extreme climatic conditions recorded in santa barbara basin laminated sediments the 1835 1840macoma event abstract the santa barbara basin sbb record of laminated sediment well known for its excellent time resolution shows the presence of an unusual once in a millenium event in the period 1835 1840 at a depth of 55 57 cm below the surface of the seafloor this macoma event is the brief flourishing of a macrofaunal community in the deep center of the sbb from which large organisms are normally excluded by the lack of oxygen shells of the pelecypodmacoma leptonoidea dall and associated fossils which congregate in a thin layer dated at 1840 provide evidence for a sudden local extinction of macrobenthos by suffocation various stratigraphic evidence suggests thatm leptonoidea colonized the basin floor for a few years prior to 1840 as a result of temporarily decreased productivity and increased oxygen content in the water spilling over the sill into the basin historical eyewitness reports indicate unusually severe winter storms from southeasterly directions during the 1830s in the sbb we suggest that coastal upwelling was severely reduced during 1835 1839 with the effect of reducing the intensity of the oxygen minimum layer themacoma event apparently is one manifestation of large scale climatic anomalies felt throughout north america and which were possibly linked to the major volcanic eruption of cosiguina nicaragua in 1835

climatically controlled marker layers in santa barbara basin sediments and fine scale core to core correlation varved sediments with annual layering can provide a detailed history of environmental conditions of marginal ocean basins and of lakes to reconstruct this history requires fine scale correlation and correct age assignment in the santa barbara basin a combination of x radiography water content profiles and microfossil stratigraphy yields correlations of the desired resolution x radiography permits identification of individual layers 1 2 mm thick that differ in porosity or pore water content and hence x ray absorption microfossils and sedimentological parameters provide the clues to the presence for el nino years e g in a d 1984 1959 and 1942 varves allowing the tie in to absolute years within this century el nino layers once identified are readily recognized on the radiographs and within the porosity profiles counting of laminae in x radiographs and core to core correlation via water content then allow establishing age scales for other cores a prominent shell layer in the first half of the 19th century is dated at a d 1838 this event macoma i is of supraregional significance

impact of micro organisms activity on the natural remanent magnetization of the young oceanic crust microbes have been widely reported in the deep subseafloor environment still it is difficult to detect a global chemical signature of bacterial activity in the oceanic crust we carried out experiments up to 355 days exposing very young oceanic basalts to anaerobe sulfate reducing organisms in an in vitro marine environment the natural remanent magnetization of samples was monitored during the whole duration of experiments and within this time frame the most magnetized sub samples lost up to 30 of their original signal scanning electron microscope observations show cycling of iron from the titanomagnetites to iron sulfide phases our results suggest that microbes can have a major and fast impact on the magnetization of young oceanic basalts and could contribute to a global signal as the central anomaly magnetic high seen along ridges axis

a 750 year fire history based on lake sediment records in central yellowstone national park usa a 750 year fire history was reconstructed for the central plateau of yellowstone national park from the deep water sediments of five lakes the charcoal record from a large lake provided a chronology of regional fires data from four small lakes were used to study local and extralocal fires the co occurrence of abundant charcoal and high magnetic susceptibility values at the same stratigraphic level was used as evidence of a local catchment fire and a charcoal peak without high magnetic susceptibility was ascribed to an extralocal fire or a local fire without a related erosion event the fire history was compared with the dendrochronologic fire record for the last 450 years and the close agreement provided the justification to extend the chronology back in time large areas of the region burned in ad 1988 c 1700 c 1560 and c 1440 from c 1220 to 1440 and c 1700 to 1987 intermediate to small areas burned the near absence of fires in the twentieth century prior to the large fires of 1988 is evi

formation of a jump by the dam break wave over a granular bed a previously unreported shock feature associated with the scouring of a horizontal granular bed by a dam break wave is discussed near the wave centre the present study shows the free surface breaks backward and a hydraulic jump forms this behaviour is described from the standpoint of shallow water theory suitably extended to deal with non equilibrium sediment transport the shock formation involves a particularly strong coupling between flow free surface evolution and bed morphodynamics support for our conclusions is sought through experimental and numerical approaches in order to magnify the observed phenomena measurements were performed for the case of light bed particles moving in sheet and debris flow modes a detailed picture of the transient two phase flow is presented based on whole field acquisition of the grain motions by particle tracking techniques corresponding shallow water solutions are constructed numerically using a shock capturing scheme finally an interpretation of the jump formation is proposed based on the theory of characteristics

three dimensional numerical model for flow and bed deformation around river hydraulic structures this paper describes a numerical model developed to simulate flow and bed deformation around river hydraulic structures the model solved the fully three dimensional reynolds averaged navier stokes equation expressed in a moving boundary fitted coordinate system to calculate the flow field with water and bed surfaces varying in time a nonlinear k e turbulence model was employed in order to predict flow near the structure where three dimensional flow is dominant the temporal change in bed topography was calculated by coupling a stochastic model for sediment pickup and deposition using a momentum equation of sediment particles in order to account for the effect of nonequilibrium sediment transport in validating the numerical model a spur dike and a bridge pier which are considered to be typical river engineering structures were selected by comparing the numerical results with observed laboratory experimental data the model was found to reproduce flow and scour geometry around these structures with

