biological remains geochemistry and stable isotopes for the reconstruction of environmental and hydrological changes in the holocene lakes from north sahara abstract a detailed reconstruction of the holocene palaeoenvironments and palaeohydrology is presented for wwamp and lacustrine calcareous sediments which lie in closed depressions along the northern margin of the great western erg algeria it is based on a multidisciplinary approach using numerous techniques the chronological framework is established on the basis of 23 14 c dates on inorganic calcite and mollusc shells ranging from ca 9300 to 3000 yr b p and on an isolated 14 c age of ca 10 900 yr b p the significance of various environmental markers is discussed biological remains are abundant and diversified about 150 taxa including diatoms chrysophytes charophytes ostracods molluscs and foraminifera they provide information on the water chemistry total salinity ionic ratio ph and partial pressure of carbon dioxide and on the habitats water depth development of aquatic plants ephemerality etc emphasis is given to the total salinity individual organisms and palaeobiological assemblages are ordered as a function of their salt requirements and tolerances calibration based on modern diatom samples allows a tentative estimate of the palaeosalinities which range from fresh to marine like and even hypersaline waters attention is drawn to the possible colonization of marine organisms in continental basins without any connection with the sea stable isotope analyses have been carried out on both inorganic and biogenic mollusc and ostracod shells charophyte gyronites carbonates the 18 o contents demonstrate wide changes in the hydrological balance fluctuations in 13 c contents inform on water mixing residence time and on the importance of biological effects on total dissolved inorganic carbon geochemical analyses were performed on inorganic calcite and on ostracod shells high contents in fe and mn indicate reducing environments during stages of high salinity sr ca and mg ca are used as salinity indicators zn mg ratios suggest that the major water supply was the aquifer of the great western erg today down to about 50 m rather than surface waters from the atlas mountains salinities resulted from the balance between inputs from the water table evaporation rate leaching of pre existing salt crusts and seepage through the lake bottom a reconstruction of the evolution of the palaeosystems through time and space has been attempted with special attention to the instability of the water and ionic balances for different time scales short term fluctuations occurring during a single hydrological cycle annual or seasonal are deduced from the apparent discrepancies between the various indicators found in a given sample long term 10 2 10 3 years changes confirm the existence of a climatic phase being wetter than today from at least ca 9300 and may be from ca 10 900 yr b p to 3000 yr b p for the first time the salinity deduced from diatoms and the 18 o content of the water deduced from that of the carbonate at 25 c are used simultaneously to model the hydrological history of the basins and to estimate the relative humidity of the atmosphere during the holocene major environmental trends recorded from ca 9300 to 3000 yr b p do not show a clear synchronims between the different profiles studied therefore they cannot be interpreted in terms of palaeoclimate but instead demonstrate the importance of local hydrology in the evolution of closed basins associated with groundwater a common situation in arid and semi arid zones

the relative effect of particles and turbulence on acoustic scattering from deep sea hydrothermal vent plumes revisited the relative importance of suspended particles and turbulence as backscattering mechanisms within a hydrothermal plume located on the endeavour segment of the juan de fuca ridge is determined by comparing acoustic backscatter measured by the cabled observatory vent imaging sonar covis with model calculations based on in situ samples of particles suspended within the plume analysis of plume samples yields estimates of the mass concentration and size distribution of particles which are used to quantify their contribution to acoustic backscatter the result shows negligible effects of plume particles on acoustic backscatter within the initial 10 m rise of the plume this suggests turbulence induced temperature fluctuations are the dominant backscattering mechanism within lower levels of the plume furthermore inversion of the observed acoustic backscatter for the standard deviation of temperature within the plume yields a reasonable match with the in situ temperature measurements made by a conductivity

influence of late quaternary climatic changes on geomorphic and pedogenic processes on a desert piedmont eastern mojave desert california radiocarbon dating of late quaternary deposits and shorelines of lake mojave and cation ratio numerical age dating of stone pavements dorn 1984 on the adjacent soda mountains piedmont provide age constraints for alluvial and eolian deposits these deposits are associated with climatically controlled stands of lake mojave during the past 15 000 yr six alluvial fan units and three eolian stratigraphic units were assigned ages based on field relations with dated shorelines and piedmont surfaces as well as on soil geomorphic data all but one of these stratigraphic units were deposited in response to time transgressive climatic changes beginning approximately 10 000 yr ago increased eolian flux rates occurred in response to the lowering of lake mojave and a consequent increase in fine sediment availability increased rates of deposition of eolian fines and associated salts influenced pedogenesis stone pavement development and runoff infiltration relations by 1 enhancing mechanical weathering of fan surfaces and hillslopes and 2 forming clay and silt rich surface horizons which decrease infiltration changes in alluvial fan source areas from hillslopes to piedmonts during the holocene reflect runoff reduction on hillslopes caused by colluvial mantle development and runoff enhancement on piedmonts caused by the development of less permeable soils inferred increased in early to middle holocene monsoonal activity resulted in high magnitude paleo sheetflood events on older fan pavements this runoff triggered piedmont dissection which in turn caused increased sediment availability along channel walls thus runoff infiltration changes during the late quaternary have occurred in response to eolian deposition of fines pedogenesis increased sheetflood activity in the holocene and vegetational changes which are related to many complicated linkages among climatic change lake fluctuations and eolian hillslope and alluvial fan processes

scintillation structure of a turbulent tidal flow we describe observations of fine structure in a turbulent tidal channel on the british columbia coast obtained with towed and vertical conductivity temperature and depth profiles and a novel acoustic technique the acoustic scheme involves the detection of the scattering of sound waves by fine structure moving through a pair of acoustic paths that traverse the channel the larger scale flow and also the fine structure can be observed in this way providing a time series measurement of the flow properties as the tidal current changes the channel is characterized by periods of intense mixing through most of the tidal cycle near slack water it becomes stratified and although it remains active the outer scale of the turbulence decreases to 2 3 m the fine scale variability is a sensitive function of flow speed and stratification

palaeoclimate in the saharan and arabian deserts during the middle palaeolithic and the potential for hominin dispersals to disperse out of sub saharan africa it was necessary for hominins to cross the deserts of either the sahara and or arabia thus understanding the palaeoclimate of the saharo arabian region is central to determining the role these deserts played in the peopling of the planet when did they act as barriers and when were they more humid opening dispersal routes across them to address these questions we have conducted a temporal and spatial evaluation of dated sites from 20 to 350 ka using combined probability density function pdf and geographical information system gis analyses of all sites dated using uranium thorium u th or optically stimulated luminescence osl methods radiocarbon dates were not considered because of contamination problems in this time range the results show that during mis 2 there is little evidence for humidity in arabia as would be expected during the height of the last glacial maximum however the sahara shows a sharp rise in probability at the beginning of mis 2 peaking near the boundary with mis 3 at 29 ka there appear to be brief periods of humidity in mis 3 and 6 though at different times in the sahara ca 37 44 138 154 and 180 ka and arabia ca 40 54 and 163 ka during mis 5 both regions show much evidence for humidity with pdf peaks corresponding to insolation maxima though not all maxima are represented in either the saharan or arabian record this situation can be explained by eccentricity modulated precession when eccentricity is strong insolation is enhanced but also more variable and the desert climate is generally more humid particularly at times of high insolation the opposite happens when eccentricity is low and deserts tend to be more arid but local factors exert more of an influence on climate affecting the timing and strength of the brief humid periods experienced so that they no longer coincide with insolation maxima the spatial distribution of humid sites is compatible with a number of different modern human dispersal theories southern arabia experienced humid periods centred on 54 ka and 125 ka and this could have facilitated dispersal from east africa to southern arabia and beyond via the bab el mandab the sahara shows considerable evidence for humidity during mis 5 and may have had dispersal across its expanse at this time

assessing geomorphological and pedological processes in the genesis of pre desert soils from southern tunisia abstract arid environments are fragile and the associated soils are subject to serious threats like water deficiency erosion salt accumulation and loss of fertility in this context understanding the processes involved in soil genesis may contribute toward protecting land from degradation this study highlights the interconnection between geomorphic and pedogenetic processes in soil formation of the jeffara plain a pre saharan area of southern tunisia to reach this goal one coastal oasis chenini nahel and two inland environments matmata nouvelle and menzel habib were studied after geomorphological and pedological surveys the soils were sampled by genetic horizons and characterized by physical mineralogical and chemical analyses and by microscope observation field observations and laboratory data suggest that soil formation in the jeffara plain was a combination of additions and losses controlled by climate changes at chenini nahel the soils developed by accumulation of wind blown sediments coming from a close area dominated by gypsum bearing rocks at matmata nouvelle the soils mainly formed from sedimentation of repeated mudflows during a rainy period between 9000 and 5000 years before present followed by drought periods finally the soils of menzel habib developed from an early gypsum formation in the presence of a salt rich water table and repeated cycles of sedimentation deflation of wind blown materials the different genesis of these pre desert soils produced characteristic b horizons byy horizons with poorly developed soil structure at chenini nahel bw horizons with a hard rupture resistance at matmata nouvelle and bk horizons at depth due to intense sedimentation with ca bc horizons at the surface due to the accretion of wind blown materials at menzel habib

