isotopic reconstruction of the african humid period and congo air boundary migration at lake tana ethiopia abstract the african humid period of the early to mid holocene 12 000 5000 years ago had dramatic ecological and societal consequences including the expansion of vegetation and civilization into the green sahara while the humid period itself is well documented throughout northern and equatorial africa mechanisms behind observed regional variability in the timing and magnitude of the humid period remain disputed this paper presents a new hydrogen isotope record from leaf waxes d wax in a 15 000 year sediment core from lake tana ethiopia 12 n 37 e to provide insight into the timing duration and intensity of the african humid period over northeastern africa d wax at lake tana ranges between 80 and 170 with an abrupt transition from d enriched to d depleted waxes between 13 000 11 500 years before present 13 11 5 ka a similarly abrupt transition from d depleted to d enriched waxes occurs ca 8 5 8 ka and is followed by generally d enriched waxes throughout the late holocene trends in d wax covary with changes in northern hemisphere summer insolation and reflect increased precipitation at lake tana during the ahp however the transition from d depleted to d enriched waxes occurs earlier at lake tana ca 8 ka vs 5 ka than in many other regional records and the amplitude of d depletion during the ahp is larger at lake tana as well we attribute this early enrichment to a reduction of moisture derived from westerly sources the congo basin and atlantic ocean which we suggest are d depleted relative to moisture sourced from the east indian ocean and the north red sea and mediterranean sea our new record highlights the importance of both the northward migration of the tropical rain belt as well as east west migration of the congo air boundary to precipitation source and amount during the african humid period

late pleistocene and holocene vegetation history of the bale mountains ethiopia abstract a sediment core recovered from garba guracha a glacial lake at 3950 m altitude in the bale mountains of ethiopia at the boundary of the ericaceous and afroalpine vegetation belts provides a 16 700 year pollen record of vegetation response to climatic change the earliest vegetation recorded was sparse and composed mainly of grasses amaranthaceae chenopodiaceae and artemisia indicating an arid climate at 13 400 cal bp amaranthaceae chenopodiaceae pollen declined sharply and cyperaceae increased suggesting a change to moister conditions the younger dryas interval is represented by a small increase in artemisia and reduced cyperaceae indicating aridity just after the start of the holocene 11 200 cal bp the upper altitudinal limit of the ericaceous belt rose and woody ericaceous vegetation extended across the sanetti plateau in response to increased moisture and temperature the marked change from clastic to organic lake sedimentation at this time reflects the increase in woody vegetation cover in the lake catchment accompanied by soil stabilisation and increased leaf litter and soil humus content from about 6000 cal bp and especially after 4500 cal bp mid altitude dry afromontane juniper podocarpus forests developed on the northern slopes of the mountains in response to reduced rainfall in a shortened wet season erica shrub and forest decreased in area and altitude and the afroalpine ecosystem expanded on the plateau podocarpus declined from about 2000 cal bp as juniperus increased to its present dominance at 2500 3300 m altitude human impact on the high altitude afroalpine and ericaceous vegetation has been relatively minor confirming that the endemic biodiversity of the ethiopian mountains is a legacy of natural holocene vegetation change following repeated expansion and contraction of the upland ecosystems during the quaternary

a record of holocene climate change from lake geochemical analyses in southeastern arabia lacustrine sediments from southeastern arabia reveal variations in lake level corresponding to changes in the strength and duration of indian ocean monsoon iom summer rainfall and winter cyclonic rainfall the late glacial holocene transition of the region was characterised by the development of mega linear dunes these dunes became stabilised and vegetated during the early holocene and interdunal lakes formed in response to the incursion of the iom at approximately 8500 cal yr bp with the development of c3 dominated savanna grasslands the iom weakened ca 6000 cal yr bp with the onset of regional aridity aeolian sedimentation and dune reactivation and accretion despite this reduction in precipitation the lake was maintained by winter dominated rainfall there was a shift to drier adapted c4 grasslands across the dune field lake sediment geochemical analyses record precipitation minima at 8200 5000 and 4200 cal yr bp that coincide with bond events in the north atlantic a number of these events correspond with changes in cultural periods suggesting that climate was a key mechanism affecting human occupation and exploitation of this region

early cretaceous bimodal volcanism in the duolong cu mining district western tibet record of slab breakoff that triggered ca 108 113 ma magmatism in the western qiangtang terrane abstract we report new zircon u pb ages and hf isotope compositions and whole rock major and trace element and sr nd isotope data for the meiriqiecuo formation mf bimodal volcanic rocks collected from the duolong cu mining district dcmd in the western qiangtang terrane qt western tibet these data provide important constraints on the petrogenetic evolution and geodynamic setting of early cretaceous magmatism in the dcmd the mf bimodal volcanic rocks are mainly basaltic andesite and andesite with subordinate rhyolite four mafic samples yielded zircon u pb ages of ca 108 2 113 0 ma and one silicic sample has an age of 109 3 2 2 ma indicating that the mafic and silicic eruptions were contemporaneous the mf bimodal volcanic rocks belong to the medium k calc alkaline to shoshonite series the rocks show arc type affinities characterized by significant enrichment in light rare earth lan ybn 7 74 12 60 and large ion lithophile elements rb cs k and pb but depletions in the high field strength elements nb ta and ti which geochemically resemble andean arc basalts therefore the mf bimodal volcanic rocks were likely emplaced at an andean type active continental margin and represent an early cretaceous magmatic arc that was located at the western qt margin moreover the mafic volcanic rocks have high initial sr isotopic ratios 0 705269 0 705413 and negative end t values of 1 5 to 0 6 compared with the silicic volcanic rocks 87sr 86sr i 0 704770 0 704903 end t 1 2 to 1 3 zircons from silicic samples have significantly higher ehf t values 11 6 to 15 5 and predominantly lower paleoproterozoic hf crustal model ages tdmc 180 428 ma than the mafic samples which have variable ehf t values of 3 4 to 13 0 and tdmc ages of 346 952 ma these results indicate that the mafic and silicic end members of the mf bimodal suite were generated from mantle and crustal sources respectively the basaltic andesite and andesite may have been derived from mantle enriched by the metasomatism of subducted fluids whereas the rhyolite could have been derived by partial melting of mafic juvenile crust that originated from an older and more depleted mantle in light of the geochemical characteristics and field relationships we propose that breakoff of the bangong nujiang oceanic lithosphere was responsible for the generation and emplacement of the mf bimodal volcanic rocks the fact that the mf bimodal volcanic arc magmatism was active at ca 108 113 ma indicates that it was associated with closure of the bangong nujiang ocean via an arc arc soft collision during the early cretaceous

tropical sea surface temperatures for the past four centuries reconstructed from coral archives most annually resolved climate reconstructions of the common era are based on terrestrial data making it a challenge to independently assess how recent climate changes have affected the oceans here as part of the past global changes ocean2k project we present four regionally calibrated and validated reconstructions of sea surface temperatures in the tropics based on 57 published and publicly archived marine paleoclimate data sets derived exclusively from tropical coral archives validation exercises suggest that our reconstructions are interpretable for much of the past 400 years depending on the availability of paleoclimate data within and the reconstruction validation statistics for each target region analysis of the trends in the data suggests that the indian western pacific and western atlantic ocean regions were cooling until modern warming began around the 1830s the early 1800s were an exceptionally cool period in the indo pacific region likely due to multiple large tropical volcanic eruptions occurring in the early nineteenth century decadal scale variability is a quasi persistent feature of all basins twentieth century warming associated with greenhouse gas emissions is apparent in the indian west pacific and western atlantic oceans but we find no evidence that either natural or anthropogenic forcings have altered el nino southern oscillation related variance in tropical sea surface temperatures our marine based regional paleoclimate reconstructions serve as benchmarks against which terrestrial reconstructions as well as climate model simulations can be compared and as a basis for studying the processes by which the tropical oceans mediate climate variability and change

