chemical and physical denudation in the amazon river basin we present major and trace element data on the suspended and dissolved phases of the amazon river and its main tributaries the sr isotopic composition of the dissolved load is also reported special attention is paid to the abundances of ree and to their fractionation between the dissolved and suspended phase the rivers of the amazon basin are among the richest in dissolved ree and are similar to the rivers of the congo system however a greater range of fractionation between lree and hree is reported here at a global scale the rivers have intermediate patterns between those of the congo system and those of high ph rivers such as the indus and mississippi rivers only few elements rb u ba k na sr and ca are mobilized by silicate weathering these elements are strongly depleted in the suspended phase with respect to upper continental crust in the dissolved load these elements are controlled by atmospheric inputs and the weathering of the main lithologies we propose a model based on mass budget equations that allow the proportions derived from the different sources to be calculated as a consequence silicate carbonate and evaporite weathering rates can be estimated as well as the consumption of co2 by weathering of each of these lithologies physical weathering rates can be estimated by two complementary approaches on the one hand the multi year average of suspended sediments yields can be used to estimate physical denudation on the other hand we have developed a steady state model of erosion that allows us to calculate physical erosion rates on the basis of the dissolved load of rivers a mean crustal composition is assumed in this model for the rock sources of the drainage basins comparison of the rates predicted by the model to the observed rates shows good agreement for the lowland rivers but a strong discrepancy for the rivers derived from the andes andean rivers solimoes madeira and amazon have observed sediment yields much greater than those predicted according to the steady state model of chemical and physical weathering two interpretations can account for this inconsistency the first is that these rivers are not in steady state and hence that the soils are being destroyed the second requires that the local continental crust is different from the average continental crust of taylor and mclennan and contains a large proportion of sedimentary rocks using the measured sediment yields and assuming a steady state we can estimate the amount of sediment recycling for each drainage basin for the amazon at santarem we find that at least 25 of the mass of the upper continental crust of the amazon drainage basin is constituted of recycled material

quantitative bedrock geology of alaska and canada 1 we quantitatively analyze the area age distribution of sedimentary igneous and metamorphic bedrock on the basis of data from the most recent geologic maps of alaska beikman 1980 1 2 500 000 and canada douglas 1969 updated 1 5 000 000 made available in digital form by the u s geologic survey and the geologic survey of canada national resources canada respectively sediments cover 72 9 and 52 4 of the surface of alaska and canada volcanic rocks comprise 11 6 and 6 of the surface area in alaska and canada respectively whereas intrusive rocks cover 6 6 and 24 of the surface respectively ultramafic rocks account for 0 20 and 0 08 of the bedrock area whereas metamorphic rocks cover 3 4 and 16 1 the average ages of major lithologic units weighted according to bedrock area are significantly younger in alaska than in canada marine sediments stratigraphic age of 206 myr in alaska versus 599 myr in canada continental sediments stratigraphic age of 81 myr versus 150 myr volcanic rocks 126 myr versus 1377 myr intrusive rocks 114 myr versus 2109 myr ultramafic rocks 261 myr versus 874 myr and metamorphic rocks 474 myr versus 2545 myr this age difference reflects the contrast between the young active margin influence on the geology of alaska and the old cratonic nature of much of the bedrock exposed in canada glacial erosion and the resulting exposure of precambrian bedrock primarily in canada contributes to this age contrast the average temporal and spatial resolution of the digital data is sufficiently high to perform age area analyses on individual river basins larger than 20 000 km2 and to evaluate the relationship between bedrock geology and river chemistry

a review of glacial geomorphology and chronology in northern spain timing and regional variability during the last glacial cycle abstract in this paper we synthesize the research in glacial geomorphology and geochronology in northern spain with special attention to the evidence of local glacier maximum extent earlier than the global lgm of mis 2 18 21 ka bp more accurate models of glacier evolution have been defined based on limnogeological geochronological and geomorphological data in the pyrenees osl optically stimulated luminescence surface exposure and radiocarbon dating techniques have identified end moraines and fluvial terraces corresponding to mis 6 about 170 ka and even to mis 8 about 260 ka and also established the timing of the last local glacial maxima as prior to global lgm mis 4 ca 50 70 ka during the global lgm a smaller re advance occurred but glaciers reached different extents in the central and the eastern pyrenees in nw iberia radiocarbon and osl techniques point to local glacial maximum prior to ca 26 ka 38 ka and probably synchronous with 45 ka although some bias might have been introduced by the dating procedures this review demonstrates that in both regions the local maximum extent occurred prior to the global lgm the asynchronies between the glacial maxima chronologies in the different mountain ranges of northern spain suggest that local climate factors exert a strong control on mountain glacier dynamics

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

10be and osl dating of pleistocene fluvial terraces along the hongshuiba river constraints on tectonic and climatic drivers for fluvial downcutting across the ne tibetan plateau margin china abstract accurate geochronologic constraints of fluvial surfaces along tectonically active mountain fronts can help clarify river responses to tectonic uplift and rapid climate change in this study we re constrained abandonment ages of four fluvial terraces along the hongshuiba river on the northern margin of the qilian shan ne tibet using 10be depth profiles to date fluvial deposits and osl dating of capping loess comparison of chronological ages of the same surface derived from both boulders and depth profiles provides an opportunity to evaluate the suitability of different sampling methods for determining landform ages in arid climate correlating terrace ages with regional climate proxies suggests that terraces formed during an interglacial stage or during glacial to interglacial transitions or during a period of strong summer monsoon or monsoonal weakening the initial period of fluvial incision may primarily result from climate changes e g monsoon evolution and other factors that influence the ratio of sediment flux to water discharge similarities between fluvial incision rates and incremental tectonic uplift rates suggest a tectonic control on continuous river downcutting before 26 ka in summary climate forcing drives the initial abandonment of fluvial terraces in this region while the amplitude of fluvial incision was mainly controlled by progressive tectonic uplift however comparison of regional fluvial incision rates e g 6 mm a of the hongshuiba river exhumation rates from the qilian shan 0 5 mm a denudation rates from the river catchments 0 3 0 8 mm a and vertical slip rates of the northern qilian thrust 1 mm a all suggest climate change driven forcing of incision since the late pleistocene to holocene

