hydrographic structure of the convection regime in the gulf of lions winter 1987 abstract deep winter convection in the northwestern mediterranean gulf of lions and the subsequent formation of mediterranean deep water were observed using advanced oceanographic instrumentation during a six week long experiment in 1987 the severe 1987 european winter forced an intense outbreak of the mistral a cold dry wind blowing down the rhone valley in mid january surface cooling and evaporation were of sufficient intensity to cause an initial episode of deep convection shortly before the experiment described here began however several more mistral events took place during the experiment during several cruises into the area ctd and absolute horizontal velocity profiles were measured in the mixed area over the rhone fan as well as across the southern front of this region in addition continuous records of shipboard meteorological and oceanographic parameters air temperature surface salinity etc were made an early february mistral apparently did not produce enough surface cooling to

late holocene surface ocean conditions of the norwegian sea voring plateau 1 late holocene sea surface ocean conditions of the eastern norwegian sea voring plateau are inferred from planktic stable isotopes and planktic foraminiferal assemblage changes in cores jm97 948 2a and md952011 66 97 n 7 64 e strong covariance between the planktic stable oxygen isotopic record and abundance changes of n pachyderma sin show that major changes in surface ocean conditions are reflected both in the geochemical composition of the foraminiferal tests as well as in the composition of the foraminiferal fauna surface ocean conditions warmer than present were common during the past 3000 years during the so called medieval warm period surface conditions were highly variable with marked changes in sea surface temperature the warmest sea surface temperatures during this period occurred between 800 and 550 years bp 0 bp ad 2000 climatic deterioration recorded as decreases in sea surface temperature occurred at about 2750 1550 400 and 100 years bp the cooling events at about 2750 and 1550 years bp appear to correlate with increases in ice rafted debris in the north atlantic based on the results from jm97 948 2a and md952011 the onset of the little ice age cooling trend seems to have occurred around 700 600 years bp faunal changes indicate two cooling events during the little ice age at 400 and 100 years bp that correspond to decreases in fennoscandian summer temperatures and increases in ice rafted debris in the eastern north atlantic index terms 3030 marine geology and geophysics micropaleontology 4267 oceanography general paleoceanography 4870 oceanography biological and chemical stable isotopes keywords paleoceanography stable isotope holocene micropaleontology citation andersson c b risebrobakken e jansen and s o dahl late holocene surface ocean conditions of the norwegian sea voring plateau paleoceanography 18 2 1044 doi 10 1029 2001pa000654 2003

evidence of 550 year and 1000 year cyclicities in north atlantic circulation patterns during the holocene sedimentological colour data obtained from north atlantic deep sea sediment core neap15k pro vide a continuous climate record for the last 11500 yr with a mean sampling interval of 15 yr variations in sediment lightness a proxy for changes in north atlantic deep water nadw circulation are characterized by high frequency fluctuations with periodicities equivalent to 550 and 1000 yr as well as a 1600 yr cyclicity comparable to that reported in other studies of holocene climate cross spectral analysis suggests that the 550 yr and 1000 yr cyclical variations in nadw circulation were coherent with fluctuations in atmospheric con ditions over greenland where both these periodicities have been identified previously in the holocene gisp2 d18o record stuiver et al 1995 a similar statistical comparison provides evidence that high frequency 550 yr variability in nadw circulation patterns may have been linked to short term variations in atmospheric 14c values during the holocene in general nadw pro

high sedimentation rates in a karstic lake associated with hydrothermal turbid plumes lake banyoles catalonia ne spain abstract in lake banyoles northeastern spain warm water enters through lake bottom seeps and generates two hydrothermal turbid plumes one of them is perennial in character and the other is active only after fluidisation events of the bottom sediment caused by reactivation of the bottom seeps after rainfall in the aquifer recharge area the vertical development of both hydrothermal plumes is very sensitive to the thermal stratification of the water column when a plume reaches a level of neutral buoyancy a turbidity current spreads laterally and transports sediment particles across the lake silt particles transported by the plume are used in adcp backscatter images to determine its maximum and equilibrium heights field results are compared and found to be in accordance with models for thermal convection from finite isolated sources when the lake is stratified the vertical transport of sediment is confined to the hypolimnion when the lake water column is mixed the plume reaches the surface of the lake the turbidity current is usually confined to the southern sub basin of the lake during stratification resulting in higher sedimentation rates during fluidisation events the sedimentation rates are one order of magnitude greater than in periods without fluidisation for the fluidisation periods turbidity has been estimated to be 10 ftu

localized convection in rotating stratified fluid we study the convective overturning of a rotating stratified fluid in the laboratory convection is induced from the surface of a salt stratified fluid by the introduction of salty fluid over a circular area the external parameters are buoyancy forcing of strength b0 applied over a circular area of radius r the rotation rate as measured by f ambient stratification n and the depth h the experiments are motivated by physical scaling arguments which attempt to predict the length and velocity scales of the convective chimney as it adjusts under gravity and rotation and breaks up through baroclinic instability the scales of interest include the number size and typical speeds of the fragments of the broken chimney the final depth of penetration of the convective mixed layer and the total volume of convectively produced water these scales are tested against the laboratory experiments and found to be appropriate in this idealized problem we have found the depth of penetration depends only on the size and strength of the forcing and the ambient stratification encountered by the convection event it does not depend explicitly on rotation the implications of the work to deep water formation in the labrador sea and elsewhere are discussed finally the study has relevance to the role and representation of baroclinic eddies in large scale circulation of the ocean

