shallow landsliding root reinforcement and the spatial distribution of trees in the oregon coast range the influence of root reinforcement on shallow landsliding has been well established through mechanistic and empirical studies yet few studies have examined how local vegetative patterns influence slope stability because root networks spread outward from trees the species size and spacing of trees should influence the spatial distribution of root strength we documented the distribution and characteristics of trees adjacent to 32 shallow landslides that oc curred during 1996 in the oregon coast range although broadly classified as a conifer dominated forest we observed sparse coniferous and abundant hardwood trees near landslide scars in an industrial forest mapleton that experienced widespread burning in the 19th century in industrial forests that were burned selectively harvested and not replanted elliott state forest swordfern was ubiquitous near landslides and we observed similar numbers of live conifer and hardwood trees proximal to landslide scarps we demonstrate that root strength quantified in landslide scarps and soil pits correlates with a geometry based index of root network contribution derived from mapping the size species condi tion and spacing of local trees indicating that root strength can be predicted by mapping the distribution and charac teristics of trees on potentially unstable slopes in our study sites landslides tend to occur in areas of reduced root strength suggesting that to make site specific predictions of landslide occurrence slope stability analyses must account for the diversity and distribution of vegetation in potentially unstable terrain

one hundred million years of geomagnetic polarity history since 1968 absolute ages have been assigned to the late cretaceous cenozoic geomagnetic reversal time scale by fixing the ages of two or more calibration points in a composite marine magnetic anomaly profile and interpolating between or extrapolating beyond these points assuming constant spreading rates in each interval previously no more than 4 calibration points were used but it is now possible to specify 11 calibration points in addition to the 0 m y datum this improvement is based on magnetostratigraphic studies in italian pelagic limestones these studies closely tie the geomagnetic reversal sequence to the foraminiferal and coccolith zonations absolute ages of calibration points are provided by the best available dates on stage boundaries which are located from the biostratigraphic zonation the greatest changes from previous scales come in the late paleocene early eocene where the new ages are as much as 3 m y younger than in the 1977 scale of labrecque and others and as much as 1 8 m y younger than in the 1980 scale of ness and others

geomagnetic reversal spurts and episodes of extraterrestrial catastrophism the possibility that the observed 26 32 myr periodicity in the records of extinction events1 and astrobleme ages2 was caused by periodic episodes of physical bombardment of the planetary surface has raised the likelihood that such an astronomical clock also affects geomagnetic reversals3 4 however the detection5 of a comparable 30 myr harmonic in the geomagnetic reversal record has now been found6 to be an accident of data length while the earlier reported7 32 34 myr harmonic of the reversal record may well be an artefact of the data base here we examine the temporal relation between geomagnetic reversals and those catastrophic episodes which occurred during periods of frequent reversals as geomagnetic history comprises long intervals of frequent reversals alternating with intervals of fixed polarity our rationale is that the geomagnetic field should be most susceptible to catastrophism during its relatively unstable periods of frequent reversals we find spurts in the frequency of geomagnetic reversals at approximately 30 myr intervals correlated with catastrophic episodes these spurts can be seen as the geomagnetic signatures of terrestrial catastrophism and we tentatively ascribe them to enhanced core turbulence during episodes of bombardment the absence of reversal spurts during long intervals of fixed polarity is attributed to the failure during these intervals of the geomagnetic dynamo s bi stable oscillations

correction of deposit ages for inherited ages of charcoal implications for sediment dynamics inferred from random sampling of deposits on headwater valley floors abstract inherited age is defined herein as the difference between times of carbon fixation in a material and deposition of that material within sediments from which it is eventually sampled in order to estimate deposit age via radiocarbon dating inheritance generally leads to over estimation of the age by an unknown amount and therefore represents unquantified bias and uncertainty that could potentially lead to erroneous inferences inherited ages in charcoal are likely to be larger and therefore detectable relative to analytic error where forests are dominated by longer lived trees material is stored for longer periods upslope and downstream post fire delivery of that material is dominated by mass movements such as in the near coastal mountains of northwestern north america inherited age distribution functions were estimated from radiocarbon dating of 126 charcoal pieces from 14 stream bank exposures of debris flow deposits fluvial fines and fluvial gravels along a headwater stream in the southern oregon coast range usa in the region these 3 facies are representative of the nearly continuous coalescing fan fill complexes blanketing valley floors of headwater streams where the dominant transport mechanism shifts from debris flow to fluvial within each depositional unit and for each charcoal piece within that unit convolution of the calibrated age distribution with that of the youngest piece yielded an inherited age distribution for the unit fits to the normalized sums of inherited age distributions for units of like facies provided estimates of facies specific inherited age distribution functions finally convolution of these distribution functions with calibrated deposit age distributions yielded corrections to published valley floor deposit ages and residence time distributions from nearby similar sites residence time distributions were inferred from the normalized sums of distributions of 30 deposit ages at each of 4 sites 2 adjacent valley reaches 103 m long and within 102 m of 2 tributary confluences mean inherited ages from the observed distributions are 666 688 and 1506 yr for debris flow deposits fluvial fines and fluvial gravels respectively on average correction reduced estimates of individual deposit age means by a factor of 0 71 0 56 0 94 and increased standard deviations by a factor of 6 1 0 97 43 across sites mean residence times decreased by 24 0 and standard deviations by 12 5 on average corrected residence time distributions have thicker tails as indicated by gamma distribution fits with smaller shape factors and these changes are significant relative to the bootstrapped 95 confidence limits representing potential error in the sampling for inherited ages the ratio of the means of sediment age and residence time ranged from 1 03 to 1 80 across sites before correction and 1 21 to 2 18 after correction where a value of one implies that probability of evacuation from the reservoir comprising valley floor deposits is independent of time since deposition corrected values of this ratio therefore indicate that evacuation favors younger deposits at all sites whereas uncorrected results implied age independent evacuation from the more downstream valley reach

determining the strength of the deep western boundary current using the chlorofluoromethane ratio abstract the dilution of a passive tracer during deep water formation and the subsequent advection mixing in a deep boundary current is modeled with application to chlorofluoromethanes cfms in the north atlantic two different types of boundary currents are considered a uniform flow and a simple shear flow in each case the core of the flow mixes with surrounding water which continually accumulates cfms in an extreme case the coupled systems predicts that the cfm ratio in the current is unaltered from the ratio of its source water save for a time lag more realistic cases however suggests that the ratio is not a conserved quantity but is substantially altered in both the overflow basin and boundary current matching the model results to cfm data collected near the grand banks gives a predicted average core speed of 5 10 cm s 1 for the deep western boundary current and provides a constraint on the transport and diffusivity of the flow as well

a model of correlated episodicity in magnetic field reversals climate and mass extinctions correlated periodicities of mass extinctions climate magnetic field reversals and other geological phenomena may best be explained by cycles of activity within the mantle and core we propose that these cycles are due to variations in the thickness of the thermal boundary layer at the base of the mantle as the layer alternates between two phases of activity in the quiescent phase little flow occurs and the layer thickens with time by thermal diffusion the active phase begins when the thickening layer becomes dynamically unstable hot material erupts from the layer causing it to become thinner as the layer thins the rate of energy supply to the geodynamo and hence its reversal frequency increases the hot material rises to the surface where it causes widespread basaltic volcanic eruptions these eruptions release large amounts of co 2 and sulfates which have a pronounced effect on the climate and biota

drainage basin sensitivity to tectonic and climatic forcing implications of a stochastic model for the role of entrainment and erosion thresholds long term average rates of channel erosion and sediment transport depend on the frequency magnitude characteristics of ood ows that exceed an erosion threshold using a poisson model for rainfall and runoff analytical solutions are developed for average rates of stream incision and sediment transport in the presence of such a threshold solutions are derived and numerically tested for three erosion transport formulas the howard kerby shear stress incision model the bridge dominic sediment transport model and a generic shear stress sediment transport model results imply that non linearity resulting from threshold effects can have a rst order impact on topography and patterns of dynamic response to tectonic and climate forcing this non linearity becomes signi cant when fewer than about half of ood events are capable of detaching rock or sediment predicted morphology and uplift gradient scaling is more closely consistent with observations and laboratory experiments than conventional slope linear or shear linear erosion laws these results imply that particle detachment thresholds are not details that can be conveniently ignored in long term landscape evolution models copyright 2004 john wiley sons ltd

fire and the evolution of steep soil mantled landscapes recent burns in the western united states attest to the signif icant geomorphic impact of fire in mountainous landscapes yet we lack the ability to predict and interpret fire related erosion over millennial time scales a diverse set of geomorphic processes is often invoked following fire the magnitude of postfire erosional processes coupled with temporal variations in fire frequency dic tate the extent to which fires affect sediment production and land scape evolution in the oregon coast range several models for long term rates of soil production and transport have been tested and calibrated although treatment of fire related processes has been limited following recent fires in the oregon coast range we observed extensive colluvial transport via dry ravel localized bed rock emergence due to excess transport and talus like accumula tion in adjacent low order valleys soils exhibited extreme but dis continuous hydrophobicity and no evidence for rilling or gullying was observed using a field based data set for fire induced dry rav el transport we calibrated a physically based transport model that indicates that soil flux varies nonlinearly with gradient the post fire critical gradient 1 03 which governs the slope at which flux increases rapidly is lower than the previously estimated long term value 1 27 reflecting the reduction of slope roughness from in cineration of vegetation by using a high resolution topographic data set generated via airborne laser swath mapping we modeled the spatial pattern of postfire and long term erosion rates postfire erosion rates exceed long term rates which average 0 1 mm yr 21 by a factor of six and subtle topographic variations generated local patches of rapid postfire erosion commonly 1 mm yr 21 our sim ulations indicate that fire related processes may account for 50 of temporally averaged sediment production on steep hillslopes our analysis provides a mechanistic explanation for the coincident early holocene timing of increased fire frequency and regional ag gradation in oregon coast range drainage basins given the sen sitivity of steep hillslopes to fire driven transport changes in cli mate and fire frequency may affect soil resources by perturbing the balance between soil transport and production