grain size dependent paleointensity results from very recent mid oceanic ridge basalts 1 we report paleointensity analyses on a suite of four samples from two axial zero age mid oceanic ridge basalt flows from the east pacific rise paleointensity experiments have been performed on several profiles each consisting of a batch of small millimeter scale subsamples going from the rapidly cooled glassy margin to the interior the coe version of the thellier double heating procedure was used with back checks performed after every other heating step most of the samples show very good behavior i e constant ratio between the lost and acquired magnetization through a large temperature range quality factor usually above 10 and positive checks which lead to unambiguous paleointensity determinations paleointensities obtained on glasses reported here and in a previously published study are very consistent and reproducible and in agreement with expected in situ values of earth s magnetic field intensity however results found within the crystalline part of the samples show values that are considerably up to 50 higher than expected these variations seem to be correlated with the cooling history of the samples and appear to be universal since all samples exhibit the same intriguing pattern an extensive study of magnetic properties allows us to link this incongruent behavior to 1 the presence of multidomain effects and 2 the cooling rate difference between laboratory experiments and in situ cooling

numerical analysis of river channel processes with bank erosion this paper presents a numerical analysis of river channel processes with bank erosion the model can be used for investigating both bed deformation and bankline shifting in 2d plan form the basic equations are used in a moving boundary fitted coordinate system and a new formulation of nonequilibrium sediment transport is introduced to reproduce the channel processes the model was applied to examine the morphological behavior of experimental channels temporal changes in the plan form in a meandering channel can be classified into two patterns meander developing and meander straightening comparison of the observed and calculated results indicates that the model is applicable to both channel changes under various hydraulic conditions on the basis of the numerical findings the paper clarifies the influence of hydraulic variables on the location of bank erosion and bed scouring the model also was used to investigate the effect of alternate bars on bank erosion and to investigate the development of channel meandering from an initially straight channel

nonhydrostatic dam break flows ii one dimensional depth averaged modeling for movable bed flows abstractthe dam break flow over a movable bed is an important problem in fluvial flow processes these flows are usually predicted by a one dimensional 1d approach based on a hydrostatic pressure distribution recent three dimensional 3d nonhydrostatic simulations of dam break waves over movable beds based on the reynolds averaged navier stokes rans equations revealed that the saint venant theory is not accurate in predicting the flow dynamics within the scour hole developed in this work a generalized 1d nonhydrostatic model for flow over movable beds is proposed assuming a linear nonhydrostatic pressure distribution the new set of 1d equations accounts for the vertical acceleration which is important in dam break waves over movable beds given the instantaneous curved beds generated over the erodible terrain these equations account for both the bed and suspended load transport modes a high resolution finite volume numerical scheme with a semi implicit treatment of nonhydrostatic terms is dev

time variations in geomagnetic intensity 1 after many years spent by paleomagnetists studying the directional behavior of the earth s magnetic field at all possible timescales detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components a significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr a second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field a third step was the juxtaposition of these signals with those derived from cosmogenic isotopes contemporaneous with the acquisition of records new techniques have been developed to constrain the geomagnetic origin of the signals much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials proper selection of samples and investigations of complex changes in magnetization during laboratory experiments altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 myr on longer timescales the field variability and its average behavior is relatively well documented for the past 400 myr section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past in each case current limits and potential promises are discussed the section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and or materials

computational dam break hydraulics over erodible sediment bed this paper presents one of the first dedicated studies on mobile bed hydraulics of dam break flow and the induced sediment transport and morphological evolution a theoretical model is built upon the conservative laws of shallow water hydrodynamics and a high resolution numerical solution of the hyperbolic system is achieved using the total variation diminishing version of the second order weighted average flux method in conjunction with the hllc approximate riemann solver and superbee limiter it is found that a heavily concentrated and eroding wavefront first develops and then depresses gradually as it propagates downstream in the early stage of the dam break a hydraulic jump is formed around the dam site due to rapid bed erosion which attenuates progressively as it propagates upstream and eventually disappears while the backward wave appears to migrate at the same speed as over a fixed bed the propagation of the forward wavefront shows a complex picture compared to its fixed bed counterpart as a result of the domination of rapid bed erosion initially the density difference between the wavefront and the downstream ambient water in the intermediate period and the pattern of the deformed bed profile in the long term it is also found that the free surface profiles and hydrographs are greatly modified by bed mobility which has considerable implications for flood prediction the computed wave structure in the intermediate period exhibits great resemblance to available experiments qualitatively and yet the existence of a shear wave is found in lieu of a secondary rarefaction postulated in an existing analysis finally the use of the complete rather than simplified conservation equations is shown to be essential for correct resolution of the wave and bed structures which suggests that previous models need reformulating

