matuyama brunhes polarity transition near bishop california summary exposed pleistocene lake sediments at three localities separated by as much as six kilometres near bishop california contain the change from reverse to normal palaeomagnetic polarity that occurred about 0 73 million years ago the matuyama brunhes m b transition a decrease in the relative field strength is used to define the length of the transition the width of that decrease exceeds by about one fifth the width where the palaeomagnetic directions show the switch in polarity because some of the usually six subsamples in a measured horizon in the central part of the transition have normal polarity while others have intermediate or reverse polarity the behaviour of the palaeomagnetic field in that region cannot be determined accurately there is however a short reversal within the upper quarter of the transition at one locality that can be documented at least 30 cm of normal polarity underlie reverse polarity at three sites spread over a distance of 150m taken as a whole the palaeomagnetic data for the full transition are a warning that some rapid short reversals within the transition that are reported in the literature might be caused by inaccurate recording of the palaeomagnetic field the palaeomagnetic polarity 17m beneath the m b transition at one locality places a minimum age of about 1 0 myr on the glass mountain d ash bed that is found in east central and southern california

comment on a relative geomagnetic paleointensity stack from ontong java plateau sediments for the matuyama by yvo s kok and lisa tauxe

if a tree falls in an urban stream does it stick around mobility characteristics and geomorphic influence of large wood in urban streams in northeastern ohio usa abstract large wood influences geomorphological ecological and biogeochemical functions of streams but in urban areas it is also considered an unaesthetic hazard quantifying the abundance and mobility of in stream wood along an urban intensity gradient and understanding the relationship between wood dynamics and stream reach network and watershed characteristics allows for a stronger understanding of the potential benefits and hazards of wood in urban streams repeat wood surveys were conducted at 11 stream reaches in the cleveland ohio area where wood removal is not practiced and forested riparian zones exist study reaches span a gradient of impervious surface cover and extent of the stream network with a forest buffer channel morphology and sediment size were also measured stream hydrology does not change when imperviousness is 12 40 in this region so quantification of impervious surfaces stands in for a broader suite of urban impacts wood abundance decreases and mobility increases as urban intensity in the watershed and along the stream network increases jams become smaller and less stable but the size and orientation of individual wood pieces do not change along an urban gradient over the 6 month study period wood mobility exceeded 100 in some of the most urbanized streams and transport distances are inferred to be on the order of 100s of meters there was a positive correlation between jam spacing and pool spacing and finer sediment sizes occurred in some wood associated deposits but overall geomorphic effects are limited where reaches are confined wood abundance is suppressed but impervious surfaces and the extent of forest along the stream network are stronger predictors of wood dynamics than reach scale characteristics these results emphasize the importance of situating understanding of reach scale wood dynamics in the context of larger scale phenomena

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

sampling surficial fluvial gravels the precision of size distribution percentile sediments abstract grid by number wolman sampling is commonly used to characterize the texture of surficial fluvial sediments assessments of sampling precision have concentrated on the median grain size but adequate representation of other grain size percentiles is paramount in a number of sedimentological contexts the field effort required to construct percentile sampling distributions by replication is prohibitive and the absence of any general size distribution for fluvial sediments precludes theoretical calculation of percentile standard errors bootstrapping is used in this paper to determine percentile standard errors for large samples from two gravel bar surfaces the improvement of percentile precision with sample size follows the theoretical expectation hence improvement in percentile recision is achieved only at the expense of much greater sampling effort once sample sizes reach about 400 stones however absolute percentile standard errors differ significantly from theoretically derived values of particular interest is the stability of coarser percentile estimates in coarse skewed distributions d95 estimates may be as precise as d50 estimates for a given sample size the sensitivity of the precision of estimated percentiles to distribution shape is examined using synthetic skewed and bimodal grain size distributions the absolute precision of percentile estimates is shown to vary with the grain size distribution of a particular sediment our field results provide a reference for further work

hydrologic variation with land use across the contiguous united states geomorphic and ecological consequences for stream ecosystems using daily discharge data from the usgs we analyzed how hydrologic regimes vary with land use in four large hydrologic regions that span a gradient of natural land cover and precipitation across the continental united states in each region we identified small streams contributing areab282 km 2 that have continuous daily streamflow data using a national database we characterized the composition of land cover of the watersheds in terms of aggregate measures of agriculture urbanization and least disturbed natural we calculated hydrologic alteration using 10 ecologically relevant hydrologic metrics that describe magnitude frequency and duration of flow for 158 watersheds within the southeast se central ce pacific northwest nw and southwest sw hydrologic regions of the united states within each watershed we calculated percent cover for agriculture urbanized land and least disturbed land to elucidate how components of the natural flow regime inherent to a hydrologic region is modified by different types and proportions of land cover we also evaluated how dams in these regions altered the hydrologic regimes of the 43 streams that have pre and post dam daily streamflow data in an analysis of flow alteration along gradients of increasing proportion of the three land cover types we found many regional differences in hydrologic responses in response to increasing urban land cover peak flows increased se and ce minimum flows increased ce or decreased nw duration of near bankfull flows declined se nw and flow variability increased se ce and nw responses to increasing agricultural land cover were less pronounced as minimum flows decreased ce near bankfull flow durations increased se and sw and flow variability declined ce in a second analysis for three of the regions we compared the difference between least disturbed watersheds and those having either n15 urban and n25 agricultural land cover relative to natural land cover in each region urbanization either increased se and nw or decreased sw peak flows decreased minimum flows se nw and sw decreased durations of near bankfull flows se nw and sw and increased flow variability se nw and sw agriculture had similar effects except in the se where near bankfull flow durations increased overall urbanization appeared to induce greater hydrologic responses than similar proportions of agricultural land cover in watersheds finally the effects of dams on hydrologic variation were largely consistent across regions with a decrease in peak flows an increase in minimum flows an increase in nearbankfull flow durations and a decrease in flow variability we use this analysis to evaluate the relative degree to which land use has altered flow regimes across regions in the us with naturally varying climate and natural land cover and we discuss the geomorphic and ecological implications of such flow modification we end with a consideration of what elements will ultimately be required to conduct a more comprehensive national assessment of the hydrologic responses of streams to land cover types and dams these include improved tools for modeling hydrologic metrics in ungauged watersheds incorporation of high resolution geospatial data

