patterns of deposition from experimental turbidity currents with reversing buoyancy turbidity currents are turbulent sediment laden gravity currents which can be generated in relatively shallow shelf settings and travel downslope before spreading out across deep water abyssal plains because of the natural stratification of the oceans and or fresh water river inputs to the source area the interstitial fluid within which the particles are suspended will often be less dense than the deep water ambient fluid consequently a turbidity current may initially be denser than the ambient sea water and propagate as a ground hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient sea water when this occurs all or part of the turbidity current lofts to form a buoyant sediment laden cloud from which further deposition occurs deposition from such lofting turbidity currents containing a mixture of fine and coarse sediment suspended in light interstitial fluid is explored through analogue laboratory experiments complemented by theoretical analysis using a box and cloud model particular attention is paid to the overall deposit geometry and to the distributions of fine and coarse material within the deposit a range of beds can be deposited by bimodal lofting turbidity currents lofting may encourage the formation of tabular beds with a rapid pinch out rather than the gradually tapering beds more typical of waning turbidity currents lofting may also decouple the fates of the finer and coarser sediment depending on the initial flow composition the coarse fraction can be deposited prior to or during buoyancy reversal while the fine fraction can be swept upwards and away by the lofting cloud an important feature of the results is the non uniqueness of the deposit architecture different initial current compositions can generate deposits with very similar bed profiles and grading characteristics highlighting the difficulty of reconstructing the nature of the parent flow from field data it is proposed that deposit emplacement by lofting turbidity currents is common in the geological record and may explain a range of features observed in deep water massive sands thinly bedded turbidite sequences and linked debrites depending on the parent flow and its subsequent development for example a lofting flow may lead to a well sorted largely ungraded or weakly graded bed if the fines are transported away by the cloud however a poorly sorted largely ungraded region may form if during buoyancy reversal high local concentrations and associated hindered settling effects develop at the base of the cloud

the interaction of ash flows with ridges using both laboratory experiments and theoretical models we examine the different flow regimes that may develop when an ash flow encounters a ridge for very small ridges all the flow may pass over the ridge for intermediate size ridges the flow may be partially blocked with a fraction of the flow reflected upstream as a travelling bore in this case the remainder of the flow which does pass over the ridge is hydraulically controlled at the ridge crest finally if the ridge is sufficiently high then the flow will be totally blocked new laboratory experiments show that the sedimentation patterns associated with these flow regimes may be very different most importantly flows that involve partial blocking and the formation of upstream propagating bores display enhanced sedimentation upstream of the ridge analogous to valley ponded and caldera fill deposits in contrast under some circumstances if the flow is able to scale a ridge the deposit may be relatively unaffected by the presence of the ridge the minimum ridge height that leads to total blocking of the flow increases with mass eruption rate and has a complex variation with distance from the source in a one dimensional channel the minimum ridge height that causes blocking increases with distance downstream this is because the flow becomes less dense through sedimentation of particles and entrainment of air and so requires less energy to scale a ridge of a particular height in axisymmetric flow the minimum ridge height initially decreases with distance downstream as the flow spreads radially but subsequently increases as the flow becomes less dense through sedimentation and entrainment a new quantitative model of dilute ash flows propagating over ridges indicates that flows with mass fluxes in excess of 108 109 kg s can partially scale barriers as high as 1000 m at distances of tens of kilometres from the source whereas smaller flows are likely to be totally blocked by such an obstacle our results shed new insight on the possible long range transport mechanism of several large flows including the ata fisher and aniakchak pyroclastic flows

an experimental investigation of density stratified inertial gravity currents turbidity currents and pyroclastic density currents may originate as stratified flows or develop stratification during propagation analogue density stratified laboratory currents are described using layers of salt solutions with different concentrations and depths to create the initial vertical stratification the evolving structure of the flow depends on the distribution of the driving buoyancy between the layers b proportional to the layer volumes and densities and their density ratio q when the lower layer contains more salt than the upper layer and so has a greater proportion of the driving buoyancy b 0 ae5 this layer travels to the nose of the current by mixing into the back of the head along the body wake density interface to produce a homogeneous flow q fi 1 or overtaking leading to streamwise stratification q fi 0 timescales describing the mixing between the layers and the streamwise separation of the layers are used to understand these flow behaviours and are in accordance with the experimental observations distance time measurements of the flow front show that strongly stratified flows initially travel faster than weakly stratified flows but during their later stages they travel more slowly in natural flows that are stratified in concentration and grain size internal features such as stepwise grading gradual upward fining and reverse grading could be produced depending on b and q stratification may also be expected to affect interactions with topography and overall fan architecture

size sorting in marine muds processes pitfalls and prospects for paleoflow speed proxies the basis for and use of fine grain size parameters for inference of paleoflow speeds is reviewed here the basis resides in data on deposited sediment taken in conjunction with flow speed measurements in the field experimental data on suspended sediment transport and deposition and theoretical treatments of the generation of size distributions of deposits from suspension controlled by particle settling velocity and flow speed in the deep sea sorting events occur under resuspension deposition events in benthic storms at flow speeds below 10 15 cm s 1 size in the noncohesive sortable silt 10 63 m range is controlled by selective deposition whereas above that range removal of finer material by winnowing also plays a role the best particle size instruments to measure a flow speed related grain size employ the settling velocity method while laser diffraction sizers can yield misleading results because of particle shape effects potential problems including source effects downslope supply on continental margins spatial variability of flow over bedforms and influence of ice rafted detritus are examined a number of studies using the sortable silt flow speed proxy are reviewed and inverse modeling of grain size distributions is examined outstanding problems are that corroboration is sparse because almost no studies have yet used the full range of proxies for flow rate and water mass identification and that the sortable silt mean size is not yet properly calibrated in terms of flow speed

a log normal spectral analysis of inorganic grain size distributions from a canadian boreal lake core towards refining depositional process proxy data from high latitude lakes better methods for interpreting grain size spectra will enhance current understanding of past transport depositional processes a high resolution inorganic grain size dataset has been measured from a freeze core extracted from alberta lake e a boreal fresh water lake 40 km east of the athabasca oil sands in north eastern alberta canada the grain size spectra are remarkably consistent throughout the core exhibiting a structure comprising six persistent grain size distributions below ca 250 m plus a rare medium sand distribution automated deconvolution of the grain size spectra produced poor results constraining the modes of two of the distributions produced deconvolution solutions that were statistically excellent and consistent with the structure of each spectrum statistical analysis of the constrained solutions indicates that deconvolution successfully extracted independent grain size populations conversely the multimodal spectra generate traditional measures for example mean grain size that are inconsistent combinations of different individual populations and thus are poor proxies of transport depositional processes alberta lake e is situated in a boreal wetland landscape where sediment delivery is dominated by overland flow transport during spring melt this context means that the alberta lake e grain size spectra can be interpreted to reflect i a bedload component transported during short duration high discharge events that reflect the intensity of the melt and ii a finer suspended load component representing material whose magnitude is controlled by the volume of the spring melt stratigraphically bedload and suspended load populations demonstrate different short wavelength and long wavelength cyclicity suggesting that spring melt is likely to be driven by cyclic external forcing factors the links between the grain size spectra and spring melt have potential for generating proxy records that better capture the external controls over spring melt in boreal systems and the risks associated with these energetic hydrodynamics this is exemplified by the coarsest alberta lake e distributions which indicate that more intense spring melt dynamics occurred in pre historical times this article is protected by copyright all rights reserved

importance of the indian ocean for simulating rainfall anomalies over eastern and southern africa the relative contributions of the indian ocean and pacific ocean sea surface temperatures ssts to the rainfall variability over eastern central and southern africa during the austral spring summer are examined the variability of african rainfall is statistically related to both oceans but the variability in the two oceans is also related to separate the effects of the indian and pacific oceans a suite of numerical model simulations is presented goga the atmosphere is forced by observed ssts globally ioga the atmosphere is forced by observed ssts only in the indian ocean basin and poga the atmosphere is forced by observed ssts only in the tropical pacific basin while the sst variability of the tropical pacific exerts some influence over the african region it is the atmospheric response to the indian ocean variability that is essential for simulating the correct rainfall response over eastern central and southern africa analyses of the dynamical response s seen in the numerical experiments and in the observations indicate that the pacific and indian oceans have a competing influence over the indian ocean african region this competition is related to the influence of the two oceans on the walker circulation and the consequences of that variability on low level fluxes of moisture over central and southern africa finally given the high correlation found between sst variability in the indian and pacific oceans with the pacific leading by 3 months we speculate on an approach to long lead dynamical climate prediction over central east and southern africa