global chemical erosion during the last glacial maximum and the present sensitivity to changes in lithology and hydrology geographically based calculations for 18 000 years ago 18 ka and today were made to examine the potential effects on terrestrial chemical erosion of changes in lithology and hydrology on glacial interglacial timescales runoff fields were derived from general circulation model predictions of precipitation minus evaporation then global lithologic maps were prepared so that empirical relationships for runoff versus bicarbonate flux for different rock types could be used to calculate global chemical erosion rates assuming that significant chemical erosion does not occur beneath ice sheets we find that chemical erosion in ice free areas at 18 ka was only slightly greater than today this result arises because the amount of land covered by ice sheets is roughly compensated for by exposed shelf areas and because there is little difference in global runoff the small 20 increase in the global chemical erosion rate during glacial conditions is due to exposure on the shelves of a relatively high proportion of carbonates which weather faster than average data on glacial interglacial movements of the calcite compensation depth ccd are inconclusive but seem to indicate little change whereas even a 20 increase in the riverine bicarbonate flux should cause an observable deepening if the ccd did not indeed change then our predicted increase in the bicarbonate flux from land during glaciations would have to be accommodated by other means such as increased carbonate productivity about half of the observed decrease in atmospheric pco2 at 18 ka could be explained if increased silicate chemical erosion accompanied increased total chemical erosion in ice free areas consideration of the maximum possible effect of meltwater at ice margins leads to an 80 increase in the global chemical erosion rate at 18 ka such an increase would lead to a larger and faster drop in atmospheric pco2 but also to an excessive deepening of the global ccd this scenario seems unrealistic our results are contrary to interpretations of a much higher silicate chemical erosion rate during glaciations based on recently published records of quaternary marine sr isotopic ratios therefore if these records are indeed globally representative and entirely due to increased silicate chemical erosion as opposed to changes in source areas or hydrothermal inputs then other factors such as a shift in the global weathering regime which we do not explicitly consider here may be involved sensitivity tests show that considering spatial heterogeneities in lithology and runoff leads to lower predictions of global chemical erosion rates than what one would obtain by considering only global averages in a non spatially resolved calculation

quantitative bedrock geology of the continents and large scale drainage regions we quantitatively analyze the area age distribution of sedimentary extrusive volcanic and endogenous plutonic and or metamorphic bedrock on the basis of data from the most recent digital geological map of the world at a scale of 1 25 000 000 the spatial resolution of the digital bedrock data averages 13 905 km2 per polygon comparison of certain regions of the world previously analyzed at higher spatial resolution with the low resolution world data reveals general consistency in the areal exposure of major rock types as well as a minor systematic bias toward older average bedrock ages in the global data set application of the global bedrock data to 19 large scale drainage regions and three large internally drained regions reveals considerable regional variability of earth s bedrock geology that is consistent with the dominant geotectonic setting of the respective drainage region

degradation of unconsolidated quaternary landforms in the western north america abstract one of the major goals of geomorphology is to understand the rate of landscape evolution and the constraints that erosion sets on the longevity of land surfaces the latter has also turned out to be vital in modern applications of cosmogenic exposure dating and interpretation of lichenometric data from unconsolidated landforms because the effects of landform degradation have not been well documented disagreements exist among researchers regarding the importance of degradation processes in the dating techniques applied to exposures here we show that all existing qualitative data and quantitative markers of landform degradation collectively suggest considerable lowering of the surface of unconsolidated landforms over the typical life span of quaternary moraines or fault scarps degradation is ubiquitous and considerable even on short time scales of hundreds of years on steeply sloping landforms these conservative analyses are based entirely on field observations of decreasing slope angles of landforms over the typical range of ages in western north america and widely accepted modeling of landscape degradation we found that the maximum depth of erosion on fault scarps and moraines is on average 34 of the initial height of the scarp and 25 of the final height of the moraine although our observations are limited to fault scarps and moraines the results apply to any sloping unconsolidated landform in the western north america these results invalidate the prevailing assumption of no or little surface lowering on sloping unconsolidated landforms over the quaternary period and affirm that accurate interpretations of lichen ages and cosmogenically dated boulder ages require keen understanding of the ever present erosion in our view the most important results are twofold 1 to show with a large data set that degradation affects universally all sloping unconsolidated landforms and 2 to unambiguously show that even conservative estimates of the total lowering of the surface operate at time and depth scales that significantly interfere with cosmogenic exposure and lichen dating

development of the truckee river terraces on the northeastern flank of the sierra nevada abstract the distribution and elevation of the abandoned strath terraces along the truckee river in the carson range on the eastern flank of the sierra nevada are mapped and quantified with measurements from lidar accompanied by field survey each mapped strath terrace is formed by incision into the hunter creek sandstone and deposition of fluvial gravel by the truckee river the gravel contains granitic boulders sourced from the glaciated headwaters of the truckee 20 km upstream from the terraces and display boulders ranging from 4 to 12 m in dimension in comparison to the

timing of the last glaciation and subsequent deglaciation in the ruby mountains great basin usa abstract the timing of the last pleistocene glaciation in western north america is becoming increasingly well understood largely due to improved methods of obtaining numerical ages of glacial deposits and landforms among these cosmogenic radionuclide surface exposure dating has been widely applied to moraines of mountain glaciers providing the framework for understanding terrestrial climate change during and since the last glaciation in western north america during the late pleistocene the great basin of the western united states hosted numerous mountain glaciers the deposits of which can provide valuable records of past climate changes if their ages can be precisely determined in this study twenty nine cosmogenic radionuclide 10be surface exposure ages from a suite of moraines in seitz canyon western ruby mountains limit the timing of the last glacial episode in the interior great basin known as the angel lake glaciation results indicate that deposition of a terminal moraine and two recessional moraines began just prior to 20 5 ka and continued until 20 0 ka retreat from the next younger recessional moraine began at 17 2 ka and final deglaciation began at 14 8 ka these ages are broadly consistent with cosmogenic surface exposure ages from the eastern sierra nevada and the western wasatch mountains in the western and eastern extremes of the great basin respectively furthermore these ages suggest that the valley glacier in seitz canyon was at or near its maximum extent before and during the hydrologic maxima of pleistocene lakes in the great basin supporting previous suggestions that a cool and wet climate persisted in this region during the early part of the last glacial interglacial transition

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

quantitative bedrock geology of the conterminous united states of america 1 we quantitatively analyze the area age distribution of bedrock based on data from the most recent geologic map of the conterminous united states of america king and beikman 1974a 1974b made available in digital form by the united states geologic survey the area age distribution agrees surprisingly well with older data higgs 1949 but provides much higher temporal resolution the mean stratigraphic age of all sedimentary bedrock is 134 myr that of tertiary cambrian sediments is 104 myr the analysis also reveals area coverage of some minor lithologies such as ultramafic rocks that cover 0 15 of the conterminous united states area coverage of 162 lithostratigraphic units is made available as an excel data sheet

glacial chronology of the ruby mountains east humboldt range nevada abstract the ruby mountains east humboldt range one of the interior mountain groups of the basin and range province lies about midway between the wasatch mountains and the sierra nevada after blackwelder s description in his review of glaciation in the western mountains sharp mapped and named the deposits of the lamoille and angel lake glaciations and correlated them with early and late wisconsin deposits of the great lakes area the refinement of relative dating rd methods the availability of airphotos and modern topographic maps and new road cuts have aided the restudy of these alpine glacial deposits and the basis for their correlation lamoille moraines are smooth ridges and show little detail of constructional topography valleys glaciated only by lamoille ice still show the characteristics of a glaciated trough but they have been greatly modified by weathering and erosion granite boulders on lamoille moraines are pitted and pegmatites have grotesque shapes with 30 cm deep pits cuts through lamoille end moraines and alluvial talus cones expose a thick soil profile with a well developed blocky structure in a reddish brown argillic b horizon subsurface granitic boulders in the b horizon of lamoille tills show much greater weathering than do those in angel lake tills in contrast angel lake moraines are irregular and rugged contain closed depressions and have been little altered since deposition surfaces scoured by angel lake ice are fresh and unweathered granites of angel lake moraines have weathered surfaces but show little pitting pegmatites have pits up to 10 cm deep the thin soil profiles on angel lake tills and alluvial talus cones display brown colors minor clay accumulation and no b horizon structure these weathering and morphological differences suggest that the lamoille deposits have been exposed to weathering and erosion for a period of time as much as an order of magnitude longer than the angel lake deposits thus only the angel lake is wisconsinan in age and the lamoille drift is more reasonably correlated with the illinoian stage of the great lakes region

