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

tectonic evolution of the gediz graben field evidence for an episodic two stage extension in western turkey western turkey is one of the most spectacular regions of widespread active continental extension in the world the most prominent structures of this region are e w trending grabens e g gediz and buyuk menderes grabens and intervening horsts exposing the menderes massif this paper documents the result of a recent field campaign field geological mapping and structural analysis along the southern margin of the modern gediz graben of pliocene 5 ma age this work provides field evidence that the presently low angle ductile brittle detachment fault is cut and displaced by the high angle graben bounding normal faults with total displacement exceeding 2 0 km the evolution of the n s extension along the gediz graben occurred during two episodes each characterized by a distinct structural styles 1 rapid exhumation of menderes massif in the footwall of low angle normal fault core complex mode during the miocene 2 late stretching of crust producing e w grabens along high angle normal faults rift mode during pliocene quaternary times separated by a short time gap the later phase is characterized by the deposition of now nearly horizontal sediments of pliocene age in the hanging walls of the high angle normal faults and present day graben floor sediments the evolution of extension is at variance with orogenic collapse and or back arc extension followed by the combined effect of tectonic escape and subduction rollback processes along the aegean cyprean subduction zone consequently it is misleading to describe the miocene sediments exhumed on shoulders of the gediz graben as simple graben fill

the importance of water column processes on the dissolution properties of biogenic silica in deep sea sediments i solubility abstract flow through experiments have been performed to study the thermodynamics of biogenic silica opal dissolution in deep sea sediments they were applied for the first time on sediment from the southern ocean van cappellen and qiu 1997a van cappellen and qiu 1997b we have extended the use of these experiments to other deep sea settings thereby covering a wide range of in situ silicic acid asymptotic concentrations casympt 200 to 900 mol l and biogenic opal content bsi 0 5 to 80 performing these experiments under in situ bottom temperatures allows for the comparison between experimental apparent solubilities and casympt concentrations low values of bsi apparent solubilities have been measured in the deepest sections of the multicores collected in the northeast atlantic 229 mol l and in the equatorial pacific 505 mol l they are only 10 to 20 higher than the in situ casympt concentrations this demonstrates a clear control of pore water silicic acid concentrations by the in situ apparent solubility of the bsi i e the solubility of bsi within a complex sedimentary matrix that includes important quantities of silicate minerals in regions where the percentage detrital percentage biogenic ratio is low the apparent solubility of the biogenic silica is close to that of in situ biogenic silica in the opposite case when the percentage detrital percentage biogenic ratio is high reprecipitation reactions induce strong interference on the dissolution properties of the opal both in situ and in flow through experiments in such a sedimentary matrix it is important to determine the appropriate opal solubility to be used in early diagenetic models i e the solubility of the biogenic silica just before deposition on the seabed this has been achieved by performing flow through experiments on sediment trap material from the north atlantic site comparison of apparent biogenic silica solubility measured by flow through experiments and the silicic acid concentrations measured in the cups of the sediment traps suggested that the solubility of biogenic silica that reaches the sediment water interface is not unique and varies spatially and temporally in fact it is the degree of coupling between surface waters and the sediment water interface that will control the aging of biogenic silica in the water column and hence the dissolution properties of the biogenic silica deposited at the sediment water interface all these results call for a strong improvement of biogenic silica early diagenetic models that should include not only a reprecipitation term that takes into account interaction with silicate minerals but also the existence of several phases of biogenic silica and thus that should operate in a non steady state mode to account for seasonal variations in the quality of deposited biogenic silica

exhumation with a twist paleomagnetic constraints on the evolution of the menderes metamorphic core complex western turkey 1 much remains to be understood about the links between regional vertical axis rotations continental extension and shortening in western turkey miocene vertical axis rotations have been reported that occur simultaneously with the extensional exhumation of the menderes metamorphic core complex which has been related to back arc extension in the eastern part of the aegean back arc in this paper we explore the spatial and temporal relationships between vertical axis rotations in southwestern turkey and extensional unroofing of the menderes massif to this end we provide a large set of new paleomagnetic data from western turkey and integrate these with the regional structural evolution to test the causes and consequences of oroclinal bending in the aegean region the lycian nappes and bey daglari are shown to rotate 20 between 16 and 5 ma defining the eastern limb of the aegean orocline this occurred contemporaneously with the exhumation of the central menderes massif along extensional detachments and after the latest oligocene to early miocene exhumation of the northern and southern menderes massifs exhumation of the latter two was not associated with vertical axis rotations the lower miocene volcanics in the region from lesbos to usak to the north of the central menderes massif underwent a small clockwise rotation insignificant with respect to eurasia this shows that exhumation of the central menderes massif was associated with a vertical axis rotation difference between the northern and southern menderes massifs of 25 30 this result is in excellent agreement with the angle defined by the trends of buyuk menderes and alasehir detachments as well as the angle defined by the regionally curving stretching lineation pattern across the central menderes massif these structures define a pivot point rotation pole for the west anatolian rotations the rotation of the southern domain including the southern menderes massif the lycian nappes and bey daglari must have led to n s contraction east of this pole the eastern limit of the rotating domain is formed by the transpressional couple of the aksu thrust and kirkkavak fault in the center of the isparta angle previously reported clockwise rotations in the volcanic fields near afyon may be the result of distributed n s shortening east of the pivot point the precise accommodation of this shortening history remains open for investigation late oligocene to early miocene extension in the eastern part of the aegean back arc was ne sw oriented and likely bounded by a discrete transform this transform may be associated with an early evolution of the eastern aegean subduction transform edge propagator fault oroclinal bending in the west anatolian region is likely related to a reconnection of the eastern part of the aegean orocline with the african northward moving plate in tandem with roll back in the aegean back arc comparable to a recently postulated scenario for western greece