persistent solar influence on north atlantic climate during the holocene surface winds and surface ocean hydrography in the subpolar north atlantic appear to have been influenced by variations in solar output through the entire holocene the evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon 14 and beryllium 10 and centennial to millennial time scale changes in proxies of drift ice measured in deep sea sediment cores a solar forcing mechanism therefore may underlie at least the holocene segment of the north atlantic9s 1500 year cycle the surface hydrographic changes may have affected production of north atlantic deep water potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally

tidal energy dissipation at the sill of sechelt inlet british columbia abstract the energy budget of a tidally active shallow silled fjord is discussed constriction of the flow over the shallow sill causes a reduction in tidal amplitude and a phase lag across the sill a generalized expression for the total power extracted from the barotropic tide by dissipation at the sill is derived as a function of the tidal amplitude difference and phase lag of the tidal elevation using tide gauge data from both sides of the sill at the entrance to sechelt inlet british columbia this generalized expression yields estimates for the energy flux of the barotropic tide which approach 100 mw during spring tides from direct measurements of the currents the estimated frictional dissipation is equal to the flux out of the barotropic tide within experimental error a small amount of the energy flux 5 is estimated to go into the generation of a tidal jet which dissipates within a few kilometers of the sill and contributes to the formation of a mid water layer

reflection of the internal tide in upper loch linnhe a scottish fjord abstract results from an observational study of the internal tide of upper loch linnhe a fjord on the west coast of scotland are presented between november 1992 and june 1993 moorings containing current meters measuring velocity temperature and salinity were deployed at two sites in the loch observations of the horizontal velocities show the presence of an energetic first mode linear internal tide at the m 2 frequency with surface amplitudes of up to 0 57 m s 1 cf 0 03 m s 1 for the barotropic component independent evidence of the m 2 internal tide is provided by vertical velocities inferred from salinity and temperature variations in june 1993 an undulating ctd was used to study the horizontal propagation of the semi diurnal internal tide the results show that over the inner reaches of the loch the internal tide behaves as a standing wave with a large proportion of the internal tide energy being reflected at the landward end of the loch using the phase relationship between the velocity components from the mooring data it is demonstrated that there is a significant standing wave component present throughout the 7 month observational period closer to the sill the undulating ctd observations show the formation of an intense hydraulic jump on the flood tide suggesting the area immediately landward of the sill to be a region of complicated dynamics and high dissipation

the comparison of a numerical model to observations of velocity and scalar data in burrard inlet and indian arm british columbia a laterally integrated time dependent numerical model of burrard inlet and indian arm is compared to time series of cyclesonde velocity and scalar data that have 10 m vertical resolution as long as deep water renewal is not occurring the model can reproduce the observed m2 and k1 tidal response in the basin of indian arm also although the rate of freshwater inflow into indian arm and burrard inlet is not large the simulated tidal response is noticeably improved if freshwater inflow is taken into account when deepwater renewal is observed to occur the model also simulates renewal but it does so poorly and the ability of the model to accurately simulate the m2 and k1 tide degrades significantly

observations of internal wave reflection off sloping bottoms current and temperature spectra exhibit intensification and polarization near sloping bottoms over a band of frequencies centered at the local internal wave critical frequency examples are drawn from a variety of island seamount and continental slope observations waves propagating down from mid depth change energy density wave number and azimuth when they reflect off the bottom providing a natural perturbation to the deep ocean interior internal wave spectrum deviations from linear theory are interpreted as evidence for dissipation through subsequent shear instability and or nonlinear interaction topographic features appear likely to be energy sinks for internal wave energy

shoreline fluctuations of lake hayk northern ethiopia during the last 3500years geomorphological sedimentary and isotope records abstract this paper presents a stratigraphical and geomorphological analysis supported with geochronological dating aiming to reconstruct the late holocene lake level fluctuations and paleohydrological history of lake hayk this lake 23 km 2 in surface is located in the eastern side of the northern ethiopia highlands at an altitude of around 2000 m above sea level along the northern margin of the lake which is mainly characterized by steep rocky slopes the studied succession consists of colluvial and stromatolitic deposits along the southern margin which is characterized by a wide coastal plain drained by the main watercourse feeding the lake the ankarka river the study deposits are mainly palustrine deltaic and fluvial in origin geomorphological and sedimentological data integrated with a radiocarbon chronology highlighted that during the last 3500 years the lake was affected by three main highstand phases which occurred at about 3250 3000 2600 950 and 650 160 cal yrs bp respectively these highstands are documented by aggradation of siliciclastic systems both along the southern and northern margins whereas isotope data highlight that most of the stromatolitic deposits developed during phases of intense evaporation this peculiar feature could be differently explained i stromatolite growth was triggered by rapid lacustrine rises but mainly occurred during the long lasting lacustrine falls ii stromatolite developed during tectonic induced transgressions which occurred during phases of intense evaporation despite the possible effects of local tectonics the lake hayk record shows a good correlation with those of several late holocene lakes of east africa lakes naivasha tanganyika victoria turkana abhe ziway shala bosumtwi and abyiata highlighting that it can be considered as a suitable climatic proxy for late holocene time span in particular by both siliciclastic and stromatolitic deposits occurring in the uppermost part of the sedimentary succession record high frequency oscillations which have been ascribed to three main droughts that occurred during the little ice age these droughts were previously documented only in lake naivasha and lake victoria

a new model of river dynamics hydroclimatic change and human settlement in the nile valley derived from meta analysis of the holocene fluvial archive in the nile catchment a growing number of site and reach based studies employ radiocarbon and more recently osl dating to reconstruct holocene river histories but there has been no attempt to critically evaluate and synthesise these data at the catchment scale we present the first meta analysis of published and publically available radiocarbon and osl dated holocene fluvial units in the nile catchment including the delta region and relate this to changing climate and river dynamics dated fluvial units are separated both geographically into the nile delta and white blue and desert nile sub regions and into depositional environment floodplain and palaeochannel fills cumulative probability density frequency cpdf plots of floodplain and palaeochannel units show a striking inverse relationship during the holocene reflecting abrupt 100 years climate related changes in flooding regime the cpdf plot of dated floodplain units is interpreted as a record of over bank river flows whilst the cpdf plot of palaeochannel units reflect periods of major flooding associated with channel abandonment and contraction as well as transitions to multi centennial length episodes of greater aridity and low river flow this analysis has identified major changes in river flow and dynamics in the nile catchment with phases of channel and floodplain contraction at c 6150 5750 4400 4150 3700 3450 2700 2250 1350 900 800 550 cal bc and cal ad 1600 timeframes that mark shifts to new hydrological and geomorphological regimes we discuss the impacts of these changing hydromorphological regimes upon riverine civilizations in the nile valley

age and backfill overfill stratigraphy of two tufa dams tigray highlands ethiopia evidence for late pleistocene and holocene wet conditions abstract geomorphological mapping of parts of the mekele outlier reveals the presence of numerous river tufa dams spring tufas and old landslides indicating high groundwater levels and karstic activity in the area during former times u th dating of river tufa dams indicates that wet conditions prevailed in the highlands as early as 15 000 bp this is 4 to 5000 years before the first signs of post last glacial maximum lgm humid conditions deduced from former lake extensions in the ethiopian and kenyan rift tufa build up and the corresponding wet conditions in the highlands gradually came to an end from 5000 bp onwards in the study area environmental deterioration started around 3000 bp and is explained as a result of forest recession evidence is presented showing that forest retreat and land degradation in the area is not climate driven but the result of human activities

holocene african droughts relate to eastern equatorial atlantic cooling here we present evidence that the holocene african monsoon system ams varied in response to the eastern equatorial atlantic sea surface temperature sst several short term episodes of decreased moisture availability as a result of low eastern equatorial atlantic sst are suggested by planktonic foraminiferal mg ca ratios these episodes promoted a weakening of the ams and thus determined the timing and intensity of arid periods local sea surface salinities also reveal regional patterns of precipitation in equatorial western africa the high eastern equatorial atlantic ssts occur in concert with seasonally increased insolation at low latitudes suggesting a strong response of african monsoonal precipitation to oceanic conditions at low latitudes