latest pleistocene glacial and climate history of the wasatch range utah abstract the geologic record of mountain glaciation is one of the most sensitive archives of terrestrial climate change during the late quaternary period pleistocene glacial deposits are exceptionally well preserved in the great basin of north america where they are found in close proximity to shoreline deposits of coeval paleo lakes representing a unique opportunity to investigate paleoclimate change using both hydrologic systems mountain glaciers advanced across much of the wasatch range utah during the same general time interval when lake bonneville expanded in the adjacent bonneville basin here we present a range wide latest pleistocene glacial history for the wasatch range based on 22 new and 41 recalculated cosmogenic 10be surface exposure ages and reconstruct glaciers and climate for the last glacial maximum lgm and lateglacial periods our results indicate three distinct range wide phases of glaciation in the wasatch including the pinedale 1 ca 21 20 ka pinedale 2 ca 17 5 ka and pinedale 3 ca 15 ka our modeling results indicate that the pinedale 1 advance near the end of the global lgm was primarily driven by decreased annual temperatures with little to no change in precipitation as compared to modern wasatch glaciers appear to have reoccupied near maximum positions after the end of the lgm during the pinedale 2 responding to increased precipitation in concert with rising lake levels across western north america and then retreated from ice distal positions near ca 17 5 ka wasatch glaciers occupied recessional positions at approximately half their maximum lengths during pinedale 3 until ca 15 ka followed by rapid i e several millennia deglaciation our glacial chronologies establish that wasatch glaciers responded synchronously across the range to global and regional climate forcings during the latest pleistocene the new wasatch glacial chronology combined with global glacial chronologies and paleoclimate proxies from western north america suggest glacier change was driven by a combination of long term global temperature forcing as well as regional to local modifications in precipitation

accuracy of cosmogenic ages for moraines analyses of all published cosmogenic exposure ages for moraine boulders show an average age range of 38 between the oldest and youngest boulders from each moraine this range conflicts with the common assumption that ages of surface boulders are the same as the age of the landform the wide spread in boulder ages is caused by erosion of the moraine surface and consequent exhumation of fresh boulders a diffusion model of surface degradation explains the age range and shows that a randomly sampled small set of boulders n 3 7 will always yield a lower age limit for the moraine the model indicates that for identical dating accuracy six to seven boulders are needed from old and tall moraines 40 000 100 000 yr 50 100 m initial height but only one to four boulders from small moraines 20 000 100 000 yr 10 20 m by following these guidelines the oldest obtained boulder age will be 90 of the moraine age 95 probability this result is only weakly sensitive to a broad range of soil erosion rates our analysis of published boulder ages indicates that 3 of all moraine boulders have prior exposure and 85 of these boulders predate the dated moraine

cosmogenic 36cl and 10be ages of quaternary glacial and fluvial deposits of the wind river range wyoming we measured cosmogenic 36 cl in 56 samples from boulders on moraines and fluvial terraces in the vicinity of the wind river range wyoming we also measured 10 be in 10 of the same samples most of the 10 be ages were in good agreement with the 36 cl ages indicating that rock surface erosion rates were very low the oldest moraine investigated the type sacagewea ridge site yielded only a limiting minimum age of 232 ka the oldest moraines in the type bull lake complex also could be constrained only to 130 ka the main sequence of type bull lake moraines yielded age distributions indicating deposition within the intervals 130 to 100 ka and 120 to 100 ka the best estimates are closer to the upper limits of these ranges and associated uncertainties are in the range of 10 to 15 these uncertainties could permit deposition in either marine isotope stage 6 or stage 5d we found no evidence of glacial deposits dating to marine isotope stage 4 both bull lake age moraines from fremont lake on the opposite side of the wind river range and boulders on a fluvial terrace above the wind river gave age distributions very similar to that of the second oldest bull lake advance ca 130 to 100 ka the distribution of boulder ages for pinedale moraines at bull lake indicated deposition between 23 and 16 ka nearly identical to the distribution of 10 be ages previously reported for the type pinedale moraines at fremont lake

climatic and topographic controls on the style and timing of late quaternary glaciation throughout tibet and the himalaya defined by 10be cosmogenic radionuclide surface exposure dating abstract temporal and spatial changes in glacier cover throughout the late quaternary in tibet and the bordering mountains are poorly defined because of the inaccessibility and vastness of the region and the lack of numerical dating to help reconstruct the timing and extent of glaciation throughout tibet and the bordering mountains we use geomorphic mapping and 10 be cosmogenic radionuclide crn surface dating in study areas in southeastern gonga shan southern karola pass and central western nyainqentanggulha shan and tanggula shan tibet and we compare these with recently determined numerical chronologies in other parts of the plateau and its borderlands each of the study regions receives its precipitation mainly during the south asian summer monsoon when it falls as snow at high altitudes gonga shan receives the most precipitation 2000 mm a 1 while near the margins of monsoon influence the karola pass receives moderate amounts of precipitation 500 600 mm a 1 and in the interior of the plateau little precipitation falls on the western nyainqentanggulha shan 300 mm a 1 and the tanggula shan 400 700 mm a 1 the higher precipitation values for the tanggula shan are due to strong orographic effects in each region at least three sets of moraines and associated landforms are preserved providing evidence for multiple glaciations the 10 be crn surface exposure dating shows that the formation of moraines in gonga shan occurred during the early mid holocene neoglacial and little ice age on the karola pass during the lateglacial early holocene and neoglacial in the nyainqentanggulha shan date during the early part of the last glacial cycle global last glacial maximum and lateglacial and on the tanggula shan during the penultimate glacial cycle and the early part of the last glacial cycle the oldest moraine succession in each of these regions varies from the early holocene gonga shan lateglacial karola pass early last glacial western nyainqentanggulha shan and penultimate glacial cycle tanggula shan we believe that the regional patterns and timing of glaciation reflect temporal and spatial variability in the south asian monsoon and in particular in regional precipitation gradients in zones of greater aridity the extent of glaciation has become increasingly restricted throughout the late quaternary leading to the preservation of old 100 ka glacial landforms in contrast in regions that are very strongly influenced by the monsoon 1600 mm a 1 the preservation potential of pre lateglacial moraine successions is generally extremely poor this is possibly because lateglacial and holocene glacial advances may have been more extensive than early glaciations and hence may have destroyed any landform or sedimentary evidence of earlier glaciations furthermore the intense denudation mainly by fluvial and mass movement processes which characterize these wetter environments results in rapid erosion and re sedimentation of glacial and associated landforms which also contributes to their poor preservation potential

