annual biogenic particle fluxes to the interior of the north atlantic ocean studied at 34 n 21 w and 48 n 21 w abstract in order to clarify the annual quality quantity and export processes of biogenic particles from the euphotic zone to the deep ocean interior an array of automated time series sediment traps were deployed for 1 year from 4 april 1989 to 17 april 1990 at 34 n 21 w and 48 n 21 w as part of the joint global ocean flux program jgofs north atlantic bloom experiment nabe three sediment traps with 13 time series sediment collectors were placed at both stations approximately 1 and 2 km below the surface and 0 7 km above the bottom they collected settling particles during 26 14 day intervals for 376 days with an 20 day hiatus in september october 1989 for changeover of the trap moorings the collection periods of the six traps were synchronized forming a spatio temporal matrix of 156 samples the annual mass flux at about 2 km deep during this experiment was 22 and 27 g m 2 y 1 at the 34 and 48 n stations consisting of biogenic particles with traceable quantities of lithogenic particle flux the spring particle bloom characterized by the sedimentation of particles relatively enriched by norg began in january at the 34 n station and in march at the 48 n station the bloom continued for 4 5 and 3 months and provided 62 and 50 of the annual biogenic particle mass flux at 2 km at the 34 and 48 n stations the surface bloom penetrated to the ocean interior within a few weeks with apparently accelerated settling speed at deeper layers the order of susceptibility of biogenic elements to mineralization while settling in the 0 7 1 km a b water column was from least to most resistant p norg corg si and ca the c n p ratio at 0 7 km a b was 154 18 1 at the 34 n station and 148 18 1 at the 48 n station

sources of sediment to the ionian sea and western levantine basin of the eastern mediterranean during s 1 sapropel times abstract the provenance of sediments to the ionian sea and western levantine basin during s 1 sapropel times was studied using 87 sr 86 sr isotopic ratios together with major elements measurements were made on sediment profiles from 5 box cores in the eastern mediterranean on saharan dust and on a core from the nile delta which included sediment deposited during s 1 sapropel times in the sediment profiles there was a decrease in 87 sr 86 sr ratio 1 sr ti al mg al and si al ratio starting at the upper mn peak and finishing at the base of the organic rich layer this showed that there was a major change in the nature of the detrital input during s 1 sapropel times the timing of the change in detrital input was similar to the change in surface salinity as measured by 18 o there was linear relationship between 87 sr 86 sr and 1 sr and for 87 sr 86 sr and ti al for all the sediment samples the favoured interpretation of this pattern was that the detrital input was a mixture of saharan dust similar to that being deposited today on corsica and a local riverine sediment source the data were incompatible with the riverine source to the ionian sea and western levantine basin being from the river nile the minor variations in sr isotopes and metal al ratios in the nile delta core was interpreted as due to changes in the proportion of blue nile to white nile inputs brought about by climatic variations it was calculated that the change in detrital input during s 1 time was caused by a simultaneous increase in riverine input and decrease in saharan dust input

the initial response of a stratified lake to a surface shear stress laboratory experiments are used to study the initial response of a stratified fluid to the action of a wind stress the experiments are described in the context of a parameterization scheme that quantifies the strength of the applied stress relative to the bulk stability of the fluid and also the duration of the wind stress relative to the periods of the waves generated by the stress this study concentrates on the first fundamental internal wave period in experiments where the fluid is considered to have upwelled i e the stratified region of the fluid reaches the surface at the upwind endwall the majority of the experiments use three layer initial density profiles as an approximation to a continuously stratified water column a linear model using normal modes proved successful prior to the commencement of upwelling and this enabled an estimate to be made of the time at which upwelling occurred at this point the wave development ceased and the flows developed via entrainment mechanisms consideration of the energy budget showed that little of the input energy was stored in the system the initial mixing efficiency defined as the ratio of the mean potential energy gained to the energy imparted by the belt never exceeded 30 peak efficiency occurred when the surface stress was just sufficient to bring the interfacial region to the surface

trace metal signatures of eastern mediterranean sapropels abstract sapropel samples of pliocene to pleistocene age recovered during odp leg 160 from sites 964 ionian basin 969 mediterranean ridge 966 967 top and northern slope of the eratosthenes seamount south of cyprus and 968 cyprus margin were analysed for major and trace elements eastern mediterranean sapropels are characterized by high organic carbon and sulphur contents and by significant enrichments in several redox sensitive and or sulphide forming trace metals bi cd co cr cu mo ni re sb tl u v enrichment factors relative to average shale are comparable to those found in ctbe black shales the re content is extremely high in the sapropels studied with maximum values up to 1000 ng g re mo ratios averaging 2 7 10 3 are close to the seawater value high trace metal enrichments and re mo ratios close to the seawater value point to an anoxic water column during sapropel formation additionally enhanced ba concentrations in the sapropels support the assumption that bioproductivity was also high during these episodes trace metal signatures of sapropels originating from different drill sites on a transect across the eastern mediterranean are not only influenced by water depth but also by their location in the eastern mediterranean metal enrichments seem to be higher at the western sites compared with the eastern sites this probably reflects a change in the circulation pattern during periods of sapropel formation pliocene sapropels exhibit elevated trace metal contents in comparison with pleistocene examples therefore different palaeoceanographic conditions may have developed during the pleistocene compared with the pliocene altering palaeoceanography was probably controlled by climatic change which may be related to the onset of glaciation of the northern hemisphere decreasing summer precipitation throughout the late pliocene and the pleistocene may also have influenced sapropel deposition

