precessional forcing of coccolithophore assemblages in the northern arabian sea implications for monsoonal dynamics during the last 200 000 years abstract the paleoceanographic potential of coccolithophores was used to decipher the monsoon induced paleoproductivity changes in the northern arabian sea during the past 200 000 years core so 90 94 kl was taken in the north eastern part of the study area representing the primarily ne monsoon influenced domain for comparison core odp117 723 a from the western side of the arabian sea off oman under the dominating influence of the sw monsoon was investigated samples were prepared using a quantitative dilution filtering technique and investigated in the sem we focus here only on the most important contributors to the assemblages gephyrocapsa oceanica gephyrocapsa ericsonii emiliania huxleyi florisphaera profunda contributing at least 63 up to 92 5 of the coccolith assemblages abundances of the two species e huxleyi and g ericsonii were summed up and depicted as a combined record ehuxgeric the 200 000 year record from the north eastern arabian sea yielded a significant precession derived signal in the abundances of ehuxgeric which could be related by cross spectral analysis to maximum low latitudinal insolation in contrast g oceanica seemed to correspond to insolation minima relative abundances of f profunda are generally rather low compared to other oceanographic regions and exhibit no distinct variations except for the thermal maxima of isotope stages ii i and v a schematic model is presented to show the changes in surface water stratification in the arabian sea driven by changing monsoon intensities over periodicities of the precessional band of milankovitch cycles this model is based on the variations of the two taxa groups g oceanica and ehuxgeric together with changes in the abundance of cocco total and f profunda probably the arabian sea remained eutrophic at least during one season of the year despite drastic changes in the summer monsoon strength lower productivities during the summer monsoon were probably compensated by higher phytoplankton growth due to stronger winter monsoons

monsoonal variability in the northeastern arabian sea during the past 5000years geochemical evidence from laminated sediments abstract a high resolution laminated sediment record from the oxygen minimum zone in the northeastern arabian sea off pakistan was studied to reconstruct climatic variations to infer the monsoon driven moisture history in the northeastern arabian sea during the past 5000 years we have used three types of independant proxies 1 varve thickness 2 stable isotope ratios of planktic foraminifera and 3 inorganic geochemical composition on the basis of these proxies we have subdivided the sedimentary record into five intervals the first time interval from 5000 to 3900 yr bp period i is dominated by summer monsoon activity deduced from ti al zr al k al and sr ca ratios and oxygen isotope data of planktonic foraminifera slightly enhanced winter monsoon conditions can be inferred for the period from 3900 to about 3000 yr bp period ii maximum precipitation during enhanced winter monsoon activity at about 3100 yr bp was followed by an onset of aridification at 3000 yr bp which continued until about 2000 2200 yr bp period iii the textural mineralogical and geochemical data suggest that this aridification was a gradual century scale process rather than an abrupt event comparison to late holocene records in nw and sw india the near east and tropical africa supports our interpretation that period iii from 3000 to 2000 yr bp is characterized by a gradual change in the atmospherical moisture budget over much of this area reduced monsoonal rainfall may be linked to variations of eurasian snow cover as deduced from varve thickness and element to al ratios period iv from 2000 to about 1500 yr bp is characterized by a summer monsoon dominated sequence with higher precipitation than during period iii the 18o record shows an abrupt shift at 2000 yr bp coinciding with an increase of varve thickness which suggests a cooling of surface waters in the northern arabian sea present day conditions in the makran area were established at 1500 yr bp period v based on inorganic geochemical data and meteorological records at several stations in the makran we conclude that period v is dominated by precipitation during the winter season here declined rainfall associated with the summer monsoon may be related to a less northward position of the intertropical convergence zone itcz during summer and a stronger influence of winter monsoon linked phenomenona

multiple monsoon controlled breakdown of oxygen minimum conditions during the past 30 000 years documented in laminated sediments off pakistan abstract late holocene laminated sediments from a core transect centred in the oxygen minimum zone omz impinging at the continental slope off pakistan indicate stable oxygen minimum conditions for the past 7000 calendar years high sw monsoon controlled biological productivity and enhanced organic matter preservation during this period is reflected in high contents of total organic carbon toc and redox sensitive elements ni v as well as by a low diversity high abundance benthic foraminiferal buliminacea association and high abundance of the planktonic species globigerina bulloides indicative of upwelling conditions surface water productivity was strongest during sw monsoon maxima stable omz conditions reflected by laminated sediments were found also during warm interstadial events preboreal bolling allerod and dansgaard oeschger events as well as during peak glacial times 17 22 5 ka all ages in calendar years sediment mass accumulation rates were at a maximum during the preboreal and younger dryas periods due to strong riverine input and mobilisation of fine grained sediment coinciding with rapid deglacial sea level rise whereas eolian input generally decreased from glacial to interglacial times in contrast the occurrence of bioturbated intervals from 7 to 10 5 ka early holocene in the younger dryas 11 7 13 ka from 15 to 17 ka heinrich event 1 and from 22 5 to 25 ka heinrich event 2 suggests completely different conditions of oxygen rich bottom waters extremely low mass and organic carbon accumulation rates a high diversity benthic fauna all indicating lowered surface water productivity during these intervals the omz was very poorly developed or absent and a sharp fall of the aragonite compensation depth favoured the preservation of pteropods the abundance of lithogenic proxies suggests aridity and wind transport by northwesterly or northeasterly winds during these periods coinciding with the north atlantic heinrich events and dust peaks in the tibetan loess records the correlation of the monsoon driven omz variability in the arabian sea with the rapid climatic fluctuations in the high northern latitudes suggests a close coupling between the climates of the high and low latitudes at a global scale

effect of mineralogy salinity and temperature on li ca and li isotope composition of calcium carbonate li ca and li isotope ratios in marine and freshwater calcium carbonates are potential tracers of past environmental variables such as weathering intensity and temperature to make use of their potential requires full understanding of the fractionation of li ca and 7 li 6 li during growth of calcium carbonate this study improves such understanding with two sets of new measurements on inorganic calcites and aragonites precipitated over a range of salinity and on the test of benthic foraminifera formed at a range of temperatures li ca in inorganic calcite increases by a factor of four as salinity increases from 10x to 50x inorganic aragonite demonstrates no significant change in li ca over the same salinity range the difference in behaviour is probably due to the incorporation mechanism of li substitution of li in the ca 2 site in aragonite makes growth solution li ca the key variable while interstitial incorporation of li in calcite means that the growth solution li concentration is more important compared to the growth solution 7 li 6 li is c3x lower in calcite and c11x lower in aragonite with no relationship to salinity for either mineral similar isotope fractionation to these inorganic experiments are observed for biogenically produced calcite foraminifera and aragonite corals this suggests little biological control during li incorporation into biogenic calcium carbonates the difference in fractionation between calcite and aragonite is consistent with an equilibrium isotope fractionation reflecting different incorporation mechanisms for li into the two minerals four samples of the benthic foraminifera uvigerina precipitated over a temperature range of 14 8c have li ca ranging from 18 5 to 13 1 amol mol the ratio is inversely related to temperature with a sensitivity of 2 5f0 8 per 8c this sensitivity is similar to that observed in a previous study of foraminiferal li ca but about half that observed during inorganic calcite growth the large variability in both li ca and temperature sensitivity exhibited between various foraminifera species suggests an additional control on calcite li ca apart from those of salinity and temperature observed in this study d 2004 elsevier b v all rights reserved

sm nd isotopic dating of proterozoic clay material an example from the francevillian sedimentary series gabon clay fractions of 0 4 m and 0 2 m and acid leachates of these clay fractions were separated from two different kerogen rich black shale samples from the francevillian sedimentary series in gabon they yielded identical sm nd isochron ages of2099 115 ma and2036 79 ma with initial nd values of 6 5 2 0 and 9 3 1 5 respectively these ages define multi episode illitization during early diagenesis as suggested by the differentsm nd ratios of the clay fractions the sm nd isotope data of the kerogen source rock up to 13 organic carbon plot between the two isochrons but close to the clay fractions suggesting that this organic matter reached nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids analyses of fracture filling bitumen showed that it had very elevated143nd 144nd and147sm 144nd ratios these values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons this indicates that the bitumen reached nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between sm and nd with an attendant increase in thesm nd ratio the results suggest that the sm nd isotope method has potential for both dating diagenetically related illitization using leachate residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil producing sedimentary basins

karelian shungite an indication of 2 0 ga old metamorphosed oil shale and generation of petroleum geology lithology and geochemistry abstract the ca 2 0 ga old 600 m thick upper zaonezhskaya formation near lake onega nw russia contains unusually high concentrations of corg up to 98 averaging around 25 the formation contains an estimated 25 1010 tonnes of organic carbon accumulated within an area of 9000 km2 organic material is represented by shungite which forms a black dense amorphous or nanocrystalline mass consisting of c with traces of n o s and h autochthonous shungite occurs as disseminated organic material 0 1 50 corg which when mixed with migrated bitumen now pyrobitumen appears as coal like seams and lenses of semilustrous and semimat layer shungite rocks oil shales 50 75 corg the migrated bitumen originally petroleum represented by the lustrous vein and layer shungite conformably fills interbedding spaces or cross cutting joints and usually contains 80 98 corg the shungite bearing rocks of the upper zaonezhskaya formation represent one of the most richest accumulations of organic material reported from the palaeoproterozoic and one of the geologically earliest stages of petroleum generation the sediments of the zaonezhskaya formation were initially deposited in brackish water in a non euxinic lagoonal environment the high c s ratio 8 1000 with a zero intercept on the c s cross plot indicates that deposition occurred in sulphur poor water intensive synchronous volcanism may have contributed to both the enhanced delivery of nutrients and elevated sedimentation rate and eventually to the high degree of preservation of organic material the integrated data suggest that the organic material has a biogenic origin most likely algal or bacterial the organic material suffered complex catagenetic and metamorphic alteration which is reflected in 1 the four modal distribution of corg content with maxima at 5 30 65 and 95 2 highly variable 13 c org 45 to 17 3 bimodal distribution of 13 c org with maxima at 28 and 39 and 4 low h c ratios 0 005 0 2 abundant diagenetic carbonates associated with shungite rocks 13 c carb 5 to 26 and the presence of pyrite 34 s 22 to 31 reflects substantial loss of organic matter via bacterial reduction of sulphate during diagenesis the shungite rocks are characterised by a further substantial loss 50 of biologically produced organic material in the course of thermal maturation and by a depletion in 12 c 10 the isotopic composition of carbonate concretions does not indicate the involvement of fermentative diagenesis conservative estimates give 13corg of 34 as the best value of the initial biomass lustrous vein and layer shungite containing more than 80 corg are considered to be allochthonous migrated bitumen originally petroleum the semilustrous and semimat layer shungite rocks containing 55 75 corg represent oil shales with both migrated bitumen originally petroleum and autochthonous kerogen residues the oil source rocks were apparently hosted in the zaonezhskaya formation the generated oil has migrated both vertically and laterally with the highest concentration in cupola structures the locality at shunga represents the most significant volume of trapped petroleum from the study area

