calcic amphibole equilibria and a new amphibole plagioclase geothermometer there is currently a dearth of reliable thermobarometers for many hornblende and plagioclase bearing rocks such as granitoids and amphibolites a semi empirical thermodynamic evaluation of the available experimental data on amphibole plagioclase assemblages leads to a new thermometer based on the aliv content of amphibole coexisting with plagioclase in silica saturated rocks the principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is left na square 1 right a left alsi 1 right t1 we have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1 edenite 4 quartz tremolite albite 1 2 pargasite 4 quartz hornblende albite 2 the equilibrium relation for both 1 and 2 leads to the proposed new thermometer t frac 0 677p 48 98 y 0 0429 0 008314 ln k and k left frac si 4 8 si right x ab plag where si is the number of atoms per formula unit in amphiboles with p in kbar and t in k the term y represents plagioclase non ideality rtln ab from darken s quadratic formalism dqf with y 0 for xab 0 5 and y 8 06 25 5 1 xab 2 for xab 0 5 the best fits to the data were obtained by assuming complete coupling between al on the t1 site and na in the a site of amphibole and the standard deviation of residuals in the fit is 38 c the thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende plagioclase assemblages with uncertainties of around 75 c for rocks equilibrated at temperatures in the range 500 1100 c the thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than an92 and with amphiboles containing less than 7 8 si atoms pfu good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry

magnetic susceptibility of soil an evaluation of conflicting theories using a national data set magnetic susceptibility values for topsoils across england arc combined with data for soil type geochemistry and concentrations of magnetotactic bacteria in order to evaluate different theories for explaining soil magnetism strongly magnetic soils in unpolluted areas are found over weakly magnetic substrates and are dominated by ultrafine superparamagnetic grains magnetotactic bacteria are present in insufficient concentrations to account for strongly magnetic soils and crop burning is discounted as a major factor a small number of samples show high values associated with either airborne magnetic particulates from pollution or residual primary ferrimagnetic minerals from igneous substrates the results are used to construct a new mechanism for the formation of secondary ferrimagnetic minerals that links abiological weathering and biological fermentation processes the fundamental driving force in the mechanism is fe supply which may be linked to climate observed causative associations between climate and the magnetic susceptibility of loess palaeosol sequences are supported by the findings

calculated greenschist facies mineral equilibria in the system cao feo mgo al 2 o 3 sio 2 co 2 h 2 o the kinetic problems associated with the experimental determination of reactions among complex solidsolution phases at low temperatures have hindered our understanding of the phase relations in greenschist facies rocks in the absence of reliable experimental data we have used the new expanded internally consistent thermodynamic dataset of holland and powell 1990 to present calculated phase equilibria for the system cao feo mgo al2o3 sio2 h2o co2 cafmasch with quartz in eccess in the range 400 500 c at low to intermediate pressures involving the minerals amphibole chlorite anorthite clinozoisite dolomite chloritoid garnet margarite andalusite and calcite by solving independent sets of non linear equations formed from equilibrium relationships we calculate not only the loci of reactions in pressuretemperature x co2 space but also the compositions of coexisting minerals in terms of the substitutions femg 1 and fe mg sial 1al 1 invariant univariant and divariant equilibria are calculated and discussed in relation to naturally occurring greenschist facies metabasic and siliceous dolomitic mineral assemblages we thus avoid the use of activity corrected curves so commonly presented in the literature as a substitute for genuine univariant phase diagram boundaries

holocene fire vegetation and climate dynamics inferred from charcoal and pollen record in the eastern tibetan plateau abstract understanding fire history and its driving mechanisms can provide valuable insights into present fire regime intensity severity and frequency the interplay between vegetation and fire and trigger of fire activities here we reconstruct the holocene fire history in the zoige basin on the eastern tibetan plateau on the basis of sedimentary micro charcoal record over the last 10 0 ka 1 ka 1000 cal yr bp and discuss the influences of vegetation and climate on fire dynamics our results show that regional fire was active at 10 0 3 3 ka and a significant decrease in fire activity characterized the period after 3 3 ka the high regional fire frequency at 10 0 3 3 ka is consistent with the forested landscape suggested by high affinity scores of cool mixed forest biome mainly consisted of spruce implying that fire dynamics during this period was generally controlled by the variations of arboreal biomass and summer temperature during 6 3 4 6 ka the prevailing asian summer monsoon provided increased moisture to this region and thus suppressed fire activities to an extent despite the availability of abundant biomass declined tree biomass after 3 3 ka probably accounted for the decreased fire activities in addition two successive fire events at ca 3 5 3 3 ka were likely responsible for the subsequent abrupt decline of forest components in the landscape

large magnitude miocene extension in the central mojave desert implications for paleozoic to tertiary paleogeography and tectonics this is the published version copyright 1990 american geophysical union all rights reserved

variations in fire frequency and climate over the past 17 000 yr in central yellowstone national park a 17000 yr fire history from yellowstone national park demonstrates a strong link between changes in climate and variations in fire frequency on millennial time scales the fire history reconstruction is based on a detailed charcoal stratigraphy from cygnet lake in the rhyolite plateau region macroscopic charcoal particles were tallied from contiguous 1 cm samples of a 6 69 m long core and the data were converted to charcoal accumulation rates at evenly spaced time intervals intervals of high charcoal accumulation rates were interpreted as local fire events on the basis of information obtained from modern charcoal calibration studies in the yellowstone region the record indicates that fire frequency was moderate 4 fires 1000 yr during the late glacial period reached highest values in the early holocene 10 fires 1000 yr and decreased after 7000 calendar yr b p the present fire regime 2 3 fires 1000 yr was established in the past 2000 yr the charcoal stratigraphy correlates well with variations in july insolation through time which suggests that regional climate changes are responsible for the long term variations in fire frequency in the early holocene summer insolation was near its maximum which resulted in warmer effectively drier conditions throughout the northwestern united states at this time the fire frequency near cygnet lake was at its highest after 7000 calendar yr b p summer insolation decreased to present values the regional climate became cooler and wetter and fires were less frequent the cygnet lake record suggests that long term fire frequencies have varied continuously with climate change even when the vegetation has remained constant