quantifying velocity and turbulence structure in depositing sustained turbidity currents across breaks in slope two dimensional experiments investigating sediment transport and turbulence structure in sustained turbidity currents that cross breaks in slope are presented as analogue illustrations for natural flows the results suggest that in natural flows turbulence generation at slope breaks may account for increased sand transport into basins and that the formation of a hydraulic jump may not be necessary to explain features such as the occurrence of submarine plunge pools and the deposition of coarser grained beds in the bottomsets of gilbert type fan deltas experimental flows were generated on 0 3 6 and 9 slopes of equal length which terminated abruptly on a horizontal bed two component velocities were measured on the slope at the slope break and downstream of the slope break flows were depositional and non uniform visibly slowing and thickening with distance downstream one dimensional continuous wavelet transforms of velocity time series were used to produce time period variance maps peaks in variance were tested against a background red noise spectrum at the 95 level a significant period banding occurs in the cross wavelet transform at the slope break attributed to increased formation of coherent flow structures kelvin helmholtz billows variance becomes distributed at progressively longer periods and the shape of the bed normal velocity spectral energy distribution changes with distance downstream this is attributed to a shift towards larger turbulent structures caused by wake stretching mean velocity reynolds shear stress and turbulent kinetic energy profiles illustrate the mean distribution of turbulence through the currents a turbulent kinetic energy transfer balance shows that flow non uniformity arises through the transfer of mean streamwise slowing to mean bed normal motion through the action of reynolds normal stresses net turbulence production through the action of normal stresses is achieved on steeper slopes as turbulence dissipation due to mean bed normal motion is limited at the slope break an imbalance between the production and dissipation of turbulence occurs because of the contrasting nature of the wall and free shear boundaries at the bottom and top of the flows respectively a rapid reduction in mean streamwise velocity predominately affects the base of the flows and steeper proximal slope flows have to slow more at the break in slope the increased turbulent kinetic energy limited bed normal motion and strong mixing imposed by steep proximal slopes means rapid slowing enhances turbulence production at the break in slope by focusing energy into coherent flow structures at a characteristic period thus mean streamwise slowing is transferred into turbulence production at the slope break that causes increased transport of sediment and a decrease in deposit mass downstream of the slope break the internal effects of flow non uniformity therefore can be separated from the external influence of the slope break

reconstructing east african rainfall and indian ocean sea surface temperatures over the last centuries using data assimilation the relationship between the east african rainfall and indian ocean sea surface temperatures ssts is well established the potential interest of this covariance to improve reconstructions of both variables over the last centuries is examined here this is achieved through an off line method of data assimilation based on a particle filter using hydroclimate related records at four east african sites lake naivasha lake challa lake malawi and lake masoko and ssts related records at six oceanic sites spread over the indian ocean to constrain the last millennium ensemble of simulations performed by cesm1 skillful reconstructions of the indian ssts and east african rainfall can be obtained based on the assimilation of only one of these variables when assimilating pseudo proxy data deduced from the model cesm1 the skill of these reconstructions increases with the number of particles selected in the particle filter although the improvement becomes modest beyond 99 particles when considering a more realistic framework the skill of the reconstructions is strongly deteriorated because of the model biases and the uncertainties of the real proxy based reconstructions however it is still possible to obtain a skillful reconstruction of ssts over most of the indian ocean only based on the assimilation of the six sst related proxy records selected as far as a local calibration is applied at all individual sites this underlines once more the critical role of an adequate integration of the signal inferred from proxy records into the climate models for reconstructions based on data assimilation

hyperpycnal plume formation from riverine outflows with small sediment concentrations a series of laboratory experiments has been conducted in order to elucidate the sediment induced mixing processes accompanying riverine outflows specifically the discharge of a warm fresh particle laden fluid over a relatively dense cool brine in a parameter regime analogous to recently acquired field measurements hypopycnal surface plumes were subject to a convective instability driven by some combination of heat diffusing out of the warm fresh sediment laden plume and particle settling within it convection was robust in the presence or absence of intense turbulence at sediment concentrations as low as 1 kg m 3 and took the form of millimetre scale sediment laden fingers descending from the base of the surface plume a consequence of the convective instability of the original hypopycnal plume is the generation of a hyperpycnal bottom riding flow the experiments presented here indicate that natural river outflows may thus generate hyperpycnal plumes when sediment concentrations are 40 times less than those required to render the outflow heavy relative to the oceanic ambient the resulting hyperpycnal plumes may play an important role in transporting substantial quantities of sediment to the continental slope and beyond

response of the benthic nepheloid layer to near inertial internal waves in southern lake michigan 1 time series measurements of water transparency water temperature and current velocity were made at a station located in 58 m of water in southern lake michigan during the summer of 1995 currents generated by near inertial internal waves are correlated with variations in the thickness and in the vertical distribution of suspended sediment in the benthic nepheloid layer although a direct causal link between internal wave action and changes in the nepheloid layer could not be established the data suggest that local resuspension by shoaling internal waves maintains the layer during the stratified period the origin and maintenance of the benthic nepheloid layer is most likely the result of local resuspension due to a combination of internal wave action and longer term processes

interaction between the enso and the asian monsoon in a coral record of tropical climate the oxygen isotopic composition of a banded coral from the western equatorial indian ocean provides a 150 year long history of the relation between the el nino southern oscillation enso phenomenon and the asian monsoon interannual cycles in the coral time series were found to correlate with pacific coral and instrumental climate records suggesting a consistent linkage across ocean basins despite the changing frequency and amplitude of the enso however decadal variability that is characteristic of the monsoon system also dominates the coral record which implies important interactions between tropical and midlatitude climate variability one prominent manifestation of this interaction is the strong amplitude modulation of the quasi biennial cycle

seasonal signals in lake malawi sediments the sedimentary record of lake malawi one of the large rift valley lakes of east africa contains thick sequences of biologically undisturbed finely laminated muds and silts laminated sequences were recovered in cores from the extensive diatom ooze and silty diatom clay facies of the lake basin that is permanently anoxic below 250 m the sequences consist of light dark laminae couplets with average thickness of 1 mm the light laminae are completely dominated by the diatom genus melosira with a relatively minor contribution from the genus stephanodiscus the dark laminae consist of 50 diatoms equal proportions of melosira and stephanodiscus few nitzschia and occasional littoral diatom genera and 50 terrestrial plant debris mineral grains and fine clay and organic material based on a 210pb and 14c calculated sedimentation rate of 1 mm yr 1 for lake malawi basin sediments and the well defined seasonal variations in fluvial input and diatom productivity levels our data suggest that the lamination couplets are annual varves representing biannual sedimentation sequences the light laminae result from diatom blooms occurring during the dry windy season april october when the upper 250 m of the lake is well mixed localized upwelling occurs and the epilimnion becomes enriched with nutrients the dark laminae represent sedimentation in the rainy low wind velocity season november march when runoff and allochthonous terrestrial input are at a maximum and primary productivity is at a minimum

playa lake sedimentation and organic matter accumulation in an andean piggyback basin the recent record from the cuenca de pozuelos north west argentina expansive playa lake systems situated in high altitude piggyback basins are important and conspicuous components of both modern and ancient cordilleran orogenic systems extant playa lakes provide vital habitat for numerous endemic species whereas sediments from these deposystems may record signals of climate change or develop natural resources over geological time laguna de los pozuelos north west argentina provides the opportunity for an actualistic sedimentological and geochemical assessment of a piggyback basin playa lake in an area of critical interest for understanding quaternary palaeoclimate dynamics silty clays and diatom ooze are the dominant playalake centre microfacies with concentrations of total organic carbon and biogenic silica commonly exceeding 1ae5 wt in this sub environment elemental and stable isotopic analyses point to a mixed organic matter composition in the playa lake centre with substantial contributions from algae and transported aquatic macrophytes bulk sediment and organic mass accumulation rates in the southern playa lake centre approach 0ae22 g cm 2 year 1 and 2ae89 mg cm 2 year 1 respectively indicating moderately rapid deposition with negligible deflation over historic time playa margin facies contain higher percentages of fragmented biogenic carbonate ostracods and charophytes and inorganically precipitated aragonite crusts due to seasonal pumping and evaporation of ground water organic matter accumulation is limited along these heavily bioturbated wet and dry mud flats fluvial lacustrine transitional environments which are key waterbird habitats are either silty terminal splay northern axis or sandy deltas southern axis containing highly oxidized and partially allochthonous organic matter modern analogue data from laguna de los pozuelos provide key insights for i environmental reconstructions of ancient lake sequences and ii improving facies models for piggyback basins