annual climate variability in the holocene interpreting the message of ancient trees abstract over vast areas of the world s landmasses where climate beats out a strong seasonal rhythm tree growth keeps unerring time in their rings trees record many climate melodies played in different places and different eras recent years have seen a consolidation and expansion of tree ring sample collections across the traditional research areas of north america and europe and the start of major developments in many new areas of eurasia south america and australasia from such collections are produced networks of precisely dated chronologies records of various aspects of tree growth registered continuously year by year across many centuries their sensitivities to different climate parameters are now translated into ever more detailed histories of temperature and moisture variability across expanding dimensions of time and space with their extensive coverage high temporal resolution and rigid dating control dendroclimatic reconstructions contribute significantly to our knowledge of late holocene climates most importantly on timescales ranging from 1 to 100 years in special areas of the world where trees live for thousands of years or where subfossil remnants of long dead specimens are preserved work building chronologies covering many millennia continues apace very recently trees have provided important new information about major modes of general circulation dynamics linked to the el nino southern oscillation and the north atlantic oscillation and about the effect of large volcanic eruptions as for assessing the significance of 20th century global warming the evidence from dendroclimatology in general supports the notion that the last 100 years have been unusually warm at least within a context of the last two millennia however this evidence should not be considered equivocal the activities of humans may well be impacting on the natural growth of trees in different ways making the task of isolating a clear climate message subtly difficult

sinkholes in hypogene versus epigene karst systems illustrated with the hypogene gypsum karst of the sant miquel de campmajor valley ne spain abstract the garrotxa banyoles artesian aquifer system in ne spain includes zones with epigene and hypogene evaporite karst and sinkhole development the sinkhole fields related to hypogene evaporite dissolution are associated with the discharge zones of the aquifer where groundwater from a confined limestone rises traversing an overlying gypsum formation capped by thick marls this work analyses the sinkhole field developed in the sant miquel de campmajor valley smcv a discharge area of the aquifer where a cartographic inventory including 94 sinkholes has been produced sinkholes in this hypogene gypsum karst are mainly large caprock collapse sinkholes rooted in deep seated cavities generated by interstratal hypogene karstification as corroborated by an electrical resistivity imaging eri survey they show a non clustered distribution within the sinkhole field and the variations in size and density across the area are mainly controlled by the thickness of the marl caprock the magnitude and frequency relationships of the sinkholes can be modelled with a high goodness of fit with a log normal function like in other regions of the world however in this deep seated hypogene system the depressions show much larger dimensions potential differences between the sinkholes developed in hypogene and epigene karst environments are discussed including their distribution with respect to the groundwater flow system their spatial patterns and dimensions

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

sediment deposition from turbidity currents in simulated aquatic vegetation canopies a laboratory flume experiment was carried out in which the hydrodynamic and sedimentary behaviour of a turbidity current was measured as it passed through an array of vertical rigid cylinders the cylinders were intended primarily to simulate aquatic vegetation canopies but could equally be taken to represent other arrays of obstacles for example forests or offshore wind turbines the turbidity currents were generated by mixing naturally sourced poly disperse sediment into a reservoir of water at concentrations from 1 0 to 10 0 gl 1 which was then released into the experimental section of the flume by removing a lock gate for each initial sediment concentration runs with obstacle arrays with solid plant fractions of 1 0 and 2 5 and control cases with no obstacles were carried out the progress of the current along the flume was characterized by the array drag term cdaxc where cd is the array drag coefficient at the frontal area of cylinders per unit volume and xc the position of the leading edge of the current along the flume the downward depositional flux of sediment out of the current as it proceeded was measured at 13 traps along the flume analysis of these deposits divided them into fine 2 2 to 6 2 m and coarse 6 2 to 104 m fractions at the beginning of their development the gravity currents proceeded in an inertia dominated regime until cdaxc 5 for cdaxc 5 the current transitioned into a drag dominated regime for both fine and coarse sediment fractions the rate of sediment deposition tended to decrease gradually with distance from the source in the inertial regime remained approximately constant at the early drag dominated regime and then rose and peaked at the end of the drag dominated stage this implies that when passing through arrays of obstacles the turbidity currents were able to retain sufficient sediment in suspension to maintain their flow until they became significantly influenced by the drag exerted by the obstacles this article is protected by copyright all rights reserved

anomalous rainfall and associated atmospheric circulation in the northeast spanish mediterranean area and its relationship to sediment fluidization events in a lake 1 changes in the dynamics of sediment transport in a mediterranean lake sediment fluidization events are linked to atmospheric circulations patterns trough monthly precipitation in the basins of lake banyoles located in the northeast of spain water enters mainly through subterranean springs and associated fluctuations in the vertical migration of sediment distribution fluidization events present episodic behavior as a result of episodic rainfall in the area the initiation of the fluidization events takes place when the monthly rainfall is 2 7 times greater than the mean monthly rainfall of the rainiest months in the area especially in spring april and may october and december the duration of these events is found to be well correlated with the accumulated rainfall of the preceding 10 months before the process initiation the rainfall in turn is mainly associated with six atmospheric circulation patterns among the 19 fundamental circulations that emerged in an earlier study focused on significant rainfall days in mediterranean spain among them accentuated surface lows over the northeast of spain general northeasterly winds by low pressure centered to the east of balearic islands and short baroclinic waves over the iberian peninsula with easterly flows over the northeastern coast of spain are found the most relevant atmospheric circulations that drive heavy rainfall events