biogenic silica dissolution in sediments of the southern ocean i solubility abstract a stirred flow through reactor technique was used to determine silica solubilities in sediments collected with a multicorer in the indian sector of the southern ocean antares i cruise the results show that the apparent silica solubility in the cores may decrease increase or remain constant with depth the silica solubility profiles are best explained by the early diagenetic interactions between biogenic silica and soluble aluminum derived from detrital material by combining the solubility data with measured dissolved silica profiles it is shown that the variable asymptotic pore water silica levels in the cores cannot be explained solely by differences in silica solubility in sediments that experience a significant detrital input the simultaneous reprecipitation of dissolved aluminum and dissolved silica prevents water silicic acid from reaching saturation with the dissolving biogenic silica the principal oceanographic control on pore water silica build up in the cores studied is the ratio of the deposition fluxes of biogenic silica and detrital material solubility differences inherited from the biomineralization process in the surface waters do not appear to have a significant effect on the observed pore water silica levels

environmental control on concretion forming processes examples from paleozoic terrigenous sediments of the north gondwana margin armorican massif middle ordovician and middle devonian and sw sardinia late ordovician abstract concretions of various compositions are common in the paleozoic terrigenous successions of the north gondwana margin this study focuses on phosphatic p and siliceous si concretions present in some successions of the armorican massif nw france and sw sardinia w italy it shows that they consist of mudstones fine to very fine grained sandstones or shellbeds with a more or less abundant p cement and form a continuum between a phosphatic end member and a siliceous biogenic end member the p 2 o 5 contents are ranging from 0 26 to 21 5 and are related to apatite the sio 2 contents vary from 25 to 82 and are linked both to a terrigenous phase and to a biogenic silica phase concretions showing the lower p contents p 2 o 5 2 al 2 o 3 5 comparison with the surrounding sediments shows that all the concretions are enriched in chlorite and in middle rare earth elements la s gd s 0 12 0 72 and some of them in y up to 974 ppm rare earth elements more than 300 ppm and sr 260 880 ppm the concretions with highest biogenic silica concentrations are contained in the outer shelf sediments whereas the other concretions are present from the proximal part of the inner shelf to the outer shelf a genetic model in two stages is proposed during early diagenesis the dissolution of shells and degradation of organic matter progressively enrich the pore water in dissolved si ca and p when the suboxic zone is reached p precipitation begins leading to the formation of protoconcretions in shallow environments the relative permeability of sediments and the winnowing or reworking of the upper few centimetres by bottom currents allow for suboxic conditions to be maintained leading to p rich concretion formation in deeper environments the anoxic zone is reached more rapidly thereby preventing extensive phosphogenesis nevertheless in the protoconcretions the early p cement preserves pore spaces from compaction in the presence of biogenic siliceous particles the fluids are enriched in dissolved silica and diffuse towards the protoconcretions silica precipitation can thus occur later in the intergranular spaces

granitoid rocks of the southern menderes massif southwestern turkey field evidence for tertiary magmatism in an extensional shear zone recent field campaign in the southern menderes massif in southwestern turkey revealed that the so called core of the massif comprises two distinct types of granitoid rocks an orthogneiss traditionally known as augen gneisses and leucocratic metagranite where the latter is intrusive into the former and the structurally overlying cover schists these differ from one another in intensity of deformation degree of metamorphism and kinematics the orthogneiss display penetrative top to the n nne fabrics formed under upper amphibolite facies conditions during the eocene main menderes metamorphism mmm whereas foliation and stretching lineation exists in the leucocratic metagranites but are not strongly developed the leucocratic metagranites show evidence of syn to post emplacement deformation in a series of weakly developed top to the s ssw fabrics formed under lower greenschist facies conditions leucocratic metagranite bodies occur all along the augen gneiss schist contact in the southern menderes massif they are emplaced as sheet like bodies into country rocks previously deformed and metamorphosed during a top to the n nne alpine orogeny along a ductile extensional shear zone located between orthogneisses and metasediments which was possibly active during emplacement the data presently available indicate that emplacement and associated ductile extensional deformation occurred during late oligocene early miocene time these results confirm previous contentions that there are tertiary granites in this part of the menderes massif