seasonal and interannual particle export in an african rift valley lake a 5 yr record from lake malawi southern east africa a time sequencing sediment trap was deployed at 300 350 m in lake malawi east africa from 1987 to 1992 to measure particulate export to the deep anoxic hypolimnion in the northern and central lake regions the monthly settling particulate samples provide a data set of seasonal and interannual export fluxes of organic carbon biogenic opal calcium carbonate and lithogenic material maximum total particle fluxes 50 400 mg m 2 d 1 occurred primarily during the dry windy season april through october when algal productivity is high because of wind induced upwelling of nutrient enriched metalimnion and hypolimnion waters peak flux particulates contained an abundance of aulacoseira and stephanodiscus diatom valves and chains the total particle mass flux during the wet austral summer months november through march was consistently one to three orders of magnitude less than that measured during the dry months and consisted of mineral shards terrestrial plant debris and scattered diatom tests the 5 yr trap data provide support for the claim that the light dark lamination couplets abundant in northern and central lake cores reflect seasonal delivery to the sediments of diatom rich particulates during the windy months and diatom poor material during the wet season however interannual and spatial variability in upwelling and productivity patterns as well as el nino southern oscillation enso related rainfall and drought cycles appears to exert a strong influence over the magnitude and geochemical composition of particle export to the deep hypolimnion of lake malawi

effects of flow density on the dynamics of dilute pyroclastic density currents abstract dilute turbulent pyroclastic density currents may have densities as large as 10 100 kg m 3 laboratory experiments using dense gases show that for such currents the speed of the head of the current depends on the density difference between the current and the surrounding air in a complex way here a model of a dilute pyroclastic density current generated by a short lived volcanic eruption is developed to account for this effect the model predicts that the head of the current travels more slowly than in existing models leading to shorter run out distances and smaller dynamic flow pressures the effects are most significant for dense pyroclastic density currents such as those produced by dome collapse events we apply the new model to the eruptions of mount st helens 1980 taupo 1800 years ago and soufriere hills 1997

hyperpycnal and buoyant plumes from a sediment laden river the interaction of river plumes and sediment transport is examined using a three dimensional numerical model which couples the hydrodynamics and sediment transport only large rivers in midlatitudes are considered so that the effect of coriolis deflection becomes important winds and tides are excluded in an initially quiescent coastal sea sediment freshwater mixture is released from the inner reaches of a river a high ratio of sediment to buoyancy input is chosen so that the initial sediment freshwater mixture is denser than the receiving seawater as the dense underflow enters the adjacent sea release of sediment to the sea floor changes the plume from hyperpycnal to buoyant the forced convection of freshwater from depths triggers an expansive buoyant plume expanding in the direction of kelvin wave propagation the resulting buoyant plume is far more expansive than a typical buoyant plume in clear seawater produced by a comparable buoyancy input circulation associated with the plume is anticyclonic throughout the water column lacking a cyclonic gyre underneath the anticyclone as one would expect from a typical buoyant plume in clear seawater a heuristic argument follows to explain the lack of baroclinic flow structure in plumes forced by the sediment freshwater mixture

a laminated hemipelagic facies from the wenlock and ludlow of the welsh basin a hemipelagic fades from the silurian succession in wales consists of finely laminated dominantly silt grade sediment it can be interbedded with sand or mud grade turbidites or can occur as an independent facies its occurrence and character was controlled mainly by low bottom water oxicity the pelagic fauna of the laminated hemipelagite supports this view many of the silt grade particles have an aggregate structure and probably represent the faecal pellets of oceanic zooplankton the fine lamination is thought to have formed by alternations of planktonic blooms and increased discharges of silt into the basin by comparison with modern examples this alternation was likely to have been annual or seasonal

the palaeolimnology of northern lake malawi over the last 25 ka based upon the elemental and stable isotopic composition of sedimentary organic matter abstract sediments accumulating in offshore areas of northern lake malawi typically contain 1 4 organic carbon this organic matter om is of mainly phytoplankton origin with varying mixtures of terrestrial and degraded reworked material high resolution stable carbon and nitrogen isotopic elemental and rock eval pyrolysis analyses of the bulk om contained in three cores have been used to characterise the om and provide detailed insights into the palaeolimnology and palaeoclimatology of northern lake malawi over the last 25 ka preservation of the om as reflected by its rock eval hydrogen index hi is highly variable periods characterised by notably low hi 15 n during the mid and late holocene suggest intervals of stable stratification due to significantly reduced wind driven mixing in the northern part of the lake when n fixing cyanophytes formed a major component of the phytoplankton the geochemical data support recent suggestions of lowstand conditions in lake malawi at the time of the lgm and provide additional evidence of periods of intense but localised aridity in the terminal pleistocene earliest holocene and at the time of the little ice age the change in mixing regime at ca 11 8 ka seems to have been the result of a radical reorganisation of the dominant surface wind flow over south eastern africa the transition was within the resolution of currently available chronologies apparently contemporaneous with marked changes in the circulation patterns of adjacent oceans and the climate of more distant parts of the northern hemisphere

measurement of the sortable silt current speed proxy using the sedigraph 5100 and coulter multisizer iie precision and accuracy the weight percentage and mean size of the 10 63 m terrigenous silt fraction termed sortable silt of fine grained marine sediments have recently been argued to be proxies for near bottom palaeocurrent intensity this paper details the accuracy and precision of the sedigraph 5100 and the coulter multisizer iie in determining the weight percentage and mean size of the sortable silt at abundances characteristic of deep ocean sediments three sortable silt standards and a fine silt clay diluent were generated from natural deep sea sediment of the iceland basin these were used to produce silty clays with standard additions of sortable silt ranging from 1 to 20 of the total by weight accuracy was measured relative to the size of the pure spike measured by each instrument at sortable silt abundance 5 the sedigraph and the coulter multisizer estimate both weight percentage and mean grain size precisely and accurately while below 5 instrumental noise degrades their performance on the basis of the sedigraph results a series of percentage errors is defined which may be applied to sortable silt mean size and weight percentage estimates used in palaeoceanographic near bottom current reconstruction it is suggested that ideally palaeocurrent reconstructions should be based on sedigraph determinations of the 0 63 m grain size distribution yielding both mean and abundance or on coulter multisizer determinations of the sortable silt grain size mean the coulter multisizer may prove especially useful in regions of very depleted 5 sortable silt component

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

organic geochemical records from lake malawi east africa of the last 700years part ii biomarker evidence for recent changes in primary productivity abstract relatively few well dated and high resolution paleoclimate records of the past few centuries presently exist from tropical east africa here we examine the bulk and molecular geochemical records of two varved sediment cores from lake malawi which together provide a continuous record of environmental variability in east africa of the last 730 years we observe a number of changes in the aquatic ecosystem of lake malawi which are likely attributed to both natural climatic forcing and anthropogenic activities biomarkers of dinoflagellates dinosterol and bacterivorous ciliates tetrahymanol display increased accumulation rates from 1900 ad to the present while a simultaneous decrease in accumulation rates of diatom biomarkers isololiolide loliolide is observed increased accumulation rates of retene a compound derived from conifers are also noted since 1930 ad and likely reflect increased soil erosion due to deforestation of the lake malawi watershed spectral analysis of the high resolution toc record indicates a periodicity of 204 years similar to the 206 year cycle noted in 14c and 10be records suggesting a link between east african climate and solar forcing

grain size areal thickness distribution and controls on sedimentation of the 1991 mount pinatubo tephra layer in the south china sea the june 15 1991 climactic eruption of mt pinatubo produced an extensive largely co ignimbrite derived airfall ash layer on luzon island and across the central south china sea the layer covers an area of 4 105 km2 with a volume of 5 5 km3 near the coast of luzon the deposit consists of two units a normally graded basal ash bed unimodal in grain size and a finer grained internally structureless upper ash bed showing grain size bimodality with increasing distance from the source the coarse particle populations of the two units merge and migrate towards a near constant fine population 11 m the distal region is covered by a fine mode dominated virtually ungraded single ash layer the reversal of the winds from easterly directions at upper tropospheric and stratospheric levels to westerly directions in the middle and lower troposphere indicates that both the coarse and fine mode components fell out from high altitude eruption clouds the high velocity upper level winds however would have transported fine grained ash particles far beyond the south china sea which suggests that their settling was accelerated by aggregation the boundary between the units thus marks a change from fallout of predominantly discrete pyroclasts to simultaneous fallout of aggregated fines and freely falling coarse grained particles the particle populations composing the upper ash bed were almost completely removed from the proximal areas by the upper level winds at lower elevations the counterclockwise circulation of a typhoon over the coastal area advected the ash south and eastward producing a thickness maximum in the medial region at about 160 km from source the strong displacement of fines possibly aided by wind turbulence led to a break in bulk tephra thinning rates close to the coastline in the distal region outside the influence of the typhoon southwest monsoonal winds caused a distinct lobe axis inflection and thickness asymmetry within this region at about 420 km from source fallout of particle aggregates created a second thickness maximum comparison of the field data with previous experimental observations and tephra flux records in the deep sea wiesner et al 1995 carey 1997 mccool 2002 implies that the transport of ash in the water column was largely determined by vertical density currents differences in the reaction of coarse and fine particles to turbulence in the descending plumes probably suppressed the segregation of fines but allowed the coarser pyroclasts to maintain their initial order of arrival at the sea surface considering typical fall rates of convective plumes modifications of the initial fallout position of the particles by the south china sea current system are on the order of only a few kilometers the results suggest that convective sedimentation processes ensure the preservation of atmospheric particle transport directions distances and fallout modes in the deep sea