15 myr periodicity in the frequency of geomagnetic reversals since 100 myr the statistical properties of the magnetic polarity time scale have been examined recently1 2 for several time scales for some of these scales the time evolution of the frequency of reversals as seen through an 8 myr moving rectangular window have been determined in particular over the past 100 myr the mean reversal frequency appears to be dominated by a linearly increasing trend on which is superimposed a rhythmic fluctuation we have attempted to analyse the mathematical structure of the reversal frequency curve in detail and to determine whether the rhythmic fluctuation was indeed periodic by modelling the time evolution of the reversal frequency we find a dominant frequency corresponding to a period of 15 myr this result represents one of the most persistent long term periodicities ever reported for a geophysical phenomenon

processes and rates of sediment and wood accumulation in headwater streams of the oregon coast range usa channels that have been scoured to bedrock by debris flows provide unique opportunities to calculate the rate of sediment and wood accumulation in low order streams to understand the temporal succession of channel morphology following disturbance and to make inferences about processes associated with input and transport of sediment dendrochronology was used to estimate the time since the previous debris flow and the time since the last stand replacement fire in unlogged basins in the central coast range of oregon debris flow activity increased 42 per cent above the background rate in the decades immediately following the last wildfire changes in wood and sediment storage were quantified for 13 streams that ranged from 4 to 144 years since the previous debris flow the volume of wood and sediment in the channel and the length of channel with exposed bedrock were strongly correlated with the time since the previous debris flow wood increased the storage capacity of the channel and trapped the majority of the sediment in these steep headwater streams in the absence of wood channels that have been scoured to bedrock by a debris flow may lack the capacity to store sediment and could persist in a bedrock state for an extended period of time with an adequate supply of wood low order channels have the potential of storing large volumes of sediment in the interval between debris flows and can function as one of the dominant storage reservoirs for sediment in mountainous terrain copyright 2003 john wiley sons ltd

large himalayan frontal thrust paleoearthquake at khayarmara in eastern nepal abstract an exposure created by excavation of a trench across the himalayan frontal thrust provides the basis to interpret that a single earthquake produced vertical separation of 7 m at khayarmara a small community 80 km southwest of kathmandu the fault trace at khayarmara is expresssed by a topographic ridge resulting from folding up dip and toward the surface at the expense of greater fault slip taking place at depth structure stratigraphy and radiocarbon data are interpreted to indicate displacement occurred after about 1050 ce to 1200 ce the timing and displacement at khayarmara are compared to that reported previously at six contiguous sites that extend from 200 km to the west and 250 km to the east of khayarmara respectively the comparison leads us to conclude that the surface rupture at khayarmara was part of a 250 km long synchronous surface rupture earthquake of magnitude approaching if not surpassing mw 9 we observe in the exposure no record of surface rupture associated with the great 1934 bihar nepal earthquake

tracing the thermohaline circulation in the western north atlantic using chlorofluorocarbons abstract measurement of chlorofluorocarbons cfcs f 11 and f 12 were obtained from vertical sections extending across the deep western boundary current dwbc along the western margin of the north atlantic ocean between 32 45 n in 1983 and 1986 these sections revealed an upper and a deep cfc maximun with highest concentrations along the continental slope the upper maximum had a potential temperature ranging from 4 2 4 6 c a core density of 1 5 34 56 and occurred at a depth of about 800m adjacent to the continental slope water with classical labrador sea water s characteristics occurs just above the cfc minimum between the upper and lower maxima and indicates that a significant volume of classical labrador sea water has not entered the region south of the grand banks for the last 3 decades the deep cfc maximum is from dense water overflowing the greenland iceland scotland ridge it occurred at about 3500m depth and had a potential temperature range of 2 02 2 18 c and a core density of 4 45 87 a source function of f 11 and f 12 for the deep maximum was estimated using temperature salinity and tracer data from previous studies the core of the dwbc defined by the deep cfc maximum was estimated to consist of roughly 2 3 gibbs fracture water and 1 3 denmark strait overflow water however 80 or more of the f 11 and f 12 signal was derived from denmark strait overflow water ages for the upper and lower cfc maxima in the dwbc were estimated from the cfc data these ages ranged from 11 12 years at the northernmost section to 18 years at the southernmost section for both features this corresponds to apparent mean current speeds of about 0 8cm s 1 for the upper maximum and 1 4cm s 1 for the deep maximum an age of 8 9 years with a corresponding apparent mean current speed of 2 2 2 5cm s 1 was also estimated for the deep cfc maximum from the observed increase in the f 11 concentration at 41 n between 1983 and 1986 the apparent mean current speeds are significantly less than current speeds measured with long term current meter moorings two reasons for this are 1 mixing between the dwbc core and adjacent water causes ages estimated from cfc data to be overestimated and hence the associated current speed to be underestimated pickart hogg and smethie 1989 and 2 water propagating along the western boundary spends some time in recirculation gyres the northern recirculation gyre south of the eastern north american continental margin is clearly apparent in the lateral distributions of f 11 and f 12 the apparent mean current speed in the northern recirculation gyre was estimated to be 1 3cm s 1 from the cfc data seaward of the dwbc north of the new england seamount chain high cfc low silica parcels of water were observed interspersed with low cfc high silica parcels of water along the bottom the high cfc low silica water is overflow water with a slightly greater component of denmark strait overflow water than contained in the dwbc deep cfc maximum the low cfc high silica water is of southern origin and has a component of antarctic bottom water

missing reversals in the geomagnetic polarity timescale their influence on the analysis and in constraining the process that generates geomagnetic reversals a major problem in defining the chronology of geomagnetic reversals is linked to the detection of short 30 kyr time intervals between reversals tibr published polarity timescales do not usually include the shortest tibr therefore the timescales are inherently incomplete the purpose of this paper is to investigate the effects of this incompleteness on the analysis of the geomagnetic polarity timescale the ultimate goal is to provide constraints on the earth s core processes the effects of inclusion exclusion of the shortest tibr are verified by comparing the results obtained by statistical analysis of real and synthetic series of events for the real sequence two basic cases are considered in which the tiny wiggles are attributed either to short tibr or to paleointensity fluctuations particular attention is paid to the influence of measurement errors estimated for the most recently published cenozoic timescale by following the minimalist philosophy of occam s razor which is particularly suitable for studying poorly known processes the reliability of the simplest model i e the poisson process which is symmetric in polarity can be checked the results indicate the plausibility of a generalized renewal process the only regularity is relative to the long term trend which is probably linked to core mantle coupling in detail a uniform exponential trend in the last 80 myr is found for the timescale it is not presently possible to estimate the influence of the inclusion of tiny wiggles because they are well resolved only in the last 30 myr a period in which both series are stationary the sequences with and without tiny wiggles are symmetric in polarity with no evidence of low dimensional chaos and memory of past configurations the empirical statistical distribution of the tibr departs slightly from a theoretical exponential distribution i e from a poisson process which can be explained by a lack of short anomalies and or by a generating process with wear out properties a more general renewal process a real exponential distribution is sustainable only if the number of missing short tibr in the last 30 myr is larger than the number of tiny wiggles observed in the same period

