dissolved redox sensitive elements re u and mo in intense denitrification zone of the arabian sea the concentrations of redox sensitive elements rhenium uranium and molybdenum have been measured in the suboxic water column of the arabian sea to determine their response behavior to intense oxygen minima and denitrifying conditions in the water column north of 12 n within the core of the intense oxygen minimum zone omz of the arabian sea the dissolved oxygen levels drop to values as low as 5 m accompanied by the presence of secondary nitrite maxima snm indicating the occurrence of denitrification in the water column the distributions of re u and mo in the arabian sea show that their dissolved concentrations in the suboxic layer are indistinguishable from those in the overlying and underlying oxic waters suggesting that there is no discernible removal of these elements from the suboxic denitrifying layers in contrast the lateral and vertical distribution of dissolved re u and mo concentrations vary as a function of salinity suggesting their conservative behavior the salinity normalized 35 re u and mo concentrations in the arabian sea are 40 pmol kg 13 8 nmol kg and 114 nmol kg respectively these concentrations are nearly identical to their abundances in the bay of bengal a basin adjacent to the arabian sea characterized by high freshwater influx and fluvial sediments the similarity in concentration suggests that the large variability in the biogenic and detrital particulate fluxes and the suboxic denitrifying conditions between these two oceanic regions do not affect the concentrations of re u and mo and that the prevailing biogeochemical conditions in these regions are inadequate for their removal from the water column

quaternary paleolake nutrient evolution and climatic change in the western qaidam basin deduced from phosphorus geochemistry record of deep drilling core sg 1 abstract phosphorus p provides an important biologically limiting nutrient in lake ecosystems and its concentrations furnish significant information on lake nutrient status climatic change and landscape development within a drainage area this study explores the potential of calcium bound p and iron bound p in calcareous lacustrine sediments to reveal variations of the paleolake nutrient status and its related climatic changes data come from the nearly 1000 m long core sg 1 from the western qaidam basin ne tibetan plateau which has been well dated by paleomagnetism to between 2 77 and 0 1 ma calcium bound p and iron bound p make up the two most frequent phases of inorganic phosphorus pools in the calcareous sediment this study compares the variations of calcium bound p and iron bound p with the independent proxy for biologic production proxy total organic carbon toc as well as a climatic proxy for lake level fluctuation from the borehole those results indicate that calcium bound p responds sensitively to paleolake nutrients and climatic changes while the iron bound p responds to more complex factors more specific to the content of iron oxides variations of the calcium bound p in the core sg 1 show a long term stepwise decrease with shifts at 2 5 ma 1 8 ma 1 2 ma and 0 6 ma indicating a long term stepwise decrease of the paleolake nutrient supply in the drainage this suggests that the ecologic systems experienced a long term deterioration and the climate in central asia became increasingly arid since after the late pliocene

dynamics of retreating slabs 2 insights from three dimensional laboratory experiments 1 a laboratory analogue of a three layer linear viscous slab upper mantle lower mantle system is established in a silicone putty honey and crystallized honey tank experiment the same setup as in the numerical investigation part 1 is used we focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions in our numerical experiments the lateral volume flux was set to zero in interpreting the results relative to the real earth the base of the box is taken as the bottom of the mantle convection system while the lateral boundaries may be associated with the presence of other nearby slabs dynamic force equilibrium assessed on the basis of an analytical review of forces is described for four different phases 1 the subduction initiation instability 2 the accelerating dynamic free fall phase of the slab 3 the dynamic interaction with the 660 km discontinuity and 4 a final phase of steady state trench retreat phase 3 is an important feature not observed in the numerical experiments this highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux on the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux one in which the lateral boundary can be considered open and the other class where it is closed closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab assuming a one to one relation between trench retreat and back arc spreading enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations

late pliocene quaternary evolution of redox conditions in the western qaidam paleolake ne tibetan plateau deduced from mn geochemistry in the drilling core sg 1 abstract manganese mn in lake sediments reacts strongly to changes of redox conditions this study analyzed mn concentrations in oxides carbonates and bulk phases of the calcareous lacustrine sediments of a 938 5 m long core sg 1 taken from the western qaidam basin well dated from 2 77 ma to 0 1 ma comparisons of extractions from diluted hydrochloric acid acetic acid and citrate bicarbonate dithionite demonstrate that variations of mn concentrations from acetic acid leaching mnhoac are mostly responsible for mn ii fluctuations in the carbonate phase taking into account the relevant processes during weathering transportation deposition and post deposition of mn bearing rocks we conclude that mn input from catchment weathering and paleolake redox condition provide the primary controls on variations in the mn records of carbonate and oxide phases we propose mnhoac as a new sensitive indicator of paleolake redox evolution and catchment scale climate change the mnhoac variations show a long term upward decreasing trend indicating a long term decrease of mn input from catchment weathering associated with increasing oxygen content in the paleolake bottom water the great similarities of the mnhoac record with other regional and global records suggest that paleolake redox changes and climatic drying in the qaidam basin may be largely related to global cooling

rare earth elements in the surface sediments of the yarlung tsangbo upper brahmaputra river sediments southern tibetan plateau abstract the yarlung tsangbo yt is the tibetan portion of the brahmaputra river it transports a large amount of sediment annually to the indo gangetic plain and thus contributes significantly to the quaternary evolution of the plain eighteen surface sediment samples were collected from the yt and the compositions of rare earth elements rees were analyzed for the bulk sediments

natural inactivation of phosphorus by aluminum in preindustrial lake sediments noncalcareous phosphorus p rich lake sediments typically release p associated with iron hydroxide fe oh 3 during the development of hypolimnetic anoxia high concentrations of aluminum hydroxide al oh 3 in such sediments e g in aluminum al treated lakes can prevent the p release here we show that sediment ability to bind p can naturally develop during lake history because of changes in the al oh 3 concentration and the al oh 3 to fe oh 3 ratio in sediment we reconstructed the development of sediment psorption characteristics through the late glacial and holocene periods on the basis of sequential fractionation analysis of p al iron calcium and magnesium in a 14 000 yr long sediment record fresh settling seston and

effect of the sumatran mega earthquake on the global magnitude cut off and event rate the great sumatran earthquake of 2004 allows us to assess the statistics and statistical stability of the global earthquake catalogue from the digital era a key question is do such mega earthquakes continue to follow the gutenberg richter g r trend1 or is there an observable cut off2

environmental implications of the magnetic record in pleistocene lacustrine sediments of the qaidam basin ne tibetan plateau abstract the application of magnetic proxies in paleoclimate studies has the advantage of fast and non destructive measurement procedures which is an important precondition for obtaining high resolution datasets within a manageable time frame here we demonstrate the advantage of using magnetic proxies for a 940 m long drill core comprising lacustrine sediments from the qaidam basin ne tibetan plateau which was previously dated by magnetostratigraphy and optical stimulated luminescence osl dating comparison of three independent mass specific magnetic susceptibility datasets confirms the horizontal homogeneity of the drill core at scales of several cm 3 the potential linkage of to astronomical forcing is suggested by spectral analysis magnetic grain size and mineralogy dependent parameters were used to evaluate possible scenarios explaining the variation moreover using additional palynological results we demonstrate the correlation of with climatic constraints thereby showing the climate sensitivity of higher and lower values are related to dry and more humid conditions respectively our results lead us to conclude that variations are caused by the interference of two driving mechanisms low temperature oxidation in the catchment area and a changing catchment area in summary this study confirms the informative value of magnetic proxies for detecting paleoenvironmental change in high resolution

interpreting benthic oxygen levels in mudrocks a new approach quantified paleoecology and gamma ray spectrometry have been applied in the analysis of the kimmeridge clay a highly organic rich british jurassic mudrock decreasing benthic oxygen trends are reflected in decreasing species richness and dominance diversity values similarly the degree of fragmentation of the benthos reflects the benthic energy levels and covaries with benthic oxygen the calculation of authigenic uranium values from data gathered by gamma ray spectrometry shows enrichment in more oxygen deficient environments the good correlation between the independently derived paleoecological and authigenic u data indicates the importance of these techniques in environmental analysis of marine petroleum source rocks

large intermediate depth earthquakes and the subduction process this study provides an overview of intermediate depth earthquake phenomena placing emphasis on the larger tectonically significant events and exploring the relation of intermediate depth earthquakes to shallower seismicity especially we examine whether intermediate depth events reflect the state of interplate coupling at subduction zones and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes historic record of large intraplate earthquakes m b 7 0 in this century shows that the new hebrides and tonga subduction zones have the largest number of large intraplate events regions associated with bends in the subducted lithosphere also have many large events e g altiplano and new ireland we compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with m 6 and depth between 40 and 200 km the final catalog includes 335 events with 47 new focal mechanisms and is probably complete for earthquakes with m b 6 5 for events with m 6 5 nearly 48 of the events had no aftershocks and only 15 of the events had more than five aftershocks within one week of the mainshock events with more than ten aftershocks are located in regions associated with bends in the subducted slab focal mechanism solutions for intermediate depth earthquakes with m 6 8 can be grouped into four categories 1 normal fault events 44 and 2 reverse fault events 33 both with a strike nearly parallel to the trench axis 3 normal or reverse fault events with a strike significantly oblique to the trench axis 10 and 4 tear faulting events 13 the focal mechanisms of type 1 events occur mainly along strongly or moderately coupled subduction zones where a down dip extensional stress prevails in a gently dipping plate in contrast along decoupled subduction zones great normal fault earthquakes occur at shallow depths e g the 1977 sumbawa earthquake in indonesia type 2 events with strike subparallel to the subduction zone and most of them with a near vertical tension axis occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km the increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and consequently the change in focal mechanism from type 1 to type 2 events events of type 3 occur where the trench axis bends sharply causing horizontal parallel to the trench strike extensional or compressional intraplate stress type 4 are hinge faulting events for strongly coupled zones we observed temporal changes of intermediate depth earthquake activity associated with the occurrence of a large underthrusting event after the occurrence of a large underthrusting event the stress axis orientation of intermediate depth earthquakes changes from down dip tensional to down dip compressional e g 1960 chile 1974 peru 1982 tonga and 1952 kamchatka earthquakes or the number of large intermediate events decreases for a few years e g 1964 alaska and 1985 valparaiso earthquakes we conclude that even though the stress changes induced by slab pull and slab distortion control the general pattern of intermediate depth seismicity spatial and temporal variations of the intraplate stress associated with interplate coupling are important in controlling the global occurrence of large intermediate depth events