tectonic environment and geodynamic significance of the neo cimmerian donqiao ophiolite bangong nujiang suture zone tibet the bangong nujiang suture zone is underlain by a discontinuous and roughly ew ophiolite belt around donqiao gyanco ophiolitic and metasedimentary palaeozoic series form nappes thrusted over jurassic flysch series all these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting although strongly dismembered a complete ophiolitic sequence comprising harzburgites with large podiform type chromite deposits wehrlites cumulate gabbros sheeted dolerite dykes foliated garnet bearing amphibolites and pillow lavas has been found

holocene tropical western indian ocean sea surface temperatures in covariation with climatic changes in the indonesian region the sea surface temperature sst of the tropical indian ocean is a major component of global climate teleconnections while the holocene sst history is documented for regions affected by the indian and arabian monsoons data from the near equatorial western indian ocean are sparse reconstructing past zonal and meridional sst gradients requires additional information on past temperatures from the western boundary current region we present a unique record of holocene sst and thermocline depth variations in the tropical western indian ocean as documented in foraminiferal mg ca ratios and 18o from a sediment core off northern tanzania for mg ca and thermocline 18o most variance is concentrated in the centennial to bicentennial periodicity band on the millennial time scale an early to mid holocene 7 8 5 6 ka warm phase is followed by a temperature drop by up to 2 c leading to a mid holocene cool interval 5 6 4 2 ka the shift is accompanied by an initial reduction in the difference between surface and thermocline foraminiferal 18o consistent with the thickening of the mixed layer and suggestions of a strengthened walker circulation however we cannot confirm the expected enhanced zonal sst gradient as the cooling of similar magnitude had previously been found in ssts from the upwelling region off sumatra and in flores air temperatures the sst pattern probably reflects the tropical indian ocean expression of a large scale climate anomaly rather than a positive indian ocean dipole like mean state

linking long term gully and river channel dynamics to environmental change using repeat photography northern ethiopia abstract in the highlands of northern ethiopia gully occurrence is linked to poverty driven unsustainable use of the land in a vulnerable semi arid and mountainous environment where intensive rainfall challenges the physical integrity of the landscape trends in gully and river channel erosion and their relation to triggering environmental changes can proffer valuable insights into sustainable development in northern ethiopia in order to assess the region wide change in gully and river channel morphology over 140 years a set of 57 historical photographs taken in tigray and clearly displaying gully cross sections were precisely repeated from 2006 till 2009 ninety two percent of the gully and river sections n 38 increased in cross sectional area during the studied period especially after 1975 two repeatedly photographed catchments of lake ashenge and atsela allowed a detailed study of gully development from 1936 until 2009 a conceptual hydrogeomorphic model was devised for these catchments and validated for the northern ethiopian highlands three major phases can be distinguished in the hydrological regime of the catchments in the first phase between 1868 or earlier and ca 1965 the relatively stable channels showed an oversized morphology inherited from a previous period when external forcing in environmental conditions had caused the channels to shape in the second phase ca 1965 ca 2000 increased aridity and continued vegetation clearance accelerated the channel dynamics of the gully and river system the third phase ca 2000 present started after the large scale implementation of soil and water conservation measures in 2009 23 of the gully and river sections were stabilizing this paper validates previous research indicating severe land degradation in the second half of the 20th century additionally it demonstrates that the recent erosive cycle started around 1965 and that at the present time improved land management stabilizes headwater streams

paleoenvironmental changes during the last 4000 yr in the tigray northern ethiopia in the tigray region at the northern ethiopian highlands paleoenvironmental reconstruction based on several infilled valley deposit sequences suggests that the past 4000 yr comprised three major wetter periods ca 4000 3500 yr b p 2500 1500 yr b p and 1000 960 yr b p during which soils were formed and two degradation episodes ca 3500 2500 yr b p and 1500 1000 yr b p during which there was an increase of sediment yield from the slopes into the valleys for the past 1000 yr and in particular since the early 17th century stratigraphic records together with historic chronicles suggest increasing aridity although difficulties arise in distinguishing between natural and human impacts particularly in a region with a long established agricultural background stratigraphical and proxy paleoclimatic data have indicated climate as the main controlling factor responsible for the environmental changes in the tigray

the interaction of tides with the sill of a tidally energetic inlet abstract the interaction of the tides with the sill of a tidally energetic inlet observatory inlet british columbia is studied because of temporal variations in the stratification of the inlet a substantial seasonal variation is observed in the power withdrawn from the barotropic tide vigorous nonlinear internal motions occur in the region of the sill but most of the withdrawn tidal power is fed into a progressive linear internal tide the first two modes which contain almost all of the energy respond very differently to changes in stratification the energy flux of the first mode is insensitive to changes in surface stratification but increases dramatically as a result of deep water renewal the energy flux of the second mode exhibits the reverse behavior being insensitive to the occurrence of deep water renewal but being a strong positive function of the surface stratification even though the inlet has a distinct surface layer in summer and appears to be a two layer system the second mode

late glacial holocene diatom record of water chemistry and lake level change from the tropical east african rift lake abiyata ethiopia abstract diatom study of a 12 6 m long sediment core ab95ii from lake abiyata ethiopian rift 8 n revealed large amplitude changes in the water level and chemistry over the past 13 4 cal kyr bp past water conductivity ph and water chemical facies reflected by ionic ratios are quantitatively reconstructed based on diatom transfer functions previously established conductivity has varied from 400 to 21 500 s cm 1 ph from 8 to 10 5 the anion hco 3 co 3 2 cl so 4 2 and cation na k ca 2 mg 2 ratios ranged between 0 9 4 and 0 5 275 respectively a high water salinity oscillation is recorded at the base of the sequence the extrapolated age of this interval 12 7 12 0 cal kyr bp corresponds to the younger dryas chronozone the early middle holocene registers a high stand freshwater lake between 12 and 5 4 cal kyr bp changes in diatom assemblages suggest minor changes in water depth lake mixing and or nutrient content during this interval post 5 4 cal kyr bp shallow saline conditions generally predominate short term regressive events are recorded at 5 6 and 3 0 cal kyr bp our results are roughly consistent with previous works in the region they refine the timing of the hydrochemical changes and quantify their amplitude thus providing inputs for modelling approaches

the time dependent hydraulic flow and dissipation over the still of observatory inlet abstract the time dependent hydraulic flow over the sill of a tidally energetic fjord observatory inlet british columbia is studied acoustic observations of streamlines and velocity were made near the sill crest during the summer of 1982 a time when freshwater runoff into the inlet had created a distinct surface layer while the tide is accelerating a simple three layer hydraulic model accurately simulates the flow near the sill crest however once the tide begins decelerating the observed flow undergoes a transition which the three layer model cannot explain it is estimated that during the period of observation the hydraulic flow utilized only about 5 of all the energy removed from the barotropic tide this suggests that other processes such as the internal tide are removing most of the energy from the barotropic tide

a numerical model of stratified circulation in indian arm british columbia a laterally integrated numerical model has been developed for the study of stratified circulation in inlets the model is time dependent and includes advective terms horizontal and vertical turbulent diffusion of momentum and salt and variations in width and depth initial and boundary conditions based on winter measurements in burrard inlet and indian arm were used to study the inlet s response to tidal forcing the friction term was tuned to match calculated and observed dissipation rates evidence for a standing wave pattern in the response of the m2 and k1 tidal constituents is discussed a comparison of model results with cyclesonde current meter time series at two locations in indian arm is consistent with resonance a modal decomposition of the calculated response shows that most of the energy is contained in the barotropic and first baroclinic modes with the former dominating near the sill and the latter dominating in the deep basin second mode energy was significant for the k1 at some locations