evidence for mediaeval soil erosion in the south hams region of devon uk a major theme of research into the causes of past and present soil erosion has been to determine the relative importance of climate and land use change in influencing holocene erosion rates previous work suggests that land use change especially the conversion of woodlands into agricultural land is the main factor influencing long term increases in soil erosion a study of an extensive minerogenic sediment deposit in a wetland at slapton ley in devon suggests that agricultural intensification occurred before the onset of sedimentation a silty clay layer c 40 cm thick in the valley bottom wetland of the slapton sewage works marsh the base of the silty clay layer lies at an altitude of between 2 2 and 2 6 m aod and has been radiocarbon dated at two locations conventional radiocarbon ages 2 were 910 160 yr and 960 140 yr bp successful radiocarbon dating of the upper surface of this minerogenic layer at one location yielded a conventional radiocarbon age of 730 120 yr bp within the errors

pennsylvanian and early permian paleogeography of east central california implications for the shape of the continental margin and the timing of continental truncation pennsylvanian and early permian paleogeographic features in east central california include a southeast trending carbonate shelf edge and turbidite basin that we infer paralleled a segment of the western margin of the north american continent this segment of the continental margin was oblique to an adjoining segment on the north that trended southwestward across nevada into easternmost california we propose that the southeast trending segment of the margin originated by tectonic truncation of the originally longer southwest trending segment in early or middle pennsylvanian to late early permian time significantly earlier than a previously hypothesized late permian or early triassic continental truncation event we interpret the truncating structure to have been a sinistral transform fault zone along which a continental fragment was removed and carried southeastward into the caborca hermosillo region of northern mexico where it is now represented by exposures of late proterozoic and paleozoic miogeoclinal rocks

the use of mineral magnetism in the reconstruction of fire history a case study from lago di origlio swiss alps abstract the use of magnetic measurements in the detection of fire signals has been neglected since the work of rummery et al 1979 yet considerable developments have been made in the interpretation of magnetic measurements over the last 16 years this paper presents a study of the fire history of lago di origlio in the southern swiss alps the study utilises the technique of mineral magnetism alongside the stratigraphic pollen spore and charcoal records correlation between the various proxy records indicates that a magnetic fire record is present within the sediments for the last 4 ka the magnetic fire record has a distinct mineralogical and magnetic grain size signature that can be recognised against the background sedimentary signal the results suggest that magnetic measurements may be usefully employed in the reconstruction of fire history their application is rapid and non destructive and the results may provide additional information in relation to the links between catchment fire events and the sedimentary record

characterisation of soils by mineral magnetic measurements processes of iron mineral authigenesis diagenesis and dissolution in interaction with primary inputs of iron minerals act to produce vertical differentiation of soil magnetic properties thus magnetic iron oxides in common with other iron forms may both respond to and reflect soil forming processes investigation of the degree of detectable and persistent magnetic differentiation within different soil materials can provide insights into the nature and direction of distinct sets of soil processes evidence of clear association between magnetic variation and discrete types of soil environment also has applied significance in several areas of environmental study for example the observed magnetic variation can be used to discriminate between individual soils and soil horizons for the purposes of soil surveying sediment tagging and tracing and empirical modelling of source sediment linkages this paper reports the use of mineral magnetic techniques to characterise the iron oxide assemblages within soils of varying type and provenance the effect of different pedogenic regimes have been investigated through measurement of a range of magnetic parameters including magnetic susceptibility frequency dependent susceptibility and anhysteretic and saturation remanences contrary to current pedological thought but in accord with other magnetically based studies e g mullins the presence of magnetite maghemite typically of superparamagnetic single domain grain size within soils has been found to be an extremely widespread phenomenon its contemporary neoformation within the soil environment is inferred however the input of artificially generated ferrimagnetic material is indicated for one of the magnetically enhanced soils examined here and the possibility of extraneous sources of magnetite in topsoils should be considered before pedogenic processes of magnetic enhancement are inferred from the data presented discrimination of industrially derived magnetic particles appears possible on the basis of their very low levels of frequency dependent susceptibility fd low anhysteretic remanence arm and dissociation from the finer clay sized not the formation but the active dissolution of ferrimagnetic minerals is indicated for soils affected by the processes of gleying and podsolisation eluvial horizons of podsolised profiles demonstrate almost total loss of detectable magnetic content variations in the form of the iron reprecipitated in the underlying illuvial layers may reflect differences in the pedological characteristics of these horizons within gleyed waterlogged soils processes of magnetite dissolution appear to be grain size specific preferential reduction of those grains of ultrafine superparamagnetic and fine single domain magnetic grain size is indicated where soil forms a major contributor to catchment sediment loads this has significance in the area of source sediment modelling soil derived from permanently wet or highly leached catchment areas is unlikely to contribute to peaks in the magnetic content of deposited sediments further only those erosional processes that operate at a faster rate than those of magnetic depletion will have the capacity to produce peaks in sediment ferrimagnetic concentrations such as have been observed within the historical magnetic record