molybdenum and uranium geochemistry in continental margin sediments paleoproxy potential abstract we measured solid phase mo and u concentrations in marine sediments from the california mexico peru and chile margins to ascertain the behavior of mo and u during early diagenesis in continental margin settings at sites along the california mexico and peru margins where there are estimates of mass accumulation rates authigenic u accumulation rates range from 0 50 nmol m 2 day 1 at the california and mexico margin sites mo accumulation rates range from 0 to 134 nmol m 2 day 1 whereas at the peru margin site rates may be as high as 550 nmol m 2 day 1 we observe relationships between metal accumulation rates and the delivery and burial rates of organic carbon corg in the case of mo there appears to be at least two relationships between metal accumulation rate and organic carbon burial for most of the data presented in this manuscript continental margin sediments have a mo corg accumulation rate ratio of 20 mol mol 1 this value is significantly lower however than those reported for anoxic basins 100 mol mol 1 but is consistent with reported mo corg rain ratios from mexico margin sediment traps in contrast to mo there appears to be a single u corg burial ratio of 5 mol mol 1 which includes a range of environments from anoxic basins and open ocean sites we interpret the relationships between metal accumulation and organic carbon to indicate that the reactions that govern authigenic metal accumulation are primarily sensitive to the delivery and burial of organic carbon in these particular settings however we note that any relationship between metal accumulation and organic carbon could be indirect in the particular case of mo based on what is known about mo geochemistry from the literature it is possible that mo authigenesis is coupled to sulfur cycling in many of the environments covered by this study and that the observed association between mo and organic carbon burial is a consequence of the coupling between carbon and sulfur burial using the observed relationships between mo and corg burial as well as constraints from the mo isotope budget we estimate that continental margin sediments are an important sink for mo the magnitude of this sink 0 4 108 mol y 1 or larger may be as much as one quarter of the oceanic removal term and is likely to be larger than the modern anoxic basin sink

the role of elasticity in slab bending previous studies showed that plate rheology exerts a dominant control on the shape and velocity of subducting plates here we perform a systematic investigation of the role of elasticity in slab bending using fully dynamic 2 d models where an elastic viscoelastic or viscoelastoplastic plate subducts freely into a purely viscous mantle we derive a scaling relationship between the bending radius of viscoelastic slabs and the deborah number de which is the ratio of maxwell time over deformation time we show that de controls the ratio of elastically stored energy over viscously dissipated energy and find that at de 10 2 substantially less energy is required to bend a viscoelastic slab to the same shape as a purely viscous slab with the same intrinsic viscosity elastically stored energy at higher de favors retreating modes of subduction via unbending while trench advance only occurs for some cases with de 1 where most zones have low de 0 1 slabs with de 10 2 either have very low viscosities or they may be yielding in which case our de estimates may be underestimated by up to an order of magnitude potentially pointing towards a significant role of elasticity in 60 of the subduction zones in support of such a role of elasticity in subduction we find that increasing de correlates with increasing proportion of larger seismic events in both instrumental and historic catalogues

palaeoenvironmental controls on the uranium distribution in an upper carboniferous black shale gastrioceras listeri marine band and associated strata england the mudrocks associated with the gastrioceras listeri g listeri marine band contain 0 to 200 ppm authigenic uranium several geochemical degree of pyritisation dop c s ratios and framboidal pyrite size distribution and palaeontological indicators oxygen restricted biofacies orb analysis suggest that the highest authigenic uranium concentrations are within marine sediments associated with the most oxygen restricted biofacies orbs 2 and 3 the uranium peaks tend to occur at the transition between biofacies rather than within the middle of more persistently anoxic intervals these horizons may have been deposited when the oxygen minimum zone impinged on the seafloor brief oxygenation of an otherwise anoxic environment was conducive to francolite precipitation which then scavenged dissolved uranium slow sedimentation rates are also important in concentrating uranium and francolite thus enriched u values are only encountered under specific conditions of low but fluctuating oxygen regime and extremely slow sedimentation rates truly euxinic facies lacking any fossils and a uniformly small framboid population do not contain high concentrations of francolite and are not u rich

minor elements in quaternary sediment from the sea of japan a record of surface water productivity and intermediate water redox conditions sediment of quaternary age from oki ridge 903 m depth in the sea of japan 3500 m deep records six episodes of high accumulation rates of cd cr cu mo ni u v and zn the high rates correspond to periods of sulfate reduction in the water column at the intermediate depth of oki ridge the intervening low values correspond to periods of denitrification and oxygen respiration the maxima have a period of 41 k y the youngest having an age of 1 10 ma the 41 k y cycle is similar to the cycle of 18 o values of open ocean plankton of the same age the similarity between the cycles of minor element accumulation in sea of japan sediment and 18 o values of atlantic ocean foraminifera indicates that redox changes in the water column of the sea of japan during the quaternary forced by major shifts in water column advection and minor shifts in photic zone productivity reflect global events

grain size dependent geochemical characteristics of middle and upper pleistocene loess sequences from the junggar basin implications for the provenance of chinese eolian deposits abstract thick loess paleosol sequences are widely distributed in the arid inland basins of northwestern china the geochemical composition of these sediments provides important constraints on the provenance of chinese eolian deposits however grain size dependent geochemical characteristics of early middle pleistocene loess deposits from these arid inland basins have not been investigated the paucity of such studies hinders a full understanding of provenance changes of loess deposits from the chinese loess plateau clp in the present study we performed a detailed major trace and rare earth element analysis of multiple grain size fractions of middle pleistocene loess deposits from the dongwan section dw in the junggar basin we found that the major trace and rare earth element concentrations of the dw loess are grain size dependent with higher concentrations in the fine fraction in addition the 75 m from the dw section changed significantly at 0 5 ma which may have arisen from intensified physical erosion or increased wind speeds in the northern tianshan area

horizontal lithosphere compression and subduction constraints provided by physical modeling physical modeling of the subduction is performed with a two layered mantle model elastico plastic lithosphere and low viscosity asthenosphere and is governed by the criteria of similarity compression of the lithosphere in the area of a passive continental margin has been shown to produce a buckling instability in the oceanic plate with wavelengths of 200 km on the average later a localization of deformation occurs in sagging at some distance from the margin where a strongly dislocated linear ridge is formed due to the thrusting the plate then experiences a failure along the inclined zone and subduction starts the inner trench slope which forms therefore has a scraped structure and include a block of crushed and dislocated oceanic crust and sediments in the lower section if there is an old inclined fault striking across the compression of an oceanic lithosphere it is on that fault that a subduction zone is initiated the inner trench slope has then a different structure and forms due to normal faulting in the frontal part of the overriding plate the formation of a subduction zone requires a compression that is smaller than that in the preceding case by a factor of 2 or 3 the subducting plate experiences an elastico plastic bending and under specific conditions thrusting along the zone dipping from under the overriding plate oceanward and crossing the entire lithosphere the best agreement between generalized relief in subduction zone in the model and nature is achieved when a shear yield limit s 1 3 108 pa modulus of elasticity e about a few times 1011 pa and a thickness h 60 km are adopted for the real lithosphere

late plio pleistocene humidity fluctuations in the western qaidam basin ne tibetan plateau revealed by an integrated magnetic palynological record from lacustrine sediments abstract deciphering the climatic evolution of the tibetan plateau region during the plio pleistocene is hampered by the lack of continuous archives and proxy datasets indicative of moisture availability here we assess the suitability of magnetic susceptibility measured on lacustrine sediments as a paleohydrological proxy based on material from drill core sg 1 2 69 0 08 ma from the western qaidam basin our assessment is based on directly comparing with the artemisia chenopodiaceae a c pollen ratio which represents a sensitive well established proxy for moisture changes in arid environments we find that higher and lower values represent drier and less dry conditions respectively for the late plio pleistocene less dry phases were likely caused by transiently increased influence of the westerlies and or decreased influence of the asian winter monsoon on glacial interglacial time scales an exception from this relationship is the interval between 1 9 and 1 3 ma when the sg 1 record exhibits a 54 ka cyclicity which may indicate summer monsoon influence on the qaidam basin during that time after 1 3 ma the summer monsoon influence may have ceased due to global cooling with the consequence that the asian winter monsoon and the westerlies exerted a stronger control on the hydrology of the qaidam basin

evaporite minerals and geochemistry of the upper 400 m sediments in a core from the western qaidam basin tibet qaidam basin is a tectonically controlled depression on the northern margin of the tibetan plateau in 2008 a long core was drilled in the qahansilatu sub basin in the western qaidam basin the sediment layers in the upper 400 m alternate between evaporite mineral layers and carbonaceous clay layers the detailed mineralogical investigation focused on evaporite minerals including halite gypsum mirabilite thenardite glauberite eugsterite and bloedite gypsum and halite make up the majority of the evaporate minerals environmentally induced variations in the mineralogy and crystal habit of the sulfates have been extensively investigated gypsum has prismatic and pyramid habits such as disc pyramid stubby prismatic slender prismatic visible isolated gypsum and aggregates rosette radial and twins are mostly scattered in carbonaceous clay layers suggesting that secondary gypsum is well developed gypsum may be a precursor mineral of glauberite and thenardite is the precursor of bloedite as a metastable and rare mineral eugsterite does not appear in other tibetan areas it forms at the expense of pre existing gypsum or thenardite in the core at an experimental temperature of higher than room temperature the presence of eugsterite indicates a warm and or hot climate at its deposition time