oceanic basalt continuous thermal demagnetization curves summary the palaeomagnetic standard technique of stepwise thermal demagnetization std long regarded as unreliable for oceanic basalts that have undergone low temperature alteration has recently been applied in a number of studies to characterize the natural remanent magnetization nrm of such rocks in order to better understand std data of oceanic basalts and to possibly identify the magnetominerals that are carrying the nrm we have carried out a number of continuous and std experiments on seven oceanic basalt samples during continuous thermal demagnetization ctd a sample is heated to a certain temperature and its nrm is measured during heating and subsequent cooling even when ctd reveals only titanomaghemite unblocking at 400 c as the remanence carrier std behaviour can be very complex and unblocking is observed at temperatures of up to 500 c and higher ctd also allowed to identify a partial or full self reversal of nrm due to interaction between two types of magnetominerals in one sample the higher degree of maghemitization of smaller titanomaghemite grains with respect to larger ones which are less efficient in carrying the remanence was seen for three samples by a shift of 80 c between the strong field thermomagnetic curve and the nrm measured at elevated temperature in several cases the identification of the nrm carrying magnetomineral was not possible from ctd data due to the ambiguity of curie temperatures in the titanomagnetite titanomaghemite system

paleointensity in hawaiian scientific drilling project hole hsdp2 results from submarine basaltic glass 1 paleointensity estimates based on the high quality thellier thellier data from the early brunhes 420 780 ka are rare only 30 in the published literature the second hawaiian scientific drilling project hsdp2 drill hole recovered submarine volcanics spanning the approximate time period of 420 550 ka these are of particular interest for absolute paleointensity studies owing to the abundance of fresh submarine basaltic glass which can preserve an excellent record of ancient geomagnetic field intensity we present here new results of thellier thellier paleointensity experiments that nearly double the number of reliable paleointensity data available for the early brunhes we also show that the magnetizations of the associated submarine basalts are dominated by viscous magnetizations and therefore do not reflect the true ancient geomagnetic field intensity at the time of extrusion the viscous contamination is particularly severe because of a combination of low blocking temperatures in the basalts and relatively high temperatures in the deeper parts of the drill core our new data when placed on the approximate timescale available for hsdp and hsdp2 are at odds with other contemporaneous paleointensity data the discrepancy can be reconciled by adjusting the hsdp timescales to be younger by about 35 kyr

modeling overfalls using vertically averaged and moment equations a new set of vertically averaged and moment equations which assumes a linear longitudinal velocity distribution and quadratic vertical velocity and pressure distributions is used for modeling flow in the vicinity of horizontal rectangular free overfall with smooth and rough beds and sharp crested weirs with sloping upstream faces these equations are modeled using a hybrid petrov galerkin and bubnov galerkin finite element scheme for the rectangular free overfalls the predicted water surface profiles upstream of the overfall and the free jet trajectory agree well with the measured data the computed vertical velocity and pressure distributions at the brink and upstream of the overfall are found to be in good agreement with the measured data while the computed longitudinal velocity distributions compare well with a two dimensional potential flow model the computed results for sharp crested weirs with sloping upstream faces agree well with the measured data for an upstream weir slope of up to 27 with the horizontal for an upstream slope of 45 and steeper and for a large weir height the predicted water surface upstream of a weir shows numerical instability

grain scale nonequilibrium sediment transport model for unsteady flow abstracta one dimensional 1d finite volume model is developed for simulating nonequilibrium sediment transport in unsteady flow the governing equations are the 1d mass and momentum conservation equations for sediment laden flow and the sediment continuity equation for both bed load and suspended load transport the rouse profile is modified to consider the nonequilibrium transport of suspended sediment the spatial lag between the instantaneous flow properties e g velocity bed shear stress and the rate of bed load transport in unsteady flow is quantified by using an adaptation length which is derived theoretically by applying the momentum principle in the bed load layer this new method for calculating the adaptation length is verified using data from several experiments and yields better results than other empirical formulas for a wide range of shear stress the nonequilibrium model is applied to simulate a series of laboratory dam break flows over erodible beds and the simulated results agree

timing of volcanism along the northern east pacific rise based on paleointensity experiments on basaltic glasses 1 samples from two adjacent and contrasting ridge segments along the east pacific rise were measured for their magnetic paleointensity in order to further explore the possibilities of dating very young volcanic samples using secular variations in the earth s magnetic field the ridge segment north of the orozco transform fault 15 22 16 20 n is the shallowest and broadest along more than 5000 km of the east pacific rise whereas the adjacent segment to the north 16 16 18 n has a typical morphology for its intermediate spreading rate both ridge segments were densely sampled during the panr01mv cruise and 36 samples of axial lava flows consisting mainly of glasses from the rims of the flows and some fragments of lobate basalts were selected from this collection for paleointensity experiments the coe version of the thellier double heating procedure in air was used twenty seven units provide internally consistent paleointensity estimates leading to precise estimates of the paleofield which range between 8 t and 57 t comparisons with reference paleointensity curves compiled from subaerial flows archeomagnetic data and sedimentary records projected to the sampling site coordinates show that the measured values can be used to constrain the volcanic history of the ridge segments over the past few thousand years a good agreement was found between apparent freshness of the glasses the geochemistry of the lavas and their magnetic paleointensity values the inflated southern segment seems characterized by recent activities as indicated by numerous flows with paleointensities clustering around today s value 39 t or around the high values typical of 2000 3000 years ago 55 t we interpret this distribution to indicate the flooding by effusive lava flows of the entire axial plateau some 2000 3000 years ago followed by a volcanic phase producing smaller volume lava flows confined to the innermost 200 m of the ridge axis the northern ridge segment is characterized by dispersed paleointensity values consistent with a series of small eruptions of diverse ages samples collected at the tips of both ridge segments across the 16 20 n axial discontinuity have the lowest paleointensities and are thus thought to be significantly older consistent with models advocating reduced magmatism near ridge axis discontinuities this study demonstrates the strong potential of paleointensity measurements as a tool to help constrain volcanic history at ridge axes