stable isotope anomalies in mediterranean pleistocene records oxygen and carbon isotope records of twelve mediterranean cores raised from west alboran sea to east off israel and of four atlantic cores raised from the western side of gibraltar and located respectively inside below and beyond the modern mediterranean outflow are compared to open ocean records of core 71 139c from the north atlantic and core v19 30 from the east pacific oxygen isotope stratigraphies can be established for deep mediterranean records and the major events of the last 150 ka are easily identifiable for instance the five substages of stage 5 or the 18 ka event stage 2 are identified but the 10 ka event younger dryas is only recorded by globigerina bulloides and benthic foraminifera suggesting that the cooling affected only winter sst 14c dates for the two terminations ia and ib of the last deglaciation which are recorded in sediment cores from areas with high sedimentation rates faro ridge alboran sea sicilian strait are in good agreement with the dates published for the same termination in atlantic cores an intermediate step however has been identified in some cores at 14 ka and divides termination ia into two phases oxygen isotope anomalies are recorded by the shallow water species globigerinoides ruber they correspond to 1 a higher amplitude glacial interglacial change in 18o values which exceeds that of the open ocean by 1 in the western basin and by 1 7 in the eastern basin 2 large isotopic depletions during the deposition of sapropel s1 s3 and s5 they are interpreted as the result of fresh water run off responsible for a strong dilution in the shallower layers of the east mediterranean waters the isotopic effect of this dilution averages 0 75 for sapropel s1 and 1 6 for sapropel s5 during the last glacial maximum the mediterranean still acted as a concentration basin and a gradient of 18o values similar to the modern one 0 75 existed from west to east in the deep waters apparently deep water temperatures were lower during the last glacial but the decrease did not exceed 4 c in the eastern basin major anomalies are also contained in the 13c records of g bulloides and benthic foraminifera the 13c records from surface intermediate and deep waters of the eastern basin and from intermediate and deep waters of the western basin are opposite to that of the open ocean during glacial time 13c values were higher in intermediate and deep waters of the whole mediterranean and also in surface water of the eastern basin at the same time the 13c gradient of surface to deep waters was low this gradient increased during the last deglaciation while 13c values of g bulloides and benthic foraminifera decreased this decrease interrupted at 10 ka may have been related to increased fresh water run off from the bordering contintents due to increased precipitation especially in the tropical belt the 13c depletion is clearly recorded in the levantine and deep water masses and can be traced westward in the mediterranean outflow oxygen and carbon isotope anomalies recorded simultaneously in the east mediterranean by the shallow water species globigerinoides ruber during sapropel deposition indicate that increased precipitation on the bordering continents and strong fresh water run off during the time of the last deglaciation may have caused a density stratification and deep stagnation in the eastern basin uncertainties arise from the fact that in the mixing model deglacial mediterranean waters are mixed with continental fresh waters the isotopic characteristics of which are not entirely understood although increased surface water production of biogenic material can not be excluded as a possible cause of 13c depletion it appears that the river discharge including the nile as well as black sea input are also plausible sources for the fresh water layering

numerical simulation of shoaling broad crested internal solitary waves abstractthe interaction between fully nonlinear miyata choi camassa mcc internal solitary waves and topographic slopes are modeled by direct numerical simulation the immersed boundary method is

an experimental study of the upwelling response of stratified reservoirs to surface shear stress an experimental study of the dynamic response of non rotating stratified reservoirs to the application of surface shear stresses is presented the experiments were made using two layered and linear stratifications a moving belt was used to supply a shear stress to the fluid detailed measurements of the density field show that upwelling occurs at all values of the wedderburn number w rather than only occurring when w 1 differences between the two layer theory and the present observations are attributed to the fact that the experimental stratifications were continuous rather than layered shearflow dispersion is observed to be an important mechanism for distributing the effects of localized upwelling over the entire length of the mixed layer a model of mixed layer deepening based on upwelling and shearflow dispersion is presented and is compared to the observations of this and other experimental studies

the sapropel record of the eastern mediterranean sea results of ocean drilling program leg 160 research on sediments recovered during ocean drilling leg 160 has concentrated on two issues the first concerned the stratigraphy of sapropel formation the second was oriented to clarify specific processes that explain sapropel origin progress has been made in the construction of stratigraphic composites out of sedimentary sequences from individual holes at each of the palaeoceanographic sites on the composites initial work has resulted in the establishment of high resolution and intermediate resolution stratigraphies for three sites 963 964 967 correlation of sedimentary cycles to astronomical insolation cycles extends the stratigraphies to sites 969 and 966 the sapropel occurrences in the marine and land sequences over the entire eastern mediterranean are correlated with the resolution that can be obtained from isotope studies groups of sapropels occurred simultaneously over the entire basin in detail however the temporal and facies patterns of sapropel sequences differ between individual sites and depositional basins the differences may be related to effects of water depth diagenesis and post depositional tectonic attenuation of sequences studies on the geochemistry and facies of sapropels agree that anoxic conditions favoured preservation of organic matter in sapropels caused the enrichment of trace metals associated with sapropels and helped to preserve primary sedimentary structures besides all evidence is consistent with elevated fluxes of organic matter and associated elements during sapropel events

geochemical and paleomagnetic evidence for the occurrence of missing sapropels in eastern mediterranean sediments the cyclic occurrence of sapropels appears to be associated with maxima in the 65 n summer insolation target curve studies of the most recent sapropel have revealed extensive oxidation of organic matter or even the complete removal of this unit geochemical and magnetic signals allow for the detection of such a missing sapropel in older sediments these signals are altered by ongoing diagenetic processes an important parameter to reveal missing sapropels is ba elevated concentrations of ba are always found in visible sapropels and even after oxidation of part of the sapropel the initial ba profile remains visible we use this tool to trace down missing sapropels in kc01b a 37 m kullenberg core from the calabrian ridge eastern mediterranean at least 8 of 11 sampled intervals around insolation maxima contain evidence of a completely oxidized sapropel ba trace metal distributions and magnetic parameters are similar to those observed around visible sapropels