a 9000 year oxygen and carbon isotope record of hydrological change in a small ethiopian crater lake the stable isotope composition of authigenic calcites in a core from lake tilo a small crater lake in the ethiopian rift valley provides a subcentury scale record of lake response to climatic change over the last 8850 14c yrs 9850 cal yrs an unusually high range of 18o and 13c values 15 is attributed in part to major changes in hydrothermal groundwater flux although hydrothermal groundwater influx to the lake was high during the early holocene its flow rate was relatively stable and thus climatically induced changes to the water budget can be inferred from variations in 18o and 13c ratios a major decline in hydrothermal groundwater input from 5500 14c yrs bp increased lakewater residence time and led to substantial increases in 18o and 13c before the mid holocene transition to more arid conditions during the last 2700 14c years diagenetic processes have resulted in extremely varied 18o and 13c values the holocene isotope record from lake tilo challenges the widely held assumpti

seasonal mixing processes in an arctic fjord system abstract studies of water movement in the aafardlikavs qaumarujuk fjord system in west greenland have barn carried out in order to understand the movement of mine tailings discharged into agfardlikavs fjord observations are explained in terms of resonance of the internal tide deep convection associated with sea ice growth and basin water exchange occurring at the time of ice breakup in early summer mixing processes are dominated by these events illustrating the limitations of any seasonal description based on conventional diffusive processes

the 8k event cause and consequences of a major holocene abrupt climate change abstract a prominent abrupt climate event about 8200 years ago brought generally cold and dry conditions to broad northern hemisphere regions especially in wintertime in response to a very large outburst flood that freshened the north atlantic changes were much larger than typical climate variability before and after the event with anomalies up to many degrees contributing to major displacement of vegetative patterns this 8k event provides a clear case of cause and effect in the paleoclimatic realm and so offers an excellent opportunity for model testing the response to north atlantic freshening has the same general anomaly pattern as observed for older events associated with abrupt climate changes following north atlantic freshening and so greatly strengthens the case that those older events also reflect north atlantic changes the north atlantic involvement in the 8k event helps in estimating limits on climate anomalies that might result in the future if warming caused ice melt and hydrologic cycle intensification at high latitudes lead to major changes in north atlantic circulation few model experiments have directly addressed the 8k event and most studies of proxy records across this event lack the time resolution to fully characterize the anomalies so much work remains to be done

human impact on the environment in the ethiopian and eritrean highlands a state of the art abstract this review analyses the environmental evolution of the ethiopian highlands in the late quaternary the late pleistocene 20 000 12 000 14c years bp was cold and dry with 1 low lake levels in the rift valley 2 large debris fans on the flanks of lake abhe basin and 3 the blue nile transporting coarse bedload then a period with abundant and less seasonal rains existed between 11 500 and 4800 14c years bp as suggested by increased arboreal pollen high river and lake levels low river turbidities and soil formation around 5000 4800 14c years bp there was a shift to more arid conditions and more soil erosion many phenomena that were previously interpreted as climate driven might however well be of anthropic origin thick sediment deposits on pediments as well as an increase of secondary forest scrub and ruderal species in pollen diagrams are witnesses of this human impact one important aspect of the late quaternary palaeoenvironment is unclear changes in nile flow discharges and rift valley lake levels have been linked to changes in precipitation depth most authors do not take into account changes in land use in the highlands nor changes in the seasonality of rain both of which can lead to a change in runoff coefficients tufa and speleothem deposition around 14 000 years ago tend to indicate that at the end of the last glacial maximum lgm conditions might have been wetter than generally accepted the most important present day geomorphic processes are sheet and rill erosion throughout the country gullying in the highlands and wind erosion in the rift valley and the peripheral lowlands based on existing sediment yield data for catchments draining the central and northern ethiopian highlands an equation was developed allowing to assess area specific sediment yields sy 2595a 0 29 n 20 r 2 0 59 where sy area specific sediment yield t km 2 year 1 and a drainage area km2 with respect to recent environmental changes temporal rain patterns apart from the catastrophic impact of dry years on the degraded environment cannot explain the current desertification in the driest parts of the country and the accompanying land degradation elsewhere causes are changing land use and land cover which are expressions of human impact on the environment deforestation over the last 2000 3000 years was probably not a linear process in ethiopia studies on land use and land cover change show however a tendency over the last decades of increasing removal of remnant vegetation which is slowed down or reversed in northern ethiopia by a set aside policy ongoing land degradation requires urgent action at different levels of society soil and water conservation swc structures are now widely implemented local knowledge and farmer s initiatives are integrated with introduced swc techniques at various degrees impact assessments show clear benefits of the soil conservation measures in controlling runoff and soil erosion in high rain areas runoff management requires greater emphasis during the design of soil conservation structures in such areas investment in swc might not be profitable at farm level although benefits for society are positive this pleads in favour of public support the present land degradation in the ethiopian highlands has a particular origin which includes poverty and lack of agricultural intensification causes of these are to be found in the nature of past and present regional social relations as well as in international unequal development this review strengthens our belief that under improved socio economic conditions land husbandry can be made sustainable leading to a reversal of the present desertification and land degradation of the ethiopian highlands

the internal tide and resonance in indian arm british columbia the nature of the internal tide in indian arm is discussed in the winter of 1983 1984 this internal tide was observed to change from a predominantly m2 internal response to a predominantly k1 one this change in the response is explained as a partial resonance response of the system during the 1983 1984 winter the resonance period steadily increased from 14 hours at the start to 22 hours at the end it is suggested that the enhanced internal response at the k1 frequency late in the winter is due to resonance the fitting of normal modes was done to look at the energy flux in indian arm about 20 30 of the energy flux is found to return from the head of the inlet a finding supporting the resonance hypothesis which requires energy to be reflected from the head

tectonic and sedimentary basin evolution of the eastern bangong nujiang zone tibet a reading cycle the ophiolite bearing bangong nujiang zone bnz traversing central tibet from east to west separates the qiangtang block in the north from the lhasa block in the south their stratigraphic development indicates that both blocks once formed a continuous continental platform until the late triassic following late paleozoic triassic rifting ocean crust formed between both blocks during the late triassic creating the dongqiao naqu basin dnb among other basins yu et al 1991 the analysis of the rift flank sequences reveals that rifting was dominated by transtension the basin was shortened by post mid cretaceous transpression thus the overall basin evolution represents a reading cycle despite some active margin processes which gave this cycle a special imprint major basin parts were preserved despite transpressional shortening suggesting that the eastern bnz represents a remnant basin our understanding of the dnb solves the prior problem of viewing the bnz as a mid late jurassic collisional suture although typical collision related deformation thickening mountain building as well as related molasse formation are lacking our model also explains the scattered linear ophiolite distribution by local transpression of remnant oceanic basin floor without having to consider problematic long range ophiolite thrusting

physical modeling of tidal resonance in a submarine canyon current observations in submarine canyons poleward of 30 are usually dominated by the semidiurnal m2 tidal frequency which is superinertial at these latitudes observations from a submarine canyon at 44 n the gully nova scotia suggest that canyons can be dominated by the subinertial k1 tidal frequency if length scales are correct for resonance of the k1 frequency a model of the gully was constructed in a tank on a rotating table and tidal currents generated to determine factors that influence resonance resonance curves were fit to measurements from the laboratory canyon for a range of stratifications background rotation rates and forcing amplitudes dense water was observed upwelling onto the continental shelf on either side of the laboratory canyon and traveled at least one canyon width along the shelf friction values measured in the laboratory were much higher than expected probably due to upwelled water surging onto the shelf on each tidal cycle similar to a tidal bore by scaling observations from the laboratory to the ocean and assuming friction in the ocean is also affected by water traveling onto the shelf a resonance curve for the ocean was created because of the broad resonance curve the diurnal tide remains strong year round at the gully even as stratification at the shelf break changes dense water surging onto the shelf on tidal frequencies may affect friction and mixing at other nonresonant canyons