dynamics of plate bending at the trench and slab plate coupling 1 the bending strength of subducting lithosphere plays a critical role in the earth s plate tectonics and mantle convection modulating the amount of slab pull transmitted to the surface and setting the boundary conditions under which plates move and deform however it is the subject of a lively debate how much of the potential energy of the downgoing plate is consumed in bending the plate and how the lithospheric strength is defined during this process we model the subduction of a viscoelastic lithosphere driven solely by the downgoing plate s buoyancy freely sinking in a passive mantle represented by drag forces to investigate the dynamics of bending 1 we vary the density and the viscosity profile within the plate from isoviscous where strength is distributed to strongly layered where strength is concentrated in a thin core and 2 we map the stress strain and dissipation along the downgoing plate the effective plate strength during bending is not a simple function of average plate viscosity but is affected by rheological layering and plate thinning earth like layered plates allow for the transmission of large fractions of slab pull 75 80 through the bend and yield a net slab pull of fspnet 1 to 6 1012 n m 1 which varies with the buoyancy of plates in all models only a minor portion of the energy is dissipated in the bending surprisingly bending dissipation hardly varies with lithospheric viscosity because in our dynamic system the plates minimize overall dissipation rate by adjusting their bending curvature as a result bending dissipation b is mainly controlled by the bending moment work rate exerted by slab pull we propose a new analytical formulation that includes a viscosity dependent bending radius which allows for assessment of the relative bending dissipation in the earth s subduction zones using parameters from a recent global compilation this yields estimates of b tot 20 these results suggest that plates on earth weakly resist bending yet are able to propagate a large amount of slab pull

u th ra fractionation during weathering and river transport the potential of radioactive disequilibria as tracers and chronometers of weathering processes has been recognised since the 1960 s e g rosholt et al 1966 hansen and stout 1968 this interest results from the dual property of the nuclides of the u and th radioactive series 1 to be fractionated during water rock interactions and 2 to have radioactive periods of the same order of magnitude as the time constants of many weathering processes and chemical transfers to ground and river waters therefore the study of radioactive disequilibria in surface environments should help to bring information about the nature the intensity but also the time scale of the water rock interactions produced during weathering and related chemical transfers these different properties have justified many of the studies on u th series in weathering profiles and river waters rosholt 1982 scott 1982 and osmond and ivanovich 1992 gave a synthesis of the studies of u series in weathering and surface hydrology up to the 1980 s and the 1990 s respectively here we present the main directions taken in these domains over the last decade they were partly stimulated by the analytical developments made in the measurement of the medium half life nuclides of 238u series i e 234u 230th 226ra in the mid 1980 s namely the use of thermal ionisation mass spectrometry tims e g chen et al 1986 edwards et al 1987 cohen et al 1991 chabaux 1993 chabaux et al 1994 and more recently the use of mc icp ms turner et al 2001 robinson et al 2002 for u and th isotope analysis details of these developments are given in this volume goldstein and stirling 2003 compared to the radioactive counting methods previously used the new techniques permit 1 a reduction by one order of magnitude or more of the size of the sample required for

environmental analysis of paleoceanographic systems based on molybdenum uranium covariation abstract modern low oxygen marine systems exhibit patterns of molybdenum uranium covariation that can be linked to specific attributes and processes of the depositional system including 1 variation in benthic redox conditions 2 the operation of particulate shuttles within the water column and 3 the evolution of watermass chemistry the importance of these factors in each depositional system can be assessed both from the degree of enrichment of authigenic molybdenum mo auth and uranium u auth and from the mo u auth ratio of the sediment relative to the seawater mo u molar ratio of 7 5 7 9 in open ocean systems with suboxic bottomwaters u auth enrichment tends to exceed that of mo auth owing to onset of u auth accumulation at the fe ii fe iii redox boundary resulting in sediment mo u auth ratios less than that of seawater as bottomwaters become increasingly reducing and at least occasionally sulfidic the rate of accumulation of mo auth increases relative to that of u auth and sediment mo u auth ratios equal or exceed that of seawater in restricted marine systems with permanently sulfidic deepwaters the relative enrichment of mo auth and u auth depends on additional factors in the cariaco basin which has an aqueous mo u ratio similar to that of seawater the operation of a particulate mn fe oxyhydroxide shuttle serves to accelerate the transfer of mo to the seafloor leading to strong enrichments in mo auth relative to u auth in the black sea the chemistry of the deep watermass has evolved to the point where its aqueous mo u ratio is only 0 04 that of seawater as a consequence of which sediments deposited under deepwater influence are depleted in mo auth relative to u auth these mo u covariation patterns can be used to gain a better understanding of the watermass attributes and processes of ancient low oxygen marine systems analysis of anoxic facies from two north american paleomarine systems the late pennsylvanian midcontinent sea lpms and the late devonian seaway lds reveals authigenic mo u relationships similar to those of the modern marine environments above implying similar redox and hydrographic controls the observed patterns are consistent with laterally unconfined circulation and strong watermass exchange within the lpms and with markedly restricted deepwater circulation in silled basins of the lds patterns of authigenic mo u covariation may prove useful in analysis of other paleoceanographic systems to reveal aspects of watermass composition and environmental dynamics

pliocene pleistocene climate change at the ne tibetan plateau deduced from lithofacies variation in the drill core sg 1 western qaidam basin china abstract the pliocene pleistocene sedimentologic evolution of the western qaidam basin at the ne tibetan plateau was studied on the nearly thousand meter deep drill core sg 1 spanning ca 2 8 0 1 ma recovered from the chahansilatu sub depression with an average recovery rate of 95 lithofacies variation records the regional climatic change in relation to asian drying and tibetan plateau uplift sedimentary textures and structures demonstrate a continuous slight coarsening and an increase of evaporites upward we recognized five lithofacies 1 gray black laminated mudstone 2 gray massive mudstone and siltstone 3 bedded halite 4 loose muddy halite and 5 brownish yellowish saliferous siltstone and mudstone representing 1 semi deep fresh to semi brackish lake 2 shallow brackish lake 3 perennial saline lake 4 playa saline lake and 5 saline mudflat conditions respectively variation of the lithofacies through the core demonstrates a clear trend of lake shrinkage since the late pliocene and the final termination of the chahansilatu paleolake accelerations of lake shrinkages are observed at ca 2 5 ma 2 2 ma 1 6 ma 1 2 ma 0 9 ma 0 6 ma and 0 1 ma our results suggest that the long term stepwise drying of the asian inland may have been forced by the change of the global ice volume

high resolution magnetostratigraphy of the neogene huaitoutala section in the eastern qaidam basin on the ne tibetan plateau qinghai province china and its implication on tectonic uplift of the ne tibetan plateau abstract the closed inland qaidam basin in the ne tibetan plateau contains possibly the world s thickest 12 000 m continuous sequence of cenozoic fluviolacustrine sedimentary rocks this sequence contains considerable information on the history of tibetan uplift and associated climatic change however work within qaidam basin has been held back by a paucity of precise time constraints on this sequence here we report on a detailed paleomagnetic study of the well exposed 4570 m huaitoutala section along the keluke anticline in the northeastern qaidam basin where three distinct faunas were recovered and identified from the middle miocene through pliocene constrained by these faunal ages the observed thirty three pairs of normal and reversed polarity zones can be readily correlated with chrons 2n 5br of the gpts this study assigns the age of the section to the interval between ca 15 7 ma to 1 8 ma in addition the widely used stratigraphic units the xia youshashan shang youshashan the shizigou and qigequan formations were formed at 15 3 ma 15 3 8 1 ma 8 1 2 5 ma and

influence of sediment redox conditions on uranium mobilisation during saline intrusion abstract in the uk several coastal nuclear sites have been identified as vulnerable to future sea level rise legacy contamination at these sites has accumulated in sub surface sediments at risk of future seawater inundation and intrusion porewater salinization changes in ph and the influx of oxygen into sediments may impact the stability of sediment associated uranium u in this study saturated column experiments were performed to compare the mobilisation of u from oxic and reduced sediments into seawater under environmentally relevant flow conditions uranium release profiles were independent of the initial geochemistry of the sediments uranium release from the sediments was kinetically controlled showing relatively slow desorption kinetics with release initially limited by the impact of the sediments on the ph of the seawater significant u release only occurred when the ph was sufficiently high for the formation of u carbonate complexes phoxic 6 3 phreduced 7 5 uranium was more strongly bound to the reduced sediments and after 400 pore volumes of seawater flow release was more extensive from the initially oxic 46 compared with initially nitrate reducing 27 and iron reducing 18 sediments the products of iron cycling appeared to act as a buffer limiting u mobilisation but the on going dissolution of the fe phases suggests that they did not form a permanent protective layer