on the frequency distribution of turbidite thickness the frequency distribution of turbidite thickness records information on flow hydrodynamics initial sediment volumes and source migration and is an important component of petroleum reservoir models however the nature of this thickness distribution is currently uncertain with log normal or negative exponential frequency distributions and power law cumulative frequency distributions having been proposed by different authors a detailed analysis of the miocene marnoso arenacea formation of the italian apennines shows that turbidite bed thickness and sand interval thickness within each bed have a frequency distribution comprising the sum of a series of log normal frequency distributions these strata were deposited predominantly in a basin plain setting and bed amalgamation is relatively rare beds or sand intervals truncated by erosion were excluded from this analysis each log normal frequency distribution characterizes bed or sand interval thickness for a given basal grain size or basal bouma division measurements from the silurian aberystwyth grits in wales the cretaceous great valley sequence in california and the permian karoo basin in south africa show that this conclusion holds for sequences of disparate age and variable location the median thickness of these log normal distributions is positively correlated with basal grain size the power law exponent relating the basal grain size and median thickness is different for turbidites with a basal a or b division and those with only c d and e divisions these two types of turbidite have been termed thin bedded and thick bedded by previous workers a change in the power law exponent is proposed to be related to i a transition from viscous to inertial settling of sediment grains and ii hindered settling at high sediment concentrations the bimodal thickness distribution of thin bedded and thick bedded turbidites noted by previous workers is explained as the result of a change in the power law exponent this analysis supports the view that a and b divisions were deposited from high concentration flow components and that distinct grain size modes undergo different depositional processes summation of log normal frequency distributions for thin and thick bedded turbidites produces a cumulative frequency distribution of thickness with a segmented power law trend thus the occurrence of both log normal and segmented power law frequency distributions can be explained in a holistic fashion power law frequency distributions of turbidite thickness have previously been linked to power law distributions of earthquake magnitude or volumes of submarine slope failure the log normal distribution for a given grain size class observed in this study suggests an alternative view that turbidite thickness is determined by the multiplicative addition of several randomly distributed parameters in addition to the settling velocity of the grain sizes present

coherence between tropical east african and south american records of the little ice age 1 comparison of high resolution paleoclimate records from lake malawi and the cariaco basin reveals a consistent tropical behavior during the little ice age these records which are interpreted to represent southward excursions of intertropical convergence zone position over southern tropical east africa and northeastern south america are characterized by subcentennial variability superimposed on a ramp of increasing intensity from ce 1400 to 1800 this response appears similar to that of cold episodes during the late glacial the synchrony of these records suggests a teleconnection in tropical climate that may be a manifestation of persistent enso like conditions

the initial giant umbrella cloud of the may 18th 1980 explosive eruption of mount st helens abstract the initial eruption column of may 18th 1980 reached nearly 30 km altitude and released 10 17 joules of thermal energy into the atmosphere in only a few minutes ascent of the cloud resulted in forced intrusion of a giant umbrella shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m s the mushroom cloud expanded 15 km upwind forming a stagnation point where the radial expansion velocity and wind velocity were equal the cloud was initiated when the pyroclastic blast flow became buoyant the flow reduced its density as it moved away from the volcano by decompression by sedimentation and by mixing with and heating the surrounding air observations indicate that much of the flow covering an area of 600 km 2 became buoyant within 1 5 minutes and abruptly ascended to form the giant cloud calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant assuming an initial temperature of 450 c and a magmatic origin for the explosion these calculations indicate that the flow became buoyant when its temperature was approximately 150 c and the flow consisted of a mixture of 3 25 10 11 kg of pyroclasts and 5 0 10 11 kg of air if sedimentation is considered these figures reduce to 1 1 10 11 kg of pyroclasts and 1 0 10 11 kg of air

predicting the geometry of channelized deep sea turbidites analytical relationships between the dynamic properties of a deposit forming turbidity current propagating across a sea floor with small slope and the geometry of the resulting deposit have been confirmed experimentally for a wide range of particle sizes initial concentrations and volumes of the driving suspension these simple expressions provide a basis for inferring the dynamics of natural flows from their deposits we surmise that the turbidity current responsible for depositing the black shell turbidite in the northwestern atlantic ocean had an initial sediment concentration on the order of 100 g of silt per litre and a volume on the order of 1000 km 3

sediment sorting in the deposits of turbidity currents created by experimental modeling of explosive subaqueous eruptions abstract deposits of turbidity currents induced by subaqueous volcanic eruptions are increasingly recognized in settings ranging from lakes to the deep sea these eruption fed density currents have high current temperatures and low density particles which affect current dynamics and deposit features field data indicate that deposit thickness and grain size decrease downflow at a changing rate this trend is similar for deposits of noneruptive turbidity currents but with volcanic activity the deposits often exhibit a break in slope of the profile of deposit thickness and grain size distribution here we present laboratory results on density currents that address the effects of variations in grain density and water temperature on flow properties and depositional processes parallel runs were performed under identical flow conditions using quartz sand and pyroclastic sediment each grain type was used in one series of runs in which a hot density current enters a cold environment and one series in which

bed thickness and grain size of turbidites in measured sequences of limestone and greywacke turbidites the bed thickness is found to vary proportionally with the fall velocity of the maximum grain size found at the base of the bed a simple theoretical model based on the decay of isotropic turbulence suggests that bed thickness should be a function not only of this fall velocity but also of bottom slope flow depth and the concentration and grain size distribution of the sediment in the turbidity current the field data do show some influence of these additional factors nevertheless for many natural sequences of turbidites the flows must have carried very poorly sorted sediments and the inferred flow volumes and densities must cluster tightly about modal values thus grain size remains the primary variable and the modal regression curve of bed thickness on maximum grain size is well defined and resembles a fall velocity curve relatively steep basin floors near to source can theoretically cause these modal regressions for distal and very proximal parts of a turbidite to diverge introducing a crudely parabolic appearance in the form of the total regression curve the form of this parabolic curve predicts the deposition of thin but relatively coarse proximal beds such beds do occur they are different from the thin but relatively fine proximal beds that have been interpreted as the result of a fractionation of a turbidity current during levee forming processes

organic geochemical records of environmental variability in lake malawi during the last 700 years part i the tex86 temperature record abstract we have applied the tex86 paleothermometer to produce a surface water temperature record for lake malawi spanning the past 700 years over much of the record temperature fluctuates from 24 27 c with a mean of 25 c however there has been a substantial increase in temperature of 2 0 c during the past 100 years the tex86 temperature record reveals a strong similarity to the instrumental record both records demonstrate warming 0 7 1 4 c over the past 50 years as well as a cooling anomaly around 1959 comparison of the tex86 temperature record with the proxy records of primary productivity suggests that wind induced upwelling and or precipitation have a strong influence on the surface temperature of lake malawi

high resolution numerical simulations of resuspending gravity currents conditions for self sustainment received 21 february 2005 revised 30 june 2005 accepted 29 august 2005 published 22 december 2005 1 we introduce a computational model for high resolution simulations of particle laden gravity currents the features of the computational model are described in detail and validation data are discussed physical results are presented that focus on the influence of particle entrainment from the underlying bed as turbulent motions detach particles from the bottom surface resuspended particles entrained over the entire length of the current are transferred to the current s head causing it to become denser and potentially accelerating the front of the current the conditions under which turbidity currents may become self sustaining through particle entrainment are investigated as a function of slope angle current and particle size and particle concentration the effect of computational domain size and initial aspect ratio of the current on the evolution of the current are also considered applications to flows traveling over a surface of varying slope angle such as turbidity currents spreading down the continental slope are modeled via a spatially varying gravity vector particular attention is given to the resulting particle deposits and erosion patterns