environmental changes in a tropical lake lake abiyata ethiopia during recent centuries lake abiyata is a small closed saline alkaline lake located in the central part of the ethiopian rift valley east africa a multi proxy study of a sediment core 116 cm long and with undisturbed mud water interface was performed to test the sensitivity of the lake system and of different proxies to the changes in climate and human activities that occurred in the catchment during the past few centuries the 210pb analyses suggest that the upper 80 cm of the core represent the past 200 years this study complements millennial scale environmental records available in the region the main freshwater climatic and biological features of the modern lake system and their variations over the past decades as known from observations are first summarised results derived from individual proxies analysed along the core are then presented successively major physical and chemical properties of bulk sediments diatoms and pollen uncertainties on the chronological framework are discussed major limnological stages are finally identified based on the multi proxy interpretation of our record our record shows large variations in the lake water and salt balances in the sediment sources and in the vegetation distribution in the basin using our 210pb chronology major changes observed in the core are tentatively compared with environmental events known from instrumental and historical records the upper 41 cm of the core 210pb age 1940 1998 ad reveal several fluctuations in diatom inferred water depth and salinity which seem to be consistent with known changes in water level human impact on vegetation clearly appears since about 30 years the interval 85 41 cm suggests a period of overall water deficit lake abiyata experienced episodes shallower and more saline than over the past decades especially around 68 66 cm 210pb dated at ca 1890 ad this level may coincide with one of the worst droughts known in the ethiopian history during 1888 1892 the lower part of the core includes a stage 108 85 cm of lake level much higher than today and which ended before 1800 ad although its base is undated so far this stage suggests that conditions much wetter than today have prevailed in the region during at least part of the 18th century lake abiyata appears to be a suitable site for a detailed environmental reconstruction over the recent past although further work is needed to reduce the uncertainties on our record as discussed in the conclusions

the effect of wave current interaction on tidally forced estuarine circulation reductions in the velocity and surface elevation of tidal flows that have been observed in the delaware bay a coastal plain estuary on the eastern coast of the united states correlate well with local wind events while there are several mechanisms that could account for this phenomenon we consider hydrodynamic wave current interaction to be a reasonable explanation in order to theoretically model the interaction between tides and surface gravity waves and eliminate the need for a reference velocity required by the grant and madsen 1979 wave current interaction model we propose a process oriented model that couples the mechanism of wave current interaction directly to the current dynamics of the tidal flow the tidal model uses an eddy viscosity concept to parameterize the turbulent stresses and further characterizes the eddy viscosity in terms of a flow dependent shear velocity within the bottom boundary layer of the surface gravity wave the turbulent stresses are also parameterized by an eddy viscosity that is coupled to the shear velocity of the tidal model results of the coupled model indicate that as expected wave current interaction increases the bottom friction felt by the tidal flow and usually reduces the volume flux however if the physical bottom roughness is sufficiently small less than 30 cm then it is possible to enhance the volume flux in a long deep estuary contrary to expectations a simple one dimensional linearized friction model shows this potentially dramatic effect can be attributed to an increase in the bottom friction that then tunes the estuary closer to its natural resonance obviously an estuary at resonance maximizes the volume flux that must pass through the estuary mouth the effect of wave current interaction is to provide a possible means of increasing the bottom friction and enhancing the volume flux

holocene palaeovegetation of the tigray plateau in northern ethiopia from charcoal and stable organic carbon isotopic analyses of gully sediments abstract a long history of supporting sophisticated but unsustainable kingdoms makes the tigray plateau of the northern ethiopian highlands a promising region for the study of relationships between palaeoenvironmental change and the trajectories of human civilizations the natural vegetation above 2200 m elevation is thought to be forests dominated by juniperus procera nonetheless this hypothesis is not supported in the vegetation cover now and is scarcely studied in the palaeorecord to examine changes in vegetation climate and land use we identified buried charcoalized wood and estimated the percentage of organic carbon from c 4 plants c 4 carbon from 13 c values of bulk organic matter in the soils of gully walls in the boundaries of the ancient aksumite empire charcoal ranged in age from ca 13 700 to 110 cal yr bp juniperus procera occurred in even the youngest samples although at lower percentages of the total charcoal than in older samples nevertheless rapidly regenerating angiosperms usually dominated or co dominated charcoal even in some of the oldest strata a shift towards higher c 4 carbon and total organic carbon toc in soils younger than 3300 cal yr bp began during a period when agricultural uses of land may have increased in order to support the needs of growing societies a shift towards higher c 4 carbon but lower toc began at ca 6000 cal yr bp however during a period when no charcoal was found and no changes in human societal complexity are known these results indicate that juniper forest types have long been present at 2200 m in the tigray plateau but that they have rarely been the dominant natural vegetation furthermore lack of repeatable correspondence between factors suggests that the causes of similar shifts in vegetation composition were not always the same

climatic change in northern ethiopia during the past 17 000 years a diatom and stable isotope record from lake ashenge lake ashenge a closed basin lake near the northernmost penetration of summer monsoon rains is well placed to provide a continental record of past changes in the strength of the african monsoon system diatom and oxygen isotope analyses of the lake sediments confirm that the overall trend of climate change during the past 17 000 years was driven by precessional forcing punctuated by abrupt shifts that may be linked to changes in atlantic surface temperatures the lake level was low from at least 17 2 to 16 2 cal kyr bp and then rose between 16 2 and 15 2 cal kyr bp which may represent a temporary reactivation of the monsoonal circulation system following its reduced activity during the last glacial maximum the lake was significantly low between 13 6 and 11 8 cal kyr bp coinciding approximately with the younger dryas but beginning 900 years before its recognised onset in the greenland ice core record a major sedimentary hiatus covering the interval 11 8 to 7 6 cal kyr bp was probably caused by an early holocene lowstand the precise timing of which cannot be determined because pre lowstand sediments were eroded from the core site the lake filled to its overflow from 7 6 cal kyr bp until 5 6 cal kyr bp when the sediments record an abrupt lake response to the regional transition to arid conditions that mark the end of african humid period evidence is also presented for climate changes which may have been associated with the rise and fall of aksum ethiopia s first great civilisation

volcanism in nascent back arc basins behind the shichito ridge and adjacent areas in the izu ogasawara arc northwest pacific evidence for mixing between e type morb and island arc magmas at the initiation of back arc rifting mineral chemistry major and trace elements and 87sr 86sr ratios are presented for 29 igneous rocks dredged from the northern portion of the izu ogasawara arc these rocks are compositionally bimodal basement gabbro and trondhjemite from the arc are extremely poor in k2o 0 05 0 19 and rb 0 48 0 62 ppm and their ree patterns and sr isotope ratios indicate that there are island arc tholeiites quaternary volcanic rocks from the present volcanic front shichito ridge active arc back arc seamounts east side inactive arc and torishima knoll between the two back arc depressions incipient back arc basins behind the active arc have the same geochemical characteristics as the above plutonic rocks though they are not as depleted in k and rb rhyolite pumice from the backarc depression is also the depleted island arc tholeiite whereas basalts from the depression have compositions that are transitional between morb and island arc tholeiites in trace element ti ni cr v y and zr and mineral chemistries the back arc depression basalts have relatively high ban cen 0 66 1 24 cen ybn 1 1 1 9 and k ba 45 105 and low 87sr 86sr 0 70302 0 70332 and ba sr 0 1 0 2 which are similar to other back arc basin basalts and e type morb but are quite unlike the depleted island arc tholeiites the diverse trace element and sr isotope compositions of basalt andesite from the back arc depressions imply the interplay between e type morb and island arc tholeiite these chemical characteristics and the relationships of ce yb n vs ba ce n and ce yb n vs 87sr 86sr suggest that the back arc depression magmas are generated by mixing of e type morb and depleted island arc tholeiite magmas geochemical characters of the associated rhyolite from the depression are compatible with partial melting of lower crust