paleoclimatic implications of magnetic susceptibility in late pliocene quaternary sediments from deep drilling core sg 1 in the western qaidam basin ne tibetan plateau 1 lake sediments are important archives of paleoclimate change and erosion history a 938 5 m long core sg 1 of lacustrine sediments dated at 2 77 ma to 0 1 ma was obtained from the western qaidam basin in the northeastern tibetan plateau consisting of dark grayish mudstone and grayish siltstone intercalated with salts and fine sandstones in the upper part magnetic susceptibility data combined with detailed rock magnetic properties were analyzed for revealing the significance of ferro i magnetic concentration for past changes of climate and erosion mass specific susceptibility shows a striking cyclic and long term variation samples with high values are dominated by magnetite and maghemite with pseudo single domain properties in contrast samples with low values contain maghemite from single domain to multidomain and additionally a significant fraction of hematite the driving mechanism of variation can be explained by three alternative models 1 different source regions with alternations of wind and cryoclastic erosion in a wider hinterland dry cold climate and surface runoff erosion from a narrower area more humid climate and 2 and 3 low temperature oxidation occurring either in the lake sediments dry climate or in the catchment area during weathering more humid climate trends of match with changes in sedimention rates and are roughly synchronous with the deep sea 18o record on a glacial interglacial timescale therefore the concentration of magnetic minerals in the western qaidam basin sediments is likely controlled by both tectonic influence and paleoenvironmental changes but can be best interpreted by alternations and trends of dry cold and more humid periods due to asian drying and global cooling

dynamics of retreating slabs 1 insights from two dimensional numerical experiments from the effects of mantle flux part 2 therefore in this paper we apply forces to the slab using simple analytical functions related to buoyancy and viscous forces in order to isolate the role of rheology on slab dynamics we analyze parameters for simplified elastic viscous and nonlinear viscoelastoplastic single layer models of slabs and compare them with a stratified thermomechanical viscoelastoplastic slab embedded in a thermal solution the near surface behavior of slabs is summarized by assessing the amplitude and wavelength of forebulge uplift for each rheology in the complete thermomechanical solutions vastly contrasting styles of slab dynamics and force balance are observed at top and bottom bends however we find that slab subduction can be modeled using simplified rheologies characterized by a narrow range of selected benchmark parameters the best fit linear viscosity ranges between 5 10 22 pa s and 5 10 23 pa s the closeness of the numerical solution to nature can be characterized by a deborah number 0 5 indicating that elasticity is an important ingredient in subduction index terms 8120 tectonophysics dynamics of lithosphere and mantle general 8159 tectonophysics rheology crust and lithosphere 8160 tectonophysics rheology general keywords subduction numerical models lithospheric rheology

global enhancement of ocean anoxia during oceanic anoxic event 2 a quantitative approach using u isotopes during the mesozoic greenhouse world the oceans underwent several oceanic anoxic events oaes characterized by intervals during which organic rich black shales were deposited indicating strong oxygen depletion in the marine realm the cenomanian turonian oae2 ca 93 ma represents one of the most prominent events of the cretaceous with significant perturbations of the global carbon cycle although oae2 likely reached a global scale the spatial extent of seawater anoxia during this oae is poorly constrained here we demonstrate that variations in the 238u 235u isotope ratio 238u a newly developed paleoredox proxy can be used to quantify the extent of marine anoxia for black shales from the mid cretaceous oae2 we find a systematic shift toward lighter 238u and lower u concentrations as compared to modern equivalent organic rich sediments from the black sea this shift translates to a global increase of oceanic anoxia during oae2 by at least a factor of three as compared to the present day or to periods before and after oae2 the constant offset in u concentrations and isotope compositions of black shales throughout oae2 compared to modern black sea sediments indicates an enhancement of oceanic anoxic conditions already prior to the onset of oae2

bending mechanics and mode selection in free subduction a thin sheet analysis summary to elucidate the dynamics of free buoyancy driven subduction of oceanic lithosphere i study a model in which a 2 d sheet of viscous fluid with thickness h and viscosity 1 subducts in an infinitely deep ambient fluid with viscosity 1 numerical solutions for the sheet s evolution are obtained using the boundary element method bem starting from an initial configuration comprising a short protoslab attached to a longer horizontal plate that is free to move laterally beneath an impermeable traction free surface interpretation of the solutions using thin viscous sheet theory shows that the fundamental length scale controlling the subduction is the bending length lb defined at each instant as the length of the portion of the sheet s midsurface where the rate of change of curvature is significant geophysically speaking lb is the sum of the lengths of the slab and of the region seaward of the trench where flexural bulging occurs the bending length in turn enters into the definition of the sheet s dimensionless stiffness s h lb 3 which controls whether the sinking speed of the slab is determined by the viscosity of the sheet itself s 1 or by that of the ambient fluid s 1 motivated by laboratory observations of different modes of subduction retreating versus advancing trench folding versus no folding etc in fluid layers with finite depth i calculate numerically the dip d of the slab s leading end as a function of and the normalized depth d h to which it has penetrated the contours of the function d d h strongly resemble the intermode boundaries in the laboratory based regime diagram of schellart supporting the hypothesis that the mode of subduction observed at long times in experiments is controlled by the dip of the slab s leading end when it reaches the bottom of the layer in particular the bem solutions explain why trenches advance in the laboratory only when lies in an intermediate range and why they retreat when is either smaller or larger than this application of the bem model to wu et al s compilation of the minimum curvature radii of subducted slabs suggests 140 510 for the earth this is too small to permit the laboratory type trench advancing mode in agreement with the lack of tomographic evidence for slabs that are bent over backwards

a marine biogeochemical perspective on black shale deposition abstract deposition of marine black shales has commonly been interpreted as having involved a high level of marine phytoplankton production that promoted high settling rates of organic matter through the water column and high burial fluxes on the seafloor or anoxic sulfidic water column conditions that led to high levels of preservation of deposited organic matter or a combination of the two processes here we review the hydrography and the budgets of trace metals and phytoplankton nutrients in two modern marine basins that have permanently anoxic bottom waters this information is then used to hindcast the hydrography and biogeochemical conditions of deposition of a black shale of late jurassic age the kimmeridge clay formation yorkshire england from its trace metal and organic carbon content comparison of the modern and jurassic sediment compositions reveals that the rate of photic zone primary productivity in the kimmeridge sea based on the accumulation rate of the marine fraction of ni was as high as 840 g organic carbon m 2 yr 1 this high level was possibly tied to the maximum rise of sea level during the late jurassic that flooded this and other continents sufficiently to allow major open ocean boundary currents to penetrate into epeiric seas sites of intense upwelling of nutrient enriched seawater would have been transferred from the continental margins their present location onto the continents this global flooding event was likely responsible for deposition of organic matter enriched sediments in other marine basins of this age several of which today host major petroleum source rocks bottom water redox conditions in the kimmeridge sea deduced from the v mo ratio in the marine fraction of the kimmeridge clay formation varied from oxic to anoxic but were predominantly suboxic or denitrifying a high settling flux of organic matter a result of the high primary productivity supported a high rate of bacterial respiration that led to the depletion of o2 in the bottom water a high rate of burial of labile organic matter albeit a low percentage of primary productivity in turn promoted anoxic conditions in the sediment pore waters that enhanced retention of trace metals deposited from the water column

magnetostratigraphy of deep drilling core sg 1 in the western qaidam basin ne tibetan plateau and its tectonic implications abstract the qaidam basin is the largest intermontane basin of the northeastern tibetan plateau and contains a continuous cenozoic sequence of lacustrine sediments a 1000 m deep drilling sg 1 with an average core recovery of 95 was carried out in the depocenter of the chahansilatu playa sub depression in the western qaidam basin aimed to obtain a high resolution record of the paleoenvironmental evolution and the erosion history stepwise alternating field and thermal demagnetization together with rock magnetic results revealed a stable remanent magnetization for most samples carried by magnetite the polarity sequence consisted of 16 normal and 15 reverse zones which can be correlated with chrons 1n to 2an of the global geomagnetic polarity time scale magnetostratigraphic results date the entire core sg 1 at 2 77 ma to 0 1 ma and yielded sediment accumulation rate sar ranging from 26 1 cm ka to 51 5 cm ka maximum sars occurred within the intervals of 2 6 2 2 ma and after 0 8 ma indicating two episodes of erosion which we relate to pulse tectonic uplift of the ne tibetan plateau with subsequent global cooling

quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols abstract paleosols fossil soils are preserved throughout the geologic record in depositional settings ranging from alluvial systems to between basalt flows until recently paleosols were studied using primarily qualitative methods in recent years paleopedology has shifted from a largely qualitative field based on comparisons with modern analogues to an increasingly quantitative endeavor some of this change has been a result of applying existing techniques to new materials but many of the innovations have been the result of applying new techniques to new materials including thermodynamic modeling of soil formation isotope geochemistry and applications of empirical relationships derived from modern soils a variety of semi quantitative and quantitative tools has been developed to examine past weathering and pedogenesis and to reconstruct both paleoenvironmental and paleoclimatic conditions at the time that the paleosols formed though it is often not possible to achieve the same temporal resolution as with marine records for paleoclimatic reconstructions proxies based on paleosols are potentially a much more direct means of making paleoclimatic reconstructions because soils form at the earth s surface in direct contact with the atmospheric and climatic conditions at the time of their formation paleoclimatic and environmental properties that may be reconstructed using the new proxies include provenance weathering intensity mean annual precipitation and temperature during pedogenesis nutrient fluxes into and out of the paleosols the atmospheric composition of important gases including co2 and o2 the moisture balance during pedogenesis the soil gas composition reconstructed vegetative covering and paleo altitude