debris dams and the relief of headwater streams in forested mountain landscapes where debris flows are common their deposition commonly forms valley spanning dams of wood boulders or complex mixtures of both in headwater valleys sediment impoundment behind these dams causes alluviation in what would otherwise be bedrock channels in this paper the effects of debris dams on the evolution of headwater valley profiles over geologic time are examined in the oregon coast range usa longitudinal profiles of three headwater channels approximately 2 km 2 maximum contributing area two of which are nominally bedrock and all debris dams were surveyed channel and valley widths were measured and surface bed material measurements pebble counts were taken at several locations along one stream cumulative relief in debris dams is highly variable within and among streams reaching a maximum of 58 of profile relief at one location the surveyed dams comprise 9 8 19 and 6 4 of total basin relief in the three corresponding drainage basins respectively a model of bedrock erosion and channel profile evolution is derived that accounts for a fractional coverage by the channel of total valley width at any given time and b shielding by sediment impounded behind debris dams the equation for incision rate assumed constant is solved for valley gradient as a function of contributing area and parameter values are estimated from or supplied by field data results are compared to stream gradient contributing area relationships obtained for the field sites these comparisons suggest a strong effect of network structure which varies significantly among the sites on profile shape and relief because of the different susceptibilities to debris flow deposition and therefore debris dam formation longitudinal profiles with and without the effects of debris dams are modeled for one forced alluvial i e nominally bedrock channel for these model profiles which extend only as far as the surveyed profile at this site 55 of the profile relief is due to debris dams and the concomitant shielding of the bed by impounded sediment the results suggest that a significant fraction of the relief of such forested mountain landscapes is due to the effects of relatively immobile wood and boulders deposited in the valleys by debris flows

the subthermocline lens d1 part i description of water properties and velocity profiles abstract one remarkable result of the local dynamics experiment in polymode is the discovery of a large number of small energetic eddies using ctd data sofar float tracks and profiles of absolute velocity we describe one of these features the subthermocline eddy d1 the feature has an advective time scale of 1 day and its primary mode of movement is advection by the ambient flow field the eddy is resolved into a coordinate system moving with the velocity of the ambient flow which ranges from 11 16 cm s 1 its phase speed is less than 2 cm s 1 the eddy is a lens centered at 1500 db velocity profiles show the lens has no detectable signal in and above the main thermocline or below about 3000 db its subsurface velocity maximum is 28 6 cm s 1 at 1500 db 15 km from the center the radial variation of the azimuthal velocity is gaussian inside the velocity maximum but decays as e br beyond there is little evidence of the eddy beyond a radius of 25 km inside the velocity maximum the eddy is characte

geomagnetic secular variation impulses the secular variation of the earth s internal magnetic field changes on time scales of the order of a year such impulses occurred in 1969 and probably in 1913 they provide constraints on deep mantle conductivity which appears to be rather low and on the motions of the top of the fluid core which are probably dominated by westward drift present data favour the existence of upwelling of core material at the core mantle boundary the impulses can be correlated with extrema in the earth s rotation rate and interpreted in terms of electromagnetic core mantle coupling

erosion of steepland valleys by debris flows episodic debris flows scour the rock beds of many steepland valleys along recent debris flow runout paths in the western united states we have observed evidence for bedrock lowering primarily by the impact of large particles entrained in debris flows this evidence may persist to the point at which debris flow deposition occurs commonly at slopes of less than 0 03 0 10 we find that debris flow scoured valleys have a topographic signature that is fundamentally different from that predicted by bedrock river incision models much of this difference results from the fact that local valley slope shows a tendency to decrease abruptly downstream of tributaries that contribute throughgoing debris flows the degree of weathering of valley floor bedrock may also decrease abruptly downstream of such junctions on the basis of these observations we hypothesize that valley slope is adjusted to the long term frequency of debris flows and that valleys scoured by debris flows should not be modeled using conventional bedrock river incision laws we use field observations to justify one possible debris flow incision model whose lowering rate is proportional to the integral of solid inertial normal stresses from particle impacts along the flow and the number of upvalley debris flow sources the model predicts that increases in incision rate caused by increases in flow event frequency and length as flows gain material downvalley are balanced by rate reductions from reduced inertial normal stress at lower slopes and stronger less weathered bedrock these adjustments lead to a spatially uniform lowering rate although the proposed expression leads to equilibrium long profiles with the correct topographic signature the crudeness with which the debris flow dynamics are parameterized reveals that we are far from a validated debris flow incision law however the vast extent of steepland valley networks above slopes of 0 03 0 10 illustrates the need to understand debris flow incision if we hope to understand the evolution of steep topography around the world

experiments in waterlogging and sedimentology of charcoal results and implications abstract fossil charcoal has a sporadic occurrence in sedimentary rocks since devonian time it is moderately common as a component of the organic material found in a wide variety of facies but there are also some notable concentrations which occur locally these occurrences have considerable palaeobotanical and palaeoecological value because the process of charring may result in excellent preservation of plant tissue if assemblages of charred material are to be used to interpret palaeoenvironments it is important to understand the behaviour of charcoal during transport from the site of the fire charcoal is an unusual sedimentary material because most fresh material floats but with prolonged immersion becomes waterlogged and sinks we carried out a series of waterlogging experiments on uncharred material and charcoal from a range of different plant types and tissues these show considerable variations in the rates of waterlogging of different charred and uncharred plant tissues and it is suggested that buoyancy contrasts are likely to result in separation during transport and deposition furthermore experiments in a flume tank have shown that the process of deposition of charcoal in sand is controlled by the rate of migration of bedforms which is in turn determined by depth flow velocity and sediment supply allochthonous assemblages of charred and uncharred plant debris will most probably be biased and not fully representative of the contemporaneous plants or vegetation the presence of charcoal concentrations may be used as an aid in interpreting palaeohydraulic conditions

evidence for regional stream aggradation in the central oregon coast range during the pleistocene holocene transition abstract low nearly continuous terraces of similar age are present along streams in drainage basins that range in size from drift creek 190 km 2 to the umpqua river 11 800 km 2 in the oregon coast range radiocarbon ages from near the bose of fluvial sediments underlying these terraces are clustered at about 9000 11 000 14 c yr b p beveled bedrock surfaces straths that underlie the fluvial sediments are 1 8 m above summer stream levels and are present along most of the nontidal reaches of the rivers that we studied where exposed the bedrock straths are overlain by 2 11 m of fluvial sediment that consists of a bottom stratum channel facies of sandy pebble cobble gravel and a top stratum overbank facies of sandy silt or silt eight radiocarbon ages from the fluvial sediments allow correlation of the lowest continuous terrace over a wide area and thus indicate that a regional aggradation episode occurred in coast range drainage basins during the pleistocene holocene transition the cause of such widespread aggradation is unknown but may be related to climate induced changes in the frequency of evacuation of colluvium from hollows which are common in all drainage basins in the region

the magnetic reversal record is not periodic statistical analysis of one version of the magnetic reversal record suggests that the frequency of reversals has changed cyclically with a period of 30 myr starting at least 165 myr ago but factors other than periodicity affect the analysis used and here i show that there is no compelling evidence for periodicity in the reversal record the results of essentially identical analyses that have been applied to other geolocial data should also be re evaluated

short polarity intervals within the matuyama transitional field records from hydraulic piston cored sediments from the north atlantic abstract detailed sampling of two short magnetozones within the matuyama chronozone recorded at dsdp site 609 49 86 n 335 77 e confirms that one the cobb mountain subchronozone 1 12 ma is a very short full normal polarity interval and that the other the older interval is a record of a geomagnetic excursion which occurred at approximately 1 55 ma the cobb mountain subchron lasted approximately 25 000 years one third the duration of the jaramillo subchron the normal polarity interval is bounded by two transition zones which document an antisymmetry in the sequence of directions in the reverse to normal and normal to reverse polarity transitions we interpret the antisymmetry as reflecting a dependence upon the sense of the reversal without significant changes in the relative contributions of non dipole terms the polarity interval recorded at 1 55 ma lasted only 8 800 years with what may be regarded as full polarity directions observed across only 3 cm of stratigraphic section this feature is interpreted as an excursion of the geomagnetic field and appears to be correlative with the gilsa subchron similarities between the transition bounding these two magnetozones suggest that these features occur as the result of the same process or triggering mechanisms in the earth s outer core

the periodicity of geomagnetic reversals abstract several periodicities have been claimed in the time series of geomagnetic reversals but none has survived subsequent methodological objections we apply the established tool of periodogram analysis to the most accurate geomagnetic scale available and obtain the following results a trend that is well fitted by an exponential curve is present in the rate of reversals occurrence and there is a 15 ma periodicity that is significant at approximately the 0 05 level no significant period is present in the series of the intervals between reversals these results indicate that geomagnetic reversals since 85 ma bp occur according to a generalized poisson process with a mean which increases exponentially and which may have a superimposed periodic modulation in physical terms this would imply a mechanism of reversal generation which is stochastic and has an energy input increasing in time modulated by a 15 ma periodicity this is consistent with reversals originated by cyclones in the outer core a long term change in the lower mantle temperature distribution and a pulsating 8 10 km thick layer at the base of the mantle