abrupt changes in the asian southwest monsoon during the holocene and their links to the north atlantic ocean during the last ice age the indian ocean southwest monsoon exhibited abrupt changes that were closely correlated with millennial scale climate events in the north atlantic region1 2 3 suggesting a mechanistic link in the holocene epoch which had a more stable climate the amplitude of abrupt changes in north atlantic climate was much smaller and it has been unclear whether these changes are related to monsoon variability here we present a continuous record of centennial scale monsoon variability throughout the holocene from rapidly accumulating and minimally bioturbated sediments in the anoxic arabian sea our monsoon proxy record reveals several intervals of weak summer monsoon that coincide with cold periods documented in the north atlantic region4 including the most recent climate changes from the medieval warm period to the little ice age and then to the present we therefore suggest that the link between north atlantic climate and the asian monsoon is a persistent aspect of global climate

formal subdivision of the holocene series epoch a discussion paper by a working group of intimate integration of ice core marine and terrestrial records and the subcommission on quaternary stratigraphy international commission on stratigraphy this discussion paper by a working group of intimate integration of ice core marine and terrestrial records and the subcommision on quaternary stratigraphy sqs of the international commission on stratigraphy ics considers the prospects for a formal subdivision of the holocene series epoch although previous attempts to subdivide the holocene have proved inconclusive recent developments in quaternary stratigraphy notably the definition of the pleistocene holocene boundary and the emergence of formal subdivisions of the pleistocene series epoch mean that it may be timely to revisit this matter the quaternary literature reveals a widespread but variable informal usage of a tripartite division of the holocene early middle or mid and late and we argue that this de facto subdivision should now be formalized to ensure consistency in stratigraphic terminology we propose an early middle holocene boundary at 8200 a bp and a middle late holocene boundary at 4200 a bp each of which is linked to a global stratotype section and point gssp should the proposal find a broad measure of support from the quaternary community a submission will be made to the international union of geological sciences iugs via the sqs and the ics for formal ratification of this subdivision of the holocene series epoch copyright 2012 john wiley sons ltd

oxygen and carbon isotope composition of authigenic carbonate from an ethiopian lake a climate record of the last 2000 years lamb henry leng m j telford r j ayenew t 2007 oxygen and carbon isotope composition of authigenic carbonate from an ethiopian lake a climate record of the last 2000 years the holocene 17 4 pp 517 526 rae2008

assembly of the lhasa and qiangtang terranes in central tibet by divergent double subduction abstract integration of lithostratigraphic magmatic and metamorphic data from the lhasa qiangtang collision zone in central tibet including the bangong suture zone and adjacent regions of the lhasa and qiangtang terranes indicates assembly through divergent double sided subduction this collision zone is characterized by the absence of early cretaceous high grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca 120 110 ma two jurassic cretaceous magmatic arcs are identified from the caima duobuza rongma kangqiong amdo magmatic belt in the western qiangtang terrane and from the along tso yanhu daguo baingoin daru tso magmatic belt in the northern lhasa terrane these two magmatic arcs reflect northward and southward subduction of the bangong ocean lithosphere respectively available multidisciplinary data reconcile that the bangong ocean may have closed during the late jurassic early cretaceous most likely ca 140 130 ma through arc arc soft collision rather than continent continent hard collision subduction zone retreat associated with convergence beneath the lhasa terrane may have driven its rifting and separation from the northern margin of gondwana leading to its accretion within asia

the response of a strongly stratified fjord to energetic tidal forcing abstract the response of upper loch linnhe a strongly stratified scottish fjord to energetic tidal forcing is studied by new analysis of two observational data sets using a ship mounted acoustic doppler current profiler adcp and an undulating conductivity temperature depth ctd we are able to repeatedly sample over a tidal cycle at sufficiently high spatial and temporal resolutions to clearly define the tidal variability in the vertical and horizontal away from the near sill region horizontal velocities are generally orientated along the longitudinal axis of the fjord analysis of the along fjord velocities shows that at the m2 frequency there is a strong 1st mode internal tide with maximum horizontal velocity amplitudes of 23 4 cm s 1 the observed vertical structure of the complex amplitudes of the horizontal velocities are in close agreement with normal mode theory along fjord variations of the fitted amplitude and phase lags from further analysis of the two independent data sets show that the 1st mode internal tide is behaving as a standing wave within the near sill region the circulation is complicated by the formation of a horizontal gyre and a large perturbation in the density field on the flood tide these features are a response to the flood tide currents advecting denser water into the fjord in the form of a strong jet due to the narrow entrance sill being off centre to the axis of the fjord the strong flood tide jet gives rise to eddy formation as the incoming flood tide water is denser than the resident surface water of the fjord the tidal jet sinks and advects the resident water landwards giving rise to the observed density perturbations

stepwise holocene aridification in ne africa deduced from dust borne radiogenic isotope records abstract transfer of tropical heat to higher latitudes is the major driving force of the earth s climate consequently sediments in regions to the north and south of the tropics potentially retain an archive of past major climate reconfigurations the climate of one such region around the arabian sea sensitively depends on the coupled asian and african monsoons that also control the dust transport here we use the sr nd isotope ratios of the dust fraction from core 905 arabian sea off somalia as a novel tool to deduce the holocene weathering history of the horn of africa with emphasis on the climate transition that took place from a wet early to a dry late holocene the highly variable sr isotope ratios are interpreted to reflect mainly changes in the evaporation precipitation balance over ne africa whilst the nd isotope measurements record no significant variations and point to a prevailing ne african dust source the sr isotope record shows that the first aridification step occurred at 8 5 kyr bp followed by an unstable transitional period up to 6 kyr bp characterized by decadal scale high amplitude variations in the evaporation precipitation balance a second aridification step began at 6 kyr bp and ceased at 3 8 kyr bp when modern day dry climate was established the combined sr and nd isotope records probably reflect north south shifts of the intertropical convergence zone controlling the evaporation precipitation balance over ne africa

reach scale river dynamics moderate the impact of rapid holocene climate change on floodwater farming in the desert nile the relationship between climate change and the development of old world riverine civilizations is poorly understood because inadequate dating control has hindered effective integration of archaeological fluvial and climate records this paper presents the most comprehensive and robustly dated archaeological and paleoenvironmental data sets yet compiled for the desert nile it focuses on the valley floor hinterland of the kingdom of kerma 2400 1450 b c in northern sudan kerma emerged as a rival to egypt during africa s first dark age drought in contrast to other irrigation based agriculturists in egypt and asia kerma flourished during the environmental crisis ca 2200 b c we have studied the stratigraphy and archaeological records of paleochannels across an 80 km reach of the nile upstream of kerma using optically stimulated luminescence to date when channels flowed and when they dried up the dynamics of the local alluvial environment were critical in determining whether climatic fluctuations and changes in river flow represented an opportunity for floodwater farmers 5000 3500 b c a hazard that could be managed 2400 1300 b c or an environmental catastrophe that resulted in settlement abandonment after 1300 b c

the simulation of deep water exchange in a fjord indian arm british columbia canada abstract a numerical model is used to simulate deep water renewals that were observed to occur in burrard inlet indian arm over a time period of about 100 days during winter 1984 85 the renewals bottom water flowing from burrard inlet into indian arm occurred or intensified during neap tide and caused a steady increase in the density near the bottom in indian arm the model successfully reproduces the renewal events for a simulation time of at least 85 days it uses a mellor yamada level two turbulence closure scheme for which the local turbulent energy production is balanced by local dissipation and for which the turbulent length scale is prescribed the length scale varies linearly with depth near the surface and bottom and is constrained to never be larger than 1 20 of the water depth and 2 the ozmidov length scale a lower bound is imposed on the vertical diffusion coefficients that depends on the brunt vaisala frequency n its magnitude for a given n is different in burrard inlet and in