ecologically relevant geomorphic attributes of streams are impaired by even low levels of watershed effective imperviousness urbanization almost inevitably results in changes to stream morphology understanding the mechanisms for such impacts is a prerequisite to minimizing stream degradation and achieving restoration goals however investigations of urban induced changes to stream morphology typically use indicators of watershed urbanization that may not adequately represent degrading mechanisms and commonly focus on geomorphic attributes such as channel dimensions that may be of little significance to the ecological goals for restoration we address these shortcomings by testing if a measure characterizing urban stormwater drainage system connections to streams effective imperviousness ei is a better predictor of change to ecologically relevant geomorphic attributes than a more general measure of urban density total imperviousness ti we test this for 17 sites in independent watersheds across a gradient of urbanization we found that ei was a better predictor of all geomorphic variables tested than was ti bank instability was positively correlated with ei while width depth a measure of channel incision bedload sediment depth and frequency of bars benches and large wood were negatively correlated large changes in all geomorphic variables were detected at very low levels of ei 2 3 excess urban stormwater runoff as represented by ei drives geomorphic change in urban streams highlighting the dominant role of the stormwater drainage system in efficiently transferring stormwater runoff from impervious surfaces to the stream as found for ecological indicators it is likely that geomorphic condition of streams in urbanizing watersheds particularly those attributes of ecological relevance can only be maintained if excess urban stormwater flows are kept out of streams through retention and harvesting the extent to which ei can be reduced within urban and urbanizing watersheds through techniques such as distributed stormwater harvesting and infiltration and the components of the hydrologic regime to be addressed requires further investigation

sawtooth variation in magnetic intensity profiles and delayed acquisition of magnetization in deep sea cores abstract recent records of normalised intensity of natural remanent magnetization nrm obtained from deep sea sediments document an asymmetrical saw tooth pattern associated with geomagnetic reversals this observation may reflect a fundamental aspect of the geodynamo time evolution or alternatively delay in acquisition of the nrm it is demonstrated here that a simple model in which a large fraction of the magnetic grains acquire nrm at deposition time while remaining grains reorientate or acquire magnetization after deposition is able to produce a saw tooth profile strikingly similar to that observed interestingly this mechanism of nrm acquisition does not damp out high frequency oscillations of the field such as those observed in the deep sea records exhibiting the saw tooth profile it provides an accurate record of field directions except for periods of very low field intensity

palaeomagnetic records of the brunhes matuyama polarity transition from odp leg 124 celebes and sulu seas summary palaeomagnetic records of the brunhes matuyama geomagnetic polarity transition were obtained from deep sea sediments of odp leg 124 in the celebes and sulu seas advanced piston core apc samples with high magnetization intensities 2 200 ma m1 and high sedimentation rates 8 4 10 cm kyr1 were recovered from this cruise rock magnetic measurements revealed the carrier of the remanence to be nearly pure magnetite in the pseudo single domain range pass through measurements at intervals of 5 mm on apc cores across the brunhes matuyama polarity transition for holes 767b 769a and 769b were deconvolved with the magnetometer sensor response using the abic minimizing method discrete samples were also taken from the polarity transition zones and subjected to either thermal or alternating field stepwise demagnetization the results were generally consistent with the pass through data after the deconvolution results from the three holes are in good agreement particularly those from the two holes 100 m apart at site 769 the transitional vgp paths from these two holes show two small loops near new zealand before the equator is crossed the vgps continue to swing eastwards to the north atlantic and then move to the northeastern margin of the pacific ocean the positions of the vgp loops obtained from site 769 are dierent from the vgp clusters obtained from both the known volcanic records and the sediment records at the boso peninsula and the north atlantic sites with high sedimentation rates such a dierence may imply the predominance of the non dipole field during the transition relatively stable mid high northern latitude vgps are recognized on all three cores just after the reversal lasting about 4000 years the plot of relative intensity versus vgp latitude for the sediment records from the north atlantic dsdp hole 609b shows a remarkable similarity with our records similar patterns were also obtained for the plot of the palaeointensity versus vgp latitude for the la palma volcanic lava records these results may suggest the existence of a metastable state of the geodynamo producing a zonal component just after the brunhes matuyama boundary which may have played a role in the change of the field intensity

an 84 kyr paleomagnetic record from the sediments of lake baikal siberia we have conducted a paleomagnetic study of sediment cores obtained from the selenga prodelta region of lake baikal russia this record which spans approximately the last 84 kyr contributes to a better understanding of the nature of geomagnetic field behavior in siberia and is a useful correlation and dating tool we demonstrate that the lake baikal sediments are recording variations in the geomagnetic field the directional record displays secular variation behavior with a geomagnetic excursion at 20 ka and additional excursions appearing as large amplitude secular variation at 41 61 and 67 ka smoothing of the geomagnetic excursion behavior occurs in lake baikal sediments owing to the intermediate sedimentation rate 13 cm kyr 1 the lake baikal relative paleointensity record correlates to absolute paleointensity data for the last 10 kyr and to relative paleointensity records from the mediterranean sea and indian ocean for the last 84 kyr this correlation suggests a strong global i e dipole component to these records and further supports the reliability of sediments as recorders of relative geomagnetic paleointensity we show that a relative geomagnetic intensity stratigraphy has a potential resolution of 7 kyr by correlating continental and marine records the geomagnetic intensity stratigraphy helps constrain the age of the difficult to date lake baikal sediments