paleoenvironmental implications of uranium concentrations in lacustrine calcareous clastic evaporite deposits in the western qaidam basin abstract uranium u provides especially useful information for reconstructing paleoredox conditions due to its unique geochemical properties and behavior in supergene environments its proxy potential under suboxic oxic conditions associated with salinization of surface water in closed hydrologic systems however has not been adequately explored this study examined the abundance and distribution of u in a sedimentary sequence of calcareous lacustrine clastics and salt layers from a high quality 938 5 m long core drilled in the western qaidam basin ne tibetan plateau this study examined u concentrations in calcareous clastic sediments including carbonates acid insoluble fractions as well as the amounts of iron and manganese oxides oxyhydroxides and salt layers in the calcareous clastic sediments variations in u concentrations within the acid insoluble fraction follow the fluctuations of reconstructed lake levels based on lithologic investigations providing a tool for tracing redox conditions in paleolake bottom water correlations between the variations in carbonate content with u and sr concentrations in the carbonate fractions demonstrate that the types and abundance of carbonate minerals control the u distribution in carbonates in the salt layers u and al concentrations correlate closely implying that both derived mainly from the detrital fraction in the salt layers however enrichments of u and mg are systematically higher in salt minerals formed in more concentrated brine and lower in those formed in less concentrated brine these results indicate that during the process of carbonate precipitation carbonate might effectively remove u from lake water during the subsequent stage of salt mineral precipitation u behavior is conservative and u resides for a long period in dissolved forms within the saline water giving rise to enhanced u enrichment in the more concentrated brines and the associated precipitated salt minerals our study suggests however that u related redox proxies should be interpreted with caution in suboxic oxic environments during the long term paleolake evolution

analysis of marine environmental conditions based onmolybdenum uranium covariation applications to mesozoic paleoceanography article i nfo article history accepted 13 september 2011 available online 17 september 2011 patterns of uranium molybdenum covariation in marine sediments have the potential to provide insights re garding depositional conditions and processes in paleoceanographic systems specifically such patterns can be used to assess bottom water redox conditions the operation of metal oxyhydroxide particulate shuttles in the water column and the degree of water mass restriction the utility of this paleoenvironmental proxy is due to the differential geochemical behavior of u and mo 1 uptake of authigenic u by marine sediments begins at the fe ii fe iii redox boundary i e suboxic conditions whereas authigenic mo enrichment re quires the presence of h2s i e euxinic conditions and 2 transfer of aqueous mo to the sediment may be enhanced through particulate shuttles whereas aqueous u is unaffected by this process in the present study we examine u mo covariation in organic rich sediments deposited mostly in the western tethyan region during oceanic anoxic events oaes of early jurassic to late cretaceous age our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regard ing water mass restriction and the operation of particulate shuttles in depositional systems these insights will help to address contentious issues pertaining to the character and origin of mesozoic oaes such as the degree to which regional paleoceanographic factors controlled the development of the oaes

computing applachian geomorphology abstract craig r g 1989 computing appalachian geomorphology in t w gardner and w d sevon editors appalachian geomorphology geomorphology 2 197 207 a computer model is used to represent the stochastic effects of geomorphic processes acting on appalachian structures and to produce the various stages of cenozoic appalachian geomorphic history that model incorporates the uncertainty in our current representation of elevation lithology dependence of slope on lithology structural dip percent of surface lithology uneroded and stratigraphic thickness four theories of appalachian history defined by initial surface relief and adjustment to underlying lithology are analyzed initial surface relief like that proposed by j t hack yields the best fit to the current surface mean elevation 410 m when erosion proceeds from a surface initialized to the modern mean elevation of the ridgetops 760 m

high frequency redox variations of the eocene cyclic lacustrine sediments in the yingxi area western qaidam basin china abstract the mineral assemblages of lacustrine mixed sediments have vital significance for reconstructing paleo lake environments and typical lacustrine mixed sediments deposited during the eocene are distributed throughout the yingxi area in the western qaidam basin accordingly the framboid size distributions and sulfur isotope 34spy characteristics of pyrite in addition to the carbon isotope 13ccarb characteristics and redox sensitive element ratios th u v v ni and v cr of authigenic carbonates were investigated to reconstruct the high frequency redox variations in cyclic lacustrine sediments corresponding to different redox conditions three types of mixed sediments developed during transgression and regression stages within the saline basin in the yingxi area during the eocene mixed terrigenous clasts formed in oxic dysoxic water column conditions during lake transgression mixed carbonates formed in anoxic water column conditions during early stages lake regression and mixed evaporites formed in anoxic sulfidic water column conditions during middle late stage lake regression the redox proxies correlate well with the water salinity exhibit little or no correlation with the water temperature and show an incongruity with the water depth suggesting that the redox conditions were predominantly controlled by the water salinity rather than the water depth or temperature therefore based on the water redox condition variations and the salinity three dynamic formation models of the high frequency sedimentary cycles in a saline basin are established a deep water low salinity oxic dysoxic model model i a medium water medium salinity anoxic model model ii and a shallow water high salinity anoxic sulfidic model model iii because the climate gradually turned dry and hot from lake transgression to regression the continued closure of the basin enhanced evaporation and salinization in addition mixed terrigenous clasts mixed carbonates and mixed evaporites deposited successively correspondingly salinity stratification gradually developed within the lake water increasing the extent of bacterial sulfate reduction in the bottom water

what drives orogeny in the andes the andes the world9s second highest orogenic belt were generated by the cenozoic tectonic shortening of the south american plate margin overriding the subducting nazca plate we use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening from the modeling we infer that the most important factor was accelerated westward drift of the south american plate changes in the subduction rate were less important other important factors are crustal structure of the overriding plate and shear coupling at the plates9 interface the model with a thick 40 45 km at 30 ma south american crust and relatively high friction coefficient 0 05 at the nazca south american interface generates 300 km of tectonic shortening during 30 35 m y and replicates the crustal structure and evolution of the high central andes the model with an initially thinner

controls on uranium distribution in lake sediments abstract uranium geochemistry has been investigated in three acid lakes located on the canadian shield and one circumneutral lake in the appalachian region of eastern canada in all shield lakes dissolved u concentrations were higher in the porewater than in the overlying water in one of them whose hypolimnion is perennially oxic u released to porewater at depths of fe remobilization was removed from the porewater at depths of fe oxyhydroxides precipitation these similarities in the u and fe profiles indicate that part of the u becomes associated to fe oxyhydroxides the dissolved u and fe profiles in the other two shield lakes whose hypolimnions were anoxic when sampled did not show any significant recycling of these elements in the vicinity of the sediment water interface and both elements diffused from the sediment to the overlying water in contrast in the appalachian lake dissolved u concentrations were higher in the overlying water than in porewater strongly decreased at the vicinity of the sediment water interface and then remained relatively constant with sediment depth diagenetic modeling of the porewater u profiles assuming steady state reveals that authigenic u always represented 3 of the total u concentration in the sediments of all lakes this observation indicates that diagenetic reactions involving u are not quantitatively important and that most of the u was delivered to the sediments at our study sites as particulate u and not through diffusion across the sediment water interface as is seen in continental margin sediments comparison of the u c org and u fe molar ratios in diagenetic material collected across the sediment water interface with teflon sheets and in surface sediments 0 0 5 cm of the lake having a perennially oxic hypolimnion suggest that solid phase u was mainly bound to organic matter originating from the watershed a strong statistical correlation between sediment non lithogenic u and c org in the appalachian lake supports this contention thermodynamic calculations of saturation states suggest that dissolved u was not removed from porewater through precipitation of uo 2 s u 3 o 7 s and u 3 o 8 s as previously proposed in the literature

late miocene intensified tectonic uplift and climatic aridification on the northeastern tibetan plateau evidence from clay mineralogical and geochemical records in the xining basin

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

great earthquakes and slab pull interaction between seismic coupling and plate slab coupling great earthquakes the few largest earthquakes that account for most of the earth s seismic energy release have occurred at only a few subduction zones around the world strong locking or seismic coupling of the interface between plates at certain subduction zones is often invoked to explain these great earthquakes although past studies have correlated strong seismic coupling with a compressional stress environment that is characterized by back arc compression and caused by trenchward motion of the overriding plate the consequences of this compressional environment for the tectonic forces that drive global plate motions are not yet clear to examine these consequences we compared subduction zone earthquake magnitudes to tectonically constrained estimates of the degree to which each slab transmits its excess weight as a direct pull force on a subducting plate at seismically uncoupled subduction zones that generate only moderate sized earthquakes we find that slabs must transmit nearly their entire upper mantle weight as a pull force on the subducting plate at seismically coupled subduction zones that produce great earthquakes however we find that slabs must be nearly completely detached from their subducting plates this suggests that slabs subducting in a compressional environment experience stress induced weakening that prevents the effective transmission of the slab pull force convergent mantle flow above a descending slab that becomes decoupled from its surface plate may induce additional surface compression that further locks the subduction zone and leads to additional slab decoupling and detachment the resulting redistribution of plate driving forces may be responsible for rapid changes in plate motion c 2003 elsevier b v all rights reserved