layer parallel shortening and related structures in zones undergoing active regional horizontal extension folds and thrust faults formed by layer parallel shortening coaxial with extensional structures such as normal dip slip faults and ductile necking structures with orthorhombic fabric symmetry are usual but little recognised structures formed within normal dip slip shear zones bounding rifts they are generated because of the shear distribution in a zone of progressive deformation and may be later extended and disrupted depending on which part of the strain ellipsoid they may be located we here describe folds and thrust faults from the southern margin of the alasehir rift in western turkey as an opportunity to discuss the properties of pureextension related structures formed by layer parallel shortening such structures are more commonly generated during the early stages of rifting when deformation rates are slow and the shear zones broader than those forming later in the life of a rift when strain rates are usually higher such structures have commonly been mistaken for witnesses documenting regional episodes of shortening rather than as integral parts of the extensional structures forming rifts not all layer parallel shortening related structures therefore indicate regional shortening we plead that hasty statements concerning the meaning of geological structures at all scales be avoided before a thorough understanding of bulk strains that have affected a region are properly understood

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

biogenic flux of al to sediment in the central equatorial pacific ocean evidence for increased productivity during glacial periods we examined the flux of al to sediment accumulating beneath the zone of elevated productivity in the central equatorial pacific ocean along a surface sediment transect at 135 w as well as downcore for a 650 kyr record at 1 3 n 133 6 w across the surface transect a pronounced broadly equatorially symmetric increase in al accumulation is observed relative to ti with al ti ratios reaching values 3 4 times that of potential detrital sources the profile parallels biogenic accumulation and the modeled flux of particulate 234th suggesting rapid and preferential adsorptive removal of al from seawater by settling biogenic particles normative calculations confirm that most al is unsupported by the terrigenous fraction the observed distributions are consistent with previous observations of the relative and absolute behavior of al and ti in seawater and we can construct a reasonable mass balance between the amount of seawater sourced al retained in the sediment and the amount of seawater al available in the overlying column the close tie between al ti and biogenic accumulation as opposed to concentration emphasizes that biogenic sedimentary al ti responds to removal transport phenomena and not bulk sediment composition thus in these sediments dominated by the biogenic component the bulk al ti ratio reflects biogenic particle flux and by extension productivity of the overlying seawater the downcore profile of al ti at 1 3 n displays marked increases during glacial episodes similar to that observed across the surface transect from a background value near al ti of average upper crust the excursions in al ti are stratigraphically coincident with maxima in both bulk and caco3 accumulation and the excess al appears to not be preferentially affiliated with opaline or organic phases consistent with the similar behavioral removal of al and 234th the latter of which responds to the total particle flux the al flux reflects carbonate accumulation only because carbonate comprises the dominant flux in these particular deposits these results collectively indicate that 1 al in biogenic sediment and settling biogenic particles is strongly affected by a component adsorbed from seawater therefore the common tenet that al is dominantly associated with terrestrial particulate matter and the subsequent use of al distributions to calculate the abundance and flux of terrestrial material in settling particles and sediment needs to be reevaluated 2 the al ti ratio in biogenic sediment can be used to trace the productivity of the overlying water providing a powerful new paleochemical tool to investigate oceanic response to climatic variation 3 the close correlation between the al ti productivity signal and carbonate maxima downcore at 1 3 n suggests that the sedimentary carbonate maxima in the central equatorial pacific ocean record increased productivity during glacial episodes