plough marks as a tool to assess soil erosion rates a case study in axum ethiopia abstract from the antiquity the hilly landscape around the city of axum in northern ethiopia has been terraced and extensively used for agriculture purposes impacts of the ard plough on protruding and or buried boulder at a depth less than the tillage depth produce scratches on their sides or their top the spatial relationship among plough marks and rock surface orientations gives information on the ploughing direction whereas the relative height from the present ground surface to the top of the marks and information on the age of cultivation can be used to assess soil loss rates parameters to define plough mark orientation were defined and measured in the field on boulders with plough marks within a study terrace located in an archaeological area with an approx of 2040 year age these parameters are height from the ground dip slope and frequency the measurements also show that the initial terrace slope was steeper than the present one confirming the effectiveness of the traditional terracing practice in soil conservation because of the low soil erosion rates assessed over a so large time interval for the main time intervals of the axumite history corresponding to the rise and fall of the axumite civilization and to variable conditions of human pressure on the land the maximum erosion rates inferred are of the order of 3 4 mg ha 1 y 1 this result is compared with erosion rates calculated with other methods reported in literature e g psiac and field measurement on terraces in the same region leading to values coinciding with those obtained by the plough marks method

late mesozoic tectonic evolution and growth of the tibetan plateau prior to the indo asian collision abstract the elevation of the tibetan plateau is a revolutionary event in the earth history which resulted in change not only of cenozoic regional and global climate but also of monsoon intensity a critical aspect for the development of the tibetan plateau that remains open to intense debate is whether the collision among the tibetan continental blocks or the subduction of the tethys prior to the indo asian collision in the cenozoic contributed to the plateau growth we propose here that an andean type orogen could have been present in southern tibet from the jurassic to early cretaceous but collapsed during the middle cretaceous based on a synthesis of existing structural sedimentary geochronological and geochemical data the orogeny is marked by north verging jurassic early cretaceous thrusting in the lhasa block a result of flat subduction of the yarlung zangpo neo tethys that is recorded by a narrow belt of 205 174 ma adakitic rocks along the southernmost margin of the lhasa block denudation of the orogen produced a thick up to 15 km sequence of jurassic turbidites and molasse type sediments on the northern lhasa block widespread middle cretaceous 135 100 ma postorogenic strongly peraluminous to calc alkaline magmatism and extensional deformation in the lhasa block and an extensive marine transgression in tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling meanwhile the north dipping subduction of the bangong meso tethys produced a wide jurassic middle cretaceous magmatic arc along the southern margin of the qiangtang block findings of 132 108 ma aged ophiolites rich with middle cretaceous radiolarians in central tibet indicate that the bangong meso tethys did not close until the late cretaceous repeated flat slab subduction of the yarlung zangpo neo tethys occurred during the late cretaceous 90 78 ma as indicated by the adakites in the southernmost lhasa block which together with the ongoing qiangtang lhasa collision could have contributed to the growth of the tibetan plateau

abrupt pre bolling allerod warming and circulation changes in the deep ocean several large and rapid changes in atmospheric temperature and the partial pressure of carbon dioxide in the atmosphere probably linked to changes in deep ocean circulation occurred during the last deglaciation the abrupt temperature rise in the northern hemisphere and the restart of the atlantic meridional overturning circulation at the start of the bolling allerod interstadial 14 700 years ago are among the most dramatic deglacial events but their underlying physical causes are not known here we show that the release of heat from warm waters in the deep north atlantic ocean probably triggered the bolling allerod warming and reinvigoration of the atlantic meridional overturning circulation our results are based on coupled radiocarbon and uranium series dates along with clumped isotope temperature estimates from water column profiles of fossil deep sea corals in a limited area of the western north atlantic we find that during heinrich stadial 1 the cool period immediately before the bolling allerod interstadial the deep ocean was about three degrees celsius warmer than shallower waters above this reversal of the ocean s usual thermal stratification pre dates the bolling allerod warming and must have been associated with increased salinity at depth to preserve the static stability of the water column the depleted radiocarbon content of the warm and salty water mass implies a long term disconnect from rapid surface exchanges and although uncertainties remain is most consistent with a southern ocean source the heinrich stadial 1 ocean profile is distinct from the modern water column that for the last glacial maximum and that for the younger dryas suggesting that the patterns we observe are a unique feature of the deglacial climate system our observations indicate that the deep ocean influenced dramatic northern hemisphere warming by storing heat at depth that preconditioned the system for a subsequent abrupt overturning event during the bolling allerod interstadial

the deepwater exchange cycle in indian arm british columbia abstract a silled fjord on the british columbia coast indian arm was used to study the exchange processes controlling deepwater exchange in fjords bottom water renewal took place during the winter of 1984 85 hydraulic control of the exchange is exerted over the long sill which restricts access to the fjord the maximum density at the sill occurs during neap tides no distinct peaks in the velocity at the sill are observed during periods of inflow reduced vertical mixing over the long sill during periods of neap tides sets the timing of the bottom water exchange during each inflow over the 1984 85 winter about 20 of the water in the fjord was replaced these exchanges occurred over periods of 5 10 days between the exchange years diffusion of the water in the basin is important in conditioning the water for the next exchange event the vertical diffusion coefficient k t was determined from an analysis of the salinity data and was found to be related to the buoyancy frequency n by the relationship k v n 1 6

shifting sediment sources in the world s longest river a strontium isotope record for the holocene nile we have reconstructed long term shifts in catchment sediment sources by analysing for the first time the strontium sr and neodymium nd isotope composition of dated floodplain deposits in the desert nile the sediment load of the nile has been dominated by material from the ethiopian highlands for much of the holocene but tributary wadis and aeolian sediments in sudan and egypt have also made major contributions to valley floor sedimentation the importance of these sources has shifted dramatically in response to global climate changes during the african humid period before c 4 5 ka when stronger boreal summer insolation produced much higher rainfall across north africa the nile floodplain in northern sudan shows a tributary wadi input of 40 50 thousands of tributary wadis were active at this time along the full length of the saharan nile in egypt and sudan as the climate became drier after 4 5 ka the valley floor shows an abrupt fall in wadi inputs and a stronger blue nile atbara contribution in the arid new kingdom and later periods in palaeochannel fills on the margins of the valley floor aeolian sediments replace wadi inputs as the most important secondary contributor to floodplain sedimentation our sediment source data do not show a measurable contribution from the white nile to the floodplain deposits of northern sudan over the last 8500 years this can be explained by the distinctive hydrology and sediment delivery dynamics of the upper nile basin high strontium isotope ratios observed in delta and offshore records that were previously ascribed to a stronger white nile input during the african humid period may have to be at least partly reassessed our floodplain sr records also have major implications for bioarchaeologists who carry out sr isotope based investigations of ancient human remains in the nile valley because the isotopic signature of nile floodplain deposits has shifted significantly over time

the late holocene geomorphic history of the ethiopian highlands supportive evidence from may tsimble abstract alluvial sedimentary archives contain important geochronological and paleo environmental information on past geomorphic processes in semi arid regions for north ethiopia in particular flashflood sediments transported by ephemeral streams can provide interesting chronological information on late holocene land degradation whether or not impacted by climate or land cover changes upstream here we compare geomorphic records with independent regional records of rainfall regime changes land use cover changes and macrohistory supported by optically stimulated luminescence osl dates for fluvial activity at a sediment sequence in the may tsimble catchment in the northeastern highlands we identified two degradation periods over the past 4000 years one broadly from 1500 500 bce and one from 500 ce onwards at least one prior incision phase is responsible for the stabilized gullies that can be seen on photographs around 1900 and another incision phase is dated to the late 20th century based on all datasets we re interpret the geomorphic history of the highlands land degradation is dominantly determined by a human impact although the impact of this human influence does get amplified during dry spells