experimental turbidity currents entering density stratified water analogues for turbidites in mediterranean hypersaline basins experimental turbidity currents entering two layer density stratified water behave differently from similar currents flowing over the same topography into non stratified water experiments were designed as analogues for flows entering mediterranean hypersaline pools in both the hypersaline pools and the experiments the water density changes abruptly across a pycnocline turbidity currents generated on a platform at the level of the pycnocline behaved in one of three ways as they flowed from the platform into deeper stratified water 1 when the bulk density of the current was less than the dense water layer the current spread at the pycnocline the head of the current advanced rapidly when it lost contact with the bed grains settling out of the current fell through the dense water layer forming an extensive deposit in nature this behaviour will lead to turbidites with sharp but non erosive bases strongly developed grading and no traction features 2 when the bulk density of the current was greater than the dense water layer the current continued as an underflow plunging into the deeper water sedimentation lowered the bulk density of the current and the low density interstitial fluid caused the head to loft low density interstitial fluid convected from the body of the current lofting particles into the water column these particles were hydraulically sorted during upward transport and subsequent settling to the floor the resulting turbidites had a more limited extent than the deposits of either non lofting underflows or interflows by inference from the experiments natural deposits of this type may have local proximal erosion and traction features at the base and strongly graded tops 3 in some of the currents with high bulk density the rising turbid water reached the pycnocline and spread at that level as a secondary interflow the tail of the turbidity current which was less dense than the head and body of the current flowed above the pycnocline adding momentum to the secondary interflow the thin non erosive graded deposit from the secondary interflow may extend beyond the deposits of the primary underflow in all three cases but more pronounced in cases 2 and 3 the interaction of the current with the pycnocline displaced that surface and generated a wave that was reflected back and forth from each end of the pool the waves remobilized sediment on the ramp

production of 15n depleted biomass during cyanobacterial n2 fixation at high fe concentrations 1 in this study we examine the effects of varying fe mo and p concentrations on 15 n fractionation during n 2 fixation in the cyanobacterium anabaena variabilis we show that when grown in fe enriched media fe 50 nm this organism produces biomass up to 3 o lower in 15 n than when grown in fe limited media fe 50 nm a compilation of our data with previous measurements of 15 n in n 2 fixing cyanobacteria reveals a general trend toward the production of more 15 n depleted biomass at higher fe concentrations we discuss our results in the context of negative 15 n values preserved in archean and some phanerozoic sediments generally attributed to the production of marine organic matter with low 15 n by n 2 fixation and potentially nh4 regeneration during periods of fluctuating nutrient dynamics we suggest that enhanced fe availability during periods of widespread ocean anoxia can further stimulate the production of 15 n depleted biomass by n 2 fixing organisms contributing to the isotopic record

source and dispersal of jokulhlaup sediments discharged to the sea following the 1996 vatnajokull eruption the october 1996 gjalp eruption beneath vatnajokull glacier led to one of the largest jokulhlaups glacial floods in iceland in the twentieth century a catastrophic discharge of meltwater and sediment swept across the skeidararsandur flood plain to the sea tephra from the eruption consists of vesicular sideromelane shards with a basaltic andesite composition 53 sio 2 3 mgo 0 8 k 2 o after the flood sediment samples were collected from the flood plain and off the southeast coast of iceland where a major sediment plume had been created by the discharge compositions of glass shards from flood plain and seafloor deposits do not match those of the gjalp magma flood plain samples consist primarily of blocky to poorly vesicular sideromelane clasts with compositions that are characteristic of grimsvotn volcanic products 50 sio 2 5 5 mgo 0 4 k 2 o marine samples collected near the jokulhlaup outflow into the sea also consist primarily of blocky to poorly vesicular sideromelane clasts with compositions that are for the most part similar to products of the grimsvotn volcanic center distal marine samples have more vesicular sideromelane clasts with compositions that are similar to products of the katla volcanic center e g 48 sio 2 4 5 mgo 0 8 k 2 o significant deposition to the seafloor was apparently limited to an area just offshore of the skeidararsandur there is no indication that juvenile volcanic material from the gjalp eruption was carried by the 1996 jokulhlaup onto the flood plain or into the ocean instead the jokulhlaup carried primarily older volcaniclastic material eroded by the flood

sediment laden gravity currents with reversing buoyancy there are many natural occurrences of sediment laden gravity currents in which the density of the interstitial fluid is less than that of the ambient fluid although the bulk density of the current is greater such currents are driven by the excess density of suspended particles however after sufficient particles have sedimented the current will become buoyant cease its lateral motion and ascend to form a plume examples of such currents include brackish underflows in deltas turbidity currents and pyroclastic flows experimental studies are described which show that due to sedimentation sediment laden gravity currents decelerate more rapidly than saline currents of the same density there is little difference in the experiments between a sediment laden current with neutrally buoyant interstitial fluid and one with buoyant interstial fluid until sufficient sediment has been lost to cause the latter kind of current to lift off a marked deceleration is then observed and a plume is generated with lift off occurring along the length of the current the resulting buoyant plume then generates a gravity current below the upper surface of the fluid in the tank the deposit from a current with buoyant fluid shows a fairly abrupt decrease in thickness beyond the lift off distance and has a flatter profile than that from a simple sediment current a theoretical model is presented which is based on the two layer shallow water equations and incorporates a model of the sedimentation in which particles are assumed to be uniformly suspended by the turbulence of the current the model shows good agreement with the observed lengths of the experimental currents as a function of time and predicts the lift off distance reasonably well these processes have implications for the behaviour of turbidity currents the interpretation of turbidites mixing processes in the oceans and the lift off of pyroclastic flows

analogue study of particle segregation in pyroclastic density currents with implications for the emplacement mechanisms of large ignimbrites analogue flume experiments were conducted to investigate the transport and sedimentation behaviour of turbulent pyroclastic density currents the experimental currents were scaled approximately to the natural environment in three ways 1 they were fully turbulent 2 they had a very wide range of particle sizes and associated rouse numbers the ratio of particle settling velocity to effective turbulent eddy velocity in the current and 3 they contained particles of two different densities two sets of surge type experiments were conducted in a 5 m long water filled lock exchange flume at five different volumetric particle concentrations from 0 6 to 23 in one set one component experiments the currents contained just dense particles in the other set two component experiments they contained both light and dense particles in equal volume proportions in both sets of experiments the population of each component had a log normal size distribution in the two component experiments the size range of the light particle population was selected in order to be in hydrodynamic equivalence with that of the dense particles dense particles were normally graded both vertically and downstream in the deposits from both sets of experiments the mass loading normalized to the initial mass of the suspension and grain size of the dense component in the deposits decreased with distance from the reservoir and were insensitive to initial total particle concentration in the currents on the other hand in the two component experiments the light particles were extremely sensitive to concentration they were deposited in hydrodynamic equivalence with the dense particles from dilute currents but were segregated efficiently at concentrations higher than a few per cent with increasing particle concentration the large light particles were carried progressively further down the flume because of buoyancy effects deposits from the high concentration currents exhibited reverse vertical grading of the large light particles efficient segregation of the light component was observed even if the bulk density of the current was less than that of the light particles in both sets of experiments marked inflexions in the rate of downstream decline in mass loading and maximum grain size of the dense component can be attributed to the presence of two different particle settling regimes in the flow 1 particles with rouse numbers 2 5 which did not respond to the turbulence and settled rapidly and 2 particles with rouse numbers 2 5 which followed the turbulent eddies and settled slowly the results are applied to the transport and sedimentation dynamics of pyroclastic density currents that generate large widespread ignimbrites field data fail to reveal significant departures from aerodynamic equivalence between pumice and lithic clasts in three such ignimbrites the particulate loads of some large ignimbrites are transported principally in turbulent suspensions of low concentration in some ignimbrites the well developed inflexions in curves of maximum lithic ml size vs distance can be attributed to the existence of distinct high and low rouse number particle settling regimes that mark the transition from an overcharged state to one in which the residual particulate load is transported more effectively by turbulence

experiments on gas ash separation processes in volcanic umbrella plumes abstract we present a series of analogue laboratory experiments which simulate the separation of ash and gas and the formation of secondary intrusions from finite volcanic umbrella plumes we examined the lateral spreading of mixtures of freshwater and particles released into a laboratory tank containing a uniformly stratified aqueous solution for times smaller than the sedimentation time of particles through the intrusion the current remains coherent and intrudes laterally as some of the particles settle into the underlying ambient fluid a layer of particle depleted fluid develops below the upper surface of the current and the density of the residual fluid is reduced over longer times the intrusion ceases to be coherent with small fingers of relatively buoyant particle depleted fluid rising from the upper part of the intrusion into the overlying fluid meanwhile the lateral motion of the injected solution induces a return flow in the ambient fluid which sweeps some of the particles sedimenting from the lower surface of the intrusion inwards as a result relatively dense particle laden fluid collects below the intrusion and then sinks into the underlying fluid eventually this fluid reaches a new neutral buoyancy height where it intrudes to form a second laterally spreading current below the original intrusion the process then repeats to form further weaker intrusions below these results of the separation of the ash and volcanic gas in an umbrella plume are consistent with field observations at sakurajima volcano where positively charged plumes thought to consist of volcanic gas have been observed above negatively charged plumes of ash this work also suggests that volcanic aerosols may form up to a kilometer above the original injection height of the ash in a strong wind shear this could result in very different trajectories of the ash and gas and so be important for evaluating the impact of ash plumes on both aviation safety and volcanic aerosol formation the separation of the ash from the umbrella plume after a time of order 10 3 10 4 s corroborates recent models of the may 18 1980 eruption at mount st helens that after a few hours the motion of the ash was simply controlled by wind dispersal and gravitational settling the complex motion of the sedimenting ash may also explain some of the observations of multiple intrusions in short lived umbrella plumes such as observed at redoubt alaska in april 1990