chemical processes in buoyant hydrothermal plumes on the east pacific rise near 21 n hydrothermal plume water containing suspended particles of precipitated black smoke has been sampled from three hydrothermal vent fields on the east pacific rise near 21 n the samples were taken within the lower 22 m of the buoyant plumes directly above 273 350 c black smoker vents using go flo bottles mounted on the submersible alvin based on li as a conservative tracer of the high temperature endmember solution mixing ratios of 102 to 104 g seawater g vent water are achieved within the lower 22 m of the plumes the particle concentrations sampled in the lower 13 m of the plumes are much larger than predicted for these mixing ratios suggesting that the particles are settling within the lower part of the plume the major minerals in the smoke are pyrrhotite fe rich sphalerite pyrite unidentified fe s si and fe si s phases chalcopyrite amorphous silica sulfur fe oxyhydroxides including goethite and anhydrite present in trace quantities are barite isocubanite wurtzite covellite marcasite and unidentified silicates and alsilicates organic matter is common the composition of the plume solutions indicates that 35 25 of the hydrothermally injected fe remains in dissolved form i e 0 45 m within the lower 22 m of the plume in spite of a 3 to 9 fold excess of h2s in the vent solutions nearly all of the mn and si and most of the ba also remain in solution h2s by contrast has been largely 70 100 removed by precipitation as sulfides and sulfur but mainly by oxidation to dissolved species with intermediate oxidation states its early removal implies that 50 30 of the injected fe will eventually precipitate as primary oxides rather than as sulfides and that fe can be fractionated from the other chalcophile metals during sulfide precipitation compared with the vent water the particles are enriched in cu co ag cd al zn pb and ni by factors of 4 to 20 relative to fe and depleted in ba s si and mn scavenging from seawater may play a role in co and cu enrichment at the ngs field and for ag cd and ni at all three fields but is negligible for pb and zn these results indicate that two distinct metalliferous components are delivered to the distal regions of a plume as a result of reactions in the lower part of the buoyant plume 1 1 a dissolved component that includes about half the fe and all of the mn and will eventually precipitate as oxides and 2 2 a particulate component consisting of fine grained sulfide minerals that are enriched in ore metals by factors of 4 to 20

correlation and dating of quaternary alluvial fan surfaces using scarp diffusion great interest has recently been focused on dating and interpreting alluvial fan surfaces as a complement to the radiometric methods often used for surface exposure dating this paper illustrates a rapid method for correlating and dating fan surfaces using the cross sectional shape of gullies incised into fan surfaces the method applies a linear hillslope diffusion model to invert for the diffusivity age t m2 using an elevation profile or gradient slope profile gullies near the distal end of fan surfaces are assumed to form quickly following fan entrenchment scarps adjacent to these gullies provide a measure of age the method is illustrated on fan surfaces with ages of approximately 10 ka to 1 2 ma in the arid southwestern united states two areas of focus are death valley california and the ajo mountains piedmont arizona gully profile morphology is measured in two ways by photometrically derived gradient slope profiles and by ground surveyed elevation profiles the t values determined using ground surveyed profiles are more consistent than those determined using photo derived t values however the mean t values of both methods are comparable the photometric method provides an efficient way to quantitatively and objectively correlate and relatively date alluvial fan surfaces the t values for each surface are determined to approximately 30 50 accuracy

two high levels of continental waters in the southern tunisian chotts at about 90 and 150 ka uranium and thorium content and activity ratios were measured on 20 samples 19 molluscs and 1 calcareous concretion from the great chotts in southern tunisia results were studied by the isochron method and by age frequency histograms they suggest that two major flood episodes took place about 150 and 90 ka uranium activity ratios indicate a ground water supply of continental origin in agreement with biological indicators which show large variations in salinity from fresh to marine like conditions the existence of a lacustrine phase radiocarbon dated to 17 40 ka as previously suggested for the northern sahara is highly questionable

use of soils and colluvial deposits in analyzing tectonic events the southern arava rift israel abstract buried soils in colluvium units developed on downthrown fault blocks have been presumed to form during periods of tectonic quiescence this assumes however that all colluvial units on a fault scarp are tectonically induced particularly for a multiple event fault scarp to test this hypothesis we compared the colluvial stratigraphy and pattern of soil development on two non tectonic scarps terrace risers and a nearby multiple event fault scarp on the same alluvial fan surface both slopes developed under extremely and climatic conditions the colluvial stratigraphy and soil catenary relationship developed on terrace risers were found to be substantially different from those on the multiple event fault scarp the fault scarp colluvium contained several colluvial units separated by soils whereas only two colluvial units without buried soils were found on the terrace risers the colluvial units on the fault scarp were triangular in shape while those on the terrace riser were concave and overlapped up the slope cumulic soils indicating a continuous and low rate of deposition during pedogenesis were formed along the terrace riser in contrast buried soils indicating episodic deposition occurred on the downthrown block of the multiple event fault scarp accordingly the colluvial stratigraphy developed on the fault scarp can be viewed as a record of distinct tectonic events with no climatically controlled deposition the data indicate that soils and sedimentary stratigraphy can serve as useful indicators to distinguish terrace risers and single event fault scarps from multiple event fault scarps these indicators can be helpful in distinguishing phases of tectonic activity and in evaluating recurrence intervals on a multiple fault scarp

high resolution fluvial record of late holocene geomorphic change in northern tunisia climatic or human impact abstract in northern tunisia the late holocene alluvial record of medjerda floodplain sediments indicates geomorphic changes due to fluctuations of past climate superimposed by the effects of human activity geomorphic activity occurred several times interrupting stable conditions enhanced fluvial dynamics occurred around 4 7 3 0 1 7 1 0 and 0 7 ka cal bp a peak of activity took place about 0 4 ka cal bp some periods of fluvial activity show event like phenomena in particular when an anthropogenic impact intensifies climatic aridification this has been the case around 1 7 1 0 0 7 and 0 4 ka cal bp with severe floods in the medjerda valley during antiquity as well as around 1 3 0 8 and 0 5 ka cal bp immature alluvial soils have developed the first one being the most distinct late holocene medjerda alluvial history shows that fluvial dynamics in northern tunisia were predominantly climate driven anthropogenic impact only intensified or weakened these processes

measuring fluid flow and heat output in seafloor hydrothermal environments we have designed built calibrated and tested new anemometer and turbine flow meter devices to measure fluid velocity at high and low temperature focused and diffuse discharge sites at oceanic spreading centers the devices perform at ocean floor depths and black smoker temperatures and can be used to measure flow rates ranging over two orders of magnitude flow velocity is determined from the rotation rate of the rotor blades or paddle assembly these devices have an open bearing design that eliminates clogging by particles or chemical precipitates as the fluid passes by the rotors the devices are compact and lightweight enough for deployment from either an occupied or remotely operated submersible the measured flow rates can be used in conjunction with vent temperature or geochemical measurements to obtain heat outputs or geochemical fluxes from both vent chimneys and diffuse flow regions the devices have been tested on 30 alvin dives on the juan de fuca ridge and 3 jason dives on the east pacific rise epr we report 102 new measurements over a wide range of discharge temperatures 5 363 c velocities 2 199 cm s and depths 1517 2511 m these include the first advective heat output measurements at the high rise vent field and the first direct fluid flow measurement at middle valley our data suggest that black smoker heat output at the main endeavor vent field may have declined since 1994 and that after the 2005 2006 eruption the high temperature advective flow at the epr 9 50 n field may have become more channelized predominately discharging through the bio 9 structure we also report first direct advective measurements in the lau basin and at the epr 9 39 5 n with flow meters that predated devices described in this work and were used in the process of their development we discuss potential error sources and how they may affect the accuracy of measurements by our and other devices in particular we use the turbulent plume theory to evaluate the effect of entrainment of ambient seawater

deep sea hydrothermal plumes and their interaction with oscillatory flows 1 the acoustic scintillation method is applied to the investigation and monitoring of a vigorous hydrothermal plume from dante within the main endeavour vent field mef in the endeavour ridge segment a 40 day time series of the plume s vertical velocity and temperature fluctuations provides a unique opportunity to study deep sea plume dynamics in a tidally varying horizontal cross flow an integral plume model that takes into account ambient stratification and horizontal cross flows is established from the conservation equations of mass momentum and density deficit using a linear additive entrainment velocity in the model e um u that is a function of both the plume relative axial velocity um and the relative ambient flow perpendicular to the plume u gives consistent results to the experimental data suggesting entrainment coefficients 0 1 and 0 6 also from the integral model the plume height in a horizontal cross flow ua is shown to scale as 1 8b1 3ua 1 3n 2 3 for 0 01 ua 0 1 m s where b is the initial buoyancy transport and n is the ambient stratification both of which are assumed constant

holocene rock varnish on the messak plateau libyan sahara chronology of weathering processes abstract rock varnish coats subaerially exposed rock surfaces on the messak settafet plateau se libya central sahara holocene surfaces host three microlayers formed by different accretion processes that record a climatic transition from a wet environment to progressively more arid conditions sem eds and optical investigations shed light on the processes leading to the formation of microlaminae while direct ams 14c dating on microlayers and relative radiocarbon dating on dark varnished pastoral neolithic stone monuments suggest a reliable chronology for varnish development the innermost microlayer includes illuvial clay and calcite incorporated during the early to middle holocene 9200 to 5500 uncal ybp under wet environmental conditions the second microlayer is more typical mn rich rock varnish biomineralized during a phase of growing aridity in the middle holocene 6000 to 4000 uncal ybp the upper microlayer is composed of mostly aeolian dust and it dates back to the last hyperarid millennia a positive correlation between varnish chronological development and climate changes in the sahara reflects also broad regional control on microlamination deposition