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

evaporite minerals of the lower 538 5 m sediments in a long core from the western qaidam basin tibet abstract qaidam basin is a tectonically controlled mesozoic cenozoic depression on the northern margin of the tibetan plateau a 938 5 m long core was drilled in the qahansilatu sub basin in the western qaidam basin with an average core recovery of 95 it revealed alternating salt layers and carbonate clay layers samples were collected at 10 40 cm intervals for mineralogical analysis by xrd and chemical analysis by icp oes the lower 538 5 m sediments are composed of halite gypsum anhydrite gaylussite calcite aragonite ankerite dolomite and an unnamed mineral mg0 92ca0 08co3 3h2o with trace eugsterite the mother brines could be na type na ca type na ca mg type ca na type ca mg na type and trace ca mg na reflux and bacterial activity could be suitable for the formation of dolomite deep burial diagenesis could have played an active role in the formation of ankerite and anhydrate the continuous presence of halite suggested the paleo lake water was highly brackish or saline with high evaporation since 2 77 ma salt layers in the lower 538 5 m long sediments were present from 2221 ka to 2052 ka corresponding to pleistocene salt formation in the qaidam basin as hydrated carbonate minerals both gaylussite and the unnamed mineral are deposited under high precipitation rates with microbial activity gaylussite was deposited from na ca enriched solutions with molar ratios of na ca more than 2 from 691 m 2226 ka to 413 6 m 1222 ka the un named mineral mg0 92ca0 08co3 3h2o was found from 523 4 m 1728 ka to 724 2 m 2308 ka anhydrite could be transformed from gypsum under deep burial from 657 42 m 2052 ka to 867 m 2556 ka alternating salt and clay layers in the lower part of the core recorded arid and relatively wet climatic oscillations and the evolution of brine as a tectonic sub basin tectonic activities could change the local climate during episodes of uplift and subsidence the dominant minerals in the chahansilatu sub basin are similar to those of the other sub basins in the western qaidam basin but have asynchronous evolutionary stages of brine

plateau uplift forcing climate change around 8 6 ma on the northeastern tibetan plateau evidence from an integrated sedimentary sr record abstract the uplift of the northern tibetan plateau tp has long been regarded as one of major factors accounting for both the enhanced process of aridification recorded in central asia and the shifts in the asian monsoonal circulations that are known to have occurred at approximately 8 ma until now there have been few sedimentary records reported on which can directly and in an integrated fashion shed light on the region s tectonic and climate regimes this can be largely attributed to the fact that the influence exerted by the presence of source materials and subsequent sedimentary sorting in this tectonically active region would be likely to bias significantly any paleoclimatic interpretation of silicate derived proxies based on detailed investigations of strontium sr distributions revealed by multi step leaching in a fluvial sequence from the linxia basin which includes paleosols we reconstructed the long term late miocene regional climatic and environmental changes likely to have occurred on the northeastern tp from 12 1 to 5 2 ma we investigated sr related proxies in two typical paleosol profiles to constrain sr mobility in paleosol weathering and thus to further prompt the interpretation of sr related proxies along the whole section we would suggest that carbonate derived sr concentrations and sr ca ratios are suitable proxies for tracing regional climate change associated with weathering and pedogenesis together they exhibit weakened chemical weathering intensity and pedogenesis at 8 6 ma the concomitant appearance of immature sediments through poor sedimentary sorting revealed by silicate derived rb sr 87 sr 86 sr and eu anomaly eu eu alongside a sharp increase in sedimentation rates noted at this time suggests that it was indeed tectonic uplift that principally controlled the dramatic climatic and environmental changes this multi proxy record of concomitant tectonic uplift and climate change in the linxia basin provides direct geological evidence for uplift driven climate change in the northeastern tp region

holocene moisture evolution in arid central asia and its out of phase relationship with asian monsoon history we synthesize palaeoclimate records from the mid latitude arid asian region dominated today by the westerlies arid central asia aca to evaluate spatial and temporal patterns of moisture changes during the holocene sediment records from i i takes with reliable chronologies and robust proxies were selected to reconstruct moisture histories based on a five class ordinal wetness index with assigned scores from the driest to wettest periods at individual sites for 200 year time slices the proxies used in these records include pollen and diatom assemblages sediment lithology lake levels and geochemistry mainly isotope data the results of our synthesis show that aca as a whole experienced synchronous and coherent moisture changes during the holocene namely a dry early holocene a wetter less dry early to mid holocene and a moderately wet late holocene during the early holocene most of the lakes experienced very low water levels and even dried out before ca 8 ka 1 ka 1000cal a bp hence the effective moisture history in aca is out of phase with that in monsoonal asia as documented by numerous palaeoclimate records in monsoonal asia a strong summer monsoon and humid climate characterized the early holocene and a weakened summer monsoon and drier climate prevailed during the late holocene which were mainly controlled by changes in low latitude summer insolation in contrast we propose that the pattern of holocene effective moisture evolution in the westerly dominated aca was mainly determined by north atlantic sea surface temperatures ssts and high latitude air temperatures that affect the availability amount and transport of water vapor also topography of the tibetan plateau and adjacent asian highlands could have contributed to the intensification of dry climate in aca during the early holocene as a result of strengthening the subsidence of dry air masses associated with stronger uplift motion on the plateau by intense heating under a stronger summer insolation summer insolation might have played a key role in directly controlling moisture conditions in aca but only after the northern hemisphere ice sheets had disappeared in the mid and late holocene c 2007 elsevier ltd all rights reserved

curvature of subducted lithosphere from earthquake locations in the wadati benioff zone estimates of slab curvature from 133 profiles across 12 major subduction zones are obtained by fitting splines through earthquakes locations several geometric quantities are computed from the spline fits including the minimum radius of curvature we also introduce a quantity called the equivalent radius of curvature to precisely define the bending dissipation in either a viscous or a plastic slab the minimum and equivalent radius of curvature are approximately equal in slabs with simple geometry i e a single bend but deviate substantially when the geometry is more complicated a compilation of estimates shows that the equivalent radius of curvature is insensitive to the age of the plate at the trench mean values are 175 km or 191 km depending on the rheology of the slab when bending occurs by viscous deformation the resulting dissipation can exceed the power available from subduction with reasonable choices for viscosity a more consistent description of the energetics is obtained by assuming a plastic rheology

trace element behavior and redox facies in core shales of upper pennsylvanian kansas type cyclothems the black shale submember of core shales of pennsylvanian kansas type cyclothems is highly enriched in redox sensitive trace elements tes relative to the overlying gray shale submember as well as to many other modern and ancient organic rich deposits controls on trace element behavior in the black shale can be inferred from trace element enrichment factors efs relative to gray shale and the relationship of tes to total organic carbon toc most tes conformed to one of two patterns 1 mo u v zn and pb exhibited moderate efs and strong covariation with toc in samples having 10 wt toc and 2 cu ni cr and co exhibited low efs and moderate to strong covariation with toc in all samples these patterns are inferred to represent different responses to benthic redox conditions the first pattern is characteristic of tes of strong euxinic affinity i e those taken up in solid solution by fe sulfide or involved in other reactions catalyzed by free h2s and resident mainly in authigenic phases whereas the second pattern is characteristic of tes of weak euxinic affinity i e those not strongly influenced by the presence of free h2s and resident mainly in the organic carbon or detrital fractions of the sediment these inferences allowed development of a multiproxy technique for assessing redox facies in black shale samples euxinic conditions were considered to have existed if at least two of four redox indicator trace elements mo u v zn showed euxinic levels of enrichment and nonsulfidic anoxic conditions were inferred otherwise the validity of the procedure is indicated by 1 agreement among all four redox indicators for a large majority of samples 69 of 185 2 among samples yielding a mixed redox signal a systematic sequence of te enrichment v zn mo u and 3 for tes of weak euxinic affinity reduced variance among samples representing each redox facies sequential enrichment may be a response to differential redox thresholds for accumulation e g v or postdepositional remobilization of trace elements e g u and may provide a basis for finer assessment of redox conditions in low oxygen paleoenvironments than permitted by simple classification schemes the multiproxy procedure for redox facies analysis developed in this study is likely to be more reliable than widely used single proxy indicators based on trace elements e g mo authigenic u or v v ni or c s fe systematics e g s toc or dop d 2004 elsevier b v all rights reserved

controls on trench topography from dynamic models of subducted slabs a finite element method with constrained elements and lagrange multipliers is used to study tectonic faults in a viscous medium a fault representing the interface between overriding and subducting plates has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included the fault causes stress to concentrate in its vicinity giving rise to a weak plate margin and a mobile plate if a power law rheology is used surface dynamic topography with either a newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin this suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow our models show that trench depth increases with fault dip angle slab dip angle slab length and age of oceanic lithosphere just prior to subduction the influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length these relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths for those subduction zones with oceanic lithosphere on both sides of the trench observed trench depths have been corrected for fault and slab dip angles based on the relationships from the dynamic models after correction to a common set of parameters trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict comparison between the trench depths corrected for fault and slab dip angles and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 mpa to 30 mpa depending on model details

wind erosion on the north eastern tibetan plateau constraints from osl and u th dating of playa salt crust in the qaidam basin the arid qaidam basin is the largest 3 88 x 10 4 km 2 basin on the north eastern tibetan plateau wind erosion in the area has been regarded as an important trigger for intra basin tectonic balance upheaval geomorphologic development and as a major supplier of dust to the chinese loess plateau downwind an initial estimate of the rate of wind erosion kapp et al 2011 based on geological cross sections has suggested up to 3 2 x 10 4 km 3 of sediments has been deflated over the past 2 8 ma lowering the landscape by an average of 0 29 mm yr in this paper we re evaluate this estimate by dating surface crusts present on three playas within the basin understanding the development of these playas is crucial to assessing the overall role of the wind in shaping the regional landscape because they are typically capped with a thick salt crust which effectively protects them from wind erosion optically stimulated luminescence osl and u series dating from a pit section and from the top of a deep drill core together with results from magnetostratigraphy and a climate proxy record correlated to the marine oxygen isotope record are used here to determine the age of the playa plains and suggest that the salt crusts have an age of c 0 1 ma this young age and the wide distribution of resistant thick salt crusts of the playa plains indicate a much lower degree of wind erosion than previously suggested the crusts protect the surface from significant surface erosion including sediment exhumation and unloading and whilst some wind erosion does occur it is unlikely to be sufficient to trigger tectonic uplift of the basin or to be a major dust source for the loess plateau as previously suggested copyright c 2013 john wiley sons ltd