geochemical evidence for enhanced productivity during s1 sapropel deposition in the eastern mediterranean the geochemistry of the youngest mediterranean sapropel layer suggests changes in productivity and water column oxygen conditions during sapropel deposition the ba enriched interval is broader than the organic carbon rich interval of this sapropel we suggest that the ba enriched horizon records the original thickness of the sapropel prior to subsequent partial oxidation the main carrier of ba is barite as microcrystals 0 5 5 m having a morphology characteristic of marine barite particularly abundant beneath high productivity regions ba concentrations do not change at the sapropel layer oxidation front and diagenetic barite crystals are absent thus the ba enriched layer reflects original oceanic conditions of increased biological productivity during sapropel deposition and not diagenetic ba remobilization paleoredox indicators point to restricted oxygenated bottom water but not to fully anoxic conditions detrital elements within this layer indicate a lower eolian terrigenous input enhanced humidity and increased precipitation runoff thus likely higher nutrient supply supporting table is available on diskette or via anonymous ftp from kosmos agu org directory apend username anonymous password guest diskette may be ordered by mail from agu 2000 florida ave n w washington dc 20009 or by phone at 800 966 2481 15 00 payment must accompany order

sensitivity studies of northern hemisphere glaciation using an atmospheric general circulation model abstract the u k university global atmospheric modeling programme gcm is used to investigate whether the growth of northern hemisphere ice sheets could have been initiated by changes of orbital parameters and sea surface temperature two different orbital configurations corresponding to the present day and 115 kyr bp are used the reduced summer solar insulation in the northern hemisphere results in a decrease of the surface temperature by 4 to 10 c in the northern continents and to perennial snow in some high latitude regions therefore the model results support the hypothesis that a deficit of summer insulation can create conditions favorable for initiation of ice sheet growth in the northern hemisphere a decreased sea surface temperature northward of 65 n during the northern hemisphere summer may contribute to the maintenance of ice sheets a simple mixed layer ocean model coupled to the gcm indicates that the changes of sea surface temperature and extension of sea ice due to insulation changes pl

modes of sapropel formation in the eastern mediterranean some constraints based on pyrite properties abstract pyrite formation within and directly below sapropels in the eastern mediterranean was governed by the relative rates of sulphide production and fe liberation and supply to the organic rich layers at times of relatively high so2 4 reduction sulphide could diffuse downward from the sapropel and formed pyrite in underlying sediments the sources of fe for pyrite formation comprised detrital fe and diagenetically liberated fe ii from sapropel underlying sediments in organic rich sapropels input of fe from the water column via fe sulphide formation in the water may have been important as well rapid pyrite formation at high saturation levels resulted in the formation of framboidal pyrite within the sapropels whereas below the sapropels slow euhedral pyrite formation at low saturation levels occurred 34s values of pyrite are 33 to 50 below the sapropels 34s is lower than within the sapropels as a result of increased sulphide re oxidation at times of relatively high sulphide production and concentration when sulphide could escape from the sediment the percentage of initially formed sulphide that was re oxidized was estimated from organic carbon fluxes and burial efficiencies in the sediment it ranges from 34 to 80 varying significantly between sapropels increased palaeoproductivity as well as enhanced preservation contributed to magnified accumulation of organic matter in sapropels

impact of the circulation on sapropel formation in the eastern mediterranean the role of the thermohaline circulation in controlling export production oxygenation of deep waters and hence possible sapropel formation in the eastern mediterranean is examined using a simple nutrient cycling model the model is driven by velocity fields from a general circulation model and receives fluxes of nutrient from river run off and atmospheric deposition the model is used to study three scenarios a strong anti estuarine circulation a weakened anti estuarine circulation and a weak estuarine circulation nutrient transports ventilation of oxygen and deposition of organic matter are investigated in each case with a present day circulation the model provides reasonable agreement with observed phosphate and oxygen profiles and for export production with the weakened anti estuarine circulation consistent with surface salinity reconstructions for the most recent sapropel s1 there is a modest increase in export production and reduced ventilation leading to anoxia in intermediate and deep waters sapropel formation is possible near the coastal margins particularly if there is enhanced river run off with an estuarine circulation there is significant increase in export production in addition to anoxia below a shallow winter mixed layer while both the latter circulations allow sapropel formation in the model the estuarine case is distinguished by higher organic carbon deposition and anoxia in the near surface waters

on the reflectance of uniform slopes for normally incident interfacial solitary waves abstract the collision of interfacial solitary waves with sloping boundaries may provide an important energy source for mixing in coastal waters collision energetics have been studied in the laboratory for the idealized case of normal incidence upon uniform slopes before these results can be recast into an ocean parameterization contradictory laboratory findings must be addressed as must the possibility of a bias owing to laboratory sidewall effects as a first step the authors have revisited the laboratory results in the context of numerical simulations performed with a nonhydrostatic laterally averaged model it is shown that the simulations and the laboratory measurements match closely but only for simulations that incorporate sidewall friction more laboratory measurements are called for but in the meantime the numerical simulations done without sidewall friction suggest a tentative parameterization of the reflectance of interfacial solitary waves upon impact with uniform slopes