reply to the discussion by denis lacelle on the biogenic origin of needle fibre calcite by cailleau et al 2009 sedimentology 56 1858 1875

early diagenetic alteration of organic matter by sulfate reduction in quaternary sediments from the northeastern arabian sea abstract hemipelagic sediments from the northeastern arabian sea off pakistan were analyzed by geochemical and microscopical methods to study the importance of bacterial degradation processes on the quantity and composition of sedimentary organic matter om on the basis of elemental analysis of total organic carbon toc and total sulfur ts the decay of om by sulfate reducing bacteria were calculated concentrations of total iron reactive iron total nitrogen tn total phosphorus tp and hydrogen indices hi were used to assess organic matter reactivity and redox conditions stable sulfur isotopic compositions provided further information about pathway timing and mechanisms of sulfate reduction microscopical observations indicate that throughout the study area om is amorphous 75 95 in almost all samples with only a small contribution of terrigenous om bacterially mediated sulfate reduction leads to continuously decreasing toc ts ratios from 0 4 m to a burial depth of 2 3 m at this depth the anaerobic oxidation of particulate organic matter ceases sulfate reduction in this narrow interval accounts for the decay of up to 70 of the om which has primarily entered the anoxic domain during the bacterially mediated diagenesis preferential consumption of nitrogen and phosphorus containing organic compounds can be observed and quantified the regeneration of up to 60 of tn and more than 50 of tp can lead to a feedback of nutrients to the seawater this recycling process may contribute to the high productivity in the northeastern arabian sea

a solar irradiance trigger for millennial scale abrupt changes in the southwest monsoon 1 marine sedimentary records of the last glacial from tropical monsoon latitudes indicate climate fluctuations comparable to rapid changes in 18o recorded in greenland synchronizing two high resolution sedimentary records from the oman and pakistan margins we resolve millennial scale reversals in sea surface temperature sst gradient sst across the arabian sea which directly correspond with the majority of dansgaard oeschger d o events for the last 65 kyr the relative amplitude of individual monsoon and d o events appears comparable suggesting coupled and at least hemispheric forcing to explore this quasi cyclic forcing we compare alkenone based geologic data with modern satellite derived sst estimates between sites interstadial conditions fall within the range of monsoons during the holocene but stadial conditions have no analogs following published associations between eurasian winter snow cover ewsc and monsoon rainfall and el nino southern oscillation enso events and anomalous ewsc we find a good correlation between the southern oscillation index soi and arabian sea sst throughout the modern data set an apparent 11 year cyclicity in the soi reveals an association between the monsoon soi and solar output variability the soi primarily tracks solar total irradiance but the soi monsoon linkage becomes nonlinear during excursions of the soi associated with el nino events strong el ninos coincide precisely with minimum solar irradiance during the solar cycle which we attribute to threshold behavior in tropical pacific ssts and associated trade wind strength we propose that both short term interannual decadal and long term centennial millennial changes in solar output are consistent with records of enso variability monsoon intensity and d o event timing

alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the oklo okelobondo and bangombe natural nuclear reaction zones franceville basin gabon abstract large scale light rare earth element lree uranium lead and phosphorus migration has been evidenced in the fa lower proterozoic sandstones of the franceville basin gabon hosting oklo natural nuclear reaction zones rz in relation with extensive accessory mineral alteration by highly saline diagenetic brines 28 7 wt nacl eq to 30 wt cacl2 eq at about 140 c and 1 kbar monazite is the most severely altered accessory mineral in the coarse grained sandstones of the basal fa formation detrital monazite crystals are altered to th oh silicate microcrystalline phase with very low concentrations of u and lree the th la ratio increase from non altered th la 0 27 to altered sandstones th la 1 14 shows that about 76 of the lree was leached this corresponds to the leaching of 2 01 109 metric tons at the scale of the fa formation in the franceville basin similarly the th u increase from monazite th u 18 6 to the th silicate phase th u 88 7 is interpreted as a result of an alteration by oxidizing brines with leaching of u together with lree and p it corresponds to the leaching of 9 06 106 metric tons of uranium this amount of uranium largely exceeds the known uranium reserves from the franceville basin in zircon crystals the cores are generally homogeneous weakly fractured and well preserved as attested by the archean ages 2867 24 and 2865 51 ma obtained by ionic microprobe analysis on zircon of the fa formation respectively from the marginal and central parts of the basin their composition corresponds to the pure end member zr hf sio4 poor in th and u th u 1 at the contrary their rims which present several growth zones with cracks fillings are enriched in ree p th and u with higher th u ratios 5 10 both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings for example in zircon the pb of galena crystals 3 23 wt largely exceeds the amount of pb maximum 0 1 wt that would have been produced in situ by radioactive decay in this mineral nearly all the lead were introduced into altered zones of accessories dissolution of accessory minerals occurred at 2000 ma producing a porous and distorted crystal structure which has allowed a later incorporation of pb galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non radiogenic pb isotopic compositions confirming the external origin of lead pb isotopic evolution models indicate a crystallization age sometime after 1000 ma both for galena located close to and far from u mineralizations and reactor zones which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms between 1000 and 750 ma at the scale of the franceville basin the present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride rich fluids during the diagenesis of a sedimentary basin

mud volcanoes at the front of the makran accretionary complex pakistan abstract conical mounds 1 1 5 km in diameter and up to 65 m high were mapped at the foot of the active makran continental margin the mounds developed seaward of the accretionary front in a relatively planar zone where the beginning of build up of tectonic pressure initiates deformation based on shallow high resolution 4 khz sediment echosounding the sedimentary sequence in this area is generally well stratified as indicated by closely spaced horizontal reflections however in the vicinity of the mounds the sediment is characterised by many acoustically transparent zones which are 100 300 m in diameter and cut near vertically through the horizontal reflectors two sediment cores from the top of the largest cone and a neighbouring acoustically transparent zone reveal small scale post depositional deformation in a stratified sequence and methane concentrations up to 40 000 ng g this deformation and disruption of potential reflectors provides a clue to explain the acoustic transparency we interpret it as caused by the rise of charged fluids and mud leading initially to the slight disturbance of the generally good acoustic reflectors and eventually to the formation of conical mud mounds mud volcanoes mcs data showing a buried mound in an analogous structural position support the idea of tectonically induced mud fluid expulsion seaward of the accretionary front

late holocene primary productivity and sea surface temperature variations in the northeastern arabian sea implications for winter monsoon variability variability in the oceanic environment of the arabian sea region is strongly 34 influenced by the seasonal monsoon cycle of alternating wind directions prominent and well studied is the summer monsoon but much less is known about late holocene changes in winter monsoon strength with winds from the northeast that drive convective mixing and highsurface ocean productivity in the northeastern arabian sea to establish the first high resolution record of winter monsoon variability for the late holocene we analyzed alkenone derived sea surface temperature sst variations and proxies of primary productivity organic carbon and 15n in a well laminated sediment core from the pakistan continental margin increased summer monsoon and weak winter monsoon intensities off pakistan are indicated from 400 b c to 700 a d by reduced productivity and relatively high sst at about 700 a d the intensity of the winter monsoon increased off pakistan as indicated by a trend to lower sst we infer that winter monsoon was still weak from 700 to 1400 a d because primary production did not increase despite decreasing sst declining sst and elevated biological production from 1400 to 1900 a d suggest invigorated convective winter mixing by strengthening winter monsoon circulation most likely a regional expression of colder climate conditions during the little ice age on the northern hemisphere the comparison of winter monsoon intensity with records of summer monsoon intensity suggests that an inverse relationship between summer and winter monsoon strength exists in the asian monsoon system during the late holocene effected by shifts in the intertropical convergence zone

an example of lower proterozoic sediments the francevillian in gabon abstract new mineralogical and isotopic data have been obtained on diagenetic fine fractions of the francevillian sediments together with the rb sr age of a syenitic intrusive and already published isotopic ages determined by means of uranium ore and uranium fission products they allow us to present a more complete geological history of the francevillian the main steps are deposition of the fa formation before 2140 ma deposition of the fb fe formations between 2140 and 2050 ma siliceous diagenesis followed by nuclear reactions between 2050 and 1930 ma carbonate diagenesis at nearly 1870 ma end of cooling at 1700 ma and dolerite intrusion at ca 970 ma the francevillian sediments are to date the oldest true sediments known they are diagenetic lower proterozoic sediments

south asian monsoon climate change and radiocarbon in the arabian sea during early and middle holocene the 1 4 c ages of planktonic foraminifers globigerinoides sacculifer bracketing the younger dryas in a 1 8 o record of globigerinoides ruber from a laminated sediment core on the pakistani continental margin suggest thatsurface reservoir ages in the arabian sea were in excess of 1000 years during the deglaciation a least squares error fit of a detailed 1 4 c chronology to the atmospheric tree ring record gave variable early holocene reservoir ages between 780 and 1120 years well above the prebomb value of 640 years mid holocene reservoir ages are less well constrained but were probably closer to the prebomb value the method used to fit individual core sections to the tree ring record was designed to require only a rough a priori estimate of the time spans which in the core presented here were taken from each section s range of 1 4 c ages a significant 220 year quasi oscillation was present in the 1 8 o record during the early holocene but not thereafter this frequency and amplitude pattern resembles an early holocene 207 to 227 year oscillation previously observed in the atmospheric 1 4 c record which is generally interpreted as reflecting solar irradiance variability an early holocene climate event at 8150 8400 calendar years b p observed elsewhere within the asian east african monsoon system was again found in our record suggesting a reduction in precipitation over nw india and pakistan

microfossiliferous cherty stromatolites in the 2000 ma franceville group gabon abstract cherty stromatolites reported here for the first time have been found in the palaeoproterozoic 2000 ma cover of the chaillu craton in gabon they occur in two different areas probably in the same formation fc which comprises widespread beds of black shales and silexites the buildups are 1 2 m thick biostromes comprising juxtaposed domes of pseudo columnar to columnar stromatolites the biostromes are transgressive over topographic highs marginal to the francevillian basins the environment was a protected shallow marine area with occasional inputs of small quantities of peloidal sediment deposited between the columns the high organic carbon content of francevillian formations fb fc and fd allows us to consider them as source rocks for the bitumen now found in the fa sandstones the stromatolites and their preserved microfossils support the hypothesis that microbial activity was at the origin of bitumen production