cosmogenic exposure age chronologies of pinedale and bull lake glaciations in greater yellowstone and the teton range usa abstract we have obtained 69 new cosmogenic 10be surface exposure ages from boulders on moraines deposited by glaciers of the greater yellowstone glacial system and teton range during the middle and late pleistocene these new data combined with 43 previously obtained 3he and 10be ages from deposits of the northern yellowstone outlet glacier establish a high resolution chronology for the yellowstone teton mountain glacier complexes boulders deposited at the southern limit of the penultimate ice advance of the yellowstone glacial system yield a mean age of 136 13 10be ka and oldest ages of 151 157 10be ka these ages support a correlation with the bull lake of west yellowstone with the type bull lake of the wind river range and with marine isotope stage mis 6 end moraines marking the maximum pinedale positions of outlet glaciers around the periphery of the yellowstone glacial system range in age from 18 8 0 9 to 16 5 1 4 10be ka and possibly as young as 14 6 0 7 10be ka suggesting differences in response times of the various ice cap source regions moreover all dated pinedale terminal moraines in the greater yellowstone glacial system post date the pinedale maximum in the wind river range by 4 6 kyr indicating a significant phase relationship between glacial maxima in these adjacent ranges boulders on the outermost set and an inner set of pinedale end moraines enclosing jenny lake on the eastern teton front yield mean ages of 14 6 0 7 and 13 5 1 1 10be ka respectively the outer jenny lake moraines are partially buried by outwash from ice on the yellowstone plateau hence their age indicates a major standstill of an expanded valley glacier in the teton range prior to the younger dryas followed closely by deglaciation of the yellowstone plateau these new glacial chronologies are indicative of spatially variable regional climate forcing and temporally complex patterns of glacier responses in this region of the rocky mountains during the pleistocene

quantitative bedrock geology of east and southeast asia brunei cambodia eastern and southeastern china east timor indonesia japan laos malaysia myanmar north korea papua new guinea philippines far eastern russia singapore south korea taiwan thailand vietnam 1 we quantitatively analyze the area age distribution of sedimentary igneous and metamorphic bedrock based on data from the most recent digital geologic maps of east and southeast asia coordinating committee for coastal and offshore geosciences programmes in east and southeast asia ccop and the geologic survey of japan 1997 1 2 000 000 published as digital geoscience map g 2 by the geological survey of japan sedimentary rocks volcanic rocks plutonic rocks ultramafic rocks and metamorphic rocks cover 73 3 8 5 8 8 0 9 and 8 6 of the surface area respectively the average ages of major lithologic units weighted according to bedrock area are as follows sedimentary rocks average stratigraphic age of 123 myr median age of 26 myr volcanic rocks 84 myr 20 myr intrusive rocks 278 myr 195 myr ultramafic rocks unknown and metamorphic rocks 1465 myr 1118 myr the variability in lithologic composition and age structure of individual countries reflects the complex tectonic makeup of this region that ranges from precambrian cratons e g northeast china and north korea to mesozoic cenozoic active margins e g japan the philippines indonesia and new guinea the spatial resolution of the data varies from 44 km2 per polygon japan to 1659 km2 per polygon taiwan and is on average 490 km2 polygon similar to our previous analyses of the united states of america and canada the temporal and spatial resolution is sufficiently high to perform age area analyses of individual river basins larger than 10 000 km2 and to quantitatively evaluate the relationship between bedrock geology and river chemistry as many rivers draining tropical mountainous islands of east and southeast asia have a disproportionate effect on the dissolved and particulate load delivered to the world oceans bedrock geology in such river drainage basins disproportionately affect ocean chemistry

too young or too old evaluating cosmogenic exposure dating based on an analysis of compiled boulder exposure ages cosmogenic exposure dating has greatly enhanced our ability to define glacial chronologies spanning several global cold periods and glacial boulder exposure ages are now routinely used to constrai

landscape evolution degrades the geologic signature of past glaciations glaciers on land leave behind characteristic landforms and bedrock topography the spatial extent of this signature and dating of the erosional and depositional features has led to a good understanding of temporal and spatial shifts of past glaciers and climate at best the landscape contains considerable information on past glacial processes in the form of moraines and other depositional landforms however evidence suggests that all unconsolidated landforms degrade manifested in the general degradation and smoothing of the topography over time to determine the relative amount and pattern of degradation on glacial moraines we use a well documented moraine degradation model that uses both local slope and a proportionality constant to determine the diamicton transport rate this formulation results in diffusion type topographic smearing and smoothing of the initially angular topographic features more specifically diamicton is eroded at the crest of a moraine transported downhill and deposited on the gentler lower flank collectively this produces moraine cross profiles that closely resemble observations in the field by assuming an initial constant boulder frequency per unit volume of the diamicton the model tracks the surface lowering and the related exhumation of boulders to the moraine surface this produces a tightly constrained set of testable predictions 1 modelled moraine cross profile 2 modelled spatial pattern of surface boulders no boulders on the lower flank and 3 relative frequency of surface boulders smallest frequency of surface boulders at the inflection point highest frequency at the crest these characteristics were measured in the field of alpine tropical and antarctic moraines the correspondence between model predictions and field data is remarkably good and strongly supports our understanding of the time dependent evolution of glacial landscapes these results highlight the evolving nature of glacial landforms and indicate the complete removal of original moraine surfaces and postglacial emergence of spatial pattern in boulder frequency

nature and timing of quaternary glaciation in the himalayan tibetan orogen abstract much effort has been made in recent years to define the timing and extent of quaternary glaciation throughout the himalayan tibetan orogen these studies are challenging because of the logistical and political inaccessibility of the region and the inherent problems associated with the application of numerical dating techniques nevertheless the studies are providing abundant evidence for significant glacial advances throughout the last several glacial cycles and are beginning to accurately define the extent and timing of glaciation in selected regions studies are showing that himalayan tibetan glaciers in arid regions during the last glacial cycle reached their maximum extent early in the cycle and that global last glacial maximum glacier advances were significantly less extensive however along the more monsoonal influenced greater himalaya there is increasing evidence to suggest that glaciation was more extensive later in the last glacial cycle but this has yet to be fully assessed in addition the new studies are showing that throughout most himalayan tibetan regions significant glacier advances occurred during the lateglacial and early holocene with minor advances in some regions during the mid holocene the still relatively poor chronological control in the himalayan tibetan orogen however makes it difficult to construct correlations across the region and with regions elsewhere in the world this in turn makes it hard to assess the relative importance of the different climatic mechanisms that force glaciation across the himalayan tibetan orogen and to quantify paleoclimate change in this high altitude subtropical region the lateglacial and holocene glacial records however are particularly well preserved in several himalayan tibetan regions glacial successions such as these have the greatest potential to be examined in detail using newly developing numerical dating and geomorphic and sedimentologic methods to derive high resolution terrestrial records of glaciation that will help in paleoclimatic reconstruction for high altitude subtropical regions