evidence for the evolution of an oxygen minimum layer at the beginning of s 1 sapropel deposition in the eastern mediterranean the sediments of the eastern mediterranean basin contain a series of organic rich sapropels intercalated with organic poor nannofossil oozes until recently the timing of the onset of sapropel formation was not known accurately because of the low resolution achievable by conventional radiocarbon dating compilation of all available 14c ams dates show that the base of s 1 the most recent sapropel was initiated 8800 years b p 14c age corrected by 400 years for reservoir effect under a 500 m water column and moved progressively into deeper water reaching depths of 3500 m at 8200 years b p the linear correlation between the age of s 1 onset and water depth r 0 71 suggests that formation of sapropels moved into deeper water at a rate of 1000 m 200 year a model is suggested in which export production which sank below the well mixed surface layers 500 m was respired consuming dissolved oxygen in the levantine deep water until a threshold value was reached when sapropels began to be preserved in the sediment this resulted in a progressively deepening oxygen minimum zone with time until eventually the entire deep water in the basin was oxygen depleted assuming that the threshold value for sapropel formation was complete anoxia it was calculated that primary productivity in the basin during the deposition of s 1 was a factor of 5 greater than that found at present

simulations of ecosystem response during the sapropel s1 deposition event abstract a one dimensional ecosystem numerical model is used to simulate the ecosystem changes that could have occurred in the open ocean areas of the eastern mediterranean sea during the climatic optimum interval 9500 6000 b p mercone et al mercone d thomson j croudace i w siani g paterne m troelstra s 2000 duration of s1 the most recent sapropel in the eastern mediterranean sea as indicated by accelerator mass spectrometry radiocarbon and geochemical evidence paleoceanography 15 336 347 in this period the s1 sapropel was deposited s1 is the most recent sapropel in the succession of organic carbon rich layers intercalated in normal neogene sedimentary sequences different theories have been invoked in order to explain the deposition of this peculiar layer our simulations seem to indicate that the modified thermohaline circulation supplying oxygen only in the first 500 m of the water column is responsible for the sapropel deposition when higher productivity is allowed in the euphotic zone the model shows the importance in this process of bacteria that consume oxygen by decomposing the particulate organic matter pom produced in the upper water column the sinking velocity of pom partially regulates the timescale of the occurrence of anoxia at the bottom and in the whole water column allowing the relatively rapid onset of sapropel deposition

internal hydraulic jumps in a long narrow lake high resolution three dimensional numerical modeling and field observations were used to describe the nonlinear response of cayuga lake to surface wind forcing the degeneration of the basin scale internal wave field was characterized according to the composite froude number g2 wedderburn number wn and lake number ln which are measures of hydraulic control and bulk and integral wind disturbance force to the baroclinic restoring force respectively the typical cayuga lake response was a nonlinear surge when 1 1 in a lake the occurrence of the hydraulic jump was correctly predicted using scaling parameters it was also shown that topographically induced internal hydraulic jumps form when the nonlinear surges interact with a sill contraction topographic feature in contrast to published literature the observed high frequency nonlinear internal waves were preferentially associated with internal jumps as opposed to steepened internal surges computed vertical diffusivities showed mixing was enhanced by two orders of magnitude within both the surges and hydraulic jumps as they propagated through the basin and interacted with topography our results can be generalized to other lakes and fjords with similar long narrow geometry and topographically separate side basins

the climatology of internal waves in a deep temperate lake kootenay lake is a long 100 km deep 140 m dimictic lake in southeastern british columbia internal waves were observed over an 18 month period using three thermistor chains moored along the major axis of the lake to characterize their seasonal aspects four types of oscillations are recognized 1 classical seiches with amplitudes of 10 30 m and periods of 1 2 weeks 2 intrabasin waves with periods of 2 4 days 3 large amplitude waves resembling nonlinear surges and 4 high frequency waves associated with both the internal surge and large amplitude internal seiche motions the periods of the major components of the internal wave field are determined mainly by morphometry but vary seasonally with stratification thus the wind which not only forces internal waves but can also modify stratification is important in large lakes like kootenay the thermal structure may vary not only spatially but also temporally at periods comparable to those of the internal waves themselves hence the shape and frequency of these waves may be altered as they pass through the lake

high frequency internal waves in lake geneva trains of high frequency internal waves have been observed in the thermocline of lake geneva within a few hours after the passing of disturbances which cause a rapid increase or jump in the depth of the thermocline three possible mechanisms for the source of the internal waves are proposed and compared with available data i that the waves are a soliton packet following the jump ii that the waves are generated as the jump passes over or around a local region of rough topography and iii that the jump produces a moving disturbance near the shoreline with internal waves forming a wake behind it the investigation of the last possibility involves the solution of the problem of the form of an internal wave wake in the presence of mean currents which vary in depth both in their strength and direction the wave pattern of the wake in the observed current distribution and stratification of the lake is described and compared with that in the absence of shear an expression for the energy of internal waves propagating in a shear flow is also derived and it is found that the rate of energy loss from the jump to the waves is small the minimum richardson number resulting from the co existence of the mean shear and the internal waves is marginally greater than 1 4 suggesting that the waves may be limited by shear flow instability however none of the mechanisms for their generation and propagation is fully compatible with the available data

significance of low symmetry fabrics in magmatic rocks abstract the significance of low symmetry fabrics monoclinic and triclinic in magmatic rocks is studied using theoretical models describing the motion of rigid markers embedded in a slowly deforming ductile matrix in agreement with curie s symmetry principle the development of these fabrics requires a non coaxial strain path simulations of monoclinic fabrics indicate that their characteristics depend on a the strain regime and the amount of strain and b the morphological composition of the population of rigid markers natural monoclinic fabrics contain precious information about the strain regime they result from type of the non coaxial strain path involved simple shear non coaxial flattening etc orientation of its axes and sense of shear the magnitude of the finite strain may eventually be estimated provided that the shape of the markers is precisely known