charcoal recognition taphonomy and uses in palaeoenvironmental analysis abstract charcoal predominantly the product of wildfires is abundant in many sedimentary rocks deposited in a wide range of environments from terrestrial to marine it also occurs in some volcanic rocks this paper outlines aspects of charcoal formation both natural and experimental and briefly considers the taphonomic processes leading to a final assemblage this is done using examples from recent fires and through experimentation the ways in which charcoal assemblages are recognized in the field and extraction in the laboratory are also considered methods of charcoal identification are presented the range of charcoalified plant organs that can be found is discussed and a wide range of study methods outlined including light microscopy dark field light microscopy reflectance microscopy scanning electron microscopy and synchrotron radiation x ray tomographic microscopy emphasis in this paper is on the study of macro and meso charcoal above180 m finally there is a consideration of the broad use of charcoal from plant evolution studies fire history studies vegetation studies anatomical studies climate and atmospheric studies and the wider importance of charcoal for the earth and biological sciences charcoal is information rich but yet is an under utilized resource

long range dependence in the cenozoic reversal record the frequency distribution of intervals between cenozoic geomagnetic reversals approximates a power law while their occurrence over time shows temporal clustering of short and long intervals application of the aggregate variance and absolute valuemethods suggest long range dependence in this time series a possible indication that the geodynamo operates as a self organised complex system this hypothesis may allow the cretaceous superchron to be considered as an integral part of the ordinary reversal regime attested in the cenozoic record 2003 elsevier science b v all rights reserved

distribution and circulation of labrador sea water abstract labrador sea water is the final product of the cyclonic circulation of subpolar mode water in the open northern north atlantic mccartney and talley 1982 the temperature and salinity of the convectively formed subpolar mode water decrease from 14 7 c 36 08 to 3 4 c 34 88 on account of the cumulative effects of excess precipitation and cooling the coldest mode water is labrador sea water which spreads at mid depths and is found throughout the north atlantic ocean north of 40 n and along its western boundary to 18 n a vertical minimum in potential vorticity is used as the primary tracer for labrador sea water labrador sea water is advected in three main directions out of the labrador sea 1 northeastward into the irminger sea 2 southeastward across the atlantic beneath the north atlantic current and 3 southward past newfoundland with the labrador current and thence westward into the slope water region crossing under the gulf stream off cape hatteras the labrador sea water core is

vegetative and climatic controls on holocene wildfire and erosion recorded in alluvial fans of the middle fork salmon river idaho the middle fork salmon river watershed spans high elevation mixed conifer forests to lower elevation shrub steppe in recent decades runoff from severely burned hillslopes has generated large debris flows in steep tributary drainages these flows incised alluvial fans along the mainstem river where charcoal rich debris flow and sheetflood deposits preserve a record of latest pleistocene to holocene fires and geomorphic response through deposit sedimentology and 14c dating of charcoal we evaluate the processes and timing of fire related sedimentation and the role of climate and vegetation change fire related deposits compose 66 of the total measured fan deposit thickness in more densely forested upper basins versus 33 in shrub steppe dominated lower basins fires during the middle holocene 8000 5000 cal yr bp mostly resulted in sheetflood deposition similar to modern events in lower basins decreased vegetation density during this generally warmer and drier period likely resulted in lower sev

effects of wood on debris flow runout in small mountain watersheds 1 debris flows have typically been viewed as two phase mixtures of sediment and water but in forested mountain landscapes wood can represent a sizable fraction of total flow volume the effects of this third phase on flow behavior are poorly understood to evaluate whether wood can have a significant effect on debris flow runout in small mountainous watersheds we used a landscape scale model combining empirical stochastic and physical submodels of storms fires forest growth tree fall wood decay soil production and diffusion landslide initiation debris flow runout and fluvial sediment transport we examined changes in the cumulative distribution function of debris flow runout lengths in a small 2 km2 watershed in the oregon coast range due to presence or absence of two hypothesized effects of wood 1 velocity reduction due to entrainment of wood in the runout path and 2 velocity reduction due to changes in flow direction angle the model was calibrated such that the distribution for simulations including both effects was similar to that measured in the study basin and amounts of wood in the simulation and the field both fallen in small valleys and incorporated by debris flows were comparable removal of either effect or both significantly shifted runout length distributions to longer lengths simulations and field observations indicate that with wood fluvial transport is a significant source of sediment output few debris flows reach the outlet and debris flow deposits are widely distributed throughout the network simulations indicate that without wood basin sediment yield greatly increases that yield is dominated by longer runout debris flows and that debris flow deposits are concentrated in the low gradient reach near the outlet

sediment reservoirs at mountain stream confluences dynamics and effects of tributaries dominated by debris flow and fluvial processes radiocarbon age estimates n 68 from bank terrace riser and in channel materials sampled from random locations near two channel confluences a debris flow dominated tributary to cedar creek and a fluvially dominated tributary to golden ridge creek in the oregon coast range are proxies for sediment transit times through tributary and mainstem sediment reservoirs separated from one another by incised bedrock risers geomorphic volumetric stratigraphic and sedimentologic data aided reservoir characterizations inferred transit time distributions for tributary deposits are right skewed and heavy tailed indicating preferential evacuation of younger deposits the debris flow fan is much larger than fluvial terraces on the other tributary but mean transit times in both reservoirs are similar 1370 2240 yr and 1660 2130 yr for fan and terrace deposits respectively whereas tributary deposits are much larger than mainstem deposits at both sites mainstem deposits adjacent to the fan have a relatively short mean transit time of 442 491 yr but mean transit time in mainstem deposits adjacent to the fluvial terrace is much greater 3870 6720 yr reservoir flux estimates indicate that most 60 of the debris flow fan tributary9s sediment yield enters fan storage but only a small part 3 of the fluvial tributary9s yield enters storage at the confluence debris flows from the debris flow fan tributary apparently promote both greater storage of mainstem sediments and more rapid unbiased evacuation of mainstem deposits whereas old mainstem deposits adjacent to the fluvial tributary have a much greater probability of preservation

geomagnetic reversal frequency since the late cretaceous models of geomagnetic reversals as a stochastic or gamma renewal process have generally been tested for the heirtzler et al 1 magnetic polarity time scale which has subsequently been superseded examination of newer time scales shows that the mean reversal frequency is dominated in the cenozoic and late cretaceous by a linearly increasing trend on which a rhythmic fluctuation is superposed subdivision into two periods of stationary behavior is no longer warranted the distribution of polarity intervals is visibly not poissonian but lacks short intervals the labrecque et al 2 polarity time scale shows the positions of 57 small wavelength marine magnetic anomalies which may represent short polarity chrons after adding these short events the distribution of all polarity intervals in the age range 0 40 myr is stationary and does not differ significantly from a poisson distribution a strong asymmetry develops in which normal polarity chrons are poisson distributed but reversed polarity chrons are gamma distributed with indexk 2 this asymmetry is of opposite sense to previous suggestions and results from the unequal distribution of the short polarity chrons which are predominantly of positive polarity and concentrated in the late cenozoic if short wavelength anomalies arise from polarity chrons the geomagnetic field may be more stable in one polarity than the other alternative explanations of the origin of short wavelength marine magnetic anomalies cast doubt on the inclusion of them as polarity chrons however the observed behavior of reversal frequency suggests that core processes governing geomagnetic reversals possess a long term memory

the intercept is a poor estimate of a calibrated radiocarbon age intercept based methods of generating a point estimate of a calibrated radiocarbon date are very popular but exhibit undesirable behaviour they are highly sensitive to the mean of the radiocarbon date and to adjustments of the calibration curve other methods give more stable results the weighted average of the probability distribution function is recommended as the best central point estimate but more consideration should be given to using the full probability distribution rather than a point estimate in developing age depth models

dynamics of the gulf stream deep western boundary current crossover part i entrainment and recirculation abstract a regional primitive equation model is applied to the study of the interaction between the gulf stream and the deep western boundary current dwbc where they cross at cape hatteras it is found that for a wide range of forcing parameters the upper core of the dwbc is split into two mean paths at the crossover one flowing toward the south along the western boundary and the other flowing toward the cast under the gulf stream the eastward branch is entrained into the southern recirculation gyre and after being diverted into the interior for up to 1500 km eventually returns to the western boundary current and continues to flow southward this recirculation and mixing is shown to have a significant impact on the separation point and mean path of the gulf stream the basin scale stratification and the properties of the dwbc south of the crossover for most configurations the lower dwbc remains largely on the western boundary and interacts only weakly with the interior the entrainment of the upp

a standard analytic curve of potential temperature versus salinity for the western north atlantic abstract an algorithm is described for computing salinity as a continuous function of potential temperature for the western north atlantic the algorithm uses historical data compiled by worthington and melcalf 1961 for the deep western north atlantic and iselin 1936 for shallow and intermediate waters of the same region