terrestrial cosmogenic surface exposure dating of glacial and associated landforms in the ruby mountains east humboldt range of central nevada and along the northeastern flank of the sierra nevada abstract deposits near lamoille in the ruby mountains east humboldt range of central nevada and at woodfords on the eastern edge of the sierra nevada each record two distinct glacial advances we compare independent assessments of terrestrial cosmogenic nuclide tcn surface exposure ages for glacial deposits that we have determined to those obtained by others at the two sites at each site tcn ages of boulders on moraines of the younger advance are between 15 and 30 ka and may be associated with marine oxygen isotope stage mis 2 at woodfords tcn ages of boulders on the moraine of the older advance are younger than 60 ka and possibly formed during mis 4 whereas boulders on the correlative outwash surface show ages approaching 140 ka mis 6 the tcn ages of boulders on older glacial moraine at woodfords thus appear to severely underestimate the true age of the glacial advance responsible for the deposit the same is possibly true at lamoille where clasts sampled from the moraine of the oldest advance have ages ranging between 20 and 40 ka with a single outlier age of 80 ka the underestimations are attributed to the degradation and denudation of older moraine crests noting that boulder ages on the older advances at each site overlap significantly with mis 2 we speculate that erosion of the older moraines has been episodic with a pulse of denudation accompanying the inception of mis 2 glaciation

himalayan glacial sedimentary environments a framework for reconstructing and dating the former extent of glaciers in high mountains abstract reconstructing paleoenvironmental change from glacial geologic evidence in the himalayas has been difficult because of the lack of organic material for radiocarbon dating and the problems of correctly identifying the origin of highly dissected landforms studies of the contemporary glacial depositional environments and ancient landforms and sediments in the hunza valley karakoram mountains the lahul and garhwal himalaya and the khumbu himalaya illustrate the variability in processes landforms and sediment types these studies can be used to interpret ancient landforms and sediments for paleoenvironmental reconstructions and aid in forming strategies for sampling sediments and rocks for the developing techniques of cosmogenic radionuclide crn surface exposure and optically stimulated luminescence osl dating many himalayan glaciers have thick covers of supraglacial debris derived from valley sides and such debris mantled glaciers exhibit important differences from clean glaciers both in terms of debris transport processes and the depositional landforms that they produce analysis of sediment landform associations can be used to reconstruct processes of sediment transport and deposition and the relationship between moraines and other landforms and climatic forcing cycles such analysis is of fundamental importance in guiding sampling and interpretation in crn and osl dating work

timing and climatic drivers for glaciation across monsoon influenced regions of the himalayan tibetan orogen abstract mapping and thirty eight 10 be terrestrial cosmogenic nuclide tcn ages help define the timing of glaciation in the monsoon influenced greater himalaya in central garwhal india glacial landforms in central garwhal are present only within a few kilometers of the present glaciers and all date to less than 12 5 ka this suggests that the lateglacial and or holocene glacial advances were more extensive than their predecessors and hence destroyed or buried evidence for earlier glaciation or that other processes such as intensive fluvial erosion and or hillslope mass movements have destroyed evidence of earlier glaciation prominent laterofrontal moraines date to the lateglacial the early holocene and the neoglaciation moraines next to the active ice and boulders on contemporary glaciers date to 10 1 10 2 years before present this suggests only a minor glacial advance during the little ice age occurred in central garhwal these young ages indicate that inheritance of tcns in areas that were recently glaciated is very small and likely has little effect when considering tcn ages on moraines older than the global last glacial maximum the new 10 be ages are combined with 1081 recalculated 10 be ages from previous studies to develop a regional framework of glaciation across the monsoon influenced and adjacent regions of the himalayan tibetan orogen we separate what appears to be continuous glaciation into 27 regional glacial stages plus 2 tentative glacial stages that are termed monsoonal himalayan tibetan stages mohits the regional glacial stages cover a wide chronologic range that includes five regional glacial stages older than the last glacial cycle mohits 13 at 483 38 ka to mohits 5e at 122 15 ka thirteen regional glacial stages within the last glacial cycle mohits 5b at 91 15 ka to mohits 2a at 12 9 0 9 ka and eleven regional glacial stages during the holocene mohits 1k at 11 4 0 7 ka to mohits 1a at 0 4 0 1 ka there are strong correlations both with periods of strong monsoons and northern hemisphere events throughout the entire chronologic range with 16 stages linked to the monsoon 11 stages linked to the mid latitude westerlies and two unassigned because of large age uncertainties overall we see a complex pattern of glaciation influenced by two climatic systems throughout the mid late quaternary

glaciation in the great basin of the western united states abstract forty individually named ranges plateaus and massifs draining wholly or partly into the great basin of the western united states show definite evidence of pleistocene glaciation the most obvious deposits are a family of moraines designated among other names tioga angel lake and pinedale such moraines generally can be traced from range to range away from described type moraines these deposits have been numerically assigned to late wisconsinan glaciation in the wasatch range white mountains boulder mountain and sierra nevada on the basis of radiocarbon and surface exposure ages and have been assigned to late wisconsinan time in several other ranges on the basis of relative age studies the type angel lake moraine and most other equivalent moraines across the great basin are thick hummocky lobate piles of till rather than looping ridges the thicknesses of the moraines often 60 m can be explained by heavy debris loads and or glacial advance retreat and readvance to the same positions a number of times which is consistent with recent evidence that multiple late wisconsinan advances possibly related to heinrich and dansgaard oeschger events occurred in the sierra nevada pre angel lake deposits occur in many great basin ranges but it is currently difficult or perhaps impossible to determine if these deposits are equivalent to each other and what their relationship is to pre tioga deposits in the sierra nevada numerical ages are rare and relative age studies suggest that pre angel lake deposits may be products of more than one glaciation mapped pre angel lake glaciers were longer than their angel lake counterparts but the length differences do not translate into large differences in ela depression there is evidence of two minor latest pleistocene or early holocene advances in some ranges judging from the presence of overlying mazama tephra and or weathering comparisons to local angel lake moraines in the latter part of the holocene ela s were sufficiently high that only the highest wettest ranges developed neoglacial glaciers there does not appear to be a consistent pattern of latest pleistocene holocene glacial fluctuations along an east west transect through the cordillera or even through the great basin

denudation rates for the southern drakensberg escarpment se africa derived from in situ produced cosmogenic 36c1 initial results cosmogenic 36 c1 concentrations in basalt samples from the drakensberg escarpment on the se african passive margin give quantitative estimates of denudation and scarp retreat rates over the 10 4 10 6 year timespan addressed by these data the calculated escarpment retreat rate has been 50 95 m ma 1 and the average summit denudation rate 6 m ma 1 the scarp retreat rate is an order of magnitude less than previously suggested the rate of summit lowering is sufficient to prevent the long term intact survival of erosion cycle surfaces formed in the mesozoic that were previously inferred for this region