quaternary continental stratigraphy landscape evolution and application to archeology jarilla piedmont and tularosa graben floor white sands missile range new mexico the upper pleistocene holocene sequence in southeastern white sands missile range was deposited in an eolian sand sheet environment that interacted with gravel depositing arroyos near the jarilla mountain piedmont and with mud depositing arroyo mouth ponds and more extensive lakes on the basin floor the sand sheet contains ridge and swale coppice dune and deflation depression subenvironments three distinctive and locally exhumed buried soils occur within these eolian alluvial and lacustrine deposits the youngest buried soil is characterized by an a to bw to bk stage i profile the second by a btk stage ii to bk stage i profile and the third by a btk stage ii to k stage iii or iv to bk stage ii profile the tops of these buried soils which in most cases are truncated represent easily identifiable regional bounding surfaces they separate the stratigraphic sequence into four units with approximate time spans of 0 to 100 yr b p unit q4 100 to 7300 yr b p unit q3 9400 to 15 000 yr b p unit q2 and about 50 000 to 250 000 yr b p unit q1 lateral variations in unit thickness delineate eolian depocenters and areas of maximal erosion during these time spans the stratigraphic sequence records three cyclic alternations between landscape instability during which erosion and sedimentation took place and landscape stability during which the soil profiles became well developed the initiation of the earliest two episodes of landscape instability represented by erosion of or deposition upon the lower two buried soils is attributed to climatic change toward drier conditions which caused the breakup of the sediment binding vegetative cover the most recent landscape destabilization episode was initiated about 100 yr b p by the destruction of the desert grassland cover by cattle overgrazing prehistoric holocene cultural remains largely are restricted to unit q3 and the surface of unit q2 or to the present surface at localities where units q2 or q1 are exhumed remains found within distinct eolian lag accumulations or within bedded arroyo sediments likely are removed from their original context the degree to which can be estimated by the extent of incision into the soil profile horizons prehistoric habitants occupied a landscape containing the ridge and swale and deflation depression eolian topography but without the current hummocky mesquite coppice dunes

mid and late holocene fluvial chronology of tunisia abstract regarding fluvial response to recent climate change it is important to identify and understand previous changes of fluvial dynamics related to past climatic shifts in this paper periods of increased flood activity in central tunisian river systems are presented changes in holocene flood dynamics are reconstructed through a comprehensive analysis of 195 radiocarbon dates from alluvial archives holocene floodplain deposits in central tunisia normally consists of cohesive overbank fines which exhibit excellent preservation conditions against lateral erosion and therefore permit chronostratigraphic records of the entire holocene at most based on published radiocarbon dates two 14 c cumulative probability density plots are created for central tunisia the former represents activity dates indicating phases of increased fluvial dynamics the latter documents stability dates showing periods of soil formation and enhanced landscape stability within the floodplain and the catchment due to geomorphological considerations activity dates and stability dates should be mutually exclusive there is a good match for the mid holocene with increased periods of flooding from 6 2 to 6 0 4 8 to 4 5 4 1 to 3 7 and 3 3 to 3 0 ka these flood periods coincide with low soil formation signals on the other hand periods of increased alluvial soil formation from 6 8 to 6 6 5 9 to 4 8 4 5 to 4 1 3 7 to 3 3 3 0 to 2 8 and 1 8 to 1 6 ka coincide with periods of low flood plain aggradation the data reveal a coupling with north atlantic cooling short term periods of enhanced fluvial dynamics in central tunisia match well with north atlantic cooling the findings indicate a strong climatic link from 4 7 ka until today compared to the holocene floodplain dynamics in tunisian drylands the temperate mid latitudes show mid holocene switches in fluvial dynamics with opposite signs especially after 5 ka north atlantic driven flooding in the temperate central european river systems contrast sharply to flood dynamics in arid to semiarid mediterranean tunisia while under temperate climate conditions floodplain sedimentation decreases it increases under more arid conditions this may indicate that fluvial archives of dissimilar climatic zones reveal different threshold values and sedimentation patterns and therefore different responses to climatic shifts

unravelling the patterns of alluvial fan development using mineral magnetic analysis examples from the sparta basin lakonia southern greece mineral magnetic analysis of b horizons of soils developing upon the surfaces of alluvial fans was undertaken in order to i differentiate and rank discrete fan surfaces by order of formation ii establish whether fan surfaces formed simultaneously in adjacent fan systems and iii deduce probable patterns of fan development the results of the analysis indicate that the greatest concentration of ferrimagnetic and anti ferromagnetic minerals occurs within soils which have developed upon the proximal fan surfaces with a progressive reduction in magnetic minerals in soils associated with medial and distal surfaces the build up of magnetic minerals in the proximal fan soils suggests that these surfaces formed first followed by the medial and distal surfaces with the exception of the kalivia sokas fan the majority of depositional events responsible for fan surface formation occurred simultaneously suggesting that adjacent fan systems share broadly similar depositional histories although the precise timing of depositional events is uncertain it is probable that by the end of the late pleistocene small largely undissected fans comprising two to three surfaces had formed at the start of the holocene fan systems experienced significant fanhead incision a net distal extension of the fan trench coupled with a progressive basinward shift of the locus of deposition during the middle and late holocene resulted in formation of medial and distal fan surfaces changes in climate are deemed to be the major control of fanhead incision fan trenching and fan surface formation however the likely effects of long term tectonic activity and approximately five thousand years of human occupation upon fan development in the sparta basin remain unclear copyright 2000 john wiley sons ltd

flow rates in the axial hot springs of the east pacific rise 21 n implications for the heat budget and the formation of massive sulfide deposits abstract the first in situ measurements of flow rates in submarine hot springs temperatures between 275 and 350 c were made in the hot springs of the east pacific rise at 21 n during november 1981 the flow rates ranged between 0 7 and 2 4 m s we estimate that the total rates at which heat and mass are transported from the three hot spring areas obs national geographic and southwest vents are ca 2 2 108 w and 150 kg of fluid s respectively the very high rate of heat loss precludes the existence of hot springs as steady state features of the east pacific rise at 21 n we estimate that hot springs are active for a maximum of ca 40 000 years chemical and or mechanical clogging of the vents can drastically reduce the lifetime of the hot springs using a simple model of plume development 1 we calculated that less than 3 of the sulfide particles that are entrained in the hydrothermal plumes at 21 n settle from the plumes before dispersal by a lateral submarine current at a height of ca 250 m above the seafloor 2 the time that is required to form the sulfide mounds that surround the hot springs by the accumulation of sulfide particles that settle from the plumes is ca 70 85 years the current rate at which sulfide particles settle from the hydrothermal plumes and accumulate near the hot springs appears to be too small to lead to the formation of large massive sulfide deposits such as the mavrouni deposit on cyprus 3 within the time limits that are suggested by our thermal energy balance calculations

evolution of quaternary alluvial fans and terraces in the intramontane pinjaur dun sub himalaya nw india interaction between tectonics and climate change quaternary alluvial fans in the tectonically active pinjaur dun an intramontane valley in the sub himalaya were deposited in front of the nalagarh thrust and were influenced both by tectonics and glacial climate fluctuations the surface morphology indicates that an earlier set of first order fans qf1 became entrenched and onlapped by a series of second order fans qf2 the younger fan segments were then cut by a pair of terraces t1 and t2 quartz optically stimulated luminescence dating establishes that the qf1 aggradational phase was initiated before 96ae 5 2 5ae3 ka and terminated after 83ae 7 1 6ae3 ka this was followed by a period of incision before qf2 fan deposition started at 72ae 4 1 3ae4 ka and continued until 24ae 5 4 ae 9k a sediment was deposited on the t1 upper and t2 lower terraces at 16ae 3 2 ae1 and 4ae5 ka respectively recording a return to overall degradation punctuated by minor deposition on terraces the period of incision separating the younger and older fan deposits coincided with enhanced sw monsoon precipitation the subsequent development of the qf2 fans and their progradation until 20 ka suggest erosional unloading of the thrust hangingwall during a tectonically quiescent phase toe cutting deposition of axial river and lacustrine facies and retreat of qf2 around 45 ka indicate fanward shift of the axial river due to tilting of the valley towards the ne in response to reactivation of the nalagarh thrust the cessation of qf2 deposition around 20 ka and the onset of through fan entrenchment suggest reduced sediment supply but relatively high stream power during the last glacial maxima lgm the prolonged stream incision since the cessation of qf2 deposition with only minor depositional phases at 16ae 3 2 ae1 and 4ae 5k a resulted from high water discharge and low sediment input during intensification of the sw monsoon and vegetation changes in the hinterland