the formation of labrador sea water part iii the evolution of oxygen and nutrient concentration abstract oxygen nutrient and tritium concentrations observed in the western labrador sea in march 1976 during deep convective renewal of labrador sea water are analyzed to show how a newly formed water mass obtains its characteristics common to other winter observations of deep mixed layers the oxygen concentrations are some 6 undersaturated even in the upper 20 m a gas transfer model coupled to a simple mixed layer model illustrates the difficulty of transferring sufficient oxygen across the air sea boundary to fully oxygenate the mixed layer when the mixed layer depth exceeds a few hundred meters the nutrient concentrations of the mixed layers are fairly well mixed as is consistent with the fairly narrow range of nutrient concentrations of the source waters only the tritium concentrations exhibit any structure within the mixed layer and it is argued that this is due to the much larger range of tritium concentrations in the source waters that make up the mixed layer

erosion rates in the southern oregon coast range evidence for an equilibrium between hillslope erosion and sediment yield the relationship of hillslope erosion rates and sediment yield is often poorly defined because of short periods of measurement and inherent spatial and temporal variability in erosion processes in landscapes containing hillslopes crenulated by alternating topographic noses and hollows estimates of local hillslope erosion rates averaged over long time periods can be obtained by analysing colluvial deposits in the hollows hollows act as local traps for a portion of the colluvium transported down hillslopes and erosion rates can be calculated using the age and size of the deposits and the size of the contributing source area analysis of colluvial deposits in nine oregon coast range hollows has yielded average colluvial transport rates into the hollows of about 35 cm3cm 1yr 1 and average bedrock lowering rates of about 007 mm yr for the last 4000 to 15000 yr these rates are consistent with maximum bedrock exfoliation rates of about 0 09 mm yr calculated from six of the hollows supporting the interpretation that exfoliation rates limit erosion rates on these slopes sediment yield measurements from nine coast range streams provide similar basin wide denudation rates of between 0 05 and 0 08 mm yr 1 suggesting an approximate steady state between sediment production on hillslopes and sediment yield in addition modern sediment yields are similar in basins varying in size from 1 to 1500 km2 suggesting that erosion rates are spatially uniform and providing additional evidence for an approximate equilibrium in the landscape

stochastic forcing of sediment supply to channel networks from landsliding and debris flow sediment influx to channel networks is stochastically driven by rainstorms and other perturbations which are discrete in time and space and which occur on a landscape with its own spatial variability in topography colluvium properties and state of recovery from previous disturbances the resulting stochastic field of sediment supply interacts with the topology of the channel network and with transport processes to generate spatial and temporal patterns of flux and storage that characterize the sedimentation regime of a drainage basin the regime varies systematically with basin area we describe how the stochastic sediment supply is generated by climatic topographic geotechnical and biotic controls that vary between regions the general principle is illustrated through application to a landscape where sediment is supplied by mass wasting and the forcing variables are deterministic thickening of colluvium random sequences of root destroying wildfires and random sequences of rainstorms that trigger failure in a population of landslide source areas with spatial variance in topography and colluvium strength landslides stop in channels or convert to scouring debris flows depending on the nature of the low order channel network sediment accumulates within these channels for centuries before being transferred downstream by debris flows time series of sediment supply transport and storage vary with basin scale for any combination of climatic topographic and geotechnical controls in a companion paper benda and dunne this issue we use simulations of timing volumes and locations of mass wasting to study the interaction between a stochastically forced sediment supply and systematic changes of storage and flux through channel networks

crossing of the equator by the deep western boundary current in the western atlantic ocean abstract property distributions and geostrophic shear from a hydrographic section near 37 w in the atlantic ocean show the deep western boundary current dwbc in the north atlantic deep water nadw established against the western boundary of the brazil basin immediately south of the equator between 2 and 5 s the dwbc thus has directly crossed the equator to the south atlantic following the east southeast trend of the continental slope isobaths the estimated dwbc transport of nadw is 35 106 m3 s 1 similar to other estimates from the tropics discussed here these large dwbc transports are opposed by flow of deep water to the north atlantic immediately offshore of the dwbc with as much as two thirds of the dwbc transport being returned as these recirculations one recirculation center is the guiana basin north of the equator but extends at least a few hundred kilometers south of the equator another is visible at 11 s in the brazil basin the degree of connection of these two observed recirculatio

revised dates of large earthquakes along the carrizo section of the san andreas fault california since a d 1310 30 1 precise knowledge of the age and magnitude of past earthquakes is essential for characterizing models of earthquake recurrence and key to forecasting future earthquakes we present 28 new radiocarbon analyses that refine the chronology of the last five earthquakes at the bidart fan site along the carrizo section of the south central san andreas fault which last ruptured during the fort tejon earthquake in a d 1857 the new data show that the penultimate earthquake in the carrizo plain occurred not earlier than a d 1640 and the modeled 95th percentile ranges of the three earlier earthquakes and their mean are a d 1540 1630 1585 a d 1360 1425 1393 and a d 1280 1340 1310 indicating an average time interval of 137 44 years between large earthquakes since a d 1310 30 a robust earthquake recurrence model of the carrizo section will require even more well dated earthquakes for thorough characterization however these new data imply that since a d 1310 30 the carrizo section has failed more regularly and more often than previously thought

sediment storage and evacuation in headwater valleys at the transition between debris flow and fluvial processes sediment from landscape disturbance often enters temporary storage in valleys and evacuates over longer times which in steeplands are poorly delimited we hypothesize that across process transitions e g debris fl ow versus fl uvial transport distributions of sediment transit times also change we use fi eld surveys and extensive radiocarbon dating to assess the distribution of transit residence times through the proxy measurement of ages of bank deposits in two mainstem reaches of a 2 23 km 2 watershed in the oregon coast range in the downstream reach debris fans impound fl uvial deposits debris fl ow fi ne fl uvial and coarse fl uvial deposits compose nearly equal parts of the valley fi ll and fl uvial erosion evacuates deposits transit times have a sample mean of 1 22 10 3 14 c yr and an exponential distribution indicating uniform probability of evacuation from storage in the upstream reach valleyspanning debris jams impound debris fl ow deposits composing 95 of the valley fi ll which is routinely scoured by debris fl ows transit times have a sample mean of 4 43 10 2 14 c yr and if 100 14 c yr a power law distribution indicating preferential evacuation of younger deposits and retention of older deposits in both reaches most sediment has short transit times 600 14 c yr but signifi cant volumes remain for millennia less than 20 of basin wide denudation passes through these reservoirs but the latter are still signifi cant buffers between hillslope disturbance and downstream aquatic habitat especially for coarse sediment

fire related sedimentation events on alluvial fans yellowstone national park u s a abstract we document initiation and flow processes deposit facies and geomorphic effects of forest fire related sedimentation on small alluvial fans in yellowstone national park brief intense convective storm precipitation on steep basins burned in the 1988 fires produced sedimentation events involving a variety of depositional processes on fans over the course of all documented events flows on fans progressed from higher to lower sediment concentration events were often dominated by either debris flows or relatively sediment poor streamflow processes in some events however flows ranging a over wide spectrum of sediment concentration produced significant fan deposits debris flows were generated by progressive sediment bulking involving pervasive surface runoff and rill erosion on st ep upper basin slopes followed by deep incision as flows progressed down channels debris flow deposits show a marked decline in thickness and coarse gravel content downfan often with extensive distal gravel poor facies we recognized a relatively minor percentage of noncohesive debris flow and hyperconcentrated flow facies with sorting and stratification intermediate between muddy debris flow and streamflow facies these were deposited where dilute flows bulked with coarse sediment by eroding channel alluvium or earlier deposits of the event below incised fan channels streamflows expanded as sheetfloods which prograded lower fans with distally fining deposits basins 3 km2 typically produced streamflow events on fans but sediment texture and availability on slopes and in channels are primary factors determining flow processes on fans of smaller basins burned soil surfaces provided abundant silt and clay for debris flow generation but because soil surface sediment was stripped and or compacted over time the lack of available fines resulted in dominance of streamflow processes in later events

extreme geomagnetic reversal frequency during the middle cambrian as revealed by the magnetostratigraphy of the khorbusuonka section northeastern siberia abstract we present new magnetostratigraphic results obtained for the drumian stage 504 5 500 5 ma epoch 3 middle cambrian from the khorbusuonka sedimentary section in northeastern siberia they complement previous data that did not allow the determination of a reliable estimate of the geomagnetic reversal frequency during this time magnetization of the samples is carried by a mixture of magnetite and hematite in various proportions thermal demagnetization makes it possible to distinguish two magnetization components the low unblocking temperature 20 reversals per myr during the drumian to 1 5 reversals per myr during the furongian upper cambrian such a sharp decrease is consistent with a transition at a 1 myr timescale probably caused by threshold effects in core processes between two geodynamo modes one characterized by reversals occurring at frequencies ranging from 1 to 5 reversals per myr and the other marked by hyperactivity of the reversing process with reversal rates 15 reversals per myr