phytodetritus at the abyssal seafloor across 10 of latitude in the central equatorial pacific fresh phytoplankton detritus or phytodetritus has been reported from numerous deep seafloor sites in the temperate north atlantic and pacific oceans following seasonal phytoplankton blooms here we report the first strong evidence for abyssal accumulations of phytodetritus in the tropics in the central equatorial pacific in november december 1992 we obtained photographs and or sediment core samples from 61 abyssal stations water depths of 4280 5012 m between 12 s and 9 n along 140 w greenish flocculent material was recovered from the top of multiple core samples from 5 s to 5 n this material was most abundant from 2 s to 2 n in some areas forming continuous layers at least 5 mm thick and individual aggregates 1 cm in diameter the greenish material was clearly visible in bottom photographs as a green veneer that covered 95 of the seafloor near the equator and as individual cm scale aggregates covering 1 of the seafloor occasionally thick accumulations of cm scale aggregates occurred in biogenic pits cleared trails and feeding traces suggest that surface deposit feeding holothurians and echiurans grazed the greenish material microscopic examination of greenish material recovered from core tops and a burrow lumen revealed relatively intact diatoms including rhizosolenia sp and other microalgae with chloroplasts containing chlorophyll the greenish material was 1 12 5 organic carbon by weight i e 5 39 times richer than associated seafloor sediments it also contained high excess activities of 234th suggesting arrival from the water column in the previous 100 days samples of the greenish flocculent material from 0 and 5 n incubated at simulated environmental pressure and temperature with 14c labeled glutamate exhibited 5 fold higher rates of microbial activity than underlying sediments or brown floc from 9 n surface sediment samples which included the greenish flocculent material from 5 s to 5 n also contained significant concentrations of chlorophyll a and other chloropigments the chloropigment concentrations were roughly comparable to deep sea phytodetritus collected in the north atlantic we conclude that fresh organic rich phytodetritus was present on the seafloor from 5 s to 5 n along 140 w in november december 1992 with highest concentrations within 2 3 of the equator this material is likely to be a concentrated high quality food resource for deep sea microbes and metazoans we estimate an upper limit for the standing stock of this phytodetritus to be 2 6 mmol corg m2 this corresponds to 3 of the annual flux of organic carbon to the seafloor at these latitudes in 1992 because the degradation rate of this material appears to be very high its presence at the seafloor for several months per year could yield significant phytodetrital contributions to the annual seafloor organic carbon budget we also suggest that the phytodetrital aggregates are formed at intense convergence zones resulting from seasonal passage of tropical instability waves within 5 of the equator if so phytodetrital accumulations are likely to recur seasonally over broad areas of the abyssal equatorial pacific

the degeneration of large scale interfacial gravity waves in lakes mechanisms for the degeneration of large scale interfacial gravity waves are identified for lakes in which the effects of the earth s rotation can be neglected by assuming a simple two layer model and comparing the timescales over which each of these degeneration mechanisms act regimes are defined in which particular processes are expected to dominate the boundaries of these regimes are expressed in terms of two lengthscale ratios the ratio of the amplitude of the initial wave to the depth of the thermocline and the ratio of the depth of the thermocline to the overall depth of the lake comparison of the predictions of this timescale analysis with the results from both laboratory experiments and field observations confirms its applicability the results suggest that for small to medium sized lakes subject to a relatively uniform windstress an important mechanism for the degeneration of large scale internal waves is the generation of solitons by nonlinear steepening since solitons are likely to break at the sloping boundaries leading to localized turbulent mixing and enhanced dissipation the transfer of energy from an initial basin scale seiche to shorter solitons has important implications for the lake ecology

internal wave weather in a stratified lake thermistor chain data collected in lake biwa japan during a period of strong stratification were used to determine the nature of the water column response to wind forcing the evolution of the basin scale waves in the lake were reproduced in three dimensional numerical simulations and the time variability of the internal wave spectrum is discussed based on spectral analysis of the thermistor chain records severe winds associated with the passage of three typhoons during the sampling period excited basin scale kelvin and poincare waves which by interaction with the topography of the lake and non linear effects generate trains of high frequency internal waves time frequency analysis of the temperature signals suggests that the 2 slope of the internal wave spectrum is probably the result of the composition of groups of waves with high intermittence travelling through the wave guide the observed rapid decay of internal wave energy after severe wind events seemed to be due mostly to high dissipation at the sloping boundaries of the lake

organic carbon accumulation in bannock basin evaluating the role of productivity in the formation of eastern mediterranean sapropels abstract three sediment cores from bannock basin a hypersaline anoxic basin in the eastern mediterranean have been studied to evaluate the importance of productivity on organic carbon accumulation and sapropel formation in this region organic carbon accumulation rates were determined for two cores from within the anoxic hypersaline zone and for one core from the oxygenated portion of this basin organic carbon accumulation rates are higher by an order of magnitude in the surficial sediments of the anoxic zone due to better preservation an empirically derived preservation factor which allows calculation of paleoproductivity during the deposition of the most recent sapropel si indicates that productivity increased by an order of magnitude during the formation of si we conclude that increased productivity was coupled with a density stratification and a circulation reversal in the eastern mediterranean which together led to sapropel formation