short term evolution of an active basin fan system aspromonte south italy abstract alluvial fans are widespread throughout calabria southern italy but only a minority of them show evidence of current activity by means of interpretation of aerial photographs orthophotos and satellite images comparison of topographic maps reconstruction of major rainfall events and geomorphologic and sedimentologic field observations we reconstructed the recent history of the geomorphic and sedimentation events and the consequent morphological changes of an active basin fan system merging with the bed of an ephemeral stream locally named fiumara the case study is located along the ionian side of the aspromonte massif a sector of calabria where mass movement intensive erosion and channel mass transport are the predominant morphodynamic processes evidence is available of large volumes of debris produced by mass movements and erosion processes affecting the upper sector of the feeder basin the aggressive climate with intense winter storms is the principal cause of erosion and mass movements largest rainfall events occurred in clusters of 4 6 events between the 1930s and the early 1970s at roughly 20 year time intervals the development of the fan appeared as incipient in 1955 air photos activity continues today with events of sediment output from the feeder basin large rainfall events are reflected with a delay of ca eight years by aggradation dissection and progradation retrogradation cycles in the feeder channel and the fan respectively mutual interference between fan development and human activity exists as well as does the interaction between the fan progradation and the floods of the receiving fiumara stream based on the aforementioned survey tools two almost 10 year cycles were recognised feeder channel aggradation owing to the debris input of the feeder subbasin and to the presence of check dams built along the feeder channel continues until the channel slope reaches a threshold value this can also be caused by the trimming of the fan toe by the fiumara the stored alluvium is therefore released resulting in progradation at the fan site until the feeder channel slope decreases subsequently a renewed cycle starts again in response to sediment input from the source area the morphological and sedimentological evolution of the active basin fan system indicates how short term trends and cyclic modifications of such systems can be important especially where tributary basins join larger stream beds of a fiumara type torrent

acoustic imaging of high temperature hydrothermal plumes at seafloor spreading centers we explore the possibility of using active sonar techniques to acoustically image high temperature black smoker hydrothermal plumes we examine recent sonar images of a hydrothermal vent field at 11 n on the east pacific rise obtained from dsrv alvin which may show the presence of plumes for vent fields on the east pacific rise estimates are obtained of the minimum detectable concentration of precipitates as a function of the range between the sonar and the plume boundary these estimates are compared with measured concentrations some of the anticipated advantages of acoustic imaging are discussed we conclude that acoustic imaging techniques have the potential of providing a coherent framework for point sampling of physical and chemical properties of hydrothermal plumes and for determining the dynamics of their injection into the surrounding water mass

late pleistocene holocene palaeoenvironmental evolution of the anamaza river valley iberian range ne spain multidisciplinary approach on the study of carbonate fluvial systems abstract the uppermost pleistocene and holocene palaeoenvironmental evolution of the anamaza river valley iberian range ne spain is deduced using multidisciplinary approach including stratigraphical mineralogical palynological geochemical geophysical methods and drilling main changes were registered in distinct subenvironments of a carbonate fluvial system including the channelled zone and wetlands in the floodplain tufa barrages dominated although pools also existed geophysical survey and coring reveal tufa build ups and pool facies also in the subsoil lower water temperature and scarce evaporation are deduced for the pleistocene fluvial system that progressively changed through the holocene with more hydrologically closed areas and higher evaporation influence a general aggrading evolution during warm stages related with increasing base level and damming due to fast carbonate precipitation characterised the holocene detrital tufa indicates erosive high energy floods or colder stages when water level would decrease favouring erosion 14 c and 230 th 234 u dating reveal high sedimentation rates and three main discontinuities related with cold episodes younger dryas middle part of the holocene climate optimum and iron age epoch during the uppermost pleistocene tufa growth would be enhanced during warmer episodes as the bolling allerod in the younger dryas scarce vegetation favoured erosion of both slopes and tufa constructions subsequent warmer temperatures during the first part of the holocene favoured vegetated slopes enhanced tufa growing although interrupted in the middle part of the holocene climate optimum and development of wetlands with riparian vegetation in the floodplain where either siliciclastics or detrital tufa incoming alternated with low energy waters stages and mud settling progressive decline in tufa is deduced for the upper holocene but it is not possible to determine whether this and other palaeoenvironmental changes were related either to climate or increasing human activities during the roman and medieval warm periods more oxidizing conditions in the wetlands and increasing erosion prevailed probably conditioned by human activities the pollen record shows for the early holocene development of pinus forest with betula and expansion of deciduous quercus xerophilous and heliophilous grassland subsequent increasing moisture supported open forests with deciduous quercus ulmus corylus and evergreen quercus ilex pistacia species from ca 4000 yrbp a dominant deciduous quercus forest with groves of corylus ulmus acer fagus and taxus expanded and human activities grazing occurred from 1200 yrbp dry grassland expanded due to intensive land use agropastoral activities almost completely deforested plateaus surround the site today with slopes covered by patchy grass with junipers groves and screeds with little soil

the path to covis a review of acoustic imaging of hydrothermal flow regimes abstract acoustic imaging of hydrothermal flow regimes started with the incidental recognition of a plume on a routine sonar scan for obstacles in the path of the human occupied submersible alvin developments in sonar engineering acoustic data processing and scientific visualization have been combined to develop technology which can effectively capture the behavior of focused and diffuse hydrothermal discharge this paper traces the development of these acoustic imaging techniques for hydrothermal flow regimes from their conception through to the development of the cabled observatory vent imaging sonar covis covis has monitored such flow eight times a day for several years successful acoustic techniques for estimating plume entrainment bending vertical rise volume flux and heat flux are presented as is the state of the art in diffuse flow detection

structural and stratigraphic perspectives on the uplift and erosional history of djebel cherichira and oued grigema a segment of the tunisian atlas thrust front

composition and dissolution of black smoker particulates from active vents on the juan de fuca ridge during two atlantis ii alvin cruises to the juan de fuca ridge in 1984 active high temperature 140 284 c vents were sampled for black smoker particulates using the grassle pump individual mineral phases were identified using standard x ray diffraction and petrographic procedures in addition elemental compositions and particle morphologies were determined by x ray energy spectrometry and scanning electron microscope x ray energy spectrometry techniques the vent particulates from the southern juan de fuca ridge vent sites were highly enriched in s si fe zn and cu and were primarily composed of sphalerite wurtzite pyrite pyrrhotite barite chalcopyrite cubanite hydrous iron oxides and elemental sulfur two additional unidentified phases which were prevalent in the samples included an fe si phase and a ca si phase the grain sizes of the individual particle phases ranged from 100 m for the fe si particles grain size and current meter data were used in a deposition model of individual phase dispersal for many of the larger sulfide and sulfate particles the model predicts dispersal to occur over length scales of only several hundreds of meters the high temperature black smokers from the more northerly endeavour segment vents were highly enriched in fe s ca cu and zn and were primarily composed of anhydrite chalcopyrite sphalerite barite sulfur pyrite and other less abundant metal sulfide minerals the grain sizes of the individual particles ranged from 10 m to slightly larger than 500 m the composition and size distributions of the mineral phases are highly suggestive of high temperature mixing between vent fluids and seawater a series of field and laboratory studies were conducted to determine the rates of dissolution of several sulfate and sulfide minerals the dissolution rates ranged over more than 3 orders of magnitude from 3 2 10 8 cm s 1 for anhydrite to 1 2 10 12 cm s 1 for chalcopyrite the results indicate that for some minerals particularly anhydrite and marcasite total dissolution occurs within a few hours to a few weeks of their formation for other more stable minerals including pyrite sphalerite and chalcopyrite the time required for total dissolution is much longer and consequently individual crystals may be expected to persist in the sediments for considerable periods of time after deposition

palhydaf palaeohydrology in africa program objectives methods major results abstract the palhydaf program aims to reconstruct the late quaternary 130 ka b p to present environmental and hydrological changes of northwest africa and to interpret them in terms of palaeoclimate for testing coupled atmospheric ocean circulation models lacustrine sequences were obtained from closed basins which lie along two meridional transects southern tunisia southern niger and algeria mali particular attention is given to groundwater storage and periods of recharge of the aquifers the chronological framework is based on 14c dates conventional and ams methods and or the th u disequilibrium method in favourable cases e g mali correlations can be made between the chronology of surface palaeohydrology and that of aquifer recharge various environmental markers mineralogy elemental chemistry stable isotopes biological remains are used for palaeoenvironmental purposes quantitative estimates of palaeoclimatic variables e g air humidity may be attempted through the combination of geochemical and palaeobiological data the major results already obtained are summarized for the following individual sectors 1 southern tunisia 1 northern algeria 2 southern algeria 3 northern niger 3 mali 4 southern niger at the present stage of investigation the following new inferences can be drown one or several humid episode s occurred in the northern sahara between 150 and 75 ka b p several factors suggest that these humid conditions also affected lower latitudes although their southern extent cannot be given precisely at present the previously postulated existence of large lakes at 20 35 ka b p is not confirmed this time interval seems to coincide with raised water tables but not to a major humid phase in all cases the 14c chronologies obtained for the late pleistocene have to be checked by comparison with other dating methods e g th u disequilibrium after the arid period of the last glacial maximum the return of humid conditions occurred in two major steps at 12 and 9 5 9 ka b p for both the southern and the northern sahara

aggradation and dissection sequences on spanish alluvial fans influence on morphological development abstract similarities in quaternary alluvial fan development are identified throughout southeast spain pre wurm aggradation phases culminate in calcrete crusted upper fan surfaces which are followed by wurm dissection phases producing deep fanhead or through trenches deposits of the aggradation phases reflect source area size relief and geology and in addition to proximal distal facies variations show vertical decreases in debris flow and increases in gravel deposition the dissection phases were complicated by cut and fill sequences forming within trench terraces which show a southward decrease in complexity the sequences are expressed in progressive changes in fan terrace and channel slopes and intersection point location fan morphological development during the quaternary reflects long term progressive change complicated by short term responses to climatic fluctuations and further comolicated by the locally variable effectiveness of trenching thresholds