post fire geomorphic response in steep forested landscapes oregon coast range usa abstract the role of fire in shaping steep forested landscapes depends on a suite of hydrologic biologic and geological characteristics including the propensity for hydrophobic soil layers to promote runoff erosion during subsequent rainfall events in the oregon coast range several studies postulate that fire primarily modulates sediment production via root reinforcement and shallow landslide susceptibility although few studies have documented post fire geomorphic response here we describe field observations and topographic analyses for three sites in the central oregon coast range that burned in 1999 2002 and 2003 the fires generated strongly hydrophobic soil layers that did not promote runoff erosion because the continuity of the layers was interrupted by pervasive discontinuities that facilitated rapid infiltration at each of our sites fire generated significant colluvial transport via dry ravel consistent with other field based studies in the western united states fire driven dry ravel accumulation in low order valleys of our sulphur creek site equated to a slope averaged landscape lowering of 2 5 mm given holocene estimates of fire frequency these results suggest that fire may contribute 10 20 of total denudation across steep dissected portions of the oregon coast range in addition we documented more rapid decline of root strength at our sites than has been observed after timber harvest suggesting that root strength was compromised prior to fire or that intense heat damaged roots in the shallow subsurface given that fire frequencies in the pacific northwest are predicted to increase with continued climate change our findings highlight the importance of fire induced dry ravel and post fire debris flow activity in controlling sediment delivery to channels

3h and 3he in the beta triangle observations of gyre ventilation and oxygen utilization rates abstract isopycnal maps of 3h 3he age with about 100 km resolution have been obtained for a 1000 km scale area in the eastern subtropical north atlantic the midscale texture of the maps is consistent with isopycnal diffusivities of order 500 m 2 s 1 analysis of the distributions within the context of advection diffusion equations reveals that the larger scale 3h 3he age gradients observed within the area are not seriously affected by mixing on those surfaces that outcrop in the region of ekman downwelling thus isopycnal velocities can be estimated from 1 but record only the velocity component normal to the outcrop the velocities thus obtained agree well with geostrophic estimates although the comparison is flawed by the fundamental mismatch in timescales between the two techniques backward extrapolation of the maps to zero age outcrop positions indicate that the contribution of unventilated older recirculated water to the gyre flow above 600 m depth is small in this area no more than 20

sediment entrainment by debris flows in situ measurements from the headwaters of a steep catchment 1 debris flows can dramatically increase their volume and hence their destructive potential by entraining sediment yet quantitative constraints on rates and mechanics of sediment entrainment by debris flows are limited using an in situ sensor network in the headwaters of a natural catchment we measured flow and bed properties during six erosive debris flow events despite similar flow properties and thicknesses of bed sediment entrained across all events time averaged entrainment rates were significantly faster for bed sediment that was saturated prior to flow arrival compared with rates for sediment that was dry bed sediment was entrained from the sediment surface downward in a progressive fashion and occurred during passage of dense granular fronts as well as water rich inter surge flow en massefailure of bed sediment along the sediment bedrock interface was never observed large magnitude high frequency fluctuations in total normal basal stress were dissipated within the upper 5 cm of bed sediment within this near surface layer concomitant fluctuations in coulomb frictional resistance are expected irrespective of the influence of pore fluid pressure or fluctuations in shear stress if the near surface sediment was wet as it was overridden by a flow additional large magnitude high frequency pore pressure fluctuations were measured in the near surface bed sediment these pore pressure fluctuations propagated to depth at subsonic rates and in a diffusive manner the depth to which large excess pore pressures propagated was typically less than 10 cm but scaled as d fi 0 5 in which d is the hydraulic diffusivity and fiis the frequency of a particular pore pressure fluctuation shallow penetration depths of granular normal stress fluctuations and excess pore pressures demonstrate that only near surface bed sediment experiences the full dynamic range of effective stress fluctuations and as a result can be more easily entrained than deeper sediment these data provide robust tests for mechanical models of entrainment and demonstrate that a debris flow over wet bed sediment will be larger than the same flow over dry bed sediment

geomagnetic reversal behaviour since 100 ma abstract the new magnetic polarity time scale of cande and kent j geophys res 97 13917 13951 1992a provides an opportunity to analyse further the random character of the geomagnetic polarity sequence since 100 ma large differences in the peak amplitudes of changes in magnetic reversal frequency are observed when the previous harland et al a geological time scale cambridge university press 1990 and new cande and kent sequences are compared these differences illustrate remaining uncertainties in the significance of these changes when the successive lengths of polarity intervals are displayed as a function of their order of occurrence in the sequence a change in behaviour is observed midway through the series flat white noise like behaviour is observed for the last 100 intervals from about 25 ma to the present for older intervals a quasi periodic pattern in interval duration is observed this evolution is more apparent in the cande and kent time scale the quasi periodicity of interval duration as a function of occurrence does not translate into periodicity of reversal frequency as a function of time from 85 ma to about 25 ma our description of the sequence is reminiscent of the findings of dubois and pambrun c r acad sci 311 643 650 1990 who suggested that low dimensional deterministic chaos between 85 and 25 ma preceded a random process occurring since then the details of a long term control or memory of the geomagnetic field remain an open question addition of independent information such as palaeointensity data will probably be necessary to uncover any determinism in reversal chronology

forest clearing and regional landsliding the influence of forest clearing on landsliding is central to longstanding concern over the effects of timber harvesting on slope stability here we document a strong topographic control on shallow landsliding by combining unique ground based landslide surveys in an intensively monitored study area with digital terrain modeling using high resolution laser altimetry and a coarser resolution regional study of 3224 landslides as predicted by our digital terrain based model landslides occur disproportionately in steep convergent topography in terrain predicted to be at low risk of slope failure a random model performs equally well to our mechanism based model our monitoring shows that storms with 24 hr rainfall recurrence intervals of less than 4 yr triggered landslides in the decade after forest clearing and that conventional monitoring programs can substantially underestimate the effects of forest clearing our regional analysis further substantiates that forest clearing dramatically accelerates shallow landsliding in steep terrain typical of the pacific northwest

complex behavior of the geomagnetic field during three successive polarity reversals 11 12 m y b p detailed paleomagnetic records of both the directional and relative field intensity changes occurring during three sequential polarity reversals have been obtained from marine clays of serravallian tortonian boundary age in the island of zackinthos greece the three reversals are characterized by a complicated geometrical and temporal structure periods of rapid changes of the geomagnetic field vector alternate with periods of near stationarity different episodes of collapse and recovery of the field intensity and the presence of rebounds have been documented overall the records obtained here are reminiscent of volcanic records and appear much more complex and quite different from those previously obtained from upper tortonian marine clays in crete in the same geographical area the bearing of these results on some of the published reversal models is discussed and it is also suggested that the greater complexity of the zackinthos records might be related to the fact that they are exactly coeval with the last reversal spurt documented by reversal frequency analyses

evolution of a natural debris flow in situ measurements of flow dynamics video imagery and terrestrial laser scanning many theoretical and laboratory studies have been undertaken to understand debris flow processes and their associated hazards however complete and quantitative data sets from natural debris flows needed for confirmation of these results are limited we used a novel combination of in situ measurements of debris flow dynamics video imagery and pre and postflow 2 cm resolution digital terrain models to study a natural debris flow event our field data constrain the initial and final reach morphology and key flow dynamics the observed event consisted of multiple surges each with clear variation of flow properties along the length of the surge steep highly resistant surge fronts of coarse grained material without measurable pore fluid pressure were pushed along by relatively fine grained and water rich tails that had a wide range of pore fluid pressures some two times greater than hydrostatic surges with larger nonequilibrium pore fluid pressures had longer travel distances a wide range of travel distances from different surges of similar size indicates that dynamic flow properties are of equal or greater importance than channel properties in determining where a particular surge will stop progressive vertical accretion of multiple surges generated the total thickness of mapped debris flow deposits nevertheless deposits had massive vertically unstratified sedimentological textures

eddies of newly formed upper labrador sea water a new type of submesoscale eddy has been observed south of the labrador sea during late winter embedded in equatorward flow along the western boundary the eddy radius is 20 km with a weak dynamic signature swirl speed of 0 5 cm s the center of the eddy is characterized by weak stratification elevated concentrations of oxygen and anthropogenic tracers and low tracer ages all indicative of newly ventilated water strong lateral intrusions distort the shape of the feature the water mass contained in the eddy is not classical labrador sea water from the central labrador sea but is significantly fresher and hence lighter it is of the correct density to be the source of the high chlorofluorocarbon layer of the shallow deep western boundary current observed further south and hence is termed upper labrador sea water using a combination of hydrographic data sets along the western boundary to implement a simple lateral diffusion model it is shown that such eddies decay of the order of several months and are difficult to observe equatorward of the grand banks of newfoundland this is in contrast to deeper lenses of classical labrador sea water which persist further equatorward tracer derived ages of the upper labrador sea water eddy range from 3 to 5 years much older than the lifetime deduced from their lateral diffusion a simple convection model of tracer age shows that this age discrepancy is caused by gas exchange being unable to maintain equilibrium between the deep convecting mixed layer and the atmosphere during formation