late holocene floodplain incision and alluvial fan formation in the central grampian highlands scotland chronology environment and implications a section cut across an alluvial fan and the underlying floodplain terrace in the central grampian highlands provides an unusually complete record of late holocene events at ca 2 7 2 4 cal kyr bp floodplain aggradation was replaced by net floodplain incision pollen evidence and charcoal counts provide no evidence for contemporaneous anthropogenic landscape change and the timing of the transition suggests that it reflects an increase in high magnitude erosive flood events following overall climatic deterioration the overlying fan was deposited by torrential hyperconcentrated flows during three brief storm generated depositional events at ca 2 2 2 1 1 9 1 8 and 0 9 0 7 cal kyr bp separated and succeeded by prolonged periods of stability and peat accumulation during these three events a cumulative total of ca 6750 m3 of sediment was deposited probably in no more than a few hours over a timescale of two millennia these findings imply that proposed links between human activity and the development of alluvial fans or debris cones require reassessment and that different elements of the holocene alluvial landscape have responded in different ways to the same climatic inputs aggregation of dating evidence relating to aggradation or incision of alluvial landforms at different scales therefore may produce misleading results copyright 1999 john wiley sons ltd

geoarchaeological records in temperate european river valleys quantifying the resource assessing its potential and managing its future throughout the quaternary episodes of glaciation and associated low sea level have resulted in the connection of the terrestrial landmasses of britain and mainland europe the river systems that established themselves across these newly emergent land surfaces of the coastal plain would have created important migration corridors for both animals and humans a point corroborated by the affinity of palaeolithic remains across britain and europe technological developments within the last decade have allowed these now submerged valley floors and adjacent terrestrial landscapes associated with the last cold stage and early and middle parts of the current holocene interglacial to be explored and their archaeological legacies unravelled providing geoarchaeologists with an opportunity to contribute to major cultural debates however in order to maximize knowledge it is essential that geoarchaeologists working within river valleys across both britain and the european continent are addressing similar research questions by collecting data using comparable methodologies this paper reviews the approach taken in different regions and provides a baseline assessment to allow the development of a coherent european wide framework for alluvial geoarchaeology and geoprospection particularly with respect to the holocene record

morava river floodplain development during the last millennium stražnicke pomoravi czech republic floodplain sediments deposited along the lower course of the morava river eastern czech 206 pb 207 pb and pop ddt pcb analysis and 137 cs activity data thecu trienmethodwasusedforstratigraphicallycorrelatingthesedepositsbasedonthevariationofexpand able clay minerals in the sediments the resulting stratigraphic pattern reveals the alluvial history of the cur rently active river channel system since the end of the first millennium ad fine overbank clayey sediments deposited during the mediaeval warm period were eroded from cultivated fields newly formed during mediaeval colonization between 1250 and 1450 these fine deposits are overlain by coarser floodplain sedi mentsofthe littleiceage indicatingachangeinthesedimentsourcesincethesixteenthcentury ad anda substantial increase in the sediment load in the second half of twentieth century the straunicke pomoravi floodplaindepositsrepresentavaluablepalaeoenvironmentalarchiveofthelastmillennium containingrecords offluvialprocessesconsiderablyalteredbyhumanactivities

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

geochemical tools for the stratigraphic correlation of floodplain deposits of the morava river in stražnicke pomoravi czech republic from the last millennium article i nfo the floodplain of the morava river in stra nicke pomoravi south east moravia czech republic contains a very valuable record of regional environmental change which goes back to several thousand years its interpretation has been limited by poor stratigraphic correlation and dating of the sediments we present a geochemical solution to this challenge we studied 8 outcrops of floodplain deposits from 4 localities along a 6 km long part of the current meander belt of the morava river in stra nicke pomoravi using geochemical proxy analyses magnetic susceptibility measurements 14 c dating of wood remnants and sediment micromorphology the proxy methods are based on elemental analysis edxrf and analysis of the cation exchange capacity of clay minerals granulometric analyses provided the basis for lithological and facies assignment of the sediments our geochemical and mineralogical interpretations have further been tested by microstratigraphically studying the optical properties of the fine fraction horizons older than about 3 centuries were 14

environmental changes since 13 000 cal bp reflected in magnetic and sedimentological properties of sediments from lake holzmaar germany abstract late and postglacial sediments from a german maar lake holzmaar in the west eifel volcanic field were investigated with rockmagnetic and sedimentological methods the concentration of magnetic minerals is very variable and zones exist where magnetic properties are dominated by paramagnetic minerals remanence carrying ferromagnetic minerals are dominated by a low coercivity fraction identified by its curie temperature and by scanning electron microscopy as titanomagnetite with a grain size range between a few microns and 25 m an additional high coercivity phase probably hematite is present throughout the entire record in low concentrations rockmagnetic results indicate the possible postdepositional dissolution of titano magnetite and the authigenic growth of greigite in one confined section a positive correlation between initial magnetic susceptibility and sediment accumulation rate and between and the non arboreal pollen percentage is observed while and organic carbon content show a negative correlation these coeval variations are due to a nearly exclusively detrital sedimentary input the variations show a broad dependence on climatic conditions for times prior to significant human activities in the region and the impact of the human settlement history afterwards early holocene climatic variations indicating two cooling events agree with ice rafted debris events of the north atlantic

