accelerator mass spectrometry radiocarbon dating of rock varnish accelerator mass spectrometry ams 14c dating off rock varnish provides minimum limiting ages for landforms and archaeological artifacts in arid and semiarid lands that are undatable by conventional radiocarbon methods experiments on sample collection the effect of different individuals preparing samples the influence of different chemical extraction procedures the incorporation of carbonate detritus possible contamination from rock underlying varnish the role of bio geochemical erosion of varnish and other influences reveal procedures that yield reproducible results conventional radiocarbon dates from charcoal beneath lava flows of hualalai volcano hawaii and from arid sites in western north america provide controls to test varnish radiocarbon dating ams radio carbon ages on the very bottom layer of varnishes are typically 10 younger than the 14c ages of these controls applications are illustrated here for fluvial aeolian peri glacial hillslope lacustrine and glacial geo morphology as well as rock art research in archaeology

cation leaching sites in rock varnish backscatter electron microscopy of subaerial rock varnish reveals porous textures that are sites of cation leaching with lowered k ca 2 ti 4 ratios and concentrations of manganese and iron laboratory studies show that rates of potassium and calcium leaching increase with temperature time and surface area and decrease when varnish interdigitates with coatings of amorphous silica only varnishes with continuous layering should be used in paleoenvironmental research and paleomagnetic studies not textures indicative of postdepositional modifications

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

particle fluxes into permeable sediments comparison of mechanisms mediating deposition fine particle deposition in coarser sediment beds plays an important role in biogeochemical cycling in sediments by 1 supplying organic matter attached microorganisms and nutrients and 2 mediating interfacial fluxes through changes in permeability in some cases particles could be associated with contaminants or particles could clog pores in the sediment resulting in ecological impacts through changes in porewater chemistry e g dissolved oxygen profiles we compare two mechanisms for fine particle deposition to permeable sediments first for flat beds particle delivery by sediment grain scale flows coupled with particle retention within the sediment could result in deposition in excess of the gravitational settling second pressure driven flows across ripples may deliver additional particles directly into the sediment although sediment transport can counteract deposition flume experiments were used to measure deposition rates over a range of particle and sediment sizes for both flat and rippled beds flat bed deposition rates were greater than the particle settling velocities and comparable to the deposition rates to rippled beds under similar flow conditions an empirical expression based on parameters for flow sediment and particle conditions predicts where each mechanism dominates the deposition rate this result illustrates the potential importance of grain scale processes for the deposition of fine particles to permeable sediments regardless of bed roughness predictions of particle fate penetration into the sediment and degradation following the deposition still require a description of bed topography in addition to sediment properties

geomorphic and hydrologic implications of the rapid incision of afton canyon mojave desert california afton canyon is a 150 m deep canyon that formed as a result of overflow and drainage of lake manix an 215 km2 late wisconsin pluvial lake in the central mojave desert because the canyon age is within the range of conventional radiocarbon dating it is possible to provide a time based chronology of events and resultant landscape changes due to a 120 m base level drop analysis of erosion volumes upstream of afton canyon and a review of public and private well logs downstream of the canyon mouth indicate that late wisconsin surfaces are deeply buried downstream of afton canyon burial depths are 55 m at the mouth of afton canyon 27 m in the cronese basin and 18 m at crucero the well logs also indicate that soda lake was far more areally extensive prior to late wisconsin time stratigraphic and geomorphic evidence suggests that afton canyon was cut rapidly sometime after 14 230 1325 yr b p the rapid draining of lake manix and subsequent basin dissection have important implications for late quaternary lake fluctuations in death valley base level changes and resultant alluvial fan adjustments in the cronese basin the sand source for the kelso dunes and the absence of old artifacts in the cronese basin and the mojave river wash region

the effect of entrained air in violent water wave impacts the effects of entrained air in cushioning water impact on a wall are estimated by using a flow which has many similarities to the severe flip through impacts that have been identified for water waves hitting a vertical wall this is a filling flow which rapidly fills a confined region such as a crack between blocks or the space beneath a deck projecting from the coast peregrine kalliadasis 1996 the main properties of the filling flow are easily calculated including the high pressure peak which corresponds to the pressure peak of a flip through this work extends the study of filling flows to the case where the filling liquid is an air water mixture thus giving explicit results for the reduction of peak pressure due to the compressibility of entrained air the behaviour of a bubbly liquid subject to substantial pressure changes is considered expressions are derived for an air water mixture treated as a compressible fluid the reduction in pressure from the incompressible case is found to be large even for relatively small air content and depends more on the reduction in fluid volume than any other feature of the pressure density relation results are presented in such a way that they may be used to estimate compressibility corrections to both the maximum and background pressures in a flip through wave impact if corresponding incompressible pressure values are available