a database for the matuyama brunhes magnetic reversal abstract we present a paleomagnetic database designated mbd97 comprising sedimentary and lava data for the matuyama brunhes reversal transition field using criteria regarding suitability of the rock samples measurement methods and temporal resolution of the data we consider 62 different studies of the matuyama brunhes reversal of these only 11 satisfy our selection criteria four lava sites one loess site three shallow marine sedimentary sites and three deep ocean sedimentary sites we discuss our reasons for accepting or disguarding each candidate data set we investigate the distribution of transitional virtual geomagnetic poles vgps they fall along american and asian australian longitudes the spatial and temporal complexity of the data indicate that the transitional field was almost certainly nonaxisymmetric and nondipolar the directional transition of the matuyama brunhes reversal took 2300 5000 years during which time the surface intensity fell to 10 20 of its pre and post transitional value we do not find any systematic bias due to site distribution or inclination shallowing although the errors in the data do vary over the duration of the reversal mbd97 is a small database but it has a fairly good geographic distribution and is suitable for studies of the matuyama brunhes transition nonetheless it is clear that further analysis of transitions will benefit from a continued program of data collection and analysis the data in mbd97 are listed

the matuyama brunhes transition recorded from lake tecopa sediments california abstract the paleomagnetic record of the matuyama brunhes reversal in the sediment of ancient lake tecopa provided the first evidence that the geomagnetic field during transition is not dipolar we resampled the section to increase the time resolution of the record our new data show that the characteristic direction could not be completely isolated by af demagnetization the transitional directions obtained from thermal demagnetization are thus quite different from those of the previous study but the primary conclusion that the field was non dipolar remains the vgp paths lying close to the site longitude is consistent with an axisymmetric transition field

saw toothed pattern of relative paleointensity records and cumulative viscous remanence several studies of relative paleointensity data derived from marine sediments spanning the last 4 myr display an asymmetrical saw toothed pattern polarity reversals of the geomagnetic field are associated with low points in paleofield intensity preceded by a long term progressive decay of the field and followed by a rapid post transitional recovery 1 3 since similar behavior is observed in far flung sites whose rock magnetic records are different it is argued that the world wide saw toothing represents geomagnetic field behavior 3 we present an alternative explanation calling on the effect of hard viscous remanence

channel morphology changes and their relationship to valley bottom geology and human interventions a case study from the vistula valley in warsaw poland the aim of the study was to determine the role of the geological setting of channel zone in the morphological variability of a river reach situated between engineered and seminatural channel sections investigations were carried out in the vistula river channel in warsaw the study included the drilling of 60 geological boreholes in the river channel and the registering of sedimentary structures of modern river channel sediments the morphology of the channel was defined based on a series of bathymetric measurements of different river levels to identify the reasons for the variation in the channel bed morphology numerical modelling of river flow was performed calculations were made using the fluent cfd computational fluid dynamics model the stabilization of bottom erosion along the engineered reach of the river was found to be associated with a shallow depth to the top of the basement of modern alluvial sediments composed of erosion resistant glacial deposits the suballuvial basement in this part of the river channel also serves as a local base level in relation to the river reach upstream

mono lake excursion in mono basin california and at carson sink and pyramid lake nevada summary a restudy of the mono lake excursion in the mono basin california using two new localities 20 km apart shows that the palaeomagnetic record in the basin is the most complete and accurate for the entire excursion the excursion is also recorded in exposed sediment from pleistocene lake lahontan in northwestern nevada about 250 km north of the mono basin sampling of the lake lahontan sediment was done at pyramid lake and in the carson sink following the initial discovery of the mono lake excursion by denham cox 1971 liddicoat coe 1979 showed that the excursion consists of two parts split by a volcanic ash bed at pyramid lake and in the carson sink the older half of the excursion is not recorded but the younger half can be recognized two factors appear to have contributed to the partial recording of the excursion at pyramid lake and in the carson sink 1 the relative strength of the palaeomagnetic field low during the older half of the excursion and high during the younger half and 2 the slower sedimentation in lake lahontan estimated to be about half the rate of deposition for the lacustrine sediment in the mono basin tephrochronology and carbon 14 dates indicate an age of 28 000 yr for the middle of the mono lake excursion on the assumption that the sedimentation rate was a constant 25cm kyr 1 in the mono basin the span of the full excursion is about 2000 yr

the upper kaena sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic and rock magnetic record of the upper kaena polarity reversal was determined from late pliocene marine marls in southern sicily two separate characteristic magnetization components were distinguished a low temperature lt component between 200 and 50 c and a high temperature ht component between 500 and 580 610 c the two components record the reversal in a slightly different way but both were acquired in an early stage following deposition most probably by authigenic biogenic formation of magnetic minerals the actual r n reversal via e and horizontal inclinations seems to take 3 3 kyr and is followed by a stable normal interval 3 3 kyr and subsequently by a significant excursion 50 60 in declination 3 6 kyr relative intensities were lower during a time span of 5 6 kyr and show a minimum of 10 of the nontransitional values but during the excursion no intensity changes are apparent the vgps of the ht component are confined to a longitude between india and australia those of the lt component oscillate between this same longitude and one passing over north and south america we conclude that most if riot all observed features are largely determined by the remanence acquisition process which is complicated by authigenic formation of magnetic minerals and cyclically fluctuating paleoredox conditions this has not only resulted in considerable smoothing but also in delayed nrm acquisition in specific sedimentary intervals we believe that the observed excursion is a sedimentary artifact and not related to geomagnetic field behavior during transitions we emphasize that many reported geomagnetic features may in fact be due to the sedimentary remanence acquisition process

historic and paleomagnetic secular variation and the earth s core dynamo process this paper attempts to summarize briefly the recent and ongoing efforts of the geomagnetism and paleomagnetism community to understand both the earth s magnetic field secular variation and its implications for the core dynamo process secular variation is defined in this paper as the general spatio temporal variation of the field secular change will refer to the first time derivative of the spatial field strict geomagnetic definition of secular variation we will emphasize here the long term 10 year or longer behavior of the historic secular variation hsv for this is the variation that is comparable with the paleomagnetic secular variation psv two important related topics the 1969 1970 geomagnetic jerk in secular acceleration and the spatial geomagnetic field components due to crustal or mantle sources are treated more fully in other reviews

a relative geomagnetic paleointensity stack from ontong java plateau sediments for the matuyama we present a stack of geomagnetic paleointensity estimates for the matuyama chron 0 778 2 582 ma sedimentary magnetizations from 4 ocean drilling program odp cores from the ontong java plateau ojp are analyzed with thermal techniques thellier thellier experiments are used to determine magnetic properties and indicate that a thermal treatment to 250 c can be used for paleointensity estimates the correlated records form a coherent ojp stack which is dated at reversal boundaries a coherent feature with two other data sets spanning the matuyama chron is the occurrence of a 150 kyr cyclicity however the time control of the ojp stack is not accurate except for the 10 tie points at the reversals this could at least in part explain why the three matuyama paleointensity records show no other coherent frequencies