distribution and formation of rock varnish in southern tunisia field observations chemical analysis electron and light microscopy and radiolabelling experiments provide information on the characteristics formation and distribution of fe and mn rock varnishes in southern tunisia unstable rocks are not surface coated but when broken they commonly reveal internal microbial solution fronts stable rocks are coated in fe and more rarely mn rock varnishes radiolabelling indicates the presence of many respiring microbes on both fe and mn varnishes microscopy shows fungi and bacteria both on the fe varnishes and within the rock only rare microcolonial fungi occur on mn varnishes suggesting that the microbes are within or beneath the varnish the abundance of microorganisms supports a biogenic origin for rock varnishes different microbial communities appear to be involved in fe and mn varnish formation microbes appear to live beneath the surface of unstable rocks but coat stable rocks which cannot be penetrated apparently to avoid u v light and desiccation this explains the concentration of elements that form u v opaque minerals and clays the origin of these elements is external to the substrate these results suggest that rock varnish dating and paleoenvironmental techniques can be applied in southern tunisia

active oblique ramp faulting in the southern tunisian atlas abstract the gafsa fault is the longest and most active structure of the fold and thrust belt achieving southeastward propagation of the atlas belt of eastern north africa onto the saharan platform the gafsa fault is a 75 km long dextral oblique basement fault ramp that poses a sizable challenge in earthquake hazard assessment because the post paleozoic sedimentary cover is decoupled from its basement above the basement fault in this study we combine seismic lines interpretation tectonic geomorphology and paleoseismological investigations to assess the level of seismic hazard of this fault and evaluate its role in the geodynamic framework of the central mediterranean we show that despite a moderate instrumental and historical seismicity the fault has produced m 6 earthquakes with a return period of ca 500 5000 years during the late quaternary the latest large event having produced a surface rupture on the fault occurred around 8000 yr bp suggesting an m 6 earthquake is overdue on the fault the fault has a minimum reverse component of slip rate of 0 21 0 34 mm yr over the past 50 ka the occurrence of m 7 paleoearthquakes on the fault may be suspected but not established such very strong earthquakes would require transient coseismic linkage of the buried basement fault with the overlying listric fault ramping off the decollement layer the level of seismic hazard may be underestimated on the gafsa fault indeed given the geometry of the basement cover fault system a number of earthquakes generated in the basement would have led to coseismic surface folding instead of to surface rupture the gafsa fault is a major structure accommodating eastward extrusion spreading of the atlas belt onto the saharan and pelagian plateforms above the retreating ionian lithospheric slab

freshwater to marine like environments from holocene lakes in northern sahara continuous swamp and lacustrine sequences from the northern edge of the great western erg are dated at 9 300 3 000 yr bp diatoms ostracods molluscs and foraminifera show great changes in salinity ranging from freshwater to marine like environments the stable isotope composition of carbonates indicates that the evaporation of groundwater supplies is responsible for these fluctuations in salt contents until now evidence for the occurrence of a wet climatic phase of early and middle holocene age in the north north west sahara was based on a restricted number of 14c dates from scattered lacustrine deposits and paleosols1 8 the present study complements this knowledge and highlights the importance of local hydrological factors in interpreting palaeolimnological changes

constraining the timing of alluvial fan response to late quaternary climatic changes southern tunisia the evolution of the oued es seffaia alluvial fan during the last 50 000 years is analyzed in the light of chronometric data derived from ams radiocarbon and optical dating techniques these ages have enabled the temporal constraint of the distalward progression of the intersection point the upper limit of the locus of deposition which has resulted in the telescopic segmentation of the fan comparison of these data with the late quaternary climatic history of the maghreb highlights several problems of interpretation due both to our limited knowledge of regional palaeoclimates and of the response of dryland fluvial systems to climate changes however our data provide some evidence to suggest that fan incision and formation of telescopic segments occurs in response to changes from arid to less arid conditions supporting the conclusions of some of the work from other areas

the rate of iron oxide enrichment in arid zone alluvial fan soils tunisian southern atlas measured by mineral magnetic techniques abstract rates of iron oxide enrichment of soils on a segmented alluvial fan in southern tunisia are examined mineral magnetic and chemical characteristics of soils developed on two dated relict segments and on contemporary alluvium suggest that the rate of fe oxide enrichment was more rapid in the holocene 2 6 10 8 g g 1 yr 1 over the last 5000 years than in the pleistocene 1 91 10 9 g g 1 yr 1 over the previous 45 000 years thus fe oxide concentration may be used to determine an ordinal chronology of some geomorphological surfaces in drylands but rates of enrichment cannot be assumed to be uniform over time

sedimentation from turbulent jets and plumes theoretical models are developed for the sedimentation from the margins of a particle laden axisymmetric turbulent buoyant plume in a still environment and for an axisymmetric turbulent momentum jet the models assume that the mass of each individual size fraction of sediment carried in a parcel of fluid decreases exponentially with time for relatively coarse particles the fallout models predict that the sediment deposition beyond a distance r on the ground expressed in log units should decay linearly with distance away from the vent for the momentum jet and should decrease with r1 3 for the buoyant plume the exponential decay constant j is proportional to the terminal fall velocity vt of the particles in both cases and inversely proportional to the square root of the initial momentum flux m0 for the jet fallout jj vtmo 1 2 and to the third power of the initial buoyancy flux fo for the plume fallout jp vtfo 1 3 smaller particles are affected by reentrainment caused by the turbulent eddies sweeping ambient fluid back into the plume or jet and thus reincorporating some particles that were released from the flow at greater heights this is taken into account by introducing a reentrainment coefficient into the theoretical models with the assumption that the coefficient has a constant value for a plume of given strength in new experiments fallout occurs from the margins of particle laden fresh water buoyant jets and plumes in a tank of salty water and sedimentation is measured on the tank floor two experiments were weakly affected by reentrainment and show excellent agreement with the simple theory for smaller particles and increasingly buoyant plumes and strong jets particle reentrainment is important the experimental data are fitted by the new reentrainment theory confirming that values of the reentrainment coefficient are approximately constant for a given flow a settling number is defined as the ratio of the characteristic velocity of the jet or plume to the particle settling velocity for 1 reentrainment seems to reach an equilibrium state for which the reentrainment coefficient is a constant of value 0 1 for jets and 0 4 for plumes irrespective of flow strength or particle size the plume experiments indicate that the value of the reentrainment coefficient is strongly dependent on plume strength and particle size for slightly less than 1 the general principles of sedimentation from turbulent plumes and jets are applied to the fallout of pumice from volcanic eruption columns and of metalliferous particles from black smokers on the ocean floor for volcanic eruptions the results provide an explanation for the near vent overthickening of tephra fall deposits and imply that lithic and pumice fragments from small lapilli up to at least 1 m diameter blocks are efficiently reentrained into eruption columns the size of particles reentrained in hydrothermal plumes is predicted to vary from less than 100 m in weakly buoyant plumes up to over 1000 m in megaplumes

entrainment and bending in a major hydrothermal plume main endeavour field juan de fuca ridge 1 we measure expansion rate and bending in a 23 hour time series of acoustic images of the lower 25 m section of a buoyant hydrothermal plume rising from grotto vent in the main endeavour field juan de fuca ridge we then calculate entrainment coefficient the constant of proportionality relating mean inflow velocity at the plume edge to maximum mean upward velocity within the plume the plume section alternately bends southwest at relatively high inclinations 37 and northeast at lower inclinations at irregular intervals twice during this time period apparently driven by current reversals in the mixed semi diurnal tidal cycle the measured expansion rates 0 11 0 25 m m and calculated entrainment coefficients 0 07 0 18 are directly proportional to the degree of bending r2 0 75 the loss of buoyancy flux related to enhanced mixing in a stratified environment during bending may contribute to reduction of potential rise height consistent with predicted 400 m and measured 300 350 m plume tops

a turbulent convection model with an observational context for a deep sea hydrothermal plume in a time variable cross flow 1 a turbulent convection model for a hydrothermal fluid discharging into a tidally modulated stratified cross flow is used to investigate time variable conditions in plumes such as the one rising from dante a sulfide mound at 2175 m depth on the endeavour segment of the juan de fuca ridge that plume is the consequence of the coalescence of 10 or more small individual plumes from chimneys discharging hot salt diminished fluid into the near bottom ocean at dante the discharge encounters ambient horizontal currents with speeds oscillating from near zero to a maximum of 7 cm s 1 speeds which can bend a plume more than 45 from the vertical model results are compatible with field measurements of the plume footprint size and vertical velocity both 20 m above the source when earlier estimates for dante s heat flux of 50 mw drive the convection the small scale short period variability of velocities and properties distributions observed in the field is mimicked in model results plumes pool above a source during periods of weak cross flows but stream away from the source with more diluted concentrations and lower rise heights at other times plume distributions at identical cross flow speeds differ whether the flow is accelerating or decelerating small changes in background hydrographic profiles create differences in rise heights comparable to those caused by large changes in source buoyancy flux if put into an entrainment context results suggest an entrainment coefficient eff that varies from 0 11 to 0 025 with increasing height 2 76 m above the source