late stone age variability in the main ethiopian rift new data from the bulbula river ziway shala basin abstract research conducted along the bulbula river ziway shala basin main ethiopian rift has yielded archaeological assemblages that document various phases of the upper pleistocene and the early holocene the oldest period documented is dated to 34 33 000 cal bp dw1 and the second is dated to the pleistocene holocene transition between 14 000 and 11 000 cal bp b1s1 dw2s2 and dw2s1 the analysis of these assemblages and their comparison offer new perspectives on early lsa industries and on the development and diversity of partially microlithic industries at the end the pleistocene and the beginning of the holocene through the characterization of techno economic changes in the context of shifting environments and their exploitation by humans our results intend to provide new insights into human behavioral evolution on a local scale and for a time period that is currently insufficiently known this study highlights the existence of major breaks in the archaeological record of the area during mis 3 1

the influence of the spring neap tidal cycle on currents and density in burrard inlet british columbia canada abstract tidal mixing through two constrictions at the entrance to a fjord type estuary in british colombia canada is studied from a mooring programme in the spring of 1995 and from ship born adcp measurements in the spring of 1997 time series from the moorings show that during spring tides currents at depth are as high as 1 5 m s 1 small floods bring outside water through the narrows relatively unmixed which sinks down to depth in contrast large floods result in intrusions of highly mixed water up to 1 kg m 3 lower in density during neap tides currents at depth are generally small density decreases slowly and turbulent vertical diffusion is believed to be the dominant process at depth analysis of vertical diffusion near the bottom at one station during neap tides produces estimates of the vertical eddy diffusivity which are somewhat higher than those from similar studies in other b c fjords results from the adcp study reveal the presence of strong vertical velocities downstream of the narrows during large floods possibly associated with hydraulic jumps the flow through the narrows at such times is indeed supercritical with respect to the first baroclinic mode

quantifying long term changes in gully networks and volumes in dryland environments the case of northern ethiopia abstract understanding historical and present gully development is essential when addressing the causes and consequences of land degradation especially in vulnerable dryland environments for northern ethiopia several studies exist on the severity of gully erosion yet few have quantified gully development in this study gully network and volume development were quantified over the period 1963 2010 for an area of 123 km2 representing the regional variability in environmental characteristics gully networks were mapped from small scale aerial photographs and high resolution satellite images for the latter visualizing google earth images in 3d proved to be very suitable to investigate gully erosion from the changes in networks and volumes over the period 1963 2010 the occurrence of one cut and fill cycle is apparent from a largely low dynamic gully system in the 1960s network expansion and increased erosion rates in the 1980s and 1990s caused the drainage density and volume to peak in 1994 the average gully density dtotal was then 2 52 km km 2 and the area specific gully volume va 60 103 m3 km 2 this coincides with soil losses by gully erosion slg of 17 6 t ha 1 y 1 over the period 1963 1994 by 2010 improved land management and the region wide implementation of soil and water conservation measures caused 25 of the gully network to stabilize resulting in a net infilling of the gully channels over the period 1994 2010 the study validates previous findings that land degradation by gullying was indeed severe in northern ethiopia in the second half of the 20th century but also shows that when proper land management is applied a gully can be transformed into a linear oasis which increases the resistance of gullies to further erosion

a modal decomposition of the internal tide in a deep strongly stratified inlet knight inlet british columbia this paper seeks to answer the question how much of the internal tide propagating up knight inlet british columbia is reflected by a right angled bend the internal tide in knight inlet is generated by the interaction of the barotropic tide with a shallow sill seaward of the bend it then propagates in both directions as a traveling kelvin wave the up inlet propagating wave then encounters the bend where some of it may be reflected the study is based upon 2 months of cyclesonde current meter data from four stations taken during the summers of 1981 and 1983 the vertical profiles of amplitude and phase of the m2 constituent of longitudinal velocity and density fluctuations are decomposed into a truncated series of normal modes for waves propagating both up inlet and down inlet at the two stations up inlet of the sill acceptable fits can be obtained using only two up inlet propagating waves indicating that the data are compatible with the low reflection found by webb and pond 1986 at the two stations seaward of the sill the up inlet energy flux is of the same order of magnitude as the down inlet flux indicating a second source of internal tide seaward of those two stations the results indicate that only 30 50 of the power removed from the barotropic tide is being fed into the internal tide in the summer

palaeoclimate end of the african humid period the sahara was more humid and habitable thousands of years ago reconstructions of north african hydroclimate show that the onset of aridity started in the north with the monsoon rains weakening progressively later at lower latitudes

late quaternary changes in climate and land cover in the northern horn of africa and adjacent areas abstract most studies of landscape dynamics in the ethiopian highlands offer linear relations of population pressure and periodic droughts as causes of environmental degradation such views rely on limited historical information of one or two centuries which is inadequate for understanding the longer term effects of climatic shifts on landscape dynamics and environmental change in the highlands in this study we conducted a meta analysis of long term climate and biogeographic change and landscape dynamics during the holocene using multi proxy evidence from lacustrine n 10 marine n 5 geomorphic n 13 and vegetation n 7 records from the highlands and the wider horn of africa and western indian ocean region based on the datasets we discern late pleistocene dry phases in the region during the last glacial maximum the heinrich 1 event and the younger dryas additionally we recognize at least six dry phases during the holocene after 6500 bce 4900 bce 3700 bce 1000 bce 450 ce and 1750 ce the hydroclimatic shifts seem associated with changing equatorial atlantic and western indian ocean sea surface temperatures further the results show nonlinear relations between major holocene climate shifts and grassland expansions in the highlands our findings challenge conventional cause effect explanations of human population growth and land degradation and point instead to potentially strong but unexplored feedbacks between climate changes human activity and landscape dynamics over longer and short term periods

sahara and sahel vulnerability to climate changes lessons from holocene hydrological data abstract the examination of more than 1500 paleohydrological dated records collected between 10 and 28 n during the last 50 years have been used to improve our knowledge and understanding of the sahara and sahel vulnerability to the atlantic monsoon changes in the long term we have analyzed the distribution of water bodies mainly lakes and wetlands over time and space the central saharan massifs played a major role in favoring water supply to the lowlands throughout the whole african humid period in addition distinct east west dynamics is recorded with humidity starting and stopping several millennia earlier to the east than to the west of the sahara a series of time lags are discussed 1 between the maximum of deep fresh water lake formation during the early holocene and the maximum of water body extensions during the mid holocene which highlight the primary role of aquifer water level in lake response to climate change 2 between the hydrological history of the sahara and the sahel and the forcings mainly insolation changes during the early and mid holocene which involves complex interactions between remnant ice sheet in the northern hemisphere open water bodies in the sahara and sahel and the atlantic monsoon system

the mai maikden sedimentary sequence a reference point for the environmental evolution of the highlands of northern ethiopia abstract this paper presents a geomorphologic stratigraphic analysis of a travertine dammed lacustrine swampy sedimentary sequence composed of clay peaty layers and phytoclastic travertine sands deeply incised by the mai maikden river on the highlands of tigray northern ethiopia radiocarbon datings of peaty deposits allow us to establish that the travertine dams have developed at least between 7310 90 yr b p and 5160 80 yr b p in the upper part of the sequence palaeochannels filled with travertine and limestone coarse gravels and blocks indicate the activation of concentrated erosion on the surrounding slopes later both the travertine dam and the lacustrine swampy deposits were buried by alluvial and colluvial sediments still mixed with large amounts of organic matter testifying to generalized slope erosion processes the end of travertine deposition may be connected with a progressive reduction of vegetation cover and the subsequent decrease of co2 in groundwater the occurrence of the latter phenomena may be related to the onset of drier climatic conditions even though the finding of a large prehistoric settlement in the area also indicates some influence of human impact