cosmogenic 3he in terrestrial rocks the summit lavas of maui we have identified terrestrial cosmic rayproduced 3he in three lava flows on the crest of haleakala volcano on maui 3 km above sea level and 0 5 million years old although these lavas like all oceanic basalts contain primordial 3he from the mantle the cosmogenic component 3hec can be identified unambiguously because it is extractable only by high temperature vacuum fusion in contrast a large fraction of the mantle helium resides in fluid inclusions and can be extracted by vacuum crushing leaving a residual component with 3he 4he ratios as high as 75 those in the atmosphere which can be liberated by melting the crushed grains cosmogenic 3he is present in both olivines and clinopyroxenes at 0 8 1 2 10 12 ml stp g and constitutes 75 5 of the total 3he present the observed 3hec levels require a cosmic ray exposure age of only some 64 000 years much less than the actual age of the lavas if there is no erosion using a model that includes effects of uplift or submergence as well as erosion we calculate an apparent erosion rate of the order of 8 5 m 106 years for the western rim of the summit crater as an example of the application of measurements of cosmogenic rare gases to terrestrial geological problems

role of in situ cosmogenic nuclides 10be and 26al in the study of diverse geomorphic processes the central premises of applications of the in situ cosmogenic dating method for studying specific problems in geomorphology are outlined for simple and complex exposure settings in the light of these general models we discuss the information that can be derived about geomorphic processes utilizing concentrations of in situ produced cosmogenic radionuclides 10be half life 1 5 ma and 26a1 half life 0 7 ma in a variety of geomorphic contexts glacial polish and tills meteorite impact craters alluvial fans paleo beach ridges marine terraces sand dunes and bedrock slopes we also compare 10be 26al data with results obtained by other dating methods we conclude that the technique of measuring in situ cosmic ray produced nuclides holds promise for quantitative studies of processes and time scales in a wide range of geomorphological problems

cosmic ray produced 10be and 26al in antarctic rocks exposure and erosion history we have measured cosmic ray produced t1 2 1 5 million years 10be and t1 2 0 705 million years 26al in purified quartz fractions of selected rock samples from antarctic mountains from these data we calculate 1 mean erosion rates for the limiting case of steady state surface exposure to cosmic rays and 2 minimum exposure ages for the limiting case of no erosion calculated mean erosion rates are very low on the order of a few times 10 5 cm yr we believe the sampling to be sufficient to generalize this result to exposed bedrock in antarctica in favorable cases it is possible to distinguish between the limiting cases steady state erosion seems a better description in such cases most samples including some taken a few meters above the present ice level seem to have been exposed for millions of years without major episodes of burial or abrasion by ice

mechanics of deep water plunging wave impacts on vertical structures abstract the pressures resulting from a critical case of plunging wave impact on a vertical wall are examined a deep water wave breaking condition with the crest elevation about a fifth of the mean water depth has been considered two distinct scales of the pressure time histories a slowly varying in the range of the wave period component in the order of stagnation pressure and a transient impact component with much higher pressures have been identified and correlated to the incident wave kinematics it is found that impact pressures can be approximately decomposed into a primary component associated with the overall wave evolution and a component influenced by the trapped air dynamics these characteristics are comparable to those observed in shallow water wave impact studies the loads at the zone of impact are also found to be comparable if the span of impact and the local incident wave kinematics are comparable

radiocarbon and cation radio ages for rock varnish on tioga and tahoe marainal boulders of pine creek eastern sierra nevada california and their paleoclimatic implications abstract accelerator mass spectrometry 14 c analyses of organic matter extracted from rock varnishes on morainal boulders yield limiting minimum ages for three crests of the tioga glaciation at pine creek in the eastern sierra nevada varnish started to form on boulders of the outermost tioga moraine before 19 000 yr b p and varnish originated on the innermost tioga moraine before 13 200 yr b p comparisons with lake level paleohydrological paleoecological colluvial and rock varnish micromorphological data indicate that central eastern california and western nevada experienced a moisture effective period during the late pleistocene but after the tioga maximum and perhaps as tioga glaciers receded from the mouth of pine creek canyon varnishes on tahoeage morainal boulders at pine creek have cation ratio ages of about 143 000 156 000 yr b p suggesting that the tahoe glaciation should not be correlated with oxygen isotope stage 4 in the early wisconsin but rather with stage 6 varnishes on morainal boulders of an older glaciation at pine creek are dated by cation ratio at about 182 000 187 000 yr b p