diagenesis of flint and porcellanite in the maastrichtian chalk at stevns klint denmark abstract flint is common in many successions but since the nodules can be the result of a series of replacement and void filling processes their origin and broader paleo oceanographic significance can remain enigmatic this study addresses the origin of flint in the campanian maastrichtian chalk of denmark using the 450 m long stevns 1 core as a natural laboratory four types of siliceous nodules occur in the core porcellanite white flint white flint with a core of dark flint and dark flint all nodules consist mainly of microquartz with subordinate lutecite spherulitic chalcedony megaquartz and dolomite the lutecite replaces macrofossil fragments mainly bryozoans and shells microquartz replaces coccoliths and the chalcedony and megaquartz are pore filling the preservation of the external shape of bryozoans shells and coccoliths suggest that siliceous nodules were formed early by a one by one volume replacement of chalk values of 18osmow of microquartz and lutecite around 33 indicate temperatures of 15 17 c at the time of precipitation in contrast the 18osmow values of megaquartz indicate precipitation at temperatures up to 48 c the occurrence of chalcedony together with more stable and ordered megaquartz in irregular voids indicates that silica gel and or opal ct transformed to the quartz phases microquartz lutecite chalcedony and megaquartz through the dissolution precipitation process ostwald ripening these results motivate a conceptual model in which the replacement process was initiated by microbial decomposition of organic matter and took place during periods of low rate of or even stopped sedimentation which fixed the redox boundary and the microbial metabolic zones at a specific depth below the sea floor biogenic opal a incorporated in the sediment column below the sea floor was dissolved and precipitated at the redox boundary as silica gel and or opal ct possibly aided by sulfide oxidizing bacteria during burial the dissolution precipitation process transformed the silica gel and opal ct to the quartz phases the results of this study indicate that precipitation of the flint precursor can be controlled by a combination of an abundant biosiliceous source and high microbial activity in the sediments immediately below the seafloor flint is therefore likely to reflect deposition in a narrow interval at relatively shallow depths and therefore might be useful in studies of shallow water carbonates

germanium silicon fractionation in a river influenced continental margin the northern gulf of mexico abstract in this study we have sampled the water column and sediments of the gulf of mexico to investigate the effects of high riverine terrigenous load and sediment redox conditions on the cycling of ge and si water column ge si ratios across the gulf of mexico continental shelf range from 1 9 to 25 mol mol which is elevated compared to the global ocean value of 0 7 mol mol the ge enrichment in the gulf of mexico seawater is primarily due to anthropogenic contamination of the mississippi river which is the main ge and si source to the area and to a smaller extent due to discrimination against ge during biogenic silica bsi production ge si 1 2 1 8 mol mol especially by radiolarians and siliceous sponges ge si 0 6 1 1 mol mol most sediment pore waters ge si 0 3 4 5 mol mol and sediment incubation experiments benthic flux ge si 0 9 1 2 mol mol indicate precipitation of authigenic phases that sequester ge from pore waters non opal sink this process appears to be independent of oxidation reduction reactions and suggests that authigenic aluminosilicate formation reverse weathering may be the dominant ge sink in marine sediments compilation of previously published data shows that in continental margins non opal ge burial flux is controlled by bsi supply while in open ocean sediments it is 10 100 times lower and most likely limited by the supply of lithogenic material we provide a measurement based estimate of the global non opal ge burial flux as 4 32 mmol yr 1 encompassing the 2 16 mmol yr 1 needed to keep the global marine ge cycle at steady state

laboratory analysis of dissolution effects on southern ocean polycystine radiolaria abstract laboratory analyses were conducted on siliceous microfauna in holocene and older interglacial samples from the southern ocean to determine dissolution effects on their skeletons examination of siliceous skeletons was made at regular intervals throughout the experiment no major differential dissolution was detected among the dominant southern ocean radiolarian species as only minor variations in relative abundance occurred with increasing degrees of silica dissolution dissolution rate is a function of age of the polycystine radiolaria with higher holocene rates than those registered in the interglacial sample these results serve to strengthen the premise that information about past southern ocean conditions can be reliably acquired from analysis of polycystine radiolarian assemblages preserved in marine sediments

imbalance in the carbonate budget of surficial sediments in the north atlantic ocean variations over the last millenium abstract fluxes contributing to the particulate carbonate system in deep sea sediments were investigated at the bengal site in the porcupine abyssal plain northeast atlantic deposition fluxes were estimated using sediment traps at a nominal depth of 3000 m and amounted to 0 37 0 1 mmol c m 2 d 1 dissolution of carbonate was determined using flux of total alkalinity from in situ benthic chambers is 0 4 0 1 mmol c m 2 d 1 burial of carbonate was calculated from data on the carbonate content of the sediment and sedimentation rates from a model age based on 14c dating on foraminifera 0 66 0 1 mmol c m 2 d 1 burial plus dissolution was three times larger than particle deposition flux which indicates that steady state is not achieved in these sediments mass balances for other components bsi 210pb and calculations of the focusing factor using 230th show that lateral inputs play only a minor role in this imbalance decadal variations of annual particle fluxes are also within the uncertainty of our average long term change in dissolution may contribute to the imbalance but can not be the main reason because burial alone is greater than the input flux the observed imbalance is thus the consequence of a large change of carbonate input flux which has occured in the recent past a box model is used to check the response time of the solid carbonate system in these sediments and the time to reach a new steady state is in the order of 3 kyr thus it is likely that the system has been perturbed recently and that large dissolution and burial rates reflect the previously larger particulate carbonate deposition rates we estimate that particulate carbonate fluxes have certainly decreased by a factor of at least 3 and that this change has occurred during the last few centuries