a dynamic concept for eastern mediterranean circulation and oxygenation during sapropel formation we propose that intermittent bottom water ventilation occurred throughout periods of sapropel deposition and that the recently reported sapropel interruptions represent centennial scale episodes of enhanced frequency intensity of that process in essence the modern high frequency variability in deep water formation affected by climatic variability over the northern basins on seasonal to longer time scales prevailed also at times of sapropel deposition although the overall ventilation state was much reduced this concept is supported by detailed multiple species isotope records for three aegean cores the presence of abundant globorotalia truncatulinoides within especially sapropels s7 and s8 in the western levantine basin observations of three rapid benthic repopulations within sapropel s6 in the deep western levantine basin a report of continuous benthic presence through sapropel s1 at intermediate deep locations offshore libya and further supporting information from the literature in the aegean records concomitant abundance of low oxygen tolerant benthic foraminifera and presence of the more oxyphilic benthic foraminifer uvigerina mediterranea with surface similar 13c values indicate repeated deep water re oxygenation events throughout the deposition of s1 the observations of a continuous benthic presence through s1 offshore from libya imply that no persistent anoxia developed at mid depth levels in that region which is far removed from direct deep ventilation influences the abundance of deep mesopelagic g truncatulinoides through several sapropels from the western levantine basin also suggests the presence of bio available oxygen at many hundreds of meters of depth moreover the rapid intermittent benthic repopulations within sapropels from the deep eastern mediterranean imply that bottom water anoxia was spatially restricted and or of a highly intermittent nature the short time scales of these repopulation events are incompatible with titration of an extensively anoxic water column and subsequent re establishment of water column anoxia we suggest that where anoxic azoic conditions were present they most likely were restricted to a veneer at the sediment water interface the extent of such an anoxic blanket depends on the balance between advective oxygen supply into the deep sea and biological and chemical oxygen demand the demand functions imply a decoupling of oxygenation from water mass advection allowing export production and corg posting rates to the sea floor to delimit the extent of the anoxic blanket in both space and time low productivity regions would develop no anoxic blanket allowing for the observed persistence of deep dwelling planktonic and bottom dwelling benthic faunas

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

calcareous nannofossils and the origin of eastern mediterranean sapropels a quantitative analysis of calcareous nannofossil assemblages was carried out with an optical microscope on four deep sea cores raised from the eastern mediterranean and selected to form a nw se transect a total of 748 samples were studied including a very detailed analysis of sapropels s5 and s6 in two out of the four cores this paper is restricted to the discussion of the results obtained on florisphaera profunda which provided important insights on the mechanisms of sapropel formation an intricated and unravelled puzzle for the scientists of the last decades on the basis of abundance variations of f profunda the only species of calcareous nannoplankton which inhabits the lower part of the photic zone i suggest that sapropel formation was triggered by increased primary production either confined to a deep chlorophyll maximum dcm a concept previously introduced by rohling and gieskes 1989 or extended to a thicker layer of the photic zone when increased primary production was confined to the dcm sapropel s3 s4 s5 s7 and s1 this latter only in the eastern part of the basin the abundance of f profunda was enhanced greatly with respect to all other species living in the higher part of the photic zone on the contrary when the increased primary production was extended to a thicker layer of the photic zone glacial sapropels s6 and s8 the production of most nannoplankton species was favored decreasing the relative proportion of f profunda

modeling the paleocirculation of the mediterranean the last glacial maximum and the holocene with emphasis on the formation of sapropel s 1 an ocean general circulation model is used to simulate the thermohaline circulation in the mediterranean sea during the last glacial maximum and the holocene when the sapropel s1 was deposited the model is forced by prescribed surface temperatures and salinities where present day values lead to very realistic surface buoyancy fluxes different paleoreconstructions for the surface salinity and temperature distributions during these periods are tested in both periods under all reconstructions antiestuarine flow is maintained at gibraltar and sicily the holocene circulation has fresh intermediate water produced in the adriatic and an upward salt flux from the old waters below help maintain its outflow at sicily the depth of ventilation around the basin is broadly consistent with the shallowest sapropel layers observed shoaling of the eastern pycnocline occurs in all experiments in both periods possibly indicating enhanced productivity although the reasons for this are different in each case

organic carbon variations of the eastern mediterranean holocene sapropel a key for understanding formation processes abstract this work reports on the organic carbon content of the s1 holocene sapropel the youngest and most extensively researched of the eastern mediterranean sapropels the peak organic carbon percentage of the s1 sapropel varies between 1 2 and 3 4 at varying locations these variations could not be correlated with sapropel thickness or with spatial location in contrast there was a close linear correlation with water depth the deeper samples containing the highest values of organic carbon within the s1 layer the highest values were found in the top of the sapropel even if primary productivity increased during the s1 sapropelic period the main factors responsible for sapropel deposition were changes in the degradation and preservation of organic matter in the water column and at the sediment water interface water column stratification created an oxygenated surficial layer 0 350 400 m thick and a stagnant deep water layer the majority of the organic material escaping the surficial layer was remineralised either at the sediment water interface or just below it organic matter preserved in sapropelic sediments can be assumed to have remained for a significantly longer time 1000 years within the benthic boundary layer than in the water column organic flux decreased with depth through the stagnant deep water layer when stagnation was established water exchanges either did not exist or were limited vertical oxidant transport depended on eddy diffusion while oxidant fluxes at the sediment water interface decreased with depth the organic matter mineralisation rate also decreased and the burial efficiency increased the linear relationship between sapropel organic content and depth was reflected in a linear decrease in oxidant flux regardless of location the same organic carbon percentage was preserved at the same depth in contrast in the same area but at different depths organic carbon content ranges between 1 2 and 3 4 this observation suggests that primary productivity was of the same order of magnitude basinwide the organic carbon concentration of sediment should be used with caution as a quantitative record of palaeoproductivity