traction carpet stratification in turbidites fact or fiction abstract the spaced stratification that characterizes some thick deep water sandstones consists of a stack of layers 5 10 cm thick that are inversely graded at their base and structureless at their top these layers have been attributed to deposition from a succession of collapsed inertia layers traction carpets prior to collapse each layer was believed to have been maintained by shear from the overriding turbidity current this depositional model is based on the mistaken assumption that the distribution of shear stress in such a traction carpet is indistinguishable from the distribution of shear stress in a grain flow on a steep slope instead shear stress in an externally sheared traction carpet should be approximately constant from the top to the base of the layer and deposition sho ld be a gradual process leaving no record of sequentially active inertia layers inertia layers that do form under large turbidity currents are probably never as thick as 5 10 cm based on available data from shear cell experiments instead inertia layers are probably about 1 cm thick similar to the thickness of the inversely graded basal part of each layer in field examples of spaced stratification the rest of each inversely graded stratum probably forms due to rapid fallout from suspension the basal inversely graded 1 cm is believed to record strong shear that may accompany impingement on the bed of large turbulent eddies hence spaced stratification may not be a structure produced beneath steady flows but may record strong y fluctuating hydrodynamic conditions and vigorous burst sweep cycles in large turbidity currents

fine grained sediment lofting from meltwater generated turbidity currents during heinrich events turbidity currents generated from sediment carrying freshwater discharges into the sea contain a fluid that is less dense than ambient seawater from experiments it is known that such currents will eventually lift from their substrate either in part or as a whole through buoyancy reversal this ascent will happen when their density is lowered below that of seawater through settling of suspended sediment from the top or deposition from the bottom of the flows evidence for large scale lofting of suspended sediment from the top of giant sand and gravel carrying turbidity currents in the labrador sea comes from two independent lines of observations 1 the first is a distinct sedimentary facies consisting of stacked centimeter thick graded mud layers that contain grains of ice rafted debris ird supported by the mud deposition of these unusual layers requires a gradedlayer forming process that is slow enough to allow the incorporation of ird this is not possible with normal mud carrying turbidity currents 2 the second observation is the presence of a huge abyssal sand and gravel plain in the central labrador sea that received its sediment from bed load rich meltwater discharges from the hudson strait outlet of the pleistocene laurentide ice sheet these discharges turned into turbidity currents that released rising columns of freshwater that carried fine grained suspended sediment and spread out at a water level where their density equaled that of ambient seawater deposition from these slow turbid interflows would allow the incorporation of ird in the accumulating graded mud deposits the ird spiked graded mud facies is restricted to heinrich layers within 300 km radius of the hudson strait ice stream terminus tying the sand carrying turbidity currents via fine grained sediment lofting to heinrich events estimated total discharge volumes of individual currents are on the order of 10 3 km 3 supporting the notion that heinrich ice rafting events were times of maximum meltwater generation

reversing buoyancy in turbidity currents developing a hypothesis for flow transformation and for deposit facies and architecture a turbidity current that contains fresher or otherwise less dense water than its surroundings may initially be denser than the ambient and propagate as a bottom hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient seawater it is proposed that this reversal in buoyancy may be a significant mechanism controlling the structure and facies of turbiditic deposits buoyancy reversal followed by lofting may directly affect the relative distribution of fine and coarse material in the deposit while buoyancy reversal itself may mediate the transformation between dilute and highly concentrated suspension flows particularly in distal regions and thus lead to the formation of complex turbiditic beds in particular the generation of distal co genetic debrites may be expected similar transformations occur within dilute pyroclastic density currents where a mobile basal concentrated flow termed a surge derived pyroclastic flow develops through rapid sedimentation from the suspended load of the overlying surge the physical mechanisms involved in these processes are discussed leading to the proposal of some associated facies models these are compared with field data from the northern apennines with some striking similarities being noted as well as some differences on the basis of this discussion some directions are suggested for future experimental and modelling work on the topic

laser vs settling velocity differences in silt grainsize measurements estimation of palaeocurrent vigour the use of grain size distribution of muds for the reconstruction of past deep ocean currents is becoming established and applied in the palaeoceanographic community the methods are also applicable to shallow marine and tidal flat muds with similar inferences concerning the energy of wave and current sorting being drawn fine sediment grain size distributions can be obtained using a variety of instruments based on fundamentally different theoretical principles these machines may give varying sometimes misleading results giving divergent interpretations of flow speed history the new evidence presented here combined with earlier work suggests that of the three most commonly used analytical methods for fine silts with clays settling velocity sedigraph should be the method and instrument of choice with electrical resistance pulse counters e g coulter counter as a suitable alternative the data also show that laser particle sizers should be avoided for palaeocurrent reconstructions because the measured size of platey minerals can be dominated by their large projected area this causes them to be recorded as the same size as larger equant grains although they have much smaller settling velocity and were deposited in aggregates this produces results with a weaker relationship to the dynamics of deposition the central problem is thus that some coarse clay fine silt is recorded as medium to coarse silt the key size in the sortable silt mean size method of inferring changes in flow speed for coarse silts e g loess the laser gives more satisfactory results

vertical density currents a review of their potential role in the deposition and interpretation of deep sea ash layers marine tephra layers form important chronostratigraphic markers and may also provide information on eruption dynamics and chronology recent experiments and field data from the 1991 pinatubo eruption challenge tacit assumptions that following atmospheric fallout on the ocean surface marine sedimentation of ash grade material occurs by stokes law settling of individual grains laboratory experiments have demonstrated that vertical density currents can be initiated at representative mass flux rates further experiments described herein involving sustained particle fluxes entering stratified environments demonstrate that ash particles can decouple from the transporting fluid whose vertical motions are limited by the stable density gradient in the ocean vertical density currents generated by ash loading and gravitational destabilization of the water column can overcome the strong stable density gradients in the ocean and transport ash particles vertically 1 3 orders of magnitude faster than possible by stokesian settling greatly reducing their residence time sea bed sedimentation is inferred to involve settling of individual coarse or dense grains through the entire water column plus suspension fallout from the polydisperse turbid layer delivered by convective plumes yielding normal distribution grading the turbid layer may continue laterally as a nepheloid current under the influence of bottom slopes and or geostrophic forces either mechanism possibly modifying the grain size distribution of the suspension and hence the resulting ash layer by transporting selected fractions into different depositional locations consequently and regardless of any subsequent reworking deep sea ash isopachs may not directly reflect an eruptive signal and the ash layers may not directly record the order rate or location of arrival of ash particles at the ocean surface

channels echo character mapping and tectonics from 3 5 khz profiles distal bengal fan abstract the distal parts of the bengal fan are spectacularly affected by tectonic deformation related to the diffuse plate boundary between the indian and australian plates here we use 3 5 khz and 12 khz echosounder profiles seismic reflection profiles and piston core results to examine sedimentary processes and their relationships to tectonism within an area 78 82 e 0 6 s just to the south of ocean drilling program leg 116 sites echo character mapping was completed using echosounder data in conjunction with results from piston coring and a total of five different echo types have been recognised four of these fall into the echo character classification scheme developed by damuth 1980a whilst the fifth is believed to represent hemiturbidite deposits several types of submarine channel were also identified from echosounder data and a correlation between echo type and channel location can be seen their abundance erosional and or depositional nature together with a complex meandering and bifurcation pattern across a wide region of average gradient around 1 km are all features characteristic of a broad channel termination zone on a large elongate fan it is clear therefore that the bengal fan extends beyond 6 s active faulting in the area has led to the development of an irregular topography of low rounded hillocks that interfere with incoming turbidity currents this has resulted in ponding between highs rather than lobe construction thinning and pinching out of turbidites against the flanks of local relief flow lofting and hemiturbidite drape and common small scale slumping in some cases uplifted channel segments can be seen abandoned and partially filled