pore waters of the central pacific ocean nutrient results interstitial waters from the manop siliceous and calcareous ooze sites in the central pacific ocean were recovered by shipboard squeezing and centrifuging techniques and an in situ harpoon sampler the pore waters were analyzed for nutrients carbonate system species mn 2 ca 2 total alkaline earth metals and dissolved oxygen siliceous ooze site only in calcareous sediments the co 2 alkalinity po 4 3 and ca 2 results obtained from shipboard techniques are significantly different than those recovered in situ the other species measured as well as all of the data from the siliceous ooze site reveal no significant dependence on the method used to sample the pore water the relative changes in o 2 no 3 mn 2 alkalinity and co 2 indicate that respiration using o 2 no 3 and mno 2 as electron acceptors is the dominant reaction affecting the pore water species we develop a model which simulates the fate of pore water oxygen and nitrate and particulate organic carbon in sediments this model includes the effects of pore water diffusion oxygen and nitrate respiration sediment accumulation sediment mixing and changes in sediment porosity we observe that factor of 2 variations in the rates of sediment mixing and accumulation have only a minor effect on the pore water profiles the major factor controlling the pore water profiles is the flux of organic carbon across the sediment water interface we conclude that the large intra site variability observed in the interstitial water nitrate profiles is probably caused by small scale variations in the flux of organic material

tertiary evolution of the central menderes massif based on structural investigations of metamorphics and sedimentary cover rocks between salihli and kiraz western turkey augen gneisses mica schists and marbles of the menderes massif and its sedimentary cover rocks are exposed south of the gediz graben the augen gneisses form the structurally lowest part of the studied lithological sequence and are overlain by a schist complex the structurally highest part is formed by a series of marbles the ages of this lithological sequence range from precambrian to early paleocene furthermore this sequence records the tectonic evolution since the precambrian the sedimentary cover of the menderes massif consists of two groups of sediments from early miocene to quaternary the lower group the alasehir group consists of early to mid miocene aged fluvial and limnic sediments which form the lower and the upper parts respectively the alasehir group is overlain by mainly fluvial sediments of the gediz group both the alasehir and the gediz groups are separated by an angular unconformity six deformational phases could be distinguished within the metamorphic rocks of the menderes massif and its tertiary cover the structures which were interpreted to belong to deformational events predating the paleocene are summarized as deformational phase d1 d1 structures were nearly completely overprinted by the subsequent deformation events the second deformational phase d2 occurred between early eocene and early oligocene d2 occurred contemporaneously with a barrovian type regional metamorphism the third deformational phase d3 is characterized by folding of the axial planes which formed at the end of early oligocene the deformational event d4 occurred during the late oligocene and is related to an extensional period the deposition of the sedimentary rocks which belong to the tertiary cover of the menderes massif that started in the early miocene was interrupted by a compressional phase d5 during the late miocene sediments which were deposited since the early pliocene record structures which were related to a young extensional phase d6 this extensional phase has continued to the present

distribution of biogenic silica and quartz in recent deep sea sediments all available quartz and biogenic silica concentrations from deep sea surface sediments were intercalibrated plotted and contoured on a calcium carbonate free basis the maps show highest concentrations of biogenic silica opal along the west african coast along equatorial divergences in all oceans and at the polar front in the southern indian ocean these are all areas where upwelling is strong and there is high biological productivity quartz in pelagic sediments deposited far from land is generally eolian in origin its distribution reflects dominant wind systems in the pacific but in much of the atlantic and indian oceans the distribution pattern is strongly modified by turbidite deposition and bottom current processes

effect of trace metal limited growth on the postmortem dissolution of the marine diatom pseudo nitzschia delicatissima we investigated the effects of iron fe and copper cu limitations on biogenic silica bsio 2 dissolution kinetics of the marine diatom pseudo nitzschia delicatissima during a 3 week batch dissolution experiment the dissolution of this species was faster during the first week than thereafter modeling results from four dissolution models and scanning electron microcopy images suggested the successive dissolution of two phases of bsio 2 with two different dissolution constants micronutrient limitation during growth affected the respective proportion of the two phases and their dissolution constants after 3 weeks of dissolution frustules from micronutrient limited diatoms were better preserved than those of replete cells our results also confirm that micronutrient limited cells may export more si relative to n than replete cells and may increase the silicate pump this may not only be due to a higher degree of silicification of the live cells but also to a decoupling between the recycling of si and n during dissolution we suggest that a mechanistic understanding of the evolution of the dissolution constant during dissolution is needed this would improve the parameterization of dissolution in ecosystem models and ultimately their predictions on the amount of bsio 2 that dissolves in the photic zone and the amount of bsio 2 that is exported to the seafloor