neogene sapropels in the mediterranean a review for 50 years the existence of sapropels organic carbon rich sediments deposited within plio pleistocene sediments of the mediterranean sea has been known initially research concentrated on material recovered in relatively short gravity piston cores taken from the eastern basins where sequences were found to be well developed preserved and had extensive spatial coverage in the main previous studies concentrated upon establishing a workable stratigraphy spatial correlation of individual layers and determining the probable depositional mechanisms however despite a plethora of research papers some issues still remain unresolved this is in part due to a lack of agreement between investigators sampling and analytical short comings restricted sample size and the fact that in many instances like was not being compared with like recently the limit of sapropels in the western basin has been further extended as a result the palaeoceanographic palaeoclimate models which had previously been developed for deposition of sapropels in the eastern basin have been modified most recently strong links have been established between astronomical cyclicity and sapropel formation this review paper provides a summary of sapropel research to date and ongoing sapropel research in the mediterranean some of which appears in this thematic issue of marine geology it is fitting that this thematic issue of marine geology be dedicated to the memory of colette vergnaud grazzini and rob kidd who in many ways helped to initiate the resurgence in sapropel studies in the 1970s in the mediterranean perhaps in 50 more years we will know all of the answers 1999 elsevier science b v all rights reserved

three million years of monsoon variability over the northern sahara we present a 3 million year record of aeolian dust supply into the eastern mediterranean sea based on hematite contents derived from magnetic properties of sediments from ocean drilling program site 967 our record has an average temporal resolution of 400 years geochemical data validate this record of hematite content as a proxy for the supply of aeolian dust from the sahara we deduce that the aeolian hematite in eastern mediterranean sediments derives from the eastern algerian libyan and western egyptian lowlands located north of the central saharan watershed 21 n in corroboration of earlier work we relate dust flux minima to penetration of the african summer monsoon front to the north of the central saharan watershed this would have enhanced soil humidity and vegetation cover in the source regions in agreement with results from green sahara climate models our results indicate that this northward monsoon penetration recurred during insolation maxima throughout the last 3 million years as would be expected this orbital precession scale mechanism is modulated on both short 100 kyr and long 400 kyr eccentricity time scales we also observe a strong expression of the 41 kyr obliquity cycle which we discuss in terms of high and low latitude mechanisms that involve southern hemisphere meridional temperature contrasts and shifts in the latitudes of the tropics respectively we also observe a marked increase in sub milankovitch variability around the mid pleistocene transition 0 95 ma which suggests a link between millennial scale climate variability including monsoon dynamics and the size of northern hemisphere ice sheets

a parametric study of the generation and degeneration of wind forced long internal waves in narrow lakes the generation and energy downscaling of wind forced long internal waves in strongly stratified small to medium sized narrow lakes are studied a two layer nonlinear model with forcing and damping is employed even though the wave field is fundamentally bidirectional in nature a domain folding technique is employed to simulate the leading order internal wave field in terms of a weakly nonlinear weakly dispersive model equation of korteweg devries type parametric effects of wind forcing and environmental conditions including variable topography and variable basin width are examined energy downscaling from basin scale waves to shorter scales are quantified by means of a time evolution of the wave energy spectra it is demonstrated that an internal wave resonance is possible when repetitive wind forcing events arise with a frequency near the linear seiche frequency an attempt is made to apply the model to describe the shoaling of long waves on sloping endwall boundaries modelling of the energy loss and energy reflection during a shoaling event is calibrated against laboratory experiments

cyclic variations in the chemical composition of eastern mediterranean pliocene sediments a key for understanding sapropel formation abstract two pliocene mediterranean sediment cores from ocean drilling program odp leg 160 sites 964 pisano plateau ionian basin and 967 lower northern slope of eratosthenes seamount south of cyprus have been analyzed for major and minor elements si ti al fe mg ca k ba cr ni v zn zr by x ray fluorescence analysis at a depth resolution of 3 to 4 cm a total of 430 samples core 964c 9h comprises five sapropels with up to 9 7 total organic carbon toc one 2 5 m thick turbidite and a 50 cm segment of a second turbidite whereas core 967b 9h contains eight sapropels with toc contents of up to 6 7 cyclic variations in carbonate content and detrital matter composition are evident at both sites at site 964 pisano plateau terrigenous detrital matter chemistry reflects periods of stronger saharan dust input higher si al and zr al ratios alternating with fluvial input from the northern borderlands higher mg al ratios cyclicity is not well expressed at this location because turbidites and winnowing occur site 967 eratosthenes seamount sediments are strongly influenced by discharge from the nile river during humid periods low k al and mg al ratios at site 967 sapropels occur only during periods of enhanced nile discharge with correspondingly lower si al ti al mg al k al and zr al ratios accumulation rates of carbonate and terrigenous detrital matter were 30 lower during such episodes because of the reduction in eolian input and carbonate production and or carbonate dissolution periodical ba enrichments which are not only seen in the sapropels but in every k al minimum document the association of enhanced bioproductivity and sapropel formation as calculated from excess ba concentrations productivity increased by up to a factor of five during intervals of intensified nile runoff fluviatile discharge seems to be responsible for an enhanced input of nutrients and suspended matter containing low mg al and k al ratios based on organic carbon concentrations episodes of sapropel formation lasted at least from 1000 to 4000 years at site 964 and from 2000 to 6000 years at site 967 periods of enhanced bio productivity as defined by ba enrichments lasted from 8000 to 12 000 years at site 967 episodes of high river discharge and corresponding nutrient input as well as conditions of sapropel formation seem to have lasted longer in the eastern part of the basin site 967 than at the pisano plateau site 964