riemann wave description of erosional dam break flows this work examines the sudden erosional flow initiated by the release of a dam break wave over a loose sediment bed extended shallow water equations are formulated to describe the development of the surge accounting for bed material inertia a transport layer of finite thickness is introduced and a sharp interface view of the morphodynamic boundary is adopted approximations are sought for an intermediate range of wave evolution in which equilibration of the sediment load can be assumed instantaneous but momentum loss due to bed friction has not yet been felt the resulting homogeneous hyperbolic equations are mathematically tractable using the riemann techniques of gas dynamics dam break initial conditions give rise to self similar flow profiles the wave structure features piecewise constant states two smoothly varied simple waves and a special type of shock an erosional bore forming at the forefront of the wave profiles are constructed through a semi analytical procedure yielding a geomorphic generalization of the stoker solution for dam break waves over rigid bed for most flow properties the predictions of the theoretical treatment compare favourably with experimental tests visualized using particle imaging techniques

three dimensional numerical modeling of dam break flows with sediment transport over movable beds abstracta three dimensional 3d numerical model has been developed to simulate dam break flows with sediment transport over movable beds the hydrodynamic model solves the 3d reynolds averaged navier stokes equations using a finite volume method on collocated hexahedral meshes the volume of fluid vof technique with the compressive interface capturing scheme for arbitrary meshes is used to track the water surface boundary the sediment transport model solves the nonequilibrium transport equations of suspended load and bed load separately and in turn calculates the resulting bed change a moving mesh technique is adopted to track the time evolution of bed topography the grid moving velocity and computational cell volume change due to the moving mesh are taken into consideration when the hydrodynamic and sediment transport equations are discretized the developed model has been tested using several experimental dam break flows over movable beds the calculated temporal and spatial variations of water a

moraine dam breach and glacial lake outburst flood generation by physical and numerical models summary the areas of glacial lakes in the himalayas continue to increase alarmingly due to global warming with consequent risk of glacial lake outburst flood glof the outburst is most common for moraine dammed lakes as a result of a failure of the dam by overtopping and erosion or piping due to seepage in the present context glof events especially for the moraine dam failure process needs to be predicted to assess the drastic and detrimental impacts on the downstream river valley this study focuses on validating an integrated model for simulating the failure of moraine dam by overtopping and erosion using a coupled numerical simulation model the proposed numerical methodology is validated with the data from experiments in laboratory scale physical models the results of the model are compared for both the erosional profiles during collapse of the dam and the resulting outflowing flood hydrograph the primary finding is that the moraine dam height and the volume of the lake upstream of the dam are the most sensitive parameters influencing the glof peak the model is also applied to the field problem of the tangjiashan lake outburst and dam failure and shows reasonably good agreement with observations the validated model is also run for the south lhonak glacial lake in the sikkim himalayas india to obtain the probable flood hydrograph in case of failure of the moraine dam

thermally induced inversion of al substituted titanomagnetite in basalts evidence for partial self reversal 1 we report properties of laboratory induced thermoremanence combined with detailed rock magnetic experiments on basaltic samples which contain aluminum substituted titanomagnetite fe2 24ti0 64al0 12o4 prior to thellier coe paleointensity experiments specimens were first demagnetized using an alternating field with a peak field of 90 mt and were then imparted an anhysteretic remanent magnetization arm to represent a natural remanent magnetization nrm results show that the partial thermal remanent magnetization ptrm gain and arm remaining are linearly correlated up to 300 c between 300 and 460 c specimens acquire thermoremanence with a direction antiparallel to the external field direction leading to intensity decreases this coincides with the onset of the mineral transformation from al titanomagnetite to a magnetic phase with higher tc our results support that phase interaction with distinct tc values in partially oxidized titanomagnetite can produce partial self reversal thermoremanence in rocks

low temperature oxidation in morb of titanomagnetite to titanomaghemite a gradual process with implications for marine magnetic anomaly amplitudes mid ocean ridge basalt morb samples varying in age from recent to jurassic were selected for electron microscopic and rock magnetic studies our observations indicate that the degree of oxidation of titanomagnetite in morb increases only gradually with sample age the titanomagnetite in recent morb 100 ka both electron microscopic observations and rock magnetic data support the notion that low temperature oxidation of titanomagnetite to titanomaghemite in morb is a gradual process moreover the rate of maghemitization is controlled by many factors on both macroscopic and microscopic scales including regional oceanic crustal structures lithological features grain size and surrounding matrices pillow lavas and the tops of massive flows tend to have higher degrees of oxidation than the interiors of massive flows owing to higher porosity and permeability of the former in contrast interstitial glass can protect fine titanomagnetite grains from alteration the natural remanent magnetization nrm intensity of morb varies as a function of age magnetic granulometry concentration of magnetic materials and the degree of alteration fine grained morb samples typically have higher nrm intensity the nrm intensity appears to decrease substantially with increasing degree of maghemitization and hence with age the envelope of the smoothed magnetic anomaly amplitudes resembles the change in nrm intensity for morb samples of the last 30 million years but the underlying assumption that the highest degree of maghemitization observed in the samples of a given age is entirely responsible for these intensity changes is not supported maghemitization of the micrometer sized and larger grains is probably only partly responsible whereas a significant contribution to nrm intensities is inferred from those submicrometer sized titanomagnetite grains that remained protected from oxidation by the surrounding matrix of interstitial glass