what controls opal preservation in tropical deep sea sediments measurements of opal preservation in deep sea sediment cores have been presented in three ways the opal concentration as a fraction of total dry weight opaltot the opal concentration normalized to calcite free dry weight opalcalcite free and me opal accumulation rate opal mar it is tempting to interpret changes in these indices as indicators of rates of biological production in past oceans based on theoretical constraints we argue that in typical tropical and subtropical sediments both opalcalcite free and opal mar reflect a significant artifact of dilution by other phases thus the band of high opalcalcite free in the equatorial pacific appears to be caused in large part by the high calcite in that region rather than by high opal productivity the best candidate for a reliable paleoproductivity proxy appears to be opaltot unfortunately present day opaltot data from tropical and subtropical regions show little or no systematic trend with the rain rate of opal pore water silica concentration data reveal that the apparent pore water opal solubility is not constant but correlates regionally with the rain rate of opal to the seafloor a model that treats opal as a single homogeneous phase with a single well defined solubility product predicts a strong dependence of opal concentration on rain rate in stark contrast to the data and a constant asymptotic pore water si two models representing opal as multiple heterogeneous phases with different solubilities are able to reproduce the observed asymptotic pore water si rain rate relationship but not the lack of rain rate trend in the opal concentration data only by assuming a systematic trend in the quality of opal i e the solubility as a function of opal production can we reproduce the observed pattern of opal preservation the implication of this study is that changes in opal preservation in the geologic record cannot simply be interpreted in terms of changes in surface ocean productivity until our understanding of opal diagenesis can be improved

the benthic silica cycle in the northeast atlantic annual mass balance seasonality and importance of non steady state processes for the early diagenesis of biogenic opal in deep sea sediments within the framework of the eu funded bengal programme the effects of seasonality on biogenic silica early diagenesis have been studied at the porcupine abyssal plain pap an abyssal locality located in the northeast atlantic ocean nine cruises were carried out between august 1996 and august 1998 silicic acid dsi increased downward from 46 2 to 213 m mean of 27 profiles biogenic silica bsi decreased from ca 2 near the sediment water interface to 1 at depth benthic silicic acid fluxes as measured from benthic chambers were close to those estimated from non linear dsi porewater gradients some 90 of the dissolution occurred within the top 5 5 cm of the sediment column rather than at the sediment water interface and the annual dsi efflux was close to 0 057 mol si m 2 yr 1 biogenic silica accumulation was close to 0 008 mol si m 2 yr 1 and the annual opal delivery reconstructed from sedimentary fluxes assuming steady state was 0 065 mol si m 2 yr 1 this is in good agreement with the mean annual opal flux determined from sediment trap samples averaged over the last decade 0 062 mol si m 2 yr 1 thus ca 12 of the opal flux delivered to the seafloor get preserved in the sediments a simple comparison between the sedimentation rate and the dissolution rate in the uppermost 5 5 cm of the sediment column suggests that there should be no accumulation of opal in pap sediments however by combining the bengal high sampling frequency with our experimental results on bsi dissolution we conclude that non steady state processes associated with the seasonal deposition of fresh biogenic particles may well play a fundamental role in the preservation of bsi in these sediments this comes about though the way seasonal variability affects the quality of the biogenic matter reaching the seafloor hence it influences the intrinsic dissolution properties of the opal at the seafloor and also the part played by non local mixing events by ensuring the rapid transport of bsi particles deep into the sediment to where saturation is reached

rapid post mortem maturation of diatom silica oxygen isotope values 1 oxygen isotope values of diatom silica 18odiatom are increasingly used as paleoclimate proxies however the magnitude and timing of post mortem alteration of 18odiatom values has been unclear in freshwater diatom silica from a human made pond in northern new mexico post mortem alteration of 18odiatom values occurs within one year of sediment burial diatom silica collected antemortem has an average 18odiatom value of 21 5 vsmow 1 3 n 23 whereas diatom silica from two sediment cores from the same freshwater environment records significantly higher 18odiatom values 28 9 0 8 n 13 the difference in ante and post mortem 18odiatom values indicate post mortem alteration of diatom silica oxygen that results in a 7 increase in 18o values this study demonstrates that 18odiatom values reach mature values within 0 5 years of frustule death initial diatom 18o values that record growing conditions are rapidly overwritten during silica maturation and the mature diatom 18o values approach isotopic equilibrium for quartz water the rapid post mortem alteration of diatom 18o values explains much of the disparate data regarding silica water fractionation for diatom silica and has a profound effect on the use of diatom silica 18o values as a paleoclimate proxy in lacustrine and marine environments