role of organic matter in the proterozoic oklo natural fission reactors gabon africa of the sixteen known oklo and the bangombe natural fission reactors hydrothermally altered elastic sedimentary rocks that contain abundant uraninite and authigenic clay minerals reactors 1 to 6 at oklo contain only traces of organic matter but the others are rich in organic substances reactors 7 to 9 are the subjects of this study these organic rich reactors may serve as time tested analogues for anthropogenic nuclear waste containment strategies organic matter helped to concentrate quantities of uranium sufficient to initiate the nuclear chain reactions liquid bitumen was generated from organic matter by hydrothermal reactions during nuclear criticality the bitumen soon became a solid consisting of polycyclic aromatic hydrocarbons and an intimate mixture of cryptocrystalline graphite which enclosed and immobilized uraninite and the fission generated isotopes entrapped in uraninite this mechanism prevented major loss of uranium and fission products from the natural nuclear reactors for 1 2 b y 24 refs 4 figs

natural nuclear fission reactors time constraints for occurrence and their relation to uranium and manganese deposits and to the evolution of the atmosphere abstract knowledge of the formation conditions of francevillian uranium and manganese ore deposits as well as natural fission reactors sheds light on the early evolution of the atmosphere between 1950 and 2150 ma ago the model explaining the formation of the oklo uranium deposits suggests that at the time of sediment deposition in the franceville basin 2150 million years ago the oxygen deficient atmosphere would have inhibited uranium dissolution dissolution of uranium was only possible during later diagenesis approximately 1950 ma reduction reactions in the presence of hydrocarbons allowed precipitation of dissolved uranium to u4 forming deposits with high enough uranium contents to trigger subsequent nuclear fission reactions such a model is in agreement with earlier suggestions that oxygen contents in atmosphere increased during a transition phase some 2450 2100 ma ago the manganese deposits were formed before the uranium deposits during the deposition of the black shales and very early diagenesis and thus at a time when oxygen content in atmosphere was very low carbon isotopes data of organic matter show decrease of 13c upward in the francevillian series 20 to 46 pdb reflecting the high ch4 and low o2 contents in the atmosphere during sediment deposition this favoured anoxic conditions during deposition of the basinal fb black shales and likewise the migration of mn over long distances the manganese precipitated first as mn oxides at the shallow edges of the franceville basin in photic zones where photosynthetic organisms flourished mn oxides were then reduced in the black shales forming mn carbonates when conditions became more reducing during transgression episodes and or the first stages of burial in the black shales reducing conditions prevailed until recent weathering allowing the good preservation of organic matter and the mn deposits the present day alteration is responsible for the dissolution of mn carbonates and precipitation of mn oxides at the water table to form the high grade mn ore 45 50 mn development of photosynthesizing organisms a volcanic source of the mn and favourable palaeogeography of the francevillian basins are all important parameters for the formation of the mn deposits for the occurrence of the natural nuclear reactors the age of 2 0 ga is the main parameter that controls the abundance of fissile 235u and the critical mass before 2 0 ga the 235u 238u ratio was sufficiently high for fission reactions to occur but conditions favourable for forming high grade uranium ores were not achieved then after 2 0 ga the increase of oxygen in the atmosphere commonly led to the formation of high grade uranium ores in which the 235u 238u ratio was too low to support criticality

productivity fluctuations in the southeastern arabian sea during the last 140 ka abstract detailed analyses of multiple geochemical productivity proxies such as calcium carbonate biogenic opal biogenic ba organic carbon and total nitrogen have been carried out on a sediment core from the southeastern arabian sea to trace the palaeoproductivity fluctuations over the last 140 ka high calcium carbonate biogenic opal biogenic ba contents wt and their mass accumulation rates g cm2 ka suggest higher productivity during major interglacials than in the glacials organic carbon content

planktic foraminifera particle flux and oceanic productivity off pakistan ne arabian sea modern analogues and application to the palaeoclimatic record

pbpe systematics on diagenetic clays an example from proterozoic black shales of the franceville basin gabon this study presents the first dating attempt of diagenetic clay material by the pb cpb isotope method pb pb isotopic measurements were performed on authigenic 0 4 m clay fractions illite 1m fe mg chlorite extracted from black shales highly enriched in organic matter of the francevillian lower proterozoic sedimentary series in gabon the clay residues and acid leachates 1 n hcl yield a pb pb isochron age of 2328 130 ma mswd 1 24 even if this age is within error limits identical with two sm nd isochron ages previously reported for the same material it is not considered as a true isochron age the pb isotopic data of the residues indicate that the clay minerals are not in isotopic equilibrium however the isotope data of the leachates separately define an isochron age of 2265 150 ma mswd 0 36 which is closer to the sm nd age it is suggested that the leachates are the most appropriate phases for dating diagenetic processes with pb isotopes the u th pb fractionation in the clay fractions is related to the occurrence of organic matter in the sediments the upb and uth ratios of the clays correlate with their organic carbon contents this fractionation is probably due to differential complexation of u th and pb by organic ligands during the dissolution of detrital mineral phases and clay mineral authigenesis

amino acid biogeochemistry and organic matter degradation state across the pakistan margin oxygen minimum zone to assess whether the oxygen minimum zone omz across the pakistan margin causes differences in the lability of sedimentary organic matter sediments were collected in the core of the omz in the upper and lower transition zones and below the omz sediment samples were analysed for total nitrogen tn and organic carbon oc contents mineral surface area sa and total hydrolysable amino acids thaa and enzymatically hydrolysable amino acids ehaa oc contents and organic carbon per unit of mineral surface area oc sa values were clearly elevated in the core and lower omz transition zone these sediments also contained more labile sedimentary organic matter as discerned by higher concentrations of thaa and the contribution of n in thaa to tn a protein amino acid based degradation index revealed that all sedimentary organic matter has undergone significant degradation but sediments in the upper omz transition zone and below the omz are more degraded than inside the omz changes in amino acid composition during diagenesis are attributed to a combination of factors 1 selective preservation in which amino acids in cell walls are better preserved than amino acids in cell plasma 2 formation and accumulation of bacterially derived organic matter there were relatively more living bacteria in the core of the omz and an accumulation of peptidoglycan derived amino acids in degraded sediments in the upper omz transition zone and below the omz and 3 bacterial transformation as the molar percentages of bacterial transformation products alanine bala amino butyric acid gaba and ornithine orn increased with increasing degradation

natural fission reactors of oklo fourteen natural fission reactors are known to date in three different ore deposits located in the franceville basin the most famous and the first discovered are those in the oklo uranium ore deposit petrographic geochemical and oxygen isotope studies have been performed in an attempt to answer these questions 1 what are the main characteristics of the uranium ores in which fission reactions took place 2 what are the processes that created such exceptionally high grade ores and 3 what are the effects of such fission reactions on the host rocks the ore which hosts the fission reactions at oklo is characterized by a gangue of clay minerals whereas the normal ore of oklo consists of sandstones these clay minerals are the result of hydrothermal alteration of sandstone hydrothermal circulation was induced by the heat produced by fission reactions the reactor ores are further characterized by very high uranium contents 20 60 and high uranium accumulation by means of silica depletion and by uranium enrichment of the normal ore without uranium depletion hydrothermal convective circulation induced by the fission reactions created very high grade ore next to the reactors hydrothermal processes are thus critical to the propagation of the fission reactions

biologically induced accumulations of caco3 in orthox soils of biga ivory coast biologically induced accumulations of calcium carbonate have been found inside orthox soils under and around the native iroko tree milicia excelsa moraceae in biga ivory coast the nature of these accumulations and their origin were studied in two soil profiles directly under the tree and at a distance of 30 cm from the trunk microscale forms of caco3 include 1 wood pseudomorphic structures such as parenchyma cells cellulose fibers and calcitic vessel infillings 2 three types of rhombohedra and 3 needle fiber calcite nfc in addition large scale blocks exhibit three types of textures 1 micritic calcite which seems to be the original material 2 light colored sparite in moldic voids and 3 asymmetrical radiaxial laminated fibrous cement some micritic aggregates and hemi spherulites vaterite were found in the sap on the trunk as well as in soils on silica grains and the wood itself the mineralogy of all these carbonate forms is mainly a stoichiometric calcite or a moderately enriched mg calcite however some samples contain monohydrocalcite as well as two polymorphs of calcium oxalate weddellite and whewellite calcite precipitation is facilitated by the oxidation of oxalate by soil bacteria that contributes to the increase in ph in biga soils this is in contrast to conventional orthox soils therefore three conditions are necessary for biologically induced precipitation of calcium carbonate in orthox soils associated with iroko trees the presence of a large amount of oxalate originating from the tree and fungi the existence of an oxalotrophic flora for oxalate oxidation into carbonate and a dry season

chlorin index a new parameter for organic matter freshness in sediments 1 total chlorins comprising degradation products of chlorophyll have been used recently to reconstruct paleoproductivity from marine sediment cores here we report on a new index the chlorin index ci that proves to be a helpful tool for rapidly estimating organic matter freshness in marine sediments the ci is a ratio between the fluorescence intensity of a sediment extracted with acetone and treated with hydrochloric acid and the original sediment extract it represents the ratio of chlorophyll and its degradation products deposited in the sediments that could still be chemically transformed and those that are inert to chemical attack the ratio is lower in sediments that include freshly deposited phytoplankton material and higher in older more degraded sediments we measured this new parameter on surface sediments and sediments from several short and a long sediment core from different oceanic settings ci values range from 0 2 for chlorophyll a to 0 36 0 56 for fresh material deposited on the shelf off namibia to values around 0 67 in sediments off chile and peru to values up to 0 97 for sediments in a deep core from the northeastern slope of the arabian sea we have compared the ci to rates of bacterial sulfate reduction as a direct measure of organic matter reactivity and to other degradation indices based on amino acid composition we conclude that the ci is a reliable and simple tool for the characterization of organic material freshness in sediments in respect to its degradation state

episodic preservation of pteropod oozes in the deep northeast atlantic ocean climatic change and hydrothermal activity an up to 13 cm thick pteropod ooze covers the ocean floor of the northeast atlantic ocean at five stations between 44 n and 48 30 n at around 25 w at water depths of between 2800 and 3200 m subsurface pteropod horizons occur at least at two different stratigraphic levels pteropod shells both from the surface layers and from the subsurface horizons are fe mn stained with an intensity of staining that increases with age ams 14c ages of single pteropods are between 1700 yrs b p and the present for the surface layer and between 7600 and 3300 yrs b p for the upper horizon in the sediment the mn fe ratios of the pteropod coatings are between 0 24 and 0 40 indicating a hydrothermal source apparently episodic pteropod preservations record pulses of hydrothermally derived elevated mn and fe concentrations we conclude that pteropod preservation is the combined result of episodic climatically induced non dissolution and subsequent hydrothermally induced protective fe mn coating