nonhydrostatic granular flow over 3 d terrain new boussinesq type gravity waves understanding granular mass flow is a basic step in the prediction and control of natural or man made disasters related to avalanches on the earth savage and hutter 1989 pioneered the mathematical modeling of these geophysical flows introducing saint venant type mass and momentum depth averaged hydrostatic equations using the continuum mechanics approach however denlinger and iverson 2004 found that vertical accelerations in granular mass flows are of the same order as the gravity acceleration requiring the consideration of nonhydrostatic modeling of granular mass flows although free surface water flow simulations based on nonhydrostatic depth averaged models are commonly used since the works of boussinesq 1872 1877 they have not yet been applied to the modeling of debris flow can granular mass flow be described by boussinesq type gravity waves this is a fundamental question to which an answer is required given the potential to expand the successful boussinesq type water theory to granular flow over 3 d terrain this issue is explored in this work by generalizing the basic boussinesq type theory used in civil and coastal engineering for more than a century to an arbitrary granular mass flow using the continuum mechanics approach using simple test cases it is demonstrated that the above question can be answered in the affirmative way thereby opening a new framework for the physical and mathematical modeling of granular mass flow in geophysics whereby the effect of vertical motion is mathematically included without the need of ad hoc assumptions

depth averaged two dimensional numerical modeling of unsteady flow and nonuniform sediment transport in open channels a depth averaged two dimensional 2d numerical model for unsteady flow and nonuniform sediment transport in open channels is established using the finite volume method on a nonstaggered curvilinear grid the 2d shallow water equations are solved by the isimple c n algorithms with the rhie and chow s momentum interpolation technique the proposed sediment transport model adopts a nonequilibrium approach for nonuniform total load sediment transport the bed load and suspended load are calculated separately or jointly according to sediment transport mode the sediment transport capacity is determined by four formulas which are capable of accounting for the hiding and exposure effects among different size classes an empirical formula is proposed to consider the effects of the gravity on the sediment transport capacity and the bed load movement direction in channels with steep slopes flow and sediment transport are simulated in a decoupled manner but the sediment module adopts a coupling procedure for the computations of sediment transport bed change and bed material sorting the model has been tested against several experimental and field cases showing good agreement between the simulated results and measured data

low temperature alteration and magnetic changes of variably altered pillow basalts summary pillow basalt fragments from the east pacific rise dredged during the phoenix expedition often show discoloured rims due to alteration a suite of nine pillow basalts with such discoloured rims and ranging in age between 200 and 820 ka has been characterized in terms of their fe ti oxide mineralogy and rock magnetic properties these large pillow fragments show relatively unaltered grey interiors surrounded by darker concentric halos which vary in thickness as measured from glassy pillow rims and surfaces caused by large cracks penetrating into the original pillow interior the discoloured zones are characterized by precipitation of abundant secondary minerals such as fe 3 rich clays that filled vesicle spaces fe ti oxides in subsamples from discoloured rims and grey interiors have been investigated with electron microscopy and rock magnetic techniques the subsamples come from traverses that are parallel to the outer glassy pillow rims allowing us to study the low temperature alteration effects and rock magnetic properties without having to take variable grain size into account not surprisingly titanomaghemites in discoloured rims are in a general sense oxidized to a higher degree z typically 0 55 than those in the relatively unaltered grey interior z typically 0 6 within 200 000 yr the difference between the oxidation states of titanomaghemite within the grey pillow interior and the discoloured rims gradually diminishes with increasing age so that for samples with ages of 800 ka the oxidation state of titanomaghemites in the grey interior approaches that of the discoloured rim our study demonstrates that visible discolouration of pillow basalts is not a suitable proxy for z because average ti content can vary from sample to sample curie temperatures are also inaccurate proxies for z i fone wants to study possible correlations between z and rock magnetic parameters the best technique is to determine z for each subsample by using trans

variable ti content and grain size of titanomagnetite as a function of cooling rate in very young morb transmission electron microscopy observations and rock magnetic measurements on a pillow from the new flow extruded in 1993 at the juan de fuca ridge demonstrate that ti content in large up to 40 m titanomagnetite grains varies as a function of the cooling rate large grains in the interior of the pillow have a narrow composition range of approximately x 0 6 where x represents the ulvospinel content in the solid solution series with magnetite in contrast the titanomagnetite grains near the pillow rim are progressively smaller and have a broad composition range average x 0 45 within 0 5 cm of the rim a significant portion of the grains is single domain sd to superparamagnetic sp and appears to have little or no ti this lower ti content provides a ready explanation for the higher curie temperature near the glassy margin of the pillow moreover determination of the oxidation state of the titanomagnetite does not show low temperature oxidation anywhere within the pillow indicating no rapid alteration to titanomaghemite as previously suggested for this very young mid ocean ridge basalt morb in order to explain the curie temperatures submicrometer titanomagnetite in interstitial glass in the interior of the pillow also shows variable x values these grains make contributions to susceptibility that show up as high up to 580 c curie temperatures in thermomagnetic analyses and they appear to be sd to sp the variation of x values for the titanomagnetite in the morb is consistent with thermodynamic and kinetic considerations because crystallization of titanomagnetite in the morb is controlled by processes depending on cooling rate and crystal melt fractionation