plumes in rotating convection part 1 ensemble statistics and dynamical balances atmospheric and oceanic convection often occurs over areas occupied by many localized circulation elements known as plumes the convective transports therefore may depend not only on the individual elements but also on the interactions between plumes and the turbulent environment created by other plumes however many attempts to understand these plumes focus on individual isolated elements and the behaviour of an ensemble is not understood geophysical convection may be influenced by rotation when the transit time of a convecting element is long compared to an inertial period for example in deep oceanic convection much recent attention has been given to the effect of rotation on individual plumes but the role of rotation in modifying the behaviour of an ensemble is not fully understood here we examine the behaviour of plumes within an ensemble both with and without rotation to identify the influence of rotation on ensemble plume dynamics we identify the coherent structures plumes present in numerical solutions of turbulent rayleigh benard convection a canonical example of a turbulent plume ensemble we use a conditional sampling compositing technique to extract the typical structure in both non rotating and rotating solutions the dynamical balances of these composite plumes are evaluated and compared with entraining plume models we find many differences between non rotating and rotating plumes in their transports of mass buoyancy and momentum as shown in previous studies the expansion of the turbulent plume by entrainment of exterior fluid is suppressed by strong rotation our most significant new result is quantification of the continuous mixing between the plume and ambient fluid which occurs at high rotation without any net changes in plume volume this mixing is generated by the plume plume interactions and acts to reduce the buoyancy anomaly of the plume by contrast in the non rotating case no such loss of buoyancy by mixing occurs as a result the total buoyancy transport by upwardly moving plumes diminishes across the layer in the rotating case while remaining approximately constant in the non rotating case at high values of rotation the net vertical acceleration is considerably reduced compared to the non rotating case due to loss of momentum through entrainment and mixing and a decelerating pressure gradient which partially balances the buoyancy driven acceleration of plumes as a result of the dilution of buoyancy the pressure gradient deceleration and the loss of momentum due to mixing with the environment in the rotating solutions the conversion of potential energy to kinetic energy is significantly less than that of non rotating plumes the combination of efficient lateral mixing and slow vertical movement by the plumes accounts for the unstable mean temperature gradient that occurs in rotating rayleigh benard convection while the less penetrative convection found at low rossby number is a consequence of the reduced kinetic energy transport within the ensemble of plumes identified by the conditional sampling algorithm distributions of vertical velocity buoyancy and vorticity mimic those of the volume as a whole plumes cover a small fraction of the total area yet account for most of the vertical heat flux

a heton perspective of baroclinic eddy transfer in localized open ocean convection a simple point vortex heton model is used to study localized ocean convection in particular the statistically steady state that is established when lateral buoyancy transfer effected by baroclinic instability offsets the localized surface buoyancy loss is investigated properties of the steady state such as the statistically steady density anomaly of the convection region are predicted using the hypothesis of a balance between baroclinic eddy transfer and the localized surface buoyancy loss these predictions compare favorably with the values obtained through numerical integration of the heton model the steady state of the heron model can be related to that in other convection scenarios considered in several recent studies by means of a generalized description of the localized convection this leads to predictions of the equilibrium density anomalies in these scenarios which concur with those obtained by other authors advantages of the heton model include its inviscid nature emphasizing the independence of the fluxes affected by the baroclinic eddies from molecular processes and its extreme economy allowing a very large parameter space to be covered this economy allows us to examine more complicated forcing scenarios for example forcing regions of varying shape by increasing the ellipticity of the forcing region the instability is modified by the shape and as a result no increase in lateral fluxes occurs despite the increased perimeter length the parameterization of convective mixing by a redistribution of potential vorticity implicit in the heton model is corroborated the heton model equilibrium state has analogous quantitative scaling behavior to that in models or laboratory experiments that resolve the vertical motions the simplified dynamics of the heton model therefore allows the adiabatic advection resulting from baroclinic instability to be examined in isolation from vertical mixing and diffusive processes these results demonstrate the importance of baroclinic instability in controlling the properties of a water mass generated by localized ocean convection a complete parameterization of this process must therefore account for the fluxes induced by horizontal variations in surface buoyancy loss and affected by baroclinic instability

palaeoclimatological and palaeolimnological records from fossil midges and tree rings the role of the north atlantic oscillation in eastern finland through the medieval climate anomaly and little ice age abstract hydrological changes and air temperature variability are reconstructed from the sediments of lake pieni kauro eastern finland during the medieval climate anomaly mca and little ice age lia using transfer functions derived from midge insecta nematocera based calibration models the reconstructions are compared with a regional tree ring chronology and sediment physical properties are determined to track depositional changes an objective of the study is to examine the long term relationship between the north atlantic oscillation nao and climate temperature precipitation our results show that the mca was characterized by warm and dry summer conditions which were accompanied by changes in the sediment magnetic susceptibility values most likely representing major forest fires during the tree ring indicated mca megadrought however the midge based stream flow reconstruction shows increased values during the mca thus implying enhanced spring floods after snowy winters during the lia the tree ring data indicate that a generally wetter climate prevailed during summers but the stream flow reconstruction indicates less snowy winters in the terms of long term climatology the present results show support to the concept that the nao has a positive correlation between winter precipitation and annual temperature and a negative correlation between summer precipitation in eastern finland thus the results may serve as important background data for global change assessments

observations of deep convection in the gulf of lions northern mediterranean during the winter of 1991 92 abstract during december 1991 to april 1992 measurements with moored acoustic doppler current profiler adcp stations and shipboard surveys were carried out in the convection regime of the gulf of lions northwestern mediterranean first significant mixed layer deepening and generation of internal waves in the stratified intermediate layer occurred during a mistral cooling phase in late december mixed layer deepening to about 400 m eroding the salinity maximum layer of saltier and warmer levantine intermediate water and causing temporary surface layer warming followed during a second cooling period of late january during a mistral cooling period from 18 to 23 february 1992 convection to 1500 m depth was observed where the size of the convection regime was 50 100 km extent vertical velocities 40 640 m deep recorded by four adcps of a triangular moored array of 2 km sidelength in the center of the convection regime exceeded 5 cm s 1 and were not correlated over the separation of the moorings hori

a laboratory evaluation of the laser in situ scattering and transmissometery instrument using natural sediments abstract the lisst laser in situ scattering and transmissometery instrument was designed by sequoia scientific to measure the particle size distributions and concentrations of sediment suspensions in the field environment to understand the lisst s performance with natural sediments from a variety of marine sources several experiments were performed to compare the lisst s results to traditional sieving filtering and weighing techniques the lisst was able to correctly locate the peak of a unimodal particle size distribution and resolve the two peaks of a bimodal distribution if they are separated by at least 1 for sediment sizes of 5 to 250 m unlike a single frequency scattering sensor for sediment concentration which require sediment size specific calibration constants the lisst is able to accurately measure sediment concentration with a single calibration constant for varying size distributions within the size range of 5 to 250 m the lisst was also found to adequately represent the particle volumetric size distribution for two different samples from marine environments