rangeland management and fluvial geomorphology in northern tanzania abstract researchers have independently documented the effects of land use on rivers and threats to river management institutions but the relationship between changes in institutional context and river condition is not well described this study assesses the connections between resource management institutions land use and rivers by integrating social science geospatial analysis and geomorphology in particular we measured hydraulic geometry sediment size distributions and estimated sediment yield for four rivers in northern tanzania and conducted semistructured interviews that assessed corresponding resource management institutions communities managed rivers through both customary traditional nonstate and government institutions but the differences in the resource management policies and practices of the study rivers themselves were fairly subtle clearer differences were found at broader scales the four watersheds exhibited substantial differences in land cover change and sediment yield associated with the location of settlements roadways and cultivation unexpectedly these recent land use changes did not initiate a geomorphic response in rivers the long history of grazing by domestic and wild ungulates may have influenced water and sediment supplies such that river channel dimensions are more resistant to changes in land use than other systems or have already adjusted to predominant changes in boundary conditions this would suggest that not all rivers will have the anticipated responses to contemporary land use changes because of antecedent land use patterns over long time scales centuries to millennia the presence of grazers may actually increase the ability of rivers to withstand changes in land use our findings point to a need for further interdisciplinary study of dryland rivers and their shifts between system states especially in areas with a long history of grazing relatively recent changes in land use and a dynamic social and institutional context

a magnetization process of sediments laboratory experiments on post depositional remanent magnetization summary palaeomagnetic studies require a theory of magnetization mechanism of sediments and a method of estimating magnetic field intensity from their remanences this paper establishes a physical basis for the generation of the remanence in deep sea and lake sediments experimentally redeposition experiments have been carried out under centrifugal force in weak magnetic fields the centrifuging method produces post depositional remanent magnetization post drm in the compacted sediment and its remanence and susceptibility are compatible with those of natural sediments and reconstituted materials of other redepositional experiments three properties of the post drm have been deduced from the experiments 1 the efficiency of acquisition of post drm decreases with increase in density during the compaction process 2 the total post drm is equal to the sum of the partial post drm addition law and 3 time is not a substantial factor for alignment of the magnetic particles these results lead to the conclusions that the magnetic particles do not rotate steadily but in a series of steps and that the density change is the crucial factor giving rise to the post drm a mathematical formula representing the remanence record in sediments is proposed on the basis of the experimental results and the model the principal equation is expressed as an integral of the product of three parameters over time when sediments have been compacted the field intensity variation characteristic function of the sediment and the time derivative of the density change

age model estimation in paleoclimatic research fuzzy regression and radiocarbon uncertainties abstract a fuzzy linear regression based method to relate the depth and age of sediment layers is described because algae moss or local plants respond to climatic change the age and depth of sediment layers are interrelated variables but the link between them is often imprecise in most cases due to the limited number of layers in a core i e small number of data points estimation of the slope and uncertainties in classical regression analysis does not take into account the uncertainties in radiocarbon dating here fuzzy linear regression which may be applied even to a very small data set is utilized the method illustrated through a practical example with eight data points appears to be a promising tool in stratigraphic studies to link sediment age to layer depth and takes into account uncertainties from radiocarbon dating

high resolution record of the matuyama brunhes transition constrains the age of javanese homo erectus in the sangiran dome indonesia a detailed paleomagnetic study conducted in the sangiran area java has provided a reliable age constraint on hominid fossil bearing formations a reverse to normal polarity transition marks a 7 m thick section across the upper tuff in the bapang formation the transition has three short reversal episodes and is overlain by a thick normal polarity magnetozone that was fission track dated to the brunhes chron this pattern closely resembles another high resolution matuyama brunhes mb transition record in an osaka bay marine core in the sangiran sediments four successive transitional polarity fields lie just below the presumed main mb boundary their virtual geomagnetic poles cluster in the western south pacific partly overlapping the transitional virtual geomagnetic poles from hawaiian and canary islands lavas which have a mean 40ar 39ar age of 776 2 ka thus the polarity transition is unambiguously the mb boundary a revised correlation of tuff layers in the bapang formation reveals that the hominid last occurrence and the tektite level in the sangiran area are nearly coincident just below the upper middle tuff which underlies the mb transition the stratigraphic relationship of the tektite level to the mb transition in the sangiran area is consistent with deep sea core data that show that the meteorite impact preceded the mb reversal by about 12 ka the mb boundary currently defines the uppermost horizon yielding homo erectus fossils in the sangiran area

municipal initiatives for managing dunes in coastal residential areas a case study of avalon new jersey usa abstract the characteristics of foredunes created in a municipal management program on a developed barrier island are evaluated to identify how landforms used as protection structures can be natural in appearance and function yet compatible with human values shoreline management zones include a naturally evolving undeveloped segment a noneroding developed segment eroding and noneroding segments of an improved beach where dunes have been built by artificial nourishment and a privately built artificially nourished dune on the shoreline of an inlet a disastrous storm in 1962 resulted in an aggressive program for building dunes using sand fences vegetation plantings purchase of undeveloped lots and sediment backpassing to maintain beach widths and dune elevations the present nourished and shaped foredune in the improved beach is higher wider and closer to the berm crest than the natural dune restricted inputs of aeolian sand keep the surface flat and poorly vegetated a stable section of this engineered shore has a wider beach and sand fences have created a higher foredune with greater topographic diversity the cross shore zonation of vegetation here is more typical of natural dunes but the environmental gradient is much narrower the privately built dune is low narrow and located where it could not be created naturally foreshore and aeolian sediments in the undeveloped segment and the improved beach are similar in mean grain size 0 16 0 21 mm and sorting 0 31 0 39 but sediment on the surface of the nourished dune is coarser 28 1 gravel with a more poorly sorted sand fraction 1 30 representing lag elements on the deflation surface willingness to enhance beaches and dunes for protection has reduced insurance premiums and allowed the municipality to qualify for funds from the federal emergency management agency fema to replace lost sediment thus placing an economic value on dunes success of the management program is attributed to i timing property purchase and dune building programs to periods immediately after storms causing residents to accept high dunes that restrict access or views ii instituting a vigorous education program reminding residents of hazards during nonstorm periods iii maintaining control over local sediment supplies to keep pace with erosion and create new shoreline environments iv investing private and municipal economic resources in landforms qualifying them for external funds for replacement and v maintaining augmenting or simply tolerating biodiversity and natural processes retaining a natural heritage