biogenic silica recycling in surficial sediments across the polar front of the southern ocean indian sector abstract the processes controlling preservation and recycling of particulate biogenic silica in superficial sediments must be understood before one uses biogenic silica as a proxy in paleooceanographic studies and in order to compute oceanic mass balances for silica in this respect the antarctic ocean is certainly a key region due to its high productivity and export of biogenic silica in order to quantify sedimentary fluxes and identify crucial processes that allow the preservation of biogenic silica pore water and solid phase silica profiles were performed on sediment cores from the southern ocean indian sector during the antares 1 cruise in combination with solubility data reported by van cappellen and qiu 1997a a process model representing the early diagenesis of silica was developed in this model a dependence with depth of the kinetic constant was introduced to allow the preservation of biogenic silica in sediment porewater undersaturated with respect to that phase using this steady state model it is proposed that a proportionality of the reactivity of the biogenic silica with its settling flux is necessary to explain the observed profiles it is then shown using this model that the preservation of biogenic silica is not a linear function of the deposited flux using a modified version of this model containing an explicit term of reprecipitation we hypothesize that reprecipitation alone cannot counterbalance dissolution and that its effect is certainly related to a decrease in either surface solubility or kinetics of dissolution

tectonic significance of synextensional ductile shear zones within the early miocene alacamdag granites northwestern turkey synextensional granitoids may have significant structural features leading to the understanding of the evolution of extended orogenic belts one of the highly extended regions the aegean region includes a number of metamorphic core complexes and synextensional granitoids that developed following the alpine collisional events the alacamdag area in northwestern turkey is one of the key areas where miocene granites crop out along the boundary of various tectonic units structural data from the early miocene alacamdag granites demonstrated two different deformation patterns that may provide insights into the development of granitic intrusions and metamorphic core complexes 1 steeply dipping ductile shear zones caused emplacement of syn tectonic granite stocks they include kinematic indicators of a sinistral top to the sw displacement this zone has also juxtaposed the izmir ankara zone and the menderes massif in the west and east respectively 2 gently dipping ductile shear zones have developed within the granitic stocks that intruded the schists of the menderes massif on the structurally lower parts kinematic data from the foliated granites indicate a top to the ne displacement which can be correlated with the direction of the hanging wall movement documented from the simav and kazdag metamorphic core complexes the gently dipping shear zones indicate the presence of a detachment fault between the menderes massif and the structurally overlying izmir ankara zone mesoscopic to map scale folds in the shallow dipping shear zones of the alacamdag area were interpreted to have been caused by coupling between ne sw stretching and the accompanying nw se shortening of ductilely deformed crust during early miocene times one of the ne trending shear zones fed by granitic magmas was interpreted to form the northeastern part of a sinistral wrench corridor which caused differential stretching between the cycladic and the menderes massifs this crustal scale wrench corridor the izmir balikesir transfer zone may have controlled the asymmetrical and symmetrical extensions in the orogenic domains the combination of the retreat of the aegean subduction zone and the lateral slab segmentation leading to the sinistral oblique slip tearing within the eurasian upper plate appears to be a plausible mechanism for the development of such extensive ne trending shear zones in the aegean region

fluctuations of glaciers in the tropical andes over the last millennium and palaeoclimatic implications a review abstract the aim of this paper is to document the evolution of glaciers in the tropical andes between 10 n and 16 s for the last millennium based on moraines dated by lichenometry and radiocarbon lake sediment records and historical documents viewed collectively several glacial advances occurred synchronously the first advance is dated around ad 1200 1350 the maximum glacial extent mge defined as the furthest down valley extent recorded synchronously by the majority of glaciers occurred around 1630 1680 in bolivia and peru the outer tropics and around ad 1730 in ecuador colombia and venezuela the inner tropics subsequently during the 18th and 19th centuries glaciers retreated continuously with only minor synchronous readvances in the outer tropics minor glacial advances occurred around 1730 1760 1800 1850 and 1870 in the inner tropics synchronous minor advances occurred around 1760 1820 and 1880 between the mge and the early 20th century glaciers lost about 30 of their total length the retreat was slow between the 17th and 18th centuries but then became more marked use of the accumulation area ratio aar method or historical observations in the different cordilleras revealed an increase in the equilibrium line altitude ela of about 300 m from the mge onward palaeoclimatic hypotheses based on glaciological models run in different countries suggest a cool and humid period in the 16 18th centuries followed by a colder and drier period in the 19th century the reduction of glaciers observed from the middle of the 19th century is due to increasingly warmer conditions than before here quantitative estimates are proposed to explain the evolution of the glaciers in venezuela results indicate for the period 1250 1820 that mean air temperatures were 3 2 1 4 c cooler and precipitation was about 22 higher than at present in ecuador temperatures of between 0 8 and 1 1 c lower than today and between 25 and 35 higher accumulation than today appear to have occurred in the 18th century followed by a short drier but colder period at the beginning of the 19th century in bolivia the mge could be a consequence of a decrease in temperature of 1 1 to 1 2 c and a 20 to 30 increase in accumulation or an increase in cloudiness of about 1 2 10 we discuss not only external climatic forcing but also the coincidence between glacier expansion and a decrease in solar irradiance dating uncertainties however have made the role of volcanism in glacier fluctuations impossible to determine finally we discuss the relationships between enso and glacier fluctuations in recent centuries which do not match directly with current knowledge of modern teleconnections between tropical pacific ssts and variations in glacier mass balance