earthquake size distribution power law with exponent beta 1 2 we propose that the widely observed and universal gutenberg richter relation is a mathematical consequence of the critical branching nature of earthquake process in a brittle fracture environment these arguments though preliminary are confirmed by recent investigations of the seismic moment distribution in global earthquake catalogs and by the results on the distribution in crystals of dislocation avalanche sizes we consider possible systematic and random errors in determining earthquake size especially its seismic moment these effects increase the estimate of the parameter beta of the power law distribution of earthquake sizes in particular we find that estimated beta values may be inflated by 1 3 because relative moment uncertainties decrease with increasing earthquake size moreover earthquake clustering greatly influences the beta parameter if clusters aftershock sequences are taken as the entity to be studied then the exponent value for their size distribution would decrease by 5 10 the complexity of any earthquake source also inflates the estimated beta value by at least 3 7 the centroid depth distribution also should influence the beta value an approximate calculation suggests that the exponent value may be increased by 2 6 taking all these effects into account we propose that the recently obtained beta value of 0 63 could be reduced to about 0 52 0 56 near the universal constant value 1 2 predicted by theoretical arguments we also consider possible consequences of the universal beta value and its relevance for theoretical and practical understanding of earthquake occurrence in various tectonic and earth structure environments using comparative crystal deformation results may help us understand the generation of seismic tremors and slow earthquakes and illuminate the transition from brittle fracture to plastic flow

the geochemistry of redox sensitive trace metals in sediments abstract we analyzed the redox sensitive elements v mo u re and cd in surface sediments from the northwest african margin the u s northwest margin and the arabian sea to determine their response under a range of redox conditions where oxygen penetrates 1 cm or less into the sediments mo and v diffuse to the overlying water as mn is reduced and remobilized authigenic enrichments of u re and cd are evident under these redox conditions with the onset of sulfate reduction all of the metals accumulate authigenically with re being by far the most enriched general trends in authigenic metal accumulation are described by calculating authigenic fluxes for the 3 main redox regimes oxic reducing where oxygen penetrates 1 cm and anoxic conditions using a simple diagenesis model and global estimates of organic carbon rain rate and bottom water oxygen concentrations we calculate the area of sediments below 1000 m water depth in which oxygen penetration is 1 cm to be 4 of the ocean floor we conclude that sediments where oxygen penetrates 1 cm release mn v and mo to seawater at rates of 140 260 60 150 and 5 10 of their respective riverine fluxes using the authigenic metal concentrations and accumulation rates from this work and other literature these sediments are sinks for re cd and u with burial fluxes of 70 140 30 80 and 20 40 respectively of their dissolved riverine inputs we modeled the sensitivity of the response of seawater re cd and v concentrations to changes in the area of reducing sediments where oxygen penetrates 1 cm our analysis suggests a negligible change in seawater re concentration whereas seawater concentrations of cd and v could have decreased and increased respectively by 5 10 over 20 kyr if the area of reducing sediments increased by a factor of 2 and by 10 20 if the area increased by a factor of 3 the concentration variations for a factor of 2 increase in the area of reducing sediments are at about the level of uncertainty of cd ca and v ca ratios observed in foraminifera shells over the last 40 kyr this implies that the area of reducing sediments in the ocean deeper than 1000 m 4 has not been greater than twice the present value in the recent past

importance of atmospheric inputs and fe oxides in controlling soil uranium budgets and behavior along a hawaiian chronosequence abstract a long term budget of uranium calculated for a chronosequence of hawaiian soils demonstrates the importance of mineral aerosol deposition as a geochemical pathway a series of sequential extractions on the same soils shows a strong association between fe oxides and uranium especially the amorphous and poorly crystalline fe oxides present in intermediate age soils 20 and 150 ka despite intense leaching conditions soil u contents were found to increase between 20 fold and 60 fold with age along the chronosequence from 9 to 17 g cm 2 at 0 3 ka sites to 344 543 g cm 2 at the 4100 ka site an open system transport function comparing u to an index least mobile element nb was used to calculate u gains or losses from the soils relative to the input from parent material weathering results show small losses or small gains 25 of u occurred in soils 150 ka and younger while gains of up to 531 u occurred in soils older than 150 ka highlighting the importance of mineral aerosol dust deposition at the older sites in the 4100 ka soil the ratio of dust derived u inputs to parent material derived u inputs is approximately 6 sequential extractions show that between 0 1 and 1 2 of the total u in the soils was extracted with ammonium acetate indicating that very little u exists as exchangeable cations a second extraction employing citrate dithionate and hydroxylamine hydrochloride was performed to extract u associated with fe oxides average percent occurrences for u in the fe oxide extraction for each profile are 7 at 0 3 ka 53 at 20 ka 40 at 150 ka and 17 at 4100 ka illustrating an increase with age to 20 ka followed by a subsequent decline this parallels the trend observed in amorphous fe oxide content suggesting that adsorption on amorphous fe oxides is the dominant control on u in the intermediate age soils

paleoclimate change since the miocene inferred from clay mineral records of the jiuquan basin nw china abstract global cooling and the uplift of the tibetan plateau have long been considered the crucial controls of the climate change in inland asia since the cenozoic however which of these factors has played the leading role is unknown to understand climate change and the controlling factors in inland asia here we present new records of clay mineralogy and trace elements of clay fractions from the neogene sedimentary sequence in the jiuquan basin of the northeastern tibetan plateau the clay mineral records reveal four stages of paleoclimate changes from ca 24 ma to 17 ma the low total smectite and smectite i s mixed layers kaolinite illite chlorite and high illite contents suggest weak chemical weathering and hence arid climatic conditions for ca 17 14 ma the high total smectite and total smectite kaolinite illite chlorite and low illite contents suggest strong chemical weathering indicating a change in climate from cold and dry to warm and humid there was no palygorskite during this warm and humid period from ca 14 ma to 8 ma the low total smectite content high illite content and the appearance of palygorskite indicate that the climate turned to cold and dry since 8 ma low total smectite as well as high illite and chlorite contents suggest a colder and drier period the records of clay mineralogy illustrate that the jiuquan basin has experienced increasing aridity since ca 14 ma synchronous with the inland asian aridification given the synchroneity with the global cooling we suggest that the climate evolution in the jiuquan basin from the late oligocene to middle late miocene was mainly controlled by global climate change since the late miocene the uplift of the tibetan plateau and global cooling have significantly influenced the decreasing temperature and humidity of the jiuquan basin the uplift of tibetan plateau may intensify the aridity by blocking moisture from reaching the jiuquan basin

dissolved mo and u in rivers and estuaries of india implication to geochemistry of redox sensitive elements and their marine budgets abstract dissolved molybdenum mo and uranium u concentrations were measured in five indian estuaries the narmada tapi mandovi and the mahi fall into the arabian sea and the hooghly falling into the bay of bengal riverine mo and u vary significantly in the range of 1 to 90 and 0 02 to 19 nmol kg respectively the lowest mo and u were observed in the mandovi river consistent with lateritic lithology exposed in its drainage higher runoff and lower water rock interaction the sabarmati has the highest u 19 nmol kg and sources from the groundwater having very high u concentration the highest mo 90 nmol kg is observed in the mahi river and probably results from anthropogenic sources hooghly river seems to have high concentrations of mo 14 nmol kg and u 8 nmol kg and are possibly derived from weathering of black shales of the himalaya behaviour of mo and u in all the estuaries analysed in this study is highly variable both mo and u in the narmada estuary during pre monsoon and monsoon seasons and u in the tapi estuary behave conservatively mo in the tapi shows its addition in the mid salinity ranges 4 to 12 and is sourced from anthropogenic activity probably related to effluent from industries situated along the estuary in this salinity range both dissolved mo and u show their removal at lower salinity ranges in the hooghly and the mandovi estuaries these estuaries are net sink of mo in which significant amount of oceanic mo along with riverine mo is being lost about 1 6 10 6 and 2 10 5 mol of mo are being removed annually in the hooghly and the mandovi estuaries uranium removal is order of magnitude lower compared to mo in both the hooghly and the mandovi estuaries the loss of mo and u in the estuaries are associated with mangrove swamps present in the estuaries which seems to be an important sink of the oceanic mo and could represent its significant sink in oceanic budget of mo the gulf of cambay has high dissolved mo concentration compared to that of global average seawater value which is attributed to its supply from anthropogenic sources industrial effluent waste waters along with polluted rivers supply 5 10 6 mol of mo annually to the gulf of cambay which could be derived from petrochemicals and pharmaceutical industries situated along the gulf coast

uranium diagenesis in sediments underlying bottom waters with high oxygen content we measured u in sediments both pore waters and solid phase from three locations on the middle atlantic bight mab from the eastern margin of the united states a northern location on the continental shelf off massachusetts oc426 75 m water depth and two southern locations off north carolina en433 1 647 m water depth and en433 2 2648 m water depth these sediments underlie high oxygen bottom waters 250 270 m but become reducing below the sediment water interface due to the relatively high organic carbon oxidation rates in sediments en433 1 212 mol c cm2 y oc426 120 10 mol c cm2 y en433 2 33 mol c cm2 y pore water oxygen goes to zero by 1 4 1 5 cm at en433 1 and oc426 and slightly deeper oxygen penetration depths were measured at en433 2 4 cm all of the pore water profiles show removal of u from pore waters calculated pore water fluxes are greatest at en433 1 0 66 0 08 nmol cm2 y and less at en433 2 and oc426 0 24 0 05 and 0 13 0 05 nmol cm2 y respectively solid phase profiles show authigenic u enrichment in sediments from all three locations the average authigenic u concentrations are greater at en433 1 and oc426 5 8 0 7 nmol g and 5 4 0 2 nmol g respectively relative to en433 2 4 1 0 8 nmol g this progression is consistent with their relative ordering of reduction intensity with greatest reducing conditions in sediments from en433 1 less at oc426 and least at en433 2 the authigenic u accumulation rate is largest at en433 1 0 47 0 05 nmol cm2 y but the average among the three sites on the mab is 0 2 nmol cm2 y pore water profiles suggest diffusive fluxes across the sediment water interface that are 1 4 1 7 times greater than authigenic accumulation rates at en433 1 and en433 2 these differences are consistent with oxidation and loss of u from the solid phase via irrigation and or bioturbation which may compromise the sequestration of u in continental margin sediments that underlie bottom waters with high oxygen concentrations previous literature compilations that include data exclusively from locations where o2 bw 150 m suggest compelling correlations between authigenic u accumulation and organic carbon flux to sediments or organic carbon burial rate sediments that underlie waters with high o2 bw have lower authigenic u accumulation rates than would be predicted from relationships developed from results that include locations where o2 bw 150 m