sedimentary processes and depositional environments of a continuous marine succession across the lower middle pleistocene boundary kokumoto formation kazusa group central japan abstract the chiba composite section of the kokumoto formation in central japan is a remarkable thick marine succession and is a candidate for formal recognition as the global boundary stratotype section and point gssp for the base of the middle pleistocene stage although there have been numerous studies of the kokumoto formation the sedimentology of the formation particularly in the chiba composite section has been unclear we use field observations and laboratory analyses to elucidate the sedimentary processes and depositional environments of the formation the chiba composite section includes the matuyama brunhes paleomagnetic reversal which is the primary guide for the lower middle pleistocene boundary along with multiple tephra deposits that provide isochronous correlational tie points one bed byk e also known as ontake byakubi tephra immediately underlies the paleomagnetic reversal and hence provides a useful widespread marker for the boundary the predominant silty beds of the chiba composite section are intensely bioturbated and lack evidence of episodic deposition such as slumps or muddy turbidites however minor sandy beds intercalated within the silty section particularly in its lower part are either 1 fining upward sequences with faint planar or ripple laminations and an erosional base or 2 sequences containing abundant mud clasts in a poorly sorted sand matrix with a sharp base and an irregular top surface the silty beds are interpreted to be hemipelagite formed by deposition of fine grained suspended material under stable and calm bottom water conditions the sandy deposits are interpreted to be sediment gravity flow deposits i e turbidites and debrites these sedimentary processes and the trace fossil assemblage indicate that the chiba composite section was formed in a continental slope environment the depositional environments of the chiba composite section together with tephrostratigraphy make it suitable for high resolution studies on the paleoenvironment paleoclimate and paleoceanography of the northwest pacific ocean across the lower middle pleistocene boundary it is therefore a suitable candidate to become the gssp for the middle pleistocene

influence of bank vegetation on channel morphology in rural and urban watersheds stream bank vegetation significantly influences the morphology of streams in the piedmont region of the united states we surveyed the morphology of 26 paired stream reaches in southeastern pennsylvania northern maryland and delaware one member of each pair has a forested riparian zone whereas the other has a riparian zone composed pri marily of grass the paired reaches are nearly contiguous so all significant channel forming variables except riparian vegetation are held constant the extent of urban development of the watersheds upstream of the paired reaches also varies considerably allowing us to determine the combined influence of riparian vegetation and urbanization on channel morphology statistical analyses indicate that 1 channels with forested riparian zones are wider than channels with nonforested riparian zones 2 channels in urbanized watersheds are wider than channels in nonurbanized watersheds and 3 the effect of riparian vegetation is independent of the level of urbanization

paleomagnetic record over the past 35 000 years of a sediment core from off shikoku southwest japan the paleomagnetism of a sediment core kt89 18 p4 recovered from the sea floor off southwest japan was studied to investigate geomagnetic field variation over the past 35 000 years alternating fields demagnetization revealed that the remanent magnetization of the unconsolidated sediments was stable and not affected by any secondary overprints 14c datings were made on 21 horizons using a tandetron accelerator mass spectrometer providing a detailed time scale to the core the inclination record agrees with that from lake biwa kawai et al 1975 and supports the existence of the geomagnetic excursions which had been found in the lake biwa core the ages of the excursions are estimated to be during ca 32 ky b p ca 28 ky b p and ca 26 ky b p ca 24 ky b p

regional impacts of urbanization on stream channel geometry a case study in semiarid southern california abstract urbanization often increases storm runoff peak discharges and rates of stream channel erosion coastal california has experienced rapid urbanization over the past several decades and has the potential for stream channel degradation several counties in california have implemented hydromodification management plans hmps to protect channels from erosion but few studies have quantified the impact of urbanization on channel geometry in diverse geological settings at the county scale a synoptic survey of field sites n 56 by the california environmental data exchange network ceden and additional field surveys n 24 were used to develop regional hydraulic geometry curves relating bankfull cross sectional area axs width w mean depth d and discharge qbf to watershed area aw in san diego county regional curves were compared for urban and reference sites and to other regional curves developed for southern california multiple regression models were used to identify dominant watershed and channel controls on geometry including aw percent impervious cover i mean annual precipitation underlying geology longitudinal slope hydrologic soil group and channel particle size for the reference streams regional curves were statistically significant for w and axs p 20 had significantly larger w d axs and qbf for a given watershed size a majority 68 of the urban channels and 78 of the small urban channels aw

long term bed load transport rate based on aerial photo and ground penetrating radar surveys of fan delta growth coast mountains british columbia sequential aerial photography sonar bathymetry ground penetrating radar gpr and sediment sampling and analysis provide the basis for calculating the volumetric and mass rate of progradation of the delta of fitzsimmons creek a steep high energy debris flow dominated channel draining about 100 km2 of the southern coast mountains of british columbia fitzsimmons creek is typical of small mountain rivers in the region gpr imaging is used to define the pre depositional morphology of the receiving basin a technique that improves the accuracy of the volumetric survey the 52 year record 1947 1999 of progradation yielded an average annual volumetric transport rate of 1 00 0 16 104 m3 year 1 for bed load corresponding to a mass transport rate of 1 60 0 28 104 mg year 1 bed load yields are consistent with those obtained in hydrogeomorphically similar basins in the region and elsewhere decade based annual rates which vary from 0 64 0 11 104 to 2 85 0 38 104 mg year 1 provide poor estimates of the 52 year average indeed the 52 year record may also not be long enough to fully integrate the significant fluctuations in the sediment efflux from fitzsimmons creek the methodology proposed in this paper can be transferred to other comparable mountain environments worldwide