stochastic processes of soil production and transport erosion rates topographic variation and cosmogenic nuclides in the oregon coast range landscapes in areas of active uplift and erosion can only remain soil mantled if the local production of soil equals or exceeds the local erosion rate the soil production rate varies with soil depth hence local variation in soil depth may provide clues about spatial variation in erosion rates if uplift and the consequent erosion rates are sufficiently uniform in space and time then there will be tendency toward equilibrium landforms shaped by the erosional processes soil mantle thickness would adjust such that soil production matched the erosion previous work in the oregon coast range suggested that there may be a tendency locally toward equilibrium between hillslope erosion and sediment yield here results from a new methodology based on cosmogenic radionuclide accumulation in bedrock minerals at the base of the soil column are reported we quantify how soil production varies with soil thickness in the southern oregon coast range and explore further the issue of landscape equilibrium apparent soil production is determined to be an inverse exponential function of soil depth with a maximum inferred production rate of 268 m ma 1 occurring under zero soil depth this rate depends however on the degree of weathering of the underlying bedrock the stochastic and large scale nature of soil production by biogenic processes leads to large temporal and spatial variations in soil depth the spatial variation of soil depth neither supports nor rejects equilibrium morphology our observed catchment averaged erosion rate of 117 m ma 1 is however similar to that estimated for the region by others and to soil production rates under thin and intermediate soils typical for the steep ridges we suggest that portions of the oregon coast range may be eroding at roughly the same rate but that local competition between drainage networks and episodic erosional events leads to landforms that are out of equilibrium locally and have a spatially varying soil mantle copyright 2001 john wiley sons ltd

periodicity of the earth s magnetic reversals reversals of the earth s magnetic field may occur with a certain regularity using observed percentages of normal and reversed polarity during different time intervals negi and tiwari1 detected several significant periodicities notably one of 32 34 myr raup2 used the dates of individual reversals to propose a 30 myr period in the frequency of reversals although mazaud et al 3 obtained a periodicity of only 15 myr like negi and tiwari s1 other detected short periods the 15 myr period may be harmonically related to a basic 30 myr period4 it has also been suggested that these are accidental periodicities arising in a short record5 6 or are harmonics of the record length itself7 in fact all of the cited studies have used different data different record lengths and different methods of time series analysis raup8 has consequently retracted his original claim here i present a much fuller analyis of the reversal record and show that a statistically significant period of 30 myr does formally exist in spite of the cited differences

the relative stabilities of the reverse and normal polarity states of the earth s magnetic field recent analyses of the geomagnetic reversal sequence have led to different conclusions regarding the important question of whether there is a discernible difference between the properties of the two polarity states the main differences between the two most recent studies are the statistical analyses and the possibility of an additional 57 reversal events in the cenozoic these additional events occur predominantly during reverse polarity time but it is unlikely that all of them represent true reversal events nevertheless the question of the relative stabilities of the polarity states is examined in detail both for the case when all 57 events are included in the reversal chronology and when they are all excluded it is found that there is not a discernible difference between the stabilities of the two polarity states in either case inclusion of these short events does however change the structure of the non stationarity in reversal rate but still allows a smooth non stationarity only 7 of the 57 short events are pre 38 ma but the evidence suggests that this is a real geomagnetic phenomenon rather than degradation of the magnetic recording or a bias in observation this could be tested by detailed magnetostratigraphic and oceanic magnetic surveys of the paleogene and late cretaceous overall it would appear that the present geomagnetic polarity timescale for 0 160 ma is probably a very good representation of the actual history and that different timescales and additional events now represent only changes in detail

the variability of root cohesion as an influence on shallow landslide susceptibility in the oregon coast range decades of quantitative measurement indicate that roots can mechanically reinforce shallow soils in forested landscapes forests however have variations in vegetation species and age which can dominate the local stability of landslide initiation sites to assess the influence of this variability on root cohesion we examined scarps of landslides triggered during large storms in february and november of 1996 in the oregon coast range and hand dug soil pits on stable ground at 41 sites we estimated the cohesive reinforcement to soil due to roots by determining the tensile strength species depth orientation relative health and the density of roots 1 mm in diameter within a measured soil area we found that median lateral root cohesion ranges from 6 8 23 2 kpa in industrial forests with significant understory and deciduous vegetation to 25 6 94 3 kpa in natural forests dominated by coniferous vegetation lateral root cohesion in clearcuts is uniformly 10 kpa some 100 year old industrial forests have s

geomagnetic reversal history the history of geomagnetic polarity reversals in the cenozoic and late mesozoic is well known since the late jurassic oxfordian a continuous record of polarity has been derived for this time interval from the interpretation of oceanic magnetic anomalies most of the polarity chrons in this oceanic record have been verified and dated in coordinated magnetostratigraphic and biostratigraphic studies this has led to the generation of progressively refined and improved geomagnetic reversal time scales that provide a framework for absolute dating of palaeontological zonations by serving as a basis for statistical analysis of reversal frequency they provide information relevant to processes in the earth s core the rate of reversals since the late cretaceous shows a steady increase on which a cyclical variation appears to be superposed a stochastic model for reversals predicts a poisson distribution of polarity interval lengths the polarity time scales contain many fewer short 50 kyr polarity chrons than a poisson distribution and it has been suggested that a gamma renewal process with index greater than unity is a more appropriate statistical model the statistical arguments give no convincing reason for abandoning the model and other physical reasons must be sought to explain the incompleteness of the reversal record the discovery and verification of short chrons in the oceanic record may best be investigated by deep tow magnetometer surveys the reversal history before the late jurassic is not well known magnetostratigraphy in coeval early jurassic sections has not given correlatable records and it has not been possible to compile a definitive polarity sequence evaluation of geomagnetic polarity history for the early mesozoic and the palaeozoic will require unambiguous magnetostratigraphy in well dated sections where verification of the polarity pattern is possible at the fossil zone or stage level

storm rainfall conditions for floods and debris flows from recently burned areas in southwestern colorado and southern california abstract debris flows generated during rain storms on recently burned areas have destroyed lives and property throughout the western u s field evidence indicate that unlike landslide triggered debris flows these events have no identifiable initiation source and can occur with little or no antecedent moisture using rain gage and response data from five fires in colorado and southern california we document the rainfall conditions that have triggered post fire debris flows and develop empirical rainfall intensity duration thresholds for the occurrence of debris flows and floods following wildfires in these settings this information can provide guidance for warning systems and planning for emergency response in similar settings debris flows were produced from 25 recently burned basins in colorado in response to 13 short duration high intensity convective storms debris flows were triggered after as little as six to 10 min of storm rainfall about 80 of the storms that generated debris flows lasted less than 3 h with most of the rain falling in less than 1 h the storms triggering debris flows ranged in average intensity between 1 0 and 32 0 mm h and had recurrence intervals of two years or less threshold rainfall conditions for floods and debris flows sufficiently large to pose threats to life and property from recently burned areas in south central and southwestern colorado are defined by i 6 5 d 0 7 and i 9 5 d 0 7 respectively where i rainfall intensity in mm h and d duration in hours debris flows were generated from 68 recently burned areas in southern california in response to long duration frontal storms the flows occurred after as little as two hours and up to 16 h of low intensity 2 10 mm h rainfall the storms lasted between 5 5 and 33 h with average intensities between 1 3 and 20 4 mm h and had recurrence intervals of two years or less threshold rainfall conditions for life and property threatening floods and debris flows during the first winter season following fires in ventura county and in the san bernardino san gabriel and san jacinto mountains of southern california are defined by i 12 5 d 0 4 and i 7 2 d 0 4 respectively a threshold defined for flood and debris flow conditions following a year of vegetative recovery and sediment removal for the san bernardino san gabriel and san jacinto mountains of i 14 0 d 0 5 is approximately 25 mm h higher than that developed for the first year following fires the thresholds defined here are significantly lower than most identified for unburned settings perhaps because of the difference between extremely rapid runoff dominated processes acting in burned areas and longer term infiltration dominated processes on unburned hillslopes