chlorine degassing constrained by cosmogenic 36cl and radiocarbon dating of early holocene rhyodacitic lava domes on erciyes stratovolcano central turkey abstract the erciyes stratovolcano in central turkey has several young rhyodacite lava domes that show evidence of clear exposure histories but have hitherto unknown ages we collected 27 surface samples from three volcanic domes namely karagullu perikartin and dikkartin for cosmogenic 36cl dating the perikartin eruption generated a pyroclastic flow which buried trees that were converted to charcoal the radiocarbon 14c dating yielded an average age of 9728 110 cal years b p calibrated using calib 7 1 we analyzed the cosmogenic 36cl ages however were found that they were probably affected by cl degassing of the magma our study suggests that the radiocarbon age indeed lies between the 0 cl degassing 6 8 0 5 ka under nucleogenic 36cl equilibrium and the 100 degassing conditions 9 9 0 6 ka under nucleogenic 36cl disequilibrium which implies a partial cl degassing 93 during the eruption this value is somewhat higher than earlier estimates and the difference may result from the miscalculated nucleogenic and or low energy neutron production of 36cl uncertainties arising from the corrections due to hydrogen rich cover i e snow of lava surfaces or due to unconsidered geological complications our findings provide a novel approach to estimate the cl degassing on young lavas and deliver a new evidence of the holocene volcano chronology of the central anatolia

rates of basin wide rockwall retreat in the k2 region of the central karakoram defined by terrestrial cosmogenic nuclide 10be abstract basin wide rockwall retreat rates are estimated using cosmogenic 10 be concentrations in supraglacial debris from the baltoro glacier basin in k2 region of the central karakoram pakistan total cosmogenic 10 be concentrations of the supraglacial debris were measured using accelerator mass spectrometry to determine 10 be concentrations inherited from the rockwalls within the glaciated basin given that rockfall induced by periglacial weathering and snow and ice avalanches is the most important source of supraglacial debris production in the high mountain glaciated basin the erosion rate of the bare bedrock can be considered to be the rate of rockwall retreat the rate of the rockwall erosion converted from the calculated inheritance of 10 be concentrations using the maximum velocity of the active glacier places an upper limit ranging from 0 65 mm year to 2 48 mm year this rate of rockwall retreat is in the same order of magnitude reported in other high himalayan mountains the rate however is an order of magnitude higher than erosion rates inferred from sediment budget studies and half that of the fluvial incision rate and exhumation rate for the same region the difference between rates of basin wide rockwall retreat and fluvial incision rates over the late quaternary suggests that in this glaciated basin fluvial incision is likely enhanced by localized differential tectonism and or isostatic uplift

denudation rates determined from the accumulation of in situ produced 10be in the luquillo experimental forest puerto rico we present a simple method for estimation of long term mean denudation rates using in situ produced cosmogenic 10be in fluvial sediments procedures are discussed to account for the effects of soil bioturbation mass wasting and attenuation of cosmic rays by biomass and by local topography our analyses of 10be in quartz from bedrock outcrops soils mass wasting sites and riverine sediment from the icacos river basin in the luquillo experimental forest puerto rico are used to characterize denudation for major landform elements in that basin the 10be concentration of a discharge weighted average of size classes of river sediment corresponds to a long term average denudation of 43 m ma 1 consistent with mass balance results

chronology of glaciations in the sierra nevada california from 10be surface exposure dating abstract we use 10 be surface exposure dating to construct a high resolution chronology of glacial fluctuations in the sierra nevada california most previous studies focused on individual glaciated valleys whereas our study compares chronologies developed throughout the range to identify regional patterns in the timing of glacier response to major climate changes sites throughout the range indicate last glacial maximum retreat at 18 8 1 9 ka 2 that suggests rather consistent changes in atmospheric variables e g temperature and precipitation throughout the range the penultimate glacial retreat occurred at ca 145 ka our data suggest that the sierra nevada landscape is dominated by glacial features deposited during marine isotope stage mis 2 and mis 6 deposits of previously recognized glaciations between circa 25 and 140 ka e g mis 4 tenaya early tahoe cannot be unequivocally identified the timing of sierra nevada glacial retreat correlates well with other regional paleoclimate proxies in the sierra nevada but differs significantly from paleoclimate proxies in other regions our dating results indicate that the onset of lgm retreat occurred several thousand years earlier in the sierra nevada than some glacial records in the western us

an explosive convective cloud system and its environmental conditions in mjo initiation observed during dynamo an unusually large explosive convective cloud system was observed over the equatorial indian ocean on 28 november 2011 during the dynamo dynamics of the madden julian oscillation mjo field campaign the significance of this mesoscale convective system mcs is its size and explosive development of cold cloud tops 96 c during the initiation of a strong mjo event observations from the dynamo show that the large mcs developed within a well defined synoptic scale cyclonic circulation associated with an equatorial low pressure system with characteristics of a mixed rossby gravity wave that dominated the flow in the dynamo array prior to the development of the mcs the equatorial flow was characterized by strong vertical wind shear with low level westerlies and upper level easterlies a region of decreased wind shear and enhanced upper level divergence emerged concurrently with the passage of the westward moving mixed rossby gravity wave related low pressure system and convective activity in situ sounding observations suggest that widespread deep convection upstream of the large mcs may have contributed to the reduction of the upper level easterlies through vertical momentum transport and convective outflow both the reduction in vertical wind shear and enhanced low level convergence induced by the equatorial low pressure system created a favorable environment for the rapid development of the mcs this study examines the development of the mcs and the associated synoptic scale equatorial low pressure system within the large scale mjo circulation using in situ sounding observations from dynamo which provide new insights into the interaction between convection and environmental flow during mjo initiation over the equatorial indian ocean

spatiotemporal patterns of fault slip rates across the central sierra nevada frontal fault zone abstract patterns in fault slip rates through time and space are examined across the transition from the sierra nevada to the eastern california shear zone walker lane belt at each of four sites along the eastern sierra nevada frontal fault zone between 38 and 39 n latitude geomorphic markers such as glacial moraines and outwash terraces are displaced by a suite of range front normal faults using geomorphic mapping surveying and 10 be surface exposure dating mean fault slip rates are defined and by utilizing markers of different ages generally 20 ka and 150 ka rates through time and interactions among multiple faults are examined over 10 4 10 5 year timescales at each site for which data are available for the last 150 ky mean slip rates across the sierra nevada frontal fault zone have probably not varied by more than a factor of two over time spans equal to half of the total time interval 20 ky and 150 ky timescales 0 3 0 1 mm year 1 mode and 95 ci at both buckeye creek in the bridgeport basin and sonora junction and 0 4 0 3 0 1 mm year 1 along the west fork of the carson river at woodfords data permit rates that are relatively constant over the time scales examined in contrast slip rates are highly variable in space over the last 20 ky slip rates decrease by a factor of 3 5 northward over a distance of 20 km between the northern mono basin 1 3 0 6 0 3 mm year 1 at lundy canyon site to the bridgeport basin 0 3 0 1 mm year 1 the 3 fold decrease in the slip rate on the sierra nevada frontal fault zone northward from mono basin is indicative of a change in the character of faulting north of the mina deflection as extension is transferred eastward onto normal faults between the sierra nevada and walker lane belt a compilation of regional deformation rates reveals that the spatial pattern of extension rates changes along strike of the eastern california shear zone walker lane belt south of the mina deflection extension is accommodated within a diffuse zone of normal and oblique faults with extension rates increasing northward on the fish lake valley fault where faults of the eastern california shear zone terminate northward into the mina deflection extension rates increase northward along the sierra nevada frontal fault zone to 0 7 mm year 1 in northern mono basin this spatial pattern suggests that extension is transferred from more easterly fault systems e g fish lake valley fault and localized on the sierra nevada frontal fault zone as the eastern california shear zone walker lane belt faulting is transferred through the mina deflection