sampling the oxygen minimum zone off pakistan glacial interglacial variations of anoxia and productivity preliminary results sonne 90 cruise during cruise sonne 90 the well developed oxygen minimum zone omz in the northeastern arabian sea was sampled in great detail to study surface ocean productivity and organic carbon accumulation preservation during the last hundred to hundred thousands of years nearly anoxic conditions on and below the seafloor are indicated by the lack of macrobenthos and nekton absence of bioturbation high marine organic carbon accumulation and preservation of laminated sediments between about 300 and 1000 m water depth on the steep continental slope this pattern approximately coincides with the zone of extremely low o2 concentrations presently observed in the water column accumulation of organic material on the deeper slope near the lower boundary of the omz is enhanced by downslope transport downcore we observe a complex alternation of dark colored laminated and light colored homogeneous facies in general laminated organic carbon rich intervals were deposited under suboxic bottom water and enhanced productivity conditions mainly during warm stages e g bolling allerod and holocene from 8000 yrs b p to recent the homogeneous bioturbated intervals indicate normal bottom water oxygen levels and reduced productivity during or just after cold stages e g terminations ia and b and younger dryas the striking facies similarities of the sedimentary record in cores collected from different areas and water depths suggest a unique pattern of late pleistocene corg accumulation in the northeastern arabian sea which depends mainly on regional surface ocean productivity and intermediate water oxygenation however the sedimentary record is also influenced by variations in sediment supply from the makran and karachi shelf and from indus river discharge during rapid sea level changes

organic matter preservation and sulfur uptake in sediments from the continental margin off pakistan abstract the bulk and molecular geochemical characteristics of a suite of kerogens isolated from organic matter rich sediments from the pakistan margin has been characterised by elemental analysis rock eval pyrolysis and pyrolysis gas chromatography py gc special attention was given to the geochemistry of alkylthiophenes which are thought to reflect the incorporation of sulfur into organic matter om in the context of early diagenetic sulfate reduction organically bound sulfur comprises up to 60 of total sedimentary sulfur the weight ratios for organic sulfur s org over organic carbon c org varies from 0 01 in some of the recently deposited sediments to 0 11 in older samples the results show that sulfur incorporation into organic matter occurs primarily within the upper few meters of the sediments the amount of sulfur which can be incorporated into the organic matter clearly depends on the quality of om the hydrogen index values hi which characterise the total amount of generated hydrocarbon equivalents of the pyrolysate in mg hc equiv g c org are related to the extent of reworking of organic matter either in the water column or at the sediment water interface or to remineralization processes by sulfate reduction itself which is strongly controlled by the availability of reactive iron high quality organic matter characterised by elevated hi values is capable of sequestering higher amounts of sulfur as expressed by the thiophene content which represent the organically bound sulfur of the pyrolysate in mg g c org as indicated by the ratio of thiophenes versus hi which characterises the proportion of thiophenes in the total pyrolysate sulfurisation of organic matter seems to be terminated at a ratio of 0 01

a 43 kyr record of protist communities and their response to oxygen minimum zone variability in the northeastern arabian sea abstract an extensive oxygen minimum zone omz occurs in the northeastern ne arabian sea where sedimentary records show evidence of alternating strong and weak omzs that correlate with north atlantic climate variability during the last glacial interglacial cycle omzs are expanding world wide but information on long term omz ecosystem interactions is mainly limited to fossilized species notably foraminifera here we provide a first comprehensive ancient sedimentary dna record of both fossilizing and non fossilizing protists and their response to omz variability in the ne arabian sea over the last 43 ka protist communities changed significantly during strong vs weak omz conditions coincident with interstadials and stadials respectively dinoflagellates were identified as significant indicator taxa for strong omzs during glacial as well as interglacial interstadials whereas diatoms were significant indicators for strong omzs only during glacial interstadials the chlorophyte chlorella was found to be the main phototrophic protist in nutrient depleted surface waters during glacial stadials notably strong omz conditions shaped past protist communities by creating isolated habitats for those capable of sustaining oxygen depletion either by adapting a parasitic life cycle e g apicomplexans or by establishing mutualistic connections with others e g radiolarians and mixotrophic dinoflagellates or by forming cysts e g colpodeans notably a long term increase in eutrophication and a decrease in the diatom dinoflagellate ratio was observed during the late holocene favoring the pelagic component of the marine food web a similar scenario could be expected in the context of predicted worldwide expansion of coastal omzs associated with global climate change

a first look at factors affecting aragonite compensation depth in the eastern arabian sea abstract water column measurements suggest shoaling of aragonite saturation depths asd throughout the world oceans due to increase in greenhouse gas concentration past records of aragonite saturation state under different climatic conditions are required to assess the impact of climatic changes on shoaling deepening of asd the preservation state of organisms having aragonite skeletons is used to assess the past changes in aragonite saturation depths with respect to the modern asd here for the first time we delineate and discuss the factors that affect the modern aragonite compensation depth acd in the eastern arabian sea by using pteropod abundance in the surface sediments a total of 78 spade core top samples collected along seven latitudinal transects covering the continental shelf slope and abyssal region of the eastern arabian sea were used pteropods were picked from coarse fraction 63 m based on the pteropod preservation we report that in the eastern arabian sea acd lies at a water depth of 525 m which matches with the chemically defined aragonite saturation depth we further report that the acd shoals from north to south the zone of high pteropod abundance coincides with low corg the increase in pteropod abundance in the outer shelf region coincides with the drop in dissolved oxygen concentration the deeper limit of pteropod abundance lies in the center of the oxygen minimum zone with higher corg therefore we suggest that the pteropod abundance in the eastern arabian sea is not always related with the lower dissolved oxygen but is strongly influenced by corg this first report of the pteropod based aragonite compensation depth estimates from the eastern arabian sea will help in assessing future changes in acd under the influence of anthropogenic green house gas emissions

nature of organic matter in surface sediments from the pakistan continental margin and the deep arabian sea amino acids the role of organic carbon input sedimentation rate and bottom water oxygenation bwo for organic matter om degradation was investigated using surface sediments collected from above within and below the oxygen minimum zone omz within varying sedimentary regimes along the pakistan continental margin for this samples from 47 box cores and multi cores were analyzed for their bulk component organic carbon total nitrogen carbonate opal and amino acid concentrations the results are compared to plankton net samples from the arabian sea sinking particles collected at the pakistan continental margin cultured sedimentary deep sea bacteria and data from the deep arabian sea basins high organic carbon and amino acid concentrations were found in the laminated sediments of the omz in bioturbated sediments immediately below the omz and below regions of high productivity in the western arabian sea the contribution of amino acids to organic carbon and total nitrogen as well as the molar contribution of certain non protein amino acids do not mirror these differences in oc contents no clear trend in relation to sediment structure or water depth is observed in contrast to these established indicators of om degradation the molar contribution of the aromatic amino acid tyrosine tyr exhibits a distinct pattern its contribution is relatively high in sediments at depths above the omz and further increases towards the center of the omz below the omz it decreases continuously with increasing water depth in addition for samples from similar depths and bottom water oxygen contents tyr shows systematic differences between varying sedimentary regimes the lack of a distinct trend for the established indicators does not support the idea of a predominant control of bwo on quantitative om preservation nevertheless the variability of tyr in relation to water depth or the sedimentary regime suggests that i the productivity related oc input ii the bulk accumulation rate and iii bwo influence the alteration of the sedimentary om by controlling its oxygen exposure time

petrography and paragenesis of organic matter associated with the natural fission reactors at oklo republic of gabon a preliminary report sixteen known uranium rich pockets in the sediment hosted uranium ore deposits in the oklo area became nuclear fission reactors 1968 50 ma ago and operated as such for up to 1 ma nuclear criticality was caused by unique and fortuitous geological environments and events these included the localized high concentrations of uranium in small pockets in the ore bodies the fact that the relative abundance of the fissile 235u isotope was five times greater in uranium 2 ga ago than present the presence of water which acted as a moderator in the natural reactors and the absence of neutron poisons which are elements that can prevent nuclear chain reactions organic matter is present in all of the oklo natural reactors but it is abundant only in reactors 7 to 16 organic matter has been studied in natural reactors 7 to 9 and at locations at various distances from these reactors bitumen in the oklo reactors is the predominant organic phase it is now solid bitumen containing dispersed cryptocrystalline graphite in crystal domains smaller than optical microscopic size its immediate precursor was liquid bitumen generated from syngenetic kerogen in organic rich sedimentary rocks in the francevillian basin of gabon and in the natural reactors themselves by hydrous reaction mechanisms prior to during and after nuclear criticality a preliminary paragenesis of organic matter at oklo may be defined through the following sequence of events a protokerogen and kerogen evolve from abundant cyanobacteria in the tidal and deltaic sediments in the basal fa formation of the francevillian series during sedimentation and burial respectively b during subsequent subsidence of the basin the first generation of bitumen and petroleum occurred c during uplift the sedimentary strata were fractured and bitumen and petroleum migrated through fractures following this oxidizing aqueous solutions carrying uranyl ions migrated through the highly fractured rocks met and were reduced by organic matter and precipitated pitchblende or uraninite at the present sites of the oklo uranium ore deposits and natural reactors the natural reactors reached nuclear criticality they were heated and produced the second generation of bitumen through hydrous reactions from the by now solid first generation bitumen and remaining kerogen finally hydrothermal effects associated with the dolerite dike swarm intrusion 750 150 ma ago produced yet another localized generation of liquid bitumen from solid organic matter possibly other bitumen generations also occurred during and following this time interval the organic matter in the natural reactors soon became a solid and was able to prevent the loss of uraninite grains enclosed in it these uraninite grains trapped and then immobilized fission products until and to an extent even after a major tectonic igneous event the intrusion of the dolerite dike swarm