the little ice age and medieval warm period in the sargasso sea sea surface temperature sst salinity and flux of terrigenous material oscillated on millennial time scales in the pleistocene north atlantic but there are few records of holocene variability because of high rates of sediment accumulation holocene oscillations are well documented in the northern sargasso sea results from a radiocarbondated box core show that sst was approximately 1 degree c cooler than today approximately 400 years ago the little ice age and 1700 years ago and approximately 1 degree c warmer than today 1000 years ago the medieval warm period thus at least some of the warming since the little ice age appears to be part of a natural oscillation 39 refs 4 figs 1 tab

direct transformation of tree ring measurements into palaeoclimate reconstructions in three dimensional space here we propose a new method for dendroclimatic reconstructions this direct reconstruction technique direct performs modelling of the age dependent association between climate and growth in thre

solution collapse depressions and suspensates in the limnocrenic lake of banyoles ne spain a high resolution seismic reflection survey in the banyoles limnocrenic solution lake allowed penetration of dense suspensates occupying cone like bottom depressions of different size the depressions result from the dissolution and collapse of underlying eocene calcareous and gypsiferous materials over which lacustrine sediments of varying thicknesses have accumulated the suspensates occupying the depressions present three main types of seismic signatures stratified semistratified and transparent the densities of the suspensates and the water depths of their tops which fluctuate continuously vary from one depression to another the maximum seismically recorded suspensate thickness is 44 m morphological and structural features seismic characters and variable degrees of hydraulic activity point to the existence of different stages of maturity in the lake bottom depressions this work brings new insight on the dynamics and evolution of limnocrenic solution lakes

a model of an atlantic and pacific hydrothermal plume a turbulent entrainment model of a hydrothermal plume was modified to include the effects of vertical structure in the background temperature and salinity fields owing to the stably stratified salinity field in the pacific relatively warm salty water spreads at an equilibrium level where the plume density equals the ambient density in contrast the unstable salinity profile in the atlantic gives the plume added buoyancy and it continues to rise to an equilibrium level where it is relatively cool and fresh these attributes result from the mixing between the plume and the surrounding seawater at each level below the spreading layer the equilibrium temperature anomaly is not directly proportional to the source strength but depends on the mixing and background salinity gradient the net entrainment of surrounding seawater is greater in the atlantic plume model than in the pacific plume model for the same buoyancy flux recent data from the tag hydrothermal field at 26 n on the mid atlantic ridge and data from a vent field on the endeavor segment of the juan de fuca ridge are compared to model results the model predicts the penetration height and s characteristics of the atlantic plume and it explains some of the differences between the observed atlantic and pacific hydrothermal plumes

hydrothermal plumes trapped by thermal stratification 1 hydrothermal plume penetration through density interfaces has been widely studied in laboratory experiments it has been found that competition between plume buoyancy and background stratification parameterised as the richardson number is crucial in determining whether or not the plume will penetrate through a density interface in this paper we present the analysis of field data concerning two hydrothermal sediment laden plumes in a stratified lake measurements of the particle concentration within these plumes have been used to determine their vertical spatial and temporal development in the water column the results show that neither of the plumes penetrate the lake thermoclines for large richardson numbers which is in accordance with the results found in laboratory experiments

summer temperature variations in lapland during the medieval warm period and the little ice age relative to natural instability of thermohaline circulation on multi decadal and multi centennial scales new tree ring based analysis for climate variability at a regional scale is presented for high latitudes of europe our absolutely dated temperature reconstruction seeks to characterise the summer temperatures since ad 750 the warmest and coolest reconstructed 250 year periods occurred at ad 931 1180 and ad 1601 1850 respectively these periods share significant temporal overlap with the general hemispheric climate variability due to the medieval warm period mwp and the little ice age lia further we detect a multi decadal ca 50 to 60 year rhythm attributable to instability of the north atlantic deep water in the regional climate during the mwp but not during the lia intensified formation of the north atlantic deep water further appeared coincident to the initiation and continuation of mwp the mid lia transient warmth occurring during the period ad 1391 1440 and to recent warming our results support the view that the internal climate variability i e thermohaline circulation could have played a role in the earlier start of the mwp in several proxy reconstructions compared to the externally forced model simulations copyright 2009 john wiley sons ltd

microburst modelling and scaling a microburst can be modeled by releasing a volume of fluid that is slightly heavier than the ambient fluid allowing it to fall onto a horizontal surface vorticity develops on the sides of this parcel as it descends and causes it to roll up into a turbulent vortex ring which impinges on the ground such a model exhibits many of the features of naturally occurring microbursts which are a hazard to aviation in this paper this model is achieved experimentally by releasing a volume of salt water into fresh water from a cylindrical dispenser when care is taken with the release the spreading rate of the surface outflow is measurable and quite repeatable despite the fact that the flow is turbulent an elementary numerical approximation to this model based on inviscid vortex dynamics has also been developed a scaling law is proposed which allows experiments with different fluid densities to be compared with each other and with the numerical results more importantly the scaling law makes it possible to compare the model results with real microbursts

tornetrask tree ring width and density ad 500 2004 a test of climatic sensitivity and a new 1500 year reconstruction of north fennoscandian summers this paper presents updated tree ring width trw and maximum density mxd from tornetrask in northern sweden now covering the period ad 500 2004 by including data from relatively young trees for the most recent period a previously noted decline in recent mxd is eliminated non climatological growth trends in the data are removed using regional curve standardization rcs thus producing trw and mxd chronologies with preserved low frequency variability the chronologies are calibrated using local and regional instrumental climate records a bootstrapped response function analysis using regional climate data shows that tree growth is forced by april august temperatures and that the regression weights for mxd are much stronger than for trw the robustness of the reconstruction equation is verified by independent temperature data and shows that 63 64 of the instrumental inter annual variation is captured by the tree ring data this is a significant improvement compared to previously published reconstructions based on tree ring data from tornetrask a divergence phenomenon around ad 1800 expressed as an increase in trw that is not paralleled by temperature and mxd is most likely an effect of major changes in the density of the pine population at this northern tree line site the bias introduced by this trw phenomenon is assessed by producing a summer temperature reconstruction based on mxd exclusively the new data show generally higher temperature estimates than previous reconstructions based on tornetrask tree ring data the late twentieth century however is not exceptionally warm in the new record on decadal to centennial timescales periods around ad 750 1000 1400 and 1750 were equally warm or warmer the 200 year long warm period centered on ad 1000 was significantly warmer than the late twentieth century p 0 05 and is supported by other local and regional paleoclimate data the new tree ring evidence from tornetrask suggests that this medieval warm period in northern fennoscandia was much warmer than previously recognized