precise u pb ages of syn extensional miocene intrusions in the central menderes massif western turkey western turkey is an area which has experienced large scale extension of continental crust here we report precise crystallization ages of two intrusions in the central menderes massif the turgutlu and salihli granodiorites using u pb dating both intrusions occur in the southern footwall of the seismically active alasehir graben and were emplaced syntectonically in an extensional top to the nne shear zone which was active at retrograde greenschist facies conditions the u pb ages of 16 1 0 2 ma monazite turgutlu granodiorite and 15 0 0 3 ma allanite salihli granodiorite document that tectonic exhumation of middle crustal rocks in the central menderes massif was already underway at the early to middle miocene transition combined with published geochronological structural and sedimentological data the new u pb ages point to a continued extension since at least 16 ma there is no convincing evidence for a late miocene pliocene phase of tectonic shortening

antares i a biogeochemical study of the indian sector of the southern ocean abstract the first expedition of the antares program the french contribution to southern jgofs investigated the biology and the chemistry of the benthic environment and the water column in a region located between 60 s 72 e and 39 s 55 e in this special issue we present a detailed description of the transports of water masses across the polar front zone which are corroborated by chemical and microbial tracer studies a description of the sediment water interface bridging biological chemical and microbial studies and new insights into the diagenesis of biogenic silica

microphyte pigments and resting spores at the water sediment interface in the subantarctic deep sea indian sector of the southern ocean during the antares i cruise april may 1993 in the indian sector of the southern ocean deep sea sediments were collected using a multiple gravity corer in the polar frontal zone pfz and frontal zone between crozet and kerguelen islands and in the permanently open ocean zone pooz south west of kerguelen and heard islands microphyte biomass indicators chl a b c and related phaeopigments measured by spectrofluorometry tests on natural fluorescence and detection of encysted diatoms observed by scanning electron microscopy sem were carried out on both sediment and overlying fluff whenever present evaluation of the phytoplankton biomass in the overlying surface water masses also was performed at each sampling station the phytoplankton biomass was on the whole very low and decreased from north to south 0 25 0 10 g 1 1 chl a with a maximum on the frontal zone whereas the sediment water interface of the southern sampling stations pooz was pigment enriched a consistent bioclastic fluffy layer was only present in the pooz stations up to 13 g 1 1 chl a 157 g 1 1 phaeo a biodeposition of this type was very thin or absent north of the pf polar front illustrating stronger advection in the northern part of the study area hydrodynamic studies of the crozet basin and the presence of benthic pennate coastal diatoms at the sediment interface of these northern stations further suggest increased advective activity in the north surficial sediments first 5 mm also were enriched with pigments in the pooz up to 0 15 g g 1 dw chl a 3 5 g g 1 dw phaeo a whereas in the pfz the chl a concentrations were insignificant pigment gradients in the sediments particularly well marked in the pooz indicate the absence of a secondary intense reworking such as bioturbation or resuspension nevertheless also in the pooz an unusual and well preserved megafaunal faecal cast was sampled and analysed sediments at southern stations 50 55 s 56 75 e 3600 4700 m depth were the most enriched with encysted microphytic cells mainly diatoms belonging to the genus chaetoceros this was related primarily to the presence of a fluffy layer composed mainly of well preserved detritic frustules of various diatom genera including robust nitzschia and thalassiosira as well as chains of delicate chaetoceros corethron and rhizosolenia well preserved coccoliths and silicoflagellates also were noticeable in the fluffs revival tests cultures on f2 medium initiated with samples from the water sediment interface were positive for the southern stations leading to low diversity diatom assemblages dominated by the genera chaetoceros and nitzschia