late archean initiation of manganese metallogenesis its significance and environmental controls abstract manganese metallogenesis was initiated in a modest scale during the late archean time selectively in india and brazil sedimentary deposits occur in both greenstone belts and high grade granulite terranes in shallow water shelf regimes only and are often intimately associated with stromatolites the changes in the rates of continental crustal growth mantle heat flux and outgassing along with the evolution of tectonic regimes sedimentary environments the state of the biosphere and finally the compositions of interacting atmosphere hydrosphere system from the early to the late archean time have been traced as interdependent parameters and interpreted to explain the first appearance of sedimentary manganese deposits promoted by possibly localized oxygenated environments compared to the spectacular manganese event represented by the giant paleoproterozoic kalahari deposit south africa which is interpreted to have been formed as a consequence of the rapid increase in atmospheric oxygen level ca 2 5 2 4 ga ago the modest initiation of the late archean manganese depositions was triggered by localized photosynthetic oxygen supply forming basin margin oxygen oases as a prologue for the succeeding stratified ocean system the dissolved manganese in the basin water that was oxidized to form shelf type shallow water deposits is suspected by analogy with contemporary banded iron formations bifs but not proved to be of hydrothermal ancestry the initiation of concentrated manganese sedimentation during the late archean by itself provides an evidence for an increase in the oxygen content in the contemporary atmosphere hydrosphere system compared to that in the earlier period supporting the geochemical indications from paleoweathering profiles

decadal resolution record of winter monsoon intensity over the last two millennia from planktic foraminiferal assemblages in the northeastern arabian sea the indian monsoon system is an important climate feature of the northern indian ocean small variations of the wind and precipitation patterns have fundamental influence on the societal agricultu

organic content and preservational controls in surficial shelf and slope sediments from the arabian sea pakistan margin abstract in the arabian sea organic rich deposits occur on the continental slope at depths roughly coincident with the impingement of an intense water column oxygen minimum zone omz this relationship has often been taken as causal and such deposits are often proposed as modern analogues of organic rich facies from the palaeorecord however there are numerous other potential preservational controls on continental margins and their relative importance remains unclear and much debated we report the results of organic inorganic and stable c isotope analyses on surficial sediments from three transects of the pakistan margin ranging in depth from well above to well below the omz organic carbon c org and total nitrogen n total contents show expected mid slope maxima although these do not mirror bottom water dissolved o 2 profiles maximal values are observed near the lower boundary of the omz or even in homogenous sediments below it together lignin yields and 13 c org values indicate a general paucity of terrigenous organic matter om this information combined with c org n total relationships suggests that the om contents of all sediments are dictated by differing accumulation of a predominant marine fraction while there are no obvious redox related differences in c org n total or om hydrogen richness across the margin there are clear minima in 13 c org within the omz in apparent contradiction with maximal accumulation of marine om the latter phenomenon is ascribed either to local source effects such as variable algal isotopic signatures or to differences in om preservation state possibly linked to bottom water redox conditions this remains to be clarified but along with the rough correspondence between mid slope sedimentary c org maxima and bottom water o 2 minima may indicate that o 2 availability is an important factor in determining sedimentary om content on this margin however evidence from redox sensitive indicators i c org mn al for persistent accumulation of om rich deposits under oxidizing conditions in homogeneous sediments below the omz clearly indicates that o 2 availability is not the sole or over riding control on om distributions

carbonaceous substances associated with the paleoproterozoic natural nuclear fission reactors of oklo gabon paragenesis thermal maturation and carbon isotopic and trace element compositions field relationships textural and reflectance characteristics and carbon isotopic compositions indicate that at least eight different categories of carbonaceous substances cs occur in the black shales uranium ores and fossil nuclear fission reactors of oklo the categories include kerogens of the mikouloungou coal in the fa formation the c1 pelites of the fa formation the black shale of the fb formation and bitumens present in veinlets in the basal black shale of the fb formation in the fa formation in normal ore and in hydraulic fractures in and near the fossil reactors and in late dolerite dikes bitumen migration occurred at oklo during at least three intervals 1 ca 2 0 ga ago when petroleum liquids derivatives of which are preserved as solid bitumen were generated by thermal maturation of black shale kerogen 2 ca 1 968 50 ma ago when 15 or more pockets of uranium ore achieved criticality and 3 977 to 981 27 ma ago when dolerite dikes intruded radionuclide migration occurred during intervals 2 and 3 bitumenization trends established for organic matter which has undergone thermal maturation fail to discriminate between heat affected and normally matured cs at oklo in and near oklo reactors where several types of solid bitumen commonly occur in close association carbon of the youngest low reflectance bitumen is isotopically heavier than that of older high reflectance bitumen due to thermal cracking this process together with enhanced radiolysis of bitumen in and near the reactors have combined to increase its 13c avg 25 8 compared to that of bitumen samples in normal ore avg 35 5 distant from the reactors graphitized bitumen inhibited migration of 235u and some fission products in uraninite of the natural reactors however alkaline earth elements such as ba sr and mo and alkaline elements rb and cs not compatible in the uo2 crystalline structure have migrated in hydrothermal solutions and in the once fluid oklo bitumens in situ analysis of solid bitumens by laser ablation icp ms detects differences in trace element composition developed as a result of their varying geological histories and evolutionary trends

new arabian sea records help decipher orbital timing of indo asian monsoon abstract a recent study suggested that indian monsoonal proxies commonly used in the arabian sea in general productivity proxies could be impacted by changes in the atlantic overturning rate amoc throughout a control on the nutrient delivery into the euphotic zone this oceanic mechanism could lead to a misunderstanding between the indian summer monsoon sm and orbital forcing and could confuse a direct comparison with other archives derived from other monsoonal sub systems such as east asian or african records here we analyze three independent proxies bromine foraminifera assemblages and grain size extracted from a marine sediment core md04 2861 covering the last 310 ka and retrieved in the northern arabian sea near the makran margin an area influenced by summer and winter indian monsoon the grain size proxy deals with the regional continental climate through fluvial and eolian processes it cannot be linked to changes in nutrient content of amoc and present the same phase relationship timing than the other sm proxies this demonstrates that the productivity signals bromine in the northern arabian sea are mainly controlled by sm dynamics and not amoc modulated nutrients at orbital scale changes we thus build a multi proxy record of sm variability i e sm stack using statistical tools principal component analysis further compiled on an age model constructed independently from orbital tuning we find that strong sm lag by 9 1 ka the nh summer insolation maximum minimum of precession june 21 perihelion and obliquity maximum in the precession band and by 6 1 3 ka in the obliquity band these results are consistent with previous studies based on marine and terrestrial records in both indian and asian regions except asian speleothems our study supports the hypothesis that internal climate forcing decreased ice volume together with the increase of latent heat export from the southern indian ocean set the timing of strong indo asian summer monsoons within both the precession and obliquity cycle the external forcing direct sensible heating initiate monsoonal circulation strong indian winter monsoon wm occurs between ice maxima and northern hemisphere sensible heat minima indicating that both act to strengthen wm circulation the summer and winter monsoons are in antiphase in the precession band suggesting that the two systems are dynamically linked

the lycopane c31 n alkane ratio as a proxy to assess palaeoxicity during sediment deposition abstract recently two of us schulte et al earth planet sci lett 173 1999 205 221 advocated the use of the c 35 c 31 n alkane ratio to reconstruct past variations in the intensity of the oxygen minimum zone omz in the northeastern arabian sea new analyses of arabian sea surface sediments and re analysis of previously investigated sediments indicate however that the marked increase of the c 35 c 31 n alkane ratio in surface sediments from within the omz is due to enhanced preservation of lycopane which on some stationary phases in gas chromatography co elutes with the n c 35 alkane under anoxic conditions re examination of published data farrington et al org geochem 13 1988 607 617 on surface sediments from the peru upwelling region also revealed a marked preservation of lycopane and thus a higher abundance relative to terrestrial n alkanes under anoxic conditions within the omz thus the lycopane c 31 n alkane ratio can be interpreted as an oxicity proxy an overview of literature data indicates that lycopane is indeed often abundant in sediments which were deposited under anoxic conditions the carbon isotopic composition of lycopane and its response to the perturbation of the global carbon cycle during the cenomanian turonian oceanic anoxic event suggests that lycopane is derived from a marine photoautotroph although its biological source has yet to be identified

coccolithophores in surface sediments of the arabian sea in relation to environmental gradients in surface waters abstract one hundred surface sediment samples of the arabian sea indian ocean were investigated and relative abundances of coccoliths were compared to mean annual gradients of temperature salinity chlorophyll po4 and mixed layer depth total coccolith concentrations ranged from 42 106 g sediment in coastal areas to more than 19 000 106 g sediment in oceanic regions the general distribution does not seem to be dependent on coccolithophore productivity in surface waters alone but also on the diluting input of terrigenous material a total of 27 taxa were identified the main species dominating the assemblages were gephyrocapsa oceanica emiliania huxleyi and florisphaera profunda with a combined average abundance of more than 70 several species and species groups reflect with their distribution the environmental parameters of the overlying water masses and may be successfully used to improve palaeoclimatic reconstructions e g a f profunda exhibits a high similarity or even positive correlation to the mean annual mixed layer depth b calciosolenids can be described as coastal or shelf species while temperature and salinity gradients do not seem to be crucial for coccolithophores in this region the mean mixed layer depth as well as the po4 concentration representative for total nutrient availability may control in part the coccolithophore assemblages according to the results of a cluster analysis and the distribution pattern of all species it was possible to differentiate three main coccolithophore assemblages a g oceanica dominated assemblage mainly occurs in the northern part of the study area and can be described as high nutrient assemblage the second assemblage dominated by f profunda may be typical for oligotrophic and stable conditions in open ocean waters a third assemblage with high amounts of coastal species characterises coastal conditions on the shelves

variations of oxygen minimum and primary productivity recorded in sediments of the arabian sea abstract two deep sea sediment cores from the northeastern and the southeastern arabian sea were studied in order to reconstruct the palaeoenvironments of the past glacial cycles core 136kl was recovered from the high productivity area off pakistan within the modern oxygen minimum zone omz by contrast modern primary productivity at the site of md900963 close to maldives is moderate and bottom waters are today well oxygenated for both cores we reconstructed the changes in palaeoproductivity using a set of biomarkers alkenones dinosterol and brassicasterol the main result is that primary productivity is enhanced during glacial stages and lowered during interstadials the proxies associated with productivity show a 23 kyr cyclicity corresponding to the precession related insolation cycle palaeoredox conditions were studied in both cores using a new organic geochemical parameter c35 c31 n alkane ratio developed by analysing surface sediments from a transect across the omz off pakistan the value of this ratio in core 136kl shows many variations during the last 65 kyr indicating that the omz was not stable during this time it disappeared completely during heinrich and the younger dryas events pointing to a connection between global oceanic circulation and the stability of the omz the c35 c31 ratio determined in sediments of core md900963 shows that bottom waters remained rather well oxygenated over the last 330 kyr which is confirmed by comparison with authigenic metal concentrations in the same sediments a zonally averaged circulation biogeochemical ocean model was used to explore how the intermediate indian ocean responds to a freshwater flux anomaly at the surface of the north atlantic as suggested by the geochemical time series both the abundance of southern ocean water and the oxygen concentration are significantly increased in response to this freshwater perturbation