the degeneration of internal waves in lakes with sloping topography in a laboratory study we quantified the temporal energy flux associated with the degeneration of basin scale internal waves in closed basins the system is two layer stratified and subjected to a single forcing event creating available potential energy at time zero a downscale energy transfer was observed from the wind forced basin scale motions to the turbulent motions where energy was lost due to high frequency internal wave breaking along sloping topography under moderate forcing conditions steepening of nonlinear basin scale wave components was found to produce a high frequency solitary wave packet that contained as much as 20 of the available potential energy introduced by the initial condition the characteristic lengthscale of a particular solitary wave was less than the characteristic slope length leading to wave breaking along the sloping boundary the ratio of the steepening timescale required for the evolution of the solitary waves to the travel time until the waves shoaled controlled their development and degeneration within the domain the energy loss along the slope the mixing efficiency and the breaker type were modeled using appropriate forms of an internal iribarren number defined as the ratio of the boundary slope to the wave slope amplitude wavelength this parameter allows generalization to the oceanographic context analysis of field data shows the portion of the internal wave spectrum for lakes between motions at the basin and buoyancy scales to be composed of progressive waves both weakly nonlinear waves sinusoidal profile with frequencies near 10 24 hz and strongly nonlinear waves hyperbolic secant squared profile with frequencies near 10 23 hz the results suggest that a periodically forced system may sustain a quasi steady flux of 20 of the potential energy introduced by the surface wind stress to the benthic boundary layer at the depth of the pycnocline

benthic foraminiferal successions across late quaternary mediterranean sapropels abstract benthic foraminiferal records across late quaternary mediterranean sapropels s6 s5 and s1 are reviewed and re considered in the light of recent advances in foraminiferal ecology it is suggested that the main factor controlling the foraminiferal successions that occur immediately before and after sapropel deposition is the amount of time involved in the onset of anoxic conditions and in the re oxygenation of the benthic environment faunas dominated by deep infaunal taxa such as globobulimina and chilostomella reflect a very gradual decrease or increase of bottom water oxygenation post sapropel faunas dominated by small biconvex trochospiral taxa which are inferred to have a much more opportunistic life strategy are typical of a very rapid re oxygenation of the benthic environment after sapropel deposition the time involved in re oxygenation depends upon the mechanism causing sapropel formation the faunal succession found above sapropel s5 suggests a basin with a strong density gradient in the upper part of the water column during sapropel deposition followed by a rapid turnover of the following column and quick re oxygenation of the benthic environment in contrast the faunal succession above sapropel s6 suggests a water column with a more gradual density gradient deeper in the basin a progressive deepening of the halocline and a very slow increase of bottom water oxygenation

review and new aspects concerning the formation of eastern mediterranean sapropels abstract in this paper it is proposed that the formation of eastern mediterranean sapropels occurred in an anti estuarine type of circulation which was to some degree weakened relative to the present in response to reduction of the eastern mediterranean excess of evaporation over freshwater input this reduction of excess evaporation would have been imposed by intensifications of 1 the indian ocean summer sw monsoon influencing the eastern mediterranean via increased nile discharge and 2 the system of mediterranean depressions an element of the westerly atlantic system causing increased precipitation and decreased evaporation both the indian ocean summer monsoon and the westerly atlantic system would be intensified in response to the occurrence of distinct minima in the cycle of precession it is demonstrated that reduced excess evaporation whether or not coinciding with global phases of deglaciation would lead to reduction of surface water salinities in the eastern mediterranean causing isolation of previously formed high salinity cooler deep water thus mixing was severely reduced possibly even restricted to eddy diffusion with only occasional convective events that because of the existing density gradient hardly ever would reach the deepest parts of the basin the consequently diminished oxygen advection down from a few hundred meters in the water column favoured preservation of the sinking organic matter this would however suffice only to enable the formation of sapropels with low organic carbon contents the high organic carbon contents observed in various sapropels are argued to reflect superimposed increases of export production the described scenario accounts for previously reported 1 increases of organic carbon content with increasing depth of deposition within individual sapropels and 2 asymmetrical sequences of sapropel deposition characterized by gradual build up and rather abrupt ending it is furthermore in agreement with 3 isotopic and faunal reconstructions of the history of exchange transports through both the strait of sicily and the strait of gibraltar and 4 faunal and floral reflections of the presence or absence of a distinct deep chlorophyll maximum with its associated increases in export production indicating the presence or absence of a shallow pycnocline within the euphotic layer moreover the described scenario 5 is in no way conflicting with the reports of sapropels in the western mediterranean and 6 seems to be endorsed by occasionally intercalated intervals suggestive of somewhat improved oxygenation amidst anoxic benthic desert levels a situation that has been observed in a few quaternary and pliocene sapropels

sulphidic mediterranean surface waters during pliocene sapropel formation sapropels organic matter rich layers are common in neogene sediments of the eastern mediterranean sea the formation of these layers has been attributed to climate related increases in organic matter production1 2 3 and increased organic matter preservation due to oxygen depletion in more stagnant bottom waters2 3 here we report that eastern mediterranean pliocene sapropels4 contain molecular fossils of a compound isorenieratene known to be synthesized by photosynthetic green sulphur bacteria suggesting that sulphidic euxinic and therefore anoxic conditions prevailed in the photic zone of the water column these sapropels also have a high trace metal content which is probably due to the efficient scavenging of these metals by precipitating sulphides in a euxinic water column the abundance and sulphur isotope composition of pyrite are consistent with iron sulphide formation in the water column we conclude that basin wide water column euxinia occurred over substantial periods during pliocene sapropel formation in the eastern mediterranean sea and that the ultimate degradation of the increased organic matter production was strongly influential in generating and sustaining the euxinic conditions