cosmogenic chlorine 36 production rates in terrestrial rocks abstract chlorine 36 is produced in rocks exposed to cosmic rays at the earth surface through thermal neutron activation of 35cl spallation of 39k and 40ca and slow negative moun capture by 40ca we have measured the 36cl content of 14c dated glacial boulders from the white mountains in eastern california and in a 14c dated basalt flow from utah effective time intergrated production parameters were calculated by simultaneous solution of the 36cl production equations the production rates due to spallation are 4160 310 and 3050 210 atoms 36cl yr 1 mol 139k and 40ca respectively the thermal neutron capture rate was calculated to be 3 07 0 24 105 neutrons kg of rock 1 yr 1 the reported values are normalized to sea level and high geomagnetic latitudes production of 36cl at different altitudes and latitudes can be estimated by appropriate scaling of the sea level rates chlorine 36 dating was performed on carbonate ejecta from meteor crater arizona and late pleistocene morainal boulders from the sierra nevada california calculated 36cl ages are in good agreement with previously reported ages obtained using independent methods

dating geomorphologic surfaces using cosmogenic 3he cosmogenically produced 3he in rocks can be used to date geomorphologic surfaces using the late pleistocene bonneville flood event the subsequent development of the provo shoreline and associated volcanic rocks a production rate of 432 atoms g yr in olivine is determined for 38 56 n latitude present geomagnetic latitude 46 5 at 1445 m altitude integrated over the last 14 400 years measurements of cosmogenic helium in flood deposits on river scoured surfaces and lava flows show that it is possible to date directly large scale events such as volcanic eruptions and catastrophic floods olivine and pyroxene are both satisfactory minerals for cosmogenic dating by the 3he method however in most cases plagioclase and quartz are unsuitable because they do not quantitatively retain helium isotopes in their lattices

cosmic ray produced neon in antarctic rocks we studied three antarctic rocks and quartz separates thereof for their records of cosmic ray produced 21ne all three samples reveal an excess of 21ne and in two of them a cosmic ray origin is documented by the spallation ratios 22ne 21ne 3he is shown to be incompletely retained in quartz 10 previous studies have shown the suitability of quartz as a monitor for cosmic ray produced radionuclides 10be and 26al and therefore as a tracer for geological processes 26al 10be ratios allow the calculation of minimum exposure ages and maximum erosion rates and measured ratios 21ne 10be and 21ne 26al add significant information to exposure histories especially in the case of a complex exposure this information is necessary for an evaluation of erosion rates and to define geological processes such as uplift and variations in the antarctic ice cover

the tempo of geomorphological change evidence from southeastern australia a revised denudational chronology based on k ar dating of basalts for much of southeastern australia shows that many major topographic features are far older than was previously realized some upland surfaces originated in the mesozoic and others in the early tertiary in some areas the last major uplift and onset of canyon cutting occurred prior to the mid oligocene apart from the slow headward extension of canyons and localized high level stripping much of the region had taken on approximately its present day form by the mid miocene paradoxically however none of the mechanisms that have been proposed to explain the survival of paleoforms hold here these instances may well be typical of large parts of the interior of tectonic plates and a revision of widely held estimates of modal rates of denudation seems required

cosmic ray produced neon and helium in the summit lavas of maui a 3he component attributed to cosmic ray induced spallation reactions 3hec in silicate minerals has recently been identified in lava flows on the summit of haleakala volcano on maui1 3 this3hec is observed in the helium extracted by fusion of olivine and pyroxene phenocrysts after first removing almost all of the mantle derived helium by vacuum crushing a procedure which releases the volatiles trapped in fluid inclusions within the phenocrysts cosmic ray produced 3he was found in the high altitude 3 000 m summit lavas which are 500 000 years old but was not observed in sub surface samples which were overlain by younger basalts2 or in a 200 yr old lava flow at sea level on maui1 2 we report here the identification of cosmic ray produced 21nec in addition to 3hec in gases extracted by fusion of olivines and clinopyroxenes after vacuum crushing the observed 3he 21ne c ratios and the ratio of 21nec in olivine to that in clinopyroxene are consistent with an in situ origin of 3hec and 21nec by cosmic ray spallation reactions this component could be important for interpreting helium isotopic data in terrestrial reservoirs geophysical applications could include determinations of erosion rates and exposure histories of terrestrial rocks

how does wave impact generate large boulders modelling hydraulic fracture of cliffs and shore platforms abstract boulder quarrying by large waves along steep high energy coastlines contributes to erosion both by causing inland migration of cliff faces and by vertical lowering of coastal platform surfaces it also leads to the formation of coastal boulder deposits cbd above and inland of the high water mark we describe mechanisms by which hydraulic fracture creates boulders from cliffs and bedrock platforms intense fluid pressures are induced by wave impact on the base of overhanging ledges and also along interior surfaces as cracks fill with water during wave runup and overtopping these processes impose large loads on the overlying rock and bending stresses thus induced can create or propagate microcracks in the rock repeated loading leads to complete fracture and detachment of bedrock slabs which are liberated as boulders we consider these processes using the aran islands ireland as a type locality both because cbd there are well documented and because flat lying strata and orthogonal joint patterns yield tabular bedrock geometry which lends itself to modelling the loaded rock is modelled as a beam with a microcrack that is fractured by a bending stress from an applied hydrodynamic load this simple approach allows us to examine general scalings and dependencies we determine a condition for fracture relating the fluid pressure loaded area fracture stress microcrack position and geometry in some situations the fluid pressure need only be a fraction of the fracture stress fracture is most likely to occur during extreme wave conditions and when the exposed rock has a large eroded area for loading the balance of forces suggests that larger beams break more easily facilitating the creation of large boulders

the accumulation of cosmogenic chlorine 36 in rocks a method for surface exposure dating because of the reaction of cosmic rays with elements in minerals chlorine 36 accumulates in rocks exposed at the earth s surface this suggests that the ratio of chlorine 36 to stable chloride can be used as a geochronometer reasonable agreement has been obtained between measurements of chlorine 36 in volcanic rocks of known age and calculations of chlorine 36 production based on rock composition and cosmicray flux the buildup of chlorine 36 should be a useful means for measuring the surface exposure time of young volcanic rocks and geomorphic features in the age range of 10 3 to 10 6 years

the pressure impulse in a fluid saturated crack in a sea wall abstract when a wave breaks against a sea wall containing a crack such as might exist within the blockwork pressure pulses can travel through the fluid and propagate into the crack this can cause high stresses to act on the sides and roof of the crack and may even cause the constituent blocks to move the pressure impulse p is used to model the effect of wave impact against a wall in which there is a fluid filled crack a two dimensional field equation is derived for p that is applicable in plane cracks of non uniform narrow width this is solved for several geometries relating to cracks between constituent blocks of sea walls in order to compare the impulsive forces with the gravitational force on a block it is shown that a large block can be lifted due to the impulse exerted by the fluid in a crack beneath it

cosmic ray production rates of 10be and 26al in quartz from glacially polished rocks we have studied the concentrations of 10be and 26al in quartz crystals extracted from glacially polished granitic surfaces from the sierra nevada range these surfaces were identified with the glacial advance during the tioga period 11 000 years ago our measurements yield the most accurate estimates to date for the absolute production rates of these nuclides in sio2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43 8 44 6 n and altitudes 2 1 3 6 km the estimates are relatively free from uncertainties in snow cover since we studied a suite of rock surfaces inclined 0 75 with respect to the horizontal the principal uncertainty arises due to the lack of a precise date for the glacial retreat event about 10 the 26al 10be ratio at production 6 0 is determined more accurately since the exposure age of the samples is 2 orders of magnitude smaller than the mean lives of the two nuclides production rates of 10be and 26al in quartz at other latitudes and altitudes in the troposphere can be determined from the present measurements by scaling using the known altitude and latitude dependence of cosmic ray fluxes of nucleons and negative muons knowledge of the production rates of these nuclides is a prerequisite for their application in erosion and geomorphological studies

in situ10be 26al exposure ages at meteor crater arizona a new method of dating the surface exposure of rocks from in situ production of 10be and 26al has been applied to determine the age of meteor crater arizona a lower bound on the crater age of 49 200 1 700 years has been obtained by this method

age and geomorphic history of meteor crater arizona from cosmogenic 36cl and 14c in rock varnish abstract using cosmogenic 36 cl buildup and rock varnish radiocarbon we have measured the exposure age of rock surfaces at meteor crater arizona our 36 cl measurements on four dolomite boulders ejected from the crater by the impact yield a mean age of 49 7 0 85 ka which is in excellent agreement with an average age of 49 3 ka obtained from thermoluminescence studies on shock metamorphosed dolomite and quartz these ages are supported by undetectably low 14 c in the oldest rock varnish sample