comment on saw toothed variations of relative paleointensity and cumulative viscous remanence testing the records and the model by l meynadier j p valet y guyodo and c richter

urban transformation of river landscapes in a global context abstract over the past 50 years considerable progress has been made in understanding the impacts of urban development on river processes and forms such advances have occurred as urban population growth has accelerated around the world using a compilation of research results from more than 100 studies conducted in a range of areas 58 addressing morphological change this paper describes how urbanization has transformed river landscapes across earth s surface emphasizing the distribution of impacts in a global comparative context urban development induces an initial phase of sediment mobilization characterized by increased sediment production on the order of 2 10 times and deposition within channels followed by eventual decline that couples with erosion from increased runoff to enlarge channels data from humid and temperate environments around the world indicate that channels generally enlarge to 2 3 times and as much as 15 times the original size although research has emphasized temperate environments recent studies of tropical areas indicate a tendency for channel reduction resulting from strong sediment erosion and deposition responses because of intense precipitation and highly weathered soils embryonic research in arid environments further suggests variable river responses to urbanization that are characterized by rapid morphological change over short distances regardless of location the persistence of the sediment production phase varies from months to several years whereas several decades are likely needed for enlarging channels to stabilize and potentially reach a new equilibrium urbanizing streams pose particular challenges for management given an inherent changing nature successful management requires a clear understanding of the temporal and spatial variations in adjustment processes

geomagnetic excursions in the past 60 ka ephemeral secular variation features geomagnetic excursions have been reported for the past 25 years in both sedimentary and igneous rocks of brunhes age and from widespread geographic localities they comprise sequences of paleo magnetic directions that are anomalous in that they depart widely from the range of geomagnetic north directions recorded through historic time they have sometimes been interpreted as records of aborted reversals of polarity of the main geomagnetic dipole the search for excursions sought to provide a set of stratigraphic markers the case of the laschamp excursion described in lava flows from the chaine des puys massif central france is analyzed here through a new sequential record of paleosecular variation recovered from sedimentary cores collected in lac du bouchet a maar lake about 100 km from the laschamp site the absence of anomalous directions indicates that this excursion lasted for only a few centuries this constitutes a warning to stratigraphers who attempt to use excursions as marker events and it gives an insight on the behavior of earth s geodynamo on the scale of 102 to 103 yr

a systematic analysis of eight decades of incipient motion studies with special reference to gravel bedded rivers data compiled from eight decades of incipient motion studies were used to calculate dimensionless critical shear stress values of the median grain size t c 50 calculated t c 50 values were stratified by initial motion definition median grain size type surface subsurface or laboratory mixture relative roughness and flow regime a traditional shields plot constructed from data that represent initial motion of the bed surface material reveals systematic methodological biases of incipient motion definition t c 50 values determined from reference bed load transport rates and from visual observation of grain motion define subparallel shields curves with the latter generally underlying the former values derived from competence functions define a separate but poorly developed field while theoretical values predict a wide range of generally higher stresses that likely represent instantaneous rather than time averaged critical shear stresses the available data indicate that for high critical boundary reynolds numbers and low relative roughnesses typical of gravel bedded rivers reference based and visually based studies have t c50 ranges of 0 052 0 086 and 0 030 0 073 respectively the apparent lack of a universal t 50 for gravel bedded rivers warrants great care in choosing defendable t c50 values for particular applications

effects of urbanization on watershed hydrology the scaling of discharge with drainage area this study examines the effects of impervious surfaces within urbanized land on the scaling of river discharge with drainage area discharge in a river channel grows as drainage basin area increases following the general equation q ka c where q is river discharge k is a measure of river base flow a is upstream drainage area and c is the scaling power dependency land use is a critical variable in the examination of river discharge discharge has significant geologic and ecologic influences on fluvial systems discharge is assumed to scale linearly or nearly linearly with drainage area c 1 but in spite of its widespread application the relationship has not been explicitly tested with respect to urbanization here we show that in small urban settings the scaling is nonlinear for peak flows it is proposed that effective water loading occurs through a combination of increased runoff and an increase in the rate of transport to the rivers these higher discharges in urban rivers have the potential to increase erosion degrade aquatic habitats and significantly alter channel forms

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

detailed paleomagnetic records during the brunhes matuyama geomagnetic reversal and a direct determination of depth lag for magnetization in marine sediments abstract detailed paleomagnetic records were studied from homoclinal bathyal siltstone deposited during the brunhes matuyama geomagnetic reversal outcropping in the boso peninsula central japan the records are based on three continuous sediment cores from three sections within a distance of 14 km the stratigraphic horizons were correlated using a volcanic ash bed which occurs about 2 m above the brunhes matuyama reversal the sedimentation rate according to the oxygen isotope stratigraphy for the three sections decreases eastward and is 3 70 2 20 and 1 83 m ka 1 the depth lag of magnetization in the sediments was calculated directly as 42 cm based on the differences in the relative position of the key ash layer in the correlated virtual geomagnetic pole vgp paths for the three sections vgp movement during the geomagnetic reversal is characterized by short intervals of very rapid motion between periods of relative stability the last swing of the vgp from the southern hemisphere to the northern hemisphere in the brunhes matuyama geomagnetic reversal took only 38 yr intensities of the remanent magnetization normalized by anhysteretic remanent magnetization indicate that the intensity of the geomagnetic field decreased to one fifth for about 5 4 ka including the geomagnetic reversal the movement of the vgp longitude is 0 36 yr 1 westward and this westward drift can be related to the rotation of the non dipole field