the use of magnetic susceptibility to measure long term soil redistribution abstract several studies have documented the severity of recent soil erosion on the canadian prairies where cultivation started about a century ago little quantitative information is available on erosion before 1960 this study attempts to quantify post and pre 1960 soil erosion in a small cultivated basin near saskatoon saskatchewan canada by measuring 137cs and magnetic susceptibility distribution with depth soil cores were collected along six transects three across closed depressions and three across the drainage channel in the cultivated field and one transect across an uncultivated depression the cores were sliced into 3 cm layers and the soil analyzed for 137cs and organic and inorganic c high variability in with depth in eroding areas as indicated by 137cs made it impossible to use to quantify past soil losses in these locations however these eroding upper and middle slope positions have a much higher than lower slope areas where soil deposition occurs and where the variability in with depth could be used to estimate soil deposition estimating soil deposition from the vs depth profiles was more successful in the closed depressions than in the drainage channel where the profiles may reflect the variable source areas of the materials rather than the pedological conditions the data indicated that soil deposition since 1960 has been about 30 to 50 of that prior to 1960 this suggests that soil deposition rates and by implication soil erosion rates have been relatively steady since cultivation of these soils started although there are clear indications that the spatial pattern of deposition has varied

non synchronous response of adjacent floodplain systems to holocene environmental change abstract two adjacent upland floodplains are compared to establish the local response of floodplains to environmental change radiocarbon dating the analysis of sedimentary exposures terrace mapping aerial photography and archaeological evidence are used to examine late quaternary valley fill sediments on the afon tanat and the afon vymwy in the upper severn basin wales uk the alluvial stratigraphy of the two floodplain systems consists of late devensian last glacial fluvio glacial sediments at the valley margins deposited under a braided outwash river regime holocene age terraces with floodplain and palaeochannel deposits composed of gravels overlain by silty sands are inset into this older unit and were formed by meandering fluvial channel systems a combination of field and laboratory data demonstrates that from the mid late holocene the two floodplain systems had divergent development the afon vyrnwy has remained vertically stable for the last ca 4000 yrs whereas the adjacent afon tanat continued to be vertically and laterally active both floodplain systems have been affected by late quaternary climatic fluctuations and anthropogenic activity from the bronze age to the roman period but local geomorphic gradient controls combined with a possibly greater focus of anthropogenic activity in the tanat catchment may explain the differential evolution of the two systems the data demonstrate that multiple reach scale studies are essential for revealing significant stages in the chronology and historical development of fluvial systems

paleomagnetism geochronology and possible tectonic rotation of the middle miocene barstow formation mojave desert southern california the barstow formation of the central mojave desert in the mud hills consists of 965 m of epiclastic and volcanic sediments deposited in an extensional rift this widespread unit is of importance because its mammalian faunas characterize the barstovian land mammal age previous isotopic ages on interbedded tuffs have been used to calibrate middle miocene time and it was deposited in a tectonically active region volcanic ash samples were collected from the barstow formation for isotopic dating using conventional 40 k 40 ar and 40 ar 39 ar single crystal laser fusion methods multiple analyses on minerals from 5 superposed tuffs yielded the following mean 40 k 40 ar ages rak tuff at 16 3 0 3 m y oreodont tuff at 15 9 0 2 m y dated tuff at 14 8 0 2 m y hemicyon tuff at 14 0 0 1 m y and lapilli tuff at 13 4 0 2 m y similar mean ages were also obtained on the following tuffs using single crystal 40 ar 39 ar laser fusion methods oreodont tuff at 15 9 0 06 m y dated tuff at 14 8 0 06 m y and hemicyon tuff at 14 0 0 09 m y in order to develop a local magnetostratigraphy 272 paleomagnetic sites were collected from 7 important sections within the mud hills saturation isothermal remanence experiments suggested the presence of either hematite or magnetite as carriers of remanence after either stepwise alternating field or thermal demagnetization characteristic directions were usually isolated between 30 and 50 mt or 300 and 500 c positive reversal and fold tests indicate a stable remanence acquired prior to folding there are 21 magnetic polarity zones interpreted from the continuous part of the barstow formation in the study area by using the 5 isotopic ages determined from interbedded tuffs the local composite polarity pattern is correlated to the top of chron c5d through the bottom of c5ab on the magnetic polarity time scale or an age range from 17 6 to 13 4 m y the boundary between hemingfordian and barstovian land mammal ages herein occurs 30 m below the oreodont tuff 15 8 0 2 m y within c5b and is taken at about 16 0 m y ago the barstow formational mean declination is 347 1 inc 47 5 95 6 51 d 7 97 suggesting negligible or possibly slight counterclockwise rotation r 4 1 r 10 5 for this part of the mojave block during the past 18 m y a 40 ar 39 ar mean age of 19 3 0 02 m y for the red tuff which is near the local base of the barstow formation and unconformably underlying our composite section constrains a minimum age for the beginning of local basin filling with principally epiclastic sediments