decadal record of climate variability spanning the past 700 yr in the southern tropics of east africa biogenic silica profiles in varved sediments from northern lake malawi nyasa east africa span the past 700 yr and reflect past primary productivity in the overlying waters on a centennial scale this has been influenced by lake level and a consequent shift in the location of high diatom productivity within the lake basin primary production was higher during the little ice age an arid period from about a d 1570 to 1850 when lake level was about 120 m lower than during the previous three centuries or the past 150 yr

significance of fine grained sediment lofting from melt water generated turbidity currents for the timing of glaciomarine sediment transport into the deep sea abstract sediment lofting from turbidity currents is a process that occurs in density currents generated from fresh water discharges into the sea and other gravity currents that can produce reversed buoyancy settling of suspended sediment from the top or deposition from the bottom of the flows may lower the density below that of ambient seawater causing the currents to lift up from their substrate either in part or as a whole through buoyancy reversal as is well known from experiments fine grained sediment lofting on a large scale during times of heinrich events has been inferred for ice proximal regions of the northwest labrador sea adjacent to the margin of the late pleistocene laurentide ice sheet lis the evidence comes from a distinct depositional facies of stacked graded mud layers containing ice rafted debris ird the significance of the lofted fine grained depositional facies arises from its genetic link to large sand and gravel transporting turbidity currents that were generated by melt water discharges from the hudson strait outlet of the lis and are the likely source of the lofted sediment the observed lofted depositional facies is exclusively found in heinrich layers through this stratigraphic relationship the lofted facies ties the main pulses of sediment supply in the labrador basin to heinrich events heinrich events thus appear to be the times of the main depositional activity in the labrador sea a hitherto unknown aspect of these phenomenal events besides their known role as intense iceberg discharges

beds comprising debrite sandwiched within co genetic turbidite origin and widespread occurrence in distal depositional environments co genetic debrite turbidite beds occur in a variety of modern and ancient turbidite systems their basic character is distinctive an ungraded muddy sandstone interval is encased within mud poor graded sandstone siltstone and mudstone the muddy sandstone interval preserves evidence of en masse deposition and is thus termed a debrite the mud poor sandstone siltstone and mudstone show features indicating progressive layer by layer deposition and are thus called a turbidite palaeocurrent indicators ubiquitous stratigraphic association and the position of hemipelagic intervals demonstrate that debrite and enclosing turbidite originate in the same event detailed field observations are presented for co genetic debrite turbidite beds in three widespread sequences of variable age the miocene marnoso arenacea formation in the italian apennines the silurian aberystwyth grits in wales and quaternary deposits of the agadir basin offshore morocco deposition of these sequences occurred in similar unchannellized basin plain settings co genetic debrite turbidite beds were deposited from longitudinally segregated flow events comprising both debris flow and forerunning turbidity current it is most likely that the debris flow was generated by relatively shallow few tens of centimetres erosion of mud rich sea floor sediment changes in the settling behaviour of sand grains from a muddy fluid as flows decelerated may also have contributed to debrite deposition the association with distal settings results from the ubiquitous presence of muddy deposits in such locations which may be eroded and disaggregated to form a cohesive debris flow debrite intervals may be extensive 26 x 10 km in the marnoso arenacea formation and are not restricted to basin margins such long debris flow run out on low gradient sea floor 0 1 may simply be due to low yield strength 50 pa of the debris water mixture this study emphasizes that multiple flow types and transformations between flow types can occur within the distal parts of submarine flow events

dependence of the structure of the northern hemisphere annular mode on the polarity of enso 1 the structure of the northern hemisphere annular mode nam is shown to be significantly different during warm and cold winters of the enso cycle during warm winters the arctic center of action of the annular mode is more prominent and extends deeper into siberia its related surface air temperature signal is stronger and the compensating outer ring in the sea level pressure field is much broader encompassing the entire tropics during cold winters the nam signature is much more pronounced over the pacific sector the sea level pressure pattern exhibits a north pacific center of action positively correlated with the atlantic center and pronounced deflections of the storm track are also evident it is suggested that the former features are associated with the equatorward displacement of the jet stream and the storm tracks during warm winters of the enso cycle and the latter is related to the the more pronounced break in the subtropical jet stream over the pacific sector during cold winters

behaviour of particle laden flows into the ocean experimental simulation and geological implications the behaviour of subaerial particle laden gravity currents e g pyroclastic flows lahars debris flows sediment bearing floods and jokulhlaups flowing into the sea has been simulated with analogue experiments flows of either saline solution simple suspensions of silicon carbide sic in water or complex suspensions of sic and plastic particles in methanol were released down a slope into a tank of water the excess momentum between subaerial and subaqueous flow is dissipated by a surface wave at relatively low density contrasts between the tank water and the saline or simple suspensions the flow mixture enters the water and forms a turbulent cloud involving extensive entrainment of water the cloud then collapses gravitationally to form an underwater gravity current which progresses along the tank floor at higher density contrasts the subaerial flow develops directly into a subaqueous flow the flow slows and thickens in response to the reduced density contrast which is driving motion and then continues in the typical gravity current manner complex suspensions become dense flows along the tank floor or buoyant flows along the water surface if the mixtures are sufficiently denser or lighter than water respectively flows of initially intermediate density are strongly influenced by the internal stratification of the subaerial flow material from the particulate depleted upper sections of the subaerial flow becomes a buoyant gravity current along the water surface whereas material from the particulate enriched lower sections forms a dense flow along the tank floor sedimentation from the dense flow results in a reduction in bulk density until the mixture attains buoyancy lifts off and becomes a secondary buoyant flow along the water surface jokulhlaups lahars and debris flows are typically much denser than seawater and thus will usually form dense flows along the seabed after sufficient sedimentation the freshwater particulate mixture can lift off to form a buoyant flow at the sea surface leading to a decoupling of the fine and coarse particles flood waters with low particulate concentrations 2 may form buoyant flows immediately upon entering the ocean subaerial pyroclastic flows develop a pronounced internal stratification during subaerial run out and thus a flow splitting behaviour is probable which agrees with evidence for sea surface and underwater flows from historic eruptions of krakatau and mont pelee a pyroclastic flow with a bulk density closer to that of sea water may form a turbulent cloud resulting in the deposition of much of the pyroclasts close to the shore dense subaqueous pyroclastic flows will eventually lift off and form secondary buoyant flows either before or after the transformation to a water supported nature

event suspended sediment characteristics and the generation of hyperpycnal plumes at river mouths east coast continental margin north island new zealand abstract steepland rivers draining small coastal watersheds often have very high suspended sediment loads but the event characteristics of suspended sediment concentration and yield in this class of river is not well documented continuous monitoring at four sites in the waipaoa river basin new zealand demonstrates that during individual and composite events suspended sediment concentration versus water discharge relations typically show clockwise hysteresis and that event maximum concentration is poorly related to event peak discharge the signature of different erosion processes is also imprinted on the event yield magnitude frequency distributions gully dominated tributary basins produce relatively high yields at all frequencies reflecting greater sediment availability whereas in tributary basins where shallow landsliding is the dominant erosion process there is a steep increase in yields in relation to return period we estimate that flood discharges from the waipaoa river approach or exceed

late quaternary vegetation history of southeast africa the molecular isotopic record from lake malawi abstract accurate reconstructions of past hydrological variability are essential for understanding the climate history of the tropics in tropical africa the relative proportion of vegetation utilizing the c3 vs c4 photosynthetic pathway is mainly controlled by precipitation and thus past hydrological changes can be inferred from the vegetation record in this study biomarkers of terrestrial plants lignin phenols and plant leaf wax carbon isotopes are examined from a well dated sedimentary record from lake malawi to provide a vegetation aridity record of the past 23 cal ka from southeast africa we suggest that the ratio of cinammyl to vanillyl c v lignin phenols in lake malawi sediments mainly reflects inputs of c3 trees woody tissue vs c4 grasses non woody tissue and find that changes in the c v ratio generally support variability noted in the n alkane carbon isotope record together these records provide evidence for increased c4 vegetation grasses surrounding lake malawi during last glacial maximum lgm the younger dryas in the early holocene and from 2 cal ka to the present suggesting drier conditions at these times elevated inputs of c3 vegetation are noted in the intervals from 17 13 6 cal ka and 7 7 2 cal ka indicating wet conditions in southeast africa a relationship is noted between the n alkane average chain length acl and temperature with longer acls associated with higher temperatures higher n alkane carbon preference index cpi values correlate with higher mass accumulation rates of biogenic silica and may result from periodic increased northerly winds over lake malawi which enhance upwelling and diatom productivity while simultaneously increasing erosion and transport of plant leaf waxes to the lake the molecular data produced in this study suggest that the carbon isotopic signature of bulk sediment 13ctoc in lake malawi is primarily a reflection of terrestrial inputs c3 vs c4 vegetation and may not mainly reflect changes in algal productivity as previously thought