bed coarsening riffle shortening and channel enlargement in urbanizing watersheds northern kentucky usa abstract stream systems naturally respond to watershed land use dynamics particularly in urban developments with unmanaged impervious areas such urban provoked alterations to channel morphology cause water quality impairments have adverse effects on aquatic biota and pose risks to adjacent public infrastructure over the past four years we have collected detailed hydrogeomorphic data at 40 unique stream locations throughout northern kentucky with at least two rounds of annually repeated surveys at 70 of the sites and three rounds of surveys at 50 of the sites analysis of this time series data encompassed measured rates of instability across three distinct dimensions including 1 channel cross sections 2 longitudinal profiles and 3 bed material particle composition regression analyses between geomorphic change and 2011 watershed imperviousness indicated stream cross sections in urban suburban watersheds tend to be getting larger their overall shape is both deepening and widening additionally stream riffle lengths are shrinking and their pools are becoming both longer and deeper and finally their bed material composition is coarsening particularly in streams in the early stages of watershed development by documenting fluvial geomorphologic dynamics in such detail this study highlights the process by which unmitigated urbanization homogenizes stream habitat and degrades aquatic ecosystems this improved process based understanding of the urban induced channel response sequence has clear implications to both stormwater management and stream ecosystem restoration particularly in stream systems where headcut migration is a primary driver of channel instability

latest olduvai short lived reversal episodes recorded in chinese loess 1 a duplicate high resolution magnetic record was obtained from two parallel loess sections separated by about 300 m near baoji city southern chinese loess plateau this reveals at least eight short lived reversed polarity episodes in the uppermost part of the olduvai normal polarity subchronozone rock magnetic experiments and anisotropy of low field magnetic susceptibility confirm that the multiple occurrence of short lived reversals is not due to rock magnetic and or sedimentary causes assuming a constant accumulation rate within the olduvai subchron the short episode zone was estimated to span about 24 30 ka and 24 31 ka for the two sections respectively the duration of each short episode ranges from about 0 3 ka to 2 1 ka the present results together with previous worldwide observations show that the occurrence of numerous short reversals is peculiar to the latest olduvai subchron the geodynamo may have been in an anomalous state at that time

a sixty thousand year paleomagnetic record from gulf of california sediments secular variation late quaternary excursions and geomagnetic implications a unique inclination record of geomagnetic secular variation for the past 60 000 years 60 ka has been obtained from continuously deposited sediments in the gulf of california recovered during deep sea drilling project dsdp leg 64 by hydraulic piston coring at site 480 the chronology of hole 480 was determined by 18o stratigraphy and varve counts indicating sedimentation rates approaching 1 m ka the paleomagnetic results of the upper 49 m show 1 the average inclination over this interval is identical to the geocentric axial dipole value at the sampling site 2 excursion directions occur between about 53 and 22 ka before present bp during this time the geomagnetic field was generally noisier than in the overlying and underlying sections with greater dispersion of the inclination 3 the laschamp excursion was apparently recorded at hole 480 between about 51 and 49 ka bp and the mono lake excursion between about 29 and 26 ka bp in addition a narrow 0 3 0 4 m zone near 23 ka bp has a very similar paleomagnetic signature as the excursion observed at summer lake oregon 1 and we suggest that the summer lake is distinct from and younger than the mono lake excursion by 3 5 ka and of considerably shorter duration lasting no more than a few hundred years 4 recurring inclination fluctuations were identified at site 480 characterized by end points with steep inclinations and shallow intermediate value s as compared with the geocentric axial dipole the inclination cycles are particularly apparent from 54 to 24 ka bp with a characteristic period of about 4 4 ka 5 the noisier inclination record between 54 and 24 ka bp might be related to the generally reduced dipole moment between about 20 and 50 ka and particularly low paleointensities for the laschamp and mono lake excursions

floods in mountain environments a synthesis floods are a crucial agent of geomorphic change in the channels and valley floors of mountains watercourses at the same time they can be highly damaging to property infrastructure and life because of their high energy mountain watercourses are highly vulnerable to environmental changes affecting their catchments and channels many factors have modified and frequently still tend to modify the environmental conditions in mountain areas with impacts on geomorphic processes and the frequency magnitude and timing of floods in mountain watercourses the ongoing climate changes vary between regions but may affect floods in mountain areas in many ways in many mountain regions of europe widespread afforestation took place over the twentieth century considerably increasing the amounts of large wood delivered to the channels and the likelihood of jamming bridges at the same time deforestation continues in other mountain areas accelerating runoff and amplifying the magnitude and frequency of floods in foreland areas in many countries in channel gravel mining has been a common practice during recent decades the resultant deficit of bed material in the affected channels may suddenly manifest during flood events resulting in the failure of scoured bridges or catastrophic channel widening during the past century many rivers in mountain and foreland areas incised deeply the resultant loss of floodplain water storage has decreased attenuation of flood waves hence increasing flood hazard to downstream river reaches on the other hand a large amount of recent river restoration activities worldwide may provide examples of beneficial changes to flood risk attained as a result of increased channel storage or reestablished floodplain water storage relations between geomorphic processes and floods operate in both directions which means that changes in flood probability or the character of floods e g increased wood load may significantly modify the morphology of mountain rivers but morphological changes of rivers can also affect hydrological properties of floods and the associated risk for societies this paper provides a review of research in the field of floods in mountain environments and puts the papers of this special issue dedicated to the same topic into context it also provides insight into innovative studies methods or emerging aspects of the relations between environmental changes geomorphic processes and the occurrence of floods in mountain rivers

reply to comment by j p valet and l meynadier on a relative geomagnetic paleointensity stack from ontong java plateau sediments for the matuyama

palaeomagnetic investigations on lake sediments from ne china a new record of geomagnetic secular variations for the last 37 ka summary detailed palaeomagnetic investigations were carried out on two 23 m long sediment cores from erlongwan maar lake ne china the sediment composition of both cores is nearly identical based on a macroscopical inspection of the cores 410 graded layers intercalated into the laminated sediments were identified measurements of the anisotropy of the magnetic susceptibility revealed that these layers have not disturbed the sediment structure in general but only in intervals where their thicknesses exceeds 50 cm the age model for erlongwan is based on 15 ams 14c datings on bulk sediment showing that the sediment profile spans the last 37 ka cal bp although one of the sediment cores has been slightly deformed during core recovery similar inclination and declination records could be obtained by standard palaeomagnetic methods the stacked inclination and declination records show both variations similar to those known from palaeosecular variation records from eastern china and japan therefore the presented study is a contribution to the ongoing process of compiling a psv mastercurve for east asia