fennoscandian summers from ad 500 temperature changes on short and long timescales quantitative estimates of 1480 years of summer temperatures in northern fennoscandia have previously been derived from continuous treering records from northern sweden here we show the results of spectral analyses of these data only a few peaks in the spectra are consistently significant when the data are analyzed over a number of sub periods relatively timestable peaks are apparent at periods of 2 1 2 5 3 1 3 6 4 8 32 33 and for a range between 55 100 years these results offer no strong evidence for solar related forcing of summer temperatures in these regions our previously published reconstruction was limited in its ability to represent long timescale temperature change because of the method used to standardize the original tree ring data here we employ an alternative standardization technique which enables us to capture temperature change on longer timescales considerable variance is now reconstructed on timescales of several centuries in comparison with modern normals 1951 70 generally extended periods when cool conditions prevailed prior to the start of the instrumental record include 500 700 790 870 1110 1150 1190 1360 1570 1750 a d with the most significant cold troughs centred on about 660 800 1140 1580 1620 and 1640 predominantly warm conditions occurred in 720 790 870 1110 and 1360 1570 with peaks of warmth around 750 930 990 1060 1090 1160 1410 1430 1760 and 1820

behaviour and dynamics of a hydrothermal plume in lake banyoles catalonia ne spain a hydrothermal plume forms in lake banyoles ne spain as a result of convection above a springwater fed suspension cloud ponded on the lake floor the plume propagates upwards reaching a level of neutral buoyancy from where a turbidity current spreads out laterally two dimensional temperature and particle concentration measurements show the fate of the hydrothermal plume and its associated turbidity current and reveal its seasonal development silt particles transported by the plume have been used as tracers to determine the maximum and equilibrium heights of the plume when the lake is stratified the vertical transport of sediment is confined to the lake hypolimnion as the thermocline limits the vertical propagation of the plume in contrast when the lake water column is mixed the plume reaches the surface of the lake the field measurements have been compared with models for thermal convection from finite isolated sources measurements of the flow velocity at the source of the hydrothermal plume i e the rim current velocity indicate that cold hypolimnetic water is entrained by the plume sedimentation rates measured from sediment traps at the zone where the turbidity current develops vary between 10 and 25 g m 2 day 1 and result from continuous silt particle sedimentation from the turbidity current sedimentation rates in traps are higher for stations situated close to the source than those further away 5 g m 2 day 1 moreover the results demonstrate that double diffusive sedimentation from the turbidity current was dominant over grain by grain settling causing a mixed distribution of sediments in the region where the turbidity current spreads the deposition of silt particles could explain the occurrence of silt layers interbedded with biocalcarenites in the littoral zones of the lake and the stratigraphy identified by seismic profiles and cores taken from the lake floor

reconstructing monitoring and predicting multidecadal scale changes in the north atlantic thermohaline circulation with sea surface temperature sea surface temperature sst observations in the north atlantic indicate the existence of strong multidecadal variability with a unique spatial structure it is shown by means of a new global climate model which does not employ flux adjustments that the multidecadal sst variability is closely related to variations in the north atlantic thermohaline circulation thc the close correspondence between the north atlantic sst and thc variabilities allows in conjunction with the dynamical inertia of the thc for the prediction of the slowly varying component of the north atlantic climate system it is shown additionally that past variations of the north atlantic thc can be reconstructed from a simple north atlantic sst index and that future anthropogenically forced changes in the thc can be easily monitored by observing ssts the latter is confirmed by another state ofthe art global climate model finally the strong multidecadal variability may mask an anthropogenic signal in the north atlantic for some decades

holocene periodicity in north atlantic climate and deep ocean flow south of iceland climate fluctuations during the past millennium are relatively well documented1 on a longer timescale there is growing evidence of millennial scale variability of holocene climate at periodicities of 2 500 and 950 years possibly caused by changes in solar flux 2 3 and 1 500 years maybe related to an internal oscillation of the climate system 4 5 6 but the involvement of deep water masses in these holocene climate changes has yet to be established here we use sediment grain size data from the iceland basin to reconstruct past changes in the speed of deep water flow the study site is under the influence of iceland scotland overflow water isow the flow of which is an important component of the thermohaline circulation that modulates european climate flow changes coincide with some known climate events the little ice age and the mediaeval warm period and extend over the entire holocene epoch with aquasi periodicity of 1 500 years the grain size data indicate afaster isow flow when the climate of northern europe is warmer however a second mode of operation is observed in the early holocene when warm climate intervals are associated with slower isow flow at that time the melting remnant of land based glacial age ice may have provided a sufficient source of fresh water to the ocean to reduce isow flow south of iceland

focusing of phytodetritus deposition beneath a deep ocean front chatham rise new zealand in october 2001 we observed a deep ocean phytodetritus deposition event on chatham rise beneath the subtropical front stf the origin of this phytodetritus was probably an extensive phytoplankton bloom that occurred in the stf in the preceding weeks we assessed the spatial distribution of the deposition event using video images from benthic lander and epibenthic trawl deployments and sediment pigment analyses at six sites on a north south transect across chatham rise high surficial sediment chlorophyll a concentrations were restricted spatially to the southern flank of chatham rise 350 1 200 m depth with the highest values centered at 750 m water depth 750 s this southern 750 s site was also the only site where macroscopic phytodetritus was observed coincident with elevated benthic biomass and sediment community respiration rates at 750 s phytodetritus resuspension was observed on video and corroborated by current meter sediment trap and optical