experiments on bidisperse constant volume gravity currents propagation and sediment deposition laboratory experiments show that the propagation and sedimentation patterns of particle laden gravity currents are strongly influenced by the size of suspended particles the main series of experiments consisted of fixed volume releases of dilute mixtures containing two sizes of silicon carbide particles 25 m and 69 m mean diameter within a 6 m flume polydisperse experiments involved mixtures of five different particle sizes and variation of the amounts of the finest and coarsest particles all variables apart from the initial relative proportions of particles were identical in the experiments the effects of mixing different proportions of fine and coarse particles is markedly non linear adding small amounts of fine sediment to a coarse grained gravity current has a much larger influence on flow velocity run out distance and sedimentation patterns than adding a small amount of coarse sediment to a fine grained gravity current the experiments show that adding small amounts of fine particles to a coarse grained current results in enhanced flow velocities because the fine sediment remains suspended and maintains an excess current density for a much longer time thus the distance to which coarse particles are transported increases substantially as the proportion of fines in the flow is increased our experiments suggest that sandy turbidity currents containing suspended fines will be much more extensive than turbidity currents composed of clean sand

transport mechanism and paleoclimatic significance of terrigenous silt deposited in varved sediments of an african rift lake we analyzed a varved sequence of sediment from a 350 m depth in the north basin of lake malawi east africa for the size distribution of the sortable silt fraction 10 64 m mean size of the sortable silt mean ss varies measurably in sediments spanning the past 650 yr and covaries with the mass accumulation rate of terrigenous silt and clay tmar over much of the interval most of the silt and clay is delivered to the offshore basin in benthic nepheloid plumes of unknown duration and frequency tmar appears to be related to annual rainfall which is related to the north atlantic oscillation because it roughly tracks the historical record of lake level that extends back to 1860 mean ss should be related to density or thickness of underflow thus related to resuspension intensity or river flood loading it also tracks lake level and regional wind strength as determined by national center for environmental prediction ncep reanalysis

simple estimates for the magnitude of bedload transport under a turbid surge characteristic length and timescales for a turbid surge are used to estimate bedload transport by the surge deriving estimates for the conditions under which deposited material will be mobilized as bedload and of the relative importance of bedload in determining the overall deposit geometry a critique is provided of the common modelling assumptions which underlie these estimates and of how their consistency can be checked for large turbidity currents such as those which emplaced the marnoso arenacea turbidites in northern italy model predictions of overall geometry are not easy to reconcile with the field data some possible reasons for this are discussed the estimates obtained from these characteristic scales are consistent with the widespread presence in turbidites of sedimentary structures which indicate bedload transport some of these structures from the marnoso arenacea formation are reviewed briefly however the estimates suggest that bedload transport is not a major factor responsible for the geometry of the large turbidites in this formation which exhibit a broad thickness maximum in their proximal region that contrasts with the downstream thinning geometry of smaller beds this effect suggests that the explanation for this theoretically unexpected geometry should be sought in other physical mechanisms

initiation of turbidity currents outcrop evidence for eocene hyperpycnal flow turbidites abstract the most widely used although not as widely documented explanation for initiation of turbidites is the conversion of catastrophic sediment failures into turbidites such instantaneous sediment failures generate short lived surge type flows that deposit turbidite beds described as bouma sequences in this communication we present data from the eocene central basin of spitsbergen that shows turbidite systems that are volumetrically dominated by thick ungraded and laminated sandy turbidite beds deposited by gradual aggradation from sustained flows sustained flows are long lived and more or less continuous they can be generated by a variety of mechanisms e g 1 instability during volcanic eruptions and the consequent remobilisation of unconsolidated material 2 seismically triggered subaerial sliding within the drainage 3 storm surges 4 retrogressive slope failure 5 breaching and 6 hyperpycnal flows our database shows that in the eocene central basin the sustained flows were generated by hyperpycnal flows i e by direct river effluent the hyperpycnal flow turbidites have been recognized on the basis of 1 physical connection between fluvial and turbidite channels at the shelf edge 2 abundance of thick turbidite sandstone beds 3 sand prone nature of the turbidite systems 4 downslope changes of individual thick sandy turbidite beds and their collapsed pinch out segments 5 great abundance of continental material leaves coal fragments in turbidite beds 6 low abundance of associated slumped or debris flow beds and 7 the occurrence of turbidites in systematically accreted shelf margins these features strongly suggest that hyperpycnal flow generated by direct river effluent deposited significant number of turbidite beds in the documented slope and basin floor turbidite successions we also argue that the hyperpycnal flows are preferentially fed into the deepwater slopes beyond the shelf edge during the falling stage and lowstand of relative sea level the turbidite beds were studied in the context of seismic scale 1 15 km dip oriented outcrops by measuring detailed vertical sections following individual beds and packages of beds downslope for distances over 5 km lateral mapping and helicopter taken photomosaics

emplacement of the taupo ignimbrite by a dilute turbulent flow an ignimbrite is a pumice rich deposit that records the passage of a ground hugging ash flow a pyroclastic flow generated by the collapse of a volcanic eruption column1 geologists study such deposits to reconstruct the parent eruptions and to predict the consequences of future eruptions the 1 800 yr old taupo ignimbrite of new zealand has been interpreted to represent en masse emplacement by an avalanche like flow with a volumetric solids concentration in excess of 30 ref 2 the evidence for this is equivocal however and here we propose the alternative view that the deposit was emplaced by a relatively dilute and turbulent density current3 we present an isothermal hydraulic model the results of which taken together with existing observations suggest that the total flow rate of solids and gas was about 40km3s 1 for around 15 minutes this intense flux resulted in a flow which had a near vent solids concentration of 0 3 by volume was about 1 km thick and travelled outward from the vent with a typical speed of 200 ms 1 in view of the good agreement between predicted and observed radial trends in the taupo deposit we suggest that the origin of other ignimbrites with similar characteristics should be reconsidered

what factors control mechanical erosion rates mechanical erosion rates are important factors in understanding how continents evolve mechanical erosion is much faster than chemical erosion especially for highly elevated regions of the earth s surface it is a principal way in which mountain ranges are removed exposing deep metamorphic roots which comprise much of the older portions of the continental masses in addition there has to be a long term balance between erosion and mountain building a new data set allows us to explore in greater detail some of the many factors which control mechanical erosion rates the most important factors are some expression of the average slope of a drainage basin some measurement of the amount of water available for erosion some environmental measurements and also a measurement of basin length for which we have no good explanation the estimate of global mechanical erosion rate obtained here is considerably lower than those obtained by some other workers some of whom have concentrated on the fact that smaller river basins tend to get eroded faster than larger basins and it is mainly smaller basins which have not been measured and which are therefore not allowed for by simple arithmetic averaging of observed erosion rates it is shown here that although smaller basins are eroded faster this is mainly because they are steeper than larger basins we also show that extrapolation of current data to smaller basins does not work because the observed continental area which is draining to the ocean cannot be attained by the simplest extrapolation scheme

late pleistocene paleohydrography and diatom paleoecology of the central basin of lake malawi africa analysis of sedimentary diatom assemblages 10 to 144 ka form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of lake malawi lake level fluctuations on the order of hundreds of meters were in ferred from dramatic changes in the fossil and sedimentary archives many of the fossil diatom assemblages we observed have no analog in modern lake malawi cyclotelloid diatom species are a major component of fossil assemblages prior to 35 ka but are not found in significant abundances in the modern diatom communities in lake malawi salinity and alkalin ity tolerant plankton has not been reported in the modern lake system but frequently dominant fossil diatom assemblages prior to 85 ka large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31 ka similarly prior to 31 ka common central basin aulacoseiroid species are replaced by species found in the shallow well mixed southern basin surprisingly tychoplankton and periphyton were not common throughout prolonged lowstands but tended to increase in relative abundance during periods of inferred deeper lake environments a high resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wetsieved fossil and mineralogical residue records prior to 70 ka fossil assemblages suggest that the central basin was periodically a much shallower more saline and or alkaline well mixed environment the most significant reconstructed lowstands are 600 m below the modern lake level and span thousands of years these conditions contrast starkly with the deep dilute dysaerobic environments of the modern central basin after 70 ka our reconstruction indicates sustained deeper water environments were common marked by a few brief but significant lowstands high amplitude lake level fluctuations appear related to changes in insolation seismic reflection data and additional sediment cores recovered from the northern basin of lake malawi provide evidence that supports our reconstruction