saw toothed variations of relative paleointensity and cumulative viscous remanence testing the records and the model a recent model kok and tauxe 1996a that attempts to explain asymmetrical sawtooth patterns of relative paleointensity from sediments valet and meynadier 1993 as the consequence of cumulative long term viscosity effects has been tested theoretically and experimentally a mathematical description of the model demonstrates that only a sharp distribution of relaxation times would be able to match the paleomagnetic record which implies that all the magnetic grains would be identical this has direct consequences for the distribution of the blocking temperatures tb which would not exceed 260 c and for the coercivities of the natural remanent magnetization nrm this tb spectrum derived from the model is inconsistent with the unblocking temperatures of the nrm and with the tb spectrum of the experimental thermoremanent magnetization trm of the sediment that recorded the sawtooth new results including thermal demagnetization and thellier thellier experiments across brunhes matuyama confirm the overall stability of the original records the stability of the older part of the record 2 to 4 ma has also been strengthed the sawtooth is unchanged between 20 mt and 65 mt and thermal demagnetization in a neutral atmosphere at 300 c and 450 c duplicates the results obtained by alternation field a f demagnetization thus viscous processes at least within the framework of a cumulative viscous model and for the existing data sets are not responsible for saw toothed paleointensity records

comparing gravel bed rivers in paired urban and rural catchments of southeastern pennsylvania surveys in eight paired urban and rural watersheds illustrate how urbanization changes fluvial morphology and processes our data also provide quantitative criteria for evaluating streamrestoration projects in urban areas bankfull depth reach averaged bed slope and median grain size are similar in urban and rural watersheds the median width of urban channels is 26 larger than the median width of rural channels the median sinuosity is 8 lower in urban channels and pools are 31 shallower the median composite manning s n based on median grain diameter pool depth and channel sinuosity is 10 lower in urban streams while the median bankfull discharge per unit drainage basin area is 131 higher in urban channels histograms of bed sediment size distributions in urban channels lack a secondary mode in the size range 2 64 mm characteristic of rural channels indicating that these sizes tend to be selectively removed from urban channels however bankfull shields stresses in urban and rural channels exceed typical threshold values at most sites indicating significant bedload transport at bankfull stage apparently increased peak discharges caused by decades of urbanization have not removed all the transportable sediment from these urban stream channels we speculate that the supply of sediment to urban channels from hillslope processes and channel erosion remains significant even though much of the upland surfaces of these urban catchments are covered with nonerodible impervious surfaces

recent and historic sediment dynamics along difficult run a suburban virginia piedmont stream abstract suspended sediment is one of the major concerns regarding the quality of water entering the chesapeake bay some of the highest suspended sediment concentrations occur on piedmont streams including difficult run a tributary of the potomac river draining urban and suburban parts of northern virginia accurate information on catchment level sediment budgets is rare and difficult to determine further the sediment trapping portion of sediment budget represents an important ecosystem service that profoundly affects downstream water quality our objectives with special reference to human alterations to the landscape include the documentation and estimation of floodplain sediment trapping present and historic and bank erosion along an urbanized piedmont stream the construction of a preliminary sediment balance and the estimation of legacy sediment and recent development impacts we used white feldspar markers to measure floodplain sedimentation rates and steel pins to measure erosion rates on floodplains and banks respectively additional data were collected for from legacy sediment thickness and characteristics mill pond impacts stream gaging station records topographic surveying and sediment density texture and organic content data were analyzed using gis and various statistical programs results are interpreted relative to stream equilibrium affected by both post colonial bottomland sedimentation legacy and modern watershed hardening associated with urbanization six floodplain channel sites from high to low in the watershed were selected for intensive study bank erosion ranges from 0 to 470 kg m y and floodplain sedimentation ranges from 18 to 1369 kg m y m refers to meters of stream reach upstream reaches are net erosional while downstream reaches have a distinctly net depositional flux providing a watershed sediment balance of 2184 kg m y trapped within the system the amounts of both deposition and erosion are large and suggest nonequilibrium channel conditions both peak discharge and number of peaks above base have substantially increased since the mid 1960s when urbanization of the watershed began deposition patterns are most closely correlated with channel gradient sinuosity and channel width floodplain width for recent and historic periods the substantial amounts of fine grained sediment deposited on the floodplain over the past two centuries or so do not appear to be closely related to historic mill pond presence or location the floodplain continues to provide the critical ecosystem service of sediment trapping in the face of multiple human alterations trends in sediment deposition erosion may react rapidly to land use practices within the watershed and offer a valuable barometer of the effects of management actions

millennial to submillennial scale features of the matuyama brunhes geomagnetic polarity transition from osaka bay southwestern japan 1 a high resolution magnetic record of the matuyama brunhes mb reversal transition was obtained from homogeneous marine clay in a 1700 m core from osaka bay the transition stretches over a core length of 6 4 m and yields four short reversal episodes two predate the main mb boundary mbb and two postdate it we made diatom analyses of sediments to estimate relative sea level changes and constructed an age model by correlation with the astronomically calibrated marine oxygen isotope curve the age model shows the short reversal episodes with durations of 1 kyr occurred at 4 5 kyr intervals within stage 19 and the mbb occurred about 5 kyr after the sea level highstand corresponding to isotopic event 19 3 it also shows the paleointensity minimum of 15 20 of the postreversal mean occurred at about 6 kyr before the mbb and another minimum at about 1 kyr after four virtual reverse to normal polarity boundaries found in this study suggest different ages for the mbb could be observed in lava flow studies some millennial to submillennial scale transition features found in this study are compatible with the previous numerical simulations of geomagnetic field reversal

saw toothed pattern of sedimentary paleointensity records explained by cumulative viscous remanence abstract the relative paleointensity of the earth s magnetic field from odp site 851 has been characterized by progressive decay towards polarity reversals followed by sharp recovery of pre reversal values 1 we resampled the gilbert gau reversal boundary of this deep sea core and show that during demagnetization this saw toothed pattern disappears further the recently published cumulative viscous remanence model 2 using the herewith obtained paleointensity record and constraints from thermal treatment replicates the saw tooth of 1 implying that it is of non geomagnetic origin