influence of station topography and moho depth on the mislocation vectors for the kyrgyz broadband seismic network knet deviations of slowness and backazimuth from theoretically calculated values the so called mislocation vectors are measured for the kyrgyz broadband seismic network knet in the tien shan region 870 events have been analysed for arrivals of p and pkp waves from all azimuths the deviations of slowness and backazimuth show a strong trend with values up to 1 s deg 1 for slowness values for waves arriving from the north and south and backazimuth deviations of in some cases more than 10 degrees for waves arriving from the east and west calculating the traveltime deviations of the stations for topography of the tien shan region and moho depth values appropriate for this area shows that most slowness and backazimuth deviations can be reduced to very small values the remaining mislocation vectors show no strong trends and are on average smaller than 0 2 s deg 1 for slowness and 2 degrees for backazimuth values which is within the error bars of these measurements results from array methods that rely on the knowledge of the backazimuth values show much improved resolution after the correction of the mislocation vectors which shows the importance of knowing and correcting for structures directly beneath arrays

pluvial climatic episodes in north america and north africa types and correlation with global climate abstract differences in late quaternary pluvial chronologies among the world s deserts can be related to fundamental differences in the climatology of those pluvial episodes paleoenvironmental records and climate model simulations for two northern hemisphere desert regions are used to illustrate two types of pluvial climatic regime in the higher latitude deserts of the american southwest pluvial conditions during the last glacial maximum were caused by the southward displacement of the westerly jet stream in turn caused by the blocking effect of laurentide ice sheet and a steepened arctic equator pressure gradient winter seasonality of precipitation and substantially lower mean annual temperatures are reflected in the paleoenvironmental record and simulated by models of 18 000 yr b p climate a second pluvial climate type which strongly influences the lower latitude deserts of north africa is characterized by enhanced summer monsoons and occurs during interglacials and perhaps interstadials the primary forcing mechanism of the last pluvial episode in the sahara and libyan deserts appears to have been increased solar radiation milankovitch forcing causing thermally enhanced ocean land pressure gradients and increased flow of maritime tropical air into the interior of the continent poleward displacement of the summer monsoon is consistent with floristic evidence for summer seasonality of precipitation and replicated by model simulations of 9000 and 6000 yr b p climate chronologies from some deserts such as the southern portion of the american southwest suggest the consecutive occurrence of both types of pluvial climatic regime pluvial conditions during the last glacial maximum were brought about by the equatorward displacement of the prevailing westerlies a subsequent more subdued pluvial period occurred later between 12 500 and 8000 yr b p caused by the poleward displacement of summer monsoons characterization of the climatology of pluvial episodes generates explanations for their temporal distribution and therefore this approach may be useful in assessing the quaternary chronology of pluvial episodes in other desert regions

correlated oceanic and continental records demonstrate past climate and hydrology of north africa 0 140 ka analysis of pollen and dinocysts in a core from the eastern atlantic core v22 196 lat 13 degree 50 n long 18 degree 57 w provides the first continuous record of paleoclimate and paleohydrology for the latest climatic cycle during interglacial intervals increases in pollen input from humid vegetation zones are correlative with high dinocyst productivity this data shows a close coincidence of northward extensions of sudanian and guinean vegetation zones in north africa and increases in fresh water input into the atlantic direct evidence indicates positive hydroclimatic phases at 135 125 103 80 47 and 12 10 ka coincident with widespread continental records of high lake levels that confirm the validity of the u th chronology

intrinsic stream capture control of stepped fan pediments in the high atlas piedmont of ouarzazate morocco abstract the ouarzazate basin is a cenozoic foreland basin located to the south of the high atlas mountains the basin has been externally drained during the quaternary with fluvial dynamics dominated by erosive processes from a progressive base level drop the current drainage network is composed of rivers draining the mountain and carrying large amounts of coarse sediments and by piedmont streams with smaller catchments eroding the soft cenozoic rocks of the ouarzazate basin the coarse grained sediments covering the channel beds of main rivers cause the steepening of the channel gradient and act as a shield inhibiting bedrock incision under such circumstances piedmont streams that incise to lower gradients evolve to large depressed pediments at lower elevations and threaten to capture rivers originating in the mountain much of the current surface of the ouarzazate basin is covered by coarse sediments forming large systems of stepped fan pediments that developed by the filling of low elevation pediments after a capture event we identified 14 capture events and previously published geochronology support an 100 ka frequency for fan pediment formation our study indicates that the reorganization of the fluvial network in the ouarzazate basin during the late pleistocene and holocene has been controlled by the piedmont stream piracy process a process ultimately controlled by the cover effect the stream capture is influenced by erosion sediment supply and transport and therefore may not be entirely decoupled from tectonic and climatic forcing indeed we show that at least two capture events may have occurred during climate changes and local tectonic structures control at most the spatial localization of capture events

impact of tectonics on alluvial landforms in the hexi corridor northwest china abstract the hexi corridor is a cenozoic foreland basin system on the northeast margin of the qinghai tibet plateau the structure is characterised by reverse faults trending nnw or wnw rivers have been affected by sinistral offset by wnw trending faults and dextral offset by nnw trending faults the cenozoic sinistral offset reaches 24 km on the north front fault trending wnw of the qilian mountains on the northeast front fault trending nnw of the yumu mountains a bluff of the hei river terrace has been dextrally offset by 12 m with a rate of 1 2 m ka the vertical movement of the faults has caused the deformation of river terraces and controlled the distribution and migration of the alluvial fans three terraces of the hei river have been vertically dislocated 9 m 4 m and 2 m respectively by a reverse fault with uplift rates of 0 4 1 7 m ka two reverse faults have caused the older alluvial fans to be dissected and the younger fans to be deposited more towards the centre of the jiuquan basin the dislocation of terraces and the migration of alluvial fans suggests that tectonic activity was more intense around 160 and 40 ka bp

tectonic controls on the morphometry of alluvial fans around danehkhoshk anticline zagros iran article i nfo alluvial fans are important landforms where their morphology and morphometry reflect changes in tectonic climate base level and drainage basin characteristics along the margins of tectonically active mountain ranges like the zagros mountains alluvial fans are generally assumed to act as useful landforms for identify ing the level of tectonic activity the purpose of this paper is to evaluate the relationship between active tec tonics and morphometric characteristics of alluvial fans around danehkhoshk anticline in the simply folded belt of zagros morphometric characteristics of alluvial fans such as area fa slope sf length of base bf width length ratio w l radius r sweep angle sa and entrenchment e as well as valley floor width to height ratio vf and strata dips of anticline limbs dal were measured the study area was sub divided into eight tectonic zones and then the mean values of the above mentioned parameters were calculated in each zone result reveals that values of sa bf and e are directly proportional to dal the poor relationships between catchment characteristics slope and area and fan parameters are probably due to extensive karstic landforms of catchments having complex hydrologic systems and hence result in complex catchment fan relations the highly entrenched fans with high sweep angles and long bases are characteristic of tectonically active fronts of danehkhoshk anticline having v shaped valleys higher vf values steep triangular facets and more rotated limbs higher dal values apart from the tectonic control on fan development the fan head entrenchment and negative accumulation spaces on most alluvial fans can be attributed to decreased sediment load and discharge the drier the present day climate regime

alluvial fan surface ages recorded by landsat 8 imagery in owens valley california abstract alluvial fans are important depositional landforms that offer valuable records of terrestrial sedimentation history if their surfaces can be mapped and dated accurately unfortunately as this often depends on detailed field mapping and intensive absolute dating techniques it can be a challenging expensive and time consuming exercise in this study we demonstrate that quantitative information about the ages of alluvial fan surfaces in owens valley california is recorded by landsat 8 multispectral satellite imagery we show that systematic changes in the wavelength dependent brightness of fan surfaces occur gradually over a timescale of 100 kyr in this semi arid setting and are highly correlated with known deposit ages using spectro radiometry and x ray diffraction analysis of sediment samples collected in the field we interpret that surface reflectance evolves primarily in response to the in situ production of secondary illite and iron oxide by weathering in this landscape furthermore we demonstrate that first order predictions of absolute fan surface age can be derived from multispectral imagery when an initial age calibration is available these findings suggest that multispectral imagery such as landsat data can be used i for preliminary mapping of alluvial fans prior to detailed field work and before choosing sampling sites for conventional dating techniques and ii to extend age models to un dated neighbouring surfaces with equivalent physical properties once an age brightness calibration has been established

acoustic imaging of hydrothermal plumes east pacific rise 21 n 109 w we report the first observations based on acoustic imaging of large scale structure and time variability of buoyant plumes emanating from black smoker type seafloor hot springs three dimensional plume reconstructions were made from a digital data set of acoustic backscattering information recorded on a prototype submersible mounted sonar system the acoustic images of two adjacent black smokers depict volume and show zones of flow organization meters to tens of meters in the lower 40 m of the buoyant plumes the two plumes coalesce bend in the prevailing current exhibit short term minutes variation in cross section and rapid seconds turbulent eddy variations at small scales 1 m the plume imaging system is being developed for determination of plume dynamics flux determinations when combined with chemical and thermal measurements and long term monitoring of the activity of seafloor hydrothermal fields