orbitally tuned age model for the late pliocene pleistocene lacustrine succession of drill core sg 1 from the western qaidam basin ne tibetan plateau

rheologic controls on slab dynamics several models have been proposed to relate slab geometry to parameters such as plate velocity or plate age however studies on the observed relationships between slab geometry and a wide range of subduction parameters show that there is not a simple global relationship between slab geometry and any one of these other subduction parameters for all subduction zones numerical and laboratory models of subduction provide a method to explore the relative importance of different physical processes in determining subduction dynamics employing 2 d numerical models with a viscosity structure constrained by laboratory experiments for the deformation of olivine we show that the observed range in slab dip and the observed trends between slab dip and convergence velocity subducting plate age and subduction duration can be reproduced without trench motion i e slab roll back for locations away from slab edges successful models include a stiff slab that is 100 1000 times more viscous than previous estimates from models of plate bending the geoid and global plate motions we find that slab dip in the upper mantle depends primarily on slab strength and plate boundary coupling with a small dependence on subducting plate age once the slab sinks into the lower mantle the primary processes controlling slab evolution are 1 the ability of the stiff slab to transmit stresses up dip 2 resistance to slab descent into the higher viscosity lower mantle and 3 subduction induced flow in the mantle wedge corner

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

exploring quaternary paleolake evolution and climate change in the western qaidam basin based on the bulk carbonate geochemistry of lake sediments abstract this study focuses on the concentration of major and trace elements in the diluted acetic acid soluble fraction of sg 1 core lacustrine sediments from the western qaidam basin tibetan plateau in order to explore the potential of applying bulk carbonate analysis to tracing paleolake evolution and climate change the sg 1 core sediments are considered highly suited to the exploration of the paleoenvironmental significance of bulk carbonate geochemistry because most carbonates in the core are authigenic in origin based on a principal component analysis of the elemental composition of diluted acetic acid leachate four major components with distinct characteristics have been extracted accounting for 80 of the total variance the first and most important component is associated with a high positive loading of mn p y sc and rare earth elements ree the second component is characterized by a high positive loading of ca sr and u the third component displays a high negative loading of mg li and b and the fourth component is associated with a relatively high positive loading of fe v and cr after considering the geochemical behavior of these element assemblies during acid leaching we have concluded that these four components contain information indicative of watershed climate conditions carbonate precipitation lake water salinity and iron oxide content respectively reconstructions of watershed climate change and paleolake evolution using the component scores are comparable to those reconstructed from other independent proxy records indicating a long term trend of lake shrinkage and climatic drying since 2 7 ma with abrupt drying events at 2 5 ma 1 8 ma 1 2 ma and 0 6 ma our results suggest that lake carbonates bear more information about watershed climate and lake evolution than expected and that occurrences of some elements in particular trace elements in the diluted acetic acid leachate are not solely and necessarily linked to carbonate minerals

late pleistocene evolution of salt lakes in the qaidam basin qinghai province china abstract the qaidam basin is a tectonically controlled depression at 2700 m altitude on the northern margin of the qinghai xizang tibet plateau rimmed by the high aljun qilian ranges to the north and kunlun peaks to the south rising above 5000 m the basin floor experiences hyper arid conditions with evaporation precipitation ratios exceeding 100 1 periglacial and glacial conditions prevail on montane peaks with permafrost above 3800 m on the basin floor small waterbodies lie on extensive halite salt flats on the largest the qarhan salt flat 5800 km2 in extent the small lakes dabuxun and taijinier represent contracted remnants of a once expansive water body throughout neogene time a succession of clastic sediments indicates lake depo centre migration from nw to se towards late pliocene evaporitic conditions developed with formation of gypsum halite celestite mirabilite and bloedite continued deformation through pleistocene time has developed a series of nw se trending domes and troughs this setting has continued to exert a controlling influence on basin and brine evolution with progressive development through sulfate to chloride brines as dry conditions intensified in late pleistocene expanded lake conditions qarhan mega lake corresponding to near the 2800 m contour indicate the existence of past climates much more humid than those of today a drilling program has defined the lateral and vertical extent of evaporitic halite deposits freshwater clastics and shell deposits which predate the halite phase indicate humid lacustral conditions until after 40 000 30 000 yr b p desiccation from about 25 000 yr until about 9000 yr b p resulted in deposition of halite to 60 m thick with thin parting muds representing brief returns to more humid environments high lake shorelines with aragonitic ooids surrounding the dry playa kunteyi and high level ostracod rich beach and near shore deposits on xiao qaidam attest to the presence of expanded late pleistocene lacustral conditions on lakes other than qarhan preceding the final evaporite stage clastic sediments associated with the halite are dominated by loessial silts equivalent to the late pleistocene malan deposits of central china simultaneous halite and loess deposition reflect the dry conditions that developed in the qaidam basin during the latter part of the last glaciation and which have persisted ever since the final evolution of brines has resulted in development of extensive potassium rich brines and salts with the mineral carnallite providing the basis for a large potash extraction industry near golmud the hydrologic history documented by the analyses of lake sediments reflects the interaction of tectonic deformation and climatic change in the development of brines to the potash stage

kinematics and flow patterns in deep mantle and upper mantle subduction models influence of the mantle depth and slab to mantle viscosity ratio 1 three dimensional fluid dynamic laboratory simulations are presented that investigate the subduction process in two mantle models an upper mantle model and a deep mantle model and for various subducting plate mantle viscosity ratios sp m 59 1375 the models investigate the mantle flow field geometrical evolution of the slab sinking kinematics and relative contributions of subducting plate motion and trench migration to the total rate of subduction all models show that the subducting plate is always moving trenchward resulting from slab pull furthermore all deep mantle models show trench retreat as do upper mantle models in the initial stage of subduction before slab tip transition zone interaction upper mantle models with a low sp m 66 217 continue to show trench retreat after interaction upper mantle models with a high sp m 378 709 show a period of trench advance after interaction followed by trench retreat upper mantle models with a very high sp m 1375 show continued trench advance after interaction the difference in trench migration behavior and associated slab geometries is attributed to both sp m and the mantle depth to plate thickness ratio tm tsp which both affect the slab bending radius to mantle thickness ratio rb tm four subduction regimes can be defined regime i with rb tm 0 3 trench retreat slab draping and a concave trench regime ii with 0 3 rb tm 0 5 episodic trench migration slab folding and a concave trench regime iii with rb tm 0 5 trench advance slab rollover geometries and minor trench curvature and regime iv with rb tm 0 8 trench retreat slab draping and a rectilinear trench in all models slab parallel downdip motion induces poloidal mantle flow structures in addition trench retreat and rollback motion of the slab induce quasi toroidal return flow around the lateral slab edges toward the mantle wedge rollback induced poloidal flow around the slab tip is not observed in any of the experiments finally comparison between the slab geometries observed in the upper mantle models and slab geometries observed in nature imply that the effective viscosity ratio between slab and ambient upper mantle in nature is less than 103 and of the order 1 7 102 with a best estimate of 1 3 102

physical characteristics of subduction interface type seismogenic zones revisited 1 based on global earthquake catalogs the hypocenters nodal planes and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007 assuming that the seismogenic zone coincides with the distribution of 5 5 m 7 earthquakes the subduction interface seismogenic zones were mapped for 80 of the trench systems and characterized with geometrical and mechanical parameters using this database correlations were isolated between significant parameters to identify cause effect relationships empirical laws obtained in previous studies were revisited in light of this more complete accurate and uniform description of the subduction interface seismogenic zone the seismogenic zone was usually found to end in a fore arc mantle rather than at a moho depth the subduction velocity was the first order controlling parameter for variations in the physical characteristics of plate interfaces determining both the geometry and mechanical behavior as such the fast subduction zones and cold slabs were associated with large and steep plate interfaces which in turn had large seismic rates the subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches events with mw 8 5 preferentially occurred in the vicinity of slab edges where the upper plate was continental and the back arc strain was neutral this observation was interpreted in terms of compressive normal stresses along the plate interface large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally

bending stress and dissipation in subducted lithosphere 1 oceanic lithosphere undergoes permanent deformation during subduction once the stresses exceed the elastic limit departures from elastic behavior occur by brittle failure in the shallow lithosphere and by a combination of low and high temperature creep at greater depths we combine laboratory based rheological models with estimates of slab shape from earthquake hypocenters to quantify the bending stress and dissipation in subduction zones the peak stress occurs at the depth of the brittle ductile transition which is controlled mainly by lithospheric age integrals of the stress over the thickness of the plate are used to evaluate the resistive bending force and the bending moment a representative value for the resistive force on old oceanic lithosphere is 3 1012 n m 1 which is comparable in magnitude to ridge push but opposite in direction both the bending force and moment are remarkably insensitive to the rate of subduction in fact the bending moment can be approximated using a simple power law rheology and a stress exponent of n 14 such a large exponent implies that the lithosphere behaves like a perfectly plastic solid for most subduction zones the bending moment saturates along the entire plate as a consequence the bending stress does not influence the development of curvature during subduction this behavior may explain why the curvature of subducted lithosphere is nearly independent of age