effects of a turbid hydrothermal plume on the sedimentation rates in a karstic lake 1 the formation of a hydrothermal plume has been recently found in the main basin of a karstic lake the hydrothermal plume carries a concentration of particles in suspension that are transported up to an equilibrium depth at the equilibrium level particles are transported horizontally as a turbidity current the particle volume concentration in the hypolimnion where the turbidity current occurs has been found to be homogeneous for points close to the source of the plume however there is a decrease in the particle concentration points from close to the source of the plume to the points far from the source which indicates that the sedimentation of particles from the turbidity current occurs this sedimentation process has been attributed mainly to double diffusive sedimentation rather than to single particle sedimentation here we demonstrate the effect of this turbidity current on the zonal sedimentation rates at the southern lobe of the lake

holocene humidity changes in northern finnish lapland inferred from lake sediments and submerged scots pines dated by tree rings conclusive evidence for a rise in water levels has been found in connection with lake sediment studies undertaken partly in collaboration with the pine megafossil sampling and dendrochronological work in northern finnish lapland the change in lake level stands is shown by slow sedimentation rate in the early to mid holocene and an increase thereafter these data indicate a regional rise in water levels during the latter part of the holocene following a relatively dry period between 8000 and 4000 bp synchronous changes also indicating rising water levels have been observed in the diatom and cladoceran assemblages of the sediment cores subfossil scots pines pinus sylvestris l have been preserved in large quantities in small lakes in lapland because in many cases the rising water level has inundated the trunks after death the position of the subfossil trunks and stumps often indicates that the pines have been growing on dry land at the sides of the lakes in which they are now submerged traces of

dilution of the northern north atlantic ocean in recent decades declining salinities signify that large amounts of fresh water have been added to the northern north atlantic ocean since the mid 1960s we estimate that the nordic seas and subpolar basins were diluted by an extra 19 000 5000 cubic kilometers of freshwater input between 1965 and 1995 fully half of that additional fresh water about 10 000 cubic kilometers infiltrated the system in the late 1960s at an approximate rate of 2000 cubic kilometers per year patterns of freshwater accumulation observed in the nordic seas suggest a century time scale to reach freshening thresholds critical to that portion of the atlantic meridional overturning circulation

sediment retention by a mediterranean posidonia oceanica meadow the balance between deposition and resuspension the role of posidonia oceanica in promoting sediment stability and accretion was studied in a 15 m deep meadow at fanals point nw mediterranean spain by comparing particle deposition within the meadow and adjacent bare sediment small sediment traps were used to measure deposition within and above the meadow and over bare sand a model based on measurements of particle deposition at increasing distances from the bottom was used to partition the total depositional flux between primary sediment particles deposited for the first time at the measuring site and resuspended deposition sediment particles that have been previously deposited at the measuring site measurements were conducted monthly over a year to establish the magnitude and seasonality of deposition and to form a balance of particle transport at the annual time scale significant differences in total deposition were found over time ranging from 1 5 to 500 g dw m 2 d 1 including those between bare and vegetated sediments the effect of p oceanica in increasing primary deposition at an annual scale was modest however p oceanica significantly buffered sediment resuspension which was reduced more than three fold compared to the unvegetated bottom the annual flux of deposition was dominated by settling of resuspended materials which represented 85 of the total flux within the meadow but 95 of the total deposition on bare sand thus seagrass meadows reduce resuspension in the nw mediterranean littoral thereby contributing to increased sediment retention and therefore reducing erosion in the coastal zone

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

resuspension of sediment by focused groundwater in lake banyoles resuspension of sediment by subterranean springs occurs in karstic multibasin lake banyoles spain the sediment at the bottom of one of the basins was consolidating when intrusion of groundwater caused resuspension here the lutocline rose nearly 20 m over 10 months during this episode temperature profiles sediment size distributions bulk density and settling rates showed that the jet generated turbulence produced strong mixture in the central core of the conic depression forming the basin data from another basin where the sediment was always in suspension are presented for comparison here the stratification in density and settling rate of the particles suggest sorting out of the aggregates with the largest flocs occupying the deepest parts of the basin

decadal variability of sea surface temperatures off north iceland over the last 2000 years abstract ocean variability at decadal time scales remains poorly described partly because of the scarcity of high temporal resolution marine records here we present a reconstruction of sea surface temperatures ssts over the past two millennia at unprecedented temporal resolution 2 to 5 years from a marine core located off north iceland alkenone paleothermometry was used to infer sst variability and tephrochronology to build the age model spectral analyses of the sst signal indicate intermittent 20 25 year oscillations with periods of strong and weak power that are likely reflecting the ocean response to wind forcing presumably the north atlantic oscillation nao warmer ssts and paleo magnetic proxy data between 1000 and 1350 year a d overlapping the medieval warm period mwp suggest enhanced heat transport across the denmark strait by the north icelandic irminger current niic this is in contrast with the subsequent period which includes the little ice age lia showing continuous cooling towards the 20th century reduced niic flow through the denmark strait likely resulting from higher freshwater and sea ice export from the arctic would account for the observed colder conditions

limnogeology ancient and modern tales of an evolving earth between the late seventies and the present day sedimentology and basin research of lakes in the geologic record expanded from a marginal academic curiosity into a spectacular groundbreaking area of multidisciplinary research few earth science disciplines have experienced such greatprogressin methodsandprinciplesin thelast three decadesasthe field of limnogeology economic interest in ancient lacustrine basins initially drove research recognizing lake environments as suppliers of natural resourcesranging from evaporiticsalts andbrines e g lithium raw materials i e limestones tufas clays and diatomites and fossil energy oil gas and coals with this focus the collection of highresolution high quality depositional records of ancient and recent lacustrine systems opened an epoch of continuous expansion of research and improved understanding of these records consequently recognition of the importance of lacustrine deposits as archives of past global changes on continents which are complementary to marine records has been growing research on lacustrine systems in a wide arrayoftectonicandclimaticsettingshasbeenchallengedbythevariety