distribution and composition of hydrothermal plume particles from the ashes vent field at axial volcano juan de fuca ridge in 1986 and 1987 buoyant and neutrally buoyant hydrothermal plume particles from the ashes vent field within axial volcano were sampled to study their variations in composition with height above the seafloor individual mineral phases were identified using standard x ray diffraction procedures elemental composition and particle morphologies were determined by x ray fluorescence spectrometry and scanning electron microscopy x ray energy spectrometry techniques the vent particles were primarily composed of sphalerite anhydrite pyrite pyrrhotite chalcopyrite barite hydrous iron oxides and amorphous silica grain size analyses of buoyant plume particles showed rapid particle growth in the first few centimeters above the vent orifice followed by differential sedimentation of the larger sulfide and sulfate minerals out of the buoyant plume the neutrally buoyant plume consisted of a lower plume which was highly enriched in fe s zn and cu and an upper plume which was highly enriched in fe and mn the upper plume was enriched in fe and mn oxyhydroxide particles and the lower plume was enriched in suspended sulfide particles in addition to the fe and mn oxyhydroxide particles the chemical data for the water column particles indicate that chemical scavenging and differential sedimentation processes are major factors controlling the composition of the dispersing hydrothermal particles short term sediment trap experiments indicate that the fallout from the ashes vent field is not as large as some of the other vent fields on the juan de fuca ridge

late pleistocene and holocene alluvial archives in the southwestern mediterranean changes in fluvial dynamics and past human response abstract cohesive floodplains from the semiarid southwestern mediterranean morocco tunisia exhibit excellent preservation toward lateral erosion and present a continuous record of late pleistocene and holocene flood dynamics holocene mean sedimentation rates and 14 c cumulative probability plots from cohesive tunisian floodplain sequences reveal a coupling with north atlantic bond events short term periods of fluvial activity match well with north atlantic coolings the findings indicate a strong climatic link from 4 8 ka until today unlike fluvial archives from the humid mid latitudes semiarid southwest mediterranean alluvial archives exhibit phases of increased fluvial dynamics under generally drier conditions the 14 c cumulative probability plot of archeological sites in mediterranean north africa indicates correlations with alluvial records therefore prehistoric societies seemed to be very sensitive to past shifts in landscape dynamics

late quaternary climatic reconstruction for the maghreb north africa abstract the paleoclimatic evolution of the maghreb since 40 000 yr b p is not yet known in great detail for that purpose data from more than 35 sites have been collected although only about 11014c dates and a few prehistoric remains can be used to correlate these data however the sites are well distributed over the maghreb and the convergent results from paleosoils paleolakes alluvian or eolian deposits pollen analysis etc induce one to draw up a certain pattern in climatic evolution for the period between 40 000 and 20 000 yr b p the data are still fragmentary but are concordant in indicating wetter environments generalized as far as the northern sahara the effect of dropping temperatures on the increasing water budget and soil moisture is not yet clear but rainfall could have been more regular and more abundant as a consequence of the shifting of middle latitude cyclone paths towards the south in both summer and winter the environmental indicators show that between about 20 and 18 000 yr b p rainfall progressively became more irregular however proofs of aridity in the south maghreb and the northern sahara is evident only for the period between 12 10 000 yr b p at that time the saharan high pressure belt was reinforced and well established at higher latitudes than at present this dry climate lasted more or less until the thermal optimum after 6000 yr b p there is some evidence that the climate became wetter again but the environmental conditions were rather different from those of before 20 000 yr b p for instance the middle holocene dark humic sois are the result of a pedogenesis different from that of the late pleistocene reddish paleosoils for the last two millennia it is not easy to distinguish between a natural trend towards aridification and desertification through man s intervention

a chronology of alluvial fan response to late quaternary sea level and climate change crete abstract to better understand how fluvial systems respond to late quaternary climatic forcing osl and u series dating was applied to stratigraphically significant sedimentary units within a small 2 alluvial fan system the sphakia fan in southwest crete the resultant chronology comprising 32 osl and u series ages makes sphakia fan one of the best dated systems in the mediterranean and suggests that cretan fans responded to climate in two ways first during the transitions between marine isotope stage mis 5a 4 and mis 2 1 sphakia fan was characterised by significant entrenchment and distal shift in the zone of deposition it is proposed that the phases of entrenchment were driven by sea level induced base level fall during mis 5a 4 and landscape stabilisation during the onset of the current interglacial mis 2 1 second with the exception of these two entrenchment episodes fan alluviation occurred across the entire last interglacial glacial cycle in all climatic settings i e interglacials interstadials and stadials it is likely that the topographic setting of the catchment supplying sediment to sphakia fan maintained high sediment transfer rates during most climatic settings enabling fan aggradation to occur except during major climatic driven transitions i e major sea level fall and postglacial vegetation development

techniques for measuring rock weathering application to a dated fan segment sequence in southern tunisia there are a number of techniques for estimating the amount of weathering a clast has undergone these usually have the objective of establishing an ordinal chronology of geomorphological surfaces or investigation of site specific conditions affecting weathering rates three such techniques are applied to a dated sequence of alluvial fan segments in southern tunisia two of these techniques depend on measuring surface roughness the micro roughness meter and a displacement approach and one uses the structural weakening of the rock fabric schmidt hammer the micro roughness meter enables calculation of standard deviation of surface height variation root mean square roughness and surface autocorrelation function of these techniques schmidt hammer rebound values standard deviation root mean square roughness and the displacement technique show systematic changes on the three fan segments which are statistically significant at the 0 05 level however the amount of variance in all datasets is very large indicating the need for caution in application of these techniques for relative dating 1998 john wiley sons ltd

tectonic and climatic implications of alluvial fan sequences along the batinah coast oman along the batinah coast in northern oman seven levels of alluvial fan remnants stretching from the oman mountains to the gulf of oman were considered evidence for quaternary uplift by previous workers we suggest that these regionally extensive fan remnants were instead caused by quaternary climate fluctuations acting on the oman mountain headwaters and the major wadi systems the gradients of these wadis were oversteepened twice by mid tertiary uplift of the mountains and by a major plio pleistocene alluvial aggradation event wadi channels are still being cut through this massive deposit the episodic nature of climatically induced downcutting has produced the fan terrace levels the southern batinah coast is a hinge zone about which the arabian plate is tilting as it collides with the eurasian plate to the north the musandam peninsula and the associated fans are subsiding in response to subduction to the south uplifted marine shorelines suggest an apparent rigid plate response to the northward subduction

centennial scale late pleistocene to mid holocene synthetic profile of the medjerda valley northern tunisia during the late pleistocene to mid holocene the medjerda river system in mediterranean tunisia shows events of increased flooding and aggradation of relatively coarse overbank deposits geomorphic activity in river systems within this region correlate with aridification of the climate but not with temperature the late pleistocene to mid holocene synthetic profile of the ghardimaou floodplain mid medjerda river indicates fluvial activity from 12 4 to 11 8 6 6 to 6 0 and after 4 8 ka cal bp external correlations imply global and regional causes of the late pleistocene to mid holocene climatic development such as north atlantic deep water formation fluctuations in saharan humidity and impacts of local mediterranean weather phenomena palaeomagnetic records offer additional opportunities to interpret fluvial activity in river systems especially for detecting chronological hiatuses in apparently homogeneous sediment sequences

fragility of western mediterranean landscapes during holocene rapid climate changes abstract in this paper we explore the evidence for holocene rapid climate changes rccs in western mediterranean records examining similarities and differences in the timing and nature of impacts on different components of the natural environment vegetation fluvial and coastal sedimentation fire activity soil formation marine lacustrine and fluvial archives of the western mediterranean iberian peninsula and northwest africa provide evidence for both pervasive millennial scale climatic variability and abrupt decadal to centennial scale transitions we focus in particular on three rccs characterised by high latitude cooling glacier advances and north atlantic ice rafting events the mid holocene rcc interval 6 5 cal ka bp the late holocene rcc interval 3 5 2 5 cal ka bp and the historical rcc interval known as the little ice age lia 1300 1950 ad evidence from multiple records indicates wide ranging impacts of rccs in the western mediterranean region the three rcc intervals were characterised however by contrasting hydrological situations in the western mediterranean with prevailing dry conditions including marked aridification events during the rcc intervals 6 5 and 3 5 2 5 cal ka bp and prevailing or recurrent wet conditions during the lia we examine issues of proxy sensitivity in palaeoecological and geomorphological records and evaluate examples of contrasting geomorphological responses to regional climatic triggers between humid and semi arid sectors of the western mediterranean finally we consider the long term sensitivity of the region to rapid climate change the role of threshold changes and the extent to which this region represents a fragile landscape

eustatic and tectonic control on late quaternary alluvial fans along the tyrrhenian sea coast of calabria south italy abstract the late quaternary evolution of the western coastal range northern calabria has been reconstructed by means of a multidisciplinary approach we have focused on the role of allocyclic mechanisms tectonics and base level change considered as factors influencing alluvial fan development in particular we identify no clear evidence for a major influence of climate age constraints were provided by a tephra layer and a paleosol coupled with geomorphological correlations middle pleistocene perched terraces and fan relicts hanging above fault scarps indicate that tectonics caused switches in fan dynamics between aggradational and erosional phases as tectonic activity declined during late middle pleistocene eustatically driven changes in base level possibly plus low rates of tectonic subsidence represent the main factor which controlled late pleistocene fan development the two major fan aggradation phases occurred during periods of climate amelioration prior to highstands whereas high sea levels ois 5 and 1 promoted channel incision into the fan surfaces this continued during periods of climate deterioration with a seaward progradation of alluvial deposition sea level rise is seen as creating accommodation space producing very open fans locally within mountain embayments the fans are characterised by a mushroom shape in plan view and by a retrograde stacking pattern of alluvial fan facies