variations in sediment yield from the upper doubs river carbonate watershed jura france since the late glacial period abstract the upper doubs river valley is a 910 km2watershed feeding into lake chaillexon the lake was formed by a natural rockfall at the end of the bolling chronozone around 14 250 cal yr b p and since then has trapped material eroded from the watershed the filling process and variations in sediment yield have been investigated by mechanical coring seismic surveys and electric soundings the detrital sediment yield of the upstream watershed can be calculated by quantifying the sedimentary stocks for each climatic stage of the late glacial period and holocene epoch and estimating the lake s entrapment capacity this enables us to determine the intensity of the erosion processes in relation to climate and environmental factors the bolling allerod interstade produced the greatest yields with mean values of 19 500 metric tons per calendar year t yr the younger dryas chronozone saw a sharp fall 8900 t yr that continued into the preboreal 2100 t yr clastic supply increased during the boreal 4500 t yr before declining again in the early atlantic 2400 t yr since then yields have risen from 4500 t yr in the late atlantic to 6800 t yr in the subboreal and 11 100 t yr in the subatlantic comparison of quantitative data with the qualitative analysis of the deposits and with the paleohydrologic curve of the watershed based on level fluctuations in lakes around chaillexon shows that climate was the controlling factor of sediment yield until the late atlantic from the late atlantic subboreal around 5400 cal yr b p 470014c yr b p and especially from the end of the subboreal chronozone and during the subatlantic chronozone 2770 cal yr b p 270014c yr b p present climatic constraints have been compounded by human activity related to forest clearing and land use

magnitude and significance of miocene crustal extension in the central mojave desert california the newly recognized waterman hills detachment fault whdf of the central mojave desert california is significant because it provides the first unambiguous evidence for large scale core complex style crustal extension in the central mojave desert and because it has significantly rearranged the pre miocene paleogeography of the mojave desert the whdf places steeply dipping to overturned miocene volcanic and sedimentary rocks upon mylonitic pre tertiary basement the mylonites which apparently formed during extension are predominantly l tectonites which manifest top to northeast shear the whdf dips to the northeast beneath dominofaulted ranges of the central mojave desert and detachment faults of the colorado river trough forming an imbricated early miocene system of detachment faults extension continued in the colorado river trough after extension had ceased in the central mojave desert tentative correlations of mesozoic intrusions suggest about 40 km of slip across the whdf which carries eugeoclinal paleozoic rocks in its hanging wall and cratonal miogeoclinal paleozoic rocks in its footwall restoration of 40 km of slip 1 removes a prominent kink in the boundary between eugeoclinal and cratonal miogeoclinal facies 2 aligns cratonal miogeoclinal strata near victorville more closely with the late paleozoic continental margin farther north 3 places cratonal miogeoclinal rocks structurally beneath eugeoclinal rocks implying that the facies were stacked by thrusting and 4 straightens the western margin of the late jurassic independence dike swarm

river sedimentation and fluvial response to holocene environmental change in the yorkshire ouse basin northern england holocene alluvial sediment sequences and floodplain geomorphological histories from three valley floor sequences in the yorkshire ouse basin have been determined using a variety of field evidence and laboratory methods geomorphological mapping sediment coring and trenching physical and chemical analysis of sedimentary units and 14c dating the application of multiple methods has revealed contrasting phases of river erosion alluviation and depositional styles during the late quaternary at each of the individual study reaches the gravel bedded river swale at catterick has been characterized by progressive valley floor incision during the late holocene with major phases of incision occurring during the cooler and wetter phase of the little ice age within the last c 245 years substantial thicknesses up to 2 m of overbank alluvium contaminated by metalliferous mining have been deposited across thefloodplain in contrast floodplain development in the lower reaches of the river swale at myton and t

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

u th pb rb sr and ar ar mineral and whole rock isotopic systematics in a metamorphosed granitic terrane southeastern california mesozoic structural domes are developed in an older proterozoic crystalline basement of granitic to granodioritic foliate metaplutonic rocks in the halloran hills southeastern california isotopic analyses of whole rocks and mineral separates from these rocks by u th pb rb sr and ar ar techniques yield a complex pattern of discordance that is the result of a fairly simple geologic history individual mineral isotopic systems have variably equilibrated with each other in response to mesozoic regional metamorphism and locally to later heating during mesozoic batholith emplacement discordant u th pb zircon data indicate that the granitic core rocks are 1 710 ma and that one dioritic phase may be slightly older rb sr whole rock model dates scatter about 1 700 ma rb sr amphibole whole rock and u th pb amphibole dates are also proterozoic potassium feldspars retain a 207 pb 206 pb signature of their proterozoic age ar ar amphibole spectra from the flank of the main dome reveal disturbed dates of 1 450 ma to 1 100 ma and the dates become younger toward the structurally deeper core of the dome all remaining isotopic determinations yield mesozoic or younger dates for mineral whole rock systems rb sr whole rock apatite feldspar biotite analyses show nonequilibration of strontium isotopes with resultant mineral pair dates from 4 foliate plutonic rocks ranging from 200 to 50 ma no single metamorphic age is indicated by the rb sr data rb sr whole rock biotite dates are consistently younger than any other determinations and may be reduced by weathering or gain of nonradiogenic strontium from ground water u pb sphene and apatite analyses from rocks that yield 1 710 ma zircon dates are nearly concordant at 140 ma an amphibole from the structurally deepest rocks of the main dome that yield 140 to 150 ma u pb sphene dates has an ar ar plateau date of 144 ma the u pb sphene and ar ar amphibole analyses are believed to be the best age estimate for the end of the highest temperature phase of regional metamorphism th pb sphene and apatite dates and ar ar biotite dates cluster at 90 5 ma as a consequence of regional cooling during late cretaceous time following extensive mesozoic plutonism in the region at 97 to 90 ma we interpret the discordant mineral date patterns to have resulted from metamorphism of 1 700 ma plutonic rocks during the jurassic 140 50 ma and subsequent uplift and cooling to 200 c at about 90 ma on the basis of this study the isotope dating systems ranked in decreasing order of resistance to resetting are u th pb zircon concordia intercept rb sr whole rock rb sr amphibole u th pb amphibole pb pb whole rock ar ar amphibole rb sr sphene u pb sphene and apatite rb sr plagioclase potassium feldspar apatite th pb sphene and apatite ar ar biotite u pb feldspars rb sr biotite