hydrothermal and supergene clays of the oklo natural reactors conditions of radionuclide release migration and retention abstract mineralogical chemical and o isotopic analyses were performed on the clay matrices surrounding two natural fission reactors of the oklo deposit in the deeply preserved reactor 10 450 m clays are composed of hydrothermal mg chlorites and sudoites al chlorites the wide range of their o isotopic compositions from 0 0 to 15 1 smow is consistent with crystallisation during conditions of criticality when the reactors created a thermal gradient of 100 c m the temperature in the core of reactor is calculated to have been approximately 400 c in contrast in the shallow reactor 9 120 m deep vermiculitized al chlorites are the products of the weathering of the hydrothermal chlorites these vermiculitized al chlorites are 18 o enriched 18 o 18 smow and underwent interaction with surface fluids 2 to 5 149 sm 147 sm isotopic ratios of the clays of reactor 9 show a wide dispersion 2 3 m of the fissiogenic sm around the reactor the migration of the fissiogenic sm which is usually retained in the deep reactors is here related to the circulation of meteoric water under oxidising conditions

organic matter preservation on the pakistan continental margin as revealed by biomarker geochemistry in order to assess whether the oxygen minimum zone omz in the arabian sea has an eaect on the preservation and composition of organic matter in surface sediments we investigated samples from three diaerent transects on the pakistan continental margin across the omz in addition to determining the total amount of organic carbon toc we analyzed the extractable lipids by gas chromatography combined gas chromatography mass spectrometry and compound specific stable carbon isotope measurements the extractable lipids are dominated by marine organic matter as indicated by the abundance of lipids typical of marine biota and by the bulk and molecular isotopic composition sediments from within the omz are enriched in organic carbon and in several extractable lipids i e phytol n alcohols total sterols n c35 alkane relative to stations above and below this zone other lipid concentrations such as those of total n fatty acids and total n alkanes fail to show any relation to the omz only a weak correlation of toc with mineral surface area was found in sediments deposited within the omz in contrast sediments from outside the omz do not show any relationship between toc and surface area among the extractable lipids only the n alkane concentration is highly correlated with surface area in sediments from the hab and makran transects in sediments from outside the omz the phytol and sterol concentrations are also weakly correlated with mineral surface area the depositional environment of the indus fan oaers the best conditions for an enhanced preservation of organic matter the omz together with the undisturbed sedimentation at moderate rates seems to be mainly responsible for the high toc values in this area overall the type of organic matter and its lability toward oxic degradation the mineral surface area the mineral composition and possibly the secondary productivity by sedimentary bacteria also appear to have an influence on organic matter accumulation and composition 2000 elsevier science ltd all rights reserved

abrupt intensification of the sw indian ocean monsoon during the last deglaciation constraints from th pa and he isotopes abstract sediments from western arabian sea core 74kl representing the last 23 ka were analyzed for helium thorium and protactinium isotopes assuming global average fluxes of extraterrestrial 3he and 230th the average 3he derived sediment mass accumulation rate mar is a factor of 1 8 higher than the average 230th derived mar 3he and 230th derived mars converge however during the younger dryas yd and during the peak of the early holocene humid interval these features not seen anywhere else in the world probably reflect a combination of climate driven changes in the flux of 230th and 3he ratios of xs231pa xs230th proxies of paleoproductivity are lowest during the last glacial maximum lgm and increase abruptly during the bolling allerod later following a sudden decrease to near lgm values during the yd they rise abruptly to maximum values for the entire record in the early holocene we hypothesize that low xs231pa xs230th ratios reflect low productivity due to the decreased intensity of the sw monsoon whereas the opposite is true for high ratios the correlation between arabian sea productivity and monsoonal upwelling on the one hand and north atlantic climate variability on the other suggests a linkage between high and low latitude climates caused by changing patterns of atmospheric circulation

a 5000 yr record of climate change in varved sediments from the oxygen minimum zone off pakistan northeastern arabian sea the upper holocene marine section from a kasten core taken from the oxygen minimum zone off karachi pakistan at water depth 700 m contains continuously laminated sediments with a sedimentation rate of 1 2 mm yr and a unique record of monsoonal climatic variability covering the past 5000 years our chronostratigraphy is based on varve counts verified by conventional and ams 14 c dating individual hemipelagic varve couplets are about 0 8 1 5 mm thick with light colored terrigenous laminae a deposited mainly during the winter monsoon alternating with dark colored laminae b rich in marine organic matter coccoliths and fish debris that reflect deposition during the high productivity season of the late summer monsoon august october precipitation and river runoff appear to control varve thickness and turbidite frequency we infer that precipitation decreased in the river watershed indicated by thinning varves after 3500 4000 yr b p this is about the time of increasing aridification in the near east and middle east as documented by decreasing nile river runoff data and lake level lowstands between turkey and northwestern india this precipitation pattern continued until today with precipitation minima about 2200 1900 yr b p 1000 yr b p and in the late middle ages 700 400 yr b p and precipitation maxima in the intervening periods as documented by spectral analysis the thickness of varve couplets responds to the average length of a 250 yr cycle a 125 yr cycle the gleissberg cycle of solar activity 95 yr and a 56 yr cycle of unknown origin higher frequency cycles are also present at 45 39 29 31 and 14 yr the sedimentary gray value also shows strong variability in the 55 yr band plus a 31 yr cycle because high frequency cyclicity in the enso band ca 3 5 and 5 yr is only weakly expressed our data do not support a straightforward interaction of the pacific enso with the monsoon driven climate system of the arabian sea

structural and metamorphic evolution of the gabon orogenic belt collision tectonics in the lower proterozoic abstract the archaean granite gneiss terrain of central gabon comprises medium to high grade gneisses metamorphosed at 3 15 ga forming the monts cristal and underlying the mitzic region intruded by and enclosed as enclaves in granitoids 3 0 2 6 ga old that form the extensive chaillu and north gabon blocks these rocks are unconformably overlain in the east of gabon by early proterozoic francevillian sedimentary rocks between 2 3 and 2 0 ga old forming the franceville and booue basins the francevillian is overthrust on its west side by the ogooue metamorphics themselves considered to be of early proterozoic age these rocks crop out in a broad synclinorium consisting essentially of paragneisses with orthogneisses appearing locally at the base the overall tectonic setting is discussed in terms of the evolution of the various domains from west to east 1 1 on the west flank of the synclinorium is a migmatitic granulitic dome the abamie dome in which structures diverge away from the core metamorphism is of high temperature intermediate to low pressure type 2 2 on the east flank of the ogooue synclinorium the medium grade ogooue metamorphics sillimanite biotite kyanite staurolite staurolite muscovite biotite have been thrust eastwards on to the low grade kyanite sericite francevillian 3 3 the francevillian is parautochthonous being widely detached from its archaean basement owing to the effects of the same eastward directed tangential tectonics 4 4 locally slices of archaean basement have been tectonically incorporated the structural cross section illustrates a history of diapiric uprise passing eastward into tangential tectonics that is typical of classical collision belts the suture marking the collision is interpreted to be located in the abamie migmatite dome where the nappes are rooted the gabon orogenic belt is correlated with the collision belts of about the same age on the eastern margin of the congo craton and with the trans amazonian orogenic belt of the guyana shield

coccolithophore fluxes from sediment traps in the northeastern arabian sea off pakistan abstract we investigated coccolithophore fluxes from four sediment traps located in the northeastern arabian sea at the continental slope of pakistan two moorings wpt and ept western isastern pakomin trap consisting of one and two sediment traps were deployed at water depths of about 500 m and 1500 m they collected settling particles from october 1993 to february 1994 and from may 1995 to february 1996 at intervals of 22 and 24 days respectively a longer sampling was prevented probably due to high sedimentation events which occluded the trap cones both areas were characterised by a strong seasonality in coccolith fluxes high fluxes were recorded with up to 2000 10 6 coccoliths m 2 d 1 in the early sw summer monsoon and up to 5400 10 6 coccoliths m 2 d 1 in the early ne winter monsoon generally coccosphere fluxes exhibited a similar trend but were some orders of magnitude lower than coccolith fluxes twenty seven taxa were identified in this study all well to moderately well preserved and with a small range of diversity the species composition was dominated by gephyrocapsa oceanica florisphaera profunda and emiliania huxleyi making up about 70 of the assemblages during the sw summer monsoon high productivity in the surface waters of the eastern arabian sea may be linked to the upwelling system off oman apparently nutrient rich surface water derived from the southwestern upwelling areas off oman with high standing stocks were carried into this area by northeast flowing currents in contrast during the ne winter monsoon the locally observed high coccolithophore production is hypothesised to respond to local injections of nutrient rich deep water into the surface water due to sea surface cooling leading to convection consequently fluxes during the winter monsoon were in the same range as during the summer monsoon

authigenic carbonates derived from oxidized methane vented from the makran accretionary prism off pakistan we report the first discovery and sampling of a methane hydrogen sulfide rich cold seep from the makran accretionary prism off pakistan arabian sea a variety of cm to m scale pockmarks and gas seepage structures were identified from underwater tv photo sled profiles crossing the oxygen minimum zone omz related to highresolution seismic parasound records from the seeps isotopically light bacterially formed methane is emanating that is partly oxidized to hco3 in the bacterial sulfate reduction zone this results in the precipitation of irregular dark gray to black crusts of indurated authigenic carbonates mainly cryptocrystalline magnesian calcite and ca rich dolomite near the sediment seawater interface within the omz downcore the crusts grade into hemipelagic carbonate poor silty clays with transitional lithologies in between similar to the pockmark carbonates from the oregon margin and in the north sea the authigenic carbonates are extremely depleted in 13c 13ccarbonate 40 this suggests that they were derived from bacterial methane 13cmethane 77 8 that was oxidized under anaerobic conditions the authigenic carbonates are associated with white fluffy mats of chemoautotrophic h2s oxidizing bacteria small pockmarks appear to be concentrated at small scale escarpments suggesting focussed fault controlled pore fluid expulsion due to the tectonic dewatering and degassing of the accretionary prism whereas diffuse discharge of pore fluids is inferred from the widespread occurrence of tiny gas bubble tubes