do superchrons occur without any palaeomagnetic warning abstract the mainly dipolar geomagnetic field generated by the geodynamo within the earth s liquid core has reversed its polarity many times in the past this succession of intervals of alternate polarity defines the geomagnetic polarity time scale gpts usually interpreted as resulting from a gamma renewal process the rate of which would be controlled by the boundary conditions imposed by the mantle on the core in this interpretation boundary conditions would have occasionally evolved towards being unfavourable to reversals leading the reversal rate of the process to progressively decrease and reach zero at least twice causing the onset of the so called kiaman and cretaceous very long polarity intervals superchrons here we reconsider this causal link analysing the latest and best constrained gpts thanks to the continuous record provided by marine magnetic anomalies describing the past 160 myr we found that contrary to earlier claims no long term behaviour over the 40 myr preceding the cretaceous superchron can be seen in the reversal rate that could explain its onset at 120 ma more generally it turns out that hardly any special behaviour can be identified in the gpts which could have announced this superchron only the occurrence of the longest of all pre superchron intervals cm1n 3 myr before the onset of the superchron could be identified as a precursor such a behaviour could possibly be the consequence of medium term on the 10 myr time scale changes in the boundary conditions imposed by the mantle on the core but we note that the sole analysis of the gpts does not allow this to be tested in fact it appears that a single stationary process could also explain the entire pre superchron gpts with the only possible exception of cm1n this suggests that the occurrence of superchrons does not necessarily require changes in boundary conditions and could simply attest for a sudden non linear transition between a reversing and a non reversing state of the geodynamo

how steady are steady state landscapes using visible near infrared soil spectroscopy to quantify erosional variability although topographic steady state is often used as a simplifying assumption in sediment yield studies and landscape evolution models the temporal and spatial scales over which this assumption applies in natural landscapes are poorly defined we used visible near infrared visnir spectroscopy to measure the weathering of hilltop soils and quantify local erosional variability in two watersheds in the oregon coast range united states one watershed appears adjusted to base level lowering driven by rock uplift in the cascadia forearc while the other is pinned by a gabbroic dike that locally slows river incision and hillslope erosion models for uniformly eroding hillslopes imply uniform soil residence times instead we observe significant variability around the mean value of 18 8 k y 31 2 11 8 k y for our adjusted watershed the magnitude of erosional variability likely reflects the time scales associated with stochastic processes that drive bedrock weathering soil production and soil transport e g tree turnover the residence time distribution for our pinned watershed has a mean value of 72 9 k y 165 6 50 6 k y and is highly skewed with a substantial fraction of long residence time soils we speculate that this pattern results from the lithologic control of base level and lateral divide migration driven by erosional contrasts with neighboring catchments our novel and inexpensive methodology enables us to quantify for the first time the magnitude of erosional variability in a natural landscape and thus provides important geomorphic context for studies characterizing regolith development more generally we demonstrate that soils can record catchment scale landscape dynamics that may arise from lithologic controls or forcing due to climate or tectonics

geodynamo energy source constraints from palaeomagnetic data abstract analyses of palaeo geomagnetic data can provide surprisingly strong constraints upon models of the geodynamo in particular it is shown here from an analysis of the geomagnetic reversal sequence that long term variation in reversal rate does not occur as a result of change in any inherent inhibition or encouragement in the reversal process instead there must be some physical process that actually triggers reversals and this process must derive from an energy source effectively independent of the energy source powering the main geomagnetic field from this two very simple generalised models referred to as the hot blob and cold blob models are suggested data from individual reversal transitions are used to discriminate between these two models the preferred model is that the geodynamo is driven primarily by convection related to freezing of the outer core at the inner core outer core boundary cold blobs descending from the core mantle interface are identified as the instabilities that trigger reversals

fire and alluvial chronology in yellowstone national park climatic and intrinsic controls on holocene geomorphic processes we employed a systemwide approach a large and robust set of radiocarbon ages and modern process analogs to interpret the holocene history of forest fire related sedimentation and overall alluvial activity in northeastern yellowstone national park debris flow and flood events following the 1988 fires provided facies models for interpreting the stratigraphic record of fire related sedimentation within valley side alluvial fans of soda butte creek fire related deposits make up approximately 30 of the late holocene fan alluvium fifty 14 c ages on fire related events cluster within the intervals of 3300 2900 2600 2400 2200 1800 and 1400 800 yr b p and suggest earlier episodes of large fires and fan aggradation around 7500 5500 and 4600 4000 yr b p a major pulse of fire related debris flow activity between 950 and 800 yr b p coincided with the height of the widely recognized medieval warm period ca a d 1050 1200 instrumental climate records over the last 100 yr in yellowstone imply that the intensity and interannual variability of summer precipitation are greater during warmer periods enhancing the potential for severe short term drought major forest fires and storm generated fan deposition along lower soda butte creek fill cut terrace treads were created by lateral migration of channels and accumulation of overbank sediments ca 8000 yr b p terrace level t1a 7000 5600 t1b 3100 2600 t2 2000 1300 t3 and post 800 yr b p t4 these periods coincide with overbank sedimentation on slough creek and the lamar river but alternate with intervals of fire related fan deposition implying a strong climatic control local paleoclimatic data suggest cooler effectively wetter conditions during terrace tread formation in warmer drier intervals reduced average runoff in axial streams results in meander belt narrowing concurrent channel incision may be caused by infrequent large floods greater resistance to downcutting however allowed fewer terraces to be formed along slough creek and the lamar river alluvial systems in northeastern yellowstone show a clear response to millennial scale climatic cycles wherein alluvial fans aggrade and prograde over flood plains in drier periods axial streams widen their flood plains and trim back the fans during wetter periods small scale climatic fluctuations of the holocene thus had substantial impact on postglacial landscapes in northeastern yellowstone

deep cross equatorial flow in the atlantic measured with sofar floats neutrally buoyant sofar floats at nominal depths of 800 1800 and 3300 m were tracked for 21 months in the vicinity of tropical boundary currents in the atlantic near 6 n and at several sites near 11 n as well as along the equator trajectories at 1800 m show a swift 50 cm s narrow 100 km wide southward flowing deep western boundary current dwbc extending from 7 n to the equator the average transport per unit depth in the dwbc was estimated to be 13 8 103 m2 s coupling this value with mean velocities measured in the dwbc by current meters gave a volume transport of 15 106 m3 s between depths of 900 m and 2800 m approximately 6 106 m3 s recirculated northward between the dwbc and the mid atlantic ridge leaving 9 106 m3 s as cross equatorial transport no obvious dwbc nor swift equatorial current was observed by the 3300 m floats a low mean velocity at this depth lay between f 11 and higher velocity cores above and below the 1800 m trajectories also suggest that at times february march 1989 the north atlantic deep water in the dwbc turned eastward and flowed along the equator and at other times august september 1990 the dwbc crossed the equator and continued southward the velocity near the equator calculated by grouping floats in a box along the equator was eastward at 4 1 cm s from february 1989 to february 1990 and westward at 4 6 cm s from march 1990 to november 1990 thus the amount of cross equatorial flow in the dwbc appeared to be linked to low frequency variability of the structure of the equatorial current system floats in antarctic intermediate water at 800 m revealed a northwestward western boundary current although flow patterns were complicated three floats that significantly contributed to the northwestward flow looped in anticyclonic eddies that translated up the coast at 8 cm s six 800 m floats drifted eastward along the equator between 5 s and 6 n at a mean velocity of 11 cm s one reached 5 w in the gulf of guinea suggesting that the equatorial currents at this depth extended at least 35 40 along the equator three of these floats reversed direction near the end of the tracking period

stochastic forcing of sediment routing and storage in channel networks the stochastic field of sediment supply to the channel network of a drainage basin depends on the large scale interactions among climatically driven processes such as forest fire and rainstorms topography channel network topology and basin scale during infrequent periods of intense erosion large volumes of colluvium are concentrated in parts of a channel network particularly near tributary junctions the rivers carry bed material and wash load downstream from these storage sites at different rates the bed material travels slowly creating transient patterns of sediment transport sediment storage and channel morphology along the channel network as the concentrations of bed material migrate along the network their waveforms can undergo changes by diffusion interference at tributary junctions and loss of mass through temporary sediment storage in fans and terraces and through particle abrasion which converts bed material to wash load we investigated how these processes might influence the sediment mass balance in channels of third and higher order in a 215 km 2 drainage basin within the oregon coast range over a simulated 3000 year period with a climate typical of the late holocene we used field measurements and a simulation model to illustrate interactions between the major controls on large scale processes functioning over long periods of time in complex drainage basins

the 13 million year cenozoic pulse of the earth abstract the geomagnetic polarity reversal rate changes radically from very low to extremely high such process indicates fundamental changes in the earth s core reorganization and core mantle boundary heat flow fluctuations however we still do not know how critical such changes are to surface geology and climate processes our analysis of the geomagnetic reversal frequency oxygen isotope record and tectonic plate subduction rate which are indicators of the changes in the heat flux at the core mantle boundary climate and plate tectonic activity shows that all these changes indicate similar rhythms on million years timescale in the cenozoic era occurring with the common fundamental periodicity of 13 myr during most of the time the periodicity is disrupted only during the last 20 myr such periodic behavior suggests that large scale climate and tectonic changes at the earth s surface are closely connected with the million year timescale cyclical reorganization of the earth s interior