segmentation of alluvial fans in death valley california new insights from surface exposure dating and laboratory modelling laboratory experiments recent paleoenvironmental analyses of rock varnish and surface exposure dating of geomorphic units have led to new insights into the process of entrenchment and segmentation of alluvial fans and into the history of quaternary sedimentation in death valley entrenchment begins at the fanhead as the trench deepens its down slope end migrates down fan taking several tens of thousands of years to reach lower parts of the fan laboratory experiments suggest however that a new segment begins to grow at the toe long before the trench reaches this part of the fan furthermore the initial slope of the segment is not the equilibrium slope field evidence supports this model the tectonic tilting that caused entrenchment and segmentation in death valley may have been triggered by loading of the valley with water sedimentation on the salt pan in southern death valley is not at present in equilibrium with that on the fans rather it seems to be adjusting to an increase in the amount of fine material reaching the playa due in part to breaching of the outlet of lake tecopa somewhat after 600 ka bp and in part to subsidence of different parts of the valley at different rates failure to recognize this disequilibrium resulted in errors in earlier estimates of the age of the segmentation events

paleohydrologie isotopique de l oued el akarit sud tunisie au pleistocene superieur et a l holocene previous data from gobert and howe coque and page on the upper quaternary deposits from wadi el akarit are expanded through radiocarbon and stable isotope measurements 18c and 13c from carbonates 18o and 34s from sulphates these new results report on detailed stratigraphic profiles and indicate fluviatile and palustrine environments during the late pleistocene vegetal dams linked to spring activity caused water stagnation fine grained sediments were deposited with appreciable amounts of organic matter which locally developed into some peaty levels in the upper part of the typical cross section located on the left bank about 100 m upstream of the railway bridge pleistocene deposits show a palustrine carbonate crust this formation is attributed to the drainage induced by the major marine regression at glacial maximum during the holocene stagnation periods still occurred interrupted by flood episodes marked by bottom transport and coarse deposits in the typical cross section deposits are well drained and oxidized however downstream a stagnation episode takes place with a rich malacofauna dominated by cerastoderma cardium the uneven distribution of rainfall is documented by the alternation of fine and coarse deposits and it is accentuated by the climatic evolution toward aridity which results in the formation of a gypsum crust after 4000 yr bp later exceptional floods removed sediments from the wadi channel cutting the upper quaternary deposits for about 13 m along the central course of the wadi radiocarbon ages on carbonates are discussed according to various possible origins of total dissolved carbon atmospheric co2 organic activity in the basins and aquifer supply a simple exchange mixing model between aquifer bicarbonate and atmospheric co2 is used for calculations of initial carbonate activities chemical isotopic and hydraulic steady states are assumed in this treatment present day 14c and 13c contents of aquifer bicarbonate are thus adopted for past values the model leads to acceptable corrected ages for pleistocene carbonate samples corrected ages are younger than apparent ages by a few thousands of years and become consistent with ages given by organic matter from nearby levels however holocene samples appear overcorrected their crystallization appears to be controlled by modern organogenic co2 holocene 14c ages are probably correct 18o contents of carbonate indicate a continental origin for environmental water nodules and carbonate concretions crystallized in bottom sediments through upward leakage of confined aquifers they show 18o contents compatible with present day stable isotope compositions of artesian springs this supports the concept of a good time constancy in stable isotope contents shell carbonate values indicate a variable evaporation rate from the same initial waters 18o and 34s contents of gypsums are also compatible with aqueous sulphates from springs the data set falls within the range of permo triassic sulphate and suggests deep hydraulic connections within aquifer systems the entire formation appears to be of continental origin the occurence of cardium rich basins can only be attributed to the initial salinity of springs waters 7 9 g 1 1 sightly increased by evaporation this signifies that one can not deduce any tectonic uplift from their location with respect to sea level

relative age dating of inorganic deposits the need for a more critical approach relative age dating rd techniques rely on indices of post depositional change principally degree of rock surface weathering or soil development in the chronosequence approach where a measured parameter displays a trend in values that parallels a trend in surface age the parameter is commonly assumed to be age dependent such an assumption may be invalid this is demonstrated with particular reference to glacier forelands where environmental and sedimentological factors that can influence commonly used rd parameters may exhibit systematic variations in parallel with but independent of surface age where some independent dating control is available regression analysis may be used to define chronofunctions providing numerical ages based on rd parameters however the error limits of such ages are often underestimated the use of modal rather than mean values is particularly problematic a more critical approach is suggested involving deductive experiments to examine the influence of non tempo

sedimentation of tephra by volcanic plumes i theory and its comparison with a study of the fogo a plinian deposit sao miguel azores sedimentation of ejecta from volcanic plumes has been studied as a function of distance from the source in the fogo a plinian deposit sao miguel azores the fogo a trachytic pumice deposit is reversely graded and can be divided into two parts on the basis of pumice colour abundance of syenite accessory lithic clasts and distribution the lower syenite poor part was dispersed to the south and was clearly influenced by wind the upper syenite rich part is coarsegrained and has a nearly symmetrical distribution around the vent elongation of isopachs to the east indicate a weak wind influence the grain size variations of lithic and crystal components in the upper coarse part were studied total accumulation and accumulation per unit area expressed in kg m2 show good fits to a gaussian function at distances greater than 7 km for grain diameters less than 2 cm these results agree with a theoretical model for a radially spreading turbulent current moving over a quiescent fluid the gaussian coefficient is shown to be a function of grain size and the flow rate of material into the umbrella region of the eruption column the coefficient is therefore also a function of column height the column height deduced from these data is 21 km which is in broad agrrement with the column height of 27 km deduced from maximum clast dispersal using the method of carey and sparks 1986 the accumulation of clasts larger than 2 cm agrees with a theory for the fallout of clasts from the margins of the ascending eruption column which treats the plume as a succession of large eddies that decrease their mass of particles as an exponential function of time calculations are also presented for the influence of the radial inflow of surrounding air into the column on the deposition of clasts these calculations constrain the wind speed during the later part of the fogo a eruption to be at most a few metres per second the study has allowed four different dynamic categories of clast behaviour to be recognised in eruption columns

on the ages of humid holocene and late pleistocene phases in north africa remarks on late quaternary climatic reconstruction for the maghreb north africa by p rognon discussion sur l article de rognon p paru en 1987 dans palaeogeogr palaeoclimatol palaeoecol vol 58 p 11 34

use of multiparameter relative age methods for age estimation and correlation of alluvial fan surfaces on a desert piedmont eastern mojave desert california numerical and calibrated age determinations of the late quaternary alluvial fan deposits of the soda mountains piedmont in the mojave desert provide an opportunity to study the utility of the multiparameter relative age ra method for distinguishing and mapping geomorphic surfaces on a desert piedmont most ra parameters could not discriminate between deposits of holocene age although pavements have formed over locally significant parts of surfaces as young as middle holocene several parameters including lithologic composition particle size soil development and varnish cover permit distinguishing between holocene surfaces and late pleistocene surfaces statistically significant differences in initial particle size and lithology of the deposits inferred to be the result of complex interaction among hillslope alluvial fan and eolian processes and climatic change create conditions unfavorable for use of most ra techniques in contrast soil profile development and varnish cover data are successful in discrimination among deposits of holocene and pleistocene age this is attributed to the development of pedogenic features and varnish that are strongly dependent on dust influx and to the relatively minor dependence of these features on differences in the depositional character of the fan

flood frequencies reveal holocene rapid climate changes lower moulouya river northeastern morocco current high resolution palaeoenvironmental records reveal short term holocene cool ings one of these major holocene rapid climate changes occurred between 3 2 and 2 7 cal ka bp the sensitivity of river systems vis a vis slight and short term holocene climatic variations is a subject of controversy in the scientific community in this paper we present a 4 0 to 1 4 cal ka bp palaeoflood record from the lower moulouya river northeastern morocco to demonstrate the high sensitivity of semiarid rivers in the southwestern mediterranean towards holocene environmental changes the lower moulouya flood deposits are characterised by thick well stratified predominantly clayey to silty overbank fine sediments these cohesive sediments show evidence of excellent preservation conditions against fluvial erosion and contain a continuous record of mid to late holocene flood sequences the moulouya palaeoflood record can be interpreted in the context of regional and global high resolution proxy data revealing a strong coupling with holocene rapid climate changes the centennial scale moulouya palaeohydrological history will be discussed with palaeoenvironmental data from the same record palaeomagnetics sedimentary charcoal record anthracological analyses snail analyses to generate new ideas about the mid to late holocene hydrological cycle in the southwestern mediterranean the deduced features of pronounced lower moulouya flooding and the decreased fire recurrence during holocene cooling remain somewhat inconsistent with the interpret ation of other palaeohydrological and paleaoecological records from the southwestern mediterranean however enhanced lower moulouya flood frequencies between 3 2 and 2 7 cal ka bp agree with increased floodplain aggradation in other major river systems of mediterranean north africa copyright 2009 john wiley sons ltd

geothermal heat flux from hydrothermal plumes on the juan de fuca ridge thesis m s massachusetts institute of technology dept of ocean engineering and woods hole oceanographic institution 1991