the record of land use change and soil erosion in the late holocene sediments of the petit lac d annecy eastern france pollen chemical and magnetic analysis of a 6 m core from the petit lac d annecy provides a record of land use change and soil erosion spanning the last 2000 years variations in the magnetic properties of the sediments especially above 3 m in the core can be linked to the history of soil inwash over the last 1000 years these changes together with parallel ones in element concentrations and pollen frequencies both concentrations and percentages can be interpreted by reference to the land use history in the catchment during medieval and post medieval times peak deforestation and human impact on vegetation and soils is recorded from the eleventh to fourteenth centuries and again during the eighteenth century the present study suggests that future detailed analyses of the laminated sediments of the lake could provide a precisely datable fine resolution environmental history of the lake and its drainage basin

pre transform early miocene extension in western california earliest miocene 20 24 ma faulting and volcanism were widespread in western california including the san joaquin la honda basin coast ranges and transverse ranges evidence for strike slip on faults of this age is lacking their arrangement on a pre san andreas restoration suggests continuity with early miocene extended terranes in eastern california and southwestern arizona deformation was probably not an effect of transform or triple junction migration tectonics because it was synchronous over 300 km of latitude and there is no direct evidence for a transform boundary until late in early miocene time magnetic anomalies in the eastern pacific permit an interpretation that a spreading ridge supplied a still active subduction zone with young oceanic crust until sometime after 20 ma if so extension evidently took place in an arc related setting a transform regime probably formed about 18 ma when most basins underwent episodes of deepening or uplift strike slip faults became active and rotation of the western transverse ranges block began

stratigraphic charcoal analysis on petrographic thin sections application to fire history in northwestern minnesota results of stratigraphic charcoal analysis from thin sections of varved lake sediments have been compared with fire scars on red pine trees in northwestern minnesota to determine if charcoal data accurately reflect fire regimes pollen and opaque spherule analyses were completed from a short core to confirm that laminations were annual over the last 350 yr a good correspondence was found between fossil charcoal and fire scar data individual fires could be identified as specific peaks in the charcoal curves and times of reduced fire frequency were reflected in the charcoal data charcoal was absent during the fire suppression era from 1920 a d to the present distinct charcoal maxima from 1864 to 1920 occurred at times of fire within the lake catchment fire was less frequent during the 19th century and charcoal was substantially less abundant fire was frequent from 1760 to 1815 and charcoal was abundant continuously fire scars and fossil charcoal indicate that fires did not occur during 1730 1750 and 1670 1700 several fires occurred from 1640 to 1670 and 1700 to 1730 charcoal counted from pollen preparations in the area generally do not show this changing fire regime simulated sampling of the thin section data in a fashion comparable to pollen slide methods suggests that sampling alone is not sufficient to account for differences between the two methods integrating annual charcoal values in this fashion still produced much higher resolution than the pollen slide method and the postfire suppression decline of charcoal characteristic of my method but not of pollen slides is still evident consideration of the differences in size of fragments counted by the two methods is necessary to explain charcoal representation in lake sediments

petrology of felsic granulites metapelites metabasics ultramafics and metacarbonates from southern calabria italy prograde metamorphism uplift and cooling of a former lower crust

estimating the mass flux of charcoal from sedimentary records effects of particle size morphology and orientation reconstructing past fire requires ways to compare concentration estimates obtained by different charcoal analysis methods these methods estimate concentrations from area measurements or particle counts on pollen slides on sieves and on thin sections and they do so at different magnifications depending on particle properties each method might give substantially different results image analysis of charred particles in thin sections of lake sediments is used to parameterize relationships among different concentration estimators we determined distributions of particle size shape and orientation in undisturbed sediments and assessed how those distributions affect concentration estimates particle diameter distributions were conservative across different magnifications particle shape was also conservative with major minor axis ratios near 2 5 our derived relationships among estimators could be used to improve comparability for all but the smallest particle classes which tended to be unpredictable

geomorphological theory of the hydrological response the geomorphological theory of the hydrological response originated by rodriguez iturbe and valdes is reviewed with reference to the general framework of the formulation of transport processes by travel time distributions this review by no means exhaustive of the publications on this subject collects in a reasonably organic manner some relevant theoretical developments and a few significant applications