the influence of subaquatic springs in lacustrine sedimentation origin and paleoenvironmental significance of homogenites in karstic lake banyoles ne spain abstract banyoles 42 08 n 2 45 e is the largest and deepest lake of karstic tectonic origin in the iberian peninsula the lake comprises two basins and six sub circularly shaped sub basins fed by subaquatic springs periods of intense groundwater inflow in the deepest sub basins lead to the fluidization and re suspension of previously deposited sediments and subsequent settling forming homogenite deposits on the southern basin intermediate platforms the multiproxy analysis of sediment cores combined with high resolution seismic stratigraphy 3 5 khz pinger and multi frequency chirp surveys allows a precise reconstruction of depositional environments and related hydrological variability and groundwater inflow during the last ca 7 6 cal kyr bp according to the age model based on 137cs 210pb and ams 14c dating homogenite deposition occurred between 7 2 and 5 5 cal kyr bp stopped during the middle holocene 5 5 2 8 cal kyr bp and greatly increased during the last two millennia with a total of 17 homogenite layers individually up to 75 cm thick the onset of this unique sedimentation mode at ca 3 cal kyr bp coincides with an increase in lake level evidenced by the onlapping of fine grained distal sediments over coarser massive carbonate rich littoral deposits a detailed multidisciplinary study of the homogenites sedimentology physical properties high resolution elemental geochemistry mineral composition grain size organic matter content and sem combined with seismic stratigraphy demonstrates that the fluidization events triggering the formation of the homogenites were caused by higher and more intense local groundwater inflow related to increased rainfall during the late holocene and likely intensified by land use changes during the last millennium

on the long term context for late twentieth century warming 1 previous tree ring based northern hemisphere temperature reconstructions portray a varying amplitude range between the medieval warm period mwp little ice age lia and present we describe a new reconstruction developed using largely different methodologies and additional new data compared to previous efforts unlike earlier studies we quantify differences between more traditional std and regional curve standardization rcs methodologies concluding that rcs is superior for retention of low frequency trends continental north american versus eurasian rcs series developed prior to merging to the hemispheric scale cohere surprisingly well suggesting common forcing although there are notable deviations e g fifteenth to sixteenth century results indicate clear mwp warm lia cool and recent warm episodes direct interpretation of the rcs reconstruction suggests that mwp temperatures were nearly 0 7c cooler than in the late twentieth century with an amplitude difference of 1 14c from the coldest 1600 1609 to warmest 1937 1946 decades however we advise caution with this analysis although we conclude as found elsewhere that recent warming has been substantial relative to natural fluctuations of the past millennium we also note that owing to the spatially heterogeneous nature of the mwp and its different timing within different regions present palaeoclimatic methodologies will likely flatten out estimates for this period relative to twentieth century warming which expresses a more homogenous global fingerprint therefore we stress that presently available paleoclimatic reconstructions are inadequate for making specific inferences at hemispheric scales about mwp warmth relative to the present anthropogenic period and that such comparisons can only still be made at the local regional scale

oxygen isotope evidence of little ice age aridity on the caribbean slope of the cordillera central dominican republic climate change during the so called little ice age lia of the 15th to 19th centuries was once thought to be limited to the high northern latitudes but increasing evidence reflects significant climate change in the tropics one of the hypothesized features of lia climate in the low latitudes is a more southerly mean annual position of the intertropical convergence zone itcz which produced more arid conditions through much of the northern tropics high resolution stable oxygen isotope data and other sedimentary evidence from laguna de felipe located on the caribbean slope of the cordillera central of the dominican republic support the hypothesis that the mean annual position of the itcz was displaced significantly southward during much of the lia placed within the context of regional paleoclimate and paleoceanographic records and reconstructions of global lia climate this shift in mean annual itcz position appears to have been induced by lower solar insolation and internal dynamical responses of the global climate system our results from hispaniola further emphasize the global nature of lia climate change and the sensitivity of circum caribbean climate conditions to what are hypothesized to be relatively small variations in global energy budgets

coherent high and low latitude climate variability during the holocene warm period a faunal record of sea surface temperature sst variations off west africa documents a series of abrupt millennial scale cooling events which punctuated the holocene warm period these events evidently resulted from increased southward advection of cooler temperate or subpolar waters to this subtropical location or from enhanced regional upwelling the most recent of these events was the little ice age which occurred between 1300 to 1850 a d when subtropical ssts were reduced by 3 to 4 c these events were synchronous with holocene changes in subpolar north atlantic ssts documenting a strong in phase link between millennial scale variations in high and low latitude climate during the holocene

observations of vertical currents and convection in the central greenland sea during the winter of 1988 1989 during the winter of 1988 1989 five acoustic doppler current profilers adcps were moored in the central greenland sea to measure vertical currents that might occur in conjunction with deep mixing and convection two adcps were looking up from about 300 m and combined with thermistor strings in the depth range 60 260 m two were looking downward from 200 m and one was looking upward from 1400 m first maxima of vertical velocity variance occurred at two events of strong cold winds in october and november when cooling and turbulence in the shallow mixed layer generated internal waves in the thermocline beginning in late november the marginal ice zone expanded eastward over the central greenland sea reaching its maximum extent in late december in mid january a bay of ice free water opened over the central greenland sea leaving a wedge of ice the is odden curled around it along the axis of the jan mayen current and then northeastward and existing well into april 1989 below the ice a mixed layer at freezing temperatures developed that increased in thickness from 60 to 120 m during the period of ice cover corresponding to an average heat loss of about 40 w m 2 through brine rejection mixed layer salinity increased steadily reducing stability to underlying weakly stratified layers roach et al 1993 during the ice cover period vertical currents were at a minimum after the opening of the ice free bay successive mixed layer deepening to 350 m occurred in conjunction with cooling events around february 1 and 15 accompanied by strong small scale vertical velocity variations upward mixing of more saline waters of atlantic origin during this phase reduced the stability further generating a pool of homogeneous water of 50 km horizontal extent in the central greenland sea preconditioned for subsequent convection to greater depths individual convection events were observed during march 6 16 associated with downward velocities at the 1400 m level of about 3 cm s l one event was identified as a plume of about 300 m horizontal scale in agreement with recently advanced scaling arguments and model results and with earlier similar observations in the gulf of lions western mediterranean the deep convection occurred in the center of the ice free bay hence brine rejection did not seem necessary for its generation plume temperatures at 1400 m were generally higher than that of the homogeneous surface pool suggesting entrainment of surrounding warmer waters on the way down mean vertical velocity over a period of convection events was indistinguishable from zero suggesting that plumes served as a mixing agent rather than causing mean downward transport of water masses however different from the surface pool that was governed by mixed layer physics the water between 400 and 1400 m was not horizontally homogenized in a large patch by the sporadic plumes overall and compared to results from the gulf of lions convection activity in the central greenland sea was weak and limited to intermediate depths in winter 1988 1989