prehistoric fires and the shaping of colonial transported landscapes in southern california a paleoenvironmental study at dune pond santa barbara county using a novel combination of paleoecologic proxies including pollen non pollen palynomorphs npps macroscopic charcoal and spheroidal carbonaceous particles scps 5000 years of landscape change fire history and land use have been reconstructed from dune pond santa barbara county california the pond was sensitive to holocene regional climatic variability showing different phases of lower 4600 3700 cal yr bp 2100 700 cal yr bp historical period and higher 3700 2100 cal yr bp 700 150 cal yr bp local moisture availability during this period the landscape was dominated by a coastal mosaic vegetation including dune mats coastal scrub and salt marshes on the dunes and backdunes with chaparral and oak woodland growing in the valley plains and foothills fire was intimately linked with such dominating mosaic vegetation and the combination of wet conditions and the presence of nearby human settlement were a trigger favoring coastal fires for at least two periods from 3100 to 1500 cal yr bp and from 650 cal yr bp until the 18th century in both cases fire was an important tool to keep an open coastal landscape attractive to hunting wildlife finally matching this varied range of high resolution paleoecological proxies with historical records we could characterize the development of colonial transported landscapes following the euro american settlement of santa barbara the introduction of livestock grazing by spanish colonists favored erosive processes and the introduction of fecal borne parasites in freshwater bodies negatively impacted salt and brackish coastal marshes and promoted the invasion of alien grasses and ruderals this agro pastoral landscape was consolidated during the american period with a greater role for cultivation the development of industrial activities and increased population despite negative environmental consequences such as the loss of native habitats exotic land uses and plants introduced during the historical period significantly contributed to the configuration of a cultural landscape which forms part of the cultural heritage of california

paleointensity record in zero age submarine basalt glasses testing a new dating technique for recent morbs abstract thellier thellier paleointensity experiments were conducted on a collection of glasses from three very recent submarine axial flows two were erupted along the juan de fuca ridge at around 46 n and one along the east pacific rise south at around 18 s the within sample dispersion of paleointensity results from the animal farm flow epr south is very low and leads to a well defined mean value of 35 6 1 t 95 error on the mean based on 11 glass chips from four independent samples today s geomagnetic field intensity in the area is 31 2 t comparing animal farm results with published field model reference curves developed for the past 400 yr suggests an eruptive date estimated between 1880 a d and 1950 a d taking into account different sources of errors this is consistent with qualitative evidence for the age of this flow and constitutes the first precise demonstration of using paleointensity as a dating tool for very recent mid ocean ridge basalts morbs however in the juan de fuca area results show a more erratic pattern with samples varying by up to 30 higher and lower from the expected value of about 55 t the dispersion is attributed to the large local crustal magnetic anomalies in this area that can lead to inconsistent intensity values over the same unit local magnetic anomalies should thus always be checked when doing paleointensity on morb samples which should also be distributed as widely as possible in a flow unit when no significant magnetic anomalies are detected the paleointensity dating tool is anticipated to be especially efficient to investigate the volcanic cyclicity along the epr axis during the last several hundred years

one dimensional modeling of dam break flow over movable beds a one dimensional model has been established to simulate the fluvial processes under dam break flow over movable beds the hydrodynamic model adopts the generalized shallow water equations which consider the effects of sediment transport and bed change on the flow the sediment model computes the nonequilibrium transport of bed load and suspended load the effects of sediment concentration on sediment settling and entrainment are considered in determining the sediment settling velocity and transport capacity in particular a correction factor is proposed to modify the van rijn formulas of equilibrium bed load transport rate and near bed suspended load concentration for the simulation of sediment transport under high shear flow conditions the governing equations are solved by an explicit finite volume method with the first order upwind scheme for intercell fluxes the model has been tested in two experimental cases with fairly good agreement between simulations and measurements the sensitivities of the model results to parameters such as the sediment nonequilibrium adaptation length manning s roughness coefficient and the proposed correction factor have been verified the proposed model has also been compared to an existing model and the results indicate the new model is more reliable

preservation of pristine titanomagnetite in older ocean floor basalts and its significance for paleointensity studies transmission electron microscopy observations and rock magnetic measurements reveal that alteration of fine and large grained iron titanium oxides can occur at different rates fine grained titanomagnetite occurs as a crystallization product within interstitial glass that originated as an immiscible liquid within a fully differentiated melt in several samples with ages to 32 ma it displays very little or no oxidation z 0 in contrast samples with ages of 10 ma or older are observed to also contain highly oxidized z 0 66 large grained titanomaghemite these large grains having originated by direct crystallization from melt are associated with pore space such pore space can serve as a conduit for fluids that promote alteration whereas fine grains may have been armored against alteration by the glass matrix in which they are embedded apparently alteration of oceanic crust is a heterogeneous process on a microscopic scale the existence of pristine fine grained titanomagnetite in the interstitial glass of older ocean floor basalts that have undergone significant alteration implies that such glassy material is capable of carrying original thermal remanent magnetization and may be suitable for paleointensity determinations