metamorphic evolution of exhumed middle to lower crustal rocks in the mojave extensional belt southern california usa the waterman metamorphic complex of the central mojave desert was exposed as a consequence of early miocene detachment dominated extension however it has evidence consistent with a more extensive geological history that involves collision of a crustal fragment s tectonic thickening by overthrusting and two periods of extension the metamorphic complex contains granitoid intrusives and felsic mylonitic gneisses as well as polymetamorphic rocks that include marble calc silicate quartzite mafic granulite pyribolite amphibolite migmatite and biotite schist the latter group of rocks was affected by an initial series of high grade metamorphic events m1 and m2 and a localized lower grade overprint m3 the initial metamorphism m1 can be separated into two stages along its high grade p t path m1a a granulite facies metamorphism at 800 850 c and 7 5 9 kbar and mlb an upper amphibolite facies overprint at 750 800 c and 10 12 kbar m1a developed mineral assemblages and textures consistent with granulite facies conditions at a reduced activity of h2o and is associated with intense ductile deformation d1 and minor local partial melting m1b overprinted the granulite assemblages with a series of hydrous phases under conditions of increasing pressure and h2o activity and is accompanied by little or no deformation m2 developed at lower pressures and temperatures 650 750 c 4 5 5 5 kbar and is distinguished by a second local overprint of hydrous phases that reflects an input of aqueous fluids probably associated with the intrusion of a series of granitic dykes and veins effects of m3 are confined to the mitchel detachment zone an anastomosing early miocene detachment fault and are characterized by local ductile brittle deformation d2 of the pre existing high grade rocks and granitoid intrusives and by the production of mylonites and mylonitic gneisses under greenschist facies conditions 300 350 c 3 5 kbar the initial overprint m1a represents metamorphism devolatilization and minor partial melting of supracrustal rocks under granulite facies conditions as a consequence of tectonic and possibly magmatic thickening the increasing pressure transition of m1a to m1b reflects a period of continued compressional tectonism thrusting and influx of h2o in part locally related to crystallization of partial melts the near isothermal decompression between m1b and m2 probably represents a pre 112 ma extensional episode that may have been the result of a decompressional readjustment of a thickened crust following the initial extensional event the metamorphic complex remained at depths of 10 17 km for at least 90 ma until it was uplifted following miocene extension m3 develops locally in response to this second extensional period resulting from the early miocene detachment faulting

titanian chondrodite and titanian clinohumite bearing metadunite from the 3800 ma isua supracrustal belt west greenland chemistry petrology and origin

the use of mineral magnetic analyses in identifying middle and late holocene agriculture a study of peat profiles in smaland southern sweden a transect of profiles from the bratamossen bog near the village of axlarp in the province of smaland southern sweden was investigated with respect to magnetic properties human impact pollen indicators and loss on ignition the bog is situated close to an area with clearance cairns an indication of abandoned arable fields there are also graves from the bronze age and the early iron age close to the bog a variety of analytical methods were used to reconstruct the land use history of this area since c 6000 14 c years bp while pollen analysis was used to detect changes in vegetation the magnetic analyses reflected how the soils in the landscape were affected by these changes the pollen analysis showed that the area became deforested and that forest vegetation was succeeded by pasture and arable land magnetic analyses indicated soil erosion as a consequence of agricultural activity the conventional technique of determining the minerogenic content by loss on ignition yielded no valuable information

permian and triassic rocks of the mojave desert and their implications for timing and mechanisms of continental truncation upper paleozoic and lower mesozoic rocks in the mojave desert and adjacent areas record the change from a passive continental margin to a subduction margin in the southwestern united states this change was apparently accompanied by truncation and reorientation of the continental margin in the mojave desert and western basin and range province from late proterozoic to mississippian time the western margin of north america had a northeast southwest trend through nevada and eastern california truncation of the margin began in pennsylvanian time when a northwest trending left lateral strike slip fault zone formed across the antler orogenic belt the cordilleran miogeocline and the more cratonal character mojave desert rocks of the antler belt were apparently displaced southward from east central california to their present position in the northwestern mojave desert sedimentary rocks deposited during this interval record an active continental borderland characterized by alternately subsided and uplifted marine basins this transcurrent faulting continued to late permian time magmatic activity and deformation started in the mojave desert in late permian time and affected both miogeoclinal cratonal and displaced antler rocks magmatic activity ceased and marine sedimentation resumed in early triassic time lower triassic rocks constitute an overlap sequence on both deformed and undeformed miogeoclinal cratonal rocks and possibly on displaced antler rocks facies trends in these rocks are northwest southeast indicating the new margin trend truncation and reorientation of the margin were completed therefore by early triassic time or possibly as early as late permian time upper permian magmatism and deformation in the mojave desert are older than and distinct from the early triassic sonoma orogeny of nevada lower triassic rocks in the mojave desert are post tectonic with respect to deformation in that region whereas correlative lower triassic rocks in nevada are pretectonic to syntectonic with respect to the sonoma orogeny this difference reflects the variation in deformation and tectonic setting along a newly developed cordilleran margin the southern part of the cordillera was apparently a subduction margin as early as late permian or middle triassic time while the central nevada segment was the site of collision tectonics

particle size evidence for source areas of charcoal accumulation in late holocene sediments of eastern north american lakes abstract two methods of analyzing charcoal in sediment reveal changes in charcoal accumulation across temperate eastern north america during the last several hundred years in one method the analyst counts mostly small particles that reflect regional emissions in the other the analyst counts only larger particles derived mostly from such local sources as catchment fires we used these methods to compare charcoal accumulation at 14 lakes from the prairie forest border in minnesota to eastern maine the two methods gave concordant accumulation rates for sediments of pre 1850 age at each of 4 lakes analyzed by both methods this concordance is consistent with the interpretation that pre 1850 emissions were controlled by broad scale factors such as climatically controlled regional differences in fuels and moisture since 1900 large particles decreased greatly and small particles decreased slightly in minnesota and wisconsin by contrast in the northeast the large particle accumulation has remained at the low values measured in pre 1900 sediments at most sites while small particles increased everywhere east of central new york and pennsylvania the observed patterns suggest that 1 large particles primarily reflect local fires that were common in the midwest before fire suppression became effective 2 large particles were rare in the northeast especially before extensive land clearance 3 small particles reflect regional combustion that increased in the northeast after extensive use of fire for land clearance and wood burning for industrial purposes of the 19th century and 4 small particles remain abundant in the midwest long after effective fire suppression probably because these well dispersed small particles have a large source area that extends beyond the local wildfires that account for large particles before european settlement