varves in marine sediments a review the global compilation of reported marine varved sedimentary records throughout the quaternary contains 52 sites marine varve deposition and preservation typically depend on environmental and sedimentological conditions such as a sufficiently high sedimentation rate severe depletion of dissolved oxygen in bottom water to exclude bioturbation by macrobenthos and a seasonally varying sedimentary input to yield a recognizable rhythmic varve pattern additional oceanographic factors include the strength and depth range of the oxygen minimum zone omz and regional anthropogenic eutrophication from point sources such as large polluted rivers quaternary marine varves are not only found in those parts of the open ocean that comply with these conditions but also in fjords embayments and estuaries with thermohaline density stratification and nearshore marine lakes with strong hydrologic connections to ocean water this review critically describes settings and sedimentological characteristics of marine sites where varves have been reported broader applications of marine varve records are discussed for example in terms of radiocarbon calibration on high resolution time scales constraining paleoceanographic variability and global change teleconnections diagnosing factors affecting modern fish population dynamics based on past performance detailing biogeochemical cycles and deciphering complex factors influencing marine productivity varves in saline water bodies without hydrologic connection to the global ocean are not included in this review and instead pertain to the companion review on lacustrine varves where also general and fundamental principles of varve deposition classification and dating are covered zolitschka et al 2015 the postulated occurrence of varves in pre quaternary rocks is critically discussed in this review with representative examples in the case of non evaporitic laminations in fine grained ancient marine rocks such as banded iron formations and black shales laminations may not be varves but instead may have multiple alternative origins such as event beds or formation via bottom currents that transport and sort silt sized particles clay floccules and organic mineral aggregates in the form of migrating bedload ripples

carbon and nitrogen elemental and stable isotopic compositions of surficial sediments from the pakistan margin of the arabian sea abstract the factors controlling sedimentary organic matter distributions are the subject of ongoing debate we report here the carbon and nitrogen contents and stable isotopic compositions for surface sediments and downcore profiles from sites ranging in water depth from 140 to 3200 m located across the indus margin of the arabian sea the sites spanned the intense midwater oxygen minimum zone omz and showed dramatic differences in bottom water redox conditions benthic communities and degrees of bioturbation five primary stations were studied at 140 300 940 1200 and 1850 m water depth with sampling in march may intermonsoon and august october late to postmonsoon 2003 high resolution sampling was also conducted across the upper 140 275 m and lower omz 600 1200 m boundaries the sampling strategy was selected to help clarify the roles of oxygen availability and other potential environmental factors in determining sediment organic matter content and composition the extent of organic matter cycling within the sediments under contrasting depositional conditions and the fate of organic matter deposited during the southwest monsoon as previously noted on this and other omz margins there was a roughly inverse spatial relationship between bottom water oxygen and sedimentary organic matter concentrations however variability in organic carbon content within the omz that was unrelated to oxygen and the fact that maximal organic carbon contents occurred under relatively oxidizing conditions towards the lower omz boundary rather than at the omz core indicate that factors other than oxygen contribute significantly as controls on carbon distributions despite a 3 fold range in organic carbon content carbon to nitrogen ratios showed no systematic trends across the omz with values reflecting uniform predominantly marine organic matter inputs nitrogen isotopic signatures 15n values in the range of 6 5 8 5 also did not show clear cross omz trends and are attributed to a 14n depleted nitrate pool created by pelagic denitrification there was little downcore change in organic matter content at any site suggesting either that organic matter decay occurs largely in the water column or that decay within the sediments at oxygenated sites is masked by bioturbation seasonal variability in organic matter content and composition was restricted to the most surficial sediments and changes were ephemeral suggesting rapid turnover of organic matter deposited during the southwest monsoon on timescales of weeks stable c isotopic compositions trended towards more negative values in surficial sediments at hypoxic sites within the omz core this is attributed to a chemosynthetic bacterial imprint

holocene monsoon and sea level related changes of sedimentation in the northeastern arabian sea abstract the indian monsoon and the westerlies strongly influence the sedimentation in the northeastern arabian sea by impacting rainfall and erosion on land and on biogeochemical processes in the ocean to disentangle the terrestrial and oceanic processes we analysed mineralogical and bulk geochemical components of a holocene sediment core offshore pakistan endmember modelling of grain sizes and principal component analyses pca of major and trace elements identify the origin of sediments and their dominant mode of transport sedimentation processes during the early holocene 10 8 8 2 ka bp were influenced by the post glacial sea level rise and orbitally forced strengthening of the indian summer monsoon ism and westerlies this led to a shift from rather terrestrial dominated towards a marine dominated sedimentation whereas the fluvial source shifted from the makran rivers to the hab river near karachi during the mid holocene 8 2 4 2 ka bp a combination of weakening ism and southward displacement of the itcz enhanced the influence of the westerlies together decreasing river discharges and enhancing aeolian input probably from the sistan basin region this trend continued during the last ca 4 ka when the increasing aridification of the hab river catchment further increased the aeolian inputs solar and lunar driven short term variations as well as bond events known from the north atlantic ocean superpose these trends they lead to a pronounced increase of fluvial inputs between 8 6 8 4 ka bp and at ca 3 ka bp as well as to dry events around 4 2 ka and 1 2 1 ka bp our study highlights the increasing influence of the westerlies on the sedimentation processes in the northeastern arabian sea towards the late holocene

organic matter preservation through the oxygen deficient zone of the ne arabian sea as discerned by organic carbon mineral surface area ratios abstract it is debated whether there is a causal or even correlative relationship between the presence of a minimum in bottom water oxygen bwo concentration and the preservation of organic matter in underlying marine sediments one relationship that has not been examined in this regard is the relationship between sedimentary organic carbon content and mineral surface area normalization of organic matter content to mineral surface area eliminates the mass based problems associated with traditional weight percent carbon measurements and thus may allow better evaluation of carbon preservation we measured organic carbon to mineral surface area ratios oc sa on 26 sediment samples from along the ne edge of the arabian sea and on 21 hydrodynamically sorted fractions using split flow lateral transport thin splitt fractionation from three of these stations samples spanned from above to well below the minimum in local bwo concentration sediments deposited under the oxygen minimum had oc sa ratios in excess of 1 1 mg oc m 2 indicative of enhanced preservation a bioturbated sediment at the edge of the oxygen minimum bwo 35 m also had a high oc sa ratio with the exception of the anomalous sample along the slope all samples from shallower and deeper than the oxygen depleted water mass bwo 35 m have oc sa ratios that fall within the typically observed range 0 5 1 1 mg oc m 2 while further work will be necessary to discern what causes this relationship our data indicate that organic matter preservation as estimated by oc sa is enhanced within the general locale of the bwo minimum in ne arabian sea sediments if this relationship proves to be robust repeatable and stable over time in other modern bwo minimum zones surface area normalized organic carbon loadings may be a useful paleoceanographic proxy for past bwo levels

detrital zircon and provenance analysis of late cretaceous miocene onshore iranian makran strata implications for the tectonic setting a multidisciplinary provenance study including sandstone framework heavy mineral analysis in situ u pb dating of detrital zircon and hf isotopic ratio analysis of dated zircons was undertaken on late cretaceous miocene deep marine turbiditic and deltaic sandstones of makran accretionary wedge se iran to determine their sedimentary provenance and tectonic setting sandstone framework modes reveal both magmatic arc rocks as a source of late cretaceous oligocene detritus and recycling of miocene sandstones heavy mineral assemblages cr spinel and blue amphibole indicate ophiolite and high pressure low temperature metamorphic rocks blueschists as a supplementary provenance in total 2931 laser ablation inductively coupled plasma mass spectrometry icp ms u pb detrital zircon ages on 21 sandstone samples yielded three major age peaks at ca 167 ma 88 7 ma and 48 9 ma also 241 in situ hf isotope analyses of dated zircons provide evidence for dominantly igneous source rocks two main detrital zircon ages are identified 1 abundant middle jurassic grains with hf isotopic compositions of continental crust suggesting a rifting related magmatic provenance and 2 late cretaceous eocene grains with hf isotopic compositions of continental crust and nondepleted mantle suggesting a continental magmatic arc provenance this change in provenance is attributed to the late cretaceous convergence between arabia and eurasia

links between palaeoproterozoic palaeogeography and rise and decline of stromatolites fennoscandian shield through a review of literature and new data we have documented two major events in the palaeoproterozoic history of stromatolites as indicated by palaeontological and palaeoenvironmental studies with a time resolution of between 40 and 200 ma we confirm semikhatov and raaben s and awramik s albeit approximately maximum in diversity and abundance of stromatolites between 2330 and 2060 ma ago jatulian diversification we suggest that this taxonomic diversity was driven by a major phase of cratonisation formation of the karelian carbonate platform and numerous rift related shallow water carbonate basins supersaturated with ca 2 mg 2 and co2 the jatulian stromatolite explosion is synchronised with a positive 13ccarb shift of jatulian age carbonates we also document stromatolite decline which occurred on the fennoscandian shield somewhere between 2060 and 1900 ma ago this decline both in abundance and in taxonomic diversity is interpreted as having been caused by the first phase of oceanisation the oceanisation led to the considerable reduction in ecological niches that could be utilised by cyanobacteria the post jatulian decline of stromatolites coincides with an abrupt downward 13corg shift from 19 to 38 and is roughly coeval with the appearance of the first eukaryotic algae documented elsewhere the systematics of the fennoscandian diversity of palaeoproterozoic stromatolites is identical to that reported from india and china and reveals a dissimilarity with abundance and diversity patterns in australia and northern america