observations of a hydrothermal plume in a karstic lake a hydrothermal plume with vertical and horizontal length scales of 18 and 300 m respectively develops in a karstic lake the plume is generated at the bottom of a basin that contains sediment in suspension which is at a higher temperature than the water immediately above the hypolimnion of the lake the rising convective plume entrains colder hypolimnetic water and develops upward until it reaches the base of the seasonal thermocline carrying an important amount of sediment particles from the bottom which are used as tracers to describe the spatial distribution of the plume at the level of neutral buoyancy the plume spreads laterally as a horizontal baroclinic intrusion

a 4500 year reconstruction of sea surface temperature variability at decadal time scales off north iceland marine paleo records acquired at high temporal resolution provide critical data for testing numerical climate models and help to understand processes underlying ocean variability this study presents a unique 4500 year reconstruction of sea surface temperature sst obtained from alkenones in the north atlantic polar front area off north iceland at an average temporal resolution of 4 5 years spectral analysis of this signal shows dominant multidecadal oscillations which occurred with a stronger amplitude between 2500 and 4200 years bp hand in hand with fluctuations of bottom currents indicated by paleomagnetic proxies contemporaneous large excursions of the inter tropical convergence zone itcz are also recorded by the distant cariaco titanium time series suggesting a link with low latitude atlantic climate we speculate that the oscillations reflect changes of the meridional overturning circulation moc induced by increased enso activity

dynamics of isolated convective regions in the ocean abstract an initially resting ocean of stratification n is considered subject to buoyancy loss at its surface of magnitude b0 over a circular region of radius r at a latitude where the coriolis parameter is f initially the buoyancy loss gives rise to upright convection as an ensemble of plumes penetrates the stratified ocean creating a vertically mixed layer however as deepening proceeds horizontal density gradients at the edge of the forcing region support a geostrophic rim current which develops growing meanders through baroclinic instability eventually finite amplitude baroclinic eddies sweep stratified water into the convective region at the surface and transport convected water outward and away below setting up a steady state in which lateral buoyancy flux offsets buoyancy loss at the surface in this final state quasi horizontal baroclinic eddy transfer dominates upright plume convection by using parcel theory to consider the energy transformations taking place it is shown that the de

rate of sedimentation in lake kinneret israel spatial and temporal variations the rate of sedimentation in lake kinneret was measured over several years by means of sediment traps in up to seven different locations in the lake gross sedimentation rates measured in the sediment traps vary from about 1 5 kg m 2 a 1 in the deepest part of the lake up to 10 kg m 2 a 1 near the mouth of the upper jordan river the rate of sedimentation near the jordan s inflow is highly correlated to flow discharge in the river while in the centre of the lake the seasonal sedimentation pattern is mainly correlated to the bloom period of peridinium gatunense during the bloom period of peridinium gatunense sedimentation rates all over the lake are very similar indicating that the peridinium is evenly distributed in the lake the average suspended sediment discharge of the upper jordan river flowing into the lake is 41 000 ton a 1 copyright 2000 john wiley sons ltd

on the presence of aggregates in the basins of lake banyoles suspension of sediment by subterranean springs is a very common phenomena in the karstic multibasin lake banyoles the flow of groundwater into the lake introduces a suspension of marly and argilaceous materials the temperature of this slurry is nearly constant throughout the year around 19 c the lake is formed by 6 basins and at the bottom of these basins there are 12 sources where the presence of suspended material signifies entering groundwater figure 1 suspension of bottom lake sediment by groundwater is a feature of lake banyoles that has not been described elsewhere here we present experimental results which show that the maximum height that the sediment can rise to is affected by the formation of aggregates

mid and late holocene tree population density changes in northern fennoscandia derived by a new method using megafossil pines and their tree ring series changes in tree density are estimated for the northern forest limit region of finnish lapland over the past seven and a half millennia this is done using dendrochronologically dated scots pine megafossils and their tree ring series direct and indirect estimates of past tree density are derived from chronology sample size css and growth trend modelling gtm respectively the latter is a new method where the past levels of growth competition the influence of nearby trees are extracted from the behaviour of growth trends in cross dated tree ring series trends that are expected to be driven by tree density control two records constructed from the same original data set but by different means independently correlate significantly both records show a tree density maximum around 3000 1750 bc indicating relative warmth during that time another positive tree density anomaly occurred in accordance with the medieval warm period preceding the thinning at the forest limit due to the little ice age copyright 2005 john wiley sons ltd

sediment fluidization events in a lake caused by large monthly rainfalls 1 in the last fourteen years 1986 1999 fluidizations of confined bed sediments due to underground springs in basin bii of lake banyoles spain have been detected as a result of periods of high precipitation sediment confined at the bottom of karstic basins migrates from 45 m depth to approximately 25 m depth fluidization processes are associated to monthly rainfall 1 5 to 4 5 times larger than the long term mean 1970 1999 as a result of a fluidization event the water column of the lake maintains the particle volume concentrations of suspended sediment pvc as high as 44 l l which is 8 29 times larger than the usual values of pvc