evidence for the dissolution of magnetite in recent scottish peats abstract magnetic hydrological and geochemical data for 11 scottish peat cores confirm previous reports that anthropogenic pollutants constitute the principal source of ferrimagnets to such environments but indicate that postdepositional fe3o4 persistence may be more limited than formerly envisaged in all cores the lower stratigraphic limit of magnetic enrichment coincides with the water table below which negative mv values and enhanced ratios of fe mn and cu zn depict an abrupt transition to a strongly anoxic regime dissolution of fe3o4 in the reducing inundated zone is also signified by the more rapid down core loss of magnetic properties in cores from topographically depressed sphagnum lawns than in those from more elevated calluna hummocks as a consequence the peak and cumulative sirm values for cores from the former settings appear disproportionately low and the magnetostratigraphic records from adjacent but topographically variant sites are frequently asynchronous in contrast to most marine and lacustrine sediments dissolution of fe3o4 in peat induces a refinement of magnetic mineral assemblages highlighted by declining sirm arm ratios with depth and a hardening of remanence near the base of the magnetically enhanced strata sem micrographs of magnetic extracts from one scottish core indicate that this refinement results from preferential dissolution along crystallographic weaknesses within multidomain phases ultimately causing their disintegration and the adoption of single domain type magnetic behavior rates of dissolution are considerably greater than those reported for marine sediments probably due to the unusually high dissolved sulfide concentrations in mire waters

major late miocene cooling of the middle crust associated with extensional orogenesis in the funeral mountains california the funeral mountains of the death valley region consist of a metamorphic core complex that underlies the miocene boundary canyon detachment fault fissiontrack age determinations on sphene zircon and apatite from the highest grade portions of the funeral mountains collected in monarch canyon indicate rapid cooling from above 285 c at 9 10 ma the steep dt dt slope implied by the cooling path envelope as well as its synchroneity with development of the boundary canyon detachment suggests tectonic unroofing of between 9 5 and 17 km since 10 ma assuming typical geothermal gradients prior to detachment development of between 20 30 c km previous studies report muscovite 40ar 39ar plateau ages from the same area indicating that the funeral mountains core may have cooled below 350 c between 110 55 ma these data when combined with the fission track ages suggest residence of the metamorphic terrain at 285 350 c from late cretaceous to late miocene time recent studies using petrologic constraints have suggested that much of the unroofing of the funeral mountains as well as development of tectonite fabrics occurred during mesozoic extension recognition of temperatures in the 300 c range during miocene time suggests some of the ductile extensional fabrics may be late miocene in age

land use change soil erosion and alluvial dynamic in the lower doubs valley over the 1st millenium ad neublans jura france abstract geochemical and particle size analyses surface scanning magnetic susceptibility microscopic charcoal counting and pollen analysis have been carried out on two cores from the lower doubs valley in order to reconstruct the land use history the interpretation of anthropogenic pollen indicators and micro charcoal deposits is discussed in relation to the lithological variation of sedimentary accumulation and evolution of the settlement the environmental changes which occurred during the 1st and the 6th century ad reflect the impact of human activities on the landscape in the immediate surroundings of the site multiproxy indicators indicate that soil erosion as a consequence of agricultural activities including fire practices a reactivation of hydrologic flow of the river recorded in the sedimentary sequences attests to a larger environmental perturbation between 1st 3rd and 7th 8th century ad

petrologic constraints on the unroofing history of the funeral mountain metamorphic core complex california this is the published version copyright 1990 american geophysical union all rights reserved

speculative tectonic model for the early mesozoic arc of the southwest cordilleran united states the early mesozoic continental arc of arizona california and western nevada is here interpreted to have occupied a continuous graben depression more than 1000 km long similar to the modern extensional are of central america high calculated rates of subsidence the presence of syndepositional normal faults and fault talus breccias the abundance of calderas and high level intrusions and interstratification of cale alkalic and alkalic volcanic rocks provide support for this model the arc graben depression acted as a long lived more than 40 m y trap for craton derived quartz sands which were funneled along the length and across the width of the depression

the evolution of mediterranean wetlands in the first millennium ad the case of les arenes floodplain tortosa ne spain abstract mediterranean wetlands are characterised by two factors 1 a remarkable seasonal fluctuation in the water table and 2 a long history of anthropisation being exceptional archives for palaeoenvironmental and archaeological research humid areas have been thoroughly studied to assess the interplay between people and the environment during the holocene we present here the study of a sedimentary sequence from a mediterranean wetland the les arenes floodplain tortosa ne spain fully dry to date les arenes is documented as a partially drained floodplain in the 12th century records with the aim of assessing the climatic and anthropic contribution to its desiccation we report here the results of bulk sediment analyses environmental magnetism soil micromorphology and ams dating of this sedimentary sequence plus a review of the available historical records the outcome is a clearer understanding of the evolution of this mediterranean floodplain during the 1st millennium ad which implications may be extrapolated to other floodplain wetlands in the iberian peninsula and the mediterranean wet conditions and a low to medium energy depositional environment are attested in les arenes between the 1st and 7th centuries coinciding with the iberian roman humid period irhp and the dark age humid period dahp drier conditions between the 7th and 10th centuries concur with enhanced fluvial dynamics the medieval climate anomaly mca and the first arrival of arabic berber populations in the iberian peninsula al andalus starting in ad 711 results suggest that although the shift from wet to dry conditions in les arenes complies with major climatic trends the construction of the first drainage canals by arab berber groups likely contributed to the enhancement of on going climate related environmental processes