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

benthic biological and biogeochemical patterns and processes across an oxygen minimum zone pakistan margin ne arabian sea abstract oxygen minimum zones omzs impinging on continental margins present sharp gradients ideal for testing environmental factors thought to influence c cycling and other benthic processes and for identifying the roles that biota play in these processes here we introduce the objectives and initial results of a multinational research program designed to address the influences of water depth the omz 150 1300 m and organic matter om availability on benthic communities and processes across the pakistan margin of the arabian sea hydrologic sediment and faunal characterizations were combined with in situ and shipboard experiments to quantify and compare biogeochemical processes and fluxes om burial efficiency and the contributions of benthic communities across the omz in this introductory paper we briefly review previous related work in the arabian sea building the rationale for integrative biogeochemical and ecological process studies this is followed by a summary of individual volume contributions and a brief synthesis of results five primary stations were studied at 140 300 940 1200 and 1850 m water depth with sampling in march may intermonsoon and august october late to postmonsoon 2003 taken together the contributed papers demonstrate distinct cross margin gradients not only in oxygenation and sediment om content but in benthic community structure and function including microbial processes the extent of bioturbation and faunal roles in c cycling hydrographic studies demonstrated changes in the intensity and extent of the omz during the sw monsoon with a shoaling of the upper omz boundary that engulfed the previously oxygenated 140 m site oxygen profiling and microbial process rate determinations demonstrated dramatic differences in oxygen penetration and consumption across the margin and in the relative importance of anaerobic processes but surprisingly little seasonal change a broad maximum in sediment om content occurred on the upper slope roughly coincident with the omz but the otherwise poor correlation with bottom water oxygen concentrations indicated that other factors are important in determining sediment om distributions downcore profiles generally showed little clear evidence of in situ om alteration and there was little sign of om enrichment resulting from the southwest monsoon in sediments collected in the late to postmonsoon sampling this is interpreted to be due to rapid cycling of labile om organic geochemical studies confirmed that sediment om is overwhelmingly of marine origin across the margin but also that it is heavily altered with only small changes in degradation state across the omz more negative stable c isotopic compositions in surficial sediments at hypoxic sites within the omz core are attributed to a chemosynthetic bacterial imprint dramatic changes in benthic community structure occurred across the lower omz transition apparently related to om availability and quality as well as to do concentrations high resolution sampling biomarkers and isotope tracer studies revealed that oxygen availability appears to exert threshold type controls on benthic community structure and early faunal c processing biomarker studies also provided evidence of faunal influence on sediment om composition together the results offer strong evidence that benthic fauna at sites across the margin play important roles in the early cycling of sediment om through differential feeding and bioturbation activities

natural fission reactors in the franceville basin gabon a review of the conditions and results of a critical event in a geologic system abstract natural nuclear fission reactors are only known in two uranium deposits in the world the oklo and bangombedeposits of the franceville basin gabon since 1982 five new reactor zones have been discovered in these deposits and studied since 1989 in a cooperative european program new geological mineralogical and geochemical studies have been carried out in order to understand the behavior of the actinides and fission products which have been stored in a geological environment for more than 2 0 ga years the franceville basin and the uranium deposits remained geologically stable over a long period of time therefore the sites of oklo and bangombeare well preserved for the reactors two main periods of actinide and radionuclides migration have been observed during the criticality under p t conditions of 300 bars and 400 500 c respectively and during a distention event which affected the franceville basin 800 to 900 ma ago and which was responsible for the intrusion of dolerite dikes close to the reactors new isotopic analyses on uranium dioxides clays and phosphates allow us to determine their respective importance for the retention of fission products the uo 2 matrix appears to be efficient at retaining most actinides and fission products such as rees y and zr but not the volatile fission products cd cs xe and kr nor rb sr and ba some fissiogenic elements such as mo tc ru rh pd and te could have formed metallic and oxide inclusion in the uo 2 matrix which are similar to those observed in artificial spent fuel clays and phosphate minerals also appear to have played a role in the retention of fissiogenic rees and also of pu

a critical evaluation of depositional parameters controlling the variability of organic carbon in arabian sea sediments abstract organic carbon distribution in the surficial sediments of the arabian sea to some extent mirrors the surface productivity being both high in the peripheral portions and low in the central region suggesting that the productivity is a primary source of organic matter however a critical examination reveals that the organic enrichment on the slope of the indian margin is two to four higher max 16 71 than on the slope of the arabian peninsula max 7 54 while the productivity in the former region is three to four times lower than the latter this observation suggests that the influence of productivity on organic enrichment is of secondary importance high concentrations 4 of organic carbon exactly coincide with the oxygen minimum zone 150 1500 m water depth evidently reflecting the crucial role played by the anoxic bottom waters in the preservation of organic carbon incidently the organic rich band along the western indian slope is wide long and highly concentrated as compared to that of the slope of the arabian peninsula and no organic enrichment is found on other continental slopes of the arabian sea although an equally intense oxygen minima impinge on the floor of the slope such a variability of organic carbon on the slopes is attributed to various geological parameters such as texture of sediments rates of fine terrigenous sediment deposition shelf width slope gradient bottom currents and adsorption capacity of individual clay minerals all these observations suggest that the productivity is not an ultimate control for the organic enrichment of bottom sediments but it is the bottom water anoxia in conjunction with various depositional parameters which determine the degree of preservation

bioturbation geochemistry and geotechnics of sediments affected by the oxygen minimum zone on the oman continental slope and abyssal plain arabian sea abstract we investigate the way the oxygen minimum zone omz alters interactions between bioturbation and sediment geochemistry and geotechnical properties sediments are compared within and below the omz on the oman continental slope and adjacent abyssal plain during the post monsoonal autumn season quantitative measurements were made of eh and ph of total organic matter tom and carbonate of water content and shear strength and of bioturbation structures in vertical profiles of subcores taken from spade box core samples the omz stations had distinctively low redox conditions and high carbonate content and different geotechnical properties and different bioturbation structures than stations below the omz on the abyssal plain these differences are related to the degree of anoxia and to water depth within the omz eh ph and carbonate increased with water depth and tom and water content decreased we also noted the presence of subsurface sediment heterogeneity on the continental slope within the omz in the omz eh water content and bioturbation decreased with increasing sediment depth there was a slight decrease in ph in the top 5 cm at all stations shear strength nearly always increased with increasing sediment depth at each water depth correlations show down core trends in these parameters while across all water depths correlations were significant at deeper sediment depths 20 30 cm an eh ph diagram identified two water depth groupings 391 1008 and 1265 3396 m cluster analysis showed the upper and lower sediment depths form separate clusters the break occurring at 4 7 5 cm while there are also distinct clusters related to water depth we relate our results to bottom water oxygen concentrations reported by other investigators and to regional scale geochemical processes

characterization of organic matter associated with uranium deposits in the francevillian formation of gabon lower proterozoic abstract elemental analysis organic petrography and high resolution transmission electron microscopy were used to study organic matter in lower proterozoic rocks of the francevillian series in africa results show a convincing relationship between solid bitumens derived from thermal alteration of crude oil and deposition of uraninite ores evidence is presented that suggests the presence of migration paths for crude oil in associated sandstones moreover the solid bitumens appear to have been further altered by radiation damage as a consequence of oxidation and uranium mineralization

influence of marginal highs on the accumulation of organic carbon along the continental slope off western india abstract twenty five surficial sediment samples were collected from the central western continental margin of india which consists of diverse physiographic entities continental slope shelf margin basin marginal high and the deep arabian basin at depths between 250 and 2050 m the sediments were studied to assess the relative importance of productivity oxygen minima grain size and bathymetry on organic carbon accumulation preservation the total organic carbon toc content in the study area varies from 0 70 to 5 86 highest values of organic carbon are recorded on the marginal highs 5 12 5 86 followed by shelf margin basin 3 53 4 22 and the continental slope 1 80 3 84 the organic carbon content is relatively low in the arabian basin 0 98 1 46 the sediments from marginal highs are characterized by high oc contents relative to upper slope at equivalent water depth and also the deeper shelf margin basin the elevated organic carbon concentrations on marginal high and upper slope are associated with coarse grained biogenic sediments while those in the shelf margin basin with fine grained silty clays the enrichment of organic carbon on the marginal highs is attributed to productivity related biogenic sediments and their faster burial it is surmised that diverse topographic features in the slope and the associated hydrodynamic processes also play an important role in the enrichment of organic carbon

the neodymium isotope system as a tool for petroleum exploration abstract mineralogical petrological geochemical and neodymium isotopic investigations strongly suggest isotopic and chemical exchange between authigenic silicate phases and petroleum alteration of organic matter sulfur reduction generation of aqueous acidic solutions with the resultant dissolution of k feldspar leads to in situ crystallization of illite and chlorite as well as neodymium isotopic equilibration between the silicate phases and petroleum these findings are of significance not only to understand the mechanisms leading to neodymium isotopic equilibration between authigenic clay minerals in detrital sediments but also to source rock petroleum in reservoir correlations in particularly when biomarker molecules i e isoprenoid hydrocarbons have been obliterated by prolonged severe diagenesis

the biogenic origin of needle fibre calcite needle fibre calcite is one of the most ubiquitous habits of calcite in vadose environments caves deposits soil pores etc its origin either through inorganic indirect or direct biological processes has long been debated in this study investigations at 11 sites in europe africa and central america support arguments for its biogenic origin the wide range of needle morphologies is the result of a gradual evolution of the simplest type a rod this rod is the elementary brick which by aggregation and welding builds more complex needles the absence of cross welded needles implies that they are welded in a mould or under a longitudinal and unidirectional constraint before being released inside the soil pores the difference between the lengthening of the needles and the c axis can be explained by the existence of needles observed under a scanning electron microscope in organic sleeves which can act as a mould during rod growth complex morphologies with epitaxial outgrowths on straight rods cannot have grown entirely inside organic microtubes they must result from soil diagenesis after the release of straight rods in a soil free medium whisker crystals are interpreted as the result of growth and coalescence of euhedral crystals on a rod rhomb chains are considered to be the consequence of successive epitaxial growth steps on a needle during variations in growth conditions isotopic signatures for needle fibre calcite vary from 16 63 to 1 10 and from 8 63 to 2 25 for 13c and 18o respectively the absence of high 18o values for needle fibre calcite precludes a purely physicochemical origin evaporative for this particular habit of calcite as epitaxial growth cannot precipitate in the same conditions as initial needles needle fibre calcite stable isotopic signatures should be used with caution as a proxy for palaeoenvironmental reconstructions in addition it is suggested that the term needle fibre calcite should be kept for the original biogenic form the other habit should be referred to as epitaxial forms of needle fibre calcite

evolution and variability of the asian monsoon system state of the art and outstanding issues the asian monsoon is comprised of the indian and east asian subsystems these two components are linked to one another in varying degrees by regions of strong sensible heating indo asian landmass and strong latent heat export the western pacific warm pool and the southern subtropical indian ocean variability within the indian and east asian subsystems interactions among them and the extent to which they interact with other climate phenomena e g enso are current topics of modern and paleoclimate research this work provides an overview of past and current paleomonsoon research on tectonic to interannual time scales with a primary focus on marine sediment records while past and current work has contributed greatly to our understanding of paleomonsoon variability at all time scales additional efforts are required to make further progress in two critical areas 1 additional efforts are needed in terms of proxy development and evaluation requiring concerted efforts at long term sediment trap deployments in key monsoon influenced regions as well as development of adequate and widely available core top databases these are necessary to assess the impact of modern oceanographic and seafloor processes on potential monsoon proxies 2 additional effort is also needed in acquiring a sufficient geographic distribution of downcore records to assess linkages among the subsystems and their role in the context of extratropical climate change these records should have high sedimentation rates including varved sections this requires substantial survey support to identify the most appropriate coring and drilling targets c 2004 elsevier ltd all rights reserved