quaternary glacial history of the central karakoram abstract the quaternary glacial history of the world s highest mountains the central karakoram is examined for the first time using geomorphic mapping of landforms and sediments and 10 be terrestrial cosmogenic nuclide surface exposure dating of boulders on the moraines and glacially eroded surfaces four glacial stages are defined the bunthang glacial stage 0 7 ma the skardu glacial stage marine oxygen isotope stage mis 6 or older the mungo glacial stage mis 2 and the askole glacial stage holocene glaciers advanced several times during each glacial stage these advances are not well defined for the oldest glacial stages but during the mungo and askole glacial stages glacial advances likely occurred at 16 11 13 5 and 0 8 ka the extent of glaciation in this region became increasingly more restricted over time in the braldu and shigar valleys glaciers advanced 150 km during the bunthang and skardu glacial stages while glaciers advanced 80 km beyond their present positions during the mungo glacial stage in contrast glaciers advanced a few kilometers from present ice margins during the askole glacial stage glacier in this region likely respond in a complex fashion to the same forcing that causes changes in northern hemisphere oceans and ice sheets teleconnected via the mid latitude westerlies and also to changes in monsoonal intensity

updated cosmogenic chronologies of pleistocene mountain glaciation in the western united states and associated paleoclimate inferences abstract surface exposure dating with terrestrial cosmogenic nuclides tcns has become the primary method for determining numerical ages of pleistocene mountain glacial deposits and landforms in the conterminous western united states u s and in numerous other glaciated settings worldwide recent updates to models of tcn production and scaling warrant a reconsideration of published exposure ages of moraines of the last two pleistocene glaciations and associated paleoclimate inferences previously reported tcn exposure ages of moraines are recalculated here using newer production rates and scaling models for nuclides helium 3 3he beryllium 10 10be aluminum 26 26al and chlorine 36 36cl in most cases yielding significant differences from originally reported ages recalculated tcn exposure ages of moraines of the penultimate glaciation display a high degree of variability for individual landforms particularly toward the younger end of age distributions suggesting that exposure history is affected by moraine denudation and that older age modes provide the best estimate of the depositional age of these moraines oldest exposure ages of penultimate glaciation moraines are well aligned among mountain ranges across the western u s and yield a mean of 138 13 ka indicating that mountain glaciation occurred in step with global ice volume maxima during marine oxygen isotope stage 6 on average terminal moraines of the last glaciation date to 19 5 2 3 ka and correspond to the latter part of the global last glacial maximum lgm down valley recessional moraines representing prolonged glacial stabilizations or readvances to 75 of maximum lengths have a mean exposure age of 17 0 1 8 ka suggesting that these moraine positions were last occupied during heinrich stadial 1 evidence for multiple glacial culminations during the last glaciation is found in several mountain ranges and likely reflects at least two phases of late pleistocene climate an earlier phase when glaciers attained their maximum length in response to cooling during the lgm and a later phase when glaciers persisted at or readvanced to near maximum lengths in response to sustained cold temperatures and or increased precipitation

natural controls of fluvial denudation rates in major world drainage basins we present a new compilation of estimates of modern rates of mechanical and chemical denudation for externally drained basins exceeding 5 105 km2 in area these estimates are based on sediment and solute load data selected in order to represent natural rates as far as possible chemical denudation rates have been calculated by deducting the nondenudational component of solute load mechanical denudation rates range from 1 mm kyr 1 for the st lawrence and dnepr basins to 670 mm kyr 1 for the brahmaputra basin chemical denudation rates vary from 1 mm kyr 1 kolyma niger nile and rio grande basins to 27 mm kyr 1 chiang jiang basin the kolyma basin has the lowest 4 mm kyr 1 and the brahmaputra basin the highest overall rate of denudation 688 mm kyr 1 relationships between denudation rates and a range of morphometric hydrologic and climatic variables are investigated through correlation and regression analysis morphometric variables such as mean local relief are accurately calculated for large basins for the first time by using the national geophysical data center 10 minute topographic database variables expressing basin relief characteristics and runoff are found to be most strongly associated with both mechanical and chemical denudation rates with more than 60 of the variance in total denudation being accounted for by basin relief ratio and runoff basin area runoff variability and mean temperature however are only weakly associated with rates of denudation although direct comparisons cannot be made it appears that rates of basin denudation derived from present day mass flux estimates are not overall significantly different from estimates of long term rates based on sediment volume and thermochronologic data it therefore appears that the key factors identified as controlling denudation rates here are also applicable to the geological time spans relevant to the interaction between tectonic and denudational processes

constraints on the late quaternary glaciations in tibet from cosmogenic exposure ages of moraine surfaces abstract this contribution provides new constraints on the timing of tibetan glacial recessions recorded by the abandonment of moraines we present cosmogenic radionuclide 10 be inventories at 17 sites in southern and western tibet 32 crests 249 samples and infer the range of permissible emplacement ages based on these analyses individual large embedded rock and boulder samples were collected from the crests of moraine surfaces and analyzed for 10 be abundance we consider two scenarios to interpret the age of glacial recession leading to the moraine surface formation from these sample exposure ages 1 erosion of the moraine surface is insignificant and so the emplacement age of the moraines is reflected by the mean sample age and 2 erosion progressively exposes large boulders with little prior exposure and so the oldest sample age records the minimum moraine emplacement age we found that depending on the scenario chosen the moraine emplacement age can vary by 50 for 100 ka old samples we consider two scaling models for estimating the production rates of 10 be in tibet which has an important although lesser effect on inferred moraine ages while the data presented herein effectively increase the database of sample exposure ages from tibet by 20 we find that uncertainties related to the interpretation of the 10 be abundance within individual samples in terms of moraine emplacement ages are sufficient to accommodate either a view in which glacial advances are associated with temperature minima or precipitation maxima that are recorded by independent paleoclimate proxies a reanalysis of published data from moraines throughout tibet shows that the variation we observe is not unique to our dataset but rather is a robust feature of the tibetan moraine age database thus when viewed in a similar way with other samples collected from this area uncertainties within moraine exposure ages obscure attribution of tibetan glacial advances to temperature minima or precipitation maxima our work suggests that more reliable chronologies of tibetan glaciations will come from improvements in production rate models for this portion of the world as well as a better understanding of the processes that form and modify these geomorphic surfaces