nonhydrostatic dam break flows i physical equations and numerical schemes abstractmodeling of dam break flows is frequently required in civil and environmental engineering because of the risk associated with this catastrophic flow typically model predictions are conducted using the saint venant hydrostatic theory which can lead to unrealistic predictions the prediction of the amplitude of nonhydrostatic waves generated during dam break flows is an important engineering problem given the risk of overtopping of flow in manmade canals or the increasing of flooding areas in natural watercourses the weakly nondispersive and fully nonlinear serre equations are a suitable choice for modeling these flows but there is a lack of a systematic assessment of this system of equations for dam break flow modeling reported in the literature in this paper the serre equations are applied to dam break flows over horizontal rigid bottoms whereas in the second part of this research the nonhydrostatic dam break waves over erodible beds are considered here a high resolution finite volume m

one dimensional numerical model for nonuniform sediment transport under unsteady flows in channel networks in this study the proposed one dimensional model simulates the nonequilibrium transport of nonuniform total load under unsteady flow conditions in dendritic channel networks with hydraulic structures the equations of sediment transport bed changes and bed material sorting are solved in a coupling procedure with a direct solution technique while still decoupled from the flow model this coupled model for sediment calculation is more stable and less likely to produce negative values for bed material gradation than the traditional fully decoupled model the sediment transport capacity is calculated by one of four formulas which have taken into consideration the hiding and exposure mechanism of nonuniform sediment transport the fluvial erosion at bank toes and the mass failure of banks are simulated to complement the modeling of bed morphological changes in channels the tests in several cases show that the present model is capable of predicting sediment transport bed changes and bed material sorting in various situations with reasonable accuracy and reliability

geomagnetic intensity variations over the past 780 kyr obtained from near seafloor magnetic anomalies knowledge of past variations in the intensity of the earth s magnetic field provides an important constraint on models of the geodynamo a record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences but a long term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust here we show that near seafloor magnetic anomalies recorded over the southern east pacific rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr moreover the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well documented dipole intensity pattern for the past 50 kyr a comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region the record of geomagnetic intensity preserved in the oceanic crust should provide a higher time resolution record of crustal accretion processes at mid ocean ridges than has previously been obtainable

tem study of the alteration of interstitial glass in morb by inorganic processes abstract scanning electron microscopy sem and transmission electron microscopy tem observations on a suite of mid ocean ridge basalts morb reveal abundant interstitial glass which differs in occurrence and composition from basaltic glass at pillow rims interstitial glass consists dominantly of sio2 and al2o3 and is depleted in other cations representing a highly differentiated residual material low temperature alteration by seawater solutions results in the formation of saponite mg rich variety of smectite replacing the interstitial glass the si al ratio of this saponite reflects that of the source glass chemical changes during this transition differ from those resulting from alteration of basaltic or rhyolitic glass alteration of interstitial glass results in uptake of mg and fe and loss of si and al but there is little or no leaching of alkalis this reflects a strong control on chemical reactions by solution composition high na and mg concentrations of seawater fluids in contrast to basaltic glass at pillow rims our observations do not reveal microbial activities in the alteration of interstitial glass

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

the reconstruction of boreal forest fire history from lake sediments a comparison of charcoal pollen sedimentological and geochemical indices abstract fossil charcoal fossil pollen sedimentological and geochemical analyses of lake sediments have been used previously to reconstruct a history of local fires and resulting vegetation change the rationale behind these approaches is described and the usefulness of each technique for reconstructing fire history in the boreal forest is assessed empirically historical and dendrochronological records provide regional and local fire histories for a site in wood buffalo national park alberta canada the local and regional history of fires is compared with the microscopic charcoal content macroscopic charcoal content elemental carbon content fossil pollen content sedimentology and geochemistry of annually laminated sediments from a small lake there is no significant correlation between the abundance of microscopic charcoal macroscopic charcoal and total elemental carbon content of the sediments automated measures of microscopic charcoal abundance made with an image analysis system are correlated significantly with optical counts of microscopic charcoal none of the charcoal measures provide unequivocal records of local fire activity and the abundance of microscopic charcoal appears to be influenced by variations in regional fire activity however the highest depositional rate of macrofossil charcoal occurred during the time of a fire that burned within the drainage basin variations in sedimentological measures and geochemistry do not correlate with local fire activity fossil pollen percentages and accumulation rates display a pattern of variation that is consistent with observed vegetation responses to fire in the boreal forest what is likely apparent in the pollen record are the results of a series of burns of sufficient extent and intensity to kill most of the above ground biomass of the vegetation in an area at least as great as the drainage basin the inability of evidence from lake sediments to provide detailed histories of past fire activity is not surprising given the wide range of variation in the spatial extent proximity intensity and impact of individual fires

strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite 1 we present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the troodos ophiolite similar to 92 ma formed during the cretaceous normal superchron cns a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all monte carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength our data suggest a dipole strength equivalent to the present field or nearly twice the post cns average the mean and standard deviation of the dipole moment 81 43 zam 2 z 10 21 from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from dsdp site 522 the new data set for the cns suggests a picture of a strong and stable field during the period of time when it stopped reversing moreover the similarity of the cns data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability

nrm intensity of altered oceanic basalts across the mar 21 n 0 1 5 ma a record of geomagnetic palaeointensity variations summary during cruise tammar may 1996 of r v nadir and submersible nautile 66 basaltic rock samples were collected along two long cross sections encompassing the oceanic crust between 0 and 1 5 myr at the centre of the mid atlantic ridge segment located at 21 n this dense sampling provides a means to investigate natural remanent magnetization nrm variability with age using rock magnetic studies high field and k t experiments palaeofield intensity determinations by the original thellier method together with the good geophysical and geological knowledge of this peculiar segment nrm intensities range from 1 3 to 25 4 a m 1 but do not display any rapid exponential decay with age as expected despite the scatter they seem to present short wavelength variations consistent on both flanks of the two lines because of the relative uniformity in grain size i e single domain sd ulvospinel content i e x 0 6 and amount of magnetic minerals in tammar samples the observed across axis nrm intensity variations may be due either to oxidation degree variations or to geomagnetic field intensity changes curie temperatures display no increase towards the flanks nor a clear relationship with nrm intensities suggesting that oxidation degree is not the major process controlling the nrm variations half of the collected samples either fresh or highly altered provide good quality palaeointensity determinations depending on the alteration degree of samples two major types of nrm trm diagrams are observed for relatively fresh materials only one line segment and the major part of nrm is used for field intensity calculations for moderately or highly altered specimens two line segments are observed with the first most probably corresponding to thermal demagnetization of the original remanence and the second to the product resulting from chemical modification i e inversion despite the difference in the amount of nrm that can be thermally demagnetized samples of similar age but of different non stoichiometric degree give coincident palaeofield strengths palaeofield values from 15 to 62 t i e vadm from 3 4 to 13 6 1022 a m2 are in good agreement with the global palaeointensity database and display coherent short wavelength undulations with age consistent assuming a 1 km wide neo volcanic zone and steady symmetrical spreading with the relative palaeointensity record deduced from sediment core analysis guyodo valet 1999 comparison between nrm and field intensity variations with age reveals a remarkable coincidence between both signals suggesting that the nrm intensities are more sensitive to the field intensity than to the alteration degree these observations suggest that magnetic particles are titanomaghemites generated during the initial magma cooling and storing a durable record of the geomagnetic field intensity the type of remanence is either 1 a chemical remanent magnetization that closely mimics the properties of the initial nrm or 2 a thermoremanent magnetization the maghemitization precedes the trm acquisition or 3 a mixture of both

inter decadal signals during the last millennium ad 1117 1992 in the varve record of santa barbara basin california annual varve thickness ad 1117 1992 from santa barbara basin has been decomposed into orthogonal components using singular spectrum analysis ssa to identify and retrieve inter decadal oscillations after removing all variability with periods greater than 150 years leading ssa eigenfunctions eofs identify four oscillatory pairs with periods of 100 58 25 and 12 years respectively based on 2500 simulated series and on two sided confidence intervals eofs 1 7 are significant at the 99 level and eofs 8 9 are significant at the 90 level oscillatory signals retrieved from the marine varves show an abrupt change in frequency and amplitude near ad 1600 the largest contribution to this environmental shift is given by the interdecadal components especially the 25 and the 12 year oscillation the near ad 1600 change may be related to multi annual events reported in the stratigraphy of the nearby santa monica basin and in dendrochronological records of the american southwest

charcoal deposition and redeposition in elk lake minnesota usa sedimentary charcoal diatom and phytolith records of the past 1500 years at elk lake minnesota in combination with sediment trap studies and a transect of surface sediment samples document the mechanisms by which previously deposited charcoal is redeposited and finally buried in this lake the frequent correspon dence of high diatom concentrations and peaks of phytolith and charcoal fragments suggests that currents and turbulence related to lake circulation are responsible for winnowing charcoal and phytoliths from shallow water depositional sites to deeper areas of the lake high diatom concentrations in the record relate to increased nutrient fluxes also supplied by circulation despite the fact that the watershed and area around elk lake has not been burned since ad 1922 charcoal continues to reach the profundal zone from littoral source areas in elk lake the variable redeposition of within lake charcoal requires evaluation before fire history records can be related to global regional or even lo

a 560 year record of santa ana fires reconstructed from charcoal deposited in the santa barbara basin california microscopic charcoal from varved santa barbara basin sediments was used to reconstruct a 560 yr record a d 1425 to 1985 of santa ana fires comparison of large 3750 m2 charcoal with documented fire records in the santa barbara ranger district shows that high accumulations correspond to large fires 20 000 ha that occurred during santa ana conditions the charcoal record reconstructed a minimum of 20 large fires in the santa barbara region during the study period the average time between fires shows no distinct change across three different land use periods the chumash period apparently characterized by frequent burning the spanish early american period with nominal fire control and the 20th century with active fire suppression pollen data support the conclusion that the fire regime has not dramatically changed during the last 500 yr comparison of large charcoal particle accumulation rates and precipitation reconstructed from tree rings show a strong relationship between climate and fire history with large fires consistently occurring at the end of wet periods and the beginning of droughts