quantitative bedrock geology of brazil 1 we quantitatively analyze the area age distribution of sedimentary igneous metamorphic and ultramafic bedrock on the basis of data from the digital geologic map of brazil published as a gis map by the brazilian geological survey bedrock units exclusively encompassing sedimentary rocks igneous rocks or metamorphic rocks cover 40 4 31 5 and 17 7 respectively of the total bedrock area these numbers have to be considered minimum estimates of the areal abundance of sedimentary igneous and metamorphic bedrock because polygons defined by mixed lithologies cover 8 5 9 5 of the total bedrock area these mixed units are sedimentary rocks with igneous and or metamorphic contributions 1 4 metamorphic rocks with sedimentary contributions 1 2 metamorphic rocks with igneous contributions 1 5 igneous rocks with sedimentary and or metamorphic contributions 4 4 and ultramafic units with sedimentary igneous and or metamorphic contributions 1 2 the average ages of major lithologic units weighted according to bedrock area are as follows sedimentary rocks average stratigraphic age of 248 5 1 myr median stratigraphic age of 87 5 myr igneous rocks 1153 13 1 myr metamorphic rocks 1678 30 1 myr and ultramafic rocks 1227 25 1 myr the average bedrock age of brazil is 946 7 1 myr the range in lithologic composition and age structure of the various bedrock units reflects the complex tectonic makeup of brazil that ranges from neogene sedimentary cover in the amazon basin to precambrian cratons guyana and brazilian shields and transamazonian greenstone belts the average spatial resolution of the data is 232 km2 polygon 1 and is sufficient to perform area age analyses of individual river drainage basins larger than 5 000 km2

estimating rates of denudation using cosmogenic isotope abundances in sediment we propose as a testable hypothesis a basin scale approach for interpreting the abundance of in situ produced cosmogenic isotopes an approach which considers explicitly both the isotope and sediment flux through a drainage basin unlike most existing models which are appropriate for evaluating in situ produced cosmogenic isotope abundance at discrete points on earth s surface our model is designed for interpreting isotope abundance in sediment because sediment is a mixture of materials in favourable cases derived from throughout a drainage basin we suggest that measured isotope abundances may reflect spatially averaged rates of erosion we investigate the assumptions and behaviour of our model and conclude that it could provide geomorphologists with a relatively simple means by which to constrain the rate of landscape evolution if a basin is in isotopic steady state and if sampled sediments are well mixed

latest pleistocene advance of alpine glaciers in the southwestern uinta mountains utah usa evidence for the influence of local moisture sources cosmogenic surface exposure 10 be dating of last glacial maximum lgm moraines indicates that glaciers in the southwestern uinta mountains remained at their maximum positions until ca 16 8 0 7 ka 2 k y after glaciers in the neighboring wind river range and colorado rockies began to retreat the timing of the local lgm in the south western uintas overlaps with both the hydrologic maximum of lake bonneville and pre liminary estimates of the local lgm in the western wasatch mountains this broad syn chroneity indicates that lake bonneville and glaciers in northern utah were responding to similar climate forcing furthermore equilibrium line altitudes elas for reconstruct ed lgm alpine glaciers increase with distance from the lake bonneville shoreline rising from 2600 m to 3200 m over the 120 km length of the glaciated uintas this pro nounced ela gradient suggests that the magnitude of the latest pleistocene glacial advance in the western uintas was due at least in part to enhanced precipitation derived from lake bonneville thus the lake acted as a local amplifier of regional climate forcing this relationship underscores the sensitivity of alpine glaciers to moisture availability during the latest pleistocene and further demonstrates the importance of local moisture sources on glacier mass balance

large magnitude late holocene earthquakes on the genoa fault west central nevada and eastern california the genoa fault a principal normal fault of the transition zone between the basin and range province and the northern sierra nevada displays a large and conspicuous prehistoric scarp three trenches excavated across this scarp exposed two large displacement late holocene events two of the trenches contained multiple layers of stratified charcoal yielding radiocarbon ages suggesting the most recent and penultimate events on the main part of the fault occurred 500 600 cal b p and 2000 2200 cal b p respectively normal slip offsets of 3 5 5 m per event along much of the rupture length are comparable to the largest historical basin and range province earthquakes suggesting these paleoearthquakes were on the order of magnitude 7 2 7 5 the apparent late holocene slip rate 2 3 mm yr is one of the highest in the basin and range province based on structural and behavioral differences the genoa fault is here divided into four principal sections the sierra diamond valley carson valley and jacks valley sections and is distinguished from three northeast striking faults in the carson city area the kings canyon carson city and indian hill faults the conspicuous scarp extends for nearly 25 km the combined length of the carson valley and jacks valley sections the diamond valley section lacks the conspicuous scarp and older alluvial fans and bedrock outcrops on the downthrown side of the fault indicate a lower activity rate activity further decreases to the south along the sierra section which consists of numerous distributed faults all three northeast striking faults in the carson city area ruptured within the past few thousand years and one or more may have ruptured during recent events on the genoa fault

worldwide distribution of continental rock lithology implications for the atmospheric soil co2 uptake by continental weathering and alkalinity river transport to the oceans 1 the silicate rock weathering followed by the formation of carbonate rocks in the ocean transfers co2 from the atmosphere to the lithosphere this co2 uptake plays a major role in the regulation of atmospheric co2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks this leads us to develop the first world lithological map with a grid resolution of 1 1 this paper analyzes the spatial distribution of the six main rock types by latitude continents and ocean drainage basins and for 49 large river basins coupling our digital map with the gem co2 model we have also calculated the amount of atmospheric soil co2 consumed by rock weathering and alkalinity river transport to the ocean among all silicate rocks shales and basalts appear to have a significant influence on the amount of co2 uptake by chemical weathering

middle pleistocene glaciation in patagonia dated by cosmogenic nuclide measurements on outwash gravels the well preserved glacial record in argentine patagonia offers a 1 ma archive of terrestrial climate extremes in southern south america these glacial deposits remain largely undated beyond the range of radiocarbon dating at ca 40 ka dating old glacial deposits several 105 a by cosmogenic surface exposure methods is problematic because of the uncertainty in moraine degradation and boulder erosion rates here we show that cobbles on outwash terraces can reliably date old glacial deposits in the lago pueyrredon valley 47 5 s argentina favorable environmental conditions e g aridity and strong winds have enabled continuous surface exposure of cobbles and preservation of outwash terraces the data demonstrate that nuclide inheritance is negligible and we therefore use the oldest surface cobbles to date the deposit 10be concentrations in outwash cobbles reveal a major glacial advance at ca 260 ka concurrent with marine isotope stage 8 mis 8 and dust peaks in antarctic ice cores a 10be concentration depth profile in the outwash terrace supports the age and suggests a low terrace erosion rate of ca 0 5 mm ka 1 we compare these data to exposure ages obtained from associated moraines and find that surface boulders underestimate the age of the glaciation by 100 ka thus the oldest boulders in this area do not date closely moraine deposition the 10be concentration in moraine cobbles help to constrain moraine degradation rates these data together with constraints from measured 26al 10be ratios suggest that all moraine boulders were likely exhumed after original deposition we determine the local last glacial maximum lgm occurred at 27 25 ka consistent with the maximum lgm in other parts of patagonia

surface dating of dynamic landforms young boulders on aging moraines the dating of landforms is crucial to understanding the evolution history and stability of landscapes cosmogenic isotope analysis has recently been used to determine quantitative exposure ages for previously undatable landform surfaces a pioneering application of this technique to date moraines illustrated its considerable potential but suggested a chronology partially inconsistent with existing geological data consideration of the dynamic nature of landforms and of the ever present processes of erosion deposition and weathering leads to a resolution of this inconsistency and more generally offers guidance for realistic interpretation of exposure ages

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr